POTASSIUM CRYOLITE AS A BASIC ELECTROLYTE
COMPONENT FOR THE LOW-TEMPERATURE
ALUMINUM PRODUCTION
Tkacheva O., Redkin A., Kataev A., Zaikov Yu.
Institute of High-Temperature Electrochemistry,
Akademicheskaya 20, Yekaterinburg 620990 Russia
ABSTRACT
A basic component of conventional electrolyte for aluminum production (the HallHeroult process) is sodium cryolite (Na3AlF6). It has a very high melting point and a
good solubility of Al2O3. Any addition (typically AlF3, CaF2, MgF2) to the molten
sodium cryolite reduces as the electrolyte liquidus temperature as the alumina
solubility. Nevertheless the operating temperature of the aluminum electrolysis is still
high (950-960 °C) that is a decisive factor for increased corrosion activity of fluoride
melts. The impossibility of using new constructional materials, including
inconsumable anodes, in traditional sodium electrolyte induces the deeper interest for
searching new low-melted electrolytes. Potassium cryolite (K3AlF6) with the molar
ratio of KF/AlF3≤1.5 possessing the melting point below 730 °C and the alumina
solubility about 5-6 wt% (at 750 °C) can be such an electrolyte. The peculiarity of
these electrolytes is that any addition of fluoride salt (LiF, NaF, CaF2, KBF4),
excepting AlF3, significantly increases the liquidus temperature and decreases the
alumina solubility.
INTRODUCTION
The classic Hall-Héroult aluminum electrolysis process consists of electrochemical
decomposition of alumina dissolved in sodium-cryolite-based electrolyte at 950-960
°C. The process is distinguished by the high energy and material consumption as well
as the significant greenhouse emission. Recently, in order to improve cell efficiency
and achieve environmental benefits, a novel low-temperature technology utilizing
inert anode and wetted cathode was evaluated (1). This process based on using a new
electrolyte, the main component of which is potassium cryolite (KF-AlF3) with
cryolite ratio (CR) 1.3, allows performing electrolysis of aluminum and its alloys at
temperature below 800 °C. The successful combination of inert anode with wetted
cathode technology has the potential to save up to 25% of the energy used in the hightemperature electrochemical production of aluminum.
However, radical changes in operating temperature and electrolyte composition can
negatively impact the process of electrolysis. That is why a reliable data regarding
properties of potassium-cryolite-based electrolytes with low CR is of particular
interest. The liquidus temperature, electrical conductivity, density, and alumina
solubility in the KF-AlF3 melt with the CR equal to 1.3 and 1.5 containing the NaF,
LiF, CaF2, and KBF4 additives are considered in this paper.
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EXPERIMENTAL
The liquidus temperature of the molten mixtures was studied using the differential
thermal analysis (DTA). A measuring cell was a tightly closed quartz container on the
bottom of which a glassy carbon crucible filled with a known amount of salt was
placed. A cap had holes for thermocouple (Pt/Pt-Rh) and gas (Ar) in/outlet.
The original technique to study the electrical conductivity of aggressive fluoride and
oxide-fluoride melts was developed (2). It consists in combination of two types cells:
capillary (BN) and with parallel electrodes (Mo). The resistivity of electrolyte was
measured by means of impedancemeter (Zahner electric IM6E) in a frequency range
from 100 Hz to 1 MHz using a signal with amplitude 5 mV/s. The electrical
conductivity of the potassium cryolite was measured in the capillary-type cell whereas
the electrical conductivity of electrolytes with additions was studied in the cell with
parallel electrodes. The temperature dependence of a cell constant was considered that
made the electrical conductivity measurements more accurate in a wide temperature
range.
Several techniques were used to measure the alumina solubility in the potassiumcryolite-based electrolytes. The alumina solubility was determined by the “cryolite –
Al2O3” phase diagram obtained by the thermal analysis. The isothermal saturation
method with electrochemical determination of a saturation point was also employed.
This method in details is described elsewhere (3). in addition, this procedure allows
for a visual observation of the solubility process.
Density of cryolite melts was measured using the Archimedean method. A platinum
sphere attached to an electronic balance unit using a platinum wire was immersed into
the electrolyte. Measurements were performed under the inert gas (Ar) atmosphere.
The temperature dependence of the Pt sphere volume determined in the calibration
tests using the molten FLiNaK was considered.
The electrical conductivity, density and alumina solubility measurements were
performed in the temperature range from liquidus to 800 °C.
It should be mentioned that the cryolite ratio, the ratio between molar concentrations
of the alkali fluoride (MF) and the aluminum fluoride (CR=NMF/NAlF3), was always
retained constant, equal to 1.3 or 1.5. For this purpose each following addition of the
LiF or NaF to the KF-AlF3 melts with CR=1.3 or 1.5 was introduced as MF-AlF3
having the same CR.
LIQUIDUS TEMPERATURE
It is well known that all general additives (AlF3, CaF2, MgF2, LiF, KF) to the sodium
cryolite reduce the liquidus temperature of the molten mixture (3). However, the
opposite tendency is observed when the same salts are introduced to the potassium
cryolite with low CR.
