Preparation of ion imprinted polymer adsorbent and application of ion imprinted polymer adsorbent in preparation of battery-grade lithium dihydrogen phosphate

Abstract

The invention discloses preparation of an ion imprinted polymer adsorbent and application of the ion imprinted polymer adsorbent in preparation of battery-grade lithium dihydrogen phosphate. The method comprises the following steps: firstly, preparing Fe3O4 coated SiO2 by taking nano Fe3O4 as a magnetic core and tetraethoxysilane, and then carrying out imprinting polymerization on lithium dihydrogen phosphate as a template and a mixed functional monomer under the action of a cross-linking agent to obtain a crude product; and finally, eluting the template to obtain the magnetic ion imprinted polymer adsorbent. The adsorbent can be used for specifically adsorbing lithium dihydrogen phosphate ions from low-concentration lithium, high-impurity brine or a lithium chloride solution, the application process of the adsorbent comprises the steps of adsorption, desorption, evaporation concentration, step-by-step cooling crystallization and drying, battery-grade lithium dihydrogen phosphate with the purity larger than or equal to 99.5% is finally obtained, and the lithium comprehensive yield reaches 90% or above. The process provided by the invention does not need to remove impurities in advance, has high selectivity, can recycle the adsorbent, has the characteristics of simple flow, low energy consumption, environmental friendliness and suitability for large-scale production, and has remarkable environmental protection and economic benefits.

Description

Technical Field

[0001] This invention belongs to the field of materials synthesis technology, specifically relating to the preparation of an ion-imprinted polymer adsorbent and its application in the preparation of battery-grade lithium dihydrogen phosphate. Background Technology

[0002] Lithium dihydrogen phosphate (LiH2PO4) is an important inorganic compound. Besides its use as an analytical reagent and catalyst, it has become a key raw material for lithium iron phosphate (LiFePO4), a lithium-ion battery material, driven by the development of the new energy industry. Its purity directly determines the performance of lithium battery electrodes; low purity can easily lead to abnormal discharge, short circuits, or even spontaneous combustion or explosion. Therefore, the preparation of battery-grade lithium dihydrogen phosphate is a core requirement of the new energy industry.

[0003] Traditional domestic production processes generally involve reacting lithium carbonate or lithium hydroxide with phosphoric acid, followed by evaporation, concentration, and cooling crystallization to obtain the product, which is mainly divided into three categories: 1. The neutralization method involves the reaction of lithium carbonate / lithium hydroxide with phosphoric acid to produce the product. Precise control of temperature and pH is required to ensure purity and yield. 2. The metathesis method involves reacting phosphoric acid solution with lithium hydroxide solution, which also requires strict reaction conditions; 3. Extraction methods use organic reagents such as methanol and ethanol for purification, but they have poor safety and cannot meet the requirements for large-scale industrial production.

[0004] Chinese invention patent CN116143091A discloses a method for producing battery-grade lithium dihydrogen phosphate from lithium chloride brine. This method still follows the traditional approach of "removing impurities first, then precipitation, and finally acid hydrolysis," failing to overcome the limitations of multi-step reactions, leading to increased process complexity and difficulty in impurity control. Furthermore, the multiple separation and purification steps required to achieve battery-grade purity inevitably reduce production efficiency and increase energy consumption.

[0005] Currently, lithium-ion adsorbents show great promise for lithium extraction from salt lakes and seawater, but they still face a series of drawbacks and challenges in practical applications. The following is a detailed analysis of the main drawbacks: 1. Limited adsorption capacity Most adsorbents have low theoretical adsorption capacities, making it difficult to meet the demands of high-concentration or large-scale lithium extraction. In practical applications, the capacity may be further reduced due to structural collapse and a decrease in active sites.

[0006] 2. Poor selectivity Mg 2+ With Li + Due to their similar properties, the adsorbents compete fiercely for adsorption, leading to a decrease in lithium separation efficiency. Some adsorbents require pretreatment or pH adjustment to enhance selectivity, increasing costs.

[0007] 3. Slow dynamic performance Some adsorbents (such as certain organic framework materials) have slow diffusion rates and long adsorption-desorption cycles, which affect treatment efficiency.

[0008] In summary, existing methods have shortcomings in terms of cost, safety, product quality, and process control. They also have low extraction efficiency for high magnesium-lithium ratio brines, which cannot meet the needs of large-scale commercialization of batteries. Therefore, developing a new low-cost, battery-grade, and easily controllable method for preparing lithium dihydrogen phosphate is an urgent need in the field of battery materials. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of existing lithium dihydrogen phosphate (LiH2PO4) preparation processes and provide a method for preparing ion-imprinted polymer adsorbents and their application in the preparation of battery-grade LiH2PO4. This invention utilizes an adsorbent to prepare battery-grade LiH2PO4. This process achieves specific adsorption of low-concentration lithium and high-concentration impurities in brine without the need for impurity removal, yielding battery-grade LiH2PO4 (purity ≥ 99.5%) with a comprehensive lithium yield exceeding 90%. This invention's process is characterized by low energy consumption, safety, environmental friendliness, and suitability for large-scale industrial production, possessing significant environmental and economic value.

[0010] To achieve the above objectives, the technical solution designed by the present invention is as follows: This invention provides a method for preparing an ion-imprinted polymer adsorbent, comprising the following steps: 1) Preparation of Fe3O4@SiO2: Weigh nano-Fe3O4 and tetraethyl orthosilicate (TEOS) at a weight ratio of 1:3~10; dissolve nano-Fe3O4 in an ethanol solution (ethanol is used as a solvent to dissolve TEOS and hydrolysis intermediates, allowing the reaction to proceed slowly and controllably) and mix thoroughly, then add tetraethyl orthosilicate; adjust the pH value to 9.5-10.2 (ammonia is used as an adjuster), stir the reaction at room temperature, wash several times with ethanol and water, and vacuum dry to obtain the reaction product Fe3O4@SiO2; 2) Preparation of crude Fe3O4@SiO2@IIPs products: First, weigh out hexadecyltrimethylammonium bromide (porogen CTAB), 1,2-bis(triethoxysilyl)ethane (crosslinking agent BTESE), mixed functional monomers, lithium dihydrogen phosphate (template molecule), and the aforementioned Fe3O4@SiO2 (magnetic carrier). Then, dissolve hexadecyltrimethylammonium bromide in an ethanol solution, and then add water, 1,2-bis(triethoxysilyl)ethane, mixed functional monomers, and lithium dihydrogen phosphate in sequence. Stir until homogeneous at room temperature, then add Fe3O4@SiO2. Continue stirring until homogeneous at room temperature, wash repeatedly with ethanol and water, and vacuum dry to obtain the crude product for later use. The weight ratio of Fe3O4@SiO2, hexadecyltrimethylammonium bromide, 1,2-bis(triethoxysilyl)ethane (crosslinking agent BTESE), mixed functional monomers, and lithium dihydrogen phosphate is 1:1~1.1:0.5~0.7:0.34~0.66:0.052~0.166; and the mixed functional monomers include acrylamide and functional monomers. 3) Preparation of Fe3O4@SiO2@IIPs: The crude product was washed multiple times with a methanol solution containing acetic acid (to remove the template lithium dihydrogen phosphate and CTAB), and then dried under vacuum to obtain the magnetic-mixed functional monomer-lithium dihydrogen phosphate ion-imprinted polymer adsorbent (Fe3O4@SiO2@IIPs, abbreviated as IIPs).

