Process for the preparation of lithium hexafluorophosphate by means of a lithium-containing aluminum electrolyte

Abstract

The application relates to a method for preparing lithium hexafluorophosphate through a lithium-containing aluminum electrolyte, and the method comprises the following steps: providing a mixed solution comprising phosphoric acid and an alcohol solvent; providing a lithium-containing cryolite-aluminum oxide molten salt electrolyte; mixing the cryolite-aluminum oxide molten salt electrolyte with the mixed solution, and then reacting at a first temperature for a predetermined time to obtain a slurry; adding a neutralizing agent to the slurry to perform a neutralization reaction until the slurry reaches neutral; cooling the slurry to a second temperature and removing water in the slurry; collecting a filtrate by filtration; removing the solvent in the filtrate; and obtaining lithium hexafluorophosphate, wherein the neutralizing agent is at least one of sodium hydroxide and aluminum hydroxide, and the alcohol solvent is at least one of methanol and ethanol. The application has the effects of simple process and low cost.

Description

Technical Field

[0001] This application relates to the field of aluminum electrolysis, and more particularly to aluminum electrolytes. Background Technology

[0002] In the Hall-Heroult process for aluminum electrolysis, a cryolite-alumina molten salt electrolysis method is employed. Cryolite serves as the electrolyte, and alumina as the solute. After a strong direct current is applied, an electrochemical reaction occurs at the electrodes within the electrolytic cell. The lithium-containing aluminum electrolyte, acting as part of the electrolyte, helps lower the melting point and increase conductivity, thereby reducing energy consumption and improving production efficiency. The lithium in the electrolytic cell electrolyte mainly comes from two sources: externally added lithium fluoride and lithium introduced by the alumina. This lithium primarily exists in the form of lithium-containing aluminum electrolyte (LiNa₂AlF₆).

[0003] In recent years, with the rising price of lithium carbonate, lithium in electrolytes has received increasing attention. Currently, the primary method for extracting and recycling lithium from surplus aluminum electrolytes is to prepare lithium carbonate, or to convert fluorides into cryolite or aluminum fluoride, generating impurities such as sodium and potassium salts in the process. These methods often involve roasting, acid leaching, and impurity removal, resulting in a lengthy extraction process. Furthermore, lithium carbonate is typically used as a raw material for lithium-ion battery electrolytes, and converting it into a lithium-ion battery electrolyte requires a complex process. Overall, the process of preparing lithium-ion battery electrolytes from surplus aluminum electrolytes is extremely complex and costly. Summary of the Invention

[0004] This application provides a method for preparing lithium hexafluorophosphate using a lithium-containing aluminum electrolyte, thereby solving the technical problem of complex and costly processes for converting lithium from surplus aluminum electrolyte into electrolytes for lithium-ion batteries.

[0005] This application provides a method for preparing lithium hexafluorophosphate using a lithium-containing aluminum electrolyte, the method comprising the following steps:

[0006] Provides a mixed solution including phosphoric acid and alcohol solvents;

[0007] A lithium-containing cryolite-alumina molten salt electrolyte is provided. The cryolite-alumina molten salt electrolyte is mixed with the mixed solution and reacted at a first temperature for a predetermined time to obtain a slurry.

[0008] A neutralizing agent is added to the slurry to carry out a neutralization reaction until the acidity or alkalinity of the slurry reaches neutrality;

[0009] The slurry is cooled to a second temperature and the water in the slurry is removed. The filtrate is collected by filtration, and the solvent in the filtrate is removed to obtain lithium hexafluorophosphate.

[0010] The neutralizing agent is at least one of sodium hydroxide and aluminum hydroxide, and the alcohol solvent is at least one of methanol and ethanol.

[0011] In some embodiments of this application, the amount of phosphoric acid added is 1.2 to 2 times the theoretical amount required for the cryolite-alumina molten salt electrolyte to react completely.

[0012] In some embodiments of this application, providing a mixed solution comprising phosphoric acid and an alcohol solvent includes: mixing concentrated phosphoric acid and an alcohol solvent to obtain a mixed solution, wherein the volume ratio of the alcohol solvent to the concentrated phosphoric acid is 3 to 20:1.

[0013] In some embodiments of this application, the first temperature is 60–200°C.

[0014] In some embodiments of this application, the predetermined time is 1 to 24 hours.

