JPH0812274B2 - Method for removing impurities from solvent - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for removing impurities from a solvent, and more particularly to a method for removing impurities from an extraction solvent containing tributyl phosphate as a main component used for reprocessing nuclear fuel.

[Conventional technology]

Tributyl phosphate (hereinafter referred to as TBP) is used as an extraction solvent for reprocessing nuclear fuel. Part of TBP is decomposed by radiolysis or chemical decomposition in the reprocessing process and forms a complex with metal ions in fission products, which interferes with normal extraction operation. In order to remove these decomposition products and metal ions to regenerate the solvent, conventionally, washing mainly by liquid-liquid extraction with a sodium carbonate solution has been carried out. However, these cleaning agents become solid waste, and, for example, about 1 ton of nuclear fuel is processed,
The problem is that 100 kg of nanolium compound waste is generated and accounts for a large proportion of the waste generated from the reprocessing process. On the other hand, according to JP-A-51-65297, a cleaning agent containing hydrazine as a main component is used,
A method for reducing the amount of solid waste generated by decomposing this has been proposed.

[Problems to be solved by the invention]

However, in the above-mentioned prior art, hydrazine undergoes a decomposition reaction even at room temperature, which causes difficulty in storage, and is also highly explosive and chemically toxic, requiring careful handling. Further, since it is less alkaline than sodium-based compounds such as sodium carbonate, it has a weak effect on the acid, which is the main component of the decomposition product of TBP such as dibutyl phosphate (hereinafter referred to as DBP). Moreover, in terms of cost, since the cost of chemicals is relatively high, it is desirable that the amount used can be reduced.

It is an object of the present invention to provide a method for producing a solid organic solvent, which produces a small amount of waste, is easy to store and handle, and can be used at a low concentration so that the amount of chemicals used can be reduced.

[Means for solving problems]

The above object is to bring an aqueous solution in which a tetraalkylammonium compound (a) and at least one compound having a complexing action (b) selected from organic acids and carbonic acid coexist by contacting the organic solvent with a liquid-liquid, This is achieved by transferring impurities in the solvent into the aqueous solution.

[Action]

DBP, which is a major decomposition product of TBP, has the property of being an acid, and easily migrates to an alkaline aqueous solution. Therefore, alkaline solutions have been used to regenerate TBP.
Conventionally, hydrazine, hydroxylamine, and the like, which have been proposed as detergents that can be decomposed and reduced in volume, have low dissociation constants. However, tetraalkylammonium hydroxide such as tetramethylammonium hydroxide shows a dissociation constant as high as that of sodium hydroxide. That is, for example, tetraalkylammonium hydroxide (R ₄ NOH) exhibits strong alkalinity and almost completely dissociates to release hydroxide ions. This hydroxide ion reacts with a decomposition product (HD) of an organic solvent, which mostly takes the form of an acid, and causes ionization dissociation of the decomposition product as shown in the following equation.

Since HD + OH ⁻ → H ₂ O + D ⁻ D ⁻ is water-soluble, it is transferred from the organic solvent to the alkaline aqueous solution (tetraalkylammonium hydroxide) which is a cleaning agent and removed. Therefore, it is expected that the cleaning action for DBP is equal to or more than that of conventional sodium carbonate.
On the other hand, other cleaning agents such as hydrazine are more expensive than sodium carbonate in terms of cost, so by using as thin a concentration as possible, the cleaning effect is sacrificed somewhat to reduce the amount used, and to improve economic efficiency. There is. However, in the present invention, tetramethylammonium hydroxide has a strong alkalinity, and in this respect as well, it shows an excellent cleaning effect even at a low concentration, which is economically advantageous. In an actual solvent, DBP forms a complex with a metal ion.
In order to remove BP, dissociation action of this complex is required, and further removal of metal ions is required. Therefore, since zirconium forms the strongest complex in metal ions, the result of measuring the zirconium removal rate in the case of forming the zirconium-DBP complex as the case that is the most difficult to wash and remove is shown in FIG. Tetramethylammonium hydroxide can be used at lower concentrations than sodium hydroxide. However, compared with sodium carbonate, the concentration is about the same, and the merit of cost reduction by using low concentration is small. This is because the action on zirconium in the metal ion-DBP complex is small with tetramethylammonium hydroxide. In order to improve this point, a compound having a complexing effect on zirconium is added.

