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

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 (MnO 2 ) 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 (C 6 H 8 O 7 .H 2 O) 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 (Na 2 S) 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.