Method for removing phosphorus from phosphorus pentafluoride in lithium hexafluorophosphate synthesis tail gas

Abstract

The invention discloses a method for removing phosphorus pentafluoride in lithium hexafluorophosphate synthesis tail gas, which is characterized by comprising the following steps of: absorbing phosphorus pentafluoride in the lithium hexafluorophosphate synthesis tail gas by using a fluorine-containing solvent; according to the method, the fluorine content in the byproduct hydrochloric acid in the lithium hexafluorophosphate industry can be effectively reduced, the bottleneck problem of downstream utilization of the byproduct hydrochloric acid in the industry is solved, and meanwhile, the utilization rate of the phosphorus pentafluoride gas is increased.

Description

Technical Field

[0001] The present invention relates to the field of lithium hexafluorophosphate synthesis, and specifically to a method for removing phosphorus pentafluoride from the tail gas in the synthesis of lithium hexafluorophosphate. Background Art

[0002] Lithium hexafluorophosphate is a key electrolyte material in lithium battery electrolytes. Currently, the mainstream process for synthesizing lithium hexafluorophosphate in the industry is to use hydrogen fluoride as a solvent and react a mixed gas of phosphorus pentafluoride and HCl with lithium fluoride. However, due to the limitations of the nature of the lithium hexafluorophosphate synthesis reaction itself, the tail gas HCl will still contain a relatively high content of phosphorus pentafluoride gas during the production process. Using this tail gas directly to prepare hydrochloric acid, the obtained hydrochloric acid will have high fluorine content and phosphorus content, which cannot meet the requirements of downstream applications, is difficult to recycle, and the phosphorus pentachloride gas in the tail gas cannot be recycled, resulting in waste of resources.

[0003] Patent CN114870589B uses carbon tetrachloride or benzene as a solvent for absorbing and recycling phosphorus pentafluoride in the tail gas of lithium hexafluorophosphate synthesis. The highest recovery rate of phosphorus pentafluoride can only reach 85.3%, and there is still a large amount of phosphorus pentafluoride loss. Moreover, the recycled phosphorus pentafluoride will contain carbon tetrachloride or benzene. These two substances are very likely to react with phosphorus pentafluoride or hydrogen fluoride during the absorption process to generate fluorinated impurities, which will cause the quality of the recycled phosphorus pentafluoride product to decline and the recovery rate to decrease, and even affect the product quality of the lithium hexafluorophosphate synthesis unit.

[0004] Therefore, how to create a new tail gas removal method with high recovery rate of phosphorus pentafluoride and without affecting the recycling of phosphorus pentafluoride is still a work worthy of in-depth study. Summary of the Invention

[0005] To solve the above technical problems, the present invention proposes a method for removing phosphorus pentafluoride from the tail gas of lithium hexafluorophosphate synthesis. In this application, the use of a fluorine-containing solvent can not only efficiently absorb phosphorus pentafluoride, but also will not react with phosphorus pentafluoride or hydrogen fluoride. Moreover, if a trace amount of fluorine-containing solvent enters the lithium hexafluorophosphate synthesis unit along with the recycled phosphorus pentafluoride, no side reaction will occur or the product quality of lithium hexafluorophosphate synthesis will not be affected. Therefore, this method has the advantages of safe and environmental protection process route and is suitable for industrial production.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A method for removing phosphorus pentafluoride from the tail gas of lithium hexafluorophosphate synthesis, comprising the following steps:

[0008] Absorb phosphorus pentafluoride from the tail gas of lithium hexafluorophosphate synthesis using a fluorine-containing solvent.

[0009] In a preferred embodiment of the present invention, the fluorinated solvent is a perfluoroalkylamine solvent, preferably one or more of perfluorotripropylamine, perfluorotributylamine, or perfluoroisopropylmorpholine.

[0010] In the present invention, the tail gas from the synthesis of lithium hexafluorophosphate can first remove HF and then PF5, or first remove PF5 and then HF. The tail gas after removing HF and PF5 can be directly used for hydrochloric acid preparation. The fluorinated solvent that absorbs PF5 is sent to the absorber regeneration rectification column, and after removing the dissolved PF5, it can be recycled to the absorption column.

