Method for preparing high-purity alkali metal hexafluorophosphate and alkali metal hexafluorophosphate prepared by method
The process of preparing alkali metal hexafluorophosphate in a cooling reactor has solved the problem of high-purity preparation, achieved an efficient and low-cost purification process, and improved battery performance and production efficiency.
Patent Information
- Application Number
- CN202480020788.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2023-03-20
- Filing Date
- 2024-03-20
- Publication Date
- 2025-11-07
AI Technical Summary
Existing technologies struggle to prepare high-purity alkali metal hexafluorophosphates, especially NaPF6, KPF6, and CsPF6. Conventional methods result in high impurity content, affecting battery performance, and also present challenges such as back pressure, clogging, and complex purification steps that are difficult to control.
A process method is employed, which involves reacting alkali metal fluorides with phosphorus pentafluoride and hydrogen chloride gas in a cooled reactor, preparing high-purity alkali metal hexafluorophosphate by controlling pressure and purification steps, using high-purity anhydrous hydrogen fluoride and phosphorus pentachloride, controlling impurity content, and improving purity through static crystallization and drying steps.
The preparation of high-purity alkali metal hexafluorophosphate was achieved, the impurity content was reduced, the battery performance problem was solved, the controllability and efficiency of the production process were optimized, and the production cost was reduced.
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Abstract
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a process for the preparation of alkali metal hexafluorophosphate. More particularly, the present invention relates to a process for the preparation of high purity alkali metal hexafluorophosphate based on Group 1 elements, preferably selected from sodium hexafluorophosphate (NaPF6), potassium hexafluorophosphate (KPF6) and cesium hexafluorophosphate (CsPF6). The alkali metal hexafluorophosphate is used as electrolyte for stationary batteries, batteries for low speed electric vehicles. BACKGROUND
[0002] Lithium ion batteries (LIBs) have been in use for years across various industries since their commercialization in 1991. Starting from their use in portable electronic products, the demand for LIBs has now shifted mainly towards electric vehicle (EV) and energy storage system (ESS) applications. This has led to an unprecedented demand for the core raw material of LIBs, i.e., “lithium”. According to a Statista report, the global demand for lithium will be 2.5 million tons by 2030, while the demand in 2021 was 0.5 million tons. But the challenge lies in the relatively less abundance of lithium in the earth’s crust, which poses a serious threat to its future availability. Therefore, there is an urgent need to find the right alternative to lithium ion batteries. Here, sodium ion batteries come into play.
[0003] Sodium and potassium belong to the same group as lithium in the periodic table and reflect similar characteristics of lithium. Therefore, electrolytes based on sodium and potassium are gaining attention.
[0004] Sodium is the sixth most abundant element in the earth’s crust, and therefore, there is no threat to its availability. Although sodium ion batteries (NIBs) currently have limitations in adapting to high speed EVs, it is perfectly suitable for energy storage systems (ESSs) and low speed vehicles. In the foreseeable future, with continuous research and development activities in sodium ion batteries (NIBs), it is expected that sodium ion batteries will also find application in high speed EVs in the future. In this way, sodium ion batteries will become a potential alternative to lithium ion batteries in the future.
[0005] Similarly, due to the abundance of potassium, potassium ion batteries (KIBs) are emerging as a promising energy storage system. Potassium ions have certain advantages over similar lithium ions (e.g., lithium ion batteries): the battery design is simple, and both the materials and the manufacturing process are cheaper. The key advantage of potassium over lithium is its abundance and low cost, which makes potassium batteries a promising candidate for large-scale batteries such as home energy storage and electric vehicles.
[0006] While NaC104 is the most studied electrolyte salt in sodium-ion batteries (NIBs), KCI04 is the most studied electrolyte salt in potassium-ion batteries (KIBs), but the strong oxidizing nature of perchlorate anion hinders commercial applications. Similar to lithium hexafluorophosphate (LiPF6), sodium hexafluorophosphate (NaPF6), potassium hexafluorophosphate (KPF6), or cesium hexafluorophosphate (CsPF6) are electrolyte salts for sodium-ion batteries, potassium-ion batteries, or cesium-ion batteries, respectively. Therefore, they can adopt the already commercialized mature battery technology, such as lithium battery salts, which have good solubility and ionic conductivity in battery solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, and mixtures thereof.
[0007] For long-term cycle life of the battery, the electrolyte salt must be extremely pure.
[0008] In the conventional preparation of NaPF6, PF5 gas is passed through an HF solution of NaF to form NaPF6. Thereafter, HF is removed and NaPF6 is crystallized. However, one of the drawbacks of the conventional method is that a large amount of impurities in the form of fluorinated solids sink together with the NaPF6 crystals and are mixed together. Further, it is difficult to reduce the amount of impurities such as fluorinated solids in NaPF6 without the aid of complex reactions, procedures, and additional purification steps.
