GB2104797A - A process for the separation of isotopes - Google Patents

Abstract

A process for the separation of isotopes in two-phase systems, for example, solid phase - liquid phase, liquid phase - liquid phase, solid phase - gaseous phase, is based on the knowledge that the distribution of isotopes can be controlled by the choice of phase composition, that is by their structural and material nature, by the concentration of isotopes and ligands, and 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, or from their gaseous mixtures, if they are contacted with solid sorbents or with liquid extraction agents, can be separated with high efficiency by the above process. The process is, in principle, one of chemical separation.

Description

SPECIFICATION A process for the separation of isotopes The invention seeks to provide a process for the separation of isotopes in two-phase systems, for example of the type solid phase-liquid phase, liquid phase-liquid phase, solid phase-gaseous phase, which process is based on the knowledge that the distribution of isotopes can be controlled by the choice of phase composition, that is by their structural and material nature, by concentration of isotopes and ligands, and 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, or, 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.It is, in principle, a chemical separation method.

Up to now published and used chemical methods for separation of isotopes (see for example: Laskorin B. N. and coworkers: Uspechi Khimii, band XLIV, no. 5, 761-780 (1975); Carman de J. and coworkers: Report AAEC/LIB/BIB -- 273, 1970) are based on isotope-exchanging reactions, equilibrium constants of which, that is elementary separation factors as a rule, vary in the limits approximately from 1.01 to 1.0001 depending on the mass weight of the relevant isotopes; the lower values are related to isotopes of heavy elements, for example of uranium. In recent latest years much attention has been paid to the separation of uranium isotopes by solid sorbents and liquid extraction agents.Namely patents and Japanese patent applications in West Germany (for example DOS 2,235.603 (1973); DOS 2,349.595 (1973); DOS 2,623.891(1977)), France (for example: French patent: 1,600.437(1968)) and Rumanian journal publications (for example: Ponta A., Calusaru A.: Isotopenpraxis 13, No. 6, 214-216(1977); ibid 11, No.12,422-426 (1975) can be cited.

The common feature of all the published processes is the maximum attempt to maintain conditions for the course of isotope-exchanging reaction of the type: 238us + 235utl 238oil + 235 where roman numerals indicate phases.

Initial work directed to the separation of uranium (Vl) isotopes was nevertheless unsuccessful due to the fact higher separation factor than approximately 1.00009 were not achieved 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/lll)-U(IV). Electron-exchanging reaction takes place simultaneously in these systems, in which reaction the higher valency state of uranium is always enriched with the lighter isotope. Further, in connection with the isotopic effect of complexing reactions and using a suitable type of sorbent, or extraction agent, selective for one uranium valency, a substantial enhancement of separation efficiency had been reached.From published data (for example: M. Seko, T. Miyake, K. Inada -- Nucl. Technology 50,178-186(1980); Report INFEC/DEP/WG. 2/31 -- "On the Technical, Economical and Safeguard Information of the Different Enrichment Techniques", 1 5.3.1 979) it is evident that a shift in the value of elementary separation factor to the level approximately 1.001 which is alleged to be the lower limit of economic utilization of the process was achieved. (For example the elementary operation factor of the process of gas diffusion, used in operational criterion for the separation of uranium isotopes, is approximately 1.0027). It can therefore be stated, that the results reached are already economically and technologically interesting.Operational utilization is not nevertheless unambiguous and that is mainly due to the limiting value of the elementary separation factor with which value building up devices with a large number of stages, with a large hold-up of enriched uranium and with relatively long time necessary for reaching a stationary state of the process is connected. The necessity of repeating many times the process of uranium reduction and oxidation is connected with substantial requirements of the relevant reduction and oxidation agents, and the energy of these reactions as well.

