Hydrogeochemical determination of the salt load from copper mine waste in the Bushveld igneous complex
Moukodi, Georges Pascal
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This dissertation characterised geochemically waste from a copper mine in the Bushveld Igneous Complex, coupled with hydrochemical interpretations. In the first part of the dissertation, the methods involved in the quantification and qualification of mine acidic drainage were discussed. This included paste and rinse pH, Acid and Base potentials, net neutralisation potentials and neutralisation potential ratio. These methods form what is known as Acid-Base Accounting, (ABA). In the second part of this dissertation, the sampling methods and representativeness of the samples were discussed. All these methods are scientifically justifiable as they are in agreement with methods used internationally. In the third and most important part of this dissertation, results where discussed and proven to correlate with visual estimates from the field. In sulphides-containing waste material, Acid Mine Drainage (AMD) is produced due to the ingress of oxygen and water mainly from rainfall, and thus subsequent oxidation of sulphide-bearing minerals with the help of bacteria. However, in the presence of sufficient neutralisation potential, alkaline drainage is favoured as carbonated minerals (mainly dolomite and calcite) buffer the acidity. For this study, copper sulphide is recovered from foskorite ore imbedded with carbonatite which helped at the non acidification of the mine site. In the dissertation, the geochemical software PHREEQC was used and it confirmed that many silicates and alumino-silicates minerals are dissolved in the groundwater. This is an important finding as the presence of silica is an indication that substantial amount of acidity could have been released. This also constitutes reasonable evidence for the fact that chalcocite and covellite which are the main sulphides of copper mined on site can be considered as “weak” sulphides unlike pyrite which can be more severe to the environment. The software PHREEQC also confirmed that dolomite and calcite control the pH of the water, thus only fairly alkaline values in the range of pH 8 to pH 9.5 are expected on site. With visual investigations, it has been noticed that natural vegetation is taking place around the site even on the most hostile tailings like the magnetite. This is a very important aspect as positive outcome is expected for the fauna after mine closure. The following serve as recommendations: - The neutralisation processes release sulphate into the groundwater, and it is recommended that tracer tests methods be conducted to estimate the travelling time of the salts. - It is also recommended that the mine speed up the re-vegetation process as many animals cross the Kruger National Park to feed themselves within the mine. - It is recommended that more regular monitoring on boreholes be conducted as many dataless. Also it is suggested that anomalous boreholes be investigated, geochemically logged and re-sampled to verify or determine likely origins and interactions. - For further research, standard values of salt loads should be implemented so as to compare acceptable values with abnormal values. - The study of ABA using static tests should always be coupled with the humidity cells methods. While the former provides with the probability of acid formation, the latter provides with the reaction rates. - Additional studies should be conducted exclusively outside the mine to corroborate with the results from inside the mine.