Quantitative evaluation of minerals in coal deposits in the Witbank and Highveld Coalfields, and the potential impact on acid mine drainage
Pinetown, Kaydy Lavern
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A mineralogical and geochemical study on the coal and coal-bearing successions of the Witbank and Highveld Coalfields in the Mpumalanga Province of South Africa was proposed in order to, firstly, investigate the quantitative distribution of minerals in the lithological units, and secondly, to correlate this data with the potential of the units to contribute to acid mine drainage conditions in the region. X-ray diffraction and X-ray fluorescence techniques were used to analyse the samples from the study area. Samples from the No.1, No.2, No.4 and No.5coal seams were collected from several mines in the Witbank Coalfield, while samples from the No.4 and No.5coal seams were collected from borehole material obtained from the Highveld Coalfield. The inorganic components make up approximately 8.00 to 35.00 wt% of a coal sample. Si02 concentrations varied between 0.00 and 35.00 wt% of a sample, AI203 between 0.50 and 16.00 wt%, Fe203 between 0.03 and 10.00 wt%, and S between 0.15 and 8.00 wt%. Minor concentrations of CaO (0.00 to 8.00 wt%) and MgO (0.00 to 1.00 wt%) were present. P205 occurred in concentrations of 0.00 to 3.50 wt% and K20 was in the order of 0.00 to 1.30 wt%. Na20 values were the lowest varying between 0.00 and 0.45 wt%. The only difference in chemistry between Witbank and Highveld coals was a slight increase in Na20 (0.00 to 0.51 wt%) in the Highveld coals. These results were confirmed by the XRD investigations. The mineral components in the XRD patterns were semi-quantitatively evaluated in terms of dominant (>40% of the mineral fraction), major (10-40%), minor (2-10%), accessory (1-2%) and rare « 1%) constituents. The mineral fraction in the coals was dominated by quartz and kaolinite, with major to minor and trace amounts of calcite, dolomite and pyrite, as well as accessory phosphates phases. XRF and XRD results for the coal-bearing units were also in good agreement. Higher K20 and Na20 concentrations were obtained in the sandstones in comparison to the siltstone and carbonaceous shale samples, and were supported by the presence of feldspars and clays such as illite in XRD interpretations. A normative program designed for Australian coals and sedimentary rocks, called Sednorm, was used to calculate normative mineralogical compositions from the geochemical results. Good \ correlations were obtained for comparisons made between the chemical composition, mineralogical interpretations and normative results for the coal and sediment samples. Acid-base accounting was used to investigate the potential of the coal and coalbearing units to produce acid mine drainage conditions. The acid and neutralising potentials are largely dependant on the abundance and availability of minerals such as pyrite and calcite respectively. According to the screening criteria proposed by Usher et al. (2001), averages for Neutralising Potential Ratio (NPR) suggest that all the coal and coal-bearing units, excluding the unit between No. 1 and No. 2 coal seams, are potentially acid generating. The latter lithological unit is considered to be inconclusive. The average Net Neutralising Potential (NNP) values suggest that the NO.5 coal seam, NO.4 Upper coal seam, and between NO.4 and NO.2 coal seams are potentially acid generating. This is a result of the weathering of carbonates in these lithological units. The other units could become either acidic or neutral. In theory it is possible to calculate the AP from the analysed S by multiplying the S value by 31.25. Assuming that all sulphide-S is available for oxidation, then the total S analysed could be used to predict the AP for samples on which no acid-base determinations has been carried out. Similarly, the excellent correlation between the NP and CaO, and between the NP and combined CaO and MgO, confirms that these chemical components are largely responsible for NP values. It is then also possible to predict the NP by using the CaO and MgO concentrations for samples for which no AP or NP data is available. The application of ABA in this study offered a major contribution to understanding the complexities governing water-rock interactions. Results provided a preview of situations that might arise regarding groundwater quality in a certain area, but also offers ample time to decide on appropriate prevention or remediation programs. The potential for these lithological units to contribute to the deterioration of groundwater is evident.