JP7711924B2 - How to recover boron - Google Patents

Description

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

In recent years, the demand for rare earth magnets containing rare earth metals has increased, and the need to recycle rare earth metals has been increasing, and techniques for separating and recovering rare earth metals by molten salt electrolysis have been considered (for example, Patent Document 1 and Patent Document 2). A "rare earth magnet" refers to a magnet whose main component is an intermetallic compound of rare earth metals and 3d transition metals such as iron (Fe) and cobalt (Co). A representative rare earth magnet, a "neodymium magnet," is a magnet whose main component is a neodymium-iron-boron (Nd-Fe-B) intermetallic compound (Nd ₂ Fe ₁₄ B), and has advantages such as excellent magnetic properties and low production costs, and is used in many applications. In addition, for applications such as electric vehicles, a neodymium magnet to which the heavy rare earth element dysprosium (Dy) is added is used because high coercive force is required at high temperatures.

However, there are no known reported methods for separating and recovering elemental boron from molten salts containing rare earth metal ions and boron ions. In the following description, elemental boron will be referred to simply as "boron" or by its element symbol "B."

It has been considered difficult to directly recover boron ions from a molten salt containing a mixture of rare earth metal ions and boron ions. 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 in which simple boron (hereinafter referred to as "simple boron") is stable in terms of thermodynamic calculations, when the molten salt is electrolyzed, a boride of _CaB6 is generated in a noble potential region. It has also been reported that rare earth metal borides such as _NdB6 , _EuB6 (Non-Patent Document 1), and _LaB6 (Non-Patent Document 2) have been confirmed to be generated in the molten salt. However, these rare earth metal borides are compounds with the same crystal structure as _CaB6 and are highly chemically stable substances, so it is difficult to separate the rare earth metal element from the boron element.

As described above, in conventional recycling technology using molten salt electrolysis, the recovered rare earth metals contain rare earth metal borides, which reduces the purity of the rare earth metals. This requires the process of removing boron after recovery. If it were possible to recover boron from the molten salt in a form separated from the rare earth metals, this would lead to improved work efficiency and reduced processing costs in the rare earth metal recycling process.

It has been reported that boron can be electrolytically deposited from a molten salt containing boron ions. According to these reports, boron and borides were obtained by electrolytic deposition using a molten salt in the high temperature range of 650°C to 950°C (for example, Patent Document 3, Non-Patent Document 3, 2003). The present inventors reported that a constant potential electrolysis of 1.3 V (vs. Li ⁺ /Li) was performed in a molten salt at 450°C using a Ni electrode on the cathode side, and a Ni-B compound was deposited on the Ni electrode (Non-Patent Document 4). However, since the molten salt used in these reports does not contain rare earth metals, they do not show electrolysis conditions for directly separating boron from a molten salt in which rare earth metal ions and boron ions coexist.

Patent No. 6823314 Patent No. 6057250 U.S. Pat. No. 3,030,284

G. A. Bukatova, S. A. Kuznetsov, and M. Gaune-Escard, Russ. J. Electrochem. , 43, 929-935 (2007) M. Kamaludeen, I. Selvaraj, A. Visuvasam, and R. Jayavel, J. Mater. Chem. , 8, 2205-2207 (1998) J. Zhou and P. Bai, Asia-Pacific J. Chem. Eng. , 10, 325-338 (2015) Y. Katasho and T. Oishi, ECS Trans. , 98, 53-59 (2020)

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

As a result of investigations aimed at achieving the above object, the inventors discovered electrolysis conditions that make it possible to separate and precipitate boron contained in a molten salt from rare earth metals, and thus completed the present invention. Specifically, the present invention includes the following aspects (1) to (6). In this specification, the expression "to" includes the numerical values at both ends. In other words, "X to Y" is synonymous with "X or more and Y or less."

(1) A method for recovering boron from a material to be treated containing rare earth metals and boron, comprising the steps of: preparing a molten salt of the material to be treated; placing a cathode electrode and an anode electrode in the molten salt; applying a voltage between the cathode electrode and the anode electrode so that 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; and separating and recovering boron from the molten salt.

(2) The method for recovering boron described in (1) above, in which the recovered boron precipitates on the surface of the cathode electrode either as boron alone or as a boride with the electrode material.

(3) A method for recovering boron according to (1) or (2) above, in which the molten salt contains a chloride, a fluoride or a mixed salt thereof, and contains rare earth metal ions and boron ions.

(4) A method for recovering boron described in any one of (1) to (3) above, in which the bath temperature of the molten salt is 300°C or higher and 950°C or lower.

(5) An electrolysis device used for the boron recovery method described in any one of (1) to (4) above, wherein the cathode electrode and the anode electrode are made of a conductive material that is inactive with respect to the molten salt.

