A Microscopic study of the transformation of sphalerite particles during the roasting of zinc concentrate
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I.
INTRODUCTION
THE increasing
amount of iron in zinc concentrates that are now being produced, despite optimal milling and flotation techniques, has directed considerable attention to the problem of the removal of this iron at a later stage in the process. The most common treatment of these concentrates at present is dead roasting in a fluidized bed reactor during which all of the iron forms a spinel-zinc ferrite (ZnFe204), while the remaining zinc is converted mainly to the oxide (ZnO) and sulfate (ZnSO4). The latter two compounds are easily leached with dilute acid from the calcine for further processing, but the ferrite requires an additional, hot, concentrated acid leaching stage.~ Although this leach results in the total dissolution of all zinc present in the ferrite, it also causes dissolution of all of the iron that was present in the original concentrate. This dissolved iron must then be removed prior to recove~ of the zinc, and several precipitation methods are used. Two of these (the jarosite and goethite processes) produce considerable quantities of residue which contain soluble, toxic impurities. Thus, this residue must be permanently retained in ponds with provision for continuous collection of seepage, and it presents an increasingly unacceptable environmental hazard. A third process which precipitates iron as hematite results in a more manageable residue but involves the use of high temperature, acid-resistant autoclaves and so is quite costly. All three of these techniques for iron removal involve additional expense as well as a loss of at least 1 pct of total zinc in the residue. Removal of the majority of the iron prior to leaching is an economically attractive alternative which can be realized if the formation of zinc ferrite during roasting is minimized. There have been many experimental investigations of the oxidation of zinc and iron sulfides, both in their pure state and together as solid solutions. 3-13 As well, sufficient J.W. GRAYDON, Research Associate, and D.W. KIRK, Assistant Professor, are with the Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, ON, Canada, M5S 1A4. Manuscript submitted July 27, 1987. METALLURGICALTRANSACTIONS B
information is available from equilibrium studies to construct phase diagrams for the Zn-Fe-S-O system in some detail. ~4.15However, more information is required about the mutual solubilities between phases, the behavior of nonstoichiometric compounds, and the possible existence of complex phases. The exact reaction pathways and the mechanisms involved during roasting also depend on kinetic factors. Most of the kinetic studies 7-~3 have been carried out on either single or multi-particle systems at a macroscopic level. The usual technique for phase identification is X-ray diffraction which provides information only on abundant, crystalline, well characterized compounds and gives no information at all on their morphology. Little attention has been directed at the microscopic changes that occur in a particle of sphal
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