Method for recycling heavy rare earth elements and method for recycling rare earth magnets

Abstract

The present application is a method for recycling heavy rare earth elements from molten salt electrolysis residue containing heavy rare earth elements, comprising: a step of fluorination treatment of coarse particles of molten salt electrolysis residue by mixing the coarse particles of molten salt electrolysis residue with fluorination material and calcining; a step of crushing the fluorination-treated coarse particles of molten salt electrolysis residue to obtain a powder 21 of molten salt electrolysis residue; and a step of mixing the powder 21 of molten salt electrolysis residue with R, R-M alloy or R-M-B alloy 31 (R is one or more rare earth elements selected from Y, La, Ce, Nd, Pr, Sm, Gd, Dy, Tb and Ho, M is a transition metal such as Fe, Co, etc., and B is boron), heating and melting, and separating into molten slag 22 and molten alloy 32, thereby selectively extracting heavy rare earth elements into the molten alloy 32. According to the present application, a recycling method of heavy rare earth elements that can effectively recycle scarce heavy rare earth elements in an alloy form close to a product, and a recycling method of rare earth magnets using an alloy obtained by the recycling method can be provided.

Description

Technical Field

[0001] This invention relates to a method for reusing heavy rare earth elements by recovering heavy rare earth elements from molten salt electrolysis residue generated in a rare earth magnet manufacturing process, and a method for reusing rare earth magnets using alloys containing heavy rare earth elements extracted by the reusing method. Background Technology

[0002] Rare earth magnets, as indispensable functional materials for energy saving and high functionality, have expanded their applications to a wide range of fields, from general household appliances such as air conditioners to automobiles such as HEVs or EVs, and their production volume has been increasing year by year. In addition, recently, due to the use of these products using magnets in high-temperature environments, there is also a demand for improving the heat resistance of rare earth magnets.

[0003] Conventional rare-earth magnets are manufactured by micro-pulverizing a raw material alloy with a specified composition in an inert gas atmosphere, pressing the powder into a certain size while applying a magnetic field, and then sintering it in a vacuum or inert gas atmosphere. The manufactured rare-earth magnets are then machined or ground into product shapes, and further surface treated with plating, coating, etc., to become finished products. In-process waste (forming powder leakage, sintering / defective products, processing defects, plating defects, etc.) generated in these processes accounts for more than ten percent of the initial raw material weight, and slag (machining and grinding chips) generated in the machining / grinding processes accounts for several tens of percent of the finished product raw material. Therefore, from the perspective of effectively utilizing scarce resources, reducing waste generation, and lowering the price of rare-earth magnets, the reuse of rare-earth elements from the aforementioned waste and slag is considered an important process.

[0004] Furthermore, adding heavy rare earth elements such as Dy and Tb is effective in improving the heat resistance of rare earth magnets. For example, high-heat-resistant rare earth magnets contain a few percent of heavy rare earth elements. Although these heavy rare earth elements are widely distributed in the Earth's crust geologically, they are concentrated in economically viable locations. Therefore, there is a high risk of unstable supply of heavy rare earth element raw materials, and the price of heavy rare earth elements fluctuates significantly with changes in supply and demand. Therefore, establishing effective reuse methods, especially for heavy rare earth elements, is of paramount importance.

[0005] Patent Document 1 describes a method for the efficient recovery of rare earth elements. When remelting R-Fe-B series rare earth magnet waste and / or sludge (R: a rare earth element containing Y, preferably one or more rare earth elements selected from Pr, Nd, Tb, and Dy), a raw material metal that does not contain rare earth elements from the raw metal used in the R-Fe-B series magnets is pre-loaded into a crucible. Then, after they are heated and melted, appropriate amounts of: the raw material metal containing rare earth elements, the R-Fe-B series rare earth magnet waste and / or sludge, and a flux containing halides of one or more metals selected from alkali metals, alkaline earth metals, and rare earth metals are added; and they are melted. The rare earth elements are then recovered in the form of an R-Fe-B series magnet alloy.

[0006] In addition, Patent Document 2 describes a method for extracting rare earth elements by immersing a rare earth-containing alloy such as a neodymium magnet in a molten salt containing a halide compound of alkali metal or alkaline earth metal, and then electrolyzing it to recover the rare earth elements in the form of metal or alloy.

