JPH07328430A - Gaseous iodine adsorbent - Google Patents

Abstract

PURPOSE:To provide an inexpensive gaseous iodine adsorbent excellent in adsorbing performance especially at high temp. or high humidity and used for adsorbing and removing gaseous iodine, e.g., radioactive gaseous iodine contained in off-gas from a nuclear power plant. CONSTITUTION:This gaseous iodine adsorbent is made of at least one kind of compd. selected from among compds. represented by the formulae, M<1> M<2>xO(2+3x)/2.mH2O and M<1>(1-y)M<2>y(OH)2A<p->y/p.nH2O [where M<1> is a divalent metal, M<2> is a trivalent metalloid or metal, 0<x<=1, 0<=m<=2.5, 0<y<=0.5, A<-p> is a p-valent anion, (p) is an integer of 1-4 and 0<=n<=0.6]. For example, magnesium carbonate is mixed with aluminum carbonate so that the atomic ratio of Mg:Al is regulated to 1:0.4 and the resultant mixture is fired at 700 deg.C to obtain the objective adsorbent represented by the formula, MgAl0.4O1.6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is particularly excellent in iodine gas adsorption performance at a high temperature of 200 ° C. or higher or a high humidity of 100% relative humidity, and is a radioactive substance contained in a gas, for example, an off gas of a nuclear power plant. The present invention relates to an iodine gas adsorbent useful for adsorption / removal of iodine (hereinafter sometimes simply referred to as an adsorbent).

[0002]

2. Description of the Related Art In a nuclear power plant such as a nuclear power plant, prevention of radioactive iodine emission is important for environmental protection. Conventionally, a nuclear power plant is provided with an iodine removal filter in order to prevent the release of radioactive iodine in the event of an accident such as the dissolution of nuclear fuel. Also,
In the spent nuclear fuel reprocessing plant, a device for removing radioactive iodine from the offgas generated in the shearing and melting processes of the spent nuclear fuel is provided. Further, recently, in order to reduce the amount of radioactive iodine released to the environment in a normal time as much as possible, an iodine removing device is provided in these emission sources, a building ventilation system and the like.

As the iodine gas adsorbent, at present, in addition to activated carbon, zeolite, alumina, silica gel and the like impregnated with silver are used. However, activated carbon has a low iodine gas adsorption performance under high temperature or high humidity, and there is a problem that the release of radioactive iodine cannot be sufficiently prevented when exposed to a high temperature or high humidity atmosphere at the time of an accident such as a nuclear power plant. Further, there is a problem that it cannot be used under high humidity such as off-gas at the time of reprocessing nuclear fuel (90% or more of relative humidity at room temperature).

On the other hand, an adsorbent such as zeolite impregnated with silver has an adsorbing performance at a temperature of about 120 ° C. However, according to JP-A-57-45328, in a high-humidity atmosphere, water condenses in the pores and inhibits the reaction between silver and iodine. Therefore, in order to maintain high adsorption performance, silver impregnation is performed. The amount has to be increased, and there is a problem that some of expensive silver is discarded without being effectively used. In addition, according to the results confirmed by the present inventors, 200 ° C
At the above high temperature, even if the amount of silver impregnated is sufficient, there is a drawback that the adsorption performance is poor. As described above, the conventional adsorbent does not always have sufficient performance for adsorption removal of iodine gas generated in a nuclear power plant, etc., and has a practical iodine gas adsorption performance under high temperature or high humidity. There is a strong demand for the development of an adsorbent that can maintain and exhibit the above properties.

[0005]

DISCLOSURE OF THE INVENTION The present invention solves the above problems of conventional iodine gas adsorbents and provides an adsorbent having a practically sufficient iodine gas adsorption performance even under high temperature or high humidity. Is an issue.

