A method for continuously producing lithium hexafluorophosphate

Abstract

The application 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: uniformly mixing excess liquid PF5 and solid LiF in a beater, feeding the mixture into a continuous reactor, controlling the temperature in the reactor to be greater than 18.95 DEG C and the pressure to be greater than 3.39 MPa so that the PF5 is in a supercritical state and is stirred to react, and feeding the reaction material into a spray drying tower for solid-gas separation under reduced pressure, wherein the solid-phase product is lithium hexafluorophosphate, and the gaseous component is PF5 which is recycled as a reaction raw material after being compressed and condensed. In the preparation method, the supercritical PF5 is used as a reaction raw material and a heat and mass transfer carrier at the same time, the reaction efficiency can be significantly improved, and no other solvent or gaseous component is introduced, so that the lithium hexafluorophosphate can be efficiently and continuously prepared in high quality.

Description

TECHNICAL FIELD

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

[0002] In the current lithium hexafluorophosphate (LiPF6) production field, the mainstream process usually adopts a batch reaction system. The process for synthesizing LiPF6 by batch reaction includes mixing hydrogen fluoride (HF) with a lithium-containing compound (such as LiF, LiOH·H2O, etc.) and a phosphorus source (such as P2O5, phosphoric acid, phosphate, etc.) in batches, and then obtaining the final product through subsequent washing, evaporation and crystallization steps. The reaction process uses HF, which requires protection with dry inert gas, and the equipment sealing requirement is strict, making it difficult to produce on an industrial scale.

[0003] In addition, there is also a process for synthesizing LiPF6 using a gas-solid method, in which PF5 gas directly contacts LiF solid for reaction. The reaction of LiF solid with PF5 gas is carried out at a relatively high temperature and pressure, and no solvent is used in the reaction process. However, since it is a gas-solid reaction, the reaction only occurs on the surface of the solid, the conversion rate is low, and as the reaction proceeds, the raw material lithium fluoride will be completely covered by the generated lithium hexafluorophosphate, preventing the reaction from further proceeding. The final product contains a large amount of LiF, which needs to be separated and purified, and the process is relatively complex, making it difficult to obtain a high-purity product.

[0004] The organic solvent method is to react PF5 gas and LiF solid in a suitable organic solvent. The organic solvent used has good solubility for lithium hexafluorophosphate, so it can reduce the covering of the raw material lithium fluoride. However, the organic solvent itself and the trace water contained therein are prone to side reactions with PF5, and the solubility of PF5 gas in general organic solvents is poor, reducing the mixing and contacting effect of PF5 and LiF solid, and the reaction efficiency is low. At the same time, it is difficult to separate and crystallize the dissolved lithium hexafluorophosphate from the organic solvent, and multiple concentration and recrystallization purifications are required, making the process complex.

[0005] Supercritical fluid refers to a special fluid whose pressure and temperature are higher than its critical pressure (Pc of PF5 is 3.39 MPa) and critical temperature (Tc of PF5 is 18.95℃), and thus has good performance in dissolving other substances. Supercritical fluid is used as a solvent in the preparation of lithium hexafluorophosphate. For example, the patent for invention with publication number CN 107697933 A discloses a preparation method of lithium hexafluorophosphate, which comprises reacting lithium fluoride with phosphorus pentafluoride, and using liquid carbon dioxide or supercritical carbon dioxide fluid as a reaction solvent. However, the properties of carbon dioxide, lithium hexafluorophosphate and raw materials (lithium fluoride and phosphorus pentafluoride) used in the reaction are quite different, the mass transfer effect of the raw material mixing and contacting reaction is poor, resulting in low reaction efficiency; and there is still a part of the product lithium hexafluorophosphate coated on the raw material lithium fluoride, resulting in high insoluble content of the product. SUMMARY

[0006] In view of the above-mentioned defects and shortcomings of the prior art, the purpose of the present application is to provide a method for continuously preparing lithium hexafluorophosphate. The preparation method of the present application uses supercritical PF5 as a reaction raw material and a heat and mass transfer carrier, which can significantly improve the reaction efficiency, and does not introduce other solvents or gas phase components, thereby realizing efficient and high-quality continuous preparation of lithium hexafluorophosphate.

