KR100351554B1 - Method for extraction and mutual separation of actinide elements and rare earth elements from radioactive liquid waste - Google Patents

Abstract

The present invention relates to a method for extracting and mutually separating actinium elements and rare earth elements from radioactive liquid waste, and specifically, using tributyl phosphate or a mixed extractant containing a metal salt of a high molecular weight alcohol and a phosphoric acid compound. The present invention relates to a method for extracting and isolating actinium elements and / or rare earth elements from radioactive liquid wastes by solvent extraction. When the mixed extractant of the present invention is used, actinium elements and rare earths are used even in radioactive liquid wastes having a concentration of 0.1 M or more. The elements can be extracted and separated with high efficiency.

Description

Method for extraction and mutual separation of actinide elements and rare earth elements from radioactive liquid waste}

The present invention relates to a method for extracting and mutually separating actinium elements and rare earth elements from radioactive liquid wastes. Specifically, a mixed extractant comprising tributyl phosphate or a metal salt of a high molecular weight alcohol and a phosphate compound is used. A method for extracting and mutually separating actinium elements and / or rare earth elements from radioactive liquid waste by solvent extraction.

Radioactive liquid waste from subsequent fuel cycles contains many species of nuclear species, many of which are industrially useful elements. For example, Americium (Am) is used for the analysis of inorganic compounds in bones in the medical field, and for soil moisture or density measurement in the soil field, and for the online analysis of minerals, the analysis of cement components in concrete, and the calibration of detectors. It is used in various fields such as radiometers and gas detectors for gas density measurement (WW Schulz ', The Chemistry of Americium , TID-26971, pp 29-32 (1976)). In addition, lanthanide elements are used to improve the physical properties of alloys because of desulfurization and deoxygenation, or as direct alloy materials, and in the ceramics field for the purpose of decoloring and coloring or improving the properties of ceramics and semiconductors. It is used as a direct fluorescent material or as a constituent of ceramic capacitors. In addition, it is not only used as a material for magnets, catalysts, or gas sensors, but also useful in the nuclear industry (Ha Young-gu, Yun-kyung, "Chemistry of Lanthanide Elements"). Chapter 'Use and Use of Lanthanides', pp 481-617, Blue Road, 1999).

Recently, in order to manage radioactive waste more efficiently, the development of a transmutation technology that separates the long-lived nuclide of long-lived nuclide among these elements and converts into short-lived nuclide or other stable element. Research is actively being conducted.

On the other hand, in order to recycle or annihilate such industrially useful elements from radioactive liquid waste, these elements must first be efficiently extracted and separated. Several methods have been developed for the extraction and separation of elements from radioactive liquid waste, and it is common to classify and extract elements with similar chemical properties into a group. Also, in most cases, extraction and separation are performed by solvent extraction using an extractant suitable for the elements belonging to the classified group.

One of the most widely used extractants in the present solvent extraction method is di- (2-ethylhexyl) phosphoric acid. When used as an extractant, it is possible to use actinium elements (Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No and Lw) at low nitric acid concentrations of 0.1 M or less. Rare earth elements (Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) can be efficiently extracted and separated from each other. In general, however, radioactive liquid waste has a very high nitric acid concentration, and when the nitric acid concentration exceeds 0.1 M, there is a problem that the extraction efficiency drops rapidly. Therefore, when extracting actinium elements and / or rare earth elements from radioactive liquid waste having a concentration of nitric acid of 0.1 M or more, a denitration process must be performed prior to the extraction process. Furthermore, when the nitric acid concentration of the radioactive liquid waste was lowered to 0.1 M or less by the denitrification process, hydrolysis occurred and precipitation of actinium elements and rare earth elements resulted in inefficient solvent extraction ( Evaluation of actinide partitioning and transmutation , International Atomic Energy Agency, Technical Report Series No. 214, Vienna, (1982); C. Keller, The chemistry of the transuranium elements , Verlag Chemie GmbH, (1971); LH Baetsle, T. Wakabayashi, and S. Sakurai, " Status and assessment report on actinide and fission product partitioning and transmutation "in OECD-NEA meeting on partitioning transmutation, 5th Information exchange meeting, Mol, Belgium, 25-27 Nov. (1988); M. Kubota and Y. Morita," Preliminary assessment on four group partitioning process developed in JAERI ", International Conference on Future Nuclear Systems, Proceedings, October 5-10, 1997, Yokohama, Japan, Global 1997 Vol 1, pp 458-468 (1997)). In addition, when the extractant was used and then regenerated with an alkaline solution, the third phase separated, making it difficult to reuse the extractant.

