Metal Recovery from Hydroprocessing Spent Catalyst: A Green Chemical Engineering Approach

The present study aims to develop an ecofriendly, chelant-assisted extraction methodology for significant recovery of heavy metals (cobalt (Co), molybdenum (Mo)) from hydroprocessing spent catalyst. Ethylene diamine tetraacetic acid (EDTA) was employed for metal mobilization in the extraction process. The possibility of internal and external mass transfer resistance was investigated to improve the diffusion rate of reactants, while kinetic aspects were studied to achieve thermodynamic equilibrium for the process. Percentage distribution of various protonation stages of EDTA was explored to understand the conjugate base and ligand precursor and to improve the effect of reaction pH on extraction efficiency. Extraction of 80.4% Co and 84.9% Mo was achieved at optimum reaction conditions. Selective precipitation of metals was attained according to maximum solubility of metal oxides at different pH regions. Efforts were also made to recycle the recovered EDTA, recovered support material, and extracted metals. Significant metal extraction efficiency (72.7% Co and 76.5% Mo) was observed with recovered EDTA even after the fourth cycle of operation which may provide economic consistency to the extraction process. The extracted metals were impregnated on recovered alumina to synthesize fresh catalyst. Structural analysis of spent catalyst, recovered support material, and synthesized catalyst from extracted metals suggested successful recovery and recycling of metals. This work offers an incentive to the industrial practice for waste minimization, recycling of the extracted metals, and the noncorrosive, ecofriendly approach for metal extraction from spent catalyst.