JP6004895B2 - Treatment method of contaminated soil - Google Patents

Description

  The present invention relates to a solidified material having excellent permeability to soil and capable of suppressing generation of dust from contaminated soil, and a method for treating contaminated soil using the solidified material.

Since the accident at the Fukushima Daiichi Nuclear Power Station, decontamination of contaminated soil with radioactive substances such as radioactive cesium has become an urgent issue in Japan. From recent survey results on the contaminated soil, it has been found that about 90% of the radioactive material in the soil stays in the surface layer part from the surface to a depth of 3 cm.
Therefore, decontamination methods currently being studied include topsoil cutting, soil agitation / removal with water, and reverse tillage. Among these, the topsoil cutting method is attracting attention because it can efficiently and reliably remove radioactive substances. The construction method includes a backhoe shaving method, a wiper method, a rotary cutter method, a turf stripper method, a skimmer method, etc., depending on the machine used for shaving.
However, all of the above methods (i) tend to generate dust containing radioactive substances as the topsoil is scraped off. In addition, (ii) dust may be generated during transport and storage of excavated soil, and radioactive materials may spread over a wide area. Furthermore, (iii) there is a problem that it is difficult to manage the thickness of the cut to suppress excessive cutting of the topsoil.

  In recent years, there have been many reports of soils such as ruins of factories and offices being contaminated with heavy metals such as selenium, arsenic, hexavalent chromium, lead and fluorine. If the concentration of heavy metals in the soil exceeds the environmental standard value, the land cannot be used as it is. In order to use the land, it is necessary to excavate and remove the contaminated soil, and to fill the land after removal with non-contaminated soil. However, the excavation, transportation and storage of this contaminated soil has the same problems as (i) to (iii) in the above construction method.

Under such circumstances, solidification materials and solidification methods for solidifying contaminated soil have been proposed.
For example, Patent Document 1 proposes a solidified metal elution-suppressing solidified material containing magnesium oxide, and Patent Document 2 proposes a solidified and insolubilizing agent for toxic substance-contaminated soil made of MgO and / or MgO-containing material. Patent Document 3 proposes a soil solidifying material mainly composed of magnesium oxide and sulfate such as gypsum, and Patent Document 4 discloses specific magnesium oxide and sulfate such as magnesium and calcium carbonate. Soil consolidation materials are proposed that contain proportions.
Furthermore, Patent Document 5 proposes a solidification / insolubilization method for contaminated soil, in which magnesium oxide having a fineness of 4000 cm ² / g or more calcined at a specific temperature is added to and mixed with contaminated soil.

  However, any of the above-mentioned solidifying materials is described in paragraph 0012, paragraph 2 0022, paragraph 3 0015, paragraph 0034, paragraph 0034, paragraph 4 0022, and paragraph 0008 of Patent Document 1. As described above, in order to uniformly disperse the solidified material in the soil, the solidified material and the soil must be mixed, and dust is generated during mixing. In addition, there is a problem that the radioactive material diffuses deep into the ground by mixing.

JP 2003-117532 A JP 2003-225640 A JP 2003-193050 A JP 2007-161839 A JP 2003-334526 A

The present invention has an object to provide a method of processing contaminated soil occurrence of dusty little, and the management of shaving thickness of the contaminated soil with easy solid reduction material upon solidifying contaminated soil .

The present inventors have examined a method for treating contaminated soil that meets the above-mentioned purpose, and the method for treating contaminated soil using a solidified material containing a specific amount of magnesium oxide and having a specific particle size distribution can achieve the above object. The present invention was completed.

That is, this invention is a processing method of the contaminated soil which has the following structures.
[1] Water / solidifying material ratio of the slurry below the solidifying material is (weight ratio) is 2.5~4.5, 1.5~3.0kg added to contaminated soil contaminated soil 1 m ² per magnesium oxide in terms of A method for treating contaminated soil, comprising solidifying and solidifying contaminated soil.
Solidified material: containing 50% by mass or more of magnesium oxide, and in the cumulative distribution of particle size, the particle size d ₂₀ with a cumulative volume of 20% is 7 μm or less, the particle size d ₅₀ with a cumulative volume of 50% is 22 μm or less, and the cumulative The particle size d ₈₀ with a volume of 80% is 40 μm or less. [2] Further, d ₅₀ / d _{20 of the} solidified material is 1.3 to 6.0 and / or d ₈₀ / d ₅₀ is 1.3 to The method for treating contaminated soil according to [1], which is 5.0 .
[3] The method for treating contaminated soil according to [ 1 ] or [2] , wherein the contaminated soil is soil contaminated with radioactive cesium.

