Method for preparing high-purity lithium hexafluorophosphate

Abstract

The invention relates to the technical field of lithium hexafluorophosphate production processes, and particularly discloses a method for preparing high-purity lithium hexafluorophosphate. Comprising the following steps: preparing hexafluorophosphoric acid, removing water through azeotropy, preparing phosphorus pentafluoride, preparing lithium hexafluorophosphate, purifying lithium hexafluorophosphate, removing water through azeotropy, separating water in a hexafluorophosphoric acid aqueous solution, and then preparing phosphorus pentafluoride through pyrolysis. The water removal agent still exists in the hexafluorophosphoric acid and participates in the later pyrolysis process of the hexafluorophosphoric acid, so that the purity of the finally produced lithium hexafluorophosphate is not high.

Description

Technical Field

[0001] The invention relates to the technical field of lithium hexafluorophosphate production technology, in particular to a method for preparing high-purity lithium hexafluorophosphate. Background Art

[0002] Lithium hexafluorophosphate (LiPF6) is an electrolyte with excellent comprehensive performance, with advantages such as high conductivity, low viscosity and good chemical stability, so it is widely used in the battery field. The performance of lithium-ion batteries is closely related to the quality of the electrolyte. The electrolyte made of lithium hexafluorophosphate containing impurities will cause the internal resistance of lithium-ion batteries to increase, the battery capacity to decay rapidly, and the safety performance cannot be guaranteed. Therefore, high-purity lithium hexafluorophosphate is particularly important for new energy batteries.

[0003] At present, most lithium hexafluorophosphate manufacturers at home and abroad use the hydrogen fluoride solvent method to prepare lithium hexafluorophosphate, that is, first prepare phosphorus pentafluoride, and then use phosphorus pentafluoride to react with anhydrous hydrogen fluoride solution containing lithium fluoride to produce lithium hexafluorophosphate. The hydrogen fluoride solvent method is simple and has gradually become the mainstream process for lithium hexafluorophosphate production. Since this method requires the preparation of phosphorus pentafluoride as a precursor for lithium hexafluorophosphate production, the purity and yield of phosphorus pentafluoride directly affect the purity and yield of lithium hexafluorophosphate.

[0004] At present, there are two commonly used processes for the preparation of phosphorus pentafluoride in the preparation process of lithium hexafluorophosphate. The first is the PCl5 method: using PCl5 and anhydrous hydrogen fluoride as raw materials to directly produce phosphorus pentafluoride. However, this method has a violent exothermic reaction in the early stage, which is difficult to control, and will introduce Cl elements, and there is a problem that by-product PF3Cl2 and tail gas HCl are easily generated and difficult to separate and treat. The other process is the hexafluorophosphoric acid pyrolysis method: first use a phosphorus source and a fluorinating agent to produce hexafluorophosphoric acid, and then pyrolyze the hexafluorophosphoric acid to prepare phosphorus pentafluoride. This process only uses a phosphorus source and hydrogen fluoride, and hydrogen fluoride is the solvent for the later lithium fluoride. Therefore, this method will not introduce other elemental impurities, and the upper limit of the purity of the preparation of lithium hexafluorophosphate is higher. No tail gas treatment is required, and the process is simpler, which has received widespread attention and research. However, in this process technology, there is a technical detail that affects the purity and yield of phosphorus pentafluoride products, that is, the hexafluorophosphoric acid produced by using a phosphorus source and a fluorinating agent exists in the state of aqueous solution, and the equation is: H7P5O 16+30HF-5HPF6+16H20; the water will not only evaporate in the subsequent pyrolysis and be brought into the subsequent process to become an impurity, but hexafluorophosphoric acid will also hydrolyze in the aqueous solution. After the hydrolysis, not only the content of hexafluorophosphoric acid is reduced, but also difluorophosphoric acid, monofluorophosphoric acid, phosphoric acid and other phosphorus-containing acid compounds are produced. Such compounds are not only unable to be pyrolyzed into phosphorus pentafluoride, but part of them also volatilizes and gasifies to mix with phosphorus pentafluoride, resulting in low yield and low purity of phosphorus pentafluoride. The low yield and low purity of phosphorus pentafluoride will inevitably lead to low yield and low purity of lithium hexafluorophosphate.

