JP6497650B2 - Cleaning method for arsenic contaminated soil - Google Patents

Description

  The present invention relates to a technique for cleaning contaminated soil, and more particularly, to a cleaning method for naturally occurring low-concentration arsenic-contaminated soil that occurs in shield construction or the like.

  Soil contamination by naturally-occurring heavy metals has been a problem in the past for excavation work in mountain tunnels through ore deposits, but recently in Japan, especially in urban areas such as the Tokyo metropolitan area, The problem of soil contamination by leaching heavy metals, especially arsenic, has become apparent. Although the contamination concentration by such natural arsenic will be described in detail later, although the concentration is low, the elution amount is about 0.01 to 0.03 mg / L, the environmental standard value (0.01 mg / L), and the soil. The problem is that it exceeds the elution amount reference value (0.01 mg / L) (hereinafter, also referred to as elution amount reference value, etc.) of the Pollution Control Law, and purification treatment is required.

On the other hand, the soil generated by shield construction in Japan is often contaminated with arsenic derived from nature mainly composed of fine particles such as Dotan (consolidated silt) and clay. In general, in the muddy water type underground shield construction, muddy water is sent from the face of the shield machine to the ground treatment facility, and cleaning processing or the like is performed. Here, since soil containing arsenic must be treated as contaminated soil even if it is naturally derived arsenic, the following problems occur when shield construction is performed on ground containing naturally derived arsenic: Will occur.
(A) When shield construction is performed on ground containing natural arsenic, if the arsenic concentration (elution amount) of the soil generated in the construction exceeds the environmental standards and elution amount standards (Soil Contamination Countermeasures Law), It cannot be disposed of as industrial waste and must be disposed of as contaminated soil. As a result, disposal costs increase significantly.
(B) Since the allowable amount of the disposal site is tight, it is difficult to dispose of a large amount of contaminated soil in the first place.

Soil contaminated with naturally-occurring heavy metals is widespread in Japan, particularly in the Tokyo metropolitan bay area and Osaka bay area. In other words, a large amount of contaminated soil is generated when large-scale public works or building rooting works are performed in the area.
In the future, large-scale shield works such as the construction of the Linear Shinkansen and the outer ring road will be carried out, and it is certain that a large amount of soil contaminated with natural heavy metals will be generated in these works.

  As a characteristic of soil contaminated with natural arsenic, the content value is about 1 to 5 mg / kg, well below the content standard value (150 mg / kg) of the Soil Contamination Countermeasures Law. It can be mentioned that it is low-concentration contamination, such as being about 1 to 3 times the elution amount reference value (0.01 mg / L) of the countermeasure method.

  As described above, contaminated soil containing naturally derived heavy metals must be disposed of as contaminated soil and cannot be reused as construction generated soil. Therefore, when conducting large-scale shield construction, etc., it is required to reduce the contaminated soil as much as possible in order to avoid the problems shown in (a) and (b) above.

Conventionally, environmental pollutants (heavy metals, mineral oil, cyanide, etc.) in artificially contaminated soil are finer, such as soil organic matter (humus), clay and silt, than coarse particles such as sand and gravel. It is known that many particles are adsorbed and retained. For this reason, when purifying the contaminated soil, it is advantageous to separate the coarse particles and the fine particles and remove the fine particles for efficient treatment.
On the other hand, in the case of pollution with naturally-occurring heavy metals, unlike artificial pollution, the contaminants were not unevenly distributed in fine particles, and it was thought that there was no difference in the amount of elution due to the soil particle size. . Therefore, excavation and removal, insolubilization, containment by impermeable walls, etc. were the main countermeasures against soil contaminated with natural heavy metals.

  The present inventors conducted experiments using naturally-derived arsenic-contaminated soils of multiple sites mainly composed of fine particles, and in many soils contaminated with natural heavy metals, contaminants are unevenly distributed in the fine particles. I found out. In other words, it is clear that if the contaminated soil is washed and classified, and only fine particles classified at an appropriate classification point are treated and disposed of as contaminated soil, the amount of disposed soil can be significantly reduced. I made it.

  Conventionally, as a method for classifying / cleaning contaminated soil, for example, a method disclosed in Patent Document 1 can be cited. In Patent Document 1, first, a fine particle portion adsorbing a pollutant is separated by a hydrocyclone using a contaminated soil as a slurry. Next, a method is disclosed in which a flocculant is added to and stirred in the suspension containing the fine particles to perform a coagulation-precipitation treatment, and the coagulated sludge is dehydrated and disposed as concentrated contaminated soil (dehydrated cake).

  Although the above-described hydrocyclone is an apparatus having a very high processing capability, a hydrocyclone having a classification point smaller than 20 to 30 μm has a demerit that the processing capability is low because the size is small. In order to process a large amount of soil slurry using such a hydrocyclone, a very large number of cyclones and pumps are required, which is not practical. In the field of soil washing, a hydrocyclone having a classification point of 60 to 125 μm is generally used from the viewpoint of efficiency and economy.

On the other hand, the classification with hydrocyclone alone does not sufficiently reduce the arsenic concentration of the entire soil particles. Therefore, some of the soil particles that exceed the elution amount standard value are also included in the primary treated soil obtained by the cyclone classification. There is a possibility of sorting.
That is, even if the contaminated soil containing arsenic derived from nature generated by the muddy water shield method is cleaned by the conventional method described in Patent Document 1, the cleaning treatment is performed while effectively reducing the amount of the secondary treated soil. It is difficult to carry out, and accordingly, it is difficult to increase the proportion of primary treated soil that can be reused.

  Here, the present inventors selectively agglomerate only soil particles of 5 to 10 μm or more in the slurry to form flocs, which is high when the hydrocyclone is used at a normal classification point. A method for efficiently separating fine particles of less than 5 to 10 μm, which has been difficult to classify with processing capability, has been proposed (see Patent Document 2).

  However, when the method described in Patent Document 2 is adopted, a large amount of a polymer flocculant for forming flocs is required, and thus there is a problem that the processing cost for cleaning increases. Moreover, since the washing soil discharged by the method described in Patent Document 2 contains flocs, it becomes difficult to reuse, and it must be disposed as industrial waste, compared with the case where it is disposed as construction generated soil. There is a problem that the disposal cost becomes high. In addition, as in the method described in Patent Document 2, the method in which the arsenic extraction method using an alkali and the incomplete coagulation precipitation method using a polymer flocculant are combined in this order, depending on the setting of the classification point. There was a possibility that some soil particles exceeding the standard value would be distributed to the washed soil.