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The effect of the KF, NaF, LiF, and KBF4 additives to the KF-AlF3 melt with CR=1.3
on liquidus temperature is shown in Fig.1. A first portion of each additive increases
the liquidus temperature in the potassium cryolite system. This fact is especially
important at a choice of electrolyte composition for aluminum electrolysis. So called
“sodium” issue emerges during the low-temperature electrolysis in potassium cryolite
(CR=1.3): In fact, the alumina continuously added to the electrolytic bath always
contains the Na2O impurities (about 0.3 wt% of Na) that results in collecting the NaF
and changing the physical-chemical properties of electrolyte (4). Thus, in accordance
with the liquidus change in the (KF-AlF3)-NaF mixture, the electrolytes with the NaF
content of about 15 mol.% and CR=1.3-1.5 are preferred for the low-temperature
electrolysis.
It is interesting to note that unlike the binary KF-AlF3 melt the LiF additive to the
ternary KF-NaF-AlF3 system decreases the liquidus temperature. So, the addition of 3
wt% LiF to the KF-NaF(10 wt%)-AlF3 electrolyte reduces the temperature of primary
crystallization on 11 degrees.
1100
KF
Temperature, C
1000
900
KBF4
800
NaF
700
LiF
600
500
0
5
10
15
20
25
30
35
Additives, mol.%
Fig.1 The effect of the KF, NaF, LiF, and KBF4 additives to the KF-AlF3 melt
(CR=1.3) on liquidus temperature
The CaF2 addition to the KF-AlF3 electrolyte with CR=1.3-1.5 results in a sharp
liquidus rise that indicates the low CaF2 solubility at temperature below 800 °C (5).
The CaF2 solubility increases in the KF-NaF-AlF3 molten mixtures, herewith the
higher the NaF content the better the CaF2 solubility. Hence, from the point of the
CaF2 presence in the KF-NaF-AlF3 electrolytes, the electrolytes enriched with the
NaF are more favored.
ALUMINA SOLUBILITY
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The Al2O3 solubility in the Na3AlF6 sodium cryolite (with additions of AlF3, LiF,
CaF2, MgF2 and some others) at temperature about 1000 °C is well known (3). For
instance, the alumina solubility in the conventional electrolyte is 10 wt% at 962 °C.
The operating alumina concentration in the electrolytic bath retains in the range of 2-4
wt%, therefore the alumina solubility issue is not so critical in practice of aluminum
electrolysis. Apparently, the significant temperature decrease, owing to the novel
electrolyte composition, results in the significant alumina solubility decrease.
The alumina solubility in the KF-AlF3 melts with CR=1.3-1.5 was found to be in the
range of 4.7-8.35 wt% (3.25-5.76 mol.%) at temperatures 700-800 °C (6). However,
both the LiF and NaF additives to the potassium system reduce the alumina. The data
obtained in the (KF-AlF3)-NaF melts with CR=1.3 and 1.5 and in the (KF-AlF3)-LiF
melts with CR=1.3 at 800 °C is presented in Fig. 2. The (KF-AlF3)-LiF electrolytes
with the LiF concentration of 3 wt% dissolves about 4.5-5.5 wt% of alumina at 700800 °C that is acceptably for the aluminum electrolysis. The LiF addition in amount
of more than 5 wt% significantly decreases the Al2O3 solubility in the (KF-AlF3)-LiF
melt.
7
Al2O3 solubility, mol.%
6
5
4
4
3
3
1
2
2
1
0
10
20
30
40
NaF or LiF, mol.%
50
60
Fig.2 The NaF and KF effect on Al2O3 solubility in KF-AlF3:
1 - (KF-AlF3)-NaF, CR=1.5; 2 - (KF-AlF3)-NaF, CR=1.3;
3 - (KF-AlF3)-LiF, CR=1.3; 4 - (KF-LiF(3 wt%)-AlF3)-NaF, CR=1.3
The CaF2 is practically insoluble in the KF-AlF3 melt with CR=1.3 at temperatures
below 800 °C therefore the KF-NaF-AlF3 electrolytes enriched with the NaF were
chosen to study the alumina solubility. It was found that the higher the KF content in
the KF-NaF-AlF3-CaF2 melts the lower the Al2O3 solubility. The XRD analysis of the
frozen samples revealed the presence of the following calcium-containing
compounds: NaCaAlF6 in the NaF-AlF3-CaF2 melt; KCaAl2F9 in the NaF-KF-AlF3CaF2; and KCaAl2F9 and KCaF3 in the KF-AlF3. The melting point of the NaCaAlF6
compound is 718 °C, it is soluble in the sodium cryolite with CR=1.3 at temperature
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above 750 °C. The melting point of KCaF3 is 1070 °C. Consequently, the highmelting compounds formed in the molten mixtures containing potassium and calcium
ions endorse the liquidus temperature increase and impede the dissolution of the
Al2O3.