In the present invention, organic acids such as oxalic acid, citric acid and tartaric acid are used as the compound having a complexing action. or,
Carbonic acid is also effective. When a strong alkali such as hydrazine is used, the addition of compounds having these complexing effects impairs the alkalinity of the detergent and reduces the washing action on DBP, but as in the case of the present invention, it is strong. DB in the presence of the alkali tetramethylammonium hydroxide
The cleaning action on P is not or hardly impaired. Oxalic acid is considered to be suitable as the complexing agent to be added because it has a small molecular weight, can reduce the amount of gas generated during decomposition, and has a large complexing effect on zirconium. First
The figure shows the data when tetramethylammonium oxalate was used as the cleaning agent. It can be seen from FIG. 1 that the zirconium removal rate is sufficient even at a low concentration of 0.01 N, and the effect of the complexing agent is great. On the other hand, in Figure 2, DBP
The results obtained by measuring the removal rate using tetramethylammonium oxalate are shown below. It shows a sufficient removal rate up to 0.1N, and the effect is small compared to the case of zirconium, but it can be used at a low concentration of about 1/5 compared to tetramethylammonium hydroxide alone, and it can be used as a zirconium-DBP complex. Improved dissociation effect D at low concentration
It was found that the removal action of BP was improved. That is, DBP, which is a decomposition product of an organic solvent forming a complex with a metal ion, is dissociated from the metal ion by the action of oxalic acid which is a complexing agent. Furthermore, the dissociated DBP is
It is ionized and dissociated by the above-mentioned reaction formula, transferred to an alkaline aqueous solution which is a cleaning agent, and removed.

As the tetraalkylammonium compound used in the present invention, water-soluble compounds such as tetraethylammonium and tetrapropylammonium are used.

As the organic solvent, for example, di2 (ethylhexyl) phosphoric acid, trioctylphosphine oxide or the like can be used in addition to the above-mentioned tributyl phosphate.

In the present invention, the organic solvent may be diluted with at least one diluent such as dodecane, paraffin and kerosene before use. This can enhance the extraction action of the organic solvent.

〔Example〕

Embodiment 1 An embodiment of the present invention will be described below with reference to FIG. The cleaning agent tank 1 stores a 0.1N tetramethylammonium oxalate solution 2. This solution is sent from the pump 4 to the mixer settler 5 through the pipe 3, and the pipe 6
And the solvent sent to the mixer settler through the mixer 7
After being agitated and mixed in, the mixture is separated into an organic phase and an aqueous phase in the mixer settler 5, and the organic phase is sent to another system through the pipe 8 for use as a regenerated solvent. The water phase is used cleaning agent and is sent to the evaporator 10 through the pipe 9.
Evaporate the water at about 100 ° C and concentrate to 1/10. The generated steam is sent to the condenser via the pipe 11. The concentrated solution is sent to the decomposition furnace 13 through the pipe 12. A heater 14 is installed in the decomposition furnace 13 to heat the temperature inside the decomposition furnace 13 to 200 ° C. Tetramethylammonium hydroxide decomposes from approximately 135 ° C and oxalic acid from approximately 110 ° C to become a complete gas,
In the decomposition furnace, these become gas and are discharged to the off-gas processing system through the pipe 15. In the decomposition furnace, only oxides produced by deoxidation of metal ions and phosphoric acid produced by decomposition of DBP remain, and the amount of fixed waste is reduced to 1/100 or less compared to the case without decomposition treatment. it can. According to this example, the added amount of oxalic acid does not have to have the same normality as that of tetramethylammonium hydroxide, but a sufficient effect was not obtained with 0.1N or less. The tetramethylammonium oxalate after washing can be completely vaporized by heating at a temperature of 200 ° C or lower, so that a large volume reduction can be performed.

Example 2 An example in which the method of the present invention is applied when the concentration of decomposition products of the solvent is high will be described with reference to FIG. A tetramethylammonium oxalate solution 22 is stored in the first cleaning agent tank 21. In this example, since the decomposition product concentration is high, it is preferable that the detergent concentration is also high, and a 0.5N concentration was used. The tetramethylammonium oxalate solution 22 is supplied by a pump 23 to a mixer-settler 25 through a pipe 24. A solvent is supplied to the mixer-settler 25 through a pipe 26, and after being brought into liquid-liquid contact, it is separated into an organic phase and an aqueous phase, and the aqueous phase is treated as a waste liquid through the pipe 27. The organic phase is supplied to the next mixer-settler 33 through the pipe 28. Dilute nitric acid solution (0.1N) in the second cleaning agent tank 29.
30 is stored and is supplied to the mixer settler 33 through the pipe 32 by the pump 31. Here, the solvent and the nitric acid solution are brought into liquid-liquid contact with each other to remove impurities that could not be removed in the previous step with a solution having a different liquid property, and also to remove tetramethylammonium oxalate remaining in the solvent. The aqueous phase separated in the mixer settler 33 is sent to the acid recovery system through the pipe 34. The organic phase is supplied to the mixer settler 41 through the pipe 35. Dilute tetramethylammonium hydroxide solution (0.01N) 37 in the third cleaning agent tank 36
Is stored and supplied to the mixer-settler 40 through the pipe 39 by the pump 38. Here, the solvent and dilute tetramethylammonium hydroxide solution are in liquid-liquid contact,
The solvent becomes slightly alkaline. This can prevent decomposition of the solvent during storage. The organic phase decomposed in the mixer settler 40 is sent to the storage facility for the regenerated solvent through the pipe 41, and the aqueous phase is treated as a waste liquid together with the aqueous phase flowing through the pipe 27 through the pipe 42. According to this example, impurities can be removed with high cleaning efficiency even from a solvent containing a high-concentration decomposition product.

