Method for recycling waste lithium iron phosphate positive electrode material

Abstract

The application discloses a method for recycling waste lithium iron phosphate positive electrode materials and belongs to the technical field of recycling of waste lithium iron phosphate batteries. + The molecular structure of the grinding aid contains O and N coordination atoms, can occur complexation with Li + in the lithium iron phosphate, and produces a synergistic effect with ammonium chloride, effectively promotes dissociation of lithium iron phosphate crystals, and is beneficial to release of Li 2+ ; meanwhile, under mechanical activation and air atmosphere, Fe 3+ exposed after the lithium iron phosphate structure is destroyed is oxidized into Fe 3+ by oxygen and immediately combines with phosphate to generate hardly soluble iron phosphate, which is firmly retained in the solid phase. 3+ Since the phosphate of Fe is extremely difficult to dissolve in an aqueous solution, and the complexation of the grinding aid with Fe is extremely weak, the grinding aid cannot compete with the lithium extraction from the iron phosphate, so that in-situ fixation of iron and selective separation of lithium are completed while lithium is efficiently extracted.

Description

Technical Field

[0001] This invention belongs to the field of waste lithium iron phosphate battery recycling technology, specifically relating to a method for recycling waste lithium iron phosphate cathode materials. Background Technology

[0002] With the rapid development of new energy vehicles and the energy storage industry, lithium iron phosphate batteries have been widely adopted due to their advantages such as high safety, long cycle life, and low cost. The resulting large amount of waste lithium iron phosphate cathode material (black powder) has become an important secondary resource. Its main component is lithium iron phosphate (LiFePO4), and it also contains graphite, conductive agents, and small amounts of metallic impurities. Efficient and green recycling technologies are of great significance for alleviating the shortage of lithium, iron, and phosphorus resources and reducing the total life-cycle cost of batteries.

[0003] Currently, the recycling technologies for waste lithium iron phosphate are mainly divided into three categories: pyrometallurgical, hydrometallurgical, and mechanochemical methods. Pyrometallurgical recycling is energy-intensive, easily generates harmful fumes, and has a low lithium recovery rate; hydrometallurgical recycling has a long process, high acid and alkali consumption, and high wastewater treatment costs; while mechanochemical methods have become a research hotspot in recent years due to their advantages such as short process, low energy consumption, and environmental friendliness. Among them, the mechanical activation-chlorination synergistic process uses high-energy ball milling to cause a mechanochemical reaction between the chlorinating agent and lithium iron phosphate, selectively destroying the Li-O bond to achieve efficient lithium dissolution while maintaining the stability of the iron-phosphorus solid phase. This is currently the mainstream technical route for achieving stepwise lithium-iron-phosphorus recovery.

[0004] However, existing mechanical activation-chlorination recovery technology still has significant bottlenecks, making it difficult to simultaneously achieve efficient lithium leaching, highly selective retention of iron and phosphorus, and high product purity requirements.

[0005] For example, patent application CN116573655A discloses a method for activating lithium iron phosphate black powder through high-energy ball milling using sodium chloride or potassium chloride as a chlorinating agent. This method utilizes mechanical force to drive the chlorination reaction, converting lithium into soluble lithium chloride. However, it has significant drawbacks: firstly, with only a chlorinating agent, the high-energy ball milling reaction activity is limited, resulting in low mechanochemical conversion efficiency and difficulty in achieving a lithium dissolution rate exceeding 90%; secondly, sodium and potassium ions in the chlorinating agent dissolve simultaneously with lithium during water leaching, making deep removal difficult and severely affecting the purity of the recovered lithium carbonate, thus failing to meet battery-grade material standards.

