Investigation of the liquidus curve of chiolite in the system Na3A1F*—AIF3

aJ. VRBENSKA and bM. MALINOVSKÝ

1Department of Electrotechnology, Slovak Technical University, 880 19 Bratislava

ь Department of Inorganic Technology, Slovak Technical University, 880 37 Bratislava

Received 3 January 1979

Accepted for publication 19 July 1979

The liquidus curve of chiolite in the system Na3AlF6—A1F3 was determined by means of thermal analysis in an open system. The experimental results were submitted to the thermodynamic analysis.

The composition and temperature of the peritectic and eutectic points were found to be (in coordinates NaF—A1F3): JCP = 41.1 mole % A1F3, Tp= 1006 K, JCE = 46.8 mole % A1F3, TE = 958 K. The experimental results obtained in an open system indicate the existence of the compound NaAlF4 in the melts, decomposing at 945 К by a eutectoid reaction into chiolite and A1F3.

The existence of NaAlF4 has also been confirmed indirectly by a ther­modynamic analysis of the experimental and calculated liquidus curves. The determination of the degree of thermal dissociation of the chiolite anion by means of an indirect method showed that the chiolite anion dissociates only to a minor extent, most probably according to the scheme

A13FÍ4-*±A1FT + 2A1F4-

Методом термического анализа в открытой атмосфере была определе­на кривая ликвидуса хиолита в системе Na3AlF6—A1F3. Результаты были анализированы термодинамически.

Были определены температура и состав перитектики и эвтектики (для координат NaF-^-AlF3): * р = 41,1 мол. % A1F3, Тр= 1006 К, хЕ = 46,8 мол. % A1F3, ТЕ = 958 К. Кроме того было доказано существование соединения NaAlF4, которое при 945 К разлагается с образованием хиолита и A1F3.

Существование NaAlF4 косвенно подтверждено и при термодинамичес­ком рассмотрении экспериментальной и рассчитанной кривой ликвидуса. Косвенное определение степени термической диссоциации аниона хиоли­та показало, что он диссоциирует в очень незначительной степени, ве­роятно по схеме

A13F?4- ?± A1F6

3- + 2A1F4-

Chem. zvesti 34 (1) 47—55 (1980) 47

J. VRBENSKÁ, M. MALINOVSKÝ

The phase equilibria in the system Na3AlF6—A1F3 have been repeatedly investigated owing to the prime importance of this system with respect to the electrodeposition of aluminium. This system, first studied in 1912 by Fedotiev et al. [1], was reinvestigated at least 15 times, mostly with respect to the liquidus curve of cryolite. On the other hand, the liquidus curve of chiolite has not been investigated so thoroughly.

For a better understanding of processes which occur in the electrolyte, it is, however, inevitable to dispose with reliable data on the liquidus curve and on the scheme of dissociation of chiolite. The attention will be paid mainly to papers describing the existence of sodium tetrafluoroaluminate, NaAlF4.

Review of the literature data

According to the different authors [2—16], the system Na3AlF6—A1F3 invol­ves the compounds Na5Al3Fi4, NaAlF4 and, probably, also NaAl2F7 [9]. Chiolite melts incongruently at a temperature reported within the range 725—745°C, the composition coordinate of the peritectic point being reported within 39.4—41.0 mole % A1F3. Chiolite forms with another constituent of the melt a simple eutectic system, the coordinates of the eutecticum being reported within 45—47 mole % A1F3 and 685—700°C. It should be pointed out that there is not a consent as to the composition of the "another constituent" in the system Na3AlF6—A1F3 which is supposed to be either NaAlF4 or A1F3.

The presence of the compound NaAlF4 in the system Na3AlF6—A1F3 was first mentioned in 1933 by Hardouin [2]. The existence of this compound was further assumed by Piontelli [3], Griinert [4], and Boner [5].

On the other hand, no maximum corresponding to the melting point of NaAlF4

was observed in the investigations of an open system which were carried out by Abramov et al. [6] and Holm [7].

The direct confirmation of the existence of the compound NaAlF4 was first given in 1954 by Howard [8] who identified it by the X-ray analysis in the condensed vapours of the NaF and A1F3 mixtures close to the composition of this compound. This finding has been confirmed shortly afterwards by Ginsberg and Böhm [9] and by Mashovets et al. [10].

Thus far, the existence of NaAlF4 in the investigated system was confirmed only in the investigation of a closed system. Using the closed-cell method of thermal analysis, Ginsberg and Wefers [11] concluded that NaAlF4 is formed by a peritectic reaction at 710°C and decomposes at 680°C according to the scheme

5NaAlF4 -+ Na5Al3F14 + 2AlF3

The existence of NaAlF4 as well as the narrow interval of its thermal stability

48 Chem. zvestí 34(1) 47—55 (1980)

LIQUIDUS CURVE OF CHIOLITE

was later confirmed by Mesrobian et al. [12] who worked under argon at ^ 2 2 5 x 105 Pa.

