Continuous crystallization system and method of lithium hexafluorophosphate

Abstract

The invention relates to a continuous crystallization system and method of lithium hexafluorophosphate, and belongs to the technical field of lithium ion battery electrolyte production. Aiming at the characteristics and requirements of a lithium hexafluorophosphate cooling crystallization process, the invention provides a continuous crystallization system of lithium hexafluorophosphate, which separates cooling nucleation from cooling crystallization. An ultrasonic generating device is introduced into the cooling nucleation module, lithium hexafluorophosphate is stimulated and induced by ultrasonic waves to nucleate, limitation of a metastable region is reduced, the nucleation rate is increased, crystal nuclei are controlled to be within a small particle size distribution range, meanwhile, introduction of impurities in the nucleation process is reduced, and the product purity is guaranteed. And the lithium hexafluorophosphate solution to be crystallized can continuously and uninterruptedly enter the cooling nucleation module, the cooling crystallization module and the separation circulation module in sequence, so that the continuous crystallization of the lithium hexafluorophosphate is realized, the crystallization time is greatly shortened, and the crystallization efficiency is improved.

Description

Technical Field

[0001] The present invention relates to a continuous crystallization system and method for lithium hexafluorophosphate, belonging to the technical field of production of lithium-ion battery electrolytes. Background Art

[0002] Lithium hexafluorophosphate (LiPF6) is a typical electrolyte salt for lithium-ion batteries, mainly used in lithium-ion power batteries, lithium-ion energy storage batteries and other daily-use batteries. At the same time, it is also an irreplaceable raw material for lithium-ion battery electrolytes. Lithium hexafluorophosphate products are divided into solid and liquid categories. Liquid lithium hexafluorophosphate usually has problems such as low purity and difficulty in transportation and storage. In contrast, solid products have a wider range of applications due to the advantages of easier product transportation and storage and fewer safety hazards.

[0003] The preparation methods of lithium hexafluorophosphate mainly include gas-solid direct reaction method, solvent method and ion exchange method. Among them, the most studied and technically mature process is the hydrogen fluoride solvent method. The specific steps of the hydrogen fluoride solvent method are to mix and react phosphorus pentafluoride / anhydrous hydrogen fluoride solution and lithium fluoride / anhydrous hydrogen fluoride solution to obtain a solution of lithium hexafluorophosphate, and then obtain lithium hexafluorophosphate crystal products through steps such as low-temperature cooling crystallization, filtration, and drying. Among them, the crystallization process is one of the key steps determining the product quality, and the difficulty of the crystallization process also increases the difficulty of large-scale production of lithium hexafluorophosphate.

[0004] At present, the commonly used crystallization methods for lithium hexafluorophosphate are mainly static crystallization and stirring crystallization. During the static crystallization process, hydrogen fluoride is in a static state, with high stability and safety, and the purity and quality of the product are relatively high. However, it requires too low a temperature and a long crystallization time, resulting in high energy consumption and increased production costs. Moreover, the crystals produced are relatively large and need to be physically crushed later, resulting in irregular and uneven product particles, which do not meet market requirements. The crystallization particles of stirring crystallization are uniform, the product quality is relatively stable, and the crystallization time is short, with lower requirements for temperature, having certain advantages in terms of production efficiency and production cost. However, during the stirring crystallization process, hydrogen fluoride is prone to volatilization, causing waste of hydrogen fluoride raw materials, lower product purity, and the stirring mechanical seal is prone to corrosion and leakage, posing a hazard to the personal safety of operators.

[0005] The continuous crystallization method has been widely adopted in the current industrial crystallization field, and it has been proven to significantly improve the crystallization efficiency, better control the product performance, and narrow the product particle size distribution. The Chinese utility model patent with the publication date of May 2, 2023 and the publication number of CN218944402U discloses a crystallization system for lithium hexafluorophosphate. This crystallization system includes a primary crystallization kettle, a secondary crystallization kettle, a tertiary crystallization kettle, and more than 1 quaternary crystallization kettle; the primary crystallization kettle is connected to the lithium hexafluorophosphate mother liquor supply source through the lithium hexafluorophosphate mother liquor supply pipeline; it is also connected to the lithium hexafluorophosphate mother liquor supply source through the mother liquor heat exchange pipeline connected with the mother liquor heat exchanger; it is also connected to the secondary crystallization kettle through the first mother liquor transfer pipeline for transporting the lithium hexafluorophosphate mother liquor. The secondary crystallization kettle is connected to the tertiary crystallization kettle through the second mother liquor transfer pipeline for transporting the lithium hexafluorophosphate mother liquor. The tertiary crystallization kettle is connected to the quaternary crystallization kettle through the crystallization particle transfer pipeline for transporting the crystallization small particles in this tertiary crystallization kettle; it is also connected to the next process device through the first discharge pipeline; the quaternary crystallization kettle is connected to the next process device through the second discharge pipeline. Based on the stirred crystallization, this crystallization system separates different crystallization stages in different crystallization kettles and connects the crystallization kettles for different stages in series, achieving continuous feeding and continuous discharging during the crystallization process. However, the crystallization system requires multiple crystallization kettles, occupying a relatively large site space, and during the crystallization process, it is necessary to precisely control the temperature range of each crystallization kettle, increasing the actual operation difficulty of the crystallization system. Moreover, from its specific embodiments, in order to improve the efficiency, a stirring device is added to the crystallization kettle, so there is a high possibility of problems such as lower product purity due to the accelerated crystallization speed, and at the same time, there is also a situation where the mechanical seal of the stirrer is vulnerable to corrosion and leakage. Summary of the Invention

