Mineralogical features of size and density fractions in Sasol coal gasification ash, South Africa and potential by-products

Bulk gasification ash (a mixture of coarse and fine ash particles), a by-product of coal gasification, is formed at elevated temperatures and pressures by the interaction of included minerals present in the coal and “stone”. From the detailed mineralogical and chemical analyses of the pulverised screened size fractions and one density float fraction (<1.9 g/cm3) a number of potential viable by-products were identified.Screening and density separation produced a high ash, low volatile carbon-rich by-product, which is potentially suitable as an energy source for the cement industry. In addition, this carbon-rich product has included devolatilised kaolinite and quartz that are a source of Al2O3 and SiO2. This product could potentially replace the amount of clay required in the cement process. This high ash carbon product is not suitable as a reductant in the metallurgical industry.The −38 + 20 μm ash size fraction is characterised by a comparatively high proportion of aluminosilicate (transformed product of kaolinite) and Ca-oxide/CaMg-oxide (transformed product of calcite/dolomite). These phases will enhance the pozzolanic reactivity of this ash size fraction and provide material suitable for the cement/concrete industry.The coarse ash size fractions are used as aggregate in road construction and in the manufacture of bricks. If economically and technically feasible, anorthite in the coarse ash size fractions could be beneficiated and used in a refractory.