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Every year large quantities of spent hydroprocessing catalysts containing high concentrations of metals such as Mo, V, Ni, and Co are discarded as solid wastes from the petroleum refining industries. The metals present in the spent catalysts can be leached by water and pollute the environment. As these catalysts are hazardous wastes, landfills or other treatment methods for disposal are not preferable when all costs and liabilities are taken into account. Intensive efforts have been made in recent years on the development of processes for recycling and utilization of the waste catalyst materials as much as possible. Recovery of the valuable metals from the spent catalysts is an attractive option for their recycling and utilization. In the present work, the influence of ultrasonic agitation in enhancing the extraction of valuable metals such as Mo, V, and Ni from spent hydroprocessing catalysts by acid leaching was investigated. Two acids, one inorganic (H2SO4) and the other organic (citric acid), were used in the study. Citric acid was more active than H2SO4 for leaching metals from the spent catalyst. A comparison of the effectiveness of ultrasonic agitation and conventional mechanical stirring in the acid-leaching experiments conducted in the present study clearly demonstrated that ultrasonic vibration was more effective for the extraction of metals present in the spent catalyst. The leaching rate was enhanced by ultrasound, and large percentages (>95%) of all valuable metals (Mo, V, and Ni) present in the spent catalyst were extracted in a short time at a relatively low temperature (e.g., 60 °C) by combined use of citric acid and ultrasound.

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