Nathalie Leclerc

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.

Hydrometallurgical recovery of zinc and lead from electric arc furnace dust using mononitrilotriacetate anion and hexahydrated ferric chloride

The purpose of this work was to study the feasibility at laboratory-scale of a new hydrometallurgical process for treating electric arc furnace dusts (EAFD). The proposed process is intended to extract zinc and lead from EAFD without destroying the iron oxides matrix. So, this material can be recycled by the steel industry. Independently of the origin of the samples, major mineralogical forms present in these wastes are Fe3O4, ZnO, ZnFe2O4 and PbOHCl. The proposed process consists of a hydrometallurgical treatment of wastes based on selective leaching of zinc and lead. Initially, a leaching is carried out utilizing a chelating agent, nitrilotriacetate anion (NTA3-), as the protonated form HNTA2-. Treatment of five EAFD samples for an hour at room temperature with a molar solution of reagent results in total leaching of the ZnO. In all cases the solubilized iron does not exceed 3 wt.%. The recovery of zinc and lead is performed by precipitation of metallic sulfides with a solution of Na2S4 sodium tetrasulfide 2M. These metallic sulfides can be used as metallurgical raw materials and the chelating reagent can be reused in the process after pH adjustment. The results of the normalized leaching test AFNOR X31-210 conducted on the leaching residues, shows that all the samples meet acceptance thresholds for hazardous wastes landfill. However, the residues contain a considerable amount of zinc as ZnFe2O4. The extraction of the zinc element requires the destruction of the ferrite structure. In this process, ZnFe2O4 is treated by FeCl3.6H2O. The reaction consists in a particle O2-/Cl- exchange allowing the recovery of zinc as ZnCl2 and iron as hematite Fe2O3. The separation of these products is accomplished by simple aqueous leaching. All of the zinc is extracted in a 8h treatment at 150 degrees C with a molar ratio FeCl3.6H2O/ZnFe2O4 equal to 10. Ultimate solid residues, which have been concentrated in iron, can be oriented towards the steel industry.