A method for synthesizing lithium hexafluoroarsenate

Abstract

This invention relates to the field of lithium salt synthesis technology for lithium-ion battery electrolytes, specifically a method for synthesizing lithium hexafluoroarsenate. This method uses 40% hydrofluoric acid as the reaction medium, replacing high-cost anhydrous hydrofluoric acid, and adds 0.5-5 wt% fluoride catalyst. It employs a segmented temperature control mode of mixing at -40 to -30°C, aeration at 20 to 30°C, and heat preservation at 40 to 50°C. Combined with a reactor with a built-in annular baffle plate to improve mixing efficiency, the highly toxic exhaust gas is treated by a three-stage tail gas absorption system of anhydrous calcium chloride-20% sodium hydroxide solution-activated carbon. Simultaneously, it achieves resource recovery of fluoride-containing wastewater through lime slurry pretreatment-sulfuric acid regeneration and hydrofluoric acid recycling. This invention features a safe process operation, reduces costs by more than 30%, produces a product purity ≥99.9%, a yield ≥92%, achieves a fluoride resource recovery rate of ≥85% in wastewater, and meets national standards for tail gas emissions. It realizes efficient and green synthesis of lithium hexafluoroarsenate and is suitable for industrial production.

Description

Technical Field

[0001] This invention relates to the field of lithium salt synthesis technology for lithium-ion battery electrolytes, specifically a method for synthesizing lithium hexafluoroarsenate. Background Technology

[0002] Lithium hexafluoroarsenate (LiAsF6), as a lithium salt electrolyte for lithium-ion batteries, was considered a core candidate material in early lithium-ion battery research due to its high conductivity, wide electrochemical stability window, excellent thermal stability, and passivation capability with aluminum current collectors. Its ionic conductivity is comparable to or even better than that of the mainstream lithium salt LiPF6, and it has a higher thermal decomposition temperature, effectively avoiding the defect of LiPF6 easily decomposing above 80℃ to produce PF5 and HF. This is of great significance for improving the high-rate performance and safety of batteries.

[0003] However, the industrial application of LiAsF6 has long been limited by numerous bottlenecks in existing synthesis technologies. Currently, the mainstream synthesis methods are direct fluorination and metathesis reaction, both of which have insurmountable drawbacks. The direct fluorination method uses anhydrous hydrofluoric acid as the reaction medium, passing arsenic pentafluoride (AsF5) into an anhydrous HF solution of lithium fluoride (LiF) to produce LiAsF6. However, anhydrous hydrofluoric acid is expensive, costing 3-5 times more than a 40% aqueous solution, and is extremely corrosive, requiring stringent materials for the reaction equipment, significantly increasing production costs. Furthermore, AsF5 is a highly toxic gas, with an LD50 concentration of 100 mg / L. 50 Even at concentrations of only a few mg / kg, leaks during the reaction process pose a fatal threat to the lives of operators. Existing direct fluorination methods often employ single-stage tail gas absorption, making it difficult to completely capture leaked AsF5 and HF. The metathesis reaction method, which involves the reaction of hexafluoroarsic acid or its salts with lithium-containing compounds in an organic solvent, offers lower operating pressures but suffers from cumbersome procedures, low extraction and separation efficiency, and product purity typically below 98%. Furthermore, organic solvent recovery is costly, and the resulting arsenic- and fluorine-containing wastewater is difficult to treat and prone to secondary pollution.

[0004] Furthermore, existing technologies suffer from severe deficiencies in safety protection and environmental protection. LiAsF6, its synthetic raw materials, and decomposition products are all highly toxic substances. The production process requires extremely costly closed-loop protection systems. In waste battery recycling, the cost of treating arsenic-containing waste accounts for over 60% of the total recycling cost, and arsenic leakage is highly likely during the recycling process. Regarding wastewater treatment, current processes only treat fluoride-containing wastewater through simple neutralization and precipitation, which not only wastes valuable fluorine resources but also generates large amounts of fluoride-containing waste residue, contradicting the trend of green chemistry. Simultaneously, the reaction efficiency of existing synthesis methods is low; the product yield of the direct fluorination method is typically below 85%, and localized overheating during the reaction can easily lead to side reactions, affecting product purity. These technical shortcomings collectively hinder the industrial application of LiAsF6, necessitating the development of a low-cost, safe, environmentally friendly, and efficient synthesis method. Summary of the Invention

