Method for directly preparing and purifying lithium fluoride from lepidolite and fluoride

Abstract

The invention discloses a method for directly preparing and purifying lithium fluoride from lepidolite and fluoride, and belongs to the technical field of lithium battery materials. According to the invention, the characteristic that lithium fluoride and potassium fluoride can continuously volatilize at 1000 DEG C or above is utilized, so that gas-solid separation of lithium fluoride from complex lepidolite is realized. The method comprises the following steps: mixing and grinding lepidolite and fluoride salt, roasting until the quality is stable, collecting volatile matters, dissolving the collected volatile matters in water, filtering, and collecting precipitates to obtain high-purity lithium fluoride. The method disclosed by the invention has the characteristics of no need of acid treatment, avoidance of a complicated step of separating lithium from elements such as aluminum, silicon and calcium, simplicity in operation and easiness in amplified production, the production cost of the lithium fluoride can be remarkably reduced, and the purity of the lithium fluoride is improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery materials technology, specifically to a method for directly preparing and purifying lithium fluoride from lepidolite and fluorides. Background Technology

[0002] Lithium fluoride (LiF), an important inorganic fluorine material, is widely used in lithium batteries, optical materials, and the nuclear industry. Currently, the price of industrial-grade lithium fluoride is approximately RMB 112,600-124,400 per ton, while battery-grade products are priced at approximately RMB 138,800-143,200 per ton. The high cost of lithium fluoride is mainly due to the high cost of its raw materials, especially high-purity lithium carbonate and hydrofluoric acid. As of October 2025, the market price of lithium carbonate was RMB 280,000-320,000 per ton, and the price of hydrofluoric acid remained high at RMB 9,250-9,450 per ton. Most lithium fluoride is indirectly produced from lithium carbonate, a process involving the extraction of lithium carbonate from spodumene or lepidolite and its subsequent conversion into lithium fluoride, which is lengthy and further increases the overall cost.

[0003] Currently, the mainstream lithium extraction process is the sulfate roasting method. This process involves mixing lepidolite or spodumene with additives such as potassium sulfate and sodium sulfate, and roasting at 900-1000℃ for 1-2 hours to generate soluble lithium sulfate through an ion exchange reaction. Subsequently, it undergoes purification (e.g., pH adjustment to precipitate Fe³⁺). + Al³ + Adding Na2CO3 removes Ca²⁺ + Mg² + (Impurities such as potassium, sodium, aluminum, silicon, and iron are removed), and then lithium is precipitated by adding sodium carbonate at pH=10 and temperature of 80-90℃, followed by drying to obtain lithium carbonate. Lithium carbonate is then neutralized (Li₂CO₃ + 2HF → 2LiF + CO₂↑ + H₂O) to prepare lithium fluoride. However, the sulfate roasting and acid leaching processes have poor selectivity for lithium and introduce impurities such as potassium, sodium, aluminum, silicon, and iron, which are difficult to completely remove even after subsequent purification, affecting the purity of the final lithium fluoride. Battery-grade lithium fluoride requires a purity of not less than 99.95%, therefore, high-purity technology remains a key development direction for the industry.

[0004] Existing methods for directly preparing lithium fluoride from lepidolite mainly include two process routes: acidification-lithium precipitation and fluorine chemistry. The acidification-lithium precipitation method is a process that comprehensively utilizes lithium and fluorine elements in lepidolite without introducing additional fluorine-containing raw materials. This method involves reacting lepidolite with sulfuric acid to simultaneously extract lithium and fluorine, ultimately producing lithium fluoride. The fluorine chemistry method utilizes the unique property of fluorine chemistry to disrupt the crystal structure of silicate minerals. It breaks down the dense layered crystal structure of lepidolite at relatively low reaction temperatures, achieving efficient lithium extraction. However, existing methods for directly preparing lithium fluoride from lepidolite all suffer from low purity of the prepared lithium fluoride.

[0005] Therefore, providing a method for directly preparing high-purity lithium fluoride using lepidolite is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] In view of this, the present invention provides a method for directly preparing and purifying lithium fluoride from lepidolite and fluorides.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for directly preparing and purifying lithium fluoride from lepidolite and fluorides includes the following steps: (1) Mix lepidolite with fluoride salt, and then grind them evenly to obtain a mixture; (2) The mixture is roasted in a flue gas furnace to 1200-1400℃ until the quality is stable, and the volatiles are collected through a filter membrane at the flue gas outlet; (3) Dissolve the collected volatiles in water of equal mass ratio, filter and collect the precipitate to obtain high-purity lithium fluoride; (4) Evaporate the filtrate from step (3) to remove water, and obtain a mixed byproduct of potassium fluoride and sodium fluoride.

