Separation of rare earth elements using supported membrane solvent extraction

Abstract

Systems and methods for the recovery and separation of rare earth elements (REEs) are provided. The systems and methods include REE-supported membrane solvent extraction and the separation of light and heavy REEs recovered from waste permanent magnets and other electronic waste. In the supported membrane solvent extraction, an organic phase consisting of an extractant and an organic solvent is immobilized within the pores of a hollow fiber. An aqueous feed solution and a stripping solution flow along the shell side and lumen side of the hollow fiber, respectively. The extractant acts as a carrier to selectively transport certain rare earth metal ions from the feed side to the stripping side. The rare earth metals are simultaneously back-extracted into the stripping solution, allowing the process to proceed continuously without equilibrium limitations.

Description

[0001] Statement regarding federally sponsored research and development

[0002] This invention was completed with government support under contract number DE-AC05-00OR22725 granted by the U.S. Department of Energy. The government holds certain rights to this invention. Background Technology

[0003] This invention relates to a method for separating rare earth elements that have been recovered from waste permanent magnets and other end-of-life products.

[0004] In recent years, rare earth elements (especially dysprosium (Dy), praseodymium (Pr), and neodymium (Nd)) have attracted significant attention worldwide and have become strategically crucial for the security of the global economy, energy, transportation, and communications. This is due to their long-term and increasing use in various high technologies, including hybrid and electric vehicles, wind turbines, mobile devices, tablets, personal computers, a wide variety of devices with electric motors, and many other technological innovations.

[0005] Rare earth magnets are strong permanent magnets made from alloys of rare earth elements. It is estimated that over 10,000 tons of waste permanent magnets from hard disk drives containing Nd, Dy, and Pr could be recycled. However, rare earth magnets are generally not recyclable. In the case of hard disk drives, over 500,000,000 hard disk drives are manufactured annually. Processing operations primarily involve shredding the hard disk drives using an electronic waste recycler. Steel and aluminum are typically electromagnetically sorted from the shredded material stream for recycling. The remaining components (including rare earth magnets) are treated as waste.

[0006] The Dy content in waste permanent magnets varies significantly across applications. For example, while hard disk drives contain approximately 1-3% by weight of rare earth elements (REEs), hybrid and electric vehicle applications include approximately 20-25% by weight of Dy. Typically, the Dy content in the REE recovered from mixed permanent magnets is expected to be at most 10% by weight. Recent methods have been developed for recovering rare earth elements from permanent magnets and other post-consumer products. For example, U.S. Patent 9,968,887 to Bhave et al. discloses a method for recovering rare earth elements from commercial waste permanent magnets, the disclosure of which is incorporated herein by reference in its entirety. This method typically involves guiding an aqueous feed solution through or around hollow fibers to extract rare earth elements into a stripping solution, wherein the pores of the hollow fibers are wetted by an immobilized organic phase. Examples of this method have shown the recovery of high concentrations of rare earth oxides, including oxides of, for example, Nd, Pr, and Dy.

[0007] While the combination of Nd and Pr is acceptable in many industrial applications, the separation of Dy from Nd and Pr is crucial for the production of powerful magnets, which are essential for virtually every major defense system, including advanced fighter jets, ships, and ground equipment. Furthermore, Dy is a fundamental component of almost all types of permanent magnets, including laptop hard drives, hybrid / electric vehicles, and wind turbines, due to their highly specific properties such as coercivity, high-temperature resistance, and corrosion resistance. Demand for Dy is estimated to be increasing annually, exceeding 800 tons in 2020, almost double the amount used in 2011. While demand for Dy continues to rise, its supply is uncertain, as most of its production is limited to a single source in southern China, and the amount of Dy available in mines is also limited. Moreover, Dy has a high criticality due to several specific high-tech applications, and its market value is almost four times higher than that of Nd and Pr. Limited production and supply, along with high economic benefits, necessitate the recovery, separation, and purification of Dy from mixed rare earth oxides recycled from waste permanent magnets. Therefore, separating Dy from other rare earth elements has significant commercial value. Finally, separating Dy from mixed rare earth oxides (especially NdPrDy) will yield two pure products (Dy and NdPr), each with significant market value in its respective form. If Dy can be separated in its pure form, it can be incorporated into any rare earth oxide mixture to meet the specifications of end-users for various applications of recycling and reuse of rare earth elements recovered from waste magnets.

