RU2444800C1 - Immobilisation method of radionuclides of alkaline-earth and rare-earth elements in mineral matrix - Google Patents

Abstract

FIELD: power industry.

SUBSTANCE: immobilisation method of radionuclides of alkaline-earth and rare-earth elements in mineral matrix on the basis of reactions of metasomatic substitution in "wet" conditions differs by the fact that it involves the following: passage of nitrate solution of the above radionuclides through the column filled with particles of minerals with sizes of 0.40÷0.63 mm of model granite mixture or natural granite with crystalline water-free sodium orthophosphate, at mass ratio of sodium orthophosphate to the mixture which is equal to 1:3÷4; dehydration of the contents of the column by heating from room temperature to 400°C; saturation of the column contents with liquid sodium silicate with silica modulus equal to 2; the next annealing of the obtained mineral composition at temperature of 900-1250°C and atmospheric pressure, or hot pressing at temperature of 815-900°C and axial pressure of not more than 700 kg/cm² till crystalline mineral matrix containing the radionuclide phosphate is obtained.

EFFECT: invention allows optimising the method that allows reliable fixation of radionuclides in crystalline mineral matrix.

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Description

The invention relates to the field of nuclear energy, in particular to methods for the immobilization of radionuclides of alkaline earth and rare earth elements from spent nuclear fuel, as well as those generated in radiochemical and metallurgical industries, and can be implemented when disposing of radionuclides by the fixation of radioactive elements (RE) in a mineral matrix.

A known method of ceramicizing a rare earth concentrate, which involves the precipitation of rare earth orthophosphates from their concentrate, calcining the resulting suspension at 400-500 ° C, mixing the precipitate with igneous rocks containing quartz and minerals whose crystallochemical structures are capable of isomorphic rare earth occurrence in them elements, grinding the mixture to a homogeneous composition, pressing it at room temperature and firing at a temperature of 950-1050 ° C to obtain ceramics, at the volume obtained after calcination orthophosphates are mixed with rocks in a weight ratio of 1: 1-3 (RU 2128377 C1, G21F 9/16, G21F 9/32, 08/10/2000).

A known method of solidification of liquid radioactive waste (LRW) with a radioactivity of from 1 to 10 Ci / l, by including them in cement placed in a container, followed by mixing, while the container is filled with cement with a density of 1.0-1.4 g / cm ³ and using a drainage device placed in the container, diffuse liquid radioactive waste from the lower layers of cement to the upper with a front velocity of LRW of 0.05-0.2 m / min, with a cement density in the range of 1.0-1.4 g / cm ³ and an excess pressure of 2.0 × 10 ² to 2.0 × 10 ³ Pa and at a ratio of mortar / cement, corresponding to 0.4-0.5, the maximum filling of the LRW compound is 33-38%, after filling the container with LRW cement, it is sealed and rotated, while the compound is mixed, homogenized and solidified (SU 1338697 A, G21F 9 / 16, 03.20.1996).

A known method of processing liquid radioactive materials, including sorption of radionuclides on natural zeolites (clinoptilolite, mordenite) and cementation of the obtained radioactive natural zeolites using an astringent system containing granulated blast furnace slag and a clay component, mixed with sodium silicate solution, while dry components of the astringent system used in the following ratio, wt.%: natural zeolite not more than 40, granulated blast furnace slag 51-85, clay component 3-13, and sodium silicate solution isp use in an amount providing a molar ratio of sodium oxide contained in sodium silicate to aluminum oxide contained in blast furnace granular slag and clay component, from 1 to 1.5, the sodium silicate solution used has a silicate module of 1.5 and a density of 1.3 -1.4 g / cm ³ , sorption is carried out under dynamic conditions, the pH of liquid radioactive waste is set in the range of 8-12 (RU 2154317 C2, G21F 9/12, G21F 9/16, 08/10/2000).

Known methods quite effectively bind RE, but have a significant drawback, because they have a stage of dry mixing, which inevitably leads to the dispersion of the components, which significantly reduces the environmental parameters of the methods and their practical use.

