JP6640028B2 - How to clean cesium-contaminated soil - Google Patents

Description

  The present invention relates to a method for cleaning cesium-contaminated soil by washing cesium-contaminated soil with a cesium extractant.

As a technique for purifying soil contaminated with cesium, a classification cleaning method, b chemical treatment method, and c heat treatment method are known.
The washing classification method is a method of separating, concentrating, and capturing contaminants or fine particles containing a large amount of contaminants from coarse soil particle surfaces by washing / lumping or physical soil polishing.
In the case of the classification washing method, if soil grinding is insufficient, cesium or fine particles containing a large amount of cesium, which is a contaminant from coarse soil particles, cannot be completely removed, and the cesium concentration cannot be reduced below the target value. There is.

In addition, the removal rate of cesium or fine particles containing a large amount of cesium from coarse soil particles is improved by lengthening the soil grinding time or applying an advanced grinding device that completely polishes the surface of coarse soil particles. It is possible.
However, in the case of cesium, unlike heavy metal, it is firmly fixed to the surface of the soil particles, so that the cesium concentration cannot always be reduced to a target value or less. Further, in this case, it is pointed out that the former increases the running cost, and the latter increases the initial cost of the apparatus.
In addition, a large amount of fine particles are generated by the grinding, and the yield of the purified soil is reduced.

  The chemical treatment method is a method of extracting cesium from soil by adding a chemical, and purifying the cesium by adsorbing the extracted cesium on an adsorbent or the like. In order to extract cesium from soil effectively, a certain amount of heat and acid are required, particularly in the case of silt and clay, which causes an increase in cost. In addition, exposed to acid in the treated soil, especially silt and clay, may cause some of the soil components to dissolve, resulting in the generation of a large amount of dissolved material residue. May be an inhibitor.

  The heat treatment method is a method in which a reaction accelerator is added to soil, and then cesium is volatilized in a rotary kiln or a melting furnace to sinter or solidify the soil. In order to make these sintering or solidification, it is necessary to heat up to 1000-1200 ° C in case of sintering, and to 1600-2000 ° C in case of solidifying, and a large amount of heat source is required This leads to an increase in running cost.

  Further, additional equipment or the like for appropriately treating cesium heated and vaporized and other generated exhaust gas components is required, leading to an increase in initial cost. In addition, it has been pointed out that when cesium as a contaminant is not completely removed by volatilization, cesium may be re-eluted if the sintering or solidification state is not completely formed.

As described above, there are drawbacks in any of the washing classification method, the chemical treatment method, and the heat treatment method. Therefore, a method combining classification washing and chemical treatment has been proposed for the purpose of improving cesium removal efficiency and reducing costs. ing.
For example, Patent Literature 1 describes a method in which sand is subjected to an acid treatment on classified sand to embrittle the surface of soil particles, and then scrubbing is performed to remove cesium.
Further, in Patent Document 2, after separating into coarse-grained soil of 5 to 25 mm and fine-grained soil of 5 mm or less, cesium is adsorbed on fine-grained soil by improving flotation by a high-speed shear mixer. The following method for separating and recovering viscous soil is described.

JP 2013-178147 A JP 2013-221819 A

  In the case of the purification method disclosed in Patent Document 1, it is possible to remove cesium firmly adhering to the sand surface due to embrittlement of the sand surface due to acid treatment without applying an excessive polishing load. However, embrittlement may cause dissolution of soil components and particle deficiency, resulting in a large amount of fine particles and residues.

Further, in the case of the purification method disclosed in Patent Document 2, silt / clay and most of the sand are treated as contaminated soil, which is extremely uneconomical as compared with a case where landfill treatment is performed without treatment. Becomes
Further, in the case of the purification methods disclosed in Patent Literatures 1 and 2, since there are many parameters such as a type of a drug such as an extractant, a drug concentration, a washing time, and a solid-liquid weight ratio, the parameters are selected from these parameters. It is very difficult to find the optimum conditions, and there are various disadvantages when implementing the technology and applying it locally.