[0006] The first object of the present invention is to provide a continuous crystallization system for lithium hexafluorophosphate, providing a continuous crystallization system for lithium hexafluorophosphate with simple equipment and control and high product purity.

[0007] The second object of the present invention is to provide a continuous crystallization method for lithium hexafluorophosphate, providing a continuous crystallization method for lithium hexafluorophosphate with high product purity and high efficiency.

[0008] In order to achieve the above objects, the technical solution of a continuous crystallization system for lithium hexafluorophosphate in the present invention is as follows:

[0009] A continuous crystallization system for lithium hexafluorophosphate includes a cooling nucleation module, a cooling crystallization module, and a separation and circulation module connected in series in sequence;

[0010] The cooling nucleation module includes a first heat exchanger, a nucleation pipeline passes through the first heat exchanger, and an ultrasonic generator for ultrasonic stimulation of the cooling medium in the first heat exchanger is also provided;

[0011] The cooling crystallization module includes a second heat exchanger, in which a crystallization column is arranged. The crystallization column has a nucleating liquid inlet at the upper part and a crystallization slurry outlet at the lower part. The nucleating liquid inlet is connected to the outlet of the nucleating pipeline.

[0012] The separation and circulation module includes a sedimentation device and a solid-liquid separation device connected in sequence. The sedimentation device is connected to the crystallization slurry outlet, and the solid-liquid separation device has a lithium hexafluorophosphate crystal outlet.

[0013] The beneficial effects of the above technical solution are as follows: The continuous crystallization system of lithium hexafluorophosphate of the present invention is a pioneering invention. In view of the characteristics and requirements of the cooling crystallization process of lithium hexafluorophosphate, the present invention proposes a continuous crystallization system of lithium hexafluorophosphate that separates cooling nucleation and cooling crystallization. The present invention introduces an ultrasonic generating device into the cooling nucleation module, uses ultrasonic stimulation to induce the nucleation of lithium hexafluorophosphate, reduces the limitation of the metastable zone, speeds up the nucleation rate, controls the crystal nuclei within a smaller particle size distribution range, and at the same time reduces the introduction of impurities during the nucleation process, ensuring the purity of the product. Moreover, the lithium hexafluorophosphate solution to be crystallized can continuously and uninterruptedly enter the cooling nucleation module, the cooling crystallization module and the separation and circulation module in sequence, realizing the continuous crystallization of lithium hexafluorophosphate, greatly shortening the crystallization time and improving the crystallization efficiency.

[0014] As a further improvement, both the sedimentation device and the solid-liquid separation device are provided with a circulating mother liquor outlet, and the circulating mother liquor outlet is connected to the nucleating pipeline through a circulating pipeline.

[0015] The beneficial effects of the above technical solution are as follows: Since there is still lithium hexafluorophosphate in the mother liquor separated by the separation and circulation module, and the separated mother liquor has a lower temperature, introducing the separated circulating mother liquor into the cooling nucleation module can, on the one hand, reduce the waste of raw materials, and on the other hand, enable the temperature of the lithium hexafluorophosphate solution to be crystallized to rapidly drop to the nucleation temperature, effectively avoiding the situation that the temperature of the lithium hexafluorophosphate solution to be crystallized drops too low in a short time and the ideal nucleation size cannot be controlled. Moreover, it can also reduce the production energy consumption and is beneficial to reducing the production cost.

[0016] As a further improvement, a mixer is arranged upstream of the cooling nucleation module. One inlet of the mixer is connected to the circulating pipeline, and the other inlet is used to introduce the lithium hexafluorophosphate solution to be crystallized.

[0017] The beneficial effects of the above technical solution are as follows: Adding a mixer in the cooling nucleation module is beneficial to the rapid and uniform mixing of the liquid separated by solid-liquid separation and the lithium hexafluorophosphate solution to be crystallized, reaching a lower temperature in a short time and providing conditions for crystal nucleation.

[0018] As a further improvement, a gas distributor for introducing an ascending bubble group is provided at the bottom of the crystallization column, and a gas outlet for discharging gas is provided at the top of the crystallization column.

