JPH0527093A - Processing of radioactive metallic sludge - Google Patents

Abstract

PURPOSE:To obtain the processing method for the radioactive metallic waste with which the quantity of the secondary waste can be reduced, the waste can be recovered as the metallic lump having the small bulkiness, and the volume reducing effect can be improved. CONSTITUTION:A radioactive metallic waste 4 whose surface is contaminated by radioactivity is electrolyte-dissolved in a neutral salt electrolytic liquid 1 such as sodium salt, and the metal hydroxide sludge 5 formed at this time is concentrated and separated in a concentrating tank 6. The concentrated sludge (metallic sludge) 7 is introduced into an electrolysis tank 9 filled with the cleaning liquid containing at least one kind of organic acid among oxalic acid, formic acid, and citric acid, and the metallic sludge 7 is dissolved as metallic ion, and the organic acid is oxidation analyzed by the electrolytic operation, and analyzed to carbon dioxide and water. The dissolved metallic ion is electrolyzed on a cathode 11, and recovered as metallic lump 14 having the small bulkiness.

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 radioactive metal sludge, and more particularly to a method for treating metal hydroxide sludge produced by electrolysis dyeing of radioactive metal waste in a neutral salt electrolyte.

[0002]

2. Description of the Related Art Radioactive metal waste generated in a nuclear power plant is mainly waste material such as pipes and valves. Conventionally, this kind of waste is put into a container such as a drum can by a method such as cutting it into small pieces and the radioactive Stored and stored in metal waste storage.
However, the number of them has been increasing year by year, and securing a storage place for them has become a big problem. From this, it is desired to reduce the volume of the waste, and a method of reducing the volume by compression or heat melting has been proposed. However, in such a method, since the contaminated part of the surface contaminated with the radioactive material and most of the non-contaminated part are collectively treated, the volume reduction effect cannot be said to be remarkable.

Therefore, various decontamination methods for removing a large amount of the surface contaminated portion and achieving a large volume reduction are being studied. Among them, the electro-deletion dyeing method is most promising in terms of decontamination ability and decontamination speed. This method uses radioactive materials ( ⁶⁰
This is intended for waste materials to which Co, ⁵⁴ _Mn, etc.) have adhered, and only the contaminated surface is electrochemically dissolved and removed. The electro-deletion dyeing method is roughly divided into a method using a strong acid electrolyte such as phosphoric acid and sulfuric acid and a method using a neutral salt electrolyte such as sodium sulfate. Among them, the method using a strong acid is used in the electrolyte solution. When metal ions accumulate in the liquid, the liquid deteriorates,
Each time the liquid has to be renewed, the entire liquid becomes a secondary waste, and the decontamination and volume reduction effect is small. On the other hand, the method using the neutral salt electrolytic solution has the advantage that the electrolytic solution can be easily reused by solid-liquid separation of the metal hydroxide sludge generated in the electrolytic solution, and the amount of secondary waste can be reduced. is there.

[0004]

However, the metal hydroxide sludge is a very fine particle (0.4 μm) and has poor sedimentation and dewatering properties, so that the sludge containing the electrolytic solution is a stable solidified body. Has a problem that it requires a large amount of a solidifying agent such as cement. The object of the present invention is to solve the above-mentioned problems of the prior art, and to recover contaminated metal hydroxide sludge generated during electroless dyeing as a bulky metal mass without increasing the amount of secondary waste due to the solidifying agent. Therefore, it is therefore an object of the present invention to provide a method for treating radioactive metal sludge, which contributes to the electrolysis dyeing method of radioactive metal waste having a more remarkable volume reduction effect.

[0005]

In order to achieve the above-mentioned object, the present invention provides a method of settling and concentrating metal hydroxide sludge produced when electrolytically dissolving radioactive metal waste in a neutral salt electrolyte. After separation, it is introduced into an electrodeposition tank having at least one organic acid selected from oxalic acid, formic acid, and citric acid and dissolved, and the organic acid is oxidatively decomposed into water and carbon dioxide by electrolysis, and dissolved metal ions The electrodeposition is carried out and the metal lump is collected as a small bulk.

[0006]

[Function] When the metal hydroxide sludge is washed with a cleaning liquid containing at least one organic acid selected from oxalic acid, formic acid and citric acid, the hydroxide sludge is dissolved in the organic acid and metal hydroxide is converted into hydroxide. Can remove sludge. The organic acid after washing is decomposed into carbon dioxide and water by electrolytic oxidation. The dissolved metal ions can be recovered as a small-sized metal mass by electrodeposition. Therefore, the volume reduction effect becomes more remarkable.

[0007]

The present invention will be described in detail with reference to the drawings. FIG. 1 is a flow sheet showing an embodiment of the method of the present invention.
The electrode 3 and the radioactive metal waste 4 are placed in an electrolytic cell 2 containing an electrolytic solution 1 of a neutral salt containing a sodium salt such as sodium sulfate as an electrolyte and electrolyzed to remove the surface-contaminated portion of the radioactive metal waste 4. The metal hydroxide sludge 5 containing a radioactive substance is produced by eluting it into the electrolytic solution 1. The metal hydroxide sludge 5 is introduced into the concentration tank 6, separated and concentrated in the concentration tank 6, and the supernatant is reused as an electrolytic solution.

