Effect of FeO on the Melting Behavior of Direct Reduced Iron and Multi-Interfacial Reactions in Slag–Refractory Systems

Efficient melting of direct reduced iron (DRI) is essential for improving the stability and productivity of low-carbon steelmaking processes. In this study, the effect of FeO content on DRI melting behavior and coupled interfacial reactions in slag–refractory systems was investigated. Synthetic slags containing 10, 20, and 28 wt.% FeO were prepared, and hot-state melting experiments, viscosity measurements, FactSage calculations, and SEM/EDS analyses were conducted to clarify the relationship among slag properties, DRI melting, and interfacial evolution. The results showed that increasing FeO content significantly accelerated DRI melting and reduced the overall melting time. This improvement was mainly attributed to the enhanced fluidity and heat-transfer capability of the slag. Temperature-centered Arrhenius fitting showed that the apparent viscous-flow activation energies varied only within a limited range when fitting uncertainties were considered, indicating that the decrease in viscosity with increasing FeO content should not be attributed solely to a reduction in activation energy. Instead, the change in slag fluidity is associated with the combined effects of FeO on melt structure, pre-exponential fitting parameters, and temperature-dependent flow behavior. Meanwhile, the calculated thermal conductivity increased with FeO content, further promoting heat transfer from the molten slag to the DRI surface. Microstructural observations revealed that, under low-FeO conditions, a relatively continuous aluminosilicate-rich reaction layer formed at the DRI–slag interface, which hindered slag penetration and delayed melting. In contrast, high-FeO slag exhibited stronger wettability and penetration ability, allowing slag to infiltrate deeply into the porous DRI structure and form an extensive slag–iron mixed reaction zone. At the slag–MgO refractory interface, FeO promoted Fe2+/Mg2+ interdiffusion and the formation of a dense magnesiowüstite, (Mg,Fe)O, reaction layer. However, excessive FeO also intensified slag penetration and refractory corrosion. These results demonstrate that FeO plays a dual role in DRI melting systems by enhancing DRI melting efficiency while simultaneously aggravating refractory degradation, highlighting the need to balance melting performance and refractory stability in FeO-containing slag design.