Method for purifying sodium fluoride by using complexing crystallization

Abstract

A method for purifying sodium fluoride by using complexing crystallization belongs to the technical field of sodium fluoride purification, and comprises the following steps: preparing a dissolving solution; complexing the dissolving solution with a complexing agent to obtain a complexing solution; the complexing solution is pumped into membrane separators, the complexing solution passes through the first membrane separator, a mixed solution of complexing precipitation and a complexing agent is filtered and intercepted through a tube pass of the first membrane separator, and a filtrate enters other membrane separators through a shell pass of the first membrane separator to be subjected to multi-stage separation to obtain a membrane separation solution; performing pure crystallization on the membrane separation liquid; and centrifugally separating to obtain purified sodium fluoride, and drying the purified sodium fluoride to remove the residual solvent, thereby obtaining the high-purity sodium fluoride. According to the method, the content of cation impurities and the content of anion impurities in sodium fluoride are effectively reduced at the same time, high-purity sodium fluoride is obtained through a pure crystallization method, the centrifugal separation liquid, the complexing agent and the alcohol crystallizing agent can be recycled, and the purposes of high efficiency, environment friendliness and low cost of purification are achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of sodium fluoride purification, and particularly relates to a method for purifying sodium fluoride by using complex crystallization. Background Art

[0002] Sodium fluoride is an inorganic compound that appears as colorless shiny crystals or white powder. It is soluble in water, hydrofluoric acid, and slightly soluble in alcohol. Sodium fluoride is an important chemical raw material and is widely used in metallurgy, ceramics, glass, electronics, medical and other industrial fields. However, in the production and use of sodium fluoride, various impurities are often mixed in, affecting the purity, stability and use effect of sodium fluoride. At present, although there are many methods for purifying sodium fluoride, most of them have problems such as complex operation, high cost, and limited purity improvement.

[0003] For example, the patent publication number CN117361574B discloses a method for preparing high-purity sodium fluoride, wherein the prepared sodium fluoride is purified, a crude sodium fluoride product is first prepared, the crude sodium fluoride product is subjected to primary purification by recrystallization, the total amount of metal cations in the crude product is reduced, and primary purified sodium fluoride is obtained, and then the primary purified sodium fluoride is purified again by anion exchange resin to reduce the total amount of anions in the primary purified sodium fluoride, and high-purity sodium fluoride is obtained, and the purity of the high-purity sodium fluoride is as high as 99.99%. However, due to the limited adsorption capacity of the exchange adsorption of cations by the anion exchange resin, the production capacity of the purified sodium fluoride by the method is limited, and large-scale production cannot be achieved, and the investment is large, and the production cost is high. Moreover, due to the adsorption of cations in the primary purified sodium fluoride by the anion exchange resin, the prepared high-purity sodium fluoride still contains more anions such as sulfate ions, chloride ions, etc., so that the high-purity sodium fluoride prepared by the method is limited in use in some specific fields. Therefore, it is of great significance to develop an efficient, environmentally friendly and low-cost method for purifying sodium fluoride. Summary of the invention

[0004] The invention purifies sodium fluoride by adopting the method of complex impurity removal and solvent crystallization, so that the content of metal and anion impurities in the sodium fluoride is effectively reduced, thereby obtaining high-purity sodium fluoride. Sodium hexafluorophosphate synthesized by using the high-purity sodium fluoride as a raw material will show better chemical properties and stability when applied to sodium ion batteries. Compared with other sodium fluoride purification methods, the method has lower cost, less equipment requirements, simple steps and significant purification effect.

[0005] To achieve the above object, the present invention provides a method for purifying sodium fluoride by complex crystallization. The purification device comprises a dissolving kettle, a filter with a vacuum pump, a complexing kettle, a plurality of membrane separators connected in series, a pure crystallization kettle, and a centrifugal separator, which are sequentially connected by pipelines. The rear of the centrifugal separator is provided with a filtration liquid tank, the filtration liquid tank is connected with a concentration tower, the top of the concentration tower is connected with a first storage tank, the first storage tank is connected with the pure crystallization kettle by pipeline, the bottom of the concentration tower is connected with a second storage tank, the second storage tank is connected with the dissolving kettle by pipeline, the membrane separator adopts a ceramic nanofiltration membrane with a microporous structure of 1 to 2 nm; the membrane separator is provided with a tube side and a shell side, and the outlet of the tube side of the first membrane separator is connected with a complexing agent tank;

