JP2013174539A - Decontamination method of soil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a decontamination method of soil, which allows the volume of contaminated soil to be reduced with improved decontamination efficiency, by finely peeling the surface of coarse soil particles to make fine soil particles to which radioactive substances are adsorbed and then by removing the fine soil particles such that no radioactive substances remain in the coarse soil particles.SOLUTION: Calcium oxide and/or calcium hydroxide, and water are added to the soil contaminated with radioactive substances, agitated and mixed to form a processed product in slurry form. The processed soil in the processed product is separated based on a predetermined particle diameter Da, so that the soil having particle diameter Da or smaller is extracted as contaminated soil and the soil having particle larger than diameter Da is extracted as decontaminated soil. The separation of the processed soil in the processed product is gradually performed on an as needed basis. The processed soil produced in the separation step is repeatedly agitated, mixed, and separated. The processed product is drained in the separation step on an as needed basis.

Description

  The present invention relates to a soil decontamination method for decontamination of soil contaminated with radioactive substances such as radioactive cesium.

In the 2011 Great East Japan Earthquake in 2011, radioactive materials were scattered in the accident at the Fukushima Daiichi nuclear power plant. In areas where the ground was contaminated with radioactive materials, the surface of the ground was scraped and contaminated. Accumulated and stored as soil. Since the accumulation amount is enormous, decontamination of this accumulated contaminated soil and its volume reduction are being studied.
As a countermeasure, for example, it is considered to extract radioactive substances using a method for treating contaminated soil described in JP-A-2008-161766 (Patent Document 1). This is because the contaminated soil is housed in a drum-type washing machine, the contaminated soil and washing water are mixed and stirred, thereby washing the radioactive material, and then mixed into this washing water by a wet vibrating sieve. The resulting soil is sieved, and a mixture of fine soil particles mixed with washing water is taken out through a mesh, dehydrated and treated as contaminated soil. On the other hand, the coarse-grained soil on the mesh is taken out separately and reused.

JP 2008-161766 A

By the way, in the above conventional method, washing with water and stirring are performed to take out fine soil particles and washing water, and the volume is reduced in anticipation of elution of radioactive substances into washing water and adhesion to fine soil particles. However, for example, radioactive cesium, which is a radioactive substance, can be removed to some extent by taking out fine soil particles and washing water, but the ratio remaining in coarse soil is extremely high, There was a problem that the decontamination efficiency was inferior and it was difficult to substantially reduce the volume.
The reason is that clay minerals contained in the soil are characterized by a structure (two-dimensional) composed of metal ions and silicic acid connected to each other. Specifically, silicon and aluminum have tetrahedral layers and octahedrons. It is well known to strongly adsorb radioactive materials, especially cesium ions, because of this structure, and is also called layered silicate. This is because it is more firmly held in the soil and harder to leave.

  The present invention has been made in view of the above-mentioned problems, utilizing the adsorption force of radioactive material to the soil, finely separated the surface of coarse soil particles finely adsorbed radioactive material In order to improve the decontamination efficiency and reduce the volume of contaminated soil, it is possible to remove the soil particles of coarse particles so that radioactive materials do not remain in the coarse soil. It aims at providing the decontamination method of the planned soil.

In order to achieve such an object, the soil decontamination method of the present invention is a soil decontamination method for decontaminating soil contaminated with radioactive substances.
To the soil, an alkali agent and water are added and mixed by stirring to obtain a muddy water-like treated product. The treated soil in the treated product is separated on the basis of a predetermined particle size Da, and the soil having the particle size Da or less is separated. Extracted as contaminated soil, the soil that exceeds the particle size Da is extracted as decontaminated soil.

