Process for the purification of lithium hexafluorophosphate by continuous flow crystallization

Abstract

This invention relates to the field of lithium battery technology and discloses a method for purifying lithium hexafluorophosphate by continuous flow crystallization. The method includes: (1) adding a first organic solvent to crude lithium hexafluorophosphate to dissolve it, preparing a saturated lithium hexafluorophosphate-organic solvent solution, filtering to remove insoluble matter, and obtaining a saturated lithium hexafluorophosphate solution; (2) feeding the saturated lithium hexafluorophosphate solution and the second organic solvent into a microchannel reactor for cooling using metering pumps, filtering the precipitated crystals, washing them with a cooled third organic solvent, and then vacuum drying. The lithium hexafluorophosphate precipitated by the method of this invention has high purity and stable quality, making it very suitable for industrial promotion and application.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and specifically to a method for purifying lithium hexafluorophosphate through continuous flow crystallization. Background Technology

[0002] Lithium hexafluorophosphate (LiPF6) is currently the most important electrolyte lithium salt used in commercially available lithium-ion batteries. Its quality determines the charge / discharge performance, lifespan, and safety of lithium-ion batteries. With the huge market demand and promising development prospects of the new energy vehicle industry and the lithium-ion battery industry, the market demand for LiPF6 will grow in tandem, indicating a very broad prospect for industrial development.

[0003] Currently, the most mature process for producing lithium hexafluorophosphate (LiPF6) mainly uses the hydrogen fluoride solution method. The main process involves reacting a phosphorus pentachloride / hydrogen fluoride solution and a lithium fluoride / hydrogen fluoride solution in a closed reactor to produce a LiPF6 solution, which still contains impurities such as incompletely converted lithium fluoride. LiPF6 needs to undergo low-temperature crystallization to precipitate from the hydrogen fluoride solution, followed by filtration and drying to obtain pure LiPF6. However, LiPF6 obtained through simple cooling crystallization often contains varying amounts of LiCl, LiF, PCl5, etc. Therefore, in the preparation of high-purity LiPF6 using the hydrogen fluoride solution method, the crystallization process is crucial for controlling product quality.

[0004] CN110182828A discloses a dynamic crystallization apparatus and method for lithium hexafluorophosphate. The method involves heating a crystallization vessel containing lithium hexafluorophosphate mother liquor to 20–35°C, evaporating 30% by weight of hydrogen fluoride, and then cooling and recovering the tail gas. Simultaneously, the crystallization vessel is cooled to 5–15°C, stirring is initiated, and the resulting solid is separated to obtain solid lithium hexafluorophosphate. However, while this method combines static and dynamic crystallization, it does not explicitly disclose the operating parameters and is considered crude.

[0005] CN102009972B discloses a method for preparing lithium hexafluorophosphate, in which lithium hexafluorophosphate mother liquor is transported to a crystallization tank at -70 to -80°C, allowed to settle and crystallize, filtered, and then dried at 50 to 70°C to obtain a crude product. The crude product is then pulverized and dried at 120 to 130°C to obtain the finished product, with nitrogen purging to replace residual hydrogen fluoride gas. The entire static crystallization process takes 48 hours, and the obtained product tends to agglomerate into large lumps, requiring crushing, and it is difficult to obtain particles with uniform particle size.

[0006] CN105600809B discloses a method and apparatus for preparing lithium hexafluorophosphate by dynamic crystallization. In a specially designed stirring device, during the initial stage of crystallization, a lithium hexafluorophosphate solution in the supersaturated metastable region is nucleated by ultrasonic induction to obtain uniform lithium hexafluorophosphate crystals, avoiding the precipitation of impurities. Simultaneously, the stirring crystallization method mitigates the instability of mass and heat transfer caused by industrial scale-up, solving the problem of crystal adhesion to the crystal walls and obtaining particles with uniform size and a purity of up to 99.99%. However, this method has complex process conditions, requires significant equipment investment, and still suffers from uneven temperature distribution within the crystallization vessel. The crystallization process takes 6–13 hours, making it an intermittent operation with low efficiency. Summary of the Invention

[0007] The purpose of this invention is to overcome the problems of low purity, uneven particle size, complex process conditions, and high equipment investment in existing lithium hexafluorophosphate solids. It provides a continuous flow crystallization method for purifying lithium hexafluorophosphate, which produces lithium hexafluorophosphate solids with high purity, stable quality, and uniform particle size, making it highly suitable for industrial promotion and application.

