JP6164631B2 - Cesium removal method and cesium removal apparatus used therefor - Google Patents

Description

  The present invention relates to a cesium decontamination method suitable for a large area contaminated with radioactive cesium due to an accident at a nuclear power plant or the like, and in particular, a cesium decontamination method for a large amount of soil, debris, etc. The present invention relates to a method for decontaminating cesium and a cesium removing apparatus used therefor.

  When radioactive materials are extensively contaminated by an accident at a nuclear power plant, a large amount of soil, rubble, etc. must be treated. There are various types of radioactive pollutants, but for example, cesium 137 and strontium 90 currently account for most of the sources of radioactivity occurring in the area surrounding the Chernobyl nuclear power plant accident. Cesium 137 has a long half-life of 30 years. When it enters the body, it rides on the blood stream and emits beta and gamma rays to the intestines and liver. Is done. Cesium 137 emits beta rays and gamma rays from 100 days to 200 days after being taken into the body and discharged outside the body, causing internal exposure. Therefore, there is a strong demand for efficiently decontaminating radioactive cesium from contaminated soil.

  However, it is important to reduce the volume and weight of the contaminated soil because it is a large amount. Therefore, it is necessary to separate and collect radioactive cesium contained in the contaminated soil. However, cesium is known to bind strongly to soil, and it is difficult to separate it effectively at low cost.

For example, Patent Document 1 proposes a method for extracting cesium. However, Patent Document 1 relates to a technique for separating rubidium from porcite (Cs (AlSi ₂ O ₆ )), which is an economically important cesium source mineral. Currently, the amount of cesium mined from the world mine is 5 to 10 tons per year, and the harvestable period is several thousand years. Therefore, even if cesium is purposely separated from radioactively contaminated soil, No success is expected.
On the other hand, improvement of radioactively contaminated soil is also proposed in Patent Documents 2 and 3.
However, the radioactive substances to be treated in Patent Documents 2 and 3 are intended for heavy metals such as plutonium and uranium, and are not intended for alkali metals such as cesium.

Therefore, in Non-Patent Document 1, a separation method by in-situ heating of soil containing cesium is studied. However, when separating cesium from the soil, the following effects (a) and (b) are present, so that a remarkable effect cannot be obtained.
(A) Cesium dissolved in water behaves as a monovalent cation in the soil and is taken in between the thin layered structure, which is a clay layer on the surface of negatively charged soil particles, and is extremely “fixed” And cannot be easily replaced by other cations.
(B) Even if the soil adsorbed with cesium is heated to 685 ° C., which is the boiling point of cesium, or about 1300 ° C. in consideration of the melting point and boiling point of the cesium compound, no remarkable volatilization behavior of cesium is observed.
On the other hand, Non-Patent Document 2 discloses a decontamination demonstration technique characterized by a high-performance reaction accelerator characterized by soil to be decontaminated and heat treatment as a method. According to the results of the verification test in the decontamination target area near the Fukushima nuclear power plant, the decontamination rate of the decontamination verification technology is 99.9%, which is much higher than wet classification, but the processing cost is also 200,000. There is a problem that the cost is more than 10 times as much as yen / ton.
In decontamination of agricultural land and forest land, organic substances contained in soil are damaged when exposed to high temperatures. Therefore, a cesium decontamination method that is normal temperature and does not use harmful chemical substances is desired.

Japanese Patent Laid-Open No. 5-005134 Japanese Unexamined Patent Publication No. 6-051096 Japanese National Patent Publication No. 10-505903

Japan Atomic Energy Agency JAEA-Research 2011-026 Report on Decontamination Demonstration Work in Evacuation Zones Related to Fukushima Daiichi Nuclear Power Station Accident Japan Atomic Energy Agency June 2012

  This invention solves the said subject, and it aims at providing the cesium removal method and cesium removal apparatus which can remove cesium from soil, hold | maintaining decontamination object soil at normal temperature.

  The cesium removal method of the present invention solves the above problems. For example, as shown in FIG. 1, in the cesium removal method for removing cesium from soil containing cesium, the soil is pulverized using a pulverizer. The clay mineral contained in the soil is mechanochemically treated (120), and the cesium trapped between the clay mineral layers is volatilized by the mechanochemical treatment.

