Method for removing fluorine from waste lithium iron phosphate leaching solution

Abstract

The application discloses a method for removing fluorine from waste and old lithium iron phosphate leaching solution, which comprises the following steps: S1, oxidizing and roasting waste and old lithium iron phosphate battery black powder at high temperature to obtain a roasting product; S2, mixing the roasting product with a dilute acid solution, and leaching to obtain a leaching solution; S3, adding a reducing agent to the leaching solution to remove copper; S4, filtering to obtain a first filtrate, adjusting the pH of the first filtrate to precipitate iron, and obtaining phosphorus iron and a second filtrate; and S5, adding OH ‑ and Na + to the second filtrate, controlling the pH to be 3-7, and aging after a precipitation reaction to obtain a purified solution and a sodium hexafluoroaluminate filter residue. The application realizes deep fluorine removal from waste and old lithium iron phosphate battery leaching solution, and plays an important role in subsequent preparation of battery-grade lithium salt and other products. In addition, the process is simple in operation, short in period, and the generated crude sodium hexafluoroaluminate can be applied to the field of aluminum smelting, has high economic benefits, and has wide industrial application prospects.

Description

Technical Field

[0001] This invention relates to the field of waste lithium-ion battery technology, and in particular to a method for removing fluoride from waste lithium iron phosphate leachate. Background Technology

[0002] Lithium-ion batteries are widely used in consumer electronics, medical devices, electric vehicles, industry, aerospace and defense, and energy storage due to their high energy density, long cycle life, lack of memory effect, high rated voltage, and low self-discharge rate. However, lithium batteries have a limited lifespan, making the green disposal of used lithium batteries a pressing issue.

[0003] Due to the variety and complexity of lithium-ion batteries, the current pretreatment of waste lithium-ion batteries mainly involves steps such as discharge, disassembly, crushing, and sieving. Some aluminum foil and electrolyte inevitably mix with the positive electrode active material. Therefore, during the leaching process, fluorine and aluminum will dissolve along with the target metal components. The presence of fluorine will cause corrosion to the equipment and shorten its service life. The content of fluorine and aluminum in the positive electrode material of the battery terminal must be controlled at the ppm level.

[0004] Currently, fluoride removal is commonly achieved through precipitation and adsorption methods. For example, patent applications CN106745626A, CN105329974A, CN105905933A, and CN110078109A all involve introducing calcium for fluoride removal. This method consumes a significant amount of calcium salt and introduces new impurities (calcium ions) into the solution. Furthermore, when sulfate ions are present in the solution, calcium sulfate can coat the surface of the target product, affecting its quality. Patent CN106268613A discloses a fluoride removal agent, which is a hybrid material of anion exchange resin and lithium and aluminum. Lithium and aluminum are deposited within the anion exchange resin as hydroxides, forming a layered structure. Patent application CN108191118A discloses a resin adsorption method for fluoride removal, separating fluoride ions in the filtrate through resin adsorption and then recovering them as calcium fluoride precipitation. While this method reduces the amount of alkali used, the high cost of the resin makes the fluoride removal process costly.

[0005] In response to the aforementioned problems, it is necessary to develop new defluorination processes. Summary of the Invention

[0006] This invention aims to at least solve one of the technical problems existing in the prior art. To this end, this invention proposes a method for removing fluoride from waste lithium iron phosphate leachate, which is low-cost and does not require the introduction of impurity removal agents containing calcium or fluorine that affect product quality or damage equipment life.

[0007] According to one aspect of the present invention, a method for removing fluoride from waste lithium iron phosphate leachate is provided, comprising the following steps:

[0008] S1. Oxidize and roast the waste lithium iron phosphate battery black powder at high temperature to obtain the roasted product;

[0009] S2. Mix the calcined product with a dilute acid solution and leach it to obtain a leachate;

[0010] S3. Add a reducing agent to the leachate to remove copper;

[0011] S4. Filter to obtain the first filtrate, adjust the pH of the first filtrate to precipitate iron, and obtain ferric phosphate and the second filtrate.

[0012] S5. Add OH to the second filtrate. - And Na + The pH was controlled at 3-7. After precipitation, the purified liquid and sodium hexafluoroaluminate filter residue were obtained by aging.

