Effect of Na2O on Viscosity, Structure and Crystallization of CaF2–CaO–Al2O3–MgO–TiO2 Slag in Electroslag Remelting

The effect of Na₂O on the viscosity, structure, and crystallization behavior of CaF₂–CaO–Al₂O₃–MgO–TiO₂ slag was studied using the rotating cylinder method, differential scanning calorimetry, Fourier transform infrared spectroscopy, and X-ray diffraction analyses. The analyses demonstrated that with increasing Na₂O content, the viscosity and melting temperature of the slag decreased; however, the crystallization temperature increased. Na₂O acted as a network modifier to decrease the degree of polymerization in the titanoaluminate structure and consequently increased the mobility of ions in the slag structure. The network structures in the [AlO_nF_{4 – n}]-tetrahedral complexes and [AlO₄]-tetrahedra are depolymerized with the addition of Na₂O; however, the depolymerization had a minimal effect on the Ti–O stretching vibration. During the continuous cooling process of the slag without Na₂O, the crystalline phases of 11CaO·7Al₂O₃·CaF₂ were dominant, followed by CaF₂ and CaTiO₃. In addition to the first three phases, a new precipitated phase of NaAlO₂ was observed in the slag when Na₂O was added, and the sequence of crystallized precipitation during the continuous cooling process was 11CaO·7Al₂O₃·CaF₂, CaF₂, CaTiO₃, and NaAlO₂. The effective activation energy of 11CaO·7Al₂O₃·CaF₂ increased with increase in Na₂O content (0–3.9 wt %) and reached a maximum; however, a further increase in Na₂O content (6.6 wt %) decreased the effective activation energy of 11CaO·7Al₂O₃·CaF₂.