A method for purifying sodium hexafluorophosphate

Abstract

The present invention relates to a method for purifying sodium hexafluorophosphate, wherein the acidic substance in a crude sodium hexafluorophosphate solution is removed, and the solution is subjected to aging treatment, filtration treatment, and spray drying and enhanced drying treatment of the filtered clear solution to obtain purified sodium hexafluorophosphate. By adopting the purification method of the present invention to purify sodium hexafluorophosphate, water in the product can be effectively removed, the content of free acid, insoluble matter and metal impurities can be reduced, the purification efficiency and yield are high, and the quality and performance of the electrolyte in a sodium ion battery can be improved.

Description

Technical Field

[0001] The invention belongs to the technical field of material chemical preparation, and particularly relates to a method for purifying sodium hexafluorophosphate. Background Art

[0002] The main ways of energy storage at present include mechanical energy storage, electromagnetic energy storage, phase change energy storage (such as batteries) and electrochemical energy storage (such as supercapacitors). In terms of energy density, the most suitable form of energy storage is chemical energy. Batteries are important carriers for storing and providing chemical energy. Batteries can not only be used as a tool for electrical energy transmission, but also have excellent performance, high conversion efficiency and no gas emissions. Therefore, people are very interested in low-cost, safe, rechargeable batteries with sufficient voltage, capacity and rate capabilities, and have done a lot of exploration on different types of batteries, such as the more mature lead-acid batteries, the more advanced lithium-ion batteries, and the sodium-ion batteries, potassium-ion batteries and various types of solid-state batteries under development.

[0003] In the research process of sodium ion batteries, the quality of the electrolyte has a great influence on the performance of the battery. In the process of synthesizing the electrolyte sodium hexafluorophosphate, it is easily affected by factors such as the error in the feeding ratio, the control of temperature and moisture during the reaction, and the control of temperature during heating and drying, which in turn affects the quality and purity of the synthesized product. In addition, in the industrial production process, the solvent system will accumulate acid after multiple cycles, long-term production interruptions, and solvent heating will also increase the generation of impurities, thereby affecting the quality and purity of the finished product. Therefore, it is necessary to increase the purity of the synthesized sodium hexafluorophosphate product as much as possible.

[0004] The prior art discloses that the purity of sodium hexafluorophosphate can reach 99.95%, but the acidity of the sodium hexafluorophosphate is still relatively high, reaching more than 20 ppm. For example, Chinese patent application CN115092944A discloses a method for synthesizing hexafluorophosphate, and its embodiment 2 is the best embodiment. Specifically, the embodiment comprises: at -20°C, 112.6 g (0.5 mol) of hexafluorophosphate complex crystal (C6H 15 NHPF6) and 106g (1mol) sodium carbonate were dissolved in a mixture of 50mL tetrahydrofuran and 10mL thionyl chloride. After sufficient stirring for reaction, the temperature was lowered to -50°C and allowed to stand for 2h. The solid was taken out by filtration and washed 3 times with 50mL n-hexane and 20mL tetrahydrofuran, and dried under vacuum at 60°C to obtain 78.3g of product with a mass yield of 93% (based on sodium hexafluorophosphate) and a purity of 99.95%. The acidity was 20ppm (equivalent to HF) after analysis. Summary of the invention

[0005] Based on the problems existing in the prior art, the object of the present invention is to provide a method for purifying sodium hexafluorophosphate, wherein the sodium hexafluorophosphate purified by the method has lower water content, acidity and insoluble matter content.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for purifying sodium hexafluorophosphate comprises the following steps:

[0008] S1. Removing acidic substances from the crude sodium hexafluorophosphate product solution;

[0009] S2. subjecting the solution obtained in S1 to aging treatment;

[0010] S3. The mixture obtained in S2 is filtered and the filtered supernatant is taken;

[0011] S4. The filtered clear liquid obtained by filtration in S3 is subjected to spray drying and enhanced drying treatment to obtain purified sodium hexafluorophosphate.

