JP6834165B2 - Method for treating radioactive cesium-containing inorganic substances - Google Patents

Description

The present invention relates to a method for removing radioactive cesium from incineration ash containing radioactive cesium, soil, purified water sludge, sewage sludge, and the like.

Of the waste discharged from facilities that handle radioactive substances such as nuclear power plants, combustible waste is incinerated, but the incineration ash generated during the incineration contains radioactive substances. Radioactive cesium has a long half-life of ¹³⁴ Cs for about 2 years and ¹³⁷ Cs for about 30 years, so great care must be taken when storing it. In particular, recently, a large amount of radioactive substances have been released due to an accident at a nuclear power plant in Fukushima Prefecture, causing widespread pollution, and the treatment of contaminated soil and incineration ash of combustibles from the contaminated area has become a problem. ..

If a large amount of radioactive material is scattered due to a large-scale nuclear power plant accident, a large amount of radioactive contaminants to be treated will be required, and these will be landfilled in the form of inorganic substances, but it is not easy to secure a place or facility for landfill. .. By the way, there are various kinds of radioactive substances, and the radioactive substances emitted from nuclear power plants and the like are mainly cesium and iodine, but iodine has a very short half-life, so the problem is cesium.

Therefore, the present inventor has found that radioactive cesium contained in fly ash and soil mainly exists in the form of oxides and the like, and melts these inorganic substances in a reducing gas atmosphere in the presence of chlorine. By doing so, it was found that radioactive cesium could be converted into a chloride form and concentrated on the fly ash side, and the remaining slag would be harmless with almost no radioactive cesium, and a patent application was filed for this. It has been patented (Patent Document 1).

It is also known that the efficiency of separating radioactive cesium from molten slag is improved by adding a melting point lowering agent to an object to be treated such as soil and incineration residue (Patent Document 2). The melting point lowering agent includes calcium compounds, alkali metal salts, alkaline earth metal salts, boron compounds, iron compounds and the like.

Japanese Patent No. 5772556 Japanese Unexamined Patent Publication No. 2013-242194

In the method of Patent Document 1, most of the radioactive cesium was concentrated in fly ash, and the remaining slag contained almost no radioactive cesium, but the residual radioactivity of the slag was 100 Bq / depending on the type of incineration ash and soil. In some cases, it exceeded kg. The method of Patent Document 2 improved this, but it was still insufficient.

An object of the present invention is to melt an inorganic substance such as incineration ash or soil containing radioactive cesium in a reducing gas atmosphere in the presence of chlorine to reduce the volume and concentrate the radioactive cesium into flying ash to have a low concentration of radioactivity. An object of the present invention is to provide a method capable of stably reducing radioactive cesium in a method for obtaining slag containing cesium.

The present inventor has diligently studied to solve the above problems, and when melting in a reducing gas atmosphere in the presence of chlorine to chloride and volatilize radioactive cesium, the viscosity is improved rather than lowering the melting point. It has been found that if the fluidity is improved, the contact efficiency between the reducing agent, chloride and slag can be improved and cesium can be chlorinated and volatilized. Alkali is effective as this viscosity lowering agent, but if this alkali is added to an inorganic substance from the beginning, it volatilizes during heating and melting, and the effect cannot be sufficiently obtained, so that the alkali is almost melted. We found that it was effective to add it later.

The present invention has been made based on these findings.
A method for treating radioactive cesium-containing inorganic substances by melting radioactive cesium-containing inorganic substances in a reducing gas atmosphere in the presence of chlorine and collecting fly ash discharged from the gas outlet.
The equivalent of the chlorine component in the inorganic substance containing radioactive cesium is equal to or more than the sum of the equivalent of lead and the equivalent of zinc contained in the inorganic substance, and an alkali which is a sodium compound or a potassium compound that lowers the viscosity in a molten state is used. A method for treating radioactive cesium-containing inorganic substances, which is characterized by being added after melting, and
A method for treating radioactive cesium-containing inorganic substances by melting radioactive cesium-containing inorganic substances in a reducing gas atmosphere in the presence of chlorine and collecting fly ash discharged from the gas outlet.
A sodium compound or potassium compound that reduces the viscosity in the molten state by adding a chlorine source to the radioactive cesium-containing inorganic substance to make the equivalent of chlorine equal to or greater than the sum of the equivalent of lead and the equivalent of zinc contained in the inorganic substance. It provides a method for treating a radioactive cesium-containing inorganic substance, which comprises adding a certain alkali after melting.