[0009] Similarly, to prepare KPF6, PF5 gas is passed through an HF solution of KF to form KPF6. Thereafter, HF is removed and KPF6 is crystallized. However, one of the drawbacks of the conventional method is that a large amount of impurities in the form of fluorinated solids sink together with the KPF6 crystals and are mixed together. Further, it is difficult to reduce the amount of impurities such as fluorinated solids in KPF6 without the aid of complex reactions, procedures, and additional purification steps.
[0010] Impurities in the form of fluorinated solids can cause electrode corrosion, which directly affects the capacity and performance of the battery.
[0011] Further, the presence of various metal impurities such as transition metal impurities is detrimental to the performance of the battery. For example, transition metal ions dissolved in the electrolyte can deposit on the anode surface. This can cause NaPF6 and KPF6 to decompose, sodium and potassium dendrites to grow from the negative electrode surface, and cause internal short circuits.
[0012] Water is another impurity of concern in sodium-ion batteries and potassium-ion batteries. The presence of water has a negative impact on the performance of the battery. The presence of excess water will also cause the electrolyte to acidify more quickly to produce HF gas, which is one of the main reasons for the swelling of the battery, which shortens the battery life.
[0013] Water can react with NaPF6 and KPF6, thereby reducing the capacity of the battery. The presence of water is also associated with poor cycle performance and loss of active material. Water can destroy the protective solid electrolyte interface layer and produce H2gas at the anode upon reduction. The presence of H2gas can increase the internal pressure of the battery and has an explosion hazard.
[0014] Further, the conventional preparation methods also have the disadvantage of not allowing the method to be implemented on a large scale, such as back pressure, clogging problems, and pressure maintenance and control for optimizing purity.
[0015] Therefore, there is a need to develop a method for preparing high purity alkali metal hexafluorophosphate (preferably selected from NaPF6, KPF6 or CsPF6) with reduced impurities and addressing one or more of the above disadvantages. SUMMARY
[0016] In one aspect, the present application provides a method for preparing ultra-high purity alkali metal hexafluorophosphate (MPF6), the method comprising the steps of: (a) charging alkali metal fluoride (MF) into a first reactor 'B' and flushing the first reactor 'B' with nitrogen gas, (b) cooling the first reactor 'B' to a predetermined temperature, followed by charging anhydrous hydrogen fluoride (AHF) gas in the first reactor 'B' and stirring the mixture comprising alkali metal fluoride (MF) dissolved in AHF, (c) cooling the mixture comprising alkali metal fluoride (MF) dissolved in AHF in the first reactor 'B' to a predetermined temperature, (d) charging phosphorus pentachloride (PC15) into a second reactor 'A' and adding AHF in multiple batches to produce high purity phosphorus pentafluoride (PF5) and hydrogen chloride (HC1) gas, (e) reacting the alkali metal fluoride (MF) dissolved in AHF in the first reactor 'B' with the mixture of phosphorus pentafluoride (PF5) and hydrogen chloride (HC1) gas obtained in step (d) to obtain an alkali metal hexafluorophosphate mother liquor dissolved in AHF under a blanket of PF5gas, (f) cooling the first reactor 'B' to a predetermined temperature and maintaining the alkali metal hexafluorophosphate mother liquor dissolved in AHF at the temperature for a predetermined time, followed by filtration and drying at a predetermined temperature to obtain alkali metal hexafluorophosphate (MPF6).
[0017] In another aspect, the present application provides an ultra-high purity alkali metal hexafluorophosphate having an improved yield of at least 99.50%, preferably at least 99.8%. BRIEF DESCRIPTION OF DRAWINGS
[0018] Reference will be made to the embodiments of the application, examples of which can be illustrated in the accompanying drawings. These drawings are intended to be illustrative and not restrictive. Although the application will be described generally in the context of these embodiments, it should be understood that the application is not limited to these specific embodiments.
[0019] Figure 1 A flow chart is shown. DETAILED DESCRIPTION
[0020] It should be noted that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a composition containing "a compound" includes mixtures of two or more compounds. It should also be noted that the term "or" is generally employed in its sense including "and / or" unless the content clearly dictates otherwise.
[0021] Unless otherwise specified, various amounts expressed in terms of the term "%" or "% w / w" mean percent by weight of the total solution or composition.
[0022] The present application relates to a process for the preparation of ultra-high purity alkali metal hexafluorophosphate (MPF6) for use as electrolyte in high-strength batteries, preferably in batteries for energy storage applications.
[0023] In one embodiment, the process for the preparation of ultra-high purity alkali metal hexafluorophosphate MPF6, preferably M = Na, K or Cs, comprises the following steps: a. loading an alkali metal fluoride (MF) into a first reactor 'B' and flushing the first reactor 'B' with nitrogen, b. cooling the first reactor 'B' to a predetermined temperature, followed by loading anhydrous hydrogen fluoride (AHF) gas in the first reactor 'B' and stirring the mixture comprising alkali metal fluoride (MF) dissolved in AHF, c. cooling the mixture comprising alkali metal fluoride (MF) dissolved in AHF in the first reactor 'B' to a predetermined temperature, d. loading phosphorus pentachloride (PC15) into a second reactor 'A' and adding AHF in multiple batches to produce high purity phosphorus pentafluoride (PF5) and hydrogen chloride (HC1) gas, e. reacting the alkali metal fluoride (MF) dissolved in AHF in the first reactor 'B' with the phosphorus pentafluoride (PF5) and hydrogen chloride (HC1) gas mixture obtained in step (d) to obtain an alkali metal hexafluorophosphate mother liquor dissolved in AHF under a blanket of PF5 gas, f. cooling the first reactor 'B' to a predetermined temperature and holding the alkali metal hexafluorophosphate mother liquor dissolved in AHF at the temperature for a predetermined time, followed by filtration and drying at a predetermined temperature to obtain alkali metal hexafluorophosphate (MPF6), g. pulverizing the dried MPF6 crystals and optionally sieving to obtain MPF6 powder.
