RU2537391C1 - Method of recycling liquid radioactive wastes - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to method of recycling liquid radioactive wastes (LRW), including their immobilisation into crystalline material, and can be used at enterprises of nuclear power industry and chemical-metallurgical enterprises. Method includes synthesis of insoluble compounds, immobilising long-lived radionuclides and further separation of sediment. LRW, containing complexes of Co with athylenediaminetetraacetic acid (EDTA), with realisation of electrochemical synthesis of insoluble cobalt compounds. For this purpose electric current with parameters corresponding to mode of microarc oxidation is connected to electrodes, located in reservoir with LRW. Process is performed under normal conditions.

EFFECT: invention provides possibility of purifying LRW, containing soluble complexes of metals with EDTA with simplification of apparatus complex, providing LRW purification.

2 cl, 7 dwg

Description

The invention relates to environmental protection, and in particular to methods of processing liquid radioactive waste (LRW), providing for their immobilization in crystalline material, acceptable from an environmental point of view, and can be used in nuclear power plants and chemical and metallurgical industries.

A known method of purification of liquid radioactive waste: Ca ⁺ and PO ₄ ^3- ions are introduced into the solution ^, as well as magnesium ions, and then alkali precipitation is carried out at pH 10-11.5. The introduction of magnesium ions together with phosphates of two or more valence ions precipitates monovalent Rb and Cs in the form of sparingly soluble salts of Cs (Rb) MgPO ₄ · H ₂ O. An increase in the degree of purification of radioactive solutions from Sr, Cs, Rb is achieved by additional introduction of Sr ² ions ⁺ in an amount of 1-300 mg / l and Cs ⁺ and (or) Rb ⁺ in an amount of 0.5-5 mg / l (see RU No. 2200354, G21F 9/06, 2003).

The disadvantage of this method is that the process at a pH of 10-11.5 does not give a high degree of purification from Pu and Am, and precipitation is characterized by low values during sedimentation and filtration. The resulting precipitation has a relatively high humidity, and, as a result, there is a need for additional energy and labor costs for the evaporation of moisture before loading the precipitation into the container. The use of relatively expensive reagents (compounds Sr, Cs, Rb) leads to an increase in the cost of the process.

There is also known a method of processing liquid radioactive waste (LRW), including the synthesis of insoluble compounds that immobilize long-lived radionuclides, and subsequent separation of the precipitate (see RU No. 2321190, G21F 9/16, 2008). In this case, the synthesis of insoluble compounds is carried out under hydrothermal conditions at a temperature of 180-250 ° C and a pressure of 20-150 atm in a stream, passing the processed LRW and the reagents necessary for synthesis through a layer of insoluble particles at a rate that ensures crystallization of the synthesized compounds containing radionuclides, on the surface of the particles of the layer. The method provides the ability to clean LRW, including containing Co.

The disadvantage of this solution is the need to use constructively sophisticated equipment designed to operate in such adverse conditions, in addition, since under these conditions the synthesis of oxides occurs by crystallization of a new phase on the surface of the particles introduced into the solution, it is clear that the content of immobilized material in the sediment mass decreases. In addition, the presence of soluble metal complexes in liquid radioactive waste, such as Co with ethylenediaminetetraacetic acid (EDTA), greatly complicates its removal from solutions.

The objective of the proposed method is to simplify the hardware complex that provides LRW purification while ensuring the possibility of purification of LRW containing soluble metal complexes with EDTA.

The technical result obtained by solving the problem is expressed in providing the possibility of purification of LRW containing soluble metal complexes with EDTA. In addition, the hardware complex providing LRW cleaning is simplified. The method is applicable for the purification of LRW containing soluble metal complexes with EDTA in a wide range of radionuclide activity.

To solve this problem, the method of processing liquid radioactive waste (LRW), including the synthesis of insoluble compounds that immobilize long-lived radionuclides, and subsequent separation of the precipitate, is characterized in that LRW containing Co complexes with ethylenediaminetetraacetic acid (EDTA) are used, while the electrochemical synthesis of insoluble cobalt compounds, for which an electric current is supplied to the electrodes placed in a tank with LRW, with parameters corresponding to the microarc oxidized mode Niya. In addition, the process is carried out under normal conditions.

A comparative analysis of the essential features of the claimed solution with the essential features of the prototype and analogues indicates the compliance of the claimed solution with the criterion of "novelty."

The features of the characterizing part of the claims provide a solution to a set of functional tasks.