  For example, when washing is performed at an excessive drug concentration, washing time, and solid-liquid weight ratio, the size of the apparatus is increased and the running cost is increased. Yield also decreases. Further, when washing is performed under conditions where the drug concentration, the washing time, and the solid-liquid weight ratio are insufficient, there is a risk that the soil after washing cannot be reduced to a target value or less.

  The present invention has been made in order to solve such a problem, and an object of the present invention is to provide a method for purifying cesium-contaminated soil which is excellent in cost performance.

  The present inventors have intensively studied a method of classifying and washing cesium-contaminated soil (hereinafter referred to as “cesium-contaminated soil”) in combination with a cesium extractant, and have examined the cesium-contaminated concentration and the binding state of cesium to soil. Paying attention to the fact that depends on the particle size of the soil, we have completed a soil purification method that adjusts and optimizes the extraction conditions and rinsing conditions used for purification for each particle size of the soil to be purified.

FIG. 4 is a graph showing the results of investigating the cesium concentration for each soil particle diameter for soil contaminated with cesium, wherein the horizontal axis represents the passing particle diameter (μm), and the vertical axis represents the cesium concentration ratio to the initial concentration. I have. The “ratio of cesium concentration to the initial concentration” is a ratio of the cesium concentration of each particle size when the cesium concentration of the present soil (bulk sample of all particle sizes) is set to 1.
As shown in the graph of FIG. 4, the cesium concentration changes depending on the soil particle diameter, and the cesium concentration significantly increases at a soil particle diameter of 0.01 mm or less.

Further, in the case of fine-grained soil such as silt / clay, soil particles, particularly clay minerals, tend to be strongly bound to contaminants such as heavy metals due to permanent charge action and mutation charge action such as allophane. Further, fine-grained soil has higher activity than coarse-grained soil and easily forms compounds and minerals which are hardly soluble with heavy metals.
On the other hand, in the case of coarse-grained soil such as sandy material, it is often relatively loosely bound by physical adsorption or adhesion to the surface of soil particles.

In view of the fact that cesium removal from fine-grained soil is extremely expensive, the present inventor classifies cesium-contaminated soil for each soil particle diameter and removes cesium-contaminated soil having a diameter of one or more soil particles. After washing with the extractant, solid-liquid separation is performed on the washed soil, and then rinsing is performed under certain conditions to prevent cesium from re-adhering to the soil, resulting in cost performance. We thought that excellent cleaning could be realized.
The present invention is based on such a concept, and specifically has the following configuration.

(1) A method for purifying cesium-contaminated soil according to the present invention is a method for purifying cesium-contaminated soil for purifying soil contaminated by cesium,
A crushing step of crushing the cesium-contaminated soil, a classifying step of classifying the crushed cesium-contaminated soil and dividing the cesium-contaminated soil into a plurality of groups of particles in a predetermined particle diameter range, and a predetermined particle diameter or more A washing step of adding and extracting an extractant for extracting radioactive substances from soil to contaminated soil of a group consisting of particles, and a solid-liquid separation step of separating soil and a washing solution with a solid-liquid separation device, Comprising a rinsing step of rinsing the soil,
The washing step is performed in an area of pH 0 or more and pH 3 or less using an extractant comprising at least one alkali salt of a potassium salt, a magnesium salt, a lithium salt, a sodium salt, and a cesium salt. is there.

(2) In the cleaning method according to the above (1), the washing step uses the washing solution separated in the solid-liquid separation step as a part of the extractant.

(3) In the above (1) or (2), the soil obtained after rinsing is adjusted to the same pH as the cesium-contaminated soil before crushing, and then the silt at or near the point where the contaminated soil is collected -It has a soil recovery step of mixing a clay component and / or humus.

