Direct leaching of molybdenum and cobalt from spent hydrodesulphurization catalyst with sulphuric acid

Sulphuric acid leaching of spent hydrodesulphurization catalyst was carried out to investigate the extraction behavior of molybdenum, cobalt and aluminum. Composition of the spent catalyst used was: 2.24% Co, 9.93% Mo, 24.09% Al and 9.12% S. Major mineral phases were CoS, CoO, MoO, MoS, Al2O3 and elemental S. Extraction of molybdenum, cobalt and aluminum by direct leaching with only H2SO4 was very low (21.3%, 56.11% and 8.2%) and this was due to the dissolution of oxide minerals. It was required to add oxidants to dissolve elemental sulphur and sulphide minerals. Various oxidants were examined to improve metal extraction. Based on the extraction efficiency, they could be ordered as; H2O2 > HNO3 > NaClO3 > FeCl3 > NaOCl. The characterization of different samples (original and residue) was examined by XRD to investigate the dissolution behavior of the spent catalyst. The particle size of ground spent hydrodesulphurization catalyst appeared to be a critical factor on enhancing the metal extraction efficiency. Under optimum leaching conditions (pulp density 1% (w/v), [H2SO4] 0.5 M, [H2O2] five times stoichiometric, particle size 51–70 μm, temperature 50 °C and time 2 h), the extraction of molybdenum, cobalt and aluminum were 99.87%, 96.25% and 11.03% respectively.