Method for extracting iron phosphate from positive electrode material of retired lithium iron phosphate battery

Abstract

The application relates to a method for extracting iron phosphate from a retired positive electrode material of a lithium iron phosphate battery, and belongs to the technical field of resource recycling of retired lithium batteries. The method comprises the following steps: S11. obtaining a lithium iron phosphate positive electrode material separated from a retired lithium iron phosphate battery; S12. mixing the lithium iron phosphate positive electrode material with an acid solution to obtain a slurry; S13. while starting ultrasonic waves, oxygen-containing gas is introduced into the system in step S12, and reaction is carried out at 25-100 DEG C for 0.5-12 h; and S14. after the reaction is completed, solid-liquid separation is carried out, the obtained filtrate is a lithium leaching solution, and the obtained filter residue is a lithium-removed iron phosphate primary product. The method has a wide pH window, and even under strong acidic conditions with pH approximately equal to 2, lithium can be selectively leached without co-dissolving iron. The method does not depend on H2O2 as an oxidant, is green and safe, can efficiently recover iron phosphate from a retired positive electrode material of a lithium iron phosphate battery, and is suitable for industrial application.

Description

Technical Field

[0001] This application relates to the field of resource recycling technology for retired lithium batteries, specifically to a method for extracting iron phosphate from the cathode material of retired lithium iron phosphate batteries. Background Technology

[0002] With the rapid development of the new energy vehicle industry, lithium iron phosphate (LiFePO4) batteries have become one of the mainstream technologies for power batteries. The recycling of retired lithium iron phosphate batteries is increasingly becoming an important issue for resource recycling and environmental protection. The core objectives of lithium iron phosphate battery recycling are twofold: first, to efficiently extract lithium; and second, to purify the residual iron phosphate slag into high-purity iron phosphate, achieving full utilization of valuable components.

[0003] In existing technologies, the delithiation methods for lithium iron phosphate battery cathode materials mainly fall into the following categories: (1) Acid leaching oxidation method: Strong acids such as sulfuric acid and hydrochloric acid are used to dissolve LiFePO4, and oxidants such as H2O2 are used to oxidize Fe. 2+ Oxidized to Fe 3+ This method achieves a high lithium leaching rate, but consumes a large amount of strong acid and generates a large amount of saline wastewater.

[0004] (2) Selective leaching with hydrogen peroxide: Lithium is selectively leached under mild conditions using acidic salts such as ammonium dihydrogen phosphate as buffers and H2O2 as an oxidant. This method achieves selective leaching of lithium, but H2O2 is a hazardous chemical with high storage and transportation costs, significant safety risks, and the problem of ineffective decomposition exists. The actual consumption far exceeds the theoretical stoichiometry.

[0005] (3) Air oxidation and water immersion method: This method utilizes oxygen in the air to oxidize LiFePO4 under acidic conditions to remove lithium. This method is green and environmentally friendly, but the oxidation rate is relatively slow. In industrial scale-up, it is limited by the gas-liquid mass transfer efficiency and is difficult to match with batch processing cycles.

[0006] Furthermore, the FePO4 residue obtained after delithiation usually contains residual lithium and metallic impurities such as aluminum, copper, calcium, and magnesium.

[0007] In addition, existing technologies (such as CN112723330A) also employ methods that use the addition of transition metal catalysts (such as oxides of nickel, cobalt, and manganese or lithium-containing transition metal compounds) and microbubbles, pressure control, etc., to generate hydroxyl radicals to enhance oxidative delithiation. Such methods introduce transition metals into the solid phase while delithiating, and the resulting product is doped isophosphorus manganese iron ore, which increases the cost of subsequent purification and recovery. Summary of the Invention

[0008] Based on this, the purpose of this application is to provide a method for efficiently recovering FePO4 from retired lithium iron phosphate battery cathode materials.