[0011] In some preferred embodiments of the present invention, the perfluorinated solvent can be added to the device through which the tail gas from the synthesis of lithium hexafluorophosphate is passed by spraying, or the tail gas from the synthesis of lithium hexafluorophosphate can be directly introduced into the perfluorinated solvent.

[0012] In a preferred embodiment of the present invention, the mass concentration of PF5 in the tail gas from the synthesis of lithium hexafluorophosphate is 5 - 20 wt%.

[0013] In a preferred embodiment of the present invention, the absorption of the tail gas from the synthesis of lithium hexafluorophosphate is carried out in an absorption column. The operating temperature of the absorption column is 0 - 20 °C, the operating pressure is 1.5 - 5.5 barg, and the liquid-gas ratio (Kg absorbent / Kg feed gas) of the absorption column is 15 - 25. The concentration of PF5 in the fluorinated solvent absorbent that has absorbed PF5 is 13 - 18 wt%.

[0014] In a preferred embodiment of the present invention, the operating pressure of the absorber desorption rectification column is 1.5 - 2.5 barg, the operating temperature at the top of the column is 10 - 30 °C, the liquid at the top of the column is fully refluxed, and the gas is withdrawn. The PF5 desorption rate can reach more than 98%.

[0015] The beneficial effects of the present invention are as follows:

[0016] 1) Adopting a new fluorinated solvent process, which is non-flammable and has low toxicity, and the process is safe, environmentally friendly;

[0017] 2) Due to the high fluorine content of the selected fluorinated solvent and its structural characteristics very similar to PF5, the solubility of PF5 in the selected fluorinated solvent is greatly improved, the amount of absorption solvent used is reduced, and the single-pass recovery rate of PF5 is high;

[0018] 3) The fluorinated solvent absorbent described does not react with PF5 to form other by-products compared with the reaction solvent, and has no impact on the quality of the lithium hexafluorophosphate product when recycled to the lithium hexafluorophosphate synthesis unit. Detailed implementation manners

[0019] The present invention will be further described below through specific embodiments. The embodiments described in the present invention are only for the purpose of explaining the present invention and do not limit the scope of the present invention.

[0020] Unless otherwise specified, the raw materials and reagents involved in the following examples can be obtained through commercial channels.

[0021] Perfluorotripropylamine (product number FC-3283; purchased from 3M), perfluorotributylamine (purchased from Sigma-Aldrich), perfluoro-N-isopropylmorpholine (product number FC-770; purchased from 3M).

[0022]

Example 1

[0023] The synthetic tail gas of lithium hexafluorophosphate after cryogenic removal of HF, with a phosphorus pentafluoride content of 5 wt%, is introduced into an absorption tower filled with 1 m of polytetrafluoroethylene Pall rings at a flow rate of 1 g / min. The operating pressure of the absorption tower is 1.5 barg, and the operating temperature is 5 °C. The circulating spray flow rate of the absorption liquid perfluorotributylamine is 25 g / min, and the concentration of phosphorus pentafluoride in the absorption liquid is maintained at 15%. The absorption liquid is continuously withdrawn at a flow rate of 0.31 g / min. The removal rate of phosphorus pentafluoride in the tail gas is 94%. The withdrawn absorption liquid is sent to the absorption liquid desorption rectification tower. The operating pressure of the rectification tower is 1.5 barg, and the operating temperature at the top of the rectification tower is 10 °C. The liquid phase at the top of the tower is fully refluxed, and the withdrawn gas phase is high-purity phosphorus pentafluoride. The desorption rate of phosphorus pentafluoride is 98.5%. The absorbent withdrawn from the bottom of the tower can be sent to the absorption tower for reuse after cooling. The regenerated high-purity phosphorus pentafluoride gas is directly used for the synthesis of solid lithium hexafluorophosphate, and the key index acidity of the obtained lithium hexafluorophosphate crystal product is 76 ppm.

