Characterisation of fly ashes for minimisation of acid mine drainage from coal mining waste rocks

Acid mine drainage (AMD) due to the oxidation of sulphide bearing waste rock (WR) is a common environmental problem associated with coal extraction. Therefore, WRs from a lignite producing coal field in Pakistan and Lignite (PK), bituminous (FI) and biomass (SE) fly ashes (FAs) were mineralogically and chemically characterised to i) estimate the potential of WRs for generating AMD ii) estimate their deteriorating effects on natural waters, and iii) evaluate the FAs for their potential to minimise the impacts of WRs by preventing and/or neutralising AMD. The WRs were composed of quartz, pyrite, arsenopyrite, kaolinite, hematite and gypsum with traces of calcite, malladerite, spangolite, franklinite and birnessite. The major elements Si, Al, Ca and Fe were in the range (wt. %) of 8 -12, 6 -9, 0.3 -3 and 1 -10, respectively, with high S concentrations (1.94 -11.33 wt. %). All FAs contained quartz, with iron oxide, anhydrite and magnesioferrite in PK, mullite and lime in FI and calcite and anorthite in SE. The Ca content in SE was 6 and 8 times higher compared to PK and FI, respectively. The WRs had considerable potential for generating AMD with net neutralisation potential corresponding to -70 to -492 kg CaCO3 tonne(-1). FAs showed to have sufficient acid neutralisation potential corresponding to 20 -275 kg CaCO3 tonne(-1), SE being the most alkaline probably due to the higher Ca content. The element leaching varied between the WRs due to their chemical and mineralogical composition and pH conditions during the weathering cell test for 28 weeks. However, in general, the leachates from the more acidic WRs were enriched about 3 to 4 orders of magnitude for certain elements compared to the less acidic WRs. The concentrations of Ca, SO42-, Na and Cl in the leachates were much higher compared to other elements from all FA samples. Iron, Cu and Hg were not detected in any of the FA leachates because of pH ranging from 9 to 13. Overall, the WRs had considerable potential for AMD generation and element leaching, therefore, deteriorate natural waters within the mining area. FAs, on the other hand, possess potential to minimise the impacts of WRs on the environment, due to their buffering capacity. However, the FAs vary in chemical composition and buffering capacity depending on their source, which makes it a possible challenge for utilisation.