Electronic Theses and Dissertations (PhDs)
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Item Hydrometallurgical extraction of metals from secondary resources using various reagents(University of the Witwatersrand, Johannesburg, 2024-04) Teimouri, Samaneh; Billing, Caren; Potgieter, HermanThe 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”)Item Co-gasification of Coal and Solid Waste to Hydrogen Enriched-Syngas in a Fixed Bed Gasifier(University of the Witwatersrand, Johannesburg, 2020-10) Ozonoh, Maxwell; Daramola, Michael O.; Oboirien, Bilainu O.The economic growth of every nation around the globe is centred on energy. Energy can be harnessed from different sources using different conversion systems, but such systems should be sustainable. Liquid fuels such as petroleum and solid fuels (e.g. coal & biomass) are largely used for energy production. Energy recovery from these fuels is usually carried out using thermal chemical processes such as combustion, pyrolysis, and gasification systems. Out of the three technologies, gasification is considered the most attractive based on its efficiency and other qualities. In the gasification process, syngas is produced. It is necessary to produce syngas of high quality such as hydrogen-enriched syngas. Hydrogen-enriched syngas can be used in fuel cells, gas turbines and engines for electricity production. This type of gas burns with little gaseous emissions to the atmosphere, but its production is dependent on the type of fuel and process conditions, and energy conversion system employed. In South Africa, around 95 % of electric power production comes from coal, and the current reserve is projected to last not more than a century [8]. Secondly, the coal is fast depleting and generates a lot of gaseous emissions (e.g. CO2, NOX & SOX) that pose a huge threat to the environment. The emission of the aforementioned gases is a very serious issue in South Africa. Presently, some Carbon Capture and Storage (CCS) projects are on-going in the country, although the CCS is not the fuse of this study. The gasification of biomass waste and coal could assist in gaseous emission reduction. Similarly, large amounts of agricultural wastes (e.g. sugarcane bagasse, corn cob & pine saw dust) and other solid waste such as tyre are in abundance in SA. It is detailed in chapter 2. Majority of the wastes are disposed indiscriminately, hence resulting in environmental pollution. Importantly, the solitary gasification of biomass is very expensive considering the prices of biomass. Besides that, biomass produces large amount of tar hence, resulting in operational difficulties in the gasifier and end user facilities. In this study, co-gasification of coal and solid wastes is considered as a crucial alternative to addressing the aforementioned problems. Particularly, the feedstocks used for this study were coal, biomass (corn cob (CC), pine sawdust (PSD), sugarcane bagasse (SCB)) and waste tyre (WT) and were pre-treated by drying, milling, sieving, and torrefaction (coal was not torrefied). The fuel samples were blended with coal at different ratios as detailed in the thesis and used for the study. For the torrefaction process, the most viable torrefaction process conditions and feedstock were determined, while the torrefaction process model for the feedstocks were developed, using Response Surface Methodology (RSM) and Artificial Neural Network (ANN), respectively. The Performance efficiency of gasification systems was evaluated using experimental data obtained from a few gasifiers (e.g. entrained, fluidised, and fixed bed) operated at varied experimental conditions using blends of feedstocks (e.g. biomass, coal, waste tyre etc.). A backpropagation Levenberg Marquardt (L-M) and Bayesian Regularisation (BR) algorithms of ANN model with Multiple Input- Multiple Output (MIMO) and Multiple Input-Single Output (MISO) layer networks were considered. The results of the MIMO and MISO layer networks obtained from the L-M algorithm were better than that of BR algorithm which is in affirmation with some of the results found in the literature. For model result improvement, Input Variables Representation Technique-by-Visual Inspection Method (IVRT-VIM) and Output Variables Representation Technique-by-Visual Inspection Method (OVRT-VIM) were developed from the study. Estimation of the gaseous emissions and profits from biomass, tyre, and coal fired co-gasification CHP Plant using Artificial Neural Network (ANN) was carried out for 20-year investment period using South Africa (SA) and Nigeria as cases studies via Artificial Neural Network (ANN). Higher profits were obtained from South African feedstocks than