Browsing by Department "Chemical Engineering"

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    Open Access
    A Study on the Applicability of Agitated Cyanide Leaching and Thiosulphate Leaching for Gold Extraction in Artisanal and Small-Scale Gold Mining
    (Multidisciplinary Digital Publishing Institute, 2022-10-14) Manzila, Archippe Ngwey; Moyo, Thandazile; Petersen, Jochen
    Mercury amalgamation is the method of choice to recover gold in artisanal and small-scale gold mining (ASGM). However, despite the low cost and simplicity of this method, the use of mercury presents serious health and environmental risks, as well as low efficiency in gold extraction. This study investigates the application of cyanide and thiosulphate leaching as alternatives to mercury amalgamation. This investigation was undertaken by conducting leach experiments using cyanide at 1 g/L, 3 g/L, and 5 g/L, and ammonium thiosulphate at 0.1 M and 0.5 M, on three ore samples originating from an artisanal mining area in Zimbabwe. The operating conditions (T = 26 °C, solids loading: 30%, particle size: −300 + 150 µm) were selected to mimic as closely as possible the conditions of artisanal mining processes. It was found that cyanide leaching was the better performing technology compared to thiosulphate leaching, as it achieved gold extractions of 71.6%, 69.7%, and 67.8% for the three ore samples (Sample 1, Sample 2, and Sample 3, respectively), whereas thiosulphate leaching achieved gold extractions of 54.1%, 35.6%, and 38.0% for the three ores, respectively. Both methods outperformed mercury amalgamation, which typically achieves gold recoveries of 30%–50%. Studying the minerology of the ores, using XRF, XRD, QEMSCAN, SEM-EDS, and a diagnostic leach, revealed the presence of sulphide minerals hosting refractory gold which contributed to the low gold extractions observed. Besides achieving higher gold extraction, cyanide leaching proved to be a system that is easier to control compared to thiosulphate leaching, making it much more attractive to artisanal miners.
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    Open Access
    Carbon-Supported KCoMoS2 for Alcohol Synthesis from Synthesis Gas
    (Multidisciplinary Digital Publishing Institute, 2021-10-30) Osman, Mohamed E; Maximov, Vladimir V; Dorokhov, Viktor S; Mukhin, Viktor M; Sheshko, Tatiana F; Kooyman, Patricia J; Kogan, Viktor M
    KCoMoS2 was supported on various carbon support materials to study the support effect on synthesis gas conversion. Next to two activated carbons with high micropore volume, a traditional alumina (γ-Al2O3) support and its carbon coated form (CCA) were studied for comparison. Coating alumina with carbon increases the selectivity to alcohols, but the AC-supported catalysts show even higher alcohol selectivities and yields, especially at higher temperatures where the conversions over the AC-supported catalysts increase more than those over the γ-Al2O3-based catalysts. Increasing acidity leads to decreased CO conversion yield of alcohols. The two activated-carbon-supported catalysts give the highest yield of ethanol at the highest conversion studied, which seems to be due to increased KCoMoS2 stacking and possibly to the presence of micropores and low amount of mesopores.
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    Investigation on the Effect of Mesomixing on Crystal Quality during Antisolvent Crystallization of Nd2(SO4)3·8H2O
    (Multidisciplinary Digital Publishing Institute, 2023-07-31) Baloyi, Tinjombo Octavious; Chivavava, Jemitias; Lewis, Alison Emslie
    Rare earth elements (REEs) are essential for permanent magnets that are vital for wind turbines and electric vehicles motors (EV), and are also used in a range of high-tech devices such as smartphones, digital cameras, and electronic displays. Nickel metal hydride (NiMH) batteries have been identified as a potential source due to their short lifespans and an anticipated boom in the production of EV. The aim of this study was to investigate the effect of mesomixing on crystal quality in a non-confined impinging jet mixer (NCIJM) during antisolvent crystallization of 3.2 from a synthetic leach solution of NiMH battery using ethanol at an O/A ratio of 1.1. The jet streams were supplied at a Reynolds number (Re) between 7500 and 15,000. The product slurry was allowed to further crystallize in a stirred batch crystallizer at a Re of 13,000 for 45 s. An average yield of 90% was achieved. Laser diffraction and scanning electron microscopy (SEM) were used for size analysis. The initial results were inconclusive due to the secondary mixing effect in the stirred batch crystallizer. Therefore, the experiments were repeated, and samples were collected immediately after mixing in the NCIJM onto a porous grid placed on a high absorbance filter paper to abruptly halt crystallization. The samples were analysed using a transmission electron microscope (TEM), and the acquired images were processed using ImageJ to obtain crystal size distributions (CSDs). It was found that the enhanced mesomixing conditions resulted in smaller crystal sizes and narrower CSDs. This was because the nucleation rate was found to be mass-transfer-limited, such that higher mesomixing intensities promoted the nucleation rate from 6 × 10 12 to 5 times; and, therefore, favoured the formation of smaller crystals. In parallel, intensified mesomixing resulted in uniform distribution of the supersaturation and, hence, narrowed the CSDs.