Japanese Patent Laid-Open No. 2006-11697 JP 2012-081386 A

  The present invention has been made in view of the above problems, and it is necessary to dispose of contaminated soil with naturally derived arsenic generated in the muddy water shield method and the like as a contaminated soil, exceeding the elution amount standard value, etc. An object of the present invention is to provide a method for cleaning arsenic-contaminated soil that can increase the proportion of reusable purified soil while effectively reducing the amount of treated soil.

First, the first invention in the present invention is a method for cleaning arsenic-contaminated soil by cleaning naturally contaminated soil with arsenic to obtain purified soil, and the contaminated soil in the form of slurry or the contaminated soil An alkali treatment step of adding an alkaline treatment agent to a part of the slurry, and extracting the arsenic adsorbed on the soil particles of the contaminated soil with alkali to extract it in a liquid state in the slurry as a dissolved state; A wet classification step of classifying the contaminated soil into coarse particles and a slurry containing arsenic in a dissolved state other than the coarse particles, and then the slurry obtained in the wet classification step with a cyclone By classifying, the slurry is classified into a slurry fine particle containing arsenic in an dissolved state having an average particle size of less than 75 μm and a coarse particle having an average particle size of 75 μm or more. And a dehydration step of dehydrating the coarse particles classified in the cyclone classification step, and mixing the coarse particles after the dehydration with the coarse particles classified in the wet classification step, And a pH adjustment step of obtaining a purified soil by adding a neutralizing agent to the coarse particles mixed in the dehydration step to make the pH value a neutral region, and further obtained in the cyclone classification step The agglomerated agent is added to the slurry-like fine particles containing arsenic as a dissolved state to agglomerate and precipitate the fine particles together with arsenic, and then the agglomerated sludge is filtered to form a dehydrated cake. And agglomeration step of obtaining a concentrated contaminated soil containing arsenic by forming.

According to the arsenic-contaminated soil washing method of the first invention described above , an alkali extraction step, a wet classification step, a cyclone classification step, a dehydration step, and a pH adjustment step are provided in this order, and further, an aggregation step A method comprising: As a result, first, arsenic adsorbed on the soil particles in the alkali extraction step is extracted into the slurry as a dissolved state, and it is possible to effectively remove arsenic from the soil particles as the purified soil by subsequent classification. Become. Moreover, in the cyclone classification process, the slurry-like contaminated soil that has undergone the alkali extraction process, or a part of the contaminated soil, is classified using a cyclone, and the slurry is dissolved in an average particle size of less than 75 μm. By separating the arsenic into fine particles by separating it into slurry fine particles containing arsenic and coarse particles having an average particle size of 75 μm or more, and using the solid as purified soil In addition to increasing the proportion of reusable purified soil, it is possible to effectively reduce the amount of concentrated contaminated soil that exceeds the elution amount reference value.

  Therefore, for example, when washing soil contaminated with natural arsenic that occurs in the muddy water shield method, while effectively reducing the amount of concentrated contaminated soil that exceeds the elution amount reference value that must be disposed of as contaminated soil, A cleaning method capable of increasing the proportion of reusable purified soil can be realized at low cost.

It is a figure which illustrates typically the washing | cleaning method of the arsenic contaminated soil which is embodiment of this invention, and is a flowchart which shows each process. It is a figure which illustrates typically the washing | cleaning method of the arsenic contaminated soil which is embodiment of this invention, and is explanatory drawing which shows the relationship between the particle size of a soil particle and the arsenic elution amount in an arsenic contaminated soil. FIG. 3 is a diagram schematically illustrating a method for cleaning arsenic-contaminated soil according to an embodiment of the present invention, in which soil particle size, arsenic elution amount, a predetermined classification point, and an alkali extraction step in arsenic-contaminated soil; It is explanatory drawing which shows each relationship of. It is a figure which illustrates typically the washing | cleaning method of the arsenic contaminated soil which is embodiment of this invention, and is a fracture | rupture figure which shows schematic structure of the decanter used at a decanter classification process. It is a figure which illustrates typically the washing | cleaning method of the arsenic contaminated soil which is embodiment of this invention, and is the schematic explaining the operation | movement and effect | action of a decanter used at a decanter classification process, (a), (b) is a decanter (C) is a figure which shows the isolation | separation state of the contaminated soil press-contacted to the inner wall of the shell with which a decanter is equipped. BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which illustrates typically the washing | cleaning method of the arsenic contaminated soil which is embodiment of this invention, and the concept which showed the particle size structure of the soil contained in the mud of dotan (consolidation silt) in each process which performs a washing process FIG. It is a figure which illustrates typically the washing | cleaning method of the arsenic contaminated soil which is embodiment of this invention, and is a graph which shows an example of the cumulative particle size distribution of Dotan.

  Hereinafter, an example of a cleaning method for arsenic-contaminated soil according to an embodiment of the present invention will be described, and the configuration thereof will be described in detail with reference to FIGS.

[Relationship between average particle size of soil particles, total arsenic content, and dissolution amount]
Conventionally, soil contaminated with natural heavy metals is different from artificially contaminated soil, and the pollutants are not unevenly distributed in fine particles. It was thought that there was no difference. However, as a result of previous experiments conducted by the present inventors using natural arsenic-contaminated soils of multiple sites mainly composed of fine particles, as shown in FIG. It has been clarified that the smaller the particle size of the soil particles constituting the soil, the higher the arsenic elution amount (the soil contamination countermeasure method). That is, it can be seen that efficient purification treatment is possible by separating and treating fine particles, particularly fine particles, rather than coarse particles.

  Furthermore, as shown in FIG. 2, soil particles having an average particle size of more than about 15 μm may have an original contamination concentration (elution amount) slightly exceeding the elution amount reference value. Can be adapted to the elution amount reference value or the like by performing an alkali extraction step, the details of which will be described later.

  On the other hand, soil particles having an average particle size of about 15 μm or smaller have an original contamination concentration (elution amount) of about 3 to 5 times the standard value (0.01 mg / L above). Even when the alkali extraction step is performed, there is a concern that it cannot be adapted to the elution amount reference value or the like, or arsenic re-elution occurs. Therefore, the present invention comprises an alkali extraction step as a previous step, and when washing arsenic-contaminated soil, first, arsenic adsorbed on the soil particles of the contaminated soil is extracted into a slurry as a dissolved state, and then subjected to cyclone classification. In the process, by separating coarse particles with low arsenic content and leaching amount to the clarified soil side, the ratio of clarified soil that can be circulated is increased and the amount of concentrated contaminated soil that exceeds the leaching amount standard value is reduced. .