[0008] To achieve the above objectives, the present invention provides a method for purifying lithium hexafluorophosphate by continuous flow crystallization, the method comprising:

[0009] (1) Add the first organic solvent to the crude lithium hexafluorophosphate to be purified to dissolve it and prepare a saturated lithium hexafluorophosphate-organic solvent solution. After filtration, remove the insoluble matter and the filtrate is a saturated lithium hexafluorophosphate solution.

[0010] (2) The saturated lithium hexafluorophosphate solution and the second organic solvent in step (1) are respectively fed into the microchannel reactor by metering pumps for cooling. The precipitated crystals are filtered, washed with the cooled third organic solvent, and then vacuum dried.

[0011] This invention uses a saturated solution of lithium hexafluorophosphate and an organic solvent to be precisely mixed in proportion using a metering pump. This ensures thorough and uniform mixing in a microchannel crystallization reactor, resulting in good mass transfer.

[0012] The method of this invention involves a continuous flow of lithium hexafluorophosphate solution in the reactor, which is a dynamic crystallization process and does not result in any adhesion to the reactor walls. The continuous crystallization process results in high purity and stable quality of the precipitated lithium hexafluorophosphate solid, making it very suitable for industrial promotion and application. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of a microchannel reactor with a pulsed variable diameter microstructure.

[0014] Figure 2 This is a schematic diagram of the connection of the microchannel reactor module. Detailed Implementation

[0015] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0016] This invention provides a method for purifying lithium hexafluorophosphate by continuous flow crystallization, the method comprising:

[0017] (1) Add the first organic solvent to the crude lithium hexafluorophosphate to be purified to dissolve it and prepare a saturated lithium hexafluorophosphate-organic solvent solution. After filtration, remove the insoluble matter and the filtrate is a saturated lithium hexafluorophosphate solution.

[0018] (2) The saturated lithium hexafluorophosphate solution and the second organic solvent in step (1) are respectively fed into the microchannel reactor by metering pumps for cooling. The precipitated crystals are filtered, washed with the cooled third organic solvent, and then vacuum dried.

[0019] like Figure 1 As shown, the microchannel reactor is a microchannel reactor with a pulsed variable diameter microstructure, which includes an enhanced mixing-type pipe containing inclined guide vanes.

[0020] According to the present invention, filtration refers to the filtration operation using a ceramic filter element with a diameter of 0.5-20 μm.

[0021] According to a preferred embodiment of the present invention, the length of the inclined guide vane is 2-6 mm, the spacing between the inclined guide vanes is 1-3 mm, and the angle between the inclined guide vane and the pipe wall is 30°-60°. By adopting the aforementioned design, the material can be fully flowed and mixed without wall adhesion, and the product can be fully extracted.

[0022] According to a preferred embodiment of the present invention, the inner diameter of the microchannel reactor is 2-6 mm.

[0023] The pulse-diameter-variable microstructure described in this invention refers to a mixing-type pipe with inclined guide vanes. This structure can improve the mixing effect of the solid-liquid system, while enhancing the heat transfer of liquid crystallization inside the pipe and preventing the crystals precipitated inside the pipe from forming large clumps.

[0024] According to a particularly preferred embodiment of the present invention, the microchannel reactor of the pulsed variable diameter microstructure comprises: a shell-and-tube module containing an enhanced mixing-type pipe with inclined guide vanes, wherein reactants are transported inside the pipe and heat exchange medium is transported in the shell side.

[0025] According to a particularly preferred embodiment of the present invention, Figure 2 A schematic diagram of a microchannel reactor module connection is shown. The symmetrically arranged inclined guide vanes form one module. The microchannel reactor includes a pressure / flow control unit, a temperature control unit, and a shell-and-tube module containing enhanced mixing pipes with inclined guide vanes.

