LITHIUM ISOTOPE SEPARATION
Ilie HODOR
National Institute for Research and Development of Isotopic and Molecular Technologies
3400 Cluj-Napoca 5, Romania
1. Introduction
In this paper, it is summarised the research made at INCDTIM concerning lithium
isotope separation.
Natural lithium consists of two isotopes 6Li (7.5 %) and 7Li (92.5 %). Lithium isotopes
have many uses but first of all they are important materials for nuclear fusion. Up to the present
time, nuclear fusion was used only in weapons but it is believed that the fusion power reactors
will become a reality in the first decades of the following century.
In weapons, lithium is used both as a means for storing deuterium (DLi contains more
deuterium per unit volume as liquid D2 does) and as an essential fusion fuel. Furthermore, the
main way to produce tritium, a remarkable fusion partner, is the reaction 6Li(n,α)T. The
countries which have developed thermonuclear weapons, have developed technologies for the
large scale 6Li production. The separation technologies and production capacities are kept under
secret.
In Romania, the domestic demand for lithium isotopes, used especially for research
purposes, was of a few hundreds grams per year. This quantity was small but it could not be
bought from foreign market because of embargo reasons. The decision was taken to achieve an
installation capable to supply the domestic needs. In 1976, after a preliminary study, the
isotopic exchange between Li-amalgam and aqueous LiOH was chosen as basic reaction for the
separation process,
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Li+ + 7Li(Hg) ↔ 7Li+ + 6Li(Hg)
(1)
The isotopic effect of this reaction at ambient temperature is ε=0.05 [1]. The multiplication of
this effect by countercurrent exchange in a column has the advantage of simple refluxes but has
the shortcoming that the side reaction (2) take place.
Li(Hg) + H2O = Li+ + OH- + ½ H2
(2)
Some years later it was disclosed that the same reaction (1) had been used in United States in
Colex process for large scale 6Li production [2]. This coincidence is very interesting and it is
possible that some other countries had used the same reaction.
2. Separation unit
Every separation unit based on the reaction (1) should have the same main components:
1) separation column, 2) electrolyser, 3) column for amalgam decomposition, and 4)
evaporator. The technological solution chosen for these components may be very different. It
seems that for large-scale equipment some special packing columns were developed. In our
case, the scale was small so that we used a spray column in which the aqueous LiOH moved up
in an unpacked cylindrical tube and a fine spray of Li-amalgam fell down.
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The separation unit is presented schematically in Figure 1. The isotope separation takes
place in column 1, which is a simple vertical Pyrex glass tube. The amalgam dispersion is
carried out in the electrolizer 2 by electrolysis of an aqueous LiOH and using mercury jet
cathodes. At the bottom of the column 3, the amalgam and water are circulated countercurrently
and the decomposition reaction (2) is catalyzed by austenitic stainless steel packing. The
formed aqueous LiOH is introduced at the lower end of the exchange column. The mercury at
the bottom of column 3 is replaced automatically with water, so that in the exchange column 1
the amalgam and the LiOH aqueous solution have the same flow rate. In column 1, the
amalgam and electrolyte have the same Li concentration of 0.23 M to 0.5 M. An evaporator 4
maintains a high electrolyte concentration at the column top placed between 2 and 4.5 M, LiOH
solubility in water being 4.7 M. The hydrogen formed by amalgam decomposition in 3 is
removed automatically in such a way that the electrolyte flow through the column1 is little
disturbed.
O2
8
2
4
1
7
5
6
H2
Fig. 1. Scheme of the
experimental unit.
1: Separation column,
2: electrolyzer,
3: column for amalgam
decomposition,
4: evaporator,
5: mercury,
6: amalgam,
7: water,
8: electrolyte.
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3. Mercury jet cathode and drop size distribution
The drop size distribution has a determinant role for the mass transfer, that is for the
column efficiency. For a given amount of dispersed phase, the smaller the drops, the larger the
interfacial area. On the other hand, the smaller the drop diameter, the smaller the drop fall
velocity. There are many experimental and theoretical studies concerning the dispersion of a
fluid phase into another one. Still, the amalgam spray is produced in such peculiar conditions
that no appropriate data are available to permit the prediction of the drop size distribution.
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In order to fulfill certain requirements for the considered spray column, several ways to
produce mercury jets were tested. Finally, small orifices made in thin Plexiglas plates were
used. As it is believed that the drop size has an important role in the process efficiency, we have
studied experimentally the drop size distribution of lithium amalgam produced by mercury jet
cathode [3].
The drop size distribution can depend on many geometrical and physicochemical
parameters. To maintain the volume of the experimental work to an acceptable level, a limited
domain of experimental parameters was taken into account. All experiments were done with a
LiOH aqueous solution of 3 mol per liter, and at a temperature of about 20 0C. These values
seemed to be suitable for practical purposes as the LiOH solubility in water is 4.8 mol/l, and the
electrolysis is usually carried out at ambient temperature.
Due to the conical form of the orifice, the diameter  m of the narrowest orifice end was
the only geometrical parameter that has a determinant role in the jet hydrodynamics. We
measured the mercury flow through orifices made in plates of thickness 0.1 to 0.3 mm and we
found that the plate thickness had no influence upon the flow rate. At a given temperature, the
diameter  m and the pressure drop through the orifice completely determined the mercury flow
rate. The pressure drop was given by the hydrostatic pressure p of the mercury that lay above
the orifices.
For an isolated jet, the remaining parameters that could influence the drop size
distribution are the diameter  m , the pressure drop p , and the electrolysis voltage. The
experiments were carried out with an orifice diameter varying from 28 to 70 m, and with a
pressure drop varying from 200 to 800 mmHg.