  Furthermore, in the present embodiment, in the decanter classification process, the details of which will be described later, soil particles (fine particles) having an average particle size of about 15 μm or smaller are separated and removed to obtain concentrated contaminated soil, and all arsenic is obtained. A method of including soil particles (solid content) having a low content and an average particle diameter of more than about 15 μm (75 μm or less) in the purified soil can be employed. As a result, in this embodiment, while the arsenic is reliably separated to the fine particle side, the solid content is used as the purified soil, so that the ratio of the reusable purified soil can be increased, and the elution amount reference value, etc. It is possible to effectively reduce the amount of concentrated contaminated soil that exceeds.

  Here, in the present invention, since a configuration including an alkali extraction step and a cyclone classification step is adopted, for example, even when the decanter classification step is not provided, only the conventional cyclone classification is used. Compared to the method of separating arsenic to the concentrated contaminated soil side, the classification point can be reduced. Therefore, in this embodiment, even if it is a case where the structure which does not provide a decanter classification process is employ | adopted, it becomes possible to reduce the generation amount of concentrated contaminated soil.

  Note that the classification point of 15 μm in FIG. 2 is described as an example of the classification point for convenience of explanation because the average particle diameter of the soil particles that easily adsorb arsenic is approximately 15 μm or less. In the actual target soil, the classification point is not limited to 15 μm. In other words, the classification point in the decanter classification step provided in the cleaning method of the present embodiment may be appropriately set in the range of more than 0 μm to 50 μm, more preferably 6 μm to 20 μm, depending on the target soil. Specifically, by changing the amount of slurry contaminated soil or a part of the contaminated soil (residence time) and the rotational speed (centrifugal force) to be input to the decanter, the amount of input and rotation to be an appropriate classification point What is necessary is just to determine a number. By setting the classification point in the decanter classification process within the above range, the slurry of fine particles containing arsenic can be reliably separated as concentrated contaminated soil, and the remaining treated soil is in a state that conforms to environmental standards such as elution amount standard values. Become.

Here, the relationship between an alkali extraction process and the predetermined classification point in a decanter classification process is demonstrated with reference to FIG.
As shown in FIG. 3, in the slurry-like contaminated soil, first, in the alkali extraction step, arsenic adsorbed on the surface of the soil particles is forcibly desorbed, and the arsenic is dissolved and extracted into the slurry. The amount of soil elution is reduced as a whole, and the predetermined classification point in the decanter classification process can be set to a smaller value. Therefore, it can be seen that the effect of reducing the amount of concentrated contaminated soil can be further enhanced by setting the predetermined classification point to a smaller value by the alkali extraction step.

[How to clean arsenic-contaminated soil]
The cleaning method for arsenic-contaminated soil according to the present invention is based on the characteristics of arsenic-contaminated soil as described above and knowledge based thereon.
That is, the method for cleaning arsenic-contaminated soil described in the present embodiment is a method for cleaning arsenic-contaminated soil by cleaning soil contaminated with natural arsenic and using it as purified soil. Alternatively, an alkali extraction step of adding an alkaline treatment agent to a part of the contaminated soil, extracting the arsenic adsorbed on the soil particles of the contaminated soil with alkali, and extracting the arsenic as a dissolved state into the liquid in the slurry, and this Slurry fine particles containing arsenic having an average particle size of less than 75 μm and dissolved in a slurry state by classifying slurry-like contaminated soil or a part of the contaminated soil that has undergone the alkali extraction step with a cyclone And a cyclone classification step of classifying the particles into coarse particles having an average particle diameter of 75 μm or more.

Moreover, in the example demonstrated by this embodiment, in the washing | cleaning method of an arsenic contaminated soil provided with said alkali extraction process and cyclone classification process, as shown to the flowchart of FIG. 1, the following (A)-(G) A method comprising each of the steps is adopted.
(A) An alkaline treatment agent is added to the slurry-like contaminated soil produced by the muddy water shield method, and the arsenic adsorbed on the soil particles of the contaminated soil is alkali-extracted and dissolved in the liquid in the slurry. The alkali extraction process to extract.
(B) A wet classification step that is provided after the alkali extraction step and classifies the contaminated soil into coarse particles and a slurry containing arsenic as a dissolved state other than the coarse particles.
(C) By classifying the slurry obtained in the wet classification step with a cyclone, the slurry is divided into slurry fine particles containing arsenic having an average particle size of less than 75 μm and dissolved, and an average particle A cyclone classification process that classifies the coarse particles having a diameter of 75 μm or more and returns a portion of the slurry fine particles as shield drilling mud in the mud shield method.
(D) A dehydration step of dehydrating the coarse particles classified in the cyclone classification step and mixing the coarse particles after the dehydration with the coarse particles classified in the wet classification step.
(E) A pH adjusting step of obtaining a purified soil by adding a neutralizing agent to the coarse particles obtained by mixing in the dehydrating step to make the pH value a neutral region.
(F) Of the fine slurry particles containing arsenic as a dissolved state obtained in the cyclone classification process, the excess portion that is not returned as shield drilling mud is set as a predetermined classification point and decanted by classification The slurry fine particles are separated into a solid and a liquid that are not more than the above-mentioned classification point and contain a supernatant, and the solid content exceeding the above-mentioned classification point, and the pH is adjusted. Decanter classification process to be transported to the process.
(G) The agglomeration agent is added to the fine particle part that is classified in the decanter classification step and is made into a slurry including the supernatant, so that the fine particle part is agglomerated and precipitated together with arsenic, and then the agglomerated sludge is filtered. Agglomeration process to obtain concentrated contaminated soil containing arsenic as a dissolved state by forming into a dehydrated cake by pressing.
Hereinafter, each process of said (A)-(G) is explained in full detail one by one.

(A) Alkali extraction step The alkali extraction step provided in the cleaning method of the present invention is the first step of treating arsenic-contaminated soil in the cleaning method described in this embodiment. That is, in the alkali extraction step, first, an alkaline treatment agent is added to the slurry-like contaminated soil generated by the muddy water type shield method, and the arsenic adsorbed on the soil particles of the contaminated soil is alkali-extracted to form a slurry. Extract into the liquid inside.