[0011] Furthermore, in step 1), the mass fraction of the ethanol solution is 85-95%; the particle size of the nano Fe3O4 is 1-100 nm.

[0012] The key advantages of the aforementioned nano-Fe3O4 are reflected in: ① Superparamagnetic: It can respond magnetically quickly under an applied magnetic field, and there is no residual magnetism after the magnetic field is removed. It does not agglomerate, making it easy to disperse and recover in fluids, which is crucial for continuous processes.

[0013] ② High specific surface area: The nanoparticles have an extremely high atomic ratio on their surface, which can load more functional monomers (such as ion-imprinted layers), greatly improving adsorption capacity and efficiency.

[0014] ③ Surface modifiability: Nanoparticles have high surface activity and are easy to chemically modify through silanization, polymer encapsulation, etc., to achieve a strong bond with the functional layer, which is the basis for constructing a stable core-shell structure.

[0015] Furthermore, in step 2), the molar ratio of acrylamide to functional monomer is 1:1; The functional monomer is any one of benzo-12-crown-4, 14-crown-4 and dibenzo-14-crown-4.

[0016] Furthermore, in step 2), the vacuum drying temperature is 70°C.

[0017] Furthermore, in step 3), the mass fraction of acetic acid in the methanol solution is 2%.

[0018] The present invention also provides an application of the above-mentioned preparation method for the ion-imprinted polymer adsorbent in the adsorption of metal ions.

[0019] This invention also provides a process for preparing battery-grade lithium dihydrogen phosphate using lithium chloride as the lithium source, comprising the following steps: a. Preparation of lithium dihydrogen phosphate mixed solution Under stirring conditions, lithium chloride solution was added dropwise to ammonium dihydrogen phosphate solution. After the addition was completed, the reaction continued for 1-2 hours to obtain a mixed lithium dihydrogen phosphate solution. b. Adsorption The above preparation method is used to add the lithium dihydrogen phosphate mixed solution to obtain the ion-imprinted polymer adsorbent. The pH value is adjusted to 4.5-5, and the reaction is stirred at room temperature. After the reaction is completed, a magnet is added (to separate the ammonium chloride and lithium dihydrogen phosphate solutions, with an adsorption efficiency of up to 95%). c. Desorption After the ion-imprinted polymer is saturated with adsorption, it can be eluted with deionized water to obtain a purified lithium dihydrogen phosphate solution (the ion-imprinted polymer can be reused repeatedly; the single desorption efficiency is 98%, and after 10 "adsorption-desorption" cycles, the saturated adsorption capacity of the adsorbent for lithium dihydrogen phosphate still remains above 93% of the initial capacity, with a desorption efficiency > 95%). d. Evaporation and concentration The lithium dihydrogen phosphate solution is evaporated and concentrated until an oil film forms on the surface of the solution, at which point evaporation can be stopped; thus, a concentrated lithium dihydrogen phosphate solution is obtained. e. Stepwise cooling crystallization (controlling crystal formation) Before the lithium dihydrogen phosphate is concentrated and cooled, lithium dihydrogen phosphate seed crystals are added and cooled in steps. After cooling, the product is filtered and separated to obtain the wet lithium dihydrogen phosphate product. f. Drying The wet lithium dihydrogen phosphate product is dried to obtain battery-grade lithium dihydrogen phosphate.

[0020] Further, in step a, the lithium concentration in the lithium chloride solution is 0.05 g / L-1 g / L (this lithium chloride solution can be brine (such as underground brine, geothermal brine, oil and gas field brine) or other waste liquid containing lithium chloride). The stirring rate is 100-200 r / min, and the lithium chloride solution is added at a rate of 300-400 mL / h.

[0021] Furthermore, in step d, the evaporation and concentration temperature is 150-200℃.

[0022] Furthermore, in step e, the first cooling involves reducing the temperature from the concentration temperature to 80-90°C at a rate of 2.0°C / min. Second cooling: The temperature is reduced from 80-90℃ to 25-30℃ at a cooling rate of 2.0°C / min; In step f, the drying temperature is 100°C.

[0023] The beneficial effects of this invention are: ① The ion-imprinted polymer adsorbent of this invention can specifically adsorb lithium ions and dihydrogen phosphate ions, and can treat solutions with low lithium concentration and high levels of other impurity ions. Brine raw materials often have a high magnesium content, and magnesium is difficult to separate from lithium. This method makes separating magnesium and lithium simpler and more convenient. Specific adsorption combines the separation of lithium dihydrogen phosphate from ammonium chloride solution and the separation (purification) of lithium dihydrogen phosphate from impurities such as calcium, magnesium, and aluminum into one process.

[0024] ② This invention enables efficient and selective recovery of lithium from complex solutions containing low-concentration lithium (0.05-1.0 g / L) and high-concentration impurities, and can prepare battery-grade lithium dihydrogen phosphate products without the need for impurity removal steps, with a yield of over 90%.

