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Influence of NaCl and LiF on the aluminium Electrolyte
Blectrochimica Acta, 1970, Vol . 15. pp . 259 to 269. Persamon Press . Printed in Northern Ireland
INFLUENCE OF NaCl AND LiF ON THE ALUMINIUM ELECTROLYTE* K . GIuonuiIM, K . MATIASOVSKY and M . MALINOVSKY Institute of Inorganic Chemistry, The Technical University of Norway, Trondheim, Norway and Institute of Inorganic Chemistry of the Slovak Academy of Sciences, Bratislava, Czechoslovakia Abstract-The influences of NaCl, LiF and other additions on the Al electrolyte are reviewed . LiF is the most favourable addition and gives a slight increase in the current efficiency of the Al reduction process . R&=6-L'influence de NaCl, LiF et d'auters additions sur les propri6t6s physicochimiques du bain est discut6. Il apparait que LiF cat ]'addition le plus favorable, par son faire augmenter le rendement de courant de 1'6lectrolyse pour la fabrication d'aluminium . Zusammenfassung--Der Einfluss von NaCl, LiF and anderer Zusftze im Aluminium-Elektrolyten wird in einer bibliographischen Arbeit untersucht . LIE ist der giinstigste Zusatzstoff and ergibt eine leichte Lrhohung der Stromausbeute im Reduktionsprozess des Aluminiums . INTRODUCTION CONSIDERING the relatively low energy efficiency of the actual electrolytic process, recently other economically more attractive methods for the production of aluminium are being studied ."' However, in all cases a series of problems, which hinder the technical utilization of these methods, remain unsolved. Therefore, research oriented towards improving of the present electrolytic process is of primary importance . The influence of the addition of different salts or oxides to the fundamental electrolyte on the different physico-chemical and electrochemical properties that are technically important are of particular interest . These properties are : 11 (1) the temperature of the primary crystallization (fusibility) of the electrolyte ; (2) the density of the electrolyte ; (3) the electrical conductivity ; (4) the electrode processes . A decrease in the temperature of the primary crystallization makes it possible to decrease the working temperature and thus the amount of energy needed for keeping the electrolyte at the working temperature . Considering the solubility of aluminium in the melts as a function of temperature,12 the decrease in the working temperature would also result in a decrease of aluminium loss and, consequently, in an increase of the current efficiency . Furthermore, phase diagrams offer information on the solubility of alumina in melts of a given composition . The density of the electrolyte is important mainly in the separation of the metal ; it should be substantially lower than the density of aluminium to ensure a good separation . The electrical conductivity of the electrolyte is one of the most important properties since it is directly connected with the energy efficiency of the electrolytic process . 13 The study of the electrode reactions is essential in a search for means to reduce the cell voltage, which in the present technical process is approximately four times higher than the theoretical decomposition potential of alumina . * Manuscript received 9 October 1968 . 259
K . GRJOTHEIM, K . MATIA90vsKY
2 60
and
M . MAGINOVSKY
A substance to be used as an addition to the aluminium electrolyte must fulfil a series of fundamental requirements :u .14 it should lower the mp, decrease the density and increase the electrical conductivity of the electrolyte . In Fig . 1, the liquida of the systems Na 3A1F 5 -MAy are presented . The liquida of the systems Na3AIF5 -NaF (A1F3, LiF, MgF 2) are those presented by Holm ; 15 for the system Na3A1F-6NaCl the liquidus is that of Matiasovsky et al;" for the system Na3A1F6-CaF 2 the liquidus is by Holm, 17 and for the systems Na 3AIFrBeF2 (BaF2, BaCI2), those of Guskov' 8 and by Belyaev et a119 are given. 1020 1010 1000 990 980 u s
970
a . E
960
W
950 940 930 920 910
BoOL BeFZ
900 -
3
6
Na AIF
1 1 1 l0 15 20 25 Additions, wt.-%
Flo. 1 . Influence of additions on the temperature of the primary crystallization of cryolite . Considering the differences of the mp of cryolite as reported by different authors, all diagrams have been shifted in order to have the common origin at 1010°C . In Fig. 2, density isotherms of the systems Na 3AlFeMA x at 1000 °C are presented . -NaF (CaF2 , AIF3) are those reported by The densities for the systems Na 3AIF6 Abramov et al ;12 for the system Na3A1F8MgF2 the values reported by Vaclavik et -NaCl the values of Matiasovsky et a121 are used aM are given ; in the system Na 3A1F6 -LiF (BeF2, BaF2, BaC12) the values from Guskov 18 and in the systems Na3A1F 6 and Belyaev2 are given . The data on the electrical conductivity of the melts of the systems Na 3AlF8 -MAa -NaF (CaF2 , AlFa) as at 1000°C, as presented in Fig . 3, are for the systems Na sAlFs presented by Edwards et al,28 for the system Na3AIF8 -NaCl those given by Matiasovsky 2, al,M for the systems Na3AIF8MgF2 (CaF BaF2, AIF3 BaC12), those of Vakhobov et 3AMF6 LiF those of Danek et al . 26 et a126 and for the system Na It is evident that only LiF and NaCl do improve the fusibility as well as the density and the electrical conductivity of the electrolyte . Sodium fluoride also improves all these important parameters ; however, it is not to be recommended as an addition
Influence of NaCl and LiF on the aluminium electrolyte
225
220
2 .15
IE
2 10
0 05
2-00
1 .95
1 20 25
90 No 3 ALFG Additions,
wt-%
fo. 2 . Influence of additions on the density of cryolite melts at 1000°C .
