Wolfgang Pietsch Agglomeration Processes Phenomena, Technologies, Equipment
Wolfgang Pietsch
Agglomeration Processes...
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Wolfgang Pietsch Agglomeration Processes Phenomena, Technologies, Equipment
Wolfgang Pietsch
Agglomeration Processes Pheno mena, Tech no Iogies, Eq uipme nt
Dr.-/ng.Wolkang Pietsch, EUR INC
COMPACTCONSULT, INC. 2614 N. Tamiami Trail, #520 Naples, Florida 34103-4409 USA In Europe: Holzweg 127 67098 Bad Durkheim, Germany
This book was carefully produced. Nevertheless, author and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.:
Applied for.
Cover illustration
British Library Cataloguingin-Publication Data:
Like an agglomerate, the picture on the cover is composed of many disparate components, all of which relate to the topics discussed in this book. The panels on the left and right are microphotographs of naturally agglomerated nano-particles The top and the bottom panels depict different products from spray drying and fluid bed agglomeration. The four sectors (between the panels and the circle) represent Scanning Electron Micrographs (SEMs) of agglomerate structures as well as photographs of coated agglomerates and of granules. The top half of the circle shows products from tumble/growth agglomeration and the lower half are briquettes from roller presses as well as product from compaction/granulation. The center square includes tablets from punch and in die presses. The originals of the individual pictures from which sections are reproduced were supplied by (in alphabetical order): Albemarle Corp., Baton Rouge, LA, USA: Cabot Corp., Tuscola, IL, USA: Eirich, Hardheim, Germany: Euragglo, Qievrkhain, France; Niro A/S, Soeborg. Denmark; Norchem Concrete Products, Inc., Fort Pierce, FL, USA: Koppern GmbH & Co, KG, Hattingen, Germany. Their support is appreciated and acknowledged
A catalogue record for this book is available from the British Library. Die Deutsche Bibliothek - CIP Cataloguingin-Publication Data:
A catalogue record for this publication is available from Die Deutsche Bibliothek.
0 Wiley-VCH Verlag GmbH, Weinheim, 2002 Printed on acid-free paper. All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form - by photoprinting, microfilm, or any other means - nor transmitted or translated into machine language without written permission from the publishers. In this publication, even without specific indication, use of registered names, trademarks, etc., and reference to patents or utility models does not imply that such names or any such information are exempt from the relevant protective laws and regulations and, therefore, free for general use, nor does mention of suppliers or of particular commercial products constitute endorsement or recommendation for use. Mittenveger 6 Partner Kommunikationsgesellschaft mbH, Plankstadt Printing betz-druck GmbH. Darmstadt Bookbinding Grogbuchbinderei J. Schaffer GmbH & Co. KG, Crunstadt
Composition
Printed in the Federal Republic of Germany. ISBN 3-527-30369-3
I"
Contents Dedication, Acknowledgements and References
VII
1
Introduction
2
A Short History o f Agglomeration
3
Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science 5
4
Glossary o f Agglomeration Terms
5
Agglomeration Theories
5.1 5.1.1 5.1.2 5.2 5.2.1 5.2.2 5.3 5.3.1 5.3.2 5.4 5.5
The Development of Strength of Agglomerates 32 Binding Mechanisms 35 Binders, Lubricants, and Other Additives 42 Estimation of Agglomerate Strength 55 Theoretical Considerations 55 Laboratory and Industrial Evaluations 61 Structure of Agglomerates 76 General Considerations 78 Porosity and Techniques That Influence Porosity 89 Other Characteristics of Agglomerates 100 Undesired and Desired Agglomeration 109
6
Agglomeration Technologies 133
7
Tumble/Growth Agglomeration
7.1 7.2 7.3 7.4 7.4.1 7.4.2
Mechanisms of Tumble/Growth Agglomeration 140 Kinetics of Tumble/Growth Agglomeration 144 Post-treatment Methods 150 TumblelGrowth Agglomeration Technologies 151 Disc and Drum Agglomerators 153 Mixer Agglomerators 164
1
3
11
29
139
VI
I
Contents
7.4.3 7.4.4 7.4.5 7.4.6
Spray Dryers 187 Fluidized Bed Agglomerators 196 Other Low Density Tumble/Growth Agglomerators Agglomeration in Liquid Suspensions 221
8
Pressure Agglomeration
8.1 8.2 8.3 8.4 8.4.1 8.4.2 8.4.3 8.4.4
Mechanisms of Pressure Agglomeration 231 Structure of Pressure Agglomerates 236 Post-treatment Methods 241 Pressure Agglomeration Technologies 252 Low-Pressure Agglomeration 253 Medium-Pressure Agglomeration/Pelleting 266 High-pressure Agglomeration 300 Isostatic Pressing 373
9
Agglomeration by Heat/Sintering 385
9.1 9.2 9.2.1 9.2.2
Mechanisms of Sintering 385 Sintering Technologies 389 Batch Sintering 390 Continuous Sintering 397
10
Special Technologies Using the Binding Mechanisms of Agglomeration 409
10.1 10.2 10.2.1 10.2.2 10.3
Coating 415 Separation Technologies 440 Gas/Solid Separation 440 Liquid/Solid Separation 442 Fiber Technologies 447
11
Engineering Criteria, Development, and Plant Design 453
11.1 11.2 11.3
Preselection of the Most Suitable Agglomeration Process for a Specific Task 462 Laboratory Equipment, Testing, and Scale-Up 468 Peripheral Equipment 492
12
Outlook
13
Bibliography 525
13.1 13.2 13.3 13.4
List of Books or Major Chapters on Agglomeration and Related Subject 526 References 530 Author’s Biography, Patents, and Publications 531 Tables of Contents of Related Books by the Author 541
14
Indexes 543
14.1 14.2 14.3
List of Vendors 543 Wordfinder Index 580 Subject Index 591
212
229
507
I
Dedication, Acknowledgements and References When this book was first planned, the idea was to combine in one volume concise descriptions of the agglomeration phenomena, technologies, equipment, and systems as well as a compilation of the applications of agglomeration techniques in industry. The latter was intended to demonstrate the widespread natural, mostly undesired occurrences of the phenomena, including possibilities to avoid them, and discuss the varied old, conventional, and new beneficial uses of the technologies. However, it soon became obvious that, in its entirety, this project became too voluminous and required much more time than anticipated. Therefore, it was decided to split the subject’s presentation into two volumes whereby both books will be “stand alone” publications that are also complementary. The first volume, available here, covers the fundamental phenomena that define agglomeration as well as the industrial technologies and equipment for the size enlargement by agglomeration. Applications are mentioned in a general way throughout the text of this presentation but without going into details. These applications will be presented in a separate book entitled “Agglomeration Technologies - Industrial Applications” that is scheduled for publication in 2003. A preliminary table of contents is given in Section 13.4. Many persons, institutions, and companies have contributed to the two volumes of this book. First and foremost, I wish to thank my wife Hannelore for her support and understanding while, thorughout my professional career, I was compiling various papers and books (see Section 13.3).All are dedicated to her. Without my wife’s active participation in preparing almost all publications, in elaborating the textbook entitled “Size Enlargement by Agglomeration” [B.42],which is a major reference for this publication (see also below), and her, if sometimes reluctant, acceptance that I was not available for long hours on many days during almost four decades, the books, in particular, could not have been completed. It is impossible to acknowledge all the help, extensive and small, that was provided by a large number of individuals and companies. In Section 14.1, a list ofvendors and other organizations is compiled which mentions those who have, in one way or another, contributed as well as some others who may be of interest as potential contacts for the readers of this book. While I have decided not to clutter the text with references, sources have been acknowledged if figures or tables were provided by or are based
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Vlll
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Dedication, Acknowledgements and References
on information from particular companies. The Discialmer at the beginning of this book (see page IV) should be referred to when using such cross-references. Regarding references to literature, Chapter 13 should be consulted. The earlier textbook “Size Enlargement by Agglomeration” [B.42] contains treatments as well as many references relation to the developing science of the unit operation and covers in some detail the sizing of and scale-up methods for agglomeration equipment. Since the emphasis of the new book is on practical considerations and industrial applications, not theory, the earlier book “Size Enlargements by Agglomeration” (Wiley, 1991) should always also be referred to. Information on the availability of reprints is available at the beginning of Section 13.1 and as a footnote later in the same Section. Since Size Enlargement by Agglomeration is one of the unit operations of Mechanical Process Technology (see Chapter 1) and, for the design and construction of agglomeration systems of any kind, many or all of the other unit operations are required, together with the associated transport and storage technologies, often even in multiplicity, and the analytical methods are applied for process evaluation and control, the reader who is interested in the topic of this book should also learn about or have access to information on the other fields of Mechanical Process Technology. This is also emphasized in Chapter 13. At this point I wish to acknowledge two books of general importance to which I have contributed chapters on agglomeration and ofwhich major portions were included in this book. They are: “Handbook of Powder Science and Technology” M. E. Fayed, L. Otten (eds,), 1st ed., Van Nostrand Reinhold Co., New York, NY (1983) and 2nd ed., Chapman & Hall, New York, NY (1997). Source references can be found in [B.21] and [B.56], Section 13.1. Finally, I like to mention with gratitude the following individuals who, as professionals and experts in their own fields, are or have been colleagues and/or partners in several continuing education courses over many years in the USA as well as in Europe. They have agreed that statements during their presentations and the elaborations for their course notes can be used directly, adopted, or modified for this book. They are, in alphabetical order: T. van Doorslaer, W. E. Engelleitner, M. E. Fayed, M. Gursch, D. C. Hicks, S. Jagnow, R. H. Leaver, R. Lobe, K. Masters, S. Mortensen, H. B. Ries, F. V. Shaw, J. Storm, R. Wicke, and R. Zisselmar. For additional references and acknowledgements please refer to Sections 13.1 and 13.2.
Naples, November 2001
Wolfgang Pietsch
Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
1
Introduction
In 1957, under the leadership of Professor Dr.-Ing. Hans Rumpf at the Technical University (TH) of Karlsruhe, Germany, Mechanical Process Technology or Particle Technology [B.11] was first introduced as a field of science in its own right. It comprises the interdisciplinary treatment of all activities for the investigation, processing, and handling of solid particles as well as the interactions of such particulate solids. Four unit operations and associated techniques were defined (Fig. 1.1).Other common English names for this field of science, which was quickly adopted around the world, are Mechanical Process Engineering, Powder Technology, or Powder & Bulk Solids Technology. Size enlargement by agglomeration is the generic term for that unit operation of mechanical process engineering which is characterized by “combination with change in particle size” (Fig. 1.1).The author of this book had the privilege to become one of the first assistants of Professor Rumpf. For several years he was responsible for the research and development of size enlargement by agglomeration at the Institute of
Fig. 1.1 Unit operations and associated techniques o f Mechanical Process Technology
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7 Introduction
Mechanical Process Engineering and earned his PhD with a doctoral thesis on specific aspects of a binding mechanism [1.1] of agglomeration. Webster’s Unabridged Third New International Dictionary [1.2] defines the verb agglomerate as: “to gather into a mass or cluster; to collect or come together in a mass; to collect into a ball, heap, or mass, specifically: clustered or growing together but not coherent”, and the noun agglomerate as: “a cluster of disparate elements; an indiscriminately (= randomly) formed mass”. A technical dictionary [ 1.31 defines agglomeration as: “sticking or balling of (often very fine) powder particles due to short range physical forces. Therefore these forces become active only if the individual particles (forming the agglomerate) are brought closely together by external effects”. These definitions distinguish the term size enlargement by agglomeration from the more general size enlargement such that particle growth occurring, for example, during crystallization or the production of particulate solids by melt solidification are not part of this unit operation of Mechanical Process Technology.
Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
2
A Short History of Agglomeration As a basic physical efect, agglomeration has existed since particulate solids were first formed on Earth. Binding mechanisms between solid particles cause the stability of wet and dry soil and (often under the influence of heat and pressure) participate in the development of rock formations. Sandstone is the most easily recognized “agglomerate”. Agglomeration as a phenomenon, e.g. the natural caking and build-up of particulate solids, must have been observed and has been used by higher developed organisms and later by humans since prehistoric times. Sea creatures covered themselves with protective coats, birds as well as other animals built nests, and humanoids formed artificial stones, all from various solids, sand, clay and different binders that were often secreted by the creature itself. As a “tool” to improve powder characteristics, agglomeration was used by ancient “doctors” in producing pills from medicinal powders and a binder (e.g. honey) or by food preparers during the making of bread from flour whereby the inherent starchy components act as binder. In spite of this long “history”, agglomeration as a technology is only about 150 years old today (excluding small scale pharmaceutical and some little-known ancient, mostly Chinese applications as well as brick and bread making). Agglomeration as a unit operation, defined within solids processing, started around the middle of the nineteenth century as a method to recover and use coal fines. Agglomeration as a science is very young. It began in the 1950s with the formal definition of the binding mechanisms of agglomeration, interdisciplinary collection of knowledge relating to all aspects of agglomeration, and fundamental research which was no longer application oriented [B.42]. At approximately that time, the first recurring series of professional meetings were organized which were exclusively devoted to the science and technology of agglomeration (International Briquetting Association (IBA), - today Institute for Briquetting and Agglomeration (IBA) -, beginning in 1949 with biennial meetings and proceedings; International Symposia on Agglomeration, initiated in 1962 with proceedings, (see also Section 13.1)). Since that time, agglomeration science, technology, and use have experienced rapid growth but still without finding a corresponding awareness at institutions of higher learning and in the technical or process engineering communities. This book is the second by the author on the general subject of size enlargement by agglomeration. While frequently referring to fundamentals and specifics which are
4
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2 A Short History ofAgglomeration
covered in more detail in the first book [B.42],this new text tries to provide an updated, comprehensive summary of the state-of-the-art of agglomeration, its basics, technologies, and applications, at the beginning of the 21st century.
Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science
As mentioned in the previous chapter, size enlargement of particulate solids by agglomeration is as old as the existence of solids themselves. Originally, agglomeration happened naturally during the development of soil, stone, and rock formations. Later, unwanted agglomeration occurred during handling and storage of particulate matter particularly when hygroscopic and/or soluble materials (such as salt) “set-up”into lumps or large, more or less solidified masses. In the animal world agglomeration was used to develop protective coatings (e.g. many marine worms, Fig. 3.1), to build nests (e.g. swallows, termites, Fig. 3.2), and to provide a nourishing and protective environment for the offsprings (e.g. dung beetles, Fig. 3.3). Humans most probably first used agglomeration during the making of bread by taking flour (= particulate solids including an inherent binder, starch) and liquid additives (= additional binder for plasticity and “green”bonding), mixing and forming the mass, and, finally, “curing”the product, the removal of much of the moisture that was added during the mixing and agglomeration steps, to obtain structure and permanent bonding during baking. The technology of bread making combines all com-
Fig. 3.1 Protective agglomerated coating of a Rhizopod, a creping marine Protozoan (Difflugia urceolata)
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3 Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science
Fig. 3.2a Nest of swallows made by agglomeration from mud, the bird’s saliva as a binder, and organic fibers for strengthening
Fig. 3.2b Nest of termites made from earth as well as the animal’s excrements and/or secretions as binder
3 Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science 17
Fig. 3.3 Dung beetle, Scarabaeus Sacer, “pelletizing” d u n g
ponents of a complex agglomeration process including preparation of solid feed particles by milling (= adjustment of particle size and activation of the inherent binder, starch), mixing of particulate solids with additional binder(s),forming the mass into a “green”agglomerate, and a “post-treatment” (curing = baking =heating and cooling) to provide strength and texture, Very early it was also found that the porosity of the final product could be modified (= increased) by making use of gases that are produced during fermentation (initiated by sour dough or yeast) and result in bubbles in the green mass. These voids are stabilized by “strengthening” the bread during post-treatment (baking). For the construction of permanent shelter, humans may have observed the activities of animals that formed nests and protective “walls” from wet clay which hardened during drying (Fig. 3.2). By copying this behavior, wet clay, which was soon reinforced and made more water resistant by mixing-in straw or other fibrous material, was filled into a framework of wood branches and let harden during natural drying. To make building activities independent of the location of clay “mines”, during prehistoric times bricks were already produced from clay and sand and, after hardening, transported to building sites. Since fire was known for providing heat, the accidental “firing” of a piece of clay most probably resulted in the adaptation of an improved posttreatment that yielded waterproof bricks for areas where rock was not easily available, thus allowing the development of villages and, during the 4th millenium B.C. in Mesopotamia, cities with large brick structures. By experience, humans learned that certain natural materials helped cure specific illnesses. Minerals as well as dried animal and plant matter were ground to powder and “formulated” to yield medicines. Since powders cannot be easily consumed orally, natural binders, such as honey which, incidentally, also masked the unpleasant taste of
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3 Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science
some of the medicinal components, were mixed with the powder and the resulting plastic mass was rolled by hand into little balls (= pills). The sticky binder(s) caused pills to adhere to each other; therefore, it was soon found that coating the pills by rolling them in flour or pollen solved this problem (see also Section 10.1). These three, well known ancient agglomeration techniques were used with little change through the ages of human history. Many other, lesser known and somewhat more recent processes could be added. However, it is not the objective here to produce a history book. Rather, these examples relating to three major modern “industries”, food, building materials, and health products, were selected to show that humans always lived with and used agglomeration. As a result, agglomeration technologies as all the other unit operations and associated techniques of Mechanical Process Technology (see Fig. 1.1)were considered to be “normal activities” which, with the beginning of industrialization in the 18th and 19th centuries A.D., were merely mechanized by simulating what was done manually before. During these early modernization efforts it was not considered necessary to question the fundamentals of the processes and “improvements” were based on empirical developments. Until very recently, agglomeration technologies had been developed independently in the particular industries in which they were applied. Because the process requirements are fundamentally different in such unlike industries handling, for example, coal and ores on one hand or food and pharmaceuticals on the other, no interdisciplinary contact and exchange of information took place. In fact, although agglomeration techniques developed along similar lines, application related “theories” were defined which were derived from investigations of specific requirements and their solutions together with a terminology that was often incomprehensible and, therefore, not useable by the “agglomeration expert” of another industry (see Chapter 4). Agglomeration as a science began when an effort was made to interdisciplinarily combine the extensive knowledge that had been accumulated during sometimes hundreds of years in specific fields of human activities. This approach showed that (in alphabetical order, not indicating importance): Baking: A thermal post-treatment process, does not only induce the development of final bonding, structure, and consistency in bread but produces similar characteristics during the heat curing of any “green” agglomerate. Briquetting: Is not predominantly a technique for the enlargement of coal fines for beneficial use but equipment which was specifically developed for that application is also suitable for such diverse uses as, for example, the briquetting of salt for the regeneration of water softeners, the briquetting of flaked DMT to decrease the bulk volume and improve handling and shipping, the briquetting of frozen vegetable pulps to be used as rations for field kitchens, the hot briquetting of sponge iron to reduce this commodity’s reactivity and allow open handling and storage, or the production of fertilizer spikes and the manufacturing of artificial fireplace logs. Coating: Is not only suitable for the modification of surface characteristics or the control of dispersion and dissolution of medicinal specialties but also to achieve similar properties in agrochemicals as well as human and animal foods, among others.
3 Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science 19
Compacting: Is not only applicable for the production of “green” bricks or other ceramic bodies prior to firing but finds many uses in powder metallurgy or for the production of battery cathodes, etc. Granulating: Is not primarily a method to improve flowability of powders and formulations in the pharmaceutical industry but also in the fertilizer and bulk chemical industries as well as for carbon black, silica fume, and many other materials. Instantizing: As an example of a relatively new agglomertion process, is not limited to applications in the food industry for easily dissolvable drink and soup mixtures but is equally important for pigments, insectizides, fungizides, and many more. Pelleting: Originally developed for the shaping of animal feed formulations by extrusion, is also applicable for the production of catalyst carriers and other materials requiring uniform size and shape together with relatively high porosity. Sinteuing: When going back to the fundamentals of this process, was found to be not only a high temperamre process for the agglomeration of ores but, at much lower temperatures, also for plastics and other man made powders with low melting points or softening ranges, and, quite obviously, for powder metallurgy, mechanical alloying, or the like at many different temperature levels including extremely high ones for refractory metals. The above is only a small selection of the many diverse applications of particular agglomeration methods which, in all the different environments, follow the same fundamentals, apply the same rules, and use essentially the same equipment and systems if looked at from an interdisciplinary point of view. Although these facts become more and more known, there is still the understandable preconceived notion of, for example, somebody working in an ultraclean environment, such as the pharmaceutical, food, or electronic industries, that developments, expertise, and know-how gained in the “dirty” plants of, for example, minerals or metals production and processing, can not be considered as valid information that may be applied for the solution of a “clean”problem - and vice versa. In the case of “dirty” industries, a typical concern is that the often more deeply and completely investigated technologies originating in “clean” industries can not be applied because the production capacities are too small, the process may be batch, the equipment too complex, the execution and the materials of construction too expensive, etc., etc. However, as will be shown among other topics in this book, methods for the selection of the most suitable agglomeration process for a specific application (see Section 11.1)are the same for all projects. While some requirements, for example in regard to equipment or system capacity, or on the shape, size, and special properties of the products, may result in the definition of “cleaner” or “more heavy-duty, rugged” processes already in the preselection phase, the normal approach is to determine the preferred method and/or technique by considering the fundamentals as well as an interdisciplinary pool of expertise and know-how first. Conditions of the particular application such as, for example, “hot and dusty large volume processing”, or the opposite, “clean, small capacity operation with cGMP (= current Good Manufacturing Practice) and CIP (= Cleaning In Place) capabilities” are special design criteria that can be added to most of the systems later during the engineering phase.
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3 Agglomeration as a Generic, Independent, a n d Interdiscip/inary Field of Science
Nevertheless, for manufacturing reasons and sometimes also because of special requirements on the company’s test facilities (see Section 11.2), some vendors specialize in equipment for one or the other industry. This is a decision of convenience by the individual supplier and does not indicate the existence of a fundamentally different technology. In fact, techniques or apparatus that were developed for a specific industry can be adopted for use in areas with different environment and requirements while still maintaining the fundamental underlying principle as well as the general machinery and process. Examples are flaking (see Section 8.4.3),instantizing (see Section 5.4),spheronizing (see Section 8.3),and spray dryer agglomerators (see Sections 7.4.3 and 7.4.4).
Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
I
4 2
Glossary of Agglomeration Terms A Short History of Agglomeration
Newly developing fields of science are organized according to universally recognized As a basic physical agglomeration existed since particulate solids were first classifications usingefect, well-defined terms has to describe the fundamentals, correlations, formed on Earth. Binding mechanisms between solid particles cause the stability equipment, procedures, and processes. This is not the case for those technologiesof wetwere and dry soil for andcenturies (often under of heat and pressure) in the that known and the haveinfluence been developed empirically andparticipate independently development of rock formations. Sandstone is the most easily recognized “agglomfor different applications (see also Chapters 2 and 3). In such cases the same process, erate”. Agglomeration as aofphenomenon, e.g. have the natural caking andinbuild-up partiprocedure, activity or piece equipment may different names differentofindusculate solids, must have been observed and has been used by higher developed organtries or the same term may have different meanings in different fields of application. isms later by humans since prehistoric times. Sea creatures covered themselves The and earlier book “Size Enlargement by Agglomeration” [ B.421 contained already a with protective coats, birds as well as other animals built nests, and formed glossary of agglomeration terms. In the following this glossary ishumanoids repeated and upartificial stones, all from various solids, sand, clay and different binders that were dated. Although the author and many others that are active in the promotion of often “agsecreted by the As a “tool” to technical improve powder glomeration” arecreature trying toitself. use scientific and termscharacteristics, consistently inagglomeraan intertion was used by ancient “doctors” in producing pills from medicinal powders disciplinary manner (terms shown bold), it is still helpful to also explain some ofand thea binder (e.g. honey) or by food preparers during the making of bread from flour wheremore common names and expressions including a few historical ones. In the followby the inherent starchy components act as binder. ing, crossreferences are indicated by italic letters. The same and many more “agglomIn spite of this long “history”, agglomeration as atrade technology is only about 150 years eration terms”, the latter mostly descriptive and/or names, are mentioned and old today (excluding small scale pharmaceutical and some little-known mostly used in the text of the book. Sections 14.2 and 14.3 help locate these ancient, words and exChinese applications as well as brick and bread making). Agglomeration as a unit pressions. operation, defined within solids processing, started around the middle of the nineteenth recoverofand use coal from fines. the surface or edges of an Removal solid matter Abrasioncentury [n.] as a method to Agglomeration as a scienceagglomerate. is very young. It began in the 1950s with the formal The matter removed is much smaller than definition of the binding mechanisms of agglomeration, interdisciplinary collection the agglomerate itself. (See also attrition, erosion.) of knowledge relating to allMeasure aspects for of the agglomeration, andfor fundamental ability of a body, example an research agglomAbrasion resistance which was no longer application oriented [B.42]. At approximately that time, the first erate, to withstand abrasion. recurring [n.] series of professional organized which were deThemeetings process ofwere growth or enlargement by a exclusively gradual buildAccretion voted to the science and technology of agglomeration (International Briquetting Asup, such as: increase by external addition or accumulasociation (IBA), - today Institute Agglomeration -, begintion, for for Briquetting example by and adhesion of external (IBA) parts or partining in 1949 with biennial meetings and proceedings; International Symposia cles. (See also agglomeration, aggregation, build-up.)on Agglomeration,[vb.] initiated in 1962 (see pile also up. Section 13.1)). Since that Towith heapproceedings, up into a mass; Accumulate time, agglomeration science, technology, and use have experienced growth The action or process of accumulating; an rapid accumulated Accumulation [n.] but still without finding a corresponding awareness at institutions of higher learning mass, quantity, or number. and in the[ n.] technical or process engineering communities. A sticking together of solids. The molecular attraction Adhesion This book is the second by exerted the author on the general subject size enlargement by between the surfaces of of solids. Distinguished agglomeration. While frequently referring to fundamentals and specifics which are from cohesion.
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4 Glossary of Agglomeration Terms
Agglomerate [vb.] Agglomerate [n.]
Agglomeration [n.] Agglomerator [n.] Aggregate
b.1
Aggregate [vb.] Aggregation [n.]
Agtation [n.] Agitator [n.] Ammoniation [n.] Angle of repose Angle of compaction Anticaking agent
Apparent density Atomizer [n.] Atomizing [vb.] Attrition [n.] Auger [n.] Axial extruder
To gather (particulatesolids) into a ball, mass, or cluster. (See also aggregate.) An assemblage of particles which is either loosely or rigidly joint together. Particles adhering to each other. (See also conglomerate.) The action or process of gathering particulate solids into a conglomerate. Specific equipment in which agglomeration is accomplished. Any of several hard, inert materials (as sand, gravel, rock, slag) used for mixing with a binding material to form concrete, mortar, plaster or, for example, road surfacing products. Also: A mass or body of units or parts somewhat associated with one another. To collect or gather into a mass. (See also agglomerate.) A group, body, or mass composed of many distinct parts or individuals;the collection of units or parts into a mass or whole; the condition of so collected. (See also agglomerate, aggregate, cluster, agglomeration, accretion, build-up.) Changes in characteristicsofparticulate solids or agglornerates that occur naturally with time. (See also post-treatment.) A state of movement of particulate solids and/or fluids induced by external effects or forces. See mixing tool, intensijer bar. The formation of fertilizer granulates using ammonia to obtain chemical modification and bonding. The basal angle of a pile of powder that has been freely poured onto a horizontal surface. See nip angle. Liquid or solid matter applied to the surface of, for example, agglomerates that prohibits sticking or growing together. (See also caking.) The weight of the unit volume of a porous mass, for example, an agglomerate. See nozzle. Finely dispersing liquids. The unwanted break-down of agglomerates. (See also abrasion, erosion.) See screw. Low, medium, or high pressure extruder with a flat die plate at the end of a barrel; the material is extruded in the same direction as it is transported by the screw(s).
4 Clossaty of Agglomeration Terms
Backmixing [n.]
Bag set
Ball [n.] Ballability [n.]
Balling [n.]
Barrel [n.] Basket extruder Beading [n.]
Bin [n.]
Binder [n.] Binding mechanism
Biomass [n.] Blade [n.] Blunger [n.] Boiling Bed Bonding [n.] Bowl [n.]
Bridging [n.]
Briquette [n.] Briquetter [n.]
During the flow of particulate solids, reverse movement of some particles due to their stochastic motion caused by turbulence or special equipment design. Typical in the fertilizer industry; unwanted agglomeration of particulate solids in a closed bag during storage. Mostly caused by recrystallization of dissolved materials. Synonymous with spherical agglomerate. (See also pellet.) Typical in the iron ore industry; the capability of particulate solids to form more or less spherical agglomerates during growth agglomeration. Originally in the iron ore industry; any method producing spherical agglomerates by tumble or growth agglomeration. (See also pelletizing.) Cylindrical (or sometimes tapered) housing for screws, e.g. offeeders or extruders. Low pressure extruder in which the die plate resembles a basket, using rotating or oszillating extrusion blades. Formation of bead-like particles; typical in solidification of melt droplets. (See also prilling, pastillation, melt solidijcation.) A container, box, frame, crib, or enclosed volume used for storage. (See also hopper, silo.) An inherent component of or an additive to particulate matter providing bonding between the disparate particles. Physical and chemical effects that cause adhesion and bonding between solid surfaces. See Section 5.1, Tab. 5.1 and Fig. 5.8. Organic plant and animal residuals. Often organic waste material that is especially used as a source for fuel. See extrusion blade. Typical in the ceramic and fertilizer industries; double shafted pug mill. SeeJluid bed. The process of binding particles together by the action of binding mechanisms. A vertical or inclined, cylindrical, conical or convex vessel enclosing and defining the operating volume of some coaters, mixers, spheronizers, etc. Unwanted arching of solid matter in a converging discharge chute or cone. Prohibits discharge of particulate solids from containers or chutes. Also briquet; agglomerate produced and shaped by highpressure agglomeration. (See also compact, tablette.) Also briquetting machine; equipment that produces briquettes.
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4 Glossary of Agglomeration Terms
Briquetting [n.] Brittleness [n.] Build-up [n.]
Bulk density
Capillary [adj.] Capping [n.]
Cake [n.] Caking [n.] Cement [n.]
Cement [vb.] Cementitious [adj.] Channel [n.]
Chelate [adj.]
Chelate [n.] Chopper [n.] Clam shelling
Closed pore Cluster [n.] Clustering [n.] Coalesce [vb.]
The process of forming briquettes or compacts. The tendency of particles or agglomerates to break down in size easily. (See also friability.) The unwanted coating of surfaces with particles which adhere naturally due to their fineness and/or inherent binding mechanisms. The weight of the unit volume of a particulate mass under non-specific condition, e.g. in storage or in a shipping container. (See also density.) Describing full liquid saturation. Separation of a thin layer from the face(s) of compacts during decompression. Defect in tablettes caused by the recovery of elastic deformation and/or expansion of compressed air. See sheet; typical in fertilizer applications. Unwanted agglomeration during storage mostly by recrystallization of dissolved materials. (See also bag set.) A powder of alumina, silica, lime, iron oxide, and magnesium oxide burned together in a kiln, finely pulverized, and used as an ingredient of mortar and concrete. Also any mixture used for a similar purpose. (See also pozzolan.) To unite or make firm by or as if by cement. Having the properties of cement. Open ended compacting tool set for high pressure extrusion in a ram press; also any elongated opening through which material is extruded. (See also pressway.) Relating to, producing, or characterized by a cyclic structure usually containing five or six atoms in a ring in which a central metallic ion is held in a coordination complex by one or more groups each of which can attach itself to the central ion by at least two bonds. To combine with a metal to form a chelate ring or rings. See h i v e head. Opening of the leading or trailing edge of briquettes discharging from roller presses; one-sided splitting along the web.Also duck billing, oyster mouthing. A pore not communicating with or connected to the surface of a porous body. A number of similar individual entities that occur together. (See also accretion, agglomerate, aggregation.) The growing together of primary agglomerates to form larger entities. (See also satellites formation.) To unite by growth.
4 Glossary of Agglomeration Terms
Coalescence [n.] Cohesion [n.]
Coating [n.]
Coating pan
Cold bonding
Compact [n.]
Compact disperse Compactibility [n.] Compacting [n] Compacting tool set Compactionlgranulation
Composite [adj.] Compressibility [n.]
Compression ratio
Conditioning [n.]
Cone agglomerator
A growing together or union in one body, form, or group. (See also growth agglomeration.) Molecular attraction by which the particles of a body (e.g. agglomerate) are united throughout the mass whether like or unlike. Distinguished from adhesion. Applying a layer of material, a film, or a finish to a substrate; in agglomeration, application of a layer of solids to a particulate unit. Specially shaped p a n in which a material layer is applied on agglomerates (such as tablettes) usually in the presence of liquid, heat, or both. Typical in the pharmaceutical and food industries. A binding process that occurs at ambient or low temperatures and uses the cementitous or pozzolanic reactions of many hydroxides; often assisted by pressure. An object of specific size and shape produced by the compression of particulate matter. Synonymous with briquette. A state of particulate solids in which individual particles are closely packed. Distinguished from discrete dispers. See compressibility. Also compaction. The method of producing sheet. The part or parts making up the confining form in which a powder is pressed. Synonymous with die. The normally dry methods of obtaining granular products by crushing and screening compacts and/or sheet into granulate. Consisting of two or more separate materials whereby each retains its own identity. The capacity of a particulate matter to be compacted. Compressibility may be expressed as the pressure or force to reach a required density or, alternately, the density at a given pressure or force. Synonymous with compactibility. The ratio of the volume of loose particulate matter in a die to the volume of the compact made from it. Synonymous withjll ratio. In low and medium pressure extruders, the total thickness of material that is under compression in a die (including any inlet chamfer) divided by the nominal hole diameter. Development of special characteristics of particulate solids by, for example, treatment with steam, kneading, heating, etc., or surface treatment by, for example, anticaking agents. Pan with relatively high conical rim.
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4 Glossary of Agglomeration Terms
Conglomerate [n.]
Contact point Coordination number
Core rod Coufinhal press
CUP b.1 Curing [n.] Cut size Decrepitation [n.] Densification [n.] Density [n.] Deposit [n.] Die In.]
Die plate Disc [n.] Discrete dispers
Dispers [adj.] Dispersibility [n.]
Distribution plate
Doctor blades Dome extruder Double action pressing
An adhering mass of particles made up of parts from different sources or of various kinds. (See also agglomerate.) Area at which two particles touch each other. Sum of all near and contact points of a particle with surrounding particles in a structure made up of particulate solids, for example an agglomerate. Member of the compacting tool set that forms a through hole in the compact. (See also mandrel.) Punch-and-die press with multiple die sets on an indexed table for making large (e.g. coal) briquettes. (No longer used.) See pocket. Induration ofgreen agglomerates by any method. (See also post-treatment.) The actual value at which separation of a particle size distribution into “coarse” and “fines” has taken place. Breakdown in the size of particles or agglomerates due to internal forces, generally induced by heat. The act or process of making dense. Mass per unit volume of matter at specific conditions. For example: apparent, bulk, or true densities. A (natural) accumulation of particles. Member of the compacting tool set that forms the periphery of the part being produced. Also open ended channels for extrusion. Plates, rings, or other machine parts with perforations for extrusion. (See also die.) See pan. A state ofparticulate solids in which individual elements can be clearly distinguished. Distinguished from compact dispers. See particulate. Measure for the ease with which, under specific conditions (e.g. in liquids), an agglomerate breaks down into primary particles. Perforated plate at the bottom of aJuid bed through which fluidizing gas enters from the plenum. (See also gil plate.) See scraper. Axial, low pressure extruder, most often with two screws, in which the die plates resemble domes. A method by which particulate solids are pressed between opposing punches which are both moving relative to the die.
4 Glossary of Agglomeration Terms
117
Downdraft [n.] Drum agglomerator Dry granulation Duck billing Dwell time Encapsulation [n.] Erosion [n.]
Equivalent diameter
Expansion [n.]
Exter press Extrudate [n.] Extruder [n.] Extrusion [n.]
Extrusion blade
Feeder [n.] Feed screw Fill ratio
Flake [n.]
Flake breaker Flashing [n.] Flight [n.]
Downward flow of gas, for example through a particle bed. Slowly rotating, slightly inclined drum for growth agglomeration. See compaction/granulation. See clam shelling, oyster mouthing. In compacting, time during which certain process conditions, for example pressure, persist or are held constant. Typically used as microencapsulation. The gradual wearing away of an agglomerate by the progressive removal of small pieces of material. (See also abrasion.) Diameter of immaginary monosized spherical particles which feature the same property as the particulate mass to be characterized. For example: surface equivalent diameter. Increase in volume of, for example, an agglomerate after production or during post-treatment. Converse of shrinkage. See ram extruder. Product of extrusion. (See also pellet.) Machinery for the production of extrudates. (See also screw and ram extruder.) The formation of (often cylindrical) agglomerates by forcing a “plastic” mass through open ended channels or holes in (perforated) dies. In low pressure extruders, the flat, curved, or engineered blade that pushes material through the openings of a die plate; it is the part closest to the die plate. Device to deliver feed material to a processing unit. (see also force feeder.) Element@)providing forces onto particulate solids in a feeder. (See also screw.) Typically used in tabletting or other confined volume compression equipment. Synonymous with compression ratio. See sheet. Also: 1. Grains or other malleable particles flattened between smooth rollers. 2. Material solidified from a melt on a rotating, cooled drum (flaker) and removed by scrapers. A primary crusher (often two rollers with teeth) used to reduce the size of sheet. See web. A continuous or semi-continuous spiral flat plate that is attached to the shaft of a screw.
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4 Glossary of Agglomeration Terms
Floc [n.] Flocculant [n.] Flocculate [vb.] Flocculation [n.] Flocculent [adj.] Fluid bed
Fluid bed agglomeration Force feeder Fraction [n.]
Fragmentation [n.]
Friability [n.] Friction plate
Funicular [adj.] Gap b.1
Gear pelleter
Gil plate Globulation [n.] Granular [adj.] Granulate [n.]
Aflocculent mass formed by the aggregation of a number of fine suspended particles. A flocculating agent. To aggregate or coalesce into small lumps or loose clusters or into aflocculent mass or deposit. The act or process offlocculating. Containing, consisting of, or occurring in the form of loosely aggregated particles or soft flocs. Also fluidized bed. A bed of particles in which the particulate solids are kept in suspension by forces caused by an upward flowing fluid. Growth agglomeration in afluid bed. A feeder that provides forces onto particulate matter by, for example, the action offeed screws. That portion of a sample of particulate solids which is between two particle sizes (see cut) or in a stated range (e.g. fine, coarse, etc.). The process whereby a particle (or agglomerate) splits into usually a large number of smaller parts with a range of sizes. The tendency of particles to break down in size during storage and handling. (See also brittleness.) In spheronizers, a circular flat disc with a rough surface or uniformly spaced grooves which rotates inside a cylindrical bowl. Describing the transitional liquid saturation. In pressure agglomeration,the distance between the surfaces of compactingtool sets; specifically: in extrusion, the distance between the pressure generating device and the die plate, in roller presses, the closest distance between the rollers. Double-rollpellet mill in which the rollers are in the shape of coarse, intermeshing gears with bores at the root sections between the gear teeth. (Also gear pelletizer.) Distributionplate in which the perforations are manufactured such that they produce a directional flow of gas. See melt solidijication. Present as particles in “grain” shape and size. Coarsely particulate. Also Granule. From Latin granula = grain, particle. Any kind of relatively coarse particulate matter. In size enlargement, synonymous with agglomeration to a size range of between approx. 0.1 and 10 mm. In size reduction, synonymous with crushing into approx. the same size range. Granulate is normally considered dustfree, free flowing, and non-segregating.
4 Glossary of Agglomeration Terms
Granulate [vb.]
Granulation [n.] Green [adj.]
Grid [n.] Growth [n.] Growth agglomeration Heat bonding Heel [n.]
Hopper [n.] Hot melt agglomeration Hot pressing Immiscible binder agglomeration Induration [n.] Inkbottle pore
Instant [adj.] Instantizing [n.]
Intensifier bar
Interconnected porosity
Producing a granular solid matter; possible by size enlargement (agglomeration, melt solidijcation [pastillation, prilling], and crystallization) or by size reduction (crushing). (See also compaction/granulation.) A general term for the production of solids in granular form by either size reduction or size enlargement. As in “green agglomerate”, “green pellet”, etc., means fresh, moist, uncured, etc. In spheronizers, the design (size and shape) of the grooves on the fnction plate surface. An increase in dimension by for example agglomeration or crystallization. (See also coalescence.) See coalescence, tumble agglomeration. See sintering. In batch processing, for example agglomeration, a percentage of the previous batch retained in or returned to the processing vessel. The funnel or chute that stores material and/or directs it into equipment. (See also bin, silo.) Granulation of a hot melt of e.g. urea or ammonium nitrate in a pan. The simultaneous heating and molding of a compact or briguetting of hot material. Selective agglomeration of particles suspended in a liquid by adding an immiscible binder during agitation. (See also oil agglomeration.) Strengthening of green agglomerates, mostly by heat. Non-cylindrical pore with varying diameter; particularly a pore with narrow entrance followed by a large, internal volume. Quickly soluble. Characteristic as, for example, in “instant coffee”. Producing agglomerated products with instant characteristics, i.e. material exhibiting, as compared with the untreated powder, particularly high solubility, even in cold liquids. In high shear mixers and agglomerators, an independently driven bar, rotating with high speed, usually carrying mixing tools and, sometimes means for atomizing liquid binder, that extends into the particulate mass to be mixed and causes an additional turbulent motion of the particles. (See also knive heads.) A network of contiguous pores in and extending to the surface of a porous body, e.g. agglomerate.
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4 Glossary of Agglomeration Terms
Interface [n.]
A plane or other surface forming a common boundary of two bodies or spaces. Isostatic pressing The densijication of a particulate mass by subjecting it to nominally equal pressure from every direction. In high shear mixers and agglomerators, independently Knive head driven high speed rotating tools which extend into the particulate mass and cause additional turbulent motion of the particles as well as desagglomeration in mixing and controlled destruction of agglomerates in agglomeration. (See also intensijier bar.) Land area The area surrounding briquette pockets on the roller surface of briquetters. (See also fiashing, web.) During briquetting in roller presses the forward edge of a Leading edge discharging briquette. A member of the compacting tool set that determines the Lower punch powder fill level and forms the bottom of the part in a punch-and-die press. Extruder in which the die plates consist of screens or thin, Low pressure extruder perforated sheets and exert small frictional resistance during extrusion. An agent mixed with or incorporated into particulate Lubricant [n.] matter or applied to the tooling to facilitate pressing and ejection of a compact, tablette, or extrudate. Lump [n.] See second meaning of aggregate. Mandrel [n.] Also mandril. A metal bar that serves as a core around which material may be bent, cast, forged, molded, or otherwise shaped. (See also core rod.) Marum [n.] Sometimes used to describe a particle which has been spheronized. Marumerizer [n.] See spheronizer. Original (Japanese)name. Mechanical alloying A technology of powder metallurgy by which powders of metals, that cannot be combined in molten stage, are mixed and compacted to form the alloy. Medium pressure extruder See pellet mill. Melt solidification A method by which molten substances are converted into particulate solids by cooling droplets of the melt. (See also beading, pastillation, prilling.) A method by which small portions of liquids, particulate Microencapsulation [n.] solids, or gases are enclosed by a shell (membrane, capsule) to form a dry, free flowing product often with spherical particle shape. The capsule shell may provide specific product characteristics (e.g. dispersibility, solubility). The formation of small agglomerates, usually not larger Micropelletizing [n.] than 3 mm, by growth agglomeration. (See also pelletizing.)
4 Glossary of Agglomeration Terms
Mixer agglomeration Mixing tool
Mix-Muller [n.] Mold [n.] Muller [n.]
Multiple pressing Near point
Nip [n.]
Nip angle
Nodulizing [n.]
Nozzle In.] Nucleus [n.], Nuclei [pl.]
Oil agglomeration
One pot processor
Open pore Orifice [n.]
Agitation and growth agglomeration in powder mixers. Any of a large number of differently shaped tools that are attached to a rotating shaft and cause irregular movement in a particle bed. See Muller. See die. Also Mix-Muller. Originally, a device that used a heavy stone roller to grind and/or mix particulate solids. Today, a blender with one, two or four large metal rollers that mix and knead (densify) material. Often used prior to pressure agglomeration. (See also p a n grinder.) A method of pressing whereby two or more compacts are produced simultaneously in separate die cavities. Area at which two particles approach each other closely enough for a binding mechanism to become effective. (See also coordination number.) In roller presses and pellet mills, converging space (volume) between two counter-rotating rollers and, respectively, the pressure generating device and the extrusion surface. (See also nip angle.) In rollerpresses, radial angle defining the line on the roller surface at which the speed of the particulate mass is identical with that of the roller; in extruders, the angle between the extrusion surface (e.g. die plate) and the pressure generating device (e.g. extrusion blade, screw, roller). Formation of nearly spherical lumps (agglomerates) from a wet mixture of particulate solids by either drying or chemical reaction during tumbling; typically accomplished in dryers or rotary kilns. Also atomizer. Means for atomizing liquids. Primary agglomerate(s) consisting of only a few particles on which further growth occurs. (See also seed.) Also spherical agglomeration. Selective agglomeration of suspended particles in water by adding a bonding oil during agitation; typical in coal preparation. (See also immiscible binder agglomeration.) A batch processing vessel in which several process steps, for example mixing, agglomeration, post-treatment, and finishing, are carried out without opening the vessel during the entire processing sequence. A pore communicating with or connected to the surface of a porous body. (See also inkbottle pore.) The mouth or opening of something, for example an extrusion channel, that forms material into defined shape.
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4 Glossary of Agglomeration Terms
Oyster mouthing Pan [n.] Pan grinder Particle [n.] Particle size Particulate [adj.] Pastillation [n.]
Pastille [n.] Pellet [n.]
Pellet mill Pelleting [n.]
Pelletizing [n.]
Pelletization [n.]
Pelletizer [n.] Pendular [adj.] Penetrating pore Pin mixer Piston press Plenum [n.]
Plow [n.] Plug flow
See clam shelling, duck billing. Also disc. An inclined rotating circular plate with low cylindrical rini for growth agglomeration. See Muller. A piece of solid material that is an entity in itself. The controlling dimension of an individual particle as determined by analysis. Of or relating to separate particles. A method of melt solidijcation by which droplets of a molten material are solidified on a cooled, moving stainless steel belt. Product of pastillation. Name for many different types of agglomerates. Most commonly used in the iron ore industry for nearly spherical agglomerates formed by growth agglomeration in pans, cones, or drums and in the animal feed industry for extrudates produced by pelleting. Often synonymous with agglomerate. Equipment for extrusion through perforated dies. Agglomeration by extrusion of plastic material or of particulate matter containing binders through bores of dies in “pelleting machines” or pellet mills. Originally, production of pellets by growth agglomeration. Today typically agglomeration by balling. Often also used as synonym for agglomeration. Typical in the (iron) ore industry; any agglomeration method involving growth agglomeration with subsequent heat induration. (See also sintering.) Usually rotating pan, drum, cone, or the like for growth agglomeration. (See also “gear pelletizer”.) Describing the liquid bridge model. A pore that connects opposite sides of a porous body, for example, an agglomerate. (See also through pore.) A stationary, cylindrical mixer using a single shaft agitator with pins. See punch-and-die press. Specially designed chamber at the bottom of afluid bed from which fluidizing gas enters the apparatus through the openings of a distribution plate. Plow shaped mixing tool. Forward movement of particulate solids to the discharge end of tumbling drums orfluid beds, caused by a continuous particle feed and optionally assisted by downsloping the drum or the application of gil plates influid beds.
4 Glossary of Agglomeration Terms
Pocket [n] Pore [n.] Pore volume Porosity [n.] Porous [adj.] Post-treatment
Pozzolan [n.]
Pozzolanic [adj.] Powder [n.] Powder metallurgy
Powder rolling Pressway [n.]
Pressure agglomeration
Prill [n.] Prilling [n.] Pug mill Punch [n.] Punch-and-die press
Radial extruder
Indentation on the surface of rollers, normally forming one half of a briquette shape. (See also cup.) An inherent or induced cavity in a particle or void space between particles within an object e.g. agglomerate. Void space (volume)in porous objects. (See alsoporosity.) The amount of pores (voids) in an object expressed as percentage of the object’s total volume. Possessing or “full of” pores. Any treatment of green agglomerates to modify moisture content, strength, structure, etc., by, for example, aging, drying, heating, sintering, etc. (See also curing.) Also Pozzolana. Finely divided siliceous or siliceous and aluminous material that reacts chemically with slaked lime at ordinary temperature and in the presence of moisture to form a strong, slow hardening cement. Having the properties of pozzolan. Particles of dry matter typically with a maximum dimension of less than approx. 1,000 pm. The art of producing metal powders and of their utilization for the production of massive materials and shaped objects as well as for mechanical alloying. See roll compacting. Also used in powder metallurgy for direct rolling of sheet from metal powders. In extruders, the (length of the) channel in which frictional resistance causes the extrusion pressure; the total distance material is compressed inside a die. Also press agglomeration. Agglomeration technique during which agglomerates are formed by pressure. Distinguished from tumble agglomeration. Product of prilling. In the fertilizer industry often (incorrectly!!) synonymous with agglomerate. The formation of spherical particles by solidification of melt droplets. (See also melt solidllfication, shotfoming.) A paddle type mixer usually with open top, single or double shafts, and trough shaped chamber. Part of a compacting tool set which transmits pressure to the particulate matter in the die cavity. A mechanically or hydraulically actuated press in which a reciprocating piston compacts particulate matter in a die. Low pressure extruder in which part of the barrel consists of a screen or perforated thin sheet through which moist, plastic material is passed by extrusion blades to form extrudates; the material is extruded radially to the direction in which it is transported.
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4 Glossary of Agglomeration Terms
Ram [n.] Ram extruder
Ram press Rim [n.] Ring die
Ring die extruder Ring roll press Roller [n.]
Roll(er) compacting
Roll(er) press Roll(er) pressing Rope [n.] Rotary press Satellites formation
Saturation [n.]
Schugi flexowall
Scraper [n.]
Synonymous with punch. Press in which a fly-wheel powered reciprocating ram densifies and extrudes particulate solids through a long extrusion channel. Particularly suitable for elastic materials (such as peat, lignite, biomass, etc.). Also ram press, exter press. See ram extruder. Cylindrical or conical wall surrounding the circular plate of pan, disc, or cone agglomerators. A usually narrow hollow cylinder that is equipped with perforations for extrusion. See pellet mill. Special roller press with one press roller within a large ring-shaped die. (No longer used.) Also Roll. Cylindrical rotating body that is: 1.Paired with an identical, counter-rotating one in a suitable frame. This arrangement is used for briquetting, compacting, pelleting, densijkation, jlaking, and granulating particulate solids. 2. Rolling close to a die plate and forces material to flow through openings, for example, in flat die pellet mills. 3. Mixing and kneading material in a cylindrical or “figure eight”-shaped bowl. (See also Muller.) Also powder rolling. The (progressive) compacting of (metal) powders in roller presses (often called “rolling mills”). (See also roll pressing.) Equipment for pressure agglomeration between two rollers. Densification between two counter-rotating rollers. (See also compacting.) In spheronization, referring to the rotating particulate material. Tabletting machine in which compacting tool sets are arranged on a rotating table (= turret). In agglomeration, the attachment of smaller solid entities, often agglomerates, to other agglomerates by binding mechanisms. (See also clustering.) Relative amount of pores in an agglomerate filled with a liquid or solid substance, as in “liquid saturation”, “binder saturation”. High speed, high shear mixer and/or agglomerator with vertical axis, adjustable mixing tools, flexible shell, flexing roller cage, and short residence time. A tool for removing build-up in agglomeration equipment. Also doctor blades.
4 Glossary of Agglomeration Terms
Screen [n.]
Screw [n.]
Screw extruder Seed [n.] Segregation [n.]
Selective agglomeration
Sheet [n.]
Shot forming Shrinkage [n.] Silo [n.]
Single action pressing Sinter [n.] Sintering [n.]
Slug [n.]
Slugging [adj.]
Slugging press
A (usually mounted) perforated thin plate or cylinder or a meshed wire or cloth fabric used to: 1. Separate coarser from finer particles or 2. Form extrudates. A mechanical device spiral in form or appearance; a conveyor working on the principle of a screw; a conveying tool in afeeder, mixer, or extruder. Also auger, worm. Extruder in which screw(s) produce the extrusion pressure. See nucleus. The desirable or undesirable separation (according to mass, shape, size, etc.) of one or more components of a particulate mass. Agglomeration of only one component of a powder mixture controlled by, for example, binding mechanism, binder, particle size. (See also immiscible binder agglomeration.) A more or less continuous band of compacted material produced in roller presses featuring smooth or shallowly profiled rollers and a gap between those rollers. Also, anything that is thin in comparison to its length and/or breadth. The solidification of a melt into little spheres in a tall form tower. (See also prilling.) A decrease in dimension. In agglomeration, usually of a compact during sintering. Converse of expansion. A trench, pit, or especially a tall cylinder (as of wood, metal, or concrete) often sealed and used for storing particulate solids. (See also hopper, bin.) A method by which a particulate mass is pressed in a stationary die between one moving and one fixed punch. Agglomerated product of sintering. Technique involving induration of green agglomerates by heat. Generally, bonding at a temperature below the melting or softening points of the main constituent of a mixture by the application of heat. (See also heat bonding.) Large, flat faced compressed disk prepared for the purpose of stabilizing the mixture of ingredients in the pharmaceutical industry. 1. Producing slugs in a sluggingpress. 2.Influid bed technology, the slow, upward movement of large, somewhat cohesive masses of particulate solids. Punch-and-die press for the production of large tablettes or slugs which are crushed to obtain granulate. Mostly in the pharmaceutical industry. (See also tabletting machine.)
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Spherical agglomeration Spheronizing [n.]
See oil agglomeration. Rounding of soft, plastic (usually green) agglomerates (usually extrudates) in a spheronizer. Vertical drum with rotating bottom for spheronizing. Spheronizer [n.] (See also Marumerizer.) Characteristic parameter for roller presses; defined as Specific force pressing forcelactive roller width. The formation of granular solids or small spherical agSpray drying glomerates by dispersing a liquid or slurry in droplet form at the top of a tower and evaporating the liquid in the presence of drying gases. The formation of small, spheroidal agglomerates in a Spray granulation fluid, circulating, or spouted bed by spraying a solution, slurry or melt onto the particles; often combined with drying. Stabilize [vb.] Avoid segregation by agglomerating a powder mixture. An elongated body; synonymous with uncut extrudate. Strand [n.] Strip [n.] See sheet. Surface equivalent diameter The diameter of immaginary monosized spherical particles, calculated from the mass related specific surface area, in m2/g,of a particle size distribution, that produce the same specific surface area as the powder. The state ofparticulate solids which are uniformly mixed Suspension [n.] with but undissolved in a fluid. Also Tablet. A compressed agglomerate made of particuTablette [n.] late solids, specifically,in pharmacy, a small compact of a medicated particulate formulation usually in the shape of a disc or a flat polyhedral body. (See also briquette.) The process of forming tablettes. Tabletting [n.] Compaction press for the manufacture of tablettes. Tabletting machine A tall tower with enlarged conical bottom. Tall form tower To drive in or down by a succession of light or medium Tamp [vb.] blows; predensify. See tamp. Tamper [n.] A receptacle for holding, storing, or transporting liTank [n.] quids. Using heat to fuse particulate solids into agglomerates. Thermal agglomeration (See also heat bonding, sintering.) The property of various materials to become fluid when Thixotropy [n.] disturbed (as by shaking, vibration, pressure, etc.). Materials tending to exhibit Thixotropy. Thixotropic [adj.] See penetrating pore. Through pore Parts making up the compacting tool set of a tabletting Tooling [n.] machine.
4 Glossary of Agglomeration Terms
Tower [n.]
Trailing edge True density Tumble agglomeration
Turret [n.] Updraft [n.] Upper punch Wear [n.]
WDG Web [n.] Wet agglomeration Withdrawal process
Worm [n.] WSG
In spray drying or prilling, a cylindrical structure in which liquid droplets that were formed at the top solidify during their descend in a gas atmosphere with suitable temperature. During briquetting in roller presses the back edge of a discharging briquette. The mass of the unit volume of a solid material that is free of pores. Agglomeration technique during which agglomerates are formed by growth during tumbling; synonymous with growth agglomeration. (See also coalescence.) Rotating table carrying the compacting tool set of some tabletting machines. Upward flow of gas, for example through a particle bed. Member of the compacting tool set that closes the die and forms the top of the part being produced. Similar to erosion, but usually refers to the surface of a solid body such as a part of machinery. (Easily) Water Dispersible Granulate. Thin flashing surrounding briquettes made in roller presses; caused by the land area. Tumble andgrowth agglomeration in which the major binder is a liquid. Operation of some tablettingpresses by which the die descends over a fixed lower punch to reduce density variation in the tablette and facilitate removal of the compact. See screw. (Easily) Water Soluble Granulate. (See also instant.)
Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
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Agglomeration Theories The distinguishing characteristic of size enlargement by agglomeration is the formation of larger entities from particulate solids by sticking particles together by short range physical forces between the particles themselves or through binders, substances that adhere chemically or physically to the solid surfaces and form a material bridge between the particles. The components of an agglomerate are often widely disparate and, except if matrix binders are applied (see Section 5.1.2) or after shrinkage during sintering (see Sections 5.3.2 and 9.1), void spaces are present between the particles forming an agglomerate. The above definition of size enlargement by agglomeration sets this unit operation of Mechanical Process Technology apart from other grain growth techniques, particularly crystallization whereby a uniform solid body grows from a mother liquor by forming a structure in which the same atoms and/or molecules have a regularly repeating internal arrangement. As will be shown later (see Section 7.4.6),agglomeration may also play a role during crystallization if nuclei or crystallites adhere to each other in the mother liquor and form macroscopically amorphous, porous structures. Size enlargement by agglomeration is also distinguished from another particle forming technique, melt solidification. In this process a molten material is divided into droplets or extruded through die plates and cut into cylindrical pellets. The product is then solidified by cooling. The melt may be directly synthesized, as in the case of urea prilling, or obtained by heating the solid. In the latter case, similar to the meaning of the term granulation, melt solidification can be a particle size reduction, if large chunks of a solid are melted and then divided into small droplets or extrudates that are solidified, or a particle size enlargement, if a powder is melted, divided into relatively larger droplets or extrudates, and solidified. Droplet formation can be by spraying through a number of differently designed nozzles (see also Section 7.4.3) or by dividing a liquid stream either naturally, by mechanical means, or by gas or liquid impingement. Solidification is accomplished during the free fall in a cooling tower (Fig. 5.la) which results in spherical “prills” (Fig. 5.2), on a cooled stainless steel belt (Fig. 5.1b) yielding flattened “pastilles” (Fig. 5.3), or in water (Fig. 5 . 1 ~ producing ) cylindrical extrudates (underwater granulation/pelleting).
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Fig. 5.la
Fig. 5.1 Schematic representations o f the three most common melt solidification processes. (a) Prilling [8.42], (b) pastillation (courtesy Sandvik, Totowa, NJ,USA), (c) underwater granulation/ pelletizing (courtesy Gala, Eagle Rock, VA, USA).
5 Agglomeration Theories
Fig. 5.2 Photograph of urea prills (courtesy KaltenbachThuring, Beauvais, France).
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Fig. 5.3 Photograph o f the discharge end o f a pastillator also showing pastillated product (courtesy Berndorf Band, Berndorf, Austria).
Since most of the commercially produced urea for fertilizer applications is prilled by the tower melt solidification process and urea is one of the most important nitrogen providing fertilizers, farmers and suppliers often wrongly name all spheroidal agrochemicals “prills” even if they were produced by true agglomeration processes, for instance on discs or in drums (see also Section 7.4.1). In the following only size enlargement by agglomeration will be covered.
5.1 The Development of Strength o f Agglomerates
Fig. 5.4 is the random cut through part of an agglomerate. Obviously, in reality, the structure is three dimensional. In such a body strength can be caused in several ways. In Fig. 5.4a the entire pore space is filled with a matrix binder. Typical examples of agglomerates held together in this manner are concrete, where the matrix between the aggregate particles consists of hardened cement (Fig. 5.5), or road surfaces, in which bitumen occupies the volume between crushed stone (Fig. 5.6). Fig. 5.4b generally looks very similar to 5.4a but shows an agglomerate structure in which the entire void volume is filled with a liquid that wets the solid particles. If concave menisci form at the pore ends on the surface of the agglomerate, a (negative) capillary pressure develops within the pores which affords strength to the body. As explained in Fig. 5.7, depicting a series of situations representing different liquid saturations in particulate bulk solids or of agglomerates, distinct distribution models exist which depend on the amount of liquid in the structure. The term liquid saturation is defined as the percentage of total void space that is filled with the liquid. A precondition for cohesiveness of particulate solids due to the presence of liquid is that the liquid wets the solids. Although, depending on the application, other liquids may be used to totally or partially fill the voids between particulate solids, in agglomeration water is most commonly used. Referring to Fig. 5.7, absolutely dry particulate bulk solids (Fig. 5.7a) are non existent under normal atmospheric conditions. The water molecules of adsorption layers (Fig.
5.7 The Development of Strength of Agglomerates
Fig. 5.4 Random cut through part o f an agglomerate or a partculate bulk solid mass and explanations o f how strength may be caused. (a) Pore volume filled with a matrix binder. (b) Pore volume tilled with a wetting liquid. (c) Liquid bridges at the coordination points. (d) Adhesion forces at the coordination points.
Fig. 5.5 Cement bonding: Structure developing during early hydration [10.5].
Fig. 5.6
Structure o f stone mastic asphalt (SMA),
a modern asphalt matrix bonded road surfacing material.
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Fig. 5.7 Schematic representations o f different liquid saturations in particulate bulk solids o r o f agglomerates. (a) Dry, (b) adsorption layers, (c) liquid bridges (“pendular” state), (d) transitional (“funicular” state), (e) fully saturated (“capillary” state), (f) droplet.
5.7b) that quickly form on the solid surfaces are bonded so strongly that they are not mobile and, therefore, do not cause “liquid saturation” or moisture content which can be measured with “normal” laboratory equipment. However, as will be shown later (see Section 5.2.1), adsorption layers can participate in the development of strength by enhancing molecular (van-der-Waals)forces. With small amounts of “free” water, i.e. producing moisture contents of little more than a few tenth of a percent and, correspondingly, very small “saturation”, liquid bridges begin to form at the contact points between particles. With increasing moisture content or saturation liquid bridges form at all coordination points (see below) in the structure (Figs 5 . 4 ~and 5.7~).Further increase in liquid saturation produces a transitional situation in which liquid bridges and void spaces that are filled with liquid coexist (Fig. 5.7d). The theoretically highest saturation (100 %) is reached, when all voids within a bulk mass or an agglomerate are filled (Fig. 5.4b) and concave menisci are formed at the pore ends (Fig. 5.7e). Beyond complete saturation, liquid droplets shaped by the surface tension may enclose solid particles (Fig. 5.7f). Slurries, bulk particulate solids containing an excess amount of water, are shapeless. All above models exist in wet agglomeration, methods that are based on the processing of slurries, suspensions, or solutions (see Sections 7.4.3 and 7.4.6) or the presence of liquids as binders (see Section 7.4).
5.1 The Development of Strength of Agglomerates
Fig. 5.4d depicts the action of solid bridges or forces at the coordination points of a particle with other particles surrounding it in the agglomerate structure. Coordination points are points of contact with other particles and near points, areas of the particle surface which are so close to a neighboring particle surface that significant adhesion forces act or bridges can form. The coordination number is the average of the sum of all contact and near points of each particle with others surrounding it in a particular agglomerate structure (see also Section 5.3.1). Typical examples of agglomerates bonded in this manner are “natural” aggregates of very fine particles which are held together by molecular forces or agglomerates with solid bridges at the coordination points which have formed during drying of originally wet agglomerates by recrystallizing materials which had been dissolved in the liquid. 5.1.1 Binding Mechanisms
The binding mechanisms of agglomeration were first defined and classified by H. Rumpf and his co-workers (see Chapter 1).According to Tab. 5.1 they are divided into five major groups, I to V, and several subgroups (see also Fig. 5.8).
Tab. 5.1
Binding mechanisms of agglomeration
1. Solid bridges
1. Sintering 2. Partial melting 3. Chemical reaction 4. Hardening binders 5. Recrystallization 6. During drying: a) Recrystallization (dissolved substances) b) Deposition (colloidal particles) 11. Adhesion and cohesion forces 1. Highly viscous binders 2. Adsorption layers ( < 3 nm thickness) rlr. Surface tension and capillary pressure
1. Liquid bridges 2. Capillary pressure
IV. Attraction forces between solids 1. Molecular forces a) Van-der-Waals forces b) Free chemical bonds (Valence forces) c) Associations (nonvalence); hydrogen bridges 2. Electric forces (electrostatic, electrical double layers, excess charges) 3. Magnetic forces V. Interlocking bonds
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I. Solid Bridges 1. If the temperature in a disperse system rises above approximately two- thirds of the melting temperature or softening range of the solids, diffusion of atoms or molecules from one particle to an other one occurs at the points of contact. The solid bridges that develop with time are called sinter bridges. The velocity of diffusion depends on temperature, size of contact area, and contact pressure. It increases with rising temperature, larger contact area, and higher pressure. Heat can be introduced from an external source or created during agglomeration by friction and/or energy conversion (see also Section 9.1). 2. At the contact points of particles, roughness peaks may melt due to heat caused by friction and/or pressure. In such cases, liquid bridges develop which solidify quickly due to the large heat sink provided by the solids themselves. This mechanism, called partial melting, is often responsible for unwanted agglomeration and caking of substances with low melting point or softening temperature. 3./4. The formation of solid bridges by chemical reaction or hardening binders depends only on the participating materials, their reactivity, and their tendency to harden. Elevated temperature and/or pressure may improve the reaction and result in a modified, potentially stronger bridge structure. These binding mechanisms are often activated by moisture.
5.1 The Development of Strength of Agglomerates
Temperature fluctuations can result in recrystallization and bridge formation within otherwise stable or sealed bulk particulate solids. The temperature induced physical recrystallization of some substances may extend through the interface at contact points causing solid particles to grow together. Salts or mixtures of salts that contain some free moisture may cake when exposed to varying temperatures, even if the amount of moisture is very small and the material is packed in airtight enclosures. This is because, often, more salt dissolves at higher temperature which recrystallizes if the temperature drops, forming crystal bridges between the solid particles in the bulk. During temperature fluctuations caused, for example, by day and night or seasonal differences, this is a continuing process that will, with time, result in more and stronger caking (see also Section 5.5). 6. The more common method of forming solid bridges by recrystallization of dissolved substances or deposition of suspended colloidal particles is to evaporate the liquid. The strength of crystal bridges depends not only on the amount of the dissolved and recrystallizing material but also on the speed of crystallization. At higher crystallization rates a finer bridge structure is formed which results in higher strength (see also Section 5.2.2). Colloidal particles form solid bridges if the liquid between the macroscopic particles of a disperse system consists of a colloidal suspension. During drying the colloidal particles concentrate in diminishing liquid bridges and the pressure caused by the liquid’s surface tension compacts the colloidal particles. After complete evaporation of the liquid, solid bridges remain which are made up of colloidal particles. Adhesion in the bridges is mostly caused by molecular forces which may be enhanced by electrical and magnetic effects (see Group IV below). 5.
11. Adhesion and Cohesion Forces
1. If highly viscous binders, such as bitumen, honey, pitch, tar, etc., are applied, adhesion forces at the solid-binder interface and cohesion forces within the viscous material can be fully exploited for agglomerate strength until the weaker of the two fails. Highly viscous binders are often used as matrix binders (see also Sections 5.1 and 5.2.1). 2. Most finely divided solids easily attract free atoms or molecules from the surrounding atmosphere. The thin adsorption layers thus formed are not mobile. However, they can contact and penetrate each other. It can be assumed that molecular forces can be fully transmitted if the adsorption layer is thinner than 3 nm. Such forces are often high enough to cause deformation of solid particles at the contact points (Fig. 5.9) thus increasing the contact area and, therefore, strength of the bond between adhering partners. The application of external forces and/or elevated temperatures may increase the contact area and strength further [B.14, pp 97-1291.
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X
Fig. 5.9 Viscoelastic deformation at the contact point between tW0 glass spheres due t o molecular attraction.
Adsorption layers may also increase adhesion forces if the layers do not contact or touch each other (see Section 5.2.1). 111. Surface Tension and Capillary Forces One of the most common binding mechanism of wet agglomeration is liquid bridges at the coordination points between the particles forming the agglomerate. Liquid bridges can develop from free water or by capillary condensation. They are often the precondition for the formation of solid bridges (see above, 1.G). If the entire pore volume between the particles of a disperse system is filled with a liquid and concave menisci form at the pore ends on the surface of the system, a negative capillary pressure exists in the interior causing strength. Wet agglomerates are very often bonded by a combination of the above two mechanisms. In that case partial volumes exist which are completely filled with the liquid while in others liquid bridges prevail. Technically it is almost impossible to attain 100 % saturation because there is a high probability that during the agglomeration process air is trapped in some pores. IV. Attraction Forces Between Solid Particles Attraction forces between solid particles are often the cause for unwanted agglomeration: bridging, caking, coating, and build-up. The most important binding mechanisms in this category are molecular, electric, and magnetic forces (Fig. 5.10). At extremely small distances between the adhesion partners these forces can be very high but, due to their short range effect, they diminish quickly with increasing distance at the coordination points. Since particles approach each other with roughness peaks (Fig. 5.11) and the absolute roughness of smaller particles is less than that of larger ones, the adhesion probability, i.e. the chance of such particles moving closer together, increases as powders become finer. High adhesion forces are obtained if fine and ultrafine or nano-sized particles are involved.
5.I The Development of Strength of Agglomerates
Molecular Forces van-der-Waals Forces
1.a.
Valence Forces at newly created surfaces (Recombination Bonding)
1. b.
Nonvalence Association e.g., Hydrogen bridges betweenoxygenand hydroxyl radicals
a: Association - H interacts with nonbinding electron pair of oxygen b: Water molecules intensify association c: Bridging by nonvalence association of bipolar (water) molecules
Electrostatic Forces
2.
-_-_--
Fig. 5.10
Attraction forces between solid surfaces or particles
1.a) Van-der-Waals forces are naturally occurring forces at the surfaces of all solid materials. The molecules, atoms, or ions in the interior of a solid interact with each other such that they retain their relative, equilibrium positions. At the surface of, for example, a particle, the molecular forces that are directed to the outside are not satisfied and produce a force field that interacts with that of other particles.
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Fig. 5.11 Model depicting the true situation at a coordination point between two particles. Roughness exists on all real particles. a9: is the representative distance between the particles.
Then, van-der-Waals forces arise because of the electric polarization induced in each of the particles by the presence of the other ones. Forces are in the order of 0.1 eV and decrease with the sixth power of the distance between the partners. The maximum range of the van-der-Waals interaction is in the order of 100 n m which, compared to chemical bonds (valence forces), is large. 1.b) During size reduction (comminution)bonds between the atoms and molecules of a solid are stressed and ultimately part creating new surfaces. Immediately after separation, unsatisfied valences exist on these newly created surfaces. Normally, the free radicals quickly combine with atoms and molecules from the surrounding atmosphere, thereby becoming neutralized. However, conditions exist where either the newly created surface area is so large at any given moment that the number of atoms and molecules that are available in the immediate vicinity is too small to satisfy all the available valences or mobile, reactive atoms and molecules that could neutralize the free radicals are not present. In those cases, the valences themselves may recombine if newly created surfaces come close to each other. Such recombination bonding occurs during fine grinding due to the first mechanism, eventually resulting in an equilibrium between size reduction by comminution and size enlargement by agglomeration (“grindinglimit”) (see also Section 5.5). Recombination bonding also occurs during high-pressure agglomeration (see Section 8.1). If brittle particles break in the compact under the influence of high forces, new surfaces are created within a densifying mass of particulate solids where the possibilities are limited to satisfy the exposed free valences with gaseous atoms or molecules. At the same time, high compaction forces cause particle surfaces, including the newly created, reactive ones, to approach each other so closely that, after some lateral movement of the fractured pieces, free valences recombine, forming strong, permanent bonds. 1.c) Nonvalence associations of certain molecular groups can also cause bonding and provide strength to a particulate bulk solid. One important phenomenon, hydrogen bridges, is, for example, the prevailing, naturally occurring binding mechanisms between organic macromolecules in coal. Hydrogen bridges form if a hydrogen atom is bonded to a strongly electronegative atom, such as oxygen in a typical OH group, and the hydrogen atom interacts with the non-binding electron pair of another electronegative atom, e.g. oxygen of a COOH group. Water
5.7 The Development of Strength of Agglomerates
(H - 0 - H) intensifies this association and the bipolar molecules can also form
nonvalence association bridges which participate in the development of strength (see also Fig. 5.10, l.c, b and c). 2. At their surfaces, ionic solids possess an unsatisfied electrostatic field which is superimposed on that produced by the van-der-Waals forces. The strength of this field diminishes rapidly with distance from the surface and is soon negligible. However, this external electrical field can induce a dipole or a higher order moment in the charge distribution of the molecules in an adsorbed layer thus participating in adhesion. When two solid surfaces come in contact with each other, electrostatic forces of attraction arise as a result of the contact potential, forming electrical double layers. The physical cause for the transfer of electrons when two solid bodies come into contact is the difference between their electron work functions. Electrons migrate from the body with the smaller work function to the one with the larger one until equilibrium is reached (double layer). The action of this mechanism is permanent. Particles also can be charged by providing electrons from external sources (e.g. spray electrodes). Such excess charges can also cause attraction (or repulsion). Because of the field character of this binding mechanism, strength is independent of particle size. Also, the strength due to excess charges is very small and the charges tend to equalize (disappear) with time. Therefore, this mechanism is, in most cases, only significant for initial, temporary bonding (typical application: electrostatic precipitatorslfilters). As mentioned before, it is also possible that bonding between two oppositely charged solid surfaces is caused by the nonvalence association of bi- or multipolar molecules or radicals. Hydrogen bonding is a well known example. 3. The attraction mechanism caused by magnetic forces is similar to that of electrostatic forces. The presence of magnetic forces is limited to ferromagnetic particles although, recently, based on the understanding ofthe nature ofmagnetism, it was reported that it is now possible to engineer completely man-made plastic materials with magnetic properties. The latter may enlarge the applicability of this mechanism in the future. V. Interlocking Bonds Normally, interlocking bonds occur if the particulate solids have the shape of, for example, fibers, threads, or lamellae that twist, weave, and bend about each other or entangle during agglomeration. Sometimes interlocking bonds of elongated, fibrous additives are used to strengthen agglomerates which are otherwise too weak (see also Section 5.3.1). In high-pressure agglomeration, another interlocking mechanism may occur if a mixture of rigid and plastic materials is compacted. In this situation, the plastic component flows into recesses and, more generally, envelopes the exterior structure of harder particles, thus producing a strong structural bond that resembles the effect of a matrix binder (see also Section 8.1).
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Figs 5.8 and 5.10 describe pictorially the binding mechanisms that were reviewed above. It should be pointed out that only the two-dimensional situation at one coordination point between two particles or solid surfaces is shown. In reality, each particle has many interaction sites (coordination points) with other particles in the three-dimensional structure. It should be further understood that in typical particulate bulk solids and agglomerates large numbers of particles are present per unit volume (see also Section 5.3.1) and participate in bonding due to the binding mechanisms presented above. With exception of capillary and matrix bonded structures of particulate solids, it is unlikely that only one binding mechanism acts on all the coordination points within a mass. If molecular and electric forces as well as liquid bridges and the solid bridges, resulting from the latter by one or the other of the mechanisms that were discussed above, are considered, it must be assumed that the effect of each binding mechanism is different at essentially every coordination point due to varying microscopic surface structures and distances at each interaction point (see also Section 5.2.1). 5.1.2
Binders, Lubricants, and Other Additives
If size enlargement by agglomeration is desired and the correct agglomeration technique is selected, many of the binding mechanisms described in the previous Section 5.1.1 are inherently available or can be activated. Under certain conditions, some binding mechanisms also act naturally to produce undesirable agglomeration phenomena. Generally speaking, if agglomeration is wanted, means to enhance the available binding mechanisms must be developed and applied, while the effect of binding mechanisms must be eliminated or reduced to avoid unwanted agglomeration. Both aspects will be covered in much more detail in Section 5.5. As will be shown in Sections 5.2 and 5.3, particle size of the particulate solids plays an important role in agglomeration. While the surface area of particles, the interface at which all binding mechanisms act, decreases with the second power of particle size, volume and, therefore also, mass, the most important particle properties which result in forces that challenge adhesion and cause separation of bonds, diminish with the third power of the particle size. If the particle size reaches a few pm or is in the n m range, the natural adhesion forces dominate and particles which contact each other or come into close proximity adhere to one another. This phenomenon can not be economically eliminated so that very fine particles always adhere and form loose agglomerates which may be desirable or undesirable (see Section 5.5). Naturally available adhesion tendencies can be considerably increased if moisture is added during the agglomeration process. Application of external forces can contribute to the enhancement of inherently present binding mechanisms. Depending on the magnitude and nature of these forces, improved structure (by shear and low to medium compression) or plastic deformation and brittle breakage (due to high external forces) can occur. Plasticity, an often preferred response to external forces that results in high agglomerate strength (see Section 8.1),increases with many solids if the temperature of the material rises. Therefore, hot densification is often a desirable agglomeration technology, particularly for minerals and metal bearing materials.
5.1 The Development of Strength of Agglomerates
Since all binding mechanisms rely on molecular interactions on and between surfaces or interfaces, the structure and distance at these points is of great importance for the ability of powders to agglomerate. Often, the presence of ultra fine particles facilitates size enlargement of coarser particulate matter. Fines that are suspended in a liquid accumulate during drying at coordination points and form solid bridges which are bonded by molecular forces. Dry fines may fill areas with high surface energy, such as holes and depressions, thus reducing the effective distance between larger particles and increasing the attraction force (similar to the influence of adsorption layers; see Section 5.2.1). With other materials, e.g. certain coals and chemicals with low softening or melting points or containing such components, mechanical energy, introduced by dynamic forces, compression, or shear and converted into thermal energy, activates the inherently available binding properties. Under this influence, momentary softening and melting can occur upon contact at minute roughness peaks which, after almost instantaneous solidification, produce a small solid bridge between the powder particles. Similar mechanisms are responsible for the bonding of soluble materials in the presence of moisture. Mechanical energy converted into heat or the direct external supply of thermal energy result first in dissolution and then in recrystallization at the coordination points. The larger the number of coordination points in a unit volume (increasing with decreasing size of the agglomerate forming particles), the higher will be the strength of the agglomerated part. In spite of the availability of all these “natural”binding mechanisms and the various possibilities to enhance them for the desirable production of agglomerates, sometimes no economic method can be found to process a specific material and form a product with sufficient strength. Grinding the particulate solid to a sufficient fineness for strong molecular bonding and/or heating it to high enough temperatures that result in either sufficient dissolution for recrystallization, plasticity for large area contact and bonding, or sintering and melt solidification, would be too expensive and, therefore, prohibit economic processing. In those cases where no bonding can be achieved, particle size is relatively large, or specific product characteristics must be obtained, binders, mostly for higher strength, lubricants, mostly for improved density and structure, and other additives, which produce special properties, can or must be used. Binders are components which are added prior or during agglomeration to increase the strength of the agglomerated product at otherwise unchanged processing conditions. They can affect strength directly or after a curing step. Binder selection depends on many considerations which are specific for the particular application. They must be compatible with the materials to be agglomerated and the proposed uses of the product. For example, for pharmaceutical and food applications only officially approved materials may be used and for the agglomeration of metal bearing dusts which are intended for recirculation into steel mills, sulfur containing binders are normally prohibited. Many such limitations can be defined for specific materials and applications.
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For those reasons, binder development can not be generally treated. Rather, each individual case must be evaluated separately. However, a few common characteristics can be considered before starting a specific development program. Binders can be divided into inorganic or organic components and their distribution in the agglomerate structure may be in the form of films and bridges or a matrix. Film or bridge type additives are normally fluids which coat particles or are drawn to the coordination points where they form bridges. If applicable, only relatively small additions are required; porosity of the agglomerates as well as their freely accessible surface area (including internal surfaces, see also Section 5.3.2) are only insignificantly changed. Water is the most well known film and bridge forming binder. Matrix forming binder components, on the other hand, more or less fill the entire pore space and, therefore drastically reduce porosity and accessible surface area. Cement is a typical matrix forming additive. Water or other liquids may act as matrix binder in fully saturated wet agglomerates (capillarystate, see Section 5.1). However, this is only a temporary binding mechanism and the liquid will disappear naturally or during a posttreatment step (see Section 7.3) so that pores open up and surfaces become accessible again. Still other binders will react chemically with different components of the additive mixture or with some or all of the materials to be processed. Such reactions can result in high strength products with, for example, waterproof bonds. Tab. 5.2 lists examples of organic and inorganic binders that were previously employed in agglomeration. It shows that many different substances and materials have already been used. Commonly available and applied binders are printed in italic letters. Investigation of by-products or wastes as binders may result in the discovery of cheap and very acceptable additives. For example, molasses, a by-product of sugar making, is an excellent and nutritionally beneficial binder for animal feed and organic wastes can be incorporated in fertilizers as nutrient and binder. Binder development must take into consideration the availability of the substance at the point of ultimate use and over time. Normally, evaluations begin at a vendor facility with traditional and/or new materials that are available at that time and location. Often such developments become unacceptable when during the final cost analysis the binder turns out to be excessively expensive due to the need for its transportation to the location of the planned industrial agglomeration facility. A recent example for the “drying out” of a binder source with time is Brewex, the somewhat modified by-product of a specific beer brewing technology. The material, a liquid starch material, which was available at reasonable cost in the USA and quickly enjoyed a relatively widespread use, had to be taken off the market when the beer brewing technology changed and the by-product source disappeared. In such a situation, the operator of an already established agglomeration process has to search for a replacement binder with similar properties, acceptable price, and good availability to be able to remain in business and continue to be profitable. Therefore, unless there is a safe and unlimited binder supply for a particular application, it is prudent to continuously observe the market, evaluate new developments, and be ready for change.
5. I The Development of Strength of Agglomerates Examples of organic and inorganic binders that were previously employed in agglomeration (in alphabetical order).
Tab. 5.2
Organic binders
Inorganic binders
Albumates (Albuminates) Alcohols Alcotaca Alginates Asphalt/Asphalt EmulsionslRefined Asphalts Brewex Carnauba Wax Caseins CAFA (Chemically Activated FlyAsh) Cellulose Compounds Chicken Manure CMC (Carbo-Methyl Cellulose) Coal Tar, Pitch, and Creosote Coke Oven Tar Covol Crude Oil Dextnne Drying Oils ElVerona Fir Tar (Pine Wood Tar) Fish Waste Gelatine GilsoniteO (Natural Asphalt) Glues Gums (e.g. Arabic) Humates (Humic Acid) Lignins (Liquor and Powder) Lignite Lignite Tar Lignosulfonates Maltose Molasses OrimulsiorP Paper Pulp (from secondary paper making) Paraffin Peat Petroleum Pitch PeriduP Pittsburgh F l u Polyvinyl Alcohol (PVA) Resins (Natural and Synthetic) Rosin Sawdust Seaweed Slaughterhouse Refuse Starches, pregelatinized (e.g. Corn, Potato, Tapioca Wheat) Straw (Ground or Pulped) Sucrose
Alkali Silicates (e.g. Sodium, Potassium) Alum Alumina (see Colloidal ..) Attapulgite (Clay) Bentonite (Montmorillonite Clay) Caustic Soda Cements (e.g. Portland, Slag) Clays Colloidal Alumina, Silica, etc. Dolomite Fuller’s Earth Gypsum Lime Lime Hydrate (often as hardener) Magnesia/Magnesium Oxide Magnesium Chloride Metal Swarf Metal Fibers Plaster of Paris Salts Silica (see Colloidal ..) Silicates (see Alkali Silicates) Sodium Borate Sulfates (e.g. Copper) Water
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Sugars
Tanning Liquors (Tannic Acid) Terravest (Liquid Polybutadiene Emulsion) Thermoplastic Powders Tree Sap Vegetable Pulp Waxes and Wax Tailings
Wood Pulp
Lubricants may be either liquid or solid additives (Tab. 5.3). They reduce the coefficient of friction between the particles of a bulk mass and, therefore, result in a somewhat higher agglomerate density or lower porosity, E . According to the relationship k E = n (see Section 5.2.1) additional adhesion sites (characterized by the coordination number, k ) are activated by which increased agglomerate strength is expected. Tab. 5.3
Examples of some typical liquid and solid lubricants.
Liquids
Solids
Glycerine Oil/Water Emulsions Water Dry Starch Molybdenum Disulfide Stearates (Metallic, e.g. Magnesium Stearate) Talc Etylene Glycol Oils Silicones
Graphite Paraffin Stearic Acid Waxes
In pressure agglomeration, lubricants also reduce the coefficient of friction between the material to be compacted and the tooling. This results in a more uniform structure of the compact and in less density variation (see also Section 8.2). During ejection from a die or release from a mold lower forces are required for separation and, therefore, higher survival rates are obtained. Development and selection of lubricants must apply the same considerations as discussed for binders above. While, in some cases, binders may be valuable ingredients of the final product or disappear during post-treatment, lubricants are almost always contaminants. For this reason and to keep costs down, the most acceptable lubricants are those that are effective in very small amounts. In former times lubricants were mixed into the formulation prior to, for example, tabletting, even if the lubricant was only meant to reduce the friction between the solids and the tooling. Newer developments came-up with applicators that deposit the lubricant on the surfaces of the tooling thus decreasing amount, cost, and product contamination considerably (see also Section 8.4.3).
5. I The Development of Strength of Agglomerates
With the growing importance of size enlargement by agglomeration for the manufacturing of engineered products (see also Chapter 12), many other additives are used as “functional” components. Particularly in the food industry (Fig. 5.12), but also in other, by the public less well known applications, materials with specific, predetermined, and controlled properties are formulated from particulate ingredients and then agglomerated to yield consumer products that feature desirable characteristics. For example, convenience foods can be easily and quickly used such as “instant” soups, sauces, and drinks or products that were recombined from fine, ground food stuffs, contain already the correct amount of spices as well as other aromas, and, after preparation, feature a texture and taste that pleases the palate. Functional foods, also called designer foods on the other hand, have been treated to eliminate unhealthy ingredients, such as fat. They are then recombined with additives that replace the removed components without sacrificing the “mouthfeel” that is expected from the untreated food. Functional foods may also contain dietary additives that make a product particularly acceptable for a special group of often chronically sick people, such as, for example, diabetics. For those reasons, the market for food additives is growing overproportionately, largely due to the increasing production of more nutritious and better balanced designer foods whereby calorie reduction agents are the largest segment. Fun foods are the wide range of modern sweets and snacks where mostly sugar and fat based binders are applied to obtain agglomerates or, for example, bar shaped products from a multitude of ingredients for the consumer martket. A more complete coverage of these fast growing technologies is far beyond the scope of this book. They are mentioned to demonstrate the wide range of applications of agglomeration in areas that are not immediately recognized as common uses of the unit operation. Still other additives are more generally introduced to overcome problems caused by the need to obtain sufficient strength for packaging, handling, and storage. Special components may have to be added to the formulation which assist in the break-up of the agglomerated product when it comes into contact with water or other liquids. Such materials are commonly starches or their derivatives and other compounds that swell when absorbing liquids. Fibers may be added for a number of reasons, for example, as a dry binder, a structural component, a moisture absorbent, and a conduit for liquid. Mixtures of carbonates will produce carbon dioxide with water and result in the well known effect of effervescence. Fig. 5.13 shows schematically the influence of wicking by fibers or swelling of suitable components on the dispersion of agglomerates in liquids. Both effects may be also used together. Produced from renewable resources, organic fibers and their derivatives have a wide range of functional applications. In the pharmaceutical and food industries, the presently best known cellulosic additive is microcrystalline cellulose (MCC). It is obtained from wood cellulose by acidic hydrolysis. The product does no longer contain lignins, hemicelluloses, or other impurities and is bleached to produce a high degree of brightness. In a cellulose molecule, approx. 15,000 D-glucose units are connected in a 1.4-pglucosidic linear arrangement to form a filamentary molecule. Individual molecules of Other Additives
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Fig. 5.12 A few examples o f modern food products that were manufactured using agglomeration technologies. (a.1-a.4) Cereals and cereal bars (courtesy Kellog Co., Battle Creek, MI, USA); (b) cubed and granulated beef bouillon, both with "instant" (see Section 5.4) characteristics (courtesy Borden Foods/Wyler's, Columbus, OH/Chicago, IL, USA); (c.1-c.3) various snack bars f r o m cereals, whole grains, nuts, dried fruit, and processed food materials (courtesy Hosokawa Bepex/Hutt, Leingarten, Germany); (c.4) various dumplings (courtesy Hosokawa Bepex/Hutt, Leingarten, Germany).
5. I The Development of Strength of Agglomerates
WlCKlNG
SWELLING
Fig. 5.13 Schematic representation o f the influence o f wicking and swelling on the dispersion of agglomerates in liquids.
cellulose are bonded together by hydrogen bridges yielding pseudo crystalline structures. Although the hydrogen bonding is not destroyed by acidic hydrolysis, the cellulose chains are depolymerized and form “microcrystallites”. Fig. 5.14 depicts the structural and molecular formulas. n is 500 and 1,000, respectively. MCC is insoluble, physiologically inert, has high microbial purity, and is no substrate for microorganisms.
Structural formula:
Molecular formula: (C,H,O,), Fig. 5.14
Chemical composition o f cellulose
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Fig. 5.15 Examples of PC cellulosic fibers (Vivapur 101, courtesy J.Rettenrnaier & Sohne, Rosenberg. Germany).
Powdered cellulose (PC) is also prepared from plant material by chemical digestion and purification processes. Further mechanical processing, without the use of chemical additives, yields high purity fibers. They are chemically inert and insoluble in water, organic solvents, and dilute acids or alkaline liquors. Depending on the specific technical requirement of a customer, different qualities of PC fibers can be developed and manufactured (Fig. 5.15). Today, both MCC and PC fiber grades are widely used in tabletting. Depending on the composition of the formulation, one or the other cellulose product results in better hardness, friability, and disintegration values. However, the quantity of MCC required to yield comparable tablet properties is normally at least one-third higher than that of PC fibers. Since, because of a more economical production process, the cost of PC fibers is also lower than that of MCC, monetary advantages can be derived from using powdered cellulose. Other organic fiber products which are mostly used in foods as “dietary” ballast additives are made from wheat, oats, tomato, apples, and citrus. Such dietary fibers are “non-starch” polysaccharides obtained from cell walls only, which can not be broken down by the digestive enzymes of the human organism and, therefore, constitute inert ballast materials. Color, taste, and odor relate to the fiber source. Unlike cereal brans or dietary fibers derived from, for example, sugar beets, which are often rejected by consumers because of their specific taste, wheat, oat, tomato, apple, and citrus fibers offer physiological properties that are much more readily accepted. Although these fibers are primarily used in foods, there are also applications in other industries. Functional characteristics of the fibers include high water binding and retention capacity (as a rule: the longer the fibre, the more water it retains), no synergy effect with thickening agents in the normal dosage range (for example, up to 10 % of wheat fiber can be added to absorb and bind liquids and oils before a thickening effect can be detected), improvement of the rheological properties of various thickening agents (e.g., improvement of the thixotropic qualities of carbomethyl cellulose (CMC)),and free flow/anticaking agent (with very low dust content). Starches and compounds derived from starches have long been known as additives in many industry. These materials improve flowability and act as binders as well as disintegrants.
5.1 The Development of Strength of Agglomerates
A particularly interesting newer starch derivative is sodium starch glycolate (SSG). It is the sodium salt of the carboxymethylether of potato starch or other starches (e.g. wheat, maize (corn), rice, etc.) and is a fine, almost white, odorless and tasteless, free flowing powder. Because of the low degree of substitution (see Fig. 5.16), the form and particle size of the original starch remains almost unchanged. SSG is practically insoluble in organic solvents and forms translucent suspensions or clear gels with water. Until recently, sodium starch glycolate has been used exclusively as a disintegrant in pharmaceutical solid dosage forms. Since it was found that the manufacturing process can be modified, specific SSG grades are produced for different new applications (Tab. 5.4). Finally, as further examples in the context of this chapter, the beneficial use of totally different fibers than discussed above shall be mentioned and reviewed. Metal swarf, fine, elongated grindings and turnings which are fibers in a generic sense, may be applied to “mechanically reinforce” briquettes made from metal bearing dusts for recycling into metal making processes. For this application, it is important to produce high strength of which at least a certain part is retained at high temperatures, until melting occurs, so that secondary contamination due to premature release of dust is avoided. The influence of these fibers on briquette strength is demonstrated in Fig. 5.17 which depicts that the strength of briquettes increases with growing addition of swarfwhile the necessary amount of chemical binder, constituting contamination and non temperature resistant bonding, decreases [B.42]. Fig. 5.17 also shows a broken cylindrical compact (a) that was manufactured with a laboratory piston press during process development (see also Section 11.2) and actual HO
Fig. 5.16
Structural formula of sodium starch glycolate (SSG).
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5 Agglomeration Theories Tab. 5.4
New applications of sodium starch glycolate (SSG), according to J. Rettenmaier 6.Sohne (JRS). Application as
Disintegrant
Quasi soluble disintegrant
Wet granulation Taste masking binder
Thickening agent Gel former
Grade P ”)
Grade P5000
Grades P1500, P5000
Grades P1000, P3500, P5000
Guarantees excellent disintegration times of tablets, (film-)coated tablets, capsules, and granules.
Swells very much in water and forms translucent gels. These are particularly suitable to improve disintegration of tablets, effervescent tablets, soluble tablets, granules, etc.
Is used because Have very good adhesion proper- of its gel forming ties and are sui- properties to table as binding mask the taste agents with in lozenges and disintegration chewable tablets. properties in wet granulation. They can be added in powder form and granulated with water.
Grade PSOOO
Grade PO100
Are used as Forms clear gels thickening and which are stable stabilizing agents within a wide in juices, suspen- range of sions, emulsions, temperatures. ointments, creams, etc.
Typical dosage
1-5 %
2-20 %
u p to 5 %
5-20 %
2-5 % in special 5-20 % cases also higher
in the grade designation refers to potato starch as origin. All grades can also be made from other starches (“M” = maize (corn), “ R ” = rice, “W” =wheat, etc.). For formulations that are incompatible with alcohol the grade designation “SF” guarantees an alcohol content below 1 %.
7’:)”P”
briquettes (b) obtained in the industrial plant. Concerns that the swarf “fibers” would prohibit separation of briquettes that are produced with a roller press did not turn out to be a problem. Similar to concrete, refractory linings and components are agglomerates in which highly temperature resistant aggregates and mortars represent a system which is shaped and fired to yield bricks or other components that are then set into mortar for mounting, or is applied by casting or gunning. Today’s high temperature processing industries demand high performance and predictable service life from the refractory. The latest generation of low cement, ultra-low cement, and self-flow castables, which are resistant to high temperatures, continue to be weak in tension and offer minimal resistance to damage from sudden changes in stress. Thermal cycling or shock as well as mechanical impact or vibration can all cause cracking, which, in turn, may lead to premature failure and substantial costs. Because the development of cracks can not be avoided in the rough environments of the typical applications of refractories, the probability must be reduced that such cracks result in failure. This is possible by reinforcement with fibers. Some materials that have been added to accomplish this are stiff, needle-like chopped wire or slit sheet fibers which are sometimes even supplied with, for example, hooks on their ends to increase anchorage. As schematically shown in Fig. 5.18 steel fibers in the refractory structure arrest the cracks and prohibit their propagation. Newer reinforcement
5.7 The Development of Strength of Agglomerates
L
3parts sutfite-waste powder a s a binder -
140
/30%
content of
Hardening duration ( d a y s )
Fig. 5.17 Cold crushing strength o f briquettes from metal bearing dust, containing different amounts of dry lignosulfonate binder (called "sulfite waste"), with and without swarf reinforcement, as a function o f (natural) curing time. Photographs o f "reinforced" metal bearing dust briquettes (scales not identical). (a) Cylindrical test briquette and fracture surface, (b) commercial pillow-shaped briquettes.
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materials use direct spun stainless fibers that are rapidly cooled. The resulting products are fully annealed and, therefore, more pliable and ductile, feature better flow characteristics and an improved aspect ratio that results in optimum dispersion due to easy disentanglement of the fibers during mixing with the wet refractory system, and offer exceptional resistance to high temperature corrosion since a beneficial metallurgical structure is “frozen” during the ultra-rapid cooling process. The photograph in Fig. 5.18 shows a representative selection of some of these stainless steel fibers for the refractory industry.
Fig. 5.18 Schematic depiction o f the crack stopping mechanism o f steel fibers in a refractory. Photographs of some typical stainless steel fibers for the reinforcement o f refractories (courtesy RIBTEC, Cahanna, OH, USA).
5.2 Estimation of Agglomerate Strength
5.2
Estimation of Agglomerate Strength
The most important property of all agglomerates, desired or undesired, is their strength. For the practical and industrial investigation of agglomerate strength, stresses that occur in reality during storage and handling are experimentally simulated (see Section 5.2.2). In addition to the frequently used crushing, drop, and abrasion tests, methods for the determination of impact, bending, cutting or shear strength are employed. All values obtained by these methods are strictly empirical and cannot be predicted by theory because it is not known which component of the applied stresses causes the agglomerate to fail. For the same reason, the experimental results from different methods can not be compared with each other. Therefore, Rumpf (see Chapter 1) proposed to determine the tensile strength of agglomerates. It is defined by the tensile force at failure divided by the cross section or, if the test body has no uniform shape, the area of the failure plane(s) of the agglomerate(s) (see Section 5.2.2). Because failure occurs in all stressing situations with great probability under the influence of the highest tensile force, this proposal is justified. Moreover, tensile force and strength can be approximated by models and theoretical calculations. 5.2.1
Theoretical Considerations
All binding mechanisms of agglomeration (see Section 5.1.1) can be described by one of three models (see Section 5.1, Fig. 5.4): 1. The entire pore volume of the agglomerate is filled with a substance that can transmit forces and, thereby, causes strength (matrix binder, Fig. 5.421). 2. The pore volume of the agglomerate is entirely filled with a liquid (Fig. 5.41). 3 . Binding forces are transmitted at the coordination points of the primary particles forming the agglomerate (Fig. 5.4d).
Liquid bridges at the coordination points (Figs 5 . 4 ~ and 5 . 7 ~are ) described by model (3) while the transitional state (Fig. 5.7d) is connected with model (2) through the liquid saturation, S (see Section 5.1). ad 1) Maximum tensile strength ifthe pore volume isjlled with a strength-transmitting substance If the pore volume of the agglomerate is completely filled with a stress transmitting substance, e.g. a hardened binder, three strength components can define agglomerate strength: (a) q, (pore volume strength) = tensile strength of the binder substance, (b) ota(grain boundary strength) = tensile strength caused by the adhesion between binder and particulate solids forming the agglomerate,
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4
n
Fig. 5.19 Two-dimensional schematic representation of the failure lines derived from the three models describing strength o f agglomerates with a matrix binder.
(c) ot(l-E) = strength of the particulate solids forming the agglomerate. The relatively lowest component determines the agglomerate strength. Fig. 5.19 depicts schematically the expected failure lines in a two dimensional schematic representation.
ad 2) Maximum tensile strength ifthe pore volume is filled with a liquid If a liquid that wets the solid(s)fills the entire pore volume of an agglomerate to such a degree that concave menisci are formed at the pore ends on the surface, a negative capillary pressure p, develops in the interior of the agglomerate. Because the membrane forces at the surface are negligibly small in relation to the capillary pressure, the tensile strength qcof agglomerates that are completely filled with a liquid can be approximated by the capillary pressure: otc
- Pc
(Eq. 5.1)
Assuming that the pore diameter is characterized by the mean half hydraulic radius of the pore system, further assuming perfect wetting and spherical monosized particles, the following formula is obtained:
-
otc
pc = a’ ( 1 - E ) / E ax
(Eq. 5.2)
The maximum tensile strength of agglomerates that are completely filled with a perfectly wetting liquid depends on the porosity of the agglomerate, characterized by the strong term (1 - E ) / E , the surface tension a of the liquid, and the size x of the particles forming the agglomerate. The empirical correction factor a’ has values between G and 8. An approximation of the agglomerate strength ott in the transitional (“funicular”) state, in which a certain percentage S (= saturation, see Section 5.1) of the pore volume is filled with liquid, is possible by multiplying the maximum strength otcwith the appropriate saturation S:
(Eq. 5.3)
5.2 Estimation of Agglomerate Strength
ad 3 ) M a x i m u m tensile strength $forces are transmitted at the coordination points of the particles forming the agglomerate Estimation of the strength of agglomerates which is caused by solid bridges at the coordination points assumes that the entire solid binder material is uniformly distributed at all coordination points and forms bridges with constant strength oB.If, in addition, failure only occurs through solid bridges, the relative cross section of that material defines the agglomerate strength: (Eq. 5.4) MBis the mass of the bridge building material and M p the mass of the agglomerate building particulate solids, pBand pp are the densities of the respective solid materials, 1 - E is the relative volume of the particulate solids building the agglomerate, E is the specific void volume (porosity) ofthe agglomerate, and vBis the fraction ofvoids in the agglomerate that is filled with the bridge building material. Strength may be also caused by adhesion forces A acting at the coordination points of the particles forming the agglomerate. Based on statistical considerations and a simple model, Rumpf [5.l] developed a general formula that is often used to describe agglomerate strength: ot = (1 -
E)/X
k A/x2
(Eq. 5.5)
E is the specific void volume (porosity) of the agglomerate and (1- E ) the respective volume of the particulate solids, x = 3.14...., k the average coordination number, and x the representative size of the particulate solids forming the agglomerate. For k an empirical approximation exists:
kE
N
x
(Eq. 5.6)
with which Equation 5.5 is simplified to: ot = (1 -
E)/E
A/x2
(Eq. 5.7)
Theoretical Approximation o f Adhesion Forces The still unknown term in Equation
5.7 is the adhesion force A. Firstly, it must be recognized that, normally, more than one binding mechanism participates in the production of agglomerate strength. Secondly, due to differences in micro conditions, it must be expected that the adhesion force Ai at each coordination point is different. Therefore, Equation 5.7 becomes in its most general form: ot = (1 -
E)/E
CA,/x2
(Eq. 5.8)
Work of many researchers concentrates on modelling and calculating adhesion forces that are caused by the different binding mechanisms IB.421. So far, all models are based on simplified conditions at the coordination points. For example, modelling of the adhesion force of a liquid bridge is based on two monosized spherical particles with a distance a from each other (Fig. 5.20). Adding the two adhesion force components, one caused by the negative capillary pressure in the bridge and the other by the boundary force at the solid/liquid/gas
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Fig. 5.20 particles
Liquid bridge between two monosized spherical
contact line, a general formula for the adhesion force of a liquid bridge AiLcan be derived: AIL= a x f(pAa/x)
(Eq. 5.9)
The adhesion force of a liquid bridge between two monosized spherical particles depends on the surface tension of the liquid a, the particle diameter x, and a function of the angle p which defines the size of the bridge, the angle of contact or “wetting angle” 6, and a dimensionless term a/x which represents the distance at the coordination point. Obviously, a large number of different partner shapes, other than sphere to sphere, are possible and, normally, the size of the partners will be different and can vary infinitely. As already mentioned in Section 5.1.1 and shown schematically in Fig. 5.11 all particles also feature rough surfaces. Proving roughness, even on the macroscopically smoothest surfaces, depends only on the magnification. Therefore, when modelling surface interactions, this can be done macroscopically, disregarding surface roughness (for example the liquid bridge model above), or microscopically. In the case of liquid bridges the latter means that the distance a is an average as depicted in Fig. 5.11 and the angle of contact depends on the microscopic topography and, therefore, results in very complicated bridge geometries which can not be modelled. Generally, the description of the true shape of a particle, including surface roughness can not yet be described unequivocally. New techniques, such as fractal dimensions [B.37], may be applied in the future to solve this problem. As another example of modelling efforts, the estimation of the van-der-Waals adhesion force will be discussed. Three different situations at the coordination point, two flat surfaces, a spherical particle opposing a flat surface, and two spherical particles, are presented. Because van-der-Waals forces are field forces, models take into consideration the atomic and molecular interactions between the two entities. A microscopic
5.2 Estimation of Agglomerate Strength
theorie (Hamaker [5.2])assumes that all interactions may be added up and obtains the van-der-Waals adhesion force AiYdWby integrating over all pairs of atoms and molecules. The characteristic term His the “Hamakerconstant” with a value ofapprox. lo-” to lo-’’ J. The macroscopic theorie (Lifshitz [5.3], Krupp [5.4]) calculates the interaction force from the energy dissipation of the electromagnetic fields that emanate from the bodies and obtains a similar van-der-Waals adhesion force. In this case ho is the Lifshitz-van-der-Waals constant with a value of approx. 1.G.10-20 to l.G.lO-l’ J. Fig. 5.21 summarizes the model conditions and the results. 0 is the respective unit area on the opposing flat surfaces. The equations in Fig. 5.21 are only valid for distances a that are less than 150 nm. However, because already at much smaller distances at the coordination points the contribution of van-der-Waals adhesion to the strength of agglomerates becomes insignificant, this limitation is of no concern. It should be also noted that for very small distances, the “Born repulsion” is predominant as shown in Fig. 5.22. In addition to the already discussed influence of the actual micro topography at the coordination point, other conditions may influence the true adhesion forces that act between the solid partners. In the case of van-der-Waals forces the average distance a as shown in Section 5.1.1, Fig. 5.11, may be changed by the presence of adsorption layers (Fig. 5.23). From an adhesion physics point of view, adsorption layers with a thickness of less than 3 n m are so strongly bonded that they are immobile and can be considered as part of the solid. Because adsorption occurs primarily at energetically favorable locations, such as in depressions or valleys, it tends to smooth-out the surface roughness resulting in a reduction of the actual distance between the particles at the coordination point
Hamaker microscopic
Lifshitz macroscopic
AlVdw/O = H/6rra’
A,,,/O
Fig. 5.21 Three model conditions forthe estimation ofthe van-derWaals adhesion force and the results o f two theories.
=
hw/8n2a3
(Eq. 5.10)
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5 Agglomeration Theories ARepulsion
A
van- der-Waals Adhesion
AAdhesion
*
Fig. 5.22
Relationship o f Born repulsion and van-der-Waals adhesion as a function o f the distance a at the coordination point
(Fig. 5.23) and an increased adhesion force. At ambient conditions the adsorption of atoms and/or molecules from the atmosphere is a natural phenomenon. Therefore, it can also happen during storage in bulk solids and even within agglomerates. While in the latter case agglomerate strength is enhanced which, in most cases, is not detrimental, the development of adsorption layers in bulk solids can lead to difficulties during discharge, feeding, and metering.
Fig. 5.23 Model explaining the increase in strength due to adsorption layers during van-der-Waals bonding [5.1].
5.2 Estimation of Agglomerate Strength
5.2.2 Laboratory and Industrial Evaluations
Major parameters determining the properties of agglomerates are: 0 0 0
0
The primary particle size, x, distribution, Ax),surface area, s(x),and shape. The agglomerate size, d, distribution, j d ) , and shape. The apparent and bulk densities as well as the porosity E (= voids between the primary particles), also the pore sizes and their distribution in the agglomerate. The strength of the agglomerate.
Primary particle size, distribution, surface area as well as micro (= surface structure) and macro shape, define the agglomerative behavior of a given type of particulate solids. The agglomerate (used as a generic term) size, distribution, and shape together with the characteristics discussed in Section 5.3 determine most of the advantages of agglomerated materials. The apparent density describes the mass of the agglomerates themselves, and the bulk density delineates the space filling behavior (e.g. the packing volume) of an agglomerated product. The porosity of agglomerates (see Section 5.3.2) is another method of describing their apparent density; it is the void volume between the primary particles forming the agglomerate and defines the accessibility of the internal surface area while the pore sizes and their distribution regulate the capillary suction which is responsible for “takingup” liquids (as in absorbents). The strength of agglomerates is one of their most important properties and may have many different meanings. In most cases the attribute “strength” defines a survival characteristic and may be defined as crushing, bending, cutting, shear, or tensile strength, as tolerance to one or several drops from a specific height, thereby reproducing stresses experienced at transfer points, or as resistance to attrition and the formation of dust [B.42].For special applications still other measures of “strength” may be elaborated that simulate the real handling or processing conditions. Scientifically the only unequivocally defined and reproducible strength, that is ultimately and with a high degree of probability responsible for all failure modes and can be also approximated by theoretical calculations, is the tensile strength. A general formula describing the tensile strength ot of agglomerates, which are held together by binding mechanisms acting at the coordination points, was given in Section 5.2.1, Equation 5.8. The equation shows, that the porosity of agglomerates plays the most important role for their strength. The lower the porosity or, in other words, the higher the apparent density of the agglomerate, the stronger is the agglomerate. Since many of the desirable characteristics of agglomerated products require high porosity, sufficient strength is obtained in such cases by selecting a suitable binding mechanism featuring high adhesion or binding forces, using a powder with a small representative particle size, applying suitable curing techniques that produce permanent bonds with high strength (e.g. by sintering), and/or incorporating temporary additives in the feed. During or after the curing step such components are removed by melting, evaporation, or combustion (see Section 5.3.2).
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Results of experimental determinations of agglomerate strength have been published in many scientific works and were summarized in numerous specific books on agglomeration or in major chapters of more general handbooks (see Section 13.1). In the following, a few examples will be presented to describe generally important trends. For a more detailed coverage, the literature, particularly also the proceedings of the International Symposia on Agglomeration [B.4, B.14, B.18, B.23, B.35, B.48, B.701 should be consulted. Because strength depends critically on porosity, this property should be always measured first. To allow a comparison of individual strength values which were determined on different agglomerates they must be adjusted to fit a representative porosity. Then a larger number of results should be averaged and presented together with the statistical standard deviation or the minimum and maximum deviation of single values. If the density (specific mass ps) of of the solid particles forming the agglomerate (composite density if more than one material participates) is known and the volume of the agglomerate can be accurately determined, the porosity can be calculated as: Some Results of Laboratory Determinations of Agglomerate Strength
Agglomerates often contain moisture. If this is the case, they must be dried prior to the determination of the solid mass, M,. Also, with the exception of flat, cylindrical tablettes (see Section 8.4.3) and similarly well defined shapes, agglomerate volume can not be easily calculated. In those cases, the buoyancy of the agglomerate in a liquid is often measured. Since, according to the principle of Archimedes, the buoyancy is equal to the mass of the displaced liquid (under the assumption that the liquid does not penetrate into the agglomerate) the volume can be calculated as: (Eq. 5.14)
M,is the mass of the liquid which is displaced by the agglomerate during the buoyancy test and pL is the liquid's specific mass. The requirement that the liquid must not penetrate into the liquid can be met by using of a non wetting liquid (mercury was applied widely, also because of its high specific mass, by coating the surface of the agglomerate with a liquid repellant (e.g. oil)), or by painting a thin film of lacquer onto the agglomerate. The error caused by any of the protective measures is insignificant. If the binding mechanism between the agglomerate forming particles is not destroyed by the liquid, it is also possible to totally saturate the porous body and then reimmerse it to determine the buoyancy. By inserting Equation 5.14 into 5.13 another formula for determining porosity is obtained: & =
1 - (PdPS)
(Ms/ML)
(Eq. 5.15)
Porosity can be also measured by pressure permeation methods [B.GO] ifthe agglomerate can not be treated and submerged in a liquid without losing its integrity. Fig. 5.24 depicts some laboratory methods for the determination of the strength of agglomerates and cohesive powders. Often, even in a scientific environment, the transversal crushing force is measured (Fig. 5.24a). This method is quite acceptable for
5.2 Estimation of Agglomerate Strength
Agglomerate
Adhesive
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R
Fig. 5.24 Laboratory methods for the determination o f t h e strength of agglomerates or cohesive powders [B.42].
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perfect cylinders, such as tablettes and some extrudates. However, any spheroidal agglomerate is so irregular that a perfect diametral loading is impossible. This results in undefined stressing with compression, shear, and tensile forces acting in unknown ratios so that a wide scatter of data is obtained from undefined sources. Also, the definition of a compression “strength”by dividing the force at failure by the projection area of the agglomerate is, from a scientific point of view, not acceptable. Normally, the statistical mean force at failure of testing a large number of agglomerates is reported. Crushing a sheet or cylindrical agglomerate by loading parallel opposite flat surfaces between plates is even more problematic because in very few cases the faces of the agglomerate are truly parallel resulting in uneven loading or the lateral expansion is blocked by friction between the agglomerate and the plates so that uncontrolled stress concentrations build up which may be the true cause for failure. As a consequence, data obtained from transversal crushing tests are seldom comparable. More reproducible results are obtained if the shear strength of a well defined, often specially prepared agglomerate is measured (Fig. 5.24b). This method was adapted from the well known shear cell for the evaluation of cohesive particulate solids (Jenike shear cell and derivatives [5.5]). During fundamental work on the binding mechanisms and strength of model agglomerates or cohesive powders, most of the laboratory evaluations determine tensile strength (Fig.s 5 . 2 4 ~- 8). Machinable agglomerates are converted into cylinders that are glued between two adapters (Fig. 5.24~)and torn apart in a standard tensile test machine (e.g. Frank, Instron, see also Section 11.2). Other methods use “split” dies, with or without mandrils, for the manufacturing of a compacted agglomerate which is then pulled apart at a cross section which is defined by the split mold. Low strength caused by various binding mechanisms with or without prior densification is measured in flat split containers of which one part is fixed and the other part is movable with insignificant frictional resistance. The load can be applied by slowly lifting up the support table (Fig. 5.24e) or providing an incrementally increasing horizontal force (Fig. 5.24f). Often, it is desirable or necessary to measure the strength of agglomerates which are partially or completely filled with a liquid. Particularly, in the high range of saturation the correlation of strength with the capillary pressure and their change during wetting or drying of the bed (hysteresis effect) is of interest. For this purpose, the simple method shown in Fig. 5.24fwas modified as shown in Fig. 5.248 [B.42]. The most reliable results of tests determining agglomerate or cohesive powder strength that can be also interpreted best are those in which the binding mechanism is caused by the surface tension of liquids and/or the resulting capillary forces (see Section 5.1.1, I11 in Tab. 5.1). With a high degree of probability the influence of other binding mechanisms can be excluded in agglomerates or powders that are bonded by a liquid. As shown in Equations 5.2, 5 . 3 , and 5.7 together with 5.9 (see Section 5.2.1), this binding mechanism in defined by the surface tension a as well as other characteristics of the liquid and the solid, such as the wetting angle 6, the porosity E, and the representative size of the particles forming the agglomerate. Fig. 5.25 depicts the tensile strength, determined in the laboratory according to the method shown in Fig. 5.24c, of nearly saturated agglomerates made from narrowly distributed quartz and limestone powders as a function of the size x of the particles
5.2 Estimation of Agglomerate Strength
; ! , !
Quartz
Limestone
0.4 -
-
N
E
. E z
0.2 -
c
0.1 0.080.06-
;;
0.OL -
e-
5 Ol
2
a, .-
,"
0.02-
a,
c Fig. 5.25 Tensile strength oI o f nearly saturated agglomerates as a function o f the size x o f the particles forming the agglomerate. Porosity adjusted to E = 0.35. oIcaccording to Eq. 5.2.
0.008 0.0°6
LL
0.OOL 1 1
I
2
I
I
l
l
I
I
L 6 8 10 20 40 P a r t i c l e s i z e x lpm)
1
60
L
forming the agglomerate. The porosities of the individual agglomerates were adjusted arithmetically to E = 35 %. The diagonal lines represent the theoretical tensile strengths according to Equation 5.2 with a' = G and a' = 8, respectively. The diagram shows that the relationship 'T, l / x is fulfilled. The actual values are lower than theoretically predicted because the agglomerates which were produced in a pan (see Section 7.4.1) are not fully saturated with water and the structure of technically manufactured agglomerates is not perfect. Although not unequivocally visible in Fig. 5.25, regression analyses of these and many more sets of data revealed that the representative particle size for agglomeration processes is the surface equivalent diameter, x,. The importance of this representative diameter for the unit operation is not surprising as structure and bonding of the products critically depend on the surfaces of the particles forming an agglomerate as well as on the surfaces' microscopic and macroscopic conditions. Of course, only the exterior particle surface is responsible for the effects; potentially internal surface area of the agglomerate forming particles must not be included when calculating the surface equivalent diameter of a particulate mass. Therefore, experimentally, surface area should be determined by permeametry, for example the well known and in the cement industry universally applied Blaine method [ B.GO]. The data in Fig. 5.26 confirm that the relationship between tensile strength q,agglomerate forming particle size x, and surface tension of the binder liquid a and the porosity function (1- & ) / I as per Equation 5.2 is correct and Fig. 5.27 proves that the (compression) strength of agglomerates increases linearly with the surface tension of the binder liquid as indicated by Eq. 5.2. Finally, Fig. 5.28 presents the tensile strength 'T,of moist and wet agglomerates as a function of liquid saturation S. At the two extremes S = 0 % and S = 100 % the strength N
I
65
5 Agglomeration Theories -c c
0
i
C
E
c
m
U a ’-
\
c“*
2tY a .-
c
-0a
21.o . !
I I :
I
I
I
0.L
0.6
0.8
I
I
I
1.0 1.2 Porosity f u n c t i o n (1 - E ) / E
l.L
Fig. 5.26 Relative tensile strength crt x/a o f agglomerates made from sperical glass powder related to the porosity function (1 --E)/E and compared with the theory (Eq. 5.2) [B.42].
1.0,
I
I
I
I
I
Surface tension a x
I
lo5
I
1
(Nlcrn)
Fig. 5.27 Compression strength (r of spherical wet agglomerates as a function o f the surface tension a o f the liquid [B.42]. Surface tensions are that o f pure alcohol and water and o f 30/70 and 10/90 vol.% mixtures o f alcohol and water.
is close to zero. “Bone dry” powders feature very low tensile strength unless they are compacted or the representative size of the agglomerate forming particles is < 1 Fm. If it is assumed that at S = 100 % concave menisci are no longer formed at the pore ends on the agglomerate’s surface, capillary pressure is zero which, according to Equation 5.1 also lets strength go to zero. Between those extremes the conditions of Fig. 5.7 (c, d, and e) (Section 5.1) exist. On the left, Fig. 5.28 shows three curves for different dimensionless values a / x that were calculated using Equations 5.7 and 5.9. It can be assumed that up to a saturation of S = 30-40 % discrete liquid bridges prevail at the coordination points between the particles forming the agglomerate. In the right part of Fig. 5.28 the experimentally determined capillary pressure is plotted (solid dots). This curve is obtained when
5.2 Estimation of Agglomerate Strength 167
N
N
.. E E E E
zz --
1
0
Fig. 5.28
Tensile strength
‘T,and
20
I
I
LO 60 L i q u i d saturation qL
I
80
100
(YO)
capillary pressure pc as functions
of liquid saturation 5 [B.42].
the liquid from a totally saturated agglomerate is drained, for example, in an arrangement as shown in Fig. 5.248. As mentioned before, at S = 100 % the capillary pressure is zero. If, starting at this point, the agglomerate is drained, the capillary pressure rises steeply (development of concave menisci at the pore ends on the surface of the agglomerate) and then turns to a much slower rate of increase. The point at which the tangents to both curves intersect is defined as the entry suction pressure pe at which liquid begins to recede into the agglomerate. It is generally located at S>90 %. At approximately that point the maximum tensile strength of wet agglomerates exists. The circular and square open symbols represent average values of experimental results of measurements of tensile strength with their standard deviation. In the bridge model (“pendular”)state the results seem to fit the curve for a / x = 0.02 best and for high saturations (“capillary”state) they approach pe. In the transition (“funicular”) range between 30 % < S < 95 % both binding mechanisms, liquid bridges and saturated pores, contribute to the development of strength. The fact that in the transition range a difference exists between the strength ofagglomerates to which liquid was added (e.g. during agglomerate growth, circular open symbols) and agglomerates from which liquid was drained (square open symbols) confirms that liquid can only be drained from saturated pores and liquid bridges are not influenced. The mechanism of capillary flow in wet agglomerates is an important factor if the liquid is a solution or becomes one (because all or some of the agglomerate forming particles are soluble) and the dissolved material recrystallizes during drying [1.1,B.421. If the agglomerate is highly saturated, drying takes place only on the surface. Liquid moves by capillary flow to the surface where evaporation occurs and recrystallizing substances deposit. The formation of a crust may influence further drying of the porous body considerably. The developing crust reduces the drying rate and may, after
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5 Agglomeration Theorks
forming a dense crust, stop drying altogether. Since the crystal structure is influenced by the drying rate, the strength of recrystallizing substances in agglomerates during drying will be controlled by either the drying temperature, the crust, or both. Fig. 5.29 presents the tensile strength ot of the core of dry agglomerates with recrystallized salt bridges which was obtained after removing a surface layer (including a crust, if applicable). The diagram shows that for agglomerates with very low initial moisture contents (curves 1 and 2) the strength increases as expected, almost linearly with an increasing amount of available salt (rising saturation) and with the drying temperature. At higher drying rate, finer and stronger crystallites grow at the coordination points in the agglomerate and, because the liquid (solution) was concentrated in discrete, immobile bridges, no crust had developed. At an initial saturation of 20 % (curve 3) the formation of a crust begins to influence strength at high drying temperatures while for the highest liquid saturations (S = 45 % and GO %) the dense crust, formed at all temperatures, is the deciding factor for drying and development of strength (curves 5 and 6). Above 175 -200 "C the temperature within the porous body rises so quickly that the vapor pressure building up below the dense crust causes the agglomerate to burst (Fig. 5.30). The unexpectedly high tensile strength obtained at a liquid saturation of 30 % and a drying temperature of 350 "C is due to the formation of a network of small cracks in the crust that did not cause the agglomerate to fracture but increased the drying rate and, thus, the tensile strength ofthe dry agglomerate core. I
I
I
0.L
-
N
. - 0.3 E E Z
L
b
5 ol C
E
+. 0.2 cn
AVO
.(u
1
VI
2
C (u
t-
0.1
7,6/ OOL
P e l l e t s burst I 0 300
Drying temperature t d
(OC)
Fig. 5.29 Tensile strength q o f the core o f agglomerates with salt bridges as a function ofthe drying temperature td for different liquid saturations S prior to drying [l.l, 6.421.
5.2 Estimation of Agglomerate Strength 169
Fig. 5.30 Photographs of cylindrical agglomerates which contained a high amount o f a nearly saturated salt solution and burst during drying.
The above mentioned incrustation may be positive or negative. On the positive side, the phenomenon can be used as a method to achieve encapsulation of agglomerates if a film forming, easily soluble polymer is dissolved in the liquid phase. On the other hand, if a dryer is controlled by sensing the moisture content in the off-gas, the process instrumentation may mistakenly identify a heavily encrusted product as being dry when, underneath of the crust, moisture still remains. Such a product can, of course, cause a whole host of problems, such as caking during storage when the liquid slowly redistributes and problems during a secondary process, for example tabletting of a still partially moist granulated pharmaceutical formulation, as well as many more difficulties. During the initial phase of drying, when all evaporation occurs on the surface o f the porous bodies, the temperature o f the material to be dried stays at or below 100 “C. For highly temperature sensitive materials this temperature can be lowered by the application of vacuum. However, if incrustation occurs, the temperature of the mass to be dried increases to the temperature of the drying gas and can cause damage to the material. A considerable amount of fundamental research is going on in many places of the world trying to increase knowledge of all binding mechanisms and develop numerical methods to calculate or at least estimate binding forces as well as agglomerate strength. In addition to the “classic” standard methods discussed above many novel technologies, such as, for example, application of the atomic force microscope (AFM) (also called lateral force microscope (LFM)or scanning probe microscope (SPM)) for the measurement o f adhesion in the micron and submicron particle range, and new theories, for instance, Fractals [B.37] and the Chaos Theory, are applied to agglomeration research. However, as mentioned earlier (see Section 5.1.1), it is unlikely that only
70
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one binding mechanism acts on all coordination points within even a single agglomerate. Moreover, the microscopic conditions at each coordination point are so diverse that bonding at virtually each individual coordination point is different. Therefore, although big advances are being made, the science of agglomeration is still far away from formulating a useful general theory. Furthermore, this book is devoted to a more practical coverage of agglomeration. Therefore, the reader is encouraged to search for and study the increasing number of publications that report on the advances in this area (see Sections 13.1 and 13.2). Industrial Evalutations of Agglomerate Strength Determination of agglomerate strength in industry is much more pragmatic [B.42]. Although knowledge and understanding of the fundamentals of agglomeration, particularly the nature and effect of the binding mechanisms and how they can be influenced, become more and more important during the development of new and for the optimization of existing agglomeration processes, agglomeration as a unit operation is still more an art than a science. While an increasing number of criteria are known for the preselection of the most suitable agglomeration process for a specific application (see Chapter 11),it is still necessary to test the selected equipment in the laboratories of vendors or development organizations (see Section 11.2). Often, if the process is a new one, it is even desirable to operate a smaller pilot plant or to involve a “toller”, an outside processor for hire, prior to an investment decision for a large scale plant (see Section 11.2). Agglomerate strength in industry is defined as a commercial or process characteristic of the particular intermediate or final product. For example, if the agglomerated material is a final product, strength may be defined as resistance to breakage, chipping, or abrasion. The definition of this property and of other strength related requirements will differ whether it is an industrial bulk material or a consumer product. While the former may break down to a certain extent, as long as it remains free flowing and dust free, a consumer product must have perfect and pleasing appearance where even minimal chipping or breakage into large chunks must be avoided. Intermediate products must have characteristics that are suitable for the intended further processing. For example, a material may have to be strong enough and abrasion resistant for storage and handling to avoid bridging, flow problems, dusting and segregation of components. If it is a feed material for tabletting or other pressure agglomeration methods, the agglomerates must break down totally under pressure and produce a uniform final product structure. Other agglomerated intermediates may have to feature the opposite property, i.e. to yield a filter with bimodal pore size distribution it must retain its shape and structure during pressing (see also Section 5.3.2). For those reasons, “strength” means many different things in industry. Typically, measurement of strength is based on a simulation of the stresses which a particular agglomerated product must withstand. Very few industrial methods for the determination of this property are standardized or even known. In a competitive environment it is of less interest to compare quality between rivals than to make sure internally that the product properties that are expected by the industrial or public consumer are maintained. Therefore, most measurements of strength are undertaken as quality assurance. A few will be described below as examples.