Cesium in the soil is captured exclusively by zeolitic clay minerals. Since this clay mineral is usually a layered material of 5 μm or less, if the layered structure of this clay material is mechanochemically destroyed by high energy grinding force, the trapped cesium will come into contact with the air and vapor phase stable compounds such as cesium oxide It can be volatilized in the form.
“Mechanochemical treatment” is a process of mixing and crushing an object to be processed with an impact crushing device such as a ball mill while applying mechanical energy. Torn apart, creating a highly active molecular surface. Therefore, by performing a mechanochemical treatment, a reaction (mechanochemical reaction) that cannot occur at room temperature can be advanced.

The cesium removal method of the present invention preferably includes a step of adding a predetermined amount of NaCl to the soil prior to the mechanochemical treatment or during the mechanochemical treatment.
Further, since NaCl has an action of adsorbing volatilized cesium, by placing NaCl in a container for performing a mechanochemical reaction, volatilized cesium can be adsorbed and dissipated.

In the cesium removal method of the present invention, preferably, for example, as shown in FIG. 1, prior to the soil pulverization treatment, the method has a step (100) of classifying the soil to 70 μm or less, and the classified particle size is It has the process of grind | pulverizing with respect to at least one of the silt part contained in fine soil, or a clay mineral, It is characterized by the above-mentioned.
Particularly preferably, when the soil classification level is 5 μm or less, the clay mineral capturing most of the cesium in the soil can be intensively pulverized, which is particularly effective.

In the cesium removal method of the present invention, preferably, a ball mill is used for the pulverization process, and a substance that adsorbs volatile cesium is arranged in a space formed in the center of the ball mill container, and the pulverization process is performed while adsorbing the volatile cesium. It is characterized by that.
The ball mill is preferably a three-dimensional ball mill that can give grinding energy to the ball without having a critical speed.
Here, when the soil is pulverized by the ball mill, there is a concern that the adsorbed material is also pulverized by the pulverization. On the other hand, however, an object that is larger than the ball diameter used for pulverization and whose apparent density is smaller than the ball has a property of being pushed out near the center inside the pulverized ball by the ball mill pot. Therefore, using this property, the adsorbed substance container is stored in a state where the adsorbent is wrapped with a breathable substance in an outer shell larger than the diameter of the ball used for pulverization and smaller in appearance density. This protects the adsorbed material from crushing.

  The cesium removal apparatus of the present invention solves the above-described problems. For example, as shown in FIG. 3, the cesium removal apparatus is a cesium removal apparatus used in the above-described cesium removal method. The adsorbing structure includes a substance that adsorbs the volatile cesium, an adsorbing substance containing that adsorbs the adsorbing substance while maintaining a ventilation state, and protects the adsorbing substance contained from an impact from the ball. The cesium removing apparatus further includes an outer shell body that maintains air permeability between the adsorbed substance container and the ball mill container.

In the apparatus configured as described above, the ball mill container 122 and the ball 124 may be equivalent to those used for normal particle crushing. The ball mill is preferably a three-dimensional ball mill that can give grinding energy to the ball without having a critical speed.
The adsorbing material may be any material as long as it can adsorb cesium in the gas phase or its compound vapor at room temperature. For example, a halogenated earth metal salt such as sodium chloride can be used.
The adsorbed material container is a container or package that does not dissipate the adsorbed material in the ball mill container, prevents the treated soil from adhering to the adsorbed material, and must have air permeability. A bag is used.
The outer shell body protects the adsorbing substance and the adsorbing substance container covering it from the collision of the ball, and has an outer diameter larger than that of the ball so that it can exist in the space inside the ball. The material of the outer shell body is either a strong substance such as a metal or a ball shock absorber such as a sponge, and both have a structure that ensures air permeability.
In the cesium removing apparatus of the present invention, preferably, the cesium volatilization accelerator is sodium chloride.

In the cesium removal method of the present invention, the soil to be decontaminated is pulverized using a pulverizer, and the clay mineral contained in the soil is mechanochemically treated to classify the soil having a high concentration of cesium, and cesium By performing the volatilization process simultaneously, the cesium removal rate can be kept high and cesium removal can be performed at room temperature. Therefore, for example, in fields where it is desired to maintain conventional utility such as farmland and forest land, organic substances contained in soil are retained, which is preferable.
Furthermore, in the cesium removal apparatus of this invention, the mechanochemical process of the decontamination object soil can be performed easily.

It is principle explanatory drawing which shows one Example of this invention. It is a principal part photograph explaining the structure of a three-dimensional ball mill. It is explanatory drawing of 3D mill grinding | pulverization in a ball mill.

