THE SOLUBILITY O F ALUMINIUM I N NaF-AlF3-A1203 MELTS

Roynl Korwegian Cozincil for Scientific and Industrial Researclz, niIetallurgica1 Conzmiltee, The Teclzzical Uniaersity of Norway, Trondheim, Norway

Received June 25, 1965

ABSTRACT

The solubility of aluminium in NaF-AIF3 melts saturated with alumina has been investigated by analyzing quenched samples of the melts to determine the contents of metallic sodium and aluminium. The total solubility a t 1000 "C given as wt. yo A1 decreases with decreasing NaF/AlF3 molar ratio from 0.33 xvt. 70 Al a t NaF/AIF3 = 6.70 to 0.10 wt. % Al in cryolite (i\'aF/AIF3 = 3) and 0.075 wt. % A1 a t SaF/AlF3 = 1.70. The change in the solubility is prnnarily due to a change in the sodium content, while the aluminium content remains almost cor~stant over the range investigated. The solubility in cryolite increases with increasing temperature from 0.10 wt. % Al a t 1000 OC to 0.14 wt. yo A1 a t 1060 OC. In cryolite a t 1000 "C the dissolved metal was found to consist of 0.065 wt. yo A1 and 0.0'30 wt. Y, Na. The fraction of the dissolved metal represented by sodium increases with increasing tetuperature.

INTRODUCTION

The solubility of aluininium in NaF-AlF3-A1203 melts is of interest in relation to the electrolytic production of aluminium, as the loss in current efficiency in that process is caused by oxidation of the dissolved metal by the anode gas. The solubility of aluminium in inelts close to the cryolite composition (molar ratio NaF/AlF3 = 3) has been measured by several ~vorlters. The results are somewhat controversial, particularly regarding the influence of the NaF/AlF3 ratio. As will be shown subsequently the dissolved metal consists of sodiuin as well as aluminium. This fact has been disregarded by most worlters and the solubilities are normally given as wt. yo Al, which then includes both the sodium and the aluininiun~ contents.

Zhurin (1) found solubilities in the range of 0.10-0.15 wt. % A1 a t 1 060 OC in cryolite- alumina melts, and 0.21 \vt. % A1 a t NaF/AlF3 = 1.67 and 0.062 wt. % A1 a t NaF/A1F3 = 4. Pruvot (2) found the solubility in cryolite a t 1000 OC to be 0.02-0.04 wt. yo Al. iUashovets and Svoboda (3) measured the solubilities in A1F3-rich melts where it increased froin approxiinately 0.12 wt. yo A1 a t NaF/AlF3 = 2.70 to 0.40 wt. yo A1 a t NaF/A1F3 = 1.80. Haupin (4) found the solubility a t 980 OC to be 0.10 wt. % A1 in cryolite, 0.11 wt. yo A1 a t NaF/A1F3 = 3.70, and 0.09 wt. % A1 a t NaF/AlF3 = 2.70. Haupin also tried to analyze separately the sodium and the aluminium contents of the dissolved metal. In cryolite (NaF/AlF3 = 3) the contents were found to be 0.16 wt. % Na and 0.05 wt. yo Al and a t NaF/AlF3 = 2.6, 0.10 wt. yo Na and 0.05 wt. % Al.

Condensates froin NaF-AlF3-A1203 melts in contact with aluminium contain inetallic sodiuin and aluminium, as well as various fluoride compounds (3, 4). The evaporating species responsible for the inetal loss from the melts are probably aluminiuin monofluoride (AIF) and sodium metal. Sodium has been found spectrographically in the vapor phase (5).

The nature of the dissolved metal in the melt has not been established. It is customary to assuine (6) that aluminium occurs as inonovalent ions, i.e.

[ I ] A13+ + 2A1 + 3i\lf,

and that sodium occurs as uncharged metal atoms or ions like Na2+, i.e.

[a] 6Naf + Al + 3Naef + A13+.

The ionic nature of the dissolved metal is supported by electroche~llical ineasurements (7).

