Process Optimization and Kinetics Study of Metals Leaching from Spent Hydrocracking Catalysts by Cell-free Medium Filtrate of Aspergillus aculeatus

Abstract

Hydrocracking catalysts (HCC) are widely used in petrochemical refinery catalytic processes. Although these catalysts are frequently reusable, the efficacy of HCC gradually declines. This procedure leads to the production of a large amount of spent hydrocracking catalyst (SHCC), which contains valuable metals as well as hazardous chemical wastes. The pyro-hydro-metallurgical approach for metal restoration from catalyst waste is insufficient for environmental sustainability. The application of biological techniques is a more environmentally friendly strategy. Fungal leaching has emerged as an alternative method for the recovery of metals. Although metal recoveries were moderate, the use of cell-free fungal filtrate enabled rapid leaching under mild conditions, highlighting its potential as a greener and operationally simpler alternative to conventional whole-cell bioleaching. Aspergillus aculeatus was isolated from contaminated soil of a gold mine located in Thailand. The filtrate of the fungal culture medium was utilized for metal leaching, and this experiment was carried out using a Box-Behnken experimental design. The optimum processes included a spent catalyst density of 10% (w/v), a temperature of 50 °C, and a shaking rate of 148 rpm for 180 min, with the catalyst powder size being less than 150 µm. The predicted highest metal recovery rates of 7.98% for Al, 5.27% for Ni, 24.55% for Mo, 11.92% for Fe, and 0.93% for Zn aligned precisely with the actual experimental results. The comprehension of kinetics through the application of the shrink core model indicates that the mechanism of reactions is predominantly influenced by the surface chemical control reaction rather than the diffusion process. The leaching rates of Mo, Fe, Al, Ni, and Zn followed a descending order from highest to lowest.