RU2425804C1 - Method of cleaning regenerated uranium - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method of cleaning regenerated uranium from thorium, technetium and neptunium, involves extraction of uranyl nitrate from aqueous nitric acid solution with tributylphosphate in an organic diluent. During multi-step countercurrent extraction at the step for output of the extract, the degree of saturation of the extraction agent with uranium is kept not higher than 92.5% of the limiting saturation and equilibrium concentration of nitrate in the aqueous phase is kept not higher than 0.7 mol/l. Concentration of nitric acid in the first stream of the feed aqueous uranyl nitrate solution, fed at the step for output of the extract, is kept not higher than 0.6 mol/l and saturation of the extraction agent with uranium is kept not higher than 92.5% of the limiting saturation at the third or next step, towards the aqueous phase, when the second feed stream of the aqueous uranyl nitrate solution is being fed.

EFFECT: high degree of purity of uranium.

5 cl, 5 tbl, 1 ex

Description

The invention relates to methods for the extraction purification of regenerated uranium and can be used in technological processes in the processing of irradiated nuclear fuel, where it is necessary to purify uranium from thorium-228, as well as from technetium-99, neptunium-237.

It is known to use complexing agents, hetero-tungstates of the composition P ₂ W ₁₇ O ₆₁ ^10- , As ₂ W ₁₇ O ₆₁ ^10- , SiW ₁₁ O ₃₉ ^8- , GeW ₁₁ O ₃₉ ^8- and PW ₁₁ O ₃₉ ^7- , for complexing thorium in extraction PUREX process to reduce the distribution coefficients of thorium between the organic and aqueous phases in the method of separation of uranium according to application No. 2005129985 for the grant of a patent of the Russian Federation, IPC G21C 19/46, publ. 04/20/2006 [1]. Contamination of the final product with foreign metals is possible.

A known method of purification of regenerated uranium from technetium according to the patent of the Russian Federation No. 2184083, IPC C01G 43/00, 57/00, publ. 06/27/2002 [2]. The method involves the restoration of technetium to non-extractable forms under conditions of acid deficiency. The method is not convenient enough, because without acid deficiency, quick extraction is required; at any delay, partial reduction of the reduced forms of technetium occurs and, as a result, a decrease in the purification of uranium.

A known method of extraction purification of uranium from thorium with 30% tributyl phosphate (TBP) in dodecane (M.Germain, D. Gourisse et M. Sougnez, Extraction en milieu nitrigue du thorium, du neptunium, du plutonium, par les solutions de phosphate de tributyle chargees en uranium // J. Inorg. Nucl. Chem., 1970, Vol. 32, pp. 245-253) [3]. The authors showed that in the single-stage extraction process with saturation of the extractant with uranium ~ 92.5% of the limit (the limit is 120 g / l) at a concentration of nitric acid in an aqueous solution (1.0-4.0) mol / l, the thorium distribution coefficient is 0.025. The disadvantage of this method is the relatively low purification of uranium from thorium. The method selected for the prototype.

The objective of the invention is to develop a method that improves the degree of purification of uranium from thorium, technetium and neptunium.

The problem is solved in that in a method for purifying regenerated uranium from thorium, technetium and neptunium, including extraction of uranyl nitrate from an aqueous nitric acid solution with tributyl phosphate in an organic diluent, in the process of multi-stage countercurrent extraction, the degree of saturation of the extractant with uranium exceeds 92.5% of the maximum saturation of the extractant with uranium and the equilibrium concentration of nitric acid in the aqueous phase is not more than 0.7 mol / l at a concentration of nitric acid in the first pi stream of an aqueous nitric acid solution of uranyl nitrate supplied to the stage of extracting the extract, not more than 0.6 mol / l and the saturation of the extractant with uranium more than 92.5% of the maximum saturation of the extractant with uranium in the third or subsequent stage of the aqueous phase when a second stream is fed to it feed aqueous nitric acid solution of uranyl nitrate.

Uranyl nitrate solutions are fed for extraction with a uranium concentration of 400 ÷ 500 g / l. The concentration of nitric acid in the first stream supplied to the stage of extracting the extract is less than the concentration of nitric acid in the second stream supplied to the third or subsequent stage. The concentration of nitric acid in the first stream is 0.03 ÷ 0.04 mol / L, and in the second stream 0.7 ÷ 0.8 mol / L.

Hydrazine stabilized uranium (IV) is introduced into the uranyl nitrate solution.

The method is as follows.

