<p>Iron recovery from nickel slag, a significant byproduct generated during nickel pyrometallurgical process, represents a pivotal aspect of advancing sustainable utilization of metallurgical solid waste. This study explores the potential of utilizing carbide slag as a fluxing modifier to enhance iron extraction from nickel slag through the melting oxidation process. Thermodynamic simulations were conducted with FactSage software to predict phase equilibria and reaction pathway, while experimental analyses were performed to investigate phase composition, microstructural morphologies, and iron recovery efficiencies. The results indicate that in the FeO–SiO₂–CaO–MgO slag system, predominant magnetite emerges as the primary iron-bearing phase in the spinel region. Increasing the slag’s basicity shifts the corresponding phase area of spinel increases first and then decreases. Thermodynamic predictions and experimental data corroborate that the addition of carbide slag facilitates the transformation of the fayalite phase into magnetite and pyroxene phases. Optimal basicity levels enhance the precipitation and crystal growth of magnetite, as well as promote the separation of iron from the slag matrix. Specifically, at a basicity of 0.5, the oxidized samples displayed the highest magnetite precipitation, with a recovery rate of 93.2%. Detailed microstructural analysis revealed the formation of well-defined dendritic magnetite crystals. These findings suggest that the introduction of carbide slag not only optimizes the phase transformation processes but also significantly enhances iron recovery, offering a viable pathway for the synergistic reutilization of nickel and calcium carbide slags.</p> Graphical Abstract <p></p>