ANALYSIS AND SIGNIFICANCE OF MINERAL MATTER IN COAL SEAMS

Abstract

The material described as ''mineral matter'' in coal encompasses dissolved salts in the pore water and inorganic elements associated with the organic compounds, as well as discrete crystalline and non-crystalline mineral particles. A range of technologies, including but not restricted to low-temperature oxygen-plasma ashing, may be used to evaluate the total proportions of minerals and other inorganic constituents in a coal sample. The relative proportions of the individual minerals in the coal may be further determined by several different techniques, including Rietveld-based X-ray powder diffractometry, computer-controlled scanning electron microscopy (CCSEM), and normative interpretation of chemical analysis data. The mode of occurrence of particular minerals may be evaluated by optical or electron microscopy techniques.The minerals in coal may represent transformed accumulations of biogenic constituents such as phytoliths and skeletal fragments, or they may be of detrital origin, introduced as epiclastic or pyroclastic particles into the peat bed. Other minerals are produced by authigenic precipitation, either syngenetically with peat accumulation or at a later stage in cleats and other pore spaces by epigenetic processes. They may represent solution and reprecipitation products of biogenic and detrital material, or they may be derived from solutions or decaying organic matter within the peat bed. Non-mineral inorganics may be derived from a range of subsurface waters, and possibly redistributed within low-rank seams by post-depositional ion migration effects. They may also be expelled in different ways from the organic matter with rank advance.Quantitative analysis of minerals and other inorganics contributes significantly to defining coal quality. It may also be useful as an aid to stratigraphic correlation, either between seams in a coal-bearing sequence or between sub-sections within an individual coal bed. Mineralogical analysis may help in identifying the mode of occurrence and mobility of particular trace elements, including potentially toxic components such as arsenic and mercury. Knowledge of the mineral matter can also be used to evaluate the behaviour of particular coals in different utilization processes, including the processes that control the characteristics of fly ashes, slags and other combustion by-products.