Method for separating neodymium and dysprosium by complex formation and crystallization

Abstract

To provide a method for simply and selectively separating and recovering high-purity dysprosium from a solution containing neodymium ion and dysprosium ion whose simple separation is difficult.SOLUTION: A dysprosium complex is created by allowing a tripod-type Schiff ligand to react with neodymium ion and dysprosium ion in a mixed solvent of an aprotic polar solvent and a protic polar solvent. By fractionally precipitating the dysprosium complex, a separation condition for precipitating the dysprosium complex as a crystal in 1 step is determined, and dysprosium is separated at purity of 99% or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This invention relates to a method for separating neodymium (Nd) and dysprosium (Dy) by complex formation and crystallization, and more particularly to a method for separating dysprosium with high purity. [Background technology]

[0002] Neodymium magnets, the strongest of all permanent magnets and capable of mass production, are used as a core and indispensable material in advanced technology products and in the realization of eco-energy technologies such as wind turbines and electric vehicles. Neodymium magnets contain neodymium, which is relatively abundant as a resource, along with rare dysprosium, which enhances their coercivity. In recent years, while the widespread use of neodymium magnets in industrial fields has increased, the concentration of dysprosium-producing countries has raised concerns about supply risks. Therefore, it is hoped that developing a technology to easily and selectively separate and recover dysprosium contained in neodymium magnet scrap would help avoid supply risks for dysprosium and contribute to a stable supply.

[0003] Currently, methods for recovering and recycling dysprosium contained in used neodymium magnets are being considered in two ways: recycling them as a mixture without separating neodymium and dysprosium, and separating neodymium and dysprosium and then recycling their respective metal salts as raw materials in the neodymium magnet manufacturing process. However, a process for recovering neodymium magnets from used electronic products and then separating and recovering the dysprosium contained in them for recycling has not yet been established. Methods for separating and recovering dysprosium contained in neodymium magnets for recycling need to be considered not only in terms of technical feasibility but also from an economic perspective and environmental impact.

[0004] Neodymium and dysprosium are both elements belonging to the lanthanide series. Lanthanides readily assume a trivalent valency in solution, and their f orbitals, which are filled with electrons as the atomic number increases, are shielded and do not participate in chemical bonding. Therefore, the chemical properties of lanthanides are similar, making it difficult to separate mixed lanthanides by simple methods. For example, fractional crystallization, which separates lanthanides by utilizing the difference in solubility of lanthanide metal salts, requires repeated cycles of crystal formation and dissolution because the difference in solubility of lanthanide metal salts is small. Another representative method for separating lanthanides, solvent extraction, separates them by utilizing the difference in complex formation constants between the extractant and the lanthanide. However, in the case of solvent extraction, the difference in complex formation constants due to lanthanides is small, so a multi-stage separation process needs to be designed. As described above, methods for separating lanthanides from mixed lanthanides that are already in practical use require multi-stage separation procedures.

[0005] In addition to the fractional crystallization method and solvent extraction method described above, a method for separating lanthanides based on the crystallization of lanthanide complexes has recently been reported. For example, Patent Document 1 discloses a method for crystallizing only one of two types of lanthanides by coordinating an amine to a lanthanide, reacting the resulting complex with salicylaldehyde to form a complex, and utilizing the fact that these crystals have different solubility. Furthermore, Zhao et al. have reported crystallizing only one of two types of lanthanides based on the fact that coordination polymers with different structures are generated depending on the ionic radius of the lanthanide (Non-Patent Document 1). However, these documents not only fail to consider the mutual separation of neodymium and dysprosium, but also do not provide evaluation indicators for the separation method, such as crystallization yield and separation coefficient.

