JP5925016B2 - Decontamination method for removing radioactive cesium from combustible materials with radioactive cesium attached - Google Patents

Description

The present invention relates to a method for safely treating incinerated ash or fly ash by removing the radioactive cesium contained in the incinerated ash or fly ash generated when the combustible material to which radioactive cesium is attached is incinerated.

A large amount of radioactive material has been released from the Tokyo Electric Power Fukushima Daini Nuclear Power Station, which was damaged by the Great East Japan Earthquake, which has become a major social problem. Among them, radioactive cesium 137 (hereinafter sometimes referred to as “ ¹³⁷ Cs”) has a long half-life of 30 years, and it is considered that the effect will continue over a long period of time, and an immediate solution to this problem is required. Yes.

The released radioactive cesium, which was carried over a long distance by the wind, was taken to the ground by rain, so it piled up on the roof, flowed into the gutters and gutters, and adhered to crops and fallen leaves. Yes. Combustible materials such as fallen leaves should be collected and incinerated in a municipal waste incinerator, but radioactive cesium is concentrated in incineration ash or fly ash. In severely damaged areas, incineration ash has been found to contain high concentrations of radioactive material exceeding 100,000 becquerels / kg. The current situation is that each local government is struggling with countermeasures because it cannot be incinerated. On August 31, 2011, the Ministry of the Environment showed that 8,000 to 100,000 Bq / kg of incinerated ash and fly ash were solidified into cement and then reclaimed. Cement solidification is intended to prevent radioactive cesium from eluting when it comes into contact with water, and is handled in accordance with conventional methods for treating radioactive waste. However, cement solidification is a treatment method in which the volume is increased by adding a solidifying agent to incineration ash and fly ash, and there are many problems in treating a large amount of ash.

Table 1 below shows the amount of radioactivity in waste generated at a municipal waste disposal facility in Sakai City, Chiba Prefecture, from the end of June to the beginning of July 2011. The data obtained in this way was published by Sakai City on July 11.

Table 1

As for Sakai City data, it is noted that high concentration of radioactive cesium was detected from the solidified fly ash. (Measurement was also performed for radioactive iodine 131, but radioiodine 131 with a short half-life is not shown in the table because it appears to have already decayed at the time of measurement and was not detected for any waste.) In Sakai City, there are two cleaning factories and one final disposal site, but the cleaning plant called the Northern Clean Center does not have a facility for melting incinerated ash, so fly ash is solidified. There is an interesting relationship between each sample to be measured and the measurement value obtained thereby.

As is well known, cesium is an alkali metal, which itself is a low-melting-point metal, and the melting point is 645 ° C., even considering the most likely chloride in the incineration ash or fly ash. The boiling point is 1245 ° C. Incineration ash / fly ash of a cleaning plant that does not have a melting facility, fly ash contains 7,000 to 10,000 Bq / kg of radioactive cesium, and main ash contains 2,000 to 3,000 Bq / kg. It is the result. This is because the combustion temperature of the incinerator is 800 to 900 ° C. and the boiling point of cesium chloride does not exceed, but in the incinerator, cesium chloride is separated from the liquid phase / gas phase, and according to the distribution rate at this temperature, It is understood that the difference in the main ash / fly ash concentration appears.

On the other hand, in the fly ash of the southern clean center having the ash melting facility, it can be seen that radioactive cesium is concentrated 10 times or more compared with the fly ash of the northern clean center. In particular, the radioactive cesium concentration in the molten slag is in the range of 300 to 400 Bq / kg, whereas that in the molten fly ash is 60,000 to 80,000 Bq / kg. This is because the ash melting process is performed at a much higher temperature than incineration, so cesium compounds such as cesium chloride are almost completely vaporized and do not remain in the molten slag, but are collected together with the fly ash collected. It is understood. In any case, this can be said to be a preferable phenomenon from the viewpoint of concentration and volume reduction as radioactive waste.

