Masters

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Open Access
Ammonium hexachlororuthenate precipitation
(2011) De Klerk, Frederick Jacobus; Westra, Arjan; Hagemann, Justin; Petersen, Jochen
Ammonium hexachlororuthenate (ACR) is a salt precipitated during a substitution reaction between ammonium and ruthenate dissolved in a strong hydrochloric acid medium. This precipitation reaction is used within the platinum industry as a means of recovering and purifying ruthenium. Application of this process at the Precious Metals Refinery of Anglo Platinum has brought to light certain inefficiencies. In recent years, volatility in the Ru market price occurred, indicating a potential to benefit financially from an improved Ru recovery. Consequently, this study was conducted to understand the effect of certain parameters on the process of precipitating ammonium hexachlororuthenate.
Open Access
An investigation into the enzymatic activity of deepsea actinobacteria in decolourising crystal violet dye
(2019) Davids, Natasha; Petersen, Joachim; Huddy Robert
Crystal Violet (CV) decolourising deep-sea actinobacteria could provide a great source of novel redox biocatalysts that can be used in various applications such as removal of triphenylmethane dyes from contaminated wastewater and soil, degradation of aromatic environmental pollutants, biotransformation of antimicrobial agents and degradation of xenobiotics. CV is a triphenylmethane dye that has various applications, including use in medical, research and industrial applications, but its release into the environment poses a threat to aquatic life as it has characteristics of a biocide. Only a limited number of microorganisms are able to decolourise and degrade CV, and one of these proposed mechanisms by which they do so is the catalytic effect of oxidoreductase enzymes, including peroxidases, polyphenol oxidases and laccases. Triphenylmethane reductase has also been reported to be involved in decolourising CV, but the reaction involving this enzyme has not been studied systematically. Eleven deep-sea actinobacteria were investigated and found to decolourise CV by either biodegradation or biosorption. Gordonia sp. JC 51 was selected as a candidate for further study as it could decolourise CV efficiently and could tolerate high concentrations (1mM) of CV. A combination of spectral scan studies, dye decolourisation, biodegradation assays, enzymatic assays, SDS-PAGE, Native PAGE, TLC and LC/MS/MS methods revealed the mechanism involved in the decolourisation of CV. Gordonia sp. JC 51 decolourised CV via enzymatic and non-enzymatic mechanisms. However, true decolourisation of CV was performed via biodegrading enzymes. Triphenylmethane reductase and polyphenol oxidase was confirmed to be the enzymes involved. Leucocrystal Violet was identified as the metabolite produced. CV also was sequentially N-demethylated, oxidised and cleaved into smaller compounds such as Michler’s Ketone. In conclusion, Gordonia sp. JC 51 has potential as a whole cell biocatalyst and should be investigated further.
Open Access
Anaerobic digestion of algal biomass for bioenergy production- a feasibility study
(2011) Inglesby, Alister Edward; Van Hille, Rob; Harrison, STL
AD technology is well developed, cost efficient and can be easily implemented in developing countries. Biogas production has become a very topical subject, with many European nations introducing initiatives to increase biogas production. AD of algal biomass was studied in detail during the 1980’s, however, with the current drive toward cleaner technology processes, there has been a renewed interest in the technology. This study investigated the feasibility of using algal biomass as a feedstock for AD.
Open Access
The anodic dissolution of covellite in acidic, chloride solutions
(2010) Basson, Petrus; Petersen, Jochen
An electrochemical study was conducted on a stationary, synthetically produced, covellite electrode in acidic, chloride solutions at ambient conditions to investigate the dissolution behaviour of the mineral over a surface potential range from the open circuit potential (OCP) to about 0.62 V (vs. SHE). The electrode was mounted in an apparatus, which was designed to resemble leaching of the mineral under conditions applicable to heap leaching of whole ores, where the mineral occurs in cracks or pores in the gangue matrix or is covered (or partially covered) by reaction products.
Open Access
Application of electrical resistance tomography in evaluating the influence of nozzle design on the gas hold-up in boiling bubble column reactors
(2012) Sudhakaran, Harikrishna
Bubble column reactors are extensively used in the petro-chemicals industry due to the combined advantages of high rates of heat and mass transfer coupled with low operating and maintenance costs. The complex hydrodynamics brought about by the multiphase nature of such systems offer significant challenges in modelling, analysis and operation, thus making experimental measurement of system response of special importance. One such measurement technique is Electrical Resistance Tomography (ERT). In this study, the development of an experimental bubble column reactor fitted with 8 rings of 16 electrodes for ERT measurements using an instrument developed at the University of Cape Town (UCT) is described.
