A preparation process for high-purity lithium fluoride

Abstract

This invention discloses a process for preparing high-purity lithium fluoride, comprising the following steps: dispersing lithium carbonate raw material in an aqueous medium, introducing carbon dioxide gas until the solution becomes clear, to obtain a lithium bicarbonate solution; purifying the lithium bicarbonate solution to obtain a high-purity lithium bicarbonate solution; continuously adding hydrofluoric acid to the high-purity lithium bicarbonate solution; heating the solution after the crystallization reaction and performing stirring and aging treatment under constant temperature conditions; performing solid-liquid separation on the aged solution, and washing and drying the solid phase to obtain high-purity lithium fluoride. This invention features a novel design, utilizing a cation exchange resin for impurity removal, combined with a two-stage crystallization and thermal aging process control and a mother liquor recycling system, significantly improving the crystal morphology of lithium fluoride and enhancing the purity of the lithium fluoride product. The process system exhibits good robustness and economic benefits for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of lithium fluoride technology, and more particularly to a process for preparing high-purity lithium fluoride. Background Technology

[0002] Lithium-ion batteries are rechargeable batteries that use carbon materials capable of absorbing or desorbing lithium ions as the negative electrode active material and metal oxides containing lithium ions as the positive electrode active material, based on the principle of chemical reaction and using an organic solution as the electrolyte. Lithium fluoride (LiF) is a core precursor material for preparing electrolytes in novel lithium-ion batteries such as lithium hexafluorophosphate (LiPF6). Its purity, impurity content, and moisture content directly determine the battery's electrochemical performance and safety stability. With the rapid iteration of high-energy-density power batteries, the industry has placed extremely stringent requirements on the physicochemical properties of battery-grade high-purity lithium fluoride.

[0003] In existing preparation processes, the impurity content in the raw material lithium carbonate directly determines the quality of the product. When lithium fluoride crystallizes, it produces fine agglomerated microcrystals. The crystal morphology is filled with closed micropores, which easily lead to solvent entrainment, affecting product purity. The overall yield of the production process is low, resulting in poor economic benefits. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies, such as the product purity being greatly affected by the impurity content in the raw materials, the low upper limit of product purity, and the low overall yield, and to propose a preparation process for high-purity lithium fluoride.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A process for preparing high-purity lithium fluoride includes the following steps:

[0007] S1. Disperse lithium carbonate raw material in an aqueous medium, and pass carbon dioxide gas through it until the solution becomes clear to obtain lithium bicarbonate solution;

[0008] S2. The lithium bicarbonate solution is passed through a purification system filled with cation exchange resin to obtain high-purity lithium bicarbonate solution.

[0009] S3. Hydrofluoric acid is continuously added to the high-purity lithium bicarbonate solution. The addition process includes a rapid nucleation stage and a crystal growth stage. The pH value of the reaction system is controlled by gradient decrease by controlling the addition rate of hydrofluoric acid to complete the crystallization reaction.

[0010] S4. Heat the liquid after the crystallization reaction and carry out stirring and aging treatment under constant temperature conditions;

[0011] S5. Solid-liquid separation is performed on the aged liquid, and the solid phase is washed and dried to obtain high-purity lithium fluoride.

[0012] S6. The liquid mother liquor produced by separation is divided into a circulating stream and a waste discharge stream in proportion. The circulating stream is returned to the system as the aqueous medium in S1 for recycling.

[0013] Preferably, in step S1, the reaction temperature for obtaining the lithium bicarbonate solution by introducing carbon dioxide gas is controlled at 15–30°C.

[0014] Preferably, in step S2, the cation exchange resin is a macroporous chelate cation exchange resin containing iminodiacetic acid groups; the flow rate of the lithium bicarbonate solution flowing through the purification system is controlled at 1.0 to 2.0 BV / h.

[0015] More preferably, in step S3, the hydrofluoric acid is electronic-grade hydrofluoric acid with a mass fraction of 40% to 50%.

[0016] Preferably, the specific operations of the addition process in step S3 include:

[0017] The rapid nucleation stage: under a temperature of 20-30°C, hydrofluoric acid is added to the system at a first feeding rate and stirred. When the pH value of the monitored system drops to 7.2-7.5, the crystal growth stage begins.

