| In recent years, there has been considerable growth in biooxidation for the pre-treatment of refractory gold ores. The application of biooxidation process has the advantages of improved efficiency and environmental problems compred with other oxidation techniques. Stirred tank reactors (STRs) equipped with a suitable impeller are widely used for the biooxidation of sulphide minerals at industrial or laboratory scale due to a number of technical and economic advantages. However, there are three problems in bioreactor:(1) high mineral concentration but low mass transfer rates;(2) high dissolved oxygen (DO) concentration but low oxygen utilization;(3) high agitation intensity but cell shear damage. These problems have become the main constraints for the biooxidation efficiency in STRs and its economic benefits.Therefore, firstly the effects of the operating parameters (temperature, initial pH and the mineral concentration) and the cell adhesion characteristics in the biooxidation process were investigated by the shake flask experiments. Secondly, the rheology of slurry and its influencing factors were determined. And then, the relationship between the oxygen mass transfer rate and the rheology of slurry, agitation intensity, aeration rates were researched in bioreactor. At last, the effect mechanisms of DO and agitation intensity on the biooxidation process, especially cell growth and metabolic activity were explored on reactor engineering scale, cellular scale and molecular (genetics) scale. These results could provide scientific guidance and technical support to improve the efficiency of the biooxidation reactor.In order to determine the basic operating conditions of the biooxidation process, the single factor shake flask experiments and the classical Hansford logic equation fitting were carried out with the results showed that the cell concentration, ore oxidation ratio and oxidation rate increased with temperature when the operating temperature was between30-44℃. Low pH would be also conducive to cell growth and improve the efficiency and ores oxidation. And the ores oxidation rate increase with inoculum amount, when the inoculum was less then10%(V/V). Thus, the optimum culture temperature, inoculum amount and initial pH for the biooxidation process were41℃, pH1.4and10%(V/V), respectively.The research of the cell adhesion characteristics indicated that the adhesion of A. ferrooxidans on the mineral surface could improve the oxidation efficiency greatly. The kinetic parameters in the rate equation including the cell growth yield YA and the specific growth rate μA of adsorption bacteria were6.11X1012cell/kg-ore and1.0d-1respectively. The electrical properties of mineral surfaces changed significantly which could enhance the electrostatic interaction between the ores and cell, when0.005-0.01wt%sodium hexametaphosphate (SH) was added. It could improve the adhesion ratio of the cell on the mineral surface. The adsorption date could fit to the Langmuir isothermal equation well. And the adsorption equilibrium constant KA was3.39×10-14m3/cell, and the maximum absorption capacity per unit weight of the ores XAM was1.80×1012cell/kg.The results of the slurry rheology experiments showed that the flow behavior of slurry was Newtonian fluids when the mineral concentration was less than40wt.%. The viscosity of slurry increased with the mineral concentration significantly. And the flow behavior of slurry turned into pseudoplastic fluids which fitted to the Casson model, when the concentration was more than40wt.%. The addition of0.005-0.05wt.%SH into the slurry of high mineral concentration could reduce their viscosity and yield stress pronouncedly. The oxygen mass transfer coefficient Kla decreased with the slurry viscosity, in the range of10-40wt.%mineral concentration. Increase the agitation intensity or aeration rates could improve the gas-liquid mass transfer rate effectively. And the relationship between Kla and the slurry viscosity, the P/V, aeration rates as follows: The cell concentration and ores oxidation rates were also decreased with the mineral concentration.In this thesis, the effects of DO on the bioxidation process were investigated in three1.5L STRs with the multi-scale methodology. The results indicated that the biooxidation process of refractory gold ores by A. ferrooxidans was controlled by the bioreaction rate. The reaction control was more important in the high DO concentration particularly. This oxidation efficiency and cell growth rate increased with the DO concentration. The biooxidation process was restricted at1.2ppm DO due to oxygen limitation. In this condition, the ore oxidation ratio and the maximum ore oxidation rate were only43.9%and0.05708g/L/h. And both of them reached60.4%and0.08417g/L/h respectively, when the DO concentration increased to3.1ppm. But the ore oxidation rate did not increase with the DO concentration when the DO concentrations increased to5.2ppm, witch indicated that the high DO could not be used by A. ferrooxidans.In order to reveal and understand the mechanism of DO concentration affecting on the biooxidation process of refractory gold ores, the studies on the cellular and molecular genetic scale were carried out. And the results showed that oxygen uptake rate (OUR), Fe2+oxidation activity and the expression of rus genes of A. ferrooxidans in the biooxidation process were improved by73%,25%and67%respectively, when the DO concentration was increased from1.2ppm to3.1ppm. Thus, improve DO concentration could upregulate gene expression of key proteins in the electron transport chain, which resulted in the electron transfer rate accelerating and the Fe2+oxidation activity increasing. But it was also found that, OUR and Fe2+oxidation activity changed little, whereas the expression of rus genes of A. ferrooxidans was increased1.3fold, when the DO concentration increased from3.1ppm to5.2ppm. It seemed that the increased of DO concentration could not improve the oxidation efficiency of the biooxidation process, but would lead to the oxygen utilization declined.The effects of agitation intensity on the bioxidation process were also investigated in three1.5L STRs with the multi-scale methodology. The results indicated that the oxidation efficiency and cell growth rate increased with the decreased of agitation intensity. At the stiring speed of300rpm, the ore oxidation ratio and the maximum ore oxidation rate were73.8%and0.09864g/L/h. And both of them dropped to60.4%and0.08417g/L/h respectively, when the agitation speed increased to700rpm. Meanwhile, the free cell concentration decreased by19%, and the adsorption equilibrium adsorption of bacteria decreased by16%, when the speed increased from300rpm to700rpm. On the cellular and molecular genetic scale, OUR and the Fe oxidation activity and the expression of rus genes of A. ferrooxidans in the biooxidation process reduced by28%and45%respectively, and the expression of rus genes of A. ferrooxidans was reduced to1/18, when the stiring speed was increased from300rpm to700rpm. The results demonstrateed that improve agitation intensity could downregulate rus gene expression which resulted in the electron transfer rate decelerating and the Fe2+oxidation activity declinding.Based on the founding of this thesis, the φ2m×10m pilot biooxidation reactor with jet ventilation device was designed. The gas distribution in the reactor was well. Thus, the impeller stirring speed was low. Finally, compared with the industrial bioreactor, the efficiency of ores biooxidation in the pilot bioreactor was increased by26-35%. |
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