A method for preparing sodium tetrafluoroborate

Abstract

The present application relates to the field of fine chemical industry, and particularly to a preparation method of sodium tetrafluoroborate; the present application uses hydrofluoric acid, boric acid and soda to prepare sodium tetrafluoroborate; the present application uses evaporation, polar organic solvent dissolution, separation, modified polytetrafluoroethylene membrane fine filter filtration, cooling crystallization and the like to purify sodium tetrafluoroborate; the present application uses a polytetrafluoroethylene membrane filter core, vinyl guanidine, sodium acrylate, 1-allyl-3-ethyl imidazole tetrafluoroborate, an initiator and deionized water to prepare a modified polytetrafluoroethylene membrane filter core; the modified polytetrafluoroethylene membrane filter core prepared by the present application can effectively improve the purity of sodium tetrafluoroborate; the present application has the characteristics of low cost, simple process, low toxicity and environmental friendliness; the present application has mild reaction conditions in the preparation process, is easy to operate, has low energy consumption and high product purity.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of fine chemicals, and more particularly to a preparation method of sodium tetrafluoroborate. BACKGROUND

[0002] Although sodium-ion batteries have the potential advantage of low cost, their cycle life and rate performance still cannot meet the needs of current large-scale energy storage technology. For sodium-ion batteries, the electrolyte is an ion charge carrier necessary for electrochemical reactions, and has an important influence on its electrochemical performance (capacity, cycle stability, rate performance, etc.).

[0003] In recent years, battery combustion and explosion incidents occur frequently. The currently used sodium hexafluorophosphate has high hygroscopicity and produces toxic HF gas, which is not conducive to the development of sodium-ion batteries. Due to the limitations of kinetics and thermodynamics, sodium-ion batteries are difficult to work under ultra-high rate and extreme temperature conditions, and are mainly limited by the decrease of ion conductivity of electrolyte, difficult desolvation process and slow ion diffusion in electrode materials at low temperature. Sodium tetrafluoroborate has low cost, high electrical conductivity and low electrolyte viscosity, and can be applied to low temperature environment. In ionic liquid electrolyte, sodium tetrafluoroborate solute is the best, mainly because its electrode interface resistance is the lowest, ionic liquid has good thermal stability, no volatility and larger electrochemical window, so it can also be stably cycled at high temperature. Sodium tetrafluoroborate can be used as a main salt or additive for sodium-ion battery electrolyte, and has good oxidation stability under high pressure.

[0004] For example, Chinese patent application No. CN202211036289.3: A method for preparing sodium tetrafluoroborate at room temperature in one step, comprising the following steps: dispersing fluoroborine compounds and sodium-containing oxygen acid salt in a non-aqueous solvent, and reacting at room temperature for 10-24 hours, then performing solid-liquid separation, and evaporating the solvent in the obtained solution to obtain high-purity solid product sodium tetrafluoroborate; wherein the non-aqueous solvent is acetonitrile, acetone, cyclohexane, etc. The method has mild reaction conditions, easy operation, short reaction time and one-step synthesis, the by-products are easy to separate, the obtained product is high-purity, the equipment requirement is low, and the production safety problem is guaranteed.

[0005] Application No. CN200980159908.4: A method for manufacturing a tetrafluoroborate salt, an electrolyte containing the tetrafluoroborate salt, and a power storage element provided with the electrolyte, which can manufacture the tetrafluoroborate salt with high yield and high efficiency through a continuous process. The method for manufacturing the tetrafluoroborate salt of the present invention is characterized by having: a first step of dissolving boron trifluoride gas in an organic solvent; a second step of adding a stoichiometric amount of a fluoride (MFn, M is a metal or NH4, 1≤n≤3) equivalent to or less than the equivalent of the above boron trifluoride to the above organic solvent to generate a solution of a tetrafluoroborate salt; and a third step of circulating the above solution of the tetrafluoroborate salt to the above first step, thereby dissolving boron trifluoride gas in the solution of the tetrafluoroborate salt instead of the above organic solvent.

