Browsing by Subject "Chemical Engineering"
Now showing 1 - 20 of 465
Results Per Page
Sort Options
- ItemOpen AccessA kinetic study of the oligomerization of propene, butene and various hexenes over solid phosphoric acid(1987) McClean, Deaghlan Martin; O'Connor, Cyril T; Kojima, Masami
- ItemOpen AccessA 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, StephenMetal 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.
- ItemOpen AccessA 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, BelindaOres 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.
- ItemOpen AccessA stepwise Study on the characterisation and processing of South African Platinum Group Tailings(2023) Manenzhe, Re?oket?we; Corin, KirstenThe 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.
- ItemOpen AccessA study of alternative techniques to mercury amalgamation for gold extraction in artisanal and small-scale gold mining(2022) Manzila, Archippe Ngwey; Petersen, Jochen; Moyo, ThandazileArtisanal 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.
- ItemOpen AccessA study of configurational alternatives of a gas-to-liquids process based on Fischer-Tropsch technology(2018) Khazali, Ashcaan Tendo; Moller, Klaus; van Steen, EricEnvironmental 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.
- ItemOpen AccessA study of the charge structure and energy utilisation in a Stirred Media Detritor using DEM-SPH(2022) Ndimande, Conrad B; Mainza, AubreyThe 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.
- ItemOpen AccessAcid catalysed alkylation of diphenyl ether with methanol over shape-selective zeolites(2005) Ntshabele, John Phenyo Pheko; Fletcher, Jack; Böhringer, WalterSelectivity 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.
- ItemOpen AccessActivated sludge process : effects of feed concentration on effluent COD(1977) Baskir, Cyril Isadore; Hansford, Geoffrey SpearingThe 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.
- ItemOpen AccessAdding ammonia during Fischer-Tropsch Synthesis: Pathways to the formation of N-containing compounds(2017) De Vries, Christian; Claeys, Michael; Petersen, MelissaThe Fischer-Tropsch synthesis (FTS) process, better known for its ability to produce synthetic fuel via the hydrogenation of CO, has shown potential to produce valuable chemicals when ammonia is added to the feed. In this work certain aspects of the pathway to the formation of N-containing compounds that form when NH₃ is added during FTS, using mostly iron based catalysts is investigated. In addition, the effect this has on the FTS reaction itself is evaluated. To achieve this goal, both theoretical and experimental techniques are used in this study. The CO adsorption and dissociation reactions are assumed to be important elementary reactions for many proposed FTS pathways. In the theoretical part of this thesis, spin-polarized periodic density functional theory (DFT) calculations are employed to study aspects of the initial stage of the pathway on a model Fe(100) surface. Considering the formation of N-containing hydro- carbons, one would assume that NH₃ initially adsorbs and dissociates on the catalyst surface, which could take place in the presence of CO. The surface chemistry of these adsorbates is well studied both experimentally and theoretically, but their co-existence has not yet been evaluated on model Fe surfaces. Initially a platform is generated by calculating the individual potential energy surfaces (PES) for the decomposition of CO and NH₃ on Fe(100) at a coverage of ϴ = 0.25 ML. These calculations provided the basis for comparing the adsorption and dissoci- ation profiles of CO and NH₃ on the Fe(100) surface via the use of the same computational methodology, and importantly making use of the same exchange correlation functional (RPBE) for both adsorbates. Furthermore, it was desired to evaluate the kinetics and thermodynamics of the NH₃ decomposition on the Fe(100) surface at relevant temperatures and pressures (by combining the DFT results with statistical thermodynamics) to better understand the role of NHₓ surface species involved in the pathway to the formation of the N-containing compounds on a model catalyst surface. The DFT results that are reported for the individual decomposi- tion PES for CO and NH₃ were generally found to be in close agreement with