JPH0324997B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for electrodeposition regeneration using diaphragm electrolysis of a phosphoric acid-based high-concentration acid decontamination electrolyte used for electrolytic decontamination of radioactively contaminated equipment and parts in nuclear power plants and the like. If a strong acid dilute aqueous solution such as 5% sulfuric acid is used as the electrolyte in the electrolytic decontamination process in which radioactively contaminated equipment and parts are decontaminated by electrolytic polishing, the polishing speed is high, and the decontamination electrolyte by diaphragm electrolysis is This electrolyte is ideal as an electrolyte for decontaminating radioactively contaminated waste because it can be easily reprocessed by electrodeposition, but the surface of the metal after decontamination is rough and may be recontaminated with radioactive materials. This poses a problem when used as an electrolytic solution for decontaminating the inner surfaces of containers or parts intended for reuse. Among the high-concentration acid electrolytes used in general electrolytic polishing, phosphoric acid-based high-concentration acid electrolytes are used as electrolytes for electrolytic de-dying of objects intended for reuse. is photo-selected and re-contamination with radioactive substances is minimized. However, it has been difficult to regenerate this electrolyte by electrodeposition using diaphragm electrolysis due to the high acid concentration of the electrolyte, and the dissolved metal ion concentration or radiation dose in the electrolyte remains at a constant level during the electrodeposition process. When the electrolyte reaches this point, it is solidified into plastic or cement and disposed of. However, this treatment method has a problem in that it increases the amount of radioactive secondary waste. The applicant of this application has conducted research and development on a method for electrodeposition regeneration using diaphragm electrolysis for such a highly concentrated acid decontamination electrolyte, and has published Japanese Patent Application No. 1981-114 (see Japanese Patent Application Laid-open No. 89600-1983) and Japanese Patent Application No. 59-37466 (Unexamined Japanese Patent Publication 1986-
(Refer to Publication No. 17900). In the former method, the electrolyte used in the electrodeposition dyeing process is stored in the cathode chamber of an electrodeposition regeneration tank separated by a diaphragm.
The anode chamber is filled with water that has been made conductive by adding a small amount of electrolyte, and the decontamination electrolyte is regenerated by electrodeposition through batch processing. In order to collect ions by electrodeposition, the hydrogen ion concentration in the cathode chamber fluid must be reduced to about PH2, so it takes a long time for electrodeposition collection to start, and during this time, the hydrogen ion concentration in the cathode chamber solution must be reduced to about PH2. Since metal ions leak from the cathode chamber to the anode chamber, this adversely affects the decontamination ability when the anode chamber solution is reused as an electrolyte. In the latter method, in order to solve the above-mentioned problems caused by batch processing, the cathode chamber of the electrodeposition regeneration tank is filled with water and the anode chamber is filled with an electrolytic solution that does not contain metal ions. Continuous processing is performed by injecting the electrolytic solution for the electrolytic dedying process into the cathode chamber while controlling the pH to maintain the ion concentration around PH2, and then withdrawing the same amount of the anodic chamber solution as the injection amount and returning it to the electrolytic dedying process. The decontamination electrolyte was regenerated by electrodeposition using a method that enabled metal ions to be collected by electrodeposition from the beginning of energization, but since the decontamination electrolyte with a high acid concentration was directly injected into the cathode chamber, Increasing the throughput of electrodeposition regeneration by increasing the injection amount poses a problem in that the equipment becomes too large. The present invention relates to a method for electrodeposition regeneration of a phosphoric acid-based high-concentration acid decontamination electrolyte that is highly effective in preventing recontamination after decontamination.
In order to improve the method of the previous application and shorten the electrodeposition regeneration time or improve processing capacity, the highly concentrated phosphoric acid decontamination electrolyte used in the electrodeposition decontamination process was transferred to an electrodeposition regeneration tank. Before sending, the phosphoric acid in the decontamination electrolyte is extracted with a solvent, the extracted separated liquid (electrolyte with reduced phosphoric acid) is sent to the cathode chamber of the electrodeposition regeneration tank, and water is extracted from the solvent after phosphoric acid extraction. phosphoric acid is back-extracted and the back-extracted solution (phosphoric acid aqueous solution containing almost no metal ions) is sent to the anode chamber of the electrodeposition regeneration tank.
