Characterization and acid leaching of spent hydroprocessing catalyst for valuable metal recovery

About 150,000-170,000 tons of spent hydroprocessing catalysts are generated by oil refining industries every year, contributing to a significant amount of solid wastes. Although they account for only 4% of the overall refinery wastes, they are classified as one of the most dangerous wastes produced according to the USEPA, Basel Convention, and OECD. The objective of this study was thus set to characterize and assess a sample of the spent catalyst with the aim of valuable metal recycling. XRF results estimated the contents of valuable metals in the spent catalyst being La (1.9%) and Ni (1.2%), while XRD patterns showed only dominant host minerals of aluminum oxide (Al2O3) and zeolite-Y ((Na2, Ca, Mg)3.5[Al7Si17O48].32 H2O). After digestion with HCl and/or HF with/without microwave treatment, it was suggested that the form of La and Ni in the spent catalyst are La2O3 and Ni2SiO4, respectively. Acid leaching tests showed that La was the most readily dissolved target metal. Among those inorganic and organic acids tested (at 1 M), the greatest La recoveries (74% and 68% at day 1) were obtained with H2SO4 and citric acid, respectively. Meanwhile, Ni recoveries remained only 5.8% and 1.7%, respectively (at day 4) under the same conditions due to the high refractoriness of Ni2SiO4. This finding was followed by further La leaching tests using a variety of organic acids. As a result, La recoveries of 75%, 35%, 0.9% and 27% were obtained with 0.05 M of citric, lactic, oxalic, and succinic acids, respectively, indicating that citric acid is the best lixiviant to leach La. Meanwhile, Ni was barely dissolved by the organic acids tested. Since the final objective of this study lies in use of biogenic organic acids, production of citric acid is currently conducted using a fungal strain to apply to the spent catalyst leaching for La recovery.