RU2313148C1 - Method for decontaminating radioactive waste and soil - Google Patents

Abstract

FIELD: environment protection; decontamination of radiochemical industry radioactive waste, sediment cake, sludge, highly concentrated acid solutions, and soils containing radionuclides.

SUBSTANCE: proposed method for decontaminating radioactive materials includes a number of sequential operations: extraction of radionuclides from solid phase by means of mineral acid; neutralization of solution with alkali to pH = 5.8-5.9; separation of sediment by sedimentation, treatment of clarified solution with alkali to pH = 9-10 and its oxidation-reduction treatment in electrolyzer incorporating soluble electrodes of manganese steel with atmospheric oxygen saturated solution. Joint precipitation products of radionuclides are removed by sedimentation together with collectors and coagulants. Solution is filtered off, corrected by means of acid to pH = 8.5-9.0, and passed through sorbent followed by solution treatment using dialysis through electrical membrane and reconditioning of decontamination agents.

EFFECT: enhanced degree of decontamination, minimized amount of radioactive sludge, ability of extracting valuable components from waste, reduced maintenance charges.

3 cl, 1 dwg

Description

The invention relates to the field of ecology, and in particular to methods of decontamination of radioactive waste from radiochemical industries, and can be used to decontaminate cake, cake, slag, pulps, as well as soils, soils, silt with a high content of radionuclides and highly concentrated acidic radioactive technological solutions.

Known methods for decontamination of waste from radiochemical production by leaching of radionuclides from the solid phase with water (see patent RU No. 2226773, class G21F 9/28, 2005), alkalis (see patent RU No. 2208852, class G21F 9/04, 2003 .), mineral acids (see patent RU No. 2207393, class C22B 60/02, 2003), followed by the precipitation of radioactive salts with chemical reagents.

Their disadvantages:

- significant consumption of reagents for decontamination;

- large volumes of radioactive fallout intended for disposal;

- do not provide removal from the filtrates of radionuclides of an alkali group of metals, for example cesium-137;

- certain restrictions on the ability to remove radioactive contaminants associated with a chemical bond to the solid phase, if water and alkalis are used as leachants.

There is a method of decontamination of solid radioactive materials and, in particular, soil by leaching of radionuclides with sodium carbonate (see patent RU No. 2142172, class G21F 9/00, 1999), followed by extraction of radionuclides from an alkaline solution by ion-exchange particles containing magnetic material. Chelate complexes are removed from the solution by magnet. The disadvantages of this method are the limited chemical ability of the leaching agent (Na ₂ CO ₃ ) in extracting the entire gamut of radioactive compounds from the solid phase and converting them to a soluble state, as well as in the high cost of the unique chemisorption material.

Known methods for processing radioactive pulps and precipitates by dissolving them in nitric acid (see patent RU No. 2249268, class G21F 9/16, 2005) or in a mixture of nitric acid with hydrazine or hydroxylamine (see patent RU No. 2234153, class. G21F 9/04, 2004) followed by evaporation of the solutions and vitrification. The disadvantages of these methods are significant economic costs associated with evaporation of solutions and utilization of nitrogen oxides.

There is a method of treating radioactive waste with separation of valuable components by dissolving the solid phase in nitric acid, followed by extraction of radionuclides by multi-stage extraction with tributyl phosphate. The disadvantage of this method is the complexity of the technological chain of processing of radioactive waste and the limited eluting ability of the eluent.

Methods of decontamination of soils and soils by gentle methods are widely known: by eluting them with water, aqueous solutions of ammonium salts and ferrous salts, ammonia solutions and ammonium salts, aqueous solutions of carbonates with complexing agents, isotopic exchange and other chemical, biochemical and physical methods. A common drawback of these methods is the low efficiency of the extraction of radionuclides associated with chemical bonds with soil.

A more effective method of soil decontamination is proposed in the work (see patent RU No. 2094887, class G21F 9/23, 1997). The soil is treated with hydrochloric or nitric acid and ammonium fluorides or silicofluorides. The degree of extraction of strontium-137 and other radionuclides is higher than in the mentioned methods of decontamination of soils and soils. This is explained by the destruction of complexes of radionuclides under the action of mineral acids and their translation into soluble forms. This method of soil decontamination is effective, but economically disadvantageous without the recycling of leach reagents.

The decontamination of highly concentrated liquid wastes from radiochemical industries is represented by three main methods for cleaning solutions from radionuclides.