what has been reported in previous DFT studies and deduced experimentally for the relevant adsorption and decomposition pathways. The resulting Gibbs free energies for the PES suggests that NH₂ may be kinetically trapped on the Fe(100) surface at a coverage of ϴ = 0.25 ML and the reaction conditions (T = 523 K and p*NH₃ = 0.2 bar) where NH₃ is co-fed with synthesis gas during FTS. The individual adsorptions of CO and NHₓ (with x = 3, 2, 1, 0) were compared to their coadsorbed states, by calculating the heat of mixing (ΔEmix) and the activation barriers (Eₐ) for CO dissociation in the presence and absence of the NHₓ surface species on the Fe(100) sur- face. Similar to the individual adsorption of NH₃, the 0 K regime inherent to DFT calculations is bridged by calculating the Gibbs free energy of mixing for CO + NH₃ on Fe(100) at higher temperatures. Both repulsive and attractive interaction energies were calculated for the various coadsorbed states (CO + NHₓ on Fe(100)) and similarly some configurations resulted in an energetically favored or unfavored heat of mixing. The activation barrier for CO dissociation was lowered when coadsorbed with NH₃ and NH₂, and raised when coadsorbed with NH and N. With all the coadsorbed structures the CO dissociation reaction became more endothermic. Previous experimental studies have shown a concomitant reduction in oxygenate selectivity with an increase in the selectivity for N-containing compounds, when NH₃ is added during FTS. It is well-known that oxygenates undergo secondary reactions when using iron-based catalysts in FTS. In addition, the catalyst used in aforementioned studies (precipitated Fe/K) are active for the amination reactions of oxygenates. It is therefore hypothesized that some oxygenates pro- duced via the primary FTS pathway are converted to N-containing compounds via a secondary reaction. The experimental part of this thesis is therefore aimed at testing this hypothesis. A base case study included a comparison between a Fe-catalyzed slurry phase FTS reaction and a FTS reaction with all parameters remaining unchanged, except for the addition of 1 vol % NH₃ to the syngas (CO + H₂) feed. The activity (CO and H₂ conversion) data collected did not reveal any appreciable loss in the rate of the FTS reaction when 1 vol % NH₃ was added and steady state was reached (, that is after 48 hours time on stream (TOS)). A slower carburization period was however observed when comparing the CO conversion during the first 24 hours TOS, and further supported by the slow increase in CO₂ selectivity during the same period. The use of two-dimensional gas chromatography (GC × GC-TOF/FID) allowed for the discovery of a formation of a range of secondary and tertiary amines, not reported in previous studies. The expected loss in oxygenate selectivity was observed and further probed by co-feeding 1-octanol with the feed (CO + 2H₂ + 1 vol % NH₃) via a saturator. These results clearly indicated a significant loss in oxygenate formation as a result of secondary conversion to N-containing compounds. Questions regarding the stability of aliphatic nitriles prompted the co-feeding of nonanitrile under similar conditions. The results obtained after co-feeding nonanitrile, sug- gests that nonanitrile is readily converted to secondary and tertiary amines and that the ratios of aliphatic alcohols and nitriles are close to equilibrium. The use of CO₂ as carbon source, the investigation of the product spectrum at higher space velocities and the use of Rh-based catalysts, when NH₃ is added during FTS were included in shorter studies. The combination of these results, adds to the knowledge pool for the case where NH₃ is present in the FTS regime, as a poison or reactant. Additional information regarding the path to the formation of N-containing compounds was obtained via the detailed analysis of the product spectra with two-dimensional gas chromatography and the subsequent co-feeding reactions. The results ob- tained via co-feeding reactions, can be used to devise strategies to increase the selectivity of the desired N-containing compounds.
- ItemOpen AccessThe addition of HZSM-5 to the Fischer-Tropsch process for improved gasoline production(2002) Botes, Frederick Gideon; O'Connor, Cyril; Böhringer, WalterThe 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%.