[0012] In step S1

[0013] Preferably, the crude sodium hexafluorophosphate product of the present invention can be a commercially available product or can be prepared in a laboratory.

[0014] Furthermore, the crude sodium hexafluorophosphate product is a reaction product of a hexafluorophosphate complex and an alkali metal compound. The specific implementation method can be to synthesize the crude product according to the embodiment in Chinese patent CN115092944A, without using a mixed solvent of an aprotic organic solvent and a second organic solvent for washing.

[0015] Preferably, an alkaline auxiliary agent is used to remove acidic substances in the crude sodium hexafluorophosphate product solution.

[0016] Furthermore, the alkaline auxiliary agent is selected from one or more of inorganic alkaline substances, organic alkaline substances, and anion exchange resins.

[0017] As a preferred embodiment, the inorganic alkaline substance is selected from one or more of an aqueous solution of sodium hydroxide, an aqueous solution of potassium hydroxide, an aqueous solution of sodium carbonate, and an aqueous solution of potassium carbonate.

[0018] As a preferred embodiment, the organic alkaline substance is selected from one or more of a methanol solution of sodium methoxide, an ethanol solution of sodium ethoxide, and an organic amine.

[0019] As a preferred embodiment, the organic amine is selected from one or more of organic primary amines, organic secondary amines, and organic tertiary amines.

[0020] Furthermore, the organic primary amines include methylamine, ethylamine, and 1-propylamine.

[0021] Furthermore, the organic secondary amines include dimethylamine, diethylamine, dipropylamine, methylethylamine, methylpropylamine, methylbutylamine, ethylpropylamine, ethylbutylamine, and propylbutylamine.

[0022] Furthermore, the organic tertiary amine is selected from one or more of trimethylamine, triethylamine, tripropylamine, tributylamine, diethylmethylamine, pyridine, and N,N-diethylethylenediamine.

[0023] As a preferred embodiment, when the organic amine is a gas, its flow rate is 1 to 10 mL / min; preferably 2 to 8 mL / min; and more preferably 3 to 7 mL / min.

[0024] Furthermore, the anion exchange resin is a weakly basic anion exchange resin, selected from one or more of AB-8 macroporous anion exchange resin, D201 macroporous anion exchange resin, D301 macroporous anion exchange resin, D318 macroporous acrylic anion exchange resin, 330 epoxy anion exchange resin, D363 macroporous anion exchange resin, and D312 macroporous anion exchange resin.

[0025] Preferably, the system of the solution obtained after removing the acidic substances in the crude sodium hexafluorophosphate product solution is weakly acidic to neutral.

[0026] Furthermore, the pH of the system after the treatment in step S1 is controlled to be 6 to 7, preferably 6.5 to 7.0.

[0027] Preferably, the solvent in the crude sodium hexafluorophosphate product solution is a polar solvent selected from one or more of CH3CN (acetonitrile), DME (ethylene glycol dimethyl ether), DMC (dimethyl carbonate), DEC (diethyl carbonate), EC (ethylene carbonate), PC (propylene carbonate), EMC (ethyl methyl carbonate), NMP (N-methylpyrrolidone), DMAC (N,N-dimethylacetamide), THF (tetrahydrofuran), methyl acetate, ethyl propionate, and ethyl acetate.

[0028] Furthermore, the polar solvent is a strong polar solvent, preferably one or more selected from CH3CN, DME, and DMC.

[0029] According to one embodiment, the water content of the polar solvent is less than 30 ppm; preferably less than 20 ppm; more preferably less than 10 ppm.

[0030] Preferably, the mass concentration of the crude sodium hexafluorophosphate product in the crude sodium hexafluorophosphate product solution is (0.1-0.5) g / mL.

[0031] Furthermore, the mass concentration of the crude sodium hexafluorophosphate product in the crude sodium hexafluorophosphate product solution is (0.15-0.2) g / mL.

[0032] Preferably, the control step S1 is performed at -20 to 50°C.

