Some relationships between coal rank and chemical and mineral composition

Some relationships between coal rank (C-daf) and the mineral and chemical composition of coals and their high-temperature ashes worldwide were studied. Low-rank coals (C-daf <75wt%) are relatively rich in moisture, volatile matter, ash, H, N, O and S, and their ashes are abundant in MgO, CaO and SO3. Coal ashes derived from higher-rank coals demonstrate increased contents of SiO2, Al2O3, Fe2O3, K2O, Na2O and TiO2. It was found that the occurrence, abundance and origin of mineral matter in coal depends on the coal rank to a certain extent. Coals enriched in illite, mica, chlorite, spinel, dolomite, siderite and hexahydrite, and partly in quartz, kaolinite and Fe oxyhydroxides, are of higher rank, while coals with increased contents of montmorillonite, feldspars, zeolite, Al oxyhydroxides, calcite, pyrite, gypsum and Fe, Al and Ba sulfates are of low rank. Higher-rank coals are abundant in ash-forming elements associated with probable detrital minerals, while low-rank coals show enrichment in ash-forming elements associated with probable authigenic minerals and organics. Various processes related to some changes in inorganic matter during coalification are discussed. The metamorphic progression and leaching behaviour of different minerals, phases and elements from low- to high-rank coals have resulted in essential changes in the mineral and chemical composition of coals. Copyright (C) 1996 Elsevier Science Ltd.