[0019] The beneficial effects of the above technical solution are as follows: A gas distributor is provided at the bottom of the crystallization device, which can introduce a bubble group flowing in the opposite direction to the flow direction of the nucleating liquid. On the one hand, it increases the disturbance, improves the heat and mass transfer efficiency, and reduces the fouling phenomenon on the wall surface. On the other hand, it plays a role in sieving the size of crystal particles, allowing small-sized crystal particles to have the opportunity to stay in the crystallizer for a longer time under the action of bubbles, making the size of the crystal product more uniform.

[0020] As a further improvement, the gas outlet is connected to a hydrogen fluoride condenser.

[0021] The beneficial effects of the above technical solution are as follows: Connecting a gas condenser to the gas outlet helps to condense the hydrogen fluoride gas carried out by the inert gas and make it flow back into the crystallization device, which helps to reduce the waste of hydrogen fluoride and lower the production cost.

[0022] In order to achieve the above object, the technical solution of a continuous crystallization method of lithium hexafluorophosphate in the present invention is:

[0023] A continuous crystallization method of lithium hexafluorophosphate includes the following steps: cooling and ultrasonic-assisted nucleation are carried out on the continuously flowing lithium hexafluorophosphate solution to be crystallized, the obtained nucleating liquid continuously enters the crystallization column for cooling crystallization, and the obtained crystallization slurry is subjected to solid-liquid separation.

[0024] The beneficial effects of the above technical solution are as follows: The invention separates the cooling nucleation and cooling crystallization processes according to the characteristics and requirements of the cooling crystallization process of lithium hexafluorophosphate. Ultrasonic treatment is introduced in the cooling nucleation. Ultrasonic treatment can stimulate and induce the nucleation of lithium hexafluorophosphate, reduce the limitation of the metastable zone, accelerate the nucleation rate, control the crystal nuclei within a smaller particle size distribution range, and separate the crystal nucleation and growth processes, which is easy to control the overall crystallization process, effectively improves the crystallization efficiency, and improves the product purity. The invention separates the processes of cooling nucleation, cooling crystallization and separation, can realize the continuity of the crystallization process, form continuous crystallization without interruption, and improve the crystallization efficiency.

[0025] As a further improvement, the recycled mother liquor obtained by solid-liquid separation is returned to be mixed with the lithium hexafluorophosphate solution to be crystallized.

[0026] As a further improvement, in the cooling crystallization stage, the nucleating liquid flows from top to bottom, and an ascending bubble group is introduced to contact the nucleating liquid countercurrently; the size of the bubbles in the bubble group is 0.5 - 5 mm.

[0027] The beneficial effects of the above technical solution are as follows: When cooling and crystallizing, an inert gas flowing in the opposite direction to the nucleating liquid is introduced, which can introduce a bubble group flowing in the opposite direction to the flow direction of the crystallization mixture. On the one hand, it increases the disturbance, improves the heat and mass transfer efficiency, and reduces the wall fouling phenomenon. On the other hand, it plays a role in sieving the size of crystal particles, allowing small-sized crystal particles to have the opportunity to stay in the crystallizer for a longer time under the action of bubbles, making the size of the crystal product more uniform.

[0028] As a further improvement, the concentration of the lithium hexafluorophosphate solution to be crystallized is 10-25%, and the temperature is -10-20°C; the concentration of the circulating mother liquor is 5-15%, and the temperature is -20--50°C; the ratio of the lithium hexafluorophosphate solution to be crystallized to the mother liquor is 1:(5-15).

[0029] As a further improvement, the ultrasonic power of the ultrasonic-assisted nucleation is 100-180 w, and the time is 2-3 min. Description of the Drawings

[0030] Figure 1 It is a module diagram of the continuous crystallization system of lithium hexafluorophosphate in Embodiment 1 of the present invention;

[0031] Figure 2 It is a schematic diagram of the cooling nucleation module in Embodiment 1 of the present invention;

[0032] Figure 3 It is a schematic diagram of the cooling crystallization module in Embodiment 1 of the present invention;

[0033] Figure 4 It is a schematic diagram of the separation and circulation module in Embodiment 1 of the present invention;

[0034] Among them, 1 - mixer, 2 - first heat exchanger, 3 - micro reaction generator, 4 - ultrasonic generator, 5 - second heat exchanger, 6 - crystallization column, 7 - gas distributor, 8 - hydrogen fluoride condenser, 9 - settler, 10 - solid-liquid separator. Detailed Embodiments

[0035] At present, the cooling crystallization method of lithium hexafluorophosphate is mostly the intermittent static cooling crystallization method. However, this process has problems such as large equipment volume, long production cycle, and poor product quality stability. In recent years, with the continuous development of the new energy industry, the market demand for lithium-ion batteries has been increasing day by day, and the demand for lithium hexafluorophosphate has also been growing continuously. The inefficient static cooling crystallization method can no longer meet the market demand, and it is urgent to improve the crystallization process of lithium hexafluorophosphate to increase production capacity. The continuous crystallization method has been widely adopted in the current industrial crystallization field and has been proven to significantly improve the crystallization efficiency, better control the product performance, and narrow the product particle size distribution. Although there is a continuous crystallization system for lithium hexafluorophosphate in the existing technology, there are still problems such as long production cycle, low efficiency, poor product quality stability, and large equipment investment.

