Browsing by Author "Govender-Opitz, Elaine"

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    Bioleaching as a unit operation for the recovery of copper and other metals of value from WEEE
    (2025) Maluleke, Dumisani Musa ; Harrison, Susan; Kotsiopoulos, Athanasios; Govender-Opitz, Elaine
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    The development of a mathematical model for the bioextraction of the critical raw material magnesium from mine waste using acidithiobacillus caldus
    (2025) Raeburn, Yael; Kotsiopoulos, Athanasios; Govender-Opitz, Elaine; Harrison, Susan
    This investigation aimed to develop a model to describe the bioleaching of magnesium mine wastes, ultimately maximising magnesium extraction while minimising reliance on synthetic acid. In the absence of experimental data on the dissolution profiles of the waste, kinetic models were developed to describe the leaching of individual minerals: periclase, calcite, and goethite. These leaching kinetics, combined with the extracted microbial growth kinetics for Acidithiobacillus (At.) caldus, formed the basis of the bioleaching model. Both batch and continuous flow reactor systems were investigated for their suitability and performance in magnesium recovery. The system was optimised using a pH control system and a two-stage flowsheet design. Chemical leaching kinetics were determined by fitting the models to experimental data from the liter-ature. The extracted reaction orders were 0.3, 3 and 1 for periclase, goethite and calcite, respectively. This indicated that iron dissolution was most sensitive to pH, while magnesium dissolution was the least. The relative magnitude of the activation energies suggested that calcium would dissolve most readily, followed by magnesium, and then iron under the same reaction conditions (EaCa = 32 kJ/mol, EaM g = 67.8 kJ/mol, average EaF e = 90.2 kJ/mol). Investigation into two goethite samples of differing crystal morphologies demonstrated negligible impact on dissolution rate, with only a 0.01 kJ/mol variation per 1 dm2/g difference in surface area. To investigate the production rate of the biogenic acid lixiviant, At. caldus, a chemoautotrophic microor-ganism that oxidises elemental sulfur, was used to facilitate the production of sulfuric acid. Both the Monod and Michaelis-Menten equations were investigated to describe the growth kinetics, assuming 55% availability of initial elemental sulfur due to limited surface area availability for microbial attachment. The Monod expression yielded a maximum specific growth rate (μmax) of 0.275 h−1 and a saturation coefficient (Ks) of 1.5 mol/L, resulting in a maximum sulfate production rate (rp,max) of 0.0009 mol/Lh. However, the model incorporating the Michaelis-Menten equation provided more accurate pH predic-tions throughout the experimental duration, making it more reliable for predicting the acid production rate. Consequently, a Michaelis constant (Km) of 2.25 mol/L, maximum utilisation rate (Vmax) of 0.008 mol/Lh, and maximum sulfate production rate (rp,max) of 0.0004 mol/Lh were incorporated in the subsequent investigations. The extracted chemical leaching and microbial kinetics were combined to describe the bioleaching system in which the acid produced by the oxidation of elemental sulfur by At. caldus leached magnesium, calcium, and iron from the magnesium mine waste. Development of the batch system showed that calcium dissolved rapidly, consuming most of the initial acid. Magnesium followed, reacting with the remaining acid and the biogenic acid as it was produced, indicating that the rate of biooxidation was the rate-limiting step. Negligible iron dissolution was observed. Systematic optimisation of the initial pH and solids loading resulted in optimised conditions of pHi = 1 and 0.75% solids loading (m/v). These conditions achieved 98.5% extraction of magnesium, complete extraction of calcium, and a processing throughput of 0.005 gM g−waste/h over a 60 day period. Two configurations were developed for the continuous flow system: one without acid addition and the other introducing acid in the reactor feed. The former resulted in similar dissolution profiles to the batch system, with complete extraction of calcium at pHi ≤ 2. However, magnesium extraction remained ∼35.6% across pHi 1-3 due to the limited rate of acid production. The optimised system achieved 100% calcium extraction with 82.3% of the magnesium going into solution at an equivalent throughput to the batch system (0.005 gM g−waste/h). Operating at 0.5% solids loading (m/v), pHi = 