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Патент USA US3025234

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March 13, 1962
A. KEPES
3,025,224
PROCESS FOR ISOLATING ISOTOPES OF ALKALI METALS
Filed Dec. 1'7, 1958
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BY
m
°°
INVENTOR.
ANDRE KEPES
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 O?ice
3,025,224
Patented Mar. 13, 1962
1
2
3,025,224
The use of apparatus of vertical type is particularly in
e?icient because of the presence of gas bubbles which in
PROCESS FOR ISOLATING ISOTOPES OF
ALKALI METALS
evitably appear during the electromigration and pass up
wardly through the electrolytic medium. These bubbles
André Kepes, Bievres, France, assignor, by mesue as 5 disturb the selective migration of the ions by their ir
signments, to Commissariat de l’Energie Atomique,
regular ascension through the molten salts. The vertical
Paris, France
disposition of the apparatus is especially ine?icient be
Filed Dec. 17, 1958, Ser. No. 781,084
cause it is difficult to establish the same temperature in
Claims priority, application France Dec. 20, 1957
all of the salts, and this also disturbs the selective migra
14'Claims. (Cl. 204-1)
10 tion of the ions.
Finally, the employment of the gaseous mixture to re
generate the nitrate at the cathode requires the use in
the gas circuit of ?ow control apparatus and of drying
apparatus of complexity and cost and which has fre
trolytic medium with different speeds of migration, and
that they can be separated. These different rates of mi 15 quently to be renewed.
It is an object of the present invention to separate iso
gration have been assessed by M. Chemla (C.R.A.S.
topes of alkali metals, and particularly of lithium, by
241, p. 1288 (1955; 242 p., 1450 (1956)), for the iso
the principle of countercurrent electromigration through
topes of sodium and lithium in molten salts of these
a molten salt and to remove or materially improve the
metals by a method of electromigration on a strip of
asbestos paper impregnated with the molten salts and 20 disadvantages and dif?culties hereinabove described.
Another object of the invention is to replace the un
placed horizontally between the electrodes.
satisfactory mixture of N02 and O2 in the regeneration
When, in the case, for example, of two isotopic cat
of nitrate salt of the electrolyte.
ions the electrolyte is moved against the ?ow of the cat
Another object is to overcome the disadvantages in
ions at a speed between the speeds of the cations, the
faster of the cations will continue to progress upstream 25 herent in the vertical apparatus heretofore employed.
In accomplishing the invention, the electrolyte may
while the slower cation will be carried downstream. This
be horizontally arranged, nitric acid should be employed
is the principle upon which the process of separating iso
to regenerate the nitrate salt by reaction with the metal
topes by electromigration against a stream of electrolyte
deposited on the cathode, and the rate of ?ow of the
is based. This is more e?icient and the yields is higher
electrolyte should be controlled either by the rate of ad
than in the case of simple electromigration of the known
mission of HNO3 to the electrode, or by inclining the
types. In this type of process the movement of the elec
electrolyte toward the anode, or both.
trolyte from the cathode toward the anode is generated
I have discovered that the use of concentrated nitric
by regenerating the electrolytic medium, which is pref
acid, which is preferably introduced into the apparatus
erably a molten salt, from the metal which has been de
posited on the cathode. The ?ow of electrolyte will be 35 near the cathode in vapor phase, is wholly useful in trans
forming the metal deposited on the electrode to regen
from the cathode toward the anode, and the flow of cat
erate the molten nitrate. A particularly advantageous
ion will‘ be from the anode toward the cathode. The
method of carrying out the invention is to put the elec
isotopes which migrate more rapidly will thus be deposited
trolyte into a horizontal apparatus of a novel type, which
on the cathode, while those which migrate less rapidly
will be carried downstream by the ?ow of the current of 40 constitutes a part of the present invention.
This apparatus is comprised essentially of anode and
electrolyte. The ?ow may be maintained by regenerat
cathode compartments connected by a substantially hori
ing the metal deposited on the cathode.
