THE ALUMINUM OXIDE SOLUBILITY
IN THE KF-NaF-AlF3 MELTS
O. Tkacheva, Yu. Zaikov, A. Apisarov, A. Dedyukhin, A. Redkin
Institute of High Temperature Electrochemistry,
S. Kovalevskaya 20, Yekaterinburg, 620219 Russia
ABSTRACT
The alumina solubility in the KF-NaF-AlF3 system depending on alkali metal fluoride
concentration in the temperature range from 800°C down to melting point of the mixture
is presented. The cryolite ratio ([KF]+[NaF])/[AlF3] was kept 1.3 and 1.5. The
substitution of cation K+ on Na+ in the KF-NaF-AlF3 melt leads to alumina solubility
decrease. The regression equitation determined on the base of literature and obtained data
allows estimating the alumina solubility in KF-NaF-AlF3 molten mixtures in the wide CR
and temperature interval.
INTRODUCTION
All existing technologies of light metals production are very harmful and resulted in
damage of our environment. In order to reduce harmful wastes production and to
improve other technological parameters some fundamental scientific investigations are
needed. The main directions are:
1. Investigation of basic physicochemical properties of molten salt mixtures;
2. Investigation of electrode processes;
3. Development of new corrosion stable compounds which can be used as
construction, lining and electrode materials.
The aim of the first direction is the development of new electrolyte having high electrical
conductivity, low viscosity, low vapor pressure and the possibility to carry out the process
at much lower temperature. In order to obtain essential decrease of the electrolyte
liquidus temperature at aluminum production the mixture of potassium and sodium
cryolites with low cryolite ratio can be used. The supposed operating temperature interval
is 750-850oC that corresponds to the KF-NaF-AlF3 electrolyte compositions at cryolite
ratio 1,3-1,7. Therefore the objective of the present work is to study the alumina solubility
in the ternary KF-NaF-AlF3 molten system at low cryolite ratio in order to define the
proper electrolyte composition.
The alumina solubility in electrolytes based on potassium cryolite (with additions of AlF3,
LiF, CaF2, MgF2 and some others) in the cryolite ratio (CR) range from 3.0 to 2.2 at
temperatures 900-1000°C is well studied (1). It is due to using electrolytes of such
compositions in industry. For aluminum electrolysis the sufficient alumina concentration
is 3-5 mas.%. The change of the conventional electrolyte composition by adding
aluminum fluoride i.e. the CR decreasing leads to the essential drop of the liquidus
temperature but at the same time it decreases the alumina solubility. The alumina
solubility in the Na3AlF6-AlF3 melt at CR=1.5 is not exceeded 2 mol.% at 700°C (2). The
Al2O3 solubility in the MeF-AlF3 melts (M=Li, Na, K) in the temperature range 8501000°C was studied by the isotherm saturation method with following LECO analysis of
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samples (3). It was found that Al2O3 solubility increases in the order of lithium-sodiumpotassium cryolites. For systems containing NaF and KF the alumina solubility increases
with molar ratio [MeF]/[AlF3] and it has a maximum value at CR=4: 8.1 mol.% - in NaFAlF3 at 1040°C and 16.8 mol.% - in KF-AlF3 at 1027°C. For the LiF-AlF3 mixtures the
solubility is much less and equal to 2.2 mol.% at CR=1.5 and 1000°C. Consequently, the
highest alumina solubility is in potassium cryolites. The authors (4) studied the alumina
solubility in the KF-AlF3 melts at CR=1.0-1.5 in the temperature range 700-800°C by
using two methods. The first one is saturation with following analysis samples by LECO
and the second one is measuring the weight loss of a rotating corundum disc. The data
obtained by LECO were low in some cases on 20%. It was shown that solubility rises
with CR and temperature increase, but small additions (about 4 mol.%) of NaF, LiF or
CaF2 decrease the solution ability of electrolytes. The investigation of Al2O3 solubility in
the low-melting KF-AlF3 system at CR=1.3 and temperature 700, 730 and 760°C was
carried out by using method of thermal saturation with potentiometric control of the
saturation point (5). These results were in a good agreement with data obtained in (4).
