JPH0671590B2 - Method for treating solution containing high concentration alkali metal salt and trace amount of harmful metal - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating a solution containing a high concentration alkali metal salt and a trace amount of harmful metal, and is particularly applied to treatment of low-level radioactive liquid waste discharged from a nuclear power plant or the like. The present invention relates to a method for efficiently selecting and separating an alkali metal salt from the above to efficiently reduce the volume of waste.

[0002]

2. Description of the Related Art A low-level radioactive liquid waste discharged from a nuclear power plant contains a large amount of non-radioactive alkali metal salts as well as trace amounts of harmful metals such as radionuclides.
Conventionally, in treating the waste liquid, in order to contain and dispose of dangerous radionuclides, the waste liquid containing the radionuclide and the alkali metal salt is directly evaporated and concentrated to reduce the volume, and then the cement or cement is used. Sealed and solidified with asphalt and stored in a designated facility.

By the way, the amount of radioactive waste is increasing year by year, and it is becoming difficult to secure a space for the storage facility, so that there is an urgent need to reduce the volume of the waste. Then, according to the method of evaporating and concentrating the waste liquid as it is as described above, an originally dangerous alkali metal salt is also concentrated together with the radionuclide, and an overwhelmingly large amount of the alkali metal salt is stored as compared with the radionuclide. This is a major obstacle to volume reduction. Further, this method has a drawback that the amount of heat for distillation and concentration becomes enormous and the processing cost becomes high.

Therefore, there is a demand for a treatment in which only the alkali metal salt is removed from the waste liquid and the radionuclide is concentrated in the waste liquid. As a method applicable to this treatment means, it is applied when selectively separating each substance from a solution in which two or more substances are mixed, and those substances have different solubilities in a solvent. In the meantime, there is a method of precipitating and precipitating a substance having a saturated concentration or more in the process of heating and evaporating the solvent, and separating each substance while leaving the remaining substance in the mother liquor. However, the precipitation selectivity in the same method using the saturation concentration is not always good, and when applied to the above-mentioned waste liquid, harmless alkali metal salts and harmful radionuclides will precipitate at the same saturation concentration. , Is not very effective for this kind of waste liquid treatment.

[0005]

Therefore, in the present invention, the alkali metal salt, which hinders the volume reduction in the low-level radioactive liquid waste, is selectively removed in advance while suppressing the mixing of radionuclides. It was created for the purpose of providing a method for concentrating only radionuclides, thereby simplifying and improving the efficiency of the waste treatment process and saving space for the storage of treated waste.

[0006]

According to the present invention, a hydrogen ion-supplying substance and alcohols are added to a solution containing a high concentration alkali metal salt and a trace amount of harmful metal to selectively remove the alkali metal salt in the solution. The present invention relates to a method for treating a solution containing a high-concentration alkali metal salt and a trace amount of harmful metal, which is characterized by sedimenting and separating and removing. Further, in the treatment method, the addition amount of the hydrogen ion supply substance is set so that the hydrogen ion concentration index (pH) of the solution is 0 to 7, and the addition amount of the alcohols is set after the addition of the hydrogen ion supply substance. When the volume ratio to the solution is set to 1 to 7 times, the alkali metal salt can be separated and removed as a complete precipitate with high selectivity while suppressing the precipitation of a trace amount of harmful metals.

[0007]

In general, alkali metal salts have high water solubility, but their solubility in alcohols is low.
However, if alcohols are simply added to a solution containing a high concentration of alkali metal salt and a trace amount of harmful metal, complete precipitation and separation cannot be performed such that the alkali metal precipitate becomes a gel, and both components cannot be separated. The selective separation of is deteriorated. .

It is a procedure for adjusting the pH with the hydrogen ion supplying substance, and it is desirable that the pH is adjusted to 0-7. Here, the pH is not set to 0 or less because the consumption amount of the acid becomes too large, and the pH is not set to 7 or more because the precipitation of the alkali metal is also incomplete in this case. Note that nitric acid, hydrochloric acid, or the like can be used as the hydrogen ion supply substance.

Regarding the amount of alcohols added,
Volume ratio to the solution after addition of hydrogen ion supply material is 1
.About.7 times, preferably about 3 to 7 times is effective. That is, when the addition amount is less than 1 time, almost no precipitate of alkali metal salt is generated, while when the addition amount is more than 7 times, the increase in the precipitation amount is not observed, and the effect is obtained only by consuming alcohol excessively. Because there is no. Water-soluble alcohols such as methanol and ethanol can be used as the alcohols.