[0013] The high temperature mentioned refers to a temperature of 900–1500℃;

[0014] The leaching includes the following conditions: 1) H in the dilute acid solution + The concentration of the leaching agent is 0.05–1.5 mol / L; the temperature is 30–90℃; the time is 30–120 min; and the leaching process is carried out under stirring at a speed of 300–600 rpm.

[0015] The precipitation reaction temperature is above 30℃, the reaction time is 10 to 180 min, and the aging time is above 30 min.

[0016] According to a preferred embodiment of the present invention, at least the following beneficial effects are achieved: The present invention first removes fluoride through high-temperature oxidation roasting. During the oxidation roasting process, most of the fluoride evaporates in the form of vapor, with only a small amount remaining in the solid powder as lithium fluoride, greatly reducing the pressure of subsequent fluoride removal. Simultaneously, during the high-temperature oxidation roasting process, the crystal form of the impurity aluminum (alumina and its alloyed state) in the lithium iron phosphate battery powder can be transformed into α-alumina. This alumina is sparingly soluble in water, acid, and alkaline solutions, ensuring that only a small amount of aluminum dissolves during leaching, with the majority remaining in the leaching residue. This significantly reduces the probability of aluminum forming spinel with other components. Furthermore, the high-temperature oxidation roasting effectively destroys the olivine-like structure of the lithium iron phosphate. The structure oxidizes iron to a high valence state, making it difficult to dissolve, while making lithium components easily soluble. Highly selective leaching of the target element lithium can be achieved using only dilute acid, while suppressing the leaching of most iron and phosphorus. Through the oxidation roasting and leaching process of this invention, the concentration of aluminum ions can be controlled below 1 g / L. Deep defluorination is achieved directly using the leaching purification mother liquor as the experimental object, without introducing impurity removal reagents that could damage equipment or products. Only simple sodium and hydroxide ions are needed. By controlling the pH value, reaction temperature, aging time, and other conditions of the solution, deep defluorination of the leaching purification liquor is achieved, greatly increasing the service life of equipment and reducing the treatment cost of fluoride-containing wastewater. In addition, crude sodium hexafluoroaluminate can be obtained as a byproduct, which can be used in the aluminum smelting industry. The defluorination method of this invention is low-cost, simple to operate, and does not negatively affect equipment or products. Furthermore, the obtained sodium hexafluoroaluminate has high economic value and good economic benefits.

[0017] A calcination temperature above 900℃ can remove more than 80% of fluorine and organic substances such as dioxins. At the same time, the increased temperature is conducive to the transformation of the crystal form of the positive electrode current collector aluminum (mainly existing in the oxidized state) mixed in the positive electrode powder into the α-type. This crystal form is difficult to dissolve in strong acids and strong bases, thus helping to reduce the dissolution of impurity aluminum during the subsequent leaching process.

[0018] In some embodiments of the present invention, the precipitation reaction temperature is above 30 to 90°C, the reaction time is 10 to 180 min, and the aging time is 30 to 300 min.

[0019] In some embodiments of the present invention, the precipitation reaction temperature is above 30 to 80°C, the reaction time is 10 to 180 min, and the aging time is 30 to 150 min.

[0020] In some embodiments of the present invention, the precipitation reaction temperature is above 30-50°C, the reaction time is 10-180 min, and the aging time is 30-60 min. Controlling the precipitation reaction temperature below 50°C and the aging time below 60 min ensures the recovery effect while also saving energy and achieving higher economic benefits.

[0021] In some embodiments of the present invention, the high-temperature calcination further includes at least one of the following conditions: 1) heating rate of 5 to 10 min / L; 2) holding time of 60 to 360 min; 3) the high temperature refers to a temperature of 900 to 1350 °C; 4) the calcination atmosphere is air or oxygen.

[0022] In some embodiments of the present invention, the high-temperature calcination further includes at least one of the following conditions: 1) heating rate of 8-10 min / L; 2) holding time of 120-360 min; 3) the high temperature refers to a temperature of 900-1250℃.

[0023] In some embodiments of the present invention, the contents of each component of the waste lithium iron phosphate battery black powder are as follows: Li: 2.0%–3.6%, P: 9.2%–15.2%, Fe: 17.8%–24.5%, Al: 0.05%–2.5%, Cu: 0.2%–6.5%, Ca: 0.05%–0.15%, Mg: 0.001%–0.02%.

[0024] In some embodiments of the present invention, the pretreatment process for waste lithium iron phosphate battery black powder includes operations such as manual screening, capacity testing, capacity separation, water washing, crushing, dismantling, crushing, magnetic separation, air separation, and dust collection.

