Catalysts are widely used in petroleum refining and chemical industries. Hydrodesulphurization (HDS) catalysts account for about one third of the total worldwide catalyst consumption. Spent HDS catalysts contain rare metals such as molybdenum, vanadium, nickel and cobalt on an alumina carrier. Among secondary resources, spent HDS catalysts are regarded the most important catalysts for recycling these metals due to not only their large amounts and economic values, but also the environmental concerns if disposed off. In most cases, spent catalysts are treated with hydrometallurgical leaching processes such as caustic leaching and acid leaching with roasting as a pre-treatment step. In the alkaline leaching processes, most of the molybdenum and vanadium are selectively leached over aluminum, nickel and cobalt. Vanadium can be separated from molybdenum with a two stage process based on solvent extraction and precipitation stripping. Molybdenum and vanadium are simultaneously extracted with tri-n-octylamine (TOA) at around pH 4 followed by stripping of molybdenum and vanadium with ammonium salt solutions. In the stripping process, vanadium alone is precipitated as ammonium salts of tetraammonium disodium decavanadate decahydrate at pH > 8.
In the present study, precipitation method was carried out to recovery of molybdenum from aqueous solutions of ammonium chloride. Molybdenum could be precipitated as tetraammonium octamolybdate pentahydrate with ammonium chloride at the pH below 4 and the pH region of precipitation increased with increase of temperature. And also, the precipitation of molybdenum with barium chloride and calcium chloride was also investigated.
