Sodium hexafluorophosphate purification method based on supercritical carbon dioxide-carrying agent system

Abstract

The invention belongs to the technical field of purification of inorganic compounds, and relates to a purification method of sodium hexafluorophosphate based on a supercritical carbon dioxide-carrying agent system. The method mainly comprises the following steps: (1) pressurizing liquid carbon dioxide, heating, and converting into supercritical carbon dioxide; (2) mixing supercritical carbon dioxide with a carrying agent to form a mixed solvent, and mixing the mixed solvent with the crude sodium hexafluorophosphate at a certain extraction temperature and extraction pressure to form a supercritical carbon dioxide mixture with an extract; (3) heating the supercritical carbon dioxide mixture with the extract, and separating the supercritical carbon dioxide mixture into carbon dioxide gas, entraining agent gas and high-purity sodium hexafluorophosphate; and (4) depressurizing, filtering and condensing the separated carbon dioxide gas to obtain recycled liquid carbon dioxide. According to the method, the purification efficiency can be greatly improved, the production cost is low, and resource waste and loss are reduced.

Description

Technical Field

[0001] This invention belongs to the field of inorganic fluoride salt purification technology, specifically relating to a purification method for sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system, which is particularly suitable for the preparation of high-purity sodium hexafluorophosphate for the new energy field. Background Technology

[0002] Sodium hexafluorophosphate (NaPF6), as an important inorganic fluoride salt, derives its core application value from the hexafluorophosphate ion (PF6). - PF6 - With its weak coordination, high electrochemical stability, and good ionic conductivity, sodium hexafluorophosphate has shown great application potential in the field of new energy, especially in novel sodium-ion batteries.

[0003] However, the large-scale application of sodium hexafluorophosphate (Sodium hexafluorophosphate) is severely limited by its purity and stability. The purity of the electrolyte directly determines the cycle life, safety window, and rate performance of the battery system. Common production processes for Sodium hexafluorophosphate often result in the presence of metal ions, chloride ions, moisture, free acid (HF), and organic residues in the finished product. These substances can trigger a series of side reactions: moisture and HF can corrode electrode active materials and current collectors, and accelerate electrolyte decomposition, leading to gas generation, battery expansion, and capacity decay; metal ion impurities may cause uneven passivation films on the electrode surface or induce harmful metal deposition, creating a risk of internal short circuits. Currently, the purification of Sodium hexafluorophosphate mostly adopts an ethanol dissolution-centrifugation-distillation-recrystallization process, which suffers from long processes, low efficiency, and high energy consumption.

[0004] Supercritical carbon dioxide extraction technology uses carbon dioxide as the supercritical extractant. It allows for targeted extraction of desired components by controlling pressure and temperature. The operating temperature is low, near room temperature, making it particularly suitable for heat-sensitive components. Extraction and separation are completed in one step, and the extracted CO2 does not remain on the extract. CO2 is non-toxic, odorless, non-flammable, inexpensive, readily available, and recyclable. The extraction speed is also relatively fast. For example, patent document CN117844571B provides a low-temperature supercritical multi-stage separation extract of tobacco malt and its preparation method. This low-temperature supercritical multi-stage separation extract of tobacco malt is obtained from malt raw materials using supercritical carbon dioxide extraction separation technology. The tobacco malt extract contains maltol, 2,5-dimethyl-4-hydroxy-3(2H)-furanone, 5-hydroxymethylfurfural, 5-methylfurfural, and 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one. The supercritical carbon dioxide extraction technology used in this paper has advantages over traditional steam distillation methods, including higher extraction efficiency and being pollution-free and environmentally friendly. Currently, applying supercritical carbon dioxide extraction technology to the purification of sodium hexafluorophosphate is a key research direction in the field of new energy. Summary of the Invention

