Extraction of metal values from waste spent petroleum catalyst using acidic solutions

► Extraction of Mo, Co and Al was carried out using H2SO4 and HNO3 solution. ► Leaching efficiency was significantly higher using H2SO4+HNO3 than only H2SO4. ► Temperature and [HNO3] are found as key factors especially on leaching of Mo. ► Under optimum conditions, 99.7% Mo, 99.6% Co and 14.9% Al were extracted. The extraction of molybdenum and cobalt from spent petroleum catalyst (Co–Mo/Al2O3) was investigated using sulphuric and nitric acid solution. Direct leaching of spent catalyst with sulphuric acid was not effective, whereas the combination of sulphuric and nitric acid was significant for the recovery of both molybdenum and cobalt. The effect of experimental factors such as reaction time, acid concentration, temperature, solid–liquid ratio and particle size were studied to determine the best conditions for the solubilisation of metal values. The parameters, temperature and nitric acid concentration are found to be critical factors especially on leaching of molybdenum from the spent catalyst. Under optimum conditions (pulp density 10% (w/v), [H2SO4] 0.5M, [HNO3] 4.0M, particle size 51–70μm, temperature 50°C and time 5h), about 99.7% of molybdenum and 99.6% of cobalt could be extracted with 14.9% extraction of aluminium.