RU2713010C1 - Method of purifying nitrate solutions from americium - Google Patents

Abstract

FIELD: radiochemical technology.

SUBSTANCE: invention relates to radiochemical technology and can be used for purification from americium of raffinate of extraction separation of production of mixed uranium-plutonium fuel. Purification of nitrate solutions from americium involves coprecipitation of americium with calcium oxalate from solutions containing compounds of accompanying metals, followed by calcination to oxides. Initial americium-containing solution is neutralized to pH in range of 0.7–1.2, after which the calcium-containing reagent is added to reach its concentration in solution of 1.5–3.5 g/l. Oxalic acid and/or salt thereof is added to the obtained solution in an amount which is not less than stoichiometric with respect to the sum of the accompanying metals, followed by holding the pulp to form a precipitate. Obtained americium-containing precipitate is filtered and washed with 0.5–50 g/l of oxalic acid solution having pH in range of 1–2.

EFFECT: invention enables to purify raffinate nitrate from americium, significantly reduces heat generation and activity of the raffinate to be recycled, and also enables to obtain an americium-containing compact product suitable for thermal treatment both for recycling and further extraction of americium.

5 cl, 1 dwg, 2 ex

Description

The invention relates to radiochemical technology and can be used for purification of americium raffinate extraction redistribution of the production of mixed uranium-plutonium fuel.

For the production of mixed uranium-plutonium fuel use the accumulated reserves of "energy" and "weapons" plutonium dioxide. During long-term storage in plutonium dioxide as a result of radioactive decays, the accumulation of americium occurs. There are limitations to the content of americium in mixed uranium-plutonium fuel. To purify plutonium from americium, a preliminary electrochemical dissolution of plutonium dioxide is carried out, followed by extraction refining. Americium as part of the raffinate is sent for burial.

According to modern environmental requirements, radionuclides must be separated (fractionated) before burial depending on their radiation characteristics, including their half-life, on which the method and, accordingly, the period of their controlled storage depends. Americium can be used in the nuclear industry, medicine and for scientific purposes. In view of this, there is often a need for purification, isolation and concentration of americium in a separate process stream, which is a prerequisite of the present invention.

From the existing level of technology there is known a method of extracting americium [patent RU No. 2477758, C22B 60/02, C22B 3/20, publ. 03/20/2013], including precipitation from an nitric acid solution containing americium and impurities, an americium-containing precipitate by introducing oxalic acid, calcining the obtained precipitate to oxides, dissolving them in nitric acid, extracting americium from the obtained nitric acid solution using a tributyl phosphate solid extractant and the following re-extraction, precipitation from a re-extract of americium oxalate and its calcination to americium dioxide. In this method, the initial nitric acid solution containing americium and impurities is concentrated to an americium content of at least 100 mg / l by repeatedly precipitating the precipitate containing americium, followed by its dissolution each time in a new portion of the initial solution.

The disadvantages of this method: the multi-stage and complexity of the organization of the deposition of americium, the use of oxalic acid only without adding an additional carrier, which is not effective at low concentrations of americium, low coefficients of purification of nitric acid solutions from americium, increased consumption of oxalic acid due to the formation of related elements (iron, aluminum and other) stable complex compounds with oxalate ion, obtaining americium dioxide in a form that makes it difficult to store it further storage or burial.

In order to reduce technological and environmental risks during long-term storage or final disposal of americium, due to its high specific activity, it is included in some solid inert matrix. In this case, the resulting matrix must be resistant to radiolysis and oxygen, and for the convenience of further processing, the calcined matrix should be easily soluble in nitric acid solutions. These requirements are fully consistent with calcium oxide, which can be obtained by calcining calcium oxalate. In turn, calcium oxalate is a good carrier in the coprecipitation of americium.

Closest to the claimed technological solution is a method of extracting americium in the form of americium dioxide [patent RU No. 2355554, C22B 60/02, C22B 3/20, publ. 10.10.2008], including the coprecipitation of americium oxalate on a carrier - calcium oxalate from an impurity solution, calcining the obtained precipitate to oxides, dissolving the obtained oxides to obtain a nitric acid solution, extracting americium from a nitric acid solution using a solid extractant based on diisooctylmethylphosphonate followed by reextraction, Obtaining americium oxalate by precipitation from one stripped off extract.

The disadvantages of this method are the low degree of deposition of americium in one stage, which interferes with the effect of the accompanying cations-acceptors of the oxalate ion on the completeness of the separation of americium, the absence of a washing operation for the precipitate, which can lead to the formation of insoluble impurities on the calcining operation.

The problem to which the invention is directed is to increase the purification from americium of the raffinate of the extraction redistribution of the production of mixed uranium-plutonium fuel while minimizing the amount of introduced co-precipitator (calcium oxalate) to obtain an americium-containing product suitable for long-term storage (after heat treatment of the obtained mixed oxalate).

The problem is solved in that in the method of purification of nitric acid solutions from americium, including coprecipitation of americium with calcium oxalate from solutions containing compounds of the accompanying metals, followed by calcination to oxides, the initial americium-containing solution is neutralized to a pH in the range of 0.7-1.2, then a calcium-containing reagent is added until its concentration in the solution reaches 1.5-3.5 g / l, oxalic acid and / or its salt are added to the resulting solution, in an amount not less than stoichiometric with respect to the total comorbid metal, followed by exposure of the pulp to form a precipitate obtained ameritsiysoderzhaschy precipitate was filtered off and washed with 0.5-50 g / l oxalic acid solution having a pH in the range 1-2.

The technical result of the invention is the purification of highly active raffinate from americium obtained at the stage of extraction refining of plutonium, in one operation of co-precipitation with calcium oxalate with a coefficient of 4 × 10 ³ with a total content of related metals (iron, chromium) up to 1 g / l.

The proposed solution allows us to purify americium from the raffinate of nitrate, significantly reduce the heat generation and activity of the raffinate to be disposed of, and also to obtain the americium-containing compact product, suitable after heat treatment for both utilization and further extraction of americium.

The essence of the invention lies in the process of coprecipitation of americium used as a carrier of calcium oxalate in the pH range of 0.7-1.2, which ensures the formation of the most developed surface and the completeness of the deposition of the carrier, which allows to reduce the amount of calcium introduced and to obtain a sufficient purification of raffinate nitrate from America in one stage. Precipitation under these conditions allows impurities to be kept in a dissolved state, and washing with a solution of oxalic acid with a given pH effectively removes soluble chemisorbed elements from the surface of calcium oxalate.

The use of an alkali metal hydroxide or aqueous ammonia solution to neutralize the initial raffinate is preferable because the alkali metal cations and the ammonium cation do not form stable complex compounds and insoluble precipitates with oxalate ions, unlike multiply charged cations, and do not adversely affect the process coprecipitation and purification of raffinate from americium.

The selected range of calcium content of 1.5-3.5 g / l in the pulp after neutralization is due to the fact that, at a concentration below 1.5 g / l, the fine particles formed do not provide the formation of a developed sediment surface, complicate the process of separation of the solid phase, thereby full one-stage coprecipitation of americium and completeness of raffinate purification are achieved. The calcium content above 3.5 g / l does not significantly affect the degree of coprecipitation of americium and the completeness of purification of the raffinate from americium.

The duration of exposure of the pulp for 15-240 minutes is due to the fact that the induction period for the formation of an oxalate precipitate is at least 15 minutes, with prolonged exposure of the pulp (more than 240 minutes) radiolysis of oxalic acid and the reverse dissolution of the formed oxalate precipitate in a nitric acid medium, in view of which single-stage quantitative purification of raffinate nitrate is not possible.

The use of a washout of 0.5-50 g / l oxalic acid solution at pH 1-2, obtained by the addition of ammonia, alkali metal hydroxide or nitric acid, preferably due to the fact that in the specified range and this content of oxalic acid does not reverse dissolve the formed oxalate sediment and no loss of americium with the wash solution.

The proposed method is implemented in accordance with the flowchart shown in FIG. 1 in the following sequence. A solution containing americium in the form of Am (III) is neutralized to a pH of 0.7-1.2. In a particular case, the neutralization of the initial solution is carried out by introducing a reagent selected from the list: alkali metal hydroxide, aqueous ammonia. After neutralization, calcium compounds are added until its concentration in the pulp obtained at the final stage of the described method is 1.5-3.5 g / l. Then oxalic acid and / or its salt are added at least stoichiometrically with respect to the sum of the accompanying elements (metals with an oxidation state of +2 and above) capable of forming strong compounds (precipitates and complexes) and characterized by a strong bond with the oxalate ion. When calculating the amount of oxalate ion introduced, the rate of precipitating reagent is taken into account for the formation of strong compounds with impurity metals - sparingly soluble metal oxalates with a charge of +2 and above, as well as for the formation of soluble complex compounds with a stability constant logK _{n of} the complexation stage according to reaction (1) is over 3.3.

where A ^{m +} is the central ion (metal ion) with a charge of m +;

n is the stage of complexation with oxalate ion.

In the particular case of oxalic acid and / or its salt is administered in the form of a solution. When the pH of the pulp is reduced after the introduction of the oxalate ion below 0.7, additional neutralization is carried out to a pH of 0.7-1.2. Next, the pulp is aged to form a precipitate of oxalates for 15-240 minutes. The precipitate is filtered off and, if necessary, washing the precipitate from metals that do not form solid phases under the indicated conditions (alkali metals, Ag ⁺ , Al ³⁺ , Fe ³⁺ , Cr ³⁺ , etc.), the precipitate is washed with 0.5-50 g / l a solution of oxalic acid, the pH of which is provided by the introduction of ammonia, an alkali metal or nitric acid to 1-2. In a particular case, to reduce the consumption of reagents and reduce liquid radioactive waste, the spent washing solution is used to prepare a solution of oxalic acid and / or its salt at the stage of introduction of the oxalate ion. The washed mixed calcium oxalate precipitate and americium are calcined to obtain oxides suitable for both utilization and further recovery of americium.

Example 1

A solution containing 0.05 g / l americium, 90 g / l nitric acid, 205 g / l sodium nitrate, 1.5 g / l silver, 0.2 g / l aluminum, 35 mg / l iron, 30 mg / l of chromium, 20 mg / l of nickel was neutralized with solutions of 400 g / l and 40 g / l of NaOH to a pH of 1.2. To the neutralized solution was added a 490 g / L solution of Ca (NO ₃ ) ₃ to a concentration of 1.65 g / L of calcium, so that the average concentration of calcium in the pulp was 1.5 g / L. A 60 g / L solution of oxalic acid was added to the resulting solution with constant stirring for 10 min to a concentration of 4.5 g / L, which corresponds to an excess of 1.1 g / L H ₂ C ₂ O ₄ over a stoichiometric amount of calcium. After the introduction of Н ₂ С ₂ О _{4, the} pulp for the formation of a precipitate was kept for 120 min and separated on a filter baffle of the MPFK-1G type with a pore size of 0.15 μm. The pH of the filtrate was 0.9. Gamma-spectrometric analysis showed that the degree of extraction of americium (the ratio of the amount of americium transferred to the precipitate to the amount of americium in the mother liquor) was 5.1⋅10 ³ .

The precipitate was washed with 10 g / l of a solution of oxalic acid, the pH was 1.5. The resulting wash solution containing less than 0.01 mg / L americium, 50 mg / L silver, 20 mg / L aluminum, less than 6 mg / L corrosion products (iron, chromium) was used to prepare the sodium oxalate solution used in Example 2. The washed precipitate of mixed oxalates was calcined. The resulting mixture of calcium oxides and americium is suitable for storage, as well as for subsequent extraction of americium.

Example 2

Used a solution of the composition analogously to example 1. The neutralization of the solution was carried out 250 g / l and 50 g / l NH ₄ OH to a pH of 0.7. A sample of CaO weighing 0.3 g was added to the neutralized solution with constant stirring, after which the sodium oxalate solution, prepared in accordance with Example 1, was prepared using the spent washing solution obtained in Example 1 to a concentration of 4 g / l in terms of oxalate ion. After the introduction of sodium oxalate, the pulp was aged for 15 minutes to form a precipitate and filtered. The pH of the filtrate was 1.1. Gamma-spectrometric analysis showed that the degree of extraction of americium (the ratio of the amount of Am transferred to the precipitate to the amount of americium in solution) was 4.5 × 10 ³ .

The precipitate was washed with nitric acid with a 20 g / l solution of oxalic acid, the pH was 2. The resulting wash solution containing less than 0.01 mg / l americium, 60 mg / l silver, 25 mg / l aluminum, less than 6 mg / l corrosion products (iron , chromium) was used analogously to example 1 at the stage of preparation of a solution of oxalic acid and / or its salt. Handling the washed precipitate was carried out analogously to example 1.

The proposed method, in contrast to the prototype method, allows to reduce the americium content in the raffinate of the extraction redistribution of mixed uranium-plutonium fuel production in one co-precipitation operation with calcium oxalate by 4 × 10 ³ times, to obtain an americium-containing precipitate, which after heat treatment is suitable for further storage or extraction of americium.

Claims (5)

1. The method of purification of nitric acid solutions from americium, including the coprecipitation of americium with calcium oxalate from solutions containing compounds of the accompanying metals, followed by calcination to oxides, characterized in that the initial americium-containing solution is neutralized to a pH in the range of 0.7-1.2, after of which calcium-containing reagent is added until its concentration in the solution reaches 1.5-3.5 g / l, oxalic acid and / or its salt are added to the resulting solution in an amount not less stoichiometric with respect to the amount of concomitant meta lows, followed by exposure of the pulp to form a precipitate, the resulting americium-containing precipitate being filtered off and washed with 0.5-50 g / l oxalic acid solution having a pH in the range of 1-2. 2. The method according to p. 1, characterized in that the neutralization of the initial solution is carried out by introducing a reagent selected from the list: alkali metal hydroxide, aqueous ammonia. 3. The method according to p. 1, characterized in that oxalic acid and / or its salts are administered in the form of a solution. 4. The method according to p. 1, characterized in that the exposure of the pulp to form a precipitate of oxalates is carried out for 15-240 minutes. 5. The method according to p. 3, characterized in that for the preparation of a solution of oxalic acid and / or its salt using the spent solution from washing americium-containing precipitate.

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