US3025224A - Process for isolating isotopes of alkali metals - Google Patents

Description

March 13, 1962 A. KEPES 3,025,224

PROCESS FOR ISOLATING ISOTOPES OF ALKALI METALS Filed Dec. 1'7, 1958 2 Sheets-Sheet 1 a I 3 l [r J 1. i. a

In j u D Q Q o i f i INVENTOR.

ATTOR EYS March 13, 1962 A. KEPES 3,025,224

PROCESS FOR ISOLATING ISOTOPES OF ALKALI METALS Filed Dec. 17, 1958 2 Sheets-Sheet 2 INVENTOR.

ANDRE KEPES United States Patent Ofiice 3,025,224 Patented Mar. 13, 1962 3,025,224 PROCESS FOR ISOLATING ISOTOPES OF ALKALI METALS Andr Kepes, Bievres, France, assignor, by mesue assignments, to Commissariat de lEnergie Atomique,

Paris, France Filed Dec. 17, 1958, Ser. No. 781,084 Claims priority, application France Dec. 20, 1957 14'Claims. (Cl. 204-1) A. Klemm (Z. Natur-forschung 2a, p. 9 (1947), 8a, p. 397 (1953)), has shown that under the action of an electric current two isotopic cations progress through an electrolytic medium with different speeds of migration, and that they can be separated. These different rates of migration have been assessed by M. Chemla (C.R.A.S. 241, p. 1288 (1955; 242 p., 1450 (1956)), for the isotopes of sodium and lithium in molten salts of these metals by a method of electromigration on a strip of asbestos paper impregnated with the molten salts and placed horizontally between the electrodes.

When, in the case, for example, of two isotopic cations the electrolyte is moved against the flow of the cations at a speed between the speeds of the cations, the faster of the cations will continue to progress upstream while the slower cation will be carried downstream. This is the principle upon which the process of separating isotopes by electromigration against a stream of electrolyte is based. This is more eflicient and the yields is higher than in the case of simple electromigration of the known types. In this type of process the movement of the electrolyte from the cathode toward the anode is generated by regenerating the electrolytic medium, which is preferably a molten salt, from the metal which has been deposited on the cathode. The flow of electrolyte will be from the cathode toward the anode, and the flow of cation will be from the anode toward the cathode. The isotopes which migrate more rapidly will thus be deposited on the cathode, while those which migrate less rapidly will be carried downstream by the flow of the current of electrolyte. The flow may be maintained by regenerating the metal deposited on the cathode.

This process has been applied by A. Lunden (These- Goteborg, 1956), to the enrichment in isotopes of a certain number of molten salts. It has been applied, notably, to the separation of isotopes of potassium and lithium (A. Lunden, Z. Naturfor-sch 10, p. 2.79- (1955); II, p. 75 (1956)), from molten nitrates of the metals in a glass apparatus of U-shape, vertically disposed. The regeneration of the nitrates was accomplished by means of a mixture of N and 0 which was introduced, through the cathode itself, into the vicinity of the metal deposited there. The cathode and the anode thus acquired a certain accretion of isotopes but the process had grave imperfections in practice. In the case of lithium, for example, both the yield and the continuity of the process were limited because the transformation of lithium to lithium nitrate by means of the mixed gases was slow and incomplete, imposing a limitation on the current density that could be used to generate the electromigration and, consequently, the yield. Furthermore, at its temperature of fusion, lithium nitrate exercises an intense corrosive action on the materials which have generally been used to construct the apparatus, glass, quartz and silica. a

The use of apparatus of vertical type is particularly inefiicient because of the presence of gas bubbles which inevitably appear during the electromigration and pass upwardly through the electrolytic medium. These bubbles disturb the selective migration of the ions by their irregular ascension through the molten salts. The vertical disposition of the apparatus is especially ineflicient because it is difficult to establish the same temperature in all of the salts, and this also disturbs the selective migration of the ions.

Finally, the employment of the gaseous mixture to regenerate the nitrate at the cathode requires the use in the gas circuit of flow control apparatus and of drying apparatus of complexity and cost and which has frequently to be renewed.

