Method for continuously preparing lithium hexafluorophosphate

Abstract

The invention belongs to the technical field of electrolyte material preparation, and particularly relates to a method for continuously preparing lithium hexafluorophosphate. The preparation method comprises the following steps: pulping and uniformly mixing excessive liquid PF5 and solid LiF in a pulping kettle, introducing the mixed material into a continuous reactor, controlling the temperature in the reactor to be greater than 18.95 DEG C and the pressure in the reactor to be greater than 3.39 MPa so that the PF5 is in a supercritical state, carrying out stirring reaction, decompressing the reaction material, feeding the reaction material into a spray drying tower, and carrying out solid-gas separation, the solid-phase product is lithium hexafluorophosphate, and the gas-phase component is PF5 which is compressed, condensed and recycled as a reaction raw material. According to the preparation method, the supercritical PF5 serves as a reaction raw material and a heat and mass transfer carrier at the same time, the reaction efficiency can be remarkably improved, other solvents or gas phase components are not introduced, and therefore efficient and high-quality continuous preparation of lithium hexafluorophosphate is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of electrolyte material preparation, and particularly relates to a method for continuously preparing lithium hexafluorophosphate. Background Art

[0002] In the current production of lithium hexafluorophosphate (LiPF6), the mainstream process typically utilizes a batch reaction system. This process involves batch-wise mixing hydrofluoric acid (HF) with a lithium-containing compound (such as LiF, LiOH·H2O, etc.) and a phosphorus source (such as P2O5, phosphoric acid, or phosphates). The final product is then obtained through subsequent washing, evaporation, and crystallization steps. This reaction utilizes HF, requiring the use of a dry inert gas atmosphere and stringent equipment sealing requirements, making industrial-scale production challenging.

[0003] Another process for synthesizing LiPF6 is the gas-solid method, which involves directly contacting PF5 gas with solid LiF. The reaction occurs at high temperatures and pressures, without the use of solvents. However, because it is a gas-solid reaction, the reaction occurs only on the solid surface, resulting in low conversion rates. Furthermore, as the reaction proceeds, the raw material lithium fluoride is completely coated by the resulting lithium hexafluorophosphate, preventing further reaction. The final product contains a large amount of LiF, requiring separation and purification, a complex process that makes it difficult to obtain a high-purity product.

[0004] The organic solvent method involves reacting PF5 gas and LiF solid in a suitable organic solvent. The organic solvent used has good solubility for lithium hexafluorophosphate, thereby reducing its coating on the raw material lithium fluoride. However, the organic solvent itself and the trace water it contains are prone to side reactions with PF5. In addition, the solubility of PF5 gas in general organic solvents is poor, which reduces the mixed contact effect of PF5 and LiF solid, resulting in low reaction efficiency. At the same time, it is difficult to crystallize and separate the dissolved lithium hexafluorophosphate from the organic solvent, requiring multiple concentration and recrystallization purification processes, resulting in a relatively complex process.

[0005] A supercritical fluid is a special fluid whose pressure and temperature simultaneously exceed its critical pressure (Pc = 3.39 MPa for PF5) and critical temperature (Tc = 18.95°C for PF5), resulting in excellent solubility of other substances. The use of supercritical fluids as solvents has been studied in the preparation of lithium hexafluorophosphate. For example, patent publication CN 107697933 A describes a method for preparing lithium hexafluorophosphate, which involves reacting lithium fluoride with phosphorus pentafluoride using liquid carbon dioxide or supercritical carbon dioxide fluid as the reaction solvent. However, the properties of carbon dioxide differ significantly from those of lithium hexafluorophosphate and the raw materials lithium fluoride and phosphorus pentafluoride used in the reaction. This leads to poor mass transfer during the mixed contact reaction, resulting in low reaction efficiency. Furthermore, some of the lithium hexafluorophosphate product still coats the raw lithium fluoride, resulting in a high content of insoluble matter in the product. Summary of the Invention

[0006] To address the shortcomings and deficiencies of the prior art, the present invention provides a method for continuously preparing lithium hexafluorophosphate. This method utilizes supercritical PF5 as both a reaction raw material and a heat and mass transfer medium, significantly improving reaction efficiency without introducing additional solvents or gaseous components, thereby achieving efficient, high-quality, continuous production of lithium hexafluorophosphate.

