In this study metallurgical processing of two different kinds of zinc-bearing residues have been performed: Zinc A and Zinc B. These residues have been stored for over 15 years in Rotterdam Harbor.
The chemical compositions of the residues have been determined and showed that zinc ferrite is a major phase present. Zinc ferrite is not soluble under normal alkaline and acidic conditions and is not recovered by the Waelz-process, which is commonly employed for such zinc-bearing residues.
An innovative flowsheet for processing zinc ferrite-bearing residues has been developed during a pre-feasibility study, with goal to selectively recover zinc, including zinc from zinc ferrite. The innovative flowsheet consists of the following steps:
(1) Water pre-washing: removing water soluble salts, in particular the chlorides in Zinc A (~9%).
(2) 1st step alkaline leaching with caustic soda (NaOH): dissolving free ZnO into solution, for both water-washed Zinc A and original (unwashed) Zinc B.
(3) Roasting of the first leach residue in the presence a suitable reagent: decomposing the zinc ferrite to free ZnO.
(4) 2nd step alkaline leaching with NaOH: dissolving all free ZnO into solution.
(5) Solution purification by cementation: removing impurities in particular lead and copper, by using zinc powder.
(6) Electrowinning of zinc in NaOH solution: the purified zinc bearing solutions are subsequently precipitated to the final product of Zn metal.
Optimal operating conditions for the processes are deduced from a literature review in which similar residues are processed. Additionally, optimal operating conditions for the conversion of zinc ferrite into zinc oxide has been investigated using synthetic zinc ferrite with addition of Mg(OH)2, Ca(OH)2, NaOH, or Na2CO3. Finally, Na2CO3 has been chosen as reagent and used in experiments with real zinc-bearing residues.
Zinc A is water washed to remove the chlorides present. Then both the water washed residue of Zinc A, and Zinc B, are leached in an alkaline solution of 5M NaOH at 90˚C for 1 hour. Both zinc and lead are selectively extracted, leaving iron oxides and zinc ferrite in the residue. The filtercake is fused with Na2CO3 at 950˚C for 2 hours to convert zinc ferrite into zinc oxide. The calcined product is leached in fresh alkaline solution of 5M NaOH to recover zinc. The final residue is then water washed to remove residual sodium. The filtrates from the first and second leaching step are purified, with use of zinc dust, or directly used for electrowinning experiments.
The removal efficiency of chloride, sodium and potassium during water washing of Zinc A were 62%, 41% and 71% respectively. Overall dissolution yields for Zinc A and Zinc B of zinc and lead were 82%, 80% and 64%, 78% respectively. Cementation of impurities (Pb, Cu, Cr) with zinc dust followed by an electrowinning step achieving a grade zinc deposit of 94%.
Finally, it can be concluded that the combined hydro -and pyrometallurgical flowsheet is technically feasible. Furthermore, results can be improved further by optimization of major operating steps.