A deep eutectic solvent and its preparation method and application
By using eutectic solvents, the problems of poor iron separation and organic solvent pollution in rare earth permanent magnet waste have been solved, achieving efficient separation and environmentally friendly extraction of rare earth and transition metals, and simplifying the process flow.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- GANJIANG INNOVATION ACAD CHINESE ACAD OF SCI
- Filing Date
- 2024-01-15
- Publication Date
- 2026-07-10
AI Technical Summary
Existing technologies for rare earth permanent magnet waste recycling have poor iron separation effects and use a large amount of toxic organic solvents, resulting in environmental pollution and low separation efficiency.
A low-melting-point liquid solvent, composed of hydrogen bond donor and acceptor compounds, is used to extract transition metals from hydrochloric acid solutions of rare earth permanent magnet waste, achieving efficient separation of rare earth and transition metals.
It achieves efficient separation of rare earth and transition metals, reduces the use of organic solvents, simplifies the process, reduces environmental pollution and biotoxicity, and improves atom utilization.
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Figure CN117778718B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of resource recycling technology, and relates to a eutectic solvent and its preparation method and application. Background Technology
[0002] As a third-generation rare-earth permanent magnet material, neodymium iron boron (NdFeB) rare-earth permanent magnets are hailed as the "king of magnets" due to their superior magnetic properties. Currently, NdFeB is widely used in the electronics and information technology field. The rapid pace of product updates generates a large amount of waste NdFeB magnets. Furthermore, the production and processing of NdFeB magnets also produces 20%–30% waste. The rare-earth element content in waste NdFeB magnets is far higher than that in primary rare-earth ores, and it lacks other complex components. Therefore, waste NdFeB magnets have become the largest potential source for rare-earth metal recycling. Recycling them not only protects the environment but can also alleviate, to some extent, the contradiction between the continuously growing demand for rare earth elements and the insufficient supply.
[0003] In recent years, the impacts of volatile, toxic, and flammable organic solvents on environmental pollution, production safety, and human health have received increasing attention. Consequently, research on green solvents has increased dramatically. Excellent green solvents for liquid-liquid extraction processes should possess characteristics such as non-volatility, low toxicity, non-flammability, low viscosity, sufficient hydrophobicity, and a large density difference with water. These characteristics are all beneficial for improving the performance and efficiency of the extraction process.
[0004] CN112853107A discloses a method for treating rare earth permanent magnet waste, including the following steps: liquid nitrogen freezing demagnetization and degreasing; dry grinding and rust removal; waste screening; decontamination; sieving; hydrogen crushing; roasting, extraction, and purification to obtain rare earth chloride solution.
[0005] CN103509952A discloses a process for recovering rare earth elements from permanent magnet waste in electronic waste. The process includes the following steps: (1) oxidizing and melting the permanent magnet waste in electronic waste at high temperature into alloy particles and grinding them finely; (2) roasting the alloy particles with chloride and carbon powder at high temperature; (3) recovering borate and chloride salts by multiple recrystallization steps in the tail gas absorption liquid; (4) washing the powder after chlorination and roasting with hot dilute hydrochloric acid in a two-stage countercurrent process to dissolve soluble metal chloride salts; (5) passing hydrogen sulfide gas through the filtrate under acidic conditions to precipitate all cobalt and nickel, and then removing iron ions by precipitation; extracting and separating Pr, Nd, Sm, and Dy, and then precipitating and roasting with oxalic acid to obtain rare earth oxides; (6) roasting the cobalt and nickel sulfide slag with sulfation, and after acid dissolution, extracting and separating cobalt and nickel and recovering cobalt and nickel.
[0006] The recovery method described above has the disadvantages of poor iron separation effect and the use of highly toxic solvents in the separation process, which limits its practical application. Summary of the Invention
[0007] The purpose of this invention is to provide a eutectic solvent and its preparation method and application. The eutectic solvent of this invention can efficiently extract transition metals from hydrochloric acid solutions of permanent magnet rare earth waste, thereby reducing the use of large amounts of organic solvents in traditional extraction processes.
