Phase relations in the Fe-rich part of the system Fe2O3(-Fe3O4)-CaO-SiO2 at 1240-1300 degrees C and oxygen partial pressure of 5 x 10(-3) atm: implications for iron ore sinter

Synopsis Equilibrium phase relations in the Fe-rich portion of the system F e2O3(-Fe3O4)-CaO-SiO2 (FCS) have been determined at 1240-1300 degrees C and under an oxygen partial pressure of 5 x 10(-3) atm, typical for iron ore s intering. Experiments were performed with the use of a rapid-quench techniq ue and the chemical compositions of the melts and coexisting condensed phas es were determined by combined optical microscopy and electron-probe microa nalysis. At temperatures greater than ca 1250 degrees C the major phase fie lds comprise Mt + Liq, C2S + Liq, C2S + C2F + Liq and C2F + Liq (where Mt r epresents magnetite, C = CaO, F = Fe2O3, S = SiO2 and Liq denotes a quench liquid phase) plus an extensive, continuous, Liq-only phase field that exte nds from Fe-rich to SiO2-rich compositions. At less than about 1250 degrees 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 a ny particular Fe-oxide phase is strongly linked to the overall basicity (Ca O/SiO2) of the charge. Experiments show that SFCss (silico-ferrite of calcia solid solution), whic h is the major ferrite bonding phase in low-Al sinter, cannot be produced a s a single, unique, crystalline phase within the FCS system at 5 x 10(-3) a tm O-2. At this oxygen partial pressure the bulk composition lies within th e partially reduced Fe2O3(-Fe3O4)-CaOSiO2 pseudo-ternary with the three-pha se assemblage Mt + SFCss + Liq(alpha) stabilized at low temperatures (1240 degrees C). Combined with data for the FCS ternary in air the results indic ate that to maximize the formation of SFCss bonding phase a low-temperature (<1260 degrees C), semi-reduced heating environment followed by a relative ly slow, oxidizing, cooling stage is desirable.