2 and τ = 41.7 days. The inclusion of acid in the reactor feed (pHin = pHi) significantly increased the extraction of magnesium from 35.6% to 80% at pHi = 1. The optimised case (pHi = pHin = 1, 1 0.5% solids loading, τ = 14.6 days) achieved complete extraction of both magnesium and calcium at a throughput of 0.014 gM g−waste/h. However, the addition of acid did not align with the project's aim of decreasing the demand for synthetic acid. With this in mind, the CSTR system was further optimised by investigating the high-impacting param-eters: pH and mean residence time. Implementation of a two-stage system allowed for the independent optimisation of the microbial oxidation stock (Vs), and mineral (V1) reactors. Using the previously de-termined mineral reactor conditions (0.5% solids loading, τ1 = 14.6 days, pHi = 1), complete extraction of magnesium was achieved at a stock reactor mean residence time (τs) ≥ 10.4 days, establishing an optimal threshold at Vs : 1.4V1. This set-up achieved the same performance as the CSTR with acid in the feed while eliminating the need for synthetic acid. pH control was achieved using a PI controller for the rate of acid addition, for which ITAE tuning parameters were optimised for both disturbance (Kc = 40.7 mol−1.L, τI = 12 h) and set-point changes (Kc = 20 mol−1.L, τI = 326 h). The control system improved the system response times from 14 days to 11.5 and 9 days in the case of a set-point change and disturbance, respectively, while maintaining stable operation. The predictive model developed provided a fundamental understanding of the bioleaching mechanisms. However, the extraction of the CRM, magnesium, was limited by the rate of biooxidation in the CSTR system (with no additional acid) and the batch system. The single-stage CSTR with acid in the reactor feed achieved complete extraction of magnesium, processing 0.014 gM g−waste/h. Implementing a two-stage process achieved the same degree of extraction as the single-stage system while eliminating the need for synthetic acid. pH control decreased the system's natural response time to both set-point changes and disturbances. While this model combined individual chemical leaching and microbial growth kinetics, it is recommended that this model be validated against experimental data for the dissolution of complex waste. This would allow for the validation of selective extraction at different pH levels and confirm whether the effects of surface area are negligible for waste samples.
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    Open Access
    The recovery of valuable base metals from electronic waste using a biological matrix extracted from Black soldier flies
    (2021) Mabuka, Thabo; Govender-Opitz, Elaine; Harrison Susan T L
    Waste streams have increased due to advancements in technology and the increase in the global population, requiring innovative strategies to recover value from them whilst reducing their negative environmental impact and human health hazards. Thus, the increase in waste has led to research focused on circular economies. E-waste is the fastest growing waste stream in the world containing valuable metals that exceed those rich in ore from mines. In Africa, e-waste metal recycling remains largely informal and small scale resulting in inefficient metal recovery, increased negative environmental impact and human health hazards. E-waste metal recycling using pyrometallurgy is limited to secondary smelter feed and there are limited industrial plants dedicated solely for this purpose. While in hydrometallurgy, research in e-waste metal recycling has been largely focused on metal extraction whilst downstream metal recovery processing studies are limited. The strategy often employed in e-waste metal recovery via hydrometallurgy is base metal (BM) extraction before precious metal (PM) recovery due to the high concentration of these metals in e-waste. This results in the production of base metal-rich-leachate solutions. The heterogeneity of these leachate solutions and the high cost of downstream processing requires a multi-disciplinary approach that considers metal recovery selectivity and associated costs. Natural sorbents, chitin and chitosan found in large quantities in industrial food waste and precipitation with sulphides have received much attention due to their high metal recovery efficiencies, metal selectivity, scalable operation and low costs. Chitin and chitosan are mainly sourced from crustacean shell waste and there are limited techno-economic studies on the extraction and production methodology