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Phase Equilibria in the CaF2-CaOP2O5 System at 1623 K SHIGERU UEDA, YUUSUKE WATANABE, TAKASHI IKEDA, and MASAFUMI MAEDA Calcium fluoride is a commonly used additive to enhance the fluidity of the flux in steelmaking. However, the fluorine emissions must be reduced because of their negative environmental effect. Most of the CaF2 in the steelmaking is used in the hot metal pretreatment process. To reduce the mass of fluoride-containing slag remaining after use, it is necessary to (1) develop of a flux system with a high phosphorus dissolution capacity and low activity of phosphorus oxide and (2) substitute some oxide that keeps the solubility of CaO for CaF2. The former will minimize the use of flux and the latter will decrease the concentration of CaF2 in flux. The solubility of CaO and the concentration of phosphorus in slag must be increased to achieve this goal. The distribution of phosphorus between slag and molten iron reportedly increases with increasing phosphorus content over 2 mass pct.[1] However, there is insufficient thermodynamic data available for a system that contains P2O5 of more than 10 mass pct. Although a phase diagram for the CaF2-CaO-P2O5 system is reported,[2] it is restricted to the 3CaO ⭈ P2O5-CaF2 join. The estimation of the distribution of phosphorus between such slags and molten steel requires knowledge of the phase equilibria and the activity of phosphorus int the slag. In the present study, the isothermal phase diagram for CaF2-CaOP2O5 at 1623 K is measured. The quench chemical analysis SHIGERU UEDA, Graduate Student, YUUSUKE WATANABE, Undergraduate Student, TAKASHI IKEDA, Research Associate, and MASAFUMI MAEDA, Professor, are with the Institute of Industrial Science, the University of Tokyo, Tokyo 153-8505, Japan. Manuscript submitted September 26, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B

technique was applied to measure the liquidus. A resistance furnace connected to a controller with a Pt-6 pct Rh/Pt30 pct Rh thermocouple was used. The temperature was controlled within ⫾ 1 K over a length of 40 mm in a mullite tube (outside diameter: 60 mm, inside diameter: 52 mm, and length: 600 mm) in the furnace. In the measurement of phase relations, a nickel crucible was used as a container in CO/CO2 ⫽ 26/1 (PO2 ⫽ 1.0 ⫻ 10⫺8 Pa); it held 2 g of the CaF2-CaO-3CaO ⭈ P2O5 mixture with 0.5 g of a pellet of saturating oxides; CaO, CaO and 4CaO ⭈ P2O5, 4CaO ⭈ P2O5, 4CaO ⭈ P2O5 and CaF2 ⭈ 9CaO ⭈ 3P2O5, and CaF2 ⭈ 9CaO ⭈ 3P2O5. The crucible containing the specimen was equilibrated for 21,600 seconds at 1623 K; preliminary experimental results had shown that 21,600 s was enough to establish the equilibrium. Compounds for saturation were prepared by sintering mixtures of CaO, 3CaO ⭈ P2O5 and CaF2 for 43,200 seconds. The formation of compounds was confirmed by X-ray powder diffraction analysis. After withdrawal from the furnace, the specimens were quenched with flushing Ar gas, and the fluxes were separated from the saturating oxide pieces. The composition of flux was determined by analyzing its Ca