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Recovery of valuable metals such as cobalt, molybdenum, nickel, and vanadium from spent catalysts has attracted much attention owing to the gradual depletion of high-grade ores. In this work, several hydrometallurgical methods have been reviewed for recovery of these metals from several spent catalysts. The main methods include adsorption, chelation, ion exchange, leaching, precipitation, and solvent extraction. The disadvantage of precipitation lies in the difficulty in recovering pure products. Acid leaching is generally preferred to basic leaching in the industry owing to high dissolution of valuable metals. Solvent extraction is effective in the separation of valuable metals. The adsorption and complexing with chelating agents offer useful means for selective metal recovery although the scale of its application in the industry is still limited. Ion exchange is suitable for the purification of metal ions. The combination of these techniques is recommended in developing a process. The heat treatment of spent catalysts facilitates the dissolution of valuable metals by the leaching and then the dissolved metal ions can be recovered by solvent extraction. The assessment of the environmental impacts, economics, and prediction of the industrial applicability of some processes have been discussed by the life cycle analysis (LCA). Employment of LCA method to evaluate the environmental aspects during the recovery of metals from spent catalysts is necessary in further research.
Le et al. (Tue,) studied this question.