The influence of thermal transformation of coal mining wastes on their petrographic and mineralogical composition

The excavation of hard coal is inseparably connected with the production of wastes. Many of these wastes are disposed of in waste dumps (tips). On the one hand this is a nuisance to the environment, while on the other hand - it represents a potential source of aggregates. Coal mining wastes disposed of in the dumps are exposed to hypergenic factors. In particular cases, self-ignition of these wastesmay occur, even becoming the cause of endogenous fires. The combustion of organic substances and interaction with heat, which is an effect of this process, causes changes in the mineral composition of wastes and their structures and textures. Examinations conducted on coal mining waste dumps - which in previous years showed thermal activity - as well as an analysis of existing data allowed for the conclusion that the grade of thermal transformation of coal mining wastes is differentiated. The waste dumps were assigned the following zones: thermally untransformed zone, moderately thermally transformed zone, and intensively thermally transformed zone. Coal mining wastes from these zones vary in colour, the grade of conservation of the primary rock structure, and mineral composition. The colour of the thermally untransformed zone is grey and black, the wastes have preserved the primary structures of the rocks from which they originated, and the mineral composition indicated small changes connected with weathering processes. Moderately thermally transformed zones are characterized by an orange-red colour, implying the combustion of organic substances and oxidation of iron compounds. In spite of the influence of high temperature, the primary structures of the rocks are well preserved. Mineral composition distinctly changed; a part of the minerals (carbonates, clay minerals) were thermally altered. The wastes in intensively thermally transformed zones are characterized by partial or total melting of the rock material. As a result, transformations of the primary structures and textures occurred. The mineral composition has also been totally changed - new minerals originated which did not occur in the wastes at the time of their disposal in the waste dump. The occurrence of glaze as well as high-temperature minerals is characteristic for the wastes from intensively thermally transformed zones, most often originating under natural conditions as an effect of contact metamorphism.