The first - physical methods - evaporation or freezing of solutions with subsequent sealing and burial of sludge, for example (see patent application RU No. 2003103213, CL G21F 9/16, 2004; patent RU No. 2171509, CL G21F 9 / 06,2001, )

The second is physico-chemical:

- extraction of radionuclides from solutions with selective extractants with further post-treatment of solutions, for example (see patent RU No. 2234549, class C22B 60/02, 2004);

- sorption of radioactive ions by natural and synthetic sorbents, for example, sorbents based on copper or nickel ferrocyanides, followed by desalination and concentration by electro-membrane method or reverse osmosis, with further purification and zeolites or chabazite (see patents RU No. 2101235, С02F 9/00, 1998 No. 21198945, class С02F 1/28, 1999). Purification of liquid radioactive waste with a sorbent co-precipitator - manganese dioxide, which is obtained by electrochemical reduction of potassium permanganate (see patent application RU No. 2003112044, 2004).

The third is chemical (reagent), the deposition of radionuclides from solutions by reagents of various nature, for example (see patent RU No. 2200354, class G21F 9/06, 2003; patent RU No. 2217823, class G21F 9/04, 2003).

Disadvantages of liquid waste decontamination methods: physical methods - high costs for evaporation and freezing of solutions; physico-chemical and chemical methods - large volumes of radioactive sludge to be disposed of, and increased salinity of solutions.

When analyzing the generally accessible and patent literature, methods close to the technical solution for decontamination of radioactive waste to the claimed method were not found. According to the totality of essential features, a method for decontamination of radioactive waste from industries can be taken as a prototype (see "Non-ferrous metals", 1985, pp. 53-56).

According to the prototype method, radioactive waste is dissolved in mineral acids. The resulting solution (pulp) is treated with barium chloride, sodium sulfate or sulfuric acid and iron scrap at elevated temperatures. The acidic solution is neutralized with a 0.5-3N alkali solution to a pH of 4-5. The precipitate was separated by filtration, and the filtrate was treated with calcium hydroxide to a pH of 8.0 ± 0.5. Radioactive fallout is buried in special waste storage facilities.

This method allows with an efficiency of up to 99.9% to carry out the decontamination of thorium waste, converting its salts into an insoluble form suitable for long-term burial.

The disadvantages of the method:

- unsatisfactory degree of decontamination from daughter radionuclides and alkali and alkaline earth metals;

- significant consumption of non-renewable reagents;

- significant amounts of sludge to be disposed of.

The objective of the invention is to develop a method to reduce operating costs for the decontamination of radioactive waste, to increase the degree of their decontamination, to minimize the amount of radioactive sludge.

The goal is solved in that in the method of decontamination of radioactive waste, soil, soil, including leaching with mineral acid, precipitation of radionuclides with bases without collectors and in the presence of collectors and coagulants, according to the invention, after leaching and removal of suspensions, the acid solution is neutralized with alkali to pH 5.8- 5.9, and the precipitated precipitate of uranium and thorium hydroxides is separated by precipitation, the clarified solution is treated with alkali to a pH of 9-10 and subjected to redox treatment in an electrolyzer with with soluble electrodes of manganese steels when the solution is saturated with atmospheric oxygen up to 10-12 mg / l, the products of the coprecipitation of radionuclides with collectors and coagulants are removed by precipitation, the solution is filtered through coarse and fine filters, adjusted with acid to pH 8.5-9.0, passed through a sorbent with selective selectivity to alkaline radioactive metal ions, followed by treatment of the solution with electro-membrane dialysis and the regeneration of deactivation reagents.

Preferably, nitric or hydrochloric acids are used as the leaching mineral acid, and manganese dioxide and ferric hydroxide produced in the electrolyzer are used as collector and coagulant.

Using the proposed technical solution will allow to obtain the following results:

1. To extract uranium, a valuable element for industry, from radioactive waste with minimal cost.

2. To reduce operating costs for the decontamination of radioactive waste and objects of radiation pollution by replacing the reagent method of radionuclide removal with electrocoagulation.

3. Minimize the amount of radioactive sludge to be disposed of.

4. To increase the degree of decontamination of radioactive waste by expanding the range of radionuclides to be removed - from actinides, lanthanides to alkali and radioactively infected metals.

The claimed technical solution differs from the prototype in that in the method, after leaching the solid phase with mineral acid, the acid solution of radionuclides is neutralized with alkali to a pH of 5.8-5.9 to precipitate not only thorium hydroxides from the solution, but also uranium and partially lanthanides; oxidation-reduction treatment of a solution of heavy metal radionuclides is carried out in an electrolyzer (electrocoagulator) with soluble manganese steel electrodes, followed by coprecipitation of the reduced radionuclide ions with manganese dioxide collectors produced in the electrocoagulator; sorption purification of the solution from salts of radioactive alkali metals with selective sorbents based on ferrocyanides; leaching and neutralization reagents are regenerated by electro-membrane dialysis. These differences allow you to achieve the claimed effect.

