A method for purifying a fluorine-containing lithium salt

Abstract

The application provides a fluorine-containing lithium salt purification method, which comprises the following steps: S1. mixing fluorine-containing lithium salt powder with water to obtain a mixed system, adding acid to the mixed system to react, and obtaining a first lithium-containing solution; S2. adjusting the first lithium-containing solution to weak acidity, then adding a composite fluorine removal agent to the first lithium-containing solution to react, and obtaining a second lithium-containing solution; the composite fluorine removal agent comprises an aluminum fluorine removal agent, calcium hydroxide and lanthanum-loaded modified biochar, and the mass ratio of the aluminum fluorine removal agent, the calcium hydroxide and the lanthanum-loaded modified biochar is 5-8:1-3:1-5; S3. adjusting the pH of the second lithium-containing solution to neutrality or weak alkalinity to continue to react, after the reaction is completed, performing solid-liquid separation, and obtaining a third lithium-containing solution; S4. removing calcium and magnesium elements in the third lithium-containing solution to obtain a fourth lithium-containing solution; and S5. adding a lithium precipitation agent to the fourth lithium-containing solution to react, and obtaining a refined lithium salt. The method can effectively remove fluorine elements in the fluorine-containing lithium salt / fluorine-containing lithium product.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery recycling technology, specifically to a method for purifying fluorine-containing lithium salts. Background Technology

[0002] Lithium-ion batteries, with their high specific capacity, high cycle life, and safe performance, have broad application prospects and their market share is increasing year by year. However, with the rising number of used lithium-ion batteries, the problem of their disposal is becoming increasingly prominent.

[0003] Currently, the main purpose of recycling spent lithium-ion batteries is to produce lithium salts and other lithium-containing products. However, when recycling lithium salts and other lithium-containing products from spent lithium-ion batteries, problems arise because these batteries contain a large amount of fluorine, which is generally difficult to remove. This can lead to problems such as excessive fluorine content and low product purity in the final lithium salts or lithium-containing products.

[0004] Current defluorination methods primarily involve forming a fluoride-containing solution from the fluoride-containing powder, and then removing the fluoride from this solution. This method is commonly known as solution defluorination. Current solution defluorination methods mainly include precipitation, ion exchange, electrocoagulation, and adsorption. However, precipitation methods generally struggle to reduce the fluoride concentration below 20 mg / L; ion exchange is extremely costly for treating high-concentration fluoride solutions; and electrocoagulation introduces a large amount of impurities. Therefore, defluorination remains a technical challenge in the industry.

[0005] In view of this, in order to address the problem of fluorine removal in the current lithium salt or lithium-containing product preparation from lithium-ion battery recycling, a more efficient, simple and low-cost purification method needs to be developed. Summary of the Invention

[0006] To address the problems and shortcomings of existing technologies, this invention provides a method for purifying fluorinated lithium salts. This method utilizes a specific composite defluorinating agent to effectively remove fluorine from fluorinated lithium salts or fluorinated lithium products recovered from spent lithium-ion batteries, controlling the fluorine content to approximately 100 ppm. Simultaneously, this purification method effectively removes other impurities from fluorinated lithium salts or fluorinated lithium products, significantly improving the purity of these products and achieving a total lithium recovery rate of 98% or higher. Furthermore, the entire purification process does not generate organic wastewater, achieving the purification of fluorinated lithium salts or fluorinated lithium products in a more economical, efficient, and environmentally friendly manner.

[0007] This invention provides a method for purifying fluorinated lithium salts, comprising the following steps: S1. Mixing fluorinated lithium salt powder with water to obtain a mixed system, adding acid to the mixed system to react and obtain a first lithium-containing solution; S2. Adjusting the first lithium-containing solution to a weakly acidic state, then adding a composite defluorinating agent to react and obtain a second lithium-containing solution; the composite defluorinating agent includes an aluminum-based defluorinating agent, calcium hydroxide, and lanthanum-modified biochar, wherein the mass ratio of the aluminum-based defluorinating agent, calcium hydroxide, and lanthanum-modified biochar is 5~8:1~3:1~5; S3. Adjusting the pH of the second lithium-containing solution to neutral or weakly alkaline and continuing the reaction, after the reaction is completed, performing solid-liquid separation to obtain a third lithium-containing solution; S4. Removing calcium and magnesium elements from the third lithium-containing solution to obtain a fourth lithium-containing solution; S5. Adding a lithium precipitation agent to the fourth lithium-containing solution to react and obtain refined lithium salt. In S3 above, the mass ratio of aluminum-based defluorinating agent, calcium hydroxide, and lanthanum-modified biochar can be, for example, 5:3:3, 8:1:2, 7:1:5, or 8:3:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0008] The acid in S1 can fully dissolve the fluorinated lithium salt, leaching out the lithium, fluorine, and other impurity metals, which is beneficial for the subsequent removal of fluorine and other impurity metals, as well as the full recovery of lithium.

