JPS59219499A - Electrolytic decontaminating method by dilute electrolyte - Google Patents

Abstract

PURPOSE:To remove approximately thoroughly the remaining metallic ion in a liquid in a titled decontaminating method of a metallic surface contaminated by a radioactive material by using a dilute aq. soln. of a sulfuric acid as an electrolyte and subjecting the liquid used for electrolytic polishing to filtering, electrolyzing and ion exchanging. CONSTITUTION:About 5vol% aq. sulfuric acid soln. 2 is filled in an electrolytic cell 1 and a decontaminating metal 3 as an anode is electrolytically polished with Ti, etc. as a cathode, by which the surface of the metal 3 is decontaminated. The suspension released in this state is passed through a filter 7 and is separated in the stage of circulating the liquid 2 by a pump 6. The decontaminated metal 3 is washed in a tank 8 and the washing water is circulated by a pump 9 and is reused through an ion exchange tank 10. The electrolyzing operation is continued in the cell 1 until the liquid 2 is used to the limit. The entire volume of the liquid 2 is then transferred by a pump 6 into an electrodeposition cell 12 and while the liquid is stirred with a circulating pump 16, the pH of the liquid 2 is adjusted by the alkali from a pipe 17 and DC is conducted to an anode 14 and a capturing electrode 13. Then the metallic ion in the liquid 2 is deposited on the electrode 13 and the liquid 2 having the radiation level decreased to a reference value or below is passed through a cation exchange tank 20 and is subjected to neutralization, dilution, etc. Such liquid is then discarded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for decontaminating metal surfaces of equipment, parts, etc. used in nuclear power plants and the like and contaminated with radioactive substances by electrolytic polishing.

Among the radioactive substances that accumulate in the electrolyte during the decontamination process, suspended matter such as metal oxides that are released from the surface of the metal to be decontaminated can be removed by solid-liquid separation such as circulation filtration of the electrolyte or sedimentation separation. However, it is not easy to remove radioactive substances that are eluted from the object to be decontaminated and are dissolved in the electrolyte as metal ions, and especially if the used electrolyte There is a need for a method for extracting metal ions from liquids in as concentrated a form as possible.

The electrolytes used in normal electrolytic polishing are often phosphoric acid-based or phosphoric acid-sulfuric acid-based, and have an acid concentration of several 10% and a high viscosity. There are the following problems when processing a static electrolyte.

For example, a part of the electrolyte in use is extracted from the system and metal ions are removed from the electrolyte using an ion exchange resin, etc.
If this solution is to be recovered and reused, it must be diluted with a large amount of water because the electrolyte is a concentrated acid solution containing many generations of metal ions. can be,
Concentration and recovery of the 8 dilute solution after metal ions have been removed requires large equipment and many hot vents, and furthermore, ion exchange resins and the like that have adsorbed metal ions become a large amount of radioactive secondary waste. Even if the used electrolyte is neutralized and disposed of as waste liquid without recovering it, a large amount of radioactive slurry is generated and its disposal is troublesome, and all of these methods generate a large amount of radioactive secondary waste. Therefore, there is a risk that the area of radioactive contamination may actually expand significantly. In addition, the method of depositing metal ions in the electrolyte on a collection electrode and extracting them from the electrolyte is a process that is normally used when using an electrolyte with a simple composition consisting of one or several types of inorganic acids. If it is a highly efficient concentrated liquid, the degree of hydrogen ion is extremely high (on the collection electrode, the hydrogen sludge generation reaction occurs 1Ω before the precipitation reaction of iron, chromium, nickel, etc.), and these Since metal precipitation does not occur, in order to recover the precipitation of these metal ions, it was necessary to use an electrolytic solution containing components that form complex salts with these metal ions.However, the metals recovered with this solution are This increases the amount of secondary waste, and the liquid composition is complex, making it difficult to manage the liquid.Therefore, at present, conventional phosphoric acid-based or sulfuric acid-based electrolytes cannot be treated by solidifying plastic or by solidifying the liquid. The situation leaves us with no choice but to use the cement assimilation method, which naturally leaves us with the problem of increasing amounts of radioactive secondary waste.

In view of the above problems, the present invention uses a dilute aqueous solution of sulfuric acid as the electrolyte used in the decontamination process, and improves polishing efficiency and solution life compared to conventional phosphoric acid-based or phosphoric acid-sulfuric acid-based electrolytes. Focusing on the fact that electrolytic de-dying can be carried out without significantly lowering the electrolyte, the pH of the used electrolyte can be adjusted and the metal ions in the solution can be extracted in a concentrated form using a collection electrode, and the electrolytic cell was developed. In this process, electrolytic decontamination is performed while circulating and filtering the suspended matter exfoliated and released from the contaminated metal surface into the electrolytic solution.In the water washing process to clean the metal surface after decontamination, the cleaning water is circulated through an ion exchange resin and used. By injecting alkali into the used electrolytic solution and adjusting the pH to a range that allows electrodeposition, the metal ions in the solution were deposited as metal solids on the collection electrode, and the radioactivity level of the electrolytic solution was reduced to the standard value. At this point, this liquid is passed through a cation exchange resin to almost completely remove any remaining metal ions in the liquid.