(4) Further, in any one of the above (1) to (3), in the rinsing of the soil separated by the solid-liquid separator, the number of rinsings N satisfies the following equation. Things.
N ≧ 1.9 × ln (α / C0) / ln [B / {A × (1-B)}] − 1
However, A: solid-liquid weight ratio of soil and rinse liquid
B: Moisture content of soil after solid-liquid separation
C0: Target cesium concentration before washing (content in soil)
α: target cesium concentration after cleaning

(5) In the method according to any one of the above (1) to (4), the classifying step includes the step of separating the cesium-contaminated soil into at least two types of particle groups having a particle diameter of less than 0.075 mm and a particle diameter of 0.075 mm or more. Classification into groups.

The method for purifying cesium-contaminated soil according to the present invention includes a crushing step of crushing cesium-contaminated soil, and a method of classifying the crushed cesium-contaminated soil into a plurality of groups including particles in a predetermined particle diameter range. A classification step of separating, a washing step of adding an extractant for extracting radioactive substances from soil to a contaminated soil of a group consisting of a group of particles having a predetermined particle size or more, and a washing step; And a rinsing step of rinsing the soil,
The cesium-contaminated soil is washed by performing the washing step in an area of pH 0 or more and pH 3 or less using an extractant comprising one or more alkali salts of potassium salt, magnesium salt, lithium salt, sodium salt, and cesium salt. After the crushing, the optimum extraction and washing conditions are applied to each classified soil, so that a large amount of cesium-contaminated soil can be purified at low cost.

It is an explanatory view explaining each process of a purification method of cesium contaminated soil concerning one embodiment of the present invention. It is an explanatory view explaining a processing process for reusing a washing solution collected from a solid-liquid separation device in a solid-liquid separation process of one embodiment of the present invention. It is a graph which shows the result of the rinse test with respect to various cesium-contaminated soil samples in an Example. It is a figure which shows the relationship between a cesium contamination density | concentration and soil particle diameter.

  The present invention is a method for purifying cesium-stained soil using an extractant, wherein a plurality of particle groups of contaminated soil classified based on a specific particle diameter are classified into particles having a certain particle diameter or more. After washing with an extractant, the washed soil is separated from the soil and the washing liquid by a solid-liquid separator, and the separated soil is rinsed under a constant rinsing condition to selectively remove cesium. The present invention is characterized in that the cesium is extracted from soil to prevent reattachment of the extracted cesium to soil.

FIG. 1 shows each step of a method for cleaning cesium-stained soil when cesium-contaminated soil is classified into two large and small particle size groups.
In FIG. 1, reference numeral 1 denotes a step of disintegrating cesium-contaminated soil into single particles (hereinafter, a first step 1), and reference numeral 2 denotes a specific step of classifying cesium-contaminated soil which has been disintegrated in the first step 1 by a classifier. In the step (hereinafter, referred to as a second step 2) of classifying the particles based on the particle diameter, a cesium is selectively extracted from a particle group having a particle diameter equal to or larger than the specific particle diameter classified in the second step 2. The step of washing with an extractant (hereinafter, the third step 3) and the step of separating the washing liquid of the soil extracted and washed in the third step 3 by a solid-liquid separator (hereinafter, the fourth step). Steps 4) and 5 are steps of rinsing the soil collected in the fourth step 4 with a rinsing liquid (hereinafter, fifth step 5), and 6 is a specific particle diameter classified into a second high and low. Coagulation sedimentation and dehydration of waste particles such as particles and cleaning liquids (hereinafter referred to as the sixth Shows the extent 6).
Hereinafter, each step will be described in detail.

[First step]
The first step 1 corresponds to the crushing step of the present invention, in which the cesium-contaminated soil is crushed by the crushing means, and the silt / clay adheres to the surface such as a state where the soil particles are agglomerated and clumped or the surface of gravel / sand. This is a step for the purpose of unraveling from a state to single particles.

According to the first step 1, the soil loosened into single particles can be classified according to the size of the soil particles. Further, in the washing with the extractant, the contact area between the soil particles and the extractant increases, the washing time is shortened, and the efficiency of removing cesium is improved. Existing equipment such as a drum washer, a paddle mixer, a lot mill, and a ball mill is used as a soil crusher used for crushing.
Before or after crushing, remove foreign substances such as plant pieces, coal flakes, and metal pieces from the soil after crushing using a specific gravity separator, a magnetic separator, or a floating separator, or remove large soil particles with a vibrating screen. Is desirable.

