Determination of Equilibrium Constant of Calcium Oxidation by Slag-Metal Equilibrium Distribution Method

Abstract

The equilibrium constant of calcium oxidation is important to evaluate the ability of deoxidation of the dissolved oxygen in metals and the reduction of metal-oxide. Therefore, in this study, the equilibrium constant for calcium oxidation has been determined by a slag-metal equilibrium distribution method by using molten iron and/or titanium-based alloys. In general, this method can be used to determine the equilibrium constant for solved elements, e.g., as expressed by “CaO_(s) = Ca_{in metal} + O_{in metal}”. When the oxygen partial pressure and the activity of calcium in the surroundings are established, the equilibrium constant of “CaO_(s) = Ca_{(l, g)} + 1/2O_2(g)” is determined to be log[K] = −17.50 for titanium-based alloys at 1473 K and log[K] = −12.91 for molten iron at 1873 K. These values are essentially medians obtained using an experimental measurement of the oxygen partial pressure and are based on the definition of the activity of calcium in the slag as given in literature. The temperature dependence of these results has been determined as follows:

log[K] = 4.814 − 32960 (1/T) (1473–1873 K)

or

ΔG° = 631 − 0.0922T [kJ/mol] (1473–1873 K)

Fig. 7 Temperature dependence of the equilibrium constant, “CaO = Ca + 1/2O₂”, combination of Fig. 4 (literature and present study), Fig. 5 (the titanium and titanium-based alloys) and Fig. 6 (molten iron).