Coupled Thermodynamic Modeling and Experimental Study of Na2O-FeO-Fe2O3-CaO-MgO-A12O3-SiO2 System

Book excerpt: "The significance of the Na2O-FeO-Fe2O3-CaO-MgO-Al2O3-SiO2 system stems from its applications in industrial processes and natural phenomena. This system in whole or in part was studied for the cooling system of fast breeder reactors, the desulfurization of hot metal and liquid steel, the production of bioactive glasses, coal-combustion slags, the reduction process of bauxite with soda for the production of Al2O3, and the production of solid-state electrodes for electrochemical cells. It also possesses many well-known minerals such as wüstite, spinel, corundum, aegirine, etc. which are of importance in geology. However, phase equilibria in this system are indeed very complex due to the change of Fe oxidation state with oxygen partial pressure and the substitution of Fe3+ by Al3+ in solid solutions. Moreover, the high vapor pressure of sodium, hygroscopicity, high viscosity of SiO2-rich melts, and high fluidity of Na2O- and FeO-rich melts make the experimental study of this system quite challenging. As a result, experimental results in this system were often inconsistent and limited in terms of composition and temperature. Therefore, the construction of a coherent thermodynamic database for the Na2O-FeO-Fe2O3-CaO-MgO-Al2O3-SiO2 system is essential to optimize existing material processes and to develop new processes and advanced materials.All solid and liquid phases of two binaries, six ternaries and two multicomponent sub-systems in the Na2O-FeO-Fe2O3-CaO-MgO-Al2O3-SiO2 system were critically evaluated and optimized in the current study. Using proper thermodynamic models considering the crystal structure of each phase reduces the number of model parameters and thus, enhances the predictive ability of models especially in high order systems. The molten oxide phase was modeled using the Modified Quasichemical Model which takes into account second-nearest-neighbor cation ordering. Extensive solid solutions such as meta-oxides, [beta]"-alumina and pyroxene were treated within the frame work of Compound Energy Formalism with the consideration of their sublattice crystal structures. The wüstite solid solution was modeled using polynomial expansions of the excess Gibbs energy. The sulfide dissolution in the molten oxide phase was modeled using the Modified Quasichemical Model in quadruplet approximation taking into account both first and second-nearest-neighbor short range ordering, simultaneously. Experimental data in the Na2O-FeO-Fe2O3-Al2O3 system were very limited. Hence, key phase diagram experiments and thermodynamic optimization were conducted in this system. Phase diagram experiments were performed using the quenching method followed by Electron Probe Micro-Analysis and X-Ray Diffraction for phase identification. Two- and three-phase equilibria of this system including solid and liquid phases were determined, and the presence of [beta]"-alumina solid solution with a large miscibility gap was revealed for the first time in this work. The developed database was applied to predict the sulfide dissolution in the Na2O-FeO-Fe2O3-CaO-MgO-MnO-Al2O3-SiO2 molten oxide phase which is of high importance for the production of low sulfur steels. Based on the present thermodynamic modeling results, it was shown, for the first time, that the sulfide capacity of Na2O-containing oxide melts is not always a unique property of a given melt composition, and can vary with the gas composition in equilibrium with the oxide melt." --