JP2019191177A - Method for cleaning radiation-contaminated soil and method for reducing volume - Google Patents

Abstract

To provide a new method for cleaning a radiation-contaminated soil and a new method for reducing the volume of the soil which can clean and reduce the volume of a radiation-contaminated soil containing a viscous soil or a radiation-contaminated soil containing much clay economically and efficiently.SOLUTION: The method includes a cleaning step of mixing a soil, water, and a dispersant and smashing the soil by a high-pressure jet flow in a cleaning device with a high-pressure jet flow introduced therein. The cleaning device includes: an introduction part for introducing a high-pressure jet flow into the cleaning device; a putting-in port for putting the mixture of the soil, the water, and the dispersant into the cleaning device; and a cleaning part for mixing the soil, the water, and the dispersant and cleaning the soil by a high-pressure jet flow. The method further includes the step of separating fine soil particles by a wet cyclone after the cleaning step.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a radioactively contaminated soil composed of cohesive soil and a cleaning treatment and a volume reducing treatment of the radioactively contaminated soil containing a large amount of clay.

Radioactive materials released by the accident at TEPCO's Fukushima Daiichi nuclear power plant diffused into a wide area and caused environmental pollution. In 2012, more than 160,000 citizens were forced to evacuate from the contaminated area. In order to repair the environmental damage caused by radioactive contamination, decontamination work has been carried out in many local governments including Fukushima Prefecture. It is stipulated that the removed soil, etc. generated by decontamination, etc. in Fukushima Prefecture will be stored in an intermediate storage facility for a certain period and then disposed of outside Fukushima Prefecture within 30 years. The amount of decontaminated soil generated from the decontamination work is estimated to be about 9 million m ^{3 of} sandy soil and about 11 million m ^{3 of} cohesive soil. Among the decontaminated soil, sandy soil is mainly generated from decontamination of residential areas, public facilities such as schools and parks, and commercial facilities. In addition, cohesive soil originates mainly from farmland and forests, and it contains organic matter such as humus and has a large aggregate structure. In the future, in order to store the decontaminated soil in the intermediate storage facility, it is necessary to have a highly economical treatment technology that separates and reduces the volume of radioactive material from the decontaminated soil.

  Recent studies have shown that radioactive cesium is fixed between clay layers such as mica contained in soil and does not elute. For this reason, in order to separate and remove radioactive cesium from the soil, the fine clay that fixes the radioactive cesium itself is separated from the soil, or treatment that separates the radioactive cesium from the clay mineral by chemical treatment, heat treatment, etc. A method is needed.

  As a method for treating radioactively contaminated soil, first, there is a soil washing technique using a trommel or the like conventionally used for treating contaminated soil such as heavy metals as a washing classifier. This method has abundant treatment results so far and is an effective economic treatment method for sandy soil with a wide soil particle size distribution. In the future, this method will be used for volume reduction treatment for intermediate storage. Is also planned to be adopted. However, existing soil washing techniques are difficult to apply to washing treatment of viscous soil having a fine particle size. For this reason, the development of economical treatment technology for radioactively contaminated soil mainly composed of cohesive soil is a major issue.

  In recent years, research and development have been conducted on methods for efficiently separating radioactive substances from contaminated soil while decontamination projects are being carried out, and related prior art documents include Patent Documents 1 to 5.

  Documents 1 and 2 describe a treatment method in which radioactive cesium-contaminated soil is heated to a temperature of 700 ° C. or higher with a rotary kiln or the like to separate exhaust gas containing radioactive cesium from soil. In this method, radioactive cesium can be efficiently separated by mixing a halogen substance and a chloride raw material, but there is a problem that a large heat energy is required and the processing cost is increased. In addition, this method cannot be applied to the treatment of contaminated soil containing radioactive substances such as strontium that cannot be volatilized and separated.

  Document 3 discloses a method for separating radioactive cesium by mixing radioactively contaminated soil with an acidic solution and performing ultrasonic treatment. In this method, a large amount of acid is required for the treatment, and a large amount of alkali raw material is required for the neutralization treatment after the washing treatment, so that the treatment cost becomes high. In recent research, seven years after the nuclear accident, radioactive cesium has changed to be strongly adsorbed in clay minerals immediately after the accident, and it is said that extraction with acid has become difficult. Yes.

  In Reference 4, the radioactive pollutant cleaning agent containing water, acid, metal salt, dispersant, and optionally fluoride was brought into contact with soil or fly ash containing radioactive material, and contained in soil or fly ash. It describes a method for transferring a radioactive substance to the radioactive pollutant cleaning agent and recovering it. This method is a method of collecting radioactive substances by mixing a detergent and a contaminated soil, which are uniquely formulated, in an acidic environment, and requires the addition of an acid or a detergent, which increases the processing cost.

  Reference 5 shows a method for concentrating and separating soil particles containing radioactive materials by mixing solidified material and iron powder containing nano-sized particles, ferromagnetic powder such as slag, etc., and magnetic separation. Has been. This method uses the principle that a film containing these is formed on the surface of the soil particles by mixing the solidified material and fine iron powder, and the soil particles having a small particle size and light weight are adsorbed on the magnet side. It is. This technique is suitable for the treatment of sandy contaminated soil having a wide particle size distribution, but is difficult to apply to the treatment of soil mainly composed of cohesive soil.

