Magnesium Electrolyte, (II) Magnesium - J-Stage

1 downloads 0 Views 11MB Size Report
The magnesium electrolyte such as the MgCl2-LiCl system has been studied for determining the density .... the melting point of the solid solution in the LiCl-.
Density

and

Electrical

Magnesium

Electrolyte,

Electrolyte By

Tomoo

Conductivity

(II)

with

Matsushima**

and

of Fused

Magnesium

LiCl* Tsutomu

Ito***

The magnesium electrolyte such as the MgCl2-LiClsystem has been studied for determining the density and electrical conductivity. The electrical conductivity of the present system is found to be more than 2.5 to 2.7 times greater than that of the Dow and I.G. electrolytes. It is possible to reduce the electrolysis temperature of the present system in comparison with the Dow and I. G. electrolytes, and the differences in density between liquid magnesium and electrolytes are about 0.03 to 0.05g/cm2 for 14.3 to 10.5 mol% MgCl2. It is of particular interest that the magnesium is recovered at the bottom of the electrolysis cell in the case of magnesium electrolysis of the MgCl2-LiClsystem. (ReceivedJuly 29, 1968) I.

Introduction

In the previous paper(1) (2) the density and the electrical conductivity of fused magnesium electrolytes such as Dow, I.G., and the electrolytes proposed by the present authors have been determined. During the electrolysis the densities of those electrolytes are greater than that of liquid magnesium. This is the reason that the liquid magnesium is tapped off at the upper part of the electrolysis cell. On the other hand, it was attempted in an early work of Grothe et al. (3)(4) to tap either liquid mannesium or a magnesium alloy from the bottom of the electrolysis cell. From the viewpoint of electrolysis devices, it is noticed that the recovery of liquid magnesium from either the upper portion or the bottom of the electrolysis cell is only dependent upon the difference in density between magnesium and electrolyte. The magnesium electrolyte such as the LiCl-MgCl2 system proposed by Grothe et al. might be feasible, particularly for the recovery of magnesium from the bottom of, the electrolysis cell. However, there have been dissenting views on the physicochemical properties

of the above system.

*(I) of this paper was published in Trans. JIM, 8 (1967), 45. ** Research Institute of Mineral Dressing and Metallurgy, Tohoku University, Sendai, Japan. *** Research Institute of Mineral Dressing and Metallurgy, Tohoku University, Sendai. Present address: Furukawa Magnesium Co. Ltd., Oyama Work, Oyama, Japan. (1) T. Matsushima, T. Ito and P. K. Som: Electrochem. Soc. (Japan), 35 (1967), 339. (2) T. Matsushima, T. Ito and P. K. Som: Trans. JIM, 8 (1967), 43. (3) H. Grothe and W. Savelsberg: Z. Elektrochem., 46 (1940), 336. (4) H. Grothe: Metall u. Erz., 36 (1939), 63. Trans.

JIM

For the MgCl2-LiCl system, particular attention has been given to the determination of the density and electrical conductivity which contribute to the applicability of magnesium electrolysis. Therefore, the present study was undertaken to clarify such properties of the MgCl2-LiCl system. II.

Experimental

Chemically pure anhydrous LiCl and MgCl2 were treated by the following procedure with precaution for the trace amount of water in reagents. The apparatus used for the treatment of samples was composed of the quartz reaction tube for the treatment of salts, the path of argon with CaCl2, cone H2SO4, and P2O5, and that of HCl with CaCl2, and H2SO4, and the trap for dry ice-alcohol on the path of HCl. As tube

shown was

in Fig. held

for

1,400 12

hr

to at

600g

150℃

of salt and

for

in 3 hr

a reaction at

400℃

under evacuation, and then the dry HCl was introduced in the reaction tube and bubbled for 2 hr by one end of the tube inserted to the molten salt. Next, dry argon was introduced and the salt was thoroughly agitated for 30 min. Then the reaction tube was held for 30 min under evacuation. One end of the sample container was introduced to the molten salt, and the ambient atmosphere of the reaction tube was replaced by argon. By the pressure difference between the reaction tube and the sample container, the molten salt was exhausted to the sample container. After solidifying the salt, the end of the sample container was sealed off. LiCl and MgCl2 thus prepared were weighed by 10 to 60 wt% MgCl2 in the reaction tube, and the sample was prepared by the procedure mentioned above. The solubility of HCl in the sample was not con1969