[0008] To achieve this objective, the present invention employs the following technical solution:
[0009] In a first aspect, the present invention provides a eutectic solvent, the eutectic solvent comprising a hydrogen bond donor compound and a hydrogen bond acceptor compound, wherein the structural formula of the hydrogen bond donor compound is as follows: R1 and R2 are independently selected from n-octyl or isooctyl, and the hydrogen bond acceptor compound includes tri-n-octylphosphine oxide (TOPO).
[0010] The eutectic solvent of this invention is composed of specific hydrogen bond donor and hydrogen bond acceptor compounds. Through hydrogen bond interactions, the hydrogen bond donor compound and tri-n-octylphosphine oxide significantly reduce the lattice energy, resulting in a melting point much lower than that of a single component, a wide liquid phase range, and existence as a liquid at room temperature. In addition to possessing many characteristics of ionic liquids, the eutectic solvent of this invention also offers advantages such as simple preparation, no purification required, and inexpensive precursors. Furthermore, due to its hydrophobicity, it can be directly used in solvent extraction without dilution by other organic solvents, making it a more environmentally friendly alternative to traditional organic solvents.
[0011] Preferably, in the eutectic solvent, the molar ratio of the hydrogen bond acceptor compound to the hydrogen bond donor compound is 1:(1 to 5), for example: 1:1, 1:2, 1:3, 1:4 or 1:5, etc.
[0012] In a second aspect, the present invention provides a method for preparing a eutectic solvent as described in the first aspect, the method comprising the following steps:
[0013] Phthalic anhydride, a secondary amine and a solvent were mixed and grafted to obtain a hydrogen bond donor compound.
[0014] The hydrogen bond donor compound and the hydrogen bond acceptor compound are mixed and heated and stirred to obtain the eutectic solvent.
[0015] Preferably, the secondary amine in step (1) includes di-n-octylamine and / or diisooctylamine.
[0016] Preferably, the hydrogen bond donor is formed by grafting phthalic anhydride with a secondary amine (such as di-n-octylamine or diisooctylamine).
[0017]
[0018] in for It has a tertiary amine structure.
[0019] Preferably, the solvent includes DCM.
[0020] Preferably, the heating and stirring temperature is 40-80°C, for example: 40°C, 50°C, 60°C, 70°C or 80°C.
[0021] Thirdly, the present invention provides a method for separating rare earth elements and transition metals from permanent magnet rare earth waste, the method comprising the following steps:
[0022] (1) After oxidizing and roasting rare earth permanent magnet waste, it is mixed with hydrochloric acid and then leached to obtain rare earth leachate and iron slag.
[0023] (2) The rare earth leachate is mixed with the eutectic solvent as described in the first aspect, the pH is adjusted, and the extract is obtained by extraction and separation.
[0024] (3) The extract is separated to obtain a rare earth chloride solution and a eutectic solvent phase loaded with iron.
[0025] The method described in this invention prepares a type of hydrophobic eutectic solvent, which is directly used in the optimal leaching solution of permanent magnet rare earth waste to separate rare earth and transition metals in the permanent magnet rare earth waste. This avoids the use of a large amount of toxic organic solvents in the traditional solvent extraction process, and can extract only trace amounts of rare earth elements while efficiently extracting transition metal ions, thus achieving efficient separation of rare earth elements and transition metal elements.
[0026] Preferably, the oxidation roasting process in step (1) is preceded by washing, drying and grinding.
[0027] Preferably, the temperature of the oxidative roasting treatment is 500-600℃, for example: 500℃, 520℃, 550℃, 580℃ or 600℃.
[0028] Preferably, the oxidative roasting treatment time is 1 to 4 hours, for example: 1 hour, 1.5 hours, 2 hours, 3 hours or 4 hours.
[0029] Preferably, the concentration of hydrochloric acid in step (1) is 0.1 to 0.3 mol / L, for example: 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.25 mol / L or 0.3 mol / L, etc.
[0030] Preferably, the leaching treatment time is 2 to 8 hours, for example: 2 hours, 3 hours, 5 hours, 6 hours or 8 hours.
[0031] Preferably, the leaching temperature is 40–90°C, for example: 40°C, 50°C, 60°C, 70°C, or 90°C.