  Document 6 describes a process for removing plant-based foreign matters such as wood chips, roots, leaves, and the like having a specific gravity smaller than the soil from radioactively contaminated soil using a cleaning device using a high-pressure jet. This is related to pretreatment for preventing clogging during classification of contaminated soil, and is not intended for cleaning treatment or volume reduction of radioactively contaminated soil. Further, in this document, soil washing is performed using a sorting solution such as calcium chloride in order to separate foreign substances. However, since calcium chloride is agglomerated, a high soil crushing and washing effect cannot be obtained. Moreover, there is a concern about the corrosion of the cleaning device, the classification device, the dehydrating device, etc. due to the addition of high concentration calcium chloride.

Thus, various techniques for reducing the volume of radioactively contaminated soil have been studied so far. However, these are based on extraction with addition of acid, alkali, chemicals, volatilization separation by firing, etc., and require a large amount of chemicals for processing or consume large heat energy, and are not economical. It is considered difficult to apply to the treatment of contaminated soil including a large amount of viscous soil estimated at 1 million m ³ .

JP 2013-19734 A JP 2013-36883 A JP2013-124960A JP, 2006-70781, A JP 2014-226588 A Japanese Patent Application Laid-Open No. 2018-17665

  The present invention is a novel radioactively contaminated soil that can be economically and efficiently cleaned and reduced in volume of soil contaminated with radioactive soil and radioactively contaminated soil containing a large amount of clay. An object of the present invention is to provide a processing method.

  The present inventor mixes a dispersant with viscous soil and performs washing using a high-pressure jet to break up the aggregate structure and separate and remove fine soil particles in the soil, so that it is contained in the soil. The present inventors have found that the content of radioactive materials to be reduced can be greatly reduced, and have arrived at the present invention. The definition of cohesive soil is soil that contains 50% by mass or more of clay and silt with a particle size of less than 75 μm, but the soil is aggregated even in soil with 30-50% by mass of clay and silt. It is often difficult to process economically. The present invention can be suitably used for the treatment of radioactively contaminated soil containing a large amount of such clay.

  The method for cleaning and reducing the volume of radioactively contaminated soil according to the present invention has a cleaning step of crushing the soil by mixing the soil, water and dispersant by the high-pressure jet in the cleaning device into which the high-pressure jet is introduced. It is characterized by.

  In addition, the cleaning device has an introduction portion for introducing a high-pressure jet into the cleaning device, an inlet for introducing a mixture of soil, water, and a dispersant into the cleaning device, and soil, water, and a dispersant by the high-pressure jet. And a washing section for washing the soil by mixing.

  Moreover, the said washing | cleaning part has a collision board inside, It is characterized by the above-mentioned.

  Moreover, in the said washing | cleaning process, it wash | cleans by adding at least 1 sort (s) further among quartz sand, sand, iron powder, and activated carbon.

  Moreover, after the said washing | cleaning process, it has the isolation | separation process which classifies washing | cleaning soil and isolate | separates a fine soil particle with a wet cyclone.

  Furthermore, after the washing step, at least one of calcium chloride, magnesium chloride, and magnesium sulfate is added to the washing water, and after the aggregation treatment, the washing water is reused.

  In the method for cleaning and reducing the volume of radioactively contaminated soil of the present invention, a dispersing agent is mixed with viscous soil and washed with a high-pressure jet, and the surface part of the aggregate structure of the viscous soil is mainly crushed and separated. , It has the function of greatly reducing radioactive materials contained in soil. By the treatment method of the present invention, radioactively contaminated soil can be treated efficiently and economically without requiring a large amount of chemicals or consuming large amounts of energy. The method for cleaning and reducing the volume of radioactively contaminated soil containing a large amount of clay of the present invention is used for treating soil contaminated with radioactive cesium, radioactive strontium, radioactive plutonium, etc. in the vicinity of a nuclear power plant. be able to.

It is explanatory drawing of the decontamination principle of this invention. It is a schematic diagram which shows an example of the high-pressure washing apparatus used in this invention. It is a schematic diagram which shows another example of the high-pressure washing apparatus used in this invention. It is a schematic diagram which shows an example of the washing | cleaning and volume reduction processing system of the viscous soil used in this invention. 2 is a particle size accumulation curve of a viscous soil used in Example 1. It is a graph which shows the Cs content accompanying the process of Example 2, and the change of a sieve residue. 4 is a graph showing the particle size distribution measurement result recovered from the cyclone classification slurry of Example 3. 6 is a graph showing the particle size distribution measurement results recovered from the cyclone classification slurry of Comparative Example 5.

  In the treatment of radioactively contaminated soil containing a large amount of clay, since the contaminated soil is composed of fine soil particles, it is difficult to apply a conventional soil cleaning technique in which the classification particle diameter is 75 μm or more. Radioactive cesium is fixed between clay layers such as mica contained in the soil and has the property of not easily eluting, and in order to extract radioactive cesium from the soil, the fine clay particles fixing the radioactive cesium itself are removed from the soil. Or a method of separating radioactive cesium from clay minerals by chemical treatment, heat treatment or the like. For this reason, various techniques have been proposed so far, such as extraction with addition of acid, alkali and chemicals, and volatilization separation by firing. However, these treatments require a large amount of chemicals, consume large heat energy, and are difficult to economically treat radioactively contaminated soil.