that of Nigerian feedstocks due to cheaper price of SA coal WFO and WOFC, but the gaseous emissions (CO, NOX, & SO2) from the Nigerian fuels were lower than that of SA because of differences in compositions of the fuels. The potentials of biomass torrefaction in terms of profitability in a co-gasification CHP plant for a 20-year-investment period was carried out using blends of Coal + SCB, Coal + CC, and Coal + PSD with coal-to-biomass ratio of 50:50, 71:29, and 80:20, respectively. The two financial cases mentioned earlier were considered. Four investment terms including: (A) 1st–5th, (B) 5th– 10th, (C) 10th– 15th & (D) 15th– 20th and two operational cost models; with feedstock costing (WFC) and without feedstock costing (WOFC) were employed. An estimated profit of between USD5.9 million - USD6.5 million and USD7.8 - USD7.9 million was earned at the end of investment plan using WFC and WOFC, respectively. The Internal Rate of Return (IRR) was 5 ± 1 %/yr. and 7 ± 4 %/yr. based on South African electricity price of 0.14 $/c kWh, respectively. The parametric effect of process variables during torrefaction of coal/biomass/waste tyre blends using ANN and RSM models were studied. The variables considered were Higher Heating Value (HHV), Enhancement Factor (EF), and Sold Yield (SY). The most effective operating process conditions (in terms of blending ratio, temperature and torrefaction time: input variables) is of the order: 50:50 at 300 OC and 45 min > 50:50 at 250 OC and 30 min >50:50 at 200 OC and 45 min. Similarly, the most viable fuel follows the order of Coal + Torrefied PSD > Coal + Torrefied SCB > Coal + Torrefied CC and > Coal + Torrefied WT. Coal + Torrefied PSD has HHV of 28.27 % and an EF of 1.41. This corresponded to around 10 % increase in the HHV of the torrefied fuel when compared to the raw fuel and about 25.23% higher than the EF of Coal + Torrefied WT of 1.03. Based on the result of the EF of Coal +Torrefied waste tyre, upgrading of the fuel quality via torrefaction is not recommended. Furthermore, a comprehensive study on tar treatment techniques was carried out using tars produced from biomass and blends of biomass and coal employing biochar based and Ni-biochar based catalysts. Box Behnken Design of Experiment (DoE) method was used. A full quadratic regression model was used to develop a mathematical model for tar treatment based on the feedstocks studied. The Pine Sawdust-Biochar Catalyst (PSD-BC) and Nickel Pine Sawdust-Biochar Catalyst (Ni-PSD-BC) were the most effective in terms of tar treatment and with an average percentage amount of tar conversion of 89.76 and 96.73%, respectively. Ni-PSD-BC was more efficient for tar cracking than PSD-BC, but PSD-BC (waste base) may be more attractive if sustainability and cost effectiveness of precursors are considered. Co-gasification of coal and pine sawdust (PSD) to hydrogen enriched syngas in a fixed bed gasifier was carried out with catalyst (WCAT) at 900 OC and without catalyst (WOCAT), at 700, 800, and 900 OC, respectively. Coal-to-PSD ratio of 1:1 was used, while Nickel-pine sawdust-biochar (Ni-PSD-BC) and pine sawdust-biochar (PSD-BC) were employed as catalysts. The gases produced at 700, 800 & 900 OC using WOCAT cannot be used in fuel cells and gas turbines due to poor quality, while others produced at 900 OC WCAT, can be used in internal combustion engines and gas turbines, but unfortunately, have lower quality to be employed in fuel cells for electricity production. However, the study provides a method of beneficiation of the high ash content South African coal for energy production. The outcome of this study is also instrumental to energy security, efficiency and sustainability as well as waste management in South Africa, Nigeria and other parts of the globe. An assessment of the economic, energy and environmental viability of a 5 MW co- gasification power plant was carried out, using blends of coal and biomass, and two financial cases were considered namely: with feedstock costing (WFC) and without feedstock costing (WOFC). Feedstock profitability in the plant for energy production was evaluated. Equipment consisting was not considered. The power plant used 20,473,451.41 kg, 20,986,049.96 kg, 18,251,806.49 kg, and 15,276,277.85 kg of Coal + SCB, Coal + CC, Coal + PSD, and Coal + WT to produce the 5 MW and 5.56 MW electric and thermal power, annually. Coal + Torrefied PSD was the most profitable of the fuels studied. The use of Coal-to-PSD ratio of 4:1 for the power generation as against Coal-to-PSD blend ratio of 1:1 resulted to an annual loss of about ZAR6, 461,301.77 ($90,458,224.70) and ZAR123,782.47 ($1,732954.58) WFC and WOFC, respectively.