Microbial leaching of a mixed sulfide ore concentrate in continuous-flow reactors: development of a mathematical model for the process

Bacterial oxidation of a refractory pyrite-arsenopyrite-gold ore concentrate was carried out in 14-liter continuous-flow slurry reactors. Steady-state operation with residence times varying from 20 to 110 hours and feed solids concentrations varying from 6% w/v to 24% w/v were studied. At each of these steady-state conditions, oxygen and carbon-dioxide consumption rates, dissolved iron, arsenic and sulfur concentrations, solution reduction-oxidation potential, pH and elemental sulfur concentrations were measured. Oxygen and carbon dioxide were seen to be essential nutrients for mineral oxidation and cell growth. The steady-state oxygen and carbon-dioxide consumption rates peaked at a residence time of about 40 hours at each feed solids concentration. Steady-state operation at residence times below 40 hours was difficult to attain due to cell washout. At residence times over 40 hours, the uptakes dropped gradually with increasing residence times. Feed air supplementation by approximately 0.5 to 1.0 volume % carbon dioxide was indicated to be essential to the maintenance of optimal cell growth rates. Sharply reduced cell growth rates were encountered in the absence of carbon dioxide supplementation. Dissolved carbon dioxide concentration in excess of 10 mg/L was seen to be inhibitory for cell growth. Sodium isopropyl xanthate present in the ore concentrate was seen to be inhibitory to bacterial growth and activity. Significant improvement in leaching kinetics can be achieved by the removal of the xanthate from the concentrate prior to bioleaching. This can be accomplished by washing the ore concentrate in the acidic solutions produced by the bioleach process. The transient response of the bioleach system to pulse and step changes in operating conditions was studied. A mathematical model based on the steady-state experimental data was developed. The model assumes ferric ion mediated leaching of pyrite and arsenopyrite, and cell growth with ferrous ion and elemental sulfur as substrates. The equilibria between the various dissolved ionic species in the system were used to predict the free ferrous and ferric ion concentrations. The model was used to study the dynamic behavior of the bioleach system; the model predictions of the various transient response tests were compared with the experimental data.