RU2410780C1 - Method of cleaning sandy soil from radionuclides - Google Patents

Abstract

FIELD: physics, nuclear.

SUBSTANCE: invention relates to environmental protection, rehabilitation of territories contaminated with medium and low specific activity level radio nuclides. The method involves initial breaking of soil aggregates, surface muddy layer covering sand grains and pebbles, with mechanical separation of the initial soil into pebbles (3-100 mm) and pulp containing particles with size (<3 mm). After separation and washing, the pebbles are added to the process cycle. Through water-gravity separation, the pulp is divided into a sand fraction (>0.1 mm) and a fine-dispersed fraction (<0.1 mm), which, after thickening and dehydration, is taken to the radioactive waste storage. The sand fraction is washed with water, dehydrated and subjected to reagent treatment with 0.5-1 M sulphuric acid solution in an autoclave at temperature 100-140°C, with ratio of the liquid to the solid phase equal to 0.5/1-1/1, at the end of which the reagent solution is separated and the soil is washed with water. The separated reagent solution and the water used to wash the soil are mixed. Radionuclides are precipitated and the precipitate is taken to the radioactive waste storage. After reagent treatment and washing with water, the sand fraction is divided into a fine-dispersed (<0.1 mm) and a sand (>0.1 mm) fraction. After thickening and dehydration, the fine-dispersed fraction is taken to the radioactive waste storage and the sand is returned to the collection site.

EFFECT: less amount of reagent required for decontamination while maintaining the value of the decontamination factor.

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Description

The invention relates to the field of environmental protection, rehabilitation of territories contaminated with medium and low specific activity radionuclides. These territories include industrial zones of nuclear power plants, enterprises that store radioactive waste, metallurgical and radiochemical plants, and territories contaminated by accidents and disasters.

A known method of restoring soil contaminated with radionuclides, based on a preliminary determination of the most contaminated fraction. A leaching solution is introduced into the apparatus with the soil, coming from the bottom up to capture and remove particles of a certain size in the upper zone that make up the most contaminated fraction. It is assumed that the leaching solution will additionally extract radionuclides from the soil and transfer them to the liquid phase (US 5045240 A 01.05.89, publ. 03.09.91, G21F 9/28, C02F 1/42) [1].

The main disadvantage of this method is the combination of the function of separation into fractions and leaching in one device. Water-gravity methods for separating particles into particle size fractions use large volumes of water, and the combination of the separation function and reagent treatment leads to an unreasonably large number of reagents, which, after decontamination, need to be cleaned of radionuclides by passing through columns with sorbents, which also requires a lot of time.

Closest to the claimed method (prototype) is a method for cleaning sandy soils from radionuclides, including the initial destruction of soil aggregates, a surface mud layer covering sand grains and pebbles, followed by mechanical separation of the original soil into pebbles (3-100 mm) and pulp containing particles with a size (<3 mm), pebbles after separation and washing are removed from the technological cycle, the pulp is separated by a water-gravity separation in the classification module into a sand fraction (> 0.1 mm) and a finely divided fraction ju (<0.1 mm), which after thickening and dehydration is sent to the radioactive waste storage, the sand fraction is washed with water, dehydrated and returned to the place of selection, the sand fraction after washing with water and dehydration is subjected to reagent treatment, after which the reagent solution is separated, and the soil is washed with water, the separated reagent solution and soil wash water are mixed, and radionuclides are precipitated from the resulting mixture, the resulting precipitate enters the radioactive waste storage, and the sand fraction after the reaction After processing and washing with water repeatedly, water-gravity separation in the classification module is divided into finely dispersed (<0.1 mm) and sand (> 0.1 mm) fractions, finely dispersed fraction after thickening and dehydration is sent to the storage of radioactive waste, and the sand fraction is returned to the aforementioned place of selection (application No. 2008134421/06 (043649) of 08/25/08, with a decision on the grant of a patent of 11/19/09) [2].