  Specifically, in the alkali extraction step, by adding sodium hydroxide and other chemicals to muddy water (slurry contaminated soil) generated by the muddy water type shield method, an alkaline region that does not exist in nature, for example, The soil is modified to pH = 8-13. At this time, heavy metals such as arsenic adsorbed on the soil particles of the contaminated soil are forcibly desorbed, and arsenic is eluted in the extract (muddy water) to extract arsenic as a dissolved state.

(B) Wet classification step Next, as shown in the flow diagram of FIG. 1, the wet classification step is a step provided after the alkali extraction step, and contaminated soil in which arsenic is extracted as a dissolved state in the slurry. Is classified into a coarse particle portion having a large particle size and a slurry portion containing arsenic as a dissolved state other than the coarse particle portion.

In the wet classification process of the present embodiment, a wet sieve or the like that has been conventionally used for treating contaminated soil can be used without any limitation, and the amount of naturally derived arsenic is low and suitable for the standard amount of dissolution. The coarse particles composed of gravel or sand having a large particle size are separated.
And in this embodiment, after mixing these coarse particle content with the coarse particle content etc. which were separated in the below-mentioned dehydration process, in the pH adjustment process, the pH is adjusted to a neutral region and separated as purified soil, It can be treated as construction generated soil that can be reused as landfill.

(C) Cyclone classification step Next, in the cyclone classification step, the slurry obtained in the wet classification step is classified with a cyclone, so that the slurry has an average particle size of less than 75 μm and is dissolved. Is classified into a slurry fine particle portion containing arsenic and a coarse particle portion having an average particle diameter of 75 μm or more. And when the technique which concerns on this embodiment is employ | adopted for a muddy water type shield construction method, as shown in the flowchart of FIG. 1, at least one part for the classified slurry-like fine particle is made into muddy water for shield excavation. Return as.

  In the cyclone classification process of this embodiment, the hydrocyclone equipment conventionally used for the treatment of contaminated soil can be used without any limitation, and by setting the classification point to 75 μm as described above, Slurry fine particles containing arsenic (see “Overflow” in the cyclone classification process in FIG. 1) and coarse particles with low arsenic content and elution (“underflow” in the cyclone classification process in FIG. 1) To see).

  As described above, alkaline water from which heavy metals such as arsenic discharged from the cyclone as overflow and remaining in the gaps between the soil particles are extracted, and fine particles (for example, 15 μm or less) having a high arsenic content / elution amount are extracted. Including the average particle diameter: The muddy water (slurry fine particles) composed of soil particles of 75 μm or less is recycled by returning a part of it as shield excavation muddy water. On the other hand, surplus muddy water that is not returned as shield excavation muddy water is conveyed to a decanter classification process, which will be described in detail later. Further, the coarse particles discharged from the cyclone as an underflow and having a low arsenic content / elution amount and conforming to environmental standards are transported to a dehydration process which is a subsequent process.

(D) Dehydration step Next, in the dehydration step, the coarse particles separated in the cyclone classification step are dehydrated, and the coarse particles after the dehydration are mixed with the coarse particles classified in the wet classification step. .
In the dehydration process of the present embodiment, a dehydration sieve or the like conventionally used for treating contaminated soil can be used without any limitation.

(E) pH adjustment step Next, in the pH adjustment step provided in the cleaning method of the present invention, a neutralizer is added to the coarse particles obtained by mixing in the above dehydration step, and the pH value is adjusted to a neutral region. And get the purified soil.

  Specifically, in the pH adjustment step described in the present embodiment, a neutralizing agent such as an acid is mixed with the coarse particles separated in the alkali extraction step or the dehydration step, and the pH value is set in a natural state. Neutralize to a certain neutral region, for example, about 6.5 to 8.5, and make it a reclaimed soil that can be reused as backfill soil. Furthermore, in the pH adjustment step of the present embodiment, the purified soil can also be obtained by mixing the solid content separated in the decanter classification step described later with the above coarse particles and then neutralizing.

(F) Decanter classification step Next, in the decanter classification step provided in the cleaning method of the present invention, as shown in the flowchart of FIG. 1, a slurry-like fine particle containing arsenic as a dissolved state obtained in the cyclone classification step is used. Among the particles, the surplus that is not returned as shield drilling mud is set to a predetermined classification point and decanted to classify the slurry-like fine particles into a slurry that is below the classification point and contains the supernatant. The solid content is separated into a fine particle content and a solid content exceeding the classification point, and the solid content is conveyed to a pH adjustment step.

Specifically, in the decanter classification process described in the present embodiment, the alkaline water in the slurry from which heavy metals such as arsenic remaining in the gaps between the soil particles of the soil are extracted, and the fine amount exceeding the elution amount reference value, etc. Particles (for example, the average particle size is 15 μm or less) are classified and separated using a decanter facility. On the other hand, the solid content (for example, the average particle diameter exceeds 15 μm) that conforms to the elution amount reference value is conveyed to the pH adjustment step.
Moreover, as a decanter equipment used by this embodiment, the decanter conventionally used for various solid-liquid separation processes can be used without any limitation.

  In this embodiment, by classifying and separating the slurry-like fine particles using a decanter, the classification point can be made smaller than the classification point (about 63 to 75 μm) using only a conventional cyclone, Specifically, it is possible to set the classification point as low as more than 0 μm and not more than 15 μm. And, in the cleaning method of the present invention, by setting the classification point in the decanter classification step to more than 0 μm and 50 μm or less, more preferably in the range of 6 to 20 μm, only the fine particle content exceeding the elution amount reference value, etc. Separate as concentrated contaminated soil. In the present invention, by providing such a decanter classification step, the amount of concentrated contaminated soil generated is about 1/4 to 1/10 compared to the conventional method in which fine particles are classified only by a cyclone. It becomes possible to lose weight.

  In addition, solid content (for example, soil particles having a particle size of about 15 to 75 μm) after separating the fine particles is dehydrated to about 20 to 30% in the decanter classification step, and is a previous step. Since arsenic eluted in the alkali extraction step is removed and it becomes in a state that conforms to the elution amount reference value, it can be handled as purified soil.

  Here, in this invention, the structure provided with said alkali extraction process and cyclone classification process is employ | adopted. As a result, as described above, for example, even if a decanter classification process is not provided, the classification point can be reduced as compared with the conventional method of separating arsenic to the concentrated contaminated soil only by cyclone classification. As a result, the amount of concentrated contaminated soil generated can be reduced.

