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INTRODUCTION

SILICA-CONTAINING slags are formed during the pyrometallurgical processing of zinc and lead. The chemistry of both the complex sinters and slags formed in the different processes used can only be fully explained by high-order systems including several different elements. The development of thermodynamic computer models enables these complex predictions of phase equilibria in both lower- and higher-order systems to be carried out but requires precise information, especially on the lower-order systems. One of the several ternary systems relevant to the zinc/lead smelting using the Imperial smelting process (ISP) includes the Al2O3SiO2-ZnO system. Both the ZnO and SiO2 are main components of the ISP sinters and slag, while Al2O3 is also present, but at lower concentrations (the main sources of Al2O3 are coke ash and, in some cases, fluxing). The previously available phase-equilibria data on the ternary Al2O3-SiO2-ZnO system have been evaluated during optimization of a thermodynamic model using the FactSage[1] computer package. Possible discrepancies in previous data have been suggested, since no reasonable thermodynamic parameters could be found to describe previous experimental information. These discrepancies prompted the present experimental study. The phase equilibria in the Al2O3-SiO2 system have been extensively studied.[2–7] The only binary compound (mullite (Al6Si2O13)) formed in the system has been suggested to melt both incongruently[2–5] and congruently.[6,7] An assessment performed by Eriksson and Pelton[8] of the Al2O3-SiO2 system concludes that mullite melts congruently at 1890 °C, resulting in eutectics between both mullite and corundum (Al2O3) and mullite and cristobalite (SiO2). Bunting[9] used a quenching technique and optical metallography to investigate phase ROBERT HANSSON, formerly Research Scholar, PYROSEARCH, Pyrometallurgy Research Centre, The University of Queensland, is Laboratory Manager, ALcontrol AB, 581 10 Linkoping, Sweden. BAOJUN ZHAO, Research Fellow, 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: e.jak@ minmet.uq.edu.au Manuscript submitted March 3, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B

equilibria in the binary SiO2-ZnO system. A complete phase diagram including the liquidus temperatures was reported. Willemite (Zn2SiO4) was found to melt congruently at 1512
3 °C, and the melting point of ZnO was found to be 1975
25 °C.[9] Eutectics were reported between both tridymite (SiO2) and willemite and between willemite and zincite (ZnO). A liquid miscibility gap forming above 1695 °C in the silica-rich part of the binary system[9] was also reported. Solidus temperatures in the Al2O3-ZnO system were measured by Bunting.[10] The solidus data in the Al2O3-ZnO system were used to suggest that gahnite (ZnAl2O4) melts congruently, at temperatures close to 195