JP2023030580A - Boron recovery method - Google Patents

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

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

In recent years, as the demand for rare earth magnets containing rare earth metals has increased, there has been an increasing need to recycle rare earth metals. 1, Patent Document 2). A "rare earth magnet" is a magnet whose main component is an intermetallic compound of a rare earth metal and 3d transition metals such as iron (Fe) and cobalt (Co). A “neodymium magnet”, which is a representative rare earth magnet, is a magnet mainly composed of a neodymium-iron-boron (Nd-Fe-B) intermetallic compound (Nd ₂ Fe ₁₄ B), and has excellent magnetic properties. It has advantages such as low production cost and is used in many applications. Also, in applications such as electric vehicles, a neodymium magnet to which dysprosium (Dy), a heavy rare earth element, is added is used because high coercive force is required at high temperatures.

However, there is no known report on a method for separating and recovering elemental boron from a molten salt containing rare earth metal ions and boron ions. In this specification, hereinafter, the boron element may be simply referred to as "boron" or indicated by the element symbol "B".

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 the property of easily forming stable compounds with other elements. For example, a molten salt containing Ca and B has a region in which elemental boron (hereinafter referred to as "elemental boron") is stable according to thermodynamic calculations. However, when the molten salt is electrolyzed, Borides of _CaB6 form in the noble potential region. In addition, it was reported that generation of rare earth metal borides such as NdB ₆ , EuB ₆ (Non-Patent Document 1), and LaB ₆ (Non-Patent Document 2) was confirmed in the molten salt. However, since these rare earth metal borides are compounds having the same crystal structure as _CaB6 and are highly chemically stable substances, it is difficult to separate the rare earth metal element from the boron element.

As described above, in the conventional recycling technology based on molten salt electrolysis, rare earth metal borides are contained in the recovered rare earth metals, which lowers the purity of the rare earth metals. Therefore, an operation for removing boron is required after recovery. 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 working efficiency and a reduction in treatment cost in the recycling treatment of rare earth metals.

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

Japanese Patent No. 6823314 Japanese 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.M. Selvaraj, A.; Visuvasam, and R.I. 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 the recycling technology using waste materials such as rare earth metal magnets, the boron contained in the molten salt containing rare earth metal ions and boron ions forms a stable compound with the rare earth metal, and the purity of the recycled rare earth metal is reduced. There is a problem of lowering the Accordingly, an object of the present invention is to provide a method for recovering boron from a molten salt containing rare earth metal ions and boron ions by separating it from rare earth metal ions.

As a result of investigations to achieve the above object, the present inventors have found an electrolysis condition that allows boron contained in the molten salt to be deposited separately from the rare earth metal, and have completed the present invention. Arrived. Specifically, the present invention includes the following aspects (1) to (6). In the present specification, the expression "~" includes both numerical values. That is, "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 a rare earth metal 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 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 the molten salt is A method for recovering boron, comprising separating and recovering boron from

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

(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.

(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 higher and 950° C. or lower.

(5) An electrolytic apparatus used for the boron recovery method according to any one of (1) to (4) above, wherein the cathode electrode and the anode electrode are inert to the molten salt An electrolytic device comprising a conductive material that is

(6) The electrolytic device according to (5) above, wherein 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. .

INDUSTRIAL APPLICABILITY According to the present invention, boron can be separated from a rare earth metal and can be directly recovered from a molten salt containing rare earth metal ions and boron ions. It is possible to omit the boron removal treatment step that has been used, and it is possible to provide a low-cost recycling technology.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows the electrolysis apparatus to which the collection|recovery method which concerns on this invention is applied. It is a schematic diagram which shows the electrolysis apparatus for the test used in the Example.

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

(Collection method)
In the method for recovering boron according to the present embodiment, 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 placed in the molten salt, and the potential of the cathode electrode is A voltage is applied between the cathode electrode and the anode electrode so that (vs. Li ⁺ /Li) is held 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. It is characterized by The cathode electrode is an electrode arranged on the anode side, and the anode electrode is an electrode arranged on the anode side. By performing electrolysis at a potential of the cathode electrode of 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 substance of boron or in the form of a compound of boron and the electrode material, and the rare earth metal is deposited. Since precipitation is suppressed, boron in the molten salt can be directly separated and recovered.