Therefore, the present inventors have tried to develop a method for efficiently extracting actinium element and rare earth element even in radioactive liquid waste, which is a high concentration of nitric acid solution. As a result, tributyl phosphate or metal salt of high molecular weight alcohol and phosphate compound The present invention has been completed by finding that a mixed extractant containing a compound can efficiently extract and separate the actinium group element and the rare earth element by a solvent extraction method even in a radioactive liquid waste containing a high concentration of nitric acid.

It is an object of the present invention to provide a method for efficiently extracting and separating actinium group elements and rare earth elements even in radioactive liquid waste which is a high concentration of nitric acid solution.

1 shows a process for extracting and separating actinium elements and rare earth elements from radioactive liquid waste.

In order to achieve the above object, in the present invention, actinium element and / or rare earth in radioactive liquid waste by solvent extraction method using tributyl phosphate or mixed extractive agent containing alcohol and metal salt of phosphate compound Provides a way to extract elements.

Actinium group element in the present invention means an element contained in the radioactive liquid waste as an element belonging to the actinium series in the periodic table. Elements belonging to the actinium series are specifically actinium (Ac), protitanium (Pa), uranium (U), neptunium (Np), plutonium (Pu), americium (Am), curium (Cm), and burkelium (Bk) And californium (Cf).

In addition, in the present invention, the rare earth element means an element contained in the radioactive liquid waste as an element belonging to Sc, Y and lanthanide series belonging to group 3A in the periodic table. Rare earth elements are specifically scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), deodymium (Nd), promethium (Pm), samarium (Sm), Europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu).

Hereinafter, the present invention will be described in detail.

The extractant of the present invention for extracting actinium elements and / or rare earth elements from radioactive liquid waste is characterized by being a tributyl phosphate or a mixed extractant consisting of metal salts of alcohols and phosphoric acid compounds. That is, in the past, only one di- (2-ethylhexyl) phosphate was used as the extractant, but the extraction efficiency was decreased in the nitric acid solution of 0.1 M or more. However, when the mixed extractant of the present invention is used, the actinium group is used in the nitric acid solution of 0.1 M or more. The extraction efficiency of elements and rare earth elements is high.

Specifically, the phosphate compound used in the mixed extractant of the present invention is an organic phosphate compound, it is preferably selected from the group containing dialkyllyphosphoric acid and alkyl phenyl phosphoric acid. In this case, di- (2-ethylhexyl) phosphoric acid may be used as the dialkyl phosphoric acid. In addition, the alkyl group in the alkylphenyl phosphoric acid is preferably a C _{8 ~} C ₁₀ linear or crushed alkyl group, for example 2-ethylhexyl phenyl phosphoric acid, n-octyl phenyl phosphoric acid (n- octyl phenyl phosphoric acid).

In addition, the metal in the metal salt of the phosphoric acid compound is preferably selected from the group containing Zr, Hf and Ti.

In addition, the alcohol which may be used instead of the tributyl phosphate is preferably a high molecular weight alcohol having 7 or more carbon atoms, and examples thereof include n-octyl alcohol and isooctanol.

In the mixed extractant according to the present invention, the composition of each component is preferably such that the molar ratio of [tributyl phosphate] / [metal salt of phosphate compound] is at least 0.09 when tributyl phosphate is used, and alcohol is used. In this case, the molar ratio of [metal salt of phosphate compound] / [alcohol] is preferably 4 or more.

The mixed extractant consisting of the tributyl phosphate or the metal salt of an alcohol and a phosphate compound according to the present invention may be composed of only these two components, or may be used after dilution in a solvent. When di- (2-ethylhexyl) phosphoric acid is conventionally used as a extractant, a third phase is separated when the extractant is regenerated by using an alkaline solution. In the case of regenerating the mixed extractant, separation of the third phase does not occur.

The separation of the third phase is a phenomenon caused by the change of the properties of the solvent in a specific liquid extraction process, when the concentration of the extracted species is higher than the solubility in the mixed solvent consisting of the extraction solvent and the diluent solvent. The third phase appears. In other words, initially there are two phases, an aqueous phase and an organic phase, but the third phase appears as the organic phase is split into two phases (the phase of the extraction solvent and the phase of the diluent solvent are split).

