Faculty of Engineering and the Built Environment (ETDs)
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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 An Investigation into the Effect of Advanced Gravity Separation on Platinum Group Metals (PGM) Flotation Concentrates(University of the Witwatersrand, Johannesburg, 2024) Nair, Taurean Jevaldo; Sibanda, Vusumuzi“It's the gravity that shapes the large-scale structure of the universe, even though it is the weakest of four categories of forces” - Stephen Hawking Gravity concentration has been around since the dawn of mankind, and just as man has evolved so too has gravity concentration. The earliest records involved ancient cultures (Greeks, Romans, Mayans and Egyptians) using water to selectively separate precious metals from gangue. Gravity concentration has remained an integral part of many processes involving the recovery of native or alluvial precious metals and minerals which are amenable to this process. Developments in gravity concentration technology have led to the inception of advanced gravity separation devices. These advanced gravity separators can overcome the challenges associated with conventional gravity concentration as they are able to induce a high gravitational force that is capable of recovering fine and ultrafine heavy particles as well as particles with a complex mineralogy. Decreasing feed grades and falling metal prices are placing an exorbitant amount of pressure on the PGM industry. As a price-taker, the industry is at the mercy of the prevailing market conditions. This means that the only levers the industry can use are cost cutting or optimization of the process to produce more PGMs at a better quality from the current feed source. The premise of this research project is to essentially find a way to optimize the PGM beneficiation process by the use of gravity concentration. This research specifically targeted the high-grade flotation concentrate stream of a UG2 tailings plant to understand how effective advanced gravity concentration would be in recovering PGMs, upgrading the resulting concentrate as well as rejecting chromite and gangue. PGMs are associated with base-metal sulphides and are inherently complex. This complexity is further exacerbated by the fact that PGMs are in the fine to ultrafine particle size range which makes recovery of PGMs challenging. Gravity concentration is primarily a function of particle size, density and mineralogy. Separation of gangue and chromite from PGMs is another added complication as the gangue minerals are present in higher concentration than the PGMs, have a complex mineralogy and are also found in the fine and ultrafine particle sizes. Fire Assay/ICP- ii MS, XRF, XRD and SEM all confirmed the complexity of the ore being treated. This ‘entanglement of complexity’ makes processing these ores very challenging. A lab-scale Falcon gravity concentrator with an unfluidized (ultrafine) bowl was used in the main experimental work. The optimal parameters to run the gravity concentrator for PGMs was found to be a flow rate of 3 L/min, percent solids of 13.22% and a gravitational force of 300 G’s. These parameters were then applied to a multi-stage gravity concentration process. The feed to the gravity concentrator was found to have a grade of 131.02 g/t 4E PGM. The results indicated that a recovery of 48.90% and a final concentrate grade of 292.04 g/t 4E was achieved at a mass pull of 21.90%. The chromite in the final concentrate of the Falcon gravity concentrator was found to be 1.99% which did not exceed the maximum allowable chrome in concentrate of 3.00%. This proved that gravity concentration was indeed capable of recovering complex PGMs and rejecting chromite. The optimal parameters experiments indicated repeatability, and the assay results were validated by a statistical outlier test in the Minitab Software to ensure data integrity. The particle size analysis revealed that 97.07% of the feed to the Falcon was below 75μm and 66.29% below 25μm, thus confirming that the feed material was fine to ultrafine particles. The final gravity concentrate had a D90 of 42.24μm which was finer than the D90 for the feed, this demonstrates that fine and ultrafine heavy particles were more mineralized and recoverable by the Falcon. This analysis was further reinforced by the fractional analysis which confirmed that the majority of the PGMs were found in the fine to ultrafine fraction. The experiments were repeated using a fluidized bowl in the Falcon to see what the impact of fluidizing water would be. These experiments had lower overall recoveries and mass pulls than those done with an unfluidized bowl. The concentrate grade was, however, higher than the unfluidized bowl experiments possibly due to this bowl recovering PGMs in the coarser fraction. This research provides a steppingstone to understanding the effect of advanced gravity concentration on PGM flotation concentrates and indeed on PGMs in general as well as providing an