[0015] In some embodiments of this application, the neutralization reaction lasts for 30 to 90 minutes.

[0016] In some embodiments of this application, the second temperature is 0–30°C.

[0017] In some embodiments of this application, the removal of water from the slurry is achieved by removing water from the slurry using an adsorption material.

[0018] In some embodiments of this application, the adsorbent material is at least one of molecular sieve, silica gel, and activated carbon.

[0019] In some embodiments of this application, removing the solvent from the filtrate includes the following steps:

[0020] The filtrate is dried at a low temperature of 10–30°C.

[0021] The technical solutions provided in this application have the following advantages compared with the prior art:

[0022] The method for preparing lithium hexafluorophosphate using a lithium-containing aluminum electrolyte provided in this application uses a mixed solution including phosphoric acid and an alcohol solvent as the reaction site. The cryolite-alumina molten salt electrolyte reacts with phosphoric acid, and the lithium hexafluorophosphate is enriched in the soluble state by utilizing the solubility behavior of the product in the alcohol solvent. The entire process only includes operations such as reaction, neutralization, dehydration, and filtration to obtain lithium hexafluorophosphate. The process is simple, low-cost, and has high conversion rate and purity of lithium hexafluorophosphate. Attached Figure Description

[0023] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic flowchart of a method for preparing lithium hexafluorophosphate using a lithium-containing aluminum electrolyte, provided as an embodiment of this application. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0027] Unless otherwise specified, the terminology used herein should be understood as having the meaning as commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. In case of any conflict, this specification shall prevail.

[0028] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0029] Currently, there are technical problems with the process of converting lithium from surplus aluminum electrolyte into electrolyte for lithium-ion batteries, including complex procedures and high costs.

[0030] The technical solution provided in this application is to solve the above-mentioned technical problems, and the general idea is as follows:

[0031] This application provides a method for preparing lithium hexafluorophosphate using a lithium-containing aluminum electrolyte, the method comprising the following steps:

[0032] S1: Provides a mixed solution including phosphoric acid and alcohol solvents;

[0033] S2: Provide a lithium-containing cryolite-alumina molten salt electrolyte, mix the cryolite-alumina molten salt electrolyte with the mixed solution, and react at a first temperature for a predetermined time to obtain a slurry;

[0034] S3: Add a neutralizing agent to the slurry to carry out a neutralization reaction until the acidity or alkalinity of the slurry reaches neutrality;

[0035] S4: Cool the slurry to a second temperature and remove the water from the slurry. Filter and collect the filtrate, remove the solvent from the filtrate, and obtain lithium hexafluorophosphate.

[0036] The neutralizing agent is at least one of sodium hydroxide and aluminum hydroxide, and the alcohol solvent is at least one of methanol and ethanol.

[0037] Those skilled in the art will understand that the cryolite-alumina molten salt electrolyte mainly includes Na3AlF6 and LiNa2AlF6.

[0038] The main chemical change occurring in step S2 of this application is the decomposition of cryolite under the action of acid to produce hydrofluoric acid and phosphate; the hydrofluoric acid and phosphate ions react to form hexafluorophosphate. The following reaction can be considered to have occurred:

[0039] Na3AlF6+2H3PO4=Na3PO4+AlPO4+6HF

[0040] 3LiNa2AlF6+6H3PO4=2Na3PO4+Li3PO4+3AlPO4+18HF

[0041] PO4 3- +6HF+2H + =PF6 - +4H2O

[0042] The neutralizing agent added in step S3 is to convert excess phosphoric acid into Na3PO4 or AlPO4 and enter the solid phase, thereby eliminating the influence of phosphate ions on the purity of lithium hexafluorophosphate and further improving the purity of the final lithium hexafluorophosphate.

[0043] The products of the entire process include sodium phosphate, aluminum phosphate, sodium fluoride, aluminum fluoride, and lithium hexafluorophosphate. Sodium phosphate, aluminum phosphate, sodium fluoride, and aluminum fluoride are insoluble in methanol and ethanol, while lithium hexafluorophosphate is soluble in either methanol or ethanol. After the solution reaches neutrality in step S3 and moisture is removed in step S4, the solvent in the slurry is at least one of methanol or ethanol. Sodium ions, aluminum ions, phosphate ions, and fluoride ions all precipitate, while lithium hexafluorophosphate exists in a dissolved state and is ultimately separated by filtration.

