Metal recovery from spent hydrodesulfurization catalysts using a combined acid-leaching and electrolysis process

This study focuses on recovering valuable metals from spent hydrodesulfurization (HDS) catalysts using a combined acid-leaching and fluidized-bed electrolysis process. The electrolytic cell was equipped with a glass bead medium, an iridium oxide mesh anode, and a stainless steel plate cathode. An acid solution consisting of concentrated HNO3/H2SO4/HCl with a volume ratio of 2:1:1 was found to be better than the other tested solution (HNO3/H2SO4 = 1:1) to leach the metals. For the three-acid mixture, the best solid/liquid ratio and leaching time were 40 g/L and 1 h, respectively, at 70 °C; under this condition, the leaching yields of target metals (Mo, Ni, and V) in the 1st stage of leaching reached 90, 99, and 99%, respectively, much higher than those in the 2nd/3rd/4th stages. When this acid leachate was electrolyzed for 2 h at 2 A constant current (current density = ∼35.7 mA/cm2), a stable cell voltage of 5 V was observed. The electrolytic recoveries of Mo, Ni, and V were ∼15, 61, and 66%, respectively, but extending the electrolysis time from 2 to 4 h did not increase the recoveries. For this operation, the total recoveries (leaching yield × electrolytic recovery) of Mo, Ni, and V were ∼14, 60, and 65%, respectively.