[0005] Therefore, the purpose of this invention is to provide a purification method for sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system, so as to solve the problems of low efficiency and high energy consumption of existing processes, and achieve improved purification efficiency, reduced cost and reduced resource consumption.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a method for purifying sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system, comprising the following steps: (1) Conversion steps: Liquid carbon dioxide is pressurized and heated to convert it into supercritical carbon dioxide; (2) Extraction steps: First, the supercritical carbon dioxide is mixed with the carrier to form a mixed solvent, and then the mixed solvent is mixed with crude sodium hexafluorophosphate at a certain extraction temperature and extraction pressure to form a supercritical carbon dioxide mixture with extract. (3) Separation step: The supercritical carbon dioxide mixture containing the extract is separated into carbon dioxide gas, carrier gas and high-purity sodium hexafluorophosphate after heating.

[0007] Furthermore, the liquid carbon dioxide described in step (1) has a purity of ≥99.999% and is stored for later use at a temperature of -20~0℃ and a pressure of 4~6MPa.

[0008] Further, in step (1), the liquid carbon dioxide is pressurized to a pressure ≥7 MPa and heated to a temperature ≥31.1℃.

[0009] Further, in step (2), the volume ratio of supercritical carbon dioxide to carrier is <21.5; the carrier includes anhydrous ethanol with a purity ≥ 99.7%.

[0010] Furthermore, the crude sodium hexafluorophosphate mentioned in step (2) is a white to slightly yellow powder or crystalline solid with a purity of 90% to 93%.

[0011] Further, in step (2), the mixed solvent and crude sodium hexafluorophosphate are mixed in an extraction vessel, wherein the amount of crude sodium hexafluorophosphate in the extraction vessel accounts for 60% to 70% of the effective volume of the extraction vessel.

[0012] Further, in step (2), the extraction temperature is 50~60℃, the extraction pressure is 25~35MPa, and the mixing time is 90~150 minutes.

[0013] Furthermore, in step (2), the volumetric flow rate of the mixed solvent required for extracting 100g of crude sodium hexafluorophosphate is 1.0~3.0L / min.

[0014] Furthermore, the heating temperature in step (3) is 65~85℃, preferably 76℃.

[0015] Furthermore, it also includes a solvent recovery step (4), specifically: the carbon dioxide gas in step (3) is depressurized, filtered, and condensed to obtain reusable liquid carbon dioxide. Preferably, the final pressure of depressurization is 4.0~5.0MPa, and the depressurization rate is controllable and gradual, usually automatically adjusted by the valve opening to ensure a smooth pressure drop curve and avoid drastic fluctuations in system pressure and flow. The temperature after depressurization is controlled at 10~25℃. There are fewer entrained solid particles, and a one-way filter valve is used. Specifically, the carbon dioxide gas (and ethanol vapor) from the separation vessel is cooled and liquefied, turning back into liquid carbon dioxide. The recovered liquid carbon dioxide is stored in a carbon dioxide storage tank, and a one-way filter valve is configured between the carbon dioxide storage tank and the separation vessel to recover and reuse the carbon dioxide in the system.

[0016] The beneficial effects of this invention are: This invention utilizes the fact that carbon dioxide is a fluid and soluble in its supercritical state. The addition of a carrier enhances the solubility and selectivity of supercritical carbon dioxide by altering its density and intermolecular interactions. Thus, in this invention, sodium hexafluorophosphate is extracted in an extraction vessel using a mixed solvent of supercritical carbon dioxide and a carrier, and then transferred to a separation vessel. After heating and depressurization, the supercritical carbon dioxide mixture containing the extract is separated into a mixed solvent gas and sodium hexafluorophosphate, yielding a sodium hexafluorophosphate product with improved purity. This invention allows for separation at lower temperatures, preserving the properties of sodium hexafluorophosphate to the greatest extent possible, eliminating solvent contamination, and significantly improving extraction efficiency through a circulating extraction system. This minimizes resource waste and loss, increases carbon dioxide utilization, and relatively reduces carbon dioxide emissions and pollution. Attached Figure Description