Canadian Journal of Cllemistry. Volume 43 (1965)

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3430 CANADIAN JOURNAL O F CHEMISTRY. VOL. 43. lgG5

I t is apparent that the available data on the solubility of aluminium in NaF-AlF3-A1203 melts are partly in conflict and furthermore that the role played by sodiuin has mostly been neglected. The present investigation has been carried out to determine the excess sodium and aluminium contents in melts having molar ratios from NaF/AlF3 = 6.70 to 1.50.

EXPERIMENTAL

The materials used were Alcan super purity aluminium, hand-picked Greenland cryolite, and reagent grade anhydrous sodium fluoride, aluminium fluoride, and alumina.

The melts were contained in recrystallized alumina crucibles of 20 ml capacity covered by close-fitting lids made of boron nitride or graphite. The composition of the melts was determined from the weighed-in amounts, as the composition changed very little during a run. The melts had to be saturated with alumina to prevent attack on the crucibles. Enough aluminium was added to cover the bottom of the crucible. The equilibration was carried out in a vertical Icanthal furnace where the tube and the radiation shields were made of Inconel. Inert nitrogen gas was passed through the furnace. An equilibration time of 2 h was found to be sufficient. Samples of the melts were taken with a small thick-walled nickel ladle that was lowered from the cold part of the furnace into the melt and the11 quickly withdrawn. The ladle and the lid of the crucible could be manipulated from the outside by thin rods without admitting air to the furnace.

After cooling the samples were taken to a drybox with inert nitrogen atmosphere. The samples were crushed to -20 mesh, weighed, and transferred to the analyzing apparatus to determine the excess metal content. To this purpose each sample was treated with absolute ethyl alcohol and dilute hydrochloric acid. The metal would react with the alcohol and the hydrochloric acid and evolve hydrogen. The hydrogen was first led through a liquid air trap and then passed over cupric oxide a t 350 "C in a tubular furnace, where it was converted to water vapor which was determined gravimetrically by absorption. The carrier gas for this determination was nitrogen.

By the addition of alcohol to the sample a reaction accon~panied by gas evolution could be observed. The gas proved to be hydrogen. The reaction would last only for a few minutes, and even prolonged treatment would not produce any more hydrogen. When hydrochloric acid was added the reaction would recommence and normally reach completion after about 20 min.

Blank runs with samples from melts that had not been in contact with aluminium gave no reaction and no weight increase of the absorption tube. These samples were white while samples containing metal had a distinctive grayish tinge.

When the samples were quenched any dissolved metal in the form of subvalent compounds would become unstable. It was assumed that the aluminium was converted to metallic aluminium, e.g., by reversal of reaction [I], while the sodium was converted to metallic sodium. Both sodium and aluminium are expected to react with alcohol giving hydrogen and aluminium and sodium ethoxide respectively. The reaction with aluminium is known to be very slow, however, and the presence of catalyzers likechlorine or mercuric chloride are needed in order to initiate the reaction (8). In order to test this information, a piece of the aluminium used in these experiments was immersed in absolute alcohol and filed to remove the oxide layer. Even after heating no visible reaction occurred. A piece of sodium, on the other hand, reacted readily with the alcohol.

The assumption was then made that alcohol would react only with the sodium in the sample, while it would not react to any measurable extent with the aluminium. Since the metal present in the quenched samples was in a state of fine dispersion and therefore presumably very reactive, the justification of this assumption may be disputed. The shape of the experimental curve (Fig. 1) indicates, however, that the aluminium did not interfere with the determination of sodium.

RESULTS AND DISCUSSION

Figure 1 shows the results obtained a t 1 000 OC with melts of varying NaF/A1F3 ratio, saturated with A1203. The results a t the highest Na!F/AIF3 ratio are somewhat uncertain, because the NaF-rich melts severely attacked the alumina crucibles. The high viscosity of these melts also made sampling difficult. Curve I sho\vs the total solubility given in the customary way as ~ v t . yo metallic aluminium. Curve I1 shows the part of the total solubility found in the alcohol treatment, i.e., the sodium content, given as mt. % metallic sodium. Because the reducing power per mole alu~ninium is three times greater than that of sodium, the numerical values for the sodium content will for some compositions be higher than the total solubility as given by curve I. Curve 111 shows the aluminium content of the melt in wt. yo Al. Results obtained with cryolite melts a t various tein-

