Browsing by Department "Department of Chemical Engineering"

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    Open Access
    A CFD Model for a Fixed Bed Reactor for Fischer Tropsch Reaction using Ansys
    (2023) Chitranshi, Vidushi; Moller, Klaus
    Fischer-Tropsch (FT) is a process which can convert synthesis gas derived from natural gas, coal or even biomass to a variety of products including saturated and unsaturated hydrocarbon chains, while keeping the emission of greenhouse gases minimum. Among various types of reactors used for commercial FT, fixed bed tubular reactors are among the most common type of reactor. However, there is a big challenge faced by these tubular reactors. FT is a highly exothermic process and therefore, heat removal in these reactors is needed to be highly efficient to avoid a thermal runaway. To improve the heat transfer in any reactor, it is necessary to estimate the heat production correctly. Therefore, the kinetics in the FT system needs to correctly represent the heat transfer behaviour in the system. This requires an effective description of the reaction kinetics. FT is a polymerisation reaction, so the rate expressions must be able to retain the chain reaction behaviour. This is not possible with a lumped approach model which is used by most researchers in literature. Therefore, a partial equilibrium approach was employed, where thermodynamic and kinetic models were coupled, and the reaction rates depended on the concentration of reactants as well as products. The kinetic model employed in the current project was taken directly from the work of Davies and Moller. The abovementioned partial equilibrium kinetics was used to develop a CFD model for the FT reaction system. This model was reproduced using COCO simulator for a plug flow reactor for same operating conditions as Ansys to compare the results from both the softwares. The results showed a close agreement and hence, assured that the CFD model could be used for further testing. The other challenge with the FT in FBRs is the heat dissipation. To avoid the thermal runaway, some innovations in reactor design have been studied in literature. In terms of heat transfer capabilities, when shell and tube heat exchangers are compared with the plate and frame heat exchangers, various sources in literature claim that the latter is found to be more effective. However, plate type reactors have not yet been explored in detail for their heat transfer capabilities. Taking an idea from this, the CFD model developed for the tubular reactors was adapted for plate type reactors. The heat transfer capabilities of the plate type reactors were compared relative to the tubular reactors. The tube reactor and plate type reactor were compared on the basis of two criteria. One criterion was based on physical similarity between the reactors. It included having equal Reynolds Number and equal surface area available per unit volume for both type of reactors. For a plate with plate spacing t and a tube with diameter D, the latter condition resulted in the expression, D = 2t. The factors that Reynolds number for a packed bed depends on were all same for both the geometries, so by default, the Reynolds Number was identical for both cases. The other criterion was based on catalyst packing. It included having equal tube-to-particle diameter ratio for both geometries. For tube reactors, diameter is an important parameter that determines the heat dissipation behaviour, so a parametric study was carried out to study the effect of a diameter and plate spacing on heat transfer behaviour for the same set of operating conditions. It was found that the plate type reactor had a hotspot temperature which was less than the hotspot temperature of corresponding tube reactor at all plate spacings. This indicated that the heat dissipation in a plate type reactor is better than in the corresponding tube reactor. Since the tube reactors observed higher temperatures than corresponding plate reactors, the CO conversion observed in the tube reactors was higher. When the product distributions for the two geometries were compared at isothermal conditions, the results almost overlapped for the two geometries. But when they were compared for non- isothermal conditions, significant differences were observed. This showed that heat dissipation mechanisms in the system had a huge role in bringing out different performances for the two geometries. Effect of temperature and conversion on the product distribution were also studied. On the basis of tube to particle diameter ratio criterion, tube reactor was found to outperform the plate reactors in terms of temperature control when compared using the tube-to-particle diameter ratio. Therefore, the superiority of one reactor over the other was dependent on the criterion they are being compared for. The plate type reactor was then represented in PFR model by tuning the heat transfer coefficient of the tubular model in COCO. The difference between the CO conversions achieved between the plate type reactor in Ansys and the representative model in COCO was found to be very little. Hence, the plate type reactor representation could be successfully achieved in COCO. There can be a lot of further research that can be done using the current model. The areas of reaction kinetics and reactor design were highlighted in this regard. The current model can be extended to a larger number of species, for a better representation of the FT product spectrum. Formation of liquids was completely neglected in the current project. It can be taken into account as presence of liquid can affect the FT reactor system by imposing internal and external mass transfer limitations to the reactions. The current model can also be used to study the HTFT process and also to study the isomeric products in the LTFT which were assumed to be not present in the current project. In the areas of reactor design, the geometry of catalyst particles can be included in the reactor geometry. This model can also be used for plates of other shapes and sizes to study the effect of shape and size on heat transfer capabilities. Different types of corrugated plates are used in the Plate and Frame Heat exchangers nowadays. The corrugations increase the surface area available and also increase mixing. Using the current model, such modifications can be studied for their effect on the reactions in a reactive system.
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    Open Access
    A cleaner production assessment of the ultra-fine coal waste generated in South Africa
    (2007) Reddick, J F; Von Blottnitz, H; Kothuis, B
    The South African coal mining industry is currently disposing of about 10 million tons of ultra-fine coal (<150 µm) per year. Once discarded, these sulphur-containing ultra-fines contribute to several environmental problems. As part of a project initiated by the Water Research Commission to investigate the use of Cleaner Production (CP) in the mining industry, a study was carried out to determine whether a CP approach could be used to identify opportunities to reduce this coal waste, and to determine which of these opportunities would be most feasible. In order to do this, a CP assessment was conducted at three case study collieries in the South African Witbank coalfield. Mass-balancing and sampling, followed by laboratory characterisation tests and site surveys, were used to determine the quantity, quality and sources of the ultra-fine coal at the three collieries. Literature reviews, brainstorming sessions and interviews then followed to generate the CP options. An environmental, economic and technical feasibility assessment was then prepared for each option, to determine the most viable interventions for implementation. A number of opportunities were identified through the assessment. By preventing coarser coal from being discarded with the ultra-fine coal, the quantity of coal disposed of could be decreased at all three collieries, and by up to 24% in one case. Increasing the crusher top size would reduce the amount of coal that is milled to less than 150 µm, so that less is wasted. The ultra-fines that have already been disposed of on slurry dams can be completely reclaimed and converted into a valuable product, which can be sold as power station feedstock. The newly processed ultra-fines could be beneficiated using flotation and exported together with the coarser coal. The results of the assessments thus suggested that workable CP opportunities to reduce ultra-fine coal wastage exist at the sites investigated, and that their feasibility is colliery-specific. The associated financial benefits of the proposed options suggested that CP is a realistic approach to addressing environmental problems.
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    Open Access
    A finite-difference solution of solute transport through a membrane bioreactor
    (2015) Godongwana, B; Solomons, D; Sheldon, M S
    The current paper presents a theoretical analysis of the transport of solutes through a fixed-film membrane bioreactor (MBR), immobilised with an active biocatalyst. The dimensionless convection-diffusion equation with variable coefficients was solved analytically and numerically for concentration profiles of the solutes through the MBR. The analytical solution makes use of regular perturbation and accounts for radial convective flow as well as axial diffusion of the substrate species. The Michaelis-Menten (or Monod) rate equation was assumed for the sink term, and the perturbation was extended up to second-order. In the analytical solution only the first-order limit of the Michaelis-Menten equation was considered; hence the linearized equation was solved. In the numerical solution, however, this restriction was lifted. The solution of the nonlinear, elliptic, partial differential equation was based on an implicit finite-difference method (FDM). An upwind scheme was employed for numerical stability. The resulting algebraic equations were solved simultaneously using the multivariate Newton-Raphson iteration method. The solution allows for the evaluation of the effect on the concentration profiles of (i) the radial and axial convective velocity, (ii) the convective mass transfer rates, (iii) the reaction rates, (iv) the fraction retentate, and (v) the aspect ratio.