alternative unique processing option for PGMs. Ultimately advanced gravity concentration has been shown to be an uncomplicated process that can be viable in the recovery of fine and ultrafine complex PGMs. It is environmentally friendly, has low capital and operational costs and has a relatively high efficiency.Item A Life Cycle Assessment of Plastic vs Cardboard Packaging in the Fast-Moving Consumer Goods Market(University of the Witwatersrand, Johannesburg, 2024) Rivett, Stephanie Anne; Harding, KevinGlobally there is a movement to mitigate the need for single-use plastics as well as the utilization of plastic materials when alternative options are available. This movement comes in response to the extensive research that has demonstrated the long-term negative environmental impact that plastics pose to our existence when disposed of into landfills. A significant contributing factor to the mass of single-use plastics is the packaging industry. This study focused more specifically on the single use plastic packaging in the fast-moving consumer goods (FMCG) market which are used to shrink-wrap bottles together to be supplied into the trade such as Pick ‘n Pay and Checkers. South Africa is facing two main challenges pertaining to the FMCG market: namely the constrained supply of energy and the socio-economic pressure to reduce the environmental impact caused by unrecycled packaging waste. This research aimed to investigate the energy requirements and environmental impact of packaging configurations that included shrink-wrap plastic and cardboard cartons versus packaging configurations that utilized only cardboard cartons to ascertain which option provides the lowest possible energy requirements, and environmental impact. This study aimed to execute a cradle-to-grave life-cycle assessment (LCA) of the two different packaging configurations by utilizing the SimaPro software. The LCA was executed with respect to one reference product that is supplied into the FMCG market year-round known as Prewash Promo. Prewash Promo is a laundry pretreatment that aids in the removal of stains. The first of the two packaging configurations under analysis was the traditional packaging configuration of Prewash Promo that has always been used. This packaging configuration consisted of six bottles that were grouped into two sets of three using rubber bands. The two sets of three were then shrink-wrapped into a group of six. Two shrink-wrapped sixes were then placed into a box that was sealed using plastic packaging tape or sellotape. The second packaging configuration under analysis mitigated the use of shrink wrap plastic and associated materials (elastic bands) thus the second packaging configuration consisted of twelve bottles placed into the box that was then sealed using packaging tape. The main objective of this LCA was to ascertain the packaging material configuration that was the most energy-efficient and environmentally responsible choice to utilize in the Stephanie Anne Rivett A Life Cycle Assessment of Plastic vs Cardboard Packaging in the Fast-Moving Consumer Goods Market iv FMCG market. This LCA was conducted utilizing the data pertaining to the year 2022 and the functional unit of this study was one year’s worth of packaging used in the production of Prewash Promo. Prewash Promo was chosen as the reference product as it does not demonstrate seasonal or geographically specific use, and it was a viable option for the change in packaging configuration. A significant factor that influenced the impact of LCA results was the waste scenarios associated with the use of different materials. In this study, the exact quantities of material that were recycled versus sent to landfills could not be definitively known. It was for this reason that the published industry standard recycling rates for the year 2022 and knowledge of socioeconomic habits were used to formulate assumptions. It was assumed that the minor materials included in the packaging configurations such as packaging tape and elastic bands conformed with social habits and did not exhibit any recycling and went directly to landfill. The recycling rate of corrugated board for the year 2022 was reported to be 61.4% and the recycling rate of plastic for the year 2022 was reported to be 42.8% (Mpact Recycling, 2019a). These recycle rates were utilized to model the packaging configurations to facilitate the comparison between the two. The validity and influence of these assumptions were assessed by means of a sensitivity analysis after the main LCA was executed. The ecoinvent 3 database library available via the SimaPro software (version 9.4.0.3, 2022) and the ReCiPe Midpoint method were used to execute the impact assessment calculations. This method consisted of eighteen impact categories that assessed the impact of each of the packaging materials with respect to the impact they posed to human health, biodiversity, and resource scarcity. The full eighteen category impact assessment was condensed into