Browsing by Author "Petersen, J"

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    Ageing of chromium(III)-bearing slag and its relation to the atmospheric oxidation of solid chromium(III)-oxide in the presence of calcium oxide
    (Elsevier, 2003) Pillay, K; von Blottnitz, H B; Petersen, J
    Slag arising in ferrochromium and stainless steel production is known to contain residual levels of trivalent chromium. As the chromium is normally bound in the slag matrix in various silicate or spinel phases, and hence not easily mobilised, utilisation or controlled disposal of such slag is generally considered unproblematic. Experimental test work with a number of slag materials indicates, however, that very gradual oxidation of trivalent to hexavalent chromium does occur when the slag is exposed to atmospheric oxygen, rendering a quantifiable but small portion of chromium in this much more mobile and toxic form. Mechanisms and rates of the oxidation reaction were investigated in a number of long-term studies using both original slag materials and artificial mixes of chromium and calcium oxides. Powders of these materials, some of them rolled into balls, were left to age under different conditions for periods of up to 12 months. In the slag samples, which contained between 1 and 3 wt.% chromium, 1000–10 000 μg Cr(VI) were found per gram of chromium within 6–9 months of exposure to an ambient atmosphere. The rate of the oxidation reaction decreased exponentially, and the reaction could generally be said to have ceased within 12 months. In mixtures of calcium and chromium oxides the oxidation reaction is presumed to occur at the boundaries between chromium oxide and calcium oxide phases through diffusion of oxygen along the grain boundaries and of Cr3+ across the boundaries, resulting in the formation of calcium chromate. In the slags, where calcium and chromium oxide can form a solid solution, the oxidation is likely to occur at the exposed surface of grains containing this solution.
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    Open Access
    An agglomerate scale model for the heap bioleaching of chalcocite
    (Southern African Institute of Mining and Metallurgy, 2006) Ogbonna, N; Petersen, J; Laurie, H D G
    A mathematical model for bioleaching at the agglomerate scale is developed. The agglomerate is defined as a unit volume of a heap which contains a solid phase (a size distribution of ore particles), a liquid phase (stagnant and flowing leaching solution) and a gas phase (flowing air and air pockets). Leaching at the level of an ore particle is modelled using a diffusion-reaction equation, and oxidation by attached and detached microbes is treated separately. With the aid of the mathematical model, the effect of particle size distribution on the bioleaching of an ore containing a mixture of chalcocite and pyrite, and a culture of iron oxidizers, is investigated.
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    An Annual and Seasonal Characterisation of Winery Effluent in South Africa
    (South African Journal for Enology, 2011) Sheridan, C M; Glasser, D; Hildebrandt, D; Petersen, J; Rohwer, J
    Winery effluent is known to have a high chemical oxygen demand (COD) and a low pH. In this study, we extensively analysed effluent from two cellars and studied the temporal changes over the duration of a harvest and the duration of a year. We found that ethanol contributes approximately 85% to 90% of the COD of raw winery effluent, with acetic acid being the next significant contributor. The pH showed some dependence on the concentration of acetic acid. The concentration of sodium in the effluent is strongly dependent on the cleaning regime in place at the cellar, and the concentration of potassium has been shown to be linked to the spillage of juice, wine or lees. The data and correlations presented here could allow for an artificial effluent to be prepared easily for research purposes.
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    An annual and seasonal characterisation of winery effluent in South Africa
    (South African Society for Enology and Viticulture, 2011) Sheridan, C M; Hildebrandt, D; Glasser, D; Petersen, J; Rohwer, J  
    Winery effluent is known to have a high chemical oxygen demand (COD) and a low pH. In this study, we extensively analysed effluent from two cellars and studied the temporal changes over the duration of a harvest and the duration of a year. We found that ethanol contributes approximately 85% to 90% of the COD of raw winery effluent, with acetic acid being the next significant contributor. The pH showed some dependence on the concentration of acetic acid. The concentration of sodium in the effluent is strongly dependent on the cleaning regime in place at the cellar, and the concentration of potassium has been shown to be linked to the spillage of juice, wine or lees. The data and correlations presented here could allow for an artificial effluent to be prepared easily for research purposes.