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    A life cycle-based investigation into the potential of a circular and low-carbon plastics economy in South Africa
    (2024) Goga, Taahira; von Blottnitz, Harro; Russo Valentina
    Plastics are multi-functional materials that, while associated with numerous applications, are becoming strongly linked with the drawbacks of a primarily fossil fuel-based linear economy model. This results in the release of greenhouse gas emissions, low material recovery rates, and the environmental impacts associated with disposal and leakage. The circular economy approach is viewed as an alternative model that aims to improve environmental and economic performance, in the case of plastic systems by replacing conventional feedstock, reducing plastic litter, and creating value from waste through reuse and recovery. However, a transition from a linear to a circular model for the plastics sector remains poorly understood. Studies are generally restricted to the analysis of waste management systems with limited information available on the connection between material circularity and environmental impacts. Although several global studies have sought to explore this link using life cycle assessment methods, they do not consider national scale factors, such as the effect of a country's waste management landscape and its energy mix. This research study investigates potential future versions of a low-carbon and circular plastics economy in South Africa. The scenarios evaluated are based on local voluntary and regulatory objectives and include increased mechanical recycling, the shift from single-use consumption to reuse, and decarbonisation of the sector by integrating renewable energy into the electricity mix as well as replacement of the fossil fuel feedstock for monomer production. Furthermore, the combination of scenarios is modelled to demonstrate the potential benefit of implementing these strategies in concert. The industrial ecology tools Material Flow Analysis (MFA) and Life Cycle Assessment (LCA) are utilised to determine the degree of circularity and assess the impacts of these strategies along the plastics life cycle. The results are presented in terms of circularity and environmental indicators including but not limited to recycling rates, quantity of leakage, Global Warming Potential (GWP), and ecosystem quality. The results for the reference year (2018) demonstrate that South Africa had a per capita plastic consumption of 36 kg/capita/year, with a plastic sector input recycling rate of 40.3%, employment opportunities totalling 77 348, and a carbon footprint equivalent to 3.9% of the country's total annual emissions. Based on current practices and with no policy interventions or measures, the short-term forecast indicates that plastic production will increase by a rate of 1.7% annually between 2018 and 2025 (Baseline scenario). This will lead to a projected increase in plastic use of 1 kg/capita and a subsequent rise in leakage of 11%. Furthermore, it is expected that the degree of environmental impact will increase between 10 and 18% with a further increase of 21% in the subsequent decade. In terms of normalised results, significant impacts across all scenarios were identified as human toxicity and ecotoxicity, fossil resource scarcity, and freshwater eutrophication. Additionally, the contribution analysis revealed that the major quantifiable environmental impacts are associated with upstream processes such as monomer and polymer production and product manufacturing with a combined share of 55- 85% of the total impacts at the endpoint level (Areas of Protection). This is primarily due to coal-based pathways for feedstock and energy production. Although all the mitigation strategies, particularly elevating recycled rates, and decarbonisation of the system, display a benefit relative to the Baseline scenario, the findings demonstrate that the greatest gain amongst materiality, circularity, and environmental indicators can be achieved under the Combination scenario in 2025 with an 11.2% increase in recycled content, 1 kg/capita reduction in leakage, and an average decrease of 16% in midpoint impacts. This benefit is further extended to an additional 40.1% increase in recycled content and a 55% average improvement in environmental effects in 2035 under the medium-term forecast with levels projected to decline below historical findings for 2018. A comparison of results with global targets shows the potential of combining scenarios beyond short-term local ambitions. Notwithstanding these significant benefits in circularity and environmental impacts, leakage to the environment would still be prevalent with an estimated 256 kt of plastic debris in 2035. Despite the identification of significant potential improvements from a material and environmental perspective through the combined application of the three modelled strategies, it is concluded that these would not transform the South African plastics sector to the extent that it could be classified as fully or even largely circular nor low-carbon in the short- to medium-term. Recommendations on the material aspects include designing for recyclability, investigating the potential for chemical recycling to complement mechanical recycling, and promoting reuse business models. From an emissions perspective, the transition to renewable energy needs to be accelerated, and the introduction of lower-carbon routes for the local polymer production process should be investigated. The findings also illustrate the need for increased coordination between upstream strategies centred around design and material innovation together with downstream processes focusing on end-of-life recycling and recovery to improve the environmental profile of the South African plastics system
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    Open Access
    A methodology for the heat of immersion as a measure of wettability of mineral mixtures
    (2023) Magudu, Anam; Mcfadzean, Belinda; Geldenhuys Armand
    The measure of the extent to which a mineral interacts with water is called wettability and this is important in flotation processes. This is because the interactions between solid particles and liquid molecules (water) are important in understanding the flotation mechanism and achieving high recoveries. Contact angles and work of adhesion can be used to determine the physical properties of a given solid-liquid system, but there are drawbacks to these techniques. The advancement of microcalorimetry instrumentation has led to the use of heat of immersion to determine the surface wettability of solid surfaces. Several calorimetric studies have proven that the heat of immersion can be used to determine the surface wettability of minerals. Previous research within the Centre for Minerals Research (CMR) has shown that the heat of immersion can provide a reliable measure of mineral surface wettability when it is measured by precision solution calorimetry. However, this was done only for single mineral systems and its application to real ores has not been investigated in depth. In this study, the heat of immersion as a measure of wettability is applied to a simple binary mineral mixture representative of a real ore. The binary mineral mixture consists of a hydrophobic sulphide mineral, galena and a hydrophilic silicate mineral, albite. The results in this study have shown that the heat of immersion measurements present challenges such as an unexpected endothermic response. This endothermic response is attributed to the dissolution of the mineral in water. This dissolution is due to the surface ions on the mineral being exposed to the wetting liquid. In order to predict flotation response through measuring wettability, the aim is to measure only the heat of wetting, which is an exothermic response. Therefore, the dissolution process needs to be suppressed. Alternative techniques such as a solution saturated with the mineral sample, using organic liquids as the wetting liquids, and pre-coating of the mineral particles with collector were explored. From the various approaches explored to suppress the dissolution, it was observed that the saturated solution approach was an effective technique for certain minerals such as albite but was ineffective at suppressing the dissolution process across a range of mineral types. It is, therefore, an ineffective technique for exploring the heat of immersion of binary mineral mixtures. Secondly, it was observed that the collector coating approach was effective for suppressing dissolution at surface coverages above 75%. The collector coating approach is not feasible for conducting the heat of immersion measurements for the binary mineral mixtures because it only successfully suppresses dissolution at excess surface coverages that are not necessarily those at which one would choose to do the experimental work. Additionally, collector coating does not allow for the natural wettability of the uncoated minerals to be measured. Thirdly, hexane was found as a good wetting liquid for suppressing dissolution but there were some experimental difficulties that led to this liquid not being used for the binary mineral mixtures. These experimental difficulties include a premature immersion of the mineral into the wetting liquid due to the beeswax used to seal the ampoule dissolving in the hexane. Finally, hexanol was found to be a good wetting liquid in suppressing dissolution, had no associated experimental difficulties and was able to distinguish relative hydrophobicities between different mineral surfaces. It can, therefore, be used as an effective wetting liquid for mineral dissolution suppression and hydrophobicity determination. Preliminary experimental work into the feasibility of using a binary mineral mixture as a simple model ore system was performed. A linear relationship was found between the heat of immersion and the fraction of pure mineral A in a binary A + B mineral mixture. The heat of immersion could be presented in various ways depending on what data is required and desired. The surface area fraction or mass composition can be used to create the linear relationship between the heat of immersion and the composition of the binary mineral mixture. It was shown that there is a linear relationship between the heat of immersion and mass composition or surface area fraction of the binary mineral mixture. From this linear relationship, the heat of immersion of the pure minerals comprising the mixture can be extrapolated. The linear relationship based on composition provides a simple and convenient way to estimate hydrophobicity of a floatable mineral in an ore where only the mass and mineral composition of the sample is known. This could be used in flotation modelling, where valuable mineral floatability is a required input parameter. To determine the relative hydrophobicity of a binary mineral mixture in hexanol where the mass composition is unknown, the heat of immersion or heat released by the binary mineral mixture is measured and this is correlated with the mixture's mineral weight composition. This linear relationship can then be extrapolated to zero and 100% respectively to obtain the heat of immersion of the pure minerals. These values can be read off a calibration curve such as that obtained by Taguta et al. (2018) to obtain a flotation rate constant.
