Effect of Fluorspar and Industrial wastes on Desulfurization efficiency of Molten steel

Abstract

Fluorspar (CaF2) has been widely used to control the physicochemical properties of slag in steelmaking processes. However, it is very important to reduce the CaF2 consumption because it causes various environmental problems. In the current study, therefore, the effect of industrial waste materials such as ferromanganese slag (FMS), red mud (RM) and white mud (WM) on the desulfurization behavior of molten steel was investigated in order to find potential substitutes for fluorspar. Compared to conventional 10 pct CaF2-containing ladle slag, when excess industrial wastes were added (i.e., more than 10 mass pct), the composition of the slag was dramatically changed (SiO2 increases, while FetO and MnO decrease). Therefore, the sulfide capacity of the slag decreased, and the viscosity of the slag increased. Thus, the desulfurization efficiency was also reduced. However, when the RM or FMS were added not more than 10 mass pct to lean-CaF2 (3 pct) slag, the sulfide capacity of the slag was equivalent to 10 pct CaF2-containing ladle slag; thus, the desulfurization efficiency was also equivalent with lower melting point and higher fraction of liquid phase. When WM was added, the desulfurization efficiency was equivalent or higher than that of the conventional ladle slag. In the slag melting experiment conducted based on DIN 51730, it was observed that the melting rate of mixed slag increased when WM added, which is considered to have a positive effect on the initial desulfurization behavior. In addition, WM does not cause a change of slag composition through chemical reactions with components in metal. Therefore, when WM is added, the sulfide capacity is equivalent compared to the CaF2 containing slag, so it does not provide a negative environment for desulfurization thermodynamically, and the viscosity decreases, so it does not negatively affect the desulfurization efficiency. Since desulfurization of molten steel has to consider both thermodynamic and kinetic factors, in this study proposes the logCS2- -log η index. If the logCS2- -log η index which is considered both thermodynamic and kinetic factor is above critical value, the desulfurization efficiency is equivalent to those of the conventional ladle slag. However, if the logCS2- -log η value is less than critical value, the ko significantly decreases and desulfurization reaction is strongly affected by the physicochemical properties of the slag. Consequently, it was confirmed that a small amount (less than 10 mass pct) of industrial wastes can be added as a desulfurization fluxing agent as a partial substitute for fluorspar