Modelling Viscosities of CaO-MgO-Al2O3-SiO2 Molten Slags

A structurally-based viscosity model (using a few optimized parameters) is proposed to represent viscosity as functions of both temperature and composition for the CaO-MgO-Al2O3-SiO2. The model represents the slag structure through the different types of oxygen ions formed in the melt. Approximate methods for calculating the concentrations of these different types of oxygen ions are proposed and are then used to describe the effect of melt structure on viscosity. The model provides a good description of the viscosity behavior varied with composition and temperature within the CaO-MgO-Al2O3-SiO2 system. This includes pure systems: Al2O3 and SiO2; binary systems: CaO-SiO2, MgO-SiO2, Al2O3-SiO2 and CaO-Al2O3; ternary systems: CaO-MgO-SiO2, CaO-Al2O3-SiO2 and MgO-Al2O3-SiO2; quaternary system: CaO-MgO-Al2O3-SiO2. The different roles of CaO and MgO on viscosity are also discussed; these tend to differ in melts with or without Al2O3. In the absence of Al2O3, CaO reduces the viscosity more than MgO. In contrast, when Al2O3 is present, the Ca2+ ions take priority over Mg2+ ions in the charge-compensation of Al3+ ions which leads to the formation of more stable Ca-AlO45- tetrahedral structures, which, in turn, results in an increase in viscosity. However, when there is enough basic oxide (CaO or MgO) present to generate non-bridging oxygens, CaO reduces the viscosity more effectively than MgO.