CA1185074A

CA1185074A - Separation of isotopes by controlled distribution

Info

Publication number: CA1185074A
Application number: CA000386288A
Authority: CA
Inventors: Josef Katzer; Josef Cabicar; Karel Stamberg
Original assignee: Czech Technical University In Prague
Current assignee: Czech Technical University In Prague
Priority date: 1981-08-28
Filing date: 1981-09-21
Publication date: 1985-04-09
Also published as: IT8123812A0; DD210517A3; IT1138580B; GB2104797B; JPS5855030A; AU7469981A; AU547079B2; JPH0253087B2; FR2512354A1; FR2512354B1; GB2104797A; DE3135540A1

Abstract

ABSTRACT OF THE DISCLOSURE:
The invention relates to a process for the separation of isotopes in two-phase systems, which process is based on the knowledge that the distribution of isotopes can be controlled by the selection of phases composition, that is by their structural and material nature, it can be controlled by concentration of isotopes and ligands, further by temperature, illumination, hydrodynamic parameters and by the time interval of contact of phases. For example uranium, lithium and nitrogen isotopes from aqueous and non-aqueous solutions, if need be, from their gaseous mixtures, if they are in contact with solid sorbents or with liquid extraction agents, can be separated with high efficiency by the process according to the invention. A chemical separation method is in question in the principle.

Description

sc~

The inven-tion relates to a process for the separa-tion of isotopes in two-phase systems, for example of the type solid phase-liquid phase, liquid phase~liquid-phase, solid phase-gaseous phase. This process is based on the knowledge that the distribution of isotopes can be controlled by the selection of phases composition, that is by their structural and material nature. It can be controlled by concentration of isotopes and ligands, further by temperature, illumination, hydrodynamic parameters and by the time interval of contact of phases. For example, according to the process of the inven-tion, uranium, lithium and nitrogen isotopes can be separated with high e~ficiency from aqueous and non-aqueous solutions or from their gaseous mixtures, when said solutions or mixtures are brought into contact with solid sorbents or with liquid extraction agents. This process according to the invention relates to a chemical separation.
The chemical methods that have been published and used for the separation of isotopes (see for example: Laskorin B.N. and col.: Uspechi Khimii, band XLIV, n 5, 761-780 (1975); Carman de J. and col.: Report AAEC/LIB/BIB - 273, 1970) are based on isotope-exchanging reactions, equilibrium constants of which, that is elementary separation factors as a rule, approximately vary in the limits from 1.01 to 1.0001 depending on mass weight of the relevant isotopes; the lower values being related to isotopes of heavy elements, for example of uranium. In the latest years, many attention has been paid to the s~paration of uranium isotopes with solid sorbents and liquid extraction agents. Namely German patents

2,235,603(February 1, l973)~2t349~595(April 18, 1974), 2,623,891t'lay 5, 1977), French patent 1,600,437 (September 4, 1970) and Ponta A. Calusaru A , ~,JD

Isotopenpraxis 13~ No. 6, 214-216 (1977); ibid 11, No. 12, ~22-~26 ~1975)) can be cite~.
An effort fo~ keeplng conditions for the course of isotope-exchangin~ reaction of the type:
238u ~ 235u _ ~ II I