[0024]

Example 2

[0025] The synthetic tail gas of lithium hexafluorophosphate after cryogenic removal of HF, with a phosphorus pentafluoride content of 7 wt%, is introduced into an absorption tower filled with 1 m of polytetrafluoroethylene Pall rings at a flow rate of 1 g / min. The operating pressure of the absorption tower is 3 barg, and the operating temperature is 0 °C. The circulating spray flow rate of the absorption liquid perfluorotributylamine is 20 g / min, and the concentration of phosphorus pentafluoride in the absorption liquid is maintained at 13%. The absorption liquid is continuously withdrawn at a flow rate of 0.52 g / min. The removal rate of phosphorus pentafluoride in the tail gas is 97%. It is sent to the absorption liquid desorption rectification tower. The operating pressure of the rectification tower is 2.5 barg, and the operating temperature at the top of the rectification tower is 30 °C. The liquid phase at the top of the tower is fully refluxed, and the withdrawn gas phase is high-purity phosphorus pentafluoride. The desorption rate of phosphorus pentafluoride is 99.4%. The absorbent withdrawn from the bottom of the tower can be sent to the absorption tower for reuse after cooling. The regenerated high-purity phosphorus pentafluoride gas is directly used for the synthesis of solid lithium hexafluorophosphate, and the key index acidity of the obtained lithium hexafluorophosphate crystal product is 62 ppm.

[0026]

Example 3

[0027] The synthetic tail gas of lithium hexafluorophosphate after cryogenic removal of HF, with a phosphorus pentafluoride content of 8 wt%, is introduced into an absorption tower filled with 1 m of polytetrafluoroethylene Pall rings at a flow rate of 1 g / min. The operating pressure of the absorption tower is 5.5 barg, and the operating temperature is 15 °C. The circulating spray flow rate of the perfluorinated N-isopropylmorpholine absorption liquid is 15 g / min, and the concentration of phosphorus pentafluoride in the absorption liquid is maintained at 18%. The absorption liquid is continuously withdrawn at a flow rate of 0.41 g / min. The removal rate of phosphorus pentafluoride in the tail gas is 93%, and it is sent to the absorption liquid desorption rectification tower. The operating pressure of the rectification tower is 2 barg, and the operating temperature at the top of the rectification tower is 25 °C. The liquid phase at the top of the tower is fully refluxed, and the extracted gas phase is high-purity phosphorus pentafluoride. The desorption absorption rate of phosphorus pentafluoride is 99.2%. The absorbent withdrawn from the bottom of the tower can be sent to the absorption tower for reuse after cooling. The regenerated high-purity phosphorus pentafluoride gas is used for the synthesis of solid lithium hexafluorophosphate, and the key index acidity of the obtained lithium hexafluorophosphate crystal product is 73 ppm.

[0028]

Example 4

[0029] The synthetic tail gas of lithium hexafluorophosphate after cryogenic removal of HF, with a phosphorus pentafluoride content of 10 wt%, is introduced into an absorption tower filled with 1 m of polytetrafluoroethylene Pall rings at a flow rate of 1 g / min. The operating pressure of the absorption tower is 4 barg, and the operating temperature is 20 °C. The circulating spray flow rate of the perfluorotributylamine absorption liquid is 18 g / min, and the concentration of phosphorus pentafluoride in the absorption liquid is maintained at 15%. The absorption liquid is continuously withdrawn at a flow rate of 0.63 g / min. The removal rate of phosphorus pentafluoride in the tail gas is 95%, and it is sent to the absorption liquid desorption rectification tower. The operating pressure of the rectification tower is 1.8 barg, and the operating temperature at the top of the rectification tower is 10 °C. The liquid phase at the top of the tower is fully refluxed, and the extracted gas phase is high-purity phosphorus pentafluoride. The desorption absorption rate of phosphorus pentafluoride is 98.8%. The absorbent withdrawn from the bottom of the tower can be sent to the absorption tower for reuse after cooling. The regenerated high-purity phosphorus pentafluoride gas is used for the synthesis of solid lithium hexafluorophosphate, and the key index acidity of the obtained lithium hexafluorophosphate crystal product is 46 ppm.