[0007] The purpose of the present application is achieved by the following technical solutions:

[0008] A method for continuously preparing lithium hexafluorophosphate, comprising the following preparation steps:

[0009] Excess liquid PF5 is mixed with solid LiF in a beater, and the mixture is introduced into a continuous reactor. The temperature in the reactor is controlled to be greater than 18.95℃ and the pressure is controlled to be greater than 3.39 MPa to make PF5 in a supercritical state for stirring reaction. The reaction material is subjected to pressure reduction and enters a spray drying tower for solid-gas separation. The solid phase product is lithium hexafluorophosphate, and the gas phase component is PF5 which is recycled as a reaction raw material after compression and condensation.

[0010] Further, in the above preparation method, the liquid PF5 is provided by a low-temperature and low-pressure storage tank. The low-temperature and low-pressure refers to that the temperature is controlled to be below the critical temperature of PF5, and the pressure is controlled to be the saturated vapor pressure of PF5 at the temperature. For example, the temperature of the PF5 storage tank is-50℃ and the pressure is 0.2 MPa. PF5 remains in a liquid state within the above temperature and pressure range.

[0011] Further, in the above preparation method, the temperature in the beater kettle is controlled to be below the critical temperature (18.95℃) of PF5, and the pressure is controlled to be the saturated vapor pressure of PF5 at the temperature. For example, the temperature is -40℃, and the pressure is 0.3Mpa, so as to maintain the PF5 in liquid form and form a good beater mixture with solid LiF.

[0012] Further, in the above preparation method, the molar ratio of the liquid PF5 mixed with solid LiF is (2~5):1. The theoretical molar ratio of PF5 and LiF to generate LiPF6 is 1:1. By adding excess PF5, on the one hand, the concentration of the reactants is increased, the reaction is promoted in the forward direction, the reaction efficiency is improved, and the LiF solid residue in the product is reduced; on the other hand, the excess supercritical state PF5 acts as a reaction heat transfer medium in the reactor, which can better stabilize the reaction heat release in the reactor, and the temperature control is more accurate. At the same time, the excess supercritical state PF5 acts as a mass transfer carrier in the reactor, which can quickly dissolve LiPF6 and disperse and penetrate LiF solid particles, significantly improving the reaction efficiency, and the reaction can be completed in minutes.

[0013] Further, in the above preparation method, the continuous reactor uses a dynamic mixer with solid gear stirring. The actual structure diagram is as shown in Figure 1 .

[0014] The application uses a dynamic mixer with solid gear stirring as a reactor, which uses the solid gear in the mixer to continuously cut and break the solid surface, increase the specific surface area of LiF, and further reduce the wrapping of LiF, and accelerate the reaction process.

[0015] Further, in the above preparation method, the supercritical state of PF5 is achieved by increasing the temperature in the reactor to above the critical temperature (18.95℃) of PF5 and maintaining the pressure in the reactor to above the critical pressure (3.39MPa) of PF5. For example, the temperature in the reactor is increased to 30~40℃, 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 Figure 2 ). The PF5 in the above supercritical state has good heat and mass transfer effect, strong penetration and solubility, which can promote the contact reaction with LiF and promote the dissolution of LiPF6 in the liquid phase, reduce the coating of LiPF6 on LiF, and solve the problems of low conversion rate of reactants and many insoluble products.

[0016] Further, in the above preparation method, the residence time of the stirring reaction is 0.5-1 min. In the continuous reactor of the present application, the excess supercritical PF5 is used as the reaction raw material and the reaction heat and mass transfer medium, and the solid gear mechanical cutting and crushing in the dynamic mixer are combined, so that the reaction can be quickly completed, and the conversion rate of more than 99.5% can be reached within 1 min.

