Physical, chemical and mineralogical characterisation of hydraulically disposed fine coal ash from SASOL Synfuels

Coal serves as the primary energy source in most parts of the world. It is a fact that coal combustion yields enormous quantities of fly ash some of which are either hydraulically placed or dry dumped. The current study attempts to provide a comprehensive characterisation of a disused alkaline fine coal ash dam (FCAD) towards assessing environmental impact, rehabilitation and utilisation potential. Fine coal ash refers to a combination of approximately 83% power station fly ash and 17% gasification and bottom ash fines (particles <250 μm) at SASOL Synfuels. The hydration products found in Weathered Fine Coal Ash (WFCA) using X-ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC) are analcime, calcite, C–S–H gel, ettringite, hydrated gehlenite (Strätlingite), magnetite, periclase, pyrrhotite and sillimanite. High resolution Scanning Electron Microscope (SEM) results provide additional proof that hydration products are present in WFCA. No indication of appreciable leaching was given by X-ray Fluorescence (XRF) results except calcium and silicon. Thus evidence exists that pollutants from saline brines are immobilised in WFCA and an insight of reaction kinetics was obtained. High content of amorphous phase and lack of alteration in some geotechnical properties suggest that WFCA can be reutilised with lime addition to increase alkalinity and activate pozzolanic reactions.

► This study reports on the weathering of a fine ash dam formed by a slurry disposal of coal fine ash with desalination brines. ► State-of-the-art equipment was used to generate reliable mineralogical information which was not achieved by previous researchers. ► A relationship is made between geotechnical and chemical/mineralogical characteristics of disposed fine ash while literature mainly focuses on geochemical aspects. ► The overall conclusion is that weathered fine coal ash can be utilised as a pozzolan and an insight of rehabilitation options exists.