zontal channel, or by a plurality of channels. The ap
This process has been applied by A. Lunden (These
paratus may have dilterent forms, elongated or otherwise,
Goteborg, 1956), to the enrichment in isotopes of a cer
tain number of molten salts. It has been applied, not 45 always on the condition that the position of the elec
trolyte between the electrodes and the channels shall,
ably, to the separation of isotopes of potassium and lith
at the beginning be on the same level, and that there
ium (A. Lunden, Z. Naturfor-sch 10, p. 2.79- (1955); II,
after permanent current shall be set up from the cathode
p. 75 (1956)), from molten nitrates of the metals in a
toward the anode which is opposed to the direction of
glass apparatus of U~shape, vertically disposed. The re
electromigration. This movement of the electrolyte is
generation of the nitrates was accomplished by means
naturally established by the electrolytic decomposition of
of a mixture of N02 and 02, which was introduced,
the nitrate which, by loss of matter on the anode in the
through the cathode itself, into the vicinity of the metal
form of nitrous vapors and by increase of the electrolytic
deposited there. The cathode and the anode thus ac
medium at the cathode by chemical recombination with
quired a certain accretion of isotopes but the process
had grave imperfections in practice. In the case of lith 55 the nitric acid, establishes a permanent inclination be
tween the opposite ends of the eleectrolyte. At the same
ium, for example, both the yield and the continuity of
time, this movement is generally retarded and is regulated
the process were limited because the transformation of
by changing the inclination of the apparatus with respect
lithium to lithium nitrate by means of the mixed gases
to the horizontal, thus making it possible to control the
was slow and incomplete, imposing a limitation on the
current density that could be used to generate the elec 60 speed of ?ow of the electrolyte.
The canal, which connects the compartments and con
tromigration and, consequently, the yield. Furthermore,
at its temperature of fusion, lithium nitrate exercises an
tains the electrode, is preferably ?lled ‘with some inert
intense corrosive action on the materials which have gen
granular material, through which the molten nitrate ex
erally been used to construct the apparatus, glass, quartz
tends in order to reduce convection currents. Inert satis—
A. Klemm (Z. Natur-forschung 2a, p. 9 (1947), 8a, p.
397 (1953)), has shown that under the action of an elec
tric current two isotopic cations progress through an elec
and silica.
'
a
65
factory materials are powders of steatite, zirconia, poly
~
3,025,224
3
4
tetra?uorethylene. No particular size of granule is neces
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.
sary, although the granules should not be too coarse,
as they would probably atfect the opposition to convec
tion currents; neither should they be too ?ne, as they
might interrupt the regularity of the ?ow. It follows that
The cathode 6 (FIG. 3) plays the roles of electrode,
the movement of the electrolyte can be controlled by
cathode and supply tube for nitric acid vapors. It is con
altering the size of the granules which ?ll the channel
stituted of a central tube 15 carrying at its lower ex
and also by the inclination of the apparatus. The anode
tremity a receptable 16 which is pierced with a number of
and cathode compartments in which the electrodes are
holes 17 and ?lled with small metallic fragments 18.
situated, immersed in the molten nitrate, are preferably 10 Ori?ces 15-a allow the tube 15 to communicate with the
deeper than the canal. The electrodes, and more particu
interior of the receptacle 16. The tube, receptacle and
larly the cathode, are preferably provided with a large
metallic fragments may be composed of refractory steel
surface. They can also be made of metals which are re
sistant to nitric acid, such as a chemically resistant steel
sul?ciently high melting to Withstand the temperature and
su?iciently inert to the acid to remain unaffected by dis
for the cathode and platinum for the anode.
15 ruption.
The cathode compartment is supplied with nitric acid,
The metallic tube of the cathode at its upper part is
preferably in vapor phase, by gravity or by siphon from
connected by connection 19 to the tube which receives
a reservoir of concentrated nitric acid (commercial grade)
the liquid nitric acid 20. This tube has a heating zone 21
through a heating tube at 120°-130° C. To control the
at the temperature which vaporizes the nitric acid. The
?ow of acid, this tube is bene?cially ?lled with a material 20 valve 22 interrupts the feed of liquid nitric acid it neces
inert to nitric acid, for instance, in granular form. In
sary. The central part of the cathode tube, the part serv
the heating tube, the nitric acid is brought to a tempera
ing for the introduction of acid extending between the con
ture at which the water azeotrope of nitric acid boils, and
nection 19 and the valve 22, is ?lled with quartz wool
in this way it is vaporized without appreciable decomposi
23. The upper part of the cathode compartment is
tion. It is advantageous to admit an excess of acid 25 covered by a cylindrical lid 24- mounted in the block
with respect to the quantity of water deposited at the
and of the same interior diameter as the compartment
cathode with which the acid is to react. This excess is
itself. A cover 25 closes the upper part except for the
decomposed at the temperature of the cathode compart
central hole through which the electrode passes and several
ment, which is that of the molten nitrate, and the nitrous
oblique holes which permit the escape of nitric acid vapors.
vapors formed are eliminated, as are also those formed 30 The cylindrical lid and its cover are of calcined steatite,
at the anode. The elimination of the nitrous vapors may
the same as the block.
be aided by aspiration.
