Viscosity and structure evolution of CaO-SiO2-based mold fluxes with involvement of CaO-Al2O3-based tundish fluxes

In this study, the effect of involvement of CaO-Al2O3-SiO2-MgO tundish flux into CaO-SiO2-Al2O3-Na2O-CaF2 mold flux on viscosity and structure of mold flux was investigated through employing the rotating viscometer and associated structure analysis including high resolution Raman spectroscopy and solid-state Al-27 magic angle spinning nuclear magnetic resonance (Al-27 MAS-NMR). The results showed that the viscosity continuously increases with increasing involvement of tundish flux. The apparent activation energy obtained from the temperature dependence of the viscosity presents an increase from 128.5 kJ/mol to 149.1 kJ/mol with increase of involvement ratio from 0 to 0.75. Semi-quantitative analysis using the deconvoluted Raman spectra and Al-27 MAS-NMR spectra revealed an increase in amounts of [AlO4] and Si-O-Al units, but in the meanwhile a decrease in amounts of [AlOnF4-n] and [AlO5] units, this indicates the structure transformation from prevalent silicate structure to complex alumino-silicate structure with increasing involvement ratio. A self-developed structurally-based viscosity model can predict the viscosity of mold flux with varying involvement ratio and presents a smallest deviation of 15.8% compared to those prevalent viscosity models despite of the great transformation of the melt from silicate system to alumino-silicate system.