[0036] The continuous crystallization system of lithium hexafluorophosphate of the present invention proposes a continuous crystallization system of lithium hexafluorophosphate according to the characteristics and requirements of the cooling crystallization process of lithium hexafluorophosphate. The present invention separates the nucleation process and the growth process of lithium hexafluorophosphate, which is easy to control the overall crystallization process and effectively improves the crystallization efficiency. Moreover, the present invention introduces an ultrasonic generating device in the cooling nucleation module, which is beneficial to accelerating the nucleation efficiency, reducing the limitation of the metastable zone, and improving the crystallization efficiency and purity. In addition, a gas distributor is arranged at the bottom of the crystallization device, which can introduce a bubble group flowing in the opposite direction to the nucleating liquid flow direction. On the one hand, it increases the disturbance, improves the heat and mass transfer efficiency, and reduces the wall fouling phenomenon. On the other hand, it plays a role in sieving the size of crystal particles, allowing small-sized crystal particles to have the opportunity to stay in the crystallizer for a longer time under the action of bubbles, making the size of the crystal product more uniform.

[0037] To further describe the present invention, it should be noted that on the premise of no conflict, the following-described embodiments or technical features can be arbitrarily combined to form new embodiments. The equipment and raw materials used can be purchased from the market or are commonly used in the art. The methods in the following embodiments are conventional methods in the art unless otherwise specified.

[0038] I. Specific embodiments of a continuous crystallization system of lithium hexafluorophosphate of the present invention:

[0039] Example 1

[0040] The module schematic diagram of the continuous crystallization system of lithium hexafluorophosphate in this embodiment is as Figure 1 shown, including a cooling nucleation module, a cooling crystallization module, and a separation and circulation module connected in series in sequence. The lithium hexafluorophosphate solution to be crystallized flows through the cooling nucleation module for cooling nucleation, the cooling crystallization module for cooling crystal growth, and the separation and circulation module for solid-liquid separation to obtain lithium hexafluorophosphate crystals.

[0041] Specifically, the cooling and nucleation module can achieve the cooling and induced nucleation of the saturated solution of lithium hexafluorophosphate to be crystallized. The schematic diagram is as shown in Figure 2 Figure 1. A mixer 1 for mixing the saturated solution of lithium hexafluorophosphate to be crystallized and the circulating mother liquor is provided upstream of the cooling and nucleation module. One inlet of the mixer receives the saturated solution of lithium hexafluorophosphate to be crystallized, and the other inlet receives the circulating mother liquor. The outlet is connected to the cooling crystallization module. The cooling crystallization module includes a first heat exchanger 2 for cooling. A micro-reactor 3 passes through the first heat exchanger 2, and an ultrasonic generator 4 for ultrasonic treatment of the cooling medium in the first heat exchanger 2 is also provided. The micro-reactor 3 has a spiral pipeline to provide a nucleation space for lithium hexafluorophosphate. Among them, the first heat exchanger is a jacketed heat exchanger. The saturated solution of lithium hexafluorophosphate to be crystallized and the circulating mother liquor are uniformly mixed in the mixer 1 to obtain a mixed solution. The mixed solution enters the micro-reactor 3 to nucleate under ultrasonic and low-temperature conditions, and the nucleated solution enters the cooling crystallization module.

[0042] The cooling crystallization module can achieve the cooling and crystal growth of the nucleated solution from the cooling and nucleation module. The schematic diagram is as shown in Figure 3 Figure 2. It includes a second heat exchanger 5. A crystallization column 6 is vertically arranged in the second heat exchanger 5. The crystallization column 6 has a nucleated solution inlet at the upper part and a crystal slurry outlet at the lower part. The nucleated solution inlet is connected to the outlet of the nucleation pipeline. A gas distributor 7 for dispersing inert gas into bubbles is provided at the bottom of the crystallization column 6, and a gas outlet for discharging inert gas is provided at the top. The gas outlet is connected to a hydrogen fluoride condenser 8 for recovering hydrogen fluoride carried out by the inert gas. Among them, the second heat exchanger is a jacketed heat exchanger. The nucleated solution from the cooling and nucleation module enters the crystallization column from the upper inlet of the crystallization column, flows down for cooling and crystal growth, and the obtained crystal slurry flows out from the lower outlet of the crystallization column and enters the separation and circulation module. The gas distributor at the bottom of the crystallization column disperses the inert gas into a bubble group and disturbs the nucleated solution upward, which is discharged from the gas outlet at the top of the crystallization column. The hydrogen fluoride gas discharged with the inert gas is cooled and refluxed by the hydrogen fluoride condenser.

