RU2414294C1 - Method of producing sorbent to remove iodine radioniuclides and/or its organic compounds - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to nuclear engineering and may be used for removal of radionuclides of iodine and/or its organic compounds in treatment and control of gaseous radioactive wastes. Proposed method comprises mixing china clay and fine magnesium oxide with water solution of sodium trisilicate in mass ratio of 10:(0.2-0.8):(10-15), mechanical granulation, treatment by mineral acid, washing, drying and calcination at 500-700°C, as well as impregnation of produced porous carrier in vacuum by silver salt.

EFFECT: efficient removal both of 1 g and CH₃I labeled by ¹³¹I and over 99,9%, with xenon-133 selectivity factor being defined as initial activity-to-sorbed activity ratio exceeding 2·10⁵.

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Description

The invention relates to nuclear energy and industry and can be used to trap sorbents of iodine radionuclides and its organic compounds in the purification and control of gaseous radioactive waste (GD), the air of working rooms, the environment, as in the normal operation of nuclear power plants (NPP) and in an emergency.

Recently, special attention has been paid to improving the safety of nuclear power plants at nuclear power plants and other nuclear facilities. One of the ways to solve this problem is to provide reliable protection of the atmosphere and the environment, especially in the event of accidents, using technologically advanced, affordable and safe inorganic sorbents.

Radioactive iodine is found in the GRO in several forms - aerosol, molecular, and in the form of organic compounds. The main difficulty in ensuring effective control and cleaning of radioactive waste from radioiodine is that they contain a subtle form of iodine - methyl iodide, the proportion of which from the radioiodine content can be 10% or more. If elemental iodine is absorbed by sorbents as a result of physical sorption, then the removal of organic iodine compounds requires impregnation (impregnation) of the sorbents with substances that form a chemical bond with iodine as a result of a chemical reaction or isotopic exchange.

The most widely used in nuclear energy for sorption of radioiodine found activated carbon [Kolyshkin D.A., Mikhailova K.K. Active carbons: a guide. - M., Chemistry, 1972, p.57]. For their impregnation, potassium iodide, elemental iodine, their mixture, silver nitrate, triethylenediamine, etc. are most often used.

The disadvantages of organic sorbents include their combustibility, which does not allow their use at temperatures above 150 ° C. The same restrictions apply to amine impregnators [Styro B.I., Nevetskaite T.N., Filistovich V.I. Iodine isotopes and radiation safety - St. Petersburg, Gidromethioizdat, 1992, p. 33-36]. In addition, the efficiency of trapping radioiodine by sorbents based on activated carbon sharply decreases at high relative humidity of the gas (more than 90%).

Known highly effective sorbent for removing radionuclides of iodine and / or its organic compounds based on amorphous silicic acid, impregnated with a metal salt, in particular silver nitrate [US Patent No. 3838554 B01D 53/02 from 06.23.71]. To prepare the sorbent (example 6), kaolin and silicic acid obtained by precipitation of calcium chloride and hydrochloric acid from water glass are suspended in an aqueous solution of a salt of silicic acid. The resulting suspension is mixed with an aqueous suspension of finely divided magnesium oxide. A mixture of two suspensions using a dispenser is fed into a column filled with orthodichlorobenzene, which is not miscible with water. Entering the organic medium, the jets separate into droplets, which harden in a few seconds, forming balls. The pellet in the form of balls is then separated and dried at a temperature of about 100 ° C. Next, aluminum and magnesium are removed from the granules by treatment with hot hydrochloric acid. Then the granules are washed, dried and calcined at a temperature of 700 ° C. The resulting carrier, resistant to hot vapors of water and acids, is impregnated under vacuum with silver nitrate. The removal efficiency of iodine compounds on such a sorbent is more than 99.9%, and the selectivity to xenon-133 is more than 2 · 10 ⁵ . This method in its technical essence and the achieved effect is closest to the claimed and selected as a prototype.

The main disadvantage of the prototype method is the difficulty of obtaining free of silicic acid salts from sodium silicate solutions (by neutralization and subsequent concentration by evaporation), as well as the use of toxic organic media during granulation (maximum permissible concentration in the air for orthodichlorobenzene 20 mg / m ³ , and for perchlorethylene 10 mg / m ³ ). In addition, this method of granulation does not provide strict fractionation of granules by size, which leads to loss of sorbent during screening.

