Improving pyrite liberation and grinding efficiency in fine coal comminution by swelling pretreatment

Experimental studies with Kentucky bituminous coals have shown that ash content has a significant influence on the coal's grindability. For the Kentucky coals studied, the specific energy consumption in grinding the coal to 10 μm, at which 90 percent liberation of the mineral matter can be achieved, exceeds 400 kwh/ton for high ash-content (> 20%) coals, while less than 100 kwh/ton energy consumption is required for low ash (< 5%) coals. An innovative technique has been investigated to improve the liberation of mineral matter and, simultaneously reduce grinding energy consumption in fine coal grinding of the coals. By treating coal with a swelling chemical agent prior to grinding, the grindability of coals can be considerably improved. The Hardgrove Grindability tests show that the HGI values for a high-pyrite Kentucky No. 9 coal is increased by 50-150 percent after swelling pretreatment, depending on the treatment reagent and time. Batch stirred ball mill grinding results demonstrate that this new technique has a great potential in reducing the energy consumption of fine coal grinding. Depending on the pretreatment conditions, the specific energy consumption of producing less than 10 μm product is reduced by 40-70 percent of that of the untreated coal feed. The grinding kinetics also increases considerably for the pretreated coal. Furthermore, pyrite liberation for the pretreated coal is improved by 50-100 percent of that of the untreated coal. Microscopic examination using scanning electron microscopy and optical microscopy shows that the swelling pretreatment induces microcracks and microfractures in the coal matrix, preferentially along the grain boundaries and are responsible for the improved grinding performance.