Elsevier

Calphad

Volume 30, Issue 4, December 2006, Pages 397-404
Calphad

Critical thermodynamic evaluation of oxide systems relevant to fuel ashes and slags Part 2: Alkali oxide–alumina systems

https://doi.org/10.1016/j.calphad.2006.08.002Get rights and content

Abstract

The associate species model was applied to the thermodynamic representation of liquids in the oxide systems Na2O–Al2O3 and K2O–Al2O3. The available experimental phase diagrams and thermodynamic data were collected and evaluated for the purpose of improving the solution database. These new data for the liquid phase, and for the solid compounds Na β- and β-alumina, are compatible with data for the solid stoichiometric compounds from the FACT Pure Substance database. The phase equilibria calculated using the new optimised solution data show good agreement with the experimental results.

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 β-alumina results in an extreme mobility of sodium ions [2]. β-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: Gm=xiGi0+RTxilnxi+i<jxixjvLij(v)(xixj)v, where xi is the mole fraction of phase constituent i (including the associated species), Gi0 is the molar Gibbs energy of the pure phase constituent i, and Lij(v) is an interaction coefficient between components i and j, according to the Redlich–Kister polynomial. Lij(v) is a temperature-dependent term according to: Lij(v)=Aij(v)+Bij(v)T+Cij(v)TlnT+Dij(v)T2+. 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 (Gi0) 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 β- and β-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)

  • D.J. Fray

    Determination of Sodium in molten aluminium and aluminium alloys using a beta-alumina

    Metall. Trans. B

    (1997)
  • R.J. Brisley et al.

    Determination of sodium in aluminium and aluminium silicon alloys using sodium beta alumina

    Metall. Trans.

    (1983)
  • D.A. Neudorf et al.

    Thermodynamic properties of Na2O–SiO2–CaO melts at 1000 to 1100 C

    Metall. Trans.

    (1980)
  • S. Yamaguchi et al.

    Activity measurement of Na2O in Na2O–SiO2 melts using the beta-alumina as the solid electrolyte

    Scand. J. Metall.

    (1982)
  • H. Itoh et al.

    Thermodynamic activity of Na2O in Na2O–SiO2–Al2O3 melt

    Trans. Japan Inst. Met.

    (1984)
  • A.D. Pelton et al.

    Thermodynamic analysis of ordered liquid solutions by a modified quasichemical approach — Application to silicate slags

    Metall. Trans.

    (1986)
  • T.M. Besmann et al.

    Thermodynamic modelling of oxide glasses

    J. Amer. Ceram. Soc.

    (2002)
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