[0006] Application No. CN201610406229.4: A method for preparing lithium tetrafluoroborate, which comprises the following steps: mixing a compound containing a lithium salt of a weak acid radical and BF3 in a non-protic non-polar or non-protic polar solvent, wherein the molar ratio of lithium, boron and fluorine is 1:1:4-1:2:5, the molar ratio of the solvent to lithium is 3:1-6:1, and the mixture is uniformly mixed at 0-70 DEG C under reflux for 1-24 hours, and then solid-liquid separation is performed. The solvent in the obtained liquid is evaporated to obtain a solid LiBF4 crude product. After one purification and separation, the yield of the LiBF4 product is greater than 96%, and the purity is higher than 99%, and the purity can be further improved after multiple purification and separation.

[0007] However, the above-mentioned patents and prior art have the following defects: the purity of the synthesized sodium tetrafluoroborate is poor, the reaction time is long, the reaction process is complex, and the energy consumption is large.

[0008] Therefore, how to improve the purity of sodium tetrafluoroborate and simplify the reaction process is a problem to be solved by those skilled in the art. SUMMARY

[0009] The purpose of the present application is to overcome the defects in the prior art, and to provide a method for preparing sodium tetrafluoroborate. The modified polytetrafluoroethylene membrane filter element prepared by the method can effectively improve the purity of sodium tetrafluoroborate, has the characteristics of low cost, simple process, low toxicity and environmental friendliness, etc.

[0010] To achieve the above-mentioned purpose, the technical scheme of the present application is to design a method for preparing sodium tetrafluoroborate, comprising the following steps:

[0011] S1: weighing 400-500 parts of hydrofluoric acid and 60-70 parts of boric acid into a reaction kettle, heating, reacting, and preparing fluoroboric acid;

[0012] S2: 90-100 parts of fluoroboric acid is added to a neutralization tank, 110-130 parts of soda ash is slowly added under stirring and cooling, the reaction temperature is controlled not to exceed 35℃, after neutralization to the specified acidity, the reaction is carried out for a certain time;

[0013] S3: after the neutralization liquid is concentrated by evaporation, 100-150 parts of a polar organic solvent is added, after warming and dissolving, the solution is filtered through a modified polytetrafluoroethylene membrane fine filter, and then crystallized by cooling, the filtrate is warmed to 50-80℃, and the polar organic solvent is removed by distillation to obtain sodium tetrafluoroborate.

[0014] In the above sodium tetrafluoroborate preparation step, the reaction temperature of S1 is 30-38℃, and the time is 2-4h.

[0015] In the above sodium tetrafluoroborate preparation step, the time of adding soda ash in S2 is 1-1.5h.

[0016] In the above sodium tetrafluoroborate preparation step, the neutralization PH of S2 is 3-4.

[0017] In the above sodium tetrafluoroborate preparation step, the reaction time of S2 is 0.5-2h.

[0018] In the above sodium tetrafluoroborate preparation step, the polar organic solvent is one or more of acetonitrile, diethyl ether, ethyl acetate or dimethyl carbonate.

[0019] In the above sodium tetrafluoroborate preparation step, the dissolving temperature of S3 is 50-70℃.

[0020] In the above sodium tetrafluoroborate preparation step, the modified polytetrafluoroethylene membrane fine filter is filled with a modified polytetrafluoroethylene membrane filter element, and the preparation method is as follows:

[0021] S1 irradiation: 500-800 parts of polytetrafluoroethylene membrane filter element is placed in a gamma ray radiation field for irradiation processing to obtain a polytetrafluoroethylene membrane containing CF2· on the surface;

[0022] S2 crosslinking grafting reaction: 0.05-0.8 parts of vinyl guanidine, 5-10 parts of sodium acrylate, 0.01-0.3 parts of 1-allyl-3-ethyl imidazole tetrafluoroborate, 2-4 parts of initiator, 500-850 parts of deionized water are added to a stirring kettle, and stirred at 50-70℃ for 30-55 minutes, then 100-200 parts of the polytetrafluoroethylene membrane filter element containing CF2· on the surface is immersed in the above materials, and stirred at 80-90℃ for 4-8h, then the polytetrafluoroethylene membrane filter element is taken out, washed with water and dried to obtain a modified polytetrafluoroethylene membrane filter element.

