Surface tension of cryolite-based melts

(‘amdrun Mrtallurgi~al Quarferiy, Vol. 34, ho. 2, pp. 129-133. 1995 Copyright i-1 1995 Canadmn Institute of Mmmg and Metallurgy Prmted m Great Britam. All rights reserved 000x-1433:95 $9 50to 00

Pergamon 0008-4433(94)00027-l

SURFACE

TENSION

OF CRYOLITE-BASED

V. DANeK,-f 0. PATARAKt tInstitute ZInstitute

of Inorganic of Inorganic

MELTS

and T. OSTVOLD$

Chemistry.

Slovak Academy of Sciences, Dubravski Bratislava, Slovakia Chemistry, The Norwegian Institute of Technology, Trondheim, N-7034 Trondheim. Norway

cesta 9, 842 36 University

of

(Received 3 MUJ~ 1994; in rel:isedform 20 October 1994) Abstract-The pin detachment method has been used to obtain data on the surface tension of liquid Na3AIF, with additions of AIF,, LiF. KF, CaFz and AI,O, as a function of temperature. In basic melts AI,O, and KF are surface active and decrease the surface tension, 0. while LiF and CaF, additions give an increased 6. Additions of AIF, to Na,AIF, decrease g. In acidic melts (CR < 3) for CR = 1.85 and 1.52. respectively, there is a marked increase in CTwith LiF additions and a smaller increase with A&O, and CaF, additions. KF is almost neutral for these melt compositions. An explanation of the observed variations in 0 in terms of the acid-base properties or the ionic-covalent character of the additives seems to be too simple.

R&um&Nous

avons utilisi: la mkthode de ditachement d’une cheville pour obtenir des don&es de tensions superficielles de Na,AIF, comprenant des additions de AIF,, LiF, KF. CaF, et de AllO,, en fonction de la tempirature. Dans les mattriaux fondus basiques AlzOj et KF sent tensioactifs et riduisent la tension superficielle. 6, alors que des additions de LiF et de CaF, on pour r&&at d’accrohre 0. Des additions de AlF, B Na,AlF, rCduisent 4. Dans les matCriaux fondus (CR < 3) pour CR = 1.85 et 1.52, respectivement, il y a une nette augmentation de g avec des additions de LiF et de plus petites augmentations pour des

additions de Al,O, et de CaFZ. Le KF est presque neutre pour la composition de ce materiau fondu. Une explication

des variations

observtes

de u, en termes de proprittCs

acides-basiques

ou de caractire

ionique--

covalent, semble &tre trop simple.

1. INTRODUCTION

A1,O1. Special emphasis is given to the highly acidic (up to 28 AlF,) region due to the low melting temperature of such melts. wt%

During electrolysis of aluminium, a lower temperature than used today may result in increased current efficiency, lower energy consumption and prolonged cell life, and may possibly promote the adaptation of inert electrode materials. One important parameter which may be affected by a reduced temperature is the surface tension of the electrolyte used. This parameter will influence the penetration of electrolyte into the carbon lining, the separation of carbon particles from the electrolyte, the coalescence of fine aluminium droplets and the dissolution rate of alumina into the electrolyte. Classical additives to cryolite in technical aluminium electrolysis are AlF,, LiF and CaF2. A promising additive may be KF, which increases the alumina solubility and decreases the temperature of primary crystallization of cryolite [l]. This additive. however, is detrimental to carbon anodes and cathodes and can only be used in substantial amounts in technical cells operating with inert electrodes. The effects of KF and K,AIF, additions on the surface tension of cryolite and some cryolite-alumina melts have been investigated by Fernandez et al. [2] who found a decrease in surface tension with increasing KF content regardless of the content of alumina in the melt. The same effect was observed by Fernandez and Mstvold [3] who also measured the surface tension of some electrolytes in the low acidic (up to 10 wt% AlF,) region. In the present work surface tension data are obtained for low temperature electrolytes containing AlF,, LiF, KF, CaF, and