  The alkali agent is selected from alkali metal compounds, alkaline earth metal compounds and the like. Strong alkali is preferred. Examples of the alkali metal compound include oxides, hydroxides, hydrogen carbonates, carbonates, silicates, aluminates, nitrates, sulfates of Li, Na, K, Rb, Cs, and Fr. Examples of the alkaline earth metal compound include Ca, Sr, Ba, and Ra oxides, hydroxides, hydrogen carbonates, and carbonates. Preferably a calcareous material is suitable. Examples of calcareous materials include quick lime (calcium oxide), quick lime lime, slaked lime (calcium hydroxide), white slaked lime, calcium carbonate, calcium carbonate, calcium silicate, by-product lime, shell fossil lime, organic lime And at least one or a mixture of two or more eggshell powder.

  Thereby, when an alkali agent and water are added to the soil and mixed with stirring, as shown in FIG. 1, radioactive particles are adsorbed on the surface mainly of coarse soil particles. While adsorbing the radioactive material, it is finely peeled to make fine particles. Therefore, when the treated soil in the treated material is separated on the basis of the predetermined particle size Da, that is, when separated into coarse soil and fine soil, it is separated from the coarse soil and finely divided. Fine-grained soil can be taken out while adsorbing radioactive substances together with the original fine-grained soil, and decontamination efficiency can be improved. For this reason, since the quantity of the contaminated soil of the particle size Da or less extracted is very small compared with the quantity of the original soil, the volume of contaminated soil can be reduced significantly. Moreover, since a radioactive substance hardly remains in the soil exceeding the particle size Da, it can be reused as decontaminated soil.

As shown in FIG. 1, the surface of the soil is finely peeled and finely divided as shown in FIG. 1, when the alkaline agent is dissolved in water, it shows a low ORP value, so that water penetrates into the clay. This is thought to be due to the fact that the strength is increased, which promotes the fragmentation of clusters on the clay surface and breaks up finely.
In the case of radioactive cesium, most is adsorbed on clay, and is adhering to the surface of the clay lump and hardly adhering to the clay in the center. Therefore, if the clay surface is peeled off, and only fine-grained clay with radiation cesium attached is recovered, the volume can be greatly reduced. And most soil can be reused by separating and isolating only high-concentration radioactive clay.
Furthermore, since most of the radioactive cesium is adsorbed on the clay, the liquid that has been drained does not substantially contain cesium and can be reused or made into a waste liquid.

  And it is set as the structure which isolate | separates the process soil in the said processed material in steps as needed. For example, when filtering or separating by sieving, the coarseness of the eyes is made finer in steps. Therefore, compared with the case where it isolate | separates at once, it becomes difficult to clog and a separation efficiency is improved.

  Moreover, it is set as the structure which repeats the said mixing stirring and isolation | separation with respect to the treated soil which arises in the process of the said isolation | separation as needed. In some cases, the radiation dose of the separated coarse-grained treated soil is high, but in that case, by repeatedly mixing and stirring and separating, the soil surface to which the radioactive material has adhered can be peeled off and subdivided. Decontamination can be performed reliably.

  Furthermore, it is set as the structure which drains the said processed material in the process of the said isolation | separation as needed. By removing the liquid, the weight of the soil is reduced, so that volume reduction can be reliably performed. In addition, since radioactive substances are adsorbed and taken up by soil and are not easily eluted into water, drainage can be reused or used as waste liquid.

  Moreover, it is set as the structure which prescribed | regulated the reference | standard Da of the particle size used as the said small diameter side soil to 30 micrometers <= Da <= 200 micrometers as needed. It is considered that radioactive cesium is abundant in soil particles up to 30 μm, and based on this, the minimum is 30 μm. When it is set to more than that, the safety factor increases, but when it exceeds 200 μm, the volume reduction rate is inferior. Desirably, 30 μm ≦ Da ≦ 100 μm.

