UBC Theses and Dissertations
Continuous microbiological leaching of a zinc sulphide concentrate Gormely, Lynton Spencer
A zinc sulphide concentrate was leached microbiologically by Thiobacillus ferrooxidans in a continuous stirred tank reactor. A model was developed to predict the leaching kinetics when the bacterial growth rate was not limited by any substrate other than the zinc concentrate, and it was modified to explain the observed results. It was possible to obtain stable steady-states over a range of dilution rates. Because a solid substrate was used, the specific growth rate of the bacteria was not a unique function of the substrate concentration, and conventional continuous culture theory based on the Monod equation therefore did not apply to this system. The bacterial concentration did not limit the growth rates under the conditions of these experiments. The leaching rates and bacterial growth rates are thus first order in mineral surface area concentration. The highest specific growth rate observed was 0.1038 hr⁻¹; the highest oxygen uptake coefficient (Q[sub O₂ ](N)) calculated was 7650. Both of these values are higher than any reported previously for T. ferrooxidans growing on a solid substrate. The highest zinc release rate obtained was 1.3 g/l-hr. None of these values is necessarily the maximum achievable. Total carbon and non-distillable ammonium ion concentrations proved to be satisfactory measures of biomass concentration. Yield coefficients calculated from these data were constant for the dilution rates investigated, indicating a low maintenance energy requirement for the organism. The value of the net ammonium ion yield constant suggests that addition of ammonium ion above the level present in the medium 9K of Silverman and Lundgren should be beneficial when zinc concentrations exceed 68 g/1. Percentage zinc extractions increased with decreasing dilution rate, but sufficiently low dilution rates to achieve extractions which would be competitive with conventional processes were not used. Recommendations are given for achieving competitive extractions by altering the process configuration. When the percentage zinc extraction was known for one dilution rate, it was possible to use a calculation method given by Levenspiel to predict the extractions at other dilution rates.
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