A process for the preparation of phosphorus pentafluoride

Abstract

This invention belongs to the field of fluorochemical technology and relates to a method for preparing phosphorus pentafluoride. The method involves reacting polyphosphoric acid and silicon tetrafluoride to obtain a reaction slurry containing hexafluorophosphoric acid and solid silica. The reaction slurry is filtered to obtain a crude hexafluorophosphoric acid solution. The crude hexafluorophosphoric acid solution is stripped to obtain a refined hexafluorophosphoric acid solution. The refined hexafluorophosphoric acid solution is pyrolyzed to obtain a mixed gas. The mixed gas is condensed and pressure distilled to obtain the final product. This invention uses silicon tetrafluoride as both the main fluorine source and the in-situ dehydrating agent, replacing the dual functions of anhydrous hydrogen fluoride and fuming sulfuric acid in the traditional process. Silicon tetrafluoride undergoes a coupling reaction with the hydroxyl oxygen in polyphosphoric acid, breaking the Si-F bond in situ to release hydrogen fluoride, completing the stepwise fluorination of phosphorus, and ultimately generating hexafluorophosphoric acid. Simultaneously, it removes oxygen from the system in the form of solid silica, effectively achieving the dehydration function of fuming sulfuric acid. The entire process is sulfur-free, completely eliminating hazardous waste sulfuric acid at the source, making it environmentally friendly.

Description

Technical Field

[0001] This invention belongs to the field of fluorochemical technology, specifically relating to a method for preparing phosphorus pentafluoride. Background Technology

[0002] Phosphorus pentafluoride is an important fluorinating agent with wide applications in the electronics industry, polymer material manufacturing, and catalysts. In recent years, with the popularization and promotion of new energy vehicles, lithium-ion batteries using lithium hexafluorophosphate as the electrolyte have been widely used due to their superior performance. As a raw material for the synthesis of lithium hexafluorophosphate, phosphorus pentafluoride has gradually become a material of great interest. However, existing traditional methods for preparing phosphorus pentafluoride have some problems, such as introducing impurities into the raw materials, generating byproducts, and cumbersome processes.

[0003] Patent document CN104261369A discloses a traditional method for preparing phosphorus pentafluoride gas. First, polyphosphoric acid and anhydrous hydrogen fluoride are used as starting materials to prepare an aqueous solution of hexafluorophosphoric acid. The hexafluorophosphoric acid aqueous solution is then reacted with sulfur trioxide to obtain a mixture of hexafluorophosphoric acid and sulfuric acid. Without separation, the mixture of hexafluorophosphoric acid and sulfuric acid is directly heated, and the resulting phosphorus pentafluoride vapor is condensed to obtain crude phosphorus pentafluoride. The crude phosphorus pentafluoride is then refined to obtain high-purity phosphorus pentafluoride with a purity of 99.5%. Although this process is mature for industrialization, it suffers from long-standing industry pain points: large amounts of hazardous waste are generated, resulting in extremely high environmental pressure. For every ton of phosphorus pentafluoride produced, 8-10 tons of fluorine- and phosphorus-containing waste sulfuric acid are generated as a byproduct, which belongs to HW34 category hazardous waste. Outsourcing treatment is costly and poses leakage and compliance risks, representing a core bottleneck restricting capacity expansion. The raw material structure is unreasonable, resulting in high costs. Large quantities of anhydrous hydrogen fluoride and fuming sulfuric acid need to be purchased externally, both of which are highly hazardous and controlled chemicals, leading to high storage, transportation, and procurement costs. Furthermore, sulfur completely enters the waste acid, rendering it economically worthless and exhibiting extremely low atom economy. Equipment corrosion is complex, and investment costs are high. The mixed system of sulfuric acid, hydrofluoric acid, and fluorophosphoric acid is extremely corrosive, requiring core equipment to use high-end materials such as Hastelloy. Initial investment and maintenance costs are significantly higher than with a single fluorination system.

