Phase equilibria in the system Fe-Zn-O at intermediate conditions between metallic-iron saturation and air
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INTRODUCTION
THE largest portion of zinc in zinc/lead smelting sinters is present as zincite (ZnO), making the volume fraction of this phase significant. The physical properties of the sinter are among the key factors that determine the efficiency of the Imperial Smelting Process (ISP); in particular, the sinter softening temperature has been shown to depend strongly on the zincite morphology.[1–3] It has been shown that the zincite morphology is directly related to the phase equilibria and, more specifically, to the chemistry of the Fe-Zn-O system at high temperatures[2] in air. The transformation from an equiaxed (rounded) to a platelike shape was related to the ferric iron concentration in the zincite.[2] Yamashita et al.[4] undertook a study to link the observed phase-equilibrium characteristics of the Fe-Zn-O system to the crystal chemistry of the zincite. Rietveld refinement of X-ray diffraction data together with high-resolution transmission electron microscopy was used in the investigation. It was shown that the morphological transition to platelike zincite occurs at ferric iron concentrations as low as 2 mol pct. The c lattice parameter of the hexagonal zincite unit cell was observed to increase with increasing ferric iron concentration on the tetrahedral sites normally occupied by zinc. The increase of the c dimension in the zincite crystal was found to be associated with the presence of stacking faults on the basal plane; this appears to affect the crystal growth in the [0 0 1] direction and result in the formation of platelike zincite. The phase equilibria in the FeO-Fe2O3-ZnO system have been extensively studied. Benner and Kenworthy[5] investiROBERT HANSSON, formerly Research Scholar, PYROSEARCH, Pyrometallurgy Research Centre, The University of Queensland, is Laboratory Manager, ALcontrol AB, 581 10 Linkoping, Sweden. PETER C. HAYES, Professor and Director, and EVGUENI JAK, Associate Professor and Research Director, are with PYROSEARCH, Pyrometallurgy Research Centre, School of Engineering, The University of Queensland, St Lucia, Queensland, 4072, Australia. Contact e-mail: [email protected] Manuscript submitted March 3, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
gated the Fe-Zn-O system at different oxygen partial pressures. Mixtures of ZnO-Fe2O3 were equilibrated in fixed atmospheres and quenched. Determination of the equilibrium concentration of ferrous iron in samples was performed using a wet-chemical method. The level of extension of the spinel solid-solution single-phase field toward the FeOx corner of the system was reported at 1100 °C, 1300 °C, and 1400 °C for pO2 levels ranging from 1 104 to 1 atm. Claude et al.[6] investigated the wustite-spinel and wustite–metallic iron twophase equilibria in the Fe-Zn-O system at 900 °C by a quenching method. Lykasov et al.[7] studied the two-phase wustite-zincite equilibria using electromotive-force measurements (EMF) with a solid electrolyte of yttrium oxide and zirconium dioxide at 847 °C, 897 °C, and 997 °C. Only the solubility of ZnO i
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