4-6 4-4 42 40 3-8 3-6 3-4 E u
3 2
0 L
30
E
2. 8 26 2-4 2. 2 2-0 18 16 Na3 ALF6
5
10
15
20
25
30
Additions, wt.-%
Fro. 3 . Influence of different additions on the specific electrical conductivity of cryolite melts at 1000°C .
261
2 62
K . GxtoTamIM, K . MATIASOVSKY
and M .
MALINOVSKY
because of the increase of the cryolite (NaFIAIF 3) ratio of the electrolyte, this being considered as unfavourable for the technical process .
PHYSICO-CHEMICAL PROPERTIES OF THE MELTS OF THE TERNARY SYSTEMS Na,A1F,A1,O,-NaCI AND Na,A1F,-A1,0,LiF 1. Liquidus Diagrams The influence of NaCl on the temperature of the primary crystallization of the melts of the system Na aAIFe A1 2 03 was studied by Phillips and co-workers8t and by Matiasovsky and Malinovsky .2 s -3o The liquidus diagram of the cryolite corner of the system Na 3A1Fa-A12 03-NaCl as reported by Matiasovsky et ales is presented in Fig. 4 .
No AIF s s
0
15e
/ I / l
I
20
wt- % A1 z 0 3 FIG . 4 .
Phase diagram of the system Na,AIF,A1 2 03-NaCI .
The liquidus of the system Na 3A1FeA1303LiF was studied by Belyaev,82 by Jenssen82 and by the authors by means of thermal analysis (TA) and X-ray phase analysis. The liquidus diagram as constructed by the authors is presented in Fig . 5. Considering the isotherms in the field of the primary crystallization of A1202, it is evident that both additions decrease the solubility of alumina in the electrolyte . The gradient of the decrease of the solubility of alumina is ca 0-3-0-4% A1203(1 % NaCI and 0-2-0-3 % A1203 /1 % LiF . From an analysis of the liquida presented in Figs . 4 and 5, it follows that the influence of LiF on the decrease of the temperature of the
No 3 AL
0
3
am0
tr e,
wt- % Lid
Do. 5. Phase diagram of the system Na,A1F,AI,O,LiF.
Influence of NaCI and LiF on the aluminium electrolyte
263
6 1'se
I 1' 90
I'9 96
I
=0q
e oe~ I
Na 3 AlF6 A6,
~ 5
0 wl- % A1 20 3
15e
20
MO . 6. Density of melts of the system Na,AJF 1-Als o,-NaCI at 1000°C.
o ^' P
i
OOp-p
9, cS
2
_~pe,~\ Na 3 At F6
10
N \
20
'9
ip
;p
;p
`9
i\\ \\ \ °\ \ \
30
40
50
wt-% LiF
Fto . 7. Density of melts of the system Na 3AIF,-A1 1O,LiF at 1000°C .
Na 3AlF6 wf-%At 0 3
Fro . 8. Specific electrical conductivity of melts of the system Na,AIF, AI,O,-NaCl at 1000 ,C,
K . GIUOTHEim, K . MATIASOVSKY and M . MALINovsKY
264
primary crystallization of the Na 3 AIF3A1203 mixtures in the field of the primary crystallization of cryolite is substantially higher than that of NaCl . In practice, it would be important that the favourable influence of LiF is most pronounced at low concentrations : eg an addition of 5 % LiF decreases the temperature of primary crystallization of the electrolyte by ca 50 ° C, the corresponding gradient for NaCl being ca 4-5°C/l % NaCl. Thus the liquidus diagrams give information on the influence of NaCl and LiF both on the fusibility as well as on the solubility of alumina in the electrolyte . 2. Density
The density isotherms of the melts of the system 21 .29 Na3 AIFgA1203NaCl at 1000° C are presented in Fig . 6. In Fig. 7 our new measured density isotherms of melts of the system Na 3 AIF6-A1 203 LiF at 1000°C are presented.
0" 0 e\°
~ >_'~
1000"C
3
17O77 -FT -°T -1--r- 7
TTT/ / T / / "'/
Np~At e
10
20
40
30
N P
I
I 50
wt-% LiF
Fia . 9 . Specific electrical conductivity of melts of the system Na,AIF,-A1,0,-LiF at 10000C.
Cot
Amp
absorption
Voltage
recorder
stabilizer
H, combustion
I
flower
I Gas chromatograph
supply
Differential voltage recorder
ampling egrato
Fro . 10 . Diagram of 10-A laboratory electrolysis cell with equipment for control and gas analysis .