[0033] Further, step S1 is controlled to be performed at -10 to 40°C;

[0034] Furthermore, the control step S1 is performed at 0-30°C.

[0035] In step S2

[0036] Preferably, the aging time is 1 to 24 hours; preferably 5 to 12 hours; more preferably 10 to 12 hours.

[0037] Preferably, the temperature of the system during the aging is -20 to 50°C.

[0038] Furthermore, the temperature of the system during the aging is 15 to 50°C;

[0039] Furthermore, the temperature of the system during the aging is 20-50°C.

[0040] In step S3

[0041] Preferably, the filtering method during the filtering treatment is selected from membrane filtration and / or multi-stage precision filtration.

[0042] According to one embodiment, the material of the filter membrane is selected from one or more of PTFE (polytetrafluoroethylene), PVDF (polyvinylidene fluoride), aqueous PES (polyether sulfone), and aqueous MCE (mixed cellulose ester); preferably PTFE.

[0043] Furthermore, the filtration is performed using a filter membrane with a pore size of 0.1 to 6 μm, preferably 0.1 to 2 μm.

[0044] According to another embodiment, the filtration accuracy of the multi-stage precision filtration is 5 μm±0.25 μm.

[0045] In step S4

[0046] Preferably, the gas used in the spray drying is selected from one or more of argon, nitrogen and carbon dioxide.

[0047] Preferably, during the spray drying, the air flow rate is controlled to be 0.5 to 5 L / min; preferably 1 to 5 L / min; and more preferably 2 to 4 L / min.

[0048] Preferably, during the spray drying, the flow rate of the filtered clear liquid is controlled to be 20-100 mL / min.

[0049] Furthermore, during the spray drying, the flow rate of the filtered clear liquid is controlled to be 20 to 90 mL / min.

[0050] Furthermore, during the spray drying, the flow rate of the filtered clear liquid is controlled to be 30-50 mL / min.

[0051] Preferably, during the spray drying, the volume flow ratio of the airflow to the filtered clear liquid is (4-100):1.

[0052] Furthermore, during the spray drying, the volume flow ratio of the airflow to the filtered clear liquid is (30-80):1.

[0053] Furthermore, during the spray drying, the volume flow ratio of the airflow to the filtered clear liquid is (50-80):1.

[0054] Preferably, the air flow temperature of the spray drying is controlled to be 100-300°C.

[0055] Preferably, the enhanced drying is vacuum drying.

[0056] Preferably, the drying temperature of the vacuum drying is 50-200°C.

[0057] Furthermore, the vacuum drying has a drying temperature of 60 to 150°C.

[0058] Preferably, the vacuum degree of the vacuum drying is -0.1 to -0.001 MPa.

[0059] Preferably, the vacuum drying time is 5 to 15 hours; preferably 5 to 12 hours; more preferably 5 to 8 hours.

[0060] Due to the application of the above technical solution, the present invention has the following advantages compared with the prior art:

[0061] (1) In the present invention, the acidic substances in the crude sodium hexafluorophosphate solution are removed, and then the solution is aged, filtered, spray-dried, and subjected to enhanced drying treatment, so that the moisture in the system can be effectively removed, the free acid content and the content of insoluble matter and metal impurities can be reduced, the purification efficiency and yield are high, and the quality and performance of the sodium ion battery electrolyte using the sodium hexafluorophosphate are improved.

[0062] (2) The method for purifying the crude sodium hexafluorophosphate product of the present invention is simple to operate, has controllable experimental conditions, has low production cost, is easy to promote, can realize industrial production, and has good social value and economic value. DETAILED DESCRIPTION

[0063] All features disclosed in this invention, or all steps in methods or processes disclosed, can be combined in any manner except mutually exclusive features or steps.

[0064] In view of the deficiencies in the prior art, the inventor of this case has proposed the technical solution of the present invention after long-term research and extensive practice. The technical solution, its implementation process and principle will be further explained as follows.

