JP2016138319A - Separation method of metal atom - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separation method of a metal element, which can reduce an amount of use of an organic solvent and, simultaneously, can separate the metal atom while ensuring high safety.SOLUTION: Provided is a method for separating a specific metal element selectively from an aqueous solution, comprising the steps of: preparing an extraction solution by mixing a first acidic aqueous solution including the specific metal atom, a temperature responsive polymer having a lower critical solution temperature and having a hydrophilic skeleton and a hydrophobic skeleton, and a ligand having a hydrophobic functional group and soluble in a nonpolar solvent at a temperature lower than the lower critical solution temperature; and extracting and separating the metal element in a first gel phase by raising the temperature of the extraction solution to a temperature higher than the lower critical solution temperature and separating the extraction solution into the first gel phase containing the temperature responsive polymer and a first aqueous phase.SELECTED DRAWING: None

Description

  The present invention relates to a method for separating a metal element.

  A large amount of rare metal elements (rare earth / rare metal) exist in urban mines and nuclear fuel waste. In Japan, where resources are scarce, these rare earths and rare metals are recovered and reused.

  For example, for nuclear fuel waste, rare earths and rare metals are present in nitric acid solutions containing high-level radioactive materials generated in the wet reprocessing process of spent nuclear fuel, and are extracted and separated by a method called liquid / liquid partition extraction method. ing. In the liquid / liquid partition extraction method, an organic solvent that is insoluble in aqueous solution and not soluble or miscible is used as an extraction medium, and a highly hydrophobic ligand (complexing agent) is dissolved in the extraction medium. By separating and mixing the phases, metal ions are extracted from the aqueous solution into the organic solvent phase and separated.

  The Purex method, a liquid / liquid partition extraction method currently in use, extracts tetravalent or hexavalent actinoids (uranium and protonium) and long half-life trivalent actinoids (americium and curium) from high-level radioactive liquid waste. To separate. Specifically, Octyl-phenyl-N, N-diisobutyryl carbamoylphosphine oxide (CMPO) which is a ligand, Tri-n-butylphosphate (TBP), and n-dodecane (n-dodecane) which is an extraction medium are mixed. Extracting and separating tetravalent, hexavalent and trivalent actinides from the nitrate solution using an extraction solvent.

  Further, in Non-Patent Document 1, trivalent actinides are obtained by a liquid / liquid partition extraction method using a mixed solvent in which CMPO and TBP are dissolved as a ligand as an extraction solvent and a DTPA-nitrate solution as an aqueous phase. A method for separating lanthanoids is disclosed.

  In Non-Patent Document 2, trivalent and tetravalent actinoids are obtained by a liquid / liquid partition extraction method using benzene in which DHDECMP and TBP are dissolved as a ligand as an extraction solvent and a nitric acid solution as an aqueous phase. A method of separating is disclosed.

  In Non-Patent Document 3, trivalent and tetravalent actinoids are obtained by a liquid / liquid partition extraction method using an ionic liquid in which DMDBTDMA and DMDOHEMA are dissolved as ligands as an extraction solvent and a nitric acid solution as an aqueous phase. Is disclosed.

  Non-Patent Document 4 discloses a method for separating actinides by a liquid / liquid partition extraction method using n-dodecane in which TODGA is dissolved as a ligand as an extraction solvent.

KOMA, Y. et al. , Et. al. , J .; Nucl. Sci. Technol. (1998) vol. 35, pp. 130-136. K. V. LOHITAKSHAN, et. al. , J .; Radioanal. Nucl. Chem. , (1994) vol. 183, pp. 359-370. AJAY B. Patil, et. al. Dalton Trans. , (2013) vol. 42, pp. 1519-1529. S. A. Ansari, et. al. , Solvent Extr Ion Exch. , (2005) vol. 23, pp. 463-479.

  By the way, in the liquid / liquid partition extraction method described above, a large amount of an organic solvent is used when separating a specific metal element contained in an aqueous solution, and the used organic solvent is purified and reused. However, there is a problem that a large amount of organic waste liquid is generated when the organic solvent is deteriorated and cannot be reused, such as when a radioactive element is handled. Furthermore, many organic solvents used in this liquid / liquid partition extraction method are harmful to the human body, and are flammable (eg, n-dodecane flash point: 74 ° C.). There was a problem that there was a danger such as.

