Bioleaching of spent refinery catalysts: A review

Large quantities of solid catalysts are routinely used in many chemical industries especially in petroleum refining and petrochemical industries. Solid catalysts contain metals, metal oxides or sulfides, and require replacement after two or three years of operation. Therefore, large quantities of spent catalysts are generated as solid wastes every year. Because of their hazardous nature and toxic chemical products, there are stringent environmental regulations for discarded spent catalysts. The recovery of metals from these catalysts is also an important economic aspect as most of these catalysts are supported, usually on alumina/silica with varying percent of metal. Bio-hydrometallurgical approaches are more economical and environmentally friendly than physicochemical metal-extraction processes. In this paper, the information available on the bioleaching fundamentals of spent catalyst wastes, as well as a focus on recent developments, is reviewed in detail.