Reevaluation of phase equilibria of the iron-rich corner of the CaO-Fe₂O₃-Al₂O₃ system at 1240°C in air

The compositional ranges of SFCA series and the phase equilibrium relationships were reevaluated in the iron-rich corner of the CaO-Fe 2 O 3 -Al 2 O 3 system at 1240 ° C in air using powder and single crystal XRD as well as EPMA: The liquidus line was reexamined using the samples with initial compositions close to the liquidus line. The observed liquidus line was shifted to the Fe 2 O 3 rich side from the previous one reported by the present authors. The liquidus compositions of the samples, the initial compositions of which were far from the liquidus line, may be affected by CF (CaFe 2 O 4 ), CFF (Ca 2 Fe 15.51 O 25 ) and C 2 F (Ca 2 Fe 2 O 5 ) precipitated during quenching. Some of the limits of compositional range of SFCA-I, SFCA-II and SFCA were determined by newly prepared samples with the three-phase equilibriums such as liquid + SFCA-I + hematite. It has been found that the Al 2 O 3 concentration is the smallest for SFCA-I and the largest for SFCA: The Al / (Al + Ca + Fe) ranges from 5.61% to 17.55% for SFCA-I in equilibrium with a liquid phase, from 14.75% or even lower to 25.00% for SFCA-II and from 19.93% to 31.42% or even higher for SFCA. It has been found that the phases of a sample having the initial composition of 10.04CaO-63.47FeO 1.5 -26.49AlO 1.5 (mol%) equilibrated at the oxygen partial pressure of 0.1 atm and at 1390 o C are SFCA-I and SFCA-III. The value of Fe 2+ /Fe in SFCA-III has been calculated to be 20.36% assuming that the structural formula of M 26 O 36 (M = Ca, Fe, Al) is satisfied by the presence of Fe 2+ : (Ca 2+ 1- x Fe 2+ x ) 6 (Fe 3+ 1- y Al 3+ y ) 20 O 36 ( x = 0.57, y = 0.33).