[0009] To achieve the above objectives, this application provides a method for extracting iron phosphate from the cathode material of retired lithium iron phosphate batteries, comprising the following steps: S11. Obtain lithium iron phosphate cathode material separated from retired lithium iron phosphate batteries; S12. The lithium iron phosphate cathode material is mixed with an acidic solution to obtain a slurry; S13. Simultaneously with starting the ultrasound, oxygen-containing gas is introduced into the system of step S12, and the reaction is carried out at 25~100℃ for 0.5~12h; S14. After the reaction is complete, solid-liquid separation is performed. The resulting filtrate is lithium leachate, and the resulting filter residue is the primary product of delithiated iron phosphate. In step S13, the energy of the ultrasonic wave is controlled to generate a cavitation effect, thereby causing the iron phosphate passivation layer on the surface of the lithium iron phosphate cathode material to continuously peel off; the oxygen-containing gas is used to oxidize the freshly exposed ferrous iron in the lithium iron phosphate cathode material in situ to ferric iron.

[0010] Preferably, the acidic solution includes a solution of a soluble acid salt, a buffer solution of a soluble acid salt, and an inorganic acid solution.

[0011] Preferably, the solution of the soluble acid salt is selected from ammonium dihydrogen phosphate solution, and the buffer solution of the soluble acid salt is selected from a mixed solution of ammonium sulfate and ammonium bisulfate.

[0012] More preferably, the process further includes a step of washing and removing impurities from the primary lithium iron phosphate product: S21. Wash the delithiated iron phosphate primary product with water at a liquid-to-solid ratio of 3 to 7:1; S22. Wash the initial product obtained in step S21 with an organic acid solution of 3-7 wt% that has a complexing effect at a liquid-to-solid ratio of 3-7:1 to obtain ferric phosphate residue after impurity removal.

[0013] Preferably, the washing solution in step S21 is combined with the lithium leaching solution in step S14 for use in subsequent lithium recovery processes.

[0014] Preferably, the organic acid is selected from one or more of citric acid, oxalic acid, and ethylenediaminetetraacetic acid.

[0015] More preferably, the method further includes the step of performing in-situ surface epitaxial growth of iron phosphate on the purified iron phosphate slag: S31. The purified ferric phosphate slag is mixed with deionized water at a liquid-to-solid ratio of 3 to 7:1 to obtain a slurry; S32. Under the conditions of starting ultrasound and introducing oxygen-containing gas, add soluble iron salt and soluble phosphate or soluble iron salt and phosphate with a Fe to P molar ratio of 0.9~1.1:1 to the system in step S31, and react for 0.5~6.0 h at 25~100℃ and pH 2.0~4.5. S33. After the reaction is complete, age for 20~120 min, then perform solid-liquid separation. The obtained solid is the iron phosphate product after repairing crystal defects.

[0016] Preferably, the iron phosphate product after repairing crystal defects is washed, dried, and calcined to obtain the final iron phosphate product.

[0017] Preferably, the soluble iron salt is selected from one or more of ferric sulfate, ferric chloride, and ferric nitrate; the soluble phosphate is selected from at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0018] More preferably, in steps S13 and S32, the frequency of the ultrasonic wave is 20~100 kHz.

[0019] Preferably, in step S12, the liquid-to-solid ratio of the acidic solution to the lithium iron phosphate is 2~10:1; and the pH of the acidic solution is 2~6.5.

[0020] The technical solution claimed in this application achieves the following beneficial effects: 1. Existing selective delithiation technologies generally rely on H2O2, a hazardous chemical (Class 5.1 oxidant, UN2014), which is strictly controlled and requires explosion-proof, light-proof, and temperature-controlled storage and transportation, resulting in high overall costs for industrial use. Furthermore, H2O2 undergoes vigorous thermal decomposition above 80℃, with an ineffective decomposition rate reaching 30%–60%, and actual consumption far exceeding the theoretical stoichiometric ratio by 2–3 times. The decomposition byproduct water increases the dilution burden on the mother liquor. In addition, residual H2O2 entering subsequent lithium recovery processes may trigger degradation of organic extractants or interfere with lithium carbonate crystallization. The proposed solution uses oxygen-containing gas (pure oxygen, oxygen-enriched air, or air) instead of H2O2 solution as the oxidant, completely eliminating the safety risks and process interference associated with using H2O2 as the oxidant. Moreover, it does not use any hazardous chemical oxidants throughout the process, resulting in mild reaction conditions, simple operation steps, low reagent consumption, and a green and efficient process suitable for industrial-scale production.

