Comprehensive utilization method of lithium hexafluorophosphate waste liquid

Abstract

The invention relates to a lithium hexafluorophosphate waste liquid comprehensive utilization method, which comprises: under an inert gas atmosphere, adding a lithium hexafluorophosphate waste liquid into a reaction kettle, adding sodium carbonate or sodium oxide powder while stirring, carrying out a reaction to prepare a slurry containing lithium difluorophosphate and sodium fluoride, and filtering the slurry while the slurry is hot to obtain a mother liquid 1 and a filter residue 1; adding the filter residue 1 into a good solvent of lithium difluorophosphate, and filtering after the lithium difluorophosphate is fully dissolved to obtain mother liquor 2 and sodium fluoride; respectively distilling the mother liquor 1 and the mother liquor 2 to remove the organic solvent, and drying to obtain a lithium difluorophosphate product. The method is simple in process, efficient in reaction and easy to industrially implement.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery material technology, specifically relating to a comprehensive utilization method for lithium hexafluorophosphate waste liquid. Background Technology

[0002] Currently, the new energy industry is experiencing explosive growth, and the market demand for lithium hexafluorophosphate, a core solute in lithium-ion battery electrolytes, is rising exponentially. However, throughout the entire lithium hexafluorophosphate industry chain, from reaction residues in industrial production and waste samples from product quality testing, to cleaning wastewater from production equipment pipelines and packaging containers, and to substandard waste generated during electrolyte preparation, a large amount of complex organic wastewater is continuously generated. This wastewater typically contains 5-15% lithium hexafluorophosphate, 0.1-1.5% hydrogen fluoride, and various organic solvents (such as esters and ethers). Its treatment has always been a technical challenge in the industry.

[0003] Currently, the treatment methods for this type of waste liquid are generally extensive, mainly involving the following three approaches: Direct incineration: This method involves directly incinerating the waste liquid as hazardous waste. While it can reduce and render the waste harmless, it results in the emission of highly valuable fluorine and lithium resources as hydrogen fluoride and lithium oxide in the form of flue gas or residue, leading to serious resource waste. Furthermore, the treatment costs are high, which is inconsistent with the development concept of a green circular economy.

[0004] Simple hydrolysis treatment: Hydrolysis is commonly used to convert lithium hexafluorophosphate into lithium fluoride and hydrogen fluoride. Lithium hexafluorophosphate decomposes readily in water, with the reaction: LiPF6 + H2O → LiF + POxFy + HF. While this method is simple to operate, it has significant drawbacks: First, the reaction is violent, releasing large amounts of highly toxic and corrosive hydrogen fluoride gas, posing significant safety and environmental risks; second, the final products are mostly low-value-added inorganic fluoride salts such as lithium fluoride, resulting in low product purity, poor economic benefits, and failing to achieve targeted, high-value recovery of fluorine and phosphorus.

[0005] Distillation separation: The waste liquid is distilled to remove organic solvents in order to recover lithium hexafluorophosphate. However, in actual applications, only 70-80% of the organic solvents can be recovered. The distilled solution is very viscous and cannot be separated and recovered from lithium fluorine resources. It can only be handed over to professional organic waste liquid treatment companies, which results in high treatment costs.

[0006] In summary, the main problems with existing technologies are: significant resource waste during the processing, safety and environmental risks, and low added value of the products. Therefore, developing a green processing method that combines high-value resource recovery, safety, environmental protection, and economic efficiency has become a major technical challenge that the industry urgently needs to solve. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a comprehensive utilization method for lithium hexafluorophosphate waste liquid. This method is simple, highly efficient, and easy to implement industrially.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A method for the comprehensive utilization of lithium hexafluorophosphate waste liquid includes the following steps: Under an inert gas atmosphere, lithium hexafluorophosphate waste liquid is added to a reaction vessel, stirring is started and sodium carbonate or sodium oxide powder is slowly added, and the reaction is carried out to obtain a slurry containing lithium difluorophosphate and sodium fluoride, and the generated carbon dioxide gas escapes; the slurry is filtered while hot to obtain mother liquor 1 and filter residue 1; a good solvent for lithium difluorophosphate is added to filter residue 1, and after the lithium difluorophosphate is fully dissolved, it is filtered to obtain mother liquor 2 and sodium fluoride; mother liquor 1 and mother liquor 2 are respectively distilled to remove organic solvents and dried to obtain lithium difluorophosphate product.

