Browsing by Author "Harrison S T L"

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    Determining the effect of acid stress on the persistence and growth of thermophilic microbial species after mesophilic colonisation of low grade ore in a heap leach environment
    (Elsevier, 2013) Tupikina, O V; Minnaar, S H; van Hille, R P; van Wyk, N; Dew, D; Harrison S T L; Rautenbach, G F
    The microorganisms involved in the bioleaching of sulphidic mineral ores are acidophilic. Generally, a pH in the range of pH 1–2.5 is applied for optimal growth in these systems. In operating heaps, perturbation of conditions could result in changes in the pH outside this “safe” window, so an understanding of the effect of changes in pH on growth and activity of bioleaching microbes is needed. Previous work has shown that some microorganisms e.g. Acidithiobacillus thiooxidans, Leptospirillum ferriphilum and Leptospirillum ferrooxidans are able to adapt to low pH environments (∼pH 0.9). However, most studies on the response of micro-organisms implicated in mineral bioleaching to pH have been conducted under submerged, aerated culture conditions, with limited performance-based studies conducted under conditions mimicking a heap environment. In this study, the effect of acid stress on the persistence of the thermophilic micro-organisms in the ore bed inoculated at mesophilic conditions and their subsequent growth on reaching thermophilic conditions is considered. Following inoculation, five columns loaded with a low grade chalcopyrite ore were irrigated at a feed pH of 1.7 at 25 °C. After a few days, the temperature was sequentially increased from 25 °C through 37 °C to 50 °C, resulting in an Eh above 850 mV across all columns. The irrigation feed pH was then varied across the range pH 1.0 to 1.7 at 50 °C. Eh values greater than 800 mV could be attained in the columns with feed pH values between pH 1.2 and pH 1.7 at 50 °C. The Eh of the column receiving feed solution at a pH of 1.0 at 50 °C dropped to below 700 mV and did not recover. The temperature was then increased gradually to 60 °C. All the columns with feed pH of 1.2 and higher achieved Eh values above 800 mV. Quantitative analyses of the microbial community on selected PLS and ore samples indicated that lower pH affected the persistence of the thermophilic micro-organisms in the ore bed and their subsequent growth on reaching thermophilic conditions. The microbial population detached from the ore sample after 120 days decreased by a factor of 5–15 and 25–100 fold on decreasing the operating pH from 1.5–1.7 to 1.4 and 1.2 respectively. Poor microbial activity was found at pH 1.0, suggesting ineffective growth or persistence of the archaea.
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    The effect of the particulate phase on coal biosolubilisation mediated by Trichoderma atroviride in a slurry bioreactor
    (Elsevier, 2008) Oborien, B O; Burton, S G; Cowan, D; Harrison S T L
    Low rank coal is currently under-utilised because of its low calorific value and high moisture and sulphur content. Its solubilisation by both bacterial and fungal cultures has been reported, the latter more commonly. Coal biosolubilisation processes have potential to convert low rank coal to either a clean, cost-effective energy source or complex aromatic compounds for biocatalytic conversion to value-added products. This can lead to an increased utilisation of low rank coal. In this study, the key variables of the slurry that affect biosolubilisation of low rank coal by Trichoderma atroviride in submerged culture were investigated. Results showed that the key operating variables that influence coal biosolubilisation in the slurry bioreactor are coal loading and particle size affecting available surface area. These factors affect the surface area available for coal biosolubilisation. The optimum coal loading occurred between 5 and 10% (w/v); an increase above this optimum led to inhibition of the fungal culture of T. atroviride (ES11) by fragmentation of the fungal mycelium. A decrease in particle size fraction led to an increase in the degree of coal solubilisation. Coal biosolubilisation was shown to increase 4-fold when particle size was decreased from 600–850 μm to 150–300 μm. A 28% biosolubilisation of coal of 150–300 μm, characterised by a surface specific area of 2.17 cm2 g−1 , was measured as coal weight loss over 14 days at solids loading at 5%. This can be compared with a 7.8% coal weight loss at 600–850 μm diameters (0.54 cm2 g−1 ). Soluble phenolic compounds are not a significant product of the coal biosolubilisation process. The change in pH observed in the presence of both coal and fungi was independent of coal loading and was not directly related to the extent of coal solubilisation. While soluble intermediates were observed as total organic, further metabolism resulted in complete oxidation of a significant fraction of the coal to CO2.
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    The use of mini-hydrocyclones for differential separations within mineral slurries subjected to  bioleaching
    (Elsevier, 1997) Harrison S T L; Cilliers J J
    Mini-hydrocyclones were used to effect differential separation of a leached slurry of sulphidic ore from a continuous bioleaching process. Separations of the sulphidic ore, other non-biological solids as well as free and attached bactetia were studied. The volumetric flow split was found to be a power law function of the ratio of the spigot to vortex finder diameters. The mass recovery was shown to be a function of the outlet diameters individually, when treating a feed of the same size distribution. The recovery of bactetial activity was directly related to the mass recovery, except at high solids concentrations in the unde$ow stream. Following passage through a two-stage circuit of mini-hydrocyclones, it was shown that a significant upgrading of the solids concentration could be attained, enhancing downstream processing such as cyanidation. In addition, the preferential upgrading of one stream with respect to bacterial activity (measured in terms of oxygen utilisation rate) was demonstrated. This holds potential in the recycling of bacteria to the continuous bioleaching tanks to retain a higher biomass concentration ana’ improve process robustness. 4, recycle of a low volume stream with reduced solids concentration, plant capacity may be improved.