Method for recovering boron

Abstract

To provide a method of separating boron from a rare earth metal in a molten salt including rare earth metal ions and boron ions to recover the boron.SOLUTION: A method of separating boron from a molten salt to recover the boron in the present invention includes: preparing a molten salt of a treatment material including a rare earth metal and boron; arranging a cathode electrode and an anode electrode in the molten salt; and applying a voltage between the cathode electrode and the anode electrode so that the electric potential (vs. Li+ / Li) of the cathode electrode is held between 1.4 V and 1.6 V.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for recovering boron. In particular, it relates to a method for separating and recovering boron in a bath of a molten salt containing a rare earth metal and boron from the rare earth metal using the molten salt.

Background Art

[0002] In recent years, with the increasing demand for rare earth magnets containing rare earth metals, the need to recycle rare earth metals has been growing, and techniques for separating and recovering rare earth metals by molten salt electrolysis have been studied (for example, Patent Document 1, Patent Document 2). The “rare earth magnet” refers to a magnet mainly composed of an intermetallic compound of a rare earth metal and a 3d transition metal such as iron (Fe) or cobalt (Co). A typical rare earth magnet, the “neodymium magnet”, is a magnet mainly composed of a neodymium-iron-boron (Nd-Fe-B) based intermetallic compound (Nd2Fe 14 B), which has advantages such as excellent magnetic properties and low production costs, and is used in many applications. Also, in applications such as electric vehicles, since a high coercive force is required at high temperatures, a neodymium magnet added with dysprosium (Dy), a heavy rare earth element, is used.

[0003] However, there are no reported examples of a method for separating and recovering boron element from a molten salt containing rare earth metal ions and boron ions. In this specification, hereinafter, the boron element may be simply described as “boron” or indicated by the element symbol “B”.

[0004] Conventionally, it has been considered difficult to directly recover boron ions from a molten salt in which rare earth metal ions and boron ions are mixed. The main reason is that boron has a property of easily forming stable compounds with other elements. For example, in a molten salt containing Ca and B, although there is a region where elemental boron (hereinafter referred to as "elemental boron") is stable in terms of thermodynamic calculations, when the molten salt is electrolyzed, a boride of CaB6 is formed in a noble potential region. In addition, it has been reported that the formation of rare earth metal borides such as NdB6, EuB6 (Non-Patent Document 1), and LaB6 (Non-Patent Document 2) has been confirmed in the molten salt. However, since these rare earth metal borides have the same crystal structure as CaB6 and are substances with high chemical stability, it is difficult to separate the rare earth metal element and the boron element.

[0005] Thus, in the conventional recycling technology by molten salt electrolysis, the rare earth metal boride is contained in the recovered rare earth metal, which reduces the purity of the rare earth metal. Therefore, an operation for removing boron after recovery is required. Therefore, if boron can be recovered from the molten salt in a form separated from the rare earth metal, it will lead to an improvement in work efficiency and a reduction in processing cost in the recycling process of rare earth metals.

[0006] In addition, it has been reported that boron is electrodeposited from a molten salt containing boron ions. According to these reports, boron and borides were obtained by electrodeposition using a molten salt in a high temperature region of 650°C to 950°C (for example, Patent Document 3, Non-Patent Document 3). The present inventors reported that a Ni-B compound was deposited on the Ni electrode by performing constant potential electrolysis at 1.3 V (vs. Li + / Li) using a Ni electrode on the cathode side in a 450°C molten salt (Non-Patent Document 4). However, since the molten salts used in these reports do not contain rare earth metals, they do not show the electrolysis conditions for directly separating boron from a molten salt in which rare earth metal ions and boron ions coexist.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Document

[0008]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0009] As described above, in the recycling technology using waste materials such as rare earth magnets, there is a problem that boron contained in a molten salt containing rare earth metal ions and boron ions forms a stable compound with the rare earth metal, reducing the purity of the recycled rare earth metal. Therefore, an object of the present invention is to provide a method for separating and recovering boron from a molten salt containing rare earth metal ions and boron ions from the rare earth metal.

Means for Solving the Problems

[0010] As a result of investigations to achieve the above object, the present inventors have found electrolysis conditions under which boron contained in a molten salt can be separated from rare earth metals and deposited, and have thus completed the present invention. Specifically, the present invention includes the following aspects (1) to (6). In the present specification, the expression "~" includes the numerical values at both ends thereof. That is, "X~Y" is synonymous with "X or more and Y or less".