five focus categories based on the target audience, the research objectives and geographical location of the study. These five focus categories were: global warming, stratospheric ozone depletion, fine particulate matter formation, freshwater ecotoxicity and water consumption. These five categories were chosen because they provide the best overview of the impact in a summarized form pertaining to factors contributing to environmental decline, changing weather conditions, reduction in air quality and the impact of freshwater resources. The LCA was first executed for each packaging configuration in isolation to ascertain the impact contributions of each of the individual factors involved in the construction of the Stephanie Anne Rivett A Life Cycle Assessment of Plastic vs Cardboard Packaging in the Fast-Moving Consumer Goods Market v packaging set-up. The analysis of each packaging configuration in isolation facilitated highlighting major contributing factors to consider replacing with alternatives or mitigating the use thereof. This assessment highlighted the drastic impact contribution that the use of electricity had on the impact score of the heat shrink-wrap plastic configuration. The full LCA comparison was then executed to compare the two packaging configurations. In each of the five focus impact categories the corrugated board only packaging configuration achieved an environmental impact score 83% lower than the heat shrink-wrap plastic packaging configuration. This drastic difference was only reduced to 79.6% when excluding long-term emissions. Upon the conclusion of the LCA comparative assessment the validity of the recycle rate assumptions for corrugated board and shrink-wrap plastic were assessed by executing sensitivity analyses. These analyses determined that the conclusion achieved at the end of the LCA comparison stage remained valid irrespective of the recycling rate of corrugated board or shrink-wrap plastic. The final objective investigated in this study was the uncertainty analysis to assess the accuracy and reliability of the data utilized in the LCA. The uncertainty analysis was executed in the SimaPro software by utilizing a Monté Carlo analysis with the ReCiPe 2016 midpoint (H) method which consisted of 1 000 fixed runs with a confidence interval of 95%. An uncertainty bar chart was generated that displays the error associated with each of the eighteen impact categories. The uncertainty analyses for both packaging configurations determined that the data for global warming, stratospheric ozone depletion, fine particulate matter formation and freshwater ecotoxicity demonstrated low error. The cardboard only configuration exhibited very low error values of between 8% and 61% as opposed to the plastic packaging configuration which exhibited errors between 16% and 214%. The water consumption data in contrast exhibited significant uncertainty for both configurations due to the difficulty in definitively determining accurate water consumption data for such extensive life cycles. Water was utilized extensively in the developmental stages of each of the materials (forestry, paper/pulp manufacturing and plastic polymer and plastic shrink manufacturing) and exhibited significant variation in volume of consumption due to high degrees of variation in plant technology and process equipment age. Stephanie Anne Rivett A Life Cycle Assessment of Plastic vs Cardboard Packaging in the Fast-Moving Consumer Goods Market vi The culmination of the results of each of the assessments executed concluded that the corrugated board only configuration is the packaging configuration that is the most environmentally friendly, and energy-efficient packaging option of the two that were considered.Item Characterizing flotation processes of Platreef PGM ores: The applicability of models based on the Weibull and γ rate constant distributions(University of the Witwatersrand, Johannesburg, 2024) Ngema, Sithandokuhle Fortune; Safari, Mehdi; Sibanda, VusumuziThis dissertation delves into a comprehensive examination of the flotation behaviour of Platreef Platinum Group Metal ores, with a specific focus on the applicability of two widely used distribution models, the Weibull and Gamma distributions. The primary objective of this study was to demonstrate the effectiveness of these models in characterizing the flotation response of the ore under varying grind sizes, ranging from 80% passing 150 μm down to 38 μm, and under varying collector dosages. In this research, a series of meticulous experiments that involved the collection of samples from Platreef PGM ore were conducted. These samples were characterized based on their mineralogical composition, and their flotation behaviour across specified particle size distributions and varying collector dosages. The gathered data were then subjected to analysis using the Weibull and Gamma models. This allowed the assessment of the flotation performance of the ore and the description of the intricate flotation sub-processes