  The present invention has been made in view of the above circumstances, and provides a metal element separation method capable of reducing the amount of an organic solvent used and simultaneously separating a metal element while ensuring high safety. This is the issue.

  In order to solve the above problems, the invention according to claim 1 is a method for selectively separating a specific metal element from an aqueous solution, the first acidic aqueous solution containing the specific metal element, and a lower critical solution temperature. A temperature-responsive polymer having a hydrophilic skeleton and a hydrophobic skeleton and a ligand having a hydrophobic functional group and soluble in a nonpolar solvent at a temperature lower than the lower critical solution temperature. Preparing an extraction solution by heating, raising the temperature of the extraction solution to a temperature higher than the lower critical solution temperature, and adding the extraction solution to the first gel phase containing the temperature-responsive polymer; And a step of separating the metal element into the first gel phase and extracting and separating the metal element into the first gel phase.

  Further, the invention according to claim 2 recovers the first gel phase at a temperature higher than the lower critical solution temperature, and the first gel phase and the second acidic aqueous solution are used for the lower critical solution. Preparing a solution for back extraction by mixing at a temperature lower than the temperature; raising the temperature of the back extraction solution to a temperature higher than the lower critical solution temperature; and bringing the back extraction solution into a second gel phase 2. The method for separating a metal element according to claim 1, further comprising a step of separating the metal element into a second aqueous phase and back extracting the metal element into the second aqueous phase. .

  The invention according to claim 3 is characterized in that the type of acid used in the first acidic aqueous solution and the type of acid used in the second acidic aqueous solution are different. It is.

  The invention according to claim 4 is characterized in that the concentration of the acid used for the first acidic aqueous solution is different from the concentration of the acid used for the second acidic aqueous solution. It is.

  In addition, the invention according to claim 5 includes the same acid as the acid used for the first acidic aqueous solution, which is higher than the lower critical solution temperature before the step of preparing the solution for back extraction. The method for separating a metal element according to any one of claims 2 to 4, further comprising a step of washing the first gel phase using an acidic aqueous solution.

  The invention according to claim 6 is characterized in that the temperature-responsive polymer has an N-substituted alkylamide skeleton, an N-substituted arylamide skeleton, an amide skeleton, a vinyl ether skeleton, or a methacrylate skeleton. 6. The method for separating a metal element according to any one of items 1 to 5.

  The invention according to claim 7 is characterized in that the temperature-responsive polymer is a copolymer containing Poly (N-isopropylamide), a derivative of the polymer, or a monomer that forms the polymer. The method for separating a metal element according to any one of 1 to 5.

  Moreover, the invention according to claim 8 is the metal according to any one of claims 1 to 7, wherein the ligand is a ligand that can be extracted into the first gel phase. This is an element separation method.

  The invention according to claim 9 further includes a step of adjusting the acidity of the extraction solution before, during, or immediately after the temperature of the extraction solution is raised. Item 9. The method for separating a metal element according to any one of Items 1 to 8.

  The invention according to claim 10 is characterized in that the addition amount of the temperature-responsive polymer (a) is 1/500 or less in mass ratio (a / b) to the first acidic aqueous solution (b). A method for separating a metal element according to any one of claims 1 to 9.

  According to the method for separating a metal element of the present invention, a specific metal element can be selectively extracted and separated using a small amount of a temperature-responsive polymer instead of a large amount of an organic solvent, so that the amount of the organic solvent used can be reduced. At the same time, the metal elements can be separated while ensuring high safety.

It is a graph which shows the nitric acid concentration in each metal element, and the relationship of an extraction rate. It is a graph which shows the relationship between the temperature in each solution, and the transmittance | permeability of light.