[0007] The manufacturing method for heavy rare earth raw materials, represented by Dy and Tb added to improve the heat resistance of rare earth magnets, generally employs the molten salt electrolysis method. In the molten salt electrolysis method, heavy rare earth fluorides, or mixed fluorides of heavy rare earth fluorides and alkali metal fluorides or alkaline earth metal fluorides, are used as the electrolytic bath. Then, the heavy rare earth fluorides or heavy rare earth oxides are reduced to obtain heavy rare earth metals or alloys. For example, Patent Document 3 describes a method for manufacturing dysprosium-iron alloys from a molten salt electrolytic bath containing dysprosium fluoride, lithium fluoride, barium fluoride, and calcium fluoride.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2003-113429,

[0011] Patent Document 2: Japanese Patent Application Publication No. 2015-120973,

[0012] Patent document 3: Japanese Patent Application Publication No. 62-146290. Summary of the Invention

[0013] The problem that the invention aims to solve

[0014] In the rare earth magnet waste recycling method of Patent Document 1, metal halides are added as fluxes. However, this flux is not derived from the raw material of the residue, but is added as a new auxiliary material. Therefore, new auxiliary materials need to be added, increasing raw material costs and environmental impact. Thus, from the perspective of economy and reuse, the rare earth magnet waste recycling method of Patent Document 1 has room for further improvement.

[0015] In the rare earth element recovery method of Patent Document 2, rare earth magnet waste or alloys are returned to the molten salt electrolysis process, which is a general rare earth metal manufacturing process. The waste is immersed in molten salt to extract rare earth elements, thereby recovering them as metals or alloys. However, since the extraction rates of light rare earths such as Nd and Pr and heavy rare earths such as Dy and Tb in molten salt are almost identical, the cheaper light rare earths contained in the rare earth magnet waste and alloys are also extracted into the molten salt. Furthermore, generally speaking, when studying the reuse of products with long manufacturing processes, the further upstream the process is, the more complex the separation process becomes, and the lower the impurity concentration required for the recycled raw materials. Therefore, the process load involved in reuse increases, potentially leading to decreased profitability.

[0016] In the aforementioned molten salt electrolysis process for manufacturing heavy rare earth raw material metals or alloys, fluorides or oxides are used as raw materials. Therefore, unreduced oxides and oxyfluorides from the reaction products, as well as Fe and carbon mixed in from the anode and cathode, remain as residue after electrolysis. Typically, this molten salt residue is finely pulverized, calcined in the atmosphere, and then impurities are removed by magnetic separation before being reused as molten salt for electrolysis. However, to increase the purity of the molten salt residue to the same impurity concentration as the pure raw material molten salt, refining costs are incurred. Therefore, to increase the purity of the molten salt residue to an impurity level that is not problematic for industrial electrolysis, it is then diluted with the pure raw material, allowing for reuse. Consequently, the industrial reuse rate of molten salt is not necessarily high, often only around ten percent.

[0017] The present invention was made in view of the above circumstances, and its object is to provide a method for reusing heavy rare earth elements by efficiently extracting rare heavy rare earth elements in a form as close as possible to the product, and a method for reusing rare earth magnets by reusing the recovered heavy rare earth elements as raw material alloys for rare earth magnets.

[0018] Methods for solving problems

[0019] In order to solve the above-mentioned problems, the inventors conducted in-depth research and found that by fluorinating the coarse particles of molten salt electrolysis residue containing heavy rare earth elements, then crushing the fluorinated coarse particles, mixing the resulting powder with R, RM alloy or RMB alloy raw materials (R is one or more rare earth elements selected from Y, La, Ce, Nd, Pr, Sm, Gd, Dy, Tb and Ho, M is transition metals such as Fe and Co, and B is boron), and heating to melt it, heavy rare earth elements can be effectively recovered as alloys, thus completing the present invention.

[0020] That is, the present invention provides the following method for reusing heavy rare earth elements from molten salt electrolysis residue containing heavy rare earth elements, and the method for reusing rare earth magnets using alloys containing heavy rare earth elements extracted by the reusing method.

[0021] [1] A method for reusing heavy rare earth elements, the method comprising: a step of fluorinating the coarse particles of the molten salt electrolysis residue containing heavy rare earth elements by mixing and calcining them with a fluorinating material; a step of crushing the coarse particles of the fluorinated molten salt electrolysis residue to obtain powder of the fluorinated molten salt electrolysis residue; and a step of mixing the powder of the fluorinated molten salt electrolysis residue with R, RM alloy or RMB alloy (R is one or more rare earth elements selected from Y, La, Ce, Nd, Pr, Sm, Gd, Dy, Tb and Ho, M is transition metals such as Fe and Co, and B is boron), heating and melting, separating them into slag and molten alloy, thereby selectively extracting heavy rare earth elements into the molten alloy.