[0026] In this invention, the number of shell-and-tube modules is not particularly important as long as the objective of the invention can be achieved. According to a particularly preferred embodiment of the invention, the number of shell-and-tube modules is 7-8, and each shell-and-tube module is connected by a connecting pipe. By adopting the aforementioned preferred solution, the purity and production efficiency of lithium hexafluorophosphate can be further improved.

[0027] According to a particularly preferred embodiment of the present invention, each of the shell-and-tube modules is placed vertically and parallel to each other, and the liquid holding capacity of each shell-and-tube module is 4.25-13.5 mL. By adopting the aforementioned preferred embodiment, the purity and production efficiency of lithium hexafluorophosphate can be further improved.

[0028] The method of this invention has excellent heat transfer effect, the mixed liquid can be cooled quickly and accurately, there is no local temperature gradient, the cooling is more uniform, and the crystal particle size is more uniform; the lithium hexafluorophosphate solution flows continuously in the reactor, which is a dynamic crystallization process and there is no wall adhesion phenomenon.

[0029] In this invention, as long as the objective of the invention can be achieved, there are no special requirements for the weight ratio of the saturated lithium hexafluorophosphate solution to the second organic solvent in step (2). According to a preferred embodiment of the invention, the volume ratio of the saturated lithium hexafluorophosphate solution to the second organic solvent in step (2) is (0.7-1):1, preferably 1:1. By adopting the aforementioned preferred scheme, lithium hexafluorophosphate can be fully precipitated and crystallized, while impurities remain dissolved in the mother liquor.

[0030] In this invention, as long as the purpose of this invention can be achieved, the cooling method described in step (2) can be a conventional choice in the art. According to a preferred embodiment of this invention, the cooling method described in step (2) is a programmed cooling method.

[0031] According to a preferred embodiment of the present invention, the conditions for the programmed cooling include: maintaining at 0–5°C for 0.5–1.5 min; maintaining at -5–-15°C for 0.5–1.5 min; maintaining at -15–-25°C for 0.5–1.5 min; maintaining at -25–-35°C for 0.5–1.5 min; and maintaining at -35–-45°C for 2–4.5 min.

[0032] By adopting the aforementioned preferred scheme, gradient cooling crystallization can be achieved, which greatly reduces the instability of mass and heat transfer caused by industrial scale-up, and allows lithium hexafluorophosphate to grow crystals at a stable rate, resulting in particles with uniform particle size.

[0033] In this invention, the vacuum drying conditions in step (2) can be conventionally chosen in the art. According to a preferred embodiment of the invention, the vacuum drying conditions in step (2) include: a vacuum degree of 5-10 Pa and drying at 50-70°C for 7-9 hours. By adopting the aforementioned preferred scheme, the purity and production efficiency of lithium hexafluorophosphate can be further improved.

[0034] In this invention, as long as the purpose of this invention can be achieved, the first and third organic solvents in step (1) and step (2) can be conventional choices in the art. According to a preferred embodiment of this invention, the first and third organic solvents in steps (1) and (2) are each selected from one or more of diethyl ether, methyl tert-butyl ether, ethylene glycol dimethyl ether, ethylene carbonate, dimethyl carbonate and diethyl carbonate.

[0035] In order to further improve the purity and production efficiency of lithium hexafluorophosphate, according to a preferred embodiment of the present invention, the first and third organic solvents in steps (1) and (2) are diethyl ether.

[0036] In this invention, as long as the purpose of this invention can be achieved, there are no special requirements for the temperature of the third organic solvent in step (2). According to a preferred embodiment of this invention, the temperature of the third organic solvent in step (2) is -20 to -40°C.

[0037] In this invention, the second organic solvent can be a conventional choice in the art as long as it can achieve the purpose of this invention. According to a preferred embodiment of this invention, the second organic solvent is selected from one or more of n-pentane, n-hexane, n-heptane, cyclohexane, tetrahydrofuran, and 1,4-dioxane.

[0038] To further improve the purity and production efficiency of lithium hexafluorophosphate, according to a preferred embodiment of the present invention, the second organic solvent is n-heptane.

