A novel sequential process of bioleaching and chemical leaching for dissolving Ni, V, and Mo from spent petroleum refinery catalyst

Two step leaching experiments were carried out to extract nickel, vanadium, and molybdenum present in spent refinery catalyst. A bioleaching process was applied in the first step. Pulp density, initial Fe(II) concentration, initial pH, particle size, and temperature were varied to optimize the bioleaching process. Ni, V, and Mo were leached out with maximum recoveries of 97%, 92% and 53%, respectively, at an optimized bioleaching condition of initial ferrous ion of 2 g/L, initial pH of 2, pulp density of 10% (w/v), particle size of (− 106 + 45) μm, and a temperature of 35 °C. As the Mo leaching rate was very low, a second leaching step for the bioleached residue was applied with different concentrations of (NH4)2CO3, Na2CO3, or H2SO4. The second step leaching was optimum at a concentration of 30 g/L (NH4)2CO3 with respect to Mo extraction. The rate of Mo dissolution with respect to concentration of lixiviant in the second leaching step was evaluated. The percentages of Ni, V, and Mo leached were 97%, 97% and 99%, respectively, by combining the first step under optimized conditions and the second step with 30 g/L (NH4)2CO3.

► We used bioleaching process for extraction of metals values from spent refinery catalyst in the 1st step.
► Molybdenum (Mo) leaching was very poor compared to other metals (Ni & V) in bioleaching process (1st step).
► 2nd step acid and alkali leaching was applied to recover remainder Mo present in the residue of 1st step.
► Mo leaching was 99% after the second step with (NH4)2CO3.
► The order of Mo leaching was evaluated for all three reagents in the 2nd step leaching.