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AuthorAphane, Velapi Venessa
SubjectInstitute for Groundwater Studies
AbstractAcid Rock Drainage (ARD) is expected to take place as soon as rock or coal that consists of sulphide minerals is exposed to the surface and comes into contact with oxygen and water. Water not only acts as catalyst, but also as a transportation medium of the yellow boy precipitates which in turn deteriorates the condition of the environment as well as water resources. Due to the long-term irreversible impact that ARD has, minimisation of this condition is an important factor to be looked at by mining companies. Worst cases have been reported by the Department of Water Affairs in and around coal and gold mines in the Witwatersrand and Witbank areas. This has brought about a necessity to explore pre-mining investigations into different lithologies to be aware of possible changes that might occur in the environment once mining commences. An evaluation of ARD was also done on the coal and rock samples to have better knowledge of what will be released in the environment and water resources as more mines are introduced in the Waterberg Coalfield.. Controlling ARD means to control the acid generating reactions of which mineralogy stands at the core of the whole reaction process. Coal deposits in South Africa largely consist of shales; mudstones, siltstones and sandstones which contain clay minerals, quartz, carbonates and sulphides. Problems that are associated with ARD (results) include decreased pH values and increased values of metals, acidity, sulphate and dissolved and suspended solids. The sulphate concentration is caused by sulphide minerals that are in the mining environment and undergo oxidation, thus bringing out the presence of sulphate. The investigation took place in four areas, namely Resgen, Grootegeluk (Exxaro), Sasol and Sekoko. A total of 214 samples were selected from the borehole core to conduct an Acid Base Accounting (ABA) (from the interburden, overburden and coal samples), 29 samples were selected for whole rock and mineralogical analysis, and 8 selected for kinetic tests. Results from the ABA and the kinetic tests leachate were further analysed for major and trace elements using the ICP-OES (Inductively Coupled Plasma- Optical Emission Spectrometer). ABA results show that the interburden and coal samples have a higher risk of producing acid upon oxidation when comparing it to overburden samples. They have a higher concentration of neutralising minerals that can neutralise the acid produced through sulphide minerals as oxidation takes place. In the Sasol samples, which were collected from the full succession that includes the Volksrust and the Vryheid formation, the closed Net Neutralising Potential (NNP) varied from -306.19 to 121.05 kg/tonne CaCO3. The results indicate that there is higher potential of acid production than neutralisation. Grootegeluk samples taken from the overburden showed a closed NNP that varied between -0.33 to 310.80 kg/tonne CaCO3 which classifies the samples as having more potential of neutralisation than acidification; the same case applies for Resgen with closed NNP that is between -68.96 and 57.74 kg/tonne CaCO3. Sekoko consists of the lowest number of samples that have a low risk of acid generation with more samples having a sulphide √ĘS that is more than 0.3%. Mineralogical analysis indicates that there is a presence of pyrite, calcite and dolomite present in accessory to minor concentration. Minerals quartz and kaolinite are found in all the samples constituted in major to dominant concentrations. Results from the whole rock analysis which correlate with mineralogical analysis were SiO2 and Al2O3 which have the highest weight percentage and is similar to the presence of quartz and kaolinite. The average Fe2O3 that is 6.73 wt.% for Sasol and 3.28 wt.% for Resgen, is higher than the average CaO and MgO which is the result of the calcite and dolomite mineral. Leachate analysis from the ABA shows that metals become more soluble as acidity increases. An increase was noted in metals such as iron, magnesium and cadmium, which is a greater threat than the acidity that results from the drainage. Further tests were carried out after the completion of the ABA analysis; kinetic tests were performed on samples that gave inconclusive NNP results. The analysis showed the long-term behaviour of different samples with the EC and pH changing over time. Samples with a lower pH continue to produce more sulphate, while calcium continues to increase until it is depleted from the samples. The Waterberg Coalfield has not yet experienced major environmental deteriorations due to ARD; this is likely to change as more mines will be added to the area.
PublisherUniversity of the Free State