involved to be possible. The parameters for both the Weibull and Gamma distributions were determined through a rigorous statistical method known as regression. This method involved fitting the experimental data to the models and iteratively adjusting the model parameters until convergence to their most accurate estimates was achieved. The results from the research reveal that both the Weibull and Gamma models demonstrate a commendable ability to describe the flotation process of Platreef PGM ores. However, the study also highlights certain limitations of these models, especially when dealing with coarser grind sizes, where the Weibull model shows a slight decrease in effectiveness. The investigation also points to the significant impact of liberation and grinding time on the accuracy of these models. These findings underscore the importance of understanding the nuances of particle interactions and liberation at different grind sizes in floatation processes. Furthermore, the effect of varying collector dosages on the performance of the models was explored. It was observed that the accuracy of the models diminishes with longer residence times, particularly when collector dosages are changed. This sensitivity to longer residence times and grind characteristics underscores the need for a holistic approach to model development. Another intriguing aspect of the study was the relationship between collector dosage and grind characteristics. It became apparent that increasing collector dosage has a more v pronounced positive effect on recoveries for both coarser and finer grinds, while the intermediate grinds exhibit a less substantial improvement. This discovery challenges the convention of a near-linear relationship between collector dosage and recovery. In conclusion, the research provides valuable insights into the complexities of the flotation process for Platreef PGM ores and underscores the significance of selecting the most appropriate distribution models based on specific ore characteristics and grind sizes. The dissertation also highlights the need for a nuanced approach to process optimization, acknowledging the interplay of factors such as liberation, grind characteristics, and collector dosage. This understanding can be applied to enhance recovery rates and efficiency in flotation processes, offering valuable contributions to the field of mineral processing and chemical engineering. Findings of this work shed light on the intricate nature of chemical processes, emphasizing the importance of a holistic approach to model development and practical applications.Item Carbon nanotubes application for lithium-ion battery anodes(University of the Witwatersrand, Johannesburg, 2024) Mhlanga, Nqobile; Raphulu, Mpfunzeni; Sibanda, VusumuziThe most commonly used anode material for lithium-ion batteries (LIBs) is graphite, however it has some shortcomings such as having a low reversible capacity and low diffusion rate which produce low-power density batteries. Thus, the purpose of this study was to examine the application of carbon nanotubes (CNTs) as an alternative LIB anode material. A bimetallic iron-cobalt catalyst supported on calcium carbonate (Fe-Co/CaCO3) was used for the synthesis of CNTs and it was prepared using the wet impregnation method. X-ray diffraction (XRD) analysis of the catalyst showed that it was highly crystalline. The specific surface area (SSA) which was determined using Brunauer-Emmett-Teller (BET) was found to be 11.3 m2/g. CNTs were prepared using the chemical vapour deposition (CVD) method at various test parameters i.e. temperature (650°C,700°C,750°C and 800°C), hydrocarbon flow rate (90 mL/min and 120 mL/min) and carbon source (acetylene and ethylene). High-resolution transmission electron microscopy (HRTEM) results for samples synthesised at 650°C and 700°C using acetylene at a flow rate of 90 mL/min (650°C-A90 and 700°C-A90) showed that CNTs which were multiwalled carbon nanotubes (MWCNTs) in nature were produced. The formation of what appeared to be non-tubular carbon and carbon nanofibers was observed when the synthesis temperature was increased from 700°C to 800°C. The average outer diameter (OD) of the tubes ranged from 20 to 89 nm. At a higher acetylene flowrate (120 mL/min), the quality of CNTs seemed to deteriorate for synthesis temperatures above 650°C. The formation of non-tubular carbon-like nanofibers was observed at synthesis temperatures above 650°C. The average OD of the tubes ranged from 22 to 81 nm. XRD analysis of all samples synthesised using acetylene showed a similar pattern with the most intense peak being that of carbon and minor peaks being of iron. The samples also contained some broad peaks which suggested that the samples contained amorphous carbon. The calculated crystallite size ranged from 3.4 to 6.4 nm for samples synthesised using acetylene at a flowrate of 90 mL/min. For samples synthesised using acetylene at a flowrate of 120 mL/min, the crystallite size ranged from 3.1 to 4.4 