[0021] 2. Ultrasonic cavitation achieves dual acceleration: it continuously strips away the FePO4 passivation layer, exposing fresh LiFePO4 reaction surfaces; it breaks up oxygen-containing gases into microbubbles, increasing the gas-liquid mass transfer area by several orders of magnitude, and significantly improving the gas dissolution rate and utilization rate. The synergistic effect of these two factors greatly enhances the oxidation rate of oxygen-containing gases such as pure oxygen or oxygen-enriched air under mesophilic conditions, achieving reaction efficiency comparable to or even better than the H2O2 approach.

[0022] 3. Washing and organic acid purification achieve precise separation of soluble salts and metallic impurities. The washing solution containing soluble lithium salts recovered from washing is directly added to the lithium-containing leachate obtained in the previous step and finally enters the subsequent lithium recovery process; in the organic acid purification process, the complexation effect of organic acids can remove water-insoluble metallic impurities, avoiding contamination of the lithium-containing solution by metallic impurities, while maximizing the lithium recovery rate.

[0023] 4. After washing and impurity removal, the lithium-depleted slag is used to repair crystal defects through epitaxial growth under the assistance of oxygen-containing gas and ultrasound. The FePO4 content is significantly increased and the metal impurity content is greatly reduced, upgrading it from industrial grade to high purity. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description are only embodiments of this application. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0025] Figure 1 This is a flowchart of the method for leaching lithium and extracting iron phosphate from the cathode material of retired lithium iron phosphate batteries used in this application. Detailed Implementation

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

[0027] The “lithium iron phosphate cathode material” mentioned in this application refers to the cathode powder obtained by crushing and sorting retired lithium iron phosphate battery cathode sheets or positive and negative electrode sheets, then calcining to remove PVDF binder, electrolyte residue and waste carbon, and removing graphite by flotation. Its main component is lithium iron phosphate.

[0028] like Figure 1 As shown, the method for selectively leaching lithium and extracting iron phosphate from the cathode material of retired lithium iron phosphate batteries in this application mainly includes three steps: Step S1. Selective lithium removal using oxygen-containing gas and ultrasound: The lithium iron phosphate cathode material separated from retired lithium iron phosphate batteries is mixed with an acidic solution to form a slurry, and then a selective delithiation reaction is carried out under ultrasonic-assisted and oxygen-containing gas bubbling conditions.

[0029] In this step, ultrasound continuously strips away the FePO4 passivation layer formed after lithium removal from the recycled material through cavitation, exposing the fresh LiFePO4 surface inside. Simultaneously, it breaks up oxygen-containing bubbles into microbubbles to increase the gas-liquid mass transfer area. The reaction temperature is 25–100℃, the pH is 2.0–6.5 (maintained by an acidic salt buffer or adjusted by controlling the amount of dilute inorganic acid added), and the reaction time is 0.5–12.0 h. After the reaction, solid-liquid separation is performed to obtain a lithium-containing leachate and delithiated iron phosphate slag.

[0030] This step does not use hydrogen peroxide as an oxidant, nor does it use transition metal catalysts (such as oxides of nickel, cobalt, or manganese, or lithium-containing transition metal compounds). The oxidation of the iron component comes solely from the oxidizing atmosphere provided by the bubbling of oxygen-containing gas. The oxygen-containing gas can be pure oxygen, air, or oxygen-enriched air. The introduction rate of the oxygen-containing gas is calculated based on the volume of pure oxygen. When using oxygen-enriched air or air, the flow rate is converted to an equivalent volume of pure oxygen based on the volume fraction of oxygen it contains. The aforementioned acidic salt is capable of ionizing to produce H₂. + The preferred soluble acid salt is ammonium dihydrogen phosphate. Other alternative acid salts include sodium dihydrogen phosphate, potassium dihydrogen phosphate, and sodium bisulfate. The acid salt maintains the reaction system within an acidic pH range of 2.0–6.5. If necessary, organic or inorganic acids such as phosphoric acid and citric acid can be optionally added as pH-adjusting additives to form a pH buffer system. The buffer system should exhibit minimal pH changes before and after the reaction, further adjusting and stabilizing the pH of the reaction system within the aforementioned range, thus maintaining a stable acidic delithiation environment.