In the method for treating contaminated soil of the present invention, the generation of dust from the contaminated soil is small, and the shaving thickness of the contaminated soil is easily managed.

As described above, the solidifying material used in the present invention contains a specific amount of magnesium oxide as an essential component and has a specific particle size distribution. In addition, the method for treating contaminated soil according to the present invention adds a specific amount of the solidified slurry having a specific water / solidifying material ratio to the contaminated soil, solidifies the contaminated soil, and removes the solidified contaminated soil. It is a method to do.
Below, it explains in detail according to the solidification material (an essential component, an arbitrary component, a particle size distribution, a specific surface area), and the processing method of contaminated soil.
The solidified material contains a specific amount of magnesium oxide as an essential component and has a specific particle size distribution. In addition, the method for treating contaminated soil according to the present invention adds a specific amount of the solidified slurry having a specific water / solidifying material ratio to the contaminated soil, solidifies the contaminated soil, and removes the solidified contaminated soil. It is a method to do.
Below, it explains in detail according to the solidification material (an essential component, an arbitrary component, a particle size distribution, a specific surface area), and the processing method of contaminated soil.

1. Solidifying material (1) essential component The solidifying material used in the present invention contains 50% by mass or more of magnesium oxide as an essential component.
The magnesium oxide is light-burned magnesia and / or light-burned magnesia partial hydrate. Among these, light-burned magnesia is obtained by baking magnesium hydroxide or the like obtained by precipitating magnesium components in magnesite, dolomite, brucite, or seawater with alkali such as slaked lime at 650 to 1300 ° C. Is. When the value is less than 650 ° C., light-burned magnesia is difficult to generate, and when the value exceeds 1300 ° C., the solidification strength may be lowered. The firing temperature is preferably 750 to 1000 ° C, more preferably 800 to 950 ° C. Moreover, although the said baking time is based also on the preparation amount of a raw material, a particle size, etc., it is 30 minutes-5 hours normally.

Further, the light-burned magnesia partial hydrate is obtained by pulverizing the light-burned magnesia, and then adding water to the pulverized product and stirring and mixing, or mixing the pulverized product in an atmosphere having a relative humidity of 80% or more It is obtained by holding for more than a week.
When the light-burned magnesia partial hydrate is defined with respect to two types of magnesium oxide and magnesium hydroxide, which are the main components, preferably 65 to 96.5% by mass of magnesium oxide and 3.5 to 35% by mass of magnesium hydroxide. %, More preferably 70 to 95% by weight of magnesium oxide and 5 to 30% by weight of magnesium hydroxide, more preferably 75 to 90% by weight of magnesium oxide and magnesium hydroxide. 10 to 25% by mass is contained. If the value is within the above preferred range, the solidification strength is higher. The light-burned magnesia partial hydrate may contain other components in addition to magnesium oxide and magnesium hydroxide as long as solidification is not inhibited.

The content rate of the magnesium oxide in the solidification material used by this invention is 50 mass% or more. When the value is 50% by mass or more, the solidification strength is sufficiently high and the generation of dust can be suppressed. The value is preferably 60% by mass or more, more preferably 70% by mass or more, further preferably 80% by mass or more, and particularly preferably 90% by mass or more.
From the viewpoint of protecting agricultural land, preferably, SO ₃ contained in the solidified material used in the present invention is 7.0% by mass or less, MnO ₂ is 1.0% by mass or less, and Na ₂ O _eq is 1.0% by mass. More preferably, SO ₃ is 5.0% by mass or less, MnO ₂ is 0.3% by mass or less, and Na ₂ O _eq is 0.7% by mass or less.

(2) Optional component The solidifying material used in the present invention is selected from blast furnace slag, magnesium carbonate, gypsum, magnesium sulfate, ferrous sulfate, calcium chloride, heavy burned phosphorus, molten phosphorus, and the like as optional components. One or more solidification accelerators, one or more cesium adsorbents selected from insoluble ferrocyanide, montmorillonite-containing material, zeolite, beidellite-containing material, and the like may be included.
The content of these optional components is preferably less than 50% by mass, more preferably 30% by mass or less, still more preferably 20% by mass or less, and particularly preferably 10% by mass or less. If the component is 50% by mass or more, the solidification strength may be lowered.

