Phase equilibria of Fe₃O₄–Cr₂O₃–CaO system: experimental measurements and thermodynamic calculations

Understanding the phase equilibria of the Fe₃O₄–Cr₂O₃–CaO system is essential for the efficient recycling of stainless steel pickling sludge. The isothermal section of this system at 1473 K under oxygen partial pressure of 0.15 Pa was investigated. Key experiments on the relevant binary systems were conducted using a combination of equilibrium-quenching techniques, X-ray diffraction, high-resolution transmission electron microscope, and electron probe microanalysis. These systems were rigorously assessed using the CALPHAD (CALculation of Phase Diagram) method, incorporating the present experimental data. The liquid phase was modeled using the ionic two-sublattice model, represented as (Ca²⁺, Cr³⁺, Fe²⁺)_P(O²⁻, Va, FeO_1.5)_Q, where Va represents vacancy, and P and Q denote the number of sites on the cation and anion sublattices, respectively. To ensure electroneutrality, the values of P and Q adjust according to the composition of the mixture. From this, the isothermal section of the Fe₃O₄–Cr₂O₃–CaO system at 1473 K under the specified oxygen partial pressure was obtained based on the thermodynamic parameters of the binary systems. The present experimental data and calculation results hold significant implications for the comprehensive recycling of stainless steel pickling sludge.