RU2373155C2 - Method of extractant purification of regenerated uranium - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method involves reducing technetium (VII) in aqueous nitric-acid solution of regenerated uranium (VI) with hydrazine, complexing reduced technetium with sodium hexametaphosphate, adjustment of nitric acid content in the obtained solution and extraction of uranium (VI) with tributyl phosphate in an organic diluent. Sodium hexametaphosphate is added until concentration of not less than 0.02 mole NaPO₃ per litre of solution. The uranium (VI) extract is washed with nitric-acid solution, which contains uranium (IV), stabilised with hydrazine. Waste washing solution is combined with uranium (VI) solution for extraction operations.

EFFECT: method allows for purifying uranium from technetium through extraction and allows for use of washing solution which contains uranium, stabilised with hydrazine, for washing the extract so as to increase degree of purity of uranium from technetium.

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Description

The invention relates to methods for the extraction purification of regenerated uranium and can be used in technological processes for the processing of irradiated nuclear fuel, in which tributyl phosphate (TBP) is used as an extractant in an organic diluent and where it is necessary to purify uranium from beta-active technetium-99.

The problem of extraction purification of uranium from technetium is that technetium in nitric acid solutions of uranium is in the heptavalent state in which it is coextracted with tributyl phosphate together with uranium in the form of an anionic complex: UO ₂ (NO ₃ ) ₂ (TBP) ₂ + TCO ₄ ^- = UO ₂ NO ₃ TcO ₄ (TBP) ₂ + NO ₃ ^-

The known processes of extraction purification of uranium from technetium using TBP solutions in hydrocarbon diluents as the extractant are based on the transfer of technetium from the heptavalent state, in which it is coextracted with uranium, into the tetravalent state, weakly extracted with tributyl phosphate. The more fully restored technetium from the heptavalent state to the tetravalent state, the higher the extraction purification of uranium from technetium.

In relation to the extraction purification process of irradiated uranium using 30% TBP, a method for purifying uranium from technetium is known, in which technetium (VII) in a nitric acid solution of uranyl nitrate is reduced with hydrazine before extraction, that is, technetium (VII) is converted into an unextractable valence form [US patent 4443413, IPC C01G 43/00; C01G 57/00, publ. 04/17/1984]. In this case, the reduced technetium compound remains in the raffinate. The disadvantage of this method is the low degree of uranium extraction during multistage extraction with 30% TBP because of the need to maintain a low concentration of nitric acid in an aqueous solution (0.01-0.1 mol / L) to stabilize technetium in a reduced state.

A known method of extraction purification of uranium (VI) from technetium in which technetium (VII) in a nitric acid solution of UO ₂ (NO ₃ ) ₂ before extraction of uranium with 30% TBP in an organic diluent is reduced with hydrazine and reduced technetium is complexed with oxalic acid, that is, translated technetium (VII) in non-extractable form [US patent 4528165, IPC C01G 43/00; C01G 57/00, publ. 07/09/1985] - a prototype. The technetium (IV) oxalate complex is formed at a 0.1N nitric acid concentration, and extraction is carried out from an aqueous solution with a 1N nitric acid concentration. Purification of uranium from technetium occurs at the extraction stage; washing the extract in the prototype is not provided.

In an aqueous solution of regenerated uranium, into which a reducing agent and a complexing agent were introduced, a mobile equilibrium is established between the two valence forms of technetium. This equilibrium is violated during the extraction process, since in order to ensure a more complete extraction of uranium into the extract, the extraction is carried out at a higher (in comparison with the aqueous solution, in which reduced technetium was complex) nitric acid content. In the process of multistage extraction, the possible oxidation of part of technetium with nitric acid again to the heptavalent state and its transition to the extract at each stage reduces the overall purification of uranium from technetium.

Uranium (VI) could be washed off from the technetium transferred to the extract during washing of the extract using a washing solution containing uranium (VI) and a reducing agent. Typically, the extract is washed with a solution containing uranium (IV) and a stabilizer of uranium (IV) hydrazine as a reducing agent. In this case, in order not to lose uranium (VI), which transferred to the washing solution, the spent washing solution is returned to the uranium (VI) extraction operation. In relation to the prototype, in the process of uranium (VI) extraction, insoluble uranium (IV) oxalates will form upon the interaction of oxalic acid with the remaining uranium (IV) in the spent washing solution. Therefore, the use of uranium (IV) for washing in the prototype is impossible.