[0022] [2] The method for reusing heavy rare earth elements described in [1] above, wherein the molten salt electrolysis residue containing heavy rare earth elements is one or more compounds selected from oxides, fluorides and oxyfluorides, and the compound contains more than 50% by mass of one or more heavy rare earth elements selected from Dy and Tb.

[0023] [3] The method for reusing heavy rare earth elements described in [1] or [2] above, wherein the fluorinated material is one or more fluorinated materials selected from NH4F, NH4FHF, HF gas and fluorine gas.

[0024] [4] The method for reusing heavy rare earth elements as described in any one of [1] to [3] above, wherein the oxygen concentration of the coarse particles of the fluorinated molten salt electrolysis residue is less than 1.0% by mass.

[0025] [5] The method for reusing heavy rare earth elements as described in any one of [1] to [4] above, wherein the carbon concentration of the coarse particles of the fluorinated molten salt electrolysis residue is less than 0.3% by mass.

[0026] [6] The method for reusing heavy rare earth elements as described in any one of [1] to [5] above, wherein the heating and melting method is electric arc melting, plasma melting or high frequency induction heating melting.

[0027] [7] The method for reusing heavy rare earth elements as described in any one of [1] to [6] above, wherein the average particle size of the powder of the fluorinated molten salt electrolysis residue is 10 to 100 μm as measured by laser diffraction based on airflow dispersion.

[0028] [8] The method for reusing heavy rare earth elements as described in any one of [1] to [7] above, wherein the R, RM alloy or RMB alloy is waste generated in the rare earth magnet manufacturing process.

[0029] [9] The method for reusing heavy rare earth elements as described in any one of [1] to [7] above, wherein the R, RM alloy or RMB alloy is a sintered body produced in the rare earth magnet manufacturing process.

[0030]

[10] The method for reusing heavy rare earth elements as described in any one of [1] to [8] above, wherein the R, RM alloy or RMB alloy is sludge generated in the processing of rare earth magnets or a processed product obtained by calcining the sludge.

[0031]

[11] The method for reusing heavy rare earth elements as described in any one of [1] to [7] above, wherein the R, RM alloy, and RMB alloy are magnetic waste recovered from products using rare earth magnets.

[0032]

[12] A method for reusing rare earth magnets, wherein an alloy containing heavy rare earth elements extracted by the method for reusing heavy rare earth elements described in any one of [1] to

[11] is used as a raw material alloy for rare earth magnets.

[0033] The effects of the invention

[0034] According to the present invention, a method for reusing heavy rare earth elements that can effectively reuse rare heavy rare earth elements in an alloy form close to that of the product is provided, as well as a method for reusing rare earth magnets using alloys obtained by the reusing method. Attached Figure Description

[0035] [ Figure 1 ] Figure 1 This is a schematic diagram illustrating an example of molten salt electrolysis.

[0036] [ Figure 2 ] Figure 2 (a) to (c) are conceptual diagrams used to illustrate the extraction of Dy from molten salt electrolysis residue into Nd-Fe alloy.

[0037] [ Figure 3 ] Figure 3 The results are XRD analysis results of the slag remaining in the crucible in Example 1. Detailed Implementation

[0038] The molten salt electrolysis method for producing the molten salt electrolysis residue containing heavy rare earth elements (hereinafter, sometimes simply referred to as molten salt electrolysis residue) of the present invention is a known method for manufacturing rare earth metals or rare earth alloys. (See also...) Figure 1 A general molten salt electrolysis method will be described. Examples of general molten salt electrolysis methods include: using a metal electrode as the cathode 6 and a graphite electrode as the anode 5, and using an electrolytic bath 4 obtained by melting rare earth oxides as raw materials with alkali, alkaline earth, and a mixture of rare earth fluorides. The bath temperature is 700–1200°C. Through electrolytic reduction of rare earth oxides or fluorides, rare earth metals are continuously deposited on the cathode surface. Since the density of the deposited rare earth metal 3 is higher than that of the electrolytic bath 4, it accumulates in a liquid state in the deposited metal receiving tank 2 at the bottom of the bath. The rare earth metal 3 accumulated in the deposited metal receiving tank 2 is recovered from the system at regular intervals using a slag removal device (not shown) that utilizes vacuum suction. During recovery, to suppress the solidification of molten rare earth metal within the device, damage to the suction pipe, and the introduction of impurities from the suction pipe, transition metals such as iron, used as magnet raw materials, can be added to the electrolytic bath 4. Thus, rare earth metals form alloys, lowering their melting points, thereby enabling safe and efficient recovery of rare earth metals. Furthermore, by using an iron electrode as the cathode 6, the rare earth metals deposited at the cathode can alloy with the iron at the cathode, producing droplets of an alloy with a lower melting point.

