Thermodynamic Study on MgO Solubility in High-Alumina-Content Slag System
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INTRODUCTION
MGO is a significant component in the generation of inclusion and affects the longevity of refractory materials. Several researchers have reported on the dissolution behavior of MgO from a thermodynamic viewpoint. They focused on the kinetic consideration of MgO dissolution[1–4] and MgO solubility[5–15] with respect to the equilibrium solubility for slag. The aforementioned previous studies on MgO solubility in slag discussed the dissolution behavior of MgO that is inversely proportional to basicity in the high-basicity region. Recently, Park et al.[16] reported that increases in the alumina content of the slag with the formation of MgAl2O4 spinel causes MgO solubility to depend on the composition of Al2O3 and basicity in the slag. The MgO dissolution behavior is interpreted as controlled by the effect of cations and the basicity of slag on the stability of MgO and MgAl2O4 spinel, and the aforementioned correspond to thermodynamic equilibria in molten slag.[17] In addition, a phenomenological study reported that spinel inhibits the physical–chemical corrosion of
CHEOL MIN YOON, YOUNGJOO PARK, and DONG JOON MIN are with the Department of Material Science and Engineering, Yonsei University, Seoul, 03722, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted December 21, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
refractory materials via the stability of MgAl2O4 spinel in the spinel equilibrium region.[18] However, despite the phenomenological study, there is a paucity of structural and thermodynamic studies on the ionic equilibrium mechanism in the liquid-phase slag and spinel interface. In contrast, the effect of Al ions on the MgO solubility indicates that alumina exists in a tetrahedral structure with a 4-coordination state due to the charge compensation of cations such as Ca2+ and Mg2+ ions, and 5and 6- coordination states that are similar to those of Ca2+ and Mg2+ cations.[19–31] Thus, it is necessary to consider the MgO solubility in the slag of the spinel equilibrium region with changes due to the amphoteric behavior of Al ions. Furthermore, based on Loewenstein’s Al avoidance rule and Pauling’s rule for stability of Al2O3 (an amphoteric oxide), it is reported that it is not possible for [4]Al-O-[4]Al BO linkage species to exist while considering binding energy and binding angle.[19] However, several recent studies on 27Al and 17O MAS NMR spectra reported that there are exceptions to the aforementioned laws.[32–35] According to Lee et al., the degree of Al avoidance is different from Al avoidance based on the cation species concentration and cation field strength in aluminosilicate melts; they predicted the fractional change in the BO species based on the compositional change. The recent discussion on thermodynamic stability of the slag with respect to the structural change in the tetrahedral Al ions in the melts is necessary to investigate the mechanism of spinel equilibrium. Hence, the structural properties of alumina
on the dissolution of MgO in the molten slag shoul
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