Reductive leaching of zinc, cobalt and manganese from zinc plant residue

• Zinc plant hot filter cake (HFC) contains ZnO, Co(OH)3, MnO2.
• Reductive leaching of cobalt and manganese using sulfuric acid and citric acid
• Leaching recovery of cobalt, manganese and zinc: 96.43%, 90.26% and 64.12%
• Sulfide precipitation stage to separate Co2 +, Zn2 +, Mn2 + with Na2S
• Conceptual flowsheet with acid leaching, reductive leaching and sulfide precipitation

Zinc plant residue known as hot filter cake (HFC) contains a significant quantity of zinc oxide (ZnO). In addition, it holds cobalt hydroxide Co(OH)3, manganese dioxide (MnO2) and minor amounts of nickel, copper, iron and lead containing species. The process consists of three unit operations: (i) selective leaching of zinc; where the effects of parameters such as acid concentration (0–1.0 M), temperature (40–70 °C), time (15–90 min) and solid to liquid ratio (S/L) (0.01–0.1 g mL− 1) are investigated. At this stage, 81.5% of zinc, 2.0% of cobalt and 1.7% of manganese were extracted at the optimum conditions determined. (ii) Reductive leaching of cobalt and manganese; where the leaching residue of step (i) was leached with 1 M sulfuric acid solution in the presence of citric acid (C6H8O7.H2O) as a reducing agent. The investigated independent leaching parameters were citric acid content (30–60% more than the stoichiometric ratio), S/L ratio (0.02–0.1 g mL− 1), temperature (75–95 °C) and time (45–75 min). Response surface methodology (RSM) was used to optimize the parameters. In the optimum conditions, recovery of cobalt, manganese and zinc was 96.43%, 90.26% and 64.12%, respectively. (iii) Sulfide precipitation; at this stage, in order to separate Co(II), Zn(II) and Mn(II), sodium sulfide (Na2S) 1 M was added to the solution from the reductive leaching, at the optimum conditions investigated. Recovery of zinc, cobalt and manganese was 93.45%, 95.15% and 100%, precipitated at pH = 2, pH = 3 and pH = 8, respectively. Finally, based on this research work, a conceptual flowsheet was developed.