[0009] In S2, a special composite defluorinating agent, combined with a specific environment of weak acid (compared to S1), enables the composite defluorinating agent to exhibit superior fluoride precipitation and adsorption effects, which is beneficial for reducing the fluoride content in lithium-containing solutions. The defluorination principle of the composite defluorinating agent (aluminum-based defluorinating agent + calcium hydroxide + lanthanum-modified biochar) in this invention is mainly coagulation precipitation + adsorption. The aluminum-based defluorinating agent primarily functions through adsorption, and under near-neutral conditions, it hydrolyzes to produce aluminum hydroxide, simultaneously exhibiting coagulation. Calcium hydroxide reacts with fluoride ions to form calcium fluoride precipitate. Lanthanum-modified biochar has the advantages of a large specific surface area and numerous adsorption sites, resulting in stronger adsorption and excellent fluoride adsorption. The combined use of the aluminum-based defluorinating agent, calcium hydroxide, and lanthanum-modified biochar accelerates the growth and sedimentation of calcium fluoride through the trapping and adsorption bridging effects of the aluminum-based defluorinating agent and the lanthanum-modified biochar, while also enhancing the defluorination effect through the adsorption of the aluminum-based defluorinating agent and the lanthanum-modified biochar itself. Therefore, the combined use of aluminum-based defluorinating agents, calcium hydroxide, and lanthanum-loaded modified biochar can exert a synergistic effect in precipitation and adsorption. This can reduce the fluorine content in lithium-containing solutions to very low levels with a small amount of defluorinating agent, making it more economical and environmentally friendly. Furthermore, it can further improve the purity of lithium salts or lithium-containing products recovered from waste lithium-ion batteries.

[0010] It should be noted that in existing technologies, aluminum-based defluorinating agents are commonly used for defluorination. If the fluoride in the lithium-containing solution needs to be reduced to below 100 ppm, a large amount of aluminum-based defluorinating agent needs to be added (the aluminum additive is 100 to 150 times the mass of fluoride in the lithium-containing solution). However, this is not economical or environmentally friendly, and the defluorination effect is also generally poor.

[0011] In step S3, further increasing the pH of the lithium-containing solution facilitates the adsorption and precipitation of fluorine by the combined defluorinating agent, and also removes iron, copper, and aluminum from the solution. After reaction S3, solid-liquid separation yields a third lithium-containing solution and a fluorine-containing residue. The fluorine-containing residue can be further processed to recover fluorine. Step S4 further removes calcium and magnesium from the lithium-containing solution, further purifying it and improving the purity of subsequent lithium salts or lithium-containing products.

[0012] Preferably, in S1, the fluorinated lithium salt is fluorinated lithium carbonate.

[0013] Preferably, in step S1, the specific operation of adding acid to the mixed system is as follows: adjusting the pH of the mixed system to 2-4 using acid, raising the temperature to 40-60°C, and reacting for 1-2 hours to obtain a first lithium-containing solution. In step S1, the pH can be, for example, 2, 3, or 4; the reaction temperature can be, for example, 40°C, 50°C, or 60°C; and the reaction time can be, for example, 1 hour, 1.5 hours, or 2 hours. The above reaction conditions are not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0014] Preferably, in S1, the acid is sulfuric acid. Sulfuric acid can quickly and thoroughly leach various elements of lithium salts or lithium-containing products into the solution, which is more conducive to the subsequent removal of impurities and recovery of lithium-containing solutions. Moreover, compared with other acids, sulfuric acid does not introduce other impurities, which is more conducive to improving the purity of the final product.

[0015] Preferably, the sulfuric acid has a mass fraction of 98%.

