In the present study, the viscosity of the CaO–SiO₂–FeO–Al₂O₃–MgO slag system was measured for the recovery of FeO in the electric arc furnace (EAF) process using Al dross. Considering the MgO-saturated operational condition of the EAF, the viscosity was measured in the MgO-saturated composition at 1823 K with varying FeO and Al₂O₃ concentrations. An increase in the slag viscosity with decreasing temperature was observed. The activation energy was evaluated, and the change in the thermodynamically equilibrated phase was considered. The changes in the aluminate structure with varying FeO and Al₂O₃ concentrations were investigated by Fourier-transform infrared spectroscopy, which revealed an increase in the [AlO₄] tetrahedral structure with increasing Al₂O₃ concentration. Depolymerization of the aluminate structure was observed at higher FeO concentrations. The Raman spectra showed the polymerization of the silicate network structure at higher Al₂O₃ concentrations. By associations between the silicate and aluminate structures, a more highly polymerized slag structure was achieved in the present system by increasing the Al₂O₃ concentration.