[0017] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the system used in the purification method of sodium hexafluorophosphate based on the supercritical carbon dioxide-carrier system of the present invention; in the figure, 1 is the first CO2 cylinder, 2 is the second CO2 cylinder, 3 is the filter, 4 is the CO2 storage tank, 5 is the carrier storage tank, 6 is the mixer, 7 is the first extraction vessel, 8 is the second extraction vessel, 9 is the first separation vessel, 10 is the second separation vessel, P1 is the CO2 pump, P2 is the carrier pump, and V1~V13 are all valve switches. Detailed Implementation

[0019] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. All mentioned embodiments are implemented based on the technical solutions of the present invention, and detailed implementation processes are given. However, it should be stated that the scope of protection of the present invention is not limited to the following embodiments.

[0020] The following embodiments provide detailed implementation procedures for the technical solutions of the present invention. Unless otherwise specified, the experimental methods used in the following experimental examples are conventional methods; unless otherwise specified, the materials and reagents used are commercially available.

[0021] The purification method for sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system in this invention mainly includes a conversion step, an extraction step, a separation step, and a solvent recovery step. The main purification system used is as follows: Figure 1 As shown.

[0022] (1) Conversion steps: Liquid carbon dioxide is pressurized and heated to convert it into supercritical carbon dioxide. Specifically: The liquid carbon dioxide in the first CO2 cylinder 1 and the second CO2 cylinder 2 is first transported to the filter 3 for filtration. After filtration, liquid carbon dioxide with a purity >99.999% is obtained and stored in the CO2 storage tank 4 for later use. The gradient pressure in the CO2 storage tank 4 is controlled to be maintained in the range of 4~6MPa, preferably 5MPa; the temperature is controlled in the range of -20℃ to 0℃, preferably -15℃. The high-purity liquid carbon dioxide after filtration can significantly improve the extraction efficiency.

[0023] Then, during use, the liquid carbon dioxide in CO2 storage tank 4 is pressurized to a supercritical pressure, higher than 7 MPa, and then heated to a supercritical temperature, higher than 31.1℃, through a heat exchanger to form supercritical carbon dioxide. Heating can be achieved through a jacketed heat transfer medium, and pressurization can be achieved using a high-pressure pump.

[0024] (2) Extraction steps: First, supercritical carbon dioxide is mixed with a carrier to form a mixed solvent. Then, the mixed solvent is mixed with crude sodium hexafluorophosphate at a certain extraction temperature and pressure to form a supercritical carbon dioxide mixture containing the extract. Specifically: First, the supercritical CO2 converted from CO2 storage tank 4 and the carrier agent from the carrier agent storage tank are pumped into mixer 6 in a specific ratio using CO2 pump P1 and carrier agent pump P2 respectively to obtain a mixed solvent. The preferred carrier agent of this invention includes anhydrous ethanol with a purity ≥99.7%, which greatly enhances the solubility of supercritical CO2 in the polar substance NaPF6. The volume ratio of supercritical carbon dioxide to the carrier agent ethanol is less than 21.5, and the preferred volume ratio is 19:1.

[0025] Then, the mixed solvent is passed into the extraction vessel and mixed with the raw materials therein to form a supercritical carbon dioxide mixture containing the extract. The extraction vessel contains crude sodium hexafluorophosphate raw material, which is the chemically synthesized product sodium hexafluorophosphate, typically in the form of a white to slightly yellow powder or crystalline solid, with an initial purity usually between 90% and 93%. The main impurities are unreacted raw material particles, amorphous impurities generated during the reaction, or adsorbed dust. The number of extraction vessels can be rationally configured according to the production needs, with each extraction vessel operating in parallel to perform extraction simultaneously. In this invention, two extraction working groups are used for extraction, each working group having one extraction vessel, namely the first extraction vessel 7 and the second extraction vessel 8. The raw material filling volume in the extraction vessel is 60% to 70% of the effective volume to ensure sufficient space for the supercritical fluid to pass through the bed and fully contact the material. During extraction, the volumetric flow rate of the mixed solvent is 1.0 to 3.0 L / min for every 100 g of crude sodium hexafluorophosphate extracted. The extraction parameters are as follows: extraction temperature is 50 to 65 °C, preferably 55 °C; extraction pressure is 25 to 35 MPa, preferably 30 MPa; and the dynamic extraction time is usually 90 to 150 minutes, preferably 135 minutes, starting from when the mixed solvent begins to continuously pass through the raw material bed.