[Second step]
The second step 2 corresponds to the classification step of the present invention, and the soil separated into the single particles in the first step 1 is divided into a group of particles in a range of a predetermined particle size from a specific particle size. The purpose is to classify into.
Classification is performed using various devices such as a screen, a spiral classifier, a centrifuge, and a cyclone, alone or in combination.

For classification, particles with a particle size of 0.075 mm or less have a particle size of 0.075 mm or less because silt and clay are below 0.075 mm and sand and particles with a particle size of more than 0.075 mm and less than 2.0 mm are sandy and have different cesium adsorption strengths. It is preferable to classify the particles into at least two types of particle groups having a size exceeding 0.075 mm and not more than 2.0 mm.
Particles having a particle diameter of 0.075 mm or less and particles having a particle diameter of more than 0.075 mm and not more than 2.0 mm may be further classified. In particular, as described above, since the cesium concentration often significantly increases at a particle diameter of 0.01 mm or less, particles having a particle diameter of 0.075 mm or less may be further classified at 0.01 mm.

[Third step]
The third step 3 corresponds to the washing step of the present invention, and the extractant is used for the soil of a group of particles having a specific particle size or more among the two groups classified in the second step 2. It is intended to be washed by using. The apparatus used for this washing may be any apparatus that can sufficiently mix the soil and the extractant, such as mixing washing with a stirring blade.

In general, the smaller the particle size, the more strongly cesium is taken into the soil, and it is necessary to add a drug or heat to forcibly desorb the cesium from the soil. The cost is expected to increase significantly. Further, since the particle diameter is small, a special device is required also in collecting the soil after the treatment, and when a plurality of washings are required, the running cost is increased.
On the other hand, in soils with a large particle size, the adhesion of cesium to the soil is relatively slow, so it is possible to suppress the addition of chemicals and heating load as compared with the case where the particle size is small, and it is also inexpensive to recover the soil after treatment. Can be handled by a simple device.

As described above, in order to suppress the processing cost, the particles having a specific particle size or more may be washed with the extractant.
As the extractant, a potassium salt, an ammonium salt, a magnesium salt, or the like, which has an effect of expanding the interlayer and replacing cesium with cesium incorporated as a fixed state between layers such as mica, is desirable. Above all, an extractant consisting of a magnesium salt and a cellulose derivative promotes the substitution reaction between cesium ions, magnesium ions, and hydrogen ions by increasing the dispersibility of the soil at normal temperature and pressure and improving the contact between the magnesium salt and the soil. can do.

Further, it is better to adjust the pH of the slurry obtained by adding and mixing the soil and the extractant to pH 3 or less from the viewpoint of supplying hydrogen ions replacing cesium ions and preventing re-substitution. That is, when cesium-contaminated soil is extracted and washed in an acidic region, cesium is extracted, and iron, aluminum, and the like, which are components of the soil, are also dissolved. Generally iron, aluminum, constant pH of more than (Fe ^2+: pH 5.2 or higher, Fe ^3+: pH 2.8 or higher, Al ^3+: pH 4.3 or higher) will the electrostatically heavy metal ions in the slurry It is considered that co-precipitation phenomena that adsorb to the soil are likely to occur, and that re-adhesion to the soil occurs due to the presence of the organic components and calcium generated by the dissolution of the soil and the adsorptivity of the soil, and remains in the soil. Therefore, it is better to set the pH to 3 or less.

However, the lower the pH, the more cesium is dissolved and extracted, and the ratio of dissolution and extraction of the main minerals and substances that make up the soil is increased. As a result, the yield of purified soil is reduced, and the cost performance such as liquid treatment is significantly reduced. Therefore, the pH is preferably 0 or more.
To adjust the pH, for example, hydrochloric acid or sulfuric acid may be added.
The cleaning time in the third step is preferably 60 minutes or less, and more preferably 30 minutes or less from the viewpoint of further reducing the initial cost of the cleaning apparatus.

