JP2014174090A - Method for removing radioactive cesium from incineration ash - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for removing radioactive cesium from incineration ash that is capable of efficiently removing radioactive cesium from incineration ash of combustible waste at low cost.SOLUTION: This method includes: (1) an addition step for adding an inorganic calcium compound or an organic calcium compound generating calcium oxide in an oxidizing atmosphere of 500°C or more so that the ratio of the inorganic calcium compound or the organic calcium compound in a mixture of combustible waste and the inorganic calcium compound or the organic calcium compound is 3 to 30 mass%, and further adding sodium chloride so that it is more than 0.5 mass% and 10 mass% or less to the mass of the mixture of the combustible waste and the inorganic calcium compound or the organic calcium compound; (2) a heating step for volatilizing radioactive cesium from the incineration ash after the addition step, by applying heat treatment for 30 to 120 minutes at a temperature of 1000 to 1200°C; and (3) a recovery step for recovering radioactive cesium volatilized from the incineration ash in the heating step.

Description

The present invention adds an inorganic calcium compound or an organic calcium compound and sodium chloride to incinerated ash containing radioactive cesium (a radioactive cesium isotope such as ¹³⁴ Cs or ¹³⁷ Cs) and then heat-treats it. The present invention relates to a method for recovering radioactive cesium by volatilizing radioactive cesium and removing it from incineration ash.

  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 the residual soil or incinerated ash are combusted by the high-temperature combustion gas blown into 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 is removing the organic content or combustible content from the soil, it does not make the inorganic substance like a cesium compound into the removal object.

  The processing method of radioactive waste of patent document 2 is a method of fixing radioactive waste as a vitrified body, and processing at high temperature is required. Moreover, since it cannot reduce the volume of contaminated soil when applied to soil, it cannot be applied to a large amount of soil. In addition, there is a similar problem when the processing target is incinerated ash.

  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.

  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 a processed material to 1300 ° C or more, and energy consumption is large. Even when the treatment target is incinerated ash, the incinerated ash may melt at such a high temperature.

  In addition, combustible waste is usually incinerated using an incinerator, but some of the radioactive cesium contained in the combustible waste is transferred to fly ash released from the incinerator along with the exhaust gas. , Some have been reported to burn or remain in incineration ash without shifting to fly ash. For this reason, it is necessary to collect radioactive cesium not only from the fly ash in the exhaust gas discharged from the incinerator but also from the burnt ash or the main ash.

  In addition, according to the study by the present inventors, the radioactive cesium remaining in the incineration ash is difficult to volatilize even if it is heated at a high temperature for a long time in the incinerator. It was confirmed that it was difficult to remove. Since radioactive cesium contaminants are required to be reduced as much as possible, a method for efficiently volatilizing radioactive cesium from incinerated ash is required.

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

  When the present inventors heat-treat after adding an inorganic calcium compound or an organic calcium compound and sodium chloride to the incinerated ash of combustible waste containing cesium, the cesium removal disclosed in Non-Patent Document 1 It was found that cesium volatilizes from incineration ash at a heating temperature lower than that of the method.

  Therefore, the present inventors, as an additive for promoting cesium volatilization during heat treatment, by combining an inorganic calcium compound or an organic calcium compound and sodium chloride, from incineration ash at a temperature lower than the prior art related to soil treatment. A method for removing radioactive cesium was investigated. As a result, the present inventors have developed at least one inorganic calcium compound selected from the group consisting of calcium oxide, calcium carbonate, calcium phosphate, calcium silicate, calcium cyanamide, calcium sulfate and calcium nitrate, or oxidation at 500 ° C. or higher. It has been found that the above-mentioned problems can be solved by using an organic calcium compound that generates calcium oxide under an atmosphere and sodium chloride in combination, and the present invention has been completed.

Specifically, the present invention
Incineration ash containing radioactive cesium so that the proportion of inorganic calcium compound or organic calcium compound in the mixture of incineration ash containing radioactive cesium and inorganic calcium compound or organic calcium compound is 3% by mass or more and 30% by mass At least one inorganic calcium compound selected from the group consisting of calcium oxide, calcium carbonate, calcium hydroxide, calcium phosphate, calcium silicate, calcium cyanamide, calcium sulfate and calcium nitrate, or calcium oxide in an oxidizing atmosphere of 500 ° C. or higher An addition step of adding sodium chloride so as to be 1% by mass or more and 10% by mass or less based on the mass of the mixture of the incinerated ash and the inorganic calcium compound or the organic calcium compound,
A heating step of volatilizing radioactive cesium from the incinerated ash 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 incinerated ash by the heating process;
The present invention relates to a method for removing radioactive cesium from incineration ash having slag.