And, if necessary, an alkaline agent and water are added to the soil, and the mixture is stirred and mixed to obtain a muddy water-like treated product. The treated product is a muddy water-like secondary treated product in which the soil having the particle size Da or less is mixed. , Separated into soil exceeding the particle size Da, and then the flocculant is added to the secondary treated product to settle the treated soil, and then the secondary treated product on which the treated soil has settled is filtered. The filtrate is divided into a residue and the residue is extracted as contaminated soil.
The method of separating the treated material into a muddy water-like secondary treated material mixed with soil having a particle size Da or less and soil having a particle size Da can be performed by methods such as filtration and sieving. Moreover, since the secondary treated product is coagulated and settled with a coagulant, it can be easily filtered, and the soil having a particle size of Da or less can be easily extracted as a residue.

  Further, if necessary, an alkaline agent and water are added to the soil, and the mixture is stirred and mixed to obtain a muddy water-like treated product. The treated product is passed through a sieve having a predetermined size larger than the particle size Da, and large. It separates into a diameter-side soil and a muddy water-like secondary treated product mixed with a small-diameter side soil, and then the secondary-treated product is dehydrated and dried to obtain a small-diameter-side soil. It is set as the structure isolate | separated into the soil of the particle size Da or less and the soil which exceeds the said particle size Da. Thereby, the large-diameter side soil can be used as the decontaminated soil as it is, or it can be agitated and mixed and separated again to obtain the decontaminated soil. Moreover, since the dehydrated and dried small-diameter side soil is separated, the final separation can be performed relatively easily such as using an air classifier.

  In addition, if necessary, calcium oxide and / or calcium hydroxide is used as the alkali agent. As shown in FIG. 1, calcium oxide and / or calcium hydroxide exhibits a particularly low ORP value when dissolved in water, so that the osmotic power of water penetrating into the clay is increased and the clusters on the clay surface are increased. Promotes subdivision and excels in breaking down finely.

In this case, the water added to the soil is made 2 to 4 times the weight of the soil by weight ratio, and the calcium oxide and / or calcium hydroxide added to the soil is 0.1 to 10 weight by weight ratio to the added water. %.
If the amount of water added is small, calcium oxide and / or calcium hydroxide will not reach the soil evenly. If the amount of water is too large, subsequent handling becomes complicated. The amount of water is preferably 2.5 to 3.5 times, although it varies slightly depending on the amount of water in the soil.
Further, if the amount of calcium oxide and / or calcium hydroxide added is less than 0.1% by weight, the effect is small, and if it exceeds 10% by weight, the properties of the recovered soil and liquid are affected. Preferably, the content is 0.2 to 5% by weight, more desirably 0.8 to 2% by weight.

  According to the present invention, even if radioactive substances are adsorbed on the surface of the soil particles, when the alkaline agent, particularly calcium oxide and / or calcium hydroxide, water is added to the soil and stirred, the surface of the soil is Finely peel and fine grain. Therefore, when the treated soil in the treated material is separated on the basis of the predetermined particle size Da, that is, when separated into coarse soil and fine soil, it is separated from the coarse soil and finely divided. Fine-grained soil can be taken out while adsorbing radioactive substances together with the original fine-grained soil, and decontamination efficiency can be improved. For this reason, since the quantity of the contaminated soil of the particle size Da or less extracted is very small compared with the quantity of the original soil, the volume of contaminated soil can be reduced significantly. Moreover, since a radioactive substance hardly remains in the soil exceeding the particle size Da, it can be reused as decontaminated soil.

It is a figure which shows the principle of the decontamination method of the soil of this invention. It is a figure which shows the decontamination method of the soil which concerns on 1st embodiment of this invention. It is a figure which shows the decontamination method of the soil which concerns on 2nd embodiment of this invention with the system. It is a table | surface figure which concerns on Example 1 of this invention and shows a radionuclide measurement result. It is a table | surface figure which concerns on Example 2 and a comparative example of this invention, and shows a radionuclide measurement result.

