Financial support: FONDECYT 1980335 and DGIP-UCV 203.780. One of them (RG) received a AGCI fellowship from the Chilean government.

Keywords: agitation, bioreactor, biooxidation, slurry.

Present address: § Department of Chemical Engineering, Iowa State University, Ames IA 50011-2230, USA. Tel: 1 515 294 1516; Fax: 1 515 294 2689.

The large-scale biooxidation of gold concentrates is usually carried on in continuous stirred tank reactors (CSTR). Attaining homogeneous slurries is a difficult task, as solids tend to stratify in the tank. The objective of this work was to determine the optimal conditions of agitation in a CSTR so to obtain the best solids suspension. The experiments were performed in a 5 litre glass tank operated with 3 litres of 6% w/v slurry. The impellers (pitched blade turbine or marine propeller) were placed at heights of 6.7 to 13.4 cm from the bottom and operated at 370 to 1040 rpm, with specific aeration rates of 0.3 to 3.7 vvm. A statistical experimental design was used which allowed the derivation of a model representing response surfaces of the exit and mean solids concentration as a function of the impeller type, impeller distance from the bottom and aeration and agitation rates. During the experiments no solids were deposited on the bottom and the solids concentration near the bottom was always higher than that of the top region. At the optimal conditions for each type of impeller, the marine propeller required agitation rates about 15 to 22% higher than the pitched blade turbine. Nevertheless it is concluded that the marine helix is preferable because it requires less power and produces a more homogeneous suspension.