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    A multifarious comparative ecotoxicological approach on a catchment scale from three mine wastelands for improved environmental management and risk assessment: integrating abiotic, biotic and agroecosystem approaches
    (2022) Mudenda, Lee; Harrison, Susan T L; Hudson-Edwards, Karen; Syampungani, Stephen
    Metal mobilization under acid or neutral rock drainage represents one of the major environmental impacts associated with mining of sulphidic minerals. To avert this, suitable handling of mine overburden material, waste rock, open pits and tailing storage facilities (TSF) is needed. This study addressed the risks associated with metal mobilization from the waste, including those resulting from the potential for ARD generation from the mine waste (tailings) following the recovery of copper from sulphidic ores, the impacts on the aquatic ecosystem, the agro-ecosystem and potential ecological restoration using phytomining technologies. The study is focused on the Kafue River basin on the Zambian Copperbelt and seeks to identify impacts and potential benefits through studying a grouping of TSFs and their impact in a single geographical region, allowing attributes of the facilities to be contrasted. In this study, we have addressed the categorisation of ARD generation of Chibuluma TSF, TSF15A and TSF14 tailing samples and associated metal mobility using the standard static tests, UCT biokinetic test and column bioleach experiments. Owing to the potential for compromised water quality and exploratory studies alluding to this, an ecotoxicology study at the catchment scale was conducted seasonally for three years on water resources (Nselaki Stream, Fikondo Stream and Mululu Streams) and food crops in close proximity to the selected TSFs. The potential of phytomining technologies using native herbaceous plants to mitigate mobilization of metals from the copper mine wastelands was investigated. Characterisation of the risk of ARD using data generated from the standard static and biokinetic tests was compared across the three samples. The biokinetic test supported the standard static test classification of non-acid forming, providing preliminary kinetic data on ARD generation. The three tailings have high neutralisation potential and are not acid forming over an initial period. Column bioleach tests allowed for differentiation of metals according to their leaching potential under conditions ranging from neutral through varying levels of acidity conditions, providing support evidence for potential ecological burdens. The results showed that low pH promoted significant release of Fe, Cu and Mn while release of metals Co, Ni, Zn and Pb remained considerably low. Low mobilization of metal species was observed under high pH, however, over time the sustained low mobilization of metal species is likely to cause significant ecological risks. The results better inform the risk posed by copper wastelands, through the combined use of a suit of tests (static, biokinetic and column leach tests). Under high acidic conditions, Fe and Cu exhibited high ecological risk while the risk was moderate under non-acidic conditions. The ecological risk under acidic conditions for Ca, Al, Mg and Mn was observed to vary from low to moderate, while negligible ecological risk profiles were observed with elements of interest Pb, Co, Zn, and Ni. Our research further expanded the studies on monitoring abiotic and biotic ecosystem drivers in adjacent streams. Selected physiochemical indicators downstream were identified in relation to the influence of the mine wastelands. No significant difference in heavy metals was observed between the three streams at the significance level (P > 0.05), however, notable changes in chemical and physical signatures for selected elements was reported downstream of the selected TSFs. Multivariate analysis such as principal component analysis, indicated prevalent TSF interferences of Cu, Co, Mn, Zn, and Pb in water and sediment samples analysed. The use of macroinvertebrates provided a useful approach to monitor the variation in the degree of impacts and characterise the ecological integrity of the streams, as well as evaluate the links with selected physiochemical contaminants. The various physiochemical markers used were useful in observing persistent impacts on macroinvertebrate taxa, which can be linked to severe anthropogenic impacts as well as timely warning indicators. Particularly, macroinvertebrate taxa tolerant to water pollution such as Talitridae and Gnathobdellidae were observed to be dominant species. The biotic monitoring results supported the abiotic test classification with regards to stream contamination. The use of macroinvertebrate community structures proved more useful to characterize the integrity of the ecosystem of the streams and determine the links with possible contaminants. Similarly, results from food crops irrigated using the selected streams reported significant elevation of metals Cu, Co, Mn, and Pb in the edible parts. The contamination load index (CLI) showed that the pollution index of Pb, at ≈43.8 in the vegetable samples, exceeded that of the other metals; equally, metal contamination was also determined in the edible vegetables for Cu, Co and Mn, but not consistently for Zn. One-way ANOVA at p≤0.05 and boxplot analysis suggests that heavy metal concentration in soils and crops did not vary significantly among the sites downstream of the TSF. In contrast, the soils in the upstream control sites showed much reduced metal content. These observations suggest that the TSFs may be the primary source of metal contamination in the selected streams. The study presents phytomining as an improvement approach towards mitigating the impacts of metal mobilization and rehabilitation of wastelands. Further, it acknowledges the benefit of vegetation of TSFs. A rich diversity of indigenous herbaceous plant species was observed to thrive on the low-grade wastelands, with 622 indigenous herbaceous species from 21 families and 46 genera identified. Through analysis of the rhizosphere and above- and belowground biomass of these plant species, the following plants reporting copper accumulation above 1000 ppm, terming them hyper-accumulators: A. eucomus, B. alata, C. floribunda, C. ductylon, C. alternifolius, H. filipendula, E. scuber and V. glabra. However, hyper-accumulation of Co, Zn and Mn was not observed despite accumulation to levels of 300, 200 and 1000 ppm respectively. Further, a number of the hyper-accumulators showed wide-spread acclimatisation to TSFs through their importance value index (IVI). Our findings suggest that phytomining using indigenous herbaceous plant species in Zambia has potential as a viable technology. Overall, the approach of comparing catchments impacted by similar land use activities, was observed to be valuable and useful in current and future management of watersheds exposed to similar challenges. The study highlights useful monitoring methods, key risks requiring mitigation and highlights the need for interventions. The comparative catchment scale study is unique and rare which few studies have utilised to assess the likely impacts of mine wastelands, while also investigating potential remedial measures.