[0011] (1) A method for recovering boron from a material to be treated containing rare earth metals and boron, comprising preparing a molten salt of the material to be treated, disposing a cathode electrode and an anode electrode in the molten salt, and applying a voltage between the cathode electrode and the anode electrode so that the potential (vs. Li + / Li) of the cathode electrode is maintained at a potential of 1.4 V or more and 1.6 V or less, and separating and recovering boron from the molten salt.

[0012] (2) The method for recovering boron according to (1) above, wherein the recovered boron is deposited on the surface of the cathode electrode as elemental boron or a boride with the electrode material.

[0013] (3) The method for recovering boron according to (1) or (2) above, wherein the molten salt contains a chloride, a fluoride, or a mixed salt thereof, and contains rare earth metal ions and boron ions.

[0014] (4) The method for recovering boron according to any one of (1) to (3) above, wherein the bath temperature of the molten salt is 300°C or more and 950°C or less.

[0015] (5) An electrolysis apparatus used for the method for recovering boron according to any one of (1) to (4) above, wherein the cathode electrode and the anode electrode are made of a conductor material that is inert to the molten salt.

[0016] (6) The conductor material in the cathode electrode is a metal or alloy containing one or more selected from the group consisting of nickel, molybdenum, iron, copper, chromium, and manganese, and the electrolysis device according to (5) above.

Advantages of the Invention

[0017] According to the present invention, since boron can be directly recovered by separating it from rare earth metals from a molten salt containing rare earth metal ions and boron ions, in a recycling technique using waste materials such as neodymium magnets, it is possible to omit the boron removal treatment step that has been conventionally required, and a low-cost recycling technique can be provided.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0019] Hereinafter, embodiments of the present invention (hereinafter referred to as "the present embodiment") will be described. The present invention is not limited to the following embodiments, and various modifications are possible without changing the gist of the present invention.

[0020] (Recovery Method) The boron recovery method according to the present embodiment prepares a molten salt of a material to be treated containing rare earth metal ions and boron ions, arranges a cathode electrode and an anode electrode in the molten salt, and the potential of the cathode electrode (vs.Li +A voltage is applied between the cathode electrode and the anode electrode so that the potential of (Li) is maintained at a potential of 1.4 V or more and 1.6 V or less, and boron is separated and recovered from the molten salt. The cathode electrode is an electrode disposed on the anode side, and the anode electrode is an electrode disposed on the anode side. By electrolyzing at a potential where the potential of the cathode electrode is 1.4 V or more and 1.6 V or less, boron is deposited on the surface of the cathode electrode in the form of a simple form of boron or a compound form of boron and the electrode material, and the precipitation of rare earth metals is suppressed. Therefore, boron in the molten salt can be directly separated and recovered.

[0021] (Material to be treated) The object to which the recovery method according to this embodiment is applied is not particularly limited as long as it contains rare earth metals and boron. For example, a molten salt obtained by anodic dissolution of a waste material of a rare earth magnet containing two or more rare earth metals, iron, and boron can be used. Examples of the rare earth magnet include a neodymium magnet containing neodymium (Nd), dysprosium (Dy), iron (Fe), and boron (B). For example, in terms of mass%, Nd: 23% to 30%, Dy: 2% to 10%, Fe: 60% to 65%, and B: 1% are included. The neodymium magnet may contain rare earth metals other than Nd and Dy, such as praseodymium (Pr). Furthermore, the neodymium magnet may contain a metal element or a non-metal element other than Fe, rare earth metals, and boron. For example, one or more elements of copper (Cu), aluminum (Al), cobalt (Co), nickel (Ni), tungsten (W), carbon (C), and nitrogen (N) may be included.

[0022] In this specification, "rare earth metals" refers to elements belonging to Group 3 of the periodic table and the lanthanoid series. For example, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

[0023] According to a preferred example of the recovery method according to this embodiment, boron can be directly separated and recovered from a molten salt obtained by anodic dissolution of waste neodymium magnets containing rare earth metals such as neodymium (Nd) and dysprosium (Dy). After boron is recovered, the molten salt is useful in that it contains a high proportion (purity) of rare earth metals such as neodymium and dysprosium, and thus the rare earth metals can be efficiently recovered in subsequent steps.

[0024] (Molten salt) The molten salt used in the recovery method according to this embodiment is not particularly limited as long as it contains rare earth metal ions and boron ions. Halide salts of alkali metals or alkaline earth metals can be used. As the halide, chloride or fluoride is preferred.