2. EXPERIMENTAL 2.1. Apparatus The surface tension was determined by the pin detachment method. The experimental device is described in detail elsewhere [4,5]. A specially shaped sinker for density measurement, made from Pt20Rh alloy with a circular rod attached on the bottom and suspended by a 0.5 mm diameter wire from an electronic balance into a furnace, was used for the measurement. The diameter of the rod (approximately 2.2 mm) was measured at room temperature with a precision of fO.001 mm using an electron microscope. The estimated experimental error in the surface tension measurement did not surpass f 2 mN/m. 2.2. Chemicals Natural hand-picked Greenland cryolite with a melting point of 1009°C was dried under vacuum at 200°C for 12 h. Aluminium fluoride was sublimed in vacua at 1200°C in a platinum crucible. Analytical grade lithium and potassium fluoride were recrystallized from the melt in a platinum crucible. Analytical grade calcium fluoride and aluminium oxide were dried in vacua at 600°C for 6 h. All handling and storage of chemicals were done in a glove box with a moisture content lower than 10 ppm. 129

130

V. DANfiK

et al.

CRYOLITE-BASED

2.3. Procedure The platinum crucible containing about 100 g salt was placed inside the furnace chamber, which was evacuated. The mixture was heated to 150°C and kept at that temperature for 1 h. Then the furnace and balance system were filled with nitrogen and a slow stream of nitrogen from the balance chamber through the furnace was maintained during the whole measurement in order to avoid fluoride vapours in the balance chamber. The temperature was then raised to approximately 120°C above the estimated liquidus temperature and kept constant for 1 h. The surface tension was measured at 5-6 different temperatures in a range of approximately 100°C starting approximately 20°C above the estimated temperature of primary crystallization. This temperature was calculated using a regression equation given by Rostum et al. [6]. The surface tension measurements were made after 1 h at constant temperature (+0.5”C). Measurements at each temperature were reproduced at least twice, and no significant change in surface tension values was observed. Electrolytes with two different cryolite ratios,

MELTS

CR = nNaP/nAIF, = 1.85 (samples I-19) and CR = 1.52 (samples 20-38) were studied; n, is the number of moles of component i in the melt. The influence of additions of LiF and KF up to 9 wt% and CaF, and A&O, up to 6 wt% on the surface tension of the melt was examined. In Table 1 the detailed melt compositions are given.

3. RESULTS

AND DISCUSSION

The temperature dependencies of the surface tension of a given melt could be described by an equation linear in temperature. c7,1mNmp’ = a- b(T/“C)

To obtain the coefficients in this equation the least squares method was used. The coefficients a and b in equation (1) as well as the standard deviations of the fit are given in Table 2.

Table 1. Composition in wt% of the investigated electrolytes Melt number

Na?AIF,

AlF,

>iF

KF

CaF,

Al203

Cwt%)

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

80.00 77.60 75.20 77.60 75.20 78.40 76.80 78.40 76.80 75.20 76.80 75.20 75.20 73.73 73.60 72.16 70.40 69.02 70.40 72.00 69.84 65.45 68.40 65.15 69.84 67.81 70.56 69.18 67.85 69.12 67.11 65.20 65.52 66.24 64.80 63.36 62.12 62.40

20.00 19.40 18.80 19.40 18.80 19.60 19.20 19.60 19.20 18.80 19.20 18.80 18.80 18.43 18.40 18.04 17.60 17.25 17.60 28.00 27.16 25.45 26.60 25.34 27.16 26.37 27.44 26.90 26.38 26.88 26.10 25.36 25.48 25.76 25.20 24.64 24.16 24.26

(1)