[0024] Whenever PF5 gas is passed into the solution of MF / AHF, generally the dip tube gets clogged and the reaction mass develops back pressure which leads to loss. Therefore, the process reaction is carried out under PF5 gas overlay, which allows complete resolution of the problem of back pressure and clogging by the process of the present application.
[0025] Further, according to the disclosed process, the pressure of the reaction mass can be controlled as required. The overpressure is controlled by slow, batch-wise addition of AHF to the reactor containing PCl5. The generated HCI and PF5 gas is overlaid in the MF / HF reactor. Therefore, the pressure can be controlled by releasing the excess gas into a scrubber with basic pH and the pressure can be maintained and controlled more effectively in the scrubber such that the flow of the acidic fumes becomes unidirectional.
[0026] In one embodiment, the alkali metal is selected from Na, K and Cs and the alkali metal hexafluorophosphate is selected from NaPF6, KPF6 or CsPF6.
[0027] In one embodiment, the alkali metal fluoride is obtained from the corresponding metal carbonate and hydrogen fluoride (HF). For example, sodium fluoride (NaF) and potassium fluoride (KF) used in the process are obtained from sodium carbonate (Na2C03) or potassium carbonate (K2C03) and hydrogen fluoride (HF) respectively.
[0028] In one embodiment of the present application, the anhydrous hydrogen fluoride (AHF) gas is a purified anhydrous hydrogen fluoride (AHF) gas comprising less than or equal to 1 ppm of cation (metal) impurities or less than or equal to 1 ppm of anion impurities or less than or equal to 1 ppm of moisture, or a combination thereof.
[0029] In another embodiment, the purified anhydrous hydrogen fluoride (AHF) gas is reacted with the PCl5 solid bed reactor under stirring.
[0030] In another embodiment, the anhydrous hydrogen fluoride (AHF) gas is purified by treatment with fluorine (F2) gas as an oxidizing agent.
[0031] In one embodiment of the process, the PF5and HCI gases from the second reactor 'A' are dosed into the first reactor 'B' through the exhaust system without any dip tube, until a constant pressure.
[0032] In one embodiment of the process, the predetermined temperature in steps (b) and (c) is in the range of 0 to 15 °C, preferably in the range of 5 to 10 °C.
[0033] In one embodiment of the process, the AHF is added in multiple batches at 25 to 40 °C, preferably in the range of 200 to 300 g or 100 to 200 g.
[0034] In one embodiment of the process, the predetermined temperature and time for cooling in step (f) is in the range of -10 to -25 °C for 5 to 7 hours.
[0035] In one embodiment of the process, the predetermined temperature and time for cooling in step (f) is in the range of -15 to -20 °C for 6 hours.
[0036] In one embodiment of the process, the predetermined temperature for drying in step (f) is in the range of 35 to 50 °C, preferably in the range of 38 to 40 °C.
[0037] In one preferred embodiment, the process involves the use of a static crystallizer with a single run cycle of about 48 hours to allow slow growth of the crystals to large size and to minimize the surface area for HF adsorption.
[0038] In another preferred embodiment, the mother liquor is reused by adding alkali metal fluoride (preferably selected from NaF, KF or CsF) at -15 to 15 °C, preferably at 10 °C.
[0039] In one preferred embodiment, the drying of the crystallized alkali metal hexafluorophosphate is carried out by hot water circulation in the dryer jacket, followed by passing dry N2through the drying crystals for about 6-7 hours.
[0040] In another embodiment, the crystallized alkali metal hexafluorophosphate is dried, followed by solvent assisted drying preferably with solvents such as ether or dichloromethane.
[0041] Another aspect of the process is the use of a mesh size to sieve out the lower HF adsorbing crystals of larger size as the final product. The smaller crystals with higher adsorption of HF are recycled through the mother liquor. This reduces the overall concentration of HF in the finished product and recovers the alkali metal hexafluorophosphate crystals for seeding in the next crystallization procedure and also reduces the production cost.
[0042] In a preferred embodiment, the dry crystals of alkali metal hexafluorophosphate are sieved by a mesh size of less than or equal to 90.