Influence of NaC1 and LiF on the aluminium electrolyte
2 65
Evidently both NaCl and LiF decrease the density of the electrolyte, the influence of NaCl being substantially higher . The corresponding values are 0 .10-0.15 %/1 % LIE and 0 . 50-0-60%/1 % NaCl . The pronounced influence of NaCI may be mainly due to two factors : (1) the introduction of the relative large ions as Cl - ( r = 1-81 A) into the melt, the melt thus being "diluted" ; (2) a decrease of the temperature of primary crystallization of the melt with increasing concentration of NaCl . The influence of the relative overheating is evident in the density isotherms of the system Na 3AIFeAl 2 03NaCI (Fig . 6), where the isodensity curves pass through a minimum that is almost identical with the line of the secondary crystallization Na 3AIFe + A 1203 . The relatively weak influence of LiF on the density of the electrolytes is probably due to the fact that the volume of the melt in the range of high concentrations of cryofite is determined by the large complex anions AIFg -.
3. Electrical conductivity The electrical conductivity isotherm 24 •2 9 .3O of the melts of the system Na 3AIFe A1 203NaC1 at 1000 ° C is presented in Fig . 8. The isotherm of the electrical conductivity of the Na3 A1FeA1203LiF mixtures at 1000°C as observed by the authors is presented in Fig . 9 . The specific conductivity of the melts of the system Na 3AIFeA1 20sNaCI is determined mainly by the concentration (activity) of the Na+ ions, which are mainly responsible for current transport in the cryolite melts 3 1 Considering the character of the dependence of the specific conductivity on the concentration of NaCl, it may be assumed that at low NaCl concentrations, beside Na+, Cl - anions play a significant part in the transport . At higher NaCl concentrations, the formation of less mobile complex anions may be assumed 2 4 From Fig . 9 it follows that an addition of LiF will considerably increase the electrical conductivity of the basic industrial electrolyte . Even at 5 % LiF, the specific conductivity is increased by 10%, this offering interesting possibilities from the technical point of view . The ohmic drop in the electrolyte, U0 , which is the major part of the total voltage drop across an aluminium cell, is
Ua
= 16 .I .J
where a is the specific conductivity of the electrolyte in mholcm, 1 the interpolar distance in cm and J the cd in A/cm2 . It follows that an increase of the specific conductivity effected by an addition produces a decrease of the ohmic voltage this having a favourable influence on the energy efficiency of the electrolysis . Furthermore, with an electrolyte with increased specific conductivity, the working temperature could be reduced, and the interpolar distance and the cd could be increased . All these factors have a favourable influence 11 .12 .93.84 on the current efficiency and/or the productivity of the technical process by reducing the influence of the secondary reactions in the electrolysis .
4. Cathodic processes It is necessary to ensure that the cathode reaction remains unaffected by any addition . Here the study of the influence of NaCl was of interest because of the
26 6
K . GIUOTxen.I, K . MATIASOVSKY
and M . MALINOVSKY
controversy in the published data on the relative nobility of Na+ and AIs+ in the conditions of the electrolysis . The work86 confirmed the hypothesis that in the normal conditions of the electrolysis A13+ is more noble than Na+. According to Feinleib et al8° the difference between the deposition potential of A1 3+ and Na+ is small and in the electrolysis probably both metals are deposited simultaneously, the amount of the deposited sodium not being significant . Using the method of E/T curves, Kubik et aP7 found that the difference between the deposition potentials of A1 3+ and Na+ is approximately 0-1 V, which agrees with Piontelli ss The influence of LiF was determined by means of I/E curves . It was found that at a concentration of 15% LiF, the decomposition potential, E d , of 5 wt % alumina in cryolite is increased by 65 mV. At lower LiF concentrations the increase of E d is correspondingly lower . The changes in the values of the deposition potentials of Na+ and Li+ effected by the addition of Li+ ions to the melt were found to be negligible . The data obtained are presented in Table 1 . TABLE 1 . OBSERVED DEPOSITION POTENTIAL IN 5 wr-% AI,O, IN LiF-Na,A1F, MELTS
Composition
wt-%
mV of
Deposition potentials, mV vs 0. electrode Al„ Na+ Li+
95 % Na,AIF, 5 % A1,0,
2065
2320
5 % LiF 90% Na,AJF, 5 % AI,O,
2080
2280
2390
10% LiF 85 % Na,AIF, 5 % AI,O,
2105
2270
2385
15 % LiF Na,AlF, 5 % A1,0,
2130
2265
2370
80 %
5 . Electrolysis of the molten Na3 AIFeA1a03NaCl and Na3AIF6A1 208LiF mixtures Laboratory cell studies may give valuable information on the influence of some parameters on the current efficiency during aluminium electrolysis . Such studies were started some years ago. 39 In 1966 a new 10-A laboratory cell was built by Votava .40 The experimental set-up is shown in Fig . 10 and a section of the cell in Fig . 11 . The influence of LiF- and NaCl-additions on the electrolysis is being studied by Fellner?r So far only preliminary results have been obtained, and some of these are shown, eg the dependency of the current efficiency on the addition of LiF, Fig . 12 . A slight increase in current efficiency with LiF addition is observed up to 16 wt LiF . CONCLUSIONS
From an analysis of the experimental results on the influence of NaC1 and LiF additions on some technically important parameters of the aluminium electrolyte, it is evident that the more effective addition is LiF, which substantially reduces the
Influence of NaCI and LiF on the aluminium electrolyte
MG. It . Section of furnace with anode and cathode parts of 10-A laboratory
electrolysis cell . (16-graphite crucible, (17)-BN lining, (18)---grtphite anode, (19)-.BN lid, (20)-sintered alumina tube, (21)-aiundum cement radiation shields .