Hereinafter, the present invention will be described with reference to the drawings.
FIG. 1 is a diagram illustrating the principle of an embodiment of the present invention.
The decontamination target soil 10 is classified to about 70 μm or less in the classification step 100. In general, gravel having a particle size of about 70 μm or more is classified as low-dose soil 12 because the content of cesium is small. Preferably, when the soil classification level is 5 μm or less, the clay mineral capturing most of the cesium in the soil can be intensively pulverized.
The fine soil 14 classified in the classification step 100 is pulverized in a milling treatment step 120, and the clay mineral contained in the fine soil 14 is mechanochemically treated. Then, as the fine soil 14 is pulverized, the cesium contained in the fine soil 14 comes into contact with the air and volatilizes in the form of cesium oxide. In this case, the cesium adsorbing substance 20 and sodium chloride 22 are preferably added to the fine soil 14 in the milling process 120. Then, the cesium adsorption material 20 and the cesium oxide volatilized in the sodium chloride 22 are adsorbed to form the cesium adsorption material 24.
Since the pulverized fine soil 14 is volatilized and removed by the cesium oxide, the fine soil 26 having a reduced dose is obtained.

FIG. 2 is a photograph of the main part for explaining the configuration of the three-dimensional ball mill.
A normal uniaxial ball mill (referred to as a rolling mill) has a critical speed, and beyond that, even if the rotational speed is increased, the pulverized ball is fixed to the wall surface by centrifugal force and the pulverization effect is lowered. On the other hand, since the three-dimensional ball mill has a rotation axis in a perpendicular direction in addition to the rotation axis of the rolling mill, there is no critical speed, and an effective crushing effect can be exhibited between the ball and the wall surface. A three-dimensional ball mill (referred to as a 3D mill) is used.

Table 1 explains the extraction of high-dose fine soil components by classification. Cesium is known to be adsorbed to fine soils, particularly clay minerals of 5 μm or less, among soils. In the example of soil used here, 95% of radioactivity can be targeted by classification of 5 μm or less. Thus, by performing classification before the ball mill treatment, the amount of treatment can be greatly reduced and efficient treatment becomes possible.

Table 2 explains the reduction of cesium in soil due to milling. Processed with a 3D mill and a stamp mill for comparison, showing the weight loss effect of radioactive cesium. Table 2 shows the amount of radioactive cesium in the soil after measurement as a ratio to that before the treatment, and the value obtained by subtracting this ratio value from 100 is the removal rate of radioactive cesium.

FIG. 3 is an explanatory diagram of 3D mill grinding in a ball mill.
As the 3D mill, a quasi-spherical pot having a diameter of 20 cm as a ball mill container 122 and 40 steel balls having a diameter of 17.4 mm as balls 124 are used. The 3D mill has an X axis that performs horizontal rotation and a Y axis that performs vertical rotation. The X axis generates lateral centrifugal acceleration, and the Y axis generates vertical centrifugal acceleration. The X-axis and the Y-axis both pulverize the treated soil 128 at a rotational speed of 240 rpm to generate crushed treated soil 126 .
The measured value of Table 2 has shown the case where the to-be-processed soil 128 is grind | pulverized with 3D mill. The treatment time of the 3D mill was 30 minutes and 60 minutes, the ground soil 126 to be treated was sampled, and the amount of radioactive cesium was measured. The stamp mill was performed by dropping from a pot diameter of 10 cm and a stamp bar height of 5 cm. A sample after one hour treatment was sampled, and the amount of radioactive cesium was measured.
Even in the stamp mill, radioactive cesium is removed by almost 1/4 in one hour, and it can be seen that the cesium can be separated from the clay mineral by mechanochemical reaction. In addition, the 3D mill is located between the ball mill pot inner wall and the ball. In order to effectively use the contact crushing force, it can be processed in a shorter time, and the amount of radioactive cesium that can be reached in the same time is greatly reduced.

Table 3 explains the effect of NaCl addition.
NaCl is known to have an effect of promoting volatilization in volatilization of radioactive cesium by high-temperature heating at about 1000 ° C. In 3D mill pulverization (steel balls, rotation speed is the same as the above), a case where NaCl was not added, a case where 10% was added to the amount of soil, and a case where 200% was added were compared. Although there is no effect at about 10%, it can be reduced by about 20% from the 40% reduction rate when 200% more than soil is added.

