Browsing by Author "Govender, Elaine"
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- ItemRestrictedEffect of physico-chemical and operating conditions on the growth and activity of Acidithiobacillus ferrooxidans in a simulated heap bioleaching environment(Elsevier, 2015-05) Govender, Elaine; Bryan, Chris; Harrison, SueRecent understanding of microbial retention within heap bioleaching systems has highlighted the importance of quantifying microbial growth and activity in both the bulk flowing solution and in the ore-associated phases. Typically, industrial heap bioleaching operations report variations in process conditions such as inoculum preparation and concentration and elevated copper concentrations in the recycled irrigation solution. In this paper, a mini-column reactor system containing pre-constructed and agglomerated, low-grade ore samples representing grab samples from a larger heap, were used to investigate the effect of a selection of physico-chemical and operating conditions on microbial growth, colonisation and substrate utilisation kinetics, considering both the planktonic and sessile populations of Acidithiobacillus ferrooxidans. The factors studied included inoculum size, inoculum cultivation conditions, availability of ferrous iron in the bulk flowing solution and copper concentration in the bulk flowing solution. The microbial population in the interstitial phase, i.e. associated with, but not bound to, the ore, remained the most abundant within the heap under all physico-chemical conditions considered. A comparison of the tests with different inoculum sizes found that a smaller inoculum size resulted in an increased delay in microbial growth and ferrous iron oxidation, but similar apparent maximum specific growth rates and iron oxidation rates. In contrast to the microbial culture grown on pyrite, a delay in microbial activity was observed for the culture grown on ferrous iron. However, greater microbial cell densities were reached, in the interstitial and attached phases compared with the pyrite grown culture. The introduction of 6 g L-1 cupric ions into the feed solution containing 0.2 g L-1 ferric iron resulted in decreased microbial growth rate in the interstitial phase but not in the attached phase. Where the pyrite culture was pre-exposed to cupric ion, the microbial growth rate in the interstitial and attached phases was significantly enhanced. Nevertheless, the presence of cupric ion in the irrigation solution resulted in a decrease in microbial ferrous iron oxidation rate, irrespective of pre-culture conditioning. This study emphasises the important role played by the stagnant interstitial phase during the colonisation of a low-grade heap, particularly under adverse conditions for microbial growth and activity. It also highlights the role of inoculum culture conditions on the potential trade-off between increased heap colonisation and increased lag periods in microbial activity during heap start-up.
- ItemRestrictedA novel experimental system for the study of microbial ecology and mineral leaching within a simulated agglomerate-scale heap bioleaching system(Elsevier, 2015-03) Govender, Elaine; Bryan, Chris; Harrison, SueHeap bioleaching systems are complex, with multiple sub-processes interacting at various scales within the heterogeneous reaction environment. This provides a challenge to determining the growth characteristics of micro-organisms and reaction characteristics of the mineral ore in a representative environment. The experimental system presented in this paper was designed to simulate heap bioleaching conditions using multiple, identically constructed agglomerate-scale mini-column reactors. Ore samples were prepared representatively as grab samples of a larger heap. Particle size distributions and agglomerate masses of the prepared ore samples were shown to be similar within acceptable variance and provided comparable surface areas for microbial colonisation and chemical reaction. The microbial abundance within the whole ore system was determined from effluent sampling for the planktonic population and the systematic and sequential sacrifice of identically operated mini-column reactors to determine the change in the ore-associated microbial population with time. Microbial colonisation and growth rate kinetics were determined from analysis of these populations. The growth curves obtained for the bulk flowing solution and ore-associated populations at the base case operating conditions were reproducible, within a 95 % confidence interval.
- ItemRestrictedQuantification of growth and colonisation of low grade sulphidic ores by acidophilic chemoautotrophs using a novel experimental system(Elsevier, 2013) Govender, Elaine; Bryan, Christopher G; Harrison, Susan T LMicrobial colonisation of low grade sulphidic ores and subsequent growth in heap bioleaching systems has not been quantified rigorously. In this study, an experimental system simulating the sub-processes that occur at the agglomerate scale was used to quantify the colonisation, growth and propagation of Acidithiobacillus ferrooxidans in an unsaturated bed of crushed and agglomerated low grade chalcopyrite ore. The relative distribution of the microorganisms in the flowing leachate solution (‘PLS’), the stagnant interstitial liquid and weakly and strongly attached to the mineral surfaces was determined at various time points during the leach. There was a distinct difference in population dynamics in each of these discrete phases. The microbial cells present in the interstitial phase dominated the microbial population in the ore bed. Particularly, the microbial concentration in the free flowing PLS was found to be a poor representation of the ore-associated microbial population. The calculated growth rate of At. ferrooxidans in the PLS was unreasonably high when modelled as a continuous system, indicating that change in cell concentration in the PLS was dominated by transfer from the mineral ore associated population. However, the transfer rate was not correlated directly to changes in either the interstitial or attached population sizes. Therefore, unless transfer rates can be accounted for, PLS population dynamics do not accurately represent those in the column as a whole. Growth rates of microorganisms in the interstitial, weakly mineral-attached and strongly mineral-attached phases better predicted growth of At. ferrooxidans on the whole ore system owing to the dominance of the microbial location in these phases.
- ItemOpen AccessA study of permeability and diffusion at the agglomerate-scale in heap (bio)leaching systems(Trans Tech Publications, 2015-11) Govender, Elaine; Kotsiopoulos, Thanos; Harrison, SueMultiple mini-column reactors, loaded with identically constructed ore samples representing grab samples of a larger heap, were used to study the behaviour of solution tracers to elucidate solution diffusion, dispersion and transport. The tracers were either introduced to the ore bed as a pulse, included during agglomeration of the ore or introduced to the system by submerging the ore bed. These methods of tracer introduction allowed for the characterisation of flow interchange in unsteady state systems. The resulting concentration-time distribution curves were analysed to allow characterisation of flow dispersion and diffusion, which facilitates exchange between the fast flowing and largely stagnant liquid phases. Preliminary results support the presence of distinct stagnant and flowing regions within the agglomerated ore bed. Agglomeration with the tracer promotes increased retention on the ore; potentially enhancing microbial transport via increased solution exchange after the initial period of attachment.