JP5894550B2 - Method for removing radioactive cesium from soil - Google Patents

Description

In the present invention, after wet classification or dry classification of soil containing radioactive cesium (a radioactive cesium isotope such as ¹³⁴ Cs or ¹³⁷ Cs), a calcium compound and sodium chloride are added, followed by heat treatment. Relates to a method for volatilizing and removing radioactive cesium from soil.

  In order to remove harmful organic components or combustible components from incineration ash generated after incineration of construction surplus soil or waste, incineration of organic components or combustible components is performed using a kiln. In Patent Document 1, the construction residual soil is continuously put into the rotary kiln by the feeder, and the residual soil thrown by the rotation of the kiln is gradually transferred to the discharge port provided on the opposite side of the feeder, and the organic matter in the residual soil is transferred. A high-temperature combustion gas with an air volume that conveys the ash produced by the combustion of the minute as fly ash is blown countercurrently into the rotary kiln, combusting the combustible organic components contained in the residual soil, and the ash with the above-mentioned combustion gas A method for firing construction residual soil is disclosed, in which the incombustible component in the remaining soil is fired and discharged by being exposed to the combustion gas while being discharged.

  In the firing method of Patent Document 1, combustible components contained in residual soil or incinerated ash are combusted by the high-temperature combustion gas blown in the countercurrent, and incombustible components such as sand, rubble, and ash are exposed to the high-temperature combustion gas. Is fired. Sediment, rubble or ash fired in the rotary kiln is discharged from the rotary kiln as sterilized high-quality baked sand (fired earth) or ash that does not contain combustible components. It is possible to arrange the particle diameters by sorting and further sifting.

  On the other hand, in the case of radioactive waste, unlike an organic component or a combustible component, it cannot be decomposed even by heating, so that a unique treatment method is required. Patent Document 2 mainly uses sodium nitrate, which is characterized by heating a radioactive waste mainly composed of sodium nitrate, a reducing agent, and a vitrifying agent to produce a vitrified body without generating nitrogen oxides. A method for treating radioactive waste as a component is disclosed. In the treatment method of Patent Document 2, even when waste is buried and contacted with groundwater, radionuclide elution is small and the volatility of the radionuclide is low during denitration and vitrification treatment.

  In Patent Document 3, activated concrete generated by dismantling a nuclear power generation facility is cut into blocks, the concrete blocks are crushed in a sealed compartment, and coarse aggregate, fine aggregate, and fine powder having a predetermined particle size are classified. Then, the recycled material manufacturing process for manufacturing the recycled material, and the fine powder of the recycled material is supplied into the thermal decomposition furnace, heated to 700 ° C. or higher with the supplied high-temperature air, and contained in the fine powder. A heat treatment step for separating tritium and carbon-14, and a decontamination step for adsorbing and removing the tritium and carbon-14 contained in the high-temperature air on a path for circulating the high-temperature air from within the pyrolysis furnace; The recycling method for activated concrete is disclosed in which each recycled material is discharged from the sealed compartment after decontamination is confirmed. The recycling method of Patent Document 3 is said to be able to regenerate and manufacture recycled aggregates and the like by removing a predetermined radioactive substance adhering to the activated concrete.

  Here, after the explosion accident at TEPCO and Fukushima Daiichi NPS in March 2011, radioactive cesium has been detected from soil in a wide area centering on Fukushima Prefecture. . For decontamination treatment of soil contaminated with radioactive cesium, many methods such as water washing, acid treatment under heating, topsoil removal, high pressure washing, or high temperature treatment in the presence of calcium salt have been studied. However, a processing method at a level that can be adopted on a practical scale has not been developed. The main cause is that the existence form of cesium in the soil, the chemical and physical properties of the cesium compound, and the reaction behavior between the cesium compound and the soil component have not been clarified.

  As a technique for removing radioactive cesium from contaminated soil containing radioactive cesium, Non-Patent Document 1 adds two types of calcium compounds as cesium volatilization promoters to contaminated soil and heat-treats them at 1350 ° C. Discloses a 99.9% volatilization removal method from soil. Non-Patent Document 2 discloses that when calcium chloride is added to soil, cesium hardly volatilizes even when the soil is heated to 1000 ° C. or higher.