[0025] ③ This invention utilizes ion imprinting technology to separate ammonium chloride and lithium dihydrogen phosphate solutions. Magnetic nanoparticles are etched onto the substrate, and magnetic ammonium chloride ion-imprinted polymers (IIPs) with dual-site synergistic recognition functional monomers are designed using electrostatic interactions and specific adsorption between ions. This ensures that the effective adsorption capacity ratio of the adsorbent for lithium and dihydrogen phosphate is close to their stoichiometric ratio, thereby achieving the separation of ammonium chloride and lithium dihydrogen phosphate solutions. The pH of the reaction solution is controlled between 4.5 and 5, at which point the amino groups attract H₂PO₄ through electrostatic attraction. - This facilitates the subsequent desorption process without introducing other impurity ions. Crown ether / cryptoether functional monomers can specifically adsorb lithium. This technology achieves the specific adsorption of lithium dihydrogen phosphate ions and combines the separation and purification of lithium dihydrogen phosphate solution into one process, eliminating the need for subsequent impurity removal. The process is simple, and the magnetic materials facilitate recovery and continuous operation.

[0026] ④ In this invention, evaporation and concentration are carried out at a temperature of 150-200℃, which causes excess ammonium dihydrogen phosphate in the solution to decompose into phosphoric acid and ammonia. The phosphoric acid remaining in the system after decomposition reacts with some of the lithium dihydrogen phosphate, which can inhibit the hydrolysis of lithium dihydrogen phosphate during heating and help maintain the stability of the system. Then, lowering the evaporation temperature to adjust the pH of the solution can remove trace metal impurities in the solution, all of which help to improve the purity of the product.

[0027] ⑤ This invention utilizes the characteristic that the solubility of lithium dihydrogen phosphate decreases significantly with decreasing temperature to perform stepwise cooling crystallization. This avoids the formation of fine crystals encapsulating impurities due to excessive saturation, thus increasing the product purity to over 99.2%. Furthermore, by adding a certain amount of lithium dihydrogen phosphate seed crystals before cooling crystallization, the solute in the solution is guided to grow orderly on the seed crystal surface, rather than spontaneously forming new crystal nuclei. This reduces the formation of fine crystals and impurity encapsulation, increasing the purity of the lithium dihydrogen phosphate product from 99.2% to over 99.5%, with a more uniform crystal size distribution. In addition, the larger crystals facilitate subsequent filtration and washing, reducing product loss. Simultaneously, by fully utilizing the characteristic that its solubility decreases significantly with decreasing temperature, lithium residue in the mother liquor is reduced, increasing the single-step lithium yield to over 98%, balancing purity and economy.

[0028] ⑥ In this invention, because ammonium chloride has a low decomposition temperature, residual ammonium chloride adhering to the surface can be decomposed during the drying process of battery-grade lithium dihydrogen phosphate wet material, so as to ensure the purity of the product. Detailed Implementation

[0029] The present invention will now be described in further detail with reference to specific embodiments, so that those skilled in the art can understand it.

[0030] Example 1 I. A method for preparing ion-imprinted polymer adsorbent 1, comprising the following steps: 1) Preparation of Fe3O4@SiO2: 1.2 g of nano Fe3O4 (particle size 1~100 nm) was added to 130 g of ethanol solution (mass fraction 85~95%) and mixed evenly. Then 7.5 g of tetraethyl orthosilicate was added. Ammonia water was added to adjust the pH value to 9.5-10.2. The reaction was stirred at room temperature, and the product was washed several times with ethanol and water and dried under vacuum to obtain the reaction product. 2) Preparation of crude Fe3O4@SiO2@IIPs products: 0.54 g of hexadecyltrimethylammonium bromide was dissolved in 30 g of ethanol solution with a volume fraction of 30-35%. Then, 100 mL of ultrapure water and 0.35 g of 1,2-bis(triethoxysilyl)ethane (crosslinking agent BTESE) were added sequentially. 0.33 g of acrylamide and benzo-12-crown-4 mixed functional monomers (molar ratio = 1:1) and 0.08 g of lithium dihydrogen phosphate were added sequentially to a three-necked flask and stirred at room temperature for 1 h. Then, 0.5 g of Fe3O4@SiO2 was added and stirred at room temperature for 6 h. The product was washed three times with ethanol and water, and dried in a vacuum drying oven at 70 °C to obtain the crude Fe3O4@SiO2@IIPs product for later use. 3) Preparation of Fe3O4@SiO2@IIPs (IIPs): The crude Fe3O4@SiO2@IIPs product was repeatedly washed with a methanol solution containing 2% acetic acid to remove the template lithium dihydrogen phosphate and CTAB. The product was then placed in a vacuum drying oven and dried at 70°C to obtain ion-imprinted polymer adsorbent 1 (Fe3O4@SiO2@IIPs, abbreviated as IIPs) of magnetic-mixed functional monomer-lithium dihydrogen phosphate, for later use. II. The process for preparing battery-grade lithium dihydrogen phosphate includes the following steps: a. Preparation of lithium dihydrogen phosphate mixed solution Under a stirring rate of 100-200 r / min, lithium chloride solution (lithium concentration 0.5 g / L) was added dropwise to ammonium dihydrogen phosphate solution (concentration 17 g / L) at a dropping rate of 300-400 mL / h. After the addition was completed, the reaction was continued for 2 h to obtain a mixed lithium dihydrogen phosphate solution; wherein the molar ratio of lithium chloride to ammonium dihydrogen phosphate was 1:1.1. Table 1 Main Components of Lithium Chloride Raw Material b. Adsorption: Add 32g of the above-mentioned ion-imprinted polymer adsorbent 1 to 100mL of lithium dihydrogen phosphate mixed solution, then add ammonia water to adjust the pH of the solution to 5, and stir the reaction at room temperature for 2h; after the reaction is completed, add a magnet to achieve the separation of ammonium chloride and lithium dihydrogen phosphate solution. c. Desorption: After the ion-imprinted polymer is saturated with adsorption, it is eluted with 100 mL of deionized water to obtain a purified lithium dihydrogen phosphate solution. d. Evaporation and concentration The lithium dihydrogen phosphate solution was evaporated and concentrated at a temperature of 200°C. Evaporation was stopped when an oil film appeared on the surface of the solution. e. Stepwise cooling crystallization (controlling crystal formation) Before cooling and crystallizing the purified lithium dihydrogen phosphate solution, lithium dihydrogen phosphate seed crystals are added, and the solution is cooled in stages. After cooling, the mixture is filtered and separated to obtain the wet lithium dihydrogen phosphate product. First cooling: The temperature is reduced from the concentration temperature to 80-90℃ at a rate of 2.0°C / min. Second cooling: The temperature is reduced from 80-90℃ to 25-30℃ at a cooling rate of 2.0°C / min; f. Dry the wet lithium dihydrogen phosphate product at a temperature of 100°C to obtain battery-grade lithium dihydrogen phosphate product 1.