[Fourth step]
The fourth step 4 corresponds to the solid-liquid separation step of the present invention, and aims at recovering the washed washing solution from the cesium-contaminated soil extracted and washed through the third step 3 by a solid-liquid separation device. I do. The solid-liquid separator is used alone or in combination with existing devices such as a screen, a spiral classifier, a centrifuge, a cyclone, and a filter press.
When the washing solution collected by the solid-liquid separator is recirculated for washing the soil, the mixing ratio with the new solution is appropriately adjusted so that the washing solution for extracting and washing the soil can maintain a predetermined washing ability. adjust.

FIG. 2 shows an example of equipment for embodying the fourth step 4, in which the extractant solution for extracting and washing the soil is mixed with a new extractant solution so that a predetermined washing ability can be secured. This is an example of equipment for appropriately adjusting the temperature.
In FIG. 2, reference numeral 7 denotes a detector for detecting the concentration and / or pH of the extractant solution, and reference numeral 8 denotes a new extraction used in the extraction / washing tank based on the concentration and / or pH of the extractant solution detected by the detector 7. A control device for adjusting the amount of the extractant solution, 9 is a flow control valve for adjusting the amount of the new solution as a new extractant solution, and 10 is a new control agent for using the extractant solution recovered from the solid-liquid separator for the soil. The piping (recirculation path) joined to the piping system for supplying the liquid is shown.

  In the equipment shown in FIG. 2, the detector 7 detects the concentration and / or pH of the extractant solution to be added (mixed liquid of the new solution and the recovered extractant solution), and controls the controller 8 based on the detected value. Calculates the amount of new liquid to be injected, and controls the flow regulating valve 9 based on the calculated value. The new liquid is injected into the pipe 10 via the flow control valve 9, and the concentration and / or pH of the extractant solution added to the soil is adjusted.

[Fifth step]
The fifth step 5 corresponds to the rinsing step of the present invention, and aims at rinsing the soil from which the extractant solution has been removed by the solid-liquid separator in the fourth step 4. In order to reuse the soil washed with the extractant solution as purified soil, it is necessary to remove cesium, a contaminant remaining in the soil, and drug components.
As for the rinsing, as in the case of the extraction and washing in the third step 3, by adjusting the pH after mixing the soil and the rinsing liquid to 3 or less, it is possible to prevent re-adhesion of soil contaminants, and A good rinsing effect can be obtained. Note that water or a weak acid is preferable as the rinsing liquid.

Further, if the number of times of rinsing of the soil is insufficient, a rinsing liquid containing a large amount of cesium as a pollutant remains and adheres to the soil again, thereby impairing the washing effect of the extractant solution.
In order to prevent cesium caused by the remaining rinse liquid from re-adhering to the soil, it is sufficient that the rinse liquid can be completely separated from the soil. Rinse remains on the soil.

Therefore, by performing solid-liquid separation and soil rinsing a plurality of times, these effects on the soil after purification can be made extremely small. In this case, however, the optimum number of times of rinsing, solid-liquid ratio, etc. If it is not carried out under the conditions, the use amount of the rinsing liquid, and the equipment costs of the solid-liquid separation device and the rinsing bath will increase.
Therefore, the present inventors have found that, based on various rinsing tests, the solid-liquid weight ratio of soil to the rinsing liquid: A, the water content of the soil after solid-liquid separation: B, the target before washing with the extractant solution. A relationship between the following cesium concentration (content relative to soil): C0, a target cesium concentration after washing: α, and the number of times of rinsing: N (an integer of 0 or more) was found by the following equation (1).
In addition, the number of times of rinsing is defined as one step in which the solid-liquid separated soil after extraction and washing with the extractant solution is rinsed and then solid-liquid separated again, and the number of repetitions.