[0039] In this invention, as long as the objective of this invention can be achieved, there are no special requirements for the composition of the crude lithium hexafluorophosphate to be purified. According to a preferred embodiment of this invention, by weight, the crude lithium hexafluorophosphate to be purified contains ≥98% lithium hexafluorophosphate, ≤0.5% DMC insoluble matter, ≤0.02% moisture, ≤0.08% free acid (calculated as HF), ≤0.005% sulfate (calculated as SO4), ≤0.005% chloride (calculated as Cl), ≤0.008% iron (Fe), ≤0.001% potassium (K), and ≤0.008% sodium (Na).

[0040] According to a preferred embodiment of the present invention, the method includes filtration parameters including: using a ceramic filter element with a filtration accuracy of 0.5-20 μm. By adopting the aforementioned preferred scheme, the purity and production efficiency of lithium hexafluorophosphate can be further improved. The working pressure is adjusted according to the specific material properties.

[0041] This invention uses a saturated solution of lithium hexafluorophosphate and an organic solvent to be precisely mixed in proportion using a metering pump. The mixture is fully and uniformly mixed in the microstructure of the microchannel crystallization reactor, resulting in good mass transfer. The prepared lithium hexafluorophosphate has a purity of up to 99.98% and a yield of up to 85%.

[0042] The method of this invention involves a continuous flow of lithium hexafluorophosphate solution in the reactor, which is a dynamic crystallization process and does not result in any adhesion to the reactor walls. The continuous crystallization process results in high purity and stable quality of the precipitated lithium hexafluorophosphate solid, making it very suitable for industrial promotion and application.

[0043] The present invention will be further described below through specific embodiments. The scope of the present invention is not limited to the scope covered by the embodiments. In this embodiment, the lithium hexafluorophosphate product is tested in accordance with the requirements of HG / T4066-2008 standard, and the yield of lithium hexafluorophosphate product is expressed as Li.

[0044] Example 1

[0045] At room temperature, crude lithium hexafluorophosphate (with a composition of ≥98% lithium hexafluorophosphate, ≤0.5% DMC insoluble matter, ≤0.02% water, ≤0.08% free acid (as HF), ≤0.005% sulfate (as SO4), ≤0.005% chloride (as Cl), ≤0.008% iron (Fe), ≤0.001% potassium (K), and ≤0.008% sodium (Na)) was dissolved in diethyl ether to prepare a saturated solution. The solution was filtered through a 0.5 μm ceramic filter at a working pressure of 0.6 MPa to remove insoluble solid impurities, yielding a saturated filtrate of lithium hexafluorophosphate-diethyl ether. The saturated filtrate and n-heptane were pumped separately into a microchannel crystallization reactor with a pulse-variable diameter microstructure at a volume ratio of 1:1 using two metering pumps for continuous mixing and cooling. The flow rate of both pumps was 5 mL / min. The microstructure of the microchannel crystallization reactor is a circular tube with inclined guide vanes for enhanced mixing. The inclined guide vanes are 4 mm long, 2 mm apart, and form an angle of 30° with the tube wall. The inner diameter of the tube is 4 mm. Seven shell-and-tube modules are used, connected by connecting tubes and placed vertically and parallel. The circular tube holds 10 mL of liquid. The mixture is held at 0℃ for 1 min; -10℃ for 1 min; -20℃ for 1 min; -30℃ for 1 min; and -40℃ for 3 min. The crystallized liquid with precipitated solids is then transferred to a vacuum filter tank for filtration. The filter cake is washed with diethyl ether at -30℃ and finally dried in a vacuum oven at 60℃ for 8 h under a vacuum of 5 Pa to obtain pure lithium hexafluorophosphate with a purity of 99.98% and a yield of 85%.

