Nickel and cadmium compounds are important impurities in zinc electrolyte . In the electrolytic production of zinc, these impurities must be removed from the zinc- bearing solution prior to electrolysis . At the Isfahan Zinc Smelting Factory, nickel and cadmium have been precipitated by sufficient amount of zinc powder and copper sulfate at 50-70 ºC. The resulting precipitate was known as nickel and cadmium filter cake and contains 30% Cd, 22.7 Zn, 3.2 Ni and 1.95 Cu.
A sample of filter cake was first identified by XRD analysis and microscopic studies. The major XRD peak was matched to Braunite (3Mn₂O₃ . MnSiO₃) and to probable existing of Cd₅Ni, Cd₂SO₄(OH)₂  ,  (Zn,Cu)SO₄(OH) and synthetic CdS. Microscopic studies proved the existence of sulfate phases, native copper and metalic zinc. Nickel sulfate (NiSO₄ . 7H₂O) was also observed in the thin section.
Leaching results showed that 10% of filter cake was soluble in water, in which 10 , 16 and 18 percent of total nickel, zinc and cadmium were dissolved in water, respectively.
Results obtained from different methods showed that nickel was present as NiS and cadmium existed in the oxide form . Finally leaching process was carried out with sulfuric acid at different    conditions . The effect of temperature, acid concentration , reaction time and pulp density were investigated and the results indicated that about 93% of nickel and cadmium were extracted under the following conditions: T=60ºC,    t=1.5-2 hr,     C=20% , Pulp density(S:L)=60 g/l.