  Hereinafter, the method for cleaning and reducing the volume of radioactively contaminated soil of the present invention will be described in detail. As described above, the target clay soil is decontaminated soil generated from farmland and forest, and soil having a aggregate structure occupies most. Agglomerated soil is an aggregate of small particles formed by combining fine clay particles, aluminum, organic matter, and the like, and has a structure having physical strength and water resistance. It is considered that the radioactive material released by the nuclear power plant accident was preferentially adsorbed on the fine clay particles present on the surface of the aggregate soil. As described above, the aggregated soil has a structure having strength and water resistance, and therefore, most of the clay particles adsorbed with the radioactive substance do not move to the inside after that, and even today, mainly the surface part of the aggregated soil. It is presumed that they are distributed.

  In the present invention, in order to separate fine clay particles adsorbing radioactive substances from the aggregate soil, the surface portion is efficiently crushed by adding a dispersant and washing with a high-pressure jet. . Soil washing using a high-pressure jet has an excellent function, but this alone is not sufficient to break up the aggregated soil, and it can be washed by adding a dispersing agent to the aggregated soil. A big crushing effect is born. Moreover, if a dispersing agent is not added at the time of soil washing | cleaning, a fine soil particle will re-aggregate at a latter stage and the crushing effect will be lost. In carrying out the present invention, it is not preferable to use a stirring type cleaning device. The reason is that the high-crushing effect as in the high-pressure washing treatment of the present invention cannot be obtained with a stirring type washing apparatus.

  In the present invention, the surface of the aggregated soil is mainly crushed and separated by washing with high-pressure jets while adding water and a dispersant to the contaminated soil to improve the cohesiveness of the aggregated soil. It depends on the principle. In addition, due to the strong cleaning effect of the present invention, the whole may be crushed in the aggregated soil with low strength. According to the present invention, fine clay particles adsorbed with a radioactive substance can be separated from the aggregate soil to perform efficient decontamination. Further, the addition of a dispersant prevents fine earth particles from reaggregating after washing and losing the crushing effect. An explanatory view of the decontamination principle of the present invention is shown in FIG. According to this treatment principle, the radioactively contaminated soil cleaning and volume reducing treatment method of the present invention is used to treat soil contaminated with radioactive cesium, radioactive strontium near the nuclear power plant, radioactive plutonium, and the like. Can be used.

  The method for cleaning and reducing the volume of radioactively contaminated soil according to the present invention has a cleaning step of crushing the soil by mixing the soil, water, and the dispersing agent by the high-pressure jet in the cleaning device into which the high-pressure jet is introduced. It is characterized by.

  Contaminated soil is, for example, added to 2 to 30 times the mass of water, preferably 5 to 10 times the mass of water, and the dispersant, and then stirred for several tens of seconds to several hours or several days, and then into the washing apparatus. It is thrown. Moreover, the slurry which added 2-20 times mass water to the contaminated soil, and a dispersing agent solution can also be thrown into a washing | cleaning apparatus by another path | route.

  Although various structures can be considered as a cleaning device, in order to perform continuous processing, as shown in FIG. 2, as shown in FIG. 2, slurry mixed with radioactively contaminated soil and dispersant solution, that is, soil and An inlet for introducing a mixture of water and a dispersant into the cleaning device, a high-pressure jet injection port through which high-pressure jet water is injected, and an intake hole, and an introduction unit for introducing the high-pressure jet into the cleaning device; It is reasonable to have a structure having a substantially pipe-shaped cleaning part with a discharge port. In the washing section, the soil is washed by mixing the soil, water, and a dispersing agent with a high-pressure jet. An apparatus having this structure is conventionally called an ejector and has been used in various industries, and has a function of transporting, mixing, and cleaning powder, liquid, and gas.

  In this device, the jet water jetted from the high-pressure jet jet port entrains the air supplied from the intake hole to form a jet with an air layer formed around the jet water, and this jet is injected. The soil is washed by mixing vigorously the soil supplied from the mouth, water and a dispersant.

  The cleaning apparatus used in the present invention is, for example, a member provided with a contaminated soil inlet, an intake hole, a high-pressure jet outlet, and an inside connected to this having a diameter of 20 to 200 mm and a length of 0.5 to 2 m. It consists of a pipe-shaped member and a discharge port. As the material of the cleaning device, steel or stainless steel is usually used. A material having excellent wear resistance such as ceramics can also be used. In order to increase the soil cleaning effect, a protrusion can be installed inside the pipe of the cleaning section to generate turbulent flow and increase the cleaning effect.

  In addition, a collision plate can be installed inside the cleaning unit of the cleaning device to obtain a structure as shown in FIG. In this structure, the installation of the collision plate causes a strong turbulent flow from the discharge port to the vicinity of the collision plate, so that a very high soil cleaning effect can be obtained. This cleaning apparatus is, for example, a member having a contaminated soil inlet, an intake hole, and a high-pressure jet injection port, and a pipe for cleaning inside a diameter 20 to 300 mm and a length 0.5 to 3 m connected thereto. It consists of a shaped member and a discharge port. As the material of the cleaning device, steel or stainless steel is usually used. A material having excellent wear resistance such as ceramics can also be used.