5.2 Estimation of Agglomerate Strength
A general problem associated with the determination of product properties in industry is sampling [B.24, B.271. Particularly the measurement of strength is in most cases based on totally or partially destructive methods. If taken during production, these “lost” samples are extracted from the product stream in a random but representative manner and either tested directly “in-line” or, sometimes after again sampling the sample, in a quality assurance laboratory which is associated with production. Afterwards they are discarded. During initial and occasionally repeated process optimization, the influence of different process parameters on agglomerate strength is determined. Results of the measurement of strength are often used to adjust process parameters as required. If there is a difference between “green”and “cured” or final strength, both strength values may have to be evaluated to allow adjustment of the respective process steps. Even bigger problems exist iflarge bulk masses (e.g. stock piles, silos, ship loads, rail cars, trucks, etc.) must be sampled. This is done to guarantee product quality prior to or after shipment and at the point of consumption. Results of those tests are only of commercial value because, typically, they can no longer be corrected but may influence acceptability or price of the commodity. Often, if quality is below standard but does not meet the guarantee, the price will have to be adjusted by offering discounts or rebates. Among the few standardized methods for determining agglomerate strength are the compression strength (IS0 TC 102/Sc 3 DP 4700 and ASTM E 382-97) and tumble ( I S 0 3271 1975 E and ASTM E 279-97)tests for iron ore pellets as well as the “tumbler test” for coke (ASTM D 294-72). In this case, a group of consumers (steel companies) forced a growing number of independent suppliers to test and guarantee agglomerated bulk commodities by formulating the standards. For iron ore the tests are on finished pellets, either prior to shipment or at the consumer’s facility and, therefore, are not intended or even suitable for process control. It will be shown later that iron ore pellets are first produced as “green” agglomerates and then indurated by sintering. For the determination of compression strength, a bulk sample is first screened and at least 600 pellets are taken from the size range in which the maximum is found. In a “Riffle” splitter [B.24, B.271 four samples, containing at least 100 pellets each are prepared. From two of the samples, individual pellets are placed between the parallel, surface-hardened platens of a compression testing machine, loaded with a constant speed, and crushed. The maximum force at which each pellet breaks is determined and recorded. After testing 100 pellets of each of the two samples, the arithmetic averages for the batches are calculated. If they deviate by more than a predetermined amount, another 100 pellets are tested to confirm one or the other value. In the tumble test the abrasion resistance of the pellets is measured. The “ASTM drum” is a cylindrical container with specific dimensions which is rotated around its horizontal axis at a predetermined speed and for a defined number of revolutions. A given mass containing a representative sample of clean pellets is filled into the drum, tumbled for so many revolutions at the constant speed, removed and screened at 600 m. The “strength”of the pellets is defined by the amount of “fines” smaller than 600 m that was abraded during the test. For coke, the “tumbler test” is carried out correspondingly in a similar drum (Fig. 5.31).
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5 Agglomeration Theories
SIOC CLLVATIOY.
Fig. 5.31
mur IN sccncw ON ma
W CCCVAIIW
.
W F I N Y R I O t I ON a A
Sketch o f the ASTM “tumbler test apparatus” for coke
Both compression and tumble tests have been widely used and modified for other applications. The crushing test can be utilized for any agglomerate that is large enough for individual testing and, sometimes, a single layer of many narrowly sized granules is crushed by this method. However, as discussed above, in most cases, due to a more or less irregular macroscopic and microscopic shape of the agglomerates, stressing is not uniform or reproducible and, therefore, the results can not be used for scientific or general purposes. If large enough numbers are crushed and the results are statistically treated and evaluated, the average values are good enough for quality control in a specific plant. It has been repeatedly shown, however, that a comparison of data between different plants, laboratories, and even between technicians in the same laboratory (often referred to as the “human effect”) is not possible. Many publications also report on the fact, that most agglomerates do not break under the influence of a single, well defined force. Rather, because agglomerates are porous bodies, which are made up from particulate solids with binding mechanisms acting between them, and often feature irregularities in their structure, they will disintegrate in steps. It is possible, that several small pieces break from the agglomerate before it finally fails catastrophically. Other products, particularly wet agglomerates, deform plastically before failure occurs. Some binding mechanism, for example those caused by highly viscous binders or capillary forces (see Section 5.1.1), can also produce a “self healing” effect after a first, smaller crack has developed. Therefore, even an unequivocal definition of the crushing strength is problematic. For the testing of other agglomerates by tumbling, the drum shape and execution is often modified. To avoid a sliding motion and produce cascading during the test, square drums have been designed or varying numbers of differently designed lifters have been built into cylindrical drums. The composition and mass of the sample to be tested, the rotational speed, the duration of the test, and the screen size defining “fines” are varied to fit particular needs.
5.2 Estimation of Agglomerate Strength
If the abrasion resistance of smaller granules, for example of fertilizers, agrochemicals, intermediate products, etc., must be tested, specific, often smaller drums can be used as described above. Recently, based on this technique, again influenced by pressure from consumers and the desire to develop a quality assurance plan, the Saskatchewan Potash Producers Association has defined a standard procedure for the determination of degradation characteristics (= “strength”)of this granulated bulk fertilizer [5.G]. More often however, a representative sample is placed on the particular test screen that defines the “fines” and vibrated or shaken in a laboratory screening machine (e.g. Rotap, Fritsch, etc.) for a predetermined time [5.7]. To produce a sufficiently significant amount of abrasion for quality control, “grinding media”, such as a specific number of steel bearing balls of a particular size or other pieces with the same purpose, are added to the granular sample. If the separation size defining “fines”is very small and, therefore, the screen is delicate, the test can be carried out in the pan. The amount of fines is then determined in a separate screening step. Because of the well defined shape of tablets, crushing tests are regularly and with great success used in the pharmaceutical industry in-line or off-line and often automatically, in combination with an automatic sampler, for monitoring tablet strength. Other modern, fully automated equipment measures tablet weight, thickness, diameter, and hardness for quality control and validation (Fig. 5.32). For some agglomerated products it is important to make sure that they meet certain strength related characteristics. For example, many animal feeds are pelleted by extruding mixtures of conditioned components through cylindrical bores in flat or cylindrical dies (see also Section 8.4.2). While pelleted food for fowl or fish is swallowed whole, products for feeding mammals need to be chewable. A compromise must be found between high strength and abrasion resistance, which allows storage, transportation, and handling without breakdown and/or the production of fines, and the requirement that pellets must be safely crushed between the teeth of the animal. A crushing test to measure this type of crushing strength was developed (Fig. 5.33) and is used for quality control in feed mills.
Fig. 5.32 Photograph of the “Schleuniger Autotest 4” tablet testing system for the quick and automatic measurement o f tablet weight, thickness, diameter, and hardness (courtesy Dr. Schleuniger Pharmatron, Manchester, NH. USA).
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Fig. 5.33 Handheld (a) and automated (b) crushingtest equipment for the determination of strength of pelleted animal feed (courtesy Amandus Kahl, Reinbek. Germany).
In many industries, the requirements on product quality are not very stringent. The agglomerates must withstand handling, including the loading of silos and transport vessels, as well as transfers. Generally speaking, they must survive several drops with only limited breakage and the creation of a minimum of fines. For the measurement of this characteristic “drop tests” are carried out. The actual equipment and procedure may vary widely and is normally a simulation of the expected “abuse” which the material will encounter on its way from production to consumption or use. Fig. 5.34 is the sketch of a typical arrangement. A drop test arrangement can be easily built and applied in the field. The “equipment” consists of a heavy plate (1)made from steel with at least 20 m m thickness, a concrete slab, or - in the most simple case - a stone laboratory floor on which a
5.2 Estimation of Agglomerate Strength
Fig. 5.34
Sketch of a drop test arrangement (explanations see text).
tube or some kind of collar (2) is placed to contain the sample after the drop and avoid material losses. Diameter, height, and material of construction of the retaining wall must be such that, after impact, pieces can dissipate without experiencing secondary breakage. A vertical pipe ( 3 ) ,the upper end ofwhich is at a distance h from the impact plate on the floor, extends into the retaining container. Length and diameter of the pipe depend on the size of the agglomerates to be tested. The diameter should be at least 5times or, even better, lotimes greater than the largest agglomerate dimension. The length is simulating the expected drops during further handling of the product. The pipe must end at a sufficient distance from the impact plate to allow free lateral movement of the mass upon impacting the plate. The test itself can be carried out in different ways. One method is to drop batches, each, for example, consisting of five large agglomerates (in most cases briquettes), one after the other, from different, increasing heights. “Strength”is defined as that height from which all five agglomerates still survive the drops without damage. This test determines the maximum drop height that can be tolerated in a plant which must produce whole agglomerates and handle them without breakage. Such a requirement may exist if products are manufactured that must have a certain appeal such as charcoal briquettes for barbecueing, salt briquettes for the regeneration of home water softeners, or, generally, consumer products. The test as described before is carriedout during system development, prior to plant design; later, for quality control during operation, representative samples are extracted in regular intervals from the product stream and dropped from the predetermined height to recheck and confirm their survival. Often it is not necessary to produce industrial agglomerates that must survive all handling completely intact. In this case, a relatively great drop height is selected
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and individual agglomerates are tested. The particle size distribution of the broken pieces is determined by screening and the result of the drop test is judged based on the amount of “fines”that is produced during impact. The definition of what constitutes fines and their permissible amount depend on the application. In still other cases, the collective behavior of a large number of agglomerates is of interest. Then, a sample, sometimes weighing several kilograms, is dropped, all at once, from a bucket through the pipe. This setup simulates the “cushioning” effect of a bed of material at the impact point. The evaluation is again carried out by screening and determining the amount of “fines”.During an alternative procedure the entire sample or only pieces larger than “fines” are dropped again or repeatedly for a specific number of times. The evaluation of the test is done in the same way as before whereby data are determined either after each drop or after a certain number of drops. These few examples of industrial methods for the determination of agglomerate strength and descriptions of how these test may be carried out shall suffice. The technicalliterature ofthe past century is full ofreports that cover industrial plants producing agglomerates and evaluations of their “strength”, whatever that characteristic may mean in a particular case. Readers who wish to know more are encouraged to seek out these publications whereby it should be recognized that, in the past, most papers have appeared in application oriented journals and proceedings of conferences. The above descriptions of alternatives that are possible in the execution of the same test to obtain specific information for agglomerate handling or use should also help to understand how existing methods, which may have been used in a completely different context for other agglomerates, can be adapted interdisciplinarily to fit new requirements.
5.3
Structure of Agglomerates
Agglomerates are bodies that are, often artificially and with purpose, produced from individual “small” particles. The term “small” is to be understood in relation to the agglomerate. Although there are agglomerates, for example in the food industry (see Section 5.1.2 and Fig. 5.12), or natural, often undesired agglomerates (see Section 5.5), which consist of only a few particles, typical agglomerates contain very large numbers of particles (see Section 5.3.1, Table 5.6) with sizes that are orders of magnitude smaller than that of the agglomerate. Binding mechanisms (see Section 5.1.1) cause these particles to temporarily or permanently stick together and form a lose or porous entity (see Section 5.3.2). Since binding mechanisms act in different ways (see Section 5.1), the structure of agglomerates is of great importance for all properties of agglomerates. The sketch in Fig. 5.35 depicts a random cut through an agglomerate. The area within the heavy solid lines is arbitrarily defined as “one”. Fig. 5.35 seems to show particles and their distribution. In reality, what is visible are cross sections through particles at a random level. If another random cut through the same agglomerate is made, a totally different picture is obtained. Moreover, particles that seem to float in space are in contact with other particles at some level. For example, the shaded cross section may be the result of cutting the particle, shown in elevation on
5.3 Structure of Agglomerates
Examples of:
@
Contact points
0 Nearpoints
Elevation (Side View)
Fig. 5.35 Sketch of a random cut through an agglomerate.
the side of Fig. 5.35, at the indicated line. Obviously this particle will have a completely different outline at another level. The same observation is true for the void spaces (= porosity) that are visible between the particle cross sections. If the heavily bordered square in Fig. 5.35, which represents the area “one”,is large enough and contains a great number of the two significant structural characteristics, i.e. outlines of cross sections through particles and of pores between the particles, a statistical evaluation of any random cut will produce generally valid results with an accuracy that can be described by the standard deviation which is associated with that statistical treatment. Therefore, for example, scanning the picture of the cut will produce information on particle size and distribution, porosity, E , solids content, 1 - E , and, with the appropriate software, a shape factor and the specific surface area of the particles (B.601. Accuracy can be increased by investigating multiple cuts through the same agglomerate and determining the statistical averages for all of them. A visual evaluation of the enlarged picture of the cut through an agglomerate also reveals certain other features, although the observations can only be used to explain phenomena and do not serve any scientific purpose. The shaded circles in Fig. 5.35 indicate, for example, some of the contact points between particles in this particular cross section and the open circles depict some of the “near points” at which a binding mechanism, such as liquid bridges or one of the field forces (see Section 5.1.1), could develop. The average of the sum of both types of interaction points for one particle defines the coordination number k . Taking into consideration the statements made above in regard to random cuts, it is of course possible that “near points” in a particular cut are actually contact points in a level slightly above or below and it is impossible to determine all the interaction points which are distributed three-dimensionally around a particle.
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5.3.1
General Considerations
The structure of agglomerates depends on many different parameters. Generally, they are parameters related to the particles building and all the processes involved in forming the green and final agglomerate. Particularly during high-pressure agglomeration (see Section 8.2) and various post-treatment (curing) processes (see Sections 5.3.2,7.3, and 8.3) parameters that relate to the original feed particles may change. Parameters Related to Particles Building an Agglomerate. The most important particle related parameters that influence agglomerate structure are:
Particle size, Particle size distribution, Macroscopic particle shape, Microscopic particle shape (surface configuration, e.g. roughness). With the exception of geometrically well defined particles, particularly spheres and cubes, it is difficult to describe with one dimension and measure particle size (Fig. 5.36). During particle size analysis [B.GO], the response of each particle to a physical effect is determined; for example, whether a particle will pass a defined opening (screening), how fast a particle will settle in a stationary fluid under the influence of gravity or centrifugal force (sedimentation), at what speed of a gas flow a particle will be entrained (sifting), how much extinction will be caused by a particle passing through a sensing zone (sensor output),what is the outline of a picture or projection of a particle (scanning),how much energy is reflected from a particle at a particular angle (scattering), etc., etc. Only absolutely spherical, microscopically smooth particles will produce results with which particle size (diameter of the sphere) is determined unequivocally and by modifying the effect which determines size, the distribution of the sizes of spherical particles can be determined, if dilute samples are analyzed where the particles do not influence each other during the test. For all other situations, particle shape has an overwhelming effect on how they behave during a test or in any process. Shape is characterized by form and proportions. Form refers to the degree to which a particle approaches a definite form, such as a sphere, cube, tetrahedron (Fig. 5.36),or higher order polyhedron. The relative proportions distinguish one spheroid, cuboid, tetrahedron, or polyhedron from another of the same class. Macroscopically, shape may be described rather subjectively by comparison with “standard shapes” or defined by coefficients (Fig. 5.37). The major problem of characterizing the three-dimensional shape of a particle by its size is that size is one-dimensional and coefficients of “standard shapes” are two-dimensional. To overcome this problem, particles may be described by polar coordinates, for example, radius vectors from the center of gravity extending to any point of the surface. By using the radius and the two polar coordinate angles, the shape of the particle surface can be described to any desired degree of accuracy. Obviously, for the time being, this technique is limited to scientific work.
5.3 Structure of Agglomerates
Microscopically, particle shape, particularly surface texture, may be defined by fractals or Fourier functions. It must be realized that in nature no absolutely smooth surfaces exist. With increasing magnification macroscopically smooth, e.g. polished, surfaces first reveal scratches, caused by the polishing media, and later “natural”roughness with peaks and valleys. Since the size of small particles which are interacting with other, larger particles extends into the nano range and, therefore, such particles are themselves similar to or potentially smaller than many surface features on other particles, it is understandable that knowledge of the microscopic particle texture is of great importance for agglomeration. However, even with the quickly growing technological advances in the nano scale it is still impossible in practice to apply the information for general theories with which agglomerate characteristics can be predicted. As will be shown below, the extremely large number of particles that are involved and their variability (it can be assumed that no two particles are exactly alike) is another reason for today’s inability to generally and unequivocally describe the interactions between particles in an agglomerate.
Fig. 5.37 “Standard set o f shapes” for the determination of particle sphericity according t o Rittenhouse [B.42].
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All that is normally known about a particle is its silhouette, projection, or profile or, in those cases where size is derived from other physical effects, such as, for example, the settling velocity, a dimension related to volume, mass, or surface texture. Therefore, methods must be found that interpret information from cuts through the particle, scans of portions of the surface area, or information from particle behavior in, for example, fluids and connect it with overall shape. Unless the measured outline of the particle misses a unique, dominant feature of the particle shape, the result will be representative of the particle. The methods are still very complicated and require a large number of discrete items of information to describe a particle signature reasonably well. Shape influences particle behavior in powder packings and the representative (particle) equivalent diameter changes with the physical situation. In agglomeration, in addition to the surface equivalent diameter of an entire particle size distribution, which is the representative value for estimating the strength of agglomerates (see Section 5.2.2), for the packing structure, the diameter of an inscribed average circle representing the particle projection is less significant than that of the circle enveloping all peaks and protrusions (Fig. 5.38). This is particularly true for loose packings (Fig. 5.38, top). A different equivalent diameter would be representative for closely packed particles (Fig. 5.38, bottom). It appears possible that, in the future, equivalent particle diameters can be computed for loose and dense packings. Furthermore, it should become feasible to calculate the work that is required to go from one packing structure to another, the resistance of a powder to penetration, and its angle of repose. It is already possible to characterize the pore structure of a particle system by using automatic scans of a cross section and employing fractals to analyze the data. Nevertheless, the characterization of particulate matter and the structure of particle systems is still at the beginning of becoming an exact and widely used science.
Particle size versus packing r a d i u s Size Packing radius-
Close packed p a r t i c l e s
Fig. 5.38
Effect o f particle shape on its packing behavior.
5.3 Structure of Agglomerates
Much of the industrial research of packing structures is still based on spherical particles. The most fundamental information is obtained if the regular packings of monosized spheres are evaluated. Fig. 5.39 shows the six regular packing structures of monosized spheres. For the packings depicted in Fig. 5.39 porosity E, the void volume between the monosized spherical particles, and the coordination number k, the number of interaction points of a sphere in the structure with neighboring spheres, can be exactly determined (Tab. 5.5). All coordination points in these structures are contact points.
Fig. 5.39 Systematic arrangements of spherical particles (“regular packings”). For explanations see text and Table 5.5.
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5 Agglomeration Theories Tab. 5.5 Porosities, coordination numbers, and approximations according to Eq. 5.6 for the “regular packings” shown in Figure 5.39. Geometric arrangement
-
Coordination number, k
k
&
Cubic (Figure 5.39a)
0.476
6
6.599
Orthorhombic, two alternatives (Figure 5.39b)
0.395
8
7.953
(Figure 5.39~)
0.395
8
7.953
Tetragonal-spheroidal (Figure 5.39d)
0.302
10
10.40
Rhombohedral (pyramidal) (Fig. 5.39e)
0.260
12
12.08
Rhombohedral (hexagonal) (Fig. 5.394
0.260
12
12.08
Porosity
ir/c
(Eq. 5.6)
Tab. 5.5 shows that the porosity of regular packings ranges from 47.6 % for the most open structure to 26 % for the densest packing. Real packings of monosized spheres are called irregular packings. Unlike the specified location of each sphere in a regular packing (deterministic system), the position of any sphere in a randomly packed bed can only be described by a probability distribution (stochastic system). Moreover, the density (or porosity) of a randomly packed bed depends on the mode of packing. Normally, in freely developing, infinite beds two structures are distinguished: a very loose random packing with a typical porosity of 40-43 % and a lose random packing with 39-41 % porosity. If packings are produced in a container they are influenced by the “wall effect” (Fig. 5.40). On and near rigid walls the positioning of the spherical particles can not occur freely and this disturbance is continuing into the packing, creating voids and other irregularities. Nevertheless, poured random packings in containers may attain 37 - 39 % porosity, depending on the dimensions of the container in relation to the size of the spheres and, if the container is vibrated or vigorously shaken, a porosity of approx. 36 % may be obtained. Tab. 5.5 also shows that the coordination numbers for regular packings of monosized spheres are 6, 8, 10, and 12 and that even for these unique conditions the approximation of Equation 5.6 is rather good. Therefore, it can be assumed that Equation 5.6 results in a close approximation of k which indicates that, based on a purely mathematical estimation, high densities of <10 %, which are, for example, obtained during high-pressure agglomeration (see Section 8.2), are associated with coordination numbers of >30, explaining, among other reasons, the immediate high strength of agglomerates produced by these methods. Even though many studies have been and are being carried out to characterize packings derived from two or more sphere sizes, there is still no theory that satisfactorily describes the structure and allows an universally valid prediction of density or porosity, pore sizes and distribution as well as the coordination number of specific packings. For particles with irregular shape and a particle size distribution, the typical case in industry, a general understanding of structure and its characteristics is still remote. If
5.3 Structure of Agglomerates
Fig. 5.40 [5.8].
Examples o f packings demonstrating the "wall effect"
packing parameters need to be known they must be determined experimentally. Nevertheless, some interesting information has evolved from the many tests that were carried out over time. It relates to so called optimum packings. The most important optimum packing is the densest regular packing of spherical particles. It can be most easily derived from the two loosest regular packings (a and c in Fig. 5.39) of the largest spherical particles in a mixture (Fig. 5.41). It is obtained by inscribing the largest possible spherical particle into the void between the four or three larger spheres (depending on the model used) and adding the appropriate amount of these smaller spherical particles to fill all the voids between the larger ones. This method is then continued as shown in Fig. 5.41.
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--r-- t---I
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Fig. 5.41 Sketches o f the systematic arrangements o f differently sized spherical particles t o obtain the densest possible packing.
Obviously, the resulting particle sizes and their relative masses do not fit a continuous distribution. The mixture consists of discrete classes of particles. In reality, where such densest packings are desired for a number of reasons, for example in the production of high quality concrete to obtain high strength and water impermeability by minimizing porosity and the presence of interconnected pores (see Section 5.3.2), narrow size ranges of aggregate particles are mixed in the appropriate amounts as
5.3 Structure of Agglomerates
prescribed by the model. Although neither particle size nor shape of the components correspond with the assumptions of the model (spherical monosized particles in different classes) the real random packing produces the desired high density and strength. To obtain the best possible impermeability in concrete, the smallest particles that are added today feature particle sizes <SO0 n m (e.g. silica fume). In those cases where it is not practical to mix discrete classes of narrowly sized particles, an approximation yields a continuous particle size distribution. Mathematically, it is described by the “Fuller distribution” [B.42],an exponential distribution in which the exponent must be between 1/3 and 213. For agglomerates, strength depends primarily on porosity and particle size (see Section 5.2.1). The smaller they are, the higher is the expected strength. This is due to the fact that, with reduced particle size, the number of particles per unit volume increases and with decreasing porosity the coordination number increases. Both effects multiply the binding forces to sometimes very high levels resulting in considerable agglomerate strength. For example, iron ore pellets which, for other reasons (see Section 11.8),are produced from concentrate particles <44 pm, are so strong after sintering that they will not be crushed by truck or front end loader tires. The effect of porosity on the coordination number has been discussed above. Tab. 5.6 shows that if a 1 g sphere, made of a material with a specific mass of 1 g/cm3, is converted into spherical particles (for example by melting, spraying, and consolidation) with 1 pm diameter, 1.9 billion (1.9 trillion according to the American system) particles are produced. Dividing the same mass into spherical particles with 0.5 m m (500 pm) diameter still yields 15,000 particles. These relations have to be kept in mind when considering the effects of binding mechanisms on agglomerates. Since the structure of agglomerates is so important for many properties of agglomerates and scientific or theoretical predictions or descriptions are not yet possible, empirical approaches must be taken to understand and evaluate agglomerate structure. This is particularly true when real particles, i.e. those with irregular shape and an often wide, sometimes multimodal size distribution are involved. As will be obvious in this book, scanning electron microscopy (SEM) is a very powerful tool for the observation of surface and/or interior structures of most solids, including agglomerates. The main advantage of the scanning electron microscope is, that it can not only enlarge the field of vision such that even submicron features are clearly visible but that it also provides depth of focus which allows a three dimensional evaluation. Tab. 5.6 Some properties o f spherical particles (density 1 g/cm3) and o f agglomerates made from these particles. Mass Diameters
Volume
Porosity
kl
Imml
[cm3i
w r
1
p--12.4 a:
1
p = 10.’ a--14.75
1
p
p
=
=
-
0.5 a-14.34
particle(s), a
=
p
=
0; a:
p : - ; a-1.67
p
=
0; a
p : - ; a-1.54
p = 0; a
agglomerate
1; a:
-
Spec. Surface
p=l
~ - 4 . 8 . 1 0 - ~a: ; -
p-l.9.10’2
p : - ; a-5.97-6
p-1.5.104
p : - ; a-1.2.10
[m2lg1
p
=
Numbers
= 40 =
35
~
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Since they were first invented the size and cost of scanning electron microscopes has come down considerably, the time for sample preparation has been shortened, and other complications of the procedure have been reduced. Today, relatively low priced desk top microscopes are available and many scientific or corporate laboratories are equipped with larger units so that this technology is accessible to almost everyone. Parameters Related to Processes Forming Agglomerates. Processes applied for the manufacturing of green or final (after post-treatment) agglomerates strongly influence agglomerate structure. As will be shown in more detail later (see Chapter G), agglomeration processes generally belong to three disparate technological fields:
Tumble/Growth Agglomeration Pressure Agglomeration Agglomeration by HeatlSintering According to the mechanisms that prevail in each of these technologies, the structure of agglomerates is different. Tumble/Growth Agglomeration. As the name implies, in tumblelgrowth agglomera-
tion small particles adhere to each other after colliding during their irregular, stochastic motion in a particle bed and form a new entity that is held together by binding forces. After passing the difficult and critical stage of seed formation (see Section 7.1) the particle assembly grows by the attachment of additional particles to its surface. The structure of agglomerates resulting from growth during tumbling depends on the density of the particle bed, the energy imparted the tumbling mass, the acting binding mechanisms, and the time, among others. Agglomerates formed in a low density cloud of particles, for example in a fluidized bed or other low density tumblelgrowth agglomerators (see Sections 7.4.4 and 7.4.5), feature a very loose structure, high porosity, far beyond that of even the most open regular packing (Fig. 5.39 and Tab. 5.5), and contain few particles. In high density tumbling beds of particles, as realized, for example, in disc, drum, or cone agglomerators (see Section 7.4.1), and even more so if mixing tools provide additional energy and shear, such as in mixer agglomerators (see Section 7.4.2), particles that have attached themselves to the surface are either torn off again or moved to an energetically more favorable location as shown in Fig. 5.42. This results in dense structures with porosities between that of the cubic and orthorhombic regular packings (Fig. 5.39a - c and Tab. 5.5). It must be realized, however, that once particles are incorporated in the structure, a change of that arrangement is only possible during secondary processing or post-treatment.
5.3 Structure of Agglomerates 187
Fig. 5.42 Conceptual model depicting how a small particle is incorporated into the surface o f a (wet) agglomerate in a high density tumbling bed o f particles during tumble/growth agglomeration.
Pressure Agglomeration During pressure agglomeration external forces act on an at least partially confined mass of particles. As shown in Fig. 5.43 two different densification phases are conceivable (see Section 8.1),influencing the structure of agglomerates obtained by pressure. First, requiring relatively little force, particle adhesion and interparticle friction are overcome and some densification of the particle mass is realized. The amount of densification depends on the bulk density of the original feed particle mass. Sometimes the bulk density is very low, corresponding to high bulk volume, because natural adhesion forces are high, for example due to the presence of very small particles, and/or because considerable interparticle friction occurs which is caused, for example, by irregular shape and/or significant surface roughness of the powder particles. During this initial densification phase, the sizes and shapes of the feed particles are not altered or only very little, for example by breaking off some roughness peaks. Some of the pressure agglomeration techniques which make use of binders or inherent binding mechanisms do not apply forces that densify to or far beyond this point (see Sections 8.4.1 and 8.4.2). Once the densest packing of the unaltered feed particles in a confined space is obtained, - which, by the way, does not correspond to the absolutely highest density because solid particles can not flow freely into still available voids as would the molecules of liquids or gases -, once this density is reached and the external forces are
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5 Agglomeration Theories
fP A
c‘ Fig. 5.43
Mechanisms of densification of particulate solids
continuing to increase, particles deform and/or break. During this phase of compaction, structure is changed drastically and void space is reduced until the “hydrostatic” state is reached at which no further densification occurs and the compact responds like an incompressible solid (see Section 8.1). Porosity of agglomerates that are produced at high pressure is in the 10 to 20 % range or may be as low as 5 % and many closed pores have developed. Therefore, for some agglomerates resulting from high-pressure agglomeration, it may be necessary to introduce porosity that may be required or desired during post-treatment processes (see Sections 5.3.2 and 8.3).
5.3 Structure of Agglomerates
During sintering, atoms and molecules move at elevated temperatures across the interface at the contact points between two solid particles and form a bridge (see Sections 5.3.2 and 9.1). This process is influenced by temperature, contact area, and pressure as well as time. As a primary technology, agglomeration by heat occurs in bulk masses which are deposited onto a stationary or moving (“travelling”)grate. Heat is provided by hot flue gases or burning solid fuel which has been mixed into the bulk mass. As a result, the structure of “sinter”is relatively open, particularly if the heat was caused by solid fuel which disappears to a large extent (only leaving ashes) during burning. Often, sintering is used as a post-treatment process to provide strength to agglomerates (see Sections 7.3 and 8.3) or modify the porosity. Depending on how the sintering process is controlled, the agglomerate shrinks and densifies (up to almost 100 % density or 0 % porosity) or porosity is maintained and, sometimes, even increased (see Section 5.3.2). Agglomeration by Heat/Sintering
5.3.2
Porosity and Techniques That Influence Porosity
The properties of materials that are produced from fine (FP) and ultrafine (UFP) or “nano”particles by agglomeration are critically influenced by the void volume between the particles forming the agglomerate. This product attribute is called porosity and is defined by the presence, size, shape, and distribution of pores. To a considerable extent, the following text is based on two recently published books [B.61, B.621 which are recommended for further reading. Generally, there are two types of pores (Fig. 5.44). Open pores are connected with the body’s surface while closed pores are isolated within and may be filled with a fluid. Penetrating pores are a special type of open pores; they feature at least two ends and connect opposing surfaces of the porous body. Pore models are typically based on cylindrical tubes (Fig. 5.45a). In reality, pores are produced by voids between particles and, therefore, feature narrow necks and wider parts (Fig. 5.45b) and form a complicated network which is strongly dependent on the
Open pores
Fig. 5.44
Schematic representation o f different pore types [8.61].
Closed pores
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Fig. 5.45
Pore models. (a) Cylindrical, (b) realistic.
size, shape, and distribution of the particles forming the agglomerate (Fig. 5.46).Another special type of open pores are “inkbottle” pores (see Fig. 5.44) which have a narrow opening and expand below the surface. Porous materials are solids which contain void spaces. Agglomerates are special in that the particles forming the porous body must not be uniform. In fact, it is more common that the primary particles which are loosely or rigidly joint together by binding mechanisms are different in size, shape, structure, and composition. For most industrial applications, for example those that make use ofthe accessibility of the large specific surface area of the primary FPs and/or UFPs in agglomerates, open pores are required. Penetrating pores are necessary if fluids must flow through agglomerates in, for example, filters, fluid distributors, or catalyst carriers. Agglomerates with closed pores are primarily used for sonic and thermal insulators and for light weight building materials. Many of the statements and explanations in the following subchapters are similar to treatments already covered in the previous section (Section 5.3.1). For reasons of clarity and to better connect the items, a certain amount of redundancy will be found. Structure and porosity of agglomerates are very closely related. Porosity and Agglomerate Strength One of the equations that is most commonly used to describe the strength of agglomerates (see Section 5.2.1) connects the porosity, E, with the coordination number, k, the sum of all adhesion forces at each coordination point, Ai, and the particle size, x, (Eq. 5.8). The term (1 - E ) / E indicates that the strength of agglomerates which are held together by binding mechanisms acting at the coordination points within the aggglomerates is strongly dependent on porosity. Unless special efforts are made, high porosity, a feature of agglomerates that is often desired, results in low agglomerate strength, which is equally as often not acceptable in agglomerated industrial products such as, for example, catalyst carriers. The same influence of porosity on strength is obtained for agglomerates that are completely saturated with a wetting liquid (for example water). In this case, the sum of all adhesion forces, Ai, is replaced by the liquid’s surface tension, a (Eq. 5.2).
5.3 Structure of Agglomerates
Fig. 5.46 Schematic representation o f pore configurations (adapted from [B.61]). (a) Most agglomerates have a geometry o f openings between particles. Fundamentally the pore shape is angular. (b) Agglomerates produced from or containing plate-like particles have a geometry of openings between plates. When the platelets pile up, porosity becomes less. (c) Agglomerates consisting o f or containing elongated particles usually feature high porosity which becomes less when fibers pile up. (d) An interconnected network o f
homogeneous pores is observed if, for example, in glassy agglomerates a leaching technique decomposes spinodal components. (e) In this pore geometry, large pores are connected by small ones. The removal of pore forming agents during a posttreatment o f agglomerates results in this structure. (f) Agglomerates that are produced from porous particles have a pore structure with networks o f both large and small pores.
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If in matrix bonded agglomerates (see Section 5.1), such as, for example, concrete (matrix: cement) and road surfaces (matrix: asphalt),a single, narrowly sized aggregate would be used, large pores result requiring much binder to fill them and yielding low quality products. In addition to the possibility of many faults in the matrix (incomplete filling, gas bubbles, etc.) which act as stress raisers and cause failure, the binder becomes the quality determining part of the system and causes low strength in concrete and insufficient hardness at elevated temperatures in road surfaces. The densest packing of aggregate (= particles forming the agglomerate structure) in such materials can be determined by considering models that are based on regular packings of monosized spheres (see Section 5.3.1, Fig. 5.41). Although, in reality, neither monosized spherical particles nor regular packings are obtained, the gradation (= size fraction) of the aggregate and the respective amounts of each to achieve dense packing or low porosity and, with it, high product quality can be determined. In addition to obtaining good strength and other mechanical quality specifications, introduction of UFPs (as, for example, nano-sized silica fume) in the above mentioned models and during formulation, results in high impermeability for water in ultra high quality concrete for e.g. prestressed structures and airport runways. Typical Agglomerate Porosities Different agglomeration methods yield different por-
osities of the resulting agglomerates. In this respect, three methods with several subgroups can be distinguished: I. Growth or tumble agglomeration (Chapter 7) 1. High density tumbling bed (Section 7.4.1) 2. High shear tumbling bed (Section 7.4.2) 3. High densitylhigh shear with abrasion or crushing transfer (Section 7.4.2) 4. Low density fluidized bed (Section 7.4.4) 5. Low density particle clouds (Section 7.4.5) 6. Agglomeration in stirred suspensions (Section 7.4.6) 7. Immiscible liquid agglomeration (Section 7.4.6) 11. Pressure agglomeration (Chapter 8) 1. Low-pressure agglomeration: Extrusion through screens (Section 8.4.1) 2. Medium-pressure agglomeration: Pelleting, extrusion through perforates die plates (Section 8.4.2) 3. High pressure extrusion: Ram presses (Section 8.4.3) 4. High-pressure agglomeration a) In confined spaces: Punch-and-die pressing, tabletting (Section 8.4.3) b) In confined spaces: Isostatic pressing (Section 8.4.4) c) In semi-confined spaces: Roller presses (Section 8.4.3) 111. Agglomeration by heat/sintering (Chapter 9) 1. Growth or Tumble Agglomeration (see Chapter 7) The mechanism of growth (see Section 7.1) of this method, the addition of single or aggregated particles to the outside of an agglomerate that increases in size, results in a typical structure with rather constant porosity. Porosity depends mostly on the size, distribution, shape, and surface
5.3 Structure of Agglomerates
morphology of the particles forming the agglomerate and very little on the growth mechanism itself. The only difference in agglomerate structure results from whether dense particle beds and suspensions (1.1, 6, and 7) or low density fluidized particle beds, clouds, and suspensions (1.4- 7) are present during agglomeration. In dense environments, where many particles are tumbling in close proximity, the separating forces are more pronounced, particularly if shear is induced by mixing tools (1.2). In this case, weakly attached particles or loosely associated particle assemblies are removed by attrition and have a chance to become reconnected in a more favorable, normally closer position resulting in higher density or lower porosity. In low density fluidized beds, clouds, or suspensions, on the other hand, even weak, loosely bonded agglomerates with very high porosity survive. Once definite positioning of particles in the structure of a growing agglomerate is obtained, it is virtually impossible to change the size and network of pores unless very high external forces are applied (see, for example, 11.3 and 4). Another possibility to increase density and decrease porosity during growth is to introduce high shear through mixing tools (1.2) or to install high speed choppers that are integrally mounted but individually powered and rotate at several thousand RPM to achieve abrasion or crushing transfer (1.3, see also Section 7.1). Even if forced abrasion and crushing transfer are applied, the feed particle size distribution is chosen to form dense structures, and selective agglomeration of the fine components is avoided, growth or tumble agglomeration very infrequently produces agglomerates with porosities of less than 40 % but may yield very loose structures with porosities as high as 95 % if UFPs agglomerate in low density particle clouds. II. Pressure Agglomeration (see Chapter 8) Pressure agglomeration applies external forces to shape and densify particle masses, with or without a binder, and to produce strength. Low-pressure agglomeration (see Section 8.4.1) is carried out by passing (extruding) plastic and sticky particle masses through screens or contoured, thin, perforated metal sheets (11.1),whereby mostly shaping with very little densification occurs. It results in agglomerates which, after drying, typically have a porosity of between 40 and 60 %. No change of this porosity is possible after extrusion. Some reshaping and surface densification is achieved during spheronizing, a rounding process that may be applied to still plastic extrudates (see Section 8.3). In medium-pressure agglomeration (see Section 8.4.2), the extrusion through perforated die plates (11.2), higher densification can be obtained by two effects. First, the feed, that must have binding characteristics and be somewhat plastic, is predensified prior to extrusion by the screw(s) or between the press roller(s) and the die. Secondly, the material is passed through holes in the die where pressure builds up due to wall friction. The length over diameter ratio of the hole (= extrusion channel) and its shape as well as the properties of the feed determine final densification. Because of the wall friction, the surface of the extrudate is always denser than the center (see also Section 8.2). After production, no change in porosity other than potentially caused by some shrinking during drying is possible. Porosity is typically in the range of 30 to 50 %.
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During high-pressure agglomeration (see Section 8.4.3) the forces may be so high that plastic deformation and flow or breakage of brittle particles followed by rearrangement can occur (see also Sections 5.3.1 and 8.2). Correspondingly, porosities can be as low as 5 % or even less, approaching theoretical density. On the other hand, it is relatively difficult to produce compacts with porosities of more than 30 % without resorting to some post-treatment method. Such possibilities (see below) are rather important because punch-and-die and isostatic pressing methods (II.4.a and b) are often used to produce green bodies, particularly in ceramics and powder metallurgy, which may require a controlled network of pores in the final product with larger porosity than originally obtained in the green part. A particular characteristic of all pressure agglomeration methods is that the density of a compacted body made from particulate solids is not uniform (see Section 8.2). This is due to the presence of adhesion and frictional forces between the particulate mass and the pressing tools, the existence of interparticle friction, and the division of force at the points of contact between the particles. This phenomenon is most pronounced in products from high-pressure agglomeration in confined spaces (11.4.a) which is the method of choice for “near-net-shape” green parts in the ceramics and powder metal industries. Because non uniform density leads to deformation during the post-treatment process that is required to obtain final strength and properties, isostatic pressing (11.4.b),both cold and hot, has been developed in which the pressure acts from all sides through a fluid in an autoclave thus producing green bodies with uniform density distribution. This does not mean that density and porosity are constant throughout the body; density is still higher at the surface and less in the center due to the dissipation of forces and interparticle friction. However their distribution is uniform and, therefore, does not cause uncontrolled deformation during post-treatment. Elastic and/or fibrous materials such as many organic materials (peat, lignite, straw, hay, wood, vegetable, and other plant materials as well as their wastes, etc.) pose a particular problem in agglomeration. Unless powdered in some way, which is expensive, such materials can not be agglomerated by growth and tumble agglomeration. It is also difficult to successfully use pressure agglomeration because after the typically very short application of force, deformation and densification are not permanent and elastic spring back occurs after pressure release (see Section 8.1).For such materials ram presses (11.3) are used. They feature a long extrusion channel in which many compacts are held under pressure by the influence of elastic forces and wall friction. These compacts undergo repeated new densification and deformation phases when moving forward with each ram stroke, thus producing stable briquettes even from highly elastic materials (see Section 8.4.3). Because of the particular shaping and densification process, porosity is very low on all surfaces but relatively high inside. Products are ideally suited as primary or secondary fuels. Ill. Agglomeration by Heat (see Chapter 9) At elevated temperatures, close to the melting point or softening range of a material, the atoms and molecules at the surface of a solid particle become so mobile that they can move across the interface at a contact point between two particles; solid bridges, so called sinter bridges, develop.
5.3 Structure of Agglomerates
Although this phenomenon may occur with any material at the appropriate temperature (including relatively low ones for e.g. some man-made plastic and other organic materials), sintering was originally invented as an agglomeration method for metal ores. In this process, ore fines are mixed with a solid fuel (e.g. coal); the mixture is then loosely deposited as a futed bed on a stationary or moving grate, ignited, and kept burning by passing air through the bed. As a result, sinter bridges develop while the fuel is consumed and leaves large voids. After cooling, the agglomerate is broken into suitable pieces and used as feed for blast furnaces. During the reduction process, the large voids and a relatively open pore network provide good access of reducing gases to the ore particles in the sinter structure. sintering is also used as a post-treatment process to obtain final and permanent strength and structure of green bodies which were formed by agglomeration (see Sections 7.3 and 8.3). Methods for Influencing the Porosity of Agglomerates It is possible to produce, for example, quickly disolvable, “instant”,or easily dispersible granules (so called water soluble or dispersible granules, WSGs and WDGs) with sufficient strength for storage, handling, and application by tumble or growth agglomeration, particularly in low density fluidized bed agglomerators. It is also feasible to manufacture agglomerates with pores of atomic scale for gas separation or catalysis from zeolites, silica gel and other similar materials featuring such small pores by sticking the nanoporous primary particles together and connecting them with the relatively large interparticle voids of the agglomerate (Fig. 5.4Gf). In one application, hardening binders, that ultimately result in high strength and also seal the large voids between the particles, are used to avoid a partially penetrating open network of coarse pores, and pressure agglomeration methods are selected to obtain well formed product shapes as designed for a particular application. In another case, a bimodal pore structure, in which a penetrating network of large pores is maintained by selecting a suitable binder that acts only at the coordination points between the particles, allows selective filtration. Such structures are, for instance, used in bioreactors. Enzymes or bacteria are immobilized in the small pores and the large pores are used as channels for transporting reactants and products
[B.bl].
The large group of agglomerated materials with high permanent strength, large porosity, and mostly open or even penetrating pores require post-treatment methods to achieve the theoretically impossible: high strength and a large amount of voids. In the following, some such methods will be discussed. For more information, the book “Porous Materials” [B.Gl] is recommended as additional reading material. An important mechanism for the production of porous products during a post-treatment process is sintering of green agglomerated bodies. The driving force for sintering is the reduction of surface area that is associated with pores. The free surface of the particles in the agglomerate has a specific surface energy which is caused by the atoms in the surface and the lack of opposing atoms. A decrease of the surface results in a reduction of surface energy. Therefore, the total free energy of an agglomerate decreases during sintering. At the same time the body often becomes denser. This phenomenon is shown in Fig. 5.47 in a very basic configuration of three spheres.
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Particles in contact
Particles adhere and form necks
Necks grow and open porosity decreases
Necks become large and pores change Open pores disappear and closed shape to spheroidal pores appear Grain boundary migration occurs leaving spherical closed pores isolated from grain boundary diffusion routes
Fig. 5.47 The phases ofsintering that are associated with shrinkage as shown in a simple model involving three spherical particles; adapted from [B.61].
Fig. 5.48 depicts schematically the sintering processes that can occur between two spherical particles. The numbers correspond to those in Tab. 5.7. It can be concluded that several mechanisms causing material transport (= diffusion paths) do not produce densification. While sintering which results in densification is well researched (see Section 9.1), diffusion phenomena which do not cause shrinkage and are, therefore, important for the production of porous products are less known. For the realisation of porous products during sintering two important preconditions must be fulfilled. One is the manufacturing of green agglomerates with homogeneous packing structure and low density. The other is the necessity to influence the sintering process such that no significant densification occurs but strong, well shaped bridges
Fig. 5.48 Model of the sintering processes that can occur between two particles [8.61]. (a) Depicts schematically bonding. X, Y, a, and p are: bridge radius, penetration depth, particle radius, and neck radius, respectively. (b) Is the bonding area in more detail showing the diffusion paths (Tab. 5.7).
5.3 Structure of Agglomerates Tab. 5.7
Description of path routes of diffusion during sintering.
No.
Diffusion path routes
Diffusion source
Diffusion sink
Densification
1
Surface diffusion
Surface
Neck
No
2
Volume diffusion
Surface
Neck
No
3
Evaporation/condensation
Surface
Neck
No
4
Grain boundary diffusion
Grain boundary
Neck
Yes
5
Volume diffusion
Grain boundary
Neck
Yes
G
Volume diffusion
Dislocation
Neck
No
develop. The first precondition is obtained by adjusting and controlling the powder characteristics and by CIPing (cold isostatic pressing; see II.4.h above, and also Section 8.4.4) with low pressure. The second is accomplished by using sintering conditions during which surface diffusion and evaporation/ condensation prevail. These requirements differ considerably from those of “normal” sintering for the production of dense products. Porous structures in green agglomerates can be achieved if the feed particles are preagglomerated (Fig. 5.49) or consist of bi- or multi-modal particle size fractions. Another method is the production of loose packings by sedimentation. Then, to retain the open porosity, the sintering process must be carried out at a low temperature and for a short time or, to obtain more open pores, pore forming additives are mixed with the powder. Pore forming additives can be either liquids or solids. Liquid additives are used if the green agglomerate is produced by either extrusion, injection molding, or casting. In those cases, the porosity of the green body is controlled by the amount of liquid in the viscous mix. Because liquids are not compressible they occupy volume elements which
Fig. 5.49 Pores in a structure made up of pre-agglomerated particles [8.61].
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remain as pores after drying. If the liquid is a solution, strength is produced during drying by recrystallization. Depending on the final application and strength requirements, the dry agglomerate can be further strengthened by sintering. Solid pore forming additives (Tab. 5.8) are mixed with the primary particles and the blend is then agglomerated with any of the different available methods. As discussed above, agglomeration by heat (111, Sintering) produces directly a porous agglomerate when the solid fuel burns and, with exception of its ashes, disappears. Green agglomerates obtained by other methods are subjected to one or more post-treatments during which final strength develops and the pore forming agent burns, evaporates, or is removed from the structure as a liquid leaving pores behind (Fig. 5.50). It is difficult to produce small pores with this technique because very fine pore forming solids tend to selectively agglomerate during mixing so that they do not distribute uniformly. The low cost method is, however, very good for the production of porous materials with large, open pores. Tab. 5.8
Examples from literature of solid pore forming additives (for refs. see [8.61]).
Ammonium tetrachloride Carbonyl Coal Iodine fluoride Paraffin Petroleum coke Spherical polymer (PMMA) Wood dust
Carbon black Charcoal Dextrin Melamine Perid& Salicylic acid Starch
Another possibility for the manufacturing of solid and permanently porous agglomerates is the utilization of binders which melt during post-treatment at high temperatures and solidify during cooling to solid bridges (so called vitrification). Fig. 5.51 explains the process schematically. Reaction sintering is another technology that can be used for the production of solid, porous materials. By this means, chemicals in the powder mixture react under specific conditions with the atmosphere and/or other particles whereby solid bridges are formed. Porous ceramics may be produced by reaction sintering which is then called reaction bonding.
Fig. 5.50 Schematic representation of pore forming with solid additives [B.61].
5.3 Structure of Agglomerates
Fig. 5.51 Schematic representation o f pore forming by vitrification [B.61].
It is often difficult to produce a highly porous body, which, in the green state, has little strength, and to sinter it without destroying its shape during handling and losing porosity. One possibility to overcome these problems is to sinter the green compact in the pressing tool by passing electric current through it. A new technology which is particularly suitable for the production of porous products is pulsed electric current sintering (PECS) which is sometimes also called spark plasma sintering (SPS). Hot isostatic pressing (HIP) is often applied to densify sintered materials and to correct casting defects. Recently a new HIP process for the production of porous materials was developed. Partially densified compacts are sintered in a high pressure atmosphere. The pressurized gas in the open pores delays densification and porosity remains largely intact. Finally, the so called Sol-gel processes should be mentioned in which porosity, pore size, and polarity of products manufactured by this method can be controlled. Processing begins with a solution (= sol) which becomes a gel. The solution can be prepared from either inorganic salts or organic compounds. It is then hydrolized to a sol or condensed to form a gel. The process can be terminated in the sol-phase, where a dispersion of colloidal particles in a liquid exists, or continued to the gel-phase, the development of a three-dimensional, linked solid structure in which the pores are filled with a liquid. In the wet gel-phase, the pores are interconnected. The process of gel forming or gelation involves first the formation of particle clusters which are held together by hydrogen bonds with silanol groups (Fig. 5.52) followed by the development of a three-dimensional network which is strengthened by bridge growth between the particles. The size of the primary particles and their coordination number determine the porosity and the average pore diameter. In addition to a large number of parameters that control “Sol-Gel’’processing, the drying conditions are most important for the production of porous materials. During “normal” drying, considerable shrinkage occurs which is attributed to the negative capillary pressure which depends on the liquid’s surface tension. To minimize this effect, the original liquid which may have high surface tension can be exchanged by one with lower surface tension prior to drying. As mentioned previously, for additional information the book “Porous Materials” [B.G1] should be consulted.
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Si
\
GEL
SOL Fig. 5.52 Schematic representation of changes during the gelation of particulate sols. Dots represent hydrogen bonds: curved lines are
micelle surfaces.
5.4
Other Characteristics of Agglomerates
As the particle size of solids decreases, many characteristics of individual units change. Particle behavior in bulk masses also changes. Tab. 5.9 illustrates the influence of particle size, listing some important examples. In this context it is immaterial how size is defined: the table considers the relative decrease of a linear particle dimension which is randomly called "particle size". The upper part of Tab. 5.9 presents Influence of particle size on some important characteristics of fine particulate solids <0.5 mm.
Tab. 5.9
Characteristicsof Single Particles
With Decreasing Particle Size
Homogeneity Elastic/plastic behavior Probability of breakage Strength Resistance to attrition Vapor pressure, solubility, reactivity, etc. Color perception and intensity
Increasing Increasing ductility Decreasing Increasing Increasing Increasing Changing
Characteristicsof Bulk Masses
With Decreasing Particle Size
Bulk density (space-filling behavior) Flow characteristics, Flowability (of particles) Mixing efficiency Separating efficiency Ease of fluidization Occurrence of undesired agglomeration Dusting - Losses - Ignition Behavior/Explosiveness
Decreasing Decreasing Decreasing Decreasing Decreasing Increasing Increasing Increasing
5.4 Other Characteristics of Agglomerates
characteristics of single (individual) particles, while the lower describes the behavior of bulk masses. The most important single particle characteristics are homogeneity and specific surface area. Both increase with decreasing particle size. Homogeneity improves because, during size reduction, breakage begins preferably at faults (pores, microcracks, imperfections, contaminants, and others) leaving the resulting smaller particle more and more without flaws until a perfect structure is obtained. Therefore, as size reduction progresses, materials that first exhibit brittle behavior become increasing ductile, the probability of breakage decreases, and strength or resistance to attrition increase. And, if fine or ultrafine particles are produced directly, for example by precipitation, the probability that imperfections develop decreases with smaller particle size. Since volume and mass decrease with the third power of the characteristic linear particle dimension and surface area is only proportional to the square of that particle property, the specific surface area, expressed in m2/cm3or m2/g, increases with decreasing particle size. This results in higher vapor pressure, improved solubility, and increased reactivity as well as related changes of other, more complex properties. Also related to size and surface characteristics are the changes of color perception and intensity. As knowledge of these and other particle characteristics became more readily available by applying new and powerful microscopes, miniaturized tools, and novel methods of investigation, interest increased in the production and use of ultra-fine particles (UFPs) with physically predictable and sometimes new properties (so-called Nan0 Technology). Many particles that require extreme cleanliness in, for example, the food, pharmaceutical, fine chemicals, and metals industries are obtained by precipitation from gases or liquids yielding UFPs [B.55]. Fig. 5.53 depicts the size ranges of particulate matter in process technologies together with some reference points (i.e. radii of atoms, sizes of viruses, wave lengths of visible light). The particle size range of mechanical processes and of all mechanical unit operations (see Chapter 1, Fig. 1.1)now extends down to nano meters. Although, in comparison with the atomic scale, ultra-fine particles <0.1 pm or
Processes with changes in atomic structure
Processes of colloid physics and biochemistry Viruses
--
---
Mechanical processes Including electrical + magnetic separation Thermal processes
--- Wave lengths of visible light
Macromolecules __-___ + -Radii of atoms
Molecular disperse
Disperse systems
Colloid ,0-13
10-12
lfm
Fig. 5.53
10-l~ 1pm
+
Fine
1 0 . ~ lo-@ 1 0 . ~ I O - ~ 1 0 . ~ lo4 10” I A Inrn 1Pm
Particle sizes of various disperse systems
.+ Coarse
10.’
10.’ l o o 10‘ 10’ [cm] Imrn Icrn Idrn I r n
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bodies, their size is less than optical wave lengths and, therefore, require electron microscopy to make them visible, i.e. individual nano-particles can not be observed optically. Since the smallest UFPs only contain approx. 100 atoms, new particle qualities may be defined by answers to questions such as: “How many atoms are required for a particle to exhibit characteristics that are common to a metal?”or “When does a particle begin to show biological interactions and functions?” [B.55]. UFPs of a particular size are the smallest units of solids, just as the smallest unit in the world of microorganisms today is thought to be the virus, a “particle” about the same size as that of UFPs. Therefore, further to the changes of properties with decreasing size of single fine particles as shown in Tab. 5.9, additional, new, and desirable characteristics are attributed to nano-particles. They also include, for example, biochemical interactions with microbes [B.55]. The disadvantages of fine and ultra-fine particles in bulk relate mostly to the storage, handling, and processing of such disperse systems. Interactions of extremely small particles with each other or various components in bulk masses cause a number of problems. Many result from the changing competition between volume and surface related forces. For example, since the weight or mass of particles decreases faster (volume related) than the adhesion force between particles (surface related), very fine particles adhere naturally to each other or to larger entities. This phenomenon causes decreased bulk density or increased bulk volume, reduced flowability, lower mixing or separation efficiencies, reduced ease of fluidization, increased tendency for undesired agglomeration (such as bridging, caking, build-up, etc.). Material losses due to dusting result from small size and mass while increased selfignition behavior and explosiveness (dust explosions) or, generally, high reactivity relate to the large specific surface area of fine powders. As shown in Tab. 5.10, size enlargement by agglomeration can improve the bulk properties of particulate solids in many ways. Agglomeration is characterized by an increase of the apparent size so that, for example, dusting or material losses are reduced and flowability is improved. At the same time the characteristics of the single particles, such as surface area and, with this, solubility, reactivity, and other
Tab. 5.10
Some of the most important advantages of agglomerated products.
No or low content of dust; therefore, increased safety during handling of, for example toxic or explosive materials, and, generally, fewer losses which may cause primary or secondary pollution. Freely flowing. Improved storage and handling characteristics. Improved metering and dosing capabilities. No segregation of co-agglomerated materials. Increased bulk density and lower bulk volume. Defined size and shape. Sometimes, defined weight of each agglomerate. Within limits, porosity or density can be controlled; thus, dispersibility, solubility, reactivity, heat conductivity, and other properties can be influenced. Improved product appeal. Increased sales value.
5.4 Other Characteristics of Agglomerates
related properties, are maintained. This is due to the fact that agglomerates are porous bodies, in which the primary particles, held together by binding mechanisms, are still identifiable and retain most of their individual qualities. Depending on the specific use of agglomerates,the typical advantages listed in Tab. 5.10 may have different importance. For example, in waste treatment, recycling, or pollution control, the low content of dust and, respectively,reduced dustiness are generally important, also to prevent secondary pollution. Flowability, on the other hand, often has little priority in these applications. The same is true for improved metering and dosing capabilities as well as for defined shape and weight. Quite significant are, however, improved handling and storage characteristics,often due to increased bulk density and lower bulk volume. Co-agglomerated materials, from mixtures containing many different components with often considerable differences in characteristics and amount, do not segregate. This plays a role in the pharmaceutical industry where the active drug component and various excipients are co-agglomerated to form a free flowing, dust free, non segregating feed material for high speed tabletting presses. It is also important when hazardous materials are incorporated into concrete blocks. In such cases it is often also necessary that the hazardous components will not leach and that the binder does not age and disintegrate. If wastes become secondary raw materials, improved characteristics resulting in enhanced appeal and increased value are often desirable. These are just a few examples with many more in existence. In the next decades, particulate solids processing will focus on a shift away from just attaining size and distribution of products toward the more fundamental area of microstructure and morphology. The emphasis of new products and processes will be on better control of primary particle physical properties, highly specific product size and composition, as well as the creation of desirable characteristics. This will lead to an increased demand for “engineered particulate materials with high value that are produced in low tonnages. In the future, for many products special consideration will be on high value and special effects rather than “simple” bulk commodities. In general, the industry will have to cater increasingly to the needs of the end user. Agglomeration science and technology will play a vital role in the search for such novel, differentiated particulate products and the means for making them. A major requirement in these new areas is for a much greater degree of flexibility in regard to agglomerate structure and morphology. This will not only require a deeper understanding of the fundamentals of agglomeration but also an interdisciplinary effort of mechanical, chemical, and process engineers with physical chemists, colloid and biochemical scientists as well as other researchers in various industries. Among the many “engineered” products that exhibit specific properties after size enlargement by agglomeration are the following: 0
Easily soluble (“instant”)products Easily dispersible products Easily degradable products Highly absorbent, stable products Highly porous, stable products
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Products with controlled reactivity Controlled high reactivity - Precisely controlled reactivity - Reduced reactivity
-
This list is by no means exhaustive and new “engineered”,agglomerated materials with specific characteristics are being developed at an accelerating pace. Many of the new products are results of or influenced by advances in nano-technologies. Easily Soluble (“Instant”) Products The manufacturing of easily soluble, “instant” granules is one of the oldest and most thoroughly researched applications of “engineered” products. The term “instant” is normally used in the food industry and related fields, such as pharmaceuticals and animal feeds, to describe characteristics of drink powders (including coffee, tea milk, milk replacers, soft drinks, vitamins, medicated powders, etc.), soups, sauces and the like. Instant agglomerates are also desirable for pigments and other chemicals that are ultimately dissolved in a diluent. All instant products must quickly dissolve in a specified solvent or, more generally, in liquids of any kind and temperature (particularly also at ambient or even cold conditions) without residue or sediment. Each industry has a more or less well defined procedure to determine the maximum allowable time. Typically, complete dissolution should be accomplished within a few seconds in warm liquid and approx. 30-GO s in cold liquid. Special drink powders will have to meet the shorter times, even in cold liquids. Such instant granules may contain certain substances that assist in the breakup of the granules during the dispersion phase (see Section 5.1.2). Easily Dispersible Products Easily dispersible products are very similar to “instant” products. The only difference is in the fact that the primary particles are not soluble. Typical examples are pigments, carbon black, silica fume, etc. There are two different applications for easily dispersible agglomerated products: 1. Products that are dispersible in liquid phase. 2. Products that are dispersible in solid phase.
In addition, transitional applications exist which require that agglomerated products must be dispersible in wet bulk solids or slurries. ad 1) Dispersion in liquid phase must achieve complete separation of the primary particles to obtain statistically uniform mixing with other solid particles that are present in the liquid phase, or, if the particles are small enough, to produce a suspension. Suspensions should be stable indefinitely or for a long time and not form sediments. To further reduce the formation of a sediment, the viscosity of the liquid can be increased by incorporating so called stabilizers into the agglomerate which are liberated during dispersion and “thicken” the liquid. Such materials may be pregelatinized starch, pectines, alginates, and the like. Because the primary particles are not soluble, the break-up of agglomerates must occur only by a weakening or destruction of the binding mechan-
5.4 Other Characteristics of Agglomerates
ism(s). For that reason, it is much more common in dispersion that high shear forces are applied by liquid mixing techniques. The effect of shearing is often enhanced by a high viscosity of the liquid or the developing suspension. ad 2) Other easily dispersible agglomerated materials are manufactured to improve the storage, transportation, and metering characteristics of an intermediate product. During the mixing with bulk solids that are dry or contain various amounts of moisture (e.g. slurry) such agglomerates must disintegrate into the primary, finely divided or nano particles. In addition to considerable shear forces, mass related forces are created which help to overcome the binding mechanisms in the agglomerates. If moisture is present (for example during the production of concrete) the influence of liquid on the strength and survival of binding mechanisms should be also considered during product design. Easily Degradable Products Typical examples are carrier materials for fertilizers, insectizides, fungizides, and many other chemicals. In liquid phase the active substance is highly concentrated and, in most cases, toxic. By adsorbing these liquids on the mostly inner surface of granules that are manufactured from fine particles by agglomeration, the toxin is diluted and the products are rendered safe for handling and application. For example, spreading of granular agro-chemicals by conventional equipment is possible. Newer applications of the technology also include special micronutrients. Easily degradable carrier materials must feature a large accessible (inner) surface area, i.e. high porosity, and sufficient strength to withstand processing, storage, and handling. The binding mechanism must survive the impregnation process with the active substance. For example, if molecular or electric forces were used for dry agglomeration, it is possible that, after impregnation, the active substance replaces (e.g. by recrystallisation during a drying step) or enhances (e.g. by viscous liquid bonding and/or chemical reaction) the original binding mechanism. On the other hand, the granules must break down easily and quickly under the influence of moisture either from the soil or the atmosphere (rain or dew). Therefore, the product must easily wet and even small amounts of moisture should reduce the strength. Surfactants improve wetting and components that swell in the presence of moisture may assist breakdown. Sometimes, particularly in the case of fertilizers and micronutrients, interactions with bacteria participate in the breakdown. Highly Absorbent, Stable Products The basic requirements for these materials are the same as for the previous ones. However, as compared with “easily degradable” products, after absorbing liquids the granules must not disintegrate. For easy application and disposal, they should not be as hard and stable as the “highly porous, stable” carrier materials (see below). Typical examples are cat litter and absorbents for nuisance liquids (e.g. spilled oil).
Most absorbents are consumer products. For that reason, a new requirement is customer appeal. For absorbents this means that the material must be uniform, easily spreadable, and completely dust free with high abrasion resistance. In both areas
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of main interest, cat litter and absorbents for spilled liquids, considerable research and development have been carried out in recent years to “engineer”the product for specific uses. One major requirement is particle size; an optimal size exists for perfect wetting. Another is particle shape. For cat litter and for some liquid absorbents spherical shape is undesirable. It has been found that cats tend to play with spherical particles, and round liquid absorbents scatter too widely during application thus, in both cases, causing widespread contamination. On the other hand, some absorbents are round to allow easier distribution into far corners and to produce a more uniform layer at the spill site. While, nowadays, cat litter is supposed to lump for easy removal of the used portion from the litter box, granules applied for the clean-up of liquid spills are supposed to remain separate for effective gathering with mechanical cleaning equipment. Although all absorbents must retain some strength and not convert to slush, mud, or slurry they must disintegrate when being stepped on to avoid accidents by slipping. Particularly cat litter must also absorb odors and/or provide a pleasent smell. Therefore, antibacterial and odor-suppressing substances or perfumes are added whereby it is more important that the smell pleases the cat than the human owner. Highly Porous, Stable Products Another group of agglomerated materials is used, for example, as carriers for catalysts. These products must feature high, easily accessible surface area before and after impregnation with the catalyst. In contrast to the “easily degradable” or “highly absorbent” products discussed above, these carrier materials must be permanently strong and stable. They should not break down during the chemical process which is often accompanied by harsh conditions, particularly high temperatures and aggressive chemicals. The primary particles forming the carrier itself need to be inert in respect to the chemicals used for impregnation and during reaction. In addition, the shape and structure of the carrier must be defined and uniform and the strength high to guarantee good permeability of the activated carrier (catalyst)bed. Little or no mechanical breakdown must occur due to the overburden pressure in the column. Products With Controlled Reactivity This group of agglomerated materials with adjustable, specific characteristics includes the widest range of choices. Products may have controlled high reactivity, for example explosives as well as specialty products, such as airbag chemicals, precisely predetermined reactivity, e.g. chemical oxygen generators and pyrotechnic components, or drastically reduced reactivity as, for instance, densified iron and titanium sponges (the products resulting from direct reduction of the oxides of these metals) or hazardous (including toxic and radioactive) wastes. In contrast to the previously discussed products, no common requirements can be defined for materials with controlled reactivity. Therefore, a few examples in the areas of:
Products with controlled high reactivity Products with precisely predetermined reactivity Products with drastically reduced reactivity will be discussed in the following.
5.4 Other Characteristics $Agglomerates
Products With Controlled High Reactivity High reactivity of particulate solids is connected with large specific surface area and, respectively, small primary particle size. Because reactions take place on the surface of solids, agglomerated products with high reactivity must feature voids which are interconnected with a continuous network of pores that is accessible from the outside. In this respect, their structure is similar to those that are easily wettable. The main difference is, that, in most cases, the chemicals reacting with the solid are gases. The most common reactions are oxidation and reduction. In the case of explosives or, for example, chemicals that activate airbags, large amounts of gas (products of combustion) are produced during a very fast reaction which, in addition, expand because of high system temperatures thereby creating the destructive forces or the pressure in an airbag. After activating a primary detonation, the chemical reaction of these explosives is a rapid decomposition by oxidation whereby the necessary oxygen may be part of the material itself or made available from oxygen-rich molecules such as chlorates. The effect of an explosive depends on its density, the energy produced during reaction, and the speed of reaction. Particularly if oxygen is made available from separate oxygen carriers in a mixture, it is necessary to provide good contact between the components. Product characteristics can be adjusted by modifying the degree of densification. At higher density the stored energy concentration increases but, because of lower surface area, the unassisted reactivity decreases. The reaction of such explosives must be initiated with primers. A primer is a highly reactive material that is easily ignited by friction, percussion, or electricity and will, in turn, set off a less reactive explosive. High reactivity is also desirable of other agglomerated products. One of the most important large scale application of size enlargement by agglomeration is the pelletization of iron oxides (see also Section 11.8).The technology was first developed for iron ores the concentration or purity of which are too low for economic processing in the blast furnace. Typical examples for such ores are the Taconites with large reserves in North America or the Itabirites. To make these ores economically useful, they must be upgraded. Prior to any of several concentration processes the ore must be ground to below separation fineness which is defined by the ore structure and the requirement that each particle contains only iron oxide or gangue. With the before mentioned ores this particle size is <45 Fm (<425 mesh). Such concentrates can not be fed to and reduced in the blast furnace. To overcome this problem, iron ore pelletizing was developed. Because the size range of the resulting pellets is very narrow, new important advantages in the blast furnace were obtained. One results from the more uniform bed structure which increases permeability and thereby process speed (production capacity). The other is due to the fact that, in comparison with the performance of dense, natural lump ore, the porous agglomerates, which consist of small primary particles, feature a significantly better reducibility. The latter is so important that, in the meantime, iron ores which do not need upgrading are also pelletized. In that case, the ore concentrate must be ground to the particle size that is required for tumble and growth agglomeration.
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Products With Precisely Predetermined Reactivity To this group belong, for example, pyrotechnic articles or components and chemical oxygen generators. These materials must feature precisely predetermined, reproducible, and often variable speed(s) of reaction. Pyrotechnic articles contain components that transmit ignition and produce light, sound, fire, and explosion effects such as coal, sulfur, milk sugar, resins, etc. as easily combustible materials, nitrates and chlorates as oxygen sources, and various chemicals for special effects. Among others, antimony sulfide and aluminum or magnesium powders emit white light while colored light is produced by sodium (yellow), strontium nitrate (red),barium nitrate (green),and caustic copper carbonate (blue); aluminum, magnesium, and iron filings exhibit sparks and mixtures of potassium nitrate and potassium picrate result in hissing sounds. In the pyrotechnic article these components are packed with materials that provide ignition, fire, explosion and, in the case of rockets, propulsion. The other product featured in this group is the chemical oxygen generator. These “oxygen candles”, which are, for example, installed in aircraft to provide each passenger with oxygen if an emergency arises, must produce a variable amount of oxygen during a period of time in which the plane descends to lower altitudes. The production of oxygen occurs during the thermal dissociation of chlorates and perchlorates. Therefore, the oxygen candle is an agglomerate containing chlorate, fuel, and catalyst and is ignited by a primer charge that is activated when the oxygen mask is pulled down. It is a requirement that immediately after decompression a large amount of oxygen is made available as it is assumed that the emergency begins at great altitude. Because it is assumed that the aircraft quickly descends to a denser atmosphere, during the “candle’s’’ 12 to 25 min of burning time, the production of oxygen can diminish. Products With Drastically Reduced Reactivity There are many products which, immediately after manufacturing, are highly reactive mainly because of their small particle size and/or large specific surface area. The greatest danger with such materials exists in potential self ignition and/or spontaneous combustion. Since many such reactions are exothermic, the reaction may accelerate and result in a catastrophic development of heat. In other cases, if, for example, water provides oxygen for the reaction, hydrogen is liberated which may produce explosive mixtures with air. In recent years an important development in the field of iron and steel production is the direct reduction of iron ore. In this technology iron oxide is reduced to virgin iron in the presence of suitable reductants (often C O and/or H,) and at elevated temperatures (but still in solid state). After the removal of oxygen a highly porous, sponge-like iron product remains that features a very high specific surface area. Depending on its microstructure, DRI (direct reduced iron) or “sponge iron” has a self ignition temperature in the order of 150 to 230°C [5.9, 5.101. Even at ambient temperaures a slow reoxidation occurs. With water as oxygen carrier, sponge iron reacts quickly thereby producing heat and hydrogen. Sea water, an electrolyte, accelerates this process. Because all reactions proceed exothermically and due to its structure, direct reduced iron is a heat insulator, in the core of a bulk mass (e.g. storage piles or ship holds) a catastrophic heating can occur that stops only if the source of oxygen (i.e. water and air) is
5.5 Undesired and Desired Agglomeration
removed or replaced. Without passivation, fine sponge iron from fluidized bed reduction processes can not be stored and handled openly. To overcome this problem, the spongy structure is destroyed and the specific surface area, particularly on the exterior of the product, is reduced so that it becomes inert at all technically relevant transport, storage, and handling conditions. In other examples of this group, toxic, radioactive, or hazardous particulate solids of any size, from large pieces with dimensions of several centimeters to dusts and nanosized particles, are agglomerated to render them safe for disposal. In those cases it is necessary to produce an “agglomerate” which is stable for long periods of time, does not emit radiation or gaseous components and is not leached by liquids.
5.5
Undesired and Desired Agglomeration
Agglomeration is a natural phenomenon. Therefore, under certain conditions, it happens, whether it is desired or not [for specific ref. see B.561. Tab. 5.11 lists the operations of Mechanical Process Technologies (see Chapter 1, Fig. 1.1)and indicates if agglomeration is desired or undesired or, sometimes, both. During separation unwanted agglomeration may occur and must be avoided if a particle collective is to be divided into well defined classes. The separation curve is a measure for separation quality. In a diagram the degree of separation (the percentage amounts of particles above and below the desired separation size) is plotted versus the particle size. Fig. 5.54 is a qualitative presentation of several separation curves. Line (a) in Fig. 5.54 represents the ideal or perfect separation of a particle size distribution x,,,<x<x,, at cut size xtl which is possible only in theory. In industrial separation equipment, curves of the type (b)are obtained. The cut size is that particle Separation
100 I I
%
Ideal separation at xl, Technical separation at Ideal separation Technical separation; separation limit xln Technical separation with agglomeration separation limit X , ~ .
6-
.E 75 I
e
Fi v) L
50 g! 0)
0”
I
.
.
I
I
I I
Fig. 5.54 Examples o f separation curves.
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5 Agglomeration Theories Tab. 5.11 The occurrence of undesired and desired agglomeration
in mechanical and related process technologies. Unit Operation
Process
Agglomeration Undesirable
Separation
Mixing
Comminution Particle size enlargement
Conveying
Storage Batching, metering Drying
Screening, sieving Classifying, sorting Flotation Dust precipitation Clarification, thickening Particle size analysis Dry mixing Wet mixing Stirring Suspending, dispersing Fluidized bed Dry grinding Wet grinding Agglomeration Briquetting Granulating Pelletizing Pelleting Sintering Tabletting Mechanical conveying Vibratory conveying Pneumatic conveying Silos, hoppers, stockpiles
Desirable
+ + + +
+ + +
+ +
++ ++ ++
-
+ + + + + +
++ ++ ++ ++
(+I (+I -
Explanations: (+) sometimes yes, + yes, ++ decisively yes, (-) sometimes no, - no, - - decisively no
size of which half end up in the coarse fraction and half in the fine. The sharpness of separation increases with the slope of the curve. If the abscissa uses a logarithmic scale, separation curves representing similar separation efficiencies at different cut sizes are parallel to each other. The influence of agglomeration must be judged differently. If the separation task is to remove all particles xmin<x<xm, from a fluid the desired cut size is xmin.Line (c) in Fig. 5.54 describes the ideal, only theoretically possible separation curve. In reality, a certain amount of smaller particles remains suspended in the fluid and the actual cut size is xt2>xmin(curve d). If agglomeration occurs, the finest particles may form larger entities or attach themselves to larger particles thereby changing the separation curve in Fig. 5.54 to (e).The new cut size xt2.is still somewhat larger than the desired xmin. where all particles would have been removed from the fluid, but agglomeration definitely helps to move the actual cut size closer to the ideal one. In general, as described above, whenever the task is to remove all particulate solids from a fluid, agglomeration will be advantageous. Since particularly the smallest, low
5.5 Undesired and Desired Agglomeration
mass particles are, on one hand, the most difficult to remove and, on the other hand, feature the highest natural adhesion tendency, the chance agglomeration of these particles improves separation efficiency. As will be shown later (see Sections 7.4.5 and 7.4.6) techniques for enhancing the natural agglomeration tendency of very fine particles are often applied during gas and liquid cleaning. Even relatively loose conglomerates of particles behave according to their combined weight during, for example, settling and particularly in the centrifugal fields of cyclone separators. For all those separation cases, however, which attempt to separate a particle collective according to certain properties of the particulate solids, agglomeration is most often undesired. Techniques for which this statement is true include screening, sifting, classification, sorting, flotation, and, as a general analytical method, particle size characterization. It should also be realized that the respective separation property is not only size; it could be density, shape, color, chemical composition, and others. During screening, unwanted agglomeration is often facilitated by the motion of the material on the screen; spherical agglomerates are frequently formed from material containing fines or featuring other binding characteristics, for example if it is moist. Among others, binding mechanisms are: 0 0
0
For finely divided solids, molecular and electrical forces and/or adsorption layers, for plastics, electrostatic forces, for ores, magnetic forces, for moist powders, liquid bridges and capillary forces, for fibers, interlocking, and for materials with low melting points, partial melting and solidification.
In some substances several bonding mechanisms may occur simultaneously. In all cases, the result of screening is distorted because agglomerated fines are classified as coarse. The immediate and complete removal by dedusting or “scalping” of the finest fraction prior to screening into the desired property classes is one practical method to avoid selective agglomeration of the fines or adhesion of fines to larger particles. During screening itself, the effect of adhesion is reduced by mechanical destruction of agglomerates with, for example, rubber cubes or balls placed on or under the screen decks, the application of brushes, air jets, or ultrasound, or the modification of screen amplitude or, respectively, frequency (e.g. ultrasonic screen excitation). During the screening of moist bulk materials, difficulties increase with moisture content, but agglomeration tendencies are almost completely eliminated during wet screening when the particles are suspended in a liquid. Since, in moist screening, particles are often held in the mesh openings by liquid bridges, by direct resistance heating, inductive heating, or by modifying the wetting angle and/or the surface tension with surfactants the separation of such materials is facilitated and blinding of the screen is avoided. In air classijcation, products from, for example, dry fine grinding are separated. Particular problems arise if the material to be separated contains agglomerates that were formed during comminution. Conglomerates would be recirculated into the mill and “overgrinding” occurs. Therefore, attempts are made to destroy them by spe-
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cia1 feeder designs. Destructive forces are caused by, for example, sudden changes in speed or direction of flow and by installing air jet mills in front of the classifier. When classifying cement, it was determined that grinding aids, used during comminution to avoid agglomeration, also improve separation in the classifier. In the classifier itself, agglomerates are formed by molecular forces that may be reinforced by adsorption layers if separation is carried out with ambient air, by liquid bridges if moist materials are processed, and by electrostatic forces in a dry environment. As an example, Fig. 5.55 depicts separation curves of various air classifiers. With decreasing particle size, the amount found in the coarse fraction increases, which is due to agglomeration, whereby fine particles adhere to larger ones and conglomerates of fines behave as if they were coarser particles. Both effects reduce the separation efficiency and can be avoided only if the causes for adhesion are removed, that is, mostly by eliminating moisture in the material and humidity in the air. Sorting processes that separate materials according to particle characteristics other than size are often carried out in liquids. During flotation, one of the technologies, the relative capacity of light components to float is enhanced by the addition of chemicals which form bubbles that attach selectively to one component. Agglomeration can again reduce the separation efficiency when smaller particles of other components stick to larger ones of the floating type or to bubbles. By using modified chemicals, processing very dilute suspensions, or applying multiple separation steps efficiency can be improved. On the other hand, agglomeration can be also beneficial in dense media sink/ swim separation, centrifugal separation, or during jigging if particles of a particular ingredient can be made to selectively adhere to each other and form larger, heavier agglomerates. During particle size analysis, in addition to screening, sifting, and counting, sedimentation techniques are often used which produce unequivocal results only if the individual particles can move without influencing each other. For that reason, very dilute suspensions are used. Nevertheless, it is possible that agglomerates form or already
Particle size x Fig. 5.55
Separation curves of different air classifiers.
5.5 Undesired and Desired Agglomeration
present conglomerates do not disperse completely. Therefore, dispersion aids are often added which reduce particle affinity (see also Section 7.4.6). The molecules of dispersion aids attach to the particles, eliminating polarities and/or reducing interfacial tensions. Separation forces, such as ultrasonic vibrations, can be also introduced for improved desagglomeration and dispersion. In connection with particle size analysis, the importance of correct sample preparation must be stressed. Because natural, unintentionally formed agglomerates always incorporate a larger than average number of the finest particles, the result of particle size analysis will be incorrect if preexisting agglomerates are not destroyed or conditions prevail during testing that promote agglomeration. Mixing Many of the previously mentioned considerations apply to the formation and prevention of undesired agglomerates during mixing. Little needs to be added concerning mixing in liquids by stirring or methods for the production of suspensions and dispersions. The addition of dispersion agents is always recommended if the tendency of the solid particles to agglomerate is high. Agglomerates or flocs that are already intentionally, for example to improve handling characteristics of fine powders, or unintentionally present prior to mixing can be destroyed by shear forces in the liquid. Consequently, the generation of the highest possible shear gradient is often considered advantageous when selecting agitators. During extended storage, the particles in (pharmaceutical) suspensions often form agglomerates that can be no longer destroyed by shaking the preparation. This is of particular concern in, for example, eye drops. The problem can be avoided by controlled flocculation of the solids. After the addition of an electrolyte,the fine particles aggregate to loose flocs that can be easily redispersed by shaking the dispenser prior to application. When mixing dry or moist bulk solids, agglomerates may form which originate from the finest components of the mixture. They are held together by molecular and electrostatic as well as capillary forces. These undesired agglomerates should be broken up by shear or frictional stresses, generated by the motion of the bulk mass, or by special disintegration devices that are built into the blender (see Section 7.4.2). Comminution Duringfinegrindingin roller crushers and tube mills containing grind-
ing media, with all materials certain problems begin to develop at a certain fineness of the solids to be milled. Two types of phenomena can be distinguished. In the first case, the finest particles start to adhere to walls and the grinding media in the mill. On this first coating,even coarser particles find excellent conditions for adhesion and massive deposits form rapidly. Experiments, during which the particle size distribution across thick layers of build-up were investigated, showed that the finest particles are indeed found in the lowest layers. Fig. 5.5G is the photograph of clean grinding balls and of those which, after a short period of operation, are already covered with a light primary deposit upon which additional layers will build during extended grinding. Fig. 5.57 are the photographs of the interior side of a manhole cover of a ball mill before and after use illustrating the extent of such deposits. These layers adhering to the inside walls and the grinding media produce a cushioning effect which lowers the intensity of stressing and, therefore, increase the duration of grinding and decrease efficiency.
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Fig. 5.56
Grinding balls before (right) and after (left) brief milling.
The second phenomenon during dry fine grinding is the occurrence of agglomerates in the freely moving charge itself. Formation of such agglomerates is associated with the so called limit of grinding. For each material a fineness exists at which, in spite of continued consumption of energy, the finest particles in the charge do not seem to become finer. Agglomeration and adhesion in mills can be attributed to various binding mechanisms (see Section 5.1.1). Since the mill housing may become highly charged by the friction between its contents and the walls, electrostatic forces are often the cause for initial build-up. This effect can be eliminated quite easily by grounding the mill. In
Fig. 5.57 Manhole cover o f a ball mill before (top) and after (bottom) grinding.
5.5 Undesired and Desired Agglomeration
other cases, wall deposits will begin with particles of the size that generally corresponds to that of the wall roughness. The strength of the layer depends on the contact pressure which is magnified by the mill charge consisting of grinding media and material to be crushed. Adhesion is largely affected by molecular forces; however, partial melting and sintering are also possible. Agglomerates are formed in the freely moving charge of a tube mill by the compaction of fine particles between the grinding media and by recombination bonding (see also Section 5.1.1). Adhesion is affected by van-der-Waals forces between the particles that have been compressed very tightly and by the recombination of free valence forces at newly created surfaces. Since these agglomerates are very strong, a grinding equilibrium exists which has been observed and described by many. It means, that in fine grinding, after a certain time, a state of equilibrium between size reduction and size enlargement by agglomeration occurs. From that point on, agglomerates are formed, crushed, and re-formed so that the apparent particle size does not change. However, since destruction of particles continues to occur, a growing amorphization of the material can be observed which also results in increasing specific surface area and is often called mechanical activation as, in many cases, higher reactivity is obtained. Every form of agglomeration during size reduction reduces the efficiency of grinding and the fineness obtained at the limit of grinding is often insufficient for many tasks, even though the agglomerates actually contain much smaller particles. Therefore, it is desirable to prevent or, at least, reduce these effects. In milling, one possibility to achieve less unwanted agglomeration is to add surface-active substances, so called surfactants or grinding aids. It has long been known that small amounts of such additives may reduce the grinding time required to reach a particular fineness by 2030 %. Molecules ofthese substances, which are present in a gas or vapor phase, quickly saturate free valences at the newly created surfaces and avoid recombination bonding. The effect of some of these grinding aids on the fineness of cement after a specific grinding time is depicted in Fig. 5.58. It can be seen that with the exception of soot, the desired effect is produced only if the amount used is very small. At higher concentrations the agglomeration tendency increases due to the formation of adsorption layers and liquid bridges. In the case of soot a greater quantity is required because it is a solid which, as compared with molecules in the form of gas or vapor, is not very mobile. As a rule, grinding aids also reduce caking. Fig. 5.59 demonstrates the effect of 0.1 % sodium stearate during the grinding of cement clinker. Other surface-active substances can delay build-up for longer periods or prevent them entirely up to a certain fineness (for cement clinker 0.1 % triethanol-amine, for example, Fig. 5.60). Fig. 5.59 also shows that the specific surface, which is a measure of the fineness of cement, increases when 0.1 % sodium stearate is added and the build-up consists of finer particles.
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5 Agglomeration Theon'es 3500
cm2/g
3000
'5v1
2500
0
5
0
2
0
2000
1500
1000
.
e a: 0 b: @ c: 8 d: 0 e:
Acetone Sodium stearate Water Naphthenic acids
0.5 .. Addition of auxiliary grinding agent
I O. h_ 1
Fig. 5.58 Effect of some "grinding aids" on the fineness o f cement after grinding clinker for the same time in a rod mill.
The formation of lamellar flakes or flat agglomerates in tube mills has been attributed to compaction between the grinding media during impact. The same mechanism occurs in all comminution processes in which the material to be crushed is subjected to stresses between two surfaces, for example in roller mills. Since the second condition
Fig. 5.59 Specific surface o f the build-up (= accretion) and o f the free charge as well as amount o f the build-up (= accretion quantity) with and without the use o f a grinding aid during size reduction o f cement clinker in a rod mill.
5.5 Undesired and Desired Agglomeration YO
100
-2
.-b C
80
W
a
F
2060 0
.-E
40
-2. E
IL20 Fig. 5.60 Changes in the amount o f freely moving charge during the grinding o f cement in a laboratory ball mill with and without the addition of triethanolamine.
Limestone
s = 122, 'l?!m
or
I
/
;
2
4
6
8
1
/
5'
' '
'rO1m
'
Cement clinker
x=96Opm
s = 12?, 5pm
pm
Aw9,5
A 4
X
De ree of re uktion
B
--
0
Grinding duration
-As I--~
A = -X S
-
Fig. 5.61 Agglomerates produced during the grinding o f limestone and cement clinker in a roller mill with a high reduction ratio.
1
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for the formation of agglomerates is a sufficient fineness of the particles that are involved, the occurrence of strong, flat flakes is mostly observed during fine grinding. One measure for the fineness, the intensity of stressing, and the unintentional formation of agglomerates is the so called reduction ratio, that is, the quotient of maximum feed particle size and gap between the rollers. Fig. 5.61 depicts typical agglomerates produced in a roller mill with a high reduction ratio. Since the fine material is immediately compacted, almost all free valences at the newly created surfaces participate in recombination bonding. Consequently, the formation of agglomerates in roller mills can be only avoided if a smaller reduction ratio is chosen or by applying friction between the rollers. More recently it was found that, as compared with the stochastic crushing process in a tube mill with grinding media, the combination of the well defined stressing in a high pressure roller mill with large reduction ratio and the ensueing desagglomeration of the conglomerated flakes that were produced in the mill result in a significantly lower overall energy consumption during fine grinding of brittle materials, such as cement clinker and many ores; in those cases the unintentional and unavoidable agglomeration of the fine particles is not only tolerable but also results in a more economical fine grinding method. Agglomerates can be also formed during impact grinding. Fig. 5.62a shows schematically the fracture lines that develop during impact stressing of a glass sphere. A cone of fine material is created at the impact point and is compacted by the pressure resulting from the kinetic energy of the system into an agglomerated mass (Fig. 5.62b and c). Here too, the effect of free valence forces on newly created surfaces is used to its almost complete extent, yielding a quite strong agglomerate. It is very difficult to avoid this type of agglomeration; it can be affected only by reducing the impact velocity
a: Restkegel b: Seilenspliffer c: Feingutkegel
Fig. 5.62 (a) Schematic representation of the fracture lines caused in a glass sphere by impact stressing. (b) Agglomerated cone o f fines created during the impact stressing of a glass sphere (sphere dia.: 8 mm, impact velocity approx. 150 m/s). (c) Agglomerated cone of fines created during impact stressing of a sugar crystal (shown on the left).
5.5 Undesired and Desired Agglomeration
which, in turn, results in a lower degree of comminution. For glass spheres, for example, the formation of agglomerates was observed only at impact speeds exceeding 80 m/s. During the impact crushing of thermoplastic materials or inorganic substances with low melting points, solidified bridges of partially melted material may further increase adhesion and the strength of agglomerated fines. Since the rise in temperature depends on the energy input and is constant for a given impact velocity, cryogenic milling, whereby the particles to be crushed are cooled prior to feeding into the mill, not only results in an increased brittleness but may also avoid partial melting and unwanted agglomeration. In wet grinding, as a rule, agglomeration is avoided by suspending the particles in liquid. Sometimes, the product of dry fine grinding is subjected to a brief final wet grinding to destroy the previously formed agglomerates. Nevertheless, some materials also tend to flocculate in wet grinding. Since the adhesion forces causing flocs are mostly electrical in nature, the addition of a small amount of electrolyte to the suspending liquid nearly always suffices to prevent flocculation. Agglomeration By definition, the unit operation size enlargement by agglomeration
makes use of all binding mechanisms and often enhances them in suitable environments and equipment. All agglomerates that are produced are made intentionally and are desired. Nevertheless, there are instances where adhesion and agglomeration are unwanted and undesired. Because, particularly in tumble/growth agglomeration, binders are added, the effect of these binders is still present on the surface of green agglomerates and during post-treatment new binding mechanisms between the agglomerates may develop which result in the formation of clusters of agglomerates. Of course, because agglomerates are larger bodies and only a few interaction points ( = coordination points) are present in a unit volume, even relatively strong solid bridges which may have developed by recrystallisation, chemical reaction, or sintering during post-treatment can be broken relatively easily. Nevertheless, such clusters could be detrimental during storage, feeding, or metering and, therefore, should be avoided or broken up prior to a following process step. More information on potential problems is given in the subchapter on Storage below. Transportation During the conveying ojparticulate solids, especially of finely dispersed powders, the unintentional formation of agglomerates and (sometimes thick) coatings on walls is often observed. Whereas agglomerates occur mostly on vibrating or shaking conveyors and inclined conveyors or chutes, wall build-up is more common in pneumatic conveyors. The main causes of agglomeration during the conveying of fine particulate solids are molecular and electric forces as well as binding mechanisms related to moisture and, as a result of mechanical or thermal energy input, binding mechanisms such as partial melting and solidification can be activated, too. Although it is very difficult to avoid agglomeration on vibrating or shaking conveyors, several possibilities exist for the prevention of wall build-up and deposits during pneumatic transport. Since the adhesion of the finest particles always begins in the roughness depres-
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sions, one of the most important conditions to avoid a common reason for initial build-up is to provide smooth inner wall surfaces of pneumatic conveying lines. Because high drag forces tend to remove particles that have already adhered to the walls, high transport velocities also reduce the danger of build-up. Deposits preferably start in dead or calm flow zones; therefore, when designing such systems, low speed areas must be avoided. On the other hand, sudden changes in the direction of flow will cause high energy impacts of particles with the wall, causing build-up. Friction between particulate solids and pneumatic conveyor walls may result in high electrostatic charges on both partners. They depend to a large extent on whether the particles and/or the walls are electrically conductive and the lines are grounded or not. System design must take these conditions into consideration. To further explain some of the phenomena that occur during pneumatic conveying, the results of some pilot investigations will be summarized and presented in the following. Pneumatic transportation tests were carried out in a 58.51 m long horizontal pipe with a diameter of 0.7 m. The pressure within the system was determined at seven locations which were distributed along the measured length of the pipe. Ap = p1- p7 is the total pressure drop in the conveying system. In Fig. 5.63 the pressures at three different location are plotted over time. Since the fan located behind the dust collector at the end of the conveyor generates a slight negative pressure in the filter housing this also can be measured in the pipe as long as it is clean. After a few seconds, however, pressure p1 rises and the other locations follow after a short delay. Part of the pressure increase is caused by loading the air with particles, but a major portion is due to the build-up of deposits in the pipe. If the pipe was inspected after runs of 20 s and 50 s, respectively, no deposit was found after the short duration but after the longer run deposits had build up in the feed end portion while the part closer to the end still remained clean.
Tube diameter: D = 0.7m "ip= 1440kg/hr mplrnf= 40
I
hrf=36kg/hr
..
U=2.11m/s
Time t [sec]
Fig. 5.63 Pressure changes at three locations o f a pilot pneumatic conveying system during the first 150 s of a test run.
5.5 Undesired and Desired Agglomeration
Fig. 5.64 Pressure drop along the measured pipe length o f a pilot pneumatic conveying system a s a function of time.
Fig. 5.64 depicts the total pressure drop between both ends of the pipe. The lower diagram represents results of the same test as shown in Fig. 5.63. After about 2.5 min, the system is in equilibrium and the total pressure drop in the systems remains almost constant. This indicates that, at least macroscopically, no further build-up takes place after this time. The upper diagram in Fig. 5.64 represents a different behavior. The total pressure drop increases more slowly. This is mostly due to a lower solids/fluid ratio and a higher transport velocity than used during the experiment depicted in the lower part of Fig. 5.64. In rather regular time intervals, however, a high pressure peak developed which was first measured at the feed end and propagated quickly to the discharge end. This behavior, together with some other observations, indicated that deposits fall off and are pushed along the pipe, thus momentarily increasing the pressure drop. If the system is opened immediately after such a pressure wave, the inner walls are found almost completely clean.
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At high conveying velocities or in vertical pipes, deposits build up uniformly and, in the following, shall be called “crusts”. In horizontal pipes, bonds between particles and the wall are stressed by the weight of the build-up in the upper part while they are strengthened in the lower part by the gravitational forces. Therefore, especially during conveying at low velocities and high solidslfluid ratios, a second type of build-up is observed in horizontal pipes that shall be called “massy” deposits. The lower part of Fig. 5.65 describes schematically the different types ofbuild-ups in horizontal pneumatic transport pipes. They are affected by gravity, grow in the direction of mass flow, and are composed of finer particles because these particles exhibit a higher adhesion tendency and a “sieving” or classification effect takes place in the charge by which finer components move to the bottom layer of the moving mass. The upper part of Fig. 5.65 is a photograph taken during a model experiment. With exception of the formation of a crust, all other stages, including a “dune” of freely mobile particles moving over the deposits, can be distinguished. Fig. 5.66 is the view into a pipe after pneumatically conveying a slightly moist, finely divided quartz powder showing the heavy build-up in crust and massy deposit as well as the remainder of a dune. Massy deposits can be also formed by the action of other influences, such as centrifugal and inertial forces at elbows. Another important agglomeration phenomenon is that deposits may be shaped such that they yield a more effective flow channel. This can be explained by the fact that separation and drag forces influence adhesion. Particles preferably build up in zones without flow or where the direction of flow is changed by, for example, eddies. A typical example is shown in Fig. 5.67. On the bottom the partial cross section of a Moller pump is presented. Such pumps are used for feeding powders into a pneumatic conveying system. Powder and air enter a mixing chamber through a screw and a nozzle, respectively, and are then forced into the piping. The photograph in the upper part of
Growing “massy“
deposits
\
‘&.sy’ deposit
Crust
Fig. 5.65 Sketch and photograph o f a model experiment explaining the different types of build-up in a horizontal pneumatic conveying Pipe
5.5 Undesired and Desired Agglomeration
Fig. 5.66 View into the pipe of a horizontal pneumatic conveyor after conveying a slightly moist, finely divided quartz powder at low velocity.
Fig. 5.67 is a view (in direction A-A) of such a mixing chamber which was opened after conveying fine zinc oxide. Opposite the nozzle a deposit in a Venturi-like shape has built up which defines the most effective flow channel at this point. Similar deposits can be found along the line in pneumatic systems that were not optimally designed and/or arranged. Storage Adhesion phenomena are involved in, for example, the bridging ofparticulate
solids in hoppers. In the case of relatively coarse materials, bridge formation is caused by dome-like structures which are supported on the inclined walls in the lower conical part of bins. With decreasing particle size, the participation of true adhesion forces in bridging and agglomeration increases. Binding mechanisms are molecular forces and adsorption layers or liquid bridges. The latter often play an important role whereby liquid bridges form by capillary condensation at the coordination points. For example, feeding warm and moist material into silos must be avoided, even if its moisture content is very low. Evaporating moisture condenses on the cooler silo walls and drips into the charge forming wet agglomerates and causes strong capillary adhesion bonding of particles on the walls. Insulation of the silos and/or forced large volume venting can be employed to avoid condensation and agglomeration problems. Bridging can totally block the discharge from silos, thus causing severe operating problems. Because adhesion even of finely dispersed DRY solids can not be avoided, agglomerates and bridges must be destroyed by special devices. For this purpose, inflatable cushions are mounted to the inside walls of silos or the material is momentarily fluidized by the (pulsed) injection of air jets. In the case or coarser solids, which tend to form domes, it is often sufficient to select a cone with steeper sides (= “mass flow” design). Small remaining flow problems due to adhesion can be overcome by installing vibrators or “hammers” on the outside silo walls. Unwanted agglomeration is often observed if the particulate materials are soluble or if chemical reactions can occur, particularly in the presence of moisture. These phenomena are very common in the fertilizer industry and are called caking if they occur in bulk masses or bag-set if material solidifies in bags. Caking of fertilizers and of other soluble materials has long been and still is a great problem. To get an idea about its importance and scale, three examples shall be pre-
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Moller pumL) (schematically) Mixture solids/air -
Fig. 5.67 Photograph and sketch of a "Moller pump". The photograph shows Venturi-like deposits after conveying finely divided zinc oxide.
5.5 Undesired and Desired Agglomeration
Fig. 5.68
Manually unloading a shiplead of caked sylvite.
sented. Fig. 5.68 depicts the unloading of a shipload of Sylvite that was expected to arrive at its destination as a free-flowing particulate mass, similar to the state it was in when it was loaded. This historical photograph shows that, instead, it had badly caked so that, owing to the limited room in the shiphold, the very expensive and time consuming method of manual unloading had to be chosen. Fig. 5.69 is another historical photograph. It was taken by TVA (Tennessee Valley Authority, Muscle Shoals, AL, USA) in 1947 and recalls the recovery of nongranulated triple superphosphate from a curing pile which had to be blasted first to break the pileset. Although today, agglomeration methods are used to produce a granular product which does no longer cake to such an extend that it can not be broken up by, for example, front-end loaders, some fertilizers, particularly those with high nitrogen content, are so hygroscopic that, additionally, they must still be stored in bulk storage facilities with controlled low humidity to prevent excessive caking. Breaking a potential pile-set by high energy input, such as blasting, is not possible for nitrate fertilizers.
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Fig. 5.69 Recovery o f nongranulated triple superphosphate from a curing pile after blasting to break the “pile-set”.
The third photograph (Fig. 5.70) demonstrates potential difficulties that may be experienced by the end user. The granular fertilizer in the left bag displays the desired free flowing behavior while, in the other bag, the same product, that was not treated with an anticaking agent, has set up (bag-set). Even after mechanically breaking up such a caked mass it may no longer exhibit the same uniform distribution characteristics as the product that did not experience secondary agglomeration. Different materials become caked during various storage and handling procedures but caking itself is almost exclusively by solid bridges or, more specifically,by chemical reaction and crystallization of dissolved substances (see also Section 5.1.1). Other binding mechanisms contribute only slightly to caking. The rate and extent to which caking takes place depends on the moisture content, the particle size or specific surface area, the pressure under which the material is stored (e.g. top or bottom of the pile), the temperature and its variation during storage, as well as the time. The effect of these parameters changes with different materials. Fig. 5.71
Fig. 5.70 Granular fertilizer treated with an anticaking agent (left) and untreated control (right) showing severe “bag-set”.
5.5 Undesired a n d Desired Agglomeration
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* - / 1
Fig. 5.71 Variation of crushing strength with caking pressure (left) and time of storage (right). (a) NaNO,, (b) (NH,),SO,, (c) Urea, (d) KCI (potash), (e) (NH,)H2P0,, (f) superphosphate. The numbers in brackets indicate the respective moisture contents.
depicts results obtained in a “caking bomb”. It can be seen that the crushing strength of caked masses rises with increasing moisture content (numbers in brackets, curves (a)), caking pressure and time of storage. The influence of temperature and temperature variations depends on the solubility of the solids. Fig. 5.72 shows four different temperature-solubility curves. Whereas the solubility of sodium chloride changes little with temperature, this is not true for potassium chloride (or potash) and potassium nitrate, for example. Especially the latter features a very steep curve. Some salts, such as sodium sulfate, exhibit various temperature dependent solubility ranges. Salts or mixtures of different salts, such as fertilizers, for example, containing a small amount of moisture, may cake during storage and/or transport if exposed to changing temperatures even if the moisture content is very small and the material is packed in airtight containers. In many cases (see Fig. 5.72) more salt will be dissolved at higher temperatures which recrystallizes and forms solid bridges between the particles when the temperature drops again. Repeated cycling, for instance due to climatic changes or differences in day and night temperatures, reinforces this bonding and causes bag-set. The crushing strength of caked materials depends on the number of bridges formed per unit volume and, therefore, decreases with increasing particle size. As mentioned earlier, mixtures of powdered soluble materials that were granulated by agglomeration may still set up somewhat due to the mechanism described above, however, normally the granulated material can be broken and desagglomerated easily. In conclusion, it can be stated that the tendency for caking of a fertilizer mixture, for example, will vary with the physical and chemical properties of the components and their proportions in the mixture. It also depends on the method of mixing, the particle size after processing, which often includes size enlargement by agglomeration, and the storage conditions to which the finished products are exposed.
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Ternperdure T f OCI Fig. 5.72
Solubility curves o f four different salts.
The answer to what can be done to avoid or, at least, lessen caking is complex but generally the same as in all other cases where unwanted adhesion or agglomeration occurs: Detect the binding mechanism that is responsible and the parameters that influence the process and then try to reduce their effect. In the following some examples will be discussed briefly. If (unobjectional) chemical reactions between components of a mixture do occur, these components should be mixed separately until the reaction is completed. The resulting intermediate product can then be blended with the other components and no longer induces caking. An example for this is any mixture that contains both ammonium sulfate and superphosphate. An almost trivial precaution is very often to lower the moisture content. However, this is not always necessary. Different maximum moisture levels exist that depend on the material. Fig. 5.71 shows that the crushing strength of caked superphosphate containing 1.1 % moisture is very low while the strength of some other
5.5 Undesired and Desired Agglomeration
Fig. 5.73 Granules o f 12-12-12 NPK fertilizer showing typical crystalline hulls o f an urea-ammonium chloride complex after storage for 3 months in bags. Uncured (left) and cured for 7 days prior to bagging (right).
caked fertilizers is much higher although they contain considerably less water. It was found during microscopic studies of several types of high-analysis fertilizers that caking usually resulted from bonding by the crystals of soluble salts. These crystals often covered the entire granule surface in the form of a “veneer” or hull. Fig. 5.73 shows typical 12-12-12 NPK fertilizer granules that were produced with an ammonia-urea solution after 3 months of storage. They were illuminated from below and photographed at a higher magnification to reveal details of the crystalline hull. Bondingphase salts identified during the study were potassium nitrate, ammonium chloride, monoammonium phosphate, ammonium nitrate, and an urea-ammonium chloride complex; all, particularly the last one, are highly soluble. Those salts migrated to the surface of the granule, leaving numerous small cavities within. This mechanism requires water and drying should, therefore, reduce caking. Fig. 5.74 is a photograph taken with crossed Nicol prisms. It shows the difference in hull thickness between undried and predried 12-12-12 grade NPK fertilizer granules. The crushing strength of caked material after storage decreases correspondingly. (c) Curve b (ammonium sulfate) in the right hand side diagram of Fig. 5.71 shows the typical behavior of materials that respond favorably to several days of bin or pile curing prior to bagging. Such products cake in a few days to their final strength but the resulting lumps are broken up before the cured materials are bagged and put into long term storage. Curing can even accelerate hull formation owing to the heat and moisture retention in the bin or pile. In products that respond well to curing, additional development of crystals on the granule surfaces during subsequent storage is not sufficient to cause significant caking.
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Difference in the hull thickness o f undried (top) and predried (bottom) granular 12-12-12 NPK fertilizer made with Ammonia-Urea solution.
Fig. 5.74
However, many products do not improve during this type of curing. Fig. 5.73 depicts the comparison of uncured (left) and cured (right; for 7 days prior to bagging) 12-12-12 NPK fertilizer granules that were made from ammonium sulfate, potassium chloride, and superphosphate with ammonium-urea solution and sulfuric acid. Although ammonium sulfate is present, the caking behavior of the other components dominates and both the cured and uncured granules exhibit continued growth of the hulls and caking during storage. Another curing method will be described under (e) below. (d) The oldest method of conditioning fertilizers is the coating with a parting agent. Storage properties are improved after the addition of up to 3 % of an extremely fine particulate solid such as diatomaceous earth, kaolin, vermiculite, pulverized limestone, magnesium oxide, and a variety of other inexpensive, very fine powders. Again, microscopic studies revealed the fundamental properties of a “conditioner” which are threefold: 1.The powder coating acts as a separator between the individual fertilizer granules and prevents intergrowth of crystals during and after drying. 2. The hulls from beneath the coating crystals rarely project beyond the layer of conditioner. 3. The moisture is distributed uniformly over the surface of the granules due to the high sorptive capacity of the finely porous coating. Thus the localized growth of crystals at the coordination points is prevented and the surface hulls are much finer grained, more intergrown, and more densely packed than those covering unconditioned products. Such anticaking conditioning agents are usually applied by mixing them with the granular fertilizer in a rotary tumbler (typically a drum) prior to bagging. (e) A modern variation of the above mentioned conditioning process is the coating with surface-activeorganic chemicals. it was found, however, that not all surfactants improve the physical conditions of mixed fertilizers. It was reported that the caking ten-
5.5 Undesired and Desired Agglomeration
dencies can be reduced by as much as 45 % if non-ionic chemicals were used but increased by as much as 37 % with the use of anionic materials. Where in the process the surface-active agents were applied was also found to be of decisive importance. Typical cationic anticaking agents are fatty amines with a general formula R-NH2 with R representing C16 and C18 chains. They are believed to attach directly to the fertilizer particles with their surface-active amine group. The fatty, hydrophobic part of the molecule extends outward, thus preventing hygroscopic products from attracting moisture. Of course, this is only true, if a monomolecular layer covers the fertilizer granules and all amine molecules extend their hydrophobic portion outward. Therefore, too much conditioner may cause rather than prevent caking. Multiple layers are alternately hydrophobic and hydrophilic. The above makes an alternative curing process advantageous. The molecules of a second molecular layer, if attached, would position themselves with the amine group extending outward. These amine groups are free to interact with other fertilizer particles, especially the phosphate portion of incompletely coated granules, to form an amine-phosphate salt. Pressure intensifies this effect. The chemical “bridge” is not as strong as a recrystallized salt bridge and the “set”can be broken easily. Since, on the other hand, the amine- phosphate bond is stronger than the RR bond, a more uniformly covered product results from a short bin cure (1- 2 days) which is unlikely to set again (see Fig. 5.70, left side). Sometimes a combination of the two types of conditioners is used. An example for this approach is finely divided kaolin treated with surfactant. ( f ) A last but not least method is granulation. Today, this technology is almost obligatory, particularly for mixed fertilizers. Size-enlarged, granular fertilizers offer fewer coordination points per unit volume where solid bridges can develop. If the strength of the bridges is low anyway, as in the case of superphosphate with 1.1 % moisture or monoammonium phosphate with 0.06 % moisture (see Fig. 5.71) granulating alone is sufficient to prevent severe caking. The above examples were selected to demonstrate how unwanted agglomeration problems can be studied and possible remediation techniques be determined. They date back to a point in time when the fundamentals of unwanted agglomeration in different industries were first investigated and means were developed to avoid some of these phenomena. While this part of size enlargement by agglomeration, unwanted agglomeration, is often very important, because its effects may result in considerable losses of production and profit, most of the past and present major publications deal only with the methods and equipment for the production of agglomerates with beneficial properties. Therefore, it is an important achievement that recently a book entitled “Cake Formation in Particulate Systems” [B.44] was published that does cover the unwanted adhesion and agglomeration phenomena. The author distinguishes four major classes of caking in particulate systems: Mechanical caking Plastic-flow caking Chemical caking Electrical caking.
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In addition, several sub-classes are defined whereby certain properties of components, either pure substances or part@) of a formulation, can be expected to cause caking under certain conditions. After describing the above, considerable emphasis is given in the book to laboratory techniques and test procedures that need to be considered by those engaged in solving caking problems. Therefore, this publication is recommended for further reading.
Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
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2 Agglomeration Technologies A Short History of Agglomeration In two previous chapters (Sections 5.3 and 5.3.2)reference was already made to the As atechnologies basic physicalthat efect, hasdesired existedsize since particulate of solids were first three areagglomeration available for the enlargement small partiformed on by Earth. Binding mechanisms between solid particles cause the stability of culate solids agglomeration, i.e. wet and dry soil and (often under the influence of heat and pressure) participate in the Tumble/growth agglomeration development of rock formations. Sandstone is the most easily recognized “agglomPressure agglomeration erate”. Agglomeration as a phenomenon, e.g. the natural caking and build-up of partiAgglomeration by heat/sintering culate solids, must have been observed and has been used by higher developed organisms later by since prehistoric times. Sea creatures covered themselves and theand division intohumans the following sub-groups: with protective coats, birds as well as other animals built nests, and humanoids formed I. artificial Tumblelgrowth (Chapter stones, allagglomeration from various solids, sand,7)clay and different binders that were often 1. High density tumbling bed (Section 7.4.1) secreted by the creature itself. As a “tool” to improve powder characteristics, agglomera2. High shear tumbling bed (Section 7.4.2) tion was used by ancient “doctors” in producing pills from medicinal powders and a 3. High with abrasion orthe crushing 7.4.2) binder (e.g.density/high honey) or by shear food preparers during makingtransfer of bread(Section from flour where4. Low density fluidized bed (Section 7.4.4) by the inherent starchy components act as binder. 5.In Low particle clouds agglomeration (Section 7.4.5)as a technology is only about 150 years spitedensity of this long “history”, 6. Agglomeration in stirred suspensions (Section 7.4.6) old today (excluding small scale pharmaceutical and some little-known ancient, mostly 7. Immiscible liquid agglomeration (Section 7.4.6) Chinese applications as well as brick and bread making). Agglomeration as a unit 11.operation, Pressuredefined agglomeration (Chapter 8) within solids processing, started around the middle of the nine1. Low-pressure agglomeration: Extrusion through screens (Section 8.4.1) teenth century as a method to recover and use coal fines. 2. Medium-pressure agglomeration: Pelleting, extrusion perforates die Agglomeration as a science is very young. It began in thethrough 1950s with the formal plates (Section 8.4.2) definition of the binding mechanisms of agglomeration, interdisciplinary collection High pressure extrusion: Ram presses (Section 8.4.3) of3.knowledge relating to all aspects of agglomeration, and fundamental research High-pressure agglomeration 4. which was no longer application oriented [B.42]. At approximately that time, the first a) In series confined spaces: Punch-and-Die pressing, tabletting 8.4.3) derecurring of professional meetings were organized which(Section were exclusively b) In confined spaces: Isostatic pressing (Section 8.4.4) voted to the science and technology of agglomeration (International Briquetting Asc) In (IBA), semi-confined spaces: Roller presses (Section 8.4.3) sociation - today Institute for Briquetting and Agglomeration (IBA) -, begin111.ning Agglomeration by heat/sintering 9). in 1949 with biennial meetings(Chapter and proceedings; International Symposia on Agglomeration, initiated in 1962 with proceedings, (see also Section 13.1)). Since that Additionally, most technologies can be subdivided into two techniques, those time, agglomeration science, technology, and use have experienced rapid growth utilizing no binder, anda those but still without finding corresponding awareness at institutions of higher learning requiring binder. or process engineering communities. and in the atechnical This book is the second by the the author the general in subject of sizeagglomeration enlargement by It should be pointed out that bindingonmechanism binderless agglomeration. While frequently referring to fundamentals and specifics often resembles that of bonding with a binder. This is due to the fact that which bindersare
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are sometimes inherently available and act during agglomeration and/or post-treatment. A typical example for this process is the wet agglomeration of materials that are easily soluble in the liquid. The modified surface tension of the liquid solution may already influence the strength of the green agglomerate and during drying, the necessary post-treatment to convert the intermediate wet agglomerate into the dry, final product, solid bridges develop by recrystallization of the dissolved substance@) (see also Sections 5.1.1, Chapter 7, and Section 7.3). Other inherently available binders have to be activated by a so called conditioning process prior to agglomeration. A typical example for this technique is in the pelleting of animal feed where the starchy component of feed grains becomes plastic and sticky during moistening and heating with steam while mixing the formulation in a kneader. After conditioning the starch provides plasticity that is required for extrusion through bores in the dies of pelleters (see Section 8.4.2) as well as green and dry product strengths. The basic mechanism of tumble/growth agglomeration is shown in Fig. 6.1. Adhesion of individual particles to each other or to solid surfaces is controlled by the competition between volume and surface related forces (see also Section 5.4). To cause permanent adhesion, certain criteria must be fulfilled. The most important of all is that any system force (e.g. caused by gravity, inertia, drag, etc.) must be smaller than the attraction forces between the adhering partners. According to Fig. 6.2 and Equation 6.1, the ratio between the binding forces Bi(x)and the sum of the active components of all ambient forces Fi,(x) is a measure for the adhesion tendency T,:
T,= C Bi(x)/C Fi,(x)>l
(Eq. 6.1)
Both the attraction and the ambient forces are mainly dependent on the size x of the powder particle(s). To cause adhesion, T,must be larger than “one”. In most cases, to keep the particle(s) adhering, the sum of all moments Q ( x ) must be zero, too:
(Eq. 6.2)
Q(x) = x/2 CFiX(x)= 0
It has been discussed (Section 5.1.1) that most of the attraction forces have only a short range; their magnitude and strength decreases quickly with increasing distance. Therefore, because the surfaces of all particulate matter are, at least microscopically, rough (see Section 5.1.1, Fig. 5.11), and the mass of the particles decreases with the third power of the particle size, the adhesion tendency increases with decreasing particle dimensions.
Nucleation
Grawlh
Coalescence
Layering
Fig. 6.1: Basic mechanism o f tumble/growth agglomeration.
6 Agglomeration Technologies
Fig. 6.2: Schematic representation of the adhesion tendency o f a spherical particle o n a flat wall.
The mechanism as depicted in Fig. 6.1 occurs naturally if the agglomerate forming particles are nano-sized. In the case of larger, micron-sized particles the adhesion forces must be produced by the addition of binders (mostly water and other liquids) or enhanced by conditioning and the probability of collision must be increased by providing a high concentration of particles. Such conditions are obtained (Fig. 6.3)
Fig. 6.3: Schematic representation of typical equipment for size enlargement by tumble/ growth agglomeration (left) and the post-treatment steps required t o obtain a final product (right). Left, f r o m the top: Inclined disc (pan), drum, continuous mixer, batch mixer, fluidized bed. (1) Liquid binder (spray), ( 2 ) fresh feed, (3) recirculating fines, (4) dryer, (5) cooler, (6) double deck screen, (7) mill, (8) conditioning drum.
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in inclined discs (pans), rotating drums, and any kind of powder mixer (see also Sections 7.4.1 and 7.4.2). Relatively lose agglomerates are obtained in fluidized beds which realize an irregularly moving particle bed with lower concentrations (see also Section 7.4.4). Sometimes, simple rolling and tumbling motions, for example on inclined stationary or moving surfaces, are sufficient for the cheap formation of crude agglomerates (see Section 7.1 and Chapter 10). In most instances, tumblelgrowth agglomeration processes yield first so called green agglomerates after growing nuclei into larger, nearly spherical aggregates by coalescense and/or layering (Fig. 6.1).These wet agglomerates are temporarily bonded by the effects of surface tension and capillary forces of the liquid binder. While, occasionally, components within the green agglomerate naturally produce permanent bonding by, for example, cementitious reactions, in most cases post-treatments consisting of all or some of the following processes are required to obtain permanent and final strength (see right hand side of Fig. 6.3):heating, potentially chemically reacting, drying, and, sometimes, sintering or partial melting, cooling, screening, adjustment of product properties by crushing and conditioning as well as recirculating undersized material. Since it is difficult, if not impossible to screen green (wet) agglomerates without blinding the screen cloths, separation of undersize material for recycle occurs normally after drying, reacting (if applicable), and cooling. Although, the mostly pre-agglomerated recirculating particles often play an important role in tumblelgrowth agglomeration because they provide most of the nuclei that are necessary for an accelerated growth of product-size agglomerates (see also Section 7.2),the sometimes very large percentage of recycle (often > 300 %) must be again activated for agglomeration by rewetting and needs to pass once more through the entire process, including heating, drying and cooling, which, in final analysis, may render this technology uneconomical. Relatively uniformly shaped and sized agglomerates can be obtained with low- and medium-pressure agglomeration (see also Sections 8.4.1 and 8.4.2).For these processes, the feed mixture must still be made up of relatively small particles and inherently available, activated, or externally added binders (see above). The moist, often sticky mass of particulate solids as well as plastic and liquid binders is extruded through holes in differently shaped screens or perforated dies (Fig. 6.4). Agglomeration and shaping are caused by the pressure forcing the mass through the holes and by the frictional forces developing during the material’s passage. Depending on the plasticity of the feed mix and the dimensions of the holes, short “crumbly”, elongated “spaghetti-like”,or cylindrical green extrudates are produced. Particularly the thin, string shaped agglomerates that are obtained from low-pressure agglomeration (Fig. 6.4, a.1 - a S ) are often spheronized, i.e. rolled into small spherical particles while the product is still plastic. In most cases a post-treatment (typicallydrying and cooling) is required to yield final, permanent strength. As far as applicability is concerned, high-pressureagglomeration (Fig. 6.5) is the most versatile technique for size enlargement of particulate solids by agglomeration (see also Section 8.4.3).If certain characteristics of the feed materials and conditions occurring during densification (see Section 8.1) are considered during equipment selection as well as plant design and operation (see also Section 11.1),particulate solids of
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b.1
Fig. 6.4 Schematic representation o f equipment for (a) low- and (b) mediumpressure agglomeration. (a.1) Screen, (a.2) basket, (a.3) radial, (a.4) dome, (a.5) axial. (b.1) screw, (b.2) flat die, (b.3 - b.5) different designs of cylindrical dies, (b.6) gear.
b.3
b.5
m b.2
b.4
b.6
any kind and size, from nanometers to centimeters, and at any condition, for example with temperatures from below freezing to 1,000"C, can be successfully processed. Typically, the products from high-pressure agglomeration feature high strength immediately after discharge from the equipment. Nevertheless, to further increase strength, addition of a small amount of binder and/or application of post-treatment methods are possible. The mechanism of densification of particulate solids (Fig. 6.6) includes, as a first step, a forced rearrangement of particles requiring little pressure followed by a steep pressure rise causing brittle particles to break and malleable ones to deform plastically. During the entire process, porosity decreases so that fluids which originally occupied the pore space of the bulk feed must be able to escape and the initial elastic deformation must have sufficient time to either cause breakage or convert into plastic deformation (see also Section 8.1).These requirements limit the speed of densification and, therefore, the production capacity. The agglomeration by heat or sintering has been developed and is mostly applied in industries processing minerals and ores for the size enlargement of fines prior to further use (see also Chapter 9). Because the technology requires large amounts of thermal energy, special efforts are made to recover heat or use sources of waste heat. The resulting agglomerates are crude but meet the requirements of the industry.
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I Fig. 6.5: Schematic representation o f equipment for high-pressure agglomeration. Ram press (upper left), punch and die press (upper right), roller presses for compaction (lower left) and briquetting (lower right).
Another large field of application of sintering in agglomeration is in post-treatment where the phenomenon (see also Section 9.1) is used to produce strong permanent bonds in many parts that may have been produced by virtually any one of the other agglomeration techniques (see also Sections 5.3.2,7.3, and 8.3). Particularly in powder metallurgy, sintering is the most important finishing process for the achievement of final strength and stucture.
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p!
Briiife
Q
/ Bulk
Deformation d
-,
Fig. 6 . 6 The mechanisms occurring during the densification o f particulate solids.
Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
7
Tumble/Crowth Agglomeration
As mentioned several times before (see Sections 5.3, 5.3.2, 5.4, and Chapter 6) and indicated in Chapters 2 and 3, the “natural” adhesion of small particles is the most basic agglomeration phenomenon. Therefore, it is that which is most often responsible for unwanted agglomeration (see Section 5.5) and the conglomeration of fine particles that is frequently observed in nature. To arrive at methods which achieve size enlargement by agglomeration in a desired and controlled manner, both a movement of particles and binding mechanisms must be created and enhanced. As the solids move in relation to each other, for example in the relatively dense bed of a rotating or otherwise actuated containment of some sort or in a low density suspension, particles of any size and kind, will collide from time to time and, if the attraction force at the collision site is high enough, coalesce. Theoretically, for this phenomenon to occur, no specific piece of equipment is necessary. As long as the solid particles are kept in irregular, stochastic motion, the probability for collision and coalescence exists. If, additionally, the binding force that has developed upon impact is strong enough to withstand the separating effects of all system forces (see Chapter 6) and does not disappear with time without being replaced by some other binding mechanism, the “seed agglomerate” will survive and eventually collide with other particles or agglomerates. At each instance of collision the bonding criterium as defined in Chapter 6, Fig. 6.2, Equations 6.1 and 6.2, will be tested leading to either growth, indifference, that is, the colliding partners will separate again and remain single, or the destruction of weaker agglomerates. To achieve growth, the individual mass of adhering particles must be small and their surface large. This is equivalent to the requirement that the size of agglomerating particles must be small. Typically, the surface equivalent diameter (see Section 5.2.2) should be in a range below approx. 100-200 pm. Micron and submicron or nano-sized solids (approx. < 10 pm), even if they are dry, will adhere naturally and form agglomerates. Larger particles necessitate the addition of binder for successful growth agglomeration. The limitation to small dimensions of the particles forming the agglomerate and the fact that, in most cases, only temporary bonds are formed constitute major drawbacks of all tumblelgrowth agglomeration methods. If particles are larger than required, crushing to achieve the necessary fineness is normally uneconomical.
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Immediately after growth agglomeration, in the green (moist or wet) stage, the main binding mechanisms are caused by bridges of freely movable liquids, capillary pressure at the surface of particle conglomerates that are filled with a freely movable liquid, or adhesion caused by viscous binders and slurries. To a lesser degree, other binding mechanisms, such as van-der-Waals, electric, and magnetic forces, may also participate. After curing, which often results also in a considerably strengthening of the agglomerates, bonding is achieved by solid bridges resulting from sintering, chemical reactions, partial melting and solidification, or recrystallization of dissolved substances. Some tumblelgrowth agglomeration equipment can handle large volumes effectively if the above requirements (small primary particle size and instantaneous bonding with high strength) are fulfilled. The apparatus is simple and the design is unsophisticated (see Section 7.4.1) but control depends largely on operator experience. Curing is normally the expensive part of plant investment and also contributes to a large extent to operating costs, both of which may render an otherwise perfect technology uneconomical. However, if very large amounts of solids must be agglomerated and the finely divided particulate form of the primary particles is required for other reasons, for example, the concentration of valuable components of ores (see Section 5.4), tumble agglomeration is the preferred technology. In those cases the main binder is water. At production capacities exceeding 1 million t/y, the curing facilities become cheaper and more economical and methods for, for example, the recuperation of heat to make the process more efficient and reduce operating costs become feasible. Other reasons for the application of tumble/growth agglomeration, even at small capacities, may be the high porosity of the agglomerates with other attendant beneficial product characteristics (see also Section 5.4),such as high surface area (e.g. for catalyst carriers) and easy solubility (e.g. for food {drink} and pharmaceutical products). These advantages may be so valuable that additional costs for grinding to obtain the necessary small particle size for agglomeration will be acceptable and high operating costs can be absorbed. In these cases, even the agglomeration liquids (binders for the formation of green agglomerates) may be so costly that they are condensed from the dryer off-gas and recirculated.
7.1
Mechanisms of Tumble/Crowth Agglomeration
With the exception of very few applications where particles are so small that they naturally agglomerate in the dry state, tumble/growth agglomeration methods utilize binders. Even if materials contain binder components inherently, this constituent is so obvious in the bulk mass that the process can not be classified as binderless. In this general section, only those tumblelgrowth agglomeration methods will be discussed in which discrete solid particles, (seed) agglomerates, and fragments of agglomerates attach themselves to each other. Other technologies, such as spray drying, use almost identical equipment as, for example, fluid bed agglomerators; however, since they are utilizing different growth mechanisms, their fundamentals will be covered in Section 7.4.3.
7.7 Mechanisms of TumblelCrowth Agglomeration
In tumble/growth agglomeration distinct process steps can be defined in which (see also Chapter 6, Fig. 6.3): 1. 2. 3.
4.
Green agglomerates are formed from solid particles and binder. Green agglomerates are cured. If necessary, the cured agglomerates are sized (undersized material is recirculated and oversized agglomerates are crushed and rescreened or recirculated). If desired, post-treatment takes place, for example, the application of anticaking agents, coating, etc.
Steps 3 and 4 may sometimes move in front of step 2 to avoid the expense of energy that is required for repeated drying and rewetting of large circulating streams of material. However, since sticking and other unwanted agglomeration problems (see also Section 5.5) may be encountered during sizing and oversize crushing, application of this alternative may not always be possible. In a broad sense, process equipment for tumblejgrowth agglomeration itself, may be divided into: Apparatus producing movement of a densely dispersed mass of particulate solids - dense phase tumble/growth agglomeration (Sections 7.4.1 and 7.4.2). 11. Apparatus producing movement while keeping solid particulate matter suspended or loosely dispersed in a suitable fluid - suspended solids agglomeration (Sections 7.4.4 and 7.4.5). I.
In both cases, finely divided binder is added in a suitable manner to the turbulently agitated mass of particles. If solid particles are suspended in a liquid, agglomerates may be formed after adding a second, immiscible bridging (binder) liquid - immiscible binder agglomeration (see also Section 7.4.6). In the widest sense, this technology belongs to the Type 11-processes. It was previously mentioned (see Chapter 6, Fig. 6.1 and 6.2) that the basic adhesion criterion of tumblejgrowth agglomeration is that two solid entities colliding with one another coalesce and the resulting bond is stronger than the combined effects of all system forces which try to separate it again. This principal process continues, causing size enlargement by agglomerate growth. However, as it proceeds, somewhat more complicated mechanisms evolve. Fig. 7.1 and 7.2 [B.42] present almost identical explanations of what is happening. While Fig. 7.1 is the more easily understandable series of sketches defining nucleation, random coalescense, abrasion transfer, as well as crushing and layering (preferential coalescense), Fig. 7.2 distinguishes between size enlargement and size reduction phenomena, both of which take place simultaneously. Nucleation, the production of primary agglomerates or, in general technical terms, of “seeds”,occurs when several individual particles adhere to each other. Nucleation is the most difficult and time consuming part of any tumblejgrowth agglomeration process. The reason for this is, that most seed agglomerates are very weak because the kinetic energy of the impact during particle collision is low which translates into the
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7 Tumble/Crowth Agglomeration
- 6b
o+o+o
9.0+ o + o- &B
Nucleat ion
Random coalescence
+o +o
c
+O
Abrasion transfer
(-J+*.
