<p>The electric arc furnace process is the main melting process in scrap- and DRI-based steelmaking. The metal–slag reactions during the flat bath stage play an important role in metal yield and removal of impurities. This paper presents a new reaction module for modeling mass transfer constrained metal–slag reactions during the flat bath stage based on modification of the effective equilibrium constant method. The model accounts for the main oxidation/reduction reactions involving multiple oxidation states of slag species, the removal of S and P to slag, and the resulting temperature evolution of the metal and slag phases. The method is shown to converge to an equilibrium dictated by the equilibrium constants. The model is validated using data from laboratory-scale experiments for three cases: reduction of FeO by Si, reduction of MnO by Si, and metal–slag reactions during powder injection into hot metal. The results indicate that the model accurately reproduces the transient evolution of the metal and slag composition in all three cases. Finally, the model is validated with data from a 150 metric ton EAF for stainless steelmaking.</p>