Hereinafter, based on an accompanying drawing, the soil decontamination method concerning an embodiment of the invention is explained in detail.
The soil decontamination method according to the embodiment performs decontamination of soil contaminated with radioactive substances. For soil, for example, the surface of the ground to which a radioactive substance such as radioactive cesium adheres is scraped, and this is collected and stored as contaminated soil.
In the embodiment, calcium oxide and / or calcium hydroxide is used. This calcium oxide and / or calcium hydroxide is produced, for example, by firing a shell. The firing temperature of the shell is 800 ° C. or higher. Desirably, it is 1000 degreeC or more. If less than 800 ° C., the activity is poor. Satisfactory activity can be maintained at 800 ° C. or higher. Calcium hydroxide is produced, for example, by adding water to calcium oxide.

  In the soil decontamination method according to the embodiment, first, calcium oxide and / or calcium hydroxide and water are added to the soil, followed by stirring and mixing to obtain a muddy water-like treated product. For the mixing and stirring, for example, a civil engineering mixer or drum washer is used. The capacity can be selected as appropriate depending on the amount of soil to be treated, the installation environment, and the like, for example, in the range of 30 L to 4000 L.

  The treated soil in the treated product is separated with reference to a predetermined particle size Da, soil having a particle size Da or less is extracted as contaminated soil, and soil exceeding the particle size Da is extracted as decontaminated soil. In the separation, for example, a filtering tool, a vibrating screen machine, a centrifuge, an air classification, or the like is appropriately used according to the conditions such as the moisture state of the treated product or the treated soil. If necessary, the treated soil in the treated material is separated in stages. Furthermore, if necessary, the above mixing and agitation and separation are repeated for the treated soil generated in the separation process. Furthermore, if necessary, the treated product is drained during the separation process.

  The amount of water added to the soil is 2 to 4 times the weight of the soil. If the amount of water added is small, calcium oxide and / or calcium hydroxide will not reach the soil evenly. If the amount of water is too large, subsequent handling becomes complicated. The amount of water is preferably 2.5 to 3.5 times, although it varies slightly depending on the amount of water in the soil.

  Calcium oxide and / or calcium hydroxide added to the soil is 0.1 to 10% by weight in terms of the weight ratio to the added water. If the amount of calcium oxide and / or calcium hydroxide to be added is less than 0.1% by weight, the effect is small, and if it exceeds 10% by weight, the properties of the recovered soil and liquid are affected. Preferably, the content is 0.2 to 5% by weight, more desirably 0.8 to 2% by weight.

  Further, the standard Da of the predetermined particle diameter is defined as 30 μm ≦ Da ≦ 200 μm. It is considered that radioactive cesium is abundant in soil particles up to 30 μm, and based on this, the minimum is 30 μm. When it is set to more than that, the safety factor increases, but when it exceeds 200 μm, the volume reduction rate is inferior. Desirably, 30 μm ≦ Da ≦ 100 μm. In the embodiment, Da = 30 μm is set. If 30 μm ≦ Da, for example, it may be set in the range of Da ≦ 40 μm, Da ≦ 50 μm, Da ≦ 60 μm, and the like. Increasing the setting value decreases the volume reduction rate, but the safety factor increases, so it can be determined according to the decontamination conditions.

FIG. 2 shows a soil decontamination method according to the first embodiment. In this method, calcium oxide and / or calcium hydroxide and water are added to the soil and mixed by stirring to obtain a muddy water-like treated product (1-1). For mixing and stirring, a mixer for civil engineering having a capacity of 50 L is used.
Next, this processed product is separated (1-2). In the embodiment, separation by a filter is performed. Specifically, a sandbag with an opening of 30 μm was used. The processed material is put in the sandbag, and the sandbag is placed in a plastic rectangular container and left for an appropriate time. As a result, the muddy water-like secondary treated material mixed with soil having a particle size of 30 μm (Da) or less flows out and collects in the container (1-3), and the soil having a particle size exceeding 30 μm (Da) It remains inside and is taken out (1-4). If the radiation dose is appropriate, this removed soil is used as decontaminated soil as it is. If the radiation dose is not appropriate, the above mixing and stirring are repeated.