(6) The electrolytic device described in (5) above, in which 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.

The present invention makes it possible to separate boron from rare earth metals and directly recover it from molten salt containing rare earth metal ions and boron ions, making it possible to omit the boron removal process that was previously required in recycling technologies that use waste materials such as neodymium magnets, and to provide a low-cost recycling technology.

1 is a schematic diagram showing an electrolysis device to which a recovery method according to the present invention is applied; FIG. 2 is a schematic diagram showing a test electrolysis device used in the examples.

The following describes an embodiment of the present invention (hereinafter, "the present embodiment"). The present invention is not limited to the following embodiment, and various modifications are possible without departing from the gist of the present invention.

(Collection method)
The method for recovering boron according to the present embodiment is characterized in that a molten salt of a material to be treated containing rare earth metal ions and boron ions is prepared, a cathode electrode and an anode electrode are disposed in the molten salt, a voltage is applied between the cathode electrode and the anode electrode so that 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, 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 the cathode electrode at a potential of 1.4 V or more and 1.6 V or less, boron is precipitated on the surface of the cathode electrode in the form of simple boron or in the form of a compound of boron and an electrode material, and the precipitation of rare earth metals is suppressed, so that boron in the molten salt can be directly separated and recovered.

(Material to be treated)
The material to be treated to which the recovery method according to the present embodiment is applied is not particularly limited as long as it contains rare earth metals and boron. For example, a molten salt obtained by anode dissolving scrap rare earth magnets containing two or more rare earth metals, iron, and boron can be used. The rare earth magnets include neodymium magnets containing neodymium (Nd), dysprosium (Dy), iron (Fe), and boron (B), and examples of such rare earth magnets include Nd-Dy-Fe-B alloys containing, by mass, Nd: 23% to 30%, Dy: 2% to 10%, Fe: 60% to 65%, and B: 1%. The neodymium magnets may contain rare earth metals other than Nd and Dy, such as praseodymium (Pr). Furthermore, neodymium magnets may contain metallic or non-metallic elements other than Fe, rare earth metals, and boron, such as one or more of the following elements: copper (Cu), aluminum (Al), cobalt (Co), nickel (Ni), tungsten (W), carbon (C), and nitrogen (N).

As used herein, "rare earth metal" refers to elements in Group 3 and the lanthanide series of the periodic table, such as Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

According to a preferred example of the recovery method of this embodiment, boron can be directly separated and recovered from the molten salt obtained by dissolving waste neodymium magnets containing rare earth metals such as neodymium (Nd) and dysprosium (Dy) at an anode. The molten salt from which boron has been recovered contains a high proportion (purity) of rare earth metals such as neodymium and dysprosium, making it useful in that the rare earth metals can be efficiently recovered in subsequent processes.

(Molten salt)
The molten salt used in the recovery method according to the present embodiment is not particularly limited as long as it contains rare earth metal ions and boron ions. A halide salt of an alkali metal or an alkaline earth metal can be used. As the halide, a chloride or a fluoride is preferable.

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 (MgCl ₂ ), calcium chloride (CaCl ₂ ), barium chloride (BaCl ₂ ), magnesium fluoride (MgF ₂ ), calcium fluoride (CaF ₂ ), barium fluoride (BaF ₂ ), etc. can be used. In the case of rare earth metal halides, neodymium chloride (NdCl ₃ ), dysprosium chloride (DyCl ₃ ), neodymium fluoride (NdF ₃ ), dysprosium fluoride (DyF ₃ ), etc. can be used. In addition, oxides, oxalates, sulfates, and carbonates may be used.

The above molten salts may be used alone or in combination of two or more. A eutectic salt in which two or more salts are mixed to lower the melting point may be used. Specifically, it is preferable to include a chloride, a fluoride, or a mixed salt thereof, and LiCl-KCl eutectic salt can be mentioned. As a molten salt containing rare earth metal ions and boron ions, a molten salt in which _DyCl3 and _KBF4 are blended with LiCl-KCl eutectic salt can be mentioned as an example. The rare earth ion source and the boron ion source can be any chloride or fluoride source as long as they are dissolved in the molten salt as rare earth metal ions and boron ions.

(bath temperature)
The recovery method according to the present embodiment is preferably carried out at a temperature of the molten salt bath (hereinafter referred to as "bath temperature") in the range of 300°C or more and 950°C or less. 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 rate of progress of the electrolytic deposition reaction decreases. On the other hand, when the bath temperature exceeds 950°C, the proportion of the molten salt that volatilizes becomes excessive, resulting in a loss of the recovered components. In addition, heating energy is required to maintain the bath temperature in a high temperature range, which increases the operating cost. The lower limit of the bath temperature may be 300°C or more, 350°C or more, or 400°C or more. The upper limit of the bath temperature may be 700°C or less, 600°C or less, or 500°C or less.

