Recycling method of waste electrolyte

Abstract

The invention relates to a method for recycling waste electrolyte, which comprises the following steps of: mixing the waste electrolyte with a solid deacidification agent to react, performing solid-liquid separation, and performing vacuum rectification to obtain an organic solvent and a solid 1; heating the solid 1 in a closed container and carrying out contact reaction on the solid 1 and high-temperature hydrogen fluoride gas flow to obtain a dry solid 2; and recrystallizing the solid 2, and drying to obtain the lithium hexafluorophosphate. According to the method, a solute lithium salt and an organic solvent of the waste electrolyte are separated through reduced pressure rectification, the solid lithium salt and high-temperature hydrogen fluoride gas flow are subjected to a contact reaction, a part of impurities are converted into a lithium hexafluorophosphate product, meanwhile, by-product water is removed, and two purposes are achieved at a time; and further recrystallizing the dried lithium hexafluorophosphate with fewer impurities to obtain a high-purity lithium hexafluorophosphate product.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium battery electrolyte, and in particular relates to a method for recycling waste electrolyte. Background Art

[0002] In recent years, the rapid development of new energy vehicles with lithium batteries as the main power has led to a rapid increase in the production and sales of lithium batteries. As one of the "new three" of China's export products, lithium batteries have become an important pillar industry in China. As a chemical energy storage device, lithium batteries are mainly composed of positive electrodes, negative electrodes, and electrolytes. The electrolyte is responsible for connecting the positive and negative electrodes inside the battery and conducting lithium ions. It is divided into three types: solid, semi-solid, and liquid. Since the first two technologies are not yet mature, liquid electrolytes, commonly known as electrolytes, are still widely used. The electrolyte is composed of an organic solvent and lithium hexafluorophosphate dissolved therein. Commonly used organic solvents include ethylene carbonate, dimethyl carbonate, diethyl carbonate, propylene carbonate, etc. Sometimes a small amount of additives are added to improve battery performance, such as vinylene carbonate, fluoroethylene carbonate, lithium bis(fluorosulfonyl)imide, etc.

[0003] Lithium battery factories often produce waste electrolytes during the batch production of batteries. For example, when the filling machine is rinsed at the start of operation, the filling machine is cleaned when the production is stopped, the electrolyte is replaced when the filling machine fails, the electrolyte is tested when the new filling machine is debugged, and various inspections are performed. These electrolytes cannot be used normally due to contamination by impurities or moisture, and cannot be discharged into the sewer at will (because the electrolyte will decompose to produce hydrogen fluoride, phosphorus pentafluoride and other toxic substances that harm the environment). They are often accumulated and then centrally processed by specialized institutions. However, it is difficult to treat this type of waste electrolyte and the cost is high.

[0004] Patent CN105417770A discloses a method for treating waste liquid containing lithium hexafluorophosphate, wherein a concentrated strong alkali solution, especially a concentrated NaOH solution, is reacted with the lithium hexafluorophosphate waste liquid to generate a precipitate, the waste liquid is desalted by filtration, and then a low-salt organic solvent is incinerated, and the generated waste gas is absorbed by water and passed through a calcium oxide precipitation tank to obtain waste water; the solid obtained by filtration is dissolved in water, and a calcium or magnesium precipitant is added to treat it into waste water and waste residue; this method treats both phosphorus and fluorine elements into waste residue, which is a waste of resources. Patent CN111816947A provides a harmless removal process for waste lithium battery electrolyte, including the following steps: step one, tearing the lithium battery into pieces; step two, in a closed state, vacuum heating and hydrating the battery fragments to decompose and decompose; step three, condensing the decomposition gas generated by the electrolyte during vacuum heating and hydrating dehydration by a vacuum pump, separating and removing the low-boiling point electrolyte; step four, spraying the condensed gas with alkaline solution to remove harmful gases, and phosphorus pentafluoride and hydrogen fluoride in the gas react with the alkaline solution to generate harmless sodium fluoride and phosphate products; step five, the remaining gas is finally discharged after purification by activated carbon adsorption; this method processes phosphorus and fluorine elements into low-value fluoride salts and phosphoric acid, and the economic benefits of industrial application are poor.