Hereinafter, embodiments of the present invention will be described in more detail with reference to the accompanying drawings.

An electrolytic solution 11 for electrolytic de-dying is filled with a 5Vo1% sulfuric acid aqueous solution as the electrolytic solution 2, the object to be decontaminated 3 is used as an anode, and a titanium or copper net electrode is used as a cathode. Then, a direct current is passed between both electrodes by the rectifier 5 to decontaminate the surface of the object 3 to be decontaminated by electrolytic polishing. The suspended solids are separated by the filter 7 while the electrolyte circulation pump 6 is stirring the liquid in the tank.The decontaminated object 3 that has been completely decontaminated in the electrolytic tank 1 is transferred to the washing tank 8. The electrolyte adhering to the surface is removed by stepwise cleaning, and after drying, it is packed in a storage can and stored.In the washing tank 8, the washing water is circulated through the ion exchange resin tank 10 by a washing water circulation pump 9, and the The washing water containing hM DJ gold metal ions attached to the dyed fabric 3 and brought into the washing tank 8 is transferred to the cation exchange resin and the anion exchange resin in the ion exchange resin tank 10, where it is contaminated with radioactive metal ions and sulfate ions. is removed and returned to the washing tank 8 as pure water for repeated use.

On the other hand, in the electrolytic cell 1, the polishing performance of the electrolyte decreases as the electrolytic dedying work continues, and at the same time, the radioactivity level increases due to the accumulation of radioactive metal ions eluted into the electrolyte.
At some point, it reaches the end of its life as an electrolyte. When the electrolyte reaches the end of its life, the circulation system is switched to the transfer system 11 and the pump 6
The entire amount of the used electrolyte is transferred to the electrodeposition tank 12. In the electrodeposition tank 12, a collection electrode 13 made of an iron plate or the like serves as a cathode, and a dissolvable electrode 14 made of titanium or the like serves as an anode. The electrolyte in the electrodeposition 4!112 is circulated and stirred by the circulation pump 1G, and the alkali injection pipe 17 on the circulation system path 18
When more caustic soda is injected to adjust the electrolytic solution to about 1) 82% and DC current is applied between both electrodes 13 and 14, metal ions in the solution are deposited as metal solids on the collection electrode 13.

The pH value of the electrolytic solution decreases due to a decrease in metal ions in the solution, and hydrogen gas is preferentially generated on the collection electrode 14, so that the precipitation and collection of metal ions is not impaired.
It is desirable to inject it in conjunction with ′. tj9 of the electrolyte due to a decrease in metal ions. When the split level falls to the reference value, the thread path is switched and the electrolytic solution is sent to the cation exchange tank 20 through the pipe line 19 to remove metal ions remaining in the solution. The electricity Me that has passed through the cation exchange tank 20 can be subjected to treatments such as neutralization and dilution, and then disposed of as mere wastewater.

As described below, in the method of the present invention, a sulfuric acid aqueous solution with a 5% fertility is used as the electrolyte, so the cleaning water can be circulated in a closed cycle, and the used electrolyte is free of metal ions. Since it can be separated as a solid, the amount of radioactive secondary waste generated is small (because the electrolyte after treatment has almost completely removed tIi sword metal ions, it does not require combined treatment with normal wastewater). It becomes possible.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the process of de-dying and electrolyte treatment in one embodiment of the present invention. 1... Electrolytic cell, 2... Electrolyte, 3... Decontaminated object,
4... Electrode, 5... Rectifier, 6... Electrolyte circulation pump, 7... Filter, 8... Washing tank, 9...
Cleaning water circulation pump, 10...Ion exchange tank, 11...
- Electrolyte transfer pipe line, 12... Electrodeposition tank, 13... Collection electrode, 14... Anode, 15... Rectifier, 16...
Electrolyte circulation pump, 17...alkali injection pipe, 18.
... Circulation system line, 18'...l) H meter, 19... Electrolysis pipe line, 20... Circular ion exchange tank.

Claims (1)

[Claims] When decontaminating a metal surface contaminated with radioactive materials by electrolytic polishing, a dilute aqueous solution of sulfuric acid is used as an electrolyte, and the contaminated metal is removed while circulating and filtering the suspended matter released in the electrolyte. In the water washing process that decontaminates the surface and cleans the metal surface after decontamination, the washing water is circulated through an ion exchange resin and an alkali is injected into the used electrolyte.
l) After reducing the electrolyte's QJ ability level to the standard value by precipitating the metal ions in the solution onto the collection electrode while adjusting H, the solution is passed through an ion exchange resin to remove the remaining metal ions. 1. A method for electrolytic de-dying using a dilute electrolytic solution, characterized in that: