• The addition of 5wt% Fe3O4 to the CaO–SiO2–TiO2 system at 1400°C in air results in a single liquid region and various liquid–solid and three-phase equilibria, including liquid–tridymite, liquid–rutile, liquid–perovskite, liquid–wollastonite, and liquid–tridymite–rutile and liquid–rutile–perovskite.
• The experimental isothermal sections of the CaO–SiO2–TiO2–5wt%Fe3O4 system at 1400°C in air were determined, providing essential thermodynamic data for optimizing the selective crystallization and phase separation (SCPS) process for titanium extraction from titania-bearing slag.
• The experimental results show good agreement with FactSage calculations, validating the thermodynamic database for this quaternary system and supporting its use in process modeling.
• This study fills a critical gap in thermodynamic data for the CaO–SiO2–TiO2–Fe3O4 system, which is essential for improving the efficiency and sustainability of titanium recovery from blast furnace slag.