[0009] Furthermore, the by-product sodium fluoride is washed and dried to produce industrial sodium fluoride. The by-product organic solvent is recovered and reused after distillation and purification.

[0010] The comprehensive utilization method for lithium hexafluorophosphate waste liquid described in this invention involves the following chemical reactions: LiPF6+ 2Na2CO3= LiPO2F2+ 4NaF + 2CO2↑ LiPF6 + 2Na2O = LiPO2F2 + 2NaF To ensure efficient reaction, the sodium carbonate or sodium oxide powder is ground into an ultrafine powder with a D50 < 10 μm and slowly and evenly added to the lithium hexafluorophosphate waste liquid under high-speed stirring conditions, with a feeding time of 0.5-1 h.

[0011] Preferably, the reaction is a two-stage heating reaction. The first-stage reaction control conditions are: reaction temperature 30-50℃, reaction time 0.5-1h, and stirring rate 500-800 rpm; the second-stage reaction control conditions are: reaction temperature 50-80℃, reaction time 1-2h, and stirring rate 500-800 rpm. The two-stage reaction is to control the reaction rate, preventing the generated carbon dioxide from agglomerating due to vigorous reaction and causing a rushing phenomenon; simultaneously, the slow formation of fine sodium fluoride particles at low temperature is beneficial for initiating the nucleation of sodium fluoride, forming larger sodium fluoride particles, and reducing product entrainment.

[0012] To promote efficient and uniform reaction and ensure the complete conversion of lithium hexafluorophosphate to lithium difluorophosphate, the mass of sodium carbonate or sodium oxide added is 1.05-1.2 times the theoretical amount required for the reaction with LiPF6 and HF in the lithium hexafluorophosphate waste liquid.

[0013] Specifically, the mother liquor 1 is an organic solvent solution containing lithium difluorophosphate. Depending on the organic solvent in the lithium hexafluorophosphate waste liquid, the solubility of the generated lithium difluorophosphate in the organic solvent also varies, resulting in different lithium difluorophosphate contents in the mother liquor 1. Furthermore, the filter residue 1 is a mixture of lithium difluorophosphate and sodium fluoride. Again, depending on the organic solvent in the lithium hexafluorophosphate waste liquid, the solubility of the generated lithium difluorophosphate in the organic solvent also varies, resulting in different lithium difluorophosphate contents in the filter residue 1.

[0014] Preferably, the good solvent for the lithium difluorophosphate is at least one selected from ethyl acetate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propyl propionate, propylene carbonate, ethylene carbonate, and ethylene glycol dimethyl ether. To ensure complete dissolution of the lithium difluorophosphate in filter residue 1, the amount of good solvent added is sufficient to completely dissolve the lithium difluorophosphate in filter residue 1 at room temperature.

[0015] Compared with the prior art, the advantages and beneficial effects of the method of the present invention are as follows: This invention develops a two-stage heating technology in an anhydrous system, achieving controllable and efficient reactions, avoiding side reactions, and improving reaction efficiency and product quality. The lithium hexafluorophosphate waste liquid recovery process of this invention is simple, highly efficient, has high resource utilization, and is easy to implement industrially. The lithium difluorophosphate and industrial sodium fluoride products recovered using this method are of high purity (lithium difluorophosphate purity above 99.6%, water content below 46 ppm, free acid (HF) below 29 ppm, and insoluble matter below 131 ppm; sodium fluoride purity above 98.5%), meeting downstream market demand and providing a new technical route for the recycling of low-grade resources in the fluorochemical industry. Detailed Implementation

[0016] The technical solution of the present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto.

