Safe and environment-friendly treatment method for tail gas pipeline residues in lithium hexafluorophosphate drying process

Abstract

The invention relates to a safe and environment-friendly treatment method for residues in a tail gas pipeline of a lithium hexafluorophosphate drying process, which comprises the following steps: connecting a tail gas buffer tank in series in the tail gas pipeline of the lithium hexafluorophosphate drying process, and collecting waste materials formed by sedimentation from the tail gas buffer tank; the waste material is treated by the following steps: 1) adding ultrapure water and the lithium hexafluorophosphate waste material into a hydrolysis kettle, hydrolyzing, and extracting hydrofluoric acid through negative pressure azeotropy; 2) centrifuging; centrifugally collecting filtrate as phosphoric acid, and washing a filter cake until effluent is neutral to form crude lithium fluoride; 3) adding ultrapure water and concentrated hydrochloric acid into the crude lithium fluoride; after stirring for 0.5-1 hour, centrifuging to remove iron ions, washing a filter cake with ultrapure water until effluent is neutral, and discharging the filter cake to obtain wet lithium fluoride; and 4) drying the wet lithium fluoride until the moisture is qualified to obtain the lithium fluoride for lithium hexafluorophosphate production.According to the method, the pipeline maintenance cost is reduced, meanwhile, the lithium fluoride and byproducts of hydrofluoric acid and phosphoric acid are recovered from the waste material formed by collection, and the method is environment-friendly and energy-saving.

Description

Technical Field

[0001] The invention relates to a safe and environmentally friendly treatment method for tail gas pipeline residues in a lithium hexafluorophosphate drying process. Background Art

[0002] With the continuous growth of global energy demand and the development of clean energy technology, the demand for lithium-ion batteries has increased dramatically, and the demand for lithium battery electrolytes has also seen a blowout. As an important component of the electrolyte, the production scale of lithium hexafluorophosphate has gradually expanded in the past decade.

[0003] The wet synthesis process of lithium hexafluorophosphate comprises the steps of dissolving lithium fluoride in an anhydrous hydrogen fluoride solution, introducing phosphorus pentafluoride gas, separating the solid and the liquid after the reaction is completed, drying the crude solid to obtain the fine lithium hexafluorophosphate, and crushing the fine lithium hexafluorophosphate to obtain the finished lithium hexafluorophosphate.

[0004] Lithium hexafluorophosphate is incompatible with water. When it comes into contact with water vapor in the air, it will decompose and release phosphorus pentafluoride and produce white smoke.

[0005] The production process of lithium hexafluorophosphate is a fully enclosed flow, involving two drying processes: crude product drying and fine product drying. In both drying processes, the lithium hexafluorophosphate inside is heated by the dryer jacket, and nitrogen is introduced to take out the free acid. At the same time, the powdered lithium hexafluorophosphate is taken out to the tail gas pipeline by nitrogen. Because the tail gas pipeline cannot be disassembled and cleaned every day, the powdered lithium hexafluorophosphate gradually accumulates in the tail gas pipeline, and the tail gas pipeline directly goes to the water absorption tower. Therefore, the environment in the tail gas pipeline is unstable, and the quality of the accumulated lithium hexafluorophosphate (hereinafter referred to as waste material) will change, and the acid value, insoluble matter, and water content will increase significantly. Moreover, because the tail gas pipeline is connected by flange bolts, the metal iron content in the waste will also increase to 20~200ppm, which is 10~100 times that of normal products.

[0006] The existing pipeline tail gas treatment method is to collect the waste materials in the pipeline and dump them into the wastewater pool for quenching, which produces fluorine-containing and phosphorus-containing acidic lithium salt wastewater, and then add lime for defluorination and dephosphorization. The solids deposited at the bottom of the wastewater pool need to be cleaned out regularly, drained and treated as hazardous solid waste outsourced. During the quenching reaction, a lot of heat and acid mist are generated, which poses a safety and environmental hazard. Although the wastewater pool has an anti-corrosion coating inside, the coating will peel off after long-term use. High-fluorine and high-phosphorus wastewater may enter the groundwater, causing environmental pollution. Summary of the invention