  Hereinafter, the relationship between the structure of the decanter used in the decanter classification process of this embodiment, the particle size structure of the soil contained in the mud of Dotan (consolidated silt), and the classification point in the decanter classification process will be described. In addition, in the following description, the conditions of the other steps described above may be described, and the unit (%) indicating the particle size distribution is volume% unless otherwise specified.

  FIG. 4 is a schematic broken view showing an example of a decanter used in the decanter classification step of the present embodiment. In the decanter 1 of the example shown in FIG. 4, a screw 3 configured so that the vicinity of the tip 3 a side is reduced in diameter toward the tip 3 a is installed in a substantially cylindrical shell 2. Further, the vicinity of the tip 2a of the shell 2, that is, the tip 3a side of the screw 3 is configured to reduce in diameter toward the tip 2a.

  For example, the shell 2 is configured to rotate in the axial direction at a rotational speed such that a high centrifugal force can be exerted by power of a motor (not shown) or the like.

  Further, the screw 3 is provided with a spiral propeller 32 around the rotation shaft 31, and is configured to be rotatable by transmitting power such as a motor (not shown) to the rear end 3b. Further, the screw 3 in the illustrated example is configured such that the distal end 3a side has a reduced diameter as described above, while the rear end 3b side extends with the same diameter.

  Further, the decanter 1 is provided with an introduction pipe 4 for introducing the contaminated soil F into the interior. The inlet pipe 4 is provided so that an inlet 41 provided on one end side is exposed from the opening 21 of the shell 2, and an outlet 42 provided on the other end side is accommodated in a diffusion pipe 5 described later. In the illustrated example, most of the introduction pipe 4 including the outlet 42 is accommodated in the diffusion pipe 5.

Further, the decanter 1 in the illustrated example is configured such that the diffusion tube 5 is accommodated inside the rotation shaft 31 of the screw 3. The diffusion tube 5 is provided to rotate together with the screw 3, and includes a diffusion portion 51 formed in a bottomed cylindrical shape and a small diameter portion 52 formed in a tubular shape with a smaller diameter than the diffusion portion 51. Yes.
The diffusion unit 51 is configured to discharge the contaminated soil F introduced into the shell 2 toward the shell 2 while stirring as the screw 3 rotates, and discharges the contaminated soil F to the side surface. A plurality of holes 51A are formed.
Most of the introduction pipe 4 including the outlet 42 described above is accommodated in the small diameter portion 52. Further, the small diameter portion 52 of the diffusion tube 5 and the introduction tube 4 are rotatable with respect to each other.

  In addition, the decanter 1 receives slurry-like contaminated soil F to be treated from the inlet 41 of the introduction pipe 4 provided on the opening 21 side provided at the tip 2a of the shell 2, that is, the overflow muddy water in the cyclone classification. be introduced. Then, the solid recovery UF separated by the decanter classification is discharged from the opening 21 of the front end 2a, and the separated muddy water OF is discharged from the opening 22 provided on the rear end 2b side. Here, on the side of the opening 22, a dam portion 23 is provided on the inner wall 2 </ b> A of the shell 2 as an annular weir for ensuring the liquid depth of the separated muddy water OF in the shell 2.

  When the contaminated soil F is decanted using the decanter 1 configured as described above and separated into solid and liquid, the shell 2 is first rotated and the screw 3 is connected to the spiral propeller 32 on the opening 21 side. The contaminated soil F is introduced from the inlet 41 of the introduction pipe 4 provided on the opening 21 side while being rotated in the direction of traveling toward. At this time, the slurry-like contaminated soil F introduced into the inside is first discharged from the hole 51A toward the inner wall 2A of the shell 2 while being stirred with the rotation of the diffusion part 51 provided in the diffusion pipe 5. The And the contaminated soil F will be in the state by which the solid collection | recovery part UF was isolate | separated so that it might be pressed by the inner surface 2A of the shell 2 by centrifugal rotation of the shell 2, and the separation mud water OF was isolate | separated on the surface.

  The solid recovery UF in close contact with the inner surface 2A of the shell 2 is sequentially discharged from the opening 21 side to the outside as the screw 3 rotates. At this time, the solid recovery portion UF is discharged to the outside while being given a dehydrating action due to the reduced diameter of the shell 2 on the tip 2 a side and the rotational difference between the shell 2 and the screw 3. The muddy water OF flows out from the opening 22 side. That is, the slurry-like contaminated soil F discharged as an overflow component in the cyclone classification process is introduced into the decanter 1 and then separated into the solid recovered component UF and the separated mud water OF.

  And the solid collection | recovery part UF solid-liquid-separated by the decanter classification | category by the decanter 1 is introduce | transduced into the pH adjustment process (E) shown in FIG. Soil) is used for re-use such as backfilling. Further, the separated mud water OF separated by solid-liquid separation by the decanter 1 is introduced into the coagulation sedimentation step (G), the details of which will be described later, and subjected to coagulation sedimentation, filter press treatment, etc. As soil), it is treated as industrial waste.

  FIG. 6 is a conceptual diagram showing the particle size configuration of soil contained in the mud of the soil and the particle size configuration of the soil in each step of the cleaning treatment. As shown in FIG. 6, arsenic-contaminated muddy water (contaminated soil) generated by the muddy water type shield construction method generally has a coarse particle content of 75 μm or more occupying about 25%, and a fine particle content of less than 75 μm is 15 μm or less. About 75% including the fine particles (10%). First, a chemical is added to the contaminated soil in the “alkaline extraction step (A)” to modify the contaminated soil into soil in an alkaline region, and arsenic and the like adsorbed on the soil particles of the contaminated soil. Heavy metals are eluted in the extract (muddy water) to extract arsenic as a dissolved state. After that, by the classification in the above-mentioned “Cyclone classification process (C)”, coarse particles (75 μm or more) occupying about 25% are separated to the purified soil side, and “(D) dehydration process” to “(E) pH adjustment”. Transport to “Process”. In the “cyclone classification step (C)”, slurry-like muddy water containing fine particles (less than 75 μm) occupying about 75% is conveyed as an overflow to the decanter classification step (F). This fine particle content includes a fine particle content of 15 μm or less. In the “(F) decanter classification step”, the muddy water introduced as the overflow in the “cyclone classification step (C)” is solid-liquid separated by decanter classification. As a result, the slurry-like muddy water containing fine particles is separated from the original muddy water (contaminated soil F) with a solid recovery (solid content) UF consisting of fine particles (over 15 μm and less than 75 μm) that occupy about 65%. Then, it is subjected to solid-liquid separation into a separated mud water OF comprising fine particles (15 μm or less) containing the supernatant liquid and occupying about 10% of the original mud water.