[0025] For example, in the case of alkali metal halides, sodium chloride (NaCl), potassium chloride (KCl), lithium chloride (LiCl), sodium fluoride (NaF), potassium fluoride (KF), lithium fluoride (LiF), etc. can be used. In the case of alkaline earth metal halides, magnesium chloride (MgCl2), calcium chloride (CaCl2), barium chloride (BaCl2), magnesium fluoride (MgF2), calcium fluoride (CaF2), barium fluoride (BaF2), etc. can be used. In the case of rare earth metal halides, neodymium chloride (NdCl3), dysprosium chloride (DyCl3), neodymium fluoride (NdF3), dysprosium fluoride (DyF3), etc. can be used. In addition, oxides, oxalates, sulfates, carbonates may be used.

[0026] The above molten salts may be used alone or in combination of two or more. A eutectic salt obtained by mixing two or more salts to lower the melting point may also be used. Specifically, it preferably contains chlorides, fluorides or mixed salts thereof, and examples thereof include LiCl-KCl eutectic salt. Examples of the molten salt containing rare earth metal ions and boron ions include a molten salt in which DyCl3 and KBF4 are blended in LiCl-KCl eutectic salt. As long as the rare earth ion source and the boron ion source are dissolved as rare earth metal ions and boron ions in the molten salt, chlorides or fluorides can be used.

[0027] (Bath temperature) The recovery method according to this embodiment is preferably carried out in the range where the temperature of the molten salt bath (hereinafter referred to as "bath temperature") is 300 °C or higher and 950 °C or lower. The bath temperature mainly depends on the type of molten salt. When the bath temperature is less than 300 °C, the ionic conductivity of the molten salt decreases, and the progress rate of the electrodeposition reaction decreases. On the other hand, when the bath temperature exceeds 950 °C, the volatilization rate of the molten salt becomes excessive, resulting in loss of the recovered components. In addition, since heating energy is required to maintain the bath temperature in the high temperature range, the working cost increases. The lower limit of the bath temperature may be 300 °C or higher, or 350 °C or higher, 400 °C or higher. The upper limit of the bath temperature may be 700 °C or lower, or 600 °C or lower, 500 °C or lower.

[0028] (Potential) In the molten salt electrolysis method, by applying a voltage to the anode electrode and the cathode electrode arranged in the molten salt bath, a reduction reaction of cations occurs on the cathode side, and a substance reduced from the anions in the molten salt is deposited on the surface of the cathode electrode. The recovery method according to this embodiment uses a molten salt containing rare earth metal ions and boron ions, and preferably performs molten salt electrolysis while maintaining the potential of the cathode electrode (vs. Li + / Li) at 1.4 V or higher. The method for measuring or setting the potential is not particularly limited. For example, as a reference electrode, an Ag +An / Ag electrode may also be used. The potential obtained with the Ag+ / Ag electrode is calibrated based on the "Li + / Li potential" shown by metallic Li, and can be set as "potential (vs. Li + / Li)". In this specification, the description of "vs. Li + / Li" in "potential (vs. Li + / Li)" may be omitted to indicate the numerical value of the potential.

[0029] When the potential of the cathode electrode is in the range of 1.4 V or higher, boron ions are reduced on the cathode side, boron is deposited on the cathode electrode, and boron is recovered as elemental boron or as a compound with the electrode material. On the other hand, when the potential of the cathode electrode is less than 1.4 V, the proportion of rare earth metal ions being reduced and rare earth metal elements being deposited together with boron elements increases, making it difficult to recover boron by separating it well from the rare earth metals. Therefore, the potential according to this embodiment is preferably 1.4 V or higher, more preferably 1.5 V or higher.

[0030] On the other hand, the higher the potential of the cathode electrode is, the lower the deposition rate of boron becomes. Specifically, it becomes difficult to recover a sufficient amount of boron at a potential exceeding 1.6 V. Therefore, the potential in the recovery method according to this embodiment is preferably a potential of 1.6 V or lower. In actual operation, an optimal potential range can be selected considering the separability and the recovery rate. Specifically, 1.4 V to 1.6 V is preferable, and 1.5 V to 1.6 V is more preferable.

[0031] (Function) The recovery method according to this embodiment involves applying a molten salt containing rare earth metals and boron to a cathode electrode with a potential of 1.4 V (vs. Li +By performing potentiostatic electrolysis within a potential range of 1.6 V or higher and 1.6 V or lower, while depositing boron in a sufficient amount, the co-deposition of rare earth metals is suppressed. It is possible to recover boron ions in the molten salt while minimizing the loss of rare earth metal ions. The reason for such separation of boron is considered to be that the redox potential of boron is in a nobler potential range than the redox potential of rare earth metals and the redox potentials of both are significantly different.

