Catalysts are widely used in petroleum refining and chemical industries. Hydrodesulphurization (HDS) catalysts account for about one third of the total worldwide catalyst consumption. Spent HDS catalysts contain rare metals such as molybdenum, vanadium, nickel and cobalt on an alumina carrier. Among secondary resources, spent HDS catalysts are regarded the most important catalysts for recycling these metals due to not only their large amounts and economic values, but also the environmental concerns if disposed of. In most cases, spent catalysts are treated with hydrometallurgical leaching processes such as caustic leaching and acid leaching with roasting as a pre-treatment step. In the alkaline leaching processes such as sodium carbonate roasting followed by water leaching, most of the molybdenum and vanadium are selectively leached and most of nickel and cobalt are left in the residue after the alkaline leaching of spent HDS catalysts. Therefore, it is important to recover nickel and cobalt from the residue. The main form of nickel and cobalt in the residue is spinel, therefore, it is difficult to extract nickel and cobalt from the residue using mineral acids. In the present study, the effect of grinding, acid concentration, leaching temperature and stirring speed on the leaching efficiency of nickel and cobalt from the residue after sodium carbonate roasting followed by water leaching of spent HDS catalysts with sulfuric acid was investigated. The grinding, sulfuric acid concentration and leaching temperature significantly affected the leaching efficiency of nickel, cobalt and aluminum. However, the extraction of nickel and cobalt was ca. 80% and ca. 60%, respectively. The spinel was not affected by only mechanical grinding, therefore, it was considered that the extraction of nickel and cobalt was relatively low.