[0008] Therefore, systems and methods for separating rare earth elements that have been recovered from rare earth magnets and other end-of-life products remain needed. In particular, improved methods for separating light rare earth elements (e.g., Nd and Pr) from heavy rare earth elements (e.g., Dy) are still needed. It is also desirable to recover rare earth elements in a high-purity form suitable for direct reuse with minimal processing. Summary of the Invention

[0009] A system and method for separating and recovering rare earth elements are provided. The system and method include loaded membrane solvent extraction of rare earth elements and rare earth element oxides recovered from permanent magnets and other electronic waste. In the loaded membrane solvent extraction, an organic phase consisting of an extractant and an organic solvent is immobilized in the pores of a hollow fiber. An aqueous feed solution and a stripping solution flow along the shell side and the inner cavity side of the hollow fiber, respectively. The extractant acts as a carrier to selectively transport certain rare earth metals from the feed side to the stripping side. The rare earth metals are back-extracted into the stripping solution, allowing the process to proceed continuously without equilibrium limitations.

[0010] In one embodiment where the oxides of Dy are separated from the oxides of Nd and Pr, the permeable hollow fibers are hydrophobic polypropylene hollow fiber membrane modules oriented in a common direction between relative tube sheets. The hollow fibers may comprise bundles of thousands of fibers with an inner diameter of about 0.24 mm and an outer diameter of about 0.30 mm, a pore size of about 30 nm, and a membrane area of ​​about 1.4 m². 2 The immobilized organic phase comprises a solvent and an extractant. The organic phase may include an isoparaffin solvent and a phosphorus-based chelating extractant, having a volume ratio between 1:1 and 3:1 or any other combination. The feed solution may include a pH maintained between 0 and 2.0, and further optionally between 1.0 and 1.5.

[0011] According to the aforementioned method, Dy with 100% purity is separated in only three stages using Cyanex 572 as the extractant. Compared with conventional techniques (e.g., hydrometallurgy, pyrometallurgy, hydrogen crushing, and solvent extraction), supported membrane solvent extraction offers several significant advantages. Key advantages of supported membrane solvent extraction include minimal chemical use and waste generation, low extractant stock, operation at ambient temperature and low pressure, and the ability to handle highly contaminated / various samples (e.g., shredded waste mixtures). Extraction and recovery are completed in a single-step process, without emulsion formation and equilibrium limitations. Modular configuration allows for process flexibility in terms of production capacity. High-purity products and high separation factors (high selectivity for REEs relative to non-REEs) are achieved due to the absence of solvent entrainment. Furthermore, supported membrane solvent extraction requires low energy consumption and low operating and capital costs.

[0012] These and other features and advantages of the invention will become apparent from the following description of the invention when viewed in conjunction with the accompanying drawings and claims. Attached Figure Description

[0013] Figure 1 This is a description of a system for loaded membrane solvent extraction used to separate rare earth elements.

[0014] Figure 2 This is a description of a membrane solvent extraction module that includes a porous hollow fiber load.

[0015] Figure 3 This is a description of a multi-stage system for loaded membrane solvent extraction.

[0016] Figure 4 This is a flowchart of a method for loading membrane solvent extraction to separate rare earth elements.

[0017] Figures 5A to 5F This is a diagram illustrating the first-stage recovery of Dy from mixed rare earth element oxides.

[0018] Figures 6A to 6F This is a diagram illustrating the second-stage recovery of Dy from mixed rare earth element oxides.

[0019] Figures 7A to 7F This is a diagram illustrating the third-stage recovery of Dy from mixed rare earth element oxides.

[0020] Figure 8 The X-ray diffraction (XRD) analysis of Dy obtained from the stripping solution recovered in the third stage is shown, with standard Dy2O3 used as a reference.

[0021] Figure 9 This is a description of a system used for membrane solvent extraction of REE and subsequent separation of Dy from Nd and Pr.

[0022] Figures 10A to 10F This is an instruction on how to use. Figure 9 The system separates Dy from rare earth elements recycled from waste magnets. Detailed Implementation

[0023] As discussed herein, a system according to one embodiment includes a loaded membrane solvent extraction for separating heavy rare earth elements from light rare earth elements, the heavy and light rare earth elements having been co-recovered from waste permanent magnets as a mixture of rare earth element oxides in substantially pure form (e.g., greater than 90 wt%, optionally greater than 99.5 wt%).