[0008] The reaction route of the method of the present invention can be any of the following: 2KLi2Al(Si4O 10 )(F,OH)2(s)+CaF2(s)→4LiF(g)+2KF(g)+CaAl2Si2O8(s)+6SiO2(s) Formula 1; 2KLi2Al(Si4O 10 )(F,OH)2(s)+2NaF(s)→4LiF(g)+2KF(g)+Na2Al2Si2O8(s)+6SiO2(s) Equation 2.

[0009] In this invention, fluoride undergoes a solid-phase reaction with lepidolite at high temperature (reaction formulas 1 and 2). When the temperature exceeds 1100°C, volatile phases such as lithium fluoride, potassium fluoride, and sodium fluoride are generated. Water is then separated by utilizing the concentration differences among lithium fluoride, potassium fluoride, and sodium fluoride. A key feature of this method is that no acid is required throughout the preparation of lithium fluoride, and there is no subsequent separation of calcium, silicon, and aluminum.

[0010] Furthermore, the fluoride salt mentioned in step (1) is calcium fluoride or sodium fluoride.

[0011] Furthermore, the mass ratio of lepidolite to fluoride in step (1) is 10:2-3.

[0012] The molar mass of lepidolite is 408.33, the molar mass of calcium fluoride is 78, and the molar mass of sodium fluoride is 42. The reaction formula calculates that the molar ratio of calcium fluoride to sodium fluoride is 2:1 (10.47:1 by mass), and the molar ratio of sodium fluoride is 1:1 (9.72:1 by mass). For a complete reaction, fluoride needs to be in excess; therefore, the preferred ratio of calcium fluoride is 10:3, and the preferred ratio of sodium fluoride is 10:2.

[0013] Furthermore, in step (2), the flue gas furnace is roasted in a nitrogen atmosphere in the absence of air, and the nitrogen blows the volatiles out to the flue gas furnace outlet, where the volatiles are collected by the filter membrane.

[0014] Furthermore, the roasting temperature in step (2) is preferably 1200-1300℃.

[0015] The beneficial effects of this invention are as follows: Compared with the indirect preparation of lithium fluoride, this invention eliminates the intermediate preparation step of lithium carbonate, resulting in a shorter preparation process. Lithium fluoride refined by indirect lithium extraction, acidification-precipitation methods, and fluorine chemical methods all employ acid leaching for extraction. The resulting lithium sulfate contains a large amount of impurities such as aluminum, magnesium, calcium, and iron. These impurities are removed by removing silicon and aluminum, adjusting the pH value, and precipitating the silicon and aluminum. For iron and aluminum removal, pH is adjusted or an oxidizing agent is added to precipitate the iron and aluminum. For calcium and magnesium removal, a fluorinating agent (such as NaF) or sodium carbonate is added to form calcium fluoride and calcium carbonate precipitates. The purity of refined lithium fluoride is generally 98%. This invention extracts lithium fluoride through volatilization, and the volatilized product contains no aluminum, magnesium, calcium, or iron impurities, eliminating the need for the aforementioned cumbersome removal processes. The purity of lithium fluoride can reach over 99.5%. Compared with all current methods for preparing lithium fluoride, this method has the advantages of not requiring acid treatment, avoiding complex separation steps of lithium from elements such as aluminum, silicon, and calcium, and being simple to operate and easy to scale up for production. It is expected to significantly reduce the production cost of lithium fluoride and improve the purity of lithium fluoride. Attached Figure Description

[0016] Figure 1. TG curves of the mixture of lepidolite and fluoride in Examples 1-2; Figure 2 TG and DSC graphs of lithium fluoride; Figure 3 Image showing the elemental leaching concentrations of residues from lepidolite and calcium fluoride at 1100°C and 1400°C in Example 1; Figure 4 The leaching concentration of residual elements in lepidolite and sodium fluoride at 1100°C and 1400°C in Example 2; Figure 5 Acid leaching diagrams of volatile elements from lepidolite and calcium fluoride, and lepidolite and sodium fluoride in Examples 1-2; Figure 6Water leaching diagrams of volatile elements from lepidolite and calcium fluoride, and lepidolite and sodium fluoride in Examples 1-2; Figure 7 SEM image of the volatiles in Example 1; Figure 8 EDS spectrum of volatiles in Example 1; Figure 9 The leaching concentrations of various elements in the solution obtained by the conventional method and the method of the present invention are shown in the diagram.

[0017] Figure 10 XRD results of potassium fluoride obtained after water purification of collected volatiles Figure 11 XRD results of lithium fluoride obtained after water purification of collected volatiles Detailed Implementation The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Example 1 A method for directly preparing and purifying lithium fluoride from lepidolite and fluorides: (1) Mix lepidolite and calcium fluoride at a mass ratio of 10:2, and then grind them evenly to obtain a mixture; (2) The mixture is transferred to a flue gas furnace and calcined at 1250°C in a nitrogen atmosphere until the quality is stable. Nitrogen blows the volatiles out to the flue gas furnace outlet, and the volatiles are collected by the filter membrane. (3) Dissolve the collected volatiles in water of equal mass ratio, filter and collect the precipitate to obtain high-purity lithium fluoride; (4) Evaporate the filtrate from step (3) to remove water, and obtain a mixed byproduct of potassium fluoride and sodium fluoride.