The main disadvantage of this method is the high amount of the desired reagent with a coefficient of deactivation (KD-70-80). To completely remove the finely dispersed substance, with which ~ 90% of radionuclides are associated, the prototype method uses 2 M sulfuric acid treatment at a liquid / solid ratio of 2/1 at a temperature of 80 ° C for 7 hours.

The technical result of the proposed method is to reduce the amount of reagent required for decontamination, while maintaining the value of the coefficient of decontamination.

To achieve the specified technical result, a method for cleaning sandy soils from radionuclides is proposed, including the initial destruction of soil aggregates, a surface mud layer covering sand grains and pebbles, followed by mechanical separation of the original soil into pebbles (3-100 mm) and pulp containing particles of size ( <3 mm), the pebbles after separation and washing are removed from the technological cycle, the pulp is separated by the water-gravity separation in the classification module into the sand fraction (> 0.1 mm) and finely divided a fraction (<0.1 mm), which after thickening and dehydration is sent to a radioactive waste storage facility; the sand fraction is washed with water, dehydrated and subjected to reagent treatment with a 0.5-1 M solution of sulfuric acid in an autoclave at a temperature of 100-140 ° C, with a ratio of liquid and solid phases of 0.5 / 1-1 / 1, at the end of which a reagent solution is separated, and the soil is washed with water, the separated reagent solution and soil wash water are mixed and radionuclides are precipitated from the resulting mixture, the precipitate is sent to the radioactive waste storage, and the sand fraction after the reagent treatment and washing with water by gravity separation in the module classification for separated fine (<0.1 mm) and sand (> 0.1 mm) fraction, fine fraction after condensation and dehydration is sent to storage of radioactive waste, as the sand fraction is returned to the sampling point.

Distinctive features of the proposed method for cleaning sandy soils from radionuclides is the reagent treatment of the sand fraction with a 0.5-1 M solution of sulfuric acid in an autoclave at a temperature of 100-140 ° C, with a ratio of liquid and solid phases of 0.5 / 1-1 / 1.

Reducing the amount of reagent during decontamination while maintaining the value of the coefficient of deactivation is possible only by increasing the processing time or processing temperature, that is, conducting it in an autoclave mode (t> 100 ° C). The increase in processing time leads to an extreme complication of the technological process, processing can be carried out only in separate cycles, after each of which the extracted radionuclide must be removed from the solution (RF Patent No. 2361301, publ. BI No. 19 of July 10, 2009) [3]. The most advisable to carry out reagent processing in autoclave mode in the range of 100-140 ° C. The reagent pressure reaches 5 atm at 140 ° C; therefore, the use of high temperatures places high demands on the composition of the reagent and the material from which the autoclave is made. According to the prototype, for the decomposition of hydroxide films (the condition for the complete separation of the finely divided fraction containing up to 90% of the finely dispersed substance), 2 M sulfuric acid is required at a ratio of liquid and solid phases of 2/1 and a processing temperature of 80 ° C (RF Application No. 2008134421/06 (043649 ) of 08.25.08, with a decision on extradition of 11.19.09) [2]. Therefore, the use of an 1/1 ratio of the liquid and solid phases in the autoclave mode at a reagent concentration of 1 M sulfuric acid solution allows a 4-fold reduction in the amount of reagent, and if the contamination of the test sample is small, a 0.5 M sulfuric acid solution can be used in the ratio of liquid and solid phase 0.5 / 1, which reduces the use of the reagent by 16 times.

An example of the purification of sandy soils contaminated with ¹³⁷ Cs radionuclides.

