Alkali-based components as slag fluidizers in steelmaking processes: A thermodynamic assessment

Abstract

Defining the optimized volume of dual saturated (CaO and MgO) slags is of utmost importance to secondary metallurgy, especially for the ladle furnace. To be effective, these slags may present high desulphurization capability, combining the ability to shield the electric arc flare, to avoid the skull formation in the ladle borders and bottom, and suitable viscosity to provide fast incorporation of additives and tapping. Some fluidizers are applied for attaining the purpose of taking CaO/MgO into solution, being fluorspar (calcium fluoride, CaF2) and silica sand (silicon oxide, SiO2) broadly used in Si-killed steelmaking plants. At the Al-killed plants, either bauxite (mainly aluminum hydroxide) or calcium-aluminates are recommended choices. Nevertheless, for various reasons, some steelmakers use alternative fluidizers containing a significant amount of Na2O and K2O, such as sodalite and nepheline. Based on thermodynamic calculations (CALPHAD method), the effects of such alkalis on the CaO/MgO solubility and the optical basicity of slags on the CaO–SiO2–MgO–Al2O3 system were explored in this paper. Moreover, the potential impact on the performance of MgO-based refractories in contact with these alkali-containing slags was also assessed by a thermodynamic approach. Although kinetic aspects must also be considered, it was seen that (Na2O)2·(K2O) additions into these systems must be carefully managed to make the most of their ability to improve desulphurization, while avoiding the earlier corrosion of the refractory lining.