A method for recovering rare earth elements from rare earth molten salt slag

Abstract

The present invention provides the use of an acylphenoxycarboxylic acid compound having the structure represented by formula (I) as an extractant in the process of recovering rare earth elements from rare earth molten salt slag. The present invention specifically uses the acylphenoxycarboxylic acid compound as an extractant in the process of recovering rare earth elements from rare earth molten salt slag. The present invention also provides a corresponding method for recovering rare earth elements from rare earth molten salt slag. The recovery method provided by the present invention has an excellent recovery rate, a simple process, mild conditions, and good controllability, making it more suitable for promotion and application in large-scale industrial production. #imgabs0#

Description

Technical Field

[0001] The present invention belongs to the technical field of rare earth molten salt slag recovery, and relates to the use of acylphenoxycarboxylic acid compounds as extractants in the process of recovering rare earth elements from rare earth molten salt slag, and a method for recovering rare earth elements from rare earth molten salt slag, and in particular to the use of acylphenoxycarboxylic acid compounds represented by formula (I) as extractants in the process of recovering rare earth elements from rare earth molten salt slag, and a method for recovering rare earth elements from rare earth molten salt slag. Background Art

[0002] The production process of rare earth metals produces a certain amount of rare earth molten salt slag. If this slag is stored, it poses a significant threat to the environment and safety, and also wastes rare earth resources. Several related research proposals have been published in the prior art. For example, patent CN201510244696.7 discloses a sulfuric acid pretreatment-normal-pressure alkali conversion-hydrochloric acid leaching process. This method utilizes the principle that rare earth fluorides can be converted into rare earth hydroxides when heated under alkaline conditions. Under normal pressure, sodium hydroxide is used to dissolve the acid-insoluble rare earth fluorides into acid-soluble rare earth hydroxides. However, this process requires multiple acid dissolutions. Furthermore, Chen Dongying et al.'s publication "Research on the Comprehensive Utilization of Waste Molten Salt from Rare Earth Electrolysis" explores a negative-pressure alkali conversion-hydrochloric acid leaching process, which uses a combination of beneficiation and smelting to treat the waste slag. While this method achieves a 90% recovery rate, it places high demands on the equipment.

[0003] Therefore, how to find a more suitable way to better recycle rare earth molten salt slag, facilitate industrialization, and solve the above-mentioned technical problems existing in the existing technology has become one of the urgent problems to be solved by many front-line researchers in the industry. Summary of the Invention

[0004] In view of this, the technical problem to be solved by the present invention is to provide the use of acylphenoxycarboxylic acid compounds as extractants in the process of recovering rare earth elements from rare earth molten salt slag and a method for recovering rare earth elements from rare earth molten salt slag, in particular, the use of acylphenoxycarboxylic acid compounds represented by the structure of formula (I) as extractants in the process of recovering rare earth elements from rare earth molten salt slag. The present invention applies acylphenoxycarboxylic acid compounds having the structure of formula (I) as extractants in the process of recovering rare earth elements from rare earth molten salt slag. The recovery method provided by the present invention has an excellent recovery rate, simple process, mild conditions, good controllability, and is more suitable for promotion and application in large-scale industrial production.

[0005] The present invention provides the use of an acylphenoxycarboxylic acid compound represented by the structure of formula (I) as an extractant in the process of recovering rare earth elements from rare earth molten salt slag;

[0006]

[0007] wherein R1 and R2 are each independently selected from a C1 to C9 alkyl group;

[0008] n is a natural number selected from 1 to 5.

[0009] Preferably, the rare earth molten salt slag contains one or more of lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium and yttrium;

[0010] The extractant includes a supported extractant;

[0011] The application includes the coordinated application of the extractant and the base.

[0012] Preferably, the combined application is that the extractant and the alkali are used simultaneously;

[0013] The mass ratio of the extractant to the alkali is (0.01-1): (0.001-0.1).