(potential)
In the molten salt electrolysis method, a voltage is applied to an anode electrode and a cathode electrode arranged in a bath of molten salt, whereby a reduction reaction of cations occurs on the cathode side, and substances reduced from anions in the molten salt are precipitated on the surface of the cathode electrode. In the recovery method according to the present embodiment, it is preferable to perform molten salt electrolysis by using a molten salt containing rare earth metal ions and boron ions, and maintaining the potential of the cathode electrode (vs. Li ⁺ /Li) at 1.4 V or higher. The method of measuring or setting the potential is not particularly limited. For example, an Ag ⁺ /Ag electrode in which an Ag wire is immersed in LiCl-KCl salt containing AgCl may be used as a reference electrode. The potential obtained by the Ag + /Ag electrode can be set as a "potential (vs. Li ⁺ /Li)" by calibrating based on the "Li ⁺ /Li potential" indicated by metal Li. In this specification, the term "potential (vs. ^{Li +} /Li)" may be omitted to indicate the numerical value of the potential.

When the potential of the cathode electrode is in the range of 1.4 V or more, boron ions are reduced on the cathode side, and boron is precipitated on the cathode electrode, and boron is recovered as elemental boron or as a compound with the electrode material. In contrast, when the potential of the cathode electrode is less than 1.4 V, rare earth metal ions are also reduced, increasing the proportion of rare earth metal elements precipitated together with boron elements, making it difficult to effectively separate boron from rare earth metals and recover them. Therefore, the potential in this embodiment is preferably 1.4 V or more, and more preferably 1.5 V or more.

On the other hand, the more noble the potential of the cathode electrode is, the slower 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 1.6 V or less. In actual operation, an optimal potential range can be selected taking into consideration the separation ability and recovery speed. Specifically, 1.4 V to 1.6 V is preferable, and 1.5 V to 1.6 V is even more preferable.

(effect)
In the recovery method according to the present embodiment, a molten salt containing rare earth metals and boron is subjected to constant potential electrolysis at a cathode electrode potential range of 1.4 V (vs. Li ⁺ /Li) or more and 1.6 V or less, thereby precipitating a sufficient amount of boron while suppressing the mixed precipitation of rare earth metals. It is possible to recover boron ions in the molten salt while minimizing the loss of rare earth metal ions. The reason why such separation of boron is possible is believed to be that the redox potential of boron is located in a more noble potential range than the redox potential of rare earth metals, and the redox potentials of the two are far apart.

(Boron recovery form)
The boron recovered by the recovery method according to the present embodiment includes boron precipitated on the surface of the cathode electrode as boron alone or as a boride with the electrode material. When the cathode electrode is made of Ni, a compound of nickel and boron (Ni ₂ B, Ni ₃ B) precipitates on the cathode electrode.

(Molten salt electrolysis device)
A known molten salt electrolysis device can be used as the electrolysis device used in the boron recovery method according to this embodiment. 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 supply 4. Molten salt 5 of the material to be treated is contained in the electrolysis cell 3, the anode electrode 1 and the cathode electrode 2 are placed 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 supply 4 to perform molten salt electrolysis.

The molten salt in the electrolytic cell is heated to a molten state at a predetermined temperature. For this purpose, a heating means (not shown) can be used, such as attaching a heating device to the electrolytic cell or placing the electrolytic cell in an electric furnace.

The atmosphere in the electrolytic cell is not particularly limited. If moisture is mixed into the electrolytic cell, oxygen may be generated during electrolysis, which may oxidize the molten salt components. For this reason, for example, an inert gas such as dry argon (Ar) may be supplied to the electrolytic cell. In addition, a sealing means may be provided in the electrolytic cell to prevent volatile gases generated from the molten salt from leaking to the outside.

(electrode material)
The cathode and anode electrodes of the electrolytic device according to this embodiment can be made of a conductive material that is inactive against the molten salt. The conductive material is preferably a transition metal, and is preferably a metal or alloy containing at least one selected from the group consisting of nickel (Ni), molybdenum (Mo), iron (Fe), copper (Cu), chromium (Cr), and manganese (Mn). In particular, Ni, Fe, and the like, which easily form compounds with boron, are preferred. The electrodes can be used in the form of a solid or liquid.

The following describes an embodiment of the present invention. The present invention is not limited to the following description.

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

Measurements and analyses are performed by the electrochemical measurement system 19 using information obtained from the working electrode 11, the reference electrode 12, and the counter electrode 13. To create a predetermined atmosphere inside the electrolysis device 16, argon gas is supplied from an argon gas supply source 20 connected to the electrolysis device 16 via a gas flowmeter 21 and a gas supply pipe 22 to the electrolysis device 16. Gas volatilized from the molten salt 17 is exhausted through a gas exhaust 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).