Open Access
Application of mineralogy in the interpretation of laboratory scale acid rock drainage (ARD) prediction tests : a gold case study
(2014) Dyantyi, Noluntu; Becker, Megan; Broadhurst, Jennifer Lee
The mining and beneficiation of gold generates large tonnages of waste, with up to 99% of mined gold ore discharged as waste. The waste generated contains unoxidized sulfides that when exposed to air and water react to form acid, which results in acid rock drainage (ARD). ARD is usually associated with low pH, high sulfate content and elevated concentrations of toxic elements. The mobility of ARD affects our scarce water resources, land and aquatic species. Methods applied to treat ARD do not provide a walk-away solution and they are either expensive or difficult to maintain. The best solution to completely eradicate ARD is to prevent it from the source. However, the effectiveness of ARD prevention depends on the accuracy of predicting future drainage quality. This can be done by using ARD prediction tests, which are generally classified as either static (acid base accounting, ABA, net acid generation, NAG) or kinetic (column leach, humidity cell, biokinetic test). There is no single test capable enough to accurately predict acid generating potential. It is therefore usual practise to conduct more than one test and cross-check results to ensure that the appropriate conclusions are made. In doing so, the reliability of the tests is improved but in some cases the different test results do not correlate. Mineralogy is an analytical technique that can be used to understand the nature of the errors and to better understand the leaching behaviour of minerals in the different tests. This study uses mineralogy to analyse both static and biokinetic test results of a Witwatersrand gold sample in order to improve the understanding of behaviour of mine wastes under different ARD prediction test conditions. A run-of-mine gold sample from the Witwatersrand region in South Africa was used as a case study to explore the mineral leaching behaviour for different ARD prediction tests.
Open Access
An approach of compartmentalisation in development of non-isothermal chemical reactor network models for the high speed simulation of iso-octane combustion
(2011) Khan, Zamier Ahmed; Rawatlal, Randhir
Every aspect of the modern day life relies on combustion, be it in motor vehicles, industrial equipment or power generation. The downside to the extensive use of combustion technology is the environmental pollution produced by the process. The lack of fast solving models to simulate combustion hampers the investigation into the optimisation of combustion processes. In this study, the compartment approach in developing a fast and accurate simulation is used to investigate combustion systems. A chemical reactor network (CRN) is proposed for the simulation of the combustion of iso-octane. The compartmentalisation of a combusting system involves proposing a reactor network based on the flow fields predicted by computational fluid dynamics (CFD). The first step in the development of such a model involves using of a reduced kinetic model representing thousands of combustion steps in a few elementary steps by lumping species. The reduced kinetic model used in this study consists of a five-step mechanism involving four pseudo species. The thermodynamic properties of the pseudo species in the system were regressed against experimental data and successfully validated using the plug flow and continuous stirred tank reactor sub-models. The reduced kinetic model was also further validated using Rapid Compression Machine data. The current study also modified the methodology for developing a CRN in order to make the CRN more predictive as compared to previous studies. This was achieved by incorporating non-isothermal sub-models into the network instead of isothermal sub-models that rely on the CFD temperature field. The network parameters were also correlated to the inlet Reynolds number in order to further increase the predictive nature of the network for industrial applications and to allow for the systems performance to be predicted over a wide range of input conditions. The investigation begins by conducting a CFD simulation of iso-octane combustion in a furnace and double inlet reactor assuming a one-step global reaction. On the basis of the CFD flow fields, a CRN was proposed and coupled to the reduced kinetics.
