A solid-state lithium hexafluorophosphate with low fine crystal content and a method for preparing the same

Abstract

This invention belongs to the field of lithium-ion battery electrolyte material preparation technology, and provides a solid lithium hexafluorophosphate with low fine crystal content and its preparation method. The preparation method involves adding lithium fluoride to anhydrous hydrogen fluoride to form anhydrous hydrogen fluoride slurry. A portion of the anhydrous hydrogen fluoride slurry and phosphorus pentafluoride gas are fed into a primary continuous reaction crystallizer. The resulting slurry is subjected to low-temperature fractionation to form a first stream and a second stream. The second stream enters a secondary continuous growth crystallizer and continues to react with phosphorus pentafluoride gas, followed by solid-liquid separation to obtain a solid wet product and mother liquor. The first stream, another portion of the anhydrous hydrogen fluoride slurry, and a portion of the mother liquor enter a pre-conditioning section, and the resulting pre-conditioned stream is returned to the feed end of the primary continuous reaction crystallizer. The solid wet product is then subjected to low-temperature washing and programmed vacuum drying to obtain the final product. This method, through the combination of fractional reflux, mother liquor participation in pre-conditioning, and continuous growth processes, reduces fine crystal entrainment and improves particle size distribution stability.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery electrolyte material preparation technology, and relates to a solid lithium hexafluorophosphate with low fine crystal content and its preparation method. Background Technology

[0002] In existing solid-state lithium hexafluorophosphate preparation processes, reaction formation and crystal precipitation often occur simultaneously in the same low-temperature system. After phosphorus pentafluoride gas enters the anhydrous hydrogen fluoride medium, it needs to undergo gas mass transfer, dissolution, and contact reaction with lithium fluoride. Lithium fluoride is mainly dispersed in the system in the form of suspended solids, resulting in obvious heterogeneous characteristics at the reaction interface and in the crystal precipitation region.

[0003] When local supersaturation is high or the slurry residence state is uneven, more fine crystals tend to form in the system, resulting in a high content of fine crystals and insufficient particle size distribution stability in the final solid lithium hexafluorophosphate product. A high content of fine crystals increases the specific surface area of ​​the product, making it easier for the product to adsorb residual acidic components and trace amounts of moisture. It also affects the powder flowability, packaging stability, and the uniformity of feeding during the subsequent electrolyte preparation process.

[0004] Existing processes typically improve crystal size by extending crystallization time, adjusting cooling conditions, filtration and sieving, or returning the mother liquor for recrystallization. However, these measures are mostly concentrated in a single crystallization stage or post-processing stage, making it difficult to simultaneously address the formation of fine crystals during the reaction stage, the migration of fine crystals in the crystal slurry, and the subsequent crystal growth process. Especially in systems where phosphorus pentafluoride mass transfer, lithium fluoride suspension reaction, and lithium hexafluorophosphate crystallization are coupled, simply relying on extending the residence time or post-processing sieving is insufficient to reduce fine crystal entrainment and stabilize the product particle size distribution from a closed-loop process perspective. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a solid lithium hexafluorophosphate with low fine crystal content and its preparation method. The method involves a primary continuous reaction crystallization of anhydrous hydrogen fluoride slurry and phosphorus pentafluoride gas. After low-temperature classification, the resulting slurry enters a secondary continuous growth stage. The primary stream, along with a portion of the anhydrous hydrogen fluoride slurry and mother liquor, is pre-conditioned and then returned to the primary feed end. The solid wet product is washed and dried to obtain the final product, thereby improving the fine crystal content and particle size distribution stability of solid lithium hexafluorophosphate.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing solid lithium hexafluorophosphate with low fine crystal content, the method comprising:

[0008] S1, Lithium fluoride is added to anhydrous hydrogen fluoride and mixed under an inert atmosphere to obtain anhydrous hydrogen fluoride slurry;

[0009] S2, the first part of anhydrous hydrogen fluoride slurry and the first part of phosphorus pentafluoride gas are passed into a first-stage continuous reaction crystallizer to obtain the first crystal slurry. The first crystal slurry is sent to a low-temperature classification device to separate the first stream and the second stream.

[0010] S3, the second material is fed into the secondary continuous growth crystallizer and the second part of phosphorus pentafluoride gas is introduced to obtain the second crystal slurry. The second crystal slurry is subjected to solid-liquid separation to obtain solid wet product and mother liquor.

[0011] S4, during continuous operation, the first stream, the second part of anhydrous hydrogen fluoride slurry and part of the mother liquor are fed into the pre-conditioning section to obtain a pre-conditioning stream, and the pre-conditioning stream is returned to the feed end of the first-stage continuous reaction crystallizer;

[0012] S5, the solid wet product is washed and dried to obtain solid lithium hexafluorophosphate.

[0013] Preferably, in S1, the mass concentration of lithium fluoride in the anhydrous hydrogen fluoride slurry is 5% to 15%; the lithium fluoride is dispersed in the anhydrous hydrogen fluoride in the form of suspended solids; the inert atmosphere is a nitrogen atmosphere; the mixing temperature is -20℃ to 5℃; and the mixing time is 10 to 60 min.

[0014] Preferably, the mass flow ratio of the first part of anhydrous hydrogen fluoride slurry to the second part of anhydrous hydrogen fluoride slurry is 100:(5-35).

