Analysis of the influence of adding CaF2 and Na2O to steel desulfurizing mixtures using computational thermodynamics

Steel desulfurization mixtures of the CaO-Al2O3 system are the most industrially used. These mixtures require an amount of solid CaO to keep them always saturated in CaO. The aim of this paper is to study the influence of adding CaF2 and Na2O on the desulfurization efficiency of CaO-Al2O3 system mixtures. Desulfurizing steel mixtures capable of producing steels with sulfur content of less than 0.0020% were formulated. Desulfurizing mixtures based on CaO-Al2O3-flux were prepared with fluxes CaF2 and Na2O. All mixtures presented 95% of liquid phase and 5% of solid CaO, which, according to the literature, are the most efficient slags of this system. The equilibrium conditions were simulated using the composition of the mixtures and the steel by means of computational thermodynamic software. In these simulations, the activities of the slag components, the percentage of solids and liquids, the slag viscosity, and the equilibrium sulfur content were determined. The experimental tests were carried out in an induction furnace at 1600 °C. In addition, data on sulfide capacity (Cs), sulfur partition (Ls) and optical basicity (ʌ) of the initial mixtures, were calculated. The results of the simulations show that all mixtures have thermodynamic potential to reach the target of sulfur content set for this work. The mixtures with CaF2 proved to be the most efficient. However, it is possible to obtain sulfur contents of 0.0020% using mixtures without CaF2.