[0005] Based on this, this application was developed. Summary of the invention

[0006] The present invention aims to overcome the above-mentioned defects of the prior art and provide a method for recycling waste electrolyte. The method first separates the solute lithium salt and the organic solvent of the waste electrolyte by vacuum distillation, and the solid lithium salt is contacted with a high-temperature hydrogen fluoride gas flow to react so that part of the impurities therein are converted into a lithium hexafluorophosphate product, and the by-product water is removed at the same time, achieving two goals at one stroke, and the dry lithium hexafluorophosphate with less impurities is further recrystallized to obtain a high-purity lithium hexafluorophosphate product.

[0007] To achieve the above object, the present invention adopts the following technical solution: A method for recycling waste electrolyte comprises the following steps: mixing the waste electrolyte with a solid acid scavenger for reaction, separating the solid from the liquid, and then performing vacuum distillation to obtain an organic solvent and a solid 1; heating the solid 1 in a closed container and reacting the solid with a high-temperature hydrogen fluoride gas flow to obtain a dry solid 2; and recrystallizing and drying the solid 2 to obtain high-purity lithium hexafluorophosphate.

[0008] Specifically, the solid acid scavenger is preferably lithium carbonate, and the amount of the solid acid scavenger can be 0.2-2wt% of the weight of the waste electrolyte.

[0009] Furthermore, the solid acid scavenger lithium carbonate is mixed with the hydrofluoric acid in the waste electrolyte for 10-60 minutes at room temperature to be converted into lithium fluoride. The organic solvent obtained by vacuum distillation of the electrolyte no longer containing hydrofluoric acid is purer, which is beneficial for subsequent applications and can generally be used to prepare the electrolyte again.

[0010] Furthermore, vacuum distillation can also be carried out by step distillation at different temperatures to obtain single-component solvents, which is more convenient for subsequent applications. The vacuum distillation conditions are preferably 80-120°C and vacuum -0.07~ -0.1MPa. Under these conditions, the lithium salt decomposes very little, reducing material loss. The solid 1 obtained after distillation is mainly composed of lithium hexafluorophosphate, and also contains a small amount of lithium fluoride, lithium difluorophosphate, lithium phosphate, lithium carbonate, etc. Under heating conditions, it is contacted with a high-temperature anhydrous hydrogen fluoride gas flow. Lithium difluorophosphate, lithium phosphate and lithium carbonate all react with hydrogen fluoride to generate lithium hexafluorophosphate, lithium fluoride, water vapor and carbon dioxide. Driven by the high-temperature hydrogen fluoride gas flow, water vapor and carbon dioxide leave the solid and are taken away, achieving a drying effect.

[0011] Furthermore, the solid 1 heating temperature is preferably 80-100°C, the temperature of the high temperature hydrogen fluoride gas is preferably 80-120°C, the high temperature hydrogen fluoride gas flow rate can be 0.1-2 m / s, and the contact reaction time is 30-180 min. Furthermore, the hydrogen fluoride gas flow rate is preferably 1-2 m / s.

[0012] Specifically, the dried solid 2 is mainly lithium hexafluorophosphate, and also contains a small amount of lithium fluoride and trace impurity metal ions. It is recrystallized and dried in a solvent to obtain high-purity lithium hexafluorophosphate. The solvent used for recrystallization can be dimethyl carbonate, diethyl carbonate, propylene carbonate, etc., or liquid anhydrous hydrogen fluoride. When liquid anhydrous hydrogen fluoride is used, lithium fluoride will accumulate in the solution. When a higher concentration is reached, phosphorus pentafluoride is added to generate new lithium hexafluorophosphate, so that fluorine and lithium resources are fully utilized.

[0013] The chemical reactions involved in the present invention are: LiPO2F2+ 4 HF → LiPF6+ 2 H2O; Li3PO4+ 8 HF → LiPF6+ 2 LiF + 4 H2O; Li2CO3+ 2 HF → 2 LiF + H2O + CO 2 。

