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Gold is found in nature as micro particles disseminated in ores bodies. Usually gold is recovered from these ores by extraction with cyanide solutions. But when applied to some types of ores, known as refractory, the cyanidation operation is turns very inefficient because the gold particles are covered with a film of insoluble sulphides that impedes an adequate contact between the cyanide and the metal. To economically benefit refractory ores, a pre-treatment stage must be included in the process aimed at weakening or eliminating the sulphide coat. Biooxidation of refractory gold concentrates has become the technology of choice for the pre-treatment of these types of minerals. It is extensively used in no less than eight large-scale mining operations in Australia, Africa and South America. Biooxidation is carried on in large continuous stirred tank reactors (CSTR). The reacting system is complex, as it consists of three phases: gas (air), liquid, and solid (mineral particles, cells adsorbed onto the particles and planktonic cells). In order to obtain an efficient continuous operation, the contents of the reactor should be homogeneous. The attainment of this goal requires careful consideration of the reactor design and operation, with special reference to agitation.

The objective of this work was to determine the optimal agitation conditions in a CSTR so to obtain the best solids suspension for the biooxidation of refractory gold concentrates.

The experiments were carried on in a 5-L acrylic reactor with round bottom operated with 3 L of slurry. Pulp concentration was 6% w/v and particle size ranged from 38 to 75 mm. The gold concentrate contained 42 g Au/ton, 42.8% pyrite (FeS₂) and 40.7% enargite (Cu₃AsS₄). The CSTR was equipped with a single impeller agitator, annular air sparger, four baffles and overflow exit. Two types of impellers were used: a 6 pitched-blade turbine pumping up (6MFU) and a marine propeller pumping up (3MPU). Independent liquid and solids feeds were used. All experiments were run at 33ºC and pH 1.5. A central composite rotatable (CCR) experimental design was used. The data are fitted to second order polynomial functions. Response surface methodology allowed the building of models, evaluation of factors and searching of optimum conditions.

Results show that in all runs the exit solids concentration was lower than the mean concentration of the complete content of the reactor. Although no solids accumulated in the bottom, complete homogeneity was not possible. The co-existence of two phenomena was observed related to the axial and radial components of the slurry flow lines. The axial component produced a pumping zone that favoured solids suspension, while the radial component produced a re-circulation zone that was detrimental. These phenomena explain the existence of optimal operation conditions that equilibrate the positive and negative effects, maximizing solids suspension.

Mathematical models were derived from the experimental data that allowed the determination of optimal agitation conditions for each type of impeller. Only the significant effects, as determined by statistical analysis, were included.

With a pulp density of 6% w/v and aeration rate of 2.0 vvm, optimal conditions for the 6MFU impeller were 760 rpm and a distance of 8.7 cm from the bottom, condition at which 84% of the solids were suspended. These conditions were 860 rpm and 9.0 cm for the 3MPU impeller, with a solids suspension of 93%.

It is concluded that although the 6MFU impeller requires a lower agitation speed at its optimal condition, the 3MPU is preferable because it produces a more homogenous suspension and requires less agitation power.