The cause of the third phase may include the kind of extractant, diluent and inorganic acid, the kind of metal element and oxidation state, extraction temperature and the like. In particular, whether or not a third phase is generated depends on the type of dilution solvent, but not all solvents generate a third phase. Generally, a substance used as a dilution solvent may be classified into an aliphatic compound, an aromatic compound, and other compounds. Examples of the solvent that is an aliphatic compound include C _10- C ₁₆ paraffin hydrocarbons and kerosene, such as dodecane, and the like. While there is a lot of loading capacity, a third phase may be generated. Examples of the solvent, which is an aromatic compound, include benzene and decalin, but the amount of samples that can be extracted at one time is small while the third phase is not produced. Other solvents include carbon tetrachloride, and in the case of carbon tetrachloride, the specific gravity is greater than that of water, which causes the organic layer to be located under the water layer during solvent extraction, and the chlorine atom (Cl) causes the device to corrode and cause pollution. As a result, their use is limited in commercial scale facilities. Therefore, the solvent used for dilution of the extractant should be selected in consideration of the above conditions.

The solvent used to dilute the mixed extractant of the present invention is preferably selected from the group containing C _10- C ₁₆ paraffinic hydrocarbons such as kerosene, dodecane, benzene, decalin, carbon tetrachloride.

In the embodiment of the present invention, when the concentration of DEHPA is 1 M, the concentration of Zr impregnated in DEHPA is 13.5 g / L, and the concentration of TBP is 0.09 M, the extraction ratio of Am and Eu in the 1 M nitric acid solution is 81.9% and 94.9, respectively. %to be. The reverse extraction rate of Am was 53% using 0.05 M DTPA (diethylene triamine pentaacetic acid) and 1 M lactic acid mixed solution in the first stage back extraction for the selective back extraction of Am in the organic phase. , The Am / Eu separation ratio was 5.2. In the second step of back extraction of Eu remaining in the organic phase, 6M nitric acid solution was used as a back extraction agent, and the Eu reverse extraction rate was 90.9%. In addition, it was possible to prevent the formation of a third phase generated when regenerating DEHPA with alkaline solution under the condition that the molar ratio of [TBP] / [DEHPA] was 0.09 or more. Thus, by re-establishing the concentration of Zr using regenerated DEHPA and TBP, a new mixed extractant can be prepared and reused.

Hereinafter, the present invention will be described in more detail with reference to Examples.

However, the following examples are illustrative of the present invention, and the content of the present invention is not limited by the examples.

Preparation Example Preparation of Mixed Extract

329 g of di- (2-ethylhexyl) phosphoric acid (DEHPA) (Merck, Germany) was added to a 1 L volumetric flask and n-dodecane was added to a 1M solution of DEHPA dodetan. Prepared.

60 g of Zr (SO ₄ ) ₂ (Aldrich, USA) was dissolved in 1000 mL of an aqueous 1 M H ₂ SO ₄ solution. The 1 M DEHPA dodecane solution prepared above and the Zr (SO ₄ ) ₂ / 1M H ₂ SO ₄ solution were mixed in a volumetric ratio of 1: 1, put into a separatory funnel, and impregnated with Zr by a solvent extraction method. DEHPA was prepared. Zr-DEHPA obtained from the organic layer was washed once with 1M sulfuric acid solution.

900 ml of Zr-DEHPA prepared above and 90 ml of 1M tributyl phosphate (TBP) (Merck, Germany) were placed in a 1 l volumetric flask, 10 ml of n-dodecane was dissolved, diluted with dodecane. One Zr-1M DEHPA / TBP mixed extractant was prepared. The Zr-1M DEHPA / TBP mixed extractant was used once in pre-equilibrium with nitric acid solution, such as the nitric acid concentration conditions used in the experiment, prior to use in the experiment.

EXAMPLES Extraction of Actinium and Rare Earth Elements

Using a nitric acid standard solution containing an actinium group element and a rare earth element, the actinium group element and the rare earth element were extracted and separated by a process as shown in FIG . 1 .