[0043] The separation and circulation module can achieve the separation of lithium hexafluorophosphate crystals in the slurry from the cooling crystallization module and the circulation of the circulating mother liquor. The schematic diagram is as shown in Figure 4As shown, it includes a sedimentation tank 9 and a solid-liquid separator 10 connected in sequence. The sedimentation tank 9 is connected to the crystallization slurry outlet, and the solid-liquid separator has a lithium hexafluorophosphate crystal outlet. Both the sedimentation tank 9 and the solid-liquid separator 10 are provided with two circulating mother liquor outlets. The circulating mother liquor outlets are connected to the nucleation pipeline through a circulating pipeline, and the circulating mother liquor is introduced into the lithium hexafluorophosphate synthesis unit through a pipeline. The crystal slurry from the cooling crystallization module enters the sedimentation tank of the separation and circulation module for thickening of the crystal slurry. The thick crystal slurry flows into the solid-liquid separator. The circulating mother liquor with fewer crystals in the upper part is discharged from the circulating mother liquor outlet and is recycled to the cooling nucleation module and / or the lithium hexafluorophosphate synthesis unit through the circulating pipeline. The thick crystal slurry is subjected to solid-liquid separation in the separator. The crystals are discharged from the lithium hexafluorophosphate crystal outlet, and the circulating mother liquor is discharged from the circulating mother liquor outlet and is recycled to the cooling nucleation module and / or the lithium hexafluorophosphate synthesis unit through the circulating pipeline.

[0044] In other implementation cases, the crystallization column of the cooling crystallization module does not contain a gas distributor, and at this time, a hydrogen fluoride condenser is not provided at the top of the crystallization column.

[0045] II. Specific embodiments of a continuous crystallization method for lithium hexafluorophosphate of the present invention:

[0046] Example 2

[0047] This example uses the continuous crystallization system for lithium hexafluorophosphate in Example 1 to conduct continuous crystallization of lithium hexafluorophosphate. The specific operations are as follows:

[0048] Cooling and nucleation: A lithium hexafluorophosphate solution to be crystallized with a concentration of 10% and a temperature of -10°C is mixed with 8 times the volume of circulating mother liquor with a temperature of -30°C and a saturated concentration of 8% to obtain a mixed mother liquor at -27°C. The mixed mother liquor is introduced into the cooling nucleation module with an internal ultrasonic vibration rod and an internal cooling jacket at a rate of 10 mL / min. The temperature of the cooling medium introduced into the cooling nucleation module is -33°C. The ultrasonic power is set to 150 w, and the residence time of the mother liquor is 2 min. Small crystal nucleus particles will precipitate from the mixed mother liquor after ultrasonic stimulation to obtain a nucleated liquid with crystal nuclei. The temperature of the nucleated liquid is -28°C.

[0049] Cooling crystallization: The nucleated liquid is introduced into a vertically placed crystallization column with a cooling jacket. The effective volume of the crystallization column is 720 mL. By controlling the flow rate, the residence time of the material is 8 min. The temperature of the cooling medium introduced into the cooling jacket is -35°C. Nitrogen is introduced from the lower part, and the gas flow rate is set to 100 sccm. It is dispersed into a bubble group with an average size of 1 mm through a gas distributor. The temperature of the crystal slurry leaving the crystallization column is about -30°C.

[0050] Separation cycle: The crystal slurry is directly fed into the separation cycle module, where the thickening of the crystal slurry is carried out in a settler with a residence time of 3 min. The crystal slurry containing thick crystals flows into a solid-liquid separator. The circulating mother liquor with fewer crystals in the upper part is discharged from the circulating mother liquor outlet and recycled to the cooling nucleation module for cooling nucleation. The crystal slurry containing thick crystals is separated into solid and liquid in the solid-liquid separator. The filtered crystal particles are dried to obtain lithium hexafluorophosphate crystal particles with a purity of 99.94% and a crystal particle size distribution range of 210 - 250 μm. The temperature of the circulating mother liquor is -30 °C. Part of it is recycled to the cooling nucleation module for cooling nucleation, and the other part is recycled to the lithium hexafluorophosphate synthesis unit.

[0051] Example 3

[0052] In this example, the continuous crystallization system of lithium hexafluorophosphate in Example 1 is used for the continuous crystallization of lithium hexafluorophosphate. The specific operations are as follows:

[0053] Cooling nucleation: A lithium hexafluorophosphate solution to be crystallized with a concentration of 24% and a temperature of 17 °C is mixed with 10 times the volume of circulating mother liquor with a temperature of -33 °C and a saturation concentration of 7% to obtain a mixed mother liquor at -28 °C. The mixed mother liquor is fed into the cooling nucleation module with an internal ultrasonic vibration rod and an internal cooling jacket at a rate of 10 mL / min. The temperature of the cooling medium fed into the cooling nucleation module is -35 °C. The ultrasonic power is set to 100 w, and the residence time of the mother liquor is 2 min. Small crystal nucleus particles will precipitate from the mixed mother liquor after ultrasonic stimulation to obtain a nucleated liquid with crystal nuclei. The temperature of the nucleated liquid is -29 °C.

