Mineralogical characterization of South African mine tailings with aim of evaluating their potential for the purposes of mineral carbonation
Master Thesis
2017
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University of Cape Town
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Abstract
South Africa is an energy intensive economy which primarily relies on the burning of fossil fuel such as coal. The South African coal energy sector accounts for approximately 420 million metric tonnes of carbon dioxide emitted per annum. With present alarming concerns with regards to the ever-increasing atmospheric carbon dioxide concentrations resulting in global warming and climate change, several mitigation strategies have to be implemented. A majority of Carbon Capture and Storage (CCS) technologies require monitoring from potential leakages, making the process expensive. However, a benign technology exists to permanently store away anthropogenic CO₂ with products obtained instantaneously. This CCS technology is known as Mineral Carbonation. The fundamental procedure is a reaction between (magnesium - calcium - iron) silicates and CO₂ to form carbonates. The products of from the reaction require no monitoring and the fear of leakage of CO₂ is eliminated. Moreover, the carbonates from this technology are useful in the road, agriculture and building industries. The CO₂ storage capacity in mineral carbonation exceeds other CCS techniques. The South African mineral industry annually produce immense tonnages of ultramafic mine tailings. Due to the generally fine nature of the tailings, no further cost would be incurred in grinding the material. The platinum group metal (PGM), nickel and copper companies are examples of industries that produce massive tonnages of which could serve as potential feedstock for the purposes of mineral carbonation. Recent studies have shown that, the potential feedstock could sequester close of 70% of the annual CO₂ produced at Secunda, South Africa. A mineralogical investigation into the mineral carbonation potential of mine tailings was conducted using samples from seven mining companies. Four of the mining operations considered (Impala, Rustenburg, Amandelbult and Mogalakwena) are PGM operations mining the Merensky, Upper Group 2 (UG-2) and Platreef. Nkomati was another operation selected for the study, with samples collected from the Main Mineralised Zone (MMZ) and Chromititic Peridotite Mineralized Zone (PCMZ). Tailings material from the dormant O'okiep operation was the seventh, chosen for the study. To determine the suitability of these tailings for mineral carbonation, the particle size, surface area and mineral for each individual operation was accounted using Malvern, BET analysis, XRF, QXRD and QEMSCAN respectively. The overall fine-grained nature of the mine tailings was manifested in the particle size distribution results were sizes ranged from d(0.5) = 33.67 (Nkomati) to d(0.5) = 231.45 (Impala). The range in surface area was 1.45 m²/g (Amandelbult) to 5.89 m²/g (Nkomati). A theoretical carbonation capacity ranking scheme was developed where the seven mining companies selected for this study were graded based on their suitability for mineral carbonation. Three distinct factors made up the classification criteria of the ranking scheme. The first was the carbonation capacity. This was determined by the mineralogy, the Rco2 value and the tonnage of mine waste produced annually. The second major factor was the reactivity (ignoring kinetics) of the tailings. In this case, the particle size distribution and surface area of the respective mine tailings were considered. Thirdly, the distance from the CO₂ (Secunda) source was taken into account as the cost of transporting CO₂ to the mineral carbonation facility should be weighed up. In using these principles, Nkomati was unquestionably ranked first while O'okiep was rated last priority among the seven operations. The motive behind was to improve upon the theoretical carbonation capacity ranking scheme and in turn examining a variety of South African mine tailings for the purposes of detailed mineral carbonation studies in South Africa.
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Amponsah-Dacosta, M. 2017. Mineralogical characterization of South African mine tailings with aim of evaluating their potential for the purposes of mineral carbonation. University of Cape Town.