[0026] (3) Separation step: The supercritical carbon dioxide mixture containing the extract is separated into carbon dioxide gas, carrier gas, and high-purity sodium hexafluorophosphate after heating, specifically: The supercritical carbon dioxide mixture containing the extract formed after extraction in step (2) is heated to 65-85℃ (preferably 76℃) and then enters a separation vessel, where it is separated into carbon dioxide gas, ethanol gas, and high-purity sodium hexafluorophosphate. The number of separation vessels can be rationally configured according to the production output requirements. In this invention, two sets of separation vessels are used for separation: the first separation vessel 9 and the second separation vessel 10. After the heated supercritical carbon dioxide mixture containing the extract enters the separation vessel, under constant operating pressure, the solubility of sodium hexafluorophosphate in supercritical carbon dioxide decreases sharply due to the increase in temperature, resulting in crystallization and precipitation, which settles at the bottom of the separation vessel. The supercritical carbon dioxide mixture in the separation vessel. The high-purity sodium hexafluorophosphate obtained by separation has a purity of 99.3-99.9%.

[0027] (4) Solvent recovery step: The carbon dioxide gas obtained in the separation step is depressurized to 4.0~5.0 MPa, filtered using a one-way filter valve, and condensed at a temperature of 10-25℃ to obtain recyclable liquid carbon dioxide, which is then stored in CO2 storage tank 4 for later use. Specifically: A pressure-reducing valve is installed between the extraction vessel and the separation vessel, and a one-way valve is installed between the separation vessel and the filter 3. The pressure-reducing valve reduces the carbon dioxide pressure in the extraction vessel to a certain value, while the one-way valve directs the carbon dioxide gas to the CO2 storage tank 4. The adsorbent material placed in the filter 3 removes any impurities that may be present in the carbon dioxide, keeping it pure. Carbon dioxide is recycled through solvent recovery, which reduces costs, increases carbon dioxide utilization, and reduces emissions, making it more environmentally friendly.

[0028] Furthermore, both the extraction vessel and the separation vessel in this invention are surrounded by heat-insulating jackets for heat preservation. Preferably, the heat-insulating jackets are insulated by a heating device, which includes a heating pump, a heating tank, and a heater; the heating tank contains a heat exchange medium, the heater heats the heat exchange medium in the heating tank to 50~80°C, and then the heated heat exchange medium is pumped into the heat-insulating jacket by the heating pump.

[0029] Example 1

[0030] Purification method of sodium hexafluorophosphate based on supercritical carbon dioxide-carrier system (1) Conversion steps: First, the liquid carbon dioxide in the first CO2 cylinder 1 and the second CO2 cylinder 2 is transported to the filter 3 for filtration. After filtration, liquid carbon dioxide with a purity of >99.999% is obtained and stored in the CO2 storage tank 4 for later use. The gradient pressure in the CO2 storage tank 4 is controlled at 5 MPa and the temperature is controlled at -15℃. Then, the liquid carbon dioxide in the CO2 storage tank 4 is pressurized to 8 MPa and then heated to 35℃ through a heat exchanger to form supercritical carbon dioxide.