[0006] Regarding the mutual separation of neodymium and dysprosium, Patent Document 2 describes using di-2-ethylhexyl phosphate as a ligand and neodymium ions (Nd 3+ ) and dysprosium ions (Dy 3+ From a mixed solution of ) Dy 3+A method for selectively precipitating as a coordination polymer is disclosed. Further, Schelter et al. reported a method for selectively crystallizing Dy as a complex from a mixed solution of Nd 3+ and Dy 3+ using tris(2-tert-butylhydroxylaminato)benzylamine as a ligand (Non-Patent Document 2). Furthermore, Yin et al. reported a method for selectively crystallizing Dy as a complex from a mixed solution of Nd 3+ and Dy 3+ using a specific borate as a ligand (Non-Patent Document 3). These documents disclose a one-step separation method for the mutual separation of neodymium and dysprosium, and the purity of the separated dysprosium is considered to be 90% or more. However, in the method described in Patent Document 2, the reaction inevitably becomes unstable stoichiometrically, and in the method described in Non-Patent Document 3, heating at 200 °C for 3 days is required. Therefore, from the viewpoints of economy and environmental load, it is not always satisfactory as a method for separating and recovering dysprosium contained in neodymium magnets and recycling it.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0008]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0009] As described above, regarding the mutual separation of neodymium and dysprosium, although a one-step method using a specific compound as a ligand to selectively precipitate dysprosium ions as a coordination polymer is known, for the obtained dysprosium, not only its purity, but also from the viewpoints of economy, stoichiometric stability, environmental load, etc., a satisfactory method for separating and recovering dysprosium has not been obtained. Based on such a situation, an object of the present invention is to provide a method for simply and selectively separating and recovering high-purity dysprosium from a solution in which neodymium ions and dysprosium ions, which are difficult to separate simply, are mixed.

Means for Solving the Problems

[0010] As a result of intensive studies to achieve the above object, the present inventors have found that the above object can be achieved by using tris[2-(5-methylsalicylideneimino)ethyl]amine (hereinafter, may be referred to as "H3L") shown in FIG. 1 as a ligand. Furthermore, by evaluating the solubility and stability constant of the complex of Nd 3+ and Dy 3+ and the solubility and stability constant of the complex of Dy 3+We determined the separation conditions for precipitating the complex as a crystal in a single step. 3+ When the purity was evaluated, the purity of dysprosium under optimal conditions was found to be over 99%, demonstrating high selectivity compared to conventional technology.

[0011] The present invention is defined by the following: [1] A method for separating and recovering dysprosium from a solution containing neodymium ions and dysprosium ions in a single step, characterized in that a dysprosium complex is generated by reacting neodymium ions and dysprosium ions with a tripod-type Schiff ligand in a mixed solvent of a non-protic polar solvent and a protic polar solvent, and the dysprosium complex is fractionally precipitated. [2] The method according to [1], characterized in that the aprotic polar solvent in the mixed solvent is dimethyl sulfoxide and the protic polar solvent is isopropanol. [3] The method according to [2], characterized in that the volume ratio of dimethyl sulfoxide to isopropanol in the mixed solvent is 1:3. [4] The method according to [1], characterized in that the tripod-type Schiff base is tris[2-(5-methylsalicylideneimino)ethyl]amine. The method according to [1], characterized by adding a strong base to the mixed solvent. [6] The method according to [5], characterized in that the strong base is diisopropylamine. [Effects of the Invention]

[0012] According to the present invention, it is possible to separate and recover high-purity dysprosium from a solution containing neodymium ions and dysprosium ions, which are difficult to separate in a simple manner, in a single step, providing not only high selectivity but also a simple method. [Brief explanation of the drawing]

[0013] [Figure 1] Structure of a tripod-type Schiff ligand (H3L) [Figure 2] Schematic of a 7-coordinate complex (LnL) between a lanthanide and a tripod-type Schiff ligand. [Modes for carrying out the invention]

[0014] The present invention provides a method for separating and recovering dysprosium in a single step by generating a dysprosium complex from a solution containing neodymium ions and dysprosium ions and then fractionally precipitating it, characterized in that a tripod-type Schiff ligand is used in a mixed solvent of a non-protic polar solvent and a protic polar solvent.

[0015] The mixed solvent of an aprotic polar solvent and a protic polar solvent used in the present invention is obtained by mixing an aprotic polar solvent and a protic polar solvent selected from the viewpoints of ease of handling, high boiling point, and non-carcinogenicity. Preferably, the mixed solvent has low solubility of the dysprosium complex and a large difference between the solubility of the neodymium complex and the solubility of the dysprosium complex. The aprotic polar solvent is not particularly limited, but solvents with a high dielectric constant are preferred, such as dimethyl sulfoxide and dimethylformamide. Dimethyl sulfoxide is preferred from the viewpoint of ease of handling. The protic polar solvent is not particularly limited, but examples include lower alcohols such as ethanol and isopropanol.