nm. Raman spectroscopy confirmed the successful synthesis of MWCNTs; however, the intensity ratio (ID/IG) was found to be above 0.7 for a majority of the samples which confirmed the presence of impurities in the samples. SSA studies revealed that an inversely proportional relationship existed between the SSA and the synthesis temperature. vi HRTEM results for samples synthesised at 650°C and 700°C using ethylene at a flow rate of 90 mL/min (650°C-E90 and 700°C-E90) revealed that CNTs which were MWCNTs in nature were formed. As the synthesis temperature increased from 700°C to 800°C, the formation of what appeared to be non-tubular carbon and carbon nanofibers was observed. The average OD of the tubes ranged from 21 to 84 nm. At a higher ethylene flowrate (120 mL/min), the quality of CNTs seemed to deteriorate for synthesis temperatures above 700°C. The formation of non-tubular carbon-like nanofibers was observed at synthesis temperatures above 700°C. The average OD of the tubes ranged from 11 to 79 nm. XRD analysis of all samples synthesised using ethylene showed a similar pattern with the most intense peak being that of carbon and minor peaks being of iron. However, the depicted minor peaks were broad which suggested that the samples contained amorphous carbon. The calculated crystallite size ranged from 3.7 to 5.7 nm for samples synthesised using ethylene at a flowrate of 90 mL/min. For samples synthesised using ethylene at a flowrate of 120 mL/min, the crystallite size ranged from 3.6 to 6.5 nm. Raman spectroscopy confirmed the successful synthesis of MWCNTs. SSA studies revealed that the SSA decreased with an increase in the synthesis temperature. Furthermore, to evaluate the electrochemical performance of the synthesised material, electrodes of selected samples were fabricated. Commercial graphite electrodes were also fabricated to compare the performance with the samples synthesised in this study. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used for the electrochemical measurements. The CV tests were conducted at scan rates of 5 mV/s and 10 mV/s, An increase in the CV curve area was observed as the scan rate was increased. The calculated specific capacity of the samples compared well with that of the electrode fabricated with commercial graphite which reported an average of ~197 mAh/g after four cycles at a scan rate of 5 mV/s. The average specific capacity of the electrode fabricated with CNTs sample synthesised at 650°C using ethylene at a flowrate of 120 mL/min (650°C-E120) reported the highest value of 214 mAh/g after four cycles at a scan rate of 5 mV/s. Overall the variation between the samples of the EIS data was marginal. These EIS results also compared well with that of the electrode fabricated with commercial graphite. The findings of this work suggest that MWCNT electrodes have a good application potential and with doping, they may provide better electrochemical performance than graphite as a viable anode material for LIBs.Item Investigation of corrosion behaviour of aluminium alloy 7075 processed by laser shock peening without coating(University of the Witwatersrand, Johannesburg, 2024) Shonhai, Natsai; Cornish, L.A.Laser shock peening (LSP) is a surface treatment to induce beneficial compressive residual stresses in metallic structures, thus improving their fatigue resistance. This technology has the potential to improve aeronautical component performance during application and maintenance. Aluminium alloys are used in the aviation industry due to their high strength-to-weight ratios and ease of design and manufacturing. However, they are susceptible to localised corrosion in some aggressive environments. This study investigated the corrosion behaviour of AA7075-T651 after LSP without a protective coating (LSPwC). Variations in power intensity (PI) of 1-6 GW/cm2, coverage (Np) of 2.5-67 spots/mm2 and spot size (SS) of 0.5-1.5 mm were explored. Surface modifications were evaluated using stereo microscopy, scanning electron microscopy (SEM), contact profilometry and Vickers microhardness tests. Three types of corrosion tests were conducted in 3.5 wt% NaCl solution on the peened and unpeened samples: potentiodynamic polarisation tests, 30-day immersion tests and stress corrosion cracking by three-point bending for 40 days. Microscopic examination revealed rough surfaces with areas of melting and solidification in LSPwC samples. Surface roughness increased in all samples post-LSPwC due to induced plastic deformation and surface ablation. Increasing PI and Np led to increased surface roughness. All peened samples had increased microhardness with a positive correlation with Np. Potentiodynamic polarisation revealed higher corrosion rates for most LSPwC samples, likely due to increased surface roughness which reduced corrosion resistance. Corrosion rate had a positive linear correlation with PI and Np with aluminium oxide formation and pitting as the dominant corrosion mechanisms. Improved corrosion resistance after