Equilibria between the rare earth elements, oxygen and sulfur, in molten iron

Abstract

The values of Kce2o2s and KCe2S 3 reported in the literature are greater than they should be. Such a situation has arisen because in the calculation of the respective equilibrium constants, the total amount of Ce, O, and S each contained in the samples has been used instead of only the dissolved amounts. The relationships between equilibrium constants and temperatures of Ce-O-S, Y-O-S, Nd-O-S, La-O-S, and Ce-S in pure liquid iron have been determined by using a previously developed experimental method. The activity interaction coefficients, e N~, e~, and es La at different temperatures have been determined. The standard free energies of formation of Nd202S, YzO2S, and Ce2S 3 have been calculated by using the relevant experimentally derived parameters. I. INTRODUCTION THE rare earth elements are strong deoxidizers and desulfurizers. In liquid steels treated by rare earth elements, rare earth oxides and sulfides and oxysulfides, such as RE2OzS, are formed. The rare earth oxysulfides are found more often in steel ~-4 than the sulfides or oxides. Nevertheless, little work has been done on the equilibrium of RE-O-S in molten steel until recently. Considerable research has dealt with the problems on the equilibrium constant of low-valency rare earth sulfide in molten iron, but there is almost no information on the determination of equilibrium constant for high-valency rare earth sulfides. Highly sensitive radioassay was adopted to determine the amount of dissolved RE and S in the iron specimens by means of electrolyzing the specimens in organic electrolyte at low temperature to remove the rare earth inclusions. The oxygen activity in molten iron was measured by a solidelectrolyte oxygen-probe made of ZrOz(MgO) tube so as to determine accurately the equilibrium constants of RE-O-S and RE-S.