[0006] The method for purifying sodium fluoride comprises the following steps:

[0007] 1) preparing a solution containing 3.8-4.2 wt% of sodium fluoride by recrystallization;

[0008] 2) The dissolved solution is pumped into the complexing kettle to complex with the complexing agent, stirred and fully mixed, and complexed for 10 to 30 minutes to obtain a complexing solution;

[0009] 3) The complex liquid is pumped into the membrane separator, and the volume flow ratio of the outlet of the tube side and the outlet of the shell side of the first membrane separator is controlled to be 1:20-50. The complex liquid passes through the first membrane separator, and the complex precipitate and the complexing agent mixed liquid are filtered and intercepted through its tube side. The filtrate passes through the shell side of the first membrane separator and enters the remaining membrane separators for multi-stage separation to obtain membrane separation liquid;

[0010] 4) the membrane separation liquid is pumped into a pure crystallization kettle, an alcohol crystallization agent is added dropwise into the pure crystallization kettle, and the mixture is stirred to obtain an alcohol liquid. After the addition is completed, stirring is stopped, and cooling is started to condense and completely crystallize the sodium fluoride;

[0011] 5) Separating and obtaining purified sodium fluoride by a centrifugal separator, concentrating the centrifuged liquid to 40-60% of its original volume by a concentrator at 70-80° C., and returning the concentrated liquid to the dissolving kettle for recycling;

[0012] 6) Drying the purified sodium fluoride to remove the residual solvent to obtain high-purity sodium fluoride.

[0013] Furthermore, the dissolving solution is prepared in the following specific steps:

[0014] Ultrapure water or the returned concentrated solution is added to the dissolving kettle, sodium fluoride of 300-800 mesh is weighed according to the solubility of sodium fluoride, and poured into the dissolving kettle, and fully stirred to fully dissolve the sodium fluoride. The temperature of the dissolving solution is controlled at 30-50° C. to obtain a mixed solution, and the insoluble impurities therein are filtered out through a filter with a vacuum pump to obtain a dissolving solution containing 3.8-4.2wt% of sodium fluoride.

[0015] Further, in step 2), the complexing agent is one or more of disodium ethylenediaminetetraacetate, trisodium nitrilotriacetate, and pentasodium diethylenetriaminepentaacetate.

[0016] Further, the input amount of the complexing agent is 2 to 5 times the total molar concentration of the impurity metal cations in the dissolution solution.

[0017] Further, there are at least three membrane separators. A pressure pump is installed at the front end of each membrane separator to pressurize it. For the remaining membrane separators after the first membrane separator, the outlet of the tube side is closed, and only the outlet of the shell side is opened. The multiple membrane separators temporarily intercept anions, and the interception rate is 80 - 85%.

[0018] Further, in step 4), the alcohol crystallization agent is one or more of methanol, ethanol, or isopropanol with a low boiling point and high water solubility.

[0019] Further, the alcohol crystallization agent is absolute ethanol.

[0020] Further, in step 4), the input amount of the alcohol crystallization agent is 0.5 to 1.5 times the volume of the membrane separation liquid by volume ratio.

[0021] Further, in step 4), the alcohol precipitation liquid is cooled to 0 - -10°C at a cooling rate of 0.1 - 0.5 min / °C, and the crystallization time is 4 - 8 h.

[0022] Further, in step 5), the concentrated liquid contains 1.2 - 1.8 wt% sodium fluoride and less than 2 wt% alcohol crystallization agent.

[0023] Further, in step 6), the drying temperature is 100 - 130°C, and the drying time is 4 - 8 h.

[0024] In the present invention, the membrane separator used has a microporous structure of 1 - 2 nm. Although the microporous structure of the ceramic nanofiltration membrane is larger than the anions to be intercepted, such as SO 4 2- (ionic radius is about 0.28 nm) and Cl - (ionic radius is about 0.181 nm), due to the tortuous internal channels, it forms a resistance to the intercepted anions, and the surface of its nano-sized micropores is charged, having a good interception effect on ions, enabling the membrane separator to temporarily intercept anions such as SO 4 2- and Cl - Thus, this also provides convenience for the reuse of the ceramic nanofiltration membrane in the membrane separator.

