JPH0452916B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating used waste liquid from chemical decontamination in which pipes, equipment, and metal waste of a nuclear power plant are cleaned with a decontamination liquid containing an organic acid or a chelating agent. In particular, it relates to a method of treatment by oxidation.

In nuclear power plants, as the operating time increases, radioactive materials accumulate on the inner surfaces of primary system piping, equipment, etc., and the dose rate increases. Therefore,
The chemical decontamination method, in which a liquid containing dissolved chemicals is poured into pipes and equipment to dissolve and remove attached radioactive substances, is said to be an effective method, and is used mainly in PWR plants and CANDU plants overseas. There are many achievements in this field, and technological development including prevention of corrosion of equipment piping, etc. is actively being carried out in Japan.

It is said that most of the radioactive substances attached to primary system piping and equipment exist in a state of being incorporated into metal oxides such as magnetite.
Therefore, in chemical decontamination, a decontamination method that combines a chelating agent, etc., with an inhibitor, etc., which reduces metal oxides and prevents the re-deposition of dissolved organic acids and dissolved metal ions, and retains them in the liquid as a stable complex, is recommended. Many dye solutions have been developed. In the case of using a decontamination liquid containing such an organic acid or a chelating agent, the decontamination waste liquid contains metals in the form of ions or complexes in addition to the organic acid and the chelating agent. Among such methods for treating decontamination waste liquid, the ion exchange resin method is said to be most suitable when the concentration of chemicals in the decontamination liquid is low, that is, in the case of the dilute solution method. Furthermore, in order to obtain high decontamination performance, when using highly concentrated chemicals, that is, in the case of the concentrated liquid method, the concentration solidification method has been often used. However, in the ion exchange resin method used in the dilute liquid method, increasing the concentration of organic acids and chelating agents in the liquid results in a huge amount of waste resin, and in the concentration solidification method, the equipment becomes large.
Application is difficult if existing equipment cannot be used.

In response to this situation, recently, decontamination has been carried out using a mixture of organic acid and a reducing agent as a decontamination solution, and in order to treat the waste solution, electrolysis of the waste solution is carried out using carbon, etc. as an electrode in an electrolytic tank, and an anode is used to treat the waste solution. A method has been proposed in which the organic acid is decomposed by the oxidation action in the oxidation process, the precipitated metal hydroxide is separated into solid and liquid, and the cake is solidified.
Furthermore, in the above method, a method of oxidizing using hydrogen peroxide as an oxidizing agent instead of oxidizing at the anode of the electrolytic cell has also been proposed.

However, in the method of oxidizing waste liquid at the anode of an electrolytic cell, for example, when an organic acid with a large number of carbon atoms, such as citric acid, is used as a decontamination liquid, it takes a lot of electricity to decompose the organic acid. It takes time and is inefficient. In addition, in the method using hydrogen peroxide, a reaction called Fuenton oxidation is expected due to the divalent iron ions contained in the decontamination waste liquid.
Although the oxidation efficiency can be improved, the concentration of iron dissolved in the decontamination waste fluid varies depending on the decontamination time and the amount of crud (metal oxides attached to the object to be decontaminated), and can be as low as several hundred ppm. Sometimes it is low. When the concentration of iron is low as described above, the efficiency of oxidation by hydrogen peroxide becomes low. In addition, in these treatment methods, when the organic acid decomposes, the pH increases and metals such as iron are precipitated as hydroxides, but hydroxides have poor dehydration properties and create a large amount of secondary waste. There is.

In this way, methods that combine oxidation operations using electrolysis or hydrogen peroxide and operations to separate precipitated hydroxides for the treatment of waste liquid from chemical decontamination have poor oxidation efficiency and are likely to cause problems in secondary waste. It has the disadvantage of being large in quantity.

The purpose of the present invention is to eliminate the drawbacks of the previous prior art, to efficiently oxidize organic acids and chelating agents in decontamination waste liquid, and to produce chemical decontamination waste liquid that produces less secondary waste. The object is to provide a treatment method, the object of which is achieved by increasing the iron concentration in the decontamination waste liquid prior to the oxidation operation with hydrogen peroxide.

Specifically, the method for treating decontamination waste liquid according to the present invention includes a step of injecting the waste liquid into an electrolytic cell equipped with an electrode mainly composed of iron and an insoluble electrode, an electrode mainly composed of iron as an anode, A process of dissolving iron ions in waste liquid by electrolysis using an insoluble electrode as a cathode, a process of electrolyzing while adding hydrogen peroxide with an iron-based electrode as a cathode and an insoluble electrode as an anode, hydrogen peroxide The method is characterized by comprising the steps of stopping the addition of the chlorine and continuing electrolysis to precipitate heavy metal components on the cathode, and filtering the electrolytically treated waste liquid and then bringing it into contact with a cation and anion exchange resin.