ls the common feature o~ all the published processes, where roman numeral indicates phase.
The initial works directed to the separation of uranium (VI) isotopes were nevertheless unsuccessful due to the fact that a higher separation factor than approximately 1.00009 was not succeeded to obtain reproducibly when ion exchangers and chromatographic processes were used. That is why attention was paid to the systems in which uranium is present in various valency states, and that is U(VI) - U(IV) and U(III) - U(IV). Electron-exchanging reaction takes simultaneously place in these systems, at which reaction higher valency state of uranium is always enriched with lighter isotopeO Further, it was really reached substantial enhancement of separation efficiency in the connection with isotope effect of complexation reactions and using suitable type of sorbent, i~ need be, of extraction agent, selective for one of uranium valency form. From published data ~for example: M. Seko, T. Miyak~, K. Inada - Nucl. Technology 50, 178-186 (1980); Report INFEC/DEP/WG. 2/31 - ~<On the Technical, Economical and Safeguard Informations of the Different Enrichment TechnlquPs~, 15.3.1979) it is evident that it was succeeded to shift the value of elementary separation factor to the level approximately 1.001 what is alleged the lower limit of economic utilization of the process. (For example elementary separatlon factor of the process of gas diffusion, used ln operational criterion for separation of uranium isotopes, is approximatel~ 1.0027). I-t can be 5~37~
therefore stated that the results reached are already economically and technologically interesting. Operational utilization is not nevertheless unambiguous and that is mainly due ~o the limiting value of elementary separation factor with which value building up devices having a large number of stages, a large hola-up of enriched uranium and a relatively long time necessary for reaching the stationary state of the process, is connected. The necessity of repeating many times the process of uranium reduction ana oxidation is connected with substantial requirements to the relevant reduction and oxidation agents and, i.f need be,~~o the energy of these reactions.
These drawbacks are overcome with a process accord-ing to the present i.nvention.
Therefore, the present invention relates to a process ~or the separation of isotopes, based on the method of controlled distribution using the concentration isotopic effect in two-phase systems. The subject matter of the process according to the in~ention is characterized in that one of the phases is an a~ueous or non-aqueous solution of isotopic components and o a ligand,having a concentration between 10 6 M and the saturated solution, or a gaseous mixture of isotopic components or a gaseous mixture of isotopic components and an inert gas, said isotopes being, in the starting solution or mixture, in molar ratio of 1:105 to 1:10 5; and the second phase is a solid sorbent, such as styrene-divinylbenzene ion exchangers, bio-sorbents on the basis of mycelium of lower fungi or sorbents on the basis of cellulose, or an extraction agent such as tributyl phosphate and trioctyl amine, supported or not on a carrier such as teflon (trademark for polytetra-fluoroethylene), silica gel or cellulose.
T.he two-phas~ system may a~tageously exhibits a non-.~ - 3 -~ ~5~4 linear equilibrium isotherm for sorption and/or desorption, or for extraction and/or re-extraction.
After bringing both phases into contact, the rate of transport of isotopic components from one phase into another is not equal and retardation o~ isotopic exchange takes place by complexation of isotopes with ligands such as carbonate, sulphate, citrate, chloride and ethylenediamine tetraacetate ions, or by using sorbents and extraction agents with chelating functional groups such as carboxyl and hydroxyl groups, groups on the basis of phosphorus, nitrogen and sulphur, and/or by operating in darkness or in the light having a wavelength between 2.5 x 102 and 109 nm. -2 The contact time o phases is selected between 10 and 106 s, at a temperature selected between 102 and 103K, under a number of stirrer revolutions selected between 10 2 and 104 revolutions per s, a flow rate at column arrangement selected between 10 6 and 10 1 m/s and with a size of particles of sorbent selected between 10 6 and 10 2 m.
For carrying out the desorption or re-extraction of isotopic compo-nents, solutions of compounds selected from the group consti-tuted by the sodium carbonate, ammonium nitrate, sodium chloride, sodium sulphate, sulphuric acid, nitric acid and hydrochloric acid and/or compounds which contain complexing ligands and which are selected from the group constituted by the citrate, ethylenediamine tetraacetate and isobutyrate ions are used,alone or in a mixture, said solutions having a total concentration from 10 3 M to saturated solution~
For example, with a process according to the inven-tion, an aqueous solution of isotopic components such as 235U(VI) and 238U(VI)and aligand such as nitrate, sulphate and carbonate ions, the total concentration being between 10 2 and 1~ ~ andsaid isotopes 235U and 238U heing in the starting solutions ,.

~5~7~
in a molar ratio of 1:500 to 1:100, can be preferably used.
Also, ion-exchan~er on the basis of styrene-divinylbenzene copolymer or sorbents on the basis of mycelium of lower fungus of the race P. Chrysogenum can be used as a solid sorbent.
The contact time of phases is selected between 102 and 104 s, in darkness or in a light having a wavelength between 250 and 650 nm. ~urthermore, when the number of stirrer revolutions is selected between 0.800 and 12.00 revolutions per s, or when the flow rate at column arrangement is selected between 10 ~ and 10 1 m/s, the solid sorbent has a size of particles selected between 10 4 and 10 3 m and the temperature is selected between 273 and 373K. Solutions of sodium chloride ~ sodium carbonate, or sodium carbonate, or of hydrochloric acid having concentration between 10 1 and 1.5 M can be preferably used for desorption.
The process according to the invention is character-ized by substantially higher separation factors than those reached up to now with isotope-exchange reactions. The maximum values of elementary separation factors vary between 1.02 and 1.10 in the course of sorption, if you like also desorption of uranium isotopes. It is especially advantageous to use uranium (VI~ solutions and solid sorbents with high selectivity but not with too fast kinetics o sorptioniprocess, if need be, desorption process. The economical and technological signifi-cance of the process for separation of isotopes according to the invention is considerable. A substantial enhancement of the value of elementary separation factor represents, namely for the separation of uranium isotopes, a decrease of the total number of separating stages by one to 1.5 order, making shorter the time necessary for the establishmentofastationary state and suppressing the hold up of enriched uranium in the device approximately by an order as well. It is possible to operate ~5~
with solutions of hexavalent uranlurn, it is not necessary to change its valency state and it is not necessary to transfer uranium of UF6 before enrichmen^t - 5a -'.P~

