Phase relations in the Fe-rich part of the system Fe 2 O 3 (-Fe 3 O 4 )-CaO-SiO 2 at 1240-1300°C and oxygen partial pressure of 5 × 10 -3 atm: implications for iron ore sinter

Abstract

Equilibrium phase relations in the Fe-rich portion of the system Fe₂O₃(-Fe₃O₄)-CaO-SiO₂ (FCS) have been determined at 1240-1300°C and under an oxygen partial pressure of 5 × 10^-3 atm, typical for iron ore sintering. Experiments were performed with the use of a rapid-quench technique and the chemical compositions of the melts and coexisting condensed phases were determined by combined optical microscopy and electron-probe microanalysis. At temperatures greater than ca 1250°C the major phase fields comprise Mt + Liq, C ₂S + Liq, C₂ S + C₂ F + Liq and C₂ F + Liq (where Mt represents magnetite, C = CaO, F = Fe₂O₃, S = SiO₂ and Liq denotes a quench liquid phase) plus an extensive, continuous, Liq-only phase field that extends from Fe-rich to SiO₂-rich compositions. At less than about 1250°C the Liq-only melt region segregates into two distinct melt fields—one at high Fe contents and basicities greater than 1.8-2.0 and a second melt at low basicities (<1.8) and high SiO₂ contents. At all temperatures examined the reduced oxygen conditions significantly enlarged the field of silicate melt present relative to that of Ca-ferrite melt. Magnetite (predominantly) or hematite may be the stable Fe-oxide phase; however, the occurrence of any particular Fe-oxide phase is strongly linked to the overall basicity (CaO/SiO₂) of the charge.

Experiments show that SFC_ss (silico-ferrite of calcia solid solution), which is the major ferrite bonding phase in low-Al sinter, cannot be produced as a single, unique, crystalline phase within the FCS system at 5 × 10^-3 atm O₂. At this oxygen partial pressure the bulk composition lies within the partially reduced Fe₂O₃(-Fe₃O₄)-CaO-SiO₂ pseudo-ternary with the three-phase assemblage Mt + SFC_ss + Liq_α stabilized at low temperatures (1240°C). Combined with data for the FCS ternary in air the results indicate that to maximize the formation of SFC_ss bonding phase a low-temperature (<1260°C), semi-reduced heating environment followed by a relatively slow, oxidizing, cooling stage is desirable.

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