According to the present invention, an inorganic substance containing radioactive cesium can be changed to slag containing almost no radioactive cesium, and only the fly ash generated at that time needs to be landfilled, so that the amount of landfill can be significantly reduced.

It is a figure which shows the schematic structure of the apparatus which performs the processing method of this invention using an electric resistance type ash melting furnace.

The inorganic substance is not particularly limited as long as it contains radioactive cesium, but incineration ash generated when combustible substances containing radioactive cesium are incinerated, incineration ash generated when sewage sludge containing radioactive cesium is incinerated, Examples include soil, rubble, and concrete contaminated with radioactive cesium. In particular, the incineration ash generated when combustibles containing radioactive cesium are incinerated includes the main ash, which is the bottom ash that collects at the bottom of the incinerator, and the main ash, which is contained in the combustion exhaust gas and is collected by a dust collector such as a bag filter. There is fly ash to be collected.

The content of radioactive cesium is not particularly limited, but it is usually not detected in areas affected by a large-scale nuclear power plant accident, about 6,000 Bq / kg, and especially within 100 km from the power plant, 3,000 Bq / kg. It is about kg.

Chlorine changes a cesium compound into a form of cesium chloride when melted, and a wide variety of chlorine can be used. Incineration ash generated when combustibles containing radioactive cesium is incinerated often contains 0.4 to 2% by weight of chlorine component, and the content of chlorine component is less than 0.4% by weight. Needs to add a chlorine source.

Examples of the chlorine source include vinyl chloride resin, alkali metal chloride, alkaline earth metal chloride and the like. Alkaline metal chlorides include sodium chloride and potassium chloride, and alkaline earth metal chlorides include calcium chloride and magnesium chloride.

The chlorine content is preferably equal to or greater than the sum of the equivalent of lead and the equivalent of zinc contained in the inorganic substance. This is because lead and zinc are easily chlorinated and more easily volatilized than cesium. The equivalents of lead and zinc can be calculated by doubling the amount of each substance (number of moles) of lead and zinc, and if the inorganic substance contains a chlorine component, this is also used as the chlorine content. It may be included in the calculation.

Further, when the chlorine content is 10% by weight or more, the quality of the slag is deteriorated, so that the chlorine content is preferably less than 10% by weight.

The melting furnace used for melting inorganic substances is not particularly limited, and examples thereof include a shaft type gasification melting furnace and an electric resistance type ash melting furnace used for treating general waste.

FIG. 1 is a diagram showing a schematic structure of an apparatus for treating radioactive cesium-containing inorganic substances using an electric resistance type ash melting furnace.

In FIG. 1, 1 is an inlet for an inorganic substance, 2 is an electrode, 3 is a gas discharge port, 5 is a molten slag discharge port, and 6 is a molten metal discharge port. The inorganic substance charged into the ash melting furnace 10 is melted on the molten slag 4 heated to a high temperature of a thousand and several hundred degrees by the electric resistance heat generated by the energization between the electrodes 2, and is accumulated in the furnace bottom as molten slag. The molten slag 4 at the bottom of the furnace is appropriately discharged to the outside of the furnace from the molten slag discharge port 5. The radioactive cesium-containing inorganic material charged from the inorganic material input port 1 is heated and melted in the furnace, the generated slag 4 is continuously or intermittently generated from the molten slag discharge port 5, and the molten metal collected under the molten metal discharge port 6 is continuously or intermittently. Is discharged. On the other hand, the exhaust gas containing fly ash is discharged from the gas outlet 3, cooled by the cooling tower 7, and the fly ash is collected by the bag filter 8 and discharged from the chimney 9.