[0018] During the crystal growth stage: reduce the stirring speed of the system and continue to add hydrofluoric acid at a second feeding rate lower than the first feeding rate until the pH value of the system stabilizes at the reaction endpoint of 6.0-6.5.

[0019] More preferably: the stirring speed during the rapid nucleation stage is controlled at 300-400 r / min; the stirring speed during the crystal growth stage is reduced to 100-150 r / min, and the second feeding rate is set to 5%-10% of the first feeding rate.

[0020] Preferably, in step S4, the aging treatment conditions are as follows: the liquid is heated to 75-85°C at a uniform rate, and stirred at a low speed of 50-100 r / min for 2-3 hours at this temperature.

[0021] Preferably, in step S5, after the solid phase is washed, it is vacuum dried under conditions of vacuum degree ≤ -0.09 MPa and temperature 100~120℃ until the product moisture content is ≤0.05%.

[0022] Preferably, in step S6, 85% to 90% of the total amount of the separated liquid mother liquor is returned to S1 as the circulating stream, and the remaining 10% to 15% is discharged from the system as the waste stream.

[0023] More preferably: in steps S2 to S6, the inner surface material of all reaction vessels, stirrers, conveying pipelines and filtration devices that come into contact with hydrofluoric acid or fluorine-containing materials is polytetrafluoroethylene or perfluoroalkoxy resin.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] 1. In this invention, metal ions in lithium bicarbonate solution are intercepted through physical adsorption process, avoiding exogenous impurities introduced by other impurity removal methods, and greatly reducing the metal ion content in the product.

[0026] 2. In this invention, by controlling the crystal morphology through two-stage crystallization and thermal aging, lithium fluoride crystals are induced to achieve dense, regular, and large-particle-size layered growth. By controlling the crystal morphology, the capillary water absorption and mother liquor entrainment phenomenon caused by fragmented crystal clusters are eliminated, reducing the drying difficulty and significantly reducing the product water content, thereby improving the product purity.

[0027] 3. In this invention, the process design of mother liquor circulation significantly improves the overall yield and greatly enhances the economic benefits of large-scale production while ensuring that no malignant accumulation of impurities occurs in the circulation system.

[0028] This invention features a novel design that utilizes cation exchange resin for impurity removal, combined with a process control system of two-stage crystallization and thermal aging treatment, and a mother liquor recycling system. This significantly improves the crystal morphology of lithium fluoride and enhances the purity of lithium fluoride products. The process system exhibits good robustness and economic benefits for large-scale production. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0030] The basic parameters of the raw materials selected in this embodiment of the invention are as follows:

[0031] Lithium carbonate can be selected from the following two types:

[0032] Raw material 1: Lithium carbonate (Li2CO3-1) with a weight content of 99.2%, sodium (Na) of 800 ppm, calcium (Ca) of 250 ppm, magnesium (Mg) of 150 ppm, iron (Fe) of 20 ppm, hydrochloric acid insoluble matter of 50 ppm, and sulfate of 2000 ppm;

[0033] Raw material 2: Lithium carbonate (Li2CO3-2) with a weight content of 99.0%, sodium (Na) of 1500ppm, calcium (Ca) of 400ppm, magnesium (Mg) of 250ppm, iron (Fe) of 35ppm, hydrochloric acid insoluble matter of 150ppm, and sulfate of 3500ppm;

[0034] The carbon dioxide used is high-purity carbon dioxide, with a purity of ≥99.999%.

[0035] The hydrofluoric acid used is electronic grade hydrofluoric acid (G3) with a purity of 49%.

[0036] All reaction vessels, stirrers, conveying pipelines, and filtration devices used in embodiments of the present invention that come into contact with hydrofluoric acid or fluorine-containing materials are made of polytetrafluoroethylene (PTFE) or perfluoroalkoxy resin (PFA) to physically prevent the leaching and secondary pollution of impurities such as silicon, aluminum, and barium in materials such as glass or stainless steel.

[0037] Example 1

[0038] In this embodiment, the aforementioned raw material one (Li2CO3-1) is used as the precursor, and the specific steps are as follows:

[0039] S1. In a 10L reactor with a PTFE liner, 400g of lithium carbonate is added to 10L of aqueous medium (deionized pure water is used for the first start-up, and recycled mother liquor from process S6 is used for subsequent batches) and stirred. The stirring speed is 200 r / min to suspend the lithium carbonate powder, and carbon dioxide gas is introduced from the bottom of the reactor. The system temperature is controlled at 25℃, and the gas introduction is stopped when the system becomes a clear and transparent solution. Insoluble impurities are removed by filtration through a microporous membrane to obtain a clear lithium bicarbonate solution.