A known method of neutralizing liquid radioactive materials, including pouring into a capsule pre-filled with porous ceramics and equipped with a trap of sublimates, liquid radioactive or toxic materials, their evaporation, thermolysis of the dry residue and capture of sublimates with high vapor pressure, and evaporation and thermolysis of the dry residue is carried out in a capsule directly in the pores of the ceramic, and after the end of the thermolysis process, the capsule is sealed, the evaporation process is carried out at a temperature of 70-90 ° C, and the thermolysis process at a temperature of 700-900 ° C, all operations are carried out in a sealed chamber at a pressure of 0.01-0.05 MPa (RU 2137230 C1, G21F 9/16, G21F 9/22, 09/10/1999).

The disadvantage of this method is the complexity of the process, the need for strict control, since the possibility of the release of solutions during evaporation is not excluded, and during heat treatment the possibility of the release of volatile components and their ingress into the atmosphere.

An object of the invention is the creation of a new effective way of fixing radionuclides of alkaline earth and rare earth elements in crystalline mineral matrix materials based on metasomatic substitution reactions in "wet" conditions.

The technical result of the invention is the optimization of the method, which allows reliable fixation of radionuclides in a crystalline mineral matrix.

One of the important steps in the manufacture of ceramic matrix materials is to obtain a homogeneous charge, which is usually achieved by thoroughly mixing finely ground calcined radioactive components in dry conditions. At the same time, the problem remains unresolved - spraying the charge.

In the proposed method, this problem is solved by carrying out the process of transferring the radionuclide into the crystalline structure of the mineral completely in "wet" conditions at room temperature and atmospheric pressure.

The developed method of immobilization of radionuclides in wet conditions is briefly presented in a message at a seminar on experimental mineralogy, petrology and geochemistry (Suvorova V.A., Tikhomirova V.I., Kovalsky AM, Akhmedzhanova G.M., Kotelnikov A.R. Synthesis matrix materials for the immobilization of radionuclides by the “wet process” method. // Materials of the annual seminar on experimental mineralogy, petrology and geochemistry ESEMPG-2006, Moscow, GEOKHI RAS. 2006. S. 70).

The specified source is the closest analogue to the claimed method.

In the General case, the "wet process" can be represented in the form of a scheme:

Water
RE solution ⇒ Sorption from aqueous solutions, metasomatic substitution reactions, precipitation reactions ⇒ Phase transformation ⇒ Mineral matrix for placement in the rocks of the Earth's crust

According to the scheme, when obtaining a stable mineral matrix for immobilizing the components of radioactive waste, it is first necessary to carry out the primary fixation of the radionuclides of elements in one of the following ways: ion-exchange sorption, metasomatic substitution or precipitation reactions. These reactions proceed at low (<100 ° C) temperatures in aqueous solutions. Then, it is required to carry out phase transformation of the obtained compound by high-temperature treatment to turn it into a stable crystalline matrix.

The claimed method of immobilizing radionuclides of alkaline earth and rare earth elements in a mineral matrix is based on such a “wet process” scheme.

The essence of the method based on metasomatic reactions is to replace the crystalline phase of sodium phosphate with a poorly soluble phosphate of a radionuclide, such as strontium or cerium, by passing a solution of this radionuclide through solid sodium phosphate. The soluble compound (sodium nitrate) is discharged into the aqueous solution according to the reactions

The technical result of the invention is achieved in that a method of immobilizing radionuclides of alkaline earth and rare earth elements in a mineral matrix based on metasomatic substitution reactions in “wet” conditions according to the invention includes: passing a solution of radionuclide nitrate of said groups of elements through a column filled with mineral particles with sizes in the range 0 , 40 ÷ 0.63 mm of a model granite mixture or natural granite with crystalline anhydrous sodium orthophosphate, with mass Ocean sodium orthophosphate to a mixture of 1: 3 ÷ 4; dehydration of the contents of the column by heating from room temperature to 400 ° C; impregnation of the contents in the column after dehydration with liquid sodium silicate with a silicate module equal to 2; annealing the obtained mineral composition at a temperature of 900-1250 ° C and atmospheric pressure, or hot pressing at a temperature of 815-900 ° C and an axial pressure of not higher than 700 kg / cm ² to obtain a crystalline mineral matrix containing radionuclide phosphate. As a model granite mixture, a mechanical mixture of quartz and albite or quartz and microcline is used, with a quartz content of 25-30 wt.%, Natural granite is represented by crushed fine-grained granite of the Spokoininskoye deposit (East Transbaikalia).