JP 2002-79234 A JP 2002-221593 A Japanese Patent No. 4471110

March 1, 2012 Asahi Shimbun article, http://www.asahi.com/national/update/0301/TKY201203010146.html The National Institute of Agricultural and Food Research, February 22, 2012, press release, "Development of technology for removing dry cesium from contaminated soil containing radioactive materials" http: //www.naro.affrc. go.jp/publicity_report/press/laboratory/narc/027564.html

  Although the baking method of patent document 1 can be utilized as a method of removing organic content or combustible content from the soil and regenerating the soil, inorganic materials such as cesium compounds are not targeted for removal.

  The processing method of the radioactive waste of patent document 2 is a method of fixing the radioactive waste as a vitrified body, and the treated soil cannot be reused. Moreover, since it cannot reduce the volume of contaminated soil when applied to soil, it cannot be applied to a large amount of soil.

  The recycling method of Patent Document 3 is intended for removal of tritium or carbon-14, and is not intended for removal of radioactive cesium in soil.

  Although the radioactive cesium removal method of nonpatent literature 1 is supposed to be able to remove the radioactive cesium in soil almost completely, it is necessary to heat soil to 1300 ° C or more, and energy consumption is large. Moreover, when heat-treated at such a high temperature, it becomes impossible to reuse the treated soil as soil. For this reason, there exists a problem that processing cost is very large and soil cannot be reduced.

  In the Great East Japan Earthquake that occurred in March 2011, part of the Tohoku region was damaged by a large-scale tsunami. For this reason, in the area damaged by the tsunami, soil contaminated with radioactive cesium (including soil containing coarse materials such as rubble) also contains seawater-derived salt (sodium chloride).

  An object of this invention is to provide the removal method which can remove radioactive cesium from soil efficiently at low cost.

  When the present inventors volatilize cesium from the soil by heating the soil containing cesium, when sodium chloride is added to the soil and heated, the cesium removal method disclosed in Non-Patent Document 1 We found that cesium volatilizes from soil at low heating temperatures.

  However, when sodium chloride is added to the soil, chlorine gas is generated when the soil is heat-treated by a heat treatment apparatus such as a kiln. Therefore, there is a problem that the heat treatment apparatus and its attached equipment (for example, exhaust pipe) are easily corroded by chlorine gas. In addition, when sodium chloride is added in a large amount to the soil, there is a problem that it cannot be used for cultivated land unless the soil after removing cesium is washed and demineralized.

Therefore, the present inventors have reduced the amount of sodium chloride by combining an inorganic calcium compound and sodium chloride as an additive that promotes cesium volatilization during heat treatment, while reducing the amount of sodium chloride at a lower temperature than before. The method that can remove a trace amount of radioactive cesium from water was examined. As a result, the present inventors have calcium oxide, and at least one inorganic calcium compound selected from calcium carbonate and calcium hydroxide or Ranaru group, by the combined use of sodium chloride, solving the above problems I found out that I could do it.

  Here, in the case of soil damaged by tsunami, sodium chloride derived from seawater is contained. Therefore, when such soil is treated, it is not damaged by tsunami and contains sodium chloride derived from seawater. The amount of sodium chloride added from the outside can be reduced compared to the case where soil that is not treated is treated.

  The present inventors calculate the sodium chloride concentration contained in the soil by measuring the sodium chloride concentration in the effluent obtained when the soil is dehydrated after wet classification, and add it in the addition step. The inventors have found that the amount of sodium chloride to be adjusted can be adjusted, and have completed the present invention.

Specifically, the present invention
A wet classification process for wet classification of soil containing radioactive cesium;
Wherein the wet classification step, as the ratio of the inorganic calcium compound in the mixture of the remainder and the inorganic calcium compound in soil coarse particles exceeding a particle diameter 1mm is removed is 3 mass% to 30 mass%, the soil At least one inorganic calcium compound selected from the group consisting of calcium oxide, calcium carbonate, and calcium hydroxide is added to the remainder of the soil, and 0.5 mass% with respect to the mass of the mixture of the remainder of the soil and the inorganic calcium compound An addition step of adding sodium chloride to exceed 5% by mass,
A heating step of volatilizing radioactive cesium from the soil after the addition step by heat treatment at 900 ° C. to 1200 ° C. for 30 minutes to 120 minutes,
A recovery process for recovering radioactive cesium volatilized from the soil by the heating process;
Have
After the wet classification step, by measuring the sodium chloride concentration in the effluent obtained when dewatering the remainder of the soil from which the coarse particles have been removed, the coarse particles are contained in the remainder of the soil from which the coarse particles have been removed. The present invention relates to a method for removing radioactive cesium from soil, wherein the amount of sodium chloride being calculated is calculated and the amount of sodium chloride added in the adding step is adjusted.