[0014] The present invention provides a method for recovering rare earth elements from rare earth molten salt slag, comprising the following steps:

[0015] 1) mixing rare earth molten salt slag with sodium hydroxide and roasting to obtain alkaline slag;

[0016] 2) washing the alkali residue obtained in the above step in water to obtain a washing residue and an aqueous phase;

[0017] 3) mixing the washing residue obtained in the above step with a mineral acid again to obtain an acid leaching solution;

[0018] 4) mixing the acid leaching solution obtained in the above step, the acylphenoxycarboxylic acid extractant having the structure represented by formula (I), and liquid alkali again and extracting to obtain a loaded extractant;

[0019]

[0020] wherein R1 and R2 are each independently selected from a C1 to C9 alkyl group;

[0021] n is a natural number selected from 1 to 5;

[0022] 5) The loaded extractant obtained in the above step is further mixed with a mineral acid to obtain a rare earth element solution.

[0023] Preferably, the content of rare earth elements in the rare earth molten salt slag, calculated as the mass content of rare earth oxides, REO is greater than or equal to 38%;

[0024] The rare earth molten salt slag includes, in addition to rare earth elements, one or more of Ca, O, F, C, Al, Fe, W, Mo and Si;

[0025] The mass ratio of the rare earth molten salt slag to sodium hydroxide is 1:(0.1-0.5).

[0026] Preferably, the calcination temperature is 300-900°C;

[0027] The calcination time is 0.5 to 3.0 hours;

[0028] The mass ratio of the alkali residue to water is 1:(1-20).

[0029] Preferably, the aqueous phase includes one or more of calcium, fluorine and liquid alkali impurities;

[0030] The mineral acid includes one or more of sulfuric acid, nitric acid and hydrochloric acid;

[0031] The concentration of the mineral acid in step 3) is 0.05-1 mol / L.

[0032] Preferably, the mass ratio of the washing residue to the mineral acid is 1:(0.5-200);

[0033] The mass ratio of the acid extract to the acylphenoxycarboxylic acid extractant having the structure represented by formula (I) is 1:(0.01-1);

[0034] The mass ratio of the acid leaching solution to the liquid alkali is 1:(0.001-0.1).

[0035] Preferably, the liquid alkali comprises one or more of ammonia water, sodium hydroxide and potassium hydroxide;

[0036] The concentration of the liquid caustic soda is 0.5 to 12 mol / L;

[0037] The extraction time is 5 to 10 days.

[0038] Preferably, the mass ratio of the loaded extractant to the mineral acid is 1:(0.1-200);

[0039] The concentration of the mineral acid in step 5) is 0.05 to 1 mol / L.

[0040] The present invention provides the use of an acylphenoxycarboxylic acid compound having the structure represented by formula (I) as an extractant in the process of recovering rare earth elements from rare earth molten salt slag. Compared to the prior art, the present invention specifically utilizes the acylphenoxycarboxylic acid compound as an extractant in the process of recovering rare earth elements from rare earth molten salt slag, and also provides a corresponding method for recovering rare earth elements from rare earth molten salt slag. The recovery method provided by the present invention has an excellent recovery rate, a simple process, mild conditions, and good controllability, making it more suitable for promotion and application in large-scale industrial production.

[0041] Experimental results show that the method for recovering rare earth elements from low-grade rare earth molten salt slag provided by the present invention has a yield greater than 92%, which is greater than 80% of the industry average. DETAILED DESCRIPTION

[0042] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than for limiting the claims of the invention.

[0043] All raw materials of the present invention are not particularly limited in their sources and can be purchased from the market or prepared according to conventional methods well known to those skilled in the art.

[0044] There is no particular limitation on the purity of all raw materials in the present invention. The present invention preferably uses industrial purity or conventional purity used in the field of NdFeB magnets.