- ItemOpen AccessAdsorption of K and KO on Hägg iron carbide surfaces and its effect on the adsorption of CO: a DFT study(2012) Cariem, Muhammad Junaid; Van Steen, Eric; Petersen, MelissaThe Fischer-Tropsch synthesis catalysed by iron is a well-established process, used for the conversion of syngas (a mixture of CO and H₂ ) to long chain hydrocarbons. Potassium is typically added as a promoter in iron-based Fischer-Tropsch to improve activity, selectivity and product distribution. The mechanism behind potassium promotion has in the past been explained as a combination of electron donation and electrostatic interaction. However, despite the importance of potassium as a promoter, the nature of the potassium species on the surface; whether it is present as metallic potassium (K) or is present as another species has received relatively little investigation. No research has been published as of yet as to the effects of potassium adsorption on a Hägg iron carbide surface or the effects on CO adsorption when co-adsorbing CO with potassium on a Hägg iron carbide surface. In this study density functional theory (DFT) has been used to investigate * The adsorption of CO on the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces. * The adsorption of K, O and KO on the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces. * The co-adsorption of K, O or KO with CO on the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces. A thermodynamic analysis was done to investigate the stability of K versus the stability of KO at Fischer-Tropsch conditions. The adsorption of CO on the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces was done as a pre-cursor to investigating the effect of co-adsorbing K, O or KO with CO on the CO adsorption energy, CO stretching frequency and CO bond length. Subsurface carbon on the Fe₅C₂(100)₀.₀₀ surface caused a decrease in the CO8 adsorption energy of 0.38eV when compared to CO adsorption on a similar site with subsurface iron. On the Fe₅C₂(100)₀.₀₈₉ surface, the lack of subsurface carbon allowed for CO adsorption in the 1F adsorption configuration on top of a valley iron site. The strength of potassium adsorption on both surfaces was calculated to be similar to that of CO in its most stable state (~1.60eV). Potassium is highly mobile across the surface, with a maximum barrier for K diffusion of 0.02eV calculated on both surfaces. A Bader analysis revealed that potassium donates electrons to the surface (~0.72) and that the electron donation from the potassium to the surface is localised and affects only the iron atoms not the carbon atoms. The co-adsorption of O with K leads to a significant increase in the stability of O adsorption on both surfaces, with increases in the O adsorption energy of O of ~0.60eV on the Fe₅C₂(100)₀.₀₀ surface and ~0.40eV on the Fe₅C₂(100)₀.₀₈₉ surface. The O also stabilises the K with the maximum barrier for diffusion of K increasing to 0.07eV on the Fe₅C₂(100)₀.₀₀ surface and 0.15eV on the Fe₅C₂(100)₀.₀₈₉ surface. However, these maximum barriers for diffusion are still extremely low, indicating that potassium is still highly mobile on the surface. The charge density difference plot showed some polarisation of the O towards the K and vice versa, indicating interaction between the two species. No orbital overlap between the adsorbed O and adsorbed K was observed in the charge density difference plot. This together with the results from a local density of states (LDOS) plot indicates that the interaction between O and K on the surface is ionic in nature. The co-adsorption of CO with either K or KO on both the Fe₅C₂(100)₀.₀₀ and Fe₅C₂(100)₀.₀₈₉ surfaces resulted in a significant increase in the calculated CO adsorption energy coupled with an increase in the CO bond length and a decrease in the CO stretching frequency. The magnitude of the increases in calculated CO adsorption energy and CO bond length as well as the magnitude of the decrease in the CO stretching frequency was virtually the same irrespective of whether CO was co-adsorbed with K or KO. The combination of these results shows that K and KO both enhance CO adsorption to a similar degree on Hägg iron carbide surfaces while possibly making CO dissociation more facile. Co-adsorbing CO with O on the Fe₅C₂(100) 0;00 surface lead to a significant decrease in the CO adsorption energies, an increase in CO bond length and an increase in the CO stretching frequency in certain cases. This negative effect on CO adsorption is very localised and restricted to CO adsorption sites which are near to the adsorbed O and have subsurface carbon which prevents CO migration away from the O to a more stable site. On the Fe₅C₂(100)₀.₀₈₉ surface where no subsurface carbon is present, the CO migrates away from the O to a site unaffected by the presence of O.
