Lithium hexafluorophosphate purification conical filtration dryer

Lithium hexafluorophosphate is an important electrolyte currently used in lithium-ion secondary batteries, which are recognized as relatively ideal green energy sources. They are small in size and have a large capacity, and have been widely applied. The performance of the electrolyte prepared from lithium hexafluorophosphate and organic carbonate solvents directly affects the charge and discharge capacity, cycle life and safety of lithium-ion secondary batteries. Due to its strong hygroscopicity and easy decomposition in the air, the requirements for its preparation conditions are very strict. Similarly, there are also strict requirements for the packaging and storage environment. It decomposes into acid when exposed to water and has a corrosive effect on body tissues. It cannot be in direct contact and must be stored in airtight containers. When in use, it must be operated in a dry atmosphere with a humidity of less than 10ppm to prevent decomposition.

The commonly used purification methods for LiPF6 mainly include thermal vacuum drying and dissolution recrystallization. The dissolution recrystallization method involves dissolving LiPF6 in ether, dimethyl carbonate or diethyl carbonate at a certain temperature, centrifuging to remove insoluble substances, then lowering the temperature to allow LiPF6 to recrystallize from the organic solvent. After vacuum drying the filtered LiPF6 crystals, the final product is obtained. This purification method is disclosed in CN1884046. However, this method is rather complicated to operate, cannot be used for continuous production, and while purifying LiPF6, the recrystallized product will carry a certain amount of organic solvents, which affects the purity of the product. Moreover, the use of a large amount of organic solvents in the purification process leads to an increase in production costs. Therefore, it is difficult to apply it to the large-scale production of LiPF6. In addition, CN1462722 discloses a method for removing moisture from LiPF6 using acidic gas. On the one hand, this method cannot remove hydrogen fluoride from the crude LiPF6 product, and on the other hand, it introduces acidic gas impurities. Compared with the above-mentioned methods, the thermal vacuum drying method, which directly heats and purifies the crude LiPF6 product under vacuum conditions, is a purification approach suitable for the large-scale production of LiPF6. It can avoid the problem of secondary contamination of LiPF6 that exists in the dissolution recrystallization method. The main problem with the thermal vacuum drying method is how to avoid the decomposition of LiPF6 during the heating process and ensure that every particle of the crude LiPF6 product is fully exposed to the vacuum atmosphere, so that the final LiPF6 product does not have the problem of hydrogen fluoride inclusions, thereby reducing the hydrogen fluoride content in the LiPF6 product to less than 80ppm.

To solve the above-mentioned technical problems, Wuxi SRE has developed a conical drying system for lithium hexafluorophosphate purification (Patent No. : ZL 202122464595.4). This new technology relates to the field of separation and purification of lithium battery electrolytes. It addresses the main issues existing in the removal of HF and impurities from crude lithium hexafluorophosphate products by nitrogen pressure filtration and pulse vacuum low-temperature drying methods, providing a dedicated device for the purification and drying of lithium hexafluorophosphate that is highly efficient, safe, and capable of continuous production.

The working principle of the lithium hexafluorophosphate purification conical filtration, washing and drying machine

The lithium hexafluorophosphate purification conical filtration, washing and drying machine adopts a cylindrical conical structure. Heating jackets are set on the cylindrical body and the lower conical body, which can heat or cool the materials. Inside the multi-functional machine, there are hollow shafts and variable-angle and variable-lead hollow spiral stirring devices for stirring and lifting the materials, and the materials are pushed out when discharging. A relatively complex conical filtering device is set in the lower cone. At the bottom of the equipment, there is a specially designed quick discharge device without dead corners or a pneumatic discharge ball valve without dead corners.

During the reaction stage, the reaction materials are added from the top. The unique stirring structure enables the materials to be evenly stirred inside the machine. Through the simultaneous heating or cooling of the cylinder jacket, conical jacket, hollow shaft and hollow spiral blades, the materials can be fully reacted inside the machine. During the filtration stage, this machine can make the material dry through pressure or vacuum filtration inside the machine, and add washing liquid to make the material undergo multiple automatic re-slurry washing inside the machine. After meeting the washing requirements, it can be pressed dry again. During the drying stage, the cylinder jacket, conical jacket, hollow shaft and hollow screw ribbon simultaneously heat the material and create a vacuum at the top to conduct vacuum sealed low-temperature drying of the material. After drying, the material is automatically sealed and discharged.