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    Biooxidation kinetics of Leptospirillum ferriphilum under heap bioleach conditions
    (Metallurgy and Exploration Inc, 2008) Ojumu, T V; Harrison, S T L; Hansford, G S; Petersen, J
    Although the kinetics of biological oxidation of ferrous to ferric iron–the key step in any bioleaching process–have been studied for a variety of organisms, the focus has always been on conditions typical of tank-bioleaching. In heaps, parameters such as pH and temperature vary widely across the heap and are usually far from optimal. Total iron concentrations are usually much lower than the tanks(except in gold heaps), and the presence of dissolved gangue mineral is usually much more significant.
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    Carbon dioxide and oxygen consumption during the bioleachingof a copper ore in a large isothermal column
    (Elsevier, 2010) Petersen, J; Minaar, S H; du Plessis, C A
    During large-scale column tests at BHP Billiton's Johannesburg Technology Centre (JTC) on a low-grade copper ore during 2005/6, the concentrations of both oxygen and CO2 were continuously monitored in feed and exit gas as well as at various intermediate positions over the height of the column. This paper describes results from a test run at 40 °C fed with an air stream enriched to between 1000 and 2000 ppm CO2. Oxygen consumption very closely tracks iron and copper leaching. CO2 is consumed rapidly from the bottom up, resulting in significant depletion midway through the column, even though an enriched feed was used. Oxidation rates decline in CO2 depleted zones, but were not observed to cease completely. This decline is postulated to be linked to a slowly decaying population unable to regenerate itself, and a relative rate of decay in the absence of oxygen has been estimated to be around 3%/day. A comparison between O2 and CO2 consumption rates shows a linear correlation beyond a minimum oxidation rate. This minimum rate corresponds to a non-growth maintenance energy requirement, and the slope of the linear correlation to the growth yield. Both are functions of available CO2 in the range 50 to 1000 ppm, with maintenance declining and yield increasing. The findings of this study imply that CO2 supplementation in bioheaps will stimulate microbial growth and CO2 consumption, but not necessarily increase the rate of oxygen uptake and hence leaching. Absence of CO2 is expected to result in gradual population decline, but a degree of oxidation continues on the basis of maintenance. In tall heaps, CO2 depletion with height is likely and may therefore result in impaired leaching in the upper zones.
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    Determination of oxygen gas-liquid mass transfer rates in heap bioleach reactors
    (Elsevier, 2010) Petersen, J
    A detailed experimental study is described which was conducted to determine the rate of oxygen gas–liquid mass transfer within the packed bed of heap bioleach reactors at different temperatures (22–68 °C), using the Na2SO3 method. The raw data was analysed using a simplified film mass transfer model, making corrections for oxygen solubility in concentrated solution and for increased water vapour partial pressure at elevated temperatures. The results compared favourably against two independent experimental leach studies, indicating kLa values between 33 and 46 h−1. The value varied with the particle size distribution of the packing, with kLa assuming larger values for those packings that had a higher fines contents. While kLa increases with temperature, the solubility of oxygen decreases simultaneously, resulting in net mass transfer rates being relatively unaffected by temperature in the range studied. This indicates that thermophile heap bioleach reactors are likely to operate under gas–liquid mass transfer limitations, especially at high altitude.
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    The influence of applied potentials and temperature on the electrochemical response of Chalcopyrite in Bacterial Leaching
    (Elsevier, 2002) Tshilombo, A F; Petersen, J; Dixon, D G
    Electrochemical techniques were conducted to clarify the role of solution potential and temperature under a variety of experimental conditions similar to those found during the mesophilic and thermophilic biooxidation of chalcopyrite (CuFeS2). Despite a large number of publications dealing with the bacterial leaching of CuFeS2, three central aspects remain unclear: How to dissolve preferentially copper from CuFeS2, the effect of temperature on the extent of CuFeS2 passivation, and the behavior of ferric ions on a polarized CuFeS2 surface. Anodic characteristics showed that CuFeS2 passivation was more severe in the potential range 0.45–0.65 V saturated calomel electrode at 25 C. However, there was no evidence of CuFeS2 passivation at higher temperatures (45 and 65 C). Cu was preferentially dissolved from CuFeS2 at lower potentials and high temperatures at a ratio copper to iron of about 3:2. Cathodic characteristics showed that the ferric ions inhibited the leaching process when the CuFeS2 surface was polarized at high potentials and low temperatures.