Open Access
Assessment of water pollution arising from copper mining in Zambia: a case study of Munkulungwe stream in Ndola, Copperbelt province
(2018) Mudenda, Lee; Harrison, Sue; Syampungani, Stephen
Water pollution is recognized as one of the major environmental problems in the mining industry. This has been compounded with an increase in agriculture activities. Water pollution is a major problem on copper and coal mines throughout the world and Zambia, the focus of this study, is no exception. Worldwide freshwater resources, which provide important ecosystem services to humans, are under threat from rapid population growth, urbanization, industrialization and abandonment of wastelands. There is an urgent need to monitor and assess these resources. In this context, the physical, chemical and ecological water quality of the Munkulungwe Stream located on the Copperbelt of Zambia, was assessed with possible contamination from Bwana Mkubwa TSF, agriculture activities and subsequent impact on the surrounding community. The chemical and physical parameters were assessed at four sampling locations. Sampling site S1 was located on the Munkulungwe stream upstream of Bwana Mkubwa TSF, S2, S3 and S4 were on the main stream downstream of Bwana Mkubwa TSF. In addition, a macroinvertebrate composition analysis was performed to estimate the quality of water using the biotic index score. Finally, the relationship between physiochemical parameters and biotic index score was analysed to interrogate their inter-relationship with respect to water quality. The results showed that the average values of dissolved oxygen (DO) of 4.52 mg/l, turbidity (40.96 NTU), Co (0.24 mg/l), Pb (0.25 mg/l), Fe (0.36 mg/l) and Mn (0.22 mg/l) downstream exceeded international standards for drinking water. Upstream, the values of Co, Pb, Fe and Mn were within acceptable standards for drinking water, DO and turbidity were above acceptable standards. The metal concentration and total dissolved solutes were impacted by closeness to the mine tailings deposit with the heavy metal concentration being highest at S2 and S3. Moreover, high turbidity levels revealed that land erosion induced by agriculture activities is a severe problem in the area. Physical parameters were high in the rainy season due erosion escalated by rains while chemical parameters were high post rainy season. During the rainy season, the chemical contaminants are diluted and thus they are not such a big impact, but they tend to concentrate up during the dry MDNLEE001 III season. The stream at sampling points S2 and S3 was dominated by species tolerant (leech, Isopod and Snail: Pouch) and semi tolerant (Blackfly larvae and Amphipod or Scud) to pollution. The change in season influenced the composition of macroinvertebrates, with the number of species increased post rainy season. The average biotic index score (2.5) showed that the stream condition is not good, it is slightly polluted. The results showed that water quality downstream was substantially affected by Bwana Mkubwa TSF, agriculture activities and is likely to affect human health and food security. It is recommended that groundwater surrounding tailings dams should be monitored in both active and abandoned mines. Curtain boreholes around a tailings dam can be drilled and the water extracted and treated so that it doesn't contaminate other water bodies. To improve the environmental management of mining related impacts in Zambia, mining areas should be completely rehabilitated. There is need for remediation strategies for abandoned mine sites. Constructed wetlands, roughing filtration and phytoremediation are highly promising techniques, as they are reliable, cheap, effective and sustainable.
Open Access
Atmospheric leaching of a saprolytic nickel laterite ore in chloride solutions
(2008) Field, Karen Louise; Gaylard, Peter
Open Access
Base metal heap and tank leaching of a platreef flotation concentrate using ammoniacal solutions
(2013) Muzawazi, Caroline; Petersen, Jochen
The technical feasibility of the ammonia leaching process of a Platreef flotation concentrate was investigated in different reactor settings i.e. shake flasks, columns and batch stirred tank reactors, respectively. The process investigated aims to use either a heap leaching environment or tank leaching of untreated low-grade concentrates under ambient conditions and mild temperatures. This process is proposed as an alternative primary treatment method for the recovery of base metal sulphides from a PGM containing concentrate that cannot be extracted economically by conventional milling, smelting and refining methods.
Open Access
Biooxidation kinetics of Leptospirillum Ferriphilum attached to a defined solid substrate
(2011) Duku, Porogo; Petersen, Jochen; Minnaar, Sanet; Harrison, STL
Bioleaching can be categorized as being either stirred tank type (i.e. bio-oxidation) or irrigation type (i.e. heap/dump bioleaching) yet studies investigating the kinetics of bioleaching systems mostly use empirical data determined from stirred tank type and initial rate experiments in batch cultures or using iso-potential devices. Rate equations deduced from such empirical data is then used to model both the stirred tank type and irrigation type bioleaching systems overlooking the possibility that there may be significant differences in their environments and therefore the kinetics. Tank bioleaching systems are well mixed suspension systems dominated by planktonic microorganisms (freely suspended in the liquid medium). Heap bioleaching systems on the contrary, are heterogeneous in nature with chemical and physical conditions changing over time and are dominated by sessile microorganisms (attached microorganisms to the surface of a solid). The heap bioleaching system is therefore highly complex compared to the stirred tank-type systems. Microbial growth in bioleaching systems significantly influence the overall bioleaching kinetics yet biological kinetic effects in sessile/ attached environments are not well understood. Heap and dump leaching account for about 20% of the world’s copper production and are becoming popular methods of copper production from leaching low grade ores. It is therefore important that the kinetics of irrigation type bioleaching systems are well understood. A strategy to determine the microbial kinetics of a sessile microbial population is enforced in this study. From this, empirical data determined from irrigation type environments can then be used to derive equations which can be used to accurately model heap bioleaching systems. Three sets of experiments were conducted to try and achieve this: i. planktonic experiments - investigating the microbial kinetics of a planktonic microbial population ii. attachment experiments - investigating the nature of growth of the microbial population to the surface of a solid substrate during attachment to create a sessile microbial population iii. sessile experiments - investigating the microbial kinetics of the sessile microbial population A pure culture of Leptospirillum ferriphilum (a mesophilic, ferrous iron oxidizing bioleachingmicroorganism) was used in this study. Planktonic experiments were conducted in a completely mixed, well aerated continuous stirred tank reactor (CSTR) with a 1 litre working volume, operating at a pH of about 1.3 and temperature of 37oC. Attachment and sessile experiments were conducted using a CSTR with similar conditions to the planktonic experimental, however the system was modified by introducing a packed bed vessel (PBR) attached as a closed loop to the CSTR. Solution drawn from the CSTR was then continuously pumped through the PBR and back to the CSTR.
