Bioleaching as a unit operation for the recovery of copper and other metals of value from WEEE
| dc.contributor.advisor | Harrison, Susan | |
| dc.contributor.advisor | Kotsiopoulos, Athanasios | |
| dc.contributor.advisor | Govender-Opitz, Elaine | |
| dc.contributor.author | Maluleke, Dumisani Musa | |
| dc.date.accessioned | 2025-09-04T13:07:05Z | |
| dc.date.available | 2025-09-04T13:07:05Z | |
| dc.date.issued | 2025 | |
| dc.date.updated | 2025-09-04T13:05:32Z | |
| dc.identifier.apacitation | Maluleke, D. (2025). <i>Bioleaching as a unit operation for the recovery of copper and other metals of value from WEEE</i>. (). University of Cape Town ,Faculty of Engineering and the Built Environment ,Department of Chemical Engineering. Retrieved from http://hdl.handle.net/11427/41695 | en_ZA |
| dc.identifier.chicagocitation | Maluleke, Dumisani. <i>"Bioleaching as a unit operation for the recovery of copper and other metals of value from WEEE."</i> ., University of Cape Town ,Faculty of Engineering and the Built Environment ,Department of Chemical Engineering, 2025. http://hdl.handle.net/11427/41695 | en_ZA |
| dc.identifier.citation | Maluleke, D. 2025. Bioleaching as a unit operation for the recovery of copper and other metals of value from WEEE. . University of Cape Town ,Faculty of Engineering and the Built Environment ,Department of Chemical Engineering. http://hdl.handle.net/11427/41695 | en_ZA |
| dc.identifier.ris | TY - Thesis / Dissertation AU - Maluleke, Dumisani AB - The biohydrometallurgical approach (bioleaching) continues to emerge as the promising option in comparison to pyrometallurgy and hydrometallurgy, in terms of both economic and environmental advantages as well as recovery efficiency, for the extraction of base metals from printed circuit boards (PCBs). The biohydrometallurgical route typically presents less corrosive reagents and lower energy demands as operating temperatures are typically low, thus lower investment is required, distributed systems have potential and the technology may be profitable even for the treatment of low-grade PCBs. Of much benefit to bioleaching is the continuous in-situ regeneration of the ferric (Fe3+) ion reagent by the iron oxidising microorganisms within the system, further lowering operating costs and minimising the disposal of toxic effluent to the environment. Bioleaching of PCBs has thus been explored since the early 2000s, with emphasis on the operating factors affecting the rate and extent of metal extraction, such as choice of microbial consortia, Fe3+ and/or Fe2+ concentrations and ratios, pH of the system, temperature, particle size of the PCBs, and solid loadings. High metal extractions are reported, especially for Cu (i.e. >90% Cu). Despite such progress, one of the major challenges in further development and commercialisation of bioleaching is the limited rate of microbial regeneration of the Fe3+ oxidant relative to the Fe3+ reduction rate as a function of metal dissolution, resulting in low leaching rate and extent of metal leached. The low Fe3+ regeneration rate is further exacerbated by the inhibition of microbial culture by accumulating metal ions and/or other non-metallic components of PCBs within the system, thus limiting the benefits of microorganisms in the system. This study thus evaluated chemical leaching (Fe3+ reduction) and microbial Fe2+ oxidation (Fe3+regeneration) kinetics during the extraction of base metals from PCBs and used such fundamental understanding to inform reactor set-up for the bioleaching of PCBs. Hence, the project was apportioned into three parts: (1) developing chemical leaching kinetics, (2) enhancing microbial Fe2+ oxidation kinetic rates through minimising inhibition and maximising volumetric oxidation rates through maximising the volumetric oxidation rate through biomass retention, and (3) bioleaching of PCBs in a bioreactor in which the balance on rates of ferric leaching of base metals from PCBs with microbial ferric regeneration is sought. The chemical leaching kinetics were evaluated through the leaching of four typical predominant base metals of interest in PCBs, i.e., Cu, Zn, Ni, and Sn, as elementary metal powder. To best evaluate relative metal leaching behavior and associated kinetics, the four metals were leached as individual metals and as mixed metals. The kinetic data obtained for ferric and acid leaching were compared to that of leaching of complex PCBs. Leaching of elementary metals prior