Hereinafter, a metal element separation method according to an embodiment to which the present invention is applied will be described in detail.
The metal element separation method of the present embodiment includes a step of preparing an extraction solution (extraction solution preparation step), a step of adjusting the acidity of the solution (acidity adjustment step), and an extraction solution as a first solution. The process of extracting and separating a specific metal element in the first gel phase simultaneously with the separation into the gel phase and the first aqueous phase (extraction process), and the process of washing the first gel phase (washing process) are reversed. A step of preparing an extraction solution (back extraction solution preparation step), and separating the back extraction solution into a second gel phase and a second aqueous phase, and simultaneously reversing a specific metal element into the second aqueous phase And a step of extracting (back extraction step).
The metal element separation method of this embodiment is a method for selectively separating a specific metal element from an aqueous solution.

(Extraction solution preparation process)
In the extraction solution preparation step, a first acidic aqueous solution containing a specific metal element, a temperature-responsive polymer having a lower critical solution temperature and having a hydrophilic skeleton and a hydrophobic skeleton, a hydrophobic functional group and nonpolar An extraction solution is prepared by mixing a ligand capable of being dissolved in an organic solvent at a temperature lower than the lower critical solution temperature (hereinafter referred to as “LCST”) of the temperature-responsive polymer. To do.

  The specific metal element to be separated is not particularly limited as long as it can form a metal complex that is extracted and separated into the first gel phase. Specifically, for example, actinide elements (An) including uranium (U), plutonium (Pu) and americium (Am), curium (Cm), and other superuranium elements (An), lanthanum (La), neodymium (Nd), europium. Lanthanoid elements (Ln) such as (Eu), lutetium (Lu), platinum group elements such as rhodium (Rh), palladium (Pd), ruthenium (Ru), cesium (Cs), strontium (Sr), technetium (Tc) , Tin (Sn), zirconium (Zr) and the like. The first acidic aqueous solution can contain at least one metal element.

  The selective separation of the metal element is performed by appropriately selecting a ligand described later. Specifically, for example, when selectively separating Ln elements such as La, Nd, Eu, and Lu, and Am and Cm, CMPO can be selected as a ligand. Moreover, when selectively separating U and Pu which are tetravalent or hexavalent An elements, TBP can be selected as a ligand. Moreover, when selectively separating Zr, Ca, Sr, Ln elements, and An elements, TODGA can be selected as a ligand.

The first acidic aqueous solution is not particularly limited as long as the temperature-responsive polymer can be changed from a colloidal shape to a gel-like shape in an extraction step described later. Specific examples include sulfuric acid (H ₂ SO ₄ ), nitric acid (HNO ₃ ), hydrochloric acid (HCl), and the like.

  The first acidic aqueous solution is preferably selected so that a metal element to be separated and a ligand (described later) can easily form a complex in the aqueous solution. Specifically, for example, when separating Ln elements such as La, Nd, Eu, and Lu using CMPO as a ligand, nitric acid is preferably used as the first acidic aqueous solution.

  In the case where Ln elements such as La, Nd, Sm, and Gd are separated using a mixed ligand of HDEHP and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEH / EHP) as a ligand. It is preferable to use sulfuric acid as the acidic aqueous solution 1.

A temperature-responsive polymer is a polymer whose solubility in water changes due to temperature changes.
The temperature-responsive polymer used in the present invention is not particularly limited as long as it has LCST and has a hydrophilic skeleton and a hydrophobic skeleton in the molecular structure, and exhibits hydrophilicity at a temperature lower than LCST. It is hydrophobic at higher temperatures. Specific examples include a polymer having an N-substituted alkylamide skeleton, a polymer having an N-substituted arylamide skeleton, a polymer having an amide skeleton, a polymer having a vinyl ether skeleton, and a polymer having a methacrylate skeleton.

Specific examples of the polymer having an N-substituted alkylamide skeleton include, for example, Poly (N-isopropylamide) (hereinafter referred to as “PNIPAAam”) represented by the following formula (1), Poly (N-ethylacrylamide), Poly (N, N′-diethylacrylamide), Poly (N-cyclopropylamide), Poly (N-methyl-N-ethylacrylamide), their derivatives, and copolymers containing monomers forming these polymers, etc. .
The copolymer may be a copolymer of a monomer that forms the polymer and a monomer that forms a polymer having another LCST, or does not have the LCST and the monomer that forms the polymer. It may be copolymerized with a monomer that forms a polymer. The same applies to the following copolymers.