[0044] Therefore, taking the neutralizing agent as an example that includes both sodium hydroxide and aluminum hydroxide, the entire process can be considered to involve the following chemical reaction:

[0045] Formula 1: 3LiNa2AlF6+8H3PO4=2Na3PO4+3AlPO4+3LiPF6+12H2O

[0046] Equation 2: Na3AlF6 + 2H3PO4 + 6NaOH = Na3PO4 + AlPO4 + 6NaF + 6H2O

[0047] Formula 3: Na3AlF6+2H3PO4+2Al(OH)3=Na3PO4+AlPO4+2AlF3+6H2O

[0048] Those skilled in the art will understand that lithium hexafluorophosphate is a widely used solute in lithium-ion battery electrolyte solutions.

[0049] This application uses a mixed solution including phosphoric acid and alcohol solvents as the reaction site to react cryolite-alumina molten salt electrolyte with phosphoric acid, and utilizes the solubility behavior of the product in alcohol solvents to enrich lithium hexafluorophosphate in the soluble state. The entire process only includes reaction, neutralization, dehydration, and filtration to obtain lithium hexafluorophosphate. The process is simple, cost-effective, and has high conversion rate and purity of lithium hexafluorophosphate.

[0050] In this application, the conversion rate of lithium to lithium hexafluorophosphate is over 95%, and the purity of the prepared lithium hexafluorophosphate is over 99%.

[0051] In some embodiments of this application, the amount of phosphoric acid added is 1.2 to 2 times the theoretical amount required for the cryolite-alumina molten salt electrolyte to react completely.

[0052] It is easy to understand that the theoretical quantities are calculated according to Formulas 1, 2, and 3. Before implementing this application, the contents of LiNa2AlF6 and Na3AlF6 in the cryolite-alumina molten salt electrolyte can be calculated in advance, and then the theoretical amount of phosphoric acid required for the cryolite-alumina molten salt electrolyte to react completely can be calculated.

[0053] As an example, the amount of phosphoric acid added can be 1.2 times, 1.4 times, 1.5 times, 1.8 times, or 2 times the theoretical amount required for the cryolite-alumina molten salt electrolyte to react completely.

[0054] In some embodiments of this application, providing a mixed solution comprising phosphoric acid and an alcohol solvent includes: mixing concentrated phosphoric acid and an alcohol solvent to obtain a mixed solution, wherein the volume ratio of the alcohol solvent to the concentrated phosphoric acid is 3 to 20:1.

[0055] The beneficial effect of having a volume ratio of 3 to 20:1 between the alcohol solvent and the concentrated phosphoric acid is that sufficient phosphoric acid solution allows the reactants to be fully ionized, accelerating the reaction efficiency. At the same time, sufficient alcohol is present to allow the generated lithium hexafluorophosphate to be fully dissolved in it, while sodium phosphate, aluminum phosphate, sodium fluoride, and aluminum fluoride are fully precipitated.

[0056] In some embodiments of this application, the first temperature is 60–200°C.

[0057] The advantage of setting the first temperature to 60–200°C is that it ensures that the reactants can react fully.

[0058] As an example, the first temperature can be 60°C, 80°C, 120°C, 150°C, or 200°C.

[0059] In some embodiments of this application, the predetermined time is 1 to 24 hours.

[0060] The first temperature increase is beneficial to the increase of reaction efficiency, and the predetermined time can be adjusted with temperature.

[0061] In some embodiments of this application, the neutralization reaction lasts for 30 to 90 minutes.

[0062] As an example, the duration of the neutralization reaction can be 30 min, 50 min, 70 min, or 90 min.

[0063] In some embodiments of this application, the second temperature is 0-30°C, which can prevent the decomposition of lithium hexafluorophosphate and improve the conversion rate of lithium hexafluorophosphate.

[0064] As an example, the second temperature can be 0℃, 5℃, 10℃, 15℃, 20℃, 25℃, or 30℃.

[0065] In some embodiments of this application, the removal of water from the slurry is achieved by removing water from the slurry using an adsorption material.

[0066] In addition, when removing water from the slurry by adsorption material, the adsorption efficiency is high when the second temperature is 0 to 30°C.

[0067] In some embodiments of this application, the adsorbent material is at least one of molecular sieve and silica gel.

[0068] In some embodiments of this application, removing the solvent from the filtrate includes the following steps:

[0069] S41: The filtrate is dried at a low temperature of 10-30°C.