[0039] Molten salt electrolysis residue is the residue remaining in the electrolytic cell after the operation of the above-described molten salt electrolysis method. It mainly contains a mixture of fluorides, oxides, and oxyfluorides of alkali metals, alkaline earth metals, and rare earths. In addition, it contains metals from the cathode material and graphite (carbon) and furnace materials from the anode material mixed in from the electrolysis device. Suitable materials for the molten salt electrolysis residue include preferably 50% by mass or more of heavy rare earth elements (especially at least one rare earth element from Dy and Tb). An example of molten salt electrolysis residue suitable for use in the method for reusing heavy rare earth elements of the present invention is shown in Table 1 below. Table 1 is an example of the composition of molten salt residue generated in molten salt electrolysis using LiF-DyF3 as the electrolytic bath and Fe material as the cathode. It should be noted that LiF in LiF-DyF3 lowers the melting point of the DyF3 bath, thus improving conductivity. The residue from molten salt electrolysis generally contains 1–12 wt% Fe and a maximum of 1.0 wt% carbon, but this content depends on the operating conditions of molten salt electrolysis and cannot be specified in general.

[0040] [Table 1]

[0041] Table 1

[0042]

[0043] For the aforementioned general molten salt electrolysis residue, the following treatment can be implemented, for example. After removing lumpy unrecovered metals, electrode components, furnace building materials, and other foreign matter, the residue is mechanically crushed to a particle size of less than 20 mm using a jaw crusher and hammer mill, resulting in coarse particles of the molten salt electrolysis residue. Then, hydrochloric acid is added to the coarse particles of the molten salt electrolysis residue to dissolve metal components such as Fe, thereby separating and recovering undissolved components such as rare earth fluorides and oxyfluorides. The dissolution conditions (hydrochloric acid concentration, dissolution temperature, time, etc.) are appropriately adjusted so that the processed weight and the metal content in rare earth fluorides and oxyfluorides are less than 1% by mass. By pre-crushing the molten salt electrolysis residue to a particle size of less than 20 mm before dissolution to produce coarse particles, the reaction interface with hydrochloric acid can be increased, improving the dissolution efficiency.

[0044] Preferably, the coarse particles of the resulting molten electrolytic residue are washed with pure water and then heated in air at 400–600°C. This removes moisture while simultaneously causing complete combustion of the graphite mixed in from the anode, thereby removing carbon as CO2 gas. By heating the residue to above 400°C, the reaction rate of the graphite is increased, shortening the time required for complete combustion. Conversely, heating the residue to below 600°C inhibits the melting of the coarse particles. It should be noted that if the coarse particles of the molten electrolytic residue melt, the resulting residue may coat the graphite, sometimes hindering complete combustion.

[0045] In this invention, it is preferable to first mix coarse particles of preferably dried molten salt electrolysis residue with a fluorinating material (e.g., acidic ammonium fluoride (NH4FHF)) and heat the mixture. This allows the coarse particles of the molten salt electrolysis residue to be fluorinated as shown in formulas (1) and (2) below.

[0046] R2O3 + 3NH4FHF → 2RF3 + 3NH3 + 3H2O ··· Equation (1)

[0047] ROF+NH4FHF→RF3+NH3+H2O·······Equation (2)

[0048] (It should be noted that, in this context, R represents rare earth elements.)

[0049] As fluorinating materials, acidic ammonium fluoride (NH4FHF) or ammonium fluoride (NH4F), HF gas, and fluorine gas can be used. These fluorinating materials can be used alone or in combination of two or more. The amount of these fluorinating materials mixed varies depending on the amount of oxides, hydroxides, and oxyfluorides contained in the coarse particles of the molten salt electrolysis residue, and is not particularly limited. For example, from the viewpoint of ensuring necessary and sufficient fluorination of the oxides and oxyfluorides in the molten salt electrolysis residue, it is generally appropriate to use an amount 1.0 to 1.3 times, preferably 1.05 to 1.2 times, of the equivalent required for fluorination of the coarse particles of the molten salt electrolysis residue. Furthermore, from the viewpoint of ensuring necessary and sufficient fluorination reaction, the heating temperature is preferably 200 to 700°C, more preferably 250 to 600°C. To prevent oxidation, the heating atmosphere is preferably an inert atmosphere such as Ar or N2. Additionally, for the above-mentioned fluorination reaction, heating can also be performed while HF gas is flowing through it; in this case, HF gas diluted with Ar can also be used. The flow rate, time, and temperature can be appropriately selected based on the oxygen content in the molten salt electrolysis residue.

