Studies on Isotope Separation of Lithium by Electromigration in Fused Lithium Bromide and Potassium Bromide Mixture, (II) Composition of Salt in Anode Compartment

Home , Lithium bromide, Potassium bromide

Journal of NUCLEARSCIENCE and TECHNOLOGY,7 (10), p. 522~526 (October 1970).

Yoshinobu YAMAMURA* and Shin SUZUKI*

Received May 13, 1970

Accurate knowledge on the salt composition in the anode compartment is indispensable when 6Li is to be highly enriched by electromigration in fused LiBr-KBr mixture. A study was made on the dependence on temperature shown by the salt composition in the anode compartment. It was observed that sustained electromigration led to a salt composition in the anode compartment that was determined by the prevailing temperature. The composition was observed for various temperatures between 380- and 740-C: In terms of the ratio Li/K in chemical equivalent, the values were 1.31 at 380-C, 1.29 at 420-C, 1.31 at 460-C, 1.41 at 500-C, 1.76 at 540-C, 1.82 at 580-C, 1.95 at 620-C, 2.16 at 680-C and 2.34 at 740-C. These results can be explained by assuming that the fused LiBr-KBr mixture is a system composed of two simple salts and their eutectic, and that at temperatures below 550-C, which is the melting point of LiBr, LiBr and KBr are dissolved in fused eutectic, while KBr is dissolved in the fused eutectic and LiBr at temperatures between 550- and 738-C, which latter is the melting point of KBr.

I. INTRODUCTION . EXPERIMENTAL In a previous study(1), one of authors suc- 1. Reagents and Apparatus ceeded in highly enriching 7Li by electromi- All the reagents and the migration equip- gration in a fused LiBr-KBr mixture at 500-C. ment were the same as those used in the It was found that at the end of the electro- previous study(1), except for the electromigra- migration the separation tube was filled with tion cell. This cell was made of quartz glass a salt of nearly eutectic composition except arranged as shown in Fig. 1. The separation in the vicinity of the cathode. From these tube and the anode compartment had a com- results, the species present in the fused salt mon diameter of 6 mm. In order to assure mixture were assumed to be in two forms: uniformity of temperature in the separation (a) simple cations which migrated toward the tube, the tube was immersed to a point above cathode propelled by the electric field, and the anode compartment in the fused salt of (b) a species of eutectic composition, which the cathode compartment. This fused salt backflowed toward the anode by capillary was agitated by bubbling argon gas intro- action. The presence of this backflowing duced from the bottom of the compartment. species was pointed out to be essential for In the present work we express the com- sustained electromigration during long periods position of the salt by the value of Li/K in to permit accumulation of the isotope effect. chemical equivalent ratio, unless otherwise The purpose of the present investigation noted. is to measure accurately the composition of 2. Procedure the salt in the anode compartment at differ- The electromigration was performed in two ent temperatures and to elaborate on the runs. In the first run, the electromigration composition and nature of the backflowing species. The salt composition thus determined was initiated with salt having a composition should provide very valuable data for the * The Research Institute for Iron, Steel and Other enrichment of 6Li by electromigration. Metals, TOhoku University, Sendai.

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ture the electromigration was continued at 300 mA for 5 days. The experimental temperature was further altered several times as marked in Fig. 2. The entire run was terminated after 56 days. In the second run, a salt with a composition of 1.50 was used and the experiment was conducted along lines similar to the first run, and terminated after 27 days. During the runs, an aliquot of the salt was sampled from the anode compartment twice a day, and the composition of the salt was determined by the same method as in the previous report(1). The characteristic of the present experiment is that separate runs were not made for each temperature, and instead the effect of changes in temperature brought upon the composition of the salt in the anode compartment was determined in only two runs in the course of which the temperature was changed in several steps, both upwards and downwards. This procedure has the merit of revealing the absence of any hysteresis that may occur in the relation between temperature and composition in the anode compartment. The temperature and the electric current with which the data have been obtained are given in Table 1.