[0004] Therefore, there is an urgent need to develop a new phosphorus pentafluoride preparation process that can effectively reduce the generation of hazardous waste, lower production costs, improve atom economy, and provide a new way for the sustainable development of enterprises. Summary of the Invention

[0005] To achieve the above objectives, the present invention adopts the following technical solution: This invention provides a method for preparing phosphorus pentafluoride. The method involves mixing polyphosphoric acid and silicon tetrafluoride and reacting them to obtain a reaction slurry containing hexafluorophosphoric acid and solid silica. The reaction slurry is filtered to obtain a crude hexafluorophosphoric acid solution. The crude hexafluorophosphoric acid solution is stripped to obtain a refined hexafluorophosphoric acid solution. The refined hexafluorophosphoric acid solution is pyrolyzed to obtain a mixed gas containing phosphorus pentafluoride. The mixed gas is condensed and pressure distilled to obtain the finished phosphorus pentafluoride product.

[0006] Furthermore, the polyphosphoric acid contains 80%~88% P2O5 and <0.5% moisture, and is used after preheating to 60~80°C. Preferably, the polyphosphoric acid is preheated to 60~80°C to reduce viscosity, and then filtered through a precision filter to remove mechanical impurities before being sent to a raw material storage tank for later use.

[0007] Furthermore, the silicon tetrafluoride should be pre-dried, and after drying, its moisture content should be ≤5ppm and its heavy metal impurity content should be <1ppm.

[0008] Furthermore, the molar ratio of phosphorus in the silicon tetrafluoride to the polyphosphoric acid is (1.5~3):1, the reaction temperature is 50~150℃, the pressure is 0.2~1MPa, and the time is 2~5h.

[0009] This invention uses silicon tetrafluoride as both the primary fluorine source and an in-situ dehydrating agent, replacing the dual functions of anhydrous hydrogen fluoride and fuming sulfuric acid in traditional processes. Silicon tetrafluoride undergoes a coupled reaction with the hydroxyl oxygen in polyphosphoric acid: on one hand, it breaks the Si-F bond to release hydrogen fluoride in situ, completing the stepwise fluorination of phosphorus and ultimately generating hexafluorophosphoric acid; on the other hand, it removes oxygen from the system in the form of solid silicon dioxide, effectively achieving the dehydration function of fuming sulfuric acid. No sulfur is introduced throughout the entire process, completely eliminating hazardous waste sulfuric acid at its source.

[0010] Furthermore, the coupled fluorination reaction employs a bubble column reactor to enhance gas-liquid-solid three-phase mass transfer. Polyphosphoric acid is sprayed into the reactor from the top; silicon tetrafluoride gas is countercurrently bubbled into the reactor from the bottom, contacting the liquid polyphosphoric acid in a countercurrent manner; the reaction outlet yields a reaction slurry containing hexafluorophosphoric acid and solid silica.

[0011] Furthermore, the reaction slurry is filtered using multi-stage filtration to remove silica filter cake; the final stage of the multi-stage filtration has a filtration accuracy of 0.1~0.5μm.

[0012] Furthermore, the stripping of the crude hexafluorophosphate solution is carried out in a stripping tower, using dry hydrogen fluoride as the stripping gas, at a temperature of 40~60℃, in a countercurrent stripping process to remove dissolved silicon tetrafluoride and fluorosilicic acid.

[0013] Furthermore, the total silicon residue in the refined hexafluorophosphate solution is ≤10ppm.

[0014] Furthermore, the pyrolysis temperature of the refined hexafluorophosphate solution is 80~160℃, the pressure is 0.1~0.3MPa, and the time is 1~5h.

[0015] Furthermore, the pyrolysis temperature of the refined hexafluorophosphate solution is 120~150℃, the pressure is 0.1~0.3MPa, and the time is 1~5h.

[0016] Furthermore, the temperature of the mixed gas condensed after the pyrolysis of the refined hexafluorophosphate solution is -40~10℃, the pressure of the pressurized distillation is 1.0~2.0MPa, and the temperature of the bottom of the column is 40~80℃.

[0017] Further, the refined hexafluorophosphoric acid solution is fed into a pyrolysis tower for continuous thermal decomposition. Hexafluorophosphoric acid decomposes upon heating into phosphorus pentafluoride gas and hydrogen fluoride vapor, and the mixed gas is collected from the top of the tower. The polyphosphoric acid impurities enriched in the bottom of the tower are returned to the upstream reaction unit for reuse. The mixed gas at the top of the tower first passes through a two-stage cryogenic condenser to condense and recover hydrogen fluoride, which can be refined into a byproduct for sale; the phosphorus pentafluoride is then subjected to pressurized distillation to finally obtain high-purity phosphorus pentafluoride as the final product.