LSPwC was observed in some samples with specific parameters. Notably, one sample had a corrosion rate four times lower than the unpeened one. The three-point bending induced tensile stresses on the peened surfaces, which led to the formation of multiple tangled cracks on the top surface of all specimens. The LSPwC samples were less susceptible to SCC and the best resistance was observed for Sample P3-SS1-Np50.38 which had no visible cracks in the sample cross-section.Item Development and experimental validation of an acid mine drainage prediction tool based on mineral particles(University of the Witwatersrand, Johannesburg, 2024) Ramatsoma, Mafeni SamuelAcid Mine Drainage (AMD) is an environmental hazard that is generated as a by-product of mining-related activities. It is an acidic metal-rich water formed when sulfide minerals react with oxygen and water. Due to different ore types at different mines, kinetic AMD models are often ‘calibrated’ with kinetic humidity tests done with the target mine site ore. However, most of these tests require several months to complete. This study aimed to investigate ways to reduce the time required to conduct kinetic humidity tests, and to develop a mineral particles-based kinetic AMD model. An Accelerated Humidity Cell (AHC) is proposed in this study. It was tested with two ore types and performed better (produced 24% more acidic-leachates) than normal humidity cells. The particles-based model was developed and tested with experimental data and gave promising results. It is recommended that the proposed model and AHC be further tested with other ore types.Item Characterization of a TW R260-60E1 Pearlite Rail Steel with a Weld Gap Size of 40 mm and 50 mm(University of the Witwatersrand, Johannesburg, 2024) Meyiwa, Lindani; Maledi, NthabisengThe continuous weld rails (CWR) tracks that are joined by thermite welding (TW) method are known to experience weld failures during their service life. In avoiding or mitigating these failures, a weld gap in joining two rail ends was introduced. The purpose of the weld gap was to accommodate rail expansion during elevated temperatures. Traditionally, a weld gap of 24 mm was created between two rail ends. However, these weld joints were observed to be experiencing challenges of not being able to withstand high contact loads during service life, leading to plastic deformation and eventual failures. This study was investigating the effect on microstructural characteristics, heat-affected zone (HAZ) dimensions, mechanical properties attributes, and wear resistance behaviour of TW R260-60E1 pearlitic rail steel when the weld gap was set to 40 mm and 50 mm. The results revealed that the TW R260-60E1 pearlitic rail microstructure had no martensitic, bainitic and intergranular Widmanstätten ferritic structures. Pearlitic grain sizes and interlamellar spacing on the weld zone of the web region were measured to be 3.6 % wider on the sample with 40 mm weld gap compared to those on the sample with 50 mm weld gap. Big surface area to volume ratio and fast cooling rate had an influence on the production of finer pearlite grain sizes and interlamellar spacing within the web region for both samples. The results further showed that the HAZ width was one of the attributors to the improvement on the mechanical and wear resistance properties. HAZ width varied from 29 mm – 35 mm for sample with 40 mm weld gap, and 27 mm – 33 mm for sample with 50 mm weld gap. The HAZ width on the sample with 50 mm weld gap was narrow than the HAZ width on the sample with 40 mm weld gap. Narrow HAZ width improves microstructural characteristics, mechanical properties and wear resistance behaviour compared to the wider HAZ width. The improvement on the microstructure characterisation, mechanical properties and wear resistance behaviour are expected to extend the service life of the rail and facilitate superior rail expansion capabilities. This, in turn, is projected to diminish the probabilitiy of rail-related incidents and mitigate the risks of human fatilies associated with rail transportationItem Modelling of the distribution of coal tar product qualities from a tar distillation plant(University of the Witwatersrand, Johannesburg, 2024) Mokoena, Lehlohonolo Christopher; Brooks, Kevin; Mulopo, JeanThis work presents the simulation modelling and optimisation of a coal tar distillation process to improve the product qualities and increase overall product revenue. The coal tar distillation process consists of three vacuum distillation units and a flash column. The system produces four distillate products: light oil, refined chemical oil (RCO), light creosote, and heavy oil, as well as the residue pitch used as a binder in the manufacturing of electrodes in the aluminium industry. The simulation model was developed in HYSYS using the actual plant mass balance and operating conditions for the production of a residue pitch product with a softening point of 115 – 118 Metller and associated distillates as