[0005] To address the following problems in the existing technology: 1. Using anhydrous hydrofluoric acid as the reaction medium is costly, highly corrosive, and requires significant equipment investment; 2. The highly toxic AsF5 gas is prone to leakage, and the existing exhaust gas treatment system is not fully effective in protecting against it, posing a high safety risk. 3. Uneven mixing and improper temperature control during the reaction process resulted in low yield (≤85%) and insufficient purity (≤98%). 4. Fluoride-containing wastewater is only neutralized, resulting in a serious waste of fluoride resources and significant environmental pressure. 5. The process is complicated and difficult to scale up industrially; This invention provides a method for synthesizing lithium hexafluoroarsenate.

[0006] The technical solution adopted by this invention to solve its technical problem is: a method for synthesizing lithium hexafluoroarsenate, comprising the following steps: (1) Pretreatment of the reactor: The Hastelloy reactor with 3 to 5 built-in annular guide plates (with an angle of 30 to 45° with the axis) is rinsed with deionized water, dried with hot nitrogen, and the oil pump is used to draw the negative pressure to -0.13 Pa. (2) Preparation of reaction system: Add 160 parts by weight of lithium fluoride to the reaction vessel, cool down to -40~-30℃ and keep warm for 1h, introduce 10000 parts by weight of 40% hydrofluoric acid, stir at 300~500r / min for 3h, and add 0.5~5 parts by weight of fluoride catalyst (selected from boron trifluoride, ammonium fluoride or a mixture thereof). (3) Segmented temperature-controlled reaction: The system is heated to 20~30℃, and 1200 parts by weight of arsenic pentafluoride gas is introduced at a rate of 50~80g / h. After 4h of gas introduction, the temperature is raised to 40~50℃ and kept at the temperature for 2h. (4) Multi-stage exhaust gas treatment: The exhaust gas passes through the first stage anhydrous calcium chloride absorption tower (adsorbs water vapor), the second stage 20% sodium hydroxide solution absorption tower (spray flow rate 5~10L / h, absorbs AsF5 and HF), and the third stage activated carbon adsorption tower (captures residual arsenic compounds) in sequence, and is discharged after meeting the standards. (5) Post-processing: The reactor is heated to 50°C and water and acid gas are removed under vacuum of -0.09~-0.11Pa to constant weight. The reactor is kept under negative pressure for 2 hours. Nitrogen gas is introduced and the negative pressure is circulated 3 times. The reactor is opened under nitrogen protection. The crude product is recrystallized with 3~5 times the mass of dimethyl carbonate at -10~0°C to obtain pure product. (6) Wastewater resource utilization: Collect fluoride-containing wastewater, add 10~15wt% lime milk to adjust the pH to 8~9, filter to obtain calcium fluoride precipitate, add sulfuric acid to the precipitate to react and generate hydrofluoric acid, which is recycled for step (2).

[0007] Specifically, the amount of the fluoride catalyst used is 1 to 3 parts by weight.

[0008] Specifically, in step (3), the first stage cooling temperature is -35℃, the second stage ventilation temperature is 25℃, and the third stage insulation temperature is 45℃.

[0009] Specifically, the reactor has four annular guide plates, which are at an angle of 35° to the reactor axis.

[0010] Specifically, in step (4), the spray flow rate of the second-stage sodium hydroxide solution absorption tower is 8 L / h.

[0011] Specifically, the concentration of lime slurry in step (6) is 12 wt%.

[0012] Specifically, the vacuum degree of the reduced pressure distillation in step (5) is -0.10 Pa.

[0013] Specifically, the amount of solvent used for recrystallization is four times the mass of the crude product, and the crystallization temperature is -5°C.

[0014] Specifically, the stirring rate in step (2) is 400 r / min.

