Integrated bio-pyro-hydro-metallurgical approach to recover metal values from petroleum refinery spent catalyst

Abstract The present study reports an integrated three-step bio-pyro- hydrometallurgical process to recover nickel (Ni) and molybdenum (Mo) from spent petroleum catalyst. High leaching yields of nickel (94%) and molybdenum (92%) along with other critical metals like rhenium, selenium, niobium, chromium, and zirconium were obtained. A toxic element like Pb was leached entirely from the spent catalyst while precious metal like palladium was concentrated during the treatment. The waste petroleum catalyst and all the residues were characterized to understand the morphology, chemical composition, and mineralogy by Scanning Electron Microscopy-Electron Dispersive X-Ray spectroscopy (SEM-EDAX), X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD) respectively, while the Ni and Fe concentration in the bioleach liquor was analyzed by Atomic Absorption Spectroscopy (AAS). In the first stage, bioleaching of spent catalyst leached up to 94% Ni and 71% Mo. The rate-limiting step in Ni bioleaching kinetics was calculated to be both chemically as well as diffusion controlled. The Ni bioleaching followed 1st order reaction kinetics. Integrating roasting (1 h) and alkaline leaching (6 h) with the bioleaching process remarkably increased the recovery of Mo by 21%. Roasting of the spent catalyst after bioleaching lessened not only harmful SO 2 emission by 64.95% but also liberated residual Mo by 23% from the aluminium silicate matrix. Therefore, employing a sequential bio-pyro-hydrometallurgical technique is a more eco-friendly and cost-effective strategy to recover metal values from spent petroleum catalyst.