CS214904B1 - Method of separation of isotopes by method of controlled distribution with utilization of particularly kinetic concentration effect - Google Patents

Abstract

The purpose of the invention is to significantly increase the elemental separation factor value in particular for the separation of uranium isotopes, reduce the total number of separation steps by one to 1.5, reduce the settling time and reduce the enriched uranium tank by about 1 order. It is possible to work with solutions of hexavalent uranium, it is not necessary to change its valence state and it is not necessary to convert uranium before enrichment to UF «, as is the case with the enrichment procedures used so far. The essence of the invention is that the separation of isotopes by the method of controlled isotope distribution in two-phase systems, such as solid sorbent-aqueous solution, solid sorbent-gas mixture. and the liquid extraction agent - the aqueous solution is controlled by the duration and hydrodynamic parameters of phase contact, temperature, illumination and phase composition in the sense that the isotope transport rates from one phase to the other, i.e. the product of the driving force and the mass transfer coefficients, are different from each other and further, that the isotope exchange rate is minimal.

Description

The invention is a method of separating isotopes in two-phase systems such as solid phase - liquid phase, liquid phase - liquid phase, solid phase - gas phase, based on the knowledge that isotope distribution can be controlled by the choice of phase composition, ie their structural and material nature, concentration of isotopes and ligands, as well as temperature, illumination, hydrodynamic parameters and duration of phase contact. The isotopes of uranium, lithium and nitrogen can be separated from aqueous and non-aqueous solutions, or gaseous mixtures thereof, when they are contacted with solid sorbents or liquid extraction agents, with high efficiency, according to the invention. It is basically a chemical separation method.

Chemical isotope separation methods published and used hitherto (see, for example: Laskorin, N. N. et al., Uspechi chimii, Vol. XLIV, No. 5, 761-780 [1975]; Garraan de J. et al., Report AAEC [LIB / BIB-273 [1970]] are based on isotope exchange reactions whose equilibrium constants, i.e., typically elemental separation factors, depending on the mass numbers of the isotopes in question, are in the range of about 1.01 to 1.0001; values refer to isotopes of heavy elements such as uranium In recent years, much attention has been paid to the separation of uranium isotopes on solid sorbents and liquid extraction agents, specifically patents and Japanese patent applications in the Federal Republic of Germany (eg: DOS 2,235,603 [1973]; DOS 2,349,595 [1973]; DOS 2,623,891 [1977]), French (e.g.: Franc, patent: 1,600.437 [1968]} and Romanian magazines (e.g.: Ponta A., Calusaru A .: Isotopenpraxis 13, Nr. 6, 214 to 216 [1977] ib id 11, No. 12, 422-426 [1975]).

The common feature of all published procedures is the maximum effort to comply with the conditions for the course of the isotopic exchange reaction type:

238uj + 235u _n 238 238u _n -J-235u _t where the Roman numeral indicates the phase.

However, the initial work on the separation of U (VI) isotopes was unsuccessful since it was not possible to reproduce reproducibly, using ion exchangers and chromatographic techniques, a separation factor greater than about 1,000 09. Therefore, attention has been turned to systems in which uranium present in different valence states, namely: U (VI) - U (IV) and U (III) —U (IV). At the same time, an electron exchange reaction occurs in these systems, in which a higher valence grade of uranium is always enriched with a lighter isotope. Furthermore, in connection with the isotopic effect of the complexing reactions and the use of a suitable type of sorbent or extraction agent selective for one of the valency forms of uranium, a significant increase in separation efficiency has indeed been achieved. It is apparent from the published data that the value of the elementary separation factor has been shifted to the level of about 1.001, which is reportedly the limit of the economic usability of the process. (For example, the elemental separation factor of the gas diffusion process used on a production scale for the separation of uranium isotopes is about 1.0027.) Therefore, it is possible to state that the results achieved are already economically and technologically interesting. mainly due to the elemental separation factor limit, which is associated with the construction of large-scale plants, the large containment of enriched uranium, and the relatively long time required to achieve a stationary process, and the multiple repetition of the uranium reduction and oxidation process. appropriate reducing and oxidizing agents, respectively, to the energy of these reactions.