[0032] Preferably, the solid-liquid ratio of the leaching treatment is 1:(25-150)g / mL, for example: 1:25g / mL, 1:50g / mL, 1:80g / mL, 1:100g / mL or 1:150g / mL, etc.
[0033] Preferably, the volume ratio of the eutectic solvent and the rare earth leachate in step (2) is 1:(2-10), for example: 1:2, 1:4, 1:6, 1:8 or 1:10, etc.
[0034] Preferably, the pH adjuster in step (2) includes hydrochloric acid and / or ammonium chloride.
[0035] Preferably, the pH is -1 to 0.5, for example: -1, -0.5, -0.2, 0 or 0.5, and more preferably the pH is -1 to 0.
[0036] The pH value of the present invention is -1, which means that the hydrogen ion concentration in the mixed liquid is 10 mol / L. The pH < 0 indicates high acidity, which can extract iron while avoiding the extraction of rare earth elements, thus achieving the separation of iron and rare earth elements. However, if the pH is too low, the extraction effect of iron will also decrease, and multiple extractions are required to separate iron to the maximum extent.
[0037] Preferably, the extraction and separation method includes shaking.
[0038] Preferably, the extraction and separation time is 8 to 12 hours, for example: 8 hours, 9 hours, 10 hours, 11 hours or 12 hours.
[0039] The extract obtained by the extraction and separation described in this invention can be repeatedly extracted using a low-temperature eutectic process until the iron extraction rate reaches the standard.
[0040] Preferably, the eutectic solvent phase loaded with iron in step (3) is back-extracted with hydrochloric acid to obtain a eutectic solvent for recycling.
[0041] Preferably, the concentration of the hydrochloric acid is 2 to 5 mol / L, for example: 2 mol / L, 2.5 mol / L, 3 mol / L, 4 mol / L or 5 mol / L, etc.
[0042] Compared with the prior art, the present invention has the following beneficial effects:
[0043] (1) The present invention prepares a novel eutectic solvent, which can efficiently extract transition metals from hydrochloric acid solution of permanent magnet rare earth waste (neodymium iron boron waste), which can reduce the use of a large amount of organic solvent in the traditional extraction process and simplify the process. The prepared eutectic solvent has the advantages of easy synthesis, low cost, high atom utilization, low environmental pollution and low biological toxicity, and therefore has both industrial application and environmental protection benefits.
[0044] (2) The eutectic solvent described in this invention is used in the method for separating rare earth and transition metals in permanent magnet rare earth waste described in this invention. It can separate iron and rare earth elements. The separation effect is significantly affected by the pH of the mixed solution obtained after mixing the eutectic solvent and rare earth leachate. The element separation can be achieved by controlling the pH to within 0.5 (a small amount of rare earth is extracted at 0.5). Specifically, the iron can be extracted when the pH is below 0, while the rare earth is not extracted, thus achieving the separation of iron and rare earth elements. Attached Figure Description
[0045] Figure 1 The infrared spectrum is that of the eutectic solvent prepared in Example 1.
[0046] Figure 2 The infrared spectrum of the eutectic solvent prepared in Example 2 is shown.
[0047] Figure 3 This is a process flow diagram of a method for separating rare earth and transition metals from permanent magnet rare earth waste, as described in an application example of the present invention. Detailed Implementation
[0048] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.
[0049] The composition of the rare earth permanent magnet waste (neodymium iron boron waste) described in the application example of this invention is as follows:
[0050] Fe 44.37wt%, Nd 12.23wt%, Pr 1.74wt%, Ce 2.36wt%, Gd 2.89wt%, Dy 1.12wt%, with the remainder being O and impurities.
[0051] Example 1
[0052] This embodiment provides a eutectic solvent, which is prepared by the following method:
[0053] Phthalic anhydride, di-n-octylamine and DCM were mixed and grafted to obtain hydrogen bond donor compound A.
[0054] Hydrogen bond donor compound A and tri-n-octylphosphine oxide were mixed in a molar ratio of 3:1 and stirred at 60°C until the mixture became a homogeneous and transparent liquid phase to obtain the eutectic solvent.
[0055] Infrared spectra of the eutectic solvent were obtained as follows Figure 1 As shown.