On the other hand, according to the data provided by the National Institute for Environmental Studies, when efforts are made to elute radioactive cesium with water, the relationship between various pollutants or wastes and the elution ratio from there is as shown in Table 2 below. is there.

Table 2

The inventors have stated that when ash is processed by an ash melting facility, radioactive cesium is concentrated to molten fly ash at a high concentration, and radioactive cesium concentrated in molten fly ash is almost completely extracted by water. In fact, it was concluded that this route is the most efficient way to decontaminate radioactive cesium. Radiocesium that has fallen to urban areas is washed with rainwater, gathers at a sewage treatment plant, and finally concentrates on sludge. The sludge is incinerated to become ash. The radioactive cesium that has fallen in the mountains is also concentrated as ash by collecting dead leaves and thinned trees and incinerating them. Since the radioactive cesium concentrated in the ash is concentrated in the molten fly ash and then eluted with water, a highly concentrated radioactive cesium aqueous solution can be obtained. The idea is that the detoxification treatment will eventually enable the treatment of radioactive cesium with a high volume reduction rate.

The purpose of the present invention is to put the above-described idea of the inventor into practical use, from the incinerated ash or fly ash generated when the combustible material to which the radioactive cesium adheres is incinerated, to efficiently contain the radioactive cesium contained therein, and The object of the present invention is to provide a method for effectively eliminating decontamination by almost eliminating the problem of the treatment plant by removing it with a high volume reduction.

The decontamination method of the present invention that achieves this object is a decontamination method for removing radioactive cesium from a combustible material to which radioactive cesium has adhered, and basically comprises the following steps:
A) Combustible material with radioactive cesium attached to it is incinerated in an incinerator to produce incinerated ash or fly ash.
B) Incineration ash or fly ash is melted in an ash melting facility, and radioactive cesium is concentrated in the molten fly ash.
C) Extracting water-soluble radioactive cesium compound by washing molten fly ash with water to obtain an aqueous solution thereof,
D) The adsorbent is brought into contact with an aqueous solution of a radioactive cesium compound and an aggregating precipitant is added to adsorb the radioactive cesium ions to the adsorbent, and then the adsorbent is agglomerated and precipitated, and
E) Separating the aggregated precipitate of the adsorbent adsorbing radioactive cesium by solid-liquid separation.
Since the liquid produced by the solid-liquid separation no longer contains harmful radioactive cesium, it can usually be discharged as it is or after necessary treatment.

The specific aspect of the decontamination method of the present invention is first implemented using an adsorbent containing mordenite-type zeolite as an active ingredient as the adsorbent. The second is carried out using a ferrocyanide adsorbent that selectively adsorbs cesium.

According to the decontamination method of the present invention, the overwhelming majority of the radioactive cesium adhering to the combustible material is present in a highly concentrated form by moving into the molten fly ash, and a very small portion is present in the molten slag. Since it only migrates, the decontamination effect is extremely high. In the case of Table 1 described above, the content of radioactive cesium was 60 to 70,000 Bq / kg in the molten fly ash, while it was only 300 to 500 Bq / kg in the molten slag. The ratio is 100: 1 or more. The radioactive cesium in the molten slag that has not been decontaminated by the present invention has a low concentration and can be disposed of as it is.

The high concentration of radioactive cesium means that the volume of secondary waste is extremely small and a high volume reduction is realized. Decontamination, as in this accident, in a situation where the impact is widespread and therefore the amount of contaminants to be treated is huge and decontamination has to be continued for a considerable number of years However, it is urgently important to deal with a large amount of secondary waste generated as a result of this, but if the volume is reduced to a high degree by the present invention, this problem is remarkably reduced. In particular, according to the second aspect of the present invention, since a higher concentration can be achieved, the amount of secondary waste generated can be further reduced.

The decontamination method of the present invention has no special equipment and chemicals necessary for its implementation, and can be easily carried out by those who have ordinary knowledge and experience. Therefore, in many local governments and clean centers under its control. Can be implemented easily and at low cost, and the above-mentioned problems can be solved.