[0031] The core of this selective delithiation step lies in the fact that the delithiation process primarily utilizes topological delithiation: lithium ions are extracted from the crystal lattice, while iron is oxidized in situ to trivalent iron in the solid phase and remains as ferric phosphate (FePO4), rather than as Fe. 2+ The crystal enters the solution in its original form. Under low pH conditions, without the synergistic effect of ultrasound and oxygen-containing gas, the strongly acidic medium will drive the crystal lattice to undergo acidic dissolution (LiFePO4 + 3H+). + → Li + + Fe 2+ + H3PO4), leading to iron co-dissolution. However, the solution in this application continuously strips the surface passivation layer and maintains a fresh active interface through ultrasonic cavitation, making the topological delithiation rate significantly faster than the lattice acid dissolution rate; at the same time, with the assistance of pH and oxidation regulation, even in the strongly acidic range of pH≈2, iron is still retained in the solid phase FePO4 with an extremely low dissolution rate.

[0032] When using dilute inorganic acid solutions for acidic solutions, dilute sulfuric acid, dilute hydrochloric acid, dilute nitric acid, dilute phosphoric acid, etc., can be used, with a concentration of 0.05~1.0 mol / L. Although dilute inorganic acids do not possess the buffering capacity of acid salts, under the synergistic effect of oxygen-containing gas bubbling and ultrasound, Fe... 2+ Rapidly oxidized to Fe by O2 3+ This process immediately forms a stable FePO4 precipitate, resulting in extremely low Fe dissolution within a low pH window (iron dissolution rate <0.5%), thus overcoming the technical limitation of existing technologies where dilute acids cannot selectively extract lithium. Furthermore, the lithium-containing leachate obtained using the dilute inorganic acid method has a simpler composition (containing only Li). + The presence of corresponding acid radicals (and their corresponding anions) simplifies subsequent lithium recovery processes. In particular, when using dilute phosphoric acid (H3PO4), because PO4... 3- The FePO4 product is composed of Fe ions, without introducing any foreign anions, and the resulting lithium-containing leachate contains only Li. + and PO4 3- / HPO4 2- / H2PO4 - In this system, the phosphorus component after lithium precipitation can be directly recycled for the epitaxial growth process in step S3, achieving closed-loop utilization of phosphorus.

[0033] It should be noted that different oxidants (such as hydrogen peroxide or oxygen-containing gases) can all oxidize Fe. 2+ Oxidized to Fe 3+ However, existing selective leaching methods using acidic salts and oxidants (such as hydrogen peroxide) typically require controlling the initial pH of the mixed solution within a narrow range of 5-6. When the pH is too low (e.g., using a free strong acid system, pH≈2), the co-solubility of iron can significantly increase to 17%-45%. This is because such methods rely on oxidizing iron and then retaining it as ferric phosphate precipitate. At low pH, the solubility of ferric phosphate increases, and strong acidity promotes the dissolution of the lattice acid, thus requiring a buffer system to maintain the pH in a higher range. This application, through the synergy of oxygen-containing gas and ultrasound, enables iron to remain as solid-phase FePO4 without co-solubility even at low pH, extending the effective pH window down to 2.0. This substantially widens the pH range for selective delithiation, reducing dependence on buffer systems and making it possible to use cheaper dilute inorganic acids (such as dilute sulfuric acid) for direct leaching.

[0034] Furthermore, this step does not add any transition metal catalysts. Instead, it uses ultrasonic cavitation effect to replace metal catalysis to enhance the oxidation of oxygen-containing gases, thereby achieving efficient and selective delithiation under zero transition metal addition conditions. The resulting delithiated iron phosphate slag is pure phase iron phosphate without foreign metals, avoiding the adverse effects of transition metal impurities on the purity of subsequent iron phosphate products, and also saving the cost and recycling burden of catalysts.

[0035] Step S2. Washing and removing impurities: The delithiated iron phosphate slag obtained in step S1 is first washed to dissolve residual soluble lithium salts and soluble impurities, obtaining a lithium-containing washing solution. This solution is collected and then proceeds to the subsequent lithium recovery process. Next, an organic acid solution is used to remove impurities, utilizing the complexing effect of the organic acid to dissolve residual metallic impurities (existing in the form of oxides / phosphates, insoluble in water), yielding pure iron phosphate slag.