[0045] The present invention provides the use of an acylphenoxycarboxylic acid compound represented by the structure of formula (I) as an extractant in the process of recovering rare earth elements from rare earth molten salt slag;

[0046]

[0047] wherein R1 and R2 are each independently selected from a C1 to C9 alkyl group;

[0048] n is a natural number selected from 1 to 5.

[0049] In the present invention, R1 and R2 are each independently selected from C1 to C9 alkyl groups, or selected from C2 to C8 alkyl groups, or selected from C3 to C7 alkyl groups, or selected from C4 to C6 alkyl groups.

[0050] In the present invention, n is selected from a natural number of 1 to 5, specifically 1, 2, 3, 4 or 5.

[0051] In the present invention, the rare earth molten salt slag preferably contains one or more of lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium and yttrium, more preferably lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium or yttrium.

[0052] In the present invention, the extractant preferably includes a supported extractant.

[0053] In the present invention, the application preferably includes the combined application of an extractant and a base.

[0054] In the present invention, the combined application is preferably that the extractant and the alkali are used simultaneously.

[0055] In the present invention, the mass ratio of the extractant to the base is preferably (0.01-1):(0.001-0.1), more preferably (0.1-0.7):(0.001-0.1), more preferably (0.2-0.4):(0.001-0.1), more preferably (0.01-1):(0.01-0.07), and more preferably (0.01-1):(0.03-0.04).

[0056] The present invention provides a method for recovering rare earth elements from rare earth molten salt slag, which is characterized by comprising the following steps:

[0057] 1) mixing rare earth molten salt slag with sodium hydroxide and roasting to obtain alkaline slag;

[0058] 2) washing the alkali residue obtained in the above step in water to obtain a washing residue and an aqueous phase;

[0059] 3) mixing the washing residue obtained in the above step with a mineral acid again to obtain an acid leaching solution;

[0060] 4) mixing the acid leaching solution obtained in the above step, the acylphenoxycarboxylic acid extractant having the structure represented by formula (I), and liquid alkali again and extracting to obtain a loaded extractant;

[0061]

[0062] wherein R1 and R2 are each independently selected from a C1 to C9 alkyl group;

[0063] n is a natural number selected from 1 to 5;

[0064] 5) The loaded extractant obtained in the above step is further mixed with a mineral acid to obtain a rare earth element solution.

[0065] The method firstly mixes rare earth molten salt slag with sodium hydroxide and then roasts the mixture to obtain alkaline slag.

[0066] In the present invention, the content of rare earth elements in the rare earth molten salt slag, calculated as the mass content of rare earth oxides, REO is preferably greater than or equal to 38%, and may be greater than or equal to 30%, or greater than or equal to 20%.

[0067] In the present invention, in addition to rare earth elements, the rare earth molten salt slag preferably includes one or more of Ca, O, F, C, Al, Fe, W, Mo and Si, and more preferably includes multiple of Ca, O, F, C, Al, Fe, W, Mo and Si.

[0068] In the present invention, the mass ratio of the rare earth molten salt slag to sodium hydroxide is preferably 1:(0.1-0.5), more preferably 1:(0.15-0.45), more preferably 1:(0.2-0.4), and more preferably 1:(0.25-0.35).

[0069] In the present invention, the calcination temperature is preferably 300-900°C, more preferably 400-800°C, and even more preferably 500-700°C.

[0070] In the present invention, the calcination time is preferably 0.5 to 3.0 hours, more preferably 1.0 to 2.5 hours, and even more preferably 1.5 to 2.0 hours.

[0071] The present invention further washes the alkali residue obtained in the above steps in water to obtain washed residue and water phase.

[0072] In the present invention, the mass ratio of the alkali residue to water is preferably 1:(1-20), more preferably 1:(5-16), and even more preferably 1:(9-12).

[0073] In the present invention, the aqueous phase preferably includes one or more of calcium, fluorine and liquid alkali impurities, more preferably calcium, fluorine or liquid alkali impurities.

[0074] The present invention then mixes the washing residue obtained in the above steps with mineral acid again to obtain an acid leaching solution.