[0007] The purpose of the present invention is to provide a safe and environmentally friendly treatment method for tail gas pipeline residues in a lithium hexafluorophosphate drying process, to recover lithium resources to the greatest extent, to reduce production costs, and to overcome safety and environmental problems caused by improper waste material treatment. The specific technical scheme is as follows: A Safe and Environmentally Friendly Treatment Method for Residues in the Tail Gas Pipeline of the Lithium Hexafluorophosphate Drying Process: A tail gas buffer tank is connected in series in the tail gas pipeline of the lithium hexafluorophosphate drying process. After the tail gas in the pipeline enters the tail gas buffer tank, its speed decreases and the solid particulate matter therein settles. The waste materials formed by sedimentation are collected from the tail gas buffer tank. Through multiple batches of detection, it is known that the acid value of the waste materials is 100 - 10,000 ppm, the insoluble matter is 200 - 20,000 ppm, the moisture is 10 - 100 ppm, and the iron ion content is 20 - 200 ppm. The waste materials are processed according to the following steps: 1) Add ultrapure water and lithium hexafluorophosphate waste materials to a hydrolysis kettle, hydrolyze, stir and heat up, control the reaction at 60 - 70 °C for 3 - 4 hours, and azeotropically distill out hydrofluoric acid under negative pressure; there are 20 - 200 ppm of iron ions in the waste materials, and iron fluoride precipitates are formed during the reaction in step 1); 2) When the HF content in the liquid in step 1) is less than 1%, centrifuge; the centrifuged filtrate is collected as phosphoric acid, and the filter cake is washed with ultrapure water until the effluent is neutral, and the filter cake is discharged as crude lithium fluoride; adding hydrochloric acid in step 2) can dissolve the iron fluoride precipitate to form water-soluble ferric chloride. After centrifugation and replacement by washing with ultrapure water, the iron content in the crude lithium fluoride can be reduced to 10 ppm and below; 3) Add ultrapure water and concentrated hydrochloric acid with a mass fraction of 36.5% to the crude lithium fluoride in step 2); stir for 0.5 - 1 hour and then centrifuge, and the filter cake is washed with ultrapure water until the effluent is neutral, and the filter cake is discharged as wet lithium fluoride; 4) Dry the wet lithium fluoride until the moisture is qualified to obtain lithium fluoride for the production of lithium hexafluorophosphate.

[0008] In the described safe and environmentally friendly treatment method for residues in the tail gas pipeline of the lithium hexafluorophosphate drying process, its further design lies in: in step 1), the mass ratio of the waste materials to the ultrapure water is 1:2.5 - 3.5.

[0009] In the described safe and environmentally friendly treatment method for residues in the tail gas pipeline of the lithium hexafluorophosphate drying process, its further design lies in: in step 1), hydrofluoric acid is azeotropically distilled out under a negative pressure of 0.06 - 0.09 MPa.

[0010] In the described safe and environmentally friendly treatment method for residues in the tail gas pipeline of the lithium hexafluorophosphate drying process, its further design lies in: the washing liquid formed during washing in step 2) is collected and applied to the next batch of hydrolysis to replace the corresponding mass of ultrapure water.

[0011] In the described safe and environmentally friendly treatment method for residues in the tail gas pipeline of the lithium hexafluorophosphate drying process, its further design lies in: in step 3), the ratio of the crude lithium fluoride to the ultrapure water is 1:4 - 5.

[0012] In the described safe and environmentally friendly treatment method for residues in the tail gas pipeline of the lithium hexafluorophosphate drying process, its further design lies in: in step 3), the ratio of the concentrated hydrochloric acid to the crude lithium fluoride is 1:20 - 25.

[0013] The described safe and environmentally friendly treatment method for residues in the tail gas pipeline of the lithium hexafluorophosphate drying process is further designed as follows: In step 3), the centrifugate and the washing liquid are collected together, and part of them is recycled to the next batch of step 3) to replace ultrapure water, while the other part is treated as waste liquid. The purpose of recycling is mainly to reduce the amount of waste liquid treatment. Usually, two-thirds can be recycled, and one-third enters the environmental protection system as waste liquid for treatment, and ultrapure water is supplemented into step 3).

[0014] The present invention has the following beneficial effects: 1. By setting a tail gas buffer tank in the tail gas pipeline in the present invention, the residues attached to the pipeline brought out by the lithium hexafluorophosphate drying process are collected, reducing the pipeline maintenance cost. At the same time, lithium fluoride, by-product hydrofluoric acid and phosphoric acid are recovered from the collected waste materials, which is environmentally friendly and energy-saving. And each step of waste material treatment is completed in a kettle reactor, which can be sealed, temperature-controlled and easily observed, overcoming the defects existing in the original safety and environmental protection.

[0015] 2. The centrifugation-washing liquid generated in step ) can be recycled and refined for utilization, saving resources and conforming to the green environmental protection production concept.