Vol.10

162

Density and Electrical

Conductivity

firmed but considered to be negligiblly bubbling argon, evacuation, and the solubility of gas during the solidification.

of Fused Magnesium Electrolyte,

small due to decrease of

1. Density

Fig. 2

within

±0.1℃.

Diagram of the electrical conductivity measurement (A) Oscillater, (B) Conductivity cell, (C) Oscilloscope

measurement

III.

The density was measured by the Archimedian method. Details of the measuring device with a specially designed sinker were already reported in the previous paper. The pyrolitic corrosion of the quartz container and the sinker was negligiblly small when the sample was subjected to the same treatment as mentioned in the preceding section.

with LiCl

optimum output potential was held below 150mV to minimize the polarization effect. In the circuit, it was assumed that the inductive capacitance was parallel to the measuring resistance, and the variable capacitator of 40 to 400 pF in the figure was used to compensate the above capacitance on the line. The determination of null point was detected by the audio-amplifier (90 dB, B and selectionof 20 c/s to 20 kc/s, Yokogawa Elec. Co., tuned null detector, Model 4403A) with an oscilloscope. Thus, the variation of the order of 0.02 ohm in the conductivity measurement was determined by measuring the capacitance and resistance. The capacitance increase in the case of resistance measurement by the aid of the capillary cell(5)(6)was also observed in the range of 80 to 150pF with increasing temperature. A noninductive furnace made of a nichrome resistance heater was used for heating and controlled

Fig. 1 Purification of salt (A) Sampling tube: pyrex, (B) HCl and A inlet: quartz, (C) Gas outlet: quartz, (D) Silicone cap, (E) Purification container: quartz, (F) Salt

(II) Magnesium Electrolyte

Only a few works properties of the the density and

Results

have been reported

on the

physical

MgCl2-LiCl system, particularly electrical conductivity. In the

present study, therefore, the density and the electrical conductivity of the magnesium electrolyte were determined. The temperature dependence of various electrolytes was

determined

in

the

temperature

range

from

610°

to 870℃.

2.

Electrical

conductivity

measurement

The design and assemblly of the conductivity cell was already given in the previous report. From the viewpoint of the electrical conductivity of LiCl in comparison with NaCl and KCl, the capillarly tube was changed as follows: The capillarly tube of 1.10 to 1.20mm in I.D., and about 150mm in length corresponded to the cell constant of 2000 to 1000cm-1 which was determined by the pure KCl melt in a wide temperature The ±0.15%, kc/sec

range

effect

of

and that was

of

790° ∼950℃.

temperature

±0.03%

of

on the frequencies

after

calibrating

cell at

consant 1, the

10

was and

20

inductive

capacitance. The block diagram of the circuit for conductivity measurement is shown in Fig., 2. The oscillator (transistor unit, wide band, variable output potential, Toadenpa K.K. Model CR-1OJK) was used. The

Fig. 3 shows the results of density measurement for each electrolyte containing 10 to 60 wt% MgCl2, in which the linear relationship between temperature and density is well determined according to Table 1. Fig. 4 shows the temperature dependence of the electrical conductivity. The empirical formula of the electrical conductivity with temperature which was established or the basis of the curves in Fig. 4 are represented in Table 2. From the knowledge of the phase diagram(7) (8) the melting point of the solid solution in the LiCl(5) E. R. Buckle and P. E. Tsaoussologlou: J. Chem. Soc., (1964), 667. (6) M. Kunitomi: Nippon Kagaku Kaishi, 64 (1943), 1242. (7) T. Matsushima and T. Ito: 34th Spring Meeting of Electrochem. Soc. (Japan). (8) W. Klemm and P. Weiss: Z. anorg. allg. Chem., 245 (1940), 281.