3.0 6.0 3.0 6.0 2.0 4.0 2.0 4.0 6.0 2.0 2.0 4.0 3.92 2.0 1.96 3.0 2.94 3.0

2.0 4.0 2.0 3.92 4.0 3.92 5.0 5.89 3.0

2.0 3.92 3.0 2.94 3.0

1.0 1.96 3.0

3.0 9.1 5.0 9.51 3.0 5.82 2.0 3.92 5.77 2.0 4.85 4.12 4.0 4.0 4.0 3.0 2.94 3.81

2.0 1.94 4.72 4.0 2.0 2.0 5.0 5.88 3.81

I .o 2.0 4.0 3.0 2.94 2.86

1.0 1.96 2.86

V. DANEK c/ al. : CRYOLITE-BASED Figure 1 gives an example of the temperature dependence of the surface tension for electrolytes containing all additives at both cryolite ratios. It may be observed that a substantial change in the surface tension. G, is caused by the change in the cryolite ratio. Among the other additives LiF seems to have the strongest influence on the surface tension. On the basis of all the surface tension data obtained, an equation for the composition and temperature dependence of the surface tension was fitted to the experimental data using a multiple linear regression analysis. Surface tension data at 900 and 1OOO’C for each composition, calculated from equation (l), were used as a basis for the calculation. To obtain an equation which fits the acidic region from the Na,AlF, to the NaAIF, composition, surface tension data for some selected NaF-AlF, compositions obtained by Fernandez and Mstvold [3], were also used. Since the data of Fernandez and Ostvold clearly indicate a second order dependence on the AlF, concentration, such a term was also introduced. Due to the technical interest in the present binary system. wt”/o is used instead

Table

Melt

1 2 4 5 8 9 10 11 12 13 14 15 16 17 1x 19 20 21 22 23 24 25 26 27

28 29 30 31 32 33 34 35 36 37 38

131

MELTS

E 100 : 7;

2 95: 90 85

1 CR=1.52 t 850

LA-d

L 900

950 T

1000

("C>

Fig. 1. Temperature dependence of the surface tension low temperature electrolytes. See Table 1 for composition. mental: (0) 17; (17) IX; (0) 19; (+) 36; (W 37; Calculated : (-) equation (1).

2. Coefficients a and h, the standard deviations of the lit to equation (1) and the surface tension investigated electrolytes at 900’C compared with that calculated from equation (2) number

(I (mN:m)(mN/m 1x1.1 187.2 194.3 193.5 191.3 183.3 186.0 188.3 186.2 183.7 188.1 206.3 214.1 225.3 190.8 207.6 199.6 212.7 178.8 152.0 16x.9 196.0 154.7 166.0 15x.x 155.8 157.2 16x.1 156.4 165.X 164.7 1x4.1 171.8 176.5 192.9 171.4 191.9 174.7

h

0.0934 0.0925 0.0940 0.1043 0.0997 0.0918 0.0908 0.0962 0.0923 0.0875 0.0927 0.1106 0.1144 0.1248 0.0918 0.1083 0.0991 0.1113 0.076X 0.0760 0.0862 0.0964 0.0805 0.0884 0.0x09 0.0770 0.0817 0.0899 0.075x 0.0866 0.0790 0.0951 0.0x55 0.0897 0.1052 0.0825 0.1044 0.0856

SD C) (mN/m)

0.8 0.6 0.5 0.6 1.2 0.7 0.6 0.6 0.5 0.3 0.9 0.6 0.5 0.8 0.3 0.6 0.7 0.6 0.6 0.9 0.7 0.4 1.1 0.5 0.8 0.3 1.5 0.6 0.7 0.5 0.4 0.2 0.7 0.3 0.4 0.3 0.7 0.2

Temperature (’ Cl

range

946~- 1047 92771026 92X-1028 95221051 951-1052 948x1049 94x 1046 92331047 922 -1022 92331023 923- 1025 923-1022 921l1022 923m 1024 9241024 923-1023 923-1026 925.- 1026 X98-997 x75-976 875 -1026 8477950 X51-950 8766978 x75-975 875~1000 8522951 853-976 87661002 X73-950 X51-953 853 951 85 l-950 850~976 85@977 876976 x75-977 x73-974

1050

of the

u Exp. (eqn 1) (mN:m)

d Calc. (eqn 2) (mN /m)

97.0 104.0 109.8 99.7 101.5 100.7 104.3 101.7 103.1 105.0 104.7 106.7 111.2 112.9 108.1 110.2 110.4 112.6 109.7 83.6 91.3 109.2 x2.2 x6.4 86.0 86.6 x3.7 87.2 X8.1 87.8 93.7 9X.6 94.9 95.x 98.2 97.1 98.0 97.6

9x.3 104.2 109.8 99.6 101.0 99.9 101.5 100.1 101.9 103.7 104.1 106.1 109.1 111.9 107.8 109.1 113.1 111.7 110.2 x2.9 90.8 105.6 85.4 x7.6 85.9 88.6 85.2 x7.4 89.5 X8.9 96.1 97.5 96.1 96.0 97.9 96.5 97.3 98.3

of some Experi(0) 38.