In the present invention, after the soil is wet-classified or dry-classified to remove coarse particles (and coarse matter), the inorganic calcium compound or organic calcium compound and sodium chloride are added to the rest of the soil, and then heat-treated. Volatilizes radioactive cesium from the soil. As cesium volatile promoter, calcium oxide, and at least one inorganic calcium compound selected from calcium carbonate and calcium hydroxide or Ranaru group, by the combined use of sodium chloride, 1300 ° C. Non-Patent Document 1 Even when the radioactive cesium which exists only in a trace amount compared with stable cesium is removed by the heating of lower 900 to 1200 degreeC for 30 minutes or more, radioactive cesium can be removed efficiently .

This is because, by adding an inorganic calcium compound to the soil, the radioactive cesium is desorbed from the soil, and the desorbed radioactive cesium is combined with chlorine atoms derived from sodium chloride, so that the radioactive cesium contained in the soil is reduced. It is assumed that it is because it volatilizes as (radioactive) cesium chloride .

In the addition step, adding an inorganic calcium compound to the soil, but further addition of sodium chloride to be equal to or less than 5 wt% exceeds 0.5% by weight, based on the weight of the mixture of soil and inorganic calcium compound, When the soil contains sodium chloride derived from seawater, the amount of sodium chloride added is excessive. Therefore, in the present invention, the concentration of sodium chloride contained in the remainder of the soil (content per mass) is calculated and adjusted so that the amount of sodium chloride added exceeds 0.5% by mass and is 5% by mass or less. Accordingly, it is possible to optimize the volatilization of radioactive cesium from the soil in the heating process, reduce the amount of sodium chloride to be added, and reduce the processing cost.

The term “soil” as used herein means not only the soil itself, but also soil mixed with noncombustible coarse materials (so-called “earthquake rubble”) such as concrete, roof tiles or boards. In addition, “soil” as used herein includes river sediment and sand. Moreover, the soil used as a reference | standard when mixing an inorganic calcium compound and sodium chloride is based on the mass in a dry state.

In the method for removing radioactive cesium from soil of the present invention, the amount of sodium chloride added in the adding step is 1% by mass or more and 3% by mass or less based on the mass of the mixture of soil and inorganic calcium compound . preferable.

Part of the added sodium chloride decomposes or volatilizes due to the effects of moisture in the soil due to heating and does not react with radioactive cesium, so from the viewpoint of more reliably volatilizing radioactive cesium in the soil, soil and inorganic calcium It is preferable to add 1% by mass or more of sodium chloride based on the mass of the mixture with the compound. On the other hand, some of the chlorine atoms derived from sodium chloride that do not react with radioactive cesium react with water volatilized from the soil or water generated by the thermal decomposition of organic substances contained in the soil to generate hydrogen chloride. From the viewpoint of suppressing the generation of hydrogen chloride and reducing the residual amount of sodium chloride in the heat-treated soil, 3% by mass or less of sodium chloride relative to the mass of the mixture of soil and inorganic calcium compound or organic calcium compound It is preferable to add at.

In the addition step, and classifying the particle size less than 1mm soil, thereby removing coarse particles (and coarse material), you compacting the soil to be subjected to a heating step.

  Since radioactive cesium is contained in soil having a small particle size, if the volume of the soil to be heat-treated is reduced by classification as pretreatment, the processing cost can be reduced and the processing efficiency can be increased. Prior to the adding step, it is more preferable to classify soil having a particle size of 500 μm or less, and it is even more preferable to classify soil having a particle size of 75 μm or less.

  As used herein, “particle size” means a particle size that has passed through a sieve. For example, “particle size of 1 mm or less” means a particle size that has passed through a sieve having a mesh width of 1 mm.

  The radioactive cesium volatilized from the soil is preferably collected by one or more means selected from the group consisting of a bag filter, a HEPA filter, and a wet scrubber.

  Simply volatilizing radioactive cesium from the soil will diffuse the radioactive cesium into the atmosphere. In the present invention, radioactive cesium volatilized from soil is collected by one or more means selected from the group consisting of a bag filter, a HEPA filter, and a wet scrubber so that radioactive cesium is not discharged out of the system. Is desirable.

  It is preferable to further include a salt removal step of removing sodium chloride from the soil after the heating step.

  When the radioactive cesium-removed soil after the heating process is reused, if sodium chloride remains in the soil, adverse effects will occur when plants are planted. Similarly, when landfilling radioactive cesium-removed debris after the heating step, salt damage may be caused to the soil around the landfill site. By performing the salt removal step, salt damage caused by the remaining sodium chloride can be prevented.