[0006] To improve lithium dissolution efficiency, patent application CN120117630A proposes a high-energy ball milling method using a mixture of ammonium chloride and hydrogen peroxide. This method utilizes the strong oxidizing properties of hydrogen peroxide to dissolve Fe in lithium iron phosphate. 2+ Oxidized to Fe 3+This process disrupts stable Fe-O bonds, thereby enhancing mechanochemical effects and significantly improving lithium release efficiency. However, this technical route also has serious problems: strong oxidation leads to the loss of selective iron production, with a large amount of ferric iron dissolving along with lithium, resulting in deep mixing of iron and lithium in the leachate. This not only complicates subsequent lithium-iron separation processes and increases costs, but also significantly reduces the iron content in iron-rich phosphorus slag, making it impossible to efficiently recover high-value iron-based products.

[0007] In summary, existing mechanical activation-chlorination technology faces the dual contradictions of "low lithium release efficiency and heavy impurity contamination" and "high lithium release efficiency but difficulty in separating lithium iron phosphate." Currently, there is still a lack of a synergistic recovery process that can simultaneously achieve highly efficient selective lithium dissolution, efficient solid-phase retention of iron and phosphorus, and the introduction of no external impurities. This severely restricts the economic viability and large-scale application of waste lithium iron phosphate resource recycling. Summary of the Invention

[0008] To address the above problems, the purpose of this invention is to provide a method for recycling waste lithium iron phosphate cathode materials.

[0009] This invention provides a method for recycling waste lithium iron phosphate cathode materials, comprising the following steps: S1. Lithium iron phosphate black powder, ammonium chloride and grinding aid are mixed and then ball-milled to obtain ball milling material and exhaust gas; wherein: the grinding aid is selected from one or more of urea, citric acid, tartaric acid, malic acid, ascorbic acid and ammonium acetate; S2. After mixing the ball milling material with water, stir and leach to obtain a lithium-containing leachate and a phosphorus-iron leachate residue. S3. After adjusting the pH of the lithium-containing leaching solution to 6-8, filter out the impurities; add phosphoric acid to the filtrate, and then concentrate, evaporate, crystallize, filter, and dry to obtain lithium dihydrogen phosphate. S4. Add oxalic acid solution to the ferric phosphate leaching residue, stir until the solids no longer dissolve, filter to obtain filtrate, add reducing agent to filtrate, stir to react, control the pH of the solution to 2~2.5 during the reaction, filter a second time to obtain ferrous oxalate.

[0010] Preferably, in step S1, the mass ratio of lithium iron phosphate black powder to ammonium chloride is 1:(0.3~1.2), and the mass ratio of lithium iron phosphate black powder to grinding aid is 1:(0.1~0.5).

[0011] Preferably, in step S1, a dispersant is added before ball milling; wherein: the dispersant is selected from one or more of polyethylene glycol, ethanol, and acetone; the mass of the dispersant is 0.5 to 3% of the total mass of lithium iron phosphate black powder, ammonium chloride, and grinding aid.

[0012] Preferably, in step S1, the grinding aid is selected from one or more of citric acid, tartaric acid, malic acid, and ascorbic acid mixed with urea.

[0013] Preferably, in step S1, the ball milling is a high-energy ball milling, the rotation speed of the high-energy ball milling is 600~1000 r / min, and the high-energy ball milling time is 1~6 h.

[0014] Preferably, in step S1, the grinding balls used in the ball mill are zirconia balls, and the ball-to-material ratio is (5~20):1.

[0015] More preferably, during the ball milling process, intermittent ball milling and / or forced cooling are used to control the temperature during the ball milling process to not exceed 60°C.

[0016] Preferably, in step S1, the exhaust gas generated during the ball milling process is absorbed by hydrochloric acid solution, and the ammonium chloride generated after absorption is returned to step S1 for use.

[0017] Preferably, in step S2, the mass ratio of ball milling material to water is 1:(1~8); the stirring and leaching temperature is 20~60℃, and the stirring and leaching time is 0.5~2h.

[0018] Preferably, in step S3, phosphoric acid is added to adjust the molar ratio of Li to P in the filtrate to (0.98~1.02):1.

[0019] Preferably, in step S3, the solid content is concentrated and crystallized to 60-75 wt%; after filtration, the mother liquor is mixed with the next batch of lithium-containing leachate and recycled for the preparation of lithium dihydrogen phosphate.

