J. Phys. Chem. B 1997, 101, 9447-9457
9447
Structure and Thermodynamics of Alkali Fluoride-Aluminum Fluoride-Alumina Melts. Vapor Pressure, Solubility, and Raman Spectroscopic Studies E. Robert,† J. E. Olsen,‡ V. Danek,§ E. Tixhon,† T. Østvold,‡ and B. Gilbert*,† Laboratoire de Chimie Analytique, UniVersite´ de Lie` ge, B-4000 Lie` ge, Belgium, Institute of Inorganic Chemistry, The Norwegian Institute of Science and Technology, N-7034 Trondheim, Norway, and Institut of Inorganic Chemistry, SloVak Academy of Sciences, 842 36 BratislaVa, SloVakia ReceiVed: NoVember 5, 1996; In Final Form: August 21, 1997X
The MF-AlF3-Al2O3 (M ) Li, Na, K) systems have been studied in the liquid state. Raman spectra were recorded to obtain information on the structure and the ionic composition of these melts, vapor pressure measurements were performed to obtain thermodynamic information, and the Al2O3 solubility has been measured as a function of the melt composition and temperature. These data show that the ions AlF4-, AlF52-, AlF63-, and F- exist together with the oxyanions Al2OF62-, Al2O2F42-, and possibly Al3O4F43-. For the NaF- and KF-containing systems, the solubility of Al2O3 increases very much with the mole ratio n°MF/ n°AlF3 and is maximum at n°MF/n°AlF3 ) 4 (8.1 mol % at 1040 °C for the NaF-AlF3 mixtures, 16.8 mol % at 1027 °C for the KF-AlF3 mixtures). For the LiF-AlF3 system the Al2O3 solubility is much smaller with a maximum of ≈2.2 mol % probably around n°MF/n°AlF3 ) 1.5 at 1000 °C. The four equilibria AlF63- ) AlF52- + F-, AlF52- ) AlF4- + F-, 2Al2OF62- + 2F- ) Al2O2F42- + 2AlF52-, 2Al2O2F42- + F- ) Al3O4F43+ AlF52-, can give a reasonable explanation of the observed Raman, vapor pressures, and solubility data.
Introduction In the Hall-He´roult process, aluminum is produced electrolytically from a molten cryolite bath containing dissolved Al2O3. Since this is a highly energy-consuming industrial process, extensive studies have been performed to improve its current and energy efficiency. One part of this effort has been research related to the basic understanding of the structure and thermodynamics of the electrolyte and the kinetics of the reduction process. Despite the large number of investigations, some problems still remain to be solved. The dissolution mechanism of alumina and the structure of the oxyfluoride complexes formed when Al2O3 dissolves are not yet known. There is also a large and not understood difference in alumina solubility in MF-AlF3 melts (M ) Li, Na, and K). There is no explanation as to why alumina is more soluble in Na3AlF6 than in Li3AlF6 and still more soluble in K3AlF6.1 When Al2O3 is added to cryolitic melts, oxide-containing species are formed since the oxide ion is not likely to be stable in melts containing trivalent cations. Grjotheim et al.2 listed 22 different complexes that had been suggested through the years to account for the solubility of Al2O3 in cryolite. The number of possible complexes was reduced considerably when Raman spectra were obtained by Gilbert et al.3 They concluded that AlOFx1-x species could not exist in solutions with more than 2 mol % Al2O3 and that only complexes with bridging Al-O-Al bonds would form. The Raman data further indicated that several oxide-containing complexes existed in equilibrium. From electromotive force (emf) measurements on saturated melts Sterten4 proposed that anions of the type Al2OFx4-x and Al2O2Fx2-x were the major oxide-containing species. Sterten also proposed distribution curves for the different species as a * To whom correspondence should be addressed. E-mail:
[email protected]. † Universite ´ de Lie`ge. ‡ The Norwegian Institute of Science and Technology. § Slovak Academy of Sciences. X Abstract published in AdVance ACS Abstracts, October 1, 1997.
