An assessment of the Ca-Fe-O system
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I.
INTRODUCTION
THE CaO-FeO-Fe203
system is of central importance for steelmaking and has been studied experimentally many times with an emphasis on various aspects. The purpose of the present study was to attempt to fit all of the experimental information by applying thermodynamic models to the individual phases and then to obtain the phase diagram and thermochemical properties by computer calculation. The hope was to obtain a more reliable phase diagram and, in addition, more reliable descriptions of the thermodynamic properties of the phases, in particular, the liquid. The numerous studies of the Fe-O system will not be covered here because that system is discussed in a parallel work. Ill A metallic phase can exist inside the CaO-FeO-Fe203 triangle, and it is thus necessary to extend the assessment to include the Fe and Ca comers. The very scarce information available on the Ca-Fe and Ca-CaO systems has been used in order to have a reasonable description of these two binaries as a basis for assessing CaO-FeO-Fe203. A special notation is generally used when discussing these oxide systems and will be used here. CaO is denoted by C, FeO by W, and Fe203 by F. All stoichiometric oxide phases and solutions based upon them will be denoted by combinations of these three components. II.
S E L E C T E D E X P E R I M E N T A L DATA
A. Ca-Fe Sponseller and Flinn ]21have investigated the solubility of Ca in liquid Fe in equilibrium with liquid Ca. This must be done in a pressurized system to avoid losing Ca by boiling. No information on the mutual solubility in the solid phases could be found. Thus, the same regular interaction parameter as in the liquid was used for the face-centered cubic (fcc) and body-centered cubic (bcc) phases.
B. Ca-CaO Above 50 pct oxygen, the Ca-O system is unknown. In the range of 0 to 50 pct oxygen, some information is
given by Bevan and Richardson. ]3] No solubility data for the solid phases could be found. Thus, the same regular interaction parameter as in the liquid was used in the fcc and bcc phases.
C. CaO-Fe20s Experimental information on the CaO-Fe203 system has been obtained by studies in air and in a pure oxygen atmosphere at 1 atm by Phillips and Muan. ]4] That information is not on the real binary CaO-Fe203 system, as there will be a significant fraction of Fe +2 present in the liquid, but these isopotential sections fall to a large degree fairly close to the true binary side. Phillips and Muan ]4J conducted experiments with the quenching technique under controlled oxygen pressure. When the results were presented, they gave the composition of the liquid in only some of the cases. The results at the two oxygen potentials were presented as diagrams resembling binary ones. They show three intermediary compounds, C2F, CF, and CF2. Two of them, CF and CF2, melt incongruently, and C2F melts congruently at both potentials. Some pieces of information on the liquid composition at 1 atm O2 will be used in the present assessment. King lSj gave values on entropy and heat capacity at 298 K
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