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    Simulating the Effect of Water Recirculation on Flotation through Ion-Spiking: Effect of Ca2+ and Mg2+
    (2020-11-19) Dzingai, Mathew; Manono, Malibongwe; Corin, Kirsten
    Froth flotation is a multifaceted complex process which is water intensive. The use of recycled water as an alternative source of water to meet water demands of the process may introduce deleterious inorganic ions that affect the mineral surface, pulp chemistry, and reagent action, hence the need to establish whether threshold ion concentrations exist beyond which flotation performance will be adversely affected. This is of paramount importance in informing appropriate recycle streams and allowing simple, cost-effective water treatment methods to be applied. Here we report that increasing ionic strengths of synthetic plant water (SPW); 3, 5, and 10 SPW respectively, resulted in an increase in water recovery in the order 0.073 mol·dm−3 (3 SPW) < 0.121 mol·dm−3 (5 SPW) < 0.242 mol·dm−3 (10 SPW), indicating an increase in froth stability as higher water recoveries are linked to increased froth stabilities. This behavior is linked to the action of inorganic electrolytes on bubble coalescence which is reported in literature. There was, however, no significant effect on the valuable mineral recovery. Spiking 3 SPW to 400 mg/L Ca2+ resulted in higher copper and nickel grades compared to 3 SPW, 5 SPW, and 10 SPW and was deemed to be the Ca2+ ion threshold concentration for this study since 3 SPW spiked with further Ca2+ to a concentration of 800 mg/L resulted in a decrease in the concentrate grade. The spiking of 3 SPW with Mg2+ resulted in higher copper and nickel grades compared to all other synthetic plant water conditions tested in this study.
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    The Use of Acid Mine Drainage (AMD) in the Flotation of a Platinum-Group-Minerals-Bearing Merensky Ore
    (Multidisciplinary Digital Publishing Institute, 2022-10-05) Taguta, Jestos; Peku, Zandile; Sehlotho, Nthapo; Corin, Kirsten
    Water scarcity is compelling mining houses to not only recycle process water but to also identify alternative sources of make-up water in concentrators. South Africa has significant volumes of acid mine drainage (AMD) generated from vast mining operations. This study investigated the viability of using AMD as a replacement for potable water in the flotation of a platinum-group-minerals (PGM)-bearing Merensky ore. Rougher and cleaner flotation testwork was conducted at laboratory scale to compare the performances of potable water (baseline water), AMD treated with Ca(OH), and AMD treated with the Veolia process. Water analysis showed that the three water types differed in pH, water hardness, conductivity, and total dissolved solids. The results showed the AMD treated with Ca(OH) was detrimental to PGM recovery compared to potable water at depressant dosages of 50 g/t. Specifically, AMD treated with Ca(OH) achieved a PGM rougher recovery of 67.8%, while potable water achieved a PGM rougher recovery of 88.4%. Depressant dosage optimisation and treatment of the AMD using the Veolia process were investigated as potential strategies to mitigate the detrimental effects of the AMD treated with Ca(OH) on the flotation performance of a Merensky ore. The AMD treated with the Veolia process achieved a PGM rougher recovery of 70.8%. Thus, treatment of the AMD was beneficial, though the PGM and base metal sulphides (BMS) recoveries were still lower than those achieved in potable water. Reducing the depressant dosage to 25 g/t in AMD treated with Ca(OH) resulted in the highest PGM, Cu, and Ni rougher recoveries of 91%, 60.2%, and 58%, respectively. The AMD treated with Ca(OH) at lower depressant dosage outperformed the potable water in terms of PGM and BMS recoveries and concentrate grades, indicating that AMD has the potential to replace potable water as make-up water in Merensky ore processing plants. The results showed that depressant optimisation is important to achieve superior metallurgical results when using AMD treated with Ca(OH). The use of AMD in Merensky ore processing plants not only conserves freshwater in minerals processing plants but also reduces high volumes of contaminated effluents.