Metadata only
Carbon dioxide mass transfer within algal raceway ponds and the potential for improvement using slopes to create wave
(2016) Burke, Matthew; Harrison, STL
The growth of microalgae has the potential to be extremely useful for the production of a wide range of products or for specific processes, such as capture and cycling of CO₂. The fast micro-algal growth rates and ability to grow on agriculturally poor land and in waste water means that bio-production using algae has many advantages over traditional agricultural processes for certain applications. The raceway pond is the most common reactor used for the growth of microalgae, due to low capital costs, low operating costs, higher energy efficiency, improved net energy recovery and ease of installation. Low carbon dioxide mass transfer, which limits algal growth and productivity, is currently one of the largest issues in photo bioreactors of all forms. The microalgae within these systems only obtain carbon from the dissolved inorganic carbon and hence sufficient carbon dioxide mass transfer is one of the most important design parameters for any photobioreactor. This is particularly evident in raceway ponds as they have a lower volumetric mass transfer rate than other photobioreactors and are typically mass transfer limited. [Please note: the full text of this thesis has been deferred until 30 September 2017]
Open Access
Characterisation of the effect of alteration on the PPM platinum ore and evaluation of selected strategies to improve metallurgical performance
(2011) Ramonotsi, Mpho; Petersen, Jochen; Becker, Megan
The aim of this study is in two parts; the first part focuses on mineralogical quantification of the extent of weathering at PPM by profiling the flotation behaviour and host rock density with spatial depth.
Open Access
Characterising the acid mine drainage potential of fine coal wastes
(2013) Kotelo, Lerato Olga; Broadhurst, Jennifer Lee; Becker, Megan; Harrison, STL; Franzidis, Jean-Paul
Acid mine drainage (AMD) is one of the major environmental challenges facing the South African mining sector. Acid mine drainage has received significant public attention in recent years. South Africa's long mining history has led to a growing concern that coal-related AMD from these mines (both operational and defunct) will continue for centuries to come. Pyrite bearing fine waste, generated during coal preparation and beneficiation, is thought to carry a significant amount of AMD pollution risk. Coal-related AMD generation has not been afforded the same exposure as AMD generation from high sulphide minerals such as gold and copper ores. This is exacerbated by the growing concern over water quality degradation in the Mpumalanga region of South Africa. The development of integrated solutions to address the management of coal-related AMD requires an understanding of the principle causes behind coal-related AMD. To date, most of the prediction methods described in literature have been derived for the prediction of AMD in metal bearing ores. Furthermore, some of these methods are based on assumptions and do not take into consideration the various sulphur species present. Additionally, some of these methods have limited applicability to coal due to the high total organic carbon content (TOC) of the material. This research project attempts to address these short comings and uncertainties by developing a systematic and meaningful framework for the characterisation of South African coal and coal waste. The research project contributes to the knowledge of coal-related AMD with particular emphasis on the characterisation methods responsible for sulphur speciation and mineralogy for coal. The approach entails carrying out a case study assessment aimed at empirically assessing a coal tailings sample according to: particle size distribution, textural reference, mineralogical characteristics, and how the aforementioned factors influence the acid potential in coal. The approach intends to address key factors which include: identifying the sulphur bearing organic and inorganic constituents related AMD generation in coal, assessing how the mineralogy, texture and particle size distribution contribute to AMD potential in coal tailings, and then identifying suitable analytical techniques and test methods which can provide data. The combination of these key outcomes will seek to provide a systematic and meaningful framework for the characterisation of coal and coal waste streams. The characterisation methods used in this case study outlined a framework focusing on four main areas of acid mine drainage characterisation for coal wastes, these included: chemical characterisation, mineralogical characterisation, sulphur speciation and AMD prediction. This comprehensive approach employed a suite of techniques, including: petrography, quantitative x-ray diffraction (QXRD) and quantitative evaluation of minerals by scanning electron spectrometry (QEMSCAN).