- ItemOpen AccessAlkylation of phenol with methanol over H-ZSM-5, H-Beta, H-Mordenite, H-USY and H-MCM-22(2003) Moon, Gillian Catherine; O'Connor, Cyril; Möller, KlausAlkylphenols are intermediates used in the synthesis of many important substances. The most significant of the commercially important lower alkylphenols are cresols (methyl phenols) and xylenols (dimethyl phenols). The main markets for alkylphenol products are nonionic detergents, phenolic resins, polymer additives and agrochemicals. Of the western world cresol market, 55% originates from synthetic cresols whereas 45% originates from so-called “natural” cresols obtained from coal tars and refinery caustics. The methylation of phenol is the only process specifically developed to produce cresols and xylenols. There are three cresol isomers, namely ortho, para and meta. It is possible to produce a high selectivity to o-cresol by alkylation of phenol with methanol over a basic or a Al2O3 catalyst or a Fe/V catalyst. A mixture of the cresol isomers can be obtained over aluminium oxides with strong acid sites, silica alumina, zeolites, aluminium phosphates and phosphoric acid-Kieselguhr catalysts. However, direct synthesis of high purity m-cresol and p-cresol is desirable due to the high costs of separating these two isomers. Thus to synthesize p-cresol, a m-cresol free or very low m-cresol concentration in the product is desirable for purification reasons and a high p/o-cresol ratio is also desirable in the product for yield reasons since separation of the p- and the o-cresols is economically possible.
- ItemOpen AccessAlumina-modified cobalt catalysts for the Fischer-Tropsch synthesis(2018) Petersen, Anna Paula; Van Steen, Eric; Claeys, MichaelIn the Fischer-Tropsch process, valuable hydrocarbons are produced using the basic starting materials hydrogen and carbon monoxide, which can be derived from alternative carbon sources such as coal, gas or biomass [1]. Although this process has been studied for almost a century, the effects of the support material on activity, selectivity and stability of the catalyst remain obscure. This study aims to gain fundamental insights into the effect of metal-support interactions in cobalt alumina based Fischer-Tropsch catalysts. To accomplish this, the effects of metal-support interactions have to be isolated from possible convoluting effects of the metal crystallite size and support porosity. This is achieved by preparing inverse-model catalysts, in which the support is deposited onto the metal, in contrast to conventional supported catalysts, in which the metal phase is deposited onto a porous support [2]. Cobalt alumina inverse-model catalysts were prepared by incipient wetness impregnation of cobalt oxide with aluminium sec-butoxide. The alumina loading was varied systematically between 0 and 2.5 wt% Al. The catalysts were characterised by X-ray diffraction (XRD), Transmission electron microscopy (TEM), H2 -chemisorption, and X-ray absorption near edge spectroscopy (XANES). The catalyst reducibility was studied by temperature programmed reduction (TPR), in situ (XRD) and in situ (XANES) experiments. The catalytic performance for the Fischer-Tropsch synthesis was studied in a slurry reactor under industrially relevant conditions. The alumina modification was found to prevent sintering and decrease the reducibility of the catalysts. With increasing alumina loading, and increasing calcination temperature, reduction peaks shifted to higher temperatures and peaks with maxima above 400 ˝C appeared in the TPR. The kinetic evaluation showed that the decreased reducibility was due to a decrease in the pre-exponential factor, which suggests that the alumina modification hindered hydrogen activation and/or nucleation of reduced cobalt phases. The activity of the catalysts for the FT reaction was found to increase with increasing alumina loading. This was likely an effect of the increase in metal dispersion upon alumina modification. Furthermore, alumina-modified catalysts had a higher C5+ and olefin selectivity, and lower methane selectivity. Pyridine-TPD experiments showed that the alumina modification introduced Lewis acid sites to the cobalt catalysts. Lewis acid sites may interact with adsorbed CO thereby weakening the C-O bond and facilitating CO dissociation. This was supported by CO-TPR experiments, which revealed that alumina-modified catalysts had an increased activity for the surface catalysed Boudouard reaction. It is concluded that the alumina modification increased the rate of CO dissociation on metallic cobalt. An increased rate of CO dissociation may lead to coverage of the metal surface with carbon thereby decreasing hydrogenation and shifting the product selectivity towards high molecular weight products. Hence, alumina may promote the selectivity of cobalt catalysts via a synergistic effect.
