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 of 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. (c) 2007 Elsevier B.V. All rights reserved.