In the case of a decrease in the rate of passage of the solution through the column as a result of hydration of sodium phosphate, it is possible to use forced dilution at the outlet of the column.

The method worked out during the synthesis of matrices containing phosphates of Sr or Ce. To obtain 1 M mineral matrices, solutions of strontium nitrate or cerium nitrate were passed through columns filled with mixtures of ground model or natural granite with particles of crystalline anhydrous sodium orthophosphate, and substitution reactions (1) or (2) took place. The completeness of metasomatic substitution was controlled by chemical analysis of the solutions leaving the columns.

To obtain a model granite mixture, the following minerals were used: 1) albite and 25-30 wt.% Quartz or 2) microcline and 25-30 wt.% Quartz. The composition of albite in wt.% (Normalization to 100%): Na ₂ O 11.91; Al ₂ O ₃ 19.48; K ₂ O 0.23 and SiO ₂ 68.39. The composition of the microcline in wt.% (Normalization to 100%): Na ₂ O 0.62; Al ₂ O ₃ 18.32; K ₂ O 16.00 and SiO ₂ 65.08. As natural granite, samples of fine-grained granite from the Spokoininskoye deposit (East Transbaikalia) containing muscovite composition, in wt.% (Normalization to 100%): Na ₂ O 0.45; MgO 0.12; Al ₂ O ₃ 36.77; K ₂ O 11.03; FeO 3.02 and SiO ₂ 48.62 and feldspar composition, in wt.% (Normalization to 100%): Na ₂ O 0.45; Al ₂ O ₃ 18.79; K ₂ O 15.60 and SiO ₂ 65.16.

For the successful implementation of metasomatic substitution reactions, the particle size distribution of mixtures of the starting components is important. In columns with a fraction of particles of the initial mixture in the range 0.40–0.63 mm, the rate of permeation of the solution through the mineral ensured the complete exchange reactions according to reactions (1) or (2).

The essence of the method: a model granite mixture or natural granite with a uniform inclusion of anhydrous sodium orthophosphate in them with a mass ratio of orthophosphate to the mixture equal to 1: 3 ÷ 4, was placed in a column of quartz glass with an inner diameter of 8 mm. 1 M solutions of Sr (NO ₃ ) ₂ or Ce (NO ₃ ) _{3 were} passed through the filled columns until sodium was completely replaced in phosphate according to reactions (1) or (2). To accelerate the filtration at the column outlet, in some cases, a vacuum (~ 10 ^-1 atm) was created using a water-jet pump. The filtrate was collected in flasks and analyzed for the content of strontium (cerium) and sodium. The appearance in solutions of Sr or Ce in amounts exceeding 0.08 mg / ml was a criterion for the completeness of the passage of the metasomatic substitution reaction. The resulting composite with a uniform inclusion of Sr or Ce phosphates was washed with distilled water, passing it through the columns until the Na content in the washings was <0.03 mg / ml. Then the contents together with the column were subjected to dehydration by heat treatment from room temperature to 400 ° C. Then, the contents of the column were impregnated with a solution of sodium silicate with silicate module 2. The obtained columns were annealed in a KO-14 laboratory furnace at a temperature of 900-1250 ° C and atmospheric pressure or subjected to hot pressing at a temperature of 815-900 ° C and an axial pressure of not higher than 700 kg / cm ² to obtain the final crystalline mineral matrix. The obtained phosphates of radionuclides of alkaline-earth and rare-earth elements are synthetic analogues of natural accessory minerals, which have shown their stability in the Earth's crust for a long (on geological scales) time. The mineral composition, corresponding in composition to granite, is an additional barrier that minimizes the possible destructive effect of an aggressive environment on phosphate. Impregnation of the composite with a solution of sodium silicate can increase the density of the matrix and increase the reliability of fixation of radionuclides in it.