〔The invention's effect〕

According to the present invention, it is possible to reduce the amount of waste generated as compared with the case of using sodium carbonate, and it is easy to store and handle, and it is effective even at a low concentration of about 0.1 N. It can be reduced to a certain degree.

[Brief description of drawings]

Figure 1 shows that 0.1% TBP is DB when the detergent concentration is changed.
A graph showing the measurement results of the zirconium removal rate for a solvent forming a complex with zirconium in place of P, Fig. 2 shows the removal of DBP under the same conditions as in Fig. 1 when tetramethylammonium oxalate was used as a detergent. FIG. 3 is a schematic diagram of the impurity removing apparatus used in one embodiment of the present invention, and FIG. 4 is a schematic diagram of the impurity removing apparatus used in the embodiment of the present invention. 1 ... Cleaning agent tank, 2 ... Tetramethylammonium oxalate solution, 3 ... Piping, 4 ... Pump, 5 ... Mixer settler, 6 ... Piping, 7 ... Mixer, 8 ... Piping, 9 ... … Piping, 10 …… Evaporator, 11 …… Piping, 12 …… Piping, 13 …… Decomposition furnace, 14 …… Heater, 15 …… Piping, 21…
… First cleaning agent tank, 22 …… Tetramethylammonium oxalate solution, 23 …… Pump, 24 …… Piping, 25 …… Mixer settler, 26 …… Piping, 27 …… Piping, 28 …… Piping,
29 …… Second cleaning agent tank, 30 …… nitric acid solution, 31 …… Pump, 32 …… Piping, 33 …… Mixer settler, 34 …… Piping, 35 …… Piping, 36 …… Third cleaning agent tank, 37 …… Tetramethylammonium hydroxide solution, 38 …… Pump, 39…
… Piping, 40 …… Mixer settler, 41 …… Piping, 42 ……
Piping.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Fumio Kawamura 1168 Moriyama-cho, Hitachi City, Hitachi, Ibaraki Prefecture Energy Research Institute, Nitrate Manufacturing Co., Ltd. (72) Yasuo Hirose 3-1-1, Saicho-cho, Hitachi City, Ibaraki Hitachi, Ltd. Hitachi factory (56) References JP-A-51-65297 (JP, A) JP-A-62-200298 (JP, A)

Claims (9)

[Claims] 1. An aqueous solution in which a tetraalkylammonium compound (a) and at least one compound having a complexing action (b) selected from organic acids and carbonic acid coexist are brought into contact with an organic solvent in a liquid-liquid contact, A method for removing impurities from a solvent, which comprises migrating impurities in an organic solvent into the aqueous solution. 2. The method for removing impurities from a solvent according to claim 1, wherein the organic solvent is diluted with a diluent. 3. The method for removing impurities from a solvent according to claim 1 or 2, wherein the organic solvent is tributyl phosphate. 4. The method for removing impurities from a solvent according to claim 1 or 2, wherein the aqueous solution is an aqueous solution in which tetramethylammonium hydroxide and oxalic acid coexist. 5. The method for removing impurities from a solvent according to claim 4, wherein an aqueous solution containing 0.1 M or more of each of tetramethylammonium hydroxide and oxalic acid is used. 6. An aqueous solution in which a tetraalkylammonium compound (a) and at least one compound having a complexing action (b) selected from organic acids and carbonic acid coexist are brought into contact with an organic solvent in a liquid-liquid contact, A method for removing impurities from a solvent, which comprises converting impurities in an organic solvent into the aqueous solution, concentrating the aqueous solution, and then thermally decomposing it into a gaseous waste. 7. The method for removing impurities from a solvent according to claim 6, wherein the organic solvent is diluted with a diluent. 8. The method for removing impurities from a solvent according to claim 6 or 7, wherein the organic solvent is tributyl phosphate. 9. The method for removing impurities from a solvent according to claim 6 or 7, wherein the aqueous solution is an aqueous solution in which tetramethylammonium hydroxide and oxalic acid coexist.