Tomoo

Matsushima

and Tsutomu

Ito

163

MgCl2 system shows a minimum value at a certain composition. In contrast to the NaCl-MgCl2 system, the diagram of the KCl-MgCl2 systems indicates the formation of complex salts by a congruent or an incongruent reaction(8)(9). Fig. 5 shows the composition dependence of the molar volumes of the LiCl-MgCl2 system, in comparison with the NaCl-MgCl2and KClMgCl2 systems. The NaCl and KCl systems with MgCl2 are referred from the data of Huber et al. (10). The molar volumes of salt mixtures are obtained in accordance with the additivity law: (1) where Xi is the molar fraction and Mi the molar weight of the i-th component.

Fig.

4

Table

Fig. 3

Density

Table 1

of the system

of MgCl2-LiCl

Density of electrolyte temperature

Electrical LiCl vs.

2

conductivity temperature

Electrical function

conductivity of temperature

of

the

of the

system

of

electrolyte

MgCl2-

as a

vs. temperature

as a function

of

(9) A. I. Ivanov: Sbornik Statei Oshcheu Khim., Akad. Nauk S. S. S R., 1(1953), 758. (10) R. W. Huber, E. V. Potter and H. W. St. Clair: Bureau of Mines RI, 4858, (U.S.A.), (I952), p.14.

Fig. 5

Comparison MgCl2

systems

of the at

molar 800℃

volume

of alkali

chloride-

164

Density

and Electrical

Conductivity

of Fused Magnesium Electrolyte,

As shown in Fig. 5, the curve of the LiCl-MgCl2 system indicates a slightly negative deviation from the ideal line. On the other hand, the curve of the KCl-MgCl2 or the LiCl-MgCl2 system has a positive deviation at a lower MgCl2 fraction and a negative deviation at a higher MgCl2 fraction. Such a tendency of the variation of the molar volume(11)(12) may correspond fairly well with the pattern of the phase diagram. The equivalent conductivity of the salt was calculated from the following equation:

mixture

(2) where

κ is the

specific

conductivity

of the

salt

mixture,

fi the equivalent mole fraction, Ei the equivalent weight of the i-th component d the density of the salt mixture. The equivalent conductivities of the MgCl2-LiCl, MgCl2-KCland MgCl2-NaClsystems were compared as shown in Fig. 6.

Fig.

6

Comparison

of

chloride-LiCl

systems

electrical at

conductivities

(II) Magnesium Electrolyte

with LiCl

is reversed in the two systems of LiCl-MgCl2 and NaCl-MgCl2. This may be attributed to either the electrical properties of LiCl in comparison with NaCl and KCl or to the hindrance effect of the formation of autocomplex. However, due to the experimental uncertainty of data referred to by Huber et al., this may not be sufficient for proceeding the discussion. Further consideration is given to the magnesium electrolyte for the electrolysis on the basis of the results mentioned above. The densities of electrolytes at various concentrations of magnesium chloride for the same temperature are compared with those of liquid magnesium. This is shown in Fig. 7 in which the density of magnesium is quoted from the data of McGonigal(15). The constituent of magnesium chloride in electrolyte at which the density of electrolyte is equal to that of

magnesium

700℃, system.