[0032] (Recovery form of boron) The boron recovered by the recovery method according to this embodiment includes deposition on the surface of the cathode electrode as elemental boron or a boride with the electrode material. When the material of the cathode electrode is Ni, compounds of nickel and boron (Ni2B, Ni3B) are deposited on the cathode electrode.

[0033] (Electrolysis device for molten salt) As the electrolysis device used in the boron recovery method according to this embodiment, a known molten salt electrolysis device can be used. As shown in FIG. 1, the basic structure of the electrolysis device 6 includes an anode electrode 1 provided on the anode side, a cathode electrode 2 provided on the cathode side, an electrolysis cell 3, and a DC power source 4. The molten salt 5 of the material to be treated is accommodated in the electrolysis cell 3, the anode electrode 1 and the cathode electrode 2 are disposed in the molten salt 5, and a predetermined voltage is applied between the anode electrode 1 and the cathode electrode 2 by the DC power source 4 to perform molten salt electrolysis.

[0034] The molten salt in the electrolysis cell is heated so as to be in a molten state at a predetermined temperature. Therefore, heating means (not shown), such as attaching a heating device to the electrolysis cell or installing the electrolysis cell in an electric furnace, can be used.

[0035] The atmosphere inside the electrolysis cell is not particularly limited. If moisture enters the electrolysis cell, oxygen may be generated during electrolysis, which may oxidize the molten salt components. Therefore, for example, an inert gas such as dry argon (Ar) may be supplied into the electrolysis cell. Further, in order to prevent the volatile gas generated from the molten salt from leaking to the outside, a sealing means may be provided for the electrolysis cell.

[0036] (Electrode material) The cathode electrode and the anode electrode of the electrolysis device according to the present embodiment can be composed of a conductor material that is inert to the molten salt. As the conductive material, a transition metal is preferable, and it is preferably a metal or an alloy containing one or more selected from the group consisting of nickel (Ni), molybdenum (Mo), iron (Fe), copper (Cu), chromium (Cr), and manganese (Mn). In particular, Ni, Fe, etc. that easily form compounds with boron are preferable. The electrodes can be used in solid or liquid form.

Example

[0037] Hereinafter, examples of the present invention will be described. The present invention is not limited to the following description.

[0038] (Test electrolysis device) A schematic diagram of the test electrolysis device (hereinafter simply referred to as the "electrolysis device") used for the test of this example is shown in FIG. 2. Inside the glove box 10 equipped with an electric furnace 18, an electrolysis device 16 equipped with a graphite crucible 15 is provided. The graphite crucible 15 is placed on a support base 25. Inside the graphite crucible 15, a test molten salt 17 is placed, and a working electrode 11, a reference electrode 12, a counter electrode 13, and a thermocouple 14 are installed so as to be immersed in the bath of the molten salt 17.

[0039] Using the information obtained from the working electrode 11, the reference electrode 12, and the counter electrode 13, measurement and analysis are performed by an electrochemical measurement system 19. In order to make the inside of the electrolysis device 16 a predetermined atmosphere, argon gas supplied from an argon gas supply source 20 connected to the electrolysis device 16 is supplied to the electrolysis device 16 through a gas flow meter 21 and a gas supply pipe 22. Further, the gas volatilized from the molten salt 17 is discharged through a gas discharge pipe 23 and flows into a chemical trap 24. The temperature of the electric furnace is monitored and controlled by a thermocouple installed in the furnace (not shown).

[0040] (Electrolysis treatment of molten salt) A molten salt prepared by adding DyCl3 (about 0.5 mol%) and KBF4 (about 2 mol%) to a LiCl-KCl molten salt (44:56 mass%) mixed in the eutectic composition was placed in a graphite crucible of an electrolysis apparatus and then heated to 450 °C. The electrolysis apparatus was filled with a dry argon gas atmosphere. Using the said molten salt, the potential shown in Table 1 below was maintained, and constant potential electrolysis was carried out for 1 hour each. The samples subjected to this electrolysis treatment are shown as "Test Examples 1" to "Test Examples 8" in Table 1.

[0041] For the electrolysis apparatus, an Ni plate or an Mo plate was used as the cathode electrode, and a glassy carbon rod (glass-like carbon rod) was used as the anode electrode. As a reference electrode, a LiCl-KCl salt containing 1 mol% AgCl was placed at the bottom of a semi-closed heat-resistant glass tube, and an Ag + / Ag electrode was used. An Mo wire electrode was used as the working electrode. The potential of the reference electrode was calibrated based on the "Li + / Li potential" indicated by the metallic Li deposited on the Mo wire electrode. The potentials described in this example are described with reference to the Li + / Li potential.