[0025] The purification of the ceramic nanofiltration membrane of the membrane separator is carried out according to the following operating steps:

[0026] a) Backwash the membrane separator from the shell side with pure anhydrous ethanol. The number of backwashing times is 5 - 10 times. During the backwashing process, close the outlet of the tube side of the membrane separator, and make the anhydrous ethanol backwashed from the shell side flow out through the opening of the tube side of the membrane separator.

[0027] b) Backwash the membrane separator from the shell side with ultrapure water. The number of backwashing times is 5 - 10 times. During the backwashing process, close the outlet of the tube side of the membrane separator, and make the ultrapure water backwashed from the shell side flow out through the opening of the tube side of the membrane separator.

[0028] c) Backwash the membrane separator from the shell side with a high-purity sodium fluoride (purity greater than 99.99%) solution. The number of backwashing times is 5 - 10 times. During the backwashing process, close the outlet of the tube side of the membrane separator, and make the high-purity sodium fluoride solution backwashed from the shell side flow out through the opening of the tube side of the membrane separator. The high-purity sodium fluoride solution has a cationic impurity content of less than 10 ppm and an anionic impurity content of less than 5 ppm.

[0029] d) For the membrane separator after the above backwashing, take out the internal ceramic nanofiltration membrane and dry it at a temperature of 60 - 80°C under vacuum conditions to obtain a reusable ceramic nanofiltration membrane.

[0030] The technical features of the present invention are as follows:

[0031] 1) Prepare a solution containing 3.8 - 4.2 wt% sodium fluoride by the recrystallization method, thereby removing insoluble impurities in sodium fluoride, achieving preliminary purification of sodium fluoride, and reducing the total amount of metal cation impurities and anionic impurities in sodium fluoride.

[0032] 2) Add a complexing agent to the solution. The metal ions in the solution and the complexing agent form a complex and precipitate, thereby further reducing the total amount of metal cation impurities in sodium fluoride.

[0033] 3) The complexing solution is filtered and intercepted by the tube side of the first membrane separator for the complex precipitate and complexing agent mixture and collected in the complexing agent tank. The complex precipitate and macromolecular complexing agent in the complexing solution are separated. The mixture in the complexing agent tank is separated into the complexing agent and complex precipitate by conventional methods. The separated complexing agent can be recycled. The filtrate enters the next membrane separator through the shell side. The small-molecule filtrate is filtered and multi-stage separated by other membrane separators. Sulfate ions, chloride ions, etc. contained in the filtrate are intercepted inside the ceramic nanofiltration membrane to achieve the removal of anions, thereby further reducing the total amount of anionic impurities mainly containing sulfate and chloride in sodium fluoride.

[0034] 4) By dropping an alcohol crystallization agent into the membrane separation liquid, since the alcohol crystallization agent is easily soluble in water and the solubility of sodium fluoride in the alcohol crystallization agent is extremely low, the solubility of sodium fluoride in the alcohol precipitation liquid is significantly reduced. During the gradual dropping of the alcohol crystallization agent, sodium fluoride slowly crystallizes out to obtain small crystal seeds. The slow precipitation of the crystal seeds can prevent impurities in the alcohol precipitation liquid from precipitating together through the adsorption of the crystal seeds. The alcohol precipitation liquid with crystal seeds is slowly cooled by turning on the circulating coolant to obtain completely crystallized sodium fluoride crystals. Through centrifugal separation and drying treatment, the residual solvent is removed to obtain high-purity sodium fluoride with a purity of over 99.9% in the present invention.

[0035] 5) The centrifugal separation liquid can be recycled. The centrifugal separation liquid is concentrated at a temperature of 70 - 80°C. Since the boiling point of the alcohol crystallization agent is low, during the concentration process, the concentration is easily achieved and the energy consumption is low. The alcohol crystallization agent volatilizes after heating and concentration and is recovered as alcohol after condensation. During the concentration and volatilization process of the alcohol crystallization agent, a large amount of the alcohol crystallization agent entrains the water in the filtrate to reduce the water content in the filtrate. Therefore, after the filtrate is concentrated to 40 - 60% of its original volume, the alcohol crystallization agent is separated. And due to the entrainment and loss of water in the filtrate, the sodium fluoride content can be increased to 1.2 - 1.8 wt%, and the alcohol crystallization agent content is less than 2 wt%. At this concentration temperature, the concentrated liquid is returned to the dissolution kettle. The addition of the hot concentrated liquid can raise the overall temperature of the dissolution liquid to 30 - 50°C. The increase in the temperature of the dissolution liquid is conducive to the dissolution liquid dissolving more sodium fluoride and increasing the sodium fluoride content in the dissolution liquid. The recovered alcohol can be further purified to obtain a higher-purity alcohol crystallization agent, and the recovered and purified alcohol crystallization agent can also be returned to the pure precipitation crystallization kettle to achieve the purpose of recycling and use.