[0070] During the low-temperature drying process, mist can be collected to obtain alcohol solvents that can be recycled; the low-temperature drying process can be carried out by vacuuming to improve the efficiency of low-temperature drying; at the same time, low-temperature drying can effectively avoid the decomposition of the generated lithium hexafluorophosphate and improve the conversion rate.

[0071] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0072] Example 1

[0073] This embodiment provides a method for preparing lithium hexafluorophosphate using a lithium-containing aluminum electrolyte, the method comprising the following steps:

[0074] A lithium-containing cryolite-alumina molten salt electrolyte is provided. The composition of the cryolite-alumina molten salt electrolyte is calculated. Based on the weight percentage of the cryolite-alumina molten salt electrolyte, the content of Na3AlF6 is 54.6 wt%, LiNa2AlF6 is 40.5 wt%, CaF2 is 2.8 wt%, and α-Al2O3 is 2.1 wt%.

[0075] Concentrated phosphoric acid and anhydrous ethanol were mixed, with the amount of concentrated phosphoric acid added being 1.2 times the theoretical value for complete reaction with cryolite. The volume ratio of anhydrous ethanol to concentrated phosphoric acid was 3:1. The mixture was thoroughly mixed to obtain a mixed solvent.

[0076] Cryolite-alumina molten salt electrolyte was mixed with a mixed solvent and reacted at 60°C for 24 hours. Then, sodium hydroxide was added as a neutralizing agent to neutralize the mixture and obtain a slurry. The reaction time was 30 minutes.

[0077] The slurry was cooled to 30°C, and after water adsorption using a molecular sieve, it was filtered. The filtrate was a saturated lithium hexafluorophosphate anhydrous ethanol solution. The filtrate was then dried at a low temperature, controlled at 30°C. During the drying process, the flue gas was cooled and collected to obtain anhydrous ethanol for recycling. The solid after low-temperature drying was lithium hexafluorophosphate.

[0078] The conversion rate of lithium in the electrolyte to lithium hexafluorophosphate was 95%, and the purity of the prepared lithium hexafluorophosphate was 99%.

[0079] Example 2

[0080] This embodiment provides a method for preparing lithium hexafluorophosphate using a lithium-containing aluminum electrolyte, the method comprising the following steps:

[0081] A lithium-containing cryolite-alumina molten salt electrolyte is provided. The composition of the cryolite-alumina molten salt electrolyte is calculated. Based on the weight percentage of the cryolite-alumina molten salt electrolyte, the content of Na3AlF6 is 61.4 wt%, LiNa2AlF6 is 32.5 wt%, CaF2 is 3.2 wt%, and α-Al2O3 is 2.9 wt%.

[0082] Concentrated phosphoric acid and anhydrous ethanol were mixed, with the amount of concentrated phosphoric acid added being 1.6 times the theoretical value for complete reaction with cryolite. The volume ratio of anhydrous ethanol to concentrated phosphoric acid was 12:1. The mixture was thoroughly mixed to obtain a mixed solvent.

[0083] The cryolite-alumina molten salt electrolyte was mixed with a mixed solvent and reacted at 120°C for 12 hours. Then, aluminum hydroxide was added as a neutralizing agent to neutralize the mixture and obtain a slurry. The reaction time was 60 minutes.

[0084] The slurry was cooled to 15°C, and after water adsorption with silica gel, it was filtered. The filtrate was an anhydrous ethanol solution of saturated lithium hexafluorophosphate. The filtrate was dried at a low temperature of 20°C. During the drying process, the flue gas was cooled and collected to obtain anhydrous ethanol for recycling. The solid after low-temperature drying was lithium hexafluorophosphate.

[0085] The conversion rate of lithium in the electrolyte to lithium hexafluorophosphate was 96.2%, and the purity of the prepared lithium hexafluorophosphate was 99%.

[0086] Example 3

[0087] This embodiment provides a method for preparing lithium hexafluorophosphate using a lithium-containing aluminum electrolyte, the method comprising the following steps:

[0088] A lithium-containing cryolite-alumina molten salt electrolyte is provided. The composition of the cryolite-alumina molten salt electrolyte is calculated, and based on the weight percentage of the cryolite-alumina molten salt electrolyte, the content of Na3AlF6 is 70.6 wt%, LiNa2AlF6 is 24.2 wt%, CaF2 is 2.2 wt%, α-Al2O3 is 1.2 wt%, and K2NaAlF6 is 1.8 wt%.

