Field study demonstrates inordinate respirable dust generation during continuous mining in rock versus coal strata

In modern room and pillar coal mines, the coal is produced by continuous miner (CM) machines. The CM is used to mine the coal seam by continuously cutting at a vertical face. Depending on the seam thickness, quality, and geotechnical properties, some roof, floor, or interburden rock is often cut along with coal. While CMs can be highly efficient in terms of production rates, they can also generate high concentrations of dust. Dust poses both safety (i.e., explosibility) and respiratory health hazards. Previous research has generally indicated that CM cutting in rock yields much more respirable dust than cutting in coal. Although in-mine studies that directly evaluate this trend have not been reported, understanding relative dust generation from different geologic strata could have important implications. In many mines, for instance, the rock is the primary source of respirable silica and silicates, which can be especially hazardous. To mitigate dust generated by the CM, mines use a variety of controls including ventilation, on-board scrubber systems, and water sprays. However, the relative effects of controls on dust generated from different strata have also not been widely investigated. In this field study, respirable dust sampling was conducted in the intake and return airways of an active CM during periods when the cutting was targeted either primarily at the coal seam (bottom cut) or primarily at the roof rock (top cut) in a standard entry. Results indicated that CM cutting in rock strata generated somewhat finer particles and respirable dust concentrations that were 2.1-26 times higher than cutting in coal strata, although the coal height being cut was about 2.2-2.9 times greater than the rock height. Additionally, the analysis of dust mineralogy generally showed a mix of both carbonaceous (coal) and mineral particles regardless of the target strata. Furthermore, the study was designed to evaluate the effects of two typical combinations of CM scrubber and ventilation conditions, and increased pressure and volume through the CM water sprays. In general, operation of the scrubber tended to yield lower and somewhat finer respirable dust concentrations, irrespective of the strata the CM was targeting. Increased water spray pressure and volume sometimes appeared to reduce the respirable dust concentration when the CM was targeting the roof rock, but no effect could be discerned when the CM was targeting the coal seam.