S1089-5647(96)03452-9 CCC: $14.00
function of melt composition. In basic solvents, where the cryolite ratio, CR, is higher than 5 (CR ) n°MF/n°AlF3, n°i being the initial number of moles of component i), the anions Al2O2F42- and Al2O2F64- were the major species while in acidic melts, CR < 3, Al2OF62- and Al2O2F42- dominate. For low alumina contents, Al2OF62- was the main species. Julsrud5 also considered the Al2OF84- complex in a thermodynamic model based on cryoscopic and calorimetric data. Kvande6 combined the results of Julsrud and Sterten and suggested large amounts of Al2OF84- for solvents with CR ) 3. His calculation of the anionic molar fraction of the different Al-O-F species indicated that three species existed in about the same concentration for dilute solutions of alumina while Al2O2F42- was the major species in Al2O3-rich melts. Recently, Dewing and Thonstad7 made a careful analysis of literature data to obtain reliable values for the activities of Al2O3 in cryolitic melts. They showed that the nonstoichiometry of solid Na3AlF6 had a major influence on the interpretation of the data. They concluded that the slope of a log(activity) vs log(concentration) plot was about 3 in dilute solutions and 1.5 in concentrated solutions. This corresponds to formation of oxide complexes with one and two oxygen, respectively. In the present work more information about the structure of alumina dissolved in MF-AlF3 (M ) Li, Na, K) melts is obtained by Raman, vapor pressure, and solubility measurements. A model is proposed for the large variation in solubility of alumina vs composition and for the variation in solubility between the different MF-AlF3 systems. Experimental Section Chemicals. The origins of the chemicals and the purification methods used are given in Table 1. The oxide contents in the fluorides are typical values from different batches, analyzed by carbothermal reduction using a LECO TC-436 nitrogen/oxygen analyzer. All chemicals were handled in a N2- or Ar-containing glovebox with less than 1 ppm of water vapor. Vapor Pressure. The boiling point method described by Motzfeldt, Kvande, and Wahlbeck8 was used since this method © 1997 American Chemical Society
9448 J. Phys. Chem. B, Vol. 101, No. 46, 1997
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TABLE 1: Origins and Purification Methods of Chemicals Used chemical
origin and purification method
Na3AlF6
hand-picked cryolite from Greenland, dried under vacuum. wt % oxygen: 0.04-0.08. E. Merck,“Zur chromatographischen adsorptions”, Al2O3, powder (min 99%), dried under vacuum. Al2O3, pieces Aldrich, (99.8%). LiF and KF E. Merck, “Zur Analyse”, recrystallized in N2 atm. wt % oxygen: 0.01-0.04. A° rdal og Sunndal Verk,“Technical grade”, resublimed AlF3 twice under vacuum. wt % oxygen: 0.1-0.4.
gives reliable data in the pressure region 10-2 to 1 atm and at high temperatures (i.e., 1000-1300 °C). An apparatus specially designed for measurements of corrosive gases was readily available at the Institute of Inorganic Chemistry and has been described in detail by Herstad and Motzfeldt.9 The main components of the equipment are a cold wall vacuum furnace with a vertically mounted graphite tube heater. Further details describing the equipment and the experimental procedure are given by Gilbert et al.10 Alumina Solubility. The solubility measurements were performed in a Kanthal wound vertical tube furnace, equipped with an Eurotherm temperature controller. The melt container was a platinum crucible placed on a steel supporter. Radiation shields of sintered alumina was used above and below the crucible to ensure constant temperature ((2 °C) along it. The melt was stirred with a graphite stirrer and the temperature was measured with a calibrated Pt/Pt 10% Rh thermocouple located in the melt. Samples for analysis were withdrawn by a specially designed graphite ladle whose inlet was located 10 mm above the bottom to avoid undissolved alumina in the samples. This graphite ladle was threaded and screwed on a steel shaft and preheated just above the melt before sampling. The temperature of the melt was determined with an uncertainty of (2 °C. The samples were analyzed for oxide content by carbothermal reduction using a LECO TC-436 nitrogen/oxygen analyzer. The chemicals were weighed and added to the platinum crucible in a glovebox to avoid moisture. For each experiment about 80-100 g of salt was used. The crucible containing the chemicals was then transferred to the furnace chamber which was evacuated ( 1 are present, the increased solubility can be understood. Again, an isolated complex is the easiest way to explain the data and to build a model. The actual structure in the melt could, however, be different and more complicated. Figure 5 indicates that the observed maximum in Al2O3 solubility is around CR ) 2 for the LiF-AlF3 system. The solubility of Al2O3 in LiF-YF3 melts has been reported to increase when decreasing the molar ratio nLiF/nYF3 down to 1.26 It is therefore not unlikely that the maximum Al2O3 solubility in the LiF-AlF3 system can be explained by a dissolution of Al2O3 as Al2OF62- ions. Ideally such an ion will give a maximum solubility at CR ) 1.5. In agreement with previous results, our measurements show that the observed solubility of alumina in MF-AlF3 melts is strongly affected by the cation nature and increases in the sequence Li < Na < K. The most probable explanation involves the fact that the alkali cation radius is decreasing considerably from K+ to Li+. This decrease results in an increase in the electric potential of the M+-F- interactions in the same sequence. This effect is also reflected in the Raman spectra that clearly show an increase in bandwidth for all the
TABLE 6: Calculated Theoretical Solubilities Based on the Formation of 100% M2Al2OF6 or M2Al2O2F4, Respectively, Together with Observed Solubilities of Al2O3 in MF-AlF3 (M ) Li, Na, and K)a theoretical solubility, mol %
observed solubility, mol %
n°MF/n°AlF3
100% M2Al2OF6
100% M2Al2O2F4
KF-AlF3-Al2O3
1.2 1.5 2 3 4
8.3 9.1 7.7 5.9 4.8
15.4 16.7 18.2 20.0 16.7
7.8 11.6 16.2 16.4
a
Experimental data at 1000 °K.