The above and further objects and novel features of
The anode compartment is closed in the same way by a
small steatite cover 26 having a central hole for the pas~
the present invention will more fully appear from the
sage of the electrode and small lateral holes for the
following detailed description when the same is read in 35 escape of gases.
connection with the accompanying drawings. It is to be
The channel 2 contains a ?ll 27 of small depth, for ex
expressly understood, however, that the drawings are for
the purpose of illustration only and are not intended as
a de?nition of the limits of the invention, reference for
ample several millimeters, constituted by calcined steatite
particles of sizes AFNOR 35 to 60.
According to one variation, the block in which the
this latter purpose being had primarily to the appended 40 electromigration takes place need not ‘be of a single piece
claims.
but of conforming elements and pro?le which are united
In the drawings, wherein like reference characters refer
with each other by plastic joints, for example of poly
to like parts throughout the several views,
tetra?uorethylene. This structure permits the dismount
FIG. 1 is a longitudinal vertical section through ap
ing and facilitates the entering of the anode and cathode
paratus representative of the invention,
45 compartments at the end of the operation.
FIG. 2 is a cross-section on the line II—II of FIG. 1,
and
FIG. 3 is an enlarged detail of the cathode end of the
Another modi?cation concerns the heating, which may
of high-frequency steatite, elongated and of rectangular
such as described above may be used in cascade to carry
be obtained by the introduction of heating wires into the
interior of the furnace and exteriorly provided with tem
apparatus of FIG. 1, in vertical-longitudinal section.
perature regulating means.
The apparatus comprises a parallel pyrolitic block 1 50
According to another modi?cation, a number of units
section, having very low porosity and ?red at about 1400°
out the process continuously.
C. Prior to the ?ring, a block was provided with a chan
a system of supply and interconnection having as an
This is accomplished by
nel 2 relatively deep compared to its width and with two
object to continue the separation of isotopes until one
cylindrical cavities 3 and 4 at its extremities. These 55 has attained an enrichment of the desired magnitude.
cavities were deeper than the canal and communicate
The following example is an illustration of the best
with it by a series of oblique holes 5.
method of carrying out the invention.
The block itself is contained in a cylindrical furnace 8
which may be made of a glass tube about which is coiled
Example
an electrical heating coil 9. The tube is closed at its 60
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 65
The apparatus having been assembled but with the elec
trodes 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 ?owed to
and ?lled the anode compartment. The addition of
molten salt was ended when the level approached the top
of the granular steatite which ?lled the channel and of
which the size of particle was between AFNOR 35 and
cathode compartments. The tubular furnace 8 above
the cathode has a large opening extended by a tube 12
which is ?xed 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 70 60. The cathode cover was replaced. A difference of
potential was applied to the electrode so as to produce
polytetra?uorethylene closes the tube at its upper end.
a ?ow of about 0.3 ampere. The nitric acid was passed
The tube itself may be covered with a heating coil if de
sired. In order to permit insulation of the steatite block
through the vaporizer and into the cathode compartment,
in the furnace, the two extremities of the furnace, or only
the rate of supply being regulated either by valve 22 or
one of them, may be separable as indicated at 8-a.
by tilting the apparatus so as to produce an inclination of
3,025,224
6
the channel. The acid was supplied in an excess with re
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 speci?c embodi
spect 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 5 ments.
the anode while the slow ions are returned to the anode.
What is claimed is:
For isotopes 6Li and 7Li, 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 elec
trode 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 spectro
graphic examination of the mass, and sampling the initial
proportion of the isotopes.
1. The method of separating isotopes that comprises
fusing lithium nitrate at about 280° C., ?owing electro
lytic 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 differ
ence in potential between the points of entrance and exit
of the current producing about 0.3 ampere, ?owing the
electrolyte toward the anode at a rate similar to that pro
15 duced by a physical gradient of 1 in 400, and regenerating
'lLi
6Li
the Li deposited on the cathode -by bringing ‘vaporized
nitric acid in contact therewith, the quantity of HNO3
tion of the apparatus was consequently equal to
that comprises forming an electrolytic bath of molten
alkali metal nitrate, ?owing the bath from the cathode
to the anode, through inert, ?nely divided material, at a
added being in excess with respect to the Li deposited on
was equal to 11.35.
the cathode by about 200 cm.3 per 24 hours of concen
At the end of the 72 hours the proportion was 11.95 at 20 trated HNO3.
the anode and 10.8 at the cathode. The factor of accre
2. The method of separating isotopes of an alkali metal
11.95/ 10.8 for a mass of lithium nitrate of the order of
20 grams.