[0005] In order to solve the above-mentioned problem of hydrolysis of hexafluorophosphoric acid, a dehydrating agent is usually used in the prior art to control the hydrolysis of hexafluorophosphoric acid. For example, in the patent document with application number CN200810049660.3, a method for preparing lithium hexafluorophosphate is disclosed, which is to react anhydrous hydrogen fluoride with concentrated phosphoric acid under the protection of inert gas to obtain hexafluorophosphoric acid; dehydrate hexafluorophosphoric acid with a dehydrating agent and pyrolyze it to obtain phosphorus pentafluoride gas; react phosphorus pentafluoride gas with a high-purity lithium fluoride anhydrous hydrogen fluoride solution; after reaction, crystallization, separation, and drying, a pure lithium hexafluorophosphate product is obtained. The above patent adds fuming sulfuric acid as a dehydrating agent before pyrolysis to control the free water in hexafluorophosphoric acid. Then the yield and purity of phosphorus pentafluoride are improved, and finally the purpose of obtaining high-purity lithium hexafluorophosphate is achieved. For example, the patent document with application number CN202311234089.3 discloses a method for preparing phosphorus pentafluoride gas, which directly adds an organic anhydride dehydrating agent during the reaction of a phosphorus source and hydrogen fluoride, thereby controlling the free water content in the production of hexafluorophosphoric acid. Therefore, controlling the water content of hexafluorophosphoric acid in the production of lithium hexafluorophosphate is an important process for preparing high-yield and high-purity lithium hexafluorophosphate.

[0006] In the prior art, all water removal methods are to use a water removal agent to directly react with free water or to form a complex adsorption. After the water removal agent in this method combines or complexes the free water, it still exists in the hexafluorophosphoric acid and participates in the subsequent pyrolysis process of the hexafluorophosphoric acid. During the pyrolysis process, the water removal agent will not only release some water, but also pyrolyze other impurities. Although the subsequent process will distill and remove impurities from phosphorus pentafluoride, some impurities still reach the subsequent process to affect the purity of lithium hexafluorophosphate. Summary of the invention

[0007] The object of the present invention is to provide a method for preparing high-purity lithium hexafluorophosphate, so as to solve the technical problem that in the above-mentioned prior art lithium hexafluorophosphate production process, free water in hexafluorophosphate is removed by using a dehydrating agent, but the dehydrating agent still exists in the hexafluorophosphate and participates in the subsequent thermal decomposition process of the hexafluorophosphate, resulting in low purity of the finally produced lithium hexafluorophosphate.

[0008] In order to solve the above problems, the technical solution adopted by the present invention is as follows: A method for preparing high-purity lithium hexafluorophosphate, characterized in that it comprises the following steps:

[0009] S1: Preparation of hexafluorophosphoric acid: under vacuum or inert atmosphere, phosphoric acid and anhydrous hydrogen fluoride liquid are introduced into the first microreactor as starting materials for reaction to obtain a hexafluorophosphoric acid aqueous solution;

[0010] S2: Azeotropic dehydration: Add a dehydration entrainer to the hexafluorophosphoric acid aqueous solution obtained in S1, heat to boiling, and evaporate the entrainer and water together to remove water to obtain hexafluorophosphoric acid liquid;

[0011] S3: Preparation of phosphorus pentafluoride: directly heating and pyrolyzing the hexafluorophosphoric acid liquid obtained in S2 to generate phosphorus pentafluoride gas, which is then condensed and distilled to obtain high-purity phosphorus pentafluoride;

[0012] S4: preparing lithium hexafluorophosphate: introducing the high-purity phosphorus pentafluoride obtained in S3 and the hydrogen fluoride solution of lithium fluoride into the second microreactor respectively for reaction to obtain a lithium hexafluorophosphate synthetic solution;

[0013] S5: Purification of lithium hexafluorophosphate: crystallizing, filtering and drying the lithium hexafluorophosphate synthesis solution obtained in S4 to obtain a high-purity lithium hexafluorophosphate product;

[0014] The S2 dehydration azeotropic agent is selected to have an azeotropic point with water not higher than 75°C.