Thus it may be concluded that the existence of NaAlF4 has been proved, though the course of liquidus of the system Na3AlF6—A1F3 does not indicate its existence if investigated in an open system. Generally, it has been assumed that the existence of this compound can be determined only by thermal analysis in a closed system.

The reported coordinates of the invariant points in the system Na3AlF6—A1F3

[1, 6, 7, 11—23] range within 39.4-41.0 mole % A1F3 and 998—1018 К for the peritectic and 43.1—47.2 mole % A1F3 and 957—973 К for the eutectic point. The temperature of the eutectoid reaction was found to be 953 К [11].

Experimental

The liquidus curve of chiolite in the system Na3AlF6—A1F3 was investigated by means of thermal analysis. This investigation was carried out in an open system with samples synthesized of NaF and A1F3. The experimental procedure was described in the previous papers [24, 25].

In total, 25 mixtures were investigated. The critical temperatures of the individual samples were determined with a reproducibility of ±2°C. The A1F3 losses owing to sublimation did not surpass the value of 0.15 wt %.

On the cooling curves within the range of the primary crystallization of chiolite three deviations from a monotonous course indicating three different phase transitions have been determined. The first (highest) transition corresponds to the equilibrium solidus—liquidus, accompanied by the separation of the chiolite crystals. The second deviation corresponds to the invariant eutectic crystallization of the couple "chiolite + another constituent of the system".

Finally, at 945K = 672°C a third deviation was observed, which corresponds to an invariant process again. The proof of the existence of this second invariant process which was not observed on the cooling curves by other investigators thus far, is considered to be the main contribution of this work. This third deviation may be attributed to the following processes:

a) Polymorphous modification of chiolite or A1F3; b) Crystallization of the ternary eutectic mixture Na5Al3F14 + A1F3 + ? (in the case if A1F3

contained minor quantities of A1203 or if a third — unknown — component entered into the system);

c) Eutectoid decomposition of NaAlF4 according to the scheme

NaAlF4 <=• Na5Al3F14 + 2AlF3

The most probable appears to be the last possibility as there were no polymorphous modifications of Na5Al3F14 or A1F3 observed at the given temperature (672°C) and the cooling curves of pure cryolite and chiolite did not indicate the presence of A1203 or another impurity.

It should be mentioned that at a heating rate of 0.5°C/min the heating curve did not give

Chem. zvesti 34 (1) 47—55 (1980) 49

J. VRBENSKÁ, M. MAUNOVSKÝ

any evidence of this eutectoid reaction. It can be assumed that the formation of NaAlF4 by the reaction

2AlF3 + Na5Al3Fu - • 5NaAlF4

in the solid state is slowed down to such an extent that it cannot be registered even, at low heating rates.

The values obtained by a graphical interpretation of the cooling curve* ife «hown in Fig. 1.

850

800

750

700

650

N a 5 A l 3 F K 20 40 60 80 NaAlF^ mole %

Fig. 1. Phase diagram of the system Na5Al3F,4—NaAlF4.

The existence of the compound NaAlF4 in the melts thus has been confirmed also in an open system, hence in the description of the investigated system in Fig. 1 the composition coordinates "chiolite—NaAlF4" are used.

Indirect determination of the degree of the thermal dissociation of the chiolite anion

The methods for a direct determination of the degree of thermal dissociation of a molten compound [26] are inaccessible to the authors thus far, therefore, an indirect method was applied, similar to the determination of the dissociation öf the complex anion in the incongruently melting compound CaAlF5 [25]. This method 18 based on the comparison of the calculated and experimental courses of the liquidl!5

<r л CAem. zve& 34 Щ 47—55 (1980)

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LIQUIDUS CURVE OF CHIOLITE

curve and, as described, e.g. by Glasstone [27], it has been applied successfully by Grjotheim [13] to the determination of the degree of thermal dissociation of the cryolite anion in molten cryolite.

The course of the liquidus of chiolite between the peritectic and the eutectic points can be described by the Le Chatelier—Shreder equation only in the case when the transformation of coordinates leads to an independent system chiolite—second constituent without solid solutions when the investigated system is close to an ideal one.

As the second constituent in the system investigated both A1F3 and NaAlF4 have been considered using the corresponding values of AH\ (i = Na5Al3Fi4) determined from the slopes of tangents to the experimental liquidus curves. Owing to the narrow temperature interval, in both cases AH\ was considered to be independent of temperature.

Altogether, three potentially possible schemes of dissociation of chiolite, both pure and in a mixture with the second component (A1F3 in the first scheme and NaAlF4 in the second and third scheme) were considered.

