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Investigating the effect of acid stress on selected mesophilic bioleaching microorganisms
Heap bioleaching is a microbially-assisted hydrometallurgical metal extraction process in which metals are solubilised from low grade ore by ferric iron and acid leach agents. Particularly for low grade ores, heap bioleaching provides several advantages over conventional technologies, it is simpler and safer to operate with low capital and operation cost and acceptable recoveries. Key challenges with heap bioleaching systems include the need to minimise leach durations and optimise both rate and extent of metal recovery. There is limited understanding of the sub-processes involved in microbially assisted bioheap leaching (nutrient transport, microorganisms attachment to mineral, effects of metal concentration on microorganisms viability, …). Mineral ore agglomeration is a pre-treatment process typically carried out in the setup of the bioheap operation to dissolve metal oxide residue, neutralise acid consuming gangue, prepare agglomerates of the mixed particle size fraction to optimise heap permeability, prepare the ore surface for microbial attachment and optimise moisture content and mineral exposure to leaching reactions. Most agglomeration processes are carried out with an acidic solution. This may create an acid stress condition for the bioleaching microorganisms when inoculated into the operation. This is particularly relevant when the inoculum is introduced during the agglomeration process and may remain under the highly acidic conditions for a protracted time. However, quantitative data on the recommended acid concentration during agglomeration processes is very limited and is dependent on the ore treated. Similarly little is reported on the response of the acidophilic bioleaching microorganisms to acid stress. This research project contributes to the bioleaching knowledge base by providing an understanding of the effect that acid stress has on the mesophilic species typically implicated in mineral sulphide bioleaching as a function of acid concentration (and resulting acidity) and duration of exposure. The study addresses the following specific key factors: The effect of acid stress due to acid concentration and exposure time on performance on mesophile microorganisms in terms of the microbial and ferrous iron oxidation. The interaction of acidity and exposure time with respect to microbial stress on the mesophilic bioleaching system performance. The nature of the stress response observed i.e. only the lag period or also the rate of ferrous iron and sulphur oxidation on the initiation of the leaching process. The observed effects on microbial activity mediated through the number of active cells or through the activity of these cells. Quick fit stirred tank reactors (STR) containing 3% pyrite concentrate and 1 litre Norris media (Norris, 1983), aerated with 2 L.min-1 compressed air and stirred at 550 rpm were inoculated with a mixed mesophilic culture mainly Acidithiobacillus ferrooxidans, Acidiplasma cupricumulans, Ferroplasma acidiphilum, and predominantly Leptospirillum ferriphilum following its pre-stress at 0.34M, 0.51M and 0.68M acid (H2SO4), whilst operating as a batch system. A Control, inoculated with an un-stressed culture, was run concurrently. The cultures were subjected to these acid stresses for a period of one hour, three hours and 24 hour and assessed for microbial growth and activity, leaching performance and microbial speciation. Findings showed an increasing period necessary for microbial recovery with increased acid stress and increased time exposure. A similar leaching performance to the Control were recorded soon after the cultures recovered from the stress when the acid concentrations used were low, but the highest acid concentration (0.68M) combined with the longer exposure time (24 h) compromised the overall leaching performance and the required time of recovery was extended to as high as 200 h. Equally the microbial growth rates were similar to that of the Control culture following the recovery period. The yield in terms of microbial cells produced per kg iron oxidised decreased with increased acid stress but not necessarily with increased exposure time. The extent of iron solubilisation, at the time the Control achieved its highest solubilisation, decreased with both increases in acid stress concentrations and in exposure time. Microbial speciation indicated that four of the initial six species in the mixed culture were sensitive to acid stress. Only three species survived the stress in the early stages of the experiment and one specie disappeared during the course of the leaching experiment leaving just two species surviving. Of the two surviving species, Fe. acidiphilum and L. ferriphilum, the latter dominated to a final ratio of 99% to 1%. Some recommendations have been made for future studies, namely: Acid stress effects should be tested on simulated heap leaching experiments using agglomerated ore. Acid concentration and exposure time should be increased to assess the extent of microbial recovery and acid tolerance levels. Similar experiment should be conducted using moderate thermophile and thermophile cultures. A conglomerate of a more defined mixed culture should be used to assess the acid resistant species. Physico-chemical conditions resulting from the acid agglomeration, such as shear stress, increase temperature, radiation should be considered to be assessed further.