is

and For

29.2

21.2 mol%

the

mol% at

at 800℃

KCl-MgCl2

650℃,

26.2

mol%

at

in the LiCl-MgCl2

system,

the

system

of

in alkali-

800℃

Fig. 7

The deviation from the ideal line for each system showed a maximum at nearly 30 to 40 mol% MgCl2. The deviation from the ideal line was 26% for the LiCl -MgCl2 system, 23% for the NaCl-MgCl2 system, and 29% for the KCl-MgCl2 system at 40 mol% MgCl2, respectively. Therefore, it is identical that the curve obtained by eq. (2) is ruled out from the ideal line. In view of the interaction parameter(13) (14), Kleppa pointed out the presence of the most prominent complex species of MgCl42- in the MgCl2-KCl and MgCl2NaCl systems, and even in the MgCl2-LiCl system. However, in terms of the equivalent conductivity, the order of magnitude in deviation from the ideal line (11) H. Bloom, I. W. Knaggs, J. J. Molloy and D. Welch: Trans. Faraday Soc., 49 (1953), 1458. (12) J. O'M Bockris: Modern Aspects of Electrochemistry, Butterworths Sci. Pub., (London), No. 2, (1959),p. 204. (13) T. Chitani: Inorganic Chemistry, Sangyo Tosho, (Japan), 1 (1959), 99. (14) B. R. Sundheim: Fused Salts, McGraw-Hill Book Co., (U.S.A.), (1964),p. 145..

Isothermal densities of chlorides vs. composition

KCl-MgCl2, 750℃,

the and

constituent

41.4

mol%

density of the system that of magnesium

magnesium of MgCla

of at

MgCl2 800℃.

chloride-alkali

32.8

mol%

at

Further, the

of NaCl-MgCl2 is greater than in the temperature range as

postulated above. So far, in comparison with the KCl-MgCl2 and LiCl-MgCl2 systems, the content of magnesium chloride of the former system is greater than that of the latter system at a particular density. However, it is noted that in the KCl-MgCl2 system the electrolysis temperature is higher and the electrical conductivity is considerably lower than those of the LiCl-MgCl2 system.

(15) R. J. McGonigal,A. D. Kirshenbaum and A. V. urosse: J. Phys. Chem., 66 (1962), 737. (16) E. F. Emley: Principles of Magnesium Technology, Pergamon Press, (London), (1966), p. 39, 43. (17) E. R. Van Artsdalen and I. S. Yaffe: J. Phys. Chem., 58 (1955), 118.

Tomoo

The

electrolysis

temperature

is

700° to

Matsushima

740℃

for

the commercial electrolyte when the average content of MgCl2 is nearly 15 mol% as in the case of Dow and I.G. electrolytes(16). On the other hand, it is possible to reduce the electrolysis temperature in the LiCl-MgCl2 system and the difference of density between liquid magnesium and

electrolyte

is to

be

0.03 to

0.05g/cm3

at

and Tsutomu

Ito

165

on the bottom of the cell. However, it is noticed that the effect of KCl on the electrical conductivity might be concerned. This subject, particularly with the ternary system of LiCl-KCl-MgCl2,will be reported elsewhere. IV. Conclusion

660℃

for 14.3 to 10.5 mol% MgCl2. This configuration will enable tapping of the magnesium from the bottom of the cell with the LiCl-MgCl2 electrolyte. In the previous reprot the electrical conductivity

(1) The molar volume and the equivalent conductivity of the system of LiCl-MgCl2are comparedwith those in the systems in KCl-MgCl2and NaCl-MgCl2. (2) The density and electrical conductivity of the LiCl-MgCl2system are determined in the composition

of the

range

and the

Dow

electrolpte

I.G.

electrolyte

was

2.16

ohm-1cm-1

1.75ohm-1cm-1

According to the present study, the ductivity of the sy3tem of LiCl-MgCl2 ohm-1cm-1

at

660℃

for the

composition

at at

700℃,

740℃,

electrical conis 4.70 to 4.95 of

14.3 to

10.5 mol% MgCl2. From this point of view thin system is applicable to the magnesium electrolysis. From the above considerations only the addition of KCl to the LiCl-MgCl2 can preferably reduce the electrolysis temperature for the tapping of magnesium

of 4.7

to

40

mole%

at 630° to

860℃.

(3) The electrical conductivity is 4.95 to 4.70 ohm-1cm-1 in the above-mentioned concentration range

of

magnesium

chloride

at

660℃.

These

values

are 2.5 to 2.7 times greater than those of such commercial electrolytes as Dow and I.G. electrolytes. (4) The use of the MgCl2-LiCl electrolyte enables the electrolysis of magnesium at a low temperature above the melting point of magnesium and also the tapping of magnesium from the bottom of the cell.