The objects that can be treated by the decontamination method of the present invention include a wide range because all of the objects that are to be incinerated are included. Specifically, it includes municipal waste with radioactive cesium, combustibles in rubble generated by the earthquake, and fallen leaves from homes, parks and street trees. The radioactive cesium transported to the sewage generated by the decontamination work on the roofs, roads, and gutters of the building is transported to the sewage and added to the sewage. Are also subject to processing. The radioactive cesium that has fallen into the forests is attached to the leaves of the trees and leaves the leaves, and the trees that have entered the groundwater from the ground are sucked up and held, but will eventually be included in the leaves. It can be expected that there will be no substantial impact in the long run by continuing processing and thinning of trees. The decontamination method of the present invention is useful for such work.

As water for extracting the radioactive cesium compound from the molten fly ash, it can be said that it is advantageous to use not only mere water but also water added with a substance that enhances the cleaning effect such as a surfactant. In the above step (C), “washing with water” is a term including washing with an aqueous solution to which such an additive is added.

As an adsorbent for separating radioactive cesium from an aqueous solution containing a radioactive cesium compound, in the first embodiment, mordenite-type zeolite is used as described above. Zeolite is a universal adsorbent due to its availability, balance between performance and cost, and ease of use. In the second embodiment, ferrocyanide is used. On the other hand, the flocculant can be selected from a wide range of polymer or inorganic flocculants. A typical one is an iron-based flocculant.

A representative of ferrocyanides is a group of inorganic pigments called “Prussian blue”. These substances are known to have the ability to selectively adsorb cesium. Even if a large amount of ions of alkali metals such as sodium and potassium and alkaline earth metals such as calcium coexist in a cesium compound aqueous solution. These adsorbents are advantageous in that they do not deprive the adsorption sites and reduce the adsorption efficiency of cesium, and can maintain a high degree of volume reduction. A typical example of “Prussian blue” is iron potassium hexacyanoferrate of the formula KFe [Fe (CN) ₆ ], which has the name “potash blue”, but K is replaced by Na or NH _4. Edge compounds can be used as well. For example, use of potassium ferrocyanide, which is a compound of the same kind, has also been proposed (Patent Document 1).

As a mechanism for selectively adsorbing cesium by Prussian blue (hereinafter abbreviated as “PB”), the hydration radius of cesium ions matches the size of the internal pores of PB and is adsorbed there. It is considered. Recently, the adsorption capacity of nanoparticulated PB increases as the specific surface area increases, and therefore the results of mass production techniques and performance tests have been published on the assumption that its use is effective (Non-patent Document 1). According to the announcement, PB nanoparticles (particle size 60 μm, 11 μm) are more adsorbed in a region where the liquid-solid ratio (volume of cesium-containing aqueous solution to be treated / weight of adsorbent) is larger than PB commercial products. Show performance. Thus, it has been proposed to use PB nanoparticles to remove radioactive cesium from municipal waste incineration ash contaminated with radioactive cesium.
JP 2008-526833 A (National Institute of Advanced Industrial Science and Technology) Press Release February 8, 2012

In the production of PB, when obtaining an inorganic pigment, a step of reacting ferrous sulfate with potassium ferrocyanide and adding an oxidizing agent is usually employed, but when ferric sulfate is used instead of ferrous sulfate, Ferric ferrocyanide Fe ₄ [Fe (CN) ₆ ] _{3 in} which potassium of ferric ferrocyanide was replaced with iron was obtained, and it was confirmed that this was also useful as a selective adsorbent for cesium. And included in the PB used in the present invention.