[0015] The beneficial effects of this embodiment are:

[0016] 1. In the prior art, after the dehydrating agent combines the water in hexafluorophosphoric acid, it cannot be separated from the hexafluorophosphoric acid. For example, the equation for the adsorption of water by fuming sulfuric acid is SO3+H2O→H2SO4. It can be seen from this that it is very difficult to separate H2SO4 from hexafluorophosphoric acid. Therefore, in the prior art, the dehydrating agent is directly heated and pyrolyzed together with the hexafluorophosphoric acid. Therefore, the dehydrating agent directly participates in the subsequent pyrolysis process after dehydrating. This not only causes the complexed water in the dehydrating agent (complex dehydrating agent: such as organic acid anhydride dehydrating agent) to be dehydrated at the high temperature of pyrolysis, but also causes the complexed water in the dehydrating agent (complex dehydrating agent: such as organic acid anhydride dehydrating agent) to be dehydrated at the high temperature of pyrolysis. At a temperature of about 140 degrees, part of it will be removed from the dehydrating agent, affecting the pyrolysis of hexafluorophosphoric acid, and causing the dehydrating agent to pyrolyze into other impurities during the pyrolysis process (such as sulfuric acid generated by oleum will pyrolyze into sulfur dioxide), ultimately resulting in low purity of lithium hexafluorophosphate. However, the present application utilizes azeotropic separation, and the azeotropic point of the entrainer and water is lower than the pyrolysis temperature. The pyrolysis is performed after the water, the entrainer and the hexafluorophosphoric acid are separated by evaporation. Therefore, the entrainer will not participate in the later process, will not introduce impurities generated by the dehydrating agent, and the final purity of the lithium hexafluorophosphate is higher.

[0017] 2. In the prior art, there is only one means to control free water in hexafluorophosphate, but the present application provides an alternative solution that can also achieve the same purpose, so that manufacturers can freely choose according to actual production and raw material price changes in the production of lithium hexafluorophosphate, which is more flexible.

[0018] 3. Since hexafluorophosphoric acid will be pyrolyzed when heated, experimental data show that the pyrolysis rate of hexafluorophosphoric acid at 80°C is 64.3%, which limits the technical idea of ​​using evaporation to remove water from hexafluorophosphoric acid. Therefore, temperature control of evaporation to remove water is very important. If the azeotropic point of the entrainer and water is too high (more than 80°C), although the hexafluorophosphoric acid will not boil and volatilize during boiling, part of the hexafluorophosphoric acid will be pyrolyzed and discharged together with the steam of the entrainer and water. Compared with the hydrolysis of hexafluorophosphoric acid in the prior art, both will cause the loss of hexafluorophosphoric acid and increase the production cost of the lithium hexafluorophosphate method. Therefore, the present application studies the change of the pyrolysis temperature of hexafluorophosphoric acid and finds that the pyrolysis rate of hexafluorophosphoric acid changes significantly between 70-80°C, and its pyrolysis rate decreases significantly when it is lower than 75°C. Therefore, the present application selects an azeotropic agent having an azeotropic point of no more than 75°C with water, and explores the azeotropic effect and ratio of different azeotropic agents with hexafluorophosphoric acid through experiments. An organic azeotropic agent is selected to ensure that when the water in the hexafluorophosphoric acid solution reaches boiling and volatilization, the loss of hexafluorophosphoric acid is low, and its cost can be kept at a low level. Preferably, an azeotropic agent having an azeotropic point with water below 60°C is selected.

[0019] Furthermore, after the azeotropic agent and water in S2 evaporate together, the vapor is collected and condensed, and after condensation, the azeotropic agent and water are separated by a water separator, and the separated azeotropic agent is circulated to the hexafluorophosphoric acid aqueous solution in S2 for azeotropy again. In the prior art, in order to prevent a large amount of hydrolysis of hexafluorophosphoric acid, a dehydrating agent is usually added to the hexafluorophosphoric acid solution to remove free water in the hexafluorophosphoric acid through a chemical reaction, thereby achieving the degree of controlling the hydrolysis of the hexafluorophosphoric acid, thereby improving the purity and yield of phosphorus pentafluoride after the thermal decomposition of the hexafluorophosphoric acid, mainly the purity. However, in this technical means, free water is usually combined with the dehydrating agent to form crystal water or chemical water, so it is difficult to separate and cannot be recycled. For example, the equation for the adsorption of water by oleum is SO3+H2O→H2SO4, from which it can be seen that it is very difficult to decompose H2SO4 to obtain oleum sulfuric acid again. The present application uses an entrainer to remove water. The entrainer reduces the boiling point of water in the hexafluorophosphoric acid solution. Therefore, by low-temperature heating, the water body and the entrainer are evaporated together to achieve the purpose of separating free water. The entrainer and the water body are separated by a water separator, so that the entrainer can be recycled, which has better economic benefits and lower production costs.