(Electrolytic treatment of molten salt)
A molten salt was prepared by adding DyCl ₃ (about 0.5 mol %) and KBF ₄ (about 2 mol %) to a eutectic mixture of LiCl-KCl molten salt (44:56 mass %), and then the molten salt was placed in a graphite crucible of an electrolysis device and heated to 450°C. The electrolysis device was filled with a dry argon gas atmosphere. The molten salt was used to perform constant potential electrolysis for one hour, while maintaining the potentials shown in Table 1 below. The samples that underwent this electrolysis process are shown in Table 1 as "Test Example 1" to "Test Example 8".

The electrolysis apparatus used a Ni plate or Mo plate as the cathode electrode and a glassy carbon rod as the anode electrode. The reference electrode was an Ag ⁺ /Ag electrode made by placing LiCl-KCl salt containing 1 mol% AgCl in the bottom of a heat-resistant glass tube with one side closed and immersing an Ag wire in the salt. The working electrode was an Mo wire electrode. The potential of the reference electrode was calibrated based on the "Li ⁺ /Li potential" indicated by metallic Li deposited on the Mo wire electrode. The potentials described in this example are described with reference to the Li ⁺ /Li potential.

(Measurement of Precipitate Components)
After performing the electrolytic deposition treatment under the above-mentioned predetermined conditions, the obtained electrode plate and the deposit on the electrode plate were entirely dissolved in aqua regia using a microwave dissolving device. The obtained solution was then subjected to component analysis using an inductively coupled plasma atomic emission spectrometer (ICP-AES device, "ICPE-9000" manufactured by Shimadzu Corporation) to measure the contents (mg) of B and Dy. Based on the measured values, the respective content ratios (at%) of B and Dy relative to the total amount of B and Dy were calculated to obtain B/Dy (molar ratio). The results of these measurements are shown in Table 1. Note that "B/Dy (molar ratio)" in Table 1 is a value with two significant digits.

(evaluation)
The precipitate obtained by the molten salt electrolysis of this example can be evaluated based on the "B/Dy (molar ratio)" value shown in Table 1. Test Examples 2 and 6 are examples in which molten salt electrolysis was performed under electrolysis conditions within the scope of the present invention. The obtained B/Dy (molar ratio) values all showed high values of 100 or more, indicating that the percentage of boron that could be directly recovered from the molten salt containing rare earth metal ions and boron ions by separation from the rare earth metal was high.

In contrast, in Test Examples 3, 4, 7, and 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 metals. Therefore, under these potential conditions, the amount of Dy remaining in the molten salt after electrolysis is reduced, and the recycling rate of rare earth metals is reduced.

In addition, in Test Examples 1 and 5, where the potential of the cathode electrode exceeded 1.6 V, the amount of boron precipitated was significantly reduced, even though the electrolysis time was the same as in Test Examples 2 and 6. Therefore, under these potential conditions, the boron recovery efficiency is reduced.

The surface of the cathode electrode plate after the electrolytic treatment was subjected to XRD analysis to identify the precipitates. According to the analysis results, Ni ₂ B was present in Test Example 2, which is included in the scope of the present invention. This confirmed that the boron precipitated on the surface of the Ni plate of the cathode electrode exists in the form of elemental boron and also exists in the form of a compound with the electrode material.

This example shows a test in which electrolytic deposition was performed 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 in the redox potential with boron is greater than in the case of Dy. Therefore, even if the molten salt contains Nd as a rare earth metal, it is possible to effectively separate and recover boron from the molten salt.

REFERENCE SIGNS LIST 1 anode electrode 2 cathode electrode 3 electrolytic cell 4 DC power supply 5 molten salt 6 electrolytic device 10 glove box 11 working electrode 12 reference electrode 13 counter electrode 14 thermocouple 15 graphite crucible 16 electrolytic 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 exhaust pipe 24 chemical trap 25 support stand

Claims (4)

A method for recovering boron from a treated material containing rare earth metals and boron using an electrolysis device, comprising :
preparing a molten salt of the material to be treated, and disposing a cathode electrode and an anode electrode of the electrolysis device in the molten salt;
A voltage is applied between the cathode electrode and the anode electrode so that 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 method for recovering boron, comprising separating and recovering boron from the molten salt. The boron recovery method according to claim 1, wherein the recovered boron precipitates on the surface of the cathode electrode either as simple boron or as a boride with the electrode material. The method for recovering boron according to claim 1 or 2, wherein the molten salt includes a chloride, a fluoride, or a mixed salt thereof, and includes rare earth metal ions and boron ions. 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.