Energy for size reduction

Processing costs include those for size reduction, size classification, minerals concentration and separations, soHd—Hquid separation (dewatering), materials handling and transportation, and tailings disposal. Size reduction, one of the most expensive unit operations in minerals processing, could account for as much as 50% of the total energy consumed. This cost varies considerably from deposit to deposit and quite often from one area of a deposit to another. Ore bodies are extremely heterogeneous and the associated minerals Hberation, complex. 

Stressing by Nonmechanical Energy. Such processes are not fully developed but examples exist of a plasma reaction (Fig. 3g) being used for size reduction. Such cases, however, are specialized and not in general use. 

Heat Sensitivity. Only 1—2% of appHed energy is effectively used for size reduction. The remainder is mainly converted to heat, which is absorbed by the grinding air, product, and equipment. 

One of the most far reaching analyzes along these lines of thought was given by Commenge [114] in the context of gas-phase reactions in continuous-flow processes. Specifically, he analyzed four different aspects of micro reaction devices, namely the expenditure in mechanical energy, the residence-time distribution, safety in operation, and the potential for size reduction when the efficiency is kept fixed. 

Since the surface of unit mass of material is proportional to 1/L, the interpretation of this law is that the energy required for size reduction is directly proportional to the increase in surface. 

Currently, besides catering to the pharmaceutical industry, Luid energy milling is employed for size reduction of agricultural chemicals, carbon black, ceramics, cosmetics, pigments, precious metals, propellants, resins, and toner materials. 

FIGURE 4.3 Average energy required for size-reduction equipment, O is typical product size, is typical feed size. 

More recent studies have shown that a magnetic method may reveal the distribution of particle sizes in supported nickel catalysts. The method appears to be effective down to near-atomic dimensions, and it permits independent determination of rates and activation energies for the reduction process as contrasted with the sintering, or particle-growth, process. The structural relationship of impurities or promoters, such as copper, in the nickel is readily determined, and extension of the method to cobalt and iron catalysts seems possible. 

In general, the smaller the particle size, the more rapidly dehydration occurs. This can be justified on the basis of surface area and/or, in some cases, the existence of crystal defects. The latter is particularly true for substances that have been subjected to high-energy particle size reduction processes. The impact of morphology may be more subtle and is certainly not independent of size considerations. The balance between the two mechanisms has been a topic of discussion, but very little quantitative work has been done to elucidate the relative contribution to the dehydration process. 

FIGURE 6.2 Relationship of specific energy to size reduction for two coal cleaning devices. (From Meyers, R.A., Ed., Coal Handbook, Marcel Dekker, New York, 1981, p. 176.)... 

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%. 

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