[0016] Preferably, in S1, the stirring rate is maintained at 250-300 r / min during the reaction. For example, it can be 250 r / min, 270 r / min, 290 r / min, or 300 r / min, but is not limited to the listed values; other unlisted values ​​within this range are also applicable. Maintaining a low stirring rate during acid leaching allows for sufficient contact between the acid and the fluorinated lithium salt, leaching out all elements from the fluorinated lithium salt. Furthermore, it avoids excessive heat generation from excessively fast stirring, thus reducing the safety factor of the production process.

[0017] Preferably, in step S2, the amount of composite defluorinating agent added is 45 to 60 times the mass of fluorine in the first lithium-containing solution. For example, it can be 45, 50, 55, or 60 times, but is not limited to the listed values; other unlisted values ​​within this range are also applicable. Insufficient amounts of composite defluorinating agent cannot effectively reduce the fluorine content in the lithium-containing solution in a short time, and the synergistic effect of aluminum-based defluorinating agents, calcium hydroxide, and lanthanum-modified biochar is not significant. Excessive amounts of composite defluorinating agent do not significantly enhance the defluorination effect, and the large-scale use of composite defluorinating agents places a significant burden on the environment and increases treatment costs, making it neither environmentally friendly nor economical.

[0018] Preferably, in step S2, the first lithium-containing solution is treated as follows: the pH of the first lithium-containing solution is adjusted to 5-6, then a composite defluorinating agent is added, and the reaction is carried out at a temperature of 25-70°C for 0.5-1 h to obtain a second lithium-containing solution. In step S2, the pH can be, for example, 5, 5.5, or 6; the reaction temperature can be, for example, 25°C, 35°C, 45°C, 50°C, 60°C, or 70°C; and the reaction time can be, for example, 0.5 h, 0.8 h, or 1 h. The above reaction conditions are not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0019] Preferably, in S2, the aluminum-based defluorinating agent includes at least one of aluminum sulfate, nano-sized aluminum hydroxide, aluminum carbonate, and activated alumina.

[0020] Preferably, in S2, the aluminum-based defluorinating agent is a mixture of aluminum sulfate and nano-sized aluminum hydroxide. The nano-sized aluminum hydroxide acts as a seed crystal, and when used in combination with aluminum sulfate, it enhances the adsorption effect of the aluminum-based defluorinating agent, thus promoting the defluorination process.

[0021] Preferably, when the aluminum-based defluorinating agent is a mixture of aluminum sulfate and nano-sized aluminum hydroxide, the mass ratio of aluminum sulfate to nano-sized aluminum hydroxide is 4~6:1. For example, it can be 4:1, 5:1, or 6:1, but is not limited to the listed values; other unlisted values ​​within the range are also applicable.

[0022] Preferably, when the aluminum-based defluorinating agent is a mixture of aluminum sulfate and nano-sized aluminum hydroxide, the mass ratio of aluminum sulfate to nano-sized aluminum hydroxide is 5:1.

[0023] Preferably, in step S3, the first lithium-containing solution is treated as follows: the pH of the second lithium-containing solution is adjusted to 7-8, and after reacting at a temperature of 25-70°C for 0.5-1 hour, solid-liquid separation is performed to obtain the third lithium-containing solution. In step S3, the pH can be, for example, 7, 7.5, or 8; the reaction temperature can be, for example, 25°C, 35°C, 45°C, 50°C, 60°C, or 70°C; and the reaction time can be, for example, 0.5 hours, 0.8 hours, or 1 hour. The above reaction conditions are not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0024] Preferably, in step S2, a first pH adjuster is used to adjust the pH of the first lithium-containing solution to 5-6; in step S3, a second pH adjuster is used to adjust the pH of the second lithium-containing solution to 7-8; the first and second pH adjusters independently include at least one of sodium hydroxide, calcium hydroxide, and lithium hydroxide. These bases are highly alkaline, enabling rapid pH adjustment of the solution, while also avoiding the introduction of excessive other impurity elements, thus ensuring a high purity of the final product.

[0025] Preferably, in step S2, the stirring rate is maintained at 250-300 r / min during the reaction. For example, it can be 250 r / min, 270 r / min, 290 r / min, or 300 r / min, but is not limited to the listed values; other unlisted values ​​within this range are also applicable. Maintaining a low stirring rate when adding the composite defluorinating agent is beneficial for accelerating the coagulation and adsorption effects, while also preventing excessively high stirring rates from failing to fully adsorb fluorine from the lithium-containing solution, thus ensuring effective reduction of fluorine levels.