[0056] Example 2
[0057] This embodiment provides a eutectic solvent, which is prepared by the following method:
[0058] Phthalic anhydride, diisooctylamine and DCM were mixed and grafted to obtain hydrogen bond donor compound B.
[0059] The hydrogen bond donor compound B and tri-n-octylphosphine oxide were mixed in a 1:1 molar ratio and stirred at 80°C until the mixture became a homogeneous and transparent liquid phase, thus obtaining the eutectic solvent.
[0060] Infrared spectra of the eutectic solvent were obtained as follows Figure 2 As shown.
[0061] Example 3
[0062] This embodiment provides a eutectic solvent, which is prepared by the following method:
[0063] Phthalic anhydride, di-n-octylamine and DCM were mixed and grafted to obtain hydrogen bond donor compound A.
[0064] Hydrogen bond donor compound A and tri-n-octylphosphine oxide were mixed in a molar ratio of 5:1 and stirred at 40°C until the mixture became a homogeneous and transparent liquid phase to obtain the eutectic solvent.
[0065] Example 4
[0066] The only difference between this embodiment and Example 1 is that the hydrogen bond donor compound and tri-n-octylphosphine oxide are mixed at a molar ratio of 0.5:1. All other conditions and parameters are exactly the same as in Example 1.
[0067] Example 5
[0068] The only difference between this embodiment and Example 1 is that the hydrogen bond donor compound and tri-n-octylphosphine oxide are mixed in a molar ratio of 8:1, while the other conditions and parameters are exactly the same as in Example 1.
[0069] Application Example 1
[0070] This application example provides a method for separating rare earth elements and transition metals from permanent magnet rare earth waste. The process flow diagram of the method is shown below. Figure 3 As shown, the method specifically includes the following steps:
[0071] (1) The NdFeB waste was washed multiple times to remove surface oil and other impurities. After drying, crushing and grinding, a powdery waste with uniform particle size was obtained. 5g of waste was evenly spread in a crucible and oxidized and roasted in a muffle furnace at 500℃ for 4h. Then it was mixed with 0.2mol / L dilute hydrochloric acid at a solid-liquid ratio of 1:50g / mL and stirred. The leaching time was 6h at 80℃. The leached mixture was placed in a centrifuge and centrifuged at 3000rpm. After filtration and separation, rare earth leachate and iron slag were obtained.
[0072] (2) The eutectic solvent prepared in Example 1 was mixed with the rare earth leachate. At pH 0 and the volume ratio of the eutectic solvent to the rare earth leachate was 1:5, the mixture was shaken and extracted at room temperature for 8 hours. After extraction, the extract was obtained by centrifugation and filtration.
[0073] (3) The extract is separated to obtain an organic phase of eutectic solvent loaded with iron and a rare earth chloride enrichment solution. The eutectic solvent loaded with iron is back-extracted with hydrochloric acid with a concentration of 3 mol / L to obtain a eutectic solvent that can be recycled.
[0074] Application Example 2
[0075] This application example provides a method for separating rare earth elements and transition metals from permanent magnet rare earth waste. The process flow diagram of the method is shown below. Figure 3 As shown, the method specifically includes the following steps:
[0076] (1) The NdFeB waste was washed multiple times to remove surface oil and other impurities. After drying, crushing and grinding, a powdery waste with uniform particle size was obtained. 5g of waste was evenly spread in a crucible and oxidized and roasted in a muffle furnace at 600℃ for 1h. Then it was mixed with 0.1mol / L dilute hydrochloric acid at a solid-liquid ratio of 1:150g / mL and stirred. The leaching time was 8h at 60℃. The leached mixture was placed in a centrifuge and centrifuged at 3000rpm. After filtration and separation, rare earth leachate and iron slag were obtained.
[0077] (2) The eutectic solvent prepared in Example 1 was mixed with the rare earth leachate. At pH -1 and the volume ratio of the eutectic solvent to the rare earth leachate was 1:2, the mixture was shaken and extracted at room temperature for 10 h. After extraction, the extract was obtained by centrifugation and filtration.
[0078] (3) The extract is separated to obtain an organic phase of eutectic solvent loaded with iron and a rare earth chloride enrichment solution. The eutectic solvent loaded with iron is back-extracted with hydrochloric acid with a concentration of 2 mol / L to obtain a eutectic solvent that can be recycled.