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    Open Access
    Investigation of particles with high crack density produced by HPGR and its effect on the redistribution of the particle size fraction in heaps
    (Elsevier, 2013) Ghorbani, Yousef; Petersen, J; Becker, Megan; Mainza, Aubrey Njema; Franzidis, Jean-Paul; Kalala, JT
    The application of comminution technology such as the High-pressure grinding rolls (HPGR), which is able to generate a high density of cracks in the ore particles, is favourable for leaching processes. Extraction of metallic values by the heap leach process, can take place on the particles with partial exposure of mineral grains, if it can provide sufficient surface front for chemical attack by leaching solution. The aim of this study was to assess the benefits of high crack density in the ore particles produced using the HPGR and how it could diminish due to inadequate percolation of the leaching agent. A zinc ore was comminuted using HPGR at three different pressure settings and with a cone crusher for the control experiment. Subsamples from the (+23/-25, +14/-16, +5.25/-6.75 mm) size fractions were characterized and packed into leach reactors. The reactors were stopped from time to time to investigate the progress of crack and micro-crack growth and its effect on metal extraction using the X-ray computed tomography (CT). The results are validated with those obtained using traditional techniques such as SEM and QEMSCAN. Investigation of the leach reactors residue indicated significant changes in the particle size distribution (PSD) of initial feed toward the fine size fraction. The residues from the reactors leaching the material prepared using the HPGR product contained more fine particles than the reactors, which were fed by cone crusher product. These differences were up to 10.3%.
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    Open Access
    Investigation of the effect of mineralogy as rate-limiting factors in large particle leaching
    (Elsevier, 2013-10) Ghorbani, Yousef; Becker, Megan; Petersen, J; Franzidis, Jean-Paul
    Although heap leaching is by now well established in the mining industry, the process remains limited by low recoveries with different rate-limiting factors that are not clearly understood. In this study, three large particle size classes (+19/-25, +9.5/-16, +4.75/-5 mm) were prepared from a sphalerite ore by two different methods of comminution (HPGR and cone crusher). The particles were then packed into leach reactors that were operated continuously for eleven months with well-mixed internal circulation of the leach solution. Characterization of the residue of the leach reactors indicated that there are areas within the ore particles where although sphalerite grains are accessible to the solution, they remain unreacted. X-ray tomography and QEMSCAN® analysis of the selected samples before, during and after leaching, showed increased leaching of sphalerite grains associated with pyrite due to galvanic interactions. Mineral chemistry (Fe, Mn content of sphalerite) and jarosite precipitation were also investigated as factors influencing sphalerite leaching.
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    The kinetics of ferrous-iron oxidation by Leptospirillum ferriphilum in continuous culture: the effect of pH
    (Elsevier, 2011) Ojumu, T V; Petersen, J
    The kinetics of ferrous ion oxidation by Leptospirillum ferriphilum were studied in continuous culture with a focus on the effect of solution pH (pH 0.8–2.0), assuming that the effect of pH on cell metabolism can be independently studied of reactor context and other reactions common in bioleach heaps. A simplified competitive ferric ion inhibition model and the Pirt Equation were used to analyze the experimental data. The results showed that the maximum specific activity of L. ferriphilum has a symmetrical bell-shaped curve relationship with pH. The maximum specific ferrous-iron oxidation rate,qFe2 +maxgave a highest value of 14.54 mmol Fe2+(mmol C h)− 1 at pH 1.3, and was described by a quadratic function. The steady state carbon biomass in the reactor and the apparent affinity constant, K′Fe2 +, also increased with increase in pH; however, a slight increase in the carbon biomass was observed beyond pH 1.6. The results also showed that ferric ion precipitation is significant beyond pH 1.3 and about 13% total iron from the feed was lost at pH 2.0. The maximum biomass yield increased linearly with pH, while the culture maintenance coefficient was significantly small in all experiments and was minimum at pH 1.3. The values are indicative of actively growing chemostat cultures. This study shows that microbial ferrous ion oxidation by L. ferriphilum may be sustained at pH lower than pH 0.8 as the microbial activity is much higher than reported values for common mesophilic acidophiles. This may have implications on how bioleach heap operations can be started-up to improve metal recovery.