of these polymers. Chitin and chitosan from Black Soldier Fly (BSF) larvae shells, a waste product from BSF farming, is thought to have high adsorptive properties due to their low crystalline index. However, studies on metal adsorption onto chitin and chitosan sourced from BSF larvae and their potential combined application with sulphide precipitation to recover metals from e-waste leachate solutions remains limited. Therefore, the dissertation aimed to develop a cost-effective method of extracting chitin and chitosan from BSF larvae shell waste and investigated the techno-economic feasibility of the application of these polymers in combination with sulphide precipitation for the recovery of base metals from e-waste leachate solutions. The potential application of chitin/chitosan from BSF larvae in e-waste metal recovery may result in a circular economy where solid waste is utilized to produce BSF larvae. While the BSF larvae shell waste generated from BSF larvae production can be used to remediate electronic waste, recovering value from these waste streams while reducing their environmental impact. The cost-effective method for the extraction of chitin and production of chitosan from BSF larvae was investigated by a study into the effects of demineralisation, deproteination, decolourisation, de-acetylation processes on the chitin and chitosan character, metal adsorption performance and techno-economics. Chitin and chitosan were extracted and produced from BSF larvae (Hermetia illucens) using a combination of the processes stated prior. Adsorption studies with the produced chitin and chitosan were conducted on base metals ferrous, ferric, copper and aluminium ions in single and bimetal solutions. The adsorbed metals were then eluted using 0.1 M H2SO4. Precipitation studies were also conducted with various concentrations of copper in a ferrous, copper and aluminium solution. The techno-economic feasibility of the application of the chitin and chitosan and sulphide precipitation with NaHS in PCB leachate solutions was investigated by the development of a model based on the ascertained individual metal recovery performance in the adsorption and precipitation studies. Extracted chitin from BSF larvae was found to be in the alpha form. 4-hour Deproteination of the BSF larvae after liberation with 4 wt % NaOH and de-acetylation of the deproteinated chitin with 40 wt% NaOH was found to produce chitin and chitosan with the highest metal sorption capacities and lowest cost of production. The maximum adsorption capacity for ferrous, ferric, copper and aluminium ions onto chitin from BSF larvae was 2.29 ± 0.0001 mmol/g, 2.07 ± 0.0001 mmol/g, 1.69 ± 0.0001 mmol/g and 1.82± 0.0001 mmol/g respectively. While for chitosan, the maximum adsorption capacity for ferric, copper and aluminium ions was 0.951 ± 0.0012 mmol/g, 1.16 ± 0.0016 mmol/g and 0.961± 0.0013 mmol/g respectively. The order of metal adsorption selectivity for ferrous, ferric, copper and aluminium on chitin from BSF larvae was determined to be Fe2+>Fe3+>Al3+>Cu 2+. While for chitosan it was determined to be Cu2+>Fe3+>Al3+ and at a low pH (below pH of 2) it was observed to be Cu2+>Al3+>Fe 3+. Ferrous ion oxidation to ferric ions was observed during the adsorption of ferrous ions onto the chitin and chitosan. Adsorption of the metals onto chitin and chitosan were best modelled by the Freundlich isotherm and Pseudo 2nd order kinetic model. The adsorption on both polymers was found to be spontaneous, favourable, chemisorption and predominantly surface complexation. Sulphide precipitation with NaHS was observed to be selective towards copper precipitation however co-precipitation with aluminium occurred. The application of chitin and chitosan on the multi-metal synthetic PCB leachate solution resulted in the production of two refined streams respectively. The application of NaHS precipitation seems to be more feasible on the refined streams produced by the application of chitin. The combined application of NaHS and chitin from BSF larvae on the multi-metal synthetic PCB leachate solution showed economic feasibility. The recovery costs were $ 116 per kg metal recovered and an overall gross profit of $ 933/ kg metal recovered. However further economic studies which include consideration of capital costs need to be conducted to conclusively determine the economic feasibility of this downstream metal recovery process. This study shows the potential of chitin and chitosan extracted from BSF larvae to upgrade PCB metal leachate solutions for further downstream processing