as it is up to noW at operationall~ employed enrichmerlt stages.
Another significant moment is the fact depleted uranium ~7hich is produced by present processes can be used as an input material and it is real to reduce content of 35U isotope in this waste at least to its half.
The subject matter of new process for separation of isotopes, namely on the example of sorption, can be further described in following way:
a) By bringing into contact solid sorbent or liquid ~0 extraction agent with aqueous solution of isotopes, for example with mixture 235U(VI) + 8U(VI), where concentration of one of the isotopes is at least by half to one order higher than concentration of the other one, sorption of both isotopes takes place simultaneously, the rate of sorption of each isotope being nevertheless proportional to the driving force of the process, that is to the relative displacement from equilibrium and to the value of mass-transfer coefficient.
This generally different rate of sorption of isotopic components, which is conditioned by retardation of isotopic exchange (see point g), is the nature of effect which we have called concentration isotopic effect. The rate of sorption can be described for example by a relationship:
R = kq ~ ~q = ~c o ~c, where R designate the rate of sorption, if need be, of desorption respectively of transport of isotope from one phase into another, kq, kc are the mass-transfer coefficients and ~q, ~c designates the driving force of the process, respect-ively concentration gradient in phase aq and c.
b) Driving force of the isotope transport from one phase (here aqueous) into another is a function of the 7 '~
initi.al concentrations of i.sotopes in phases on the one hand and a function of shape of equilibrium isotherm in the regions of variations of isotope concentrations on the other hand, which variations take place in the course of sorption~
c) In relation to isotope concentrations in the solution and to the -type of limiting process of sorption it is advantageous to chose such a sorbent which exhibits in the region of concentration variations as far as possible difEerent slopes of section of equilibrium isotherm, cor-responding to single isotopes, as possible. It seems ad-vantageous that the part of the equilibrium sorption isotherms which corresponds to the isotope with lower concentration, would have a gradient approximately between 10 and 103 while the part corresponding to the isotope with higher concentra-tion :is approximately of 10.
d) As the choice of sorbent has its limitation in properties of sorbents alone, it is also possible to adjust the composition of the solution to the properties of sol~ent~
In this sense~vari~tion of the absolute concentration of isotopes comes into consideration, for example by dilution, for keeping isotopic ratio at the original value, further adjustment of solution cc~2osition being obtained by addition of further components such as co~plexing agents.-e) The mass-transfer coefficient is a function of solution composition (isotope concentration) and o~ the kind of sorbent, of extraction agent, if need be, on the one hand and a function of temperature and hydrodynamic parameters (the rate of stirring, flow rate, size of sorbent particles and similarly) on the other hand; due to the fact ~.at concentration dependence is one of the most important, these coefficients have generally for each isotope different value, the absolute value b~ing neV2rtheless determined mainly by a kind of sorbent.

5 ~3 ~1~

We have found that it is advantageous to operate with sorbents for which the mass-transfer coefficient is between 10 5 and 10 3 s 1 f) For example the so-called bio-sorbents (see czechoslovak author's certificates n 155.833 of September 15, 197~ to K. Stamberg, J. Stamberg, J. ~atzer, H. Prochazka, R.
Jilek, P. Nemec and P. Hulak; n 184.471 of March 20, 1975 to V. Votapek, E. Marval, R. Jilek,and X. Stamberg and n 189,198 of January 29, 1976 to R. Jilek, Z. Slovak, M. Smrz and K.
Stamberg), further ion exchangers on the basis of styrene-divinylbenzene copolymers, ~xtraction agents alone or`supported on a carrier such as teflon (trademark), cellulose, silica gel and similar, can be employed as sorbents having the above-cited equilibrium and kinetic properties.
g) Simultaneously with sorption, if need be, with extraction, of isotopes, isotopic exchange starts to take place practically immediately,which exchange suppresses the differ-ences in sorbed amounts of single isotopes resulting from various rates of sorption. Negative effect of the process of isotopic exchange, the rate of which is approximately equal to the rate of sorption of isotope with higher concentration, can be compensated, if you like interferred, on the one hand by the selection and proper choice of sorbent type and solution com-position and on the other hand by operating at lower tempera-ture and under limited light access. Retardation of isotopic exchange can be also reached by complexation of the isotopic component in one of the phases, that means either by the addi-tion of suitable liga.nd in~o solution or with the help of sorbent, if need be, of extraction agent, having complex-0 forming functional groups.h) What was given above as an example for kinetics or sorption of isotopes, is also true, in its principle, for 3t`~ 7~
kinetics of desorpti.on, if you like re-extraction ~nd it ~ccomp~nylng lsotopic exch~nge.