0 a 0
65
5 iQ
Naz At6
4 75
wt--%IIF
Flo . 12. Current efficiency vs Li? content. Electrolyte : Na,ALF, + 7 wt-% A1,0, . 0 .8 A(cm' ; I -_ 4 cm ; 970"C
2 67
268 temperature
K.
GRtoTHEIM,
K.
MATm sovs Y and
M.
MAUnovsnn
of
the primary crystallization and increases the electrical conductivity the electrolyte. It is of primary technical importance that the influence of LiF is pronounced even at low concentrations -an addition of 5 % LiF decreases the tem-
of
of primary crystallization by ca 50°C and increases the specific conductivity 10 °% . In this concentration range the unfavourable influence of this addition, increase of the decomposition potential of alumina by ca 15 mV, and decrease of the solubility of alumina, is fairly small . In contradiction to the generally accepted view 12 that current efficiency decreases with increasing basicity of the electrolyte, our measurements show that an addition of LiF slightly increases current efficiency . This favourable influence of LiF has been confirmed in a pilot-plant scale electrolysis 42 perature by ca
Acknowledgements-This work was mainly done in the summer 1968 when Professor Grjotheim stayed as a guest at the Institute of Inorganic Chemistry of the Slovak Academy of Sciences, Bratislava Czechoslovakia .
REFERENCES 1. H . GROrIIE, Z. Elekirochem . 54, 210 (1950) . 2. H. GRorHE, Z. Erzbergbau 12,213 (1950) . 3 . A . T . BELAYEV and L. A. FntsANOVA, Odnovaleninyi Alyumlnu v Metallurgicheskikh Protsessakh . Metallurgizdat, Moscow (1959) . 4. H. Rent', Anorganicka chemie. SNTL, Praha (1961) . 5 . H. GINsweRG and V . SPARwALD, Aluminium 41, 181, 219 (1965) . 6. G . GITLESEN, O. HERSrAD and K . MorzFELDT, in Selected Topics in High Temperature Chemistry, ed . T. Forland, K. Grjotheim, K. Motzfeldt and S. Umes, p . 179 . Universitetsforlaget, Oslo (1966) . 7. E . HERRMANN, Aluminium 37, 143, 215 (1961). 8. E . HERRMANN, Z. Metallk . 53, 617 (1962) . 9. A. Roy, Tekn. Ukeblad 109, 1127 (1962) . 10. P. T. STROUP, Trans. Met. Soc. Am. Inst . Mech . Engrs. 230,356 (1964) . 11 . K. GRJOTHEIM, J. L. HOLM, C. KRoaN and K . MATnAsovsKY, Svensk Kem. Tidskr . 78, 547 (1966) . 12 . G. A . AERAMov, M. M . VEr'uxov, I. P . GuPALo, A. A. Kosrnntov and K. N . LozHKIN, Teoreticheskie osnovy elektrometallurgli alyuminla. Metallurgizdat . Moscow (1953) . 13 . K. GRroTHEIM and K . MATIASOVSKY, Tidsskrift for Kjemi, Bergvesen og Metallurgi 12, 226 (1966) . 14 . K . MATTAsovsKY and M . MALINOVSKY, Chem, zvesti 14, 258 (1960). 15, . 1 L. HOLM, Thesis, Technical University of Norway, Trondheim (1963). 16 . K . MATIAsovsKY and M . MALINovsKY, Chem. west! 14,353 (1960). 17 . J. L. HOLM, Acta them . stand. 22,1004 (1968) . 18 . V . M . Gusicov, Elektroliticheskoye rafmirovanie alyuminia . Metallurgizdat, Moscow (1945) . 19 . A . 1. BELAYEV, E. A . ZHEMCHUGLNA and L . A . FIRSANOVA, Fizicheskaya Khimia Basplavlennykh Solei. Metallurgizdas, Moscow (1957) . 20 . E . VACLAVIK and A . 1 . BELYAEV, Zh . neorg. Whim . 3, 1044 (1958) . 21 . K. MATUSOVSKY, A . JASZOVA and M. MALINOVSKY, Chem . zvesti 17, 605 (1963) . 22 . A . I. BELYAEV, Elekrrolit alyuminievykh vann . Metallurgizdat, Moscow (1961) . 23 . J. D . EDwARDS, C . S . TAYLOR, L . A . CosoRova and A. S . RUSSELL, J. electrochem . Soc. 100, 508 (1953) . 24 . K. MATIASOVSKY, M . MALINOVSKY and S . ORDZOVENSKY, J. electrochem . Soc. 111, 973 (1964) . 25 . A . V . VAKHOBOV and A . 1 . BELYAEV, in Fizicheskaya khimia rasplavlennykh sole!, p . 99. Metallurgizdat, Moscow (1965) . 26. V. DANEK, M . MALINOVSKY and K. MATTASOVSKY, Chem . zvesti 22, 641 (1968) . 27 . N . W . F . Pumps, R . H . SINGLETON and E . A . HOLDNOSHEAD, J. electrochem . Soc. 102, 690 (1955) . 28 . K. MAnAsovsKY and M. MALINOVSKY, Chem. zvesti 14, 551 (1960) . 29 . K. MATIAsovsxY and M_ MALINovsicY, Hutnicke listy 19, 37 (1964) . 30. K. M .ArlAsovsIY and M . MALINovsIY, Izv. vysshikh uchebnykh zavedenit, razdel Tsvetnaya metallurgia, No . 3, 87 (1964) . 31 . W . B . FRANK and L . M . FosrER, J. Phys . Chem . 61,1531 (1957).