[0079] Application Example 3
[0080] This application example provides a method for separating rare earth elements and transition metals from permanent magnet rare earth waste. The process flow diagram of the method is shown below. Figure 3 As shown, the method specifically includes the following steps:
[0081] (1) The NdFeB waste was washed multiple times to remove surface oil and other impurities. After drying, crushing and grinding, a powdery waste with uniform particle size was obtained. 5g of waste was evenly spread in a crucible and oxidized and roasted in a muffle furnace at 550℃ for 1h. Then it was mixed with 0.3mol / L dilute hydrochloric acid at a solid-liquid ratio of 1:25g / mL and stirred. The leaching time was 8h at 60℃. The leached mixture was placed in a centrifuge and centrifuged at 3000rpm. After filtration and separation, rare earth leachate and iron slag were obtained.
[0082] (2) The eutectic solvent prepared in Example 1 was mixed with the rare earth leachate. At pH 0.5 and the volume ratio of the eutectic solvent to the rare earth leachate was 1:5, the mixture was shaken and extracted at room temperature for 8 hours. After extraction, the extract was obtained by centrifugation and filtration.
[0083] (3) The extract is separated to obtain an organic phase of eutectic solvent loaded with iron and a rare earth chloride enrichment solution. The eutectic solvent loaded with iron is back-extracted with hydrochloric acid with a concentration of 5 mol / L to obtain a eutectic solvent that can be recycled.
[0084] Application Example 4
[0085] The only difference between this application example and application example 1 is that the eutectic solvent prepared in example 2 is used, while the other conditions and parameters are exactly the same as in application example 1.
[0086] Application Example 5
[0087] The only difference between this application example and application example 1 is that the eutectic solvent prepared in example 3 is used, while the other conditions and parameters are exactly the same as in application example 1.
[0088] Application Example 6
[0089] The only difference between this application example and application example 1 is that the eutectic solvent prepared in example 4 is used, while the other conditions and parameters are exactly the same as in application example 1.
[0090] Application Example 7
[0091] The only difference between this application example and application example 1 is that the eutectic solvent prepared in example 5 is used, while the other conditions and parameters are exactly the same as in application example 1.
[0092] Application Example 8
[0093] The only difference between this application example and application example 1 is that the pH in step (2) is 1, while the other conditions and parameters are exactly the same as in application example 1.
[0094] Application Example 9
[0095] The only difference between this application example and application example 1 is that the extract obtained in step (2) is extracted twice, while the other conditions and parameters are exactly the same as in application example 1.
[0096] Comparative Application Example 1
[0097] The only difference between this comparative application example and application example 1 is that the eutectic solvent is replaced with P204 extractant; all other conditions and parameters are exactly the same as in application example 1.
[0098] Performance testing:
[0099] The extraction rates of iron and rare earth elements in each application example and the comparative application example were tested, and the test results are shown in Table 1:
[0100] Table 1
[0101]
[0102] As can be seen from Table 1, and from Application Examples 1-5, the eutectic solvent described in this invention, when used in the method for separating rare earth and transition metals in permanent magnet rare earth waste, can achieve the separation of iron and rare earth elements. The separation effect is significantly affected by the pH of the mixed solution obtained after mixing the eutectic solvent and the rare earth leachate. Controlling the pH below 0.5 can achieve element separation (a small amount of rare earth is extracted at pH = 0.5). Specifically, a pH below 0 can extract iron while avoiding the extraction of rare earth, thus achieving the separation of iron and rare earth elements.
[0103] A comparison of Application Examples 1 and 6-7 shows that the ratio of the eutectic solvent affects its performance and thus the separation effect. Maintaining a molar ratio of hydrogen bond donor compound to tri-n-octylphosphine oxide between 1 and 5:1 yields better results with the eutectic solvent. If the proportion of the hydrogen bond donor compound is too high, it will cause co-extraction of a small amount of rare earth elements while extracting iron, leading to the loss of rare earth elements. If the proportion of the hydrogen bond donor compound is too low, the eutectic solvent's ability to extract iron decreases, resulting in insufficient impurity removal and reduced purity of the rare earth enrichment solution.