[0026] Preferably, in step S3, the stirring rate is maintained at 250-300 r / min during the reaction. For example, it can be 250 r / min, 270 r / min, 290 r / min, or 300 r / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0027] Preferably, in S4, an ion exchange resin is used to remove calcium and magnesium elements from the third lithium-containing solution.

[0028] Preferably, in step S4, the ion exchange resin is a sodium-type cation exchange resin, and the column flow rate of the third lithium-containing solution is controlled to be 3~7 bv / h. For example, it can be 3 bv / h, 4 bv / h, 5 bv / h, 6 bv / h, or 7 bv / h, but is not limited to the listed values; other unlisted values ​​within the range are also applicable. Controlling a certain column flow rate ensures that calcium and magnesium elements in the lithium-containing solution are removed quickly and completely.

[0029] Preferably, in step S5, the amount of lithium precipitation agent added is 1 to 1.2 times the mass of the fourth lithium-containing solution. For example, it can be 1, 1.1, or 1.2 times.

[0030] Preferably, the lithium precipitation agent includes at least one of sodium carbonate and sodium bicarbonate.

[0031] Preferably, the specific operation of S5 is as follows: a lithium precipitant solution is added dropwise to the fourth lithium-containing solution at a uniform rate of 5-10 mL / min, the reaction temperature is 85-100℃, and the reaction time is 1-2 h. Subsequently, the mixture is filtered, washed with water, and dried to obtain refined lithium salt. In S3, the drop rate can be, for example, 5 mL / min, 6 mL / min, 7 mL / min, 8 mL / min, 9 mL / min, or 10 mL / min; the reaction temperature can be, for example, 85℃, 90℃, 95℃, or 100℃; and the reaction time can be, for example, 1 h, 1.5 h, or 12 h. The above reaction conditions are not limited to the listed values; other unlisted values ​​within the range are also applicable. Under a certain drop rate and reaction temperature, lithium elements in the solution can be fully precipitated, and the precipitated substance contains very few impurities. During this reaction step, crude lithium salt and lithium precipitation mother liquor are obtained by filtration.

[0032] Preferably, in S5, the stirring rate is maintained at 250~300 r / min during the reaction. For example, it can be 250 r / min, 270 r / min, 290 r / min, or 300 r / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0033] Preferably, the washing temperature is 85~100℃ and the time is 0.5~1h; the drying temperature is 100~150℃ and the time is 3~6h. For example, the washing temperature can be 85℃, 90℃, 95℃, or 100℃, and the time can be 0.5h, 0.8h, or 1h; the drying temperature can be 100℃, 110℃, 120℃, 130℃, 140℃, or 150℃, and the time can be 3h, 4h, 5h, or 6h. The above reaction conditions are not limited to the listed values; other unlisted values ​​within the range are also applicable.

[0034] Preferably, during the water washing process, the stirring rate is maintained at 260~300 r / min. For example, it can be 260 r / min, 270 r / min, 290 r / min, or 300 r / min, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0035] Preferably, during the washing process, the crude lithium salt obtained from filtration is added to pure water at a solid-liquid ratio of 3 to 8:1 to form a slurry, and then the temperature is raised to 85 to 100°C for washing. The solid-liquid ratio can be, for example, 3:1, 4:1, 5:1, 6:1, 7:1, or 8:1, and the temperature can be, for example, 85°C, 90°C, 95°C, or 100°C. The reaction conditions described above are not limited to the listed values; other unlisted values ​​within the range are also applicable.

[0036] In summary, the purification method for fluorinated lithium salts provided by this invention can achieve the purification of fluorinated lithium salts more economically, efficiently, and environmentally friendly. The fluorine content can be reduced to a very low level, and other impurities such as metallic elements can be effectively removed. Therefore, the final refined lithium salt or lithium-containing product has very high purity, meeting battery-grade standards, and the lithium recovery rate is also very high. This method has practical guiding significance for the current field of lithium-ion battery recycling technology. Attached Figure Description

[0037] Figure 1 This describes the purification process of fluorinated lithium carbonate in Example 1. Detailed Implementation

[0038] To enable those skilled in the art to better understand the present invention, 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.