Municipal waste incineration ash and fly ash often contain harmful heavy metals such as chromium, lead, cadmium or arsenic. For the purpose of preventing the heavy metals from leaching and damaging as ash is disposed of, heavy metal immobilization treatment using a liquid chelating agent is often performed. In the practice of the present invention, if the molten fly ash present in the presence of concentrated radioactive cesium contains harmful heavy metals in addition to it, it is harmful if the above steps (A) to (E) are simply carried out. Since there is a risk that heavy metals will be mixed into the discharged water, before the step (C) of washing the molten fly ash with water,
F) Applying a liquid chelating agent to the molten fly ash to fix heavy metals,
It is desirable to join the process. Exemplary liquid chelating agent, there are salts of di-thiocarbamate, even in this case they are useful.

The decontamination method of the present invention can further reduce the volume of secondary waste by repeating the operation. That is, the sludge containing the adsorbent and the flocculant recovered in any of the decontamination consisting of the steps (A) to (E) or the decontamination performed in the step (F) prior to the step (C). The molten fly ash in which radioactive cesium is further concentrated is obtained by melting again in the ash melting facility, and an operation for further concentrating the radioactive cesium by treating this by steps (C) to (E) is performed as necessary. By repeating, the volume of secondary waste is significantly reduced. This aspect is an embodiment that should be recommended in some situations.

Reference example

Since the leachate showed a radiation dose of 145 Bq / L (mostly derived from ¹³⁷ Cs and ¹³⁴ Cs) at the final disposal site for municipal waste, mordenite zeolite was added to 2.5, 5 or 25 g / Attempts were made to add L and 50 ppm of an iron-based flocculant for aggregation and precipitation. The radiation dose was measured using a germanium semiconductor detector (Canberra Model GC2520), and the results shown in FIG. 1 were obtained. The radiation dose of the leachate to which 25 g / L of mordenite zeolite was added was below the detection limit of 10 Bq / L.

A combustible material with radioactive cesium was incinerated at a municipal waste incineration plant with an ash melting facility. The main ash and fly ash of the incinerator were transferred to an ash melting facility and melted to obtain molten slag and dust collected fly ash. 3 kg of water was added to 2 kg of the molten fly ash, stirred for 20 minutes, and filtered to obtain an extract. The radiation dose of the extract (also derived from ¹³⁷ Cs and ¹³⁴ Cs) was 4,000 Bq / L. To this, 3, 5, 25, or 50 g / L of the mordenite zeolite used in the Reference Example and 50 ppm of an iron-based flocculant were adsorbed and coagulated and precipitated. Since precipitation proceeded rapidly, the radiation dose after 10 minutes of each extract was measured to obtain the graph shown in FIG.

As a precaution, regarding the exhaust from the ash melting facility, the presence or absence of radioactive substances was confirmed by three test methods: the cylindrical filter paper method, the drain method, and the activated carbon filter method. The results were as follows: radioactive ¹³¹ I, radioactive ¹³⁷ Cs and ¹³⁴ Cs. None were detected.

The molten fly ash was treated in the same manner as in Example 1 to obtain a water extract containing a radioactive cesium compound. The radiation dose of this water extract was 8,900 Bq / kg. To this solution, 0.05, 0.1, 0.2 or 1 g / L of PB (potassium ferrocyanide) was added and stirred sufficiently to adsorb and remove radioactive cesium, and then PB was separated by filtration. The relationship between the radiation dose of the filtrate and the amount of added PB is shown in the graph of FIG. When 1 g / L was added, the radiation dose of the filtrate was 550 Bq / kg, and the removal rate of radioactive cesium reached 93.8%.

If the second aspect of the present invention showing data according to the second embodiment is implemented on a large scale, a system as shown in FIG. 4 can be considered. The molten fly ash is put into a water washing tank where the radioactive cesium compound is extracted with water. The slurry in which fly ash is dispersed in water is filtered and dehydrated by a filtering device such as a filter press. Cake, that is, washed fly ash sludge, can be disposed of in landfill if it is confirmed that the radiation dose is 8,000 Bq / kg or less, which is the landfill reference value. The filtrate is once stored in a storage tank, and then passed through an adsorbent layer in an adsorption tower filled with a PB adsorbent, so that radioactive cesium is adsorbed and removed. Although the adsorption tower has two stages in FIG. 4, if necessary, a multistage structure of three or more stages is adopted. If the first stage adsorbent is saturated with radioactive cesium, the adsorbent is removed and the subsequent stage adsorbent is moved to the previous stage in sequence. In the final stage, a new adsorbent may be applied to finally perform a high degree of adsorption removal. It is recommended that the adsorbent be used by filling a container made of a material capable of shielding radiation and storing it as it is.