[0016] 3. The lithium fluoride prepared by the method provided by the present invention meets the national standard of battery-grade lithium fluoride and can be used for feeding in the lithium hexafluorophosphate production system. Description of the Drawings

[0017] Figure 1 It is a comparison table of the national standard parameters of the lithium fluoride obtained in the embodiment of the present invention and battery-grade lithium fluoride. Detailed Embodiments

[0018] To make the purpose, technical solutions and technical effects of the implementation of the present invention clearer, the technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should know that the embodiments are only for helping to understand the technical content and technical effects of the present invention and should not be regarded as a limitation of the present invention. Embodiment

[0019] A tail gas buffer tank is connected in series in the tail gas pipeline of the applicant's lithium hexafluorophosphate drying process. After the tail gas in the pipeline enters the tail gas buffer tank, its speed slows down and the solid particles therein settle; the waste materials formed by the settlement are collected from the tail gas buffer tank.

[0020] (1) Add 6000 L of ultrapure water into the hydrolysis kettle, then add 2 t of waste lithium hexafluorophosphate. Stir and heat up, control the reaction at 60 °C for 3 - 4 hours, and azeotropically distill out 3800 L of hydrofluoric acid under a negative pressure of 0.06 - 0.09 MPa. Detect that the HF content in the hydrolysis solution is 0.92%. Centrifuge and collect 3700 L of the centrifugate. Rinse with 6000 L of ultrapure water until the centrifuged water is neutral, collect 6000 L of the washing solution, and obtain 0.45 t of crude lithium fluoride.

[0021] (2) Add 0.45 t of crude lithium fluoride, 2000 L of ultrapure water, and 20 L of hydrochloric acid. Stir for 10 min and then centrifuge. Rinse with 1000 L of ultrapure water until the centrifuged water is neutral, collect 3000 L of the centrifugate - washing solution, and obtain 0.45 t of moist lithium fluoride.

[0022] (3) Dry 0.45 t of moist lithium fluoride to obtain 0.34 t of dry product. Take a sample for testing, and if the moisture content and other indicators are qualified, it can be fed into the lithium hexafluorophosphate production system.

[0023] The following examples only describe the treatment steps for the waste materials. Example

[0024] (1) Add 6000 L of the washing solution from step (1) of Example 1 into the hydrolysis kettle, then add 2 t of waste lithium hexafluorophosphate. Stir and heat up, control the reaction at 60 °C for 3 - 4 hours, and azeotropically distill out 3850 L of hydrofluoric acid under a negative pressure of 0.06 - 0.09 MPa. Detect that the HF content in the hydrolysis solution is 0.91%. Centrifuge and collect 3650 L of the centrifugate. Rinse with 6000 L of ultrapure water until the centrifuged water is neutral, collect 6000 L of the washing solution, and obtain 0.45 t of crude lithium fluoride.

[0025] (2) Add 0.45 t of crude lithium fluoride, 2000 L of the centrifugate - washing solution from step (2) of Example 1, and 20 L of hydrochloric acid. Stir for 10 min and then centrifuge. Rinse with 1000 L of ultrapure water until the centrifuged water is neutral, collect 3000 L of the centrifugate - washing solution, and obtain 0.45 t of moist lithium fluoride.

[0026] (3) Dry 0.45 t of moist lithium fluoride to obtain 0.34 t of dry product. Take a sample for testing, and if the moisture content and other indicators are qualified, it can be fed into the lithium hexafluorophosphate production system. Example

[0027] (1) Add 6000 L of the washing solution from step (1) of Example 2 into the hydrolysis kettle, then add 2 t of waste lithium hexafluorophosphate. Stir and heat up, control the reaction at 60 °C for 3 - 4 hours, and azeotropically distill out 3820 L of hydrofluoric acid under a negative pressure of 0.06 - 0.09 MPa. Detect that the HF content in the hydrolysis solution is 0.91%. Centrifuge and collect 3700 L of the centrifugate. Rinse with 6000 L of ultrapure water until the centrifuged water is neutral, collect 6000 L of the washing solution, and obtain 0.45 t of crude lithium fluoride.

[0028] (2) 0.45 t of crude lithium fluoride is added to 2000 L of the centrifugation-washing liquid in step (2) of Example 2, along with 20 L of hydrochloric acid. After stirring for 10 min, centrifugation is carried out, and 1000 L of ultrapure water is used for rinsing until the centrifuged effluent is neutral. 3000 L of centrifugation-washing liquid and 0.45 t of moist lithium fluoride are collected.