(Step 1) Primary Extraction of Actinium Elements and Rare Earth Elements

The nitric acid standard solution used for extraction was Am-241 (tracer), Eu-152 (radioisotope, IPL (Isotope Product Laboratories), USA) (tracer), Np-237 (radioisotope, AEA Technology, USA) (tracer), 0.002 M Eu (NO ₃ ) ₃ 5 H ₂ O (Aldrich, USA), 0.04 M Nd (NO ₃ ) ₃ 6 H ₂ O (Aldrich, USA), 0.008 MY (NO ₃ ) ₃ 5H ₂ O (Aldrich, USA), 0.033 M Ce (NO ₃ ) ₃ 6H ₂ O (Aldrich, USA), 0.037 M CsNO ₃ (Aldrich, USA), 0.0165 M Sr (NO ₃ ) ₂ (Aldrich Inc., USA).

50 ml of the mixed extractant prepared in the above preparation and 50 ml of the mixed metal solution (nitric acid concentration: adapted to the experimental conditions) prepared above were put into a separatory funnel (or similar glass container) and extracted to separate the organic layer. Liquid scintillation counter (Packard, Model 2500TR / AB), multi-channel analyzer (Multi channel analyzer, Oxford, PCA-Multiport-Plus, HPGe detector) and Induction Coupling Plasma Analyzer, Jobin Yvon , Model JY 38 Plus), the water layer contained Sr and Cs. In addition, it was confirmed that other elements were included in the organic layer, and in the case of the 1.0 M nitric acid solution, the extraction rates were 81.9% Am, 94.9% Eu, 66% Nd, 84% Nd.

Among the actinium group elements and the rare earth elements extracted and separated from the nitric acid standard solution through step 1, extraction rates of Am and Eu are shown in Table 1 below. The extraction rate is based on the amount of elements contained in the original nitric acid standard solution. As a comparative example, numerical values reported in the prior art were cited for the extraction method using only DEHPA (Svantesson et al. , J. Inorg. Nucl. Chem . Vol 42, 1037-1043 (1980)).

Extraction Rate of Am and Eu Nitrate concentration Am (extraction rate,%) Eu (extraction rate,%) DEHPA TBP + Zr-DEHPA DEHPA TBP + Zr-DEHPA 0.1 M 99.9 99.6 99.4 99.9 0.5 M 86.3 94.7 46.2 98.6 1.0 M 41.2 81.9 8.84 94.9 1.5 M 16.6 55 3.2 85.3 2.0 M 9.9 31.2 1.77 68.4

As shown in Table 1 , in the case of using only DEHPA according to the conventional method, the extraction ratio of Am and Eu is very high as 99.9% and 99.4%, respectively, in 0.1 M nitric acid solution. However, as the concentration of nitric acid solution increased, the extraction rate of each element was significantly decreased. That is, the extraction rates of Am and Eu were 86.3% and 46.2%, respectively, at a concentration of 0.5 M nitric acid, and rapidly decreased to 9.9% and 1.77%, respectively, at a concentration of 2.0 M nitric acid.

On the other hand, in the case of using the mixed extractant TBP + Zr-DEHPA according to the present invention, the extraction rates of Am and Eu in the 0.1M nitric acid solution were 99.6% and 99.9%, respectively, which were similar to those of the conventional case using only DEHPA. In addition, even if the concentration of nitric acid was increased to 0.5 M, the extraction rates of Am and Eu were still excellent as 94.7% and 98.6%, respectively, and even if the concentration of nitric acid was increased to 2.0 M, the extraction rates of Am and Eu were 31.2% and 68.4%, respectively. Compared with the case, it increased by 310 times and 3800 times, respectively. In particular, in the conventional method using only DEHPA, the extraction rate of Eu was significantly decreased when the concentration of nitric acid was high. However, the extraction rate of Eu was still maintained even in a high concentration of nitric acid solution when the mixed extractant according to the present invention was used.

(Step 2) Back Extraction of Actinium Group Am and Cm

In order to wash and remove nitric acid contained in the organic layer in step 1, the organic layer obtained in step 1 was separated and washed once with a volume ratio of organic layer: 0.1M nitric acid solution = 1: 2.