[0006]

Means for Solving the Problems The present inventors have evaluated and examined the iodine gas adsorption performance of various adsorbents at high temperature or high humidity. As a result, they have found that an adsorbent composed of an inorganic compound having a specific chemical composition has excellent iodine gas adsorption performance capable of solving the above problems, and completed the present invention.

The iodine gas adsorbent of the first invention is characterized by comprising at least one compound selected from the group of compounds represented by the following composition formulas (1) or (2). M ¹ M ² _x O _{(2 + 3x) / 2} · mH ₂ O (1) (where M ¹ is a divalent metal, M ² is a trivalent semimetal or metal, and x and m Are 0 <x ≦ 1, 0 ≦ m ≦, respectively.
It is a number that satisfies 2.5. ) M ¹ _(1-y) M ² _y (OH) ₂ Ap ^- _{y / p} · nH ₂ O (2) (where M ¹ is a divalent metal and M ² is a trivalent semimetal Or metal and y and n are 0 <y ≦ 0.5 and 0 ≦, respectively.
It is a number that satisfies n ≦ 0.6. A ^p- is a p-valent anion, and p is a positive integer. The second invention is characterized in that in the composition formula (1) or (2), M ¹ is Mg and M ² is Al.

The above "M ¹ " is Mg, Mn, Fe, Co,
Although it is a divalent metal such as Ni, Cu and Zn, M ¹ is M
When it is g, it is particularly preferable because it is excellent in iodine gas adsorption performance, and Mg can be easily obtained and is also versatile and economical. Further, the above "M ² " is a trivalent semimetal such as Al or In or a trivalent metal such as Fe, Cr, Co or Sc, but when M ² is Al, the iodine gas adsorption performance is particularly high. It is excellent and can be easily obtained like Mg.

In the composition formula (1), 0 <x ≦ 1, but a range of 0.1 ≦ x ≦ 0.5 is particularly preferable because it has excellent iodine gas adsorption performance and can be easily obtained. On the other hand, in the composition formula (2), 0 <y ≦ 0.5, and y of 0.33 or less is particularly preferable because it has excellent iodine gas adsorption performance and can be easily obtained. A ^{p −} in the composition formula (2) is OH ⁻ , F ⁻ , Cl ⁻ , Br ⁻ , N.
It is a p-valent anion such as O ₃ ⁻ , CO ₃ ²⁻ , SO ₄ ²⁻ , Fe (CN) ₆ ³⁻ , CH ₃ COO ⁻ , oxalate ion and salicylate ion.
p is a positive integer, usually an integer of 1 to 4.

The compound represented by the composition formula (1) is a compound having M ¹ and a compound having M ² , such as carbonate or hydroxide, and the atomic ratio of M ^{2 to} M ¹ is 1 or less. Thus, it is obtained by baking and preferably firing at 400 to 900 ° C, more preferably 500 to 700 ° C. It can also be obtained by firing the compound represented by the composition formula (2) in the above temperature range. The compound represented by the composition formula (2) is known as a hydrotalcite-type compound, and includes a compound having M ¹ and a compound having M ² such as carbonate or hydroxide at a pH of 9 to 12. It is obtained by a method such as hydrolysis in an aqueous solution of.

The iodine gas adsorbent of the present invention is capable of adsorbing iodine gas in a gas by contacting it with a gas containing iodine gas and removing the iodine gas from the gas. Therefore, the adsorbents of the present invention can be used in conventional gas-solid contact devices such as fixed beds and fluidized beds. Also,
In order to facilitate the contact between the adsorbent and the gas containing iodine gas, the adsorbent may be used in the form of granules or pellets,
Alternatively, the adsorbent can be used by supporting it on a carrier.
Furthermore, the iodine gas adsorbent of the present invention can be used by molding it into a filter by a method of filling it in a cartridge, a method of sandwiching it between a woven fabric or a non-woven fabric, and the like.