reference. A mass balance reconciliation technique using an optimiser in HYSYS was applied to fit the plant quality and distillate rate data through adjustment of the Murphee tray efficiencies for each column. The simulation model was validated by simulating the manufacturing of a softer pitch product of softening point 68 – 73 Ring and ball using conditions specified for this particular product and its related distillate products. Through this process, the base conditions were established for the hard and soft pitch production processes. The resultant pitch yield of softening point 115 – 118 M was 42 %, with the light creosote distillate yield at 27 %, as for the softer pitch, the initial yield was estimated at 65 %, and the light creosote at 9,6 %. Following the model development and the establishment of base conditions, a sensitivity analysis focusing on product quality distribution was done to develop an operating philosophy of the process followed by an optimisation process carried out using HYSYS original optimiser to maximise the objective function defined as the sum of product revenue sales with constraints placed on product qualities and adjustable parameters selected as column reflux and boil up ratio as well as the top and bottom temperatures. From the optimisation results, the general adjustment on the first two columns was the drop-down of column top and bottom temperatures by increasing the reflux ratio and reducing the boil-up rate. The light oil product quality in the simulation of a 115 – 118 M pitch improved by decreasing the naphthalene content from 48 % to less than 8,0 % as required by standard operation, with the naphthalene recovery in the RCO stream increasing from 44 % to 67 %. The optimisation process had a large impact on the product yields, where the pitch product 115 – 118 M showed an increase in yield from 42 % to 49 %, which is close to the general yield of 50% mentioned in the literature and normally expected from a coal tar distillation process. and the light creosote distillate product had a positive yield increase of 14 % from the initial value. The overall revenue benefit for the production of a hard pitch improved by an estimated figure of 3,1 % per annum from the initial value (non-optimised condition). In the production of a softer pitch product, the total revenue benefit was 3,2 % higher per annum in comparison to the non-optimised condition.Item Design of an Industrial Process for Enzymatic Cannabidiol Conversion(University of the Witwatersrand, Johannesburg, 2024) Flavell, Erin Reece; Harding, Kevin; Rumbold, KarlThis work aims to model a theoretical enzymatic bioreactor and all the necessary surrounding processes required to facilitate the bioremediation of THC into CBD, to produce a CBD product with THC levels below the legal concentration limits (0.001%). The primary purpose is to explore whether further research into the potential biochemical remediation of THC into CBD would be worth pursuing in terms of both functionality and profitability within the CBD industry. Two primary process designs were modelled using SuperPro Designer, one producing a CBD isolate, and another producing a full spectrum CBD blend containing other cannabinoids beyond CBD, as well as other compounds like flavonoids and terpenes. The CBD isolate model is composed of four parts: the extraction of the crude oil (including the pre-extraction process); the upstream processing of the oil; the reaction of THC into CBD; and the downstream processing of the oil. The full spectrum CBD model is similarly structured but with a different fourth process stage (downstream processing). Cultivating one’s own cannabis was calculated to be more economical than purchasing it from a third-party supplier, and thus a drip-based irrigation system of 1.2108 hectares was used, requiring capital costs of $ 24 641.90 and a yearly cultivation cost of $ 7629.76. Both processes begin with milling to increase the surface area of the cannabis, followed by passing through two consecutive cold ethanol mixer-settler extraction units. Next, the oil-plant matter mixture passes through a plate-and-frame filtering system and then a decarboxylation oven, which will convert the cannabinoids into their neutral forms, producing CBD from CBDA and THC from THCA, and releasing CO2 as a co-product. The oil then passes into a PFR, where CLEAs catalyse the reaction of THC into CBD. Due to the theoretical nature of the as-yet-unknown enzyme, conversion was assumed to be 85 %, where 37.46 kg of the enzyme was calculated to be required per year, assuming replacement is required after seven days of operation. The possibility of either producing or purchasing the enzyme was considered, but producing the enzyme worked out more economically viable, at a yearly cost of $ 2.84. This is where the full spectrum process halts. In the isolate process, the oil is then mixed with an acetonitrile-TBME stream before entering the CPC, alongside a heptane stream. Most of the CBD passes into