It is an object of the present invention to separate isotopes of alkali metals, and particularly of lithium, by the principle of countercurrent electromigration through a molten salt and to remove or materially improve the disadvantages and difficulties hereinabove described.

Another object of the invention is to replace the unsatisfactory mixture of N0 and O in the regeneration of nitrate salt of the electrolyte.

Another object is to overcome the disadvantages inherent in the vertical apparatus heretofore employed.

In accomplishing the invention, the electrolyte may be horizontally arranged, nitric acid should be employed to regenerate the nitrate salt by reaction with the metal deposited on the cathode, and the rate of flow of the electrolyte should be controlled either by the rate of admission of HNO to the electrode, or by inclining the electrolyte toward the anode, or both.

I have discovered that the use of concentrated nitric acid, which is preferably introduced into the apparatus near the cathode in vapor phase, is wholly useful in transforming the metal deposited on the electrode to regenerate the molten nitrate. A particularly advantageous method of carrying out the invention is to put the electrolyte into a horizontal apparatus of a novel type, which constitutes a part of the present invention.

This apparatus is comprised essentially of anode and cathode compartments connected by a substantially horizontal channel, or by a plurality of channels. The apparatus may have dilterent forms, elongated or otherwise, always on the condition that the position of the electrolyte between the electrodes and the channels shall, at the beginning be on the same level, and that thereafter permanent current shall be set up from the cathode toward the anode which is opposed to the direction of electromigration. This movement of the electrolyte is naturally established by the electrolytic decomposition of the nitrate which, by loss of matter on the anode in the form of nitrous vapors and by increase of the electrolytic medium at the cathode by chemical recombination with the nitric acid, establishes a permanent inclination between the opposite ends of the eleectrolyte. At the same time, this movement is generally retarded and is regulated by changing the inclination of the apparatus with respect to the horizontal, thus making it possible to control the speed of flow of the electrolyte.

The canal, which connects the compartments and contains the electrode, is preferably filled with some inert granular material, through which the molten nitrate extends in order to reduce convection currents. Inert satis factory materials are powders of steatite, zirconia, polytetrafluorethylene. No particular size of granule is necessary, although the granules should not be too coarse, as they would probably atfect the opposition to convection currents; neither should they be too fine, as they might interrupt the regularity of the fiow. It follows that the movement of the electrolyte can be controlled by altering the size of the granules which fill the channel and also by the inclination of the apparatus. The anode and cathode compartments in which the electrodes are situated, immersed in the molten nitrate, are preferably deeper than the canal. The electrodes, and more particularly the cathode, are preferably provided with a large surface. They can also be made of metals which are resistant to nitric acid, such as a chemically resistant steel for the cathode and platinum for the anode.

The cathode compartment is supplied with nitric acid, preferably in vapor phase, by gravity or by siphon from a reservoir of concentrated nitric acid (commercial grade) through a heating tube at 120-130 C. To control the flow of acid, this tube is beneficially filled with a material inert to nitric acid, for instance, in granular form. In the heating tube, the nitric acid is brought to a temperature at which the water azeotrope of nitric acid boils, and in this way it is vaporized without appreciable decomposition. It is advantageous to admit an excess of acid with respect to the quantity of water deposited at the cathode with which the acid is to react. This excess is decomposed at the temperature of the cathode compartment, which is that of the molten nitrate, and the nitrous vapors formed are eliminated, as are also those formed at the anode. The elimination of the nitrous vapors may be aided by aspiration.

The above and further objects and novel features of the present invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention, reference for this latter purpose being had primarily to the appended claims.

In the drawings, wherein like reference characters refer to like parts throughout the several views,

FIG. 1 is a longitudinal vertical section through apparatus representative of the invention,

FIG. 2 is a cross-section on the line IIII of FIG. 1, and

FIG. 3 is an enlarged detail of the cathode end of the apparatus of FIG. 1, in vertical-longitudinal section.