[0042] (Measurement of the components of the deposit) After performing electrolytic deposition treatment under the above-mentioned predetermined conditions, the obtained electrode plate and the deposit on the said electrode plate were completely dissolved in aqua regia using a microwave dissolution apparatus. Then, the obtained solution was subjected to component analysis by an inductively coupled plasma optical emission spectrometer (ICP-AES apparatus, "ICPE-9000" manufactured by Shimadzu Corporation), and the contents (mg) of B and Dy were measured. Based on the measurement values, the respective content ratios (at%) of B and Dy with respect to the total amounts of B and Dy were calculated to obtain the B / Dy (molar ratio). These measurement results are shown in Table 1. Note that the "B / Dy (molar ratio)" in Table 1 is a numerical value with two significant figures.

[0043]

Table 1

[0044] (Evaluation) The precipitate obtained by molten salt electrolysis in this example can be evaluated based on the numerical value of "B / Dy (molar ratio)" shown in Table 1 for its content. Test Example 2 and Test Example 6 are examples in which molten salt electrolysis was carried out under electrolysis conditions included in the scope of the present invention. Since the obtained B / Dy (molar ratio) all showed high values of 100 or more, the ratio at which boron could be directly recovered by being separated from the rare earth metal from the molten salt containing rare earth metal ions and boron ions was high.

[0045] On the other hand, in Test Example 3, Test Example 4, Test Example 7, and Test Example 8 where the potential of the cathode electrode was less than 1.4 V, a mixture of B and Dy was recovered and boron was not sufficiently separated from the rare earth metal. Therefore, under this potential condition, the amount of Dy remaining in the molten salt after the electrolysis treatment decreased, and the recycling rate of the rare earth metal decreased.

[0046] In addition, in Test Example 1 and Test Example 5 where the potential of the cathode electrode exceeded 1.6 V, the deposition amount of boron significantly decreased despite the same electrolysis time as in Test Example 2 and Test Example 6. Therefore, under this potential condition, the recovery efficiency of boron decreases.

[0047] Regarding the electrode plate of the cathode electrode after the electrolysis treatment, XRD analysis of its surface was performed to identify the precipitate. According to the analysis results, in Test Example 2 included in the scope of the present invention, Ni2B was present. Thereby, it was confirmed that the boron deposited on the surface of the Ni plate of the cathode electrode exists in the form of elemental boron and also in the form of a compound with the electrode material.

[0048] This example showed a test in which an electrolytic deposition treatment was carried out using a molten salt containing Dy as a rare earth metal. Since Nd is an element that is reduced at a lower potential than Dy, the difference from the redox potential of boron is larger than in the case of Dy. Therefore, even for a molten salt containing Nd as a rare earth metal, it is possible to satisfactorily separate and recover boron from the molten salt.

Explanation of symbols

[0049] 1 Anode electrode 2 Cathode electrode 3 Electrolytic cell 4 DC power supply 5 Molten salt 6 Electrolysis device 10 Glove box 11 Working electrode 12 Reference electrode 13 Counter electrode 14 Thermocouple 15 Graphite crucible 16 Electrolysis device 17 Molten salt 18 Electric furnace 19 Electrochemical measurement system 20 Argon gas supply source 21 Gas flow meter 22 Gas supply pipe 23 Gas discharge pipe 24 Chemical trap 25 Support stand

Claims

A method for recovering boron from a material to be treated containing rare earth metal and boron using an electrolysis device, comprising: preparing a molten salt of the material to be treated, disposing a cathode electrode and an anode electrode of the electrolysis device in the molten salt, The potential of the cathode electrode (vs. Li ＋ / Li) is maintained at a potential of 1.4 V or more and 1.6 V or less, a voltage is applied between the cathode electrode and the anode electrode, separating and recovering boron from the molten salt. A method for recovering boron. **Claim 2** The method for recovering boron according to claim 1, wherein the recovered boron is deposited on the surface of the cathode electrode as elemental boron or a boride with the electrode material. **Claim 3** The method for recovering boron according to claim 1 or 2, wherein the molten salt contains a chloride, a fluoride or a mixed salt thereof and contains rare earth metal ions and boron ions. **Claim 4** The method for recovering boron according to any one of claims 1 to 3, wherein the bath temperature of the molten salt is 300 °C or higher and 950 °C or lower.

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