Silicon-Oxygen equilibrium and nitrogen distribution between CaO-SiO 2 slags and liquid iron
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I.
INTRODUCTION
F O R better control of ladle refining processes, such as deoxidation and desulfurization, it is of crucial importance to obtain accurate equilibrium data on slag-metal reactions. For this purpose, there have been a number of studies on sulfur distribution between slag and metal, sulfide capacities, and activities of the components in CaO-based slags. The present authors previously measured the nitrogen distribution ratios between CaO-A1203 slags and liquid iron as a function of AI content m and nitride capacities in CaO-A1203, CaO-SiO2, and CaOSIO2-A1203 slags, using a gas-slag equilibration technique. I2,31 On the basis of these results, the activities of A1203 in the CaO-A1203 system were assessed, tll As a result, the activities thus obtained at the compositions saturated with a CaO crucible were significantly different from Rein and Chipman's reported value, I41 which is generally accepted for the thermodynamic calculation for ladle refining. Activity data in the CaO-SiO2 system are not well established, particularly at the saturated slag compositions of 2CaO. S i O 2. The present investigation is part of a series of studies aimed at clarifying the conflicting activity data in the CaO-SiO2-A1203 system. For this purpose, nitrogen distribution ratios between CaO-SiO2 slags and liquid iron were measured using a CaO crucible in the temperature range of 1823 to 1923 K. Using these results and those of nitride capacity obtained in previous slag-gas experiments, 131 the activities of SiO2 were evaluated.
A. Materials The master Fe-Si alloy was prepared by melting mixtures of high-purity electrolytic iron [99.99 mass pct, 30 mass parts per million (ppm) O, 3 mass ppm A1, by equilibrating CaO-SiO2 slag with a CO-Nz(-Ar) gas mixture at 1823 K in a graphite crucible. The results obtained by extrapolating the results of Ito and Fruehan I]61 to compositions saturated with a CaO crucible, as well as the values of Martinez and Sano, E17} are indicated in Figure 4 by an arrow mark. The present slag-metal experimental results are in good agreement with those obtained in a gas-slag experiment by Tomioka and Suito.{3} In the present work, the oxygen potential in slag-metal reaction is controlled by the Si/(SiO2) equilibrium given by Eq. [ 1]. Substituting the free-energy change for the reaction given by Eq. [1], AG~, into Eq. [7], one can express log CN, by Eq. [8], in which the values for log CN~- can be obtained without using the measured values for oxygen contents. log CN3
log [mass-% Si]
METALLURGICAL TRANSACTIONS B
[5]
D3/4/DI/2
3/4 "aft)} [8]
The log CN~ values calculated from Eq. [8] were estimated to be +-0.004. The values for asio2{41 and the respective interaction parameters/hi for f~ and fs_ were used for calculating CN~ . These results are plotted in Figure 5. It is clear that the present results agree well with those obtained in a gas-slag experiment of Tomioka and Suito {31 at 1823 K, except for those obtained at very low Si contents, but they tend to slightly deviate at 1873 and 1923 K. T
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