[0020] Preferably, in step S4, the concentration of the oxalic acid solution is 1~1.5mol / L, and the mass-to-volume ratio of the phosphorus-containing iron leaching residue to the oxalic acid solution is 1g:(6~12)mL.

[0021] Preferably, in step S4, the reducing agent is one or both of ascorbic acid and sodium sulfite; the mass ratio of ferrophosphorus leaching residue to reducing agent is 100:(50~70).

[0022] Preferably, in step S4, the stirring reaction time is 0.5~2h.

[0023] Compared with the prior art, one or more of the above technical solutions can achieve at least one of the following beneficial effects: This invention selects one or more of urea, citric acid, tartaric acid, malic acid, ascorbic acid, and ammonium acetate as grinding aids. The molecular structure of these grinding aids contains O and N type coordination atoms, which can react with the Li in lithium iron phosphate. + Complexation occurs, and a synergistic effect is produced with ammonium chloride, effectively promoting the dissociation of lithium iron phosphate crystals, which is beneficial to Li...+ The dissolution and release of Fe2+; simultaneously, under mechanical activation and an air atmosphere, the Fe2+ exposed after the lithium iron phosphate structure is destroyed. 2+ Oxidized by oxygen to Fe 3+ and immediately with PO4 3- It combines to form insoluble iron phosphate (FePO4), which is firmly retained in the solid phase. Because of Fe... 3+ Phosphates are extremely difficult to dissolve in aqueous solutions, and grinding aids have a negative effect on Fe. 3+ The complexation effect is extremely weak, making it impossible to competitively dissolve it from FePO4. Thus, while achieving efficient lithium extraction, in-situ fixation of iron and selective separation of lithium are also achieved. The recovery method of the present invention yields lithium dihydrogen phosphate with a purity of ≥99.5% and ferrous oxalate with a purity of ≥99.2%, which can be directly used to prepare battery-grade lithium iron phosphate cathode materials. Attached Figure Description

[0024] Figure 1 The image shows the XRD pattern of lithium dihydrogen phosphate recovered in Example 1.

[0025] Figure 2 The image shows the XRD pattern of ferrous oxalate dihydrate recovered in Example 1. Detailed Implementation

[0026] To facilitate understanding of the present invention, the invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0027] As mentioned above, this invention provides a method for recycling waste lithium iron phosphate cathode materials, comprising the following steps: S1. Lithium iron phosphate black powder, ammonium chloride and grinding aid are mixed and then ball-milled to obtain ball milling material and exhaust gas; wherein: the grinding aid is selected from one or more of urea, citric acid, tartaric acid, malic acid, ascorbic acid and ammonium acetate; S2. After mixing the ball milling material with water, stir and leach to obtain a lithium-containing leachate and a phosphorus-iron leachate residue. S3. After adjusting the pH of the lithium-containing leaching solution to 6-8, filter out the impurities; add phosphoric acid to the filtrate, and then concentrate, evaporate, crystallize, filter, and dry to obtain lithium dihydrogen phosphate. S4. Add oxalic acid solution to the ferric phosphate leaching residue, stir until the solids no longer dissolve, filter to obtain filtrate, add reducing agent to filtrate, stir to react, control the pH of the solution to 2~2.5 during the reaction, filter a second time to obtain ferrous oxalate.

[0028] The principle of the method of the present invention is mainly as follows: In step S1, lithium iron phosphate black powder is mixed with ammonium chloride, grinding aid, and dispersant, and then ball-milled. The impact, shearing, and friction of the grinding balls refine the lithium iron phosphate particles, distort the crystal lattice, and generate a large number of defects and active sites. Ammonium chloride undergoes lattice distortion and partial decomposition under mechanical force, generating HCl gas; HCl reacts with lithium iron phosphate, causing Li... + It is converted into soluble LiCl, while exposing Fe. 2+ It is oxidized to Fe by oxygen in the air inside the ball mill jar. 3+ and with PO4 3- This combination forms sparingly soluble iron phosphate (FePO4). FePO4 has a stable structure and is almost insoluble in aqueous solution, thus the iron is firmly fixed in the solid phase. The grinding aid binds to Li through its O and N coordination atoms. + A weak complexation reaction occurs, which synergistically promotes the disintegration of the crystal structure and the release of lithium by ammonium chloride. However, the complexation reaction with iron is extremely weak, and iron cannot be competitively dissolved from FePO4, thereby achieving selective extraction of lithium and in-situ fixation of iron.