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    The kinetics of ferrous-iron oxidation by Leptospirillum ferriphilum in continuous culture: the effect of temperature
    (Elsevier, 2009) Ojumu, T V; Hansford, G S; Petersen, J
    A typical bioleach heap is characterized by wide variation of temperature across the heap bed, leading to oxidation of target minerals occurring at different rates. Previous studies on the effect of temperature on the microbial oxidation of ferrous-iron were limited to a narrow range of temperatures (30–40 °C) near optimum conditions and mostly toAcidithiobacillus ferrooxidans. The kinetics of ferrous-iron oxidation by Leptospirillum ferriphilum were studied in continuous culture. In this paper we focus on the effect of temperature (18–45 °C) on these kinetics. The study was based on the assumption that the effect of temperature can be studied independently of other, equally important factors such as pH, dissolved salts, etc. and independent of the reactor context. The experimental data were correlated using both, a simplified ferric-iron inhibitory model and the Pirt Equation. The results showed that the maximum specific ferrous-iron oxidation rate, increased with increasing temperature to a maximum at 42 °C. This trend can be described adequately by the Arrhenius Equation with an activation energy, Ea of 34.46 kJ mol−1 and frequency factor,K0 of 1.05 × 107 mmol Fe2+(mmolC)−1 h−1. An increase in temperature slightly reduces the steady state carbon biomass in the reactor, while the apparent affinity constant, K′Fe2+ increases. The investigation further suggests that at low temperature (18 °C) and beyond the maximum temperature (42 °C), the culture cannot be sustained in a continuous mode. The maximum biomass yield followed a linear decline with increasing temperature, while cell maintenance on ferrous-iron followed a quadratic trend, although the small values indicates that it is not significant, as would be expected in continuous culture. The results indicate that L. ferriphilum is likely to perform optimally, at warm temperatures (25–42 °C) in heap bioleach operations before being taken over by thermophiles at higher temperatures.
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    Microbial leaching of a low-grade sphalerite ore using a draft tube fluidized bed bioreactor
    (Elsevier, 2009) Soleimani, M; Hosseini, S; Roostaazad, R; Petersen, J; Mousavi, S M; Vasiri, Kazemi A
    The use of a draft tube fluidized bed bioreactor (DTFBB) has been successfully demonstrated for the bioleaching of a chalcopyrite concentrate (Mousavi, et. al., 2005). In the present paper this technique was used for the bioleaching of zinc from a sphalerite bearing low-grade ore. A strain of the thermophilic bacterium, Sulfobacillus, has been isolated from the Kooshk lead and zinc mine near the city of Yazd (Iran) and is tested at temperatures 47 °C–72 °C. This is compared to leaching with a strain of the mesophileAcidithiobacillus ferrooxidans, which has been adapted to the high levels of zinc and the presence of the low-grade ore, and which was tested at temperatures 18 °C–42 °C. The effects of using the two temperature ranges, as well as varying pH in the range 1.2 to 1.8, and solid concentration in the range 1–20% w/v, on Zn recovery, ferrous, ferric and total iron concentrations and cells number were investigated in two batch DTFBBs with external recycling streams. The best conditions for leaching by mesophilic and thermophilic strains occurred at 10% w/v pulp density, giving 87% and 91.4% Zn recovery respectively, after 9 days.
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    Open Access
    Modelling and Assessment of the Long-term Leachate Generation Potential in Deposits of Ferro-chromium Slags
    (The Southern African Institute of Mining and Metallurgy, 2000) Petersen, J; Petrie, J G
    The environmental impact of solid waste materials is closely linked to their potential to produce a hazardous leachate once deposited in a landfill. This potential is conventionally assessed with simple laboratory batch leach tests, which find more and more frequent application also in the minerals and metals industry. Such tests are intended to help distinguish between high and low risk wastes, but are completely unsuitable to allow quantitative prediction or modelling of the long-term leachate generation potential. In the present approach the leachate generation process is perceived as a combination of bulk convective transport, stagnant zone and inner particle diffusion, as well as reaction kinetic effects. Model equations are formulated accordingly. A modelling framework, WASTESIM, has been developed at UCT, which allows flexible combination of the model components and simulation of leach scenarios. Quantitative model parameters are obtained from a systematic laboratory study on the waste material in question. The combined assessment and modelling approach is demonstrated with a study on Cr(VI) leaching from a ferro-chromium slag material. Although not straightforward, the approach is particularly useful in determining long-term liabilities in the context of waste disposal sites with considerably more reliability than has previously been possible. Keywords: waste deposits, waste leaching, heap leaching, environmental, risk assessment, Cr(VI), ferro-chromium, slag.
