Metallurgical processing of zinc bearing residues
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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. From a pre-feasibility study a innovative flowsheet for processing these residues has been developed. The chemical compositions of the residues have been determined. XRPD analysis showed that zinc ferrite was a major phase in both residues. Zinc ferrites are not soluble under normal alkaline and acidic conditions. To achieve sufficient recovery of zinc, zinc ferrite is converted into soluble zinc oxides by a roasting step with addition of a reagent. This has been investigated using synthetic zinc ferrite. Na2CO3 has been chosen as reagent and used in further experiments. Zinc A is first water washed to remove halides and (earth) alkali elements. Then both residues are leached in an alkaline solution of 5M NaOH at 90˚C for 1 hour, in which both zinc and lead are selectively extracted, leaving iron oxides in the residue. The residue 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 water washed to remove residual sodium salts. The filtrates from the first and second leaching step are purified, with use of zinc dust, or directly used for electrowinning experiments. Halides and sodium salts were insufficiently removed during water washing. Overall recoveries of zinc and lead of Zinc A were 74% and 55%, and of Zinc B 74% and 71%, respectively. These recoveries were insufficient. Cementation of heavy metals and electrowinning of zinc was feasible. However, optimization is needed to obtain products of wanted purity. The final residue is not suitable as an aggregate or feedstock for steel industries and cannot be disposed of by landfilling without further specialized treatment. Finally, recommendations for major operating steps and flowsheet development are given.