SIZE REDUCTION OF SOLIDS

In the materials processing industry, size reduction or comminution is usually carried out in order to increase the surface area because, in most reactions involving solid particles, the rate of reactions is directly proportional to the area of contact with a second phase. Thus the rate of combustion of solid particles is proportional to the area presented to the gas, though a number of secondary factors may also be involved. For example, the free flow of gas may be impeded because of the higher resistance to flow of a bed of small particles. In leaching, not only is the rate of extraction increased by virtue of the increased area of contact between the solvent and the solid, but the distance the solvent has to penetrate into the particles in order to gain access to the more remote pockets of solute is also reduced. This factor is also important in the drying of porous solids, where reduction in size causes both an increase in area and a reduction in the distance 

There are a number of other reasons for carrying out size reduction. It may, for example, be necessary to break a material into very small particles in order to separate two constituents, especially where one is dispersed in small isolated pockets. In addition, the properties of a material may be considerably influenced by the particle size and, for example, the chemical reactivity of fine particles is greater than that of coarse particles, and the colour and covering power of a pigment is considerably affected by the size of the particles. In addition, far more intimate mixing of solids can be achieved if the particle size is small. 

Whilst the mechanism of the process of size reduction is extremely complex, in recent years a number of attempts have been made at a more detailed analysis of the problem. If a single lump of material is subjected to a sudden impact, it will generally break so as to yield a few relatively large particles and a number of fine particles, with relatively few particles of intermediate size. If the energy in the blow is increased, the larger particles will be of a rather smaller size and more numerous and, whereas the number of fine particles will be appreciably increased, their size will not be much altered. It therefore appears that the size of the fine particles is closely connected with the internal structure of the material, and the size of the larger particles is more closely connected with the process by which the size reduction is effected. 

The exact method by which fracture occurs is not known, although it is suggested by I r r i i 51 that the compressive force produces small flaws in the material. If the energy concentration exceeds a certain critical value, these flaws will grow rapidly and will generally branch, and the particles will break up. The probability of fracture of a particle 

The method of application of the force to the particles may affect the breakage pattern. Prasi 11 r 6 suggests that four basic patterns may be identified, though it is sometimes difficult to identify the dominant mode in any given machine. The four basic patterns are  

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Institution of Chemical Engineers, 1975. In Comminution, Institution of Chemical Engineers Working Party concerned with the theory and practice of the size reduction of solid materials. Ed. V.C. Marshall, Institution of Chemical Engineers, Rugby, 83pp. 

Comminution, or particle size reduction of solids, is considerably different from that of the breakup of one liquid by dispersal as small droplets in another. Particle size reduction is generally achieved by one of four mechanisms (1) compression, (2) impact, (3) attrition and (4) cutting or shear. Equipment for particle size reduction or milling includes crushers (which operate by compression, e.g., crushing rolls), grinders (which operate principally by impact and attrition, although some compression may be involved, e.g.,... 

Carstensen J. Advanced Pharmaceutical Solids. New York Marcel Dekker, 2000. Austin LG. Size reduction of solids crushing and grinding equipment. In Fayed ME, Otten L, eds. Handbook of Powder Science and Technology. New York Van Nostrand Reinhold, 1984 562-606. 

Cowan reported that significant size reduction of solids occurred when cellulose acetate was dried in a spouted bed, indicating its possible limitations for handling other friable particles. 

Although the term shredders is sometimes used as a general term to include the equipment that causes size reduction of solid waste, it fits best to the equipment that uses mainly shear force to reduce the size of the materials. In such a sense, hammer mills are sometimes included in the category of shredders, as, in addition to impact force, materials are also subjected to shear force for their size reduction, in case that they are also equipped with stationary knives fixed at the wall. In this chapter, shredders are considered only the equipment that use shear... 

The granulation of powders by size enlargement, whereby agglomerates are first formed, then crushed into the desired particle size range, and finally classified to obtain the desired product size distribution, is even more difficult to analyze than the complex size reduction of solids. The reason for this is twofold. 

Introduction. In size reduction of solids, feed materials of solid are reduced to a smaller size by mechanical action. The materials are fractured. The particles of feed are first distorted and strained by the action of the size-reduction machine. This work to strain the particles is first stored temporarily in the solid as strain energy. As additional force is added to the stressed particles, the strain energy exceeds a certain level, and the material fractures into smaller pieces. 

Power required in size reduction. The various theories or laws proposed for predicting power requirements for size reduction of solids do not apply well in practice. The most important ones will be discussed briefly. Part of the problem in the theories is that of estimating the theoretical amount of energy required to fracture and create new surface area. Approximate calculations give actual efficiencies of about 0.1 to 2%. 

Particles distributed according to their mass (or volume) are frequently encountered in applications. The size reduction of solid materials is an example of such a breakage process. The evolution of drop size distributions... 

Size reduction of solid pesticide products can be accomplished with either mechanical energy or fluid energy mills (Perry, 1973). The choice depends on the physical behavior of the product, on the fineness of grind which must be achieved, and the concentration of active ingredient in the end-use product. 

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