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    Open Access
    A Process Mineralogical Study on the effect of Alteration on the Flotation of Great Dyke Platinum Group Element (PGE) Ores
    (2018) Dzingai, Theophilus C; Becker, Megan; Tadie, Margreth; McFadzean, Belinda
    Ores from the same deposit may exhibit extensive variability in their mineralogy and texture. The ability to quantify this variability linked to metallurgical performance is one of the primary goals of process mineralogy and geometallurgy. Ultimately this information can be used to inform decisions around all core activities of mining and processing. This study focusses on identifying the key mineralogical differences between three Great Dyke platinum group element (PGE) ores in Zimbabwe. These ores are known to be characterized by extensive oxidation and alteration resulting in numerous metallurgical challenges in recovering the PGE. The behaviour of three different ores sampled along the strike of the Great Dyke is compared, focusing on mineralogical composition, rheological characteristics and batch flotation performance. The contribution of the differences in mineralogy (bulk mineralogy, base metal sulfide (BMS) liberation and association, and naturally floating gangue) to processing challenges and potential opportunities to manage these was considered. It was noted that slight differences in mineralogy, particularly BMS liberation and association, yielded notable differences in copper, nickel, platinum and palladium recoveries. The most oxidized ore was found to have lower recoveries due to the oxidation of the BMS, though a deeper understanding of the oxidation and flotation behaviour of PGEs (and platinum group minerals - PGMs) is still necessary. Through the mineralogical analysis of the batch flotation concentrates it was observed that more finely disseminated and yet locked (unliberated) talc resulted in higher amounts of naturally floating gangue (NFG). The effect of 3 polymeric carboxymethyl cellulose (CMC) depressants, differing in degree of substitution, was also evaluated in terms of their ability to depress the naturally floating gangue and mitigate any rheological complexities that may be associated with these ores, through the electrostatic repulsion of the negatively charged carboxylate groups. There was no significant advantage of one depressant over the others in the batch flotation tests or in the rheology tests. The more oxidized ore was found to contain relatively low amounts of phyllosilicate minerals and, therefore, no rheological problem was present that would have required a chemical solution. There were no noticeable differences in the rheology of the slurries of the 3 ores. This was likely to be due to the dampening or buffering effect of the high proportion of minerals that do not contribute to rheological complexity. Changing of depressant type also had no effect in this case possibly due to the same reasons. In addition to this, the region after which the rheological complexity of all 3 ore types begins to increase exponentially is from 30-35 vol.% solids concentration (60-65 wt.% for an ore with a specific gravity of 3.3). It is therefore advisable for Great Dyke operations not to exceed these solids concentrations as this would exacerbate the processing challenges associated with rheological complexity. The use of such solids concentrations during flotation is however unlikely though this may be the case in other parts of the processing circuit, e.g. comminution, and should thus be noted. The decoupling of the terms referring to alteration (that is oxidation and hydrolysis/hydration) is also presented in this study together with the effects of these different types of alteration on the processing of PGE ores. Oxidation affects the valuable minerals and thus flotation recoveries whilst hydrolysis/hydration acts on the gangue minerals and therefore mainly affects concentrate grade. The more oxidized ore sample in this study had undergone the oxidation type of alteration, rather than hydrolysis/hydration and the processing challenge associated with it lies not in the gangue but with the valuable minerals. Finally, it was shown that investigating an ore’s characteristics solely on mineralogy may not necessarily give a full prediction of the ore’s response but the linking of the mineralogical characterization with metallurgical test work gives a more holistic view.
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    A prospective comparative lifecycle assessment for green and grey hydrogen production and utilisation in the South African context
    (2022) Mbaba, Ongezwa; von Blottnitz, Harro; Fadiel, Ahjum
    Green hydrogen has gathered increasing interest as a medium in the transition to a carbonneutral economy, with several large, export-focused projects currently under development in Southern Africa. However, the environmental implications of hydrogen production and utilisation are not well understood. To address this challenge, a comprehensive literature review for hydrogen production and utilisation lifecycle assessment studies was conducted, and two prospective comparative lifecycle assessments are presented for green and grey hydrogen production and utilisation in the South African context. The first LCA aims to quantify the environmental impacts of producing green hydrogen, relative to grey hydrogen, and determine the production route with the least environmental impacts. The scenarios investigated for hydrogen production are water electrolysis powered by wind, solar PV or concentrated solar power, steam methane reforming, and water electrolysis powered by a 2040 grid electricity mix. Furthermore, the impacts of three available electrolysis technologies; viz. polymer electrolytic membrane (PEM), alkaline, and solid oxide electrolysis were compared. The second LCA aims to compare two systems of utilisation for the green hydrogen that would be produced in South Africa to determine the option where the highest level of decarbonisation could be achieved. The application considered for the assessment is the fuelling of heavy-duty truck transportation. The systems considered are local utilisation for fuelling heavy-duty trucks and hydrogen exportation to Germany also to fuel heavy-duty trucks. These two systems were expanded to include conventional fuel utilisation, making the functional units of the systems equal and thus the systems comparable. SimaPro was used to conduct the two LCAs, and the ReCiPe 2016 midpoint method was used for the lifecycle impact assessments. Grid-powered water electrolysis is found to have the highest potential impacts across most impact categories, even for the case of the significantly decarbonised 2040 grid mix, with SMR second. Solar PV-powered electrolysis leads to the highest potential human non-carcinogenic toxicity impact caused, by the supply chains of PV panels. Wind-powered water electrolysis is the least impactful option across most categories. However, it has the highest potential human carcinogenic toxicity impact among the renewable production options, though it is less than half compared to the value for non-renewable hydrogen production. This toxicity is caused by the supply chains of wind turbines. Considering optimal electrolyser utilisation, combined wind and solar PV-powered electrolysis is the best option. When comparing the water electrolysis technologies, PEM electrolysis leads to the highest environmental impacts. The energy input for production dominates all the impacts. In terms of utilisation, the environmental impact reductions achievable by the export case outweigh the environmental impact reductions achievable by using the green hydrogen locally, across all impact categories. The highest level of decarbonisation is achieved by replacing the most environmentally harmful fuel; South African coal-based diesel used to fuel heavy-duty trucks. The results of the first LCA confirm that green hydrogen is indeed significantly less environmentally impactful compared to grey hydrogen, but with one hotspot for each of the PV and wind-powered electrolysis, which require attention by project developers. The environmental impacts of all the production scenarios are dominated by the energy required for the production processes. The main finding for the second LCA is that local hydrogen utilisation for heavy-duty truck transportation leads to a larger environmental benefit compared to hydrogen exportation in the case of usage for heavy-duty truck transportation in another country. The highest level of decarbonisation is achieved by displacing South African coal-based diesel first.