[0025] In some embodiments of the present invention, the particle size of the crushed material is at the micrometer level.

[0026] In some embodiments of the present invention, the particle size of the crushed material is between 80 and 200 μm. At the atomic level, this is approximately 0.1 nm. Therefore, during the calcination process, the aluminum current collector and lithium iron phosphate powder are not mixed at the atomic level, reducing the probability of spinel formation and allowing them to largely separate into small particles.

[0027] In some embodiments of the present invention, the particle size of the crushed material is between 80 and 160 μm.

[0028] In some embodiments of the present invention, the leaching further includes the following conditions: a liquid-to-solid ratio of 3 to 20 mL / g.

[0029] In some embodiments of the present invention, the leaching includes at least one of the following conditions: 1) H in a dilute acid solution +The concentration of the solvent is 0.1–1 mol / L; the temperature is 30–50℃; the liquid-to-solid ratio is 3–5 mL / g; the time is 60–90 min; and the leaching process is carried out under stirring at a speed of 400–600 rpm.

[0030] In some embodiments of the present invention, the concentration of aluminum ions in the leachate is 0.1–1 g / L. The leaching process of the present invention can achieve an aluminum ion content below 1 g / L.

[0031] In some embodiments of the present invention, the concentration of aluminum in the leachate is 0.5 to 0.8 g / L.

[0032] In some embodiments of the present invention, the reducing agent is iron powder, and the amount of iron powder added is 1 to 1.8 times the theoretical molar amount. Iron powder or other reducing agents can be used as the reducing agent. Using iron powder avoids the introduction of impurity ions.

[0033] In some embodiments of the present invention, the amount of iron powder added is 1.0 to 1.5 times the theoretical molar amount.

[0034] In some embodiments of the present invention, the copper removal reaction temperature is 25–75°C.

[0035] In some embodiments of the present invention, the iron precipitation process includes at least one of the following conditions: 1) the pH of the first filtrate is adjusted to 1.5 to 2.0; 2) the reaction time is 30 to 120 min.

[0036] In some embodiments of the present invention, the iron deposition reaction time is 30 to 60 minutes.

[0037] In some embodiments of the present invention, the purification liquid is the filtrate obtained after waste lithium iron phosphate battery black powder has undergone oxidation roasting, dilute acid leaching, copper removal, and precipitation of iron phosphate.

[0038] In some embodiments of the present invention, the pH adjusting solution is at least one of sodium hydroxide, sodium carbonate, and sodium bicarbonate.

[0039] In some embodiments of the present invention, the precipitation reaction temperature is 30–60°C and the reaction time is 10–60 min.

[0040] In some embodiments of the present invention, the main equation for the precipitation reaction is as follows:

[0041] (1)

[0042] (2)

[0043] (3)

[0044] (4)

[0045] (5)

[0046] (6)

[0047] (7)

[0048] (8)

[0049] (9)

[0050] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. Attached Figure Description

[0051] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0052] Figure 1 This is a flowchart of the defluorination method for leachate from waste lithium iron phosphate batteries in an embodiment of the present invention.

[0053] Figure 2 This is a graph showing the relationship between F-Al groups and pH.

[0054] Figure 3 The image shows the XRD characterization of the filter residue obtained in Example 1 of this invention. Detailed Implementation

[0055] The following will clearly and completely describe the concept and technical effects of the present invention in conjunction with embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention. Unless otherwise specified, the experimental methods used in the embodiments are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available. Unless otherwise specified, the same parameter value is the same in all embodiments. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0056] In the description of this invention, the terms "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0057] The waste lithium iron phosphate battery black powder (particle size D) used in the embodiments and comparative experiments of this invention 50 The main components and contents of particles with a diameter of 120 μm and a particle size between 80 and 160 μm are shown in Table 1 below:

[0058] Table 1

[0059] element Fe P Li Al Cu Ca Mg F wt% 19.82 11.87 3.5 2.36 4.05 0.05 0.010 1.59

[0060] Example 1

[0061] This embodiment describes a method for removing fluoride from leachate of spent lithium iron phosphate batteries, such as... Figure 1 As shown, the specific process is as follows:

[0062] Weigh 500g of waste lithium iron phosphate battery black powder, spread it evenly in a magnetic boat, place it in a muffle furnace, and calcine it in an oxygen atmosphere. The calcination temperature is set to 1250℃, the heating rate is 10℃ / min, and the temperature is maintained for 4h to obtain the calcined product.