[0046] Example 2

[0047] At room temperature, crude lithium hexafluorophosphate (with the following composition: lithium hexafluorophosphate content ≥98%, DMC insoluble matter ≤0.5%, moisture ≤0.02%, free acid (calculated as HF) ≤0.08%, sulfate (calculated as SO4) ≤0.005%, chloride (calculated as Cl) ≤0.005%, iron (Fe) ≤0.008%, potassium (K) ≤0.001%, sodium (Na) ≤0.008%) was dissolved in diethyl ether to prepare a saturated solution. The solution was filtered using a 0.5 μm ceramic filter at a working pressure of 0.6 MPa to remove insoluble solid impurities, yielding a saturated filtrate of lithium hexafluorophosphate-diethyl ether. Two metering pumps were used to continuously mix and cool the saturated filtrate and n-heptane in a volume ratio of 0.7:1 into a microchannel crystallization reactor with a pulse-variable diameter microstructure. The pump delivering the lithium hexafluorophosphate-diethyl ether saturated solution had a flow rate of 3.5 mL / min, and the pump delivering n-heptane had a flow rate of 5 mL / min. The microstructure of the microchannel crystallization reactor was a circular tube with inclined guide vanes for enhanced mixing. The inclined guide vanes were 2 mm long, 1 mm apart, and the angle between the vanes and the tube wall was 30°. The inner diameter of the tube was 2 mm. There were 8 shell-and-tube modules, which were connected by connecting tubes and placed vertically and parallel to each other. The liquid holding capacity of the circular tube was 4.25 mL. The mixture was kept at 0℃ for 0.5 min; -5℃ for 0.5 min; -15℃ for 0.5 min; -25℃ for 0.5 min; and -35℃ for 2 min. The precipitated solid was then transferred to a vacuum filter tank for filtration. The filter cake was washed with diethyl ether at -20℃ and then placed in a vacuum oven at 50℃ for 9 h under a vacuum of 8 Pa to obtain pure lithium hexafluorophosphate with a purity of 99.98% and a yield of 81.5%.

[0048] Example 3

[0049] At room temperature, crude lithium hexafluorophosphate (with the following composition: lithium hexafluorophosphate content ≥98%, DMC insoluble matter ≤0.5%, moisture ≤0.02%, free acid (calculated as HF) ≤0.08%, sulfate (calculated as SO4) ≤0.005%, chloride (calculated as Cl) ≤0.005%, iron (Fe) ≤0.008%, potassium (K) ≤0.001%, sodium (Na) ≤0.008%) was dissolved in diethyl ether to prepare a saturated solution. The solution was filtered using a 0.5 μm ceramic filter at a working pressure of 0.6 MPa to remove insoluble solid impurities, yielding a saturated filtrate of lithium hexafluorophosphate-diethyl ether. Two metering pumps were used to continuously mix and cool the saturated filtrate and n-heptane in a volume ratio of 0.8:1 into a microchannel crystallization reactor with a pulse-variable diameter microstructure. The pump delivering the lithium hexafluorophosphate-diethyl ether saturated solution had a flow rate of 4 mL / min, and the pump delivering n-heptane had a flow rate of 5 mL / min. The microstructure of the microchannel crystallization reactor was a circular tube with inclined guide vanes for enhanced mixing. The inclined guide vanes were 6 mm long, 2 mm apart, and at a 60° angle to the tube wall. The inner diameter of the tube was 6 mm. There were seven shell-and-tube modules, all connected by connecting pipes and placed vertically in parallel. The liquid holding capacity of the circular tube was 13.5 mL. The mixture was kept at 5℃ for 1.5 min; -15℃ for 1.5 min; -25℃ for 1.5 min; -35℃ for 1.5 min; and -45℃ for 4.5 min. The precipitated solid was then transferred to a vacuum filter tank for filtration. The filter cake was washed with diethyl ether at -40℃ and then placed in a vacuum oven at 60℃ for 8 h under a vacuum of 10 Pa to obtain pure lithium hexafluorophosphate with a purity of 99.96% and a yield of 84.5%.

[0050] Example 4

[0051] The method is the same as in Example 1, except that:

[0052] The second organic solvent, "n-heptane", is replaced with "n-pentane".

[0053] Operating under these conditions yielded pure lithium hexafluorophosphate with a purity of 99.95% and a yield of 83%.

[0054] Example 5

[0055] The method is the same as in Example 1, except that:

[0056] The first organic solvent, "diethyl ether", is replaced with "methyl tert-butyl ether";

[0057] The second organic solvent, "n-heptane", is replaced with "n-pentane".

