Browsing by Author "Minnaar, S H"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Restricted
    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.
  • Thumbnail Image
    Item
    Restricted
    Effect of inoculum size on the rates of whole ore colonisation of mesophilic, moderate thermophilic;thermophilic acidophiles
    (Elsevier, 2014) Tupikina, O V; Minnaar, S H; Rautenbach, G F; Dew, D W; Harrison, S T L
    Bioheap leaching of low grade copper sulphides has been applied successfully at the commercial scale for the extraction of copper from secondary sulphide minerals. It is important to optimise the inoculation of heaps in order to minimise the residence time required for the heap and to maximise extraction. Thermophilic bioleaching of the primary sulphide chalcopyrite poses an additional challenge of rising temperatures inside the heap demanding microbial succession. After heap start up, rising heap core temperatures make conditions less favourable for mesophilic microbial species, and the moderately thermophilic community succeeds them in the consortium. In turn, thermophilic microorganisms succeed the moderately thermophilic microbes as the higher temperatures are reached. A detailed understanding of the microbial colonisation of whole ore is necessary to optimise microbial succession during thermophilic bioleaching, as is that of microbial growth kinetics on whole ore. Most published research is focused on microbial growth rates of bioleaching organisms in liquid cultures; little work is reported on microbial colonisation of whole ore and subsequent microbial activity. To extend the information available on the microbial diversity and succession in a bioleaching habitat, a study of bioleaching microbes colonising the ore body is required. The aim of this work was to explore aspects of colonisation of low grade chalcopyrite ore at 23 °C, 50 °C and 65 °C by acidophilic micro-organisms. Laboratory column packed bed reactors were designed to simulate heap leach environments and to provide a systematic way of studying microbial dynamics on whole ore. The effect of inoculum size and inoculation strategies on microbial activity established and the subsequent leaching performance were investigated under conditions that support mesophilic, moderately thermophilic and thermophilic microorganisms. A relationship was shown between the inoculum size and the culture time required to achieve Eh values greater than 700 mV, especially at 23 °C and 65 °C. However, the culture time required to establish an active iron- (and sulphur-) oxidising culture was also influenced by ore type, irrigation rate and inoculum adaptation. The effect on effluent Eh, pH and dissolved iron levels is also discussed.
  • Thumbnail Image
    Item
    Restricted
    Microbial colonisation in heaps for mineral bioleaching and the influence of irrigation rate
    (Elsevier, 2012) Chiume, R; Minnaar, S H; Ngoma, I E; Bryan, C G; Harrison, S T L
    Microbial colonisation is important for mineral dissolution in heap bioleaching of low grade ore. Colonisation studies to date have focused on the microbial attachment of single species to mineral concentrates in batch and flow systems. Hydrology and soil engineering studies suggest interaction between microbial colonisation and fluid flow in porous systems that result from solution-ore and microbe-mineral contacting (Wan et al., 1994 and Yarwood et al., 2006). The effect of the irrigation rate on microbial colonisation was assessed using columns packed with acid agglomerated low grade copper-containing ore. Continuous flow, unsaturated, aerated bed reactors were inoculated by pulse irrigation with iron and sulphur oxidising mesophilic microorganisms (1012 cells/ton ore), followed by operation at irrigation rates of 2, 6 and 18 l/m2/h. A novel in-bed sampling technique allowed the extraction of ore samples from the bed during the leaching process. Novel insights regarding microbial growth, interstitial and weakly and strongly attached microbial populations were obtained. Bacterial adherence and cell number retained in the ore bed increased over the 32 day leaching period. Average specific growth rates of ore-associated micro-organisms of 0.161 ± 0.0045, 0.155 ± 0.026 and 0.120 (± 0.00) 1/h were obtained at 2, 6 and 18 L/m2/h respectively. Faster colonisation occurred at lower irrigation rates. At higher irrigation rates, higher detachment and cell removal were apparent, based on PLS cell numbers. The interstitial cells from the stagnant fluid in the ore bed formed the dominant contribution to the microbial population within all the heap systems.