JP6957368B2 - How to remove radioactive cesium - Google Patents

Description

The present invention relates to a method for removing radioactive cesium from waste contaminated with radioactive cesium.

There is a problem that radioactive cesium released into the external environment due to a major accident at a nuclear power plant may be contained in waste or soil. Radioactive cesium (cesium-137) has a half-life of 30 years and can adversely affect the human body for a long period of time. Therefore, it is required to remove radioactive cesium from waste and the like.
As a method for removing radioactive cesium, Patent Document 1 describes a waste contaminated with radioactive cesium, a calcium oxide source, and a chlorine source, and heats the preparation to remove the radioactive cesium in the waste. After volatilizing, the gas generated by heating the formulation is cooled, the dust contained in the cooled gas is classified into coarse powder and fine powder, the coarse powder after classification is granulated, and the granulated product is prepared. A method for removing radioactive cesium, which is characterized by heating with an object, is described.
Further, in Patent Document 2, a waste contaminated with radioactive cesium, a calcium oxide source, and a chlorine source are mixed, and the preparation is heated to volatilize the radioactive cesium in the waste, and the preparation is described. The gas generated by the heating is cooled, the dust contained in the cooled gas is classified into coarse powder and fine powder, the coarse powder after classification is washed with water and separated into solid and liquid, and the coarse powder after solid and liquid separation is separated. A method for removing radioactive cesium is described, which comprises heating the solid side containing the above with the formulation.

JP-A-2017-62141 JP-A-2017-62143

An object of the present invention is to provide a method capable of removing more radioactive cesium from waste contaminated with radioactive cesium.

As a result of diligent studies to solve the above problems, the present inventor has obtained a mixed material by mixing a waste contaminated with radioactive cesium and a CaO source having a specific particle size distribution, and then obtains the mixed material. The present invention has been completed by finding that the above object can be achieved by a method of heating to volatilize radioactive cesium in waste.
That is, the present invention provides the following [1] to [4].
[1] Radiocesium including a mixing step of mixing a waste contaminated with radioactive cesium and a calcium source to obtain a mixed material, and a heating step of heating the mixed material to volatilize the radioactive cesium in the waste. The calcium source contains a CaO source, and the CaO source has a 100 μm sieving residual amount of 30 to 80% by mass, a 300 μm sieving residual amount of 15 to 50% by mass, and 600 μm. A method for removing radioactive cesium, characterized in that the amount of residual sieving is 15% by mass or less.
[2] After cooling the exhaust gas containing volatile radioactive cesium generated in the heating step, the dust contained in the exhaust gas is classified into coarse powder and fine powder, and the coarse powder obtained by the classification is mixed. The method for removing radioactive cesium according to the above [1], which is mixed with the waste and the calcium source in the step.
[3] The method for removing radioactive cesium according to the above [1] or [2], wherein the CaO source is one or more selected from the group consisting of quicklime, slaked lime and calcium carbonate.
[4] The method for removing radioactive cesium according to any one of [1] to [3] above, wherein the calcium source contains calcium chloride in addition to the CaO source.

According to the method for removing radioactive cesium of the present invention, radioactive cesium can be volatilized and removed by heating (for example, a high temperature of 1,200 ° C. or higher) from waste contaminated with radioactive cesium, and radioactive cesium can be removed. Since the concentration of radioactive cesium in the fired product obtained after removal is small, the fired product can be used more safely as an earthwork material or the like.
In addition, dust containing volatilized radioactive cesium becomes radioactive waste, but since the concentration of radioactive cesium in the dust is high and the amount of dust generated is small, long-term management as radioactive waste is required. The volume of by-products can be reduced.

The method for removing radioactive cesium of the present invention includes a mixing step of mixing a waste contaminated with radioactive cesium and a calcium source to obtain a mixed material, and heating the mixed material to volatilize the radioactive cesium in the waste. A method for removing radioactive cesium including a heating step, wherein the calcium source includes a CaO source, and the CaO source has a 100 μm sieving residual amount of 30 to 80% by mass and a 300 μm sieving residual amount of 15 to 50 mass. % And 600 μm sieving residual amount is 15% by mass or less.
The details will be described below.