[0063] 200g of the above-mentioned roasted product was weighed and subjected to full-component leaching under the following conditions: sulfuric acid concentration of 0.25mol / L, liquid-to-solid ratio of 10mL / g, leaching temperature of 60℃, stirring speed of 600r / min, and reaction time of 90min. After leaching, the product was filtered to obtain the leachate (which, upon analysis, contained Fe 8.52g / L, P 6.23g / L, Li 3.48g / L, Al 0.31g / L, F 0.64g / L, Na 0.15g / L, Cu 3.95g / L, Ca 0.01g / L, and Mg 0.002g / L).

[0064] Add 9.56g of iron powder to the leachate, react for 30min, and filter under pressure to obtain the first filtrate (the first filtrate contained Fe 12.5g / L, P 6.21g / L, Li 3.47g / L, Al 0.29g / L, F 0.64g / L, Na 0.18g / L, Cu 0g / L, Ca 0.008g / L, and Mg 0.0019g / L).

[0065] The pH of the first filtrate was adjusted to 2.0 to carry out the precipitation of ferric phosphate. After the pH reached 2, the reaction was carried out at room temperature (30±5℃ is acceptable) for 30 min. The mixture was then filtered to obtain the second filtrate (the second filtrate contained Fe 0.25 g / L, P 0.18 g / L, Li 3.44 g / L, Al 0.28 g / L, F 0.62 g / L, Na 4.6 g / L, Ca 0.005 g / L, and Mg 0.0012 g / L).

[0066] Take 400 mL of the second filtrate, adjust the pH of the mother liquor to 3.0 with 15 wt% NaOH solution, react at 50℃ for 1 h, let stand, age for 1 h, then filter to separate, obtaining purified liquid and filter residue. Analyze the concentration of the purified liquid (lithium purified liquid, tested to be Fe 0.21 g / L, P 0.015 g / L, Li 3.40 g / L, Al 0.23 g / L, F 0.56 g / L, Na 5.81 g / L, Ca 0.0001 g / L, Mg 0.0001 g / L). The filter residue contains Al 5.51%, F 25.82%, Na 45.07%, Li 2.98%, and other compounds 20.62%.

[0067] Example 2

[0068] This embodiment is a method for removing fluoride from the leachate of waste lithium iron phosphate batteries. The process of preparing the leachate into the second filtrate is exactly the same as in Example 1, except that the purification liquid operation step is to adjust the pH of the second filtrate to 5.0.

[0069] The purified solution contained Fe 0.03 g / L, P 0.013 g / L, Li 3.35 g / L, Al 0.12 g / L, F 0.40 g / L, Na 19.59 g / L, Ca 0.0001 g / L, and Mg 0.0001 g / L. The filter residue contained Al 9.14%, F 34.35%, Na 48.14%, Li 1.67%, and other components 6.7%.

[0070] Example 3

[0071] This embodiment is a method for removing fluoride from the leachate of waste lithium iron phosphate batteries. The process of preparing the leachate into the second filtrate is exactly the same as in Example 1, except that the purification liquid operation step is to adjust the pH of the second filtrate to 7.0.

[0072] The purified solution contained 0.0013 g / L Fe, 0.010 g / L P, 3.30 g / L Li, 0.003 g / L Al, 0.025 g / L F, 7.35 g / L Na, 0.0001 g / L Ca, and 0.0001 g / L Mg. The filter residue contained 11.39% Al, 52.98% F, 35.14% Na, 0.46% Li, and 0.03% other components.

[0073] Example 4

[0074] This embodiment is a method for removing fluoride from the leachate of waste lithium iron phosphate batteries. The process of preparing the leachate into the second filtrate is exactly the same as in Example 1, except that the operation steps for the purification liquid are to adjust the pH of the second filtrate to 9.0.

[0075] The purified solution contained Fe 0.0015 g / L, P 0.03 g / L, Li 3.25 g / L, Al 0.0015 g / L, F 0.12 g / L, Na 18.5 g / L, Ca 0.0001 g / L, and Mg 0.0001 g / L. The filter residue contained Al 10.54%, F 44.05%, Na 43.24%, Li 1.15%, and other components 1.02%.

