JP4120794B2 - Efficient extraction and separation of calcium and strontium in nitric acid solution - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for efficiently extracting and separating calcium (Ca) and strontium (Sr) in a nitric acid solution with a diglycolamide (DGA) compound that is a tridentate ligand. The invention also relates particularly to a method for recovering the long half-life, exothermic ⁹⁰ Sr present in high level radioactive liquid waste (HLLW) resulting from reprocessing of spent nuclear fuel.
[0002]
[Prior art]
The need to separate Sr from nitric acid solutions on an industrial scale has arisen particularly in the treatment of HLLW resulting from spent nuclear fuel reprocessing. Specifically, removal of Sr (including ⁹⁰ Sr, a pyrogenic nuclide) present in the storage and disposal of HLLW facilitates the processing and disposal of HLLW. It has been studied.
[0003]
In the existing separation method, for example, (1) an adsorption separation method using a mixed inorganic ion exchanger of mordenite and hydrous titanic acid or an inorganic ion exchanger such as zeolite or antimonic acid (for example, see Non-Patent Document 1). ), (2) Solvent extraction using a solvent in which dibutylcyclohexano 18-crown-6 and tributyl phosphate (TBP) are dissolved in paraffinic hydrocarbons or an organic solvent in which cobalt dicarbollide is dissolved in highly polar nitrobenzene (For example, see Non-Patent Documents 2 and 3.).
[0004]
[Non-Patent Document 1]
Marsh (SF Marsh), 2 others, (Distributions of 14 elements on 63 adsorbers from three simultant solutions (acidic-dissolved sludge, acidified supernate and alkaline supernate) for Hanford HLW Tank 102-SY.), “LA-12654”, (United States), Los Alamos National Laboratory, 1994, p. 1-96
[Non-Patent Document 2]
Rayleigh (SD Reilly), the outer two persons, cobalt (III) Jikaruboraido, ¹⁸⁷ the possibility of Cs and ⁹⁰ Sr waste extractant (Cobalt (III) Dicarbolide A potential 187 Cs and 90 Sr Waste Extraction Agent.), "LA- 11695 ", (USA), Los Alamos National Laboratory, 1992, p. 1-19
[Non-Patent Document 3]
Evaluation of the srex solvent extraction process for removal of ⁹⁰ Sr and toxic metals from acid wood effluent containing DJ Wood, high concentration of interfering alkali metal ions the removal of ⁹⁰ Sr and hazardous metals from acidic nuclear waste solutions containing high concentrations of interfering alkali metal ions., “Separation Science and Technology”, (USA), Marcel Dekker Inc. .), 1997, 15, p. 65-78
[0005]
[Problems to be solved by the invention]
However, the various inorganic ion exchangers and extraction solvents conventionally used to separate Sr have several disadvantages as described below. That is, when an inorganic ion exchanger is used, since it is a solid-liquid reaction, there are disadvantages that the reaction time until equilibration is long and the operation time is prolonged to some extent. Moreover, in the condition where acidity is high, there exists a fault that adsorption | suction performance reduces remarkably. For this reason, in the HLLW treatment, it is necessary to reduce the acid concentration of the stock solution to be treated. However, since the hydrolyzable element is precipitated as the acid concentration is reduced, additional work such as filtration is also required. Furthermore, the exchanger generally has a drawback that it is not highly selective with respect to Sr and is easily affected by coexisting elements.
[0006]
On the other hand, the conventional extraction solvent has the following problems. That is, dibutylcyclohexano 18-crown-6 is expensive, and when the diluent TBP-paraffin hydrocarbon is used, there is a high possibility that a third phase is generated when the metal concentration is high. Furthermore, there is also a problem that phosphorus compounds (solid waste) remain after incineration. In addition, in cobalt dicarbollide series, harmful diluents such as ditrobenzene are used and it is difficult to obtain the extraction reagent because the source of the extraction reagent is limited. There is a problem that (waste) remains.