In the present invention, calcium oxide, calcium carbonate, calcium hydroxide, calcium phosphate (lime phosphate), calcium silicate (lime silicate), calcium cyanamide (lime nitrogen: CaCN ₂ ), calcium sulfate (gypsum) ) And at least one inorganic calcium compound selected from the group consisting of calcium nitrate (lime nitrate), or an organic calcium compound that produces calcium oxide in an oxygen atmosphere at 500 ° C. or higher, and sodium chloride. The radioactive cesium can be efficiently removed from the incinerated ash by heating at 900 ° C. to 1200 ° C., which is lower than 1300 ° C. in Non-Patent Document 1, for 30 minutes or more. This is because by adding an inorganic calcium compound or an organic calcium compound to the incineration ash, radioactive cesium is desorbed from the incineration ash, and further, the desorbed radioactive cesium is combined with chlorine atoms derived from sodium chloride, It is assumed that the contained radioactive cesium volatilizes as (radioactive) cesium chloride. As the inorganic calcium compound, at least one inorganic calcium compound selected from the group consisting of calcium oxide, calcium carbonate, and calcium hydroxide is particularly preferable.

  By heat-treating the incineration ash at 900 ° C to 1200 ° C for 30 minutes to 120 minutes, radioactive cesium volatilizes from the incinerated ash after the addition process, but the volatilized radioactive cesium is in the form of cesium chloride, It moves with the dust generated by the heat treatment. For this reason, if the generated dust is collected, radioactive cesium can be efficiently recovered and the amount of radioactive cesium remaining in the main ash can be reduced.

  Here, “incinerated ash” is used for agricultural waste (for example, rice straw or wheat straw), sawn timber, dewatered cake of sewage sludge, pruned branches, dead leaves, grass, paper, plastics, decontamination work. Ash (main ash) produced by incineration of combustible waste such as tyvek or clothing.

  Further, the “organic calcium compound that generates calcium oxide in an oxidizing atmosphere of 500 ° C. or higher” may be an organic calcium compound that generates calcium oxide in an oxidizing atmosphere of 500 ° C. or higher, and is less than 500 ° C. (for example, 300 Also included are organic calcium compounds that produce calcium oxide in an oxidizing atmosphere of ° C.

  In the recovery step, it is preferable to collect radioactive cesium volatilized from the incinerated ash by one or more means selected from the group consisting of a bag filter, a HEPA filter, and a wet scrubber.

  Simply volatilizing cesium from incineration ash will diffuse radioactive cesium into the atmosphere. In the present invention, radioactive cesium volatilized from incineration ash 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. It is desirable.

  According to the present invention, it is possible to remove radioactive cesium from incineration ash, which has been difficult in the past.

An example of the process flowchart of this invention 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.

<Incineration of combustible waste>
The combustible waste containing radioactive cesium is equipped with a collection means (for example, a bag filter, a HEPA filter or a wet scrubber) for collecting dust (fly ash) containing radioactive cesium that volatilizes into exhaust gas. Incinerated by an incinerator. Examples of such an incinerator include a stoker furnace, a fluidized bed furnace, and a kiln furnace.

<Addition process>
Incinerated ash containing radioactive cesium, at least one inorganic calcium compound selected from the group consisting of calcium oxide, calcium carbonate, calcium hydroxide, calcium phosphate, calcium silicate, calcium cyanamide, calcium sulfate and calcium nitrate, or 500 An organic calcium compound that generates calcium oxide in an oxidizing atmosphere at or above ° C is added. The addition amount of the inorganic compound or the organic calcium compound is adjusted so that the ratio of the inorganic compound or the organic calcium compound in the mixture of the incinerated ash and the inorganic compound or the organic calcium compound is 3% by mass or more and 30% by mass or less. Next, the incineration ash to which the inorganic calcium compound or the organic calcium compound is added becomes more than 0.5% by mass and 10% by mass or less with respect to the total amount of the mixture of the radioactive cesium-containing incineration ash and the inorganic calcium compound or the organic calcium compound. Add sodium chloride as follows.