[0058] Operating under these conditions yielded pure lithium hexafluorophosphate with a purity of 99.95% and a yield of 81%.

[0059] Example 6

[0060] The method is the same as in Example 1, except that:

[0061] The first organic solvent, "diethyl ether," is replaced with "ethylene glycol dimethyl ether."

[0062] The second organic solvent, "n-heptane", is replaced with "n-pentane".

[0063] Operating under these conditions yielded pure lithium hexafluorophosphate with a purity of 99.95% and a yield of 84%.

[0064] Example 7

[0065] The method is the same as in Example 1, except that:

[0066] The second organic solvent, "n-heptane", is replaced with "n-hexane";

[0067] Operating under these conditions yielded pure lithium hexafluorophosphate with a purity of 99.95% and a yield of 83%.

[0068] Example 8

[0069] The method is the same as in Example 1, except that:

[0070] The first organic solvent, "diethyl ether", is replaced with "methyl tert-butyl ether";

[0071] The second organic solvent, "n-heptane", is replaced with "n-hexane".

[0072] Operating under these conditions yielded pure lithium hexafluorophosphate with a purity of 99.90% and a yield of 81%.

[0073] Example 9

[0074] The method is the same as in Example 1, except that:

[0075] The first organic solvent, "diethyl ether," is replaced with "ethylene glycol dimethyl ether."

[0076] Operating under these conditions yielded pure lithium hexafluorophosphate with a purity of 99.92% and a yield of 83%.

[0077] Example 10

[0078] The method is the same as in Example 1, except that:

[0079] The first organic solvent, "diethyl ether," is replaced with "ethylene carbonate."

[0080] The second organic solvent, "n-heptane", is replaced with "tetrahydrofuran".

[0081] Operating under these conditions yielded pure lithium hexafluorophosphate with a purity of 99.79% and a yield of 83.2%.

[0082] Example 11

[0083] The method is the same as in Example 1, except that:

[0084] The first organic solvent, "diethyl ether," is replaced with "diethyl carbonate."

[0085] The second organic solvent, "n-heptane", was replaced with "cyclohexane".

[0086] Operating under these conditions yielded pure lithium hexafluorophosphate with a purity of 99.86% and a yield of 83.6%.

[0087] Example 12

[0088] The method is the same as in Example 1, except that:

[0089] The first organic solvent, "diethyl ether," is replaced with "dimethyl carbonate."

[0090] The second organic solvent, "n-heptane", is replaced with "1,4-dioxane".

[0091] Operating under these conditions yielded pure lithium hexafluorophosphate with a purity of 99.80% and a yield of 83.4%.

[0092] Example 13

[0093] The method is the same as in Example 1, except that:

[0094] The first organic solvent, "diethyl ether," is replaced with "dimethyl carbonate."

[0095] The second organic solvent, "n-heptane", is replaced with "a mixture of n-heptane and 1,4-dioxane in a volume ratio of 1:1".

[0096] Operating under these conditions yielded pure lithium hexafluorophosphate with a purity of 99.75% and a yield of 81.4%.

[0097] Example 14

[0098] The method is the same as in Example 1, except that:

[0099] The phrase “the volume ratio of saturated lithium hexafluorophosphate to n-heptane is (1:1)” should be replaced with “the volume ratio of saturated lithium hexafluorophosphate to n-heptane is (0.5:1)”. The flow rate of the pump delivering the saturated filtrate is 2.5 mL / min, and the flow rate of the pump delivering the n-heptane is 5 mL / min.

[0100] Operating under these conditions yielded pure lithium hexafluorophosphate with a purity of 99.98% and a yield of 79.6%.

[0101] Example 15

[0102] The method is the same as in Example 1, except that:

[0103] The programmed cooling mode "hold at 0℃ for 1 min; hold at -10℃ for 1 min; hold at -20℃ for 1 min; hold at -30℃ for 1 min; hold at -40℃ for 3 min" is replaced with "hold at 10℃ for 2 min; hold at 0℃ for 1 min; hold at -10℃ for 2 min; hold at -20℃ for 2 min; hold at -30℃ for 4 min".