1. a) Pure chiolite

Na5Al3Fi4 —> 5Na+ + Al3Fu (electrolytic dissociation)

( 1 - 6 ) Al3Fu *=* b A\Fl~ + 2b A1F; (thermal dissociation)

where b is the degree of dissociation of the chiolite anion in pure molten chiolite. b) Mixture of x moles of chiolite and (1 -x) moles of A1F3

x Na5Al3F14 -» 5JC Na+ + jt A13FÍ4"

JC(1 - c) A13F?; <± xc AlFr + 2JCC A I F ;

(1 -x) A1F3 + AlFr -+ | (1 -JC) A1F4-

where с is the degree of dissociation of the chiolite anion in the mixture. 2. a) Dissociation of pure chiolite is considered to be the same as in 1.

b) Mixture of x moles of chiolite and (1 —x) moles of NaAlF4

x Na5Al3F14 -• 5JC Na+ + * A13F?4~

JC(1 - c) A13FM +± xc A\Fl~ + 2xc A1F4"

(1 -x) NaAlF4 -+ (1 -JC) Na+ + (1 -JC) A1F4"

3. a) Dissociation of the chiolite anion in pure chiolite

Na 5 Al 3 F 1 4 ^ 5Na+ + Al3F?;

(1 - b) Al3Fu +± ЪЬ A1F4- + 2b F"

Chem. zvesti 34 (1) 47—55 (1980) 51

J. VRBENSKÁ, M. MALINOVSKÝ

b) Mixture of x moles of chiolite and (1 -x) moles of NaAlF4

x Na5Al3F14 -• 5JC Na+ + * A13FM

JC(1 - c) A13F?4" ^ 3xc A1F4- + 2xc F"

(1 —JC) NaAlF4 -> (1 -x) Na+ + (1 -x) A1F4"

The degree of dissociation of the chiolite anion, c, in the system was calculated from the equality of dissociation constants of this anion in pure chiolite and in a mixture of chiolite with the second component

KAľ3F?4-= KAľ3F?4-/AlF3 ^ ^ )

~dis - b - d i s [ 7 1 ( 2 ) A A l 3 F Í ; " Ä A l 3 F Í 4 - / N a A I F 4 ^ J V )

After expressing the activities of the corresponding ions by means of the products of ionic ratios of cations and anions, and introducing into eqn (1) or eqn (2), the following equations were obtained:

For model 1

For model 2

For model 3

l + 2xc 8JC(1-C)(2JCC + 1) 2 ( 5 )

x(\ - c) = (2xc + x - l)(4xc - 3x + 3) \xc 8jt(l-c)(2jtc +

x ( l - c ) с(2хс + 1-д:)2

1 + 2JCC (1-C)(2JCC + 1) 2 l ;

1+4JCC (1-C)(4JCC + 1) 4 ( j

For the first and second model a series of equations of the third degree and for the third model a series of equations of the fifth degree with respect to the unknown quantity с were obtained; the values of b and x were chosen sequentially within the interval (0, 1). These equations have been solved using the iteration method. In all cases, the solution gave a single root within the interval (0, 1), which is in agreement with the physical reality. It should be mentioned, however, that with the first model assuming chiolite and A1F3 as components the solution does not cover the entire concentration interval of JC, corresponding to the liquidus curve. This indicates indirectly the inadequacy of the composition coordinates corresponding to the system chiolite—A1F3 and supports the assumption on NaAlF4 being the second component.

52 Chem. zvestí 34 (1) 47—55 (1980)

LIQUIDUS CURVE OF CHIOLITE

The liquidus curves calculated by means of a method described elsewhere [27, 28] were compared with the experimental curve. The best fitting was assumed to be criterion for the correctness of the model considered. brum;* '•

The value ot AH\ were obtained from the slopes of tangents, which means that the AH\ values might imply also the ^respective values oř the enthalpy of dissociationiiln the determination of the degree of thermal dissociation of NaíAtlF6

andjiiUíAIF i tt,iWasfffotmdys>howeyet, hauUhisfiäaesJinöt affect» the/teVöltSf iti asigriíficaáťwáyvialLléastíatal degree of dissociation of «да Ш%г[2ЩЬ Ißrmarii

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component for the first model.

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C/iem. zvesti 34 (1) 47—55 (1980) 53

J. VRBENSKÁ, M. MALINOVSKÝ

model for the degree of dissociation of the chiolite anion in pure chiolite b = 0.05 4Fi&,2i) ^ща mixture this Value further decreases with increasing concentration of the second componeiithCFtgi.a). Ь i> ' iw/nvo -it iot поп,но *H <

rteiíKoťrectness of, thei first nloddt wasi confirmed as nóhfe/ of ;the' calculated j curves; (tould be fitted itö the experimental liquidui curve and, besides, Mth this itl0deHhe degree öf diteociatioti;c, extrfemdty increases with inciieasing-concentra-

itfoniÄbABrii m thfe mixturftvihoilghiátiíis highly improbable;that thtí/degrebiof thermal dj&öfciattöh might iramásefwbiTka value:close;toiObpito/iOOflicwiihiii^he oam>w eortcettfralkmi {iFigt<4)iíiiw orí í

Acfriow/edgeiiiente. Tfte authors wish to express their gratitude to ŔNDr. M. Galova for computing the numerical data and to Ing. E. Cibiková for performing the experimental measurements.

References

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54 Chem. zvesti 34(1) 47—55 (1980)

UQUIDUŠ CURVE OF CfflOUTE

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28. Malinovský, M., unpublished results.

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Chem. zvestí 34 (1) 47—55 (1980) 55