  Specific examples of the polymer having an N-substituted arylamide skeleton include Poly (N-phenylacrylamide), derivatives thereof, and copolymers containing monomers that form these polymers.

  Specific examples of the polymer having an amide skeleton include, for example, Poly (N-vinylisobutylamide), Poly (N-acryloylpyrrolidine), Poly (N-acryloyl-N'-alkylpiperazine) represented by the following formula (2), cyclic Examples thereof include Poly (N-vinylcaprolactam) having a lactam structure which is an amide, derivatives thereof, and copolymers containing monomers forming these polymers.

  Specific examples of the polymer having a vinyl ether skeleton include, for example, Poly (methyl vinyl ether), Poly (oxymethyl vinyl ether), Poly (oxyethylene vinyl ether), derivatives thereof, and monomers forming these polymers. And a copolymer.

  Specific examples of the polymer having a methacrylate skeleton include Poly [2- (dimethylamino) ethylmethacrylate], derivatives thereof, and copolymers containing monomers that form these polymers.

  The addition amount of the temperature-responsive polymer is not particularly limited as long as it is an amount that can sufficiently extract the metal element to be separated. Regarding the mass ratio (a / b) of the temperature-responsive polymer (a) to the first acidic aqueous solution (b), the upper limit of the mass ratio is preferably 1/500 or less, more preferably 1/750 or less. / 1000 or less is more preferable. Moreover, as a lower limit of mass ratio, 1/10000 or more is preferable, 1/50000 or more is more preferable, and 1/10000 or more is further more preferable. By setting the mass ratio to the upper limit value or less, use of unnecessary temperature-responsive polymers can be suppressed. Further, by setting the mass ratio to be equal to or higher than the lower limit value, it is possible to sufficiently extract the metal element to be separated.

A ligand is a compound that coordinates to a metal element to form a complex.
The ligand used in the present invention is not particularly limited as long as it is a compound that forms a complex by coordination with a metal element to be separated and can be extracted into the first gel phase. It is not something. Specifically, it has a hydrophobic group that can be extracted in the first gel phase, and a phosphoryl group (P = O), an oxygen atom (O), a nitrogen atom (N), or a sulfur atom (S) is an electron. Examples thereof include a ligand as a donor group. More specifically, for example, phosphate ligands, amide ligands, malonamide ligands, crown ether ligands, calixarene ligands and the like can be mentioned.

  Specific examples of the phosphoric acid-based ligand include, for example, Octyl- (phenyl) -N, N-diisobutylcarbamoylmethylphosphine oxide (hereinafter referred to as “CMPO”), Diphenyl-N, N-di-n-butylphenylphenol. Oxide, Dibutyl-N, N-diethyl carbamoylmethylphosphonate, Dibutyl-N, N-dibutyl carboxymethylphosphonate, Dihexyl-N, N-diethyl carbylate lphosphine oxide, Bis (2-ethylhexyl) phosphate, Di-2-ethylhexyl phosphoric acid (HDEHP), include Diisodecyl posphoric acid (DIDPA) and the like.

  Specific examples of the amide-based ligand include N, N, N ′, N′-tetraoctyl diglycolide (TODGA), N, N, N ′, N′-tetradecyl dicolamide (TDdDGA), and the like. .

  Specific examples of the malonamide-based ligand include, for example, N, N′-dimethyl-N, N′-dibutyltetradecylmalonamide, N, N′-dimethyl-N, N′-dioctyl-2- (2 ′). -(Hexyloxy) ethyl) -malonamide and the like.

  Specific examples of the crown ether ligand include, for example, Benzo-18-crown-6-ether, Dibenzo-18-crown-6-ether, Cyclohexano-18-crown-6-ether, and Diclohexeno-18-. crown-6-ether and derivatives thereof, specifically, Di-tert-butylcyclohexano-18-crown-6-ether and the like.

  Specific examples of the calixarene ligand include calix such as Calix [4] arene-bis (crown-6-ether) which is a calix crown which is a compound obtained by crosslinking phenolic hydroxyl groups with polyether. Arene derivatives.