      N ≧ 1.9 × ln (α / C0) / ln [B / {A × (1−B)}] − 1 (1)

Here, a method of deriving the above equation (1) will be described.
<Preconditions>
(i) A certain rinsing liquid adheres to the soil after solid-liquid separation, and the soil content becomes larger than the true value due to the contaminants contained in the adhering water.
(ii) At the time of the next rinsing, the attached water is once released from the soil, and is uniformly mixed with the rinsing liquid.
(iii) Precipitation of contaminants shall not occur during rinsing.
(iv) The symbols used are as follows.
Solid-liquid weight ratio of soil to rinse liquid: A [liquid kg / soil kg]
Soil moisture content after solid-liquid separation: B [-]
Soil content of the pollutant of interest before washing with acidic solution: C0 [mg / kg soil]
True soil content of pollutants after washing with acidic solution: C [mg / kg soil]
Apparent soil content of contaminants after washing with rinsing liquid: Z [mg / soil kg]
Criteria for specifying soil content: α [-]
Number of rinses: N [positive integer]
Soil weight: [Soil kg]
Weight of attached water: X [liquid kg]
Contaminant concentration in waste liquid and rinsing liquid: L [mg / liquid kg] (Specific gravity = 1)
Pollution amount brought out as soil water: V [mg]

<Calculation formula>
The soil water content B, soil weight W, and attached water weight X are represented by the following relationship.
B = X / (W + X) [−] ··· (I)
X = B ・ W / (1-B) [liquid kg] ・ ・ ・ ・ (II)
Also, the contaminant concentration L0 in the waste liquid is
L0 = (C0-C) / A0 [mg / kg of liquid] ・ ・ ・ ・ (III)
Therefore, the amount of contamination V0 taken out as the water attached to the soil in the rinsing 1 is
v0 = (C0-C) / A0 × B · W / (1-B) [mg] ··· (IV)
If the amount of contaminants taken out as attached water is diluted with the rinse liquid, the contaminant concentration L1 becomes
L1 = (C0-C) / A0 × B ・ W / (1-B) × 1 / (W ・ A1)
= (C0-C) / A0 x B / [(1-B) x A1] [mg / kg of liquid] ... (V)
Similarly, the pollutant concentration in the rinsing 2 The pollutant concentration L2 is
L2 = (C0-C) / A0 × B / [(1-B) × A1] × B / [(1-B) × A2] [mg / liquid kg] (VI)
Therefore, the contaminant concentration Ln in the rinsing N is:
Ln = (C0-C) / A0 × B / [(1-B) × A1] × ・ ・ ・ × B / [(1-B) × An] [mg / liquid kg] ・ ・ (VII)
When converted to the soil content Z of apparent contaminants after rinsing and dehydration,
ZC = [contamination concentration in the rinsing liquid in rinsing N] × [weight of attached water] / [weight of soil]
= (C0-C) / A0 × B / [(1-B) × A1] × ・ ・ ・ × B / [(1-B) × An] × B ・ W / (1-B) × 1 / W
= (C0-C) / A0xB / [(1-B) xA1] x
... × B / [(1-B) × An] × B / (1-B) [mg / soil kg] (VIII)
If the apparent contaminant soil content Z after dehydration is smaller than the content designation standard α, the rinsing from the viewpoint of the content is completed,
α ≧ Z
≧ (C0-C) / A0 × B / [(1-B) × A1] ×
... × B / [(1-B) × An] × B / (1-B) + C ・ ・ ・ (IX)
Now, if A = A0 = ... = An,
α ≧ (C0-C) / A × [B / ｛(1-B) × A｝] ⁿ × B / (1-B) + C ・ ・ ・ (X)
α-C ≧ (C0-C) × [B / ｛(1-B) × A｝] ^{n + 1}・ ・ ・ (XI)
[(α-C) / (C0-C)] ≧ [B / ｛(1-B) × A｝] ^{n + 1}・ ・ ・ (XII)
ln [(α-C) / (C0-C)] ≧ (n + 1) · ln [B / ｛(1-B) × A｝] ・ ・ ・ (XIII)
From ln [B / ｛(1-B) × A｝] <0,
n + 1 ≧ ln [(α-C) / (C0-C)] / ln [B / ｛(1-B) × A｝] (XIV)
n ≧ ln [(α-C) / (C0-C)] / ln [B / ｛(1-B) × A｝] -1 (XV)
Assuming (α-C) / (C0-C) = (α / C0) ^Q ,
n ≧ Q × ln (α / C0) / ln [B / ｛(1-B) × A｝]-1 ··· (1)

