Stability of SFC (silico-ferrite of calcium) solid solution limits, thermal stability and selected phase relationships within the Fe2O3-CaO-SiO2 (FCS) system

Abstract

Quenching experiments have been performed to investigate the thermal stability, solid solution limits, and selected phase relationships of SFC (silico-ferrite of calcium) within the Fe2O3-CaO-SiO2 (FCS) system. Experiments were performed in air over the temperature interval 1050–1260°C using a combination of synthetic oxide mixtures and SFC compositions which had been pre-synthesized at 1200°C. SFC forms a solid solution along a trend line between the theoretical end-members CF3 and C4S3. The maximum solid solution range occurs between compositions containing approximately 7.0 through to 11.7 wt% C4S3 component. The solution range is valid between 1060°C and 1240°C. Above 1240°C the compositional range narrows until the liquidus is reached. The maximum liquidus temperature for SFC is composition dependent with the highest melting point (T = 1252°C) recorded from a sample containing 9.0 wt% C4S3. Determination of ferrous iron content in SFC shows a range between 0.24 −0.37 wt% at 1200°C compared to 0.40-0.64 wt% at 1250°C. The absolute Fe2+ content is both temperature and composition dependent, with higher ferrous iron values measured at high temperature and high C4S3 contents. EPMA data, combined with the ferrous iron measurements, indicate a coupled substitution mechanism in SFC represented by the reaction 2(Fe3+) = (Ca2+, Fe2+) + Si4+. Data obtained in the present investigation combined with those available in the literature enable the construction of a series of isothermal sections showing phase relationships within the broader FCS system. These diagrams may be used as a guide to improving the understanding of fundamental sintering phase relations in the high iron corner of the FCS ternary system, as well as providing some insight into the compositional and thermal conditions required to maximize the stability of SFC phase in iron ore sinter.