[0043] In another embodiment, the alkali metal fluoride (sodium fluoride, potassium fluoride or cesium fluoride) used in the process is obtained by dissolving an alkali metal carbonate (preferably selected from sodium carbonate or potassium carbonate or cesium carbonate) in water to obtain an alkali metal carbonate solution, reacting the alkali metal carbonate solution with ultrapure hydrogen fluoride to obtain an alkali metal fluoride such as sodium fluoride or potassium fluoride or cesium fluoride, Na2C03+ 2HF→ 2NaF + C02+ H20; or K2C03+ HF→ KF + C02+ H20 In another embodiment, the alkali metal fluoride (sodium fluoride, potassium fluoride or cesium fluoride) used in the process is obtained by dissolving a treated pure alkali metal hydroxide in ultrapure water to obtain an alkali metal hydroxide solution or lye (caustic lye and caustic potash solution); reacting the alkali metal hydroxide solution / caustic lye / caustic potash solution with ultrapure hydrogen fluoride to obtain an alkali metal fluoride (sodium fluoride or potassium fluoride or cesium fluoride), NaOH + HF→ NaF + H20; or KOH + HF→ KF + H20 The obtained alkali metal fluoride (NaF or KF or CsF) of the desired particle size range is then dried and pulverized.
[0044] In another embodiment, the off-gas from the reactor comprising PF5, HCI, phosphorous oxide and HF is redirected to the mother liquor for a second stage reabsorption process, which improves the overall efficiency of the process.
[0045] In another embodiment, the gaseous off-gas from the mother liquor tank comprising HCI and HF is redirected to the recovery system, which improves the overall efficiency of the HF recovery.
[0046] Another aspect of the present application relates to an ultra-high purity alkali metal hexafluorophosphate selected from the group consisting of NaPF6, KPF6and CsPF6, having a purity of at least 98.50%, preferably at least 99.8%.
[0047] In a preferred embodiment of the present application, the ultra-high purity alkali metal hexafluorophosphate (preferably selected from the group consisting of NaPF6, KPF6or CsPF6) comprises: - insoluble substances in an amount of less than or equal to 200 ppm, or - metal impurities, each present in an amount of less than or equal to 2 ppm, or - hydrogen fluoride (HF) in an amount less than or equal to 70 ppm, or - sulfate ions (SO42) in an amount less than or equal to 10 ppm, or 2- - nitrate ions (NO3) in an amount less than or equal to 5 ppm, or - - chloride ions (CI ) in an amount less than or equal to 5 ppm, or - water / moisture in an amount less than or equal to 10 ppm, or a combination thereof.
[0048] In a preferred embodiment of the present application, the ultra-high purity alkali metal hexafluorophosphate salt (preferably selected from NaPF6, KPF6or CsPF6) comprises the following metal impurities: Na < 2 ppm K < 2 ppm Fe < 2 ppm Zn < 2 ppm Ni < 2 ppm Mg < 2 ppm Ca < 2 ppm Pb < 2 ppm Cr < 2 ppm.
[0049] According to the process of the present application, high purity phosphorus pentachloride (PC15) is obtained from commercial sources. Gaseous high purity anhydrous hydrogen fluoride (AHF) is reacted with solid phosphorus pentachloride (PC15) to produce phosphorus pentafluoride (PF5) and hydrogen chloride (HCI).
[0050] Impurities in the alkali metal hexafluorophosphate salt are directly derived from the impurities present in the main raw materials, i.e. AHF and NaF or KF. Therefore, as discussed below, the AHF and NaF or KF or CsF used in the present application are purified to minimize moisture, insoluble impurities and metal impurities. In other words, only high purity battery grade AHF and NaF or KF or CsF are used in the present application.
[0051] PC15 is generally dissolved in AHF to make PF5. This leads to the production of impurities such as AsF5, BF4, etc. However, in the present invention, gaseous AHF is reacted with solid PCl5 in a packed bed reactor. Apart from the gases produced by arsenic and boron mixing with PF5, most of the impurities brought in with PCl5 react with PF5, form high boiling point fluorides precipitate in the reactor and remain as solid at the bottom. Thereafter, the produced PF5 is passed through a filter to remove any PCl5 particles that can float in the gas stream to obtain high purity PF5 gas. Thus, due to the said process by carrying out in a packed bed reactor with high purity gaseous AHF, impurities and water in PF5 gas can be eliminated or minimized to the maximum extent.
[0052] The PF5 produced in step (d) enters the first reactor 'B' where it reacts with NaF or KF dissolved or suspended in AHF. This leads to the formation of NaPF6 or KPF6 or CsPF6 dissolved in AHF, also known as mother liquor. Preferably, step (d) is carried out at a temperature range of -15 to 15 °C, more preferably at 10 °C.
[0053] The exhaust from the second reactor 'A' containing PF5, HCI, phosphorous oxide and HF is redirected into the mother liquor for a second stage reabsorption process which particularly improves the overall efficiency of the reaction by improving the recovery efficiency of PF5. After every 1 hour, the excess pressure due to HCI gas is scrubbed by a scrubber with alkaline pH. After the complete consumption of HF and PCl5 (identified by no PF5 gas pressure production), the reactor is cooled to -15 to -20 °C.
