Evaluating Composition Dependence in Surface Tension of Si–Ca–Na–O–F Reciprocal Oxide–fluoride Melts

Abstract

Surface tension of molten ionic mixtures such as slag is an important physical property related directly to various surface or interfacial phenomena in high-temperature industrial processes. In particular, surface tension of oxide–fluoride melts is of interest to understand interfacial phenomena in continuous casting in steelmaking. However, the composition dependence of surface tension in reciprocal ionic melts has not been well understood because of its complexity related to the reciprocal reaction between input components in the melt. In this study, we measured the surface tension of reciprocal oxide–fluoride melts in a Si–Ca–Na–O–F multicomponent system, to evaluate its composition dependence. We used a maximum bubble pressure method to determine the surface tension of these melts with high accuracy. Our results indicate that (1) the surface tension of the SiO₂–CaO melt decreases by NaF addition more steeply than by Na₂O addition, and (2) the surface tension of the SiO₂–CaO–CaF₂ melt decreases with Na₂O addition but depends on NaF formation because of the reciprocal reaction CaF₂ + Na₂O = CaO + 2 NaF. When this reaction is taken into account, the surface tension of the SiO₂–CaO–CaF₂–Na₂O melt matches that of the SiO₂–CaO–NaF melt with increased CaO concentration from the above melt composition.