The LiF additions (up to 10 mol.%) into the KF-AlF3 melts essentially reduce the alumina
solubility. In general one could say that the information concerning alumina solubility in
the low-melting electrolytes are bare, and the systematic study of the KF-NaF-AlF3
molten mixture was not carried out at all.
EXPERIMENTAL
Chemicals
Electrolytes under investigation were prepared from individual salts (chemically pure)
AlF3 and NaF. KF was taken as KF·HF. At first electrolytes KF-AlF3 and NaF-AlF3 with
the molar ratio required were prepared. Component AlF3 was initially heated in mixture
with NH4F and kept for 6 hours at 450-500 °C for performing oxide fluorination. In order
to obtain KF-AlF3 electrolyte the mixture of KF·HF and AlF3 was heated up to 700°C in
the glassy carbon container and kept at this temperature over four hours. At that HF was
removed from the melt as a result of the thermal KF·HF decomposition (T=238.7°C).
The NaF-AlF3 system was prepared by melting the mixture of AlF3 and NaF in presence
of NH4F. The ternary system KF-NaF-AlF3 was obtained by mixing binaries NaF-AlF3
and KF-AlF3. Preliminary dried Al2O3 (Achinsk alumina plant, Russia) was used. Before
experiments the electrolytes obtained were analyzed on potassium, sodium and aluminum
content by ICP method.
Principles
The solubility was determined by the isothermal saturation technique with potentiometric
control of the saturation concentration (5). The EMF of the concentration galvanic
element Pt| melt+Al2O3 (saturated) || melt+Al2O3 (dissolved) |Pt depends on the
concentration of alumina dissolved in the electrolyte. The EMF equal zero corresponds
the equality of the alumina concentration in both parts of the element.
The thermal analysis method was used in order to obtain the part of quasi-binary phase
diagram of the (KF-AlF3)-Al2O3 system. The glassy carbon crucible with salt was placed
into the quartz cell tightly closed by the vacuum rubber cover with holes for Pt-Pt/Rh
thermocouple and gas in/outlet. The measurements were carried out in argon atmosphere.
176
The temperature control and the data processing were performed using computerized
measuring device APPA-109N.
Results
The aluminum oxide solubility in the KF-AlF3 and KF-NaF-AlF3 molten mixtures at
fixed values of the ([KF]+[NaF])/[AlF3] ratio, equal to 1.3 or 1.5, in the temperature
range from 800°C down to melting point of the mixture have been measured.
KF-AlF3-Al2O3
Alumina solubility values obtained in the KF-AlF3 melts in temperature range 700-800°С
at CR=1.3 and 1.5 are presented in Fig.1. Results of the present work are in satisfactory
agreement with those reported in (4). The increase of the cryolite ratio improves the
Al2O3 solubility. For example at 800 °С in the KF-AlF3 electrolytes alumina solubility is
4.76 and 5.76 mol.% at CR=1.3 and CR=1.5 correspondently. Zhang et. al. (6) explains
the alumina solubility rise with the CR increase in sodium cryolites by the change of the
Al - O - F complexes composition. The alumina is dissolved mainly as Al2OF62- anion in
electrolytes with low CR, but in less acidic melts the di-oxygen solute Al2O2F42- is
dominant. Possibly the same explanation can be applied for the potassium systems.