Countercurrent multistage extraction is carried out. The feed solution of uranium (aqueous nitric acid solution of uranyl nitrate) is supplied in two streams: to the first stage (numbering of the steps is given along the aqueous phase) and to the third or any subsequent stage (fourth, etc.). An increase in the sequence number of the stage to which the second feed solution stream is fed leads to an increase in the total number of stages of the extraction unit (an acceptable uranium content in the raffinate is provided by the length of the extraction unit from the input stage of the second feed solution stream to the raffinate output stage), therefore, it is advisable not to feed the second stream further than the third step. From the first stage, the final product is selected - uranyl nitrate extract in tributyl phosphate in an organic diluent. An extractant, tributyl phosphate in an organic diluent, is fed to the last stage, and raffinate is taken from the last stage. To stabilize technetium and neptunium in a tetravalent weakly extractable state, a reducing agent solution is introduced into the feed solution before being fed to the extraction unit or directly to the first and third steps.

Example 1. Single-stage extraction (determination of equilibrium thorium distribution coefficients).

In the experiments, conditions were selected to reduce the distribution coefficient of thorium between the organic and aqueous phases compared with the value of 0.025 achieved in [3].

Two series of stock aqueous solutions of uranyl nitrate were prepared. In each series, the solutions had a different concentration of nitric acid; the uranium concentration in the experiments of the first series differed from the concentration of uranium in the second series. In the experiments, uranium was extracted from each initial solution with 30% TBP in a hydrocarbon diluent at a phase ratio of O: B = 1: 1 and room temperature. In the first series, the saturation of the extractant with uranium was ≈92.6% of the limit, in the second series ≈94% (depending on the concentration of uranium in the initial aqueous solutions).

The results of the distribution of uranium and thorium between phases depending on the saturation of the extractant with uranium and the equilibrium concentration of nitric acid in the aqueous phase are presented in table 1.

Table 1 Component to be analyzed Component concentration one 2 3 Series No. 1 [U] extract, g / l (saturation of the extractant with uranium,%) 111.5 (92.9%) 111.1 (92.6%) 111.1 (92.6%) [U] raffinate, g / l 361.60 362.10 365,10 [HNO ₃ ] raffinate, g / l 22.5 48.0 90.5 [Th] extract, mg / l 1,63 2.67 2.64 [Th] raffinate, mg / l 95.0 100.0 95.0 To distrib. Th 0.0172 0,0244 0,0278 Series No. 2 [U] extract, g / l (saturation of the extractant with uranium,%) 113.30 (94.4%) 113.20 (94.3%) 112.50 (93.8%) [U] raffinate, g / l 441.70 433.70 447.10 [HNO ₃ ] raffinate, g / l 19.00 29.00 46.00 [Th] extract, mg / l 0.99 1.05 1.95 [Th] raffinate, mg / l 85.00 75.00 80.00 To distrib. Th 0.0116 0.0140 0,0227

From table 1 it is seen that the increase in the distribution coefficients of thorium, i.e. deterioration in the purification of uranium from thorium occurs with an increase in the concentration of nitric acid in the equilibrium aqueous phase. When the concentration of nitric acid in series No. 1, not exceeding 48 g / l (or 0.76 mol / l) with saturation of the extractant with uranium 92.6% of the maximum saturation of the extractant with uranium (120 g / l), and in series No. 2, not exceeding 46 g / l (or 0.73 mol / l) when the extractant is saturated with uranium 93.8%, the distribution coefficient of thorium is below the value of 0.025 achieved in [3].

The authors limited the acid content in the equilibrium aqueous phase to 0.7 mol / L, provided that the extractant was saturated with uranium more than 92.5% of the limit to increase the degree of purification of uranium from thorium. With multistage countercurrent extraction, this concentration ≤0.7 mol / L should be in the equilibrium aqueous phase at the stage of extracting the extract. For this, in practice, taking into account the transfer of acid to the first stage with the extract, the concentration of acid in the flow of the uranium feed solution supplied to the stage of extracting the extract should not exceed 0.6 mol / L. As further experiments showed (see Example 2), under the same conditions a high purification of uranium from technetium and neptunium is achieved.

Example 2. Multistage countercurrent extraction.

Three experiments were conducted on the extraction cleaning of irradiated uranium from thorium-228, technetium-99 and neptunium-237, two of which were carried out according to the claimed method.

The extraction unit included six steps. The first (step numbering is shown along the water phase) was fed with uranium feed solutions (aqueous nitric acid solutions of uranium in the form of uranyl nitrate containing thorium, neptunium and technetium), uranium extract was taken from the same step, extractant was fed to the sixth step - 30% - solution of TBP in a hydrocarbon diluent, raffinate was removed from the same step. The aqueous phase moved along the extraction block in countercurrent to the organic phase at room temperature. The saturation of the extractant with uranium was regulated by the concentration of uranium in the feed solutions and the phase ratio.

