Browsing by Author "Mashale, Kedibone Nicholine"

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Chemical analysis of low grade gold from mine tailings after size fractionation and acid digestion using reverse aqua regia
(Nature Research, 2025-03) Chimuka, Luke; Tshilongo, James; Mashale, Kedibone Nicholine; Sehata, James; Ntsasa, Napo Godwill
The growing interest in reprocessing mine tailings for gold recovery requires a suitable quantification method that is accurate, rapid, and not harsh to the environment. Acid digestion is often used to determination of gold; however, it often faces the challenge of incomplete digestion due to the presence of minerals such as quartz, and homogeneity is compromised due to small sample masses, which can result in low bias. This study investigated a shorter acid digestion method employing reverse aqua regia, both in the presence and absence of hydrofluoric acid. Before digestion, the sample was subjected to gold depot analysis, which showed that 78% was free-milling gold and that only 0.8% was associated with pyrite, increasing the chances of accurate quantifications. Furthermore, the size screening test showed that most of the gold could be recovered on the −38 μm screen. This proposed method provided good linearity (5–100 µg. L−1) and low detection limits (0.139–0.183 µg.kg−1). The concentrations obtained by the acid digestion was 0.258 g.t−1 with the recoveries ranging between 80% and 82%, which fit the criteria set. The method also worked well for the certified reference materials (CRM), AMIS 610 (accurate value=0.068 g.t−1) and AMIS 646 (accurate value=0.166 g.t−1), which are of a similar matrix and are also lower in grade compared to the sample. The method was also evaluated for uncertainty (±value) using the bottom-up approach, and the expanded uncertainty (k=2) was reported to be 0.258±0.092 g.t−1, which was comparable to that offered by the fire assay with the ICP‒OES finish, which was 0.28±0.10 g.t−1. This implies that the acid digestion method is suitable for quantifying gold from mine tailings without large uncertainties.
Quantitative analysis of gold in low-grade tailings from different matrices, coupled with a study into the associated uncertainties
(University of the Witwatersrand, Johannesburg, 2023) Mashale, Kedibone Nicholine; Tshilongo, James; Chimuka, Luke
Gold is one of the precious group elements that is used for various purposes, such as jewellery, auto catalysts and as a form of investment. Various countries have gold reserves, with South Africa being the leading gold producer between 1980 and 2007. However, as of 2022, it is ranked as the eighth largest producer of gold, contributing 3% to the global contribution. The majority of gold is mainly mined from the Witwatersrand Basin in Johannesburg. It is well known that mining has been ongoing for decades, which means that a significant amount of land has been mined across the country. During gold mining, a large proportion of the ore material from which the gold is extracted is waste, together with the chemicals that were used, and this waste is termed mine tailings. This implies that based on the years that gold mining has occurred for and the depth of mining, a significant amount of the tailings have been deposited into free land around the mines, some of which are close to communities. The tailings consist of traces of gold that were left due to inefficient extraction processes and other components, such as base metals. The disadvantage of this is that due to the other chemical composition of these tailings, they have the potential to be dangerous to the environment. Some tailings contain minerals such as jarosite (KFe2(SO4)2(OH)6) that cause acid mine drainage, while heavy metals such as lead, mercury, arsenic and chromium can leach into surface and ground waters, causing pollution. Furthermore, they pose a danger if the dams that they are stored in collapse, which was recently witnessed in South Africa. Because of these factors, there have been various advances made towards the beneficiation of tailings, such as utilizing them to make glass or bricks for construction. A major advancement was the reprocessing of these mine tailings to recover or extract the remaining gold, which benefits both the environment and the mining houses. Therefore, in a move to support this initiative, scientists have taken to the laboratory to develop new or optimize existing methods for the extraction and quantification of gold, which is expected to be of a low grade over time. Various methods can be used for the quantification of gold, including the conventional fire assay, wet and dry chlorination and acid digestion. Most of these are suitable for medium- to high-grade gold ores but are known to experience challenges in regard to low-grade ores. The aim of this research was therefore to find the optimum method for the quantification of gold from mine tailings emanating from the Ventersdorp Contact Reef (VCR) and Barberton Greenstone Belt (GBS). Subsequent to chemical analysis, the samples were characterized for mineralogy using X-ray diffraction (XRD) and Brunauer‒Emmett‒Teller (BET) surface area