6,0
1
2
Alumina solubility , mol.%
5,5
5,0
3
4,5
4
4,0
3,5
3,0
690
710
730
750
Temperature, оС
770
790
810
Fig. 1. Alumina solubility in the KF-AlF3 system
1 – (7) CR=1.3; 2 – (7) CR=1.5; 3 – present work CR=1.3; 4 – present work CR=1.5
The alumina solubility in the melts can be also presented as liquidus of the quasi-binary
phase diagram (KF–AlF3)–Al2O3 (Fig.2). The part of this diagram, where the primary
crystalline phase is cryolite, was obtained by the thermal analysis method. The liquidus
in that part of diagram where the primary crystalline phase is Al2O3 was plotted on the
data obtained by the isothermal saturation technique. The quasi-binary phase diagram of
the (NaF–AlF3)–Al2O3 system with CR≈1.5 determined by measuring the weight loss of a
rotating corundum disc (7) is also shown in Fig.2.
177
The alumina solubility increases with CR rise but at that the temperature of primary
crystallization increases significantly. For example, in potassium system with small Al2O3
addition (3 mol.%) the CR change just on 0.2 increases the liquidus temperature on 100
degrees that may be very important at selecting the low melting electrolyte for the
alternative aluminum reduction technology. It is necessary to note that at the same CR the
liquidus temperature in potassium system essentially low then in sodium one. The Al2O3
solubility in the KF–AlF3 melts is higher then that in NaF–AlF3. So, it is 5.8 and 2.1
mol.% at t=800°С and CR=1.5 correspondently in the KF–AlF3 and NaF–AlF3 melts.
1060
960
860
t, °C
1
760
2
660
3
560
0,0
1,0
2,0
3,0
4,0
5,0
6,0
Al2O3, mol. %
Fig.2. Liquidus temperature in the (KF-AlF3)-Al2O3 system: 3 – CR=1.3; 2 – CR=1.5.
Open marks – the thermal analysis method data; filled marks – the isothermal saturation
method data. Liquidus temperature in the (NaF-AlF3)-Al2O3 system: 1 - CR=1.5 (7).
KF-NaF-AlF3-Al2O3
The alumina solubility in the KF-NaF-AlF3 system depending on the substitution of KF
by NaF at fixed values of the ([KF]+[NaF])/[AlF3] ratio (1.3 or 1.5) was measured. The
results obtained are listed in Table 1.
The alumina solubility in the KF-NaF-AlF3 mixture at 800°C is presented in Fig.3. The
solubility values in the KF–AlF3 and NaF–AlF3 melts with CR=1.5 obtained at 1000°C
(3) was extrapolated on 800°C and plotted on the same figure for comparison. The Al2O3
solubility increases with rise of CR and KF content. In the electrolytes with high NaF
concentration ([NaF]/[KF]+[NaF]=0.9-1) the solubility has close values (about 2 mol.%)
at 800°C.
The substitution of cation K+ on Na+ in the KF-NaF-AlF3 melt leads to alumina solubility
decrease. This fact can be explained by the difference of alkali metals ionic potentials.
Sodium cations with higher ionic potential make more strong bonds with fluorine in
fluoride-aluminum complexes that results in difficulties at the oxide-fluoride-aluminum
178
complexes formation at alumina dissolving in such electrolytes. Assuming that the
alumina dissolving in cryolite mixtures with low CR occurs according following
equitations
Al2O3 + 4AlF4- + 2F- = 3[Al2OF6]2-
[1]
2Al2O3 + 2AlF4- + 4F- = 3[Al2O2F4]2-
[2]
one may conclude that the equilibrium will be shifted to the left at strengthening
interaction of the alkali metal ions with fluorine. Consequently the alumina solubility will
be decreased in the row K – Na – Li cryolites.
