The relationship between slag composition andoxygen partial pressure of dilute Fe_xO-CaO-Al_2O_3 slag inequilibrium with iron the activity of which is below 1 wasextrapolated to give the oxygen partial pressure of a slagequilibrated with iron of the activity 1. In the process ofextrapolation, the oxygen partial pressure and the activity ofiron were unified to give a single thermodynamic quantitya_(Fe_XO) which is defined as: a_(Fe_XO) =K(x)a_(Fe)~xP_(O_2)~(1/2), where K(x) is the equilibriumconstant for the formation reaction of a_(Fe_XO) and isconsidered to be a function of x. The dependence of a_(Fe_XO)on the slag composition was obtained experimentally. On theother hand, it was found that the plot of log(X_(Fe~(3+)))~2/(X_(Fe~(2+)))~3 against log a_(Fe) has theslope -1 as expected from the theoretical consideration. Therelation has been employed to estimate the composition of slagin equilibrium with liquid iron. Thus, the obtained compositionis used to calculate the values of a_(Fe_XO) and P_(O_2) ofthe slag in equilibrium with liquid iron.
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