[0036] Step S3. In-situ surface epitaxial growth of iron phosphate: The purified ferric phosphate slag obtained in step S2 was slurried with deionized water. Under conditions of oxygen-containing gas bubbling and ultrasonic assistance (the frequency of the ultrasound is not explicitly limited, but the energy is controlled to promote the stable formation of an epitaxial ferric phosphate layer from the ferric phosphate crystal nucleus, for example, a range of 20~100 kHz), soluble iron salts and soluble phosphates were added (maintaining an Fe / P molar ratio of 0.9~1.1:1). This caused in-situ epitaxial growth on the surface of FePO4 particles, repairing crystal defects generated during the delithiation process, filling vacancies, and improving the yield. The reaction temperature in this step was 25~100℃, the pH was 2.0~4.5, and the reaction time was 0.5~6.0 h. After the reaction, aging, solid-liquid separation, washing, drying, and calcination were performed to obtain the high-purity final product FePO4.

[0037] The above scheme will be further explained below with different specific parameter conditions. In the following examples and comparative examples, the content of iron phosphate products, the content of impurity elements, crystallinity and other indicators were all determined by the HG / T 4701 series standard methods.

[0038] <Example 1> This embodiment provides a method for recovering FePO4 from lithium iron phosphate cathode material separated from retired lithium iron phosphate batteries. The process flow is as follows: Figure 1 As shown. Follow these steps: S1. Selective delithiation of oxygen-containing gas using ultrasound: Approximately 1 kg of lithium iron phosphate cathode material was mixed with an NH4H4PO4 solution with a pH of 4.5-6.5 to form a slurry (liquid-solid ratio 4:1, NH4H2PO4 concentration 50 g / L, total slurry volume approximately 4 L), and then added to a reactor equipped with a mechanical stirrer, an ultrasonic transducer, and a gas dispersion device. The temperature was raised to 70°C, and ultrasonic waves (25 kHz continuous mode) and pure oxygen bubbling (flow rate of 100 mL / min) were started. After reacting for 80 min, solid-liquid separation was performed.

[0039] This step yielded approximately 3.9 L of lithium-containing leachate and approximately 1.33 kg of delithiated iron phosphate slag filter cake (with a moisture content of approximately 28%). The lithium leaching rate was 99.2%, and the iron dissolution rate was 0.29%.

[0040] S2. Washing and removing impurities: The delithiated iron phosphate slag filter cake obtained in S1 was washed twice with deionized water (liquid-solid ratio 5:1, 45℃, stirring and washing for 30 min each time). After solid-liquid separation, the water washing liquid was combined into about 8.8 L and then incorporated into the lithium-containing leachate obtained in step S1 for subsequent lithium recovery process. The obtained solid was then washed with citric acid solution (concentration 5 wt%, liquid-solid ratio 5:1, stirred at 45℃ for 45 min). After solid-liquid separation, it was washed three times with deionized water to obtain approximately 1.30 kg of pure iron phosphate residue (approximately 0.94 kg on a dry basis).

[0041] S3. In-situ surface epitaxial growth: The pure ferric phosphate slag obtained in step S2 was mixed with deionized water (liquid-solid ratio 5:1), added to the reactor, and pure oxygen bubbling and ultrasonic assistance were started (25 kHz continuous mode). Fe2(SO4)3 solution and NH4H2PO4 solution were slowly added over approximately 45 min (maintaining Fe / P = 1:1), at a reaction temperature of 68℃ and pH 3.0. After the addition was complete, the mixture was aged for 45 min. The product was subjected to solid-liquid separation. The solid was washed three times with deionized water, dried at 120°C for 4 h, and finally calcined at 350°C for 2.5 h to obtain the final product, namely high-purity FePO4.

[0042] In this step, approximately 0.96 kg (about 960 g) of high-purity FePO4 product was obtained. Of this, FePO4 accounted for 99.2% of the total mass, Li 0.003%, Al 0.003%, Cu 0.001%, Ca 0.003%, and Mg 0.002%. The final product had a crystallinity of 96.5% and a measured specific surface area of ​​12.3 m² / g.

[0043] <Example 2> The difference between this embodiment and Embodiment 1 is that: In step S1: the reaction temperature is 75℃, the ultrasonic frequency is 40 kHz, and the reaction time is 60 min; the lithium leaching rate is 99.1%, and the iron leaching rate is 0.31%.

[0044] In step S3: the reaction temperature is 70℃, and the aging reaction time after the feeding is completed is 1 h.

[0045] Step S2 is the same as in Example 1.