[0007] The purpose of the present invention is achieved through the following technical solutions: A method for continuously preparing lithium hexafluorophosphate comprises the following steps: Excess liquid PF5 and solid LiF are slurried and mixed evenly in a slurry kettle, and the mixture is introduced into a continuous reactor. The temperature in the reactor is controlled to be greater than 18.95°C and the pressure is greater than 3.39 MPa to make PF5 in a supercritical state for stirring reaction. The reaction materials are decompressed and enter a spray drying tower for solid-gas separation. The solid phase product is lithium hexafluorophosphate, and the gas phase component is PF5, which is compressed and condensed and then recycled as a reaction raw material.

[0008] Furthermore, in the above preparation method, the liquid PF5 is provided by a low-temperature, low-pressure storage tank. This refers to a temperature controlled below the critical temperature of PF5 and a pressure controlled to the saturated vapor pressure of PF5 at that temperature. For example, a PF5 storage tank at -50°C and a pressure of 0.2 MPa maintains PF5 in a liquid state within the above temperature and pressure ranges.

[0009] Furthermore, in the above preparation method, the temperature in the beating kettle is controlled to be below the critical temperature of PF5 (18.95°C), and the pressure is controlled to be the saturated vapor pressure of PF5 at that temperature. For example, the temperature is -40°C and the pressure is 0.3 MPa, so as to maintain a good beating mixture of PF5 in liquid form with solid LiF.

[0010] Furthermore, in the above preparation method, the molar ratio of the liquid PF5 to the solid LiF is (2-5):1. The theoretical molar ratio of PF5 and LiF to produce LiPF6 is 1:1. The present invention, by adding excess PF5, increases the reactant concentration, promotes the forward reaction, improves reaction efficiency, and reduces LiF solid residue in the product. Furthermore, the excess supercritical PF5 acts as a heat transfer medium in the reactor, effectively stabilizing the exothermic reaction within the reactor and providing more precise temperature control. Furthermore, the excess supercritical PF5 acts as a mass transfer carrier within the reactor, rapidly dissolving LiPF6 and dispersing and penetrating the LiF solid particles, significantly improving reaction efficiency and allowing the reaction to be completed in just minutes.

[0011] Furthermore, in the above preparation method, the continuous reactor adopts a dynamic mixer with solid gear stirring. Its physical structure is shown in the figure Figure 1 shown.

[0012] The present invention adopts a dynamic mixer with solid gear stirring as a reactor. The solid gears in the mixer are used to continuously cut and crush the solid surface, thereby increasing the specific surface area of ​​LiF, further reducing the situation of LiF being wrapped, and accelerating the reaction process.

[0013] Furthermore, in the above preparation method, the PF5 is brought into a supercritical state by increasing the temperature in the reactor to above the critical temperature of PF5 (18.95°C) and maintaining the pressure in the reactor above the critical pressure of PF5 (3.39MPa). For example, the temperature in the reactor is increased to 30-40°C and the pressure in the reactor is maintained at 3.5-5MPa. Under the above temperature and pressure conditions, PF5 is in a supercritical state (the saturated vapor pressure curve of PF5 is as shown in FIG. Figure 2 PF5 in the supercritical state has good heat and mass transfer effects, strong penetrability and solubility, and can promote the contact reaction with LiF and promote the dissolution of LiPF6 in the liquid phase, reducing the coating of LiPF6 on LiF, and solving the problems of low reactant conversion rate and high insoluble matter in the product.

[0014] Furthermore, in the above preparation method, the residence time of the stirring reaction is 0.5 to 1 minute. The use of excess supercritical PF5 in the continuous reactor of the present invention as both the reaction raw material and the reaction heat and mass transfer medium, combined with the mechanical cutting and crushing of solid gears in the dynamic mixer, enables the reaction to be completed rapidly, achieving a conversion rate of over 99.5% within a reaction time of 1 minute.