Fig. 7.1: Sketches explaining the different processes taking place during tumblelgrowth agglomeration [8.42].
0
Crushing and layering (preferential coalescence)
Size enlargement
jP,-
INucleation
Coalescence
Layering
I
g.
9 7 --Pi+j
+
jp,-P.
Abrasion transfer +
?-1
+
pi-1
9.1
Shatter
Breakage
t
Pi
Size reduction
I
*I
.
Pi - P . J +
-F-j
Attrition
9-j +Jp,
0 0
Pi
Fig. 7.2: Schematic representation o f the mechanisms involved in size changes during tumble/ growth agglomeration [B.42].
7. I Mechanisms of TumblelGrowth Agglomeration
development of only small adhesion forces, only a few interactive adhesion sites exist, and individual primary particles are not yet embedded in a structure where forces at several coordination points participate in the bonding. As a result, under the effects of system forces, nuclei tend to disintegrate again into individual particles. Since only a small number of nuclei survives at any given time, this initial part of the growth process is time consuming. As long as individual particles are available they tend to adhere, trying to form nuclei or attach themselves to larger agglomerates. The latter becomes the preferential process because the larger entities with more mass and higher kinetic energy easily “pick up” individual particles and incorporate them into their surface structure (see also Section 5 . 3 and, for example, Fig. 5.42). Therefore, to accellerate the tumble/growth agglomeration process specific operating strategies that influence the nucleation stage are commonly applied. If the agglomeration is carried out batch, at the end of the process only 2/3 to 3 / 4 of the agglomerated mass is removed. The rest, often called a “heel” remains in the apparatus which is then filled to the predetermined level with fresh powder mix. As soon as tumbling and, if applicable, the addition of binder fluid start, the preagglomerated material, that remained from the previous batch, participates actively in agglomerate growth by random coalescense, abrasion transfer, as well as crushing and layering (see Fig. 7.1) and, at the same time, but not influencing the agglomeration rate, nuclei are being formed. In some cases, where low force natural bonding of submicron- or nanoparticles is used and, therefore, nuclei are particularly weak, the time for completion of the agglomeration process has been reduced from days to hours by this measure, for example, during the densification of silica fume in a batch fluidized bed (see also Section 7.4.5). In continuously operating tumble/growth agglomeration processes, the recirculating fines take the place of seeds. Although the recirculating particles are smaller than the lower product size limit, most of them are preagglornerated entities which become rewetted, if they had passed the drying stage, and then easily pick-up single feed particles while “regular” nuclei are also developing in the tumbling charge. Therefore, recirculating fines are not only a burden on productivity and operating cost, in many tumblelgrowth agglomeration systems they also play a vital part in agglomeration efficiency. Particularly in the pharmaceutical industry and other applications requiring high cleanliness, modern batch agglomeration equipment is now often operated as a “one pot processor”. This means that all agglomeration steps, including dry powder mixing, is done in the same vessel without opening the containment between steps and transferring intermediate materials. A “one pot process” blends the components, agglomerates (stabilizes) the mixture by adding binder liquid, potentially adjusts agglomerate size by crushing, further agglomerates while introducing additional binder liquid, and finally dries and cools the agglomerated mass. In such processes, fines may be removed by screening from the one pot processor discharge and returned into the processor with the appropriate amount of fresh feed components to act as seeds. Retaining a heel or returning fines which act as seed material may also effectively prevent the potential of selective agglomeration of the finest component(s) of the for-
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mulation. Because the adhesion tendency increases with decreasing particle size (see also Sections 5.1.1 and 5.4) very small particles adhere easily to larger ones but also preferentially to each other. Such selective agglomeration may change the uniformity of the mix and/or composition of individual agglomerates. Because of the larger mass of the preagglomerated returning fines they participate in the destruction of selectively growing agglomerates and, thus, help in maintaining a uniform distribution of all components of a formulation in an agglomerated, mix stabilized product. As the mass of the growing agglomerates increases they may break apart at structurally weaker areas or as a result of the force of impact. Abrasion will also take place resulting in newly liberated primary particles or small conglomerates which then try to attach themselves to entities offering better binding conditions. Particularly in batch operations, both mechanisms help to prevent the growth of a few agglomerates to excessively large sizes. To make sure that the production of oversized agglomerates is prevented or, at least, reduced, individually controlled cutting or shredding devices are often installed which will continuously or intermittently operate and mechanically assist the breakdown of agglomerates (see also Section 7.4.2). Such operational methods may lead to a relatively uniform and narrow agglomerate size distribution if their application has been developed for a particular process in the laboratory. However, these procedures need to be redefined in the large scale commercial unit because scale-up is extremely difficult, if not impossible. Depending on the density of the tumbling material, the (changing) mass of the individual agglomerates, and the type of equipment causing agitation, the growth phenomena and, herewith, the agglomerate properties will differ. One reason for change is the varying extent of the previously mentioned naturally occurring or mechanically induced abrasion, break-down, and reagglomeration. Another is how new particles are attached and incorporated into the structure (see also Section 5.3 and, for example Fig. 5.42). It is obvious that particle beds, tumbling in rotating equipment or agitated by mixing tools, will produce denser agglomerates than obtained in the low density particle clouds of fluidized beds. These effects will be described in more detail in the appropriate chapters of this book.
7.2 Kinetics of Tumble/Crowth Agglomeration
For all methods of tumble/growth agglomeration, during which size enlargement occurs in an irregularly moving mass of particulate solids by the adhesion of single particles, nuclei, conglomerates, and pieces of agglomerates to each other, growth is a function of time. Investigation of the kinetics of tumble/growth agglomeration, i.e. the change of sizes and their distribution with time, is of direct and practical importance, particularly in regard to the determination of the end-point of the process at which the desired product properties are attained. Kinetic studies deal with theoretical and experimental investigations of the motion in agglomeration equipment, the growth mechanisms caused by these movements, and the operating and equipment parameters influencing the process. To be able
7.2 Kinetics of Tumb/e/Growth Agglomeration
to explain the phenomena, knowledge of the binding mechanisms acting in the charge and between the particles is necessary. It is the task of these studies to correlate the equipment parameters and the material characteristics, both ofthe feed and the product as well as ofthe binder(s),ifapplicable, such that the conditions in the apparatus and the agglomerated product quality can be predicted. And it is the ultimate goal ofthese studies to provide fundamental input for the automatic control of the processes. At the beginning, very much influenced by the emerging large scale iron ore pelletization processes in the early 196Os,batch operating drums were employed to investigate the growth of agglomerates during “balling”. Typical results of such tests are shown in Fig. 7.3 and 7.4 [B.42]. In Fig. 7.3 agglomerate growth is plotted over time. The latter is characterized by the number of revolutions of the drum. Four tests were carried out with moist (42.5 vol.% water) limestone powder and the curve confirms good reproducibility. In such a batch process, at first small nuclei are formed which grow into larger agglomerates as time progresses. The absolute number of agglomerates diminishes during the process as
E E
I
Fig. 7.3: Average pellet diameter as a function of agglomeration time (measured in drum revolutions) (A) Region o f seed (nuclei) formation; (B) transition region; (C) agglomerate growth region.
Fig. 7.4: Agglomerate size distribution after different times. n = number o f drum revolutions.
al
rn
P
> Q
Pellet diameter d (mm]
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small and weak conglomerates are crushed and, then, the pieces adhere to larger a g glomerates. This is depicted in Fig. 7.4 where the cumulative mass distribution curves of agglomerate sizes are plotted after different processing times which, again, are characterized by the total number of drum revolutions. In continuously operating equipment, these growth regions take place simultaneously. As mentioned previously (see Section 7.1) the critical phase of the process is the formation of nuclei. This must always take place at a sufficient rate to guarantee the transformation of the often highly adhesive feed material into a freely flowing granular mass. Investigations of the kinetics of growth agglomeration continues using more modern laboratory equipment and modeling techniques [e.g. 7.1 -7.31. Population and mass balance equations are widely used to describe the conditions with rather mixed success. For example, Sastry states [7.1]:“Formal derivations of these model equations can be found in the literature, including details on deriving rate terms for the individual mechanisms. Such equations can be adapted appropriately to specific applications, for example, in describing plug flow drum agglomeration, fluidized bed particle coating, high speed mixer agglomeration, or fully mixed flocculation systems. Relevant initial and boundary conditions must be introduced. Then, of course, we need analytical expressions or correlations for the nucleation, coalescense, and layering process parameters (among others) as a function of all material, machine, and operating conditions. After all this, one finds that analytical solutions for the model equations are most unlikely because of the complex nature of the system equations. Consequently, one resorts to numerical solutions”. The problems encountered in mathematical modeling of tumble/growth agglomeration do not relate to the theories, formulas, and possibilities to solve the ever more complicated equations. With modern computing possibilities, a whole series of assumptions can be introduced into the model equations and responses to certain imaginary process conditions can be predicted. However, the real system often produces unexpected results intermittently or even consistently without offering a clear indication of why such deviations occur. Introduction of new mathematical methods, such as, for example, fuzzy logic or chaos theory, produce more complicated model equations and “closer to life” results but still are not able to serve as unequivocal bases for control schemes. The real problem is, of course, the determination of and correlations between process data as input for the model and its solutions. Such expressions are different for each situation, i.e. they depend on feed material and binder characteristics, equipment design and operation, process variations, final product properties, and many more. Data that can serve as input for the model equations must be obtained experimentally. Since access to commercial, often restricted or large scale operations is not available or possible, typically the determination of data and their correlation is based on model experiments. In addition to the difference in size and operation between the laboratory model and the real system, the gathering of data is interrupting and critically changing the process. For example, referring to Fig. 7.3 and 7.4, to obtain each data point in Fig. 7.3 and curve in Fig. 7.4 the batch laboratory drum agglomerator was stopped after the indi-
7.2 Kinetics of Tumb/e/Crowth Agglomeration
cated number of revolutions, the charge was removed and screened. After that, the material was placed back into the drum which then rotated for the additional number of revolutions until the next set of data was determined. It is obvious that the interruptions of the process and the handling will have some, but indeterminable influence on how agglomeration as a whole proceeds. Of even more concern is that the laboratory model experiment is carried out in much smaller scale and almost exclusively in batch mode. Even small continuous tests require so much material to reach equilibrium during the multiple test runs that are necessary to evaluate different process parameters that their use is very infrequent. To overcome the need for large amounts of material and the associated handling problems, agglomerates are sometimes crushed and reused as “fresh feed” which introduces a completely new set of problems. If, on the other hand, a continuous process is simulated in a batch operation, the influence of recycle and the question of process equilibrium remain unsolved problems. Scale-up, even from laboratory batch to commercial batch operations, is very difficult and can not be totally predicted by tendencies or “rules” that are obtained as results of systematic variations of assumptions in model equations. As a result, vendors maintain an often extensive test facility, potentially with differently sized equipment (see Section 11.2) to more accurately predict the behavior of the commercial system. Particularly in the more regulated industries, which process materials such as food and pharmaceuticals with high profit margins, a new trend is towards tolling operations (see Sections 11.2 and 14.1) either during the development phase or for the manufacturing of an intermediate or a particular final product during its entire life. Equally difficult is the definition of suitable, easily measurable process variables that can be used to control the performance of a commercial installation. In continuous operations it is often sufficient to measure the inputs, such as the mass flows of fresh and recycling material and the amount of binder as well as its location and means of application at defined operating conditions of all system components and keep them constant. If, after reaching equilibrium, product yield and quality are not acceptable, all process variables must be evaluated and changed until optimal conditions are reached. Sampling plays an important role during this endeavor. For that reason it is most important to plan for and provide sampling points at crucial points of the process when first designing the system (see also Section 11.3). Determination of the conditions in and the need for modifications of process variables of batch tumble/growth agglomerators are normally not possible. It is not feasible to sample the contents of batch operating equipment during a process run to determine the progress and state of agglomeration. Therefore, methods have been proposed to control the performance of such equipment by measuring, for example, the sound of the tumbling charge and its change or the momentary energy consumption of the drive motor. Measurement and analysis of the power consumption during tumble/growth agglomeration in a drum have been carried out by many researchers [e.g. 7.4, 7.51. Fig. 7.5 is a typical power consumption curve obtained during the agglomeration of lactose with water in a batch high energy mixer/granulator as a function of time
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or, respectively, binder content defined as liquid saturation S in [mL liquid/100 g of powder]. Although the actual shape of the curve does change with material, binder, process, and product characteristics, the overall result can be characterized by the schematic representation of Fig. 7.6. The tumblelgrowth agglomeration process is carried out by placing the dry components into the apparatus. As described previously the equipment could be operated as “one pot processor” (see Section 7.1). Then the goal of interpreting the power consumption curve would be to determine the endpoint of agglomeration, that is the time at which agglomeration is completed and drying begins. During “standard”processing, where curing (drying, cooling and, if applicable, other post-treatment procedures) are carried out externally, the “endpoint of agglomeration” defines the time when the agglomerator should be emptied. Referring to Fig. 7.5 and 7.6, in phase I, powder mixing takes place. During this process s, is “zero” (or very low) and tumbling conditions are not altered; therefore, the power consumption remains constant. After mixing has been completed, which is defined by the previously, experimentally determined time that is required to uniformly blend the powder components, addition of binder liquid begins. Typically, binder is added as a fine spray and at constant rate. For a short time (see Fig. 7.6), the power consumption curve will not be influenced until, at S,, it begins to increase. In phase 11, liquid bridges develop between the powder particles and nuclei as well as smaller agglomerates are formed. The increase in power consumption is, on one hand, due to the added mass of binder liquid and, on the other hand, caused by the increasing cohesiveness of the moist powder. In phases I11 and IV, from saturations S3
Phase V 1
Phase I V
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h
I
c
I
0
I
7
I
I
I
1L 21 28 Total l i q u i d addition Iml IiquidllOOg powder)
Fig. 7.5: Power consumption o f a batch high energy mixer/granulator during the agglomeration o f lactose with water as binder liquid, plotted vs. time and, respectively, the amount o f liquid added.
)
7.2 Kinetics of Tumble/Crowth Agglomeration
' POWER OR ENERGY CONSUMPTlOh
s1
PHASE1
PHASE11
PHASE111
PHASE
m
PHASEP
I I I I I I I I
I I I I I I I I
I 1 I
I I I
I I I
IS 2
I I
I I I
I I 1
I I
t+l
0 Fig. 7 . 6 Schematic representation o f the general shape of power consumption curves as a function of time or, respectively, liquid saturation, 5,.
to S,,the interparticle void volume in the agglomerates fills up with liquid until, at Ss, complete saturation is achieved. From a control point of view, for practical purposes, phase 111 is the most important one as only in this range of saturations (between S3and S,) agglomerates are obtained that feature acceptable size distribution and quality. If a batch granulator is equipped with both mixing tools and cutter heads (see Section 7.4.2) it is in this phase that particle size adjustment is accomplished by intermittently activating the high speed cutter heads. In phase IV, particularly towards its end which is characterized by S,, local ovenvetting takes place which may be corrected to some extent by operating the cutter heads and redistributing the moisture more uniformly (approaching the straight line depicted in Fig. 7.6), however it is often recommended not to exceed saturation S, if a well controllable operation of the process is desired. Phase V should be avoided altogether. Operation becomes erratic due to excessive sticking and the build-up of large, wet conglomerates. While, with identical feed materials, the overall shape of curves obtained with various types of mixer/agglomerators is the same, as shown in a very generalized manner in Fig. 7.6, different amounts of water are required to reach the specific saturations that define phases I to V. For example, Fig. 7.7 depicts results obtained with three equipment designs [B.42]. Referring to saturations Sz and S5, the absolute variation in liquid requirement is very small but for S,, the minimum amount of liquid that is required to achieve an agglomerated state of the powder, the difference is approx. 100 % between two of the granulators chosen for the experiment. Equally as large is the difference between two of the granulators at S,, the safe maximum amount
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Fig. 7.7: Liquid requirements for obtaining the specific saturation levels 5, as defined by the energy consumption curve in Fig. 7.6 for different mixer designs [B.42, 7.41. o Intensive, high energy mixer with vertical axis of rotation o f the mixing tools (Mfg. DIOSNA). # Intensive, high energy mixer with horizontal axis o f rotation ofthe mixing tools (Mfg. LODICE). :'; Low shear planetary mixer (Mfg. LOEPTHIEN).
ofbinder liquid addition, but with a change in relation. This means, in reference to this particular example, that the very turbulent environment in the intensive, high energy mixer with vertical axis of rotation of the mixing tools produces good agglomerates with a relatively small amount of liquid in a narrow range, while the intensive, high energy mixer with horizontal axis of rotation of the mixing tools requires somewhat more liquid before larger, more uniformly tumbling agglomerates are formed but also tolerates a larger amount of binder before entering the increasingly unstable phase IV. For the low shear planetary mixer it is particularly important to point out that the difference between S, and S5 is the smallest which means that the charge may become easily ovenvetted resulting in erratic and uncontrollable process conditions. In the end, when considering the kinetics of tumble/growth agglomeration, which is really the art of controlling an agglomeration system such that high quality products with the desired properties are obtained, external consultants who use fundamental and/or interdisciplinary knowledge and know-how are often the only ones that can help to bring non-performing plants into compliance or optimize under-performing systems. For further information on the kinetics of agglomeration in pans a study by W. Dotsch [B.33] should be referred to also.
7.3 Post-treatment Methods
Several times before it had been mentioned that green agglomerates that grew during tumbling in the presence of a liquid binder are bonded by temporary binding mechanisms and must be cured, using post-treatment methods to achieve permanent bonding (see, for example, Chapter G and Fig. 6.3). Tab. 7.1 is a general summary of the effects of post-treatment methods that are commonly applied during size enlargement by agglomeration. In addition to the
7.4 Tumb/e/GrowthAgglomeration Technologies
achievement of the final binding mechanism after the removal of liquid binder components, which is necessary in almost all applications of tumblelgrowth agglomeration, other accomplishments are feasible and may be used in the design of any agglomeration system. Effects of post-treatments (curing) in size enlargement by agglomeration.
Tab. 7.1:
~~
~
Achievement
offinal binding mechanisms (by recrystallization, sintering, chem. reaction,..)
Development
offinal agglomerate characteristics (by drying, hardening, disinfection, impregnation,..)
Improvement
offinal agglomerate characteristicts (by coating, polishing, conditioning,..)
Modijcation Change
of agglomerate characteristicts (by removal of temporary ingredients [porosity],..) of the applicability Of agglomerates (by secondary agglomeration [tabletting, spheronizing, instantizing,..], encapsulation,..)
Many of the curing methods and their effects were already mentioned in previous chapters (see, for example, Sections 5.3, with subchapters, and 5.4) while others willbe covered later in the appropriate chapters. Therefore, the summary in Tab. 7.1 shall suffice at this location to highlight the topic.
7.4 Tumble/Crowth Agglomeration Technologies
In the following six subchapters the technologies and the equipment for beneficial agglomeration by growth during tumbling will be described. In the context of these chapters tumbling means the irregular, turbulent (stochastic) movement of all participating particulate matter in a suitable environment. As mentioned before (see Chapter 6) no specific equipment is necessary for this movement to occur. Any means that will produce the stochastic movement of the solids in any environment can be utilized and adapted to become an “agglomerator”. For practical reasons, four different types of agglomeration technologies will be distinguished and covered in this part of the book: High density tumbling particle beds (Sections 7.4.1 and 7.4.2), drying of solutions and suspensions (Section 7.4.3), low density particle clouds (Sections 7.4.4 and 7.4.5), and agglomeration in liquid suspensions (Section 7.4.6).
page page page page
153-187 187-196 196-221 221-227
Drying of solutions and suspensions in so called spray dryers (Section 7.4.3) make use of the binding mechanisms of agglomeration which develop during drying, but secondary agglomeration, that is the adhesion of semi-dry and/or dry particles during collisions in the drying tower, is not the primary objective of the process. Nevertheless, the technology will be covered in these chapters because it is sometimes combined with fluidized bed agglomeration (Section 7.4.4).
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High Density Tumbling Particle Beds (Sections 7.4.1 and 7.4.2) These tumblelgrowth agglomerators are characterized by the presence of an agitated particle bed in which, during the addition of a binder, collisions and coalescence occur. Typical equipment for realizing this technology include rotating inclined pans, cones, and drums as well as any mechanical powder mixer that has been modified to allow the addition of binder into the live blend. Particularly the latter group of equipment, powder mixers, is applied to an ever increasing extent because particle movement that is caused by specific container shapes as well as movements or by mixing tools together with separately driven cutter bars or heads allows to exert considerable control over the process. According to the type of agitation, mixers can be classified into several design groups. For agglomeration, the most important distinguishing characteristic is the amount of shear that is created in the particle bed. Low shear particle mixers (Section 7.4.1) typically employ rotating containers featuring shapes that produce irregular particle flow which, sometimes, is enhanced by baffles or similar structures that are built into the machine. For use as agglomerators, liquid spray arrangements and often cutter bars or heads are installed. High shear particle mixers (Section 7.4.2) are usually equipped with stationary shells and rotating mixing tools inside. The axis of these mixing tools may be either vertical or horizontal. For processing reasons some shells may be mounted at an angle to the horizontal with a corresponding inclination of the axis of the mixing tools. Again, for agglomeration spray systems are added and additional shear may be introduced by shear plates and/or cutter heads. Drying of Solutions and Suspensions (Section 7.4.3) Although binding mechanisms of agglomeration are developing during the process, these technologies are primarily dryers that are often used to produce the primary particles for agglomeration by some other method. Depending on the feed to the dryer, either solutions or suspensions, including slurries of many consistencies and filter cakes, the structure of the solid particles after drying is quite different. Low Density Particle Clouds (Sections 7.4.4 and 7.4.5) If agitation of particulate solids occurs by gas flows or jets, the speed of the suspending fluid is normally so high that the so called fluidization point or incipient bubbling velocity is exceeded and an expanded, low density particle bed develops. In this condition the stochastic movement of the solids is attained and coalescense may occur during particle collisions. Because the separating forces in the low density particle cloud are relatively little and, at a given gas velocity, only a narrow distribution of particle sizes can be retained in the fluidized state, small, very porous agglomerates are obtained.
Solid particles that are suspended in liquids may agglomerate as a result of two basically different phenomena: Flocculation is the aggregation of solid particles into relatively loose conglomerates (flocs) after collision and coalescense have occurred. Adhesion may be enhanced by the addition of polymers (so called flocculants).
Agglomeration in Liquid Suspensions (Section 7.4.6)
7.4 TumblelGrowth Agglomeration Technologies
Immiscible liquid agglomeration is using the affinity to certain particulate solids of a binder liquid that is dispersed in the form of tiny droplets in and is immiscible with the suspending liquid. Particles are selectively bonded with the immiscible binder liquid, form agglomerates, and can be separated as enlarged entities from the suspension. 7.4.1
Disc and Drum Agglomerators Disc, Dish, or Pan Agglomerators The basic disc agglomerator is a simple, inclined, flat-bottomed, shallow pan that, owing to the particular pattern of particle motion, features a distinctive classification effect whereby only the largest agglomerates discharge over the rim (Fig. 7.8). To achieve special effects, modified pan designs are available (see below).
A typical shallow disc “pelletizer” is shown in Fig. 7.9. It consists of the the pan (A) which is often equipped with an expanded metal liner (B) to reduce wear. The height h of the rim forming the pan is small compared with the diameter D of the disc ( h / D 0.10-0.20). Normally, the pan angle can be adjusted between 40 and GO” to the horizontal, in this case with a handwheel operated jacking screw (C). The disc with angle adjustment and drive is mounted on a heavy base structure (D). Also attached to the base is a frame (E) which carries the plows or scrapers (F), the spray nozzles (G), the positioning of which can, in this case, be adjusted by flexible metal hoses, and, if applicable, the dust cover. The particulate feed is delivered to the pan by suitable metering equipment and dropped onto the wetted moving mass. As shown in Fig. 7.10, shallow inclined discs are manufactured with diameters of less than 0.5 m to more than 7.5 m or, in special cases, exceeding 10 m in diameter. The design of shallow inclined pans which, if left uncovered, allows to observe the movement of the charge during operation and its response to changes, such as rotational speed and pan angle as well as method(s), position(s),and amount(s) of fresh N
Fig. 7 . 8 Photograph of a disc “pelletizer” with narrowly sized finished agglomerates discharging over the rim.
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Fig. 7.9: Photographs o f a typical shallow disc “pelletizer” depicting the structural components (courtesy FEECO International, Green Bay, WI, USA).
feed and binder additions. This has resulted in extensive and successful scientific evaluation of that type of growth/tumble agglomerator. The most striking feature of the shallow inclined pan is a very defined pattern of particle motion and a size classification both over the pan area and in the tumbling particle bed. As sketched in Fig. 7.11 and discernible in Fig. 7.8, in a clockwise rotating disc the material is lifted up from around the G o’clock position through 9 o’clock to near the top from where it fans out and cascades back down to the lower portion: here, the cycle begins again.
Fig. 7.10 Photograph showing a range o f differently sized shallow disc “pelletizers” (courtesy FEECO International, Green Bay, WI, USA).
7.4 Turnble/Crowth Agglomeration Technologies
12
3
Dis
Cross section: Development of different layers
Surfoce: Movement of feed in various stages
Fig. 7.11: Sketch of the pattern of particle motion in a shallow inclined disc agglomerator (adapted from [B.42]).
Observation and/or sampling of the contents of the operating pan reveals that in the left half of the clockwise rotating disc, where material is lifted up, a particle bed exists that extends to the edge of the rim. Due to a natural segregation effect, the largest particles move on the top of the bed and discharge over the rim as more material is added to the pan. The smallest particles, unagglomerated feed and seed agglomerates, are concentrated on the bottom ofthe bed. Under the weight of the bed, the lowest layer of moist feed and small agglomerates tends to stick to the bottom and is cleaned off by a series of staggered scrapers. As mentioned above, to minimize wear, in many cases an expanded metal liner is installed in the pan to encourage the build-up of a layer of material, the thickness of which is controlled by the plows; at the same time, the frictional characteristics which are required for uniform movement of the charge are optimized. In the right half of the clockwise rotating pan, directed and partially produced by the scraper plows, a curtain of small and seed agglomerates moves down over the disc bottom. Considering the growth mechanism of tumble agglomeration (see Chapter 6, Fig. 6.1, and Section 7.1, Fig. 7.1 and 7.2),control of agglomerate growth in an inclined, shallow pan can be affected by the relative positions of binder liquid and fresh feed addition. For example, if the liquid sprays impinge the moving courtain of small agglomerates roughly along the 3 o’clock radius and fresh, relatively dry powder feed is added on the 5 o’clock radius, powder particles will adhere to the wetted seeds and make them grow. When entering the bed at roughly the 6 o’clock position the newly adhering powder particles will be either “compacted” onto the growing agglomerate or removed by attrition and transferred to the surface of other agglomerates, thus causing growth. Adding liquid and fresh feed according to this pattern results in the production of many relatively small agglomerates. Nevertheless, the largest ones
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will move on the top of the oval shaped “tumbling center” close to the rim in the left half of the pan and discharge by overflow at the 7 to 8 o’clock position. On the other hand, if liquid is added to the top of the bed at, say, the 11 o’clock radius and fresh feed is dumped into the turbulently moving foot of the bed at approximately 6 o’clock, large agglomerates will be grown. Angle of tilt, rim height, and rotational speed (influencing the pattern of motion) together with adjustments of method and location of binder and fresh feed addition, can further modify the growth behavior and may be used for control. It is no surprise that, after the inclined shallow pan agglomerator was invented around the middle of the first half of the 20th century for the agglomeration of fertilizers, cement raw meal, and other mineral powders, it quickly attracted the interest of many researchers, including the author of this book, because it lent itself to easy scientific scrutiny and evaluation. Rather extensive coverages of the subject are included in earlier books [e.g. B.9, B.16, B.331 to which the interested reader should refer. Unfortunately, the tumble/growth agglomerator that is best described and understood when operated in a laboratory environment, is very difficult to control when used in large scale and an industrial environment. Small fluctuations in binder liquid and fresh feed addition as well as in frictional conditions in the charge and between the charge and the equipment components, most of which are caused by temporary overor under-wetting, render the operation of inclined shallow pan agglomerators an art. There are a few large scale applications, for example the pelletization of iron ores, where the highly uniform and fine feed (particle size <40 pm) and the desired pellet size (12.5 m m average) produce an optimum system response and require little operator interference. The benefit of such an operation is the manufacturing of narrowly sized, spherical “balls” which do not need extensive sizing and handling of large amounts of recycle. Most other applications, particularly those attempting the production of often desired “micro agglomerates” (approx. < 5 mm), require the continuous attendance
Fig. 7.12 Schematic representation o f scraper arrangements in inclined shallow pan agglomerators. Left: Stationary wall scraper and rotating bottom scraper. Right: Stationary wall and bottom scrapers.
7.4 Jumble/Crowth Agglomeration Technologies
of an experienced operator who can remedy problems by identifying, understanding, and correcting excessive and/or ongoing fluctuations. Because fluctuations are often beginning at the scrapers where moisture may accumulate and build-up can occur which, from time to time breaks off as large slabs, manufacturers of inclined shallow pan agglomerators sometimes propose to use motor-powered rotating scrapers. As shown schematically in Fig. 7.12 they not only scrape the bottom but also somewhat extend into the particle bed to serve a similar purpose as the cutter heads in mixer agglomerators (see Section 7.4.2). The rotating scrapers can be operated co- or counter-currently. For some applications a more uniform, less operator intensive production is achieved. Modified Disk or Pan Agglomerators Inclined shallow disk agglomerators lend themselves easily to modifications. By employing innovative pan designs (Fig. 7.13 and 7.14), the particle motion in parts of the disk may be changed to achieve special effects. Unfortunately the already mentioned sensitivity to operational upsets is even more pronounced with these tumblelgrowth agglomerator designs and, therefore, these interesting process variations are seldomly used. Nevertheless, they are mentioned here as examples of ideas that could be beneficially applied for the manufacturing of special products. Fig. 7.13 shows schematically four pans with collars or other re-roll designs. The purpose of the addition of collars (Fig. 7.13a and b) is to provide a separate re-roll space for agglomerates discharging from the main operating area of the pan. The re-roll can serve to smooth the surface of the agglomerates, densify their outer layer, or, generally, produce a more spherical shape. In certain applications, the re-roll space is also utilized for additional processing. This may include the coating with components for subsequent use, such as coke breeze on green iron ore “balls” (as a fuel or a solid reductant), limestone powder on coal pellets (for desulfurization of the combustion gas), or anticaking agents (to improve the storage characteristics of, e.g., fertilizers). Fig. 7.15 is the photograph
Fig. 7.13: Sketches of inclined shallow pan agglomerators that are equipped with collars or partitions t o achieve special effects. (a) Basic collar design, (b) collar with r i m baffle, (c) concentric vertical partitioning rings, (d) peripheral rings o n the rim.
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(C)
Fig. 7.14 Sketches of inclined shallow pan agglomerators with stepped side wall or bottom. (a) Basic onestepped design, (b) multistepped design, (c) partially raised bottom.
of uncoated (white) and coated (gray) as well as of cut pellets (to show the coating) which were produced by this technology. The collar or re-roll ring can be fastened flush (Fig. 7.13a) or recessed to provide a baffle (Fig. 7.13b) over which the agglomerates must discharge. Similar effects can be obtained by installing concentric vertical partitioning rings or peripheral rings on the rim (Fig. 7 . 1 3 ~and d). To enhance the segregation of fines and of smaller or larger agglomerates, stepped side wall designs have been suggested (Fig. 7.14). Proponents of the multistep sidewall (Fig. 7.14b) claim that stronger, more uniform agglomerates are produced because the larger pellets impact and roll on the stepped sidewall rather than on a “soft bed” of agglomerates and fines. The inventor of an inclined shallow pan with a concentric raised central bottom part (Fig. 7.14~)reports that a more stable operation is obtained because larger lumps, which are normally breaking off the scrapers in a flat bottom design, are not present. This is due to the fact that liquid binder and fresh feed are sprayed and, respectively, fed onto the raised bottom part which is held
Fig. 7.15: Photograph of uncoated (white) and coated (gray) as well as cut pellets (to show the coating). The coating was applied on a re-roll ring (courtesy EIRICH, Hardheim, Germany).
7.4 JumblelCrowth Agglomeration Technologies
clean by a stationary scraper. The “seeds” thus produced are directed into the annular space between the raised bottom and the rim where uniform growth occurs without surging. Deep Disk or Pan Agglomerators Some manufacturers offer inclined pans with a diameter to rim height ratio exceeding 0.25 and claim that the increased mass (hold-up) in the pan results in additional strengthening of the agglomerates due to overburden pressure and longer residence time. One design in particular (EIRICH, see Section 7.4.2) also uses an eccentrically mounted, rotating scraper/mixer which causes partial disintegration of already formed agglomerates, thus forcing crushing transferllayering and producing more uniform structure and sizes of pellets.
/ Water
connect ion
operat ing level
Fig. 7.16 M M C Mars Mineral drum pelletizer. (a) Schematic representation of the design principle, (b) photograph o f a large industrial M M C drum pel. letizer (courtesy Mars Mineral, Mars, PA, USA).
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Fig. 7.17:
Sketch of a “cone pelletizer”.
Of particular interest is the deep pan agglomerator with integral rear auger feeder (Fig. 7.16). By transporting the material to be agglomerated directly into the “seeding area” at the bottom of the pan, this design promotes the production of new seeds and eliminates the free fall of fine feed from the top which may cause dusting. In top-fed inclined pan agglomerators the installation of covers and dust collection is necessary to avoid particulate pollution. This impairs physical and visual access to the pan and is avoided by bottom feeding. Since it is difficult to reach the smaller seeds with the binder spray, most of the liquid impinges on the larger agglomerates, and the material has a longer residence time in the deep pan, larger pellets are typically being produced. As shown in Fig. 7.16b, which depicts an “MMC drum pelletizer”, the deep pan begins to resemble a drum agglomerator (see below). The main difference is a still rather steep tilt angle with a 30” adjustment range. The charge moves upward from the feed to the discharge end, which results in a relatively short “drum”,requiring little space, and still features the production of narrowly sized pellets due to the natural segregation that is typical for inclined pan agglomerators. The “MMC drum pelletizer” is also available in a multiple-depth design that provides three different drum depths in one unit. Another, no longer available inclined pan agglomerator, which was approaching the design of a drum but still featured size classification in the charge, used a truncated cone rotating around its axis (Fig. 7.17). Feed and binder were added from the top but the higher peripheral speed, which was necessary to retain the charge in the cone, resulted in additional surface “compaction” of the larger agglomerates as they traveled to the base of the cone prior to discharge. Drum Agglomerators They represent the most simple type of equipment for growth agglomeration by tumbling. They are used in industries for the processing of large amounts of bulk solids where in the relatively crude and rough environment unsophisticated machinery performs best. Drum agglomerators consist normally of a cylindrical steel tube with a slight (typically up to 10” from the horizontal) slope a toward the discharge end (see Fig. 7.18). Retaining rings are often fitted to the feed and discharge ends of the drum to avoid spill-back and, respectively, to increase the bed depth of material and/or its residence time.
7.4 JumblelCrowth Agglomeration Technologies
Liquid
/
Yper
Fig. 7.18: Sketch o f a d r u m agglomerator.
Fig. 7.19 is the photograph of a typical drum agglomerator for the wet granulation of fertilizers. The tubular shell (1)is fitted with steel tires (2). The drum rests on forged trunnions (3) with antifriction bearings which are mounted on a heavy structural frame (4).Sturdy, adjustable thrust rolls (5) keep the inclined drum in place. A roller chain girth drive (6) guarantees a “soft”start and smooth running of the equipment. It consists of a hardened drive sprocket, sectionalized girth sprocket, and rugged roller chain. Gear drives are also available as an alternative. Retaining rings are fitted on both ends of the drum. Material enters by way of an inclined chute (not shown) and exits through an enclosed manifold. A liquid feed assembly (8) serves to introduce the binder into the tumbling mass in the drum. Similar to the practice used for inclined discs (see above), the interior of drums may be covered with cement or expanded metal to encourage build-up of material as an
Fig. 7.19:
Photograph o f a typical d r u m agglomerator for fertilizers (courtesy A.J. S A C K E n & Sons, Baltimore, MD, USA). For explanations see text.
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“autogenous” wear liner. To control the thickness of this coating, different designs of scrapers are employed [B.42]. As in the case of the inclined pan agglomerator, depending on the tube slope and diameter, the material, and the moisture content of the charge, the rotational speed of the drum must be adjusted such that the bed begins to separate from the wall at approximately the 10 to 11o’clock position or, respectively, the 1to 2 o’clock position in a counter-clockwise rotating drum (see Fig. 7.18) so that the particles tumble down the inclined bed. Binder liquid is sprayed onto the bed surface and fresh material is added to the turbulent zone at the foot of the tumbling mass. More extensive coverages of the theoretical treatment, including calculations and scale-up procedures, are published in earlier books [e.g. B.42, B.651 to which the interested reader should refer. Although, within the kidney shaped tumbling mass of solids, production of seeds as well as growth of agglomerates takes place and some segregation by size occurs, the major difference between drum and pan is, that the entire bed moves forward in a plug flow fashion. To avoid discharge of overwetted material, binder addition is limited to the first l/4 (minimum) to 3/4 (maximum)length of the tube. In the remaining portion of the tube, moisture distribution is equalized by abrasion and crushing of conglomerates as well as reagglomeration ofthe fines (see Section 7.1, Fig. 7.1 and 7.2). Nevertheless, many fines and pieces remain in the drum discharge which consists of a wide Fresh feed
Additive(s)
n u I (Optional)
Mixer (and Binder Iprewetter) liquid
%. I
Alternative 1 Vibrating screen
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Drum agglomerator
Vibrating screen&-----
Shredder&
1
Fig. 7.20 Sketch of the flow sheet of a drum agglomerator that operates in closed circuit prior to curing the green pellets.
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7.4 TumblelCrowth Agglomeration Technologies
distribution of sizes including over-, product-, and undersized agglomerates as well as some still unagglomerated material. Since the wide distribution of sizes in the discharge from drum agglomerators is not acceptable for most applications, they typically work in closed circuit (Fig. 7.20). Green agglomerates exiting the drum are sized on vibrating or roller screens. Large pieces are shredded and returned to the drum agglomerator together with the screen fines. The correctly sized product is sent to curing. Because green agglomerates are somewhat sticky and tend to blind vibrating screens (alternative I in Fig. 7.20), most of the fines are first “scalped off” whereby the mass of the larger pieces helps to pass the fines and keeps the screen cloth open. Product is separated from the oversized material after all the fines are eliminated. More reliable are self-cleaning roller screens (see Section 11.3) with individually driven rollers and steadily increasing gap. In most cases the rotation of the screen rollers is against the gravitational flow of solids on the downward sloping machine which induces a rolling movement of the material on the “deck” and causes an additional rounding of the spherical agglomerates. Inspite of these screening “tricks” it is often not possible to separate the moist discharge from a drum agglomerator and prevent blinding of the screen cloth for an acceptable time. In those cases, curing, which always includes the removal of water, must be carried out first, as shown in Chapter 6, Fig. 5.3, so that classification occurs in a dry state. The disadvantage is, of course, that the recycle, often amounting to several hundred percent of the system’s production capacity, must be rewetted and again cured which results in increased operating costs. To enhance the natural segregation behavior in drums and ultimately avoid the need for agglomerate sizing, a number of alternative drum designs have been proposed, all of which mimic the inclined pan or cone. For example, Fig. 7.21 shows a drum agglomerator that consists of a cylindrical tube with a slight (typically up to 10” from the horizontal) rise toward the discharge end [B.42, B.65l.A retaining ring at the feed end avoids spill-back.The operating principle is that of the deep pan agglomerator (see above). The difference is in the type and arrangement of accessories. Referring to Fig. 7.21, powder to be agglomerated is metered into the lower feed end of the drum by, for example, a screw conveyor (1)and liquid binder is sprayed onto the tumbling charge by suitable means (2). As usual, a scraper (3) controls the build-up on the wall. Finished agglomerates discharge at point (4) which is the highest point of the drum. Therefore, only closely sized agglomerates are produced. The rise of the drum can be adjustedlchanged at point (5) and the drive is connected near the drum’s thrust support (6).
Fig. 7.21: Sketch of a segregating drum agglomerator.
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Other similar proposals include multi-partitioned drums with weirs that let only closely sized agglomerates discharge into the next chamber for controlled growth, or a series of conical inserts that serve the same purpose. Scale-up of these modified drums more closely resembles that of inclined pan agglomerators. 7.4.2
Mixer Agglomerators
The irregular, stochastic movement that is required for mixing particulate solids also produces ideal conditions for growth agglomeration by coalescense. Therefore, unwanted agglomeration is often observed in powder mixers, especially if the particle size of the powder is small and/or some moisture is present. Considerable problems can arise if components of the bulk mass have different characteristics and/or sizes because, in that case, particles that have higher adhesion tendency and/or the smaller size fraction@)may selectively agglomerate, thus making it impossible to obtain an ideal mixture. Such selective agglomeration is of particular concern in the pharmaceutical industry where often an extremely small amount of finely divided active substance (the drug) must be mixed uniformly and reliably with a relatively large amount of inert filler material (excipient). On the other hand, if the conditions in the mixer can be adjusted such that a statistically uniform blend is obtained, segregation can be avoided through “stabilizing” the relative distribution by agglomeration. Often, this can be achieved in the same apparatus during or after the mixing phase by a controlled addition of binder. Since, normally, the binder is a liquid, green agglomerates are formed which require curing before they can be safely stored and handled. As mentioned before (see Section 7.1), in batch, ultraclean applications, particularly in the pharmaceutical industry, all of these process steps can be carried out in the same vessel (“onepot processing”) thus avoiding contamination of the product and/or the environment. Literally all powder mixers can be modified to operate as agglomerators. The relative movement of the particles that is required to obtain uniform mixing causes particles to collide with each other. Such particle to particle contact may result in coalescense and agglomeration if the adhesion criterion (see Chapter 6, Fig. 6.2) is fulfilled. In some cases, this occurs due to van-der-Waals or other attraction forces ifthe particles are very small (typically,particles must be nano-sized). In all other instances, binders (mostly liquids) must be added to achieve the growth of agglomerates. Therefore, the major and often only modification that is required to convert a powder mixer into an agglomerator is the installation of suitable means for binder addition. Introducing a liquid uniformly into the tumbling powder mass to entice agglomeration is not as easy as it sounds. For certain simple, batch operating mixing tasks where, at the end, the blend should be agglomerated, it is feasible to dump the entire predetermined amount of binder liquid into the mixer and cause it to distribute uniformly by the application of shear. Eventually, agglomerates will form. More commonly, however, liquid is slowly metered into the tumbling mass whereby it is important that no ovenvetting of certain volume elements occurs and agglomerate growth is obtained as the amount of binder liquid increases.
7.4 Tumble/Crowth Agglomeration Technologies
Fig. 7.22 Cross sections through two phase (pressurized gas assisted) spray nozzles. (a) External and (b) internal mix set-up (courtesy BET€ Fog Nozzle, Greenfield, MA, USA); (c) photograph of an operating nozzle, cross section through the nozzle, and spray patterns that are obtainable with such nozzles (courtesy Spraying Systems Co., Mheaton, IL, USA).
To avoid ovenvetting, liquid binder is introduced by means of more or less complicated and sophisticated spray systems which employ nozzles to atomize the liquid. Particularly in batch operating mixers it is important that the nozzles do not drip during the blending phase and are not clogged when atomizing begins. In many cases, tightly closing external valves and the application of two phase nozzles, in which atomization is caused or assisted by compressed gas (Fig. 7.22), is sufficient. In other cases, nozzles are required that seal themselves mechanically while they are not in use. Binder liquid should wet the solid particles and not the interior walls or the mixing tools of the equipment. If parts of the mixer are wetted, build-up occurs which is difficult to remove. Different spray patterns are available with commercially available single or two phase nozzles (Fig. 7 . 2 2 ~and 7.23) and a suitable pattern should be selected to assure that liquid impinges only on the moving powder. Since the spray pattern depends critically on the cleanliness of the orifice area, as mentioned before, nozzles must be drip free and installed such that they remain clean or are blown free by the action of atomizing gas. This is particularly important for low capacity nozzles which are often preferred because the binder liquid should be always completely ab-
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Fig. 7.23: Spray patterns ofsingle phase (liquid only) spray nozzles (courtesy Spraying Systems Co., Wheaton, IL, USA).
sorbed by the growing agglomerates to generate a “dry” tumbling mass and avoid build-up. For the purpose of agglomeration, mixers can be classified into:
0
Low shear mixers, high shear mixers, low or high shear mixers with intensifiers.
Low Shear Mixers The most simple low shear mixers use horizontally oriented cylindrical drums that rotate about their axis. Many of these mixers are operating in a batch mode and feature differently shaped lifters or internal baffles or the vessels are shaped such that the otherwise regular flow of material is modified to a stochastic movement. Continuously operating cylindrical drum blenders resemble the previously described drum agglomerators (see Section 7.4.1).
One of the earliest drum mixers that was applied in the fertilizer industry for agglomeration (granulation) produced a curtain of solid materials as sketched in Fig. 7.24. If acid and ammonia are sprayed onto the curtain and/or added by sparger tubes granulation occurs during ammoniation. The co-processing (ammoniation and granulation) makes this technique a most beneficial one.
7.4 TumblelGrowth Agglomeration Technologies
Fig. 7.23:
(continued)
Other low shear mixers use double or slanted cone and V-shaped vessels. Fig. 7.25 depicts schematically these always batch operating blenders which rotate about a horizontal axis. The lines and arrows in the sketches try to explain the particle paths that cause mixing. In Fig. 7.26 photographs of typical equipment are presented. The modification to a mixer agglomerator involves the installation of spray nozzles in the axis of rotation and often a so called intensifierbar, a separately driven shaft with paddles or other dispersion means that rotates at high speed and serves to produce additional turbulence. It also helps to disintegrate large, loose, overwetted agglomerates. The intensifier bar may also carry the spray nozzles or other distribution means
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Fig. 7.24 Sketch of a drum mixer with internal baffles which produce curtains of material during rotation as well as spray and Sparger tubes that modify the mixer to a co-processing agglomerator for the granulation of fertilizers.
for liquid binder (Fig. 7.27). With the intensifier bar design as shown in Fig. 7.27 the liquid spray issues through an adjustable tiny slot around the entire periphery of the dispersion blades. Although it can not be avoided that the walls are wetted to some extent, the periodic covering and uncovering with material together with the sliding action of the mass tend to keep the interior clean. The bame inserts of low shear drum mixers can also have other designs than the simple radial lifters shown in Fig. 7.24. Fig. 7.28 is a cut-away view of a rotary batch blender with internal mixing flights. Liquid manifolds with nozzles can be installed along the imaginary central axis to convert the blender to a mixer agglomerator. As depicted in Fig. 7.29, this mixer agglomerator can be also operated continuously whereby cleaning and accessibility are guaranteed by the quick and easy removal
Fig. 7.25:
Schematic representation o f different batch operating low shear mixers with indication of basic particle movements. (a) Double cone, (b) slanted cone, (c) V-shape (adapted from CEMCO, Middlesex, NJ, USA).
7.4 Turnble/Growth Agglomeration Technologies
Fig. 7.26 Photographs of typical batch operating low shear mixers. (a) Double cone (courtesy Abbe, Little Falls, NJ, USA.), (b) slanted cone (courtesy CEMCO, Middlesex. NJ,USA.), (c) V-shape (courtesy Abbe, Little Falls, NJ, USA.).
or exchange of the internals (Fig. 7.29b). The internals can be specifically designed for the changing process requirements of each application. As shown in Fig. 7.30, agglomerates from low shear mixers are loosely assembled and bonded entities of irregular shape (Fig. 7.30a) which become somewhat denser and more rounded as they grow to larger sizes (Fig. 7.30b). The combination of a high shear blender and a low shear agglomerator is the P-K Zig-Zag continuous blender/a&omerator (Fig. 7.31). It consists of a slowly turning eccentric drum with a dispersion head inside that rotates at high speed (Fig. 7.32) and a V-shaped tumbling shell that is attached to the drum and causes multiple internal material recyclings to produce uniform blends. If the equipment is operated as an agglomerator, liquid is introduced by slots (similar to what is shown in Fig. 7.27) and atomized; the powder that is aerated by the action of the high speed dispersion head is uniformly wetted and seeds form in the mass which grow to loosely bonded agglomerates in the low shear Zig-Zag portion of the machine. Fig. 7.33 is the photograph of such a machine.
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Size of aperture controls spray fineness, from a mist to droDlets
Spray issues through tiny slot oround entire periphery. Width of sproy bond
Liquid inlet to central tube
Oipersion blades to aerote and suspend solids in orea of sproy bond.
Liquid odditionol detail Fig. 7.27: Sketches ofthe design ofa specific intensifier bar and the liquid addition detail (courtesy Patterson-Kelley, East Stroudsburg, PA, USA).
7.4 Jumble/Crowth Agglomeration Technologies
Fig. 7.28 Cut-away view o f a batch rotary blender with internal mixing flights (courtesy Munson Machinery Co., Utica, NY, USA).
High Shear Mixers High shear mixers are characterized by a normally stationary vessel with mixing tools inside that cause a stochastic movement of the particulate charge. The most simple blenders with mixing tools are single or twin shafted pug mills. They consist of a bottom portion with semi-cylindrical trough(s), connecting to vertical side, feed and end walls, and feature a flat cover (Fig. 7.34). The latter is often kept at least partially open for observation and the mounting of liquid manifolds. The shaft(s) carries(y) a series of paddles and rotate(s) relatively slowly to cause the mixing action. Because of the slow movement of the paddles and the charge, pug mills constitute a transition between low and high shear mixers. They are often used as part of a transportation system for bulk solids and, if liquid is added to the charge, form crude
Fig. 7.29 Artists rendering o f a continuous rotary blender with internal mixing flights. (a) Cut-away view, (b) different internal mixing flight designs (courtesy Munson Machinery Co., Utica, NY, USA).
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Fig. 7.30 Photographs of two typical agglomerates that were produced in a batch low shear agglomerator with V-shaped shell. Left: Agglomerate size approx. 750 pm, Right: agglomerate size approx. 4 mm (courtesy Patterson-Kelley, East Stroudsburg, PA, USA).
agglomerates which are suitable for dustfree deposition of particulate waste in a land fill or for granulated products with low quality requirements. The first true high shear mixer that got patented in Germany in 1934 to carry out agglomeration was the Eirich granulating mixer. The patent describes the mixing and rolling of particulate solids on the flat bottom of a rotating pan by means of eccentrically arranged, counter-currently moving mixing tools. The mixing elements of these tools are either blades or bars which extend into the material to be agglomerated (Fig. 7.35). The patent claims that, due to the intensive shearing, practically all
Fig. 7.31: Schematic representation of a P-K Zig-Zag continuous blender/agglomerator (courtesy Patterson-Kelley, East Stroudsburg, PA, USA).
7.4 Tumb/e/Crowth Agglomeration Technologies
Fig. 7.32: (a) Sketch depicting how liquid is dispersed into aerated solids in the eccentric drum o f a P-K Zig-Zag blender/agglornerator. (b) The close-up photograph shows a dispersion head inside the eccentric drum of a P-K Zig-Zag blender agglomerator featuring multiple dispersion blades and liquid spray apertures between discs (courtesy Patterson-Kelley, East Stroudsburg, PA, USA).
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Fig. 7.33: Photograph of a P-K Zig-Zag blender/agglomerator (courtesy Patterson-Kelley, East Stroudsburg, PA, USA).
Fig. 7 . 3 4 Photograph o f a double trough (twin shafted) pug mill (courtesy FEECO International, Green Bay, WI, USA).
Fig. 7.35: (a) High shear mixing and agglomeration tools o f an Eirich counter-current granulating mixer. (b) Schematic representation of the pattern o f movement in the Eirich counter-current mixer (courtesy EIRICH, Hardheim, Germany).
7.4 Tumb/e/CrowthAgglomeration Technologies
Fig. 7.36 (a) Photograph ofan Eirich inclined, deep pan granulator with counter-currently operating high shear mixing and agglomeration tools. (b) Schematic cut-away view o f the equipment shown in (a) describing its function (courtesy EIRICH, Hardheim, Germany).
solid powders can be agglomerated into relatively uniform granules without, if the material features sufficient natural binding characteristics, or with the addition of a binder. While, originally, the Eirich granulating mixer used a deep, flat pan with vertical axis and operated in a batch mode, it was later modified by tilting the axis, thus yielding a continuously operating deep pan agglomerator but maintaining the high shear, counter-currently rotating mixing tools. Fig. 7.36 is the photograph of such a piece of equipment and a cut-away view describing its function. In 1949, also in Germany, Lodige invented another important new tool for high shear mixing, the plow (Fig. 7.37). This element is fastened, in multiplicity, to a horizontal shaft as shown in Fig. 7.38 and rotates in a stationary drum (Fig. 7.38a). The rotational speed depends on the material, the diameter ofthe drum as well as the shape of the plows and is empirically determined to produce an intensive, three-dimensional particle motion as shown schematically in Fig. 7.3%. Often, the condition of the turbulently moving, aerated particle mass is described by the term “mechanically fluidized”.
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Fig. 7.37: The plow-shaped mixing tool as invented by Lodige at the middle of the 20th century (courtesy LODICE,Paderborn, Germany).
Fig. 7.38: (a) Shaft with plow-shaped mixing tools. The half plows at each end serve t o keep the mixer end walls clean. (b) Cut-away view of plow-shaped mixing tools installed in a drum. (c) Cut-away view o f a batch mixer indicating the particle movement caused by the plowshaped mixing tools (courtesy LODIGE, Paderborn, Germany)
After expiration of the patent protection for Lodige, the plow-shaped element has become one of the most common mixing tools for high intensity, high shear batch and continuous mixers and mixer/agglomerators with stationary cylindrical shell and horizontal axis. Other high shear mixers and mixer/agglomerators include pin mixers (Fig. 7.39) employing pins and paddles of many different shapes (Fig. 7.39b), whereby the elements can be arranged in straight lines (Fig. 7.39a) or resembling a spiral (Fig. 7.39c), and ribbon blenders with a large number of varying mixing tool designs (Fig. 7.40) as well as single or double shaft execution. Particularly in batch mixers and when cohesive powders had to be blended or if the mixer was modified to operate, at least during certain phases, as an agglomerator, the problem always existed to avoid unwanted agglomeration or the formation of oversized conglomerates. Although the application of mixing tools and the resulting shear in the
7.4 Tumb/e/Crowth Agglomeration Technologies
Fig. 7.39: (a) Pin mixer with open cover showing the built-in screw feeder (foreground) and detail o f a the pin mixer shaft (courtesy FEECO, Green Bay, WI, USA) (b) Detail o f different mixing elements (courtesy LODIGE, Paderborn, Germany) (c) Detail o f a pin mixer shaft with elements arranged in a staggered, overlapping, double helical pattern (courtesy Mars Mineral, Mars, PA, USA).
tumbling mass caused some desagglomeration or destruction of oversized agglomerates, both undesired phenomena still persisted. To overcome this problem, in 1957 in Germany, Lodige, the same company that had earlier invented the plow-shaped mixing tools, invented the independently driven, high speed “knife heads” or choppers (Fig. 7.41).As shown in Fig. 7.41b the knife head extends into the vessel such that it does not
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Fig. 7.40 Five different element designs for the ribbon mixers of one supplier. (a) Double ribbon, (b) interrupted outer ribbon, (c) double ribbon with flanged shaft, (d) split ribbon, (e) "mullers" can be added to any ribbon design to yield a spatula-like action (courtesy Abbe, Little Falls, NJ, USA).
interfere with the mixing tools. Again, after expiration ofthe patent protection for Lodige essentially all manufacturers of high intensity mixers and their modifications for agglomeration offer and use knife heads. They are called accelerators, intensifiers, turbines, mills, choppers, and many similar names. Fig. 7.42 depicts various single and multiple action elements of another mixer manufacturer. As long as the mixer is used for blending powders, the knife heads are used as accelerators and intensifiers for the mixing action. They may be operating continuously at a speed of 1,800 rpm and more to destruct undesired agglomerates which hamper mixing. If the process enters the agglomeration mode or if a mixer is used primarily for agglomeration, the operating parameters of the knife heads must be modified. In that case the choppers are applied from time to time to mechanically destruct agglomerates, thus arriving at a more uniform agglomerate structure and size distribution by crushing and layering (see Section 7.1, Fig. 7.1).
Fig. 7.41: (a) Multiple knife head. (b) Schematic representation of how a knife head is installed in an intensive mixer with plow-shaped mixing tools (courtesy LODICE, Paderborn, Cermany).
7.4 Jurnb/e/Growth Agglomeration Technologies
Fig. 7.42: Various modern single and multible action chopper elements of a specific mixer manufacturer (courtesy DRAIS, Mannheim, Germany).
The purpose of the knife heads is to reduce the size of agglomerates in a controlled fashion whereupon the fragments reagglomerate with still available and/or freshly produced fines. The method also serves to distribute the binder liquid more uniformly. The mode of operation to control agglomeration by this manner is characterized by applying the following consecutive steps: 1. Fillinglmetering of components. Mixing, potentially with knife heads operating. Spraying of some binder and agglomeration. Tumbling without binder addition (optional). 5. “Chopping” with knife heads. 6. Spraying of some additional binder and agglomeration. 7. Repeat(s) of steps 4-6 until the final amount of binder has been added and uniform agglomeration has been obtained. 8. Begin of the next mode of operation, for example drying/cooling or discharge of green agglomerates. 2. 3. 4.
Agglomeration occurs by alternating growth and disintegration while slowly adding binder. Within limits, the final size and density of the agglomerates can be controlled by the number and relative duration of the individual steps. It should be realized that scale-up of successful sequencing, that was determined in a small laboratory mixer, can not occur linearly. Rather, the purpose of the different steps and the amounts of powder and liquid involved in every step must be considered. It is often necessary to again carry out optimization trials with the full scale equipment. The above mentioned mechanisms for achieving agglomerate size and density control have only limited applicability for continuous mixer/agglomerators. In most cases their mixing chamber is subdivided into imaginary zones in which the processes mixing, binder addition and agglomeration as well as “finishing” take place. Choppers may be applied in the agglomeration and early finishing sections to accomplish similar modifications as described above for batch mixers.
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Fig. 7.43: Schematic representation ofthe vortex flow pattern in a Henschel high intensity mixer (courtesy HENSCHEL Mixers America, Houston, TX).
The mixer/agglomerators which were discussed so far employ tools that rotate around a horizontal axis. A large, important group of mixers, which are also used as agglomerators, is equipped with vertical tool shafts. They always operate in a batch mode and are equipped with mixing blades that are located close to the bottom. One of the first of these types of machines, the “Henschel” mixer, was introduced around 1955. It provides a (mechanically) fluidized vortex pattern of movement (Fig. 7.43) in which all particles move freely, essentially independent of size, density, or coefficient of friction. Therefore, originally, the equipment was applied for fast and optimal desagglomeration of powders and dispersion of additives. Later, it was found that the highly turbulent particle movement is also causing agglomeration if liquid binders are added to the charge. To obtain one or the other effect, not only the correct additives must be introduced into the batch but also the mixing tools must be adapted. For that purpose a variety of mixing tools is available, including special tools of different design. However, even the so called “standard tools” are very versatile (Fig.
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! Fig. 7.44: Design of a “standard mixing tool” (courtesy HENSCHEL Mixers America, Houston, TX).
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Fig. 7.45: Sketches oftypical designs o f bowl mixer/agglornerators. (a) Basic design, (b) equipment for “one pot” processing (courtesy DIOSNA, Osnabruck, Germany).
7.44);they are of modular design thus allowing adjustment of the energy input into the batch by the installation of different spacers. Fig. 7.45 depicts schematically typical designs of bowl mixers which, with the exception of the bowl shape, feature similar designs as the “original” Henschel equipment. The shape of the bowl promotes formation of a vortex flow and the mixing tool has minimum clearances to the inner equipment walls for maximum product yield. Often, as shown in the sketches of Fig. 7.45a and b, the impeller can be lifted, sometimes even hydraulically, for improved cleaning. A chopper (or multiple ones) is located such that it extends into the zone of greatest material velocity to perform the same functions as described previously. Particularly in the pharmaceutical industry, cleaning requirements may necessitate installation of the chopper@)through a removable roof as shown in Fig. 7.45b. This apparatus is designed for efficient mixing, granulating, gas stripping, and vacuum drying in a one-pot manner. To avoid condensation the so called “auto-lift lid” is also heated in this case. Fig. 7.46 depicts the design of a modern one-pot mixing, granulating, and drying system in which the particular advantages of microwave drying are utilized. In this drying method, the internationally standardized microwave energy of 2,450 MHz
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Fig. 7.46 Schematic o f a “one-pot’’ mixing, granulating, and drying system featuring microwave drying (courtesy FUKAE Powtec Corp., Kobe City, Japan).
causes the water molecules in the moist agglomerates to vibrate at high speed. Heat that results in evaporation is generated by friction between the water molecules throughout the mass to be dried. Therefore, drying does no longer occur from the outside by transfer and conduction of heat; it now proceeds at a faster rate, particularly if the solids exhibit poor heat conductivity. Although bowl mixers and agglomerators always operate in a batch mode, new equipment has sometimes a rather large volume (up to 2,000 L bowl volume) of which, depending on the material and application, between 30 and 80 % are useable per batch thus allowing large production rates because, typically, processing times are short. If, in addition, the drying process is carried out externally, as shown in Fig. 7.47 in which an external fluidized bed dryer is applied, a quasi-continuous process is obtained. In such an arrangement, a closed system, from loading the raw materials to the discharge of dry granular product, is also achieved. Other batch or continuous mixers with an almost vertical axis of the mixing tool are the orbiting type screw blenders (Fig. 7.48) often also called “Nauta” mixers. The total contents of the conical silo is mixed by the orbiting screw which rotates such that it transports material from the depth of the bin to the surface. Although not often
7.4 Tumb/e/CrowthAgglomeration Technologies
Fig. 7.47: Quasi-continuous system for mixing, granulating, and drying featuring a bowl mixer/granulator with 600 L bowl volume and external fluidized bed dryer. A schematic of the entire system is shown in the inset (courtesy DIOSNA, Osnabruck, Germany).
used as granulator, agglomerate growth can be accomplished in the turbulently moving center of the material surface if the screw speed is high. Under those circumstances, material exiting the screw at the bed surface is thrown towards the whirling center were it begins to descend. If binder liquid is introduced at this location, agglomeration occurs. This example is mentioned not because of the importance of this blender type for agglomeration but to demonstrate that any mixer that causes irregular and
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Fig. 7.48: Partial cut through a vertical orbiting screw blender (courtesy JAYCO, Inc., Union, NJ).
7.4 TumblelGrowth Agglomeration Technologies
Fig. 7.49 Partial cut away artist’s conception of the “Schugi Flexornix” (courtesy HOSOKAWA SCHUCI, Lelystad, The Netherlands).
independent particle movement, which is the precondition for good mixing, can also be used as an agglomerator. Another mixer that is very often used to produce agglomerates and features a vertically arranged tool shaft is the Schugi Flexornix. This machine, shown schematically in Fig. 7.49, has a vertical open-ended cylindrical mixing chamber in which a shaft,
(a)
(4
Fig. 7.50 (a) Schematic representation of a cross section through the operating parts of a “Schugi Flexornix”. (b) Photograph o f the opened-up roller cage of a “Schugi Flexornix” also showing the vertical shaft with the mixing blades (after removal o f the flexible sleeve that defines the mixing chamber) (courtesy HOSOKAWA SCHUCI, Lelystad, The Netherlands).
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Granule diameter (pm)
Fig. 7.51: Effect of “Schugi” rotor speed on granule size distribution.
equipped with adjustable blades, rotates at high speed (1,000to 3,000 rpm). The number and position of the knife blades and their angle of attack are selected to suit the particular process needs. The shaft is suspended from heavy duty bearings in the drive system above the vertical cylindrical mixing chamber, resulting in a continuous, completely unobstructed discharge of the moist, agglomerated product. The mixing chamber of the “Schugi” consists of a flexible sleeve that is continually deformed from the outside by rollers which are moving up and down, thus preventing build-up on the interior (Fig. 7.50). The rollers are mounted in a cage which is pneumatically operated. Roller cage and mixing chamber are easily accessible for cleaning and servicing (Fig. 7.50b). Powders are dropped into the upper end of the mixing chamber such that a low concentration of solids in the mixing chamber develops. Hold-up or retention time, which is only approx. one second or less, can be, to a certain extent, influenced by the angle of the knife blades; they may increase or decrease the free fall speed component of the rotating charge. Agglomeration of the solid particles occurs either by desagglorneration of a wet feed and reagglomeration using the liquid that is available in the feed or by wetting dry powder with binder liquid. For liquid addition, a wide range of spray or atomizing nozzles is available; selection and installation of these wetting arrangements depends on the liquid and the desired product characteristics. Agglomerates from the “Schugi” normally feature a small but somewhat adjustable particle size in the range from 0.2 to 2 m m and narrow distribution. With increasing rotor speed the width of the particle size distribution tends to become smaller [B.42] (Fig. 7.51). Fig. 7.52 is the flow sheet of a continuous granulation system using a Schugi Flexomix as the agglomerator. Although this is only a process schematic it shows that the
7.4 Tumb/e/Growth Agglomeration Technologies
Flow sheet o f a continuous granulation system using a “Schugi Flexornix” as the agglomerator (courtesy HOSOKAWA SCHUCI, Lelystad, The Netherlands).
Fig. 7.52:
heart of the plant, the “Schugi”,is almost inconsequential in size. This figure, in addition to partially depicting what is necessary to obtain a complete granulation process using a “Schugi”,demonstrates again, that in most wet agglomeration plants, the peripheral equipment and installations, such as raw material receiving and storage, powder preparation -which often comprises metering and premixing of several components -, liquid receiving, storage, preparation, metering, and addition, green agglomerate drying, product sizing - often including crushing of oversized agglomerates in a closed milling loop -, dust collection, and fines recirculation, are much larger and more expensive, both in regard to investment and operation, than the agglomerator itself (see also Chapter 11). 7.4.3 Spray Dryers
Spray drying is the drying of a spray [B.43]. As the name implies, spray dryers are primarily used to obtain dry products from liquid or wet feed stocks. In all of these methods, feed in a liquid or semiliquid form is dispersed in a gas stream to produce granular solids through heat and/or mass transfer. Features that are common to these techniques are: 1. The feed must be pumpable and dispersible into droplets. 2. Product size is limited to particles with approx. 1 m m diameter and is often much smaller.
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3.
Uncontrolled adhesion resulting in oversized clusters and fines carryover are often a problem but the systems are designed to recover and/or recycle them. 4. The processes are normally amenable to continuous, automated large scale operation. Spray drying is one of the most important continuous drying methods for the conversion ofsolutions, emulsions, or slunies into powders. The pumpable feed is dispersed into droplets which are dried in a controlled flow of hot gas. Particles are formed as moisture evaporates from each droplet. Particularly in regard to agglomeration, there is a fundamental difference between producing dry particles from solutions or emulsions and from suspensions or slurries. If droplets of solutions or emulsions are dried, a solid hull if formed in the supersaturated layer on the surface. This solid casing soon gains structural integrity and defines the size of the final product particle which is only slightly smaller than that of the original droplet. During further drying, solids deposit onto this hull mostly from the inside so that, finally, hollow spherical particles are formed (Fig. 7.53a). With some materials it is possible that the casing becomes impermeable to steam which results in cracked hollow spheres (7.5%). In other cases the particle hulls may collapse forming doughnut-like shapes (7.53~). In any case the bulk density of these particles is extremely low. Also, the formation of the solid is not characterized by an agglomeration process but by crystallization and/or deposition.
Fig. 7.53: Photographs of typical particles (explanations see text) that are obtained during the spray drying o f solutions or emulsions (courtesy CEA/NIRO, Soeborg, Denmark).
7.4 TumblelGrowth Agglomeration Technologies
Fig. 7.54 Model explaining (see text) the drying o f droplets o f suspensions or slurries (courtesy CEA/NIRO, Soeborg, Denmark).
The solids formation process is much different if suspensions or slurries are dried. In this case, the droplets already contain the solid in the form of small particles. As shown in the model depicted in Fig. 7.54 drying proceeds as follows: Due to the surface tension of the liquid, suspensions first form a spherical droplet while slurries, depending on their solids content, obtain a more or less (less in the case of high solids loading) spheroidal shape. At the beginning, liquid evaporates only on the surface. If particles in the droplet are still somewhat movable, i.e. if particles are suspended in relatively low concentration, some densification occurs when the droplet becomes smaller due to evaporation of the liquid. This is normally not the case if the dryer feed is a slurry. Since agglomerates of the particles that are contained in the droplet develop during drying, some binding mechanism must develop between the particles when the capillary forces disappear. This binding mechanism is typically the recrystallization of dissolved substances. Most commonly it is the solid material itself or, in the case of a mixture of solids, at least one of the components which is soluble in the liquid and recrystallizes during drying. Since drying only occurs at the surface, supersaturated solution develops there and some crystals form near the outer pore ends between the particles. b) When the excess liquid has evaporated, the capillary bonding state is obtained. The solid particles are now packed as tightly as they will ever be and are held together by the negative capillary pressure of the liquid that completely fills all the void spaces (pores) between the particles (see also Sections 5.1 and 5.1.1). a)
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Evaporation continues to take place on the surface of the agglomerate, where dissolved material recrystallizes, and solution is made available from the interior by capillary flow. c) At a certain stage of the drying process, some pores become empty and the residual liquid concentrates in liquid bridges at the coordination points between the particles, while some other pores are still filled with liquid. Capillary flow can only occur in completely filled pores. Therefore, as drying progresses, the drying zone moves into the interior of the agglomerate and recrystallization now also occurs inside the structure. d) Just before reaching the dry state, all remaining moisture is in the form of liquid bridges between the particles which form crystal bridges as drying is completed. Therefore, while during the spray drying of solutions and emulsions, mostly hollow spherical particles are produced by a process which is not directly defined as agglomeration, the production of dry particles from suspensions and slurries uses the binding mechanisms of agglomeration to yield true agglomerates. Besides the gas handling, heating, and cleaning facilities, the two most important system components are the equipment for dispersion of the wet feed and the containment in which hot gas contacts the dispersed wet material and drying takes place. Dispersion of the wet feed is typically accomplished by specially designed nozzles. Although different designs of nozzles (see also Section 7.4.2) are offered and used by competing manufacturers of spray dryers, below, as examples, only two often applied nozzle designs will be described. The most widely applied type of spray dryers by one supplier is provided with a rotary atomizer (Fig. 7.55 and 7.56). The system disperses fluids by centrifugal force. It has a high degree of flexibility, with capacities from small (a few kg/h) to very large (over 200 t/h). Various wheel designs (Fig. 7.55) have been developed for the handling of different types of liquid feed such as solutions, emulsions, suspensions, slurries, pastes, and melts. Fig. 7.56 is the photograph of a rotary atomizer in operation. The main operating parameter affecting droplet size is the peripheral speed of the atomizer wheel.
Fig. 7.55: Schematic representation and partial cut through one design o f a rotary atomizer wheel. (1) (2) (3) (4) (courtesy CEA/NIRO, Soeborg, Denmark).
7.4 Tumb/e/Crowth Agglomeration Technologies
Fig. 7.56
Photograph of a rotary atomizer in operation (courtesy CEA/NIRO, Soeborg, Denmark).
ADS
=
cx F
R ~ ~ x~ disc ~ vx Lnd ~
(Eq. 7.1)
with ADS = average droplet size, C = a constant, FR = feed rate, rpm = speed of the atomizer wheel, dia = wheel diameter, n = number of vanes, and a, b, c, and d = empirical exponents. Often, the droplet size is inversely proportionate to the peripheral wheel speed to the power of 0.8. The size of the dry particles depends on the primary droplet size as well as the above mentioned factors that affect size during drying, such as shrinkage, expansion, rupture, and agglomeration, whereby the latter may include some densification. The normal way to obtain dry powders or granules that are not dusty is to produce larger droplets. However, as droplet size is increased, the diameter of the drying chamber must be enlarged, too, to avoid the formation of deposits on the wall. Especially for smaller plant capacities, this requirement makes it impractible to use only spray drying to obtain this product characteristic. In those cases a combination between spray drying and fluidized bed agglomeration is being used (see Section 7.4.4). Spray dryers with nozzle atomizers use two different types of dispersers: singlephase, hydraulic pressure nozzles and two-phase, pressurized gas assisted nozzles (see also Section 7.4.2). The latter has a relatively limited application in spray drying because of the relatively large flow of atomizing gas, which influences the flow pattern of the drying gas in the tower, and the broader particle size distribution that is produced by this type of nozzle.
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Fig. 7.57: Photograph of a single phase, hydraulic pressure nozzle in operation (courtesy GEA/NIRO, Soeborg, Denmark).
The feeds for spray dyers with nozzle atomizers are normally solutions, emulsions, and dilute suspensions. As with rotary atomizers the product particle size of a “nozzle plant” depends on primary droplet size. For a given feed and nozzle type and size, the droplet diameter is inversely proportionate to the liquid pressure to the power of e.g. 0.3 and directly proportionate to the square root of the orifice diameter. Typical liquid pressures are 5 to 50 bar and the orifice diameter is between 1 and 4 mm. With these
c
r Fig. 7.58: Different drying chamber designs. (a) Co-current, (b) counter-current, (c) mixed flow patterns (courtesy GEA/NIRO, Soeborg, Denmark).
r
7.4 Jumble/Crowth Agglomeration Technologies
parameters, average particle sizes range from 50 to 350 pm. As droplet size is enlarged the height of the cylindrical section of the drying chamber must be increased to avoid the formation of deposits. The capacity of a single pressure nozzle is only up to 1 t/h; therefore, high tonnage plants use multiple nozzles. Since the spray pattern from these nozzles can be relatively narrow (see Fig. 7.57) many such nozzles can be installed in the roof of the drying chamber with minimal interference between the sprays. Drying chambers are designed to realize co-current, counter-current, and mixed flow patterns of drying gas and droplets or wet particles (Fig. 7.58). Especially the counter-current and, to some extent, the mixed flow patterns increase the residence time of the descending solids in the drying chamber, create a certain amount of turbulence, and, therefore, result in collisions between partially dried solid units. These conditions lead to coalescense and to the formation of agglomerates. Fig. 7.59 is the simplified flow sheet of a co-current spray drying system with recirculation of fines that is another possibility to accomplish agglomeration in a spray dryer. Often, the fines are returned to the liquid feed bin and redissolved or redispersed (see, for example Fig. 7.68b and c). However, if fine particles are returned to the spray zone, as shown in Fig. 7.59, they are captured by the droplets (similar to a wet scrubbing effect) and incorporated in the product. In regard to gas handling and flow, the most involved and costly part of any spray drying system, the flow sheet of Fig. 7.59 shows an open system. However, for economical or other reasons, closed, semi-closed or even aseptic plant gas flow schemes may be applied (Fig. 7.60). In Fig. 7.61 a photo of some spray dried powders is presented. Successful drying of sticky, hygroscopic, thermoplastic, or slowly crystallizing products into free flowing agglomerated powders requires powder temperatures at much lower levels than possible in conventional spray dryers. Under those conditions, such materials would tend to adhere to the discharge cone of the tower and potentially produce massive build-up which results in process upsets. Also, completion of drying
Fig. 7 . 5 9 Simplified flow sheet o f a co-current spray drying system with rotary atomizer and recirculation o f fines. (1) Feed container, ( 2 ) feed pump, (3) rotary atomizer, (4) drying chamber (tower), (5) dust collector, (6) blower, (7) dry product (courtesy CEA/NIRO, Soeborg, Denmark).
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Fig. 7.60 Schematic representation of different spray drying system designs. (a) Open, (b) closed, (c) semi-closed, (d) aseptic plant gas flow schemes (courtesy GEA/NIRO, Soeborg, Denmark)
Fig. 7.61: Photo of some spray dried powders (courtesy LEA/ NIRO, Soeborg, Denmark).
7.4 Jumble/Crowth Agglomeration Technologies
Fig. 7.62 Artist’s conception and flow sheet o f a “filtermat” spray dryer together with the photograph of an agglomerated powder product sample (courtesy GEA/NIRO, Soeborg, Denmark).
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requires that the material is held at the (lower) drying temperature for a much longer time. This is accomplished in the integrated belt spray dryer termed ‘ffiltermat” spray dryer ( F M D ) . In Fig. 7.62 the flow sheet and an artist’s conception of this system is shown. To avoid build-up, the drying chamber is wider towards the lower end. In this tower, agglomeration and the first drying phase are accomplished. The semi-dried material accumulates as an agglomerated, porous layer on a moving belt that is permeable to gas. The still warm gas from the tower is sucked through the material and the belt so that drying continues (1)while the material is transported into three other hooded chambers that are divided from each other by baffles. Further processing occurs in the efficient downdraft gas flow pattern. In chamber (2) hot gas is provided for high rate drying, a mixture of hot and cold gas completes drying in chamber (3), and in chamber (4) the product is cooled with dehumidified gas prior to its discharge. The agglomerated powder sample in Fig. 7.62 reveals that the dry product consists of relatively large, loose (porous) agglomerates. For further information on spray drying and associated matters it is recommended to consult the book authored by K. Masters [B.43]. 7.4.4
Fluidized Bed Agglomerators
As a result of the mechanical action of mixing tools in high intensity mixers (see Section 7.4.2) an aerated, turbulent particulate matter system with stochastic particle movement develops. Similar conditions exist if the particles are suspended in a fluidized bed. The main difference between the two methods is that in the mixers particle movement is caused by mechanical forces while in fluidized beds drag forces, that are induced by a flow of gas, are the main reason for the movement of the particulate matter. Therefore, fluidized beds are not only used as excellent environments in which gas efficiently and intimately contacts particles but also for dry mixing of particulate solids and coalescence of particles which, in the presence of binding mechanisms, causes agglomeration. Originally, the fluidized bed technology was developed during pioneering work in the mid 1920s with the “Winkler generator” for the gasification of bituminous coal in Germany and of Standard Oil and Kellogg in the United States. The latter improved the catalytic break-up of heavy oil by replacing the less efficient fixed bed crackers [B.42]. Almost immediately it was also recognized that cooling and drying of particulate solids was easily accomplished in fluidized beds if the carrier gas was dry and cool or heated. As an alternative to fluidization, the spouted bed was developed for the drying of wheat where the particles are too coarse and uniform for the development of a good, regular fluid bed. Fluidization begins when the upward flow of gas through the voids between the particles in the bed attains a frictional resistance equal to the weight of the bed. This condition is called incipient buoyancy. However, at that flow rate, the particles are still so closely together that they do not have any appreciable mobility. The desired bed homogeneity and particle mobility is achieved if the gas velocity is increased
7.4 Turnble/Crowth Agglomeration Technologies
further until the particle mass seems to behave like a boiling liquid (therefore, sometimes, the term boiling bed is used to describe this condition). To obtain agglomeration, the bed movement must be more vigorous which is accomplished by the formation of bubbles [B.42].The gas velocity at which bubbles first appear is referred to as incipient bubbling velocity. From the description of conditions that are required to obtain a fluidized bed it is obvious that the size distribution of the particles in the fluidized bed must be narrow. Larger particles will not be sufficiently lifted by the gas and sink down. This effect can be and often is used to accomplish discharge of agglomerates that have grown to the proper size. At the other extreme, if the particles are too small, they are entrained in the gas and carried out of the chamber. Such solids must be removed from the off-gas in dust collectors and are normally recirculated, either to the liquid feed stock for redispersion and drying or directly into the fluidized bed for reagglomeration. Inspite of the obvious, good conditions for agglomeration, a high probability of coalescence between particles, it took several decades and the beginning of interdisciplinary evaluation of processes before, by design, fluidized and spouted beds were utilized for agglomeration [B.42, B.571. Particularly if dry powder is produced in a spray dryer plant from suspensions or slurries, agglomeration can be accomplished if the partially solidified but still wet particles are tumbled in an associated fluidized bed where, in most cases, final drying also takes place. Fig. 7.63 is the schematic flow sheet of a fluidized spray dryer (FSD). As compared with Fig. 7.59 (Section 7.4.3), the conventional spray dryer, a somewhat modified gas handling system is the most obvious new feature of the FSD. Drying gas (9) not only enters the top of the tower for co-current drying but also a so called “plenum”, a specially designed chamber at the bottom of the tower, from which the hot gas enters the tower through a distribution plate (for further details see below). The amount of drying gas entering with the dispersed feed (3) is controlled such that, while the droplets descend in the tower, only partial drying is accomplished. The still wet, slightly sticky particles are captured in a fluidized bed (5) where they collide and form larger agglomerates. Fines (7)that are removed from the off-gas (10)in a dust collector (6)are recirculated to the fluidized bed where they are attached
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Fig. 7.63: Schematic of a fluidized spray dryer (FSD) with open plant gas flow (courtesy GEA/NIRO, Soeborg, Denmark). Explanations see text.
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rm 17
lo +
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Fig. 7.64 Two-stage back mixing/plug flow fluidized bed according t o European patent EP 0 749 560 81.
(C) Conventional spray dryer with rotary atomizer Fig. 7.65: Comparison of FSD-dried products with those obtained in other systems (courtesy CEA/NIRO, Soeborg, Denmark).
7.4 Turnble/Crowth Agglomeration Technologies
to the growing agglomerates. While the solids tumble in the fluidized bed and grow by agglomeration they are also dried with the hot fluidizing gas. Dry, agglomerated product (8)is removed from the fluidized bed in a suitable manner. Fig. 7.64 depicts a patented possible discharge system of a fluidized bed spray dryer. It realizes back mixing and plug flow (for details on these characteristics see below) and avoids short circuiting of wet particles from the primary fluidized bed (13) into the product discharge (17). Further control of the discharge rate is accomplished by, for example, a conventional rotary vane valve. The photographs in Fig. 7.65 demonstrate the larger powder particle size that can be obtained with the FSD in comparison with other equipment and Fig. 7.66 depicts a typical particle size distribution from a fluidized spray dryer (FSD) as compared with those from a conventional spray dryer with rotary atomizer (SD) and a “tall form spray dryer” (TFD) (see Fig. 7.68c, below). In Fig. 7.67 a patent drawing shows the integration of the special fluidized bed design of Fig. 7.64 in an FSD. Other special features of this FSD are that final drying occurs in the central fluid bed portion with hot gas (Ga),the product is cooled in the annular fluid bed portion with cold air (Gb),product discharge is by overflow (Gc),and the spent air exits the chamber through highly efficient filters (11)which are mounted within the tower so that fine particles that are dislodged during the cleaning cycle of the filters are directly recirculated. Originally, the gas distributors in fluidized beds were made of perforated steel plates (see Fig. 7.70, top). The size of the holes, in most cases the diameter of circular bores, the percent open area, defined by the sum of all hole areas, sometimes the distribution pattern of the holes in the plate, and the gas pressure in the plenum below the distribution plate, which, together with the other dimensions, defined the gas flow rate, were major design parameters. To obtain a good, stable fluidized bed, the gas velocity has to be uniform across the entire area of the bed and must be adjusted such that, as a result, the solid particles are in a suspended state. Perforated plate gas distributors, although relatively cheap and easy to manufacture, have a number of disadvantages (Tab. 7.2). Characteristics of perforated plate gas distributors for fluidized beds.
Tab. 7.2
~~
~
+
Simple construction, therefore, relatively cheap.
-
Particles remain static between holes. - Tendency to form deposits. - Dead volumes.
-
Vibration needed to prevent air channeling Particle sifting through holes
-
Poor directional flow control.
For structural reasons, holes must have a certain distance from each other causing static particles in dead volumes on the “land areas” and potential deposits. Also, particles tend to sift through the holes necessitating periodic cleaning of the plenum.
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7 Tumble/Growth Agglomeration 9
9
.
9
1
Fig. 7.66 Typical particle size distribution of a dry powder produced in the FSD and comparison with powders from SD and TFD (courtesy CEA/NIRO, Soeborg, Denmark).
Fig. 7.67: Schematic o f a fluidized spray dryer with a modified two stage fluidized bed (Fig. 7.64) and integrated filters according to WO 97/14288 (courtesy CEA/NIRO, Soeborg. Denmark).
Furthermore, the fluidized bed can not be shut down while material is still in the chamber. The desire for directional flow control will be discussed below. Agglomeration and/or drying canbe more effectivelycontrolled, ifthe fluidized bed is arranged externally. Fig. 7.68 shows three different spray dryer systems with external fluidized bed agglomerator/dryer/cooler. Although the designs were originally introduced only for final drying and product cooling, additional agglomeration can be accomplished if the tower discharges still moist and slightly sticky material. In all three examples fines are entrained in the off-gasesfrom the spray tower and the fluidized bed, removedinafines separator (dustcollector)priortoexhaust, andrecirculatedeithertothe
7.4 Tumble/Crowth Agglomeration Technologies
sprayzoneorintothewetfeed.Thesketchofthe spraytowerin Fig. 7.68crepresents theso calledtallformspraydryer(TFD)inwhich powder separation from the off-gasoccursvery efficiently in the enlarged cone section of the tower. Continuous fluidized beds are normally characterized by the residence time distribution of individual particles in the unit. A wide residence time distribution is obtained in a fluidized bed with a relatively small length to width ratio. A circular fluidized bed at the bottom of a spray tower (see, for example, Fig. 7.63) is a perfect example of this concept. Such an arrangement is called back-mixed fluidized bed. In
6
n
Fig. 7.68 Schematic representation o f three different spray dryer systems with external fluidized bed agglomeratorldryerl cooler (courtesy GEA/ NIRO, Soeborg, Denmark). (a) "Standard" spray dryer (SD) with fines recycling to the spray zone, (b) "standard" spray dryer (SD) with fines recycling into the wet feed, (c) "tall form spray dryer" (TFD) with fines recycling into the wet feed. (1) Feed container, (2) feed pump, (3) spray nozzle, (4) drying chamber, (5) dust collector, (6) external fluidized bed agglomerator/dryer/cooler, (7) blower, (8) re-dissolution or dispersion tank.
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terms of Chemical Engineering it can be modeled by an agitated tank with an overflow (Fig. 7.69a). The residence time distribution is wide because individual units (powder particles or agglomerates) can “short circuit” to the discharge with extremely low residence time, while others may be retained in the bed for a very long time. A narrow residence time distribution is obtained if the length to width ratio of a fluidized bed is large. Such a design corresponds to the external fluidized beds that are schematically shown in Fig. 7.68. The residence time distribution is narrow because all particles are pushed forward towards the discharge by the material that enters the feed end. This arrangement is called plug flow fluidized bed and in terms of Chemical Engineering it can be modeled by a large number of agitated tanks in series (Fig. 7.69b) whereby every single agitated tank represents a “mini”back-mixed fluidized bed. As mentioned above, even and stable distribution of the fluidizing gas is a prerequisite for sustained operation. Distribution plates ensure optimal fluidization and powder movement. The original distribution plate design, the perforated plate (top of Fig. 7.70), has a number of disadvantages (see Tab. 7.2). It was also discovered that it could be advantageous if the direction of particle flow in fluidized beds can be influenced. This, and the desire to overcome some of the other disadvantages
t
I
Fig. 7.69: Explanation of the conditions in back-mixed (a) and plug flow (b) fluidized beds.
7.4 Tumble/Crowth Agglomeration Technologies
Perforated plate
4-w-
Gill plate
Brigde plate
Flex plate
Non-sifting plate Fig. 7.70: Sketches of different gas distribution plates for fluidized beds (courtesy CEA/NIRO, Sceborg, Denmark).
of the simple perforated plate design, led to the development of modified distribution plates. Although all companies that design and/or offer fluidized bed particle processing units have developed such new distribution plate designs that differ in details from each other, the development of one vendor of fluidized bed equipment will be discussed in the following as an example. Fig. 7.70 are sketches of the basic perforated plate design and of new and improved distribution plates. Tab. 7.3 lists the most important characteristics of some new plate designs. The most important features are that, because air enters at an angle (see Fig. 7.70), no deposits form at the land areas between the holes and the fluidized particles are moving in the direction of the airflow which can be used for lateral powder transport. Depending on the application, the latter can be also employed to develop circular or linear flow patterns which are particularly advantageous for the design of fluidized beds that require plug flow. The non-sifting plate designs feature hole covers which are larger than the holes themselves so that, if the gas flow stops and the powder settles onto the plate, a powder surface develops near the holes which depends on the angle of repose. If the geometry of the hole covers and the respective size of the holes are selected correctly, powder will not sift through the holes. Fig. 7.71 is a photograph showing three of the new gas distribution plates for NIRO fluidized bed processors. Particularly if used for size enlargement by agglomeration, totally back-mixed fluidized beds are often not suitable because the discharging product is still somewhat moist since it is near equilibrium with the entire humid exhaust air. Furthermore, because the particles from a back-mixed fluidized bed will have had different residence times (see above), a large portion of the material is either over- or under-dried.
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Characteristics of NIRO Distribution Plates for Fluidized Bed Processors (courtesy CEA/NIRO, Soeborg, Denmark). Table 7.3:
Gill Plate'"
+ +
+ +
+
Less tendency for deposit formation. Improved directional air flow. Improved powder transport control. Good bed emptying properties. Vibration normally not necessary. Particles are still sifting through holes
Bridge Plate'"
+ +
Essentially the same performance as Gill PlateTM, but: Creates slower fonvard particle movement. Particle sifting prevented when bridges overlap the holes. Expensive fabrication procedure, particularly if bridges overlap.
Flex Plate'"
+
+
Similar features as Gill Plate'", but: Further improvement in powder transport and movement control as gill openings can be located to face in any direction resulting in high flexibility. Good bed emptying properties.
Non-SijIing Gill Plate'" and Non-Sifting Flex Plate"' Essentially the same as Gill and Flex Plates'", but: + Because "Gills" are overlapping with the plate, particles are no longer sifting through.
Fig. 7.71: Photograph o f three advanced gas distribution plates for fluidized beds (courtesy CEA/ NIRO, Soeborg, Denmark).
To overcome these problems, plug flow fluidized beds with controlled particle residence time are selected. By using directional airflow, through the application of specific gas distribution plates, optimally operating fluidized beds are obtained. They are either circular or rectangular. In circular fluidized beds with plug flow (Fig. 7.72), the feed, which must be directly fluidizable, is introduced into the center of the fluid bed and the fluidized particles are forced to follow a long narrow path (between the spiralshaped baffle in Fig. 7.72) to the periphery of the fluid bed where they discharge over a weir. In this apparatus, good control of the residence time of each particle is achieved and, on discharge, the product is near equilibrium with the dry hot gas so that very low
7.4 Tumble/Crowth Agglomeration Technologies
Fig. 7.72: Schematic representations of a circular fluidized bed with plug flow (courtesy CEA/NIRO, Soeborg. Denmark).
residual moisture can be obtained without overheating the material. In rectangularly shaped fluidized beds, the plug flow of the fluidized particles is achieved with transversely arranged bames (Fig. 7.73). Since, as mentioned before, the feed to such fluidized bed processors must be directly fluidizable, they are charged with particulate solids. Ideally suited materials may be formed in independent processing steps, such as precipitation, crystallization, coagulation, and polymerization, followed by drying, or by grinding which may be followed by upgrading, again potentially drying, and/or mixing. Particles may also be formed by spray drying. In those cases where solutions are spray-dried, which results in the production of hollow particles (see Section 7.4.3), it may be desirable to destroy the hollow structures prior to feeding them into an agglomerator to obtain denser products. Size enlargement in fluidized beds that are charged with dry particles occurs after the addition of atomized binder liquid in the turbulently moving charge by particle collisions and coalescense. Therefore, this process is called rewet agglomeration in fluidized beds and, as far as the growth mechanism is concerned, is equivalent to the mechanisms of tumble/growth agglomeration (see Section 7.1, Fig. 7.1 and 7.2). As discussed earlier, a particular characteristic of fluidized beds is that, because of the necessity that lifting forces, resulting from the gas flow, and the weight of the particles must be in equilibrium, only a narrow size distribution can be kept in the fluidized state. Therefore, only an incremental growth can be obtained in a given fluidized bed. It is also difficult to obtain agglomeration in a fluidized bed featuring plug
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Fig. 7.73: Sketch o f a rectangular fluidized bed with plug flow (courtesy CEA/NIRO, Soeborg, Denmark).
flow. The relatively uniformly advancing flow of the tumbling mass is not as amenable to particle collisions, which constitute the fundamental mechanism of tumblelgrowth agglomeration, as the turbulently agitated back-mixed fluidized bed. Therefore, the plug flow designs are primarily used for the final drying of narrowly sized green agglomerates that were formed in a previous agglomeration step. A combination that can be used for agglomeration and drying applies a two-stage circular fluidized bed apparatus (Fig. 7.74). By arranging a totally back-mixed fluid bed on top of a plug flow fluid bed, agglomeration and predrying can be achieved in the first stage if moist powders (e.g. filter cakes) or fine particles, small agglomerates, and fines as well as atomized binder liquid are fed to this back-mixed fluidized bed. In the case of feeding moist powders, a rotary distributor (see Fig. 7.74) disperses small chunks of the material evenly over the back-mixed section. In the fluidized bed, attrition, abrasion transfer, crushing, andlayering (see Section 7.1, Fig. 7.1 and 7.2) as well as predrying take place. Fig. 7.75 is the photograph of the turbulently moving surface of a fluidized bed above which the atomizing nozzle for binder liquid is located. Other designs use atomization nozzles that are submerged in the bed to achieve more intimate contact between the liquid and the solids. Solids, fresh feed and recycle, are typically fed into the bed below its surface to allow collisions with other particles, adhesion, and agglomeration before they may be entrained in the off-gas leaving the apparatus. The fines are then removed from the gas, collected, and once more recirculated to the back-mixed fluidized bed.
7.4 Tumb/e/Crowth Agglomeration Technologies
Fig. 7.74 Schematic drawings of two 2-stage fluidized bed processors. (a) Plug flow (bottom) with radial baffles and alternating transfer ports. (b) Contact heating panels in the back-mixed (top) section (courtesy CEA/NIRO, Soeborg, Denmark).
In both cases, the feeding of moist powders or the wetting of dry particles, moist agglomerates are collecting near the bottom of the back-mixed bed and discharge into the beginning of the plug flow fluidized bed where final drying occurs. Since the backmixed fluidized bed is always on top of the plug flow fluidized bed, the solids flow counter-currently to the drying gas; thus, space requirement, installation cost, and energy consumption are minimized. Fluidized beds of this design may also feature more than two fluid beds which are stacked upon each other. Generally, all equipment with two or more stacked fluidized beds are called multi-tier fluidized beds. Another multistage fluidized bed processor is depicted in Fig. 7.76. It is a rectangular apparatus in which the different chambers are separated by walls with transversely arranged connections or (as shown in Fig. 7.76) openings at the bottom, close to the gas distribution plates, both providing plug flow. Although in the sketch a common
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Fig. 7.75: Photograph o f the surface o f a fluidized bed and an atomizing nozzle supplying liquid binder (courtesy LEA/ NIRO, Soeborg, Denmark).
Fig. 7.76 Rectangular plug flow fluidized bed apparatus with spray nozzles and internal dust filters (courtesy CEA/NIRO, Soeborg, Denmark).
7.4 Tumble/Crowth Agglomeration Technologies
plenum for the fluidization gas is shown, more typically the plenum is subdivided to supply gas of different pressure (or speed) and temperature to the individual chambers. As a result several different operating modes are possible. For example, solutions or suspensions may be spray-dried whereby the product size increases in consecutive chambers. The feed port on the left may supply recycle as nuclei and for incorporation into the product, although the schematic in Fig. 7.76 indicates optionally that in each chamber internal dust filters are installed which directly recirculate any fines. Through the feed port dry powder (plus, potentially, recycle) may be also introduced. In this case, binder liquid is sprayed onto the fluidized bed to entice agglomeration and, again, consecutive chambers produce ever larger agglomerates. Finally, it is possible
Fig. 7.77: Two sketches of a rectangular two-stage "contact fluidizer". The numbers in (b) indicate: (1) Feed inlet, (2)/(3) process gas inlet (2) back-mixed section, (3) plug flow section], (4) product discharge, (5) process gas outlet, (6) heating panels, (7) wet feed rotary distributor (cour. tesy GEA/NIRO, Soeborg, Denmark).
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to cool the material and/or provide some product treatment (e.g. coating, application of anticaking agent, impregnation, etc.) in one or more of the final chambers. Fig. 7.77 shows two schematic representations of a rectangular contact fluidizer which, in this case, also incorporates back-mixed as well as plug flow sections. Therefore, it can be utilized in much the same way as discussed before. Plug flow is achieved with baffles arranged transversely (see also Fig. 7.73). As shown, a rotary distributor disperses the wet feed evenly over the back-mixed section (dry feed and atomized binder liquid could be also used) which, in addition, is equipped with contact heating surfaces that are immersed in the fluidized bed (see also Fig. 7.74b). As shown in Fig. 7.77b, the heating panels can be easily removed for cleaning and maintenance. The supply of thermal energy is selected such that a substantial portion of the required heat is provided by the panels. Therefore, it is possible to reduce both the temperature and the amount of gas through the system significantly which is particularly important if the material to be treated is heat sensitive. In those cases where the solids to be processed can not be easily fluidized due to a broad particle size distribution, highly irregular particle shape, or low abrasion resistance (requiring relatively low fluidization gas velocity), a shallow vibrated fluidized bed apparatus (also called “vibro fluidizer”) is applied (Fig. 7.78). It is normally designed as a long rectangular trough with (natural) plug flow and is vibrated with a frequency of 5 - 25 Hz and a half amplitude of a few millimeters. The vibration vector is applied at an angle to the vertical (typically <45”) so that the material is easily transported along the trough by the combined effects of fluidization and vibration. If the feed is moist and has self-agglomerating tendency or by providing atomized binder liquid near the feed end of the vibrated fluidized bed agglomeration can be achieved which is followed by drying in the same apparatus. While all of the examples that were discussed so far featured continuously operating equipment, it should be pointed out, that in some industries, for example the pharmaceutical industry, fluidized bed processors are used in batch modes. Fig. 7.79 is the
7.4 TumblelCrowth Agglomeration Technologies
simplified flow sheet of a batch fluid bed granulator system. Continuous processes that could include premixing, wetting, granulation (agglomeration),classifying, collection and recirculation of fines, drying, cooling, and post-treatment (e.g. coating) may carry out each step or a few combined steps in separate pieces of equipment requiring transport, potentially intermediate storage, and handling in between. A batch fluidized bed apparatus, as shown in Fig. 7.79, can realize all these steps in one unit providing one-pot processing (see also Section 7.4.2). During a processing cycle, the powder constituents are weighed-out and dispensed into the chamber which is then closed. First, a fluidized powder bed is established and the different components of the formulation are uniformly mixed. When this condition is reached, binder liquid is sprayed onto the turbulently moving bed and agglomeration begins. As particle size increases, the fluidizing gas flow must be adjusted (increased) to compensate for the larger particle mass. All along, fines are entrained in the off-gas and collected in the bag filters that are mounted within the chamber. During the automatic cleaning cycle of the filters, fines are recirculated into the bed were they get another chance to adhere to the growing agglomerates. After reaching the optimal particle size, drying begins which is followed by cooling and, potentially, a post-treatment (e.g. coating, see Section 10.1). The apparatus is only opened after the last processing step is completed so that a true one-pot operation has been realized.
Fig. 7.79: Batch fluid bed spray granulator system. (1) Air filter and heater, (2) powder batch/fluidized bed, (3) spray nozzle, (4)/(5) liquid feed tank and pump, (6) bag filter, (7) exhaust fan (courtesy GEA/NIRO, Soeborg, Denmark).
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Further system improvements may be obtained by recirculating the fluidizing gas and, after condensing the evaporated fluid during the drying cycle, also returning the liquid. The first allows an aseptic operation and recovers energy during the drying cycle and the second is particularly advantageous if the agglomeration liquid is expensive (for example, alcohol). Without going into more detail, Tab. 7.4 lists parameters in fluidized bed agglomerationlgranulation which are grouped into material variables and process variables. In summary, Fig. 7.80 provides an overview over different fluidized bed processes, the product structure obtained with these processes, and the average particle size (50 % point of the distribution in mm) of the product. The first five methods (entries 1-5) relate to processes for the conversion of liquid feed (solutions, emulsions, suspensions, thin slurries) into dry powders and the second six methods (entries 6-11) make use of moist or dry powders and liquids as binders. The last two of the second group (entries 10, 11)apply powder mixers (see Section 7.4.2) for agglomeration and fluidized beds as dryer/cooler/post-treatment equipment. Rewet agglomeration (the 3rd last in Fig. 7.80) may use mixers or fluid bed processors for agglomeration.
~
Feed
Process
Abbr.
~
~
Liquid
Spray DryeriRotary htomizer
SD-R
Spray DryeriNozzle Atomizer
TFD-N
1
P r o r c t 5tr;ture
1
'I
D5Omm
0.02-0.2
0 1-0.3
Fluidized Spray Dryer
FILTERMAT Dryer
Spray Fluidizer ~
Fluid Bed Agglomerator Fluid Bed Granulator
Powde, t
Spray Fluidizer
Liquid Rewet Agglomerator Tumbler Agglomerator
-
0.3-2.0
RWA TA
a
0.5-3.0
Tumbler Agglomerator
Fig. 7.80 Overview over different fluid bed processes (courtesy CEA/NIRO, Soeborg, Denmark).
7.4 TumblelCrowth Agglomeration Technologies
7.4.5 Other Low Density Turnble/Crowth Agglornerators
There are further, less well known but often important techniques that make use of one or the other or all of the previously discussed agglomeration methods in low density turbulently moving particle beds and clouds. Parameters in fluid bed agglomeration/granulation (based on information from GEA/NIRO, Soeborg, Denmark).
Tab. 7 . 4
Material Variables
Feed materials
Binder
Product
Process Variables
Particle size and distribution Surface area Chemistry, wetting characteristics Type, chemistry Quantity If solution, concentration and quantity Viscosity Particle size and distribution/dust Strength/abrasion resistance Bulk density/porosity Dispersibility/solubility/reactivity
Schematic representation of the PCS rotary-valved pulse combustor/atomizer. Explanations see text (courtesy Pulse Combustion Systems, San Rafael, CA, USA). Fig. 7.81:
Load Moisture content Nozzle position/spray shapeldroplet size Liquid flow rate
Fluidized bed Binder distribution
Fluidizing gas Processing time
0
Velocity/pressure Inlet/outlet temperatures
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An alternative to spray drying of solids that are suspended in a liquid using high pressure single and two phase or rotary nozzles (see Section 7.4.3) is the break-up of a low pressure stream of slurry in gas dynamic atomization. In this process, the fluid is pumped to an orifice where it is released into a pulsating flow ofhot gas (Fig. 7.81) and atomized. The heart of this drying system is a rotary-valved pulse combustor. Referring to Fig. 7.81, combustion air (1)is pumped at low pressure into the pulse combustor’s outer shell and flows through an unidirectional air valve (2) into a tuned combustion chamber [“Helmholtz Resonator” (3)]where fuel (4) is added. The air valve closes. The fuelair mixture is ignited by a pilot (5) and explodes, creating hot air which is pressurized to approx. 0.2 bar above combustion air fan pressure. The hot gas exits the chamber through a pipe (6)towards the atomizer area (7).Just above the atomizer, quench air (8) is blended in to achieve the desired process temperature. The orifice releases the liquid (9)into the carefully balanced gas flow which dynamically controls the break-up of the liquid stream (atomization), drying, and particle trajectories. The pulse cycle can be up to 100 per second. After break-up, the particles enter a conventional tall-form drying chamber (10)and the downstream equipment is also identical to that used with spray drying as well as fluidized bed agglomeration (see Sections 7.4.3 and 7.4.4). In gas dynamic atomization, a liquid stream is broken-up into small entities which are dried very quickly. Because the formation of droplets occurs in aerodynamic suspension, the material experiences no shear and the liquid temperature does not rise above the local dew point, despite high gas temperatures. Since drying and subsequent cooling are rapid, organic materials do not have time to oxidize, degrade, or experience any other damage. Food powders often exhibit better flavor, texture, and instant characteristics than comparable powders from other spray dryers. Because a low pressure stream of slurry is pumped and dispensed, the system can also handle corrosive and abrasive products easily. Control over particle size is normally better. Fig. 7.82 depicts SEM photographs of some typical products. In designing pulse combustor/atomizer drying systems, the pulse intensity as well as the temperature and velocity of the gas at the point of atomization are optimized for each product. A particular advantage of the technology is, that the plant’s control system can modify the process conditions such that a variety of dry powder characteristics are met without physically changing the equipment. These characteristics primarily include particle size, flowability, texture, temperature history, residual moisture content, flavor, and ease of reconstitution. An alternative to wetting by atomized liquid for rewet agglomeration in low density turbulently moving particle clouds is steam jet agglomeration [7.6] (Fig. 7.83). In this process a dry powder, consisting of single particles or weak aggregates of primary particles that are bound by van-der-Waals or electrical forces (see Section 5.1.1), is fed into the apparatus. Since such powders do not usually flow freely, the method of conveying and dispensing is of great importance to the result of agglomeration as, at this stage, the number, size, and morphology of the dry feed conglomerates are determined. As shown in Fig. 7.83a, for more easily handleable powders a vibratory feeder could be used while for cohesive powders a powder dispenser with two rotary cylinders (Fig. 7.8313) is proposed.
7.4 Jumble/Crowth Agglomeration Technologies
Fig. 7.82 SEM photographs o f some typical product from dryers with PCS pulse combustion atomizers (courtesy Pulse Combustion Systems, San Rafael, CA, USA). (a) non-fat dry milk, 200x; (b) non-fat dry milk, 2,000~;(c) sodium alkene sulfonate, 350x; (d) sodium alkene sulfonate, 2,000~;(e) soy sauce, 2,000~;(f) silica alumina, 2,ooox.
In the chamber, steam jets are directed parallel to or slightly impinging the flow of feed. This serves two purposes: to wet the solids and to cause particle movement resulting in collisions and coalescense. Contact of the steam with cold particle surfaces results in condensation and thermal energy transfer; also, droplets can form in the vapor phase. Therefore, two different mechanisms contribute to the wetting processes in the agglomeration zone: 1. Condensation of steam on cold particle surfaces and collision of particles with liquid droplets.
2.
Calculations and measurements have shown that condensation is the dominant effect. As condensation only occurs until the temperature of the solids reaches the dew point temperature that corresponds to the relative humidity of the gas atmosphere, particle inlet temperature has an important influence on the amount of water that can condense. Steam jet agglomeration is normally applied for the size enlargement of water soluble materials in the food industry to obtain products with instant characteristics (see also Section 5.4). Since during the condensation of steam not only a thin, uniform coating of water is formed on the solids but also thermal energy is transmitted, a maximum of the water soluble material is dissolved which produces recrystallization bonds during drying (see Section 5.1.1)
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Solids
.($
) ( Feeder
($ Powder stirrer Solids supply
Exhaust gas + fines
agglomerator
I
Agglomerated product
Fig. 7.83: (a) Schematic representation of a steam jet agglomerator. (b) Sketch o f a dispenser with rotating cylinders for cohesive powders [7.6].
If the solid particles are very small, typically in the submicron or nanometer range, the natural molecular attraction forces are large in comparison with the particle mass so that particles that collide with each other in a fluidized bed will adhere to each other and form weak agglomerates. As discussed earlier (Section 7.1), formation of nuclei is the most difficult and time consuming step. Therefore, development of the earliest applications of binderless agglomeration in fluidized beds occurred by chance until, quite some time later, interdisciplinary application of the fundamentals of agglomeration explained the phenomena and allowed the design of simple, reliably functioning fluidized bed agglomerators. To avoid the time consuming nucleation step, a heel of agglomerated product is now retained in batch operating equipment or “seeding” with recirculating, undersized but pre-agglomerated material is carried out. In either case, growth of the already available nuclei occurs by the mechanisms described in Section 7.1 while new nuclei are also formed in the bed. Typical applications of this technology are for the agglomeration of carbon black and silica fume. Silicafume, in contrast to fumed silica which is produced by purposefully volatilizing silica, is a by-product from the manufacturing of ferrosilicon and silicon metal. From the violently turbulent bath of the submerged-arc furnaces, that are used in these production processes, tiny droplets of Silica (SO2)emerge which, under the influence of surface tension, become spherical, solidify, and are removed from the furnace offgas in bag-house filters. Because of the very fast cooling effect, the spherical particles consist of amorphous silica, have extremely small particle size (Fig. 7.84), and feature large specific surface area. These characteristics of silica fume make it an excellent
7.4 TumblelCrowth Agglomeration Technologies
1oc
80
s % 60 m
4-
$
40
Q
20 Fig. 7.84 Typical silica fume particle size distribution (courtesy Norchem Concrete Products, Fort Pierce, FL, USA).
0
'
0.05 0.1
0.2
0.5p m l
admixture to, for example, high strength concrete as well as high performance grouts and mortars. The amorphous structure and high surface area render the particles highly reactive causing pozzolanic effects and the small particle size also results in highly impermeable structures of building materials (see also Section 5.3.1, Fig. 5.41). In prestressed concrete structures, the latter protects the rebars from attack by water.
Fig. 7.85: Schematic of a fluidized bed agglornerator for dry silica fume (courtesy Norchern Concrete Products, Fort Pierce, FL, USA).
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Fig. 7.86 The principle o f pressure swing granulation.
As produced, directly from the bag house, silica fume is very dusty, difficult to handle, self agglomerating, - causing bridging, build-up, and lumping -, and can not be transported and handled economically. Because of its very low bulk density (160- 240 kg/m3),bulk tanker trucks only hold 8- 10 t and require long pump-off times and if bags are used they are large and bulky. A simple dry fluid bed agglomeration process (Fig. 7.85) converts silica fume by dry agglomeration into a product which is less dusty, flows well, and can be handled pneumatically. Product density may be such that the tanker truck now holds approx. 25 t and can be adjusted to fit different handling and end use applications. At the same time, agglomerate bonding is so weak that the product disperses easily, for example in cement mixers. Similar considerations, which, through observation, investigation, and understanding of the processes that are involved, led to the development of the dry agglomeration
Fig. 7.87: Photograph of a pressure swing granulator, Model DQ-350 (courtesy Fuji Paudal, Osaka, Japan).
7.4 TumblelCrowth Agglomeration Technologies
technique for ultrafine particles that was described above, yielded another novel dry agglomeration method for fine solid particles: the pressure swing granulator (PSG) [7.7; B.70, pp. 15-24].The principle of this intensive, dry, fluidized bed agglomeration process is shown in Fig. 7.86. It consists of a cylindrical chamber with a perforated gas distribution plate at the bottom and operates in batch mode using cyclic fluidization and compaction of the material. During the fluidization and upward motion, agglomerates are formed, mostly due to molecular (e.g. van-der-Waals) forces (see Section 5.1.1). These agglomerates have a rather wide particle size distribution. During the downward flow, the bed of material is compacted with compressed air; weak agglomerates collapse and larger agglomerates break due to compaction forces or they loose small particles due to attrition. The overall result is that the larger agglomerates will become smaller and more spherical while the smaller agglomerates pick-up fines and become larger and more spherical during each compaction step. After a number of compaction and fluidization cycles granules with relatively uniform size and spherical shape are obtained. Fig. 7.87 is the photograph of a Fuji Paudal pressure swing granulator, model DQ-350, for a batch size of 25 kg WCco-wax. Fig. 7.88 depicts different states of particle beds that develop above a gas distribution plate. In the first two on the left, particles are immobile. At the incipient fluidization point, the forces caused by the flowing gas and the particle mass are in equilibrium and the bed volume has reached its maximum before, at somewhat higher gas velocities, particles begin to move freely and randomly in the fluidized beds that are shown on the right.
Fixed bed
Incipient f h i d izat ion
Particulate or smooth f Iu idizat i on
Aggregate orbubbling fluidization
. .. .. . . . . ..: . . _ . I
.....
.. ,. ... ._: . . .. -.: '..\
:
.
.. . . . . . .
;
,
..
* '
I
.. .. ..... .
'
I
.
.
....... .................... 6 .
t' I Fluidization air or gas Fig. 7.88: Sketches depicting the different states of particle beds above a gas distribution plate.
Slugg i ng
spouted bed
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Fig. 7.89 (a) Schematic representation of an industrial spouted bed. (b) Flow sheet o f a continuously operating spouted bed agglomeration system.
For natural, dry agglomeration of very fine particles as described above, destructive forces, which are, for example, caused by bubbling or slugging, must be avoided. Therefore, the gas distribution plate is a finely pored diaphragm such as a sintered glass frit or a cloth which is supported by screens (see Fig. 7.85). Rewet agglomeration in fluidized beds, on the other hand, often requires the turbulent movement and wakes caused by the rising gas bubbles [B.42]. The sketch on the far right of Fig. 7.88 identifies a spouted bed [B.56]. Typically, to obtain the spouted bed condition, gas is introduced in a relatively narrow area in the bottom center of the particle bed (Fig. 7.89a). Movement of the particles in such an arrangement is caused by a vertical, steady axial jet and is rather regular. The bed particles circulate much like a water fountain; they are carried-up in the central spout as a dilute phase until they loose their momentum and fall back onto the top of the bed towards the outer periphery. The particles then recirculate back down as a dense moving bed, are directed back into the gas stream by the normally conical base of the apparatus, and begin again their upward flow. Liquid, injected as a spray into the base of the chamber together with hot spouting gas, deposits a thin layer of liquid onto the recirculating seeds and particles so that fresh feed particles adhere causing agglomerate growth. Permanent bonding oc-
7.4 TumblelCrowth Agglomeration Technologies
curs by recrystallization during drying or solidification (see Section 5.1.1) in the spout. The gas-solids contacting efficiency of fluidized systems becomes impaired at particle sizes larger than, say, 1m m because more and more gas bypasses the solids in the form of large bubbles. Spouted beds avoid that problem and are, therefore, suited for the formation of larger agglomerates. Other advantages of the spouted bed over regular fluidized beds are: Materials with caking tendencies can be processed. Higher gas inlet temperatures are permissible. Layer-by-layergrowth favors well rounded and uniform granules. Due to the absence of a gas distribution plate none can get scaled or plugged. A classification effect at the top of the bed allows selective removal of the largest particles through the outlet pipe, yielding a relatively narrow product particle size distribution. Inspite of the last statement, continuous operation of spouted beds normally does require the separation of product from over- and undersized agglomerates; both off-scale material streams, the oversized after crushing to below maximum product size, are recirculated (Fig. 7.89b). If back-flow of particles into the spout is avoided by enclosing the spout with a concentric pipe and the spray liquid is a hot melt or a saturated solution, an excellent coater (“Wurster”coater, see Section 10.1) is obtained. 7.4.6
Agglomeration in Liquid Suspensions
Finely divided solids that are suspended in liquids may be difficult to deal with. The sizes of the individual particles are often so small that conventional methods to capture and remove them, such as different types of filters, are not effective unless some sort of size enlargement is applied first. The traditional methods for the agglomeration of fine particles in liquids, such as flocculation (see Section 10.2.2), rely on relatively small interparticle bonding forces to form rather weak agglomerates. The objective of these size enlargement procedures is simply to remove the fine particles from the liquid, thereby cleaning a contaminated effluent. In contrast, the present discussion deals with those techniques in which stronger bonding and specialized equipment are used to form generally larger and more permanent agglomerates in liquid suspensions [B.17, B.21, B.42, B.56, B.581. In addition to the capture and removal of particles from suspensions, these methods have other broad objectives as shown in Tab. 7.5. They include the production of granular, often spherical material, displacement of as much suspending liquid as possible from the product, direct agglomeration of crystals during crystallization, and selective agglomeration of one or more components of a wet multiparticle mixture. During crystallization, solid particles are formed in the solution (the so called “motherliquor”)by reducing the concentration of the solvent (evaporation), solvent
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Agglomeration processes that are carried-out in liquid systems (according to Capes and Darcovich [B.58]).
Tab. 7.5:
Process Objective
Material treated and Process used
Formation of spheres and production of coarse granular product
Nuclear fuel and metal powder production by sol-gel processes. Manufacturing of small spheres from refractory and high melting point solids (e.g. tungsten carbide) by immiscible liquid wetting. Spherical crystallization: direct agglomeration of crystals during crystallization for drug delivery systems.
Removal and recovery of fine solids from liquids
Removal of soot from refinery waters by wetting with oil. Recovery of fine coal from coal preparation plant effluents to allow recycling of water.
Displacement of the suspending liquid
Dewatering of various sludges by flocculation followed by mechanical drainage on belt filters, in rotary drums, etc. Displacement of moisture from fine coal by wetting with oil.
Selective agglomeration of certain components from mixtures of particles in liquids
Removal of ash-forming impurities from coal and tar-sands. Coal-gold agglomeration to recover very low valuable concentrations in gold ore. Solvent extraction and simultaneous soil agglomeration to remediate oil-contaminated soil.
(ex-)change,neutralization, or temperature decreasing methods. It is often difficult to control crystal growth and avoid undesired clustering by the growing together of numerous crystallites into an amorphous, irregular particle aggregate. As a result, sizing or even sorting of the product of crystallization may become necessary to arrive at suitable crystal sizes, compositions, and/or structures for specific uses. A new technology, spherical crystallization [7.8],has been developed to design the properties of (pharmaceutical)crystals and improve their yield as well as powder handling. Because agglomeration occurs simultaneously with the crystallization process and does not require binders, processing steps, such as separation of crystallites and drying prior to external agglomeration with binders, are avoided. The product particles resulting from this process are uniform, spherical agglomerates of fine crystals. Fine particles in liquid suspension can be transformed into often large and dense agglomerates of considerable integrity by adding a second binder liquid while suitably agitating the system. The second liquid must be immiscible with the suspending liquid and must wet the particles to be agglomerated. Therefore, this technology is often called immiscible liquid agglomeration. An example is the addition of oil to the aqueous suspension of fine coal. The oil is immiscible with the water and adsorbs preferentially on carbon particles; it forms oil bonds between these particles when they
7.4 Tumb/e/Growth Agglomeration Technologies
collide in the agitated liquid system and coalesce. Inorganic impurities, i.e. those particles that would form ash, are not wetted by the oil and remain in suspension. A similar result would be obtained if silicious particles suspended in oil are agglomerated with water as the immiscible binder. In immiscible liquid agglomeration, the same structural and bonding conditions exist as discussed for tumble/growth agglomeration (see Section 5.1, Fig. 5.7, and Section 7.1). At low levels of binder liquid, only bridges can form between the particles (“pendular”state) which results in an unconsolidated floc structure. If these agglomerates were allowed to settle, a loose mass with a volume that is larger than that of the un-flocculated particles would be obtained. As more oil is added, a transitional (“fuar”) state is reached and the flocs consolidate somewhat. Soon, agglomerates appear and increase in number until, about midway in the transitional region, all the agglomerating particles have been transformed into “micro-agglomerates”.As the amount of binding liquid is further increased, the agglomerates grow in size, using a mechanism that is again similar to that of tumble/growth agglomeration (see Section 7.1, Fig. 7.1 and 7.2), and reach a peak of strength and sphericity near the “capillary”region (compare Section 5.2.2, Fig. 5.28). Beyond this point, additional binder liquid addition results in the formation of pasty lumps in which the solids are dispersed in the binder liquid. A most useful feature of immiscible liquid agglomeration is that the size of extremely fine, suspended solid particles, including those in the nanometer range, can be enlarged. This allows wet, ultrafine grinding of minerals in which valuable components or impurities are very finely distributed and the recovery of agglomerates with much higher purity or, respectively, the removal of gangue components as agglomerates. A number of such applications have reached either commercial or semicommercial operation in the minerals industry. The general relationships described above are not specific to a certain system. However, given the need for optimal separation of valuable particles from the associated gangue, the colloid and surface chemistries that are involved may be quite complex. As in the flotation process, selective agglomeration by immiscible liquids depends strongly on the relative wettability of surfaces, and the same fundamentals of surface chemistry apply to the conditioning of particles to yield the required affinity for the wetting liquid. For a variety of applications, spherical particles are required. Many of these are associated with the field of powder metallurgy. While it is relatively easy to produce spherical particles from low melting materials by conventional techniques, such as shot or prilling towers (see Chapter 5), refractory solids in general and, specifically, high melting point metals can not be converted by these techniques. However, if the solid is available in powder form, various methods are available to produce spherical particles by agglomeration. One such spherical agglomeration process uses an immiscible binder liquid to form spheroidal products from particles that are suspended in a second liquid. These highly specialized materials are required in small amounts and, therefore are carried-out in small, high energy, batch shaking devices as shown in Fig. 7.90. In this apparatus tungsten carbide spheres are manufactured which, after sintering, yield ball pen
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tips [BSG, B.581. The closely sized particles with 1 m m diameter are prepared by agitating tungsten carbide and cobalt powders in a closed teflon container with hemispherical ends on a high-speed reciprocating shaker. Halogenated solvents are used as the suspending liquid and water is the binder. The addition of approx. G % cobalt to the tungsten powder is required to lower the sintering temperature to more acceptable levels. In the batch process, compaction and rounding occurs during many collisions between the agglomerates and with the container walls. The advantage of products from this process is that finishing operations, such as lapping and grinding after a preliminary sintering step, are greatly reduced if compared to those necessary for, for example, spherical compacts from press molding. When comparatively high (but still relatively small) production rates are required, continuous processes are better suited. Fig. 7.91, for example, depicts a drum agglomerator featuring an internal screen classifier for the formation of uniform spheres by immiscible liquid agglomeration [BSG]. In these tumbling agglomerators, the pre-
TO Q E A R REDUCER AND E L E C T R I C MOTOR
TEFLON VESSEL
-
$in. STROKE R E C I P R O C A T I N O ACTION
Fig. 7.90 Teflon cylinder with hemispherical ends, mounted in a reciprocating shaker, and used to form small spheres by the "spherical agglomeration process" [8.561.
7.4 JumblelGrowth Agglomeration Technologies
sence of a liquid slurry is useful to reduce dusting, especially if toxic powders are processed. The liquid environment also avoids avalanging because particles and immicible binder liquid are uniformly distributed throughout the agglomerating mass thus allowing conglomerates to grow into larger entities in a much more controlled manner. The liquid charge also helps in the development of a desirable tumbling and cascading motion in the equipment because it is more voluminous and better interparticle lubrication occurs than would be the case if no suspending liquid were present. Furthermore, the solids are carried with the liquid which makes internal classification possible. As shown in Fig. 7.91 a spiral screen that rotates at a slower speed than the drum passes through the charge and picks-up agglomerates. Undersized particles fall through the screen openings and return to the agglomerating liquid mass. Larger material moves along the spiral until it reaches a tube at the axis of the drum which directs the finished agglomerates to a discharge point. In immiscible liquid agglomeration, particles with a small amount of adsorbed binding liquid on their surfaces collide and coalesce to form larger entities. In the sol-gel process (see also Section 5.3.2, Fig. 5.52), another agglomeration technique that occurs in liquid phase, fine particles are initially suspended in a binder liquid. The suspension is then formed into spherical droplets and the excess binder is removed to solidify the droplets into a particulate product.
Fig. 7.91: Drum agglomerator with internal spiral screen classifier for the formation o f uniform spheres by immiscible liquid agglomeration [ B.5 61.
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The sol-gel process has been developed for the production of spherical oxide fuels with a particle size of up to 1 m m for use in nuclear reactors [B.56]. The following operations are involved in converting the initially aqueous sol of colloidal particles into calcined microspheres: 1. Dispersion of sol into droplets.
2. 3. 4.
5.
Suspension of sol droplets in an immiscible fluid that will extract water to cause gelation. Separation of gel microspheres. Recovery of the immiscible liquid for reuse. Drying, calcining, and sintering of the microspheres.
Equipment used to accomplish steps 1-4 in a continuous operation is shown in Fig. 7.92. In this process, the aqueous sol of colloidal particles is dispersed into drops at the top of a tapered vessel. The droplets are fluidized by the upward flow of the waterextracting fluid, such as 2-ethyl-1-hexanol. Interfacial tension holds the droplets in a spherical shape, but there is a maximum size since larger drops tend to more distortion. A surfactant is added to the immiscible liquid to prevent coalescense of the sol droplets with each other or sticking to the walls of the vessel and lumping of partially dehydrated drops. As water is removed and the sol is converted to a gel, the particles become denser and their settling velocity increases. Vessel design and flow rates are controlled such that the densified gel particles continuously drop-out to the product
Fig. 7.92 Flow diagram o f a sol-gel process for the formation of gel microspheres and the cleaning o f the water-extracting fluidizing liquid [B.56].
7.4 Tumb/e/Crowth Agglomeration Technologies
receiver while fresh sol droplets are added at the top. The extracting liquid is separated from the product and a portion of it is sent to the distillation system for purification to maintain a sufficiently low water content in the fluidizing liquid. Agitation in baffled vessels can also be used to disperse and suspend sol droplets in an extracting fluid. Compared with the system of Fig. 7.92, the more vigorous agitation produces smaller microspheres of less than 100 Fm in diameter.
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Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
8
Pressure Agglomeration During the agglomeration by growth in tumbling particle beds (see Chapter 7 and subchapters), “natural” adhesion forces, which are either totally inherent, enhanced by suitable methods, introduced by binders, or acting as a combination of two or all of these effects, cause particles to stick together when they collide in a stochastically moving mass of particulate solids. With exception of forces that are exerted during the interaction between the particles, the surrounding atmosphere and equipment walls as well as, in some cases, various mixing tools, no externally induced directional forces or pressures act on the growing agglomerates and no shaping, other than caused by attrition, occurs. As a result, depending on the level of interaction, which is largely influenced by the tumbling bed density, more or less spherical agglomerates are grown. Because of the relatively small forces caused by interactions in and with the tumbling charge, porosity of the agglomerates is high and increases as bed density decreases. Also, since the adhesion forces are small, too, and separation forces which try to destruct the growing agglomerates are mass related, the particles forming the agglomerate must be little (see Section 5.2.2). Typically, with a few exceptions, temporarily bonded “green” agglomerates are produced which require post-treatment to achieve permanent, final strength. In pressure agglomeration, new, enlarged entities are formed by applying external forces to particulate solids in more or less closed dies. In contrast to tumble/growth agglomeration, pressure agglomeration is used to achieve one or more and sometimes all process conditions and product characteristics that are summarized in Tab. 8.1. Of course, as will be shown in the following chapters, certain conditions and characteristics are better obtained with one or the other pressure agglomeration process and, sometimes, one or more of the parameters can not be met with a specific technique and/or equipment, system, or plant. Pressure or press agglomeration, using a large number of different extrusion machines, punch-and-die presses, isostatic pressing equipment, and roller presses as well as some lesser known machinery, represents a major share among commercial applications of size enlargement by agglomeration. This technology is largely independent of feed particle size and the forces acting upon the particulate solids can be small with some or may be very high with other equipment. Therefore, it constitutes the most versatile field of size enlargement by agglomeration.