These drawbacks are eliminated by a new process for the separation of isotopes according to this invention, which process is based on the method of controlled distribution using a concentration isotopic effect in two-phase systems. The subject matter of the process according to the invention consists in that one of the phases is an aqeuous or non-aqueous solution of isotopic components and a ligand having a concentration from 1 O-e M to saturated solution, or one of the phases is a gaseous mixture of isotopic components or gaseous mixture of isotopic components and an inert gas and separated isotopes are in the starting mixture, or, in solution in molar ratio from 1:105 to 1::10-5 and the second phase is solid sorbent, such as styrene-divinylbenzene ion exchangers or bio-sorbents based on the mycelium of lower fungi and sorbents on the basis of cellulose, or the second phase is an extraction agent such as tributyl phosphate and trioctyl amine, or on a carrier such as teflon, silica gel or cellulose and the two-phase system advantageously exhibits a non-linear equilibrium isotherm for sorption and/or desorption, for extraction and/or re-extraction, or and after bringing both phases into contact the rate of transport of isotopic components from one phase into another is not equal and retardation of isotopic exchange takes place by a complexing of isotopes and 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 based on phosphorus, nitrogen and sulphur, and,'or by operating in darkness, or in light having a wavelength between 2.5 x 102 and 109 nm, further in that the time interval of contact of phases is chosen between 10-2 and 1 06 S, temperature between 102 and 103 K, the number of stirrer revolutions between 10-2 and 104 revolutions per s, flow rate at the column arrangement between 1 of6 and 10-' m/s and the size of particles of sorbent between 10-6 and 10-2 m, further in that for desorption or re-extraction of isotopic components, solutions of such compounds are used like sodium carbonate, ammonium nitrate, sodium chloride, sodium sulphate, sulphuric acid, nitric acid and hydrochloric acid, further such compounds which contain complexing ligands like citrate, are ethylenediamine tetraacetate and isobutyrate ions and that is alone or in a mixture with total concentration from 10-3 to saturated solution. In the process according to the invention an aqueous solution of isotopic components such as 235U(VI) and 238U(VI) and a ligand such as nitrate, sulphate and carbonate ions, the total concentration being between 10-2 and 10-' M, in which solution isotopes 235U and 238U are in the starting mixture in molar ratio between 1:500 and 1:100, for example can be used. Ion-exchangers on the basis of styrene-divinylbenzene copolymer or sorbents on the basis of the mycelium of lower fungi of the race P. Chrysogenum can be used as a solid sorbent.The time interval of contact of phases is chosen between 102 and 104 S, operating in darkness or in the light having wavelength between 250 and 650 nm, further when the number of revolutions of the stirrer is between 0.800 and 12.00 revolutions per s, or when the flow rate at the column arrangement is between 10-4 and 10-1 m/s and with a solid sorbent having size of particles between 10-4 and 10-3 m and at temperature between 273 and 373 K. Solutions of sodium chloride and sodium carbonate, or sodium carbonate, or of hydrochloric acid having concentration between 10-1 and 1.5 M can be used for desorption.

The process according to the invention is characterised by substantially higher separation factors than are up to now reached in isotope-exchange reactions. The maximum value 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 of the sorption process or desorption process. Economical and technological significance 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 the order of one to 1.5, making shorter the time necessary for the establishment of a stationary state and suppressing the hold up of enriched uranium in the device approximately by an order as well. It is possible to operate with solutions of six-valent uranium, it is not necessary to change its valency state and it is not necessary to transfer uranium to UF6 before enrichment as it is up to now at operationally employed enrichment stages. Another significant point is the fact that depleted uranium which is produced by present processes can be used as an input U-material and it is reasonable to reduce the content of 235U isotope in this waste at least by a half.

The subject matter of the new process for separation of isotopes, namely on the example of sorption, can be further described in the following way: a) By bringing into contact a solid sorbent or liquid extraction agent with aqueous solution of isotopes, for example with mixture 235U(VI) + 238U(Vl), where concentration of one of the isotopes is at least by half to one order higher than the 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 masstransfer 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 the effect which we have called concentration isotopic effect. The rate of sorption can be described for example by a relationship: R=kq.Aq=kc.Ac, where R designates the rate of sorption, or, of description respectively of transport of isotope from one phase into another, ka, kc are the mass-transfer coefficients and Aq, Ac designate the driving force of the process, respectively concentration gradient in phase "q" and c b) Driving force of the isotope transport from one phase (here aqueous) into another is a function of the initial concentration of isotopes in phases on the one hand and a function of the shape of equilibrium isotherm in the regions of variation of isotope concentrations on the other hand, which variation takes place in the course of sorption.

c) In view of isotope concentrations in the solution and of the type of controlling action of the sorption it is advantageous to choose such a sorbent which exhibits in the region of concentration variation, as far as possible, gradients as different as possible of the sections of an equilibrium isotherm, corresponding to the respective isotopes. It is advantageous, that the part of the equilibrium sorption isotherm, which corresponds to the isotope with lower concentration, should have a gradient approximately between 1 02 and 103 and the part corresponding to the isotope with higher concentration, approximately 1 0.