[0015] Furthermore, in the above-mentioned preparation method, the decompression refers to reducing the pressure of the material entering the spray drying tower to below the critical pressure of PF5 (3.39 MPa), causing the PF5 to evaporate into a gaseous state and thereby separate it from the product lithium hexafluorophosphate. For example, the reaction materials can be decompressed through a pressure reducing valve before entering a spray drying tower at a pressure of 0.2-0.5 MPa and a temperature of -20-0°C, causing the PF5 to evaporate into a gaseous state, thereby separating the excess PF5 from the product lithium hexafluorophosphate. By controlling the low pressure and low temperature conditions within the spray drying tower, the LiPF6 released from the supercritical PF5 fluid can be rapidly agglomerated at low temperatures to form a uniform spherical powder with a particle size distribution of 200-500 μm. The resulting solid product has a fine and uniform particle size, which can reduce the entrapment of impurities and reduce the acidity of the product.

[0016] Furthermore, in the above preparation method, the obtained lithium hexafluorophosphate solid phase product is spherical powder particles with a particle size distribution of 200-500 μm, a purity of >99.95%, an insoluble matter content of <300 ppm, and an acidity of <20 ppm.

[0017] Compared with the prior art, the present invention has the following beneficial effects: (1) High reaction efficiency: The critical density of PF5 in the supercritical state is between that of gas and liquid, closer to that of liquid. It has strong penetrability and solubility, can quickly dissolve LiPF6 and disperse and penetrate LiF solid particles, promote contact reaction with LiF and promote LiPF6 to dissolve in the liquid phase, reduce LiPF6 coating on LiF, thereby significantly improving the reaction efficiency and solving the existing problems of low reactant conversion rate and high insoluble product.

[0018] (2) High production efficiency: Based on the high reaction efficiency of the present invention, the synthesis reaction can be completed in a continuous reactor. The excess PF5 in the reaction is separated by a spray drying tower and then continuously compressed and condensed and returned to the raw material storage tank for recycling, making the entire process continuous. At the same time, there are only two reaction media and one product in the reaction process, and no other solvent separation and treatment is involved. There is no need for separation and recovery operations such as crystallization or evaporation of solvents, which greatly simplifies the production process and improves production efficiency.

[0019] (3) High reaction stability: The reaction between PF5 and LiF releases heat significantly. PF5 in the supercritical state has good heat transfer performance, which can ensure that the heat can be transferred in time during rapid reaction, making the temperature control more stable and the reaction stability high.

[0020] (4) High product purity: The present invention uses excess supercritical PF5 as both a reaction raw material and a heat and mass transfer carrier, which can significantly promote the reaction conversion of solid-phase LiF raw materials without introducing other solvents or gas phase components. It has few side reactions, low insoluble matter content, and high product purity. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A physical structural diagram of the dynamic mixer used in the present invention; Figure 2 is the saturated vapor pressure curve of PF5; Figure 3 This is a process flow chart for the continuous preparation of lithium hexafluorophosphate in the examples. DETAILED DESCRIPTION

[0022] The present invention will be described in further detail below with reference to the embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0023] Example 1 A method for continuously preparing lithium hexafluorophosphate, the process flow chart of which is as follows Figure 3 As shown, the preparation steps include: A PF5 storage tank 1 is used to store liquid PF5 material with a temperature of -50°C and a pressure of 0.2Mpa. The liquid PF5 and solid LiF are transported to a beating kettle 2 in a molar ratio of 2:1 for beating and mixing evenly. The temperature in the beating kettle is controlled to -40°C and the pressure is 0.3Mpa. The mixed material in the beating kettle is connected to a dynamic mixer 3 with solid gear stirring. The temperature in the dynamic mixer 3 is controlled to 35°C and the pressure is maintained at 3.5Mpa, so that PF5 is in a supercritical state for stirring reaction. After the material in the dynamic mixer stays for 0.75min, it is decompressed through a pressure reducing valve 4 and enters a spray drying tower 6. The drying tower is equipped with an insulation jacket. The pressure in the drying tower is controlled to 0.4MPa and the temperature is controlled to -10°C. The solid phase product at the bottom of the drying tower is lithium hexafluorophosphate, which is discharged and collected through the product outlet 5; the gaseous phase component at the top of the drying tower is PF5, which is compressed by a compressor 7 and condensed by a condenser 8 and then enters the PF5 storage tank 1 for circulation as a reaction raw material.

[0024] The lithium hexafluorophosphate product obtained in this example was spherical powder particles with an average particle size of 405 μm, and the yield was 99.5%. The product purity (as determined by ion chromatography) was 99.98%, the insoluble matter content (as determined in dimethyl carbonate (DMC) solvent) was 210 ppm, and the acidity (as determined by potentiometric titration) was 8 ppm.

