Effect of CaF2 and CaO/Al2O3 on viscosity and structure of TiO2-bearing slag for electroslag remelting

The relationship between the viscosity and structure of CaF2-CaO-Al2O3-MgO-TiO2 slag with different CaF2 contents and CaO/Al2O3 ratios was studied using the rotating cylinder method, Fourier transform infrared spectroscopy, and Raman spectrometry. The activity coefficients of CaF2 and the CaO/Al2O3 ratio were determined to understand the correlation between viscosity and structure of the slag. The results suggest that the slag viscosity reduces gradually with an increase in CaF2 content from 14.1 to 28.1 wt.% or CaO/Al2O3 ratio from 0.9 to 1.5, and correspondingly apparent activation energy for viscous flow reductions. The addition of CaF2 does not change the structure of the molten slag; however, the relaxation effect of the anionic species and the hindrance effect of the cationic species are promoted by substituting part of the non-bridging oxygens (NBO) with F- ions from CaF2, which is attributed to the formation of NBO-Ca2+-F- and NBO-Ca2+-NBO, respectively. However, as the CaO/Al2O3 ratio increases, some of the Q(4) units in the aluminate structure are depolymerized into Q(2) units, so that the relative strength of the Al-O-Al linkage decreases, and the relative fraction of Ti2O64- chains increases, whereas that of O-Ti-O chains decreases slightly, resulting in depolymerization on the slag structure. Additionally, the effect of the CaO/Al2O3 ratio on the structure was greater than that of CaF2 because of the greater depolymerization effect. The variation in the activity can indirectly explain the relationship between the viscosity and structure of the aluminate structural units based on thermodynamic analysis.