[0029] In step S2, LiCl in the ball mill will dissolve during the leaching process, while iron-containing substances such as iron phosphate and ferrous hydrogen phosphate are relatively stable and will not dissolve. Therefore, efficient separation of lithium iron phosphate can be achieved by water leaching.

[0030] In step S3, by adjusting the pH of the solution to neutral, trace amounts of metal ions such as aluminum or iron can be precipitated, removing impurity metal ions from the leachate and improving the purity of lithium dihydrogen phosphate.

[0031] In step S4, oxalic acid solution is used to dissolve the phosphorus-containing ferric leaching residue, and impurities such as carbon powder that are insoluble in acidic solutions are separated and removed. A reducing agent is added to the filtrate, and the pH of the system is adjusted to a strongly acidic range of 2 to 2.5. While ensuring the formation of ferrous oxalate, the precipitation of hydroxide impurities such as ferric hydroxide and ferrous hydroxide can be effectively avoided, significantly improving the purity of the ferrous oxalate product.

[0032] In this invention, a grinding aid is used to synergistically enhance the mechanical ball milling effect. Under air atmosphere, mechanical activation disrupts the lithium iron phosphate structure, exposing the Fe... 2+ It is gently oxidized by oxygen to Fe 3+ and immediately with PO4 3- By combining with the formation of insoluble ferric phosphate (FePO4), iron is firmly fixed in the solid phase. Compared to systems using strong oxidants such as hydrogen peroxide (H2O2), the air oxidation process of this invention is more gentle and controllable, avoiding excessive dissolution of iron or the formation of soluble complexes during the leaching stage. Since iron exists in the stable FePO4 form after ball milling, it is essentially undissolved in the subsequent water leaching step, thereby significantly reducing the iron content in the leachate.

[0033] This invention uses ammonium chloride as the main dissociation agent for lithium iron phosphate. Compared with traditional sodium or potassium salt dissociation systems such as sodium chloride and potassium chloride, no sodium or potassium ions are introduced during the entire leaching process. This fundamentally avoids the problem of sodium or potassium impurities being carried over into the subsequent lithium salt products, greatly simplifies the lithium purification and impurity removal process, reduces the separation and purification cost, and is conducive to obtaining high-purity lithium products.

[0034] The recovery method of the present invention yields lithium dihydrogen phosphate with a purity of ≥99.5% and ferrous oxalate with a purity of ≥99.2%, which can be directly used to prepare battery-grade lithium iron phosphate cathode materials.

[0035] In some embodiments, in step S1, the mass ratio of lithium iron phosphate black powder to ammonium chloride is 1:(0.3~1.2), including but not limited to: 1:0.3, 1:0.5, 1:0.8, 1:1, 1:1.2, etc.; the mass ratio of lithium iron phosphate black powder to grinding aid is 1:(0.1~0.5), including but not limited to: 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5, etc.

[0036] In some embodiments, in step S1, a dispersant is added before ball milling; wherein: the dispersant is selected from one or more of polyethylene glycol, ethanol, and acetone; the mass of the dispersant is 0.5 to 3% of the total mass of lithium iron phosphate black powder, ammonium chloride, and grinding aid; including but not limited to: 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, etc.

[0037] This invention adds a dispersant that adsorbs onto the surface of lithium iron phosphate black powder and ammonium chloride particles, reducing interfacial tension, decreasing agglomeration, improving fluidity, promoting uniform mixing of the two, enhancing the uniformity of the mechanical chlorination reaction, thereby strengthening the disintegration of the crystal structure and increasing the lithium dissolution rate.