[0040] S2. The above-mentioned lithium bicarbonate clarified solution was pumped into a purification chromatography column packed with macroporous iminodiacetic acid chelate cation exchange resin using a peristaltic pump. The flow rate of the clarified solution penetrating the resin layer from top to bottom was controlled at 1.5 BV / h. The effluent was collected to obtain high-purity lithium bicarbonate clarified solution. Utilizing the specificity of macroporous iminodiacetic acid chelate cation exchange resin for divalent metal cations, the solution was purified for Ca... 2+ Mg 2+ The process effectively intercepts sodium while avoiding exogenous sodium contamination caused by the use of sodium salt complexing agents (such as EDTA-4Na) in traditional processes. Samples of the purified solution were taken and tested, and the levels of Ca and Mg in the solution were reduced to below 3 ppm.

[0041] S3. Add hydrofluoric acid to the above high-purity solution. The crystallization process is divided into two stages:

[0042] Rapid nucleation stage: Maintain the system temperature at 25 ℃ and set the stirring speed to 350 r / min. Add hydrofluoric acid rapidly dropwise at a first feeding rate of 50 mL / min. Monitor the system pH value; when the system pH value drops to 7.3, immediately proceed to the next stage.

[0043] Crystal growth stage: The stirring speed is reduced to 120 r / min to decrease fluid shear force, while the hydrofluoric acid feeding rate is abruptly reduced to a second feeding rate of 4 mL / min (i.e., 8% of the first feeding rate) and continued dropwise. Crystals are allowed to grow slowly under extremely low local supersaturation until the system pH stabilizes at 6.2 with no significant drift within 10 minutes, reaching the crystallization endpoint, at which point acid addition is stopped. Through this two-stage crystallization morphology control process, thanks to the dense lattice rejection effect of lithium fluoride, sodium ions will not be squeezed into the densely and slowly grown crystals; the sodium ions will remain free in the mother liquor and will not affect the purity of the final product.

[0044] S4. After the crystallization reaction is complete, turn on the jacket heating of the reactor to uniformly raise the temperature of the liquid to 80°C. Under this constant temperature condition, age the mixture for 2.5 hours with low-speed stirring at 80 r / min. This promotes the dissolution and deposition of fine crystal fragments on the surface of larger crystals, significantly improving crystal morphology and crystallinity.

[0045] S5. After the aged liquid is naturally cooled to room temperature, it is filtered. The resulting filter cake is rinsed three times with ultrapure water until the washing liquid is neutral. Then, the solid phase is transferred to a vacuum drying oven and dried for 4 hours under a vacuum of -0.095 MPa and a temperature of 110℃. The moisture content of the product is measured by a moisture analyzer and is 0.03%, thus obtaining a high-purity lithium fluoride product. The material is washed with ultrapure water to remove the trace amounts of background sodium free on the crystal surface, thereby removing sodium ions and ensuring product purity.

[0046] S6. Collect the liquid mother liquor separated in S5, measure its total volume, and use 90% of the total mother liquor as a circulating stream, pumping it back to the S1 process as an aqueous medium to replace pure water for recycling; the remaining 10% is discharged as waste stream, increasing the overall lithium fluoride yield of the system to over 95% in a single run.

[0047] Example 2

[0048] It is basically the same as Example 1, except that:

[0049] In step S3, during the rapid nucleation stage, acid is added until the system pH drops to 7.2, then the process is switched; during the crystal growth stage, the second feeding rate is set to 5% of the first feeding rate, and acid is added until the system pH stabilizes at 6.0, reaching the endpoint.

[0050] In step S4, the aging conditions are: uniformly heated to 75°C, and aged with constant temperature stirring for 2 hours.

[0051] Example 3

[0052] It is basically the same as Example 1, except that:

[0053] In step S3, during the rapid nucleation stage, acid is added until the system pH drops to 7.5, at which point the process is switched. During the crystal growth stage, the second feeding rate is set to 10% of the first feeding rate, and acid is added until the system pH stabilizes at 6.5, at which point the endpoint is reached.