[0023] The irradiation condition in the preparation step of the modified polytetrafluoroethylene membrane filter core is 5kGy-50kGy of absorbed dose.

[0024] The initiator in the preparation step of the modified polytetrafluoroethylene membrane filter core is at least one of potassium persulfate and ammonium persulfate.

[0025] The technical mechanism of the present application is:

[0026] The polytetrafluoroethylene membrane containing CF2· reacts with vinyl guanidine, sodium acrylate and 1-allyl-3-ethyl imidazole tetrafluoroborate to obtain a modified polytetrafluoroethylene membrane filter core, the guanidine group of the modified polytetrafluoroethylene membrane filter core can absorb carbon dioxide, the tetrafluoroboric acid functional group is conducive to improving the compatibility of sodium tetrafluoroborate and the membrane filter core, and the above is conducive to forming selective filtration, blocking carbonate impurities on one side of the filter core, so that the purity of sodium tetrafluoroborate is improved.

[0027] The present application has the following advantages:

[0028] The present application has the following advantages compared with the prior art:

[0029] 1. The modified polytetrafluoroethylene membrane filter core prepared by the present application can effectively improve the purity of sodium tetrafluoroborate;

[0030] 2. The present application has the characteristics of low cost, simple process, low toxicity and environmental friendliness;

[0031] 3. The preparation process of the present application has mild reaction conditions, is easy to operate, has low energy consumption and high product purity. DETAILED DESCRIPTION

[0032] The present application will be further described below by describing the embodiments to help those skilled in the art to have a more complete, accurate and in-depth understanding of the inventive concept and technical solution of the present application

[0033] The purity calculation method of sodium tetrafluoroborate in the specific implementation method of the present method is as follows:

[0034] Purity: the purity of sodium tetrafluoroborate = the mass of sodium tetrafluoroborate in the mixture ÷ the mass of the mixture x 100%;

[0035] Example 1

[0036] A preparation method of sodium tetrafluoroborate, the operation steps of which are:

[0037] S1: weigh 400g of hydrofluoric acid and 60g of boric acid into a reaction kettle, heat, react, and prepare fluoroboric acid;

[0038] S2: 90g fluoroboric acid is added to a neutralization tank, 110g sodium carbonate is slowly added under stirring and cooling, the reaction temperature is controlled not to exceed 35℃, after neutralization to the specified acidity, the reaction is carried out for a certain time;

[0039] S3: after the neutralization liquid is concentrated by evaporation, 100g polar organic solvent is added, after warming and dissolving, the solution is filtered through a modified polytetrafluoroethylene membrane fine filter, and then crystallized by cooling, the filtrate is warmed to 50℃, and the polar organic solvent is removed by distillation to obtain sodium tetrafluoroborate.

[0040] The reaction temperature of S1 is 30℃, and the time is 2h.

[0041] The sodium carbonate of S2 is added for 1h.

[0042] The neutralization PH of S2 is 3.

[0043] The reaction time of S2 is 0.5h.

[0044] The polar organic solvent is acetonitrile.

[0045] The dissolving temperature of S3 is 50℃.

[0046] The modified polytetrafluoroethylene membrane fine filter is internally provided with a modified polytetrafluoroethylene membrane filter element, and the preparation method is as follows:

[0047] S1 irradiation: 500g polytetrafluoroethylene membrane filter element is placed in a gamma ray irradiation field for irradiation processing to obtain a polytetrafluoroethylene membrane containing CF2· on the surface;

[0048] S2 crosslinking grafting reaction: 0.05g vinyl guanidine, 5g sodium acrylate, 0.01g 1-allyl-3-ethyl imidazole tetrafluoroborate, 2g initiator, 500g deionized water are added to a stirring kettle, and stirred at 50℃ for 30 minutes, then 100g polytetrafluoroethylene membrane filter element containing CF2· on the surface is immersed in the above material; stirring at 80℃ for 4h, taking out the polytetrafluoroethylene membrane filter element, washing with water, and drying to obtain a modified polytetrafluoroethylene membrane filter element.

[0049] The irradiation condition is that the absorbed dose is 5kGy.

[0050] The initiator is potassium persulfate.