The alumina additions to the (KF-AlF3)-KBF4 and (KF-NaF-AlF3)-KBF4 melts
decrease the liquidus temperature of the molten mixtures as it is shown in Fig.3. The
quasi-binary “cryolite mixture”– alumina phase diagrams are simple eutectics. The
KBF4 presence in the KF-NaF-AlF3-Al2O3 melt positively impacts the alumina
solubility. The eutectic points were found equal to 759 and 761°C at the Al2O3
content of 4 and 5 mol%, respectively, in the [KF-NaF(15.9 mol%)-AlF3+KBF4 (3
mol%)]-Al2O3 and [KF-NaF(15.9 mol%)-AlF3+KBF4(5 mol%)]-Al2O3 mixtures with
CR=1.3. The Al2O3 addition of 6 mol% to the both electrolytes results in the
significant liquidus rise. During the DTA measurements the bend point on the cooling
curves corresponding to the temperature of primary crystallization was not detected
because the beginning temperature of the cooling process in these tests was below the
liquidus. The presence of a not dissolved precipitate on the bottom of crucible at 850
°C was also confirmed by visual observation.
850
Temperature, ◦C
830
810
790
3
770
2
750
1
730
0
1
2
3
4
Al2O3, mol%
5
6
7
Fig.3 The Al2O3 effect on the liquidus temperature in the
KF-NaF(15.9 mol%)-AlF3 electrolytes (CR=1.3) containing KBF4 (mol.%):
1 – 0; 2 – 3; 3 – 5
ELECTRICAL CONDUCTIVITY
One of the main disadvantages of the low-melted cryolite electrolytes is a relatively
low electrical conductivity. It decreases significantly with the CR and temperature
lowering. Among all valid additives to the sodium electrolytes used for the aluminum
production the LiF is the only one increasing the electrical conductivity. The electrical
conductivity of the KF-AlF3 melt can be increased by as the LiF as the NaF due to
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enhanced mobility of the lithium and sodium ions. Obviously, the LiF impact is
greater. Yet, the KF-LiF-AlF3 electrical conductivity dependence on the LiF
concentration is not linear: the electrical conductivity increase decelerates with each
addition of LiF. It can be explained by the high ionic potential of the lithium cation
which is a basic carrier of the electrical charge in electrolyte. The Li+ ion forms strong
electrostatic bonds with complex anions of the cryolite melt, thereby losing the
mobility that affects the electrical conductivity value. The addition of 12 wt% NaF to
the KF-AlF3 cryolite (CR=1.3) increases the electrical conductivity on 6% whereas
the addition of 3 wt% LiF to the same melt improves the electrical conductivity on
8%. Both LiF and NaF additives in potassium cryolite can reimburse the electrical
conductivity fall caused by the alumina introduction (7).
The CaF2, in general, decreases the electrical conductivity of the KF-AlF3 and KFNaF-AlF3, and KF-LiF-AlF3 molten cryolite mixtures. Nevertheless, the small CaF2
additions (about 1-2 mol.%) to the KF-LiF-AlF3 melts with the LiF content up to 25
mol.% slightly increase the conductivity (5). Such a phenomenon is not indicated in
solutions with the higher LiF concentration.
DENSITY
The density isotherms in the potassium and sodium cryolites and their mixtures at
temperatures 760 and 800 °С are presented in Fig.4. The replacing potassium fluoride
by sodium fluoride results in the density increase. Thus, the density of a possible lowmelted electrolyte KF-NaF-AlF3 with the high concentration of KF has lower density
in comparison with the conventional electrolyte (2.05-2.1 g/cm3). This creates better
conditions for the liquid aluminum separation from the electrolyte.
The density of the NaF-KF-AlF3 melts was revealed to decrease with the cryolite ratio
rise and to increase with the CaF2 additions.
The KBF4 reduces the density of the KF-AlF3 melt. So, the density of the KF-AlF3
and KF-AlF3-KBF4(3mol.%) is 1.78 and 1.71 g/cm3 at 750 °С, accordingly.
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1.98
1.94
2
Density, g/cm3
1.9
1.86
1
1.82
1.78
1.74
1.7
1.66
0
10
20
30
NaF, mol.%
40
50
Fig.4 Density of KF-NaF-AlF3 (CR=1.3) at temperature (°С): 1 – 800, 2 – 760;
x – density of KF-AlF3 and NaF-AlF3 (CR=1.3)
calculated on equations given in (8)
CONCLUSION
The physical-chemical properties of the potassium cryolite (KF-AlF3, CR=1.3-1.5)
with the LiF, NaF, CaF2, and KBF4 additions were studied. Based on obtained data
the composition of the potassium-cryolite-based electrolyte applicable for utilizing in
the low-temperature technology of the aluminum and its alloys’ production can be
determined. The KF-AlF3 melt provides the acceptable alumina solubility at
temperatures below 800 °С and gives an opportunity to perform the electrolysis at
temperature as low as 700 °С.
ACKNOWLEDGEMENT
This work is a part of the Integration Project funded by the Ural Branch of Russian
Academy of Sciences. Project N 12-I-3-2056.
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