Open Access
Characterizing the potential environmental risks of South African coal processing wastes
(2018) Moyo, Annah; Broadhurst, Jennifer; Amaral-Filho, Juarez; Harrison, Susan
The environmental impacts of coal processing wastes are a challenge in South Africa as large amounts of coal wastes are produced annually, pegged at 60 million tons per year according to Eberhard (2011). Whilst the fossil fuel-based industry is in decline globally, coal is likely to remain the dominant source of power in South Africa. The major environmental impacts reported in several studies are water pollution and soil quality degradation due to acid rock drainage (ARD) and its associated elevated levels of elements and salts. Several studies have shown the environmental performance of the wastes to be dependent on the geochemical properties of the wastes. Owing to the complex nature of coal wastes, their characterisation using tools developed for hard rock ores is associated with inconsistency and uncertainty. As a result, the South African coal processing wastes are poorly characterized and the associated risks not well understood. This study investigates the reliability of relevant characterisation techniques and interpretation of characterisation data in terms of the environmental risk potential of coal wastes. The outcomes of the study address some of the uncertainties and deficiencies arising from the current characterisation tools and evaluate potential environmental risks posed by coal processing wastes. Laboratory-scale characterisation of the physio-chemical properties and of ARD and elemental risk potential of two ultrafine coal waste and one discard waste sample were conducted. Evaluation of accuracy and repeatability of selected analyses was conducted on a certified coal standard. The selected analyses tested for accuracy and repeatability were total sulphur analysis by Leco and Eschka methods in addition to elemental analysis by wavelength dispersive x-ray fluorescence (WDXRF), inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The ISO 157:1996 and ACARP C15034 protocols for assessment of sulphur forms were also compared and evaluated for precision using the coal standard and coal waste samples. Conversions of the sulphur species under static ARD tests were also studied to understand the sulphur species behaviour and implication on ARD potential. The mineralogy of the coal wastes was evaluated from a quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) and quantitative x-ray diffraction (QXRD) analysis. In addition, conventional net acid generating (NAG) and acid-base accounting (ABA) static tests were enhanced through extended boil NAG tests to assess the organic acids effect on the NAG capacity. The static tests were validated by theoretical ARD calculated from mineralogy as well as biokinetic shake flask tests which gave the timerelated acid generating behaviour of the coal waste samples. Sequential chemical extractions combined with a simple score and ranking protocol were subsequently used to evaluate the potential water and soil-related risks associated with environmentally available elements and salts in the coal wastes. The results showed both the Leco and Eschka methods to be highly precise (±0.01-0.03 % standard error) but the Leco was more accurate (±3.1 % compared to ±12.5 % relative standard error (RSE)). The total sulphur content of the coal processing waste was less than 2 %. The ISO157:1996 and ACARP C15034 protocols gave comparable and slightly different results but the latter was more precise in sulphate analysis. Furthermore, the ACARP protocol could differentiate the acid forming sulphates from the soluble sulphates giving a better theoretical maximum acid producing potential. The sulphur species from the two chemical methods and QEMSCAN mineralogy showed 52-61 %, 12-26 % and 21-43 % to be sulphide, sulphate and organic/low-risk sulphur respectively. The conversion of the sulphur species showed that partial solubilisation of sulphides in ANC and partial conversion of organic/low-risk sulphur under NAG tests can cause an over or underestimation of ARD potential. The static ARD tests has shown the Witbank coal discards sample to be potentially acid forming (PAF) (9.2-25.9 kg H2SO4/Ton), Waterberg coal slurry to be non-acid forming (NAF) (-68.6 to -46.8 kg H2SO4/Ton) and Witbank coal slurry to be uncertain (-12.1 to 9.9 kg H2SO4/Ton). The extended boil NAG tests showed organic acids effect on the Witbank coal slurry likely caused an overestimation of the NAG capacity. Validation of the static tests by biokinetic tests and ARD calculated from mineralogy classified both Witbank samples as PAF and the Waterberg sample as NAF. The results also showed the net acid producing potential of the coal wastes to depend on the mineralogy of the samples. The elemental results showed WDXRF and LA-ICP-MS analysed most of the elements accurately within ±10 % RSE and that a combination of techniques provides more reliable and accurate results. The analyses showed the coal waste to contain significant amounts of environmentally sensitive elements like Cr, As, Mo, Sb, Se. The ranking and scoring of potentially available elements under oxidising leach conditions evaluated Fe in Waterberg coal slurry and Witbank coal discards to pose high risk in drinking water while S (as sulphate), Pb, Sb, Mn, As, Al and Hg in the three samples pose moderate risk. This case study evaluated the accuracy and precision of commonly used analytical techniques and applicability of risk evaluation protocols for coal processing wastes. The research outcomes underlined some factors that cause uncertainty and inconsistency with the evaluation of ARD potential of coal wastes. The findings highlighted the need to validate and complement the characterisation data using various tools and risk evaluation protocols to overcome specific limitations. The results also indicated the coal wastes have the potential to cause environmental impacts from ARD and elevated concentration of elements and salts, thus providing a basis for designing and implementing waste management strategies which minimise these risks. The mineralogy and elemental composition of coal wastes showed enrichment of elements and presence of potentially usable and economically valuable constituencies for future studies on value recovery. Characterisation of coal processing wastes for air pollution impacts is recommended for future studies as well as a study of ARD behaviour under continuous flow systems to more closely represent the conditions in dump disposal scenario.
