BACKGROUND: Solid catalysts containing metals or metal oxides play a key role in the chemical process industries to produce valuable products and fuels and consequently are left as solid wastes after a certain period of use. Disposal of these spent catalysts requires compliance with stringent environmental regulations because of their hazardous nature and content of toxic chemicals. Therefore recovery of the metals by various methods has been explored. In the present study recovery of nickel from spent nickel catalysts using ultrasonication-assisted leaching has been investigated. RESULT: The effect on nickel recovery of acid concentration, temperature, solid to liquid (S : L) ratio, and time of digestion were studied in detail and optimized for the ultrasonication route. The results obtained are compared with the chelation route and conventional acid leaching technique. Using ultrasonication-assisted leaching 95% extraction of nickel was achieved at 90 degrees C, 40% nitric acid concentration and S : L ratio 1 : 10 (g : mL) in 50 min from the spent nickel-alumina catalysts. CONCLUSION: Using an ultrasonication technique 95% recovery of nickel was significantly faster (50 min) than the chelation route (7 h), while with conventional acid leaching a maximum of 93% nickel recovery was obtained in 9 h. Compared with conventional acid leaching the purity of leached nickel salts was good and they could be recycled for the preparation of fresh catalysts after removing Al impurities. (C) 2011 Society of Chemical Industry