The preferable time for contacting the gas containing iodine gas with the adsorbent of the present invention is not decided unconditionally depending on the object to be treated or the atmosphere at the time of treatment such as temperature and humidity, but it is from several seconds to several hours. It will take a few days. Further, the use ratio of the gas containing iodine gas and the adsorbent can be appropriately adjusted within a range in which the adsorbed amount of iodine gas in the adsorbent does not exceed the adsorption capacity of the adsorbent. For example, iodine (I ₂ ) 1 The amount of the adsorbent is preferably 1.0 g or more per millimole. This usage ratio can also be adjusted by the conditions for removing iodine gas, for example, the contact time or the contact method between the gas containing iodine gas and the adsorbent.

Since the adsorbent which has adsorbed iodine gas by the above-mentioned operation stably adsorbs and holds iodine, when it is disposed of as waste as it is, even if it comes into contact with groundwater, the adsorbed iodine absorbs iodine. A stable adsorption state is maintained for a long period of time without being released. In addition, when the adsorbent after iodine gas adsorption is treated as secondary waste, the adsorbent is solidified with an appropriate solidifying agent to facilitate its handling, so that it is safely treated as a solidified product that does not release iodine. can do. Preferable solidifying agents include cement, asphalt, plastic, glass and the like, and if desired, a bone agent such as sand and crushed stone, and a filler such as calcium carbonate may be used.

Of the above solidifying agents, cement is particularly preferable, and a solidified product having durability can be obtained at low cost and easily. Preferred cements include calcium silicate-based Portland cement and calcium aluminate-based alumina cement. All of these solidifying agents are already known, and the solidifying method may be a conventional method depending on the type of solidifying agent. The used adsorbent is 500 to 900 ° C., more preferably 600 to 700 ° C.
By calcination at ℃ and releasing iodine, it is regenerated as a compound containing little or no water of crystallization, or after calcination in the same temperature range, water is added by adding it to pure water. Then, by drying at 200 ° C. or lower, more preferably 150 ° C. or lower, the compound containing water of crystallization can be regenerated and reused.

[0015]

EXAMPLES The present invention will be described in more detail below with reference to examples. Example 1 An adsorbent represented by the composition formula (MgAl) _0.4 O _1.6 by mixing magnesium carbonate and aluminum carbonate in an amount such that the atomic ratio of Mg and Al is 1: 0.4 and firing the mixture at a temperature of 700 ° C. Got 0.1 g of this adsorbent was put in a flask with a capacity of 250 ml and heated to 120 ° C. in the heater, and then the flask was taken out from the heater once and distilled water 5
The glass tube containing ml was left standing. After that, the flask with a lid was capped, placed again in the heater to 120 ° C. (this creates a high-humidity atmosphere with a relative humidity of 100% in the flask), and then air containing 50000 ppm of I ₂ gas was added.
ml was injected using a syringe into a flask with a lid. 1
After a minute, the gas in the flask was brought into contact with 10 ml of a 0.1 M sodium thiosulfate aqueous solution, and then the I ⁻ (iodine ion) concentration in the sodium thiosulfate aqueous solution after the contact was measured with a sequential high-frequency plasma emission spectrometer manufactured by Shimadzu Corporation. By measuring, the iodine concentration in the gas in the flask was determined.

Example 2 A composition formula (MgAl) _0.4 O _1.6 obtained in the same manner as in Example 1
· 2.4H except for using an adsorbent represented by ₂ O, Example 1
The iodine concentration in the gas in the flask after contact with the adsorbent was determined in the same manner as in. Example 3 Composition formula (MgAl) _0.3 O _1.45 obtained in the same manner as in Example 1
The iodine concentration in the gas in the flask after contact with the adsorbent was determined in the same manner as in Example 1 except that the adsorbent represented by

Example 4 Compositional formula (MgAl) _0.3 O _1.45 obtained in the same manner as in Example 1
· 2.2H except for using an adsorbent represented by ₂ O, Example 1
The iodine concentration in the gas in the flask after contact with the adsorbent was determined in the same manner as in. Example 5 Except that the temperature in the heater was set to 200 ° C. and the glass tube containing distilled water was not placed at the bottom of the flask with a lid (the flask has a high temperature but is not in a high humidity atmosphere).
In the same manner as in Example 1, the iodine concentration in the gas in the flask after contact with the adsorbent was determined.