  According to the present invention, compared with the conventional method for removing radioactive cesium from soil, it is possible to achieve a similar removal rate at a lower heating temperature.

The process flowchart of Embodiment 1 is shown. The process flowchart of Embodiment 2 is shown.

  Embodiments of the present invention will be described with reference to the drawings as appropriate. The present invention is not limited to the following description.

[Embodiment 1]
<Classification process>
FIG. 1 shows a process flow diagram of the first embodiment. The process flow in FIG. 1 is a flow for performing wet classification as soil classification. The soil (removed soil) containing radioactive cesium is preferably classified so as to have a particle size of 1 mm or less, preferably 500 μm or less, more preferably 75 μm or less before the addition step.

  Classification may be performed in multiple stages. For example, coarse classification may be performed as a pretreatment for wet classification, and coarse particles may be removed by classification so that the particle size is 40 mm or less. For coarse classification, a classification device such as a vibrating sieve or a trommel can be used.

  A non-combustible coarse material such as rubble or an organic coarse material such as wood mixed in soil is removed by coarse classification. The soil from which the coarse particles have been removed is further subjected to a wet classification step or a dry classification step, and coarse particles larger than a predetermined particle size are classified as coarse particles in the wet classification step or the dry classification step. Since radioactive cesium is mainly present in soil with a small particle size, the soil used for the heating process can be reduced by removing large particles and coarse particles such as rubble by wet classification. Is possible. As a result, the cost required for the heat treatment is reduced. Known wet classification can be applied to wet classification of soil.

  After wet classification of the soil, the coarse particles are washed with washing water (eg industrial water, river water or lake water). By this cleaning, fine particles (containing radioactive cesium) on the surface of the coarse particles are removed. The cleaned processed product (coarse particles) is discharged as a decontaminated processed product. Suspended water to be drained is mixed with classification water containing finely divided soil classified by wet classification.

  In the case of using a wet classifier, cleaning is also performed at the same time as wet classification. Therefore, when fine particles on the surface of coarse particles are sufficiently removed, only wet classification may be performed and the washing operation of coarse particles may be omitted.

  The coarse product obtained by coarse classification is also washed in the same manner as the coarse particles described above, and the particles adhering to the surface of the coarse product are removed. The washed treated product (coarse product) is discharged as a decontaminated treated product, and the suspended water to be drained is mixed with classification water containing finely divided soil classified by wet classification.

[Measurement of sodium chloride concentration]
The classification water containing fine particles is subjected to solid-liquid separation by a dehydrating device such as a filter press or a belt press. If necessary, precipitation separation such as coagulation precipitation may be performed before applying to the dehydrator. Moisture is reused as wet separation water for classification as a desorbing solution, and the salt concentration is measured before being reused. The salt concentration (sodium chloride concentration) in the detachment liquid can be measured by, for example, a known salt concentration meter or an electric conductivity meter. Then, the sodium chloride concentration in the classified particulate soil (including particulate organic matter) is calculated from the salinity concentration in the desorbed liquid.

  Since sodium chloride is easily dissolved in water, it is considered that most of the sodium chloride derived from seawater is present in the desorbed liquid by wet classification. On the other hand, by measuring the moisture content in the solid content, it is possible to calculate the amount of water supplied to the adding step together with the classified fine particle soil. For this reason, it becomes possible to calculate the sodium chloride concentration in the particulate soil subjected to the addition step by measuring the salt concentration in the desorbed liquid. After measurement of the salinity, the particulate soil that is a solid content is subjected to an addition step.

  The case where the salinity concentration in the desorbed liquid is measured has been described, but by measuring the salinity concentration contained in the dehydrated cake (solid content after solid-liquid separation), it can also be measured in the particulate soil used for the addition process. The sodium chloride concentration of can be calculated. Specifically, it is possible to calculate the salinity concentration in the dehydrated cake by collecting a part of the dehydrated cake, putting this in water and eluting sodium chloride into the water, and measuring the sodium chloride concentration in the water. it can. From this salt concentration and the amount of dehydrated cake, the concentration of sodium chloride contained in the dehydrated cake can be calculated. When collecting the dehydrated cake and measuring the salt concentration in the dehydrated cake, it will be measured batchwise, but it may be measured every predetermined time or measured for each processing object. Good.