[0031] Example 2 I. A method for preparing ion-imprinted polymer adsorbent 2, comprising the following steps: 1) Preparation of Fe3O4@SiO2: 1.2 g of nano Fe3O4 (particle size 1~100 nm) was added to 130 g of ethanol solution (mass fraction 85~95%) and mixed evenly. Then 7.5 g of tetraethyl orthosilicate was added. Ammonia water was added to adjust the pH value to 9.5-10.2. The reaction was stirred at room temperature, and the product was washed several times with ethanol and water and dried under vacuum to obtain the reaction product. 2) Preparation of crude Fe3O4@SiO2@IIPs products: 0.54 g of hexadecyltrimethylammonium bromide was dissolved in 30 g of ethanol solution with a volume fraction of 30-35%. Then, 100 mL of ultrapure water and 0.35 g of 1,2-bis(triethoxysilyl)ethane (crosslinking agent BTESE) were added sequentially. 0.33 g of acrylamide and benzo-12-crown-4 mixed functional monomers (molar ratio = 1:1) and 0.05 g of lithium dihydrogen phosphate were added sequentially to a three-necked flask and stirred at room temperature for 1 h. Then, 0.5 g of Fe3O4@SiO2 was added and stirred at room temperature for 6 h. The product was washed three times with ethanol and water, and dried in a vacuum drying oven at 70 °C to obtain the crude Fe3O4@SiO2@IIPs product for later use. 3) Preparation of Fe3O4@SiO2@IIPs (IIPs): The crude Fe3O4@SiO2@IIPs product was repeatedly washed with a methanol solution containing 2% acetic acid to remove the template lithium dihydrogen phosphate and CTAB. The product was then placed in a vacuum drying oven and dried at 70°C to obtain ion-imprinted polymer adsorbent 2 (Fe3O4@SiO2@IIPs, abbreviated as IIPs) of magnetic-mixed functional monomer-lithium dihydrogen phosphate, for later use. II. The process for preparing battery-grade lithium dihydrogen phosphate includes the following steps: a. Preparation of lithium dihydrogen phosphate mixed solution Under a stirring rate of 100-200 r / min, a lithium chloride solution (lithium concentration 0.5 g / L, main components as described in Example 1) was added dropwise to an ammonium dihydrogen phosphate solution (concentration 17 g / L) at a dropping rate of 300-400 mL / h. After the addition was completed, the reaction was continued for 2 h to obtain a mixed lithium dihydrogen phosphate solution; wherein the molar ratio of lithium chloride to ammonium dihydrogen phosphate was 1:1.1. b. Adsorption: Add 32g of the above-mentioned ion-imprinted polymer adsorbent 2 to 100mL of lithium dihydrogen phosphate mixed solution, then add ammonia water to adjust the pH of the solution to 5, and stir the reaction at room temperature for 2h; after the reaction is completed, add a magnet to achieve the separation of ammonium chloride and lithium dihydrogen phosphate solution. c. Desorption: After the ion-imprinted polymer is saturated with adsorption, it is eluted with 100 mL of deionized water to obtain a purified lithium dihydrogen phosphate solution. d. Evaporation and concentration The lithium dihydrogen phosphate solution was evaporated and concentrated at a temperature of 200°C. Evaporation was stopped when an oil film appeared on the surface of the solution. e. Stepwise cooling crystallization (controlling crystal formation) Before cooling and crystallizing the purified lithium dihydrogen phosphate solution, lithium dihydrogen phosphate seed crystals are added, and the solution is cooled in stages. After cooling, the mixture is filtered and separated to obtain the wet lithium dihydrogen phosphate product. First cooling: The temperature is reduced from the concentration temperature to 80-90℃ at a rate of 2.0°C / min. Second cooling: The temperature is reduced from 80-90℃ to 25-30℃ at a cooling rate of 2.0°C / min; f. Dry the wet lithium dihydrogen phosphate product at a temperature of 100°C to obtain battery-grade lithium dihydrogen phosphate product 2.

[0032] Example 3 I. A method for preparing ion-imprinted polymer adsorbent 3, comprising the following steps: 1) Preparation of Fe3O4@SiO2: 1.2 g of nano Fe3O4 (particle size 1~100 nm) was added to 130 g of ethanol solution (mass fraction 85~95%) and mixed evenly. Then 7.5 g of tetraethyl orthosilicate was added. Ammonia water was added to adjust the pH value to 9.5-10.2. The reaction was stirred at room temperature, and the product was washed several times with ethanol and water and dried under vacuum to obtain the reaction product. 2) Preparation of crude Fe3O4@SiO2@IIPs products: 0.54 g of hexadecyltrimethylammonium bromide was dissolved in 30 g of ethanol solution with a volume fraction of 30-35%. Then, 100 mL of ultrapure water and 0.35 g of 1,2-bis(triethoxysilyl)ethane (crosslinking agent BTESE) were added sequentially. 0.33 g of acrylamide and benzo-12-crown-4 mixed functional monomers (molar ratio = 1:1) and 0.08 g of lithium dihydrogen phosphate were added sequentially to a three-necked flask and stirred at room temperature for 1 h. Then, 0.5 g of Fe3O4@SiO2 was added and stirred at room temperature for 6 h. The product was washed three times with ethanol and water, and dried in a vacuum drying oven at 70 °C to obtain the crude Fe3O4@SiO2@IIPs product for later use. 3) Preparation of Fe3O4@SiO2@IIPs (IIPs): The crude Fe3O4@SiO2@IIPs product was repeatedly washed with a methanol solution containing 2% acetic acid to remove the template lithium dihydrogen phosphate and CTAB. The product was then placed in a vacuum drying oven and dried at 70°C to obtain ion-imprinted polymer adsorbent 3 (Fe3O4@SiO2@IIPs, abbreviated as IIPs) of magnetic-mixed functional monomer-lithium dihydrogen phosphate, for later use. II. The process for preparing battery-grade lithium dihydrogen phosphate includes the following steps: a. Preparation of lithium dihydrogen phosphate mixed solution Under a stirring rate of 100-200 r / min, a lithium chloride solution (lithium concentration 0.5 g / L, main components as described in Example 1) was added dropwise to an ammonium dihydrogen phosphate solution (concentration 17 g / L) at a dropping rate of 300-400 mL / h. After the addition was completed, the reaction was continued for 2 h to obtain a mixed lithium dihydrogen phosphate solution; wherein the molar ratio of lithium chloride to ammonium dihydrogen phosphate was 1:1.1. b. Adsorption: Add 32g of the above-mentioned ion-imprinted polymer adsorbent 3 to 100mL of lithium dihydrogen phosphate mixed solution, then add ammonia water to adjust the pH of the solution to 4.5, and stir the reaction at room temperature for 2h; after the reaction is completed, add a magnet to achieve the separation of ammonium chloride and lithium dihydrogen phosphate solution. c. Desorption: After the ion-imprinted polymer is saturated with adsorption, it is eluted with 100 mL of deionized water to obtain a purified lithium dihydrogen phosphate solution. d. Evaporation and concentration The lithium dihydrogen phosphate solution was evaporated and concentrated at a temperature of 200°C. Evaporation was stopped when an oil film appeared on the surface of the solution. e. Stepwise cooling crystallization (controlling crystal formation) Before cooling and crystallizing the purified lithium dihydrogen phosphate solution, lithium dihydrogen phosphate seed crystals are added, and the solution is cooled in stages. After cooling, the mixture is filtered and separated to obtain the wet lithium dihydrogen phosphate product. First cooling: The temperature is reduced from the concentration temperature to 80-90℃ at a rate of 2.0°C / min. Second cooling: The temperature is reduced from 80-90℃ to 25-30℃ at a cooling rate of 2.0°C / min; f. Dry the wet lithium dihydrogen phosphate product at a temperature of 100°C to obtain battery-grade lithium dihydrogen phosphate product 3.