Influence of NaCl and LiF on the aluminium electrolyte
269
32 . B . JENSSEN, Dissertation, Institute of Inorganic Chemistry, Technical University of Norway, Trondheim (1965) . 33 . M. FERBER, Alluminio 34,453 (1965) . 34 . K . MATIAsovsKV and M. MALINOVSKY, in Collection of papers presented at the Conference of the College of Mining, Ostrava, September 1967, in press . 35 . K . GRJOTHEIM, Alluminio 22, 679 (1953) . 36. M . FEINLEUE and F. PORTER, J. Electrochem . Soc . 103, 231 (1956) . 37 . C . Kusuc, K . MATIASOVSKY, M . MALINOVSxv and J . ZEmAN, Electrochim. Acta 9,1521 (1964) . 38 . F. PIoNrELLn, Atti simposio elettrolisi Balifusi a produzione metalil speciali in Italia, Milano, p . 11 . (1960) . 39 . K . GRJOTHEIm, J. THONSTAD and J . K . Tusrr, Can . Met . Q . 7,173 (1968) . 40. I. VOrAVA, internal report, Institute of Inorganic Chemistry, NTH, Norway (1967) . 41 . P . FELLNER, internal report, Institute of Inorganic Chemistry, NTH, Norway (1968) . 42 . A . R . LEwis, Evaluation of Five Percent Lithium Fluoride Modified Hall Bath in 10 kA Experimental Aluminium Reduction Cells, presented at AIME Annual Meeting, Los Angeles (1967) .
INFLUENCE OF NaCl AND LiF ON THE ALUMINIUM ELECTROLYTE* K . GIuonuiIM, K . MATIASOVSKY and M . MALINOVSKY Institute of Inorganic Chemistry, The Technical University of Norway, Trondheim, Norway and Institute of Inorganic Chemistry of the Slovak Academy of Sciences, Bratislava, Czechoslovakia Abstract-The influences of NaCl, LiF and other additions on the Al electrolyte are reviewed . LiF is the most favourable addition and gives a slight increase in the current efficiency of the Al reduction process . R&=6-L'influence de NaCl, LiF et d'auters additions sur les propri6t6s physicochimiques du bain est discut6. Il apparait que LiF cat ]'addition le plus favorable, par son faire augmenter le rendement de courant de 1'6lectrolyse pour la fabrication d'aluminium . Zusammenfassung--Der Einfluss von NaCl, LiF and anderer Zusftze im Aluminium-Elektrolyten wird in einer bibliographischen Arbeit untersucht . LIE ist der giinstigste Zusatzstoff and ergibt eine leichte Lrhohung der Stromausbeute im Reduktionsprozess des Aluminiums . INTRODUCTION CONSIDERING the relatively low energy efficiency of the actual electrolytic process, recently other economically more attractive methods for the production of aluminium are being studied ."' However, in all cases a series of problems, which hinder the technical utilization of these methods, remain unsolved. Therefore, research oriented towards improving of the present electrolytic process is of primary importance . The influence of the addition of different salts or oxides to the fundamental electrolyte on the different physico-chemical and electrochemical properties that are technically important are of particular interest . These properties are : 11 (1) the temperature of the primary crystallization (fusibility) of the electrolyte ; (2) the density of the electrolyte ; (3) the electrical conductivity ; (4) the electrode processes . A decrease in the temperature of the primary crystallization makes it possible to decrease the working temperature and thus the amount of energy needed for keeping the electrolyte at the working temperature . Considering the solubility of aluminium in the melts as a function of temperature,12 the decrease in the working temperature would also result in a decrease of aluminium loss and, consequently, in an increase of the current efficiency . Furthermore, phase diagrams offer information on the solubility of alumina in melts of a given composition . The density of the electrolyte is important mainly in the separation of the metal ; it should be substantially lower than the density of aluminium to ensure a good separation . The electrical conductivity of the electrolyte is one of the most important properties since it is directly connected with the energy efficiency of the electrolytic process . 13 The study of the electrode reactions is essential in a search for means to reduce the cell voltage, which in the present technical process is approximately four times higher than the theoretical decomposition potential of alumina . * Manuscript received 9 October 1968 . 259
K . GRJOTHEIM, K . MATIA90vsKY
2 60
and
M . MAGINOVSKY
