Comminution energy requirements

In comminution processes, energy consumption is often the most important design consideration. In fact, it has been estimated that 1 per cent of global energy consumption is used in comminution. Energy consumption is a function of the size and hardness of the material and the required degree of breakage or surface area formation. Empirically... 

Since comminution involves new surfaces, the energy required to produce a given amount of surface can be decreased by reducing the solid surface energy through the adsorption of surfactants. 

The capacities of comminuting machines are best discussed when the individual types of equipment are described. The fundamentals of product size and shape and of energy requirements are, however, common to most machines and can be discussed more generally. 

CHARACTERISTICS OF COMMINUTED PRODUCTS. The objective of crushing and grinding is to produce small particles from larger ones. Smaller particles are desired either because of their large surface or because of their shape, size, and number. One measure of the efficiency of the operation is based on the energy required to create new surface, for as shown in Chap. 28, the surface area of a unit mass of particles increases greatly as the particle size is reduced. 

Energy requirements for mechanical dispersion are determined by the mechanical properties of solid phase and are dictated by the desired degree of dispersion. Brittle materials are usually broken down easily while it is rather difficult to crush ductile ones. With respect to the particular degree of dispersion, it is possible to distinguish crushing (coarse dispersion to sizes of several centimeters or millimeters), comminution (to sizes on the order of tens of microns), and fine dispersion. Crushing usually obeys an empirical... 

The advantages of steam explosion pretreatment include the low energy requirement compared with mechanical comminution and no recycling or environmental costs. 

The diversity of designs of the above-described machinery implies that their operating variables differ quite considerably. While energy requirements are generally governed by the comminution laws previously discussed, as illustrated in Example 4.1, some other features such as capacity and rotational velocity are particular for each imit and may be related, in some way, to the predominant force performing the reduction action. Two types of equipment with critical particularities of operation would be roller mills and tumbling mills. 

On the basis of stress analysis theory for plastic deformation. Kick (1885) proposed that the energy required in any comminution process was directly proportional to the ratio of the volume of the feed particle to the product particle. Taking this assumption as our starting point, we see that ... 

The energy required for the comminution, or size reduction, of a material to a certain required fineness (characterized by the specific surface of the product obtained) will depend on the hardness of the material, its compressive strength, its brittleness (or its elasticity or its plasticity), the size and shape of its particles, its temperature and moisture content, and of course also on the nature of the comminuting action exerted by the grinding process employed These factors in combination determine the resistance that the material offers to size reduction and can be regarded as specific of the material. 

Size reduction (qv) or comminution is the first and very important step in the processing of most minerals (2,6,10,20—24). It also involves large expenditures for heavy equipment, energy, operation, and maintenance. Size reduction is necessary because the value minerals are intimately associated with gangue and need to be Hberated, and/or because most minerals processing/separation methods require the ore mass to be of certain size and/or shape. Size reduction is also required in the case of quarry products to produce material of controlled particle size (see Size measurement of particles). In some instances, hberation of valuables or impurities from the ore matrix is achieved without any apparent size reduction. Scmbbers and attritors used in the industrial minerals plants, eg, phosphate, mtile, glass sands, or clay, ate examples. 

In providing an overall picture of the Griffith theory applied to the comminution process, it must be pointed out that the theory requires that a tensile stress should exist across a crack to open it further. While a uniform compressive force can close a crack, a nonuniform one can lead to the occurrence of localized tensile stresses. In a comminution process the particles are subjected to nonuniform loading, and therefore it can be surmised that they normally break in tension and not in compression. However, the tensile component of loading in comminution does not form the major loading component and this contributes towards a lowering of the overall energy efficiency of comminution. 

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