[0038] In some embodiments, in step S1, the grinding aid is selected from one or more of citric acid, tartaric acid, malic acid, and ascorbic acid mixed with urea.

[0039] In this invention, the grinding aid, composed of organic acid and urea, can ensure the dissolution effect of lithium while further inhibiting the dissolution of iron, thus achieving effective separation of iron and lithium.

[0040] In some embodiments, in step S1, the ball milling is a high-energy ball milling, and the rotation speed of the high-energy ball milling is 600~1000 r / min, including but not limited to: 600 r / min, 700 r / min, 800 r / min, 900 r / min, 1000 r / min, etc.; the high-energy ball milling time is 1~6h, including but not limited to: 1h, 2h, 3h, 4h, 5h, 6h, etc.

[0041] In some embodiments, in step S1, the grinding balls used in the ball mill are zirconia balls, and the ball-to-material ratio is (5~20):1, including but not limited to: 5:1, 8:1, 10:1, 12:1, 15:1, 18:1, 20:1, etc.

[0042] In some embodiments, intermittent ball milling and / or forced cooling are used to control the temperature during the ball milling process to not exceed 60°C.

[0043] Ammonium chloride is relatively stable at room temperature, but high-energy ball milling can generate local high temperatures (especially exceeding 100°C), causing ammonium chloride to decompose rapidly. Although the HCl produced by decomposition can react with LiFePO4 to form LiCl, if the temperature is too high, a large amount of HCl and NH3 will escape and leave the ball mill jar before they can fully contact the solid phase. This leads to a significant reduction in the effective amount of ammonium chloride actually participating in chlorination, resulting in a decrease in lithium conversion rate.

[0044] In some embodiments, in step S1, the exhaust gas generated during the ball milling process is absorbed by hydrochloric acid solution, and the ammonium chloride generated after absorption is returned to step S1 for use.

[0045] In some embodiments, in step S2, the mass ratio of the ball milling material to water is 1:(1~8), including but not limited to: 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, etc.; the stirring and leaching temperature is 20~60℃, including but not limited to: 20℃, 30℃, 40℃, 50℃, 60℃, etc.; the stirring and leaching time is 0.5~2h, including but not limited to: 0.5h, 1h, 1.5h, 2h, etc.

[0046] In some embodiments, in step S3, phosphoric acid is added to adjust the molar ratio of Li to P in the filtrate to (0.98~1.02):1.

[0047] In some embodiments, in step S3, during the evaporation, concentration, and crystallization step, the solid content is concentrated to 60-75 wt%; after filtration, the mother liquor is mixed with the next batch of lithium-containing leachate and recycled for the preparation of lithium dihydrogen phosphate.

[0048] In some embodiments, in step S4, the concentration of the oxalic acid solution is 1~1.5 mol / L, including but not limited to: 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, etc.; the mass-volume ratio of the ferrophosphate leaching residue to the oxalic acid solution is 1 g:(6~12) mL.

[0049] In some embodiments, in step S4, the reducing agent is one or both of ascorbic acid and sodium sulfite; the mass ratio of phosphorus-containing iron leaching residue to reducing agent is 100:(50~70).

[0050] In some embodiments, the stirring reaction time in step S4 is 0.5 to 2 hours, including but not limited to: 0.5 hours, 1 hour, 1.5 hours, 2 hours, etc.

[0051] The waste lithium iron phosphate black powder used in this embodiment of the invention has the following main element mass percentages: Li 4.21%, Fe 33.15%, P 18.56%, C 2.18%, O 38.32%, with the remainder being unknown impurities.

[0052] Example 1 S1. Mix 100g of waste lithium iron phosphate black powder, 65g of ammonium chloride, 15g of citric acid, and 2g of polyethylene glycol, and add them to a ball mill jar. Add zirconia grinding balls at a ball-to-material ratio of 10:1. Then, install the ball mill jar on a planetary high-energy ball mill, set the ball mill speed to 800r / min, and mill for 3 hours. During the ball milling process, the temperature inside the ball mill jar is controlled by intermittent operation and air cooling to ensure that it does not exceed 50℃. The ball milling is carried out in an air atmosphere. After the ball milling is completed, the milled material is obtained.

