BACKGROUND A large number of spent catalysts are regularly discharged by petrochemical industries. The spent catalysts contain some valuable metals, such as molybdenum, nickel, and vanadium. Therefore, it is necessary to recover these high purity metals from the spent petroleum catalysts with environmentally friendly processes. RESULTS In order to recover valuable metals such as molybdenum, vanadium, nickel, and aluminum from spent petroleum catalysts, a hydrometallurgical process consisting of leaching followed by solvent extraction was developed in the present work. First, molybdenum was separated from the leaching solution by extraction with D2EHPA and the loaded organic phase was stripped by a mixture of NH4OH and (NH4)(2)CO3. Second, the molybdenum free raffinate was contacted with Aliquat 336-decanol to separate the vanadium, and then a hydrochloric acid solution was employed to strip vanadium from the corresponding loaded extractant. Since the acidity of the leach solution was high, TEHA (tris-2-ethylhexylamine) was chosen for the extraction of hydrochloric acid in order to increase the pH of solution. The loaded TEHA phase was then stripped by distilled water, which enabled the recovery of HCl. The nickel and aluminum in the molybdenum and vanadium free raffinate were separated by the extraction of nickel with saponified Cyanex 301 at a solution pH of 2.5. It was possible to strip nickel from the loaded organic phase by a strong hydrochloric acid solution. CONCLUSIONS A flow sheet of the proposed process for the separation of Al(III), Mo(VI), Ni(II), and V(V) was presented. The current process can be applied to aqueous solutions containing these four metals from diverse material resources. (c) 2020 Society of Chemical Industry