Electric Arc Furnace (EAF) black slags are a major by-product of steelmaking, rich in iron oxides (FeO, Fe₂O₃) and containing silicates and other oxides. Although EAF black slags are already widely recycled as aggregates in construction applications, as reported by Pellegrino et al. [1], this represents a form of downcycling. Despite their favourable properties, such as abrasion resistance, their high density limits broader application. Therefore, deferrization aimed at selectively removing iron from the slag remains crucial for maximizing resource recovery, reducing waste, and enhancing the potential for higher-value reuse of the residual material. This study investigates deferrization using thermodynamic modeling with FactSage software, focusing on a post-slagging process applied directly to molten slag. The approach evaluates the reduction of iron oxides using metallic silicon (Si) or aluminum (Al) as reducing agents, which also act as slag modifiers. Simulations were carried out to predict phase equilibria under varying temperature and slag composition conditions, highlighting the dual role of Si and Al in both reducing iron oxides and modifying the slag’s properties. The results show that, under optimized conditions, iron oxides can be effectively reduced to metallic iron or magnetite, enabling separation through magnetic or gravitational methods. The use of Si and Al not only improves the reduction process but also modifies the slag’s melting behavior and phase stability, allowing a better division between ferrous and non-ferrous fractions. The residual non-ferrous slag, after processing, has significant reuse potential, particularly as a clinker substitute in the cement industry. This reuse aligns with circular economy principles and can greatly reduce the carbon footprint of cement production. This work emphasizes the need to integrate thermodynamic modeling with experimental validation to optimize deferrization strategies. By leveraging the properties of Si and Al and ensuring consistent slag quality, the findings contribute to resource-efficient and sustainable solutions for managing steelmaking by-products, supporting a more circular approach to slag utilization and valorization.