The sulphuric acid leaching of spent alumina-based catalysts from dehydrodesulphuration processes, previously treated by alkaline solutions to remove vanadium and/or molybdenum, yields a solution rich in Ni(II), Co(II) and Al(III). In order to recover the two heavy metals the aluminum impurity must be eliminated. Several separation routes, including precipitation and liquid-liquid extraction, were first tested by computer simulation. Three of them were then checked at a laboratory stage. The separation of the three metals can be accomplished by selective extraction, as a function of pH, using CYANEX 272 (Cyanamid Co.) dissolved in hexane. Aluminum is first quantitatively removed in one extraction step at pH = 4.7 with a phase ratio V(aq)/V(org) = 5 followed by one scrubbing step at pH = 3.1 with V(aq)/V(org) =1. Cobalt is then quantitatively removed in two extraction steps, at pH = 6.3 with V(aq)N(org) = 5 followed by one scrubbing step at pH = 5.0 with V(aq)/N(org) = 1. The purified nickel in the final aqueous solution can be recovered by electrowinning with special cells of high cathode surface or concentrated by cation exchange. The extracted cobalt can be stripped by a H2SO4 + CoSO4 solution and sent to a conventional electrowinnig unit where cobalt is reduced to the metallic form and die stripping solution is regenerated.