  Furthermore, dry classification of soil containing radioactive cesium, measure the sodium chloride concentration in the remainder of the soil from which coarse particles have been removed, and calculate the concentration of sodium chloride in the remainder of the soil from which coarse particles have been removed By doing, it is good also as a structure which adjusts the amount of sodium chloride added in an addition process. In this case, the sodium chloride concentration in the rest of the soil is determined by taking a part of the soil, eluting the sodium chloride in the soil into water, and measuring the sodium chloride concentration in the eluate. The amount of sodium chloride contained can be calculated.

<Addition process>
Soil containing radioactive cesium (particulate soil), calcium oxide, adding at least one inorganic calcium compound selected from calcium carbonate and calcium hydroxide or Ranaru group. The addition amount of the inorganic calcium compound is adjusted so that the proportion of the inorganic calcium compound in the mixture of soil and inorganic calcium compound is 30 mass% or more 3 wt%.

Then, the soil by adding an inorganic calcium compound and sodium chloride are added such that 5% by mass or less than 0.5% by weight relative to the total amount of the mixture of the radioactive cesium-containing soil and inorganic calcium compound.

(1) From the viewpoint of allowing the treated soil to be reused without salt removal or by simple salt removal, and (2) from the viewpoint of reducing the amount of sodium chloride-derived hydrogen chloride generated. it is preferably 3 mass% or less based on the total amount of the mixture of the addition amount of sodium and radioactive cesium-containing soil and inorganic calcium compound. Further, from the viewpoint of reliably volatilized radioactive cesium from the soil, the amount of sodium chloride is preferably 1 mass% or more with respect to the total weight of the mixture of the radioactive cesium-containing soil and inorganic calcium compound.

  When the added amount of sodium chloride is about 1% by mass, most of the added sodium chloride is decomposed or volatilized in the heating process and taken out from the heating device together with the exhaust gas. It may be possible to reuse without processing.

When adding an inorganic calcium compound , in order to reduce the heat processing amount after addition, it is more preferable that the addition amount to a soil shall be 10 mass% or less.

The inorganic calcium compound and sodium chloride may be added to the rest of the soil in a solid state, or may be dissolved in water and added to the rest of the soil in a liquid state.

Here, the addition amount of sodium chloride is adjusted based on the amount of sodium chloride in the particulate soil calculated after the wet classification step or the dry classification step. That is, when sodium chloride is contained in the soil, the amount of sodium chloride contained per unit mass of soil is subtracted from the original, and 0.5% with respect to the mass of the mixture of soil and inorganic calcium compound . Sodium chloride is added so as to be more than 5% by mass and less than 5% by mass.

The order of addition to the soil may be either sodium chloride or inorganic calcium compound , and both may be added to the soil simultaneously.

A general blender can be used for mixing the soil, the inorganic calcium compound and sodium chloride. Also, when adding a solid inorganic calcium compound and sodium chloride, soil when classifying into the following particle size 1mm by dry classification, under particle diameter or classifying inorganic calcium compound and sodium chloride and classification before An inorganic calcium compound and sodium chloride may be mixed with the soil simultaneously with classification by adding to the soil and performing classification with the additive. By doing in this way, it becomes possible to mix soil, an inorganic calcium compound , and sodium chloride uniformly, without using a special mixing apparatus.

In addition, the soil to which the inorganic calcium compound and sodium chloride are added may be dried as necessary before being subjected to the heating step. By drying the soil prior to the heating step, it is possible to reduce heat loss in the heating step and to suppress a temperature drop in the furnace accompanying water evaporation. The drying of the soil uses either an indirect contact type drying device that makes the heating medium and the soil contact indirectly to dry, or a direct contact type drying device that makes the heating medium and the soil contact directly to dry. Also good. The heat medium is not limited to air, and water vapor can also be used. The drying of the soil is not limited to immediately before the heating step, and may be performed before the adding step.

<Heating process>
The soil to which sodium chloride has been added is supplied to a heating device such as a heating furnace, an incinerator, or a rotary kiln. Then, it is preferably heated at 900 ° C. or higher and 1200 ° C. or lower, preferably 950 ° C. or higher and 1100 ° C. or lower. The heating time is preferably from 30 minutes to 120 minutes. By the heat treatment, radioactive cesium is volatilized and removed from the soil to which the inorganic calcium compound and sodium chloride are added. The method for removing radioactive cesium from soil of the present invention is characterized by using an inorganic calcium compound and sodium chloride as a radioactive cesium volatilization promoter. In Non-Patent Document 2, when two types of inorganic reaction accelerators are used in combination, heating to about 1300 ° C. is necessary to make the radioactive cesium volatility 80% (Non-Patent Document). 2 of FIG. 1).