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8 Pressure Agglomeration Tab. 8.1:
Some reasons for the selection of pressure
agglomeration. Process conditions
Larger feed particle size High initial strength Dry processing No or little binder Hot processing No post treatment Processing of elastic materials Automatic operation Easy clean-out Quick turn-over Product characteristics
Specific shape Large pieces Specific mass (or weight) High density Low porosity High final strength Long shelf life Amenable to the production of near net-shape parts
The equipment in which pressure agglomeration occurs is a machine that operates with well defined mechanical parameters that are independent of the performance and characteristics of the particulate solids to be processed. Therefore, pressure agglomeration techniques lend themselves readily to automation and remote control and are essentially independent of operator presence and/or skill. Because the equipment is relatively complex and the throughput per unit is often limited, this technology finds its largest usage in low to medium-sized applications (approx. 1to 50 t/h). Of course, this statement is relative. Specialty products, such as those in the pharmaceutical industry, may be processed in very small and sophisticated machinery, handling only a few kilograms per hour, while certain high-tonnage materials, for example some fertilizer, refractory, and mineral materials, are briquetted or compacted in large facilities employing multiple units. An advantage of high-pressure agglomeration is that, in most cases, essentially dry solids are processed which do not tend to set-up, so that the process can be stopped at almost any time and re-started easily; also, the amount of material in the system is relatively small. Therefore, pressure agglomeration methods, specifically those applying high pressure, lend themselves particularly well to batch or shift operation and to applications in which several products must be manufactured from different feed m h r e s in the same unit. At the end of a campaign, the system can be easily and completely emptied in a relatively short time. If the danger for cross- contamination is unimportant, for example in the fertilizer industry, a new campaign with a different feed can be started immediately (see also Chapter 12). Another possibility for fast and quick change-over is to install different feed and product discharge/handling systems
8.1 Mechanisms of Pressure Agglomeration
as, in most cases, the expensive pressure densificationlshaping equipment itself can be easily cleaned or adapted to the manufacturing of a new product. In general, if it is the task to agglomerate several million tons per year of always the same feed composition, - as is often the case in ores or minerals mining, upgrading, and processing -, pressure agglomeration will not normally be the preferred choice. In all other cases, one of the different methods of pressure agglomeration should be considered. All pressure agglomeration processes have in common that externally provided forces act on particulate solids and that some sort of a tool or die defines the shape of the agglomerated product. All other process conditions and product characteristics that are listed in Tab. 8.1 are more or less well fulfilled or, sometimes, do not apply at all. The level of force that is applied during densification and shaping is the most distinguishing factor in pressure agglomeration. Therefore, the technology is subdivided into low (Section 8.4.1),medium (Section 8.4.2), and high (Section 8.4.3) pressure techniques. As will be shown in Sections 8.1 and 8.2 it is another important distinguishing characteristic whether the material to be pressed is subjected to forces in open ended (extrusion presses), totally confined (punch-and-dieand hydrostatic presses), or semi-confined (roller presses) compacting tool sets.
8.1 Mechanisms of Pressure Agglomeration
There is a great variety of pressure agglomeration methods, each corresponding to one or more ofthe binding mechanisms of agglomeration (see Section 5.1.1,Tab. 5.1, Fig. 5.8 and 5.10).While in low and medium-pressure agglomeration all binding mechanisms are equally possible, in high-pressure agglomeration attraction forces (Fig. 5.10) provide the most common bonding. According to the mechanisms involved, the processes can be further categorized as those using binders and those without binders (see also Chapter 6 ) . Binderless pressure agglomeration comprises all size enlargement processes that apply high compaction forces and use one or more of the binding mechanisms: Solid bridges caused by chemical reaction, partial melting, or sintering; adsorption layers; molecular, electrical, and magnetic forces; recombination bonds; non-valence associations; and interlocking. Several materials contain binders naturally, e.g. the bituminous components in most coals or starch in many foods. Since this inherent binder was not added specifically, pressure agglomeration methods processing such materials are considered binderless. Some particulate solids, especially those that are relatively coarse, do not exhibit inherent binding tendencies; therefore, a binder must be added to secure adhesion of the particles. The main binding mechanisms for such cases are: Bridges of highly viscous media and the negative capillary pressure in liquid bridges caused by the surface tension of freely movable liquids that wet the solids. Processes that use added viscous and liquid binders apply low to medium-pressure agglomeration (see Chapter
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6, Fig. 6.4). Products from such methods are initially of low or medium strength. To obtain stronger bonds, subsequent aging, drying, firing, or curing is necessary. Then, the following binding mechanisms apply: Solid bridges caused by crystallization of dissolved substances, chemical reaction, hardening, or sintering. The most versatile techniques of pressure agglomeration use high forces to compact essentially dry particulate matter into tablets and briquettes of specific size and shape or into compacts. Under the acting pressure, the solid particles approach each other closely which results in strong bonds due to molecular adhesion forces; fusion may even take place at the grain boundaries and many of the free valence forces on newly created surfaces will recombine. Normally, high-pressure agglomeration (see Chapter 6, Fig. 6.5) yields products that exhibit high strength instantaneously. Binders are not necessary in most cases. Other pressure agglomeration techniques use lower forces; this medium-pressure agglomeration technology is often called pelleting (see Chapter 6, Fig. 6.4b.2 - b.6). In these processes, feed materials with sufficient natural binding tendency, often after an appropriate conditioning step, or those containing binders are forced through open die channels or through holes in dies of different shapes. Typically, the briquettes or pellets formed in this way are cylindrical with a predetermined diameter but variable length. Such products may still require curing to gain final strength. In low-pressure agglomeration (see Chapter 6, Fig. 6.4a.l - a.5), moist masses or particulate solids are processed which are plastic or have been plasticized by various conditioning methods (see also Section 8.4.2). Similar requirements exist in regard to size of the feed particles and binding characteristics as for tumble/growth agglomeration (see also Chapter 7). Since very low forces act on the particle mass while it passes the openings in screens or perforated dies that are made of thin sheets, a small degree of densification and little strengthening, other than that caused by the binder, occur. The difference to tumble/growth agglomeration is, that the shape of the green agglomerates does not result from growth and attrition in a tumbling mass of particles but is defined by the openings through which the material is passed. Because a considerable “stickiness” is one of the preconditions for successful agglomeration by this technique, often vermicelli-like extrudates are first produced (see Section 8.4.1), which break or are deformed during post-treatment (see also Section 8.3) into the final product shape. Corresponding to the previous discussion, pressure agglomeration can be carried out by a number of techniques. Each method results in the manufacture of different types of products with respect to size, shape, and physical properties. However, all have in common a basic compaction mechanism. Fig. 8.1 is another presentation of what has been discussed and illustrated before (see Section 5 . 3 and Chapter 6, Fig. 5.43 and 6.6). The upper part of Fig. 8.1 shows with four model sketches the structural change of a bulk mass of particulate matter during densification in a die, the attendant change in volume, and an indication of the modifications of particle shape and size that occur at high pressure. The lower part of Fig. 8.1 depicts the build-up of pressing force with time under the assumption that the forward movement of the punch occurs with a constant rate until pma is reached at which point the direction of movement reverses and the punch now retracts with the same constant (or a higher) speed.
8.1 Mechanisms of Pressure Agglomeration
Bulk
Densified
Elastic springback
Time Sketches explaining the mechanism of pressure agglomeration.
Fig. 8.1:
Referring to Fig. 8.1, as a first step, pressure agglomeration achieves a rearrangement of particles which requires little force and does not change particle shape and size. This is followed by a steep rise of pressing force during which brittle particles break and malleable particles deform. Sketches 3 (brittle) and 4 (plastic) occur either/ or and often simultaneously if both brittle and malleable particles are present in the mix. Two important phenomena limit the speed of compaction and, therefore, the capacity of any pressure agglomeration equipment: compressed residual gas (air) in the pores and elastic springback. Both cause different, equipment specific cracking and a weakening or, sometimes, total destruction of the products (see also [B.12b; B.42; B.561 and Sections 8.4.1 through 8.4.3). Low and medium-pressure agglomeration apply small forces (up to approx. the beginning of the steep slope of the curve in Fig. 8.1) but, nevertheless, the removal of a relatively large amount of air must be guaranteed (the time-axis is directly proportional to the volume change since the punch advances with constant speed). Development of compressed air pockets within the densifying product can be avoided if compaction occurs slowly enough so that all gas is able to escape from the diminishing pore space. High-pressure agglomeration extends into the steep increase of the pressing force. In this range, particle size and shape change by breakage and or deformation and porosity is further reduced. The maximum pressing force is normally defined by an overload feature of the equipment. Since a predetermined final strength and structure (see Section 8.2) must be reached, equipment selection must take into consideration that a sufficiently high maximum pressing force can be attained.
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After arriving at the maximum pressing force, pressure is released. If, as shown in Fig. 8.1, compaction is performed by a punch in a die, the direction of travel of the piston reverses and, when no expansion of the densified body occurs, the pressing force should drop to zero immediately (vertical line). In reality, there is always a more or less pronounced “spring-back which is caused by the expansion of compressed gas and the relaxation of elastic deformation. As mentioned before, this effect becomes more pronounced with increasing speed of densification until, at a certain compression rate, the compacted body disintegrates partially or totally upon depressurization. Therefore, it is often necessary to find an optimal compromise between densification speed (= capacity) and product integrity (= quality). The problem becomes greater with finer particle size because such materials are naturally more cohesive and, therefore, in the feed state, feature lower bulk density or, respectively,higher bulk volume. In these cases, cohesive arches will collapse at low pressure whereby large amounts of gas are driven out. At the same time, pores between fine particles are small which results in low diffusivity so that it takes a relatively long time for the large amount of displaced gas to escape. To help overcome problems associated with degassing or deaeration, special design features, such as force feeders and/or various provisions for venting, are applied with all pressure agglomeration methods, particularly if fine powders must be processed. If the mechanism of densification is considered (refer to the sketches in the upper part of Fig. 8.1),it becomes clear that the pores in the feed for a pressure agglomeration process of any kind must not be filled completely (saturated) with a liquid. An example of such a material would be a normal filter cake, i.e. one that has not been blown dry or otherwise further dewatered. Since liquids are incompressible, the pressing force would increase quickly and mechanical dewatering would have to occur, which further reduces the speed of densification. It would also require an effective separation of solids and liquid during the densification process; this is a task which, so far, has not been solved satisfactorily. Therefore, with increasing pressure applied to the particulate solids, which typically results in higher densification or lower porosity, the moisture content of the feed must diminish. In high-pressure agglomeration the feed must be essentially dry! The destructive effects of expanding compressed air and relaxation of elastic deformation can be also reduced if the maximum pressure is held for some time, called dwell time, before it is released. Fig. 8.2 shows, that, without special technical provisions, this is only achieved in ram extruders (Fig. 8.2b, see also Section 8.4.3). In such equipment, a number of briquettes is retained in the long pressing channel and is redensified during each stroke. After the wall friction is overcome and the entire line of briquettes moves forward, the pressing force remains almost constant. A similar, but much smaller effect is obtained in pellet presses (see also Section 8.4.2). Since, as mentioned before, a dwell time and, particularly, the application of several densification cycles also helps to convert temporary elastic deformation into permanent plastic deformation, these techniques are especially suitable for the densification of elastic materials such as, for example, biomass. If required or desired, in the case of punch-and-die presses (Fig. 8.2a) special drive systems must be used to accomplish a dwell time (see also Section 8.4.3). It is obvious
8. I Mechanisms of Pressure Agglomeration
Fig. 8.2 Cycles o f force build-up in the three different high-pressure agglomeration techniques.
(c) Converging die (roller press)
from Fig. 8 . 2 that ~ no such possibility exists in roller presses (see also Section 8.4.3) where a continuous rolling action densifies the material between approaching surfaces until, immediately after passing the point of closest approximation, the relative motion is reversed, the surfaces retract, and the pressing force drops, ideally to zero (see also Section 8.4.3).
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As compared with the structure of tumble/growth agglomerates (see Chapter 6, Fig. 6.1) where particles adhere to each other and form a porous body by natural coalescense and in which the agglomerate forming particles largely retain their individuality, size, shape, and characteristics, during pressure agglomeration, depending on the level of densification and the applied forces, this structure may change dramatically. Referring again to the upper part of Fig. 8.1 (Section 8.1) it can be seen that, as densification progresses (represented by the x-axis) and the pressing force increases (represented by the y-axis), the volume of the particulate matter becomes smaller which indicates a decrease in the void volume between the individual particles. In low-pressureagglomeration (see Chapter 6, Fig. 6.4a), only the porosity of the mass is changed while the original particle sizes and shapes are retained. Typically, wet particle mixtures or those exhibiting a sufficient amount of natural plasticity and lubricity are passed through the openings of a screen or thin, perforated sheet by the wiping action of a suitably designed tool (see Section 8.4.1).The product is a crumbly mass or small extrudates, one dimension of which is defined by the screen or perforated sheet openings. Porosity is still high and the fresh (“green”)strength is normally obtained by liquid or “sticky” binders. Because, as experienced in tumble/growth agglomeration as a result of the competition between adhesive and destructive forces, preferential coalescense does not take place in low-pressure agglomeration, a system that densifies feed by passing it through openings and relying on inherent or added binders for green strength, porosity of the product particles is higher than in agglomerates obtained in high density tumbling beds and is comparable with that of granules from low density fluidized beds. The advantage of low-pressure agglomerators over fluidized bed agglomerators is that the size of the product granules is better controllable and that larger agglomerates can be made directly from a moist powder mixture. The green granules from low-pressure agglomeration are also well suited for spheronizing (see Section 8.3). Final strength is achieved by curing which normally involves drying. Medium-pressure agglomeration or “pelleting” comprises processes in which sufficiently plastic and “lubricated” particle mixtures are extruded through perforated dies (see Chapter 6, Fig. 6.4b). In contrast to the “dies” used in low-pressure agglomeration, the openings feature significant length and the densification pressure is caused by the frictional resistance in the orifice during extrusion (see Section 8.4.2). As a result, lower porosity, higher green strength, and a better defined product shape are obtained. As shown in Fig. 8.1 (Section 8.1), often some deformation of the agglomerate forming particles is obtained if they are sufficiently plasticized during a conditioning step (often by “steaming” to activate starchy components, see also Section 8.3) and a high enough pressing force results from the dimensions of the extrusion channels. Brittle breakage of agglomerate forming particles does not normally take place because plasticity is a precondition for successful pelleting. The fresh, green strength of products from medium-pressure agglomeration is caused by capillary, adhesion, cohesion, and attraction forces as well as by interlocking due to plastic
8.2 Structure of Pressure Agglomerates
flow. In most cases a drying step follows during which additional bonding, mostly by recrystallization of dissolved substances, occurs. High-pressure agglomeration (see Chapter 6, Fig. 6.5) extends densification into the reduction of void spaces by changing particle size and shape. As product density comes near to the true density of the solids and porosity approximates zero, the pressing force curve asymptotically approaches a vertical line. At this state, very large increases in force result from small changes in volume which endanger the structural integrity of the equipment. For that reason, densification is only carried out to a save level in Fig. 8.1) and overload protection is included in the equipment design. In size enlargement by pressure agglomeration, the aim of densification is to bring the primary particles into sufficiently close contact so that the forces acting between them become large enough to yield adequate strength for the agglomerate’s intended use. This may be achieved directly and/or after a post-treatment. In dry, high-pressure agglomeration, it is often necessary to carry compaction into the bulk compression stage in which stressing is hydrostatic in character. Then, broken or deformed particles are no longer able to change position because only few, small voids remain and a certain degree of particle conformity has been achieved. The rate at which the apparent density approaches theoretical density depends on the yield point of the solids. It is more difficult to compact brittle materials to high density by pressure alone because fragmentation decreases due to the development of hydrostatic pressure conditions and smaller particles exhibit higher strength (see also Section 5.4). When voids become fully disconnected, a considerable internal gas pressure may develop in the isolated pores which, together with stored energy from residual elastic deformation, contribute to the potential weakening or destruction of compacts when the pressure is released (see also Section 8.4.3). Compact density and its distribution is also strongly influenced by interactions between the solid particles and of the particulate mass with equipment features (e.g. die walls, punch surfaces, roller press pockets, etc.). If a perfectly lubricated particulate solid (i.e. featuring no interparticle friction) were compacted in a cylindrical die with frictionless walls, it could be expected that the force exerted by the smooth, flat punch is transmitted through the entire volume of material resulting in uniform pressure and, therefore, uniform density throughout the compact. In reality, the presence of frictional and shear forces leads to a non-uniform pressure distribution and irregular particle movement (displacement) causing variations in compact density. Density variations are present in products from most pressure agglomeration techniques and may lead to a weakening of the compact (see Sections 8.4.2 and 8.4.3).If post-treatment includes crushing to yield a granular product, particles with different hardness and strength are obtained, and if sintering is applied, distortion of the final product is caused by the density variations (see Section 8.3). To demonstrate some of the possible effects of real particulate masses and equipment features on the outcome of densification, results of tests are shown in Fig. 8.3 which depict lines of constant solids content in a cylindrical compact after uniaxial compaction at different pressures [B.42].Since, as explained in Section 5.2.1, solids content in an agglomerate can be related with the term (1- E ) to the porosity E , a solids content of, for example, 39.6 % corresponds to a porosity of 60.4 %. Therefore, the
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d 39 M N I ~ ~
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Fig. 8.3: Density distribution in cylindrical compacts at progressive stages of densification [B.42].
feed, even though not shown, had a solids content of approx. 30 % or a porosity of 70 % which is indicative of and rather typical for a fine bulk solid. After subjecting identical bulk volumes of the same powder to the indicated compaction forces, as a first result, the volumes of the densified cylindrical bodies were reduced to the sizes that are represented by the corresponding rectangles. For structural evaluation, the residual pores were filled with a highly fluid, perfectly wetting polymer which hardened without shrinkage by chemical reaction. Upon solidification, rings were machined from the cylindrical body and, after leaching out the polymer, the solids content in each ring was determined by weighing the solids and relating their mass to the volume of the ring. Points of identical solids content were connected to obtain the lines shown in Fig. 8.3. The differences in density (or porosity, respectively)can be explained by interparticle friction, wall friction, force dissipation from particle to particle, and sliding under shear. Referring to the three highest densified samples in Fig. 8.3 (the lower row of compacts) in which the effects are most distinctly expressed, the high density in the upper corners is caused by the downward movement of the punch and the frictional resistance of the powder mass on the die wall while the low density in the lower
8.2 Structure of Pressure Agglomerates
corners is the result of lower pressure due to force dissipation, interparticle friction, and friction on the wall. The low density in the top center indicates a lateral frictional arrest of the powder mass because of its intimate contact with the punch. Finally, the high density in the lower center is obtained where the shear plane faults, which typically occur at an angle of approx. 45” in pressurized particulate masses, intersect. Similar explanations of density variations are possible for all other products of pressure agglomeration. In pelleting, for example, the extrusion through longer, mostly cylindrical openings in dies, the pressing force develops as a result of the friction between the extruding mass and the extrusion channel walls. Therefore, in most cases, the extrudate features a distinct, highly densified “skin” of defined thickness on the outside while the center is much less densified. As already mentioned in Section 5.2.2 this characteristic is advantageous for animal feed, the main field of application of this technology, because the highly densified surface provides good abrasion resistance for transport, handling, and application while, at the same time, the transverse crushing strength is relatively low yielding a feed that is easily chewable by the animals for which it is intended. On the other hand, crumbling or granulating an extruded material by crushing, destroys the strengthening effect of the skin and often results in a dusty product with low abrasion resistance (see also Section 8.3). In general, density variations during pressure agglomeration increase with higher pressing force and with greater height or thickness of the compact, decrease for cylindrical compacts with increasing diameter, even if the height to diameter ratio is constant, are slightly reduced by the addition of a lubricant to the powder, and are considerably reduced by lubricating the die walls and/or pressing tools. To avoid uneven densification, originally to alleviate distortion of pressed parts during sintering, isostatic pressing was developed. In this process, particulate material is shaped and compressed in a flexible mold by a pressurized fluid. By this method, the pressing force acts uniformly (isostatically) from all sides on the powder to be densified. Of course, prior to the application of pressure, the powder in the mold must be well evacuated to avoid the build-up of compressed gas in the densifying material. The compacts resulting from isostatic pressing still feature a different density, for example on the surface and in the center of the parts, but, because the density gradient is uniform, no distortion occurs during sintering (see Sections 8.4.4 and 9.1). Segregation during feeding and/or the filling of die cavities also results in density variations in the compacted product owing to localized changes in particle size and/or distribution and, in the case of mixtures, due to different distributions of plastic and friable components. There is ample evidence in Mechanical Process Technology that macroscopic flow of solid particles within powder masses is negligible. Particulate systems do not behave like gases or liquids in which molecules are mobile and can freely move. Therefore, it must be expected that variations in overall or localized density of the particulate feed before compaction will have a definite and significant effect on the uniformity and quality of the compacts.
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Fig. 8.4: Structures ofcompacts f r o m high-pressure agglomeration after processing equal feed materials (recrystallized KCI, Potash) at low (left, brittle disintegration) and elevated (right, plastic deformation) feed temperatures in identical roller presses with the same operating parameters. (For more information see text.).
In the foregoing, it was mentioned several times that the structure (and the characteristics) of products from high-pressure agglomeration depends on whether the particles are brittle and break or deform plastically during compaction. The structure of composit materials can be influenced not only by their mechanical and morphological features but also by shape and size. Since most materials become more malleable with increasing temperature and may be brittle at low temperature, heating or refrigerating particulate solids prior to feeding them to high-pressure agglomeration equipment can be also used to influence product structure. For example, Fig. 8.4 illustrates the influence of feed temperature on the structure of specific compacts. These scanning electron microscope (SEM) photographs depict microstructures of recrystallized (upgraded/concentrated) fertilizer grade potassium chloride (potash) after high-pressure agglomeration in a roller press (see also Section 8.4.3).The pictures on the left demonstrate brittle behavior, while the structure on the right results from plastic deformation. All parameters of the feed, the process, and the operation, with exception of the temperature, were kept constant. The feed temperature was at ambient on the left, a temperature at which potash crystals respond brittle to high speed loading, and approx. 130°C on the right, indicating that at this feed temperature plastic deformation occurs. To appreciate the effects fully, it should be pointed out, that the feed particle size is in a range from 10 to several hundred Fm, with most of the particles larger than 100 pm and only a very small percentage of fines < 10 pm. Thus, the pictures on the left (the one above at higher magnification for better scrutiny) prove that all potash feed particles have disintegrated into small, cubic crystallites. The photograph on the right (same magnification as the picture on the lower left) indicates that at the higher feed temperature the original crystals have survived but were deformed such that they now contact each other with large surface areas while fines are also plastically deformed and packed tightly into the pore space between the larger particles.
8.3 Post-treatment Methods
Although, the structure obtained from either brittle or plastic behavior of feed materials during high-pressure agglomeration is quite different, as demonstrated in Fig. 8.4, compression strength of the compacts is often equal. In the case of brittle response, a much larger number of considerably smaller particles is produced, which results in increased strength (see Section 5.2.1) and recombination bonding (see Section 5.1.1) participates importantly in the development of strength. If plastic deformation occurs, high strength is obtained inspite of retaining the larger particles because large surfaces contact each other intimately and interlocking bonds can occur when plastic components partially or completely flow around harder solids. In regard to residual porosity, it is often observed that brittle materials still retain a considerable amount of narrow but open porosity, which can be useful if the material must easily disintegrate and disperse in a liquid, while most of the porosity in compacts from plastically deformed feed particles is isolated, rendering a product with low dispersibility. As shown in Section 5.2.1, the tensile strength of agglomerates, in which the bonding occurs by binding mechanisms acting at the coordination points of the agglomerate forming particles, is directly proportionate to the solids content (1- E ) as well as the sum of all adhesion forces Ai caused by these bonds, and inversely proportionate to the porosity E as well as a representative equivalent particle size. If in tumble/growth agglomeration, where the size of the agglomerate forming particles does not change during the process, all acting adhesion forces are known, as, for example, in the case of capillary bonding, strength can be estimated. This is not possible for high-pressure agglomeration during which, as indicated in Fig. 8.4, particle sizes within the compact change if brittle solids are involved or unknown plastic deformation and approaches of surfaces take place; the extent of these modifications can not be estimated. Therefore, much more work needs to be done to learn about the structure of compacts from pressure agglomeration, particularly of those from high-pressure agglomeration. To that end it is necessary to understand the development of structure and to determine the parameters that control the structure as well as all product characteristics that are influenced by structure. 8.3 Post-treatment Methods
Referring to Section 7.3, where post-treatment methods were first covered in connection with tumble/growth agglomeration techniques, Tab. 7.1 summarized the effects of post-treatments on the final characteristics of agglomerates. As mentioned there, the methods and their effects “are feasible and can be used in the design of any agglomeration system”, which includes those based on pressure agglomeration technologies. In tumble/growth agglomeration, most frequently green (moist) agglomerates are produced which require drying as a post-treatment method to gain final, permanent strength. Only a few “natural” tumblelgrowth agglomeration technologies, involving ultrafine (nano-sized)particles, yield dry agglomerates which do not need further processing for strength. Additional post-treatment of aggregates from any tumblelgrowth
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agglomeration method is dictated by the desired or required final product characteristics. In pressure agglomeration, the properties of freshly produced compacts depend on the level of forces that have been exerted during densification and shaping. Low-pressure agglomeration requires feeds that are moist and plastic to allow passing the material through the structurally weak die plates (Section 8.4.1). As explained in Section 8.2, the structure of the resulting agglomerates resembles that of products obtained in low density tumbling particle beds. That means, they feature high porosity as well as low strength and are initially held together by bonds based on the surface tension of liquids and/or the adhesion of viscous substances. Therefore, they always need to be dried whereby final strength is obtained. The feed to medium-pressure agglomeration or pelleting equipment contains much less moisture. Plasticity and lubricity are normally obtained by the activation of feed components during a conditioning step. Such conditioning may be the mixing and kneading with small amounts of solid or liquid lubricants, or the contacting with steam to moisten and heat as well as gelatinize starchy materials, or may simply mean heating to soften certain components. An additional rise in temperature results from working the feed with the extrusion tools and by friction in the extrusion channels. In many cases, the products of pelleting need only cooling to remove the heat and gain final strength. The small amount of moisture that is present typically evaporates naturally prior to cooling. In some cases where for various reasons more moisture had to be added prior to pelleting, drying may be also necessary. As mentioned in Section 8.1, high-pressure agglomeration requires essentially dry feed materials due to the high densification to low residual porosities and the incompressibility of liquids. Typically products from high-pressure agglomeration feature high strength immediately and very seldom need post-treatment for strengthening. Excluded from this statement are applications in ceramics and powder metallurgy where final structure and strength are obtained during sintering (see also Section 5.3.2 and Chapter 9). Therefore, while in tumble/growth agglomeration at least one post-treatment step is almost always a necessary part of any system, the same is true for all low-pressure agglomeration but only for some medium-pressure agglomeration and very seldom for highpressure agglomeration processes. Increasing with the applied force during the process, post-treatment methods in pressure agglomeration are directed towards modifications of product size, shape, and/or structure. At first it seems that one of the most distinguishing features of pressure agglomeration, the production of a particular shape by densifying and forming particulate solids in various dies, is very desirable. However, for physical and technical reasons (see also Sections 8.4.1 through 8.4.4)it is only feasible to produce relatively large compacts. For a number of considerations it may be preferred to apply one of the pressure agglomeration techniques but still seek a granular product with particle sizes of only a few millimeters in diameter or even below that dimension. With the exception of low-pressure agglomeration, where extrudates with diameters of as little as 0.8 mm can be produced from selected feed materials, in medium or high-pressure agglomeration such sizes can not be obtained directly.
8.3 Post-treatment Methods
Most of the methods for the reduction and adjustment of size are based on crushing the compacts and screening the crusher discharge to yield a particle size distribution of predetermined width and accuracy. The narrower the distribution and the greater the desired accuracy of the sizing cuts, the lower is the yield of acceptable product and the greater are the amounts of over- and undersized particles. The rejected material streams (the one containing the larger particles after milling) are normally recirculated to the agglomerator. However, to increase or optimize product yield, a closed loop recrushing of the oversized particles is often preferred (see below and Section 11.3). This technology is called compaction/granulation. Agglomerates are assemblages of solid particles. Compared with the solid itself, in which atoms and molecules are held together by valence forces and form regular arrangements with an organization that depends on the types of the atoms and/or molecules, agglomerates are made up of smaller particles with different size and shape that are arranged in irregular structures and joined together by binding mechanisms. Because small particles contain only a few irregularities and flaws, their strength is relatively high (see also Section 5.4) and always exceeds that of the binding forces acting between the particles (see also Section 5.1). Agglomerates also feature void spaces between the particles, so called porosity (see Section 5.3.2). As a result of these characteristic properties of all agglomerates, in most cases it is easy to convert them back into the primary particles from which the agglomerate was originally made. Therefore, breaking larger compacts into smaller, but still agglomerated granules, which is the task of this particular post-treatment method, and avoiding the formation of excessive amounts of fines requires a different set of crushing parameters than those used for the “normal” size reduction of solids. For optimal granulation of agglomerates by crushing, it is most important to control the input of crushing energy, minimize the interaction between particles during crushing, and immediately remove fines from the crushing chamber. Taking into consideration the well known fracture mechanics [8.1]and interdisciplinarily applying crushing know-how, the following applies for compaction/granulation: 1. A low energy input must be maintained during crushing to avoid total destruction of agglomerates into or beyond the original powder particles. 2. Fines from any source, either leaking through the pressure agglomerator or produced during handling, should be removed to avoid overloading the crusher and, thereby, overgrinding the agglomerated material. 3. Crushing must be carried out gently to produce only few fines, a certain amount of product, and a considerable amount of oversized material; after screening, the latter is being recrushed in the same mill (closed circuit) or in another one (second crushing step). 4. Even in a separate second crushing step, energy input should be such that still some oversized material is produced, removed during screening, and recrushed to further optimize the yield of granular product.
The shape of granular material from compaction/granulation is irregular and angular (Fig. 8.5). This is in stark contrast to the spherical or spheroidal shapes of agglomerates
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Fig. 8.5: Photographs o f granular products from compaction/granulation showing the irregular, angular shape o f the particles.
from tumble/growth agglomeration. On an industrial scale, the first agglomerated granular materials were produced by rolling and growth. Since some of the major requirements on the characteristics of granular products are good flowability and distribution (as, for example, in mechanical spreaders for granulated fertilizers) as well as superior metering capabilities and high abrasion resistance, to avoid the production of nuisance dust, i.e. fine, airborne particles produced by erosion, it was thought that spherical or spheroidal agglomerates fulfill these requirements better than angular ones. Therefore, initially, granular products obtained by compaction/granulation were judged to be inferior to micro-agglomerates from tumble/growth agglomeration. This perception changed when it was shown in the pharmaceutical and the fertilizer industries that dry, high-pressure agglomeration and granulation by crushing and screening offered definite advantages (see also Chapter 11). Processing of dry particulate materials proved to be more economical because of the absence of a liquid binder that is costly and can also cause unwanted chemical reactions, the immediate obtaining of strength, the much lower energy requirement because drying of large amounts of material is not necessary, the easier house keeping, cleaning, and changeover possibilities with dry powders, and many other reasons. It was also shown that, for example, the spreading of granular fertilizers from compaction/granulation is accomplished as well and uniformly as that of “conventional” granulated products. A valid concern is, however, that the edges, corners, and peaks on the irregular broken granules tend to rub-off during handling, producing the dreaded nuisance dust. Because such dust manifests itself only at the distributors or users it leads to complaints as it is widely visible and does not settle quickly. The manufacturer faces claims even though the actual amount of airborne material is minimal. On the other hand, if this dust is produced by rounding the material during a post-treatment process in the plant prior to packing or intermediate storage, loading, and shipping, it can be separated and recirculated to the agglomerator for reuse. Therefore, in those cases
8.3 Post-treatment Methods
where granules are very irregular in shape and feature a strength that “favors” the production of dust, abrasion drums (see Chapter 11.3) or other suitable equipment are sometimes used to erode and/or round the particles. Rounding can be achieved by spraying a liquid onto the surface of a tumbling bed of granules after which the peaks on the now softened surface are flattened and/or dust particles are re-attached. While after erosion a dedusting step and fines collection as well as recirculation are necessary, no fines are produced during rounding but some drying may be required. A further possibility to round irregularly shaped granules from a crushing step is coating (Section 10.1). Particularly in the fertilizer industry, melt coated particles, in which the coating material constitutes at least one of the nutrients of the final multi component product, are an excellent solution of the problem with additional beneficial improvements. As always, interdisciplinary application of such methods can result in similar benefits for other granular products in different industries. Another rounding technology in agglomeration with quickly increasing, varied application is spheronization. It is the technique of converting plastic extrudates or particles that were formed otherwise into a rounded spherical or spheroidal shape. Approx. 40 years ago, the Japanese inventors coined the name marumerizer for this device which means translated “round maker”. In many industries, the technique is still called “marumerizing”although equipment by other vendors may be in use. The original apparatus looked very similar to the machines that are in use today; the changes since then have centered around auxiliaries and improvements to the internal structure which now allows a wider range of applications and offers the availability of competitive suppliers. In the beginning, spheronization had been primarily used in the pharmaceutical industry as a final forming method for formulations with high active loading. A spherical particle was needed for coating with a thin polymeric material for controlled release (see also Section 10.1).Today, spheronization is also applied for animal medicines, herbicides, enzymes, specialty fertilizers, advanced ceramics, and many more. The ability of an agglomerated material to be spheronized depends mainly on its rheology. Extrudates must break into shorter pieces and those as well as other agglomerates must have the right amount of plasticity to deform by impact and during rolling. The rheology can be adjusted by the addition of binders and lubricants or more wetting agent (usually water). With this feed characteristic and the frequent desire to produce small (i.e. in the millimeter range) spherical or spheroidal products, low-pressure agglomeration is the predominant technology preceding spheronizing. In fact, many wet mixtures and thixotropic materials that, during handling, processing, or compaction, become too pasty for use in any other agglomeration equipment, can be successfully densified in extruders, shaped into discrete agglomerates, and further treated to yield uniform rounded particles. In modern applications, perfectly spherical particles are often not required. Spheronization is then being used if extrudates do not break into short enough lengths or can not be cut in uniform pieces when exiting the die plate. In these cases the function of the spheronizer equipment is to reduce the size of long extrudates (Fig. 8.6a) into short cylinders with rounded edges (Fig. 8.Gb,c). After processing the same material for several minutes (Fig. 8.6d), almost perfectly rounded particles are obtained. In
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Fig. 8.6: Extrudates (a) and products from spheronization treatments after 5 s (b), 30 s (c), and several m i n (d) (courtesy LCI Corp., Charlotte, NC, USA).
regard to Fig. 8.6, it should be pointed out that, with more plastic materials, the spherical shape is often reached after much shorter times. A very round particle is produced if the end use dictates the need for this shape, for example for uniform coating in the pharmaceutical industry or well defined fixed bed packing of catalyst carriers. Spheronization begins in most cases with wet extrudates obtained from low (Section 8.4.1) or medium (Section 8.4.2) pressure agglomeration. To retain a maximum of plasticity, the elongated, often spaghetti-like extrudates are immediately charged into the spheronizer (Fig. 8.7) which consists of a vertical hollow cylinder (called “bowl”)with a horizontal rotating disc (called “friction plate”) located inside. The friction plate is the most important component of a spheronizer. It features a variety of different textures designed for specific purposes [ B.421. Upon contacting the friction plate, which rotates with several hundred (up to approx. 1,600)revolutions per minute, the extrudates break almost instantly into short pieces of uniform length. As shown in Fig. 8.8, the segments are flung outwards by the centrifugal force that is exerted by the friction plate and form a rotating mass that contacts the wall of the bowl. The proper motion of the moving mass of particles should resemble a twisting rope (Fig. 8.9)
8.3 Post-treatment Methods
Fig. 8.7: Sectional representation o f a marumerizer-type sphero. nizer (courtesy LCI Corp., Charlotte, NC, USA).
which turns at a significantly slower speed than that of the spinning friction plate. Mechanical energy is transformed into kinetic energy and the still plastic particles in the mass are being worked by contact with the friction plate as well as by collisions between particles and of particles with the wall. Continued processing causes a gradual deformation into a more and more spherical shape (see Fig. 8.6). Several auxiliary devices have been developed to improve and/or accelerate the rounding process (Fig. 8.10). During deformation and densification, excess moisture may migrate to the surface or the mass can exhibit thixotropic behavior. In such cases, a slight dusting by means of a suitable powder feeder reduces the likelihood of particles sticking together. Warm or cool dry air can be also introduced under the plate to remove some of the surface water from the particles. Other special features may in-
Fig. 8.8 Photograph of a marumerizer, Type QJ-400,in operation (courtesy LCI Corp., Charlotte, NC, USA).
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Fig. 8.9 Schematic presentations of the flow of particles that are being spheronized (courtesy LCI Corp., Charlotte, NC, USA).
Fig. 8.10 Auxiliary devices for a rnarurnerizer-type spheronizer (courtesy LCI Corp., Charlotte, NC, USA).
8.3 Post-treatment Methods
clude cooling or heating of the bowl through a jacket or cleaning of the friction plate with brushes. A moving baffle, consisting of several arms with pitched blades that are placed close to the wall and to the friction plate, serves to increase the agitation by wiping the inside wall and directing product into better contact with the friction plate. Variable speed drives are standard options since process conditions vary widely between applications (see below, Fig. 8.14). Also, formulation changes or different production rates will require modified rotational speeds of the friction plate for best performance. Spheronization equipment is designed for batch operation (see Fig. 8.7, 8.10, and 8.11). Continuous operation is possible by employing multiple units or cascade flow [B.42]. Both methods use two or more spheronizers. Multiple batch operation, for example with two spheronizers (Fig. 8.12), is sequenced such that one unit discharges
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Fig. 8.11: Schematic representation o f a batch spheronizing system including mixer, extruder, and spheronizer (courtesy LCI Corp., Charlotte, NC, USA).
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Fig. 8.12: Schematic flow diagram of a complete (quasi-) continuous spheronization system employing multiple (two) spheronizers (courtesy LCI Corp., Charlotte, NC. USA).
and subsequently begins with loading while the other one is in the second half of its spheronization cycle. A reversing belt or, as shown in Fig. 8.12, a diverter gate can be applied to alternately feed each machine. In cascade operation, two or more spheronizers are linked in series to extend the total processing (residence)time while, overall, realizing continuous flow through the system. Feed is continuously charged into the first unit resulting in partially spheronized material being displaced and overflowing into the next one@). Scale-up of spheronizers depends on the mode of operation. For batch units, it is volumetric. Each machine has a typical operating volume as shown in Fig. 8.13 as a function of bowl diameter. These relationships also apply for each batch spheronizer in a multiple batch, (quasi-)continuous system.
Fig. 8.13: Working volume of marurnerizer-typespheronizers as a function of bowl diameter (courtesy LCI Corp., Charlotte, NC, USA).
8.3 Post-treatment Methods
For continuous cascade operation, the friction plate is lowered in the bowl so that a volume of material always remains inside while excess overflows. The residual volume can be either measured experimentally or calculated, assuming that the cross-section of the rope may be approximated by a fourth of a circle (quarter torus). To obtain a particular spheronization effect, an overall residence time must be maintained. The processing time in each machine can be calculated as the ratio of residual volume divided by the volumetric feed rate (= volumetric throughput). Since bowl diameters are predetermined and fixed by the design, the position of the friction plate in the bowl is the only variable which can be modified to match a certain feed rate or system capacity to the desired or necessary residence time. The friction plate speed is scaled-up by maintaining the tangential (or circumferential) accelleration constant. The formula for scale-up is: (rotational speed,)*/(rotational speed,)* = bowl radius,/bowl radius,
(Eq. 8.1)
Results of Equation 8.1 are plotted in Fig. 8.14. In many cases it is an advantage that products of pressure agglomeration have uniform shape and often also feature the same size. For example, in the pharmaceutical industry it is a requirement that all tablettes made from the same formulation are of the same shape and size because they also represent the dosage form. Since, during therapy, individual tablettes are to be taken, packing of such products is easy and selection as well as retrieval are errorfree. Many agglomerated consumer products, such as foods, snacks and sweets, some flavoring mixtures, certain detergents, etc., have similar requirements. In the metallurgical industry, sometimes alloying elements are agglomerated such, that the large briquettes represent the quantity to be added to a certain amount of liquid metal, and for home heating, briquettes have long been easily chargeable solid fuels.
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-E P8
1400
1200
1000 800
m
a,
i;j
600
h 400
Fig. 8.14 Relationship between plate diameter o f a spheronizer and r p m t o maintain constant tangential accelleration (courtesy LCI Corp., Charlotte, NC, USA).
200
0 ; 200
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Monosized pieces in bulk, on the other hand, particularly if they are not spherical, do not reach high density or mass. For example, run-of-mine,coarsely crushed coal that is loaded into trucks, railroad cars, or ship holds results in relatively high bulk mass, as smaller particles fill the voids between larger ones. If that same coal is briquetted, the identical transport device holds much less and, therefore, in comparison, renders shipping less economical. Crushing briquettes and sizing the broken pieces to be within the same limits as run-of-mine coal results in identical bulk mass and meets standard loading requirements. If agglomerates that were produced by any of the pressure agglomeration methods are broken, it must be realized, however, that individual pieces may exhibit different density and strength (see Section 8.2, Fig. 8.3 and corresponding text). In contrast to what has been said before (see above), sometimes it may, therefore, be inappropriate to crush agglomerates too gently. Weak particles may survive the stressing and then disintegrate during handling, producing excessive amounts of fines. Such breakdown of the granulated product does not normally result in airborne “nuisance dust” as defined above, but may still not be accepted by a particular user because the amount of “fines”,whatever that term means in a particular case, is greater than specified and again results in claims to the supplier. An important post-treatment method for many parts obtained from powders by high-pressure agglomeration is sintering (see Chapter 9). This technology produces final strength and structure in most ceramic and powdermetallurgical parts. Finally, because during high-pressure agglomeration porosity is often reduced to very low levels, post-treatment methods may be required to regain a more open pore structure and larger voids (see Section 5.3.2).
8.4 Pressure Agglomeration Technologies
In the following four subchapters the technologies and the equipment for the beneficial agglomeration by pressure will be described. As already mentioned in Chapter 8, in pressure agglomeration, new, enlarged entities are formed by applying external forces to particulate solids in differently shaped and operating dies. There are two major distinguishing characteristics which define different pressure agglomeration techniques: Pressure and Die or Tool Configuration. The first three sections are organized according to the level of pressure that is applied, while the fourth one describes a method that is set apart from the others by how the pressure is applied. Although it has been decided not to break the sections into further subsections, the different die configurations are so important that they will be described and collected in specific paragraphs with appropriate group headlines. The following is a summary of the methods and how they will be identified:
8.4 Pressure Agglomeration Technologies
Pages 8.4.1 Low-pressure agglomeration (Extrusion) 253 - 266 Gravity feed and extrusion blade@) (screen and basket extruders) 253-256 Screw feeder@)and extrusion blades (radial, axial, and dome extruders) 257-262 Gravity feed and roller(s) (flat die extruders) 262 - 266 8.4.2 Medium-pressure agglomeration (Pelleting) Hollow, perforated cylinder(s), feed from the outside Hollow, perforated cylinder, feed from the inside 0 Flat, perforated die plate with press roller(s) Gear-shaped press rollers, feed from the outside Medium pressure axial screw extruders
2GG - 299 273-276 277 - 283 284- 289 290 - 294 294-299
8.4.3 High-pressure agglomeration High pressure axial screw and ram extruders Punch-and-die presses Roller presses
300 - 373 300-315 315-335 33s - 373
8.4.4 Isostatic Pressing
373 - 383
Another criterion that might be used to distinguish between the different pressure agglomeration techniques could be whether or not they operate continuously. However, although systems, overall and in practical terms, can be almost always designed to discharge product continuously, with the exception of screw extruders and roller presses, they do not actually perform continuously. From a fundamental point of view, in relation to the process and the formation of each individual agglomerate, most pressure agglomeration equipment operates discontinuously. For example, in low and medium-pressure agglomeration extrusion takes place only as long as pressure is exerted on a particular row of orifices and in ram extrusion as well as punchand-die presses the pressing tool reciprocates, forming each compact in a separate densification cycle, etc.
8.4.1
Low-Pressure Agglomeration
The techniques of low-pressure agglomeration may be the oldest method for the production of granular material by agglomeration. Originally, a moist mass was passed through a sieve by the eminence of the hand, a spatula, specially designed hand tools, or a sturdy brush. The “crumbled” mass was dried and yielded a granular product that was used directly, mostly in the food and pharmaceutical industries, or further processed, for example by pressing it into cubes or tablettes. This method, the use of which dates back several hundred years, was carried out to obtain free flowing and non segregating powder mixtures, which still is the major task for most granulation methods. In more recent times, the procedure was mechanized and what had been done by hand is now accomplished by motorized rotating or oscillating extrusion blades (Fig. Gravity Feed and Extrusion Blade(s)
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8.15a and b, see also Chapter 6, Fig. 6.4a.l and a.2). The oldest and most basic equipment is the screen extruder (Fig. 8.15a). A system for the production of dry agglomerates is shown in Fig. 8.16. Wet feed (1)is fed by gravity onto a circular flat screen and wiped through the mesh openings with a rotating blade. The agglomerates with an approx. size of the screen openings are dried in a suitable piece of equipment. Since
(b) Pressing
i 11
Screen
'Lreen
Pressing
\
Screen Fig. 8.1 5:
Schematic representation of low-pressure agglornerators using gravity feed and screens or thin perforated sheets. (a) Screen extruder, (b) trough extruder, (c) basket extruder.
Fig. 8.16
2
I
Sketch of a low-pressure agglorneration system with screen extruder, dryer, and (optional) mill. (1) Wet feed, (2) granular product and tines.
8.4 Pressure Agglomeration Technologies
Fig. 8.17: Photograph of a small trough-type granulator with horizontal rotor axis (courtesy Erweka, Heusenstamm, Germany).
the feed must be very plastic, it is possible that agglomerates stick together in the dryer and form large lumps. If this is the case an optional mill is used to break the dry material into the final granular form (2) which contains a certain amount of fines that, depending on the application, may have to be separated and recirculated (not shown). Fig. 8.17 is the photograph of a small trough granulator (see also Fig. 8.15b). Inside the screen trough is a rotating or oscillating cage with wiper bars that passes the material through the screen. Most modern low-pressure agglomerators using gravity feed and screens are basket extruders (Fig. 8.15~).Fig. 8.18 depicts more detailed sketches ofthis machine and its
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Fig. 8.19: Photograph of a basket extruder, model BR-450, and detail o f extrudates being formed (courtesy LCI Corp., Charlotte, NC, USA).
function. In basket extruders, the meshed or perforated cylinder sits upright. Feed material falls into the chamber and in front of the extrusion blades (Fig. 8.18a). The material is compressed in the nip between the rotating blades and the cylinder and forced through the screen openings, forming extrudates (Fig. 8.18a). The extrudates break-off naturally or are cut-off by a rotating knive on the outside of the cylinder and fall into an inclined collection chute or, as depicted in Fig. 8.18a, onto a rotating plate. In both cases, the green agglomerates leave the equipment through a discharge chute. Because pressure is only exerted by the extrusion blade, basket extruders feature the least compaction of the various low pressure extrusion devices and are especially suited for easily extruding materials yielding products with high porosity. Fig. 8.19 and 8.20 are photographs of industrial basket extruders and of extrudates being formed during operation. Fig. 8.20 also depicts the major components of a basket extruder showing feed hopper, extrusion blades, and perforated basket die. To achieve higher densification, additional compressive forces must act on the feed material. This can be attained by means of integrated screw feeders or by heavy and/or pressurized rollers. At the same time, the perforated die must become structurally
Fig. 8.20 Photograph o f the major components (feed hopper, extrusion blades, and perforated die) o f a small basket extruder (Bextruder BX 150, courtesy Hosokawa Bepex CmbH, Leingarten, Germany).
8.4 Pressure Agglomeration Technologies
stronger to withstand these forces which results in somewhat longer extrusion channels and higher frictional resistance that also yield higher densification. Screw Feeder(s) and Extrusion Blades A modern machine that may, alternatively, apply low or medium pressure to a wet or moist particulate mass of solids is the screw extruder. In these machines the phenomenon of movement caused by the flights of rotating screws in more or less tightly fitting barrel-shaped housings is used to produce the necessary pressure to overcome the friction in open-ended channels. Screw extruders may feature single or twin (= two) screws. In low pressure screw extruders extrusion blades are used to create a wiping effect at the die plate. Two fundamentally different arrangements are possible: extruders with peripheral (or radial) and axial discharge (see also Chapter 6, Fig. 6.4a.3 and a.5). Pressures in extruders with radial discharge are typically less than 1 MPa. The extrudates exit radially from a screen cage located at the end of the screw(s) (Fig. 8.21). The extrusion blades are either tapered cylinders with vanes in single screw extruders or intermeshing blades if twin screws are applied (Fig. 8.22). While the screw@)transport(s) the particulate mass through the barrel and provide(s) pressure, the extrusion blades push the material through the screen in a similar way as shown in Fig. 8.18b. In addition to the blade forces and the frictional resistance to slip, the force exerted by the screw(s) acts on the mass. If the feed mix to be extruded features unfavorable flow characteristics, some of the material may collect in front of the extrusion blades and rotate across the screen. Then the screw(s) must impart more driving force (work), increasing the internal pressure which participates in pushing the mass through the openings. For that purpose different screw designs are available (see be-
Fig. 8.21: Schematic cross section through a peripheral or radial low pressure screw extruder.
Fig. 8.22 Conical, vaned and intermeshing extrusion blades for radial low pressure extruders (courtesy LCI Corp., Charlotte, NC, USA).
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Fig. 8.23: Photograph of the extrusion section o f a low pressure extruder with radial discharge (courtesy LCI Corp., Charlotte, NC, USA).
low, Fig. 8.26). The direction of extrudate flow is perpendicular to the axis of the equipment as shown in Fig. 8.23. In an axial extruder, pressure is again developed by a screw or screws which forces the particulate mass through uniform openings in a die plate that seals the end of the barrel (Fig. 8.24). According to the principal mechanical operations that are performed along the screw(s),three major zones are defined in an axial extruder: 0 0
Feed zone, transport and compression zone, extrusion zone.
The feed zone is the area where the moistened formulation is first introduced into the extrusion device. It includes a hopper to channel and distribute the flow of material into the chamber containing the screw(s). Most screw extruders will be operated with only a slight excess of feed or even in a somewhat starved state. Because, for extrusion, the material must be plastic, too much feed tends to build up over the screws and bridging is likely to occur. The screw(s) move the mass from the feed zone into the compression zone. In some machines, liquid can be introduced in this zone and the material is kneaded to form a Feed hopper
2
5 / 5
Fig. 8.24
2 Feed zone
Screw
$ 2
5 Compression 5 *Extrusion / ’ 5/ zone $ 2
Schematic cross section through an axial extruder.
zone
8.4 Pressure Agglomeration Technologies
Feed
4
Vacuum
moist homogeneous mass. Some mixing of different powders can be also accomplished. Most manufacturers of screw extruders offer both single and twin screw designs. The twin screw extruder has the advantages of less bridging in the feed zone due to the larger open area and better transport into the compression and extrusion zones. Also, a greater throughput is achieved. On the other hand, a single screw extruder is capable of delivering more power per unit mass of the material to be processed which may help to produce a harder and/or denser extrudate. In the compression zone, as a result of the specific design of the screw(s) in this part of the equipment, the void volume between the particles is reduced as particles are forced to approach each other more closely and gas (in most cases wet air) is expelled from the loose mass (see also Sections 8.1 and 8.2). As shown in Fig. 8.25, some extruders have vents, which may be open or connected to a vacuum and exhaust treatment, to remove the displaced gas. Screw design varies in accordance to how much compression is needed. A very low pressure extruder may feature screws with regularly spaced flights and a straight shaft. They will provide some compression but the main function of such screws is just to transport the material along the barrel of the extruder to the extrusion zone. Other designs use progressively closer screw flights, i.e. variable pitch, and/or tapered screw shafts (Fig. 8.26).
Fig. 8.26 screws.
Sketches of various types of extrusion
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Fig. 8.27: Schematic representation o f a low pressure axial extruder with extrusion blades, also showing a typical drive arrangement
(6.421.
Sometimes, a space is left between the end of the screw(s) and the die plate. If rheological properties of the material are such that further densification takes place in this transition space, a denser extrudate is produced. In most cases, a large gap is used in high pressure screw extruders (see Section 8.4.3)where the necessary forces to obtain extrusion are solely developed by the screw@)and high hydrostatic pressure is required to induce hydraulic flow through the extrusion channel(s). In low pressure axial extruders, extrusion blades are commonly attached to the end of the screw shaft (Fig. 8.27). In those cases, the gap is small and the plastic mass is compressed in the
Fig. 8.28: Photograph o f strands discharging from a twin screw low pressure axial extruder without cutters (courtesy LCI Corp., Charlotte, NC, USA).
8.4 Pressure Agglomeration Technologies
Material inlet /Extrusion blade
\
Fig. 8.29 Schematic cross section thrcu g h a low pressure dome extruder.
Dome die
f
nip between the blades and the die face, forcing the material to flow through the openings utilizing a localized “drag flow” pressure. With axial extruders, it is difficult to cut the extrudates into uniform lengths because the material extrudes faster on the outside of the die plate than nearer to the center (Fig. 8.28).This phenomenon is a result of the complex flow and pressure patterns that are developed in the extrusion zone. A rotating knife or an oscillating wire can be used to separate the extruded strands into pellets but, nevertheless, their length will usually vary considerably. The dome extruder, also called extended die extruder, is a hybrid between the axial and radial low pressure extruders (Fig. 8.29, see also Chapter 6, Fig. 6.4a.4). This design was developed while trying to find a way to make the flat die plates of axial extruders last longer. The latter tend to bend out when they are overloaded if, for example, difficult to extrude materials are processed. Geometrically, the best balance of forces happens on a sphere and the extrusion stresses are spread over a three-dimensional area. An additional advantage is the increase in production capacity even if compared to radial extruders. Although the extrusion area is smaller than that of
Fig. 8.30 Dome type low pressure extruder in operation showing extrudates emerge from the semi-spherical die plates. Also visible is a double shafted pug mill for mixing and conditioning the feed prior to the extruder (courtesy LCI Corp., Charlotte, NC, USA).
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the radial extruder, a steeper nip angle at the extrusion blade can be used which results in a more efficient wiping motion that overcomes the increase in pressure due to the smaller screen area. Typically, dome type low pressure extruders are equipped with two screws and, consequently, with two domes. The extrudates from all low pressure extruders, but particularly those from low pressure dome extruders, are slightly curved since they bend under their own weigth after leaving the orifice (Fig. 8.30). Gravity, acting on the overhanging mass of extrudates, causes strands to break irregularly wherever and for whatever reason a cross section is weaker. Accordingly, extrudates feature different lengths. Gravity Feed and Roller(s) Flat die extruders (see Chapter 6, Fig. 6.4b.2) are commonly designed for medium pressure extrusion or pelleting. They consist of a horizontally arranged flat die plate on which normally two (or more) rollers move along (Fig. 8.31). Feed material is charged by gravity from the top into a chamber surrounding the circular die plate, pressed by the rotating rollers, and squeezed through the die openings. Typically, a rotating knife cuts off the extrudates below the die plate. Because of the high forces that are normally exerted by the rollers and involved in densifying and extruding the plastic particulate feed, the die plate must be thick to offer sufficient structural integrity and, consequently, the extrusion channels (in most cases cylindrical bores) are long, requiring medium pressure for extrusion (see Section 8.4.2). Since the principle is of interest for the agglomeration of some materials by low pressure extrusion, a method was found to support a die which is made from perforated thin sheet and, thus, obtain a low pressure flat die extruder. This machine resembles a low pressure screen extruder (see above, Fig. 8.15a and 8.16) in which the extrusion blade(s) has (have) been replaced by rollers. Fig. 8.32 is the photograph of a small, low pressure flat die extruder in operation. In extrusion, material flow through the openings of a die is very complex and related not only to the physical characteristics of the particulate mass but also to the die plate or screen configuration. As an example, Fig. 8.33 shows how the extrusion rate varies in relation to the dimension of the orifices (defined by the die thicknesses and hole diameters) as well as the percentage of free area (defined as the sum of all hole cross
0;. Roller
1-1-111
1 7 cutter Side view
Top view
Fig. 8.31: Schematic representation and working principle o f a flat die extruder.
8.4 Pressure Agglomeration Technologies
Fig. 8.32 Photograph o f a small, low pressure flat die extruder in operation. The front half o f the extrusion chamber is transparent t o make the press rollers visible (courtesy LCI Corp., Charlotte, NC. USA).
sections divided by the total area of the die plate . 100). In low pressure extrusion, a radial discharge extruder with screens will typically have more than six times the extrusion area than an axial extruder with the same barrel diameter. If a given material can be processed on either type of machine, this relationship translates into a higher capacity and cost advantages for the extruder with radial discharge. The differences between a screen and a perforated sheet or plate are quite substantial. Up to 1 mm, screens are usually of the same thickness as the hole diameter; they are rarely thicker than 1.5 m m due to physical limitations of mounting. Perforated sheets and plates are > 1 mm (see also Section 8.4.2). The moisture level of the feed directly influences extrusion capacity and material flow (Fig. 8.34). In low pressure extrusion, most materials can only be extruded within a relatively narrow moisture range, typically 2 to 5 %. Even then, the extrusion characteristics may vary dramatically. At the lower end, the extrudate may have a rough surface, cause high power
500
450 400 n
2
350
\
rn x 300
W
250 ip
K
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150 100 CA
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'
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.
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.
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Die thickness (mm) Fig. 8.33: Extrusion rate as a function o f die thickness, hole diameter, and percent free area (courtesy Fuji Paudal, Osaka, Japan)
*
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1200 1100
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m Q)
c
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0 11
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%
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Moisture
Fig. 8.34 Extrusion rate o f a rubber chemical as a function of the moisture content and hole diameter (courtesy LCI Corp., Charlotte, NC, USA).
consumption, exhibit high temperature rise, and may become excessively dense (see also Section 8.4.2). At the same time, the machine responds with a high demand of power and the screen or die plate may become overloaded. At moisture levels approaching the upper limit, moisture may be squeezed from the material, the extrudates may stick together, and the material may adhere to the feed screw thus reducing its transport efficiency (see below). Scale-up of low pressure extruders usually begins in the laboratory with testing on smaller equipment. After extensive experimentation with the formulation and equipment, an optimal set of parameters is defined which includes information on the material’s bulk density (before and after extrusion), the extrusion rate, the power consumption during extrusion, and the product’s temperature rise. An efficiency factor is then determined by ratioing the actual extrusion rate obtained on the small equipment to the calculated theoretical maximum extrusion rate. Efficiency factors are in the range of 5-35 % for axial, 15-55 % for radial, and 35-85 % for dome extruders. This efficiency factor is then applied to the theoretical extrusion rates of the industrial extruder. Many manufacturers of extruders will also include an application related “experience factor” for the determination of a safe but reasonable expected extrusion rate. Fig. 8.35 depicts relative levels of extrusion pressure and shear that are applied by the various low pressure extrusion equipment. For all machines that rely on a screw or screws for material movement and the development of pressure, it is important to understand how a conveyor screw works. In general, the mass flow rate dm/dt of, for example, a screw extruder is de-
8.4 Pressure Agglomeration Technologies
t
Axial
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/-Aiiai
Roll
I
f Roll Extrusion pressure
Fig. 8.35: Relative levels o f extrusion pressure and shear applied i n different low pressure extruders (courtesy LCI Corp., Charlotte, NC, USA).
Shear applied per weight of protliict
’1
i -Basket
Radial
Dome
Radial
t ~
LBasket
1)ome
~
_
termined by the combined influences of screw transport and die resistance. The operating point, defining pressure and capacity, is obtained in a mass flow/pressure diagram (Fig. 8.36) as the intersection point between the lines characterizing the screw and, respectively, the die performances. Because of the influence of both, the theory of screw extruders is rather complex (see also Section 8.4.3).The actual operating point results from the superposition of two extremes: of screw conveying with no back pressure and pumping/mixing against a completely closed end [B.42]. The difficulty to theoretically describe the conditions in a screw extruder becomes even more complicated if, as described above, special kneading, densification, and deaeration sections are included in the design. To further understand the function of a screw, whether used for feeding (see also Section 8.4.3) or to build up pressure, it should be always realized that a “perfect” performance of a pressure creating transport screw requires that, along its length, the screw and barrel diameters as well as the pitch and flight thickness are constant, no build-up occurs on either the screw or the housing, the volume between the flights is completely filled, and the particulate solids do not rotate and/or densify by deaeration. In reality, all these conditions are not fulfilled. Shaft and barrel (or housing) dimensions often change for process reasons, build-ups may appear with time, the space between flights may partially empty due to rearrangements of particles, the solids may begin to rotate with the screw, mostly because of surface blemishes and build-up on the screw flights, and uncontrolled separation of solids and gas may occur. In screw design, a correction factor is trying to correct these inefficiencies.
Nozzle characteristic
Point of operation Screw characteristic Fig. 8.36 Extrusion rate dm/dt o f a screw extruder as a function o f the (back-)pressure o f the mass t o be extruded [B.42].
Pressure p
_
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Most critical is the fact, that the particulate solids often begin to rotate with the screw. In an ideal case the screw flights are so smooth that no friction occurs with the particulate solids and, in contrast, the barrel walls exhibit so much frictional resistance that the particulate mass remains stationary in relation to the housing and is “screwed” forward efficiently. Sometimes this condition is also lost if the screw turns too fast. Then the solids may begin to exhibit turbulent movement and lose contact with the barrel wall, thus initiating rotation. Once this situation has occurred, the screw or screws may have to be stopped, emptied, and restarted with lower speed. Occasional cleaning of the screw flights, the frequency of which depends on the characteristics of the material and the condition of the screw, may also be necessary. Coating the screw flights with a “non stick” surface may improve efficiency. 8.4.2
Medium-Pressure Agglomeration/Pelleting
Referring back to Section 8.1, Fig. 8.1, and the accompanying text, medium-pressure agglomeration or pelleting is characterized by forces that are in the transition range. Structurally, most or all of the rearrangement of particles has taken place in the compact and, at the high end of medium-pressure agglomeration, some particle deformation may take place. Since pelleting is carried out by extrusion, the materials to be agglomerated need to be plastic or deformable. Therefore, the occurrence of brittle disintegration, the other mechanism that causes changes in particle shape (and size) in pressure agglomeration, is very unlikely. As in low-pressure agglomeration, differently shaped extrusion dies are used in which agglomerates with cross sections that are defined by the orifices are formed. The forces for densification, extrusion, and shaping are provided by the movement of the dies themselves and/or by press rollers and by the frictional resistance in the die channels. To accomplish higher densification, the ratio “lengthldiameter” or, respectively, “length/cross sectional area of the extrusion channels” must become greater, resulting in a higher frictional resistance. Since the extrusion force must be larger than the frictional resistance, considerable pressures develop in front of the extrusion channels. Therefore, the die body thickness must be selected such that it will not break during operation even if, occasionally, overloading takes place. Also, the open area as defined in Section 8.4.1, Fig. 8.33 (“% free area”) is normally relatively small, also to achieve structural integrity; that means that the “land” between the holes is large. These conditions influence the design of the extrusion channels. The cheapest execution of an orifice, the straight cylindrical bore, is very seldom applied. Fig. 8.37 depicts six commonly used designs of extrusion channels for medium-pressure agglomeration; in all cases the direction of extrusion is from top to bottom. As shown in Fig. 8.37a, in the simplest case there is at least a small inlet chamfer to compensate for the large land area that is necessary between the orifices; it serves to guide the material into the extrusion bore. Sketches 8.37b and 8 . 3 7 ~indicate that this feature may be more pronounced and varied to fit specific applications. Because the die must be thick, an extrusion channel through the entire body may be too long, the channel
8.4 Pressure Agglomeration Technologies
Fig. 8.37: Sketches of six typical extrusion channel designs for medium-pressure agglomeration.
(d)
(e1
If1
ratio may be too large, and, as a result, the frictional resistance may be too high. In such a case, the orifice length is reduced by increasing (relieving) the size on the inlet (Fig. 8 . 3 7 ~or ) the exit (Fig. 8.37d). In pelleting, organic materials are often processed which feature a certain amount of elasticity. It is possible that, when such products reach the end of the orifice, some of the initial elastic deformation has not yet transformed into permanent plastic deformation. If this is the case, the green densified pellet strands expand when they leave the extrusion channel (see also Section 8.1, “elastic springback). If the channel exit is designed with a sharp edge, the sudden expansion results in cracking and the pellets exhibit a surface structure that is called “Christmas tree shape”; the cracks caused by the expansion upon exiting the bores impart a very ragged pellet surface which resembles the shape of a pine (Christmas) tree. This “defect” can be avoided if, instead of ending with a sharp edge, the exit is tapered (Fig. 8.37e),thus allowing a controlled expansion with no cracking. Sketch 8.37fis a channel with inlet chamfer, tapered exit, and relieve bore. Fig. 8.38 is used for a more detailed description of all die hole characteristics. The diagram assumes that the die body is a cylindrical ring with thickness T and that the direction of material flow is from the inside of the ring die to its outside (for more details on this design, see below). As mentioned before and as will be further discussed in this section, dies may be also flat or machined into a gear shape and, in cylindrical or gear shaped ring dies, the flow of material may be also from the outside to the inside. Referring now again to Fig. 8.38, with exception of the elastic recovery or “springback, d represents the pellet diameter and L is the effective length in which work is performed on the material during extrusion. T is the total, overall thickness of the die body which has been selected to withstand all the stresses that may develop within the equipment. Xis the counter bore depth; it reduces the die thickness T to the effective channel length L. The counter bore may feature a tapered part with angle B to obtain a gradual elastic expansion of the exiting strand and avoid structural defects on the surface of the pellets. Other counter bores have only a cylindrical bottom; this design, or straight bores with no counter bore, can be used for plastic materials with no or negligible elastic expansion. The tapered inlet (or chamfer) from diameter D to channel diameter d with angle @ is required to either increase the open area without sacrificing the strength of the die body and/or to obtain additional compression according to D2/ d2. The first effect is particularly important for fibrous material which may produce matting and ultimately cause clogging of the die if too large land areas exist between
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Inside diameter
Fig. 8.38:
B
Die hole characteris-
tics [B.42].
the holes. The second reason for a tapered inlet may be dictated by the need for a high overall densification of feeds with low bulk density. Dies deteriorate by “washing out” the inlet and discharge areas as well as the cross section of the extrusion channels. This is due to the high friction between the extruding mass and the walls of the orifices. Die life is determined by the increase of channel cross sections to such dimensions that either the product size is no longer acceptable or the back pressure due to decreasing frictional resistance becomes too low. In the latter case, compression is reduced and, therefore inadequate product density and/or strength are obtained. If neither pellet size, density, or strength are critical, the limiting die life is defined by the decreasing structural integrity of the die body. Particularly if extrudates with small cross section and high density and/or strength must be produced, the necessary thickness of the die body and the effective length of the extrusion channel may be rather incompatible. In this case, replaceable insert plates with the required short length of the bores may be used (Fig. 8.39a). Other inserts may be utilized (Fig. 8.39b) if extrusion channels have worn out to salvage the massive, expensive die body which, for strength reasons, is often made from forged high quality steel. On the left side of Fig. 8.39, in both cases the inserts are pictured as being installed in gear type dies. However, it is obvious to those skilled in the art that the principle is universally applicable in medium-pressure agglomeration by extrusion. If thin inserts are used in thick die bodies to obtain short bores for extrudates with small cross section (Fig. 8.39a), there is sometimes a problem in discharging the pellets from the recessed die plates if the product is too sticky and does not easily break off. The formulation may have to be adapted for successful operation.
8.4 Pressure Agglomeration Technologies
(b)
Fig. 8.39 Replaceable inserts for medium-pressure agglomeration by extrusion with (a) short and (b) long bores (courtesy HOSOKAWA BEPEX/Hutt. Leingarten, Germany).
Of course, in the case of inserts for wear replacement, any bore including a single channel, for example as depicted in Fig. 8.38, can be applied. Fig. 8.40 depicts the basic principle of medium-pressure agglomeration by extrusion or “pelleting”. Although sketch (a) shows the situation obtained between an internal press roller and a perforated ring die and (b)represents a press roller moving on a flat extrusion die, the following discussions are valid, with corresponding adjustments, for all arrangements of medium pressure extrusion. In Fig. 8.40, in both cases a cylindrical pressing tool rolls over a layer of material that was deposited by some feeding and distribution means on a perforated (only a few holes with simplified design are shown) support (= die). In the wedge-shaped nip, material is first compressed and then extruded through the holes. Fig. 8.40b includes some additional information. First, it should be recognized that the nip geometry between the roller (1)and the flat die (2), characterized by (3), (4),and (S), is different from that depicted in Fig. 8.40a where the roller is within a ring-shaped die. Because of nip geometry, more volume is densified in the case of a flat die and, if two perforated hollow rollers contact each other and form a nip (see below, Fig. 8.42c), this volume and its change during rotation is even greater. These are the types of adjustments that have to be made when applying the principles to different arrangements and designs. With the changing nip volume and compression rate, the pressure in the material as well as on the roller and the die, characterized by the curve 3-m-6which is equivalent to the curve that was first presented in Section 8.1, Fig. 8.1, also becomes different. The material layer is first densified between (3) and (4).In this regime, the increasing pressure is still smaller than the frictional resistance in the bores and no extrusion takes place. At point (4)the static frictional force is overcome and the maximum pressing force is reached (m).Between (4) and (5) extrusion occurs through the die holes and the pressure remains almost unchanged (see Section 8.1, Fig. 8.2b [although representing the conditions in a ram extruder (see Section 8.4.3), the conditions in the channels of medium pressure extruders are similar]). At the point of closest approach
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Fig. 8.40
Sketches explaining the basic principle of medium-pressure agglomeration by extrusion [B.42]. For explanations see text.
(5) a gap remains and, therefore, a predensified layer of feed covers the extrusion die which helps to avoid damage by metal to metal contact. Later, this coating serves to obtain optimal drag of and improved bonding with the new feed and to reach better densification. As already discussed in Section 8.1, if materials to be pelleted exhibit a certain elasticity, the residual layer expands between (5) and (6). The curve 3-m-6 represents a typical profile of all the forces that act in the compression, extrusion, and expansion zones. Because of product and equipment design considerations, the forces that can be exerted on the mass to be pelleted are higher than those in low-pressure agglomeration but are still relatively small. Therefore, binders and lubricants (see Section 5.1.2) play an important role for the technology and the product is normally not highly densified.
8.4 Pressure Agglomeration Technologies
Fig. 8.41: Sketches depicting the nip area of a medium pressure extruder with concave die and press roller explaining the forces at work [B.42]. For explanations see text.
Overfeeding is a common problem of medium-pressure agglomeration equipment. If, as shown in Fig. 8.41a for a concave die, the thickness of the layer of fresh material in front of the pressing tool increases from TF to 2TFas the feed rate is doubled, the force component which is directed forward and tends to push material away increases and the downward (compression and extrusion) force decreases. As a result of these conditions feed material may build up in front of the roller to a point where the pressing tool can no longer entrain it. The entire die cavity may then fill with material and the equipment will plug up. The same can occur in a fluctuating feed situation even if the machine is only temporarily overfed. This effect decreases for the flat die (Fig. 8.42a), is even less pronounced in machines with convex rings (Fig. 8.42c),and almost never occurs with gear type dies (see Fig. 8.66).
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Fig. 8.42 Schematic representations o f the three major designs of medium-pressure agglomerators also called “pelleting machines” or “pellet mills”. (a) Flat, (b) concave, and (c) convex die configuration.
Nevertheless, machines with concave die rings and internal press rollers do have advantages. For example, if the feed material exhibits a certain elastic behavior, because the forces in the relatively long and slender nip increase slowly, a more complete conversion of temporary elastic into permanent plastic deformation takes place. Fig. 8.41b is another presentation of the forces at work. Feed, ideally deposited in a uniform layer on the die, is pulled into the space (nip) between roller and die and compressed. Friction between roller, die, and material as well as interparticle friction in the mass are responsible for the “pull” of the feed into the nip and for densification. Smooth surfaces of roller and/or die may result in slip. Axial grooves in the roller, which may also favor build-up of a thin layer of material, and the above mentioned residual layer of densified feed on the die effectively reduce slip. Low interparticle resistance to flow or a distinct plasticity result in a more or less pronounced tendency of the mass to “avoid the squeeze” (back-flow),thus reducing densification and potentially choking the machine (see above). In medium-pressure agglomeration equipment (Fig. 8.42), the perforated support (die) can be either flat (a), concave (b),or convex (c).For all three designs, intermeshing, toothed executions have been proposed in the patent literature [B.42] to avoid slippage and improve extrusion as well as extrudate quality, but only the so called “gear pelleter”, in which the convex dies (Fig. 8.42~)are hollow gears with extrusion channels between the teeth, has reached commercial importance (see below). In the following the different, commercially available equipment and some of their characteristics are described in more detail. Often, these machines are called pellet mills.
8.4 Pressure Agglomeration Technologies
Machines With Hollow, Perforated Cylinder(s) and Feed From the Outside The prin-
ciple of these machines is shown in Fig. 6.4b.3 and b.4 (Chapter 6). Although, in the historic literature, equipment featuring the design of Fig. 6.4b.4 is mentioned, today’s commercial offerings are limited to the execution depicted in Fig. 6.4b.3. Fig. 8.43 is a current schematic sketch as published by the manufacturer. Contrary to the drawing of Fig. 6.4b.3 the press roll is neither solid nor of the same size as the perforated die cylinder. As shown in a photograph ofthe two operating parts of such a machine (Fig. 8.44), the hollow press roller is a cylinder with roughened surface and features a diameter that, in most cases, is somewhat smaller (between approx. 81 and 87 %, see Tab. 8.2) than that of the perforated die. It is claimed that the difference in circumferential speed, that causes a certain amount of shear in the nip, improves the extrusion characteristics and, thereby the quality of the granulated product. The working tools (Fig. 8.44) of the Alexanderwerk “moist granulator” are two counter-rotating cylinders. One is perforated and acts as a die while the press cylinder is solid. Because the press roll is normally hollow, it can be equipped for cooling or heating. Feed is fed from above, mostly by gravity, and pressure is build up in the nip. When the pressure is high enough to overcome the static frictional resistance in the bores, the densified moist, plastic material passes through the appropriate perforations and extrudates are formed within the die cylinder. A scraper plate, located inside the perforated cylinder, cuts the cylindrical ropes into granules. Fig. 8.45 is the front view of such a machine, also showing the scraper plate support extending into the open die. A common problem of all ring die extruders in which material passes from the outside to the inside of a cylinder was and is that the extruded material must reliably discharge from the inside of the die without becoming entangled and/or stuck. To allow successful extrusion, the particulate mass must be moist and, to produce
Fig. 8.43: Schematic o f the Alexanderwerk “moist granulator” (courtesy Alexanderwerk, Remscheid, Germany).
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8 Pressure Agglomeration Technical data of ‘standard’ “moist granulators” (according to Alexanderwerk, Remscheid, Germany).
Tab. 8.2:
~
~~
Parameter
Working length Die diameter Orifice diameter Press roll dia. Throughput Drive power Approx. weight
Model
Unit
mm mm mm mm kg/h kW kg
CA 65
C1/100/1605 C1/148
80 70 1-5 60 30-50 1 90
160 110 1-8 90 100-500 4
420
160 110 1-8 186 100-500 4 800
C1/168
Gl/244
250 180 2-10 156 500-1.000 10
240 270 2-10 218 1,000-3,000 15 1,300
1,100
strength, it must exhibit good binding characteristics. Although cut into short lengths, sticking of the product granules to each other, to the scraper blade support, and to the inside of the die cylinder is a definite possibility, particularly if the die cylinder is small in diameter. As shown in Fig. 8.46, to facilitate discharge, the machines are typically mounted on a slanted support such that gravity assists in product removal from the die interior. Because the nip between the rollers is fed by gravity, a too steep mounting platform will preferably feed orifices near the front of the die cylinder which causes an increase in the variation of extrudate length. Therefore, a compromise must be found between good feed distribution and acceptable discharge when designing the sloping support structure. A further problem of all ring die extruders in which the material is fed from the outside and passes to the inside is that at the closest line of approach between the
8.4 Pressure Agglomeration Technologies
Fig. 8.45: Front view of an Alexanderwerk “moist granulator”, also showing a view into the die cylinder and the scraper blade (courtesy Alexandewerk, Remscheid, Germany).
roller and the die, theoretically, there should be no gap to avoid that material extrudes through this space or is compacted without entering the extrusion channels or simply leaks through. However, in reality, a gap can not be avoided. First, from a design point ofview, metallic contact between the two tool parts must be avoided. Second, clearance in the support bearings opens up a small gap when the operating pressure is acting on the two cylinders. Third, even though the operating pressure is low, because both the roller and the die are hollow cylinders, it is conceivable that some small deformation occurs in the area of highest force which coincides with the line of closest approach. Furthermore, over time, both the roller and die will wear which increases the gap that exists by necessity due to the three previously mentioned reasons. As a result, as sketched in Fig. 8.47, some material will always pass in between the two cylinders and collect below. This leakage or production of “fines” will increase with time due to wear. Provisions must be made to clean out the housing in regular intervals or to recirculate the leakage to the feed mixer/conditioner for reprocessing. In some cases, where the presence of fines in the product is not a problem, the small amount (typically <10 %) of this off-grade material can also be combined with the product. Because of the need to keep the clearance between the two cylinders to a mini-
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Fig. 8.46: Three “moist granulators” on a common base (courtesy Alexanderwerk, Remscheid. Germany).
mum, they are mounted in fmed relative positions. If a larger piece of tramp material enters the nip between the cylinders (which should be avoided at all cost, for example by sizing the feed just before it enters the equipment), overload protection can be only achieved by stalling the drive. Nevertheless, if this should happen, the perforated cylinder is often damaged. Although this part is easily replaced, the spare extrusion die is costly. In spite of the problems that are associated with this design, it is often applied, particularly in clean environments and for easily deformable materials which require small extrusion pressure. Tab. 8.2 lists the “standard” models offered by one manufacturer and summarizes their most important technical details. Among the mediumpressure agglomerators or pellet mills, the Alexandenverk “moist granulator” represents equipment that operates with the lowest forces. In Fig. 8.48 three examples of products are shown. As can be easily seen, granules or pellets can be well formed (a and c) or somewhat crumbly (b). If the latter is not acceptable, the extrusion characteristics of the feed may be adjusted by changing (in this case increasing) the moisture, binder, and/or lubricant contents.
Fig. 8.47: Sketch explaining the operation of a medium pressure extruder i n which a feed material is densified on the outside o f a cylindrical extrusion die and passes into the interior. For further explanations see text.
8.4 Pressure Agglomeration Technologies
Fig. 8.48 Three examples ofproducts that were manufactured with the Alexanderwerk “moist granulator”, (a) 3 mm dia., (b) 4 mm dia., (c) 5 mm dia (courtesy Alexanderwerk, Remscheid, Germany).
“Pellet-Mills” With Hollow Perforated Cylinder and Feed From the Inside This design represents the most commonly installed medium-pressure agglomeration equipment (see Chapter 6, Fig. 6.4b.5). It is primarily used in feed mills for the granulation of animal feed and associated products. A large number of vendors in many countries manufacture and offer these machines. Fig. 8.49 is another schematic representation, published by one of the manufacturers (CPM), showing the typical design of a pellet mill with cylindrical die and two internal press rollers. The execution suggested by Fig. 6.4b.5 (Chapter G), 8.40, and 8.41 with one press roller is only used in laboratory and small production machines. For structural and process reasons, the perforated concave die can not be very wide (Fig. 8.50). Therefore, to increase the capacity of a given press and more uniformly distribute the forces acting on the ring, up to three rollers (see below, Fig. 8.52) are installed. With two rollers (Fig. 8.49) the capacity of a machine doubles as compared with a single roll pellet mill and triples with three rollers; from a ring loading point of view, the latter also results in the best distribution of the pressing forces.
Fig. 8.49 Schematic representation by one o f the manufacturers (CPM) of the operating principle o f medium pressure extrusion in a “pellet mill” with ring die and internal press rollers.
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Fig. 8.50 A selection o f typical concave die rings for pellet mills with internal press roller(s) (courtesy Sprout-Matador, Muncy, PA, USA).
It is clear from Fig. 8.49, that pellet mills with ring dies and internal press roller(s) have the great advantage over the previous design, in which the material passed from the outside to the inside of the cylindrical die, that the pellets exit on the outside periphery of the ring. Therefore, they can be easily cut and discharged and no leakage of feed material occurs. However, to obtain uniform pellet quality, minimize pellet length variations, avoid uneven die wear, and ascertain constant power demand, it is necessary to distribute the feed evenly across the entire working width (= perforated area) of the die. Since the particulate feed can only enter the operational area of the machine from the open front of the die ring and, additionally, the interior is to a large extent occupied by the press rollers, this requirement is not easily met. Fig. 8.51 is a partial cut through a pellet mill showing the most important internal parts as designed and offered by this particular manufacturer. The machine is a directly gear driven model. For pellet mills that are used in the animal feed and similar industries, it is rather common that the feeding arrangement includes a conditioner (top), in which the different components are mixed and the feed characteristics are adjusted to exhibit optimum plasticity and extrusion properties by adding moisture (binder and/or lubricant) as well as steam (for example, for the activation of starchy ingredients by heating and moistening during condensation). The conditioned mass is then fed to the operating area of the pellet mill. In Fig. 8.51, a special feeder is used to overcome most of the previously mentioned distribution problems. The feed enters this “centri feeder” after, as a safety provision, a magnet in the feed chute catches any tramp metals. A transport device with a screw and paddles is used to advance an annular flow of material uniformly to the entire conical cover area of the die (Fig. 8.52). As shown in the schematic and in the photograph, adjustable plows, that are located between the (stationary) press rollers, divide the advancing feed stock into equal portions and direct it evenly in front of each press roller. For easy maintenance and cleaning, all modern pellet mills are equipped with a hinged door in the front of the machine which can be opened to access the internal machine parts (Fig. 8.53). To also meet the demands of short production runs, a “quick change pelleting cartridge” (Fig. 8.54), which contains the die housing, the die, the
8.4 Pressure Agglomeration Technologies
Fig. 8.51: Partial cut through a directly gear driven pellet mill (courtesy Sprout-Matador, Muncy, PA, USA). For further explanations see text.
Fig. 8.52: Schematic and photograph o f a "Centri-Feeder" distributor (courtesy Sprout-Matador, Muncy, PA, USA). See text for explanations.
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conical die covers, the roller assemblies with the feed plows and the main shaft, can be removed as a whole in directly gear driven machines (Fig. 8.51) and replaced. This is important if, when changing products, no cross-contamination is tolerated (for example, in the case of medicated animal feed) or different pellet sizes and/or shapes are required. Fig. 8.55 shows a ring die and three press rollers with different surface configurations. The surfaces of the rollers are roughened to improve the drag and make the pressing tools roll on the material. If the material is too slippery and/or the rollers are too smooth the material to be pelleted will not be entrained and the equipment may stall. If this tends to happen with a particular material or application, it is preferable to select a pellet mill with V-belt drive rather than a directly gear driven machine, as slippage of the belts may act as additional safety precaution. Often the rollers are also furnished with an abrasion resistant coating to extend their life. Pellet dies should resist wear, corrosion, and breakage. Because the extrusion bores must be machined economically, selection of the material of construction to meet all require-
Fig. 8.53: Open door view of a pellet mill demonstrating easy accessibility for cleaning and maintenance (courtesy SproutMatador, Muncy, PA, USA).
Fig. 8.54 "Quick change pelleting cartridge" on a transport and mounting cart (courtesy SproutMatador, Muncy, PA, USA).
8.4 Pressure Agglomeration Technologies
Fig. 8.55: Ring die and three press rollers with different surface configuration (courtesy CPMRoskamp Champion, Waterloo, IA, USA).
ments is limited. Nevertheless, the use of modern alloyed steels, which are selected with a particular application in mind, has increased the life expectancy of dies considerably, particularly if also good equipment maintenance is provided. Tab. 8.3 lists, as examples, the technical data of one manufacturer of pellet mills. Rather typical equipment characteristics are: die inner diameters (ID) from 419 to 1,143 mm, working widths from 79 to 360 mm, pelleting surface areas from 0.1 to 1.29 rn’, number of press rollers: (l), 2, or 3, roller outer diameters (OD) from 206 to 457 mm, and power requirements from 160 to 620 kW.
Fig. 8 . 5 6 Typical cylindrical animal feed pellets.
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Tab. 8.3:
Parameter
Unit
Characteristic
Comment
149 Ultra-V All grease 419 or 533 79-152 0.1 -0.26 2 or 3 254(2) or 206(3) Gravity Centrifeeder
Max. allowable Single reduction V-belt (No messy oil changes)
186 Direct gearing Cartridge Tapered die fit Gearbox Main shaft 533 108 or 152 0.18 or 0.26 2 or 3 254(2) or 206(3) Gravity Centrifeeder
Max. allowable Single reduction helical Optimum wear Inherent piloting effect Circulating oil cooled/filtered Grease
375 V-Belt
All grease
Max. allowable Single reduction Eliminates belt pull on motor shaft Permits use of std. short shaft motors Inherent piloting effect (No messy oil changes)
660 or 812 102 - 305 0.21-0.78 3 254 or 305 Centrifeeder Heavy duty Electric hoist
For severe stress operation Optional
Model V-200 Series - V-Belt Drive Pellet Mills Main drive motor Drive type Lubrication method Die sizes
Number of rollers Roller size Die feeding
kW
ID Operating width Working area
mm mm m2
OD 2 roll design 2/3 roll design
3 minimize stress
Model 21 - 250 Series - Gear driven Pellet Mills Main drive motor Drive type Quick die change
kW
Lubrication method Die sizes
Number of rollers Roller size Die feeding
ID Operating width Working area
mm mm m2
OD 2 roll design 2/3 roll design
mm
3 minimize stress
Model V-500 Series - V-Belt Drive Pellet Mills Main drive motor Drive type Jackshaft design
Quick die change Lubrication method Die sizes
Number of rollers Roller size Die feeding Mechanical design Maintenance
kW
Tapered die fit ID Operating width Working area OD
mm mm m2
8.4 Pressure Agglomeration Technologies Tab. 8.3:
continued
Parameter
Unit
Characteristic
Comment
375 Direct gearing Cartridge Tapered die fit Gearbox Main shaft 660 or 812 108-305 0.22-0.78 2 3 254 or 304 Centrifeeder Cast Electric hoist
Max. allowable Single reduction helical For 660 m m ID dies For 660 or 812 mm ID dies Circulating oil cooled/filtered Grease
Model 500 Series - Gear driven Pellet Mills ~~
~
kW
Main drive motor Drive type Quick die change Lubrication method Die sizes
ID Operating width Working area
mm mm m2
Number of rollers Roller size Die feeding Mechanical design Maintenance
OD
Light duty Heavy duty
Internals: heat treated steel Optional
Model 800 Series - Gear driven Pellet Mills kW
Main drive motor Drive type Quick die change Lubrication method Die sizes
Number of rollers Roller size Die feeding Mechanical design Maintenance
ID Operating width Working area OD
mm mm m2
597 Direct gearing Tapered die fit Gearbox Main shaft 812 or 1,143 203 - 360 0.52- 1.29 3 305 or 457 Centrifeeder Cast Electric hoist
Max. allowable Single reduction helical Inherent piloting effect Circulating oil cooled/filtered Grease
Internals: heat treated steel Optional
All features depend on the application and the feed properties. Capacities, although strongly influenced by the material as well as the pellet size and shape, may be as low as a few hundred kg/h and, on the high end, exceed 80 t/h. As visible from the dies shown in Fig. 8.50, most of the extrusion channels in pellet mills are cylindrical bores; however, as depicted in the center of this Fig. (8.50), square openings (and others) are provided for specific applications (for example, the manufacturing of catalyst carriers). As a typical product of pellet mills, Fig. 8.56 presents some cylindrical animal feed pellets.
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Equipment Using Flat, Perforated Die Plates and Press Rollers The above general designs of medium-pressure agglomerators (extruders) “suffered” from a potential lack of structural integrity if larger and/or denser and stronger pellets had to be produced, from feeding difficulties when trying to distribute the mass to be agglomerated evenly, and, sometimes, from leakage of “fines” as well as a limited possibility to control the densification of the feed and influence the extrusion process. Equipment that uses flat, perforated dies and press rollers can overcome most of these shortcomings. The basic principle of flat die pelleting machines was already shown in Chapter 6, Fig. 6.4b.2 and above, Fig. 8.40b as well as Fig. 8.42a. Fig. 8.57 is the photograph of some flat dies and of several press roller arrangements. The drawing in Fig. 8.58 demonstrates how the individual items fit together. As can be deduced from both figures, the machine may be constructed in a very heavy duty execution which allows the build-up of high pressures for the production of highly densified pellets, even from difficult to extrude materials. The perforated flat dies can be quite massive with good structural support. The roller arrangements consist of a minimum of two rollers. With larger machines, this number can increase to a max-
Fig. 8.57: Photograph depicting several flat dies and different press roller assemblies (courtesy Amandus Kahl, Hamburg, Germany).
Fig. 8.58 Drawing of the arrangement of the operating parts ofa Kahl flat die pelleting machine (courtesy Amandus Kahl, Hamburg, Germany).
8.4 Pressure Agglomeration Technologies
Range of technical characteristics of flat die pelleting machines [not including low pressure applications (Section 6.4.1) with perforated thin sheet dies] (according to Amandus Kahl, Hamburg, Germany).
Tab. 8.4
Parameter ~~~~
Unit
Characteristic
mm mm mm mm
175 - 1,250 125-1,026 22-200 130-450 2-6 90-6,220 20-30 2-40 20 - 200 0.6-2.7 50-160 2-400 5-80 50-> 30,000
~
Outer diameter of the flat die Medium roller track diameter Track width = Roller width Roller diameter Number of rollers Open track area = Z hole area Specific open track area Extrusion channel (= hole) dia. Flat die plate thickness Circumferential roller speed Speed of roller assemblies Motor power (press only) Specific press motor power Throughput (dep. on product)
-
cm2 cmz/kW mm mm
mls rpm of main shaft kW kW/t kg/h
-
imum of six. As the number of rollers is directly proportionate to the capacity (throughput)of the machine, depending on the material and the pellet diameter, production rates per machine can reach more than 30 t/h. Including a laboratory press, the drive motor power may be between 2 and 400 kW. Tab. 8.4summarizes the range of technical characteristics of flat die pelleting machines. Fig. 8.59 is the partial cut through a Kahl flat die pelleting machine with explanations. Because the principle of rollers running on the bottom of a flat pan has been known for grinding long before the flat die pelleting machine was invented and, in milling, this device was called “pan grinder” or “muller” the press rollers are sometimes identified as “pan grinder rollers” or “muller wheels”. Referring to Fig. 8.59, feed (called product) enters the machine at the top and falls down by gravity. The flat top of the hydraulic roller adjustment device rotates with the speed of the roller assembly and diverts the material into the annular space that is bordered by the cylindrical guide plate. In some cases a cone is mounted on the top to avoid build-up of material. The feed, thus diverted, falls rather evenly onto the rollers and the track of the die, i.e. that portion of the plate that is perforated and over which the pressing tool rolls (see also Tab. 8.4),so that uniform densification and extrusion occur. A scraper removes feed that may have been pushed to the housing wall and directs it back to the track. Below the die plate, knives (called cutting devices) rotate with the roller assembly and shear the protruding strands into pellets. The product falls on a plate, is collected by a wiper arm and deposited into a discharge chute through which it leaves the machine. Fig. 8.59 shows the drive situation that is most often used and is applicable for “normal” materials which form discrete pellets with little stickiness. The main shaft that carries and rotates the wiper arm, the cutting devices, the roller assemblies, the scraper, and the hydraulic roller adjustment is driven by a worm gear from below.
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Fig. 8.59 Partial cut through a Kahl flat die pellet press (courtesy Amandus Kahl, Hamburg, Germany). Further explanations see text.
If the material to be extruded is very moist and/or sticky the cutting device would lump the strands into globs of wet material and the discharge plate with wiper bar would further aggravate this situation. A special design is available in which the main shaft is driven from the top. Then, the wet strands fall directly into a dryer or on a belt, thus minimizing aggregation (Fig. 8.60). As in all medium-pressure agglomerators that use rollers for the densification and extrusion of moist and/or plastic materials, the primary cause for roller rotation is friction between the roller surface and the material to be processed. The roller is mounted on a shaft with sealed antifriction bearings and is brought to close proximity
8.4 Pressure Agglomeration Technologies
Fig. 8.60 Top driven Kahl flat die pelleting machine for the extrusion o f wet or pasty materials. (a) Schematic, (b) discharge ofwet strands during operation, (c) press model 24-390 featuring an overhead drive (courtesy Arnandus Kahl, Hamburg, Germany).
with the die plate. Feed is wedged between the roller and the die (see Fig. 8.40and 8.41)and the roller rotates due to the friction between its surface and the material. Roller rotation is necessary for proper operation (see above). It has been discussed previously, that, for several reasons, a gap is provided between the roller and the die. An important process feature is the layer remaining after extrusion which binds easily with fresh feed and is predensified thus increasing pellet density and, with it, quality.
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Fig. 8.61: Sketch of the Kahl hydraulic roller adjustment device (“hydraulic nut”). O n the left, the roller has been lifted to maximum clearance and, on the right, hydraulic pressure has reduced the gap against the force of the springs (courtesy Amandus Kahl, Hamburg, Germany).
In Kahl flat die pellet presses, a unique feature is the hydraulic roller adjustment device. As depicted in Fig. 8.61, the roller assembly can slide up and down the main shaft and is supported by strong springs. A so called “hydraulic nut”, a hydraulic cylinder arrangement, with which the rollers can be pushed down or lifted, is installed at the end of the shaft. This allows to adjust the clearance or gap between the rollers and the die to optimize densification and extrusion characteristics for different materials and to modify the thickness of the predensified layer after the extrusion step. Since adjustments can be made anytime, even during operation, it is possible to optimize press performance as needed. To avoid slip ofthe rollers, the surface is axially grooved as shown in Fig. 8.62. Other surface configurations are also available; all serve to increase the drag and avoid slippage. For some difficult to process, very slippery materials rotation of the rollers can not be reliably guaranteed. In those cases, the rollers can be driven by a direct bevel gear drive (Fig. 8.63). Obviously, a vertical roller adjustment is not possible with this drive arrangement and, therefore, is not shown in Fig. 8.63. If a cylindrical pressing tool rolls over a circular track, shear is caused in the material that is wedged between the roller and the die because the circumferential speed of the main shaft rotation only matches that of the roller(s) at one point of the roller surface. The inside edge turns faster and the outside edge slower than the overall rotation of the tool. This additional shear may be of advantage for some materials to be processed, but for others, for example those exhibiting thixotropic properties, this speed gradient must be avoided. The uneven speeds also cause more wear than would be experienced with a uniform movement. To overcome this problem, conical press rollers are used when necessary (Fig. 8.64). As shown in the schematic these rollers as well as cylindrical ones can be equipped with channels for cooling with a liquid. Another, rather unique process feature of flat die pellet presses is that excessive amounts of liquid can be mechanically removed from the feed during the predensification step. By providing a space where liquid can collect within the housing at the periphery of the flat die and installing drain lines, some liquid can be squeezed out. Although this is not a major application for this type of equipment it is worth mentioning.
8.4 Pressure Agglomeration Technologies
Fig. 8.62: Photograph o f a Kahl flat die with the hopper housing removed showing a roller arrangement with four grooved rollers, the die plate, the scrapers, and the hydraulic nut (courtesy Amandus Kahl, Hamburg, Germany).
Tab. 8.5 lists the technical data of machines that are offered by a manufacturer of flat die pellet presses. For ranges of typical equipment characteristics, reference should be made to Tab. 8.3 above. While this type of equipment is also widely used for the production of animal feed, the ability to exert higher forces and the availability of special machine features (not all of which have been mentioned in this section) make these pellet presses amenable to applications that can not normally be handled with the ring die models which were discussed earlier. Of particular interest in this respect are many difficult to handle waste materials that need to be transformed into a relatively large particulate shapes for reuse as secondary raw materials. Fig. 8.65 shows a few examples.
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Fig. 8.63: Photograph and schematic o f a positively driven roller arrangement for the processing o f slippery materials (courtesy Arnandus Kahl, Hamburg, Germany).
Pellet Presses With Gear-Shaped Press Rollers and Feed From the Outside So far, all medium pressure extruders or pellet mills had in common, that the feed material is able to “avoid the squeeze” and, as a consequence, will not always extrude, resulting in clogging and stalling of the machine. Different surface configurations of the pressing tools are used to increase the drag into the nip and changes in the feed formulation may be made to render the material less slippery. However, in any case there is a more or less pronounced shear and back-flow in the nip that causes frictional heat which may be objectionable and normally can not be controlled by cooling the operational parts, which is possible with some machines. In the patent literature, several toothed, intermeshing pressing tool and die arrangements have been proposed [B.42]. However, to the knowledge of the author, only one has gained commercial importance and is offered as standard equipment. Fig. 8.66 shows as a schematic, published by the manufacturer, what had already been presented in Chapter 6, Fig. 6.4b.6 and mentioned several times before. This pellet mill, the so called gear pelletizer, features two intermeshing hollow gears with large modulus and extrusion channels at their roots between the teeth. Feed enters by gravity from the top, is caught by the teeth, entrapped between the teeth, densified and then extruded into the gear’s interior by the punch-like action of the opposing tooth. Inside,
8.4 Pressure Agglomeration Technologies
Fig. 8.64: Photograph and schematic o f a roller arrangement with conical pressing tools (courtesy Amandus Kahl, Hamburg, Germany).
protruding strands are often cut into pellets by scraper blades or they break under their own weight. Because of the positive feeding and displacement that is caused by the teeth, the material to be processed experiences very little shear due to back flow. Therefore, the gear pelletizer is ideally applied for temperature sensitive and thixotropic materials as well as for low melting point and waxy products. Widely used for the pelleting of rubber chemicals, foodstuffs, and pharmaceuticals, it is also particularly effective in applications requiring medium pressure and small diameters (see also Fig. 8.39 and corresponding text). Smaller extrudates may be spheronized to obtain final agglomerate shape. As shown in Tab. 8.6, for manufacturing and process reasons, the gear diameters are relatively small and the working width is limited due to potential difficulties in discharging extruded products from the depths of wider hollow gears. Throughput capacity is also relatively low because only one extrusion occurs per revolution of the gear. In this respect the H U T gear pelletizer is similar to the Alexandenverk moist granulator (see above). With some feeds it can also not be excluded that, after extrusion, some material drops from the space between the teeth, thus requiring periodic removal of densified fines from under the machine. As depicted in Fig. 8.67 the gears are connected to a double output-shaft gear reducer which is normally powered
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8 Pressure Agglomeration Technical data o f pellet presses with rollers and a flat die (according to Amandus Kahl, Hamburg, Germany).
Tab. 8.5:
Parameter
Model
Unit
Designation Die diameter Roller diameterlwidth Number of rollers Power of drive motor Roller speed Perforated die area Approx. machine weight
mm mm kW m/s cm2 kg
14-175
17-250
24-390
25 - 500
Laboratory 175 130127
Small Prod. 250 160135
L
L
3
7.5 1.27 203 450 with motor
Overhead 390 200175 2 37 2.11 617 1,800 w/o support
Overhead 5 00 200/75 4 37 2.7 844 1,800 w/o support
38-600
38-780
0.5-1.3 106 260 with motor
33-390 or -500 34-GOO ~
~
Designation Die diameter Roller diameterlwidth
mm mm
Number of rollers Power of drive motor Roller speed Perforated die area Approx. machine weight
kW m/s cm2 kg
Designation Die diameter Roller diameterlwidth Number of rollers Power of drive motor Roller speed Perforated die area Approx. machine weight ;k
mm mm kW m/s cm2 kg
~
~
~~
~
Standard 390 or 500 230177
Standard 600 2801102
Standard 780 280/102
2 or 3 or 4 15-30+< 2.2 617 or840 990 or 1,300 with motor
3 or 4 45 - 553:” 2.7 1,382 max. 2,430 with motor
3 or 4 55-75” 2.6 1,382 max. 2,430 with motor
Standard 780 280 or 3501 102 4 or 5 90-100” 2.6 1,916 3,400 w/o motor
37-850
39-1000
45-1250
60-1250
Standard 850 3501130 3-5 132” 2.5 2,695 4,600 w/o motor
Standard
Standard 1,250 450/192 or 1156 4 or 5 200-250” 2.7 5,900 9,000 w/o motor
Double drive 1,250 4501192 4 or 5 2.160- 200” 2.6 5,900 9,370 w/o motors
Motor rpm 1,500, other speeds possible;
$<+<
1,000 450/156 3-5 160- 200“ 2.6 5,400 5,400
w/o motor
Motor rpm 750
by a mechanical (shown) or electrical variable speed drive. The gears are mounted to the cantilevering shafts (CS models) which, for larger extrusion forces, are held parallel by additional support bearings in front (as shown in Fig. 8.67),further obscuring the discharge area. The largest gear pelletizer is built as a mill shaft design (MS) machine (for details on such designs see Section 8.4.3, roller presses).
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8 Pressure Agglomeration Tab. 8.6 Some technical data of standard “Gear Pelletizers” (according t o Hosokawa Bepex/HUm, Leingarten. Germany). Parameter
Gear diameter Gear width Drive power Weight
Model
Unit
mm
mm kW kg
CCS200/40
CCS200/80
CCS300/80
CCS300/120 CMS300/200
200 40 4
200 80
300 80 11 3,000
300 120 15 3,200
800
7.5 1,000
300 200 22 3,800
Fig. 8.67: Photograph of a typical CS model “gear pelletizer” (courtesy Hosokawa Bepex/ Hum, Leingarten, Germany).
Medium Pressure Axial Screw Extruders Axial screw extruders normally operate with low (see Section 8.4.1) or high (see Section 8.4.3) pressure. The basic principle of an axial screw extruder was shown in Fig. 6.4b.l (Chapter 6) and Fig. 8.24 (Section 8.4.1). The pressure that is developed by the screw(s) depends on the power of the drive and the frictional resistance in the extrusion channel or other discharge device. Axial screw extruders that rely solely on the pressure developed by the rotating screw@)employ hydrostatic pressure as the driving mechanism for extrusion. Such machines generally use high pressure. However, under certain conditions and for specific applications, some can be classified as medium-pressure agglomerators. Three different medium pressure axial screw extruders will be discussed in the following as examples. Fig. 8.68 is a schematic representation of an “Extrud-0-Mix”.It is designed to process plastic masses or generate its own suitable conditions by mixing and working solids and additives (binders and/or lubricants, see Section 5.1.2) prior to extrusion. The “Extrud-0-Mix”features a single horizontal shaft which carries rows of paddles that are arranged in a spiral pattern and move the material(s) to be processed through a cylindrical housing. Stationary blocks (called anvils) attached to the inside wall of the housing are inserted between the rotating paddles. As the blades pass the anvils, a portion of the material is moved forward and the remainder lags behind. The
8.4 Pressure Agglomeration Technologies
blocks also prevent the material from rotating with the shaft so that a continuous kneading and mixing action occurs. Furthermore, as indicated in Fig. 8.68, various orifice plates may be installed within the barrel to increase the uniformity of the feed mixture. Fig. 8.69 is a view into the open housing of an “Extrud-0-Mix”showing the different parts. At the end of the barrel a final die plate is located through which the completely processed material is extruded into pellets with various shapes and sizes. A cutting device may be used to control the length of the product particles. Normally, extrudates are cylindrical with diameters between 0.5 and 6 mm. Several equipment sizes are offered with capacities ranging from approx. 350 kgjh to 4.5 t/h and drive power ratings from 7.5 to 100 kW. Other medium-pressure agglomerators that are offered by several manufacturers and are mostly used in the food and animal feed industries are pressure cooker extruders. They apply medium pressure because the mostly grain and/or vegetable based starchy, organic feeds are conditioned by pressure and heat into easily deformable and extrudable masses. Fig. 8.70 is a drawing depicting the functional components. At the inlet on top of the equipment, the slightly premixed feed components enter first a mixing and predensification screw. Pressure is build up by the changes in shaft diameter, sometimes variable pitch of the screw flights, and by a collar at the end of the screw shaft (see inset in Fig. 8.73, below) or a “pressure piece”, both forming an annular space with reduced area through which the material must pass. Such a screw conveyor, mixer, and processor is often called expander. The so processed material drops into the cooker in which pressurized steam is injected and paddles or screws move the material to accomplish optimal contacting. The
Fig. 8.68 Schematic representation of an “Extrud-0-Mix” medium pressure axial extruder (courtesy Hosokawa Bepex, Minneapolis, MN, USA).
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8 Pressure Agglomeration
Fig. 8.69 Open top view o f an "Extrud-0-Mix" showing the working parts exposed (courtesy Hosokawa Bepex, Minneapolis, MN, USA).
previously mentioned plug at the end of the mixing and predensification screw acts as a dynamic seal so that the cooker is kept under pressure. Fig. 8.70 depicts a single cooker vessel with the indicated dimensions and paddles. However, depending on the capacity and cooking time required for a specific application, the cooker vessel dimensions and/or number (multiple ones are mounted on top of each other, see Fig. 8.73 below) as well as the type of agitators may be changed. By varying the speed of the agitator(s) and the operating pressure in the cooker, almost infinite time/temperature combinations can be obtained. Pressurized steam cooking decreases work and power use, cuts production costs, and increases production capacity as much as 50 % of other extruders that accomplish heating by the conversion of mechanical into thermal energy through friction. The pressure (steam)cooker accomplishes much
Fig. 8.70 Schematic drawing o f a typical pressure cooker extruder (model TME 2000) with main dimensions in feet (') and inches (") (courtesy Sprout-Matador, Muncy, PA, USA) 1' = 0.3048 rn, 1" = 25.4 mm.
8.4 Pressure Agglomeration Technologies
Examples of technical data of pressure cooker extruders (according to Sprout-Matador, Muncy, PA, USA).
Tab. 8.7:
Parameter
Model
Unit TMEZOOO
Dia. of mixing screw Mixing screw drive Cooker assembly Cooker drive Water injection system Steam addition system Electric control panel Extruder drive assembly Extruder diameter Segmented extruder barrel Insert type extrusion die Adjustable knive cutter assbly Cutter drive Construction Total electrical req. Steam requirements Water requirements
mm kW mm kW
kW mm
kW kW
TME1500
MDL450
200 (240 optional) 200 160 11 (7.5) 7.5 3.5 760 dia. x 3,020 760 dia. x 2,100 530 dia. x 2,130 5.5 5.5 2 Standard Standard Standard Standard Standard Standard Standard Standard Standard 150 110 55 200 200 110 Standard Standard Standard Standard Standard Standard Standard Standard Standard 3.5 5.5 5.5 Stainless Steel in Product Zone 167 (170) 130 67 10 kg steam/1,000 kg material, dry basis 8 to 12 % of dry feed, weight basis
(Dimensions were converted to the metric system from nonmetric figures.)
of the work that is conventionally done by the extruder; therefore, the life span of the extruder screw, inserts, barrel, die plate, and bearings is significantly increased. The cooker is easy to maintain and operate and, because the agitators are of simple design, these may be rebuilt numerous times to regain critical clearances.
Fig. 8.71: Photograph of the pressure cooker extruder depicted in Fig. 8.70 (courtesy Sprout-Matador, Muncy, PA, USA).
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Fig. 8.72: Front view of another pressure cooker, similar to the one presented in Figs. 8.70 and 8.71, with the cutting assembly removed, showing the extruder discharge (courtesy Sprout-Matador, Muncy, PA, USA).
Fig. 8.73: Schematic representation o f a pressure cooker with three barrels and cell wheels. The inset (left) shows the restraining collar at the end ofthe mixing screw shaft (courtesy Sprout-Matador, Muncy, PA, USA).
Pet and fish food
Textured soya protein
Fig. 8.74: Some examples o f products obtained with pressure cooker extruders (courtesy Sprout-Matador, Muncy, PA, USA).
Puffed snacks
8.4 Pressure Agglomeration Technologies
The processed mass is transferred into the screw extruder where hydrostatic pressure is developed which causes axial extrusion through the openings of the die plate. The orifices produce extrudates which often feature different cross sections (for example [see also Fig. 8.741, for processed cereals tubes yielding rings and for dog food a bone shape may be used). The ropes are cut with an adjustable and, for cleaning purposes, replaceable device (lower right in Fig. 8.70) from which they discharge for further post-treatment (e.g. drying and cooling). Fig. 8.71 is a photograph ofthe equipment presented in Fig. 8.70. Tab. 8.7 summarizes, as examples, the technical data of three pressure cooker extruders. Fig. 8.72, the front view of a similar machine with the cutter device removed, shows the end of the extruder and Fig. 8.73 is the schematic of a three barrel pressure cooker with screws as agitators. Fig. 8.74 depicts some products obtained with pressure cooker extruders. Sometimes, the lumpy shape of the processed (often called “expanded”)mass can be directly used in a post-treatment facility (for example, “puffing” snack pieces during drying); in that case rotating cell wheels, that also act as pressure seals, are applied to discharge the processed mass (lowerright end in Fig. 8.73). This idea has been recently modified by another manufacturer of expanders. Fig. 8.75a is an artist’s conception of the annular gap extruder, another medium-pressure agglomeration device, showing the steam manifold and feed lines, indicating the turbulent movement of the particles inside the barrel as well as their expansion and mass densification, and demonstrating the extrusion of material through the annular space at the discharge end. The cross section of the annular gap can be changed hydraulically, even during operation, by moving the conical piece that creates the back pressure in or out of a beveled seat. Fig. 8.75b depicts an optional execution of the discharge end whereby two different, hinged discharge configurations can be attached alternatively to the extruder.
Fig. 8.75: (a) Artist’s conception o f the operation o f an annular gap extruder. (b) Optional configuration o f the discharge end: (1) beveled seat and perforated die plate (retracted) with cutter, (2) “standard” conical resistance piece and seat, defining the annular space and backpressure (courtesy Amandus Kahl, Hamburg, Germany).
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8 Pressure Agglomeration 8.4.3 High-pressure Agglomeration
Once again referring back to Section 8.1, Fig. 8.1, and the related text, high-pressure agglomeration is characterized by densification that is accompanied by particle deformation and destruction, often requiring very high specific forces. Therefore, particulate solids of any kind and size can be processed, making this technology the most versatile of all the various agglomeration techniques. As long as feed particles can be fed into the high pressure zone of the agglomeration equipment and high enough forces can be exerted without destroying the machine, there is no maximum size limit. Large pieces will either be deformed or broken down and incorporated in the structure of the agglomerate. However, if the feed only consists of large particles and no or not enough fines are present to fill the large pores, deaeration may require a long time if brittle pieces suddenly collapse or may be not at all possible if the deformation of large plastic particles results in closed pores in which gas becomes trapped. On the other hand, if very fine particles are densified into the bulk compression stage (see Section 8.2), gases can not be easily expelled from the diminishing pore space because of high resistance in pores with extremely small diameters and, since small particles feature high strength (see Section 5.4, a considerable elastic deformation occurs. Therefore, generally, when pressure release begins after compaction, gas must have been completely removed from the structure to avoid destructive expansion of compressed gas pockets and the energy of elastic deformation must have been converted into other forms of energy as much as possible to avoid detrimental elastic springback. As mentioned several times before, the speed of densification or the rate of pressure rise is the most important parameter for successful high-pressure agglomeration. In some cases, repeated densification prior to final discharge or an extended period during which the maximum pressure is applied will reduce the effects of high speed densification while such corrective measures are not possible with other high-pressure agglomeration methods (see also Section 8.1). Since speed of densification is directly proportionate to the production capacity, its limitation, determination, modification, and optimization is of the greatest importance for all high-pressure agglomeration techniques. High Pressure Axial Screw and Ram Extruders Extrusion is possible by low, medium, and high pressure. In all three cases the same underlying basic principle is responsible for the agglomeration process. Tab. 8.8 lists the requirements. The major difference between low and medium pressure extruders is the execution of the machine. To exert and contain the forces that are necessary for high-pressure agglomeration of masses which consist to a large extent of particulate solids, the drives must be capable of delivering high torque, the processing chambers as well as mixing and densification tools must be heavy duty, and the orifice(s)must produce a high back pressure. To avoid the development of closed pores that are filled with residual pressurized gas, the particulate mass must be kneaded, densified, and degassed, often by applying vacuum, prior to entering the hydrostatic zone in which the pressure in the mass is equalized.
8.4 Pressure Agglomeration Technologies Tab. 8.8:
Requirements for the agglomeration o f particulate solids by
extrusion. Material
Particulate solids must be premixed to match formulation. Size of solid particles should be smaller than cross section of orifice. Particles to be plastic or becoming deformable during conditioning. Interparticle friction must be small or decreased by lubricants. Conditioned mixture to be de-gassed and plastic (deformable) Binding characteristics must be inherent or caused by binders. Equipment
Mixing, conditioning, and degassing must be accomplished by suitable tools Pressure must be built up in the conditioned and degassed mass. Special extrusion tools may be necessary to feed the mass to the orifice(s.) Orifice dimensions define resistive force and cross section of extrudate. Orifice design must enable flow and assist in pressure release at discharge. Cutting devices may be necessary or desirable to divide strand into pellets. Drive must be capable of sustaining mixing, conditioning and extrusion Individual process steps may be carried out in-line in separate equipment.
Fig. 8.76a is a schematic cross section through an extruder with vacuum degassing. It shows first the end of the mixing/conditioning equipment (in this case an open pug mill but more commonly a closed paddle or screw mixer/ conditioner [see, for example, Fig. 8.70, Section 8.4.21) in which the feed is prepared for extrusion. The mass is then densified in a pug sealer in which a compacted plug of material is formed that provides a dynamic seal against the vacuum that is created in the feed chamber of the extruder. While dropping into the extruder screw by gravity, gas is removed from the particulate mass. Alternatively, there is a vertical feederlsealer (Fig. 8.76b) for smaller capacities which mounts directly to the vacuum chamber of the extruder and accomplishes a similar degassing effect. The characteristic pressure distribution within the axial high pressure screw extruder is depicted in Fig. 8.77. The pressure zones that are identified in this figure can be described as follows [8.2]. Zone 1: Up to line A, this is the feeding area. In this conveying zone, the material is still relatively loose and moves along the barrel essentially without any densification. There is little pressure build-up and the bulk density remains largely unchanged. Zone 2: From lines A to B is the densification region of the screw and the loose material becomes compacted. Zone 3: From line B to the end of the continuous screw flights metering takes place. This results from the number of partial flights or “wings” on the screw shaft tip. The double or “split” wing, as shown in Fig. 8.77, is by far the most common execution. Split wing tips are used to attenuate the unbalanced flow which is coming off the screw.
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Fig. 8.76 Schematic cross section through an extruder with vacuum degassing [8.2].(a) Horizontal p u g sealer, (b) vertical pug sealer.
8.4 Pressure Agglomeration Technologies
Rf = Resistive force due to sliding friction of material on barrel, augers, and die surfaces. Rs = Resistive force due to shear of material.
Fig. 8.77: Depiction o f the characteristic pressure distribution within an axial high pressure screw extruder [8.2].
Zone 4: This zone, from lines C to D, is a space in which the pressure in the material that is delivered by the metering device is more evenly distributed. In this volume, the previously mentioned hydrostatic pressure is achieved. Zone 5: The resistance to flow in the die, from D to E, results in a pressure drop to atmospheric pressure at its front (discharge) face. This pressure drop depends on the frictional resistance of the die, the volumetric rate of the material flowing through the die, and the rheological properties of the mass.
Fig. 8.78: Sketches of extrusion plates. (a) Single, machined die plate, (b) sandwich die plate [8.2].
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Fig. 8 . 7 9 Two methods o f dividing single strands with large cross section into smaller ropes [8.2].
In high pressure axial screw extruders, dies and their orifice@)can have different shapes and sizes. Their selection depends on the plasticity and flow properties of the materials to be extruded and, obviously, on the desired product shape and size. Two basic types of dies are used in extrusion. For the formation of small agglomerates, such as strands or pellets, extrusion plates are used. Such plates can be simple or more complex, the latter, so called sandwich types, consist of multiple plates that are made of different materials. Fig. 8.78 presents sketches of two extrusion plate designs. Parameters are based on manufacturer know-how as well as vendor experience from different applications and can not be discussed here.
Fig. 8.80 Schematic presentation o f the lubri. cation o f single stream tapered dies (8.21.
8.4 Pressure Agglomeration Technologies
Single stream tapered dies, as shown in Fig. 8.76 and 8.77, are used for shapes such as bricks and, using mandrels, for the production of tubes or cored blocks. However, this type of die may be also acceptable for the production of smaller agglomerates. As shown in Fig. 8.79, some simple methods exist by which an extruded strand with large cross section can be divided into several ropes. Cutters can then be employed to yield smaller extrudates. If the material to be processed exhibits properties that make it suitable for successful extrusion with a single large orifice it is an advantage that less wear occurs because relatively little material contacts the orifice wall and it is further possible to lubricate the die (Fig. 8.80) resulting in still lower power requirement, still less wear, and a better surface quality of the extruded strand. Plasticizers or “extrusion aids” are commonly used in preparing materials for extrusion which are otherwise not sufficiently flowable and deformable. Many of these also act as binders and lubricants (see Section 5.1.2) and, quite often, water may be a cheap choice. The photographs in Fig. 8.81 show, as examples, two different executions of J.C. Steele’s (see Section 14.1) Model 90AD extruder. Fig. 8.81a includes a horizontal mixer/pug sealer and dual hinged dies while Fig. 8.81b shows a hydraulic die changer in which the die plates are not installed in the holders. Die changers are used to allow the exchange of plates with worn orifices with a minimum of downtime or to switch over from one cross section to another. Tab. 8.9 presents technical information on some
Fig. 8.81:
Photographs of two high pressure axial screw extruders showing different die changers. (a) Extruder (model 90AD with horizontal mixer/pug sealer and dual hinged mouthpieces (= dies), (b) extruder (model 90AD with hydraulic horizontal die changer; dies are not installed in the die holders (courtesy J.C. Steele, Statesville, NC, USA).
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8 Pressure Agglomeration Tab. 8 . 9 Technical information on some typical high pressure axial extruders and pug sealers (according to J.C. Steele, Statesville, NC. USA). Parameter
Unit
Model
(Model: rprn)
25A Extr.
75ADExtr.
90AD Edr.
(Hor./VertPug sealer)
Screw shaft
rpm
10-37
18-38
22-41
(25A 10-37/25A: 10-37) (75AD: 21 -41) (90BD: 25-44)
Motor power
rPm rPm kW
22-75
110-260
225 - 375
(25A 11-45/25A: 7.5-30) (75AD: 75-85) (90BD: 150-300)
12,
22,
kW kW
Std. US bricks Capacity
/h
3, - 9,000
t/h
Machine weight kg
5,000-6,000
-
24,500
-
41,000
25-54
48 - 90
10,000-20,000
11,000-21,000
typical high pressure axial extruders. Because one of the main uses of the equipment is the production of bricks, the number of “standard US brickslh” is included as a capacity figure. Other high pressure axial extruders are mostly applied for the processing of pastes and of plastic materials, particularly thermoplastic polymers. Because much specific work is done during the mixing, kneading, and plasticizing of these materials, instead of conventional continuously flighted screws, they often apply specially configured tools that are attached to single or twin shafts. They accomplish processing tasks as well as transportation and the development of extrusion pressure (Fig. 8.82). If the materials are sticky and/or plastic, the tools are intermeshing with stationary parts in the barrel of single shafted machines or with each other if twin shafts are applied. Close tolerances prevent build-up and result in a self-wiping cleaning action. With the use of highly wear resistant internal parts and dies, modern applications include those for the processing of masses consisting of or containing large amounts of particulate solids. Such machines can be classified as equipment for high-pressure agglomeration.
Fig. 8.82 Photograph of the open barrel o f a double shafted high pressure extruder showing the processing elements for mixing and conveying (courtesy Readco, York, PA, USA).
8.4 Pressure Agglomeration Technologies
Extrusion, particularly if high pressures are applied, is particularly well amenable to the processing of elastic materials. As mentioned before (Section 8.1, Fig. 8.2b), the densification process builds-up pressure in the mass itself. With increasing length of the extrusion channel, which is synonymous with the application of higher force, it becomes more and more likely that extrudates remain in the die for an extended period before they discharge at the mouth. During this time additional degassing and conversion of elastic into plastic deformation can and, in most cases, will occur. The use of a “pressurechamber” between the end of the tool that provides forward transportation and the extrusion channel and the application of a long extrusion channel will result in the production of extrudates with minimum springback and/or expansion even if the feed was loose (i.e. containing much gas or, in other words, requiring a large percentage of densification) and had elastic properties. The above phenomenon is further enhanced if, instead of utilizing continuously operating screws or screw-like devices, the reciprocating movement of a punch is applied. Consequently, the so called Exter press, a horizontal ram extrusion press, was developed for the briquetting of peat. This highly elastic organic material which, after appropriate drying, becomes also very loose, was greatly desired as a cheap fuel when such material was thought after in large amounts for the quickly expanding use of the steam engine in industry and for locomotion [8.3]. The principle of the ram extrusion press (Exter press) is depicted schematically in Fig. 8.83. A typical feature is the horizontal extrusion channel which first converges somewhat to allow the development of sufficient pressure in the mass for initial bonding. The reciprocating punch presses feed against briquettes which were formed during previous strokes and remain wedged in the channel. During each stroke, fresh feed as well as all the other briquettes in the channel are compressed until the axial force becomes high enough to overcome the wall friction and a potential back pressure acting at the mouth of the channel. When this happens, shortly before the end of each stroke, the entire column of briquetted material moves forward and a briquette emerges at the discharge end of the machine. Fig. 8.84 shows the sequence of events during a briquetting cycle. The reciprocating motion is, for example, produced by an eccentric drive which is symbolized by the circular representation on the left. The diagram on the right indicates the development of force that is exerted by the ram onto the material to be briquetted. The figure is self-explanatory.Only a few important operating stages will be mentioned below.
Fig. 8.83: Schematic representation of the Exter, reciprocating ram, or ram extrusion press [B.42].
Briquettes
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Fig. 8.84: Sequence o f events during a briquetting cycle in the r a m press [8.3].
At position (3),lower left, the force produced by the ram has reached the level that is required to overcome the friction of all briquettes in the channel as well as, if applicable, the back pressure caused, for example, by the briquettes in a cooling channel. The entire line of briquettes moves forward with the force remaining approx. constant (position 4,upper right). During the back stroke, the energy of the drive is stored in a flywheel and made available later to overcome the deceleration/acceleration at the return points of the reciprocating motion and also to help during compaction.
1 Bulk teed
Fig. 8.85: Schematic representation of the decrease i n elastic recovery and increase of density o f a briquette during consecutive press cycles in a ram press [8.3].
I Briquene
8.4 Pressure Agglomeration Technologies
At the beginning of the back stroke (when the eccentric drive has passed position 4) and if elastic materials, such as peat or, generally, biomass are being processed, at first the ram face does not separate from the newly created briquette because of its considerable elastic recovery and expansion. At a typical rotational speed of the eccentric drive of 90 rpm, the duration of the compression phase is only 0.04 s. This time is too short to achieve total conversion of elastic into plastic deformation. Therefore, the elastic recovery during the back stroke is high. Without the characteristic of ram extrusion presses that, during each compression stroke, many briquettes that remain wedged in the extrusion channel are again loaded and compacted, whereby more and more permanent plastic deformation is obtained, successful briquetting of elastic materials would not be economically feasible. For example, in a punch-and-die press (see below) densification would have to occur very slowly and pressure would have to be held at maximum for a long time (dwelltime) to make sure that the conversion takes place during the single stroke and be completed prior to pressure release and removal of the compact from the die. As a result, only few compacts could be made per unit time in an expensive machine resulting in uneconomical operation. Fig. 8.85 is the schematic representation of the increasing density and the decreasing elastic recovery of a particular briquette during repeated pressing as it moves forward in the extrusion channel. It is important to note that, in contrast to the conditions in medium pressure extrusion presses where in longer extrusion bores of pellet mills material may similarly pass during more than one pressing event (see Section 8.4.2) but bind into a continuous extrudate structure, even after the first stroke the surface produced by the ram
Detail
Z
Fig. 8.86 Cross section through a modern ram extrusion press (courtesy ZEMAG, Zeitz, Germany).
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face is so highly densified that, during the next stroke and for phases 2, 3, and 4 in Fig. 8.84, it acts as the solid bottom of a confined volume densification chamber. During the entire production process, the surfaces of adjacent briquettes do not develop significant bonding. Therefore, upon discharge from the press mouth or, if applicable, the cooling channel, the product will separate into single briquettes. There are physical limits to the design of such machines because friction and drive power as well as overall stressing of the equipment increase quickly with channel length. In a technically feasible channel reaching the conditions of Fig. 8.85 may not be possible and, consequently, the briquettes will retain a certain elastic deformation. If suddenly released, the elastic recovery may be large enough to damage or even destroy the integrity of the product. Therefore, for most applications, a gradual release is provided by slowly increasing the cross section of the channel prior to product discharge (see also Section 8.4.2, discussion of Fig. 8.37 and 8.38). Fig. 8.86 is the cross section through a modern ram extrusion press. The upper channel wall is adjustable such that different release angles can be obtained. In addition, a flexible support system at this point serves as a safety device to avoid overloading due to tramp material in the feed or “overcompaction”. As compared with a closed mold (punch-and-diepresses, see below) in which a predetermined pressure is reached with no difficulty, in high pressure ram extrusion presses the situation is complex (see Fig. 8.85). The peak pressure that is developed at each stroke depends not only on the force exerted by the ram but also on the resistance to the forward movement of the briquettes in the extrusion channel as well as a potential back pressure. The two latter ones are influenced by the shape and length of the channel, the changes in cross section in relation to length, the smoothness of the channel walls, the nature of the material to be processed, including parameters such as temperature, structure, plasticity, etc., and, if applicable, the type and length of the curing (cooling) channel. The rate of pressure increase is also important. It depends on the stroke frequency and length as well as, again, the rather complicated relationship between the movement of the ram and the magnitude of the resisting frictional force between briquette and die wall and the force caused by the column of already compressed material that is
Fig. 8.87: Photograph showing different traditional (coal) briquette shapes that were produced with ram extrusion presses.