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    A stepwise Study on the characterisation and processing of South African Platinum Group Tailings
    (2023) Manenzhe, Re?oket?we; Corin, Kirsten
    The endurance of the mining industry has led to the near-depletion of some of the most processible ore types. This has resulted in a unique challenge that necessitates unique innovation for the industry. Firstly, existing technologies are increasingly geared towards improved efficiency in processing lower grade ores. Secondly, ores that were previously processed with older, and in some cases, inefficient technologies, have emerged as potentially viable solutions that would help maintain concentrator capacities. On the other hand, as the industry has endured, so has its waste accumulation. Tailings dumps continue to grow, and continue to pose various environmental issues. But although they are a waste product, tailings also have several merits to them. They are already mined and readily available, and reprocessing them immediately addresses two conundrums i.e. how can the industry source alternative ores, and how can it deal with the steady accumulation of waste? It is expected that as a result of their initial processing from so-called fresh ores, and their stay in their respective dumps, the tailings will be altered or tarnished. The surface properties of their compositional minerals will be oxidised and layered with other compounds that might hinder their interaction with flotation reagents such as collectors and hence, hinder their flotation response, presenting a challenge for their proposed reprocessing. More obviously, as they are a waste product, their grades will be far lower than the fresh ores that produced them. Numerous studies aim to elucidate the viability of the metallurgical reprocessing of tailings. However, this flurry of innovation thus far extends to the vast stretches of the Witwatersrand dumps, and thus, to gold. The issue with processing other tailings types then becomes threefold. Is there enough value in the dumps to justify tailings beneficiation? What beneficiation method would be suitable? Would the method yield economically viable results? This study acquired three bulks of PGM tailings to investigate these questions. The first bulk was Merensky, the second was UG2, and the third was a deslimed version of UG2 in which a portion of denser minerals were separated out. For the sake of convenience, the bulks are referred to by their shorthands throughout the thesis. Merensky became MER, the normal or raw UG2 became UG2R, and the deslimed UG2 became UG2D. The study conducted the investigations along two lines: first, by characterising the tailings, and secondly, by floating them. The first characterisation step was a full elemental and mineralogical analysis that quantified the amount of valuables as well as gangue minerals; this was done via XRD and QEMSCAN. The second step was to determine the degree of oxidation by using qualitative, in-situ experimental methods; these are EDTA extraction and oxygen reactivity. It was hypothesised that EDTA extraction would measure the concentration of secondary and oxidised materials on the mineral surfaces; and oxygen reactivity would measure the demand of oxygen in each tailings, and therefore the tailings' capacity to react with species in the pulp phase. The qualitative methods have only ever been used for fresh ores, and have shown to be reliable in predicting and/or explaining the flotation behaviour of those ores. They have never been used to predict an/or explain the flotation behaviour of an ore material that has already been processed, and is therefore very low grade, and oxidised. If they are as viable for tailings as they are for fresh ores, they would determine different EDTA extraction indices and oxygen demand constants. QEMSCAN and XRD provided the different concentrations of the different minerals across the tailings, and showed that some minerals were present in one tailings but not the other. For instance, MER contained the highest fraction of chalcopyrite, as well as the highest fraction of sulphides. UG2R and UG2D each had more than ten times the fraction of chromite seen in MER. MER, on the other hand, had more pyroxenes, plagioclase and amphibole. It was expected that these differences in composition would be the cause of the different extraction indices and oxygen demand constants. The robustness of both methods thus had to be tested. This was done by altering each of the tailings and testing whether the extraction indices and oxygen demand constants would change. The surface and composition alteration was induced with ultrasonication and desliming. The study thus answered the question: can the EDTA extraction and oxygen reactivity methods detect when a change has occurred on the mineral surfaces of the same tailings? The tailings were then floated, and it was here that the question of economic viability was assessed. For the concentrators from which MER, UG2R and UG2D were collected, a reprocessing venture would be deemed economically viable if 30% of the copper present in the tailings was recovered. The flotation performance was thus analysed with copper recovery as the primary positive indicator. Nickel behaviour was also tracked in case of any supporting elucidations, and also because pentlandite is the primary PGE-carrier for Merensky and UG2. The tailings were floated with DOW200 as the frother, SIBX as the collector, and CMC as the depressant. The results showed that the presence of the depressant resulted in very low solid quantities being recovered to the concentrate. In fact, less than 2% of each tailings was recovered, and less than 10% of the present copper (and therefore chalcopyrite) was extracted. When the depressant was removed from the reagent scheme, recovery of the solids improved to 10%, but the copper yield was still below 30%. So, the collector dosage was increased under the fundamental assumption that the hydrophobicity of the valuables would be improved, and in this way, the efficiency of separating the hydrophilic gangue from the valuables would also improve. The plan worked, and UG2R finally achieved the industrial objective of recovering 30% of the present copper. While MER and UG2D failed to do the same, their performance was also at its best under these conditions, with each tailings yielding roughly 23% copper recovery. In an effort to improve floatation, the tailings were cleaned via ultrasonication and desliming. These cleaning methods both had a detrimental effect on copper recovery. However, nickel (and therefore pentlandite) behaviour improved, showing that while the methods were disadvantageous to one mineral, they were favourable to another, and they might be useful for a study that uses a different metal as its positive performance indicator. The study also showed that MER, UG2R and UG2D have different copper EDTA extraction indices. UG2D had the highest index, followed by UG2R, and then MER. The copper minerals associated with UG2D can therefore be concluded to be more oxidised than those associated with UG2R and MER. Moreover, UG2D was the least reactive to oxygen, having an oxygen consumption rate constant of 0.113 min-1 when compared to 0.198 and 0.152 min-1 for UG2R and MER, respectively. Ultrasonication and desliming decreased each of these constants, indicating that when cleaned with the chosen methods, the mineral surfaces became less reactive to oxygen. And so, it was concluded that of the investigated tailings, UG2D was more oxidised than the other two, reacted the least with oxygen, and yielded the lowest copper recoveries. When UG2R achieved the highest reactivity with oxygen, it also yielded the lowest copper extraction index and the highest copper recovery. Overall, nickel behaved contrary to copper.
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    A study of alternative techniques to mercury amalgamation for gold extraction in artisanal and small-scale gold mining
    (2022) Manzila, Archippe Ngwey; Petersen, Jochen; Moyo, Thandazile
    Artisanal and small-scale gold mining (ASGM) has many definitions depending on the context. However, the common theme that characterises gold mining operations that fall within this category is that they make use of rudimentary methods to mine and process gold. The ASGM sector is a source of livelihood for millions of people worldwide and continues to grow due to the ever-rising demand for gold, and high unemployment rates which have been exacerbated by the Covid-19 pandemic, particularly in developing countries. Mercury amalgamation is the method of choice to recover gold in ASGM. This method consists of contacting the gold found within an ore with mercury to form an alloy i.e., the mercury-gold amalgam and subsequently burning off mercury to recover the gold in a form known as sponge gold. The popularity of this method has to do with its simplicity of application, low cost, and quick returns. However, mercury is a highly toxic substance; therefore, its use presents serious health risks for artisanal miners and their communities, and environmental risks for the ecosystems surrounding their operations. These risks arise primarily from the amalgam burning stage, whereby mercury is vapourised, and the dumping of mercury-rich tailings into local rivers. This mercury release affects human health by causing serious diseases that may lead to death. From an environmental perspective, mercury has been reported to severely pollute river ecosystems, inevitably finding its way to food chains. Due to these issues, alternative technologies such as borax smelting, the Gemini table, thiosulphate, cyanide, chlorine, and urea leaching, to name a few, have been developed or adapted over the years to substitute mercury. However, most of these technologies have not been successfully implemented in artisanal mining operations. This lack of success is primarily due to their complexity and high cost, making them unattractive to artisanal miners. This study investigates the application of cyanide and thiosulphate leaching as alternatives to mercury amalgamation for the recovery of gold in ASGM operations. Although cyanidation is practiced in ASGM, in some regions, it is only employed to treat tailings from the mercury amalgamation process. This is undesirable due to the fact that exposing mercury to cyanide results in the mobilisation of elemental mercury found in the tailings as mercury cyanide. This project investigates gold extractions that can be achieved with cyanide and alkaline thiosulphate systems and compares the results to those of mercury amalgamation. This investigation was undertaken by conducting leach experiments using cyanide at 1 g/L, 3 g/L and 5 g/L, and ammonium thiosulphate at 0.1 M and 0.5 M, on 3 ore samples originating from artisanal mining locations. The experiments were conducted using batch stirred tanks reactors and the operating conditions (T= 26°C, solids loading: 30%, particle size: --300 +150 µm) were selected to mimic as closely as possible the conditions of artisanal mining processes. The findings of the study revealed that cyanide leaching was the better performing technology compared to thiosulphate leaching as it achieved gold extractions of 71.6%, 69.7% and 67.8% for the 3 ores samples (Sample 1, Sample 2, and Sample 3, respectively) while thiosulphate leaching achieved gold extractions of 54.1%, 35.6% and 38.0% for the 3 ores, respectively. Studying the minerology of the ores, using XRF, XRD, QEMSCAN, SEM-EDS and diagnostic leach, revealed the presence of sulphide minerals hosting refractory gold which contributed to the low gold extractions observed. Cyanide leaching proved to be a system that is easier to control compared to thiosulphate leaching, making it much more attractive to artisanal miners. It is recognised that cyanide is a toxic chemical, however, the method is already practiced in ASGM and cannot be simply wished away. Instead, steps must be taken for its safe and responsible use. Hence, this research makes recommendations on avenues that can be explored to reduce the risks associated with cyanide use. It was also found that cyanide leaching outperformed mercury amalgamation which typically achieves gold recoveries of 30-50%. Thiosulphate leaching may be capable of achieving better gold recoveries than mercury amalgamation as well, as one of the ore samples achieved a gold extraction of 54.1%. However, this would depend on the ore type and reagent conditions as it was found that the 3 ore samples responded differently to leaching.