In this operation, the faster migrating isotopes flowed 25 speed between the speeds of migration of the isotope
upstream and reached the cathode while the isotopes of
ions, passing an electrolytic current through the electro
slower migration were carried downstream by the flow
lyte between the electrodes, regenerating substantial quan
of the electrolyte and were deposited at the anode. The
tities of the metal deposited at the cathode by reacting
nitric acid vapors which were admitted to the cathode
it with nitric acid vapor, and removing the gases released
compartment reacted with the isotopes deposited there to 30 by the electrolysis and the reaction.
regenerate lithium nitrate.
The lithium nitrate thus re
generated flowed again into the channel and was sub
jected once more to electrolysis and separation of iso
topes. By this method, adequate quantities of electrolyte
3. In the method of separating isotopes of an alkali
metal that comprises forming an electrolytic bath of
molten alkali metal nitrate between electrodes, ?owing
the electrolyte toward the cathode at a speed between the
are maintained in the system despite the electrolysis, the 35 speeds of migration of the isotopes and passing an elec
isotope deposited at the anode is undisturbed, and the
trolytic current between the electrodes through the elec
gases released by the electrolysis and by the reaction are
trolyte, the step of regenerating substantial quantities of
carried off by aspiration.
the alkali metal nitrate from the metal deposited at the
The invention includes a process for separating iso
cathode, by introducing nitric acid in the cathodic com
topes of the alkali metals, of which lithium has been 40 partment.
given hereinabove as a speci?c example. The separation
4. In the method according to claim 3 the step of in
is carried out by migration against a stream of electrolyte
.troducing in the cathodic compartment nitric acid pre
?owing at ‘a speed between the speeds of migration of the
heated at a temperature corresponding to the boiling
isotope ions. The electrolyte is the nitrate of the metal
point, of the azeotropic mixture nitric acid/water.
whose isotopes are desired.
5. In the method according to claim 3 the step of in
A {distinguishing feature of the invention is the regenera 45 troducing in the cathodic compartment nitric acid vapor.
tion of the alkali metal nitrate in the cathode compart
6. The method according to claim 2 in which the alkali
men-t by means of nitric acid acting on one of the depos
metal nitrate is lithium nitrate.
ited 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 ?ow of the electrolyte.
7. A method of separating isotopes which comprises
fusing an alkali metal nitrate, passing an electrolytic cur
rent through the molten salt, ?owing 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
Yet another is the use of inert ‘granular material for 55 at the cathode with HNO3, and subjecting the regenerated
control of the flow in the channel.
alkali metal nitrate to electrolysis in a stream of an alkali
The invention also involves novel apparatus in which
metal nitrate proceeding toward an anode at a speed be
anode and cathode compartments are connected by one
tween the speed of migration of the isotopes.
or more horizontal channels in which a current of electro
8. The method of separating the metal isotopes of an
lyte ?ows from the cathode to the anode, the speed of
?ow being controlled by tilting the channels appropriate
alkali metal nitrate that comprises fusing the nitrate,
passing an electrolytic current therethrough, ?owing the
ly. The channels are lined with an inert granular ?ller
fused nitrate toward the anode at a speed between the
which reduces convection currents, controls horizontal
speeds of migration of the isotopes, and reacting the iso
?ow, and tends to maintain uniformity of current ?ow
topes at the cathode with nitric acid, thereby restoring
through the electrolyte. The anode and cathode com
alkali metal nitrate to the fused electrolyte and subjecting
65
partments are preferably deeper than the connecting canal.
the cathodic concentration of isotopes to further separa
The cathode may conveniently be in refractory steel and
tion.
the anode in platinum. The nitric acid is supplied in
9. In a process for separating the isotopes of an alkali
vapor form from an acid reservoir containing concen
metal by electromigration through a molten bath of alkali
trated nitric acid and ?ows to the cathode compartment 70 metal nitrate, the step of adding to the bath in the im
through a heating tube at a temperature of l20°-l30°
mediate vicinity of the cathode a quantity of nitric acid
C. The apparatus is enclosed in- a heated tube which
in excess of that which is theoretically required to react
maintains the molten condition of the electrolyte and
with the alkali metal deposited at the cathode, and thereby
from which the vapors rising from the system can be re
regenerating the alkali metal nitrate.
moved.
10. In a process for separating the isotopes of an alkali
75
3,025,224
7
'metal by ele'ctromigration through a molten alkali metal
8
-
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 ?le of this patent
of the cathode_
12. The process of claim 9 in which aqueous nitric acid
is heated to the temperature at which it forms an azeotrope with water and the azeotrope is conducted to the 10
immediate vicinity of the cathode.
UNITED STATES PATENTS
1,018,802
2,566,308
2,645,610
2,813,064
Acker _______________ __ Feb. 27,
Brewer ______________ __ Sept. 4,
Madorsky et a1. ______ __ July 14,
Clark ______________ _.. Nov. 12,
1912
1951
1953
1957
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