Table.1 The melt composition and alumina solubility in the KF-NaF-AlF3 system
Melt composition, mol.%
Alumina solubility
CR
[NaF]/([NaF]+[KF])
NaF
KF
AlF3
T, °C mas.%
mol.%
0.00
47.35
52.65
0.00
700
4.70
3.24
0.00
47.35
52.65
0.00
750
5.50
3.81
0.00
47.35
52.65
0.00
800
6.85
4.76
1.3
10.00
35.33
54.67
0.28
800
5.20
3.47
20.00
23.31
56.69
0.54
800
4.50
2.89
30.00
11.29
58.71
0.79
800
3.51
2.17
0.00
50.92
49.08
0.00
750
6.85
4.70
0.00
50.92
49.08
0.00
800
8.35
5.76
10.00
39.04
50.96
0.26
800
6.75
4.47
1.5
20.00
27.16
52.84
0.50
800
5.20
3.31
30.00
15.28
54.72
0.73
800
4.40
2.70
40.00
3.40
56.60
0.94
800
3.62
2.14
Alunina solubility, mol.%
6
CR=1,3
CR=1,5
CR=1,5 (3)
5
4
3
2
1
0
0.2
0.4
0.6
0.8
[NaF]/([NaF]+[KF])
Fig.3 Alumina solubility in the KF-NaF-AlF3 mixture at 800°C
179
1
DISCUSSION
In order to describe the regularities in changing the alumina solubility in the KF-NaFAlF3 melts within wide temperature and concentration range it is possible to take the
value of the mixture molar volume (V). The molar volume is a thermodynamic
characteristic of a liquid and it is connected with density by equitation:
V=M/d
[3]
where M – molar mass of the cryolite mixture, g/mol; d – density, g/cm3. The molar
volume change in dependence on the different parameters of a system represents the
source of valuable information about inter-partical distances that in its turn reflects the
nature of forces acted between the components. To elucidate an influence of the cation
composition on the physical-chemical properties of the KF-NaF-AlF3 mixture it is also
convenient using the value of the molar volume fraction occupied by the alkali fluorides
(VMeF). It can be presented as follows:
VMeF 
M  n MeF
d
[4]
Here M – the molar mass of the cryolite mixture, g/mol; nMeF - the alkali fluorides molar
fraction; d – the molten mixture density, g/cm3. The cation content of the melt determines
the value VMeF and it also effects on the alumina solubility in the melt.
In order to calculate the molar volume it is necessary to know the density of the (KFAlF3)-(NaF-AlF3) mixtures. However there is available only one paper (8) dedicated to
study the density of the sodium and potassium cryolites mixture at CR=3 and
temperatures 930-1090°C. It was shown that density of such melts is non-additive
magnitude but the molar volume changes in the linear low. One can assume that the molar
volume for mixtures with another molar ratio (([NaF]+[KF])/[AlF3]) will be change
additively. There are available data on density both as the KF-AlF3 as the NaF-AlF3
mixtures with low CR at 1000-1100°C (9). Therefore for the molar volume calculation of
the studied mixtures the values of density (9) extrapolated on temperature 800°C were
used.
The dependence of alumina solubility on the molar volume fraction occupied by the alkali
fluorides in the KF-NaF-AlF3 system at constant molar ratio ([NaF]+[KF])/[AlF3] equal
to 1.3 or 1.5 is given in Fig.4. There is also shown similar dependence obtained from the
data of work (3) for the MeF-AlF3 melts (Me = Li, Na, K) with the CR rang 1.5-4.0 at
1000°C.
The tendency similarity of alumina solubility change with VMeF allows describing all data
by the empirical equitation:
S=56.87-0.043·T+2.74·V-1,12·VMeF - 0.0037·V2 –3047.8·V/T +0.0533·V2MeF ± 0.14
180
[5]
Here S – alumina solubility, mol.%; VMeF - the molar volume fraction occupied by the
alkali fluorides, cm3/mol; V – the mixture molar volume, cm3/mol; T – temperature, K.
The equation [5] allows estimating the Al2O3 solubility in the potassium and sodium
cryolite mixtures within the CR range from 1.3 to 4.0 at temperatures 700-1000°C. The
comparison of the experimental and calculated solubility is shown in Fig 5. The results
are in a good agreement for melts with low CR. Since the alumina solubility in the mixed
K/Na cryolites at high CR has not been studied yet the calculated data for melts with
CR=3 is presented together with the solubility values obtained in pure sodium and
potassium cryolites (3).
