Powder Mixing

Powder Mixing Brian H. Kaye

Kluwer Academic Publishers, Norwell, MA, 276 pages, $129.95 Although the mixing of powders is a commonly practiced unit operation in many processes, this is the first book devoted entirely to this subject in English, to the best of my knowledge. The author is an internationally known researcher in the physics of powder mixing, and has written extensively on particulate technology. The book contains nine chapters, an author index, a subject index, and a wordfinder index at the beginning to enable the reader to find the first occurrence and definition of technical terms used throughout the book.

Chapter 1 is a general review of mixing technology that includes: an historical overview of powder mixing science and technology; a holistic approach to powder mixing, mixers, and powder mixing mechanisms; techniques for describing the structure of a powder mixture; and graphical and experimental descriptions of a powder mixture structure. The technology for characterizing single powders and powder mixtures is discussed in Chapter 2. Prof. Kaye thoroughly describes methods for sampling a powder mixture, techniques for characterizing the grain sizes of a powder, quantitative evaluation of the shape of powder grains, fingerprinting of powder mixtures using an aerosol spectrometer, and characterizing a powder mixture by its permeability. In Chapter 3, the principles of powder rheology are reviewed, among them: powder flow characterization (angles of repose and sliding or drain, effects of flowassisting agents, and flow characterization funnels); the use of flowassisting agents; settling in a vibrated bed (evaluation of tapped bulk density and bed voidage); and characterizing the flow behavior of a powder by studying avalanching behavior (a major contribution by the author to powder rheology).

Chapter 4, "Can Ingredient Modification Expedite Mixing Strategies?," discusses alternative ingredient strategies for solving powder mixing problems, modifying the size distribution (some interesting actual solutions to industrial problems are presented), microencapsulation of ingredients, and techniques for producing microcapsules (a number of industrial processes and equipment are described). The monitoring of mixers and mixtures is thoroughly reviewed in Chapter 5, which covers topics such as distinguishing between chaos-creating operations and dispersion mechanisms, Poisson tracking as a technique for studying chaotic conditions in a powder mixer, using radioactive tracers to follow powder dispersion in powder mixing equipment, monitoring mixture structure by optical reflectance, fingerprint sizing of powder mixtures to monitor the performance of powder mixing equipment, characterizing the structure of consolidated mixtures by optical inspection, auto- and cross-correlation of mixture structure, and infrared fingerprinting of powder mixtures.

The impact of chaos theory and experimental mathematics on powder mixing theory and practice is discussed in Chapter 6. It opens with an introduction to chaos theory as it applies to powder mixing, and then discusses random walk models of powder mixing. This short chapter will be very useful to those engineers not familiar with chaos theory, which is becoming more widely used in analyzing mixing of liquids, powders, and pastes. Chapter 7 is devoted to active mixing machines (ribbon, tumbling, and high-shear mixing and multimechanism mixers), while Chapter 8 covers passive powder mixing systems (baffled passive mixers and gravity in-bin mixing devices). Both of these chapters describe a number of industrial mixers and provide the names and addresses of manufacturers. Some widely used industrial mixers with which I am familiar and have used are only briefly mentioned, or not even cited. It would have been helpful if the author had also included tables comparing the performance characteristics and applications for various types of mixers. The final chapter on turning powder mixtures into crumbs, pastes, and slurries reviews how liquid bridges hold together the constituents of a powder mixture and how pastes are formed for use as extrudates.

An excellent review of one aspect of mixing technology, this highly readable book should prove useful to chemical engineers involved with powder mixing processes (both in development and plant operations), powder technology researchers, and grad students.

[Author Affiliation]

Stanley S. Grossel

S. S. Grossel is president of Process Safety & Design, Inc., Clifton, NJ.