Note: Descriptions are shown in the official language in which they were submitted.
1144343
The present application relates to a method for separating isotopes.
Isotopes of an element exhibit different physical properties which can be used
to detect and separate isotopes.
The most effective method for the separation of isotopes is the mass
spectrographic method which,permits complete separation of the isotopes in one
stage. This method uses the differences in electrical and magnetic deflect-
ability of identically charged ions of different masses. Due to the fact
that a mass spectrograph uses a stream of ions, however, the quantity of
matter that can be recovered is negligible. Consequently, mass spectrographs
are used only as detectors but not for isotope enrichment.
One method which can produce, under suitable conditions, separation
almost as effective as the mass spectrograph is based on the principle that
monochromatic, laser-produced light can be directed to excite one isotope of
a mixture and thus convert it into a reactive state, while the other isotopes
remain in the nonreacti~e basic state. This method is not yet widely used in
industryJ however, since the necessary highly monochromatic, tunable lasers
of high intensity are fairly expensive and not widely available, and the yield
of the process is also relatively small.
Diffusion separation methods, namelyJ partition diffusion, thermo-
diffusion, and pressure diffusion, are used in industry for isotope enrichment.
Partition diffusion is based on the concept that within a mixture of
gases, molecules of a lower molecular weights will, on the averageJ travel at
higher velocities than molecules of higher molecular weight. Consequently a
porous barrier is used, which permits passage of individual molecules, but
does allow bulk flow of the entire gas. The probability of a gas molecule
crossing this partition is proportional to its velocity. As a result, the gas
which diffuses through the barrier contains a higher proportion of the lighter
isotope than the undiffused gas.
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~443~3
In thermodiffusion, a temperature gradient is established in a fluid
isotope mixture. Due to this gradient, one isotope will concentrate in the
hot region, and another in the cold region.
In pressure difEusion, isotopes are separated by allowing a gaseous
compound to flow through a properly shaped separating nozzle.
In these diffusion methods, an individual separating stage produces
only a slight change in the composltion of the isotope mixture. Consequently,
it is necessary in order to obtain the desired enrichment, to connect a
plurality of stages in series, which is known as a cascade. Several thousand
stages may be necessary to obtain the desired enrichment.
It is also known to separate isotopes by means of centrifuging a mix-
ture, a method which requires extremely high speeds to obtain any substantial
separation from a single stage. This centrifuging can also be combined with
a diffusion method.
Also used in industry are isotope separations by means of fractional
distillation and exchange reactions.
The fractional distillation method is based on the dependence of the
vapor pressure of a substance on its mass.
The exchange reaction method utilizes the dependence of an equili-
brium constant on the mass of a compound. Thus, if equilibrium is established
between for instance, a gas and a liquid, the concentration of a particular
isotope will be different in the gas and the liquid. This method is used
mainly in the production of heavy water.
Heavy water is also produced in smaller quantities during the electro-
lysis of water. Due to kinetic effects, the ratio of hydrogen to deuterium is
much larger in the gas leaving the cell than in the water and consequently,
deuterium accumulates in the aqueous phase.
Chromatographic methods are also suitable for the separation of iso-
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topes but are uneconomical as enrichment methods.
Moreover, isotopy effects are known between two liquid phases of a
substance, and these constitute the basis for a corresponding enrichment
method. For example, lithium isotope enrichment can be effected by means of
extraction of lithium ions in water or organic solvents and from metallic
lithium in amalgam with liquid mercury.
It can be seen that the known methods for separating isotopes are
not entirely satisfactory, especially in terms of economy.
It is known that gases in the supercritical state have the capacity
to absorb certain compounds depending on their precise thermodynamic state.
A change in thermodynamic state can result in the release of the absorbed
substances.
The critical temperature of a gas is well known to physical chemists
as the temperature above which the gas cannot be liquified regardless of the
pressure to which it is subjected. The pressllre necessary to liquify the
gas at its critical temperature is the critical pressure.
German Auslegeschrift No. 1,493,190 discloses the separation of liq-
uid and/or solid mixtures of substances containing organic compounds and/or
compounds containing organic groups by treating the mixtures with supercriti-
cal gases and, after separating the charged supercritical gas phase, recover-
ing the compounds contained therein by relaxation and/or changes in temperature.
This method is used with difficulty volatile and temperature sensitive mix-
tures. The separation of mixtures with similar boiling points is also
possible.
The present invention is directed to providing a method for the
separation of isotopes which operates as economically as possible.
The present invention is also directed to providing a method for the
separation of isotopes which are present in solid form or present in a liquid.
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114~343
According to the present invention there is provided a method for
separating isotopes of an element comprising contacting a material comprising
the isotopes with a gas or gas mixture under supercritical conditions to form
a gaseous extraction phase which is richer in one isotope than the material
comprising the isotopes and a raffinate phase which is poorer in the one
isotope than the material comprising the isotopes, and separating the gaseous
extraction phase from the raffinate phase.