8a -Example 1 2 g of sorbent of the trademark Ostsorb MV 6/5 (a sorben-t on -the basis of fungus of the race P. chryso~enum), activated with hydrate titanium (IV) oxide, in Na-form Iswollen and centrifuged) has been under mixing, at stirrer speed 5 revolutions per s, at temperature 293K and withou-t admission of ligh-t brought into contact with 60 ml of solu-tion of 0.01 M
uranyl nitrate, pH of which had been adjusted to 3.5. The ratio of isotopes U : U in the initial solution was 0,725x10 2 (a compound prepared from na-tural uranium was dealt with). In the course of sorption reaction, samples of liquid phase were taken off and the total concentration of uranium has been determined in these samples (by spectro-photometric method using reagent arsenazo I (trademark)) and the ratio of isotopes 235U : 38U was determined by means of mass spectrometer of the Aldermaston-Micromass 30 Company (trademark). The value of separation factor has been calculated as a function of time from the results obtained:

~ e ; 0 o .6xl03 1.2~lo3 2.~xlo3 3.6xlo3 7.2xlo3 14.4x103 The total concen-tration 0.01 0.006 0.0053 0.0046 0.0042 0.0037 0.0035 of U (M) t~ ~ 0 997 0.976 0,993 ~ Ll.o05 The separation factor is defined as the ratio of isotopic ratios U : U in sorbent phase -to the same isotopic ratio in liquid phase.
Example 2 Wlth the same operating process as in Example 1, but with the d:Lfference that the experime~t had been led under admission of d~ily light, the time variation of the total uranium concentration and of the isotopic ratio in li.clui.d phase were determined. From these resultsth~ dependence of the separation factor on -the time was calculated:

romteI~s~ 0 0.6x103 1.2x103 2.4x103 3.6xl03 7.2x103 1~

tration 0.01 0.0058 0~0052 0.0044 0.0042 0.0039 0.0037 of U(M) .
_ _ .. _ Separ-atlon 1.000 0.980 0.980 0.997 1.016 1.020 1.025 factor . _ Example 3 Wi-th the same operating process as in the example 1, bu-t with the diEference that a strongly acidic cation exchanger of the mark ~mberlite IR-120 Itrademark) in a H - form has been used, the time variation of the total uranium concen-tration and of the isotopic ratio in liquid phase were determined. From these results, the dependence of the separa-tion factor on the time was calculated:

. .
Times (s) 0 0.3x10 0.6x10 1.8x10 3.6x10 10.8x10 jTotal _ tration 0.01 0.0031 0.0013 0.0003 0.0002 0.0002 of U (M) _ Separa -tion 1.000 0.997 1.025 1.007 1.000 1.001 factor _ Example 4 5 g of sorbent of the mark Ostsorb MV 6/5 (trademark) (swollen, centrifuged) saturated with uranium to the capacity

3~7~

4xl.0 5 M of U/g was under stir~ing~ at st:irrer speed of 5 revolutions per s, at temperature of 293l< and under admissi.on of light brought into con-tact with 20 ml of the solution of l M NaCl + 0.1 M Na2CO3A The rati.o of uranium isotopes in the sorbent at t.he beginning of the experiment was 0.725xlO 2.
In the course of desorption operation, samples of liquid phase were taken off and the total concen-tration of uranium and of isotopic ratio U : U were determined in them. The dependence of the separation factor on the time was calculated from these results:

