An investigation into the effects on leaching select low-grade gold ores in the Witwatersrand Basin
Date
2024
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Abstract
The mining industry currently faces challenges including, but not limited to: gold price volatility, declining ore grade, complex ores, escalating production costs and increasing pressure to be environmentally responsible. As green mining becomes a strong focus of the 21st Century mining industry, an opportunity exists to explore non-cyanide green leaching alternatives. In the past, several non-cyanide reagents such as thiosulfate, chloride, bromide, ammonia, thiourea have been proposed to replace cyanide, however all present unique shortcomings. Glycine is a promising green alternative due to its ease in handling, affordability and recyclability. This research project aims to determine how complex and low-grade Middelvlei Reef from the Witwatersrand Basin in South Africa can be successfully leached using the glycine system.
The selected Witwatersrand ore was characterised by using optical microscopy, scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDS), quantitative X-ray diffraction (QXRD), Tescan integrated mineral analyser (TIMA) and micro-X-ray computed tomography (MicroXCT). Experimental results shows that gold is semi-refractory due to it being physically associated with gangue minerals (e.g., pyrite, carbonaceous matter) and the presence of electrum has been detected which is known to influence leaching kinetics. The occurrence of minerals such as quartz, pyrite and carbonaceous matter in the ore impedes leaching by displaying preg-robbing behaviour, reagent consumption or lowering gold recoveries. A multi-elemental analysis, conducted prior to glycine leaching, highlighted the presence of deleterious base metals such as of Cu = 67.57 ppm and Co = 97.80 ppm. These results highlight that the traditional method of processing needs to be reconsidered.
Leaching experiments were aimed at optimizing gold recoveries while remaining sustainable. Results demonstrate that the optimum parameters that yielded high gold recoveries are: 1) glycine to permanganate ratio of 1:1, 20% solids (ore) and 80% liquids, pH 10. 5 and a temperature of 25 °C. Experiments demonstrate that glycine as a reagent yields average recoveries of 66.7% from all the different experiments conducted. The lowest recovery was 56% and the highest recovery was 92%. The implications of the results are that the Middelvlei Reef shows great potential for glycine leaching, however vital pre-treatment steps should follow prior to leaching. An ore pre-treatment will ensure that the correct measures are put in place to counteract the refractory and complex nature of the ore and obtain higher recoveries.
Description
A research report submitted in partial fulfilment of the requirements for the degree Master of Science to the Faculty of Science, School of Geosciences, University of the Witwatersrand, Johannesburg, 2023
Keywords
Mining industry