[0104] A comparison of Application Example 1 and Application Example 8 shows that when the eutectic solvent is mixed with the rare earth leachate, the pH of the resulting mixed solution is too high, and a large amount of rare earth elements will be leached out, resulting in poor separation of iron and rare earth elements, making separation impossible.
[0105] By comparing Application Example 1 and Application Example 9, it can be seen that by controlling the pH of the mixed solution obtained after mixing the eutectic solvent and the rare earth leachate to be 0 (or below) and repeating the extraction, iron can be completely extracted without extracting rare earth elements, showing a very good separation effect.
[0106] As can be seen from the comparison between Application Example 1 and Comparative Application Example 1, compared with commonly used extractants, the extractant used in this invention has a better extraction effect on iron, the separation efficiency of rare earth and iron is more efficient, the process flow is shortened, the consumption of chemical reagents is reduced, and it is more suitable for the removal of impurities from rare earth leachate.
[0107] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.
Claims
1. A method for separating rare earth elements and transition metals from permanent magnet rare earth waste, characterized in that, The method includes the following steps: (1) After oxidizing and roasting rare earth permanent magnet waste, it is mixed with hydrochloric acid and then leached to obtain rare earth leachate and iron slag. (2) The rare earth leachate is mixed with a eutectic solvent, the pH is adjusted, and then the extract is obtained by extraction and separation; (3) The extract is separated to obtain a rare earth chloride solution and a eutectic solvent phase loaded with iron; The eutectic solvent includes a hydrogen bond donor compound and a hydrogen bond acceptor compound, wherein the structural formula of the hydrogen bond donor compound is as follows: R1 and R2 are independently selected from n-octyl or isooctyl, and the hydrogen bond acceptor compound includes tri-n-octylphosphine.
2. The method as described in claim 1, characterized in that, In the eutectic solvent, the molar ratio of the hydrogen bond acceptor compound to the hydrogen bond donor compound is 1:(1~5).
3. The method as described in claim 1 or 2, characterized in that, The method for preparing the eutectic solvent includes the following steps: Phthalic anhydride, a secondary amine and a solvent were mixed and grafted to obtain a hydrogen bond donor compound. The hydrogen bond donor compound and the hydrogen bond acceptor compound are mixed and heated and stirred to obtain the eutectic solvent.
4. The method as described in claim 3, characterized in that, The secondary amine in step (1) includes di-n-octylamine and / or diisooctylamine.
5. The method as described in claim 3, characterized in that, The solvent includes DCM.
6. The method as described in claim 3, characterized in that, The heating and stirring temperature is 40~80℃.
7. The method as described in claim 1, characterized in that, Before the oxidation roasting process described in step (1), the ingredients are washed, dried and ground.
8. The method as described in claim 1, characterized in that, The temperature of the oxidative roasting treatment is 500~600℃.
9. The method as described in claim 1, characterized in that, The oxidative roasting treatment takes 1 to 4 hours.
10. The method as described in claim 1, characterized in that, The concentration of hydrochloric acid in step (1) is 0.1~0.3 mol / L.
11. The method as described in claim 1, characterized in that, The leaching treatment time is 2-8 hours.
12. The method as described in claim 1, characterized in that, The leaching treatment temperature is 40~90℃.
13. The method as described in claim 1, characterized in that, The solid-liquid ratio of the leaching treatment is 1:(25~150)g / mL.
14. The method as described in claim 1, characterized in that, The volume ratio of the eutectic solvent and the rare earth leachate in step (2) is 1:(2~10).
15. The method as described in claim 1, characterized in that, The pH adjuster mentioned in step (2) includes hydrochloric acid and / or ammonium chloride.
16. The method as described in claim 1, characterized in that, The pH ranges from -1 to 0.
5.
17. The method as described in claim 1, characterized in that, The extraction and separation method includes shaking.
18. The method as described in claim 1, characterized in that, The extraction and separation time is 8-12 hours.
19. The method as described in claim 1, characterized in that, In step (3), the eutectic solvent phase loaded with iron is back-extracted with hydrochloric acid to obtain a eutectic solvent for recycling.
20. The method as described in claim 19, characterized in that, The concentration of the hydrochloric acid is 2~5 mol / L.