8.4 Pressure Agglomeration Technologies
Fig. 8.88: Partial view of a row of twin (two channel) ram extrusion presses in a lignite briquetting plant (courtesy KRUPP Fordertechnik, Essen, Germany).
being pushed forward. These forces change with both the state of compaction and the rate of movement. As with all reciprocating equipment and after consideration of the above conditions related to forces, densification, permanent plastic deformation, and development of strength, capacity of ram extrusion presses is restricted. To overcome this limitation, multiple extrusion channels are used in a single machine and relatively large briquettes are made. Fig. 8.87 is the photograph of several traditional (coal) briquettes demonstrating the size and shape of such products. The approx. dimension of the one on the top left is 153 m m long x 67 m m wide x 45 m m thick and, if made binderless from German brown coal (lignite),it weighs approx. 500 g. Inspite of using multiple channels and producing large briquettes, industrial plants employ numerous presses (Fig. 8.88). The partial view of the “press house” of this lignite briquetting plant shows the discharge ends of eight double channel (twin) ram extruders. Fig. 8.89 demonstrates the crank shaft drive mechanism of a three channel ram extrusion press after removal of the crank case cover.
Fig. 8.89 Photograph o f a triple (three channels) ram extrusion press with the crank case opened t o show the three crank shafts (courtesy KRUPP Fordertechnik, Essen, Germany).
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Tab. 8.10 summarizes some technical information for high pressure ram extrusion presses. Tab. 8.10a presents machine details and Tab. 8.10b indicates the approximate briquette output per channel of the shapes shown at the top. In Tab. 8.1Ob “impact area” means the face area of the ram that is contacting (impacting) the material to be briquetted; it is approximately equivalent to the face area of the briquettes as represented by the shapes specified in the first three lines of Tab. 8.10b. As mentioned before, at typical ram speeds the contact time is so short for each cycle (0.04 s was mentioned, see above) that compacting is often referred to as being carried out by a blow. From its invention in 1857 to approx. the 19GOs, the Exter press was almost exclusively applied for the binderless briquetting of processed (partially dried) peat and soft coal (mostly lignites). For several reasons, during the second part of the 20th century use of such briquettes became no longer acceptable and many of the classic manufacturers of ram presses, who were mostly located in Europe, folded or gave up Technical information on some typical high pressure extrusion presses. (a) machine data, (b) briquette output (according to ZEMAG, Zeitz, Germany).
Tab. 8.10
(a) Model
PSA 200-1 PSB 200-1 PSC 200-1 PSA 400-1 PSB 400-1 PSC 400-1 PZA 300
Max. load
Fly wheel Largest speed width
IMNl
[rpml
Imml
2.5 2.5 2.5 3.8 3.8 3.8 3.0
120 120 120 120 120 120 120
211 211 211 315 315 315 273
- o m
(b) Briquet output
Salon briquette
shape
Semi briquette
shape
Industrial briquette
shape
Impact area
No. of rams
cm2
used with presses of rated size
No. of flywheels [approx. tons]
Req. motor
Press weight Ikwl
102 51 51 132 67 67 147 0
om
450 250 250 630 280-355 280-355 630
m w r n r n
G1/182 H2/182
H2/210
)4/209
H3/273
H3/315
)5/261
125
115
120
136
158
170
200
200
200
200
300 400
300 400
300 400
400
Largest permissible contact pressureg
MPa
200
217,s
208
279
240
223,5
190
in contin. operation
MPa
140
140
140
195
168
155
133
Briquette output at 100/min and 45 mm thickness
t/h
7,45
636
7.2
8.1
9.4
10
12
Briquette output at 75/min and 40 mm thickness
t/h
5
4.55
4.8
5.4
6.25
6.65
a
8.4 Pressure Agglomeration Technologies
this sector of their businesses [8.3]. However, since the technology is available, mature, and has continued to be developed until a few decades ago, some companies found new niche markets, particularly in the field of biomass based solid fuels. The particular characteristic of this press type, is the possibility to successfully and permanently densify and shape materials that feature high elasticity or require the removal of large amounts of interstitial gas. Therefore, the reciprocating ram extrusion press and, after some modifications which particularly provide a longer extrusion channel, screw extrusion presses are also amenable for the briquetting of very fine powders that feature a certain amount of “lubricity” and of inert elastic materials which inherently contain binders or to which binders have been added. Typical examples for the latter are lignin in or for wood based products, either as a natural ingredient or as an added waste product (lignosulfonates from paper making), and natural sugars or the byproduct of sugar making (molasses)for bagasse or spent slices of sugar beets. For these applications new, typically less heavy duty machines were developed [B.22, B.411. Fig. 8.90 depicts the cross section through a ram extrusion press that has been modified for the briquetting of organic waste materials. In all major major components it resembles the machines that were described above and were based on the Exter press.
The most obvious difference is the application of a vertical feed screw which is required to stuff the typically loose, voluminous feed material into the press chamber. Providing predensified material allows to produce briquettes with sufficient size (thickness) without excessive stroke length of the ram.
Fig. 8.90
Cross section through a ram extrusion press for the briquetting o f biomass [B.41].
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In contrast to the intermittent build-up of pressure in reciprocating ram presses, screw extrusion presses operate by continuously forcing material into and through a long die channel. As a result, the material is experiencing pressure, that is caused by wall friction, for a long time while being pushed through the extrusion channel. During this period, densification, deaeration, and conversion of elastic into plastic deformation continue. The product is a continuous strand of compacted material that is normally cut into cylindrical pieces. For the briquetting of biomass or similar material, different designs of screw extrusion presses are on the market. They are: machines with conical or cylindrical screw(s) with or without externally heated dies. The conical screw press provides predensification before the material to be processed enters the extrusion channel where final density is obtained. The main disadvantage of this design is the severe wear in the screw if the material consists of or contains abrasive components. Fig. 8.91 is the cross section through a screw extrusion press with externally heated die. Most commonly, heating is accomplished by an electric resistance heater which is wired around the die. Heat is either used to activate the binder or a lubricant, to achieve more plasticity of the material during densification, or, at least, to begin drying off moisture. If heating is applied for the latter, a system of vents allows the steam that is generated to escape from the material. In this case, the process can accept raw materials with a free moisture content of up to 35 % without mechanically squeezing water from the structure or limiting densification by the development of hydrostatic pressure in the pores.
-,
I
Fig. 8.91: Cross section through a screw extrusion press for biomass with heated die [B.41].
SCREW
3- 4 MOTOR
I
8.4 Pressure Agglomeration Technologies
Punch-and-Die Presses Punch-and-die presses for the compaction of particulate solids
are the oldest (high) pressure agglomeration machines [B.42]. The densification of powders in a totally confined volume is a well defined process (see also Section 8.1, Fig. 8.2a) and the products resulting from such compaction can and most often do feature excellent uniformity in size, shape, and even mass. Punch-and-die presses are used by numerous industries for a wide variety of purposes [B.42]. Today, the largest application, in terms of numbers of machines and compacts produced, is most probably in the pharmaceutical industry for the production of solid dosage forms. However, punch-and-die presses are also widely used by the ceramic, powder metal, confectionary, catalyst, and, to an increasing extent, the general chemical industries. Principally, the equipment can be divided into two main groups: Vertical and horizontal presses and the vertical equipment can be categorized into: Reciprocating or single-stroke machines and Rotary machines. Vertical Punch-and-Die Presses
Reciprocating Machines Reciprocating punch-and-die presses operate with one upper and one lower punch in a single die (see Chapter 6, Fig. 6.5, upper right). They are mainly used for the production of large compacts or complex shapes where high pressure and/or low output are required (typically less than 100 compressions per minute). Machines using the reciprocating punch-and-die arrangement can be subdivided into two types: ejection and withdrawal presses. Ejection presses are among the most versatile machines. They are built as simple hand-operated equipment, with pressure capabilities of <20 MN/m*, and as highly complex units which may exert pressures of more than 1,000 MN/m2 and produce compacts with a very high degree of density and accuracy. Even hand-operated machines (Fig. 8.92) incorporate the features that are common to all ejection presses. Contrary to the situation shown in Fig. 8.92 where the pressing force is acting from below, Fig. 8.93 depicts a die (d), containing the material to be pressed (e),with an upper punch (c) as well as lower closing (r) and bottom plates (g) that are placed between the hydraulic cylinder (a),operating from above, and a fixed press table (i)below. When the upper punch moves downward, compaction is carried out in the die cavity until a predetermined pressure is reached. After releasing the hydraulic pressure, the bottom plate (g) is removed and the compact is extracted from the die through an exit port (h)in the press table by again activating the hydraulic cylinder. For more specific, always recurring applications the die is mounted in the fixed press table and upper and lower punches are attached to moving rams. Fig. 8.94 demonstrates schematically the sequence of events in an eccenter driven ejection press. In such machines, the lower punch descends in the die to allow its filling with powder from a fill shoe (A). Often, all the compression is accomplished by the upper punch which is moving toward the stationary lower one (B+C). Later the lower punch ejects
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Fig. 8.92: Hand-operated hydraulic platen press with the pressing force acting from below (courtesy Carver, Wabash, IN, USA).
n
1
Fig. 8.93: Schematic representation o f a simple ejection press [B.32]. For explanations see text.
8.4 Pressure Agglomeration Technologies
the compact upward from the die, the fill shoe moves it to a discharge chute (D+A),and the cycle begins again. As discussed in Section 8.2 (Fig. 8.3),interparticle and wall friction, force dissipation from particle to particle, and sliding under shear cause differences in density distribution in a compact that is produced in a die with one-sided compression by one of the punches. To obtain a somewhat more uniform structure, compaction can be carried out by both punches. Ifboth move at the same rate and for the same stroke length, thus exerting identical forces, and assuming uniform filling of the die, a mirror image of the density distributions that were shown in Fig. 8.3 (Section 8.2) develops along a neutral plane which, under those conditions, is located in the middle of the compact. Machines that operate in this manner may be identified as presses with “double pressure” (see also below). A large variety of ejection presses has been developed for different applications. They vary in the size and complexity of products that can be made and in the amount of pressure that may be exerted during the formation of compacts. Another differentiation is the type of drive that is used to move the punches. Small machines are often hand-operated hydraulic presses (see Fig. 8.92) or the platen is actuated by moving it up and down with a ball screw drive. Larger machines use mechanical or hydraulic drives. Fig. 8.95 shows schematically the principles of the most common drive arrangements [B.25, B.421. In practical terms, apart from the output, the effectiveness of mechanical and hydraulic systems is equal. The cycle time of hydraulic presses varies with the stroke. The low pressure portion of each stroke can be made quite fast by using a multistage pump but, as the higher pressure cuts in, the remainder of the stroke becomes progressively slower. The length of the high pressure stroke depends directly on the thickness of the piece which is being pressed. In addition, when it is used near maximum pressure, the pumping system of the hydraulic press can rarely achieve a cycle time that is comparable with that of a mechanical press.
\(;\(&\\,; 0..
::. ..::...... ..*:. ... .... .>
:
*
. .... ..*...
I . .
Fig. 8.94 Diagram depicting the operational stages of an ejection press [B.32].
...:
... ..._ .
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8 Pressure Agglomeration ,-Main
bearings
(a.1)
Crankshaf t
Eccentric shaft
Crosshea
Accumulator
Safety valve
\
/
,Hydraulic
cylinder
Piston shaft Crossheod Control valve Gibs
‘Oil
pump and reservoir
Fig. 8.95: Schematic representations of the m o s t c o m m o n drive systems for reciprocating powder presses [B.25, 8.421. (a) Mechanical drives, (a.1) eccentric o r crank drives, (a.2) toggle or knuckle drive, (b) hydraulic drive.
8.4 Pressure Agglomeration Technologies
Therefore, mostly because of their low output, the use of hydraulic presses is restricted to the ceramic and powder metal industries where high compacting pressures are required. The disadvantage of all mechanical punch drives that are shown in Fig. 8.95 (a.1 and a.2) is that, while the compression speed becomes smaller as the eccentric connection of the rotating drive member approaches dead center, overall compaction takes place very quickly and is associated with a sudden release of force after reaching the maximum. This is a particular problem if the material to be processed is very fine and aerated or features elastic properties. Such products reach sufficient deaeration or permanent plastic deformation and strength only after comparatively slow densification and/or remaining under pressure for some time (see also Section 8.1).Fast compaction and/or premature pressure release result in excessive expansion of the product which may destroy its structural integrity and result in well known failure modes (cracking, lamination, etc.) indicating “overpressing”. The only reliable means to overcome these problems in punch-and-die presses is to employ hydraulic actuation of the punch(es) (Fig. 8.95b). The timing of the punch strokes as well as the rate of increasing or decreasing pressure and the “dwell time” can be easily adjusted. In addition, hydraulic presses typically feature overload protection by gas filled accumulators and, because there is no physical limit to the length of the stroke, densification ratios can be very high, thus allowing successful compaction of large amounts of feed even if its initial bulk density is low. New activities in the development of press drives are directed towards hybrid punchand-die presses. This equipment combines mechanical and hydraulic components with electronics to yield machines that incorporate the advantages of both drive systems together with easy process control and data logging. The concept is particularly advantageous for withdrawal presses (see below) in which, for example, the die is moved hydraulically while the fill shoe and top punch are mechanically actuated. As a result, by utilizing the high cycle numbers of mechanical presses and the freely programmable characteristics of the hydraulic drive, a very flexible, reliable, and reproducible operation for the manufacturing of parts with unsurpassed, high quality is obtained. Smaller machines are employed in many fields, including the pharmaceutical, confectionary, and fine chemicals industries, if only a limited output is required and, to a certain extent, for development work in all areas of high-pressure agglomeration (see Section 11.2). Larger ejection type presses are mainly used in the powder metal and ceramic industries. However, even there, the applications are in most cases limited to compacts that feature no or little change in cross section. Withdrawal presses always operate with two (independent)drives. One controls the movement of the upper punch and the other moves the die (Fig. 8.96). Whereas the majority of ejection presses is mechanically operated, for the withdrawal type both the mechanical and hydraulic drives are commonly used. In a withdrawal press, compaction and ejection take place with a continuous downward movement of the upper punch and the die. As shown in Fig. 8.96, at the beginning of the press cycle, the die is positioned on top of the lower punch which remains stationary at all times. Material is filled into the die cavity and compressed while both
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8 Pressure Agglomeration Movement of upper punch
I
t
I
-t
Ready
Filled
Compact Ion
Eject ion
I Return
Fig. 8.96 Diagram indicating the operating phases of a withdrawal press [8.42].
the upper punch and the die travel downward. The simultaneous movement of the die with the punch minimizes the influence of wall friction on compact structure and results in highly uniform product density. At the end of the compression stroke, the upper punch is lifted while the die continues to move down until it has been completely separated from the compact. During this ejection procedure, the compact is supported by the stationary lower punch. After removal of the densified product, the die moves up and the cycle begins again. Tooling for withdrawal presses is much more expensive and complex than that required for ejection presses. It consists of a die set which is removable from the machine as a complete unit. This has the advantage that the tooling is exchangeable between presses. Further advantages lie mainly in its adaptability to the production of complex components. It is also possible to obtain greater accuracy. Compacts can be made on this type of tooling with dimensional tolerances of less than 4 x m m and uniform density. This makes withdrawal presses particularly well suited for powder metallurgical and ceramic applications where uneven shrinkage during sintering must be avoided to make the production of “near net shape” parts possible. Some of the most common shapes of products from reciprocating presses are solid and perforated cylinders (i.e. bushings), rectangular (i.e. bricks) or cubic pieces, and structured shapes (Fig. 8.97). In terms of variety of applications, complexity of shapes, and accuracy of parts, punch-and-die pressing is the most versatile agglomeration method. All other agglomeration techniques offer either only one more or less defined shape with different sizes and little accuracy (for example all tumble/growth agglomeration methods, see Chapter 7) or a relatively small number of shapes with some accuracy (for example roller presses and pelleting, see Sections 8.4.2 and 8.4.3, below).To achieve this versatility, the basic principle of punch-and-die pressing is often modified [B.25, B.421. On the other hand, capacity of reciprocating punch-and-die presses becomes quite low if they are used for the manufacturing of, for example, high definition ceramic and powder metal parts. Since these products are made from expensive materials and almost always undergo post-treatment by sintering, it is of utmost importance that they feature uniform structure to avoid uneven shrinkage and rejects.
8.4 Pressure Agglomeration Technologies
Fig. 8.97: (a) Some parts made from metal powders, metal oxides, ferrites, ceramic materials abrasives, and other particulate solids in punch-and-die presses (courtesy Komage, Kell am See, Germany); (b) A selection of different products made with hydraulic presses [6.42]
One possibility to improve the structure of compacts is to reduce friction (see also Section 8.2, Fig. 8.3). The addition of lubricants (see also Section 5.1.2) was first introduced in the pharmaceutical industry for the improvement of tablette quality from high speed punch-and-die presses (see below). Even today, lubricants are often a considerable part of the formulation of solid drug dosage forms. Mixed into the entire powder mass such lubricants must be included as inert excipients but always constitute additives that need to be accounted for. Because interparticle friction is normally very little affected by mass lubrication, the amount of lubricants, which, in reality, is meant mostly to reduce wall friction, must be much higher than is commensurate with its effect. Therefore, developments were directed toward the lubrication of only the tool surfaces. For this task, nozzles and solenoid valves which operate reliably in millisecond intervals were used and, later, the lubricant was also electrostatically charged to become attracted by and adhere to the tooling walls [B.42]. This technology was also adapted for other applications. Particularly the pressing of metal powders is much improved by lubricating the die cavity. Parts become more uniform, die wear is reduced, and the lubricant, which is often a contaminant, is eliminated from the metal powder mixture. Fig. 8.98 shows schematically and as a photograph a die wall lubrication system which uses an electrostatically charged powder for
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Schematic and photograph o f a die wall lubrication system using a n electrostatically charged powder (courtesy Gasbarre, DuBois, PA, USA).
Fig. 8.98
8.4 Pressure Agglomeration Technologies
WlthoUt 'prepress'
High
Too m u c h
'Correct amount of 'prepress'
Mid
'prepress'
Low
Fig. 8.99 Sketches describing possibilities to influence the location of the "neutral plane" in upper punch pressing and controlled withdrawal die [B.25, 6.421.
lubricating the die cavity of larger presses in powder metallurgy. A timed pneumatic function conveys the powder lubricant from the hopper to the charge gun where an electrostatic charge is induced. A second timed pneumatic function transfers the charged powder through a nozzle, discharge hose, and discharge block into the die where it adheres to the cavity walls. After powder pressing and discharge of the compacted part, the cycle begins again. Particularly in complex parts, it is also necessary to control the position of the neutral plane, the low density zone that is approximately perpendicular to the direction of pressing. This is achieved by the relative motions of the tooling members (Fig. 8.99). It is also important to understand that, particularly under pressure, particles will not move from one level or position in the developing structure of a part to another one. As a consequence, if parts are pressed that feature more than one level, separate pressing forces must be applied simultaneously for each level. As a result, neutral planes will exist for each part level (Fig. 8.100). Fig. 8.101 is the photograph of a typical large vertical hydraulic press for the manufacturing of refractory brick and the three presses in Fig. 8.102 depict examples of a mechanical (a), a hydraulic (b), and a hybrid press that represent special powder presses of one German manufacturer. As an example of the wide variability, Tab. 8.11 summarizes technical information indicating the ranges of design data from that same manufacturer. Obviously a large number of other suppliers will offer a wide range of other presses.
Fig. 8.100 The different "neutral planes" in single and multi-
level parts 18.25, B.421.
m I
One
Two
Three
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Fig. 8.101: Photograph o f a typical vertical hydraulic press for the manufacturing o f refractory brick [B.42]
Fig. 8.102: Examples o f different special powder presses. (a) Hydraulic, (b) mechanic, and (c) hybrid drives (courtesy Komage, Kell am See, Germany).
8.4 Pressure Agglomeration Technologies
Summary of technical information on the design ranges offive different special powder presses (according to Komage, Kell am See, Germany).
Tab. 8.11:
Parameter
Unit
TYPe Pressing force kN m a . mm Stroke (top punch) Die hold. force k N Strokes” min-’ Die fill depth mm Std. height mm Space req. mm Width Depth mm mm Height Weight kg Power req. kW ;k
Models K
KHA
s
KFMA
KMA
0- 50/0- 500“
20- 1200
20-1200
20-250
30
mechanic 50-500 110-200
hydraulic 200- 12,000 225-425
hydraulic 200- 12,000 100-300
hybrid 200-2,500 140-218
hybrid 300 140
20-80 (250“”) 10-80/6-15 0-50/0-120” 650-920
140-8,400 14/2 100-300 1,100-2,700
140-8,400 15/3 100-300 1,100-2,700
140-1,350 7-50/3-15 140-250 1,045-1,895
170 10-40
1,400- 2,100 1,600-2,300 2,000-2,700 600 - 4,200 4.0-7.5
2,800-6,300 3,000-4,500 2,600-6,800 5,000 - 46,000 25-130
2,800 - 6,300 3,000 -4,500 2,600-6,800 5,000- 46,000 25-130
2,700- 3,500 3,500- 6,000 2,600-5,500 3,450 - 18,800 15-100
Small machines/large machines: $:’:
special machines.
Rotary machines Rotary punch-and-die presses were developed to meet the ever increasing demand for higher outputs of relatively small tablettes, primarily in the pharmaceutical industry. Their basic principle of operation is similar to that of simple reciprocating machines. The difference lies in the fact that a series of dies is mounted into a circular steel table (the so called turret) near its periphery (Fig. 8.103) and that two punches (one upper and one lower) are associated with each die. The punches are moved by stationary cams while the turret with the dies and punches is rotating. An evoluted presentation of one pressing cycle is shown in Fig. 8.104. Feed is supplied to the table by an open frame, often called “feed shoe”, which is connected to a hopper above. While the feed frame momentarily covers a particular die, the bottom punch that is associated with that die is pulled down to the lowest position by its cam thus allowing the die to fill with powder. It then rises up on adjustable ramp to eject excessive powder from the die. The powder surplus is scraped off flush with the top of the turret at the highest point ofthe “weight adjustment ramp”. Assuming uniform fill density, this always leaves the same volume of powder to be compacted in the die. It is common practice to let the lower punch drop down slightly after the surplus material has been scraped off. This is done to prevent uncontrolled displacement or “blow-out” of powder from the die when the upper punch enters. Both punches are then moved together by their respective cams to achieve densification and compaction. If, optionally, the ramps moving the punches remain parallel for some distance after reaching
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ROTATIONAL DIRECTION
FEED F R A M E Fig. 8.103: Sketch o f the layout
DIE T A B L E
o f a rotary punch-and-die press [B.56].
maximum densification, a so called “dwell time” is introduced. During this time, the compact remains under pressure so that additional deaeration and conversion of elastic deformation into permanent plastic deformation can occur and expansion upon pressure relief is minimized. The overall opposite movement of both punches during densification and compaction produces the effect of double pressure and, therefore results in a relatively uniform structure of the tabletted product. Finally, the upper punch is lifted from the die and the lower punch travels up to eject the finished compact. As shown in Fig. 8.105, another evoluted presentation of a typical high speed, high pressure rotary tabletting machine, quite often, the maximum pressure is produced by two press rollers that oppose each other. One or both are supported by springs to provide overload protection. In such machines, the final compaction takes place very quickly and is followed by a sudden pressure relief. This is similar to what happens in roller presses (see below) but, because in tabletting machines the roller diameter is very small and the table speed is high, this process takes place extremely fast. Therefore, capping (see below) is a commonly observed problem if high speed tabletting is desired and the preparation of specially prepared particulate feeds by pre-granulation (see below) is frequently a necessity to overcome this defect.
Evoluted (straightened) schematic of a rotary punchand-die press.
Fig. 8.104
8.4 Pressure Agglomeration Technologies
Fig. 8.105: Paths o f the punches in a rotary punch-and-die (tabletting) press in evoluted presentation [8.56].
The simplest type of rotary machine is “single sided” with one feed location and a certain number (as few as four) of “stations” (= dies) on the table. One rotation of the turret produces as many compacts as there are dies (and punch sets) on the machine. Therefore, the output of single sided rotary machines depends on the maximum allowable speed of and the number of stations on the table. It is normally in the range of 300 - 800 tablettes per minute and can be doubled by installing two feed locations. In this case, the stations are filled twice on opposite sides of the rotating table and two compressions are carried out in each die per revolution of the turret. Obviously, to maintain the rate of densification and compaction the number of stations on the correspondingly larger table would have to be doubled, too. Outputs of more than 3,000 tablettes per minute can be obtained from well compacting material with double sided machines. Although the above production numbers also seem to indicate large volumetric capacities this is not the case because the individual compacts often weigh less than 1 g each. For example, at a tablet weight of one gram an output of 3,000 tablettes per minute translates into a capacity of 180 kg/h. A further increase in numbers of (typicallysmall) compacts produced per minute in rotary punch-and-die presses can be achieved by dual or multiple tooling (two or more die sets) per station (see below). In many reciprocating and most rotary presses for the pharmaceutical and similar industries, the original and still most common “standard shape of compacts is a more or less cylindrical tablet (also tablette). As depicted in Fig. 8.106, this description in-
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cludes flat, faceted, and crowned products. For these shapes, simple die and punch configurations are applicable. Since these agglomerates are consumer products aesthetics, requirements that are dictated by the medical application (i.e. an easy identification of a particular formulation by the user), and the marketing driven desire to distinguish between manufacturers have more recently resulted in the development of special shapes, some ofwhich are shown in Fig. 8.107. Additionally, the punches may be engraved as demonstrated, for example, in Fig. 8.108. Finally, as already mentioned above, the tooling for smaller tablettes can be designed such that in a single pressing station two or more die cavities can be associated with correspondingly shaped punches to produce several compacts at once (Fig. 8.109). Of course, such punchand-die designs are very delicate and require high precision press designs as well as excellent maintenance. Expulsion of entrapped gas (air) from granulated or (particularly) powder feeds is very important because it reduces lamination and capping of the tablettes. As repeatedly mentioned (see, for example, Section 8.1), if gas is entrapped in compacts where it becomes compressed in the residual pore spaces and/or elastic deformation is still present when the compaction pressure is released, products from pressure agglomeration methods are partially or totally destroyed during ejection. In the high speed rotary tabletting presses, capping, the separation of a thin layer of material from the main body of the tablette on one or both faces (Fig. 8.110),is a particular problem. In regard to processing it is caused by particulate solid feeds that are not suitable for quick, high pressure densification or, in other words, by too high compaction forces and/or excessive speed of densification. Raw particulate solids for tabletting may be described by three types: 1. Noncompressible powders, 2. compressible powders possessing poor flow characteristics, and 3. compressible powders featuring good flow properties. Noncompressible powders are either pregranulated wet, which adds a binder component that also renders the granulate
Flat
Fig. 8.106
Faceted "Standard" tablette shapes.
Crowned
8.4 Pressure Agglomeration Technologies
Fig. 8.107: Designs and photo. graph of some special tablette shaDes.
compressible, or, if the dosage level is sufficiently low, they are mixed with a powder excipient of type 3 so that the blend becomes compressible and free flowing. The same methods are used to improve the characteristics of type 2 whereby, if pregranulation is selected, application of the dry compaction/granulation methods (see below) may be advantageous. Type 3 powders are called directly compactible. While for punch-and-die presses with relatively few strokes per minute (see, for example, Tab. 8.11) accurate and reproducible filling of the die is normally not a problem and the rate of densification in those machines can be adjusted to match the compactibility of the particulate feed, the very high speed of rotary presses often
Fig. 8.108: Photograph o f an assortment of engraved punches (courtesy Kilian, Koln, Germany).
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Fig. 8.109 Photograph of dies, several o f which feature multiple cavities, and of corresponding punches (courtesy Kilian, Koln, Germany).
Approximate separation lines
a -____---
Fig. 8.110 Photograph of tablettes with "capping" and sketch explaining the capping phenomenon [B.42].
8.4 Pressure Agglomeration Technologies
Fig. 8.111: Schematic representation o f a force feeder for rotary tabletting machines (courtesy Kilian. Koln, Germany).
causes problems. If powders are pregranulated, owing to their now larger apparent (agglomerate) size, flow characteristics are usually superior to those of naturally free flowing powders, compactibility can be adjusted, and a good granule size and distribution can be selected that yields an optimal feed bulk density. Nevertheless, rotary presses often require more than the equivalent of a simple shuttle feeder or fill shoe [B.25, B.421. Fig. 8.111 shows schematic representations of a force feeder for the accurate high speed feeding of rotary tabletting presses and Fig. 8.112 is the photograph of a modern machine on which such a feeder as well as a tablette
Fig. 8.112: Photograph of a modern rotary tabletting machine with force feeder and tablette discharge unit (courtesy Kilian, Koln, Germany).
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discharge unit are installed. Depending on the equipment to which they are connected, the latter feature good tablette unloading, bad tablette channel, and sample extractor. Electromagnetic gates may also separate tablettes that are off-specification during machine start-ups and shut-downs. Validation and cleanliness requirements considerably burden the designs of equipment for the pharmaceutical industry. To avoid cross contamination it is necessary to include CIP (cleaning in place) or at least WIP (washing in place) features on modern machines. It is easily understandable that such techniques are difficult, at least, when considering the complicated mechanical design of multistation (up to 79 per turret, see Tab. 8.12) rotary tabletting presses. Nevertheless, WIP is one of the latest features of such machines (Fig. 8.113) and often, to meet the stringent requirements of the regulatory authorities, from machines that were designed during the last decade, the entire turret assembly, complete with die table, upper and lower punches as well as upper and lower cam tracks (Fig. 8.114a) can be removed for cleaning, exchange, or maintenance. Smaller machines are equipped with integrated handling and mounting devices (Fig. 8.114b) while the assemblies of larger machines require remote handling systems (Fig. 8.114c,d). Tab. 8.12 summarizes some technical specifications of rotary punch-and-die tabletting presses of one manufacturer to demonstrate the range of sizes and capabilities.
Fig. 8.113: Glove box design o f the processing part of a rotary punch-and-die tabletting machine demonstrating (WIP) "washing in place" (courtesy Fette, Schwarzenbek, Germany).
8.4 Pressure Agglomeration Technologies
Horizontal Punch-and-Die Presses Some punch-and-die presses are arranged horizontally. Normally, such machines use a hydraulic drive. Because they are typically used for the briquetting of voluminous materials, such as metal turnings and borings or biomass, hay, straw, wood shavings, bark, saw dust, etc., shredded plastic, cardboard, paper, etc., and fine dusts with low bulk density and, therefore, require a large Tab. 8.12 Some technical specifications by one manufacturer o f three families o f rotary punch-and-die tabletting presses for the pharmaceutical industry (according t o Fette, Schwarzenbek, Germany) Parameter
Unit
Model family
Punch types Tablet output
Max. compr. Max. precomp. Max. tablet 0 Table speed Die 0 Die height Punch shank 0 ;k
h ‘min. ma*. kN kN mm min mm mm mm ~
’
P 1200
PT 2090
PT 3090
EU 19/1”/1” - 441 IPT 1911” 30-48 T” 120-230.4 T;c 80 50 11/13/16/25 25 - 100 22/24/30.16/38.1 22.221233 19125.35
EU 19/1”/1” - 441135 IPT 19/1” 19.8-42.3 T“ 105.6-338.4 T“ 100 100 11/13/16/25/34 15 - 120115-80 22/24/30.16/38.1/52 22.22/23.8/30 19/25.35/35
EU 19/1”/1“ - 441135 IPT 19/1”
T = thousands, for example: 30 T
=
30,000; 1,004.88 T
=
1,004.880
355.2- 1,004.88 T” 100 100 11/13/16/25/34 30 - 100/15 - 80 22/24/30.16/38.1/52 22.22/23.8/30 19/25.35/35
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degree of densification, they feature long, slow speed strokes, high forces, and almost always include a certain dwell time at maximum pressure, Hydraulic drives are ideally suited for these tasks. Fig. 8.115a depicts the principle design of a large hydraulic die press with horizontal ram movement for the briquetting of metal scrap. In this machine both the “chip box” and the punch move. At the beginning of the cycle the ram is retracted and the chip box with the integrated die is pressed forward against the anvil plate, closing the bottom of the die which is part of the chip box. Next, the punch moves forward and pushes feed material into the die. If the material is very loose, a “tamping device” can be added which predensifies the chips and holds them down. When the punch enters the die the pressure stroke begins during which the briquette is made. The finished briquette is held against the anvil while the chip box retracts. As soon as the ram begins to retract the briquette falls into a discharge chute. The cycle begins again when the ram is fully retracted and the chip box is fully moved forward and pressed against the anvil plate. Fig. 8.11% is the photograph of a typical press and Fig. 8 . 1 1 5 ~shows some actual briquettes from this type of press.
Fig. 8.115: (a) Drawings showing the principle design of large hydraulic ram presses with horizontal punch movement; (b) photograph o f a typical press for the briquetting of metal scrap; (c) briquettes from such a press (courtesy Svedala Lindemann, Dusseldorf, Germany).
8.4 Pressure Agglomeration Technologies
Fig. 8.116 (a) Schematic of a horizontal hydraulic punch-and-die press for the briquetting of loose stripper dust: (b) photograph of the press showing the horizontal ram and the vertical predensification channel: (c) photograph of loose feed and the highly densified briquette (courtesy Pneumafil, Charlotte, NC, USA).
While machines for the briquetting of metals and many other recyclables require high forces, special machines have been developed for lower force processing, particularly of biomass. In many such applications, the material to be briquetted is difficult, because large amounts of densification and air removal are necessary, they have, however, good binding characteristics. For example, the grinding dust from smoothing the surfaces of wood chip boards contains the chip board binder as well as a small amount of water from dust suppression. The biggest problem for the processing of such dusts is its fineness and looseness. Fig. 8.116a depicts the schematic of a press for the production of cylindrical briquettes from stripper dust (Fig. 8.116~). As can be seen, the loose dust is transported with a screw from the feed hopper into a vertical rectangular channel in which it is predensified by a hydraulically operated “tamper”. The material, thus densified, is briquetted by a horizontal, hydraulically actuated punch in a die with a force of, in this particular case, up to 42 tons. The product can be picked up with the dumpster of a trash service for regular disposal or it can be burned as a man-made solid fuel. Fig. 8.11Gb shows the hydraulic press part with the vertical tamping channel. Roller Presses Traditionally, roll pressing is of greatest interest for industries in which large amounts of finely divided solids, both valuable and worthless (wastes), must be converted into larger, agglomerated pieces. The most widely used machines feature two rolls of identical size which rotate countercurrently and achieve compaction by squeezing the feed in the nip area (Fig. 8.117), much in the same manner as in rolling mills [B.S].Around the middle of the 19th century, roller presses were originally developed as an economic method to agglomerate coal fines [B.12b, B.421. More recently this method of size enlargement by high-pressure agglomeration is applied for a large number of materials in the chemical, pharmaceutical, food processing, mining, minerals, and metallurgical industries. The versatile technology lends itself to such different uses as compaction/granulation of highly heat and pressure sensitive pharmaceutical materials as, for example pancreatin or penicillin; the briquetting of
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Fig. 8.1 17:
The basic principle of roll pressing.
extremely corrosive and hazardous materials as, for example, chlorinated or brominated biocides and sodium cyanide; or the briquetting of crude, hot materials as, for example, metal chips and turnings, ores, and “sponge iron” with temperatures of up to 1,000“C. An important new application is in the vast field of environmental control where often micron- or submicron-sized particulate solids must be economically enlarged for recycling or disposal. The rollers themselves or pockets and indentations that are machined into the working surfaces of the rolls form compacts or briquettes. Between smooth, fluted, corrugated, or waffled rollers, material is compacted into dense sheets (Fig. 8.118, see also Chapter 6 , Fig. 6.5, lower left). Normally, these sheets are crushed and screened to yield a granular product. This process is called compaction/granulation. If the two rollers carry rows of identical pockets or moulds and the rolls are timed such that the pockets, representing roughly one half of the final product shape, match exactly (Fig. 8.119),so calledbriquettes are produced (see also Chapter 6, Fig. 6.5, lower right). Roller presses do not produce compacts with the same fine detail and uniformity as tabletting machines or other punch-and-die presses. The “web” or “flashing” that is caused by the land area around each pocket is usually found on the outer edges of all briquettes from roller presses. Even if they are thin and brittle, they can not be totally removed, for example on a screen, and, if they do break off, a clean edge is seldom obtained. In most cases some webbing remains on the product which may be objectionable in itself and because it may produce fines during handling. Because of these characteristics, briquetting roller presses find their natural field of application where relatively large scale production with low investment and operating costs is more important than the absolute uniformity of the product. The other important application is for the essentially dry compaction of powders into sheets followed by crushing and screening into a granular product of almost limitless average particle size and distribution. Granule size and distribution depend critically on crushing and screening which must be optimized for maximum yield and economical operation (see also below as well as Sections 8.3 and 11.1). Originally, roller presses were not conceived to exert high pressure and use high forces for briquetting. As mentioned before, they were invented for the economical conversion of fine coal into briquettes that could be applied as solid fuel for the quickly expanding use of the steam engine [B.1, B.21. For that purpose, coal fines were mixed with milled coal tar pitch and heated in a vertical pug mill by direct impingement with
8.4 Pressure Agglomeration Technologies
(b)
Fig. 8.118: Sketches showing different roller surface configurations for compaction and representations o f the corresponding products. (a) Smooth, (b) corrugated, (c) fluted offset, (d) fluted peak-to-peak, (e) waffled.
x (e)
h
b
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t
t---
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Fig. 8.120 Drawing o f an early double roll briquetting press designed by Schuchtermann and Kremer (a company no longer existing). w, w, = Roller shafts; f, f, = Rollers with briquette pockets: z = Internal coupling gears; v = Feeder, pan with rotating distributor arms.
steam. The resulting blend was warm, moist, and sticky but still reasonably free flowing. As shown in Fig. 8.120, early roller presses consisted of two large, hollow drums with briquette pockets machined or cast into sleeves or segments that were shrunk or bolted onto the drum body. They used gravity feeding. The conditioned blend was transferred into a feeder pan with rotating distributor arms which moves and passes the blend through rectangular slots above the nip area between the rolls (Fig. 8.121~). Feed control was accomplished by “tongues”(Fig. 8.121) which were moved, mostly by hand, to increase or restrict the flow of material into the nip. Briquetting was more a forming than a compacting process. Final product strength was obtained during cooling by solidification of the coal tar pitch. For this duty, the machines were powered with flat belts and speed adjustment was accomplished by means of pulleys and crude open gear reduction. Only one roller was driven and timing occurred by open (“naturally”lubricated by coal dust) coupling gears
8.4 Pressure Agglomeration Technologies
Fig. 8.121: Drawings ofdifferent gravity feed controls. (a) Standard tongue, (b) tongue with parallel movement, (c) mechanical distribution (see also Fig. 8.120) with standard tongue.
that were fastened to the drum bodies between two pocketed rings. No pressurizing system existed; the two rollers were fixed in the frame and supported by sleeve bearings. Sometimes, one bearing housing was located in the frame by shear pins to provide overload protection in case tramp material entered the nip. From the beginning, the coal tar pitch which made briquetting so easy was also a constant source of concern and, eventually, the down fall of this technology. Burning of briquettes resulted in the production of excessive amounts of sooty, acrid smoke and, therefore, efforts were made to, at least, reduce the amount of coal tar pitch that was necessary for the production of good briquettes. Correspondingly, more and more pressure was required for briquetting to overcome the lack of binder which slowly changed the mechanical design of the machines. Nevertheless, although the drives were now by electric motors through enclosed gear boxes, the original coupling gears were still used (Fig. 8.122a) which were later moved to a location outside the frame, enclosed in a sheet metal housing and lubricated by dipping into an oil bath (Fig. 8.122b), and springs were added to support one set of bearing housings to result in a floating roller for pressure control and overload protection. Since more and more machines were used for compaction where, during operation, a larger gap opens up which defines the sheet thickness, coupling gears, although manufactured with very large modulus, tended to disengage and frequently teeth were damaged or broke. Therefore, practically all modern machines are equipped with a synchronized double output-shaft gear reducer and misalignment (e.g. gear tooth) couplings between the fixed gearbox shafts and the adjustable or moving roller shafts (Fig. 8.122~). As will be shown below, other drive arrangements and hydraulic pressurizing systems are being used today. Since roller presses were originally gravity fed (see Fig. 8.121), the rolls had been always arranged side-by-side. Later, when a few roller press manufacturers tried to
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(C)
Fig. 8.122: Schematic representations depicting three different roll drives (explanation see text).
overcome the loss of business, after coal briquetting was phased out, and some new companies entered the market, mostly to supply small, ultraclean equipment to the pharmaceutical industry, force feeders became necessary to supply the often very fine and aerated powders to the nip between the rollers and higher pressures were required. Fig. 8.123 describes the compaction of a particulate solid in the nip between two gravity fed counter rotating rolls. For clarity, the roller diameter D and the distance between the rolls hAare not to scale. In reality the roll gap is much smaller as compared with the roller diameter (e.g. D/hA-100/2 to 100/5).Compaction between two smooth rolls may be explained by dividing the nip area into three zones: The feed zone, the compaction zone, and the extrusion zone.
8.4 Pressure Agglomeration Technologies
Fig. 8.123: Conditions in the nip between two smooth, counter rotating rollers during the compaction o f particulate solids. Definitions o f geometry, angles, and roll force.
The feed zone is defined by the two angles aE’and aE.In the feed zone, the material is pulled into the roller nip by friction on the roller surface and between the feed particles. Densification is solely due to rearrangement of particles (see Section 8.1, Fig. 8.1).The density of the feed is characterized by the bulk density yo and reaches the tap density yt at aE.The peripheral speed of the rolls is higher in this zone than the downward velocity of the material to be compacted. a, is the so called “angle of delivery” which is defined by the width h, of the rectangular feed opening above the rollers. The angle that is enclosed by the two tangents on the rollers at a, is called “angle of entry” af (not shown). The compaction zone follows after the heavy solid line (Fig. 8.123), defined by the angle aEwhich is known as the “angle of rolling”, the “gripping angle”, the “angle of nip”, or the “angle of compaction”. In the compaction zone the full pressing force becomes quickly effective and the feed particles deform plastically and/or break if they are brittle (see Section 8.1, Fig. 8.1).ag is the “neutral angle” where the sign (direction) of the frictional force changes. At this point, the pressure in the material and the density reach their highest values and, for many materials, the velocity of the densified solids increases towards the centerline; therefore, this zone is called extrusion zone and this phenomenon assists in the release of the compacted material from the rollers.
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a, (assumed zero in Fig. 8.123) is the “angle of elastic compression of the rolls” which determines the thickness h, of the compacted sheet. a, becomes zero and the sheet thickness is equal to h, the theoretical gap width, if the elastic deformation of the rollers is insignificant. However, in most cases, the sheet is even thicker than h, due to the elastic recovery of the compacted material and the expansion of entrapped compressed gas (see Section 8.1). The actual sheet thickness is h, and the angle aR corresponding to the release plane is called “angle of release”. In the case of briquetting, the conditions in the nip of roller presses become much more complex. The distance between the outer contour of the rollers approaches zero to produce briquettes with only thin webs that easily break into singles. Nevertheless, in many cases, briquette separators are required to accomplish the task of producing individual briquettes (see Section 11.1).Fig. 8.124 depicts the mechanism of briquetting in roller presses. Only the final compaction phase is of particular interest. It begins when the leading (lower in Fig. 8.124) axial land area between successive pockets passes through the line connecting the centers of the two rollers. At this point, the pocket forming the briquette is practically closed at the leading (lower in Fig. 8.124) edge while the trailing (upper in Fig. 8.124) edge is still open and connected with the material in the nip. Immediately following this condition, during the continuous rolling action of the briquetting rolls, the formerly closed leading edge of the pocket opens while, now, the trailing edge closes and completes the compaction of the briquette. Above the final compaction phase, depicted in Fig. 8.124, similar conditions exist as shown in Fig. 8.123 which are modified by the fact that the roller surfaces are not smooth and, therefore, an “interlocking effect” assists in pulling material into the nip. The feed and compaction zones are less clearly defined, only determined by interparticle friction, and no longer depend on the friction between material and roller surfaces. However, it has been determined in a roll press simulator [B.l2b, B.421 that, as a result of insufficient interparticle friction, with certain particulate solids, large portions of material, that was initially contained between one pair of pockets, are squeezed out and move back into the following space. Feed
1
1 Discharge
Fig. 8.124 Five successive momentary conditions o f briquetting between two counter currently rotating rollers with matching pockets.
8.4 Pressure Agglomeration Technologies
The specific compaction process, described in Fig. 8.124, may result in beneficial or in negative effects. As the leading edge of the pocket opens, the force acting vertically to the line connecting the roller centers tries to “extrude” the briquette from the pocket, thus assisting in the release of the briquette, provided the pocket shape is correctly designed [B.42].On the other hand, since this affects mostly that part which is already completely densified, it may also cause a number of product imperfections (e.g. cracks, soft trailing edge, etc., Fig. 8.125) which also depend mostly on pocket shape and size. Another common defect ofbriquettes produced with roller presses is that they open up at the plane of pocket contact. In the vast majority of cases, this opening is at the trailing (last compacted) edge, but, occasionally, opening at the leading (first compacted) edge of the briquettes has been described and explained by overcompaction as well as volume recovery upon pressure release. Independent of their positioning, these latter faults are known as “clam-shelling”, “oyster-mouthing”, “duck-billing”,or similarly descriptive terms. In order to influence the conditions in the nip area of any roller press, provide a certain amount of control to the process, and allow higher pressures and densification rates, even in machines with relatively small roller diameter and if the feed consists of fine, aerated powder mixtures, in more recent times, gravity feeding has been replaced by force feeders, which are mostly based on the application of feed screws (Fig. 8.126, see also discussion below). With this modification, it is no longer necessary to position the rollers side by side. As will be shown later, vertical and even diagonal roller arrangements are offered today, particularly with small machines for specialty applications, such as in the pharamceutical industry. However, it should be mentioned at this point, that, in the opinion of the author, vertical feeding, which translates into horizontal roller positioning, is always preferable, as gravity, the natural force acting always and everywhere, assists in uniform vertical feeding while it may negatively influence the flow of solids in horizontal feed arrangements. Similarly to what is true for any of the other technologies of size enlargement by agglomeration, much of the knowledge about roller press operation is phenomenological in nature. Because of the change of particle sizes and shapes during high pressure densification and compaction of particulate solids (see Section 8.1, Fig. 8.1),a comprehensive theory is not even available for the “simple” punch-and-die process. The complex conditions in the nip between two counter rotating rollers (Fig. 8.123 and 8.124) makes theoretical predictions even more difficult, although certain similarities exist with the much better defined, investigated, and understood deforma-
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’ I
i
& Fig. 8.126 Schematic representation o f some typical force (screw) feeders [B.lZb]. (a) Vertical straight o r slightly tapered, (b) inclined straight, (c) vertical tapered (conical), (d) horizontal straight.
tion of metal (a continuum) in rolling mills [B.8]. Details of the more scientifically oriented treatment can be gleaned from two earlier books by the author [B.l2b, B.421. In keeping with the emphasis of this book “industrial applications, not theory” readers who are interested in those aspects are referred to the previously mentioned literature (see also Chapter 2). In addition to what has been already covered and can be extracted from several publications that are listed in Section 13.1, the following discusses, from a practical point of view, the parameters which are required and used to size and scale up roller presses. Tab. 8.13 is a summary of these items: Roller presses were invented for the economical size enlargement of coal fines and built as large machines with gravity feed and rolls that often exceeded 1 m and sometimes were as large as 2 m in diameter, typically carrying two sets of rings with a set of coupling gears in the middle (see above). Modern roller presses are descendants of these machines. Therefore, descriptions as well as design and scale-up considerations will be first based on equipment which, as far as size is concerned, has been directly
8.4 Pressure Agglomeration Technologies
developed from those earlier presses (as examples of early machine designs see Fig. 8.120 and 8.127). Although, almost everything which will be said for large machines also applies for the smaller ones, specific executions of the new generation of small roller presses meet the requirements of particular industries. Such details will be covered later. Fig. 8.128 and 8.129 present a series of photographs and artist’s renderings of different roller presses. These pictures, together with the contents of Tab. 8.13, will be used to explain important items. Fig. 8.128a.1 and a.2 show photographs of recent roller presses for the briquetting of coal fines with binder. Both still feature gravity feeders. The smaller one (Fig. 8.128a.1) uses a simple movable plate for the control of feed volume (Fig. 8.130), a single pair of wide, pocketed rollers, hydraulic pressurization of the floating roller, and a drive system featuring variable speed (frequency modulated, SCR) electric motor, double output-shaft gear reducer, and misalignment couplings. Since in a gravity feeder, the downward flow of material is retarded near the walls of the chute by friction, the edges of wider rollers tend to be underfed, producing partially soft briquettes or softer bands on the sides of sheets, if roller compaction is employed. To compensate for this, the tongue(s) or movable plate(s) may be curved such that the feed opening in the middle of the roller is restricted (Fig. 8.131a). It is also possible to modify the volume of the briquette near the edge of the roller as shown in Fig. 8.132. The larger press (Fig. 8.128a.2), equipped with 1.4 m diameter rollers, has two gravity feed chutes, with manual and, after switching to automatic, electrically actuated tongue controls inside, feeding two pairs of pocketed rollers (not shown in Fig. 8.128a.2). Actually, when installed, the rollers will be connected to a double output-shaft gear reducer via misalignment couplings with a timing feature for matching the pockets. Therefore, the split of each roller into two separate rings is not done to accommodate the coupling gears of the old designs (see, for example, Fig. 8.120) but to accomplish a more uniform feed across the now narrower working faces and to provide additional deaeration possibilities. The first (uniform feed), requires a constant level of the material column in the feed chute which, in this case, is guaranteed by an overflow chute. In this chute a particulate solids flow meter (see Section 11.1)is installed and the feed streams to the presses are adjusted such that always a trickle overflow is measured, thus keeping the level in the feeder constant. Regarding the requirement to reliably and completely remove all gas from the particulate solids during densification (see Section 8.1)in roller presses, several routes for the escape of gas exist. Fig. 8.133 shows the different deaeration paths in roller presses. The two left representations depict side views of a narrow and a wide roller and the right sketch is a front view showing the rollers, the nip, the product sheet (which may also be a string ofbriquettes),and the feed hopper. When gas (air) is squeezed from the densifying material in the nip, it can leave between the feeder base and the top of the rollers (a),between the rollers and the cheek plates sealing the nip on the sides (b),and against the flow of feed through the loose bulk material (c). No escape is possible through the roller gap other than as compressed trapped gas; but this must be avoided. The sketch suggests several important considerations for the design and operation of roller presses. 1. The open space between the feeder base and the the top of the
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Summary of design features and parameters of roller
presses. Design feature or parameter
Feeder
Design
Various levels of details or subparameters
Gravity
Unrestricted Manual adjustment Force Screw/other Screw Vertical/horizontal Drive Gear reducer w. motor Direct E-motor variable Parameters Gravity Standpipeloverburden Force Screw Speed Design Fixed cheekplates Other features Design Bearings Antifriction (ball/roller) Conical/selfaligning Tapered/withdr. sleeve Shafts Solid, forged Heatedlcooled Rollers/general Single pair Solid Heated/cooled Compacting Side-by-side Smooth Briquetting Side-by-side Number of pockets Pocket shape Parameters Diameter/width Speed Gap Design None Fixed rollers No overload protection Elasticlgeneral One roller Spring Helical Hydraulic Handpump W. accumulator Parameters Specific force [kN/cm] Force characteristic Accumulator pressure Cantilever Design Cast Fixed/bolted Enclosed (dust-/airtight) Parameters Strength (cold) Design Gearing Open Gear box (sing]. outp.) Roll synchr. No or coupling gears Uneven no. of teeth Couplings Regular Misalignm. couplings W. or w/o. roll timing Tongue
Feeder base
Roll arrangement
Pressurizing
Frame
Drive
Flow stimulators Automatic control Manual/automatic Singlelmultiple Fixedlvariable Direct hydraulic Flight and pitch Adjustable cheekplates Sleeve Other Cylindrical Composite Multiple pairs Tires/segments Other Profiled (specif.) Other Size of pockets Shoulder (seal) (Force) Gap control Shear pin or similar Both rollers Other Automatic W/o. accumulator Max. force Accum. volume Mill shaft Fabricated Hinged Open Strength (hot)
G.b. (doubl. outp.) By gear box Spec. synchr. device Universal joint
8.4 Pressure Agglomeration Technologies Tab. 8.13 cont'd:
Summary of design features and parameters
of roller presses. Design feature or parameter
Various levels of details or subparameters
Power supply Transmission Parameters Execution
General
Electric motor Hydraulic drive Belt (flat or V) Power (kW] Speed reduction ratio Heavy duty Clean (CIP, WIP) Rough environment Ambient
Variable speed Other Direct Speed Torque Light duty Stainless steel Special mat. constr. Hot
rollers is a major deaeration feature in roller presses ((a)in Fig. 8.133). However, if the material is very fine and aerated and this space is too big, large amounts of material may flush over the rollers and end-up in the discharge of the press as fines, thus reducing efficiency. It is possible to instal baffles in this area which allow the escape of gas and retain solids by depositing them onto the roller surfaces for transport back into the nip. 2. The escape of gas between the rollers and the cheek plates ((b)in Fig. 8.133) is the most important and often also the most misunderstood deaeration path. Cheek plates (for design and more detailed descriptions see below), the heart-shaped pieces sealing the side of the nip between the rollers, can not be in rubbing contact with the rollers as excessive wear would take place and the constant friction, which is aggravated by the presence of fine powder particles, causes the rollers and the cheek plates to quickly become red hot. Rather, well adjusted cheek plates have a finite clearance which is selected such that the leakage of fine material is minimized. In this
Fig. 8.127: Early roller press for the briquetting of caol fines, built by Zimmermann & Hanrez (a company no longer existing).
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Fig. 8.128: Photographs and artist's renderings o f different roller presses. (a.1, courtesy Lewis, Pocatello, IA, USA) and (a.2, courtesy Koppern, Hattingen, Germany): modern presses for the briquetting o f coal; (b, courtesy Koppern, Hattingen, Germany): roller press with external gear reducer and coupling gears; (c, courtesy Otsuka, Tochigi-City, japan): roller press with pressurization by helical springs; (d, courtesy Sahut-Conreur, Raismes, France): narrow faced large roller press with screw feeder; (e.1, courtesy Koppern, Hattingen, Germany) and (e.2, courtesy Hosokawa BEPEX. Minneapolis, MN, USA): artist's conceptions o f roller presses exposing important internal parts; (f, courtesy Koppern, Hattingen, Germany): large roller compactor o f latest design for mineral fertilizers; (g.1, 8.2, courtesy Alexanderwerk, Remscheid, Germany): roller presses with horizontal feed and vertical roller arrangement
8.4 Pressure Agglomeration Technologies
Fig. 8.128 cont’d
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Fig. 8.128: cont’d
8.4 Pressure Agglomeration Technologies
Fig. 8.128 cont’d
Fig. 8.129 Hinged frame. (a) Drawing depicting the principle, (b) photograph o f a roller press with hinged frame, opened on one side for maintenance (courtesy Koppern, Hattingen, Germany).
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Fig. 8.130 Sketch o f movable plate volume control in the gravity feeder o f a roller press for briquetting. One or two plates may be used; actuation is manual (shown) or automatic.
position, large volumes of gases that are squeezed out during densification can escape. Since this gas flow entraps fine particles, leakage of material seemingly becomes excessively large and, since some pressure is lost in the material to be compacted at the edge of the nip, the already present effect of underfeeding at this point (see above) increases, causing soft briquettes or edges or compacted sheet. However, as became obvious when, due to improved designs and materials of construction, the rollers could become increasingly wider, the center portion of the nip between these rolls lost its ability to let squeezed out gas flow to and escape at the cheek plates. If this happens, more and more of the gas tries to flow against the flow of material ((c) in Fig. 8.133), particularly in the central portion of wide rollers. In a gravity feed situation, this results in a cycling of the flow of feed and the performance of the press. Gas escaping upwards, against the flow of feed aerates the still loose material above the nip so that the particulate solid’s condition becomes similar to that of a fluidized bed (see Section 7.4.4).The bulk density of feed to the nip diminishes to such an extent that full densification is no longer possible and aerated, little compacted material passes through the rolls. At the same time, the roll pressure and the drive torque drop to near no load conditions. Since in this state, most of the gas passes the rollers with the less compacted material, the upward flow of air ceases and the “fluidized bed” collapses. Immediately afterwards, material with “normal”, high bulk density enters the nip and good compaction takes place. At the same time pressure and torque peak. However, because gas is now again squeezed out and flows upwards, the cycle begins again. This operating condition is not acceptable because not only the yield of good compacts drops considerably, thus reducing process economics, but, equally important, the chattering, caused by the large fluctuations in pressure and torque, may result in serious damage to the roller assemblies, bearings, couplings, gear reducers, and drives, often to the point of destruction. To overcome the above mentioned problems, wide rollers are subdivided into two or more rings with one or more gap in between where cheek plates are arranged and deaeration can take place. After leaving the original and more recent designs of roller presses for the briquetting of coal (see also several papers by the author, listed in Section 13.3) and before going on to more modern roller presses that were conceived for the processing of a
8.4 Pressure Agglomeration Technologies
i Section A-A ( a1
1
Fig. 8.131: Some feeder designs for wide faced roller presses. (a) Curved tongue (or plate) o f gravity feeders, (b) independent and (c) overlapping multiple screws [B.12b].
variety of new materials, Fig. 8.128b shows that, in certain cases and for a number of reasons, the application of older design principles may still be preferred. The machine depicted in this photograph makes use of a simple, cheap, fabricated frame and directly attached coupling gears. The press produces briquettes so that fluctuations in gap width, the main problem for machines with coupling gears, are minimal. Because the material to be processed is not abrasive, there is practically no need to exchange rollers or do other maintenance which makes a bolted frame acceptable. In spite of a large speed reduction, the arrangement is compact and requires little floor space (compare with a machine of similar frame design, roller diameter, and capacity but with double output-shaft gear reducer and misalignment coupling that is shown in Fig. 8.128e.1. On the other hand, the press features such modern details as force (screw) feeders and hydraulic pressurization of the floating roller.
Fig. 8.132: Cross section through a roll sleeve with reduced volume o f the border pockets t o improve quality of these briquettes.
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b
---I-
7Fig. 8.133: Schematic representation o f the different deaeration paths in roller presses. Also describing the difference between narrow and wide rolls.
Fig. 9.128~is the photograph of a modern machine with a spring loaded pressurization system that avoids the use of hydraulic fluid in and potential contamination of the processing area. Also, combinations of springs, for example nesting helical springs with different spring characteristics or pakets of springs of other designs (e.g. disc springs),create forces in response to gap changes that are not achievable with hydraulic systems and may be of particular interest for certain processing reasons. Fig. 8.128d is the photograph of a hydraulically pressurized roller press, driven through a double output-shaft gear reducer and featuring a (force) screw feeder with variable speed (SCR) electric motor. Force feeders are used to provide a controlled amount of particulate solids with a defined bulk density to the nip area between the rollers to, according to the conditions explained in Fig. 8.123, accomplish the desired densification and compaction. Force feeders are also applied to overcome the previously mentioned problem of fluidization of fine feed particles during deaeration. They provide a downward pressure, thus prohibiting development of the fluidized bed condition, but this action can also hinder proper deaeration and may cause failure of the compacted product due to the expansion of compressed gas. It is important to consider the relationship between screw diameter and roller width. As shown in Fig. 8.134, top views, there is a fundamental problem in feeding the nip between two rollers, which is rectangular in cross section, with a rotating screw that features circular projection and material delivery areas. Such an arrangement tends to overfeed the center portion of the nip while the edges, which are lacking feed anyway due to wall friction and deaeration as well as leakage at the cheek plates, become severely starved. Since additionally, particles in a bulk mass, particularly if it is under pressure and interparticle contact has been established, do not have the ability to move from one area to another as, for example, the molecules of gases or liquids, screw feeders can provide the necessary mass flow, bulk density, and feed pressure but they may also cause uneven feeding. A simple partial remedy is to use a screw diameter which is slightly larger than the roller diameter and push excess material into the nip by suitably shaped cheek plates.
8.4 Pressure Agglomeration Technologies
A further characteristic of screw feeders, which results in uneven compaction, is the fact that, in single-flighted screws, the end of the blade extends farthest into the nip and exerts a rotating point-force onto the particulate mass. While, in briquetting machines, the consequence of this moving pressure point can be shown only indirectly by the varying density and strength of individual briquettes within a product batch, it can be easily demonstrated with roll compactors, particularly if smooth rollers are used for compaction. As shown in Fig. 8.134, a sine-wave pattern of somewhat more densified material is visible on the compacted sheet. The frequency of this sine-wave depends on the screw and roller speeds. If such sheet is crushed and screened into a granular product, density and strength variations are commonly obtained. Double-flighted screws with blade ends on opposite sides and two rotating pressure points produce offset, overlapping sine-waves and, as a whole, a more uniformly briquetted or compacted product. Finally, again referring to Fig. 8.134, as the width of the rollers gets bigger the diameter of the screw must be increased correspondingly. As shown in the front views (lower part of Fig. 8.134),a screw feeder diameter which approaches the diameter of the rollers, which is synonymous with rollers that feature the same diameter and width, the screw more and more “applies breakes” to the rollers. A considerable
Fig. 8.134 Sketches showing the relationship between the diameter o f a single-screw feeder and the width o f the rollers [ 6.12 (b)].
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amount of the screw force acts on the roller surface and not on the particulate material in the nip. Such a situation results in wasted energy (of the main and screw drives), exaggerated uneven feeding, and insufficient predensification of the particulate mass in the nip. As a rule of thumb, the diameter of the screw should not exceed GO- 70 % of the roller diameter and, preferably be in the 50 % range. As shown in Fig. 8.128e.1, e.2, and f, wider rollers that, for process reasons, require screw feeders are equipped with multiple screws. In Fig. 8.131b and c, the reasoning behind this choice is explained. To provide material to the nip between rollers with growing width, the longer rectangular feed area is approximated by a series of small diameter screws. Each of these screws, whether single- or double-flighted, will produce a sine-wavepressure pattern (Fig. 8.131b) which can be further modified by the use of overlapping screws (Fig. 8.131~).As depicted in Fig. 8.128e.1, e.2, and f, multiple screws are often arranged under an angle to allow easier loading of the feed hopper (see also Fig. 8.12Gb). Although coming from different directions, the screws in the artist's conception of Fig. 8.128e.2 are working in a common hopper extension and, therefore, feature the characteristics of overlapping screw flights, while each screw in Fig. 8.128e.1 is housed in separate pipes. Sometimes, all screws are angled from the same side, particularly if the overlapping feature is used. Then, the screw axes will be directed towards the futed roller to avoid excessive movement of the floating roller as a result of always possible bulk density variations. As visible through the open doors in the photograph of Fig. 8.128f the need for optimal deaeration may also direct the use of multiple rings as discussed above. The main difference between the artist's conceptions in Fig. 8.128e.1 and e.2 are the choices of bearings for supporting the rollers. Fig. 8.128e.1 represents what the majority of manufacturers of large roller presses use today. As shown in more detail in Fig. 8.135a the bearings are selfaligning roller bearings which are mounted onto cylindrical shaft journals with conical withdrawal sleeves. The advantages of this design are that bearings can be mounted and removed quickly without the danger of doing any damage to the seats, cylindrical (straight) shaft journals are easily manufactured accurately and maintained, even after repeated removal seating is well reproducible, and, if the floating roller momentarily moves out of parallel, due to the spherical (selfaligning) design no forces act on the bearings and no bending of the shaft is experienced. Another bearing arrangement, which was suggested by metal rolling applications where the rolls, by design, do not cock, uses heavy duty conical (Timken) roller bearings. These bearings are typically directly seated on conical shaft journals (Fig. 8.13% shows the removal procedure), therefore require reshimming during every renewed installation, and do cause bending if cocking of the floating roller occurs. Fig. 8.135a also suggests the design and shows the simple labyrinth seal of a sheet metal housing surrounding the rollers (see photograph in Fig. 8.1284, mostly for dust containment. Other details will not be discussed, as they exceed the scope ofthis book. Reference should be made to [B.12(b)]and [B.42]as well as to other literature in Sections 13.3 and 14.1. Another feature of most modern roller presses, that merits a specific mentioning, is the hydraulic pressurization system. It is used on all presses shown in Fig. 8.128 with
8.4 Pressure Agglomeration Technologies
Fig. 8.135: Bearing design: (a) Cross-sectional drawing depicting a roller assembly with selfaligning roller bearings and other modern features (see Section 13.3 [102]); (b) conical (Timken) roller bearing with removal device.
the spring loaded machine, pictured in Fig. 8.128c, being the only exception. The hydraulic pressurization of the floating roller serves to provide a controllable operating pressure and an overload feature if and when tramp material enters the rollers. A typical advanced schematic is shown in Fig. 8.136. The hydraulic pressure is produced by a pump which, in this system, is motorized and often submerged in the oil reservoir. In the most simple case, the pressurized fluid actuates hydraulic cylinders that push against the bearing blocks of the floating roller. In a no load situation, the bearing blocks are held in place by shimmed stops, avoiding metallic contact of the rollers and defining the “no load gap”. Since hydraulic fluid is incompressible, an accumulator, a partially gas-filled pressure vessel, is associated with each bearing block. Before pumping up the hydraulic system, the accumulator is filled with gas, typically nitrogen, to a pressure that is lower than the expected operating pressure. As shown in Fig. 8.137, if an empty hydraulic system begins to be filled up at time marker (0),the hydraulic pressure in the system begins to increase until it reaches the gas pressure in the accumulator (A) at time marker (1).Before the system pressure increases further the volume of the compressible gas in the accumulator is reduced while pumping continues and, for some time, no pressure change is observed. (This is an easy
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A
Contact pressure
Pressure/ accumulator
Floating roller
A
Nonreturn valve
I
L - 4
I I
Motor pump
\
Safety valve
Fig. 8.136 Schematic diagram o f a hydraulic pressurization system for the floating roller o f a high pressure roller press (see Section 13.3 [102]).
on-site check to determine the functioning and pressure of the accumulator(s) if no pressure gauge for the gas filling is available). After the equilibrium gas volume is reached at time marker (2), the pressure in the system rises again until it reaches a predetermined pressure (B) at time marker (3) and pumping is discontinued. The no-load pressure (B) has been determined during tests such that, if operation of the roller press begins at time marker (4)and material opens the “no load gap” to the operating gap, the corresponding movement of the hydraulic piston in the closed hydraulic system increases its pressure to the operating pressure (C). While material is being densified and compacted, the pressure will fluctuate around the operating pressure depending on momentary changes in feed bulk density. Normally, the hydraulic system is then switched to “automatic” whereby the signal from the contact pressure gauge (Fig. 8.136) turns the pump on if the minimum pressure is reached and off when the maximum is obtained. If the pressure increases beyond the maximum, the safety valve dumps hydraulic fluid back into the reservoir. While many machines use only one hydraulic accumulator (as shown, for example, in Fig. 8.1288.1) and the two hydraulic cylinders are connected with common hydraulic tubing, the more sophisticated (and better) system uses at least one accumulator per bearing block and pressure lines that are separated from each other by non-return valves. In this design (as shown in Fig. 8.136), ifthe floating roller cocks, the hydraulic pressure in that part of the system that moves more also increases more and tries to push the bearing block back to regain a parallel position. Systems that are always in
8.4 Pressure Agglomeration Technologies
(0)- (4) (A)
: Time markers : Accumulator pressure
(B)
: no load ( pre) pressure : Operating pressure
(C)
(A)
(0)
(1)
(2)
(3)
(4)
d
Time
Fig. 8.137 Operational diagram of a hydraulic pressurization system for the floating roller of a high pressure roller press.
equilibrium, do not offer this possibility so that a non parallel position of the floating roller can persist forever or until the situation is manually remedied and may reoccur again. The gas accumulator(s) in the hydraulic system of a roller press is (are) often considered not very important by the operators. This is far from true. First and foremost, the hydraulic accumulator provides flexibility to the system and avoids overload situations. Peak loads due to variations of the bulk density of the feed can not be avoided and are compensated by the accumulator(s). If an accumulator is damaged and does not contain a gas cushion, the system is rigid, because hydraulic fluid is incompressible, and very short high peak loads, which are not picked up and displayed by standard instrumentation, reduce the life of many critical machine components (e.g. bearings, shafts, couplings, gearing, etc.). The hydraulic accumulators also influence compaction by defining the response to overload situations. If the volume of the gas cushion is large, the floating roller moves easily in response to load changes. Vice versa, if the volume of gas at the operating pressure is small, the response is more rigid. This performance can be influenced by the size of the accumulator and the gas pressure (determined at the no-load condition). The larger the volume and/or the closer the (noload) gas pressure to the operating pressure the softer the response. A “standard” starting accumulator pressure is normally set at GO- 75 % of the operating pressure but if, for example, the material to be processed easily overcompacts, i.e. showing signs of lamination, clam-shelling, cracking, etc., accumulator volume and/or pressure should be changed which may result in improved product quality.
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Modern roller presses often use very high forces (up to and beyond 1 MN) which, through the hydraulic system, act on the bearing blocks of the floating roller. Although the hydraulic fluid is under pressure (presented, for example, in kN/cm2)the piston or pistons exert a force onto the bearing block which is defined by the area of the piston. These high forces may deform the bearing block and damage the bearing or, at least, reduce bearing life. Therefore, high pressure roller presses feature two punches per bearing block which act above and below the centerline of the rollers and the bearing block is designed asymmetrically (i.e. more steel on the side where the force is acting). Fig. 8.138 is the photograph of a hydraulic block featuring to cylinders and an accumulator between a frame member and the bearing block. At this point it should be mentioned that, for roller presses, a densification and/or compaction pressure can not be calculated. Referring to Fig. 8.123 it is easily understood that a volume element on which the maximum pressure acts can not be defined. Fig. 8.139 translates the curve of Fig. 8.1 (Section 8.1)to a smooth roll, gravity fed high pressure roller press. Because there are no defined, unequivocal volume elements in which the pressure can be determined, the curve represents force. For comparison of press performance, the specific force, a physically meaningless designation, which is, however, useful for, for example, scale-up, has been developed. It is defined as the (normally operating) force divided by the operating width of the roller and measured in kN/cm. If, for example, the roller width between the cheek plates is 0.75 m and the average total force exerted by all hydraulic cylinders onto the bearing blocks of the floating roller is 5,000 kN, the press operates with a specific force of approx. 66.7
Fig. 8.138: Photograph o f the hydraulic block in the frame of a modern high pressure roller press (courtesy Koppern, Hattingen, Germany).
8.4 Pressure Agglomeration Technologies
kN/cm. The same specific force characteristic is used to identify the capabilities of a particular press. Fig. 8.140 shows typical specific forces which are required by some different materials for successful compaction in roller presses. At and above approx. 150 kN/cm the local yield pressure of even the best materials for the manufacturing of roller surfaces (at that level almost exclusively segments [B.42])is overcome; therefore, for technical reasons, the limit for roller pressing is reached at that specific force. With increasing pressure, wear of the roller surfaces becomes more and more of a concern, particularly if the material to be processed is also abrasive, such as many metals, minerals, and ores. In such cases, the rollers must be frequently removed from the frame for maintenance. Because the rollers must be always larger in diameter than the bearings and those become increasingly larger with higher specific forcerequirements, in early machines, such roller maintenance meant opening the frame after removing most everything above the press and lifting out the roller sets. For fabricated, bolted frames (see for examples Fig. 8.128a.2, b, and e.1) this is quite time consuming. A big improvement was made when the hinged frame was invented (Fig. 1.829).As can be seen, the frame can be opened easily and quickly and the rollers can be removed to the sides of the machine where they are picked-up by a lifting device (Fig. 8.129a). If the installation is planned properly, nothing above, below, or around the press must be removed other than items that are directly related to the rollers (e.g. cooling, lubrication), the frame (e.g. hydraulic pressurizing sys-
(d-8)
. I
d08k
rprhgLurck
Fig. 8.139 Pressure conditions in the nip between two counter rotating smooth rollers during the passing o f particulate solids.
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Pressure agglomerallon
150-
Hinerals Ores, Burnt Ihe Melollurgical dusts. sponge iron bol) Causlk MgO
110120-
loo80
-
:i 20 10
High pressure comminution
Ammonium sulphale NPK-mixed fertilizers Cool uilhout bmders Rock salt
Ores
Soda
Cement clinker
FGD gypsum NPK-mixed fertilizers with urea Briquetling with bmders Polosh. Oil shale Ceramic raw materials Maleic anhydride. DMT Coal wilh binders Shales, Cloy Earlhy ores
Cool
Fig. 8.140 Specific forces required for the successful compaction o f some different materials with roller presses (courtesy Koppern, Hattingen, Germany).
tem), and the press (e.g.roller housing). A real maintenance situation is shown in Fig. 8.129b. So far, only presses with rollers arranged side-by-side, were presented. Although most of the presses with horizontal feed and rollers on top of each other are small, manufactured by relative newcomers to the field, and applied in the pharamaceutical industry for ultraclean processing, Fig. 8.1288.1 and 8.2 are examples of large machines of this design. Everything that has been said about roller presses so far and will still be discussed below is valid for presses of any design and size. Selection and design details are mostly influenced by the application. As mentioned repeatedly, roller presses that were originally developed for the economic size enlargement of coal fines by briquetting have, after losing their own field of application, found many new uses and, accordingly, were modified to fit the particular requirements. As shown in Fig. 8.141 modern roller presses are currently used in three different fields. Particularly for the new high pressure comminution,where efficient brittle breakage during compaction in the nip is the primary reason for passing minerals through the rollers and the agglomerates that are formed are later dispersed into individual small particles by total destruction of the bonds (in mills), large throughput capacities were desired. This led to the construction of very large roller presses (Fig. 8.142) which incorporate the newest developments of roller press design plus some specific ones which were suggested by the size of this equipment. If, given the extremely torque requirements, the typical mo-
8.4 Pressure Agglomeration Technologies
tor-gear reducer-misalignment coupling drive system would have been chosen, the “foot print” of such installations would have become excessively large. Therefore, as shown in Fig. 8.142 direct hydraulic drives are often being employed. Just for comparison, Fig. 8.143 is the photograph of a complete compaction/ granulation system that is used in the pharmaceutical industry for the conversion of dry powder mixtures into dust free, free flowing, non-segregating granular intermediate products for subsequent tabletting. The roller press is shown in the upper center. So far, all machines that were presented used the mill shaft design in which the roller shafts are supported on both sides by bearings that are mounted in a frame. Another, relatively new design of roller presses uses cantilevered shafts and rolls. With these machines, the shafts are supported in a frame and the rollers are mounted on the front of that frame. Fig. 8.144 shows four examples. Because the feeder, the rollers, the cheek plates, and the nip are relatively freely accessible from the outside, these machines are used for frequently changing small applications, for development work, and in the pharmaceutical industry.
lo)
‘
Brtqueites
(b)
a
(C)
I
Compacled Sheet l a
i
Fig. 8.141: Schematic representation o f the three current fields of application o f modem roller presses. (a) Pressure aggtomeration/ briquetting, (b) pressure agglomeration/compaaing, (c) high-pressure comminution.
Fig. 8.141: Schematic representation of the three current fields of application of modern roller presses (a) Pressure agglomeration/ briquetting, (b) pressure agglomeration/compacting, (c) high-pressure com m inution.
Fig. 8.142 Large roller press for high pressure comminution (courtesy Koppern, Hattingen, Germany).
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Fig. 8.143: Dry, high pressure compaction/ granulation system employing a small roller press (courtesy Fitzpatrick, Elmhurst, IL, USA)
Selection, sizing, and scale-up of roller presses are based on experience and testing. In addition, some very basic formulae support these efforts. As has been discussed above (see also Fig. 8.140),the specific force, which is required for the production of an acceptable product, obviously represents one of the most important results of testing. For its determination, a representative sample of the material to be processed is compacted in a roller press. The roll diameter of this test press should be as close to that which foreseen for the large scale application. The roller diameter can not be easily scaled up. This is due to the conditions in the nip which become unpredictably complicated if the roller surface is not smooth and vary with the changes in structure, size, and shape of the particulate solids during densification and compaction. As shown in Fig. 8.145, the nip shape and size are quite different between rollers with different diameters. In addition to the modifications caused by the changes in diameter, the gap would have to become narrower as the rollers are getting smaller. It can be easily recognized that, if, for example, the circumferential speed v, is kept constant and, therefore, the angle of nip, a, remains approximately the same, the densification and compaction behavior must be quite different. Between larger rollers, the process occurs much less suddenly and such important processes as deaeration and conversion into plastic deformation are achieved more completely. Even though circumferential speeds are normally only in the range of 0.5 - 0.9 m/s (for easily compactible materials, such as common salt, Y, maybe as high as 1.5 -2.0 m/s and in high pressure comminution it can be in excess of 3.0 m/s) it shouldbe realized that the entire process in the nip happens in fractions of a second.
8.4 Pressure Agglomeration Technologies
+ I
c
Fig. 8.144: Four examples o f roller presses featuring cantilevered shafts and rolls. (a) Side view drawing (a.1) and photograph (a.2) o f a model CS machine with vertical feeder (courtesy Hosokawa BEPEX, Minneapolis, MN, USA): (b) schematic drawing (b.1) and photograph (b.2) o f a model CS machine with horizontal feeder (courtesy Komarek, Elk Grove Village, IL, USA); (c) outline drawing (c.1) and photograph (c.2) o f a small cantilevered roller press with horizontal feeder and integrated flake breaker: the photograph also includes an integrated granulator (mill) (courtesy Alexanderwerk, Remscheid, Germany): (d) overall photograph (d.1) and detail (d.2) o f a cantilevered roller press with vertical feeder and integrated granulator (mill) (courtesy Sahut Conreur, Raismes, France).
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8.4 Pressure Agglomeration Technologies
Fig. 8.144: cont'd
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Fig. 8.144: cont’d
Fig. 8.145: diameters.
Nip shape and size between rollers with different
8.4 Pressure Agglomeration Technologies
The following are useful practical equations for sizing and understanding roller presses: Circumferential speed:
vc = TI 2r rpm 1/60 [m/s]
Throughput (compaction)
Q
(briquetting) Relationships: D - p D-s
With:
= TI
2r s 1 rpm 60 y [kg/h]
Q = z V rpm GO y [kg/h] ( D 2 l W 2 (D2/D1)”2
.PI =p2 . SI = ~2
.PI
= P2
S - P
(S2/Sd1’2
2r= D
Roller diameter [cml [I/min] Roller speed Sheet thickness (avg.) [cm] Roller width [cml Apparent density?; [kg/cm31 Number of briquette pockets per roller Briquette volume (avg.) [cm3] Specific force [kN/cmI
rpm S
1
Y z
V
P
(Eq. 8.2)
(Eq. 8.3) (Eq. 8.4) (Eq. 8.5) (Eq. 8.6) (Eq. 8.7)
(“ average of the compacted sheet or, respectively, the briquettes)
More equations can be found in the literature [e.g. B.8, B.12b, B.42, B.47, B.561. If the specific force, the average density of the product, and the desired throughput capacity of the machine are known, with the sheet thickness or the briquette size a roller size can be calculated and a machine can be selected based on its maximum specific force capability. The drive power is determined from the torque requirement during testing and safety factors are added to all machine and process parameters. After installation it is normally necessary to readjust and optimize all conditions in-line. In Tab. 8.14 some parameters of roller presses from different suppliers are summarized and Fig. 8.146 shows photographs of typical products that are obtained with roller presses.
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Some typical parameters of roller presses from different
suppliers. Vendor (see Section 14.1)
Type Roller
Frame design Feeder design
Accessories Execution Drive power Throughput Weight
Orientation Arrangement Diameter Width Surface Design Support Pressurizing system Specific press. force Drive Motor Speed Standard Multiple Special Standard Special Standard Rollers Approx. typical Approx.
Vendor (see Section 14.1)
Type Roller
Orientation Arrangement Diameter Width Surface Design support
Unit
mm
mm
Alexandewerk
Lab/Pharm Large H H M C 250-1,000 120-300 25-120 120- 1,000 C Rs/Si/wc Rs/Si/wc AFB/cyl AF B Icy1 H/wAcc H/wAcc
Frame design Feeder design
Accessories
Standard Multiple Special Standard Special
Lab/Pharm Large V V M C/M 200-400 200 - 400 C C/B Rs Rs/Si/wc AFB AFB/cyl HAc H/wAcc
kN/cm G/do El/vS
G/do El/vS
G/do EI/vS
Gdo El
CF sc Yes
HF sc Yes
HF/CF sc No
hg sc No
md/hd/De
Id/ss/De
md
to 60,000
to 2,000
m/s
md/ss kW kg/h to 400 1,000kg to 4.0
Hosokawa BEPEX
Unit
mm mm
B/C Rs/wc A F B ,B ,R
kN/cm
m/s
Koppern
Lab/Pharm Large V V M M/C 300- 1,100 200-400 75 - 1,100 50-350
CY1
Pressurizing system Specific press. force Drive Motor Speed
Fitzpatrick
No 55 G/do El/ss, vs 0.1-1.5
HF/tb(hg) Sc/co,(Gr) Yes ‘b/V Hvd
Large V M 300-1,400 100- 1,600 B/C B/C Si/ Rs/ Sg/wc Rs/Si/Sg/wc AFB,R AFB,R slvlsph con H/wAcc H/wAcc 150 (160) 125 G/do,Dd G/do El/ss,Hy/vs El/ss, vs 0.1-1.2 0.1-1.5 HF HF/hg Sc/co,Gr/ag Sc/cy,Gr/ag Yes Yes PlV/WVP
Lub
P/V/CO Lub Hot exec./ HYd
Commin. V M 500-2,400 200- 1,700 C Si/Sg/wc AFB,R slv/sph H/wAcc 150 G/do,Dd Hy/vs,El <3.0 HF/hg Gr/ag No
Lub
8.4 Pressure Agglomeration Technologies Tab. 8.14
continued. Hosokawa BEPEX
Vendor (see Section 14.1)
Unit
Execution Drive power Throughput Weight
Id/md/ss/Sa md/hd/De/Sa md/hd/De kW 3-45 7.5-1,100 to 1,000 to 160,000 kg/h to 100.0 1,000kg to 8.0 2.0-150.0
Standard Rollers Approx. typical Approx.
Vendor (see Section 14.1)
Type Roller
Orientation Arrangement Diameter Width Surface Design support
Unit
KR Komarek
Large
H C 130-457 10-150
V
B/C Rs/wc AFB,R H/wAcc 20-168 G/do Ellvs 0.08 -0.89 CF sc
B/C Rs/Sg/wc AFB,R con H/wAcc 40-130 G/do El/vs 0.09- 1.1 OF/HF Sc/Gr
No
Yes
No
No
CYI
Speed
Accessories Execution Drive power Throughput Weight
m/s
Standard Multiple Special Standard Special Standard Rollers Approx. typical Approx.
Type Roller
M 330- 1,371 50-685
Lub HYd ld/md/ss/De md/hd/ss/De kW 2.2-60 30-220 kg/h to 6,000 to 80,000 1,000kg 0.8-12.0 5.0-28.0
Vendor (see Section 14.1)
Orientation Arrangement Diameter Width Surface Design Support Pressurizing system Specific press. force Drive
Lab H mm mm
kN/cm
Lab/Pharm Large V V C M 150 -400 400 - 1,400 30-100 150-1.600 B/C B/C Rs/wc Rs/Si/Sg/wc AFB,R AFB,R CYl slvlsph H/wAcc H/wAcc 40-90 90-170 G/tg,G/do G/do El/vs El/ss,vs 0.02-0.42 0.04-1.40 CF OF/HF sc/co/vp, Gr/ag No Yes vacuum deaeration No Lub Id/ss/De 3-37 to 1,500 0.5-4.0
Turbo Kogyo (Lic. Alexanderwerk)
Unit
C/M 90-230 30-170 C Si/Rs/wc AFB H/wAcc to 36 G/do
md/hd/De 150-3,000 to 2 x lo6 to 400.0
Sahut Conreur
Lab
mm mm
Pressurizing system Specific press. force kN/cm Drive Motor Frame design Feeder design
Koppern
Large H M 265 -662 170-660 C Si/Rs/wc AFB H/wAcc to 45 G/do
hd/ss/De 37-1,000 to 120,000 5.0-120.0 Zemag
Large V M 250- 1,000 150-1,250 B/C Rs/Sg/wc AFB,R H/wAcc 24-150 G/do, Dd
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8 Pressure Agglomeration Tab. 8.14
continued.
Vendor (see Section 14.1)
Unit
Motor Speed Frame design Feeder design
Execution Drive power Throughput Wright
Rollers Approx. typical Approx.
Explanations Roller .... Orientation Arrangement Diameter Width Surface Design
Support
kW kg/h 1OOOkg
H V C M
[mml [mml B C Rs Si sg wc AFB CYl con slv
Pressurizing system
Specific press. force Drive
Motor
SPh H S No wAcc [kN/cmI G Dd tg do El HY vs ss
Speed Frume design
Wsl HF OF CF
Zemag Ellvs, Hy/vs 0.04-1.0 OF/HF Sc/cyl, Gr/ag,v Yes
El/vs
El/vs
CF/HF sc Yes
HF sc Yes
HYd
HYd
ld/md/ss/De/ Sa 0.2-7.5 12-600 to 2.8
md/hd/ss/De
hd/sslDe
5.5-200 800- 20,000 to 42.0
14-630 200-55,000 to 86.0
m/s
Standard Multiple Special Standard Special Standard
Accessories
Turbo Kogyo (Lic. Alexanderwerk)
Horizontal feed; rolls on top of each other Vertical feed: rolls horizontal to each other Cantilevered rolls outside frame Rolls between two frame members (= mill design) If applicable, range Range Briquetting Compacting Ring (sleeve) Solid (integral shaft/roll) Segmented Watercooled Antifriction bearings ( B = ball; R = roll; N = needle) Cylindrical bore and shaft journal Conical bore and shaft journal Sleeve and cylindrical shaft journal Self aligning (spherical) Hydraulic Spring No pressurizing system (fixed rollers) With hydraulic accumulator Max., range Gear reducer Direct drive (each roller directly driven) Timing gear Double outputshaft (synchronized) Electric Hydraulic Variable speed Single speed Circumferential speed of rollers (range) H - frame 0 - frame Cantilever frame
8.4 Pressure Agglomeration Technologies Tab. 8.14
continued.
Explanations (cont’d)
Feeder design
hg tb Gr ag sc CY
co P V
rb Accessories
VP Lub HYd
Execution
Id md hd ss De Sa
Throughput
[kg/hl
Hinged Tie bars Gravity Adjustable gate Screw Cylindrical screw Conical screw Parallel screw shafts V - arrangement, inclined Ribbon flights Variable pitch screw Automatic lubrication system Hydraulic support functions (feederlframe dismantling) Low duty Medium duty Heavy duty Stainless steel Dust enclosure Sanitary Typical, range
8.4.4 Isostatic Pressing
One of the potential problems of all pressure agglomeration methods is, that friction between the material and tooling walls, interparticle friction, and the often one sided or, generally, directional introduction of the compression force result in density variations in the compacted product (see Section 8.2, Fig. 8.3). For many applications this is not a problem. In fact, the highly densified “skin” of pelleted or briquetted products acts almost like a packaging of the compacted material. The dense surface provides higher strength and, particularly, good abrasion resistance while the interior is somewhat softer. This may be of advantage for, for example, certain pelleted or briquetted animal feeds or for metal briquettes which must feature an inert surface to avoid reoxidation. Particularly ceramic and metal powders are formed and densified by pressure agglomeration techniques to yield intermediate products which are heat treated (sintered) to reach final shape, density, and strength. Since many of these materials can not be easily machined after sintering due to their abrasiveness and/or hardness, “near net shape” articles are often desired. This means, that, after taking into consideration the dimensional changes caused by, during, and after the sintering process, the part should posses dimensions which need no or minimal adjustment for its intended use. In isostatic pressing, the consolidation pressure is applied by a fluid that is pressurized and acts uniformly from all sides onto particulate solids that are enclosed in a
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Fig. 8.146 Photographs o f different products from roller presses. (a) Briquettes, (b) compacts and granules, (c) feed and product o f comminution.
flexible container (“mold”)which is immersed in the fluid [B.l2a]. This results in as uniform a consolidation as is technically possible. Of course, there is still a density gradient across the part, the center is always less compacted than the surface (see also Section 8.3), but the gradient is uniform and, typically, does not cause distortion during sintering. Although isostatic pressing was already mentioned and patented around 1910, only approx. 30 years later the technology found a general and large scale application for the direct isostatic pressing of spark plug insulator preforms [B.42]. Technically it is quite immaterial whether or not sintered ceramic spark plug insulators are totally straight. However, as a consumer product which, at that time, needed replacement often, the part, in addition to functioning well, had to look good. After introducing isostatic
8.4 Pressure Agglomeration Technologies
pressing into the manufacturing process, the number of rejects became very low. Later, the technology was used for the production of many other ceramic parts, particularly of the high performance variety, and for many applications in the metal powder industry, including mechanical alloying, for composite parts, consisting of ceramic, metallic, and plastic components, and for plastics (especially PTFE), explosives, chemicals as well as pharmaceutical specialties. Isostatic pressing is carried out cold or hot. The most common application is still cold isostatic pressing (CIP) which is performed at ambient temperatures. In hot isostatic pressing (HIP) the forming and densification process is achieved uniformly by heated high-pressure gas in an autoclave. The material to be processed itself is often also brought to elevated temperature prior to loading. Contrary to CIP, in which powders are always containerized, HIP may be applied for containerized powders and also for preformed metal, ceramic or plastic components. Manufacturing of the preforms may be by any method, including, for example, casting. In the latter case, porosity and structural flaws can be virtually eliminated and the properties as well as the service life of the parts may be markedly improved. Related functions are near net shape forming and diffusion bonding of dissimilar materials. The term hydrostatic pressing is often used as a synonym of isostatic pressing. Isostatic pressing is the generic term covering liquids and gases as the pressure transmitting medium whereas hydrostatic pressing is best reserved for liquids. However, the two are used interchangeably to cover both aspects. If flexible containers are used, their arrangement may be such that they contract or dilate by the application of pressure. Whether the tooling is an integral part of the press or loaded and removed during each compaction cycle determines if it is a “dry bag” or “wetbag” process. The difference between the two methods is illustrated in Fig. 8.147.
Fig. 8.147: Schematic representation o f the differences between dry bag and wet bag pressing [6.12a, 6.421.
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Fig. 8.148: Operational sequence o f a dry bag isostatic press for the manufacturing o f spark plug blanks [B.12a, 6.421.
In the dry bag process, the flexible container is fixed in the pressure vessel and the powder is loaded directly. The tool forms a membrane between the fluid and the powder. Optionally, the flexible container may be placed inside of a primary diaphragm so that the powder never comes in contact with the fluid, even if the flexible mold is damaged or breaks. Therefore, dry bag tooling also has the advantage that the fluid is not contaminated with the powder. However, because the container must stand up to many pressing cycles and since changing it is time consuming, it has to be made of a very durable material. The wet bag process, in which the container, that has been filled externally with the powder, is entirely submerged in the fluid inside of a pressure vessel, utilizes the simplest type of equipment. This process is commonly applied in the laboratory or for pilot plants and is commercially used for the production of single large components or a large number of small parts. While dry bag tooling can be fitted with means to remove the gas that is being displaced during densification (for example, through internal rigid formers and the breech plug, see Fig. 8.148),the material in containers for wet bag pressing must be consolidated and evacuated prior to closing and loading into the autoclave to avoid compressed air pockets which, upon pressure release, may damage the structure of the compacted parts (see Section 8.1).
8.4 Pressure Agglomeration Technologies
The basic principles of isostatic powder pressing are: 1.
2.
3. 4.
5.
The wet bag pressing of large and/or complex shapes in which the flexible container is filled and prepared outside the pressure vessel and then immersed in the fluid and compacted. The dry bag pressing of smaller, regular shapes in which the tooling forms an integral part of the pressure vessel. The use of internal or external rigid formers to produce accurate surfaces. Pressurization by systems using pumps or by direct compression with pistons in a die. Handling systems for the filling, preparation, loading and unloading of powders and parts as well as for the tooling and pressing equipment.
Therefore, isostatic powder compaction equipment consists of powder storage and dispensing facilities, at least one pressure vessel with means for loading and unloading the tooling or parts, pressure generator(s),and related items that enable effective and safe operation of the process. Particularly dry bag pressing is used for the production of small components at a high rate. As shown in Fig. 8.148, which demonstrates the compaction, ejection, and filling of a dry bag press during the manufacturing of a spark plug insulator, it is relatively easy to automate the operation of this process. The permanent location of the tool and small fluid volume surrounding it contribute to a fast operation. Production rates in the neighborhood of 100 parts per minute are common. Where mass production of simple, small compacts from powder is the task (e.g.
Fig. 8.149 Operational sequence of an automatic isostatic press with round, timed tooling table [B.lZa, €3.421.
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-.+-
--
Fig. 8.150 The principle of intensifiers [B.l2a, 8.421.
t-
Secondary
Pressure
t
Fig. 8.151: Photograph and schematic (inset) of a modern high pressure intensifier pump (courtesy EPSI, Haverhill, MA, USA).
spark plug insulator blanks, other electrical insulators, grinding media, carbide tool bits, etc.), the equipment usually takes the form of a multi cavity press or a series of small machines which operate similarly to conventional hydraulic presses. The actual production rates depend on powder properties, size of the part, maximum pressure and potential dwell time requirements, the number of tool cavities as well as preparation and handling needs. Large automatic units have been developed. Some such installations include a round, sequenced pressure chamber system (Fig. 8.149). Generally, the time needed to reach the required pressure is of great concern in isostatic pressing. It also depends on a number of factors, including the volume of the autoclave vessel, the compaction ratio of the powder, the compressibility of the fluid, and, most importantly, the delivery rate of the pumping system. To speed up pumping, it is possible to apply a number of pumps in parallel. Alternatively,
8.4 Pressure Agglomeration Technologies
Fig. 8.152 Photograph of an automated wet bag cold isostatic press (pressure chamber dimension: 430 mm dia. . 1,000 mm high, 200 MPa max. pressure) with mold washing and handling system (courtesy EPSI, Haverhill, MA, USA).
a pump system using different types of pumps to reach different pressure levels may be designed. Intensifiers (Fig. 8.150) increase the primary pressure according to the relationship of the piston areas. Fig. 8.151 is the photograph of a modern high pressure pump system with intensifier and double action design (see inset). The pump provides a constant flow of pressurized liquid from two opposed cylinders as the central hydraulic piston cycles back and forth. Fig. 8.152 is the photograph of an automated wet bag cold isostatic press. As mentioned before, one of the disadvantages of the wet bag technology is the danger of contaminating the pressure transmitting fluid with powder that either adheres to the outside of the bag, as a result of the filling, processing, and handling procedures, or arises from a failure of the container. Up to approx. 400 MN/m2 (= MPa),
Fig. 8.153: View into the cavity of a large wet bag production CIP (pressure chamber dimension: 1,220 m m dia. . 2,500 m m high, 70 MPa max. pressure) (courtesy EPSI, Haverhill, MA, USA, and Seagoe Adv. Ceramics, N.-Ireland).
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Fig. 8.154 Pressure levels [in MPa] required during cold isostatic powder pressing for the manufacturing of certain product groups (courtesy EPSI, Haverhill, MA, USA).
most cold isostatic presses operate satisfactorily on an oillwater emulsion or hydraulic oil which can be easily cleaned with conventional means or discarded after contamination. At higher pressures, special liquids may have to be used for which compatible tooling materials are required and with which problems may develop when it becomes necessary to dispose of the often toxic fluids. To avoid or delay contamination, mold washing may become part ofthe handling system (Fig. 8.152). Fig. 8.153 is a view into the open pressure chamber of a cold isostatic press and also shows the breech plug handling frame. As illustrated in Fig. 8.154, different pressure levels are required for the consolidation ofvarious powders. Pressures below 200 MPa are sufficient for the manufacturing
Fig. 8.155: Some typical parts (see also Fig. 8.154) that were manufactured by cold isostatic powder pressing (courtesy EPSI, Haverhill, MA, USA).
8.4 Pressure Agglomeration Technologies
Fig. 8.156 Photograph ofan open production scale (pressure chamber dimension: 850 mm dia. . 1,700 mm high, 200 MPa max. pressure, 1.400'C) hot isostatic pressing (HIP) unit (courtesy EPSI, Haverhill, MA, USA).
of parts from carbon/graphite, refractories, certain ceramics and, particularly, plastics (PTFE). Cemented carbides and powder metals, on the other hand, may necessitate pressures of up to GOO MPa. Fig. 8.155 depicts some typical examples ofparts that were produced by cold isostatic powder pressing. During hot isostatic pressing, parts to be HIPed are loaded into a pressure vessel which contains a modular electric furnace. A thermal barrier is placed around the furnace to direct the heat toward the parts and away from the water cooled vessel walls. Argon or other (inert) gases are used as the pressure transmitting medium. Depending on the temperature requirement and the atmosphere, interchangeable plug-in furnaces are available. Iron-chromium-aluminum (FeCrAl) furnaces create temperatures of up to 1,200"C and are capable of operating with a concentration of up to 20 % oxygen in a balance of argon; molybdenum furnaces, with temperatures
Fig. 8.157: Schematic lay-out of a complete industrial HIP installation (courtesy EPSI, Haverhill, MA, USA).
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PM
Castings
(
Fig. 8.158 Typical examples of HIP applications (courtesy EPSI, Haverhill, MA, USA).
Ceramics
HIGHPRESSUREGAS
1
Fig. 8.159 Schematic representation ofthree different types o f hot isostatic pressing [B.61]. (a)/(a’): Capsule HIP/sintering; (b)/(b’): sintered preforrn/HIP (c)/(c’): HIP/sintering for making porous products.
8.4 Pressure Agglomeration Technologies
to 1,450 "C, are suitable for applications requiring a clean environment; and graphite furnaces are designed for use in argon and nitrogen up to 2,000 "C. Graphite furnaces have a high resistivity and are, therefore, very well suited for operation in vacuum where the use of low voltage is necessary. As with all modern technologies, a computer control system is a standard feature. It is particularly desirable in hot isostatic pressing for the flexible programming of cycles, for process supervision, and data logging. Typically, all current system conditions are displayed and each cycle is stored under a specific name for review at a later time and quality assurance. Fig. 8.156 is the photograph of an open hot isostatic press and Fig. 8.157 is the schematic lay-out of a complete installation. Ancillary equipment may include load preparation stations, electrohydraulic compressor(s),vacuum pump(s),high pressure valve system, and closed loop vessel cooling system. Cooling of the parts and the interior of the furnace after finishing the consolidation cycle is particularly important in hot isostatic pressing. Cooling gas must be circulated uniformly through the entire work zone so that thermal distortion and grain growth are minimized. Fig. 8.158 shows three typical HIPed parts. While, originally, hot isostatic pressing was developed and used to remove defects and/or produce parts with minimum porosity and, consequently, ultimate density from powders and preforms, the study and understanding of the mechanisms of pressure agglomeration also led to a modification of the process which is then actually used to produce parts with a controlled high porosity [B.61] (see also Section 5.3.2). Fig. 8.159 illustrates schematically the different types of hot isostatic pressing. (a)/ (a') and (b)/(b')represent the conventional applications of hot isostatic pressing which ultimately result in dense products. HIPing which, because of the temperature of the process is associated with sintering (see Section 9.1), is applied to powders that are encapsulated in containers made from metal or glass (a) or to presintered bodies with closed pores (see also Section 5.3.2, Fig. 5.47). During the new HIP for making porous products, open, non-containerized powder compacts or loosely sintered bodies are HIPed directly at high temperatures in a high pressure gas atmosphere. In this situation, densification is avoided or delayed by the high pressure gas that fills the open pores. High open porosity of sintered materials, which experience considerable densification during conventional sintering alone, can thus be obtained by the combination of HIP and final sintering.
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Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
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9
Agglomeration by Heat/Sintering
At a certain elevated temperature, which is different for various materials, while still in solid state, atoms and molecules begin to migrate across the interface where particles touch each other. Depending on temperature, time, and intensity of contact (caused by pressure during the manufacturing of the preform or the sintering process itself) diffusion of matter forms bridge-like structures between the surfaces which solidify upon cooling. The process may also result in a densification of a compact which is associated with a densification by the elimination of pores and shrinkage. This entire group of phenomena is called sintering. Sintering is a binding mechanism (see Section 5.1.1), a size enlargement process by agglomeration (see Sections 9.2, 9.2.1, and 9.2.2), and a method of post-treatment to create final characteristics of agglomerated products (see Sections 7.3 and 8.3). In the following, after a short introduction into the sintering mechanisms, sintering as a size enlargement process by agglomeration will be covered. Different parts of the book should be consulted for information on the other meanings of the word. 9.1
Mechanisms of Sintering
As already mentioned earlier, particles in a powder mass can be bonded in solid state at elevated temperatures below the melting or softening temperature of the material@) (see Section 5.1.1). This process is called (solid state) sintering. If the powders are predensified in a compact, sintering is typically accompanied by further densification of the body until only few and isolated pores remain, but other processes, which retain some open porosity can occur, too [B.Gl] (see also Section 5.3.2). The geometrical arrangement of the particles largely determines what can happen during sintering although it does not predict whether or how fast particular changes occur [B.l2c]. The driving force for sintering is the diminution of surface area of the assembly of original particles. The accessible internal surface of the particles or surface of the pores between the particles features a specific surface energy. This specific surface energy is due to the fact that surface atoms have no neighbors. The reduction of free surface also leads to a reduction in surface energy. Therefore, sintering occurs with a reduction in total surface energy and, accordingly, the total free energy of the powder decreases with sintering.
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by Heat/Sintering
Most attention has been focused on the kinetics and mechanisms of sintering and less on the geometry of the process [B.l2c]. In general, a particulate multiphase body consists of a series of cells (pores) and grains which meet, in stable configurations, three to an edge and four to a point. These three-grain edges and four-grain corners are the fundamental units of structure and their shapes are determined by the equilibrium between the various surface energies involved. When there are two phases, one of which is porosity, it is energetically favorable for the second phase (porosity)to occupy, in order of preference, four-grain corners, threegrain edges, two-grain faces, and, lastly, sites in the interior of grains, because the total amount of interface or of interfacial energy is thereby reduced. Basically, sintering occurs in stages. During the first stage of sintering, the points of contact in a collection of particles grow into necks by mass transport (see also Section 5.3.2, Fig. 5.48). The difference in free energy or chemical potential between the surface of the neck area and the surface of the particle provides the driving force [B.Gl]. After a time and at a point where porosity becomes approx. 15 %, the grain boundary energy begins to be a significant contributor to the total energy of the system and the grain boundaries begin to rearrange themselves to minimize their total area. The geometry in this second stage of sintering becomes that of an assembly of polyhedral grains with pores along the three-grain edges (Fig. 9.1) and the tendency to minimize the grain boundary area results in grain growth. The remaining pores continue to shrink as sintering proceeds and, in the third stage of sintering, become unstable as approximate cylinders along the three-grain edges and pinch off to become isolated pores at four-grain corners. This third stage commences at approx. 5 % total porosity and may continue until all porosity is eliminated. Fig. 9.2 depicts typical curves of the change of open and closed porosity during the second and third stages of sintering as functions of relative density [B.l2c]. A new phenomenon may occur during the third stage of sintering. If gas is trapped in the closed pores its pressure will rise as the pore size decreases until further shrinkage stops when an equilibrium is reached. It is, however, energetically favorable from the point of energy stored in the compressed gas and neutral from the point of surface energy if gas transfers from small pores with high pressure to large pores with low pressure. If this is possible, the result is an increase in the volume of the part, a phenomenon called bloating.
\ Particle
Fig. 9.1: Schematic drawing o f the pore structure developing in the second stage of sintering.
9.7 Mechanisms of Sintering 15-
-zc i
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Fig. 9.2: Example of the change of open and closed porosities during the second and third stages of sintering as a function o f relative density (B.12~1.
Relative density (%)
The adjustments that are needed during sintering to minimize the surface energy in a body necessarily involve the movement of matter. This has been previously discussed in some detail in Section 5.3.2 (Fig. 5.48 and Tab. 5.7) but shall be repeated here in a somewhat different presentation. In the case of a single solid phase, matter can be moved under the influence of surface energy from the neighborhood of convex surfaces to that of concave surfaces by several mechanisms. These are: 1. Evaporation and subsequent recondensation, influenced by vapor pressure and
surface curvature. Diffusion over the surface, atom by atom. Plastic flow by dislocation movement in a crystalline material. 4. Viscous flow in an amorphous material, requiring a liquid phase. 5. Bulk diffusion through the solid, down to the vacancy gradient produced by the pressure gradient resulting from variations of the surface curvature. 2. 3.
Of these, the first two are not capable of moving the centers of particles closer together and, therefore, do not cause densification. They are, however, causing neck growth and strengthening of the initial assembly or compact of particles. The other processes can cause both neck growth and shrinkage. In addition to solid state sintering, liquid phase sinteringis possible. From a binding mechanism point of view, this process could be described as one using partial melting (see Section 5.1.1). In connection with sintering, an appreciable volume of a liquid phase must be present, the solid must be soluble in the liquid, and the liquid phase must completely wet the solid. These are criteria that are also required for growth agglomeration (see Section 7.1), although the liquid source and the structural consequences are different in sintering. Liquid phase sintering is, for example, used for hard metal alloys from powders [B.l2c]. Because of the great hardness of the carbide particles, which constitute the
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bulk of hard metal powders, it is impossible to press the powder to a density >GO % of theoretical. Yet, by liquid phase sintering a perfect pore-free compact can be obtained. The detailed metallurgical description of the sintering operation is highly complex because of the complicated nature of the phase equilibria that are involved. Very simply explained, sintering begins by solid diffusion, frequently aided by a carburizing atmosphere, and results in the formation of a tungsten-cobalt-carbon eutectic with a relatively low melting point at suitable points of contact between particles. Once small amounts of liquid have formed, further liquefaction proceeds rapidly and surface tension forces cause rapid flow and wetting of all the other solid surfaces. This is accompanied by considerable contraction (shrinkage), often 40 % by volume. The whole process occurs with great speed at only a few degrees above the eutectic temperature. Very small quantities of liquid phase, approx. 1 % by volume, which is much less than one might assume necessary, are sufficient to result in rapid and near complete densification. The high packing density must be caused by a considerable rearrangement of the positions of carbide particles. However, because of the small volume of liquid that is involved it is not possible that this densification is obtained by particle rearrangement alone. Even with the best packing of the solid particles, the liquid phase would be insufficient to completely fill the interstices. Therefore, some change in particle shape must also occur and there must be considerable material transfer at the contact points to bring about this shape change. For this, the only possible phenomenon is the solution-reprecipitation process. Ceramic parts are often made from finely powdered components which are shaped by a pressure agglomeration technique and then sintered by the application of heat. In some cases this simple technique is not applicable because the powders may not sinter together, the sintering temperature or atmosphere requirements may be impractical or uneconomic, or a base material may not be readily available or may decompose under normal sintering compositions. In some of these cases, reaction sintering may offer the possibility of making sound ceramic bodies, often also with high density, which are impossible or at least difficult to produce by other methods [B.l2c]. The term reaction sintering is not very well defined. In the simplest case, the powder that decomposes during heating is substituted although, in the true sense of the word, this is not actually reaction sintering. In true reaction sintering processes, two (or more) components of the desired ceramic compound are selected such that they react with each other during sintering. In a first method, the powder components are mechanically mixed, shaped, and reaction-sintered. It is particularly suited for those materials which are solids at room temperature and feature relatively high melting points. Another process is applicable when one of the constituents is gaseous at room temperature (e.g. oxides, nitrides) or of low melting point relative to the other components (e.g. phosphides, sulphides and some silicides or aluminides). Then the more refractory powder components are shaped and afterwards reacted hot with the other constituent in gaseous or liquid form. The preshaped body must be porous to allow for entry of the reactant(s) and for extra volume (if any) of the product of reaction. As mentioned before, sintering may be beneficially carried out under pressure (see also Section 8.4.4, HIP). During pressure sintering of a powder, an external pressure is applied and the initial stage of compaction, probably up to a relative density of approx.
9.2 Sintering Technologies
85 %, includes the complex mechanisms of pressure agglomeration, i.e. particle packing, sliding, fragmentation, and deformation (see Section 8.1).The subsequent intermediate (featuring connected porosity) and final (with closed porosity) stages both involve a solid matrix and a definite pore system. A theory of pressure sintering [B.12c]is valuable because it allows to extrapolate experimental data for a given material, for predicting performance under changed conditions, and also allows the calculation of viscosity or diffusion data, thus affording a means of assessing possible results when changing the composition of the material. In pressure assisted sintering, the more accurate name for pressure sintering, pressure and heat are applied simultaneously to a powder that is enclosed in a die. Generally, it permits the use of lower temperatures and pressures and shorter processing times than those required for cold pressing and subsequent sintering. It can also assist in the production of parts with finer grain size, lower porosity, and higher purity. As mentioned in Chapter 9, sintering is a binding mechanism and the different technologies may be used for size enlargement by agglomeration and as methods for the creation of final characteristics of various products. Depending on their applications, the required properties of pieces or parts after the sintering process may vary widely. Correspondingly, different methods for powder preparation, the manufacturing of preforms, and the application of heat must be chosen. For example, iron ore pellets, which must feature high strength to guarantee the excellent transportation and handling characteristics required for a bulk commodity, uniform size and shape for good and reproducible packing in shaft and blast furnaces, and a large percentage of open porosity for optimum reducibility, are made by tumble/growth agglomeration in discs and drums (see Section 7.4.1), dried and sintered to produce necks between the ore particles but retain the high porosity of the agglomerates. Powder metal or most ceramic parts, on the other hand are formed into dense compacts by pressure agglomeration (see Sections 8.4.1 through 8.4.4) and then to yield well sintered, possibly with the assistance of pressure (HIP, see Section 8.4.4) (sometimes near net) shaped parts with nearly theoretical density and high strength. Other powder metal or ceramic parts may have to become filters or catalyst carriers requiring large numbers of penetrating pores (see Section 5.3.2), uniform structure, and high strength. In those cases sintering of preforms is carried out such that no densification occurs and porosity remains unchanged.
9.2 Sintering Technologies
To remain within the scope of this book, the description of industrial agglomeration processes, only those sintering technologies will be reviewed which are used either directly for the size enlargement of particulate solids or for the post-treatment of agglomerates to gain final product properties. The many highly sophisticated sintering techniques that have been developed during the past few decades for the production of new, engineered, often composite materials with novel characteristics, will be covered in a future book [B.71].
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9 Agglomeration by HeatlSintering
9.2.1 Batch Sintering
Batch sintering in stationary furnaces of different kinds is used primarily for the posttreatment of agglomerated products to gain final strength and properties but also for laboratory work in connection with the development of continuous sintering technologies and for pilot plants [B.l2c, B.lG, B.251. The two most important applications of batch sintering are in the ceramic and powder metal industries. Muffle, bell, elevator, or pit type furnaces are used. As shown in Tab. 9.1, the processes occurring during sintering are somewhat different for ceramic and powder metal parts. During sintering, ceramic ware is heated to a temperature between 700 and 2,OOO"C. Because raw ceramic parts are almost always "green" (= moist), removal of water is the first post-treatment step. Following or simultaneously and immediately preceeding firing, binders and plasticizers, which have provided the properties needed for forming, are also removed. The amount of residual moisture and/or additives that can be tolerated in the part when firing begins depends on its shape and structure as well as on the heating rate of the furnace. If the part is made by dry pressing, removal of additives can be incorporated into the heat-up stage of the sintering furnace if the time for this process is not too long. However, since additives are often cellulose, wax, or starch type products, they can be conveniently decomposed by oxidation in air at low temperature prior to loading the parts into the furnace. In the furnace, clay minerals usually dehydroxylizebetween 500 and GOO "C whereby steam is produced. The loss of strength that occurs at this stage may result in cracking. Often, carbon and sulphur compounds are present in unfired ceramics. They must be oxidized before sintering densification has advanced too far to avoid black cores. Oxidation can be accomplished by holding the temperature at a certain level which varies with the manufacturing method that was used for and the type of the body but is often in the range of 900 "C. A decomposition of carbonates and sulphates may produce bloating in vitrified parts. Silica, which exists in many different crystalline forms, is an important constituent of most ceramics. The conversion from one form into another is accompanied by sometimes large volume changes. Because this occurs during heating and cooling, the rate of temperature change must be considered and may have to be adjusted to avoid deformation and/or cracking. Processes occurring during the sintering of ceramic and powder metal parts [B.lZc].
Tab. 9.1:
Ceramic Parts
Powder Metal Parts
Removal of water Removal of binder and organic media
nla Burn-off of pressing lubricant
Dehydroxylation Oxidation Decomposition
nla Heating to sintering temperature Soaking
Phase transformation Cooling
nla Cooling
9.2 Sintering Technologies
Fig. 9.3: Photographs o f simple atmospheric muffle furnaces (courtesy Casbarre, Sinterite Furnace Div., St. Marys, PA, USA).
Most ceramic products are fired in air, i.e. under oxidizing conditions. The ideal kiln for the firing of ceramics is capable of heating and cooling the parts uniformly at the maximum rate of temperature change for each of the stages mentioned in Tab. 9.1. Simple mume furnaces are typically used for batch sintering in the ceramics industry (Fig. 9.3). For high quality wares, temperature control is very important to avoid the previously mentioned potential problems in different processing stages. It can be accomplished easiest and most accurately in batch furnaces although many bulk ceramic products must be of such low cost that continuous furnaces are used which operate more economically (see Section 9.2.2). Some materials must be, at least during certain stages, fired in reducing atmosphere which can be also easily provided in batch kilns. In powder metallurgy, sintering requirements are different. Although the volatilization of pressing lubricant from the compact prior to sintering is sometimes carried out separately, it is more typically an integral part of the process. During sintering itself, temperature is held constant so that no distortion takes place and full bonding is obtained. Therefore, temperature control and soaking periods are most important. Additionally, it is necessary to retain the composition of the atmosphere to ensure reproducibility of strength, carbon content, dimensional stability, etc. of the final part. Therefore, ingress of air into the furnace during sintering must be avoided. This is achieved by using either a gas tight furnace shell or a muffle or retort which is usually manufactured from a nickel-chromium alloy. Batch sintering furnaces are employed for: 1. Low output production,
2.
3.
special duties, (because there are no moving parts, batch furnaces can be designed for higher temperatures; furthermore, since it is possible to seal the interior more effectively, purer atmospheres can be realized and maintained) and experimental work.
For (1)and/or (2), a small manual pusher furnace can be applied in which parts on a tray are moved through a furnace, one tray at a time (Fig. 9.4). If it features gas tight
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9 Agglomeration by Heat/Sintering
UN- I I LOAD I
I
COOL
I
l
l
I 1 lEXlT .DOOR
ELEMENTS
I I
CURTAIN
Fig. 9.4 Sketch o f a longitudinal section-box or manual pusher furnace [B.25].
interlocks and/or doors for charging and discharging, it can also be used for sintering processes in which the atmosphere must be well controlled. The bell type furnace (Fig. 9.5) is widely used for P/M parts requiring long sintering cycles. Typical equipment consists of one or more supporting bases with removable sealed retorts, to cover the loads and to retain the protective atmosphere around them throughout the entire heating and cooling cycles, a portable heating bell, and a standby (idling) base, a hoist, and an optional (not shown) cooling bell. The elevator type furnace (Fig. 9.6) is useful for sintering heavy and/or bulky loads. It has an elevated heating chamber with open bottom in a fEed position, a mechanism for raising and lowering load supporting cars into and out of the furnace, a stand-by car to plug the kiln opening during idling periods, and optional cooling chambers. It is also applicable if protective atmospheres of exceptionally high purity are required. Flexible hoses carry atmosphere gas and cooling water to and from the cars. -/LIFTING
IDLING BASE
HEATING BELL ON RETORT
EYE
RETORT ON LOAD
Fig. 9.5: Schematic representation o f a bell furnace [B.25].
LOAD ON BASE
9.2 Sintering Technologies
ELEMENTS
ON 4 SIDES-
LOAD-SUPPORTING Fig. 9.6
CAR
1
Schematic representation o f an elevator furnace [B.25].
Batch kilns can be operated under vacuum and as direct-resistance furnaces for the sintering of refractory metals (e.g. for tungsten at 3,000 "C). For hardmetal processing, lower temperatures are used. Fig. 9.7 shows typical pressure and temperature profiles. A complete sintering cycle may take from G to 14 h or longer for thick parts. Therefore, such furnaces are often connected in pairs to a common vacuum pumping system and power supply as well as a single set of controls because approx. 50 % of the cycle is required for heating under vacuum and the balance for cooling in inert gas. During various phases of the heating cycle, inert or active gases may be injected into the vacuum system, thereby changing the partial pressure in the sintering chamber, to
150013001000
e,
1100-
-
900-
100
P)
5
10
E
700-
E I-
Pressure
500-
/
1.o
!--\
.1
\ //
\
I
'.
.01 Pressure 2 :Torr)
_I
1
2
3
4
5
6
7 8 9 1 Time (Hours)
Fig. 9.7: Typical pressure and temperature profiles o f a vacuum sintering cycle in a batch furnace [B.25].
0
1
1
1
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9 Agglomeration by Heat/Sintering
Pat grate Insulating materiol : fired @lets
1.) Green pellets
2.1 hsulating side walls : pellet fragments 3.1 Grote bars :cast alloys 4 .1 Insulating hearlh layer
Pot grate Insulating material : rehactory lining
1.) Green pellets 2.) hsulating side walls: bricks or rammed mas: 3.) Grate bars: usually silicium corbide
Fig. 9.8: Two-pot grate furnaces for the heat treatment o f (iron ore) pellets [8.16].
achieve special effects which influence the structure and properties of the sintered part. Since batch furnaces are normally relatively small and can be controlled easily, they are also commonly used for development work. For the sintering of ceramic or powder metal parts the results from small scale testing can be directly transferred to larger or continuously operating kilns. As will be shown in Section 9.2.3, large continuous sintering facilities are used in the minerals industry for the size enlargement of fine ores and the induration of “green balls”, spherical agglomerates made by tumble/growth agglomeration from fine ore concentrates (see Section 7.4.1). Since, in the development phase, the heating and gas flow conditions in industrial plants must be simulated, batch pot grate sintering equipment (Fig. 9.8) is being used [B.lG]. Representative tests for determining the performance of travelling grate, grate-kiln, and shaft furnace processes (see Section 9.2.3) can be carried out in these laboratory and pilot facilities. According to the actual conditions in industry, pot grates are operated either with insulating side walls of fired pellet fragments and a hearth layer (Fig. 9.8, left) or are equipped with a corrugated refractory brick lining (Fig. 9.8, right). In the first case, the pot grate itself is a metal container with a bottom of metallic grate bars. By placing indurated pellets between the hot wall and, respectively, the grate and the green pellets which are to be sintered, the charge is protected by a refractory envelope and overheating of the metallic parts is avoided. Other pot grates feature a refractory lining and a high temperature resistant grate (Fig. 9.8, right). To eliminate the wall effect
9.2 Sintering Technologies
in these relatively small furnaces and to be able to test samples of different ores during a single test run, stainless steel baskets, which are filled with the appropriate ores or pellets, are embedded in the charge. After the test, they are retrieved and the characteristics of the fired materials are determined. During a pot grate test the following parameters are determined and may be varied: Direction of the air flow, gas volume, suction, and pressure in the wind box, preheating rate and temperature profile, fuel type (gas or oil), gas atmosphere (using additional oxygen, if necessary). Fig. 9.9 shows the operation of a pot grate and the locations of thermoelements. In this case, drying is carried out first with updraft warm air (flowing up through the pellet bed), followed by downdraft sintering with hot air from the burner above the bed, and, finally, cooling is accomplished by an updraft flow of ambient air. It is also possible to design the system such that during sintering separate up- and downdraft stages can be used. During the entire process, the temperatures are continuously monitored as they are decisive for the quality of the fired product. In a pot grate as shown in Fig. 9.9, the different process stages occur intermittently, one after the other. When the flow of gas is reversed, the temperature ofthe peripheral
If Burner Buret hood movable
I2 M i l e GIpellet bed
Fig. 9.9 Operation and temperature control o f a pot grate [B.16].
I - lemperoture control points
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Fig. 9.10:
Movable pot grate system for sintering tests [B.16]
parts, which are associated with the pot grate itself, must first change to the new condition which can critically modify and impair the test conditions. To avoid this, a movable pot grate system as shown in Fig. 9.10 was developed [B.16] in which all equipment for the different steps is kept at process conditions and the pot grate is moved as necessary into the various positions. It should be mentioned that pot grate sintering machines may be also used for the industrial sintering of small amounts of metal ores. Fig. 9.11 shows the flow diagram of a pan sintering plant [B.23]. In such sintering systems, heat is provided by the burning of carbonaceous solid fuels that were mixed with and are uniformly distributed in the charge. After ignition of the bed surface, for example by depositing red-hot CYCLONE SEPARATOU
RETURN SINTER
COKE ORE
")
SINTERING
MIXER
Fig. 9.11:
PAN,
Flow diagram of a pan sintering plant [B.23, p. 2121.
9.2 Sintering Technologies
charcoal or coke, oxygen from the air, which is pulled through by a suction fan, produces the intense heat that is necessary for sintering. During sintering, the entire charge becomes one large cake which, after cooling, is broken and screened into the desired sinter size. Fines are recirculated and blended with fresh fine ore and fuel. Such plants typically produce 30-50 t/day of sized sinter for use in reduction furnaces. 9.2.2 Continuous Sintering
As mentioned in Section 9.2.1, sintering of ceramic wares normally occurs in oxidizing atmosphere and without a special gas environment. Therefore, continuous sintering furnaces are often directly flame heated. Fig. 9.12 is the side elevation of a tunnel furnace for the firing of ceramic parts, indicating the direction of material (car) and gas movement as well as the process zones. The diagram below depicts the temperature profile over the length of the furnace and shows that temperature control is normally quite unpretentious. Most tunnel kilns for ceramics are of the car type. Cars are more rugged and reliable than belts and other continuous methods of movement. Fig. 9.13 is a schematic cross section through the sintering zone of a directly fired, atmospheric tunnel furnace. The tunnel is enclosed by refractory walls and a simple sand seal prohibits the exit of hot combustion gases at the car base and wheels. Operation of modern furnaces is computer controlled and continuous movement is accomplished with automatic loading and unloading systems. Fig. 9.14 is the partial view of a state-of-the-art tunnel kiln for the firing of table ware and also shows an automated car handling system. [krection of cars
Reheat zone---Firing
zone-----
1200I 100
-
Temperature wore/'C
1OOO900 800 -
700 -
Fig. 9.12 Side elevation (schematic) o f a directly flame heated tunnel kiln for the firing o f ceramic parts and typical temperature profile [B.l2c].
Cooling zone-
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9 Agglomeration by Heat/Sintering
Combhion spcce
Cor
/
wheel
Fig. 9.13: [B.l2c].
\
Car base
‘Sand
seal
Cross section through the sintering zone o f a tunnel kiln
In comparison, continuous furnaces for the sintering of P/M parts are more complicated because of the needs for a more differentiated temperature profile (Fig. 9.15) as well as for controlled gas environments (see Section 9.2.1). The latter requires some sort of separation of the atmospheres in different sections along the kiln, either by oscillating doors or by gas curtains.
Fig. 9.14 Photograph o f a modern tunnel kiln for the tiring of table ware with a fully automated car handling system (courtesy Eisenmann, Boblingen, Germany).
9.2 Sinterjng Technologies
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BURNOFF HEAT
+SINTER -+-? SLOW
I SOAK
I HEAT I SOAK lCOOLl
TI ME Fig. 9.15: Typical temperature profile o f a continuous furnace for the sintering o f powder metallurgical parts [8.25].
Fig. 9.16: Schematic longitudinal section through a continuous mesh-belt sintering furnace [B.25].
Fig. 9.17: Schematic and photograph o f a horizontal mesh-belt sintering furnace including an optional “accelerated delube system’’ (ADS) (courtesy Casbarre, Sinterite Furnace Div., St. Marys, PA, USA).
COOL
-- 7
I
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9 Agglomeration by HeatlSintering
The most commonly used conveyor in continuous furnaces for the sintering of small, light parts is the mesh-belt. Fig. 9.16 is a schematic longitudinal section through a mesh-belt sintering furnace and includes an indication of the different zones. The doors, which can be manually operated, are usually left open and the atmospheric conditions within the furnace are created by and between the gas inlets. This puts a certain strain on the atmosphere generating equipment as ample capacity must be available. Fig. 9.17 shows both the schematic and a photograph of a horizontal mesh-belt sintering furnace which includes an optional "accelerated delube system" (ADS) for the rapid removal of powder metal lubricants. A variation ofthe straight mesh-belt furnace is the humpback kiln (Fig. 9.18), which is used where high purity of the atmosphere in the sintering zone is required. The belt in a long, inclined, gas tight purge chamber carries the work from the charge area up to the elevated hot zone. This design is particularly advantageous if light gases are used in the sintering zone because these gases tend to naturally rise to the highest point of the furnace. Fig. 9.19 is a schematic representation of a roller hearth furnace. Loaded trays are conveyed through the kiln by riding on individually driven rolls (Fig. 9.20). Depending on roll spacing, a section is capable of holding a substantially greater load than an equivalent length of mesh-belt. The grade of alloy used for the rolls limits furnace temperature to between 1,150 and 1,260 "C. The charge and discharge doors are automatically opened or closed and are interlocked with the tray handling system. Because they are only opened when a tray is charged or discharged, the amount of atmosphere gas is optimized and heat losses are minimized. HEATING CHAMBER
COOLING CHAMBER E X I T INCLINE
ENTRANCE INCLINE
UNLOADING
LOADING
Fig. 9.18
Schematic representation o f a hump-back furnace [B.25].
. " . I . , * .,a 11"...1".C
~UI.1.
.I..
C*."01
<.D
U"'l.
<..
..," "1.1 C"..l.
'C... ..I.. ,111 P
,
u* I-
Y I C I . U "
Cc.l.0,
I.I"Y1CO
Fig. 9.19 Schematic longitudinal section through a continuous roller hearth sintering furnace [B.25].
..TI.
COO,IOC".I"
.,**"s ./l.,.-.r.l
DllC"."Ol
IlD
9.2 Sintering Technologies
Fig. 9.20 Roller drive system using cogged belts which offer durability, low maintenance, and quiet operation. Roller hearth sintering furnaces with temperatures o f up to 1,450"C can be equipped with this design (courtesy Eisenmann, Boblingen, Germany).