[0046] The final product obtained was approximately 0.96 kg (approximately 960 g) of FePO4, accounting for 99.4% of the total mass, and 0.004% of Li. The crystallinity of the final product was 97.2%.

[0047] <Example 3> The difference between this embodiment and Embodiment 1 is that: In step S1: the reaction temperature is 60℃, the ultrasonic frequency is 20 kHz, and the reaction time is 90 min; the lithium leaching rate is 98.8%, and the iron dissolution rate is 0.27%.

[0048] Steps S2 and S3 are the same as in Example 1.

[0049] The final product contains approximately 0.96 kg (about 960 g) of FePO4, accounting for 99.0% of the total mass, and 0.008% of Li. The crystallinity of the final product is 95.8%.

[0050] <Example 4> The difference between this embodiment and Embodiment 1 is that: In step S1: 0.2 mol / L dilute sulfuric acid was used to replace the NH4H2PO4 solution (liquid-solid ratio 4:1), and the pH of the reaction system was maintained in the range of 2.0~4.0 by controlling the amount of dilute sulfuric acid added; the reaction temperature was 70℃, and ultrasonic waves (25 kHz continuous mode) and oxygen-enriched bubbling (the oxygen volume ratio in the oxygen-enriched gas was 30%, and the oxygen flow rate in the oxygen-enriched gas was maintained at 100 mL / min during the bubbling process) were started, and the reaction was carried out for 80 min; the lithium leaching rate was 98.5%, and the iron dissolution rate was 0.33%.

[0051] Steps S2 and S3 are the same as in Example 1.

[0052] The final product, FePO4, was approximately 0.96 kg (about 960 g), accounting for 99.0% of the total mass, while Li accounted for 0.008%. The crystallinity of the final product was 95.5%. This indicates that dilute inorganic acids can also achieve selective delithiation under the synergistic effect of oxygen-containing gas and ultrasound.

[0053] <Example 5> The difference between this embodiment and Embodiment 1 is that: In step S1, a mixed solution of ammonium sulfate and ammonium bisulfate was used instead of NH4H2PO4 solution as the base solution (pH 4.0~5.5); the reaction temperature was 70℃, and ultrasonic waves (25 kHz continuous mode) and pure oxygen bubbling (flow rate 100 mL / min) were started, and the reaction was carried out for 80 min; the lithium leaching rate was about 98.6%, and the iron dissolution rate was 0.34%.

[0054] The mixture contains approximately 40 g / L of ammonium sulfate and 30 g / L of ammonium bisulfate, with a liquid-to-solid ratio of 4:1. This mixture is acidic, and the ionization of ammonium bisulfate provides H₂. +To maintain the acidity of the reaction system, the pH of the reaction system was finely controlled within the range of 4.0 to 5.5 by adjusting the amount of ammonium bisulfate. During the reaction, LiFePO4 delithiation consumes H₂. + This ensures that the pH of the system remains stable within the aforementioned range.

[0055] Steps S2 and S3 are the same as in Example 1.

[0056] After washing and impurity removal in step S2 and in-situ epitaxial growth in step S3, the final FePO4 product obtained weighs approximately 0.96 kg (approximately 960 g); FePO4 accounts for more than 99% of the total mass, the Li content is less than 0.01%, and the contents of major metallic impurities (Al, Cu, Ca, Mg) are all less than 0.01%; the crystallinity of the final product is approximately 96%, and the specific surface area is approximately 12 m². 2 / g, meeting the requirements for battery-grade iron phosphate.

[0057] <Comparative Example 1> The difference between this comparative example and Example 1 is as follows: In step S1, ultrasound is not activated; only pure oxygen is used for bubbling.

[0058] Under these conditions, the reaction time in step S1 was extended to approximately 5 hours, with a lithium leaching rate of 94.2% and an iron leaching rate of 0.06%. This indicates that ultrasound plays a crucial role in lithium leaching efficiency and significantly shortens the reaction time.

[0059] Steps S2 and S3 are the same as in Example 1.

[0060] The final product obtained was approximately 0.96 kg (approximately 960 g) of FePO4, accounting for 98.5% of the total mass, and 0.040% of Li, with a crystallinity of 94.0%. The purity and crystallinity of the final product were both lower than those of Example 1.

[0061] <Comparative Example 2> The difference between this comparative example and Example 1 is as follows: In step S1, air bubbles are used instead of pure oxygen.