- ItemOpen AccessAmmonia leaching as a pre-treatment for the processing of oxidised PGM ores(2015) Musonda, Kabwe Julianna; Petersen, JochenThe exploitation of near-surface deposits has resulted in the need to adjust current conventional mineral processing technologies for the treatment of low grade oxidised PGM ores. The exposure of the ore to the atmosphere leads to the formation of an oxidation layer (consisting of base metal oxides) which inhibits the flotation process that requires a clean mineral surface to be effective. Previous studies have shown that an acid pre-treatment could lead to a 20% PGM recovery increase. Due to the corrosive nature of acid, such treatment would require additional equipment and a pre-wash. Therefore, this project investigates the use of ammonia as a pre-treatment of oxidised low grade PGM ores. Ammonia leaching has shown success in the extraction of base metals (94% and 91% Cu and Ni extraction) and is used as a wash reagent in an attempt to dissolve the oxidation layer and expose a cleaner mineral surface. Both column leaching of whole ore and batch stirred tank reactor leaching of milled ore were investigated as pre-treatment methods for varying length of process times, and the material was subsequently tested for its flotation behaviour. The total solids, base metal and PGM recoveries were monitored in order to determine the effect of the treatment.
- ItemOpen AccessAmmoxidation of propene over iron promoted bismuth molybdate for the production of acrylonitrile(2001) Maripane, Kgolole David; Van Steen, EricThe fundamental aim of this study is to prepare pure bismuth molybdate (α-phase) followed by iron impregnation at different ratios of iron to bismuth. The influence of iron in the ammoxidation of propene for the formation of an acrylonitrile is to be investigated.
- ItemOpen AccessAn exploration of lived experiences of 11 resettled families in Mazabuka district, Zambia, by a Nickel mine project(2019) Tumbama, Lewis; von Blottnitz, Harro; Donovan, ElspethMining-induced displacements and resettlements are a global phenomenon. However, how these are done and the implications that they have on the displaced and resettled are shaped by micro, local level dynamics. These include existing regulations, socio-economic and cultural situations of communities involved, level and type of compensations offered to the communities, among others. These shape how communities live the experiences of being displaced and resettled, as more powerful companies with financial resources seek out mining opportunities. For communities that are displaced and resettled, this process becomes a socio-economic one as change of place has implications on people’s ability to access resources on which they depend. This may lead to a transformation of their livelihood assets in the short and long term, which requires changing of their livelihood strategies. Zambia is endowed with natural resources, and the mining sector is the mainstay of the country’s economy. In particular, Zambia ranks among the top 15 producers of copper in the world. The government of Zambia (GRZ) gives mining licenses to companies that in return, pay royalties in form of taxes that support the government’s development programs nation-wide. Therefore, mining is said to be in the broader interest of the nation (GRZ, 2006). This means that the rights to the access and use of land by a particular group of people, individuals or ethnic grouping will be superseded by an economic undertaking that is seen as having a broader development interest of the nation. This includes economic activities such as building bridges or hydro-electric plants, road construction, building economic zones and mining. In Zambia, below ground surface mineral rights belong to the state. Given the global nature and scope of mining induced displacements and resettlements, micro-level dynamics easily remain invisible. In Zambia, research in lived experiences of displaced and resettled communities remain sparse and often unpublished. The aim of the research was to explore the psychological and socio-economic implications for the displaced and resettled households by a Nickel Mining Company in Mazabuka, southern Zambia. Families were purposively sampled and interviewed. Positive and negative lived experiences emerged from the interviews and have been presented as themes. Improved access to services; increased employment opportunities; improved production and acquisition of productive assets; and guaranteed security of land tenure and improved quality of houses were the positive lived experiences of resettled families. Negative lived experiences were: poor quality of