Chemical and mineralogical characterization of a South African bituminous coal and its ash, and effect on pH of ash transport water

Sasol utilizes a low-rank bituminous coal for steam and synthetic gas production. The ash produced is transported by water for further processing. Understanding the chemistry of coal and the resulting ash properties would allow a better understanding of the origin of high pH of water used during ash handling or transport. A main objective of this study was to identify the species responsible for high pH values of the ash transport water, when burning coal from the Sasolburg Mooikraal mine. The coal contained proportions of extraneous carbonates, i.e. CaMg(CO3)2 and CaCO3, that decompose at elevated temperatures to form CaO and MgO. The ash samples in this work contained free CaO, as verified by X-ray diffraction and ethylene glycol leaching. ICP-OES analysis of water leachates of the ash showed calcium dissolution into the water. The high pH values of the leachates were consistent with the expected pH of a saturated water solution of calcium hydroxide. Contact of the ash with water produced Ca(OH)2. For combustion of this coal, the high pH of the water used for ash transport can be attributed to the free CaO in the ash. The methodological approach to this study could be directly applicable to other coals and their ashes, even of different composition than the specific coal and ash used in this study.