Acid-base potential characterisation in the Southern highveld coalfield of Mpumalanga
Ntwaeaborwa, Gaonkile Molly
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Pyrite, iron disulphide is the most common mineral in the metal sulphite and coal deposit. The oxidation of Pyrite and other metal-sulphide minerals by oxygen has a large environmental impact and plays a key role in Acid Mine Drainage (AMD). Environmental impact shows that soil acidity, toxic metal concentrations and vegetation damage are the predominant negative impact of AMD. AMD is a major concern for the mining industry because mining activities tend to increase the amount of rock surface exposed to air and water. Mining companies are increasingly required to evaluate the AMD potential at future mine sites and provide detailed plans to prevent or minimise AMD at all phases of mine operation as part of the environmental Impact assessment (EIA) process. An investigation was conducted in two mine (Mine 1 and Mine 2) areas in the Southern Highveld Coalfield of Mpumalanga. Mine 1 is an underground coal mine and is situated 10 km outside Trichardt on the road to Bethel. It was established in May 2012 and its shaft supply coal to Sasol Synfuels. Mine 2 is an open-cast coal mine and is situated between 2 towns, amely Trichardt and Kriel. Construction activities started in 1990 and the mine reached full production in September 1992. Both Mines fall in the Karoo Supergroup which comprises of Ecca group formation and consist dominantly of sandstone, siltstone, shale and coal. The aim of this study was to investigate the acid-base potential of these two Mines. 118 samples were collected from Mine 1 borehole core and 71 samples were collected from Mine 2 borehole core to conduct mineralogical and Acid Base Accounting (ABA) analysis. Acid- base potential leachate were further analysed for major and trace elements using Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES). Most of the samples analysed for Mine 1 have been found to have High AMD risk at the interburden and these samples have low NNP values, all these samples consisted of coal and they contained pyrite mineral as indicated in the mineralogical analysis. These samples must be taken into consideration to minimise oxidation. Samples (Mine 1) that contained layers of sandstone, siltstone and shale have the highest NNP values and these layers can be used as a buffer to neutralise the AMD. Therefore Mine 1 will produce an AMD at the coal seam once exposed during mining. Mine 2 shows that 3 samples have High AMD risk at the interburden while 2 boreholes show the risk between 20m and 60m, and these samples were coal samples and others were sandstone samples and they contained pyrite. These samples that have coal showed low NNP. Therefore most of the acid generation would be mainly found at the coal seams. Layers that show high NNP values consisted of sandstone, siltstone and shale. They showed no indication of acid generation, therefore they will work as buffers to neutralise the any AMD that will be produced. Mine 2 also indicate that AMD will be produced at the interburden at layers that contains coal, therefore more consideration is needed when these layers are exposed during mining. With such condition it is possible for the mines to predict the types of situations that might arise concerning groundwater quality, and implement proper prevention or remediation programs.