[0015] Preferably, in S2, the temperature of the first-stage continuous reaction crystallizer is -45℃ to -20℃, and the residence time is 15 to 50 minutes; the first portion of phosphorus pentafluoride gas accounts for 70% to 90% of the total phosphorus pentafluoride gas introduced.

[0016] Preferably, in S2, the operating temperature of the low-temperature grading device is -45℃ to -15℃; the low-temperature grading device is a hydrocyclone or a gravity sedimentation grading tank; and the grading particle size of the low-temperature grading device is 15 to 40 μm.

[0017] Preferably, in S2, the mass percentage of lithium hexafluorophosphate crystals with a particle size smaller than the graded particle size in the first stream is higher than the mass percentage of lithium hexafluorophosphate crystals with a particle size smaller than the graded particle size in the second stream; the mass percentage of lithium hexafluorophosphate crystals with a particle size larger than the graded particle size in the second stream is higher than the mass percentage of lithium hexafluorophosphate crystals with a particle size larger than the graded particle size in the first stream.

[0018] Preferably, in S3, the temperature of the secondary continuous growth crystallizer is -40℃ to -15℃, and the residence time is 30 to 180 min; the second part of phosphorus pentafluoride gas accounts for 10% to 30% of the total phosphorus pentafluoride gas flow; the first part of phosphorus pentafluoride gas and the second part of phosphorus pentafluoride gas constitute the total phosphorus pentafluoride gas flow.

[0019] Preferably, in S4, the mass ratio of the first material, the second part of the anhydrous hydrogen fluoride slurry, and part of the mother liquor is 100:(5-40):(20-120).

[0020] Preferably, in S4, the temperature of the pre-conditioning section is 5-25°C higher than the temperature of the first-stage continuous reaction crystallizer, and the temperature of the pre-conditioning section is not higher than 0°C; the residence time of the pre-conditioning section is 30-300s.

[0021] Preferably, the primary continuous reaction crystallizer and the secondary continuous growth crystallizer are respectively equipped with a jacketed heat exchange structure or an external circulation heat exchange structure.

[0022] Preferably, in step S3, the mass flow rate of the mother liquor returned to the pre-conditioning section as part of the mother liquor accounts for 10% to 60% of the mass flow rate of the mother liquor obtained in step S3; another part of the mother liquor obtained in step S3 enters the mother liquor treatment section; the temperature of the solid-liquid separation is -40℃ to -15℃.

[0023] Preferably, the anhydrous hydrogen fluoride slurry is conveyed by a slurry conveying device resistant to anhydrous hydrogen fluoride corrosion, wherein the slurry conveying device is a diaphragm pump, a screw pump, or a magnetically driven slurry pump.

[0024] Preferably, in step S5, the washing process uses anhydrous hydrogen fluoride at a temperature of -40℃ to -15℃; the drying process includes nitrogen replacement, a first vacuum drying, and a second vacuum drying. The first vacuum drying is performed at a temperature of 40℃ to 50℃, an absolute pressure of 1 to 5 kPa, and a drying time of 2 to 6 hours. The second vacuum drying is performed at a temperature of 50℃ to 60℃, an absolute pressure of 0.1 to 1 kPa, and a drying time of 0.5 to 3 hours.

[0025] When lithium fluoride is added to anhydrous hydrogen fluoride, it does not completely enter a homogeneous dissolved state, but exists as a suspended solid with a small amount of dissolved fluoride and lithium ions. Anhydrous hydrogen fluoride provides the anhydrous reaction medium and participates in the wetting and ionization processes on the surface of the lithium fluoride particles. Upon entering the system, phosphorus pentafluoride first dissolves in the anhydrous hydrogen fluoride phase and diffuses around the lithium fluoride particles. As a Lewis acid, phosphorus pentafluoride accepts fluoride ions to form hexafluorophosphate, and lithium ions combine with hexafluorophosphate to form lithium hexafluorophosphate. This reaction occurs at the interface region where gas mass transfer, liquid-phase diffusion, and solid particle surface dissolution coexist. The concentration of the products varies around the bubbles, on the surface of the lithium fluoride particles, and in the crystal precipitation region.

[0026] After the first portion of anhydrous hydrogen fluoride slurry and the first portion of phosphorus pentafluoride gas enter the primary continuous reaction crystallizer, the dissolution of phosphorus pentafluoride and the reaction on the surface of lithium fluoride particles occur simultaneously. Lithium hexafluorophosphate reaches the crystallization conditions in a localized area and forms crystal nuclei. Because lithium fluoride exists in the form of suspended particles, the reaction is not homogeneous and instantaneous, but rather a concentration gradient is formed between the particle surface, the gas-liquid contact area, and the main crystal slurry. The primary continuous reaction crystallizer is responsible for the formation of crystal nuclei and the establishment of the initial crystal slurry. The supply of the first portion of phosphorus pentafluoride provides the system with the reaction driving force required for the formation of lithium hexafluorophosphate, while reserving the reaction margin required for subsequent secondary continuous growth, thus avoiding the lithium hexafluorophosphate formation process being concentrated in a single reaction space.

