Particle size enlargement

Disc-type attrition mills are employed in the food industry for grinding cereals and grains as well as in the pharmaceutical industry. Other applications include the grinding of paint pigments and inks. Buhrstone mills are an ancient example of a disc attrition mill. 

Colloid mills are used to grind and disperse solids in hquids and to prepare emulsions. They operate on the principle of high-speed fluid shear to grind the feed material. Another application is in the manufacture of lubricating greases by dispersion of calcium stearate in hydrocarbon oils. In the paint industry, colloid mills are used to incorporate pigments in liquid vehicles. In the food industry, the mills are used to make purees, sauces, ointments, creams, lotions, and other products. 

Size enlargement is generally considered to be a process in which small particles are combined into larger masses but the individual particles can still be distinguished. Agglomeration is the formation of aggregates of material due to the fact that particles tend to stick together. 

For many purposes, materials of intermediate sizes are the most desirable forms, neither too small nor too large. Examples are catalyst beds of very small granules that exhibit too great resistance to the flow of reacting fluids or too small particles in suspensions that settle out or filter too slowly. The benefits of size enlargement and examples are presented in Table 12.13. 

Binders are frequently used to assist in the agglomeration process. They make a difference in the performance of the end product. Gantner (2003) pointed out that there are six types of binding mechanisms  

TABLE 12.10. Mill Performance Data for Grinding of Specific Products 

Cement clinker three-compartment wet tube mill 42 

Note CC is closed circuit grinding the ring-roller mill has built-in air classification. 

Ammonium phosphate, potassium chloride, potassium bicarbonate, and salt. 

---

FIG. 20-81 Atypical disc granulator Capes, Particle Size Enlargement, Elsevier, 1980). 

Because interparticle forces in very fine powders make them very difficult to handle, the effective particle size is frequently increased by agglomeration. This topic is discussed in Section 2.4 on particle size enlargement in Chapter 2. 

Capes, C. E. Particle Size Enlargement (Handbook of Power Technology, Volume 1 eds. Wilbams, J. C. and Allen, T.) (Elsevier Scientific Pubfishing Company, 1980). 

Capes, C.E., Particle size enlargement, Elsevier, London, 1979. 

In the following, the main equipment for particle size enlargement will be illustrated and discussed. 

Methods to improve properties of bulk powders by particle size enlargement... 

Particle size enlargement can occur either by the collision and successful adherence of the primary feed into a granule or by the continual growth of a small layer of solids around a nucleus, e.g., like an onion skin. In this latter case, the main attractive force is still the initial particle collision but growth is a slow and uniform process. Therefore purity of the... 

Particle size enlargement equipment can be classified into several groups, with advantages, disadvantages, and applications summarized in Table 20-36. Comparisons of bed-agitation intensity, compaction pressures, and product bulk density for selected agglomeration processes are highlighted above in Fig. 20-71. 

---