Geochemistry of high-temperature coal ashes and the sedimentary environment of the New South Wales coals, Australia

Abstract

Chemical analyses of high-temperature coal ashes were used to establish the distribution, association and relationship between major inorganic elements such as Si, Al, Ti, Fe, Mn, Mg, Ca, Na, K, P, S and CO 2 in a number of New South Wales economic coal seams and to study the composition and character of mineral matter in these coals. The methods used for the evaluation of the data were statistical analysis (univariate and bivariate), ratios, normative mineral composition and variation diagrams. The distribution of major and minor inorganic elements in coal appears to be related to the amount of mineral matter occurring in coal (determined as ash yield) and its mineralogical composition. The quantitative variations in levels of these elements can be classified as in-seam and inter-seam variations. In-seam variations are largely ash yield dependent, i.e. the levels of an element (wt.%) in coal increase along with the increase of its ash content (wt.%). The inter-seam variations are more complex and are related to both ash yield and to the mineralogical composition of mineral matter. The principal components of New South Wales coal ashes are silicon and aluminium. Silicon may be present as silica or combined with aluminium in different proportions to form clay minerals, such as kaolinite, illite, mixed-layer clay minerals, and smectite. Thus, the concentration levels of aluminium in relation to silicon in coal may give an indication about the character of clay minerals present in coal. Ratios and correlation coefficients of element pairs such as Al and Ti, Na and K, and Na and Al were used to determine differences in the chemical composition of high-temperature coal ashes of seams from various stratigraphic positions and provinces. In some seams the nature of associations of these elements is more significant than in others. This is interpreted as being a product of specific environmental conditions controlling the deposition of these seams. The nature of clay mineral content in coal is believed to be a major reason for chemical dissimilarities found between seams of various stratigraphic levels and geographic areas. For example, in some seams kaolinite, in others expandable clay minerals are dominant. The vertical distribution of these minerals has a stratigraphic significance. Within the Upper Permian Newcastle Coal Measures a trend from kaolinite-rich through to expandable minerals-rich and to kaolinite-rich assemblages can be observed from the bottom to the top. These changes are noticeably gradual. All significant variations in the clay mineral assemblages could relate to the long-term changes in the provenance of sedimentary material, weathering conditions in the source area and the rate of subsidence in the place of deposition. These changes are associated with major tectonic events controlling the history of sedimentation within the paralic Sydney and Gunnedah Basins during the Permian.