  Next, a coagulant is added to the secondary treated product to settle the treated soil (1-5). As a flocculant, the commercially available thing which uses a calcium compound, coal ash, an aluminum compound etc. as a component is used, for example. Thereafter, the secondary treated product on which the treated soil has settled is filtered and separated into a filtrate and a residue (1-6). In this case, the residue becomes a lump having a size of 30 μm (Da) or more and settles. In this filtration, a secondary treated product in which treated soil is settled is put in a sandbag similar to the above, and the sandbag is placed in a plastic rectangular container and left for an appropriate time. As a result, the filtrate flows out and accumulates in the container (1-7), and a residue that is a lump of soil of 30 μm (Da) or less remains in the sandbag and is taken out (1-8). Since the filtrate is a calcium hydroxide solution with no soil runoff and almost no radioactive material, it can be reused or used as a waste solution. The residue is a lump of soil having a particle size Da or less and is contaminated soil. The contaminated soil is then stored by, for example, compressing it into pellets and placing it in a concrete container. Since the amount of the contaminated soil having the particle size Da or less to be extracted is extremely small compared to the amount of the original soil, the volume of the contaminated soil can be greatly reduced.

  In the first embodiment, the treated soil in the treated product may be separated stepwise. For example, first, primary filtration is performed using a sandbag with a mesh opening of, for example, 100 μm, and then a muddy water-like processed material mixed with soil of 100 μm or less is used with a sandbag with the opening of 30 μm. Secondary filtration is performed. Thereby, compared with the case where it isolate | separates at once, it becomes difficult to clog fine-grained soil and a separation efficiency is improved.

FIG. 3 shows a soil decontamination method according to the second embodiment. In this method, calcium oxide and / or calcium hydroxide and water are added to the soil and mixed by stirring to obtain a muddy water-like treated product. For mixing and stirring, a drum washer 1 having a capacity of 400 to 3600 L is used in a batch system.
Next, the treated material is passed through a sieve having an opening having a predetermined size (100 μm in the embodiment) larger than a particle size of 30 μm (Da), and a large-diameter side soil having a particle size exceeding 100 μm and a particle size of 100 μm or less It separates into a muddy water-like secondary treated product mixed with small-diameter side soil. This separation is performed using a known vibrating screen 2. The separated large-diameter side soil is directly decontaminated soil if the radiation dose is appropriate. If the radiation dose is not appropriate, the above mixing and stirring are repeated.

Next, the secondary treated product is dehydrated and dried to obtain a small-diameter side soil having a particle size of 100 μm or less. For this dehydration drying, a well-known dehydration dryer 3 is used. If there is a liquid that can be recovered, it is a calcium hydroxide solution with little radioactive material, so it can be reused or used as a waste liquid.
Then, the small-diameter side soil having a particle size of 100 μm or less is separated into soil having a particle size of 30 μm (Da) or less and soil having a particle size exceeding 30 μm (Da). In this separation, a well-known air classifier 4 is used. Since the air classifier 4 is used for the final separation, the separation can be performed relatively easily. Soil having a particle size exceeding 30 μm (Da) can be decontaminated soil. Further, soil having a particle size of 30 μm (Da) or less is stored as contaminated soil in a concrete container. Since the amount of the contaminated soil having the particle size Da or less to be extracted is extremely small compared to the amount of the original soil, the volume of the contaminated soil can be greatly reduced.

Next, examples will be described.
Example 1
Using the decontamination method according to the first embodiment described above, contaminated soil of radioactive material accumulated in the 14 district of Hiei, Iizumi-mura, Soma-gun, Fukushima Prefecture was decontaminated.
10 kg of soil (untreated soil) was put into a mixer for civil engineering having a capacity of 50 L, and 3 times the amount of water (30 kg) was added. Calcium oxide was added to 1% by weight with respect to water. And it stirred and mixed for 30 minutes.
The treated product obtained by stirring and mixing was placed in a sandbag having an opening of 30 μm, and the sandbag was placed in a plastic rectangular container and allowed to stand for an appropriate time. As a result, the muddy water-like secondary treated product mixed with the soil having a particle size of 30 μm or less flows out and accumulates in the container, and the soil having a particle size exceeding 30 μm remains in the sandbag. The soil of this sandbag was taken out and used as decontaminated soil. Next, a commercially available flocculant was added to the secondary treated product to settle the treated soil, which was put in a sandbag and filtered in the same manner as above to separate the filtrate and the residue, and the residue was taken out as contaminated soil.