A substance to be used as an addition to the aluminium electrolyte must fulfil a series of fundamental requirements :u .14 it should lower the mp, decrease the density and increase the electrical conductivity of the electrolyte . In Fig . 1, the liquida of the systems Na 3A1F 5 -MAy are presented . The liquida of the systems Na3AIF5 -NaF (A1F3, LiF, MgF 2) are those presented by Holm ; 15 for the system Na3A1F-6NaCl the liquidus is that of Matiasovsky et al;" for the system Na3A1F6-CaF 2 the liquidus is by Holm, 17 and for the systems Na 3AIFrBeF2 (BaF2, BaCI2), those of Guskov' 8 and by Belyaev et a119 are given. 1020 1010 1000 990 980 u s
970
a . E
960
W
950 940 930 920 910
BoOL BeFZ
900 -
3
6
Na AIF
1 1 1 l0 15 20 25 Additions, wt.-%
Flo. 1 . Influence of additions on the temperature of the primary crystallization of cryolite . Considering the differences of the mp of cryolite as reported by different authors, all diagrams have been shifted in order to have the common origin at 1010°C . In Fig. 2, density isotherms of the systems Na 3AlFeMA x at 1000 °C are presented . -NaF (CaF2 , AIF3) are those reported by The densities for the systems Na 3AIF6 Abramov et al ;12 for the system Na3A1F8MgF2 the values reported by Vaclavik et -NaCl the values of Matiasovsky et a121 are used aM are given ; in the system Na 3A1F6 -LiF (BeF2, BaF2, BaC12) the values from Guskov 18 and in the systems Na3A1F 6 and Belyaev2 are given . The data on the electrical conductivity of the melts of the systems Na 3AlF8 -MAa -NaF (CaF2 , AlFa) as at 1000°C, as presented in Fig . 3, are for the systems Na sAlFs presented by Edwards et al,28 for the system Na3AIF8 -NaCl those given by Matiasovsky 2, al,M for the systems Na3AIF8MgF2 (CaF BaF2, AIF3 BaC12), those of Vakhobov et 3AMF6 LiF those of Danek et al . 26 et a126 and for the system Na It is evident that only LiF and NaCl do improve the fusibility as well as the density and the electrical conductivity of the electrolyte . Sodium fluoride also improves all these important parameters ; however, it is not to be recommended as an addition
Influence of NaCl and LiF on the aluminium electrolyte
225
220
2 .15
IE
2 10
0 05
2-00
1 .95
1 20 25
90 No 3 ALFG Additions,
wt-%
fo. 2 . Influence of additions on the density of cryolite melts at 1000°C .
4-6 4-4 42 40 3-8 3-6 3-4 E u
3 2
0 L
30
E
2. 8 26 2-4 2. 2 2-0 18 16 Na3 ALF6
5
10
15
20
25
30
Additions, wt.-%
Fro. 3 . Influence of different additions on the specific electrical conductivity of cryolite melts at 1000°C .
261
2 62
K . GxtoTamIM, K . MATIASOVSKY
and M .
MALINOVSKY
because of the increase of the cryolite (NaFIAIF 3) ratio of the electrolyte, this being considered as unfavourable for the technical process .
PHYSICO-CHEMICAL PROPERTIES OF THE MELTS OF THE TERNARY SYSTEMS Na,A1F,A1,O,-NaCI AND Na,A1F,-A1,0,LiF 1. Liquidus Diagrams The influence of NaCl on the temperature of the primary crystallization of the melts of the system Na aAIFe A1 2 03 was studied by Phillips and co-workers8t and by Matiasovsky and Malinovsky .2 s -3o The liquidus diagram of the cryolite corner of the system Na 3A1Fa-A12 03-NaCl as reported by Matiasovsky et ales is presented in Fig. 4 .
No AIF s s
0
15e
/ I / l
I
20
wt- % A1 z 0 3 FIG . 4 .
Phase diagram of the system Na,AIF,A1 2 03-NaCI .
The liquidus of the system Na 3A1FeA1303LiF was studied by Belyaev,82 by Jenssen82 and by the authors by means of thermal analysis (TA) and X-ray phase analysis. The liquidus diagram as constructed by the authors is presented in Fig . 5. Considering the isotherms in the field of the primary crystallization of A1202, it is evident that both additions decrease the solubility of alumina in the electrolyte . The gradient of the decrease of the solubility of alumina is ca 0-3-0-4% A1203(1 % NaCI and 0-2-0-3 % A1203 /1 % LiF . From an analysis of the liquida presented in Figs . 4 and 5, it follows that the influence of LiF on the decrease of the temperature of the
No 3 AL
0
3
am0
tr e,
wt- % Lid
Do. 5. Phase diagram of the system Na,A1F,AI,O,LiF.
Influence of NaCI and LiF on the aluminium electrolyte
263
6 1'se
I 1' 90
I'9 96
I
=0q
e oe~ I
Na 3 AlF6 A6,
~ 5
0 wl- % A1 20 3
15e
20
MO . 6. Density of melts of the system Na,AJF 1-Als o,-NaCI at 1000°C.
o ^' P
i
OOp-p
9, cS
2
_~pe,~\ Na 3 At F6
10
N \
20
'9
ip
;p
;p
`9
i\\ \\ \ °\ \ \
30
40
50
wt-% LiF
Fto . 7. Density of melts of the system Na 3AIF,-A1 1O,LiF at 1000°C .