soils; loss of locational advantage; emotional depression; discontinuation of gardening activities; and inability to buy drugs for livestock. The findings revealed that despite the cultural homogeneity of the sampled families, lived experiences after resettlement were different based on socio-economic situation of households. This was determined by who the head of the household was, literacy levels and family labour availability, because agriculture is the main livelihood activity. This research used a qualitative single case study approach to understand the ‘how, what and why’ of the lived experiences of the resettled families. It sought to respond to three related research questions: i) Did loss of access manifest in the studied case, and if so, how? What are the psychosocio-economic implications of the Munali Nickel mining-induced displacement and resettlement; ii) What characterizes the livelihoods of resettled families following the establishment of the Munali Nickel Mine, which assets were positively or negatively affected by resettlement and compensation; and iii) What are the coping mechanisms of the displaced and resettled communities following the establishment of the Munali Nickel Mine? The results of this research indicate that the level of compensation paid to resettled family cannot make up for what communities give up so that mining activities can start. Cultural values and the sense of belonging, for example are not compensated for when these are the factors that ensure psychological well-being of communities. Compensation and fulfillment of development promises were‘delayed and not necessarily denied.’ Access to education and health facilities was not achieved until nearly 10 years later, partially attributed to frequent closures of the mine that prevented the mine from honouring its promises in time. The uncertainty in the operations of the mine, loss of access to livelihood strategic resources was unsettling and created a sense of anxiety among resettled community members. While access to schools and clinics as physical assets was facilitated following resettling, the resettled community was not wholly part of the mining operation to the level that ensured human capacity development. For a highly technological undertaking as mining, resettled households could not benefit from any knowledge transfer. Furthermore, interviewed households reported increased distance to the tarred road; loss of sources of livelihood to closure of the mine, and loss of business opportunities in resettlement site; sub-standard houses with leaking roofs and cracks, as well as non-uniformity in compensation; inadequate grazing land; reduced agroproduction due to water logging in the fields and poor soils; scarcity of firewood; and nonreinstatement of churches, among negative experiences that they have lived. Coping with the experiences of ‘delay but not denial of access’ and a ‘mixed bag’ of changes to different asset classes was differential based on age, sex and household composition that determined the level of available labour in the household.
- ItemOpen AccessAn investigation into the complementary capabilities of X-ray computed tomography and hyperspectral imaging of drill core in geometallurgy(2022) Mashaba, Dineo; Becker, MeganThe mining industry is faced with the challenge of mining and processing low grade, heterogeneous, and complex ores, a phenomenon known as ore variability. These ores need to be managed at an early operational stage, ideally during drill core exploration, to avoid risks during the project phase (such as project delays and failure) and operational phases (such as plant instabilities), ultimately affecting the cash flow. The discipline of geometallurgy has arisen to manage the risks associated with ore variability by acquiring upfront knowledge of the mineral assemblage and texture before mining and processing. As we head towards the fourth industrial revolution (4IR), machine learning, intensive and automated data derived from drill cores are becoming more common. In this case, using non-destructive, rapid, and inexpensive automated scanning techniques such as 2D hyperspectral imaging (HSI) and 3D Xray computed tomography (XCT) have the potential to be incorporated into the machine learning dataset. Hyperspectral imaging is a critical component of continuous drill core scanning in geometallurgy for identifying problematic minerals in downstream mineral processing, such as the phyllosilicates (e.g., kaolinite, serpentine and talc). However, it only provides 2D imaging of the core, and its mineral identification is limited to minerals that show a definitive spectral response. On the other