Phase equilibria for the Fe2O3-rich part of the system Fe2O3-CaO-SiO2 in air at 1240-1300 degrees C

Equilibrium phase relations in the Fe2O3-rich portion of the system Fe2O3-C aO-SiO2 (FCS) have been determined at 1240-1300 degrees C in air. Experimen ts were performed using a rapid-quench technique and the chemical compositi ons of the melts and coexisting condensed phases were determined by combine d optical microscopy and electron-probe microanalysis. At temperatures above 1270 degrees C the major phase fields comprise Hem Liq, C2S + Liq, C2S + C2F + Liq and C2F + Liq, plus an extensive, continuou s liquid-only phase field extending from Fe2O3-rich to SiO2-rich compositio ns. The existence of the single, large liquid-only phase field is not consi stent with previous investigations of the FCS system, which predict two dis continuous melt fields stable up to at least 1315 degrees C. At temperatures between 1270 and 1255 degrees C the liquid-only melt region segregates into two distinct melt fields: one at high Fe2O3 contents and b asicities (CaO/SiO2) greater than B = 2.0, which results in calcium ferrite crystallization on cooling; and a second melt field present at low basicit ies (B < 2.0) and high SiO2 contents, which is characterized by low-tempera ture crystallization of calcium silicate(s) and hematite. The composition o f the first melt to develop in the FCS system will therefore depend strongl y on the basicity. At 1240 degrees C, the lowest temperature examined in the study, the stable existence of a SiO2-substituted Ca-ferrite solid solution, denoted SFCss, has been demonstrated. The SFCss phase lies on a trend line between the ide al end-members CF3 and C4S3 and is structurally and chemically related to t he iron ore sinter phase SFCA.