[0024] Now for reference Figure 1 This describes a system for separating rare earth elements (REEs), such as separating heavy rare earth elements (e.g., Dy) from light rare earth elements (e.g., Nd and Pr), and is generally designated as 10. The system generally includes a feed line 12, a stripping line 14, and a hollow fiber membrane module 16. As discussed below, the hollow fiber membrane module 16 includes a bundle assembly of hollow fibers oriented in a common direction between relative tube sheets. The REE feed solution is contained in a feed reservoir 18 and mixed to ensure a uniform concentration. The feed solution is circulated in a closed loop through the hollow fiber membrane module 16 under pressure from a first pump 20 (e.g., a peristaltic pump), optionally ensuring that the feed line pressure is greater than the stripping line pressure. The stripping line 14 includes a reservoir 22 and a pump 24 (e.g., a peristaltic pump) to ensure continuous flow of the stripping solution through the module 16. Both the feed line 12 and the stripping line 14 are... Figure 1 In all embodiments, the feed line and / or stripping line are shown as a closed loop, allowing for continuous recirculation of the feed solution and stripping solution. However, in other embodiments, the feed line and / or stripping line form an open loop.

[0025] exist Figure 2The description refers to a membrane module containing fiber bundles, generally designated as 16. Membrane module 16 includes a housing 26 defining a feed inlet 28, a feed outlet 30, a stripping inlet 32, and a stripping outlet 34. A plurality of fibers 36 are encapsulated at their opposing ends to a first tube sheet 38 and a second tube sheet 40 such that the fibers 36 extend in a common direction within module 16. Each fiber 36 includes an inner cavity side 42 and a housing side 44. The inner cavity side 42... Figure 2 The description indicates exposure to the stripping solution; however, in other embodiments, the inner cavity side 42 is exposed to the feed solution. Similarly, the shell side 44 is... Figure 2 The description indicates that the shell side 44 is exposed to the feed solution; however, in other embodiments, the shell side 44 is exposed to the stripping solution.

[0026] As used herein, the “lumen side” includes an inner surface defining a long channel extending longitudinally through the hollow fiber, and the “shell side” includes the outer surface of the fiber, such that the lumen side and the shell side are spaced apart from each other by the thickness of the membrane sidewalls. The side in contact with the feed solution defines the “feed interface,” while the side in contact with the stripping solution defines the “stripping interface.” Thus, in some embodiments, the lumen side is the feed interface, while in other embodiments it is the stripping interface. Similarly, in some embodiments, the shell side is the stripping interface, while in other embodiments it is the feed interface.

[0027] The REE feed solution includes rare earth elements pre-separated from non-rare earth elements. For example, the feed solution can be extracted according to a membrane-assisted solvent extraction method as described in U.S. Patent 9,968,887 to Bhave et al., in which rare earth elements (e.g., Nd, Dy, and Pr) are recovered from commercial waste magnets (which also contain non-rare earth elements such as Fe and B). The feed solution comprises a dry mixture of two or more different rare earth element oxides as a first component (e.g., composed of more than 90% by weight, optionally more than 99.5% by weight of rare earth element oxides), and a solution of dilute nitric acid (e.g., 0.02 M nitric acid) as a second component. Examples of rare earth element oxides (REO) include Nd₂O₃, Pr₂O₃, and Pr₆O₃. 11 And Dy2O3, although other rare earth element oxides may be used in other embodiments. In still other embodiments, the feed solution may comprise a mixture of two or more different rare earth elements combined with a dilute nitric acid solution, wherein the rare earth elements are not in oxide form but have a purity of at least 90% by weight, optionally at least 99.5% by weight.

[0028] The pH of the feed solution is typically maintained between 0 and 2.0, consistent with the optimal operating mode for cationic extractants, and optionally further between 1.0 and 1.5. The stripping solution is typically chosen to strip heavy rare earth element complexes that have diffused from the feed interface to the stripping interface. The stripping solution may include, for example, HNO3, HCl, or H2SO4 at higher molar concentrations than in the feed solution. For example, the stripping solution may include 3.0 M HNO3, compared to 0.02 M HNO3 in the feed solution. Figure 1 As shown, the stripping solution is contained in the second reservoir 22 and circulated in a closed loop through the hollow fiber membrane module 16 under the pressure of the second pump 24.

