JP3845883B2 - Treatment method of chemical decontamination waste liquid - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for treating a decontamination waste liquid generated when decontamination is performed with a chemical decontamination liquid containing a chelating agent. Here, the “chelating agent” means not only a chelating agent such as EDTA (ethylenediamine tetraacetate) and NTA (nitriloacetate) but also a chemical that can form a chelate compound with a metal such as citric acid and oxalic acid.
[0002]
[Prior art]
For example, when the surface of a metal product such as a pipe of a nuclear power plant or a tool is contaminated with radioactive substances, a mixed solution of a chelating agent as described above and a reducing agent such as hydrazine or ascorbic acid is brought into contact with the surface. Decontamination work to remove the pollutants.
[0003]
In such decontamination work, a relatively large amount of radioactive waste liquid is generated although it is at a low level. Low-level radioactive waste is usually treated by cement consolidation after necessary concentration, but concentration consumes a lot of energy and is uneconomical, with or without concentration. There is a concern in treating the waste liquid containing selenium because of the reaction between the Ca component in the cement and the chelating agent.
[0004]
Decontamination using a chelating agent is a preferable decontamination method for facilities without having the strong decontamination power and the disadvantage of damaging the object to be decontaminated in contrast to the decontamination using a strong acid. However, since the decontamination power is mild, there is a problem that the amount of waste liquid generated increases, and the waste liquid has a high COD concentration, so that the treatment is troublesome.
[0005]
[Problems to be solved by the invention]
The purpose of the present invention is to treat chemical decontamination waste liquids using chelating agents so that the final waste is of no concern for the cement solidification process and to mitigate environmental issues . Another object is to provide a treatment method that reduces the cost of treatment and reduces the amount of waste generated by collecting and reusing the chelating agent .
[0007]
[Means for Solving the Problems]
As shown in the flowchart of FIG. 1 , the processing method of the decontamination waste liquid that achieves the above-described object of the present invention occurs when decontamination is performed with a chemical decontamination liquid containing a chelating agent, which includes the following steps. How to treat waste liquid :
A metal hydroxide precipitation separation step of adding a caustic alkali to a chemical decontamination waste liquid containing a metal chelate to precipitate the metal as a hydroxide, and separating the precipitate;
A chelating agent recovery step of adding a mineral acid to the remaining waste liquid from which the metal hydroxide has been separated to precipitate a chelating agent, and separating and recovering the deposit;
The chelating agent present in the remaining liquid was separated chelating agent is decomposed by the electrolytic process for the anodic oxidation reaction, dissolved chelating agent treatment step;
A decontamination liquid preparation step in which caustic and water are allowed to act on the chelating agent recovered in the chelating agent recovery step to regenerate the decontamination chelating agent to replenish the component loss of the decontamination solution; and
The electrolytically treated liquid is filtered to remove solids, and the liquid is treated with reverse osmosis or ion exchange resin to remove inorganic ions to obtain reclaimed water, and this reclaimed water is directed to the decontamination liquid preparation process. Process.
[0008]
The means for oxidizing the chelating agent remaining in the liquid is based on electrolytic oxidation as described above. That is, the chelating agent is oxidized by an anodic reaction and converted into a simple compound such as CO ₂ or H ₂ O.
[0010]
The hydroxides were separated by a metal hydroxide precipitation separation step, and washed with recycled water obtained in the regeneration water obtaining step, by washing water is treated with electrolytic treatment step, metal hydroxide is substantially attachment of chelating agents The subsequent processing becomes easier.
[0011]
The pH value to be realized in each step is as follows. Addition of caustic alkali in the metal hydroxide precipitation separation step is performed until the pH of the solution becomes 11 or more, and addition of mineral acid in the chelating agent recovery step is performed until the pH of the solution becomes 2 or less. The addition of caustic alkali in the preparation process is performed so that the pH of the liquid is in the range of 3-5.
[0012]
[Action]
The waste liquid obtained by decontaminating piping or the like in a nuclear facility with a decontaminating agent containing a chelating agent contains a radionuclide, so this is first separated from the chelating agent. This is achieved by increasing the pH of the solution to lose the chelating effect. The amount of caustic added for this purpose is in the range of 0.1 to 0.2 mol / l, and the pH of the solution is selected to be 11 or more as described above. It is sufficient that the pH is 11.5 or more. If the pH is set to about 12.5, the precipitation rate of iron hydroxide by decomposition of the iron chelate can be 99% or more in terms of removal rate.