[0054] In step S4, the aging conditions are: uniformly heated to 85°C, and aged with constant temperature stirring for 3 hours;

[0055] Example 4

[0056] It is basically the same as Example 1, except that:

[0057] In step S3, acid is added during the rapid nucleation stage until the system pH drops to 7.5, at which point the process switches to the next stage; during the crystal growth stage, acid is added until the system pH stabilizes at 6.0, at which point the process reaches its endpoint.

[0058] Example 5

[0059] It is basically the same as Example 1, except that:

[0060] In step S4, the aging conditions are: uniformly heated to 75°C, and aged with constant temperature stirring for 2 hours;

[0061] In step S6, 85% of the total amount of the separated liquid mother liquor is used as a circulating stream.

[0062] Example 6

[0063] It is basically the same as Example 1, except that:

[0064] In step S4, the aging conditions are: uniformly heated to 85°C, and aged with constant temperature stirring for 2 hours;

[0065] In step S6, 90% of the total amount of the separated liquid mother liquor is used as a circulating stream.

[0066] Comparative Example 1

[0067] This is basically the same as Example 1, except that this comparative example uses raw material 2 (Li2CO3-2) as the precursor.

[0068] Comparative Example 2

[0069] It is basically the same as Example 1, except that:

[0070] In step S2, without going through a cation exchange resin purification system, the chelating agent tetrasodium ethylenediaminetetraacetate (EDTA-4Na) is directly added to the solution for complexation.

[0071] Comparative Example 3

[0072] It is basically the same as Example 1, except that:

[0073] In step S3, hydrofluoric acid is continuously added dropwise to the high-purity lithium bicarbonate solution at a single constant flow rate of 50 mL / min until the pH of the system drops to 6.2 and then the acid addition is stopped.

[0074] The obtained dried product was subjected to spectral analysis, and the data are recorded in Table 1:

[0075] Table 1. Product ICP Analysis Report

[0076] standard / ≥ 99.9% <5.0 <5.0 <2.0 ≤0.05% Example 1 96.2% 99.98% 1.2 0.8 0.4 0.03% Example 2 93.8% 99.94% 2.0 1.6 1.1 0.04% Example 3 95.9% 99.96% 1.5 1.2 0.7 0.03% Example 4 96.0% 99.97% 1.3 1.0 0.5 0.03% Example 5 93.5% 99.95% 1.8 1.5 0.9 0.04% Example 6 95.7% 99.96% 1.4 1.1 0.6 0.04% Comparative Example 1 95.8% 99.93% 4.5 1.2 0.5 0.04% Comparative Example 2 94.5% 99.78% 7.22 0.9 0.6 0.05% Comparative Example 3 94.1% 99.85% 13.5 1.05 0.55 0.08%

[0077] The data in Table 1 fully demonstrates the significant advantages of this invention in terms of product purity and yield. On the one hand, this invention utilizes a mother liquor recycling design, employing a saturated lithium fluoride solution instead of pure water to dissolve lithium carbonate, thereby stabilizing the overall system yield at 93.5%–96.2%. Furthermore, the 10%–15% waste discharge design prevents the malignant accumulation of impurities, ensuring product purity. On the other hand, this invention employs cation exchange resin for impurity removal, avoiding exogenous contamination caused by methods such as EDTA-4Na (as in Comparative Example 2), further reducing the impurity content in the product and improving product purity.

[0078] The two-stage crystallization and thermal aging process used in this invention ensures effective control over the morphology of lithium fluoride. Compared to the nucleation and particle agglomeration caused by a single constant-rate acid addition (as in Comparative Example 3), this invention induces the formation of dense, regular, large-particle-size crystals through a mechanism of rapid nucleation, slow growth with low supersaturation, and high-temperature aging. This eliminates solvent-entrained dead zones within the crystals, reducing moisture content to 0.03%–0.04%. Crucially, this regular, dense crystal form also prevents the malignant enrichment of impurities within the mother liquor circulation system. The large-particle-size crystals significantly reduce the specific surface area, decreasing the adsorption of trace metal ions. Surface free water also readily evaporates, significantly reducing the moisture content of the dried product. In contrast, during crystal formation in Comparative Example 3, the explosive nucleation caused by a single rapid acid addition results in severe agglomeration of the generated microcrystals and the creation of numerous internal micropores. This easily traps the reaction mother liquor rich in background sodium ions deep within the crystals, making it impossible to remove them through subsequent ultrapure water washing, ultimately leading to an increase in sodium content.