[0025] Comparative Example 1 and Examples 2 to 4 Comparative Example 1 and Examples 2 to 4 provide a method for continuously preparing lithium hexafluorophosphate. Compared with Example 1, the molar ratios of PF5 and LiF are adjusted to 1:1, 3:1, 4:1, and 5:1, respectively. The remaining steps are the same as those in Example 1.

[0026] The test results of the lithium hexafluorophosphate products obtained in Comparative Example 1 and Examples 2 to 4 under different molar ratios of PF5 to LiF are shown in Table 1 below.

[0027] Table 1 Test results of lithium hexafluorophosphate products obtained under different molar ratios of PF5 to LiF

[0028] It can be concluded from the results of Table 1 and Example 1 that the present invention uses excess PF5 as both a reaction raw material and a heat and mass transfer carrier, which can achieve efficient, high-quality continuous preparation of lithium hexafluorophosphate.

[0029] Example 5 A method for continuously preparing lithium hexafluorophosphate is provided. Compared with Example 1, the temperature in the dynamic mixer is controlled to 30° C., the pressure is 4 MPa, the residence time of the material in the dynamic mixer is 1 min, and the rest are the same.

[0030] The yield of lithium hexafluorophosphate obtained in this example was 99.5%, the product purity was 99.97%, the insoluble matter content was 250 ppm, the acidity was 14 ppm, and the particle size was 385 μm.

[0031] Example 6 A method for continuously preparing lithium hexafluorophosphate is provided. Compared with Example 1, the temperature in the dynamic mixer is controlled to 40° C., the pressure is 5 MPa, the residence time of the material in the dynamic mixer is 0.5 min, and the rest are the same.

[0032] The yield of lithium hexafluorophosphate obtained in this example was 99.7%. The product purity was 99.99%, the insoluble matter content was 60 ppm, the acidity was 12 ppm, and the particle size was 375 μm.

[0033] Example 7 A method for continuously preparing lithium hexafluorophosphate. Compared with Example 1, the pressure in the drying tower is controlled to 0.5 MPa, the temperature is controlled to -15°C, and the rest are the same.

[0034] The yield of lithium hexafluorophosphate obtained in this example was 99.5%, the product purity was 99.96%, the insoluble matter content was 195 ppm, the acidity was 12 ppm, and the particle size was 446 μm.

[0035] Example 8 A method for continuously preparing lithium hexafluorophosphate. Compared with Example 1, the pressure in the drying tower is controlled to 0.2 MPa, the temperature is controlled to -5°C, and the rest are the same.

[0036] The yield of lithium hexafluorophosphate obtained in this example was 99.5%, the product purity was 99.97%, the insoluble matter content was 215 ppm, the acidity was 7 ppm, and the particle size was 230 μm.

[0037] Example 9 A method for continuously preparing lithium hexafluorophosphate. Compared with Example 1, the temperature in the drying tower is controlled to room temperature, and the rest is the same.

[0038] The yield of lithium hexafluorophosphate obtained in this example was 99.5%. The product purity was 99.94%, the insoluble matter content was 224 ppm, the acidity was 54 ppm, and the particle size was 670 μm.

[0039] By comparing the results of this embodiment with those of embodiment 1 and embodiments 5-6, it can be seen that the present invention can control the low temperature condition of -20-0°C in the spray drying tower to obtain a smaller product particle size and significantly reduce the acidity of the product.

[0040] Comparative Example 2 A method for continuously preparing lithium hexafluorophosphate. Compared with Example 1, a supercritical carbon dioxide fluid with a mass five times that of the lithium fluoride is introduced into the dynamic mixer 3 as a reaction solvent, and the pressure in the dynamic mixer is increased to 8 MPa to make the carbon dioxide in a supercritical state. The rest of the method is the same.

[0041] The product yield of this comparative example after the same residence time was tested to be 99.0%, the product purity was 99.91%, the insoluble matter content was 880 ppm, the acidity was 25 ppm, and the particle size was 175 μm.