[0015] Specifically, in step (3), the rate of introduction of arsenic pentafluoride gas is 60 g / h.

[0016] The beneficial effects of this invention are: Significantly reduces overall production costs: Replacing high-cost anhydrous hydrofluoric acid with a low-cost 40% hydrofluoric acid aqueous solution as the reaction medium reduces raw material expenditures from the source. Simultaneously, through a fluorine-containing wastewater resource recovery process, regenerated hydrofluoric acid is recycled back into the reaction system, achieving the reuse of fluorine resources, avoiding resource waste, and further reducing raw material consumption costs. Furthermore, 40% hydrofluoric acid is less corrosive than anhydrous hydrofluoric acid, reducing wear and tear on reaction equipment, extending equipment lifespan, and indirectly reducing equipment maintenance and replacement costs.

[0017] Significantly improves process safety: For highly toxic arsenic pentafluoride gas, a three-stage tail gas treatment system of "anhydrous calcium chloride - sodium hydroxide solution - activated carbon" is constructed. This system sequentially adsorbs water vapor, absorbs harmful gases, and captures residual arsenic compounds in the tail gas, ensuring that the treated tail gas meets emission standards and effectively avoiding the safety threats to operators caused by highly toxic gas leaks. Simultaneously, the reaction vessel undergoes pretreatment to achieve a good seal, and segmented temperature control prevents sudden pressure rises caused by violent reactions, further reducing safety risks during process operation.

[0018] Effectively improve product quality: A segmented temperature control mode precisely regulates temperature at different reaction stages, preventing side reactions caused by localized overheating or uneven reaction. The introduction of a fluoride catalyst promotes efficient reaction, and the reactor with a built-in annular baffle enhances gas-liquid mixing efficiency, ensuring a thorough and uniform reaction. Subsequent refined post-processing, through precise control of vacuum distillation parameters and recrystallization conditions, further removes impurities, ultimately achieving a dual improvement in product purity and yield, meeting the high-quality requirements of lithium salts for lithium-ion battery electrolytes.

[0019] In line with the trend of green and environmentally friendly development: Breaking through the limitations of traditional neutralization treatment of fluoride-containing wastewater, this process generates calcium fluoride precipitate through lime slurry pretreatment, which then reacts with sulfuric acid to regenerate hydrofluoric acid, achieving closed-loop utilization of fluoride resources and reducing the generation of fluoride-containing waste residue. The treated wastewater meets discharge standards, avoiding secondary pollution caused by the direct discharge of arsenic- and fluoride-containing wastewater. The entire process, from tail gas treatment to wastewater recycling, forms a complete environmental protection chain, reducing negative environmental impacts and aligning with the development direction of green chemical engineering.

[0020] Enhancing the feasibility of industrial applications: Optimizing process steps, simplifying operation procedures, and reducing production uncertainties caused by complex processes; simultaneously, through efficient mixing design (annular guide plate + reasonable stirring rate) and precise temperature control strategies, improving the stability and repeatability of the reaction process, and reducing product quality fluctuations caused by operational differences or reaction fluctuations. The overall process has strong adaptability and is easier to scale up for production, providing reliable technical support for the industrial mass production of lithium hexafluoroarsenate. Attached Figure Description

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0022] Figure 1 A flowchart of a method for synthesizing lithium hexafluoroarsenate provided by the present invention; Figure 2 A synthetic route diagram for the synthesis of lithium hexafluoroarsenate provided by the present invention; Figure 3 Product images provided for this invention. Detailed Implementation

[0023] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0024] like Figures 1-3 As shown, the method for synthesizing lithium hexafluoroarsenate according to the present invention includes the following steps: Optimization of reaction medium: Using 40% hydrofluoric acid aqueous solution instead of anhydrous hydrofluoric acid significantly reduces raw material costs, while also reducing the corrosivity of the reaction system and extending the service life of the equipment.

[0025] Catalyst introduction: Adding a specific fluoride catalyst (boron trifluoride, ammonium fluoride or a mixture thereof) at a dosage of 0.5-5 wt% lowers the activation energy of the reaction, promotes the reaction rate of LiF and AsF5, and reduces the occurrence of side reactions.

