JPS6348572B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field: The present invention relates to a method for almost completely removing iodine and/or iodine compounds from a gas mixture.

Conventional technology: Various methods have already been used to remove iodine contained in off-gas (hereinafter abbreviated as DOG) generated from the spent nuclear fuel melting process at spent nuclear fuel reprocessing plants. Proposed.

These methods include alkaline cleaning, Mercurex, and Iodox.
There are two methods: wet methods such as methane and dry methods using various solid adsorbents. First, regarding the wet method, in the alkaline cleaning method, DOG is washed with a 1-2M caustic soda aqueous solution, and the iodine in DOG is removed by NaI, NaOI, NaIO ₃
This is a method of absorbing and removing it by changing it to something like In addition to precipitation of the iodide produced, this method
The reaction between CO ₂ and NOx contained in DOG and caustic soda generates a large amount of sludge, which significantly increases the amount of radioactive waste that must be post-processed, which increases the burden of processing. (Reference Holladay, DW, 1979, A literature
survey：methods of the removal of iodine
species from off-gases and liquid waste
streams of nuclear power and nuclear fuel
reprocessing plants, with emphasis on solid
sorbents.ORNL／TM―6350；International
Atomic Energy Agency, 1980, Radioiodine
removal in nuclear facilities.Technica
reports series No.201, IAEA, Vienna;
Benedict, M., THPigford and HWLevi,
1981, Nuclear Chemical Engineering,
McGraw-Hill NY) Mercurex method uses a nitric acid solution of mercury nitrate (1-14 mol/l) as an absorption liquid, and converts iodine into HgI ₂ , Hg(IO ₃ ) _2, etc., and absorbs and removes it. . Since this method uses mercury, sufficient measures are required to eliminate pollution caused by mercury (see the above references and). In addition,
According to the published Mercurex method, during its operation, the mercury nitrate solution that has absorbed iodine is sent to an electrolytic cell where it is decomposed into iodine and mercury, fixing the iodine as copper iodide and converting it into mercury. is used cyclically. As such, it has the disadvantage that its operating process must be extremely complex (Reference Collad, GE
R. et al., 1978, Iodine trapping and
conditioning in the mercurex system，16th
DOE Nuclear Air Cleaning Conference,
p552.). Finally, the iodox method uses 20-
This method uses nitric acid at a high concentration of 22 mol/l as an absorption liquid to precipitate iodine as HI ₃ O ₈ , which is then separated and recovered. This method uses concentrated nitric acid, which has the undesirable disadvantage of corroding the equipment materials (see the above references,
and ).

Next, regarding dry methods that use various solid adsorbents to remove iodine, most of them utilize the reaction of silver or silver salt with iodine. for example,
AC6120 is an adsorbent made of amorphous silicic acid as a carrier and impregnated with silver nitrate, and can efficiently adsorb and remove iodine even in the coexistence of NOx (see Japanese Patent Publication No. 53-22077). However, in order to cause the silver nitrate to react with iodine and iodine compounds rapidly enough, the DOG must be heated to about 150°C. (See references and). Also,
The Hanford factory in the US has had good success using an adsorbent containing silver nitrate on unglazed Berl saddles. However, when using this adsorbent, DOG must be heated,
It is said that the reaction between silver and iodine does not occur at temperatures below 110°C (see references and Hiroshi Yamamoto, 1976, Nuclear Chemical Engineering, Nikkan Kogyo Shimbun).

In addition, various other silver-exchanged zeolites can be used.
A method for removing iodine from DOG has also been reported;
Both were heated to 100℃ or higher, similar to the former.
The objective cannot be achieved without the use of DOG (References and Thomas, TR, BA Stapeles
and LPMurphy, 1978, The development of
AgZ for bulk ¹²⁹ I storage.Proc.15th DOE
Nuclear Air Cleaning Conference, p394.) In addition, when using a solid adsorbent, AC6120 that has absorbed iodine or silver-exchanged zeolite, etc.
If it is intended to be stored as is or to be subjected to final disposal, it has the decisive disadvantage that the extra volume of the carrier must also be handled.

OBJECTS OF THE INVENTION: The object of the present invention is to simplify the operational steps without having the drawbacks of the wet and dry methods mentioned above, without involving any extra waste other than iodine compounds. The purpose of the present invention is to provide a method that does not require the use of corrosive chemicals such as concentrated nitric acid, does not require preheating DOG, and is almost unaffected by NOx and moisture coexisting in DOG. .

Structure and effect of the invention: According to the present invention, a mixed gas or vapor containing at least iodine and/or an iodine compound is brought into contact with an aqueous solution containing at least silver ions, and the iodine in the gas is converted into insoluble iodine. This was achieved by removing the compound as a precipitate.