NaF-AlF3-Al2O3
LiF-AlF3-Al2O3 2.8
5.7 6.2 6.4
3.1 2.1
9456 J. Phys. Chem. B, Vol. 101, No. 46, 1997
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TABLE 7: Model Parameters a
n°MF/n°AlF3 T, °C M 2 3 2-4
K1 10
K2 0.06410
1020 Na 4.3 1020 Na 3.310 0.04910 1027 K 4.814 0.1114
γMAlF4 γMFa
p°MAlF4, p°MF , mbar mbar
1.810
47.515
2.810 1.514
47.515 125.914
0.614
TABLE 8: Equilibrium Constants from the Model Calculations T, °C
M
K11
K12
K5
1020 1027
Na K
15 6
0.004 0.02
0.79 ( 0.07 44 ( 9
7.320
a
Activities of NaF for NaF-AlF3-Al2O3 are used from Sterten and Mæland.14
AlF63-, AlF52-, and AlF4- bands in the same sequence (K+ to Li+), indicating distortions in the regular structure of these ions when the M+-F- interactions become stronger. These interactions will influence the oxide solubilities since stronger Li+AlF52- interactions will shift equilibria 4 and 5 to the left and reduce the Al-O-F complex formation and in this way will reduce the Al2O3 solubilities. More explanations will be given below along with our discussion of the nature of the oxide species as a function of the melt cation. A Solubility Model. On the basis of the above results it is likely that Al2O3 dissolves in MF-AlF3 melts as Al2OF62-, Al2O2F42-, and Al3O4F43-. We may therefore be able to build a model for the Al2O3 solubility in these melts using the equilibria which determine the concentrations of the different oxide-containing ions. Equations 1 and 2 and a combination of eqs 4-6 give us four independent equilibria which are related to the concentrations of oxide-containing species.
AlF63- ) AlF52- + F-
(1)
AlF52- ) AlF4- + F-
(2)
2Al2OF62- + 2F- ) Al2O2F42- + 2AlF52-
(11)
2-
2Al2O2F4
3-
+ F ) Al3O4F4 -
+ AlF5
2-
(12)
This set of equations contains seven unknown mole fractions and four equilibrium constants. With the three mass balances for Al, F, and O and the assumption that the equilibrium constants K1 and K2 have the same values as in their respective binary systems, the two last constants, K11 and K12, may be fitted to the experimental vapor pressures. All calculations are made with the stoichiometric equilibrium constants obtained from Raman data in the respective binary systems.10,12 The assumption that K1 and K2 are constant is based on the nearly constant ratio between the Raman bands of AlF4- and AlF52with alumina addition. The pressures are calculated using the relation pi ) xiγip°i where pi is pressure, xi is mole fraction, γi is the activity coefficient, and p°i is the pressure above pure component i. The activity coefficients used are those obtained for the binary systems.10,12 Due to lack of data these activity coefficients are assumed constants when alumina is added. This may be a reasonable approximation as we found that Al2O3 behaves as a close to neutral additive, indicating a constant activity coefficient of Al2O3.10 The model parameters are given in Table 7. The fitting procedure was performed as follows: (i) Equations 1, 2, and 11 together with the mass balances for Al, F, and O are used to estimate K11 at CR ) 2. At this ratio the concentration of Al3O4F43- is very small and can be neglected. K11 is adjusted until calculated and measured vapor pressures fit over the whole Al2O3 concentration range. K1 and K2 are kept constant as outlined above. (ii) The same procedure is repeated at CR ) 4 using eqs 1, 2, and 11. Now K12 is adjusted until calculated and measured vapor pressures match. At such CR, and mainly for medium
Figure 17. Measured and calculated Al2O3 solubilities for the two MF-AlF3-Al2O3 (M ) Na and K) systems as a function of CR at 1000 °C. b, experimental; s, model. The two lines represent the uncertainty.