[0062] Under these conditions, the reaction time was approximately 4 hours, with a lithium leaching rate of 95.1% and an iron leaching rate of 0.05%. This indicates that while air oxidation is feasible, the rate is relatively slow, and pure oxygen or oxygen-enriched air is a better choice.

[0063] Steps S2 and S3 are the same as in Example 1.

[0064] The final product obtained was approximately 0.96 kg (approximately 960 g) of FePO4, accounting for 98.8% of the total mass, and 0.035% of Li, with a crystallinity of 95.0%.

[0065] In this comparative example, because air bubbling was used in step S1, the oxygen partial pressure was lower and the oxidation rate was slower, which prolonged the reaction time. The purity and crystallinity of the FePO4 final product were also lower than those in Example 1, which used pure oxygen, indicating that pure oxygen or oxygen-enriched air is a better choice for oxygen supply.

[0066] <Comparative Example 3> The difference between this comparative example and Example 1 is as follows: In step S1, instead of using oxygen-containing gas for bubbling, approximately 0.7 L (actual consumption is approximately twice the theoretical stoichiometry) of a 30 wt% H2O2 solution is slowly added dropwise as an oxidant.

[0067] Under these conditions, the reaction time for step S1 is approximately 80 minutes. After solid-liquid separation, the lithium leaching rate was measured to be 98.3%, and the iron dissolution rate was 0.06%. However, a residual H2O2 concentration of approximately 120 mg / L was detected in the lithium-containing leachate. This H2O2 will decompose and generate oxygen microbubbles during the subsequent lithium recovery process, potentially interfering with the crystallization process of lithium carbonate precipitation. Therefore, an additional heating and holding deoxygenation step is required to ensure the stability of the mother liquor.

[0068] In addition, during the reaction, violent bubble overflow was observed (caused by the thermal decomposition of H2O2). The H2O2 decomposition loss at the vent of the reactor accounted for about 40% of the added amount. The amount of deionized water required to wash the obtained filter cake to neutral was about 1.5 times that of Example 1 (the pH of the washing water was too high).

[0069] The FePO4 final product obtained in step S3 is approximately 0.94 kg (approximately 940 g), accounting for 98.6% of the total mass, with Li accounting for 0.012%, and the crystallinity of the final product is 94.1%.

[0070] Although the lithium leaching rates of the scheme in Example 1 and the scheme in Comparative Example 3 are basically the same (99.2% vs 98.3%), the scheme in Example 1 has the following substantial advantages: Higher oxidant utilization: H2O2 has an actual utilization rate of only about 60% due to thermal decomposition, while oxygen-containing gases can achieve high utilization and no decomposition loss through ultrasonic microbubble solubilization. Higher purity of mother liquor: The lithium-containing leachate obtained by the scheme using H2O2 as an oxidant requires additional deoxygenation treatment, which increases the number of processes and reagent consumption; Higher product quality: The FePO4 content and crystallinity of the main product obtained in Example 1 are better than those in Comparative Example 3. It is speculated that the decomposition products of H2O2 (HO· free radicals) produce local over-oxidation defects on the particle surface, which is not conducive to uniform nucleation of subsequent epitaxial growth.

[0071] Greater safety: Completely eliminates the need for H2O2 hazardous chemical approval, explosion-proof storage, and transportation procedures.

[0072] In addition to the above-described embodiments and comparative exceptions, this application further tests the lithium leaching, iron dissolution, and the mass percentage of FePO4 in the final product under different pH systems. The results are summarized in the table below: 1 dilute sulfuric acid 0.2 mol / L 2.0 70 25 pure oxygen 80 98.2 0.41 98.6 2 dilute sulfuric acid 0.2 mol / L 2.5 70 25 pure oxygen 80 98.7 0.38 98.9 3 dilute sulfuric acid 0.2 mol / L 3.0 70 25 pure oxygen 80 99.1 0.35 99.0 4 (corresponding to Example 4) dilute sulfuric acid 0.2 mol / L 4.0 70 25 <![CDATA[Oxygen-enriched gas (O₂ 30% VOL)]]> 80 98.5 0.33 99.0 5 dilute sulfuric acid 0.2 mol / L 5.0 70 25 pure oxygen 80 98.3 0.22 99.2 6 dilute sulfuric acid 0.2 mol / L 6.0 70 25 pure oxygen 80 98.0 0.15 99.3 7 (corresponding to Example 1) <![CDATA[NH4H2PO4]]> 50 g / L 4.5~6.5 (self-buffered) 70 25 pure oxygen 80 99.2 0.29 99.2 8 (response ratio 3) <![CDATA[H2O2]]> 0.2 mol / L 5.0 70 25 <![CDATA[Using H₂O₂ as the oxidant]]> 80 98.3 0.06 98.6 In experiments 1-3 and 5-6 above, all parameters except pH were the same as in Example 4. The results in the table above further demonstrate that the method of this application can achieve efficient recovery of iron phosphate and maintain extremely low iron co-solubility within a wide pH range of 2.0-6.5, effectively broadening the pH operating range for selective lithium extraction.