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    A study of configurational alternatives of a gas-to-liquids process based on Fischer-Tropsch technology
    (2018) Khazali, Ashcaan Tendo; Moller, Klaus; van Steen, Eric
    Environmental concerns, associated legislation, limited oil reserves and fluctuating crude oil prices are some of the factors that highlight the need for alternative and environmentally friendly routes to fuels. One alternative is to use Fischer-Tropsch Synthesis (FTS) as the major technology in conversion of carbon containing feedstock to transportation fuels. The FTS product, called syncrude, can be refined to high quality transportation fuels in Coal, Gas or Biomass to liquid plants (denoted as CTL, GTL, and BTL, respectively, and collectively referred to as XTL). The economic viability of XTL processes is generally subject to the present price of crude oil and past studies show that traditional refining is generally more economically viable. However, XTL processes have been shown to be more economical and in some cases more environmentally friendly than conventional options when legislative measures aiming to curb traditional fossil fuel usage are considered. This study explores XTL process configurations that can improve plant carbon efficiency to diesel and liquids. The configuration encompasses technologies used, operating conditions, and layout of unit operations. A basic GTL process configuration consists of an Air Separation Unit (ASU), Auto-Thermal Reforming (ATR), syngas cleaning, full conversion Low Temperature Fischer-Tropsch (LTFT) and wax hydrocracking (WHC). These operations are modeled individually and combined to produce a plant model for study with the aim of determining the effects of configurational alternatives on the process efficiency to liquids and diesel. Furthermore, given that the ASU is a major contributor to costs the effect of using oxygen-enriched or pure air is investigated. Since production of heavy wax is prioritized, FTS represents the use of cobalt catalyst in LTFT operation. Where air is used, FTS is run to high conversion in once through mode to avoid the unfavorable economics of recycling nitrogen. After separation of the syncrude, the light fraction is reformed back to syngas in order to maximize carbon efficiency. The heavy wax is hydrocracked to maximize distillate range material. The light products from the WHC are combined with the lights from FTS and the heavy wax is recycled. Carbon efficiency, liquid selectivity and diesel yield are the means of assessing performance. The Scilab programming language is used along with physical properties estimated using the COCO/ChemSep pure component database as a starting point. Estimation of properties for alkanes and olefins of carbon chain length up to C200 has been carried out. The presence of 25% nitrogen in the ATR was found to beneficial to the H2 : CO ratio in the resulting syngas. Furthermore, in FTS the presence of 10-20% nitrogen produced the lowest reduction in carbon monoxide conversion and _FTS. In general, the introduction of nitrogen resulted in decreased conversion of methane in the ATR and both decreased _FTS and conversion in FTS. WHC performance was found to benefit from alpha being as high as possible; however, when the heavy wax recycle was inactive the optimal value was 0.92. The OOT80 configuration was found to have the highest liquid selectivity, while the efficiency to diesel was maximized for the OIRC40 configuration.
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    A Study Of Cyanide-Glycine Synergistic Lixiviant And The Igoli Process As Suitable Replacements For Mercury Amalgamation In Artisanal And Small-Scale Gold Mining
    (2023) Masuku, Wilson; Moyo, Thandazile; Petersen Joachim
    Artisanal and Small-scale Gold Mining (ASGM) operations are characterized by the use of rudimentary tools and technologies owing to limited access to capital. ASGM is predominantly a poverty-driven exercise practiced as a source of livelihood, typically in rural communities where people lack other employable skills. Globally, ASGM accounts for 20-25% of gold production, while at local scales, this number varies and can be as high as 65% in countries such as Ghana and Zimbabwe. Mercury is used in ASGM to capture gold from free-milling ores, in a process called mercury amalgamation. This is the go-to technology in most ASGM operations owing to its availability and ease of operation. However, mercury amalgamation has low recoveries in the range of 30-33% of gold from the otherwise rich gold ores typically mined in ASGM. In the amalgamation process, about 70% of the mercury used is lost to the environment with the amalgamation tailings and during the roasting process. Mercury is a toxic heavy metal, and mercury poisoning can lead to neurological and behavioural disorders and has been a major concern globally, leading to the signing of the Minamata Convention treaty. Mercury-free gold concentration and extraction methods such as shaking tables and roasting with borax have been put forward over the years, but their uptake has been very limited. The reasons for this poor uptake have never been systematically studied but it is thought that, among other reasons, it has to do with that some technologies are too complex for the ASGM context. Beyond the mercury-free technologies proposed for the ASGM sector, gold extraction and recovery in the large-scale mining sector has attracted researchers' attention for years, with a plethora of technologies having been proposed and tested. Little effort has been made to establish if any of these technologies could be a good fit for ASGM. In this study, two mercury-free technologies (cyanide-glycine lixiviant and the iGoli process) were tested to establish their effectiveness in the leaching of gold from ores sourced from two ASGM sites. The ores were characterized using QEMSCAN, XRD and XRF to identify mineral phases, and quartz was found to be the most dominant mineral. Sulphide minerals in both ores host the largest percentage of gold. The cyanideglycine lixiviant uses a combination of cyanide and glycine to improve gold extraction. The results from this showed that dissolution rate increases with an increase in glycine concentration in non-agitated systems at 3g/l NaCN while the reverse was true in agitated systems at the same cyanide concentration and when it was varied. The percentage of gold extracted in the non-agitated system after 72 hours was 36% at 5 g/l glycine, 21% at 2 g/l glycine and 19% when no glycine was added. In agitated systems at 5g/l cyanide, the highest extraction after 24 hours was 81% at 2 g/l glycine. Increasing glycine concentration led to lower gold extractions with 5 g/l and 10 g/l glycine extracting 74% and 68% respectively. This trend of decreasing extraction with an increase in glycine concentration was observed at different fixed cyanide concentrations i.e., at 1 g/l, 3 g/l and 5 g/l. The iGoli process uses hydrochloric acid and sodium hypochlorite to leach gold. The extractions were very low and reported below the detection limit of the analytical instrument, and thus they cannot be reported with confidence. However, iron was analyzed and showed a 55% extraction of the total iron in the ore. Results from these two technologies were compared to those of mercury amalgamation and benchmarked against the conventional cyanide process. Beyond the purely technical, a case study of two ASGM sites was done with the objective to observe and understand the day-to-day operations in a typical ASGM site and identify limitations and opportunities for mercury-free technology adoption. Based on insights drawn from the case studies, it was concluded that the cyanide-glycine lixiviant is relatively easy to implement given the current process operation in the ASGM sector which makes use of vat tanks that do not agitate the slurry (lixiviant + ore). However, the observed poor recoveries associated with the technology in non-agitated systems would be a limitation. When more profits are realized, the ASGM practitioners can upgrade to agitated systems and add hydrogen peroxide as an oxidizing agent to improve extraction.