d) As the choice of sorbent has its limitation in the properties of sorbents alone it is also possible to adjust the composition of the solution to properties of the solvent. In this sense variation of the absolute concentration of isotopes comes into consideration, for example, by dilution, while keeping the isotopic ratio at the original value, and further adjustments of the solution composition by the addition of further components, including complexing agents.

e) The mass-transfer coefficient is a function of solution composition (isotope concentration) and 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 concentration dependence is one of the most important, these coefficients generally have a different value for each isotope, the absolute value is nevertheless determined mainly by the kind of sorbent.We have found it is advantageous to operate with such sorbents for which masstransfer coefficients lie between 10-5 and 10-3 1, f) For example the so-called bio-sorbents (see Czechoslovak author's certificate No. 155.833 and Czechoslovak author's certificates No. and No. (PV 186775 and PV 588-76)), further ion exchangers on the basis of styrene-divinylbenzene copolymers, extraction agents alone or on a carrier such as teflon, cellulose, silica gel and similar, can be employed as sorbents of the above-cited equilibrium and kinetic properties.

g) Simultaneously with sorption, if need be, with extraction, of isotopes, practically immediately isotopic exchange also starts to take place, which exchange suppresses the differences 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 interfered with, on the one hand by the selection and proper choice of sorbent type and solution composition and on the other hand by operation at lower temperatures and under limited light access.Retardation of isotopic exchange can be also reached by complexing of the isotopic component in one of the phases, that means either by the addition of suitable ligand into solution or with the help of a sorbent, or, of an extraction agent, having complex-forming functional groups.

h) The same as given above as an example of kinetics or sorption of isotopes, is also in principle true for the kinetics of desorption, if you like re-extraction and its accompanying isotopic exchange.

EXAMPLE 1 2 g of sorbent of the trade mark "OSTSORB MV 6/5" (a sorbent based on the fungus of the race P. chrysogenum), activated with hydrated titanium(lV) oxide, in Na-form (swollen and centrifuged) and mixed, at a stirrer speed of 5 revolutions per s, at a temperature of 293 K and in the absence of light was brought into contact with 60 ml of solution of 0.01 M uranyl nitrate, pH of which had been adjusted to 3.5. The ratio of isotopes 235Utt 235U in the initial solution was 0.725 x 10-2 (a compound prepared from natural uranium was involved).In the course of the sorption reaction samples of liquid phase were taken off and the total concentration of uranium was determined in the samples (by spectrophotometric method using reagent "arsenazo I") and the ratio of isotopes 235U :238U was determined by means of the mass spectrometer of the Aldermaston-Micromass 30 Company.The value of separation factor was calculated as a function of time from the results obtained: Time (s) 0 0.6 x 103 1.2x103 2.4x103 3.6x103 7.2x103 14.4x1O The total 0.01 0.006 0.0053 0.0046 0.0042 0.0037 0.0035 concentration of U(M) Separation 1.000 0.997 0.976 0.993 1.024 1.030 1.005 factor The separation factor is defined as the ratio of isotopic ratios 235U 235U in the sorbent phase to the same isotopic ratio in liquid phase.