The apparatus comprises a parallel pyrolitic block 1 of high-frequency steatite, elongated and of rectangular section, having very low porosity and fired at about 1400 C. Prior to the firing, a block was provided with a channel 2 relatively deep compared to its width and with two cylindrical cavities 3 and 4 at its extremities. These cavities were deeper than the canal and communicate with it by a series of oblique holes 5.

The block itself is contained in a cylindrical furnace 8 which may be made of a glass tube about which is coiled an electrical heating coil 9. The tube is closed at its extremities by closures; at the anode side, two appendages 10 permit the entrance of air, or of a thermometer if desired. At the cathode end there is an appendage 11 communicating with an aspirator which evacuates the gases or vapors which are released in the anode and cathode compartments. The tubular furnace 8 above the cathode has a large opening extended by a tube 12 which is fixed or not to the tube 8 which holds the cathode in place and permits the introduction of lithium nitrate at the beginning and during the operation. A cover 13 of polytetrafluorethylene closes the tube at its upper end. The tube itself may be covered with a heating coil if desired. In order to permit insulation of the steatite block in the furnace, the two extremities of the furnace, or only one of them, may be separable as indicated at 8-a.

The anode 7 may be composed of a platinum wire ending in a spiral immersed in the molten salt. Its upper end is connected to a generator of direct current, either through the end of the tube or through the tube itself. The cathode 6 (FIG. 3) plays the roles of electrode, cathode and supply tube for nitric acid vapors. It is constituted of a central tube 15 carrying at its lower extremity a receptable 16 which is pierced with a number of holes 17 and filled with small metallic fragments 18. Orifices 15-a allow the tube 15 to communicate with the interior of the receptacle 16. The tube, receptacle and metallic fragments may be composed of refractory steel sulficiently high melting to Withstand the temperature and sufiiciently inert to the acid to remain unaffected by disruption.

The metallic tube of the cathode at its upper part is connected by connection 19 to the tube which receives the liquid nitric acid 20. This tube has a heating zone 21 at the temperature which vaporizes the nitric acid. The valve 22 interrupts the feed of liquid nitric acid it necessary. The central part of the cathode tube, the part serving for the introduction of acid extending between the connection 19 and the valve 22, is filled with quartz wool 23. The upper part of the cathode compartment is covered by a cylindrical lid 24- mounted in the block and of the same interior diameter as the compartment itself. A cover 25 closes the upper part except for the central hole through which the electrode passes and several oblique holes which permit the escape of nitric acid vapors. The cylindrical lid and its cover are of calcined steatite, the same as the block.

The anode compartment is closed in the same way by a small steatite cover 26 having a central hole for the passage of the electrode and small lateral holes for the escape of gases.

The channel 2 contains a fill 27 of small depth, for example several millimeters, constituted by calcined steatite particles of sizes AFNOR 35 to 60.

According to one variation, the block in which the electromigration takes place need not be of a single piece but of conforming elements and profile which are united with each other by plastic joints, for example of polytetrafluorethylene. This structure permits the dismounting and facilitates the entering of the anode and cathode compartments at the end of the operation.

Another modification concerns the heating, which may be obtained by the introduction of heating wires into the interior of the furnace and exteriorly provided with temperature regulating means.

According to another modification, a number of units such as described above may be used in cascade to carry out the process continuously. This is accomplished by a system of supply and interconnection having as an object to continue the separation of isotopes until one has attained an enrichment of the desired magnitude.

The following example is an illustration of the best method of carrying out the invention.

Example The apparatus having been assembled but with the electrodes not yet energized, the heating coil was energized and the apparatus was brought to the temperature at which lithium nitrate had been fused and was poured into the cathode compartment, from which it flowed to and filled the anode compartment. The addition of molten salt was ended when the level approached the top of the granular steatite which filled the channel and of which the size of particle was between AFNOR 35 and 60. The cathode cover was replaced. A difference of potential was applied to the electrode so as to produce a flow of about 0.3 ampere. The nitric acid was passed through the vaporizer and into the cathode compartment, the rate of supply being regulated either by valve 22 or by tilting the apparatus so as to produce an inclination of the channel. The acid was supplied in an excess with respect to the lithium deposited at the cathode, which is to say, about 200 cm? concentrated acid for 24 hours.

