Doctoral Degrees (Earth Sciences)

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Browsing Doctoral Degrees (Earth Sciences) by Author "Chingwaru, Steve Jason"

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    Gold deportment and ore characterisation of the historical Witwatersrand tailings dams with emphasis placed on the sulphides
    (Stellenbosch : Stellenbosch University, 2024-03) Chingwaru, Steve Jason; Von der Heyden, Bjorn; Tadie, Margreth; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The Witwatersrand Basin, discovered in 1881, has yielded over 53,000 tons of native gold hosted in quartz pebble conglomerates. Present gold extraction methods involve comminution and direct cyanidation, resulting in the retention of 5-10 % of residual gold in tailings. Re-mining primarily targets native gold recovery, achieving an average of 30-50 % gold recovery through direct cyanidation. This implies that the remaining 50-70 % of unrecovered gold, along with approximately 30 million tons of residual sulphide waste, is re-dumped to the tailing stream. The Witwatersrand tailings significantly contribute to South Africa's pollution, causing acid mine drainage and deleterious element effluent due to sulphide waste oxidation. The mineralogical distribution of unrecovered gold during tailings reprocessing is inadequately characterized and absent from existing literature. Furthermore, the current body of literature lacks a comprehensive environmental assessment specifically addressing the movement and dispersion of deleterious elements during sulphide oxidation within the Witwatersrand tailings. This PhD research focuses on the ore characterization and mineralogical deportment of unrecovered gold within the Witwatersrand tailings from Klerksdorp, Carletonville, Evander, and Central Rand goldfields. Simultaneously, an environmental assessment explores deleterious element behaviour, retention, and release mechanisms during sulphide oxidation in Witwatersrand tailings dumps. The methodology utilized employs the multi-element, high- resolution, and low-detection capabilities of laser ablation inductively coupled plasma mass spectrometry analysis, combined with an automated mineralogical technique to analyse bulk tailings samples and their operationally defined mineral fractions. Additional analytical methods include aqua regia digestion combined with ICP-MS analysis and fire assay. Mineralogical analysis using optical microscopy, electron microscopy, and X-ray diffraction analysis provides comprehensive insights into the composition and physical properties of minerals. Metallurgical test work, incorporating gravity separation, direct cyanidation, and diagnostic leaching, validates Witwatersrand tailings behaviour in processing scenarios. Results indicate cyanide amenable gold recovery at 14- 61 %, primarily in the form of micron to nano-sized native gold particles. Non-cyanide amenable phases contributing to the refractory behaviour of gold include sulphides (mainly pyrite) and silicates at 4-38 % and 11- 32 %, respectively. In situ pyrite analysis reveals 'invisible' gold within detrital pyrite and arsenian pyrite. These Archean detrital pyrites exhibit grades comparable to auriferous sulphides from surrounding Archean granite- greenstone gold ores on the Kaapvaal craton (up to 2700 ppm). Pyrite also hosts a substantial concentration of deleterious elements, with Co, Au, and Pb strongly leached during pyrite oxidation. However, a significant proportion of As, Ni, Cu, and Zn is retained in the iron oxyhydroxide alteration rims through incorporation and absorption. The study underscores the necessity of addressing 'invisible gold' during beneficiation, proposing tailored leaching parameters and a pretreatment strategy to potentially recover up to 420 tons of gold, thereby contributing to the local economy. Furthermore, the recovery of metals can not only reclaim valuable 'sweetener' by-product metals such as Cu, Co, and Ni but also directly mitigate issues related to deleterious element pollution and acid mine drainage associated with surface tailings dumps. The findings presented in this dissertation represent significant and original contributions to multiple scientific disciplines, notably including geometallurgy, geology, mineral processing, and environmental sciences.