RU2257626C2 - Method for recovering alpha-active nitric acid solutions containing trivalent iron - Google Patents

Abstract

FIELD: recovery of liquid radioactive wastes.

SUBSTANCE: proposed method for recovering alpha-active nitric acid solution containing trivalent iron includes pre-concentration of solution being recovered by its evaporation to produce regenerated nitric acid in still bottoms, whereupon still bottoms are neutralized to pH of 1 - 2 and trivalent iron is partially recovered by sodium sulfate until valence forms ratio Fe³⁺ : Fe² = 2 : 1 is attained. This is followed by next neutralization with alkali to pH = 10 - 11. Magnetite sediment obtained after settling down is conveyed for curing. Then solution is decanted, clarified solution is magnetically separated and additionally cleaned.

EFFECT: reduced volume of secondary wastes, reduced consumption of chemical agents used for the purpose.

2 cl, 1 tbl, 1 ex

Description

The invention relates to the field of processing and disposal of liquid radioactive waste (LRW) of an average level of activity and can be used mainly in radiochemical plants, where solutions are formed with a high iron content. In the current process of chemical and metallurgical production of plutonium, acidic solutions are formed with a high (1-7 g / l) iron (III) content. Currently, these wastes are processed by neutralizing the solution with alkali. The precipitate of hydrated sludge pulp is sent for storage for further processing. The disadvantage of this method is the formation of a large number (up to 30% of the total volume) of hydrated-slurry pulps, subject to further localization.

A known method of neutralizing liquid radioactive waste, based on the deposition of iron hydroxide [Kuznetsov Yu.V., Schebetkovsky V.N., Trusov A.G. Fundamentals of water decontamination, 1968, M., Atomizdat, pp. 80-88]. However, the iron hydroxide pulp has a relatively large volume. The precipitate of mixed iron oxide (magnetite) has a crystalline structure and therefore has a higher density, better sedimentation and filtration properties than the precipitate of iron hydroxide, which allows to obtain a smaller volume of secondary waste [Radovenchik V.M., Korostyatinets V.D., Ivanenko E .AND. Study of the efficiency of the separation of iron ions from aqueous solutions by the ferritic method — Chemistry and Water Technology, 2001, v.23, No. 2, pp. 172-176; Klenisheva L.D., Zadorozhnaya A.B., Bunturi I.N. Methods of intensifying the separation of a suspension of iron hydroxides - Ecotechnology and resource saving, 1994, No. 5-6, p.87-91].

Closest to the proposed method is the purification of alpha-active LRW, which consists in the formation of a magnetite precipitate by mixing solutions of ferrous and ferric iron in a ratio of Fe ³⁺ : Fe ²⁺ = 2: 1, introducing the obtained magnetite precipitate into the processed solution, followed by adjusting the pH of the treated solution to a pH of more than 9, sedimentation of a precipitate of magnetite and decantation of the purified solution [Simidzy K. Waste water treatment by ferritization. - Chemical Economy And Engineering Review, 1975, vol. 7, No. 7, p.32-37].

The disadvantage of this method is that for the purification of acidic solutions of chemical and metallurgical production, the introduction of a significant amount of ferrous iron will be required, which will lead to an increase in the total amount of waste and will minimize the advantages of this method.

The objective of the invention is the purification of alpha nitric acid active solutions, reducing the volume of secondary waste to be further localized, reducing the amount of alkali required to neutralize.

The problem is solved by a method including evaporation of the processed solution, as a result of which regenerated nitric acid is returned, which is returned to the process, and still bottoms, in which the chemical and radioactive components are localized. To adjust the pH to 1-2, alkali is metered into the bottom residue, and then sodium sulfite is introduced to restore Fe ³⁺ to Fe ²⁺ . The reducing agent is dosed in an amount that allows you to get the ratio of the valence forms of Fe ³⁺ : Fe ²⁺ = 2: 1. The resulting solution is neutralized with alkali to a pH of 10-11. As a result of neutralization, a magnetite precipitate is formed and the solution is purified from radionuclides. After sedimentation, the precipitate is sent for curing, and the solution is decanted and sent for further purification by known methods, for example, sorption. Since magnetite has good magnetic properties, it is proposed to use magnetic separation before further purification to ensure that decantate is cleaned of magnetite particles.

The table shows the deposition characteristics of mixed iron oxides prepared chemically for various ratios between Fe ²⁺ and Fe ³⁺ and for various pH values.

Table
Change in pulp volume depending on the ratio between the valence forms of iron and the pH of the solution PH Fe ³⁺ : Fe ²⁺ ratio Pulp volume,% of the initial solution volume 10 10 35 10 4 22 10 2 thirteen 10 0.67 thirty 10 0.29 35 8 2 thirty nine 2 27 10 2 thirteen eleven 2 16

A distinctive feature of this method from the prototype is the partial recovery of ferric iron present in the solution, which makes it possible to refuse the introduction of ferrous salts and reduce the amount of secondary waste to be cured, and hence the cost of subsequent storage (disposal). In addition, due to the regeneration of nitric acid, the amount of alkali required to neutralize the solution is reduced.

Example

Alpha nitrate active solution with the content of [НNO ₃ ] = 380 g / l, [Fe ³⁺ ] = 4 g / l, total alpha activity ~ 1 · 10 ⁸ Bq / l, total beta activity ~ 1 · 10 ⁷ Bq / l is fed to the evaporator in order to partially return nitric acid to production and reduce alkali consumption for subsequent neutralization.

As a result of evaporation, two solutions are obtained: regenerated nitric acid returned to production and still bottom residue sent for further processing.

To the bottom residue with an average composition of [HNO ₃ ] = 180 g / l, [Fe ³⁺ ] = 6 g / l, alpha activity ~ 1.5 · 10 ⁸ Bq / l, beta activity ~ 1.5 · 10 ⁷ Bq / l is injected with alkali to adjust the pH to 1-2 (free acid concentration of 0.6-6.0 g / l). A solution of sodium sulfite is introduced into the resulting solution to partially restore ferric iron. The amount of reducing agent is introduced so that the ratio between the valence forms of iron corresponds to the ratio of Fe ³⁺ : Fe ²⁺ = 2: 1. The resulting solution is then neutralized with alkali to pH = 10-11. After sedimentation of the magnetite precipitate, the purified solution with an average composition [Na ₃ ] = 250 g / l, alpha activity ~ 1 · 10 ² Bq / l, beta activity ~ 1 · 0 ² Bq / l decanted, subjected to magnetic separation for complete removal particles of magnetite and sent for additional cleaning together with low-level waste, and pulp with alpha activity ~ 1 · 10 ⁹ Bq / l, beta activity ~ 1 · 10 ⁸ Bq / l sent for curing by known methods, for example, cementing.

Thus, the proposed method allows to obtain regenerated nitric acid, returned to production, to reduce the amount of alkali required to neutralize the processed solution, and to reduce the amount of secondary waste in the form of pulp, to be further localized.

Claims (2)

1. A method of processing nitric acid alpha-active solutions containing ferric iron, including pre-evaporation of the processed solution to obtain regenerated nitric acid and bottoms, its neutralization to pH 1-2, partial restoration of ferric iron with sodium sulfite to the ratio between the valence forms of Fe ³⁺ : Fe ²⁺ = 2: 1, subsequent neutralization with alkali to pH 10-11, the formation of a precipitate of magnetite, which after settling is sent to the curing, decantation of the solution, magnetic separation for Ekantata and the direction of the decantate for additional cleaning. 2. The method according to claim 1, characterized in that the additional purification is carried out by ion exchange.