The inclination that one gives to the apparatus assures the movement of the electrolyte from the cathode toward the anode while the slow ions are returned to the anode. For isotopes Li and Li, this inclination is on the order of 1 mm. in 400. I

Tests made in apparatus of this type, operated for 72 hours, containing 10 grams of lithium nitrate in each electrode compartment and 20-25 grams of lithium nitrate in the canal, showed accretion on the anode and the cathode of their respective isotopes. Methods used were spectrographic examination of the mass, and sampling the initial proportion of the isotopes.

Li was equal to 11.35. At the end of the 72 hours the proportion was 11.95 at the anode and 10.8 at the cathode. The factor of accretion of the apparatus was consequently equal to 11.95/ 10.8 for a mass of lithium nitrate of the order of 20 grams.

In this operation, the faster migrating isotopes flowed upstream and reached the cathode while the isotopes of slower migration were carried downstream by the flow of the electrolyte and were deposited at the anode. The nitric acid vapors which were admitted to the cathode compartment reacted with the isotopes deposited there to regenerate lithium nitrate. The lithium nitrate thus regenerated flowed again into the channel and was subjected once more to electrolysis and separation of isotopes. By this method, adequate quantities of electrolyte are maintained in the system despite the electrolysis, the isotope deposited at the anode is undisturbed, and the gases released by the electrolysis and by the reaction are carried off by aspiration.

The invention includes a process for separating isotopes of the alkali metals, of which lithium has been given hereinabove as a specific example. The separation is carried out by migration against a stream of electrolyte flowing at a speed between the speeds of migration of the isotope ions. The electrolyte is the nitrate of the metal whose isotopes are desired.

A distinguishing feature of the invention is the regeneration of the alkali metal nitrate in the cathode compartmen-t by means of nitric acid acting on one of the deposited isotopes. The nitric acid is preferably vaporized before introduction into the cathode compartment, the preferred temperature of vaporizaiton being that of the boiling point of its water azeotrope.

Another distinguishing feature of this invention is the horizontal flow of the electrolyte.

Yet another is the use of inert granular material for control of the flow in the channel.

The invention also involves novel apparatus in which anode and cathode compartments are connected by one or more horizontal channels in which a current of electrolyte flows from the cathode to the anode, the speed of flow being controlled by tilting the channels appropriately. The channels are lined with an inert granular filler which reduces convection currents, controls horizontal flow, and tends to maintain uniformity of current flow through the electrolyte. The anode and cathode compartments are preferably deeper than the connecting canal. The cathode may conveniently be in refractory steel and the anode in platinum. The nitric acid is supplied in vapor form from an acid reservoir containing concentrated nitric acid and flows to the cathode compartment through a heating tube at a temperature of l20-l30 C. The apparatus is enclosed ina heated tube which maintains the molten condition of the electrolyte and from which the vapors rising from the system can be removed.

As many apparently widely diiferent embodiments of the present invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments.

What is claimed is:

1. The method of separating isotopes that comprises fusing lithium nitrate at about 280 C., flowing electrolytic current through the lithium nitrate, between an anode and a cathode immersed in the electrolyte, thereby dissociating it and providing ions Li and Li, the difference in potential between the points of entrance and exit of the current producing about 0.3 ampere, flowing the electrolyte toward the anode at a rate similar to that produced by a physical gradient of 1 in 400, and regenerating the Li deposited on the cathode -by bringing vaporized nitric acid in contact therewith, the quantity of HNO added being in excess with respect to the Li deposited on the cathode by about 200 cm. per 24 hours of concentrated HNO 2. The method of separating isotopes of an alkali metal that comprises forming an electrolytic bath of molten alkali metal nitrate, flowing the bath from the cathode to the anode, through inert, finely divided material, at a speed between the speeds of migration of the isotope ions, passing an electrolytic current through the electrolyte between the electrodes, regenerating substantial quantities of the metal deposited at the cathode by reacting it with nitric acid vapor, and removing the gases released by the electrolysis and the reaction.