to high alumina contents, the Al2OF62- concentration is small and can be neglected as a first approximation. (iii) A program calculates the ionic distribution based on K1, K2, K11, and K12. Starting with the data obtained in (i) and (ii), K11 and K12 are progressively adjusted to fit the vapor pressure data over the whole composition range (content in alumina and CR). (iv) The concentration of the different Al2O3-containing species are then calculated at CR ) 3 using the obtained constants, K11 and K12, and the measured solubility at CR ) 3 for the respective MF-AlF3 systems. With this information, eq 5, with aAl2O3 ) 1, is used to calculate K5. The uncertainty on the value of the solubility at CR ) 3 is used to calculate the error on K5. Equation 5 is used preferentially because, at the solubility limit in the Na+ and K+ systems, Al2O2F42- is the major oxide-containing species. Knowing K5, K11, and K12, we are able to calculate the vapor pressures of any melts and the solubility at any CR. The calculations show that measured and predicted pressures agree within 5% for NaF-AlF3-Al2O3 melts. For the KFAlF3-Al2O3 system at CR ) 4, the model pressure was 70% of the measured pressure while at CR ) 2 and 3 and the deviation was within 5-10%. The values of the different constants K5, K11, and K12 for the Na+ and K+ systems are given in Table 8. The calculated and measured solubilities of alumina are compared in Figure 17. The two solid lines represent the upper and lower limits of the calculated solubility, considering the error on K5. The agreement between model and experimental
Alkali Fluoride-Aluminum Fluoride-Alumina Melts
J. Phys. Chem. B, Vol. 101, No. 46, 1997 9457 in the LiF-AlF3 system and thus for the lower Al2O3 solubility in this binary than in the other two MF-AlF3 (M ) Na, K) binaries. Concluding Remarks Structural and thermodynamic data have been presented for alkali fluoride-aluminum fluoride melts containing alumina up to the solubility limit of the oxide. These data indicate that alumina behaves as a close to neutral additive in an acid/base sense in these melts. Moreover, several oxide-containing anionic complexes seem to form with the oxygen atom in a bridging position between aluminum atoms. We have suggested that Al2OF62- forms at low oxide concentration followed by Al2O2F42- at higher concentrations. We have also suggested that a complex richer in oxygen, such as Al3O4F43-, forms at concentrations close to the solubility limit in KF-AlF3 melts. The interpretation of the experimental data is, however, not straightforward since it is impossible to establish unequivocally both the structure and the concentration of the ionic species in the melt directly from the Raman spectra.
Figure 18. Calculated mole fractions of oxide species in the systems MF-AlF3-Al2O3 (M ) Na, K) at 1020 °C and 1027 °C, respectively. Filled symbols, Al2OF62-; open symbols, Al2O2F42-; dotted symbols, Al3O4F43-. b and O, CR ) 2; 9 and 0, CR) 3; [ and ], CR ) 4.