Soil samples were provided by the Russian Research Center Kurchatov Institute. At the installation of this center (Volkov V.G., Zverkov Yu.A., Ivanov O.P. et al. Decontamination of radioactively contaminated soil at the Kurchatov Institute RSC. Atomic Energy, 2007, v.103, issue 6, s .381-387) [4] the following were performed: the water-gravity separation of the finely dispersed fraction, including the initial destruction of soil aggregates, the surface silt layer covering sand grains and pebbles, the subsequent mechanical separation of the initial soil into pebbles (3-100 mm) and pulp, containing particles of size (<3 mm), separation of the pulp into a sand fraction (> 0.1 mm) and chalk dispersed fraction (<0.1 mm). At an initial concentration of ¹³⁷ Cs in the pulp 1120 ± 25 kBq / kg after separation of the finely dispersed fraction, the specific activity of ¹³⁷ Cs of the sand fraction was 299 ± 3 kBq / kg, the finely dispersed fraction, by weight 15.5% of the pulp weight, was sent to the radioactive waste storage . Reagent treatment of the sand fraction was carried out with a sample of 6 g, placed in a porcelain cup, into which 6 g of a 1 M solution of sulfuric acid was poured (2 M sulfuric acid was used in the prototype with a ratio of liquid and solid phases of 1/1, this corresponds to a decrease in the amount of reagent in 4 times). The cup was placed in an autoclave, which is a cylindrical capsule made of stainless steel and sealed with a fluoroplastic gasket. Capsules were placed in a high-temperature furnace, in which reagent treatment took place for 7 hours at a temperature of 100 ° C. After the treatment, the soil with the reagent solution was transferred to a 250 ml conical glass flask using wash water, the amount of which should provide 1 cm excess of the liquid phase in the flask over sandy soil. By vigorous circular motions, the soil and the solution were mixed for 1 min, 2 seconds after the end of mixing, the settled solution with the finely dispersed substance suspended in it was transferred to a 1 liter glass, in subsequent operations, water was added to the residual soil and the process was repeated until after 2 seconds of settling, the solution did not become transparent.

After 24 hours of settling the finely dispersed substance in the beaker, initially by decantation and then filtering through a paper filter, the liquid and solid finely divided components of the soil were separated and the mass of the finely dispersed fraction was measured and the content of ¹³⁷ Cs in this fraction and in the total volume of the liquid phase containing the remaining reagent solution and washings water 1 ml of potassium ferrocyanide with a concentration of 10 ^-3 M is introduced into the liquid component (residues of the reagent solution and washing water), the solution was mixed, and after settling by decantation, it was separated from the precipitate of potassium ferrocyanide, which entered the radioactive waste storage.

According to the above scheme, decontamination was carried out at temperatures in the range of 100 ° C-140 ° C, the results are shown in table 1.

Table 1.
The results of decontamination according to the proposed method of sandy soil with the initial specific activity of the radionuclide ¹³⁷ Cs 299 ± 3 kBq / kg, A _i is the specific activity of the soil after the i-th stage of processing, KD _i is the coefficient of decontamination after the i-th stage of processing, δ _i - the residual amount of soil after the i-th stage of processing, reagent treatment was carried out with 1 M sulfuric acid with a ratio of liquid and solid phases (w / t) 1/1 for 7 hours. Water-gravity separation of pulp Reagent Processing Water-gravity separation of the sand fraction A ₁ , kBq / kg KD ₁ δ ₁ ,% T, ° С A ₂ , kBq / kg KD ₂ δ ₂ ,% A ₃ , kBq / kg KD ₃ δ _3, 299 ± 3 4.4 ± 1.1 85.5 one hundred 59.3 ± 3.5 22.9 ± 1.3 83.6 41.5 ± 3.3 32.6 ± 2.6 83.0 110 47.3 ± 2.8 28.6 ± 1.8 83,2 33.3 ± 2.6 40.9 ± 3.1 82.6 120 10.9 ± 0.7 123 ± 8 83.8 7.9 ± 0.7 172 ± 15 82.9 130 12.1 ± 0.8 110 ± 7 84.0 7.9 ± 0.6 171 ± 18 83,2 140 10.7 ± 0.8 125 ± 9 83,2 7.0 ± 0.8 195 ± 18 82.3

When processing in autoclave mode, it was possible to achieve and exceed the KD value (almost 2 times) in comparison with the prototype at a processing temperature in the temperature range 120-140 ° C.