[0039] Example 1

[0040] The purification of fluorinated lithium salts is carried out according to the following steps, with fluorinated lithium carbonate as an example in this embodiment:

[0041] S1. Mix fluorinated lithium carbonate powder with water, then adjust the pH of the resulting mixture to 3 using 98% sulfuric acid, and heat to 50°C. React for 1.5 hours, maintaining a stirring rate of 280 r / min during the reaction to obtain the first lithium-containing solution.

[0042] S2. The pH of the first lithium-containing solution was adjusted to 5.5 using sodium hydroxide solution, and then a composite defluorinating agent was added. The reaction was carried out at 40°C for 0.75 h to obtain a second lithium-containing solution. The composite defluorinating agent includes an aluminum-based defluorinating agent, calcium hydroxide, and lanthanum-modified biochar. The mass ratio of the aluminum-based defluorinating agent, calcium hydroxide, and lanthanum-modified biochar is 6.5:2:3. The amount of composite defluorinating agent added is 50 times the mass of fluorine in the first lithium-containing solution. In this embodiment, the aluminum-based defluorinating agent is a mixture of aluminum sulfate and nano-sized aluminum hydroxide, and the mass ratio of aluminum sulfate to nano-sized aluminum hydroxide is 5:1.

[0043] S3. The pH of the second lithium-containing solution was adjusted to 7.5 using sodium hydroxide solution, and the reaction was continued at 45℃ for 0.75 h. Solid-liquid separation was then performed, with the stirring rate maintained at 280 r / min during the reaction. After the reaction was completed, solid-liquid separation was performed to obtain the third lithium-containing solution and fluorine-containing slag. Fluorine can be further recovered using the fluorine-containing slag.

[0044] S4. Use sodium-type cation exchange resin to remove calcium and magnesium elements from the third lithium-containing solution, and control the column flow rate to 5 bv / h to obtain the fourth lithium-containing solution.

[0045] S5. A lithium precipitant solution (a saturated solution of sodium carbonate or sodium bicarbonate) is added dropwise at a uniform rate to the fourth lithium-containing solution. The amount of lithium precipitant added is 1.1 times the mass of the fourth lithium-containing solution. The dropping rate is set to 7 mL / min, the reaction temperature is 93℃, the reaction time is 1.5 h, and the stirring rate is maintained at 280 r / min during the reaction. After the reaction, crude lithium carbonate and lithium precipitation mother liquor are obtained by filtration. The crude lithium salt obtained by filtration is washed as follows: pure water is added at a solid-liquid ratio of 5:1 to adjust the slurry, and then the temperature is raised to 93℃ for washing for 0.75 h. The stirring rate is maintained at 280 r / min during the washing process. After washing, the solution is filtered (here, refined lithium carbonate and washing liquid are obtained by filtration) and dried at 120℃ for 4.5 h to obtain refined lithium carbonate.

[0046] The purity and fluorine content of the refined lithium carbonate prepared in this embodiment were tested, and the specific test methods are as follows:

[0047] Purity test: The test is performed using acid-base titration, and the method is in accordance with standard GB / T 11064.1-2013;

[0048] Fluorine content test: The test is performed using the ion-selective electrode method, and the method is in accordance with the standard GB / T 11064.15-2013.

[0049] After the above tests, the purity of the refined lithium carbonate was found to be 99.56% and the fluorine content was 76.32 ppm.

[0050] In addition, the lithium recovery rate in this embodiment was calculated as follows: lithium recovery rate = 1 - lithium percentage in fluorine-containing slag. The final calculated lithium recovery rate in this embodiment was 98.75%.

[0051] Example 2

[0052] In the purification of fluorinated lithium salt in this embodiment, the difference from that in Example 1 is that in S2, the mass ratio of aluminum-based defluorinating agent, calcium hydroxide, and lanthanum-modified biochar is 5:3:4; the rest is the same as in Example 1.

[0053] The purity and fluorine content of the refined lithium carbonate prepared in this embodiment were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this embodiment was also calculated, and the calculation method is also as in Example 1. Finally, the purity of the refined lithium carbonate in this embodiment was 99.52%; the fluorine content was 74.01 ppm; and the lithium recovery rate was 98.49%.

[0054] Example 3

[0055] In the purification of fluorinated lithium salt in this embodiment, the difference from that in Example 1 is that in S2, the mass ratio of aluminum-based defluorinating agent, calcium hydroxide, and lanthanum-modified biochar is 8:1:3; the rest is the same as in Example 1.