[0007]
[Means for Solving the Problems]
In order to solve the problems of the prior art as described above, the present inventors have conducted intensive research.As a result, Sr can be directly extracted even from a solution having a high acid concentration (that is, it is necessary to reduce the acid concentration of the stock solution to be treated). Thus, the present inventors have developed an extraction solvent having high selectivity to Sr and completed the present invention relating to a method for efficiently extracting and separating Ca and Sr in a nitric acid solution. The extraction solvent used in the present invention has a composition that does not contain metal or phosphorus, which is a source of solid waste, dissolves well in nonpolar solvents, does not produce a third phase, and is stable to hydrolysis and radiolysis. It has the characteristic of being. The extraction solvent that can be used in the present invention is a tridentate DGA, for example, TODGA (N, N, N ′, N′-tetraoctyl 1,3-oxapentanediamide).
[0008]
In addition, the present inventors added a monoamide compound (for example, DHOA (dihexyl octaamide), (C ₆ H ₁₃ ) ₂ —N—CO—C ₇ H ₁₅ ) to such a TODGA extraction solvent, thereby adding a third amide compound. It has been found that suppression of phase formation and improved stability against radiolysis can be achieved.
[0009]
That is, the present invention uses Ca in a nitric acid solution containing HLLW, wherein a solvent obtained by diluting a tridentate ligand DGA having a long alkyl chain and high lipophilicity in dodecane is used as an extraction solvent. A method for efficiently extracting and separating Sr is a problem solving means.
[0010]
Further, the present invention provides a method for efficiently extracting and separating Ca and Sr, characterized in that, in the above problem solving means, a monoamide compound is further added to an extraction solvent to suppress the formation of a third phase. Let it be a problem solving means.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
The present invention uses Ca and Sr in a nitric acid solution containing HLLW, characterized in that a solvent obtained by diluting a tridentate ligand DGA having a long alkyl chain and high lipophilicity in dodecane is used as an extraction solvent. It is an invention related to a method for efficient extraction and separation. The extraction solvent that can be used in the present invention is a tridentate DGA, for example, TODGA.
[0012]
According to the method of the present invention, sufficiently large extractability with respect to Ca and Sr is shown by using a mixture of two extraction solvents of DGA and dodecane. The synthesis of these two extraction solvent agents is simple for those skilled in the art.
[0013]
In one embodiment of the present invention, a monoamide compound can be added to the extraction solvent to suppress the formation of the third phase. A monoamide compound that can be used in the present invention is, for example, DHOA.
[0014]
The advantages of the extraction and separation method of the present invention are summarized as follows with respect to the disadvantages resulting from the use of the conventional ion exchanger and the use of the extractant.
(1) The composition of the extraction solvent contains only carbon, hydrogen, oxygen, and nitrogen and can be completely incinerated, so there is almost no solid waste generated.
[0015]
(2) By using two types of extraction solvents in combination, it is possible to suppress the formation of the third phase due to the increase of metal ions in the organic phase (with extraction), and to extract more metal ions into the organic phase. Become. Simplification of the process and cost reduction can be expected by increasing the amount of metal (load capacity) that can be extracted.
[0016]
(3) The combined use of two extraction solvents of DGA and monoamide compound can suppress the radiolysis of DGA, and the influence of decomposition products can be almost ignored. Furthermore, as a result, the extractant can be used repeatedly.
[0017]
(4) DGA-dodecane solvent has a higher partition ratio than other extractants previously reported for Ca and Sr, so it is excellent for extraction from highly acidic solutions, and has a low acidity aqueous nitric acid solution. Also excellent in back extraction from
[0018]
【Example】
(Example 1)
Various concentrations of N, N ', N, N'-tetraoctyl-3-oxapentanediamide (TODGA) -normal from nitric acid solutions containing tracer amounts of Ca (II), Sr (II), Ba (II) The metal was extracted with dodecane solvent. FIG. 1 shows the TODGA concentration dependence of the extraction partition ratio (D) of Ca (II), Sr (II), and Ba (II) at equilibrium. As a result, the extraction partition ratio of Ca (II) and Sr (II) at equilibrium was about 100 and 20 when the nitric acid concentration in the aqueous phase was 2.9M, respectively.
[0019]
From this result, it is considered that a metal complex having a molar ratio of Sr: TODGA of 1: 2 can be formed in the extraction reaction between the metal and TODGA.
