Influence of sulfuric acid baking on leaching of spent Ni–Mo/Al2O3 hydro-processing catalyst

A low temperature sulfuric acid baking and mild acid leaching process was explored for maximum dissolution of metal values from a spent Ni–Mo/Al2O3 hydro-processing catalyst obtained from a Korean refinery. The spent catalyst was found to be a matrix of γ-Al2O3. containing 27.1% Al, 11.6% Mo, 2.5% Ni, 9.7% S, 4.8% C, 1.9% P, 0.4% Si and trace amounts of Co and Fe. Various baking parameters such as baking temperature, sulfuric acid concentration and baking duration were optimized according to 23 full factorial method for maximum dissolutions of Mo, Al and Ni during leaching. Under optimum baking conditions (400 °C; 1.28 stoichiometric H2SO4; 1 h), > 96% Mo, 99% Al and 95% Ni were readily dissolved from the spent catalyst by 2% v/v H2SO4 at 80 °C. Carbon and sulfur analyses of the baked samples and leaching residues indicated only 10–15% of residual hydrocarbons reacted during acid baking, while most of the sulfur (assumed to be metal sulfides) was converted into soluble sulfates/oxy-sulfates. By comparison, direct sulfuric acid leaching of the catalyst resulted in poor dissolution of Mo and Al, even with large excess of acid, thus demonstrating the beneficial effect of sulfuric acid baking.