[0056] The purity and fluorine content of the refined lithium carbonate prepared in this embodiment were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this embodiment was also calculated, and the calculation method is also as in Example 1. Finally, the purity of the refined lithium carbonate in this embodiment was 99.60%; the fluorine content was 63.33 ppm; and the lithium recovery rate was 98.26%.

[0057] Example 4

[0058] In the purification of fluorinated lithium salt in this embodiment, the difference from that in Example 1 is that in S2, the mass ratio of aluminum-based defluorinating agent, calcium hydroxide, and lanthanum-modified biochar is 6.5:3:1; the rest is the same as in Example 1.

[0059] The purity and fluorine content of the refined lithium carbonate prepared in this embodiment were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this embodiment was also calculated, and the calculation method is also as in Example 1. Finally, the purity of the refined lithium carbonate in this embodiment was 99.65%; the fluorine content was 68.90 ppm; and the lithium recovery rate was 98.41%.

[0060] Example 5

[0061] In the purification of fluorinated lithium salt in this embodiment, the difference from that in Example 1 is that in S2, the amount of composite defluorinating agent added is 40 times the mass of fluorine in the first lithium-containing solution; the rest is the same as in Example 1.

[0062] The purity and fluorine content of the refined lithium carbonate prepared in this embodiment were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this embodiment was also calculated, and the calculation method is also as in Example 1. Finally, the purity of the refined lithium salt in this embodiment was 99.52%; the fluorine content was 103.34 ppm; and the lithium recovery rate was 99.24%.

[0063] Example 6

[0064] In the purification of fluorinated lithium salt in this embodiment, the difference from that in Example 1 is that in S2, the amount of composite defluorinating agent added is 65 times the mass of fluorine in the first lithium-containing solution; the rest is the same as in Example 1.

[0065] The purity and fluorine content of the refined lithium carbonate prepared in this embodiment were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this embodiment was also calculated, and the calculation method is also as in Example 1. Finally, the purity of the refined lithium carbonate in this embodiment was 99.49%; the fluorine content was 57.25 ppm; and the lithium recovery rate was 98.03%.

[0066] Example 7

[0067] In the purification of fluorinated lithium salt in this embodiment, the difference from that in Example 1 is that in S2, the aluminum-based defluorinating agent used is nano-sized aluminum hydroxide + aluminum carbonate, and the mass ratio of nano-sized aluminum hydroxide to aluminum carbonate is 1:5; the rest is the same as in Example 1.

[0068] The purity and fluorine content of the refined lithium carbonate prepared in this embodiment were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this embodiment was also calculated, and the calculation method is also as in Example 1. Finally, the purity of the refined lithium carbonate in this embodiment was 99.51%; the fluorine content was 84.40 ppm; and the lithium recovery rate was 98.88%.

[0069] Example 8

[0070] In the purification of fluorinated lithium salt in this embodiment, the difference from that in Example 1 is that, in S2, the aluminum-based defluorinating agent used is activated alumina; the rest is the same as in Example 1.

[0071] The purity and fluorine content of the refined lithium carbonate prepared in this embodiment were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this embodiment was also calculated, and the calculation method is also as in Example 1. Finally, the purity of the refined lithium carbonate in this embodiment was 99.53%; the fluorine content was 95.41 ppm; and the lithium recovery rate was 99.00%.

[0072] Example 9

[0073] In the purification of fluorinated lithium salt in this embodiment, the difference from that in Example 1 is that in S2, the aluminum-based defluorinating agent used is aluminum sulfate + activated alumina, and the mass ratio of aluminum sulfate to activated alumina is 1:1; the rest is the same as in Example 1.

[0074] The purity and fluorine content of the refined lithium carbonate prepared in this embodiment were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this embodiment was also calculated, and the calculation method is also as in Example 1. Finally, the purity of the refined lithium carbonate in this embodiment was 99.51%; the fluorine content was 81.20 ppm; and the lithium recovery rate was 98.11%.

[0075] Comparative Example 1

[0076] In the purification of the fluorinated lithium salt in this comparative example, the difference from Example 1 is that in S2, the mass ratio of the aluminum-based defluorinating agent, calcium hydroxide, and lanthanum-modified biochar used is 3:2:3, while the rest is the same as in Example 1.

[0077] The purity and fluorine content of the refined lithium carbonate prepared in this comparative example were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this comparative example was also calculated, following the same calculation method as in Example 1. Finally, the purity of the refined lithium carbonate in this comparative example was 99.61%; the fluorine content was 145.96 ppm; and the lithium recovery rate was 98.71%.