EXAMPLE 2 By the same operating process as in Example 1, with the difference that the experiment was conducted in the presence of daylight, time variation against the total uranium concentration and the isotopic ratio in liquid phase were determined. From the result dependence of separation factor on time was calculated:: Time (s) 0 0.6 x 103 1.2x103 2.4 x 103 3.6 x 103 7.2x103 14.4x103 Total con- 0.01 0.0058 0.0052 0.0044 0.0042 0.0039 0.0037 centration of U(M) Separation 1.000 0.980 0.980 0.997 1.016 1.020 1.025 factor EXAMPLE 3 By the same operating process as in Example 1, with the difference that a strongly acidic cation exchanger of the trade mark AMBERLITE lR-1 20 in H±form was used, time variation against the total uranium concentration and the isotopic ratio in liquid phase were determined.From the results dependence of separation factor on time was calculated Time (s) 0 0.3 x 103 0.6x103 1.8x103 3.6x103 10.8x103 Total con- 0.01 0.0031 0.0013 0.0003 0.0002 0.0002 centration of U(M) Separation 1.000 0.997 1.025 1.007 1.000 1.001 factor EXAMPLE 4 5 g of sorbent of the trade mark OSTSORB MV 6(5) (swollen, centrifuged) saturated with uranium to the capacity 4 x 10-5 M of U/g stirred, at a stirrer speed of 5 revolutions per s, at a temperature of 293 K in the presence of light was brought into contact with 20 ml of the solution of 1 M NaCI + 0.1 M Na2CO3. The ratio of uranium isotopes in the sorbent at the beginning of the experiment was 0.725 x 10-2.In the course of the desorption operation samples of liquid phase were taken off and the total concentration of uranium and of isotopic ratio 235U : 238U was determined in them. Dependence of separation factor on time was calculated from the results: Time(s) 0 0.3 x 103 0.9 X 103 1.8x103 3.6x103 10.8x103 Total con- 0 3.5 x 10-3 3.7 x 10-3 4.3 X 10-3 4.5 X 10-3 5.2 x 10-3 centration of U(M) Separation 1.000 0.984 1.033 1.045 1.050 1.032 factor EXAMPLE 5 By the same operating process as in Example 4, with the difference that the experiment was conducted in the absence of light, time variation against the total uranium concentration and the isotopic ratio in liquid phase were determined. Dependence of separation factor on time was calculated from the results: Time(s) 0 0.3 x 103 0.9 X 103 3.6x103 10.8x103 Total con- 0 2.7 x 10-3 3.5 x 10-3 4.4 x 10-3 4.9 x 10-3 centration of U(M) Separation 1.000 1.059 1.038 1.071 1.066 factor EXAMPLE 6 2 g of sorbent of the trade mark OSTSORB MV 6/5 (a sorbent based on the mycelium of fungus of the P. chrysogenum race), in H±form, swollen and centrifuged, stirred, at a stirrer speed of 5 revolutions per s, at a temperature of 293 K and in the presence of daylight was brought into contact with 50 ml of the solution of 0.01 M uranyl nitrate.The ratio of isotopes 235U 235U in the starting solution was 0.725 x 10-2. Both phases were separated off after three hours and the solution, in the second stage, was brought into contact with a fresh 2 g supply of the same sorbent again for three hours. Four such sorption stages were accomplished on the whole. The total concentration of uranium in liquid phase (by spectrophotometric method using the "arsenazo I" reagent) was determined after each stage and the isotopic ratio 235U :23by in the sorbent phase (using the mass spectrometer of the Aldermaston-Micromass Company, model 30) was determined after the last stage. The elementary separation factor of one stage was calculated from the results. The result: 1.020.

Separation factor is defined as the ratio of the isotopic ratios 235U 235U in the sorbent phase to the same isotopic ratio in liquid phase - in our case after three hours of contact of phase.

EXAMPLE 7 By the same operating process as in Example 1, with the difference that the sorbent of trade mark OSTSORB MV 6/5 activated with titanium(lV) oxide was used, four sorption stages were accomplished.

The elementary separation factor of one stage was calculated from the result. The result: 1.025.

EXAMPLE 8 5 g of sorbent of the trade mark OSTSORB MV 6/5 in H±form (swollen and centrifuged), saturated with uranium to the capacity 4 x 10-5 M of U/g, stirred, at a stirrer speed of 5 revolutions per s, at a temperature of 293 K and in the presence of daylight was brought into contact with 20 ml of a solution of 1 M sodium chloride +0.1 M sodium carbonate. The ratio of isotopes 235U 235U in the sorbent at the beginning of the experiment was 0.725 x 10-2. Both phases were separated off after 1 hour and a new solution of the above-mentioned composition was added to the sorbent. Four such desorption stages were accomplished altogether. After each stage the total concentration of uranium in solution was determined and the isotopic ratio 235U 233U 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 By the same operating process as in Example 3, with the difference that a solution 0.1 M sodium carbonate was used for desorption, four desorption stages were accomplished. The elementary separation factor of one stage was calculated from the results. The result: 1.120.

EXAMPLE 10 By the same operating process as in Example 3, 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: 0.0980.

EXAMPLE 11 20.0 ml of a solution of 0.1 M tri-n-octylamine in the form of a base was brought into contact with 100.0 ml of a solution of 0.01 M UO2SO4 + 0.1 M Na2SO4 while stirring in a laboratory shaker at a temperature of 293 K. The ratio of isotopes 235U 233U in the starting solution was 0.725 x 10-2 (a compound prepared from natural uranium was involved). In the course of extraction samples of aqueous phase were taken off, in which phase the total concentration of uranium (by spectrophotometric method using reagent "arsenazo I") and the isotopic ratio by means of the mass spectrometer of the Aldermaston -- Micromass (model 30) Company were determined.The value of the separation factor as a function of time was calculated from the results: Time (s) 0 15 30 60 120 240 Total con- 0.01 0.0083 0.0069 0.0063 0.0071 0.0071 centration of U(VI)-M Separation 1.000 1.029 1.003 1.004 1.000 1.000 factor The principle of the method of controlled distribution can be further generally used for the separation of components either of various concentrations or of various transport rates between phases, that is for those cases where the rate of sorption, or of desorption, is different for individual components. Besides isotopic mixtures, separation of materials with similar properties, such as rare earths e.g. Zr-Hf, 226Ra-Ba and others are for applicable to the present invention. Preparation of pure materials like materials of semiconductors, nuclear and analytical purity can advantageously be realised by this method as well.