The thermodynamic state of the separated gaseous extraction phase
from the gaseous extraction phase then can be changed in order to recover a
substance which is richer in the one isotope. A second substance which is
poorer in the one isotope can be recovered from the raffinate phase.
Thus, it has now been found that treatment with supercritical gases
can be used to separate not only chemically different substances but also the
isotopes of one element.
It is to be understood that both the foregoing general description
and the following detailed description are exemplary, but are not restrictive
of the invention.
The sole drawing figure is a schematic view of an apparatus used to
separate isotopes by the method of the present invention.
The process of the present invention comprises contacting a material
containing at least two isotopes of an element with a gas under supercritical
conditions, to form a gaseous extraction phase and a raffinate phase. The
gaseous extraction phase and the raffinate phase then are separated from each
other. The extracted material in the separated gaseous extraction phase is
then recovered by changing the thermodynamic state of the gaseous extraction
phase, such as by reducing the temperature and/or pressure of the gas, or by
reducing the pressure and increasing the temperature of the gas.
Importantly, it has been found that the isotope ratio in the gas
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39~3
phase and in the raffinate phase will be different from the ratio in the
original material. In the case of a two isotope system~ the gaseous extrac-
tion phase will be richer in one isotope than the original material and the
raffinate phase will bc richer in the other isotope. By proper selection of
extraction conditions, and/or by extracting a number of times, it is possible
to considerably concentrate a particular isotope.
The method of the present invention can be used, in principle, to
separate any desired isotope mixture, including those containing more than two
isotopes. The method can be advantageously used for separation of commercially
important isotopes, such as hydrogen-deuterium, Li6 _ Li7, and U 5 - U238.
The starting materials comprising the isotopes, for use in the
present process, can be elements, but are generally compounds containing sever-
al isotopes. An example of such a compound would be NaCl, containing both
C135 and C137.
The method of the present invention is suitable for use with start-
ing materials which are present in solid or finely comminuted form, in
solution in water, in dissolved form in organic or inorganic solvents, or in
a melt.
In order to separate isotopes, the method of the present invention
uses an extraction with a gas or gas mixture at a temperature above the criti-
cal temperature and a pressure at or above the critical pressure, a state
which will be referred to as "supercritical conditions". Advantageously,
the extraction will take place at a temperature up to 100C above the critical
temperature and a pressure up to 350 bar, above the critical pressure.
A variety of gases and gas mixtures may be used as the gas which is
under supercritical conditions and which serves as an extraction agent. Sever-
al guides should be used in the selection of an appropriate gas. The gas
should not be reactive with any of the materials to be separated. The critical
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1~443~3
temperature for the gas should not be so high that heating the material ex-
tracted to above the critical temperature will cause undesirable effects,
such as decomposition. The critical temperature must also not be so low that
cooling to subcritical condition is difficult for standard industrial equip-
ment. For these reasons, the critical temperature of the gas chosen will
generally be between about 10C and about 300C. Some typical gases suitable
for extractions, along with their critical temperatures and pressures are:
gas TC P(atm)
Carbon dioxide 31 73.9
Ethane 32.2 48.2
Propane 96.7 42
Ch F3 25.9 46.9
~2 36.5 71.7
CClF3 28.9 38.2
C2H2F2 30.1 43.8
CHClF2 96 48.5
Ethene 9.9 51.2
Propene 91.9 45.4
A preferred gas for use as an extraction agent in the present method
is carbon dioxide.
If desired, a suitable entrainer can be added to the pure gas or gas
mixture to improve the absorbability and selectivity of the gaseous extraction
agent with respect to certain materials. The entrainer need not be under
supercritical conditions. Water and benzene are examples of typical
entrainers.
In the present process, the material to be extracted is contained in
an apparatus suiLable for withstanding high temperature and pressure, such as
an autoclave. Gas under supercritical conditions is supplied to the autoclave
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4~43
and allowed to thoroughly contact the material to be extracted. Preferably,
the gas will be supplied and removed continuously.
The gas removed from the autoclave is brought to a new thermodynamic
state, usually by reducing temperature and/or pressure. The gas in its new
state, partially or completely loses its capacity to absorb the extracted
components, which will separate from the gas and may be recovered. The gas
may then be recycled to the autoclave.
A particular material will be subjected to the extraction process for
a period of time which may be minutes or hours. The material may be supplied
to the autoclave continuously or it may be supplied in batches. After the
extraction period, the material is removed from the autoclave and separated
from any absorbed gas which can then be returned to circulation. The parti-
cularly desired isotope may be contained in this raffinate phase, or in the
gas phase, depending on exactly which isotopes are being extracted.
As with other methods of isotope separation, a plurality of extrac-
tion systems may be set up in a series, known as a cascade, to further enrich
the desired isotope in the raffinate or in the extract. In that case, the
starting material for the second and subsequent extraction stages will be
either the material recovered from the gas phase of the previous extraction
or the material recovered from the raffinate phase of the previous extraction.