hand, XCT provides 3D imaging of drill cores, but is more routinely used in research applications and does not independently give the mineral assemblage. Mineral identification and discrimination for XCT is limited and requires prior mineralogical knowledge and sufficient mineral density and attenuation coefficient variation greater than 6%. No systematic study to date appears to have explored how the results from these two techniques can be integrated using a local South African magmatic nickel-copper-platinum group element (Ni-Cu-PGE) ore case study. This opened an opportunity to couple the two techniques to address and emphasize the image scanning techniques for drill core in geometallurgy and to provide further knowledge on the practicality of the HSI and XCT in drill core from image acquisition to processing. Ultimately, the aim is to investigate how well the techniques complement each other for mineral and texture identifications and, if combined, will produce additional mineralogical and textural information. The objective of this study was achieved by moving HSI cores to smaller samples than standard practice to produce 25 mm diameter mini cores instead of standard cores (e.g., 50 mm in diameter). For accurate mineral assemblage and textural characterisation of the drill cores, manual core logging, quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) and quantitative X-ray diffraction (QXRD) were used as supporting techniques. The results showed HSI scanning on the magmatic Ni-Cu-PGE drill core to be challenging because of pervasive mineral alteration and the nature of the rock types (mafic and ultra-mafic rocks) - providing limited information on the mineral assemblage and texture due to low scanning resolution and pervasive alteration (serpentinisation and chloritization) in the rocks. The limited mineral identification includes mixed-phases (such as serpentine-olivine in visible-shortwave infrared and plagioclase-chlorite in the longwave infrared) and unclassified minerals in the core. The resultant mineral assemblage was comparable to QEMSCAN and QXRD in terms of minerals present with generally similar abundances. However, useful information on the alteration mineralogy can still be extracted, such as the presence of serpentine, chlorite and talc and their association with other silicate minerals. Other parameters such as mineral grades and grain sizes were quantified on MATLAB using specially developed scripts. The interconnected grains could not be separated due to invisible boundaries on the HSI maps. Therefore, only a small number of grains were generated with larger grain size values, likely underestimating the real grain numbers. XCT provided information on valuable high-density minerals (including possible platinum-group minerals (PGMs)) and mineral texture in the cores. Due to extensive alteration in the rocks, discrimination between grey values was, however, challenging. Grey level segmentation into the different mineral groups was also noted to be dependent on the rock type. For example, plagioclase and orthopyroxene were more easily discriminated in the less altered rocks (feldspathic pyroxenite and anorthosite) than the more altered rocks (altered harzburgite and pegmatoidal pyroxenite). The high scanning resolution allowed for the extraction of mineral texture, such as mineral association and grain size distribution (GSD). The 3D XCT derived GSD was slightly coarser than the 2D QEMSCAN derived GSD. The differences in GSD are attributed to a combination of both stereological and sampling effects. However, sufficient information on ore variability can be obtained when using the pertinent scanning parameters and careful segmentation processes. These two techniques provide variable information on the mineral assemblage and texture, such as the identification of silicate minerals (particularly alteration minerals) in HSI and high-density minerals in XCT and good textural information on XCT than HSI. With the information provided, possible image overlapping scenarios of the two techniques were identified: (1) using XCT for high-density minerals, and HSI for silicate identification, (2) using XCT data with good mineral and texture discrimination (silicate associated with sulphides) to map unclassified areas in HSI, (3) is the opposite of the second scenario. Ultimately, the two scanning techniques will likely offer complementary information, although the application of this combined technique for routine work will be limited in practicality. Additionally, more work needs to be carried out with revised scanning and processing to improve the sustainability of the techniques in geometallurgy.