These disadvantages are overcome by the novel method of isotope separation according to the invention, based on the controlled distribution method, using in particular the kinetic concentration effect and the isotopic exchange retardation in two-phase systems. The process according to the invention is characterized in that one of the phases is an aqueous or non-aqueous solution of isotopic components and a ligand at a concentration of 10 ^-6 M to a saturated solution, or is a gaseous mixture of isotopic components or a gaseous mixture of isotopic components and an inert gas. is a solid sorbent such as styrene-divinylbenzene ion exchangers, lower fungal mycelium biosorbents and cellulose sorbents, or is an extraction with an agent such as tributyl phosphate and trioctylamine optionally anchored on a carrier such as teflon, sillcagel and cellulose In both phases, the rate of transport of isotopic components from one phase to another is not the same and the complexation of isotopes by ligands such as carbonate, sulfate, citrate, chloride and ethylenediaminetetraacetic, or by the use of sorbents and extracting agents with chelating functional groups such as carboxyl, hydroxyl , groups based on phosphorus, nitrogen and sulfur, and / or work in the dark or in light with a wavelength in the range of 250 mm to 1 m, isotopic exchange retardation occurs, the temperature being in the range of -173 to +727 ° C, sorbent particle size in the range of 1 μτη — 0.01 m, further that solutions of compounds such as sodium carbonate, ammonium nitrate, sodium chloride, sodium sulfate, sulfuric acid, nitric acid and hydrochloric acid are used for desorption or re-extraction of isotopic components containing complexing ligands such as citrate, ethylenediaminetetraacetic and isobutyric ions alone or in a mixture having a total concentration of 10 ^-3 M to a saturated solution. For example, an aqueous solution of isotopic components such as ²³⁵ U (VI) and ²³⁸ U (VI) and a ligand such as nitrate, sulfate and carbonate ions in a total concentration of 0.01 to 0.1 M may be used in the method of the present invention. wherein the isotopes ²³⁵ U and ²³³ U are in the starting mixture in a molar ratio of 1: 500 to 1: 100. As a solid sorbent, strongly acidic cation exchangers based on styrene-divinylbenzene copolymer or sorbents based on mycelium of the lower fungus strain P. chrysogenum can be used. The phase contact duration is chosen in the range of 100 s to 16.7 min., Working in the dark or in the light with a wavelength of 250 to 650 nm, further with the stirrer speed in the range of 0,800 to 12,00 s ^_1 , or high-speed pellets at a column size range of 10 ^-4 to 10 ^-2 m / s with a solid sorbent having a particle size in the range of 0.1 to 1 mm and at a temperature between 0 ° C and 100 ° C. Sodium chloride + sodium carbonate or sodium carbonate or hydrochloric acid solutions of 0.1 to 1.5 M may be used for desorption.

The process according to the invention is characterized by considerably higher separation factors than hitherto achieved in isotopic exchange reactions. During the kinetics of sorption or desorption of uranium isotopes, the maximum values of elemental separation factors range from 1.02 to 1.10. It is particularly advantageous to use solutions of hexavalent uranium and solid sorbents with high selectivity, but not too large a capacity for uranium and not with too rapid kinetics of the sorption or desorption process. The economic and technological significance of the isotope separation process according to the invention is considerable. A significant increase in the value of the elemental separation factor represents, in particular for the separation of uranium isotopes, a reduction of the total number of separation stages by one to 1.5 orders, a shortening of the stationary state and a reduction of the enriched uranium retention in the plant by about one order. It is possible to work with solutions of hexavalent uranium, it is not necessary to change its valence state and it is not necessary to convert uranium to UF before enrichment, as is the case with the previously used enrichment processes. Another important point is that depleted uranium can be used as input U-material, which is "produced" by existing processes and it is realistic to reduce the isotope content of ^23S D in this waste by at least half.

The essence of the new isotope separation process, namely the sorption process, can be described in more detail as follows:

a) By contacting the solid sorbents or liquid extracting agent with an aqueous isotope solution, e.g. a mixture of ²³⁵ U (VI) + ²³³ U (VI), where the concentration of one of the isotopes is at least half to one order higher than the other, The sorption rate of each isotope is proportional to the driving force of the process, ie the relative distance from equilibrium and the value of the mass transfer coefficient. This speed can be described by the formula: R = k _q . And _q = k _c . A _c where denotes

R - rate of sorption, eventual desorption, respectively transport of isotopes from one phase to another, k _q , k _c - mass transfer coefficients,

Δ _ς , Δ _ο - the driving force of the process, resp. concentration gradient in phases “q” and “c”.

b) The driving force of isotope transport from one phase (here aqueous) to another is a function of both the initial isotope concentrations in phases and the shape of the equilibrium isotherm in the area of isotope concentration changes that occur during sorption.