[0054] In one embodiment of the said process, the crystallization from the mother liquor is preferably carried out in a static crystallization tank, wherein the mother liquor is cooled. The mother liquor is cooled for a long period of time.
[0055] The single run cycle of the static crystallizer used in the present invention is 48 hours. The alkali metal hexafluorophosphate crystals are allowed to grow slowly to large size. The larger the size of the NaPF6 or KPF6 or CsPF6 crystals, the smaller the surface area for adsorption of HF, thereby resulting in the lowest content of HF in NaPF6, KPF6 or CsPF6. The crystals are separated by a screen. The separated larger NaPF6 or KPF6 or CsPF6 crystals are crushed and dried under vacuum at a temperature of 60-70 °C for about 6 hours to maximize the removal of HF to obtain high purity powdered NaPF6 and KPF6.
[0056] The smaller NaPF6, KPF6 or CsPF6 crystals are reused and the process is repeated to obtain larger size crystals by adding an amount of sodium fluoride (NaF) or potassium fluoride (KF) or cesium fluoride (CsF) to the mother liquor at -15 to 5°C, preferably at -10°C, respectively.
[0057] If a conventional rotating crystallizer is used instead of a static crystallizer, smaller crystals with HF content in the range of 150-300 ppm are produced. If drying is performed at higher temperatures, the NaPF6 or KPF6 or CsPF6 crystals decompose and HF is produced.
[0058] Some representative embodiments of the present application are discussed below.
[0059] The present application in its broader aspects is not limited to specific details and representative methods described. This section describes one illustrative example in connection with the provided embodiments and methods.
[0060] Figure 1 Figure illustrates a process flow diagram for the preparation of NaPF6 according to one embodiment of the present application. According to the process, a HF solution (101) is passed through an evaporator (V1) to convert HF to gaseous HF (102). The gaseous HF (102) obtained from the evaporator (V1) is reacted with PCl5 powder (103) in a first reactor (R1) to obtain an intermediate product PF5, HCI (g) and unreacted trace HF (105).
[0061] The intermediate product (105) is passed through a filter (Fl) and fed into a second reactor (R2) to obtain a product solution (106) comprising NaPF6 and HF solution, and the product solution is fed into a hold tank (V2). The product solution (106) from the hold tank (V2) is fed into a crystallizer (V3) where the product NaPF6 (107) is recovered and a mother liquor (108) is separated. The product (107) is passed through a screen (SI) to obtain the product NaPF6 of the desired particle size and the remainder is recycled to the mother liquor tank (V4). The mother liquor (108) comprising NaF and HF solution is fed into the mother liquor tank (V4) where further HF solution and NaF (104) is added and the treated mother liquor (109) is recycled to the second reactor (R2) to obtain the product solution (106) comprising NaPF6 and HF solution. The off-gas (111) from the mother liquor tank (V4) is fed to a recovery system (S2) to reuse and recycle the HF gas (101) and the unrecovered off-gas (110) is fed to a 3-stage scrubber (S3) for disposal.
[0062] The impurities in NaPF6are directly derived from the impurities present in the main raw materials, i.e. AHF and NaF. Therefore, as discussed below, the AHF and NaF used in the present application are purified to minimize moisture, insoluble and metal impurities. In other words, only high purity AHF and NaF are used in the present application.
[0063] In commercial HF, metals such as Fe, Ca, Mg react with active PF5in the synthesis reaction vessel to form FeF3, CaF2, MgF2, which adds to the impurities in the NaPF6 / KPF6crystals. Therefore, the AHF used in the present application is purified before it is used in the process of the present application.
[0064] Commercial AHF is purified by treating it with fluorine (F2) as an oxidizing agent to fluorinate the AHF impurities, thereby removing most of the metals, including gaseous impurities arsenic compounds and boron compounds. If the AHF is not purified by reaction with F2, gaseous impurities such as metal fluorides, AsF5and BF4may contaminate the mother liquor. The use of purified AHF in the present application further minimizes the impurities in the NaPF6 / KPF6crystals of the present application.
[0065] Preparation of high purity AHF: Pure fluorine gas is passed into commercial AHF in a reaction vessel at a temperature of 25°C and a pressure of 3 kg / cm 2 for 3 minutes. After the impurities are precipitated as fluorides, the pure AHF is transferred from the reaction vessel to a distillation column. The impurity levels of commercial AHF and purified AHF are as follows: Impurity levels: Commercial AHF vs. Purified AHF
[0066] Insoluble impurities such as silicates, fluorosilicates and sulfates of metals such as Fe, Ca, K, Na, Ni, Pb, Zn, Cr, Mg, Cu and Al are present in NaF. Some metals are insoluble in AHF and are present as solid particles. Some metals react with AHF to form fluoride species, e.g. FeF3, NaF, CaF2, MgF2. In these cases, the fluoride species precipitate as solids at the bottom of the crystallizer and get mixed with the NaPF6crystals. This is the main cause of impurities in NaPF6. These insoluble impurities end up in NaPF6, thereby increasing the combined levels of Ca, Mg, Na, K, etc. to more than 100 ppm. Therefore, to produce high purity NaPF6, the NaF used in the process must be free of the said metal impurities.