The signs “..use aqueous LRW containing complexes of Co with ethylenediaminetetraacetic acid (EDTA)” provide the possibility of processing LRW produced as decontamination waste from equipment and facilities of nuclear reactors. It is known that at present the presence of soluble Co complexes with EDTA in LRW greatly complicates the removal of Co ions from solutions. For this, cobalt complexonate is oxidized with hydrogen peroxide, ozonation, permanganate and electrochemical oxidation, but all of the above methods are not effective enough.

Signs indicating that "carry out the electrochemical synthesis of insoluble cobalt compounds", provide the possibility of precipitation of compounds of Co in the precipitate, without the use of special reagents or high temperatures or high pressure, which allows you to separate from the solution compounds containing radionuclides by filtration.

Signs indicating that the electrochemical synthesis is carried out by supplying electric current to the electrodes placed in a tank with LRW, provide the implementation of the electrochemical synthesis of insoluble cobalt compounds.

Signs indicating that the parameters of the electric current must correspond to the “microarc oxidation regime” allow drastic changes in pH, temperature (up to several thousand degrees) and pressure (due to the formation of gas bubbles, the pressure can reach 1000 atm), which ensures effective destruction Co complex with EDTA.

Signs of an additional claim specify conditions ensuring the implementation of the method, minimizing requirements for operational parameters (the method is implemented at a pressure of 760 mm Hg and a temperature of 20 ° C).

The claimed method is illustrated by materials where in figure 1 shows table 1 "Process Parameters in a model solution of complexonate (Co-EDTA)"; figure 2 shows the x-ray sample of the dry precipitate before annealing (after drying at a temperature of 100 ° C); figure 3 shows the x-ray sample of the precipitate after annealing to 1050 ° C; figure 4 shows a thermogram of a sample of sediment; 5-7 show the morphology of the sample after drying at 100 ° C, respectively, with a local content of Co and Al in weight and (atomic)%: 1.44 (0.45) and 33.2 (22, 8); 18.12 (5, 79) and 18.89 (1, 18); 9.43 (6, 8) and 23.86 (18, 23).

To implement the method, a known installation for microarc oxidation of metals and their alloys (not shown) can be used, containing a galvanic bath, a thyristor converter, including thyristors installed on the output terminals of a current source, a control computer connected via a microprocessor to the control inputs of the thyristor converter thyristors , measuring transducers, at least one of which is made in the form of a current sensor, and the other in the form of a voltage sensor , according to the type described in patent RU No. 2181392, 2002, or its modification described in patent RU No. 2333299, 2008.

It is understood that, for use in decontamination purposes, the described installation must have appropriate protection ensuring radiation safety (not shown).

The basis of the claimed method are the following considerations. The development of promising methods and materials for the purification of liquid radioactive waste (LRW) from long-lived ¹³⁷ Cs, ⁶⁰ Co, ⁹⁰ Sr radionuclides remains one of the urgent tasks that many researchers are working on. The solution to the problem is complicated by the fact that when decontaminating surfaces contaminated with radionuclides, they often use not only alkalis and acids, but also complexones such as weak tetrabasic ethylenediaminetetraacetic acid or its dihydrate-disodium salt (Na ₂ H ₂ Y ₂ H ₂ O, Trilon B, EDTA), a doubly charged anion of which is capable of forming six bonds with metal ions: two due to nitrogen atoms and four due to acetate groups. The reaction between metal ions and Trilon B is written in the general form:

$$M^{n+}+H_2{Y}^{2-}=M{Y}^{n-\mathrm{four}}+2H^{+}\text{. (one)}$$

According to the theory, only the ionized form of the ligand Y ^4- takes part in the reaction of complexation of metal ions with a complexon:

$$M^{n+}+Y^{\mathrm{four}+}=M{Y}^{n-\mathrm{four}}\text{. (2)}$$

Reaction (1) can be shifted both towards the formation and destruction of the complexonate with a change in the pH of the solution. A decrease in pH shifts the reaction equilibrium to the left - to the formation of starting reagents. Alkalization leads to the formation of complexonates.

The stability constant of reaction products (2) does not depend on the pH and concentration of the reaction components and is determined by the nature of the metal ion, its charge, radius, electronic structure, ionic strength of the solution, nature of the solvent, and temperature.

The presence of soluble metal complexes with EDTA in liquid radioactive waste, according to reaction (1), for example, Co-EDTA, greatly complicates its removal from solutions. For this, cobalt complexonate is oxidized using hydrogen peroxide (H ₂ O ₂ ), ozonation, permanganate and electrochemical oxidation, but all of these methods are not effective enough (see Nikiforov A.S., Kulichenko V.V., Zhikharev M.I. LRW disposal // M .: Atomizdat, 1985. - 356 p., 184 p. Or Combined Methods for Liquid Radioactive Waste Treatment, IAEA-TECDOC-1336, Vienna, 2003).