(Material to be treated)
The object to be treated to which the recovery method according to the present embodiment is applied is not particularly limited as long as it contains a rare earth metal and boron. For example, a molten salt obtained by anodically dissolving waste materials of rare earth magnets containing two or more kinds of rare earth metals, iron and boron can be used. As the rare earth magnet, there is a neodymium magnet containing neodymium (Nd), dysprosium (Dy), iron (Fe), and boron (B). %-10%, Fe: 60%-65%, B: 1%. The neodymium magnet 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 copper (Cu), aluminum (Al), cobalt (Co), nickel (Ni). , tungsten (W), carbon (C) and nitrogen (N).

As used herein, the term "rare earth metal" refers to an element belonging to Group 3 and the Lanthanide series of the periodic table. For example, 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 according to the present embodiment, boron is directly separated and recovered from molten salt obtained by anodically dissolving waste materials of neodymium magnets containing rare earth metals such as neodymium (Nd) and dysprosium (Dy). be able to. The molten salt from which boron has been recovered has a high ratio (purity) of rare earth metals such as neodymium and dysprosium, and is useful in that the rare earth metals can be efficiently recovered in subsequent steps.

(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. Salts of alkali metal or alkaline earth metal halides can be used. Chlorides or fluorides are preferred as halides.

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. Available. For alkaline earth metal halides, magnesium chloride ( _MgCl2 ), calcium chloride ₍ CaCl2), barium chloride ( _BaCl2 ), magnesium fluoride ( _MgF2 ), calcium fluoride ( _CaF2 ), barium fluoride (BaF ₂ ) and the like 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.

One kind of the molten salt may be used alone, or two or more kinds may be used in combination. A eutectic salt, in which two or more salts are mixed to lower the melting point, may be used. Specifically, it preferably contains a chloride, a fluoride, or a mixed salt thereof, and an LiCl—KCl eutectic salt can be mentioned. An example of the molten salt containing rare earth metal ions and boron ions is a molten salt in which DyCl ₃ and KBF ₄ are mixed with LiCl—KCl eutectic salt. As the rare earth ion source and boron ion source, both chlorides and fluorides can be used 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 molten salt bath temperature (hereinafter referred to as "bath temperature") in the range of 300°C or higher and 950°C or lower. The bath temperature mainly depends on the type of molten salt. If the bath temperature is lower than 300° C., the ionic conductivity of the molten salt is lowered and the rate of progress of the electrolytic deposition reaction is lowered. On the other hand, if the bath temperature exceeds 950° C., the rate of volatilization of the molten salt becomes excessive, resulting in loss of recovered components. Moreover, since heating energy is required to maintain the bath temperature in a high temperature range, the working cost is increased. The lower limit of the bath temperature may be 300° C. or higher, 350° C. or higher, or 400° C. or higher. The upper limit of the bath temperature may be 700°C or lower, 600°C or lower, or 500°C or lower.

(potential)
In the molten salt electrolysis method, a voltage is applied to an anode electrode and a cathode electrode placed in a bath of molten salt, whereby a reduction reaction of cations occurs on the cathode side, and the molten salt in the molten salt is deposited on the surface of the cathode electrode. Substances reduced from anions are deposited. The recovery method according to the present embodiment uses a molten salt containing rare earth metal ions and boron ions, and performs molten salt electrolysis while maintaining the potential of the cathode electrode (vs. Li ⁺ /Li) at 1.4 V or more. is preferred. A method for measuring or setting the potential is not particularly limited. For example, an Ag ⁺ /Ag electrode with an Ag wire soaked in LiCl—KCl salt containing AgCl may be used as a reference electrode. The potential obtained by the Ag+/Ag electrodes can be set as "potential (vs. Li ⁺ /Li)" by calibrating based on the "Li ⁺ /Li potential" indicated by metal 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.