The metal ions in the solution are collected by electrodeposition in the cathode chamber, and the phosphoric acid in the solution is concentrated in the anode chamber to the initial electrolyte concentration and reused as a decontamination electrolyte. Solvents for liquid-liquid extraction of phosphoric acid from an aqueous phosphoric acid solution include isopropyl ether, ethylene menomethyl ether, n-butyl alcohol, isoamyl alcohol, methyl isobutyl ketone,
Although there are many solvents such as butyl acetate, these organic solvents are evaporated by the heat of the decontamination electrolyte and are flammable, so none of them are suitable as phosphorus extractants for use in the electrodeposition regeneration process. As a result of searching for various phosphorus extractants, it was found that tributyl phosphate (TBP), which is insoluble in water and nonflammable, is the most effective phosphorus extractant for use in the electrodeposition regeneration process of decontamination electrolytes. Tributyl phosphate is known as a metal extractant and is used to extract uranium from nitric acid, but it hardly extracts metal ions such as iron, nickel, chromium, and cobalt from the decontamination electrolyte, and is Acid is well extracted, and phosphoric acid and metal ions are almost completely extracted by back extraction with water. Since it has a high boiling point and no evaporation loss, it can be used repeatedly without replenishment. Hereinafter, an embodiment of a method for electrodeposition regeneration of a decontamination electrolyte using extraction of phosphoric acid with a solvent and back extraction will be described in detail based on the drawings. In the electrolytic dedying process, a highly concentrated phosphoric acid is used as an electrolytic solution, and the object to be decontaminated 2 whose surface is radioactively contaminated is placed in the electrolytic solution in the electrolytic dedying tank 1, and then it is passed through a rectifier (Fig. The surface of the object 2 to be decontaminated is decontaminated by electropolishing by connecting it to the anode (not shown) and passing a direct current between it and the cathode 3 in the electrolytic solution. In the electrodeposition regeneration process of the embodiment shown in FIG. After separating into an electrolytic solution with reduced phosphoric acid and a phosphoric acid aqueous solution containing almost no metal ions by phosphoric acid extraction with a solvent and phosphoric acid back-extraction with water, these solutions are respectively applied to the cathode of the electrodeposition regeneration tank 5. chamber 6 and anode chamber 7, and a direct current is passed between the collection electrode 9 in the cathode chamber 6 and the insoluble electrode 10 in the anode chamber 7 through the diaphragm 8, and the electrodeposition regeneration tank 5 is subjected to batch processing. The decontamination electrolyte is regenerated by electrodeposition, and the liquid in the anode chamber 7, in which phosphoric acid has been concentrated to the initial electrolyte concentration, is transferred to the decontamination tank 1 using a pump 11.
Send it back to The extraction and back extraction of phosphoric acid described above are performed in an extraction separation tank 12 in which the solvent S is stored in the lower half. The operation will be explained with reference to FIGS. 2A to 2D. From the decontamination tank 1, an amount of decontamination electrolyte 13 equal to the capacity of the anode chamber 7 is extracted using a pump 4 and transferred to a separation tank 12.
The agitator 14 is started to stir and mix the liquid, and the phosphoric acid in the electrolyte 13 is extracted with the solvent S.
After stopping the stirrer 14, the extracted liquid 15 separated into the lower layer is discharged from the discharge valve 16 to the cathode chamber 6. The extraction and separation liquid 15 is discharged while being monitored with an electrical conductivity meter 17 to prevent the solvent S from being discharged into the cathode chamber 6. The discharged extracted separation liquid 15 is transferred to the cathode chamber 6
Since the amount is less than the capacity of the cathode chamber 6, the water supply valve 18 is opened to supply water up to the upper limit level of the cathode chamber 6. Next, the water supply valve 20 of the water storage tank 19 is opened to supply back extraction water 21 to the extraction separation tank 12, and the stirrer 14 is started to stir and mix the liquids, thereby back extracting the phosphoric acid in the solvent S. After the stirrer 14 is stopped, the reverse extraction liquid 22 separated into the lower layer is injected into the anode chamber 7 up to the upper limit level by opening the discharge valve 23. Since the amount of the back extraction liquid 22 is larger than that of the decontamination electrolyte 13, most of it remains in the extraction separation tank 12. When DC current is applied between the collection electrode 9 and the insoluble electrode 10 in this state, in the cathode chamber 6, hydrogen ions, which are cations in the liquid, are released as a large amount of hydrogen gas on the surface of the collection electrode 9, and the liquid is As the hydrogen ion concentration decreases, phosphate ions, which are anions, move to the anode chamber 7 through the diaphragm 8. In the anode chamber 7, hydrogen ions increased by the generation of oxygen gas on the surface of the insoluble electrode 10 combine with the phosphate ions transferred from the cathode chamber 6, and the phosphoric acid in the liquid is gradually concentrated. When the hydrogen ion concentration of the liquid in the cathode chamber 6 decreases to around PH2 due to continued energization, the amount of hydrogen gas generated on the surface of the collection electrode 9 decreases, and electrodeposition of metal ions in the liquid begins. During this energization process, water is released by electrolysis and evaporation in the electrodeposition regeneration tank 5, and the liquid level in both the cathode chamber 6 and the anode chamber 7 decreases due to the loss of water. Makeup water is automatically supplied from the anode chamber 7 to adjust the liquid level, and in the anode chamber 7, the back extraction liquid 22 remaining in the extraction separation tank 12 is automatically supplied from the discharge valve 23 to adjust the liquid level. When almost complete discharge of the back extraction liquid 22 from the extraction separation tank 12 is detected by the electrical conductivity meter 17,
The liquid level adjustment function is canceled and the discharge valve 23 is closed, and the current is continued to be applied. When almost no metal ions or phosphate ions are present in the liquid in the cathode chamber 6, the current is stopped. At this point, the electrolytic solution 13 sent from the de-dyeing tank 1 has finally been regenerated as highly concentrated phosphoric acid with almost the same volume.