Fig. 1 Electromigration cell III. RESULTS of 1.46 (Li/K) and an electric current of 240 mA The results from the first and the second at 460-C until the composition of the salt in the runs are presented in Figs. 2 and 3, respec- anode compartment reached a constant value. tively. It is clear from these figures that In this case it took 12 days. Then the tempera- the salt compositions in the anode compart- ture was raised to 540-C, and at this tempera- ment attain constant values that are charac-

Fig. 2 Variation of composition (Li/K) Fig. 2 Variation of composition of salt in anode compartment with (Li/K) of salt in anode temperature (Experiment I) compartment with temperature (Experiment II)

— 37 — 524 J. Nucl. Sci, Technol., teristic of the temperature. The data are summarized in Table 1 and plotted in Fig. 4 W. DISCUSSION on a phase diagram for the LiBr-KBr sys- In the fused salt electromigration, the tem(2). It is seen that up to 460-C the salt lighter isotope is enriched in the cathode composition, represented in mol/o KBr in the compartment because of its larger mobility. salt, remains roughly constant (about 43.5 In% However, the fractionation of elements cannot KBr), and beyond this temperature, there be treated with the same simple approach as appears a transition range lasting to about for isotope separation, because we have seen 540-C, followed by a region of linear decrease that the salt composition in the anode com- with rising temperature. partment reaches a constant value, beyond which it changes no further with continued electromigrationwm(1)(3).Chemla et al.(3)(4) has interpreted this phenomenon as follows: Lithi- um ions migrate faster than potassium ions but in a fused LiBr-KBr mixture ion associa- tion species (Li(n-1)n and KB(n'-1)-R')-)are formed which do not migrate by the action of the electric field. The fraction of each metal contributed by the corresponding complex species depends on the composition of the fused salt mixture. Since the transport rate of a metal is proportional to the product of the effective cation concentration and its mobility, the transport rates of Li and K species should. equalize in the fused salt of a certain composition. As one evidence for the existence of a complex species, they gave the deviations from ideal behavior in the variation of electric conductance of the fused LiBr- KBr mixture(5). The reason for the deviations is attributed to the complicated structure of the fused salt solution. Generally speaking, the complex compounds existing in the solid state are retained, though in an imperfect form, in the liquid state, and thus electrical conductance data have frequently been used in conjunction with the phase diagram, to probe the presence and the form of ion asso- ciation complex in a fused salt. The LiBr- KBr system, however, has no complex compound even in its solid state because it has a simple phase diagram(2) with only one eutectic point. In the electromigration in a fused LiBr-KBr mixture at 500-C the salt in the anode compartment have been reported(1) to be approximately eutectic composition at the final stage of the experiment. This finding Fig. 4 Final composition of salt in anode compartment at each was interpreted by supposing a backflowing temperature (plotted on species of eutectic composition present in phase diagram) addition to the simple salts.

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It is apparent from Fig. 4 that the relation which LiBr and KBr are dissolved in fused between the composition and the temperature eutectic. In this range, the composition of differs appreciably between the two sides of the backflowing species should logically be the melting point of LiBr. The prevailing only the eutectic, and therefore constant, but temperature would separate the three species it is apparent from Fig. 4 that this holds true composing the fused salt mixture into species only in the lower temperature range CD(460--- X — with their melting point above the prevail- 380-C), and does not apply at temperatures ing temperature, and species Y— with melting immediately below the LiBr melting point point lower than the temperature in con- (range BC). In this intermediate range BC sideration. The system can be likened to a (540--460-C) the composition of the backflow- solution in which species X is dissolved in ing species depends on temperature, but in a species Y, species X thus being the solute manner different from the range AB. The and Y the solvent, respectively. The melting fused salt mixture in this range BC can be points of eutectic, LiBr and KBr are 328-, 550- presumed to have properties intermediate be- and 738-C, respectively, and so each component tween those of the ranges AB and CD. The can be classified into: composition of the backflowing species (Li/K =1.30, 43.5 mol/0KBr) observed in the range CD is constant but is not strictly equal to the eutectic composition found in literature(2) (Li/K =1 .50, 40 mol/0KBr). The reason for the differ- ence may possibly be the effect of the im- Each compound dissociates with its charac- purities introduced by the dissolution of the teristic dissociation constant, but, as pointed Br2 and of impurities present in the Br2 gas*. out above the melt generally keeps the struc- If the fused LiBr-KBr mixture is con- ture of its solid state, unless the temperature sidered to be a system such as described is very much higher than the melting point. above, the mechanism of electromigration in Since the degree of dissociation can be ex- this system could be the same as the counter- pected to be lower for the molten compound current electromigration in an aqueous solu- than for the dissolved compounds in the fused tion reported by Brewer & others'''. This salt mixture, it may be assumed that the permits us to explain electromigration in the solvent corresponds to the backflowing species fused salt perfectly by the treatment proposed mentioned in the previous report(1). by these authors(8). On the basis of the above model we can It has already been observed that, for the explain the experimental results as follows. enrichment of 6Li by electromigration in fused In the temperature range AB(740-~540-C) in salt, the cathode compartment of the electro- Fig. 4 which agrees approximately with the migration cell should be as small as possible range between the melting point of KBr in comparison with the anode compartment(1). (738-C) and of LiBr (550-C), the fused salt With electromigration at a temperature below mixture can be regarded as solution in which the melting point of the two simple salts, the KBr is present as solute dissolved in a solvent composition of the fused salt must be care- composed of the mixture of fused LiBr and fully adjusted. If we use a salt of composi- eutectic. The solvent is transported toward tion different from that of the backflowing the anode by capillary action. The degree * After sustained electromigration with ample Br2 of dissociation of the eutectic compound into the simple salts, LiBr and KBr, increases gas supplied to the fused salt in the cathode compartment, a black solid substance was found ad- with rising temperature. Thus the concentra- hering to the wall of the gas outlet. This possibly tion of LiBr in the backflowing species in- comes from grease used to prevent leakage of Br2 creases with temperature, as is observed in from the system. Bromine dissolved grease and carried it into cathode compartment where the Fig. 4. On the other hand, at temperatures grease was decomposed into carbon in the fused below the melting point of LiBr, the fused salt. Carbon thus formed deposited on the wall salt mixture can be regarded as a system in of the gas outlet.

— 39 — 536 J. Nucl. Sci. Technol., species obtained in this investigation, the salt species X is dissolved in species Y, the be- in the cathode compartment could shift to a havior described above is easily explained. composition that would not remain fluid at It is essential for successful enrichment the prevailing temperature. This would break of 6Li to let the composition of the salt mix- down the migration cell. Therefore the data ture be as close as possible to the composi- obtained in the present study are very valu- tion of the backflowing species in order to able for determining the composition of the prevent solidification in the migration cell. salts to be used for the enrichment of 5Li by The results reported here are useful for de- fused salt electromigration. termining the temperature and the composition of the salt to be adopted in the enrich- V. CONCLUSION ment of 6Li. Sustained electromigration in a fused LiBr- KBr was observed to lead to a salt composi- ACKNOWLEDGMENT tion in the anode compartment -that was The authors wish to thank Dr. Y. Inoue determined by the prevailing temperature. for his helpful advice, and Mr. T. Nihei and This dependency of salt composition in the anode compartment on temperature can be his co-workers for glassblowing. roughly stated to be: —REFERENCES — (1) 740--540-C Lithium content in the salt increases (1) YAMAMURA,Y.: J. Nucl. Sci, Technol., 6 [12], linearly with the temperature rise. 698 (1969). (2) 460--380-C (2) AUKRUST,E., BJORGE, B., FLOOD,H., et al.: The salt composition remains constant Ann. N.Y. Acad. Sci., 79, 830 (1960). at a value of 1.30 (Li/K in chemical (3) PERIE, J., CHEMLA,M., GIGNOUX,M.: Bull. equivalent ratio). Soc. Chim. France, 1961, 1249 (1961). (3) 540--460-C (4) CHEMLA,M.: Discuss. Faraday Soc., 32, 63 Transition between (1) and (2). (1961). The relation between the salt composition (5) MEHTA, 0.P., LANTELME,F., CHEMLA,M.: and temperature changes markedly at the Electrochim. Acta, 14, 505 (1969). melting point of LiBr. Fused LiBr-KBr mix- (6) Van ARTSDALEN,E.R. : J. Phys. Chem., 60, 172 (1956); ture is composed of the two simple salts and SAKAI, K., HAYASHI,S.: J. Chem. Soc. Japan, their eutectic. The prevailing temperature (in Japanese), 76, 101 (1955). would separate these three components into (7) BREWER,A.K., et al.: J. Res. Nat. Bur. Stad., species X— with melting point above this tem- 38, 137 (1947); perature, and species Y— with melting point MADORSKY, S.L., STRAUS,S. : ibid., 38, 185. below the same. Assuming that the fused (8) WESTHAVER,J.W. : ibid., 38, 169 (1947); LiBr-KBr mixture is a solution in which BREIT,G., FRIEDMAN,F.L.: ibid., 39, 397 (1947).

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