[0065] The lower the content of free acid, water content, metal impurities, etc. in the sodium hexafluorophosphate to be purified, the more difficult it is to further purify it. The purification method of the present application can further effectively reduce the acidity, water content and metal impurity content of sodium hexafluorophosphate while taking into account the yield.

[0066] In the present invention, the crude sodium hexafluorophosphate product is firstly treated with an alkaline auxiliary agent to remove acidic substances in the system, and then the purified crude sodium hexafluorophosphate product is obtained by aging, filtering, spray drying, and enhanced drying. The alkaline auxiliary agent is dissolved in an organic solvent system with a relatively low moisture content and can be combined with free hydrofluoric acid in the crude sodium hexafluorophosphate product, thereby adjusting the acidity of the system to a certain pH range. Subsequently, aging at a certain temperature can cause colloidal and submicron particles in the solution to aggregate and grow, thereby facilitating the removal of solid impurities in the system by filtering. Finally, the filtered sodium hexafluorophosphate liquid is spray dried at a certain temperature and the airflow rate and the flow rate of the filtered clear liquid are strictly controlled during spray drying, so as to better remove the moisture and solvent in the system, shorten the time of purification treatment, and the organic solvent and moisture remaining in the sodium hexafluorophosphate solid particles can be further removed by enhanced drying, thereby further improving the degree of purification of the sodium hexafluorophosphate. The sodium hexafluorophosphate obtained by the purification method of the sodium hexafluorophosphate of the present invention has high purity, and the moisture, acidity, and insoluble impurity content in the product are low, so that it can be better used in secondary batteries.

[0067] The invention is particularly suitable for purifying sodium hexafluorophosphate to be purified with a free acid content of no more than 200 ppm. The purity of the crude sodium hexafluorophosphate product in the invention is 98.9% to 99.5%, the acidity is 2.3‰ to 6.3‰ (equivalent to HF), the moisture content is 12ppm to 25ppm, and the metal impurities (iron, nickel, zinc) are 1.0ppm to 2.7ppm.

[0068] Furthermore, the alkaline auxiliary agent is selected from one or more of an inorganic alkaline substance, an organic alkaline substance, and an anion exchange resin, and will not react with sodium hexafluorophosphate, and trace residues will not have a negative impact on the performance of the electrolyte. The organic base includes one or more of a methanol solution of sodium methoxide, an ethanol solution of sodium ethoxide, and an organic amine, preferably a methanol solution of sodium methoxide or an organic amine. The anion exchange resin is preferably a weakly basic anion exchange resin.

[0069] When the alkaline auxiliary agent is a weakly alkaline anion exchange resin, the reaction formula for reducing the acidity in the system is as follows:

[0070] R3NH+HF→R3NH +F -

[0071] Furthermore, by using spray drying at a temperature of 100 to 300°C, and strictly controlling the liquid flow rate used during spray drying to be 20 to 100 mL / min, the air flow rate to be 0.5 to 5 L / min, and the ratio of the air flow rate to the liquid flow rate to be (4 to 100): 1, the solution absorbs enough heat and completely evaporates to form completely dry particles, effectively reducing the moisture in the system and increasing the yield of sodium hexafluorophosphate. Moreover, by adjusting the aperture of the liquid nozzle, a product of a certain particle size can be obtained, so that the product has a certain fluidity to meet the needs of different uses.

[0072] The present invention is further described below in conjunction with the examples. However, the present invention is not limited to the following examples. The implementation conditions used in the examples can be further adjusted according to the different requirements of specific use, and the implementation conditions not specified are conventional conditions in the industry. The technical features involved in each embodiment of the present invention can be combined with each other as long as they do not conflict with each other.

