An Investigation of Particle Size Variation in Stirred Mills in Terms of Breakage Kinetics

Abstract

The structure and particle size of solid fossil fuels have effects on linear or nonlinear movements encountered in breakage rate, besides petrography types, in-organic materials, extensive network of pores, macro chemical forms, etc. Nonlinear breakage behavior in mono size groups of −3,350 + 2,360 μm, −2,360 + 1,700 μm, and −1,180 + 850 μm was a result of heterogeneous structure of the material along with being coarser for grinding medium. The part having low-ash content (coal-like) in the heterogeneous structure was exposed to faster breakage than the part having high-ash content (shale-like). As a result, over-sieve material was composed of shale, which is more difficult to grind, breakage behavior slowed down, and there occurred a deviation in linear breakage behavior with increasing grinding period. Linear breakage behavior in mono size groups of −425 + 300 μm and −212 + 150 μm was considered as a result of homogeneity of the material along with its adequate fineness for grinding medium. Variation of particle size in grinding is an important parameter in terms of breakage behavior. Maximum breakage rate was reached for −425 + 300 μm size group in the stirred mill. The ratio of “grinding medium size/size of the material ground” (6.0/0.425) was calculated as approximately 14. A decrease in this ratio, i.e., an increase in particle size of material, caused engagement difficulties of particles by balls. An increase in the ratio, however, resulted in a pillowing effect of fine materials in the medium and, thus, a decrease in breakage rate.