133 Table

DANEK

V.

et ul. : CRYOLITE-BASED

MELTS

LI,~(I,, and the standard deviation of the fit of the experimental data to the model equation (2)

3. Coefhcients

Coeficient, a,,

u,+SD

cr,:mVm I -1 a,/mNm-’ u,/m&rn-’

266.69*10.15 (-1.257+0.011)X

C

10-l

-4.154+0.4x

n,:mNm-’ C a,:mNm-’ a,imNm~ ’ C

(1.546i0.439)~ (3.002+0.165)x (8.738+0.520)x

u?/mNm n,:mNm a,:mNma,,,/mXm u,,:mNm-“C

(2.078+0.555)x

~’ C “C ’ -’ C

SDlmYm-’ I j

(2.702_+0.569)~

10-j IO-lo-’ IO ’ 10 ’

1.007~0.175 (-3.507~0.614)xlO

(-4.3IXf0.794)x 1.76

in (r

0

’

10 i

of mole fractions. The regression analysis was performed with a confidence level of 95%. By omitting the statistically nonimportant terms, equation (2) below was obtained for the surface tension, li : = u, t-cl?. T+a,

*wt%AIF,

+ ulq * wt%AlF;

*T

+u, . (wt%AlF;)’ +a, .wt%AlF,

*wt%LiF*

T

+ u, . wt%AlF:

. wt%CaF2

.T

+a, *wt%AIF,

*wt%Al,O,

.T

+a, .wt%LiF*wt%KF +a,,, *wt%CaF1

.wt%Al,O,.

+a,,

.wt%LiF*wtO/oKF*

*wt%AlF,

T 1 1

4

6

8

10

wt% additive

Equation (2) is valid in the temperature range 850 < Tj C < 1050 and the concentration range 0 < wt% AIF; < 30. 0
g/(mNm-‘)

2

(2)

where wt%(i) is the content of the additive. i. and T is the temperature in “C. The wt% AIF? is the amount in excess relative to CR = 3. The coefficients ~,-a,, together with their standard deviations as well as the standard deviation of the fit are given in Table 3. Figure 2 shows a comparison between

Fig. 3. Calculated effects of different additives on the surface tension of the NaF +AlF, mixtures at 9OO’C. The concentration of all the additives except the one to be studied is 0.1 wt%. Equation (2) : (--) LiF ; (----) AllO,: (---) CaF2: (- -) KF. Extrapolated from equation (1) : (0.0) LiF; (a,A)A120J; (O,+) CaF,; (O.m)KF.

calculated (equation 2) and measured surface tension for some selected compositions. Such a comparison is also shown in Table 2. where data calculated using equations (I) and (2) are compared with reasonable agreement. The effect of the individual additives on the surface tension of NaF-AIF; melts with high acidity (high AIF? concentration) is shown in Fig. 3. The lines shown are calculated on the basis of equation (2), and extrapolated experimental data, using equation (I), are marked with different symbols for each additive. The results shown in Fig. 3 are given for constant CR. keeping the content of all the additives except the one to be studied equal to 0.1 wt%. It may be observed that equation (2) predicts an increase in the surface tension in acidic cryolitebased melts with increasing amount of each additive. LiF has the strongest effect while KF is almost neutral. For comparison, the effects of the same additives on the surface tension of pure cryolite (CR = 3) is shown in Fig. 4. In both the basic (Fig. 4) and the acidic (Fig. 3) melts, LiF and CaFz additions give increasing surface tension. Al,O, and KF additions, however, give decreasing surface tension in basic melts and increasing surface tension in acidic melts. The observed decrease in the surface tension of NaF-AlF, melts with increasing AlF, may be caused by the increasing content of NaAlF,. All acidic compounds or complexes in the

E

Lb 135 130

KF 1

125 800

X50

900

950

1000

IO.50

Fig. 2. Comparison between calculated, imental

surface

tension

data shown

(...)lS;(----)19:(~~~~)36;(--

0

5

10

15

20

25

30

wt% additive

T W>

cquatton (2). and the experin Fig. I. Calculated : (m-P --) I7 ;

-)37;(--)38.