  And about the untreated soil, decontaminated soil, contaminated soil (residue), and the filtrate, the radionuclide measurement by the germanium semiconductor detector was performed. The results are shown in FIG. From this result, the decontaminated soil of Example 1 was only mixed and stirred once with calcium oxide and water, but the total amount of radioactive cesium in the untreated soil was 57000 Bq / Kg. It is significantly reduced to 19600 Bq / Kg, and it can be seen that the decontamination rate is extremely high. This is also clear from the fact that radioactive cesium in the contaminated soil is condensed and increased to 77000 Bq / Kg, and it can be seen that a high concentration of clay is recovered. Moreover, the density | concentration of the radioactive cesium of a filtrate (calcium hydroxide solution) is substantially zero, and it turns out that it is harmless even if it reprocesses or drains. In addition, although the decontamination soil of Example 1 only mixed and stirred calcium oxide and water once, if mixing and stirring are repeated, the radiation concentration can be further reduced.

(Example 2)
Using the same decontamination method as in Example 1, the contaminated soil of radioactive material accumulated in Hiei 415, Iitate Village, Soma-gun, Fukushima Prefecture was decontaminated. In addition, as a comparative example, untreated soil was treated in the same process as in Example 1 using water to which calcium oxide was not added. And the radionuclide measurement by the germanium semiconductor detector was performed about untreated soil, decontaminated soil (Example 2), and decontaminated soil (comparative example). The results are shown in FIG. From this result, the total amount of radioactive cesium in the untreated soil was 27000 Bq / Kg, whereas in the decontaminated soil simply treated with water (Comparative Example), it was 15500 Bq / Kg, and the decontamination rate was low (decontamination). It can be seen that the dyeing rate is 43%) and the practicality is poor. On the other hand, in the decontamination soil (Example 2) which concerns on the process of Example 2, it falls significantly to 1370 Bq / Kg, the decontamination rate is very high (decontamination rate 95%), and it is rich in practicality. I understand.

<Experimental example>
Next, experimental examples will be described. The experiment was conducted on the activity of calcium oxide used in the above embodiment. A treatment solution was prepared by dissolving 1 g of calcium oxide in 2 L of water, and the effect was confirmed by using it in various ways.
(Experimental example 1)
The tissue paper impregnated with iodine and turned brown was immersed in the treatment solution and stirred. As a result, the brown color disappeared and it became colorless and transparent. It was found that the treatment liquid was reduced in iodine and changed to a completely different substance called hydrogen iodide, and its activity was rich.
(Experimental example 2)
Tea leaves were put into a mesh-shaped tea strainer, and the above-mentioned treatment liquid was poured into the tea leaves to extract tea leaf components. Compared to extraction with water at room temperature, a brown extract was obtained. This is presumed to support the mechanism by which the outermost highly-concentrated radioactive cesium-containing clay (corresponding to the pigment component of tea leaves) forming the clay mass (corresponding to tea leaves) is subdivided. In addition, it is estimated that the mechanism by which extraction power becomes large is due to the ORP (redox potential) being lowered by the action of calcium oxide, the water clusters becoming smaller, and the penetration power being increased.
(Experimental example 3)
The above treatment liquid was added to a commercially available tea beverage contained in a PET bottle. The tea color changed to brown, and it was found that the tea components were further extracted. This is also presumed to support the further fragmentation of the alkaline aqueous solution with increased penetrating power through small clay blocks.

