Hydrometallurgical treatment of zinc ash from hot-dip galvanizing process

The oxide fraction of industrial zinc ash was characterized in terms of chemical and phase composition, leaching behavior in sulfuric acid solutions, and leaching thermal effect. The waste product contained about 68 percent zinc (Zn), 7 percent chlorine (Cl), 2 percent aluminum (Al) and less than 1 percent other metals, such as iron (Fe), manganese (Mn), calcium (Ca) and silicon (Si). It consisted mainly of zinc oxide contaminated with metallic zinc and zinc hydroxide chloride. The dissolution of metals from the zinc ash was determined for solid-to-liquid ratios ranging from 100 to 500 kg/m3, acid concentrations of 10 and 20 percent, and temperatures of 20 and 40 °C. The best result — 130 g/dm3 Zn(II) at 95 percent zinc recovery — was obtained at a solid-to-liquid ratio of 200 kg/m3, sulfuric acid concentration of 20 percent and initial temperature of 40 °C. The final solutions were contaminated mainly by Mn(II) and Fe(III) ions, but the presence of cadmium (Cd(II)) and lead (Pb(II)) ions enabled electrowinning of zinc of about 99 percent purity at high current efficiency of 85 to 99 percent. Leaching of the material was an exothermic process with reaction heat of about 520 kJ/kg.