Phase Equilibria in the System “FeO”-CaO-SiO₂-Al₂O₃-MgO at Different CaO/SiO₂ Ratios

The “FeO”-containing slags play an important role in the operation of an ironmaking blast furnace (BF), in particular the primary slags such as the system “FeO”-CaO-SiO₂-Al₂O₃-2 mass pct MgO with CaO/SiO₂ weight ratios of 1.3, 1.5, and 1.8 saturated with metallic iron. To investigate the characteristics of such a slag system and its behavior in BF, the phase equilibria and liquidus temperatures in the slag system have been experimentally determined using the high-temperature equilibration and quenching technique followed by an electron probe X-ray microanalysis (EPMA). Isotherms between 1553 K and 1603 K (1280 °C and 1330 °C) were determined in the primary phase fields of dicalcium silicate, melilite, spinel, and monoxide [(Mg,Fe²⁺)O]. Pseudo-ternary phase diagrams of (CaO + SiO₂)-Al₂O₃-“FeO” with a fixed MgO concentration at 2 mass pct and at CaO/SiO₂ ratios of 1.3, 1.5, and 1.8 have been discussed, respectively, simplifying the complexity of the slag system for easy understanding and applying in BF operation. It was found that the liquidus temperatures increase in melilite and spinel primary phase fields, but decrease in dicalcium silicate and monoxide primary phase fields with increasing Al₂O₃/(CaO + SiO₂) ratio. In addition, the liquidus temperatures decrease with increasing “FeO” concentration in dicalcium silicate and melilite primary phase fields, while showing an increasing trend in the spinel and monoxide primary phase fields. The data resulted from this study can be used to improve and optimize currently available database of thermodynamic models used in FactSage.