[0053] S2. Transfer the ball-milled material to a reactor, add 400 mL of deionized water, and stir and leach at 40°C for 1 hour. After leaching, filter and separate to obtain 320 mL of lithium-containing leachate and 121 g of iron-rich phosphorus slag (wet powder). The lithium concentration in the leachate was measured to be 12.8 g / L, and the lithium leaching rate was calculated to be 97.3% (total lithium was 4.21 g, and the amount of lithium leached was 4.10 g).

[0054] S3. Adjust the pH of the lithium-containing leachate to 7 with ammonia solution, and filter to remove trace amounts of hydroxide precipitate. Slowly add phosphoric acid to the filtrate to adjust the Li / P molar ratio to 1.01:1. Then concentrate the filtrate at 80°C until supersaturated, cool to crystallize (solid content 70 wt%), filter and dry to obtain lithium dihydrogen phosphate product with a yield of 97.3%. Testing showed that the obtained lithium dihydrogen phosphate had a purity of 99.6% and a sodium content of 6 ppm (sodium is a trace impurity that is unavoidably introduced into reagents or products). The mother liquor obtained from filtration was returned and combined with the next batch of lithium-containing leachate for recycling in the preparation of lithium dihydrogen phosphate.

[0055] S4. Iron-rich phosphorus slag and 1.2 mol / L oxalic acid solution were mixed at a mass-to-volume ratio of 1 g / 8 mL and stirred until no more solids dissolved. The mixture was then filtered to obtain a filtrate. Ascorbic acid was added to the filtrate at a mass ratio of 100:60 (iron-rich phosphorus slag to ascorbic acid), and the mixture was stirred to allow precipitation for 2 hours. During the precipitation reaction, ammonia was added to maintain the pH of the reaction solution at 2-2.5. After the reaction was complete, the mixture was filtered to obtain ferrous oxalate dihydrate precipitate. The precipitate was washed and dried to obtain ferrous oxalate dihydrate. The purity of the product was 99.4%, and the iron recovery rate was 96.5%.

[0056] The XRD pattern of lithium dihydrogen phosphate obtained in this embodiment is as follows: Figure 1 As shown: The XRD absorption peak of the lithium dihydrogen phosphate recovered in this embodiment corresponds completely with the absorption peak of the standard card, and there are no impurity peaks, indicating that its purity is high.

[0057] The XRD pattern of ferrous oxalate dihydrate obtained in this embodiment is as follows: Figure 2 As shown: The XRD absorption peak of the recovered ferrous oxalate dihydrate in this embodiment corresponds completely with the absorption peak of the standard card, and there are no impurity peaks, indicating that its purity is high.

[0058] Comparative Example 1 It is basically the same as Example 1, except that in step S1, sodium chloride is used instead of ammonium chloride.

[0059] In step S2, the lithium leaching rate is 96.8%.

[0060] In step S3, the obtained lithium dihydrogen phosphate has a purity of 99.3% and a sodium content of 185 ppm.

[0061] In step S4, the purity of ferrous oxalate dihydrate was 99.2%, and the iron recovery rate was 96.2%.

[0062] Comparative Example 2 It is basically the same as Example 1, except that citric acid and ammonium chloride are not added in step S1.

[0063] In step S2, the lithium leaching rate is <1%; subsequent steps cannot be performed.

[0064] Comparative Example 3 It is basically the same as Example 1, except that: in step S1, citric acid is not added, and the corresponding amount of ammonium chloride added is 80g.

[0065] In step S2, the lithium leaching rate is 82%.

[0066] In step S3, the obtained lithium dihydrogen phosphate has a purity of 99.2% and a sodium content of 5 ppm.

[0067] In step S4, the purity of ferrous oxalate dihydrate was 99%, and the iron recovery rate was 91%.

[0068] Comparative Example 4 It is basically the same as Example 1, except that: in step S1, ammonium chloride is not added, and the corresponding amount of citric acid added is 80g.