[0030]

Comparative Example 1

[0031] The synthetic tail gas of lithium hexafluorophosphate after cryogenic removal of HF, with a phosphorus pentafluoride content of 10 wt%, is introduced into an absorption tower filled with 1 m of polytetrafluoroethylene Pall rings at a flow rate of 1 g / min. The operating pressure of the absorption tower is 4 barg, and the operating temperature is 20 °C. The circulating spray flow rate of the absorption liquid carbon tetrachloride is 18 g / min, and the concentration of phosphorus pentafluoride in the absorption liquid is maintained at 4%. The absorption liquid is continuously withdrawn at a flow rate of 2.0 g / min. The removal rate of phosphorus pentafluoride in the tail gas is 81%, and it is sent to the absorption liquid desorption rectification tower. The operating pressure of the rectification tower is 1.8 barg, the operating temperature at the top of the rectification tower is 10 °C, and the liquid phase at the top is fully refluxed. The extracted gas phase is phosphorus pentafluoride with a higher purity, and the desorption rate is 90.3%. The absorbent withdrawn from the bottom of the tower can be sent to the absorption tower for reuse after cooling. The regenerated high-purity phosphorus pentafluoride gas is used for the synthesis of solid lithium hexafluorophosphate, and the key index acidity of the obtained lithium hexafluorophosphate crystal product is 162 ppm.

[0032]

Comparative Example 2

[0033] The synthetic tail gas of lithium hexafluorophosphate after cryogenic removal of HF, with a phosphorus pentafluoride content of 8%, is introduced into an absorption tower filled with 1 m of polytetrafluoroethylene Pall rings at a flow rate of 1 g / min. The operating pressure of the absorption tower is 5.5 barg, and the operating temperature is 15 °C. The circulating spray flow rate of the absorption liquid benzene is 15 g / min, and the concentration of phosphorus pentafluoride in the absorption liquid is maintained at 6%. The absorption liquid is continuously withdrawn at a flow rate of 1.13 g / min. The removal rate of phosphorus pentafluoride in the tail gas is 85%, and it is sent to the absorption liquid desorption rectification tower. The operating pressure of the rectification tower is 2 barg, the operating temperature at the top of the rectification tower is 25 °C, and the liquid phase at the top is fully refluxed. The extracted gas phase is phosphorus pentafluoride with a higher purity, and the desorption rate is 91.6%. The absorbent withdrawn from the bottom of the tower can be sent to the absorption tower for reuse after cooling. The regenerated high-purity phosphorus pentafluoride gas is used for the synthesis of solid lithium hexafluorophosphate, and the key index acidity of the obtained lithium hexafluorophosphate crystal product is 189 ppm.

[0034] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the method of the present invention, several improvements and supplements can be made, and these improvements and supplements should also be regarded as the protection scope of the present invention.

Claims

1. A method for removing phosphorus pentafluoride from the synthetic tail gas of lithium hexafluorophosphate, characterized in that, It includes the following steps: Absorb phosphorus pentafluoride in the tail gas of lithium hexafluorophosphate synthesis with a fluorine-containing solvent.

2. The removal method according to claim 1, characterized in that, The fluorine-containing solvent is a perfluoroalkylamine solvent, preferably one or more of perfluorotripropylamine, perfluorotributylamine or perfluoroisopropylmorpholine.

3. The removal method according to claim 1 or 2, characterized in that The tail gas of lithium hexafluorophosphate synthesis first removes HF and then removes phosphorus pentafluoride, or first removes phosphorus pentafluoride and then removes HF.

4. The removal method according to any one of claims 1 to 3, characterized in that, The perfluoro solvent is added to the device through which the tail gas of lithium hexafluorophosphate synthesis passes by spraying, or the tail gas of lithium hexafluorophosphate synthesis is directly introduced into the perfluoro solvent.

5. The removal method according to any one of claims 1-4, characterized in that, The mass concentration of phosphorus pentafluoride in the tail gas of lithium hexafluorophosphate synthesis is 5-20 wt%.

6. The removal method according to any one of claims 1-5, characterized in that, The absorption of the tail gas of lithium hexafluorophosphate synthesis is carried out in an absorption tower. The operating temperature of the absorption tower is 0-20 °C, the operating pressure is 1.5-5.5 barg, and the liquid-gas ratio (Kg absorbent / Kg feed gas) of the absorption tower is 15-25.

7. The removal method according to any one of claims 1-6, characterized in that, The operating pressure of the absorbent desorption rectification tower is 1.5-2.5 barg, the operating temperature at the top of the tower is 10-30 °C, the liquid phase at the top of the tower is fully refluxed, and the gas phase is withdrawn.