FIG. 3 shows the results of a rinsing test for various cesium-contaminated soil samples under the condition that the pH after mixing the soil and the rinsing liquid was 2 or less. The target value of the cesium concentration after the treatment was 3,000 (Bq / kg).
As shown in FIG. 3, regardless of the cesium concentration, when the relationship of the above equation (1) is satisfied, the cesium concentration of the soil after purification is lower than the target value (open symbol). Later the cesium concentration in the soil exceeded the target value (solid symbol).
Before the rinsing, the soil sample was washed for 30 minutes by using an extractant composed of a magnesium salt and setting the pH after mixing to 1 to 2 for coarse-grained soil composed of sand of 0.075 to 2.0 mm.

  In the case where the rinsing is performed a plurality of times, when the conditions such as the solid-liquid ratio: A and the water content: B in each rinsing are different, N under each condition is calculated by the above equation (1), and the N is calculated. The arithmetic or harmonic mean of the numerical values may be used. The effect of the above formula (1) in the rinsing is as follows: the soil to be rinsed only needs to be obtained by extracting and washing cesium-contaminated soil with an extractant solution and then using a solid-liquid separator, and the washing method is limited. is not.

[Sixth step]
The sixth step 6 aims to coagulate, sediment and dehydrate the particles having a particle diameter smaller than the specific particle diameter classified in the second step 2 and the waste liquid of the third and fifth steps 5. And
As the flocculant in this step, any flocculant, such as an inorganic flocculant or a polymer flocculant, which forms and grows flocs and promotes precipitation can be used.

The dehydrated particles having a particle diameter smaller than a specific particle diameter, suspended matters in the waste liquid, and released cesium are solidified, and after the solidification, they may be stored in a predetermined place. At this time, cement, quicklime, gypsum and the like may be added as an auxiliary for solidification.
Further, the cesium extracted and released by the extractant is adsorbed by a component having high cesium adsorptivity such as clay contained in a particle group having a particle diameter smaller than a specific particle diameter during transportation and in the sixth step 6. This is very advantageous because no additional adsorbent is required.

  According to the present invention, the cesium-contaminated soil can be purified at a low cost by the first step to the sixth step 6 described above. Although FIG. 1 used for describing each process shows a case where each process is performed once, the present invention performs any one of the processes multiple times as necessary until a desired cleaning effect is obtained. It is possible.

[Soil recovery process]
When the purified soil is reused after the sixth step 6, the pH may be readjusted to the same value as the soil before excavation and backfilled. Note that the same value here is only required to be such that the ecosystem of the surrounding environment can be quickly restored, and does not mean that it is the same.
As a means for readjusting the pH, one or more of the following 1 to 3 may be performed.
1. Perform soil washing with water several times.
2 Add an alkaline agent such as calcium hydroxide, calcium oxide, calcium carbonate.
3 Add alkaline soil and / or coal ash that has been confirmed to be non-contaminated.

If the soil at the point of backfilling is adapted to the surrounding environment at an early stage, after re-adjustment of the soil pH, the silt / clay components and / or humus at or near the point where the contaminated soil was collected are mixed. do it.
Although the above description has described the case where the soil is classified into two large and small particle diameters after crushing, the present invention does not limit the number of groups of the particle groups to be classified after crushing, and after crushing, The soil may be classified into three or more groups, and one or more groups of soil may be extracted and washed with an extractant.

An experiment for confirming the effect of the present invention was performed, and will be described below.
After adding 15% of water to the cesium-contaminated soil and crushing the mixture with a paddle mixer at 100 rpm for 10 minutes, water was added so as to have a solid-liquid weight ratio of 1: 1. This slurry was washed with water at a rotation speed of a stirring blade of 300 rpm for 10 minutes, and classified with a vibration sieve of 0.075 mm. An extractant solution consisting of 5% by weight of magnesium chloride, 5% by weight of hydrochloric acid, and a cellulose derivative at a solid-liquid weight ratio of 1: 3 was added to the soil on the sieve after classification, and stirred for 30 minutes (stirring conditions: stirring blades). Rotation speed of 300 rpm). At this time, the pH after the mixing of the soil and the extractant solution was 1.5 immediately after the start of the washing, and 2.5 before the end of the washing.

