Bioleaching kinetics of a spent refinery catalyst using Aspergillus niger at optimal conditions

The kinetics of bioleaching of Mo, Ni, and Al from spent hydrocracking catalyst, using Aspergillus niger was studied. The four most effective bioleaching variables were selected in accordance with the Plackett–Burman design and were further optimized via central composite design (CCD). The optimal values of the variables for maximum multi-metal bioleaching were as follows: particle size 150–212μm, sucrose 93.8g/L, pulp density 3%w/v, and pH 7. The maximum metal recoveries corresponding to these conditions were 99.5±0.4% Mo, 45.8±1.2% Ni, and 13.9±0.1% Al. The relatively low Ni extraction was attributed to the precipitation of Ni in the presence of oxalic acid. Under the optimal conditions, the fungus growth was found to be higher in the presence of spent catalyst than that in the catalyst-free medium. Determinations of the organic acid concentration showed noticeable variation during bioleaching, particularly for gluconic acid. Accordingly, a modified form of shrinking core model was used to take these variations into account. The predictions by the model showed good consistency with the experimental results, suggesting that diffusion of bioleaching agent through the solid matrix was the rate-controlling step.