Open Access
Combining froth flotation and reflux classification to mitigate ARD generating potential of the Waterberg and Witbank coal ultrafines via sulfide removal
(2014) Iroala, Onyinye Judith; Franzidis, Jean-Paul; Harrison, STL
In South Africa, over 10 million tons of ultrafine coal wastes are discarded every year, typically in the form of ultrafine slurries. These fines have a high calorific value, and contain sulfur minerals, particularly pyrite. The high calorific value of these discards leads to a waste of energy that could be harnessed and used, while the high sulfur content contributes to adverse environmental effects such as acid rock drainage (ARD). The University of Cape Town (UCT) has developed a two-stage flotation process, which involves coal flotation in the first stage and pyrite flotation of the tailings in the second stage, for mitigating the ARD potential of ultrafine wastes. Research has shown that this two stage froth flotation process was sufficient to render the tailings non-acid forming. At the same time, North West University (NWU) has been carrying out research on coal fines using the recently invented reflux classifier. The reflux classifier is claimed to be capable of separating particles down to 38 ìm in size; however, no work has been done using the reflux classifier to separate pyrite from coal. This dissertation investigates the effectiveness of combining flotation and reflux classification for removing sulfide minerals from two South African coal ultrafines, whilst recovering valuable coal, and compares the results to those obtained using the UCT two-stage flotation process. As no previous work has been done using reflux classification to remove sulfide minerals from coal, this is the first time that the reflux classifier will be investigated for this purpose. Two process routes were investigated: (i) froth flotation followed by reflux classification of the tailings (process route 1), and (ii) reflux classification followed by froth flotation of the overflow (process route 2). Coal flotation, sulfide flotation and reflux classification were conducted on samples of Waterberg and Witbank coals, using a 3 L Leeds-type flotation cell and a 10 L batch reflux classifier constructed at NWU. Acid base accounting (ABA) and net acid generating (NAG) static characterization tests were performed on the products and feeds from all three process routes.
Open Access
Control and optimization of a multiple-effect evaporator
(2000) Smith, Patrick D; Swartz, Chris; Harrison, STL
Falling commodity prices have reduced the profit margins of Southern African sugar producers. Although these price falls have been severe, they reflect a long-term trend of reducing margins for basic commodity producers during the 20th Century. This trend has forced. producers to closely examine their processes and to look for areas in which improvements in productivity, yield and efficiency can be achieved. Evaporation is the most energy intensive unit operation in the sugar factory, and it is responsible for the removal of most of the water from sugar solution, or juice, which is extracted from the sugar cane. There is also a large potential to lose sucrose at the evaporators due to the high temperatures and long residence times employed there. The smooth control of the evaporators is thus vital to consistent factory operation, and the evaporators are commonly a sugar factory bottleneck. This study developed a control strategy for the particular evaporator configuration found at Triangle Sugar Mill in south eastern Zimbabwe. There are currently several evaporator control strategies being used in the sugar industry. Most of these are an assembly of single loop Proportional Integral Derivative (PID) controllers, which cannot optimally account for the interactions encountered in most evaporator stations. Ideally, any evaporator control system should be able to handle the multiple input multiple output problem while anticipating and handling constraints on inputs and outputs. Several multivariable approaches have been tried, but these usually require a great deal of expensive instrumentation.After a review of the multivariable control literature and testing of several alternative control systems, Dynamic Matrix Control (DMC) was chosen as the bestwsuited control algorithm for the Triangle control problem. A dynamic model of the Triangle evaporator station was, developed to formulate and test the DMC and other controllers. The model was based on a set of differential equations involving mass and energy balances through the evaporators. Real plant data were collected from the SCADA system and the model was tested against this data. After validation the model was. used to record step responses of the process to key input variables. The control system had nine (9) measurable inputs, and three (3) controlled outputs. The objective of the control system was to deliver the maximum amount of consistently high quality symp, within plant constraints. This was formulated in an objective function which seeks to minimize a weighted sum of the errors of syrup concentration from a setpoint, and the fluctuations in juice flowrates. Two alternative formulations were developed, and tested on the plant model.
