Interdiffusivities and mass transfer coefficients of NaF gas
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INTRODUCTION
INTERDIFFUSIVITIES (mutual diffusion coefficients) of gases have been extensively investigated by both experimental and theoretical methods. Among the many empirical and theoretical expressions,[1–33] the Chapman–Enskog theory has been successful in the prediction of transport coefficients such as viscosity, thermal conductivity, and diffusivity. The Chapman–Enskog equation is in good agreement with observation, especially in the temperature range 200 to 1000 K. Quantum effects become important below 200 K, and above 1000 K, the Lennard–Jones potential function is no longer applicable.[7] The accuracy for the viscosity of nonpolar gases at low density is of the order of 1 pct or less. However, for thermal conductivity and the diffusion coefficient, the accuracy is within 6 pct for polar gases; however, there are little data on the gaseous fluorides in the literature and no Lennard–Jones parameters exist for fluorides.[1–9] In high-temperature processing, gaseous fluoride species can evolve from the slags and fluxes used during processing and can lead to environmental contamination and enhanced corrosion of the structural members surrounding the areas where fluorides are used. This is especially true in the secondary cooling chamber of a steel continuous casting machine, where the mold flux, which is clinging to the cast slab, is exposed to a highly humid environment at temperatures above 1100 K. The problem of fluoride vaporization has recently become a topic of research in the steelmaking process, as there are several types of fluoride off-gases, such as SiF4, NaF, AlF3, and KF, which can evolve from process slags containing CaF2. In a humid atmosphere, HF Y. KASHIWAYA, Visiting Professor, formerly with the Faculty of Engineering, Hokkaido University, Sapporo 060, Hokkaido, Japan, is with the Department of Materials Science and Engineering, Carnegie Mellon University. A.W. CRAMB, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted August 12, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
can also form by the reaction between the fluorides present either in the gas or slag phase and water. As there is little fundamental physiochemical or transport data for fluorides, the equilibrium vapor pressures of SiF4 from several kinds of slags were studied by Zaitsev et al.,[34] Shinmei et al.,[35] and Neufeld et al.[36] SiF4 is a gas phase from ambient to high temperature. However, when a slag contains Na2O, as is common in the slags used in the mold of a steel continuous caster, the vaporization species are not only SiF4 but also NaF. Overall reactions for SiF4 and NaF vaporization from a mold slag are as follows: (SiO2) 1 2(CaF2) 5 2(CaO) 1 SiF4 (g)
[1]
(Na2O) 1 (CaF2) 5 (CaO) 1 2NaF (g)
[2]
To estimate the extent of environmental contamination and the variation of slag properties due to composition change during operation, it is necessary to understand the kinetic factors that affect the rate of evolution of thes
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