[0027] After the first crystal slurry enters the low-temperature classification unit, the particles in the slurry separate into the first stream and the second stream according to sedimentation, swirling, or clarification. The first stream carries more small-diameter crystals and the accompanying mother liquor, while the second stream has a relatively higher proportion of larger crystals. Low-temperature classification is not simply a downstream screening process, but rather a separation of the fine crystals generated in the primary reaction crystallization stage from the direct entry into the product growth path, preventing the fine crystals from directly entering the subsequent solid-liquid separation and drying processes with the second stream. Through this separation, the fine crystals in the slurry are guided into a pre-conditioning path, while the larger crystals are guided into a secondary continuous growth path. The crystal particle size evolution path is divided into a fine crystal reflux conditioning path and a crystal continued growth path.

[0028] After the second stream enters the secondary continuous growth crystallizer, the second portion of phosphorus pentafluoride continues to react with lithium fluoride in the system. The supersaturation in the secondary continuous growth crystallizer primarily affects the existing crystal surfaces. Larger crystals have available deposition faces, and lithium hexafluorophosphate molecules or ions continue to align on the crystal surface and enter the crystal lattice, causing the crystal size to continue to increase. Since the primary stage has already provided the nucleation basis, the phosphorus pentafluoride supply in the secondary stage no longer serves a centralized nucleation function, but instead cooperates with the crystals in the second stream for surface growth. In this way, the mass transfer of phosphorus pentafluoride, the reaction of lithium fluoride particles, and the growth of lithium hexafluorophosphate crystals are distributed across different continuous segments, reducing competition for reactants between nucleation and growth within the same space.

[0029] After the first stream, the second part of the anhydrous hydrogen fluoride slurry, and part of the mother liquor enter the preconditioning section, the fine crystals in the first stream are in a different temperature and compositional environment than those in the first-stage continuous reaction crystallizer. The temperature in the preconditioning section is higher than that in the first-stage continuous reaction crystallizer, causing the dissolution equilibrium of lithium hexafluorophosphate in anhydrous hydrogen fluoride to shift towards dissolution. Due to their smaller radius of curvature and higher surface chemical potential, the fine crystals are more easily introduced into the mother liquor phase. The second part of the anhydrous hydrogen fluoride slurry introduces suspended lithium fluoride particles, while part of the mother liquor introduces dissolved lithium hexafluorophosphate components that are close to the system composition, along with the anhydrous hydrogen fluoride medium. Both participate in the compositional buffering in the preconditioning section. The fine crystals are not directly mechanically removed, but rather undergo partial dissolution, surface passivation layer renewal, and slurry composition redistribution in the preconditioning section.

[0030] After the pre-conditioned stream returns to the feed end of the primary continuous reaction crystallizer, the fine-crystal dissolved components from the first stream, the lithium fluoride suspended particles from the second section of anhydrous hydrogen fluoride slurry, and the lithium hexafluorophosphate dissolved components from the mother liquor all enter the primary reaction environment. When this stream comes into contact with the newly introduced phosphorus pentafluoride gas, the system is no longer simply a reaction starting point formed by fresh lithium fluoride slurry and phosphorus pentafluoride gas, but contains the pre-conditioned mother liquor composition and residual crystal components. The pre-conditioned stream acts as a buffer for the composition at the primary feed end, diluting and absorbing the local formation rate and crystal slurry composition changes after the entry of phosphorus pentafluoride. The fine crystals, separated in the previous cycle and entering the pre-conditioning section, subsequently return to the primary reaction environment as dissolved components or surface-reformed crystal residues, thus participating in a new lithium hexafluorophosphate generation and crystal growth cycle.

[0031] The role of the mother liquor in the preconditioning section is not merely material recovery. After secondary continuous growth, the mother liquor undergoes a crystal surface deposition process, and its lithium hexafluorophosphate solubility, anhydrous hydrogen fluoride composition, and acidic component state correspond to the secondary growth environment. Introducing a portion of the mother liquor into the preconditioning section reduces abrupt compositional changes when the first stream and the second portion of the anhydrous hydrogen fluoride slurry come into direct contact, allowing fine crystal dissolution and slurry redistribution to occur in a medium close to the process system. The remaining portion of the mother liquor enters the treatment section, preventing the continuous accumulation of impurities and byproducts during continuous operation and maintaining the compositional boundaries of the crystal growth environment.

[0032] The heat exchange structures in the primary continuous reaction crystallizer and the secondary continuous growth crystallizer are used to remove the heat released from the reaction of lithium fluoride with phosphorus pentafluoride. If the heat of reaction accumulates locally in the gas-liquid-solid contact area, it will alter the solubility and crystallization state of lithium hexafluorophosphate, causing alternating periods of localized dissolution and recrystallization. Jacketed heat exchange or external circulation heat exchange maintains the reactor temperature within a set range along the axial and radial directions, preserving the matching relationship between phosphorus pentafluoride mass transfer, lithium fluoride particle surface reaction, and lithium hexafluorophosphate crystallization. Temperature control, combined with segmented phosphorus pentafluoride introduction, creates different solubility environments in the nucleation zone, growth zone, and pre-conditioning zone.