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, boron is deposited on the cathode electrode, and boron is recovered as a single boron or a compound with the electrode material. On the other hand, when the potential of the cathode electrode is less than 1.4 V, the rare earth metal ions are also reduced and the rate of deposition of the rare earth metal element together with the boron element increases, so that boron is well separated from the rare earth metal and recovered. It is difficult to Therefore, the potential according to this embodiment is preferably 1.4 V or higher, more preferably 1.5 V or higher.

On the other hand, the more noble the potential of the cathode electrode, the lower the boron deposition rate. Specifically, it becomes difficult to recover sufficient amounts of boron at potentials above 1.6V. Therefore, the potential in the recovery method according to this embodiment is preferably 1.6 V or less. In actual operation, the optimum potential range can be selected in consideration of separability and recovery rate. Specifically, 1.4V to 1.6V is preferable, and 1.5V to 1.6V is more preferable.

(Action)
In the recovery method according to the present embodiment, a molten salt containing a rare earth metal and boron is subjected to constant potential electrolysis in a potential range in which the potential of the cathode electrode is 1.4 V (vs. Li ⁺ /Li) or more and 1.6 V or less. As a result, while a sufficient amount of boron is deposited, mixed deposition of rare earth metals is suppressed. Boron ions in the molten salt can be recovered with minimal loss of rare earth metal ions. The reason why such boron separation is possible is thought to be that the redox potential of boron is positioned in a nobler potential range than the redox potential of rare earth metals, and that the redox potentials of the two are far apart. be done.

(Form of recovery of boron)
Boron recovered by the recovery method according to the present embodiment includes deposition on the surface of the cathode electrode as boron alone or as a boride with an electrode material. When the material of the cathode electrode is Ni, compounds of nickel and boron (Ni ₂ B, Ni ₃ B) are deposited 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 the present embodiment. As shown in FIG. 1, the basic structure of the electrolytic device 6 includes an anode electrode 1 provided on the anode side, a cathode electrode 2 provided on the cathode side, an electrolytic cell 3 and a DC power supply 4 . The molten salt 5 of the material to be treated is placed in the electrolytic bath 3, the anode electrode 1 and the cathode electrode 2 are placed in the molten salt 5, and a DC power supply 4 is applied between the anode electrode 1 and the cathode electrode 2. A predetermined voltage is applied to perform molten salt electrolysis.

The molten salt in the electrolytic bath 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 electrolytic cell or installing the electrolytic cell in an electric furnace, can be used.

The atmosphere in the electrolytic cell is not particularly limited. If water is mixed in the electrolytic cell, oxygen may be generated during electrolysis to oxidize the molten salt component. Therefore, for example, an inert gas such as dry argon (Ar) may be supplied into the electrolytic cell. Moreover, in order to prevent the volatile gas generated from the molten salt from leaking to the outside, the electrolytic cell may be provided with sealing means.

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

Examples of the present invention will be described below. The invention is not limited to the following description.

(Electrolytic device for testing)
FIG. 2 shows a schematic diagram of a test electrolysis device (hereinafter simply referred to as "electrolysis device") used for the tests of this example. An electrolytic device 16 with a graphite crucible 15 is provided in a glove box 10 with an electric furnace 18 . The graphite crucible 15 is placed on a support base 25 . A test molten salt 17 is placed in the graphite crucible 15 , 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 the molten salt 17 bath.

Using the information obtained by working electrode 11 , reference electrode 12 and counter electrode 13 , measurements and analyzes are performed by electrochemical measurement system 19 . Argon gas supplied from an argon gas supply source 20 connected to the electrolytic device 16 is supplied to the electrolytic device 16 through a gas flow meter 21 and a gas supply pipe 22 to create a predetermined atmosphere inside the electrolytic device 16 . be. Also, the gas volatilized from the molten salt 17 is discharged through the gas discharge pipe 23 and flows into the 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 prepared by adding DyCl ₃ (about 0.5 mol%) and KBF ₄ (about 2 mol%) to LiCl—KCl molten salt (44:56% by mass) mixed in a eutectic composition was added to the electrolytic device. and then heated to 450°C. The electrolyser was filled with a dry argon gas atmosphere. Using the molten salt, constant potential electrolysis was performed for 1 hour while maintaining the potential shown in Table 1 below. Samples subjected to this electrolytic treatment are shown in Table 1 as “Test Example 1” to “Test Example 8”.