  In the method for cleaning and reducing the volume of radioactively contaminated soil according to the present invention, in order to further improve the cleaning effect of the contaminated soil, a plurality of cleaning devices using a high-pressure jet can be used in combination.

  The specifications of the specifications of the cleaning equipment, the pressure and flow rate of the high-pressure jet are determined based on the amount of soil treated per unit time, and further optimized through a bench scale test. For example, the high-pressure jet introduced into the cleaning device can be supplied from a high-pressure pump having a discharge port pressure of 2 to 30 MPa, and is suitable in many cases by using a high-pressure jet having a pump discharge port pressure of 2 to 30 MPa. A cleaning effect is obtained.

  Note that the high-pressure jet introduced into the cleaning device may be any one that can crush and peel the soil surface portion. Therefore, it is not limited to the one formed by water and air as described above, and may be a jet of water only or air.

  In the present invention, the surface of the soil can be efficiently crushed and washed by mixing the dispersant with the radioactively contaminated soil containing a large amount of clay and performing high-pressure washing. The dispersant is sometimes called a peptizer, and has a function of promoting the crushing of the aggregated soil particles and a function of uniformly dispersing the soil particles in water. Dispersants and peptizers used in the present invention include known sodium phosphates such as sodium hexametaphosphate, sodium pyrophosphate, sodium tetraphosphate, sodium tripolyphosphate, potassium metaphosphate, pyrophosphorus. Potassium phosphate such as potassium phosphate, potassium tripolyphosphate, potassium metaphosphate, sodium silicate, lithium silicate, sodium carbonate, sodium aluminate, sodium oxalate, sodium tannate, sodium hydroxide, sodium citrate, At least one of inorganic dispersants such as sodium tartrate, sodium sulfate, sodium acetate, lithium hydroxide, lithium aluminate, and lithium citrate can be used. Among these, sodium phosphates such as sodium hexametaphosphate and sodium pyrophosphate and sodium potassium phosphates such as potassium pyrophosphate have a strong dispersion effect and are particularly preferably used in the present invention. Sodium silicate is a dispersant that has been used in the ceramic industry for a long time, has a high treatment effect, and is particularly preferably used in the present invention. Note that when sodium hydroxide or lithium hydroxide is used, the pH of the slurry is greatly increased, which makes subsequent treatment difficult. For this reason, adoption of sodium hydroxide or lithium hydroxide is not preferable.

  In addition, in the present invention, known kaolin and organic dispersants for clay can be used. Examples of the organic dispersant include polycarboxylic acid polymers, polyacrylates, polyacrylic acid polymers, naphthalene sulfonic acid polymers, etc., and at least one of these, or at least one of these is the above-mentioned It can be used in combination with an inorganic dispersant.

The amount of dispersant added depends on the composition of the clay, cation exchange capacity, and the dispersant used, so it is difficult to determine uniformly, and it is necessary to determine by conducting a test evaluation with a bench scale test using the chemical used. is there. Although depending on the composition of the cohesive soil, the amount of the dispersant added can be, for example, 5 × 10 ⁻³ mol / kg or more with respect to the mass of the soil.

  In the method for cleaning and reducing the volume of radioactively contaminated soil of the present invention, at least one of additive materials consisting of silica sand, sand, iron powder, and activated carbon can be added during the cleaning with a high-pressure jet. Silica sand, sand, and iron powder correspond to abrasives, and by adding this, the effect of crushing the aggregated soil can be improved. In addition, hard granular materials such as ceramic powder and metal powder such as stainless steel can be used as an abrasive in the present invention. Activated carbon has a relatively hard property and has the function of improving the pulverization effect by adsorbing and removing organic matter contained in the soil in the wash water, and should be used as an additive for the washing treatment of the present invention. Can do. The particle size of abrasives such as silica sand, sand and iron powder is preferably 500 μm or more, which can be easily recovered by sieving. About activated carbon, the product of various particle sizes can be utilized. The addition amount of these additives is preferably in the range of 3 to 20% by mass with respect to the mass of the contaminated soil.

  In the method for cleaning and reducing the volume of radioactively contaminated soil of the present invention, the radioactive material and the soil from which radioactive material has been removed are adsorbed after washing and pulverizing the soil using a high-pressure jet after adding a dispersant. It is preferable to configure a series of systems by installing fine soil containing clay, equipment for separating and collecting organic substances, equipment having a function of dewatering washed soil, and equipment for concentrating the separated fine soil.

  By this system, clay particles and organic substances adsorbed with radioactive substances contained in contaminated soil can be separated from the soil, and the volume of radioactively contaminated soil can be reduced and the concentration of radioactive substances can be reduced. In addition, by dehydrating the washed soil, the volume of the washed slurry can be reduced, and handling of soil containing a radioactive substance and high water content can be facilitated.