to complex PCBs provided useful fundamental leaching kinetics, which remains scarce in the literature. Such data is critical in informing the modeling and design of process flowsheets for the leaching of PCBs. For example, understanding relative metal leaching rates can help in determining residence time and number of leaching tanks required. As metal leaching is also through the attack by proton (H+), all leaching experiments were also carried out with H+ oxidant only by using acidified water, in addition to Fe3+ leaching. Across all leaching experiments, the Fe3+ oxidant exhibited faster leaching rates than H+, suggesting that Fe3+ was a dominant oxidant. Amongst individual elementary metal leaching, the relative Fe3+ leaching rates were in the order of Zn > Sn > Cu > Ni and were the same as that of Fe3+ leaching of mixed metals. For acid leaching systems, the relative leaching for both individual and mixed metals was in the order of Zn > Cu > Ni > Sn. This suggested that the difference in the relative leaching rates between Fe3+ and H+ leaching systems was on Sn. This was attributed to the low solubility of Sn in the sulfuric acid (H2SO4) medium. Similar order in relative leaching rates between individual and mixed metals was critical as it meant that the kinetic leaching data for individual metals can be useful in understanding mixed metal leaching systems. The Fe3+ leaching of individual Cu and Sn was 2nd order with respect to Fe3+ concentration and approximately 5th and 1st order for Zn and Ni, respectively. Our estimated leaching kinetics showed no dependence on Fe2+ concentration across all four metals. Leaching kinetics for mixed metals were assumed to be dominantly controlled by Cu leaching and the leaching order remained 2nd order with respect to Fe3+. The apparent activation energy for the Fe3+ leaching of Cu, Zn, Ni, and Sn was, respectively, 32.7, 24.3, 84.5, and 18.6 kJ.mol-1, suggesting that the mechanism for leaching of Cu and Zn was mixed-controlled, Ni was chemically controlled, and Sn was diffusion-controlled. The apparent activation energy for the leaching of mixed metal was 16.3 kJ.mol-1, implying a diffusion-limited mechanism. More than 75% Cu, 85% Zn, 49% Ni, and 4% Sn were successfully leached over 40 min from PCBs by the Fe3+ oxidant, but at a lower leaching rate compared to fully liberated elementary metal powders. The relative metal leaching rate was comparable with that of elementary metals, i.e., Zn > Cu > Ni > Sn. The apparent low leaching rate of Sn may have been due to its precipitation. As with the leaching of the elementary metals, Fe3+ leaching of PCBs was 2nd order with respect to Fe3+ and showed no dependency on Fe2+. The leaching mechanism was mixed-controlled with an apparent activation energy of 26.8 kJ.mol-1. Acid leaching of PCBs was also successfully achieved with 72% Cu, 99% Zn, 73% Ni, 9% Sn solubilised over 1440 min and a leaching order of Zn>Ni>Cu>Sn. This first part of this PhD thesis provided critical fundamental data on the leaching of base metals from PCBs and importantly, demonstrated that the kinetic data for leaching of individual metals can be useful in understanding the leaching of complex multi-metal systems. In the second part of this PhD thesis, microbial Fe2+ oxidation kinetics is investigated, mainly focusing on Fe2+ oxidation rates in the presence of potential inhibitory metal ions during the bioleaching of PCBs. Microbial Fe2+ oxidation rates were evaluated in the presence of the four metals of interest, Cu, Zn, Ni, and Sn, as individual and in combination, and PCB leachates. Metal concentrations tested, added as metal ions, were informed by the expected concentration of each metal per PCB solid loading (0 – 20% w/v, weight PCB per volume of solution) determined from the characterisation of PCBs carried out and were in the range of 0 – 50 g/L Cu2+ and 0 - 10 g/L for Zn2+, Ni2+, and Sn2+. PCB leachates tested were prepared by chemically (Fe3+) leaching the custom-made PCB samples and filtering through a 0.45 μm filter to obtain a metal ion-rich (including liberated non-metallic components) solution. The effect of varying concentrations of PCB leachates, 0 – 50% v/v (volume PCB leachate to volume of medium with inoculum), on microbial Fe2+ oxidation rate was then studied. Owing to the Fe3+ reduction rate exceeding the microbial Fe3+ regeneration rate in bioleaching systems, this study explored both the use of microbial adaptation and microbial