Roasting of La Oroya Zinc Ferrite with Na 2 CO 3
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ODUCTION
ZINC ferrite (ZnFe2O4) is produced globally as a byproduct of the oxidative roasting of iron-bearing zinc sulfide concentrates through the reaction of zinc oxide (ZnO) with hematite (Fe2O3).[1] Zinc ferrite is not soluble in the low acid conditions of the neutral leach and, thus, its formation can cause significant insoluble zinc losses in zinc electrolytic plants that operate with iron-bearing concentrates and can necessitate the storage, or secondary treatment, of the zinc ferrite residues produced. Both hydrometallurgical and pyrometallurgical methods for the recovery of zinc from these residues are used industrially. The most common approach commercially is PRESTON C. HOLLOWAY, formerly a Ph.D. student with the Department of Chemical and Materials Engineering, University of Alberta, is currently a Research Metallurgist and Alberta Ingenuity Industry Associate, Sherritt Technologies, Fort Saskatchewan, AB, Canada T8L 4K7. Contact e-mail: [email protected] THOMAS H. ETSELL, Professor, and ANDREA L. MURLAND, Undergraduate Student, are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6. Manuscript submitted February 10, 2007. Article published online September 7, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
to leach the zinc ferrite in a hot, concentrated H2SO4 solution, followed by precipitation of the dissolved iron values from solution as either jarosite, goethite, or hematite, prior to zinc dust purification and electrowinning.[2] The Waelz process is the dominant pyrometallurgical technology for treating these materials and involves the reduction of the zinc ferrite using coke or coal in a horizontal rotary kiln above 1000 C.[2] Metals, such as Zn, Cd, and Pb, are reduced and volatilized and are reoxidized to metal oxides in the outlet gas stream and collected as dust or fume, while iron and other gangue elements remain in the solid residue, where they are reduced to a combination of oxides, sulfides, silicates and metallic iron.[3] (Zinc ferrites are also processed commercially using Ausmelt technology, in which the ferrite is mixed with SiO2 and coal or coke and similarly reduced, to form a metal oxide fume and an iron silicate residue.[4]) Both methods have significant advantages and disadvantages. High zinc recoveries, as well as the potential recovery of In, Ga, and Ge from the ferrite residues, are possible with hot acid leaching, but both the volume and the environmental stability of the iron residues produced are significant concerns. Waelz kiln processing and Ausmelt processing are much more energy intensive, but VOLUME 38B, OCTOBER 2007—781
both produce smaller volumes of more stable solid waste than does hot acid leaching. With iron precipitation, hot acid leaching can produce a leach solution that is readily integrated into existing zinc electrolytic flowsheets, while the low purity and high Cd, Cl, Pb, and F content of the dusts produced from Waelz kiln or Ausmelt processing would make the production of high-purity electrolyt
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