[0033] Example 4 I. A method for preparing ion-imprinted polymer adsorbent 4, comprising the following steps: 1) Preparation of Fe3O4@SiO2: 1.2 g of nano Fe3O4 (particle size 1~100 nm) was added to 130 g of ethanol solution (mass fraction 85~95%) and mixed evenly. Then 7.5 g of tetraethyl orthosilicate was added. Ammonia water was added to adjust the pH value to 9.5-10.2. The reaction was stirred at room temperature, and the product was washed several times with ethanol and water and dried under vacuum to obtain the reaction product. 2) Preparation of crude Fe3O4@SiO2@IIPs products: 0.54 g of hexadecyltrimethylammonium bromide was dissolved in 30 g of ethanol solution with a volume fraction of 30-35%. Then, 100 mL of ultrapure water and 0.35 g of 1,2-bis(triethoxysilyl)ethane (crosslinking agent BTESE) were added sequentially. 0.17 g of acrylamide and benzo-12-crown-4 mixed functional monomers (molar ratio = 1:1) and 0.08 g of lithium dihydrogen phosphate were added sequentially to a three-necked flask and stirred at room temperature for 1 h. Then, 0.5 g of Fe3O4@SiO2 was added and stirred at room temperature for 6 h. The product was washed three times with ethanol and water, and dried in a vacuum drying oven at 70 °C to obtain the crude Fe3O4@SiO2@IIPs product for later use. 3) Preparation of Fe3O4@SiO2@IIPs (IIPs): The crude Fe3O4@SiO2@IIPs product was repeatedly washed with a methanol solution containing 2% acetic acid to remove the template lithium dihydrogen phosphate and CTAB. The product was then placed in a vacuum drying oven and dried at 70°C to obtain ion-imprinted polymer adsorbent 4 (Fe3O4@SiO2@IIPs, abbreviated as IIPs) of magnetic-mixed functional monomer-lithium dihydrogen phosphate, for later use. II. The process for preparing battery-grade lithium dihydrogen phosphate includes the following steps: a. Preparation of lithium dihydrogen phosphate mixed solution Under a stirring rate of 100-200 r / min, a lithium chloride solution (lithium concentration 0.5 g / L, main components as described in Example 1) was added dropwise to an ammonium dihydrogen phosphate solution (concentration 17 g / L) at a dropping rate of 300-400 mL / h. After the addition was completed, the reaction was continued for 2 h to obtain a mixed lithium dihydrogen phosphate solution; wherein the molar ratio of lithium chloride to ammonium dihydrogen phosphate was 1:1.1. b. Adsorption: Add 32g of the above-mentioned ion-imprinted polymer adsorbent 4 to 100mL of lithium dihydrogen phosphate mixed solution, then add ammonia water to adjust the pH of the solution to 5, and stir the reaction at room temperature for 2h; after the reaction is completed, add a magnet to achieve the separation of ammonium chloride and lithium dihydrogen phosphate solution; c. Desorption: After the ion-imprinted polymer is saturated with adsorption, it is eluted with 100 mL of deionized water to obtain a purified lithium dihydrogen phosphate solution. d. Evaporation and concentration The lithium dihydrogen phosphate solution was evaporated and concentrated at a temperature of 200°C. Evaporation was stopped when an oil film appeared on the surface of the solution. e. Stepwise cooling crystallization (controlling crystal formation) Before cooling and crystallizing the purified lithium dihydrogen phosphate solution, lithium dihydrogen phosphate seed crystals are added, and the solution is cooled in stages. After cooling, the mixture is filtered and separated to obtain the wet lithium dihydrogen phosphate product. First cooling: The temperature is reduced from the concentration temperature to 80-90℃ at a rate of 2.0°C / min. Second cooling: The temperature is reduced from 80-90℃ to 25-30℃ at a cooling rate of 2.0°C / min; f. Dry the wet lithium dihydrogen phosphate product at a temperature of 100°C to obtain battery-grade lithium dihydrogen phosphate product 4.