[0073] Example 1

[0074] At 20°C, take 20g of crude sodium hexafluorophosphate product, add 50mL of dehydrated dimethyl carbonate (water content less than 20ppm), and add the prepared sodium methoxide dropwise under stirring until the solution is neutral. Sampling and measuring the pH value is 6.52, aging for 10h, filtering with a filter membrane (PTEE filter membrane with a pore size of 0.1μm) to obtain the clear liquid, at 200°C, nitrogen intake volume 3L / min, liquid flow rate 40mL / min, and then drying at 120°C, -0.05MPa vacuum conditions for 5h to obtain 18.47g of product, sampling and analysis, purity of 99.98%, pH value 6.60, acidity 2.51ppm (equivalent to HF), water 4.72ppm, metal impurities (iron, nickel, zinc) less than 0.5ppm.

[0075] Example 2

[0076] At 20°C, take 20g of crude sodium hexafluorophosphate product, add 50mL of dehydrated dimethyl carbonate (water content less than 20ppm), pass ethylamine gas into the sample at a rate of 5mL / min, until the pH value of the sample is 6.30, age for 10h, filter with a filter membrane (PTEE filter membrane with a pore size of 0.1μm) to obtain the clear liquid, spray dry at 200°C, the drying conditions are: nitrogen intake 3L / min, liquid flow rate 40mL / min, use spray drying sample, and then dry at 120°C, -0.05MPa vacuum conditions for 8h to obtain 18.20g of product, sample analysis, purity of 99.98%, pH value 6.25, acidity 5.6ppm (equivalent to HF), moisture 5.0ppm, metal impurities (iron, nickel, zinc) less than 0.5ppm.

[0077] Example 3

[0078] At 20°C, take 20g of crude sodium hexafluorophosphate product, add 50mL of dehydrated dimethyl carbonate (water content less than 20ppm), and drip diethylamine solution under stirring until the sample is neutral. The pH value of the sample is 6.54, and it is aged for 10h. The clear liquid is filtered through a filter membrane (PTEE filter membrane with a pore size of 0.1μm), and spray-dried at 200°C. The drying conditions are: nitrogen intake volume 3L / min, liquid flow rate 40mL / min, and the sample is spray-dried, and then dried at 120°C and -0.05MPa vacuum conditions for 5h to obtain 18.89g of the product. Sampling and analysis showed that the purity was 99.97%, the pH value was 6.52, the acidity was 3.0ppm (equivalent to HF), the moisture was 5.7ppm, and the metal impurities (iron, nickel, zinc) were less than 0.5ppm.

[0079] Example 4

[0080] At 20°C, 20g of crude sodium hexafluorophosphate product was taken, 50mL of dehydrated dimethyl carbonate (water content less than 20ppm) was added, the pretreated D301 macroporous weak alkaline anion exchange resin was loaded into the column, the sample was repeatedly passed through the column until the exchange was complete, and the pH value of the final sample was 6.25. After aging for 10h, the clear liquid was filtered through a filter membrane (PTFE filter membrane with a pore size of 0.1μm), and spray-dried at 200°C. The drying conditions were: nitrogen intake volume 3L / min, liquid flow rate 40mL / min, and then dried at 120°C, -0.05MPa vacuum conditions for 5h to obtain 18.12g of the product. After sampling and analysis, the purity was 99.96%, the pH value was 6.31, the acidity was 4.9ppm (equivalent to HF), the moisture was 8.9ppm, and the metal impurities (iron, nickel, zinc) were less than 0.5ppm.

[0081] Comparative Example 1

[0082] At 20°C, take 20g of crude sodium hexafluorophosphate product, add 50mL of dehydrated dimethyl carbonate (water content less than 20ppm), and add the prepared sodium methoxide dropwise under stirring until the solution is neutral. The pH value of the sample is 6.52, and the solution is aged for 3.5h. The clear liquid is filtered through a filter membrane (PTEE filter membrane with a pore size of 0.1μm), and dried under vacuum conditions of 120°C and -0.05MPa for 12 hours to obtain 17.31g of the product. The sample was analyzed and the purity was 99.85%, the pH value was 6.19, the acidity was 8.5ppm (equivalent to HF), the water content was 12.5ppm, and the metal impurities were less than (iron, nickel, zinc) 1.20ppm.