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    A study of impact breakage of single rock specimen using discrete element method
    (2020) Oladele, Temitope Philip; Bbosa, Lawrence; Weatherley, Dion
    Comminution is a critical stage of mineral processing which aims to reduce the size of ore particles through breakage, consequently increasing the likelihood of the liberation of valuable minerals. However, comminution is highly energy-intensive and an understanding of the key breakage mechanisms has been identified as an important factor in improving the efficiency of the process. Several factors, such as pre-existing cracks, mineralogical composition, ore shape and size are known to affect ore breakage behaviour during breakage. To investigate breakage mechanisms, it is important to be able to determine how individual factor influences the breakage behaviour of rock specimens. However, isolating and investigating individual factors under experimental conditions is challenging and typically impractical. Numerical techniques such as the Bonded Particle Model-Discrete Element Method (BPMDEM) have been developed as a means of investigating in isolation, the effects of different factors on ore breakage behaviour under closely controlled breakage conditions using synthetic rock specimens. This study investigates how individual factors influence rock specimen breakage using BPM-DEM numerical methods. Numerical simulations were conducted using ESyS-particle 2.3.5, an open-source discrete element method (DEM) software package which uses Python-based libraries to generate geometries and simulations and a C++ engine for mathematical computations. Empirical calibration relationships were developed to relate microstructural model parameters to the macroscopic mechanical properties that are typically obtained from standard geotechnical breakage experiments. The robustness of the model was evaluated by considering the sensitivity of fracture measures to the variation of model resolution, size-dependency and macroscopic mechanical properties (Young's modulus and uniaxial compressive strength) of the numerical specimens. A comparative study of single rock specimen breakage using the current BPM-DEM and laboratory SILC experiments carried out by Barbosa et al. (2019) was conducted. The measured fracture force and fracture patterns at different sizes for both cylindrical and spherical synthetic rock specimens were examined. Furthermore, the model was used to study, in isolation, the influence of pre-existing cracks in rock specimens and differing mineralogical compositions upon measurable breakage properties. Numerical rock specimens with pre-existing cracks were constructed using a microcrack approach, while a unique approach with the insertion of "seed points" was developed and demonstrated to construct numerical rock specimens with varying mineralogical compositions. Results from the numerical simulations showed that a high model resolution with a sufficiently large number of DEM-spheres exhibited results with the least deviation and error with respect to fracture measures, and, was therefore considered numerically stable. The dependency of fracture measurements on specimen size showed an expected increase in the measured fracture force as the specimen size increases. The variation of the macroscopic Young's modulus and uniaxial compressive strength against the fracture measures emphasised that the locus of these mechanical properties against the fracture measure can be used to specify a calibration relationship. Results of the comparative study showed that for both cylindrical and spherical rock specimens, the DEM consistently predicted the fragment patterns as well as the increase in the measured fracture force as the specimen size increased. The investigation on the effect of pre-existing cracks revealed that an increasing number of pre-existing cracks in rock specimens necessitated lower fracture force and consequently produced a low amount of new fracture surface area. For the binary phase mineralogical composition in the study, it was found that the fracture force decreased with an increase in the concentration of the softer component due to the increased percentage of weakness in the specimen. It was concluded that, with an appropriate calibration exercise and a realistic specification of material properties from the evaluation study, the DEM as a tool was sufficient to act as a "virtual laboratory" to isolate and study the individual effects of factors that influence ore breakage. The understanding of these results highlighted two important points. Firstly, this study was able to unravel some of the possible causes of the inefficiency in comminution practices, whereby significant amounts of energy can be expended to achieve minimal gains in respect of enhancing liberation due to pre-weakening and mineralogical composition. Secondly, it emphasised some of the causes of the variation observed during ore characterisation on a laboratory breakage device, attributable to pre-weakening and mineralogical composition.
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    A study of the charge structure and energy utilisation in a Stirred Media Detritor using DEM-SPH
    (2022) Ndimande, Conrad B; Mainza, Aubrey
    The Stirred Media Detritor, (SMD), is a grinding device used for fine and ultra-fine grinding applications in mineral processing. The SMD has a vertically orientated shell that supports a shaft, with protruding impeller arms for agitating the charge. There is currently limited understanding of charge structure and motion in the SMD, particularly the interaction of the media and the slurry. Additionally, the number of arms and their arrangement on the shaft, are important aspects of the impeller that determine flow, energy consumption and grinding efficiency. Impeller geometry choices affect these characteristics of the process. This work focuses on studying the flow of grinding media and slurry for the industrial scale SMD 1100- E. This information is used to explore charge dynamics and energy utilisation in the SMD. To investigate the effect of impeller arm configuration on the operational behaviour of the SMD, the commercially available impeller configuration of the industrial scale SMD 335-E is used as the base case. Mill charge dynamics, transport and mixing, patterns of energy absorption on the mill surfaces are examined for the base case and compared to three different impeller arm arrangements. A two-way transient coupled Discrete Element Method (DEM) and Smoothed Particle Hydrodynamics model is used to achieve this. The ceramic grinding media is represented by the DEM component of the model, which is fully resolved, while the slurry (water and fine particles) is represented by the Smoothed Particle Hydrodynamics (SPH) model. The focus is on steady state operation therefore discharge from and feed into the mill are omitted. A nominal media size of 8 mm is used. The rotational action of the impeller forces the charge to the mill wall creating vortex centred on the mill shaft. The vortex is conical with a large diameter at the bottom, which decrease towards the bottom of the mill. Abrasion is found to be the dominant breakage mechanism in the SMD. Mixing behaviour is complex with media transfer past layers of impeller arms being influenced by the fall distance of media between impeller arm encounters.