[0079] Furthermore, the process of this invention still has a very wide tolerance for low-grade and low-cost precursor raw materials (as in Comparative Example 1), demonstrating good robustness and good economic benefits. Even when using a relatively inferior precursor (Raw Material 2) with a high initial impurity content, the final product's indicators can still meet battery-grade standards, demonstrating the good impact resistance of the process system of this invention.

[0080] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A process for preparing high-purity lithium fluoride, characterized in that, Includes the following steps: S1. Disperse lithium carbonate raw material in an aqueous medium, and pass carbon dioxide gas through it until the solution becomes clear to obtain lithium bicarbonate solution; S2. The lithium bicarbonate solution is passed through a purification system filled with cation exchange resin to obtain high-purity lithium bicarbonate solution. S3. Hydrofluoric acid is continuously added to the high-purity lithium bicarbonate solution. The addition process includes a rapid nucleation stage and a crystal growth stage. The pH value of the reaction system is controlled by gradient decrease by controlling the addition rate of hydrofluoric acid to complete the crystallization reaction. S4. Heat the liquid after the crystallization reaction and carry out stirring and aging treatment under constant temperature conditions; S5. Solid-liquid separation is performed on the aged liquid, and the solid phase is washed and dried to obtain high-purity lithium fluoride. S6. The liquid mother liquor produced by separation is divided into a circulating stream and a waste discharge stream in proportion. The circulating stream is returned to the system as the aqueous medium in S1 for recycling.

2. The preparation process of high-purity lithium fluoride according to claim 1, characterized in that: In step S1, the reaction temperature for preparing the lithium bicarbonate solution by introducing carbon dioxide gas is controlled at 15–30°C.

3. The preparation process of high-purity lithium fluoride according to claim 1, characterized in that: In step S2, the cation exchange resin is a macroporous chelated cation exchange resin containing iminodiacetic acid groups; the flow rate of the lithium bicarbonate solution through the purification system is controlled at 1.0 to 2.0 BV / h.

4. The preparation process of high-purity lithium fluoride according to claim 1, characterized in that: In step S3, the hydrofluoric acid is electronic-grade hydrofluoric acid with a mass fraction of 40% to 50%.

5. The preparation process of high-purity lithium fluoride according to claim 1, characterized in that: The specific operations of the addition process in step S3 include: The rapid nucleation stage: under a temperature of 20-30°C, hydrofluoric acid is added to the system at a first feeding rate and stirred. When the pH value of the monitored system drops to 7.2-7.5, the crystal growth stage begins. During the crystal growth stage: reduce the stirring speed of the system and continue to add hydrofluoric acid at a second feeding rate lower than the first feeding rate until the pH value of the system stabilizes at the reaction endpoint of 6.0-6.

5.

6. The preparation process of high-purity lithium fluoride according to claim 5, characterized in that: The stirring speed during the rapid nucleation stage is controlled at 300-400 r / min; the stirring speed during the crystal growth stage is reduced to 100-150 r / min, and the second feeding rate is set to 5%-10% of the first feeding rate.

7. The preparation process of high-purity lithium fluoride according to claim 1, characterized in that: In step S4, the aging treatment conditions are as follows: the liquid is heated to 75-85°C at a uniform rate, and stirred at a low speed of 50-100 r / min for 2-3 hours at this temperature.

8. The preparation process of high-purity lithium fluoride according to claim 1, characterized in that: In step S5, after the solid phase is washed, it is vacuum dried under conditions of vacuum degree ≤ -0.09 MPa and temperature 100~120℃ until the product moisture content is ≤0.05%.

9. The preparation process of high-purity lithium fluoride according to claim 1, characterized in that: In step S6, 85% to 90% of the total amount of the separated liquid mother liquor is returned to S1 as the circulating stream, and the remaining 10% to 15% is discharged from the system as the waste stream.

10. The preparation process of high-purity lithium fluoride according to any one of claims 1 to 9, characterized in that: In steps S2 to S6, the inner surface material of all reaction vessels, agitators, conveying pipelines and filtration devices that come into contact with hydrofluoric acid or fluorine-containing materials is polytetrafluoroethylene or perfluoroalkoxy resin.