Fig. 4. Measured effects of different additives on the surface cryohte at IOOO’C. Data from Ref. 3 : (-) curve fitted; (+) AIZO, : W) CaF2; (H) KF.

tension of (0) LiF;

V. DANEK PI al. : CRYOLITE-BASED NaF-AIF melt will have higher tendencies to form molecular species than the basic compounds. These covalent species will concentrate on the surface of the melt, resulting in a positive surface adsorption of these species and a reduction in surface tension. The basic additives, LiF, KF and CaF,, increase the AlFz-,AlF:and F- ion concentrations in the melt and make the melt more ionic [779]. This should lead to higher surface tensions, which are indeed observed for LiF and CaFZ additions. Since KF is a more basic additive than both LiF and CaF, we should have observed a more pronounced increase in (T with KF than LiF and CaF, additions. This is not the case, however, and our explanation seems to be too simplistic to give a complete understanding of the observed changes in the surface tension of these melts. The surface tension of the pure components, LiF. KF and CaF,, may also be of importance [3]. When A&O, is dissolved in Na,AIF, the surface tension decreases rapidly at low A&O, contents as can be observed from the data of Fernandez and Mstvold [3] shown in Fig. 4. In the acidic region A&O1 becomes surface non-active and the surface tension of the electrolyte increases. This can be observed from Fig. 3. When AI,O, dissolves in cryolite based melts, the melt structure changes considerably. Raman data obtained by Gilbert and coworkers [lo] indicate that AllO3 behaves as a neutral species relative to the acid base properties of the cryolite melt. This means that the reactions AIF;--

= AIF;-

+F-

(3)

AIF;-

= AlF;

+ F-

(4)

do not shift when AllO, is added. Even if the data indicate more than one A&O,-containing complex ion in the melt, the

MELTS

133

dominating species seems to be A1,02Fz This is also one of the ions proposed by Sterten [I I]. It is reasonable to believe that compounds like Na,Al,O,F, will be more covalent in character than the typical ionic salts NaF and Na,AIF,. In view of the complex character of the NaF-AIF,-Al,O, liquid, however, a proper explanation of the surface property of this melt has to be postponed until further and more conclusive experimental data are available. Arknowledgrments-Associate Prof. Dr Vladimir Danek and Ing. Ondrej Patarak stayed at the Institute of Inorganic Chemistrv, NTH, in Trondheim, during three months in 1992 while this work was performed. This stav was supported by The Strong Point Center of Light Metals Production at The-Instituteof Inorganic Chemistry, NTH, T;ondheim, and COMALCO Aluminium, Australia.

REFERENCES 1. R. Fernandez, K. Grjotheim and T. 0stvold. L&$zt ?Metals 501 (1985). 2. R. Fernandez, K. Grjotheim and T. 0stvold, Light Metals 1025 (1986). 3 R. Fernandez and T. 0stvold, Acta Chem. Stand. 43, 15 1 (I 970). 4: B. Lillebuen, Actu Chem. Stand. 24. 3287 (1970). C. M. Ferro and T. Ostvold, Acta Chem. Stand. 37. 5. D. Bratland, 4x7 (1983). 6. A. Rostum. A. Solheim and A. Sterten, Light Metals 311 (1990). 7. B. Gilbert and T. Materne, Appl. Spectroscopy 44. 299 11990). 8. J. Guzman, K. G. Grjotheim and T. Ostvold, &h~ hetak 425 (1986). 9. E. Dewing, Proc. Electrochem. Sot. 86-1. 262 (1986). Light 10. B. Gilbert, E. Robert, E. Tixhon, F. E. Olsen and T. 0stvold, Me/& (1995). Il. A. Sterten, Electrochim. Acta 25, 1673 (1980).