[0017] Further, in the above preparation method, the reduced pressure refers to reducing the pressure of the material entering the spray drying tower to below the critical pressure (3.39 MPa) of PF5, so that PF5 is evaporated into a gaseous state to realize separation from the product lithium hexafluorophosphate. As an example, the reaction material can be reduced in pressure through a pressure reducing valve and then enter a spray drying tower with a pressure of 0.2-0.5 MPa and a temperature of -20-0°C, so that PF5 is evaporated into a gaseous state, thereby realizing separation of excess PF5 from the product lithium hexafluorophosphate. By controlling the low pressure and low temperature conditions in the spray drying tower, LiPF6 separated from the supercritical PF5 fluid can quickly form uniform spherical powder with a particle size distribution of 200-500 μm at low temperature, the solid product has fine and uniform particle size, the inclusion of impurities can be reduced, and the product has lower acidity.

[0018] Further, 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 content of <300 ppm, and an acidity of <20 ppm.

[0019] Compared with the prior art, the present application has the following beneficial effects:

[0020] (1) High reaction efficiency: the critical density of PF5 in the supercritical state is between gas and liquid, which is closer to liquid, has strong penetration and solubility, can quickly dissolve LiPF6 and disperse and penetrate LiF solid particles, promote the contact reaction with LiF and make LiPF6 dissolve in the liquid phase, reduce the coating of LiPF6 on LiF, thereby significantly improving the reaction efficiency, and solving the problems of low conversion rate of existing reaction materials and high insoluble content of products.

[0021] (2) High production efficiency: based on the high reaction efficiency of the present application, the synthesis reaction can be completed in a continuous reactor, the excess PF5 is separated by a spray drying tower and then continuously compressed and condensed back to the raw material tank for recycling, so that the whole process is continuous. At the same time, only two reaction media and one product exist in the reaction process, and no other solvent separation and treatment is involved, and no crystallization or evaporation operation and recovery operation is needed, which greatly simplifies the production process and improves the production efficiency.

[0022] (3) High reaction stability: the reaction of PF5 and LiF is obviously exothermic, and the supercritical state of PF5 has good heat transfer performance, which can ensure that the heat can be transferred in time when the reaction is fast, the temperature control is more stable, and the reaction stability is high.

[0023] (4) High product purity: the supercritical state PF5 is used as a reaction raw material and a heat and mass transfer carrier at the same time, which can significantly promote the reaction conversion of the solid LiF raw material, and does not introduce other solvents or gas phase components, has few side reactions, low insoluble content, and high product purity. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The physical structure diagram of the dynamic mixer used in the present application;

[0025] Figure 2 The saturation vapor pressure curve of PF5 is shown in the figure;

[0026] Figure 3 The process flow chart of the continuous preparation of lithium hexafluorophosphate in the example is shown. DETAILED DESCRIPTION

[0027] The present application will be further described in detail below in combination with examples and drawings, but the embodiments of the present application are not limited thereto.

[0028] Example 1

[0029] A method for continuously preparing lithium hexafluorophosphate, the process flow chart of which is shown in Figure 3 The method comprises the following preparation steps:

[0030] The PF5 storage tank 1 is used to store liquid PF5 material with a temperature of-50℃ and a pressure of 0.2Mpa, the liquid PF5 and the solid LiF are conveyed into the beater 2 in a molar ratio of 2:1, the temperature in the beater is controlled to be-40℃, and the pressure is 0.3Mpa, the mixed material in the beater is connected to the dynamic mixer 3 with solid gear stirring, the temperature in the dynamic mixer 3 is controlled to be 35℃, and the pressure is maintained at 3.5Mpa, so that the PF5 is in a supercritical state for stirring reaction, the material in the dynamic mixer stays for 0.75min, then passes through the pressure reducing valve 4 to enter the spray drying tower 6, the drying tower is provided with a heat preservation jacket, the pressure in the drying tower is controlled to be 0.4MPa, and the temperature is controlled to be-10℃, the solid phase product at the bottom of the drying tower is lithium hexafluorophosphate, which is collected through the product outlet 5; the gas phase component at the top of the drying tower is PF5, which is compressed by the compressor 7 and condensed by the condenser 8, then enters the PF5 storage tank 1 to circulate as a reaction raw material.