YIWAKAL
wswn
P U R M IPIIE-UEAl CHLYICN
WW R t L f C R W I I
U l E R CODLED C*&mER LI117*-1-.111*¶
u I u I * P I M * PL.lTWY
WIYrnht
Fig. 9.21: Longitudinal section through a continuous pusher furnace [8.25].
The pusher type furnace that was shown schematically as manually operated equipment in Fig. 9.4 (Section 9.2.1) can be mechanized and then becomes a continuous kiln. Fig. 9.21 is the longitudinal section through a continuous mechanical pusher furnace. It is suitable for sintering metal parts which are too heavy for the meshbelt and production rates do not warrant the roller hearth. It can also be used for sintering temperatures of up to 1,GSO"C which are too high for the mesh-belt and the roller hearth furnaces. Mechanically or hydraulically operated, intermittent or continuous, stoker type pushers are available. Fig. 9.22 is the photograph of a pusher furnace for the high temperature sintering of powder metal compacts. The final typical design of continuous sintering furnaces for powder metallurgy and similar applications is the walking beam furnace (Fig. 9.23). With this furnace the weight of the work that can be conveyed safely is practically unlimited and the maximum continuous operating temperature is only limited by the refractory material used to line the hot zone chamber and by the compatibility of the atmosphere with the heating element. The sintering temperature may be as high as 1,800"C. Fig. 9.24 is a schematic cross section through the hot zone of a walking beam furnace. A comparison with Fig. 9.13, above, shows the hermetically closed furnace housing which is typical for all metal sintering furnaces and the vertical (left) or horizontal (right) noncontaminating electrical resistance heating elements. Both are required to
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Fig. 9.22: Photograph o f a high temperature pusher furnace (courtesy Gasbarre, Sinterite Furnace Div.. St. Marys, PA, USA).
Fig. 9.23: Longitudinal section through a walking beam sintering furnace [B.25].
maintain a particular composition of the atmosphere in the kiln. Movement of the charge in a walking beam furnace is accomplished by a mechanism that lifts and pushes forward, by only a few centimeters each, a bottom tray with the beams below (see hydraulic cylinders (A) and (B) in Fig. 9.23). The two, timed displacements produce a rectangular motion which conveys the work through the furnace at the required speed. The use of sintering for the induration of ceramic products (bricks, pots, vases, etc., see also Chapters 2 and 3) is quite old but through the centuries, even though empirically improved, was exclusively carried out in batch kilns. Continuous heat treatment of ceramic, powder metal, and other pre-agglomerated parts is less than 150 years old. Another original, also quite old application of sintering is found in the minerals industry that is associated with technologies for the production of metals. There, agglomeration by heat was introduced many centuries ago for the size enlargement of fine ores. Primitive versions of the pan sintering process (see Section 9.2.1, Fig. 9.11) were used to produce sinter from fine ores which had been mixed with a particulate solid fuel. The necessary heat was produced by blowing air through a bed of ore particles with bellows and burning charcoal that was uniformly distributed within (Fig. 9.25). Continuous sinter plants for ores were developed at the beginning of the 21st cenb r y for the size enlargement of fine ores, flue dust, mill scale and other fine metal bearing materials [B.42].At the beginning, metal cars with perforated or slotted bottom
9.2 Sintering Technologies
Fig. 9.24 Sketch showing a cross section through the hot zone o f a walking beam furnace (left: vertical heating element; right: horizontal heating element) [8.25].
were pushed through a directly fired tunnel furnace. As discussed in Section 9.2.1, Fig. 9.8, left, screened, fired fines were placed between the metal walls and bottom and the feed containing the solid fuel to avoid overheating of the mechanical parts. Later, the carts were connected to form a continuous grate belt which was moved continuously through a tunnel furnace by a motorized drive. Fig. 9.26 shows schematically the operating principle of such a travelling grate sintering machine. First, recirculated, fired (= sintered) fines from the sinter screens are deposited on the grate as a hearth layer. Then, feed, consisting of a blend of fine ore
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Fig. 9.25:
Ceorgius Agricola's (De Re Metallica, 1556) representation o f a small ore sintering furnace.
and solid fuel, is placed onto the insulating hearth layer with a swinging conveyor (for uniform bed depth across the wide grate carts) and leveled with a roll feeder. Next, the charge thus built passes through an ignition furnace which is a short hood with burners inside. The flames impinge the surface ofthe bed and ignite solid fuel that is close to the hot interface. Beginning at this point and after leaving the hood, air is pulled through the completely open bed in a downdraft fashion; combustion of the solid fuel is sustained and enhanced by providing excess oxygen and the burning hot zone travels downward through the bed. The amount of air which is pulled through the entire length of the bed and the speed of the grate are adjusted such that, at the end of the machine, the fuel has disappeared and the entire charge has sintered together. The porous solidified mass is broken in a (hot) sinter breaker before the pieces are screened into the desired particle size distribution and externally cooled. Fines are recirculated to provide the hearth layer and potentially oversized pieces are recrushed in closed loop with a screen. Solid particles that are entrained in the combustion air, settle in bins which are part of the main suction duct and fine dust is removed in a dust collector. These solids are recirculated to the burden preparation plant and ultimately fed back to the sintering machine.
Feed ( f r o m H e a r t h l a y e r btn
Swinging
Ignition furnace
n t * r 5 reoker
:ly
A,, collec
screer, S
n
C o u n t e r w e i g h t e d d u s t valves
Fig. 9.26
Schematic o f an early travelling grate sintering machine.
9.2 Sintering Technologies
Although, after the development of pelletizing (see below), sinter has lost some of its importance as a sized feed for reduction furnaces, the technology is still used, particularly in the iron and steel industry, and occasionally new sinter plants are installed. However, overall, since the 19GOs, worldwide production of sinter is decreasing in favor of pelletizing (Fig. 9.27). Nevertheless, even today reports dealing with improved sinter machine designs are published [B.48].While the basic process is still the same, new developments are directed towards better burden preparation, particularly in connection with unusual ore compositions, improved temperature control, minimization of pollution, general process optimization, and reduced energy consumption (Fig. 9.28). During the middle of the 20th century in several locations, particularly in the USA, development work started to render the large reserves of Taconite and Itabirite, low grade iron ores, useable for iron and steel making. In the early 1960s iron ore pelletizing was developed. The iron ore concentrates which, after upgrading, have a particle size <44 pm (see also Section 5.4) are agglomerated by tumble/growth methods (pan, drum, cone, Section 7.4.1) into nearly spherical “green” agglomerates with a narrow particle size distribution around 12 mm. These so-called iron ore pellets are held together by surface tension and capillary forces. During a post-treatment, which consists of drying, induration (sintering),and cooling, final strength and structural properties (e.g. porosity) are obtained. Fig. 9.29 shows schematically the three induration furnaces and the corresponding plant lay-outs that have been developed [B.42].The furnace types are 1. the shaft furnace, 2. the straight (or sometimes circular) travelling grate (or strand), and 3. the Grate-Kiln.
Fig. 9.27: Development o f blast furnace charge compositions in Germany p.48, p.1611.
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Main ton
Blast furnoce
Electric power saving
Fig. 9.28:
Flow sheet o f a modern sintering plant with irnprovernents and cost saving features [B.42]
Green agglomerates Burner chambers
Shaft Pe Ilets
D Drying
B Firing C Cooling
Fig. 9.29
Schematic representation o f the three most common heat induration (sintering) furnaces for iron ore pellets and sketches depicting the corresponding plants. (a) Shaft furnace, (b) straight (travelling) grate, (c) Grate-Kiln [B.42].
Ballingdrum clrcuits
9.2 Sintering Technologies
Fig. 9.29 cont’d
During post-treatment of the green iron ore pellets, they must be first dried and preheated before induration by sintering occurs. As in many similar curing processes that are used for the strengthening of moist agglomerates, the problem of this sequence of events is that after drying the original binding mechanisms (capillary forces
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and surface tension) have disappeared while sintering, which requires approx. twothirds of the melting or softening temperature, has not yet begun. Therefore, there is a time interval during which the dry agglomerates exhibit almost no strength. Theoretically, only the travelling grate exerts low enough stresses in the essentially stationary bed that the weak pellets can survive. In reality, even these machines, because of their relatively crude design, which must be suitable for hot operation, introduce vibrations and other dynamic forces which endanger the survival of the dry agglomerates. To overcome this problem, additives are used during tumble/growth agglomeration (see Section 5.1.2) which retain some bonding characteristics in the dry state and improve the chance of survival until sintering begins. In iron ore pelletizing, the additive is traditionally bentonite, a natural montmorillonite clay. Unfortunately, bentonite not only increases the cost of the process but also introduces impurities (slag components in iron making). Therefore, efforts to optimize the process have yielded additives which do retain strength in the dry stage but burn out or otherwise disappear at high temperature, thus avoiding unwanted contamination. Considering that the first commercial plants for iron ore pelletizing were put into operation in the early 1950s [B.42], this technology has experienced quick acceptance and growth. Fig. 9.30 shows the worldwide development of installed capacity. If operating capacity is considered, the trend after 1990fell short of expectations because some of the older plants were phased out, the associated mines have reached the end of their economical life, and cheaper high quality lump ore became available together with other iron sources, notably direct reduced iron (DRI). While, in the past, approx. two-thirds of the iron ore pelletizing plants were located at the mine sites, approx. one-quarter at the shipping or receiving ports, and the remaining approx. 10 % at integrated steel works, new systems are often built in connection with direct reduction plants, which are located were energy and reductant are cheaply and abundantly available. I
Fig. 9.30 Graph showing the world-wide growth of iron ore pelletizing [8.42].
I
Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
10
Special Technologies Using the Binding Mechanisms o f Agglomeration As mentioned several times before, agglomeration is a natural phenomenon and methods for the size enlargement of solids by agglomeration date back to the beginning ofhuman life on earth (see particularly Chapters 2 and 3 ) . A new era began when, during the past century, the Mechanical Process Technologies and, among the others (see Chapter 1, Fig. l.l),the unit operation “Size Enlargement by Agglomeration” were recognized as fields of science in their own right and applied interdisciplinarily. It was then found that a common body of concepts and techniques exists which applies to any of many different processes and situations in all industries that handle and process particulate solids. The basic phenomenon agglomeration applies to all situations where at least two solid particles are joined together under the action of a binding mechanism (see Section 5.1.1). Thereby, one type of the particulate solid may be very small and the other extremely large. Such a condition exists, for example, if a single particle or, in other cases, a small, well controlled number of particles, which typically are or may be microor nanometer-sized, adhere(s) to a surface. The result of this manner of agglomeration is either detrimental if the particles are contaminants on, for example, electronic circuit boards or chips [B.34], or individual particle deposition is highly desirable if it is accomplished in a controlled fashion to achieve special material properties (see Section 10.1 and Chapter 12). If hundreds, thousands, millions or more solid particles are involved and stick together due to binding mechanisms (see Section 5.1.1) to form agglomerates, the product characteristics are defined by the final size, shape, and strength as well as the structure and porosity of the parts thus produced. In such cases, it is not necessary that the entire product features the same structure throughout. As will be shown in Section 10.1, agglomeration may be used on the surface and is then responsible for only a small part of a final product. With the above in mind, agglomeration phenomena exist always and in all cases were solid particles are bonded to each other or to another surface and, no matter how the product looks like and what it is used for, its characteristics are defined and controlled by the same fundamentals that can be collected and applied on an interdisciplinary basis. It should be also recognized that a solid “particle” can be many different things (see Sections 5.3.1 and 10.3),from a sphere or cube to an elongated filament or fiber. All feature surface configurations which may be anything from
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10 Special Technologies Using the Binding Mechanisms of Agglomeration
. - -__ _ _ _. _ _ _ __-_.
.................... .................... .................... ~
------
Fig. 10.1: Droplet granulation system. (1) Powder, (2) granular product, (3) powder recycle, (4) drop former, (5) screen, (6) dryer [B.42].
microscopically smooth to macroscopically rough with all kinds of protrusions which may enhance or hinder adhesion. In the following as well as in Sections 10.1-10.3 a few special technologies will be discussed as examples. The presentation is by no means complete and, as is true for the entire contents of this book, should serve as “food for thoughts” to trigger new ideas and/or developments. Similar to the agglomeration ofparticles by heat (see Section 9.1), where aggregation often occurs by the action of the binding mechanism alone, there is a possibility to obtain agglomerates by direct capillary action. Fig. 10.1 shows a process that uses this principle. Although it was used by the inventors (IG Farben, Germany [B.42])for the production of spherical, easily dispersible granules, so far, the method did not gain major industrial importance. Agglomerates are formed by droplets of a suitable, wetting liquid with high surface tension (for example water) which fall into a bed of powder that is transported on a belt conveyor. A spherical wetted area is formed within the powder bed and the powder particles in that volume are held together by the negative capillary pressure (see also Section 7.1). For successful operation, the frequency of droplet production and their spacing as well as the conveyor speed must be controlled such that the volume elements which are wetted by the droplets remain separated in the powder bed. The green agglomerates are separated from the excess amount of powder by screening and are further processed by suitable post-treatment methods (see Section 7 . 3 ) .Unagglomerated powder is recirculated and redeposited, together with fresh feed, on the belt conveyor. Even though the technique never found large scale applications it is a good example of an interesting alternative approach to size enlargement by agglomeration, particularly if loosely bonded, easily dispersible granules must be produced from corrosive or highly abrasive powders and a minimal equipment cost for these conditions is desired. Since this process was invented and first used, reliable droplet formers have been developed which make it easily applicable if a desire for such products arises (see “melt solidification”, Chapter 5). Macroscopically, the direct effect of molecular forces can be observed in a layer of dust on surfaces. Small, light particles that settle on, for example, furniture are held to the substrate and to each other by the always present molecular forces and removal requires wiping (e.g. with a dust cloth) or suction (e.g. with a vacuum cleaner). Microscopic natural adherance of submicron particles due to molecular forces will be discused in Sections 10.2.1 and 10.2.2.
10 Special Technologies Using the Binding Mechanisms of Agglomeration
Other special technologies use tumble/growth agglomeration for the realization of simple, low cost particle size enlargement processes, in which shape and quality are of minor importance. Such applications are, for example, in the fields of recovery of small amounts of valuable constituents by leaching from low grade ores or waste [B.42] and the agglomeration of refuse during processing for disposal. Since, in both cases, large amounts of material with low or no value must be processed, the cheapest possible method that fulfills the process requirements must be selected. Some of such low cost solutions will be described in the following as examples of how the binding mechanisms of agglomeration can be used in innovative ways [B.42]. Fig. 10.2 shows stockpile agglomeration. In this process, an inclined conveyor discharges particulate solids that were mixed with appropriate amounts of CaO and cement (= dry binder) from several meters above onto a stockpile. The stream of material which is falling from the end of the conveyor is wetted with water (and/or other liquid) sprays (= liquid binder). Below the spray area, several heavy (dispersion) bars are suspended in the falling curtain and act as simple, stationary mixer. The wetted mass than tumbles down the slopes of the pile and agglomerates into lumps. Strengthening occurs by natural curing, begins immediately, and continues until the cementitious reactions are completed. Around the foot of the pile, a front-end loader picks up the agglomerates and transfers them into a dump truck or directly onto the leaching heap of a metal (e.g. gold) recovery plant [B.42].
C a O a n d cement
Fig. 10.2 Sketch of stockpile agglomeration [B.42]. NaCN solution or H20
Fig. 10.3: Sketch o f belt conveyor agglomeration [B 4 4
N a C N solution o r H20 s p r a y s
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Another simple agglomeration method is belt conveyor agglomeration (Fig. 10.3).It is a modified stockpile agglomeration system with additional sprays and mixing at each transfer point. The inclination and speed of the belt can be chosen such, that some agglomerates tend to roll down (backwards) from time to time until held by a mass of material. During such rolling, additional fines may be picked up and agglomerate growth can occur (see also below: reversed belt agglomeration). The number of transfer points depends on the amount of fines that must be bonded onto larger particles or agglomerates. Fig. 10.4 depicts shaking trough or vibrating deck agglomeration. (a) Shows the principle of the shaking trough agglomerator. In the wave or surf like motion, particles collide and coalesce if binding properties are favorable. Agglomeration occurs if the powder particles are very fine and/or if the mass is moist. (b) Is a sketch of the vibrating deck agglomerator. Dry binder, if applicable, is added prior to the vibrating conveyor and liquid is sprayed onto the particle bed at the beginning of the downward inclined vibrating deck. The conveyor deck is executed with a number of steps over which the material tumbles whereby mixing and agglomeration take place. A further low cost method is the reversed belt agglomerator (Fig. 10.5). It uses a steeply inclined conveyor belt to which the material, possibly including dry binder, is fed near the upper end. Liquid binder sprays are located at the upper one third of the steeply inclined belt. Belt movement is such that it attempts to convey the mass to the top of the equipment but, due to the steep inclination, material rolls downward. Depending on the angle and the speed of the conveyor the material can be retained on the belt long enough to provide for adequate mixing and agglomeration.
'1
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Finished agglomerates Fig. 10.4 Schematic (a) o f shaking trough and sketch (b) of vibrating deck agglomeration [B.42].
(b)
70 Special Technologies Using the Binding Mechanisms of Agglomeration
Belt A
1 5 0 - 3 0 0 ftlmin
Fig. 10.5:
Reversed belt agglomeration [8.42].
The basic principle of tumble/growth agglomeration is that solid particles move irregularly and independently resulting in collisions (see Section 7.1). They coalesce upon impact if the binding forces which are present or activated at that moment are higher than the separating forces acting on the newly created entity. Therefore, in addition to what has been discussed previously (see Chapter 7 and subchapters) and above, many alternative methods of particle excitation can be used. Of particular interest is sonic (or acoustic) agglomeration [B.42].This technique is being developed for the agglomeration of submicron particles in flue gases and process off-gases which otherwise remain airborne as respirable solid contaminants. Acoustic pressure and velocity are superimposed on the natural Brownian movement causing collisions between even the smallest particles as well as agglomeration. The larger entities can then be removed with conventional methods. Since all of the methods of pressure agglomeration (Chapter 8 and subchapters) require specific equipment in which external forces are exerted onto particles to densify and shape the mass into an agglomerated product, absolutely new special techniques of pressure agglomeration are difficult to envisage. Most probably, any novel pressure agglomeration technology will be a modification of the already known ones. However, an alternative, unusual binding mechanism which is applied in pressure agglomeration for a specific task shall be mentioned in this context to demonstrate the great variety of ideas and phenomena that are part of the science of agglomeration. If supercooled ice or other frozen particles are compressed, the conversion of mechanical energy into heat causes roughness peaks and a thin layer of the particles to melt momentarily and produce liquid bridges or, generally, a liquid phase. However, because the bulk of the material remains at temperatures that are substantially below freezing, immediately after pressure release the liquid solidifies and the resulting shaped body becomes solid and features high density or a certain amount of porosity. Before continuous ice makers for the quick production of “cubed” ice became available, this binding mechanism was used in large commercial ice production plants to recover undersized ice or shape fines into small pieces by ice briquetting. More re-
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Fig. 10.6: Photographs o f (a) briquetted frozen vegetable pulp and (b) an example o f reconstituted food f r o m such briquettes (courtesy Sahut Conreur, Raismes, France).
10.1 Coating
cently, roller presses were applied to shape frozen, pulped vegetables into ration-sized briquettes for deep freeze storage and use in field kitchens (Fig. 10.6).
10.1
Coating
The binding mechanisms of agglomeration (Section 5.1.1) can be also applied for coating. In most cases the method of choice for the application of powder coatings is tumble/growth agglomeration (see Chapter 7 and subchapters). As discussed during the review of the mechanisms of tumble/growth (Section 7.1), layering or preferential coalescense occurs often during size enlargement by agglomeration. If layered agglomerates are produced, this growth mechanism takes place during the entire process from nucleation until the final agglomerate is obtained. In coating, nuclei or cores are provided from elsewhere and layering occurs in irregularly, often turbulently moving beds of relatively large particles and coating powder. In most cases, a liquid binder is added to assist in the formation of a layer. Furthermore, coating materials can be brought in by means of suspensions. Although not directly identifiable as an agglomeration method, coatings can be also applied by spraying a solution or a melt onto a bed of stochastically moving particles and simultaneously drying (from a solution) or cooling (from a melt) the mass. Since the latter technologies are frequently used to coat agglomerates they will be also covered in the following. The first coatings used by humans were manually applied layers of wet minerals onto shaped clay items (e.g. bricks or containers such as vases or pots). During firing these minerals reacted chemically and glazed thereby producing color and a dense surface. The coatings were applied to achieve surface hardness, for water resistance and tightness as well as for decorative purposes. Pill making, another ancient agglomeration technology (see also Chapter 3), used viscous binders, such as honey, which rendered the pills themselves sticky so that they tended to adhere to each other, forming clumps during storage. To remove the stickiness such pills were coated with adsorptive fine powders, such as talcum or pollen. The coating was applied by shaking the freshly made pills with the powder in a bag and, potentially, rerolling them by hand to increase the bond and smoothen the surface. Beginning in the Middle Ages, sweets were coated with, often colored sugar layers to make them look neat, polished, and shiny. These coatings were applied by either spraying sugar solutions onto heated cores or dipping the sweets into molten sugar. The first mechanized machines were rotating pan coaters. This equipment is still used almost unchanged today (Fig. 10.7).The generally round or pear shaped rotating vessels are operating in a batch mode. Cores (e.g.tablettes but also nuts, raisins, or similar food and other products) are placed into the rotating bowl, sprayed with a solution or suspension, and dried with hot air, or coated with a melt and solidified with cold air. Hot or cold air is blown into the pan and circulates over the surface of the tumbling bed. The products are well polished, normally sugar coated pieces (Fig. 10.8).Today, larger units are also available,for example as shown in Fig. 10.9,which, depending on the material to be coated, can process between 50 and 500 kg per batch.
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Fig. 10.7: Simple revolving coating and polishing pans with open style base and round or pear shaped bowls (courtesy LMC Int’l, Elmhurst, IL, USA).
Fig. 10.8 Sugar/cocoa coated raisins and almonds (courtesy LMC Int’l. Elmhurst, IL, USA).
Fig. 10.9 Artist’s conception o f a large modern bowl coater (courtesy Trybuhl, Dassel-Markoldendorf, Germany).
10.I Coating
More recently it was found, that coating is not only suitable for taste masking or the enhancement of flavor, for the improvement of surface conditions, yielding a smooth, polished exterior, and for better oral administration of, for example, solid dosage forms because they can be swallowed easier, but that coating can also provide important functional properties. Functional coatings may be soluble or insoluble in water, soluble only in liquids with specific characteristics and/or composition, permeable, impermeable, or partially permeable, permanently plastic or elastic, elastic featuring a well defined burst pressure, insulating or conductive, etc., etc. While many of the functional properties are used in medicine, other applications are feasible; many of them are already used or are being envisaged for a multitude of products and a virtually unlimited number may be developed in the future (see also Chapter 12). The dimensions, structure, and uniformity of functional coatings must be much better controlled than those of the “classic” enrobing of particles. Sugar and other coatings of particulate foods often disguise the irregular shape of the core particles (see, for example, Fig. 10.8). To accomplish functionality, it is important to cover the surface with a uniform coating (= film coating),which is often only a few molecular or powder particle layers thick and must not have holes, due, for example, a shadow effect, or “blobs” of coating material which make the coating ineffective at these points. Therefore, it is most important to apply strict process control and modern drum coating equipment features at least four support areas as shown in Fig. 10.10. To obtain uniform coverage, the cores (typically tablettes or spheronized agglomerates) must tumble in the apparatus, the liquid sprays must cover the entire length of the particle bed, and the flow of warm or hot air must be directed such that each particle is instantaneously dried to guarantee the production of a smooth surface. Correct movement of the core particles is achieved by installing baffles or lifters or by using polygonally shaped drums. Spray systems have become very sophisticated whereby the stainless steel spray arms with nozzles are often telescopic and can be extracted through the front door for cleaning (Fig. 10.11).If slurries are used, spraying is air assisted to unplug the nozzles and keep them clean (Fig. 10.12).
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=D
Fig. 10.10 Diagram depicting schematically the flow sheet of a typical (film) coating facility [8.42]. (a) Programmable Logic Controller (PLC), (b) storage tanks for spray liquid(s) and metering/ pumping system, (c) equipment for supplying and processing air, (d) air cleaning and exhaust system.
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Depending on the application, the flow of air may be directed in different ways to obtain specific effects. Fig. 10.13 shows three alternatives which are possible in the same polygonal coating drum of one manufacturer by simply switching some valves. In such drum coaters some or all of the panels are double walled and perforated to allow air inlet and exhaust in a controlled manner. Another design utilizes stationary, hollow, perforated paddles (Fig. 10.14)which are immersed in the product and create an unidirectional, constant, and homogeneous flow of air in the tumbling particle bed. Similar to what has been shown in Fig. 10.13, in drum coaters using paddles air can be directed in different ways, too (Fig. 10.15). Most of the drum coaters are used in ultraclean industries (for foods and pharmaceuticals). Therefore, modern coaters are made in sanitary, seamless design from stainless steel and feature smooth exterior housings (see, for example, Fig. 10.16). Additionally, in accordance with the rules of cGMP (current good manufacturing practice), the equipment parts which are contacting product must be capable of CIP (cleaning in place). Fig. 10.17 shows schematically the automatic cleaning of a drum coater according to these requirements. The drum and the cleaning tub that is built into the housing are separated from the “technical part” by water tight seals. After wet cleaning in four steps, the machine’s own air system is used for drying.
10.7 Coating I419
Product
Top view
Manual shut-off
~
'.
1
Standard system equipped with four stations
'
/j I ,
1
Air- f low valve
-I 375 12 0
I.
,80 8 0 8 0
Tumble drum length varies,
12 0
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Fig. 10.12 Top and side view of a slurry spray system [B.42]
As everywhere else in modern industrial technology, continuous operation of drum coating is desired. Fig. 10.18 depicts schematically a continuous drum coater. Although the long, perforated drum is divided into three independent zones for spraying, distribution/polishing, and drying, the results of coating are not as uniform and reproducible as in batch equipment. Fig. 10.19 is the photograph of the machine shown schematically in Fig. 10.18. To enhance flow of solids through the drum, its axis is inclined and the slope is adjustable. CIP is also available. Continuous coating is also possible in balling pans (see Section 7.4.1) if they are equipped with re-roll collars (see, for example, Fig. 7.13). Coating is much cruder and applied, for example, for fertilizer products (see e.g. Fig. 7.15). It should be pointed out in this context, that coating is by no means limited to the clean and ultraclean industries and products which were mentioned previously. Among the many and varied recent developments, a large, relatively new field of use is in the agricultural industry for the coating of plant seeds with fertilizer, fungizide, and insectizide. These materials may be applied in combination or individually; they feed and/or pro-
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Fig. 10.13: Three alternative air flows which are possible in the polygonal drum of one supplier (courtesy Driam, Eriskirchl Bodensee, Germany). (a) Reverse air flow from the bottom through the particle bed with air exhaust at the top; (b) direct air flow from the top through the particle bed with air exhaust at the bottom; (c) axial reverse flow from the bottom through the particle bed with air exhausting through the hollow shaft.
tect plant seedlings during sprouting and initial growth. The continuous drum coater as shown in Fig. 10.19 is an example of typical machinery that is being used for this purpose. While during most of the previous discussions much emphasize was given to the development of a relatively thin, uniform coating, other reasons for enrobing particles are also possible. Fig. 10.20 shows cross sections through melt coated fertilizer granules. In this case, the cores of conventionally tumble/growth granulated TSP (= triple super phosphate) were melt coated with sulfur, to provide an additional nutrient for sulfur deficient soils and/or obtain a slow release fertilizer. A flow sheet ofthe process which may, for example, be used to achieve this result is shown in Fig. 10.21. In the fluid drum granulator (FDG)TSP granules are sprayed with liquid sulfur and coated in a fluidized pan and the tumbling bed (Fig. 10.22).A similar flow sheet can be also used
70.7 Coating
Fig. 10.14
Photographs o f two types of air-blowing paddles (courtesy CS Coating Systems, Osteria Crande (Bologna), Italy).
to round crystals (Fig. 10.23) or the irregularly shaped granules from compaction/ granulation (see Section 8.3) as shown in Fig. 10.24 or to “fatten” smaller granules (Fig. 10.25).Therefore, the purpose of such coating processes is to provide functional layers (e.g. for controlled release of nutrients), to add a component (e.g. sulfur), to round irregular granules (e.g. from compaction/granulation), or to enlarge small particles (fattening of urea prills). Another melt coating method, the rotocoat process, uses a turbine in which a finely divided molten coating material contacts solid particles which are at room temperature (Fig. 10.2Ga).Due to the surface tension of the melt, the individual particles are enrobed with the liquid which cools to form the coating. Depending on the amount of solids passing radially through the turbine, secondary agglomeration may also occur if particles come into contact with each other while the coating material is still sticky. Fig. 10.261, is a simplified flow sheet of the process. The above method is suitable for the coating of smaller particles, down to 0.1 mm. However, more commonly, small particles, either powders, crystals, or agglomerates, the shape of which may be irregular, spheroidal, tabletted, or spheronized, are typically coated in specially designed fluid bed equipment (see also Section 7.4.4). As with all other coaters, the heart of fluid bed processes is the type and location of the delivery
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Fig. 10.15: Schematic flow sheets o f two different air flow regimes using air-blowing paddles (courtesy CS Coating Systems, Osteria Crande (Bologna), Italy). (a) Hot air through the paddles into the particle bed with air exhaust through the hollow shaft; (b) hot air through the hollow shaft onto the particle bed with air exhaust through the paddles. (1) Inlet air handling unit, (2) control panel, (3) solution tank, (4) dosing system for liquid to be sprayed, (5) sliding support arm for spray nozzles, (6) coating pan, (7) air-exhaust or -blowing paddle device, (8) dust collector, (9) outlet air fan, (10) powder dosing device.
70.7 Coating I423
Fig. 10.16 Photograph of two d r u m coaters, installed in an ultraclean (pharmaceutical) environment (courtesy Driam, Eriskirch/Bodensee, Germany).
Fig. 10.17: Sketch showing cleaning in place (CIP) o f a d r u m coater (courtesy Driam, Eriskirch/Bodensee, Germany). Cleaning phases: (1) D r u m inside by cleaning spray bar, ( 2 ) d r u m outside and air distributor, (3) air channels inside, (4) rinsing of tub.
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Fig. 10.18 Schematic representation o f a continuous drum coater (courtesy Driam, Eriskirch/Bodensee, Germany).
Fig. 10.19: Photograph o f the continuous drum coater shown schematically in Fig. 10.18 (courtesy Driam, Eriskirch/ Bodensee, Germany).
Fig. 10.20 Broken (cross sections through) melt coated fertilizer granules. Cores: triple superphosphate (TSP) granules; coating: sulfur (courtesy Kaltenbach-Thuring, Beauvais, France).
70.7 Coating 1425
Fig. 10.21: Flow sheet o f a fluid drum granulating (FDC) process for the coating or fattening o f granules with sulfur and/or purge solution (courtesy Kaltenbach-Thuring, Beauvais. France).
seeds and recycled product
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Fig. 10.22: Sketch describing the principle of the fluid granulation drum (FCD) for coating particles (courtesy Kaltenbach-Thuring, Beauvais, France).
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Fig. 10.23: Photographs depicting the rounding o f crystals by coating (courtesy Kaltenbach-Thuring, Beauvais, France).
Fig. 10.24 Granules from cornpaction/granulation (left) and rounded particles (right) after coating (courtesy Kaltenbach-Thuring, Beauvais, France).
Fig. 10.25: “Fattening” o f granules (right to left) by coating (courtesy Kaltenbach-Thuring, Beauvais, France).
70.7 Coating I 4 2 7
Fig. 10.26 Principle (a) and simplified flow sheet (b) o f the rotocoat process (courtesy Sandvik, Totowa, NJ, USA).
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Product container
Fig. 10.27: Sketches of the material processing sections o f three fluidized bed coaters [B.42]. (a) Top spray, (b) tangential spray (rotating disc fluidized bed coater), (c) bottom spray (Wurster coating system).
Nozzle
70.1 Coating
Fig. 10.28 Artist’s conception of a t o p spray fluidized bed coating system (courtesy Vector, Marion, IA, USA).
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system for the liquid coating material. For this, three methods are available: top, tangential, and bottom spraying (Fig. 10.27). The nozzles are often binary, i.e. liquid is supplied at low pressure to an orifice and is atomized by pressurized gas. Such pneumatic nozzles produce finer droplets, an advantage when coating smaller particles. However, it is also an important requirement of coating that the liquid, solution, or suspension droplets impact the core particles and uniformly distribute on the surface before the liquid is dried off. Since very fine droplets evaporate quickly as they travel from the nozzle to the particle bed, solids concentration and viscosity of solutions and suspensions increase. Therefore, droplets may fail to spread satisfactorily when they contact the substrate surface, resulting in an imperfect coating. This drying of the coating spray can be severe in topspray coaters (Fig. 10.27a) in which the most random particle movement exists and liquid is sprayed against the flow of drying air. Nevertheless a substantial share of coating is performed in this type of equipment because larger amounts can be processed per batch and the design is simple. Fig. 10.28 is an artist’s conception of a top-spray coater showing the fully integrated processing systems and matching accessories. The rotating disc fluidized bed coater (Fig. 10.27b) combines centrifugal, high intensity mixing with the efficiency of fluid bed drying. A major advantage of this method is its ability to layer large amounts of coating materials onto cores consisting either of robust granules, crystals, or nonpareil nuclei. Because of the unit’s high drying rate, relative large gains in product weight can be achieved in a short period of time. In this respect it is similar to the fluid drum granulator (Fig. 10.22) if this is used for “fattening” cores. Another advantage of the rotating disc fluidized bed coater is the possibility to layer dry powders that are dispersed in the fluid bed onto nuclei which have been wetted with a liquid. Because the spray nozzle(s) is (are) located below the bed surface, the above mentioned problems with early drying are not experienced. The same is true of the Wurster coating process (Fig. 10.27~). This is the only bottom-spray fluid bed coating method which is applicable for tablettes, pellets, and coarse granules as well as fine particles. The Wurster coating chamber is cylindrical and the basic model contains a concentric inner tube with approximately half the diameter of the outer chamber. At the base of the apparatus is a perforated plate which features larger holes underneath the inner tube. The liquid spray nozzle is located in the center of the orifice plate and the tube is positioned at a certain distance above the plate to allow the movement of material from the outside annular space to the higher velocity airstream inside the tube. This design creates a very organized flow of material which is similar to that of the spouted bed (see Section 7.4.5, Fig. 7.89). Solids move upwards in the center where coating and highly efficient drying occur. Contrary to what happens in the spouted bed, where some mass exchange occurs between the solids moving upwards in the center and those in the outside downward flow, the high speed upward flow regime in a Wurster coater is contained in the center tube, so that no backmixing occurs. At the top of the tube, the material discharges into an expansion area and then flows down, as a near-weightless gaslsolids suspension, in the annular space outside the tube. Design variations include different chamber configurations for use in coating tablettes, coarse granules, or fine particles (Fig. 10.29). For scale-up, the outer vessel diam-
70.7 Coating I 4 3 1 Filler housing
Product retention screen
Fig. 10.29 Sketches o f the different chamber configurations o f single tube Wurster coaters as used for (a) tablettes, (b) coarse granules, and (c) fine particles (8.421.
Fig. 10.30
1(
Schematic explanation o f Wurster coater scale-up (courtesy Aeromatic-Fielder, Bubendorf, Switzerland).
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eter and the number - rather than the size - of inner tubes increase. Fig. 10.30 is the simplified explanation of how the capacity of a (laboratory or pilot) Wurster coater (Fig. 10.29)is scaled up 3, 5,8, and lotimes and Fig. 10.31 is a view into the coating section of a Wurster coater with ten tubes; each tube features its own gas distributor and spray nozzle and is essentially identical with that in the test equipment which was used during development work. During studies of particle and gas flow patterns in traditional Wurster coaters (Fig. 10.32a) it was found that flow paterns were dominated by the particles rather than the gas. This explains why sometimes, even in the well defined traditional Wurster coaters, uniform coating is difficult to control. To overcome this problem, the precision coater (Fig. 10.32b) was developed. An essential feature of this new Wurster coater design is the highly controlled gas flow pattern in the coating zone. This is achieved by application of the so-called Swirl Accelerator, a guiding system in which the gas is accelerated, stabilized, and given a precise amount of swirl which eliminates slugging, often seen in traditional coaters, and stabilizes multi-tube systems. Particles are entrained into the swirling air on an individual basis. This results in an optimized probability of impact with the droplets of atomized liquid and to an even application of coating material.
Tra d it i o na I C o a t e r
Coating column
Down flow bed
Air distributor plate Two-fluid spray nozzle
Air inlet Fig. 10.32 Operating principles o f a traditional Wurster coater (a) and (b) a precision coater (courtesy Aeromatic-Fielder, Bubendorf, Switzerland).
Another coating technique is encapsulation. Although this is a relatively new technology, many different processes have been developed, a large number of applications has evolved, and many new uses are conceivable and found, literally on a daily base (see also Chapter 12). In Section 5.2.2, the mechanism of capillary flow in wet agglomerates or, more generally, in porous bodies that are filled with a liquid (= continuous phase) was described. During drying, in a first drying phase, evaporation takes place on the surface and the liquid is replenished by capillary flow of the continuous phase from the interior of the porous body. If the liquid is a solution or suspension, solids are deposited at the pore ends on the surface and causes more or less severe incrustation (see Section 5.2.2). If a film forming, easily soluble polymer is dissolved in the continuous phase as emulsion or dispersion, encapsulation occurs during drying. These encapsulation processes can be carried-out with agglomerates of any size and shape and result in a large number of special effects which, depending on the type and composition of the coating or incrustation, modify final product properties (see also Section 7.3). More commonly used and widely researched is microencapsulation. In this context, the partial word “micro” refers to the dimension of the encapsulated product which is typically <1-2 m m in size or, increasingly, in the 10 to a few 100 pm range. If a slurry containing a solution, emulsion, or suspension of polymer is dispersed into small particles and dried in a spray dryer (see Section 7.4.3) microencapsulated particles
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Coherent. Homoaeneous element. Simole microcaosule uCoating can be porous to various levels of molecular weight chemicals or impermeable, of various solubility, and rigid or soft.
Simole multilaver microcaosule
*Early season herbicide Fertilizer Late season herbicide
Simole Heteroaeneous microcaosula [Carrier, cohesion phase can b e continuous or discontinuous] Heterogeneousstructure can be multiplexedwith capsules within capsules ad nauseam
d) Heteroaeneous coatinq [Sparse or Dense; Tenacious or Temporary.]
-0
Capsule
Encapsulation
Blending
Ordered Mixture Fixing
(embedding) ____)
Composite
Fig. 10.33: Schematic description o f possible structures if micro. capsules (courtesy Brian Kaye Associates, Sudbury, Ont. Canada).
of the type described above are formed. Such a process yields a dry, free flowing powder which, in most cases, satisfies the criteria defined for instant products (see also Section 5.4). However, microencapsulation becomes more and more a sophisticated packaging method in which the “packing material” (= coating) features a specific, well defined functionality. With this technology small agglomerates or tiny portions of powders, liquids, and even gases are individually wrapped into a shell (= capsule) to form
70.1 Coating ( 4 3 5
Fluid wall deposition
Wall solidifies
Fig. 10.34 Schematic representation of four stages of the coacervation process (courtesy Brian Kaye Associates, Sudbury, Ont. Canada).
free flowing particles which are often spheroidal. Fig. 10.33 describes schematically possible structures of microcapsules. Originally, drugs, chemicals, toner materials, pigments, and the like were encapsulated to facilitate or improve handling and to bring about special product characteristics. In most cases, the capsules according to (a),(b),and (c) are produced by sol-gel processes (see also Section 5.3.2) or coacervation, an electrostatically assisted coating process. As shown in Fig. 10.34, the core material to be encapsulated (often a liquid) is placed in a (immiscible) carrier liquid (to form small, individual droplets). The coating material is also suspended or is present in a dissolved state in this carrier liquid. To induce the process that is known as coacervation, the temperature, pH, or other conditions of the system are changed in such a way that the wall material comes out of solution, the resulting particles or those which were originally suspended as solids aggregate around the cores, and continuous encapsulating walls are formed. In a final stage of the process the capsules are hardened. While many other microencapsulation techniques also use surface and in situ polymerization methods or, generally, interfacial reactions to produce soluble or insoluble and impermeable or permeable capsule walls, in addition to the coating and spray drying methods that were discussed previously, a growing number of processes de-
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Coating Fineparticles
Core Fineparticles
Ionizer
Air at 30 psi
Heater Section
Reaction Chamber
(b)
Vent
EncapsulationSystem developed by IlTRl
Fig. 10.35: (a) Principle of microencapsulation by electrostatic, aerosol based coating [ ( l ) coating particles, ( 2 ) core particles] [B.42] and (b) sketch of a possible equipment configuration (courtesy Brian Kaye Associates, Sudbury, Ont., Canada).
posit particles onto cores or solid surfaces whereby the i n d i n g mec.-anisms of agglomeration are utilized. Three such methods will be mentioned below as examples. Fig. 10.35 are sketches of the principle (a) and the equipment (b) of electrostatic, aerosol based microencapsulation. The two components to be turned into a microencapsulated product, the coating and, respectively, the core particles, are given an ionic charge of the appropriate sign using a sub-corona discharge system. To achieve sufficient encapsulation, the apparatus must be designed such that a high rate of collisions between the two components occurs in a turbulent supportive gas system. In this process, the coating materials must be selected so that they will uniformly and completely cover the core particles. They include softened wax particles which solidify upon cooling or polymers which form a skin by interfacial action between a component in the core and another in the coating material or upon exposure to a suitable gas phase, etc. The coating (capsule) can be also finished by heating (= sintering). Another method of manipulating coating particles uses magnetic forces. In the magnetically assisted impaction coating process (MAIC) the coating material is applied onto the cores by the actions of the coating material (i.e. impacts due to turbulent particle movement) and by magnetic attraction if either coating or core materials or both are magnetic in character or by the action of magnetic elements in a bipolar
Fig. 10.36 Sketch depicting the principle o f the magnetically assisted impaction coating (MAIC) process (courtesy Aveka, Woodbury, MN, USA).
Fig. 10.37: Schematic diagram describing the deformation mechanism which results in improved adhesion o f hard/soft coatings produced by the MAIC process (courtesy Aveka, Woodbury, M N , USA).
Fig. 10.38 Sketches explaining the mechanisms o f mechanofusion [B. 421.
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Effect
Spherical largelf i ne particles (inorganic / plast ic 1
Surface fusion
Surf ace f u s i o n and
I r regu l a r
large/ f i ne particles
sp heronizin! effect
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Grinding / s u r f ace fusion
0.
D ispersion I surface fusion
With agg Iom erat e d particles
I rregular Iy shaped plastics
Fine powders (dyelpigment 1
c3 fi(9
-
00
Spheronizing effect
0 Precision mixing
oscillating magnetic field which fluidizes the coating material, the cores, and the magnetic elements (Fig. 10.36). Particle to particle impacts cause peening of the coating material onto the cores. If neither the coating particles nor the cores are magnetic, the bipolar oscillating magnetic field causes impingement of the magnetic elements into the coating particles which forces them onto the core material with a peening action (Fig. 10.37). With this method coatings can be developed which do not require a separate binder. Adhesion is accomplished by molecular forces which are enhanced by drastically reducing the distance at the contact point and by partially embedding the coating particles in the surface of the core. In the end, the mechanism that causes strong adhesion of the coating particles to the cores is due to mechanical forces acting in the system. It works best, if the coating particles are somewhat harder than the core material. Because the coating particles are often so small, that no dislocations are present in their structure, they behave as very hard entities (see Section 5.4). Therefore, it is for example possible to partially embed submicron-sized Ti particles in the surface of glass. A similar method uses mechanical forces outright. The process was developed in the early 1970s by Hosokawa in Japan and is called mechanofusion [B.42]or hybridization. In the high energy environment of a special mill, particles are held by centrifugal force against the inner wall of a fast rotating cylinder and wedged into the space between a
Fig. 10.40 Microphotographs of four products from hybridization (courtesy Nara, Tokyo, Japan),
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stationary, curved inner piece (Fig. 10.38a). During densification in the converging clearance, the particles experience complex forces such as compression, attrition, shearing, and rolling (Fig. 10.38b). Depending on the characteristics of the materials that are involved and on the operating conditions of the equipment one or more of the following effects can occur: Solid-solid mechanochemical reaction (mechanofusion), microspheronizing, intensive dispersion (precision mixing), and granulation (agglomeration). Fig. 10.39 illustrates these effects pictorially [B.42]. Coating processes, particularly MAIC, mechanofusion, and hybridization are often used to produce materials with new, engineered physical properties (see Chapter 12). First and foremost flowability is typically improved but other characteristics such as abrasion resistance, electrical conductivity or insulation, magnetic properties, and many others may be attained in a controlled fashion. Fig. 10.40 shows photographs of four products from hybridization.
10.2 Separation Technologies
As discussed in Section 5.5, separation technologies may suffer from unwanted agglomeration when particles, that should be separated according to a certain property (including size, shape, density, composition, color, etc.), stick together and can not be dispersed sufficiently well that a good separation efficiency is obtained. On the other hand, many separation technologies depend on agglomeration for the effective removal of particulate solids, especially those featuring micron and submicron size, from gases and liquids in environmental control. Without going into details, in the following two subsections a few remarks are made and some examples will be presented which are intended to show the importance of agglomeration phenomena in connection with separation technologies. As is generally true for this book, no claim is made for completeness. Rather, ideas should be instilled in those readers who are faced with developing, using, and/or optimizing separation technologies. 10.2.1 Gas/Solid Separation
In the field of dust collection or the separation of ultrafine particles (UFPs) from gas streams the mechanisms of tumble/growth agglomeration in low density fluidized beds and particle clouds are frequently applied. As particle size decreases, i.e. if it is in the micron or submicron (nano) range, particle mass becomes very small. As a result, for example, centrifugal forces, which are the underlying effect for particle separation in cyclone separators,or inertia, which controls uniform particle movement in a gas stream and determines the collision probability with e.g. filter media, are neglectable. Therefore, such particles follow the stream lines of the flowing gas and exit cyclones or packed bed filters with the “clean” gas (see also Section 10.3). Since, at the same time, these particulate solids are respirable and, owing to their
7 0.2 Separation Technologies
very large surface area, exhibit high reactivity, they represent the most dangerous particulate contamination from the human health point of view. If ultrafine particles can be agglomerated, the mass of the new entity is equal to the sum of all particles in the structure and mass related forces as well as inertia increase proportionately. After agglomeration, ultrafine particles, in their new form, can be removed in standard dust collection devices such as cyclones and packed bed filters. As mentioned several times before, the natural adhesion forces (see Section 5.1.1), caused, for example, by molecular (e.g. van-der-Waals) or electrical forces (e.g. due to asymmetric molecular structures), may become much larger than the separation forces which are mass and shape related. Therefore, if collisions or, generally speaking, contact between ultrafine particles occur, a rather strong bond will develop. This phenomenon is also responsible for the fact that most nano-sized particles do not exist as individuals but as assemblies of many particles (Fig. 10.41);this might be a problem in those applications where ultrafine particles must be deposited individually or in monolayers (see Chapter 12). For the effective separation of such particles from gases, however, agglomeration is desired and must be promoted. Ultrafine particles that are suspended in a fluid exhibit Brownian motion, a random movement resulting from the impact with molecules of the fluid surrounding the particles. Inspite of the randomness of the motion, it is very unlikely that particle-
Fig. 10.41: Microphotographs of naturally agglomerated nano particles (courtesy CABOT, Tuscola, IL, USA).
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to-particle impacts occur because the amplitudes of the movement and the particle sizes are very small. Therefore, other methods must be applied to entice particle collisions which will result in coalescence and agglomerate growth. Such techniques use artificially induced turbulences, for example in front of filters and cyclones, uni- or bipolar charging in so-called electrostatic precipitators, magnetism, and ultrasound [B.42].Although, from a process point of view, it is very advantageous that with these methods particle accretion occurs in the free flowing fluid, a major draw-back is that the collision probability changes with the square of the particle concentration. Therefore, as the number of particles becomes smaller, also during the cleaning process itself by the incorporation of dust particles into the growing agglomerates, the collision probability decreases and the desired low concentrations of ultrafine particles in the off-gas may not be reached. A different, new strategy for the effective removal of aerosols from gases uses a principle which is already well known from the capture of ultrafine particles by comparatively large liquid droplets in wet scrubbers. In contrast to using liquid droplets, which combine in a sump together with the collected solids and transfer at least part of the separation problem to a secondary cleaning process, i.e. the removal of fine particulate solids from a liquid (see Section 10.24, it is proposed to pass the contaminated gas through a fluidized bed of solids [10.1, 10.2, 10.31. In such a fluidized bed dust collector, the aerosol particles adhere to the large surface area of the fluidized collector medium and form a coating which is densified when coated collector particles collide with each other. It has been determined that, later, attrition of the coating may result in the formation of secondary dust particles. However, these secondary particles are normally agglomerates and substantially larger than the original aerosol so that they can be easily separated in conventional dust collectors. 10.2.2
Liquid/Solid Separation
Aggregation of fine particulate solids also takes place in liquids. Some of the phenomena have already been described and were discussed in Section 7.4.6. In environmental control, the removal of particulate solids from liquid process effluents is of great importance. As in the case of gaslsolid separations (see Section 10.2.1),when the size of the solids diminishes and reaches the micron or submicron (nano) range, mass of the individual particles becomes so small that they remain in suspension and can not be removed by settling. Because of the fineness, membranes would be required to retain particles on or in a diaphragm which is uneconomical for the cleaning of large volumes of contaminated liquids from industrial plants or waste water treatment facilities. However, remembering the mechanisms of growth agglomeration (Section 7.1), if particles can be forced to impact with each other, it is possible that they adhere to one another. Therefore, when water, that is contaminated with suspended fine solids, is stirred, flocs may form naturally. If this happens, the size and shape of these aggregates depend on the circumferential speed of the stirrer and the processing time. Fig. 10.42 shows that flocs are larger if the shear forces are low and the processing time is
7 0.2 Separation Technologies Ctrcum'erential speed of propeller Original sample
1 m/s
0 6 m/s
0 27 m / s
0 18 m/s
30 min
60min
90 rnin
Stationory sample ofter 1 5 h mixed
Pro-essmg
time
Fig. 10.42 Natural flocculation ofsolid contaminants in river water [B.42]. Parameters are the circumferential speed o f t h e stirrer and the processing time.
short. But further investigation revealed that higher speed of the stirrer and/or longer duration of mixing ultimately result in denser and more stable agglomerates. This is due to the previously discussed mechanism of growth agglomeration (see Section 5.3, Fig. 5.42) whereby loosely attached particles are removed under the influence of ambient forces (in this case, for example, shear) and later have the chance to become reattached in energetically more favorable positions thus yielding denser and stronger products. It depends on the process that will be used for solids removal, which of the two agglomerate structures is required, the loose flocs resulting from relatively gentle movement or stronger agglomerates from a more vigorous stirring for a longer time. In the large diameter circular thickener/clarifier (Fig. 10.43), which is commonly used in municipal water treatment plants and in many industrial applications, water, flowing slowly from the feedwell in the center to the overflow around the periphery of the circular tank, is gently moved by a slowly rotating arm. Loose flocs are formed which settle by gravity to the slightly conical bottom. Differently shaped scrapers (rakes)are used to move the sludge to the discharge cone at the lowest point from where it is transported to conventional liquid filters. The more vigorous stirring which produces stronger agglomerates is used when the resulting agglomerates are moved with the water to an off-site filtering system and, therefore, must survive transport. In many cases, even if collisions between particles do take place, the naturally available binding mechanisms, mostly molecular forces which are considerably lower in a
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suprrrtructuro
0
Fig. 10.43: Partial top view and elevation (a) (according to EIMCO, Div. Baker Hughes, South Walpole, MA, USA) and photograph (b) o f a circular thickener/clarifier.
10.2 Separation Technologies
liquid environment than in a gas atmosphere, do not create bonds with sufficient strength to withstand the various separating effects and sufficient flocculation does not occur. For quite some time it has been known that polymers, added to liquid based particulate systems, have a dramatic influence on particle interaction. Molecules may attach themselves to solid surfaces and, depending on the characteristics of the exposed radicals, can cause particle attraction [B.26] or dispersion [B.51].The second, dispersion, is applied to avoid agglomeration or enhance disintegration of aggregates. There are two ways in which polymers can promote aggregation: 1. By making particles more susceptible to salts or 2. by flocculating the system without the aid of electrolytes. These processes are known as sensitization and adsorption flocculation, respectively. The second is more common. To create aggregates or flocs, the polymer adsorbs on different particles simultaneously which is best accomplished by using substances with high molecular weight and a strong affinity to the particles to be agglomerated. Fig. 10.44 explains the principle. In nearly all applications of polymeric flocculants, the polymer addition and the subsequent flocculation process are carried out under conditions where the suspension is agitated in some way, for example by stirring. This way, the polymer molecules are distributed uniformly throughout the system and adsorb onto the particles which are then encouraged to collide and form aggregates. As described earlier in other contexts, bridging may be followed by break-up if the bond is not strong enough and, later, re-attachment during another impact. Fig. 10.45 is the sketch of a flocculate. Care must be taken not to oversaturate the suspension with polymer. If too much polymer is adsorbed, the particles may become restabilized (= deactivated) because of surface saturation or by steric stabilization [B.26]. Fig. 10.46 demonstrates schematically bridging, which results in the desired flocculation, and restabilization. Commercial flocculants are used extensively in practice, for instance in water purification. By influencing the affinity ofthe polymer, it is also possible to obtain selective agglomeration. This method is used in the upgrading of certain minerals and ores, for example during flotation. Less well known is the fact that, more often than not, solids and immiscible droplets dispersed in aqueous solution are electrically charged due to preferential adsorption of certain ion species, charged organics, and/or dissociation of surface groups [B.42]. Depending on such variables as nature of the material, its pretreatment, pH, and composition of the solution, these charges can be either positive or negative. Since the
Fig. 10.44: Principle o f polymer adsorption and flocculation [B. 261. (a) Adsorption o f polymer molecule on the particle, (b) rearrangement o f adsorbed chain, (c) collisions between destabilized particles and bridging to form aggregates (flocs), (d) break-up o f flocs.
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Fig. 10.45: Structure o f a flocculate (floc) bonded by a polymer [8.42].
(b)
Fig. 10.46 Schematic illustration o f (a) polymer bridging between particles and (b) restabilized particles [8.26].
+
*
+
-
- + +
Fig. 10.47: Schematic representatiofl of two particles with electrical double layers in a liquid
-
+
[8.42].
surface charges on particles are compensated by an equal but opposite countercharge surrounding them (Fig. 10.47) an electrical double layer develops (see also Section 5.1.1). Even though, as a whole, the system is electrically neutral, repulsion between the particles occurs. Upon addition of indifferent (= non adsorbing) electrolyte (e.g. a salt),the double layers become less active and, as a consequence, the particles can now approach each other more closely before repulsion sets in. If enough salt is added, the particles may eventually come so close that van-der-Waals attraction binds them together. This is, in principle, the explanation of the sensitivity of colloids and suspensions to salts and may, in other environments, be used to destroy stable colloids or suspensions and cause flocculation. For technical applications, electrocoagulators are used [B.42] to charge the solids in contaminated effluents. Metal hydroxides are produced by a system of soluble electrodes (anodes) which, in suitable electrolytes, cause coagulation of particles into larger flocs.
70.3 Fiber Technologies 10.3
Fiber Technologies
The influence of fibers on the strength, structure, and characteristics of agglomerates was discussed in Section 5.1.2. The binding mechanism “interlocking bonds” (see Section 5.1.1), the intertwining of fibers and threads, is also used directly to produce “agglomerates” by producing non-wovens, felts, filters, webs, paper, etc. Each of these applications, which produce agglomerates from fibers featuring interlocking bonds, are technological fields that are covered in the literature, in books and scientific as well as commercially oriented papers. Therefore, they will not be covered here. The purpose of this short chapter is, to remind the reader that many products are in reality some sort of agglomerates and their characteristics are based on and controlled by the fundamentals of agglomeration, particularly the binding mechanisms of agglomeration (Section 5.1.1). In the following, one specific technology will be discussed in more detail, because it describes first how the “agglomerate” is made and then strengthened by post-treatment and, secondly, indicates that, during application, agglomeration plays an important role again. This technology is the manufacturing and use of fiber based filter media. For pleated bag and cartridge filters for example [10.4],there are wet-laid filter media, made from cellulose or synthetic fibers, needled felt or spunbonded, metallized or carbon impregnated polyester media, or acrylic, Nomex, Ryton, and P-84 (designations by TDC Filter Manufacturing, see Section 14.2) needled felt (Fig. 10.48). Wet-laid media (Fig. 10.48a) is made by mixing a slurry of cellulose or synthetic fibers (= particles), or of both, with a chemical resin (= binder) that holds the fibers together and protects them from picking up excessive moisture. The mixture is pressed flat, dried, and cured in a high temperature oven (= post-treatment). Wetlaid cellulose fiber, typically made from hardwood, softwood, or grass, produces filter media which is suitable for moderate filtering efficiency, but is not applicable in high temperature, high moisture, or oily environments. Filters from wet-laid synthetic fiber, however, offers excellent filtering efficiency in moist and/or abrasive conditions up to 135 “C. Needled felt media (Fig. 10.48b) consists of intertwined short fibers, pressed together, and mechanically fixed with a needle punch machine. The efficiency of such filter media varies with its density, composition, and relative thickness. Needled felt media is strong and durable, but, for maximum filtering efficiency requires the build-up of a dust layer (see below). Spunbonded media (Fig. 10.48~) is made from continuous polyester filaments. During manufacturing, polyester is melted, extruded, spun, and drawn to form laminated webs. The webs are rolled, thermally bonded, and embossed to form sheets. Spunbonded media feature a hard, slick surface that is ideal for increasing the filtering efficiency and, at the same time, retains little dust during cleaning. Also, spunbonded media provides 30 to 50 % more surface area than other filter media. Such filter media can be either used to capture dust by building a layer on their surface or to collect particles on the fibers in the interior. The build-up of a dust layer is controlled by the relative open (pore) area on the surface. Cleaning is accomplished
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70 Special Technologies Using the Binding Mechanisms of Agglomeration a. Wet-laid media
Fig. 10.48 Photomicrographs of different fiber based filter media (courtesy TDC, Cicero, IL, USA).
10.3 Fiber Technologies
Fig. 10.49: Enlarged view of the interior o f a filter mat with particles sticking t o the fiber surfaces.
by back blowing or mechanical shaking. It is advantageous if strong enough bonds (which may be enhanced by adding a suitable “binder”to the gas stream) have developed in the dust layer so that during cleaning some agglomeration remains for ease of transportation to disposal, avoiding re-entrainment of dust particles (secondary pollution). Media that collects dust on the fibers in the interior consist mostly of empty space. Typically,the fiber volume represents only a few percent of that of the filter mat. Therefore the voids are orders of magnitude larger than the dust particles. Fig. 10.49 is the enlarged view of the interior of a filter medium with small particles sticking to the fiber surfaces. The fiber diameter in this picture is approx. 50 pm and the size of the particles ranges from 3 to 10 pm. There are a number of problems associated with this type of dust collection. First, the particles to be attached to the surface (Fig. 10.50) may not stick immediately because they bounce back due to the elastic energy that develops during the impact. Secondly, if particles adhere, they may be torn off again by the drag force of the flowing gas: this effect is often increased by the development of a momentum if the particles are not closely attached to the fiber surface and extend into the gas stream (Fig. 10.50).
Fig. 10.50 Detail of a dust laden fiber from Fig. 10.49 showing how particles extend into the gas stream.
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A further problem is that differently sized particles follow stream lines in a different way. It must be a goal during filtering that the particles collide with the fiber and adhere upon impact. As shown by the results of model calculations (Fig. 10.51) various particle sizes not only move differently but modified system conditions also influence the behavior of the particulate solids [10.5]. Fig. 10.51a depicts the lines of motion for 1, 2, 3, 5, and 10 pm particles. Originating at the points on the left, all do not impact the fiber and show well the influence of particle size. Even if the paths ofthe three smallest particle sizes begin almost at the center line, they still pass around the fiber without colliding. Often, particles or fibers carry an electrical surface charge which causes electrostatic forces (see also Section 10.2.1). Fig. 10.511, shows the flow lines and origination points of the previously defined particles for the case that the product of particle charge q and fiber charge per unit length Q is equal to Cou12/cm, a typical value which has been determined by measurements. In this case the smallest particles move farthest away from the fiber. For a similar case, i.e. the existence of electrical charges, Fig. 1 0 . 5 1depicts ~ the paths that result in collisions with the fiber. As can be seen, small particles may even collide on the back of the fiber if electrical charges are present. Fig. 10.52 shows that the natural agglomeration of very fine particles (see also Chapter 10 and Section 10.2.1) and the resulting larger mass of the conglomerate also contribute to the separation mechanism on filter fibers. Filter media which collects particles on fibers within the media are either discarded after saturation with dust or washed with suitable liquids. Particles may be also dissolved by the action of solvents which do not attack the fibers and/or modify their surface characteristics.
(b)
(c)
Fig. 10.51: Results of model calculations showing the paths of differently sized particles around a fiber at different conditions [lO.S]. Explanations see text.
70.3 Fiber Technologies
Fig. 10.52 Naturally formed agglomerates o f small (8 pm) glass spheres adhering to a filter fiber [10.5, B.421.
Finally, in closing this chapter, a relatively new fiber based agglomeration technology shall be briefly introduced. It was already mentioned at the beginning of this chapter that paper making is one of the techniques that uses predominantly fibers, together with fillers, if applicable, and sometimes binders to yield products with widely differing qualities, from newsprint to papers for applications in the arts, the building industry, for decorative purposes, and many more. The common base of all these products is that fibers, mostly cellulosic from plant material, are used which intertwine and interlock in a thin slurry layer. The wet individual sheets or continuous bands are pressed, dewatered, and dried to yield the final product. Strength, mostly defined as the resistance against tearing, depends directly on the type and, particularly, the length of the fibers. The least stringent requirements on fiber length are for the manufacturing of newsprint. Triggered by growing environmental concerns, the already well established recycling of paper offers several advantages. It reduces the requirement for fresh fiber, saving forests, and makes cheaper secondary raw materials available. It also eliminates the need for large new disposal sites or the construction of expensive waste incineration plants. During recycling, waste paper is deinked and redissolved to form a slurry from which most of the fillers and binders are removed. However, to obtain secondary paper (mostly newsprint, tissue, and packing paper) with acceptable quality, the slurry must be separated by suitable means into a stream containing long fibers, suitable for recycling, and one in which fiber length is below specification. Originally, the stream with the short fibers had been dewatered for disposal in landfills or burning in incinerators. It was found that, after filtering, the waste fibers can be easily agglomerated by tumble/growth methods (see Section 7.4 and subsections). After drying, a granular product is obtained which has excellent absorbtion characteristics for liquids and, due to a structure of interlocking fibers, retains strength after wetting. This material can be used, for example, as absorbent to bind spilled liquids or as cat litter (see also Section 5.4). Additives can produce special characteristics such as extra strength, odor control, avoidance of bacterial growth, and others.
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Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
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11
Engineering Criteria, Development, and Plant Design
Size enlargement by agglomeration is one of the unit operations of Mechanical Process Technology (see Chapter l),the technical field that deals with the processing and handling of solids. When developing, designing, constructing, installing, and operating an industrial plant that includes size enlargement by agglomeration, many or all of the other unit operations of Mechanical Process Technology, each sometimes more than once, as well as the associated techniques and the analytical support functions (see Fig. 1.1, Chapter 1) are required and used. Since Mechanical Process Technology encompasses the oldest techniques serving mankind and because these methods are all based on natural phenomena, they were applied by various users in parallel so that similar but separate techniques evolved in different fields. For centuries, development was purely empirical until recently, beginning less than 150 years ago, one after the other, the unit operations were recognized and treated as generic fields of engineering science and approx. 50 years ago they were evaluated and used interdisciplinarily (see also Chapter 3). At that time, efforts began to apply experience and know-how that was available in one field to solving problems in another, potentially with totally different requirements on, for example, plant size, process cleanliness, and product characteristics. While in the newer fields of industrial technologies, for example chemistry, electronics, communications, etc., process research and plant development started from first principles and many of the equipment and system designs had to be newly elaborated for a particular purpose using modern thought processes as well as manufacturing and industrial methods, even the new Mechanical Process Technologies mostly still rely on fundamentals that are rooted in the purely empirical past enhanced by the know-how of expert persons and/or companies. In addition, since mechanical processes and installations are widely considered simple and the techniques are viewed as being dirty, - because particulate solids in different size ranges, including dusts and slurries, are always involved and present -, having little technical and scientific appeal, and requiring only the combination of known conventional equipment for successful operation, very often designs are crude, old fashioned, and far from optimal. Furthermore, the different disciplines that are today involved in the elaboration of new industrial plants often lack a common background and an understanding of the availability of new, improved technologies so that the resulting designs commonly include stunning misconceptions.
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Fig. 11.1:
Cartoon satirizing the frequently experienced development, design, manufacturing, and installation of a new industrial plant.
The above is demonstrated in a cartoon-like fashion in Fig. 11.1.The idea and execution of this series of sketches was not conceived by the author. It was given to him many years ago during one of his numerous industrial contacts and consultations. The originator is unknown. However, whoever dreamed up and drafted this sequence understood well what is often found in today’s industrial practice. His knowledge and draftsmanship deserve credit. Referring to Fig. 11.1,at the beginning there is a site and an idea which is discussed between the customer, owning or having control of the site, and a supplier of engineering, equipment, and services. (1)Is the supplier’s engineering; (2) is the specification of the purchasing department; (3) is what was actually built; (4)are the instructions for installation; (5) shows how the installation was executed; (6) is the modification which was made in the field prior to start-up; (7) depicts what the customer really wanted; and (8) is what the supplier shows as reference in his advertisements. Although this picture story is exaggerated, it indicates that finally, in the field, even under adverse circumstances, modifications attempt and typically succeed in making the installation work. The result is, however, far from optimal and from what was really intended and/or desired. A fix of all problems, for which outside experts and consultants are typically called in, which should, after the fact, result in a well designed system and an economical operation, is normally not feasible. Some improvements can always be made but an optimal solution is only possible with a completely new installation and expert project management.
7 7 Engineering Criteria, Development, and Plant Design
To improve the development, design, and selection process, some guidelines will be developed below and in the two subsections that follow this introduction. As will be seen and is easily understandable, selection of the proper agglomeration equipment, that which is best suited for a specific task, as well as procurement of the optimal peripheral equipment and system layout depend greatly on the application and the industry for which the process and plant are destined. Tab. 11.1 summarizes the most important parameters in determining the best suited approach for a particular project. The characteristics to be considered fall into four main categories: Particulate feed, agglomerated product, method options, and site.
Considerations during the selection of a suitable agglomeration process for a particular project (see Section 13.3, e.g. [128, 141, 143, 1521).
Tab. 11.1:
Parameters of the particulate feed Feed particle size and shape (dimensions and distribution, surface area, shape factor, fractals, etc.) Moisture content (free, encapsulated, crystal water) Material characteristics (chemistry, density, porosity, plasticity, brittleness, elasticity, wettability, abrasivity, etc.) Special material characteristics (heat and/or pressure sensitivity, toxicity, reactivity, etc.) Bulk Characteristics (temperature, density, flowability, etc.) Binding characteristics Parameters of the agglomerated product Agglomerate size and shape (dimension(s), distribution, volume, weight, tolerances, etc.) Strength - Green strength (if applicable) - Cured (final) strength Structure and other characteristics (porosity, specific surface area, dispersibility, solubility, reactivity, abrasion resistance, etc.) Parameters of the agglomeration method Batch or continuous operation (interruptions or downtimes tolerated or not) Capacity per hour and per year or per campaign Wet or dry operation Simultaneous processing Space and energy requirements Investment and operating costs Site, supply, and environmental conditions, infastructure Relative location to suppliers and consumers (raw materials, additives and binders, energy, users, etc.) Site accessibility and transportation facilities Climatic conditions Availability of skilled and other labor Availability of support functions Regulations ( e g EPA, OSHA, etc.)
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Particulate feed Particle size and distribution should be determined and considered first. A limit in the range of a few hundred micrometers defines the applicability of methods using growth mechanisms based on coalescence in moving beds of particles (tumble/growth agglomeration). Larger particles, which may also constitute seed agglomerates or recycle, can only be incorporated if an adequate amount of binder or sufficiently small particles are present. Small particles tend to embed larger ones (see Sections 5.3 and 7.1). Agglomerate strength is defined by the matrix of fine powder in this case. Generally speaking, it is difficult to agglomerate narrow particle-size distributions or monosized particles. Adding binder can cause relatively large particles to agglomerate. However, it may be more economical to crush larger particles to render material suitable for tumblelgrowth agglomeration. This is particularly true if the product must feature high porosity. Pressure agglomeration can be applied for larger feed sizes, e.g., sandlike material and particles of up to 20 - 30 mm. Since the external forces acting upon the mass result in particle disintegration or deformation, the upper limit of feed particle size is determined more by geometrical restrictions of the feeder than the ability of the material to agglomerate. Inmost cases, consolidation takes place ina short time. To obtain sound agglomerates, considerable amounts ofair must be removed during compaction. Because ofincreasing resistance to flow with decreasing particle size (due to smaller pore radii), very fine bulk solids (below about 150 pm) can be agglomerated by pressure methods only if certain preconditions, particularly low speed and dwell time, are fulfilled (see Section 8.1). Moisture content, especially, free moisture, can play an important role in growth agglomeration by coalescence. Here, moisture provides the binder or prevailing binding mechanism. The maximum volume ofliquid must not be more than about 95 % of anticipated agglomerate porosity. Wet (tumble/growth) agglomeration is sensitive to this limit because a small excess of moisture beyond 100 % saturation causes the entire charge to turn to mud (see Section 5.22, Fig. 5.28). Further, since the addition of moisture in the agglomerator is an important tool to control growth (see Section 7.2) and other agglomeration parameters, the feed moisture should be several percentage points below the critical maximum moisture content, as defined above. The moisture content is less critical in fluidized-bed agglomerators which also act as dryers. Here, the moisture must often be high enough to make the feed pumpable (see Section 7.4.4). In pressure agglomeration, moisture must be kept low. In most cases, dry feed is a precondition for high-pressure agglomeration. The reasons are that, due to the high compaction forces, crushing, rearrangement, and deformation of the solid take place which result in a considerable reduction of porosity. Excess water either is squeezed out or remains in the mass as an incompressable component (see Section 8.1). Both result in low strength. Filter cakes turn into mud upon discharge from filter presses unless moisture is reduced by applying, for example, vacuum or compressed air. Material characteristics, such as chemistry, particle density or porosity, brittleness, elasticity, plasticity, wettability, and abrasivity, etc., play important roles in the choice of an agglomeration method. A particular chemistry may be necessary to bring about
7 7 Engineering Criteria, Development, and Plant Design
the required chemical bonding or may be incompatible with certain conditions of a method (such as the necessary addition of water or other liquids in most tumble/ growth agglomeration techniques). Density (or porosity) of the feed particles, through particle weight, determines gravitational and other field forces which may be counteractive to adhesion by coalescence. Brittleness, elasticity, plasticity, and abrasivity are most important for pressure agglomeration but are of less concern for tumble/growth methods. Wettability, on the other hand, is the single most significant parameter for all agglomeration methods using surface tension and capillary forces in the growth regime. Wetting is required for green strength. In some industries, special material characteristics, such as heat or pressure sensitivity, toxicity, and reactivity must be considered. These parameters are particularly important in the pharmaceutical industry. Pressure agglomeration has only limited applicability for heat- or pressure-sensitive materials. Toxicity may limit the appeal of tumble agglomeration because of the difficulty to contain dust.
Fig. 11.2: Simplified flow sheet of dry granulation (as used typically in the pharmaceutical industry) using slugging or a roller press and optional circuit alternatives.
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11 Engineering Criteria, Development, and Plant Design
Bulk characteristics, such as temperature, bulk density, and flowability, may be adjusted in a preparatory step to improve size enlargement. Prior to briquetting into ration-sized agglomerates, vegetables, food pulps, and fruit juices may be frozen. Metal dusts and powders, as well as certain minerals, are often heated and briquetted hot to make use of their increased malleability at elevated temperatures. High bulk volume and unacceptable flowability are sometimes corrected by using two agglomeration methods in series. For example, fine feed powders are pre-agglomerated (Fig. 11.2) to reduce the compaction stroke and improve the flow of feed into a die. This increases, for example, the speed of the turret of rotary tabletting machines. At the same time, this technique avoids segregation of the feed mix by stabilizing the blend in a granular form (see Section 8.4.3). Roller presses are particularly sensitive to the problems associated with high bulk volume and the need for sufficient deaeration. As shown in Fig. 11.3, bulk density can be increased by either using two presses in line (Fig. 11.3a) or by producing a sufficient amount of predensified recycle (Fig. 11.3b). Finally, the possibility of obtaining bonding with a candidate method must be determined to decide whether agglomeration can be carried out binderless (potentially, by making use of an inherent binder in the material and/or high pressure or elevated temperature) or requires the addition of binders. Agglomerated product The desired shape, dimensions, and size distribution of the ag-
glomerated product also influence the selection of a suitable method. Results of size enlargement may be, for example, free flowing, dustfree, granular products with strict requirements on the limits of size distribution, accurately shaped compacts with extreme demands on tolerance, or large, highly densified and strong briquettes. Many other requirements, including predetermined volume and/or weight, are conceivable. Granular,fiee-fowing, dustfiee products can be manufactured using almost all methods of size enlargement. The (often necessary) task of narrowing the size distribution of the discharge from tumble/growth agglomeration is done by screening whereby under- and oversized components are produced. While the fines are recirculated to the agglomerator, oversized particles are crushed and either rescreened (to recover additional product) or directly recirculated with the fines (see Chapter 7). Granular products can be also obtained by crushing and screening large agglomerates such as tablettes, cylindrical pellets, compacted sheets from smooth roll compactors, and briquettes or tablettes (slugs) from various equipment (Fig. 11.2). In this application, selection criteria are often defined by other parameters, such as product porosity or density, solubility, reactivity, or inertness. Yield of the end product may be very small (less than 25 % for narrow size distributions) or relatively high (75-80 % for wide distributions) but, in most cases, sizable amounts of fines must be recirculated to the agglomerator (alternatives 1 and 2 in Fig. 11.2) unless granulating (crushing or milling) yields a product that can be used directly (granular product, normal, in Fig. 11.2). Only the confined die pressing in punch-and-die machines is suitable for producing accurately shaped compacts with extreme demands on tolerance. Such requirements exist, for example, for dry dosage forms in the pharmaceutical industry and for near net-shape preforms in powder metallurgy.
7 7 Engineering Criteria, Development, and Plant Design
Fig. 11.3: Two flow sheets o f precompaction arrangements with roller presses.
The reciprocating movement of the pistons and the often small volume of the die cavity restrict the capacity of these machines, even if modern, multistation, rotary tabletting presses are considered. With the production of larger pieces in, for example, hydraulic presses or roller briquetting machines, accuracy in weight, shape, and dimensions sometimes is no longer obtained. Shape is often also important. In many cases, spherical products of size enlargement are desired. The approximate shape can be obtained with all tumble/growth agglomeration methods. On the other hand, unless extremely accurate feed control is established in some punch-and-die machines or by using wet bag isostatic pressing, spherical products cannot be produced with pressure agglomeration. The nearest approximation would be pillow-, lens-, or almond-shaped compacts. Strength is significant for the final product, but also plays a role during size enlargement itself. Particularly in growth agglomeration, green agglomerates are first formed which then must be cured to obtain permanent bonding. In most cases, a weak intermediate state exists when the binding mechanism of the green agglomerate disappears and before the permanent, cured bond sets in. Unless large amounts of matrix binders are used, or agglomerates are cured at extremely high temperatures (e.g. sintering, partial melting) or by some chemical reactions, growth agglomeration will result in weaker products than most pressure agglomeration methods. Porosity plays a major role, too. Different strength levels develop primarily because agglomerates growing by coalescence feature higher porosity than those from pressure agglomeration. However, strength may not be the only determining characteristic. In fact, materials which must be easily dispersible and are only agglomerated to improve handling of the intermediate product should have just enough strength to survive their short existence. In other cases, a large spec@ sufuce (e.g. catalyst carriers) is more important than high density and strength. Generally, with increasing external forces acting upon the particulate matter during size enlargement, porosity and characteristics related to this parameter decrease, while density and strength increase.
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Method options Depending on the requirements of the application, agglomeration may be batch or continuous. Batch processes are normally low capacity, but feature a high degree of control. Most large-volume applications operate continuously with corresponding variations in quality. Maintenance and cleaning reduce the annual capacity. In growth agglomeration, uncontrolled build-up must be removed, and in pressure agglomeration, worn parts require replacement. In plant design, the annual production is often calculated on the basis of 330 days per year and 20-22 hours per day to allow for scheduled and unscheduled downtime. Sometimes other processes that are closely related with the agglomeration system operate in campaigns which may last for a few months or more than a year. In those cases, it is necessary to select an agglomeration method or instal suitable redundancies to meet this requirement. Most of the growth methods are wet processes using liquids as binders for forming green agglomerates. In contrast, most high-pressure techniques are dry processes. These operating conditions play an important role, if, for whatever reason, a particulate solid must not be wetted during processing. On the other hand, addition of liquids may bring about desirable effects such as specific binding mechanisms or chemical reactions and, therefore, require tumble/growth techniques. The latter qualify as simultaneous processing if agglomeration and chemical reactions occur concurrently. Some classic fertilizer agglomeration methods (ammoniators)operate in this fashion. More often, simultaneous processing happens in mixer-granulators, granulator-dryers, or even mixer-granulator-dryers. Mixers are often also granulators in which both processes occur but in different zones. However, in fluid-bed granulators, agglomeration and drying take place simultaneously. Space and energy requirements as well as investment and operating costs frequently render an otherwise perfectly feasible process uneconomical. Incorrectly, these factors also sometimes direct interest toward methods, which after superficial investigation seem to offer cheaper alternatives because indirect or hidden costs are not recognized. The entire process must be considered. For example, in the granulation of fertilizers a granulation drum may seem cheaper than a roller compactor. If only investment costs are considered, this may still be true. However, if space requirements of the complete system, which includes dryer(s) and cooler(s),as well as the energy and operating costs for entire processes are compared, in many cases a different conclusion may be drawn. New applications for size enlargement by agglomeration include the recycling wastes which contain valuable ingredients, and the disposal of particulate wastes without value in an environmentally safe and acceptable way. Particularly in the latter case, an economical solution seldom can be found. However, because legislation forbids dumping or landfilling in a quickly increasing number of countries, agglomeration must be applied. Sometimes, economic justification can not be obtained and the process is used solely in compliance with laws. In other cases, the application of bonuses (e.g. incentive payments by communities) or credits (e.g hazardous materials become nonhazardous due to size enlargement) may result in an economical or even profitable operation.
I 7 Engineering Criteria, Development, and Plant Design
Site, supply, and environmental conditions, infrastructure The relative location of a
processing plant and its influence on economics is of great importance. Many pitfalls can endanger the success of a project. Since, in most cases, one of the major reasons for size enlargement is the improvement of material handling characteristics, plants should be built at the source of the particulate solid. Therefore, a suitable method must consider the availability and cost es and auxiliary materials, such as binders. The availability of waste steam or byproduct gas for heating may render wet granulation economical. On the other hand, where energy must be purchased, the same task may best be accomplished using roller presses for dry compaction, and granulation by crushing and screening. If binders are required for strength, the binder, often developed during tests in an equipment vendor’s laboratory, must be available in sufficient quantity and at an acceptable cost where the plant will be located. Feasibility of many well thought out projects disappeared when it became clear that the necessary binder was not available at the proposed plant site and existing processes had to close when the binder source “dried up” (see also Section 5.1.2). When wastes containing valuable components are processed for recycling, it is necessary to not only identify the need for and method of use of the agglomerated secondary raw material, but also an actual user and its relative location. The cost for transportation from the source to the potential consumer often becomes prohibitive. Since higher strength of the product is normally associated with higher production cost (due to more binder, higher curing temperature, higher compaction force, and higher wear), unless there are special requirements, agglomerate strength should be just high enough to survive handling and transportation. Therefore, products for a more distant user may need higher strength, costing more, and resulting in reduced economics. The accessibility of the site, including infrastructure, availability of a labor force and nce as well as support facilities, availability or lack of already existing transs, and climatic conditions must be considered early during project development. Regarding climate for example, long periods of freezing may require excessively high costs for winterization and hot and humid summer months may make air conditioning necessary, not for the comfort of the workers but to avoid condensation and/or adsorption of moisture onto powders with large surface area and attendant changes in flowability and binding characteristics that result from this condition. If not considered and resolved during project implementation, agglomeration plants may show marked differences in performance and product quality during winter and summer which, in the worst case, prohibit a successful year-round operation. Environmental regulations can influence the selection of agglomeration equipment in several ways. A first concern is always the finely divided particulate nature of the feed materials. Often, these are precipitated dusts or solids removed from fluids in pollution control devices. Recontamination of the environment is an obvious concern and is typically regulated. The equipment must provide dust control and be completely enclosed, particularly if the material is hazardous or toxic. Many of the growth agglomeration methods, for example, drums or discs, do not easily fulfill these requirements. In the USA, for example, all installations handling, treating, and/or processing fine
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1 1 Engineering Criteria, Development, and Plant Design
particulate solids must obtain permits (e.g. from OSHA and EPA as well as state and local authorities) prior to planning an installation.
11.1 Preselection of the Most Suitable Agglomeration Process for a Specific Task
Since a large variety of techniques and equipment is available to carry out size enlargement by agglomeration, one must first determine which method is the most likely choice for a specific task. Fig. 11.4 is a simplified preselection guide for picking a suitable method. The arrow tips indicate the direction of an increasing feed or product property and point to the technology that is most suitable for treatment of that material or obtaining a specific product characteristic. For example: With increasing feed particle size, tumblelgrowth agglomeration becomes less applicable until only high-pressure agglomeration remains. Or: Products from high-pressure agglomeration have low residual porosity, while agglomerates from tumblelgrowth methods typically feature porosities of approx. 40 % and more. Although there are many exceptions to these rules, Fig. 11.4 provides a simple and often valuable guide to preselection. Particularly post-treatments (see Sections 7.3 and 8.3) or the use of matrix binders (see also Section 5.1) may change final product strength and reactivity such that they become independent of the agglomeration method.
Fig. 11.4 method.
Simplified selection guide for choosing an agglomeration
7 1 . 7 Preselection of the Most Suitable Agglomeration Process for a Specific Task Techno1 Process
Tumble/Growth Low density High density : Pan D r u m Mixer Fluid with me&. Bed agit.
TOLERANCE TO FEED MATERIALS: f ines H H H coarse partic. L L L wide d i s t r . A A A low bulk dens. H H H low flowabil. L/A L/A A
Pressure
low
medium
high
Screen w i t h extrud. spheron.
Dellet./
ram
die extrud.
ex-
A/H
A D
H
H
VL
VL
A
VL
L
L
A/H H H A
A/H H
H
H
H
A W
H
VH
VH
H H
H H
e l a s t . p a r t i c . VH moisture cont. H high teap. VL
VH H VL
VR H VL
VH
VH
A
A
VH VL
B I N D E R REQUIR.:H
H
H
A/H
H
H
A L L
H L L/A
DANGER OF: segregation H d u s t (pollut.1 H temp. r i s e VL
H VL
H
tablet.
trud.
r o l l . Dress b r i q . &nap/ gran
-
H A H
H
VL
L
A/H
H
A/H
H H L
L L L
L L L
A L L/A
L/A
L
L L L/A L A A/H
A/= A A A A/H A
L A/L A
V L A/L A
L(m.1
L(vL) L/A H V H H V L
V L A/L A/H
V
L A/L A/H
PRODUCT I
capacity max. s i z e density strength ( a f t e r curing1 solub./disp. Uniform S i r e uniform shape abrasion r e s . ( a f t e r pOSt treatment)
H H A A L V L L L H H H H A A A L/A A L/A A/H L/A
A L L L
H H A L/A L/A L/A
V L Y L V L L L L L/A L/A
VB
H
L/A L/A L L/A
A A L/A L/A
L L L L H A L/A L L
VH
H H H
L H
H H
H
H
H
A A/H
A A
A/H A/H
VH
VH H H
VH(L) A/H H V H H V V L H H H H
VH(L1 A H H H L A/H A/H A/H
H
Fig. 11.5: Comparison o f some o f the most important considerations for different agglomeration methods. Notations: A = average, H = high, L = low, V = very (as, for example, in VH = very high).
Fig. 11.5 is another attempt to compare some of the most important selection considerations for different agglomeration methods. It must be realized that general, always valid statements for all processes and all conditions can not be made. The notations in Fig. 11.5 represent the most likely result or observation in each category. Also, it should be pointed out that all indications are relative to the typical application of each process. When using an interdisciplinary approach to process selection, where certain knowhow and experience from one industry is applied to solving problems in another field, somewhat different results may be obtained than are commonly known from the traditional application of a particular method. For example, product dispersibility (and solubility) is normally considered to be low if high-pressure agglomeration is used for size enlargement. It has been found, however, that powder formulations that contain dry binders and/or dispersants can be compacted with roller presses, which normally apply high pressure (see Section 8.4.3, Fig. 8.140), now using a linear force which may be considerably less than 1kN/cm (e.g. 0.2 kN/cm). Inspite of the low force, sheets are formed that can be crushed and screened to produce a narrowly sized granular material with good yield, sufficient strength, and excellent dispersibility both in dry and wet environments (see also Section 5.4). The notations in Fig. 11.5 can only be used to “get a feeling” for what a particular agglomeration method needs and is able to produce. For the actual preselection of a process, relative ratings must be expressed by numbers which can be added up to allow overall comparisons. Fig. 11.6 represents such a numerical ranking format. It is only
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7 7 Engineering Criteria, Development, and Plant Design
feed choracteriitii (method's toleronce of): 101
10
h , e
2
2
hlOi5I
Abioiive tlom
8 9 7 9
Plortic
10
8iinle Law bulk deositv Poorly lbwing
10 8 8 3
8 9 7 9 10 10
ftno Hoid
High lmpeiotuie
10 2 8 5 3 8 9 9
10
9
1
1
8 2
1 9
1 7 7 1 0
9 5
9 1 0 4 1 0
9 6
4 1 3
I 7 2
1 5 4
2 10 10 ~ 8 9 6
1 10
5 8 10 10
3
1
0
9 10 10 5
9 9 9 a 8 9 8 ~ 1 3 4 2 1 0 N A
2
9
9
5 l 4 3
5 i 3 3
2 9 8 10 10 10 10 10 7 7 8 8 1 0 1 0 1~ 3 8 8
8 10 7 9 8
5 7 5
5
5
5
5/9' 7
3 5
3 5
5
5
5
5
5
I 1 0
3 10 9 2 5 1 0 10 8 10 5 10 7 10 9 10 9 3 8 9 5 10 9
9
9
1
1
10
5
2 3
2 2 2
10
10
8 9 8 9
8 9 8 9
10 10 N# NA NA 5 5 NA
8 3 3 4 1 1
8 4 3
I
I 2
5
Method's typical
ogglomerale choracteristic:
Smoll Yze icrge Iize Tight Yie toleronce High rnengh Hgh pno$r(
High dubiir( Equipment Cost Inrermnf Oprohng ReQured mantemrxe
(opatiryhrotqhpui Pc idmon Lore d operunon tore d o u t m o m Requiid eneigy
10 3
8 2 5 2
10
10
10
10
4 7 3 8
4 3 3
3 3 3
2 2 2
7
7 1 0 1 0 7 1 0 1 0
9
7
5
5
5 5
7 8
4
5 5 5
?' ?' 1"
a 5
P 5 10
lo/?
9
4
4
I
5 5
5
5
5 6 10 9 1013' 9/3' 7 7
5
5 5/3' 5 2 3
5 2
4
8 8
5
5 5 5 3 3
5/84 5 8 5 9 10 10 7
5
9
8
5
5
2 2
2 5
2 3
2 2
2 1
4 4
6 5 6 3 3 8
8 8 5 2 8 4
5 b 6 9
5 l 5 5 9/51 5 8 5
5 7 5 5 5
5 5 7 4 5 5
8
9
8
7
7
6
5
5
5
5
5
5
5
5/7'. 8 10 1 10 5
2 3 4
8
1
5 5
9
8
4
7
5 10 10 8 1 10
4
NA NA 3 4 NA I N A 1 9
5
5
4 10
9
5
5
NA
N 5
5
5
5
'
5 s
s 5
I 5 5
5 0
?
5
5
A 5 8
5
Fig. 11.6: Ranking o f some agglomeration methods by common feed, agglomerate, and equipment factors. Notes: (b) rankings: 1 = low, 5 = average, 10 = high; (c) NA = not applicable; (d) these (second) rankings are valid for the production o f granular material. When manufacturing granular products, a fairly large amount o f undersized material is recycled which can limit production capacity; the lower numbers apply for those projects that require a narrow size distribution (= large amount ofrecycle). (e) ? = these characteristics are unknown.
an example and rates some of the most common factors (similar to what has been presented in Tab. 11.1, Chapter 11) for a selection of agglomeration methods. Also, as mentioned before, the ranking is based on traditional applications. If a specific project needs to be evaluated, after determining own prior experience, related published work, and vendor input as well as potentially employing the services of an unbiased consultant in the field, a knowledgeable person within the company must review the list of factors, adding or subtracting specific items, and revise the individual ranking numbers. By using a scale of 1 to 10, with 1 = low, 5 = average, and 10 = high, a more subtle distinction of responses to the different factors by individual agglomeration methods can be obtained. It should be pointed out, however, that the numbers always mean what they indicate, literally. Therefore, they may sometimes imply different conclusions. For example, if the tolerance to fines is 10, this
~
7 1.1 Preselection ofthe Most Suitable Agglomeration Process for a Specifc Task
indicates excellent performance of the method if the feed is a fine powder. But if the energy requirement is rated 10, this means that much energy is consumed which, normally, represents a negative process performance. To demonstrate the preselection procedure that is based on the aforementioned selection guides, two examples will be presented in Tables 11.2 and 11.3. The evaluations are based on the rankings in Fig. 1l.G, and for the elaboration of these examples, the numbers reflecting certain important factors are taken without any adjustment to particular conditions of the two projects. As mentioned before, additional factors could be defined and ranked which take into consideration the special projects and, based on published information or experience, the numbers, indicating responses of different agglomeration methods to these factors, could be modified. Contrary to what has been done in the following, referring to Tab. 11.1, in most cases, the influences of site, supply, and environmental conditions as well as infrastructure considerations should be included. Example 1 (Tab. 11.2) refers to the production of an easily dispersible granular material from dry powders (see Section 5.4). The following factors were selected for consideration during the preselection exercise: 0
Feed characteristics: fine, brittle, low bulk density (3 x ) . Agglomerate characteristics: small size, high porosity (3 x). Equipment specifications: low operating cost, low maintenance, ease of operation, ease of automation (3 x ) .
The individual response numbers of Fig. 11.6 are added up for each category and then totaled with and without the opposite influence of costs. The highest respective number indicates the most promising choice for carrying out this task by agglomeration. In comparing the results with the agglomeration methods in Fig. 11.6, without the influence of costs (operating and maintenance), for the first G choices the following ranking is obtained: 1. fluidized bed, 2. mixer, 3. disc or cone, 4. deep disc or drum, 5. spray drying, and 6. vibrating trough. As expected in Example 1, all suitable methods belong to the tumble/growth group oftechniques. If costs are included in the evaluation, the disc or cone and spray drying are tied for third place and the vibrating trough moves to the last of the six choices. Sometimes, a better preselection can be obtained if, in each category, the most important characteristic is weighted by multiplying it with a factor (for example 3 x). After doing this, not much change in ranking occurs in this example. The only remarkable modification is, that spray drying moves up in rank and looks now as good as or better than the mixer. Also, pelleting, a medium-pressure agglomeration technique, becomes a feasible alternative. While, generally, the results of this exercise agree with what is commonly used in existing industrial applications, modifying the basis, by adding new techniques and/or introducing new factors and determining how they fare with the individual agglomeration methods, enhances the preselection process. For example, low pressure extrusion with and without spheronizing could be included as a viable alternative method and, under certain conditions (e.g. the use of low specific force), compaction/granulation could become feasible for the project.
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7 7 Engineering Criteria, Development, and Plant Design Tab. 11.2 Preselection Exercise: Example 1. Production of easily dispersible granular material from dry powders. Feed Characteristics: Fine, brittle, low bulk density (3 x ) . poorlyflowing Agglomerate Characteristics: Small size, high porosity (3 x ) Equipment: Low operating and maintenance, ease ofoperation, ease o f automation (3 x )
Feed Characteristics 36 36 Product 18 17 Characteristics 9 Easeofoperation/ 9 Automation 14 14 (Cost + Maintenance
35 17
30 20
37 20
17 17
31 7
26 7
28 7
34 5
34 5
33 9
25 10
32 16
34 9
34 9
14
7
12
8
16
13
16
12
12
11
18
13
10
10
11
9
13
-
11
16
12
12
12
12
10
12
10
10)
Total(exc1. C&M) Ranking
63 3
62 4
66 2
57 6
69 1
42
54
46
51
51
51
53
53
61
53
53
Total (incl. C&M) Ranking
49 3
48 4
55 2
46 5
56 1
-
43
30
39
39
39
41
43
49 3
43
43
Weighted Total (excl. C&M) Ranking
105 102 110 91
123
-
89
84
87
89
89
91
89
111 83
83
4
5
3
6
1
Weighted Total (incl.C&M)
91
88
99
82
110
Ranking
3
4
2
5
1
5
6 -
78
68
75
77
77
2
79
79
99
6
(6) 2
73
73
Note: During weighting the most important responses in the three categories are multiplied with three (3 x )
Example 2 (Tab. 11.3) evaluates the production of highly densified, inert (passivated) briquettes from hot sponge iron (see Section 5.4). The following factors were selected for consideration during this preselection exercise: Feed characteristics: coarse, abrasive, plastic, high temperature ( 3 x). Agglomerate characteristics: large size, high density = 10 - high porosity ( 3 x). Equipment specifications: low operating, maintenance, and energy ( 3 x ) costs as well as ease of operation and automation. The individual response numbers from Fig. 11.6 are again added up for each category and then totaled with and without the opposite influence of costs. The totals without the influence of costs (operating, maintenance, and energy) indicate that the ranking for the first 6 choices is: 1. sintering, 2. roller press, 3. punch-and-die pressing, 4. ram extrusion, 5. screw extrusion, and 6. pelleting tied with plastification. As expected, all methods belong to the heat and pressure agglomeration groups of techniques. If costs are included in the evaluation there is only a small change in that pelleting drops out. If the weighting procedure is performed, plastics granulation as well as plastification become highly rated apparent choices.
11.1 Preselection ofthe Most Suitable Agglomeration Process for a Specific Task Preselection Exercise: Example 2. Production of highly densified, inert (passivated) briquettes from hot sponge iron. Feed Characteristics: Coarse, abrasive, plastic, high temperature (3 x ) ; Agglomerate Characteristics: Large size, high density = 10 - high porosity (3 x ) ; Equipment: Low operating, maintenance, and energy (3 x ) costs, ease of operation and automation.
Tab. 11.3:
Feed Characteristics 22 Product 6 Characteristics Ease of Operation/ 9 Automation (Cost + 19 Maintenance [C&M]
22 7
18 6
18 2
24 3
-
24 16
20 13
35 15
25 13
23 13
23
3
11
39 15
25 4
22 12
22 13
9
14
7
12
8
16
13
16
12
12
11
18
13
10
10
19
16
12
21
-
16
21
17
17
17
17
20
22
15
15)
72 1
42
44
45 G
52
20
29
30 G
Total(exc1.C&M) Ranking
37
38
38
27
39
-
56 3
46
66 2
50 4
48 5
45 6
Total(inc1. C&M) Ranking
18
19
22
15
18
-
40 3
25
49 2
33 4
31 5
28
WeightedTotal (excl. C&M) Ranking
47
50
52
31
59
76
64
102 76
74
67
102 64
80
81
1
2
3
Weighted Total ( i d . C&M) Ranking
18
21
26
12
22
40
62
55
56
4
3
~~~~
~
4 -
50 5
43
1
4
5
75
49
47
1
G
1
2
22
~
Note. During weighting the most important responses in the three categories are multiplied with three (3 x )
Example 2 demonstrates some of the potential problems and pitfalls of this preselection method. As mentioned before, the response numbers of Fig. 11.6 have been collected without a specific project in mind and, therefore, represent the most common applications of the individual agglomeration methods. The hot densification of directly reduced iron (DRI) to achieve passivation of this initially highly reactive material (see Section 5.4) features and requires very specific responses to the different factors so that, prior to carrying out the preselection process, the response numbers should have been reviewed and adjusted. In the industry, hot densification with roller presses has emerged as the exclusive agglomeration method for this task. As can be seen from Tab. 11.3, even without an adjustment of the response numbers, the roller press is selected in the #2 spot and moves to the #1 selection if the most important factors are weighted and the requirement for low costs is included. However, the rankings seem to indicate that sintering is another desirable method for the task. This is due to the fact, that this technology can not only handle but requires high temperatures, is a very rugged technique, and, in some sintering applications which were actually considered during the determination of the response factors in Fig. 11.6 (for example HIP, see Section 8.4.4), high density is achieved.
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Nevertheless, for several reasons, a discussion of which exceeds the scope of this book, sintering does not become the preferred method. The emergence of plastics granulation and plastification in the top rankings if the most important factors are weighted is due to the fact that they represent easy responses of plastics processing. Since hot DRI is by no means comparable with plastics, these results of the preselection process will be disregarded by anybody who is somewhat skilled in the art.
11.2 Laboratory Equipment, Testing, and Scale-Up
As has been shown in Section 11.1,knowledge of the binding mechanisms of agglomeration, the parameters controlling the processes of agglomeration, the characteristics of the equipment that is available for size enlargement by agglomeration, and the requirements on product quality as well as plant design, together with the availability of interdisciplinary research and operational know-how and experience, does allow a certain preselection of the most suitable method(s) of size enlargement by agglomeration. Nevertheless, development of all agglomeration techniques is still more an art than a science. Having done the preselection, which, after collecting all the information that is summarized in Tab. 11.1 (Chapter l l ) ,is a desk job, it becomes normally necessary to carry out further investigations and do testing, particularly for example, to determine if and potentially what kind of a binder must be added and how much of it is required. After that, tests with actual equipment must be conducted to find limitations regarding capacity as well as product size, shape, and characteristics and needs for peripheral equipment and/or post-treatment. Closed loop processing and recirculation must be evaluated, too. Unfortunately, in many cases the availability of the actual material that needs to be processed in a new installation is limited at the time of plant design. The testing of similar materials from different sources, even if they are chemically identical and seem to be physically comparable, is not recommended because traces of impurities and minuscule changes of surface structure, for example, can decisively change many or all aspects of a material’s agglomerative behavior. Therefore, a common desire is to use small laboratory, often desk top equipment in an effort to develop the sizing and parameters of a large scale industrial plant. Referring to Tab. 11.1 (Chapter ll), to be able to evaluate a particulate solid or the mixtures of particles and/or powders, the chemical and, particularly, the physical properties of this material must be known. A description of all the methods and procedures that are available today to completely characterize particulate solids and determine all the quality attributes that are necessary for a meaningful determination of their behavior under different process conditions is a book project in itself. Therefore, in the context of this book, only a few will be mentioned. Those selected represent relatively new laboratory equipment that combines the analysis of different powder characteristics in one piece of equipment and/or improve the gathering of data, that is of particular interest for agglomeration.
7 7.2 Laboratory Equipment, Testing, and Scale-Up
Fig. 11.7: Photograph of the micron powder tester (courtesy Hosokawa Micron Powder Systems, Summit, NJ, USA).
Fig. 11.7 is the photograph of the Micron powder tester by Hosokawa Micron Powder Systems. It is a multi purpose instrument that provides seven mechanical and three supporting measurements in a single unit. It has been developed to improve the handling efficiency and accuracy in testing the bulk properties of dry, fine particulate solids. The mechanical measurements determine the angle of repose, the compressibility, the angle of spatula, the cohesiveness, the angle of fall, the dispersibility, and the angle of difference. Supporting measurements provide the aerated bulk density, the packed bulk density, and the uniformity of the powder. The instrument employs a microprocessor, an electronic balance, and a built-in dust collection system. The results can be printed or, with a data communication port which is provided, they may be sent directly to a computer file. With this feature, the apparatus can be also used later for quality control (QC) and quality assurance (QA) in the industrial plant. Fig. 11.8 are photographs of two instruments offered by Amherst Process Instruments (API). Fig. 11.8a is the API aerosizer, a high resolution particle size analyzer for fine powders (range 0.2- 700 pm), which is based on aerodynamics. A gas containing the entrained particles expands through a nozzle at supersonic velocities into a partial vacuum which is contained within a barrel shock envelope. The exit velocity of a particle depends on its density and size. Two laser beams, separated by a defined distance, and two photomultipliers form the measurement zone. From the velocity and the known density of the particulate solid, the aerosizer determines size, one by one, with a speed of up to 100,000 particles per second and an accuracy of better than 1 % which make this instrument rather unique. The system uses interchangeable dispersers for different types of particles and, in total (refer to Fig. 11.8a), consists of particle dispersers (A), sensor unit (B), vacuum system (not shown), and computer (C).
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Fig. 11.8: Photographs of (a) the API aerosizer and (b) the API aero-flow. (a) also includes a schematic presentation of the measuring principle (courtesy API, Amherst, MA, USA).
7 1.2 Laboratory Equipment, Testing, and Scale-Up
Many other particle size analyzers are available for a variety of particulate solids in all particle size ranges. They are based on a large number of different physical principles. For an up-to-date coverage of this topic the literature should be consulted [B.GO]. API also offers an automated powder flowability analyzer (Fig. 11.8b)which features real time, low pressure, non intrusive powder flow analysis. It is being used both in the laboratory and for process control. The Aero-Flow utilizes the deterministic chaos theory [B.59] to measure the time intervals of a series of catastrophic avalanches. The particular advantage of this system is that it eliminates the need for any operator subjective measurement. Other, already conventional methods to measure flowability and adhesion tendencies of particulate solids are based on the shear cell developed by Jenike and Johanson [B.11] and adapted or modified by many other researchers. Particle shape is another characteristic that is of particular importance for agglomeration (see Section 5.3.1) and, for a long time, could not be determined easily. Fractals [ B.371 have become one possibility of describing macroscopic and microscopic particle shape. The company Particle Characterization Measurements is now offering the Powder WorkBench 32 (Fig. 11.9), a particle size and shape analyzer, that identifies, differentiates, and categorizes powders and particles based on advanced morphological characteristics, such as shape, size, roughness, microroughness, and partial symmetry; it also performs a complete Fourier analysis. In all, the Powder WorkBench 32 offers over 50 specific morphological particle characterization features. It recognizes that not all particles are spheroidal. By applying advanced mathematical solutions it uniquely identifies and characterizes each particle whereby their orientation will not effect the results. This orientation independence is achieved by two simple operations. First the center of gravity of each particle is located and then each profile is rotated coincident with its principal axis. This is called “standard orientation”. With it, results are reproducible since identical particles which are only oriented differently produce the same data. Finally, another new laboratory technique, that is particularly important for the evaluation of agglomerates, measures porosity. Fig. 11.10 is the photograph of a Poremaster automatic pore size analyzer which is based on mercury porosimetry. While this technology is not new, the automatic features and computer control, including
Fig. 11.9: Photograph o f the Powder WorkBench 32 (courtesy Particle Characterization Measurements, Iowa City, IA, USA).
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Fig. 11.10 Photograph o f a Poremaster automatic pore size analyzer (courtesy Quantachrome, Boynton Beach, FL, U S A ) .
data transmittal and reduction software, make the instrument particularly easy to use. Representative for many other modern analyzers in different fields of applications, Tab. 11.4 demonstrates what data reduction software can do with a basic set of results. Such conversion features are included in most modern computer assisted laboratory instrumentation and allow, with a mouse click, to evaluate many related properties of what has been investigated in the first place. While mercury porosimetry still ranks among the most commonly used methods for the determination of pore structure, volume, size, and related data, concerns associated with the presence of elemental mercury, although well contained, triggered the development of alternative equipment. For example, Fig. 11.11 shows different
Poremaster data reduction software (according to Quantachrome, Boynton Beach, FL, USA).
Tab. 11.4
Cummulative pore volume vs. pressure or pore diameter Cummulative surface area vs. pressure or pore diameter Differential pore volume vs. pressure or pore diameter Differential pore area vs. pressure or pore diameter Pore number fraction vs. pressure or pore diameter Particle size distribution (Mayer-Stowe and Smith-Stermer Theories) Pore tortuosity Permeability Throat/pore ratios Fractal dimension Statistics Sample compressibility
7 7.2 Laboratory Equipment, Jesting, and Scale-Up
Fig. 11.11: Photographs showing non-mercury instruments for the determination o f porosity and related data. (a) Capillary flow porometer for gas as well as liquid permeability and the testing offilter integrity; (b) gas pycnometer; (c) bulk/absolute density analyzer; (d) BET sorptometer (courtesy PMI, Ithaca, NY, USA).
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instruments in non-mercury technology for the determination of gas and liquid permeability in a capillary flow porometer (a),which can be also used for the testing of filter integrity; for the measurement of the absolute (true) density of particulate solids with a gas pycnometer (b),applying helium or other non-reacting gas; for the measurement of bulk density (c), using wetting fluids; and for performing physisorption and chemisorption in a BET sorptometer (d)with an extremely low range of adsorption pressures to obtain surface area, micropore and mesopore information, isotherms, and density. In addition many traditional and other new laboratory technologies are available for the determination of those parameters that were mentioned in Tab. 11.1(Chapter 11) and which are necessary to evaluate feed materials and agglomerated products during the first development phase.
7 7.2 Laboratory Equipment, Testing, a n d Scale-Up
Fig. 11.12:
Photograph o f an all purpose drive stand for the attachment o f different work modules (courtesy Erweka, Heusenstamm, Germany).
After having determined all or at least the most important characteristics of the feed material, the preselected agglomeration method must be used to produce agglomerated products. In the laboratory, such experimental work is relatively easy and meaningful for all tumblelgrowth agglomeration methods. Small discs, drums, mixers, and fluidized bed processors are available that can simulate the growth process satisfactorily. Sometimes, small scale equipment is available in a modular design. For example, to the all-purpose drive stand, shown in Fig. 11.12, a multitude of attachments can be fitted that allow the testing of most tumblelgrowth agglomeration methods in the laboratory. Fig. 11.13 depicts the most important work modules; they are a disc or pan agglomerator (a), a coating pan (b), a bowl blender (c), a planetary bowl blender (d),which is similar to the no longer available Loepthien planetary mixer that was used for one set of data points in Fig. 7.7 (Section 7.2), a double cone blender (e), a cube mixer (4, a high shear mixer (g),and a pug milllkneader (h).The corresponding large scale equipment was discussed in Sections 7.4.1 (a),7.4.2 (c through h), and 10.1 (b). The fluidized bed technology (Section 7.4.4) can be easily scaled down. Fig. 11.14 is a picture and the dimensional layout of an MP-micro fluidized bed apparatus. It features variable process control of airflow, temperature, and liquid addition as well as interchangeable product containers and is equipped with a blowback filter system. It can be easily stripped down for cleaning and may be applied for dry mixing and liquid addition with a spray nozzle, for particle size enhancement by spray granulation/agglomeration, for spray coating, and for fluid bed drying. All processes can be carried out in sequence to obtain one-pot operation or simulate a continuous system. Another table top laboratory fluid bed is depicted in Fig. 11.15. In three schematic representations the different main uses of this equipment are also shown which are granulating, coating, and drying. Finally, a further flexible modular system is presented in Fig. 11.16 together with a table which indicates the different process modules that can be realized with this equipment. The terms granulating and pelletizing are alternative names for size enlargement processes by agglomeration. In the context of this presentation, granulating may also include powder mixing with a variable speed
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Fig. 11.13: Photograph of different tumblelgrowth agglomeration work modules that are attached to the all purpose drive stand of Fig. 11.12. (a) disc or pan agglomerator; (b) coating pan; (c) bowl blender; (d) planetary bowl blender; (e) double cone blender; (f)cube mixer; (g) high shear mixer; (h) pug mill/kneader (courtesy Erweka, Heusenstamm, Germany).
11.2 Laboratory Equipment, Jesting, and Scale-Up
Fig. 11.15: Tab. t o p laboratory fluid bed and schematic representations o f the various possible applications (courtesy AeromaticFielder, Columbia, MD, USA).
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__--
PROCESSMODULES PLANT TYPE
Fig. 11.16:
iRANULATING
MELT PELLETIZING
WET PELLETIZING
Flexible modular system for RSLD powder processing projects (courtesy Aeromatic-Fielder, Columbia, MD, USA).
VACUUM IRANULATINI
MICROWAVE DRYING
77.2 Laboratory Equipment, Testing, and Scale-Up
impeller. The parameters can be registered on optional chart recorders or data display and recording systems. Power, water, and air are simply plugged in and, after finishing a test, the whole system can be easily cleaned. Of course, if considering the mechanisms (Section 7.1) and kinetics (Section 7.2) of tumble/growth agglomeration it becomes obvious, that smaller containments and masses of tumbling particulate solids translate into lower forces acting during impact and coalescense as well as in the system as separating forces. This results in weaker bonds and, because the forces of the moving environment are small, in less destruction and, therefore, more porous structures (see also Sections 7.1 and 7.2). These conditions, in turn, may and generally will result in higher binder requirements, lower strength, quicker dispersibility, and differences in a whole host of other agglomerate characteristics if they are later compared with products from larger scale industrial operations. While no easy solution can be offered, this problem must be mentioned at this point to alert researchers and project developers to the differences that will exist between the products from small scale laboratory and large scale industrial operations (also see below). It is much more difficult to carry out small laboratory tests for most of the pressure agglomeration methods and obtain results which are meaningful and can be used for process development. The technique that lends itself best to small scale laboratory development and evaluation is low-pressure agglomeration (Section 8.4.1) which may be followed by spheronization (see Section 8.3). Since in this method of pressure agglomeration a wet mixture is passed through the openings of a screen or a thin perforated sheet, very little pressure is exerted and it is essentially a shaping process. Therefore, even if tests are performed on a small perforated die, in regard to product characteristics, the results are also representative for larger units. The same is true for the spheronizing process. As an example for laboratory size equipment, Fig. 11.17 shows the photograph and a dimensional drawing of one system which uses a low pressure axial extruder and a small spheronizer. The machines are mounted on a common base cabinet which includes the controls and the display of process parameters (i.e. power consumption and screw speed of the extruder, extrusion pressure, temperatures of the product and of cooling or heating mediums, and speed of the spheronizer). Small radial, flat screen, and basket extruders are available as well. Medium-pressure agglomeration in pellet mills can be easily simulated because even a single bore with the correct diameter to length ratio and featuring all other details of the orifice (e.g. inlet chamfer, discharge cone, relief bore, etc., see Section 8.4.2) can be used to determine the extrusion characteristics of a particular feed material and the properties of the extrudate. A laboratory set-up can be easily deviced and used for process development. A standard motor powered test stand (Fig. 11.18)can be selected and modified for this purpose. Fig. 11.19 is the photograph of another all purpose drive stand together with the five modules that can be installed for laboratory evaluations of different processes. The attachments provide planetary mixing (a); rotary granulation (b) in which a bar-cage rotor gently passes dry (compacted) or moist materials through a screen to produce a consistent particle size distribution; grating, shredding, milling, and
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f-
Fig. 11.17: Photograph and dimensional drawing of a laboratory low-pressure agglomeration and spheronizing system utilizing an axial extruder (courtesy WLS Cabler, Ettlingen, Germany).
granulating (c) of dry and moist materials by a rotating wiper blade which pushes the products through a perforated plate; moist granulating (d) in which moist feed is mechani- cally extruded through a perforated cylinder to produce uniform pellets; and mincing/extruding (e) in a simple screw extruder to form strands of compacted moist material.
7 7.2 Laboratory Equipment, Testing, and Scale-Up
Fig. 11.18 Motorized single (a) and two (b) column test stands for exerting tensile or compression forces (courtesy Chattllon, Largo, FL. USA).
Samples of punch-and-die pressing can be produced in a variety of home made or purchased small machines. The previously mentioned force/pressure test stands (Fig. 11.18),which may also use hydraulic actuation with hand or motor pumps, can be applied in connection with home made punch-and-die arrangements. Many laboratories are equipped with automatically or hand operated hydraulic laboratory presses, for example as shown in Fig. 11.20 (see also Fig. 8.92, Section 8.4.3). From the suppliers of such machines a large number of simple or sometimes highly sophisticated and automated presses are available. They are used for the determination of a variety of strength and force or pressure related product characteristics and, although the densification and compaction mechanisms are quite different from those of roller presses and can not be correlated, punch-and-die compacts are often made and evaluated to preliminarily investigate the compactibility of different feed materials or powder mixtures and to determine the type and amount of potential binders. Tabletting research and development can be carried out in single station punch-anddie presses which use different drive mechanisms. One such possibility is the application of an attachment to the drive stand that was depicted in Fig. 11.12 (Fig. 11.21).
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Fig. 11.19 All purpose drive stand with five laboratory modules that can be installed. (a) Planetary mixer; (b) rotary fine granulator; (c) grater/shredder; (d) moist granulator; (e) rnincer/extruder (courtesy Alexandetwerk, Remscheid, Germany).
77.2 Laboratory Equipment, Testing, and Scale-Up
Fig. 11.20: Photograph of an automatically operated hydraulic "Carver" laboratory press with explanations o f the components (courtesy Carver, Wabash, IN, USA).
Fig. 11.21: Tablet press attachment to the universal drive stand of Fig. 11.12 (courtesy Erweka, Heusenstamm, Germany).
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Fig. 11.22: Small single station eccentric tabletting press. (a) press with stand; (b) selection of different die assemblies; (c) data measurement, display, and storage system; (d) example of a pressing force vs. displacement (densification) diagram (courtesy Korsch, Berlin, Germany).
11.2 Laboratory Equipment, Testing, and Scale-Up
Fig. 11.23: Photograph o f a laboratory electro-hydraulic four column press in which a cold isostatic press modul has been installed (courtesy Weber, Remshalden, Germany).
Most manufacurers of tabletting machines also offer a small machine that is suitable for laboratory and development work. Fig. 11.22 is an example of such a machine in which any kind of die assemblies can be installed, including those with unusual shapes and multiple punches (Fig. 11.22b). If used for high precision tabletting and/or R&D work, instrumentation is added (Fig. 11.22~) with which data can be recorded, stored, and processed. For tabletting research, determination of the pressing force over displacement (densification) diagram (Fig. 11.22d) is of great value (see also Section 8.4.2). For evaluating isostatic pressing in the laboratory a specially designed press chamber can be inserted between the platens ofa suitable press. Fig. 11.23 shows a powerful laboratory electro-hydraulic four column press in which a cold isostatic press module has been installed. The most difficult laboratory evaluation is that of high pressure roller presses. As discussed in Section 8.4.3 the conditions in the nip between the two counter rotating, converging roller surfaces depends on so many parameters, that it is practically impossible to accurately predict press performance. To simulate the process and get an insight into the macroscopic and microscopic events that occur during densification in the nip, a roller press simulator (Fig. 11.24) was designed and extensively used. For details, an earlier book by the author should be consulted [B.12b]. One of the disadvantages of this roller press simulator was that it still needed a relatively large amount of material and movement was slow and limited. There is a special need in the pharmaceutical industry to accurately predict the compaction behavior of dry powder formulations in roller presses during the development of
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(b) Fig. 11.24 Photograph (a) and diagrammatic representation o f t h e NCB/CRE (National Coal Board/Coal Research Establishment, UK) roller press simulator; (b) shows the link and drive mechanism [ 6.12 b].
71.2 Laboratory Equipment, Jesting, and Scale-Up
Fig. 11.25: Photograph of the Polygran Micropactor and close-ups of the arrangement (left) and the nip (right) during a test (courtesy Certeis, Jona, Switzerland).
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new drugs. Since roller presses that are used in the pharmaceutical industry are typically small, testing could be carried-out in actual machines. However, because, particularly in the development stage, very little active substance is available which, in addition, is extremely expensive, even the smallest machines require too much material especially if the limiting roller speed (= capacity) is to be determined. The Polygran Micropactor (see Section 14.1, Gerteis) tries to solve this problem; it is claimed that with only a few grams of material, meaningful results are obtained. Fig. 11.25 is a photograph of the equipment and also shows close-ups of the arrangement (left) and the nip during a test (right);the front cheek plate, which seals the nip, has been removed to show the material. A narrow roller faces a straight metal strip which, during the test moves downward with a speed that is identical to the circumferential speed of the roller. This way one half of the nip produces '/* compacted strip which can be tested for strength and other characteristics (e.g. porosity) as well as granulation and tabletting behavior. During the test, gap, force, and torque are measured which can be used for designing an industrial machine. Another modular system is often called laboratory equipment; it can be used for small scale production and for the laboratory evaluation of small samples. Fig. 11.26 depicts the design and some of the accessories. In the most simple execution a hopper feeds a pair of rollers which are driven by a hand crank. The rollers can be solid and may be equipped with compacting or briquetting surfaces (see Section 8.4.3) or two perforated, geared, intermeshing pelleting rolls (see Section 8.4.2)are installed to accomplish medium pressure extrusion. In a modular fashion the rollers can be motorized, screw feeders can be added, and the rolls may be oriented vertical or horizontal or in any other direction. As shown in the photographs of Fig. 11.27 the roller frame can be totally enclosed for dust control if toxic or hazardous materials are processed. A panel includes controls and instrumentation for data display and collection. As has been demonstrated, test equipment is available in all areas of interest for the determination of feed and product characteristics, including new techniques that have been developed in response to advancements in modern Mechanical Process Technol-
Fig. 11.26 Schematic representation ofthe design and some o f the accessories of a modular laboratory roller press (courtesy Bepex/HUm, Leingarten, Cermany) ([38] in Section 13.3).
11.2 Laboratory Equipment, Testing, and Scale-Up
Fig. 11.27: Two photographs of totally enclosed laboratory roller presses (courtesy Bepex/HUm, Leingarten, Germany).
ogy and to new applications for the manufacturing of novel, for example, engineered products (see Section 5.4). However, testing is only as good and predicts industrial performance of the projected plants as correctly as test conditions reflect what will be found later in the actual installation. A common problem during the development of any new process of Mechanical Process Technology, particularly also including size enlargement by agglomeration is the requirement to investigate a representative sample of the future feed material. As mentioned before, in many cases the actual feed material which must be agglomerated in the course of a new plant flow sheet is not yet available in large quantities. Often, it is manufactured within the same or another new project, either by processing a natural resource, changing the characteristics of already available solids, or synthesizing from various raw materials. In those cases, the feed for laboratory tests is itself the results of tests or of a small scale pilot plant. The properties of such materials may change considerably when they will be produced in the industrial scale and/or in-line. The results of tests with such material are questionable, at best, which must be considered when evaluating the data and designing the plant. In other circumstances the feed material is already available from an industrial installation, either, for example, mines or chemical and mechanical processing plants. It may also be a waste stream or by-product from an unrelated manufacturing facility. In those cases, it has become necessary at a later time to modify material characteristics and improve their properties (see Section 5.4, Tab. 5.10). For testing, this material must be sampled so that it accurately represents what needs to be continuously pro-
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cessed in the planned agglomeration system. Such sampling must take into consideration potential segregation as well as normal, unavoidable fluctuations in material consistency and properties. Sampling of particulate solids is a complex problem in itself [B.24, B.271. Its coverage is beyond the scope of this book. Another, often overlooked influence on the results of testing and their applicability is that, in practically all cases, the test facility is located at avendor or research facility, often hundreds or thousands of kilometers away from the source of the material to be evaluated. Assuming correct sampling, i.e. excluding this problem, the material is bagged in a suitable way,which, today, often includes “big (bulk)bags”, so called FIBCs (flexible intermediate bulk containers). During handling and transshipment, particulate solids, in addition to potential chemical changes, may segregate, break, and/or cake and generally will change their bulk characteristics. This is particularly true if the modern FIBC is used because one of the characteristics of this packing method is its flexibility. However, the “old fashioned” packing in drums or packages may cause at least some of the same problems. To obtain the best possible feed for testing, the original bulk properties must be reinstated which, as is easily recognizable, is difficult or even impossible. Segregation and settling may be reversed by tumbling and mixing, but changes in particle size and shape, either during transshipment and handling or the breaking of lumps, and other particle modifications are irreversible. Furthermore, aging of materials is a common, but little recognized problem. This term refers in most cases to a modification of the surfaces of the particulate solids by adsorption of moisture and other atoms or molecules and/or oxidation and other chemical reactions. Sometimes, new (often whisker-like) crystal growth is also observed (see Section 5.5). The product(s) of aging have a marked effect on the results of testing, because, in agglomeration, binding mechanisms rely on chemical and physical interactions at and between surfaces of the particles to be agglomerated and, if applicable with the binder component(s). In conclusion, even if a representative sample is provided, a material which is several days, weeks, or months old and may have had to be reheated, dried, rewetted, delumped, mixed, fluffed, etc. to bring it back to conditions that are corresponding to or comparable with those found or expected in the real plant environment may yield completely different results than obtained later “in-line”.Although, as described in the previous chapters of this book, as reviewed in other publications (see Section 13.1),and as made available by vendors (see Section 14.1) in their brochures and newsletters, certain characteristic relationships have been developed for most agglomeration methods and performance factors can be collected in charts for the preselection of methods which are most likely suitable for a particular application (for example, Fig. 11.5 and 11.6, Section ll.l),determination ofthe actual design parameters remains a serious problem. This means that, as a general rule, tests must be carried out with representative samples of the specific, unaltered particulate solids which need to be processed by an agglomeration method and installation of a pilot plant on-site and/or in-line should be considered if the risks, which are always connected with new installations, are to be minimized. Furthermore, even if plants are already successfully operating in other places and “the same material” from new or existing sources or particulate solids with essentially
1 1.2 Laboratoty Equipment, Testing, and Scale-Up
identical chemical composition must be agglomerated in a new location, experience teaches that it can not be safely assumed that the new installation can use the same design and operating parameters to obtain a product with comparable quality. Minute differences in feed characteristics, such as particle shape, size distribution, surface roughness, wettability, porosity, physical contamination with nanometer dust or adsorbed layers, chemical modification with trace elements, etc. may result in significantly different process and operating conditions. A plant which is comfortably sized in a “reference location” may, at another site, handling “the same material”, be grossly underperforming if for the design and execution of the new project only data from the “reference plant” were utilized. While the pilot plant approach during project development must be investigated and decided upon on an individual basis, testing of equipment is always necessary. For that reason, nearly all manufacturers and/or vendors maintain sometimes rather elaborate facilities [ B.421 which normally include machines of different sizes, including large scale equipment, to avoid scale-up problems. These test facilities must also include peripheral equipment such as mixers, heaters or coolers, conveyors, crushers, dryers, screens, etc. although the variations that are available in these special areas can not be offered. Therefore, additional tests for the evaluation of the best peripheral equipment are often necessary at different facilities. All of these tests have the same problems as mentioned above. Furthermore, for cost reasons, even if only a limited selection of process equipment would be used, the continuous operation of an entire production line is normally not possible during testing. In those cases where in the actual plant recycle will be produced and recirculated in one way or another, product is first made from the fresh feed, the expected type(s) of recycle is (are) produced, and for further testing the anticipated amount(s) is (are) mixed with the fresh feed or other material streams, such as, for example, the crusher or screen feeds, to simulate the conditions in a continuously operating system. All data from tests and evaluations at often many different locations and with various equipment, sometimes also including information from pilot plant operations, is collected, compared with related know-how and experience, if available, and used for designing the new plant and the selection of process equipment. In spite of all the efforts that normally go into the determination of the design data, it is prudent to include safety factors in the design stage which will allow to optimize the system if and when it comes on stream. Such optimization is a normal requirement for all installations of Mechanical Process Technology. Selecting equipment for the lowest expected material flow rates and/or product qualities, which often seems necessary to meet project cost limitations, later leaves no room for modifications and often results in underperforming plants. Remediation of this situation may not be possible or becomes much more costly than a small project cost overrun would have been. A relatively new development is the emergence of tolling companies (see Section 14.1).These are installations which maintain production lines for contract manufacturing, co-manufacturing, and back-up manufacturing. Originally, most of the “tollers” were formed when the sometimes very large and extensive test and development facilities of diversified companies became profit centers as the head office’s management philosophy was changed. The growing desire for out-sourcing instead of new
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investments supported this development. More recently, tolling companies are specifically formed to accept waste materials and by-products from the industry for conversion into secondary raw materials. And, especially in the pharmaceutical industry, intermediate products are produced by specialized tollers and co-production or backup production is established to meet often seasonal peak demands. At certain times, these companies also offer their processing capabilities during project development for the testing of new materials and the evaluation of products. Although, in most cases, a major percentage of the manufacturing capabilities are dedicated to supporting a limited number of customers with particular needs and to the production of specific products, tolling companies typically also offer their services on the open market and can, for example, do the pilot plant stage in the development of a large project. Another possibility is that new materials are made for exploratory marketing purposes and/or to bridge the gap between product demand and supply during a new facilities’ start-up phase.
11.3
Peripheral Equipment
If a complete installation, which includes size enlargement by agglomeration, is considered, the entire range of equipment and technologies that is related to the unit operations and associated fields of Mechanical Process Engineering (see Chapter 1, Fig. 1.1) may be utilized in different locations. Which of the various alternatives are optimal solutions for specific tasks depends on the application and the requirements that are defined by specific needs or regulations of a particular industrial field. It would go beyond the scope of this book to discuss details, pros and cons as well as selection criteria of all of the machines and techniques of Mechanical Process Technology. For the purpose of this publication, only some typical equipment, which is closely related to the agglomeration process or constitutes a necessary part of the agglomeration system itself, will be discussed. Particular emphasis will be on characteristics and operating behavior that is or needs to be different if the equipment is applied as part of an agglomeration method. More detailed information will be provided in a future book by the author on the industrial applications of agglomeration [B.71]. Excluding agglomeration by heat (sintering), which, as discussed in Chapter 9 and subsections,has conditions and requirements of its own, the industrial agglomeration technologies can be roughly divided into tumble/growth and pressure agglomeration techniques. The various agglomeration methods work best with different feed characteristics and product parameters. Thus, agglomerators and their components must be designed with these conditions in mind. Fig. 11.28 shows generic tumble/growth (a)and pressure (b)agglomeration systems. The flow sheets are simplified but include the areas in which special considerations must be observed. During a “walk”through these block diagrams, different requirements will be pointed out.