[0051] According to the analysis and calculation, the purity of sodium tetrafluoroborate in this example is 99.992%.

[0052] Example 2

[0053] A preparation method of sodium tetrafluoroborate, the operation steps are as follows:

[0054] S1: take 440g hydrofluoric acid and 64g boric acid into the reaction kettle, heat, react, and prepare fluoroboric acid;

[0055] S2: add 94g fluoroboric acid into the neutralization tank, slowly add 115g sodium carbonate under stirring and cooling, control the reaction temperature not to exceed 35℃, and neutralize to the specified acidity, and react for a certain time;

[0056] S3: after the neutralization liquid is concentrated by evaporation, 110g polar organic solvent is added, and after heating and dissolving, the solution is filtered through a modified polytetrafluoroethylene membrane fine filter, and the filtrate is cooled to crystallize; the filtrate is heated to 60℃, and the polar organic solvent is removed by distillation to obtain sodium tetrafluoroborate.

[0057] The reaction temperature of S1 is 32℃, and the time is 3h.

[0058] The sodium carbonate adding time of S2 is 1h.

[0059] The neutralization PH of S2 is 3.

[0060] The reaction time of S2 is 1h.

[0061] The polar organic solvent is diethyl ether.

[0062] The dissolving temperature of S3 is 55℃.

[0063] The modified polytetrafluoroethylene membrane fine filter is internally provided with a modified polytetrafluoroethylene membrane filter element, and the preparation method is as follows:

[0064] S1 irradiation: place 600g polytetrafluoroethylene membrane filter element in a gamma ray irradiation field for irradiation processing to obtain polytetrafluoroethylene membrane containing CF2· on the surface;

[0065] S2 crosslinking grafting reaction: add 0.2g vinyl guanidine, 6g sodium acrylate, 0.1g 1-allyl-3-ethyl imidazole tetrafluoroborate, 3g initiator, and 600g deionized water into a stirring kettle, and stir and react at 55℃ for 35 minutes; then immerse 140g polytetrafluoroethylene membrane filter element containing CF2· on the surface into the above materials; stir and react at 85℃ for 5h, take out the polytetrafluoroethylene membrane filter element, wash with water, and dry to obtain a modified polytetrafluoroethylene membrane filter element.

[0066] The irradiation condition is that the absorbed dose is 10kGy.

[0067] The initiator is potassium persulfate.

[0068] According to the analysis and calculation, the purity of sodium tetrafluoroborate in this example is 99.995%.

[0069] Example 3

[0070] A preparation method of sodium tetrafluoroborate, the operation steps are as follows:

[0071] S1: weigh 480g of hydrofluoric acid and 68g of boric acid into a reaction kettle, heat, react, and prepare fluoroboric acid;

[0072] S2: add 98g of fluoroboric acid into a neutralization tank, slowly add 125g of soda under stirring and cooling, control the reaction temperature to be not more than 35℃, after neutralization to the specified acidity, react for a certain time;

[0073] S3: after the neutralization liquid is concentrated by evaporation, 140g of polar organic solvent is added, after heating and dissolving, the mixture is filtered through a modified polytetrafluoroethylene membrane fine filter, and then the filtrate is cooled to crystallize; the filtrate is heated to 70℃, and the polar organic solvent is removed by distillation to obtain sodium tetrafluoroborate.

[0074] The reaction temperature of S1 is 36℃, and the time is 3h.

[0075] The adding time of soda in S2 is 1.5h.

[0076] The neutralization PH of S2 is 4.

[0077] The reaction time of S2 is 1.5h.

[0078] The polar organic solvent is ethyl acetate.

[0079] The dissolving temperature of S3 is 65℃.