Studies of the effectiveness of various methods for the destruction of cobalt complexonate: by ultrasound, pulses of varying pressure, and ultraviolet irradiation have not yet allowed the creation of effective methods for the destruction of cobalt complexonate.

However, it is known that when metals and alloys polarization in aqueous electrolytes in the electrode regions, resulting in the discharge of OH ^- groups of protons of water is changed pH, particularly manifested at high potentials, when the formed oxide layers on anodically polarized electrode surface are destroyed in as a result of the formation of local breakdown zones of the oxide layer with high current densities (see, for example, P. Gordienko, Coating formation on anode-polarized electrodes in aqueous electrolytes at sparking potentials and breakdown // Vladivostok: Dalnauk Publishing House, 1996. - 216 pp. or Gordienko PS, Vasilenko OS, Panin ES and others. Formation of coatings on valve metals and alloys in electrolytes with inductive energy regulation during microarc oxidation // Metal Protection, 2006. - T.43. - No. 5. - S.300-305.). In the breakdown zone, not only the pH changes sharply, but the temperature reaches several thousand degrees, gas bubbles are formed, the pressure in which can reach 1000 atm. (see, for example, Nugolnykh K.A., Roy N.A. Electric discharges in water // M: Nauka, 1971. - 155 p.). The electrochemical process of microarc oxidation of metals is used in the formation of coatings on metals and alloys.

The claimed method is implemented as follows: the volume of liquid radioactive waste containing complexes of radioactive Co isotopes with ethylenediaminetetraacetic acid is introduced into the grounded galvanic bath of the installation, an electrode made of aluminum, for example, is placed in it and power is supplied from a power source to this electrode and to a bath. The voltage value is monitored by a voltage sensor of an installation (not shown) installed between the current supply to the bath body and the current supply to the aluminum electrode. The control machine programmatically provides the setting of the number of pulses, their polarity, voltage and current of the pulses, and also generates a given cycle of this program of pulses. After passing a given number of cycles, which are determined, for example, empirically, the control machine disables the installation from work. The result is the synthesis of insoluble compounds that immobilize cobalt radionuclides and their precipitation, the separation of which is carried out by one of the known methods, for example by filtration.

To establish the operational parameters of the method, a set of works was carried out, including studying the composition of the reaction products of an aqueous solution of Co-EDTA complexones taken as an electrolyte with the products of an electrochemical process in an electrochemical cell at high potentials, determining the degree of purification of solutions (model solutions at this stage) from cobalt ions depending on the electrolysis conditions.

Aluminum electrodes ADO (99.5% Al) were taken as electrodes. A solution containing cobalt salts was prepared by dissolving CoCl ₂ · 6H ₂ O (analytical grade) in distilled water (concentration 3.19 g / l). The Na ₂ H ₂ C ₁₀ H ₁₆ O ₈ N ₂ · 2H ₂ O salt (analytical grade) was taken as a complexing agent, the pH of the initial solution with Co-EDTA complexes was 1.19 (table 1). The ratio of the areas S _a / S _to was 2.4 and 3.5, the initial temperature of the electrolyte was 20 ° C. The yield of aluminum ions in the electrolyte was calculated based on the amount of electricity consumed (without taking into account losses of the amount of electricity due to gas evolution at the anode).

An automatic installation with computer processing of results was used, developed for microarc processing of metals and alloys at high potentials (see Gordienko PS, Vasilenko OS, Panin ES and others. Formation of coatings on valve metals and alloys in electrolytes with inductive energy regulation during microarc oxidation // Metal Protection, 2006. - T.43. - No. 5. - S.300-305). Table 1 presents the experimental data, including and energy costs per unit weight of the cobalt solution removed from the solution, because This is an important criterion for the practical application of this method.

With a decrease in the initial concentration of the electrolyte by Co-EDTA, the amount of electricity spent on the separation of Co-EDTA from the solution increased, and the specific energy consumption for the “deposition” of complexones with cobalt ions increased accordingly.

From the data of Table 1 it follows that 1 μg Co spent from “2.75 W · sec to 17 W · sec” to “precipitate”. During the electrochemical treatment, the electrochemical cell and the electrolyte were not cooled. The electrolyte temperature during the electrolysis 360 seconds increased to 80-90 ° C.