[0031] (2) Extraction Steps: Supercritical CO2 converted from CO2 storage tank 4 and anhydrous ethanol with a purity ≥99.7% from carrier tank 4 are pumped into mixer 6 at a volume ratio of 17:3 via CO2 pump P1 and carrier pump P2 to obtain a mixed solvent. This mixed solvent is then passed into extraction vessels 7 and 8, which are filled with crude sodium hexafluorophosphate, to form a supercritical carbon dioxide mixture containing the extract. The initial purity of the commercially available crude sodium hexafluorophosphate is 91%, and the filling volume accounts for 65% of the effective volume of the extraction vessel to ensure sufficient space for the supercritical fluid to pass through the bed and fully contact the material. During extraction, the volumetric flow rate of the mixed solvent is 2.0 L / min for every 100 g of crude sodium hexafluorophosphate extracted. The extraction parameters are: temperature 55℃, pressure 30 MPa; dynamic extraction time is 135 minutes, starting from when the mixed solvent begins to continuously pass through the raw material bed.

[0032] (3) Separation Step: The supercritical carbon dioxide mixture containing the extract formed after extraction in step (2) is heated to 76°C and then enters the separation vessel, where it is separated into carbon dioxide gas, ethanol gas, and high-purity sodium hexafluorophosphate. Two sets of separation vessels, the first separation vessel 9 and the second separation vessel 10, are used. The purity of the separated high-purity sodium hexafluorophosphate is measured to be 99.8%.

[0033] (4) Solvent recovery step: The carbon dioxide gas obtained in step (3) is depressurized to 4.5 MPa, filtered and cooled to 20°C to obtain liquid carbon dioxide that can be recycled. It is then returned to CO and stored in CO2 storage tank 4 for later use.

[0034] In this invention, both the extraction vessel and the separation vessel are surrounded by insulating jackets for heat preservation. Preferably, the insulating jackets are insulated by a heating device, which includes a heating pump, a heating tank, and a heater. The heating tank contains a heat exchange medium, and the heater heats the heat exchange medium in the heating tank to 50-80°C. The heated heat exchange medium is then pumped into the insulating jacket by the heating pump.

[0035] Example 2

[0036] The difference between this embodiment and Example 1 is as follows: (2) In the extraction step, the volume ratio of supercritical CO2 and anhydrous ethanol is 17:1; the loading amount of crude sodium hexafluorophosphate accounts for 60% of the effective volume of the extraction vessel; the volumetric flow rate of the mixed solvent required for extracting 100g of crude sodium hexafluorophosphate is 1.0L / min; and the extraction working parameters are: temperature 50℃, pressure 25MPa; dynamic extraction time 100 minutes. The purity of the high-purity sodium hexafluorophosphate finally separated in this embodiment is 99.5%.

[0037] Example 3

[0038] The difference between this embodiment and Example 1 is as follows: (2) In the extraction step, the volume ratio of supercritical CO2 and anhydrous ethanol is 17:4; the loading amount of crude sodium hexafluorophosphate accounts for 70% of the effective volume of the extraction vessel; the volumetric flow rate of the mixed solvent required for extracting 100g of crude sodium hexafluorophosphate is 3.0L / min; and the extraction working parameters are: temperature 60℃, pressure 35MPa; dynamic extraction time 150 minutes. The purity of the high-purity sodium hexafluorophosphate finally separated in this embodiment is 99.8%.

[0039] Example 4

[0040] The difference between this embodiment and Embodiment 2 is that the supercritical carbon dioxide mixture containing the extract formed after extraction in step (2) is heated to 85°C and then enters a separation vessel, where it is separated into carbon dioxide gas, ethanol gas, and high-purity sodium hexafluorophosphate. The high-purity sodium hexafluorophosphate finally obtained in this embodiment has a purity of 99.7%.

[0041] Example 5

[0042] The difference between this embodiment and Embodiment 3 is that the supercritical carbon dioxide mixture containing the extract formed after extraction in step (2) is heated to 65°C and then enters a separation vessel, where it is separated into carbon dioxide gas, ethanol gas, and high-purity sodium hexafluorophosphate. The high-purity sodium hexafluorophosphate finally obtained in this embodiment has a purity of 99.6%.