[0017] In the following examples, all raw materials used are commercially available products that can be purchased directly, or can be prepared using conventional techniques in the art. The sodium carbonate and sodium oxide powders used are ultrafine powders with a D50 < 10 μm.

[0018] Example 1 Lithium hexafluorophosphate waste liquid: LiPF6 12.5%, HF 0.12%, solvent is dimethyl carbonate.

[0019] A method for the comprehensive utilization of lithium hexafluorophosphate waste liquid, the method specifically includes the following steps: 100g of lithium hexafluorophosphate waste liquid was added to a reactor purged with high-purity nitrogen. Stirring was started at 500 rpm, and 18.63g of sodium carbonate powder was slowly added over 1 hour. The temperature was raised to 30℃ for a first-stage reaction for 1 hour, followed by a second-stage reaction at 50℃ for 2 hours. After the reaction was completed, the reaction solution was hot-filtered to obtain mother liquor 1 and filter residue 1. Add 25g of ethyl acetate to a dissolving vessel purged with high-purity nitrogen, then add filter residue 1, stir to dissolve, and filter to obtain mother liquor 2 and sodium fluoride.

[0020] Mother liquor 1 was distilled at 45℃ and a vacuum of -0.09 MPa to recover dimethyl carbonate; mother liquor 2 was distilled at 37℃ and a vacuum of -0.09 MPa to recover ethyl acetate. The distilled materials were dried at 90℃ under nitrogen protection to obtain 8.66 g of lithium difluorophosphate product.

[0021] 13.7g of industrial sodium fluoride was obtained by washing and drying the by-product sodium fluoride.

[0022] Example 2 Lithium hexafluorophosphate waste liquid: LiPF6 15.1%, HF 1.05%, solvent is methyl ethyl carbonate.

[0023] A method for the comprehensive utilization of lithium hexafluorophosphate waste liquid, the method specifically includes the following steps: 200g of lithium hexafluorophosphate waste liquid was added to a reactor purged with high-purity nitrogen. Stirring was started at 600 rpm, and 33.46g of sodium oxide powder was slowly added over 45 minutes. The temperature was raised to 40℃ for a first-stage reaction for 45 minutes, followed by a second-stage reaction at 70℃ for 1.5 hours. After the reaction was completed, the reaction solution was hot-filtered to obtain mother liquor 1 and filter residue 1. Add 63g of propyl propionate to a dissolving vessel purged with high-purity nitrogen, then add filter residue 1. After stirring and dissolving at room temperature, filter to obtain mother liquor 2 and sodium fluoride.

[0024] Mother liquor 1 was distilled at 60℃ and -0.09MPa vacuum to recover methyl ethyl carbonate; mother liquor 2 was distilled at 70℃ and -0.09MPa vacuum to recover propyl propionate. The distilled materials were dried at 90℃ under nitrogen protection to obtain 20.9g of lithium difluorophosphate product.

[0025] 20.5g of industrial sodium fluoride was obtained by washing and drying the by-product sodium fluoride.

[0026] Example 3 Lithium hexafluorophosphate waste liquid: LiPF6 5.2%, HF 1.5%, solvent is dimethyl carbonate.

[0027] A method for the comprehensive utilization of lithium hexafluorophosphate waste liquid, the method specifically includes the following steps: 200g of lithium hexafluorophosphate waste liquid was added to a reactor purged with high-purity nitrogen. Stirring was started at 800 rpm, and 48.63g of sodium carbonate powder was slowly added over 30 minutes. The temperature was raised to 50℃ for a first-stage reaction for 30 minutes, followed by a second-stage reaction at 70℃ for 1 hour. After the reaction was completed, the reaction solution was hot-filtered to obtain mother liquor 1 and filter residue 1. Add 73g of ethylene glycol dimethyl ether to a dissolving vessel purged with high-purity nitrogen, then add filter residue 1, stir to dissolve, and filter to obtain mother liquor 2 and sodium fluoride.