7 7.3 Peripheral Equipment
Fig. 11.28: Simplified block diagrams of tumble/growth (a) and pressure (b) agglomeration systems.
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(a) Turnble/growth agglomeration
If more than one feed powder is agglomerated, the components must be metered and premixed. Homogenization may be necessary since there is a pronounced danger of selective agglomeration, e.g., if the sizes and distributions of the particulate solids are very different. This could involve separate or joint milling of the components. During mixing, some of the liquid or dry binders could be added. It is also possible to feed all or part of the recycle into the mixer. The premixed material should still be loose and aerated such that, after feeding it into the agglomerator, the solid particles are able to move individually and randomly, pick up more binder, and agglomerate upon impact. A metered addition of recycle to the agglomerator improves and accelerates agglomerate growth by seeding the charge. This is because recycle, in spite of its representing undersized product, consists largely of somewhat preagglomerated material. Control of the growth mechanism also requires addition of at least some of the liquid or dry binders in the agglomerator. Tumblelgrowth units discharge green agglomerates that are preferentially bonded by liquids. These units include drums, inclined pans, all kinds of mixers, and fluidized beds (see Chapter 6, Fig. 6.3). With the exception of products from inclined pans (see Section 7.4.1), the agglomerate sizes and shapes vary within wide limits. Green agglomerates are often weak and sticky, tending to blind conventional screens. Therefore, separation of over- or undersized material at this point must be frequently bypassed. In some cases, it is possible to screen the green agglomerates and feed only a narrow distribution to the post-treatment step. For this process step, special screens are required. Fig. 11.29 shows schematic representations and the photograph of a roller (conveyor) screen in operation. Originally, this type of equipment was used for the rounding of spherical “pellets” and the removal of fines in iron ore pelletizing plants prior to feeding clean agglomerates to the drying and sintering machine (see Section 9.2.2). Each of the rollers, which together form a downward slanted surface, is individually driven and, as shown in the upper left, the spherical agglomerates are rounded when moving down the slope while fines fall through the gaps between the rollers. The moist fines (recycle) are sent directly back to the agglomerator. Modern roller screens feature a horizontal deck. Fig. 11.30a depicts the principle of such a machine. Each of the rollers is still individually driven, as shown in the photograph (Fig. 11.30b), but the movement of the material is caused by intermeshing triangular lobes (discs) which, at the same time, loosen up the bed to free the fines which drop through the gaps. Since the materials are often tacky, scrapers, which in their entirety resemble combs, clean the screen openings. During post-treatment, the temporary liquid bonding is transformed into a permanent bond. In this step, liquid is removed and permanent bonding is obtained by recrystallization of dissolved substances, sintering, partial melting, or chemical reactions. The discharge from the post-treatment should be screened or rescreened because fines are still present or may have formed by abrasion and breakage, and oversized agglomerates may have developed by secondary agglomeration of the still moist and sticky green agglomerates. Oversized agglomerates must be crushed. This recycle is
7 7.3 Peripheral Equipment
Movement 01 geen pellets
Fig. 11.29: Schematic presentation of the principle (a) o f a roller screen in iron ore pelletization and photographs o f such screens (b) showing the design (left) and operation (right) [6.16, 6.421.
dry and, therefore, should be returned to the mixer. A preferred feature of this flow sheet is a surge bin in which undersized material and dust from the dedusting system are collected. This allows the metered addition of recycle, thus contributing to a better defined process performance in the agglomerator and a more uniform product. (b) Pressure agglomeration On first sight, the block diagram for pressure agglomeration looks almost identical to that of tumble/growth agglomeration, particularly when post-treatment is required. Typically, this is the case if low- and medium-pressure agglomeration methods are used. These methods require liquid binders to guarantee easy formability (see Sections 8.4.1 and 8.4.2). A typical high-pressure agglomeration system does not include posttreatment and in most applications, the addition of binders is limited to dry additives.
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width oi screen bed
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roller packager
Fig. 11.30 Principle (a, side and top views) and photograph (b) o f a modern horizontal roller screen for the separation o f sticky fines (courtesy ZEMAG, Zeitz, Germany).
7 1.3 Peripheral Equipment
Fig. 11.30
cont'd
(b)
In contrast to tumble/growth agglomeration, which requires a feed with a surface equivalent diameter of less than a few hundred micrometers (see Section 7.1),as well as excellent dispersion and aeration, pressure agglomeration tolerates a wide particle size distribution. The maximum allowable particle size increases with increasing pressure, and aeration of the feed prior to agglomeration must be avoided by all means. As mentioned repeatedly (see for example Section 8.1) and also discussed below, air in the feed must be removed from the compaction area during densification. Large particles are easily incorporated during the forming of an agglomerate under pressure. If high forces are applied, brittle disintegration and plastic deformation occur (see Fig. 8.1, Section 8.1). In any case, a considerable volume reduction takes place which is largest for high-pressure agglomeration. Densification ratios of 1: 2 and 1: 3 are common and may be as high as 1: 5. Since gases (air) in the bulk feed must be totally removed to avoid compressed air pockets, blending prior to agglomeration should not be carried out in high speed powder mixers. A good piece of equipment for this task is the batch or continuous mixmuller (Fig. 11.31). In pressure agglomeration, it is preferable to meter recycle into the mixer. However, when product quality does not need to be tightly controlled, recycle can be added to the fresh feed in the agglomerator. The ratio of fresh feed to recycle should be kept constant, however, because variations in feed composition change the agglomerate quality which, in turn, influences the recycle rate. Therefore, a surge bin for the recycle must be provided from which the fines are metered back into the system. (Note: This statement is valid for all agglomeration methods.)
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LpLIJl-7
Sampling Port
Discharge opening Drive motor
b
/
U
Inner and chute
Crib
Fig. 11.31: Sketches o f a batch (a) and continuous mix-muller (courtesy National Engineering, Aurora, IL, USA).
While surging is normal for most tumble/growth agglomeration techniques, in pressure agglomeration, particularly at high pressures, uncontrolled surging, caused by varying recirculation rates, may totally disrupt the process. The feed/recycle ratio must be changed if there is a change in the amount of recycle produced in pressure agglomeration as determined by level probes in the surge bin. This often requires changing operating parameters of the agglomerator and of downstream equipment. Agglomerates leaving pressure agglomerators first hang together then break into pieces due to their own weight. Agglomerate strength increases with higher pressures during densification and forming. Knives must be used to cut extrudates, and various separators are used to break a string of briquettes into singles (Fig. 11.32). Pressure agglomeration can also be applied to produce a granular product (Fig. 11.33). In such use, the separator, if required, is often a prebreaker. Granular product is obtained between the two decks of double-deck screens. Oversized material is crushed in a suitable mill and rescreened; undersize is recirculated. Multistep crushing and screening operations are used to improve the yield and obtain a cleaner granular product (see also Section 8.3).
I 1.3 Peripheral Equipment
roller presses [for details refer'to 8.421.
(d)
(el
Yield can be also increased by installing mills using a gentler crushing mechanism. However, because compacted materials are not uniformly dense and strong, the product may be softer and produce more fines during storage and handling. Granular materials which must be shipped in bulk, such as fertilizers, should be produced with mills using high energy input, thus giving a somewhat lower yield but high product strength. Nevertheless, as discussed in Section 8.3, the particles should be stressed only once or multiple times individually during unrestricted movement,
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Fig. 11.33: Flow sheet o f an optimized compaction/granulation system for the manufacturing ofgranular product (for example mixed [NPK] fertilizer) ([147] Section 13.3).
i.e. without being retained in the crushing chamber by, for example, exit screens. The discharge from the size reduction equipment is then screened and the oversize is recrushed to produce more product while minimizing fines production (see Fig. 11.33). An excellent crusher design with almost unlimited adjustability of the crushing process is the cage mill (Fig. 11.34).As shown in the artist's conception (a) and the representation of the operating principle (b),in this design two independently driven bar-cages, each consisting of two cylindrical rows or bars which interlink with the opposite ones, operate in a common housing. The material to be crushed enters through a chute in the center and, driven by centrifugal force, travels through the bar cages to the periphery and drops by gravity out of the housing which is open below. There are literally unlimited possibilities for varying the degree of stressing by selecting co- or counter-rotation and varying the absolute and relative speeds of the cages. The number of cages can be also changed with different models and only one single or multiple cage may intermesh with stationary opposing bars.
7 7.3 Peripheral Equipment
Fig. 11.34 Artist's conception (a) and principle (b) o f a cage mill (courtesy Cundlach, Belleville, IL. USA) .
The irregular particle shape resulting from crushed agglomerates may be rounded in post-treatment steps. Since screening is always a part of the process, the particular material movement on the extremely flat screen decks of gyratory screens (Fig. 11.35) is sometimes providing some such rounding. In general, screens of this design are often preferred for agglomeration and, particularly, compaction/granulation plants because residence time on the deck is relatively long and movement is gentle without the impacts between solid particles and the screen that are typical for vibrating or mechanically excited machines and may cause fines through secondary breakage or attrition. On the other hand, loosely bonded particles and corners or edges on the irregularly shaped granules from crushed compacts, are abraded during the rolling action on the screen. As discussed in several parts of this book, agglomerates, particularly immediately after their production, are weaker than solid particles with comparable size. Therefore, transportation equipment should be selected to handle agglomerates gently. Normally
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Fig. 11.35: Schematic presentation ofthe particle path (a) on a flat, gyratory screening machine and photograph (b) o f such equipment (courtesy Rotex, Cincinnati, OH, USA) In both pictures, feeding is on the right and discharge on the left.
mechanical belt conveyors are used which, to avoid sliding and tumbling if the direction of transport is up or down, are preferably pocketed (Fig. 11.36). Smaller agglomerates can be transported pneumatically if alternatives to the high velocity, low density system, which causes destruction by impact and results in fines and/or build-up, are selected (Fig. 11.37). In Section 11.2 it was pointed out that, with any new agglomeration system, after installation and start-up it is normally necessary to adapt its operation to the in-line and site conditions and to perform optimization. Unless batch or small, for example pharmaceutical, installations are considered, the often large continuous plants feature considerable internal mass flows (see, for example, Fig. 6.3, Chapter G, and Fig. 11.33, above) in closed or recirculation loops. Since in installations that handle and process particulate solids, dusting and particulate contamination is a common and objectionable problem, the equipment of more recently designed plants is enclosed and aspiration points are connected to dust collection systems. As a result, it is surprising that personnel, operating systems for quite some time, has no idea what happens inside the plant and how mass flows are influenced when processing parameters are changed. This is particularly true for the closed loop screening and crushing cycles of granulation plants where the final product size and distribution are adjusted. Optimization requires a good knowledge of the mass flow rates at critical points of the plant and of the characteristics of the particulate solids at these points. Until relatively recently, flow rates of solids had to be determined by opening a system, bypassing and collecting the entire amount of material for a measured time, and weighing it. Today, many different mass flow meters for solids are available on the market. Fig. 11.38 shows the principle (a) and two executions (b) of such an instrument. All measuring principles are based on determining the force that results from the mass of particulate solids impacting onto or flowing over a plate which momentarily supports a fraction of the material. Installation of such solids flow meters at as many points as
7 7.3 Peripheral Equipment
Fig. 11.36 Photograph o f an open, pocketed mechanical conveyor (a), detail o f the pocketed belt (b). and some typical configurations (c) (courtesy Unitrac, Port Hope, Ont., Canada).
possible indicates the flow patterns in the system and allows to influence system performance by changing process parameters and evaluating their effect. In addition, sampling equipment or, at least, access points must be provided to draw material samples for evaluating feed, agglomerate, intermediate, and product characteristics [8.24, B.271. Although, the above represents only a very limited discussion of the peripheral equipment in agglomeration plants, these comments shall suffice for this publica-
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Fig. 11.37: Schematic representations of the operation of different pneumatic conveyors. (a), (b), and (c) show high-velocity conveying systems. (d), (e), and (f) depict low-velocity systems for gentle conveying (courtesy Biihler, Uzwil, Switzerland).
11.3 feripherd Equipment
Fig. 11.38: Principle (a) and two alternative configurations (b) o f a solids mass flow meter (courtesy El, Wilmington, NC, USA).
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tion. Special equipment considerations and the selection of the best suited machinery for a particular flow sheet depend largely on the application and the characteristics of feed and product as well as on the agglomeration method. Coverage of the applications of agglomeration in industry is planned in an additional book [B.71]. A more detailed evaluation of the peripheral equipment for various applications will be presented there.
Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
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Outlook Although agglomeration is a natural phenomenon (see Chapters 2 and 3) and was used by animals and humans for a number of purposes for millions and, respectively, thousands of years (see Chapter 3 ) , it has been recognized as a technology only at the beginning of industrialization, approximately two hundred years ago, and has been defined as a unit operation of Mechanical Process Technology and a field of science in its own right during the 20th century. The understanding of the fundamentals of agglomeration (see Chapter 5 and subchapters) has quickly led to a large number of new and improved processes, some of which, such as the briquetting of coal, the pelletizing of iron ore, the pelleting of animal feed, the development of a large number of solid dosage forms in the pharmaceutical industry, the shaping of new food products, the granulation of fertilizers, agrochemicals, and, generally, of many chemicals for the most diverse uses, the compaction of wastes for recycling, the production of ceramic and metallic materials for high strength and high temperature applications, and many more, have been produced in large bulk quantities and have helped revolutionize particulate solids technologies. More recently, requirements for the vast novel field of life sciences and for many other modern applications have started a new trend in the manufacturing and/or manipulation of solids. As already discussed in Section 5.4, particulate solids processing is shifting away from just attaining a particular shape, size, and distribution of products and moves toward the realization of improved composition, microstructure, morphology, and characteristics. In other words, the emphasis of new products and processes will be on better control of primary particle physical properties, highly specific product size and composition, as well as the creation of desirable properties. Many products have high value, feature special effects, and are being produced in small quantities. There is a shift away from the production of simple bulk commodities with average, widely useable, but not always optimized quality to specifically engineered materials that respond directly to the particular needs of the end user. A few examples of such processes which use the fundamentals of agglomeration and/or conventional or modified agglomeration processes will be discussed to illustrate what future developments may entail.
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Granulation of Fertilizers
For many readers, fertilizer granulation may be a rather unlikely topic responding to the above description of new trends. Shortly after postulating the need for the feeding phosphorus (P), of commercially grown plants with the basic nutrients - nitrogen (N), and potassium (K) - by Justus von Liebig during the middle of the 19th century, large scale production of fertilizers and their application by farmers everywhere began. While up to this time, empirically developed fertilization with organic wastes was practiced in agriculture, mineral sources of nutrients were defined, mined, and processed in large amounts, and shipped in bulk to the growers. Since availability of the elements depends to a large extend on their solubility, fertilizers must be soluble and, therefore, if transported or stored for longer periods of time they tend to develop unwanted agglomeration (see also Section 5.5) resulting in lumping, pile set, or, more generally, an uncontrolled size enlargement which hinders or prohibits uniform distribution on the fields. To overcome this problem and to be able to serve the quickly increasing needs of growers around the world, during the first half of the 20th century granulation of fertilizers was introduced. As described in Section 5.4, Tab. 5.10, size enlargement by agglomeration produces freely flowing products with improved handling and storage characteristics and low content of dust, with defined size and shape, featuring no segregation of different components, if applicable, and often with increased bulk density. At the beginning, granulation was accomplished by tumblelgrowth agglomeration methods, mostly in drums (see Section 7.4.1) with water and post-treatment, strengthening the particles by recrystallization during drying (see Section 7.3). Later, particularly in the potash industry, dry compaction/granulation was used (see Sections 8.3 and 8.4.3). In both cases bulk volumes were produced in facilities with increasingly large capacities. As agrochemical research and the understanding of the needs of growing plants in various environments expanded, it became clear that plant species in different soils and climates have very distinct requirements. Not only does a particular planting perform best if it is fed with a very defined NPK relationship but a large number of so-called trace elements, such as copper, iron, or sulfur, to name a few, are required for optimum results. In addition, the fertilizer granules may be coated with insectizide and/or fungizide to protect the new plant growth or availability may be varied by coatings which provide for delayed or slowed dissolution. Fertilizers will also feature distinctly different composition if they are applied during various growth stages. To avoid overfertilization or the application of only partially suitable fertilizers and achieve all the other desirable features, an agrochemical evaluation of the soil is combined with climatological data and the needs of the particular plant species. Then, a special multicomponent fertilizer is defined, mixed from a multitude of ingredients, granulated, and potentially subjected to specific post-treatment methods, for example coating, to arrive at the best possible plant food. For this modern method of fertilization, which is particularly desirable for tropical, subtropical, and other high performance farming areas, even if the farming area is relatively large, due to varying soil qualities which necessitate adjustment in fertilizer quality, only small amounts
7 2 Outiook
of the fertilizer are desired. This material must be produced on demand and by economical means. The classic method of fertilizer agglomeration using tumble/growth methods is not suitable for this complex task of particulate solids processing. To produce designer plant foods as described above, which are often no longer called fertilizers but are identified as agrochemicals, compaction/granulation is being applied (for a typical flow sheet refer to Section 11.3, Fig. 11.33 and for representative references see Section 13.3 [29, 35, 40, 41, 95, 96, 101, 104, 106-108, 110, 112, and, particularly, 1441).
The most important advantage of fertilizer granulation by compaction is its versatility as demonstrated by the following list: 1.
2.
3. 4.
5.
6.
7.
8.
With the exception of a few materials (such as urea or TSP (triple superphosphate) for which maximum amounts exist that can be used in a formulation) literally all solid particulate plant nutrients can be processed. This includes, for example, dry digested sludge from municipal waste treatment plants and also the addition of small amounts (typically <10 %) of liquid additives. To minimize the cost of the product, raw materials can be purchased on the world markets without specific requirements on particle size, shape and mass. Fines which are off-specification for, for example, fertilizer bulk blending can be used and are often even preferred. Compaction/granulation plants can be designed for economic operation at any feed rate. Production capacities per line are feasible between 0.1 and 50 t/h. Larger plants are preferably equipped with two or more lines fed by only one large compounding (= batching or formulation) system. The rest of the plant is designed with separate lines to improve availability; only one line is shut down during maintenance and emergency shut-downs. If a plant is equipped with multiple lines and features separate day bins for fresh material, recirculating fines, and granulated product, each line can process different formulations. Economical production of small batches is feasible. Depending on the extent of cleaning that is necessary during change-over (determined by how much crosscontamination can be tolerated), up to three different formulations (batches) can be produced in an 8 hour shift. Fertilizer granulation plants using compaction can be combined with either custom designed batching systems or standardized formulation or bulk blending units. Particularly the latter allows easy expansion of bulk blending to include mixed fertilizer granulation. Any fertilizer compactionjgranulation system can be also utilized as a regional production facility for the manufacturing of bulk blending grade material from off-specification fines. This capability also includes special formulations which may be required by the local market such as indigenous fillers, with or without major nutrients or carriers incorporating micronutrients. Such products can then be used together with imported bulk blending grade materials in bulk blending.
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9. To demonstrate the ultimate in flexibility it should be mentioned that plants using roller presses for compaction can be easily modified to also produce unusual fertilizer materials. For example, urea supergranules for deep placement in wetland rice production may be manufactured by changing the rollers to a pocketed (= briquetting) configuration, bypassing the flake breaker (9 in Fig. 11.33) and obtaining the product as oversize discharge from the scalping screen (10 in Figure 11.33) which is also used for separating the briquettes. The previously mentioned coating of the granulated fertilizer to achieve the other product characteristics may be accomplished with almost any of the methods described in Section 10.1. Production o f Agglomerated Materials with Instant Characteristics
As briefly discussed in Section 5.4, the term “instant” is normally used in the food industry, for drink powders, soups, sauces, and the like, as well as in related fields, for example for pharmaceuticals and animal feeds, and describes the easy solubility of these products. Instant agglomerates are also desirable for pigments and other finely divided chemicals which are ultimately applied with and in a diluent. Many of these materials are insoluble and, therefore, only require complete dispersion. All instant products must be able to quickly disperse and, if applicable, dissolve in a specific liquid at any temperature, particularly also at ambient or even cold conditions, without residue and sediment. Instant characteristics of an agglomerate are defined by the three (dispersion) or four (dissolution) mechanisms (see Section 13.3 [147, 149-151, 153, 1581) listed in Tab. 12.1. All three or four phases of dispersion or dissolution are proceeding individually whereby some overlapping may occur, depending on the amount of material involved. Instant properties are a function of time. Each industry has a more or less well defined procedure to determine the maximum allowable time. Typically, complete dispersion or dissolution should be accomplished within a few seconds in warm liquid and in approx. 30-GO s in cold liquid. Particularly in the food and pharmaceutical industries, instant agglomerates may contain certain substances that assist in the break-up during the dispersion phase (see Section 5.1.2, Fig. 5.13). In the pigment and chemical industries dispersion is often assisted by some sort of agitation [ B S l ] . It is interesting to note, that, for many of these modern and very likely for most of the future engineered products, quality control procedures must be newly established. The characteristics which control their performance have previously been unknown, were Dispersion and, respectively, dissolution mechanisms of “instant” agglomerates.
Tab. 12.1:
1. 2.
3. (4.
Penetration of liquids into the pores of the agglomerates (also called wetting) Submergence of the agglomerates in the liquid (also called sinking behavior) Break-up of the agglomerates into the primary particles (also called dispersibility) If the solid is soluble, dissolution of the primary particles (also called solubility))
72 out'ook
unimportant, and/or could not yet be determined because of scientific or technical limitations at that time. For example, to evaluate the instant properties of agglomerates their size, which influences the wetting or suction (= take-up of liquid) behavior, and the dispersion characteristics must be measured. Fig. 12.1 depicts a technique for the measurement of the dynamic wetting behavior. The force measured by the weigh cell is proportional to the liquid volume that has entered the particle bed by suction and wetting. With the method shown in Fig. 12.2 the speed of penetration of a liquid into a particle mass can be determined. After forming a layer of material with thickness h on the bottom diaphragm and adding a plexiglass cylinder to avoid wetting from above, the test cell is submerged in the liquid. The time is measured until the entire bed is wetted and liquid appears on the surface. If for each measurement differently fractionated (= sized) agglomerates are used to build the bed, the optimal granule size for quick wetting and liquid penetration can be determined. The results of this and other tests are influenced by various potential particle behaviors as shown in Fig. 12.3. Fig. 12.4 represents a method by which both wetting and dispersion behavior can be determined. The apparatus consists of a 250 mL, temperature controlled beaker with flip top in which wetting and dispersion occur and a photometric device with a flowthrough cell (= cuvette) for the measurement of the degree of dispersion. Fig. 12.5, in which transmission is plotted vs. time is a typical result of this test. a
Weighing cell
3 Fig. 12.1: Measurement o f the dynamic wetting behavior or particulate materials [B.20]. Left: ready position, right: measuring position.
Plexiglass
Powder Fig. 12.2: Determination of the speed of penetration of a liquid through a powder bed [12.1, 12.21.
Screen
I
511
512
I
12 Outlook
-&b --J.I
___ J
__
c
0'''
1.0
I
I,
-c
0.8
-
-Stirrer o n
I
I
1I
Fig. 12.4 Apparatus for measuring the wettability and dispersibility o f "instant products" [B.42]. (1) Instant product, ( 2 ) cuvette, (3) amplifier with lens, (4) thermostat, (5) magnetic stirrer, (6) beaker, (7) light source, (8) light conductor, (9) photodiode; / / l o = Transmission, H = 40 mm.
+ I
Ultrasound
\ LI
L1
5 0.6-
._
.-ul v)
5 0.1 -
I
e
I
I-
I
0.2 -We!
I
ting
-+
Dispersion I
OO
0.5
1 Time ( m i n l
I I
5
10
I
Fig. 12.5: Transmission vs. time of an agglomerated instant 1 powder evaluated using the apparatus of Fig. 12.4 [B.42].
First, the sample of material to be investigated is uniformly deposited onto the surface of the liquid by the flip top. During the entire test, a small amount ofliquid, which does not change the stationary behavior of the liquid mass in the beaker, is transported by a pulse-free peristaltic pump through the cuvette. At the beginning, the magnetic stirrer is not operated. If, after wetting the sample, particles sink in the liquid and enter the suction port of the pump, they are transferred to the photometric test cell where a reduction of light intensity is observed. The measurement determines the transmission, i.e. the actual light intensity I referred to the light intensity I, ofthe clear liquid, as
I
' 2 Outlook 513
a function of time. Materials that wet more easily show a decrease in transmission earlier than less readily wettable solids. Later, the magnetic stirrer is operated in a defined manner and additional dispersion is obtained. The graph plotting transmission vs. time (Fig. 12.5) provides a measure of dispersion velocity. A plateau of the transmission curve indicates the best (stationary) dispersion. By applying ultrasound, the degree of dispersion can be further improved in most cases. The test results may be used to determine several characteristic dispersion conditions of a particulate solid. The results define, for example, the speed of dispersion, dispersion without and with different levels of stirring, and final dispersion after the application of ultrasound. The level of ultrasound that is necessary to disperse aggregated particles in a liquid can also be used as a measure of agglomerate strength of, for example, granulated silica fume in a somewhat modified test arrangement [12.3]. This discussion shall suffice at this point to demonstrate the need for the development of novel technologies and methods for new generation, agglomerated, engineered materials. While the techniques that were discussed above use relatively simple fundamentals and equipment, many others are based on highly sophisticated electronic and physical methodology. Products with instant characteristics, for example from powdered food materials, can be obtained from a variety of different, rather conventional processes (Fig. 12.6); most of these use agglomeration techniques (A in Fig. 12.6). Because granule size should be small and porosity must be high, instant food products are most commonly manufactured by rewet agglomeration in mechanically agitated beds (see Section 7.4.2) or fluidized beds in which turbulent movement is induced by flowing gas (see Section 7.4.4). Spray drying (see Section 7.4.3) combined with mixer agglomeration (see Section 7.4.2) or fluidized bed agglomeration (see Section 7.4.4) are also often used. A more recent development in instantizing utilizes compaction/granulation. One of the most important binding mechanisms of high-pressure agglomeration (see Section 8.1) is caused by van-der-Waals forces. This short range molecular attraction does not develop solid bridges between the agglomerate forming particles. Since in wet environments van-der-Waals forces are lower by a factor of approx. 10, agglomerates thus bonded disperse easily in liquids. This knowledge led to the adaptation of compaction/
A: Aaalomeration Techniaues A.1
A.2 A.3 A.4
6 : Techniques utilizing other processes
Rewetting of Powders in Fluid B1 Beds A. 1 .amechanically induced 8.2 A. 1 .b: gas induced Spray Drying and Agglomeration 8.3 Combinationsof (A.1) and (A 2) Press Agglomeration A.4.a: CompactionlGranulation A.4.b: ExtrusionlCrumbling
Improvement of Wetting with Additives (Surfactants) Improvement of Wetting by Extraction (e.g. of Fat) Improvement of Solubility (e.g. amorphous structure)
Fig. 12.6 Principles that are most commonly used to manufacture instant products from powdered food materials 112.21.
514
I
72 Outlook
granulation for the manufacturing of instant materials. Originally, this method was considered an unlikely candidate technology for the production of easily dispersible agglomerates because the product particles are dense and feature low porosity. However, it turns out that the higher density of the material, which translates into larger mass of each product particle, coupled with the fact that agglomeration occurs in the dry state, make this technique uniquely applicable for, for example, the granulation of detergents. The heavier products can be fitted into smaller packages and the smaller dispensing cup suggests a more economical use as well as less environmental damage. Since submergence (= sinking behavior, Tab. 12.1) is no problem with these heavy particles, quick wetting must be guaranteed by, for example, the addition of surfactants (see Section 5.1.2) prior or after compaction/granulation. Although, as mentioned above, the binding forces are lower in a liquid environment, additional dispersion may be realized by employing aids such as using amorphous components or adding materials such as micro crystalline cellulose (MCC, see Section 5.1.2), or by producing effervescent granules. In some cases fluidized bed agglomeration is not possible, because high enough strength can not be produced or the resulting agglomerates are too loose and friable, and compaction/granulation is also not feasible, because material components are pressure sensitive or become too dense. In such instances, low pressure extrusion (see Section 8.4.1) followed by drying, cooling, “crumbling”, and screening can be used. Undersized fines are recirculated. The resulting instant products are somewhat denser and less friable than those obtained from fluidized beds but are typically easier dispersible without employing disintegration aids and are lighter than compacted particles. Coating, Microencapsulation, Mechanofusion, and Hybridization
These technologies were already covered in some detail in Section 10.1. All are used to modify the surface characteristics of pieces, particles, granules, or agglomerates of any kind and shape. Coating is the covering of relatively large pieces with macroscopic layers of solids originating from either powders, suspensions, solutions, melts, or vapors. In plasma vapor deposition (PVD) solids pass directly into the vapor phase and deposit by sublimation as nano particles onto surfaces where they sinter together to form layers. Because these ultrafine particles feature no dislocations they are extremely hard, a characteristic which is retained in the coating. Hard coatings have significantly improved the performance of tools and other parts that are exposed to friction and wear. New developments are directed at producing protective coatings with a combination of advantageous material characteristics, such as enhanced hardness or wear resistance, while other properties, e.g. friction and high temperature behavior, are at least kept at their established levels. Such demands can be met by new combinations of materials in a single layer where phases with the desired characteristics are combined (multiphase coatings) or by depositing multiple layers which, in combination, produce the desired effect [12.4, 12.51. Traditionally, many powder color coatings are applied as suspensions in organic liquids. Anything from small parts, such as toys, to large items, such as automobile
I
7 2 Outlook 515
bodies, were painted in this way. Recently, clean air legislation in many countries has required to sharply reduce the emission of VOCs (volatile organic compounds). As a result, to avoid using solvent based paint systems, most manufacturers have turned to the application of dry powders [12.6]. Todays powder coatings come in many colors, textures, and finishes, but only in two basic types - thermosetting and thermoplastic. Thermoset powders consist of lower molecular weight resins, such as polyesters and acrylics, that chemically crosslink (cure) to form a coating which, in its chemical stmcture, is quite different from the initial powder. Once cured, a thermoset coating will not resoften. In contrast, thermoplastic powders, which do not crosslink, can remelt. Until recently, thermoset powder coatings have been used primarily for metals. The challenge is to lower the temperature needed to fuse powders into a smooth, continuous layer which then crosslinks so that powder coatings can be used on woods and plastics. Alternative curing methods using, for example, ultraviolet (UV) radiation are being developed also. Encapsulation and, particularly, microencapsulation produces a thin coating typically on smaller particles, granules, and agglomerates but is also used to enclose powders, liquids, and even gases. The particular characteristic of these capsules is normally that they feature properties which are enhancing the application of the encapsulated material. For example in pharmacy, microencapsulation often creates a drug delivery system. By being soluble only in specific bodily liquids and/or under specific ambient conditions, drugs can be transported to the location of need where they will be released. Other delivery systems use the slow release performance of a semipermeable capsule. In carbonless copying papers microcapsules which contain the dry or liquid ink are uniformly distributed in the paper structure which consists mostly of fibers (see Section 10.3). The individual capsules must be small and arranged individually but so closely together that by applying pressure with the type of a typewriter or the tip of a writing tool individual capsules will burst and release the ink. To obtain a clear copy, the capsules must have a diameter of between approx. 0.5 to 10 pm. Quality of the paper is determined by the uniformity of size and distribution of the microcapsules as well as their strength. The capsules must be so strong that the paper does not smudge by mere contact during storage and handling but must reliably burst when contacted by a type or the tip of a pen. The challenge is to produce the microcapsules according to this specification with a very narrow bursting pressure which is in the range of 200 pN and, for quality control, to be able to measure the bursting force [12.7]. Another exciting new development is the manufacturing of hollow capsules in the submicrometer (= nano) to micrometer size range [12.8]. Such hollow capsules constitute an important class of new particulate materials that are employed in very diverse technological applications. Uses include the encapsulation and controlled release of different substances (e.g. drugs, cosmetics, dyes, inks, etc.), application in catalysis and in acoustic insulation as lightweight composite materials as well as the development of piezoelectric transducers, of materials with low dielectric constant as fillers in electronic components, and of the manufacturing of “advanced materials”. Today, hollow capsules comprising composites of polymers, glasses, metals, and ceramics are routinely produced by using various chemical and physicochemical methods.
516
I '*
Outlook
A novel, versatile technique for the synthesizing of uniform hollow capsules from a broad range of materials is based on a combination of colloidal templating and selfassembly processes [ 12.81. Fig. 12.7 describes schematically the concept. Colloidal templates of different composition, size, and geometry (although spheroidal shape is preferred) can be employed. Materials range from spherical polymer particles to nonspherical biocolloids, all with diameters in the nano- to micrometer range. The first step, (1) in Fig. 12.7, involves the deposition of a charged polymer layer (+) onto the colloidal particles. In a next step, oppositely charged (-) polymer, (2) in Fig. 12.7, or nanoparticles, ( 3 ) in Fig. 12.7, resulting in another layer, are added. Additional layers can be produced as shown in Fig. 12.7 by repeated deposition which
Fig. 12.7: Schematic diagram describing hollow capsule production by exploiting colloidal ternplating and self-assembly methods [12.8]. Explanation see text.
72 out'ook
makes use of the surface charge reversal occurring every time a layer is adsorbed. Colloidal core/multilayer-shell particles are manufactured. After the desired thickness of the layer is obtained, excess unadsorbed polyelectrolyte or nano particles are removed by repeated centrifuging or filtering and wash cycles. Finally, hollow capsules are produced by the removal of the core from the composite colloids. This is achieved either by chemical or thermal means. If a solvent is used, only the core is dissolved which results in hollow polymer, (4)in Fig. 12.7, or composite, (6) in Fig. 12.7, capsules. Heat treatment (= calcination) of the coated particles, (5) in Fig. 12.7, removes both the colloidal core and the bridging polymer, thereby producing hollow inorganic spheres. By combining colloidal templating with self-assembly, the manufacturing of a broad range of coated colloids and, ultimately, of hollow capsules in the nano- and micrometer range is possible, featuring various and defined composition. Capsule geometry, size, and wall thickness can be controlled with nanometer precision by the use of colloids with given shape and dimensions and by varying the number of coating cycles.
Core powder: Nylon 12 Fine Powder: Ti 0 2
Fine powder (50-0.01 u m)
P
sr'
I
Embedding process (Inorganic surfaced microcapsule)
i
/ Core powder
(500-0.1urn)
Q\
-+o/ ---O-' Ordered Mixture
a
Filming process (Microcapsule)
Core Powder: Styrene resin Fine powder: PMMA
Fig. 12.8: Diagram demonstrating the process of hybridization and showing photomicrographs of intermediate as well as final particles (courtesy Nara, Tokyo, Japan). Explanation see text.
%
T~ advanced technology (multi-layer)
I
517
518
I
7 2 Outlook
Fig. 12.9 Flow sheet and photograph o f a hybridization system and dimensioned outline o f the NHS-1 (see Tab. 12.2) laboratory system (courtesy Nara, Tokyo, Japan).
Uniformity in the size of the hollow capsules is defined by the monodispersity of the colloidal templates. Mechanofusion and hybridization modify the surface structure and the characteristics of fine particles by embedding nano-sized particles into or coating such particles onto the core substrates. Both technologies were described earlier in this book (see Section 10.1) and mechanofusion was already covered in an earlier book by the author [B.42]. Fig. 12.8 demonstrates again how hybridization works and Fig. 12.9 is a schematic flow sheet. Core particles with a size between 0.1 and 500 k m are mixed with nano particles that, depending on the size of the cores, may feature sizes between 0.01 and
12 Outlook I 5 1 9
800XS50
I
’
-
t-----.-j Fig. 12.9
cont’d
Tab. 12.2 Power requirements and approximate production capacities of standard Hybridization systems (according to Nara, Tokyo, Japan).
Model
NHS-1 (Laboratory) NHS-2 NHS-3 NHS-4 NHS-5
Power Requirement
Production Capacity
Ikwl
Ikglhl
3.7-5.5 7.5-11 15-22 30-45 55-90
approx. approx. approx. approx.
3.5 6.0 15.0 35.0 approx. 50.0
520
I
12 Outlook
50 pm, (1)in Fig. 12.9. After mixing a coating has developed on the cores (see left photomicrographs in Fig. 12.8). These prepared particles are metered, (2) in Fig. 12.9, into the hybridizer, (3) in Fig. 12.9. During a short time (1-5 min), the hybridizer introduces so much mechanical and thermal energy into the product, that the fine particles are imbedded in or are permanently bonded to the surface of the core material (see right photomicrograph in Fig. 12.8). The whole process is controlled from a panel, (5) in Fig. 12.9, and the finished product is transferred to a product collection container (4)in Fig. 12.9. As shown in the center right of Fig. 12.8, multiple surface layers can be produced. If the core material of such products is removed with solvents or during calcination, hollow capsules as described above may be produced. As shown in Tab. 12.2, the production capacity ofhybridization systems is measured in kg/h and is quite small. However, since many of these particles with modified characteristics of their surfaces are used for very specialized applications, where numbers rather than mass count, at this point in time, larger production units are not necessary. Deposition and Bonding of Individual Particles on Surfaces
A very new group of methods for the deposition and bonding of particulate solids onto surfaces assembles nano- to micrometer-sized particles in a predetermined and orderly fashion onto a substrate [12.9]. Such techniques produce an organized structure, made up of particles, on solid surfaces which can bring about many interesting properties; for example, it is possible to create microdevices and microstructures with multiple functions.
Fig. 12.10: Overview schematically describing the manipulation techniques for small solid particles (12.91.
'* Outlook I So far, only a few methods can produce such materials. Fig. 12.10 is an overview of the two groups of techniques that are available for the manipulation of nano- to micrometer particles. With the methods of one group, single particles are deposited on the substrate, one by one. The scanning probe microscope, laser, and microneedles are used for particle manipulation (upper part of Fig. 12.10). Particles can be deposited at specific positions with high accuracy but only at a very low rate. In contrast, the techniques of the other group can deposit a great number of particles by using particle jets (lower part of Fig. 12.10).The disadvantage is a low accuracy for the positioning of each particle. A new method aims to overcome these limitations. Fig. 12.11 shows a concept that can be used to assist the deposition of fine particles by an electron beam drawing [12.9]. The technique, electrification assisted controlled particle deposition, is based on the fundamentals of electrophotography. As shown in Fig. 12.11, at first, the electron beam produces an electrified pattern on the substrate surface. Next, positively electrified particles are attracted by electrostatic forces to the electrified pattern and adhere there. By repeating the electron beam drawing and the adhesion steps, using different types of particles, composite deposits can be created. Fig. 12.12 shows the steps that are required for the electrically assisted deposition of small particles. The oppositely electrically charged particles are made available in a suspension into which the substrate, carrying the electron beam drawing, is dipped. After remaining in the suspension for a predetermined time, the substrate
Electrification by. electron beam drawing
Powder particle arran ement on electrifed patterns
Id!
Electron beam drawing
t
I *A** *.*+ 7': +
Powder particle arrangement
Fig. 12.11: Concept o f a process for the assembly of small powder particles on a substrate that is assisted by electron beam drawing (12.91.
/
\
3-D structure
material
521
522
I
12 Outlook
Drawing an electrified pattern on a substrate
Dipping the substrate into a suspension
Rinsing the substrate and then drying it
Fig. 12.12: Schematic representation o f the steps that are required for the electrically assisted deposition of small particles by electron beam drawing [12.9].
is removed, rinsed, and dried. Stronger bonds between particles and substrate can be achieved with a suitable post-treatment (for example sintering). The micrographs in Fig. 12.13 show how silica spheres with a diameter of approx. 5 pm were arranged on two negatively electrified lines (substrate is CaTiO,). The upper photograph (a) depicts the two lines which are 1,600 pm long and 800 pm apart and are composed of silica spheres. The lower photograph (b)is an enlargement of one of the lines in Fig. 12.13a and shows how the silica spheres are arrayed along the electrified line on the substrate.
(b)
Fig. 12.13: SEM photographs of silica spheres arranged along negatively electrified lines [12.9].
I
7 2 Outlook 523
Many persons, scientists, and experts in Mechanical Process Technology, may not agree that some of the examples, particularly the latter ones, are new applications of agglomeration. However, the adhesion, bonding, and final structure are all controlled by the fundamentals of agglomeration, especially the binding mechanisms. Therefore, ideas for new products consisting of more or less defined particle assemblies of various size, structure, and properties can be derived from an in depth knowledge of agglomeration mechanisms.
Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
I
13
Bibliography Contrary to the format of the author’s earlier book [B.42], in which numerous individual bibliographical references were listed, many of which referred to the fundamentals and the scientific treatment of the unit operation “Size Enlargement by Agglomeration”, the present work is trying to offer a complete, up-to-date compilation of the various agglomeration techniques and their applications. To that end, in addition to introducing the properties of agglomerates and the specific characteristics of the different technologies, descriptions of equipment and their special features for particular uses are the main topic of the book. Emphasis is on industrial applications, not theory. The explanations of details of equipment, processes, systems, plants, and applications as well as the descriptions of products and of their uses are largely based on information from vendors, the experience of the author as well as input from many of his colleagues that are active in this field. Therefore, it was decided that it is not necessary to collect the numerous individual publications that, in one way or another, report on technical and practical developments and review specific industrial features, applications, and products. Rather, with the exception of a few annotations (Section 13.2), reference is made to books or major chapters dealing with all facets of agglomeration and related subjects (Section 13.1) and to the vendors (Section 14.1) who, either by direct communication or through their technical sales literature and/or brochures, supplied the information that has been processed by the author to yield an unbiased presentation. Size enlargement by agglomeration is a unit operation of Mechanical Process Technology, the science which is concerned with all activities that are related to the processing and handling of particulate solids (see also Chapter 1).As has been repeatedly shown in the book, all unit operations of Mechanical Process Technology as well as the peripheral techniques (see Chapter 1, Fig. 1)are being used, sometimes several times, in the design and execution of agglomeration systems and plants. Therefore, in addition to what has been presented in Section 13.1 it should be pointed out, that some of the books in which major chapters deal with Agglomeration are also valuable sources of information on other topics of Mechanical Process Technology. Specifically, those references are (in numerical order): Winnacker-Kuchler “Chemical Technology” [B.11], “Handbook of Powder Technology” [B.17],“Handbook of Powder Science and Technology” [B.21, B.561, “Series on Bulk Materials Handling” [B.24, B.291, “Developments in Mineral Processing” [B.31], “Ullmann’s Encyclopedia of Industrial Chemistry”
525
526
I
73 Bibliography
[B.32], “Fortschr. Ber. VDI” (Reihe 3, Verfahrenstechnik) [B.33, B.38 - B.40, B.571, “Drugs and Pharmaceutical Sciences” [B53, B.541, Kirk-Othmer “Encyclopediaof Chemical Technology” [B.58].Obviously, there are numerous others, for example, Perry’s Chemical Engineer’s Handbook, Dubbel, and Hutte as well as many more, particularly those published in different parts of the world and in other languages.
13.1
List of Books or Major Chapters on Agglomeration and Related Subject (With exception of the more recent ones, most of the following references are “out-of-print”.However, today, there is a growing number of suppliers and/or publishers of out-of-printbooks available on the internet. They either offer antiquarian publications for sale or on loan or will try to obtain books for “on demand” reproduction. One such service is, for example, the reprinting service for out-of-print books of Bell & Howell information and Learning Company, Ann Arbor, MI, USA, at http://w.bellhowell.infolea~ing.com (see also footnote on page 528). Other sources are listings of antiquarian book such as h t t p : / / ~ . b i b l i o f i n d . c o mor http:// www.zuab.com The Library of the US Congress ( h t t p : / / w . l o c . g o u ) , listing almost all books which were published at any time, anywhere in the world, and the German Library ( h t t p : / / m . d d b . d e ) make books available at their reading rooms.) G. Franke, Handbuch der Brikettbereitung (Handbook of [Coal] Briquetting), Verlag Ferdinand Enke, Stuttgart, Germany (1909). B.2 K. Kegel, Aufbereitung und Brikettierung (Processing and Briquetting [of Coal]),Wilhelm Knapp Verlag, Halle/Saale, Germany (1948). B.3 Proceedings of the Biennial Conferences of the institute for Briquetting and Agglomeration (IBA), vol. 1-27 (1949, 1951, 1953, ....., 2001). B.4 W.A. Knepper (ed.),Agglomeration, Proc. 1st international Symp. Agglomeration, Philadelphia, PA, USA, John Wiley & Sons, New York, NY, USA, and London, UK (1962). B.5 W.A. Ritschel, Die Tablette. Grundlagen und Praxis des Tablettierens, Granulierens und Dragierens (The tablette. Fundamentals and applications of tabletting, granulating and coating), Editio Cantor KG, Aulendorf, FR Germany (1966). B.6 W. Pietsch, Kornvergrogerung (Agglomerieren),(Size enlargement by agglomeration), in: “Fortschritte der Verfahrenstechnik. VDI-Verlag GmbH, Dusseldorf, FR Germany, vol. 9 (1971), 831-872; V O ~10 . (1972), 223-235; V O ~11. (1973), 162-172; V O ~1 .2 (1974), 133146; V O ~ 13. (1975), 143-163; V O ~14. (1976), 149-160; V O ~16. (1978), 73-89. B.7 A.S. Goldberg (ed.),Compaction ’73, Proc. 1st International Conf. on Compaction and Consolidation of Particulate Matter, Powder Technology Publ. Series No. 4, Powder Advisory Centre, London, U K (1972). W. Herrmann, Das Verdichten von Pulvern zwischen zwei Walzen (The densification of B .8 powders between two rollers), Verlag Chemie GmbH, Weinheim, FR Germany (1973). B.9 D.F. Ball, J. Darhell, J. Davison, A. Grieve, and R. Wild, Agglomeration of iron Ores, American Elsevier Publishing Co., New York, NY, USA (1973). B.10 S.K. Nikol (ed.),Pellets and Granules, Proc. Symp. Pellets and Granules, The Australian Inst. of Mining and Metallurgy, Newcastle, NSW, Australia (1974). B.ll H. Rumpf, Mechanische Verfahrenstechnik (English ed.: Particle Technology), Monograph in Winnacker-Kiichler,Chem. Technology,vol. 7,3rd ed., Carl Hanser Verlag, Munchen, FR Germany/Wien, Austria (1975): Engl. ed.: F.A. Ball (Translator), Particle Technology, Chapman & Hall, London, UK (1990). B.12 A.S. Goldberg (series ed.), Monographs in Powder Science and Technology, Heyden & Son Ltd., London, UK/Rheine, FR Germany/NewYork, NY, USA; a) P. Popper, Isostatic Pressing B.l
13.1 List of Books or Major Chapters on Agglomeration and Related Subject
B.13
B.14 B.15 B.16 B.17
B.18
B.19
B.20 B.21
B.22 B.23 B.24 B.25 B.26 B.27 B.28 B.29
B.30
B.31
B.32
(1976);b) W. Pietsch, Roll Pressing (1976), 2nd ed. (1987);c) M.B. Waldron and B.L. Daniell, Sintering (1978);d) J.K. Beddows, The production ofmetal powders by atomization (1978);e) P.J. Sherrington and R. Oliver, Granulation (1981). H.C. Messmann and T.E. Tibbets (eds.), Elements of Briquetting and Agglomeration vol. 1, The Institute for Briquetting and Agglomeration (IBA), Hudson, WI, USA (1977);R.N. Koerner and J.A. McDougall (eds.), Elements of Briquetting and Agglomeration vol. 2, The Institute for Briquetting and Agglomeration (IBA), Hudson, WI, USA (1983). K.V.S. Sastry (ed.), Agglomeration 77, vols. 1 and 2, Proc. 2nd International Symp. Agglomeration, Atlanta, GA, USA, AIME, New York, NY, USA (1977). H. Schubert et al., Mechanische Verfahrenstechnik (Mechanical Process Technology), Deutscher Verlag fur Grundstoffindustrie, Leipzig, DR Germany (1977). K. Meyer, Pelletizing of Iron Ores, Springer-Verlag, Berlin/Heidelberg. FR Germany/New York, NY, USA - Verlag Stahleisen mbH, Dusseldorf, FR Germany (1980). C.E. Capes, Particle Size Enlargement. In: j.C. Williams andT. Allen (series eds.) “Handbook of Powder Technology” vol. 1, Elsevier Scientific Publishing Co., Amsterdam, The Netherlands/Oxford, UK/New York, NY, USA (1980). 0. Molems and W. Hufnagel (eds.), Agglomeration 81, vols. 1 and 2, Proc. 3rd International Symp. Agglomeration, Nurnberg, FR Germany, Nurnberger Messe- und Ausstellungsgesellschaft, Nurnberg, FR Germany (1981). W. Pietsch, Agglomeration. In: “Fortschritte der Verfahrenstechnik (in English), VDI-Verlag GmbH, Dusseldorf, FR Germany, vol. 19 (1981), 133-149; vol. 21 (1983), 121-139; vol. 23 (1985), 125-139. H. Schubert, Kapillaritat in porosen Feststoffsystemen (Capillarity in porous solid systems), Springer-Verlag, Berlin/Heidelberg. FR Germany/New York, NY, USA (1982). C.E. Capes, W. Pietsch, et al., Size Enlargement Methods and Equipment. In: M.E. Fayed and L. Otten (eds.) “Handbook of Powder Science and Technology”,ch. 7, Van Nostrand Reinhold Co., New York, NY/Cincinnati, OH, USA/Toronto, Canada/London, UK/Melboume, Australia (1983). U. Bossel (ed.), Brikettieren und Pelletieren von Biomasse (Briquetting and pelleting of biomass), SOLENTEC Fachbuchvertrieb, Adelebsen, FR Germany (1983). C.E. Capes (ed.),Agglomeration 85, Proc. 4th International Symp. Agglomeration, Toronto, Ont., Canada, The Iron & Steel Society, Inc. (ISS), Warrendale, PA, USA (1985). J.W. Merks, Sampling and Weighing of Bulk Solids. Series on Bulk Materials Handling, vol. 4, Trans Tech Publications, Clausthal-Zellerfeld, FR Germany (1985). S. Bradbury (ed.),Powder Metallurgy Equipment Manual, 3rd ed., Metal Powder Industries Federation, Princeton, NJ, USA (1986). B.M. Moudgil and P. Somasundaran (eds.), Flocculation, Sedimentation & Consolidation, Proc. Engineering Foundation Conference, United Engineering Trustees, Inc., USA (1986). K. Sommer, Sampling of Powders and Bulk Materials, Springer-Verlag. Berlin, Heidelberg, FR Germany (1986). Committee on Raw Materials, Sinter and Pellets. Production and Use Capacities (State: 1987), International Iron and Steel Institute (IISI), Brussels, Belgium (1987). British Materials Handling Board, Particle Attrition - State-of-the-ArtReview, Series on Bulk Materials Handling, vol. 5, Trans Tech Publications, Clausthal-Zellerfeld, FR Germany (1987). P. Ridgeway-Watt, Tablet Machine Instrumentation in Pharmaceuticals - Principles and Practice, Ellis Hanvood Series in Pharm. Technology, John Wiley & Sons, New York, NY, USA (1988). J. Srb and Z. Ruzickova, Pelletization of Fines (Minerals, Ores, Coal). In: D.W. Fuerstenau (advisory ed.) “Developments in Mineral Processing”, vol. 7, Elsevier Science Publishers B.V., Amsterdam, The Netherlands (1988). K. Sommer, Size Enlargement. In: “Ullmann’s Encyclopedia of Industrial Chemistry”, 5th ed., vol. B.2, ch. 7, Verlag Chemie GmbH, Weinheim FR Germany (1988), 1-37.
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B.33 W. Dotsch, Agglomerationskinetik zur Simulation von Agglomerationsprozessen im Agglomerierteller (Kinetics of agglomeration for the simulation of agglomeration processes in the pan granulator), Fortschr.-Ber. VDI, Reihe 3, Nr. 157, VDI-Verlag GmbH, Dusseldorf, FR Germany (1988). B.34 K.L. Mittal (ed.),Particles on Surfaces 1 , 2 , and 3: Detection, Adhesion, and Removal, Plenum Publishing Corp., New York, NY, USA (1988, 1989, 1990). B.35 Agglomeration 89, Proc. 5th International Symp. Agglomeration, Brighton, UK, The Institution of Chemical Engineers (IChemE), Rugby, UK (1989). 8.36 I. Ghebre-Sellassie (ed.), Pharmaceutical Pelletization Technology, The Pharmaceutical Sciences Series, No. 37, Marcel Dekker, New York, NY, USA (1989). B.37 B.H. Kaye, A Random Walk Through Fractal Dimensions, VCH Verlagsgesellschaft mbH, Weinheim, FR Germany (1989). B.38 B. Schetter and J. Funcke, Agglomeration der dispersen Phase von Aerosolen durch starke Schallfelder (Agglomeration of the disperse phase of aerosols by strong sound fields), Fortschr.-Ber. VDI, Reihe 3 (Verfahrenstechnik/Process Technology), Nr. 196, VDI-Verlag GmbH, Dusseldorf, Germany (1990). B.39 P. Schultz, Trocknung kapillarporoser Korper bei Anwesenheit auskristallisierender Stoffe in der Gutsfeuchte/Trocknungmit Krustenbildung (Drying of wet porous bodies which contain dissolved substances/Drying with incrnstation), Fortschr.-Ber. VDI, Reihe 3 (Verfahrenstechnik/Process Technology), Nr. 201, VDI-Verlag GmbH, Dusseldorf, Germany (1990). B.40 C.-J. Klasen, Die Agglomeration partikelformiger Feststoffe in Matrizenpressen (The agglomeration of particulate solids in pellet presses), Fortschr.-Ber. VDI, Reihe 3 (Verfahrenstechnik/Process Technology), Nr. 220, VD1-Verlag GmbH, Dusseldorf, Germany (1990). B.41 S. Eriksson and M. Prior, The briquetting of agricultural wastes for fuel, FA0 Environment and Energy Paper 11,Food and Agriculture, Organization ofthe United Nations, Rome, Italy (1990). B.42 W. Pietsch, Size Enlargement by Agglomeration, John Wiley & Sons Ltd., Chichester, UK/ New York, NY, USA/Brisbane, Australia/Toronto, Canada/Singapore - Otto Salle Verlag GmbH & Co., Frankfurt/M, Germany - Verlag Sauerlander AG, Aarau, Switzerland (1991). This book is out-of-printand the copyright is now held by the author.” B.43 K. Masters, Spray Drymg Handbook, 5th ed., Longman, London, UK/John Wiley & Sons, New York, NY, USA (1991). B.44 E.J. Griffith, Cake Formation in Particulate Systems, VCH Publishers, Inc., New York, NY, USA (1991). B.45 Agglomerations-und Schuttgut-Technik (Agglomerationand Bulk SolidsTechnologies), Preprints, GVC, VDI Gesellschaft Verfahrenstechnik und Chemieingenieunvesen, Dusseldorf, Germany (1991). B.46 F. Loffler and J. Raasch, Grnndlagen der Mechanischen Verfahrenstechnik (Fundamentalsof Mechanical Process Engineering), Friedr. Viehweg & Sohn Verlagsgesellschaft mbH, Braunschweig/Wiesbaden, Germany (1992). B.47 2. Drzymala, Industrial Briquetting. Fundamentals and Methods, Studies in Mechanical Engineering, vol. 13, Elsevier Science Publishers, Amsterdam, The Netherlands/London. UK/New York, NY, USA/Tokyo, Japan - PWN Polish Scientific Publishers, Warzawa, Poland (1993). B.48 AGGLOS, Proc. 6th International Symp. Agglomeration, Nagoya, Japan, The Society of Powder Technology, Japan - The Iron and Steel Institute of Japan - The Society of Chemical Engineers, Japan (1993).
* Arragements have been
made with “Books on tional information, also on the availability o f another Demand”, the reprinting service for out-of-print book by the author (“Roll Pressing” (B. 12b], Order # books of Bell EL Howell Information and Learning AU00526) and further out-of-print books that may be Company, to make the publication available to those listed in Section 13.1, can be obtained through the who are interested in it (Order # 2067035). Addi- web site www.bellhowell.infolearning.com.
73.7 List ofBooks or Major Chapters on Agglomeration and Related Subject
B.49 First International Particle Technology Forum (1st IPTF), Denver, CO, Proceedings, PTF of AIChE, New York, NY, USA (1994). B.50 W. Herman de Groot, I. Adami, and G.F. Moretti, The Manufacture of modern Detergent Powders, Herman de Groot Academic Publishers, Wassenaar, The Netherlands (1995). B.51 B.M. Moudgil and P. Somasundaran (eds.), Practical Dispersion. A Guide to Understanding and Formulating Slurries, VCH Publishers, Inc., New York, NY, USA (1996). B.52 The 5th World Congress of Chemical Engineering and 2nd IPTF (Int’l Particle Technology Forum), San Diego, CA, Proceedings, AIChE and PTF ofAIChE, New York, NY, USA (1996). B.53 G. Alderborn and Ch. Nystrom (eds.), Pharmaceutical Powder Compaction Technology, Drugs and Pharmaceutical Sciences, vol. 71, Marcel Dekker, Inc., New York, NY, USA (1996). 8.54 D.M. Parikh (ed.), Handbook of Pharmaceutical Granulation Technology, Drugs and Pharmaceutical Sciences, vol. 81, Marcel Dekker, Inc., New York, NY, USA (1997). B.55 Ch. Hayashi, R. Uyeda, and A. Tasaki (eds.), Ultra-Fine Particles. Exploratory Science and Technology, Noyes Publications, Westwood, NJ, USA (1997). B.56 W. Pietsch, Size Enlargement by Agglomeration, In: “Handbook of Powder Science and Technology”, M.E. Fayed and L. Otten (eds.), 2nd ed., ch. 6, Chapman & Hall, New York, NY, USA (1997), 202-377. B.57 R.-D. Becher, Untersuchung der Agglomeration von Partikeln bei der Wirbelschicht-Spriihgranulation (Investigation of the agglomeration of particles during fluidized bed spray granulation), Fortschr.-Ber. VDI, Reihe 3 (Verfahrenstechnik/ProcessTechnology), Nr. 500, VDIVerlag GmbH, Diisseldorf, Germany (1997). B.58 C. Edward Capes and K. Darcovich, Size Enlargement, In: Kirk-Othmer, “Encyclopedia of Chemical Technology”, 4th ed., vol. 22, John Wiley & Sons, Inc., New York, NY, USA (1997), 222-255. B.59 B.H. Kaye, Powder Mixing, Chapman Sr Hall, London, UK (1997). B.60 T. Allen, Particle Size Measurement, 5th ed., vol. 1: Powder Sampling and Particle Size Measurement, vol. 2: Surface Area and Pore Size Determination, Chapman Sr Hall, London, UK (1997). B.61 K. Ishizaki, S. Komarneni, and M. Nanko, Porous Materials. Process Technology and Applications, Kluwer Academic Publishers, Dordrecht, NL, Boston, USA, London, UK (1998). B.62 R.W. Rice, Porosity of Ceramics, Marcel Dekker, Inc., New York, USA, Basel, Switzerland, Hong Kong (1998). B.63 World Congress on Particle Technology 3, Brighton, UK, including 3rd IPTF (“Emerging Particle Technologies: A Vision to the Future”), IChemE, Rugby, UK (1998). B.64 J. Scheirs, Polymer Recycling: Science, Technology, and Applications, John Wiley & Sons, Ltd., Chichester, West Sussex, England (1998). B.65 G. Heinze, Handbuch der Agglomerationstechnik (Handbook of Agglomeration Technology), Wiley-VCH Verlag, Weinheim, Germany (2000). B.66 J.F. Scamehorn and J.H. Hanvell (eds.), Surfactant-based Separations. Science and Technology, American Chemical Society, Washington, DC, USA (2000). B.67 D. Ramkrishna, Population Balances. Theory and Applications to Particulate Systems in Engineering, Academic Press, San Diego, CA, USA (2000). B.68 E.M. Petrie, Handbook of Adhesives and Sealants, McCraw-Hill, New York, NY, USA (2000). B.69 H. Uhlemann and L. Morl, Wirbelschicht-Spriihgranulation(Fluidized bed spray granulation), Springer Verlag, Heidelberg, Germany (2000). B.70 Preprints 7th Int’l Symposium Agglomeration, Albi, France, Progep, 18 chemin de la loge, F-31078 Toulouse, Cedex 4, France (2001). B.71 W. Pietsch, Agglomeration Technology - Industrial Applications. Wiley-VCH Verlag, Weinheim, Germany (2003).
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13.2 References
1.1
W. Pietsch, “Festigkeit und Trocknungsverhalten von Granulaten, deren Zusammenhalt durch bei der Trockung auskristallisierende Stoffe bewirkt wird.” (Strength and drying behavior of agglomerates, the induration of which is caused by the recrystallization of dissolved solids during drying.), Diss. (Ph.D. thesis) Universitat (T.H.) Karlsruhe, FR Germany (1965). 1.2 “Webster’sThird New International Dictionary of the English Language” (unabridged),Merriam-Webster, Inc. Springfield, MA, USA (1986). 1.3 K. Schiefer (ed.), “RoRoRo Techniklexikon, Verfahrenstechnik, Bd. 3” (RoRoRo Technical Lexicon, Process Technology, vol. 3), Rowohlt Taschenbuch Verlag GmbH, Hamburg, FR Germany (1972). 5.1 H. Rumpf, “The strength of granules and agglomerates”,in [B.4],379-418. 5.2 H.C. Hamaker, “The London - van der Waals attraction between spherical particles”, Physica 4 (1937) 10, 1058-1072. 5.3 E.M. Lifshitz, “The theory of molecular attractive forces between solids”,Soviet Phys. JETP 2 (1956)1, 73-83. 5.4 H. h p p , “Particle adhesion - Theory and experiment”, Adv. Coll. & Interf. Sci. (1967)1, 112 -239. F.M. Thomson, “Storage and flow of particulate solids”,In: Handbook of Powder Science and 5.5 Technology, M.E. Fayed and L. Otten (eds.),2nd ed., ch. 8, Chapman & Hall, New York, NY, USA (1997),389-486. 5.6 B. Wist, “Ball-milldegradation test for quality control of granular potash products”, (revised), PCS Potash, Saskatoon, Sask., Canada (1997). 5.7 H. Rieschel, K. Zech, “Comparisonofvarious test methods for the quality control of Potash granulate”, Phosphorous & Potassium, Brit. Sulphur Corp., Sept./Oct. (1981). S. Debbas, “Uber die Zufallsstruktur von Packungen aus kugeligen oder unregelmassig ge5.8 formten Kornern” (The random structure of packings of spherical and irregular particles), Diss. (Ph.D. thesis) Universitat (T.H.) Karlsruhe, FR Germany (1965). 5.9 W. Pietsch, “Storage, shipping, and handling of direct reduced iron”, AIME/SME Transactions 262 (1977)3, 225-234. 5.10 W. Pietsch and W. Schutze, “HBI - A safe DRI-based source of iron units”, Paper at World Iron Ore 96, Orlando, FL (1996),Skillings’ Mining Review 86 (1997)18, 4-9. 7.1 K.V.S. Sastry, “Process Engineering of Agglomeration Systems”,in [B.46], 37 -44. 7.2 A.A. Adetayo et al., “Drum granulation of fertilizers: Modelling and circuit simulation”, in [B.46],105-110. 7.3 J.R. Wynnyckyl, “Microstructure and growth mechanisms in pelletizing - A critical re-assessment”, in (B.461, 143-159. 7.4 H. Leuenberger, “Designand optimization approaches in the field of granulation, drying, and coating”, in “Topics in Pharmaceutical Sciences 1993”, Proc. 53rd Int’l. Congr. of Pharmaceutical Sciences of F.I.P., Tokyo, Japan, D.J.A.Crommelin, K.K. Midha, and T. Nagai (eds.), Scientific Publishers, Stuttgart, Germany (1994),493-510. 7.5 T.S. Chirkot, “Characterization of a pharmaceutical wet granulation process in a V-type low shear granulator”,Ph.D. thesis, The Union Institute, Cincinnati, OH, publ. by UMI Co., Ann Arbor, M I , USA (1998). S. Hogekamp, H. Schubert,and S. Wolf, “Steam jet agglomeration ofwater soluble material”, 7.6 Powder Technology 86 (1996), 49-57. 7.7 K. Nishii, Y. Itoh, and N. Kawakami, “The characteristics of pressure swing granulation”,in [B.46], 111-116. 7.8 Y. Kawashima, “Spherical crystallization technique: A new tool for micromeritic design of crystals and preparation of drug delivery systems”, in [B.46],487-492. 8.1 S.P. Shah, S.E. Swartz, Ch. Ouyang, “Fracture Mechanics of Concrete: Applications of Fracture mechanics to concrete, rock, and other quasi-brittle materials”.John Wiley & Sons, Inc., New York, NY, USA, and Toronto, Ont., Canada (1995).
13.3 Author’s Biography, Patents, and Publications
8.2 8.3
10.1 10.2
10.3
10.4 10.5
12.1
12.2 12.3
12.4 12.5 12.6 12.7
12.8 12.9
R.B. Steele, “Agglomeration of Steel Mill By-productsvia Auger Extrusion”. Proc. 231d Biennial Conf. IBA (see also [B.3]),Seattle, WA, USA (1993) 205-217. W. Pietsch, “Ram Pressing - An almost extinct technology with interesting new applications in coal and other solid fuel processing”. Proc. 2Sth Int’l. Technical Conf. On Coal Utilization & Fuel Systems, Clearwater, FL (2000), 37-48 (see also Section 13.3). Y. Doganoglu, V. Jog, K.V. Thambimuthu, and R. Clift, “Removal of fine particulates from gases in fluidised beds”, Trans IChemE 56 (1978),239-248. R. Clift, M. Ghadiri, and K.V. Thambimuthu, “Filtration of gases in fluidised beds”, In: Progress in Filtration and Separation, vol. 2, R.J. Wakeman (ed.), Elsevier, Amsterdam, The Netherlands (1981),75-123. S. Fleck and U. Riebel, “Einfluss der Fluidisierungsbedingngen auf Abscheidung und Agglomeration von Aerosolen beim Durchgang durch Wirbelschichten” (Influence of the fluidization conditions on removal and agglomeration of aerosols during the passage through fluidized beds), Chemie Ingenieur Technik 71 (1999)4, 361 -364. V.A. Bielobradek, “Selecting a better media for your pleated bag and cartridge filter”, Powder and Bulk Engineering 14 (2000)10, 77-81. K. Schonert, “Mechanische Verfahrenstechnik - Insbesondere Umgang mit feinen Partikeln” (Mechanical Process Technology - Particularly processing of fine particles), Fridericiana, Z. der Univ. Karlsruhe, Verlag C.F. Muller, Karlsruhe, FR Germany (1977)21,12-33. H. Schubert, “Eine Schnellmethode zur Messung der Instanteigenschaften pulverformiger Stoffe” (A fast method for measuring the instant characteristics of products from powder materials), Z. Lebensmitteltechnologie und -verfahrenstechnik 36 (1985)5, 149- 152. H. Schubert, “Instantisieren pulverformiger Lebensmittel” (Instantizing of powdered food materials), Chem.-1ng.-Tech.62 (1990)11, 892-906. J. Wolsiefer, Sr., “The measurement and analysis of silica fume particle size distribution and dispersion”,Norchem Concrete Products, lnc. (see Section 14.1),Paper at 5th CANMETiACI Int’l Conf. on Durability of Concrete, Barcelona, Spain, June 4, 2000. Th. Zehnder and J. Patscheider, “Nanocomposite TiC/a-C:H hard coatings deposited by reactive PVD”, Surface and Coatings Technology 133- 134 (2000), 138-144. P.H. Mayrhofer and C. Mitterer, “High-temperatureproperties of nanocomposite TixBN,and TiB,C, coatings”, Surface and Coatings Technology 133- 134(2000),131- 137. Ch. Crabb, “Powder coatings find cures”, Chemical Engineering (2000)2, 54, 56, 57. Z. Zhang, R. Saunders, and C.R. Thomas, “Mechanical strength of single microcapsules determined by a novel micromanipulation technique”, J. Microencapsulation 16 (1999)1, 117 - 124. F. Caruso, “Hollow capsule processing through colloidal templating and self-assembly”, Chem. Eur. J. 6 (2000)3, 413-419. H. Fudouzi, M. Kobayashi, and N. Shinya, “Arrangement of microscale particles by electrification”, Kona (1999)17, 55-63.
13.3 Author’s Biography, Patents, and Publications
Dr. Pietsch is a Senior Consultant in the general fields of Mechanical Process Engineering (Powder & Bulk Solids Technologies) and, particularly, Size Enlargement by Agglomeration for COMPACTCONSULT, Inc. of Naples, FL, USA. He received the equivalents of B.S. and M.S. degrees in Mechanical and, respectively, Chemical Engineering from the Technical University (TH) of Karlsruhe, West Germany, in 1959 and 1962 (Dip1.-Ing.). In 1965 he earned his Ph.D. (Dr.Ing.) at the same University with work on the Fundamentals of Binding Mechanisms of Agglomeration. Prior to his industrial career, Dr. Pietsch did further research in the
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general field of Size Enlargement by Agglomeration at the Institute of Mechanical Process Engineering at the Technical University (TH) of Karlsruhe until 1967. Later, while in industry, he taught the “Unit Operations of Mechanical Engineering” at the University of Stuttgart, Heilbronn Branch, Heilbronn, West Germany. Beginning in 1967, his industrial positions included: Research Scientist, Allis-Chalmers, Milwaukee, WI, USA; Staff Consultant, Komarek Greaves (today HOSOKAWA BEPEX), Rosemont, IL, USA; Technical Director, HU7T GmbH (today HOSOKAWA BEPEX), Leingarten, West Germany; Managing Director, Technical, Lemforder Kunststoff GmbH, Lemforde, West Germany; Director Agglomeration Systems and Product Technology, MIDREX Corp., Charlotte, NC, USA; General Manager Large Metallurgical Systems, Leybold-Heraeus GmbH, Hanau, West Germany; Senior Technical Manager, Maschinenfabrik KOPPERN GmbH & Co KG, Hattingenl Ruhr, Germany; Executive Vice President and, later, President, KOPPERN Equipment, Inc., Charlotte, NC, and Pittsburgh, PA, USA, until 1995 when he retired from industry. As of the publication date of this book, Dr. Pietsch is the author of more than 150 papers, 3 books, including the textbook “Size Enlargement by Agglomeration” published by Wiley & Sons in cooperation with Salle + Sauerlander in 1991 (now outof-print but available from “Bookson Demand”, see Sections 13.1 and 13.2),and holds 9 patents. He is a member of six professional organizations in the USA and Germany and is active in a number of technical committees. He is a frequent lecturer of workshops, short courses, and continuing education events in the fields of Mechanical Process Technology and Agglomeration in the USA and Europe. COMPACTCONSULT, Inc., was founded in 1983 and incorporated in the state of North Carolina, USA, in early 1984. It is 100 % owned and operated by Mrs. Hannelore Pietsch and, therefore, qualifies as a woman-owned small business concern. The primary purpose of COMPACTCONSULT, Inc. is, to make international experts available to industries as well as private and government agencies. The fields of expertise of consultants are the Unit Operations of Mechanical Process Technology (see Chapter 1, Fig. 1.1) in all areas producing, handling, and processing particulate solids (particles, powders, bulk masses, etc.) as well as hot and cold metal bearing particulate matter, including direct reduced iron (DRI). Specific expertise exists in Size Enlargement by Agglomeration. Other important activities are in the fields of processing and recirculating particulate wastes as secondary raw materials. In 1991 COMPACTCONSULT,Inc. moved temporarily to the State of Pennsylvania, USA, where it operated as “foreign” enterprise while still incorporated in North Carolina. After relocating to Naples, Florida, USA, the company was reincorporated in the State of Florida on August 17, 1995. Ownership has remained unchanged. Dr.-Ing. Wolfgang Pietsch (Ph.D.),EUR ING, joined COMPACTCONSULT, Inc. in 1983 as Senior Consultant and, after leaving KOPPERN Equipment, Inc. of Pittsburgh, PA, in 1995, continues working in this position as an unaffiliated consultant and independent contractor. During his entire professional career, from becoming a student helper to Prof. Dr.Ing. Hans Rumpf at the Institute of Mechanical Process Engineering of the Technical University (TH)of Karlsruhe, West Germany, in 1960 to now exclusively working as a
13.3 Author’s Biography, Patents, and Publications
consultant, Dr. Pietsch was always involved in Mechanical Process Technology, particularly the unit operation of Size Enlargement by Agglomeration and fields related to any aspect of agglomeration, as a researcher, teacher, process developer, designer, and user on two continents. While in industry (from 1967 to 1995) as a vendor representative, he has travelled to almost all countries on our planet to evaluate customer’s needs, develop suitable solutions, offer equipment and systems, and, if successful, help with the implementation, process optimization, and maintenance. Therefore, this book is based on that long and varied experience to which innumerable professionals have contributed. Even though they remain anonymous, these persons deserve credit. Also acknowledged should be the countless “students”that partook in the seminars and continuous education programs which Dr. Pietsch has either personally conducted or in and to which he has actively participated and contributed as a faculty member. At these sessions as well as during hundreds of consulting assignments, discussions with those faced with technical problems and with the development of solutions, to which Dr. Pietsch often contributed, have played a significant role in collecting the know-how that has been partially presented in this book. Much of the experience and know-how that was gathered by Dr. Pietsch during forty years of professional work has also been published in books, papers, and patents as well as in semipublic course notes. Although many of the more important statements and conclusions are reproduced in various parts of this book, it may be of interest to refer to the complete listing of these publications. The titles, summarized below, are always clear indications of the contents and, therefore, may complement what is submitted briefly in this book by directing the reader to a more detailed coverage.
Patents
Priority patents only: related patents filed or issued in many foreign coun-
tries.) 1.
2.
3.
4. 5. 6. 7. 8.
9.
C. Buchholz and W. Pietsch, Verfahren zur Aufiereitung von feuchten Metallspanen zum Wiedereinschmelzen (Process treating moist metal chips for melting), German patent DP 2 151 819, filed Oct. 18, 1971, issued Oct. 24, 1974. H.-J. Pitzer and W. Pietsch, Verfahren und Vorrichtung zur Ausnutzung von bei der Spanplattenherstellung anfallenden Sagespane- und Schleifstaubteilchen (Process and equipment for the recovery of wood dust and chips produced during chipboard manufacturing), Swiss patent SP 530 262, filed Oct. 22, 1971, issued Nov. 15, 1972. W. Pietsch, Verfahren zur Pressgranulation von in Entstaubungsanlagen abgeschiedenen Industriestauben (Briquetting process for industrial dusts), German patent DP 2 314 637, filed March 23, 1973, issued March 6, 1975. W. Pietsch, Binder Composition, US patent No. 4,032,352, filed May 3, 1976, issued June 28, 1977. W. Pietsch, Apparatus for continuous passivation of sponge iron material, US patent No. 4,033,559, filed April 5, 1976, issued July 5, 1977. W. Pietsch, Method for continuous passivation of sponge iron material, US patent No. 4,076,520, filed April 5, 1976, issued Feb. 28, 1978. W. Pietsch, Compacted, passivated metallized iron product, US patent No. 4,093,455, filed December 22, 1976, issued June 6, 1978. W. Pietsch and Ch.A. Schroer, Briquet and method of making same, US patent No. 4,105,457, filed June 22, 1977, issued August 8, 1978. W. Pietsch, Metallized iron briquet, US patent No. 4,116,679, filed March 24, 1977, issued, Sept. 26, 1978.
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Publications 1.
2.
3.
4.
5.
6.
7.
8.
9.
10. 11.
12. 13.
14. 15. 16. 17. 18. 19. 20.
W. Pietsch, Festigkeit und Trocknungsverhalten von Granulaten, deren Zusammenhalt durch bei der Trocknung auskristallisierende Stoffe bewirkt wird. (Strength and drymg behavior of agglomerates, the induration of which is caused by solids crystallizing during the drying operation.) Diss. (Ph.D. thesis) Universitat (TH) Karlsmhe (1965). W. Pietsch, Die Beeinflussungsmoglichkeiten des Granuliertellerbetriebes und ihre Auswirh n g e n auf die Granulateigenschaften. (The possibilities of influencing the pelletizing pan operation and their effects on the properties of the pelletized material.) Aufbereitungs-Technik 7 (1966)4, 177-191. H. Rumpf and W. Pietsch, Festigkeit und Trocknungsverhalten von Granulaten mit Salzbriickenbildung. (Strength and drying behavior of granules with salt bridges.) Chemie-hgenieur-Technik 38 (1966)3, 371-372. W. Pietsch, Neue Entwicklungen auf dem Gebiet der Granuliertechnik und die Festigkeitseigenschaften von Granulaten. (New agglomeration procedures and physical properties of the agglomerates produced.) Revue Technique Luxembourgeoise (1966)2, 68 - 90. W. Pietsch and H. Rumpf, Transport phenomena, crystallization and development of tensile strength during the drying of moist agglomerates containing NaC1-Solutions. Comptes-Rendues, Coll. Int. CNRS Nr. 169, Paris (1966), 213-235. W. Pietsch, Zweites Europaisches Symposium “Zerkleinern”. (2nd European Symposium on Comminution.) a) Aufiereitungs-Technik 7 (1966)11, 655 -665; b) Chemie-Ingenieur-Technik 38 (1966)12, 1307-1309; c) Staub-Reinhalt. Luft 27 (1967)1, 52-55. W. Pietsch, Das Agglomerationsverhalten feiner Teilchen. (The agglomeration tendencies of fine particles.) Staub-Reinhalt. Luft 27 (1967)2, 64-65; English ed.: 27 (1967)1, 24-41. W. Pietsch, Einfluss der Verkmstung auf die Trocknung kapillar-poroser Korper. (Influenceof the incrustation on the drying of porous bodies.) Staub-Reinhalt. Luft 27 (1967)2, 64-65; English ed.: 27 (1967)2, 10-11. W. Pietsch, Die Festigkeit von Granulaten mit Salzbriickenbindung und ihre Beeinflussung durch das Trocknungsverhalten. (The strength of granules with salt bridges and its change due to their drying behavior.) Aufiereitungs-Technik 8 (1967)6, 297- 307. W. Pietsch, Die Festigkeit von Agglomeraten. (Thestrength ofagglomerates.)Chemie Technik 19 (1967)5,259-266. W. Pietsch, Die Gmndlagen der Kornvergrogerung, ihre wissenschaftliche Untersuchung und technische Anwendung. (The fundamentals of size enlargement, its scientific investigation and technical application.) Revue Technique Luxembourgeoise 59 (1967)2, 53 -65. H. Rumpf and W. Pietsch (Hrsg./eds.), Zerkleinern. (Comminution.) Dechema-Monographien Nr. 993- 1026 (2 Bde.), Verlag Chemie, GmbH, Weinheim/Bergstrasse (1967). W. Pietsch and H. Rumpf, Haftkraft, Kapillardmck, Flussigkeitsvolumen und Grenzwinkel einer Fliissigkeitsbriicke zwischen zwei Kugeln. (Bindingforce, capillary pressure, liquid volume and critical angle of a liquid bridge between two spheres.) Chemie-Ingenieur-Technik 39 (1967), 885-893. J.E. Moore and W. Pietsch, Briquetting and compacting of lime and lime-bearing materials. Proc. 10th Biennial Conf. of IBA, Albuquerque, N M (1967), 38-50. W. Pietsch, Tensile strength of granular materials. Nature, 217 (1968)130, 736-739. W. Pietsch, Stand der Eisenerzpelletiemng. (Pelletizing of iron ore, worldwide.) Aufbereitungs-Technik 9 (1968)5, 201-214. W. Pietsch, An evaluation of techniques for particle size analysis, Part I and 11. Minerals Processing 11 (1968), 6-11; 12 (1968), 12-14, 24. W. Pietsch, Adhesion and agglomeration of solids during storage, flow and handling-A survey. Journal of Engineering for Industry (Trans. ofthe ASME), Series B, 91 (1969)2,435-449. W. Pietsch, E. Hoffman, and H. Rumpf, Tensile strength of moist agglomerates. I Sr EC Product Research and Development 8 (1969), 58-62. W. Pietsch, The strength of agglomerates bound by salt bridges. The Canadian Journal of Chemical Engineering 47 (1969), 403-409.
73.3 Author’s Biography, Patents, and Publications
21. W. Pietsch, Improving powders by agglomeration. Chem. Engng. Progress 66 (1970)1,31- 35. 22. W. Pietsch, Die Bedeutung der Walzenkonstruktion von Brikettier-, Kompaktier- und Pelletiermaschinen fur ihre technische Anwendung. (The importance of roll design for roll-type briquetting, compacting, and pelleting machines as defined by their technical application.) Aufbereitungs-Technik 11 (1970)3, 128- 138. 23. W. Pietsch, Brikettieren, Kompaktieren und Kompaktieren/Granulieren von Kalk und kalkhaltigen Stoffen. (Briquetting, compacting and compacting/granulatingof lime and lime bearing materials.) Zement-Kalk-Gips 59 (1970)5, 210 - 215. 24. W. Pietsch, a) Granulierverfahren fur die pharmazeutische Industrie. Die Pharmazeutische Industrie 32 (1970)5, 383- 389; b) Granulation techniques for pharmaceutical applications. Drugs made in Germany 13 (1970)2, 58-66. 25. W. Pietsch, Erwtinschte Agglomeration mit Granulatformmaschinen. (Wanted agglomeration with pelleting machines.) Maschinenmarkt M M - Industriejournal 77 (1971)10, 193- 196. 26. W. Pietsch, Size enlargement of solids. Particulate Matter (Bulletin of the Powder Advisory Center) 2 (1971)1, 15-22. 27. W. Pietsch, Roll designs for Briquetting-Compacting Machines. Proc. 11th Biennial Conf. of IBA, Sun Valley, Idaho (1969),145-163. 28. W. Pietsch, Kornvergrogerung. (Size enlargement.) Abschnitt 26, “Fortschritte der Verfahrenstechnik”, Bd. 9, VDI-Verlag GmbH, Dusseldorf (1971),831 -872. 29. W. Pietsch, Das Kornen von Dungemitteln mit dem Kompaktier-Granulierverfahren. (Granulating of fertilizers by means of the compacting/granulating procedure.) Aufbereitungs-Technik 12 (1971)11, 684-690. 30. W. Pietsch, Possibiliti di miglioramento delle qualita fisiche delle polveri tramite di agglomerazione. (Possibilities to improve the physical characteristics of powders by agglomeration methods.) Ing. Chim. Ital. 7 (1971)11, 161-166. 31. W. Pietsch, Granulieren durch Kornvergrogerung. (Granulation by size enlargement.) CZChemie Technik 1 (1972)3, 116-119. 32. W. Pietsch, Anwendungen und Vorteile von Walzdmck-Brikettiermaschinen bei der Aufbereitung mineralischer Rohstoffe. (Applications and advantages of roll type briquetting machines for mineral processing.) Proc. IXth Int. Min. Processing Congr., Prag (1970)3, 255-259, Ustav Pro Vyzkum rud, Praha (1972). 33. W. Pietsch, Torque mill studies. A new approach in grinding research. “Particle Technology”, Proc. of Seminar, Indian Institute of Technology (IIT), Madras (1971),203-232. 34. W. Pietsch, Size enlargement. Lit. (33), 276-290. 35. W. Pietsch, Granulation of fertilizers using compacting/granulationmethods. Lit. (33),335 348. 36. W. Pietsch, Agglomerieren problemlos - Kompaktiervorgang in Walzdruckbrikettier- und Kompaktiermaschinen. (Agglomeration without problems - The process of compaction in roll-type briquetting and compacting machines.) Maschinenmarkt MM - Industriejournal 78 (1972)88, 2036-2040. 37. W. Pietsch, Wet grinding experiments in a torque ball mill. In “Zerkleinern” Symposion in Cannes 1971, Dechema-Monographien, Band 69, Nr. 1292-1326, 751 -779, Verlag Chemie GmbH, Weinheim/Bergstrasse (1972). 38. W. Pietsch and H. Liebert, Design and application of a laboratory machine for briquetting, compacting and pelleting research. Proc. 12th Biennial Conf. of IBA, Vancouver, Brit. Columbia (1971), 19-30. 39. W. Pietsch, Kornvergrogerung. (Size enlargement.) In: “Fortschritte der Verfahrenstechnik (1970/71), Bd. 19, Abt. B: Mechanische Verfahrenstechnik I, 221 -235, Hrsg. VTG im VDI, VDI-Verlag GmbH, Dusseldorf. 40. W. Pietsch, Das Kornen von Dungemitteln mit dem Kompaktier-Granulierverfahren. (Granulation of fertilizers using compaction/granulationmethods.) Proc. 2. Wissenschaftlich-Technische Konferenz “Mineraldunger”,Varna, Bulgarien, Hrsg.: Wissenschaftlich-Techn. Verband fur chem. Industrie, Sofia (1972),359-379.
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44.
45. 46.
47.
48.
49.
50. 51. 52.
53.
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57. 58. 59. 60.
W. Pietsch, A granulcao de adubos pel0 sistema de granulacao-compactacao. (Granulation of fertilizers using compaction/granulation methods.) Productos Quimicos 12 (1972)9/12,3- 10. W. Pietsch, Agglomerieren problemlos - Kompaktiervorgang in Walzdruck-Brikettier- und Kompaktiermaschinen. (Agglomeration made easy - The process of compaction in roll type briquetting and compacting machines.) Europa Industrie Revue (1973)1, 28- 31. W. Pietsch, Der Kompaktiervorgang in Walzdruck-Brikettier- und Kompaktiermaschinen. (The process of compaction in roll type briquetting and compacting machines.) Proc. Symposium Pracovniku Binskkho Pnimyslu, Hornicka P r h a m ve Vede a Technice, Prhram, CSSR (1972), 683-710. C. Buchholz and W. Pietsch, Neue Anwendungen der Kompaktiertechnik - Aufbereitung von Abfallmaterialien zu hochwertigen Rohstoffen. (New applications of compacting - Production of secondary raw materials from waste materials). CZ-Chemie-Technik 2 (1973)8, 319-321. W. Pietsch, Mechanische Verfahrenstechnik i m Dienst der Umwelttechnik. (Mechanical Process Engineering in Pollution Control.) CZ-Chemie-Technik 2 (1973)9, 351 -354. W. Pietsch, The many versatile applications of size enlargement in pollution control. Proceedings ofthe First Int’l Conf. o n the Compaction and Consolidation of Particulate Matter, Brighton, England, The Powder Advisory Centre, London (1973), 227-235. W. Pietsch, Granulieren, Agglomerieren und Kornvergrogerung in der Pharmazeutischen Industrie. (Granulation, agglomeration and size enlargement in the Pharmaceutical Industry.) APV-Informationsdienst, Mainz 19 (1973)2/3, 147- 182. W. Pietsch, KornvergroBemng. (Size enlargement). In: “Fortschritte der Verfahrenstechnik, Bd. 11, 1972, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Diisseldorf (1973), 162 172. W. Pietsch, Anwendung der Brikettiemng i m Umweltschutz a m Beispiel der Riickfuhrung von Filter- und Erzstauben in metallurgischen Anlagen. (Application of briquetting in pollution control as demonstrated by recycling of filter- and ore-dusts from metallurgical plants.) Aufiereitungs-Technik 14 (1973)12, 818 - 821. W. Pietsch, Application of briquetting in pollution control-recycling of filter and ore dusts in metallurgical plants. Proc. 13th Biennial Conf. of IBA, Colorado Springs, CO (1973), 1-12. W. Pietsch, The new HUTT laboratory Kompaktor and the Pharmapaktor. Proc. 13th Biennial Conf. of IBA, Colorado Springs, CO (1973), 113-117. W. Pietsch, Kornvergrogemng. (Size enlargement.) In: “Fortschritte der Verfahrenstechnik, Bd.12, 1973, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Diisseldorf (1974), 131 146. R.H. Snow, B.H. Kaye, C.E. Capes, R.F. Conley, J. Sheehan, F. Schwarzkopf, and W. Pietsch, Size reduction and size enlargement. Section 8. In: R.H. Perry, C.H. Chilton “Chemical Engineer’s Handbook, 5th ed., McGraw-Hill (1973), 1-65. W. Pietsch, Kornvergrogemng. (Size enlargement.) In: “Fortschritte der Verfahrenstechnik, Bd. 13, 1974. Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Dusseldorf (19754,143163. W. Pietsch, Kornvergrogemng mit Walzenpressen - Eine alte Technologie mit neuen Anwendungen. (Roll pressing -An old technology with new applications.) Aufiereitungs-Technik 17 (1976)3, 120-127. W. Pietsch, Kornvergrogemng. (Size enlargement.) In: “Fortschritte der Verfahrenstechnik”, Bd. 14, 1975, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Dusseldorf (1976),149160. W. Pietsch, Roll pressing. Heyden & Son Ltd., London/New York/Rheine (1976). W. Pietsch, Use of sponge iron in foundries. AFS Int’l Cast Metals Journal 1 (1976)2,43-50. W. Pietsch, Storage, shipping, and handling of MIDREX iron. Preprint Nr. 76-B-317, SMEAlME Meeting & Exhibit, Denver, CO (1976). W. Pietsch and G.A. Mott, Face to face interview: Direct reduced iron .....p ast and present. Modern Casting 66 (1976)9, 50-52.
73.3 Author’s Biography, Patents, and Publications
61. W. Pietsch, The use of sponge iron in foundries. Modem casting 66 (1976)9, 53-55 (condensed form of [%]). 62. W. Maschlanka and W. Pietsch, Aplicacion del hierro esponja como material de carga e n fundiciones. (Application of sponge iron as charge material in foundries.) Proc. Congreso Fundicion, ILAFA (1976), 75-85. 63. W. Pietsch, Charging with direct-reduced iron may reduce costs, improve chemistry. Foundry Operation Planbook (McGraw-Hill lnc.) (1977)4, 45 -48. 64. D.L. Keaton and W. Pietsch, An update of MIDREX Direct Reduction techniques and innovations. Proc. 50th Annual Meeting Minnesota Section AIME, Duluth, MN (1977), 5-23. 65. W. Pietsch and R. Kreimendahl, Us0 de hierro esponja e n la elaboracion de hierro. (Use of sponge iron in iron making.) In: “Us0 y comercializacion del hierro esponja”. Proc. Congreso ILAFA - Reduccion Directa, Macuto, Venezuela (1977), 233-239. 66. W. Pietsch, MIDREX- Direktreduktion - Standder Technik. (MIDREX direct reduction -The state of the art.) Aufbereitungs-Technik 18 (1977)8, 410-416. 67. J.E. Bonestell and W. Pietsch, MIDREX direct reduction - State of the art. Proc. SEAS1 Direct Reduction Conf., Bangkok, Thailand (1977). 68. W. Pietsch, Technical development of a merchant direct reduced iron facility. Annual Convention and Iron and Steel Exposition, Cleveland, OH (1977). 69. W. Pietsch, Storage, shipping, and handling of direct reduced iron. AIME/SME Transactions 262 (1977)3, 225-234. 70. W. Pietsch, Pressure agglomeration - State ofthe art. In: K.V.S. Sastry (ed.) Agglomeration 77, AIME New York (1977), 649-677. 71. W. Pietsch, The MIDREX cold briquetting system: An economic answer to direct reduced iron fines recovery. Iron and Steel Int’l 51 (1978)2, 119, 121-123. 72. W. Pietsch, Direct reduced iron: A new charge material for iron and steel foundries. The British Foundrymen 71 (1978)4, 89-93. 73. W. Pietsch, Agglomerieren. (Agglomeration.) In: “Fortschritte der Verfahrenstechnik, Bd. 16, 1978, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Diisseldorf (1978), 73 -89. 74. W. Pietsch, Agglomeration and direct reduction: A technical symbiosis. Mining Magazine 139 (1978)4, 414-421. 75. J.E. Bonestell and W. Pietsch, The floating direct reduction plant - A feasible future reality. a) Preprint, 3rd Int’l Iron and Steel Congress, Chicago, IL, USA (1978); b) Continental Iron & Steel Trade Reports 18 (1978),707-709; c) Proc. 3rd Int’l Iron & Steel Congress, Chicago, IL, USA (1978), 186-194. 76. W. Pietsch, The availability of direct reduced iron - An assessment of the technology and production capabilities through 1985. 82nd AFS Casting Congress and Exposition, Detroit, MN, USA (1978). 77. W. Pietsch, Direct reduced iron - A new charge material for iron and steel foundries. SEAISI 1978 Singapore Seminar “Modern Foundry Practice” (1978). 78. W. Pietsch, Development, installation, and operation of a briquetting system for direct reduced iron fines. Proc. 15th Biennial Conf. of IBA, Montreal, Canada (1977), 83-96. 79. W. Pietsch, The influence of raw material and reduction temperature on the structure and characteristics of direct reduced iron. SME of AIME Transactions 264 (1978), 1784- 1789. 80. W. Pietsch, The role of vacuum metallurgy in the production and processing of non-iron metals. Proc. VII. Ritkafkm Konferencia, Budapest, Hungary (1979), 75 -99. 81. W. Burgmann and W. Pietsch, Modern technologies in steel degassing and ladle metallurgy. Proc. Int’l Symposium Modern Developments in Steelmaking, Jamshedpur, India (1981), 7.8.1 -7.8.26. 82. W. Pietsch, Vakuumverfahren in der Metallurgie. (Vacuum process technology in metallurgy.) a) Vortrag Messe “Pulvermetallurgie”,Minsk, BSSR (1981) (in Russian); b) Fachberichte, Huttenpraxis, Metallverarbeitung 19 (1981)10, 808-817 (in German); c) World Steel and Metalworking Export Manual (1981), 93-101 (in English).
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13 Bibliography 83. W. Pietsch, Agglomeration. In: “Fortschritte der Verfahrenstechnik, Bd. 19, 1981, Abtlg. B: Mechanische Verfahrenstechnik, VDI-Verlag, Diisseldorf (1981), 133- 149. 84. W. Pietsch, Agglomeration. 3rd Int’l Symposium. Aufbereitungs-Technik 22 (1981)9, 488494. 85. W. Pietsch, New production technologies for metal and alloy powders. 1981 Int’l Industrial Seminar on Pilot Plant Experiences, Amelia Island, FL, USA (1981). 86. W. Pietsch, Agglomeration. 17th Biennial Conf. of IBA, Reno, Nevada, USA (1981).Aufbereitungs-Technik 23 (1982)2, 92-99. 87. W. Pietsch, New production technologies for metal and alloy powders. In: “Competing in the World Market - New technology for the Metals Industry”, Proc. 35th Annual Conf., Sydney, Australia (1982),17-24. 88. H. Stephan, W. Pietsch, H. Ettl, and H. Aichert, Degassing ofmetal powders and the filling of degassed powders into capsules for the manufacturing of ingots and discs. Proc. Int’l Powder Metallurgy Conf. Florence, Italy (1982), 179- 191. 89. W. Pietsch, H. Stephan, A. Feuerstein, J. Heimerl, and R. Ruthardt, Some new results of the atomization of reactive and refractory metals with the EBRD Process. Proc. Int’l Powder Metallurgy Conf. Florence, Italy (1982), 481-499. 90. W. Pietsch, New production technologies for metal and alloy powders. Proc. Int’l Powder Metallurgy Conf. Florence, Italy (1982),739- 754. 91. W. Pietsch, Titanium - From sponge to powder. VII Yugoslav Conf. on Contemporary Materials, Subotica, Yugoslavia (1982). 92. W. Pietsch, Die KornvergroBerung in der Verfahrenstechnik und ihre industrielle Anwendung am Beispiel der Direktreduhon von Eisenerzen. (Sizeenlargement in process engineering and its industrial application as exemplified by the direct reduction of iron ores.) Aufbereitungs-Technik (part 1) 23 (1982)4, 193-200; (part 2) 23 (1982)5, 248-257. 93. R. Ruthardt, W. Pietsch, and H. Stephan, Atomization techniques for high quality metal powder production. Unpublished manuscript (1982). 94. W. Pietsch, H. Stephan, A. Feuerstein, J. Heimerl, and R. Ruthardt, Atomization of reactive and refractory metals by the electron beam rotating disc process. Powder Metallurgy Int’l 15 (1983)2, 77-83. 95. W. Pietsch, Energy conservation in the fertilizer industry - The compaction/granulation process for mixed (NPK) fertilizers. a) Proc. 18th Biennial Conf. of IBA, Colorado Springs, CO, USA (1983),243-265; b) Proc. Int’l Conf. Fertilizer ’83, London, UK 2 (1983),467-479. 96. W. Pietsch, Modern equipment and plants for potash granulation. Proc. 1st Int’l Potash Technology Conf. Potash ’83, Saskatoon, Sask., Canada (1983), 661-669. 97. W. Pietsch, Einsatz grosser Walzenbrikettiermaschinen in der Koksherstellung. (Large roller briquetting machines in coke production.) Aufbereitungs-Technik 25 (1984)1, 29- 38. 98. W. Pietsch, Agglomerate bonding and strength. Section 7.2 in M.E. Fayed and L. Otten (eds.) “Handbookof Powder Science and Technology”, Van Nostrand Reinhold Co., Inc., New York (1984), 231-252. 99. W. Pietsch, Roll pressing, isostatic pressing and extrusion. Section 7.4 in M.E. Fayed and L. Otten (eds.)“Handbookof Powder Science and Technology”,Van Nostrand Reinhold Co., Inc., New York 11984). 269-285. 100. W. Pietsch,‘Aggiomeration. In: “Fortschritte der Verfahrenstechnik. Bd. 21, 1983, Abtlg. B: Mechanische Verfahrenstechnik. VDI-Verlag, Diisseldorf (1983), 121 - 139. 101. W. Pietsch, Granulation of mixed fertilizers by compaction. Proc. 34th Annual Meeting, Fertilizer Round Table, Baltimore, M D (1984),48-58. 102. H.-G. Bergendahl and W. Pietsch, Hot briquetting with roller presses. Proc. 4th Int’l Symposium on Agglomeration, Toronto, Canada, Iron and Steel Society, Inc. (1985), 543-550. 103. W. Pietsch, Agglomeration - Key to reycling of metal bearing fines. Proc. Int’l Symposium on Recycle and Secondary Metals, Fort Lauderdale, FL (1985),683-699. 104. W. Pietsch, Compaction/granulationof dry, digested sludge from municipal waste treatment plants. Proc. 19th Biennial Conf. of IBA, Baltimore, MD, USA (1985), 179-194.
73.3 Author’s Biography, Patents, and Publications
105. W. Pietsch, Agglomeration. In: “Fortschritte der Verfahrenstechnik, Bd. 23,l Abtlg. B: Mechanische Verfahrenstechnik, VD1-Verlag, Diisseldorf (1985), 125- 139. 106. W. Pietsch and C. Rodriguez, Granulation of fertilizers by compaction. Proc. 20th Biennial Conf. of IBA, Orlando, FL (1987), 113-126. 107. P.Schafer, Ph. Wolstenholme, W. Pietsch, and R. Holland, Compaction and granulation of dried sludge at Ocean County, NJ. 60th Annual Conf. WPCF (Water Pollution Control Federation), Philadelphia, PA (1987). 108. W. Pietsch, Mixed fertilizer granulation by compaction. History, application, and present status of mixed fertilizer granulation by compaction. Proc. Int’l Conf. Fertilizer South America, Caracas, Venezuela. The British Sulfur Corp. Ltd., London, England (1989), 153-173. 109. W. Pietsch and R. Zisselmar, Pressure agglomeration with roller presses for waste processing and recycling. Proc. 5th Int’l Symposium Agglomeration, Brighton, UK, IChemE, Rugby, UK (1989), 117-130. 110. W. Pietsch, Granulation offertilizers by compaction. Proc. IFDC Workshop “Supplyingquality multinutritional fertilizers in the Latin American and Caribbean Region - Emphasizing bulk blending and the complementary role of agglomeration”, Guatemala City, Guatemala (1989), 183- 196. 111. W. Pietsch, Briquetting of coal (Can a n ancient technology be modified for the production of environmentally safer smokeless fuels?). Proc. 21st Biennial Conf. of IBA, New Orleans, LA (1989), 303-320. 112. W. Pietsch, Granulation of fertilizers by compaction. Proc. IFDC Workshop “Urea-based NPK plant design and operating alternatives”, Muscle Shoals, AL (1990), 89 -98. 113. W. Pietsch, Briquetting of non-ferrous waste for economic recycle. Proc. 2nd Int’l Symposium “Recycling of metals and engineered materials” (J.H.L. van Linden, D.L. Stewart, Y. Sahai, eds.), TMS, Warrendale, PA (1990), 667-670. 114. W. Pietsch, Size enlargement by agglomeration. John Wiley & Sons Ltd. - Salle + Sauerlander, Chichester, UK/New York, NY, USA/Brisbane, Australia/Toronto, Canada/Singapore - Aarau, Switzerland/Frankfurt/M., Germany/Salzburg. Austria (1991). 115. M.E. Fayed and W. Pietsch, Particulate solids characterization and agglomeration. Course notes, AIChE Continuing Education, Miami Beach, FL (1992) (revised). 116. W. Pietsch and H. Ries, Agglomerieren - Granulieren. (Agglomeration - Granulation.) Course notes, Technische Akademie Wuppertal e.V., Wuppertal-Elberfeld, Germany (1992) (revised). 117. W. Pietsch, Fundamentals of agglomeration. Course notes, Workshop at Powdex NJ, Somerset, NJ (1992) (revised). 118. W. Pietsch, Pressure agglomeration: Fundamentals and applications. Course notes, Workshop at Powdex NJ, Somerset, N / (1992). 119. H.O. Kono and W. Pietsch, Tumblelgrowth agglomeration of fine powders: Present state and new developments. Course notes, Industrial awareness seminar at Powdex NJ, Somerset NJ (1992) (revised). 120. W. Pietsch, Briquetting of aluminum swarf for recycling. Light Metals ’93, TMS, Warrendale, PA (1993), 1045-1051. 121. W. Pietsch, Size enlargement by agglomeration in the pharmaceutical industry with special emphasis on pressure agglomeration. Course notes, Workshop at Interphex - USA New York, NY (1993). 122. W. Pietsch and H. Gunter, New applications of roller presses in coal-related technologies. Proc. 18th Int’l Techn. Conf. on Coal Utilization Sr Fuel Systems, Clearwater, FL (1993), 837-852. 123. W. Pietsch, Agglomeration technologies for environmental protection and recycling. Proc. 6th Int’l Symposium on Agglomeration, Nagoya, Japan (1993), 837-847. 124. W. Pietsch, Size enlargement by agglomeration for solid waste treatment or minimization and for hazardous waste stabilization. Preprints: 4th Pollution Prevention Topical Conference, Seattle, WA, USA, AIChE, New York, NY, USA (1993), 202-208.
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125. H . 4 . Bergendahl and W. Pietsch, Roller presses, their applications, sizes, and capacities as well as their limitations. Proc. 23rd Biennial Conf. “The Institute for Briquetting and Agglomeration”, Seattle, WA (1993), 185-204. 126. W. Pietsch and H. Gunter, Briquetting as a n upgrading process for different types of coals. Proc. 19th Int’l Techn. Conf. on Coal Utilization & Fuel Systems, Cleanvater, FL (1994), 181195. 127. W. Pietsch, Granulation by agglomeration in the pharmaceutical industry. Course notes, Workshop at Interphex - USA, New York, N Y (1994) (revised). 128. W. Pietsch, Parameters to be considered duringthe selection, design, and operation ofagglomeration systems. Proc. 1st Int’l Particle Technology Forum, AIChE, Denver, CO (1994), Part I, 248-257. 129. W. Pietsch, Economical and innovative methods for the agglomeration of dusts and other wastes from metallurgical plants for recycling. In: H.Y. Sohn (ed.) “Metallurgical Processes for the Early Twenty-First Century”, vol. 11, Technology and Practice, TMS, Warrendale, PA (1994), 487-495. 130. W. Pietsch, A review of agglomeration fundamentals and industrial techniques to enhance productivity. Course notes, Workshop at Powdex ’94, Houston, TX (1994). 131. W. Pietsch, Aglomeracion en plantas para reciclado: Metodos economicos e innovativos para la aglomeracion de polvos y otros desechos. Siderurgia Latinoamericana 10 (1994) 414, 27-34. 132. F.-H. Grandin and W. Pietsch, Compaction of aluminum chips and turnings and of other particulate aluminum scrap with roller presses for secondary aluminum melting without losses. Light Metals ’95, TMS, Warrendale, PA (1995), 799-802. 133. W. Pietsch, Agglomeration: Controlling pollution and permitting waste recycling. Powder & Bulk Engng. 9 (1995)2, 53, 54, 56, 57, 59-62. 134. W. Pietsch, Briquetting of coal with roller presses. An important technology for the production of coal-based compliance fuel. Proc. 20th Int’l Techn. Conf. on Coal Utilization & Fuel Systems, Cleanvater, FL (1995), 87-95; also in: S.D. Serkin (ed.) “Coal Fines: The Unclaimed Fuel”, Coal & Slurry Technology Assoc., Washington, DC (1995), 91-99. 135. W. Pietsch, Agglomeration technologies for environmental protection and recycling. Course notes, Workshop at Powder & Bulk Solids ’95, Rosemont, IL (1995). 136. W. Pietsch, Review of particle formation by compaction processes. 16th IFPRI Annual Meeting, Urbana, I L (1995). 137. F.-H. Grandin, W. Pietsch, and G. Medina y Espafia, Compaction of aluminum scrap on high pressure roller presses. Proc. 4th Australian, Asian, Pacific Course & Conference o n Alumin u m Cast House Technology, Sidney, Australia (1995). 138. W. Pietsch, J.Jagnow, and R. Lobe, Schuttgtiter pelletieren, exhudieren, granulieren, brikettieren, kompakieren. Problemlosungen fur industrielle Anwendungen. (Bulk solids pelletizing, extruding, granulating, briquetting, compacting. Solutions for problems of industrial applications.) Course notes. Seminar of the Technical Akademie Wuppertal, Altdorf, Germany (1995). 139. W. Pietsch, Roller presses - Versatile equipment for mineral processing. Proc. XIX IMPC, San Francisco, CA (1995), vol. 1, 59-66. 140. W. Pietsch, Roller presses for secondary metal recycling. Proc. 3rd Int’l Symp. Recycling of Metals and Engineered Materials. P.B. Queneau and R.D. Peterson (eds.), Point Clear, AL (1995), 233-241. 141. W. Pietsch, Evaluation of parameters for the selection, design, and operation of agglomeration systems. Proc. 24th Biennial Conf. “The Institute for Briquetting and Agglomeration”, Philadelphia, PA (1995), 175-189. 142. W. Pietsch, The briquetting of coal in Europe. Proc. COAL PREP 96, 13th Int’l Coal Preparation Conf., Lexington, KY (1996), 167-183. 143. W. Pietsch, Successfully use agglomeration for size enlargement. Chem. Engng. Progr. 92 (1996)4, 30-45.
13.4 Tables of Contents of Related Books by the Author
144. W. Pietsch, Recent developments in dry granulation of fertilizers by compaction. Proc. 5th World Congress of Chemical Engineering, San Diego, CA (1996), vol. V, 552-558. 145. W. Schiitze and W. Pietsch, HBI - Survey ofthe significance and development of sponge iron hot briquetting and the application of this technology in various plants and reduction processes. Proc. Conf Pre Reduced Products and Europe, Milan, Italy (1996). 146. W. Pietsch and W. Schutze, HBI - A safe DRI-based source of iron units. Paper at World Iron Ore 96, Orlando, FL (1996), Skillings Mining Review 86 (1997)18, 4-9. 147. W. Pietsch, Granulate dry particulate solids by compaction and retain key powder particle properties. Chem. Engng. Progr. 93 (1997)4, 24-46. 148. W. Pietsch, Size enlargement by agglomeration. ch. 6 (175 pages) In: M.E. Fayed and L. Otten (eds.) “Handbook of Powder Science Sr Technology”, 2nd ed., Chapman & Hall, New York, NY (1997). 149. W. Pietsch, Agglomeration techniques for the manufacturing of “instant” granules from fine powder mixhues. In: R. Hogg, C.C. Huang, and R.G. Cornwall (eds.) “Fine Powder Processing Technology”, The Pennsylvania State University (1998), 233 -242. 150. W. Pietsch, Agglomeration techniques for the manufacturing of granular materials with specific product characteristics. Proc. 25th Biennial Conf. “The Institute for Briquetting and Agglomeration”, Charleston, SC (1997), 149- 164. 151. W. Pietsch, Particle engineering by agglomeration in the pharmaceutical industry. Course notes, Workshop at INTERPHEX ‘99, New York, NY (1999). 152. W. Pietsch, How to select an agglomeration method. (part I) Powder and Bulk Engineering 13 (1999)2, 60-65; (part 11) Powder and Bulk Engineering 13 (1999)3, 21-31. 153. W. Pietsch, Readily engineer agglomerates with special properties from micro- and nanosized particles. Chem. Engng. Progr. 95 (1999)8, 67-81. 154. W. Pietsch, The porosity ofagglomerates. Proc. 26th Biennial Conf. “The Institute for Briquetting and Agglomeration”, San Diego, CA (1999), 1-14. 155. W. Pietsch, Granulation of pharmaceutical formulations to improve handling, processing, and use. Course notes. Workshop at INTERPHEX 2000, New York, NY (2000). 156. W. Pietsch, Compaction methods for granulation and the manufacturing ofdry dosage forms. Course notes. Workshop at INTERPHEX 2000, New York, NY (2000). 157. W. Pietsch, Ram pressing - An almost extinct technology with interesting new applications in coal and other solid fuel processing. Proc. 25th Int’l Technical Conf. on Coal Utilization Sr Fuel Systems, Clearwater, FL (2000), 37-48. 158. W. Pietsch, Agglomeration methods in particle engineering. Proc. XXI Int’l Mineral Processing Congress, Rome, Italy (2000),A4: 87-96. 159. W. Pietsch, An interdisciplinary approach to size enlargement by agglomeration. Proc. 7th International Symposium on Agglomeration, Albi, France (2001). 160. W. Pietsch, Agglomeration - An old and key technology serving mankind. Proc. 3rd European Congress of Chemical Engineering (3rd ECCE), Nurnberg, Germany (2001). 13.4 Tables of Contents of Related Books by the Author
To help the reader decide whether related books by the author may contain additional information, below the tables of contents of three related books are reproduced. W. Pietsch, Roll Pressing. 1st ed., Heyden Sr Son Ltd., London/New York/Rheine (1976); 2nd ed., Powder Advisory Centre, London (1987). Introduction/Fundamentals of Roll Pressing/Phenomenology of Roll Briquetting/Phenomenology of Roll Compacting/Design Principles/Machine Designs/Roll Designs/Feeder Designs/Design of the Discharge System/Instrumentation and Control of Roll Presses/Product Design/Assessment of Compact Quality/Auxiliaries/ Binders and Lubricants/Applications/TheCost of Roll Pressing/Application of Solids Flow Properties to Roll Presses/Recommendations/Equipment.
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W. Pietsch: Size Enlargement by Agglomeration. John Wiley & Sons Ltd. - Salle + Sauerlander, Chichester, UK/New York, NY, USA/Brisbane, Australia/Toronto, Canada/Singapore - Aarau, Switzerland/Frankfurt/M., Germany/Salzburg, Austria. (1991).Introduction/Fundamentals of Agglomeration/Experimental Investigations/IndustrialSize Enlargement Equipment and Processes/Industrial Applications of Agglomeration/Past, Present, and Future of Size Enlargement by Agglomeration. W. Pietsch, Agglomeration Technologies - Industrial Applications. VCH-Wiley,Weinheim (2003). Introduction/Agglomeration as a Generic, Independent, and Interdisciplinary Field of Science/Glossary of Application-related Agglomeration Terms/Undesired Agglomeration and Methods to avoid or lessen it/lndustrial Applications of Size Enlargement by Agglomeration/Powder Metallurgy/Applicationsin Evironmental Control/Development of Industrial Applications/Optimization and Troubleshooting of Agglomeration Systems and Plants/Applications of Agglomeration Phenomena for Single Particles and in Nano-Technologies/Engineering Criteria, Development, and Plant Design/Outlook/Bibliography/Indexes.
Agglomeration Processes Wolfgang Pietsch Cowriqht 0Wilev-VCH Verlaq GmbH, Weinheim. 2002
14
Indexes As discussed in Chapter 4, one of the problems encountered in connection with the presentation of technologies that were known for centuries and henceforth have been developed empirically and independently for different applications and industries (see also Chapter 2) is that the reader of books and the student of the methods and processes find an often confusing terminology. Even the suppliers of equipment may present themselves in a manner that does not unequivocally define their activities. To help locate information, three different indexes are offered in the following: Section 14.1 is a list of addresses of vendors of equipment for size enlargement by agglomeration and of peripheral techniques as well as of their telefone and telefax numbers. It is subdivided into fields of activities and, if a vendor is active in different areas, its listing is repeated again under the appropriate heading. Of course, no claim for completeness is made and mentioning a specific vendor does not constitute an endorsement by the author of this book. Also included is a listing of some tollers with a description of their activities. Section 14.2 is a “Wordfinder Index”. It is provided to give the reader a ready reference to the way in which words and terminologies are used in this book. Throughout this publication, historical, modern, and application oriented terms of agglomeration can be found (including certain important trade names, refer to “Disclaimer” at the beginning of the book). In the “Wordfinder” approach, the location is indicated were a word or term is explained and in the text it is highlighted by bold print. Section 14.3 contains references to the locations of words and terms in a “classic” Index of Subjects. This index often provides many page numbers for the same topic. When a word is encountered without specific explanations in the text, the reader can go to the Wordfinder Index, which gives direction to the definition location, to learn about it or refresh his or her memory.
14.1 List of Vendors
When planning the book, the author intended to prepare a worldwide, comprehensive list of vendors of agglomeration equipment as well as of associated resources and services. To that end, he collected technical and process information, particularly
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in Europe, North America, and Japan. Also, the author’s extensive personal files and library, many of which were already reviewed for and incorporated in his earlier books [B.12b,B.421 and articles (see Section 13.3) were referred to. Furthermore, participants of the author’s many international continuing education courses were interviewed and work of several of his colleagues, who covered specific topics at these courses, was used as basis for some of the chapters and/or sections of this book (see Acknowledgements). At some point of his work, all of the vendors that he and others knew were contacted and asked for input to be used in this publication. Unfortunately, only very few provided information that fitted formats which the author had prepared so that an unbiased comparison of data and technical details, publication of which does have the supplier’s approval, can be provided. Therefore, the latter idea was given up and those vendors that were or became known to the author while preparing the book are included in the following listing. Companies and individuals that have provided support beyond the normal are identified in the Acknowledgements and are referred to with gratitude in figure and table captions. To include also vendors, resources, and services in countries that are less well known in the western hemisphere, particularly Russia and the former Eastern Block countries, China and the countries of the Far East, India, the Near East, Africa, and South America, technical and trade organizations were contacted and asked to provide lists of vendors and specialists working in the field. Unfortunately, again, most did not even respond so that information was exclusively gleaned from a few freely available brochures and publications. It was most disappointing to the author that, with his limited possibilities and the language as well as accessibility problems in other continents, it was virtually impossible to adequately research and cover the undoubtedly vast resources of the “Eastern” countries (particularly Russia, China, and the Far East, excluding Japan). Since the topic of this book deals particularly with “agglomeration” it was found, however, that in Australia, India, the Near East, South America as well as Africa and in many smaller countries, sources of agglomeration equipment are primarily local subsidiaries or foreign and home office representatives of those that are mentioned in the list below. Other sources are international engineering companies, their local subsidiaries and representatives which are specifying and using European, North American, and Japanese equipment. The following listing is subdivided according to methods, technologies, resources and technologies. It was decided to list vendors that are active in different field in each ofthe classifications to facilitate the search if a particular method has been preselected, for example, by methods that were described in Section 11.1. (The author of this book is working on another, complementary publication which is tentatively entitled: ‘?4gglomeration Technologies - Industrial Applications”, WileyVCH (2003) LB.711; see also Section 13.4. Suppliers of equipment, technologies, and services in the field are invited to submit information to the authorfor possible inclusion in theforthcoming book.)
14. I List of Vendors
The entries are organized according to the following “Tab. of Contents”: pages Growth/Tumble Agglomeration - Disc (Pan)/Drum/Mixer - Fluid Bed Spray Nozzles and Systems - Agglomeration in Suspensions Pressure Agglomeration - Low Pressure Extrusion Spheronizing - Medium Pressure Extrusion (Pelleting) - High Pressure Extrusion (Ram Presses, Extruders) - High Pressure Agglomeration (Punch-and-Die, Tabletting, Isostatic) - High Pressure Agglomeration (Roll) Sintering Coating Melt Solidijcation Applications Binders Test Equipment and Peripherals Organizations Tollers
Crowth/Tumble Agglomeration
Disc (Pan)/Drum/Mixer
Paul 0. AbbC, Inc. 139 Center Avenue Little Falls, NJ 07424, USA
Tel.: +1-973- 256- 4242 Fax.: +1- 973- 256- 0041
Aeromatic-Fielder Div. Niro Inc. 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1- 410- 997- 7070 Fax.: +1- 410- 997- 5021
Allgaier-Werke GmbH &L Co. KG Bereich Sieb- und Aufbereitungstechnik Ulmerstr. 75 D-73066 Uhingen, Germany
Tel.: +49- (0)7161- 301- 313 Fax.: +49- (0)7161- 301- 440
Hosokawa Bepex Corporation 333 N.E. Taft Street Minneapolis, MN 55413, USA
Tel.: +1- 612- 331- 4370 Fax.: +1- 612- 627- 1444
545 545 549 55 1 552 552 552 553 554 554 556 559 561 562 563 564 565 567 574 575
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Hermann Berstofl Maschinenbau GmbH An der breiten Wiese 3 - 5 D-30625 Hannover, Germany
Tel.: +49- (0)511-5702- 0 Fax.: +49- (0)511- 561916
L.B. Bohle, Inc. 1504 Gmndy Lane Bristol, PA 19007, USA
Tel.: +1- 215- 785- 1121 Fax.: +1- 215- 785- 1221
H.C. Davis Sons Mfg. Co., Inc. Box 395 Bonner Springs, KS 66012, USA
Tel.: +1- 913- 422- 3000 Fax.: +1- 913- 422-7220
Dierks & Sohne, GmbH & Co, KG. (DIOSNA) Sandbachstr. 1 D-49074 Osnabriick, Germany
Tel.: +49- (0)541- 3310- 0 Fax.: +49- (0)541- 3310- 410
Draiswerke GmbH Speckweg 43 - 51 D-68305 Mannheim, Germany
Tel.: +49- (0)621- 7504- 00 Fax.: +49- (0)621- 7504- 233
Maschinenfabrik Gustav Eirich Walldurner Str. 50, Postfach 1160 D-74736 Hardheim, Germany
Tel.: +49- (0)6283- 51- 310 Fax.: +49- (0)6283- 51- 304
Eirich Machines, Inc. American Process Systems Div. Delany Business Center 4033 Ryan Rd. Gurnee, IL 60031, USA
Tel.: +1- 847- 336- 2444 Fax.: +1- 847- 336- 0914
FEECO International 3913 Algoma Rd. Green Bay, WI 54311, USA
Tel.: +1- 920- 468- 1000 Fax.: +1- 920- 469- 5110
Palex Corp. (Fukae Powtec Corp) P.O. Box 65 Tajimi, Gifu-Pref. 507-0033, Japan
Tel.: +81- (0)572- 229152 Fax.: +81- (0)572- 242722
GEMCO The General Machine Company of New Jersey 301 Smalley Avenue Middlesex, NJ 08846, USA
Tel.: +1- 908- 752- 7900 Fax.: +1- 908- 752- 5857
74.1 List of Vendors
Hayes & Stolz Ind. Mfg. Co., Inc. 3521 Hemphill Street Fort Worth, TX 76110,USA
Tel.: +1- 817-926-3391 Fax.: +1- 817-926-4133
Henschel Mixers America, Inc. P.O. Box 800607 Houston, TX 77280-0607, USA
Tel.: +1-713-690-3333 Fax.: +1- 713-690-3353
Hosokawa Micron Powder Systems 10 Chatham Road Summit, NJ 07901,USA
Tel.: +1-908-273-6360 Fax.: +1- 908-273-7432
Italvacuum Srl. (Criox) Via Stroppiana 3 1-10071Borgaro (Turin), Italy
Tel.: +39-11- 470-4651 Fax.:+39-11-470-1010
Jaygo, Inc. 675 Rahway Ave. Union, NJ 07083,USA
Tel.: +1-908-688-3600 Fax.: +1-908-688-GO60
Brian Kaye Associates Ltd.
30 Courtney Hill Sudbury, Ont. P3E 5W5,Canada
Tel.: +1- 705-688-1432 Fax.: +1- 705-688-1432
Key International, Inc.