  In the extraction solution preparation step, the extraction solution is prepared at a temperature lower than the LCST of the temperature-responsive polymer. Specifically, a method for preparing a solution for extraction by stirring with a magnetic stirrer or the like while circulating a refrigerant in a jacketed beaker and maintaining a temperature lower than that of LCST can be mentioned. Since the temperature-responsive polymer exhibits hydrophilicity at a temperature lower than LCST, it exists in a dissolved state in an aqueous solution.

(Acidity adjustment process)
In the acidity adjustment step, the acidity is adjusted. By adjusting the acidity, a specific metal element can be efficiently extracted into the first gel phase in an extraction step described later, and the extraction rate of the metal element can be increased. The acidity adjustment step may be performed by adjusting the acidity of the first acidic aqueous solution before the extraction solution preparation step, or immediately after the extraction solution preparation step, the acidity of the extraction solution is adjusted. You may carry out by adjusting, and you may carry out by adjusting the acidity of the solution for extraction in the extraction process mentioned later during temperature rising or immediately after temperature rising.

(Extraction process)
In the extraction step, the extraction solution is heated to a temperature higher than the LCST of the temperature-responsive polymer to separate the extraction solution into the first gel phase and the first aqueous phase, and at the same time, the first gel phase. A specific metal element is extracted and separated. Specifically, a method of raising temperature and extracting by stirring with a magnetic stirrer or the like while circulating warm water through a jacketed beaker and maintaining the temperature higher than LCST can be mentioned.

  By raising the temperature in the extraction process, the temperature-responsive polymer changes from hydrophilic to hydrophobic and becomes colloidal in the extraction solution. Furthermore, since the extraction solution is acidic, the temperature-responsive polymer changes from colloidal to gelled in the extraction solution. As a result, the extraction solution can be separated into the first gel phase and the first aqueous phase. At that time, a part of the specific metal element is adsorbed to the temperature-responsive polymer by hydrophobic interaction in a state of forming a complex with the ligand, and is extracted and separated into the first gel phase. Thereby, a part of specific metal element can be selectively extracted and separated into a gel phase.

(Washing process)
In the washing step, the first gel phase is washed using an acidic aqueous solution that is higher in temperature than the LCST of the temperature-responsive polymer and contains the same acid as that used for the first acidic aqueous solution.
The number of times of washing may be one time or a plurality of times.

(Back extraction solution preparation process)
In the solution extraction step for back extraction, the first gel phase is recovered at a temperature higher than the LCST of the temperature-responsive polymer, and the first gel phase and the second acidic aqueous solution are combined with the temperature-responsive polymer. A solution for back extraction is prepared by mixing at a temperature lower than the LCST.
By mixing at a temperature lower than the LSCT of the temperature responsive polymer, the temperature responsive polymer changes from hydrophobic to hydrophilic, so that the first gel phase decomposes and the temperature responsive polymer, ligand, and A specific metal element is dispersed in the second acidic aqueous solution.

The second acidic aqueous solution is not particularly limited as long as it is difficult for the metal element to be separated and the ligand to form a complex in the aqueous solution. Specific examples include sulfuric acid (H ₂ SO ₄ ), nitric acid (HNO ₃ ), hydrochloric acid (HCl), and oxalic acid (H ₂ C ₂ O ₄ ). Moreover, even if it is the same as the acid used for a 1st acidic solution, the acidic aqueous solution from which a density | concentration differs corresponds also to this.

Specifically, for example, when the La, Nd, Eu, and Lu are separated using CMPO as the ligand, hydrochloric acid is preferably used as the second acidic aqueous solution. Moreover, when separating Pu and U using TBP as a ligand, it is preferable to use oxalic acid as the second acidic aqueous solution.
In addition, when separating trivalent ions of An element using TODGA as a ligand, 1 to 3 M nitric acid is used as the first acidic aqueous solution, whereas, as the second acidic aqueous solution, for example, 0 1M dilute nitric acid may be used.
In addition, when extracting Sr using Di-tert-butylcyclohexano-18-crown-6-ether as a ligand, 2 to 3 M nitric acid is used as the first acidic aqueous solution. For example, 0.1 M dilute nitric acid may be used as the acidic aqueous solution.