[0020] Furthermore, the water separator is a pervaporation membrane, the pervaporation membrane technology is mature, and the separation of organic matter and water is stable, so that the overall cycle of the present application is stable.

[0021] Furthermore, the phosphoric acid in S1 is selected as pentapolyphosphoric acid with a content of 117% based on orthophosphoric acid, and the pentapolyphosphoric acid is preheated to 40-50° C. before entering the microreactor. Pentapolyphosphoric acid has a high viscosity, and preheating in advance reduces its viscosity, making it easier to fully mix in the microreactor.

[0022] Furthermore, the azeotropic agent is selected from ether, which accounts for 10% of the mass of the hexafluorophosphoric acid aqueous solution. Since the azeotropic point of ether is as low as 34° C., the pyrolysis loss of hexafluorophosphoric acid can be avoided to the maximum extent.

[0023] Further, the entrainer is chloroform, and the entrainer chloroform accounts for 15% of the mass of the hexafluorophosphoric acid aqueous solution. When the entrainer and water boil, they will boil at a fixed composition ratio, so when the azeotropic point of the entrainer is high, in order to avoid the thermal decomposition of the hexafluorophosphoric acid, its dosage should also be increased to avoid insufficient entrainer, resulting in the temperature of the hexafluorophosphoric acid aqueous solution rising, and the thermal decomposition of the hexafluorophosphoric acid increases the loss.

[0024] Furthermore, the reaction temperature of the first microreactor in S1 is controlled to be 10-15° C., and the weight ratio of phosphoric acid to anhydrous hydrogen fluoride is 1:1.45-1.7.

[0025] Furthermore, the pyrolysis temperature of the hexafluorophosphoric acid in the S3 is 140 to 160°C, and the pyrolysis time is 1 to 2.5 hours; the condensation temperature in the S3 is -35 to -25°C, and the distillation temperature is controlled at -75 to -60°C.

[0026] Furthermore, in S4, the molar ratio of phosphorus pentafluoride to lithium fluoride-hydrogen fluoride solution is 1:1.2-1.5, and the reaction temperature is controlled at -5-5°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0028] The following is a further detailed description of the technical solutions in the embodiments of the present invention: the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] The process of the present invention is as follows Figure 1 As shown:

[0030] Example 1

[0031] S1: Under vacuum or inert atmosphere, phosphoric acid is first introduced into the microreactor, and then anhydrous hydrogen fluoride is introduced. The reaction temperature is controlled at 15°C, and the weight ratio of phosphoric acid to anhydrous hydrogen fluoride is 1:1.45.

[0032] S2: After replacing the inert gas in the kettle with a stirring device, the hexafluorophosphoric acid aqueous solution obtained in S1 is directly transferred into the kettle with a stirring device, the kettle temperature is maintained at 15°C, and an appropriate amount of entrainer is introduced under stirring. The entrainer is selected from ether, and the entrainer ether accounts for 10% of the mass of the hexafluorophosphoric acid aqueous solution. It starts to boil when heated to about 34°C. The entrainer and water are evaporated together and separated by a water separator. The water separator uses a pervaporation membrane. The separated entrainer can be reused. Therefore, the entrainer separated from the water will be circulated into the hexafluorophosphoric acid aqueous solution. The entrainer circulation is used to continuously remove water from the hexafluorophosphoric acid aqueous solution. Water is removed multiple times until the hexafluorophosphoric acid aqueous solution is completely removed. The liquid level in the tank is detected by a liquid level detector. If the change in the liquid level within ten minutes is less than 0.1%, it is determined that the water is completely removed.

