Structural Understanding for Thermodynamic Behavior of TiO₂ in CaO-Al₂O₃-MgO-TiO₂ Slag

Abstract

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 p_O2=10-16 atm at 1 823 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 10wt%) in the slag. It was confirmed that titanium could be stabilized as (TiO₃^2-) 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 Q_Al³ structure units, resulting in a further stabilization of TiO₂ in the calcium aluminate network structure.