[0050] From the viewpoint of maintaining a good alloy recovery rate obtained by mixing and heating with the R, RM, or RMB alloys described later, the oxygen concentration of the coarse particles in the fluorinated molten salt electrolysis residue is preferably 1.0% by mass or less. Under the aforementioned fluorination treatment conditions, the oxygen concentration in the fluorinated molten salt electrolysis residue increases. When heated and melted, oxygen combines with rare earth elements in the R, RM, or RMB alloys to form oxides or oxyfluorides, thus reducing the alloy recovery rate.

[0051] From the viewpoint of using the alloy obtained by mixing and melting it with the R, RM, or RMB alloys described later as a raw material for magnets, the carbon concentration of the coarse particles in the fluorinated molten salt electrolysis residue is preferably 0.3% by mass or less. It should be noted that atmospheric calcination alone is insufficient to sufficiently reduce the carbon concentration in the molten salt electrolysis residue, while further fluorination treatment can sufficiently reduce the carbon concentration in the molten salt electrolysis residue.

[0052] Regarding carbon, due to the small separation coefficient between the alloy and the slag during heating and melting, the carbon concentration of the alloy obtained after heating and melting increases. Therefore, if the carbon concentration in the molten salt electrolysis residue is not sufficiently reduced, when this alloy is recycled into a raw material alloy for magnets, carbon concentrates in the magnet raw material alloy, thereby deteriorating the magnetic properties of the final magnet product.

[0053] In this invention, the coarse particles of the fluorinated molten salt electrolytic residue, after being crushed, are used as a flux and heated together with the R, RM alloy or RMB alloy to melt. The heating and melting can be performed using known apparatus, such as an electric arc melting furnace, a plasma melting furnace, or a high-frequency induction melting furnace, which can heat the R, RM alloy or RMB alloy and the fluoride to above the melting point in an inert atmosphere.

[0054] As R, RM, or RMB alloys, ordinary pure R, RM, or RMB alloys can be used. Alternatively, R, RM, or RMB alloys can also be waste generated during the rare earth magnet manufacturing process, sintered bodies generated during the rare earth magnet manufacturing process, sludge generated during the rare earth magnet processing process, or processed products obtained by calcining the sludge, or magnet waste recovered from products using rare earth magnets. For example, among the waste generated during the rare earth magnet manufacturing process, scraps generated during the forming, sintering, or processing processes, solid waste generated due to poor size / shape, breakage / defects, or poor magnetic properties, grinding chips or sludge generated during the grinding of rare earth magnets, or processed products obtained by calcining the sludge, etc., can also be utilized. Furthermore, magnet waste recovered from products using rare earth magnets can also be utilized.

[0055] The composition of the waste generated in the rare earth magnet manufacturing process can be described using R, RM, or RMB. R represents one or more rare earth elements selected from Y, La, Ce, Nd, Pr, Sm, Gd, Dy, Tb, and Ho; M represents transition metals such as Fe and Co; and B represents boron. The content is generally R: 27–35% by weight, B: 0.9–1.3% by weight, and M: balance. It should be noted that the waste generated in the rare earth magnet manufacturing process is pre-crushed before melting. From the viewpoint of preventing oxidation during crushing and suppressing molten residue, its shape is preferably 5 mm or larger.

[0056] The amount of fluorinated molten salt electrolysis residue added is preferably 5 to 50% by weight of the total raw material metal, more preferably 10 to 30% by weight. From the viewpoint of maintaining a high alloy recovery rate, the addition amount is preferably 5% by weight or more. Unrecovered alloy forms a mixture with slag and remains in the crucible. In addition, from the viewpoint of preventing melting damage to the inner wall of the crucible due to the reaction between the molten salt electrolysis residue and the crucible material, or from the viewpoint of preventing the adverse effects on the magnetic properties and surface treatment properties of the sintered magnet caused by the incomplete separation of molten salt electrolysis residue into the alloy, the addition amount is preferably 50% by weight or less. Alternatively, a portion of the fluorinated molten salt electrolysis residue can be replaced with fluoride used for electrolysis raw materials. From the viewpoint of reducing raw material costs, the replacement amount is preferably 50% by weight or less.