(Experimental example 4)
The reduction potential of the above treatment liquid was measured. The redox potential of tap water was 680 mv, whereas the treatment liquid was 79 mv. The tap water contains a chlorine component, and is generally +600 mV to +800 mV, and even the mineral water called so-called famous water is in the range of +150 mV to +250 mV, but the treated water is extremely low and may be rich in reducing power. I understood.
From the above results, it was inferred that calcium oxide and / or calcium hydroxide exhibited an exfoliating action in the form of fine soil particles with the radioactive substance adsorbed in the action on the soil.

  In the embodiment of the present invention, the soil separation means and the separation step are not limited to the above, and may be appropriately determined according to the form of the soil.

  As mentioned above, since radioactive materials were scattered in the accident at Fukushima Daiichi Nuclear Power Plant, in areas where the ground was contaminated with radioactive materials, the surface of the ground was scraped and stored as contaminated soil. However, the amount of accumulation is enormous. At present, in order to decontaminate and reduce the volume of these stored soils, it is considered to simply wash with water or use strong acid (sulfuric acid or hydrochloric acid). It is inferior in decontamination efficiency, and when acid is used, the soil becomes acidic and cannot be used as agricultural land. According to the present invention, since the decontamination rate is extremely high, the decontamination efficiency is excellent, and since it is treated with an alkaline agent, calcium hydroxide is particularly useful for neutralizing acidic soil. Can be reused as agricultural land as it is, which makes it extremely useful.

1 Drum Washer 2 Vibrating Screen 3 Dehydrating Dryer 4 Air Classifier

Claims (9)

In soil decontamination methods for decontamination of soil contaminated with radioactive substances,
To the soil, an alkali agent and water are added and mixed by stirring to obtain a muddy water-like treated product. The treated soil in the treated product is separated on the basis of a predetermined particle size Da, and the soil having the particle size Da or less is separated. A soil decontamination method, wherein the soil is extracted as contaminated soil, and the soil having the particle size Da is extracted as decontamination soil.   2. The soil decontamination method according to claim 1, wherein the treated soil in the treated material is separated stepwise.   The soil decontamination method according to claim 1 or 2, wherein the mixing and stirring are repeatedly performed on the treated soil generated in the separation process.   4. The soil decontamination method according to any one of claims 1 to 3, wherein the treated product is drained in the separation process.   The soil decontamination method according to any one of claims 1 to 4, wherein the predetermined particle diameter Da as the reference is defined within a range of 30 µm ≤ Da ≤ 200 µm.   To the soil, an alkaline agent and water are added and mixed by stirring to obtain a muddy water-like treated product. The muddy water-like secondary treated product obtained by mixing the treated material with the soil having the particle size Da or less and the particle size Da. Then, the flocculant is added to the secondary treated product to settle the treated soil, and then the secondary treated product on which the treated soil has settled is filtered to be separated into a filtrate and a residue, 6. The soil decontamination method according to claim 1, wherein the residue is extracted as contaminated soil.   To the soil, an alkaline agent and water are added, and mixed by stirring to obtain a muddy water-like treated product. The treated product is passed through a sieve having a predetermined size larger than the particle size Da, and the large-diameter side soil and the small-diameter It separates into a muddy water-like secondary treated product mixed with the side soil, and then the secondary treated product is dehydrated and dried to obtain a small-diameter-side soil, and then the small-diameter-side soil is a soil having a particle size Da or less. The soil decontamination method according to any one of claims 1 to 5, wherein the soil is separated into soil having a particle size Da exceeding the particle size Da.   8. The soil decontamination method according to claim 1, wherein calcium oxide and / or calcium hydroxide is used as the alkaline agent.   The water added to the soil is 2 to 4 times the weight of the soil, and the calcium oxide and / or calcium hydroxide added to the soil is 0.1 to 10% by weight with respect to the added water. The soil decontamination method according to claim 8.

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