[0054] Cooling crystallization: The nucleated liquid is fed into a vertically placed crystallization column with a cooling jacket. The effective volume of the crystallization column is 720 mL. By controlling the flow rate, the residence time of the material is 8 min. The temperature of the cooling medium fed into the cooling jacket is -40 °C. Nitrogen is fed into the lower part, and the gas flow rate is set to 150 sccm. The gas is dispersed into a bubble group with an average size of 2 mm through a gas distributor. The temperature of the crystal slurry leaving the crystallization column is about -33 °C.

[0055] Separation cycle: The crystal slurry is directly fed into the separation cycle module, where the thickening of the crystal slurry is carried out in a settler with a residence time of 3 min. The crystal slurry containing thick crystals flows into a solid-liquid separator. The circulating mother liquor with fewer crystals in the upper part is discharged from the circulating mother liquor outlet and recycled to the cooling nucleation module for cooling nucleation. The crystal slurry containing thick crystals is separated into solid and liquid in the solid-liquid separator. The filtered crystal particles are dried to obtain lithium hexafluorophosphate crystal particles with a purity of 99.92% and a crystal particle size distribution range of 210 - 270 μm. The temperature of the circulating mother liquor is -33 °C. Part of it is recycled to the cooling nucleation module for cooling nucleation, and the other part is recycled to the lithium hexafluorophosphate synthesis unit.

[0056] Example 4

[0057] In this example, the continuous crystallization system of lithium hexafluorophosphate in Example 1 is used for the continuous crystallization of lithium hexafluorophosphate, and the specific operations are as follows:

[0058] Cooling and nucleation: A lithium hexafluorophosphate solution to be crystallized with a concentration of 19% and a temperature of -5°C is mixed with a circulating mother liquor with a volume 8 times that of the solution, a temperature of -50°C, and a saturation concentration of 5% to obtain a mixed mother liquor at -43°C. The mixed mother liquor is introduced into a cooling and nucleation module with an internal ultrasonic vibration rod and an internal cooling jacket at a rate of 10 mL / min. The temperature of the cooling medium introduced into the cooling and nucleation module is -45°C. The ultrasonic power is set to 130 W, and the residence time of the mother liquor is 2 min. Small crystal nucleus particles will precipitate from the mixed mother liquor after ultrasonic stimulation to obtain a nucleated liquid with crystal nuclei. The temperature of the nucleated liquid is -43°C.

[0059] Cooling crystallization: The nucleated liquid is introduced into a vertically placed crystallization column with a cooling jacket. The effective volume of the crystallization column is 720 mL. By controlling the flow rate, the residence time of the material is 8 min. The temperature of the cooling medium introduced into the cooling jacket is -55°C. Nitrogen is introduced from the lower part, and the gas flow rate is set to 150 sccm. The gas is dispersed into a bubble group with an average size of 1 mm through a gas distributor. The temperature of the crystal slurry leaving the crystallization column is about -50°C.

[0060] Separation and circulation: The crystal slurry is directly introduced into a separation and circulation module. The crystal slurry is thickened in a settler, and the residence time is 3 min. The thickened crystal slurry flows into a solid-liquid separator. The circulating mother liquor with fewer crystals in the upper part is discharged from the circulating mother liquor outlet and recycled to the cooling and nucleation module for cooling and nucleation. The thickened crystal slurry is solid-liquid separated in the solid-liquid separator. The filtered crystal particles are dried to obtain lithium hexafluorophosphate crystal particles with a purity of 99.96% and a crystal particle size distribution range of 240 - 270 μm. The temperature of the circulating mother liquor is -50°C. Part of it is recycled to the cooling and nucleation module for cooling and nucleation, and the other part is recycled to the lithium hexafluorophosphate synthesis unit.

[0061] Example 5

[0062] In this example, the continuous crystallization system of lithium hexafluorophosphate in Example 1 is used for the continuous crystallization of lithium hexafluorophosphate, and the specific operations are as follows:

[0063] Cooling nucleation: A lithium hexafluorophosphate solution to be crystallized with a concentration of 10% and a temperature of -10°C is mixed with a circulating mother liquor with a temperature of -45°C and a saturated concentration of 6% in a volume 15 times that of the solution, to obtain a mixed mother liquor at -43°C. The mixed mother liquor is introduced into a cooling nucleation module with an internal ultrasonic vibration rod and an internal cooling jacket at a rate of 5 mL / min. The temperature of the cooling medium introduced into the cooling nucleation module is -45°C. The ultrasonic power is set to 100 w, and the residence time of the mother liquor is 2 min. Small crystal nucleus particles will precipitate from the mixed mother liquor after ultrasonic stimulation, and a nucleated liquid with crystal nuclei is obtained. The temperature of the nucleated liquid is -43°C.

