Structural Understanding for Thermodynamic Behavior of TiO<sub>2</sub> in CaO–Al<sub>2</sub>O<sub>3</sub>–MgO–TiO<sub>2</sub> Slag

The thermodynamic behavior of TiO2 in CaO–Al2O3–based vacuum oxygen decarburization (VOD) slag must be understood to quantify the equilibrium of the reaction between titanium and oxygen in the molten steel and TiO2 in the VOD slag for the refining of stainless steels. Accordingly, in the present study, the thermodynamic activity of TiO2 in the CaO–Al2O3–MgO–TiO2 slag was measured through equilibrium experiments between molten silver and slag with pO2 = 10−16 atm at 1823 K. The stability of TiO2 in the calcium aluminate melts was investigated from a structural perspective through the Raman spectroscopy. The titanium oxide in the present VOD slag mainly exists as TiO2, which was determined by performing an X-ray photoelectron spectroscopy analysis. The activity coefficient of TiO2 in the CaO–Al2O3–MgO–TiO2 slag decreased with increasing slag basicity (CaO/Al2O3 = 0.7 to 1.3) and TiO2 content (2 to 10 wt%) in the slag. It was confirmed that titanium could be stabilized as ionic form in the present calcium aluminate melts by employing the titanate capacity concept. When the basicity of the slag increased, the [TiO3]–units were stabilized due to a decrease in the Al–O–Al bonds, resulting in an enhancement of TiO2 incorporation within the aluminate network. When the TiO2 content increased, the [TiO3]–units replaced the non-bridging oxygen of structure units, resulting in a further stabilization of TiO2 in the calcium aluminate network structure.