[0034] Comparative Example 1 I. A method for preparing ion-imprinted polymer adsorbents, comprising the following steps: 1) Preparation of Fe3O4@SiO2: 1.2 g of nano Fe3O4 (particle size 1~100 nm) was added to 130 g of ethanol solution (mass fraction 85~95%) and mixed evenly. Then 7.5 g of tetraethyl orthosilicate was added. Ammonia water was added to adjust the pH value to 9.5-10.2. The reaction was stirred at room temperature, and the product was washed several times with ethanol and water and dried under vacuum to obtain the reaction product. 2) Preparation of crude Fe3O4@SiO2@IIPs products: 0.54 g of hexadecyltrimethylammonium bromide was dissolved in 30 g of ethanol solution with a volume fraction of 30-35%. Then, 100 mL of ultrapure water and 0.35 g of 1,2-bis(triethoxysilyl)ethane (crosslinking agent BTESE) were added sequentially. 0.1 g of acrylamide and benzo-12-crown-4 mixed functional monomers (molar ratio = 1:1) and 0.08 g of lithium dihydrogen phosphate were added sequentially to a three-necked flask and stirred at room temperature for 1 h. Then, 0.5 g of Fe3O4@SiO2 was added and stirred at room temperature for 6 h. The product was washed three times with ethanol and water, and dried in a vacuum drying oven at 70 °C to obtain the crude Fe3O4@SiO2@IIPs product for later use. 3) Preparation of Fe3O4@SiO2@IIPs (IIPs): The crude Fe3O4@SiO2@IIPs product was repeatedly washed with a methanol solution containing 2% acetic acid to remove the template lithium dihydrogen phosphate and CTAB. The product was then placed in a vacuum drying oven and dried at 70°C to obtain the magnetic-mixed functional monomer-lithium dihydrogen phosphate ion-imprinted polymer adsorbent (Fe3O4@SiO2@IIPs, abbreviated as IIPs), which was ready for use. II. The process for preparing battery-grade lithium dihydrogen phosphate includes the following steps: a. Preparation of lithium dihydrogen phosphate mixed solution Under a stirring rate of 100-200 r / min, a lithium chloride solution (lithium concentration 0.5 g / L, main components as described in Example 1) was added dropwise to an ammonium dihydrogen phosphate solution (concentration 17 g / L) at a dropping rate of 300-400 mL / h. After the addition was completed, the reaction was continued for 2 h to obtain a mixed lithium dihydrogen phosphate solution; wherein the molar ratio of lithium chloride to ammonium dihydrogen phosphate was 1:1.1. b. Adsorption: Add 32g of the above-mentioned ion-imprinted polymer adsorbent to 100mL of lithium dihydrogen phosphate mixed solution, then add ammonia water to adjust the pH of the solution to 4.5~5, and stir the reaction at room temperature for 2h; after the reaction is completed, add a magnet to separate the ammonium chloride and lithium dihydrogen phosphate solutions. c. Desorption: After the ion-imprinted polymer is saturated with adsorption, it is eluted with 100 mL of deionized water to obtain a purified lithium dihydrogen phosphate solution. d. Evaporation and concentration The lithium dihydrogen phosphate solution was evaporated and concentrated at a temperature of 200°C. Evaporation was stopped when an oil film appeared on the surface of the solution. e. Stepwise cooling crystallization (controlling crystal formation) Before cooling and crystallizing the purified lithium dihydrogen phosphate solution, lithium dihydrogen phosphate seed crystals are added, and the solution is cooled in stages. After cooling, the mixture is filtered and separated to obtain the wet lithium dihydrogen phosphate product. First cooling: The temperature is reduced from the concentration temperature to 80-90℃ at a rate of 2.0°C / min. Second cooling: The temperature is reduced from 80-90℃ to 25-30℃ at a cooling rate of 2.0°C / min; f. Dry the wet lithium dihydrogen phosphate product at a temperature of 100°C to obtain battery-grade lithium dihydrogen phosphate product.

[0035] Comparative Example 2 I. A method for preparing ion-imprinted polymer adsorbents, comprising the following steps: 1) Preparation of Fe3O4@SiO2: 1.2 g of nano Fe3O4 (particle size 1~100 nm) was added to 130 g of ethanol solution (mass fraction 85~95%) and mixed evenly. Then 7.5 g of tetraethyl orthosilicate was added. Ammonia water was added to adjust the pH value to 9.5-10.2. The reaction was stirred at room temperature, and the product was washed several times with ethanol and water and dried under vacuum to obtain the reaction product. 2) Preparation of crude Fe3O4@SiO2@IIPs products: 0.54 g of hexadecyltrimethylammonium bromide was dissolved in 30 g of ethanol solution with a volume fraction of 30-35%. Then, 100 mL of ultrapure water and 0.35 g of 1,2-bis(triethoxysilyl)ethane (crosslinking agent BTESE) were added sequentially. 0.33 g of acrylamide and benzo-12-crown-4 mixed functional monomers (molar ratio = 1:1) and 0.08 g of lithium dihydrogen phosphate were added sequentially to a three-necked flask and stirred at room temperature for 1 h. Then, 0.5 g of Fe3O4@SiO2 was added and stirred at room temperature for 6 h. The product was washed three times with ethanol and water, and dried in a vacuum drying oven at 70 °C to obtain the crude Fe3O4@SiO2@IIPs product for later use. 3) Preparation of Fe3O4@SiO2@IIPs (IIPs): The crude Fe3O4@SiO2@IIPs product was repeatedly washed with a methanol solution containing 2% acetic acid to remove the template lithium dihydrogen phosphate and CTAB. The product was then placed in a vacuum drying oven and dried at 70°C to obtain the magnetic-mixed functional monomer-lithium dihydrogen phosphate ion-imprinted polymer adsorbent (Fe3O4@SiO2@IIPs, abbreviated as IIPs), which was ready for use. II. The process for preparing battery-grade lithium dihydrogen phosphate includes the following steps: a. Preparation of lithium dihydrogen phosphate mixed solution Under a stirring rate of 100-200 r / min, a lithium chloride solution (lithium concentration 0.5 g / L, main components as described in Example 1) was added dropwise to an ammonium dihydrogen phosphate solution (concentration 17 g / L) at a dropping rate of 300-400 mL / h. After the addition was completed, the reaction was continued for 2 h to obtain a mixed lithium dihydrogen phosphate solution; wherein the molar ratio of lithium chloride to ammonium dihydrogen phosphate was 1:1.1. b. Adsorption: Add 32g of the above-mentioned ion-imprinted polymer adsorbent to 100mL of lithium dihydrogen phosphate mixed solution, then add ammonia water to adjust the pH of the solution to 3.5, and stir the reaction at room temperature for 2h; after the reaction is completed, add a magnet to achieve the separation of ammonium chloride and lithium dihydrogen phosphate solution; c. Desorption: After the ion-imprinted polymer is saturated with adsorption, it is eluted with 100 mL of deionized water to obtain a purified lithium dihydrogen phosphate solution. d. Evaporation and concentration The lithium dihydrogen phosphate solution was evaporated and concentrated at a temperature of 200°C. Evaporation was stopped when an oil film appeared on the surface of the solution. e. Stepwise cooling crystallization (controlling crystal formation) Before cooling and crystallizing the purified lithium dihydrogen phosphate solution, lithium dihydrogen phosphate seed crystals are added, and the solution is cooled in stages. After cooling, the mixture is filtered and separated to obtain the wet lithium dihydrogen phosphate product. First cooling: The temperature is reduced from the concentration temperature to 80-90℃ at a rate of 2.0°C / min. Second cooling: The temperature is reduced from 80-90℃ to 25-30℃ at a cooling rate of 2.0°C / min; f. Dry the wet lithium dihydrogen phosphate product at a temperature of 100°C to obtain battery-grade lithium dihydrogen phosphate product.