[0080] The modified polytetrafluoroethylene membrane fine filter is internally provided with a modified polytetrafluoroethylene membrane filter element, and the preparation method is as follows:

[0081] S1 irradiation: 700g of a polytetrafluoroethylene membrane filter element is placed in a gamma ray irradiation field for irradiation processing to obtain a polytetrafluoroethylene membrane containing CF2· on the surface;

[0082] S2 crosslinking grafting reaction: 0.6g of vinyl guanidine, 9g of sodium acrylate, 0.2g of 1-allyl-3-ethyl imidazole tetrafluoroborate, 3g of an initiator, 700g of deionized water, and 65℃ stirring reaction for 50 minutes are added into a stirring kettle, and then 180g of the polytetrafluoroethylene membrane filter element containing CF2· on the surface is immersed in the above materials; 85℃ stirring reaction for 7h, taking out the polytetrafluoroethylene membrane filter element, water washing, and drying to obtain a modified polytetrafluoroethylene membrane filter element.

[0083] The irradiation condition is that the absorbed dose is 40kGy.

[0084] The initiator is ammonium persulfate.

[0085] According to the analysis and calculation, the purity of sodium tetrafluoroborate in this example is 99.998%.

[0086] Embodiment 4

[0087] A preparation method of sodium tetrafluoroborate, the operation steps are:

[0088] S1: weigh 500g of hydrofluoric acid and 70g of boric acid into a reaction kettle, heat, react, and prepare fluoroboric acid;

[0089] S2: add 100g of fluoroboric acid to a neutralization tank, slowly add 130g of soda under stirring and cooling, control the reaction temperature not to exceed 35℃, and after neutralization to the specified acidity, react for a certain time;

[0090] S3: after the neutralization liquid is concentrated by evaporation, 150g of a polar organic solvent is added, after heating and dissolving, filtered through a modified polytetrafluoroethylene membrane fine filter, and then cooled to crystallize, the filtrate is heated to 80℃, and the polar organic solvent is removed by distillation to obtain sodium tetrafluoroborate.

[0091] The reaction temperature of S1 is 38℃, and the time is 4h.

[0092] The soda addition time of S2 is 1.5h.

[0093] The neutralization PH of S2 is 4.

[0094] The reaction time of S2 is 2h.

[0095] The polar organic solvent is dimethyl carbonate.

[0096] The dissolving temperature of S3 is 70℃.

[0097] The modified polytetrafluoroethylene membrane fine filter is internally provided with a modified polytetrafluoroethylene membrane filter element, and the preparation method is:

[0098] S1 irradiation: 800g of a polytetrafluoroethylene membrane filter element is placed in a gamma ray irradiation field for irradiation processing to obtain a polytetrafluoroethylene membrane containing CF2· on the surface;

[0099] S2 crosslinking grafting reaction: 0.8g of vinyl guanidine, 10g of sodium acrylate, 0.3g of 1-allyl-3-ethyl imidazole tetrafluoroborate, 4g of an initiator, 850g of deionized water, and 70℃ stirring for 55 minutes are added to a stirring kettle, then 200g of the polytetrafluoroethylene membrane filter element containing CF2· on the surface is immersed in the above materials; 90℃ stirring reaction for 8h, the polytetrafluoroethylene membrane filter element is taken out, washed with water, and dried to obtain a modified polytetrafluoroethylene membrane filter element.

[0100] The irradiation condition is an absorbed dose of 50kGy.

[0101] The initiator is ammonium persulfate.

[0102] The purity of the sodium tetrafluoroborate in this example is 99.996% by analysis calculation.

[0103] Comparative Example 1

[0104] A preparation method of sodium tetrafluoroborate, the operation steps of which are as follows:

[0105] S1: weigh 400g of hydrofluoric acid and 60g of boric acid into a reaction kettle, heat, react, and prepare fluoroboric acid;

[0106] S2: add 90g of fluoroboric acid into a neutralization tank, slowly add 110g of soda under stirring and cooling, control the reaction temperature not to exceed 35℃, and after neutralization to the specified acidity, react for a certain time;

[0107] S3: after the neutralization liquid is concentrated by evaporation, 100g of polar organic solvent is added, heated to dissolve, separated, filtered, and cooled to crystallize, the filtrate is heated to 50℃, and the polar organic solvent is removed by distillation to obtain sodium tetrafluoroborate.

[0108] The reaction temperature of S1 is 30℃, and the time is 2h.

[0109] The adding time of soda in S2 is 1h.

[0110] The neutralization PH of S2 is 3.

[0111] The reaction time of S2 is 0.5h.

[0112] The polar organic solvent is acetonitrile.