  Here, as shown in FIG. 1, in the separation process of fine particles using decanter classification, the muddy water containing fine particles of less than 75 μm, which is discharged as an overflow part in the cyclone classification, is targeted. The muddy water is classified into a solid recovered portion containing fine particles of more than 15 μm and less than 75 μm, and a separated muddy water containing fine particles of 15 μm or less. At this time, assuming a decanter classification of Dotan considered to have the particle size configuration shown in FIG. 6, the fine particle content (over 15 μm and less than 75 μm): fine particle content (15 μm or less) = 65: 10 = 87: 13 It is thought that it becomes possible to achieve a volume reduction of nearly 90%.

  On the other hand, depending on the particle size composition of the ground excavated in the muddy water type underground shield construction, it has been clarified that there is a soil having a high proportion of fine particles, particularly fine particles of 15 μm or less. For example, when the muddy water generated from the soil is classified into decanters with the classification point kept at 15 μm, the proportion of fine particles that become contaminated soil becomes too large as shown in the graph of FIG. There is a problem in that the effect of volume reduction in the cleaning process of the water is significantly reduced.

  For this reason, in this embodiment, in the decanter classification step, the classification point is controlled to an optimum value, and in particular, in a case where mud water generated from soil having a high proportion of fine particles of 15 μm or less is washed, etc., the classification point is appropriately selected. It is preferable to adopt a configuration that can change the above. Specifically, the centrifugal force G accompanying the rotation of the shell 2 provided in the decanter 1 and the supply amount S of the contaminated soil F into the shell 2 shown in the schematic diagrams of FIGS. And a method of controlling the classification point by adjusting the liquid depth D of the contaminated soil F pressed against the inner wall 2A of the shell 2 by the centrifugal force G within a predetermined range.

Hereinafter, with reference to FIG. 5 (a)-(b), the effect | action at the time of performing decanter classification and solid-liquid separation of the contaminated soil F, controlling the classification point in the decanter 1 is demonstrated.
As shown in FIGS. 5 (a) and 5 (b), the contaminated soil F introduced into the shell 2 with the supply amount S is centrifuged as the shell 2 rotates (in the direction of arrow R in FIG. 5 (b)). The pressure G is pressed so as to be pressed against the inner wall 2A. At this time, as shown in detail in the enlarged view of FIG. 5 (c), the solid recovery portion UF composed of fine particles having a large particle size settles toward the inner wall 2A side in the direction in which the centrifugal force G acts, A slurry-like separated muddy water OF containing fine particles stays on the surface. That is, the contaminated soil F is solid-liquid separated into the solid recovery portion UF and the separated mud water OF. In the present embodiment, the conditions of the centrifugal force G of the shell 2, the supply amount S of the contaminated soil F, and the liquid depth D of the contaminated soil F are adjusted comprehensively, as shown in FIG. Further, the classification point between the solid recovered fraction UF composed of fine particles settled on the inner wall 2A of the shell 2 and the separated mud water OF containing the fine particles retained on the surface thereof is controlled and set with a desired classified diameter. It becomes possible to do.

  The centrifugal force G of the shell 2 is preferably set in the range of 500 to 1500 (G). If the centrifugal force G is within this range, the classification point can be controlled, that is, the classification diameter can be adjusted with high accuracy within the optimum range. If the centrifugal force G is too small, the classified diameter becomes too large, and a portion of fine particles that can be purified soil that satisfies the elution amount reference value is included in the separated mud water OF, and the volume reduction rate is high. May be reduced. Further, if the centrifugal force G is too large, the classification diameter becomes too small, and there is a possibility that a part of fine particles containing a contaminant such as arsenic is included in the solid recovery portion UF.

It is preferable to set the supply amount S of the contaminated soil F consisting of the overflow in the cyclone classification process into the shell 2 in the range of 0.6 to 300 (m ³ / h). If the supply amount S of the contaminated soil F is within this range, the classification diameter can be adjusted with high accuracy within the optimum range. If the supply amount S is too small, the classification diameter becomes too small, and there is a possibility that a part of fine particles containing contaminants such as arsenic is included in the solid recovery part UF. In addition, if the supply amount S is too large, the classified diameter becomes too large, and a portion of fine particles that can be purified soil that satisfies the elution amount reference value is included in the separated mud water OF, and the volume reduction rate May be reduced. In addition, the supply amount S of the contaminated soil F into the shell 2 is more preferably in the range of 50 to 100 (m ³ / h) in consideration of the processing efficiency in the actual site.

  The liquid depth D of the contaminated soil F, that is, the liquid depth D obtained by combining the solid recovered portion UF and the separated mud water OF can be determined in consideration of the capacity of the decanter 1 as a whole. In the inside, it is preferable to keep the liquid depth D constant. Here, since the liquid depth D when the separated muddy water OF overflows from the dam portion 23 provided on the inner wall 2A of the shell 2 is larger than the height H of the dam portion 23, the dam portion is taken into consideration. Preferably, a height H of 23 is determined.

  In addition, when the liquid depth D is made too deep, the solid content rate (ratio of soil contained in the soil slurry) of the solid recovery component UF is reduced, and the solid recovery component UF becomes viscous soil having a high water content. For this reason, it is more preferable to set the liquid depth D to such a depth that the solid content of the solid recovered component is maintained at a predetermined level or higher so that the solid recovered component UF does not become sticky with high viscosity. .

Next, a demonstration experiment conducted by the present inventors will be described with respect to the control of the classification points in the decanter classification described above.
First, a small decanter 1 having a structure as shown in FIG. 4 was prepared, and two types of soil (Cray-A and Clay-B) containing soil soil were prepared as objects to be cleaned.
Then, by changing the classifying conditions within the scope of the following conditions, followed by classification of the soil were examined classification diameter D ₅₀ at 50% classification point.
(1) Centrifugal force G: 500-1500 (G)
(2) Supply amount S: 0.6 to 3.0 (m ³ / h)
(3) Liquid depth D: constant (however, considering the viscosity of the solid recovery portion UF, the solid content rate of the solid recovery portion UF is adjusted to be about 70%)

As a result of the demonstration experiment under each of the above conditions, the following was revealed.
First, as the centrifugal force G is large, it was found that the classification diameter D ₅₀ at 50% classification point decreases. This is considered to be because when the centrifugal force G increases, even fine particles are separated as a solid recovery component on the inner wall 2A side of the shell 2 provided in the decanter 1.
Also, the larger the amount of supply of soil (contaminated soil F) to be cleaned, it was found that the classification diameter D ₅₀ at 50% classification point increases. This is because when the supply amount S of the contaminated soil F increases, the residence time of the pollutant F (including the solid recovery UF and the separated mud water OF after the solid-liquid separation) in the shell 2 is shortened. This is considered to be because the soil particles cannot be settled and the amount of soil particles transferred to the separated mud water OF side increases.