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    Modelling of bioleach processes: connection between science and engineering
    (Elsevier, 2010) Petersen, J
    This paper gives a brief introduction to the modelling of bioleach processes, developed from a careful analysis of the fundamental process steps at the gas–liquid, biological and mineral interfaces, and how these interact in a given reactor environment (tanks and heaps). The insights gained from such modelling work can guide both engineers in the optimisation of processes and scientist in directing their research at areas not yet well understood. From this perspective, some future directions of the bioleaching field are discussed.
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    Modelling zinc heap bioleaching
    (Elsevier, 2007) Petersen, J; Dixon, D G
    A comprehensive modelling study of the HydroZinc™ heap bioleach process, using the HeapSim modelling tool, is described. The model was calibrated on the basis of a small number of column leach experiments and compared against pilot heap test results. The model calibration thus confirmed, a detailed sensitivity study was conducted in order to establish the key parameters that determine the overall rate of Zn extraction. In the present case these were found to be oxygen gas–liquid mass transfer, various factors affecting the delivery of acid into the heap, and factors affecting the temperature distribution within the heap. A set of improved design parameters are proposed that would almost double the zinc conversion rate measured in the pilot plant – from 83% in 740 days to 78% in 383 days – and increase zinc production rate from 1.77 to 4.35 kg/m2/day. However, this improvement must be evaluated in the context of various implications for the downstream process.
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    A review of rate equations proposed for microbial ferrous-iron oxidation with a view to application to heap bioleaching
    (Elsevier, 2006) Ojumu, T V; Petersen, J; Searby, G E; Hansford, G S
    In view of the fact that the microbial oxidation of ferrous iron to the ferric form is an essential sub-process in the bioleaching of sulphide minerals, the development of a comprehensive rate equation for this sub-process is critical. Such a rate equation is necessary for the design and modelling of both tank and heap bioleach systems. Most of the rate equations presented in the literature define the specific microbial growth rate using a Monod-type form for ferrous substrate limitation, with further terms added to account for ferric product inhibition, ferrous substrate limitation and inhibition. A few of the published rate equations describe the specific substrate utilization rate in terms of a modified Michaelis–Menten equation and include the maximum yield constant and cell maintenance via the Pirt equation. Other rate equations are based on chemiosmotic theory or an analogy with an electrochemical cell. In the present paper a selection of rate equations are compared against each other by calibrating them against the same set of data and comparing the fits. It was found that none fits the data particularly well and that some of the underlying assumptions need to be questioned. In particular, it appears that ferric inhibition is perhaps not as significant a factor than previously assumed and that rate control by the availability of ferrous is more significant. Some rate equations include terms to account for the effects of temperature, pH, biomass concentration, ionic strength as well as inhibition due to arsenic. In general these effects have been studied in isolation and in ranges not too far off the optimum. Few rate equations combine more than 2 effects and there is no clarity on how a comprehensive model to account for all effects should be constructed. Rate equations have been applied to tank bioleach systems, which usually operate under controlled conditions near the optimum. Heap bioleach systems, on the other hand, often operate far from optimum conditions with respect to temperature, pH, solution conditions, etc., at the same time. The kinetics of such sub-optimal systems are still poorly understood. Future studies should be directed towards the development of a comprehensive rate equation useful for describing the kinetics of heap bioleaching over a wide range of conditions.
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    Open Access
    Thermophilic Heap Leaching of a Chalcopyrite Concentrate
    (Elsevier, 2002) Petersen, J; Dixon, D G
    Thermophiles have been shown to be the only micro-organisms to leach chalcopyrite successfully. Heap leaching may be a feasible alternative to conventional bio-reactors, providing a high temperature environment can be maintained within the heap without external heating. In the present study thermophilic heap leaching of a chalcopyrite concentrate coated onto inert support rocks (the GEOCOAT™ process) was studied in sets of small heated columns. The temperature was gradually increased to 70 °C, while successively introducing various mesophile and thermophile cultures. Individual columns were dismantled after progressively longer leach periods and the residual concentrates analysed. Copper extractions in excess of 90% were achieved within 100 days. On the basis of head and residue analyses the rate of reaction heat generated was calculated. A comprehensive heap heat conservation model was used to determine whether the experimental temperatures can be achieved and maintained in a full scale heap. Results indicate that operating hot heaps successfully is possible within a certain range of process parameters.