[0014] The present invention is dedicated to recycling lithium hexafluorophosphate in waste electrolyte and producing high-purity products. However, waste electrolyte usually contains 80-85% organic solvents, 15-20% lithium salts, and metal ion impurities such as iron, copper, nickel, and trace amounts of water. The water will react with lithium hexafluorophosphate (LiPF6) to generate lithium difluorooxyphosphate (LiPO2F2), lithium phosphate (Li3PO4), lithium fluoride (LiF), etc. These impurities, especially lithium difluorooxyphosphate, are similar in nature to lithium hexafluorophosphate and are difficult to separate. It is more difficult to produce high-purity lithium hexafluorophosphate products. In order to overcome the above disadvantages of difficult treatment of waste electrolytes, the present application provides a method for recycling waste electrolytes. Compared with the prior art, the method of the present invention has the following beneficial effects: The present application first separates the solute lithium salt and organic solvent of the waste electrolyte by vacuum distillation, and the solid lithium salt is contacted with a high-temperature hydrogen fluoride gas flow to react, so that part of the impurities therein are converted into lithium hexafluorophosphate products, and the by-product water is removed at the same time, killing two birds with one stone, and the dry lithium hexafluorophosphate with less impurities is further recrystallized to obtain a high-purity lithium hexafluorophosphate product. In addition, deacidification before vacuum distillation makes the output organic solvent purer and can be used again to prepare the electrolyte, and another by-product lithium fluoride can also continue to react with phosphorus pentafluoride to obtain new lithium hexafluorophosphate. In general, the valuable components of the waste electrolyte are well utilized.

[0015] It is understandable that if solid 1 does not react with high-temperature hydrogen fluoride gas but directly reacts with liquid anhydrous hydrogen fluoride, part of the impurities can also be converted into lithium hexafluorophosphate, but at the same time, the water generated will remain in the solution. When the temperature is reduced and crystallized, the water is entrained in the lithium hexafluorophosphate crystals and is difficult to remove by high-temperature drying, thereby reducing the quality of the product. DETAILED DESCRIPTION

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

[0017] In the following examples, the waste electrolyte used was from a domestic lithium battery manufacturer. It was found to contain 82 wt % of carbonate solvent, 17.8 wt % of lithium hexafluorophosphate and 0.2 wt % of other impurities.

[0018] Room temperature refers to 25±5°C.

[0019] Example 1 A method for recycling waste electrolyte comprises the following steps: 10kg of waste electrolyte was mixed with 0.1kg of lithium carbonate at room temperature for 30 minutes, and then the remaining lithium carbonate was filtered out. The filtrate was transferred to a vacuum distillation tower, set at 90°C and pressure -0.085MPa, and fully distilled to obtain a mixed carbonate solvent and a mixed lithium salt. The mixed lithium salt was heated to 100°C in a closed container and contacted with a 105°C hydrogen fluoride gas flow for reaction at a hydrogen fluoride gas flow rate of 1.2m / s for 120 minutes to obtain dry crude lithium hexafluorophosphate. Solid 2 was dissolved in liquid anhydrous hydrogen fluoride, recrystallized, and dried to obtain 8.04kg of high-purity lithium hexafluorophosphate, with a purity of 99.98% and a moisture content of 4ppm.

[0020] Example 2 A method for recycling waste electrolyte comprises the following steps: 10kg of waste electrolyte was mixed with 0.05kg of lithium carbonate at room temperature for 60min, and then the remaining lithium carbonate was filtered out, and the filtrate was transferred to a vacuum distillation tower, set at 80℃ and -0.095MPa, and fully distilled to obtain a mixed carbonate solvent and a mixed lithium salt. The mixed lithium salt was heated to 90℃ in a closed container and contacted with a 96℃ hydrogen fluoride gas flow for reaction at a hydrogen fluoride gas flow rate of 0.6m / s for 180min to obtain dry crude lithium hexafluorophosphate. Solid 2 was dissolved in liquid anhydrous hydrogen fluoride, recrystallized, and dried to obtain 8.06kg of high-purity lithium hexafluorophosphate, with a purity of 99.97% and a moisture content of 7ppm.

[0021] Example 3 A method for recycling waste electrolyte comprises the following steps: 10kg of waste electrolyte was mixed with 0.18kg of lithium carbonate at room temperature for 10min, and then the remaining lithium carbonate was filtered out, and the filtrate was transferred to a vacuum distillation tower, set at 120℃ and -0.08MPa, and fully distilled to obtain a mixed carbonate solvent and a mixed lithium salt. The mixed lithium salt was heated to 100℃ in a closed container and contacted with a 100℃ hydrogen fluoride gas flow for reaction, with a hydrogen fluoride gas flow rate of 2m / s, and continued for 30min to obtain dry crude lithium hexafluorophosphate. Solid 2 was dissolved in liquid anhydrous hydrogen fluoride, recrystallized, and dried to obtain 8.01kg of high-purity lithium hexafluorophosphate, with a purity of 99.98% and a moisture content of 3ppm.