Technical improvement of the conical filtration dryer for lithium hexafluorophosphate purification

(1) The mother liquor of crystallization and lithium hexafluorophosphate crystals are sealed and filtered for efficient separation, effectively reducing the content of impurity metal ions.

Conventional thermal drying is widely adopted, but this method takes a long time to dry crystals and has drawbacks such as incomplete drying. As a result, HF in the product cannot be effectively and quickly removed, which in turn affects product quality. The purity of the lithium hexafluorophosphate product obtained through filtration and purification is over 99.9%. Moreover, this process is highly safe, the mother liquor can be recycled and reused, and the cost is relatively low.

(2) Fully enclosed operation, carried out under positive pressure protection of nitrogen.

Lithium hexafluorophosphate is highly prone to decomposition when exposed to water. Due to its strong hygroscopicity, it decomposes easily in the air. Therefore, the preparation conditions for it are very strict. This requires that the production process of lithium hexafluorophosphate involves conditions such as high purity, low temperature, and no water or dust. As a result, all processes within the system are carried out under the protection of inert gas. The preferred inert gas is nitrogen. Filling the system with nitrogen ensures a positive pressure within the system, which can effectively prevent air from coming into contact with the product.

(3) During the drying process, multi-zone and multi-temperature heating is adopted; At the initial stage of drying, low temperature and slow heating are adopted to remove the residual acid on the crystal surface. Reduce the time of the high-temperature drying zone in the later stage, effectively separate the free acid, and minimize the decomposition degree of LiPF6.

(4) In the later stage of drying, a nitrogen pulse vacuum drying method is adopted, which shortens the drying time of LiPF6 crystals, ensures uniform drying, improves drying efficiency and quality, and prevents material accumulation and blockage in the drying chamber after drying is completed.

The material is forcibly heated in the conical cylinder. When the material moves in an irregular reciprocating motion in all directions, the heat exchange between the material and the heating screw ribbon as well as the cylinder wall is completed. Under vacuum conditions, the purpose of mixing and drying is achieved in a short time. The new pulse mixing design uses high-pressure nitrogen to impact the materials for rapid mixing, making the mixing of materials faster and more uniform, and quickly removing the acid inside the product crystals and a small amount of residual acid on the surface. Compared with the traditional thermal drying method, nitrogen pulse vacuum drying can significantly shorten the drying time under the same drying degree, reduce the content of free acid in the product, effectively remove mixed HF, improve product quality, and be more conducive to production operations.

(5) The thermal stability of lithium hexafluorophosphate is inferior to that of other lithium salts. At 60℃, it can decompose into a small amount of LiF and PF5. However, to promote the volatilization of free acids inside the crystal, a short period of high-temperature drying is adopted in the later stage of drying. Therefore, after the free acid index meets the requirements, rapid cooling is carried out to reduce the influence of the high-temperature zone on insoluble substances.

(6) Both the heat source and the cold source adopt precisely temperature-controlled heat transfer oil mold temperature control machines.

The hot oil mold temperature control machine acquires low-pressure high-temperature heat medium, is easy to adjust, provides uniform heat supply, and can meet the precise process temperature requirements. When the temperature of the heat transfer oil exceeds 80℃, it must be isolated from the air; otherwise, the heat transfer oil will be rapidly oxidized and deteriorate. Heat transfer oil is flammable. Effective fire prevention and extinguishing measures should be set up.

(7) Adopt air-dry double-end face mechanical seals.

Replace the traditional "liquid blocking" principle with "gas blocking", that is, use pressurized sealing gas instead of pressurized sealing liquid to ensure that the process medium achieves "zero escape". The entire set of sealed non-contact operation has a power consumption of only 5% of that of traditional double-end face seals, and its service life is more than five times longer than that of traditional seals. The simple-structured auxiliary system ensures that the process medium is not contaminated and does not leak into the atmosphere, completely getting rid of the traditional double-end face mechanical seal's reliance on the cooling water and oil systems. The sealing gas adopts industrial nitrogen or industrial instrument air, and its pressure is 0.15 to 0.2MPa higher than that of the medium.

(8) Automated control system, intelligently checking the operating status of the equipment and the drying degree of the materials; There is no need for manual operation to frequently connect equipment pipelines, reducing labor intensity. Hot nitrogen is used to isolate the materials from the air, keeping the acidity of the product stable. Due to the fully enclosed transportation process, the product quality is more stable and the operation is safer and more convenient.

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