The electrolytic apparatus used a Ni plate or a Mo plate as a cathode electrode and a glassy carbon rod (glass-like carbon rod) as an anode electrode. As a reference electrode, an Ag ⁺ /Ag electrode was used, which was prepared by putting LiCl—KCl salt containing 1 mol % of AgCl in the bottom of a heat-resistant glass tube with one side closed and dipping an Ag wire therein. A 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 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 predetermined conditions, the obtained electrode plate and the deposit on the electrode plate were completely dissolved in aqua regia using a microwave dissolving device. After that, the obtained solution was subjected to component analysis by an inductively coupled plasma 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 content ratio (at %) of each of B and Dy relative to the total amount of B and Dy was calculated to obtain B/Dy (molar ratio). These measurement results are shown in Table 1. In addition, "B/Dy (molar ratio)" in Table 1 is a numerical value with two significant digits.

(evaluation)
The content of the deposit obtained by the molten salt electrolysis of this example can be evaluated based on the numerical value of "B/Dy (molar ratio)" 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) all showed high numerical values of 100 or more, indicating that the rate of boron being separated from the rare earth metal and the molten salt containing rare earth metal ions and boron ions and being able to be directly recovered was high. rice field.

On the other hand, in Test Examples 3, 4, 7, and 8 in which the potential of the cathode electrode is less than 1.4 V, a mixture of B and Dy is recovered, and boron is sufficiently separated from the rare earth metal. I didn't. Therefore, under this potential condition, the amount of Dy remaining in the molten salt after electrolysis decreases, and the recycling rate of rare earth metals decreases.

In addition, in Test Examples 1 and 5, in which the potential of the cathode electrode exceeded 1.6 V, despite the same electrolysis time as in Test Examples 2 and 6, the amount of deposited boron was significantly reduced. Therefore, at this potential condition, the recovery efficiency of boron decreases.

In addition, regarding the electrode plate of the cathode electrode after the electrolytic treatment, the surface was subjected to XRD analysis to identify the deposits. According to the analysis results, Ni ₂ B was present in Test Example 2, which is within the scope of the present invention. As a result, it was confirmed that the boron deposited on the surface of the Ni plate of the cathode electrode existed in the form of elemental boron, and further existed in the form of a compound with the electrode material.

This example shows a test in which an electrolytic deposition treatment was performed using a molten salt containing Dy as a rare earth metal. Since Nd is an element that is reduced at a potential less noble than Dy, the difference from the redox potential of boron is greater than that of Dy. Therefore, even in a molten salt containing Nd as a rare earth metal, it is possible to separate and recover boron satisfactorily from the molten salt.

REFERENCE SIGNS LIST 1 anode electrode 2 cathode electrode 3 electrolytic cell 4 DC power supply 5 molten salt 6 electrolyzer 10 glove box 11 working electrode 12 reference electrode 13 counter electrode 14 thermocouple 15 graphite crucible 16 electrolyzer 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

Claims (6)

A method for recovering boron from a material to be treated containing a rare earth metal and boron, comprising:
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;
A method for recovering boron, comprising separating and recovering boron from the molten salt. 2. The method for recovering boron according to claim 1, wherein the recovered boron is deposited on the surface of the cathode electrode as boron alone or as a boride with an electrode material. 3. The method for recovering boron according to claim 1, wherein said molten salt contains chloride, fluoride or a mixed salt thereof, and contains 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. An electrolytic apparatus used for the boron recovery method according to any one of claims 1 to 4, wherein the cathode electrode and the anode electrode are made of a conductive material that is inert to the molten salt. electrolysis device. 6. The electrolytic device according to claim 5, wherein said conductive material in said cathode electrode is a metal or alloy containing one or more selected from the group consisting of nickel, molybdenum, iron, copper, chromium and manganese.

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