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    A systemic study of mining accident causality: an analysis of 100 accidents from a copper mining company in Zambia
    (2021) Mabeti, Daniel; Isafiade, Adeniyi
    The mining industry has remained Zambia's dominant industry for almost a century. According to the report by International Council for Mines and Minerals (ICMM) for 2013, Zambia is highly dependent on copper mining as the core productive industry. Mining contributes to direct employment (approximately at 1.7%), foreign direct investment (approximately at 86%), gross domestic product (more than 12%) and government revenue (more than 25%). Regardless of these economical enactments, the accident frequency across the mines is very significant. In general, the mining industry is perceived to be a high-risk industry. The increase in the number of mining accidents is extremely costly, whether measured in terms of medical expenses and disability compensation, loss of production and wages or damage to plant and equipment. The human cost, in terms of death and suffering, is beyond calculation. In recent years, there has been some innovations in terms of technology regarding mining methods, and this has resulted in decreased accident occurrence in the mines. The human factors involved in the mine accidents need to be addressed further to reduce these rates. Therefore, the best approach is first to understand mine accident causality, and then this will be a foremost step in a pursuit to diminish the high rate of accidents. Effective remedies and measures can be designed if only accident process is properly understood. The understanding and interpretation of causes of accidents at workplaces can only be achieved by accident modelling techniques. The effective way of analysing industrial accidents has been proven by the Swiss Cheese Model, which is also applicable to this study. The Swiss Cheese Model describes an accident as an event which happen within organization due to the combination of different unsafe acts which may include latent conditions and front-line operators. The purpose of this study was to determine how systemic factors contribute to accidents at a copper mining company in Zambia. The analysed results were compared with those of other local mines as well as mines from developed and developing countries. The approach in this study involves using the existing framework developed by Bonsu (2013). The framework had used the concepts from the Mark III of the Swiss Cheese Model, Incident Cause Analysis, safety management principles and the Nertney Wheel. The sections involved in the existing framework of Bonsu (2013) are metadata, accident barrier analysis and causal analysis. The accident causality section is designed and described in the same way as the Mark III version of the SCM. This section is used for analysis of accident causality and is categorized into proximal, work place and systemic factors. The metadata section offers explanations on different factors that influence the happening of accidents at this copper mining company in Zambia. Metadata section captures the information on accidents analysed under the barriers and causing agency section of the framework. The variables under the metadata are time and date of accident, place of the accident, accident type, activity involved which resulted in the accidents, task schedule of the accidents, age of the victim, experience of the victim, job status, etc. The last section of the existing framework is the agency and barrier analysis and was designed by Bonsu (2013) to capture data on the safety barriers which were breached and accident causing agents in the accident report. The accident reports collected from the copper mining company in Zambia were used in the existing framework and the analysed results were presented as unsafe acts, workplace and systemic factors with linkages to each other. The most prominent type of unsafe acts recognized were routine violation (recognized in 38% of all the accident analysed), closely followed by slips and lapses (identified in 30%) and then mistakes (21%). Exceptional violation and non-human cause were the lowest at 9% and 2% respectively. Systemic and workplace factors were involved in 78.2% of the accident reports that were analysed. The most prominent workplace factor recognized was behavioural environment (25.8% of all cases analysed), closely followed by physical environment (23.4% of all cases analysed), then unsafe work practices (18.8% of the accidents analysed), then fit-for purpose equipment (16.4% of the accidents analysed) and finally competent people (15.6% of the accidents analysed). In general, under the category of accident analysis on workplace factors, all the five factors were significantly contributing to the causes of accidents at the mine site that was investigated as demonstrated by the closeness in percentages. In the case of systemic factors, inadequate supervision or leadership was the most prominent factor identified (22.6% in all accidents analysed). It was also found that physical environment (23.4% of all cases considered) was the second most dominant workplace factor recognized. The results obtained also revealed that some systemic factors were associated with specific workplace factors more than others. For instance, the result of behavioural environment (workplace factor) was usually due to poor leadership problem (systemic factor), problems seen in housekeeping (systemic factor), hazard identification (systemic factor), risk management (systemic factor), and designs (systemic factor), these were also the causes of poor physical environment. In the unsafe work practices (workplace factor), hazard identification was the most common systemic factor that was recognized whereas in fit for purpose equipment (workplace factor) the most common associated systemic factors were risk management, leadership, hazard identification and design. The results obtained in this study were compared to those obtained in the study of Mwansa (2021), which also applied the framework used in this study to the analysis of accident reports from another mine site of the same mining company in Zambia as used in this study. Similarities and differences were obtained under the accident characterization and causation sections. The operations in both studies are different in terms of mining methods and metallurgical processing plants. This may be responsible for some of the differences in the results obtained in both studies. For instance, in Mwansa's (2021) study, the most dominant unsafe act recognized was also routine violation (36% of all cases considered) whereas the most prominent workplace factors recognized were physical environment (36% of all cases considered) and unsafe work practices (27% of all cases considered). In Mwansa's (2021) study, the most prominent systemic factors recognized as contributing to physical environment were hazard identification, work schedule, risk management, maintenance management, leadership, housekeeping, and contractor management. The results obtained in this study were also compared with previous studies from different commodities across the globe. This was done to have broader picture when dealing with mine accidents. The causes of accidents identified in this study are of significance to the safety of the industry. Overall, based on the analysis carried out in this study for the copper mining site investigated, it can be concluded that systemic factors are the main causes of accidents rather than human error and violations.
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    The accuracy of linear flux models in predicting reaction rate profiles in a model biochemical reaction system
    (2014) Hughes, Alistair Paul; Möller, Klaus; Harrison, STL
    Metabolic flux analysis is commonly used in the modelling of biochemical reactions. The use of MFA models has gained large amounts of interest due to the simplicity of the computational procedures required for the model, and the exclusion of difficult to measure intracellular reaction data. There are many examples of the use of MFA models in literature studies in a number of applications, ranging from the medical industry through to the development of novel biochemical processes. Little to no mention is provided in literature studies regarding the applicability of the MFA model to a specified set of reaction data. Furthermore, the techniques and routines used to compute the flux models are not well described in these studies. The objectives of this research were to determine the sensitivity of the MFA models to various operating and kinetic parameters and to highlight the considerations required when setting up the computational routine used to solve the flux balances. The study was conducted using a model pathway populated with a set of hypothetical elemental reactions and branch points. The model pathway was used in this study to negate the affects of complex regulatory biochemical architectures which are not well described in literature. The use of the model pathway ensured that the reaction system was thermodynamically feasible and there was consistency in the mass balances. The exclusion of the complex regulatory reactions did not affect the accuracy of the results generated in this study. A set of reaction mechanisms were used to describe each reaction step and were populated with parameters reference from literature. The cellular and reactor mass balances were generated using correlations presented in literature.
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    Acid catalysed alkylation of diphenyl ether with methanol over shape-selective zeolites
    (2005) Ntshabele, John Phenyo Pheko; Fletcher, Jack; Böhringer, Walter
    Selectivity in methylation of phenol over acidic catalysts depends primarily on temperature and the acidity of the catalyst. It is reported that direct methylation of phenol with methanol over weakly acidic catalysts yields predominantly o-cresol, whilst catalysts with medium acidity yield primarily o- and p-cresol, with significant anisole formation at lower temperatures (≈ 200°C). m-Cresol is, however, formed with significant selectivity over strongly acidic catalysts or at higher temperatures (typically 300°C and more). To date, only one industrial process is known which is selective to p-cresol but has the disadvantage of co-formaiton of inorganic salt, which is costly to dispose of.
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    Activated sludge process : effects of feed concentration on effluent COD
    (1977) Baskir, Cyril Isadore; Hansford, Geoffrey Spearing
    The concentration of substrates in the feed to an activated sludge process was found to exert a significant effect upon its effluent COD. A mathematical model was proposed to explain this effect and was successful in correlating the data of this study. The model was based on the hypothesis that COD measures both substrate and product concentrations. It was found that an optimum sludge age exists for achieving minimum effluent COD. At sludge ages longer than the optimum, effluent COD increased due to product formation; at shorter sludge ages the effluent COD increased due to an increased concentration of degradable substrate.
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    Activity and selectivity of transition metal (Fe, Mo and W) carbides in the Fischer-Tropsch synthesis
    (2007) Patterson, Veronica A; Van Steen, Eric
    This study focused on the Fischer-Tropsch activity and selectivity of transition metal (iron, molybdenum and tungsten) carbides. The carbide catalysts were prepared by a temperature programmed method. The properties of the materials were characterised by X-ray diffraction (XRD), transmission electron microscopy (TEM), BET measurements and temperature programmed desorption of carbon monoxide (CO-TPD). The performance of the materials was tested in a Berty reactor. A reduced, precipitated iron oxide catalyst was used as a reference catalyst.
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    The addition of HZSM-5 to the Fischer-Tropsch process for improved gasoline production
    (2002) Botes, Frederick Gideon; O'Connor, Cyril; Böhringer, Walter
    The Fischer-Tropsch process has two important disadvantages with regard to the production of gasoline. Firstly, the carbon number distribution of the product spectrum follows the statistical Schulz-Flory function. This means that principally hydrocarbons of all lengths are produced and that the gasoline selectivity is limited to a theoretical maximum value of about 48%.