STUDY OF THE ACTIVITIES AND FREE ENERGY OF COMPONENTS DURING THE FORMATION OF A SOLID SOLUTION OF MGO · AL2O3

Abstract

The work shows that spinel (MgO ⋅ Al2O3), which is formed in the process of steel production, has a high melting point and forms undeformed non-metallic C-type inclusions. It is proved that avoiding spinel formation in the process of deoxidation of high-quality steels is an urgent task. It is highlighted that the mechanism of spinel formation in liquid steel has not been sufficiently studied due to the lack of reliable thermodynamic information on spinel formation.It is shown that in previous studies, thermodynamic equilibrium calculations were carried out in the process of deoxidation of some metals in liquid iron, where the formation of spinel inclusions in liquid steel was a function of the oxygen, aluminum and magnesium content. In this case, the temperature of steelmaking and the lack of data on the activity of components in the spinel solid solution were not taken into account. It is shown that the purpose of the research was to conduct experimental studies to determine the free energy of formation of solid magnesium oxide, as well as the free energy of spinel formation. The method of conducting an experiment where the free energy of the system was determined by equilibrating liquid copper containing graphite in an MgO crucible with a CO–Ar atmosphere is presented. The equilibrium reaction of magnesium oxide formation in molten copper is established, which allowed establishing an equation for calculating the equilibrium constant of the heterogeneous process. It is shown that during the measurement of the activity of the spinel phase components, the spinel solid solution, which has a non-stoichiometric composition, was in equilibrium with liquid copper in a graphite crucible at a constant CO:Ar gas ratio. Equations are determined that allow calculating the activity of MgO and Al2O3 in the spinel solid solution and the activity of stoichiometric spinel in the solid solution. It is shown that a vertical electric furnace with LaCrO4 heating elements was used to conduct the experiment in the work. The method of conducting the experiment is described in detail. It is shown that to measure the free energy of spinel formation and the activity of the components of the spinel solid solution, 5 g of Cu–(0.005–0.03 wt.%)Mg–(0.1–1.1 wt.%)Al ligature was loaded into a graphite crucible with a stoichiometric spinel ratio in the form of a tablet. The formation of spinel was confirmed by X-ray diffraction, electron probe microanalyzer and wet chemical analysis. Additional experiments were carried out to accurately determine the phase boundaries of the spinel phase. For this purpose, the diffusion vapor method was used. It was proved that the thickness of the spinel layer formed between MgO and Al2O3 was from 300 to 500 µm depending on the temperature. To calculate the free energy of the reaction of the formation of solid manganese oxide and spinel, the equation of the dependence of the change in Gibbs energy on temperature was derived, and the activity coefficients were also determined.