As a result of the above-described demonstration experiment, the centrifugal force G accompanying the rotation of the shell 2 and the supply amount S of the contaminated soil F into the shell 2 are appropriately adjusted comprehensively while keeping the liquid depth D constant. the results demonstrated that can be arbitrarily controlled classifying diameter D ₅₀ at 50% classification point.

  That is, by controlling the classification point in the decanter classification process by the above method, even if the soil soil contained in the contaminated soil has a high particle ratio of 15 μm or less, the contaminated soil It is possible to increase the proportion of reusable purified soil while effectively reducing the amount of concentrated contaminated soil that exceeds the elution amount reference value that needs to be disposed of.

(G) Aggregation step Next, in the aggregation step, the agglomeration agent is added to the slurry-like fine particles classified in the decanter classification step to aggregate the fine particles together with arsenic. After the precipitation treatment, the aggregated sludge is dehydrated and then formed into a dehydrated cake by filter pressing to obtain concentrated contaminated soil containing arsenic.

  Specifically, in the flocculation step, first, a precipitation treatment by a flocculation precipitation method is performed. In the flocculation step of the present embodiment, by adding an inorganic flocculant such as polyaluminum chloride, a sulfuric acid band, an iron-based agent, and a polymer flocculant to a fine particle portion made into a slurry including a supernatant, The fine particles are agglomerated and precipitated.

Next, the agglomerated sludge obtained by agglomeration sedimentation is formed into a dehydrated cake by performing a filter press using a conventionally known filter press device.
Thereby, the concentrated contaminated soil containing arsenic is obtained. Since this concentrated contaminated soil cannot be reused as backfill soil, etc., it is treated as contaminated soil. However, the cleaning method of the present invention can significantly reduce the amount of concentrated contaminated soil, thereby reducing the burden on the environment. The processing cost can be reduced.

  In the present embodiment, when the above decanter classification process is not provided, it is not returned as shield drilling mud out of fine particles containing arsenic as a dissolved state obtained in the cyclone classification process. The surplus is introduced as it is into the aggregation process.

(Complex action by alkali extraction process and decanter classification)
In the arsenic-contaminated soil cleaning method of the present invention, the alkali extraction step as described above is provided as a preceding step, and further, a cyclone classification step is provided as a subsequent step. Thus, when washing arsenic-contaminated soil, first, in the alkali extraction step, after heavy metals such as arsenic adsorbed on the soil particles of the contaminated soil are forcibly desorbed and extracted into the liquid in the slurry, In the wet classification process and the cyclone classification process, it is possible to increase the ratio of purified soil that can be recycled by separating the coarse particles with low arsenic elution amount to the purified soil side, and the volume reduction rate is significantly improved. To do.

  Furthermore, in the present embodiment, a fine particle fraction with a high arsenic elution amount that cannot be adapted to the elution amount reference value only by the alkali extraction step and cannot be classified even in the cyclone classification step. There may be fine particles contained. Such fine particles containing fine particles with a high arsenic elution amount are separated on the concentrated contaminated soil side, and then in the decanter classification step, alkaline water in the slurry from which heavy metals such as arsenic are extracted, And the method of classifying and separating the fine particle content exceeding the elution amount reference value or the like using a decanter facility (see decanter 1 in FIG. 4) can be employed. On the other hand, in the decanter classification step, the solid content that conforms to the elution amount reference value or the like is transferred to the pH adjustment step, that is, transferred to the purification soil side. As a result, it is possible to greatly reduce the amount of concentrated contaminated soil that exceeds the elution amount reference value, for example, fine particles having an average particle size of 15 μm or less, and to greatly increase the proportion of purified soil that can be reused. Can be increased.

  That is, in this embodiment, when a decanter classification step is provided in addition to the alkali extraction step and the cyclone classification step, arsenic is removed from the arsenic particles by decanter classification for fine particles that cannot meet the elution amount reference value by alkali extraction. On the other hand, as for the solid content, a part of arsenic is extracted by alkali extraction, so that the soil particles become purified soil that meets the elution amount reference value and the like.

  Furthermore, in this embodiment, in addition to the alkali extraction step and the cyclone classification step described above, a pH adjustment step for finally neutralizing the purified soil is provided. A more natural state can be achieved.

  As described above, in the muddy water treatment facility in the muddy water shield method targeted by the present invention, by incorporating a process for purifying naturally-occurring heavy metals, etc., the proportion of the purified soil that conforms to the elution amount standard value, etc. is increased. Therefore, compared with the case where many contaminated soils are disposed as concentrated contaminated soils as in the past, the disposal cost can be reduced by up to about 70%.

  Moreover, according to this embodiment, in the conventional method, at least a part of fine particles that have been disposed as contaminated soil can be reused as purified soil. Accordingly, as described above, the amount of treated soil containing arsenic that is contaminated soil can be reduced to about 1/4 to 1/10, and arsenic contaminated soil can be washed at low cost. Become.

Further, in the present embodiment, when a method including a decanter classification process is adopted, and the classification conditions in the decanter classification process are optimized as described above, the conventional cleaning method includes the decanter classification process. Therefore, a large amount of fine particles (for example, average particle size: more than 15 μm) that had to be disposed of can be collected as reusable purified soil. Thereby, the generation amount of the concentrated contaminated soil can be greatly reduced, and the cost required for the disposal can be greatly reduced.
As shown in FIG. 2, the arsenic elution amount is higher as the average particle size of the soil particles is smaller. Therefore, as in the present invention, by selectively treating the fine particles, the amount of arsenic is higher than that of the conventional method. More efficient processing is possible.

  From the above, the cleaning method of the present invention is a method capable of efficiently treating low-concentration arsenic-contaminated soil derived from nature by including an alkali extraction step, and further includes an alkali extraction step and a decanter classification step. Is combined with a combination of the two, it is a very rational and effective method because more efficient processing is possible.