Claims (14)

1. A process for the separation of isotopes by a method of controlled distribution using a concentration istopic effect in two-phase systems in which one of the phases is an aqueous or nonaqueous solution of isotopic components and a ligand having a concentration of from 10-6 M to a saturated solution or a gaseous mixture of isotopic components or a gaseous mixture of isotopic components, an inert gas and separated isotopes and the second phase is a solid sorbent or an extraction agent, so that after bringing both phases into contact the rate of transport of isotopic components from one phase into the other is not equal and retardation of isotopic exchange takes place by the complexation of the isotopes with the ligands or by sorption or extraction and/or by operating in darkness, or in light having a wavelength of between 2.5 x 102 and 109 nm, with a time interval of contact between the phases of between 10-2 and 106 S at a temperature of between 102 and 103 K and wherein the number of stirrer revolutions is between 10-2 and 104 revolutions per s, the flow rate at the column arrangement is between 1 O-s and 10-1 m/s and size of sorbent particles is between 10-6 and 10-2 m followed by desorption or re-extraction of the isotopic components.

2. A process according to Claim 1 in which the separated isotopes are present in a molar ratio of from 1:105 to 1:10".

3. A process according to Claim 1 or Claim 2 in which the solid sorbent is based on the mycelium of lower fungi, cellulose or on a styrene-divinylbenzene copolymer ion exchange resin.

4. A process according to Claim 3 in which the mycelium of lower fungi is of the race P. chrysogenum.

5. A process according to any one of Claims 1 to 4 in which the extraction agent is tributyl phosphate or trioctyl amine.

6. A process according to any one of Claims 1 to 5 in which the two phase system exhibits a nonlinear equilibrium isotherm for sorption and/or desorption and for extraction and/or re-extraction.

7. A process according to any one of Claims 1 to 6 in which the ligand is selected from the group consisting of nitrate, carbonate, sulphate, citrate, chloride and ethylene diamine triacetate ions.

8. A process according to Claim 7 in which the total concentration of ions is from 10-3 to a saturated solution.

9. A process according to any one of Claims 1 to 8 in which the sorption or extraction agent has chelating functional groups and is selected from the group consisting of carboxyl and hydroxyl groups and groups based on phosphorous, nitrogen and sulphur.

10. A process according to any one of Claims 1 to 9 in which compounds selected from the group consisting of sodium carbonate, sodium sulphate, sulphuric acid, nitric acid and hydrochloric acid are used for desorption or re-extraction.

11. A process according to any one of Claims 1 to 10 in which an aqueous solution of isotopic components 235U(VI) and 238U(VI) and ligand nitrate, sulphate or carbonate ions are used, the total concentration being between 10-2 and 10-1 M.

12. A process according to any one of Claims 1 to 11 in which separated isotopes 235U and 233U and present in a molar ratio of between 1:500 and 1 :100.

1 3. A process according to any one of Claims 1 to 1 2 in which the time interval of contact of the phases is between 102 and 104 S.

14. A process according to any one of Claims 1 to 13 when' operated in darkness or under light having a wavelength of between 250 and 650 nm.

1 5. A process according to any one of Claims 1 to 14 in which the number of stirrer revolutions is between 0.80 and 12.0 revolutions per s, the flow rate at the column arrangement is between 10-4 and 10-1 m/s and the solid sorbent has a particle size of between 10-4 and 10-3 m.

1 6. A process according to any one of Claims 1 to 1 5 when operated at a temperature of between 273 and 373 K.

1 7. A process according to any one of Claims 1 to 1 6 in which a solution of sodium chloride and sodium carbonate or of hydrochloric acid having a concentration of between 10-1 and 1.5 M is used for desorption or re-extraction.

1 8. A process for the separation of isotopes substantially as hereinbefore described with reference to any one of Examples of the invention.

1 9. Isotopes which have been separated by a process as claimed in any one of Claims 1 to 1 8.