[0018] Compared with the traditional polyphosphoric acid-fuming sulfuric acid process, the beneficial effects of this invention are: 1. Environmental benefits: The output of fluorine- and phosphorus-containing waste sulfuric acid is reduced to zero, and the amount of hazardous waste emissions is reduced by more than 99%, completely solving the pain points of environmental compliance; there is no wastewater discharge, and the cost of treating waste gas, wastewater and solid waste is reduced by more than 90%.

[0019] 2. Economic benefits: Replacing all fuming sulfuric acid and purchased anhydrous hydrogen fluoride with cheap by-product silicon tetrafluoride reduces raw material costs by 35% to 50%; by-product silica and surplus hydrogen fluoride can contribute additional revenue, resulting in an overall cost reduction of 40% to 60%.

[0020] 3. Resource efficiency: Total phosphorus conversion rate ≥98%, total fluorine recovery rate ≥95%, and atom utilization rate increased by more than 30%. Detailed Implementation

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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. Unless otherwise specified, the experimental methods used in the following experimental examples are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available reagents and materials; and unless otherwise specified, the room temperature or room temperature refers to 25±5℃.

[0022] Example 1

[0023] A method for preparing phosphorus pentafluoride, comprising the following steps: (1) Raw material preparation: 200g of polyphosphoric acid with a mass fraction of 85% P2O5, preheated to 70℃; high-purity silicon tetrafluoride, moisture ≤3ppm, heavy metal impurities <1ppm; (2) Coupled fluorination reaction: A bubble column reaction device was used. Polyphosphoric acid was sprayed into the column from the top. Silicon tetrafluoride gas was bubbled into the column from the bottom countercurrently and contacted the liquid phase countercurrently. The reaction temperature was controlled at 70℃ and the pressure at 0.1MPa. Silicon tetrafluoride gas was continuously introduced and reacted for 3h. The molar ratio of phosphorus in silicon tetrafluoride to polyphosphoric acid was 2.2:1. A reaction slurry containing hexafluorophosphoric acid and solid silica was obtained. (3) Post-processing: The reaction slurry in step (2) was filtered through a 0.22μm PTFE membrane to remove silica, resulting in a clear crude hexafluorophosphate solution. The crude hexafluorophosphate solution was then fed into a stripping tower and stripped with HF gas at 50℃ for 1 hour to remove dissolved silicon tetrafluoride and fluorosilicic acid, resulting in a refined hexafluorophosphate solution. The refined hexafluorophosphate solution was then fed into a pyrolysis tower and pyrolyzed at 140℃ under normal pressure for 2 hours to produce a mixture of phosphorus pentafluoride gas and hydrogen fluoride vapor. The mixture was condensed at -30℃ to remove HF, and then subjected to pressurized distillation (distillation tower temperature 78℃, pressure 1.8MPa) to finally obtain a high-purity phosphorus pentafluoride product. The high-purity phosphorus pentafluoride product was tested, and the results were as follows: phosphorus conversion rate: 98.7%; phosphorus pentafluoride product purity: 99.96%; total silicon content: 8ppb; moisture content: 0.8ppm.

[0024] Example 2

[0025] A method for preparing phosphorus pentafluoride, comprising the following steps: (1) Raw material preparation: 200g of polyphosphoric acid with a mass fraction of 85% P2O5, preheated to 75℃; high-purity silicon tetrafluoride, moisture ≤3ppm, heavy metal impurities <1ppm; (2) Coupled fluorination reaction: A bubble column reaction device was used, and polyphosphoric acid was sprayed into the column from the top; silicon tetrafluoride gas was bubbled into the column from the bottom countercurrently and contacted the liquid phase countercurrently; the reaction temperature was controlled at 90℃ and the pressure at 0.1MPa, and silicon tetrafluoride gas was continuously introduced for 1h of reaction. The molar ratio of silicon tetrafluoride to phosphorus in polyphosphoric acid was 1.8:1; a reaction slurry containing hexafluorophosphoric acid and solid silica was obtained. (3) Post-processing: The reaction slurry in step (2) was filtered through a 0.22 μm PTFE membrane to remove silica, resulting in a clear crude hexafluorophosphate solution. The crude hexafluorophosphate solution was then fed into a stripping tower and stripped with HF gas at 50°C for 1 hour to remove dissolved silicon tetrafluoride and fluorosilicic acid, yielding a refined hexafluorophosphate solution. The refined hexafluorophosphate solution was then fed into a pyrolysis tower and pyrolyzed at 150°C under normal pressure for 2 hours to produce a mixture of phosphorus pentafluoride gas and hydrogen fluoride vapor. The mixture was condensed at -20°C to remove HF, and then subjected to pressurized distillation (distillation tower temperature 80°C, pressure 1.9 MPa) to finally obtain a high-purity phosphorus pentafluoride product. The high-purity phosphorus pentafluoride product was tested, and the results were as follows: phosphorus conversion rate: 98.1%; phosphorus pentafluoride product purity: 99.95%; total silicon content: 11 ppb; moisture content: 1 ppm.