data is very good. Nevertheless, the assumption of constant activity coefficients at very high alumina contents may be unreasonable. The model equations may therefore not be realistic at very high values of X°Al2O3. It is, however, difficult to specify the concentration range where the model is applicable. Figure 18 shows the distributions (in mole fraction) of oxide species resulting from the present model calculations. Sterten4 and Kvande6 have also given distribution curves for the mole fractions of anions in the NaF-AlF3-Al2O3 system. However, as pointed out by Dewing,27 both Sterten4 and Kvande6 have assumed that the binary NaF-AlF3 system can be described by an ideal mixture of the F-, AlF63-, and AlF4anions only. This is not consistent with the present results. The main oxide-containing species suggested by Sterten and Kvande are, however, the same as those used in the present work. For the LiF-AlF3-Al2O3 system, vapor pressure data are not available, and model calculations could not be done. When comparing Raman and solubility data, it seems reasonable, however, to assume that the very low solubility of alumina in these mixtures is due to the formation of Al2OF62- only. As already mentioned, Li-based melts are indeed characterized by stronger interactions between the cation and the F- anion than in Na+- or K+-containing melts. In a sense, this also means that, at identical overall composition, the F- activity is lower. This lower F- activity affects the Al2O3 solubility in two ways. First, as seen from eq 4, the formation of Al2OF62- will release F-, and this process should then stabilize Al2OF62- more in the LiF-AlF3 binary than in the other ones. Second, when Al2O2F42- is formed from Al2OF62-, reaction 11 requires that free F- ions are consumed. If the F- anion activity is already lower, reaction 11 will have less tendency to occur. Consequently, this competition between the alkali cation and the Fanion may be the reason for the low concentration of Al2O2F42-
Acknowledgment. This project has benefited scientifically and financially from our participation in a Human Capital and Mobility Network of the European Communities. Financial support from The Research Council of Norway and the Norwegian aluminum industry is gratefully acknowledged. The FNRS of Belgium is also acknowledged for a doctoral fellowship given to one of the authors (E.R.). References and Notes (1) Dewing, E. W. Can. Met. Q. 1991, 30, 153. (2) Grjotheim, K.; Krohn, C.; Malinovsky, M.; Matiasovsky, K.; Thonstad, J. Aluminium Electrolysis-Fundamentals of the Hall-Heroult Process, 2nd ed.; Aluminium-Verlag: Dusseldorf, 1982. (3) Gilbert, B.; Mamantov, G.; Begun, G. M. Inorg. Nucl. Lett. 1976, 12, 415. (4) Sterten, Å. Electrochim. Acta 1980, 25, 1673. (5) Julsrud, S. Thesis, Inst. Inorg. Chem., University of Trondheim, NTH, 1983. (6) Kvande, H. Light Met. 1986, 23. (7) Dewing, E. W.; Thonstad, J. Metall. Trans. B, submitted for publication. (8) Motzfeldt, K.; Kvande, H.; Wahlbeck, P. G. Acta Chem. Scand. 1977, A31, 444. (9) Herstad, O.; Motzfeldt, K. ReV. Haut. Temp. Refract. 1966, 3, 291. (10) Gilbert, B.; Robert, E.; Tixhon, E.; Olsen, J. E.; Østvold, T. Inorg. Chem. 1996, 35, 4198. (11) LECO Corporation. Instruction manual, TC-436 Nitrogen/Oxygen determinator, 1992. (12) Robert, E.; Gilbert, B.; Olsen, J. E.; Østvold, T. Acta Chem. Scand. 1997, 51, 379 (13) Skybakmoen, E.; Solheim, A. ; Sterten Å. Light Met. 1990, 317. (14) Tixhon, E.; Robert, E.; Gilbert, B. Appl. Spectrosc. 1994, 48, 1477. (15) Dewing, E. W. Met. Trans. 1989, 20B, 675. (16) Sterten, A.; Mæland, I. Acta Chem. Scand. 1985, A39, 241. (17) Kvande, H. Thesis. Inst. Inorg. Chem., University of Trondheim, NTH, 1979. (18) Guzman, J. Thesis. Inst. Inorg. Chem., University of Trondheim, NTH, 1986. (19) Zhou, H. Thesis. Inst. Inorg. Chem., University of Trondheim, NTH, 1991. (20) Sidorov, L. N.; Belousov, V. I.; Scholtz, V. B. AdV. Mass Spectrom. 1971, 5, 394. (21) JANAF Thermochemical Tables 14, 1985. (22) Tixhon, E.; Robert, E.; Gilbert, B. Vibr. Spectrosc. 1996, 13, 91. (23) Holm, J. Thesis. Inst. Inorg. Chem., University of Trondheim, NTH, 1963. (24) Foster, P. A. J. Am. Ceram. Soc. 1975, 58, 288. (25) Phillips, N. W. F.; Singleton, R. H.; Hollingshead, E. A. J. Electrochem. Soc. 1955, 102, 690. (26) Reddy, R. G.; Kumar, S. G. Met. Trans. 1994, 25B, 91. (27) Dewing, E. W. In Proceedings of the Fifth International Symposium on Molten Salts. Saboungi, M. L., Newman, D. S., Johnson, K., Inman, D., Eds.; Proc. 86, The Electrochemical Society: New York, 1986.