[0076] Example 5

[0077] This comparative example provides a method for removing fluoride from leachate of waste lithium iron phosphate batteries. The preparation of the leachate to the second filtrate is exactly the same as in Example 3, except that the purification liquid operation steps are different. The only difference is that the precipitation reaction temperature is 90°C.

[0078] The purified solution contained Fe 0.0010 g / L, P 0.007 g / L, Li 3.25 g / L, Al 0.001 g / L, F 0.12 g / L, Na 6.55 g / L, Ca 0.0001 g / L, and Mg 0.0001 g / L. The filter residue contained Al 11.43%, F 50.98%, Na 35.86%, Li 1.30%, and other components 0.43%.

[0079] Example 6

[0080] This comparative example provides a method for removing fluoride from leachate of waste lithium iron phosphate batteries. The preparation of the leachate to the second filtrate is exactly the same as in Example 3, except that the operation steps of the purification solution are different. The only difference is that the aging time is 3 hours.

[0081] The purified solution contained 0.0010 g / L Fe, 0.005 g / L P, 3.25 g / L Li, 0.012 g / L Al, 0.032 g / L F, 7.58 g / L Na, 0.0001 g / L Ca, and 0.0001 g / L Mg. The filter residue contained 10.16% Al, 52.03% F, 37.20% Na, 0.59% Li, and 0.02% other components.

[0082] Comparative Example 1

[0083] This comparative example provides a method for removing fluoride from leachate of waste lithium iron phosphate batteries. The process of preparing the leachate to the second filtrate is exactly the same as in Example 1, except that no oxidative roasting is performed before leaching.

[0084] The test results showed that the contents of the target elements iron, phosphorus, lithium, aluminum and fluorine in the leachate were 15.68 g / L, 20.60 g / L, 3.95 g / L, 1.35 g / L and 1.10 g / L, respectively.

[0085] Comparative Example 2

[0086] This comparative example provides a method for removing fluoride from leachate of waste lithium iron phosphate batteries. The process of preparing the leachate into the second filtrate is exactly the same as in Example 1, except that the calcination temperature is 800°C.

[0087] The leachate was tested and found to contain Fe 11.53 g / L, P 8.96 g / L, Li 3.15 g / L, Al 1.07 g / L, F 0.98 g / L, Na 0.10 g / L, Cu 3.61 g / L, Ca 0.01 g / L, and Mg 0.001 g / L.

[0088] Comparative Example 3

[0089] This comparative example provides a method for removing fluoride from leachate of waste lithium iron phosphate batteries. The process of preparing the leachate into the second filtrate is exactly the same as in Example 1, except that the leaching temperature is 25°C during the leaching process of the roasted product.

[0090] The test results showed that the contents of the target elements iron, phosphorus, lithium, aluminum and fluorine in the leachate were 5.85 g / L, 4.56 g / L, 2.56 g / L, 0.12 g / L and 0.85 g / L, respectively.

[0091] Comparative Example 4

[0092] This comparative example provides a method for removing fluoride from leachate of waste lithium iron phosphate batteries. The process of preparing the leachate into the second filtrate is exactly the same as in Example 1, except that the reaction time is 180 min during the leaching process.

[0093] The test results showed that the contents of the target elements iron, phosphorus, lithium, aluminum and fluorine in the leachate were 16.05 g / L, 10.13 g / L, 3.42 g / L, 1.08 g / L and 0.98 g / L, respectively.

[0094] Comparative Example 5

[0095] This comparative example provides a method for removing fluoride from leachate of waste lithium iron phosphate batteries. The process of preparing the leachate into the second filtrate is exactly the same as in Example 1, except that the sulfuric acid concentration is 1.0 mol / L during the leaching process.

[0096] The results showed that the contents of the target elements iron, phosphorus, lithium, aluminum and fluorine in the leachate were 18.68 g / L, 11.52 g / L, 3.49 g / L, 1.97 g / L and 1.2 g / L, respectively.

[0097] Comparative Example 6

[0098] This comparative example provides a method for removing fluoride from leachate of waste lithium iron phosphate batteries. The process of preparing the leachate into the second filtrate is exactly the same as in Example 1, except that the stirring frequency is 200 r / min during the leaching process.

[0099] The results showed that the contents of the target elements iron, phosphorus, lithium, aluminum and fluorine in the leachate were 3.05 g / L, 4.37 g / L, 2.1 g / L, 0.11 g / L and 0.48 g / L, respectively.

