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Mineral Matter in Coal and the Environment |
Department of Earth Sciences, University of Sheffield, Brookhill, Sheffield S3 7HF, UK
Both detrital and diagenetic minerals are quantitatively important in UK coals. The detrital minerals include quartz, and the clay minerals illite, mixed-layer illite-smectite, kaolinite and chlorite. The diagenetic minerals are dominated by pyrite/marcasite, carbonates, mainly ankerite and calcite, and kaolinite. Pyrite makes an important contribution to the S contents of the coals and is largely responsible for the variation in total S. The S content of coals delivered to power stations could be reduced by selective extraction during mining and improved physical coal-cleaning. Pyrite is a major location for trace elements of environmental concern and the elimination of pyrite would reduce both SO2 and trace element emissions. Pyrite is also important in the weathering environment due to its instability. Its breakdown in coal stocks and colliery discard heaps may lead to spontaneous combustion. In discard heaps, pyrite breakdown is also responsible for acid porewaters and toxic elements in solution which inhibit colonization by vegetation and thus hinder reclamation of derelict land. If carbonates are sufficiently abundant, acid conditions may be eliminated. Acid porewaters are one facet of the general problem of acid mine drainage, which has a major impact on water quality, particularly in streams and rivers. In the UK acid mine drainage is a problem of growing concern due to reduction of deep-mine pumping following closure of collieries. Although pyrite breakdown in colliery spoil has an important influence on porewater compositions, it does not greatly influence the engineering stability because the major component, the mudrocks, contain very little pyrite. Upon exposure, the mudrocks break down relatively rapidly due to sedimentary structures to form an aggregate. Loss of interparticle bonding is a much slower process but there are a few mudrocks associated with low rank bituminous coals with both a high mixed-layer clay content and porosity which disintegrate rapidly during cycles of wetting and drying. There are also regional variations in the relative proportions of the clay minerals which influence the engineering properties. Kaolinite is dominant in the northern coalfields. This is also detected in the composition of fly ash produced by coal-fired power stations. Fly ash is enriched in those trace elements which have an environmental impact. The volatile elements, many of which were sulphide-associated in the coal, are enriched on the surface of the ash particles where they are accessible to leaching in the weathering environment, resulting in the contamination of natural waters.