If necessary, water is added from the water supply valve 24 to adjust the concentration of phosphoric acid to the initial electrolyte concentration, and the entire amount of the anode chamber 7 solution is returned to the decontamination tank 1 using the pump 11 to prepare the decontamination electrolyte. The electrodeposition regeneration process ends. Water is replenished from the water supply valve 25 to the water storage tank 19 in preparation for the next back extraction operation, but in this electrodeposition regeneration process, water close to fresh water is generated in the cathode chamber 6.
After the electrodeposition regeneration is completed, the liquid in the cathode chamber 6 is sent to the water storage tank 19 by the pump 26, and is circulated and used as part of the water for back extraction. In this method, the extracted separated liquid 15 supplied to the cathode chamber 6 is an electrolyte with reduced phosphoric acid, and is diluted with make-up water from the water supply valve 18, so that the hydrogen ion concentration is reduced in the cathode chamber 6. Since energization is started, the time required to start electrodeposition is significantly shortened compared to the method of the prior application in which electrodeposition regeneration is performed by storing a high concentration decontamination electrolyte in the cathode chamber 6 as it is. . In the example shown in Figure 3, the steps of phosphoric acid extraction, back extraction, and electrodeposition regeneration are serialized, and items common to those in Figure 1 are given the same reference numerals, and explanations will be repeated. omitted. In the process of phosphoric acid extraction and back extraction, extraction tank 3
0. The extract liquid separation tank 31, the back extraction tank 32, and the back extraction liquid separation tank 33 are installed separately so that the liquids sequentially overflow and flow, and the solvent S is extracted from the back extraction liquid separation tank 33 with the pump 34. The liquid is constantly circulated in the tank 30, and the liquid is constantly stirred in the extraction tank 30 and the back extraction tank 32 by agitators 35 and 36, respectively. The decontamination electrolyte from the decontamination tank 1 is continuously fed to the extraction tank 30 by the pump 4, where phosphoric acid is continuously extracted by the solvent S, and the extracted liquid is separated in the extract separation tank 31 and collected in the lower layer. The separated liquid 15 is supplied from the supply valve 37 to the cathode chamber 6 while controlling the pH so that the liquid in the cathode chamber 6 is always maintained at around PH2.
The solvent S in the extract liquid separation tank 31 is transferred to the back extraction tank 32, where phosphoric acid is back extracted by water supplied from the water supply valve 20, and the back extraction liquid is separated into a lower layer in the back extraction liquid separation tank 33. The liquid 22 is supplied to the anode chamber 7 while controlling the liquid level with a supply valve 38. Reverse extraction tank 3
2, by controlling the liquid level in the back extraction liquid separation tank 33, water for back extraction is automatically supplied from the water storage tank 19 through the water supply valve 20 in an amount corresponding to the discharge amount of the extraction separation liquid 15 and the back extraction liquid 22. . In the cathode chamber 6, the liquid is constantly circulated and stirred by the pump 39, and the liquid is constantly circulated by automatic injection of the extraction and separation liquid 15.
Since the pH is maintained around 2, the metal ions in the extracted and separated liquid 15 are collected by electrodeposition at the collection electrode 9 without delay. In response to the loss of water due to electrolysis and evaporation, water is replenished from the water supply valve 40 to adjust the level. However, if the amount of extracted and separated liquid 15 injected is greater than the amount of water lost, the liquid level in the cathode chamber 6 gradually rises, so when the liquid level reaches the upper limit, the PH adjustment is canceled, injection of the extraction separation liquid 15 is stopped, and electricity is continued.
