<p>In the current study, the established thermodynamic model coupled the multicomponent mixed transport control theory and the local equilibrium theory within a single time step, considering real-time updates of the density, mold flux viscosity, mass transfer coefficients, and bulk layer thickness. The model assumed the presence of a transition layer on the mold flux side to better align with the mass transfer behavior. There was a good correlation between the predicted and experimental results. The influence of the parameters used in the model on the prediction results was thoroughly assessed. It was indicated that the concentration distribution coefficient and the thickness of the steel reaction layer had little effect on prediction results. The evolution of the weight, viscosity, density, and bulk layer thickness during the steel–flux reaction process was tracked. The analysis of the mass transfer behavior throughout the reaction indicated that the Al mass transfer was insufficient to solely control the steel–flux reaction rate. It was difficult and unrealistic to systematically evaluate the influence of the initial composition on the reaction through laboratory experiments. Hence, the current model was employed to predict the composition evolution during the reaction at 1550 °C under various initial Al content, Si content, CaO/Al₂O₃ ratio, CaO/SiO₂ ratio, Al₂O₃/SiO₂ ratio, and SiO₂/Na₂O ratio. It was noteworthy that the Al–Na₂O reaction barely occurred when the SiO₂/Na₂O ratio surpassed 6.33.</p>