In this attempt recovery of heavy metals from a hydrocracking spent catalyst was studied in a slurry bubble column bioreactor using Acidithiobacillus thiooxidans. Firstly adaptation of microorganisms to a spent catalyst was performed in a shaking flask. In the next stage the experiments were done accordance with a central composite design to find the optimal conditions; the three factors of particle size, pulp density, and aeration rate were chosen for examination. A full quadratic as well as two reduced cubic models were established to identify the relationship between the percentage of metals recovered and the variables. The optimal values of variables were a pulp density of 0.9% (w/v), particle size of 60.7 mu m, and aeration rate of 209 ml/min to obtain simultaneous maximum recovery of three metals. The results showed that at optimal conditions maximum extraction of 87% +/- 5.5% Mo, 37% +/- 2.6% Ni and 15% +/- 1.3% Al were achieved after only 7 days of batch processing in the bioreactor. The recovery of these metals can decrease the environmental impact of the waste catalysts and according to the results of this research slurry bubble columns, due to decrease the time of bioleaching process, can be further used for industrial purposes. (C) 2014 Elsevier B.V. All rights reserved.