Critical thermodynamic evaluation of oxide systems relevant to fuel ashes and slags, Part 4: Sodium oxide–potassium oxide–silica

doi:10.1016/j.calphad.2008.06.002

Calphad

Volume 32, Issue 3, September 2008, Pages 506-513

https://doi.org/10.1016/j.calphad.2008.06.002 Get rights and content

Abstract

A thermochemical assessment was performed for the system K₂O–Na₂O–SiO₂. The modified associate species model was applied to the ternary liquid in the system. All binary subsystems remained unchanged. The new databank was used for the representation of the phase equilibria in the ternary system including the quasi-binary sections of the ternary diagram. The calculated phase relations are in good agreement with the experimental data. The phase equilibria in the experimentally uninvestigated region near the alkali oxide edge are proposed as extrapolations using the new databank.

Introduction

In continuation of our previous works, [1], [2], [3] the present work is dedicated to the critical thermodynamic evaluation of the ternary system containing potassium, sodium and silicon oxide. This system is important for technical materials, e.g. slags and glasses, and common silicate minerals occurring in rocks. An almost complete phase diagram of the ternary system K₂O–Na₂O–SiO₂ was proposed by Kracek as early as 1932 [4], and it still remains in the focus of attention. Recently, the mixed alkali silicates and a novel single layer silicate have been studied using modern spectroscopic techniques [5], [6]. Because of the experimental difficulties due to the volatility and hygroscopicity of the alkalis, the model approach can be useful for the description of the thermodynamic properties in the available systems. CALPHAD type modelling now permits to treat the phase equilibria and the thermodynamic properties of the complex system.

The binary systems Me₂O–SiO₂ (Me=Na, K) have been thermodynamically evaluated previously [1] and the new database has been successfully applied for the representation of phase relations and activity data. The liquid phase was described using the modified associate species model [7], [8], which was considered as the most appropriate owing to the fact that it allows an adequate representation of the thermodynamic properties as well as the phase equilibria and can comparatively easily be adjusted to the needs of the respective systems.

The aim of the present work was the generation of a databank extended to the ternary system K₂O–Na₂O–SiO₂. The ternary liquid is described using the above mentioned modified associate species model. The Gibbs energy parameters of the ternary liquid have been determined by optimisation on the basis of available ternary experimental phase relationships. The binary solution data, which were obtained earlier [1], remained unmodified in order to keep consistency of the complete databank.

Section snippets

Phase diagram data

The phase relations in the ternary system K₂O–Na₂O–SiO₂ were studied by Kracek [4] using the quenching method for the determination of liquidus points. The author has only considered the region of the phase diagram from 0 to 50 mole% of the alkali oxides. The binary compounds K₂SiO₃, Na₂SiO₃, K₂Si₂O₅, Na₂Si₂O₅, K₂Si₄O₉ and crystalline modifications of silica quartz, tridymite, cristobalite were included. There are no ternary compounds in the system.

Kracek considered the full ternary in terms of

Assessment of Gibbs energy parameters

The Gibbs energy of the liquid phase in the Na₂O–K₂O–SiO₂ system is represented by the modified associate species model [8]. Since there are no ternary compounds in the system, the ternary liquid is represented by a solution containing only the pure component liquids and binary constituents. These are the pure liquid potassium, sodium and silicon oxide and also the binary associate species Na₄SiO₄⋅2/5, Na₂SiO₃⋅2/3, Na₂Si₂O₅⋅1/2, K₂SiO₃⋅2/3, K₂Si₂O₅⋅1/2 and K₂Si₄O₉⋅1/3 which have been applied

Results and discussions

The solution data for the liquid described using the modified associate species model [8] is given in Table 1a. These data concern the interaction parameters which were introduced for the ternary system. The remaining parameters, which relate to the binary sub-systems Me₂O–SiO₂ (Me=Na, K) were published earlier [1].