[0089] Concentrated phosphoric acid and anhydrous ethanol were mixed, with the amount of concentrated phosphoric acid added being 2.0 times the theoretical value for complete reaction with cryolite. The volume ratio of anhydrous ethanol to concentrated phosphoric acid was 20:1. The mixture was thoroughly mixed to obtain a mixed solvent.

[0090] The cryolite-alumina molten salt electrolyte was mixed with a mixed solvent and reacted at 200°C for 1 hour. Then, aluminum hydroxide was added as a neutralizing agent to neutralize the mixture and obtain a slurry. The reaction time was 90 minutes.

[0091] The slurry was cooled to 0°C, and after water adsorption by activated carbon, it was filtered. The filtrate was an anhydrous ethanol solution of saturated lithium hexafluorophosphate. The filtrate was dried at a low temperature of 10°C. During the drying process, the flue gas was cooled and collected to obtain anhydrous ethanol for recycling. The solid after low-temperature drying was lithium hexafluorophosphate.

[0092] The conversion rate of lithium in the electrolyte to lithium hexafluorophosphate was 97.5%, and the purity of the prepared lithium hexafluorophosphate was 99.2%.

[0093] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the range referred to.

[0094] In this application, unless otherwise stated, directional terms such as "upper" and "lower" specifically refer to the drawing directions in the accompanying drawings. Furthermore, in the description of this application, the terms "comprising," "including," etc., mean "including but not limited to." Moreover, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. For associations involving three or more related objects described using "and / or", it indicates that any one of the three related objects can exist alone, or at least two of them can exist simultaneously. For example, for A, and / or B, and / or C, it can mean that any one of A, B, and C exists alone, or any two of them exist simultaneously, or all three of them exist simultaneously. In this document, "at least one" means one or more, and "more than one" means two or more. "At least one", "at least one of the following", or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can each be single or multiple.

[0095] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing lithium hexafluorophosphate using a lithium-containing aluminum electrolyte, characterized in that, The method for preparing lithium hexafluorophosphate using a lithium-containing aluminum electrolyte includes the following steps: Provides a mixed solution including phosphoric acid and alcohol solvents; A lithium-containing cryolite-alumina molten salt electrolyte is provided. The cryolite-alumina molten salt electrolyte is mixed with the mixed solution and reacted at a first temperature for a predetermined time to obtain a slurry. The first temperature is 60~200℃ and the predetermined time is 1~24h. A neutralizing agent is added to the slurry to carry out a neutralization reaction until the acidity or alkalinity of the slurry reaches neutrality; The slurry is cooled to a second temperature and the water in the slurry is removed by an adsorption material. The filtrate is collected by filtration, and the solvent in the filtrate is removed to obtain lithium hexafluorophosphate. The second temperature is 0~30℃. The neutralizing agent is at least one of sodium hydroxide and aluminum hydroxide, and the alcohol solvent is at least one of methanol and ethanol.

2. The method for preparing lithium hexafluorophosphate via a lithium-containing aluminum electrolyte according to claim 1, characterized in that, The amount of phosphoric acid added is 1.2 to 2 times the theoretical amount required for the cryolite-alumina molten salt electrolyte to react completely.

3. The method for preparing lithium hexafluorophosphate via a lithium-containing aluminum electrolyte according to claim 1, characterized in that, The provision of the mixed solution comprising phosphoric acid and an alcohol solvent includes: mixing concentrated phosphoric acid and an alcohol solvent to obtain the mixed solution, wherein the volume ratio of the alcohol solvent to the concentrated phosphoric acid is 3 to 20:

1.

4. The method for preparing lithium hexafluorophosphate via a lithium-containing aluminum electrolyte according to claim 1, characterized in that, The neutralization reaction lasts for 30 to 90 minutes.

5. The method for preparing lithium hexafluorophosphate via a lithium-containing aluminum electrolyte according to claim 1, characterized in that, The adsorption material is at least one of molecular sieve, silica gel, and activated carbon.

6. The method for preparing lithium hexafluorophosphate via a lithium-containing aluminum electrolyte according to claim 1, characterized in that, Removing the solvent from the filtrate includes the following steps: The filtrate is dried at a low temperature of 10~30℃.

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