immobilisation to improve the Fe3+ regeneration rate while minimising inhibition of microbial culture. A mixed mesophilic culture consisting of Leptospirillum (L.) ferriphilum (38%), Acidibacillus (At.) caldus (45%) and Acidiplasma (Ap.) cupricumulans (15%), with additional other species present at <1% abundance, was adapted to 6 g/L Cu2+ and immobilised on polyurethane foam (PUFs) as a biomass support structure. Microbial immobilisation was achieved by repeated sub-culturing of the microbial culture, in the presence of PUFs over a period of at least two months. Adaptation was achieved by incrementally adding Cu2+ (in increments of 0.5 g/L Cu2+ at every subculturing cycle up to 6 g/L Cu2+). Microbial colonisation on PUFs was successfully confirmed by visual analysis of the surface of the PUFs using scanning electron microscopy (SEM). The immobilised cells were then exposed to increasing concentrations of inhibitory metal ions and their performance was compared to that of immobilised and Cu-adapted cells. Control experiments for both these sets of inocula were that of typically used planktonic cells, such that four sets of inocula were studied: 1) non-adapted planktonic cells (NA-PC), 2) Cu-adapted planktonic cells (A-PC), 3) non-adapted immobilised cells (NA-IC), and 4) Cu-adapted immobilised cells (A-IC). Exploring the four sets of inocula allowed screening of the best performing in terms of tolerance to toxic metal ions and microbial Fe2+ oxidation rates, and the best was used in Part 3 (reactor studies) of this thesis. Across all inhibitory metal ions tested, Cu-adapted planktonic cells exhibited better tolerance than non-adapted planktonic cells. This suggested that adapting to Cu2+ not only improved their tolerance to Cu2+ but also to other individual metal ions (Zn2+, Ni2+, and Sn2+), mixed metals, and PCB leachates. Further, immobilised cells exhibited better metal tolerance than both NA-PC and A-PC. Of much interest was the high metal tolerance exhibited by Cu-adapted immobilised cells compared to all inocula studied. This was attributed to the synergistic benefits of prior adaptation to Cu2+, biofilm formation, and high cell density achievable in immobilised systems. A-IC not only exhibited better tolerance but also maintained their Fe2+ oxidation rates better, even at increasing metal concentration. When the culture was subjected to individual metals, microbial Fe2+ oxidation was impeded from 3 g metal ions/L and severity increased with an increase in metal ion concentration. Considering the individual inhibitory metal ions, inhibition was in the order of Sn2+>Ni2+>Cu2+>Zn2+. In comparison, mixed metal ions were more inhibitory, attributed to a synergistic inhibitory effect with respect to individual metal ions. Even at low metal ion concentrations, PCB leachates were more inhibitory than both individual and mixed metal ions, postulated to be due to synergistic action with the dissolved non-metallic component of the PCBs and/or the other dissolved metal ions (Cr, Co.). Nevertheless, complete Fe2+ oxidation was achieved over 600 h at up to 40% v/v PCB leachates by Cu-adapted immobilised cells. The improved metal ion tolerance and higher microbial Fe2+ oxidation rates observed by A-IC validated the novel approach of exploring the compounded benefits of microbial adaptation and immobilisation. As such, Cu- adapted immobilised cells were used as an inoculum in the bioleaching of PCBs in the bioreactor system (Part 3). The third and final part of this PhD thesis explored a one- and two-stage bioleaching reactor system for the bioleaching of PCBs. The two-stage reactor system was set up by coupling a stirred tank reactor for the chemical leaching of PCBs to a series of packed-bed column reactors for microbial regeneration of the Fe3+ leaching agent. The column bioreactor was packed with PUFs immobilised with Cu-adapted cells. The two-stage reactor system was operated as a continuously re-circulating closed system such that the Fe2+-rich stream (including leached metal ions) generated on Fe3+ leaching of PCBs in the stirred tank reactor was circulated through the packed-bed bioreactor for the microbial regeneration of the Fe3+ oxidant. While the focus was on the two-stage reactor system, a one-stage reactor system where both Fe3+ leaching and microbial Fe2+ oxidation occurred in one pot was run concurrently. In both reactor systems, PCBs solid loading was increased incrementally from 3% to 18% w/v in 