Use of CaO-Al2O3 Synthetic Slag on EAF Tapping for Steel Desulfurization

ABSTRACT Steel desulfurization is usually performed by CaO-Al2O3-based top slags. However, in the steelmaking process at a steel mill in the city of Cariacica-ES, Brazil, deoxidation is carried out on tapping with the addition of ferro-silicon-manganese and silicon carbide, and desulfurization is carried out on ladle furnace with the addition of lime and calcium carbide. The present work aims to change this process by adding a CaO-Al2O3 system slag on the EAF steel tapping, so that the desulfurization occurs already in this step, thus enabling the CaC2 economy in the ladle furnace. Three different lime/alumina briquettes ratio were determined using the software ThermoCalc software, where the CaO activity, liquid slag ratio and viscosity were the main properties analyzed. Then, the desulfurization yields of these heats were compared to the conventional route used at this steelmaking process. The influence of additions and of the slag carry-over on desulfurization efficiency were analyzed by correlation charts and a mathematical model developed using the software Minitab 19. It was found that the slag carry-over from the EAF impairs the desulfurization, causing sulfur reversion from tapping until the ladle furnace arrival, and decreasing the desulfurization rate (desulfurization kinetics) on the ladle furnace treatment by promoting high equilibrium sulfur content, close to the real reached value. The most efficient slags were the ones with the greatest calcium oxide activities, which is in line with the Steel Desulfurization Factor.