However, in the method for removing radioactive cesium of the present invention, by using an inorganic calcium compound and sodium chloride as a radioactive cesium volatilization promoter, about 80% of the radioactive cesium is obtained by heat treatment at 1200 ° C. or lower where the soil does not melt. It can be removed from real soil. Further, since the heating temperature is 1200 ° C. or lower, preferably 1100 ° C. or lower, components such as Si (silicon) in the soil are difficult to melt, and the possibility that the treated soil is sintered is low. Therefore, the soil after heat processing can be utilized similarly to the original soil.

  In order to volatilize radioactive cesium from the soil, it is necessary to heat to 900 ° C. or higher, and to increase volatilization efficiency and shorten the heat treatment time, it is necessary to heat to 950 ° C. or higher.

The amount of sodium chloride added to the soil is 1 to 3% by mass with respect to the mixture of soil and calcium compound, and the heating temperature is 950 ° C. or higher, preferably 1000 ° C. or higher. Further, since the compound containing chlorine atoms derived from sodium chloride is volatilized or decomposed and hardly remains in the soil, it is not necessary to perform a salt removal step on the heat-treated soil. If you do not salt removal step, an inorganic calcium compound remaining, since even when the amount is less acts as a soil improvement material, can be suitably reused soil after treatment.

<Recovery process>
The radioactive cesium volatilized from the soil by the heating process is discharged from the heating device together with dust and the like while being contained in the exhaust gas during the heating process. A secondary combustion device is usually provided downstream of the heating device, and the exhaust gas is retained in an atmosphere at 850 ° C. for 2 seconds or more in order to suppress the generation of dioxins and the like. The exhaust gas is supplied to a temperature reducing tower as necessary and cooled, and then supplied to dry dust collecting means such as a bag filter. As the dry dust collecting means, a HEPA filter or a cyclone can also be used. Further, the exhaust gas may be supplied to a wet dust collecting means such as a wet scrubber. Further, a plurality of dust collecting means may be used in combination. Since dust is generated from the soil by the heating process, the exhaust gas also contains dust. And radioactive cesium is contained in dust. By being supplied to the dry dust collecting means or the wet dust collecting means together with the exhaust gas, the dust in the exhaust gas is collected by the dry dust collecting means or the wet dust collecting means.

  When dry radioactive cesium is recovered using a bag filter, etc., the recovered material and the filters attached to the recovered material are stored in a container together with other harmful substances, etc. Alternatively, it may be vitrified by melting treatment and contained so as not to be eluted, and may also be buried in a managed disposal site. Further, after being mixed with concrete and solidified, it may be buried in a managed disposal site.

  The dust containing radioactive cesium collected by the dry dust collecting means may be washed, dissolved in the washing water, and adsorbed with radioactive cesium in the concentrated washing water using an adsorbent such as zeolite.

  In addition, radioactive cesium volatilized from the soil is volatilized as a compound that is easily dissolved in water, such as cesium chloride. Instead of a bag filter or a HEPA filter, such a cesium compound is dissolved in water using a wet scrubber. It is also possible to make it. The washing water of the wet scrubber may be filtered to separate a solid component that does not dissolve in water, and the radioactive cesium compound in the washing water may be separated by an adsorption treatment using an adsorbent. By performing the adsorption process, the amount of radioactive waste that must be disposed of can be reduced. As in the case of recovering radioactive cesium in a dry manner, the adsorbent may be stored in a container, subjected to a solidification process using a melting process or concrete, and may be buried in a managed disposal site.

  The exhaust gas that has passed through the bag filter is preferably supplied to a HEPA filter to remove very fine particles and then exhausted to the atmosphere.

<Salt removal process>
You may make it remove the sodium chloride contained by making the soil after a heating process contact with water. By adjusting the amount of sodium chloride added and the heating temperature, the compound containing chlorine atoms derived from the added sodium chloride is removed from the soil in the heating step, while it is derived from organic matter contained in the soil. In order to remove chlorine compounds contained in the soil after the heating process when there are many compounds containing chlorine atoms, when there are many chlorine atoms derived from seawater, or when the amount of sodium chloride added to the soil is large In addition, the salt removal step may be optionally performed.

