Enhancement of metals dissolution from spent refinery catalysts using adapted bacteria culture — Effects of pH and Fe(II)

Leaching studies of Ni, V, Mo and Al from spent refinery catalysts were conducted using both adapted and unadapted bacterial cultures. The effects of variations in leaching parameters such as pH and Fe(II) concentration on the leaching rate of both bacterial cultures were evaluated. The leaching kinetics was characterized by an initial faster rate followed by a slower rate, which corresponded to surface and pore diffusion model, respectively. Additionally, the leaching efficiencies of Ni and V were found to be ~ 95% and ~ 85% in the case of adapted and unadapted bacteria cultures, respectively. The lower leaching rate of Mo than Ni and V was due to the mutual effects of the hydrophobic sulfur layer over the molybdenum matrix and refractory nature. The leaching rates increased as the pH increased from 1.5 to 2.0, then decreased at pH 3.0 for all metals. The variation in the concentration of Fe(II) supplement also had a substantial effect on the leaching rates. The dissolution of all metals followed first order reaction kinetics; therefore, unified dissolution rate equations were developed using the first order rate equation for different parameters. The pore diffusion mechanism was found to be the rate determining step.