From the patent and scientific literature there is no known method for the decontamination of radioactive waste from soils and soils, in which the following sequence of waste treatment operations: leaching of radionuclides from the solid phase with nitric acid; precipitation from an acidic solution of valuable components - hydroxides of uranium and thorium, by alkalizing the solution of radionuclides with alkali to a pH of 5.8-5.9; removal from the solution of heavy metal radionuclides by redox treatment of the solution at pH 9-10 in an electrocoagulator with electrodes of manganese steels, followed by coprecipitation of the reduced radionuclides with collectors based on manganese dioxide and ferric hydroxides produced in the electrocoagulator; sorption of ions of radioactive alkali metals by selective sorbents; regeneration of leaching and neutralization reagents.

The method of producing a collector (co-precipitator) based on manganese dioxide by electrochemical dissolution of electrodes from manganese steels to remove radionuclides from solutions has no analogues in open literature.

The claimed technical solution can be used in the field of ecology for the decontamination of radioactive waste contaminated with radionuclides of soils and soils.

The inventive method of decontamination is carried out using a device, a diagram of which is shown in the drawing.

A device for the decontamination of highly concentrated radioactive waste contains four blocks - A, B, C, G.

Block A — leaching block — includes drum 1 (a tank-type reactor with a stirrer), measuring tanks for nitric acid 1-1 and water 1-2, a filter container for deactivated product 1-3, a storage tank for washing water 1-4, a sump precipitator 2, clarifier 3, sludge collector 3-1.

Block B - block of uranium deposition, thorium - includes a reactor 4, a measuring tank for an alkaline solution 4-1, a clarifier 5, a sediment collector of uranium-thorium 5-1.

Block B - electrocoagulation unit - includes an electrocoagulator 6, an air compressor 6-1, a sump 7, a clarifier 8, a sludge collector 8-1, a filter cartridge unit 9, a reactor 10, an acid meter 10-1, a selective sorption filter 11.

Block G - block of electro-membrane dialysis - includes electro-membrane dialyzers 12, storage tanks for alkali 12-1, acids 12-2.

The method of decontamination is as follows.

The method provides for the sequential removal of radionuclides of different chemical nature from radioactive waste by directed operations, including leaching of radionuclides from the solid phase with mineral acid; precipitation from an acidic solution of valuable components - hydroxides of uranium and thorium, neutralization of the solution with alkali to a pH of 5.8-5.9; removal from solution of heavy metal radionuclides in the form of insoluble complexes of their hydroxides with collectors based on manganese dioxide and iron (III) hydroxide, by redox treatment of the solution at pH 9-10 in an electrolyzer with soluble electrodes from manganese steels and saturation of the solution with atmospheric oxygen, adjustment the pH of the solution with an acid to pH 8.5-9.0, followed by removal of salts of radioactive alkali metals from it by sorption them on selective sorbents; regeneration of leaching reagents and neutralization by electro-membrane dialysis.

Correction of the acidity of the treated solution to a pH of 5.8-5.9, after the “leaching” operation, creates optimal conditions for almost complete (~ 100%) precipitation of uranium and thorium hydroxides from the solution (see F. Cotton, J. Wilkinson. Modern Inorganic Chemistry, Moscow: Mir, 1969, v. 3, p. 540).

The use of an electrolyzer (electrocoagulator) with soluble electrodes of manganese steels in decontamination has two goals: reducing the operating costs of deactivation reagents and producing efficient collectors (co-precipitators) for radionuclides in the electrolyzer.

до четырехвалентных катионов М⁴⁺ по уравнениюUnder the conditions of electrolysis of aqueous solutions, along with the usual products of the electrochemical destruction of water to oxygen and hydrogen, the electrochemical reduction of hexavalent oxo-cationic actinides occurs

up to tetravalent M ⁴⁺ cations by the equation

The tendency to the formation of complex compounds and the strength of complexes with complexing agents in the tetravalent cations of M ⁴⁺ actinides is the highest in the series of actinides. This property is determined by the size and charge of the ion (see F. Cotton, J. Wilkison. Modern inorganic chemistry. M: Mir, 1969, v. 3, p. 536).