[0100] As can be seen from Example 1 and Comparative Examples 1-6, after high-temperature calcination, most of the fluorine in lithium iron phosphate battery powder can be effectively removed beforehand. Subsequent leaching processes can effectively suppress the leaching of iron, phosphorus, and fluorine, especially significantly reducing the leaching of aluminum impurities, possibly related to the crystal transformation of alumina during calcination. Furthermore, after high-temperature oxidative calcination, iron is oxidized to a higher valence state, thus low-concentration acid and room-temperature reactions can reduce the leaching of iron and phosphorus. Conversely, because lithium is uniformly embedded in the iron phosphate lattice, high-temperature calcination allows the lithium component to dissolve quickly under low acid conditions and at room temperature. Comparative Example 3 shows that as the leaching temperature of the calcined material decreases, the chemical reaction kinetics slow down. Although the leaching rate of aluminum impurities remains low, the leaching rates of other components also decrease significantly, such as valuable elements iron, phosphorus, lithium, aluminum, and fluorine, resulting in substantial resource waste. Simultaneously, the fluorine-aluminum molar ratio exceeds 10, making direct recovery in the form of sodium fluoroaluminate difficult. As can be seen from Comparative Example 6, the stirring rate has a significant impact on the leaching efficiency of the target element. The leaching process is usually a core shrinking model, and the stirring rate has a significant impact on the solid-liquid mass transfer effect. When the stirring rate is low, the leaching rate of each material is also low.

[0101] Furthermore, based on the detection results of Examples 1-4 and Figure 2 The effect of pH on F-Al groups shows that, under conditions of low initial mother liquor pH, the main F-Al complexing group in the solution is AlF. 2+ AlF2 + As the pH of the reaction system increases, the F-Al groups in the solution react to form AlF. x (OH) 3-x · y H2O. As the amount of liquid alkali added continues to increase, the concentration of sodium ions in the solution increases, AlF5... 2- Converted to AlF6 3- Under the influence of sodium ions, the reaction gradually produces Na3AlF6 (cryolite). The initial fluorine-to-aluminum molar ratio in the third filtrate was 3.15, while the fluorine-to-aluminum molar ratio in cryolite was 6. This means that after fluorine precipitates in the form of cryolite, a large amount of aluminum cannot precipitate in the same form. As the pH increases from 3 to 7, the aluminum concentration decreases from 0.23 g / L to 0.0015 g / L, and aluminum ions precipitate as Al(OH)3. Amorphous Al(OH)3 is generated simultaneously with the formation of cryolite. It can also be observed that the fluoride ion concentration in the defluorination mother liquor increases significantly with increasing pH. This is because excessively high pH causes cryolite to dissolve back. Therefore, pH 7 was selected as the optimal defluorination condition, with a fluorine content of 25 ppm in the purified solution.

[0102] As can be seen from Example 5, the solubility of cryolite increases with increasing temperature. Under high-temperature conditions, a trace amount of cryolite underwent back-dissolution. However, high temperatures require more energy; therefore, it is preferable to control the temperature below 50°C.

[0103] As can be seen from Example 6, the residual fluoride concentration in the defluorination mother liquor first decreases and then increases with the extension of aging time. This may be because the increased aging time leads to the redissolution of a small amount of precipitate, resulting in an increase in fluoride concentration. Therefore, it can be seen that when the aging time is extended to 180 min, trace amounts of fluoride in the precipitate will redissolve, causing a certain increase in the fluoride-aluminum molar ratio, which has a certain impact on the recovery effect and is also detrimental to overall economic efficiency. Therefore, an aging time of less than 3 h is preferred.

[0104] The filter residues obtained in each embodiment were characterized by XRD, wherein the results of Example 1 are as follows: Figure 3 As shown in the figure, the present invention yielded relatively pure sodium hexafluoroaluminate. The test results of other embodiments are similar, and to avoid redundancy, they are not shown individually.