Time ~s~ 0 0.3xlO 0.9x103 1.8xlO 3.6xlO 10.8x103 Total .
concen- 0 3.5xlO 3 3.7xlO 3 4.3xlO 3 4.5xlO 3 5.2xlO 3 of U ~M) .
Separa-tion 1.000 0.984 1.033 1.045 1.050 1.032 factor _ Example 5 With the same operating process as in Example 4, but with the difference that the experiment was led without admission of light, the time variation of the total uranium concentration and of the isotopic ratio in liquid phase were determined. The dependence of the separation factor on the time was calculated from these results:
~-- _ ~ o . 9xl03 3 . 6xlo3 `

o~ I ~M~ t ~ 2.7xlO 3.5xlO 4.4xlO ~ 3~lD

Separation factor 1.000 1.059 1.038 1.071 1.068 _ _ 7i~
Exam~ 6 ___ _ 2 g oE sorbent of the mark Ostsorb MV 6/5 (-trademark) a sorbent on the basis of mycelium of funqus of the P.
chrysogenum race), in H - form, swollen and centrifuged, was under stirring, at stirrer speed of 5 revolu-tions per s, a-t a -temperature of 293K and under the admission of daily light brought into contact with 50 ml of the solution of 0.01 M uranyl nitrate. The ratio of isotopes U : IJ in the starting solution was of 0.725x10 . Both phases were separated off after three hours and the solution, in the second stage, was brou~ht into contact with fresh 2 g of the same sorbent for again three hours. Four such a sorptlon stages were accomplished on the whole. The total concentration of uranium ln li~uid phase (by spectrophotometric method using the <~arsenazo I (trademark) reagent) was determined after each stage and isotopic ratio U : U in the sorbent phase (using mass spectrometer of the Aldermaston - Micromass Company, model 30 ~trademark)) was determined af-ter the last stage. The elementary separation factor of one stage was calculated from the results.
The result: 1.020 The separation factor is defined as the ratio of isotopic ratios U : 8U in the sorbent phase to the same isotopic ratio in liquid phase in our case after three hours of contact of phases.
Example 7 With the same operating process as in the Example 1, but with the difference that a sorbent of the mark Ostsorb MV
6/5 (trademark) activated with titanium (IV) oxide was used, four sorption stages were accomplished. Elementary separation factor of one stage was calculated from the results.

The result: 1.025.

Example 8 5 g of sorbent of the mark Ostsorb MV 6/5 (trademark) in H - Eorm (swollen and centriEuged)~ saturated wlth uranlum to -the capacity 4x10 5 M of U/g, was under s-tirr:Lng, at sitrrer a speed of 5 revolutions per s, at a temperature of 293K
and under the admission of daily light brought into con-tact with 20 ml of solution of 1 M soidum chloride ~ 0.1 M sodium carbonate. The ratio of iso-topes U : U in the sorbent at the beginning of the experiment was 0.725x10 . Both phases were separated off after 1 hour and new solution of the above-mentioned composition was added to the sorbent. Four such a desorption staCJes were accomplished altogether. After each stage the to-tal concentration of uranium in solution was determined and the isotopic ratio U : U in the sorbent phase was determined after the last stage. The elementary separation factor of one stage was calculated from the results.
The result: 1.150.
Example 9 With the same operating process as in Example 3, but with the difference that a solution of 0.1 M sodium carbonate was used for desorption, four desorption stages were accomplished. The elementary separation factor of one stage was calculated Erom the results.
The result: 1.120.
Example 10 With the same operating process as in Example 3, but with the difference that a solution of 0.1 M hydrochloric acid was used for desorption, four desorption stages were accomplished. The elementary separation factor of one stage was calculated from the results.
The result: 0.980.

Exampl _ 20.0 ml of the solution of 0.1 M tri-n-octylamine in the form of base was brought into contact with 100.0 ml of -the solution of 0.01 M UO2SO4 ~ 0.1 M Na2SO4 under stirrlng with a laboratory shaker at a temperature 293K. The ratio oE isotopes U : U ln the startlng solutlon was 0.725xlO 2 (a compound prepared from natural uranium was dealt with).
In the course of extraetlon, samples of aqueous phase were taken off, in which phase the total eoncentration of uranium (by spectrophotometrie method using reagent arsenazo I
(trademark)) and isotopic ratio by means of mass spectrometer of the Aldermaston - Mieromass (model 30) Company (trademark) were determined. The value of separation factor as a function of time was calculated from the results:

~i 0 15 30 concen-tration 0.01 0.0083 0.0069 0.0063 0.0071 0.0071 of U/VI/-M

Separa- .
tion 1.0001.0291.003 1.004 1.000 1.000 factor l rrhe princi.ple of the method of eontrolled distribu-tion ean be further generally used for separation of components ei-ther of various eoncentration or of various transport rates between phases, that is for those eases when the rate of sorption, if need be, of desorption, is for single components different.