[0033] S3: The hexafluorophosphoric acid obtained in S2 is directly heated to 140°C for decomposition, and the pyrolysis time is 2.5 hours. The obtained phosphorus pentafluoride gas is condensed and distilled. The condensation temperature is -30°C, and the distillation temperature is controlled at -60°C to obtain high-purity phosphorus pentafluoride with a purity greater than 99.5%;

[0034] S4: Under vacuum or inert atmosphere conditions, firstly introduce a hydrogen fluoride solution containing lithium fluoride into the microreactor, and then slowly introduce high-purity phosphorus pentafluoride gas, the molar ratio of phosphorus pentafluoride to lithium fluoride-hydrogen fluoride solution is 1:1.2, and the reaction temperature is controlled at 0°C.

[0035] S5: The lithium hexafluorophosphate synthesis solution obtained in S4 is crystallized, filtered, and dried to obtain high-purity lithium hexafluorophosphate.

[0036] After testing, the purity of lithium hexafluorophosphate reached 99.96%, the DMC insoluble content was ≤200ppm, the free acid was ≤90ppm, the moisture was ≤20ppm, the metal impurity ion was ≤1ppm, and the content of other impurities (SO42-≤5ppm, Cl-≤2ppm).

[0037] Experimental Example 2

[0038] S1: Under vacuum or inert atmosphere, phosphoric acid is first introduced into the microreactor, and then anhydrous hydrogen fluoride is introduced. The reaction temperature is controlled at 10°C, and the weight ratio of phosphoric acid to anhydrous hydrogen fluoride is 1:1.7.

[0039] S2: After replacing the inert gas in the kettle with a stirring device, the hexafluorophosphoric acid aqueous solution obtained in S1 is directly transferred into the kettle with a stirring device, the kettle is maintained at 15°C, and an appropriate amount of entrainer is introduced under stirring. The entrainer is chloroform, and the entrainer chloroform accounts for 15% of the hexafluorophosphoric acid aqueous solution. It starts to boil when heated to 56°C. After the entrainer and water evaporate together, they are separated by a water separator. The water separator uses a pervaporation membrane. The separated entrainer can be reused. Therefore, the entrainer separated from the water will be circulated into the hexafluorophosphoric acid aqueous solution. The entrainer is circulated to continuously remove water from the hexafluorophosphoric acid aqueous solution, and the water is removed multiple times until the hexafluorophosphoric acid aqueous solution is completely removed. The liquid level in the tank is detected by a liquid level detector. If the change in the liquid level within ten minutes is less than 0.1%, it is determined that the water is completely removed.

[0040] S3: The hexafluorophosphoric acid obtained in S2 is directly heated to 160°C for decomposition, and the pyrolysis time is 1 hour. The obtained phosphorus pentafluoride gas is condensed and distilled. The condensation temperature is -25°C, and the distillation temperature is controlled at -70°C to obtain high-purity phosphorus pentafluoride with a purity greater than 99.5%;

[0041] S4: Under vacuum or inert atmosphere conditions, firstly introduce a hydrogen fluoride solution containing lithium fluoride into the microreactor, and then slowly introduce high-purity phosphorus pentafluoride gas, the molar ratio of phosphorus pentafluoride to lithium fluoride-hydrogen fluoride solution is 1:1.5, and the reaction temperature is controlled at -5°C.

[0042] S5: The lithium hexafluorophosphate synthesis solution obtained in S4 is crystallized, filtered, and dried to obtain high-purity lithium hexafluorophosphate.

[0043] After testing, the purity of lithium hexafluorophosphate reached 99.98%, the DMC insoluble content was ≤200ppm, the free acid was ≤90ppm, the moisture was ≤20ppm, the metal impurity ion was ≤1ppm, and the content of other impurities (SO42-≤5ppm, Cl-≤2ppm).

[0044] Example 3

[0045] S1: Under vacuum or inert atmosphere, phosphoric acid is first introduced into the microreactor, and then anhydrous hydrogen fluoride is introduced. The reaction temperature is controlled at 12.5°C, and the weight ratio of phosphoric acid to anhydrous hydrogen fluoride is 1:1.6.