3% increments while the rate and extent of metal leaching and the microbial Fe3+ regeneration rate were studied. Comparably, high metal bioleaching efficiency was achieved in both reactor systems across PCBs loadings tested, shown by the average efficiency achieved across 3 – 18% w/v PCBs in the two-stage reactor system as follows: 98% Al, 58% Ca, 93% Cr, 96% Cu, >100% Mg, 79% Ni, 80% Pb, >100% Sn, >100% Zn, and low efficiency of 7% Co and 11% Sr. Of particular interest was the ability of the Cu-adapted immobilised cells to maintain their activity at increasing PCB solid loading in both reactor systems, with the two- stage system exhibiting faster volumetric Fe2+ oxidation rates. The Fe3+ reduction rate as a function of metal dissolution (0.211 g Fe3+.L-1.hr-1) was 8.71 times the microbial Fe3+ regeneration rate (0.0242 Fe3+.L-1.hr-1) in the one-stage reactor system, while in the two-stage system, the Fe3+ reduction rate of 0.206 g Fe3+.L-1.hr-1 was 3.46 times faster than the Fe3+ regeneration rate (0.0594 g Fe3+.L-1.hr-1). This suggested an improvement of about 60% in the matching of the Fe3+ reduction to Fe3+ regeneration rates in the two-stage reactor system. Overall, this study contributed to the understanding of the bioleaching of PCBs by providing fundamental data for both sub-processes, i.e., the chemical leaching and the microbial Fe2+ oxidation, involved in bioleaching. It demonstrated that study of the two sub-processes independently can be insightful to the complex integrated system. This is important as the data can be useful in process modelling to aid both system design and improvement of the overall bioleaching performance of PCBs. This study provided as yet unreported data on the kinetics of ferric and acid leaching of elemental base metals and their mixtures. It successfully demonstrated that the inhibitory effect of accumulated metal ions and other dissolved non- metallic components of the PCBs on the microbial culture can be minimised using microbial adaptation and immobilisation systems. In addition to the mitigated inhibition, the microbial Fe2+ oxidation rate can be improved through biomass retention, bridging the discrepancy between the high Fe3+ reduction rate and slow microbial Fe3+ regeneration rate. These results present a novel and promising approach to maximising metal recovery while maintaining and improving microbial activity in the bioleaching of PCBs. In particular, the improved Fe3+ regeneration in the two-stage reactor systems presents a promising result for its application in the commercialisation of PCB bioleaching. Further investigation of the reactor configurations, through their modelling to further balance rates of oxidation and reduction, optimisation of operating parameters such as residence time and the number of packed-bed column bioreactors, and conversion of the system to a continuous open system for ongoing metal recovery, amongst others, are recommended to contribute to the refinement of the bioleaching system for base metal recovery from PCBs, prior to recovery of platinum group metals (PGMs). DA - 2025 DB - OpenUCT DP - University of Cape Town KW - Bioleaching KW - Copper KW - Metals KW - WEEE LK - https://open.uct.ac.za PB - University of Cape Town PY - 2025 T1 - Bioleaching as a unit operation for the recovery of copper and other metals of value from WEEE TI - Bioleaching as a unit operation for the recovery of copper and other metals of value from WEEE UR - http://hdl.handle.net/11427/41695 ER - | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/11427/41695 | |
| dc.identifier.vancouvercitation | Maluleke D. Bioleaching as a unit operation for the recovery of copper and other metals of value from WEEE. []. University of Cape Town ,Faculty of Engineering and the Built Environment ,Department of Chemical Engineering, 2025 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/41695 | en_ZA |
| dc.language.rfc3066 | eng | |
| dc.publisher.department | Department of Chemical Engineering | |
| dc.publisher.faculty | Faculty of Engineering and the Built Environment | |
| dc.publisher.institution | University of Cape Town | |
| dc.subject | Bioleaching | |
| dc.subject | Copper | |
| dc.subject | Metals | |
| dc.subject | WEEE | |
| dc.title | Bioleaching as a unit operation for the recovery of copper and other metals of value from WEEE | |
| dc.type | Thesis / Dissertation | |
| dc.type.qualificationlevel | Doctoral | |
| dc.type.qualificationlevel | PhD |