Experimental study of phase equilibria in the PbO-MgO-SiO 2 system
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I. INTRODUCTION
INTEREST in the phase equilibria of the PbO-MgOSiO2 system stems from two principal sources. Magnesia refractories are used commercially to contain lead smelting slags during high-temperature processing. Magnesium is also a common impurity element found in the primary smelting of lead/zinc ores. Characterization of the phase equilibria in the PbO-MgO-SiO2 system is therefore useful in prediction and interpretation of phases formed in these reaction systems. The MgO-SiO2 system has been investigated by previous researchers.[1,2,3] The principal features of the MgO-SiO2 system include a region of liquid immiscibility on the silicarich side, a congruently melting compound forsterite, Mg2SiO4, and an incongruently melting compound protoenstatite, MgSiO3, which form binary eutectic liquid ⫹ MgO ⫹ Mg2SiO4 and peritectic liquid ⫹ Mg2SiO4 ⫹ MgSiO3, respectively. Wu et al.[4] have reported the thermodynamically calculated and optimized MgO-SiO2 phase diagram based on all available experimental results. The PbO-SiO2 system was systematically investigated by Smart and Glasser,[5] who reported compounds with PbO:SiO2 mole ratios of 4:1, 3:1, 2:1, 1:1, and 5:8. Subsequently, however, only the existence of the compounds Pb4SiO6, PbSiO3, and Pb2SiO4 have been confirmed by other researchers.[6–9] Jak et al.[10] have summarized the available experimental results and reported the thermodynamically optimized PbO-SiO2 system. No data were found in the literature for the MgO-PbO system. Argyle and Hummel[11] investigated the PbO-MgO-SiO2 system as the part of a study of the PbO-BaO-MgO-SiO2 system. Their investigations were concentrated on the subsolidus region, but some liquid was reported to form at temperatures below 1173 K. The ternary compound Pb2MgSi2O7 was reported to melt congruently at 1105 K, S. CHEN, Associate Professor, is with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, the People’s Republic of China. B. ZHAO, Postdoctoral Research Fellow, E. JAK, Senior Research Fellow, and P.C. HAYES, Associate Professor, are with the Department of Mining, Minerals and Materials Engineering, The University of Queensland, St. Lucia, Queensland, 4072, Australia. Manuscript submitted July 25, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
based on results of quenching and differential thermal analysis experiments. The ternary compound Pb2MgSi2O7 was reported to form extensive solid solutions with Pb2SiO4, PbSiO3, MgSiO3, Mg2SiO4, and SiO2. However, the method used to measure the solid solution and supporting details was not given in the article.[11] In a later study, Billhardt[12] pointed out from X-ray powder diffraction (XRD) study that this ternary compound Pb2MgSi2O7 belongs to the Pbbarysilite group, and the formula of this ternary compound should be Pb8Mg(Si2O7)3. Sugimoto and Kokusa[13] measured the activity of PbO in molten PbO-MgO-SiO2 slag at 1273 K. The objective of the present investigation is to characterize the liquidus and the phase relationships in the PbO-MgOSiO2 system
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