[0026] Example 3

[0027] A method for preparing phosphorus pentafluoride, comprising the following steps: (1) Raw material preparation: 200g of polyphosphoric acid with a mass fraction of 85% P2O5, preheated to 75℃; high-purity silicon tetrafluoride, moisture ≤3ppm, heavy metal impurities <1ppm; (2) Coupled fluorination reaction: A bubble column reaction device was used, and polyphosphoric acid was sprayed into the column from the top; silicon tetrafluoride gas was bubbled into the column from the bottom countercurrently and contacted the liquid phase countercurrently; the reaction temperature was controlled at 110℃ and the pressure at 0.1MPa, and silicon tetrafluoride gas was continuously introduced for 2h of reaction. The molar ratio of silicon tetrafluoride to phosphorus in polyphosphoric acid was 2:1; a reaction slurry containing hexafluorophosphoric acid and solid silica was obtained. (3) Post-processing: The reaction slurry in step (2) was filtered through a 0.22 μm PTFE membrane to remove silica, resulting in a clear crude hexafluorophosphate solution. The crude hexafluorophosphate solution was then fed into a stripping tower and stripped with HF gas at 43°C for 1 hour to remove dissolved silicon tetrafluoride and fluorosilicic acid, yielding a refined hexafluorophosphate solution. The refined hexafluorophosphate solution was then fed into a pyrolysis tower and pyrolyzed at 150°C under normal pressure for 2 hours to produce a mixture of phosphorus pentafluoride gas and hydrogen fluoride vapor. The mixture was condensed at -30°C to remove HF, and then subjected to pressurized distillation (distillation tower temperature 80°C, pressure 1.9 MPa) to finally obtain a high-purity phosphorus pentafluoride product. The high-purity phosphorus pentafluoride product was tested, and the results were as follows: phosphorus conversion rate: 98.8%; phosphorus pentafluoride product purity: 99.95%; total silicon content: 9 ppb; moisture content: 2 ppm.

[0028] Example 4

[0029] A method for preparing phosphorus pentafluoride, comprising the following steps: (1) Raw material preparation: 200g of polyphosphoric acid with a mass fraction of 85% P2O5, preheated to 75℃; high-purity silicon tetrafluoride, moisture ≤3ppm, heavy metal impurities <1ppm; (2) Coupled fluorination reaction: A bubble column reaction device was used, with polyphosphoric acid sprayed from the top of the column; silicon tetrafluoride gas was bubbled in countercurrently from the bottom of the column and contacted with the liquid phase in countercurrent; the reaction temperature was controlled at 110℃ and the pressure at 0.1MPa, and silicon tetrafluoride gas was continuously introduced for 2h of reaction. The molar ratio of silicon tetrafluoride to phosphorus in polyphosphoric acid was 1.7:1; a reaction slurry containing hexafluorophosphoric acid and solid silica was obtained. (3) Post-processing: The reaction slurry in step (2) was filtered through a 0.22 μm PTFE membrane to remove silica, resulting in a clear crude hexafluorophosphate solution. The crude hexafluorophosphate solution was then fed into a stripping tower and stripped with HF gas at 50°C for 1 h to remove dissolved silicon tetrafluoride and fluorosilicic acid, yielding a refined hexafluorophosphate solution. The refined hexafluorophosphate solution was then fed into a pyrolysis tower and pyrolyzed at 147°C under normal pressure for 2 h to produce a mixture of phosphorus pentafluoride gas and hydrogen fluoride vapor. The mixture was condensed at -30°C to remove HF, and then subjected to pressurized distillation (distillation tower temperature 80°C, pressure 1.9 MPa) to finally obtain a high-purity phosphorus pentafluoride product. The high-purity phosphorus pentafluoride product was tested, and the results were as follows: phosphorus conversion rate: 98.2%; phosphorus pentafluoride product purity: 99.96%; total silicon content: 7 ppb; moisture content: 3 ppm.