Example 6 The iodine concentration in the gas in the flask after contact with the adsorbent was determined in the same manner as in Example 5 except that the adsorbent obtained in Example 2 was used. Example 7 The iodine concentration in the gas in the flask after contact with the adsorbent was determined in the same manner as in Example 5 except that the adsorbent obtained in Example 3 was used.

Example 8 The iodine concentration in the gas in the flask after contact with the adsorbent was determined in the same manner as in Example 5 except that the adsorbent obtained in Example 4 was used. Example 9 Magnesium carbonate and aluminum carbonate in an amount such that the atomic ratio of Mg and Al was 7/3 were mixed and hydrolyzed in an aqueous solution having a pH of 10 to form a composition formula Mg _0.7 Al _0.3 (OH) ₂ (C
O ₃ ) An adsorbent represented by _0.15 · 0.5H ₂ O was obtained. The iodine concentration in the gas in the flask after contact with the adsorbent was determined in the same manner as in Example 1 except that this adsorbent was used.

Example 10 Composition formula Mg _0.75 Al _0.25 (O) obtained in the same manner as in Example 9
H) ₂ (CO ₃ ) _0.125 · 0.5H ₂ O was used, and the iodine concentration in the gas in the flask after contact with the adsorbent was determined in the same manner as in Example 1 except that the adsorbent was used. . Example 11 The iodine concentration in the gas in the flask after contact with the adsorbent was determined in the same manner as in Example 5 except that the adsorbent obtained in Example 9 was used.

Example 12 The iodine concentration in the gas in the flask after contact with the adsorbent was determined in the same manner as in Example 5 except that the adsorbent obtained in Example 10 was used. Examples 1 to 4 (adsorbent of composition formula (1), high humidity atmosphere) and Examples 9 to 10
The results of [adsorbent of composition formula (2), high humidity atmosphere] are shown in Table 1.
Table 2 shows the results of Examples 5 to 8 [adsorbent of composition formula (1), high temperature atmosphere] and Examples 11 to 12 [adsorbent of composition formula (2), high temperature atmosphere].

[0022]

[Table 1]

[0023]

[Table 2]

Comparative Example 1 The procedure of Example 1 was repeated, except that activated carbon (Kuraray Chemical Co., Ltd., trade name "Kuraray Coal GC-32-60") was used as the adsorbent, and the contents of the flask after contact with the adsorbent were used. The iodine concentration in the gas was determined. Comparative Example 2 The same procedure as in Example 1 was carried out except that silver zeolite having a silver impregnation amount of 60% by weight (zeolite was 10X type) was used as the adsorbent, and the iodine concentration in the gas in the flask after contact with the adsorbent was adjusted. I asked.

Comparative Example 3 The concentration of iodine in the gas in the flask after contact with the adsorbent was determined in the same manner as in Example 1 except that silver silica gel having a silver loading of 10% by weight was used as the adsorbent. Comparative Example 4 The iodine concentration in the gas in the flask after contact with the adsorbent was determined in the same manner as in Example 1 except that silver alumina having an adhering amount of silver of 10 wt% was used as the adsorbent.

Comparative Example 5 The iodine concentration in the gas in the flask was determined in the same manner as in Example 1 except that the adsorbent was not placed in the flask with a lid. Comparative Example 6 Iodine in the gas in the flask after contact with the adsorbent was performed in the same manner as in Example 5 except that activated carbon (Kuraray Chemical Co., Ltd., trade name "Kuraray Coal GC-32-60") was used as the adsorbent. The concentration was determined.

