Spent hydroprocessing catalyst management: A review: Part II. Advances in metal recovery and safe disposal methods

Petroleum refineries must pay attention to spent hydroprocessing catalysts particularly since they are hazardous toxic wastes. This review focuses on the progress in metal reclamation and disposal methods used for dealing with the environmental problem of spent hydroprocessing catalysts. Studies have been conducted with the aim either to increase the efficiency of metals recovery using established methods or to develop novel methods. Leaching studies used inorganic agents such as solutions of strong acids and bases, ammonium containing compounds and water soluble organic acids. The roasting of spent catalysts with sodium and/or potassium salts significantly enhanced solubility of some metals in water from where they could be recovered, in a pure form, either by selective precipitation or extraction using various extracting agents. The interests in bioleaching and electrochemical dissolution of metals were also reviewed. Commercial processes, involve either leaching out metals or roasting to make metals soluble in water. Markets and price trends for the metals in spent hydroprocessing catalysts are also discussed.To ensure the environmental acceptance, spent catalysts may require some pretreatments if disposal in landfills, is chosen as the last option. The aim is to make metals non-leachable by immobilizing them using thermal treatments with encapsulating agents. Consequently, the leachability of the metals is minimized. A number of methods and various agents have been evaluated for these purposes.