Multicomponent diffusion in molten slags
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Multicomponent Diffusion in Molten Slags M.D. DOLAN and R.F. JOHNSTON A method for investigating the diffusion of metal cations in multicomponent slag systems was developed. This method used microprobe analysis and allowed the analysis of the diffusion of multiple species within a single system. This project focused on the diffusion behavior of manganese, iron, calcium, and silicon in silicate slags, in order to simulate industrial steel and ferromanganese production. The molecular structure of CaO-Al2O3-SiO2 slags was investigated with Raman spectroscopy, and oxidation states of manganese and iron in slags of varying composition were determined. This study identified the variation in diffusivity of slag components with changes in composition and temperature of multicomponent slag systems. An empirical model based on the correlation between optical basicity and diffusivity was developed to predict the multicomponent diffusivity of ionic species in molten silicate slags. The model takes into account properties of the bulk slag and the network forming ability of the diffusing species. The relative rate of diffusion of metal cations is proportional to the optical basicity coefficient of that species, while the rate of diffusion of all species increases exponentially with the calculated optical basicity of the bulk slag.
I. INTRODUCTION
SINCE the article by Towers et al.[1] on the diffusion
of Ca2 in molten CaO-SiO2 slags, many studies employing the radiotracer method (for example, References 2 through 8) have been devoted to the diffusion of ionic and molecular species through molten slags and glasses. However, many aspects of this field of study are still unexamined. This project aims to enhance current knowledge by providing a comprehensive understanding of the diffusion of metals cations (including transition metals) and silicon (as silicate anions) in multicomponent systems. Transport processes and kinetics of interphase reactions determine the rate of production in a smelting system. Because the diffusivity of an element through liquid metal is shown to be approximately an order of magnitude greater than diffusivity of the same element through liquid slag,[9] diffusion through the slag phase is a rate-limiting step in the overall process. Consequently, studying the rate of diffusion of manganese and iron through the slag phase is important in determining kinetics of the smelting processes. Tracer studies only apply to the limiting case in which the concentration gradient of all nontracer species is zero. Once a significant concentration gradient of a second species is introduced, cross-effects from one species impart a flux on other diffusing species, causing a departure from the ideal tracer diffusivity to multicomponent diffusivity. Because industrial smelting systems are complex, with multiple species and multiple concentration gradients, evaluation of the cross-effects between slag components is necessary. The diffusivity of manganese, iron, calcium, and silicon will be thoroug
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