Modeling Viscosities of CaO-MgO-FeO-MnO-SiO2 Molten Slags

A model for estimating the viscosity of silicate melts is proposed in this article. The structural characteristics of a silicate slag can be described by the numbers of the bridging oxygen, nonbridging oxygen, and free oxygen present in the slag. A method of calculating the numbers of the different types of oxygen ions is presented in this article, which involves a simple approximation of “complete bridge breaking.” With just a few parameters, the model provides both the temperature and composition dependencies of viscosity for the pure component: SiO2; the binary systems: MgO-SiO2, CaO-SiO2, FeO-SiO2, and MnO-SiO2; the ternary systems: CaO-MgO-SiO2, CaO-FeO-SiO2, MgO-FeO-SiO2, and CaO-MnO-SiO2; and the quaternary systems: CaO-MgO-MnO-SiO2 and CaO-FeO-MnO-SiO2. It was found that the ability of different basic metal oxides to decrease viscosity varies and is in the following hierarchy: FeO > MnO > CaO > MgO. Two factors influence the viscosity: The first is related to the mutual interaction among different ions, and the stronger the interaction, the higher the viscosity. The second factor is the size (radius) of basic oxide cation, with viscous flow becoming increasingly more difficult (i.e., viscosity increases) as the cation size increases. However, there is a paradox in the effect of cation radius (of the basic oxide) on the two factors. Thus, varying cation size causes competitive effects; smaller cationic radii give stronger interactions among ions but less hindrance to viscous flow (and vice versa for large cation radii).