[0013]
However, other nuclides contained in trace amounts cannot always be removed at the same rate. According to the results obtained by the inventors, NaOH was added to the chelating agent-containing decontamination waste liquid having a contamination (radioactivity) density of 24.4 Bq / cm ³ to increase the pH to 12.5, thereby precipitating the metal. After that, the contamination density is 1.6 Bq / cm ³ , and the removal rate as the contamination density is only 93.4%. The treatment of low-level radioactive waste, pollution density of 10 ^-2 Bq / cm ³ or less, must be a value or less close to the detection limit.
[0014]
In view of this experience, the remaining metal chelates must be further processed. However, if a chelating agent is present, it may form a complex complex with the nuclide metal, and the process will not proceed unless the chelate is destroyed.
[0015]
In the present invention, in order to destroy the chelate, a mineral acid is added to the liquid from which the metal hydroxide precipitate has been removed to precipitate the chelating agent in the form of a sparingly soluble acid. By setting the pH to a low value of 1-2, the solubility of the chelating agent that is an acid is minimized. This effect is particularly noticeable in chelating agents such as EDTA and NTA, and citric acid and oxalic acid have considerable solubility even at low pH, so the recovery rate is limited.
[0016]
In any case, by adding mineral acid to precipitate the chelating agent, the radionuclide that could not be separated as a hydroxide in the metal hydroxide precipitation separation step is left on the liquid side and separated from the chelating agent. be able to.
[0017]
Although the amount of the chelating agent recovered by separating the precipitate by filtration is small, the radionuclide is attached to the chelating agent. The fastest treatment is incineration, which can significantly reduce the volume. The ash generated by incineration contains radioactive materials, so it must eventually be solidified, and it is necessary to use a high-performance filter that prevents the ash from scattering, but it is also suitable for other incinerations. If it is performed in conjunction with waste disposal, it can be implemented without requiring special equipment.
[0018]
Chelating agents, especially EDTA, NTA, etc., which are hardly soluble in a strong acid and can be recovered at a high rate, are not inexpensive and therefore are desired to be reused as much as possible. Therefore, in a preferred embodiment of the present invention, regeneration is performed as a processing method superior to incineration. If caustic and water are added to the recovered chelating agent to adjust the pH to 3 to 5, the chelating agent having a decontamination ability is regenerated. By depleting the loss and preparing the ingredients, a decontamination solution can be prepared for the next operation.
[0019]
The chelating agent dissolved in the remaining liquid obtained by filtering the deposited chelating agent is oxidized. The electrolytic oxidation recommended as an advantageous oxidation means is preferably carried out with the liquidity being alkaline with a pH of 11 or higher. The electrolysis can be performed according to known techniques. A noble metal plate or a noble metal-coated titanium plate is used for the anode, a stainless steel plate is used for the cathode, and direct current is applied.
[0020]
When the chelating agent is oxidatively decomposed by the electrolytic treatment, the metal bonded thereto precipitates as a hydroxide. Since this precipitate is small in amount and dispersed in the liquid, it is separated by filtration. The separated solid content is sent to the solidification system together with the metal hydroxide precipitated by the first caustic addition, and solidified. Thereby, the radionuclide can be disposed of in a confined form.
[0021]
The filtrate is reduced to such a level that the radioactive contamination meets the emission criteria, but it is preferably reused to complete the closed system. In that case, an acid is first added, and the inorganic ions are removed by performing reverse osmosis or ion exchange resin treatment after expelling the dissolved CO ₂ produced by oxidative decomposition. If CO ₂ remains, a carbonate is formed with the alkali, and the life of the reverse osmosis membrane and the ion exchange resin is remarkably shortened. Recycled water is used to prepare a decontamination solution by dissolving the regenerated chelating agent as described above. Such recycling can reduce the total amount of decontamination waste liquid to be processed. A portion of the reclaimed water can also be used to wash the first precipitated metal hydroxide. If the chelating agent is substantially removed from the metal hydroxide by washing, the influence on cement solidification can be eliminated.
[0022]
【Example】
To 600 liters of the following waste liquid generated as a result of decontamination work,
EDTA-2Na concentration about 1% by weight
Fe concentration 200ppm
Contamination density 24.4 Bq / cm ³
The pH of the liquid was adjusted to 12.5 by adding 2.4 kg (4 g / l) of NaOH. The resulting precipitate (mainly ferric hydroxide Fe (OH) ₃ ) was separated by filtration to obtain a solid content having a water content of 80%. By washing about 100 g of this precipitate four times with 500 ml of water each time, the residual chelate concentration in the water contained in the solid content could be lowered from 10,000 ppm to a concentration below the detection limit.
[0023]
When the pH was lowered to 1.5 by adding sulfuric acid to the filtrate, EDTA precipitated and was separated by filtration. Changes in the EDTA concentration and COD of the liquid before and after the addition of sulfuric acid are as follows.