(3) Particle Size Distribution The particle size distribution of the solidifying material used in the present invention, in the cumulative distribution of the particle size of the solidifying material, the particle diameter d ₂₀ of the cumulative volume becomes 20% 7μm or less, the particle size of which cumulative volume of 50% The d ₅₀ is 22 μm or less, and the particle size d _{80 at} which the cumulative volume is 80% is 40 μm or less. If the particle size distribution is within this range, the permeability of the solidified material to the soil is high. Further, in order to further increase the permeability to the soil in the solidified material used in the present invention , d ₅₀ / d ₂₀ is preferably 1.3 to 6.0 and / or d ₈₀ / d ₅₀ is 1.3 to 5. 0.
The cumulative distribution curve is an upward curve with zero on the fine particle side, such as a laser diffraction / scattering particle size / particle size distribution measuring device, such as 9320-X100 (model number) manufactured by Nikkiso Co., Ltd. The solvent can be measured using ethanol (special grade) manufactured by Kanto Chemical Co., Inc.
When the solidifying material is used as a slurry (suspension using water as a medium), the component dissolved in water does not exist as a powder in the slurry, so the particle size distribution of the solidifying material excludes the component dissolved in water. To measure.

Further, a more preferable range of the particle size distribution is as follows. That is,
d ₂₀ is preferably 6μm or less, more preferably 5μm or less, more preferably 4.5μm or less, particularly preferably 4μm or less.
d ₅₀ is preferably 21 μm or less, more preferably 20 μm or less, still more preferably 19 μm or less, and particularly preferably 18 μm or less.
d ₈₀ is preferably 38μm or less, more preferably 35μm or less, more preferably 32μm or less, particularly preferably 30μm or less.
_Further, d 50 _{/ d 20,} more preferably 1.5 to 5.5, more preferably 1.6 to 5.0, more preferably 1.8 to 4.5, particularly preferably 2.0 to 4 .0.
d ₈₀ / d ₅₀ is more preferably 1.5 to 4.5, further preferably 1.6 to 4.0, particularly preferably 1.6 to 3.5, and most preferably 1.6 to 3.0. It is.
If the particle size distribution of the solidified material is within these ranges, the permeability of the solidified material is further increased. In order to achieve the effect of the present invention that the permeability to soil is high, it is important to have the particle size distribution. From the particle sizes described in Patent Documents 4 and 5, the particle size distribution is specified. I can't.

(4) Blaine specific surface area of the solidifying material used in the specific surface area present invention, to enhance the expression of the solidification strength, preferably 6000 cm ² / g or more, more preferably 8000 cm ² / g or more, more preferably 10000 cm ² / G or more, particularly preferably 12000 cm ² / g or more.
Further, the BET specific surface area of the solidified material used in the present invention is preferably 6 m ² / g or more, more preferably 8 m ² / g or more, further preferably 10 m ² / g or more, particularly from the viewpoint of the fixing effect of harmful substances. Preferably it is 15 m ² / g or more.
However, considering the production cost (such as grinding cost) of the solidified material, the upper limit of the Blaine specific surface area is 20000 cm ² / g or less, and the upper limit of the BET specific surface area is 50 m ² / g or less.
In addition, when using a solidification material as a slurry, the specific surface area of a solidification material is measured except the component which melt | dissolves in water similarly to the measurement of the said particle size distribution.

2. Next, a method for treating contaminated soil will be described.
The treatment method is performed by adding 1.5 to 3.0 kg of the solidified material slurry having a water / solidified material ratio (mass ratio) of 2.5 to 4.5 in terms of magnesium oxide per 1 m ^{2 of} contaminated soil. In this method, the contaminated soil is solidified and the solidified contaminated soil is removed.
Examples of the contaminated soil include soil contaminated with radioactive substances such as radioactive cesium, radioactive iodine and plutonium, heavy metals such as selenium, arsenic, chromium and lead, and various harmful substances such as fluorine.
Moreover, if the water / solidification material ratio (mass ratio) of the slurry is less than 2.5, the slurry has insufficient permeability to the soil, and if it exceeds 4.5, the solidification strength may decrease. The value is preferably 2.6 to 4.4, more preferably 2.7 to 4.3, still more preferably 2.8 to 4.2, and particularly preferably 3.0 to 4.0.
Further, if the amount of the slurry added to the contaminated soil is less than 1.5 kg in terms of magnesium oxide per 1 m ^{2 of} the contaminated soil, the amount of penetration into the soil (penetration depth) is not sufficient, and the solidification strength is low. If it exceeds 0 kg, the cost increases and it is not economical. The amount of addition is adjusted in accordance with the depth that needs to be removed by examining the depth of the soil where harmful substances such as radioactive substances are present in advance.
In addition, the contaminated soil that is the target of the solidification method of the present invention preferably has a dry density of 0.8 to 1.2 g / cm ³ and a water content ratio (the mass ratio of water / soil is expressed in percentage). It is as follows. If the dry density and water content ratio are within the above ranges, the permeability of the slurry is increased. Moreover, the kind of the said soil is not specifically limited, For example, sandy soil, viscous soil, loam, organic soil etc. are mentioned.

