6. Electrowinning of Zinc and Lead from Alkaline Solutions

Electrowinning of zinc and lead from alkaline solutions depends on zinc or lead concentrations, temperature, current density, NaOH concentration, electrode distance and electrode material on current efficiency, and energy consumption of zinc or lead electrowinning. The optimum conditions for zinc electrowinning are zinc concentration 30–40 g/L, current density 800–1000 A/m², NaOH concentration 180–200 g/L, and temperature 30–50 °C, and those for lead electrowinning are lead concentration 20 g/L, 5 g/L lead concentration left in the spent electrolyte, below 50 °C, 400 A/m² current density, 5 mol/L NaOH concentration, and stainless steel of electrode material. Impurities ions have great effect on the quality of the metallic zinc or lead powder in alkaline leaching electrowinning process. The electrolysis deteriorates when the concentration of arsenide is over 500 mg/L, but \( {\mathrm{CO}}_3^{2-} \), \( {\mathrm{SO}}_4^{2-} \), \( {\mathrm{SiO}}_3^{2-} \), and F^- have no effects on zinc electrowinning. Copper will be electrodeposited prior to lead in the cathode. Tin, \( {\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{O}}_6^{2-} \), \( {\mathrm{CO}}_4^{2-} \), and gluten in the solution decrease the current efficiency of the lead electrowinning process, while As, Sb, and W, as well as \( {\mathrm{SO}}_4^{2-} \), \( {\mathrm{PO}}_4^{3-} \), \( {\mathrm{SO}}_3^{2-} \), \( {\mathrm{SiO}}_3^{2-} \), SCN⁻, \( {\mathrm{CO}}_3^{2-} \), Cl⁻, and AC⁻, have no effects.