[0057] The coarse particles of the fluorinated molten salt electrolysis residue are pre-crushed using a hammer mill, braun mill, or fluidized bed mill to prepare molten salt electrolysis residue powder, which is then used as a flux for heating and melting. From the viewpoints of preventing contamination of the furnace during vacuum venting, and preventing deterioration of the recovery yield due to unmelted material, and deterioration of the magnetic properties and surface treatment characteristics of the sintered magnets due to impurities mixed into the recycled alloy, the average particle size is preferably 10–100 μm, more preferably 20–80 μm. It should be noted that the average particle size of the fluorinated molten salt electrolysis residue powder refers to the value measured by laser diffraction based on airflow dispersion.

[0058] If the raw materials of R, RM, or RMB alloys and the fluorinated molten salt electrolytic residue added as a flux are heated to a temperature above the melting point of the R, RM, or RMB alloys, then through the reaction described in equation (3), the rare earth oxides (R2O3) contained in the R, RM, or RMB alloys react with the rare earth fluorides (RF3), which are the main components of the fluorinated molten salt electrolytic residue, to form rare earth oxyfluorides (ROF). The generated rare earth oxyfluorides (ROF) form slag due to their high melting point. Since the density of the slag is lower than that of the molten alloy, the slag and the molten alloy can be separated.

[0059] R2O3 + RF3 → 3ROF ··· Equation (3)

[0060] Here, in the case where the fluoride contains a large amount of heavy rare earth elements, the heavy rare earth elements are selectively reduced and extracted into the alloy through the reaction of the following formula (4). Light rare earth elements can form oxyfluorides and be separated in the form of slag.

[0061] LR2O3+HRF3+LR 在R-M-B中 →3LROF+HR···Equation (4)

[0062] In equation (4), HR represents heavy rare earth elements and LR represents light rare earth elements.

[0063] This is believed to be because light rare earth elements are thermodynamically more likely to form stable phases than heavy rare earth elements in rare earth fluorides, and heavy rare earth elements are more likely to form stable intermetallic compounds with transition metals. As a result, heavy rare earth elements can be regenerated in the form of rare earth magnet raw materials in the form of alloys without going through a molten salt electrolysis process.

[0064] It should be noted that rare earth oxides and rare earth fluorides are included in the slag because they have higher melting points and lower densities compared to alloys. Therefore, unreacted light rare earth oxides, unreacted heavy rare earth fluorides, oxides of light and heavy rare earths obtained by extracting heavy rare earths from light rare earth oxides, and oxides of light and heavy rare earths obtained by extracting light rare earths from heavy rare earth fluorides are also included in the slag.

[0065] As an example, refer to Figure 2 The extraction of Dy from the Nd-Fe alloy in the case where the heavy rare earth element Dy is in the fluorinated molten salt electrolysis residue and the RM alloy is an Nd-Fe alloy is explained.

[0066] like Figure 2 As shown in (a), a fluorinated molten electrolytic residue 21 containing Dy as a heavy rare earth element is added as a flux to the Nd-Fe alloy 31. In the fluorinated molten salt electrolytic residue, Dy exists in the form of DyF3. In addition, the Nd-Fe alloy 31 contains Nd2O3 as an impurity in Nd-Fe.

[0067] The fluorinated molten salt electrolysis residue and Nd-Fe alloy were heated to a temperature above the melting point of the Nd-Fe alloy. The result was as follows: Figure 2 As shown in (b), the rare earth oxides (Nd₂O₃) contained in the Nd-Fe alloy react with rare earth fluorides (DyF₃), which are the main components of the molten salt electrolysis residue after fluorination treatment, to form rare earth oxyfluoride ((Nd,Dy)OF) and rare earth fluoride ((Nd,Dy)F₃). The generated rare earth oxyfluoride ((Nd,Dy)OF) and rare earth fluoride ((Nd,Dy)F₃) form slag 22 (refer to) because their melting points are higher than those of (Nd,Dy)-Fe alloy 32. Figure 2 (c)).

[0068] Heavy rare earth elements (Dy) are selectively reduced, such as Figure 2 As shown in (b), it was extracted into the Nd-Fe alloy. As a result, the Nd-Fe alloy became (Nd, Dy)-Fe alloy 32 (refer to). Figure 2 (c) The specific gravity difference can be used to separate the (Nd,Dy)-Fe alloy 32 from the slag 22 of rare earth oxyfluoride ((Nd,Dy)OF) and rare earth fluoride ((Nd,Dy)F3).