The use of a washing solution containing only hydrazine from technetium reducing agents will not allow purification of uranium from technetium, since hydrazine, unlike tetravalent uranium, is not extracted, and therefore the restoration of heptavalent technetium with hydrazine in the extract will not occur during the washing operation.

The objective of the invention is to expand the arsenal of methods for the purification of uranium from technetium and to increase the degree of purification of uranium from technetium.

The problem is solved by the fact that in the method of extraction purification of regenerated uranium (VI) from technetium (VII), including the restoration of technetium (VII) in an aqueous nitric acid solution of regenerated uranium (VI) with hydrazine, complexing the restored technetium, adjusting the resulting solution according to the content of nitric acid by increasing its concentration and extraction of uranium (VI) with tributyl phosphate in an organic diluent, the combination of reduced technetium is carried out with sodium hexametaphosphate, which is introduced to a concentration of and not more than 0.02 mol of NaPO ₃ per liter of solution, the uranium (VI) extract is washed with a nitric solution containing uranium (IV) stabilized with hydrazine, the spent wash solution is combined with a solution of uranium (VI) in an extraction operation.

The claimed method allows to increase the coefficient of purification of uranium from technetium in the operation of uranium extraction and to ensure the purification of uranium from technetium in the operation of washing the extract.

The method uses sodium hexametaphosphate (NaPO ₃ ) _n · nH ₂ O (other names - Graham salt, sodium polyphosphate, GOST 20291-80).

In the examples, the results of four experiments on the extraction cleaning of regenerated uranium from technetium are presented. In experiments 1-3, hydrazine and sodium hexametaphosphate were introduced into the initial solution of regenerated uranium to specified concentrations, and in experiment 4, hydrazine and oxalic acid were introduced. Then, the concentration of nitric acid was raised in the solutions (to increase the distribution coefficients of uranium and its more complete extraction during the extraction operation) and extraction was carried out with 30% TBP in a hydrocarbon diluent. Extraction of regenerated uranium from an aqueous solution prepared for extraction in all four experiments was carried out in a countercurrent three-stage extraction variant.

In the first two experiments, after extraction cleaning of regenerated uranium from technetium, a single-stage washing of the extract of regenerated uranium was carried out with a nitric acid solution of uranyl nitrate containing tetravalent uranium. In the third experiment, the extract was washed with the same washing solution, but already in two steps.

In the fourth experiment, simulating the extraction purification of regenerated uranium according to the prototype method, the extract was not washed.

The results of the experiments are presented in the table.

Table Name of solution [U (VI)], g / l [U (IV)], g / l [HNO ₃ ], mol / L [N ₂ H ₄ ], g / l [NaPO ₃ ], mol / L [H ₂ C ₂ O ₄ ], mol / L [Tc], mg / l Coeff. very good U from Tc Experience 1 Regenerated Uranium Stock Solution 466.0 0 0.1 0 0 0 5.08 - Prepared
n to
extraction solution 435.0 0 0.75 2.0 0,020 0 4.74 - Extract after 3-stage uranium extraction 83.5 0 He determined 0 0 0 0,033 27.6 Raffinate after 3-stage uranium extraction 0.72 0 0.55 2.0 0,020 0 4.66 - Stock Wash Solution 109.3 1.86 0.44 0.84 0 0 0 - Single stage washed extract 89.6 0 Not determined 0 0 0 0,00082 1191 Experience 2 Regenerated Uranium Stock Solution 398.4 0 0.1 0 0 0 5.10 - Prepared
n to
extraction solution 371.5 0 0.40 2.0 0.004 0 4.76 0 Extract after 3-stage uranium extraction 78.9 0 Not determined 0 0 0 0.0038 266 Raffinate after 3-stage uranium extraction 1,1 0 0.35 1.9 0.004 0 4.74 Stock Wash Solution 113.1 1.95 0.50 1.28 0 0 0 Single stage washed extract 76.0 0 Not determined 0 0 0 0,00033 2950 Experience 3 Regenerated Uranium Stock Solution 411.4 0 0.1 0 0 0 5.15 Prepared
n to
extraction solution 383.6 0 0.85 2.0 0,0008 0 4.85 0 Extract after 3-stage uranium extraction 87.5 0 Not determined 0 0 0 0.012 92 Raffinate after 3-stage uranium extraction 0.23 0 0.65 1.8 0,0007 0 4.80 Stock Wash Solution 107.1 1.95 0.60 1.70 0 0 0 Single stage washed extract 87.0 0 Not determined 0 0 0 0.0011 1000 Two-stage washed extract 86.0 0 Not determined 0 0 0 0,00023 4730 Experience 4 Regenerated Uranium Stock Solution 421.0 0 0.1 0 0 0 4.34 Prepared
n to
extraction solution 393.0 0 0.60 2.0 0 0,022 4.05 Extract after 3-stage uranium extraction 81.0 0 Not determined 0 0 0 0.012 69.6 Raffinate after 3-stage uranium extraction 0.83 0 0.45 1.95 0 0,021 3.95