[0078] Comparative Example 2

[0079] The difference between this comparative example and Example 1 in the purification of fluorinated lithium salt is that, in S2, the mass ratio of aluminum-based defluorinating agent, calcium hydroxide, and lanthanum-modified biochar is 6.5:4:7; the rest is the same as in Example 1.

[0080] The purity and fluorine content of the refined lithium carbonate prepared in this comparative example were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this comparative example was also calculated, following the same calculation method as in Example 1. Finally, the purity of the refined lithium carbonate in this comparative example was 99.48%; the fluorine content was 122.38 ppm; and the lithium recovery rate was 98.13%.

[0081] Comparative Example 3

[0082] In the purification of the fluorinated lithium salt in this comparative example, the difference from Example 1 is that in S2, the composite defluorinating agent is replaced with a separate aluminum-based defluorinating agent; the rest is the same as in Example 1.

[0083] The purity and fluorine content of the refined lithium carbonate prepared in this comparative example were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this comparative example was also calculated, following the same calculation method as in Example 1. Finally, the purity of the refined lithium carbonate in this comparative example was 99.51%; the fluorine content was 393.79 ppm; and the lithium recovery rate was 97.89%.

[0084] Comparative Example 4

[0085] In the purification of the fluorinated lithium salt in this comparative example, the difference from Example 1 is that in S2, the composite defluorinating agent is replaced with a separate aluminum-based defluorinating agent, and the amount of aluminum-based defluorinating agent added is 100 times the mass of fluorine in the first lithium-containing solution; the rest is the same as in Example 1.

[0086] The purity and fluorine content of the refined lithium carbonate prepared in this comparative example were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this comparative example was also calculated, following the same calculation method as in Example 1. Finally, the purity of the refined lithium carbonate in this comparative example was 99.64%; the fluorine content was 66.35 ppm; and the lithium recovery rate was 93.77%.

[0087] Comparative Example 5

[0088] The purification of fluorinated lithium salt in this comparative example differs from that in Example 1 in that the composite defluorinating agent used in S2 does not contain calcium hydroxide, and the mass ratio of aluminum-based defluorinating agent to lanthanum-modified biochar is 6.5:3; the rest is the same as in Example 1.

[0089] The purity and fluorine content of the refined lithium carbonate prepared in this comparative example were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this comparative example was also calculated, following the same calculation method as in Example 1. Finally, the purity of the refined lithium carbonate in this comparative example was 99.61%; the fluorine content was 276.51 ppm; and the lithium recovery rate was 97.90%.

[0090] Comparative Example 6

[0091] In the purification of the fluorinated lithium salt in this comparative example, the difference from Example 1 is that, in S2, the composite defluorinating agent used does not contain lanthanum-modified biochar, and the mass ratio of aluminum-based defluorinating agent to calcium hydroxide is 6.5:2; the rest is the same as in Example 1.

[0092] The purity and fluorine content of the refined lithium carbonate prepared in this comparative example were tested, and the specific testing methods are as follows (refer to Example 1). The lithium recovery rate in this comparative example was also calculated, following the same calculation method as in Example 1. Finally, the purity of the refined lithium carbonate in this comparative example was 99.53%; the fluorine content was 231.09 ppm; and the lithium recovery rate was 97.83%.

[0093] Test Result Analysis

[0094] The purity, fluorine content, and lithium recovery rate of refined lithium carbonate in all the above embodiments and comparative examples were statistically analyzed, and the results are shown in Table 1.

[0095] Table 1. Statistical results of purity, fluorine content, and lithium recovery rate of refined lithium carbonate in the examples and comparative examples.

[0096]

[0097] As shown in Table 1, the purification method provided by this invention can effectively reduce the fluorine content in fluorinated lithium salts, such as fluorinated lithium carbonate, to about 100 ppm. Furthermore, this purification method can remove other impurity elements from fluorinated lithium salts or fluorinated lithium products, greatly improving the purity of these products and enabling the total lithium recovery rate to reach 98% or higher. For details, please refer to Examples 1 to 9.