In the first experiment, the feed solution — nitric acid solution of uranyl nitrate — was supplied only to the first stage; in the feed solution of uranyl nitrate (solution No. 1), the concentration of nitric acid was 0.52 mol / L.

In the second and third experiments, the feed solution was supplied in two streams: in the first and third stages of extraction. In the second experiment, the concentration of nitric acid in the feed solutions of uranyl nitrate (solutions No. 1 and No. 2) was the same, equal to 0.52 mol / L. In the third experiment, the concentration of nitric acid in the feed solutions of uranyl nitrate was different: in the feed solution supplied to the first stage (solution No. 1) - 0.03 mol / l, in the feed solution supplied to the third stage (solution No. 2) - 0, 76 mol / l.

In all experiments, to restore neptunium (V) and technetium (VII) to a tetravalent state, a solution of a reducing agent, tetravalent uranium and hydrazine, was introduced into the initial aqueous solutions of uranium immediately before extraction. The reducing agent was introduced based on the calculation of the concentration of uranium (IV) equal to 1.0 g / l in a solution of uranyl nitrate.

The compositions of the feed aqueous solutions of uranium, before the introduction of reducing agents, are shown in table 2.

table 2 Experience number No. of feed solution The composition of the feed solution U (VI), g / l [Th (IV)], Bq / g U [Np (V)], mcg / g U [Tc (VII)], μg / g U [HNO ₃ ], mol / L one one 448.5 1,5 · 10 ⁺³ 0.45 8.0 0.52 No solution No. 2 - - - - - 2 one 448.5 1,5 · 10 ⁺³ 0.45 8.0 0.52 2 448.5 1,5 · 10 ⁺³ 0.45 8.0 0.52 3 one 450.1 1,5 · 10 ⁺³ 0.45 8.0 0,03 2 448.1 1,5 · 10 ⁺³ 0.45 8.0 0.76

The costs of the products served in the extraction unit are shown in table 3

Table 3 Experience number Product costs, volumetric relative units extractant feed solution No. 2 feed solution No. 1 one 24 0 6.9 2 24 4,5 2,4 3 24 4,5 2,4 four 24 4,5 2,4

The distribution of uranium and nitric acid over the steps of the extraction block is shown in table 4.

Table 4 Experience number Step number one 2 3 four 5 6 one [U] aq. solution, g / l 419.1 312.1 167.2 27.5 3.2 0.38 [HNO ₃ ] aq. solution, mol / l 0.92 0.98 0.83 0.64 0.56 0.49 [U] org. solution, g / l, (saturation of the extractant with uranium,%) 116.3 (96.9) - 95.5 (79.6) - - - 2 [U] aq. solution, g / l 415.6 374.1 257.8 29.8 3.6 0.49 [HNO ₃ ] aq. solution, mol / l 0.56 0.76 0.81 0.92 0.79 0.52 [U] org. solution, g / l, (saturation of the extractant with uranium,%) 116.4 (97.0) - 114.8 (95.6) - - - 3 Step number one 2 3 four 5 6 [U] aq. solution, g / l 433.7 362.2 231.0 29.8 2,4 0.32 [HNO ₃ ] aq. solution, mol / l 0.19 0.44 0.87 0.92 0.75 0.49 [U] org. solution, g / l (saturation of the extractant with uranium,%) 116.2 (96.8) - 112.0 (93.3) - - -

As can be seen from table 4, in experiments 2 and 3, the degree of saturation of the extractant with uranium at the first stage is 97.0 and 96.8%, and at the third stage - 95.6 and 93.3%, that is, at both steps saturation greater than 92.5%. In experiment 1 with a degree of saturation at the first stage of 96.9%, i.e. almost the same as in experiments 2 and 3, the degree of saturation in the third stage was 79.6%.

Acid transfer, consisting in the fact that the acid, partially extracted by the organic phase from the aqueous phase at the steps of the extraction unit, is transferred by the extract back to the first steps, where, due to the high concentration of uranium in the aqueous and organic phases, its reextraction takes place, noticeably less in experiments 2 and 3 in which the supply flows at the first and third stages provide saturation of the extractant with uranium at these stages more than 92.5%, compared with experiment 1, in which there is no second supply stream at the third stage, and the degree of saturation I'm in the third stage less than 92.5%.