18
16
Alumina solubility, mol.%
14
12
NaF-AlF3
10
1
8
LiF-AlF3
КF-AlF3
6
2
4
КF-NaF-AlF3
2
0
12
14
16
18
20
VMeF, cm3/mol
22
24
26
28
Fig.4 Alumina solubility dependence on the molar volume fraction occupied by the alkali
fluorides: 1 – MeF-AlF3 (Me=Li, Na, K), t=1000°С (3) and 2 – KF-NaF-AlF3 CR=1.3
and 1.5, t=800 °С.
20.0
Alumina solubility, mol.%
15.0
CR=3.0 t=1000 oC, (3)
10.0
5.0
CR=1.6 t=800 oC
CR=1.3 t=800 oC
CR=1.5 t=800 oC
0.0
0
0.2
0.4
0.6
[NaF]/([NaF]+[KF])
181
0.8
1
Fig.5. Alumina solubility in the KF-NaF-AlF3 mixture: filled marks – experiment; lines –
calculation (equitation [5])
CONCLUSION
The investigation of the aluminum oxide solubility in the KF-NaF-AlF3 molten mixtures
allows us to make the following conclusions.
 Temperature and CR increasing lead to the alumina solubility rise.
 The substitution of cation K+ on Na+ in the KF-NaF-AlF3 melt leads to alumina
solubility decrease. The solubility has close values (about 2 mol.%) in the
electrolytes
with
CR=1.3-1.5
and
high
NaF
concentration
([NaF]/[KF]+[NaF]=0.9-1) at 800°C.
 The regression equitation determined on the base of literature and obtained data
allows estimating the alumina solubility in KF-NaF-AlF3 molten mixtures in the
range of CR=1.3-3.0 and temperature 800-1000°C.
REFERENCES
1. Grjotheim K., Krohn C., Malinovsky M. et al. Aluminium Electrolysis.
Fundamentals of Hall-Heroult Process. 2-nd Edition. Dusseldorf. Aluminium
Verlag. 1982.
2. Fenerty A, Hollingshead E.A. ‘Liquidus curves for aluminum cell electrolyte. II
Systems cryolite and cryolite-alumina with aluminum fluoride and calcium
fluoride’. Journal of the electrochemical society 1960 107 (1) 993-996
3. Robert E., Olsen J.E. et al. ‘Structure and Thermodynamics of Alkali Fluoride–
Aluminum Fluoride–Alumina Melts. Vapor Pressure, Solubility, and Raman
Spectroscopic Studies’. J. Phys. Chem. B. 1997 101 9447-9457.
4. Yang J., Graczyk D., Wunsch C. ‘Alumina solubility in KF-AlF3-based
low-temperature electrolyte system.’ Light metals 2007 537-541.
5. Kryukovsky V., Frolov A., Tkatcheva O., Apisarov A., Zaikov Yu., Redkin A.,
Khokhlov V., ‘The study of the potassium cryolite physical-chemical properties’.
Proc. 7th int. conf. ‘Aluminium of Siberia’, 5-7 Sept. 2006, Krasnoyarsk, Russia,
2006 46-49.
6. Zhang Y., Rapp R. ‘Modeling the dependence of alumina solubility on temperature
and melt composition in cryolite-based melts’. Metallurgical and materials
Transactions B. 2004 35B 509-515.
7. Skybakmoen E., Solheim A., Sterten A. ‘Alumina solubility in molten salt systems
of interest for aluminum electrolysis and related phase diagram data’ Metallurgical
and materials Transactions B. 1997 28B 81-86.
8. Fernandez R. and Ostvold T. ‘Surface tension and density of molten fluorides and
fluoride mixtures containing cryolite’. Acta Chemica Scandinavica. 1989 43 151159.
9. Fernandez R., Grjotheim K. and Ostvold T. ‘Physicichemical properties of cryolite
and cryolite alumina melts with KF additions. 2. Density and surface tension.’
Light Metals 1986 1025-1032.
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