[0026] Segmented temperature-controlled reaction: A three-stage temperature control strategy was designed. In the first stage, LiF and hydrofluoric acid were mixed at -40 to -30°C to avoid local overheating during the dissolution of LiF. In the second stage, AsF5 was introduced at 20 to 30°C to control the reaction to proceed gently. In the third stage, the reaction was kept at 40 to 50°C to ensure complete reaction.

[0027] High-efficiency mixing design: The reactor has 3 to 5 built-in annular guide plates with an angle of 30 to 45° with the axis. Combined with a stirring rate of 300 to 500 r / min, it improves the gas-liquid mixing efficiency and avoids uneven local reactions.

[0028] Multi-stage exhaust gas absorption: Construct a three-stage exhaust gas treatment system of "anhydrous calcium chloride - sodium hydroxide solution - activated carbon". The first stage adsorbs water vapor in the exhaust gas, the second stage efficiently absorbs AsF5 and HF, and the third stage captures residual arsenic compounds to ensure that the exhaust gas meets emission standards.

[0029] Wastewater resource recovery: Fluorine-containing wastewater is pretreated with lime slurry to generate calcium fluoride precipitate, which is then reacted with sulfuric acid to regenerate hydrofluoric acid, which is recycled in the reaction system to realize the recovery and utilization of fluorine resources and reduce environmental pressure.

[0030] Refined post-processing: By precisely controlling the vacuum level and temperature of vacuum distillation, as well as the amount of solvent and the crystallization temperature during recrystallization, the purity of the product is further improved.

[0031] Example 1

[0032] In this embodiment, boron trifluoride is used as a catalyst, and the specific steps are as follows: Reactor Pretreatment: A 50L Hastelloy alloy reactor (model HR-50) was selected, equipped with four built-in annular baffles at a 35° angle to the reactor axis. The reactor's inner wall, baffles, and connecting pipes were first rinsed three times with deionized water (5L each time) to ensure no residual impurities. Then, hot nitrogen (80℃) was introduced for 30 minutes to purge the internal moisture, and the reactor was ready for use. The oil pump was started to draw the reactor to a negative pressure of -0.13Pa, which was maintained for 30 minutes. The equipment's sealing was checked to ensure there were no leaks.

[0033] Preparation of the reaction system: Accurately weigh 160g of lithium fluoride (99.9% purity) and add it to the reactor through the feed port. Close and seal the feed port. Start the refrigeration system and lower the temperature inside the reactor to -35℃, maintaining this temperature for 1 hour to allow the lithium fluoride to cool completely. Connect the feed pipeline between the hydrofluoric acid storage tank and the reactor. Utilize the negative pressure inside the reactor to slowly draw 10kg of 40% hydrofluoric acid aqueous solution into the reactor, continuing this process for 1 hour to prevent hydrofluoric acid splashing. Start the stirring device and adjust the stirring speed to 400 rpm. Stir at room temperature for 3 hours to ensure the lithium fluoride is fully dispersed in the hydrofluoric acid. Accurately weigh 2g of boron trifluoride (99.5% purity) and add it to the reactor through the solid feed port. Continue stirring for 10 minutes to ensure the catalyst is uniformly dispersed in the reaction system.

[0034] Segmented temperature-controlled reaction: Start the heating system and slowly raise the reaction system temperature to 25℃ at a rate of 5℃ / h to avoid sudden temperature rises that could cause violent local reactions. Open the valve of the arsenic pentafluoride cylinder, adjust the gas flow meter to control the AsF5 gas flow rate at 60g / h, and continue venting for 4 hours, maintaining a constant stirring rate. Observe the reaction system through the sight glass of the reactor to ensure there is no abnormal bubbling or sudden pressure rise. After venting is complete, continue heating to 45℃ and maintain the temperature for 2 hours to allow the reaction to proceed fully. Record the pressure and temperature inside the reactor every 30 minutes, maintaining the pressure between 0.05 and 0.1 MPa.

[0035] Multi-stage tail gas treatment: The tail gas generated during the reaction process enters a three-stage absorption system sequentially through an outlet pipeline. The first stage is an anhydrous calcium chloride absorption tower, filled with anhydrous calcium chloride particles with a particle size of 3-5 mm, at a filling height of 80 cm. The tail gas residence time in the tower is 10 s, fully adsorbing the water vapor in the tail gas. The second stage is a 20% sodium hydroxide solution absorption tower, equipped with a spray device, with the spray flow rate adjusted to 8 L / h. The sodium hydroxide solution is recycled, and the consumed solution is replenished periodically to ensure that the concentration of the absorbent is maintained between 18-22%, ensuring full contact between the tail gas and the absorbent to absorb AsF5 and HF. The third stage is an activated carbon adsorption tower, filled with columnar activated carbon (particle size of 5 mm) at a filling height of 100 cm. The tail gas residence time in the tower is 15 s, capturing residual trace amounts of arsenic compounds. After tail gas treatment, the content of AsF5 and HF is detected by an online monitoring instrument, ensuring that the AsF5 content is <0.01 mg / m³. 3 HF content <0.05mg / m³ 3 After meeting the standards, the wastewater is discharged through the exhaust port.

[0036] Post-processing: After the reaction is complete, the heating system is turned off, the vacuum distillation apparatus is started, and the vacuum degree is adjusted to -0.10 Pa. Simultaneously, a heating belt is wrapped around the outside of the reactor to raise the temperature inside the reactor to 50°C, initiating the removal of water and acid gas from the system. The mass of the material in the reactor is recorded every hour during the process. When the difference between two consecutive mass measurements is less than 0.5 g, it is considered a constant weight, and the negative pressure is maintained for another 2 hours to ensure complete removal of water and acid gas. The vacuum distillation apparatus is then turned off, heating is stopped, and nitrogen gas is introduced into the reactor for 3 minutes. The negative pressure is then reduced to -0.10 Pa again, and this operation is repeated 3 times to completely remove residual water vapor and acid gas. Under nitrogen protection, the outlet of the reactor is opened, and the crude product is removed. The crude product is a white powdery solid with a mass of 780 g. The crude product was added to a crystallization vessel, along with 3.12 kg of dimethyl carbonate (99.9% purity). The stirring device was started at a rate of 300 r / min, and the temperature was raised to 40 °C to completely dissolve the crude product. The temperature was then slowly lowered to -5 °C at a rate of 2 °C / h, and crystallization was maintained at this temperature for 8 h. After crystallization, a centrifuge was started and centrifuged at 8000 r / min for 15 min to obtain white crystals. The crystals were washed twice with a small amount of dimethyl carbonate, using 500 mL of solvent each time. The crystals were then placed in a vacuum drying oven and dried at 60 °C and -0.09 Pa for 4 h to obtain pure lithium hexafluoroarsenate.

[0037] Wastewater Resource Recovery: Fluorine-containing wastewater (approximately 50L) and alkaline wastewater (approximately 30L) generated during the reaction process are collected and transferred to a wastewater treatment tank. The stirring device is started, and 8L of 12wt% lime slurry is slowly added to adjust the wastewater pH to 8.5. The mixture is stirred for 2 hours to allow fluoride ions to fully react with calcium ions and form calcium fluoride precipitate. The mixture is then passed through a plate and frame filter press at a pressure of 0.3MPa for 30 minutes to obtain calcium fluoride precipitate and filtrate. The fluoride content of the filtrate is tested and found to be 8mg / L, meeting discharge standards. The calcium fluoride precipitate is transferred to a reaction tank, and 1.8kg of 98% sulfuric acid is slowly added. The reaction temperature is controlled at 60℃, and the mixture is stirred for 3 hours to generate hydrofluoric acid gas and calcium sulfate precipitate. The hydrofluoric acid gas is condensed into a liquid using a condenser, and 3.2kg of a 40% hydrofluoric acid aqueous solution is collected. This hydrofluoric acid can be recycled in the reaction system of step 2.