  In the method for cleaning and reducing the volume of radioactively contaminated soil according to the present invention, as a facility for separating and recovering clay adsorbing radioactive substances and organic substances after the high-pressure washing apparatus into which the dispersant has been introduced. The classification and separation apparatus can be used in combination. Known classification devices include vibrating sieves based on the classification based on the particle size and specific gravity of soil particles, screw conveyors, wet cyclones, sedimentation separation, pressurized flotation, and centrifugal separation. Further, a flotation process based on the principle of separation based on differences in hydrophilicity and hydrophobicity can be used as a separation device. According to the flotation process, it is possible to suitably separate soil organic matter in the contaminated soil.

  In the method for cleaning and reducing the volume of radioactively contaminated soil according to the present invention, at least one of these separation devices is installed after the high-pressure washing device to adsorb the radioactive substances contained in the radioactively contaminated soil. Efficient separation and removal of clay and soil organic matter can produce excellent radioactive substance content reduction and volume reduction effects. In the treatment method of the present invention, the soil is effectively crushed and dispersed by high-pressure washing with a dispersing agent introduced, so that the functions of these classification and separation devices can be effectively utilized.

  FIG. 4 shows an example of a viscous soil cleaning and volume reduction processing system used in the present invention. In the example of this system, first, soil particles having a particle size of 75 μm or more, for example, are collected from the washed soil particles by a vibration sieve. The soil particles having this particle diameter are processed to a low radiation concentration by separating fine particles containing a radioactive substance, and can be used effectively, for example, as a banking material for embankment construction.

  The soil that has passed through the vibrating screen is treated with a wet cyclone, and soil particles having a particle size of 20 to 75 μm are collected. Here, since the wet cyclone has a small processing flow rate, a plurality of combinations can be processed. The soil particles of this particle size are also usually processed at a low radiation concentration and can be used effectively.

  In this example of the system, processing is performed with a particle size of 20 μm by a wet cyclone, but with the current wet cyclone technology, separation processing can be performed up to about 2 μm. When the treatment particle size is reduced, the radiation concentration of the separated fine soil becomes extremely high. The processing particle size by the wet cyclone may be set so as to be an advantageous condition in consideration of subsequent processing.

  As a method for dehydrating and recovering soil particles having a particle diameter of 20 to 75 μm separated by a wet cyclone, various known filtration / dehydration techniques can be used. Specific examples include filter press, membrane vacuum dehydration, belt press, screw press, and centrifugal dehydration.

  In the treatment of fine soil particles with a particle size of less than 20 μm contained in water separated by a wet cyclone, after adding a flocculant, it is concentrated and stored in a container after filtration and dehydration. A method such as storing in a storage is conceivable. This aggregation processing method will be described in detail later. Further, as a method for concentrating a slurry containing fine soil particles, a method of applying evaporation concentration may be considered. Since these operations need to be performed in a considerably high dose environment, it is necessary to determine an appropriate treatment method together with unmanned equipment.

  Furthermore, it is known from recent research that clay minerals adsorbing radioactive materials have weak magnetism. A method of applying high gradient magnetic separation and recovering fine soil particles separated in water by magnetic force can be applied after the washing step of the present invention.

  In the method for cleaning and reducing the volume of radioactively contaminated soil of the present invention, wash water having a mass of 5 to 10 times the mass of contaminated soil is usually used. In order to improve the economics of this treatment method, it is necessary to remove suspended substances in the wash water and recycle fresh water. In the washing water generated from this treatment method, the zeta potential is lowered by the addition of the dispersant, and a state in which many fine soil particles are suspended in the washing water is maintained. For example, a cyclone over recovered slurry containing the finest soil particles in the wash water shows a state where fine soil particles remain suspended and do not settle even after one day after classification. In this washing water, the zeta potential is lowered and the fine soil particles are stably dispersed at a high concentration, so that the agglomeration cannot be easily performed. It is necessary to add 10 times the amount. In order to efficiently perform this aggregation treatment, it is effective to add polyelectrolyte ions such as magnesium and calcium to the washing water. Specifically, the cohesiveness of the washing water can be greatly improved by adding magnesium chloride, magnesium sulfate, and calcium chloride to the washing water and stirring.

  In the treatment of the washing water generated from the treatment method of the present invention, the coagulation treatment can be performed by combining at least one of magnesium chloride, magnesium sulfate and calcium chloride with a known inorganic coagulant and polymer coagulant. . By adding polyelectrolyte ions to the slurry, the treatment effect of the inorganic flocculant and polymer flocculant can be improved. Known inorganic flocculants include aluminum flocculants such as PAC and aluminum sulfate, and iron flocculants such as ferric chloride and polyiron sulfate. Examples of known polymer flocculants include polyacrylamide flocculants, sodium polyacrylate, polyacrylic ester flocculants, and the like. In the treatment of the washing water of the treatment method of the present invention, after performing the coagulation treatment efficiently by these, the washing water is treated by a known filtration and dehydration technique to contain fine suspended soil particles and radioactive substances. Since the clay particles are removed, the washing water can be suitably recycled.

  As described above, in the method for cleaning and reducing the volume of radioactively contaminated soil of the present invention, the dispersion structure is mixed with the viscous soil and washed with a high-pressure jet, so that the main structure of the aggregate structure of the viscous soil is obtained. The radioactive material contained in the treated soil by crushing, peeling off the surface part, collecting the soil particles by vibrating sieve, screw conveyor, wet cyclone, sedimentation separation, specific gravity separation, centrifugal floatation, and beneficiation separation Can be greatly reduced, and the volume of contaminated soil can be reduced.