Na 3AlF6 wf-%At 0 3
Fro . 8. Specific electrical conductivity of melts of the system Na,AIF, AI,O,-NaCl at 1000 ,C,
K . GIUOTHEim, K . MATIASOVSKY and M . MALINovsKY
264
primary crystallization of the Na 3 AIF3A1203 mixtures in the field of the primary crystallization of cryolite is substantially higher than that of NaCl . In practice, it would be important that the favourable influence of LiF is most pronounced at low concentrations : eg an addition of 5 % LiF decreases the temperature of primary crystallization of the electrolyte by ca 50 ° C, the corresponding gradient for NaCl being ca 4-5°C/l % NaCl. Thus the liquidus diagrams give information on the influence of NaCl and LiF both on the fusibility as well as on the solubility of alumina in the electrolyte . 2. Density
The density isotherms of the melts of the system 21 .29 Na3 AIFgA1203NaCl at 1000° C are presented in Fig . 6. In Fig. 7 our new measured density isotherms of melts of the system Na 3 AIF6-A1 203 LiF at 1000°C are presented.
0" 0 e\°
~ >_'~
1000"C
3
17O77 -FT -°T -1--r- 7
TTT/ / T / / "'/
Np~At e
10
20
40
30
N P
I
I 50
wt-% LiF
Fia . 9 . Specific electrical conductivity of melts of the system Na,AIF,-A1,0,-LiF at 10000C.
Cot
Amp
absorption
Voltage
recorder
stabilizer
H, combustion
I
flower
I Gas chromatograph
supply
Differential voltage recorder
ampling egrato
Fro . 10 . Diagram of 10-A laboratory electrolysis cell with equipment for control and gas analysis .
Influence of NaC1 and LiF on the aluminium electrolyte
2 65
Evidently both NaCl and LiF decrease the density of the electrolyte, the influence of NaCl being substantially higher . The corresponding values are 0 .10-0.15 %/1 % LIE and 0 . 50-0-60%/1 % NaCl . The pronounced influence of NaCI may be mainly due to two factors : (1) the introduction of the relative large ions as Cl - ( r = 1-81 A) into the melt, the melt thus being "diluted" ; (2) a decrease of the temperature of primary crystallization of the melt with increasing concentration of NaCl . The influence of the relative overheating is evident in the density isotherms of the system Na 3AIFeAl 2 03NaCI (Fig . 6), where the isodensity curves pass through a minimum that is almost identical with the line of the secondary crystallization Na 3AIFe + A 1203 . The relatively weak influence of LiF on the density of the electrolytes is probably due to the fact that the volume of the melt in the range of high concentrations of cryofite is determined by the large complex anions AIFg -.
3. Electrical conductivity The electrical conductivity isotherm 24 •2 9 .3O of the melts of the system Na 3AIFe A1 203NaC1 at 1000 ° C is presented in Fig . 8. The isotherm of the electrical conductivity of the Na3 A1FeA1203LiF mixtures at 1000°C as observed by the authors is presented in Fig . 9 . The specific conductivity of the melts of the system Na 3AIFeA1 20sNaCI is determined mainly by the concentration (activity) of the Na+ ions, which are mainly responsible for current transport in the cryolite melts 3 1 Considering the character of the dependence of the specific conductivity on the concentration of NaCl, it may be assumed that at low NaCl concentrations, beside Na+, Cl - anions play a significant part in the transport . At higher NaCl concentrations, the formation of less mobile complex anions may be assumed 2 4 From Fig . 9 it follows that an addition of LiF will considerably increase the electrical conductivity of the basic industrial electrolyte . Even at 5 % LiF, the specific conductivity is increased by 10%, this offering interesting possibilities from the technical point of view . The ohmic drop in the electrolyte, U0 , which is the major part of the total voltage drop across an aluminium cell, is
Ua
= 16 .I .J
where a is the specific conductivity of the electrolyte in mholcm, 1 the interpolar distance in cm and J the cd in A/cm2 . It follows that an increase of the specific conductivity effected by an addition produces a decrease of the ohmic voltage this having a favourable influence on the energy efficiency of the electrolysis . Furthermore, with an electrolyte with increased specific conductivity, the working temperature could be reduced, and the interpolar distance and the cd could be increased . All these factors have a favourable influence 11 .12 .93.84 on the current efficiency and/or the productivity of the technical process by reducing the influence of the secondary reactions in the electrolysis .
