Fundamentals of Silico-Ferrite of Calcium and Aluminium (SFCA) Iron Ore Sinter Bonding Phase Formation: Effects of Basicity and Magnesium on Crystallisation during Cooling

The effects of basicity and Mg addition on the crystallisation during cooling of complex Ca-rich ferrite iron ore sinter bonding phases SFCA and Fe-rich SFCA was investigated using in situ synchrotron X-ray diffraction. In synthetic iron ore sinter mixtures, cooling of a high temperature (T = 1623 K) assemblage comprising magnetite and melt showed that decreasing basicity from B = 4.0 to 3.0 and 2.5 resulted in the formation of a Fe-rich SFCA phase being suppressed, with SFCA being the only Ca-rich ferrite phase to form initially from the melt at B = 2.5. Increasing the Mg concentration in the sinter mixtures to 1 and 3 wt% MgO resulted in an overall suppression of the amount of Ca-rich ferrite phase formation. However, Mg addition caused the formation of Fe-rich SFCA to be favoured over SFCA, with SFCA not observed to form at all during cooling in the 3 wt% MgO mixture. The absence of SFCA in the high-Mg experiment was rationalised on the basis that the Fe-rich SFCA structure accommodates more Fe2+ than the SFCA structure, thereby stabilising Fe-rich SFCA relative to SFCA through replacement substitution of Fe2+ by Mg2+. Observation of Fe-rich SFCA in sinter may be an indicator of localised high basicity, and/or, high Mg concentration, within a sinter blend.