OXYGEN MASS-TRANSFER LIMITATION OF BATCH BIOOXIDATION AT HIGH SOLIDS CONCENTRATION

In contrast to the cyanide leaching of gold and acid leaching of uranium, performed at solids concentrations of 40 to 50 per cent, the concentrations used in the bio-oxidation of refractory gold-bearing sulphide concentrates have been limited to 18 to 20 per cent solids. Oxygen availability has been identified as a key factor limiting bio-oxidation at high solids concentration. Mass transfer correlations for agitated, aerated slurries have been used to predict the maximum oxygen transfer potential in typical industrial stirred tank reactors. Previously reported bio-oxidation rates have been used to predict the oxygen demand for typical operating conditions. In the region at lower solids concentrations, where the oxygen supply is not limiting, the bio-oxidation rate increases linearly with solids concentration. In the region where oxygen demand exceeds oxygen supply, the bio-oxidation rate is defined by the oxygen transfer potential of the system and the rate declines with increasing solids concentration. Predicted rates and limiting solids concentrations have been compared with data from the literature and found to be in agreement. A series of laboratory batch stirred tank reactor tests have been conducted to investigate the effect of sulphide grade and solids concentration on the specific bio-oxidation rate. The highest solids concentration for bio-oxidation of a particle sulphide, at the maximum specific bio-oxidation rate, is determined by the oxygen demand in relation to the oxygen transfer potential of the system. Low grade sulphides, with a low oxygen demand per unit mass, can be oxidized at far higher solids concentrations than high grade material