The organic layer obtained in step 1 was prepared using 40 ml of aqueous solution containing 0.05 M DTPA (di-ethylene triamine pentaacetic acid, Aldrich, USA) and 1 M lactic acid (lactic acid, Junsei, Japan). Back extraction was performed to separate the organic layer (A layer) and the water layer (B layer). Part of the separated water layer (B layer) is taken and analyzed by liquid scintillation counter (Packard, Model 2500TR / AB) and multi-wavelength analyzer (Multi channel analyzer, Oxford, PCA-Multiport-Plus, HPGe detector) As a result, it was confirmed that Am which is an actinium group element was contained in the water layer (B layer). In the case of 1.0 M nitric acid solution, the reverse extraction ratio of Am was 53% and the ratio of Am / Eu was 5.2. The organic layer (A layer) remaining after separation was used in the next step.

(Step 3) Back Extraction of Rare Earth Elements Eu, Y, Nd, and Ce

40 ml of the organic layer (A layer) obtained in step 2 was back extracted using 40 ml of 6 M nitric acid solution to separate the organic layer (C layer) and the water layer (D layer). A solution of a certain amount in the separated water layer (D layer) was analyzed by a liquid scintillation analyzer and an induction coupling plasma analyzer (Induction Coupling Plasma Analyzer, Jobin Yvon, Model JY 38 Plus). It was confirmed that Eu, Y, Nd, and Ce were included. In the case of 1.0 M nitric acid solution, Eu's reverse extraction rate was 90.9%. The organic layer (C layer) remaining after separation was used in the next step.

(Step 4) Back Extraction of Actinium Group Np

30 ml of the organic layer (C layer) obtained in step 3 was back-extracted using 30 ml of an aqueous solution of 0.5 MH ₂ C ₂ O ₄ (oxalic acid, Kanto Chemical, Japan) to obtain an organic layer (E layer) and a water layer (F layer). ) Was separated. As a result of analyzing the solution taking a certain amount of the separated water layer (F layer) with a liquid scintillation analyzer and a multi-wavelength analyzer, it was confirmed that the water layer (F layer) contains the actinium element Np. In the case of 1.0 M nitric acid solution, the reverse extraction rate of Np was 91.4%.

(Step 5) Regeneration of the Mixed Extract

In step 4, the actinium element and the rare earth element were extracted, and 20 ml of the remaining organic layer (E layer) was taken into a separatory funnel, and 0.5 M (NH ₄ ) ₂ CO ₃ and 0.05 M (CH ₂ OH) ₂ (CHOH) 20 ml of the mixed solution of ₄ were mixed and shaken vigorously to separate (Zr-DEHPA + TBP) / dodecane. By this process, all actinium elements and rare earth elements included in the extractant were removed, and no separation of the third phase occurred.

As described above, when the mixed extractant according to the present invention is used, the actinium group element and the rare earth element can be efficiently extracted and separated from each other even in radioactive liquid waste having a nitric acid concentration of 0.1 M or more. In addition, the mixed extractive agent according to the present invention has the advantage that the third phase does not separate even after regeneration with alkaline solution after use. Therefore, the mixed extractant according to the present invention can be usefully used for the process of extracting and separating actinium element and rare earth element from radioactive liquid waste.

Claims (7)

A method of extracting and mutually separating actinium elements or rare earth elements from radioactive liquid waste by solvent extraction using a mixed extractant comprising tributyl phosphate and a metal salt of a phosphate compound. The method of claim 1, wherein the phosphoric acid compound is a dialkyl (phosphoric acid) containing di- (2-ethyl hexyl) phosphoric acid (dilklylphosphoric acid) and a straight chain of C ₈ ~ C ₁₀ or A method for extracting and separating an actinium group element or a rare earth element from a radioactive liquid waste, characterized in that it is selected from the group comprising alkyl phenyl phosphoric acid having a ground alkyl group. 2. A process according to claim 1, wherein the metal in the metal salt of the phosphoric acid compound is selected from the group comprising Zr, Hf and Ti. delete The actinium group element or rare earth element is extracted and recited in the radioactive liquid waste according to claim 1, characterized in that the molar ratio of [tributyl phosphate] / [metal salt of phosphate compound], which is the mixed extractant, is 0.09 or more. How to separate. The method of claim 1, wherein the mixed extractant is diluted with a solvent selected from the group consisting of C _10- C ₁₆ paraffin hydrocarbon, kerosene, benzene, decalin and carbon tetrachloride. A method for extracting and mutually separating actinium elements or rare earth elements from a radioactive liquid waste. A mixed extractant for extracting and mutually separating actinium elements or rare earth elements from a radioactive liquid waste, comprising metal salts of tributyl phosphate and a phosphoric acid compound.