[Mixing process]
This step is a step of mixing a waste contaminated with radioactive cesium and a calcium source to obtain a mixed material.
Examples of waste contaminated with radioactive cesium include soil, sewage sludge dry powder, municipal waste incineration ash, waste-derived molten slag, shells, vegetation and other general waste, sewage sludge, sewage slag, purified water sludge, etc. Examples include industrial waste such as construction sludge and disaster waste such as debris containing radioactive cesium. One of these may be used alone, or two or more thereof may be used in combination. In addition, the concentrated radioactive cesium (intermediate treatment) obtained by removing the portion containing almost no radioactive cesium (for example, sand and stone in the case of soil) in advance is also contaminated with "radioactive cesium" in the present invention. It shall be included in the concept of "waste".

In the present invention, the radioactive cesium means cesium-134 and cesium-137, which are radioactive isotopes of cesium. These radioactive cesiums are radioactive substances released into the external environment due to the accident at a nuclear power plant, and have half-lives of about 2 years and about 30 years, respectively.
Radioactive cesium, which is the object to be removed in the present invention, is released from the nuclear power plant that caused the accident into the external environment together with radioactive iodine in the form of cesium iodide, etc., and descends from the sky to the ground surface. Cesium iodide has a boiling point of 1200 ° C. or higher and has a property of being less likely to volatilize than cesium alone having a boiling point of about 700 ° C. Moreover, the radioactive cesium that has fallen to the ground surface is trapped in the clay minerals contained in the soil, making it difficult to separate from the soil, and the morphology may change. In addition, radioactive cesium that adheres to disaster waste such as debris or has fallen to the ground surface is washed away by rain and concentrated in the process of sewage treatment, resulting in sewage sludge containing radioactive cesium at a high concentration. Furthermore, vegetation is radioactively contaminated by absorbing radioactive cesium contained in the soil, and in the incinerator ash generated by incinerating those containing such radioactively contaminated vegetation, the radioactive cesium is trapped in glass or the like. Sometimes it is. The present invention attempts to separate and recover these difficult-to-treat radioactive cesium compounds.

The calcium source used in the present invention includes a CaO source.
Examples of CaO sources include quicklime (CaO), slaked lime (Ca (OH) ₂ ), calcium carbonate (CaCO ₃ ) and the like. These may be used individually by 1 type, and may be used in combination of 2 or more type. In the present specification, the term "calcium carbonate" shall include industrially produced calcium carbonate and natural limestone.
The 100 μm sieving residue of the CaO source used in the present invention is 30 to 80% by mass, preferably 40 to 70% by mass, more preferably 50 to 60% by mass, and particularly preferably 52 to 68% by mass. If the residual amount is less than 30% by mass, the amount of radioactive waste generated when removing radioactive cesium increases. When the residual amount exceeds 80% by mass, the volatilization amount of radioactive cesium becomes small.
The amount of the remaining 300 μm sieve of the CaO source is 15 to 50% by mass, preferably 20 to 40% by mass, more preferably 25 to 35% by mass, and particularly preferably 28 to 32% by mass. If the residual amount is less than 15% by mass, the amount of radioactive waste generated when removing radioactive cesium increases. When the residual amount exceeds 50% by mass, the volatilization amount of radioactive cesium becomes small.
The 600 μm sieve residual amount of the CaO source is 15% by mass or less, preferably 12% by mass or less, more preferably 10% by mass or less, and particularly preferably 9% by mass. When the residual amount exceeds 15% by mass, the volatilization amount of radioactive cesium becomes small.
The amount of the CaO source having a 1.18 mm sieve residue is preferably 0% by mass.
By using the CaO source having the above-mentioned particle size distribution, more radioactive cesium can be volatilized from the waste contaminated with radioactive cesium, and a calcined product having a low concentration of radioactive cesium can be obtained.
Further, when a heating device such as an internal combustion type rotary kiln is used, the amount of radioactive waste generated in the removal of radioactive cesium can be further reduced.