(Back extraction process)
In the back extraction step, the back extraction solution is heated to a temperature higher than the LCST of the temperature-responsive polymer, thereby separating the back extraction solution into the second gel phase and the second aqueous phase, and at the same time A part of the metal element is left in the second aqueous phase. The temperature increase / separation can be performed by the same method as in the extraction step described above.

  By raising the temperature in the back extraction step, the temperature-responsive polymer changes from hydrophilic to hydrophobic and becomes colloidal in the back extraction solution. Furthermore, since the back extraction solution is acidic, the temperature-responsive polymer changes from colloidal to gel in the back extraction solution. As a result, the back extraction solution can be separated into the second gel phase and the second aqueous phase. At that time, a part of the specific metal element forms a complex with the ligand, is adsorbed to the temperature-responsive polymer by hydrophobic interaction, and is extracted and separated into the second gel phase. 2 remains in the aqueous phase. Thereby, a 2nd aqueous phase will be in the state mainly containing 2nd acidic aqueous solution and a specific metal element. By recovering the second aqueous phase, the specific metal element can be recovered. The recovery of the second aqueous phase can be performed using a syringe or the like.

  According to the metal element separation method of the present embodiment, a specific metal element can be selectively extracted and separated using a small amount of a temperature-responsive polymer instead of a large amount of an organic solvent. In addition, the metal element can be separated while ensuring high safety.

  In addition, according to the method for separating a metal element of the present embodiment, since it further includes a step of preparing a back extraction solution and a step of back extracting the metal element, it is possible to reduce the amount of organic solvent used, Metal elements can be recovered while ensuring high safety.

  Moreover, according to the separation method of the metal element of this embodiment, since it has the structure including the process of wash | cleaning a 1st gel phase, the purity of the specific metal element collect | recovered can be raised.

  In addition, according to the method for separating a metal element of the present embodiment, the process includes a step of adjusting the acidity of the extraction solution. A complex composed of a ligand can be efficiently extracted into the first gel phase. As a result, the extraction rate of the specific metal element can be increased.

  The technical scope of the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, in the method for separating a metal element according to the embodiment described above, after recovering a specific metal element, the gel phase remaining in the back extraction step is used instead of the first gel phase, and the solution for back extraction is prepared again. The specific metal element is recovered by performing the process and the back extraction process, and the recovery rate of the specific metal element can be increased by repeating this process.

<Separation and recovery of metal elements>
Example 1
According to the metal element separation method of the present invention, La, Nd, Eu, and Lu were selectively separated and recovered from a nitrate solution containing La, Nd, Eu, Lu, Cs, and Sr.
Specifically, first, a nitrate solution containing 0.076 mM of La, Nd, Eu, Lu, Cs, and Sr was prepared as a nitrate solution containing a metal element. Furthermore, it adjusted so that the density | concentration of nitric acid of the said nitrate solution might be set to 1M (acidity adjustment process).

  Next, 11.1 mg (0.027 mmol) of CMPO and 30.8 mg of PNIPAAm (corresponding to 0.27 mmol of NIPAAm monomer unit) were added to the jacketed beaker. The refrigerant was circulated through the jacketed beaker and kept at 20 ° C. Next, 15 mL of the nitrate solution was injected and stirred at 700 rpm with a magnetic stirrer. Stirring was continued at 20 ° C. for 1 hour (extraction solution preparation step).

  Next, it switched to the 40 degreeC flow line and stirred at 700 rpm. Stirring was continued for 1 hour at 40 ° C. Immediately after switching to the 40 ° C. flow line, it became cloudy, but after stirring for 1 hour, PNIPAAm gels and adheres to the beaker wall, so that the solution is completely colorless and transparent (extraction process). A syringe was used at 40 ° C. to recover the aqueous phase. The recovered aqueous phase is designated as sample A.

  Next, the gel phase remaining in the beaker was washed four times with 40 mL of 1 M nitric acid at 40 ° C. (washing step).