Open Access
Design of an optimised fed-batch process for insulin precursor production in Pichia pastoris
(2011) Bhardwaj, Vinayak; Harrison, STL; Minnaar, Sanet
The increasing prevalence of diabetes worldwide has greatly increased the demand for insulin, a key type of treatment for many diabetics. For this purpose, the methylotrophic yeast Pichia pastoris has emerged as an additional microbial host for recombinant insulin production. A genetically modified Pichia pastoris MutS strain, engineered to produce the insulin precursor, was used as the experimental system in this study in order to optimise the insulin production process. The experimental system developed in this study employed a two-stage fed-batch feeding strategy in which growth was optimised by feeding glycerol to boost biomass followed by induction of the gene encoding insulin precursor by feeding methanol.
Open Access
Desulphurisation flotation for the selective removal of pyrite from coal discards using microorganisms
(2018) Msipa, Winfull Jaconia; Harrison, Susan; Fagan-Endres, Marijke
Mineral beneficiation processes such as base metal and coal mining produce large amounts of waste rock and coal discards that contain significant quantities of sulphide minerals with Acid Rock Drainage (ARD) generating potential. ARD is caused by the exposure of sulphide minerals, primarily pyrite (FeS2), to both water and oxygen, and microorganisms. This is a naturally occurring process, but the exposure of the sulphide containing mining wastes greatly accelerates ARD formation. Thus, ARD is a major issue associated with inactive mines, waste rock dumps and tailings impoundments, which over time presents a major environmental risk. The desulphurisation of coal discards, mine tailings and finely divided waste rock prior to their disposal has been proposed as a method of preventing ARD formation. This involves the selective separation of residual values from the waste rock, followed by selective separation of sulphide minerals – especially pyrite – from the residual waste material using a two-stage froth flotation to obtain a values stream, a low volume sulphide-rich concentrate that can be easily contained, and a high volume benign tailings fraction that can be safely disposed of. The technical feasibility of this two-stage process has been demonstrated; however, the cost of the flotation reagents used in this process are particularly high in comparison to the other operating costs, contributing as much as 75% of the operating costs for desulphurisation of coal fines. Furthermore, apart from being expensive, many of the inorganic flotation reagents are relatively toxic and could be hazardous to the environment due to their slow degradation rate. Microorganisms and their metabolic products have been identified in literature as potential reagents that can be used in the selective separation of sulphide minerals using froth flotation. Just like conventional chemical flotation reagents, the microorganisms assist separation through surface chemical alterations that modify a mineral’s hydrophobic properties, thus facilitating bioflotation. The aim of this study was to investigate the prevention of ARD formation through the desulphurisation of pyrite-containing coal discards and base metal hard rock samples using microbial cultures as alternative bioflotation reagents. In this study the feasibility of using P. polymyxa, R. palustris, R. opacus, B. subtilis, and B. licheniformis as biocollectors for the removal of pyritic sulphur in the second stage of the two-stage desulphurisation froth flotation process was investigated. Microbial screening tests were performed using a pyrite concentrate to assess each microbial culture’s affinity to pyrite and their ability to float the mineral in a batch flotation cell. Attachment experiments and batch bioflotation tests were carried out to screen for a microbial culture that showed potential. Following attachment experiments at pH 4 and pH 7, all microorganisms except B. licheniformis exhibited attachment to pyrite. The level of attachment was different for each microbial culture. P. polymyxa had the highest percentage attachment of 95.6 ± 1.0 % at pH 4 and 97.1 ± 0.7 % at pH 7 after 20 minutes of interaction. Subsequent results from the pyrite-only bioflotation tests revealed that R. opacus, R. palustris and B. subtilis did not affect the floatability of pyrite. P. polymyxa, however, showed a significant effect on the floatability of pyrite, achieving a cumulative mass recovery of 7.0 ± 0.42 % at pH 4 and 81.3 ± 0.4 % at pH 7. Zeta-potential tests revealed that P. polymyxa had the most neutral net surface charge across the pH range tested, while the other microorganisms had a large net positive or negative charge. Based on this result, it was deduced that the hydrophobicity of P. polymyxa as a consequence of its near neutral surface strongly made it seek out a surface to attach to rather than remaining suspended in water. Hence, P. polymyxa was chosen as the bio-collector candidate for the bioflotation separation of pyritic sulphur from coal discard and base metal hard rock samples. Despite the positive batch pyrite bioflotation tests, P. polymyxa was not successful for the flotation of pyrite from the coal discards nor did it upgrade pyritic sulphur to the concentrate, with the bioflotation results not significantly different from the negative control without collector. P. polymyxa did affect the floatability of the base metal hard rock, achieving cumulative mass recoveries comparable with the chemical control using PAX. However, there was no significant upgrade of pyritic sulphur content, with the biofloat achieving 22.6 % total sulphur in the concentrate which was significantly less than the 66.4 % total sulphur recovered with PAX. The study thus yielded positive results from fundamental studies of P. polymyxa’s ability to enhance the flotability of pyrite. However, tests using actual samples were less successful. Although P. polymyxa enhanced the floatability of the base metal hard rock, it did not achieve the aim of obtaining a low volume sulphide-rich concentrate as the PAX did. Recommendations for the continuation of this work include contact angle measurements and FT-IR spectroscopy to better understand the effects of P. polymyxa attachment, as well as performing a kinetic study on the growth of P. polymyxa alongside adaptation of the microbial culture to a pyrite mineral concentrate in order to test if this can improve selective flotation of the desired mineral owing to modified surface properties.