[0104] Operating under these conditions yielded pure lithium hexafluorophosphate with a purity of 99.20% and a yield of 81.6%.

[0105] Example 16

[0106] The method is the same as in Example 1, except that:

[0107] The third organic solvent, “diethyl ether at -30°C”, should be replaced with “n-pentane at -10°C”.

[0108] Under these operating conditions, pure lithium hexafluorophosphate was obtained with a purity of 99.90% and a yield of 84.6%.

[0109] Example 17

[0110] The method is the same as in Example 1, except that:

[0111] The composition of crude lithium hexafluorophosphate, which was "lithium hexafluorophosphate content ≥98%, DMC insoluble matter ≤0.5%, moisture ≤0.02%, free acid (calculated as HF) ≤0.08%, sulfate (calculated as SO4) ≤0.005%, chloride (calculated as Cl) ≤0.005%, iron (Fe) ≤0.008%, potassium (K) ≤0.001%, sodium (Na) ≤0.008%", should be replaced with "lithium hexafluorophosphate content 92.5%, DMC insoluble matter 1%, moisture 0.2%, free acid (calculated as HF) 0.8%, sulfate (calculated as SO4) 0.05%, chloride (calculated as Cl) 0.05%, iron (Fe) 0.08%, potassium (K) 0.01%, sodium (Na) 0.08%".

[0112] The crude lithium hexafluorophosphate at this concentration was continuously purified to finally obtain pure lithium hexafluorophosphate with a purity of 99.20% and a yield of 83.6%.

[0113] Example 18

[0114] The method is the same as in Example 1, except that:

[0115] The number of shell-type modules should be changed from "7" to "5".

[0116] Under these operating conditions, pure lithium hexafluorophosphate was obtained with a purity of 99.60% and a yield of 84.3%.

[0117] Example 19

[0118] The method is the same as in Example 1, except that:

[0119] The liquid holding capacity of the circular tube should be changed from "10mL" to "15mL".

[0120] Under these operating conditions, pure lithium hexafluorophosphate was obtained with a purity of 99.70% and a yield of 84.6%.

[0121] Example 20

[0122] The method is the same as in Example 1, except that:

[0123] The "microstructured reactor with inclined guide vane-enhanced mixing circular pipe structure" was replaced with a "microstructured reactor with disc-type pulse-variable rectangular flat pipe structure". The resulting product had a purity of 99.10%, a yield of 80.2%, and a free acid content (calculated as HF) of 0.04%.

[0124] Comparative Example 1

[0125] Under a nitrogen protective atmosphere, crude lithium hexafluorophosphate (with the following composition: lithium hexafluorophosphate content ≥98%, DMC insoluble matter ≤0.5%, moisture ≤0.02%, free acid (calculated as HF) ≤0.08%, sulfate (calculated as SO4) ≤0.005%, chloride (calculated as Cl) ≤0.005%, iron (Fe) ≤0.008%, potassium (K) ≤0.001%, sodium (Na) ≤0.008%) was dissolved in the organic solvent diethyl ether, filtered, and the filtrate was obtained. Silazane was added to the filtrate to carry out an acid removal reaction, crystallized, and dried to obtain the final pure product.

[0126] According to this method, lithium hexafluorophosphate with a purity of 98.9% and a yield of 81% was obtained, with an insoluble content of 0.022%, a free acid (calculated as HF) of 0.008%, a water content of 0.0005%, an alkali metal content (calculated as Na and K) of 0.0002%, and a heavy metal ion content (calculated as Fe) of 0.0002%.

[0127] Comparative Example 2

[0128] The method is the same as in Example 1, except that no filtering step is performed.

[0129] The final lithium hexafluorophosphate product had a purity of 98.90% and a yield of only 76.0%.

