The effect of CaF 2 on thermodynamics of CaO-CaF 2 -SiO 2 (-MgO) slags
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6/1/04
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The Effect of CaF2 on Thermodynamics of CaO-CaF2-SiO2(-MgO) Slags CHUL-HWAN CHOI, SUNG-KOO JO, SEON-HYO KIM, KWANG-RO LEE, and JEONG-TAE KIM To address the role of CaF2 in the CaO-CaF2-SiO2(-MgO) slag system employed for the production of low-pressure rotor steels, the thermodynamic aspects of the slag were investigated by equilibrating it with liquid iron at 1873 K in CaO or MgO crucibles. Presaturation of slag with an oxide block piece of CaO or MgO in a Pt crucible and application of a carbon paste to the outside of an oxide crucible were designed to prevent crucible failure during the slag-metal experiments. The liquidus isotherm and phase boundary of the preceding slag system were investigated using the slag-metal equilibria. Also, the effect of CaF2 on the sulfide capacity and the activity coefficient of FetO were of particular interest in controlling the sulfur level and cleanliness of low-pressure rotor steels.
I. INTRODUCTION
RECENTLY, there has been a tendency to enhance the energy capacity of nuclear power plants. Doosan Heavy Industries has abundant experience in commercializing a 500-ton ingot of low-pressure rotor steel. The ingot is manufactured by applying slag deoxidation and vacuum multipouring (four ladles) casting processes. As the initial hydrogen content in steel before the vacuum pouring stage is increased, the mean diameter of steel droplet under vacuum is reduced.[1] The oxygen content should be maintained low to secure the cleanliness of ingot. The refining process of rotor steels using the CaO-CaF2 slag system is devised for the purpose of obtaining a steel grade with low oxygen and high hydrogen contents. Therefore, it is very important to know the effect of CaF2 on the slag’s activities, phase equilibria, and sulfide capacity for steel cleanliness. There have been few reports on the effect of CaF2containing slags at steelmaking temperatures (1823 to 1923 K) by equilibrating them with liquid iron in oxide crucibles. This is probably caused by experimental difficulties such as crucible failure by slag dissolution. Generally, at steelmaking temperatures, oxide crucibles such as CaO and MgO crucibles can withstand the liquid slag if it is nearly saturated with crucible materials in advance. Reliable information on the solubilities of the oxides in CaF2-containing slags is needed for such slag-metal experiments. This information is also essential to control refractory attack in the steelmaking process with CaF2-containing slags. Mukerji has established the phase equilibria in the system CaO-CaF2-SiO2 using differential thermal analysis (DTA) and quenching experiments.[2] The 1573 K isotherm in the CaOrich region of the system was reinvestigated by Selin et al.[3] They performed the saturation experiments between slag and a CaO single crystal. As shown in Figure 1, there is considerCHUL-HWAN CHOI, Graduate Student, SUNG-KOO JO, Research Associate, and SEON-HYO KIM, Professor, are with Department of Materials Science and Engineering, POSTECH, Pohang, Kyungbuk, 790-
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