[0075] In the present invention, the mineral acid preferably includes one or more of sulfuric acid, nitric acid and hydrochloric acid, more preferably sulfuric acid, nitric acid or hydrochloric acid.

[0076] In the present invention, the concentration of the mineral acid in step 3) is preferably 0.05 to 1 mol / L, more preferably 0.1 to 0.7 mol / L, and even more preferably 0.2 to 0.4 mol / L.

[0077] In the present invention, the mass ratio of the washing residue to the mineral acid is preferably 1:(0.5-200), more preferably 1:(5-150), and even more preferably 1:(50-100).

[0078] The present invention then mixes the acid leaching solution obtained in the above step, the acylphenoxycarboxylic acid extractant having the structure represented by formula (I) and liquid alkali again and extracts to obtain a loaded extractant;

[0079]

[0080] wherein R1 and R2 are each independently selected from a C1 to C9 alkyl group;

[0081] n is a natural number selected from 1 to 5;

[0082] In the present invention, the mass ratio of the acid extract to the acylphenoxycarboxylic acid extractant of the structure represented by formula (I) is preferably 1:(0.01-1), more preferably 1:(0.1-0.7), and even more preferably 1:(0.2-0.4).

[0083] In the present invention, the mass ratio of the acid leaching solution to the liquid alkali is preferably 1:(0.001-0.1), more preferably 1:(0.01-0.07), and even more preferably 1:(0.02-0.04).

[0084] In the present invention, the liquid alkali preferably includes one or more of ammonia water, sodium hydroxide and potassium hydroxide, more preferably ammonia water, sodium hydroxide or potassium hydroxide.

[0085] In the present invention, the concentration of the liquid alkali is preferably 0.5 to 12 mol / L, more preferably 1 to 8 mol / L, and even more preferably 2 to 4 mol / L.

[0086] In the present invention, the extraction time is preferably 5 to 10 days, more preferably 6 to 9 days, and even more preferably 7 to 8 days.

[0087] Finally, the present invention further mixes the loaded extractant obtained in the above steps with a mineral acid to obtain a rare earth element solution.

[0088] In the present invention, the mass ratio of the loaded extractant to the mineral acid is preferably 1:(0.1-200), more preferably 1:(1-150), and even more preferably 1:(50-100).

[0089] In the present invention, the concentration of the mineral acid in step 5) is preferably 0.05 to 1 mol / L, more preferably 0.1 to 0.7 mol / L, and even more preferably 0.2 to 0.4 mol / L.

[0090] The present invention is to complete and refine the overall technical solution, better ensure the stability and controllability of the process of recovering rare earth elements from rare earth molten salt slag, and further improve the recovery rate. The above method for recovering rare earth elements from rare earth molten salt slag can specifically include the following steps:

[0091] A method for recovering rare earth elements from rare earth molten salt slag comprises the following steps:

[0092] (1) Mixing rare earth molten salt slag and sodium hydroxide in a weight ratio of 1:(0.1-0.5), and calcining at 300-900 degrees Celsius for 0.5-3.0 hours to obtain alkaline slag.

[0093] (2) Alkali residue is mixed with water in a weight ratio of 1: (1-20) to obtain washing residue and water phase. Impurities such as calcium, fluoride and liquid caustic soda enter the water phase, while rare earth elements remain in the washing residue.

[0094] (3) Mixing the washing residue with a mineral acid in a weight ratio of 1:(0.5-200) to obtain an acid leaching solution.

[0095] (4) mixing the acid leaching solution, the acylphenoxycarboxylic acid extractant represented by formula (I), and liquid alkali in a weight ratio of 1:(0.01-1):(0.001-0.1), and performing extraction to obtain a loaded extractant;

[0096] (5) The loaded extractant and the mineral acid are mixed in a weight ratio of 1:(0.1-200) to obtain high-purity rare earth elements.

