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Physicochemical Consideration of Interfacial Reaction between Steelmaking slag and MgO Refractory

Physicochemical Consideration of Interfacial Reaction between Steelmaking slag and MgO Refractory
Han, Jin Sung
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In steel shops, intense reactions occur at the interfaces between the metal-slag-refractory phases and can occur in operating vessels such as electric arc furnaces (EAF), ladle furnaces (LF), and vacuum degassers (VD). The refractory is attacked by complex reactions including gas-slag-metal multiphase reactions, which occur simultaneously, and must have a superior resistance to thermal shock, mechanical abrasion, and chemical corrosion caused by slag under reducing and/or oxidizing atmospheric conditions. Magnesia-based refractories are widely used for LF or EAF due to their relatively high corrosion resistance and high temperature strength. Therefore, it is important to identify the reactions that occur between the magnesia-based refractory and various slag systems. In the first part, high temperature experiments were carried out using an induction furnace to simulate the refractory-slag-metal reactions in conjunction with thermodynamic computations using FactSage7.0 software. The Al-killed steel and the CaO-Al2O3-8SiO2-5MgO-xCaF2 (CaO/Al2O3=1.9, x=0-15 wt%) slag were brought to equilibrium in a magnesia refractory at 1823 K. Moreover, the solubility of MgO in the slag was quantitatively measured using a resistance furnace as a function of CaF2 content and temperature to confirm the thermodynamic driving force of the dissolution of MgO. The penetration depth of a slag into the magnesia refractory increased with increasing CaF2 content, which originated from a decrease in slag viscosity. Also, it was confirmed that MgO grains were dynamically detached from the slag/refractory interface by increasing the content of CaF2. In the second part, this research investigated the reaction between CaO-SiO2-Al2O3-xFeO-MgO-MnO (CaO/SiO2=1.2, x=20-50 wt%) slag and magnesia refractory. A theoretical slag-refractory reaction was also simulated using the FactSageTM 7.0 software, and the results were compared to the experimental findings. Using SEM-EDS analysis, we confirmed the formation of a (Mg,Fe)Oss(solid_solution), called magesiowüstite (MW), intermediate layer at the slag-refractory interface. MgO dissolved from refractory and reacted with the bulk slag to form MW layer at the interface. Simultaneously, slag penetrated through micro-pores and reacted with the refractory to form MW layer. In other words, the MW layer built up in both directions from initial refractory-slag interface. The thickness of the MW layer increased as the FeO content in the slag increased, and using EDS line scanning, a Mg and Fe concentration gradient was confirmed within the MW layer. The slag, which penetrated into the refractory, had a chemical composition of the CaO-SiO2-Al2O3-MgO system without FeO, indicating that FeO was consumed by forming a MW layer at the refractory hot face. The slag-refractory interfacial reaction was simulated using thermochemical software. The results predicted a MW monoxide composed of MgO and FeO. A spinel phase was formed when FeO was greater than 40 wt%. These thermochemical computations were comparable to our experimental findings
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