The concentration of cobalt ions in the solution decreased by about two orders of magnitude, the pH of the electrolyte after electrochemical treatment increased to 3.5-45 (the pH of the solutions was measured with an ionometer of known design).

The concentration of Co ²⁺ in the solution before and after electrochemical treatment was determined by atomic absorption spectrometry on a Solar GM spectrometer using a cobalt analytical line. The precipitate (sample) was separated by filtration. A “blue ribbon” paper filter was used as a filter.

For the quantitative determination of the elemental composition of the precipitate (Co, Al), the energy dispersive X-ray fluorescence method (Shimadzy EDX 800 HS spectrometer) was used, the measurement time was 200 sec. For this, the precipitate from the solution after the MAO process was dried (~ 100 ° C) to constant weight, the sample was ground in an agate mortar together with finely dispersed Teflon (2: 1 by weight), placed in a mold with a diameter of 200 mm and a tablet was pressed for 2 min at a pressure of 5000 kg. Without light elements: O, N, C, H in the sample contains,% weight .: Al - 40.17; Co - 56.5. According to energy dispersive analysis, without taking into account impurity contents, the Co / Al weight ratio in the sample is ~ 1.4, and the molar ratio is 0.95: 1.4.

The X-ray diffraction pattern of the dry precipitate before annealing (Fig. 2) was recorded on a D8 ADVANCE automatic diffractometer with rotation of the sample in Cu Kα radiation. X-ray phase analysis of the samples was carried out using an EBA search program with a powder data bank. Peaks are assigned: (1) - CoAl ₂ O ₄ ; (2) - CoO.OH (heterogenite); (3) - CoO and (4) - Al (OH) _3.

The roentgenogram of the initial sample after annealing to 1050 ° С does not contain peaks related to spinels of the type Al _0.27 Co _0.73 (Al _0.73 Co _1.27 ) O ₄ or Co ₃ O ₄ (these compounds are isostructural) and halo related to the x-ray amorphous phase (figure 3). All peaks are assigned to spinel Al _0.27 Co _0.73 (Al _0.73 Co _1.27 ) and Co ₃ O _4.

Thermogravimetric analysis of the sample was carried out on a Q-1500D derivatograph system F. Paulik-P, Paulik-L, Erdei firm MOS (accuracy of determining the temperature of 5 ° C). In this case, the sample was heated at a rate of 5 ° C / min to 900 ° C in a platinum crucible, the mass of the sample was 100 mg. The thermogram is shown in figure 4.

The total weight loss of the sample is 71%. Four temperature ranges are clearly distinguished on the thermogram: (90-135) ° С - 20.4%; (135-350) ° C - 14.3%; (350-450) ° С - 33.2% and (450-850) ° С - 3.1%. Peaks in the thermogram at ~ 130 ° C should be attributed to the loss of water, and exo effects at temperatures of 380-450 ° C - to the oxidation of the sample.

The initial sample containing hydrated forms of the compound based on aluminum and cobalt had a pink (reddish) color, after annealing to 800 ° C - black, and after high-temperature annealing - blue.

The morphology of the crystalline precipitate (Fig. 5) and the local Al / Co ratio in the precipitate after electrochemical processing according to elemental microanalysis performed on an EVO 50 XVP scanning electron microscope with an energy dispersion attachment for INCA 350 microanalysis varies widely from 2.5 to 50 , i.e. it can be assumed that some particles mainly consist of aluminum hydroxide (peaks related to the crystalline modification of aluminum hydroxide were detected in the X-ray diffraction pattern), but some peaks are attributed to the crystalline phases of hetegenite, aluminocobalt spinel, cobalt oxide (Fig. 2), but most intense x-ray peaks are not assigned to any of the compounds available in the database. The composition of the unidentified crystalline phase has not been determined. After electrochemical treatment, the sediment contains about 25% (wt.) Of light elements, i.e. they contain compounds based on the "products of the destruction of cobalt complexonate (Co-EDTA) and the products of electrochemical dissolution of the anode-polarized electrode.

Claims (2)

1. A method of processing liquid radioactive waste (LRW), including the synthesis of insoluble compounds that immobilize long-lived radionuclides, and subsequent separation of the precipitate, characterized in that they use LRW containing Co complexes with ethylenediaminetetraacetic acid (EDTA), while performing electrochemical synthesis of insoluble cobalt compounds why, to the electrodes placed in a tank with LRW, an electric current is supplied with parameters corresponding to the microarc oxidation mode. 2. The method according to claim 1, characterized in that the process is carried out under normal conditions.

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