(c) In view of the concentration of isotopes in the solution, a sorbent shall be chosen which shows, as far as possible, the concentration differences in the equilibrium isotherm sections corresponding to the individual isotopes in the area of concentration changes. It is preferable that the part of the sorbent equilibrium isotherm which belongs to the lower concentration isotope has a slope of about 10 ² to 10 ³ , and the part corresponding to the higher concentration isotope is about 10.

d) Since the choice of sorbents has its limitations on the properties of the sorbents themselves, it is also possible to adapt the composition of the solution to the properties of the sorbents. In this sense, it is possible to change the absolute concentration of isotopes, e.g. by dilution, while maintaining the isotopic ratio at the original level, and to adjust the composition of the solution by adding other components, including complexing agents.

ej The mass transfer coefficient is a function of the composition of the solution (isotope concentration) and the type of sorbents or extracting agent, as well as the temperature and hydrodynamic parameters of the system (mixing speed, flow rate, sorbent particle size, etc.); since the concentration dependence is one of the most significant, these coefficients generally have different values for each isotope. However, the absolute value of the coefficient is given mainly by the type of sorbents. We have found it advantageous to work with a sorbent for which the mass transfer coefficients range from 10 ⁵ to 10 ^-3 s ^-1 .

f) The sorbents of the above equilibrium and kinetic properties can be used, for example, so-called biosorbents (see the author's certificate No. 184 471 and the author's certificate No. 189 198), as well as ion exchangers based on styrene-divinylbenzene copolymers, extraction agents alone or anchored on a carrier such as teflon, cellulose, silica gel and the like.

g) Along with the sorption or extraction of isotopes, the isotopic exchange begins practically immediately, which reduces the differences in the adsorbed amounts of the individual isotopes resulting from different sorption rates. The negative influence of the isotopic exchange process, the rate of which is approximately the same as the rate of sorption of the higher concentration isotope, can be compensated or canceled, both by selecting and selecting the type of sorbents and the composition of the solution. Isotopic exchange retardation can also be achieved by complexing the isotopic component in one phase; either by adding a suitable ligand to the solution, or by using sorbents, optionally an extracting agent with complexing functional groups.

h) The same, as exemplified above for the isotopic sorption kinetics, also applies in its principle to the desorption or reextraction kinetics and its accompanying isotopic exchange.

Example 1 g of sorbent Ref. Ostsorb MV 5.6 (based sorbent fungi P. chrysogenum strain) activated hydrated titanium dioxide, in the Na form (swollen and skimmed) with stirring at a stirrer spin speed of 5 s ^-1, at at 293 Kelvin and, in the absence of light, contacted with 60 ml of a 0.01 M uranyl nitrate solution adjusted to pH 3.5. Isotope ratio 235u _; 238u in the starting solution was 0.725.10 ^-2 (it was a compound prepared from natural uranium). During the sorption reaction, liquid phase samples were taken to determine the total uranium concentration (by the spectrophotometric method using arsenazo I reagent) and on an Aldermaston-Micromass 30 mass spectrometer ^Z35 U: ²³⁸ U ratio. factor as a function of time:

Cas (s) 0 0,8.103 1,2. 10 ³ 2.4.10 ³ 3.6. 10 '7.2. 10'14,4.10 ³ Total U (M) 0.01 0.808 0.0053 0.0.046 0.0042 0.0037 0.0035 Separation factor · 1,000 0.997 0.978 0.993 1.024 i, 030 1.005

The separation factor is defined as the ratio of isotopic ratios of ²³⁵ U: ²³⁸ U in the sorbent phase to the same isotopic ratio in the liquid phase.

Example 2

Using the same procedure as in Example 1. In contrast to the fact that the experiment was conducted under daylight approaches, the time change in the total uranium concentration and the isotopic ratio in the liquid phase was determined. The dependence of the separation factor on time was calculated from the result:

Cas (s) 0 0.6. 10 ³ 1.2. 10 ³ 2.4. 10 ³ 3.6. 10 ³ 7.2. 10 ³ 14.4. 10 ³ Total U (M) 0.01 0,0058 0.0052 0,0044 0,0042 0,0039 0,0037 Separation factor 1,000 0,980 0,980 0.997 1.016 1,020 1,025

Example 3

Using the same procedure as in Example 1, except that the strongly acidic cation exchanger Amberlit IR-120 was used in the H + form, the time change of the total uranium concentration and the isotopic ratio in the liquid phase was determined. The dependence of the separation factor on time was calculated from the results:

Cas (s) 0 0,3.10 ³ 0,6. 10 ' 1,8. 103 3.6. 10 ³ 10.8. 10 ³ total concentration U (M) 0.01 0.0031 0,0013 0.0003 0.0002 0.0002 separační factor 1,000 0.997 1,025 1,007 1,000 1,001

Example 4 g of sorbents of the brand Ostsorb MV 6/5 (swollen, centrifuged) saturated with uranium to a capacity of 4.10 ^-5 MU / g, were contacted with stirring at a speed of 5 s ^-1 , at a temperature of 20 degrees Celsius and 20 ml of 1M NaCl + + 0.1M Na 2 CO 3 solution. The ratio of isotopes of uranium in the sorbent at the start of the experiment was 0.725.10 ^-2 . During the desorption operation, liquid phase samples were taken and the total uranium concentration and the isotopic ratio of ²³⁵ U: ²³⁸ U were determined. The dependence of the separation factor on time was calculated from the results:

Time (s), 0 0.3. 103 0.9. 103 1,8. 10 ³ [3.8. 10 ³ 10.8. 10 ^:! total concentration U (M) 0 3.5. 10-3 3.7. 10- ³ 4.3. 10- ³ 4,5. 10- ³ 5.2. 10-3 separační factor 1,000 0,984 1,033 1,045 1,050 1,032

Example 5

Using the same procedure as in Example 4, except that the experiment was conducted in the absence of light, the time change of the total uranium concentration and the isotopic ratio in the liquid phase was determined. The dependence of the separation factor on time was calculated from the results:

Cas (s) p, 0.3 ... 10 ³ - 0,9.10 ³ 3,8..10 ³ .10,8.10 » Total concentration Ú (M) 0 · .- - 2.7: 10-3 3,5.10-3 4,4.10-3 4.3. io: » separační factor 1,000 1,059; · 1,038 j; 1,, 0.77% 1,080

The principle of the controlled distribution method is further applicable in general to the separation of components either at different concentrations or at different transport rates between phases, i. for those cases where the rate of sorption or desorption is different for individual components. Apart from isotopic mixtures, this involves the separation of substances of close properties, such as rare earths, Zr - Hf, Ra 226 - Ba, etc. Also, the preparation of pure substances, such as semiconductor, nuclear and analytical purity materials, can be advantageously realized .

Claims (9)

SUBJECT A method of isotope separation by a controlled distribution method using in particular a kinetic concentration effect and isotopic exchange retardation in two-phase systems, characterized in that one of the phases is an aqueous or non-aqueous solution of isotopic components and a ligand of 10 ^-6 M to saturated solution, or is a gaseous mixture of isotopic constituents or a gaseous mixture of isotopic constituents and an inert gas and the second phase is a solid sorbent such as styrene-dlvinylbenzene ion exchangers, lower fungal mycelium biosorbents and cellulose sorbents, or is an extraction agent such as tributyl phosphate and trioctylamine optionally supported on a carrier such as teflon, silica gel and cellulose, and after contacting the two phases, the rate of transport of isotopic components from one phase to another is not the same and the ligand complexation of isotopes such as carbonate, sulfate, citrate, chloride and ethylenediaminetetraacetic or pou By absorbing sorbents and extracting agents with chelating functional groups such as carboxyl, hydroxyl, phosphorus, nitrogen and sulfur based groups and / or washing in the dark, or in light with a wavelength between 250 nm and 1 m, isotopic retardation occurs Exchanges, with a temperature in the range of -173 to +727 ° C and sorbent particle size in the range of 1 μτη to 0.01 m, and solutions of compounds such as sodium carbonate, ammonium nitrate, chloride are used to desorb or re-extract the isotopic components. sodium, sodium sulfate, sulfuric acid, nitric acid and hydrochloric acid; and compounds containing complexing ligands such as citrate, ethylenediaminetetraacetic ions and isomerase, either alone or in a mixture having a total concentration of 10 ^-3 M to a saturated solution. 2. A method according to claim 1, wherein an aqueous solution of isotopic components such as ²³⁵ U (VI) and ²³⁸ U (Vij and a ligand such as nitrate, sulfate and carbonate ions) is used at a total concentration of 0.01 to 0,1 M. 3. The process of claim 1 wherein the separated isotopes, such as ²³⁵ U and ²³⁸ U, are in the starting mixture in a molar ratio of 1: 500 to 1: 100. 4. The method of claim 1, wherein the mycelium-based solid sorbent of the lower fungus strain P. chrysogenum, or the strongly acidic cation exchangers based on the styrene-divinylbenzene copolymer is used. 5. The method of claim 1 wherein the phase contact duration is selected from 100 s to 16.7 min. 6. The method according to claim 1, characterized in that it is operated in the dark or in light with a wavelength of 250 to 650 nm. 7. A method according to claim 1, characterized in that it is carried stirrer speed ranging from 0.80 to 12.0 s ^-1, at a flow rate in the column arrangement in the range of 10 ~ ^{_2 4-10} m / s and the solid absorbent having a particle size in the range of 0.1 to 1 millimeter. 8. The process of claim 1 wherein the temperature is between about 0 ° C and about 100 ° C. 9. A process according to claim 1 wherein the desorption or reextraction is carried out using a sodium chloride solution + sodium carbonate or sodium carbonate, or a hydrochloric acid solution having a concentration of 0.1 to 1.5 M.