[0067] Preparation of high purity NaF: Commercial Na2CO3 was mixed with pure deionized water to prepare a solution. Insoluble impurities were filtered out by passing the Na2CO3 solution through two cartridge filters in series. Soluble impurities were then removed by passing the Na2CO3 solution through cation and anion exchange resin columns.
[0068] The purified Na2CO3 solution was then reacted with 50% HF to effect neutralization to obtain NaF. The NaF so formed was filtered and dried by evaporation of water at 130°C.
[0069] The metal impurities in the purified NaF were each less than 2 ppm, and the anionic impurities such as chloride and sulfate were each less than 5 ppm.
[0070] Alternatively, purified sodium hydroxide (NaOH) or lye can be used in place of sodium carbonate (Na2CO3).
[0071] Preparation of NaPF6- The high purity AHF prepared as described above was added to solid PCl5, and the PF5+HCl+HF mixture resulting from the reaction was fed into a solution of NaF+AHF to prepare NaPF6 in the AHF solvent. NaPF6 was crystallized out of the solution, filtered and dried to obtain the pure product.
[0072] According to the present process, the impurity levels of the ultra-high purity NaPF6 are provided as follows: Impurity levels of ultra-high purity NaPF6: Fe < 2 ppm Ca < 2 ppm K < 2 ppm Na < 2 ppm Ni < 2 ppm Pb < 2 ppm Zn < 2 ppm Cr < 2 ppm Cu < 2 ppm Mg < 2 ppm.
[0073] The NaPF6 obtained by the process described above contains insoluble material in an amount less than or equal to 200 ppm, and / or contains metal impurities each present in an amount less than or equal to 1 ppm, and / or contains HF in an amount less than or equal to 70 ppm, and / or contains SO4 2- in an amount less than or equal to 10 ppm, and / or contains NO3 - in an amount less than or equal to 5 ppm, and / or contains Cl -and / or water in an amount less than or equal to 10 ppm, or a combination thereof.
[0074] The purity of NaPF6obtained by the process of the present application is at least 99.5%, preferably at least 99.8%.
[0075] In the absence of the AHF purification process, the preparation of high purity NaF, and the crystallization and drying steps, metal impurities are carried over and become part of the final product NaPF6. The total metal impurities of the NaPF6impurity profile are then 30-80 ppm, instead of less than or equal to 1 ppm.
[0076] Along similar lines, potassium hexafluorophosphate (KPF6) can be prepared by using potassium fluoride (KF) and phosphorus pentafluoride (PF5). Ultimately, potassium fluoride (KF) can be prepared using potassium carbonate (K2CO3) or potassium hydroxide (KOH).
[0077] PC15 + 5HF → PF5 + 5HC1 PF5 + KF → KPF6 Likewise, cesium hexafluorophosphate (CsPF6) can be prepared by using cesium fluoride (CsF) and phosphorus pentafluoride (PF5). Ultimately, cesium fluoride (CsF) can be prepared using cesium carbonate (Cs2CO3) or cesium hydroxide (CsOH).
[0078] Furthermore, high purity alkali metal hexafluorophosphates of other Group 1 elements (Rb and Fr) can be prepared following the above scheme.
[0079] Example The following example illustrates the present application without limiting its scope: Example 1: Synthesis of sodium hexafluorophosphate using a 1 : 1.2 molar ratio of sodium fluoride and phosphorus pentachloride.
[0080] Two Hastelloy high pressure autoclaves (A and B) with non-metallic wetted parts were used to react NaF and PCl5 in a 1 : 1.2 molar ratio. Reactor B was charged with high purity sodium fluoride (100 g). The equipment was flushed with nitrogen. The equipment was cooled to 5-10 °C. Cell grade AHF (800 g) was charged from a cylinder under controlled conditions. The mixture was stirred.
[0081] Reactor A was charged with PCl5(594 g). AHF was added slowly in portions of 100 to 200 g over a time frame of 24 to 30 hours at 25 to 40 °C (total 380 g). During this period, the pressure build-up in reactor A due to the formation of PF5 and HCI gas was let out in portions to reactor B (without any dip tube) through a venting system until constant pressure. The PF5 gas reacted with NaF in AHF to give NaPF6 under a blanket of PF5 gas. Every hour, the excess pressure due to HCI gas generation was released into a scrubber with basic pH.
[0082] After complete consumption of HF and PCl5 (identified by no PF5 gas pressure generation), the reactor was cooled to -15 to -20 °C, the reaction mass was maintained at -15 °C for 16 hours, and the solid was filtered under pressure. The solid was dried and removed. The weight of the isolated sodium hexafluorophosphate was 120 g. The isolated solid was assayed for content by ion chromatography based on sodium content, which showed a content of 99.85%.
[0083] Example 2: Synthesis of sodium hexafluorophosphate using 1 : 1.4 molar ratio of sodium fluoride and phosphorus pentachloride.