[0046] S2: After replacing the inert gas in the kettle with a stirring device, the hexafluorophosphoric acid aqueous solution obtained in S1 is directly transferred into the kettle with a stirring device, the kettle temperature is maintained at room temperature, and an appropriate amount of entrainer is introduced under stirring. The entrainer is carbon tetrachloride, and the weight percentage of the entrainer in the hexafluorophosphoric acid aqueous solution is 20%. When heated to 65°C, it starts to boil. After the entrainer and water evaporate together, they are separated by a water separator. The water separator uses a pervaporation membrane. The separated entrainer can be reused. Therefore, the entrainer separated from the water will be circulated into the hexafluorophosphoric acid aqueous solution. The entrainer is circulated to continuously remove water from the hexafluorophosphoric acid aqueous solution, and the water is removed multiple times until the hexafluorophosphoric acid aqueous solution is completely removed. The liquid level in the tank is detected by a liquid level detector. If the change in the liquid level within ten minutes is less than 0.1%, it is determined that the water is completely removed.

[0047] S3: The hexafluorophosphoric acid obtained in S2 is directly heated to 140°C for decomposition, and the pyrolysis time is 2.5 hours. The obtained phosphorus pentafluoride gas is condensed and distilled. The condensation temperature is -30°C, and the distillation temperature is controlled at -60°C to obtain high-purity phosphorus pentafluoride with a purity greater than 99.5%;

[0048] S4: Under vacuum or inert atmosphere conditions, firstly introduce a hydrogen fluoride solution containing lithium fluoride into the microreactor, and then slowly introduce high-purity phosphorus pentafluoride gas, the molar ratio of phosphorus pentafluoride to lithium fluoride-hydrogen fluoride solution is 1:1.2, and the reaction temperature is controlled at 0°C.

[0049] S5: The lithium hexafluorophosphate synthesis solution obtained in S4 is crystallized, filtered, and dried to obtain high-purity lithium hexafluorophosphate.

[0050] After testing, the purity of lithium hexafluorophosphate reached 99.94%, the DMC insoluble content was ≤200ppm, the free acid was ≤90ppm, the moisture was ≤20ppm, the metal impurity ion was ≤1ppm, and the content of other impurities (SO42-≤5ppm, Cl-≤2ppm).

[0051] Example 4

[0052] S1: Under vacuum or inert atmosphere, phosphoric acid is first introduced into the microreactor, and then anhydrous hydrogen fluoride is introduced. The reaction temperature is controlled at 12.5°C, and the weight ratio of phosphoric acid to anhydrous hydrogen fluoride is 1:1.6.

[0053] S2: After replacing the inert gas in the kettle with a stirring device, the hexafluorophosphoric acid aqueous solution obtained in S1 is directly transferred into the kettle with a stirring device, and a proper amount of entrainer is introduced under stirring. The entrainer is 1,2-dichloroethane, and the weight percentage of the entrainer in the hexafluorophosphoric acid aqueous solution is 25%. It starts to boil when heated to 65°C. After the entrainer and water evaporate together, they are separated by a water separator. The water separator uses a pervaporation membrane. The separated entrainer can be reused. Therefore, the entrainer separated from the water will be circulated into the hexafluorophosphoric acid aqueous solution. The entrainer is circulated to continuously remove water from the hexafluorophosphoric acid aqueous solution, and the water is removed multiple times until the hexafluorophosphoric acid aqueous solution is completely removed. The liquid level in the tank is detected by a liquid level detector. If the change in the liquid level within ten minutes is less than 0.1%, it is considered that the water is completely removed.

[0054] S3: The hexafluorophosphoric acid obtained in S2 is directly heated to 140°C for decomposition, and the pyrolysis time is 2.5 hours. The obtained phosphorus pentafluoride gas is condensed and distilled. The condensation temperature is -30°C, and the distillation temperature is controlled at -60°C to obtain high-purity phosphorus pentafluoride with a purity greater than 99.5%;

[0055] S4: Under vacuum or inert atmosphere conditions, firstly introduce a hydrogen fluoride solution containing lithium fluoride into the microreactor, and then slowly introduce high-purity phosphorus pentafluoride gas, the molar ratio of phosphorus pentafluoride to lithium fluoride-hydrogen fluoride solution is 1:1.2, and the reaction temperature is controlled at 0°C.

[0056] S5: The lithium hexafluorophosphate synthesis solution obtained in S4 is crystallized, filtered, and dried to obtain high-purity lithium hexafluorophosphate.

[0057] After testing, the purity of lithium hexafluorophosphate reached 99.97%, the DMC insoluble content was ≤200ppm, the free acid was ≤90ppm, the moisture was ≤20ppm, the metal impurity ion was ≤1ppm, and the content of other impurities (SO42-≤5ppm, Cl-≤2ppm).