Melting is performed in a reducing gas atmosphere. This reducing gas atmosphere can be formed by the presence of hydrogen gas or carbon monoxide gas. In order to maintain the atmosphere of the reducing gas in the furnace, it is necessary to make the furnace a closed structure so that only a small amount of air from the outside enters the furnace.

Further, the atmosphere in which carbon monoxide gas is used as a reducing gas is the amount of air required for the carbon in coke to be converted to carbon monoxide when coke is used as a heat source, that is, approximately 4.44 Nm ³ -air / kg. -It is formed by adding the amount of carbon air, and when heated by electricity, it is formed by the reaction between the graphite of the electrode and the oxide in the charged inorganic material.

The concentration of carbon monoxide gas is preferably about 5 to 40% by volume, and more preferably about 20 to 30% by volume. Further, the concentration ratio (CO / CO ₂ ) of carbon monoxide gas to carbon dioxide gas is preferably about 0.1 to 100, and more preferably about 0.5 to 20.

When the concentration of carbon monoxide gas is less than 5% by volume, the oxidative property is enhanced, so that cesium chloride does not proceed. On the other hand, when the concentration of carbon monoxide gas exceeds 40% by volume, the effect of cesium chloride is almost unchanged.

Further, when the concentration ratio (CO / CO ₂ ) of carbon monoxide gas and carbon dioxide gas is less than 0.1, the oxidizing property is strengthened, so that cesium chloride does not proceed. On the other hand, when the concentration ratio (CO / CO ₂ ) of carbon monoxide gas to carbon dioxide gas exceeds 100, the effect of cesium chloride is almost unchanged.

The melting temperature may be about 1200 to 1600 ° C., as long as the inorganic substance can be melted, and the melting may be continued for about 1 to 2 hours.

In this reducing gas atmosphere, _{when CaCl 2} is used as the chlorine source, the overall reaction is Cs ₂ O + CaCl ₂ → 2 CsCl + CaO, and when carbon is added, O is easily separated by a reaction such as _{Cs 2 O + C → 2 Cs + CO, and 2 Cs + CaCl 2} + CO. _It is considered that the reaction of 2 → 2CsCl + CaO proceeds.

By adding an alkali here, the viscosity of the slag is lowered, the fluidity of the slag is improved, the contact efficiency of the carbonaceous material, chloride and cesium is increased, and the chloride volatilization of cesium can be promoted.

Alkali is a basic oxide, and sodium compounds and potassium compounds are mainly used. Specific examples include sodium carbonate, baking soda, sodium hydroxide, potassium hydroxide, and borax, boric acid and the like. The appropriate amount of alkali added is 1 to 80% by weight based on the slag. This alkali should be added after the inorganic substance containing radioactive cesium has been substantially melted, and should be added as soon as possible after melting. Addition may be done by dropping from above or blowing with a lance.

By this melting treatment, the inorganic substance is slagged, and since most of the radioactive cesium is removed from this slag, it can be disposed of as it is or can be effectively used.

On the other hand, most of the radioactive cesium contained in the inorganic material is transferred to the fly ash generated from the furnace during the melting process. Therefore, this fly ash is produced, for example, by the Ministry of the Environment, Ring Abolition No. 110831001, The landfill will be disposed of in accordance with the guidelines of No. 110831001 and August 31, 2011, but the amount of fly ash is much smaller than that of inorganic substances, so it is easy to secure a landfill site.

The incineration ash (main ash) discharged from the nuclear power plant was melted using an electric resistance type ash melting furnace as shown in FIG.

The composition of the incineration ash used was Si: 13.2% by weight, Al: 5.8% by weight, Fe: 2.8% by weight, Ca: 16.0% by weight, Na: 1.8% by weight, K :. It was 1.3% by weight, Cl: 0.1% by weight, Pb: 0.1% by weight, Zn: 0.4% by weight, and the concentration of radioactive cesium ( ^{total of 134} Cs and ¹³⁷ Cs) was 610 Bq / kg. is there.

100 kg of the above incineration ash and 1.15 kg of calcium chloride as a chlorine source were mixed and put into an electric resistance type ash melting furnace. Here, the reason why the amount of calcium chloride was set to 1.15 kg is as follows.