[0043] In summary, this invention utilizes the fact that carbon dioxide is a fluid and has solubility in the supercritical state. The addition of a carrier enhances the solubility and selectivity of supercritical carbon dioxide by altering its density and intermolecular interactions. Thus, this invention uses a mixed solvent of supercritical carbon dioxide and a carrier to extract sodium hexafluorophosphate in an extraction vessel and then transfers it to a separation vessel. After heating and depressurization, the supercritical carbon dioxide mixture containing the extract is separated into a mixed solvent gas and sodium hexafluorophosphate, yielding a sodium hexafluorophosphate product with improved purity. This invention allows for separation operations at lower temperatures, preserving the properties of sodium hexafluorophosphate to the greatest extent possible, eliminating solvent contamination, and significantly improving extraction efficiency through a circulating extraction system. This minimizes resource waste and loss, increases carbon dioxide utilization, and relatively reduces carbon dioxide emissions and pollution.

[0044] The terms such as “upper,” “inner,” “middle,” “left,” “right,” and “one” used in this specification are for clarity of description only and are not intended to limit the scope of the invention. Any changes or adjustments to their relative relationships, without substantially altering the technical content, shall also be considered within the scope of the invention.

[0045] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for purifying sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system, characterized in that, Includes the following steps: (1) Conversion steps: Liquid carbon dioxide is pressurized and heated to convert it into supercritical carbon dioxide; (2) Extraction steps: First, the supercritical carbon dioxide is mixed with the carrier to form a mixed solvent, and then the mixed solvent is mixed with crude sodium hexafluorophosphate at a certain extraction temperature and extraction pressure to form a supercritical carbon dioxide mixture with extract. (3) Separation step: The supercritical carbon dioxide mixture containing the extract is separated into carbon dioxide gas, carrier gas and high-purity sodium hexafluorophosphate after heating.

2. The purification method for sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system as described in claim 1, characterized in that, The liquid carbon dioxide mentioned in step (1) has a purity of >99.999% and is stored for later use at a temperature of -20~0℃ and a pressure of 4~6MPa.

3. The purification method for sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system as described in claim 1, characterized in that, In step (1), the liquid carbon dioxide is pressurized to a pressure > 7 MPa and heated to a temperature > 31.1°C.

4. The purification method for sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system as described in claim 1, characterized in that, The volume ratio of supercritical carbon dioxide to carrier in step (2) is <21.5; the carrier includes anhydrous ethanol with a purity ≥99.7%.

5. The purification method for sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system as described in claim 1, characterized in that, The crude sodium hexafluorophosphate mentioned in step (2) is a white to slightly yellow powder or crystalline solid with a purity of 90% to 93%.

6. The purification method for sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system as described in claim 1, characterized in that, In step (2), the mixed solvent and crude sodium hexafluorophosphate are mixed in an extraction vessel, wherein the amount of crude sodium hexafluorophosphate in the extraction vessel accounts for 60% to 70% of the effective volume of the extraction vessel.

7. The purification method for sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system as described in claim 1, characterized in that, The extraction temperature in step (2) is 50~60℃, the extraction pressure is 25~35MPa, and the mixing time is 90~150 minutes.

8. The purification method for sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system as described in claim 1, characterized in that, In step (2), the volumetric flow rate of the mixed solvent required for extracting 100g of crude sodium hexafluorophosphate is 1.0~3.0L / min.

9. The purification method for sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system as described in claim 1, characterized in that, The heating temperature in step (3) is 65~85℃.

10. The purification method for sodium hexafluorophosphate based on a supercritical carbon dioxide-carrier system as described in claim 1, characterized in that, It also includes a solvent recovery step (4), which is to depressurize, filter and condense the carbon dioxide gas in step (3) to obtain liquid carbon dioxide for recycling.

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