Influence of sulfuric acid baking on leaching of spent Ni–Mo/Al 2O 3 hydro-processing catalyst

Abstract

A low temperature sulfuric acid baking and mild acid leaching process was explored for maximum dissolution of metal values from a spent Ni–Mo/Al 2O 3 hydro-processing catalyst obtained from a Korean refinery. The spent catalyst was found to be a matrix of γ-Al 2O 3. 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 2 3 full factorial method for maximum dissolutions of Mo, Al and Ni during leaching. Under optimum baking conditions (400 °C; 1.28 stoichiometric H 2SO 4; 1 h), > 96% Mo, 99% Al and 95% Ni were readily dissolved from the spent catalyst by 2% v/v H 2SO 4 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.