[0010]

EXAMPLES The present invention will be described in detail below based on various examples according to the present invention and reference comparative examples.

Example 1 Simulating a low-level radioactive waste liquid generated from a nuclear power plant or the like, 3 mol of sodium nitrate as a main component of an alkali metal salt, 1 mol of sodium carbonate and 1 mol of sodium nitrite as other components. Mole,
1 mol of caustic soda was dissolved in pure water to make 1000 ml, and Cs of 100 was added to this solution as a simulated nuclide.
A solution was prepared by adding only 0 ppm. Then, 50 ml of the solution (hereinafter referred to as "simulated waste liquid") is collected, nitric acid is added to adjust the pH to 7, and ethanol 150 is added.
When ml was added, the alkali metal salt separated and settled. Next, the precipitated alkali metal salt is detected by filtration,
After washing it with ethanol, it was dissolved in pure water, and nitric acid was added to the solution to heat it in an acid state to remove ethanol. Further, the filtrate and the above-mentioned washing liquid were heated in a state of being acidified by adding nitric acid to remove ethanol.
Then, the concentration of the alkali metal salt and the concentration of Cs in each solution after finally removing ethanol were measured and used as the analysis result.

Example 2 50 ml of the simulated waste liquid was sampled, nitric acid was added to adjust the pH to 3, and 150 ml of ethanol was added to precipitate and separate the alcohol metal salt. Then, the precipitated alkali metal salt is the same as in Example 1 above.
Was treated in the same manner as in.

Example 3 50 ml was taken from the simulated waste liquid, pH was adjusted to 1 by adding nitric acid, and 150 ml of ethanol was added to precipitate and separate the alcohol metal salt. Then, the precipitated alkali metal salt is the same as in Example 1 above.
Was treated in the same manner as in.

Comparative Example 1 50 ml was taken from the simulated waste liquid, and in this comparative example, nitric acid was not added (pH 14).
State), 150 ml of ethanol was added. As a result, although the alcohol metal salt was precipitated and separated incompletely,
The precipitated alkali metal salt was treated as in Example 1 above.

Comparative Example 2; 50 ml was taken from the simulated waste liquid, pH was adjusted to 7 by adding nitric acid, and the mixture was heated to a volume ratio of about 1/2 without adding ethanol to evaporate and concentrate. As a result, the alkali metal salt was precipitated and separated. Then, the precipitated alkali metal salt is the same as in Example 1 above.
Was treated in the same manner as in.

Comparative Example 3 50 ml was taken from the simulated waste liquid, pH was adjusted to 1 by adding nitric acid, and the mixture was heated to a volume ratio of about 1/2 without adding ethanol to evaporate and concentrate. As a result, the alkali metal salt was precipitated and separated. Then, the precipitated alkali metal salt is the same as in Example 1 above.
Was treated in the same manner as in.

Comparative Example 4; 50 ml was taken from the simulated waste liquid, and in this comparative example, when no nitric acid or ethanol was added and the mixture was heated to a volume ratio of about 1/2 to evaporate and concentrate, alkali metal salts were formed. Segregated. Then, the precipitated alkali metal salt was treated in the same manner as in Example 1 above.

The solutions obtained in Examples 1 to 3 and Comparative Examples 1 to 4 and the analysis results thereof are shown in Table 1 below.

[0019]

[Table 1]

As is clear from Table 1, in Examples 1 to 3, 50 to 57% of the alkali metal salts contained in the simulated waste liquid were compared with Comparative Examples 1 to 4 in terms of the amount of alkali metal salt settling.
In the above, 48 to 76% of the alkali metal salt is removed as a precipitate. Therefore, the amount of alkali metal salt removed from the simulated waste liquid is not so different. However, looking at the column of Cs in Table 1, in Comparative Examples 1 to 4, 20% or more of Cs in the simulated waste liquid was precipitated together with the alkali metal salt in the sediments, whereas in Example 1 In Nos. 3 to 3, the precipitation amount of Cs is suppressed to 8.65% or less. That is, in Examples 1 to 3, it is possible to separate the alkali metal salt while leaving Cs in the solution by the difference in the amount of precipitation thereof as compared with Comparative Examples 1 to 4. In other words, the selective separation property between the alkali metal salt and Cs is excellent. Further, the precipitates in Examples 1 to 3 are precipitated as completely solidified precipitates, whereas in Comparative Example 1 in which nitric acid is not added, they are precipitated only as gel-like incomplete precipitates.

