Bioleaching of vanadium rich spent refinery catalysts using sulfur oxidizing lithotrophs

Bioleaching process was attempted by using chemo-lithotrophic sulfur oxidizing bacteria to recover valuable metals from vanadium-rich spent refinery catalysts. Prior to the bioleaching process, the spent refinery catalyst was pre-treated with acetone as the solvent. The bioleaching process was carried out in one-step and two-step methods and the leaching efficiencies in both the cases were compared. Bacteria were grown in the presence of up to 50 g/L of spent catalyst using elemental sulfur as the major substrate in the leaching medium. Varying the spent catalyst concentration in the bacterial growth medium in the one-step process conducted at pH 2–3 resulted in different amounts of solubilized metals. The maximum extraction yield of metals (88.3% Ni, 58.0% Mo and 32.3% V) was obtained with a concentration of 15 g/L spent catalyst. In the two-step process, the sulfur oxidizing bacteria were first cultivated in the presence of elemental sulfur in the growth medium, and the bacterially produced acid medium at pH 0.9–1.0 was subsequently used as the leaching agent. In this study, at 50 g/L spent catalyst concentration, 88.3% Ni, 46.3% Mo and V 94.8% were recovered after 7 days. Chemical leaching with commercial sulfuric acid was compared with the two-step process and the suitability o f the two-step process was demonstrated. In all the processes studied, however , a significant amount of molybdenum was found to be lost to the solid phase as MoO3.