4. Cathodic processes It is necessary to ensure that the cathode reaction remains unaffected by any addition . Here the study of the influence of NaCl was of interest because of the
26 6
K . GIUOTxen.I, K . MATIASOVSKY
and M . MALINOVSKY
controversy in the published data on the relative nobility of Na+ and AIs+ in the conditions of the electrolysis . The work86 confirmed the hypothesis that in the normal conditions of the electrolysis A13+ is more noble than Na+. According to Feinleib et al8° the difference between the deposition potential of A1 3+ and Na+ is small and in the electrolysis probably both metals are deposited simultaneously, the amount of the deposited sodium not being significant . Using the method of E/T curves, Kubik et aP7 found that the difference between the deposition potentials of A1 3+ and Na+ is approximately 0-1 V, which agrees with Piontelli ss The influence of LiF was determined by means of I/E curves . It was found that at a concentration of 15% LiF, the decomposition potential, E d , of 5 wt % alumina in cryolite is increased by 65 mV. At lower LiF concentrations the increase of E d is correspondingly lower . The changes in the values of the deposition potentials of Na+ and Li+ effected by the addition of Li+ ions to the melt were found to be negligible . The data obtained are presented in Table 1 . TABLE 1 . OBSERVED DEPOSITION POTENTIAL IN 5 wr-% AI,O, IN LiF-Na,A1F, MELTS
Composition
wt-%
mV of
Deposition potentials, mV vs 0. electrode Al„ Na+ Li+
95 % Na,AIF, 5 % A1,0,
2065
2320
5 % LiF 90% Na,AJF, 5 % AI,O,
2080
2280
2390
10% LiF 85 % Na,AIF, 5 % AI,O,
2105
2270
2385
15 % LiF Na,AlF, 5 % A1,0,
2130
2265
2370
80 %
5 . Electrolysis of the molten Na3 AIFeA1a03NaCl and Na3AIF6A1 208LiF mixtures Laboratory cell studies may give valuable information on the influence of some parameters on the current efficiency during aluminium electrolysis . Such studies were started some years ago. 39 In 1966 a new 10-A laboratory cell was built by Votava .40 The experimental set-up is shown in Fig . 10 and a section of the cell in Fig . 11 . The influence of LiF- and NaCl-additions on the electrolysis is being studied by Fellner?r So far only preliminary results have been obtained, and some of these are shown, eg the dependency of the current efficiency on the addition of LiF, Fig . 12 . A slight increase in current efficiency with LiF addition is observed up to 16 wt LiF . CONCLUSIONS
From an analysis of the experimental results on the influence of NaC1 and LiF additions on some technically important parameters of the aluminium electrolyte, it is evident that the more effective addition is LiF, which substantially reduces the
Influence of NaCI and LiF on the aluminium electrolyte
MG. It . Section of furnace with anode and cathode parts of 10-A laboratory
electrolysis cell . (16-graphite crucible, (17)-BN lining, (18)---grtphite anode, (19)-.BN lid, (20)-sintered alumina tube, (21)-aiundum cement radiation shields .
0 a 0
65
5 iQ
Naz At6
4 75
wt--%IIF
Flo . 12. Current efficiency vs Li? content. Electrolyte : Na,ALF, + 7 wt-% A1,0, . 0 .8 A(cm' ; I -_ 4 cm ; 970"C
2 67
268 temperature
K.
GRtoTHEIM,
K.
MATm sovs Y and
M.
MAUnovsnn
of
the primary crystallization and increases the electrical conductivity the electrolyte. It is of primary technical importance that the influence of LiF is pronounced even at low concentrations -an addition of 5 % LiF decreases the tem-
of
of primary crystallization by ca 50°C and increases the specific conductivity 10 °% . In this concentration range the unfavourable influence of this addition, increase of the decomposition potential of alumina by ca 15 mV, and decrease of the solubility of alumina, is fairly small . In contradiction to the generally accepted view 12 that current efficiency decreases with increasing basicity of the electrolyte, our measurements show that an addition of LiF slightly increases current efficiency . This favourable influence of LiF has been confirmed in a pilot-plant scale electrolysis 42 perature by ca
Acknowledgements-This work was mainly done in the summer 1968 when Professor Grjotheim stayed as a guest at the Institute of Inorganic Chemistry of the Slovak Academy of Sciences, Bratislava Czechoslovakia .
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Influence of NaCl and LiF on the aluminium electrolyte
269
32 . B . JENSSEN, Dissertation, Institute of Inorganic Chemistry, Technical University of Norway, Trondheim (1965) . 33 . M. FERBER, Alluminio 34,453 (1965) . 34 . K . MATIAsovsKV and M. MALINOVSKY, in Collection of papers presented at the Conference of the College of Mining, Ostrava, September 1967, in press . 35 . K . GRJOTHEIM, Alluminio 22, 679 (1953) . 36. M . FEINLEUE and F. PORTER, J. Electrochem . Soc . 103, 231 (1956) . 37 . C . Kusuc, K . MATIASOVSKY, M . MALINOVSxv and J . ZEmAN, Electrochim. Acta 9,1521 (1964) . 38 . F. PIoNrELLn, Atti simposio elettrolisi Balifusi a produzione metalil speciali in Italia, Milano, p . 11 . (1960) . 39 . K . GRJOTHEIm, J. THONSTAD and J . K . Tusrr, Can . Met . Q . 7,173 (1968) . 40. I. VOrAVA, internal report, Institute of Inorganic Chemistry, NTH, Norway (1967) . 41 . P . FELLNER, internal report, Institute of Inorganic Chemistry, NTH, Norway (1968) . 42 . A . R . LEwis, Evaluation of Five Percent Lithium Fluoride Modified Hall Bath in 10 kA Experimental Aluminium Reduction Cells, presented at AIME Annual Meeting, Los Angeles (1967) .