[0097]

[0098] In formula (I), R1 and R2 are independently selected from C1-C9 alkyl groups, and n is selected from natural numbers of 1-5.

[0099] Specifically, the rare earth molten salt slag contains one or more rare earth elements selected from the group consisting of lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium and yttrium.

[0100] Specifically, the mineral acid is selected from one or more of sulfuric acid, nitric acid and hydrochloric acid, and the total concentration of the mineral acid is 0.05 to 1 mol / L.

[0101] Specifically, the liquid alkali is selected from one or more of ammonia water, sodium hydroxide and potassium hydroxide, and the total concentration of the liquid alkali is 0.5 to 12 mol / L.

[0102] Furthermore, the compound is selected from the structures represented by formula (I-1) and / or formula (I-2);

[0103]

[0104]

[0105] The present invention provides the use of an acylphenoxycarboxylic acid compound having the structure of formula (I) as an extractant in the process of recovering rare earth elements from rare earth molten salt slag, as well as a method for recovering rare earth elements from rare earth molten salt slag. The present invention specifically uses the acylphenoxycarboxylic acid compound as an extractant in the process of recovering rare earth elements from rare earth molten salt slag, and also provides a corresponding method for recovering rare earth elements from rare earth molten salt slag. The recovery method provided by the present invention has an excellent recovery rate, a simple process, mild conditions, and good controllability, making it more suitable for promotion and application in large-scale industrial production.

[0106] Experimental results show that the method for recovering rare earth elements from low-grade rare earth molten salt slag provided by the present invention has a yield greater than 92%, which is greater than 80% of the industry average.

[0107] To further illustrate the present invention, the following examples describe in detail the use of the acylphenoxycarboxylic acid compounds provided by the present invention as an extractant in the process of recovering rare earth elements from rare earth molten salt slag, as well as a method for recovering rare earth elements from rare earth molten salt slag. However, it should be understood that these embodiments are implemented on the premise of the technical solution of the present invention, and detailed implementation methods and specific operating procedures are provided only to further illustrate the features and advantages of the present invention, rather than to limit the claims of the present invention. The scope of protection of the present invention is not limited to the following examples.

[0108] Example 1

[0109] The chemical structure of the acylphenoxycarboxylic acid extractant is shown in formula (I-1), that is, in formula (I), R1 and R2 are independently selected from C9 and C1 alkyl groups, and n=1.

[0110] The source of acylphenoxycarboxylic acid is self-synthesized. The synthesis method is as follows: (1) Add 100 mL of petroleum ether to a 250 mL single-necked flask, then add 0.1 mol of nonylphenoxyacetic acid, 0.1 mol of octanoyl chloride and 0.1 mol of anhydrous aluminum chloride, and react at 50 degrees Celsius for 4 hours. Stop the reaction and cool to room temperature. (2) Slowly add 100 mL of water to the single-necked flask to obtain an oil phase and an aqueous phase. (3) The oil phase is subjected to rotary evaporation at 75 degrees Celsius to remove the solvent, and vacuum distilled at 120 degrees Celsius to obtain the acylphenoxycarboxylic acid extractant product. After acid-base titration and nuclear magnetic resonance detection, the yield reached 95% and the purity reached 98.5%.

[0111] The rare earth molten salt slag is obtained from dysprosium-calcium slag, and its main element composition is shown in Table 1. Table 1 shows the main element composition of the rare earth molten salt slag in Example 1 of the present invention.

[0112] Table 1

[0113] Ca O F C Of 22.1％ 34.3％ 26.0％ 17.0％ 0.60％

[0114] A method for recovering rare earth elements from rare earth molten salt slag comprises the following steps:

[0115] (1) Take 250 g of dysprosium calcium slag, add 25 g of sodium hydroxide solid, mix well, and roast at 300 degrees Celsius for 0.5 hours to obtain 60 g of alkaline slag.