The products of experiments on the synthesis of a crystalline mineral matrix were studied by the X-ray method on a HZG-4 diffractometer in the constant scanning mode. To assess the compositions of the synthesized matrices for the content of Na, K, Ca, Mg, Sr, Ce, Al, P, Si, the method of local X-ray spectral microanalysis was used. A CamScan MV2300 digital scanning electron microscope (VEGA TS 5130MM) was used, equipped with YAG detectors of secondary and reflected electrons and an energy dispersive X-ray microanalyzer with a semiconductor Si (Li) Link INCA Energy detector. The calculation of the results of X-ray spectral microanalysis was performed using the ENCA Energy 200 program, followed by recalculation of the results using programs developed at the IEM RAS.

Analysis of the filtrate for strontium content was carried out by atomic absorption spectroscopy on an AAS-1N instrument, cerium on a Spekol 11 instrument using photocolorimetry using arsenazo III (Marchenko Z. Photometric determination of elements. M .: Mir, 1971. P.315) .

The essence of the method is confirmed by examples.

Example 1

A model granite mixture of 75 wt.% Albite and 25 wt.% Quartz with a particle size in the range of 0.40 ÷ 0.63 mm, in which the same particle size of anhydrous crystalline sodium orthophosphate is evenly distributed, is poured into a column of quartz glass with a diameter of 8 mm . The mass ratio of sodium orthophosphate to the granite mixture is 1: 3. A 1 M solution of strontium nitrate is passed through the column for a time sufficient to completely replace the sodium ions with strontium ions to form sparingly soluble strontium phosphate in accordance with reaction (1). The appearance of strontium in the filtrate in an amount exceeding 0.08 mg / ml is a criterion for the completeness of the passage of the metasomatic substitution reaction. Column dehydration was carried out by heat treatment from room temperature to 400 ° C to constant weight. The dehydration time depends on the volume of the column and the amount of nitrate passed through and under experimental conditions is an average of 3 hours. Next, the obtained composite is impregnated in a column with liquid sodium silicate with a silicate module equal to 2, and sent to annealing, which is carried out at a temperature of 1250 ° C for 1 hour. The result is a crystalline mineral matrix containing strontium phosphate with a density of 2.42 g / cm ³ .

Example 2

Everything, as in example 1, but after dehydration and impregnation with sodium silicate, the mineral composition was hot pressed at a temperature of 850 ° C and an axial pressure of 680 kg / cm ² , while the matrix density increased to 2.46 g / cm ³ .

An additional increase in matrix density (by 5-10%) is achieved by increasing the hot pressing temperature of the composition to 900 ° C while maintaining pressure.

Example 3

A model granite mixture of 70 wt.% Microcline and 30 wt.% Quartz with particle sizes in the range of 0.40 ÷ 0.63 mm, in which the same particle size of anhydrous crystalline sodium orthophosphate is evenly distributed, is poured into a column of quartz glass with a diameter of 8 mm . The mass ratio of sodium orthophosphate to granite mixture is 1: 4. A 1 M solution of cerium nitrate is passed through the column for a time sufficient to completely replace sodium ions with cerium ions to form sparingly soluble cerium phosphate in accordance with reaction (2). The appearance of cerium in the filtrate in an amount exceeding 0.07 mg / ml is a criterion for the completeness of the passage of the metasomatic substitution reaction. Column dehydration was carried out by heat treatment from room temperature to 400 ° C to constant weight. The dehydration time depends on the volume of the column and the amount of nitrate passed through and under experimental conditions is an average of 3 hours. Then carry out the impregnation of the mixture in the column with liquid sodium silicate with a silicate module equal to 2. After which the resulting composite is sent for annealing at a temperature of 1100 ° C for 2 hours. The result is a crystalline mineral matrix containing cerium phosphate.

Example 4

Everything is as in example 3, however, after the impregnation step, hot pressing is carried out at 815 ° C and an axial pressure of 700 g / cm ² for 3.5 hours. As a result, a crystalline mineral matrix containing cerium phosphate was obtained, with a matrix density of up to 2.50 g / cm ³ .