the heptane, which moves iv to a distillation column after exiting the CPC process, with CBD isolate emerging from the bottom stream. The acetonitrile-TBME stream exiting the CPC will contain other cannabinoids and remaining cannabis compounds and will flow into a separate distillation. The bottom stream of the distillation column provides other valuable cannabinoid isolates, forming ancillary products (CBG, CBN, and THC isolates). In the full spectrum CBD model, the oil flows directly into a series of three consecutive distillation columns upon exiting the PFR, designed to reduce the THC concentrations in the oil to acceptable levels. The CBD oil emerging from both processes must then be incorporated into an MCT carrier oil. CBD isolates were assumed to be sold for $ 39.00 per 30 ml, with each unit containing 600 mg of CBD. Full spectrum CBD oil was assumed to be sold for $ 40.00 per 30 ml, where every 30 ml contains 1500 mg of cannabis oil. Once the costs of the MCT oil were deducted from the theoretical revenue values, the net revenue values came to $ 64 089.93 per kilogram for CBD isolate and $ 26 302.64 per kilogram for the full spectrum CBD oil. Collectively, the ancillary cannabinoid products (CBG, CBN, and THC) yielded a net revenue value (less the cost of the required MCT oil) of $ 17749.48 /kg. CBD isolate was produced at a rate of 637 kg/year, at 99.47 % purity, with ancillary cannabinoid products being produced at 494 kg/year by the isolate process. The full spectrum CBD blend was produced at a rate of 656 kg/year and did not contain any solvent residues. The isolate process was found to have a gross margin of 83.86%, an ROI of 101.58%, a payback time of 0.98 years, and an NPV of $ 190 458 000. The full spectrum blend has a gross margin of 50.89%, an ROI of 21.75%, a payback time of 4.60 years, and an NPV of $ 14 199 000. Thus, the isolate process was deemed the more economically viable of the two processes. An additional CBD isolate design involving supercritical CO2 extraction was also modelled for comparison. In this variation, the cannabis buds undergo milling before passing through the supercritical CO2 extraction unit. The CO2-ethanol solvent feed enters a CO2 storage unit wherein is pressurised to achieve supercritical conditions before entering the extraction unit alongside the cannabis stream. The bottom stream from the extraction unit then passes into a plate-and-frame filtration system, which removes the plant matter from the stream; the recovered cannabis oil is then reunited with the top stream from the extraction unit. The combined oil stream then undergoes winterisation, in 24-hour cycles, before moving into a distillation column which removes any remaining solvent. The bottom stream from the column then enters the decarboxylation oven; the remainder of the process continues in the same manner as the original, cold ethanol extraction isolate process. The CO2 extraction process produced CBD isolate at a rate of 600.54 kg/year. CBD purity of 99.29 % was achieved. An economic analysis produced project indices of a gross margin of 84.07%, an ROI of 95.28%, a payback time of 1.05 years, and an NPV of $ 173 404 000. Thus, with gross margin being the sole exception, all project indices indicate the cold ethanol process being the process with greater potential profitability to produce CBD isolates. Because the isolate process proved the most profitable of the alternatives, its potential profitability when scaled up to industrial size was also assessed. The process feed rate was increased to 79 200 kg of cannabis buds per year, solvent input streams were proportionally scaled up, and several equipment units were multiplied as required. Additionally, the quantity of the enzyme required for catalysing the reaction was recalculated based on the increased plant material in the process, coming to a yearly mass of 576.25 kg, for $ 345 750. The scaled-up process produced a CBD isolate product with a purity of 99.47% and a production rate equivalent to 9800 kg per year and the ancillary cannabinoid product at a rate of 7587 kg per year. The NPV of the scaled-up process came to $ 3.99 billion and a gross margin of 99 % was achieved, with an ROI of 1340 % and a payback time of 0.07 years. Therefore, from the simulated model and the economic analyses, the production of CBD oils using THC-to-CBD bioremediation was found to be a potentially profitable, as-yet-untapped production method that would benefit from further research. It is worth noting, however, that the research is limited by its reliance on theoretical models and assumptions, which may not fully reflect real-world conditions, potentially affecting the generalisability of the findings. The lack of empirical validation and practical factors not captured by simulations, such as enzyme stability, further constrain the applicability. Future work should focus on empirical testing and exploring a wider range of parameters to improve the results' relevance and generalisability.