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

[摘要] A mineralogical and geochemical study on the coal and coal-bearing successions ofthe Witbank and Highveld Coalfields in the Mpumalanga Province of South Africawas proposed in order to, firstly, investigate the quantitative distribution of mineralsin the lithological units, and secondly, to correlate this data with the potential of theunits to contribute to acid mine drainage conditions in the region.X-ray diffraction and X-ray fluorescence techniques were used to analyse thesamples from the study area. Samples from the No.1, No.2, No.4 and No.5coalseams were collected from several mines in the Witbank Coalfield, while samplesfrom the No.4 and No.5coal seams were collected from borehole material obtainedfrom the Highveld Coalfield. The inorganic components make up approximately 8.00to 35.00 wt% of a coal sample. Si02concentrations varied between 0.00 and 35.00wt% of a sample, AI203 between 0.50 and 16.00 wt%, Fe203 between 0.03 and10.00 wt%, and S between 0.15 and 8.00 wt%. Minor concentrations of CaO (0.00 to8.00 wt%) and MgO (0.00 to 1.00 wt%) were present. P205 occurred inconcentrations 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 onlydifference in chemistry between Witbank and Highveld coals was a slight increase inNa20 (0.00 to 0.51 wt%) in the Highveld coals. These results were confirmed by the XRD investigations. The mineral components inthe XRD patterns were semi-quantitatively evaluated in terms of dominant (>40% ofthe 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 andkaolinite, with major to minor and trace amounts of calcite, dolomite and pyrite, aswell as accessory phosphates phases.XRF and XRD results for the coal-bearing units were also in good agreement. HigherK20 and Na20 concentrations were obtained in the sandstones in comparison to thesiltstone and carbonaceous shale samples, and were supported by the presence offeldspars and clays such as illite in XRD interpretations. A normative program designed for Australian coals and sedimentary rocks, called Sednorm, was used tocalculate normative mineralogical compositions from the geochemical results. Good \correlations were obtained for comparisons made between the chemicalcomposition, mineralogical interpretations and normative results for the coal andsediment samples.Acid-base accounting was used to investigate the potential of the coal and coalbearingunits to produce acid mine drainage conditions. The acid and neutralisingpotentials are largely dependant on the abundance and availability of minerals suchas pyrite and calcite respectively. According to the screening criteria proposed byUsher et al. (2001), averages for Neutralising Potential Ratio (NPR) suggest that allthe coal and coal-bearing units, excluding the unit between No. 1 and No. 2 coalseams, are potentially acid generating. The latter lithological unit is considered to beinconclusive. The average Net Neutralising Potential (NNP) values suggest that theNO.5 coal seam, NO.4 Upper coal seam, and between NO.4 and NO.2 coal seamsare potentially acid generating. This is a result of the weathering of carbonates inthese 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 Svalue by 31.25. Assuming that all sulphide-S is available for oxidation, then the totalS analysed could be used to predict the AP for samples on which no acid-basedeterminations has been carried out. Similarly, the excellent correlation between theNP and CaO, and between the NP and combined CaO and MgO, confirms that thesechemical components are largely responsible for NP values. It is then also possibleto predict the NP by using the CaO and MgO concentrations for samples for whichno AP or NP data is available.The application of ABA in this study offered a major contribution to understanding thecomplexities governing water-rock interactions. Results provided a preview ofsituations that might arise regarding groundwater quality in a certain area, but alsooffers ample time to decide on appropriate prevention or remediation programs. Thepotential for these lithological units to contribute to the deterioration of groundwateris evident.

[发布机构] University of the Free State