[0033] When the solid wet product is washed with anhydrous hydrogen fluoride, the mother liquor, free acidic components, and attached fine particles on the crystal surface are replaced by the washing medium. The washing process maintains an anhydrous system to prevent decomposition of lithium hexafluorophosphate upon contact with moisture. Subsequent nitrogen purging and staged vacuum drying are performed. The first stage of drying removes volatile media remaining in the intercrystalline spaces and on the surface, while the second stage treats residual adsorbed components at lower pressure. The vacuum environment reduces the resistance to removal of residual anhydrous hydrogen fluoride, and staged heating avoids prolonged exposure of the crystals to high temperatures. Maintaining an anhydrous chemical environment for the crystals during washing and drying minimizes surface decomposition and the formation of secondary fine powder.

[0034] In a second aspect, the present invention provides a solid lithium hexafluorophosphate with low fine crystal content prepared by the preparation method described in the first aspect; in the solid lithium hexafluorophosphate with low fine crystal content, the volume distribution of lithium hexafluorophosphate crystal particles with a particle size of less than 20 μm is not greater than 8%, the particle size distribution span is 0.8 to 1.8, and the particle size distribution span is (D90-D10) / D50.

[0035] Compared with existing technologies, the beneficial effects of this invention are as follows: This invention involves a segmented reaction of anhydrous hydrogen fluoride slurry with phosphorus pentafluoride gas, and a low-temperature stage after the first-stage continuous reaction crystallization. This allows the fine-grained components and larger-grained components in the first-stage crystal slurry to flow in different directions, preventing fine crystals from directly entering subsequent product paths. The first stream, along with the second portion of anhydrous hydrogen fluoride slurry and a portion of the mother liquor, undergoes composition and temperature adjustments in the pre-conditioning section before returning to the feed end of the first-stage continuous reaction crystallizer. This allows the fine-grained components to re-enter the reaction crystallization cycle, reducing the entrainment of fine crystals in the final solid product. The second stream, after entering the second-stage continuous growth crystallizer, continues crystal growth under the condition of supplemented phosphorus pentafluoride gas, which helps improve the consistency of crystal particle size distribution. The participation of a portion of the mother liquor in pre-conditioning and mother liquor treatment reduces the risk of compositional fluctuations and impurity accumulation during continuous operation, thereby obtaining solid lithium hexafluorophosphate with low fine-grained content and stable particle size distribution. Attached Figure Description

[0036] Figure 1 The flowchart illustrates the preparation method provided by this invention. Detailed Implementation

[0037] The technical solutions of the present invention will be described in detail below with reference to specific embodiments and accompanying drawings. The embodiments described herein are specific implementations of the present invention, used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary, and should not be construed as limiting the implementation methods or the scope of protection of the present invention. In addition to the embodiments described herein, those skilled in the art can employ other obvious technical solutions based on the content disclosed in the claims and specification of this application. These technical solutions include those that make any obvious substitutions and modifications to the embodiments described herein.

[0038] The chemical reagents used in the embodiments and comparative examples of this invention are all commercially available products and have not undergone any further purification treatment.

[0039] Example 1

[0040] This embodiment provides a solid lithium hexafluorophosphate with low fine crystal content and its preparation method. The preparation method is as follows: Figure 1 As shown, the preparation method specifically includes the following steps:

[0041] S1, lithium fluoride is added to anhydrous hydrogen fluoride and mixed under a nitrogen atmosphere to obtain anhydrous hydrogen fluoride slurry; the mass concentration of lithium fluoride in the anhydrous hydrogen fluoride slurry is 5%, and the lithium fluoride is dispersed in the anhydrous hydrogen fluoride in the form of suspended solids; the mixing temperature is -20℃ and the mixing time is 60 min; the anhydrous hydrogen fluoride slurry is delivered by a diaphragm pump.

[0042] S2, the first portion of anhydrous hydrogen fluoride slurry and the first portion of phosphorus pentafluoride gas are introduced into a primary continuous reaction crystallizer to obtain a first crystal slurry. The first crystal slurry is then sent to a low-temperature classification device to separate the first stream and the second stream. The mass flow ratio of the first portion of anhydrous hydrogen fluoride slurry to the second portion of anhydrous hydrogen fluoride slurry is 100:5. The temperature of the primary continuous reaction crystallizer is -45℃, and the residence time is 50 min. The first portion of phosphorus pentafluoride gas accounts for 90% of the total phosphorus pentafluoride gas flow. The low-temperature classification device is a gravity settling classification tank with an operating temperature of -45℃ and a classification particle size of 15 μm.

[0043] S3, the second material is fed into the secondary continuous growth crystallizer, and a second portion of phosphorus pentafluoride gas is introduced to obtain a second crystal slurry. The second crystal slurry is subjected to solid-liquid separation to obtain a solid wet product and a mother liquor. The temperature of the secondary continuous growth crystallizer is -40℃, and the residence time is 180 min. The second portion of phosphorus pentafluoride gas accounts for 10% of the total phosphorus pentafluoride gas introduced. The solid-liquid separation temperature is -40℃. The primary continuous reaction crystallizer and the secondary continuous growth crystallizer are respectively equipped with jacketed heat exchange structures.