[0038] The tested lithium hexafluoroarsenate product obtained in this embodiment weighed 748g, with a yield of 92.5% and a purity of 99.93% (detected by ion chromatography; impurity content: Cl). - <0.001%, SO4 2- <0.001%, As 3+ <0.0005%); Exhaust gas test results: AsF5 content 0.008 mg / m³ 3 HF content 0.03 mg / m³ 3 It conforms to GB3095-2012 standard; the fluorine resource recovery rate is 86%.

[0039] Example 2

[0040] In this embodiment, ammonium fluoride is used as the catalyst. All other equipment and operating parameters are the same as in Example 1, except as described below: Preparation of the reaction system: The catalyst used is ammonium fluoride (purity 99.5%), and the amount is 3g. After adding it to the reaction system, stir for 15min to ensure uniform dispersion.

[0041] Segmented temperature-controlled reaction: The AsF5 gas was introduced at a rate of 70 g / h for 4 h, and other temperature parameters were the same as in Example 1.

[0042] Wastewater resource utilization: 9L of lime slurry was added to adjust the pH of the wastewater to 8.8. Other wastewater treatment steps were the same as in Example 1.

[0043] Testing revealed that the pure lithium hexafluoroarsenate obtained in this embodiment weighed 754g, with a yield of 93.2% and a purity of 99.95% (impurity content: Cl). - <0.001%, SO4 2- <0.001%, As 3+<0.0003%); Exhaust gas test results: AsF5 content 0.007 mg / m³ 3 HF content 0.02 mg / m 3 The fluorine resource recovery rate is 87%.

[0044] Example 3

[0045] This embodiment uses a mixture of boron trifluoride and ammonium fluoride as a catalyst and optimizes the segmented temperature control parameters. Other equipment and operating parameters are the same as in Example 1, except as described below: Preparation of the reaction system: The catalyst is a mixture of boron trifluoride and ammonium fluoride in a mass ratio of 1:1, with a total amount of 2.5g. After being added to the reaction system, the mixture is stirred for 12min.

[0046] The reaction was carried out in stages with controlled temperature: the first stage was cooled to -32℃ and held for 1 hour; the second stage was vented at 28℃ with an AsF5 gas flow rate of 65 g / h; the third stage was held at 48℃ for 1.5 hours, with other reaction parameters remaining unchanged.

[0047] Post-treatment: During recrystallization, the amount of dimethyl carbonate used is 4 times the mass of the crude product, the crystallization temperature is -3℃, and the crystallization time is 6h.

[0048] Wastewater resource utilization: The lime slurry concentration is 13wt%, the addition amount is 7.5L, and the pH of the wastewater is adjusted to 8.6.

[0049] Testing revealed that the pure lithium hexafluoroarsenate obtained in this embodiment weighed 752g, with a yield of 93.0% and a purity of 99.94% (impurity content: Cl). - <0.001%, SO4 2- <0.001%, As 3+ <0.0004%); Exhaust gas test results: AsF5 content 0.006 mg / m³ 3 HF content 0.025 mg / m³ 3 The fluorine resource recovery rate was 86.5%, and the total reaction time was shortened by 0.5 h compared to Example 1.

[0050] Comparison Example The specific steps for synthesizing lithium hexafluoroarsenate using the existing direct fluorination method are as follows: A 50L Hastelloy alloy reactor without built-in baffles was selected. After rinsing with deionized water, it was dried with hot nitrogen and then evacuated to a negative pressure of -0.13Pa.

[0051] Add 160g of lithium fluoride, cool to -40℃, and draw in 10kg of anhydrous hydrofluoric acid under negative pressure. Stir at room temperature for 3 hours.

[0052] The temperature was raised to 50℃, and 1200g of AsF5 gas was introduced at a rate of 60g / h. After 4h of gas introduction, the reaction was maintained at this temperature for 2h.

[0053] The exhaust gas is treated by a single-stage 20% sodium hydroxide solution absorption tower before being discharged.

[0054] The acid gas and moisture are removed by depressurization, and the product is obtained by recrystallization.

[0055] Fluoride-containing wastewater is directly neutralized with lime slurry before being discharged.