[0058] The above is only an embodiment of the present invention, and the common knowledge such as the known specific structure and characteristics in the scheme is not described in detail here. It should be pointed out that for those skilled in the art, several deformations and improvements can be made without departing from the structure of the present invention, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing high-purity lithium hexafluorophosphate, characterized in that: The following steps are involved: S1: Preparation of hexafluorophosphoric acid: under vacuum or inert atmosphere, phosphoric acid and anhydrous hydrogen fluoride liquid are introduced into the first microreactor as starting materials for reaction to obtain a hexafluorophosphoric acid aqueous solution; S2: Azeotropic dehydration: Add a dehydration entrainer to the hexafluorophosphoric acid aqueous solution obtained in S1, heat to boiling, and evaporate the entrainer and water together to remove water to obtain hexafluorophosphoric acid liquid; S3: Preparation of phosphorus pentafluoride: directly heating and pyrolyzing the hexafluorophosphoric acid liquid obtained in S2 to generate phosphorus pentafluoride gas, which is then condensed and distilled to obtain high-purity phosphorus pentafluoride; S4: preparing lithium hexafluorophosphate: introducing the high-purity phosphorus pentafluoride obtained in S3 and the hydrogen fluoride solution of lithium fluoride into the second microreactor respectively for reaction to obtain a lithium hexafluorophosphate synthetic solution; S5: Purification of lithium hexafluorophosphate: crystallizing, filtering and drying the lithium hexafluorophosphate synthesis solution obtained in S4 to obtain a high-purity lithium hexafluorophosphate product; The S2 dehydration azeotropic agent is an organic azeotropic agent having an azeotropic point with water not higher than 75°C.

2. The method for preparing high-purity lithium hexafluorophosphate according to claim 1, characterized in that: After the azeotropic agent and water in S2 evaporate together, the steam is collected and condensed. After condensation, the azeotropic agent and water are separated by a water separator, and the separated azeotropic agent is circulated to the hexafluorophosphoric acid aqueous solution in S2 for azeotropy again.

3. The method for preparing high-purity lithium hexafluorophosphate according to claim 2, characterized in that: The water separator is a pervaporation membrane.

4. The method for preparing high-purity lithium hexafluorophosphate according to claim 1, characterized in that: The phosphoric acid in S1 is pentapolyphosphoric acid with a content of 117% based on orthophosphoric acid. The pentapolyphosphoric acid is preheated to 40-50° C. before being introduced into the microreactor.

5. The method for preparing high-purity lithium hexafluorophosphate according to claim 1, characterized in that: The azeotropic agent in S2 is selected from any one of 1,2-dichloroethane, ethyl acetate, ether, chloroform, carbon tetrachloride, benzene, and acrylonitrile, and the amount of the azeotropic agent is 10-25% of the mass of the hexafluorophosphoric acid aqueous solution.

6. The method for preparing high-purity lithium hexafluorophosphate according to claim 5, characterized in that: The azeotropic agent is ether, and the azeotropic agent ether accounts for 10% of the mass of the hexafluorophosphoric acid aqueous solution.

7. The method for preparing high-purity lithium hexafluorophosphate according to claim 5, characterized in that: The entrainer is chloroform, and the entrainer ether accounts for 15% of the mass of the hexafluorophosphoric acid aqueous solution.

8. The method for preparing high-purity lithium hexafluorophosphate according to claim 1, characterized in that: The reaction temperature of the first microreactor in S1 is controlled to be 10-15° C., and the weight ratio of phosphoric acid to anhydrous hydrogen fluoride is 1:1.45-1.

7.

9. The method for preparing high-purity lithium hexafluorophosphate according to claim 1, characterized in that: The pyrolysis temperature of the hexafluorophosphoric acid in the S3 is 140 to 160°C, and the pyrolysis time is 1 to 2.5 hours; the condensation temperature in the S3 is -35 to -25°C, and the distillation temperature is controlled at -75 to -60°C.

10. The method for preparing high-purity lithium hexafluorophosphate according to claim 1, characterized in that: In the step S4, the molar ratio of phosphorus pentafluoride to lithium fluoride-hydrogen fluoride solution is 1:1.2-1.5, and the reaction temperature is controlled at -5-5°C.

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