[0105] This invention controls aluminum concentration below 1 g / L, achieving fluoride removal and aluminum conversion through high-temperature oxidation calcination. Simultaneously, dioxins are also removed. The synergistic effect of the preceding steps ensures a fluoride-to-aluminum molar ratio greater than 1, and the introduction of sodium ions effectively removes both fluoride and aluminum. Compared to conventional methods involving the addition of sodium fluoride, this invention avoids the problem of excessive fluoride due to the difficulty in controlling sodium fluoride content, which can significantly impact equipment and increase lithium loss. Most fluoride exists in the form of cryolite, and aluminum also exists as aluminum hydroxide. This invention achieves deep fluoride removal from leachate from spent lithium iron phosphate batteries, playing a crucial role in the subsequent preparation of battery-grade lithium salts and other products. Furthermore, the process is simple to operate, has a short cycle time, and the generated crude sodium hexafluoroaluminate can be used in aluminum smelting and other fields, offering high economic benefits and broad industrial application prospects.

[0106] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for removing fluoride from waste lithium iron phosphate leachate, characterized in that: Includes the following steps: S1. Oxidize and roast the waste lithium iron phosphate battery black powder at high temperature to obtain the roasted product; S2. Mix the calcined product with a dilute acid solution and leach it to obtain a leachate; S3. Add a reducing agent to the leachate to remove copper; S4. Filter to obtain the first filtrate, adjust the pH of the first filtrate to precipitate iron, and obtain ferric phosphate and the second filtrate. S5. Add OH to the second filtrate. - And Na + Control the pH to 3-7, and after precipitation reaction, age to obtain purified liquid and sodium hexafluoroaluminate filter residue; The high temperature refers to a temperature of 900~1500℃; The leaching includes the following conditions: 1) H in the dilute acid solution + The concentration of the leaching agent is 0.05~1.5mol / L; the temperature is 30~90℃; the time is 30~120min; and the leaching process is carried out under stirring at a speed of 300~600rpm. The precipitation reaction temperature is above 30℃, the reaction time is 10~180min, and the aging time is above 30min.

2. The method for removing fluoride from waste lithium iron phosphate leachate according to claim 1, characterized in that: The precipitation reaction temperature is 30~50℃, the reaction time is 10~180min, and the aging time is 30min~60min.

3. The method for removing fluoride from waste lithium iron phosphate leachate according to claim 1, characterized in that: The high-temperature roasting also includes at least one of the following conditions: 1) heating rate of 5~10 min / L; 2) holding time of 60~360 min; 3) the high temperature refers to a temperature of 900~1350℃; 4) the roasting atmosphere is air or oxygen.

4. The method for removing fluoride from waste lithium iron phosphate leachate according to claim 1, characterized in that: The high-temperature calcination also includes at least one of the following conditions: 1) heating rate of 8~10 min / L; 2) holding time of 120~360 min; 3) the high temperature refers to a temperature of 900~1250℃.

5. The method for removing fluoride from waste lithium iron phosphate leachate according to claim 1, characterized in that: The pretreatment process for waste lithium iron phosphate battery black powder includes manual screening, capacity testing, capacity separation, water washing, crushing, dismantling, crushing, magnetic separation, air separation, and dust collection. The particle size of the crushed material is between 80 and 200 μm.

6. The method for removing fluoride from waste lithium iron phosphate leachate according to claim 1, characterized in that: The leaching includes at least one of the following conditions: 1) H in a dilute acid solution + The concentration is 0.1~1mol / L; the temperature is 30~50℃; and the liquid-to-solid ratio is 3~5mL / g. 4) The time is 60~90min; 5) The leaching process is carried out under stirring, and the stirring speed is 400~600rpm.

7. The method for removing fluoride from waste lithium iron phosphate leachate according to claim 1, characterized in that: The reducing agent is iron powder, and the amount of iron powder added is 1 to 1.8 times the theoretical molar amount.

8. The method for removing fluoride from waste lithium iron phosphate leachate according to claim 1, characterized in that: The copper removal reaction temperature is 25~75℃.

9. The method for removing fluoride from waste lithium iron phosphate leachate according to claim 1, characterized in that: The iron precipitation process includes at least one of the following conditions: 1) the pH of the first filtrate is adjusted to 1.5~2.0; 2) the reaction time is 30~120 min.

10. The method for removing fluoride from waste lithium iron phosphate leachate according to claim 1, characterized in that: In step S5, OH is added by adding a sodium-containing pH adjuster. - And Na + The pH adjuster includes at least one of sodium hydroxide, sodium carbonate, and sodium bicarbonate.

11. The method for removing fluoride from waste lithium iron phosphate leachate according to claim 1, characterized in that: The precipitation reaction temperature is 30~60℃, and the reaction time is 10~60min.

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