480 Route 9 Englishtown, NJ 07726,USA
Tel.: +1-908-536-1500 Fax.: +1- 908-972-2630
Littleford Day, Inc. 7451 Empire Drive Florence, KY 41042-2985, USA
Tel.: +1- 606- 525-7600 Fax.: +1-606- 525-1446
Gebrtider Liidige Maschinenbau GmbH Elsener Str. 7-9 D-33102Paderborn, Germany
Tel.: +49-(0)5251309-0 Fax.: +49-(0)5251309-123
MAP S.R.L Via Cavour, 388/B 1-41030Ponte Motta, Cavezzo (MO), Italy
Tel.: +39-535-49911 Fax.: +39-535-49900
Mars Mineral P.O. Box 719 Mars, PA 16046,USA
Tel.: +1-724-538-3000 Fax.: +1- 724-538-5078
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C.G. Mozer GmbH & Co. KG Postfach 943 D-73009 Goppingen, Germany
Tel.: +49- (0)7161- 6735- 0 Fax.: +49- (0)7161- 6735- 35
MTI, Mixing Technology, Inc. 3303 FM 1960 West, Suite 490 Houston, TX 77068, USA
Tel.: +1- 281- 583- 8610 Fax.: +1- 281- 583- 0190
Munson Machinery Co., Inc. P.O. Box 855, 210 Seward Ave. Utica, NY 13503-08555, USA
Tel.: +1- 315- 797- 0090 Fax.: +1- 315- 797- 5582
Nara Machinery Co., Ltd. 5-7, 2-chome, Jonan-Jima Ohta-ku, Tokyo 143, JAPAN
Tel.: +81- (0)3- 3799- 5011 Fax.: +81- (0)3- 3790- 8055
Nara Zweigniederlassung Europa Europaallee 46 D-50226 Frechen, Germany
Tel.: +49- (0)2234- 23063 Fax.: +49- (0)2234- 23067
GEA/NIRO A/S Gladsaxevej 305, P.O. Box 45 DK-2860 Soborg, Denmark
Tel.: +45- 3954- 5454 Fax.: +45- 3954- 5800
NIRO, Inc. 9165 Rumsey Road Columbia, MD 21045, USA
Tel.: +1-410- 997- 8700 Fax.: +1- 410- 997- 5021
Patterson-Kelley Co. 100 Burson Street P.O. Box 458 East Stroudsburg, PA 18301, USA
Tel.: +1-570- 421- 7500 Fax.: +1- 570- 421- 8735
Phlauer, A&J Mixing International, Inc. 8-2345 Wyecroft Road Oakville, Ont. L6L 6L4, Canada
Tel.: +1- 905- 827- 7288 Fax.: +1- 905- 827- 5045
Processall, Inc. 10596 Springfield Pike Cincinnati, OH 45215, USA
Tel.: +1- 513- 771- 2266 Fax.: +1- 513- 771- 6767
Robot Coupe USA, Inc. Scientific-Industrial Division P.O. Box 16627 Jackson, MS 39236-6627, USA
Tel.: +1- 601- 956- 3216 Fax.: +1- 601- 956- 5758
74.7 List of Vendors
ROMACO, Inc. 104 American Road Morris Plains, NJ 07950, USA
Tel.: +1-973- 605- 5370 Fax.: +1- 973- 605- 1360
Charles Ross and Son Co. 710 Old Willets Path Hauppauge, NY 11788, USA
Tel.: +1-631- 234- 0500 Fax.: +1- 631- 234- 0691
The A.J. Sackett & Sons Co. 1701 South Highland Ave. Baltimore, MD 21224, USA
Tel.: +1- 301- 276- 4466 Fax.: +1- 301- 276- 0241
Hosokawa Schugi B.V. Chroomstraat 29 NL-8211 AS Lelystad, Netherlands
Tel.: +31- (0)320- 28 66 66 Fax.: +31- (0)320- 24 47 94
Sejong Machinery Co., Ltd. #159-11 Dodang-dong Wonmi-ku 420-130 Puchon-city Kyunggi-do, Korea
Tel.: +82- 32- 672- 7811/2 Fax.: +82- 32- 672- 7813
Vector Corporation 675 44th Street Marion, IA 52302, USA
Tel.: +1- 319- 377- 8263 Fax.: +1- 319- 377- 5574
Zanchetta & C. S.R.L (see also Key/ROMACO) Via della Contea, 24 1-55010 S. Salvatore (Lucca), Italy
Tel.: +39- (0)583- 934626 Fax.: +39- (0)583- 217317
ZE'ITL GmbH & Co. KG Oldenbourgstr. 11 D-81247 Munchen, Germany
Tel.: +49- (0)89- 81809- 0 Fax.: +49- (0)89- 81809- 55
Fluid Bed
Aeromatic Ltd. (member GEA/NIRO) Hauptstr. 145 CH- 4416 Bubendorf, Switzerland
Tel.: +41- (0)Gl-931- 2575 Fax.: +41- (0)Gl- 931- 2678
Aeromatic-Fielder Div. Niro Inc. (NICA) 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1- 410- 997- 7070 Fax.: +1-410- 997- 5021
Applied Chemical Technology, Inc. (ACT) 4350 Helton Drive Florence, AL 35630, USA
Tel.: +1-256- 760- 9600 Fax.: +1- 256- 760- 9638
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Allgaier Werke GmbH & Co. KG Ulmerstr. 75 D-73066 Uhingen, Germany
Tel.: +49- (0)7161- 301- 0 Fax.: +49- (0)7161- 34268
Allgaier Verfahrenstechnik GmbH A-4492 Hofkirchen 93, Austria
Tel.: +43- 45 67 22 59 01 25 Fax.: +43- 72 25 64 23
AMMAG Dahlienstr. 11 A-4623 Gunskirchen. Austria
Tel.: +43- 7246- 6408- 0 Fax.: +43- 7246- 6408- 39
APV Anhydro AS 0stmarken 7 DK-2860 Soborg, Copenhagen, Denmark
Tel.: +45- 3969- 2811 Fax.: +45- 3969- 3880
APV Anhydro 182 Wales Avenue Tonawanda, NY 14150, USA
Tel.: +1- 716- 692- 3000 Fax.: +1- 716- 692- 6416
Babcock-BSH GmbH August Gottlieb Str. 5 D-36222 Bad Hersfeld, Germany
Tel.: +49- (0)6621- 81449 Fax.: +49- (0)6621- 81393
DMR Prozesstechnologie Rinaustr. 380 CH-4303 Kaiseraugust, Switzerland
Tel.: +41- 61- 813- 10- 60 Fax.: +41- 61- 813- 10- 62
Fuji Paudal Co. Ltd. 2-30, 2-chome, Chuoh Joto, Osaka 536-0005, Japan
Tel.: +81- 06- 933- 1511 Fax.: +81- 06- 933- 1531
Glatt GmbH Process Technologie Buhlmiihle D-79589 Binzen, Germany
Tel.: +49- (0)7621- 664- 0 Fax.: +49- (0)7621- 647- 23
Glatt Air Technique, Inc. 20 Spear Road Ramsey, NJ 07446, USA
Tel.: +1- 201- 825- 8700 Fax.: +1- 201- 825- 0389
A. Heinen AG Anlagenbau Achternstr. 1-17 D-26316 Varel, Germany
Tel.: +49- (0)4451- 122- 0 F a . : +49- (0)4451- 122- 159
14. 1 List of Vendors
BWI Huttlin Daimlerstr. 7 D-79585 Steinen, Germany
Tel.: +49- (0)7627- 9117- 0 Fax.: +49- (0)7627- 8851
GEA/NIRO A/S Gladsaxevej 305, P.O. Box 45 DK-2860 Soborg, Denmark
Tel.: +45- 3954- 5454 Fax.: +45- 3954- 5800
NIRO, Inc. 9165 Rumsey Road Columbia, MD 21045, USA
Tel.: +1-410- 997- 8700 Fax.: +1-410- 997- 5021
NIRO, Inc. (Food & Dairy Industries) 1600 O’Keefe Road Hudson, WI 54016, USA
Tel.: +1-715- 386- 9371 Fax..: +1- 715- 386- 9376
Pulse Combustion Systems 135 Eye Street, Suite B San Rafael, CA 94901, USA
Tel.: +1-415- 457- 6500 Fax.: +1- 415- 723- 3727
SprayDryConsult Int’l. ApS Krathusparken 2 DK-2920 Charlottenlund, Denmark
Tel.: +45- (0)3964- 5030 Fax.: +45- (0)3964- 6050
Vector Corporation 675 44th Street Marion, IA 52302, USA
Tel.: +1-319- 377- 8263 Fax.: +1-319- 377- 5574
Spray Nozzles and Systems
Bete Fog Nozzle, Inc. P.O. Box 1438, 50 Greenfield Street Greenfield, MA 01302-1438, USA
Tel.: +1-413- 772- 2166 +1-413- 772- 0846 Fax.: +1-413- 772- 6729
BEX, Inc., Spray Nozzles 37709 Schoolcraft Rd. Livonia, MI 48150-1009, USA
Tel.: +1-734- 464- 8282 Fax.: +1- 734- 464- 1988
Lechler GmbH & Co. KG P.O. Box 1323 D-72544 Metzingen, Germany
Tel.: +49- (0)7123- 962- 0 Fax.: +49- (0)7123- 962- 333
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Lechler, Inc. 445 Kautz Road St. Charles, IL 60174, USA
Tel.: +1-630- 377- 6611 Fax.: +1- 630- 377- 6657
Processall, Inc. 10596 Springfield Pike Cincinnati, OH 45215, USA
Tel.: +1- 513- 771- 2266 Fax.: +1- 513- 771- 6767
Spray Dynamics 108 Bolte Lane St. Claire, MO 63077, USA
Tel.: +1-314- 629- 7366 Fax.: +1-314- 629- 7455
Spraying Systems Co. P.O. Box 7900 Wheaton, IL 60189-7900, USA
Tel.: +1-630- 665- 5000 Fax.: +1-630- 260- 0842
Agglomeration in Suspensions
Dr. Ed Capes/ Dr. Ken Darcovich National Research Council/NRC-CNRC Chemical Division/ICPET M 12-15, Montreal Road Ottawa, Ontario K1A OR6, Canada
Tel.: +1- 613- 993- 6848 Fax.: +1-613- 941- 2529
EIMCO, Div. of Baker Hughes Dillenburger Str. 100 D-51105 Koln, Germany
Tel.: +49- (0)221- 9856- 0 Fax.: +49- (0)221- 9856- 102
Baker Hughes Co. 100 Neponset Street South Walpole, MA 02071, USA
Tel.: +1- 508- 668- 0400 Fax.: +1-508- 668- 6855
Pressure Agglomeration
Low Pressure Extrusion
Aeromatic-Fielder Div. Niro Inc. (NICA) 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1- 410- 997- 7070 Fax.: +1-410- 997- 5021
Alexandenverk AG Kippdorfstr. 6-24 D-42857 Remscheid, Germany
Tel.: +49- (0)2191- 795- 216 Fax.: +49- (0)2191- 795- 350
14.1 List of Vendors
Alexanderwerk, Inc. 415 Sargon Way, Unit H Horsham, PA 19044, USA
Tel.: +1- 215- 442- 0270 Fax.: +1- 215- 442- 0271
Hosokawa Bepex Corporation 333 N.E. Taft Street Minneapolis, M N 55413, USA
Tel.: +1- 612- 627- 1412 Fax.: +1- 612- 627- 1444
Hosokawa BEPEX GmbH (HUTT) Postfach 1152 / Daimlerstr. 8 D-74207 Leingarten, Germany
Tel.: +49- (0)7131- 907-0 Fax.: +49- (0)7131- 907-301
Caleva Process Solutions Ltd. Butts Pond Industrial Estate Sturminster Newton, Dorset DTlO l A Z , England
Tel.: +44- (0)1258- 471122 Fax.: +44- (0)1258- 471133
Fuji Paudal Co. Ltd. 2-30, 2-chome, Chuoh Joto, Osaka 536-0005, Japan
Tel.: +81- 06- 933- 1511 Fax.: +81- 06- 933- 1531
WLS GABLER Maschinenbau KG Nobelstr. 16 a D-76275 Ettlingen, Germany
Tel.: +49- (0)7243- 5431- 0 Fax.: +49- (0)7243- 5431-54
LCI Corporation P.O. Box 16348 Charlotte, NC 28297-8804, USA
Tel.: +1- 704- 394- 9474 Fax.: +1- 704- 392- 8507
Spheronizing
Aeromatic-Fielder Div. Niro Inc. (NICA) 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1-410- 997- 7070 Fax.: +1-410- 997- 5021
Caleva Process Solutions Ltd. Butts Pond Industrial Estate Sturminster Newton, Dorset DTlO lAZ, England
Tel.: +44- (0)1258- 471122 Fax.: +44-(0)1258- 471133
Fuji Paudal Co. Ltd. 2-30, 2-chome, Chuoh Joto, Osaka 536-0005, Japan
Tel.: +81- 06- 933- 1511 Fax.: +81- 06- 933- 1531
I
553
554
I
7 4 Indexes
WLS GABLER Maschinenbau KG Nobelstr. 16 a D-76275 Ettlingen, Germany
Tel.: +49- (0)7243- 5431- 0 Fax.: +49- (0)7243- 5431-54
LCI Corporation P.O. Box 16348 Charlotte, NC 28297-8804, USA
Tel.: +1- 704- 394- 9474 Fax.: +1-704- 392- 8507
Medium Pressure Extrusion (Pelleting)
Andritz, Inc. (Sprout-Waldron-Bauer-Matador) 35 Sherman Street Muncy, PA 17756, USA
Tel.: +1- 570- 546- 8211 Fax.: +1- 570- 546- 1306
Buhler AG CH-9240 Uzwil, Switzerland
Tel.: +41- (0)71- 955- 1111 Fax.: +41- (0)71- 955- 3379
Buhler Inc. Box 9497 Minneapolis, MN 55440, USA
Tel.: +1- 612- 545-1401 Fax.: +1- 612- 540- 9296
California Pellet Mill Co. 1114 E. Wabash Avenue Crawfordsville, I N 47933, USA
Tel.: +1- 765- 362- 2600 Fax.: +1- 765- 362- 7551
CPM (California Pellet Mill) Roskamp Champion 2975 Airline Circle Waterloo, IA 50703, USA
Tel.: +1-319- 232- 8444 Fax.: +1- 319- 236- 0481
Amandus Kahl Nachf. Dieselstr. 5 / P.O. Box 1246 D-21465 Reinbek b. Hamburg, Germany In USA see: LCI, Corp.
Tel.: +49- (0)40-72771- 0 Fax.: +49- (0)40-72771- 100
High Pressure Extrusion (Ram Presses, Extruders)
Buhler AG CH-9240 Uzwil, Switzerland
Tel.: +41- (0)71- 955- 1111 Fax.: +41- (0)71- 955- 3379
Buhler Inc. Box 9497 MinneaDolis. M N 55440. USA
Tel.: +1- 612- 545- 1401 Fax.: +1-612- 540- 9296
1
,
74. I List of Vendors
The Bonnot Co. 1520 Corporate Woods Pkwy. Uniontown, OH 44685, USA
Tel.: +1- 330- 896- 6544 Fax.: +1- 330- 896- 0822
ENTEX Fust & Mitschke GmbH Heinrichstr. 67 D-44805 Bochum, Germany
Tel.: +49- (0)234- 85636 Fax.: +49- (0)234- 85638
Handle GmbH Industriestr. 47 D-75417 Miihlacker, Germany
Tel.: +49- (0)7041- 891- 1 Fax.: +49- (0)7041- 821- 232
Krupp Werner & Pfleiderer 663 East Crescent Avenue Ramsey, NJ 07446, USA
Tel.: +1- 201- 372- 6300 Fax.: +1- 201- 825- 6494
Krupp Fordertechnik GmbH Altendorfer Str. 120 D-45143 Essen, Germany
Tel.: +49- (0)201- 828- 04 Fax.: +49- (0)201- 828- 2566
Svedala Lindemann GmbH Erkrather Str. 401 D-40231 Diisseldorf, Germany
Tel.: +49- (0)211- 2105- 0 Fax.: +49- (0)211- 2105- 376
List AG CH-4422 Arisdorf. Switzerland
Tel.: +41- (0)Gl- 811- 3000 Fax.: +41- (0)61- 811- 3555
List, Inc. 42 Nagog Park Action, MA 01720, USA
Tel.: +1- 978- 635- 9521 Fax.: +1- 978- 263- 0570
Readco Manufacturing, Inc. 901 S. Richland Avenue York, PA 17405-0552, USA
Tel.: +1-717- 848- 2801 Fax.: +1- 717- 848- 2813
Spanex BHSU Luft- und Umwelttechnik GmbH Otto-Brenner-Str. 6 D-37170 Uslar, Germany
Tel.: +49- (0)5571- 304- 0 Fax.: +49- (0)5571- 304- 111
J.C. Steele & Sons, Inc. 715 S. Mulberry Street, Box 951 Statesville, NC 28677, USA
Tel.: +1- 704- 872- 3681 Fax.: +1- 704- 878- 0789
I
555
556
I
74 Indexes
Theysohn Maschinenbau GmbH J.-F. Kennedy Str. 48 D-38228 Salzgitter, Germany
Tel.: +49- (0)5341- 551- 110 Fax.: +49- (0)5341- 551- 177
ZEMAG GmbH Paul Rohland-Str. 1 D-06712 Zeitz, Germany
Tel.: +49- (0)3441- 880- 205 Fax.: +49- (0)3441- 212993
High-pressure Agglomeration (Punch-and-Die, Tabletting, Isostatic)
AIP American Isostatic Presses, Inc. 1205 South Columbus Airport Road Columbus, OH 43207, USA
Tel.: +1- 614- 497- 3148 Fax.: +1- 614- 497- 3407
Best Press Corp. 102 Crowatan Road, Northside Industrial Park Castle Hayne, NC 28429, USA
Tel.: +1- 910- 675- 2429 Fax.: +1-910- 675- 1395
CAPPlus Technologies, Inc 21622 N. 7th Avenue #7 Phoenix, AZ 85027, USA
Tel.: +1- 623- 582- 2800 Fax.: +1- 623- 582- 4099
Carver Inc. 1569 Morris Street Wabash, IN 46992-0544, USA
Tel.: +1- 219- 563- 7577 FLU.: +1- 219- 563- 7625
GEI Courtoy N.V. Bergensesteenweg 186 B-1500 Halle, Belgium
Tel.: +32- (0)2- 3638300 Fax.: +32- (0)2- 3560516
Dorst Maschinen & Anlagenbau GmbH & Co., KG Tel.: +49- (0)88Sl- 188- 0 Mittenwalder Str. 61 D-82431 Kochel a. See, Germany Fax.: +49- (0)8851- 188- 310
Elizabeth Carbide Die Co., Inc. 601 Linden Street, PO Box 95 McKeesport, PA 15135, USA
Tel.: +1- 412- 751- 3000 Fax.: +1- 412- 754- 0755
Elizabeth Carbide Europe NV Av. du roi Albert 134 B-1082 Bmelles, Belgium
Tel.: +32 - (0)2-46900- 30 Fax.: +32- (0)2- 46900- 1 5
14. J List of Vendors I 5 5 7
Elizabeth - Hata International, Inc. 14559 Route 30, 101 Peterson Drive North Huntingdon, PA 15642, USA
Tel.: +1-412- 829- 7700 Fax.: +1-412- 829- 9345
EPSI, Engineered Pressure Systems, Inc. 165 Ferry Road Haverhill, MA 01835, USA
Tel.: +1-978- 469- 8280 Fax.: +1-978- 373- 5628
EPSI, Engineered Pressure Systems International NV Tel.: +32- (0)3- 711- 2464 Walgoed Straat 19 B-9140 Temse, Belgium Fax.: +32- (0)3- 711- 1870 Wilhelm Fette GmbH Grabauer Str. 24 D-21484 Schwarzenbek, Germany
Tel.: +49- (0)4151- 12-0 Fax.: +49- (0)4151- 3797
Fette America, Inc. 400 Forge Way Rockaway, NJ 07866, USA
Tel.: +1-973- 586- 8722 Fax.: +1-973- 586- 0450
Flow Pressure Systems AB SE-721 66 Vaster&, Sweden
Tel.: +46- 21- 32- 7000 Fax.: +46- 21- 14- 1817
Flow Autoclave Systems, Inc. 3721 Corporate Drive Columbus, OH 43231, USA
Tel.: +1- 614- 891- 2732 Fax.: +1-614- 891- 4568
Gasbarre Products, Inc. 590 Division Street Dubois, PA 15801, USA
Tel.: +1- 814- 371- 3015 Fax.: +1-814- 371- 6387
Horn & Noack, Pharmatechnik ROMACO GmbH Am Heegwald 11 D-76229 Karlsruhe, Germany
Tel.: +49- (0)721- 4804- 0 Fax.: +49- (0)721- 4804- 225
I.M.A. SPA Via Emilia 428-442 1-40064 Ozzano Emilia (BO), Italy
Tel.: +39- (0)51- 651- 4111 Fax.: +39- (0)51- 651- 4666
Key International, Inc. 480 Route 9 Englishtown, NJ 07726, USA
Tel.: +1- 201- 536- 1500 Fax.: +1-201- 972- 2630
558
I
74 lndexes
Kilian & Co., GmbH Emdener Str. 10 D-50735Koln, Germany
Tel.: +49-(0)2217174-02 Fax.: +49-(0)2217174-110
Kilian & Co, Inc. 415 Sargon Way, Unit 1 Horsham, PA 19044,USA
Tel.: +1- 215-957-1871 Fax.: +1-215-957-1874
Kikusui Seisakusho Ltd. 104,Minamikamiai-cho Nishinokyo, Nakagyo-ku Kyoto, 604,Japan
Tel.: +81- (0)75841- 6326 Fax.: +81- (0)75803-2077
KOMAGE Gellner GmbH & Co. Maschinenfabrik KG Dr. Hermann-Gellner Str. 1 D-54427Kell am See, Germany
Tel.: +49-(0)65899142-0 Fax.: +49-(0)65899142-19
Korsch Pressen GmbH Breitenbachstr. 1 D-13509Berlin, Germany
Tel.: +49-(0)3043576-0 Fax.: +49-(0)3043576-350
Krupp Fordertechnik GmbH Altendorfer Str. 120 D-45143Essen, Germany
Tel.: +49-(0)201828-04 Fax.: +49-(0)201828-2566
Laeis Bucher GmbH Schiffstr. 3 D-54293Trier, Germany
Tel.: +49-(0)6519492-0 Fax.: +49-(0)6519492-200
BWI Manesty Evans Road, Speke Liverpool L24 9LQ,Great Britain
Tel.: +44-(0)151- 486-1972 Fax.: +44-(0)151-486-5639
Pentronix, Inc. (PTX) 1737 Cicotte Lincoln Park, MI 48146,USA
Tel.: +1-313- 388-3100 Fax.: +1- 313- 388-9171
Pneumafill P.O. Box 16348 Charlotte, NC 28297-8804, USA
Tel.: +1-704-399-7441 Fax.: +1- 704-393-2758
14. I List of Vendors
Riva S.A. Libertador San Martin 431 1702 Ciudadela, Pcia. Buenos Aires, Argentina in USA: SMI P.O. Box 219 Whitehouse, NJ 08888, USA
Tel.: +1- 908- 534- 1500 Fax.: +1-908- 543- 1546
Ruf GmbH & Co KG Tussenhausener Str. G D-86874 Zaisertshofen, Germany
Tel.: +49- (0)8268- 9090- 0 Fax.: +49- (0)8268- 9090- 90
SAMA Machinenbau GmbH Schillerstr. 21 D-95136 Weissenstadt, Germany
Tel.: +49- (0)9253- 8890 Fax.: +49- (0)9253- 1079
Sejong Machinery Co., Ltd. 159-11 Dodang Dong, Wonmi-Gu Buchun-City, Kyunggi-Do, Korea 421-130
Tel.: +82- 32- 672- 781112 Fax.: +82- 32- 672- 7813
DT Industries, Stokes Division 1500 Grundy’s Lane Bristol, PA 19007, USA
Tel.: +1- 215- 788- 3500 Fax.: +1-215- 781- 1122
Tel.: +54- 1- 653- 870518392 y 488- 5181 Fax.: +54- 1- 441- 7142
Paul-Otto Weber, Maschinen-Apparatebau GmbH Fuhrbachstr. 4 - 6 Tel.: +49- (0)7151- 72600 Fax.: +49- (0)7151- 72509 D-73630 Remshalden, Germany High-Pressure Agglomeration (Roll)
Alexanderwerk AG Kippdorfstr. 6 - 24 D-42857 Remscheid, Germany
Tel.: +49- (0)2191- 795- 216 Fax.: +49- (0)2191- 795- 350
Alexanderwerk, Inc. 415 Sargon Way, Unit H Horsham, PA 19044, USA
Tel.: +1-215- 442- 0270 Fax.: +1- 215- 442- 0271
Hosokawa BEPEX GmbH (HUTT) Postfach 1152 1 Daimlerstr. 8 D-74207 Leingarten, Germany
Tel.: +49- (0)7131- 907- 0 Fax.: +49- (0)7131- 907- 301
Hosokawa Bepex Corporation 333 N.E. Taft Street Minneapolis, M N 55413, USA
Tel.: +1- 612- 627- 1412 Fax.: +1- 612- 627- 1444
I
559
560
I
14 lndexes
The Fitzpatrick Company 832 Industrial Drive Elmhurst, IL 60126, USA
Tel.: +1-630- 530- 3333 Fax.: +1-630- 530- 0832
Gerteis Maschinen- + Processengineering AG Stampfstr. 74 CH-8645 jona, Switzerland
Tel.: +41- (0)55- 212- 1121 Fax.: +41- (0)55- 212- 1140
K.R. Komarek, Inc. 1825 Estes Avenue Elk Grove Village, IL 60007, USA
Tel.: +1-847- 956- 0060 Fax.: +1-847- 956- 0157
Maschinenfabrik KOPPERN GmbH & Co. KG Konigsteinerstr. 2 - 12 D-45529 Hattingen/Ruhr, Germany
Tel.: +49- (0)2324- 297- 0 Fax.: +49- (0)2324- 207- 207
Koppern Equipment, Inc. 2201 Water Ridge Parkway Charlotte, NC 28217, USA
Tel.: +1- 704- 357- 3322 Fax.: +1-704- 357- 3350
Ludman Machine Co., LLC. S. 82 W. 18664 Gemini Dr. Muskego, WI 53150, USA
Tel.: +1-414- 679- 3120 Fax.: +1- 414- 679- 9272
Lewis Corporation 15134 West Hunziker Pocatello, ID 83202, USA
Tel.: +1- 208- 237- 1314 Fax.: +1- 208- 238- 1834
Matsubo Co., Ltd. 8-21 Toranomon 3-chome Minato-Ku, Tokyo, 105-0001, Japan
Tel.: +81-3- 5472- 1733 Fax.: +81- 3- 5472- 1730
Powtec Maschinen und Engineering GmbH Berghauserstr. 62 D-42859 Remscheid, Germany
Tel.: +49- (0)2191- 389- 194 Fax.: +49- (0)2191- 389- 196
Prater Industries, Inc. 1515 South 55th Court Cicero, IL 60804, USA
Tel.: +1-708- 656- 8500 Fax.: +1- 708- 656- 8576
14.7 List of Vendors
Riva S.A. Libertador San Martin 431 1702 Ciudadela, Pcia. Buenos Aires, Argentina in USA: SMI P.O. Box 219 Whitehouse, NJ 08888, USA
Tel.: +1-908- 534- 1500 Fax.: +1- 908- 543- 1546
Sahut Conreur S.A. 700 Rue Corbeau, BP 49 F-59590 Raismes, France
Tel.: +33- (0)3-27- 46 90 44 Fax.: +33- (0)3- 27- 29 97 65
Turbo Kogyo Co., Ltd. 2-10, Uchikawa I-chome Yokosuka-Shi, Kanagawa, 239-0836, Japan
Tel.: +81- (0)468- 36- 4900 Fax.: +81- (0)468- 35- 6516
Vector Corporation 675 44th Street Marion, IA 52302, USA
Tel.: +1-319- 377- 8263 Fax.: +1- 319- 377- 5574
ZEMAG GmbH Paul Rohland-Str. 1 D-06712 Zeitz, Germany
Tel.: +49- (0)3441- 880- 205 Fax.: +49- (0)3441- 212993
Tel.: +54- 1- 653- 870518392 y 488- 5181 Fax.: +54- 1- 441-7142
Sintering
Deltech, Inc. 750 W. 39th Ave. Denver, CO 80216, USA
Tel.: +1- 303- 433- 5939 Fax.: +1- 303- 433- 2809
Eisenmann Maschinenbau KG Postfach 1280 D-71002 Boblingen, Germany
Tel.: +49- (0)7031- 78- 0 Fax.: +49- (0)7031- 78- 1000
Eisenmann Corp. USA 150 East Dartmoor Drive Crystal Lake, IL 60014, USA
Tel.: +1-815- 455- 4100 Fax.: +1- 815- 455- 1018
Fuller Company Member of the F.L. Smidth-Fuller Engineering Group 2040 Avenue C Tel.: +1-610- 264- 6011 Bethlehem, PA 18017-2188, USA Tel.: +1-610- 264- 6170
I
561
562
I
' 4 Indexes
Gasbarre Products, Inc. 590 Division Street Dubois, PA 15801, USA
Tel.: +1- 814- 371- 3015 Fax.: +1- 814- 371- 6387
Gasbarre Sinterite Furnace Div. 310 State Road St. Marys, PA 15857, USA
Tel.: +1-814- 834- 2200 Fax.: +1-814- 834- 9335
L&L Special Furnace Co., Inc. 20 Kent Road Aston, PA 19014-1494, USA
Tel.: +1- 610- 459- 9216 Fax.: +1-610- 459- 3689
Lurgi Metallurgie GmbH Ludwig-Erhard-Str. 21 D-61408 Oberursel, Germany
Tel.: +49- (0)69693- 0 Fax.: +49- (0)69693- 1234
HED International, Unique/Pereny 449 Route 31 Ringoes, NJ 08551, USA
Tel.: +1-609- 466- 1900 Fax.: +1- 609- 466- 3608
Coating
Aeromatic-Fielder Div. Niro Inc. (NICA) 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1- 410- 997- 7070 Fax.: +1- 410- 997- 5021
AVEKA, Inc. 2045 Wooddale Drive, Bldg. 553-C Woodbury, MN 55125, USA
Tel.: +1- 651- 730- 1729 Fax.: +1- 651- 730- 1826
Dinnissen bv Horstenveg 66 NL-5975 NB Sevenum, Netherlands
Tel.: +31- 77- 467- 3555 Fax.: +31- 77- 467- 3785
DRIAM Metallprodukt GmbH & Co, KG Aspenweg 19- 21 D-88097 Eriskirch a. Bodensee, Germany
Tel.: +49- (0)7541- 9703- 0 Fax.: +49- (0)7541- 9703- 10
Fluid Air, Inc. 2550 White Oak Circle Aurora, I L 60504-9678, USA
Tel.: +1- 630- 851- 1200 Fax.: +1- 630- 851- 1244
14.7 List of Vendors
GS Coating Systems Via Friuli 38/40 1-40060 Osteria Grande (Bologna), Italy
Tel.: +39- (0)51- 94- 6608 Fax.: +39- (0)51- 94- 5624
BWI Hiittlin Daimlerstr. 7 D-79585 Steinen, Germany
Tel.: +49- (0)7627- 9117- 0 Fax.: +49- (0)7627- 8851
IMA Solid Dose Div., Kilian & Co., Inc. 415 Sargon Way, Unit 1 Horsham, PA 19044, USA
Tel.: +1-215- 957- 1871 Fax.: +1- 215- 957- 1874
Kaltenbach-Thiiring 9, rue de 1’Industrie F-60000 Beauvais, France
Tel.: +33- 44- 02- 8900 Fax.: +33- 44- 02- 8910
LMC (Latini) International 893 Industrial Drive Elmhurst, IL 60126, USA
Tel.: +1- 630- 834- 7789 Fax.: +1- 630- 834- 9473
O’Hara Technologies 65 Skagway Ave. Toronto, Ont. M1M 3T9, Canada
Tel.: +1-416- 265- 1800 Fax.: +1- 416- 265- 6658
Sandvik Process Systems, Inc. 21 Campus Road Totowa, NJ 07512, USA
Tel.: +1- 201- 812- 1066 Fax.: +1- 201- 812- 0733
Thomas Engineering, Inc. 575 West Central Road Hoffmann Estates, IL 60195-0198, USA
Tel.: +1- 847- 358- 5800 Fax.: +1-847- 358- 5817
Trybuhl Dragiertechnik GmbH Obere Torstr. 20 D-37586 Dassel-Markoldendorf, Germany
Tel.: +49- (0)5562- 91101 Fax.: +49- (0)5562-91127
Vector Corporation 675 44th Street Marion, IA 52302, USA
Tel.: +1-319- 377- 8263 Fax.: +1-319- 377- 5574
Melt Solidification
Berndorf Band GesmbH A-2560 Berndorf, Austria
Tel.: +43- (0)2672- 2930 Fax.: +43- (0)2672-4176
I
563
564
I
74 Indexes
Berndorf ICB, Inc. 820 Estes Ave. Schaumburg, I L 60193, USA
Tel.: +1- 847- 891- 8650 Fax.: +1-847- 891- 7563
Gala Industries, Inc. 181 Pauley Street Eagle Rock, VA 24085, USA
Tel.: +1- 540- 884- 3160 Fax.: +1- 540- 884- 2310
Goudsche Machinefabriek B.V. Coenecoop 88 NL-2740 AJ Waddinxveen, Netherlands
Tel.: +31- 182- 623723 Fax.: +31- 182- 619217
Gebr. Kaiser, Chem Verfahrenstechnik Magdeburger Str. 17 D-47800 Krefeld, Germany
Tel.: +49- (0)2151-474051 Fax.: +49- (0)2151-474053
Kaltenbach-Thiiring 9, rue de 1’Industrie F-60000 Beauvais. France
Tel.: +33- 44- 02- 8900 Fax.: +33- 44- 02- 8910
Sandvik Process Systems, Inc. 21 Campus Road Totowa, NJ 07512, USA
Tel.: +1- 201- 812- 1066 Fax.: +1- 201- 812- 0733
Applications
Babcock-BSH GmbH August Gottlieb Str. 5 D-36222 Bad Hersfeld, Germany
Tel.: +49- (0)6621- 81449 Fax.: +49- (0)6621-81393
Dynamit Nobel GmbH Explosivstoff- und Systemtechnik Forschung und Entwicklung Kronacher Str. 63 D-90765 Furth, Germany
Tel.: +49- (0)911- 7930- 0 Fax.: +49- (0)911- 7930- 655
Fleissner GmbH & Co Wolfsgartenstr. 6 D-63329 Egelsbach, Germany
Tel.: +49- (0)6103- 401- 0 Fax.: +49- (0)6103-401- 440
Kellogg Co./W.K. Kellogg Institute for Food and Nutrition Research Tel.: +1-616- 961- 2000 2 Hamblin Ave. East Fax.: +1- 616- 660 6557 Battle Creek, MI 49016-3232, USA
74.7 List of Vendors
Norchem Concrete Products, Inc. 985 Seaway Drive Fort Pierce, FL 34949, USA
Tel.: +1-561- 468- 6110 Fax.: +1- 561- 468- 9702
NRS, National Recovery Systems 5222 Indianapolis Boulevard East Chicago, IN 46312, USA
Tel.: +1-219- 397- 0200 Fax.: +1-219- 392- 1419
Puritan Bennet Aero Systems 10800 Pflumm Road Lenexa, KS 66215, USA
Tel.: +1-913- 469- 5400 Fax.: +1-913- 469- 8419
Sintec Keramik GmbH Romantische Str. 18 D-87642 Buching, Germany
Tel.: +49- (0)8368- 9101- 0 Fax.: +49- (0)8368- 9101- 30
TDC Filter Manufacturing, Inc. 1331 S. 55th Court Cicero, I L 60804, USA
Tel.: +1-708- 863- 4400 Fax.: +1- 708- 863- 4472
Binders
Allied Colloids Cleckheaton Road Low Moor, Bradford West Yorkshire BD12 OJZ, UK
Tel.: +44- (0)124- 41700 Fax.: +44- (0)124- 606499
Borregaard Lignotech P.O. Box 31 NL-7213 ZG Gorssel, Netherlands P.O. Box 162 N-1701 Sarpsborg, Norway
Tel.: +31- (0)5759- 3488 Fax.: +31- (0)5759- 4575 Tel.: +47- (0)6911- 8000 Fax.: +47- (0)6911- 8790
Lignotech USA 100 Highway 51 South Rothschild, WI 54474-1198, USA
Tel.: +1- 715- 359- 6544 Fax.: +1- 715- 355- 3648
CABOT Corp., Cab-0-Sil Division 700 E. US Highway 36 Tuscola, IL 61953-9643, USA
Tel.: +1-217- 253- 9643 Fax.: +1-217- 253- 4334
DuPont (UK) Ltd. (Elveron@) P.O. Box 401 Wilton, Middlesbrough TSG 8JJ,England
Tel.: +44- (0)1642- 445521 Fax.: +44- (0)1642- 445510
I
565
566
I
74 Indexes
FMC Corp., Pharmaceutical Division 1735 Market Street Philadelphia, PA 19103, USA
Tel.: +1- 215- 299- 6534 Fax.: +1- 215- 299- 6821
GPC, Grain Processing Corp. 1600 Oregon Street Muscatine, IA 52761, USA
Tel.: +1- 319- 264- 4265 Fax.: +1- 319- 264- 4289
Green Wood Canada, Inc. 239 Russel Street, P.O. Box 2559 Sturgeon Falls, Ontario POH 2G0, Canada
Tel.: +1- 705- 753- 2822 Fax.: +1- 705- 753- 1270
Hoogovens Technical Services Wenkebachstraat 1 1951 J Z Velsen Noord, P.O. Box 10.000 NL-1970 CA Ijmuiden, Netherlands
Tel.: +31- (0)2514- 97847 Fax.: +31- (0)2514- 70030
Koch Minerals Company P.O. Box 2219 Wichita, KS 67201-2219, USA
Tel.: +1- 316- 832- 6662 Fax.: +1- 316- 832- 8028
Penwest Pharmaceuticals, Mendell 2981 Rt. 22 Patterson, NY 12563-9970, USA
Tel.: +1-914- 878- 3414 Fax.: +1- 914- 878- 3484
J. Rettenmaier & Sohne GmbH & Co. Faserstoff-Werke Holzmuhle 1 D-73494 Rosenberg, Germany
Tel.: +49- (0)7967- 152- 0 Fax.: +49- (0)7967- 152- 222
J. Rettenmaier USA LP Manufacturers of Fibers 16369 US Hwy. 131 Schoolcraft, MI 49087, USA
Tel.: +1-616- 679- 2340 Fax.: +1-616- 679- 2364
Reed Lignin, Inc. 100 Highway 51 South Rothschild, WI 54474-1198, USA
Tel.: +1- 715- 359- 6544 Fax.: +1-715- 355- 3648
RDE, Inc. 101 N. Virginia St. Crystal Lake, IL 60014, USA
Tel.: +1- 815- 459- 0470 Fax.: +1- 815- 439- 8043
74.7 List of Vendors
RIBTEC Ribbon Technology Corp. P.O. Box 30758 Gahanna, OH 43230, USA Schuurmans Sr van Ginneken Keizersgracht 534 NL-1017 EK Amsterdam, Netherlands Wyo Ben, Inc. 3044 Hesper Road, P.O. Box 1979 Billings, Montana 59103, USA
Tel.: +1-614- 864- 5444 Fax.: +1- 614- 864- 5305
Tel.: +31- 20- (0)626- 0711
Tel.: +1-406- 652- 6351 Fax.: +1- 406- 656- 0748
Test Equipment and Peripherals
AC Compacting LLC 1577 Livingston Ave. North Brunswick, NJ 08902-7266, USA
Tel.: +1- 732- 249- 6900 Fax.: +1-732- 249- 6909
Schenck AccuRate Corp. 746 E. Milwaukee St. Whitewater, WI 53190, USA
Tel.: +1-262- 473- 2441 Fax.: +1-262- 473- 4384
Aeromatic-Fielder Div. Niro Inc. 9165 Rumsey Rd. Columbia, MD 21045, USA
Tel.: +1-410- 997- 7070 Fax.: +1-410- 997- 5021
Alexandenverk AG Kippdorfstr. 6 - 24 D-42857 Remscheid, Germany
Tel.: +49- (0)2191- 795- 216 Fax.: +49- (0)2191- 795- 350
Alexanderwerk, Inc. 415 Sargon Way, Unit H Horsham, PA 19044, USA
Tel.: +1-215- 442- 0270 Fax.: +1-215- 442- 0271
API Amherst Instruments, Inc. Mountain Farms Technology Park Hadley, MA 01035-9547, USA
Tel.: +1-413- 586- 2744 Fax.: +1- 413- 585- 0536
Babcock-BSH GmbH August-Gottlieb-Str. 5 D-36222 Bad Hersfeld, Germany
Tel.: +49- (0)6621- 81449 Fax.: +49- (0)6621- 81393
I
567
568
I
74 Indexes
Hosokawa BEPEX GmbH ( H U T ) Postfach 1152 / Daimlerstr. 8 D-74207 Leingarten, Germany
Tel.: +49- (0)7131- 907- 0 Fax.: +49- (0)7131- 907- 301
Hosokawa Bepex Corporation 333 N.E. Taft Street Minneapolis, M N 55413, USA
Tel.: +1-612- 627- 1412 Fax.: +1- 612- 627- 1444
Brabender Technology 6500 Kestrel Road Mississauga, Ont. L5T 1Z6, Canada
Tel.: +1- 905- 670- 2933 Fax.: +1-905- 670- 2557
Bristol Equipment Co. 210 Beaver Street Yorkville, IL 60560-0696, USA
Tel.: +1- 630- 553- 7161 Fax.: +1- 630- 553- 5981
Biihler AG CH-9240 Uzwil, Switzerland
Tel.: +41- (0)71-955- 1111 Fax.: +41- (0)71- 955- 3379
Buhler Inc. Box 9497 Minneapolis, MN 55440, USA
Tel.: +1- 612- 545- 1401 Fax.: +1- 612- 540- 9296
Carrier Vibrating Equipment, Inc. Box 37070 Louisville, KY 40233, USA
Tel.: +1- 502- 969- 3171 F a . : +1- 502- 969- 3172
Chatillon Products, Ametek, Inc. 8600 Somerset Drive Largo, FL 33773, USA
Tel.: +1-813- 536- 7831 Fax.: +1- 813- 539- 6882
Carver, Inc. 1569 Morris Street Wabash, IN 46992-0544, USA
Tel.: +1-219- 563- 7577 Fax.: +1- 219-563- 7625
Derrick Manufacturing Corp. 590 Duke Road Buffalo, NY 14225, USA
Tel.: +1-716- 683- 9010 Fax.: +1- 716- 683- 4991
Despatch Industries P.O. Box 1320 Minneapolis, MN 55440-1320, USA
Tel.: +1-612- 781- 5363 Fax.: +1- 612- 781- 5353
14.1 List of Vendors
Dings Magnetic Group 4740 W. Electric Avenue Milwaukee, WI 53219-9990, USA
Tel.: +1-414- 672- 7830 Fax.: +1-414- 672- 5354
EI, Eastern Instruments 416 Landmark Drive Wilmington, NC 28412, USA
Tel.: +1- 910- 392- 2490 Fax.: +1- 910- 392- 2123
Eriez Magnetics 2200 Asbury Road Erie, PA 16508, USA
Tel.: +1-814- 835- 6000 Fax.: +1- 814- 838- 4960
Erweka GmbH Ottostr. 20 - 22 D-63150 Heusenstamm, Germany
Tel.: +49- (0)6104- 6903- 0 Fax.: +49- (0)6104- 6903- 40
Erweka Instrument, Inc. 56 Quirk Rd. Milford, CT 06460, USA
Tel.: +1-203- 877- 8477 Fax.: +1- 203- 874- 1179
Flexicon Corp. 1375 Stryker's Road Phillipsburg, NJ 08865-5269, USA
Tel.: +1- 908- 859- 4700 Fax.: +1- 908- 859- 4826
Flexicon Europe Ltd 89 Lower Herne Road Herne, Herne Bay, Kent CT6 7PH, England
Tel.: +44- (0)1227- 374710 Fax.: +44- (0)1227- 365821
Flottweg GmbH (Member of the Krauss-Maffei Group) Industriestr. 6-8 D-84137 Vilsbiburg, Germany
Tel.: +49- (0)8741- 301- 0 Fax.: +49- (0)8741- 301- 300
Gerteis Maschinen- + Processengineering AG Stampfstr. 74 CH-8645 Jona, Switzerland
Tel.: +41- (0)55- 212- 1121 Fax.: +41- (0)55- 212- 1140
T.J. Gundlach Machine Co. One Freedom Drive Belleville, IL 62226, USA
Tel.: +1- 618- 233- 7208 Fax.: +1- 618- 233- 6154
Gustafson Sampling Systems, Inc. 7290 Golden Triangle Drive Eden Prairie, MN 55344, USA
Tel.: +1-612- 941- 1630 Fax.: +1-612- 941- 9371
I
569
570
I
14 Indexes
Hagglunds Drives, Inc. 2275 International Street Columbus, OH 43228, USA
Tel.: +1-614- 527- 7400 Fax.: +1-614- 527- 7401
Hardy Instruments 3860 Calle Fortunada San Diego, CA 92123-1825, USA
Tel.: +1-619- 278- 2900 Fax.: +1- 619- 278- 6700
Hi Roller Enclosed Belt Conveyors 5100 W. 12th Street Sioux Falls, SD 57107-0514, USA
Tel.: +1-605- 332- 3200 Fax.: +1- 605- 332- 1107
IMI Industrial Magnetics, Inc. 1240 M-75 South Boyne City, MI 49712-0080, USA
Tel.: +1- 231- 582- 3100 Fax.: +1- 231- 582- 2704
Intersystems Sampling Systems 17330 Preston Road, Suite 105D Dallas, TX 75252, USA
Tel.: +1- 972- 380- 0791 Fax.: +1- 972- 250- 4135
Kason Corp. 67-71 E. Willow St. Millburn, NJ 07041-1416, USA
Tel.: +1-973- 467- 8140 Fax.: +1- 973- 258- 9533
Korsch Pressen GmbH Breitenbachstr. 1 D-13509 Berlin, Germany
Tel.: +49- (0)30- 43576- 0 Fax.: +49- (0)30- 43576- 350
K-Tron America Routes 55 & 553 Pitman, NJ 08071-0888, USA
Tel.: +1-609- 589- 0500 Fax.: +1- 609- 598- 8113
K-Tron Switzerland Industrie Lenzhard CH-7202 Niederlenz. Switzerland
Tel.: +41- 62- 885- 7171 Fax.: +41- 62- 891- 6661
Krauss-Maffei Verfahrenstechnik GmbH Krauss-Maffei-Str. 2 D-80997 Munchen, Germany
Tel.: +49- (0)89- 8899- 0 Fax.: +49- (0)89- 8899- 3299
Krauss-Maffei Corp. 7095 Industrial Road Florence, KY 41022-6270, USA
Tel.: +1- 859- 283- 0200 Fax.: +1- 859- 283- 1878
74.7 List ofvendors
MP Machine and Process Design, Inc. 820 McKinley Street Anoka, MN 55303, USA
Tel.: +1- 763- 427- 9991 Fax.: +1-763- 427- 8777
Mark-10 Corp. 458 West John Street Hicksville, NY 11801, USA
Tel.: +1-516- 822- 5300 Fax.: +1-516- 822- 5301
Hosokawa Micron Powder Systems 10 Chatham Rd. Summit, NJ 07901, USA
Tel.: +1- 908- 273- 6360 Fax.: +1- 908- 273- 7432
Minox Siebtechnik GmbH Interpark D-76877 OffenbachlQueich, Germany
Tel.: +49- (0)6348- 9828- 0 Fax.: +49- (0)6348- 4086
Minox/Elcan Industries, Inc. 59 Plain Avenue New Rochelle, NY 10801, USA
Tel.: +1- 914- 235- 0161 Fax.: +1- 914- 654- 9835
Modern Process Equipment, Inc. 3125 South Kolin Ave. Chicago, IL 60623, USA
Tel.: +1- 312- 254- 3929 Fax.: +1- 312- 254- 3935
Monitor Manufacturing, Inc. 44W320 Keslinger Road Elburn, IL 60119-8048, USA
Tel.: +1-630- 365- 9403 Fax.: +1- 630- 365- 5646
Natoli Engineering Co., Inc., Tableting Accessories Tel.: +1- 314- 926- 8900 28 Research Park Circle Fax.: +1- 314- 926- 8910 St. Charles, MO 63304, USA Nerak Systems, LP 6 Debbie Lane Cross River, NY 10518, USA
Tel.: +1- 914- 763- 8259 Fax.: +1- 914- 763- 9570
Nicolet Instrument Corp. 5225 Verona Road Madison, WI 53711-4495, USA
Tel.: +1-608- 276- 6100 Fax.: +1- 608- 273- 5046
Nordberg Group P.O. Box 307 33101 Tampere, Finland
Tel.: +358- 20- 484- 140 Fax.: +358- 20- 484- 141
I
571
572
I
14 Indexes
Nordberg Americas 3073 South Chase Avenue Milwaukee, WI 53207, USA
Tel.: +1-414- 769- 4300 Fax.: +1- 414- 769- 4730
Particle Characterization Measurements Div. of Business Systems International, Inc, 453 Highway 1 West Iowa City, IA 52246, USA
Tel.: +1-319- 354- 5889 Fax.: +1-319- 354- 0526
Pennsylvania Crusher Co. GOO Abbott Drive Broomall, PA 19008-0100, USA
Tel.: +1-610- 544- 7200 Fax.: +1- 610- 543- 0190
PMI Porous Materials, Inc. Cornell Business & Technology Park 83 Brown Rd. Ithaca, NY 14850, USA
Tel.: +1-607- 257- 5544 Fax.: +1-607- 257- 5639
Quadro, Inc. 55 Bleeker Street Millburn, NJ 07041-1414, USA
Tel.: +1-973- 376- 1266 Fax.: +1-973- 376- 3363
Quantachrome Corp 1900 Corporate Drive Boynton Beach, FL 33426, USA
Tel.: +1-561- 731- 4999 Fax.: +1- 561- 732- 9888
Quantachrome GmbH Rudolf-Diesel Str. 12 D-85235 Odelzhausen, Germany
Tel.: +49- (0)8134- 9324- 0 Fax.: +49- (0)8134- 9324-25
The Rapat Corp. 919 O’Donnell Street Hawley, MN 56549-4310, USA
Tel.: +1- 218- 483- 3344 Fax.: +1-218- 483- 3535
Rhewum GmbH Rosentalstr. 24 D-42899 Remscheid, Germany
Tel.: +49- (0)2191- 98306- 0 Fax.: +49- (0)2191- 51840
Rotex, Inc. 1230 Knowlton Street Cincinnati, OH 45223, USA
Tel.: +1-513- 541- 1236 Fax.: +1-513- 541- 4888
Russel Finex Ltd. Russel House, Browells Lane Feltham, Middlesex l W 1 3 7EW, England
Tel.: +44- (0)181- 818- 2000 Fax.: +44- (0)181- 818- 2060
14.7 List of Vendors
Russel Finex, Inc. 10709-A Granite Street Charlotte, NC 28273, USA
Tel.: +1- 704- 588- 9808 Fax.: +1- 704- 588- 0738
Carl Schenck AG D-64273 Darmstadt, Germany
Tel.: +49- (0)6151- 32- 0 Fax.: +49- (0)6151- 32- 1100
Dr. Schleuniger Pharmatron AG Schongriinstrasse 27 CH-4501 Solothurn, Switzerland
Tel.: +41- (0)32- 624- 4080 Fax.: +41- (0)32- 624- 4088
Dr. Schleuniger Pharmatron, Inc. One Sundial Avenue, Suite 214 Manchester, NH 03103, USA
Tel.: +1- 603- 645- 6766 Fax.: +1-603- 645- 6726
Sepor, Inc. P.O. Box 578 Wilmington, CA 90748, USA
Tel.: +1- 310- 830- 6601 Fax.: +1- 310- 830- 9336
Shimadzu Scientific Instruments, Inc. 7102 Rivenvood Drive Columbia, MD 21046, USA
Tel.: +1- 410- 381- 1227 Fax.: +I- 410- 381- 1222
Simpson Technologies Corp. 751 Shoreline Drive Aurora, IL 60504-6194, USA
Tel.: +1- 630- 978- 0044 Fax.: +1-630- 978- 0068
Simpson Technologies Baarerstrasse 77 CH-6300 Zug, Switzerland
Tel.: +41- (0)41-711- 1555 Fax.: +41- (0)41- 711- 1387
GR Sprenger Engineering, Inc. 736 West Hemlock Circle Louisville, CO 80027, USA
Tel.: +1- 303- 665- 7069 Fax.: +1- 303- 665- 5346
S.S.T. Schiittguttechnik Lechwiesenstr. 21 D-86899 Landsberg am Lech, Germany
Tel.: +49- (0)8191- 335951 Fax.: +49- (0)8191- 335955
Stedman Machine Co. P.O. Box 299 Aurora, IN 47001, USA
Tel.: +1- 812- 926- 0038 Fax.: +1- 812- 926- 3482
I
573
574
I
74 fndexes
SVS Sauk Valley Systems, Inc. P.O. Box. 1013 Sterling, IL 61081, USA
Tel.: +1- 815- 625- 5573 Fax.: +1- 815- 625- 5593
SWECO 8029 US Hwy 25 Florence, KY 41022-1509, USA
Tel.: +1- 606- 283- 8400 Fax.: +1- 606- 283- 8469
Tecnetics Industries, Inc. 1811 Buerkle Road St. Paul, MN 55110, USA
Tel.: +1- 612- 777- 4780 Fax.: +1-612- 777- 5582
W.S. Tyler 8570 Tyler Boulevard Mentor, OH 44060, USA
Tel.: +1-440- 974- 1047 Fax.: +1-440- 974- 0921
W.S. Tyler Germany Ennigerloher Str. 64 D-59302 Oelde, Germany
Tel.: +49- (0)2522- 30- 0 Fax.: +49- (0)2522- 30- 404
Unitrac Corp. Ltd. Box 330, 299 Ward Street Port Hope, Ontario L1A 3W4, Canada
Tel.: +1-905- 885- 8168 Fax.: +1-905- 885- 2614
Paul-Otto Weber, Maschinen-Apparatebau GmbH Fuhrbachstr. 4-6 Tel.: +49- (0)7151- 72600 D-73630 Remshalden, Germany Fax.: +49- (0)7151- 72509 ZEMAG GmbH Paul Rohland-Str. 1 D-06712 Zeitz, Germany
Tel.: +49- (0)3441- 880- 205 Fax.: +49- (0)3441- 212993
Organizations
Institute for Briquetting and Agglomeration (IBA) 721 Indian Springs Lane Tel.: +1-847- 229- 6126 Buffalo Grove, IL 60089-1403, USA Fax.: +1- 847- 541- 8947 Institute of Coal Preparation (Ion) Panki, District of Lubertzi Moscow oblast 14004, Russia
14. I List of Vendon
Institute “Mechanoz Engineering” 2/linia, dom 8-a St. Peterburg B-26, Russia Metal Powder Industries Federation (MPIF) 106 College Road East Princeton, NJ 08540-6692, USA
Tel.: +1-609- 452- 7700 Fax.: +1-609- 987- 8523
PennState, The Pennsylvania State University, College of Engineering Materials Characterization, P/M Lab. 118 Research West Tel.: +1- 814- 863- 6809 University Park, PA, 16802-6809, USA Fax.: +1- 814- 863- 8211 Particle Technology Forum (PTF) American Institute of Chemical Engineers (AIChE) 3 Park Avenue Tel.: +1-212- 591- 8100 Fax.: +1-212- 591- 8888 New York, NY 10016, USA Society for Mining, Metallurgy, and Exploration, Inc. (SME), (Member American Institute of Mining, Metallurgical, and Petroleum Engineers [AIME]) P.O. Box 625002 Tel.: +1- 303- 973- 9550 Fax.: +1- 303- 973- 3845 Littleton, CO 80162-5002, USA University of Florida Engineering Research Center for Particle Science and Technology 418 Weil Hall, Box 116135 Tel.: +1-352- 846- 6135 Gainesville, FL 32611-6135, USA Fax.: +1- 352- 846- 1196 (A National Science Foundation Engineering Research Center for Particle Science & Technology depending on active industry support through its Industrial Partners Program [IPP]). Verfahrenstechnische Gesellschaft (VTG) Verein Deutscher Ingenieure (VDI) Graf Recke Str. 84 D-40239 Dusseldorf, Germany
Tel.: +49- (0)211- 6214- GOO Fax.: +49- (0)211- 6214- 169
Tollers
(Out-sourcing by contract manufacturing, co-manufacturing, and “backup” manufacturing.)
(Note: In addition to the companies listed below which specijcally o$r at least some contract manufacturing services that are related to Size Enlargement by Agglomeration, essentially all manufacturers and suppliers/distributors of equipment for the unit operations of Mechanical
I
575
576
I
74 Indexes
Process Technology and related industrial and analytical techniques (see Chapter 1, Fig. 1.1)maintain a more or less extensivefacility and laboratolyfor testing materials, developing applications, and determining process parameters). Abbott Laboratories, Contract Manufacturing Services 1401 Sheridan Road Tel.: +1-847- 937- 1009 North Chicago, IL 60064-6321, USA Fax.: +1- 847- 938- 2875 (Pharmaceuticals: powders and granules, tablettes, etc.) The ASC Group 309 E. Yates St., Box 200 Tel.: +1- 217- 834- 3301 Allertown, IL 61810, USA Fax.: +1- 217- 834- 3655 (Custom (pan) pelletizing, full-scale testing, process design and engineering) AVEKA, Inc., Headquarters, R&D Tel.: +1- 612- 730- 1729 2045 Wooddale Dr. (Building 553-C) Fax.: +1-612- 730- 1826 Woodbury, M N 55125, USA AVEKA Manufacturing, Large-Scale Manufacturing 279 Woodward Avenue Tel.: +1- 319- 237- 5010 Fredericksburg, IA 50630, USA Fax.: +1-319- 237- 5056 AVEKA Foods, Food Processing 106 Bremer Avenue Tel.: +1-715- 962- 9106 Colfax, WI 54730, USA Fax.: +1- 715- 962- 3129 (Originating from 3M’s Fine Particle Pilot Plant and after adding large-scale manufacturing as well as food processing, AVEKA has become an R&D, service, and smallor large-scale particle processing group. Capabilities include: Agglomeration, blending, classification, compounding, dispersion preparation, granulation, grinding, microencapsulation, particle characterization, particle coating, particle surface modification, prilling, screening, and spray drying) Catalytica Pharmaceuticals P.O. Box 1887 Tel.: +1- 252- 707- 2330 Fax.: +1- 252- 707- 2450 Greenville, NC 27835-1887, USA (Development, scale-up, and large scale manufacturing of complete packaged pharmaceutical products, dosage form includes tablettes and granules.) Coating Place, Inc. 200 Paoli St., P.O. Box 930310 Verona, WI 53593, USA (Coating, encapsulation, fluid bed processing.)
Tel.: +1- 608- 845- 9521 Fax.: +1- 608- 845- 9526
14. 1 List of Vendors
Custom Granular, Inc. Tel.: +1-608- 868- 3838 4846 Hwy. 26 North F a . : +1-608- 868- 5448 Janesville, WI 53546, USA (Roll compaction, briquetting, milling, particle classification, blending)
Custom Powders Ltd. Tel.: +44- 1270- 530020 Gateway, Crewe Fax.: +44- 1270- 500250 Cheshire, CW1 6YT, England Custom Powders BV Tel.: +31- 492- 598598 Grasbeemd 10 F a . : +31- 492- 598591 NL-5705 DG Helmond, Netherlands (Size enlargement, size reduction, particle separation, dry blending, liquid addition, drying (water), hot melt processes.) The Dow Chemical Co., Contract Manufacturing Services Tel.: +1- 517- 636- 1000 100 Larkin Center Fa.: +1-517- 832- 1465 Midland, M I 48674, USA (Development, scale-up, and manufacturing of products from agricultural, pharmaceutical, and intermediates to specialty chemicals.) Erie Foods International, Inc. Tel.: +1-309- 659- 2223 401 Seventh Ave. Fax.: +1- 309- 659- 2822 Erie, IL 61250, USA (Developing and manufacturing specialty milk proteins for use in the food and pharmaceutical industries; agglomeration at Rochelle, IL, facility.) Fuller Company, Research & Development Member of the F.L. Smidth-Fuller Engineering Group Tel.: +1-610- 266- 5035 2040 Avenue C Bethlehem, PA 18017-2188, USA F a . : +1- 610- 266- 5109 (Crushing/classification, material preparation [including drum conditioners/pelletizers, pans, extruders, compaction/granulation], pyroprocessing [including calcination, high temperature processing, mineral roasting, drying and cooling, reduction], rotary kilns, flash calciners/dryers, physical and chemical laboratories, bulk material handling, pneumatic conveying, etc.) GEA NIRO, Inc. (Food & Dairy Industries) Tel.: +1-715- 386- 9371 1600 O’Keefe Road Fa..: +1-715- 386- 9376 Hudson, WI 54016, USA (Testing facility and pilot plants for liquid processing, spray drying and agglomeration, evaporation and concentration, product handling and packaging.) Howard Industries, Inc. 1840 Progress Avenue Columbus, OH 43207, USA
Tel.: +1-614- 444- 9900 F a . : +1- 614- 444- 4571
I
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578
I
14 Indexes
(Custom processing includes blending, milling, classification, agglomeration [with pans, pin mixers, tablet presses, roller presses, exhuders], calcining, centrifuging, double arm mixing, compounding, flaking, drying, packaging.)
IFP, Inc. Tel.: +1- 507- 334- 2730 2125 Airport Drive, Hwy. 21 & 1-35 Fax.: +1-507- 334- 7969 Faribault, M N 55021-7798, USA (Contract food processing and packaging, including spray drying, agglomeration, particle coating/encapsulation, instantizing, milling, blending.) International Processing Corp., Member of the GLATT Group Tel.: +1-859- 745- 2200 1100 Enterprise Drive Fax.: +1-859- 745- 6G36 Winchester, KY 40391-9888, USA Glatt Air Techniques, Ramsey, NJ, USA IPC Processing Center, Dresden, Germany (Blending and granulating, tabletting, extrusion and spheronizing, coating.) L. Robert Kimball & Associates, Bituminous Coal Research Facility Tel.: +1- 814- 472- 7700 615 W. Highland Ave. Fax.: +1- 814- 472- 7712 Ebensburg, PA 15931, USA (Processing and briquetting of coal.)
K.R. Komarek Briquetting Research, Inc. Tel.: +1- 256- 831- 5741 20 Wm. F. Andrews Drive Fax.: +1-256- 831- 1331 Anniston, AL 36207, USA (Roller press briquetting and compaction/granulation.) Materials Processing Technology, Inc. Tel.: +1-973- 279- 4132 95 Prince Street Fax.: +1-973- 279- 4435 Paterson, NJ 07501, USA (Agglomeration, Coating, Encapsulation, Granulation, Mixing/Blending, Screening/ Classifying). M.I.E. (Marietta Industrial Enterprises, Inc.) Tel.: +1- 740- 373- 2252 Rt. 4, BOX 179-1A Fax.: +1- 740- 373- 6369 Marrietta, OH 45750, USA (Custom crushing, grinding, milling, screening, and roll briquetting.) Metrics, Inc. Tel.: +1- 252- 752- 3800 1240 Sugg Parkway Tel.: +1- 252- 757- 2573 Greenville, NC 27834, USA (Contract and manufacturing services for the pharmaceutical industry, including encapsulation, fluid bed processing, granulation, mixinglblending, etc.)
14.7 List of Vendors
Quintiles Tel.: +1-818- 767- 3900 10245 Hickman Mills Drive Fax.: +1-818- 767- 3950 Kansas City, MO 64137, USA Tel.: +44- 131- 451- 2074 Research Avenue South Fax.: +44- 131- 451- 2063 Riccarton, Edinburgh EHl 4AP, UK (Contract and manufacturing services for the pharmaceutical industry, includes wet granulation, direct compression, fluid bed processing, film coating, encapsulation, bead manufacture, mixinglblending, etc.)
R.Tech (Results Technology) Tel.: +1- 612- 481- 2207 4001 Lexington Ave. N. Fax.: +1- 612- 486- 0837 Arden Hills, MN 55126, USA (Technical, analytical, development, and manufacturing services for the food industry, including, among many others, spray drying, agglomeration, instantizing, extrusion, dry blending, etc.) J. Rettenmaier & Sohne GmbH Sr Co. Contract Service Tel.: +49- (0)7967- 152- 0 Holzmuhle 1 Fax.: +49- (0)7967- 152- 222 D-73494 Rosenberg, Germany (Mixing/homogenizing, sifting/air classification, grinding/cryogenic grindinglpulverizing, dryinglconditioning, encapsulating/coating, agglomerating/compacting/ granulating/pelleting, filling/refilling.)
ScheringPlough Third Party Business Tel.: +1-908- 629- 3200 1095 Morris Avenue Fax.: +1- 908- 629- 3164 Union, NJ 07083-7137, USA (Pharmaceutical tabletted products at Kenilworth, N J: granulation and blending, compression and coating, in-process testing.) Stellar Manufacturing Co. Tel.: +1- 618- 337- 4747 1647 Sauget Business Blvd. Fax.: +1- 618- 337- 0003 Sauget, IL 62206, USA (Blending, compacting, granulating, milling, sizing, tabletting, pouching, bagging, packaging.) Svedala Industries, Inc., Process Research and Test Center (PRTC) Tel.: +1- 414- 762- 1190 9180 Fifth Avenue Fax.: +1-414- 764- 3443 Oak Creek, WI 53154, USA (Fully equipped facility with the capabilities to perform complex material and process testing and evaluations as well as simulating complete flowsheets that can be assembled to represent a commercial plant with many different unit operations. The test center is designed to perform comminution, agglomeration, and thermal processing studies.)
I
579
580
I
14 Indexes
Toll Compaction Service, Inc. 14 Memorial Drive Tel.: +1-732- 776- 8225 Fax.: +1-732- 776- 8306 Neptune, NJ 07753, USA (Roll compaction, pan agglomerating, screening, blending, and grinding of pharmaceuticals and chemicals.) Vector Corporation Tel.: +1-319- 377- 8263 675 44th Street Fax.: +1- 319- 377- 5574 Marion, IA 52302, USA (Agglomeration, coating, encapsulation, fluid bed processing, granulation including roller press compaction/granulation, mixinglblending.) Welch Laboratories, Inc. Tel.: +1-616- 399- 2711 4270 Sunnyside Drive Holland, MI 49424, USA Fax.: +1-616- 399- 6889 (Compaction services for the pharmaceutical, food, and chemical industries.)
14.2 Wordfinder Index
ChapterlSection
Page
8.3 5.4 10.2.2 5.1.1 5.4 11.2 11.2 11.2 1 7.2 5.2 5.2.2 5.2.2 1 12 7.4.1 8.4.2 5.5 5.2.2 5.1.1 8.4.1 8.4.3
245 105 445 37 102 47 1 469 490 2 145 55 62 70 2 509 163 299 130 69 38 258 301
A
Abrasion drums Absorbents Adsorption flocculation Adsorption layers Advantages of agglomerated products Aero-Flow Aerosizer Aging Agglomerate Agglomerate growth Agglomerate strength (Definition) Agglomerate strength (Determination) Agglomerate strength in industry Agglomeration Agrochemicals Alternative drum designs Annular gap extruder Anticaking conditioning agents Atomic Force Microscope (AFM) Attraction forces between solid particles Axial extruder Axial high pressure screw extruder
14.2 Wordfinder Index
B
Back-mixed fluidized bed Baffle inserts Bag-set Basic compaction mechanism Basic mechanism of tumblelgrowth agglomeration Basket extruder Batch sintering Bell type furnace Belt conveyor agglomeration Binder development Binderless agglomeration Binderless pressure agglomeration Binders Binder selection Bridge type additives Briquette Bulk compression stage By-products as binders
7.4.4 7.4.2 5.5 8.1 6 8.4.1 9.2.1 9.2.1 10 5.1.2 6 8.1 5 5.1.2 5.1.2 5.1.2 8.4.3 8.2 5.1.2
201 168 123 232 134 255 390 392 412 44 133 231 29 43 43 44 336 237 44
5.5 8.4.3 5.2.2 5.1.1 8.4.3 12 5.4 5.4 5.4 8.4.3 8.4.3 5.4 5.1.1 5.4 8.4.3 8.4.3 8.4.4 8.3 8.4.3 8.1 8.4.2 5.5 8.3
123 363 67 38 328 515 106 106 101 352 347 108 36 103 338 356 375 243 336 233 278 130 242
C
Caking Cantilevered shafts and rolls Capillary flow in wet agglomerates Capillary pressure Capping Carbonless copying paper Carriers for catalysts Cat litter Characteristics of single particles Chattering Cheek plates Chemical oxygen generator Chemical reaction Co-agglomerated materials Coal tar pitch Cocking of the floating roller Cold isostatic pressing (CIP) Compaction/granulation Compressed residual gas Conditioner Conditioning
I
581
582
I
14 Indexes
Contact fluidizer Continuous drum coater Continuous fluidized beds Continuous mechanical pusher furnace Continuous mixer/agglomerators Control of feed volume Convenience foods Conveyor screw Coordination number Coordination points Coupling gears Crystal bridges Crystallization Cut size Cyclone separators
7.4.4 10.1 7.4.4 9.2.2 7.4.2 8.4.3 5.1.2 8.4.1 5.1 5.1 8.4.3 5.1.1 5 5.5 10.2.1
210 419 201 401 179 345 47 264 35 35 337 37 29 109 440
8.4.3 5.5 5.1.1 5.1.2 12 5.2.2 5.2.2 5.1.2 5.1.2 8.4.3 5.3.2 10 10 5.4 7.4.1 5.1.2 7.4.4 8.4.1 8.4.3 7.4.1 10.1 10.1 8.4.4 12 8.1
345 109 37 47 509 70 71 46 50 321 96 410 410 108 153 50 197 261 338 160 418 417 376 515 234
D
Deaeration paths in roller presses Degree of separation Deposition of suspended colloidal particles Designer foods Designer plant foods Determination of agglomerate strength in industry Determination of product properties in industry Development of lubricants Dietary ballast additives Die wall lubrication system Diffusion path Direct capillary action Direct effect of molecular forces Direct Reduced Iron (DRI) Disc agglomerator Disintegrants Distribution plate Dome extruder Double output-shaft gear reducer Drum agglomerator Drum coaters using paddles Drum coating equipment Dry bag process Drug delivery system Dwell time
14.2 Wordfinder Index
E
Easily degradable carrier materials Easily dispersible products Effervescence Efficiency of grinding Ejection press Elastic springback Electrical double layers Electrification assisted controlled particle deposition Electrocoagulators Electrostatically assisted coating process Electrostatic field Electrostatic forces Electrostatic precipitators Elevator type furnace Encapsulation Encapsulation of agglomerates Endpoint of agglomeration Engineered particulate materials Engineered products Entry suction pressure Excess charges Expander Expansion of compressed gas Experimental determination of agglomerate strength Explosives Exter press Extruder with radial discharge
F Feed control by tongues Feeder pan Fertilizer granulation Fiber based filter media Fibers Fill shoe Film coating Film type additives Fine particles First stage of sintering Flat die pelleting machine Flexible intermediate bulk container Floating roller Flocculation
5.4 5.4 5.1.2 5.5 8.4.3 8.1 5.1.1 12 10.2.2 10.1 5.1.1 5.1.1 10.2.1 9.2.1 10.1 5.2.2 7.2 5.4 5.1.2 5.2.2 5.1.1 8.4.2 8.1 5.2.2 5.4 8.4.3 8.4.1
105 104 47 115 315 233 41 521 446 435 41 41 442 392 433 69 148 103 47 67 41 295 234 62 107 307 257
8.4.3 8.4.3 12 10.3 5.1.2 8.4.3 10.1 5.1.2 5.1.2 9.1 8.4.2 11.2 8.4.3 7.4 10.2.2
337 337 508 447 47 315 417 44 42 386 284 490 338 152 ###
I
583
584
I
74 Indexes
Fluid drum granulator Fluidized bed dust collector Fluidized bed technology Fluidized spray dryer (FSD) Food additives Force feeder Formation of a crust Functional components Functional coatings Functional foods Fun foods
10.1 10.2.1 7.4.4 7.4.4 5.1.2 8.4.3 5.2.2 5.1.2 10.1 5.1.2 5.1.2
420 442 196 197 47 338 67 47 417 47 47
7.4.5 8.4.2 12 9.2.2 8.4.3 5.5 5.5 7.2 7.1
214 290 514 405 337 112 115 145 144
5.1.1 5.1.1 8.4.3 8.4.3 5.5 7.4 7.4.2 8.4.4 12 8.4.3 5.1.2 5.3.2 8.4.4 9.2.2 10.1 8.4.3 8.4.3 8.4.3 8.4.3 5.1.1 8.4.4
36 37 300 362 118 152 171 383 515 333 42 99 375 400 439 319 359 319 356 40 375
c Gas dynamic atomization Gear pelletizer Granulation of detergents Grate Kiln Gravity feeding Grinding aids Grinding equilibrium Growth of agglomerates Growth phenomena H
Hardening binders Highly viscous binders High-pressure agglomeration High pressure comminution High pressure roller mill High shear (particle) mixers HIP for making porous products Hollow capsules Horizontal punch-and-die presses Hot densification Hot isostatic pressing (HIP) Hump-back kiln Hybridization Hybrid punch-and-die presses Hydraulic accumulator Hydraulic presses Hydraulic pressurization system Hydrogen bridges Hydrostatic pressing
14.2 Wordfinder Index
I
Ice briquetting Immiscible binder agglomeration Immiscible liquid agglomeration Incipient bubbling velocity Incipient buoyancy Influencing the nucleation stage Inherently available binding properties Innovative pan designs Instant products Integrated belt spray dryer Intensifier bar Interdisciplinary approach to process selection Interlocking bonds Iron ore pellets Isostatic pressing
10 7.4 7.4 7.4.6 7.4.4 7.4.4 7.1 5.1.2 7.4.1 5.4 7.4.3 7.4.2 11.1 5.1.1 9.2.2 8.4.4
413 153 153 222 197 196 143 43 157 104 196 167 463 41 405 373
7.2
144
5.5 5.1.1 9.1 5.1 8.4.1 8.4.1 8.4.1 7.4 7.4.2 5.1.2
114 38 387 32 253 262 257 152 166 46
8.2 10.1 5.1.1 9.2.1 8.3 5.5 5.4
239 436 41 391 245 123 106
7.2 5.1 5.1.2
146 32 44
K
Kinetics of tumblelgrowth agglomeration L
Limit of grinding Liquid bridges Liquid phase sintering Liquid saturation Low-pressure agglomeration Low pressure flat die extruder Low pressure screw extruders Low shear particle mixers Lubricants
M Macroscopic flow of solid particles Magnetically assisted impaction coating Magnetic forces Manual pusher furnace Marumerizer Mass flow design Materials with controlled reactivity Mathematical modelling of tumble/growth agglomeration Matrix binder Matrix forming binder components
I
585
586
I
14 lndexes
Maximum pressing force Mechanical activation Mechanical dewatering Mechanical Process Engineering Mechanical Process Technology Mechanical punch drives Mechanism of briquetting in roller presses Mechanofusion Medium-pressure agglomeration Medium pressure axial screw extruder Melt solidification Mesh-belt sintering furnace Metal swarf Microcrystalline cellulose (MCC) Microencapsulation Microwave drying Mill shaft design Misalignment coupling Muffle furnace Multi-tier fluidized beds
8.1 5.5 8.1 1 1 8.4.3 8.4.3 10.1 8.4.2 8.4.2 5 9.2.2 5.1.2 5.1.2 10.1 7.4.2 8.4.3 8.4.3 9.2.1 7.4.4
233 115 234 1 1 319 341 439 266 294 29 400 51 47 433 181 363 338 391 207
Nan0 technology Natural adhesion of small particles Natural binding mechanisms Near net shape articles Neutral plane New generation of small roller presses Nip area Nonvalence associations Nozzle atomizer Nucleation Nuisance dust
5.4 7 5.1.2 8.4.4 8.4.3 8.4.3 8.4.3 5.1.1 7.4.3 7.1 8.3
101 139 43 373 317 345 338 40 191 141 244
0 Optimum packings Organic fibers Overcompaction
5.3.1 5.1.2 8.4.3
83 47 343
10.2.1 5.3.1 9.2.1 10.3 7.4.1
440 81 396 45 1 157
N
P
Packed bed filters Packing structure Pan sintering plant Paper making Pans with collars
74.2 Wordfinder Index
Parameters determining the properties of agglomerates 5.2.2 5.1.1 Partial melting 5.3.1 Particle shape 5.3.1 Particle signature 5.4 Particles in bulk 5.3.1 Particle size analysis 1 Particle technology Pelleting 8.4.2 Pellet mills 8.4.2 Pelletization of iron oxides 5.4 Pile-set 5.5 Plasma vapor deposition (PVD) 12 Plasticity 5.1.2 7.4.4 Plug flow fluidized bed 10.1 Polygonal coating drum 5.3.2 Pore forming additives 11.2 Pore size analyzer Pores of atomic scale 5.3.2 Porosity 5.3.2 Porosity of agglomerates 5.2.2 Pot grate 9.2.1 Powder Sr Bulk Solids Technology 1 Powder coatings 10.1 Powder color coatings 12 Powdered Cellulose (PC) 5.1.2 11.2 Powder flowability analyzer 1 Powder Technology 11.2 Powder tester 11.2 Powder Workbench 32 Precision coater 10.1 8 Pressure agglomeration Pressure cooker extruders 8.4.2 Pressure sintering 9.1 Pressure swing granulator (PSG) 7.4.5 Prilling 5 Production of primary agglomerates 7.1 Product properties in industry 5.2.2 5.3.2 Pulsed electric current sintering (PECS) Pyrotechnic articles 5.4
R Ram extrusion press Reaction bonding Reaction sintering
8.4.3 5.3.2 5.3.2 9.1
61 36 78 80 102 78 1 266 272 107 125 514 42 202 418 97 47 1 95 89 61 394 1 415 514 50 471 1 469 471 432 229 295 388 219 29 141 71 99 108
307 98 98 388
I
587
588
I
74 Indexes
Reciprocating punch-and-die presses Recombination bonding Recrystallization Recrystallization at the coordination points Recycling of paper Reduction ratio Refractory linings and components Regular packings Relaxation of elastic deformation Release of briquette Representative (particle) equivalent diameter Representative sample Reversed belt agglomerator Rewet agglomeration in fluidized beds Roller hearth furnace Roller presses Roller screen
Rotary atomizer Rotary punch-and-die presses Rotating disc fluidized bed coater Rotating pan coaters Rotating point-force Rotocoat process
8.4.3 5.1.1 5.1.1 5.1.2 10.3 5.5 5.1.2 5.3.1 8.1 8.4.3 5.1.3 11.2 10 7.4.4 9.2.2 8.4.3 7.4.1 11.3 8.4.3 11.2 7.4.3 8.4.3 10.1 10.1 8.4.3 10.1
315 40 37 43 45 1 118 52 81 234 343 80 489 412 205 400 335 163 494 342 485 190 325 430 415 355 42 1
S Sample preparation Sampling Scale-up Scanning Electron Microscopy (SEM) Screw diameter Screw feeder Second stage of sintering Selection considerations Selection of lubricants Selection process Selective agglomeration Self aligning roller bearings Self ignition Sensitization Separation curve Shaft furnace Shaking trough agglomerator
5.5 11.2 11.2 5.3.1 8.4.3 8.4.3 9.1 11.1 5.1.2 11 7.1 8.4.3 5.4 10.2.2 5.5 9.2.2 10
113 489 49 1 85 354 354 386 463 46 455 143 356 108 445 109 405 412
Roll press simulator
14.2 Wordfnder Index
Sharpness of separation Sheet thickness Simplified preselection guide Sintering Size enlargement (general) Size enlargement by agglomeration Sinter bridges Sol-gel processes Solid bridges Solids flow meter Solid state sintering Sonic agglomeration Spark plasma sintering (SPS) Specific force Specific surface energy Spherical agglomeration process Spherical crystallization Spheronization Split of each roller Sponge iron Spontaneous combustion Spouted bed Spray dryers Spray pattern Spray systems Spring loaded pressurization system Steam jet agglomeration Stockpile agglomeration Strength of agglomerates Structure of agglomerates Structure of agglomerates obtained by pressure Struct. of agglomerates resulting from growth during tumbling Structure of sinter Surface-active substances Surface texture
5.5 8.4.3 11.1 9 5 1 5.1.1 5.3.2 7.4.6 5.1.1 11.3 9.1 10 5.3.2 8.4.3 9.1 7.4.6 7.4.6 8.3 8.4.3 5.4 5.4 7.4.5 7.4.3 7.4.2 7.4.2 8.4.3 7.4.5 10 5.2.2 5.3 5.3.1
110 340 462 385 29 1 36 99 225 36 502 385 413 99 360 385 223 222 245 345 108 108 220 187 165 165 354 214 411 61 76 87
5.3.1 5.3.1 5.5 5.3.1
85 89 115 79
7.4.4 3 5.2 12 9.1 8.3
201 5 55 515 386 245
T
Tall form spray dryer (TFD) Technology of bread making Tensile strength of agglomerates Thermoset coating Third stage of sintering Thixotropic materials
I
589
590
I
14 Indexes
Tolling companies Top spray coaters Transport screw Transversal crushing force Travelling grate sinter machine Tunnel kilns for ceramics Turret
11.2 10.1 8.4.1 5.2.2 9.2.2 9.2.2 8.4.3
491 430 265 62 403 397 325
5.4 5
101 29
5.1.1 5.1.1 8.2 8.4.3 8.4.3 7.4.4 10 5.3.2
40 39 237 336 315 210 412 98
9.2.2 5.1.2 5.1.2 5.1.1 8.4.4 10.2.1 5.1.2 8.4.3 10.1
401 44 50 38 376 442 47 319 430
U
Ultra-fine particles (UFPs) Underwater granulation/pelleting V
Valences Van-der-Waals forces Variations in compact density Vertical pug mill Vertical punch-and-die presses Vibrated fluidized bed Vibrating deck agglomerator Vitrification W
Walking beam furnace Wastes as binders Water binding and retention capacity of fibers Wet agglomerates Wet bag process Wet scrubbers Wicking by fibers Withdrawal press Wurster coating process
74.3 Subject Index 14.3
Subject Index a abrasion - resistance 73 - transfer 141 absorbent 105, 451 accellerators 178 accumulator - hydraulic 359 - pressure 358 - - standard 360 action, of the coating material 436 additives 43 - dietary 47, 50 - disappear at high temperature 408 -food 47 - functional 47 - impurities 408 - organic fiber 50 - pore forming 97 - solid pore forming 98 - starches 50 - swelling 47 adhesion - physics 60 - permanent 134 adhesion forces 57 - mechanism 439 - natural 42, 229 - van-der-Waals 34, 40, 58, 59, 513 adsorption - charged organic 445 -ion 445 -layer 32, 60 - preferential 445 advanced material 515 agglomerate -angular 244 - breakdown 144 -broken 252 -buoyancy 62 - characteristics 70, 243, 409, 479 - completely filled with a liquid 56 - components 29 -density 179 - dispersion 47 - failure 55 - final size 179 - freely accessible surface area 44 - green 136, 140, 150, 241 - growth 183, 442, 494
-
growth or tumble, porosity 92
- higher purity 223 -
instant properties 511
- level of ultrasound 513 liquid saturation 32 mass of 144 - matrix bonded 92 -micro 156 - more stable 443 - optimal granulation 243 - oversized 144, 495 - particle size adjustment 149 - plasticity 245 - porosity 44, 61, 62, 92, 140, 229, 242 - properties 61, 525 -quality 497 - random cut 32, 76 - ration-sized 458 - recrystallizing substance 68 - roller mills 118 -seed 141 - solid bridges 57 - spherical 229, 244 - strength 32, 55, 61, 62, 90, 242, 461 - - determination, industrial method 76 - - in industry 70 - - standardized methods 71 - structure 76-100, 300 -volume 62 - wet - - development of strength 67 --drying 68 agglomerated products, characteristics 61 agglomeration - acoustic 413 - ancient technique 8 - applications 9 - - n e w 523 -art 70 - basic phenomen 139 - basic physical effect 3 - batch 143, 460 - beneficial - - pressure 252 - - growth, technology / equipment 151 - binding mechanisms 35, 523 --models 55 -concept 409 - continuous 143, 460 - controlled 139 -
-
I
591
592
I
7 4 Indexes -
during crystallization 222
- efficiency 143 - equipment --
motion 144
- - procurement 455 - - selection 455 -expert 8 - field of science 507 - fundamentals 70, 507, 523 - fundamentals, interdisciplinary application 216 - by heat 94, 95, 137 - history 3, 4 - immiscible binder 141 - immiscible liquid 223 - in the dry state 514 - industrial 389, 492 - innovative 411 - interdisciplinary 8 - liquid system 222 - mechanism 523 - method, preselected 475 - most versatile technique 136, 300, 320 - naturally 5, 109, 507 - on the surface 409 - phenomenom 3, 409 - plant, peripheral equipment / wet 187 - pressure agglomeration (see there) - principle, fundamental 10 - processes 86 - representative particle size 65 - rewet agglomeration 214, 215, 513 - science 3 - secondary 126, 421 - selective 111, 144, 222, 445, 494 - simplified selection guide 462 - single particle 409 - size enlargement 119 - - alternative approach 410 - - methods 468 - - most versatile 229 - - preselection 468 - steam jet 216 - symposia 3 - system - - art of controlling 150 - -design 151 - - installation 502 - - necessary part 492 - - start-up 502 - - new 502 - technique - - development 468 - - different 525
instant characteristics 513 preselection 468 - - special features 525 - - specific characteristics 525 - technology 3, 409, 507 - terms, glossary 11-27 - theories 29-131 - tool configuration 252 - tool to improve powder characteristics 3 - tumble / growth (see there) - undesirable 42 - unit operation 507 - unwanted 5, 36, 176, 508 - - remediation technique 131 agglomerator 151 - drum - - retaining rings 160 - low-pressure 254 - P-K Zig-Zag continuous 169 -pan 155 - - additional processing 157 - - bottom feeding 160 - - collar 158 - - d e e p 160 - - growth behavior 156 - - industrial 156 - - large scale 156 - - modifications 157 - - operations 156 - - process variations 157 - - re-roll designs 157 - - rear auger feeder 160 - - rim, / classification effect 153 - - scraper arrangement 156 - - scraper plows 155 - - stepped side wall designs 158 agitator, type 296 agricultural industry 419 agrochemicals 105, 507 - research 508 air conditioning 461 air pockets, compressed 233, 497 air removal 334 airbag chemicals 106 airflow - direction 203 - powder transport 203 alloying, mechanical 375 alloying elements, agglomerated 251 ambient forces 134 ammaniation 167 amorphization 115 angle - of compaction 341 --
--
- of difference 469 - of fall 469 ofnip 341 of release 342 - of repose 469 - of rolling 341 - of spatula 469 animal feed 73,104,134,239,277,278,283, 289, 295, 373, 510 - medicated 280 - pelleting 507 annual production 460 anticaking agent, cationic 131 anular gap 299 anvil 294 - plate 334 application - development work 364 -small 364 aspiration point 502 ASTMdrum 71 atomization 214 atomizer wheel, peripheral speed 191 autoclave 376 -volume 378 auxiliary materials 461 -
b baffles 205, 417 bag filter 218 bag-set 126 bagasse 313 bahng 8 ball pen tips 224 balling 145 bark 333 basket test 395 batch operations 144 - equipment 147 -vacuum 393 batching system 509 bearing -blocks 358 - life 360 - reshimming 357 - with conical withdrawal sleeves 356 bentonite 408 BET sorptometer 474 binder 43-46, 133, 139, 458, 461, 468 - addition 494 - availability 44 - bridges 44 -cost 44 - development 44
evaluation 44 exbusion 305 -films 44 - immiscible bridging liquid 141 - inherent 231 - inherently available 134 - inorganic 44 - liquid 136, 140, 205 - - atomizing 165 - matrix 44 -organic 44 - replacement 44 - supply 44 -viscous 415 - wet granulation 52 binding - characteristics 334 - mechanisms 35, 55 - - adhesion forces (see there) - - destruction 105 - - effects 85 - - field forces 58 --final 151 - - inherently available 42 - - liquid bridge 57 - - means to enhance 42 biomass 234, 309, 333, 334 bitumen 32, 37 bituminous components 231 black core 390 Blaine method 65 blender - cylindrical drum 166 - orbitting type screw 182 - P-K Zigzag continuous 169 -ribbon 176 bloating 386, 390 boiling bed 197 bonding - chemical 457 - criterium 139 - recombination 115 Born repulsion 59 bread, making of 5 breech plug 376 brick 7, 305 bridging 123 briquette 335 - for deep frozen storage 415 - highly densified 466 -inert 466 - large 311 - metal 373 - ration-sized 415 -
-
594
I
74 lndexes
separators 342, 498 single 310 -thin web 342 briquetting 8 - coal 338, 507 - fine powders 313 - highly elastic organic material 307 -hot 8 - inert elastic material 313 - metal 334 -peat 307 brominated biocide 335 Brownian movement 413, 441 build-up 113, 122, 165, 196 bulk -bags 490 - characteristics 458 - commodity 389, 507 - compression stage 300 - density 87, 188, 234 - - aerated 469 - - feed 331, 353, 358 - - measurement 474 --packed 469 - masses, behavior 101 - original properties 490 - - changes 490 -volume 234 burden preparation plant 404 by-product 492 -
C
cage mill 500 caking 36, 37, 69, 115 - avoid 128 - fertilizers 123, 127 - laboratory technique 132 - particulate system 131 - temperature variations 127 - test procedure 132 campaigns 460 cams 325 capacity 277, 285, 320, 460 capillary - condensation 38 - flow 67, 190, 433 - forces 38, 136 - pressure 32, 56, 67, 190 - region 223 - state 34 capping 326, 328 capsule 434 carbide tool bits 378 carbides, cemented 381
carbon 380 carbon black 104, 217 card-board 333 carrier liquid, immiscible 435 carrier material 105, 106 cat litter 105, 451 catalyst 106 - carrier 246, 283, 389 cereals, processed 299 cellulose - chemical composition 49 - molecules 49 -powdered 50 - tabletting 50 -wood 47 cement 32, 156 - clinker 118 cementitious reaction 136 centrifugal forces 440 ceramics 94, 242, 319, 380, 388, 390, 402, 507 - applications 320 - high performance 375 -industry 391 -porous 98 cGMP 9,418 channel (see also extrusion channel) 257, 266, 267 - cooling -flow 122 - length 267, 307 chaos theory 146 characteristics - differences 492 -product 507 - relationship 490 cheek plates 352 chemical 435 chemical reaction 128 chemistry, incompatible 457 chip box 334 chlorinated biocide 335 choppers 177 - mode of operation 179 CIP (cleaning in place) 9, 418, 423 circular thickener / clarifier 443, 444 circumferential speed 364 clam-shelling 343 cleaning 278, 460 - CIP (cleaning in place) 9, 332, 418, 423 cleanIiness 332 climatic conditions 461 closed circuit 163 clusters 119
74.3 Subject Index 1595 coacervation 435 coal tar pitch 336 coal 231 - briquetting 340, 507 - bulk mass 252 -fines 347 - organic macromolecules 40 - run-of-mine 252 - Soft 312 coalescence 442 - preferential 236 - random / preferential 141 coating 8, 119, 157, 245, 415, 514 - agglomerates 415 - blobs of material 417 - by magnetic attraction 436 -cores 415 - delivery system, material 421 - drum --batch 419 - - continuous 419, 424 - fluidized bed 429 - - bottom-spray 430 - functional 421 - functional properties 417 -hard 514 - harder particle 439 - imperfect 430 - liquid binder 415 - manually applied 415 - mechanism 439 - mechanized 415 - melt 420 - multiphase 514 - multiple layer 514 - nuclei 415 - peening of the material 439 - plant seeds 419 - powder materials 415 - protective 5, 514 - separator 130 - single layer 514 - sorptive capacity 130 - spray 430 - surface-active organic chemicals 130 -thin 420 - uniform 417, 420 - Wurster 221, 430 cobalt 224 cohesiveness 469 collision - probability 442 - high rate 436 colloid, sensitivity 446
colloidal templating 516 compact 335 - accuracy 315 - accurately shaped 458 -density 315 - complex shape 315 -crushed 501 - densification 385 - large 315 - of pressure agglomeration 242 - structure 320 compacted sheet 356 compactibility 329 compaction / compacting 9, 115, 329 - brittle breakage 363 - behavior, predict 485 - compaction / granulation 513 - - economic operation 509 - - multiple lines 509 - - small batches 509 - shape of granular material 243 - zone 340 components - bituminous 231 -porosity 375 - preformed 375 - pyrotechnic 106, 108 - structural flaw 375 composite material 240 composite parts 375 composition, product 507 compounding 509 compressed - air, expanding 234 -gas 355 compressibility 469 compression - force 373 - one-sided 317 - phase, duration 309 - rate 234 - strength 64, 241 - testing 71 concrete 32 - high quality 85 - high strength 218 conditioning 134, 232, 236 conditions in-line -optimize 369 - readjust 369 consolidation, uniform 374 consultants 454 consumer product 47, 106, 251, 328, 375 contact potential 41
596
I
' 4 Indexes
contamination, particulate 502 continuous system, simulation 475 conveyor - belt 502 - mesh-belt 400 - pneumatic 119, 504 - pocket mechanical 503 cooker 295 - pressure 296 cooking, pressurized steam 296 cooling 196, 214 - rapid 214 coordination - number 77 - point 34 co-processing 167 cored block 305 cosmetics 515 costs - investment 140 - operation 140, 163 coupling gear 3 39 - fluctuation in gap width 354 - problem 354 cross contamination 280, 332, 509 crushing - chamber, exit screen 499 - know-how 243 - mechanism 499 - test, transversal 64 cryogenic milling 119 crystal bridges -strength 37 -structure 37 crystalline hull 129 crystallization 2, 29 - crystal growth 222 - undesired clustering 222 cubed ice 413 curing 129, 140 curing method 151 - ultraviolet radiation 515 customer appeal 106 cyclones 441 cylinder, perforated 276
d data - logging 383 - reduction software 472 deaeration 352, 355, 364 dedusting 111, 245 degassing 301 dense sheet 336
densest packings 84, 87 densification - cycles 234 - mechanism 234 -outcome 237 -ratio 497 - - very high 319 - speed of 137, 234, 300 density -apparent 61 - differences 238, 317 - distribution 237 - gradient, uniform 374 - localized 239 - overall 239 - pressure agglomeration 239 - theoretical 237 -uniform 237 - variations 237, 239, 373 deposit 114, 119, 122, 191, 199, 203 desagglomeration 118, 186 descriptive names 11 design data, determination 491 design parameter 490 - new location 491 detergent 251, 514 development 410 - empirical 453 - phase, first 474 -work 394 diaphragm - cloth 220 - finely pored 220 - sintered glass 220 die 325 - assemblies 485 -changer 305 - channels 232 - counter bore 267 - cylinder - - discharge from the inside 273 - - internal press rollers 277 - - no leakage 278 - - small in diameter 274 - cylindrical ring 267 -extrusion 267 - - flat 269, 284 - - perforated ring 269 - gear shaped 267 -holes 232 - - land area 267 -insert 2689 - life 268 - life expectancy 281
14.3 Subject Index 1597 -
lubrication 305
- openings 262 perforated 136 plate 258 - - circular 262 --domed 261 - - flat 261, 262 - ring, open front 278 - screens 136 -track 285 diffusion 387 - of matter 385 digested sludge 509 direct reduction plant 408 discharge system, sub-corona 436 disintegrant 51, 52 disintegration aid 514 dispersibility 469, 510 dispersion 510, 511 -aid 113, 514 displaced gas 234 disposal sites 451 distribution plates 202 -modified 203 - non-sifting 203 diversified companies, test and development facilities 492 dog food 299 dosage form 251 -dry 459 - solid 315, 417 downdraft 196, 395, 404 downtime 460 drag 288 -flow 261 - forces 120 DRI (direct reduced iron) 408 drink powder 510 drive, variable speed 292 drop test 74 droplet formation 29 drugs 435, 515 drums, polygonally shaped 41 7 dry bag - pressing 377 - tooling 376 dry powders 214 - characteristics 215 -flavor 215 dryer 69 - external fluidized bed 182 - spray dryer (see there) drylng 242 - continuous method 188 -
-
- emulsions 188 -fluid bed 430 - high rate 430 - rapid 214 - solutions 188 - slurries 189 - spray drying (see there) - suspensions 189 drying chamber 191 - Filtermat 196 - flow patterns 193 - height 193 - roof 193 - turbulence 193 drying zone 190 duck-billing 343 dung beetle 7 dust 495, 502 - collecting system 201, 440, 449, 502 -control 462 - layer 449 dwell time 309, 326, 378 dyes 515 dynamic seal 296, 301 e eccentric drive 309 economic justification 460 effective distance 43 efficiency factor 264 EIRICH granulating mixer 172 elastic - deformation 137 - - relaxation 234 - expansion 267 - recovery 310 - - characteristic, ram 309 - spring-back 267, 300 electric resistance heater 314 electrical double layer 446 electrolyte 119 electron beam drawing 521 electron work functions 41 electronic circuit board 409 electrostatic precipitators 41 emergency shut-downs 509 encapsulation, during drying 433 energy 386 - requirements 460 engineered products 103 - agglomerated 513 - new generation 513 engineering science, generic fields 453 entire process 460
598
I
74 Indexes
environmental - conditions 461 - control 335, 440, 442 - regulations 461 EPA 462 equation, population / mass balance 146 equipment - description 525 -desk top 468 -features 237 - maintenance 281 - new laboratory 468 - overload features 233 - overload protection 237 - parameters 145 - small laboratory 468 - small scale, modular design 475 - special 506 -testing 491 - transportation 502 eutectic 388 - temperature 388 evaporation 387 - porous bodies 433 excipients 321 expansion - compressed gas pockets 300 -elastic 267 experience 453, 463 - factor 264 explosives 106, 375 extrudate 93, 232, 255 - converting plastic 245 - different cross section 299 - discharging 268 -length 261 - size reduction 245 - skin 239 - spaghetti-like 246 - sticky 268 - with minimum spring-back 307 - with small cross section 268 extruder - anular gap 299 -axial 257 -basket 255 - cost advantage 263 -dome 262 - extrusion area 263 - gravity feed 255 - high pressure 304 - - configured tools 306 - - pastes 306 - - plastic materials 306
thermoplastic polymers 306 life span 297 - optional execution 299 - peripheral 257 - pressure cooker, technical data 299 - r a m 234 - ring die 274 - screen 254, 294 - screw 257, 299 - - axial 304 - - characteristic pressure distribution 301 - - conditions 265 - - conveying zone 301 - - densification region 301 - - design 265 - single screw 259 -trough 255 - twin screw 259 Extrud-0-Mix 294 extrusion zone 338 extrusion - aid 305 - blade 253, 255 - channel 93, 256, 257, 262 - - cylindrical bore 283 --design 266 - - frictional resistance 294 - - length 267, 307 - - multiple 311 - - release 310 - - relieving 267 - - square opening 283 - - straight cylindrical bore 266 - - tapered inlet 268 - - with inlet chamfer 267 - dies - - differently shaped 266 - - % free area 266 - forces to obtain 260 - high pressure 294 - hydrostatic pressure 294 - material flow 262 - medium-pressure 294 - plate, design 304 - pelleting 266 - pressure 276 - rate 264 -zone 340 --
f farming area - high performance 508 -large 508 fatty amines 131
74.3 Subject Index feeder 278 feeder base, baffles 352 feeding - high speed 331 - horizontal 344 - positive 291 - rotary tabletting presses 331 -vertical 344 feed screws 343 feed zone 340 felt 447 ferromagnetic particles 41 ferrosilicon 218 fertilizer 156, 161, 230, 419, 499 - bulk blending 509 - caking 123, 127 -coating 508 - defined NPK relationship 508 - granulation 126, 127, 131, 167, 244, 420, 460, 507, 508 - - coating 510 - - compaction / granulation 509 - - on demand 509 - high-analysis 129 -industry 245 - multicomponent 508 - nitrate 125 - partially suitable 508 - slow release 420 - trace elements 508 fiber 41, 409, 447 - briquette strength 51 - cellulose 447 - direct spun stainless 52 - metal 51 - reinforcement 52 -synthetic 447 -wire 52 filament 409 fill shoe 317, 331 filter / filtering 389, 447 -cake 234 -cartridge 447 - efficiency 447 - manufacturing 447 - media 440, 447, 450 - off-site system 443 - packed bed 441 - pleated bag 447 - u s e 447 fine -coal 223 -dust 333 - metal
fines 252 - excessive amounts 243 - recirculating 143 firing 390 flashing 336 flavor, enhancement 417 flexible containers 375 - fixed 376 - for wet bag pressing 376 floating roller - hydraulic pressurization 354 - springs 354 floc 223, 442 flocculant, commercial 445 flocculation 113, 221, 445 flotation 223, 445 flow -channel 122 - characteristics 331 - meter, solids 347, 502, 505 - ofair 418 - patterns in the system 503 - rates, mass - zones, calm 120 flowability 458 flue dust 402 fluid - contamination 379 - pressure transmitting 379 - toxic 380 fluidized bed 86 - agglomeration 196, 211, 219 - - size enlargement 204 - arranged externally 200 - back mixed 202 - bag filters 212 -batch 211 - binder liquid 210, 214 - circular 201, 204 - combination 206 - condensing, evaporated fluid 212 - contact heating surface 210 - continuously operating 211 - conversion, liquid feed 214 - cooling 196, 214 - discharge, agglomerates 197 - dry mixing 196 - dry powders 214 - drying 196, 214 - energy consumption 207 - heat sensitive material 210 - heating panels 210 - incremental growth 206 - installation cost 207
I
599
600
I
74 lndexes
- internal dust filter 210
- laboratory fluid
475 - low residual moisture 205 - multistage 209 - narrow size distribution 206 -overview 214 - particles - - flow direction 203 - - size distribution, 197 - plug flow 202, 206 - post-treatment equipment 214 - product treatment 210 - recirculation, fluidizing gas 212 - rectangular 204 - recycle 210 - residence time distribution 201 - scaled down 475 - space requirement 207 - stacked 209 - table top 475 - turbulently moving surface 206 - two-stage 206 - vibrated 211 -wet feed 210 fluid drum granulating 425 flywheel 308 food / feed 231, 251, 253, 295 -actual 489 - additives 47 - animal (see there) - built-up 271 - bulk density 331, 353, 358 - characteristics 278 - composition, variations 497 -control 459 -dog 299 - elastic 272 - essentially dry 234 - feed / recycle ratio 498 -fine 458 - formulation 290 -frame 325 - future 489 - grain 295 - homogenisation 494 -hopper 352 - industry 104, 216, 510 - material 513 - - characteristics 456, 475 - - compactibility 481 - - evaluation of 474 - mill 277 - moisture level / range 263 - new 270
-organic 295 - plasticity 242, 262 -powders 214 - predensified layer 270 - products 48, 507 - rate 271 -shoe 325 - uniform layer 172 - vegetable based 295 -zone 338 force feeder 331, 343, 354 forces - adhesion (see there) -ambient 134 - attraction 134 - capillary 38, 136 - centrifugal 190 - compaction 238 - dissipation 238 - free valence 232 - molecular (van-der-Waals) 34, 40, 58, 59 - short range 134 - sudden release 319 formulation system 509 Fourier - analysis 471 -functions 79 fractal dimension 58 fractals 79 fracture mechanics 243 frame, bolted 354 free radicals 40 friction plate 246 -speed 251 friction resistance 269 - higher 256 fuels, agglomerated 94 full scale equipment, optimization trials 179 Fuller distribution 85 functionality 417 fundamentals, interdisciplinary 409 fungizide 419 funicular state 34, 223 furnace - atmospheric conditions 400 - graphite 381 - ignition 404 - iron-chromium-aluminium 381 - molybdenum 381 -shaft 394 -tunnel 403 - walhng beam 402 -vacuum 383 fuzzy logic 146
14.3 Subject Index I601
h
g gas - compressed 352, 386 - entrapped 328 - escaping upwards 353 -flue 413 - process off-gas 413 -trapped 300 gas distributors - design parameters 199 - in fluidized beds 199 - perforated steel plates 199 gas pycnometer 474 gear reducer, double output-shaft 291 gel former 52 gelation 99, 226 glossary, agglomeration terms 11-27 gold 411 grain boundary 386 granulation 9, 29, 329 - compaction / granulation 508 - fertilizer 126,127,131,167,244,420,460, 507-510 - shape of granular material 243 - tumble / growth agglomeration 508 granule - agglomerated 243, 514 - fattening 426 - from compaction / granulation 421 graphite 380, 381 grate belt, continuous 403 grate-kiln 394 gravity 344 - feed chutes 345, 353 green strength 457 grinding -aid 115 -fine 113, 118 - impact 118 - limit 40 -media 378 -wet 119 gripping angle 341 grounding 115 grouts, high performance 218 growth - mechanism, control 494 - process, simulation 475 - regions 146 - requirement 139 guidelines, for a particular project 455
Hamaker constant 59 hard metal - alloy 387 - processing 393 hay 333 hearth layer 404 heat - energy conversion 36 - friction 36 heating element, noncontaminating 402 heel 143 Helmholtz-Resonator 214 hinged frame 362 hollow capsules - composite 517 - inorganic 517 - polymer 517 - removal of the core 517 - uniformity 518 home heating - briquettes 251 - solid fuels 251 honey 37, 415 hoppers, discharge 123 hot briquetting 8 hot zone 404 hybridization 440, 518 hybridizer 518 hydraulic - cylinder 315, 358 - fluid, contamination 354 - laboratory presses, hand operated 481 hydrostatic pressure 260, 375 - condition 237
i IBA (International Briquetting Association) 3 ice production plants, commercial 413 ideas, new 410 ignition furnace 404 impregnation 105 incrustation 433 industrial - applications 90 - products, agglomerated 90 inertia 440 infrastructure 461 inks 515 insectizide 419 installation - complete 492 -new 454
602
I
74 lndexes instant - characteristic 510 - product 216, 434, 510 instantizing 9 instrument, multi purpose 469 insu1ator - electrical 377 - spark plug 374, 377 intensifiers 178 interlocking bond 447 interparticle friction 237, 238, 272 investment costs 460 ionic change 436 iron - DRI (direct reduced iron) 408 - industry 405 - sponge iron 335 - - h o t 466 - - passivation 109 iron ore 107 - concentrate 405 - direct reduction 108 - pelletizing 71, 85, 145, 389, 405, 408, 507 isostatic pressing 239 - ancillary equipment 383 -hot 381 - programming of cycles 383 isostatic pressing 94 - evaluation 485 - in the laboratory 485 itabirite 107, 405
k kiln - direct-resistance 393 - grate-luln 394 kinetic studies 144 knife heads 177 know-how 453, 463 - different applications 304
I laboratory 376 - instrumentation, computer assisted 472 -model 146 - test, feed for 489 -work 390 lamellae 41 lamellar flakes 115 land area 336 land fill 172, 451 layer, predensified 287 layering, growth 415
leaching 411 heap 411 leading edge 342 leakage - of material 352 - reprocessing 275 life sciences 507 Lijhitz-van-der-Wads constant 59 lifters 417 lignin 313 lignites 312 lignosulfonate 313 liquid -bridges 190 - contaminated 442 - excessive amount 288 - immiscible 223 - manifolds 171 - penetration 511 - - speed of 511 loop -closed 502 - recirculating 502 low melting point, substances 119 lubricant / lubrication 321, 391 - development 46 - die wall 322 - electrostatic charge 323 -extrusion 305 - in pressure agglomeration 46 - liquid 46 - powder 323 - rapid removal 400 -selection 46 - solid 46 - tool surface 321 lumping 508 -
rn machinery, best suited 506 magnetic field, oscillating 439 magnetic properties 41 maintenance 278, 328, 460, 461, 509 malleability, increased 458 manufacturer know-how, different applications 304 marketing 328 marumerizing 245 mass, expanded 299 material - carrier 105, 106 - characteristics 145 - compacted - - abrasion resistance 373
14.3 Subject Index
-- - continuous strand 314 -- - densified skin 373 - composit 240, 389 - easy soluble 134 - easily extruding 256 - elastic 319 - - processing 307 - engineered 507 - extruded - - crumbling 239 - - granulating 239 -fibrous 267 -fine 319 - granular - - easily dispersible 465 - - excellent dispersibility 463 - - good yield 463 - - sufficient strength 463 - handling characteristics 461 - hazardous 462 - heat sensitive 210 - high elasticity 313 - h o t 335 - instant characteristics 510 - interstitial gas, large amount 313 - low melting point 291 - metal bearing 42 - metallic 507 - mineral 230 - organic 94 - plastic 258 - processed 468 - properties 489 - rheology 245 - similar 468 - temperature sensitive 291 - thermoplastic 119 - thixotropic 291 -toxic 462 -tramp 276, 310 - undersize 136 - water soluble 216 -waxy 291 - with caking tendencies 221 - with new, engineered physical properties 440 mechanical process technology 239, 453 mechanofusion 440, 518 medicine 7 melt soldification 2 melting, partial 387 mercury porosimetry 471 metal - bearing materials 42, 403
-chips 335 - high melting point 224 - ore 396 - part, heavy 401 - rolling 357 - scrap 334 -turnings 333 metallurgical industry 251 method - interdisciplinary application 245 - most likely 462 -options 460 - preselection 490 - selection 458 - suitable 462 microencapsulation 515 - aerosol based 436 - electrostatic 436 - functionality 434 - packaging method 434 micronutrients 105 microstructure 240 -product 507 mill / milling 178 - cage 500 - cryogenic 119 -feed 277 - gentler 499 - high energy 439 - pellet (see there) - p u g 171, 301 - roller mill (see there) mill scale 402 minerals 42, 137, 230 minerals industry 393, 402 misconception 453 mixer - agglomerators 86 - - accessibility 169 - - cleaning 169 -bowl 181 --batch 182 - - large volume 182 - - quasi-continuous 182 - cylindrical drum 166 - EIRICH granulating 172 - Schugi Flexornix 185 - type of agitation 152 - vortex pattern of movement 180 mixing action 178 mixing chamber - flexible sleeve 186 - S c h ~ g i 186
I
603
604
I
14 Indexes
mixing took 171 mechanical action 196 -paddles 17G -pins 176 - PIOW 175 - plow-shaped element 176 -varying 176 miwing - back 199, 430 - high intensity 430 - in liquids 113 mix-muller -batch 497 - continuous 497 moist granulator 273, 276 moist powder, cohesivness 148 moisture - content 456, 456 --maximum 128 - free 456 - residual 390 molasses 313 mold washing 380 molecule / molecular - bipolar 41 - force field 39 - forces (van-der-Wads) 34, 40, 58, 59, 513 monosized pieces, in bulk 252 montmoriflonite clay 408 morphology, product 507 mortars, high performance 218 motion -random 441 - tumbling and cascading 225 moving bed 221 muller wheels 285 multiphase body, particulate 386 -
n near net-shape 320, 459 - green parts 94 needs, end user 507 nest 7 - of swallows 6 - of termites 6 neutral -angle 341 -plane 323 newsprint 451 nip - back-flow 290 - compression rate 269 -geometry 269
-shape
364
- size 364 -volume 269 - wedge-shaped 269 no load - conditions 353 -gap 358 nonreturn valve 359 non-wovens 447 novel technology, development 513 nozzles - centrifugal force 190 - dispersion 190 - multiple 193 - pneumatic 430 - single phase 191 - spray nozzle (see there) - two-phase 191 nuclear reactor, spherical oxide fuel 226 nucleation 141 nuisance dust 244 nut, hydraulic 288 nutrient 508 - mineral source 508 - soluble 507 0
one-pot processing 143, 211 operating / operations - behavior, different 492 - COStS 460 - large scale industrial 479 - parameter, new location 491 - pressure 357, 358 - smale scale laboratory 479 - system, continuously, simulate the conditions 491 optimization 502 - system 491 oral administration 417 ores 118, 137, 335, 402 - concentrate 394 - low grade 411 organic material 94 organized structure, particle 520 OSHA 462 outside experts 454 out-sourcing 492 overcompaction 310 overfertilization 508 overflow chute 347 overheating, mechanical parts 403 overload 310 -feature 357
14.3 Subject Index I605
- protection 276, 319, 339 - situation, response 360 overpressing 319 oxygen candle 108 oxygen generators, chemical 106, 108 oyster-mouthing 343
P packing 389 - parameters 83 - solid particles 388 paddles 294 paint system - dry powder 515 - solvent based 515 - thermoplastic 515 - thermosetting 515 pan, fluidized 420 pancreatin 335 paper 333, 447 -making 313 - packing 451 - secondary 451 - strength 451 -waste 451 particle mass, mechanically fluidized 175 particle - aerosol 442 - aggregate, amorphous 222 - airborne 244 -attraction 445 - avalanging 225 - bed, state 220 - behavior 100 - biocolloids, non-spherical 5 16 - brittle 232, 300 - - desintegration 497 - capture and removal 222 - carbide 387 - characteristics 100 - collision 139 - colloidal 227, 517 - composite deposit 521 - deformation 300 -dense 514 - destruction 300 - dispersion 445 - distribution 456 - electrified 521 - feed, preagglomerated 97 -fine 89, 300 -flow 355 - from suspensions 222
- homogeneity 101 imperfections 101 interaction between 80, 237 - - polymers 445 - jet 521 -large 300 - loosly bonded 501 - low porosity 514 - malleable 233 - manipulation 521 - melt coated 245 - micro topography 59 - microencapsulated 433 - multilayer-shell 517 - nano, by sublimitation 514 - new quality 102 - no dislocations 514 - partially solidified 197 - plastic 300 - - deformation 497 - points of contact 388 - powder packing, behavior 80 -primary 90 - - nanoporous 95 - product, uniform, spherical agglomerates of fine crystals 223 - random movement 430 - rearrangement 233, 388 - respirable 440 - roughness 58, 78, 471 - rounding 245, 501 - shape 78, 232, 471 - silicious 223 - size 78, 100, 232, 456 - - analysis 113, 469, 471 - - characterization 111 - - distribution 78 - - modifications 241 - spherical 224, 245, 516 - standard orientation 471 - stochastic motion 139 - surface --charge 446 - - configuration 409 -symmetry 471 - system 81 - texture 79 - ultrafine 89, 514 - - collision 441 - - contact between 441 - - contamination 441 - - high reactivity 441 - - human health 441 - - suspended 441 -
-
606
I
14 Indexes
particulate - matter, characterization 81 - products, novel 103 - solids, agglomerative behavior 61 parts - high definition 320 - near net-shaped 389 pastillator 32 pastilles 29 peat 307, 312 pellet / pelleting 9, 232, 239, 242, 405 - animal feed 507 - cartridge, quick change 280 - cylindrical 29 -density 287 - exit on the outside 278 - flat die, top driven 287 - iron ore 71, 85, 145, 389, 405, 408, 507 - mill 277 - - directly gear driven 280 --gear 290 - - partial cut through 278 - - stalling 290 - - technical data 281 - - V-belt drive 280 - presses 234 - - flat die 289 - roughened 280 - size 280 - surface, defect 267 pelletizer -cone 160 - disk 153 - M M C d m m 160 pendular state 34, 223 penicillin 335 performance - factor 490 - control of a commercial installation 147 - differences 461 pharmaceutical industry 73, 143, 230, 245, 246, 251, 253, 315, 321, 325, 332, 340, 344, 362-364, 457, 459, 485, 492, 507 - cleaning requirements 181 pharmaceuticals 104, 510 phase equilibria 388 phenomena, great variety 413 pigment 104, 435, 510 pile set 508 pill - coating 8 - making 415 pilot plant 70, 376, 390, 489, 491, 492 pitch 37
plant - burden preparation 404 - commercial ice production 413 - continuous 502 - design 468 - development 453 - direct reduction 408 - food, best possible 508 - internal mass flow 502 - municipal water treatment 443 - nutrients, solid particulate 509 - pilot 70, 376, 390, 489, 491, 492 - sinter 402, 405 - species 508 - underperforming 491 - waste incineration 451 plastic 375, 381 - deformation 137, 234, 364 - flow 387 plasticizer, extrusion 305 plenum 197, 199 plow, adjustable 278 plug flow 199 pneumatic conveyors 119, 504 pocket shape 343 pollen 415 pollution, secondary 103, 449 polyester filament 447 polymer 436 - film forming 433 - high molecular weight 445 - influencing the affinity 445 polymerization, in situ 435 pore system 389 pores - closed 89, 300 - elimination 385 - inkbottle 90 - isolated 237 -open 89 - penetrating 89 - related data 472 - size 472 - structure 472 - tortuosity 472 -volume 472 porometer, capillary flow 474 porosity 137, 405, 456, 471 - closed 389 - compacts 241 - connected 389 - controlled high 383 - high 389 -open 389
74.3 Subject Index I 6 0 7
post-treatment 78, 134, 299, 390, 494, 501, 508, 522 -- characteristics of agglomerates 241 -methods 150 -- pore structure 252 - sintering 252 - tumble / growth agglomeration 241 pot grate test 395 potash 73 - cubic crystallites 240 powder - cohesive 176 - compressible 328 - directly compactable 329 - dispenser 215 - excipient 329 - flow analysis 471 -good flow 328 - noncompressible 328 -poor flow 328 - pregranulated 328 - refractory 388 - spray dried 194 -toxic 225 powder metall / metallurgy 94, 224, 242, 319, 323, 381, 391, 401, 459 - applications 320 - pressing of 321, 377 powder mixture - free flowing 253 - non segregating 253 power consumption 264 - analysis 147 - measurement 147 pozzolanic effect 218 pregranulation 326 preselection 463 press performance 288 press roller 278 - hollow 273 press table, fixed 315 presses / pressing - confined die 458 - confined volume 315 - double pressure 317 - drive 317 - - development 319 - - new 319 -ejection 315 - force, each level 323 - hand-operated 315, 317 - high precision 328 -horizontal 315 - hydraulic 317, 459
- - die 334 - laboratory presses 481
-
large vertical 323 isostatic 485 - mechanical 317 - modul, cold isostatic 485 - pellet 234 - pressure capabilities 315 - punch-and-die 234, 345, 481 - r a m 94 - reciprocating punch 307, 315 - roller presses (see there) - single stroke 315 - tool 269 - - cylindrical 288 - vertical 315 - withdrawal 315 pressure - developed by the screw 294 - laboratory test 479 - levels 380 - process development 479 - rise, rate 300 -vessel 377 pressure agglomeration 70, 87, 93, 229ff., 242, 492, 495 - applications 230 - binding mechanism 231 - different technique 253 - equipmemt 230 - granular product 498 -high-pressure 41, 78, 82, 94, 136, 231, 232, 242 - - d r y feed 456 - - methods 300, 495 - - speed of densification 300 - level of force 231 - low-pressure 136, 232, 242, 245 - - laboratory development 479 - - methods 495 --porosity 93 - medium-pressure 136, 242 - - basic principle 269 - - flat dies 284 - - laboratory set-up 479 - - methods 495 - - most commonly installated 277 - - overfeeding 271 - - p l u g UP 271 - - porosity 93 - - process development 479 - - roller rotation 286 - moisture 456 - multiple units 230 --
-
608
I
14 Indexes -
- granular 242, 245, 252, 336
-
- - crumbled 253 --density 356 - - intermediate 363 - - strength variations 356 - high porosity 256 - highly reactive 108 - high value 507 - instant 216, 434 - intermediate 105, 492 - narrow particle 221 - near net-shape 320, 459 - off-grade material 275 - particulate, novel 103 - porosity 459 - pressure cooker extruder 299 - quality 74, 147, 273, 461 - reactivity 463 - removal 274 -shape 459 - size 502, 507 - small quantities 507 - special 435, 507 - specific surface 459 - spherical 459 - sticking 274 - strength 268, 338,459, 463 - - h i g h 499 - tablette 326 -uses 525 - with comparable quality 491 - yield 147 production - capacity, hybridization system 520 - facility, regional 509 - rate 285 -short run 280 production line - back-up manufacturing 491 - co-manufacturing 491 - contract manufacturing 491 project - cost limitation 491 - development 461 - management 454 properties, desirable 507 Pug - mill 171, 301 - sealer 301 pulse combustor 214 Pump - double action 379 - high pressure 379
new special technique 413 particle size 456 - - distribution 497 - potential problems 373 -reasons 230 - technique 230 - pressure release 343 prills 29 - urea prills 31 process - controll 417 - effluents, liquid 442 - equilibrium 147 - research 453 - parameters / data 146 - variables 147 - - modifications 147 processing - for disposal 411 - parameters 502 product - abrasive 214 - agglomerated 90 - - characteristics 61 - - dimensions 458 - - dustfree 458 - - evaluation 474 - - free-flowing 458 - - granular 458 - - in the laboratory 475 - - industrial 90 --shape 458 - - size distribution 458 - changing 280 - characteristics 107, 435 - - influenced by structure 241 - composition 507 - consumer 47, 106, 251, 375 - - aesthetics 328 - corrosive 214 - density 268, 320 - description 525 - design 105 - discharge 310 - dispersibility 241 - distribution 502 - easy soluble 510 - encapsulated 433 - engineered 103 -final 232 - - characteristics 242, 389 - - properties 433 -food 48 - from different feed mixtures 230
74.3 Subject Index I609 pumping system - delivery rate 378 - pumps in parallel 378 - using different types of pumps 378 punch-and-die presses 234, 345 - laboratory evaluation of 481 - samples of 481 punches 325 pusher, stoker type 401 pycnometer, gas 474 pyrotechnic components 106, 108
9 quality - assurance 70, 469 - control 72, 469 -product 74
r ram - extrusion press, capacity 311 - force 310 - moving 315 - presses 94 rate terms 146 raw material - off-specification 509 - secondary 289, 461, 492 reaction, interfacial 435 real system, response 146 recombination bonding 241 recondensation 387 reconstitution, ease of 215 recontamination 462 recrystallization 134 - disolved substances 189 recycle / recycling 136, 460, 461, 495, 497 - rate 497 reducibility 108 reducing atmosphere 391 reduction, direct 106 - iron ore 108 redundancies 460 refactory 380 - brick 323 - high performance 52 - metal 393 refuse 411 regulated industrie 147 regulatory authorities 332 release - compacted material 341 - pressure 343
residence time distribution 201 - controlled 204 - narrow 202 - wide 202 riffle splitter 71 rigid former, internal 376 risk minimation 490 roller 262 - abrasion resistant coating 280 - axial grooves 272 - clean out 275 - clearance / gap 288 - cooling 288 - diameter 364 - exchange 354 - floating 358 - - non parallel position 359 -gap 118, 275 -leakage 275 - mills 115, 345 - narrow 347 - pan grinder 285 - subdivided 354 - two or more rings 354 -wear 275 -wide 347 - with different diameter 364 roller presses 235, 325 - applications 362 - briquetting 336 - compaction / granulation 335 - comprehensive theory 345 - design details 362 - dust control 488 - gravity feed 345 - high pressure 485 - hydraulic pressurization 357 - laboratory evaluation 485, 488 - large machines 345 - mechanical design 339 -modern 345 - operation, phenomenological 345 - parameters 345 - scale-up 364 - selection 362, 364 - simulate 485 - sizing 364 - - equation 369 - small scale production 488 - spring loaded 357 - versatile technology 335 - very large 363 roller screens 163 -modern 494
610
I
14 fndexes
roller surface, interlocking effect 342 rotary distributor 206 roughness peaks 38 rounding - crjstals 426 - granules from compaction / granulation 426
s safety
- factor 491 - precaution 280 -valve 359 sample / sampling 71 - access point 503 - equipment 503 - extractor 332 - particulate solids 490 - point 147 - representative 490 sauce 510 saw dust 333 scale-up 144, 361, 430 - low-pressure extruders 264 - optimization trials 179 scalping 111 scarabaeus sacer 7 Schugi Flexornix 185 scrapers 162 screen - green agglomerates 494 - gyratory 501 - mechanically excited 501 - roller 163, 494 - vibrating 163, 501 screening 111 -trick 163 - wet screening 111 screw - arranged under an angle 356 - design 257, 259, 265 - diameter 356 - flights - - cleaning / coating 266 - - overlapping 356 - - variable pitch 295 - force 343 - metering 301 - multiple 356 - predensification, plug at the end 296 - press, conical 314 - rotating 257
-transport 257 vertical feed 313 screw feeders - uneven feeding 355 - uneven compacting 355 seasonal peak demands 492 sediment 104, 113 seeding the charge 494 seeds 141 segregation 239, 458, 490 selection criteria 492 self-assembly process 516 separation - efficiency 440 - - improvement 111 service life 375 shaft - diameter, changes 295 - journals - - conical 357 - - cylindrical 356 shaft furnace 394 shear - cell 471 - forces 113, 238 -plane 239 shelter 7 shredded plastic 333 shrinkage 385, 388 shut-dom 332 shuttle feeder 331 silica fume 85, 104, 143, 217, 218, 513 - enduse 219 - handling 219 silica spheres 522 silicon metal 218 silos, discharge 123 simultaneous processing 460 sinking behavior 510 sintering / sinter 9, 89, 95, 137, 242, 383 - batch - binding mechanism 385 -breaker 404 -cake 397 - conditions 97 - densification 387 - diffusion phenomena 96 - dimensional changes 373 - directly flame heated 397 - distortion 374 - driving force 385 - furnace, continuous f car type 397 - induration 405 -kinetics 386 -
74.3 Subject Index
- mechanism 386 metallurgical description 388 movement of matter 387 - neck growth 387 - plants - - continuous 402 - - new developments 405 - post-treatment 138 - pot grate machine 396 - in powder metallurgy 138 - pressing tool 99 - pressure assisted 389 - processes 96 -reaction 388 - requirements 388 - shrinkage 96, 387 - size 397 - - enlargement process 385 - solid state 385 - stages 386 - technology 389 - temperature 385 - under pressure 388 sintering temperature 225 site 461 size - enlargement - - by agglomeration 119, 141 - - desired 133 - reduction 40, 101 slugging 432 snack pieces, puffing 299 sodium cyanide 335 sodium starch glycolate 51 sol, aqueous 227 sol-gel process 99, 435 solid bridges 35 - 37 - chemical reaction 36 - colloidal particles 37 - crystalization 37 - hardening binders 36 - recrystallization 37 - temperature fluctuations 37 solid fuel 335 - biomass based 313 solids - contamination, airborne / respirable 413 -fine bulk 238 - finely divided 221 -heating 240 - manipulation 507 - particulate - - adhesion tendencies 471 -
- bridging 123 - - bulk characteristics 490 -
--change 490 - characteristics 502 - - compaction, confined volume 315 - - conveying 119 - - flowability 471 - - high reactivity 107 - - interactions 1 - - rotate with a screw 266 - - source 461 - - surface, modification 490 - powder form 224 - processing, particulate, future 103 - refrigerated 240 - size reduction 243 - spherical, by agglomeration 224 - suspended in liquids 221 - yield point 237 solubility 127, 510 solution-reprecipitation process 388 solvent, halogenated 225 soup 510 space requirements 460 sparger tubes 167 special technology, examples 410 specific force, very high 300 spheronization / spheronizer 93, 136, 236, 246 - auxillary device 247 - batch operation 249 - cascade operation 250 - effect 251 - laboratory development 479 - residence time 251 scale-up 250 - system capacity 251 sponge iron, passivation 109 spouted bed 196, 430 - condition 221 - gas-solids contacting efficiency 22 1 spray drying / dryer 140, 151, 187, 199 - Filtermat 195 - formation of agglomerates 193 - gas handling and flow 193 - hollow particles 205 - recirculation of fines 193 - with nozzle atomizer 192 spray electrode 41 spray nozzles - atomizing 165, 186 - drip free 165 - liquid 165 -
-
Im1
612
I
74 Indexes -
spray pattern 165
- two phases 165 spray systems telescopic 417 - with nozzles 417 spring back 234 stabilizers 104 starch 231 start-up 332 starved state 258 statistical evaluation 77 steam - condesation 215, 216 -engine 336 - thermal energy transfer 215 steel industry 405 steric stabilization 445 sticking 141, 149, 247 stone mastic asphalt 33 straw 333 stream lines 450 strength - agglomerate (see there) - compression 64 - development 241 - high 241 - tensile 64 stripper dust 334 stroke - excessive 313 - frequency 310 - high pressure 317 -length 310 - low pressure 317 structure - compacts 241 - during pressure agglomeration 236 - fundamental units 386 - high-pressure agglomeration 237, 240 - interlocking fibers 451 - low-pressure agglomeration 236 - medium-pressure agglomeration 236 - microstructure 240 - modifications 242 - o f a part 323 - pelleting 236 -product 240 - tumble / growth agglomerates 236 suface charge, electrical 446, 450 sugar - beet, spent slice 313 - natural 313 sulfur 420 - liquid 420 -
supply 461 support facilities 461 surface - accessible 105 - conditioning, improvement 417 -curvature 387 -energy 385 - equivalent diameter 65 - free 385 - newly created 40, 232 - roughness 60 - specific area 101 - specific energy 385 - structure, microscopic 42 - tension 38, 136, 218 - wettability 223 surfactant 105, 227 - aid 514 surging / surge bin 495, 497, 498 suspending liquid, displacement 222 suspension 104, 446 - agitated 445 swallows, nest 6 symposia, agglomeration 3 system - layout, selection 455 - optimization 491 t tablette / tabletting - capping 328 -crowned 328 - faceted 328 -flat 328 -quality 321 - research and development 481, 485 - rotary machine, high speed / high pres. sure 326, 458, 459 -shapes 328 taconite 107, 405 talcum 415 tamping device 334 tar 37 taste masking 7, 52, 417 technology, new / improved 453 temperature - change, rate of 390 - fluctuations 37 tensile strength 64 termites, nest 6 test facility 147 - location 490 testing 468 - bulk properties 469
74.3 Subject /ndex
- continuous operation 491 - entire production line 491 - equipment 491 - result, effect 490 thermoplastic
- materials 119 - powder 515 thickening agent 52 thixotropic properties 288 threads 41 tissue 451 tolling companies - evaluation of products 492 - marketing 492 - new facilities start-up phase 492 - pilot plant 492 - project development 492 - testing of new materials 492 tolling operation 147 toner material 435 tongue controls 347 tool, intermeshing 306 too1ing -dry bag 376 - ejection presses 320 - exchangeable 320 - multiple 327 - withdrawal presses 320 trade names 11 trailing edge 342 tramp material 276, 310, 357 transfer points 412 transport facilities 461 trash 335 travelling grate 394 tray handling system 400 triple superphosphate 125, 420, 509 tube 305 tumble 1 growth agglomeration 86, 136, 139ff., 229, 241, 492, 494 - alternative method 413 - equipment 140 - feed particle size 456 - growth --control 456 - - mechanism 144 - kinetics of 479 - mechanism 415, 479 - moisture 456 - operation strategies 143 - process steps 141 - requirements 140 tumble test 71 tumbling mass, solids 162
I
613
tungsten 393 tungsten carbide 224 tunnel furnace 403 turbines 178 turret 325 - assembly 332 U
undersize material - recycle 136 - separation 136 unit operation - agglomeration 3 - mechanical process engineering 1 updraft 395 urea 509 - p r i k 31 - supergranules 510 utilities 461 V
validation 332 valve, rotary vane 199 van-der-Wads interactions 34, 40, 58, 513 - macroscopic / microscopic theorie 59 vapor pressure vegetable pulp, frozen 414 vendor - experience, different application 304 - specification 10 vibrating screens 163, 501 vibro fluidizer 210 viscous flow 387 volatile organic compounds 515 W
walking beam furnace 402 wall - build-up 119 - friction 234, 238, 307, 321 washing in place 332 waste 172, 289, 411, 492 - disposal 460 - incineration plants 451 - organic 313 - paper 451 - recycling 460, 507 - water treatment 442 water - purification 445 - treatment plant, municipal 443 wax 436 wear 305, 314 wear liner, autogenous 162
614
I
74 Indexes
web 336, 447 weight adjustment ramp 325 wet - conglomerates, build-up 149 - screening 111 wettability 457 wetting 510, 511 - angle 58 - dynamic 511 winterization 461
wood - chip board 334 -shaving 333 Wurster coater 221. 430
Y yield 458, 499 Z
zeolites 95