[0029] As described above, the hollow fiber membrane module 16 comprises an organic phase consisting of an extractant and an organic solvent immobilized in the pores of the hollow fibers. In the current embodiment, the extractant is a phosphorus-based chelating extractant, such as Cyanex 572 from Cytec Industries, Inc. Alternatively, the extractant may be a neutral extractant, such as tetraoctyl diethylene glycol amide (“TODGA”). However, in other embodiments, other extractants may be used, such as trialkylphosphine oxide, 2-ethylhexylphosphonate mono-2-ethylhexyl ester, carbamoyl phosphoryl oxide, sec-octylphenoxyacetic acid, or Cyanex 272. The organic solvent includes isoparaffin solvents, such as Isopar L from ExxonMobil Chemical; however, in other embodiments, other solvents may be used, such as tributyl phosphate, xylene, hexane, octanol, or kerosene. In this embodiment, the pores of the hollow fibers are pre-impregnated with an organic phase consisting of an isoparaffin solvent and a phosphorus-based chelating extractant, wherein the volume ratio is between 3:1 and 1:1, and more preferably 2:1. The pore size is selected based on the capillary force necessary to retain the organic phase in the pores of the membrane fibers, for example, between about 0.01 micrometers and about 1.0 micrometers, or about 30 nm in this embodiment.

[0030] As in Figure 3As shown, the separation of heavy rare earth elements (e.g., Dy) from light rare earth elements (e.g., Nd and Pr) can be carried out in multiple stages. In one embodiment, the first stage separation is carried out for a first predetermined time period, such as 24 hours, wherein the feed solution and stripping solution are continuously recycled through membrane module 16. The feed solution containing REO is recovered in advance from waste magnets dissolved in a 0.02 M nitric acid feed solution. In this first stage, the pH of the feed solution can be maintained between 0 and 2.0. For the second separation stage, the pH of the stripping solution enriched with Dy (heavy REE) from the first stage is adjusted (increased) to between 0 and 2.0, optionally 1.5, using ammonium hydroxide, and used as the feed solution for the second stage. The stripping solution can be filtered to remove any precipitated salts. The second separation stage can occur for a second predetermined time period, optionally shorter than the first predetermined time period, such as 10 hours, wherein the feed solution and stripping solution are continuously recycled through membrane module 16. For the third separation stage, the pH of the stripping solution enriched with Dy (heavy REE) can be adjusted (increased) to between 1.0 and 2.0, optionally 1.5. The third separation stage can occur over a predetermined time period, such as 10 hours, during which the feed solution and stripping solution are continuously recycled through membrane module 16. In each separation stage, membrane module 16 comprises an organic phase of Cyanex 572 (33 v / v%) and Isopar L (67 v / v%), or other combinations thereof, and the stripping solution comprises 3.0 M nitric acid or other suitable concentrations based on the feed solution conditions. At the end of the third separation stage, the heavy rare earth element Dy, separated from the lighter rare earth elements (e.g., Nd and Pr), is in substantially pure form, optionally greater than 99.5% by weight.

[0031] To reiterate, the system according to one embodiment includes a loaded membrane solvent extraction for separating heavy rare earth elements from light rare earth elements, wherein the heavy and light rare earth elements have each been co-recovered from waste permanent magnets as a mixture of rare earth element oxides in substantially pure form (e.g., greater than 90% by weight, optionally at least 99.5% by weight). Figure 4The diagram presents a flowchart of membrane solvent extraction according to one embodiment. In summary, the method may include the following steps: a) recovering mixed rare earth element oxides from waste permanent magnets or other electronic waste (50); b) pre-impregnating the pores of a plurality of permeable hollow fibers with an organic phase comprising an extractant and an organic solvent (52); c) applying an acidic aqueous feed solution comprising dissolved rare earth element oxides at a continuous flow rate along the inner or outer side of the plurality of permeable hollow fibers (54); d) applying an acidic stripping solution at a continuous flow rate along another of the inner or outer side of the plurality of permeable hollow fibers (56); and e) repeating steps c) and d) after a predetermined time period, using the stripping solution from the previous stage as the feed solution for the subsequent stage, to perform another stage (58) for rare earth element separation. The steps of applying the feed solution at step c) and the application of the stripping solution at step d) are performed simultaneously to provide simultaneous extraction (through the organic phase) and stripping (through the stripping solution) of heavy rare earth elements (e.g., Dy).