[0113] The dissolving temperature of S3 is 50℃.

[0114] The purity of the sodium tetrafluoroborate in this example is 86.64% by analysis calculation.

[0115] Comparative Example 2

[0116] A preparation method of sodium tetrafluoroborate, the operation steps of which are as follows:

[0117] S1: weigh 400g of hydrofluoric acid and 60g of boric acid into a reaction kettle, heat, react, and prepare fluoroboric acid;

[0118] S2: add 90g of fluoroboric acid into a neutralization tank, slowly add 110g of soda under stirring and cooling, control the reaction temperature not to exceed 35℃, and after neutralization to the specified acidity, react for a certain time;

[0119] S3: After the neutralization liquid is concentrated by evaporation, 100g of a polar organic solvent is added, and after being dissolved by heating, it is filtered through a modified polytetrafluoroethylene membrane fine filter, and then crystallized by cooling. The filtrate is heated to 50℃, and the polar organic solvent is removed by distillation to obtain sodium tetrafluoroborate.

[0120] The reaction temperature of S1 is 30℃, and the time is 2h.

[0121] The soda ash of S2 is added for 1h.

[0122] The neutralization pH of S2 is 3.

[0123] The reaction time of S2 is 0.5h.

[0124] The polar organic solvent is acetonitrile.

[0125] The dissolution temperature of S3 is 50℃.

[0126] The modified polytetrafluoroethylene membrane fine filter is internally equipped with a modified polytetrafluoroethylene membrane filter element, and the preparation method is as follows:

[0127] S1 irradiation: 500g of polytetrafluoroethylene membrane filter element is placed in a gamma ray irradiation field for irradiation processing to obtain a polytetrafluoroethylene membrane containing CF2· on the surface;

[0128] S2 crosslinking grafting reaction: 5g of sodium acrylate, 0.01g of 1-allyl-3-ethylimidazole tetrafluoroborate, 2g of initiator, 500g of deionized water are added to a stirred tank, and stirred at 50℃ for 30 minutes, then 100g of polytetrafluoroethylene membrane filter element containing CF2· on the surface is immersed in the above material; 80℃ stirring reaction for 4h, take out the polytetrafluoroethylene membrane filter element, water washing, drying, get modified polytetrafluoroethylene membrane filter element.

[0129] The irradiation condition is that the absorbed dose is 5kGy.

[0130] The initiator is potassium persulfate.

[0131] After analysis and calculation, the purity of sodium tetrafluoroborate in this example is 90.35%.

[0132] Comparative example 3

[0133] A preparation method of sodium tetrafluoroborate, the operation steps are as follows:

[0134] S1: 400g of hydrofluoric acid and 60g of boric acid are weighed into a reaction kettle, heated, and reacted to prepare fluoroboric acid;

[0135] S2: 90 g of fluoboric acid is added to a neutralization tank, 110 g of soda ash is slowly added under stirring and cooling, the reaction temperature is controlled not to exceed 35℃, after neutralization to the specified acidity, the reaction is carried out for a certain time;

[0136] S3: After the neutralization liquid is concentrated by evaporation, 100 g of a polar organic solvent is added, after warming and dissolving, it is filtered through a modified polytetrafluoroethylene membrane fine filter, and then crystallized by cooling, the filtrate is warmed to 50℃, and the polar organic solvent is removed by distillation to obtain sodium tetrafluoroborate.

[0137] The reaction temperature of S1 is 30℃, and the time is 2h.

[0138] The addition time of soda ash in S2 is 1h.

[0139] The neutralization PH of S2 is 3.

[0140] The reaction time of S2 is 0.5h.

[0141] The polar organic solvent is acetonitrile.

[0142] The dissolving temperature of S3 is 50℃.