  In the present embodiment, the case where the contaminated soil that is the object to be cleaned is contaminated soil caused by natural arsenic or the like generated in the muddy water shield method is described. However, the cleaning process is performed by the cleaning method according to the present invention. However, the object to be cleaned is not limited to this. In addition to the above, the cleaning method according to the present invention includes, for example, mud soil composed of non-slurry lumps generated in the mud pressure shield method, dredged soil, and natural heavy metals other than arsenic (for example, fluorine-based materials). Even when applied to cleaning treatment of contaminated soil with compounds, cyanide compounds, lead-containing compounds, etc.), the effect can be exhibited. Here, when the mud generated in the above mud pressure shield method is washed by the washing method according to the present invention, it is preferable to add the water to the mud to remove the slurry and make it into a slurry.

[Function and effect]
As described above, according to the method for cleaning arsenic-contaminated soil that is an embodiment of the present invention, the arsenic adsorbed on the soil particles is made dissolved by adding an alkaline treatment agent to the slurry-contaminated soil. By classifying the slurry-like contaminated soil extracted through the liquid component in the slurry and the slurry-like contaminated soil or a part of the contaminated soil using the cyclone, the slurry portion has an average particle diameter. And a cyclone classification step of classifying the fine particles into a slurry form containing arsenic as a dissolved state and coarse particles having an average particle size of 75 μm or more.
In this way, first, arsenic is dissolved in the alkali extraction step and extracted into the liquid component in the slurry, so that it is classified into a slurry fine particle portion and a coarse particle portion in the cyclone classification step in the subsequent step. Arsenic can be effectively removed from contaminated soil.

  Furthermore, in the present embodiment, when a method including a decanter classification step is adopted, the surplus portion of the slurry-like fine particles obtained in the cyclone classification step is converted into a fine particle containing a supernatant. By separating the arsenic into solid and solid components, the arsenic can be reliably separated into the fine particle portion in the form of a slurry containing the supernatant, and the ratio of reusable purified soil is increased. Concentrated contaminated soil consisting of fine particles containing arsenic can be reduced more effectively.

Furthermore, in this embodiment, when the decanter classification process is provided, the centrifugal force G accompanying the rotation of the shell, the supply amount S of the contaminated soil, and the liquid depth of the contaminated soil pressed against the inner wall of the shell By adopting a method of controlling the classification point by adjusting D, this classification point can be accurately controlled. Further, the centrifugal force G is adjusted in the range of 500 to 1500 (G), and the supply amount S is adjusted in the range of 0.6 to 300 (m ³ / h), more preferably in the range of 50 to 100 (m ³ / h). In addition, by keeping the liquid depth D constant, the classification point in the decanter can be controlled more accurately. As a result, arsenic is reliably separated into fine particles, and the amount of treated soil (purified soil) that can be reused is increased while effectively reducing the amount of treated soil that exceeds the elution amount reference value. The effect of improving the volume reduction rate can be obtained more remarkably. In addition, since the equipment related to the above decanter classification process can be incorporated into the ground treatment equipment of the muddy water type shield method, it is not necessary to prepare a wide site for installation separately, and it is adapted to the shield excavation speed. Purification treatment is possible.

  On the other hand, according to the present invention, by adopting a configuration including an alkali extraction step and a cyclone classification step, even if a decanter classification step is not provided, arsenic is concentrated and contaminated with only a conventional cyclone classification. Compared with the method of separating to the side, the classification point can be reduced, and the amount of concentrated contaminated soil generated can be reduced.

  Therefore, for example, when washing soil contaminated with natural arsenic that occurs in the muddy water shield method, while effectively reducing the amount of concentrated contaminated soil that exceeds the elution amount reference value that must be disposed of as contaminated soil, A cleaning method capable of increasing the proportion of reusable purified soil can be realized at low cost.

  According to the cleaning method for arsenic-contaminated soil of the present invention, by adopting the above configuration, the soil contaminated with natural arsenic generated by excavation work using the mud shield method in Japan, particularly in the Tokyo metropolitan area, is cleaned. In doing so, it is possible to increase the proportion of purified soil that can be reused while effectively reducing the amount of concentrated contaminated soil that must be disposed of as contaminated soil. Therefore, the present invention greatly contributes to the realization of a recycling society by greatly reducing the amount of contaminated soil generated.

DESCRIPTION OF SYMBOLS 1 ... Decanter, 2 ... Shell, 2A ... Inner wall, 2a ... Tip, 2b ... Rear end, 21, 22 ... Opening, 23 ... Weir part, 3 ... Screw, 31 ... Rotating shaft, 32 ... Propeller, 3a ... Tip, 3b ... rear end, F ... contaminated soil (slurry contaminated soil), UF ... solid recovery (solid content), OF ... separated muddy water (fine particle fraction in the form of slurry below the classification point and containing the supernatant) .

Claims (1)

A method for cleaning arsenic-contaminated soil by cleaning contaminated soil with natural arsenic,
An alkaline treatment agent is added to the contaminated soil in a slurry state or a part of the contaminated soil, and the arsenic adsorbed on the soil particles of the contaminated soil is alkali extracted to form a dissolved state in the slurry. An alkali extraction step of extracting into the liquid;
Next, a wet classification step of classifying the contaminated soil into coarse particles and a slurry containing arsenic as a dissolved state other than the coarse particles,
Next, by classifying the slurry obtained in the wet classification step with a cyclone, the slurry is divided into slurry fine particles having an average particle size of less than 75 μm and containing arsenic as a dissolved state, A cyclone classification step of classifying into coarse particles having an average particle size of 75 μm or more;
Next, a dehydration step of dehydrating the coarse particle portion classified in the cyclone classification step, and mixing the coarse particle portion after the dehydration with the coarse particle portion classified in the wet classification step;
Then, a pH adjustment step of obtaining a purified soil by adding a neutralizing agent to the coarse particles mixed in the dehydration step to make the pH value a neutral region, and
Furthermore, after adding the aggregating agent to the slurry-like fine particles containing arsenic as a dissolved state obtained in the cyclone classification process, the fine particles are coagulated and precipitated together with arsenic, and then the aggregated sludge is filtered. And a coagulation step of obtaining concentrated contaminated soil containing arsenic by forming into a dehydrated cake by pressing, and a method for cleaning arsenic-contaminated soil.

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