Study of Sulfide Capacity of Slag and Sulfur Removal from Hot Metal

Abstract: The present study aims at investigating the desulfurization process in hot metal and sulfide capacity of slags. The missing experimental data of sulfide capacities in Al2O3-CaOMgO-SiO2 system at 1713 K, 1743 K, 1773 K, 1823 K, 1873 K and in the Al2O3-CaO-SiO2 system at 1873 K were investigated under wellcontrolled oxygen potentials. These data along with the reliable and accurate data carefully selected from the literature were employed for KTH model optimization. The model was successfully optimized and produced good predictions of sulfide capacity between 1700 K and 1873 K for all liquid slags in the Al2O3-CaO-MgO-SiO2 system, especially for ladle slags and blast furnace slags, with an average relative deviation of approximately 15%. The partition of sulfur in the blast furnace at tapping was investigated. The results of re-melted slag and hot metal at temperatures of 1743 K and 1773 K showed that the two phases were not in equilibrium with respect to sulfur at tapping. Furthermore, about 30 min was required to reach equilibrium. The optimum equilibrium sulfur partition in the range of blast furnace slag was determined using sulfide capacity data calculated with the newly-optimized model. The results showed clearly that the best area which provides a good equilibrium sulfur partition is between 10 to 14 mass% MgO. The effect of aged CaC2 on the desulfurization of hot metal was investigated at 1673 K and 1773 K. The results show that the use of aged CaC2 does not have significant effect on the desulfurization process since all the samples used exhibited almost the same performance, despite being subject to different aging treatments. The possibility of re-sulfurization during the long waiting time in the transfer ladle before the BOF was investigated. From the results, II different solid phases were found present in the slag along with small portion of liquid slag. In addition, majority of sulfur in the slag is in the solid phase in the form of CaS, which is only a small fraction of the slag. The liquid slag and solid CaS was found to have a limited contact with the hot metal leading to very poor kinetics for re-sulfurization. Therefore, the amount re-sulfurization that is observed in the transfer ladle was found to be very low.