[0036] Comparative Example 3 I. A method for preparing ion-imprinted polymer adsorbents, comprising the following steps: 1) Preparation of Fe3O4@SiO2: 1.2 g of nano Fe3O4 (particle size 1~100 nm) was added to 130 g of ethanol solution (mass fraction 85~95%) and mixed evenly. Then 7.5 g of tetraethyl orthosilicate was added. Ammonia water was added to adjust the pH value to 9.5-10.2. The reaction was stirred at room temperature, and the product was washed several times with ethanol and water and dried under vacuum to obtain the reaction product. 2) Preparation of crude Fe3O4@SiO2@IIPs products: 0.54 g of hexadecyltrimethylammonium bromide was dissolved in 30 g of ethanol solution with a volume fraction of 30-35%. Then, 100 mL of ultrapure water and 0.35 g of 1,2-bis(triethoxysilyl)ethane (crosslinking agent BTESE) were added sequentially. 0.33 g of acrylamide and benzo-12-crown-4 mixed functional monomers (molar ratio = 1:1) and 0.08 g of lithium dihydrogen phosphate were added sequentially to a three-necked flask and stirred at room temperature for 1 h. Then, 0.5 g of Fe3O4@SiO2 was added and stirred at room temperature for 6 h. The product was washed three times with ethanol and water, and dried in a vacuum drying oven at 70 °C to obtain the crude Fe3O4@SiO2@IIPs product for later use. 3) Preparation of Fe3O4@SiO2@IIPs (IIPs): The crude Fe3O4@SiO2@IIPs product was repeatedly washed with a methanol solution containing 2% acetic acid to remove the template lithium dihydrogen phosphate and CTAB. The product was then placed in a vacuum drying oven and dried at 70°C to obtain ion-imprinted polymer adsorbent 2 (Fe3O4@SiO2@IIPs, abbreviated as IIPs) of magnetic-mixed functional monomer-lithium dihydrogen phosphate, for later use. II. The process for preparing battery-grade lithium dihydrogen phosphate includes the following steps: a. Preparation of lithium dihydrogen phosphate mixed solution Under a stirring rate of 100-200 r / min, a lithium chloride solution (lithium concentration 0.5 g / L, main components as described in Example 1) was added dropwise to an ammonium dihydrogen phosphate solution (concentration 17 g / L) at a dropping rate of 300-400 mL / h. After the addition was completed, the reaction was continued for 2 h to obtain a mixed lithium dihydrogen phosphate solution; wherein the molar ratio of lithium chloride to ammonium dihydrogen phosphate was 1:1.1. b. Adsorption: Add 32g of the above-mentioned ion-imprinted polymer adsorbent to 100mL of lithium dihydrogen phosphate mixed solution, then add ammonia water to adjust the pH of the solution to 5, and stir the reaction at room temperature for 2h; after the reaction is completed, add a magnet to achieve the separation of ammonium chloride and lithium dihydrogen phosphate solution; c. Desorption: After the ion-imprinted polymer is saturated with adsorption, it is eluted with 100 mL of deionized water to obtain a purified lithium dihydrogen phosphate solution. d. Evaporation and concentration The lithium dihydrogen phosphate solution was evaporated and concentrated at a temperature of 120°C. Evaporation was stopped once an oil film appeared on the surface of the solution. e. Stepwise cooling crystallization (controlling crystal formation) Before cooling and crystallizing the purified lithium dihydrogen phosphate solution, lithium dihydrogen phosphate seed crystals are added, and the solution is cooled in stages. After cooling, the mixture is filtered and separated to obtain the wet lithium dihydrogen phosphate product. First cooling: The temperature is reduced from the concentration temperature to 80-90℃ at a rate of 2.0°C / min. Second cooling: The temperature is reduced from 80-90℃ to 25-30℃ at a cooling rate of 2.0°C / min; f. Dry the wet lithium dihydrogen phosphate product at a temperature of 100°C to obtain battery-grade lithium dihydrogen phosphate product.

[0037] Comparative Example 4 I. The process for preparing battery-grade lithium dihydrogen phosphate includes the following steps: a. Preparation of lithium dihydrogen phosphate mixed solution Under a stirring rate of 100-200 r / min, a lithium chloride solution (lithium concentration 0.5 g / L, main components as described in Example 1) was added dropwise to an ammonium dihydrogen phosphate solution (concentration 17 g / L) at a dropping rate of 300-400 mL / h. After the addition was completed, the reaction was continued for 2 h to obtain a mixed lithium dihydrogen phosphate solution; wherein the molar ratio of lithium chloride to ammonium dihydrogen phosphate was 1:1.1. b. Adsorption: Add 32g of commercial aluminum-based lithium adsorbent (LiCl·2Al(OH)3·nH2O) to 100mL of lithium dihydrogen phosphate mixed solution, then add ammonia water to adjust the pH of the solution to 5, and stir the reaction at room temperature for 2h; after the reaction is completed, add a magnet to separate the ammonium chloride and lithium dihydrogen phosphate solutions. c. Desorption: After the ion-imprinted polymer is saturated with adsorption, it is eluted with 100 mL of deionized water to obtain a purified lithium dihydrogen phosphate solution. d. Evaporation and concentration The lithium dihydrogen phosphate solution was evaporated and concentrated at a temperature of 200°C. Evaporation was stopped when an oil film appeared on the surface of the solution. e. Stepwise cooling crystallization (controlling crystal formation) Before cooling and crystallizing the purified lithium dihydrogen phosphate solution, lithium dihydrogen phosphate seed crystals are added, and the solution is cooled in stages. After cooling, the mixture is filtered and separated to obtain the wet lithium dihydrogen phosphate product. First cooling: The temperature is reduced from the concentration temperature to 80-90℃ at a rate of 2.0°C / min. Second cooling: The temperature is reduced from 80-90℃ to 25-30℃ at a cooling rate of 2.0°C / min; f. Dry the wet lithium dihydrogen phosphate product at a temperature of 100°C to obtain battery-grade lithium dihydrogen phosphate product.