The voltage applied to the input wires of the electrolysis cell was 4.5 and 6 V. For an
orifice with  m  56.5 m, and in the experiment with three orifices, the measurements were
also carried out for zero voltage.
For the electrolyses to occur, the applied voltage had to be larger than the reversible
potential that is about 3 V. The voltage difference in excess of 3 V essentially represented
ohmic losses in the electrolyte. As the surface of the thin mercury jet was small in comparison
with the anode surface, the largest fraction of the ohmic losses was in the jet vicinity.
Lithium concentration in the amalgam depended on many parameters. For the
experimental conditions mentioned above, at a voltage of 4.5 V, the lithium concentration in
amalgam ranged between 0.23 and 0.45 mole/l. At a voltage of 6 V, the lithium concentration in
amalgam was between 0.8 and 1.6 mole/l. The variation in that interval depended on the orifice
diameter  m , and on the mercury pressure p . Larger lithium concentration corresponded to
smaller orifice diameter and to a lower mercury pressure.
Most experiments were done with isolated jets, that is with plates containing a single
orifice. However, in a practical application [4] hundreds of orifices arranged in rows were used.
Neighbouring jets could interact hydrodynamically between themselves and, on the other hand,
around jets the symmetry of the electric field is disturbed. It follows that the number of jets and
their spatial arrangement could influence the drop size distribution. For this reason two more
plates were made and experimented. One plate contained two orifices having diameters  m of
58.1 and 59.8 m, and the distance between the orifice centers of 0.5 mm. Another plate
contained three orifices with centers placed in their corners of an equilateral triangle with sides
of 1 mm, and orifice diameters  m of 60.2, 58.9 and 57.3 m. When more then one jet was
present, they screened one another reducing the access of the electrical field and lowering the Li
concentration in the formed amalgam. For example at a voltage of 6 V and a pressure of 800
mmHg, Li-concentration was 0.79 mol/l for one orifice (diameter 59.3 m), 0.61 mol/l for two
orifices, and 0.44 mol/l for three orifices.
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For drop size determination an optical method was developed [3]. A total of 51 distinct
experiments were made. In the limits of experimental errors, the same drop size distribution
was obtained for all experiments so that all results were put together, producing the general
diagrams presented in Figure 2.
Dotted Curve
Gaussian Fit
DN=183.7 m
N=42.8 m
1.0
Volume Fraction, %
0.8
Number Fraction, %
b)
a)
1.0
0.6
0.4
0.2
0.0
Dotted Curve
Gaussian Fit
DV=209.7 m
V=39.1 m
0.8
0.6
0.4
0.2
0
100
200
300
400
0.0
0
Drop Diameter, m
100
200
300
400
Drop Diameter, m
Figure 2. General drop size distribution of all experiments.
The abscissa of an experimental point is the middle of an interval of 12 μm;
the ordinate cumulates the results from that interval.
The high preservation degree of the drop size distribution is a surprising result if one
takes into account the large variation of the experimental parameters. The orifice diameter was
increased more than two times; the pressure was increased four times. The independence of
drop size distribution of the voltage is most curious. At zero voltage, the pure mercury jet was
dispersed into droplets. At 6 volt, the jet was charged strongly at its surface with lithium, giving
rise to a substance with different physico-chemical properties. In addition, over the usual
surface tension, an electrostatic tension was superimposed.
It is noted that for small orifices, when the jet started with a diameter as small as 28 m,
the diameter of the mean drop volume was comparatively huge. The mean drop volume
corresponds to a cylindrical jet segment having its height more then two hundred times longer
than its diameter.
Some practical significance of the results is to be noted:
a) Varying the implied parameters in the experimented range can not modify the drop
size distribution, given in Figure 7.
b) When orifices for a multiple jet cathode are made, it is not critical that the orifice
diameters be made constant.
c) When a multiple jet cathode is operated, varying the mercury pressure above the
orifices may modify the mercury flow rate, amalgam dispersion remaining unchanged. We have
established empirically the following relationship for the mercury flow rate through an orifice
Q  0.001253  m2
p  0.1538 m  2.385
(3)
where Q is the mercury flow rate in ml/h, p is the mercury pressure in cm Hg, and  m is the
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diameter of the narrowest orifice end in μm. Because of the interfacial forces, the mercury
passes through an orifice only when pressure surpasses a positive threshold that depends on the
orifice diameter and on the presence of the electrical field.
4. Characteristics of the separation unit
The separation unit has a series of important advantages:
a) favourable report between the velocities of reactions (1) and (2),
b) great separation power density,
c) small mercury inventory, and
d) small equilibration time.
The separation capacity of a column was 570 mol swu/year but we believe that a column with
at lest 200 times greater capacity may be realised.
There are no published data regarding the plant capacity realised in other countries.
Still, we can have an idea about that from the following information. A quantity of about 117
tone Hg was released in air and water at the lithium isotope separation plant at Oak Ridge,
Tennessee, USA. The whole Hg quantity used in our separation unit was about 0.1 tone.
In the last time we have begun to consider some other separation processes based on Li
complexation and on the combination of Li complexation with electromigration.
REFERENCES
[1] A. A. Paklo, J. S. Drury and G. M. J. Begun, Chem. Phys. 64, 1828 (1976).
[2] E. A. Symons, Sep. Sci. Technol. 20, 633 (1985).
[3] I. Hodor, G. Mihăilescu, A. Chezan, and D. Radu, Chem. Eng. Comm. 177, 231 (2000).
[4] I. Hodor, Proceedings of The International Symposium on Isotope Separation and Chemical
Exchange Uranium Enrichment, Tokyo 1990, (Y. Fujii, T. Ishida, K. Takeuchi, eds., Bull.
Res. Lab. for Nucl. Reactors, Tokyo Institute of technology, Tokyo, 1992), pp. 333-335.
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