Comparative Example 7 The procedure of Example 5 was repeated except that silver zeolite having a silver impregnation amount of 60% by weight (zeolite was 10X type) was used as the adsorbent. The iodine concentration was determined. Comparative Example 8 The iodine concentration in the gas in the flask after contact with the adsorbent was determined in the same manner as in Example 5 except that silver silica gel having a silver loading of 10% by weight was used as the adsorbent.

Comparative Example 9 The concentration of iodine in the gas in the flask after contact with the adsorbent was determined in the same manner as in Example 5 except that silver alumina having an adhering amount of silver of 10% by weight was used as the adsorbent. Comparative Example 10 The iodine concentration in the gas in the flask was determined in the same manner as in Example 5 except that the adsorbent was not added to the flask with a lid. The results of Comparative Examples 1 to 5 (high humidity atmosphere) are shown in Table 1 above, and the results of Comparative Examples 6 to 10 (high temperature atmosphere) are shown in Table 2 above.
Are also described in each.

Table 1 shows the results of performance evaluation in a high humidity (100% relative humidity) atmosphere at a temperature of 120 ° C. The iodine concentration in each Example was less than 5 ppm, and the iodine gas adsorbent of the present invention was obtained. It can be seen that has excellent adsorption performance. On the other hand, the activated carbon of Comparative Example 1 has an iodine concentration of 2
High as 0 ppm and inferior in adsorption performance, Comparative Examples 2 to 4
In the case of each of the adsorbents impregnated with silver, although the adsorption performance is excellent, it is necessary to impregnate a large amount of silver in order to maintain the excellent performance, and the handling is complicated and the cost is high. There is.

Table 2 shows the evaluation results in a high temperature (200 ° C.) atmosphere (low humidity. Humidity: less than 1% relative humidity). In each Example, the same excellent adsorption performance as the results in Table 1 was exhibited. You can see that it is done. On the other hand, the activated carbon of Comparative Example 5 had a further lower adsorption performance than the results of Comparative Example 1, and the adsorbents impregnated with silver of Comparative Examples 7 to 9 were compared with those in Table 1. It can be seen that its performance is greatly reduced. As described above, the iodine gas adsorbent of the present invention has excellent adsorption performance even at a high temperature of 200 ° C. or higher. Further, the performance remains the same even under a high humidity of 90% or more in relative humidity.

[0031]

INDUSTRIAL APPLICABILITY The iodine gas adsorbent of the first invention is excellent in iodine gas adsorption performance particularly under high temperature or high humidity as compared with conventional adsorbents such as activated carbon, and is expensive such as silver. It is economical, containing no components, and is useful in various fields as an adsorbent for adsorbing and removing iodine gas in a gas, for example, radioactive iodine contained in offgas of a nuclear power plant. Further, in the case of a compound made of a specific metal as in the second invention, an adsorbent having particularly excellent performance is obtained, which is preferable.

Claims (2)

[Claims] 1. An iodine gas adsorbent characterized by comprising at least one compound selected from the group of compounds represented by the following composition formulas (1) or (2). M ¹ M ² _x O _{(2 + 3x) / 2} · mH ₂ O (1) (where M ¹ is a divalent metal, M ² is a trivalent semimetal or metal, and x and m Are 0 <x ≦ 1, 0 ≦ m ≦, respectively.
It is a number that satisfies 2.5. ) M ¹ _(1-y) M ² _y (OH) ₂ Ap ^- _{y / p} · nH ₂ O (2) (where M ¹ is a divalent metal and M ² is a trivalent semimetal Or metal and y and n are 0 <y ≦ 0.5 and 0 ≦, respectively.
It is a number that satisfies n ≦ 0.6. A ^p- is a p-valent anion, and p is a positive integer. ) 2. The iodine gas adsorbent according to claim 1, wherein M ¹ is Mg and M ² is Al.