When the required chlorine equivalent is determined based on the content of lead and zinc in 100 kg of incineration ash,
Required chlorine equivalent = (0.1 / [Pb atomic weight] + 0.4 / [Zn atomic weight]) x 2
= 0.013 [kmol]
Here, the Pb atomic weight is 207.13 and the Zn atomic weight is 65.37.

Next, since the incineration ash contains 0.1% by weight of the chlorine component, the equivalent of this chlorine component is
Chlorine component equivalent = 0.1 / [Cl atomic weight] = 0.0028 [kmol]
Here, the Cl atomic weight is 35.453.
Therefore, the chlorine equivalent to be added is 0.013-0.0028 = 0.0104 [kmol], and when the amount of calcium chloride to be added is calculated,
Calcium chloride amount = ([Ca atomic weight] + [Cl atomic weight] x 2) x 0.0104
= 1.15 [kg]
Here, the Ca atomic weight is 40.08.

In this way, calcium chloride as a chlorine source is charged into an electric resistance type ash melting furnace, and after the incineration ash is melted at 1200 ° C., sodium carbonate is charged from above 10 kg via a hopper and then from inside the furnace. The concentration of CO gas discharged was 5 to 40% by volume, and the _{inside of the furnace was kept in a reducing gas atmosphere in the range of (CO / CO 2} ) of 0.6 to 1.0, and melted at 1500 ° C. for 1 hour. ..

As a result, 52 kg of slag was obtained from the furnace, and the concentration of radioactive cesium contained in the slag was 180 Bq / kg. On the other hand, the amount of fly ash discharged from the furnace and collected was 1.1 kg, and the concentration of radioactive cesium contained in the fly ash was 27,000 Bq / kg. In addition, 1.3 kg of metal (mainly iron) was obtained, and its radioactivity was less than 5 Bq / kg.

If the alkali is added from the beginning of the treatment, the alkali has a low melting point and a high vapor pressure, and the alkali evaporates before the cesium volatilizes. Therefore, the concentration of radioactive cesium in the slag after the treatment is 300 Bq / kg. It has become expensive.

According to the method of the present invention, radioactive cesium can be separated from the radioactive cesium-containing inorganic substance and concentrated in fly ash, so that the amount of landfill can be significantly reduced, and therefore, it can be used for the treatment of various radioactive cesium-containing wastes.

1 Inorganic material inlet 2 Electrode 3 Gas outlet 4 Molten slag 5 Molten slag outlet 6 Molten metal outlet 7 Cooling tower 8 Bag filter 9 Chimney

Claims (4)

A method for treating radioactive cesium-containing inorganic substances by melting radioactive cesium-containing inorganic substances in a reducing gas atmosphere in the presence of chlorine and collecting fly ash discharged from the gas outlet.
The equivalent of the chlorine component in the inorganic substance containing radioactive cesium is equal to or more than the sum of the equivalent of lead and the equivalent of zinc contained in the inorganic substance, and an alkali which is a sodium compound or a potassium compound that lowers the viscosity in a molten state is used. A method for treating a radioactive cesium-containing inorganic substance, which comprises adding it after melting. A method for treating radioactive cesium-containing inorganic substances by melting the radioactive cesium-containing inorganic substances in a reducing gas atmosphere in the presence of chlorine and collecting fly ash discharged from the gas outlet.
A sodium compound or potassium compound that reduces the viscosity in the molten state by adding a chlorine source to the radioactive cesium-containing inorganic substance to make the equivalent of chlorine equal to or greater than the sum of the equivalent of lead and the equivalent of zinc contained in the inorganic substance. A method for treating a radioactive cesium-containing inorganic substance, which comprises adding a certain alkali after melting. The method for treating a radioactive cesium-containing inorganic substance according to claim 1 or 2, wherein the alkali is any one of sodium carbonate, baking soda, sodium hydroxide, potassium hydroxide and borax. The method for treating a radioactive cesium-containing inorganic substance according to claim 1 or 2, wherein the radioactive cesium-containing inorganic substance is incineration ash generated when a combustible substance containing radioactive cesium is incinerated.

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