[0036] In the present invention, cation impurities in sodium fluoride are removed by complexation precipitation, and anion impurities in sodium fluoride are removed by membrane separation. The contents of both cation impurities and anion impurities in sodium fluoride are effectively reduced. Then, high-purity sodium fluoride is obtained by the method of pure precipitation crystallization. The centrifugal separation liquid, complexing agent, and alcohol crystallization agent can all be recycled, achieving the purpose of high-efficiency, environmental protection, and low cost in purification. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a process flow chart of the sodium fluoride purification process of the present invention.

[0038] Among them, 1 - dissolution kettle, 2 - filter, 3 - vacuum pump, 4 - complexation kettle, 5 - membrane separator, 6 - complexing agent tank, 7 - pure precipitation crystallization kettle, 8 - centrifuge separator, 9 - filtrate tank, 10 - concentration tower, 11 - heater, 12 - condenser, 13 - first storage tank, 14 - second storage tank. DETAILED DESCRIPTION OF THE INVENTION

[0039] To make the technical problems, technical solutions and beneficial effects to be solved by the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0040] A method for purifying sodium fluoride by complexation crystallization, see Figure 1 , the purification device thereof includes a dissolution kettle 1, a filter 2 with a vacuum pump 3, a complexation kettle 4, multiple series-connected membrane separators 5, a pure crystallization kettle 7, and a centrifugal separator 8 that are connected in sequence by pipelines. A filtrate tank 9 is provided at the rear of the centrifugal separator 8. The filtrate tank 9 is connected to a concentration tower 10. The top of the concentration tower 10 is connected to a first storage tank 13. A condenser 12 is provided between the first storage tank 13 and the top of the concentration tower 10. The first storage tank 13 is connected to the pure crystallization kettle 7 by a pipeline. The bottom of the concentration tower 10 is connected to a second storage tank 14. A heater 11 is provided between the second storage tank 14 and the bottom of the concentration tower 10. The second storage tank 14 is connected to the dissolution kettle 1 by a pipeline. The membrane separator 5 is a ceramic nanofiltration membrane with a microporous structure of 1-2 nm. There are at least three membrane separators 5. To improve the filtration rate of the membrane separator 5, a pressure pump is installed at the front end of each membrane separator 5 to pressurize it. The membrane separator 5 is provided with a tube side and a shell side. The tube side has inlet and outlet ports, and the shell side has only an outlet port. During filtration, the liquid enters from the inlet of the tube side of the membrane separator 5 and flows out from the outlet of the shell side or the outlet of the tube side of the membrane separator 5. Among them, the outlet of the tube side of the first membrane separator 5 is connected to a complexing agent tank 6. For the remaining membrane separators 5 after the first membrane separator 5, the outlet of the tube side is closed, and only the outlet of the shell side is opened.

[0041] Preparation of the dissolution solution: Ultra-pure water is pumped into the dissolution kettle 1. According to the solubility of sodium fluoride, the amount of sodium fluoride with a particle size of 300-800 mesh is weighed and poured into the dissolution kettle 1. After sufficient stirring, sodium fluoride is fully dissolved. To improve the dissolution of sodium fluoride, the dissolution kettle 1 is heated to 40 °C. The dissolved mixture is vacuum filtered through the filter 2 with a vacuum pump 3 to filter out insoluble impurities, and a dissolution solution containing 4.16 wt% of sodium fluoride is obtained.

[0042] In the dissolution solution, after detection, the total amount of metal cation impurities contained is 39.15 ppm. Among them, calcium ions are 9.81 ppm (2.453×10 -7 mol / g), aluminum ions are 9.46 ppm (3.504×10 -7 mol / g), iron ions are 4.16 ppm (7.43×10 - 8 mol / g), potassium ions are 15.72 ppm (4.031×10 -7 mol / g), and the total molar concentration of metal cation impurities is 1.073×10 -6mol / g, the total amount of anionic impurities is 38.15 ppm, among which, sulfate ion is 19.11 ppm and chloride ion is 19.05 ppm.