[0032] As described above, the step of recovering the mixed rare earth element oxides in step a) can be carried out according to the membrane-assisted solvent extraction method described in U.S. Patent 9,968,887 to Bhave et al., to obtain the mixed rare earth element oxides in substantially pure form (e.g., substantially 90% by weight, optionally at least 99.5% by weight, of a mixture of two or more rare earth element oxides). Pre-impregnating the pores of the plurality of hollow fibers with an organic phase in step b) can include wetting the pores with an isoparaffin solvent and a phosphorus-based chelating extractant or any other combination in a volume ratio between 3:1 and 1:1 (further optionally 2:1). Applying a continuously flowing acidic aqueous feed solution along the inner lumen side or shell side of the plurality of permeable fibers in step c) can include providing an acidic aqueous feed solution comprising dissolved rare earth elements from post-consumer products, end-of-life products, and other rare earth element sources. The acidic aqueous feed solution can include, for example, HNO3, HCl, or H2SO4 or other inorganic acids at the desired molar concentration. The acidic stripping solution applied at a continuous flow rate in step d) may include, for example, HNO3, HCl, or H2SO4 at a molar concentration higher than that of the feed solution. Steps c) and d) are repeated after a given time period at each separation stage, wherein the stripping solution used in the previous stage is diluted to increase the pH for use as the feed solution in subsequent stages.

[0033] Example 1

[0034] The following non-restrictive examples describe Figure 4 A method for selectively separating essentially pure Dy from Nd and Pr.

[0035] An aqueous feed solution was prepared by dissolving REO previously recovered from waste magnets in a 0.02 M nitric acid solution. The pH of the feed solution (1000 mL) was maintained between 1.5 and 2.0. The organic phase consisted of Cyanex 572 (33 v / v%) and Isopar L (67 v / v%). The organic phase was loaded into the pores of the fiber membrane through the bottom of the module's inner cavity side. 1000 mL of 3.0 M nitric acid was used as the stripping solution. The feed solution and stripping solution were continuously recirculated through the bottom of the module along the shell side and inner cavity side, respectively. The feed and stripping flow rates were approximately 250 mL / min and 70 mL / min, respectively. No pressure differential was applied between the feed and stripping sides. The first stage separation was carried out for 24 hours. For the second stage separation, the pH of the stripping solution from the first stage was adjusted to 1.5 by adding ammonium hydroxide and used as the feed for the second stage. The solution was filtered using 2.2 μm filter paper to remove any precipitated salts. The second stage separation was carried out for 10 hours. For the third-stage separation, the pH of the second-stage stripping solution was adjusted to 1.5 by adding ammonium hydroxide and used as the feed for the third stage. The third-stage separation was carried out for 10 hours. After the recovery of Dy from the feed solution was complete, the third-stage stripping solution was treated with oxalic acid to precipitate Dy. The Dy was washed with deionized water and dried overnight at ambient temperature. The Dy was annealed at 860°C for 10 hours at a gradual rate of 3°C / min to obtain Dy₂O₃.

[0036] The initial feed solution contained 27,000 ppm Nd, 8,200 ppm Pr, and 12,800 ppm Dy. 100% pure Dy was obtained in three stages of the membrane solvent extraction process. The recoveries of Dy in stages 1, 2, and 3 were 66%, 97%, and 96%, respectively, resulting in a cumulative recovery of 61%. The Dy extraction rates in stages 1, 2, and 3 were 0.42 g / m³. 2 / hr, 0.87 g / m 2 / hr and 1.2 g / m 2 / hr. A high separation factor is achieved in each stage, indicating high separation efficiency of the loaded membrane solvent extraction system. Figures 5A-5F Describe the first-stage composition of (A) the purity (%) of Dy in the feed solution, (B) the stripping solution, (C) the feed solution and the stripping solution, (D) the Dy separation factor, (E) the REE recovery rate, and (F) the REE recovery rate. Figures 6A-6F The second-stage composition is described as follows: (A) purity (%) of Dy in the feed solution, (B) stripping solution, (C) purity (%) of the feed solution and stripping solution, (D) Dy separation factor, (E) REE recovery rate, and (F) REE recovery rate. Finally, Figures 7A-7FThe third stage composition describes the purity (%) of Dy in (A) feed solution, (B) stripping solution, (C) feed solution and stripping solution, (D) Dy separation factor, (E) REE recovery rate and (F) REE recovery rate.