The means for adding the solidified material slurry of the present invention to the contaminated soil is preferably sprayed because dust does not easily occur. Further, the spraying can be performed using a shower, a sprinkler, a watering wheel, or the like.
After the solidifying material slurry is sprayed on the soil, it is preferable to cure for a certain period of time in order to obtain sufficient solidification strength. The curing period is preferably 1 day or longer, more preferably 3 days or longer, still more preferably 5 days or longer, and particularly preferably 7 days or longer, although it depends on the presence or absence of the solidification accelerator and its content.
Since the depth of penetration of the solidified material of the present invention into the soil is about 3 to 6 cm, a method of scraping off the contaminated soil is exemplified as a method of removing the solidified contaminated soil (topsoil). A backhoe, a wiper, a rotary cutter, a turf stripper, a skimmer, etc. are mentioned.
The solidified material of the present invention containing 50% by mass or more of magnesium oxide is white. When the color tone of the solidified material is represented by Hunter L, a, b color system, the L value is preferably 70 or more, the a value is 5 or less, and the b value is 20 or less, more preferably the L value is 80 or more. The a value is 3 or less, and the b value is 15 or less. Therefore, since the topsoil solidified by the penetration of the solidifying material of the present invention is almost white, the layer to be scraped can be easily visually recognized and excessive scraping can be prevented. Therefore, when the decontamination target is agricultural land, if the contaminated soil solidification method of the present invention is used, it can be scraped off leaving a large number of soil layers, resulting in a decrease in crop productivity of soil after decontamination etc. Can be suppressed.

EXAMPLES Hereinafter, although an Example demonstrates this invention, this invention is not limited to these Examples.
1. The penetration depth of the solidified material and the occurrence of dust particles Magnesium carbonate was calcined at 850 ° C. for 2 hours to obtain light calcined magnesia. The light-burned magnesia had a particle size distribution shown in Table 1, the magnesium oxide content was 92% by mass, the L value was 85, the a value was 1.9, and the b value was 10.4. .
Next, after preparing the slurry of water / solidification material ratio shown in Table 1 using the light-burned magnesia, the amount of the slurry shown in Table 1 (in terms of magnesium oxide) was added to the slurry with radioactive cesium from the surface to a depth of 3 cm. It was sprayed on farmland existing in the area.
After covering the spreading surface with a sheet and curing for 7 days, the farmland was excavated, and the penetration depth of the solidified material (the depth of the white portion of the vertical section of the soil) was measured to obtain the average. The results are shown in Table 1.

As shown in Table 1, the penetration depths of Examples 1 to 6 were 3.1 to 5.8 cm, and in all of the Examples, the entire topsoil in which radioactive cesium was present could be solidified. In particular, d ₂₀ is 7 μm or less, d ₅₀ is 22 μm or less and d ₈₀ is 40 μm or less, and d ₅₀ / d ₂₀ is 1.3 to 6.0 and d ₈₀ / d ₅₀ is 1.3 to 5.0. The penetration depth of the solidified materials of Examples 1 to 4 within the range is 3.6 cm or more, and the permeability to soil is even higher.
On the other hand, the penetration depth of Comparative Examples 1 to 5 was 2.5 cm or less, and in this comparative example, only a part of the layer in which radioactive cesium was present could be solidified.
Moreover, when the surface layer solidified in Examples 1-6 was scraped off using a backhoe, no dust was generated.

Claims (3)

Water / solidifying material ratio (mass ratio) slurry below the solidifying material is 2.5 to 4.5, the contaminated soil was solidified by 1.5~3.0kg added contaminated soil 1 m ² per magnesium oxide in terms of A method for treating contaminated soil, comprising removing solidified contaminated soil.
Solidified material: containing 50% by mass or more of magnesium oxide, and in the cumulative distribution of particle size, the particle size d ₂₀ with a cumulative volume of 20% is 7 μm or less, the particle size d ₅₀ with a cumulative volume of 50% is 22 μm or less, and the cumulative The particle size d _{80 at} which the volume is 80% is 40 μm or less. Furthermore, the d ₅₀ / _{d 20} of solidifying material is 1.3 to 6.0 and / or _d 80 _{/ d 50} is 1.3 to 5.0, the processing method of the contaminated soil according to claim 1. The method for treating contaminated soil according to claim 1 or 2 , wherein the contaminated soil is soil contaminated with radioactive cesium.