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Stability of SFC (silico-ferrite of calcium)

CSIRO Minerals, Box 312, Clayton South VIC 3169, Australia

^* E-mail address for corresponding author: mark.pownceby{at}minerals.csiro.au

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.

Key-words: SFC solid solution, phase relations, thermal stability, iron ore sinter.

Abbreviations: CF₃ = CaO.3Fe₂O₃ • C₄S₃ = 4CaO.3SiO₂ • CF = CaO.Fe₂O₃ • CF₂ = CaO.2Fe₂O₃ • C₂F = 2CaO.Fe₂O₃ • C₂S = 2CaO.SiO₂ • C₃S₂ = 3CaO.2SiO₂ • CS = CaO.SiO₂ • Hem = hematite (Fe₂O₃) • Liq = quench liquid phase • T = tridymite • hexagonal high-temperature form of SiO₂