Open Access
Desulphurisation of fine coal waste tailings using algal lipids
(2018) Chiodza, Kudzai Godknows; Harrison, Sue; Fagan-Endres, Marijke
The South African economy is an energy-driven economy which relies on coal to meet most of its energy demands. Coal mining has resulted in the generation of coal waste over 60 million tonnes, annually. Apart from the huge footprint of this waste, the sulphide minerals contained in the waste have resulted in the generation of acid rock drainage (ARD). A lot of techniques have been developed to prevent and mitigate ARD, however most of these techniques have fallen short in terms of meeting their desired objectives due to the long-term nature of ARD generation which can persist for hundreds of years after mine closure. This has resulted in emphasis being put on long-term prevention techniques that remove ARD risk over treatment techniques. One prevention technique which has shown good technical potential is the two-stage flotation method developed for desulphurisation of hard rock tailings and coal fines, developed at the University of Cape Town. On desulphurising coal, the first stage produces an upgraded coal product that may be sold, with the second stage used to separate the tailings from the first stage into targeted high-sulphide and low-sulphide fractions which may then be appropriately used or disposed of. An economic assessment of the process showed across a wide range of coal wastes the high cost of oleic acid used in the first stage of the process as a collector was a major contributor to the operating costs. The investigation undertaken in this thesis looked at the potential of algal lipids and their derivatives as biocollectors to replace the oleic acid collector in the desulphurisation process at the laboratory scale. A review of cost was carried out for a process that used raw algal lipids (RALs) or fatty acid methyl esters (FAMEs), which are derived from RALs through transesterification. Batch flotation experiments were used to assess the performance of the two bioflotation reagents in comparison to oleic acid and dodecane, an alternative but less successful chemical collector. The algal lipids cost review was a desktop study which was done by adapting literature data from Davis et al. (2014) which focused on economic evaluation of algal lipid biofuels production pathways. Results from laboratory experiments for two different coal waste feed samples showed that the performance of RALs and FAMEs was similar to that of oleic acid for the sample that was high in ash and sulphur, and better than oleic acid for the sample that was low in ash and sulphur. For example, the product from Site 1 discards from Waterberg had 24.37% ash and 2.76% sulphur using FAMEs, 26.13% ash and 2.56% sulphur with RALs, and 23.48% ash and 2.41% using oleic acid, at a reagent dose of 2.8 kg/t for all reagents. For Site 2 waste tailings from the Witbank area, the product had 23.17% ash and 0.72% sulphur when FAMEs were used as collector, 22.75% ash and 0.75% sulphur with RALs, and 20.18% ash and 0.74% sulphur using oleic acid, at the same reagent dose. Discards from Site 1 had an initial ash and sulphur content of 47.61% and 5.71%, respectively. Site 2 waste tailings had 25.56% ash and 0.91% sulphur before flotation. Increasing biocollector dosage resulted in higher yields with a compromise on the upgraded coal quality. The pH tests showed that the performance of the two bioflotation reagents was best at pH 4 in terms of yield. However, increasing the pH of the process from the natural pH of the sample (pH 2.7) to 7 resulted in collection of more ash and sulphur, thus reducing the product quality. The algal lipids cost review showed that RALs and FAMEs were potentially 20 to 21% cheaper than oleic acid, with more room for improvement. Both the laboratory experiments and the technical evaluation showed that algal lipids and their derivatives have the potential to replace oleic acid in the two-stage desulphurisation process for coal waste to obtain a saleable quality coal product while simultaneously decreasing the impact of ARD from coal waste.

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