It is the data of the reference example of this invention, Comprising: Mordenite-type zeolite and iron-type flocculant are added with respect to the leachate containing the radioactive substance ( ¹³⁷ Cs and ¹³⁴ Cs) which came out from the final disposal site of municipal waste. The graph which showed the relationship between a chemical | medical agent addition amount and a radiation dose when performing aggregation precipitation. A data of Example 1 of the present invention, the molten slag by the bottom ash and fly ash leaving the incineration treatment plants of urban waste melt treated with ash melting equipment, containing radioactive material ⁽¹³⁷ Cs and ¹³⁴ Cs) The graph which showed the relationship between a chemical | medical agent addition amount and a radiation dose when adding a mordenite type zeolite and an iron type flocculant similarly to a reference example with respect to the fly ash which collected dust, and performing coagulation precipitation. A data of Example 2 of the present invention, the molten slag by the bottom ash and fly ash leaving the incineration treatment plants of urban waste melt treated with ash melting equipment, containing radioactive material ⁽¹³⁷ Cs and ¹³⁴ Cs) The graph which showed the relationship between the amount of chemical | medical agent addition and the radiation dose when adding the Pursian blue and the iron-type flocculant to the fly ash which collected dust and performing coagulation precipitation. The conceptual diagram which showed the system configuration | structure in the case of implementing the aspect of Example 2 of this invention on a large scale.

Claims (7)

A decontamination method for removing radioactive cesium from a combustible material to which radioactive cesium has adhered, and comprising the following steps:
A) Combustible material with radioactive cesium attached to it is incinerated in an incinerator to produce incinerated ash or fly ash.
B) Incineration ash or fly ash is melted in an ash melting facility, and radioactive cesium is concentrated in the molten fly ash.
C) Extracting water-soluble radioactive cesium compound by washing molten fly ash with water to obtain an aqueous solution thereof,
D) The adsorbent is brought into contact with an aqueous solution of a radioactive cesium compound and an aggregating precipitant is added to adsorb the radioactive cesium ions to the adsorbent, and then the adsorbent is agglomerated and precipitated.
E) separating the aggregated precipitate of the adsorbent adsorbing the radioactive cesium by solid-liquid separation; and
The sludge containing the adsorbent and the flocculant separated and recovered is again melted in an ash melting facility to obtain molten fly ash further enriched with radioactive cesium, which is obtained in steps (C), (D) and (E). treated by to repeat an operation to further concentrate the radioactive cesium, decontamination method further volume reduction of secondary waste by.   The decontamination method according to claim 1, wherein the combustible material to which radioactive cesium is attached is combustible material in municipal waste or sewage treatment sludge, debris generated by the earthquake, fallen leaves or thinned trees.   The decontamination method according to claim 1 or 2, wherein an adsorbent containing mordenite-type zeolite as an active ingredient is used as the adsorbent.   The decontamination method according to claim 1 or 2, wherein the adsorbent is an adsorbent containing, as an active ingredient, a ferrocyanide adsorbent that selectively adsorbs cesium.   The decontamination method according to any one of claims 1 to 4, wherein the coagulant is a polymer or inorganic coagulant. If the molten fly ash contains harmful heavy metals, the molten fly ash is washed with water prior to the step (C),
F) Applying a liquid chelating agent to the molten fly ash to fix heavy metals,
The decontamination method according to claim 1 or 2, wherein   The decontamination method according to claim 6, which is carried out using a salt of dithiocarbamic acid as the liquid chelating agent.

Images