[0042] The above results show that under the same reaction time (0.75 min), the use of supercritical carbon dioxide fluid as the reaction solvent significantly reduces the product yield and purity, and significantly increases the insoluble matter content, compared to the present invention's direct use of excess supercritical PF5 as both the reaction raw material and the reaction medium. The reason for this is that the presence of a large amount of supercritical carbon dioxide dilutes the PF5, which is also in a supercritical state, resulting in a significant reduction in its concentration as a reactant. This also reduces the permeability of PF5 to LiF and its solubility in LiPF6, resulting in a significant reduction in reaction efficiency and the presence of some of the product, lithium hexafluorophosphate, coating the raw material lithium fluoride, thereby significantly reducing the yield of the obtained product and significantly increasing the insoluble matter content.

[0043] Comparative Example 3 A method for continuously preparing lithium hexafluorophosphate. Compared with Example 1, the temperature in the dynamic mixer 3 is controlled at 18°C ​​(below the critical temperature of PF5 (18.95°C)), and the PF5 is kept in a liquid state for the reaction. The rest of the process is the same.

[0044] The product yield of this comparative example after the same residence time was 86.5%, the product purity was 98.87%, the insoluble matter content was 1.12%, the acidity was 33 ppm, and the particle size was 560 μm.

[0045] The above results show that the reaction efficiency is significantly reduced when using liquid PF5 at a temperature below the critical temperature. At the same reaction time (0.75 min), the solid-phase LiF raw material in the subcritical state is difficult to completely convert. This indicates that the higher temperature, strong penetration, and solubility of the supercritical state of the present invention can better promote reaction conversion.

[0046] The above embodiments are preferred implementations of the present invention, but the implementations of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for continuously preparing lithium hexafluorophosphate, characterized in that: The method comprises the following preparation steps: Excess liquid PF5 and solid LiF are slurried and mixed evenly in a slurry kettle, and the mixture is introduced into a continuous reactor. The temperature in the reactor is controlled to be greater than 18.95°C and the pressure is greater than 3.39 MPa to make PF5 in a supercritical state for stirring reaction. The reaction materials are decompressed and enter a spray drying tower for solid-gas separation. The solid phase product is lithium hexafluorophosphate, and the gas phase component is PF5, which is compressed and condensed and then recycled as a reaction raw material.

2. The method for continuously preparing lithium hexafluorophosphate according to claim 1, wherein: The liquid PF5 is provided through a low-temperature and low-pressure storage tank, and the low temperature and low pressure means that the temperature is controlled below the critical temperature of PF5, and the pressure is controlled to the saturated vapor pressure of PF5 at this temperature; the temperature in the beating kettle is controlled below the critical temperature of PF5, and the pressure is controlled to the saturated vapor pressure of PF5 at this temperature.

3. The method for continuously preparing lithium hexafluorophosphate according to claim 1, wherein: The molar ratio of the liquid PF5 to the solid LiF is (2-5):

1.

4. The method for continuously preparing lithium hexafluorophosphate according to claim 1, wherein: The continuous reactor adopts a dynamic mixer with solid gear stirring.

5. The method for continuously preparing lithium hexafluorophosphate according to claim 1, characterized in that: The temperature in the reactor is controlled at 30-40° C., and the pressure is controlled at 3.5-5 MPa.

6. The method for continuously preparing lithium hexafluorophosphate according to claim 5, characterized in that: The residence time of the stirring reaction is 0.5 to 1 min.

7. The method for continuously preparing lithium hexafluorophosphate according to claim 1, characterized in that: The decompression refers to reducing the pressure of the material entering the spray drying tower to below the critical pressure of PF5, so that PF5 evaporates into a gaseous state and is separated from the product lithium hexafluorophosphate.

8. The method for continuously preparing lithium hexafluorophosphate according to claim 7, characterized in that: The decompression refers to reducing the pressure of the reaction materials through a pressure reducing valve and then entering a spray drying tower with a pressure of 0.2~0.5MPa and a temperature of -20~0°C to evaporate PF5 into a gaseous state, thereby achieving separation of excess PF5 and the product lithium hexafluorophosphate.

9. The method for continuously preparing lithium hexafluorophosphate according to claim 1, characterized in that: The obtained lithium hexafluorophosphate solid phase product is spherical powder particles with a particle size distribution of 200-500 μm, a purity of more than 99.95%, an insoluble matter content of less than 300 ppm, and an acidity of less than 20 ppm.

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