That is, the present invention belongs to the so-called wet method, in which a mixed gas or vapor containing iodine and/or an iodine compound is brought into contact with an aqueous solution containing silver ions at room temperature. It is not a dry method in which it is supported on a solid carrier.

Various mineral acid salts, especially nitrates or sulfates, can be used as the aqueous solution containing silver ions, and the concentration range can be wide ranging from 0.0001 to 5 mol/l, especially 0.001 to 1 mol/l. It's convenient. Moreover, the aqueous solution of these mineral salts may contain a mineral acid. Equipment for carrying out gas-liquid contact between the gas to be treated and the aqueous solution containing silver ions includes a bubble column, a spray column, a tray column,
Conventional equipment such as wet wall towers, and other new
Any type of equipment can be used, including liquid contact equipment.

In addition, the mixed gas or vapor containing iodine and/or iodine compounds to be treated in the present invention may include, in addition to any concentration of iodine to be removed, nitrogen dioxide and/or
It has been shown that it is okay to contain about 5000 ppm of water, and that it does not have any particular negative effect on the absorption efficiency of iodine.

Therefore, the device for carrying out the present invention allows the iodine-containing gas to be directly introduced into the gas-liquid contacting device without being treated in advance, and combined with the simplicity of the post-processing device, it is extremely easy to use. Can be configured. That is, according to the method of the present invention, iodine is precipitated as an insoluble iodide, which can be easily separated by conventional methods. Since this precipitate is the only thing that is removed as waste in the method of the present invention, the amount of waste is not only compared to the conventionally known dry method but also compared to any wet method. This is a very small amount, which is very advantageous.

Hereinafter, the present invention will be explained in more detail with reference to Examples. However, the present invention is not limited thereto.

Example 1 Dry air containing 200 ppm of iodine was added to 50 ml of a 10 mmol/l silver nitrate aqueous solution in constant temperature water at 20°C.
When aerating at a flow rate of 0.5 l/min, the iodine concentration in the outlet gas changed over time as shown in attached Figure 1, and the iodine removal rate at the gas flow rate of 26% was 99.885%.
It was hot. Converting this value into a decontamination coefficient (=inlet iodine concentration/outlet iodine concentration=DF), DF=
It becomes 6.7× ¹⁰³ . The concentration of silver ions remaining after breakthrough was 0.05 ppm or less. This means that over 99.9977% of the silver that originally existed was put to good use.

Example 2 10, 50 and 100 mmol/l in a constant temperature bath at 20°C
Add 200ppm of iodine and nitrogen dioxide to the aqueous solution of silver nitrate.
DOG simulated gas containing 2000ppm and water 5000ppm,
When aerating at a flow rate of 0.5 l/min, the iodine concentration in the outlet gas showed a change over time as shown in Figure 2 attached. From this figure, it can be seen that the amount of iodine absorbed is approximately proportional to the concentration of silver nitrate. The effective utilization rate of silver was 99.9977% or more in all cases.
Further, when comparing the cases of 10 mmol/l in Example 1 and Example 2, it is found that the coexisting NOx does not affect the absorption of iodine.

Example 3 Under the same experimental conditions as Example 2, the nitric acid concentration was
The experiment was conducted using an aqueous solution with a silver nitrate concentration of 4 mol/l and a silver nitrate concentration of 50 mmol/l. The change over time in the iodine concentration in the outlet gas is shown in attached Figure 3, and compared to the case of 50 mmol/l in Example 2, the absorption of iodine is hardly affected by nitric acid coexisting in the liquid. I understand. This result shows that even if NOx in DOG is absorbed and nitric acid is produced in a fairly large amount according to the reaction 3NO ₂ +H ₂ O=2HNO ₃ +NO, the effect is small.

Example 4 Under the same experimental conditions as Example 2, the concentration
When an experiment was conducted using a 12.5 mmol/l silver sulfate aqueous solution, the iodine concentration in the outlet gas showed a change over time as shown in attached Figure 4. It was found that silver sulfate aqueous solution has the same iodine removal ability as silver nitrate aqueous solution. In addition, the concentration of residual silver ions after breakthrough is
It was below 0.05ppm. This is a silver usage rate of 99.998%
This corresponds to the above.

[Brief explanation of the drawing]

FIGS. 1 to 4 are explanatory diagrams illustrating the effects of the method of the present invention.

Claims (1)

[Claims] 1. A method for removing iodine, which comprises bringing a mixed gas or vapor containing at least iodine and/or an iodine compound into contact with an aqueous solution containing at least silver ions, and removing the iodine in the gas as a precipitate of insoluble iodide.