[0043] Example 1:

[0044] Take the above-mentioned sodium fluoride-containing solution with a mass fraction of 4.16 wt%, pump it into the complexation kettle 4, add the amount of trisodium nitrilotriacetate (molecular weight 275) solution which is twice the total molar concentration of impurity metal cations in the solution, stir and mix well. The complexation time is 30 min, and the complexing agent and metal ions in the solution form a precipitate through complexation.

[0045] Pump the complexation solution into the membrane separator 5, and control the volume flow ratio of the outlet of the tube side to the outlet of the shell side of the first membrane separator 5 to be 1:30. When the complexation solution passes through the first membrane separator 5, the molecular structures of the complexation precipitate and the complexing agent are relatively large. The tube side of the first membrane separator 5 filters and intercepts the mixture of the complexation precipitate and the complexing agent. When the complexing agent mixture passes through the outlet of the tube side of the first membrane separator 5, it simultaneously takes the complexation precipitate out of the inner wall of the tube side of the membrane separator 5, avoiding the accumulation of the complexation precipitate on the inner wall of the tube side and affecting filtration. The obtained mixture of the complexation precipitate and the complexing agent is collected in the complexing agent tank 6, and the complexing agent and the complexation precipitate separated from the mixture in the complexing agent tank 6 by conventional methods. The separated complexing agent can be recycled.

[0046] Since the molecular structure of the filtrate is relatively small, it enters the remaining membrane separators 5 from the shell side of the first membrane separator 5 for multi-stage separation to obtain the membrane separation liquid. After separation by the first membrane separator 5, the obtained separation liquid does not contain solid impurities and the liquid to be separated. Therefore, for the remaining membrane separators 5, close the outlet of the tube side and only open the outlet of the shell side. Use the filtration of the remaining membrane separators 5 to mainly filter large-radius anions such as sulfate ions and chloride ions in the separation liquid, thereby realizing the removal of impurity anions to obtain the membrane separation liquid. In this example, three membrane separators 5 are provided.

[0047] Pump the above-mentioned membrane separation liquid into the pure precipitation crystallization kettle 7, add anhydrous ethanol with a volume ratio of 1:1 dropwise in the pure precipitation crystallization kettle 7, stir and mix to obtain an alcohol precipitation liquid. After the dropping is completed, stop stirring and start cooling. The alcohol precipitation liquid cools down to -5 °C at a cooling rate of 0.5 min / °C, and the crystallization time is 8 h to completely condense and crystallize sodium fluoride. Under the condition of -5 °C, it is separated by a centrifugal separator 8 to obtain a solid crystal, that is, purified sodium fluoride. The purified sodium fluoride is dried at 130 °C for 4 h to remove the residual solvent, and a product 1 of high-purity sodium fluoride with a purity of 99.990% is obtained.

[0048] The centrifugal separation liquid is concentrated in the concentration tower 10 at 70 - 80 °C to 40 - 60% of its original volume. The resulting concentrated liquid contains 1.2 - 1.8 wt% sodium fluoride and less than 2 wt% of the alcohol crystallization agent, and the concentrated liquid is returned to the dissolution kettle 1 for recycling.

[0049] Example 2:

[0050] The implementation method is the same as that of Example 1, except that the complexing agent is disodium ethylenediaminetetraacetate, and the dosage of disodium ethylenediaminetetraacetate is 5 times the total molar concentration of the impurity metal cations in the added dissolution liquid, obtaining Product 2 of high-purity sodium fluoride with a purity of 99.992%.

[0051] Example 3:

[0052] The implementation method is the same as that of Example 1, except that the complexing agent is pentasodium diethylenetriaminepentaacetate, and the dosage of pentasodium diethylenetriaminepentaacetate is 3 times the total molar concentration of the impurity metal cations in the added dissolution liquid, obtaining Product 3 of high-purity sodium fluoride with a purity of 99.986%.