[0016] Regarding the mixing ratio of the aprotic polar solvent and the protic polar solvent in the mixed solvent, a mixing ratio that results in low solubility of the dysprosium complex and a large difference between the solubility of the neodymium complex and the solubility of the dysprosium complex is preferred. For example, in the case of a mixed solvent using dimethyl sulfoxide as the aprotic polar solvent and isopropanol as the protic polar solvent, the solubility of the dysprosium complex is low and the difference between the solubility of the neodymium complex and the solubility of the dysprosium complex is large, so a volume ratio of dimethyl sulfoxide to isopropanol of 1:3 is preferred.

[0017] Examples of tripod-type Schiff ligands used in the present invention include tris[2-(5-methylsalicylideneimino)ethyl]amine (H3L). The aromatic ring of H3L may be unsubstituted, and substituents other than methyl groups, such as ethyl groups and propyl groups, which are not bulky, may be substituted. The substitution position may be any position other than the 5th position.

[0018] In this invention, the separation selectivity of dysprosium is synergistically amplified by combining two phenomena: the simultaneous acid dissociation of H3L and complex formation, which results in the formation of a dysprosium complex preferentially over a neodymium complex; and the fact that the solubility of the formed dysprosium complex is lower than that of the neodymium complex. To promote the complex formation reaction, a strong base can be added to the solution containing neodymium ions, dysprosium ions, and H3L, and Nd 3+ Dy 3+ By adjusting the molar ratio of H3L and the strong base, the ratio of neodymium complex to dysprosium complex contained in the resulting crystal can be controlled. The strong base is not particularly limited, but examples include triethylamine, tripropylamine, and diisopropylethylamine, with diisopropylethylamine being particularly preferred.

[0019] In this invention, a solution containing neodymium ions and dysprosium ions is mixed with a mixed solvent consisting of an aprotic polar solvent and a protic polar solvent, and a tripod-type Schiff ligand is added. The resulting solution is left to stand at 25°C (room temperature) for 3 days to generate neodymium complexes and dysprosium complexes, and the complexes precipitated in the mixed solvent are separated by filtration. [Examples]

[0020] The present invention will be described below with reference to examples, but the present invention is not limited in any way by these examples.

[0021] [Synthesis of 7-coordinate mononuclear complexes] Hepta-coordinate mononuclear complexes of neodymium and dysprosium (hereinafter, the hepta-coordinate mononuclear complex of neodymium will be referred to as "NdL" and the hepta-coordinate mononuclear complex of dysprosium as "DyL") were synthesized. DyL was synthesized according to a known method (Polyhedron, 2015, 85, 76-82). NdL was synthesized by mixing H3L and Nd(CF3SO3)3 in methanol heated to 50°C for 5 minutes, followed by the addition of diisopropylethylamine (DIEA) as a base. The general shape of the synthesized complexes is shown in Figure 2. The properties of the synthesized complexes were evaluated to determine the conditions that would enable separation by crystallization. The solvents used in the following experiments were dehydrated using molecular sieves to suppress side reactions and ensure reproducibility.

[0022] [Preparation of mixed solvents and investigation of solubility] To separate neodymium and dysprosium based on the crystallization of complexes formed with ligands, a mixed solvent was prepared in which the solubility of one of the two complexes, NdL or DyL, was low, and the difference in solubility with the other complex was large. Dimethyl sulfoxide (DMSO) was used as the aprotic polar solvent, and isopropanol (IPA) was used as the protic polar solvent to prepare the mixed solvent. The results are shown in Table 1. The volume ratio of dimethyl sulfoxide to isopropanol in the mixed solvent was adjusted from 1:1 to 1:6. It was found that the solubility of DyL in the mixed solvent (sDyL) was lower than the solubility of NdL (sNdL) at all volume ratios, indicating that this range of volume ratios can be used for the separation of DyL. Furthermore, it was found that a volume ratio of dimethyl sulfoxide to isopropanol in the mixed solvent of approximately 1:3 was preferable in terms of the balance between yield and purity.

[0023] [Table 1]

[0024] By utilizing the difference in solubility between NdL and DyL, NdL is incorporated into the crystal. 3+ and Dy 3+The change in the ratio of NdL and DyL was investigated. NdL and DyL were dissolved in a mixed solvent of dimethyl sulfoxide and isopropanol in a volume ratio of 1:3 and crystallized. The Nd in the resulting crystals 3+ and Dy 3+ When the ratio was measured using a scanning electron microscope and energy-dispersive X-ray elemental analyzer, it was found that the crystal contained dysprosium ions (Dy 3+ ) is ~80%, neodymium ions (Nd 3+ It was found that it contains approximately 20% of [the substance]. It was also found that significant selectivity for dysprosium ions can be obtained even with only the difference in solubility between NdL and DyL.

