**Thermodynamics of Transient Oxygen Carrier (FeO) Behavior in Dephosphorization during Basic Steelmaking: A Critical Review with Comparative Analysis of Industrial Oxygen Carrier Systems**

### **Alternative Titles**1. **The Transient Oxygen Carrier Mechanism of FeO in Basic Steelmaking: Thermodynamic Constraints, Slag Chemistry, and a Comparative Analysis with Chemical Looping Systems**2. **From Oxygen Delivery to Self-Reduction: A Thermodynamic Review of FeO as a Transient Oxygen Carrier in Dephosphorization and Its Parallels in Industrial Redox Processes**3. **Transient Oxygen Carriers in High-Temperature Metallurgy: A Case Study of FeO in BOF Dephosphorization and Comparative Insights from Chemical Looping Combustion and Reforming** ### **Subtitle***A comprehensive thermodynamic analysis of FeO as a transient oxygen carrier in basic steelmaking—examining its simultaneous role in oxidizing phosphorus and reducing back to pure metallic iron—alongside a comparative evaluation of analogous metal oxide oxygen carriers (Fe₂O₃, CeO₂, CuO, Mn oxides) used in chemical looping combustion, reforming, and hydrogen generation.* --- ### **Detailed Description** **Overview:**This research paper provides a critical thermodynamic review of dephosphorization in basic steelmaking, introducing a precise new conceptual framework: FeO as a **transient oxygen carrier**. Unlike conventional oxidants that persist in the slag as stable oxides (e.g., Al₂O₃) or conventional catalysts that remain chemically unchanged, FeO temporarily delivers oxygen to the phosphorus impurity, undergoes self-reduction to pure metallic iron, and returns to the steel bath—purifying both the metal and itself. The paper synthesizes classical and contemporary thermodynamic models to elucidate this dual-action refining step and extends the analysis to analogous transient oxygen carrier systems across high-temperature process industries. **Key Themes & Findings:** - **The Transient Oxygen Carrier Concept:** A central contribution of this review is the reconceptualization of FeO's function. FeO is not a permanent flux but a transient carrier that exists only for the duration of the oxygen transfer. The Fe²⁺ ion acts as the electron acceptor for phosphorus oxidation; upon accepting electrons, it is reduced to Fe⁰, which returns to the metal bath. This behavior distinguishes FeO from both permanent alloying additions and true catalysts. - **Reaction Representations—Molecular and Ionic Models:** The paper examines the evolution of dephosphorization equations, from simple molecular formulations (2[P] + 5(FeO) ⇌ (P₂O₅) + 5[Fe]) to advanced ionic representations ([P] + 4(O²⁻) + 2.5(Fe²⁺) ⇌ (PO₄³⁻) + 2.5[Fe]). The ionic model explicitly shows Fe²⁺ being reduced to [Fe], confirming the transient oxygen carrier mechanism. A complete formulation incorporating solid solution formation near the dicalcium silicate saturation surface is also presented. - **Slag Fluidity and Mass Transfer:** Beyond its oxygen carrier role, the paper discusses FeO as a network modifier in the silicate melt. By breaking Si–O–Si bonds, FeO improves slag fluidity and enhances the mass transfer kinetics of phosphorus from the metal to the slag phase. Recent industrial studies confirming the non-monotonic relationship between FeO content and dephosphorization efficiency—attributed to competition between P₂O₅ and FeO for CaO—are analyzed. - **Thermodynamic Constraints—Basicity and Temperature:** The paper analyzes the influence of slag basicity (CaO/SiO₂) and temperature. While higher basicity enhances phosphate capacity, excessive basicity increases viscosity and impairs kinetics. Lower temperatures favor the oxidation thermodynamics, but higher temperatures improve fluidity—necessitating a precise industrial operating window. Recent findings that dephosphorization ability is better predicted by slag phase constitution (liquid phase and phosphorus-containing solid solution) than by basicity alone are discussed. - **Comparative Analysis with Chemical Looping Systems:** A major novelty of this review is the comparative evaluation of FeO against analogous transient oxygen carriers used in Chemical Looping Combustion (CLC), Reforming (CLR), Gasification (CLG), and Water Splitting (CLWS). Iron oxides (Fe₂O₃/Fe₃O₄), cerium-based oxides (CeO₂, Ce–Cu), copper, manganese, cobalt, and nickel oxides, and low-cost natural ores (Brazilian iron ore, mill scale) are examined. The key structural divergence is identified: in chemical looping, the reduced carrier must be regenerated by air oxidation, whereas in steelmaking dephosphorization, the reduced carrier (Fe⁰) is the desired product—making FeO a **truly transient** carrier with no regeneration step required. - **Equilibrium vs. Kinetic Realities:** The paper critically examines the gap between theoretical equilibrium predictions and actual industrial BOF performance. Industrial partition ratios are far from equilibrium, varying with blowing practice, stirring intensity, and the risk of phosphorus reversion during tapping. Recent kinetic models confirming that oxygen activity at the slag/metal interface agrees with equilibrium oxygen activity with FeO in slag—validating the transient oxygen carrier framework—are discussed. - **Implications for Process Design:** The paper offers practical recommendations: maintaining FeO within an optimal range to balance oxygen potential, fluidity, and CaO competition; controlling basicity to avoid high liquidus temperature regimes; and employing thermodynamic modeling software (e.g., FactSage) for multi-constraint slag design. Insights from chemical looping research—particularly regarding the relationship between carrier conversion and selectivity—are proposed as potentially useful for BOF optimization. **Conclusion:**The paper concludes that the transient oxygen carrier framework provides a unifying conceptual lens for understanding redox-active metal oxides across high-temperature process industries. FeO in steelmaking occupies a unique position: it is the only major industrial system in which the reduced carrier is the desired product rather than a species to be regenerated. Future advances in dephosphorization efficiency will likely come from better integration of thermodynamic databases with kinetic models and real-time process control, rather than from further refinement of equilibrium descriptions alone.