[0053] Example 4:

[0054] The implementation method is the same as that of Example 1, except that in the membrane separator 5, the volume flow rate ratio of the outlet of the tube side to the outlet of the shell side of the first membrane separator 5 is controlled to be 1:20, obtaining Product 4 of high-purity sodium fluoride with a purity of 99.995%. Comparative Example 1:

[0055] The implementation method is the same as that of Example 1, except that the dissolution liquid is directly separated by the membrane separator 5 without complexation, obtaining Product 5 of sodium fluoride with a purity of 99.871%.

[0056] Comparative Example 2:

[0057] The implementation method is the same as that of Example 1, except that the complexing liquid is not separated by the membrane separator 5 but filtered through an ordinary centrifugal separator, obtaining Product 6 of sodium fluoride with a purity of 99.852%.

[0058] After detection, the impurity conditions of cations and anions in each product of the obtained high-purity sodium fluoride are shown in Table 1.

[0059] Table 1

[0060]

[0061] As can be seen from Table 1, Products 1 - 4 are obtained by the method of using complexation crystallization to purify sodium fluoride in Examples 1 - 4 of the present invention. Through the purification method of complexation and membrane separation, the amounts of various cation impurities and various anion impurities in sodium fluoride are effectively reduced, making the purity of the prepared sodium fluoride reach over 99.9%, with high purity.

[0062] In Example 1 compared with Comparative Example 1, since no complexation treatment was carried out in Comparative Example 1, the data shows that by using the complexing agent, the amounts of various cation impurities in the dissolution solution can be effectively reduced, and the removal rate reaches 70-75%, greatly improving the purity of sodium fluoride. In Example 1 compared with Comparative Example 2, since ordinary centrifugal filtration was used in Comparative Example 2, the data shows that for the membrane separator 5 with a ceramic nanofiltration membrane having a microporous structure of 1-2 nm, the rejection rate of the amounts of various anion impurities by the membrane separator 5 reaches 80-85%, effectively reducing the amounts of various anion impurities in the dissolution solution. At the same time, the complexing agent is also synchronously separated out by the membrane separator 5, which not only solves the problem of new impurities brought by the complexing agent to the dissolution solution, but also can effectively utilize the excess complexing agent, achieving the purpose of cost reduction.

[0063] In the present invention, for the used membrane separator 5, by utilizing the temporary retention effect of the ceramic nanofiltration membrane of the membrane separator 5 on anions, the purpose of recycling the membrane separator 5 can be achieved by means of three-step elution with absolute ethanol, ultrapure water, and sodium fluoride dissolution solution.

[0064] In the present invention, the centrifuged separation liquid can be concentrated to increase the content of residual sodium fluoride contained in the centrifuged separation liquid and returned to the dissolution kettle 1, achieving the purpose of recycling the residual sodium fluoride contained in the centrifuged separation liquid.

[0065] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for purifying sodium fluoride using complex crystallization, characterized in that: The purification device comprises a dissolving kettle, a filter with a vacuum pump, a complexing kettle, a plurality of membrane separators connected in series, a pure crystallization kettle, and a centrifugal separator, which are sequentially connected by pipelines. The rear of the centrifugal separator is provided with a filtration liquid tank, the filtration liquid tank is connected with a concentration tower, the top of the concentration tower is connected with a first storage tank, the first storage tank is connected with a pure crystallization kettle pipeline, the bottom of the concentration tower is connected with a second storage tank, the second storage tank is connected with a dissolving kettle pipeline, the membrane separator adopts a ceramic nanofiltration membrane with a microporous structure of 1 to 2 nm; the membrane separator is provided with a tube side and a shell side, and the outlet of the tube side of the first membrane separator is connected with a complexing agent tank; The method for purifying sodium fluoride comprises the following steps: 1) preparing a solution containing 3.8-4.2 wt% of sodium fluoride by recrystallization; 2) The dissolved solution is pumped into the complexing kettle to complex with the complexing agent, stirred and fully mixed, and complexed for 10 to 30 minutes to obtain a complexing solution; 3) The complex liquid is pumped into the membrane separator, and the volume flow ratio of the outlet of the tube side and the outlet of the shell side of the first membrane separator is controlled to be 1:20-50. The complex liquid passes through the first membrane separator, and the complex precipitate and the complexing agent mixed liquid are filtered and intercepted through its tube side. The filtrate passes through the shell side of the first membrane separator and enters the remaining membrane separators for multi-stage separation to obtain membrane separation liquid; 4) the membrane separation liquid is pumped into a pure crystallization kettle, an alcohol crystallization agent is added dropwise into the pure crystallization kettle, and the mixture is stirred to obtain an alcohol liquid. After the addition is completed, stirring is stopped, and cooling is started to condense and completely crystallize the sodium fluoride; 5) Separating and obtaining purified sodium fluoride by a centrifugal separator, concentrating the centrifuged liquid to 40-60% of its original volume by a concentrator at 70-80° C., and returning the concentrated liquid to the dissolving kettle for recycling; 6) Drying the purified sodium fluoride to remove the residual solvent to obtain high-purity sodium fluoride.