[0022] In a dry environment, 100 ml of the carbonate mixed solvent obtained by distillation and 18 g of the obtained high-purity lithium hexafluorophosphate were uniformly mixed and dissolved to prepare a lithium battery electrolyte with an acidity of 14 ppm, a moisture content of 6 ppm, and a conductivity of 11.2 mS / cm, which met the conventional requirements for electrolytes.

[0023] Example 4 A method for recycling waste electrolyte comprises the following steps: 10kg of waste electrolyte was mixed with 0.15kg of lithium carbonate at room temperature for 20min, and then the remaining lithium carbonate was filtered out, and the filtrate was transferred to a vacuum distillation tower, set at 100℃ and -0.08MPa, and fully distilled to obtain a mixed carbonate solvent and a mixed lithium salt. The mixed lithium salt was heated to 100℃ in a closed container and contacted with a 100℃ hydrogen fluoride gas flow for reaction at a hydrogen fluoride gas flow rate of 1.8m / s for 40min to obtain dry crude lithium hexafluorophosphate. Solid 2 was dissolved in dimethyl carbonate, recrystallized, and dried to obtain 7.95kg of high-purity lithium hexafluorophosphate, with a purity of 99.96% and a moisture content of 8ppm.

[0024] Comparative Example A method for recycling waste electrolyte comprises the following steps: 10 kg of waste electrolyte and 0.1 kg of lithium carbonate were mixed and reacted at room temperature for 30 minutes, and then the remaining lithium carbonate was filtered out, and the filtrate was transferred to a vacuum distillation tower, set at 90°C and -0.085MPa, and fully distilled to obtain a mixed carbonate solvent and a mixed lithium salt. The mixed lithium salt was dissolved in 50 L of liquid anhydrous hydrogen fluoride at 15°C in a closed container and reacted for 30 minutes, cooled and crystallized, and then dissolved in liquid anhydrous hydrogen fluoride for secondary crystallization and dried to obtain 8.00 kg of lithium hexafluorophosphate, which was tested to have a purity of 99.90% and a moisture content of 29 ppm.

[0025] Although the embodiments of the present invention have been shown and described above, and although the present invention is designed for the recycling of waste electrolyte, it is understandable to those skilled in the art that various changes, modifications, substitutions and variations may be made to these embodiments without departing from the principles and spirit of the present invention, all of which are within the scope of protection of the present invention.

Claims

1. A method for recycling waste electrolyte, characterized in that: The waste electrolyte is mixed with a solid acid scavenger for reaction, and then the solid and liquid are separated, and then the organic solvent and solid 1 are obtained by vacuum distillation; the solid 1 is heated in a closed container and contacted with a high-temperature hydrogen fluoride gas flow to react, so as to obtain a dry solid 2; the solid 2 is recrystallized and dried to obtain lithium hexafluorophosphate.

2. The method for recycling waste electrolyte according to claim 1, characterized in that: The solid acid scavenger is lithium carbonate, and the amount of the solid acid scavenger is 0.2-2wt% of the weight of the waste electrolyte.

3. The method for recycling waste electrolyte according to claim 1, characterized in that: The hydrofluoric acid in the waste electrolyte is mixed with the solid acid scavenger and reacted at room temperature for 10-60 minutes to be converted into lithium fluoride.

4. The method for recycling waste electrolyte according to claim 1, characterized in that: The vacuum distillation temperature is 80-120℃, and the vacuum degree is -0.07~ -0.1MPa.

5. The method for recycling waste electrolyte according to claim 1, characterized in that: The solid 1 is heated to a temperature of 80-100°C.

6. The method for recycling waste electrolyte according to claim 1, characterized in that: The temperature of high temperature hydrogen fluoride gas is 80-120°C.

7. The method for recycling waste electrolyte according to claim 5 or 6, characterized in that: The high-temperature hydrogen fluoride gas flow rate is 0.1-2 m / s, and the contact reaction time is 30-180 min.

8. The method for recycling waste electrolyte according to claim 7, characterized in that: The flow rate of high temperature hydrogen fluoride gas is 1-2m / s.

9. The method for recycling waste electrolyte according to claim 1, characterized in that: The solvent used for recrystallization is dimethyl carbonate, diethyl carbonate, propylene carbonate, or liquid anhydrous hydrogen fluoride.

10. The method for recycling waste electrolyte according to claim 9, characterized in that: When liquid anhydrous hydrogen fluoride is used, lithium fluoride will accumulate in the solution. When a certain concentration is reached, phosphorus pentafluoride is added to generate new lithium hexafluorophosphate.