3. In the method of separating isotopes of an alkali metal that comprises forming an electrolytic bath of molten alkali metal nitrate between electrodes, flowing the electrolyte toward the cathode at a speed between the speeds of migration of the isotopes and passing an electrolytic current between the electrodes through the electrolyte, the step of regenerating substantial quantities of the alkali metal nitrate from the metal deposited at the cathode, by introducing nitric acid in the cathodic compartment.

4. In the method according to claim 3 the step of in- .troducing in the cathodic compartment nitric acid preheated at a temperature corresponding to the boiling point, of the azeotropic mixture nitric acid/water.

5. In the method according to claim 3 the step of introducing in the cathodic compartment nitric acid vapor.

6. The method according to claim 2 in which the alkali metal nitrate is lithium nitrate.

7. A method of separating isotopes which comprises fusing an alkali metal nitrate, passing an electrolytic current through the molten salt, flowing the salt from a cathode toward an anode at a speed between the speeds of migration of the isotopes, regenerating an alkali metal nitrate at the cathode by reacting the metal deposited at the cathode with HNO and subjecting the regenerated alkali metal nitrate to electrolysis in a stream of an alkali metal nitrate proceeding toward an anode at a speed between the speed of migration of the isotopes.

8. The method of separating the metal isotopes of an alkali metal nitrate that comprises fusing the nitrate, passing an electrolytic current therethrough, flowing the fused nitrate toward the anode at a speed between the speeds of migration of the isotopes, and reacting the isotopes at the cathode with nitric acid, thereby restoring alkali metal nitrate to the fused electrolyte and subjecting the cathodic concentration of isotopes to further separation.

9. In a process for separating the isotopes of an alkali metal by electromigration through a molten bath of alkali metal nitrate, the step of adding to the bath in the immediate vicinity of the cathode a quantity of nitric acid in excess of that which is theoretically required to react with the alkali metal deposited at the cathode, and thereby regenerating the alkali metal nitrate.

10. In a process for separating the isotopes of an alkali 7 8 'metal by ele'ctromigration through a molten alkali metal 13. The process of claim 9 in which the alkali metal is bath, the step of regenerating alkali metal nitrate in the lithium. bath by reacting the alkali metal, as it is deposited at the 14. The process of claim 9 in which the nitric acid is cathode, with substantial amounts of nitric acid. concentrated.

11. The process of claim 9 in which the nitric acid is 5 admitted as a vapor to the bath in the immediate vicinity References Clted m the file of this patent f the cathode UNITED STATES PATENTS 12. The process of claim 9 in which aqueous nitric acid 1,018,802 Acker Feb. 27, 1912 is heated to the temperature at which it forms an azeo- 2,566,308 Brewer Sept. 4, 1951 trope with water and the azeotrope is conducted to the 10 2,645,610 Madorsky et a1. July 14, 1953 immediate vicinity of the cathode. 2,813,064 Clark Nov. 12, 1957

Claims (1)

1. THE METHOD OF SEPARATING ISOTOPES THAT COMPRISES FUSING LITHIUM NITRATE AT ABOUT 280*C., FLOWING ELECTROLYTIC CURRENT THROUGH THE LITHIUM NITRATE, BETWEEN AN ANODE AND A CATHODE IMMERSED IN THE ELECTROLYTE, THEREBY DISSOCIATING IT AND PROVIDING IONS 6LI AND 7LI, THE DIFFERENCE IN POTENTIAL BETWEEEN THE POINTS OF ENTRANCE AND EXIT OF THE CURRENT PRODUCING ABOUT 0.3 AMPERE, FLOWING THE ELECTROLYTE TOWARD THE ANODE AT A RATE SIMILAR TO THAT PRODUCED BY A PHYSICAL GRADIENT OF 1 IN 400, AND REGENERATING

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