2. A method for purifying sodium fluoride using complex crystallization according to claim 1, characterized in that: The specific preparation steps of the dissolving solution are as follows: Ultrapure water or the returned concentrated solution is added to the dissolving kettle, sodium fluoride of 300-800 mesh is weighed according to the solubility of sodium fluoride, and poured into the dissolving kettle, and fully stirred to fully dissolve the sodium fluoride. The temperature of the dissolving solution is controlled at 30-50° C. to obtain a mixed solution, and the insoluble impurities therein are filtered out through a filter with a vacuum pump to obtain a dissolving solution containing 3.8-4.2wt% of sodium fluoride.

3. A method for purifying sodium fluoride using complex crystallization according to claim 1, characterized in that: In step 2), the complexing agent is one or more of disodium ethylenediaminetetraacetate, trisodium nitrilotriacetate and pentasodium diethylenetriaminepentacarboxylate.

4. A method for purifying sodium fluoride using complex crystallization according to claim 1, characterized in that: The amount of the complexing agent added is 2 to 5 times the total molar concentration of the impurity metal cations in the solution.

5. A method for purifying sodium fluoride using complex crystallization according to claim 1, characterized in that: There are at least three membrane separators, and a pressure pump is installed at the front end of each membrane separator to pressurize it. The remaining membrane separators after the first membrane separator close the outlet of the tube side and only open the outlet of the shell side. Multiple membrane separators can temporarily intercept anions with an interception rate of 80-85%.

6. A method for purifying sodium fluoride using complex crystallization according to claim 5, characterized in that: The purification of the ceramic nanofiltration membrane of the membrane separator is carried out in the following steps: a) The membrane separator is backwashed from the shell side with analytical grade anhydrous ethanol for 5 to 10 times. During the flushing process, the outlet of the tube side of the membrane separator is closed to allow the anhydrous ethanol backwashed from the shell side to flow out through the opening of the tube side of the membrane separator; b) using ultrapure water to reversely flush the membrane separator from the shell side for 5 to 10 times. During the flushing process, the outlet of the tube side of the membrane separator is closed to allow the ultrapure water reversely flushed from the shell side to flow out through the opening of the tube side of the membrane separator; c) using a high-purity sodium fluoride solution (with a purity greater than 99.99%) to reversely flush the membrane separator from the shell side for 5 to 10 times, during which the outlet of the tube side of the membrane separator is closed to allow the high-purity sodium fluoride solution reversely flushed from the shell side to flow out through the opening of the tube side of the membrane separator, wherein the high-purity sodium fluoride solution contains less than 10 ppm of cationic impurities and less than 5 ppm of anionic impurities; d) After the membrane separator is rinsed, the ceramic nanofiltration membrane inside is taken out and dried at a temperature of 60 to 80° C. under vacuum conditions to obtain a reusable ceramic nanofiltration membrane.

7. A method for purifying sodium fluoride using complex crystallization according to claim 1, characterized in that: In step 4), the alcohol crystallization agent is one or more of methanol, ethanol or isopropanol which have low boiling points and are easily soluble in water.

8. A method for purifying sodium fluoride using complex crystallization according to claim 1, characterized in that: In step 4), the input amount of the alcohol crystallization agent is 0.5 to 1.5 times the volume ratio of the membrane separation liquid.

9. A method for purifying sodium fluoride using complex crystallization according to claim 1, characterized in that: In step 4), the alcohol solution is cooled to 0-10°C at a cooling rate of 0.1-0.5 min / °C, and the crystallization time is 4-8 hours.

10. A method for purifying sodium fluoride using complex crystallization according to claim 1, characterized in that: In step 6), the drying temperature is 100-130° C. and the drying time is 4-8 hours.

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