The objective of the invention is to create a simpler and toxicologically safe technology for producing a sorbent without reducing the efficiency of removal of iodine radionuclides and its organic compounds by this sorbent, as well as increasing the yield of the finished product.

The technical result of the proposed method is to simplify the technology and increase the yield of the finished product.

The essence of the invention lies in the fact that in a method for producing a sorbent for removing iodine radionuclides and its organic compounds, comprising mixing a binder, kaolin and finely divided magnesium oxide, forming granules, treating them with hot mineral acid, washing from acid, drying and calcining the granules, followed by impregnation under vacuum of the obtained porous support with a silver salt, according to the invention, an aqueous solution of sodium trisilicate is used as a binder, and granules are formed by om mechanical forming. Kaolin and magnesium oxide are mixed with an aqueous solution of sodium trisilicate in a mass ratio of 10: (0.2-0.8) :( 10-15).

The method is as follows.

As a binder for dry components use liquid sodium glass with module 3 (an aqueous solution of sodium trisilicate Na ₂ O · 3SiO ₂ ). This form of sodium silicate, on the one hand, provides a relatively easy solubility, and on the other hand, allows a sufficient amount of SiO ₂ to be added to the sorbent. Dry non-calcined kaolin and dry finely divided magnesium oxide are mixed with liquid sodium glass in a mass ratio of 10: (02-0.8) :( 10-15). The resulting mass in the inorganic phase is formed mechanically on a precision molding granulator, followed by rolling in the form of particles of the desired shape and size. Granular material to increase pore volume and obtain the desired pore size distribution is treated for 5-10 hours with an excess of hot (50-90 ° C) azeotropic (15-25%) hydrochloric acid, based on the quantitative solubility of the oxides. Then the granules are washed from chlorides, dried to remove excess moisture and calcined at a temperature of 500-700 ° C.

The calcined granules are evacuated at 50-100 mm Hg. and while maintaining a vacuum, they are impregnated with a silver nitrate solution, stirring for 15-20 minutes. The impregnated sorbent is dried at normal pressure for 6-8 hours at a temperature of 100-150 ° C.

Compared with the known method for producing a sorbent for the removal of iodine radionuclides and its organic compounds in the purification and control of gaseous radioactive waste from nuclear power plants based on silica, the use of dry kaolin and finely divided magnesium oxide in a mixture with an aqueous solution of sodium trisilicate allowed mechanical molding of this mixture under pressure in the inorganic phase, which greatly simplifies the technology (compared with the formation of granules from a suspension in an organic phase, not miscible with water) and increases its toxicological safety. In addition, mechanical molding ensures strict fractionation of granules by size and improves their operational characteristics. The use of starting materials without preliminary preparation - neutralization of sodium silicate to obtain silicic acid, preparation of suspensions for each component separately excludes intermediate stages and reduces the complexity of sorbent production.

Examples of specific performance.

Example 1 (prototype). Five mass parts of kaolin of the Glukhovetsky deposit (60-65% SiO ₂ , 35-40% Al ₂ O ₃ ) were suspended in 12 mass parts of an aqueous sol of silicic acid precipitated from sodium trisilicate with calcium chloride and hydrochloric acid (30% SiO ₂ ), stirring fast. Ten parts by weight of this suspension were mixed with 1.5 parts of an aqueous suspension of magnesium oxide containing 80 g / L MgO. A mixture of the two suspensions was fed to a star-shaped distributor located above the column of orthodichlorobenzene. The jets were crushed into droplets in the organic phase and these droplets were solidified in the form of granules due to sol-gel conversion. The granules, which still had the ability to form, were separated from the organic phase, dried by a stream of air at a temperature of 20 ° C, and then another hour at a temperature of 120 ° C. The granular material was activated with acid to increase pore volume. For this, the granules were treated for 8 hours with an excess of hot azeotropic hydrochloric acid, based on the quantitative solubility of the oxides. Then the granules were washed from the chlorides, dried and calcined at 700 ° C for 2 hours.

The sieved fraction of granules with sizes of about 1.5-2 mm was evacuated at 50 mm Hg. and while maintaining the vacuum, it was poured with a neutral solution of silver nitrate so that it covered the entire layer of granules. After evacuation, the impregnated material was boiled in this solution for 2 hours at normal pressure, suctioning the solution on the filter and drying at a temperature of 150 ° C.