[0037] Figure 8 The XRD pattern shows the phase and purity of the recovered Dy. Specifically, the XRD pattern of the feed solution (before the first-stage separation) shows that the feed solution contains Nd₂O₃, Pr₂O₃, and Dy₂O₃. The XRD pattern of the stripping solution (after the third-stage separation) indicates that the recovered product contains only Dy₂O₃, and the characteristic peaks perfectly match the standard Dy₂O₃ reference data. No praseodymium and neodymium oxides from the starting material were detected in the product obtained from the stripping solution after the third stage.

[0038] The inventors also discovered that the separation and recovery of rare earth elements depend on the pH of the feed solution. Feed solutions with a pH between 0 and 2.0, or approximately 1.5, were found to be well-suited for separating Dy using the Cyanex 572. In loaded membrane solvent extraction methods, the pH of the feed solution decreases over time as hydrogen ions are transferred from the stripping solution to the feed solution, while metal ions are transferred from the feed solution to the stripping solution. To improve Dy recovery, the pH of the feed solution can be maintained between 1.0 and 1.5, for example, by adding ammonium hydroxide solution every three hours. The aforementioned pH adjustment resulted in higher Dy recovery and extraction rates without significantly affecting purity.

[0039] Example 2

[0040] In the improved method described above, Dy is recovered directly from waste permanent magnets without intermediate steps (precipitation, annealing, and redissolving in acid), which are typically performed after the extraction and mixing of REEs from the waste magnets. For example, in Figure 9 As shown, for example, a stripping solution (“REE in stripping”) (using a neutral or cationic extractant, such as TODGA or Cyanex) is applied directly to the feed solution to separate Dy from Nd and Pr. Specifically, the Dy separation is carried out from a solution of 38,753 ppm Nd, 11,540 ppm Pr, and 4,932 ppm Dy recovered from a mixed waste permanent magnet feedstock. The feed solution (9 wt% Dy) comprises 1000 mL of 0.02 M HNO3, and the stripping solution comprises 1000 mL of 3.0 M HNO3. The membrane module (1.4 m... 2 This includes 33% by volume Cyanex 572 as the extractant and the balance Isopar L. In the first stage, as in... Figures 10A-10FAs shown, separation was performed for 11 hours, with the pH of the feed solution adjusted every hour. For the first stage, the recovery, purity, and extraction rate of Dy were 97%, 29.8% by weight, and 0.31 g / m³, respectively. 2 / hr. As shown in Example 1, an additional separation stage can be performed to obtain 100% Dy. The purity of NdPr retained in the feed solution is 99.6% by weight. These results demonstrate that this method can directly recover Dy from waste permanent magnets without intermediate precipitation, annealing, and redissolution in acid.

[0041] The above description is a description of the current embodiment of the invention. Various changes and variations may be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which will be interpreted in accordance with patent law principles including the doctrine of equivalents. For example, any reference to a singular element using the articles “a,” “an,” “the,” or “the” should not be construed as limiting that element to the singular.