- ItemOpen AccessAn investigation into the effect of potential modifiers on the flotation of a copper sulphide ore(2018) Dzinza, Lucia; Corin, Kirsten; Wiese, JenniferOxidation, adsorption and reduction reactions are electrochemical in nature in the flotation of sulphide minerals which have semiconducting properties. Electrochemical mechanisms have two valuable implications in flotation, the potential across the mineral/solution interface determines flotation recovery and the anodic oxidation reaction involving the collector is an important parameter in imparting floatability. The reactions are dependent on the redox conditions in the pulp phase. Chemical control of redox potential (Eh) using potential modifiers may be exploited in flotation processes of sulphide minerals to improve their floatability, recoveries and grades; owing to the formation of a hydrophobic dithiolate or metal thiolate in the case of thiol collectors. In addition, chemical control of Eh is advantageous as it renders a more uniform electrochemical environment around the sulphide particles as compared to the external control of pulp potential. The adjustment of pulp potential using potential modifiers is being exploited as one of the main control parameters in sulphide flotation studies as it provides a diagnostic tool to develop flotation strategies and alleviate flotation challenges. Though potential modifiers have been previously investigated, no literature has addressed the correlation between their flotation performances on copper sulphides to their respective rest potentials at different concentrations. The present study explored the use of potential modifiers such as sodium hypochlorite (NaClO), potassium permanganate (KMnO₄) and potassium dichromate (K₂Cr₂O₇) on the flotation of a copper sulphide ore from Kansanshi Copper mine in Zambia. The potential modifiers were investigated at 1x10⁻⁴, 1x10⁻³ and 1x10⁻² mols which gave rise to various Eh values for each modifier. Batch flotation and froth stability tests were carried out at the ore’s natural pH whilst varying Eh. The dynamic stability factor (Σ) was used to quantify froth stability. Electrochemical techniques have been considered as an appropriate approach in the study of collector-mineral interactions. To complement results obtained from batch flotation and froth stability tests, rest potential measurements were carried out to determine the characteristic species formed on the chalcopyrite mineral surface at specific conditions. The potential modifier-collector-mineral interactions were investigated through rest potential measurements using the aforementioned potential modifiers, a thiol collector sodium iso-butyl xanthate (SIBX) and a pure chalcopyrite mineral. It was hypothesized that assuming an X⁻/X₂ equilibrium potential below 100 mV for SIBX, a redox potential range of 100-400 mV promotes good copper floatability due to the formation of dixanthogen and thus hydrophobic mineral particles which would result in a moderately stable froth. Rest potentials above 500 mV were hypothesized to reduce copper floatability due to the presence of very hydrophobic mineral particles, which would increase bubble coalescence and bubble breakage or result in highly stable froth. In this study, the equilibrium potential of SIBX at 6.24x10⁻⁴ M was measured to be 80 mV. Furthermore, equilibrium potential of SIBX was determined to be concentration dependent. Rest potential measurements for all conditions investigated were in excess of the measured equilibrium potential, therefore implying that the dixanthogen species was formed as postulated. It was found that an increase in concentration of potential modifiers increased froth stability or bubble coalescence depending on the potential modifier used. Furthermore, concentrations of potential modifiers resulting in Eh values of 137-476 mV resulted in high copper recoveries >88%, with 1x10⁻² mols of KMnO₄ at 540 mV giving a very low copper recovery of 4.8%. However, though high copper recoveries were obtained between concentrations that gave rise to an Eh range of 137-476 mV, a slight decrease in copper recoveries of approximately <4%, was observed with even larger increases in concentrations of potential modifiers. The findings of this study showed that the use of potential modifiers improved copper grades as a result of the reduction in gangue material recovery. In addition, the present study has shown that though concentration or Eh induced by potential modifiers may affect the flotation performance of sulphide ores, the most dominant factor that has shown to have a greater impact is the nature of the potential modifier. Comparing the findings of this work to literature findings for NaClO, it was determined that different sulphide minerals indeed exhibit different rates of redox reactions at given conditions. Ultimately, an inverse relationship was determined to exist between copper recoveries and rest potential measurements. This study has provided insight into the use of potential modifiers in the flotation of copper sulphides from an electrochemical perspective.
- ItemOpen AccessAn investigation into the feasibility of bioleaching copper sulphide ore of Nchanga mine, Chingola, Zambia(2007) Manchisi, James; Hansford, Geoff; Simukanga, SBioleaching is now well established for extraction of copper from its low-grade, mixed oxide/sulphide ores in bioheap operations (Watling, 2006). It is claimed bioleaching may be a cheaper alternative to other traditional metal extraction techniques such as smelting and therefore more suitable to treat marginal or low-grade ores where high-grade mineral reserves have been depleted. But the degree of microbial oxidation for different copper sulphides varies significantly partly because of different types and level of impurity minerals associated with copper sulphides from different sources. Several authors have reported different mineral leaching kinetics data on account of variability of mineralogical composition and different solution conditions and leaching systems being employed. This has indicated that further work was necessary to determine the kinetics of copper extraction, extent of mineral dissolution and effects of some leaching variables on copper dissolution for an ore from Nchanga mine under various solution conditions.