The tests were carried out on a sorption column with a volume of 100 cm ³ and a height of the sorption layer of 6 cm. The air flow through the sorption column at 90% humidity was 20 l / min at a contact time of 0.3 s. The air pumping time was 30 minutes. The removal efficiency of both I ₂ and CH ₃ I labeled ¹³¹ I was more than 99.9%, and the selectivity coefficient for xenon-133, defined as the ratio of the ratio of iodine to xenon in the initial sample to the ratio of iodine to xenon on the sorbent, amounted to more than 2 · 10 ⁵ . This allows the use of this sorbent not only in the purification of HGF from radioactive iodine, both in molecular and organic form, but also during radioactive iodine control with its separation from radioactive noble gases.

Example 2. Five mass parts of kaolin of the Glukhovetsky deposit (60-65% SiO ₂ , 35-40% Al ₂ O ₃ ) were mixed with 0.1 parts of finely divided magnesium oxide (MgO) and suspended in 5 mass parts (in terms of Na ₂ O · 3SiO ₂ ) sodium silicate to obtain a homogeneous mass. The resulting pasty mass was molded by pressing in the form of cylindrical granules with their subsequent rolling into a spherical shape with a diameter of 0.5-2 mm. The size of the granules is determined by the diameter of the dies in the granulator precision molding. The granular material was treated with acid to increase pore volume. For this, the granules were treated for 5 hours with an excess of hot azeotropic hydrochloric acid, based on the quantitative solubility of aluminum and magnesium oxides. Then the granules were washed from the chlorides, dried and calcined at 700 ° C for 2 hours.

Annealed granules with sizes of about 0.5-2 mm (the yield of which was approaching 100%) were evacuated at 50 mm Hg. and while maintaining the vacuum, the amount of silver nitrate calculated from the moisture capacity was supplied. The sorbent was mixed with an impregnator for 20 minutes, and then the granules were dried for 8 hours at normal pressure and a temperature of 150 ° C.

Tests of impregnated sorbents were carried out under the same conditions as in Example 1. The removal efficiency of both I ₂ and CH ₃ I labeled with ¹³¹ I was more than 99.9%, and the selectivity coefficient for xenon-133 was more than 2 · 10 ⁵ .

Example 3. It differs from example 2 in that 5 mass parts of kaolin from the Glukhovetsky deposit (60-65% SiO ₂ , 35-40% Al ₂ O ₃ ) were mixed with 0.4 parts of finely divided magnesium oxide (MgO) and suspended in 7, 5 mass parts (in terms of Na ₂ O · 3SiO ₂ ) of sodium silicate, and the mixture was formed by extrusion, forcing through holes with a diameter of 2 mm followed by cutting granules up to 2 mm long).

Tests of impregnated sorbents were carried out under the same conditions as in Example 1. The removal efficiency of both I ₂ and CH ₃ I labeled with ¹³¹ I was more than 99.9%, and the selectivity coefficient for xenon-133 was more than 2 · 10 ⁵ .

The proposed method allows mechanical molding of granules under pressure, both by pressing and extrusion, which simplifies the technology compared to molding granules from a suspension in an organic phase that is not miscible with water, provides toxicological safety of the technology, ensures accurate fractionation of granules by size and improves them operational characteristics, and also increases the yield of the finished sorbent. The inventive method allows to obtain a sorbent with virtually no waste, which is of great environmental importance.

The proposed method can be carried out on domestic equipment (granulators of exact formation are produced in the Russian Federation by NPP “Ekol NN” in Dzerzhinsk, Nizhny Novgorod Region), i.e. industrially applicable.

Claims (1)

A method for producing a sorbent for removing iodine radionuclides and / or its organic compounds, comprising mixing a binder, kaolin and finely divided magnesium oxide, forming granules, treating them with hot mineral acid, washing, drying and calcining the granules, followed by vacuum impregnation of the obtained porous carrier with a silver salt, characterized in that an aqueous solution of sodium trisilicate is used as a binder, while kaolin and magnesium oxide are mixed with a solution of sodium trisilicate in mass co equipped with 10: (0.2-0.8) :( 10-15), and granule formation is effected by mechanical forming.