(Example 2)
The metal was extracted from a nitric acid solution containing tracer amounts of Ca (II), Sr (II), and Ba (II) with 0.1 M TODGA-normal dodecane solvent. FIG. 2 shows the nitric acid concentration dependence of D of Ca (II), Sr (II), and Ba (II) at equilibrium. The distribution ratio of metal ions increased with increasing acid concentration in the range of nitric acid concentration (initial concentration) 0.1-3M. From this result, after extracting Ca (II), Sr (II) to organic phase under conditions of high nitric acid concentration using TODGA, Ca (II), Sr can be easily obtained by using an aqueous phase with low acidity. It can be seen that (II) can be back extracted into the aqueous phase.
[0020]
(Example 3)
The metal was extracted from a nitric acid solution containing 10-200 mM Sr (II) with a normal dodecane solvent in which 0.1 M or 0.2 M TODGA and 1 M dihexyl octaamide (DHOA) were dissolved. FIG. 3 shows the relationship between the Sr (II) concentration extracted in the organic phase and the Sr (II) concentration in the aqueous phase before extraction. As the Sr (II) concentration in the aqueous phase increased, the Sr (II) concentration in the organic phase also increased. However, the system without DHOA showed a maximum value and then tended to decrease with increasing aqueous phase concentration. This is thought to be due to the formation of the third phase. On the other hand, it was observed that the Sr (II) concentration in the organic phase gradually increased without showing the maximum in the system to which DHOA was added. This indicates that the extraction amount approaches the theoretical extraction capacity value as the Sr (II) concentration in the aqueous phase increases without generating a third phase in the system to which DHOA is added. As a result, the extraction capacity becomes larger than the system without adding DHOA.
[0021]
In the above example, the extraction partition ratio of Ca (II) and Sr (II) from 3M nitric acid solution by TODGA is considerably high from FIG. 1, and both Ca (II) and Sr (II) are acid from FIG. It was found that extraction and back-extraction were easy by changing the concentration. The nitric acid concentration of 3M in the aqueous phase assumes the concentration of nitric acid in the simulated waste liquid of high-level radioactive liquid waste. Furthermore, FIG. 3 shows that when DHOA is added, the third phase is not generated and the extraction capacity increases.
[0022]
【The invention's effect】
According to the method of the present invention, it is possible to extract and separate Ca and Sr contained in various concentrations in a nitric acid acidic solution using a mixed extraction solvent of diglycolamide alone or monoamide.
[0023]
Further, according to the method of the present invention, ⁹⁰ Sr having a long half-life of about 30 years and a large decay heat can be directly separated and removed from HLLW by a simple solvent extraction method. As a result, the specification conditions (thermal restriction conditions) of the disposal site for the HLLW solidified material are relaxed, and the disposal costs can be significantly reduced.
[Brief description of the drawings]
FIG. 1 shows the extraction partition ratio (25 ° C.) and TODGA concentration of Ca (II), Sr (II), Ba (II) from 2.9M nitric acid solution when TODGA / n-dodecane solvent is used. It is a figure which shows the relationship.
[Fig. 2] Fig. 2 shows the extraction distribution ratio (25 ° C) of Ca (II), Sr (II), Ba (II) from nitric acid solution and nitric acid solution when 0.1M TODGA / n-dodecane solvent is used. It is a figure which shows the relationship with the density | concentration of.
FIG. 3 shows the Sr (II) concentration of the initial aqueous phase and the organic phase after extraction when 0.1M (or 0.2M) TODGA + (OM or 1M) DHOA / n-dodecane solvent is used. It is a figure which shows the relationship (25 degreeC) with Sr (II) density | concentration.

Claims (4)

Which comprises using a tridentate ligand of diglycol amide (DGA) diluted in dodecane solvent as an extraction solvent, calcium nitrate solution containing high-level radioactive liquid waste (HLLW) (Ca) and strontium (Sr ) For efficient extraction and separation.   The method for efficiently extracting and separating calcium and strontium according to claim 1, further comprising adding a monoamide compound to the extraction solvent to suppress the formation of the third phase.   The method for efficiently extracting and separating calcium and strontium according to claim 1 or 2, wherein the DGA is TODGA (N, N, N ', N'-tetraoctyl 1,3-oxapentanediamide). Monoamide compound DHOA (dihexyl octa _{amide, (C 6 H 13) 2} -N-CO-C 7 H 15) is a method for efficiently extract and separate calcium and strontium as claimed in claim 2 or 3.

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