[0031] The product of lithium hexafluorophosphate obtained in this example is spherical powder particles with an average particle size of 405 μm, and the yield is 99.5%. The product is detected to have a purity (determined by ion chromatography) of 99.98%, an insoluble content (tested in dimethyl carbonate (DMC) solvent) of 210 ppm, and an acidity (determined by potentiometric titration) of 8 ppm.

[0032] Comparative Example 1 and Examples 2-4

[0033] A method for continuously preparing lithium hexafluorophosphate is compared with Example 1, and the molar ratio of PF5 to LiF is adjusted to 1:1, 3:1, 4:1 and 5:1 respectively, and the remaining steps are the same as those of Example 1.

[0034] The detection results of the lithium hexafluorophosphate products obtained in Comparative Example 1 and Examples 2-4 under different molar ratios of PF5 to LiF are shown in Table 1 below.

[0035] Table 1 Detection results of lithium hexafluorophosphate products obtained under different molar ratios of PF5 to LiF

[0036]

[0037] From the results of Table 1 and Example 1, it can be concluded that the use of excess PF5 as a reaction raw material and a heat and mass transfer carrier in the present application can achieve efficient and high-quality continuous preparation of lithium hexafluorophosphate.

[0038] Example 5

[0039] A method for continuously preparing lithium hexafluorophosphate is compared with Example 1, and the temperature in the dynamic mixer is controlled to 30°C, the pressure is 4 MPa, and the residence time of the material in the dynamic mixer is 1 min, and the rest is the same.

[0040] The product of lithium hexafluorophosphate obtained in this example has a yield of 99.5%. The product is detected to have a purity of 99.97%, an insoluble content of 250 ppm, an acidity of 14 ppm, and a particle size of 385 μm.

[0041] Example 6

[0042] A method for continuously preparing lithium hexafluorophosphate is compared with Example 1, and the temperature in the dynamic mixer is controlled to 40°C, the pressure is 5 MPa, and the residence time of the material in the dynamic mixer is 0.5 min, and the rest is the same.

[0043] The product of lithium hexafluorophosphate obtained in this example has a yield of 99.7%. The product is detected to have a purity of 99.99%, an insoluble content of 60 ppm, an acidity of 12 ppm, and a particle size of 375 μm.

[0044] Example 7

[0045] A continuous method for preparing lithium hexafluorophosphate, compared with Example 1, the pressure in the drying tower is controlled to be 0.5 MPa, the temperature is controlled to be -15℃, and the rest is the same.

[0046] The yield of lithium hexafluorophosphate product obtained in this example is 99.5%. The purity of the product is 99.96%, the insoluble content is 195 ppm, the acidity is 12 ppm, and the particle size is 446 μm.

[0047] Example 8

[0048] A continuous method for preparing lithium hexafluorophosphate, compared with Example 1, the pressure in the drying tower is controlled to be 0.2 MPa, the temperature is controlled to be -5℃, and the rest is the same.

[0049] The yield of lithium hexafluorophosphate product obtained in this example is 99.5%. The purity of the product is 99.97%, the insoluble content is 215 ppm, the acidity is 7 ppm, and the particle size is 230 μm.

[0050] Example 9

[0051] A continuous method for preparing lithium hexafluorophosphate, compared with Example 1, the temperature in the drying tower is controlled to be room temperature, and the rest is the same.

[0052] The yield of lithium hexafluorophosphate product obtained in this example is 99.5%. The purity of the product is 99.94%, the insoluble content is 224 ppm, the acidity is 54 ppm, and the particle size is 670 μm.

[0053] From the comparison results of this example with Example 1 and Examples 5-6, it can be seen that by controlling the low temperature condition of -20-0℃ in the spray drying tower, the product particle size is lower and the product acidity is significantly reduced.

[0054] Comparative Example 2

[0055] A continuous method for preparing lithium hexafluorophosphate, compared with Example 1, a supercritical carbon dioxide fluid with 5 times the mass of 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 supercritical, and the rest is the same.

[0056] The yield of the product tested after the same residence time reaction in this comparative example is 99.0%. The purity of the product is 99.91%, the insoluble content is 880 ppm, the acidity is 25 ppm, and the particle size is 175 μm.