  From the viewpoint of reducing the amount of hydrogen chloride derived from sodium chloride, the amount of sodium chloride added may be 10% by mass or less based on the total amount of the mixture of the radioactive cesium-containing incinerated ash and the inorganic calcium compound or the organic calcium compound. preferable. Moreover, from a viewpoint of volatilizing radioactive cesium from incineration ash reliably, the addition amount of sodium chloride shall be 1 mass% or more with respect to the total amount of the mixture of radioactive cesium containing incineration ash and an inorganic calcium compound or an organic calcium compound. It is preferably 3% by mass or more, more preferably 5% by mass or more.

  When adding an inorganic calcium compound or an organic calcium compound, the amount added to the incinerated ash is more preferably 10% by mass or less in order to reduce the amount of heat treatment after the addition.

  The additive may be added to the incinerated ash in a solid state, or may be dissolved in water and added to the incinerated ash in a liquid state.

  In addition, as for the addition order to incineration ash, either sodium chloride, an inorganic calcium compound, or an organic calcium compound may precede, and you may add both to incineration ash simultaneously.

  A general blender can be used for mixing the incinerated ash with the inorganic calcium compound or organic calcium compound and sodium chloride.

<Heating process>
Incineration ash to which inorganic calcium compound or organic calcium compound (organic calcium compound that produces calcium oxide in an oxidizing atmosphere of 500 ° C or higher) and sodium chloride is added is a heating device such as a heating furnace, an incinerator, or a rotary kiln. Supplied to. And it heats at 900 degreeC or more and 1200 degrees C or less, Preferably it is 950 degreeC or more and 1100 degrees C or less. The heating time is preferably from 30 minutes to 120 minutes. By the heat treatment, radioactive cesium is volatilized and removed from the incinerated ash to which the inorganic calcium compound or organic calcium compound and sodium chloride are added. The method for removing radioactive cesium from incinerated ash according to the present invention is characterized in that an inorganic calcium compound or an organic calcium compound and sodium chloride are used as a radioactive cesium volatilization accelerator. 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, the method for removing radioactive cesium from the incinerated ash of the present invention uses an inorganic calcium compound or an organic calcium compound and sodium chloride as a radioactive cesium volatilization accelerator, and heat treatment at 1200 ° C. or less, about 70% or more. It is possible to remove the radioactive cesium from the incinerated ash, and it is possible to reduce energy consumption.

  In order to volatilize radioactive cesium from incineration ash, 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.

<Recovery process>
The radioactive cesium volatilized from the incinerated ash by the heating process is discharged from the heating device into the exhaust gas together with dust while being contained in the exhaust gas during the heating process. Note that a secondary combustion device is usually provided downstream of the heating device, and is retained in the secondary combustion device in an atmosphere of 850 ° C. for 2 seconds or longer in order to suppress 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 collection means, a cyclone, a HEPA filter or an electric dust collector can be used in addition to the bag filter. Moreover, you may use a dry collection means combining multiple stages. By collecting the dust by the dry collection means or the wet collection means, it is possible to recover the radioactive cesium released from the incinerator.

  Instead of the dry collection means, a wet collection means such as a wet scrubber may be used. Moreover, you may use combining a dry-type collection means and a wet-type collection means. Further, a dry-type collecting means such as a bag filter and an exhaust gas treatment agent (for example, slaked lime, baking soda or activated carbon) may be used in combination. The exhaust gas that has passed through the collecting means is subjected to advanced treatment such as denitration treatment as necessary, and then exhausted to the atmosphere.

  In addition, when dust containing radioactive cesium is collected by a dry collection means such as a bag filter, the collected material and the filters attached to the collected material are stored in a container together with other harmful substances, It may be landfilled in a shielded management-type disposal site, or it may be vitrified by melting and contained so as not to elute, and similarly buried in the management-type disposal site. Further, after being mixed with concrete and solidified, it may be buried in a managed disposal site.

  Furthermore, the dust recovered in the recovery process may be mixed with the dust (fly ash) collected at the time of incineration of the combustible waste and then disposed of finally.

  If the concentration of radioactive cesium in the incinerated ash after the heat treatment is measured and it is determined that the radioactive cesium is not sufficiently removed, the incinerated ash may be subjected to the heating process again.

  ADVANTAGE OF THE INVENTION According to this invention, it is possible to volatilize radioactive cesium further from the cesium containing incineration ash which generate | occur | produces by incinerating the combustible waste containing a radioactive cesium.