[0116] (2) 60 g of alkali residue was mixed with water in a weight ratio of 1:10 to obtain 30 g of washing residue and an aqueous phase. Impurities such as calcium, fluoride, and liquid caustic soda entered the aqueous phase, while rare earth elements remained in the washing residue.

[0117] (3) 30 g of the washing residue was mixed with a mineral acid in a weight ratio of 1:200, and filtered to obtain 600 g of an acid leaching solution. The mineral acid was obtained from hydrochloric acid with a total concentration of 0.05 mol / L.

[0118] (4) 600 g of the acid extract, 12 g of the acylphenoxycarboxylic acid extractant represented by formula (I-1), and 2.56 g of a 10.8 mol / L sodium hydroxide solution were mixed and extracted to obtain 15 g of a loaded extractant.

[0119] (5) 15 g of the loaded extractant was mixed with 150 g of 0.20 mol / L hydrochloric acid to obtain a high-purity rare earth element solution. The contents of calcium and fluorine impurities were 50 ppm and 2 ppm, respectively.

[0120] Through this method, rare earth elements are efficiently recovered, with a recovery rate of 93.5%.

[0121] Example 2

[0122] The chemical structure of the acylphenoxycarboxylic acid extractant is shown in formula (I-2), that is, in formula (I), R1 and R2 are independently selected from C8 and C2 alkyl groups, and n=1.

[0123] The source of acylphenoxycarboxylic acid is self-synthesized. The synthesis method is as follows: (1) Add 100 mL of petroleum ether to a 250 mL single-necked flask, then add 0.1 mol of octylphenoxypropionic acid, 0.1 mol of octyl chloride and 0.1 mol of anhydrous aluminum trichloride, and react at 75 degrees Celsius for 2 hours. Stop the reaction and cool to room temperature. (2) Slowly add 200 mL of water to the single-necked flask to obtain an oil phase and an aqueous phase. (3) The oil phase is subjected to rotary evaporation at 75 degrees Celsius to remove the solvent, and vacuum distilled at 120 degrees Celsius to obtain the acylphenoxycarboxylic acid extractant product. After acid-base titration and nuclear magnetic resonance detection, the yield reached 94% and the purity reached 96.5%.

[0124] Rare earth molten salt slag is obtained from terbium calcium slag, and its main element composition is shown in the following table:

[0125] The rare earth molten salt slag is obtained from terbium calcium slag, and its main element composition is shown in Table 2. Table 2 shows the main element composition of the rare earth molten salt slag in Example 2 of the present invention.

[0126] Table 2

[0127] Ca O F C Tb 43.9％ 27.4％ 15.5％ 12.7％ 0.39％

[0128] A method for recovering rare earth elements from rare earth molten salt slag comprises the following steps:

[0129] (1) Take 250 g of terbium calcium slag, add 25 g of sodium hydroxide solid, mix well, and roast at 300 degrees Celsius for 0.5 hours to obtain 50 g of alkaline slag.

[0130] (2) 50 g of alkali residue was mixed with water in a weight ratio of 1:10 to obtain 20 g of washing residue and an aqueous phase. Impurities such as calcium, fluoride, and liquid caustic soda entered the aqueous phase, while rare earth elements remained in the washing residue.

[0131] (3) 20 g of the washing residue was mixed with a mineral acid in a weight ratio of 1:18, and filtered to obtain 360 g of an acid leaching solution. The mineral acid was obtained from hydrochloric acid with a total concentration of 0.05 mol / L.

[0132] (4) 360 g of the acid extract, 8 g of the acylphenoxycarboxylic acid extractant represented by formula (I-2), and 18.4 g of a 1.0 mol / L sodium hydroxide solution were mixed and extracted to obtain 10 g of a loaded extractant.

[0133] (5) 10 g of the loaded extractant was mixed with 70 g of 0.28 mol / L hydrochloric acid to obtain a high-purity rare earth element solution. The contents of calcium and fluorine impurities were 40 ppm and 1.5 ppm, respectively.

[0134] Through this method, rare earth elements are efficiently recovered, with a recovery rate of 92.1%.