The average particle size of the CaO source is preferably 50 to 200 μm, more preferably 80 to 150 μm, and particularly preferably 100 to 140 μm. When the average particle size is 50 μm or more, the amount of radioactive waste generated when removing radioactive cesium can be further reduced. When the average particle size is 200 μm or less, the amount of radioactive cesium volatilized can be increased.
In the present specification, the "average particle size" refers to the particle size of particles measured by using a laser diffraction type measuring device (for example, "MT3300EXII" manufactured by Microtrac Co., Ltd.). It refers to the particle size (median diameter; d50) at a volume cumulative distribution of 50% obtained based on the diameter.
The volume cumulative distribution means a distribution in which all particles are integrated and accumulated from the smallest side in volume order, and the particle size (median diameter; d50) at 50% of the volume cumulative distribution is the volume cumulative distribution value. The diameter of the particles with respect to the cumulative ratio of 50%.

In a mixed material obtained by mixing a waste contaminated with radioactive cesium and a calcium source, the mass ratio (CaO / SiO _{2 ) of calcium oxide (CaO) and silicon dioxide (SiO 2} ) in the mixed material is preferable. 1.0 to 3.7, more preferably 1.2 to 3.5, still more preferably 1.4 to 3.3, still more preferably 1.6 to 2.7, particularly preferably 1.8 to 2. It is preferable to determine the type and blending ratio of the waste and the calcium source so as to be 5.
When the mass ratio is 1.0 or more, the volatilization amount of radioactive cesium can be increased. When the mass ratio is 3.7 or less, the amount of free lime contained in the obtained fired product is small, and the fired product does not become powdery, so that the fired product can be used as a civil engineering material. Can be a thing. When the mass ratio is 2.7 or less, the amount of potassium and sodium contained in the mixed material is reduced, and the amount of radioactive waste generated in the removal of radioactive cesium is reduced. Can be done.

From the viewpoint of promoting the volatilization of radioactive cesium and reducing the amount of radioactive waste generated in the removal of radioactive cesium, calcium chloride (CaCl ₂ ) is used as the calcium source in addition to the CaO source described above. You may.
The molar ratio (Cl / k) of chlorine and potassium in the mixed material of calcium chloride is preferably 1.4 or less, more preferably 0.01 to 1.3, and particularly preferably 0.1 to 1.2. It is preferable to use an amount that becomes. When the molar ratio is 1.4 or less, potassium and sodium are not volatilized and more radioactive cesium is volatilized, so that the amount of radioactive waste generated in the removal of radioactive cesium can be further reduced.
In addition, chlorides such as potassium chloride and sodium chloride may be used for the purpose of keeping the molar ratio (Cl / k) of chlorine and potassium in the mixed material within the above numerical range.

In a mixed material obtained by mixing a waste contaminated with radioactive cesium and a calcium source, silicon dioxide: SiO ₂ and aluminum oxide: Al ₂ O ₃ and iron (III) oxide (III): Fe ₂ O _{3 in the mixed material.} The total mass ratio (SiO ₂ / (Al ₂ O ₃ + Fe ₂ O ₃ )) is preferably 2.0 or more, more preferably 2.2 or more, and particularly preferably 2.5 or more. When the mass ratio is 2.0 or more, it is possible to prevent the mixed material from being difficult to melt during heating and to prevent the radioactive cesium from being easily volatilized by being incorporated into the liquid phase generated by the melting. can.

The mixing method of each material is not particularly limited, and examples thereof include a method of mixing each material using a blender.
Further, when firing using a rotary kiln described later, since each material is rotationally mixed in the rotary kiln, all or a part of each material may be directly put into the kiln tail of the rotary kiln.
The order in which each material is mixed is not particularly limited, and waste contaminated with radioactive cesium, a CaO source, and other materials (calcium chloride, crude powder (described later), etc.) may be mixed at the same time. After mixing the waste contaminated with radioactive cesium and the CaO source to obtain a mixture, the mixture may be mixed with other materials (calcium chloride, crude powder (described later), etc.).
The mixed material is preferably smaller than a granular material having a size of about 5 mm from the viewpoint of volatilizing more radioactive cesium. Further, stones having a size of more than 5 mm do not contain a large amount of radioactive cesium, and may be removed by washing with water in advance.