[0019] Example 2 The scheme is basically the same as that in Example 1, except that calcium fluoride is replaced with sodium fluoride and mixed at a mass ratio of 10:3.

[0020] The melting points of the fluorides are: LiF 848℃, KF 858℃, and NaF 993℃. Above these melting points, the vapor pressure increases, causing the fluorides to volatilize from the raw materials. TG-DSC curves ( Figure 2 The results show that LiF is volatile at high temperatures. Figure 2The results show that the mixtures of lepidolite-calcium fluoride and lepidolite-sodium fluoride begin to lose weight above 1100℃, with weight loss rates of 14.82% and 19.56% at 1400℃, respectively, and the maximum weight loss rate occurs at 1220℃.

[0021] ICP and ion chromatography were used to analyze volatiles and residues. Figure 3-6 It was found that the concentrations of lithium, potassium, and sodium in the residue at 1400℃ were significantly lower than those in the sample at 1100℃, while the concentrations of calcium, silicon, and aluminum increased due to relative enrichment. Further analysis of the lithium, potassium, and sodium content after acid or water leaching of the volatiles confirmed the effectiveness of selective volatilization at high temperatures. Taking advantage of the fact that lithium fluoride is poorly soluble in water (0.29 g / 100 mL, 20℃) while potassium fluoride and sodium fluoride are readily soluble in water, solid lithium fluoride can be directly obtained through water washing separation.

[0022] SEM-EDS images ( Figure 7-8 The feasibility of the process was verified by collecting volatiles on the silicon wafer. Since lithium could not be detected by energy dispersive spectroscopy, the volatiles contained large amounts of fluorine and potassium.

[0023] Example 3 A method for directly preparing and purifying lithium fluoride from lepidolite and fluorides: (1) Mix lepidolite and calcium fluoride at a mass ratio of 10:2, and then grind them evenly to obtain a mixture; (2) The mixture is transferred to a flue gas furnace and calcined at 1400°C in a nitrogen atmosphere until the quality is stable. Nitrogen blows the volatiles out to the flue gas furnace outlet, and the volatiles are collected by the filter membrane. (3) Dissolve the collected volatiles in water of equal mass ratio, filter and collect the precipitate to obtain high-purity lithium fluoride; (4) Evaporate the filtrate from step (3) to remove water, and obtain a mixed byproduct of potassium fluoride and sodium fluoride.

[0024] Example 4 A method for directly preparing and purifying lithium fluoride from lepidolite and fluorides: (1) Mix lepidolite and sodium fluoride at a mass ratio of 10:3, and then grind them evenly to obtain a mixture; (2) The mixture is transferred to a flue gas furnace and calcined at 1200°C in a nitrogen atmosphere until the quality is stable. Nitrogen blows the volatiles out to the flue gas furnace outlet, and the volatiles are collected by the filter membrane. (3) Dissolve the collected volatiles in water of equal mass ratio, filter and collect the precipitate to obtain high-purity lithium fluoride; (4) Evaporate the filtrate from step (3) to remove water, and obtain a mixed byproduct of potassium fluoride and sodium fluoride.

[0025] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for directly preparing and purifying lithium fluoride from lepidolite and fluorides, characterized in that, Includes the following steps: (1) Mix lepidolite with fluoride salt, and then grind them evenly to obtain a mixture; (2) The mixture is roasted in a flue gas furnace until it is stable in quality, and the volatiles are collected at the flue gas outlet through a filter membrane; (3) Dissolve the collected volatiles in water of equal mass ratio, filter and collect the precipitate to obtain high-purity lithium fluoride; (4) Evaporate the filtrate from step (3) to remove water, and obtain a mixed byproduct of potassium fluoride and sodium fluoride.

2. The method for directly preparing and purifying lithium fluoride from lepidolite and fluoride according to claim 1, characterized in that, The fluoride salt mentioned in step (1) is calcium fluoride or sodium fluoride.

3. The method for directly preparing and purifying lithium fluoride from lepidolite and fluoride according to claim 1 or 2, characterized in that, The mass ratio of lepidolite to fluoride in step (1) is 10:2-3.

4. The method for directly preparing and purifying lithium fluoride from lepidolite and fluoride according to claim 1, characterized in that, In step (2), the flue gas furnace is roasted in a nitrogen atmosphere in the absence of air, and the nitrogen blows the volatiles out to the flue gas furnace outlet, where the volatiles are collected by the filter membrane.

5. The method for directly preparing and purifying lithium fluoride from lepidolite and fluoride according to claim 1 or 4, characterized in that, The roasting temperature in step (2) is 1200-1300℃.

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