Example 5

Everything is as in example 1, but particles of crushed fine-grained natural granite of the Spokoininskoye deposit (East Transbaikalia), the main rock-forming minerals of which are represented by feldspar, quartz and muscovite, are used as a mineral. A 1 M solution of strontium nitrate was passed through the column, dehydrated at 400 ° C to constant weight, and the mixture in the column was impregnated with liquid sodium silicate with a silicate module of 2. Heat treatment of the column was carried out by hot pressing at 850 ° C and an axial pressure of 680 kg / cm ² for 3 hours. The result is a crystalline mineral matrix containing strontium phosphate.

Example 6

Everything, as in example 5, but a 1 M solution of cerium nitrate was passed through the column. The result is a crystalline mineral matrix containing cerium phosphate.

X-ray studies and determination of the chemical composition of the phases of the synthesized crystalline matrices confirmed the presence of crystalline strontium phosphate α-Sr ₃ (PO ₄ ) ₂ or cerium phosphate (Ce-monazite) in them.

To illustrate, Fig. 1 shows a photograph of the polished surface of a mineral matrix based on granite from the Spokoininskoye deposit (East Transbaikalia) with uniformly distributed Sr-containing phosphate, impregnated with sodium silicate and compacted by hot pressing at 850 ° C and axial pressure of 680 kg / cm ² , filmed in the backscattered electron mode by local x-ray spectral microanalysis. The photograph shows strontium phosphate α-Sr ₃ (PO ₄ ) ₂ newly formed as a result of metasomatic substitution (light phases) uniformly distributed in a dense mineral granite matrix. For comparison, figure 2 presents a photograph of a similar matrix obtained without impregnation with sodium silicate and having a higher porosity.

Phosphates obtained by synthesis according to the claimed method are analogues of natural accessory minerals that have shown their high resistance to leaching processes in the earth's crust for a long (on geological scales) time. So, strontium orthophosphate is an analogue of apatite, the high stability of which was studied in the work (Kotelnikov A.R., Akhmedzhanova G.M., Suvorova V.A. Minerals and their solid solutions - matrices for immobilization of radioactive waste // Geochemistry. 1999. No. 2. S.192-200). Cerium phosphate is an analogue of the monazite mineral - a common accessory mineral of granites and other rocks, and its high resistance to leaching processes is shown in (Ringwood A.E., Kesson SE, Reeve KD, Levins DM, Ramm EJ SYNROC // Radioactive waste forms for the future. Eds .: W. Lutze and RCEwing, Elsevier Sci. Publ., 1988. Ch. 4. 324 p.).

Thus, the method presented in the invention allows to reliably fix in the crystal structure of the mineral, which, in turn, is in the mineral matrix, radionuclides of alkaline earth and rare earth elements and to optimize the synthesis process by eliminating the stage of grinding of the initial components of the matrix in dry conditions, accompanied by spraying charge. In the proposed method, this problem is solved due to the ability to transfer the radionuclide into the crystalline structure of the mineral in "wet" conditions at room temperature and atmospheric pressure.

Claims (4)

1. A method of immobilizing radionuclides of alkaline earth and rare earth elements in a mineral matrix based on metasomatic substitution reactions in “wet” conditions, characterized in that it includes: passing a nitrate solution of said radionuclides through a column filled with mineral particles with sizes in the range 0.40 ÷ 0 , 63 mm of a model granite mixture or natural granite with crystalline anhydrous sodium orthophosphate, with a mass ratio of sodium orthophosphate to the mixture equal to 1: 3 ÷ 4; dehydration of the contents of the column by heating from room temperature to 400 ° C; impregnation of the contents of the column with liquid sodium silicate with a silicate module equal to 2; subsequent annealing of the obtained mineral composition at a temperature of 900-1250 ° C and atmospheric pressure, or hot pressing at a temperature of 815-900 ° C and an axial pressure of not higher than 700 kg / cm ² to obtain a crystalline mineral matrix containing radionuclide phosphate. 2. The method according to claim 1, characterized in that a mechanical mixture of quartz and albite or quartz and microcline is used as a model granite mixture. 3. The method according to claim 2, characterized in that the quartz content in the mixture is 25-30 wt.%. 4. The method according to claim 1, characterized in that the crushed fine-grained granite of the Spokoininskoye deposit (East Transbaikalia) is used as natural granite.