A hydrometallurgical process has been developed for recovery of cadmium from the zinc plant sewage sludge. The residue obtained from drying of the sludge contains great quantity of zinc and cadmium and also along with lead, iron, nickel, copper and cobalt. The process consists of following five major unit operations: (1) leaching: 2 mol L-1 sulfuric acid, L/S ratio of 4:1, 25 oC and 1.5 h; (2) precipitation of iron, copper and lead: pH 5.4, 2h, ambient temperature; (3) precipitation of nickel and cobalt: at pH 5.4, 80-85 oC, 20 min by an addition of dimethylglyoxime; (4) cementation of cadmium metal: at pH 4.7-5.0, 40-50 oC, 30 min by an addition of zinc metal; (5) purification of cadmium by heating the impure cadmium with sodium hydroxide in a blowing oven at temperature of 500-600 oC and then washing it with hot water. The total recovery of cadmium was found to be approximately 89%.