[0073] The embodiments and application examples described above are merely illustrative descriptions of this application and are not intended to limit the scope of this application. Any modifications and improvements made to the technical solutions of this application by those skilled in the art without departing from the spirit of this application should fall within the protection scope defined by this application.

Claims

1. A method for extracting iron phosphate from the cathode material of retired lithium iron phosphate batteries, characterized in that, Includes the following steps: S11. Obtain lithium iron phosphate cathode material separated from retired lithium iron phosphate batteries; S12. The lithium iron phosphate cathode material is mixed with an acidic solution to obtain a slurry; S13. Simultaneously with starting the ultrasound, oxygen-containing gas is introduced into the system of step S12, and the reaction is carried out at 25~100℃ for 0.5~12h; S14. After the reaction is complete, solid-liquid separation is performed. The resulting filtrate is lithium leachate, and the resulting filter residue is the primary product of delithiated iron phosphate.

2. The method according to claim 1, characterized in that, The acidic solution includes solutions of soluble acid salts, buffer solutions of soluble acid salts, and inorganic acid solutions.

3. The method according to claim 2, characterized in that, The solution of the soluble acid salt is selected from ammonium dihydrogen phosphate solution, and the buffer solution of the soluble acid salt is selected from a mixed solution of ammonium sulfate and ammonium bisulfate.

4. The method according to claim 1, characterized in that, It also includes a step of washing and removing impurities from the primary lithium iron phosphate product: S21. Wash the delithiated iron phosphate primary product with water at a liquid-to-solid ratio of 3 to 7:1; S22. Wash the initial product obtained in step S21 with an organic acid solution of 3-7 wt% that has a complexing effect at a liquid-to-solid ratio of 3-7:1 to obtain ferric phosphate residue after impurity removal.

5. The method according to claim 4, characterized in that, The washing solution in step S21 is combined with the lithium leaching solution in step S14 for use in subsequent lithium recovery processes.

6. The method according to claim 4, characterized in that, The organic acid is selected from one or more of citric acid, oxalic acid, and ethylenediaminetetraacetic acid.

7. The method according to claim 4, characterized in that, It also includes the step of performing in-situ surface epitaxial growth of iron phosphate on the purified iron phosphate slag: S31. The purified ferric phosphate slag is mixed with deionized water at a liquid-to-solid ratio of 3 to 7:1 to obtain a slurry; S32. Under the conditions of starting ultrasound and introducing oxygen-containing gas, add soluble iron salt and soluble phosphate or soluble iron salt and phosphate with a Fe to P molar ratio of 0.9~1.1:1 to the system in step S31, and react for 0.5~6.0 h at 25~100℃ and pH 2.0~4.

5. S33. After the reaction is complete, age for 20~120 min, then perform solid-liquid separation. The obtained solid is the iron phosphate product after repairing crystal defects.

8. The method according to claim 7, characterized in that, The iron phosphate product after repairing crystal defects is washed, dried, and calcined to obtain the final iron phosphate product.

9. The method according to claim 7, characterized in that, The soluble iron salt is selected from one or more of ferric sulfate, ferric chloride, and ferric nitrate; the soluble phosphate is selected from at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

10. The method according to claim 7, characterized in that, In steps S13 and S32, the frequency of the ultrasonic wave is 20~100 kHz.

11. The method according to claim 1 or 2, characterized in that, In step S12, the liquid-to-solid ratio of the acidic solution to the lithium iron phosphate is 2~10:1; the pH of the acidic solution is 2~6.5.

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