  By the treatment method of the present invention, it is possible to economically and economically treat radioactively contaminated soil without requiring a large amount of chemicals or consuming a large amount of energy. According to the treatment principle, the method of cleaning and reducing the volume of radioactively contaminated soil containing a large amount of clay according to the present invention includes not only soil contaminated with radioactive cesium but also various nuclides such as radioactive strontium and radioactive plutonium near the nuclear power plant. It can also be used to treat contaminated soil.

  Using the cohesive soil with aggregate structure collected from farmland in western Kanagawa Prefecture, the crushing effect by high pressure washing was evaluated by the following procedure.

  First, the particle size of the target clay was measured by a laser diffraction particle size distribution analyzer after pretreatment by ultrasonic dispersion with the addition of a dispersant. The particle size accumulation curve by measurement is shown in FIG. According to the measurement results, the arithmetic average diameter of the original soil was 17.98 μm, and soil particles of 10 μm or less accounted for 44%.

  Next, according to the test conditions of Table 1, the crushing effect of the viscous soil by high-pressure washing was evaluated according to the following procedure.

  (1) 50 g of clay soil and 100 g of water were put into a PP resin bottle, and after shaking the bottle, it was shaken lightly. In Example 1a, a predetermined amount of a 20% sodium hexametaphosphate solution serving as a dispersant was added, and then the bottle was closed and shaken lightly. In Example 1b, a predetermined amount of potassium pyrophosphate 20% solution serving as a dispersant was added, and then the bottle lid was closed and lightly shaken. In Example 1c, a predetermined amount of a 20% sodium silicate powder solution as a dispersant was added, and then the bottle was closed and lightly shaken.

  (2) The slurry was put into a high-pressure washing apparatus having the structure shown in FIG. 3 and washed under conditions of a pressure of 4.5 MPa and a high-pressure water flow rate of 7 liters / minute.

  (3) The washed slurry was received in a container and collected. In Comparative Example 2, a predetermined amount of the dispersant solution was put in a collection container in advance, and the slurry discharged from the high-pressure washing apparatus after washing was poured therein.

  (4) The recovered slurry was transferred to a beaker, stirred for 1 minute with a jar tester to uniformly disperse the soil particles, and the slurry was collected with a pipette.

  (5) The collected slurry was gently added to a polyvinyl chloride tube filled with water and having an inner diameter of 40 mm, and soil particles having a particle size of less than 10 μm and soil particles having a larger particle size were separated by a sedimentation method.

  (6) 0.1% by mass of a flocculant composed of aluminum sulfate and a medium anionic polymer flocculant was added to the sedimented and separated slurry and stirred for 1 minute.

  (7) The whole amount of the slurry to which the flocculant was added was filtered using a qualitative filter paper having a pore size of 4.5 μm, and the soil particles in the slurry were collected and dried in an electric furnace at 120 ° C. for 24 hours, and the mass was measured. In measuring the mass, the mass of the added flocculant was corrected.

  (8) For each test condition, the mass ratio of soil particles of 10 μm or less in the dry soil was calculated and used as an index for evaluating the crushing effect.

  (9) As Comparative Example 1, only a high-pressure washing was performed under the same conditions without adding a dispersant, and the recovered washed slurry was separated by the same sedimentation method, and soil particles of 10 μm or less in the dried soil The mass ratio of was calculated.

  (10) As Comparative Example 3, an equivalent amount of a dispersant was added, and the viscous soil was added to the beaker at a solid-liquid ratio (mass ratio) of 1: 100, and strong stirring was performed for 1 hour using a jar tester. And a slurry was prepared. This slurry was separated by the same sedimentation method, and the mass ratio of soil particles of 10 μm or less in the soil after drying was calculated.

Here, in this test, as a dispersant, manufactured by Kanto Chemical Co., Inc., sodium hexametaphosphate reagent (purity 65 to 70% (as P ₂ O ₅ ), molecular weight 611.7), potassium pyrophosphate reagent (purity) 98%, molecular weight 330.3), Junsei chemical Co., sodium silicate powder reagent _{(SiO 2: 50~55%, Na} 2 O: 23~27%, were used molecular weight 122.1).

  The test results are shown in Table 2. When the crushing effect of the viscous soil was evaluated by the mass ratio of the soil particles of 10 μm or less, the best result was obtained in the treatment in Example 1a in which water and a dispersant were added to the soil before washing. In this case, it can be seen that the mass ratio of the crushed soil particles of 10 μm or less is crushed to a level of about ½ of the original soil, which is quite effective. In Example 1b and Example 1c to which another type of dispersant was added, almost the same crushing effect was recognized.

  When high-pressure washing was performed without adding the dispersant of Comparative Example 1, the crushing effect was hardly obtained. Further, in Comparative Example 2 in which the dispersant was added after washing, the effect of the case where the dispersant was added in advance was not reached. Furthermore, even in the case of Comparative Example 3 in which a dispersant was added and strong stirring was performed with a stirring blade under a dilute solid-liquid ratio, the mass ratio of soil particles of 10 μm or less was low, and the crushing effect was as in Example 1a to It did not reach 1c.

