Emerging role of organic acids in leaching of valuable metals from refinery-spent hydroprocessing catalysts, and potential techno-economic challenges: A review

Petroleum refineries are known to generate huge quantities of spent hydroprocessing catalysts as solid waste every year. Spent hydroprocessing catalysts contain significant quantities of different toxic metals, and are designated as hazardous. Besides environmental concerns, the metals (Ni, V, Mo, Co, etc.) present in the spent hydroprocessing catalysts are also a valuable commodity and thus effective and eco-friendly methods are required to recover them. In recent years, different hydrometallurgical processes have been developed to leach these metals from spent hydroprocessing catalysts in an effective manner. Among various methods, the use of different organic acids is gaining attention as a promising green leaching method for spent catalyst due to their effectiveness, selectivity, and biodegradable nature. In this article, the properties and mechanism of different organic acids in leaching of metals from the refinery-spent hydroprocessing catalysts are reviewed. The advantages of organic acids as compared to other hydrometallurgical processes are emphasized. The potential of each organic acid in the leaching of metals from the spent hydroprocessing catalyst is elaborated. The key techno-economic challenges (efficiency, process economy, and safety) associated with the use of organic acids as leaching agents are identified, and future perspective are elaborated. Based on the analysis, it can be concluded that organic acid-based leaching might play a key role in the development of eco-friendly hydrometallurgical processes for the treatment of petroleum refinery spent hydroprocessing catalysts.