Effect of Liquid Fraction on Reaction Rate of Molten Steel Desulfurization Using Solid-Liquid Coexisting Slag

In the high purification refining processes, a CaO-containing flux or slag is used commonly for the desulfurization of molten steel. Because the slag generated during the actual process is not always a homogeneous liquid, the quantitative relation between the liquid fraction of the slag and its mass transfer coefficient must be clarified. Therefore, to elucidate the effects of the liquid fraction on the desulfurization rate and the mass transfer coefficient in the slag phase, kslag, desulfurization experiments were performed on molten steel using a resistance heating furnace with solid CaO-liquid coexisting slag corresponding to the CaO–Al2O3 and CaO–CaF2 systems. The overall mass transfer coefficient and sulfur partition ratio at the slag-metal interface were evaluated based on the regression analysis of the experimental data using the reaction rate equation, and kslag could be determined. It was found that kslag decreased slightly with a decrease in the liquid fraction of the slag for both the CaO–Al2O3 and CaO–CaF2 systems. This is because the effective diffusion coefficient decreased with the decrease in the liquid fraction. Two empirical equations for kslag for the solid-liquid coexisting compositions were formulated; these were based on a previously reported equation for the effective diffusion coefficient and the regression analysis of the experimental data obtained in this study. The threshold value of the liquid fraction between the equations for kslag was determined to be 0.87. This is possibly because the state of the solid in the slag changes because of the percolation transition.