[0029] (3) 0.45 t of moist lithium fluoride is dried to obtain 0.34 t of dry product. Samples are taken for testing, and the moisture content and other indicators are qualified, which can be used for feeding into the lithium hexafluorophosphate production system. Example

[0030] (1) 6000 L of the washing liquid in step (1) of Example 3 is added to a hydrolysis kettle, and then 2 t of lithium hexafluorophosphate waste is added. Stir and heat up, control the reaction at 60 °C for 3 - 4 hours, and azeotropically distill 3800 L of hydrofluoric acid under a negative pressure of 0.06 - 0.09 MPa. The HF content in the hydrolysis liquid is detected to be 0.91%. Centrifugation is carried out, and 3720 L of centrifuged liquid is collected. 6000 L of ultrapure water is used for rinsing until the centrifuged effluent is neutral. 6000 L of washing liquid and 0.45 t of crude lithium fluoride are collected.

[0031] (2) 0.45 t of crude lithium fluoride is added to 2000 L of the centrifugation-washing liquid in step (2) of Example 3, along with 20 L of hydrochloric acid. After stirring for 10 min, centrifugation is carried out, and 1000 L of ultrapure water is used for rinsing until the centrifuged effluent is neutral. 3000 L of centrifugation-washing liquid and 0.45 t of moist lithium fluoride are collected.

[0032] (3) 0.45 t of moist lithium fluoride is dried to obtain 0.34 t of dry product. Samples are taken for testing, and the moisture content and other indicators are qualified, which can be used for feeding into the lithium hexafluorophosphate production system.

Claims

1. A method for safely and environmentally friendly treating residues in the tail gas pipeline of a lithium hexafluorophosphate drying process, characterized in that: In the tail gas pipeline of the lithium hexafluorophosphate drying process, a tail gas buffer tank is connected in series. After the tail gas in the pipeline enters the tail gas buffer tank, its speed decreases and the solid particulate matter therein settles. The waste materials formed by sedimentation are collected from the tail gas buffer tank, and the waste materials are processed according to the following steps: 1) Add ultrapure water and lithium hexafluorophosphate waste materials to a hydrolysis kettle, hydrolyze, stir and heat up, control the reaction at 60 - 70 °C for 3 - 4 hours, and azeotropically distill out hydrofluoric acid under negative pressure; 2) When the HF content in the liquid material in step 1) is less than 1%, centrifuge. The centrifuged filtrate is collected as phosphoric acid, and the filter cake is washed with ultrapure water until the effluent is neutral, and the filter cake is discharged as crude lithium fluoride; 3) Add ultrapure water and concentrated hydrochloric acid with a mass fraction of 36.5% to the crude lithium fluoride in step 2). After stirring for 0.5 - 1 hour, centrifuge, and the filter cake is washed with ultrapure water until the effluent is neutral, and the filter cake is discharged as moist lithium fluoride; 4) The moist lithium fluoride is dried until the water content is qualified to obtain lithium fluoride for the production of lithium hexafluorophosphate.

2. A method for safely and environmentally friendly treating the residues in the tail gas pipeline of a lithium hexafluorophosphate drying process according to claim 1, characterized in that: In step 1), the mass ratio of the waste materials to ultrapure water is 1:2.5 - 3.

5.

3. A method for safely and environmentally friendly treating residues in the tail gas pipeline of a lithium hexafluorophosphate drying process according to claim 1, characterized in that: In step 1), hydrofluoric acid is azeotropically distilled out under a negative pressure of 0.06 - 0.09 MPa.

4. A method for safely and environmentally friendly treating residues in the tail gas pipeline of a lithium hexafluorophosphate drying process according to claim 1, characterized in that: In step 2), the washing liquid formed by washing is collected and applied to the next batch of hydrolysis to replace the corresponding mass of ultrapure water.

5. A method for safely and environmentally friendly treating residues in the tail gas pipeline of a lithium hexafluorophosphate drying process according to claim 1, characterized in that: In step 3), the ratio of the crude lithium fluoride to ultrapure water is 1:4 - 5.

6. A method for safely and environmentally friendly treating residues in the tail gas pipeline of a lithium hexafluorophosphate drying process according to claim 1, characterized in that: In step 3), the ratio of the concentrated hydrochloric acid to the crude lithium fluoride is 1:20 - 25.

7. A method for safely and environmentally friendly treating residues in the tail gas pipeline of a lithium hexafluorophosphate drying process according to claim 1, characterized in that: In step 3), the centrifuged liquid and the washing liquid are combined and collected. Part of it is applied to the next batch of step 3) to replace ultrapure water, and the other part is treated as waste liquid.

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