[0044] S4, during continuous operation, the first stream, the second portion of anhydrous hydrogen fluoride slurry, and a portion of the mother liquor are fed into the pre-conditioning section to obtain a pre-conditioned stream, which is then returned to the feed end of the first-stage continuous reaction crystallizer; the mass ratio of the first stream, the second portion of anhydrous hydrogen fluoride slurry, and a portion of the mother liquor is 100:5:120; the temperature of the pre-conditioning section is -20℃, and the residence time is 30s; the mother liquor mass flow rate returned to the pre-conditioning section as a portion of the mother liquor accounts for 10% of the mother liquor mass flow rate obtained in step S3, and the other portion of the mother liquor obtained in step S3 enters the mother liquor treatment section;

[0045] S5, the solid wet product is washed and dried to obtain solid lithium hexafluorophosphate; the washing is performed using anhydrous hydrogen fluoride at a temperature of -40°C; the drying includes nitrogen replacement, a first vacuum drying and a second vacuum drying, the first vacuum drying is performed at a temperature of 40°C, an absolute pressure of 5 kPa and a drying time of 6 h; the second vacuum drying is performed at a temperature of 60°C, an absolute pressure of 0.1 kPa and a drying time of 0.5 h.

[0046] Example 2

[0047] This embodiment provides a solid lithium hexafluorophosphate with low fine crystal content and its preparation method. The preparation method specifically includes the following steps:

[0048] S1, lithium fluoride is added to anhydrous hydrogen fluoride and mixed under a nitrogen atmosphere to obtain anhydrous hydrogen fluoride slurry; the mass concentration of lithium fluoride in the anhydrous hydrogen fluoride slurry is 15%, and the lithium fluoride is dispersed in the anhydrous hydrogen fluoride in the form of suspended solids; the mixing temperature is 5°C and the mixing time is 10 min; the anhydrous hydrogen fluoride slurry is delivered by a screw pump;

[0049] S2, the first portion of anhydrous hydrogen fluoride slurry and the first portion of phosphorus pentafluoride gas are introduced into a primary continuous reaction crystallizer to obtain a first crystal slurry. The first crystal slurry is then sent to a low-temperature classification device to separate the first stream and the second stream. The mass flow ratio of the first portion of anhydrous hydrogen fluoride slurry to the second portion of anhydrous hydrogen fluoride slurry is 100:35. The temperature of the primary continuous reaction crystallizer is -20℃, and the residence time is 15 min. The first portion of phosphorus pentafluoride gas accounts for 70% of the total phosphorus pentafluoride gas flow. The low-temperature classification device is a hydrocyclone with an operating temperature of -15℃ and a classification particle size of 40 μm.

[0050] S3, the second material is fed into the secondary continuous growth crystallizer, and a second portion of phosphorus pentafluoride gas is introduced to obtain a second crystal slurry. The second crystal slurry is subjected to solid-liquid separation to obtain a solid wet product and a mother liquor. The temperature of the secondary continuous growth crystallizer is -15℃, and the residence time is 30 min. The second portion of phosphorus pentafluoride gas accounts for 30% of the total phosphorus pentafluoride gas introduced. The solid-liquid separation temperature is -15℃. The primary continuous reaction crystallizer and the secondary continuous growth crystallizer are respectively equipped with an external circulation heat exchange structure.

[0051] S4, during continuous operation, the first stream, the second portion of anhydrous hydrogen fluoride slurry, and a portion of the mother liquor are fed into the pre-conditioning section to obtain a pre-conditioned stream, which is then returned to the feed end of the first-stage continuous reaction crystallizer; the mass ratio of the first stream, the second portion of anhydrous hydrogen fluoride slurry, and a portion of the mother liquor is 100:40:20; the temperature of the pre-conditioning section is -15℃, and the residence time is 300s; the mother liquor mass flow rate returned to the pre-conditioning section as a portion of the mother liquor accounts for 60% of the mother liquor mass flow rate obtained in step S3, and the other portion of the mother liquor obtained in step S3 enters the mother liquor treatment section;

[0052] S5, the solid wet product is washed and dried to obtain solid lithium hexafluorophosphate; the washing is carried out using anhydrous hydrogen fluoride at a temperature of -15℃; the drying includes nitrogen replacement, first vacuum drying and second vacuum drying, the first vacuum drying is carried out at a temperature of 50℃, an absolute pressure of 1kPa and a drying time of 2h; the second vacuum drying is carried out at a temperature of 50℃, an absolute pressure of 1kPa and a drying time of 3h.

[0053] Example 3

[0054] This embodiment provides a solid lithium hexafluorophosphate with low fine crystal content and its preparation method. The preparation method specifically includes the following steps:

[0055] S1, Lithium fluoride is added to anhydrous hydrogen fluoride and mixed under a nitrogen atmosphere to obtain anhydrous hydrogen fluoride slurry; the mass concentration of lithium fluoride in the anhydrous hydrogen fluoride slurry is 9%, and the lithium fluoride is dispersed in the anhydrous hydrogen fluoride in the form of suspended solids; the mixing temperature is -8℃ and the mixing time is 35min; the anhydrous hydrogen fluoride slurry is transported by a magnetically driven slurry pump;

[0056] S2, the first portion of anhydrous hydrogen fluoride slurry and the first portion of phosphorus pentafluoride gas are introduced into a primary continuous reaction crystallizer to obtain a first crystal slurry. The first crystal slurry is then sent to a low-temperature classification device to separate the first stream and the second stream. The mass flow ratio of the first portion of anhydrous hydrogen fluoride slurry to the second portion of anhydrous hydrogen fluoride slurry is 100:18. The temperature of the primary continuous reaction crystallizer is -32℃, and the residence time is 30 min. The first portion of phosphorus pentafluoride gas accounts for 80% of the total phosphorus pentafluoride gas flow. The low-temperature classification device is a hydrocyclone with an operating temperature of -30℃ and a classification particle size of 25 μm.

