Transformation of inherent and extraneous minerals in feed coals of commercial power stations and their density-separated fractions

Production of Al(HI)-K(I)-Ti(IV)-sulphate-containing leach liquor from metakaolinite-containing ash derived from South African coal fines

“…The occurance of extraneous calcite and extraneous dolomite discrete particles indicates that these minerals could transform at temperatures of 700°C and 1000°C to CaO (quicklime) and a mixture of MgO (periclase) and CaO respectively (Matjie et al, 2018;van Dyk, Waanders, and Hack, 2008). The inherent calcite or dolomite associated with clays (kaolinite, illite, muscovite) and microcline and carbon matrix in the coals can transform at 1200-1400°C to form a molten solution or partial melt (Rautenbach et al, 2021). Furthermore, either the extraneous calcite and dolomite react with each other at 1200-1400°C to form melts during heat treatment.…”

“…The high-temperature products (CaO and MgO) derived from transformation of either extraneous calcite or dolomite have been found to be responsible for the in-situ capturing of sulphur dioxide to form calcium sulphate (Rautenbach et al, 2021). These oxides account for the reduction of gas emissions during coal combustion.…”

“…Higher proportions of total amorphous materials formed at elevated temperatures in the ashes are associated with the mode of occurrence of the fluxing minerals (calcite, dolomite, and pyrite), clays (kaolinite, illite etc.) -and microcline in the coals (Rautenbach et al 2021). The fluxing minerals lower the ash fusion temperature (AFT) values of clays, promoting the formation of melts during heat treatment of coals.…”

“…The SA1 ash sample prepared at 1050°C contains a lower or zero percent of metakaolinite, and a higher percentage of silicon spinel and pseudomullite (sparingly soluble in mineral acid solution, apart from HF) compared to the SA1 ash samples prepared at 700°C (Tables III and IV). Rautenbach et al (2021) found that mullite crystallizes from the molten aluminosilicate solution formed at 1200-1400°C during the combustion of coals in a laboratory and gasification study of other South African coals. Furthermore, mullite, which is associated with the transformation of the extraneous kaolinite, can also be formed at elevated temperatures of 1200-1400°C via the solid-state reaction (Rautenbach et al, 2021).…”

“…Rautenbach et al (2021) found that mullite crystallizes from the molten aluminosilicate solution formed at 1200-1400°C during the combustion of coals in a laboratory and gasification study of other South African coals. Furthermore, mullite, which is associated with the transformation of the extraneous kaolinite, can also be formed at elevated temperatures of 1200-1400°C via the solid-state reaction (Rautenbach et al, 2021). The presence of trace periclase in some of the SA1, and SA2 blend ash samples (Table IV) is due to the transformation of the extraneous dolomite at 700°C to form a mixture of CaO and MgO.…”