[Heating process]
This step is a step of heating the mixed material obtained in the mixing step to volatilize the radioactive cesium contained in the mixed material in the waste contaminated with radioactive cesium.
The heating temperature of the mixed material is preferably 1,200 to 1,550 ° C, more preferably 1,240 to 1,450 ° C, still more preferably 1,260 to 1,350 ° C, and particularly preferably 1,280 to 1. , 340 ° C.
When the heating temperature is 1,200 ° C. or higher, the amount of radioactive cesium volatilized can be increased. When the heating temperature is 1,550 ° C. or lower, the formation of a liquid phase can prevent the incorporation of radioactive cesium into the liquid phase.
The heating time is preferably 15 minutes or longer, more preferably 30 minutes or longer, from the viewpoint of obtaining a sufficient amount of radioactive cesium volatilized. The upper limit of the heating time is not particularly limited, but is preferably 180 minutes or less, more preferably 120 minutes or less. When the heating time exceeds 180 minutes, the amount of potassium and sodium volatilized increases together with the radioactive cesium in the mixed material.
When the raw material rolls, such as in a rotary kiln, the contact rate between the gas and radioactive cesium increases and the thermal conductivity also improves, so a high volatility can be obtained in a shorter firing time than in a standing condition. can.

As the heating means, either a continuous type or a batch type can be used.
An example of a continuous heating means is a rotary kiln or the like.
Examples of batch-type heating means include an incinerator, an electric furnace, a microwave heating device, and the like.
Above all, the continuous heating means is preferably used in the present invention from the viewpoint of increasing the efficiency of treatment. Further, the rotary kiln is suitable from the viewpoint that a heating temperature suitable for volatilization of radioactive cesium and a residence time of waste can be easily provided.

Exhaust gas containing volatile radioactive cesium generated in the heating step is usually cooled to a solid and then recovered by a dust collector, a scrubber, or the like. The recovered radioactive cesium is further reduced in volume by washing with water, adsorption, etc. as necessary, and then stored in a concrete container or the like in a sealed manner. As a result, the volume of radioactive waste can be reduced and stored without leaking to the outside.

In addition to the volatilized radioactive cesium, the exhaust gas contains volatilized alkali metals and materials (mainly calcium and silica components) scattered during heating. After cooling the exhaust gas, it is preferable to classify the dust (material scattered by heating, etc.) contained in the exhaust gas into coarse powder and fine powder.
Here, the coarse powder is referred to as dust having a relatively large particle size, which contains a large amount of material scattered by heating. The coarse powder has a large amount of calcium and silica components and a small amount of radioactive cesium as compared with the fine powder. The particle size of the coarse powder varies depending on the ability of the classification means (for example, cyclone), but is usually 0.005 mm or more, preferably 0.3 to 1.0 mm, and particularly preferably 0.5 to 0.5 mm. When the particle size is 0.005 mm or more, the amount of radioactive cesium contained in the coarse particles can be reduced and the amount of radioactive waste can be reduced.
Further, the fine powder refers to dust containing a large amount of high-concentration radioactive cesium, which is recovered after removing the coarse powder by classification.
The value of the particle size corresponds to the opening size of the sieve.

From the viewpoint of reducing the amount of radioactive waste, the coarse powder obtained by classification is mixed with other materials (waste contaminated with radioactive cesium, etc.) in the above-mentioned mixing step, and then in the heating step again. It is preferable to heat it. The fine powder obtained by classification is treated as radioactive waste.
More specifically, in an internal combustion type rotary kiln, the exhaust gas discharged from the rotary kiln is cooled in a cooling tower and then classified using a cyclone or the like, and then the collected coarse powder is put into a blender or the like. The amount of radioactive waste finally generated can be reduced by mixing the materials of the above, or by adding the above-mentioned coarse powder from the kiln tail of the rotary kiln and heating it again together with other materials. ..