[0143] The modified polytetrafluoroethylene membrane fine filter is internally equipped with a modified polytetrafluoroethylene membrane filter element, and the preparation method is as follows:

[0144] S1 irradiation: 500 g of polytetrafluoroethylene membrane filter element is placed in a gamma ray irradiation field for irradiation processing to obtain a polytetrafluoroethylene membrane containing CF2· on the surface;

[0145] S2 crosslinking grafting reaction: 0.05 g of vinyl guanidine, 5 g of sodium acrylate, 2 g of initiator, 500 g of deionized water are added to a stirring kettle, and stirred at 50℃ for 30 minutes, then 100 g of the polytetrafluoroethylene membrane filter element containing CF2· on the surface is immersed in the above material; 80℃ stirring reaction for 4h, take out the polytetrafluoroethylene membrane filter element, water washing, drying, to obtain a modified polytetrafluoroethylene membrane filter element.

[0146] The irradiation condition is that the absorbed dose is 5kGy.

[0147] The initiator is potassium persulfate.

[0148] After analysis and calculation, the purity of sodium tetrafluoroborate in this example is 91.05%.

[0149] The present application is described above by way of example with reference to specific embodiments. It is apparent that the specific implementation of the present application is not limited to the above-described manner, and various non-essential improvements or direct application of the concept and technical solution of the present application to other occasions without modification are within the protection scope of the present application. The protection scope of the present application should be defined by the protection scope of the claims.

Claims

1. A method for preparing sodium tetrafluoroborate, comprising the following steps: S1: Weigh 400-500 parts by weight of hydrofluoric acid and 60-70 parts by weight of boric acid into a reaction vessel, heat up, react, and obtain fluoroboric acid; S2: Add 90-100 parts of fluoroboric acid to the neutralization tank, and slowly add 110-130 parts of soda ash while stirring and cooling. Control the reaction temperature to not exceed 35℃. After neutralizing to the specified acidity, react for a certain period of time. S3: After the neutralized liquid is evaporated and concentrated, 100-150 parts of polar organic solvent are added, and the solution is heated to dissolve. After separation, the solution is filtered through a modified polytetrafluoroethylene membrane fine filter, cooled and crystallized. The filtrate is heated to 50-80℃ and distilled to remove the polar organic solvent, yielding sodium tetrafluoroborate. The modified polytetrafluoroethylene membrane fine filter, which contains a modified polytetrafluoroethylene membrane filter element, is prepared by the following method: S1 irradiation: 500-800 parts by weight of polytetrafluoroethylene membrane filter element are placed in a gamma-ray radiation field for irradiation processing to obtain a polytetrafluoroethylene membrane with CF2· on the surface. S2 crosslinking grafting reaction: Add 0.05-0.8 parts of vinylguanidine, 5-10 parts of sodium acrylate, 0.01-0.3 parts of 1-allyl-3-ethylimidazolium tetrafluoroborate, 2-4 parts of initiator, and 500-850 parts of deionized water to a stirred tank and stir for 30-55 minutes at 50-70℃. Then immerse 100-200 parts of polytetrafluoroethylene membrane filter element with CF2· on the surface into the above materials; stir for 4-8 hours at 80-90℃. Take out the polytetrafluoroethylene membrane filter element, wash with water, and dry to obtain the modified polytetrafluoroethylene membrane filter element.

2. The method for preparing sodium tetrafluoroborate according to claim 1, characterized in that: The reaction temperature of S1 is 30-38℃, and the reaction time is 2-4h.

3. The method for preparing sodium tetrafluoroborate according to claim 1, characterized in that: The soda ash in S2 is added at a time of 1-1.5 hours.

4. The method for preparing sodium tetrafluoroborate according to claim 1, characterized in that: The neutralization pH of S2 is 3-4.

5. The method for preparing sodium tetrafluoroborate according to claim 1, characterized in that: The reaction time of S2 is 0.5-2 hours.

6. The method for preparing sodium tetrafluoroborate according to claim 1, characterized in that: The polar organic solvent is one or more of acetonitrile, diethyl ether, ethyl acetate, or dimethyl carbonate.

7. The method for preparing sodium tetrafluoroborate according to claim 1, characterized in that: The dissolution temperature of S3 is 50-70℃.

8. The method for preparing sodium tetrafluoroborate according to claim 1, characterized in that: The irradiation conditions are: absorbed dose of 5 kGy-50 kGy.

9. The method for preparing sodium tetrafluoroborate according to claim 1, characterized in that: The initiator is at least one of potassium persulfate and ammonium persulfate.