Critical thermodynamic evaluation of oxide systems relevant to fuel ashes and slags Part 2: Alkali oxide–alumina systems
Introduction
In continuation of our previous work on the critical thermodynamic evaluation of oxide systems relevant to fuel ashes and slags [1], the alkali oxide–silica alumina systems were evaluated.
CALPHAD-type modelling of complex systems is considered to be a useful approach for the description of the thermodynamic properties and calculated phase diagrams of such systems. A consistent thermodynamic description, by means of an appropriate database and solution models, can give results allowing free variation of parameters, such as temperature and chemical composition of the system; extrapolation into extended regions is also possible.
The oxide systems consisting of alumina–alkali oxides (Na or K) are important for many technological and scientific fields. The crystal structure of -alumina results in an extreme mobility of sodium ions [2]. -Alumina is applied as Na-electrolyte for batteries [3], and for potentiometric measurements of sodium activity in molten Al [4], [5], slags [6], [7], [8] or molten glass [9].
Appropriate solution models for the liquids are the modified quasichemical model [10] and the modified associate species model [11]. Data on the thermodynamic properties (e.g. as heat of formation, entropy etc.) and phase diagrams for both the Na2O–Al2O3 and K2O–Al2O3 systems were critically evaluated by Eriksson et al. [12], who used quasichemical model parameters to represent non-ideal interactions in the liquid phase. These authors have generated thermodynamic data for the liquid phase in the alumina-rich part of the diagram. The thermodynamically assessed phase diagrams, however, do not extend into the composition range with more than 50 mol% of Me2O (Me=Na,K).
The basis of the modified associated species model by Besmann and Spear [11] is that the strong chemical interactions in complex oxide solutions can be represented by the formation of intermediate associate species from the pure oxide end-member components. The associate species and the end-member components are assumed to form an ideal solution. Spear and Allendorf evaluated thermodynamic and phase diagram data from both binary systems [13] to determine self-consistent sets of accurate thermodynamic information, using the modified associated species model. These data are in considerable disagreement with the available experimental data. On the other hand, they do predict the phase diagram in the compositional range not covered by Eriksson et al. [12].
In the present work, the modified associate species model was chosen also, since it allows an adequate representation of the thermodynamic properties for the entire concentration range of both the Na2O–Al2O3 and the K2O–Al2O3 systems. However, the Gibbs energy solution data were improved by taking into account binary interactions between the solution constituents. Moreover, using the modified associated species model, the new dataset for these alkali oxide–alumina systems is compatible with the dataset for the alkali oxide–silica systems published earlier [1]. This is necessary for the development of a consistent database for the complete oxide system.
According to the modified associate species model [11], the molar Gibbs energy of the solution is expressed by an equation in the form of the sum of its reference part, its ideal, and its excess part as: where is the mole fraction of phase constituent (including the associated species), is the molar Gibbs energy of the pure phase constituent , and is an interaction coefficient between components and , according to the Redlich–Kister polynomial. is a temperature-dependent term according to: To provide equal weighting of each associate species with regard to its ideal mixing entropic contribution, each species contains a total of two non-oxygen atoms in its formula [11]. It should also be noted that ternary and higher order interactions terms can easily be added to Eq. (1) if needed.
Section snippets
Data assessment
The Gibbs energies of the liquid phase in the binary systems Me2O–Al2O3 (Me=Na,K) were re-assessed using the modified associate species model. Both, data for the Gibbs energies of the pure associates and data for the interaction terms in the Redlich–Kister equation have been assessed.
A database based on the associate species model has been created by Besmann, Spear, Allendorf [13]. Their basic thermodynamic functions, involving the pure solid and liquid compounds, were different from
System Na2O–Al2O3
In the literature, the systems containing alumina and sodium or potassium oxide are studied only for alumina concentrations above 50 mol%. The phase diagram for the binary system Na2O–Al2O3 has been studied by Rolin and Thanh [16], using cooling curve techniques in argon atmospheres. Later, it was critically analysed by De Vries and Roth [17], and further modified by Weber and Venero [18].
Weber and Venero reported a melting point of NaAlO2 of 1867 ∘C, which is much higher than the values of 1582 ∘
Conclusion
In the present work, new Gibbs free energy data for the phases in the binary systems Me2O–Al2O3 (Me=Na,K) are generated. The liquid phase data for these systems have been re-assessed in the framework of the modified associate species model. Suitable solution data (formation enthalpies and entropies of associate species and interaction parameters) for the sodium - and -aluminates have been optimised, taking into account the available phase diagrams and activity measurements. The new database,
Acknowledgements
This work is part of two projects supported by Bundesministerium für Wirtschaft und Arbeit (FKZs 0326844E/9 and 0326885). The authors are grateful to Dr. Ted Besmann, Oak Ridge National Laboratory and Prof. K. Spear, Pennsylvania State University, for the kind provision of their database.
References (35)
-Alumina electrolytes
Progr. Solid State Chem.
(1972)- et al.
The activity of sodium oxide in molten float glass by EMF measurements
Thermochim. Acta
(2004) - et al.
Critical evaluation and optimisation of the thermodynamic properties and phase diagrams of the MgO–Al2O3, MnO–Al2O3, FeO–0Al2O3, Na2O–Al2O3 and K2O–Al2O3 systems
CALPHAD
(1993) - et al.
FactSage thermochemical software and databases
CALPHAD
(2002) - et al.
Potentiometric determination of activities in the two-phase fields of the system Na2O–(α)Al2O3
Electrochim. Acta
(1991) - et al.
Synthesis of sodium and -alumina
Mater. Res. Bull.
(1975) - et al.
The application of NASICON (Na3Zr2Si2PO12) to the thermodynamic study of beta-alumina
Electrochim. Acta
(1983) - et al.
The equilibrium between Na-- and Na--alumina as a function of the phase composition
Electrochim. Acta
(2000) - E. Yazhenskikh, K. Hack, M. Müller, Critical thermodynamic evaluation of oxide systems relevant to fuel ashes and...
- et al.
Development of sodium-sulfur batteries
Int. J. Appl. Ceram. Technol.
(2004)
Determination of Sodium in molten aluminium and aluminium alloys using a beta-alumina
Metall. Trans. B
Determination of sodium in aluminium and aluminium silicon alloys using sodium beta alumina
Metall. Trans.
Thermodynamic properties of Na2O–SiO2–CaO melts at 1000 to 1100 ∘C
Metall. Trans.
Activity measurement of Na2O in Na2O–SiO2 melts using the beta-alumina as the solid electrolyte
Scand. J. Metall.
Thermodynamic activity of Na2O in Na2O–SiO2–Al2O3 melt
Trans. Japan Inst. Met.
Thermodynamic analysis of ordered liquid solutions by a modified quasichemical approach — Application to silicate slags
Metall. Trans.
Thermodynamic modelling of oxide glasses
J. Amer. Ceram. Soc.
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