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Hydrometallurgical extraction of metals from secondary resources using various reagents
(University of the Witwatersrand, Johannesburg, 2024-04) Teimouri, Samaneh; Billing, Caren; Potgieter, Herman
The advancement and widespread applications of metals in the modern world have led to a growing demand which outstrips their supply. This has resulted in a vital need for recovering precious and critical metals from waste materials, known as secondary resources. Recovering precious, critical and heavy metals not only improves the circularity of metals, but also minimises the deleterious effects of waste materials on the environment. To achieve this crucial aim, research in this thesis focuses on improving gold (Au) yield by finding different ways for pretreatment to break down pyrite, the predominant sulfidic mineral that encapsulates gold in mine tailings. In addition, the research focuses on extracting critical metals such as indium (In) and gallium (Ga) from industrial waste, in this case electric arc furnace dust (EAFD). The results achieved in this research are presented in five publications as explained in brief below: The dissolution of pyrite – the predominant host mineral encapsulating gold – to improve gold extraction from mine tailings was studied in a nitric acid (HNO3) solution. The study showed that when the concentration of HNO3 is above 2 M, it acts as a powerful acid and oxidant to break down the pyrite structure, while simultaneously exposing the enclosed gold through oxidative dissolution. The conducted experiments confirmed that within 2 h, 3 M HNO3 effectively dissolved 95% of FeS2 to release the remaining gold in pyrite at 75 °C. The kinetics of pyrite dissolution was also examined in the temperature range of 25 to 85 °C. The results indicated that the mixed controlled model (1/3Ln (1–X)+[(1–X)–1/3–1)] = k.t, where X is the fractional conversion, k the apparent reaction rate constant, and t leaching time) describing the interfacial transfer and diffusion was governing the kinetics of pyrite dissolution in nitric acid. The activation energy required at low temperatures (25-45 °C) was 145.2 kJ/mol and it reduced at higher temperatures (55-85 °C) to 44.3 kJ/mol. Therefore, nitric acid pretreatment is an effective method for mine tailings containing pyrite with enclosed gold. Nitric acid can be recovered in an eco-friendly manner by capturing the emission of NOx gases from the nitric acid decomposition and can be economically attractive when regenerating the starting acid/oxidant (see publication: “The Kinetics of Pyrite Dissolution in Nitric Acid Solution”). To gain insight into the dissolution of minerals encapsulating gold, such as pyrite and chalcopyrite, an electrochemical study was conducted in nitric acid media using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV scans were measured to identify the oxidation-reduction peaks for pyrite and chalcopyrite. Based on the corresponding CV scans and visual observations, anodic and cathodic reactions for oxidised and reduced species were deduced for each identified peak which occurs at a specific potential. An EIS study was then conducted at the particular oxidative potentials, to gain further indications on the relevant reactions, hence providing supporting evidence of the dissolution mechanism. The EIS study at low potential (i.e. 0.5 V vs Ag/AgCl (3 M KCl) reference electrode) showed that the dissolution reaction was controlled by a diffusion process due to the accumulation of certain species, i.e. Fe(OH)3 and S0, on the pyrite electrode, and Cu1-xFe1-yS2-z, CuS2, and S0 in the case of chalcopyrite. This was confirmed in the EIS curve through the appearance of the linear Warburg diffusion effect. Increasing the potential beyond 0.7 V leads to reactions at which the previously formed species covering the surface of the electrodes and causing a diffusion barrier, oxidised further converting them to soluble species. This was reinforced by the omission of Warburg-like effects in the EIS data (see publication: “A comparision of the Electrochemical Oxidative Dissolution of Pyrite and Chalcopyrite in Diluted Nitric Acid Solution”). Due to environmental awareness, neoteric eco-friendly solvents like ionic liquids (ILs) and deep eutectic solvents (DESs), which can be used as alternatives to conventional leaching reagents for recovering metals, are gaining increasing attention among researchers. Hence, a new hydrometallurgical method using ILs to extract Zn, In, and Ga, along with Fe as a common impurity from EAFD, that was spiked with 5% of both In and Ga, was examined. EAFD is a valuable metal containing waste generated in significant amounts during the process of steelmaking from iron