[0069] The alloy obtained by the heavy rare earth element recycling method of the present invention, after confirming its composition and impurity concentration by ICP luminescence analysis or gas analysis, can be reused as a raw material alloy for rare earth magnets. If the desired composition and impurity concentration are obtained, it can be processed into a sintered magnet according to the conventional magnet manufacturing process. For example, the alloy obtained by the heavy rare earth element recycling method of the present invention is coarsely pulverized by hydrogenation / dehydrogenation treatment, micro-pulverized (especially by fluid energy milling, etc.) in an inert gas atmosphere such as nitrogen or argon to an average particle size of 1-5 μm, shaped in a magnetic field, sintered in a vacuum or inert atmosphere at 1000-1200°C, cooled to below 400°C, and then heat-treated in a vacuum or inert atmosphere at 400-600°C to obtain a rare earth sintered magnet. In addition, for high coercivity, grain boundary diffusion treatment can be performed on the obtained sintered magnet.

[0070] On the other hand, if the alloy composition obtained by the method of reusing heavy rare earth elements according to the present invention is not the desired composition or the impurity concentration exceeds the specified range, it is also possible to mix in separately prepared pure raw materials, reheat and melt them to obtain the desired alloy for magnet raw materials. In addition, the casting method after heating and melting can be applied by stacked mold casting, thin strip continuous casting, melt spinning, etc.

[0071] Example

[0072] The present invention will be described in more detail below with examples and comparative examples, but the present invention is not limited to the following examples.

[0073] [Examples 1-4, Comparative Examples 1-2]

[0074] A Dy-Fe alloy was produced by molten salt electrolysis using a graphite electrode as the anode, Fe material as the cathode, a mixed fluoride of DyF3 (85 wt%) and LiF (15 wt%) as the electrolytic bath, and Dy oxide as the Dy feedstock. The molten salt electrolysis residue generated during the Dy-Fe molten salt electrolysis process was then used as the starting material. This residue was crushed to below 5 mm using a hammer mill. Next, a 1N hydrochloric acid aqueous solution was added, and after stirring for 4 hours, the residue was washed with pure water. The molten residue was dried and calcined at 600°C in atmospheric air, and then NH4FHF was added as a fluorinating material. The residue was placed in a stainless steel container and heated at 600°C for 8 hours in an Ar atmosphere for fluorination treatment. The compositional analysis values ​​of the coarse particles of the molten salt electrolysis residue before and after fluorination treatment are shown in Table 2. As shown in Table 2, the oxygen and carbon concentrations decreased to below 1.0 wt% and 0.3 wt%, respectively. In addition, X-ray diffraction (XRD) confirmed that DyF3 was generated in the coarse particles of the fluorinated molten salt electrolysis residue via the reaction shown in the following formula (5).

[0075] DyOF+NH4FHF→DyF3+NH3+H2O···Formula (5)

[0076] It should be noted that, for reference, NdF3, the electrolytic raw material used to manufacture rare earth magnets, is also listed in Table 2.

[0077] [Table 2]

[0078] Table 2

[0079]

[0080] Next, the coarse particles of the fluorinated molten salt electrolysis residue were pulverized to an average particle size of 20 μm using a hammer mill to prepare fluorinated molten electrolysis residue powder. This powder, along with rare earth magnet blocks, was then added to a high-frequency induction heating melting furnace and melted at temperatures above 1400°C. After confirming the magnet blocks had melted, the melt was poured into a crucible and cast only into a Cu mold for RMB alloy recovery. The recovery rate and composition analysis results of the recovered alloy are shown in Tables 3 and 4. The composition was analyzed using high-frequency inductively coupled plasma optical emission spectrometry (ICP-AES). The results showed that Dy components not present in the rare earth magnet blocks could be extracted into the alloy. Furthermore, the Dy extraction rate was proportional to the amount of fluorinated molten salt electrolysis residue powder added; by adding more than 10% of fluorinated molten salt electrolysis residue powder, a Dy extraction rate of more than 50% could be achieved. XRD analysis was performed on the slag remaining in the crucible, and the results are as follows: Figure 3 As shown, it is confirmed to consist of unreacted fluorides and oxyfluorides.

[0081] [Table 3]

[0082] Table 3

[0083]

[0084] [Table 4]

[0085] Table 4

[0086]

[0087]

[0088] [Example 5, Comparative Example 3]

[0089] Nd metal (purity: 99.6% by mass) was mixed with powdered molten salt electrolysis residue that had undergone perfluorination treatment using the same method as in the above embodiments, and then fed into a high-frequency induction heating melting furnace and melted at a temperature above 1200°C. After confirming that the Nd metal had melted, the crucible was poured out, and only the melt was cast into a Cu mold to recover the alloy. The recovery rate of the recovered alloy and the results of compositional analysis are shown in Tables 5 and 6. As a result, Dy can be extracted into the alloy. In addition, the Dy extraction rate reaches more than 69%.