  The refrigerant was circulated through the jacketed beaker and kept at 20 ° C. While maintaining at 20 ° C., 15 mL of 1M hydrochloric acid was added and stirred for 1 hour (step for preparing a solution for back extraction).

  Next, it switched to the 40 degreeC flow line and stirred at 700 rpm. The mixture was stirred at 40 ° C. for 30 minutes. A syringe was used at 40 ° C. to recover the hydrochloric acid solution (back extraction step). The recovered hydrochloric acid solution is designated as sample B.

(Example 2)
The metal element was separated and recovered in the same manner as in Example 1 except that the concentration of nitric acid in the nitrate solution containing the metal element was adjusted to 0.1M.

(Example 3)
The metal element was separated and recovered in the same manner as in Example 1 except that the concentration of nitric acid in the nitrate solution containing the metal element was adjusted to 0.5M.

Example 4
The metal element was separated and recovered in the same manner as in Example 1 except that the concentration of nitric acid in the nitrate solution containing the metal element was adjusted to 2M.

(Example 5)
The metal element was separated and recovered in the same manner as in Example 1 except that the concentration of nitric acid in the nitrate solution containing the metal element was adjusted to 3M.

<Extraction rate of each metal element>
In Example 1, the extraction rate of each metal element was calculated. Table 1 shows the extraction rate of each metal element extracted in the gel phase after the extraction step and the aqueous phase after the back extraction step. Each value represents an average value of three batch tests. The extraction rate of each metal element extracted in the gel phase after the extraction step was calculated by the following formula (1). Moreover, the extraction rate of each metal element extracted in the aqueous phase after the back extraction step was calculated by the following formula (2). “Concentration of extracted metal element” in the following formulas (1) and (2) indicates the concentration of metal element contained in sample A and sample B, respectively, and “concentration of metal element in initial solution” is The density | concentration of the metal element contained in the nitrate solution after an acidity adjustment process is shown. The concentration of the metal element was measured using an ICP-MS analyzer (NexION 300X, manufactured by PerkinElmer) for a solution obtained by diluting each sample 250 times.
Extraction rate = (1- [concentration of extracted metal element] / [concentration of metal element in initial solution]) × 100 (1)
Extraction rate = [concentration of extracted metal element] / [concentration of metal element in initial solution] × 100 (2)

  The extraction rates of the metallic elements La, Nd, Eu, Lu, Cs, and Sr contained in the aqueous phase (sample B) recovered by the back extraction step were 44.0%, 58.3%, and 51.1, respectively. %, 5.69%, 0.980%, and -0.0498%. From the above results, it can be seen that La, Nd, Eu, and Lu could be selectively separated by using the metal element separation method of this example without using an organic solvent as an extraction solvent.

  Table 2 shows the ratio of the extraction rate of each metal element extracted in the aqueous phase after the back extraction step to the extraction rate of each metal element extracted in the gel phase after the extraction step, that is, the gel in the extraction step. The ratio (the ratio of back extraction) of the metal element which could be back-extracted among the metal elements extracted to the phase is shown.

  The ratio of back extraction of each metal element of La, Nd, Eu, and Lu was 60.7%, 78.4%, 76.6%, and 37.9%. From the above results, it can be seen that the metal elements of La, Nd, Eu, and Lu could be back-extracted at a high rate by the metal element separation method of this example.

<Relationship between extraction rate and acidity>
In Examples 1-5, the extraction rate of each metal element extracted by the extraction process was calculated. The extraction rate was calculated by the same method as that described above. FIG. 1 shows the extraction rate of each metal element in a nitrate solution having nitric acid concentrations of 0.1M, 0.5M, 1M, 2M, and 3M.

  From FIG. 1, La, Nd, Eu, and Lu could be extracted at a high extraction rate in a nitrate solution having a nitric acid concentration of 1M. From the above results, it can be seen that the extraction rate of each metal element of La, Nd, Eu, and Lu can be increased by adjusting to an appropriate acidity.