Organic acids and chelating agents are efficiently oxidized when the iron concentration in the liquid is high. In the method of the present invention, in order to increase the iron concentration, electrolysis is performed in an electrolytic bath using a material containing iron as an anode to dissolve iron. After increasing the iron concentration in this way, electrolysis is performed by switching the anode to an insoluble electrode such as a carbon plate, and at the same time oxidizing organic acids and chelating agents while adding hydrogen peroxide, the concentration of these organic substances is reduced to a certain level. Once this is done, the addition of hydrogen peroxide is stopped, the molten iron and most of the pre-dissolved metals are deposited on the cathode, and the secondary waste can be discharged as a solidified product with a small volume.

Suspended substances present in the liquid are removed by filtration, and small amounts of dissolved organic substances and metals can be easily removed by ion exchange.

Next, the present invention will be explained in detail based on the drawings.

The drawing is a flow sheet of an apparatus for carrying out the method of the invention. This device mainly consists of an electrolytic cell 1, a DC power supply 2 that supplies direct current to the electrolytic cell 1, a filter 8 that removes suspended matter contained in the liquid treated in the electrolytic cell 1 at a later stage, and a filter 8 connected at a later stage. It consists of a bed-type ion exchange resin tower 9 and a hydrogen peroxide adjustment tank 10 connected to an electrolytic tank 1. Further, in the electrolytic cell 1, an electrode 3 whose main component is iron, for example, an electrode 3 made of SS41 carbon steel, and an electrode 4 made of a carbon plate are installed, and a stirrer 6 is installed between these electrodes. is provided. The electrodes 3 and 4 are connected to a DC power supply by wiring, and a polarity switching device 5 is disposed between them.

When processing using this waste liquid treatment device, a certain amount of decontamination waste liquid is allowed to flow into the electrolytic cell 1 through the inflow pipe 21. In addition to the organic acids and chelating agents used as the decontamination liquid, the decontamination waste liquid contains low-molecular-weight organic substances produced by decomposition of these organic substances during decontamination operations and metals mainly consisting of iron. The temperature of the liquid may be the state immediately after the decontamination operation, that is, high temperature, or after the decontamination operation, it may be temporarily stored in a storage tank,
It may be at room temperature. Electrodes 3 and 4 are installed in the electrolytic cell 1, and a polarity switching device 5 is set so that the electrode 3 containing iron serves as an anode and the electrode 4 made of a carbon plate serves as a cathode. When a current is applied by the DC power supply 2, the anode is a plate mainly composed of iron as described above, and since the decontamination waste liquid is strongly acidic, a dissolution reaction of iron occurs at a relatively low current density. This process is called the iron melting process.

The step of dissolving iron is carried out until the iron concentration in the liquid is effective against the oxidation step described later and is maintained at a level where precipitation on the cathode is kept small.

When iron reaches a desired concentration, the polarities of electrodes 3 and 4 are switched, and electrolysis is performed with electrode 3 serving as a cathode and electrode 4 serving as an anode. In particular, while hydrogen peroxide is added little by little from the hydrogen peroxide adjustment tank 10 to the electrolytic tank 1 via the pipe 22, the inside of the tank is stirred using the stirrer 6.
This operation is called an oxidation step. That is, since electrolysis is carried out using an insoluble carbon plate as the anode, no metal is eluted from the anode except for a small portion of the metal precipitated on the carbon plate of the previous melting device.
Oxygen generation or oxidative decomposition of organic acids and chelating agents takes place. In addition, throughout the tank, organic acids and chelating agents are oxidized and decomposed by hydrogen peroxide, but since the iron concentration in the waste liquid has increased in the previous iron dissolution process, as shown in the equation below, Hydroxyl free radicals, OH, promote the oxidation of organic acids through the Fuenton reaction.

H ₂ O ₂ +Fe ²⁺ →Fe ³⁺ +HO ^- +・OH During this process, hydrogen is generated and dissolved metals such as iron are deposited, that is, electrodeposition occurs at the cathode.

When the concentration of the organic acid or chelating agent in the waste liquid decreases to a certain level, the supply of hydrogen peroxide is stopped, and the electrolytic operation is continued with the polarities of the electrodes 3 and 4 of the electrolytic cell 1 unchanged. This operation is called an electrodeposition process. In this step, oxygen continues to be generated and the remaining organic acid is decomposed at the electrode 4, which serves as the anode, and hydrogen is generated and metal, such as iron, is deposited at the cathode 3. In this electrodeposition step, as the concentration of organic acid, chelating agent, and metal in the solution decreases, the voltage applied between the electrodes increases, and power consumption rapidly increases. Then, the electrolytic operation is stopped and the electrodeposition process is completed.