[0084] Two Hastelloy high pressure autoclaves (A and B) with non-metallic wetted parts were used for reacting NaF and PCl5 in 1 : 1.4 molar ratio. Reactor B was charged with high purity sodium fluoride (100 g). The equipment was flushed with nitrogen. The equipment was cooled to 5-10 °C. Cell grade AHF (400 g) was charged from a cylinder under controlled conditions. The mixture was stirred.
[0085] Reactor A was charged with PCl5(694 g). AHF was added slowly in portions of 100 to 200 g over a time frame of 24 to 30 hours at 25 to 40 °C (total 443 g). During this period, the pressure build-up in reactor A due to the formation of PF5 and HCI gas was let out in portions to reactor B (without any dip tube) through a venting system until constant pressure. The PF5 gas reacted with NaF in AHF to give NaPF6 under a blanket of PF5 gas. Every hour, the excess pressure due to HCI gas generation was released into a scrubber with basic pH.
[0086] After complete consumption of HF and PCl5 (identified by no PF5 gas pressure generation), the reactor was cooled to -15 to -20 °C, the reaction mass was maintained at -15 °C for 16 hours, and the solid was filtered under pressure. The solid was dried and removed. The weight of the isolated sodium hexafluorophosphate was 321 g. The isolated solid was assayed for content by ion chromatography based on sodium content, which showed a content of 99.8%.
[0087] Example 3: Synthesis of potassium hexafluorophosphate using 1 : 1.4 molar ratio of potassium fluoride and phosphorus pentachloride.
[0088] Hastelloy autoclave (A and B) was used for the reaction of KF and PCl5 in 1 : 1.4 molar ratio. Reactor B was charged with high purity potassium fluoride (58 g). The equipment was flushed with nitrogen. The equipment was cooled to 5-10 °C. Cell grade AHF (200 g) was charged from cylinder under controlled conditions. The mixture was stirred.
[0089] Reactor A was charged with PCl5 (312 g). AHF (220 g in total) was added in portions of 20-50 g at 25-40 °C over a time frame of 24-30 hours. During this period, the pressure build-up in reactor A due to the formation of PF5 and HCI gas was vented to reactor B (without any dip tube) by a venting system until constant pressure. The PF5 gas reacted with KF in AHF to give KPF6 under a blanket of PF5 gas. Every hour, the excess pressure due to HCI gas generation was released into a scrubber with basic pH.
[0090] After the complete consumption of HF and PCl5 (identified by the absence of PF5 gas pressure generation), the reactor was cooled to -15 to -20 °C, the reaction mass was maintained at -15 °C for 16 hours, and the solid was filtered under pressure. The solid was dried and taken out. The weight of the isolated potassium hexafluorophosphate was 158 g.
[0091] The above description of the present application is for the purpose of illustrating the application only, and not intended to limit the application thereto. Since those skilled in the art could make modifications to the disclosed embodiments in light of the spirit and principles of the application, the present application should be construed as including all such modifications within the scope of the disclosure. CLAIM (Amendment in accordance with Article 19 of the Treaty) 1. A process for the preparation of ultra-high purity alkali metal hexafluorophosphate (MPF6) comprising the steps of: a. charging an alkali metal fluoride (MF) into a first reactor 'B' and flushing the first reactor 'B' with nitrogen, b. cooling the first reactor 'B' to a predetermined temperature, followed by charging anhydrous hydrogen fluoride (AHF) gas in the first reactor 'B' and stirring the mixture comprising alkali metal fluoride (MF) dissolved in AHF, c. cooling the mixture comprising alkali metal fluoride (MF) dissolved in AHF in the first reactor 'B' to a predetermined temperature, d. charging phosphorus pentachloride (PC15) in the second reactor 'A' and adding AHF in multiple batches to produce high purity phosphorus pentafluoride (PF5) and hydrogen chloride (HCI) gas, e. reacting the alkali metal fluoride (MF) dissolved in AHF in the first reactor 'B' with the phosphorus pentafluoride (PF5) and hydrogen chloride (HCI) gas mixture obtained in step (d) to obtain an alkali metal hexafluorophosphate stock solution dissolved in AHF under a blanket of PF5 gas, f. cooling the first reactor 'B' to a predetermined temperature and holding the alkali metal hexafluorophosphate stock solution dissolved in AHF at the temperature for a predetermined time, followed by filtration and drying at a predetermined temperature to obtain alkali metal hexafluorophosphate (MPF6), g. obtaining dried solid MPF6, wherein the alkali metal 'M' is selected from Na, K and Cs and the alkali metal hexafluorophosphate is selected from NaPF6, KPF6 or CsPF6. 2. The process as claimed in claim 1, wherein the PF5 and HCI gas from the second reactor 'A' is added in a controlled manner to the first reactor 'B' through an exhaust system without any dip tube till a constant pressure. 3. The process as claimed in claim 1, wherein the anhydrous hydrogen fluoride (AHF) gas is purified before being used in the process; and wherein the alkali metal fluoride used in the process is obtained from alkali metal bicarbonates and hydrogen fluoride. 4. The process as claimed in claim 1, wherein the predetermined temperature in steps (b) and (c) is in the range of 5 to 15 °C. 5. The process as claimed in claim 1, wherein the addition of AHF is carried out in multiple batches in the temperature range of 25 to 40 °C. 6. The process as claimed in claim 1, wherein the predetermined temperature and the predetermined time for cooling in step (f) is in the range of -10 to -25 °C for 5 to 17 hours. 7. The process as claimed in claim 1, wherein the predetermined temperature for drying in step (f) is in the range of 35 to 50 °C. 8. Ultra-high purity alkali metal hexafluorophosphate selected from NaPF6, KPF6 and CsPF6, purity at least 99.8% w / w obtained by the process as claimed in claim 1. 9. Ultra-high purity sodium hexafluorophosphate (NaPF6) as claimed in claim 8, comprising the following metal impurities: Fe < 2 ppm Ca < 2 ppm K < 2 ppm Na < 2 ppm Ni < 2 ppm Pb < 2 ppm Zn < 2 ppm Cr < 2 ppm Cu < 2 ppm Mg < 2 ppm.
