2012 Volume 128 Issue 12 Pages 633-637
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 of. 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.
In the present study, tri-n-octylamine (TOA) was used to investigate the separation of molybdenum and vanadium from a synthetic alkaline leach solution of spent HDS catalysts with a two stage process based on solvent extraction and precipitation stripping. Molybdenum and vanadium are simultaneously extracted at around pH 4 followed by stripping of molybdenum and vanadium with ammonium salt solutions. In the stripping process, vanadium is precipitated as ammonium salts containing tetraammonium disodium decavanadate decahydrate at pH>8. The precipitation stripping efficiency was in the order of ammonium chloride > ammonium nitrate > ammonium acetate > ammonium sulfate. Meanwhile, molybdenum exists in the form of ionic species in the stripping solution at pH>8. Therefore, the separation of molybdenum and vanadium from alkaline leach solutions based on a two stage process of solvent extraction with TOA and precipitation stripping with ammonium salt can be achieved.