As the salt removal method, a known salt removal treatment method can be employed. For example, a salt removal method of washing the soil after heat treatment with water can be employed. In this case, the sodium chloride eluted in the washing water can be collected by concentrating using an RO membrane device, and then deposited by evaporation. By salt removal step is also removed some of the added inorganic calcium compound or we generated calcium oxide in the soil.

  If the salt removal step is not performed, calcium oxide will remain in the soil after the heating step, but if there is a large amount of calcium oxide remaining, the calcium oxide will be sprayed with water during or after cooling. May be converted to calcium hydroxide. By doing in this way, it becomes possible to prevent the heat_generation | fever by the calcium oxide which is not intended outside a processing plant.

  In addition, when calcium carbonate or calcium hydroxide is added to the soil in the addition step, calcium carbonate or calcium hydroxide changes to calcium oxide in the heating step, so that calcium oxide remains in the soil after the heating step. .

  In addition, by contacting the soil with water vapor at a high temperature (for example, on the rear stage of the rotary kiln), the compound containing chlorine atoms derived from sodium chloride contained in the soil reacts with water vapor, and as chlorine gas A removal salt removal method may also be employed. In this case, the discharged chlorine gas may be processed by being mixed with the exhaust gas discharged in the heating process, or may be processed by the same method as the exhaust gas as a separate exhaust system.

[Embodiment 2]
In the first embodiment, the case where wet classification is performed in the classification process has been described. In the present embodiment, the case where dry classification is performed in the classification process will be described. FIG. 2 shows a process flow diagram of the second embodiment. Since the basic processing flow is the same as that of the first embodiment, the difference from the first embodiment shown in FIG. 1 will be described here.

  In the dry classification, classification may be performed in multiple stages as in the wet classification. For example, coarse classification may be performed as a pretreatment for dry classification, and coarse particles may be removed by classification so that the particle size is 40 mm or less. The removed coarse product is washed in the same manner as coarse particles described later, whereby fine particles adhering to the surface are removed, and a cleaned coarse product is obtained. The soil from which coarse substances have been removed is classified by a dry classifier so that the particle size is 1 mm or less.

  After dry classification of the soil, the coarse particles are washed with washing water. The washing method, the effect of washing, and the handling of coarse particles after washing are the same as in the first embodiment. On the other hand, in Embodiment 1, the water for classification containing fine particles obtained by wet classification and suspension water are mixed and dehydrated by a dehydrator, whereas in this embodiment, the suspended water after washing is single. Since the dehydration process is performed, there is an advantage that the dehydration apparatus itself can be downsized.

  About the remainder of the soil from which the coarse particles have been removed, the salinity concentration is measured using a known measuring device such as a soil salinity meter. Based on the measured salt concentration, the amount of sodium chloride added in the addition step is adjusted.

(Effect of added amount of radioactive cesium volatilization accelerator)
As soil, 1 kg of field soil in Fukushima Prefecture was collected. The radioactivity concentration of radioactive cesium in the soil was measured using a Ge semiconductor detector and found to be 10000 Bq / kg. To this radioactive cesium-containing soil, calcium carbonate which is 0 to 30% by mass of the soil amount was added. Furthermore, sodium chloride which becomes 0.5-5 mass% of the total amount of calcium carbonate and soil was added to the soil after the addition of calcium carbonate.

  Thereafter, a horizontal heating furnace was used as a soil heating apparatus, and 5 g of each soil sample was heated at 1000 ° C. for 60 minutes. During heating, each soil sample was in contact with air. After the heating, each soil sample was allowed to cool to room temperature. The removal rate of radioactive cesium in the soil was calculated from the radioactive concentration of Cs-134 and Cs-137 in the soil before heating and the radioactive concentration of Cs-134 and Cs-137 in the soil after heating.

Table 1 shows the relationship between the mixing ratio of calcium carbonate (CaCO ₃ ) and sodium chloride (NaCl), which are radioactive cesium volatilization promoters, and the radioactive cesium removal rate. When 70% by mass of calcium carbonate was added to 70% by mass of soil and 5% by mass of sodium chloride was added to this mixture, the radioactive cesium removal rate was 90% or more. The numerical value of the radioactive cesium removal rate in Table 1 represents a relative value when the removal rate of radioactive cesium in the case of 30% by mass of calcium carbonate and 5% by mass of sodium chloride is 1.00. When the amount of sodium chloride added is reduced, the radioactive cesium removal rate is confirmed to decrease. When the amount of sodium chloride added is 0.5% by mass, the rate of radioactive cesium removal is about 5% more than when 5% by mass is added. 40% lower. For this reason, it was considered necessary for the mixing ratio of sodium chloride to exceed 0.5% by mass and 5% by mass or less, more preferably 1% by mass to 5% by mass with respect to the soil and calcium carbonate mixture.