  From the above, it was found that if a dispersant and water are added to a viscous soil having a aggregate structure, then cleaning with a high-pressure jet is performed, an extremely high crushing effect can be obtained. This effect could not be obtained only by washing with a high-pressure jet, and could not be reproduced by stirring treatment with a dispersant added.

  Simulated contaminated soil was prepared using a cesium chloride reagent by the following procedure, and the effects of washing and volume reduction of the contaminated soil of the present invention were examined. Moreover, according to the test conditions of Table 3, the washing | cleaning effect of the viscous soil by a high pressure washing | cleaning was investigated in the following procedures.

  (1) After immersing the viscous soil in a cesium chloride solution having a concentration of 250 mg / liter for a solid-liquid ratio of 1: 4 for 1 day, the solid solution is separated and air-dried indoors for 3 days. A simulated contaminated soil having 36 g / kg and a water content of 18% was prepared.

  (2) 100 g of simulated contaminated soil and 200 g of water were put into a PP resin bottle, a predetermined amount of dispersant was added, the bottle lid was closed, and shaken lightly.

  (3) Washing was performed with the above-described high-pressure washing apparatus under conditions of a pressure of 4.5 MPa and a high-pressure water flow rate of 7 liters / minute, and the washing soil was recovered with a 75 μm sieve.

  (4) The collected soil was dried, and the mass ratio of the soil remaining on the sieve was calculated.

  (5) Dried soil was added to a strong acid having a composition of hydrofluoric acid 1: nitric acid 3: hydrochloric acid 1 (volume ratio) under the condition of a solid-liquid ratio (mass volume ratio) 3: 100, and then heated at 150 ° C. This was dissolved.

  (6) After the said solution was further adjusted for analysis, the cesium concentration in the solution was measured using ICP-MS, and the soil content after the washing treatment was examined.

  The results are shown in Table 4 and FIG. Also in Comparative Example 4 in which only high pressure cleaning was performed, the contaminated soil surface portion was crushed by high pressure cleaning, and fine particles adsorbing cesium were separated and remained on the sieve, so the content was reduced by 40%. Yes. Furthermore, in the case where the dispersant is added, the content is further reduced, and in Example 2b, the cesium content is reduced to 1/3, and by adding the dispersant amount, high-pressure washing treatment is performed. It was confirmed that fine clay particles containing Cs were separated and removed from the aggregate soil and had a function of greatly reducing the cesium content.

  According to the following procedure, a dispersant is added to the same viscous soil as in Example 1 to perform high-pressure washing, and the soil particles being washed after passing through a vibrating sieve (mesh 0.15 mm) are used with a wet cyclone. A 20 μm classification test was conducted. Table 5 shows the conditions for adding the dispersant in this test.

  (1) 5 kg of clay soil and 30 kg of water were put into a resin container having a capacity of 50 liters, and stirred with a stirrer for 5 minutes. Next, in Example 3, a predetermined amount of a 20% sodium hexametaphosphate reagent solution as a dispersant was added and further stirred for 3 minutes. In Comparative Example 5, no sodium hexametaphosphate reagent was added.

  (2) This slurry was put into a high-pressure washing apparatus having the structure shown in FIG. 3 and washed under conditions of a high-pressure water supply pressure of 4.5 MPa and a high-pressure water flow rate of 60 liters / minute, and passed through a vibrating screen having a mesh size of 0.15 mm. . The washing water that passed through the sieve was received in a container and collected.

  (3) It was 4.3 mass% when solid content concentration of the washing water which passed the sieve was measured. This was classified using a cyclone (product name: cyclone catcher) manufactured by Nikuni Co., Ltd. under the condition of a particle size of 20 μm. The cyclone classification conditions were adjusted such that the pump supply pressure was 0.2 MPa, the flow rate was 70 liters / minute, and the cyclone over and cyclone under discharge flow rate ratio was 40: 1.

  (4) The particle size distribution of the soil particles contained in the two types of slurry recovered from the cyclone over and cyclone under was measured using a laser diffraction / scattering particle size distribution measuring apparatus LA920 manufactured by Horiba.

  (5) The particle size distribution measurement results recovered from the cyclone classification slurry of Example 3 are shown in FIG. Moreover, the particle size distribution measurement result collect | recovered from the cyclone classification slurry of the comparative example 5 was shown in FIG.

  From the particle size distribution diagram of the cyclone over (on the cyclone) and the cyclone under (under the cyclone) shown in FIG. 7, in Example 3 in which the dispersant was added, the content of soil particles of less than 20 μm in the cyclone under was 25%, and the cyclone. The content ratio of soil particles of less than 20 μm in the over is 98%, and good classification is achieved. Next, in Comparative Example 5, which is high-pressure washing without adding a dispersant, there is no significant difference in the particle size distribution of the cyclone over and cyclone under recovered soil, and it is judged that the cyclone classification did not function effectively. As described above, in the treatment method of the present invention, the clay is effectively crushed and dispersed, so that 20 μm classification using a wet cyclone is possible. By this function, the clay mineral adsorbing the radioactive substance can be separated and recovered, whereby the radioactive substance concentration in the radioactively contaminated soil can be reduced and the volume can be reduced.