[0135] The above describes in detail the use of the acylphenoxycarboxylic acid compound represented by the structure of formula (I) provided by the present invention as an extractant in the process of recovering rare earth elements from rare earth molten salt slag, and a method for recovering rare earth elements from rare earth molten salt slag. Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only intended to help understand the method and core concept of the present invention, including the best mode, and also to enable any person skilled in the art to practice the present invention, including making and using any device or system, and implementing any combined method. It should be noted that for those skilled in the art, without departing from the principles of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention. The scope of patent protection of the present invention is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements that are not different from the literal wording of the claims, or if they include equivalent structural elements that are not substantially different from the literal wording of the claims, then these other embodiments should also be included in the scope of the claims.

Claims

1. Use of an acylphenoxycarboxylic acid compound represented by formula (I) as an extractant in the process of recovering rare earth elements from rare earth molten salt slag; in, R1 and R2 are each independently selected from a C1 to C9 alkyl group; n is a natural number selected from 1 to 5.

2. The use according to claim 1, characterized in that The rare earth molten salt slag contains one or more of lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, scandium and yttrium; The extractant includes a supported extractant; The application includes the coordinated application of the extractant and the base.

3. The use according to claim 2, characterized in that The combined application specifically includes using the extractant and the alkali simultaneously; The mass ratio of the extractant to the alkali is (0.01-1): (0.001-0.1).

4. A method for recovering rare earth elements from rare earth molten salt slag, characterized in that: The following steps are involved: 1) mixing rare earth molten salt slag with sodium hydroxide and roasting to obtain alkaline slag; 2) washing the alkali residue obtained in the above step in water to obtain a washing residue and an aqueous phase; 3) mixing the washing residue obtained in the above step with a mineral acid again to obtain an acid leaching solution; 4) mixing the acid leaching solution obtained in the above step, the acylphenoxycarboxylic acid extractant having the structure represented by formula (I), and liquid alkali again and extracting to obtain a loaded extractant; wherein R1 and R2 are each independently selected from a C1 to C9 alkyl group; n is a natural number selected from 1 to 5; 5) The loaded extractant obtained in the above step is further mixed with a mineral acid to obtain a rare earth element solution.

5. The method according to claim 4, characterized in that The content of rare earth elements in the rare earth molten salt slag, calculated as the mass content of rare earth oxides, REO is greater than or equal to 38%; The rare earth molten salt slag includes, in addition to rare earth elements, one or more of Ca, O, F, C, Al, Fe, W, Mo and Si; The mass ratio of the rare earth molten salt slag to sodium hydroxide is 1:(0.1-0.5).

6. The method according to claim 4, characterized in that The calcination temperature is 300-900°C; The calcination time is 0.5 to 3.0 hours; The mass ratio of the alkali residue to water is 1:(1-20).

7. The method according to claim 4, characterized in that The aqueous phase includes one or more of calcium, fluorine and liquid alkali impurities; The mineral acid includes one or more of sulfuric acid, nitric acid and hydrochloric acid; The concentration of the mineral acid in step 3) is 0.05-1 mol / L.

8. The method according to claim 4, characterized in that The mass ratio of the washing residue to the mineral acid is 1: (0.5-200); The mass ratio of the acid extract to the acylphenoxycarboxylic acid extractant having the structure represented by formula (I) is 1:(0.01-1); The mass ratio of the acid leaching solution to the liquid alkali is 1:(0.001-0.1).

9. The method according to claim 4, characterized in that The liquid alkali comprises one or more of ammonia water, sodium hydroxide and potassium hydroxide; The concentration of the liquid caustic soda is 0.5 to 12 mol / L; The extraction time is 5 to 10 days.

10. The method according to claim 4, characterized in that The mass ratio of the loaded extractant to the mineral acid is 1:(0.1-200); The concentration of the mineral acid in step 5) is 0.05 to 1 mol / L.

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