[0024]
EDTA concentration 9622 → 186 (ppm) Recovery 98.1%
COD _Mn 6690 → 216 (ppm) Removal rate 96.8%
The EDTA recovered by filtration was 5,886 kg (the recovery rate was 98.1% as described above). This was dispersed in 600 liters of water, and the pH was adjusted to 4.0 by adding NaOH, followed by stirring. Since it melt | dissolved, 0.114 kg corresponding to loss (1.9%) was replenished, and the decontamination liquid used for the next decontamination work was able to be prepared.
[0025]
NaOH was again added to the filtrate to increase the pH to 12.5, and the dissolved chelate was oxidatively decomposed by electrolytic treatment. The electrolysis conditions are as follows.
[0026]
Electrode: Both anode and cathode Pt / Ir alloy-coated Ti plate facing area 0.6 m ² each
Distance between electrodes: 5 mm Parallel plate current density: ² kA / m ²
COD could be reduced to 20 ppm or less by electrolysis for 2 hours and 45 minutes. In the case where a liquid having a COD value of about 7000 ppm after separation of the metal hydroxide without EDTA precipitation due to the addition of sulfuric acid is treated as it is by electrolysis, it is necessary to reduce the COD to 20 ppm or less. It takes time. FIG. 2 shows a decomposition curve when electrolysis is performed from 6690 ppm of COD.
[0027]
Using the decontamination solution prepared from the recovered EDTA as described above, the comparison before and after the addition of sulfuric acid is as follows:
EDTA concentration 9732 → 181 (ppm) Recovery 98.1%
COD _Mn 7102 → 199 (ppm) Removal rate 97.2%
From this result, it can be seen that EDTA should be supplemented with a loss of 1.9%, that is, 0.114 kg each time.
[0028]
Considering the decontamination work that uses 5 liters of 600 liters of decontamination solution for a total of 3 m ³ , what used to be 30 kg in total for 6 kg of EDTA each time would be Although 6kg is used for the times, it is sufficient to supplement the losses for the second to fifth times, so a total of 6.57kg is sufficient. In the electrolytic treatment, when EDTA is not collected, it takes 9 hours each time, and it takes 45 hours in total. However, if EDTA is collected and CDO is reduced, 2.75 hours each time, What is necessary is just to perform 13.75 hours by 5 times.
[0029]
【The invention's effect】
By performing the process of chemical decontamination waste liquid by the method of the present invention, even though waste is able to eliminate the influence of the cementation containing chelating agent, less damage to the decontamination object, chelate The concern associated with the use of the agent is eliminated.
[0030]
In addition , expensive chelating agents can be used over and over again with a few percent loss, reducing decontamination costs and reducing the amount of waste that must be finally disposed of, as well as the burden on the environment. Can be reduced.
[Brief description of the drawings]
FIG. 1 is a flowchart showing a preferred embodiment in the chemical decontamination waste liquid treatment method of the present invention.
FIG. 2 is a graph showing a decrease in CDO over time when a liquid containing a chelating agent is electrolytically treated in the process of decontamination waste liquid.

Claims (3)

A method for treating a waste liquid generated when decontamination is carried out with a chemical decontamination liquid containing a chelating agent, comprising the following steps:
A metal hydroxide precipitation separation step of adding a caustic alkali to a chemical decontamination waste solution containing a metal chelate to precipitate the metal as a hydroxide, and separating the precipitate;
A chelating agent recovery step of adding a mineral acid to the remaining waste liquid from which the metal hydroxide has been separated to precipitate a chelating agent, and separating and recovering the deposit;
A dissolved chelating agent treatment step in which the chelating agent present in the remaining liquid from which the chelating agent has been separated is decomposed by electrolytic treatment that performs an anodizing reaction ;
A decontamination solution preparation step in which caustic and water are allowed to act on the chelating agent recovered in the chelating agent recovery step to regenerate the decontamination chelating agent and replenish the component loss of the decontamination solution; and
The electrolytically treated liquid is filtered to remove solids, and the liquid is treated with reverse osmosis or ion exchange resin to remove inorganic ions to obtain reclaimed water, and this reclaimed water is directed to the decontamination liquid preparation process. Process. The metal hydroxide separated in metal hydroxide precipitation separation step, and washed with recycled water obtained in the regeneration water acquisition process, processing method according to claim 1, the washing water is treated with an electrolytic process. In the hydroxide precipitation separation process, caustic is added until the pH of the liquid becomes 11 or higher, and in the chelating agent recovery process, mineral acid is added until the pH of the liquid becomes 2 or lower, in the decontamination liquid preparation process. The processing method according to claim 1 , wherein the caustic alkali is added so that the pH of the solution is in the range of 3 to 5.

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