The present invention will now be explained with reference to the
apparatus shown schematically in the drawing figure.
The apparatus comprises an autoclave 1, a liquid pump 2 to turn over
and increase the pressure of a gas, a conveying device (e.g. pump) 3 for the
continuous supply of the isotope mixture, a separator 4 for the raffinate, a
separator S for separating the extract from the gas, a heat exchanger 6 for
heating the starting isotope mixture, a condenser 7 to liquify the extraction
gas, a heat exchanger 8 to heat it to the supercritical state, a choke valve
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11~43~:~
9 for relaxing the raffinate phase, a choke valve 10 associated with the
separator 5 for relaxing the extract phase, a reservoir 11 for the isotope mix-
ture and a reservoir 12 for the gas.
With continuous operation, a gas which is under supercritical con-
ditions of pressure and temperature, enters into the isotope mixture at the
bottom of the autoclave 1 through a line 13. The isotope mixture is intro-
duced into the autoclave through a line 14. Due to its lower density, the gas
flows through the isotope mixture and is thus charged with one or more
components of the mixture, thereby forming a charged gaseous extraction phase
and a raffinate phase. lhe charged gaseous extraction phase is removed from
the upper region of the autoclave 1 and then relaxed throughchoke valve 10.
Due to the reduction in pressure and cooling, the gas, now in a new thermo-
dynamic state, completely or partially loses its capability to absorb the ex-
tracted components, so that these components partially or completely separated
from the material in the autoclave, can be recovered at the bottom of separator
5 in liquid or solid form as extract. The relaxed and regenerated, gaseous
extraction agent leaves separator 5 through line 16 and, after being condensed
in condensor 7, is again fed to liquid pump 2 with connected heat exchanger 8
so that a supercritical gas can again be produced. However, it is also pos-
sible to increase the pressure of the gas while in the gaseous state, rather
than in the liquid state as is done in the embodiment of the apparatus shown
in the drawing.
In the area at the bottom of autoclave 1, the mixture now poorer in
extracted substances is removed together with the extraction agent dissolved
therein as the raffinate phase, and is relaxed in separator 4 to recover the
gaseous extraction agent. The resulting raffinate is extracted at the bottom
of separator 4 and the gas is returned to circulation through line 15 to con-
tinue the separation process.
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114~39~3
Continuous operation of the system is made possible by liquid pump 2
and heat exchanger 8. The addition of gas from reservoir 12 is required to
start up the system and to replenish gas losses occurring during operation.
The autoclave can also be filled in batches with a sample of the
isotope mixture to be separated. The gas circulation again produces a reduc-
tion of the isotope mixture in autoclave 1 in a particular component which
collects in separator 5 by way of relaxation of the gaseous extraction phase.
After a certain turnover period for the extraction agent ~extraction period)
the raffinate phase is removed from autoclave 1. Operation in batches is thus
particularly suitable for determining the sequence of the extraction of sub-
stances and the changes in the composition of the extract or raffinate, over
a period of time. It is therefore suitable for use with solid matter.
The method according to the present invention was used to perform the
experiment set forth in the following example in the apparatus described above.
The following example is given by way of illustration to further ex-
plain the principles of the invention. This example is merely illustrative
and is not to be understood as limiting the scope and underlying principles of
the invention in any way. All percentages referred to herein are by weight
unless otherwise indicated.
EXAMPLE
A quantity of 1000 g of a pure, aqueous 25% NaCl solution with a
natural isotope ratio of Cl35 : Cl37 equal to 3.124 was gassed with a stream
of C2 at a rate of 9.53 kg/h at 280 bar and 60C. In separator 5, the charg-
ed gas was separated from the extracted substances at 56 bar and 29C and the
gas was returned to circulation. During the experiment, samples of the sodium
chloride solution were taken from the autoclave every two hours and were
examined in a mass spectrometer for their ratio of Cl35 : C137 (mass 58 : mass
60). The mass spectrometer analysis resulted in the isotope ratios in the
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3~
following table:
Sample No. Extraction Isotope ratio Separating Factor
Time (h) Cl /C13 (rh) ~ h/ o
. _ .
1 0 3.12 (rO)
2 2 3.32 1.06
3 4 3.38 1.08
In the above-described experiment, a clearly noticeable shift in the
isotope ratios could be detected after an extraction period of two hours.
With batchwise operation of the system, separating factors c~ = 1.06
and 1.08, were realized after two and four hours, respectively. At extraction
periods of greater than four hours, for example after six hours, the C135
isotope is extracted with preference, in contrast to Samples 2 and 3 in the
above table, in which C137 is extracted preferentially.
Although in the above sample supercritical CO2 was selected as the
extraction agent, other supercritical gases as well as mixtures of super-
critical gases with any desired mixing ratios and without the addition of an
uncritical entrainer can be used as the extraction agent.
It will be understood that the above description of the present
invention is susceptible to various modifications, changes and adaptations,
and the same are intended to be comprehended within the meaning and range of
equivalents of the appended claims.
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