Selective zinc alkaline leaching optimization and cadmium sponge recovery by electrowinning from cold filter cake (CFC) residue

Annually, a great amount of Cold Filter Cake (CFC) residue is produced in lead and zinc plants in Iran. CFC, the Ni–Cd purification step residue, with elemental analysis of 45–55% Zn, 11–16% Cd, 2.5–4% Ni and 0.2–1.5% Cu can be used as a secondary source of zinc, nickel and cadmium. Unfortunately, despite its hazardous content, the waste is treated in such a way that most of the hazardous constituents get easily exposed to the environment. In the present study, treatment of CFC for separation of zinc and recovery of cadmium has been conducted employing alkaline leaching and electowinning processes respectively. Affecting factors, such as alkaline concentration, leaching temperature, holding time, and liquid:solid ratio (volume/weight, L/S), and stirring rate were studied on zinc leaching from CFC with NaOH solution. Results showed that under two-stage alkaline leaching, when CFC was leached for 1 h at 75 °C in the presence of 8 mol/l sodium hydroxide and liquid:solid ratio of 10:1, the leaching rate of Zn was 98% while Ni and Cd remained intact in residue. Thus, zinc was separated from Ni and Cd selectively. In the next step, sulfuric acid leaching of alkaline leach residue, with 1 mol/l H2SO4 at 70 °C and liquid/solid ratio of 10:1, provided a stock sulfate solution containing 49.92 g/l Cd, 2.77 g/l Zn, 12.76 g/l Ni, and 1.18 g/l Cu. The Cd electrowinning experiments and the effects of current density (A/m2) and electrolysis time (h) were studied on stock sulfate solution in the presence of Ni and Zn. The electrowinning was carried out at various current density (100, 180, 250 and 400 A/m2) and electrolysis time (5, 8, and 18 h). Concentrations of Cd2 +, Ni2 + and Zn2 + were determined every hour. Cadmium recovery of 9.8% at 100 A/m2 increased sharply to 87% at 400 A/m2 in 5 h. Maximum total recovery of cadmium from solution was 99.9% achieved at 25 °C and 400 A/m2 in 18 h. Final product was cadmium sponge of high purity 98% and copper was considered the main impurity in cadmium sponge deposit (1.7%). A complete flow sheet of process for the selective separation of zinc and cadmium recovery is presented.

► Cold filter cake (CFC) residue containing Zn and Cd is produced in zinc plants.
► Recovery of Zn and Cd from CFC was studied by alkaline leaching and electrolysis.
► Affecting factors on Zn alkaline leaching and Cd electrowinning were studied.
► 98% of Zn was selectively leached with optimized conditions of alkaline leaching.
► Cadmium recovery was 99.9% by electrolysis in the presence of Ni and Zn.