[0028] Mother liquor 1 was distilled at 45℃ and a vacuum of -0.09 MPa to recover dimethyl carbonate; mother liquor 2 was distilled at 42℃ and a pressure of -0.09 MPa to recover ethylene glycol dimethyl ether. The distilled materials were dried at 90℃ under nitrogen protection to obtain 17.85 g of lithium difluorophosphate product.

[0029] 33.6g of industrial sodium fluoride was obtained by washing and drying the by-product sodium fluoride.

[0030] The lithium difluorophosphate and industrial sodium fluoride products obtained in Examples 1-3 above were tested, and the specific test results are shown in Table 1 and Table 2, respectively.

[0031] Table 1. Test results of the high-purity lithium difluorophosphate products obtained in Examples 1-3 Table 2. Test results of industrial sodium fluoride products obtained in Examples 1-3 As shown in Table 1-2, the lithium difluorophosphate and industrial sodium fluoride products obtained by the method of this invention have high purity. The lithium difluorophosphate product has a purity of over 99.6%, a moisture content of less than 46 ppm, a free acid content of less than 29 ppm (calculated as HF), and an insoluble content of less than 131 ppm. The sodium fluoride product has a purity of over 98.5%, which fully meets the needs of the downstream market and provides a new technical route for the recycling of low-grade resources in the fluorochemical industry.

Claims

1. A method for the comprehensive utilization of lithium hexafluorophosphate waste liquid, characterized in that, Includes the following steps: Under an inert gas atmosphere, lithium hexafluorophosphate waste liquid is added to a reaction vessel, and sodium carbonate or sodium oxide powder is added while stirring. The reaction produces a slurry containing lithium difluorophosphate and sodium fluoride. The slurry is filtered while hot to obtain mother liquor 1 and filter residue 1. A good solvent for lithium difluorophosphate is added to filter residue 1. After the lithium difluorophosphate is fully dissolved, it is filtered to obtain mother liquor 2 and sodium fluoride. Mother liquor 1 and mother liquor 2 are distilled to remove organic solvents and dried to obtain lithium difluorophosphate product.

2. The comprehensive utilization method of lithium hexafluorophosphate waste liquid as described in claim 1, characterized in that, The by-product sodium fluoride is washed and dried to obtain industrial sodium fluoride; the by-product organic solvent is purified by distillation and then recycled.

3. The comprehensive utilization method of lithium hexafluorophosphate waste liquid as described in claim 1, characterized in that, The sodium carbonate or sodium oxide powder is ground into an ultrafine powder with a D50 < 10μm and slowly and evenly added to the lithium hexafluorophosphate waste liquid over a period of 0.5-1h.

4. The comprehensive utilization method of lithium hexafluorophosphate waste liquid as described in claim 1, characterized in that, The reaction is a two-stage heating reaction. The first-stage reaction control conditions are: reaction temperature 30-50℃, reaction time 0.5-1h, and stirring rate 500-800rpm; the second-stage reaction control conditions are: reaction temperature 50-80℃, reaction time 1-2h, and stirring rate 500-800rpm.

5. The comprehensive utilization method of lithium hexafluorophosphate waste liquid as described in claim 1, characterized in that, The mass of sodium carbonate or sodium oxide added is 1.05-1.2 times the theoretical amount required for the reaction with LiPF6 and HF in the lithium hexafluorophosphate waste liquid.

6. The method for comprehensive utilization of lithium hexafluorophosphate waste liquid as described in claim 1, characterized in that, The mother liquor 1 is an organic solvent solution containing lithium difluorophosphate; the filter residue 1 is a mixture of lithium difluorophosphate and sodium fluoride.

7. The comprehensive utilization method of lithium hexafluorophosphate waste liquid as described in claim 1, characterized in that, The good solvent for the lithium difluorophosphate is at least one of ethyl acetate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propyl propionate, propylene carbonate, ethylene carbonate, and ethylene glycol dimethyl ether.

8. The method for comprehensive utilization of lithium hexafluorophosphate waste liquid as described in claim 7, characterized in that, The amount of good solvent added to lithium difluorophosphate is the amount that can completely dissolve the lithium difluorophosphate in filter residue 1 at room temperature.

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