Hydrometallurgical extraction of zinc from zinc ferrites

The production of steel in electric arc furnaces (EAF) generates a by-product called EAF dusts. Due to the presence of significant amounts of leachable compounds of zinc, lead, cadmium, chromium and nickel, EAF dusts are classified as hazardous wastes. The quantity of EAF dust generated per year around the world represents a possible recovery of about 900 t of zinc. The major obstacle in the hydrometallurgical extraction and beneficial reuse of zinc is the presence of highly stable zinc ferrite ZnFe2O4. The amount of zinc in this form is about 50% of the total zinc. The objective of the process studied is to extract zinc from zinc ferrites contained in EAF dusts without destroying the iron oxide matrix, which can be recycled in the steel industry. The process is a hydrometallurgical treatment of waste based on the destruction of the ferrite structure. For this, ZnFe2O4 is treated by FeCl3·6H2O. The reaction consists of O2−/Cl− exchange allowing the recovery of zinc as ZnCl2 and iron as hematite αFe2O3. The separation of these products is obtained by aqueous leaching. In a first step, the process was studied on zinc ferrites synthesised in the laboratory, then extended to real samples. All the zinc is extracted after a 8 h treatment at 150 °C with a molar ratio FeCl3·6H2O/ZnFe2O4 of 10. The ultimate solid residues, which have been concentrated in iron, should be oriented towards the steel industry. The process has been also applied to roasted zinc concentrates containing zinc ferrite.