The calculated isotherms of the liquidus surfaces in the ternary system Na₂O–K₂O–SiO₂ are given in Fig. 1b and compared with the experimental phase diagram (Fig. 1a) studied by

Conclusions

In the present work the modified associate species model was successfully applied to represent the phase relations in the ternary system Na₂O–K₂O–SiO₂. The Gibbs energy data for the liquid phase were generated for this ternary system by the addition of interaction parameters between the constituents already existing in the liquids of the binary sub-systems. The interaction parameters for the binary systems obtained earlier were kept without change. This is necessary for the development of a

Acknowledgements

This work is part of two projects supported by Bundesministerium für Wirtschaft und Technologie (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 (33)

E. Yazhenskikh et al.
Critical thermodynamic evaluation of oxide systems relevant to fuel ashes and slags, Part 1: Alkali oxide–silica systems
CALPHAD
(2006)
E. Yazhenskikh et al.
Critical thermodynamic evaluation of oxide systems relevant to fuel ashes and slags, Part 2: Alkali oxide–alumina systems
CALPHAD
(2006)
E. Yazhenskikh et al.
Critical thermodynamic evaluation of oxide systems relevant to fuel ashes and slags, Part 3: silica–alumina system
CALPHAD
(2008)
S. Rakić et al.
Single crystal structure investigation of twinned NaKSi₂O₅ — A novel single layer silicate
Solid State Sciences
(2001)
S. Rakić et al.
Hydrothermal synthesis and structural characterization of $κ$ - Na₂Si₂O₅ and Na_1.84K_0.16Si₂O₅
Solid State Sci.
(2003)
G.N. Greaves
Structural studies of the mixed alkali effect in disilicate glasses
Solid State Ion.
(1998)
J.O. Isard
The mixed alkali effect in glass
J. Non-Cryst. Solids
(1969)
H. Maekawa et al.
Effects of temperature on silicate melt structure: A high temperature 29Si NMR study of Na₂Si₂0₅
Geochim. Cosmochim. Acta
(1997)
M. Kanzaki et al.
Phase relations in Na₂O–SiO₂ and K₂Si₄O₉ systems up to 14 GPa and Si-29 NMR study of the new high-pressure phases: implications to the structure of high-pressure silicate glasses
Phys. Earth Planet. Inter.
(1998)
M. Fleet et al.
Epsilon sodium disilicate: A high-pressure layer structure Na₂Si₂O₅
J. Solid State Chem.
(1995)

V. Kahlenberg et al.

The crystal structure of $δ$ - Na₂Si₂O₅

J. Solid State Chem.

(1999)

C.W. Bale et al.

FactSage Thermochemical software and databases

CALPHAD

(2002)

R. Chastel et al.

Excess thermodynamic functions in ternary Na₂O–K₂O–SiO₂ melts by Knudsen cell mass spectrometry

Chem. Geol.

(1987)

F.C. Kracek

The ternary system K₂SiO₃–Na₂SiO₃–SiO₂

J. Phys. Chem.

(1932)

M.D. Allendorf et al.

Thermodynamic analysis of silica refractory corrosion in glass-melting furnaces

J. Electrochem. Soc.

(2001)

T.M. Besmann et al.

Thermodynamic modelling of oxide glasses

J. Am. Ceram. Soc.

(2002)

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Critical thermodynamic evaluation of oxide systems relevant to fuel ashes and slags, Part 4: Sodium oxide–potassium oxide–silica

Abstract

Introduction

Section snippets

Phase diagram data

Assessment of Gibbs energy parameters

Results and discussions

Conclusions

Acknowledgements

CALPHAD

CALPHAD

CALPHAD

Solid State Sciences

Solid State Sci.

Solid State Ion.

J. Non-Cryst. Solids

Geochim. Cosmochim. Acta

Phys. Earth Planet. Inter.

J. Solid State Chem.

J. Solid State Chem.

CALPHAD

Chem. Geol.

The ternary system K2SiO3–Na2SiO3–SiO2

J. Phys. Chem.

Thermodynamic analysis of silica refractory corrosion in glass-melting furnaces

J. Electrochem. Soc.

Thermodynamic modelling of oxide glasses

J. Am. Ceram. Soc.

The ternary system K₂SiO₃–Na₂SiO₃–SiO₂