[0025] [Consideration of stability constants] Furthermore, we investigated the possibility of further improving selectivity for dysprosium ions by combining differences in complex solubility and differences in reactivity during complex formation. We examined the formation equilibrium of NdL and DyL in a mixed solvent with a volume ratio of dimethyl sulfoxide and isopropanol of 1:3. The complex formation equilibrium of LnL (where Ln is neodymium or dysprosium) is expressed by the following equation (1). [ka] The stability constant K of this equilibrium LnL This is expressed by the following equation (2).

number

number

[0026] [Table 2]

[0027] [Examples] Based on the above findings, we conducted a separation experiment to selectively crystallize DyL by demonstrating the synergistic effect of the low solubility of DyL in the mixed solvent and the ease with which DyL is formed. DIEA was added to a solution containing Nd(NO3)3, Dy(NO3)3, and H3L, using a solvent prepared by mixing dimethyl sulfoxide and isopropanol in a 1:3 ratio. The mixing ratio of the reagents was Nd 3+ :Dy 3+ The :H3L:DIEA mixture was adjusted to 1:1:2:6, 1:1:2:3, and 1:1:2:2.5, and the prepared solutions were allowed to stand in sealed containers at 25°C for 3 days. The resulting crystals were filtered and washed with a mixed solvent of DMSO:IPA=1:3. The recovered crystals were dried under reduced pressure overnight and dissolved in nitric acid to extract neodymium ions (Nd 3+ ) and dysprosium ions (Dy 3+ The concentration of neodymium ions (Nd) in the solution was quantified by inductively coupled plasma emission spectrometry (ICP-AES). The filtrate and washing solution were collected in their entirety, and the neodymium ions (Nd) contained in the solution were quantified. 3+ ) and dysprosium ions (Dy 3+ The concentration of neodymium ions (Nd) in the crystal was quantified using ICP-AES. From the values ​​obtained from the above operations, the concentration of neodymium ions (Nd) in the crystal was determined. 3+ ) and dysprosium ions (Dy 3+ ) ratio, neodymium ions (Nd) contained in the solution 3+ ) and dysprosium ions (Dy 3+ The ratio with ) was determined. Furthermore, the separation coefficient (S) was calculated using equation (4).

number

[0028] [Table 3]

[0029] As shown in Table 3, in the separation experiments at all mixing ratios, the dysprosium ions (Dy) contained in the crystal were found to be present. 3+ The proportion of ) was over 96%, and it was found that by using a combination of differences in solubility and differences in the complex formation process, the selectivity of dysprosium was significantly improved. The separation coefficient (S) value was an order of magnitude larger than the separation coefficient obtained by one-step solvent extraction, indicating high selectivity. In particular, Nd 3+ :Dy 3+ In the case of :H3L:DIEA=1:1:2:2.5, the dysporosium ions (Dy) contained in the resulting crystal are 3+ The purity of the substance was 99.8 ± 0.1%. [Industrial applicability]

[0030] By using a tripod-type Schiff ligand in a mixed solvent of aprotic and protic polar solvents, dysprosium ions could be selectively separated with high purity through a single crystallization step. Therefore, this technology can provide an economical and environmentally friendly lanthanide separation technique that uses readily available solvents, does not require heating, and is applicable not only to neodymium and dysprosium combinations but also to neodymium and other lanthanides.

Claims

1. A method for separating and recovering dysprosium from a solution containing neodymium ions and dysprosium ions in a single step, characterized in that a dysprosium complex is generated by reacting neodymium ions and dysprosium ions with a tripod-type Schiff ligand in a mixed solvent of a non-protic polar solvent and a protic polar solvent, and the solubility of the dysprosium complex in the mixed solvent is lower than that of the neodymium complex, and the difference in solubility selectively precipitates and separates the dysprosium complex.

2. The method according to claim 1, characterized in that the aprotic polar solvent in the mixed solvent is dimethyl sulfoxide and the protic polar solvent is isopropanol.

3. The method according to claim 2, characterized in that the volume ratio of dimethyl sulfoxide to isopropanol in the mixed solvent is 1:

3.

4. The method according to claim 1, characterized in that the tripod-type Schiff base is tris[2-(5-methylsalicylideneimino)ethyl]amine.

5. The method according to claim 1, characterized by further adding a strong base to the mixed solvent.

6. The method according to claim 5, characterized in that the strong base is diisopropylamine.

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