[0083] In Comparative Example 1, the spray drying operation step is removed, that is, the preparation of sodium hexafluorophosphate powder solid is achieved directly by vacuum drying, which cannot make the sodium hexafluorophosphate dry quickly, and thus fails to more thoroughly remove the moisture and organic solvent in the system, which not only increases the moisture content, but also leads to an increase in the acidity and the content of metal impurities.

[0084] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes made according to the spirit of the present invention.

Claims

1. A method for purifying sodium hexafluorophosphate; characterized in that: The steps include: S1, removing acidic substances from a crude sodium hexafluorophosphate solution, wherein the crude sodium hexafluorophosphate solution has a purity of 98.9% to 99.5%, an acidity equivalent to HF of 2.3‰ to 6.3‰, a moisture content of 12ppm to 25ppm, and a content of metal impurities such as iron, nickel, and zinc of 1.0ppm to 2.7ppm, and removing acidic substances from the crude sodium hexafluorophosphate solution by an alkaline auxiliary agent, wherein the alkaline auxiliary agent is selected from one or more of an inorganic alkaline substance, an organic alkaline substance, and an anion exchange resin, and the pH of the system after the treatment in step S1 is controlled to be 6 to 7; S2, subjecting the solution obtained in S1 to aging treatment, wherein the aging time is 5 to 12 hours, and the temperature of the system during aging is -20 to 50°C; S3, filtering the mixture obtained in S2, and taking the filtered clear liquid, and filtering it using a filter membrane with a pore size of 0.1 to 6 μm; S4, subjecting the filtered clear liquid obtained by filtration in S3 to spray drying and enhanced drying treatment to obtain purified sodium hexafluorophosphate, wherein the air flow temperature of the spray drying is controlled to be 100-300° C., the volume flow ratio of the spray drying air flow to the filtered clear liquid is (4-100):1, the air flow rate is controlled to be 0.5-5 L / min, and the flow rate of the filtered clear liquid is controlled to be 20-100 mL / min.

2. The method for purifying sodium hexafluorophosphate according to claim 1, wherein: The gas used in the spray drying is selected from one or more of argon, nitrogen and carbon dioxide.

3. The method for purifying sodium hexafluorophosphate according to claim 1, wherein: The solvent in the crude sodium hexafluorophosphate product solution is a polar solvent selected from one or more of CH3CN, DME, DMC, DEC, EC, PC, EMC, NMP, DMAC, THF, methyl acetate, ethyl propionate, and ethyl acetate.

4. The method for purifying sodium hexafluorophosphate according to claim 1, wherein: The inorganic alkaline substance is selected from one or more of an aqueous solution of sodium hydroxide, an aqueous solution of potassium hydroxide, an aqueous solution of sodium carbonate, and an aqueous solution of potassium carbonate; and / or, the organic alkaline substance is selected from one or more of a methanol solution of sodium methoxide, an ethanol solution of sodium ethoxide, and an organic amine; and / or, The organic amine is selected from one or more of organic primary amines, organic secondary amines and organic tertiary amines; and / or, The anion exchange resin is a weakly basic anion exchange resin, selected from one or more of AB-8 macroporous anion exchange resin, D201 macroporous anion exchange resin, D301 macroporous anion exchange resin, D318 macroporous acrylic anion exchange resin, 330 epoxy anion exchange resin, D363 macroporous anion exchange resin, and D312 macroporous anion exchange resin.

5. The method for purifying sodium hexafluorophosphate according to claim 1, characterized in that: The control step S1 is performed at -20 to 50°C.

6. The method for purifying sodium hexafluorophosphate according to claim 1, characterized in that: The enhanced drying is specifically vacuum drying, wherein the drying temperature of the vacuum drying is 50 to 200° C., the vacuum degree of the vacuum drying is -0.1 to -0.001 MPa, and the drying time of the vacuum drying is 5 to 15 hours.