(Effect of added amount of radioactive cesium volatilization accelerator)
As the incineration ash, the incineration ash generated when the general waste was incinerated and the dewatered cake incineration ash of agglomerated sludge were used. Cesium hydroxide (non-radioactive, manufacturer: MP Biomedicals, LLC) was added so that the cesium concentration in general waste incineration ash and dehydrated cake incineration ash (sludge incineration ash) was 10 ppm, respectively, and used as incineration ash samples . In addition, cesium hydroxide was added to the horticultural soil so that the concentration of cesium was 10.2 mg / kg to obtain a soil sample (reference sample). To these incinerated ash samples and reference samples, calcium carbonate was added so that the proportion of calcium carbonate in the mixture of sample and calcium carbonate would be 10-30% by mass. Furthermore, to the mixture after the addition of calcium carbonate, sodium carbonate which is 5% by mass of the total amount of calcium carbonate and the sample was added.

  Thereafter, a horizontal heating furnace was used as a heating device, and 5 g of each sample was heated at 1000 ° C. for 60 minutes. During heating, each sample was in contact with air. After heating, each sample is allowed to cool to room temperature, the sample is acid-decomposed, and the concentration of cesium in the sample (mg / kg) is determined by analyzing the cesium concentration in the decomposition solution using ICP-MS. It was measured. Furthermore, the cesium removal rate was computed from the cesium density | concentration in the sample before and behind heat processing.

Table 1 shows the relationship between the mixing ratio of calcium carbonate (CaCO ₃ ) and sodium chloride (NaCl), which are cesium volatilization promoters, and the cesium removal rate. When incineration ash (general waste incineration ash or sludge incineration ash) is the target, the cesium removal rate should be more than 75% if the amount of sodium chloride added is 3% by mass or more, and 5% by mass or more. Over 80%. When soil was the target, if the amount of sodium chloride added was 5% by mass, the cesium removal rate was over 80%, similar to incinerated ash. As shown in Table 1, it was confirmed that the method of removing cesium from the incineration ash according to the present invention volatilizes cesium from the incineration ash that is the target object to the same extent as the case where the soil is treated.

  When incineration ash was used as the object, when the amount of sodium chloride added was 3% by mass, the cesium removal rate was 76%. Moreover, it was confirmed that the cesium removal rate is 70% or more when the addition amount of sodium chloride is 1% by mass, and when the calcium carbonate is 10% by mass and 30% by mass.

(Effects of the type of radioactive cesium volatilization accelerator)
In addition, using soil as a sample, instead of calcium carbonate, 30% by mass of calcium oxide (CaO) is added, 5% by mass of sodium chloride is added to the mixture, and the radiation when heated at 1000 ° C for 60 minutes The cesium removal rate was almost the same as when calcium oxide was used. Since incinerated ash and soil show the same tendency, it is considered that calcium oxide can also be used in the treatment of incinerated ash.

(Influence of heating temperature)
Calcium carbonate was added to the soil so that the proportion of calcium carbonate in the mixture of soil and calcium carbonate was 30% by mass, the amount of sodium chloride was 5% by mass, and the heat treatment was 900 ° C. for 60 minutes. In some cases, the radioactive cesium removal rate was reduced by about 10% compared to the case of 1000 ° C. for 60 minutes. 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 incineration ash melting prevention, it was considered practical to set the heating step to 1200 ° C. or lower. In addition, it was considered that the same tendency was observed when incineration ash was treated.

  The method for removing cesium from incinerated ash according to the present invention is useful in the field of combustible waste treatment.

Claims (2)

Incineration ash containing radioactive cesium so that the proportion of inorganic calcium compound or organic calcium compound in the mixture of incineration ash containing radioactive cesium and inorganic calcium compound or organic calcium compound is 3% by mass or more and 30% by mass At least one inorganic calcium compound selected from the group consisting of calcium oxide, calcium carbonate, calcium hydroxide, calcium phosphate, calcium silicate, calcium cyanamide, calcium sulfate and calcium nitrate, or calcium oxide in an oxidizing atmosphere of 500 ° C. or higher An addition step of adding sodium chloride so as to be 1% by mass or more and 10% by mass or less based on the mass of the mixture of the incinerated ash and the inorganic calcium compound or the organic calcium compound,
A heating step of volatilizing radioactive cesium from the incinerated ash 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 incinerated ash by the heating process;
Method for removing radioactive cesium from incinerated ash having   In the said collection | recovery process, the radioactive cesium volatilized from the incineration ash is collected by one or more means selected from the group consisting of a bag filter, a HEPA filter, and a wet scrubber. Radiocesium removal method.

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