Besldes isotopic mi~tures separation of materials with similar properties, such as rare earths, further Zr-~f, Ra-Ba and similarly is concerned at this place. Preparation of pure materials like materials of semiconductor, nuclear and analytical purity can be advantageously realized by this method as well.

Claims

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A process for the separation of isotopes by the method of controlled distribution based on the concentra-tion isotopic effect in two-phase systems, characterized in that one phase is an aqueous or non-aqueous solution of isotopic components and of a ligand,having a concentration between 10-6 M and the saturated solution, or a gaseous mixture of isotopic components or a gaseous mixture of isotopic components and an inert gas, said isotopes being, in the starting solution or gaseous mixture, in a molar ratio of 1:105 to 1:10-5; and the second phase is a sorbent selected from the group consisting of solid sorbents, bio-sorbents on the basis of mycelium of the lower fungi and sorbents on the basis of cellulose, or an extraction agent;
that after bringing both phases into contact, the rate of transport of isotopic components from one phase into another is not equal and the retardation of isotopic exchange takes place i) by the complexation of the isotopes with the ligands selected from the group consisting of carbonate, sulphate, citrate, chloride and ethylenediamine tetraacetate ions; or ii) by using sorbents and extraction agents with the chelating functional groups selected from the group consisting of carboxyl and hydrocarboxyl groups, and the groups on the basis of phosphorus, nitrogen and sulphur; and/or iii) by operating in darkness or in the light having a wavelength selected between 2.5 x 102 and 109 nm;

this process being carried out with a contact time of phases selected between 10-2 and 10-6 s, at a temperature selected between 102 and 103°K, a number of stirrer revolutions selected between 10-2 and 104 revolutions per s, a flow rate at column arrangement selected between 10-6 and 10-1 m/s and a size of particles of sorbent selected between 10-6 and 10-2m; and that for the desorption or the re-extraction of isotopic components, solutions of compounds selected from the group consisting of carbonate, sodium sulphate, sulphuric acid, nitric acid and hydrochloric acid, or of compounds which contain complexing ligands selected from the group consisting of citrate and ethylene-diamine tetraacetate ions, are used, alone or in a mixture, in a total concentration.between 10-3 M and the saturated solution.

2. A process according to claim 1, characterized in that the two-phase systems exhibits a non-linear equilibrium isotherm for sorption and/or desorption or for extraction and/or re-extraction.

3. A process according to claim 1, characterized in that the solid sorbents of the second phase is a styrene-divinylbenzene ion exchanger.

4. A process according to claim 1, characterized in that the extraction agent is a tributyl phosphate or a trioctyl amine.

5. A process according to claim 1 or 4, charac-terized in that the extraction agent is supported on a carrier selected from the group consisting of teflon (trade mark), silica gel and cellulose.

6. A process according to claim 1, characterized in that an aqueous solution of isotopic components and a ligand is used, and that the total concentration is selected between 10-2 and10-1 M.

7. A process according to claim 6, characterized in that the isotopic components are 235U(VI) and 238(VI).

8. A process according to claim 6 or 7, characterized in that the ligand is selected from the group consisting of nitrates, sulfates and carbonates.

9. A process according to claim 1, characterized in that the separated isotopes are,in the starting mixture, in molar ratio comprises between 1:500 and 1:100.

10. A process according to claim 9, characterized in that the separated isotopes are 235U(VI) and 238U(VI).

11. A process according to claim 1, characterized in that the second phase of the two-phase system is a solid sorbent on the basis of mycelium of lower fungus of the race P. chrysogenum or a strongly acidic cation exchangers on the basis of styrene-divinylbenzene copolymer.

12. A process according to claim 1, characterized in that the contact time of the phases is selected between 102 and 104s.

13. A process according to claim 1, characterized in that it is carried out in darkness or under a light having a wavelength selected between 250 and 650 nm.

14. A process according to claim 1, characterized in that it is carried out with a number of stirrer revolu-tions selected between 0.80 and 12.0 revolutions per s,with a flow rate at column arrangement selected between 10-4 and 10 1 m/s and with a solid sorbenthaving a size of particles selected between 10-4 and 10-3 m.

15. A process according to claim 1, characterized in that it is carried out at a temperature selected between 273 and 373°K.

16. A process according to claim 1, characterized in that the desorption or re-extraction is carried out with a solution of sodium chloride + sodium carbonate or a hydrochloric acid having a concentration selected between 10-1 and 1.5 M.