Equilibrium pressures of silicon tetrafluoride in contact with both CaO-SiO_2-CaF_2 synthesized slags and industrial mold slags were measured as a function of temperature and slag composition. CaO-SiO_2-CaF_2 slags emit primarily SiF_4 (g) under the conditions used in this study. The equilibrium pressure of SiF_4 strongly depends on temperature and slag basicity. The examination of the offgas from industrial mold slags revealed that not only was SiF_4 a stable gas in equilibrium with the industrial mold slag, but also NaF was a significant component of the offgas. Although the equilibrium partial pressures of SiF_4 are of the order of 10~(-3) atm for a prefused flux with low calcium fluoride contents, in raw mixed powders, achieving quite high equilibrium partial pressures (> 0.5 atm) is possible if the first liquid to form contains a high calcium fluoride content and is in contact with a silica at a high activity.
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