[0064] Cooling crystallization: The nucleated liquid is introduced into a vertically placed crystallization column with a cooling jacket. The effective volume of the crystallization column is 720 mL. By controlling the flow rate, the residence time of the material is 15 min. The temperature of the cooling medium introduced into the cooling jacket is -50°C. Nitrogen is introduced from the lower part, and the gas flow rate is set to 90 sccm. The gas is dispersed into a bubble group with an average size of 0.5 mm through a gas distributor. The temperature of the crystal slurry leaving the crystallization column is about -45°C.

[0065] Separation and circulation: The crystal slurry is directly introduced into a separation and circulation module. The crystal slurry is thickened in a settler, and the residence time is 3 min. The thickened crystal slurry flows into a solid-liquid separator. The circulating mother liquor with fewer crystals in the upper part is discharged from the circulating mother liquor outlet and recycled to the cooling nucleation module for cooling nucleation. The thickened crystal slurry is solid-liquid separated in the solid-liquid separator. The filtered crystal particles are dried to obtain lithium hexafluorophosphate crystal particles with a purity of 99.88% and a crystal particle size distribution range of 170 - 230 μm. The temperature of the circulating mother liquor is -45°C. Part of it is recycled to the cooling nucleation module for cooling nucleation, and the other part is recycled to the lithium hexafluorophosphate synthesis unit.

[0066] III. Comparative Example

[0067] Comparative Example 1

[0068] Compared with Example 1, the continuous crystallization system of lithium hexafluorophosphate used in this comparative example does not have an ultrasonic generating device in the cooling nucleation module. The process of continuous crystallization of lithium hexafluorophosphate is as follows:

[0069] Cooling nucleation: A lithium hexafluorophosphate solution to be crystallized with a concentration of 25% and a temperature of 20°C is mixed with a circulating mother liquor with a temperature of -40°C and a saturated concentration of 6.5% in a volume 5 times that of the solution, to obtain a mixed mother liquor at -36°C. The mixed mother liquor is introduced into a cooling nucleation module with an internal cooling jacket at a rate of 15 mL / min. The temperature of the cooling medium introduced into the cooling nucleation module is -40°C. Almost no crystal nuclei are generated in the mother liquor without ultrasonic stimulation, and a high-concentration slurry at -37°C is introduced into the cooling crystallization module.

[0070] Cooling crystallization: The nucleating liquid is introduced into a vertically placed crystallization column with a cooling jacket. The effective volume of the crystallization column is 720 mL. By controlling the flow rate, the residence time of the material is 7 min. The temperature of the cooling medium introduced into the cooling jacket is -50 °C. Nitrogen is introduced from the lower part, and the gas flow rate is set at 95 sccm. The gas is dispersed through a gas distributor into a bubble group with an average size of 0.5 mm. The temperature of the crystal slurry leaving the crystallization column is about -40 °C.

[0071] Separation cycle: The crystal slurry is directly introduced into the separation cycle module. The thickening of the crystal slurry is carried out in the settler, and the residence time is 3 min. The crystal slurry containing thick crystals flows into the solid-liquid separator. The circulating mother liquor with fewer crystals in the upper part is discharged from the circulating mother liquor outlet and recycled to the cooling nucleation module for cooling nucleation. The crystal slurry containing thick crystals is separated into solid and liquid in the solid-liquid separator. The filtered crystal particles are dried to obtain lithium hexafluorophosphate crystal particles with a purity of 95.55% and a crystal particle size distribution range of 400 - 500 μm. The temperature of the circulating mother liquor is -40 °C. A part of it is recycled to the cooling nucleation module for cooling nucleation, and the other part is recycled to the lithium hexafluorophosphate synthesis unit.

[0072] Comparative Example 2

[0073] Compared with Example 1, the continuous crystallization system of lithium hexafluorophosphate used in this comparative example does not have a gas distributor and an inert gas inlet in the cooling crystallization module. The process of continuous crystallization of lithium hexafluorophosphate is as follows:

[0074] Cooling nucleation: A lithium hexafluorophosphate solution to be crystallized with a concentration of 18% and a temperature of 0 °C is mixed with 15 times the volume of circulating mother liquor with a temperature of -20 °C and a saturated concentration of 11% to obtain a mixed mother liquor at -19 °C. The mixed mother liquor is introduced into the cooling nucleation module with an internal ultrasonic vibration rod and an internal cooling jacket at a rate of 7 mL / min. The temperature of the cooling medium introduced into the cooling nucleation module is -25 °C. The ultrasonic power is set at 180 w, and the residence time of the mother liquor is 2 min. Small crystal nucleus particles will precipitate from the mixed mother liquor after ultrasonic stimulation to obtain a nucleating liquid with crystal nuclei. The temperature of the nucleating liquid is -19 °C.

[0075] Cooling crystallization: The nucleating liquid is introduced into a vertically placed crystallization column with a cooling jacket. The effective volume of the crystallization column is 720 mL. By controlling the flow rate, the residence time of the material is 10 min. The temperature of the cooling medium introduced into the cooling jacket is -30 °C. No bubbles are introduced from the lower part of the crystallization column. The temperature of the crystal slurry leaving the crystallization column is about -20 °C.

[0076] Separation cycle: The crystal slurry is directly fed into the separation cycle module, and the thickening of the crystal slurry is carried out in the settler with a residence time of 3 minutes. The thick crystal slurry flows into the solid-liquid separator. The circulating mother liquor with fewer crystals in the upper part is discharged from the circulating mother liquor outlet and recycled to the cooling nucleation module for cooling nucleation. The thick crystal slurry is solid-liquid separated in the solid-liquid separator. The filtered crystal particles are dried to obtain lithium hexafluorophosphate crystal particles with a purity of 99.89% and a crystal particle size distribution range of 190 - 350 μm. The temperature of the circulating mother liquor is -20°C. Part of it is recycled to the cooling nucleation module for cooling nucleation, and the other part is recycled to the lithium hexafluorophosphate synthesis unit.

[0077] The above is only a preferred embodiment of the present invention and is not intended to limit the present invention. The patent protection scope of the present invention is subject to the claims. Any equivalent structural changes made by using the content of the specification of the present invention should equally be included in the protection scope of the present invention.

Claims

1. A continuous crystallization system for lithium hexafluorophosphate, characterized in that: It includes a cooling nucleation module, a cooling crystallization module and a separation circulation module which are connected in series in sequence; The cooling nucleation module comprises a first heat exchanger, a nucleation pipeline runs through the first heat exchanger, and an ultrasonic generator is provided for ultrasonically stimulating the cooling medium in the first heat exchanger; The cooling crystallization module comprises a second heat exchanger, wherein a crystallization column is arranged in the second heat exchanger, wherein the crystallization column has a nucleation liquid inlet located at the upper part and a crystal slurry outlet located at the lower part, and the nucleation liquid inlet is connected to the outlet of the nucleation pipeline; The separation circulation module comprises a sedimentation device and a solid-liquid separation device which are connected in sequence; the sedimentation device is connected to the crystal slurry outlet, and the solid-liquid separation device has a lithium hexafluorophosphate crystal outlet.

2. The continuous crystallization system of lithium hexafluorophosphate according to claim 1, characterized in that: The sedimentation device and the solid-liquid separation device are both provided with a circulating mother liquid outlet, and the circulating mother liquid outlet is connected to the nucleation pipeline through a circulating pipeline.

3. The continuous crystallization system of lithium hexafluorophosphate according to claim 2, characterized in that: A mixer is arranged upstream of the cooling nucleation module, one of the inlets of the mixer is connected to the circulation pipeline, and the other inlet is used to introduce the lithium hexafluorophosphate solution to be crystallized.

4. The continuous crystallization system of lithium hexafluorophosphate according to any one of claims 1 to 3, characterized in that: A gas distributor for introducing a rising bubble group is arranged at the bottom of the crystal column, and a gas outlet for exhausting gas is arranged at the top of the crystal column.

5. The continuous crystallization system of lithium hexafluorophosphate according to claim 4, characterized in that: The gas outlet is connected with a hydrogen fluoride condenser.

6. A continuous crystallization method for lithium hexafluorophosphate, characterized in that: The following steps are involved: The continuously flowing lithium hexafluorophosphate solution to be crystallized is cooled and ultrasonic-assisted nucleation is performed, the obtained nucleation liquid continuously enters the crystallization column for cooling and crystallization, and the obtained crystal slurry is subjected to solid-liquid separation.

7. The continuous crystallization method of lithium hexafluorophosphate according to claim 6, characterized in that: The circulating mother liquor obtained by solid-liquid separation is returned and mixed with the lithium hexafluorophosphate solution to be crystallized.

8. The continuous crystallization method of lithium hexafluorophosphate according to claim 6, characterized in that: In the cooling crystallization stage, the nucleation liquid flows from top to bottom, and a rising bubble group is introduced to contact the nucleation liquid in countercurrent; the size of the bubbles in the bubble group is 0.5 to 5 mm.

9. The continuous crystallization method of lithium hexafluorophosphate according to claim 7, characterized in that: The concentration of the lithium hexafluorophosphate solution to be crystallized is 10-25%, and the temperature is -10-20°C; the concentration of the circulating mother liquor is 5-15%, and the temperature is -20--50°C; the ratio of the lithium hexafluorophosphate solution to be crystallized to the mother liquor is 1:(5-15).

10. The continuous crystallization method of lithium hexafluorophosphate according to claim 9, characterized in that: The ultrasonic power of the ultrasonic-assisted nucleation is 100-180W, and the time is 2-3min.

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