Table 4 explains the effect of the adsorbent.
An adsorbent package in which an adsorbent (vermiculite) is wrapped with filter paper in a protective bag in a protective case and a dummy package that does not contain the adsorbent are placed, a 3D mill with 10% NaCl (Experiment 1) and 200% NaCl It processed with the added 3D mill (experiment 2), and measured the cesium adsorption amount after a process. Here, vermiculite is a raw ore meteorite produced in China, South Africa, Australia, Zimbabwe, the United States, etc., sintered at about 800 ° C. and expanded 10 times or more.

The dummy bag was used because the packaging material, protective bag, and protective case were made to be breathable, so the finely pulverized soil particles adhered to the package even though they were very small. It is to do.
At present, no cesium adsorbent for gas phase has been developed, so vermiculite, which is considered to have high cesium adsorption capacity in liquid, was used, but both Experiment 1 and Experiment 2 were significant for the dummy without adsorbent. Adsorption occurs with a great difference.

Table 5 is a figure explaining the implementation result of this invention, and has shown the weight ratio and dose ratio in each process. Here, the soil is sieved, a steel ball ball g having a diameter of 17 mm is used in a three-dimensional ball mill in which NaCl is added to a clay material of 5 μm or less and an adsorbent is placed inside, and both the X axis and the Y axis are used. An example of processing at 240 rpm for 1 hour is shown. In the soil classification process, approximately 98% of the soil remains on the sieving sieve, but the dose can be reduced to 5% or less. In the milling treatment, the dose of about 40% of the soil having a weight of 2.3% after the three-dimensional ball mill treatment was about 40%.

  Adsorbents that are suitable for gas phase adsorption have not yet been developed, but some of the vermiculite used for adsorption in the liquid phase is also adsorbed from the gas phase. If so, the dissipated nearly 60% of cesium can be adsorbed at the pulverization stage, and it can be prevented from dissipating into the environment without a containment vessel.

  Next, the mechanochemical effect will be described. If the pulverization operation is continued with respect to the crystalline material, the number of surface atoms / molecules that have lost the joint increases as the nascent surface area increases. The disorder of the bonding state extends in the vicinity of the surface layer, and the powdered solid becomes active. In dry pulverization, fine particles are aggregated and the apparent surface area is reduced. In addition, moisture and gas in the air are adsorbed on the active surface. These fine particle agglomeration and gas surface adsorption are stabilized by lowering the chemical potential of the activated fine particle surface. As a result of stabilization, various chemical reactions occur at various phase transitions and solid surfaces. These changes are collectively called mechanochemical effects.

  The above mechanochemical treatment shows the case where the soil to be decontaminated is pulverized in the air, and the present invention is not limited to this. In short, the mechanochemical treatment is performed at room temperature without performing any special heat treatment. Any chemical treatment can be used. Therefore, as a cesium gas phase stable compound at room temperature, in addition to cesium oxide, compound cesium by other elements may be generated. For example, a gas phase stable compound may be generated using nitrogen gas or carbon dioxide gas. Also good.

10 Decontamination target soil 14 Fine soil 20 Adsorbed material 26 Reduced-dose fine soil 100 Soil classification 120 Milling treatment (mechanochemical treatment)

Claims (5)

In the cesium removal method of removing the cesium from soil containing cesium,
Crushing the soil using a pulverizer, comprising a step of mechanochemical treatment of clay minerals contained in the soil;
The cesium removal method characterized by volatilizing the cesium captured between the clay mineral layers by the mechanochemical treatment. In the cesium removal method of Claim 1,
A method of removing cesium, comprising a step of adding a predetermined amount of NaCl to the soil prior to the mechanochemical treatment or in the middle of the mechanochemical treatment. The method for removing cesium according to claim 1 or 2, further comprising a step of classifying the soil to 70 μm or less prior to the pulverization treatment of the soil.
A method for removing cesium, comprising a step of pulverizing at least one of a silt component and a clay mineral contained in soil having a fine classified particle size. In the cesium removal method of any one of Claims 1 thru | or 3,
A method for removing cesium, wherein a ball mill is used for the pulverization treatment, a substance that adsorbs volatile cesium is disposed in a space formed in the center of the ball mill vessel, and the pulverization treatment is performed while adsorbing the volatile cesium. A cesium removal apparatus used for the cesium removal method according to claim 4,
A ball mill container, a ball and an adsorption structure;
The adsorbing structure protects the substance adsorbing the volatile cesium, the adsorbing substance containing the adsorbing substance while maintaining a ventilation state, and the adsorbing substance containing from the impact from the ball, An apparatus for removing cesium, comprising: an outer shell body that maintains air permeability between the adsorbed substance container and the ball mill container.