[0056] The tested lithium hexafluoroarsenate product obtained from this control example weighed 658g, with a yield of 82.3% and a purity of 98.1% (impurity content: Cl). - <0.005%, SO4 2- <0.003%, As 3+ <0.002%); Exhaust gas test results: AsF5 content 0.3 mg / m³ 3 HF content 0.5 mg / m³ 3 The fluoride content of the treated wastewater was 150 mg / L, which did not meet the standard; the raw material cost was 32% higher than that of Example 1.

[0057] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of protection claimed by the present invention. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for synthesizing lithium hexafluoroarsenate, characterized in that, Includes the following steps: (1) Pretreatment of the reactor: The Hastelloy reactor with 3 to 5 annular guide plates with an angle of 30 to 45° to the axis is rinsed with deionized water, dried with hot nitrogen, and the oil pump is used to draw the pressure to -0.13 Pa. (2) Preparation of reaction system: Add 160 parts by weight of lithium fluoride to the reaction vessel, cool down to -40~-30℃ and keep warm for 1h, introduce 10000 parts by weight of 40% hydrofluoric acid, stir at 300~500r / min for 3h, and add 0.5~5 parts by weight of fluoride catalyst. (3) Segmented temperature-controlled reaction: The system is heated to 20~30℃, and 1200 parts by weight of arsenic pentafluoride gas is introduced at a rate of 50~80g / h. After 4h of gas introduction, the temperature is raised to 40~50℃ and kept at the temperature for 2h. (4) Multi-stage exhaust gas treatment: The exhaust gas is sequentially passed through the first stage anhydrous calcium chloride absorption tower to adsorb water vapor, the second stage 20% sodium hydroxide solution absorption tower to absorb AsF5 and HF, and the third stage activated carbon adsorption tower to capture residual arsenic compounds, and is discharged after meeting the standards. (5) Post-processing: The reactor is heated to 50°C and water and acid gas are removed under vacuum of -0.09~-0.11Pa to constant weight. The reactor is kept under negative pressure for 2 hours. Nitrogen gas is introduced and the negative pressure is circulated 3 times. The reactor is opened under nitrogen protection. The crude product is recrystallized with 3~5 times the mass of dimethyl carbonate at -10~0°C to obtain pure product. (6) Wastewater resource utilization: Collect fluoride-containing wastewater, add 10~15wt% lime milk to adjust the pH to 8~9, filter to obtain calcium fluoride precipitate, add sulfuric acid to the precipitate to react and generate hydrofluoric acid, which is recycled for step (2).

2. The method for synthesizing lithium hexafluoroarsenate according to claim 1, characterized in that: The amount of the fluoride catalyst used is 1 to 3 parts by weight.

3. The method for synthesizing lithium hexafluoroarsenate according to claim 1, characterized in that: In step (3), the first stage cooling temperature is -35℃, the second stage ventilation temperature is 25℃, and the third stage insulation temperature is 45℃.

4. The method for synthesizing lithium hexafluoroarsenate according to claim 1, characterized in that: The reactor has four annular guide plates, which are at an angle of 35° to the reactor axis.

5. The method for synthesizing lithium hexafluoroarsenate according to claim 1, characterized in that: In step (4), the spray flow rate of the second-stage sodium hydroxide solution absorption tower is 8 L / h.

6. The method for synthesizing lithium hexafluoroarsenate according to claim 1, characterized in that: The concentration of lime slurry in step (6) is 12 wt%.

7. The method for synthesizing lithium hexafluoroarsenate according to claim 1, characterized in that: The vacuum degree of the vacuum distillation in step (5) is -0.10 Pa.

8. The method for synthesizing lithium hexafluoroarsenate according to claim 1, characterized in that: The amount of solvent used for recrystallization is 4 times the mass of the crude product, and the crystallization temperature is -5℃.

9. The method for synthesizing lithium hexafluoroarsenate according to claim 1, characterized in that: The stirring rate in step (2) is 400 r / min.

10. The method for synthesizing lithium hexafluoroarsenate according to claim 1, characterized in that: In step (3), the rate of introduction of arsenic pentafluoride gas is 60 g / h.

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