[0057] S3, the second material is fed into the secondary continuous growth crystallizer, and a second portion of phosphorus pentafluoride gas is introduced to obtain a second crystal slurry. The second crystal slurry is subjected to solid-liquid separation to obtain a solid wet product and a mother liquor. The temperature of the secondary continuous growth crystallizer is -28°C, and the residence time is 90 min. The second portion of phosphorus pentafluoride gas accounts for 20% of the total phosphorus pentafluoride gas introduced. The solid-liquid separation temperature is -28°C. The primary continuous reaction crystallizer is equipped with a jacketed heat exchange structure, and the secondary continuous growth crystallizer is equipped with an external circulation heat exchange structure.

[0058] S4, during continuous operation, the first stream, the second portion of anhydrous hydrogen fluoride slurry, and a portion of the mother liquor are fed into the pre-conditioning section to obtain a pre-conditioned stream, which is then returned to the feed end of the first-stage continuous reaction crystallizer; the mass ratio of the first stream, the second portion of anhydrous hydrogen fluoride slurry, and a portion of the mother liquor is 100:20:70; the temperature of the pre-conditioning section is -17℃, and the residence time is 120s; the mother liquor mass flow rate returned to the pre-conditioning section as a portion of the mother liquor accounts for 35% of the mother liquor mass flow rate obtained in step S3, and the other portion of the mother liquor obtained in step S3 enters the mother liquor treatment section;

[0059] S5, the solid wet product is washed and dried to obtain solid lithium hexafluorophosphate; the washing is performed using anhydrous hydrogen fluoride at a temperature of -28°C; the drying includes nitrogen replacement, a first vacuum drying and a second vacuum drying, the first vacuum drying is performed at a temperature of 45°C, an absolute pressure of 3 kPa and a drying time of 4 h; the second vacuum drying is performed at a temperature of 55°C, an absolute pressure of 0.5 kPa and a drying time of 1.5 h.

[0060] Example 4

[0061] This embodiment provides a solid lithium hexafluorophosphate with low fine crystal content and its preparation method. The preparation method specifically includes the following steps:

[0062] S1, lithium fluoride is added to anhydrous hydrogen fluoride and mixed under a nitrogen atmosphere to obtain anhydrous hydrogen fluoride slurry; the mass concentration of lithium fluoride in the anhydrous hydrogen fluoride slurry is 12%, and the lithium fluoride is dispersed in the anhydrous hydrogen fluoride in the form of suspended solids; the mixing temperature is -15℃ and the mixing time is 45min; the anhydrous hydrogen fluoride slurry is delivered by a diaphragm pump;

[0063] S2, the first portion of anhydrous hydrogen fluoride slurry and the first portion of phosphorus pentafluoride gas are introduced into a primary continuous reaction crystallizer to obtain a first crystal slurry. The first crystal slurry is then sent to a low-temperature classification device to separate the first stream and the second stream. The mass flow ratio of the first portion of anhydrous hydrogen fluoride slurry to the second portion of anhydrous hydrogen fluoride slurry is 100:28. The temperature of the primary continuous reaction crystallizer is -38℃, and the residence time is 40 min. The first portion of phosphorus pentafluoride gas accounts for 85% of the total phosphorus pentafluoride gas flow. The low-temperature classification device is a gravity settling classification tank with an operating temperature of -38℃ and a classification particle size of 32 μm.

[0064] S3, the second material is fed into the secondary continuous growth crystallizer, and a second portion of phosphorus pentafluoride gas is introduced to obtain a second crystal slurry. The second crystal slurry is subjected to solid-liquid separation to obtain a solid wet product and a mother liquor. The temperature of the secondary continuous growth crystallizer is -35°C, and the residence time is 120 min. The second portion of phosphorus pentafluoride gas accounts for 15% of the total phosphorus pentafluoride gas introduced. The solid-liquid separation temperature is -35°C. The primary continuous reaction crystallizer and the secondary continuous growth crystallizer are respectively equipped with jacketed heat exchange structures.

[0065] S4, during continuous operation, the first stream, the second portion of anhydrous hydrogen fluoride slurry, and a portion of the mother liquor are fed into the pre-conditioning section to obtain a pre-conditioned stream, which is then returned to the feed end of the first-stage continuous reaction crystallizer; the mass ratio of the first stream, the second portion of anhydrous hydrogen fluoride slurry, and a portion of the mother liquor is 100:30:90; the temperature of the pre-conditioning section is -18℃, and the residence time is 210s; the mother liquor mass flow rate returned to the pre-conditioning section as a portion of the mother liquor accounts for 50% of the mother liquor mass flow rate obtained in step S3, and the other portion of the mother liquor obtained in step S3 enters the mother liquor treatment section;

[0066] S5, the solid wet product is washed and dried to obtain solid lithium hexafluorophosphate; the washing is performed using anhydrous hydrogen fluoride at a temperature of -35°C; the drying includes nitrogen replacement, a first vacuum drying and a second vacuum drying, the first vacuum drying is performed at a temperature of 48°C, an absolute pressure of 2 kPa and a drying time of 5 h; the second vacuum drying is performed at a temperature of 58°C, an absolute pressure of 0.3 kPa and a drying time of 2.5 h.