The supply of power to the third (or subsequent) stage is facilitated by the creation of a high concentration of uranium on it, the redistribution of acid over the steps of the block and a decrease in the transfer of acid to the step of extracting the extract.

As can be seen from tables 2 and 4, at the same concentration of nitric acid in the feed solutions supplied to the first stage in experiments 1 and 2 (0.52 mol / l), in experiment 1 there was an increase in the equilibrium concentration of acid in the aqueous phase of the first stage by 0 , 4 mol / l (the concentration became 0.92 mol / l), while in experiment 2, the increase in acid concentration in the aqueous phase of the first stage was only 0.04 mol / l (concentration is 0.56 mol / l) . In experiment 3, the acid concentration in the aqueous phase of the first stage was 0.19 mol / L, the transfer of acid to the first stage is also significantly less than in experiment 1.

A decrease in acid transfer reduces the acid concentration in the extract stage, which in turn increases the purity of the extract being dispensed, i.e. increases the purification of uranium.

Thus, maintaining the saturation of the extractant with uranium in the third stage of more than 92.5% reduces the transfer of nitric acid with the extract to the first stage, ensuring that the content of nitric acid in the aqueous phase is not more than 0.7 mol / l in the first stage, provided that the acid content in the first a solution step of not more than 0.6 mol / L, and when the extractant is saturated with uranium more than 92.5% in the first stage, it provides thorium distribution coefficients in the extract delivery stage of less than 0.025.

The extracts of experiments 1, 2, and 3 were analyzed for the content of thorium, technetium, and neptunium. The coefficients of uranium purification from the above elements are calculated as the ratio of the element content in the initial feed solution per 1 g U in the initial feed solution to its content in the extract per 1 g U in the extract.

The results of the experiments are shown in table 5.

Table 5 Experience number [U] in products, g / l [HNO ₃ ] in the raffinates, mol / l Coefficients of cleaning U from Extract
1 stage Raffinate
6 step 1 stage 6 step Th Tc Np one 116.3 0.38 0.92 0.49 22 60.9 12.3 2 116.4 0.49 0.56 0.52 34 94 14.1 3 116.2 0.32 0.19 0.49 46 139 16

As can be seen from table 5, in experiment 2, higher coefficients of purification of uranium from impurities were obtained than in experiment 1 with one traditional feed stream supplied to the (first) stage of extracting the extract. In experiment 3, due to a decrease in the concentration of nitric acid in the aqueous phase of the stage of extracting the extract as compared with the concentration at the same stage in experiment 2, the coefficients of purification of uranium from impurities are even higher.

According to the prototype, the thorium distribution coefficient becomes equal to 0.025 at an equilibrium concentration of nitric acid in aqueous solutions of 1-4 mol / L and saturation of the extractant with uranium ~ 92.5% of the limit (120 g / L). In the method claimed by the authors, when the concentration of nitric acid is lower than in the prototype, and the extractant is saturated with uranium more than 92.5%, the distribution coefficient of thorium becomes less than 0.025, which allows to increase the purification of uranium from thorium, as well as from neptunium and technetium.

Claims (5)

1. The method of purification of regenerated uranium from thorium, technetium and neptunium, including the extraction of uranyl nitrate from an aqueous nitric acid solution with tributyl phosphate in an organic diluent, characterized in that during the multi-stage countercurrent extraction, the degree of saturation of the extractant with uranium is more than 92.5% from the limiting saturation of the extractant with uranium and the equilibrium concentration of nitric acid in the aqueous phase of not more than 0.7 mol / l at a concentration of nitric acid in the first feed water stream a nitric acid solution of uranyl nitrate supplied to the stage of extracting the extract is not more than 0.6 mol / l and the saturation of the extractant with uranium is more than 92.5% of the maximum saturation of the extractant with uranium in the third or subsequent stage of the aqueous phase when a second stream is fed to it feed aqueous nitric acid solution of uranyl nitrate. 2. The method according to claim 1, characterized in that the solutions of uranyl nitrate are fed for extraction with a concentration of 400-500 g / l by uranium. 3. The method according to claim 1, characterized in that the concentration of nitric acid in the first stream supplied to the stage of delivery of the extract is less than the concentration of nitric acid in the second stream supplied to the third or subsequent stage. 4. The method according to claim 3, characterized in that the concentration of nitric acid in the first stream is 0.03-0.04 mol / L, and in the second stream 0.7-0.8 mol / L. 5. The method according to claim 1, characterized in that uranyl (IV) stabilized with hydrazine is introduced into the uranyl nitrate solution.