scrap material in an electric arc furnace. With critical metal recovery as the main goal, two ILs: [Bmim+HSO4–] and [Bmim+Cl–], were studied in conjunction with three oxidants Fe2(SO4)3, KMnO4, and H2O2, to determine which IL and oxidant combination performs best for extracting the target metals. Following the initial tests, the influence of parameters such as the IL concentration, oxidant concentration, solid-to-liquid ratio (S/L), time, and temperature were optimised to achieve the maximum extraction of the target metals. Results from a series of experiments found the optimum condition to be; 50 ml 30% v/v [Bmim+HSO4-], 1 g of Fe2(SO4)3 oxidant (2%), S/L ratio of 1/20, at 85 °C for 240 min leaching time, resulting in extractions of 92.7% Zn, 80.2% Fe, 97.4% In, and 17.03% Ga. The dissolution kinetics of the studied metals over a temperature range of 55–85 °C was diffusion-controlled (see publication: “A New Hydrometallurgical Process for Metal Extraction from Electric Arc Furnace Dust Using Ionic Liquids”). Environmental and safety concerns about traditional methods for gold extraction, and the potential volume of enclosed gold in mine tailings in sulfidic minerals (i.e. pyrite), were the motivations to find effective, efficient and ecologically benign ways to break down the pyrite structure to expose the locked gold to improve its extraction. Hence, the feasibility of the dissolution of pyrite was studied in a deep eutectic solvent (DES) as a novel solvent. DESs are an analogue of ILs, which are gaining increasing attention as a potential solvent with eco-friendly features. Therefore, the viability of pyrite dissolution in a DES containing choline chloride – a quaternary ammonium salt [(CH3)3NCH2CH2OH]+Cl−] – and ethylene glycol [HO-CH2-CH2-OH], named ethaline, was examined both theoretically through density functional theory (DFT) calculations and experimentally. DFT calculations determined whether Cl– and/or [C2H4O2] 2−, the two ligands provided by ethaline, can make the most probable and stable complex with Fe2+ and/or Fe3+. To do so, the reaction Gibbs free energies (-G) for possible complexes that Cl– and [C2H4O2] 2− can form with Fe were calculated. In addition, the energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO gap) were considered. Among the proposed complexes, the tetrahedral complex [Fe(C2H4O2)2]−, with Fe3+ chelation taking place through the O-donors of the ligand [C2H4O2]2−, ad the lowest -G (–71.4 eV) indicating the simultaneous formation of the complex, as well as the largest HOMO LUMO gap (1.3 eV) specifying the most stable complex. For experimental evaluation, the effect of the pH of the ethaline solvent mixed with hydrogen peroxide (H2O2) oxidant, and the different S/L ratios on Fe extraction (indicating pyrite dissolution) were examined. The results show that a pH of 8 provides the desired condition at which ethylene glycol is deprotonated to [C2H4O2]2−, was the favorable ligand for Fe complexation. It was found that the S/L ratio of 1/20 was optimal and achieved 23.7% Fe extraction. The theoretical and experimental work correlated in indicating [C2H4O2]2 − as the favourable ligand. However, the ethaline solvent as the leaching solution did not achieve adequate Fe extraction, as it did not succeed in properly breaking down pyrite to expose the locked gold (see publication: “The Feasibility of Pyrite Dissolution in a Deep Eutectic Solvent Ethaline: Experimental and Theoretical Study”). DFT modelling was also applied to theoretically calculate the possibility of the extraction of In and Ga, in the IL medium. To investigate this aim, three imidazolium-based ILs, namely [Bmim+HSO4–], [Bmim+Cl–], and [Bmim+NO3–] were selected for DFT calculations. They all have the same cationic part [Bmim+], but different anionic parts, i.e. [SO42–], [Cl–], and [NO3–], which are similar to the most commonly used mineral acids H2SO4, HCl, and HNO3, respectively. The -G for different complexes were calculated to determine which of the available ligands, i.e. sulfate (SO42–), chloride (Cl–), and nitrate (NO3–), provided by each IL most likely form the most stable complex with In and Ga. The obtained values for -G confirm that IL [Bmim+HSO4–], owing to the [SO42–] O-donor ligand, resulted in the dimer complexes of [In2(SO4)3] and [Ga2(SO4)3] having the lowest G and the largest HOMO-LUMO gap, indicating the most probable and stable complexes (see publication: “Indium and Gallium Extraction Using Ionic liquids: Experimental and Theoretical Study”)
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Exploring middle ear pathologies in adults with diabetes mellitus A scoping review of available evidence and research gaps
(MDPI AG) Ben Sebothoma; Katijah Khoza-Shangase; GIFT KHUMALO; Boitumelo Mokwena