[0067] Comparative Example 1

[0068] This comparative example provides a solid lithium hexafluorophosphate with low fine crystal content and its preparation method. The difference between this example and Example 1 is that in S2, the first crystal slurry is not sent to a low-temperature classification device for classification. Instead, all the first crystal slurry obtained from the first-stage continuous reaction crystallizer is sent to the second-stage continuous growth crystallizer. No first and second streams are formed, and no pre-conditioning reflux of the first stream is performed. Other process parameters and operating conditions are exactly the same as in Example 1.

[0069] Comparative Example 2

[0070] This comparative example provides a solid lithium hexafluorophosphate with low fine crystal content and its preparation method. The difference between this example and Example 1 is that the first stream obtained in S2 is directly returned to the feed end of the first-stage continuous reaction crystallizer; the second part of anhydrous hydrogen fluoride slurry and part of the mother liquor are not pre-mixed with the first stream. Other process parameters and operating conditions are exactly the same as in Example 1.

[0071] Comparative Example 3

[0072] This comparative example provides a solid lithium hexafluorophosphate with low fine crystal content and its preparation method. The difference between this example and Example 1 is that in step S4, a portion of the mother liquor is not fed into the preconditioning section. Instead, the first stream and the second portion of anhydrous hydrogen fluoride slurry are fed into the preconditioning section and then returned to the feed end of the first-stage continuous reaction crystallizer. All the mother liquor obtained in step S3 enters the mother liquor treatment section. Other process parameters and operating conditions are exactly the same as in Example 1.

[0073] Comparative Example 4

[0074] This comparative example provides a solid lithium hexafluorophosphate with low fine crystal content and its preparation method. The difference between this example and Example 1 is that a secondary continuous growth crystallizer is not set in S3, the second stream obtained in step S2 is directly subjected to solid-liquid separation, and no second part of phosphorus pentafluoride gas is introduced. Other process parameters and operating conditions are exactly the same as in Example 1.

[0075] Performance testing:

[0076] Fine crystal content test: Samples were taken under dry nitrogen atmosphere with a dew point below -40℃. The solid lithium hexafluorophosphate sample to be tested was sealed and transferred to the dry powder sampler of a dry laser particle size analyzer. Dry nitrogen was used as the dispersion gas for the test, with a dispersion pressure of 0.05 MPa and a sample volume of 2 g. The particle size distribution of the sample was recorded. The cumulative percentage of particles with a diameter less than 20 μm was taken as the fine crystal content. Each group of samples was tested in triplicate, and the average value was taken.

[0077] Particle size distribution span test: The same dry laser particle size distribution test conditions as the fine grain content test were used. D10, D50, and D90 were recorded, and the particle size distribution span was calculated according to the formula (D90-D10) / D50. Each group of samples was tested in parallel 3 times, and the average value was taken.

[0078] Angle of repose test: The test was conducted in a dry nitrogen environment with a dew point below -40℃. 50g of solid lithium hexafluorophosphate sample was weighed and added to a smooth-walled polytetrafluoroethylene funnel with an outlet inner diameter of 10mm. The distance between the lower end of the funnel outlet and the horizontal test platform was 50mm. The sample was allowed to fall naturally onto the horizontal test platform without external vibration, forming a stable powder accumulation cone. After all the sample had flowed out, the height H and bottom diameter D of the accumulation cone were measured, and the angle of repose was calculated using the formula θ=arctan(2H / D). Each group of samples was tested in parallel three times, and the average value was taken.

[0079] The test results are shown in Table 1.

[0080] Table 1. Test results of solid lithium hexafluorophosphate in Examples 1-4 and Comparative Examples 1-4

[0081]

[0082] As shown in Table 1, compared with Example 1, Comparative Example 1 has increased fine grain content, increased particle size distribution range, and increased angle of repose; Comparative Example 2 has increased fine grain content, increased particle size distribution range, and increased angle of repose; Comparative Example 3 has increased fine grain content, increased particle size distribution range, and increased angle of repose; and Comparative Example 4 has increased fine grain content, increased particle size distribution range, and increased angle of repose.

[0083] This is because Comparative Example 1 did not have a low-temperature classification, so the fine crystals and larger crystals in the first crystal slurry were not separated, and the fine crystals directly entered the subsequent secondary growth and solid-liquid separation path. The secondary growth stage could not preferentially consume and reorganize the fine crystal components, resulting in increased fine crystal entrainment in the product, a wider particle size distribution, increased interparticle friction during powder accumulation, and a higher angle of repose. Although Comparative Example 2 retained low-temperature classification, the first stream was returned directly to the feed end of the first-stage continuous reaction crystallizer without pre-conditioning. The fine crystals in the first stream lacked a heating and dissolution process and a composition buffering process. After returning, they easily formed local compositional fluctuations with the newly introduced phosphorus pentafluoride gas, reducing the matching between crystal nucleation and growth, and increasing both the fine crystal content and the particle size distribution range.