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TIIOSSTAD: SOLUBILITY OF .-\LU>lINIUM IN NaF-AIFs-r11?03 MIJLTS

I

I - 15 20 25 30 35 40

Mole % AIF, - L,, , u

O.OO 10'00 ' - 1-

9.0 2 0 5.0 4.0 3.0 2.5 2.0 1.5 1030 1060 N ~ F / A I ~ ratio - "C -

FIG. I. The solubility of metal in KaF-AIF3-A1?03 melts. Curve I: Total solubility of sodium and aluminium as wt. 70 Al. Curve 11: Sodium content in wt. yo Na. Curve 111: Alulni~i i~~rn content in wt. yo Al.

FIG. 2. The solubility of metal in cryolite-alumina melts versus temperature. Curve I : Total solubility of sodium and aluminium as wt. % Al. Curve 11: Sodium content in wt. % Na. Curve 111: Aluminiui~l content in wt. % Al.

peratures are presented in Fig. 2. Curves I , 11, and I11 give the total solubility, the sodium content, and the aluminium content respectively.

The precision of the measurements is about f 5% for the total solubility and f 1 0 % for the sodium and aluminium contents. Methods involving slow cooling of the samples gave low and irreproducible results. This was probably due to metal loss by evaporation during sampling. The vapor pressures of sodium over the melts are considerable, e.g., 0.1-0.3 atm in cryolite a t 1 000 OC (5, 6, 9). The reproducibility of the results indicate, however, that the rapid quenching that was performed in these experiments permitted effective itfreezing" of the existing equilibria in the melts so that possible evaporation losses were negligible.

The total solubility of sodium and aluminium in cryolite a t 1 000 OC is 0.10 wt. yo Al. The results for melts close to the cryolite composition are in good agreement with the data of Haupin (4). The results for A1F3-rich melts disagree with those given by Zhurin (I) and by 3,Iashovets and Svoboda (3), as these authors found the solubility to increase with increasing AlF3 content. The observed total solubility in cryolite a t 1060 OC is slightly lower than that given by Zhurin (I). The results for the sodium content are lo\ver than those found by Haupin (4) for melts with molar ratios NaF/AlF3 = 3 and 2.6. Accordingly the results for the aluminium contents are higher, i.e., 0.065 wt. % A1 in the present investigation as against 0.05 wt. yo A1 found by Haupin for both compositions.

In a previous investigation (10) the reactivity of the dissolved metal versus COI was found to decrease with decreasing NaF/AIF3 ratio as would be expected from the solubility data presented here. On the other hand, the metal loss due to evaporation (11) increases with decreasing NaF/AlF3 ratio for melts with NaF/AlF3 < 2.7. This is probably due to kinetic effects governing the evaporation.

The existing equilibrium between the sodium and aluminium contents of the melt may tentatively be described by a reaction as follows

[31 A13+ + 2Na2+ ~t Al+ + 4Na+

providing the dissolved species are Al+ and Na?+. As long as the activities of the various constituents of these melts are virtually unknown, the Al+/Na2+ ratio cannot be estimated from thermodynamic data. If eqs. [I], [Z], and [3] are considered, however, the aluillinium

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3432 CANADIA?: JOURNAL OF CHEMISTRY. VOL. 43, 1065

content \\-ould be expected to increase and the sodium content to decrease with decreasing NaF/AlF3 ratio.

The apparent constancy of the alulninium content as given by curve 111 in Fig. 1 is therefore somewhat surprising. However, for melts with NaF/A1F3 < 3 saturated with alumina a t 1 000 OC, i t can be shown that the cation fraction of A13+ (regardless of the structural entities in which it occurs) changes very little with decreasing NaF/A1F3 ratio, e.g., from 0.36 a t NaF/AlF3 = 3 to 0.42 a t NaF/AlF3 = 1.85. This is due to the decreasing solubility of alumina in this range. The saturation concentrations of alumina a t 1 000 OC are about 13 wt. % A1203 for melts in the range NaF/A1F3 = 3 to 4.2 and decreases to 7 wt. % A1?o3 a t NaF/Alf3 = 6.4 (12) and 9.7 wt. % A1203 a t NaF/AlF3 = 1.85 (13).