[0030] In summary, this invention provides a method for preparing phosphorus pentafluoride using silicon tetrafluoride coupled with polyphosphoric acid. Silicon tetrafluoride serves simultaneously as the main fluorine source and in-situ dehydrating agent, replacing the dual functions of anhydrous hydrogen fluoride and fuming sulfuric acid in traditional processes. The silicon tetrafluoride couples with the hydroxyl oxygen in polyphosphoric acid, breaking the Si-F bond to release hydrogen fluoride in situ, completing the stepwise fluorination of phosphorus and ultimately producing hexafluorophosphoric acid. Simultaneously, oxygen in the system is removed in solid form as silicon dioxide, effectively achieving the dehydration function of fuming sulfuric acid. The entire process is sulfur-free, completely eliminating hazardous waste sulfuric acid at the source, making it a green production process for preparing high-purity phosphorus pentafluoride.

[0031] The high-purity phosphorus pentafluoride prepared using the method of this invention can be widely used as a fluorinating agent in the fields of lithium battery electrolyte (lithium hexafluorophosphate / sodium hexafluorophosphate) precursor, semiconductor phosphorus doping source, and organofluorine catalyst.

[0032] 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 preparing phosphorus pentafluoride, characterized in that, Polyphosphoric acid and silicon tetrafluoride are mixed and reacted to obtain a reaction slurry containing hexafluorophosphoric acid and solid silica. The reaction slurry is filtered to obtain a crude hexafluorophosphoric acid solution. The crude hexafluorophosphoric acid solution is stripped to obtain a refined hexafluorophosphoric acid solution. The refined hexafluorophosphoric acid solution is pyrolyzed to obtain a mixed gas containing phosphorus pentafluoride. The mixed gas is condensed and pressure distilled to obtain the finished phosphorus pentafluoride product.

2. The method for preparing phosphorus pentafluoride according to claim 1, characterized in that, The polyphosphoric acid contains 80%~88% P2O5 and <0.5% moisture. The polyphosphoric acid is used after being preheated to 60~80℃.

3. The method for preparing phosphorus pentafluoride according to claim 1, characterized in that, The silicon tetrafluoride contains ≤5ppm moisture and <1ppm heavy metal impurities.

4. The method for preparing phosphorus pentafluoride according to claim 1, characterized in that, The molar ratio of phosphorus in silicon tetrafluoride to polyphosphoric acid is (1.5~3):1, the reaction temperature is 50~150℃, the pressure is 0.2~1MPa, and the time is 2~5h.

5. The method for preparing phosphorus pentafluoride according to claim 1, characterized in that, The reaction slurry is filtered using multi-stage filtration, with the final stage of filtration having a filtration accuracy of 0.1~0.5μm.

6. The method for preparing phosphorus pentafluoride according to claim 1, characterized in that, The crude hexafluorophosphate solution was stripped using dry hydrogen fluoride as the stripping gas at a temperature of 40-60°C.

7. The method for preparing phosphorus pentafluoride according to claim 1, characterized in that, The total residual silicon in the refined hexafluorophosphate solution is ≤10ppm.

8. The method for preparing phosphorus pentafluoride according to claim 1, characterized in that, The purified hexafluorophosphate solution is pyrolyzed at a temperature of 80~160℃, a pressure of 0.1~0.3MPa, and a time of 1~5h.

9. The method for preparing phosphorus pentafluoride according to claim 1, characterized in that, The purified hexafluorophosphate solution is pyrolyzed at a temperature of 120~150℃ and a pressure of 0.1~0.3MPa.

10. The method for preparing phosphorus pentafluoride according to claim 1, characterized in that, The temperature at which the mixed gas after pyrolysis of the refined hexafluorophosphate solution condenses is 10~-40℃, and the pressure of the pressurized distillation is 1.0~1.8MPa, and the temperature is 40~80℃.