Doctoral Degrees (Chemical Engineering)

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Browsing Doctoral Degrees (Chemical Engineering) by browse.metadata.advisor "Bradshaw, Steven Martin"

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    Selective recovery of metals from citric acid leach solutions during the recycling of lithium-ion batteries
    (Stellenbosch : Stellenbosch University, 2022-04) Punt, Tiaan; Akdogan, Guven; Bradshaw, Steven Martin; van Wyk, Andries Pieter; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH SUMMARY: Recycling has become an imperative part of the lithium-ion battery (LIB) life cycle due to growing demand for energy storage in applications like electric vehicles and renewable energy technologies, as well as government legislations requiring the recycling of LIBs to reduce environmentally harmful waste. LIB recycling processes must therefore aim to provide a secondary source for strategically scarce metals, like lithium and cobalt, while seeking to reduce the environmental impact of LIB waste. This project aimed to develop a hydrometallurgical process based on environmentally-friendly reagents to recover manganese, lithium, cobalt, and nickel in separate product streams from end-of-life lithium-ion batteries. Organic acids are effective lixiviants in hydrometallurgical recovery of metals from scrap LIBs, having the added benefit of being more environmentally benign than mineral acids. Among these organic acids, citric acid exhibits similar extraction performance when compared to mineral acids. Leaching LiCoO2 (LCO) and LiNixMnyCozO2 (NMC) cathode powder following dismantling and aluminium removal with 1.5M citric acid, 2 vol.% H2O2 at 95°C and 20 g/L for 20 minutes, achieved 93% Al, 90% Co, 96% Li, 94% Mn, and 94% Ni dissolution, confirming citric acid’s performance as lixiviant. A combination of solvent extraction and precipitation technologies was then used to sequentially separate cobalt, lithium, manganese, and nickel from the citric acid leach solution. A diverse range organic extractants, namely: Versatic 10, Cyanex 272, PC-88A, D2EHPA, LIX 84-IC, LIX 984N-C, TBP, Alamine 308, Alamine 336, and Aliquat 336TG was screened to determine which metals can be selectively separated from the citrate leach solution. It was concluded that manganese and residual aluminium are best separated from the PLS under strong acidic conditions with D2EHPA, after which lithium can be separated under weak acidic conditions with D2EHPA in a second subsequent extraction. The cobalt and nickel were separated poorly by the organic extractants and would thus be separated by precipitation from the lithium extraction raffinate. The first separation of manganese and trace aluminium was optimized with 12 vol.% D2EHPA in kerosene at a pH of 2.5 and O/A ratio of 2 when using 3 counter current stages, which separated 99.9% Mn and 80% Al from the PLS. The co-extraction of other metals under optimum conditions was determined to be 7.7% Co, 12.1% Li, and 4.9% Ni. Comparable stripping performance was achieved with sulphuric acid and citric acid from the loaded organic and thus citric acid was chosen as stripping agent. Optimal stripping of the aluminium and manganese loaded organic was achieved with 1.5M citric acid at an A/O ratio of 2, where 78% Mn and 20% Al was stripped in a single stage. The novel second, sequential extraction separated 93.6% Li to a reversible 3rd phase under weak acidic conditions where the optimal lithium separation was achieved with 23 vol.% D2EHPA in kerosene at a pH 5.5 and O/A ratio of 4 with 3 counter-current stages. The co-extraction during the optimum lithium separation included 6.6% Co and Ni. The lithium loaded 3rd phase and diluent emulsion was selectively stripped with 1.5M citric acid and an A/O ratio of 1 to recover 71% Li with 24% Co and Ni in one stage. Optimal nickel precipitation from the lithium extraction raffinate using DMG was achieved with a Ni/DMG ratio of 0.2 at a pH of 8, which enabled 98.5% Ni precipitation with 20% Co co-precipitation. The final effluent from the process had a 96.1 wt.% cobalt purity (metal basis) in the aqueous phase. This hydrometallurgical process was therefore capable of effectively separating the LIB metals from an organic acid PLS to individual metal product streams.
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    A thermosiphon photobioreactor for photofermentative hydrogen production by Rhodopseudomonas palustris.
    (Stellenbosch : Stellenbosch University, 2023-03) Bosman, Catharine Elizabeth; Pott, Robert William M.; Bradshaw, Steven Martin; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: Hydrogen has widely been identified as a commodity chemical. Currently, however, hydrogen is primarily produced through non-renewable methods. Biological hydrogen production through microbial photofermentation offers an environmentally friendly and potentially economically feasible alternative. Although this technology is promising, the costs associated with photofermentation systems need to be reduced and hydrogen productivity increased, to make the technology a competitive alternative to non-renewable hydrogen production methods. This can potentially be realised through cost-reduction strategies in combination with bioremediation – purifying wastewater whilst simultaneously producing a valuable chemical. This work applied a combination of techniques to develop and evaluate a novel thermosiphon photobioreactor (TPBR) for photofermentative hydrogen production, using Rhodopseudomonas palustris (R. palustris). The TPBR implements the thermosiphon effect to passively circulate biomass – the first and currently the only photobioreactor with the potential of operating without any external energy inputs. The TPBR was successfully implemented for photofermentative hydrogen production using R. palustris, achieving maximum hydrogen production rates of up to 0.310 mol·m−3 ·h−1 in the growing state. The effects of light intensity, temperature and biomass concentration on hydrogen production and passive circulation of biomass were investigated. The effects of biomass concentration were found to be most pronounced (0.4 to 1.2 g·L−1 ), while light intensities of 400 to 600 W·m−2 and an internal operating temperature of 31 to 44 °C were found to be suitable for hydrogen production. Exploring the effects of geometry, two novel TPBR designs were proposed – a tubular loop TPBR and a flat-plate TPBR. Using computational fluid dynamics (CFD) simulations, these designs were characterized in terms of fluid flow patterns, temperature profiles and radiation fields. Both TPBR designs showed potential for hydrogen production, achieving temperature gradients sufficient to ensure adequate circulation and velocities to maintain biomass in suspension. CFD simulations indicated light distribution as a possible area for improvement in the existing TPBR. Consequently, a reflector system was developed and implemented for the enhancement of light distribution and hydrogen production in the experimental TPBR – achieving a more uniform light field and an associated 48% increase in hydrogen production. Evaluating the feasibility of outdoor operation, the effects of diurnal light cycles and the emission spectrum of light were investigated. R. palustris was able to produce hydrogen under a sunlight-mimicking light emission spectrum achieving maximum hydrogen production rates of 0.790 mol·m−3 ·h−1 , albeit slightly lower as compared to under near-infrared light where it reached production rates up to 0.891 mol·m−3 ·h−1 . Hydrogen production was found to cease during dark periods in the diurnal light cycles; however, continuing again in the presence of light and achieving maximum Stellenbosch University https://scholar.sun.ac.za iii hydrogen production rates of ~0.015 mol·m−3 ·h−1 . This demonstrated promising potential towards outdoor operation of the TPBR, circumventing the requirement for external energy inputs. This dissertation has successfully demonstrated the application of a novel thermosiphon photobioreactor for photofermentative hydrogen production with minimal external energy input. The research comprised determination of suitable operating conditions for hydrogen production, a CFD modelling method for the design of PBRs, two novel TPBR designs and characterization thereof, a light distribution strategy for the enhancement of hydrogen productivity in PBRs, and insight into the passive circulation of biomass in a TPBR and the behaviour of R. palustris under simulated outdoor conditions. Collectively, this research provides knowledge not only improving the TPBR, but which could also be extended to other systems in the biohydrogen field.