Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the CrO-Cr 2 O 3 -SiO 2 -CaO
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I.
INTRODUCTION
THIS is the third in a series of three articles on the inclusion of chrome oxides in a database of optimized thermodynamic properties of oxide systems. In the first article,[1] the methodology was described, the modified quasichemical model, which is used for the molten slag phase, was outlined, and optimizations were performed for the CrO-Cr2O3, CrO-Cr2O3-Al2O3, and CrO-Cr2O3-CaO systems. In the second article,[2] the systems CaO-Cr2O3-SiO2 and CrO-Cr2O3-SiO2-Al2O3 were optimized. Evaluated and optimized thermodynamic properties for all compounds as well as optimized parameters of the quasichemical model for the liquid phase for the CrO-Cr2O3 and CrO-Cr2O3-SiO2 systems are given in these articles.[1,2] For the CaO-SiO2 system, optimized properties of all compounds, the liquid phase, and the nonstoichiometric Ca2SiO4 phase are given in an earlier publication.[3] The calculated optimized phase diagram of the CrOCr2O3-CaO-SiO2 system, resulting from the present study, is shown in Figures 1 through 5. The results are presented now in order to facilitate comprehension of the following description of the available experimental data and the thermodynamic optimization. Figure 1 is a calculated projection of the liquidus surface of the Cr-Si-Ca-O system in equilibrium with metallic Cr from the Cr corner of the tetrahedron onto the CaO-CrOSiO2 plane. That is, if CaO, CrO, and SiO2 are mixed in a proportion given by a point on this diagram and are equilibrated with excess Cr, then the equilibrium phase assemblage can be read from the diagram. Figures 2 and 3 are calculated liquidus projections in air and at pO2 5 1026 atm, respectively. The composition axes are to be interpreted as XSiO2 5 nSi/nTOT, XCaO 5 nCa/nTOT, and X(CrO1.51CrO) 5 (nCr21 1 nCr31)/nTOT, where ni is the number of moles of species i and nTOT 5 (nSi 1 nCa 1 nCr21 1 nCr31). Figures 4 and 5 show calculated sections for the pseudobinary systems CaSiO3-Cr2O3 and Ca2SiO4-CaCr2O4, respectively. SERGEI DEGTEROV, Senior Research Associate, and ARTHUR D. PELTON, Professor, are with the Department of Metallurgy and Materials Engineering, E´cole Polytechnique de Montre´al, Montreal, QC, Canada H3C 3A7. Manuscript submitted May 2, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
Fig. 1—Calculated CrO-Cr2O3-SiO2-CaO liquidus in equilibrium with Cr (T in 7C).
Under oxidizing conditions, and in the presence of high concentrations of CaO, oxidation states of Cr greater than (31) occur in significant concentrations in the liquid phase. In the present study, it is assumed that only Cr2+ and Cr3+ occur in the liquid. Hence, the CaO-rich part of this system (i.e., compositions in the CaO-Ca2SiO4-CaCr2O4 triangle) in air is not treated in the present optimization.
II.
AVAILABLE DATA
The liquidus under strongly reducing conditions was studied by de Villiers and Muan[4] by the quenching method followed by micrography, X-ray diffraction (XRD), and electron microprobe analysis. The latter technique was used mainly to determine compositions of the liquid near
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