When there are almost no metal ions or phosphate ions in the liquid in the cathode chamber 6, the circulation line valve 4 is closed.
1 is switched to send a part of the liquid in the cathode chamber 6 to the water tank 19 to lower the liquid level in the cathode chamber 6. In the anode chamber 7, the back extraction solution 22 is automatically supplied by the amount of water lost, and the phosphoric acid concentration in the solution is measured using an electrical conductivity meter 1.
The liquid in the anode chamber 7 is returned to the electrolysis dyeing tank 1 as a recycled electrolyte by adjusting the amount of liquid sent by the pump 11 while being monitored by the electrolysis dyeing tank 1 at pump 4. The electrodeposition regeneration process is performed continuously by sending the sample to the extraction tank 30. According to this method, similar to the embodiment of FIG.
Since the extracted separated liquid 15 injected into the cathode chamber 6 is an electrolytic solution with reduced phosphoric acid, the electrolytic solution sent from the electrodepositing tank 1 is directly injected into the cathode chamber 6 to perform electrodeposition regeneration. Compared to the above method, the throughput of electrodeposition regeneration can be improved. In these embodiments, the phosphoric acid in the decontamination electrolyte sent from the electrodeposition dye tank 1 is transferred to the anode chamber 7 side by extraction and back extraction, so that it is removed from the cathode chamber 6 during the electrodeposition regeneration process. The electrical energy for transferring the phosphoric acid through the diaphragm 8 into the anode chamber 7 is saved. However, if a large amount of back extraction water is used to enhance the effect of phosphoric acid extraction back extraction, the amount of back extraction liquid 22 to be treated in the anode chamber 7 increases accordingly.
The rate of electrodeposition regeneration is limited by the rate of water loss in the anode chamber 7 due to electrolysis and evaporation.
This problem can be solved by combining phosphoric acid concentration in the anode chamber 7 with evaporative concentration, but heating the liquid in the anode chamber 7 reduces the function of the diaphragm 8, causing hydrogen ions to flow from the anode chamber 7 to the cathode chamber 6. If evaporation and concentration are used together, the reverse extraction liquid 22 to be injected into the anode chamber 7 must be concentrated in advance, or the liquid in the anode chamber 7 can be sent to an evaporator to remove the liquid. The concentrated and cooled concentrate must be received in the anode chamber 7. FIG. 4 shows an example in which evaporative concentration is used in conjunction with phosphoric acid concentration in the back extraction liquid 22, in which the back extraction liquid 22 discharged from the separation tanks 12 and 33 to the receiving tank 50 is sent to the vapor compression type concentrator 51 to phosphoric acid. Concentrate the acid. The condenser 52 uses a container or pipe with a glass lining on the inside, and the compressor 53 sucks in steam to reduce the pressure inside the vessel.At the same time, the sucked vapor is compressed and the heat of compression is transferred to a jacket 54 or a heat exchanger tube or the like with a glass lining on the outside. Since the reverse extraction liquid 22 is evaporated and concentrated at a low temperature by being transferred to the liquid in the vessel, the energy consumption for evaporation and concentration is small, and neither a heat source nor cooling water is required. The concentrated liquid in the container is sucked by the pump 55 and sent to the anode chamber 7, and the condensed water is sent to the water storage tank 19 and used for circulation as back extraction water. In this way, by combining evaporative concentration with phosphoric acid concentration of the back extraction solution 22, the extra power consumption for water electrolysis is reduced in the process of electrodeposition regeneration, and sufficient back extraction water is used to achieve a more complete process. Since extraction and back extraction are possible, the speed or throughput of electrodeposition regeneration can be further improved. In order to confirm the effectiveness of phosphoric acid extraction and back extraction in the method of the present invention, 75% phosphoric acid was used as the electrolyte.
Using the used electrolyte that destained the SUS304 plate and tributyl phosphate (TBP) as the solvent,
The following extraction and back-extraction tests were performed. Add 40c.c. of TBP to 10c.c. of the above electrolyte to extract phosphoric acid in the electrolyte, and add water to the TBP from which the phosphoric acid has been extracted.
50c.c. is added to back-extract phosphoric acid, and 10c.c. of the above electrolyte is added again to the TBP regenerated by phosphoric acid back-extraction. Repeating the extraction and back-extraction operations yields 10c.c. of electrolyte. is reduced to 7 c.c. as phosphoric acid is extracted, and when 50 c.c. of water is added to TBP43 c.c. from which phosphoric acid has been extracted and phosphoric acid is back-extracted, the regenerated TBP returns to the initial 40 c.c. , 53 c.c. of back extract containing phosphoric acid can be obtained. The electrolyte A10c.c. used, and the second and fifth
Extracted liquid obtained from the second extraction and back extraction operation
The components contained in B7c.c., reverse extract C53c.c., and recycled TBPD40c.c. are shown in the table below.