[0084] In Comparative Example 3, no mother liquor was introduced into the preconditioning section; the first stream was only mixed with the second part of anhydrous hydrogen fluoride slurry before being refluxed. Due to the lack of buffering effect of the secondary growth mother liquor on the system composition, the lithium hexafluorophosphate dissolved component in the preconditioning section was insufficient, the fine crystal dissolution and redistribution process was unstable, and the fine crystal content of the final product increased, and the angle of repose increased. In Comparative Example 4, no secondary continuous growth crystallizer was set up; the second stream directly entered the solid-liquid separation process. The crystals lacked the subsequent surface growth process, and the smaller crystals formed in the first stage could not continue to grow. Due to the segmented supply of phosphorus pentafluoride and the elimination of the crystal growth section, the concentration of product particle size distribution decreased, the proportion of fine crystals increased, and the powder flowability decreased accordingly.

[0085] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing solid lithium hexafluorophosphate with low fine crystal content, characterized in that, The preparation method includes: S1, Lithium fluoride is added to anhydrous hydrogen fluoride and mixed under an inert atmosphere to obtain anhydrous hydrogen fluoride slurry; S2, the first part of anhydrous hydrogen fluoride slurry and the first part of phosphorus pentafluoride gas are passed into a first-stage continuous reaction crystallizer to obtain the first crystal slurry. The first crystal slurry is sent to a low-temperature classification device to separate the first stream and the second stream. S3, the second material is fed into the secondary continuous growth crystallizer and the second part of phosphorus pentafluoride gas is introduced to obtain the second crystal slurry. The second crystal slurry is subjected to solid-liquid separation to obtain solid wet product and mother liquor. S4, during continuous operation, the first stream, the second part of anhydrous hydrogen fluoride slurry and part of the mother liquor are fed into the pre-conditioning section to obtain a pre-conditioning stream, and the pre-conditioning stream is returned to the feed end of the first-stage continuous reaction crystallizer; S5, the solid wet product is washed and dried to obtain solid lithium hexafluorophosphate.

2. The method for preparing solid lithium hexafluorophosphate with low fine crystal content according to claim 1, characterized in that, In S1, the mass concentration of lithium fluoride in the anhydrous hydrogen fluoride slurry is 5% to 15%; the lithium fluoride is dispersed in the anhydrous hydrogen fluoride in the form of suspended solids; the inert atmosphere is a nitrogen atmosphere; the mixing temperature is -20℃ to 5℃; and the mixing time is 10 to 60 min.

3. The method for preparing solid lithium hexafluorophosphate with low fine crystal content according to claim 1, characterized in that, The mass flow ratio of the first part of anhydrous hydrogen fluoride slurry to the second part of anhydrous hydrogen fluoride slurry is 100:(5-35).

4. The method for preparing solid lithium hexafluorophosphate with low fine crystal content according to claim 1, characterized in that, In S2, the temperature of the first-stage continuous reaction crystallizer is -45℃ to -20℃, and the residence time is 15 to 50 minutes; the first part of phosphorus pentafluoride gas accounts for 70% to 90% of the total phosphorus pentafluoride gas introduced.

5. The method for preparing solid lithium hexafluorophosphate with low fine crystal content according to claim 1, characterized in that, In S2, the operating temperature of the low-temperature classification device is -45℃ to -15℃; the low-temperature classification device is a hydrocyclone or a gravity sedimentation classification tank; the classification particle size of the low-temperature classification device is 15 to 40 μm.

6. The method for preparing solid lithium hexafluorophosphate with low fine-grain content according to claim 1, characterized in that, In S3, the temperature of the secondary continuous growth crystallizer is -40℃ to -15℃, and the residence time is 30 to 180 minutes; the second part of phosphorus pentafluoride gas accounts for 10% to 30% of the total phosphorus pentafluoride gas introduced. The first portion of phosphorus pentafluoride gas and the second portion of phosphorus pentafluoride gas constitute the total amount of phosphorus pentafluoride gas introduced.

7. The method for preparing solid lithium hexafluorophosphate with low fine-grain content according to claim 1, characterized in that, In S4, the mass ratio of the first material, the second part of the anhydrous hydrogen fluoride slurry, and part of the mother liquor is 100:(5-40):(20-120).

8. The method for preparing solid lithium hexafluorophosphate with low fine-grain content according to claim 1, characterized in that, In S4, the temperature of the pre-conditioning section is 5-25°C higher than the temperature of the first-stage continuous reaction crystallizer, and the temperature of the pre-conditioning section is not higher than 0°C; the residence time of the pre-conditioning section is 30-300s.

9. The method for preparing solid lithium hexafluorophosphate with low fine crystal content according to claim 1, characterized in that, In S5, the washing process uses anhydrous hydrogen fluoride at a temperature of -40℃ to -15℃; the drying process includes nitrogen replacement, a first vacuum drying, and a second vacuum drying. The first vacuum drying is performed at a temperature of 40 to 50℃, an absolute pressure of 1 to 5 kPa, and a drying time of 2 to 6 hours. The second vacuum drying is performed at a temperature of 50 to 60℃, an absolute pressure of 0.1 to 1 kPa, and a drying time of 0.5 to 3 hours.

10. A solid lithium hexafluorophosphate with low fine crystal content, characterized in that, The solid lithium hexafluorophosphate with low fine crystal content is prepared by the preparation method according to any one of claims 1 to 9; the volume distribution of lithium hexafluorophosphate crystal particles with a particle size of less than 20 μm is not greater than 8%, the particle size distribution span is 0.8 to 1.8, and the particle size distribution span is (D90-D10) / D50.

Images