Phase-diagram study for the Al 2 O 3 -CaF 2 -SiO 2 system
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I. INTRODUCTION
CALCIUM fluoride is a convenient additive to enhance the fluidity of the flux in steelmaking. It decreases the melting point of a mixture of calcium oxide and silicates and increases its reactivity. However, a reduction in the emission of fluoride is also mandated, because of its negative environmental effects. The vaporized species of high-temperature liquid flux may result in pollution of the environment, and the disposal of after-use slag may result in fluoride seeping into the earth. The ultimate objective of the study was to decrease the use of fluoride and, thus, minimize its impact on the environment. In order to reduce fluoride, it must be replaced by other additives that can keep the reactivity of the flux. Aluminum oxide may be one candidate for such a substitute. Although the phase diagram for the Al2O3-CaF2-SiO2 system has been reported,[1] it is restricted to the region of a two-liquid phase; as is well known, the CaF2-SiO2 system has a miscibility gap from about 1663 to 1873 K. The region of a homogeneous liquid and a miscibility gap in the Al2O3-CaF2-SiO2 system was studied to seek the possibility of substituting CaF2 with Al2O3. II. EXPERIMENTAL PROCEDURE A. Hot-Filament Technique The hot-filament technique was used to measure the temperature of phase transformation (the liquidus and congruent transformation point of two liquid phases). Details of the technique are described elsewhere.[2,3] A 0.5-mm-diameter Pt-6 pct Rh/Pt-30 pct Rh thermocouple was used as a heating device, as a holder of the sample, and to measure temperature. The temperature was calibrated by measuring the known melting points of KBr, NaCl, K2SO4, and 12CaO ? 7Al2O3. Figure 1 shows the relationship between the reading of measured temperature and the reported melting points of pure substances. Two grams of reagent-grade SiO2, CaF2, and Al2O3 were carefully weighed and thoroughly mixed in an agate mortar.
SHIGERU UEDA, Graduate Student, is with the Department of Metallurgy, the University of Tokyo, Tokyo, 113-8656, Japan. MASAFUMI MAEDA, Professor, is with Metal Source Technology, Institute of Industrial Science, the University of Tokyo, Tokyo, 106-8558, Japan. Manuscript submitted January 20, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
The mixture, of about 10 mg, was set on a filament, where it was melted and quenched in advance. The hot-filament cell was purged with argon gas to eliminate water vapor; the gas was introduced at a flow rate of 50 mL/min. During the measurement, the temperature of the sample was increased by about 30 K/min, and the measurement was completed within 90 seconds. The difference between the composition of the initial mixture and that after melting was less than 61 mass pct, as confirmed by chemical analysis. The phase transformation of the melt on the filament was visually observed by a microscope with a video camera, placed above the hotfilament cell. B. Chemical Equilibrium Technique A silicon carbide electric resistance furnace connected to a proportional-integral-differential
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