The mechanism of ferrite formation from iron sulfides during zinc roasting
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INTRODUCTION
FORMATION of the spinel zinc ferrite ZnFe:O4 during the roasting of iron-bearing zinc concentrate in industrial fluidized-bed reactors decreases the amount of zinc that is easily leachable. Recovery of the 5 to 15 pct of original zinc that is present in the leach residue requires substantial further processing, both for dissolution of the residue and for subsequent iron removal by precipitation. In addition, disposal of the final precipitate in dumps constitutes an increasingly unacceptable environmental hazard because of its physical and chemical character. These problems will become more pressing as the nature of the ores that are available for mining renders them more difficult to benefieiate, thus resulting in increased amounts of iron in the concentrate. I1'2] Clearly, prevention of the formation of zinc ferrite in the roaster would greatly facilitate early iron removal from the process stream by simple methods and the resulting residue would be environmentally stable. Iron is present in zinc concentrates in two major forms. One is as a solid solution with zinc in the mineral sphalerite (Zn, Fe) S; the other is as the iron sulfide minerals pyrite FeS2, pyrrhotite FeS, and chalcopyrite CuFeS2. The processes that result in the formation of ferrite from iron dissolved in the zinc sulfide have been described previously. [31 This paper addresses the contribution to the formation of ferrite that is made by the iron sulfides pyrite and pyrrhotite. A number of aspects of this process have been investigated by others, including the oxidation of iron sulfides, the reaction between iron and zinc oxides to form ferrite, and the subsequent transformation of this ferrite to stoichiometric ZnFe:O4. There is considerable information on the kinetics and mechanism of the oxidation of iron sulfides[4-81 and on the thermodynamics and phase relationships of the J. W. GRAYDON, Research Associate, and D. W. KIRK, Associate Professor, are with the Department of Chemical Engineering, University of Toronto, Toronto, ON, Canada, M5S 1A4. Manuscript submitted December 2, 1987.
METALLURGICALTRANSACTIONSB
Fe-S-O system. [8'91 Due to the importance of zinc ferrite, not only in the extractive metallurgy of zinc, but also to the electronics industry for magnetic uses, numerous thermodynamic and kinetic studies of the Zn-Fe-O system have been made. [1~ There also have been several investigations of the reaction mechanisms involved in the formation of zinc ferrite from its constituent oxides.j16'17'18]The crystal chemistry of iron-bearing zinc ferrite under oxidizing conditions also has been studied. [19'2~ Most of this information was obtained using reagent grade chemicals in atmospheres with high oxygen partial pressures and with only three of the four components of interest present. Thus, its applicability to the process of ferrite formation in zinc concentrate under conditions prevailing in a commercial roaster is limited. In particular, although the behavior of hematite particles in a zinc roaster has been investigated,
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