The superiority of the selective separation in Examples 1 to 3 is extremely significant. The reason is that in the treatment of low-level radioactive liquid waste, it is ideal to reduce the volume of a solution containing only harmful metals such as radionuclides to seal and solidify, and use harmless alkali metal salts to reduce the volume. This is because it is necessary to separate them efficiently.

By the way, in the precipitates obtained in the above Examples 1 to 3, Cs still adhered to the precipitates of alkali metal salts
Is included and needs to be removed. For that purpose, when it is again dissolved in water and the same procedure as in Examples 1 to 3 is repeated, Cs can be finally left on the solution side and only the alkali metal salt can be separated as a precipitate. Even in this case, due to the excellent selective separation properties of Examples 1 to 3, it is possible to separate only the alkali metal salt in a number of times much smaller than in the case of repeating the procedure of the comparative example. Further, in this case, since the number of times of dissolution in water is reduced, the amount of heat for evaporating and condensing the solution side to reduce the volume is also reduced, and energy can be saved.

Further, in this embodiment, the simulated waste liquid (1
Nitric acid (4.4 ml) was added to 5.6 ml) to adjust the pH to 1, and 8 sets of solutions (solution corresponding to Example 3) were prepared, and 10 ml to 200 ml for each solution.
Range (range of 0.5 to 10 times the volume of the solution)
The amount of ethanol was changed by adding with and the concentrations of the alkali metal salt remaining in the solution and the precipitated alkali metal salt were analyzed. The analysis results are shown in Table 2 below, and based on the table, the relationship between the amount of ethanol added and the concentration of the precipitated alkali metal salt is shown in a graph of FIG.
become that way.

[0024]

[Table 2]

As is clear from Table 2 and FIG.
When the amount of ethanol added is 1 to 7 times the volume ratio of the solution, the alkali metal salt can be precipitated, and especially 3
When it is set to ˜7 times, a large amount can be precipitated and deposited. When the addition amount was less than 1 time, almost no sedimentation was observed, and when the addition amount was more than 7 times, the sedimentation amount was not increased and only the consumption amount of ethanol was increased.

[0026]

INDUSTRIAL APPLICABILITY As described above, according to the present invention, a high-concentration alkali metal salt is precipitated and separated with high selectivity from a solution containing a high-concentration alkali metal salt and a trace amount of harmful metal, and only the harmful metal is efficiently contained in the solution. It is possible to concentrate it.
Therefore, by applying the treatment method of the present invention to the treatment process of low-level radioactive waste liquid discharged from a nuclear power plant, etc., from the waste liquid containing a large amount of alkali metal salt, which is safe with dangerous radioactive nuclides, etc. Enables efficient separation of only alkali metal salts, making it possible to significantly reduce the volume of radioactive waste compared to the conventional case where alkali metal salts are also included in the treatment of radioactive waste. To solve the urgent issue of securing storage facilities. In addition, since the alcohols used in the present invention volatilize at an extremely low temperature and the number of repeated treatment steps for complete separation is small due to the excellent selectivity of the present invention, all the waste liquid is evaporated and concentrated as in the conventional method. It will be possible to process with a small amount of heat compared to the case where
Achieve energy savings in the radioactive waste treatment process.

[Brief description of drawings]

FIG. 1 is a graph showing the concentration of precipitated alkali metal salt with respect to the amount of ethanol added to a solution.

Claims (2)

[Claims] 1. A method of adding a hydrogen ion supplying substance and alcohols to a solution containing a high-concentration alkali metal salt and a trace amount of harmful metal to selectively precipitate and separate and remove the alkali metal salt in the solution. A method for treating a solution containing a characteristic high-concentration alkali metal salt and a trace amount of harmful metal. 2. The addition amount of the hydrogen ion supplying substance is set so that the hydrogen ion concentration index (pH) of the solution is 0 to 7, and the addition amount of alcohols is set to the solution after the addition of the hydrogen ion supplying substance. On the other hand, the method for treating a solution containing a high-concentration alkali metal salt and a trace amount of harmful metal according to claim 1, wherein the volume ratio is set to 1 to 7 times.