From the sodium content of pig aluminium, Grjotheim (6) estimated the sodium pressure of a cryolite melt a t 1 000 OC to be about 0.1 atm, while Feinleib and Porter (9) from measurements with Pb-Na cathodes found i t to be about 0.15 atm. The sodium pressure above the melt has been measured spectroscopically by Stoltes and Frank (5) and was found to be 0.3 atm a t 1 000 OC, and 0.5 atm a t 1050 OC. h.Ieasurements by Jander and EIermann (14) and Grube and Hantelrnann (15) indicate that the sodium pressure reaches 1 atm in the range 1-6 mole yo AlF3 in NaF a t 1000 OC.

The sodium content of the melts as given by curve I1 in Fig. 1 increases rapidly with increasing NaF/AlF3 ratio as would be expected from the vapor pressure data. This is also in excellent agreement with the data presented by Pearson and Waddington (16) for the sodium content of aluminium in equilibrium with NaF-AlF3 melts a t 1 000 OC. The sodium content of the rnetal thus decreases from 0.031 wt. yo Na a t NaF/A1F3 = 4.25 to 0.005 \\it. % Na a t NaF/A1F3 = 1.85, while the sodium content of the melt, as deter- mined in the present work, varies from 0.25 to 0.04 \vt. yo Na within the same range. I t is readily seen that the relative changes in sodium content are the same in the metal and the melt.

The temperature dependence of the sodium solubility shown in Fig. 2 is in agreement with that of the vapor pressures found by Stoltes and Frank (5). From Fig. 2 i t call also be seen that the fraction of the total solubility represented by sodium increases with in- creasing temperature. This seems to agree with the fact (9) that the chemical potential of sodium approaches that of aluminium as the temperature increases.

REFERENCES

1. A. ZHURIN in Y. I<. DELIMARSKII and B. F. MARKOV. Electrochemistry of fused salts. The Sigma Press, IVashington. 1961. p. 212.

2. E. P a u v o ~ . Alluminio, 22, 699 (1953). 3. IT. P. MASHOVETS and R. V. SVOBODA. J. Appl. Chem. USSR Eng. Transl. 32, 2210 (1959). 4. W. E. HAUPIN. J. Electrochem. Soc. 107, 232 (1960). 5. J, J. STOKES and W. B. FRANK. Extractive metallurgy of aluminium. Vol. 11. Interscience Publishers,

New York. 1063. p. 3. 6. I<. GI~JOTHEIU. I<gl. Norske Vidensltab. Selskabs Skrifter, 1956. No. 5. pp. 65-80. 7. L. A. FIRSANOVA. Sb. Nauchn. Tr . Inst. Tsvetn. Metal. 35, 68 (1963). S. E. MULLER. Metoden der organischen Chemie. Vol. IV. Georg Thieme Verlag, Stuttgart. 1955. p. 234. 9. M. FEINLEIB and B. PORTER. J. Electrochem. Soc. 103, 231 (1956).

10. J . THONSTAD. J . Electrochem. Soc. 111, 959 (1964). 11. A. I. BELJAJEV, M. B. RAPOPORT, and L. A. FIRSANOVA. Metallurgie des aluminiums. Vol. I. VEB

Verlag Technik, Berlin. 1956. p. 114. 12. P. A. FOSTER, JR. J . Chem. and Eng. Data, 9, 200 (1964). 13. A. FENERTY and E. A. I-IOLLINGSHBAD. J. Electrochem. Soc. 107, 993 (1960). 14. W. JANDEI~ and H. HERMANN. Z. Anorg. Allgem. Chem. 239, 65 (1938). 15. G. GI~UBE and P. HANTELMANN. The reactions of A1 with melts of the system NaF-AIF3 and

NaF-AIF3-A1203. Icaiser Wilhelm Institut fiir Metallforschung. Feb. 1945. 16. T. G. PEARSON and J. ~VADDIXGTON. Discussioils Faraday Soc. 1, 307 (1947).

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