  In addition, when the mixing ratio of calcium carbonate was 3% by mass, the radioactive cesium removal rate did not decrease so much, but when calcium carbonate was not added, the radioactive cesium removal rate significantly decreased. For this reason, the mixing ratio of calcium carbonate was considered to be 3% by mass or more and 30% by mass or less.

  Even if the heating time was 30 minutes, 60 minutes, 90 minutes and 120 minutes, no significant change was observed in the removal rate of radioactive cesium, but when the heating time was 15 minutes, the removal rate of cesium decreased by 10% or more. did. For this reason, it was thought that a heating process needs to be 30 minutes or more in the case of 1000 degreeC.

(Calculation example)
When wet classification results in a dehydrated cake with a solid content (particulate soil) of 60% and a moisture content of 40%, and the sodium chloride concentration in the detachment liquid is 2% by mass, the sodium chloride concentration in the dehydrated cake is Calculated as 0.8 mass%. When this dehydrated cake is dried and the water content is 0%, the ratio of sodium chloride to soil is 1.3%. When 30% by mass of calcium carbonate is added to 70% by mass of the soil, the sodium chloride concentration in the mixture becomes 0.91% by mass. Here, in order to achieve a sodium chloride concentration of 5% by mass, it is necessary to mix sodium chloride corresponding to 4.09% by mass. In this way, by adjusting the amount of sodium chloride to be added in the addition process according to the concentration of sodium chloride remaining in the soil, approximately 1 point (5 mass% → 4 mass%) of sodium chloride equivalent Can be saved.

(Effects of the type of radioactive cesium volatilization accelerator)
In addition to calcium carbonate, 30% by mass of calcium oxide (CaO) is added, 5% by mass of sodium chloride is added to the mixture, and the radioactive cesium removal rate is 90% or more when heated at 1000 ° C for 60 minutes. there were.

(Influence of heating temperature)
When the addition amount of calcium carbonate is 30 mass%, the addition amount of sodium chloride is 5 mass%, and the heat treatment is 900 ° C for 60 minutes, the removal rate of radioactive cesium is 1000 ° C for 60 minutes compared to Decreased about 10%. On the other hand, the radioactive cesium removal rate did not increase so much even when the temperature was 1000 ° C. or higher. From the viewpoint of energy saving and soil melting prevention, it was considered practical to set the heating step to 1200 ° C. or lower.

  The method for removing cesium from soil of the present invention is useful in the field of soil treatment.

Claims (4)

A wet classification process for wet classification of soil containing radioactive cesium;
Wherein the wet classification step, as the ratio of the inorganic calcium compound in the mixture of the remainder and the inorganic calcium compound in soil coarse particles exceeding a particle diameter 1mm is removed is 3 mass% to 30 mass%, the soil At least one inorganic calcium compound selected from the group consisting of calcium oxide, calcium carbonate, and calcium hydroxide is added to the remainder of the soil, and 0.5 mass% with respect to the mass of the mixture of the remainder of the soil and the inorganic calcium compound An addition step of adding sodium chloride to exceed 5% by mass,
A heating step of volatilizing radioactive cesium from the soil after the addition step by heat treatment at 900 ° C. to 1200 ° C. for 30 minutes to 120 minutes,
A recovery process for recovering radioactive cesium volatilized from the soil by the heating process;
Have
After the wet classification step, by measuring the sodium chloride concentration in the effluent obtained when dewatering the remainder of the soil from which the coarse particles have been removed, the coarse particles are contained in the remainder of the soil from which the coarse particles have been removed. A method for removing radioactive cesium from soil, characterized in that the amount of sodium chloride is calculated and the amount of sodium chloride added in the adding step is adjusted. The method for removing radioactive cesium from soil according to claim 1 , wherein the amount of sodium chloride added in the adding step is 1% by mass or more and 3% by mass or less based on the mass of the mixture of soil and the inorganic calcium compound. . In the said collection | recovery process, the radioactive cesium volatilized from soil is collected by the 1 or more types of means selected from the group which consists of a bag filter, a HEPA filter, and a wet scrubber, The soil from the soil of Claim 1 or 2 Radiocesium removal method. The method for removing radioactive cesium from soil according to any one of claims 1 to 3 , further comprising a salt removal step of removing sodium chloride from the soil after the heating step.

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