The fired product obtained after heating has a low concentration of radioactive cesium. The fired product can be crushed as necessary and used as a cement mixture, aggregate (aggregate for concrete, aggregate for asphalt), civil engineering material (backfill material, embankment material, roadbed material, etc.), etc. can.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to these Examples.
[Material used]
The materials used are as shown below.
(1) Waste A; Fly ash (incinerated ash) recovered from the fluidized bed furnace, radioactive cesium concentration: 27,215 Bq / kg (cesium 137: 23,476 Bq / kg, cesium 134: 3,739 Bq / kg) The chemical composition is shown in Table 1. (2) Waste B; Fly ash (incineration ash) recovered from a fluidized bed furnace having a radioactive cesium concentration of 38,800 Bq / kg and a water content of 15% by mass. Was dried using a dryer until the water content was 2% by mass or less, and then crushed using a crusher so that the residual amount of the 100 μm sieve was about 20% by mass. (3) Quick ash Powders A to B (indicated as quicklimes A to B in Tables 2 to 4); particle size distribution and average particle size of each quicklime powder measured using a laser diffraction type particle size distribution measuring device (MT3300EXII manufactured by Microtrac). (4) Calcium oxide powder: Reagent (5) Silica powder

[Example 1]
299 g of waste A, 389 g of quicklime powder A, and 12.2 g of calcium chloride powder were put into a polyethylene bag and then mixed by hand for 1 minute to obtain a mixed material. The mass ratio of CaO and SiO ₂ (CaO / SiO ₂ ) in the obtained mixed material is 2.5, the molar ratio of Cl and K (Cl / K) is 1.1, and SiO ₂ and Al. The total mass ratio of ₂ O ₃ and Fe ₂ O _{3 (SiO 2} / (Al ₂ O ₃ + Fe ₂ O ₃ )) was 2.3.
After putting 50 g of the obtained mixed material into an alumina crucible, put it in a box-shaped electric furnace preheated to the temperature shown in Table 3 and heat it for 1 hour while keeping the temperature at the temperature shown in Table 3. went.
Immediately after heating, the crucible was taken out, and the concentration of radioactive cesium in the fired product (mixed material after heating) was measured using a germanium semiconductor detector.
[Comparative Example 1]
A calcined product was obtained in the same manner as in Example 1 except that quick lime powder B was used instead of the quick lime powder A, and then the concentration of radioactive cesium in the calcined product was measured. The quicklime powder B is generally commercially available.
The results of each are shown in Table 3.

From Table 3, when the fresh lime powder A was used (Example 1), the fresh lime powder B was used as the radioactive cesium concentration (1,300 ° C: 81 Bq / kg, 1,325 ° C: 57 Bq / kg) of the calcined product. It can be seen that it is smaller than the radioactive cesium concentration (1,300 ° C.: 121 Bq / kg, 1,325 ° C.: 69 Bq / kg) of the fired product in the case (Comparative Example 1). From this, it can be seen that according to the method for removing radioactive cesium of the present invention, more radioactive cesium can be removed from the waste contaminated with radioactive cesium.

[Example 2]
Waste B is put into the blending tank, and quicklime powder A, calcium chloride powder, and silica stone powder are put into the blending tank by a route different from that of waste B, and each raw material is mixed in the blender to obtain a mixed material. rice field.
In the mixing of the raw materials, the _{mass ratio of CaO and SiO 2} (CaO / SiO ₂ ) in the mixed material is 3.2, and the total mass ratio of _{SiO 2} and Al ₂ O ₃ and Fe ₂ O _{3 (SiO 2).} / (Al ₂ O ₃ + Fe ₂ O ₃ )) was 3.0, and the blending amount of each raw material was determined so that the molar ratio of Cl to K (Cl / K) was 1.1.
The obtained mixed material was put into a rotary furnace (recycling furnace) having an inner diameter of φ1.0 m (brick thickness of 150 mm) × length of 15 m and an inclination of 3 °, and heated using a heavy oil burner.
The heating is performed under the conditions that the amount of the mixed material fed into the rotary furnace: 290 kg / h, the rotation speed of the rotary furnace: 90 rf, the amount of heavy oil: 78 to 82 liters / h, and the firing temperature: 1,250 to 1,300 ° C. I went there.