[0057] From the above results, it can be seen that, under the same reaction time (0.75 min), the product yield and purity are significantly reduced and the insoluble content is significantly increased when supercritical carbon dioxide fluid is used as the reaction solvent compared with directly using excess supercritical PF5 as the reaction raw material and reaction medium. The reason 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 the concentration of PF5 as a reactant, and reducing the permeability of PF5 to LiF and the solubility of PF5 to LiPF6, resulting in a significant reduction in reaction efficiency and the presence of some product lithium hexafluorophosphate coating on the raw material lithium fluoride, thereby significantly reducing the product yield and increasing the insoluble content.

[0058] Comparative Example 3

[0059] A method for continuously preparing lithium hexafluorophosphate, compared with Example 1, the temperature in dynamic mixer 3 is controlled at 18°C (below the critical temperature of PF5 (18.95°C)), keeping PF5 in a liquid state for reaction, and the rest is the same.

[0060] The product yield of this comparative example after the same residence time reaction is 86.5%. The product detection purity is 98.87%, the insoluble content is 1.12%, the acidity is 33 ppm, and the particle size is 560 μm.

[0061] From the above results, it can be seen that the reaction efficiency is significantly reduced when liquid PF5 below the critical temperature is used for reaction, and the solid phase LiF raw material is difficult to be completely converted under the same reaction time (0.75 min). This shows that the higher temperature, strong penetration and solubility in the supercritical state of the present application can better promote the reaction conversion.

[0062] The above examples are preferred embodiments of the present application, but the embodiments of the present application are not limited by the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application are equivalent replacement methods and are included in the protection scope of the present application.

Claims

1. A method for the continuous production of lithium hexafluorophosphate, characterized in that, The preparation comprises the following steps: The excess liquid PF5 is mixed with solid LiF in a beater to form a mixture, the mixture is introduced into a continuous reactor, the temperature in the reactor is controlled at 30-40℃ and the pressure is controlled at 3.5-5MPa to make PF5 in a supercritical state, the reaction is stirred, the reaction mixture is introduced into a spray drying tower for solid-gas separation under reduced pressure, the solid product is lithium hexafluorophosphate, and the gaseous component PF5 is recycled as a reaction raw material after compression and condensation; The molar ratio of the liquid PF5 to the solid LiF is (2-5):

1.

2. The method of claim 1, wherein the method is a continuous method of producing lithium hexafluorophosphate, characterized in that, The liquid PF5 is provided by a low-temperature and low-pressure storage tank, the low-temperature and low-pressure refers to that the temperature is controlled below the critical temperature of PF5 and the pressure is controlled at the saturated vapor pressure of PF5 at the temperature; the temperature in the beater is controlled below the critical temperature of PF5 and the pressure is controlled at the saturated vapor pressure of PF5 at the temperature.

3. The method of claim 1, wherein the method is continuous. The continuous reactor is a dynamic mixer with solid gear stirring.

4. The method of claim 1, wherein the method is a continuous method of producing lithium hexafluorophosphate. The residence time of the stirring reaction is 0.5-1min.

5. The method of claim 1, wherein the method is a continuous method of producing lithium hexafluorophosphate. The reduced pressure refers to that the pressure of the material introduced into the spray drying tower is reduced to below the critical pressure of PF5, so that PF5 is evaporated into a gaseous state to realize separation from the product lithium hexafluorophosphate.

6. The method of claim 5, wherein the method is continuous. The reduced pressure refers to that the reaction material is reduced in pressure by a pressure reducing valve and then introduced into a spray drying tower with a pressure of 0.2-0.5MPa and a temperature of -20-0℃, so that PF5 is evaporated into a gaseous state to realize separation of the excess PF5 from the product lithium hexafluorophosphate.

7. The method of claim 1, wherein the method is continuous. The obtained lithium hexafluorophosphate solid product is spherical powder particles with a particle size distribution of 200-500μm, the purity is >99.95%, the insoluble content is <300ppm, and the acidity is <20ppm.

Images