[0090] [Table 5]

[0091] Table 5

[0092]

[0093] [Table 6]

[0094] Table 6

[0095] Since the raw materials used in this project are all derived from the recycling of industrial waste, not only are production costs reduced, but also the amount of waste generated and the energy required for waste treatment are decreased, thus significantly reducing the environmental impact.

[0096] Symbol Explanation

[0097] 1. Electrolytic cell,

[0098] 2. Metal precipitation receiving tank,

[0099] 3. Precipitation of metal,

[0100] 4. Molten salt bath,

[0101] 5. Graphite anode,

[0102] 6. Metal cathode,

[0103] 7 rooms,

[0104] 21. Fluorinated molten salt electrolysis residue.

[0105] 22 molten slag,

[0106] 31Nd-Fe alloy,

[0107] 32(Nd,Dy)-Fe alloy.

Claims

1. A method for reusing heavy rare earth elements, the method comprising recovering heavy rare earth elements from molten salt electrolysis residue containing heavy rare earth elements for reuse, including: The process of fluorinating the coarse particles of the molten salt electrolysis residue containing heavy rare earth elements by mixing and calcining them with a fluorinating material. The process of crushing the coarse particles of the fluorinated molten salt electrolysis residue to obtain the powder of the fluorinated molten salt electrolysis residue; and The process involves mixing the powder of the fluorinated molten salt electrolysis residue with R, RM alloy or RMB alloy, heating and melting it, and separating it into slag and molten alloy, thereby selectively extracting heavy rare earth elements into the molten alloy, wherein R is one or more rare earth elements selected from Y, La, Ce, Nd, Pr, Sm, Gd, Dy, Tb and Ho, M is a transition metal, and B is boron.

2. The method for reusing heavy rare earth elements according to claim 1, wherein, R in the R, RM alloy and RMB alloy is one or more rare earth elements selected from La, Ce, Nd, Pr and Sm, or is one or more rare earth elements selected from La, Ce, Nd, Pr and Sm and one or more rare earth elements selected from Y, Gd, Dy, Tb and Ho.

3. The method for reusing heavy rare earth elements according to claim 1 or 2, wherein, The molten salt electrolysis residue containing heavy rare earth elements is a compound selected from oxides, fluorides, and oxyfluorides, wherein the compound contains more than 50% by mass of a heavy rare earth element selected from Dy and Tb.

4. The method for reusing heavy rare earth elements according to claim 1 or 2, wherein, The fluorinated material is selected from one or more fluorinated materials, including NH4F, NH4FHF, HF gas, and fluorine gas.

5. The method for reusing heavy rare earth elements according to claim 1 or 2, wherein, The coarse particles of the fluorinated molten salt electrolysis residue have an oxygen concentration of less than 1.0% by mass.

6. The method for reusing heavy rare earth elements according to claim 1 or 2, wherein, The coarse particles of the fluorinated molten salt electrolysis residue have a carbon concentration of less than 0.3% by mass.

7. The method for reusing heavy rare earth elements according to claim 1 or 2, wherein, The heating and melting method is electric arc melting, plasma melting, or high-frequency induction heating and melting.

8. The method for reusing heavy rare earth elements according to claim 1 or 2, wherein, The average particle size of the fluorinated molten salt electrolysis residue powder, as measured by laser diffraction based on airflow dispersion, is 10~100 μm.

9. The method for reusing heavy rare earth elements according to claim 1 or 2, wherein, The R, RM alloys or RMB alloys mentioned are waste products generated during the manufacturing process of rare earth magnets.

10. The method for reusing heavy rare earth elements according to claim 1 or 2, wherein, The R, RM alloy or RMB alloy is a sintered body produced during the manufacturing process of rare earth magnets.

11. The method for reusing heavy rare earth elements according to claim 1 or 2, wherein, The R, RM alloy or RMB alloy mentioned above is sludge produced during the processing of rare earth magnets or a processed product obtained by calcining the sludge.

12. The method for reusing heavy rare earth elements according to claim 1 or 2, wherein, The R, RM alloys, and RMB alloys mentioned are magnetic waste recycled from products that used rare earth magnets.

13. Recycling methods for rare earth magnets, among which, Alloys containing heavy rare earth elements extracted by the recycling method of any one of claims 1 to 12 are used as raw material alloys for rare earth magnets.

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