Effect of different milling processes on the mineral distribution in a coal powder

Coal samples obtained from Wanbei (WB), Pingxiang (PX), Liupanshui (LP), and Datong (DT) mines were pulverized, using either a vibration mill or a ball mill, to different degrees of fineness. The effect of the different grinding methods on the mineral distribution within the pulverized coal was investigated by using proximate analysis, particle size analysis, and float-sink tests. The results show that the ash content in WB, PX, and DT coal increases with increasing particle size overall, while the ash content of the LP coal remain almost the same within each size fraction. In that case the ash in each fraction is similar to that of the raw coal. The ash versus size distributions for the same coal sample milled with the same grinding method to different degrees of fineness are similar. The ash versus size distribution of the coal powder with a 15% screen residue that was prepared with the vibration mill is different from the distribution of a similar sample prepared with a ball mill. The curves also vary between different coal samples. The grinding method has a great influence on the distribution of minerals across the various particle sizes. The float-sink tests and the laser particle size analysis results on PX and DT coal samples show that fines dominate the higher density fractions although the large +2.0 g/cm3 fraction was dominated by coarse particles. The size distribution of the low density fraction and +2.0 g/cm3 density fraction is bimodal. The size distribution of the intermediate density fraction is multimodal.