<About the types of acids>
1, 2, 3M nitric acid solution in which PNIPAAm was dissolved, and 1, 3M hydrochloric acid solution in which PNIPAAm was dissolved were prepared. The relationship between temperature and light transmittance was examined for each solution. The light transmittance was measured by transmitting light having a wavelength of 500 nm to the solution using an ultraviolet / visible spectrophotometer (manufactured by JASCO Corporation, UV-visible spectrophotometer V-630). FIG. 2 shows the relationship between temperature and light transmittance for each solution.

  As can be seen from FIG. 2, the light transmittance of each solution rapidly decreases as the temperature is increased. This is presumably because PNIPAAm in the solution changes from hydrophilic to hydrophobic, colloidal, and becomes cloudy when the temperature rises higher than the LCST as the temperature rises. Further, it can be seen that the light transmittance is increased by further raising the temperature after the light transmittance is rapidly decreased. This is probably because PNIPAAm in the solution changed from colloidal to gelled. However, in FIG. 2, since the horizontal axis is temperature, it seems that the transmittance is recovered as the temperature rises. However, in actuality, when the temperature is maintained at a temperature exceeding the LCST, gelation proceeds with time. Therefore, the transmittance is restored even when maintained at a constant temperature. It has been confirmed that the transmittance is rapidly decreased and recovered for all the solutions, and this result shows that PNIPAAm can be gelled regardless of the type of acid.

  The metal element separation method of the present invention is useful in nuclear fuel cycle (Ln / MA separation), Fukushima contaminated water treatment, rare earth recovery from city mines, removal of harmful metals from environmental water, environmental water analysis, and the like.

Claims (10)

A method for selectively separating a specific metal element from an aqueous solution,
The first acidic aqueous solution containing the specific metal element, a temperature-responsive polymer having a lower critical solution temperature and having a hydrophilic skeleton and a hydrophobic skeleton, and having a hydrophobic functional group and being soluble in a nonpolar solvent A step of mixing a ligand at a temperature lower than the lower critical solution temperature to prepare an extraction solution;
The extraction solution is heated to a temperature higher than the lower critical solution temperature, and the extraction solution is separated into a first gel phase containing the temperature-responsive polymer and a first aqueous phase. And a step of extracting and separating the metal element in the first gel phase. The first gel phase is recovered at a temperature higher than the lower critical solution temperature, and the first gel phase and the second acidic aqueous solution are mixed at a temperature lower than the lower critical solution temperature and reversed. Preparing a solution for extraction;
The back extraction solution is heated to a temperature higher than the lower critical solution temperature, and the back extraction solution is separated into a second gel phase and a second aqueous phase to form the second aqueous phase. The method for separating a metal element according to claim 1, further comprising a step of back-extracting the metal element.   The method for separating a metal element according to claim 2, wherein the acid used for the first acidic aqueous solution is different from the acid used for the second acidic aqueous solution.   The method for separating a metal element according to claim 2, wherein the concentration of the acid used for the first acidic aqueous solution is different from the concentration of the acid used for the second acidic aqueous solution.   Before the step of preparing the solution for back extraction, the first aqueous solution containing an acid that is higher than the lower critical solution temperature and contains the same acid as the acid used for the first acidic aqueous solution. The method for separating a metal element according to any one of claims 2 to 4, further comprising a step of washing the gel phase.   The temperature-responsive polymer has an N-substituted alkylamide skeleton, an N-substituted arylamide skeleton, an amide skeleton, a vinyl ether skeleton, or a methacrylate skeleton, according to any one of claims 1 to 5. Method for separating metal elements.   6. The temperature-responsive polymer is Poly (N-isopropylamide), a derivative of the polymer, or a copolymer including a monomer that forms the polymer. Of separating metal elements.   The method for separating a metal element according to any one of claims 1 to 7, wherein the ligand is a ligand that can be extracted into the first gel phase.   The method according to any one of claims 1 to 8, further comprising a step of adjusting the acidity of the extraction solution before, during, or immediately after the temperature of the extraction solution is increased. The metal element separation method described.   The addition amount of the temperature-responsive polymer (a) is 1/500 or less in mass ratio (a / b) to the first acidic aqueous solution (b). The method for separating a metal element according to one item.

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