[0130] According to the method of the present invention, the high-purity lithium hexafluorophosphate product obtained in Example 1 is of good quality. Tested according to the requirements of HG / T4066-2008 standard, the product of the present invention exhibits excellent quality, with the following characteristics: lithium hexafluorophosphate content ≥99.98%, DMC insoluble matter ≤0.003%, moisture ≤0.001%, free acid (calculated as HF) ≤0.006%, sulfate (calculated as SO4) ≤0.0005%, chloride (calculated as Cl) ≤0.0002%, iron (Fe) ≤0.0004%, potassium (K) ≤0.0001%, sodium (Na) ≤0.0001%, and yield (calculated as Li) 85%.

[0131] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for purifying lithium hexafluorophosphate by continuous flow crystallization, characterized in that, The method includes: (1) Add the first organic solvent to the crude lithium hexafluorophosphate to be purified to dissolve it and prepare a saturated lithium hexafluorophosphate-organic solvent solution. After filtration, remove the insoluble matter and the filtrate is a saturated lithium hexafluorophosphate solution. (2) The saturated lithium hexafluorophosphate solution and the second organic solvent in step (1) are respectively fed into the microchannel reactor by metering pumps for cooling. The precipitated crystals are filtered, washed with the cooled third organic solvent, and then vacuum dried. The microchannel reactor is a microchannel reactor with a pulsed variable diameter microstructure, which includes: an enhanced mixing pipe containing inclined guide vanes; the inner diameter of the microchannel reactor is 2-6 mm; the length of the inclined guide vanes is 2-6 mm, the spacing between the inclined guide vanes is 1-3 mm, and the angle between the guide vanes and the pipe wall is 30°-60°. The microchannel reactor with the pulsed variable diameter microstructure includes: A shell-and-tube module of an enhanced mixing pipeline with inclined guide vanes, wherein reactants are transported inside the pipeline and heat exchange medium is transported in the shell side, wherein the symmetrically arranged inclined guide vanes constitute a module. In step (2), the cooling adopts a programmed cooling method. The programmed cooling conditions include: maintaining 0~5 °C for 0.5~1.5 min; maintaining -5~-15 °C for 0.5~1.5 min; maintaining -15~-25 °C for 0.5~1.5 min; maintaining -25~-35 °C for 0.5~1.5 min; and maintaining -35~-45 °C for 2~4.5 min.

2. The method according to claim 1, wherein, The spacing between adjacent inclined guide vanes is 1-3 mm; and / or The number of modules is 7-8, and each module is connected by a connecting pipe.

3. The method according to claim 1, wherein, Each of the shell-and-tube modules is placed vertically and parallel to each other, and the liquid holding capacity of each shell-and-tube module is 4.25-13.5 mL.

4. The method according to claim 1, wherein, In step (2), the volume ratio of the saturated lithium hexafluorophosphate solution to the second organic solvent is (0.7-1):

1.

5. The method according to claim 1, wherein, In step (2), the vacuum drying conditions include: vacuum degree of 5-10 Pa and drying at 50-70 °C for 7-9 h.

6. The method according to claim 1, wherein, In steps (1) and (2), the first organic solvent and the third organic solvent are each one or more of diethyl ether, methyl tert-butyl ether, ethylene glycol dimethyl ether, ethylene carbonate, dimethyl carbonate and diethyl carbonate.

7. The method according to claim 6, wherein, In steps (1) and (2), the first organic solvent and the third organic solvent are each diethyl ether.

8. The method according to claim 1, wherein, In step (2), the temperature of the third organic solvent is -20 to -40°C.

9. The method according to claim 1, wherein, The second organic solvent is selected from one or more of n-pentane, n-hexane, n-heptane, cyclohexane, tetrahydrofuran, and 1,4-dioxane.

10. The method according to claim 9, wherein, The second organic solvent is n-heptane.

11. The method according to claim 1, wherein, By weight, the crude lithium hexafluorophosphate to be purified contains ≥98% lithium hexafluorophosphate, ≤0.5% DMC insoluble matter, ≤0.02% moisture, ≤0.08% free acid (HF), ≤0.005% sulfate (SO4), ≤0.005% chloride (Cl), ≤0.008% iron (Fe), ≤0.001% potassium (K), and ≤0.008% sodium (Na).

12. The method according to claim 1, wherein, The filtration parameters include: using a ceramic filter element, with a filtration accuracy of 0.5-20μm.

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