[0098] In Comparative Examples 1 and 2, the mass ratios of aluminum-based defluorinating agent, calcium hydroxide, and lanthanum-modified biochar were not within the range of 5-8:1-3:1-5, resulting in a relatively poor synergistic effect of these three substances and ultimately leading to an increased fluorine content in the refined lithium carbonate. Comparative Examples 3 and 4 both contained only aluminum-based defluorinating agents. The dosage in Comparative Example 3 was the same as in Example 1, while the dosage in Comparative Example 4 was much higher than in Example 1. Consequently, the fluorine content in the refined lithium carbonate of Comparative Example 3 was very high, while the aluminum recovery rate in Comparative Example 4 was significantly reduced. This indicates that when using only aluminum-based defluorinating agents, a small dosage is insufficient for effective defluorination, while an excessive dosage leads to a decrease in lithium recovery. Comparative Example 5's composite defluorinating agent did not contain calcium hydroxide, and Comparative Example 6's composite defluorinating agent did not contain lanthanum-modified biochar, both resulting in excessively high fluorine content in the refined lithium carbonate, i.e., a significantly reduced fluorine removal effect. This suggests that only through the combined action of aluminum-based defluorinating agents, calcium hydroxide, and lanthanum-modified biochar can the defluorination effect be further improved.

[0099] Further comparison of Example 1 with Examples 5-6 revealed that the amount of composite defluorinating agent used in Examples 5 and 6 was too small and too large, respectively. Too small an amount resulted in a higher fluorine content in the refined lithium carbonate, while too large an amount led to a decrease in lithium recovery rate.

[0100] Comparing Example 1 with Examples 7-9, the types of aluminum-based defluorinating agents in Examples 7-9 are different from those in Example 1, and the fluorine content in the refined carbonate is higher than that in Example 1. This indicates that the type of aluminum-based defluorinating agent will also affect the defluorination effect.

[0101] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention, but such modifications or substitutions are all within the scope of protection of the present invention.

Claims

1. A purification method of a fluorine-containing lithium salt, characterized by, Fluorinated lithium salts are fluorinated lithium carbonates, and the process includes the following steps: S1. Mix fluorinated lithium salt powder with water to obtain a mixed system, add acid to the mixed system to adjust the pH of the mixed system to 2-4, and obtain a first lithium-containing solution; S2. Adjust the pH of the first lithium-containing solution to 5-6, then add a composite defluorinating agent to react and obtain a second lithium-containing solution; the composite defluorinating agent includes an aluminum-based defluorinating agent, calcium hydroxide, and lanthanum-loaded modified biochar, the mass ratio of the aluminum-based defluorinating agent, the calcium hydroxide, and the lanthanum-loaded modified biochar is 5-8:1-3:1-5, the aluminum-based defluorinating agent is a mixture of aluminum sulfate and nano-sized aluminum hydroxide, the mass ratio of the aluminum sulfate and the nano-sized aluminum hydroxide is 4-6:1; S3. Adjust the pH of the second lithium-containing solution to 7-8 and continue the reaction. After the reaction is completed, perform solid-liquid separation to obtain the third lithium-containing solution. S4. Remove calcium and magnesium elements from the third lithium-containing solution to obtain a fourth lithium-containing solution; S5. Add a lithium precipitating agent to the fourth lithium-containing solution to react and obtain refined lithium salt.

2. The purification method of the fluorine-containing lithium salt according to claim 1, characterized by, In step S1, the specific operation of adding acid to the mixed system further includes: heating the mixed system to 40~60℃ and reacting for 1~2 hours to obtain the first lithium-containing solution.

3. The purification method of the fluorine-containing lithium salt according to claim 1, wherein In step S2, the first lithium-containing solution is treated as follows: after adding the composite defluorinating agent, the solution is reacted at a temperature of 25~70℃ for 0.5~1h to obtain the second lithium-containing solution.

4. The purification method of the fluorine-containing lithium salt according to claim 1, characterized by: In step S2, the amount of the composite defluorinating agent added is 45 to 60 times the mass of fluorine in the first lithium-containing solution.

5. The purification method of the fluorine-containing lithium salt according to claim 1, wherein In step S3, the first lithium-containing solution is treated as follows: after reacting at a temperature of 25~70℃ for 0.5~1h, solid-liquid separation is performed to obtain the third lithium-containing solution.

6. The purification method of the fluorine-containing lithium salt according to claim 1, characterized by: In step S5, the amount of lithium precipitation agent added is 1 to 1.2 times the mass of the fourth lithium-containing solution.

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