On the other hand, the concentrated sludge 7 is introduced into an electrodeposition tank 9 containing an organic acid 8 such as oxalic acid. Here, a cleaning liquid containing at least one organic acid selected from oxalic acid, formic acid, and citric acid can be used in the electrodeposition tank 9. This is because it is difficult to perform the subsequent treatment with sulfuric acid or hydrochloric acid, which is a mineral acid, as the cleaning liquid. That is, in the case of an organic acid, it can be decomposed into carbon dioxide and water by electrolytic oxidation, but in the case of a mineral acid, it is necessary to neutralize the used cleaning liquid, resulting in an increase in secondary waste. . Further, it is desirable that the temperature of the cleaning liquid be 80 ° C. or lower. If the temperature of the cleaning liquid exceeds 80 ° C., the cleaning liquid may boil and the operation may become difficult.

An insoluble anode 10 and a cathode 11 are placed in the electrodeposition tank 9.
And a stirring pump 12 is further provided. When treating the radioactive metal sludge concentrated using this electrodeposition tank 9, the metal ion concentration, pH, and the concentration of organic acids such as oxalic acid are adjusted in a timely manner. A high ion concentration is desirable. The liquid temperature may be room temperature or high temperature, but about 80 ° C. is preferable from the viewpoint of improving electrolytic efficiency and electrodeposition efficiency. Also, the current density is higher at higher current density, but the current rate is low, but current is low. The density is desirable, and about 0.01 A / cm ² is practical.

The electrolytic reaction of organic acid is C _X H _y O _Z → H ₂ O + CO ₂ (1) and the electrodeposition reaction of metal ions is Fe ²⁺ + 2e → Fe (2). Can be represented.

FIG. 2 is a graph showing the decomposition of oxalic acid by electrolysis, and FIG. 3 is a graph showing the deposition rate of iron. Oxalic acid 0.4 ml /
When metal hydroxide sludge was dissolved in 1 and electrolyzed at a current density of 0.1 A / cm ² in a state of 0.3 ml / l, the oxalic acid decomposition rate in about 2 hours was about 90%, The electrodeposition rate was about 60%. When the electrolysis time is 2 hours or more, both the oxalic acid decomposition rate and the iron electrodeposition rate are less likely to increase, but if oxalic acid and iron ions are supplemented at any time, specifically, the concentrated sludge 7 flows into the electrodeposition tank 9. By doing so, it is possible to prevent a decrease in the electrodeposition rate. Also, concentrated sludge 7
With the introduction of the above, the neutral salt solution is mixed in the electrodeposition tank 9, which is preferable because it increases the conductivity and prevents an increase in the required voltage.

On the other hand, since radioactive metal ions are mainly composed of ⁶⁰ Co and the amount thereof is very small, whether or not the electrodeposition of ⁶⁰ Co ions is carried out efficiently becomes a problem, but as shown in FIG. Under the Fe ²⁺ electrodeposition condition, a trace amount of ⁶⁰ Co ²⁺ could be electrodeposited. This is because the standard electrode potential of Co ²⁺ is nobler than that of Fe ²⁺ , so even if the amount of ⁶⁰ Co ²⁺ is very small,
It is considered that the electrodeposition efficiency was equal to or higher than that of e ²⁺ .

The metal ingot 13 deposited on the cathode 11 in the electrodeposition tank 9 is collected from the electrodeposition tank 9 and stored in the drum 14. In the above examples, the electrodeposition tank 9 filled with oxalic acid was simply used for dissolving the metal hydroxide sludge, but it may be used as a chemical decontamination tank as a pretreatment for the electroless dyeing. In addition, in the above-mentioned examples, a specific experimental example of oxalic acid was shown, but the same results are obtained in the case of formic acid and citric acid other than oxalic acid.

[0014]

As described above, according to the present invention, the radioactive metal sludge produced when the surface-contaminated metal waste is electro-decontaminated with a neutral salt electrolyte can be recovered as a radioactive metal mass. It is possible to significantly reduce the amount of secondary waste.

[Brief description of drawings]

FIG. 1 is a flow sheet showing an embodiment of the present invention.

FIG. 2 is a graph showing the decomposition rate of oxalic acid.

FIG. 3 is a graph showing an iron deposition rate.

FIG. 4 is a graph showing the deposition rate of Co.

[Explanation of symbols]

1 ... Electrolyte 2 ... Electrolyzer 3 ... Electrode 4 ... Radioactive metal waste 5 ... sludge 6 ... concentrator 7: Concentrated sludge 8 ... Organic acid 9 ... Electrodeposition tank 10 ... Insoluble electrode 11 ... Cathode 12 ... stirring pump 13 ... Metal block 14 ... drum

Claims (3)

[Claims] 1. A metal hydroxide produced when a radioactive metal waste whose surface is contaminated with radioactivity is electrolytically dissolved in a neutral salt electrolytic solution to remove the radioactive substance on the surface of the radioactive metal waste. In the method of treating electrolytic wastewater containing sludge,
The metal hydroxide sludge is dissolved as metal ions by a cleaning liquid containing at least one organic acid selected from oxalic acid, formic acid, and citric acid, and the organic acid is oxidatively decomposed by electrolytic operation, and the dissolved metal ions are used as a cathode. A method for treating radioactive metal sludge, characterized in that it is electro-deposited on and recovered as a small metal mass. 2. The method for treating radioactive metal sludge according to claim 1, wherein the metal hydroxide sludge is treated with a neutral salt electrolyte containing sodium salt as a main component. 3. The radioactive metal sludge according to claim 1, wherein the metal hydroxide sludge produced after the electro-deletion dyeing is immersed in the cleaning liquid containing the organic acid and washed at a temperature condition of 80 ° C. or lower. Processing method.