In parallel with actinide reduction, oxidation processes occur on the electrodes with the formation of the coagulant Fe (OH) 3 and the proven collector (co-precipitator) for radionuclides - MnO ₂ , according to the equations

Coagulant formation:

[Fe (H ₂ O) ₆ ] ²⁺ + ² OH ^- → Fe (OH) ₂ + 6H ₂ O

[Fe (H ₂ O) ₆ ] ³⁺ + 3OH ^- → Fe (OH) ₃ + 6H ₂ O

Collector Education:

[Mn (H ₂ O) ₆ ] ²⁺ + ² OH ^- → Mn (OH) ₂ + 6H ₂ O

Manganese (II) hydroxide is easily oxidized to manganese dioxide by atmospheric oxygen according to the equation

2Mn (OH) ₂ + O ₂ → 2MnO ₂ ↓ + 2H ₂ O

Air oxygen is supplied to the electrolysis chamber by a compressor.

The method of decontamination is illustrated by examples.

Example 1. The decontamination of solid radioactive materials.

The solid radioactive material to be decontaminated is crushed, loaded into drum 1 (Block A), treated with 3-5 M nitric or hydrochloric acid from the measuring unit 1-1 during rotation of the tank. The acidic solution is separated by centrifugal force and pumped into the settling tank 2. The deactivated material is repeatedly washed with water from the measuring cup 1-2, centrifugally dried and discharged into the container 1-3. Wash water is collected in a storage tank 1-4.

From sump 2, an acidic solution of radionuclide salts is pumped to continuous clarifier 3 to separate suspended particles. The sludge is discharged into the sludge collector 3-1. The clarified solution is fed into reactor 4 (Block B) and, with stirring and cooling, it is neutralized with a solution of 3-5 M alkali from 4-1 measurer to pH 5.8-5.9.

The precipitated precipitate of uranium, thorium and partially lanthanide hydroxides is precipitated in clarifier 5, collected in a collector 5-1, washed, dried and sent for processing.

After removal of uranium and thorium from the solution, the weakly acid solution is pumped into the electrolyzer (electrocoagulator) 6 (Block B) with soluble electrodes from manganese steels, it is treated with alkali to pH 9-10 and, when the solution is saturated with oxygen, air up to 10-12 mg / l from compressor 6 -1 subject it to redox treatment at a current density on the electrodes of 50-150 A / dm ² , the interelectrode distance of 10-40 mm, the processing time of the solution in the electrocoagulator 5-15 minutes The pulp with a suspension of coagulated complexes and hydroxides of radioactive heavy metals is sequentially sent to the sump 7, clarifier 8, coarse and fine filter cassette 9. Radioactive fallout is dumped into sludge collector 8-1.

The solution, purified from suspensions and organics, is fed into the reactor 10, neutralized with an acid from a mernik 10-1 to pH 8.5-9.0, and filtered through a filter 11 with a selective sorbent of radioactive alkali metal ions.

The solution purified from radionuclides enters Block G, into the electro-membrane dialyzers 12, for the regeneration of deactivation reagents - acids and alkalis. Regeneration products are collected in storage tanks of alkali 12-1 and acid 12-2.

Example 2. Decontamination of soils.

The soil, soil layer infected with radionuclides is removed, crushed, separated from the plant material, loaded into the drum 1, processed, as in example 1.

Example 3. Pulp decontamination.

The pulp is pumped into the reactor 1 (Block A) of the tank type with a stirrer, cooled and treated with mineral acid. Further processing of the acidic solution is similar to example 1.

Example 4. The deactivation of acidic solutions is carried out according to the scheme of example 1, with the exception of the leaching unit.

Claims (3)

1. The method of decontamination of radioactive waste, soil, soil, including leaching with mineral acid, precipitation of radionuclides with bases without collectors and in the presence of collectors and coagulants, characterized in that after leaching and removal of suspensions, the acid solution is neutralized with alkali to pH 5.8-5.9 the precipitated precipitate of uranium and thorium hydroxides is separated by precipitation, the clarified solution is treated with alkali to a pH of 9-10 and subjected to redox treatment in an electrolyzer with soluble manganese electrodes steel at saturation of the solution with atmospheric oxygen up to 10-12 mg / l, the products of the coprecipitation of radionuclides with collectors and coagulants are removed by precipitation, the solution is filtered through coarse and fine filters, adjusted with acid to pH 8.5-9.0, passed through a sorbent with selective selectivity to alkaline radioactive ions, followed by treatment of the solution with electro-membrane dialysis and the regeneration of deactivation reagents. 2. The method according to claim 1, characterized in that nitric or hydrochloric acids are used as the leaching mineral acid. 3. The method according to claim 1, characterized in that manganese dioxide and ferric hydroxide produced in the electrolyzer are used as the collector.