Mineral matter in low-rank coals and associated strata of the Mae Moh basin, northern Thailand

Abstract

The mineral and other ash-forming constituents in lignites of the Mae Moh basin in northern Thailand have been identified and evaluated in relation to the minerals present in the associated interseam non-coal strata and the intra-seam non-coal bands. Treatment of the coal with water, ammonium acetate and hydrochloric acid shows that a considerable part of the ash-forming inorganic matter occurs in water-soluble, acid-soluble or ion-exchangeable form. Although the distribution of elements may be modified by the effects of pyrite oxidation, the mobile fraction of the fresh coal is dominated by water-soluble, ion-exchangeable and acid-soluble calcium, water-soluble and exchangeable magnesium, water-soluble sodium and sulphur, and acid-soluble iron and aluminium. Calcium is most abundant, especially in coals where a low proportion of silicate minerals (quartz, clay, etc.) is present; it combines readily with the different sulphur forms on combustion to produce an ash with a distinctively high CaO and SO 3 content. The main minerals in the coals are quartz, pyrite, and a clay fraction consisting of kaolinite, illite and an expandable, irregular mixed-layer mineral. The mixed-layer clay appears to be more abundant in the coal, and illite less so, than in the mudrocks of the associated non-coal strata. The non-coal partings within the main seams contain quartz, pyrite and a similar clay mineral assemblage, but they also contain an abundance of calcite and/or aragonite, and in some cases consist entirely of carbonate minerals. Shell fragments and possibly plant replacement structures are present in some of these materials. Gypsum, bloedite, hexahydrite and other sulphate minerals, and calcite, are deposited as fracture fillings and surface coatings on some exposed open-cut mine faces.