[0038] The content of each component in the battery-grade lithium dihydrogen phosphate products prepared by Examples 1-4 and the comparative examples was tested.

[0039] Table 2 As can be seen from the above, the adsorbents prepared in Examples 1-4 can achieve an adsorption efficiency of over 95% by controlling the amount of functional monomers and template molecules added. The pH of the reaction process is controlled between 4.5 and 5, at which point the amino group attracts H2PO4 through electrostatic attraction. - This facilitates subsequent desorption, with a single desorption efficiency of 98%. The saturated adsorption capacity of this adsorbent for lithium dihydrogen phosphate remains above 93% of the initial capacity, with a desorption efficiency of >95% and strong cycle stability.

[0040] The adsorbents prepared in Comparative Examples 1-3 had low adsorption efficiency, were difficult to desorb, and had low efficiency; their performance dropped sharply after several cycles, and their cycle stability was poor.

[0041] Comparative Example 4 showed poor specific adsorption effect using commercial adsorbents, resulting in products with excessive impurity content and low yield.

[0042] The battery-grade lithium dihydrogen phosphate products prepared in Examples 1-4 all achieved lithium yields ≥90% and purity ≥99.5%, especially the battery-grade lithium dihydrogen phosphate product prepared in Example 1. In contrast, the comparative examples struggled to achieve both high yield and high purity due to severe impurity co-adsorption.

[0043] In summary, the adsorbent of this invention has advantages such as high adsorption capacity, easy desorption, and good cycle stability, and is suitable for the efficient separation and recovery of lithium dihydrogen phosphate.

[0044] All other parts not described in detail are existing technologies. Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing an ion-imprinted polymer adsorbent, characterized in that: Includes the following steps: 1) Preparation of Fe3O4@SiO2: Weigh nano-Fe3O4 and tetraethyl orthosilicate at a weight ratio of 1:3~10; dissolve nano-Fe3O4 in ethanol solution and mix evenly, then add tetraethyl orthosilicate; adjust the pH value to 9.5-10.2, stir the reaction at room temperature, wash several times with ethanol and water, and vacuum dry to obtain the reaction product Fe3O4@SiO2. 2) Preparation of crude Fe3O4@SiO2@IIPs products: First, weigh out hexadecyltrimethylammonium bromide, 1,2-bis(triethoxysilyl)ethane, mixed functional monomers, lithium dihydrogen phosphate, and the above-mentioned Fe3O4@SiO2. Then, dissolve hexadecyltrimethylammonium bromide in an ethanol solution, and add water, 1,2-bis(triethoxysilyl)ethane, mixed functional monomers, and lithium dihydrogen phosphate in sequence. Stir until homogeneous at room temperature, then add Fe3O4@SiO2. Continue stirring until homogeneous at room temperature, wash repeatedly with ethanol and water, and vacuum dry to obtain the crude product for later use. The weight ratio of Fe3O4@SiO2, hexadecyltrimethylammonium bromide, 1,2-bis(triethoxysilyl)ethane, mixed functional monomers, and lithium dihydrogen phosphate is 1:1~1.1:0.5~0.7:0.34~0.66:0.052~0.166; and the mixed functional monomers include acrylamide and functional monomers. 3) Preparation of Fe3O4@SiO2@IIPs: The crude product was washed multiple times with a methanol solution containing acetic acid and then dried under vacuum to obtain the ion-imprinted polymer adsorbent Fe3O4@SiO2@IIPs, which is a magnetic-mixed functional monomer-lithium dihydrogen phosphate.

2. The preparation method according to claim 1, characterized in that: In step 1), the mass fraction of the ethanol solution is 85-95%; the particle size of the nano Fe3O4 is 1-100 nm.

3. The preparation method according to claim 1, characterized in that: In step 2), the molar ratio of acrylamide to functional monomer is 1:1; The functional monomer is any one of benzo-12-crown-4, 14-crown-4 and dibenzo-14-crown-4.

4. The preparation method according to claim 1, characterized in that: In step 2), the vacuum drying temperature is 70°C.

5. The preparation method according to claim 1, characterized in that: In step 3), the mass fraction of acetic acid in the methanol solution is 2%.

6. The application of the ion-imprinted polymer adsorbent obtained by the preparation method of claim 1 in the adsorption of metal ions.

7. A process for preparing battery-grade lithium dihydrogen phosphate using lithium chloride as the lithium source, characterized in that: Includes the following steps: a. Under stirring conditions, lithium chloride solution is added dropwise to ammonium dihydrogen phosphate solution. After the addition is complete, the reaction continues for 1-2 hours to obtain a mixed lithium dihydrogen phosphate solution. b. Add the ion-imprinted polymer adsorbent prepared by the method described in claim 1 to the lithium dihydrogen phosphate mixed solution, adjust the pH value to 4.5-5, stir the reaction at room temperature, and add a magnet after the reaction is completed. c. After the ion-imprinted polymer is saturated with adsorption, it can be eluted with deionized water to obtain a purified lithium dihydrogen phosphate solution. d. Evaporate and concentrate the lithium dihydrogen phosphate solution until an oil film forms on the surface of the solution, then stop evaporation; to obtain a concentrated lithium dihydrogen phosphate solution. e. Before concentrating and cooling lithium dihydrogen phosphate, add lithium dihydrogen phosphate seed crystals, perform stepwise cooling, and after cooling, filter and separate to obtain the wet lithium dihydrogen phosphate product. f. Dry the wet lithium dihydrogen phosphate product to obtain battery-grade lithium dihydrogen phosphate.

8. The process according to claim 7, characterized in that: In step a, the lithium concentration in the lithium chloride solution is 0.05 g / L-1 g / L; the molar ratio of lithium chloride in the lithium chloride solution to ammonium dihydrogen phosphate in the ammonium dihydrogen phosphate solution is 1:1.05-1.1; and the concentration of the ammonium dihydrogen phosphate solution is 17-20 g / L. The stirring rate is 100-200 r / min, and the lithium chloride solution is added at a rate of 300-400 mL / h.

9. The process according to claim 7, characterized in that: In step d, the evaporation and concentration temperature is 150-200℃.

10. The process according to claim 7, characterized in that: In step e, the first cooling is performed by cooling the temperature from the concentration temperature to 80-90°C at a rate of 2.0°C / min. Second cooling: The temperature is reduced from 80-90℃ to 25-30℃ at a cooling rate of 2.0°C / min; In step f, the drying temperature is 100°C.

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