The next series of decontamination experiments according to the proposed method was carried out with a reagent content 8 times lower than the prototype — 0.5 M sulfuric acid solution with a ratio of liquid and solid phases (w / t) of 0.5 / 1 at the processing time 7 hours and processing temperatures of 100 ° C and 140 ° C. The results are shown in table 2.

Table 2.
Reagent treatment was carried out with 0.5 M sulfuric acid with a ratio of liquid and solid phases (w / t) of 0.5 / 1 for 7 hours, 260 ± 3 kBq / kg - specific activity of the sand fraction after water-gravity separation of the pulp A _i - specific soil activity after the i-th processing stage, K _i is the decontamination coefficient after the i-th processing stage, δ _i is the residual amount of soil after the i-th processing stage, reagent treatment was carried out with 1 M sulfuric acid at a ratio of liquid and solid phases (w / r) 1/1 for 7 hours. Water-gravity separation of pulp Reagent Processing Water-gravity separation of the sand fraction A ₁ , kBq / kg KD ₁ T, ° C% T A ₂ , kBq / kg KD ₂ A ₃ , kBq / kg KD ₃ 260 ± 3 4.4 ± 0.1 one hundred 23.0 ± 3.1 7.7 ± 1.1 14.4 ± 3.0 21.1 ± 2.9 140 7.0 ± 1.1 41.3 ± 6.7 2.4 ± 0.4 129 ± 22

From table 2 it follows that the value of the coefficient of decontamination in the proposed method at a temperature of 140 ° C is almost 1.5 times higher than the results obtained for the prototype.

The above examples show the high efficiency of the proposed method of soil cleaning, allowing reagent treatment with a significantly smaller amount of reagent while maintaining, and in some cases, increasing the coefficient of decontamination.

The proposed "Method for the purification of sandy soil from radionuclides" is included in the R&D plan of the GUL MosNPO "Radon", laboratory and bench tests of the proposed method were carried out.

Literature

1. US 5045240 A 01.05.89, publ. 09/03/91, G21F 9/28, C02F 1/42.

2. Application No. 2008134421/06 (043649) of 08/25/08, with a decision on the grant of a patent of 11/19/09.

3. RF patent No. 2361301, BI No. 19 dated 07/10/2009.

4. Volkov V.G., Zverkov Yu.A., Ivanov O.P. et al. Decontamination of radioactively contaminated soil at the Kurchatov Institute, Nuclear Energy, 2007, vol. 103, issue 6, pp. 381-387.

Claims (1)

A method of cleaning sandy soils from radionuclides, including the initial destruction of soil aggregates, a surface mud layer covering sand grains and pebbles, followed by mechanical separation of the original soil into pebbles (3-100 mm) and pulp containing particles of size (<3 mm), pebbles after separation and washing, they are removed from the technological cycle, the pulp by water-gravity separation in the classification module is divided into a sand fraction (> 0.1 mm) and a fine fraction (<0.1 mm), which are sent after thickening and dehydration the radioactive waste storage facility, the sand fraction is washed with water, dehydrated, subjected to reagent treatment, after which the reagent solution is separated, and the soil is washed with water, the separated reagent solution and soil washing water are mixed, and radionuclides are precipitated from the resulting mixture, the resulting precipitate enters the radioactive waste storage and the sand fraction after reagent treatment and washing with water repeatedly by water-gravity separation in the classification module is divided into finely divided (<0.1 mm) and sand ( > 0.1 mm) fractions, the finely dispersed fraction after thickening and dehydration is sent to the radioactive waste storage facility, and the sand fraction is returned to the aforementioned collection point, characterized in that the reactive treatment of the sand fraction is carried out with a 0.5-1 M sulfuric acid solution in an autoclave at a temperature of 100-140 ° C with a ratio of liquid and solid phases of 0.5 / 1-1 / 1.