After heating, it was cooled using a cooler to obtain a fired product. On the other hand, the exhaust gas generated in the rotary furnace containing alkali metals and radioactive cesium volatilized by heating is cooled in the cooling tower and coarse powder in the cyclone (in the rotary furnace, those containing a large amount of each raw material scattered during heating). And fine powder (those containing a large amount of volatilized radioactive cesium). The coarse powder classified in the cyclone was mixed with the above-mentioned mixed material in front of the rotary furnace and put into the rotary furnace again. The particle size of the coarse particles classified by the cyclone was 5 to 150 μm. In addition, the fine powder classified in the cyclone was collected as a by-product in the bag filter.

The concentration of radioactive cesium in the obtained fired product was measured for each batch using a clearance monitor, with each batch being 40 kg. After measuring for all batches (313 batches), the average value was calculated.
In addition, the concentration of radioactive cesium in the by-product (fine powder) collected by the bag filter was measured using a germanium semiconductor detector. In addition, the Ca concentration in the by-product was measured by XRF order analysis.
Furthermore, the amount of by-products generated per day when the above heating was continuously performed was calculated.

[Comparative Example 2]
A fired product was obtained by heating in the same manner as in Example 2 except that quicklime powder B was used instead of quicklime powder A.
The concentration of radioactive cesium in the obtained fired product was measured in the same manner as in Example 2. In the calculation of the concentration of radioactive cesium, the total number of batches was 325 batches.
The results are shown in Table 4.

From Table 4, the average value (40 Bq / kg) per batch of the radioactive cesium concentration of the calcined product when the fresh lime powder A was used (Example 2) was the case where the fresh lime powder B was used (Comparative Example 2). It can be seen that the concentration of radioactive cesium in the fired product is smaller than the average value per batch (55 Bq / kg).
Further, the radioactive cesium concentration (462,000 Bq / kg) of the by-product when the fresh lime powder A was used (Example 2) is the radioactive cesium concentration (402,) of the by-product when the fresh lime powder B was used (Comparative Example 2). It can be seen that it is larger than 000 Bq / kg).
From this, it can be seen that according to the method for removing radioactive cesium of the present invention, more radioactive cesium can be removed from the waste contaminated with radioactive cesium.
Further, the by-product generation amount (169 kg / day) when the quicklime powder A is used (Example 2) is smaller than the by-product generation amount (182 kg / day) when the quicklime powder B is used (Comparative Example 2). I understand.
From this, it can be seen that according to the method for removing radioactive cesium of the present invention, the amount of radioactive waste generated in the removal of radioactive cesium can be further reduced.

Claims (4)

A mixing process that mixes radioactive cesium-contaminated waste with a calcium source to obtain a mixed material,
A method for removing radioactive cesium, which comprises a heating step of heating the mixed material to volatilize the radioactive cesium in the waste.
The calcium source contains a CaO source, and the CaO source has a 100 μm sieve residual amount of 30 to 80% by mass, a 300 μm sieve residual amount of 15 to 50% by mass, and a 600 μm sieve residual amount of 15% by mass. A method for removing radioactive cesium, which is characterized by the following. After cooling the exhaust gas containing volatile radioactive cesium generated in the heating step, the dust contained in the exhaust gas is classified into coarse powder and fine powder, and the coarse powder obtained by the classification is obtained in the mixing step. The method for removing radioactive cesium according to claim 1, which is mixed with the waste and the calcium source. The method for removing radioactive cesium according to claim 1 or 2, wherein the CaO source is one or more selected from the group consisting of quicklime, slaked lime and calcium carbonate. The method for removing radioactive cesium according to any one of claims 1 to 3, wherein the calcium source contains calcium chloride in addition to the CaO source.