HYPERFINE GRINDING OF COAL

Coal was subjected to intensive grinding to produce submicrometer part icles. This was done in two stages. Ultrafine coal produced by fluid e nergy mill grinding was further ground in a planetary ball mill or att ritor mill. The grinding process was monitored by density gradient sep aration and surface area measurements. Conditions were established to produce coal in the 0.05-1 mum size range with 3 h of grinding. Severa l different slurry solvents were examined for grinding efficiency. Ext ensive accretion of coal particles to produce homogeneous particles wa s found to be a major problem. In several instances there appeared to be a low melting phase produced (approximately 65-degrees-C) which cem ented the fine particles together during vacuum drying. The submicrome ter coal also interacted with the surfactant used in density gradient analysis resulting in particle size dependent density distributions. T wo cases were found that may have circumvented accretion problems: gri nding in tetrabutylammonium hydroxide/pyridine/methanol and attritor m illing in a methanol slurry at <-50-degrees-C.