Claims
1. A process for the preparation of ultra high purity alkali metal hexafluorophosphate (MPF6) comprising the steps of: a. charging alkali metal fluoride (MF) into a first reactor 'B' and flushing the first reactor 'B' with nitrogen, b. cooling the first reactor 'B' to a predetermined temperature, followed by charging anhydrous hydrogen fluoride (AHF) gas in the first reactor 'B' and stirring the mixture comprising alkali metal fluoride (MF) dissolved in AHF, c. cooling the mixture comprising alkali metal fluoride (MF) dissolved in AHF in the first reactor 'B' to a predetermined temperature, d. charging phosphorus pentachloride (PC15) into a second reactor 'A' and adding AHF in multiple batches to produce high purity phosphorus pentafluoride (PF5) and hydrogen chloride (HC1) gas, e. reacting alkali metal fluoride (MF) dissolved in AHF in the first reactor 'B' with the mixture of phosphorus pentafluoride (PF5) and hydrogen chloride (HC1) gas obtained in step (d) to obtain alkali metal hexafluorophosphate mother liquor dissolved in AHF under a blanket of PF5 gas, f. cooling the first reactor 'B' to a predetermined temperature and maintaining the alkali metal hexafluorophosphate mother liquor dissolved in AHF at the temperature for a predetermined time, followed by filtration and drying at a predetermined temperature to obtain alkali metal hexafluorophosphate (MPF6), g. pulverizing the dried MPF6 crystals and optionally sieving to obtain MPF6 powder, wherein the alkali metal 'M' is selected from Na, K and Cs and the alkali metal hexafluorophosphate is selected from NaPF6, KPF6 or CsPF6. The PF5 and HC1 gas from the second reactor 'A' is added to the first reactor 'B' through an exhaust system in a controlled manner without any dip tube until a constant pressure.
3. The process as claimed in claim 1, wherein the anhydrous hydrogen fluoride (AHF) gas is purified before being used in the process; and wherein the alkali metal fluoride used in the process is obtained from alkali metal bicarbonate and hydrogen fluoride.
4. The process as claimed in claim 1, wherein the predetermined temperature in steps (b) and (c) is in the range of 0 to 15 °C, preferably in the range of 5 to 10 °C.
5. The process as claimed in claim 1, wherein the addition of AHF is in multiple batches at 25 to 40 °C, preferably in the range of 200 to 300 g or 100 to 200 g in multiple batches.
6. The process as claimed in claim 1, wherein the predetermined temperature and the predetermined time for cooling in step (f) is in the range of -10 to -25 °C for 5 to 7 hours.
7. The process as claimed in claim 1, wherein the predetermined temperature and the predetermined time for cooling in step (f) is in the range of -15 to -20 °C for 6 hours. 2. The method of claim 1, wherein, 8. The process according to claim 1, wherein the predetermined temperature for drying in step (f) is in the range of 35 to 50 °C, preferably in the range of 38 to 40 °C.
9. The process according to claim 1, wherein the alkali metal fluoride (MF) used in the process is obtained by the following steps: (1) treating an alkali metal carbonate with carbon dioxide to obtain an alkali metal bicarbonate, M2CO3 + H2O + CO2 → 2 MHCO3; (2) reacting the alkali metal bicarbonate with ultrapure hydrogen fluoride to obtain the alkali metal fluoride (MF), MHCO3 + HF → MF + CO2 + H2O; and (3) drying and pulverizing the alkali metal fluoride (MF).
10. An ultrahigh purity alkali metal hexafluorophosphate selected from the group consisting of NaPF6, KPF6 and CsPF6, wherein the purity is at least 98.50%, preferably at least 99.8%.
11. The ultrahigh purity sodium hexafluorophosphate (NaPF6) according to claim 10, comprising the following metal impurities: Fe ≤ 2 ppm Ca ≤ 2 ppm K ≤ 2 ppm Na ≤ 2 ppm Ni ≤ 2 ppm Pb ≤ 2 ppm Zn ≤ 2 ppm Cr ≤ 2 ppm Cu ≤ 2 ppm Mg ≤ 2 ppm.