  In the cyclone over recovered slurry obtained in Example 3, the zeta potential was lowered by the addition of the dispersant, and even after 1 day after classification, fine earth particles were suspended and did not settle. In the high-pressure washing treatment method of the present invention, the clay particles are effectively crushed and dispersed, and the fine earth particles are stably dispersed in the washing water, and the aggregation treatment is difficult. Of course, since 70 to 80% of the soil particles are soil particles having a particle size of less than 10 μm, it is difficult to perform filtration.

  In the following manner, a polyelectrolyte was added, and an agglomeration test of the cyclone over recovery slurry was performed. Table 6 shows the conditions for adding the flocculant in the coagulation test. The solid content concentration of the slurry used in this test was 3.8% by mass.

  (1) 200 g of cyclone over recovered slurry was collected in a 500 ml beaker.

  (2) Regarding the magnesium chloride hexahydrate, calcium chloride, and aluminum sulfate 16 hydrate, which are inorganic additives, each was dissolved in purified water to prepare a 20% solution. The polymer flocculant was dissolved in purified water to prepare a 0.1% solution.

  (3) In the aggregation test using magnesium chloride hexahydrate and calcium chloride, a beaker was set on a jar tester, a predetermined amount of inorganic additive was added to the slurry weight according to Table 6, and the mixture was stirred for 5 minutes. Subsequently, a predetermined amount of the polymer flocculant solution was added and stirred for 5 minutes.

  (3) In the agglomeration test using aluminum sulfate, a beaker was set on a jar tester, an aluminum sulfate solution was added according to Table 6, and then sodium hydroxide solution was added to neutralize, followed by stirring for 5 minutes. Subsequently, a predetermined amount of the polymer flocculant solution was added and stirred for 5 minutes.

  (4) The mixture was allowed to stand for 20 minutes, and the state of aggregation of soil particles under each test condition was observed. The test results are shown in Table 7.

Here, as an inorganic flocculant for this test, magnesium chloride hexahydrate (purity> 99%), calcium chloride (purity> 95%), aluminum sulfate 14-18 hydrate (purity 51. 0% to 57.5% (as Al ₂ (SO ₄ ) ₃ ) was used, and as a polymer flocculant, an aquatic polymer flocculant A? used.

  In the agglomeration test results in Table 7, by adding calcium chloride and magnesium chloride to the cyclone over recovery slurry containing the finest soil particles in the washing water of the present invention and stirring, the agglomeration can be achieved with a small amount of drug added. It can be processed, and the aggregation effect can be further enhanced by adding a polymer flocculant. Although not described in the examples, an aggregation effect was observed under the conditions of 0.05% by mass of calcium chloride and 0.05% by mass of magnesium chloride, but the effect was insufficient. In the agglomeration treatment of washing water, calcium chloride and magnesium sulfate were more effective than magnesium chloride. In Comparative Example 6, 4 times the amount of the inorganic flocculant as in Example 4b was added and the polymer flocculant was added, but a sufficient flocculant effect was not obtained.

As described above, the method for cleaning and reducing the volume of radioactively contaminated soil containing a large amount of viscous soil according to the present invention is performed by adding a dispersing agent to the contaminated soil and performing washing with a high-pressure jet to thereby remove the aggregated soil. By crushing, exfoliating, removing the soil particles by vibrating sieve, screw conveyor, wet cyclone, sedimentation separation, specific gravity separation, centrifugal separation by flotation, the content of cesium contained in the contaminated soil is greatly reduced, It has a function to perform volume reduction processing. In addition, the method of the present invention can be used to recycle cleaning water. According to the cleaning and volume reduction processing method of the present invention, it is possible to efficiently and economically treat radioactively contaminated soil without requiring a large amount of chemicals or consuming a large amount of energy.

Claims (6)

A method for cleaning and reducing the volume of radioactively contaminated soil, comprising a cleaning step in which soil, water and a dispersing agent are mixed by a high-pressure jet in a cleaning device into which a high-pressure jet has been introduced to disintegrate the soil. The cleaning device mixes the soil, water, and dispersant with a high-pressure jet, an inlet for introducing a high-pressure jet into the cleaning device, an inlet for introducing a mixture of soil, water, and dispersant into the cleaning device. And a cleaning unit for cleaning the soil, and the method for cleaning and reducing the volume of radioactively contaminated soil according to claim 1. The method for cleaning and reducing the volume of radioactively contaminated soil according to claim 2, wherein the cleaning unit has a collision plate therein. In the said washing | cleaning process, at least 1 sort (s) is further added and washed among siliceous sand, sand, iron powder, and activated carbon, Washing | cleaning and volume reduction of the radioactive contamination soil in any one of Claims 1-3 characterized by the above-mentioned. Processing method. The method for cleaning and reducing the volume of radioactively contaminated soil according to any one of claims 1 to 4, further comprising a separation step of separating fine soil particles by a wet cyclone after the washing step. The washing water is reused after adding at least one of calcium chloride, magnesium chloride, and magnesium sulfate to the washing water after the washing step and coagulating the suspended soil particles. 5. The method for cleaning and reducing the volume of radioactively contaminated soil according to any one of 5 above.