After the electrodeposition step is completed, the decontamination waste liquid is sent by the pump 7 through the filter 8 to the mixed bed type ion exchange resin tower 9, and then discharged as treated water through the outflow pipe 23.

The filter 8 removes suspended matter in the liquid, such as small flakes of metal deposited on the electrode 3 during the electrodeposition process, particles of compounds of organic acids and metals, and particles of metal hydroxides.

Further, in the mixed bed type ion exchange resin tower 9, organic acids, chelating agents, metal ions, complexes, etc. remaining in the liquid are removed.

Note that the drawing shows a case in which the dissolution process, oxidation process, and electrodeposition process are all performed in one electrolytic tank, but two
Using two electrolytic tanks, the melting process is performed in one electrolytic tank,
On the other hand, an oxidation step and an electrodeposition step may be carried out, in which case an electrode polarity switching device is no longer required and continuous operation is possible.

By the above operation, it is possible to efficiently oxidize the organic acid or chelating agent in the decontamination waste liquid in a short time with low power consumption and hydrogen peroxide consumption, and to reduce the amount of secondary waste discharged after treatment. The amount can also be significantly reduced.

Next, the present invention will be explained in detail based on examples.
The present invention is not limited to this.

Example A mixed solution of 0.3M oxalic acid and 0.2M citric acid was used as a decontamination solution, and this was circulated at a temperature of about 90° C. for 24 hours to decontaminate the inner surface of SUS piping. The waste liquid discharged after this decontamination was treated according to the flow sheet shown in the drawing. This decontamination waste liquid has an iron concentration of approximately 800mg/, COD-Mn of approximately 22000mg/, and a pH of approximately 1.5.
showed that. In the electrolytic cell, an electrode made of SS41 carbon steel was installed as electrode 3, and an electrode made of carbon plate was installed as electrode 4, and a current was passed using the carbon steel electrode as the anode so that the current density in the cell was about 5 A/dm ² . did. Iron was dissolved by electrolysis until the iron concentration in the liquid was approximately 3000 mg/.

Next, the polarity of the electrodes in the electrolytic cell was switched, and electrolysis was started using the carbon steel electrode as the cathode, and
While stirring 35% hydrogen peroxide solution little by little into the electrolytic cell, a current was applied so that the current density was about 10 A/dm ² . When the organic acid concentration significantly decreased, the addition of hydrogen peroxide solution was stopped, and the electrolysis operation was continued, and stopped when the power consumption suddenly increased. At this stage, COD−Mn in the waste liquid
The concentration was about 500 mg/metal, and the metal concentration, including dissolved iron, was about 400 mg/min.

Furthermore, the remaining COD components and metals could be almost completely removed by passing the mixture through a filter and an ion exchange resin column.

The power consumption required to oxidize oxalic acid and citric acid in this example is 1/5 of that in the case of only electrolytic oxidation, and the amount of hydrogen peroxide used is 1/5 of that required for oxidation with only hydrogen peroxide. It was 1/3 of that.
Furthermore, the amount of secondary waste was approximately 1/2 that in the case of electrolytic oxidation alone or hydrogen peroxide oxidation alone, in which the hydroxide sludge discharged was separated by sedimentation and solidified with cement.

[Brief explanation of drawings]

The drawing is a flow sheet of an apparatus for carrying out the method of the invention. 1... Electrolytic layer, 3, 4... Electrode, 5... Polarity switching device, 8... Filter, 10... Hydrogen peroxide adjustment tank.

Claims (1)

[Claims] 1 In a method for treating used waste liquid of chemical decontamination liquid containing organic acids, chelating agents, and heavy metals, the process of injecting the waste liquid into an electrolytic tank equipped with an electrode mainly composed of iron and an insoluble electrode, A process in which iron ions are dissolved in waste liquid by electrolysis using an electrode containing iron as an anode and an insoluble electrode as a cathode, while adding hydrogen peroxide with an electrode mainly composed of iron as a cathode and an insoluble electrode as an anode. Consisting of a step of electrolyzing, a step of stopping the addition of hydrogen peroxide and continuing electrolysis to precipitate heavy metal components on the cathode, and a step of filtering the electrolytically treated waste liquid and then contacting it with a cation and anion exchange resin. A method for treating chemical decontamination waste liquid, characterized by: