Quantitative evaluation of minerals in coal deposits in the Witbank and Highveld Coalfields, and the potential impact on acid mine drainage

This study has investigated the quantitative distribution of minerals in coals and other lithological units in the coal-bearing successions of the Witbank and Highveld Coalfields in the Mpumalanga Province of South Africa, using low-temperature oxygen-plasma ashing (LTA), high temperature ashing, X-ray diffraction, and X-ray fluorescence spectrometry techniques. Mineral matter makes up approximately 8 to 35 wt.% of the coal samples. SiO2 concentrations in the coal vary between 0.0 and 35.0 wt.%, Al2O3 between 0.5 and 16.0 wt.%, Fe2O3 between 0.03 and 10.0 wt.%, and S between 0.15 and 8.0 wt.%. Minor concentrations of CaO (0.0 to 8.0 wt.%) and MgO (0.0 to 1.0 wt.%) are also present. P2O5 occurs in concentrations of 0.0 to 3.5 wt.% and K2O is in the order of 0.0 to 1.3 wt.%. Na2O varies between 0.0 and 0.45 wt.%, with the Highveld coals showing an enrichment in Na2O (0.0 to 0.51 wt.%) in comparison with the Witbank coals. The minerals in the coals are dominated by quartz and kaolinite, with varying proportions of calcite, dolomite and pyrite, as well as accessory phosphate phases. Higher (normalised) K2O and Na2O concentrations are present in the sandstones than in the associated siltstones and carbonaceous shales, partly reflecting the presence of feldspars and clay minerals such as illite.Acid–base accounting was used to investigate the potential of the coal and coal-bearing units to produce acid mine drainage conditions. According to the screening criteria, all the coal and coal-bearing units, except the unit between the No. 1 and No. 2 coal seams, are potentially acid generating. The average Net Neutralising Potential (NNP) values suggest that the No. 5 coal seam, the No. 4 Upper coal seam, and the unit between the No. 4 and No. 2 coal seams are potentially acid generating. With such techniques it is possible to predict the types of situations that might arise concerning groundwater quality, and implement proper prevention or remediation programs.