Next, the slurry soil is dehydrated with a centrifuge (3000 rpm, 10 minutes), and the obtained soil is rinsed with an acidic solution having a solid-liquid weight ratio of 1: 3, and then centrifuged (3000 rpm, 10 minutes). Dehydration and this rinsing operation were repeated twice.
The pH of the rinsing solution at this time was 1.8 and 2.3. Further, slaked lime was added and mixed to adjust the soil to the same pH of 6.7 as the original soil, and air-dried. The purified soil was measured for isotope cesium concentration (sum of Cs134 and Cs137) with a germanium semiconductor detector. Table 1 shows the results.

As Comparative Example 1, the soil on the 0.075 mm classification sieve of the cesium-contaminated soil used in the example was subjected to extraction, washing, and dehydration under the same conditions as in the example.
The dehydrated soil was not rinsed, but slaked lime was added and mixed to make the soil the same pH of 6.7 as the original soil and air-dried. The purified soil was measured for isotope cesium concentration (sum of Cs134 and Cs137) with a germanium semiconductor detector. The results are shown in Table 1 above as Comparative Example 1.

  In addition, as another comparative example, the results in which the rinsing operation in Example 2 was performed once and the other conditions were the same as those in the example are also shown in Table 1 above as Comparative Example 2.

  As shown in the results in Table 1, according to the present invention, an excellent purification effect on cesium-contaminated soil could be confirmed. Furthermore, an excellent rinsing effect was obtained by rinsing under the condition of pH 3 or less after washing with the extractant solution. Also, by defining the optimum number of times of rinsing of the soil, the amount of rinsing liquid (water) used in rinsing could be reduced.

DESCRIPTION OF SYMBOLS 1 1st process 2 2nd process 3 3rd process 4 4th process 5 5th process 6 6th process 7 Detector 8 Controller 9 Flow control valve 10 Recirculation flow path

Claims (4)

A method for purifying cesium-contaminated soil that purifies soil contaminated by cesium,
A crushing step of crushing the cesium contaminated soil, a classification step of classifying the pulverized cesium contaminated soil above particle diameter 0.075mm and less than the particle diameter 0.075mm of at least two particle groups groups, particle size A washing process in which an extractant that extracts radioactive substances from soil is added only to contaminated soil of a group consisting of particles of 0.075 mm or more and washing is performed, and solid-liquid separation is performed by using a solid-liquid separation device to separate the soil and the washing solution And a rinsing step of rinsing the soil,
The cesium contamination, wherein the washing step is performed in an area of pH 0 or more and pH 3 or less using an extractant comprising at least one kind of alkali salts of potassium salt, magnesium salt, lithium salt, sodium salt, and cesium salt. How to clean the soil.   The method for purifying cesium-contaminated soil according to claim 1, wherein the washing step uses the washing solution separated in the solid-liquid separation step as a part of the extractant.   After the soil obtained after the rinsing is adjusted to the same pH as the cesium-contaminated soil before crushing, the soil recovery step of mixing the silt / clay component and / or humus at or near the point where the contaminated soil is collected, The method for purifying cesium-contaminated soil according to claim 1. The method for purifying cesium-contaminated soil according to any one of claims 1 to 3, wherein the number of times of rinsing N satisfies the following equation in the rinsing of the soil separated by the solid-liquid separation device.
N ≧ 1.9 × ln (α / C0) / ln [B / {A × (1-B)}] − 1
However, A: solid-liquid weight ratio of soil and rinse liquid
B: Moisture content of soil after solid-liquid separation
C0: Target cesium concentration before washing (content in soil)
α: target cesium concentration after cleaning