Browsing by Author "Mutimutema, Paul"
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- ItemValue recovery from mine calcine tailings(Stellenbosch : Stellenbosch University, 2021-03) Mutimutema, Paul; Tadie, Margreth; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: As gold mining progresses, rich and free-milling ores which are metallurgically easy to process eventually get depleted. More metallurgically complex ores are encountered. These ores yield gold recoveries below 80% and are termed refractory ores. The South African gold mining industry has left a legacy of abundant tailings dams – some of which are an environmental hazard due to the formation of acid mine drainage (AMD), discharge of metalloids, and or radioactive dust storms. Processing of tailings dams has slowly attracted the attention of investors as it is potentially cheaper to mine on the surface than shaft mining, in addition to having a lower exposure to risk associated with fatalities from mine shaft incidents. The possibility of recovering gold from a calcine tailings heap was considered in this study. Due to the refractory nature of the host ore from which they were generated, a mineralogical characterization was performed to understand the nature and occurrence of the gold. Fire assay revealed a gold grade of 2.96 ± 0.26 g/t. X-ray diffraction (XRD) and quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) both confirmed that silicates were the most abundant phase followed by iron oxide. Scanning electron microscopy (SEM) showed that gold existed in submicron and micron size, as free gold, and also as refractory gold associated with arsenic, sulfur and silicates like quartz and talc. These mineralogical findings influenced the selection of extraction routes for investigation to recover the gold. Therefore, ultrafine milling, microwave roasting, microwave assisted cyanide leaching and sodium hydroxide pre-leach treatment were the methods potentially considered to recover the gold. Firstly, in order to confirm the degree of refractoriness of the tailings a series of direct cyanidation tests were conducted on the calcine tailings which resulted in average 17.3% gold recovery. Because of the confirmed high refractoriness, ultrafine milling of the original tailings (P80 — 53 μm) to P80 — 16 μm followed by cyanidation increased the gold recovery to 66.5%. Breaking of the host phases by ultrafine milling was beneficial. Microwave roasting pre-treatment on the as received sample (original calcine tailings) proved effective as compared to the ultrafinely ground calcine tailings sample. A 43.7% gold cyanidation recovery was obtained for the as received sample after microwave roasting for 60 minutes. Addition of 6% water was also effective on the as received sample, and yielded a 42.6% gold cyanidation recovery after undergoing 30 minutes of microwave roasting. A cyanidation gold recovery of 68.4% was attained after 30 minutes of microwave roasting the ultrafine sample. Microwave assisted cyanidation had rapid kinetics. In 50 minutes, 56.8% gold recovery was attained on the ultrafine sample at a sodium cyanide dose of 8 kg/t. Application of sodium hydroxide pre-leach treatment was beneficial by leaching gangue related elements in gold carrier phases (e.g. quartz and talc) into solution. Ambient temperature pre-leach at 1 M sodium hydroxide gave a 51.6% cyanidation gold recovery on the as received tailings sample after 24 hours. The ultrafine milled sample, after ambient pre-leach at 1 and 3 M sodium hydroxide, exhibited fast leaching rates, and maximum cyanidation gold recoveries of 71.9% and 77.6% respectively were obtained after 8 hours. Heat assisted pre-leach on a hot plate, however, did not result in improved gold cyanidation recoveries for both grinds and sodium hydroxide concentrations. Based on the experimental findings, three process flowsheets were considered. The gold mass balance for each flowsheet was completed. The flowsheet which incorporated sodium hydroxide pre-leach treatment was chosen as the best as it achieved the highest cyanidation gold recovery of 77.6%.