From the data presented in the table, it is seen that the indicators of uranium removal from technetium in experiment No. 1 are lower than in experiment No. 2, although the concentration of sodium hexametaphosphate in experiment No. 1 is higher (0.02 mol of NaPO ₃ / L or in terms of technetium in the initial solution, 420 mol of NaPO ₃ per 1 mol of technetium) than in experiment No. 2 (0.004 mol of NaPO ₃ / L or 83 mol of NaPO ₃ per 1 mol of technetium). This is due to the fact that in experiment No. 1, in the process of uranium extraction at steps 2 and 3 (along the water phase), precipitation of uranium (VI) phosphates and technetium was observed, sediments were partially transferred with the extract, which reduced the purification of uranium from technetium.

In experiment No. 2, the formation of precipitation of phosphates was not observed. The purification of uranium from technetium in this experiment is maximum - during the extraction operation, the coefficient of purification of uranium from technetium is 266, and in general at the redistribution of extraction – washing, the purification coefficient was 2950.

In the third experiment, at a concentration of sodium hexametaphosphate 0.0008 mol NaPO ₃ / L (or 16 mol NaPO ₃ per 1 mol of technetium), the coefficient of uranium removal from technetium turned out to be 92 at the extraction, and 4730 after washing the extract at two stages (after the first leaching stage, the coefficient of purification of uranium from technetium was 1000). A decrease in the coefficient of purification of uranium from technetium during extraction is associated with a decrease in the concentration of sodium hexametaphosphate in an aqueous solution, and an increase in the total coefficient of purification is explained by an increase in the number of washing steps to two.

The coefficient of purification of uranium from technetium in experiment No. 4 was equal to 69.6.

In an industrial embodiment of the method, the spent wash solution containing residues of uranium (IV) will be combined with the uranium (VI) solution prepared for extraction during the extraction operation. There is no precipitation on extraction of tetravalent uranium with sodium hexametaphosphate when the uranium (VI) solution prepared for extraction is combined with the spent wash solution in the studied concentration range. In a separate experiment, when a washing solution containing uranium (IV) at a concentration of 2.0 g / l was mixed with a solution of uranium (VI) prepared for extraction containing 0.02 mol of NaPO ₃ / l, no precipitation of uranium phosphates formed (IV )

Thus, due to the use of sodium hexametaphosphate as a complexing agent, the claimed method ensures the purification of uranium from technetium in the extraction operation and allows the use of a washing solution containing uranium (IV) stabilized with hydrazine in the washing operation of the extract to increase the degree of purification of uranium from technetium in the washing operation .

Claims (1)

The method of extraction purification of regenerated uranium, including the reduction of technetium (VII) in an aqueous nitric acid solution of regenerated uranium (VI) with hydrazine, the combination of reduced technetium, the adjustment of the resulting solution according to the content of nitric acid by increasing its concentration and the extraction of uranium (VI) with tributyl phosphate in an organic diluent, characterized the fact that the combination of reduced technetium is carried out with sodium hexametaphosphate, which is introduced to a concentration of not more than 0.02 mol of NaPO ₃ per liter of solution ora, the uranium (VI) extract is washed with a nitric acid solution containing uranium (IV) stabilized with hydrazine, the spent washing solution is combined with the uranium (VI) solution in the extraction operation.