Claims

1. A method for separating rare earth elements from a premixture of rare earth element oxides at a weight percentage greater than 90%, the method comprising: A premixture of rare earth element oxides of greater than 90% by weight is introduced into an acid-containing liquid, wherein the premixture includes oxides of dysprosium, oxides of neodymium and oxides of praseodymium, thereby obtaining an aqueous feed solution containing dysprosium as a heavy rare earth element and containing neodymium and praseodymium as light rare earth elements. A plurality of hollow fibers are provided, each hollow fiber including an inner lumen side spaced apart from the shell side to define a membrane therebetween, the membrane including a plurality of pores dispersed therein; The membrane of each of the plurality of hollow fibers is pre-impregnated with an organic phase comprising an organic solvent and an extractant; One of the inner cavity side or the shell side of the plurality of hollow fibers is brought into contact with the aqueous feed solution containing a certain concentration of the heavy rare earth element and a certain concentration of the light rare earth element. One of the inner cavity side or the shell side of the plurality of hollow fibers is brought into contact with a stripping solution, wherein the stripping solution includes HNO3, HCl or H2SO4, and its molar concentration is higher than that in the aqueous feed solution, wherein the heavy rare earth element is simultaneously back-extracted from the organic phase into the stripping solution, and the light rare earth element has been retained in the aqueous feed solution, such that the concentration of dysprosium, which is the heavy rare earth element, in the stripping solution is greater than that in the aqueous feed solution; and The stripping solution is precipitated to recover a precipitate having more than 99.5% by weight of one or more dysprosium oxides. The method further includes actively maintaining the pH of the aqueous feed solution between 0 and 2.

0.

2. The method according to claim 1, wherein the organic solvent and the extractant have a volume ratio between 1:1 and 3:

1.

3. The method according to claim 2, wherein the organic solvent comprises isoparaffin, tributyl phosphate, xylene, hexane, octanol or kerosene, and the extractant is a chelating extractant.

4. The method according to claim 1, wherein the stripping solution comprises nitric acid, hydrochloric acid, or sulfuric acid.

5. The method according to claim 1, wherein the concentration of the light rare earth element in the stripping solution is maintained at less than 5% by weight.

6. The method of claim 1, wherein the plurality of hollow fibers define an average pore size of 10-1000 nm.

7. A method for separating rare earth elements, the method comprising: A mixture of more than 90% by weight of Nd2O3, Pr2O3 and Dy2O3 is dissolved in a mineral acid to obtain a feed solution, the feed solution comprising Dy2O3 as an oxide of a heavy rare earth element and comprising Nd2O3 and Pr2O3 as oxides of light rare earth elements, the pH of the feed solution being between 0 and 2.

0. A membrane module comprising a plurality of hollow fibers is provided, the plurality of hollow fibers including an inner lumen side spaced apart from a shell side to define a membrane therebetween, the membrane including a plurality of pores dispersed therein; The membrane of each of the plurality of hollow fibers is pre-impregnated with an organic phase comprising an organic solvent and an extractant; The feed solution is recirculated at a continuous flow rate along one of the inner cavity side or the shell side of the plurality of hollow fibers; A stripping solution is continuously circulated along the inner cavity side or the shell side of the plurality of hollow fibers at another rate, wherein the stripping solution comprises HNO3, HCl or H2SO4 at a molar concentration higher than that in the aqueous feed solution, wherein the heavy rare earth elements are back-extracted from the organic phase into the stripping solution, and the light rare earth elements are retained in the feed solution. and The stripping solution is precipitated to recover a solid having more than 99.5% by weight of Dy2O3.

8. The method of claim 7, further comprising actively maintaining the pH of the feed solution between 0 and 2.

0.

9. The method of claim 7, wherein the organic solvent and the extractant comprise a volume ratio between 1:1 and 3:

1.

10. The method according to claim 7, wherein the organic solvent comprises isoparaffin, tributyl phosphate, xylene, hexane, octanol or kerosene, and the extractant is a chelating extractant.

11. The method of claim 7, wherein the stripping solution comprises nitric acid, hydrochloric acid, or sulfuric acid.

12. The method of claim 7, wherein the concentration of the light rare earth element in the stripping solution is maintained at less than 5% by weight.

13. The method of claim 7, wherein the plurality of hollow fibers define an average pore size of 10-1000 nm.

14. The method of claim 7, wherein the feed solution and the stripping solution, which are directed at a continuous flow rate during the first stage separation, are subjected to a first predetermined time period, the method further comprising converting the stripping solution from the first stage separation to a feed solution for the second stage separation by adjusting the pH of the stripping solution to between 0 and 2.0, and subsequently: The second-stage feed solution, guided at a continuous flow rate along one of the inner cavity side or the shell side of the plurality of hollow fibers; and A second-stage stripping solution with a continuous flow rate along the inner cavity side or the shell side of the plurality of hollow fibers, wherein the concentration of heavy rare earth elements in the second-stage stripping solution is greater than the concentration of heavy rare earth elements in the stripping solution separated from the first stage.

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