[Table] As is clear from this table, when a 75% phosphoric acid decontamination electrolyte is extracted with phosphoric acid using 4 times the amount of recycled TBP, and the TBP after phosphoric acid extraction is back-extracted with approximately the same amount of water,
More than 1/2 of the phosphoric acid in the electrolyte is transferred to the back extraction solution,
TBP is almost completely regenerated. When the extracted separated liquid and reverse extracted liquid obtained in this way are sent to the cathode chamber and the anode chamber of the electrodeposition regeneration tank and subjected to electrodeposition regeneration treatment, the processing time is more than twice that of the method of the previous application. ability is obtained. As described above, according to the method of the present invention,
Extracts phosphoric acid from electrolyte solution for electrodeposition regeneration treatment,
By back-extracting the electrolyte beforehand, it is transferred to the anode chamber side of the electrodeposition regeneration tank, which can shorten the electrodeposition regeneration time or increase the amount of electrodeposition regeneration throughput.The electrolyte and solvent can be regenerated and used repeatedly. Because you can
This solves the problem of wastewater treatment in the electrolytic dedying process, which uses a highly concentrated phosphoric acid electrolyte that is highly effective in preventing recontamination, and allows for a significant reduction in the amount of radioactive secondary waste.

[Brief explanation of the drawing]

Figure 1 is a flow sheet of the electrodeposition regeneration process in one embodiment of the present invention, Figure 2 A is the first stage in the extraction separation tank, Figure 2 B is the second stage, and Figure 2 C is the second stage. 3 stages, FIG. 2 D is the fourth stage, and FIG. 3 is a flow sheet of the electrodeposition regeneration process of another embodiment of the present invention.
FIG. 4 is a flow sheet for the case where evaporation and concentration are used together. 1... Electrode-dye tank, 2... Decontaminated object, 3... Cathode, 4
... Pump, 5... Electrodeposition regeneration tank, 6... Cathode chamber, 7... Anode chamber, 8... Diaphragm, 9... Collection electrode, 10... Insoluble electrode, 11... Pump, 12... Extraction separation tank, 13... Decontamination electrolysis Liquid, 14... Stirrer, 15... Extraction separation liquid, 1
6... Discharge valve, 17... Electrical conductivity meter, 18... Water supply valve, 19... Water tank, 20... Water supply valve, 21... Water for back extraction, 22... Back extraction liquid, 23... Discharge valve, 24... Water supply valve, 25 ...Water supply valve, 26...Pump, 30...Extraction tank, 31...Water extraction liquid separation tank, 32...Back extraction tank, 33
... Reverse extraction liquid separation tank, 34 ... Pump, 35 ... Stirrer, 36 ... Stirrer, 37 ... Supply valve, 38 ... Supply valve, 39 ... Pump, 40 ... Water supply valve, 41 ... Valve, 5
0... Receiving tank, 51... Evaporation concentration device, 52... Concentrator,
53... Compressor, 54... Jacket, 55... Pump, S... Solvent.

Claims (1)

[Scope of Claims] 1. In a method for electrodepositing and regenerating a phosphoric acid-based high-concentration acid decontamination electrolyte solution used for electrolytic removal of radioactive contaminants by diaphragm electrolysis, a decontamination electrolyte to be subjected to electrodeposition regeneration treatment. Phosphoric acid is extracted from the liquid using a solvent, and the extracted and separated liquid is sent to the cathode chamber of an electrodeposition regeneration tank separated by a diaphragm.
Phosphoric acid is back-extracted with water from the solvent used to extract phosphoric acid, and the back-extracted liquid is sent to the anode chamber of the electrodeposition regeneration tank, which is separated by a diaphragm, between the collection electrode in the cathode chamber and the insoluble electrode in the anode chamber. The method is characterized in that metal ions in the solution are electrodeposited and collected in the cathode chamber by applying direct current to the phosphoric acid solution, and the phosphoric acid solution is concentrated to the initial electrolyte concentration in the anode chamber and reused as a decontamination electrolyte. Electrodeposition regeneration method of phosphoric acid decontamination electrolyte. 2. The method according to claim 1, wherein tributyl phosphate is used as the solvent for extracting phosphoric acid. 3. The method according to claim 1, wherein the cathode chamber liquid after completion of electrodeposition regeneration is recycled as water for back extraction. 4. The method according to claim 1, in which evaporative concentration is used in combination with phosphoric acid concentration of the back extract.