[0069] In step S2, the lithium leaching rate is 8%. The leaching rate is too low to proceed with subsequent steps.

[0070] Comparative Example 5 It is basically the same as Example 1, except that in step S1, citric acid is replaced with an equal mass of 30wt% hydrogen peroxide.

[0071] S1. Mix 100g of waste lithium iron phosphate black powder, 65g of ammonium chloride, 15g of 30wt% hydrogen peroxide, and 2g of polyethylene glycol, and add them to a ball mill jar. Add zirconia grinding balls at a ball-to-material ratio of 10:1. Then, install the ball mill jar on a planetary high-energy ball mill, set the ball mill speed to 800r / min, and ball mill for 3 hours. During the ball milling process, the temperature inside the ball mill jar is controlled by intermittent operation and air cooling to ensure that it does not exceed 50℃. After ball milling, the ball-milled material is obtained.

[0072] In step S2, the lithium leaching rate is 98.9%.

[0073] In step S3, the obtained lithium dihydrogen phosphate has a purity of 97.3% and a sodium content of 6 ppm.

[0074] In step S4, the purity of ferrous oxalate dihydrate was 99.4%, and the iron recovery rate was 85.2%.

[0075] Example 2 It is basically the same as Example 1, except that polyethylene glycol is not added in step S1.

[0076] In step S2, the lithium leaching rate is 96.5%.

[0077] In step S3, the obtained lithium dihydrogen phosphate has a purity of 99.5% and a sodium content of 7 ppm.

[0078] In step S4, the purity of ferrous oxalate dihydrate was 99.2%, and the iron recovery rate was 94.8%.

[0079] Example 3 It is basically the same as Example 1, except that temperature control is not performed during the ball milling process; In step S2, the lithium leaching rate is 89.2%.

[0080] In step S3, the obtained lithium dihydrogen phosphate has a purity of 99.4% and a sodium content of 8 ppm.

[0081] In step S4, the purity of ferrous oxalate dihydrate was 99.4%, and the iron recovery rate was 89.5%.

[0082] Example 4 It is basically the same as Example 1, except that in step S1, urea of ​​equal mass is used instead of citric acid.

[0083] In step S2, the lithium leaching rate is 95.5%.

[0084] In step S3, the obtained lithium dihydrogen phosphate has a purity of 99.5% and a sodium content of 7 ppm.

[0085] In step S4, the purity of ferrous oxalate dihydrate was 99.2%, and the iron recovery rate was 94.5%.

[0086] Example 5 It is basically the same as Example 1, except that in step S1, the grinding aid is 10g of citric acid and 5g of urea.

[0087] In step S2, the lithium leaching rate is 99.1%.

[0088] In step S3, the obtained lithium dihydrogen phosphate has a purity of 99.7% and a sodium content of 6 ppm.

[0089] In step S4, the purity of ferrous oxalate dihydrate was 99.5%, and the iron recovery rate was 98.4%.

[0090] Example 6 It is basically the same as Example 1, except that in step S1, the grinding aid is 5g of citric acid and 10g of urea.

[0091] In step S2, the lithium leaching rate is 98.5%.

[0092] In step S3, the obtained lithium dihydrogen phosphate has a purity of 99.6% and a sodium content of 5 ppm.

[0093] In step S4, the purity of ferrous oxalate dihydrate was 99.4%, and the iron recovery rate was 98.1%.

[0094] Example 7 S1. Mix 100g of waste lithium iron phosphate black powder, 30g of ammonium chloride, 25g of malic acid, 25g of urea, and 0.9g of ethanol, and add the mixture to a ball mill jar. Add zirconia grinding balls at a ball-to-material ratio of 5:1. Then, install the ball mill jar on a planetary high-energy ball mill, set the ball mill speed to 1000r / min, and mill for 1 hour. During the ball milling process, the temperature inside the ball mill jar is controlled by intermittent operation and air cooling to ensure that it does not exceed 60℃. The ball milling is carried out in an air atmosphere. After the ball milling is completed, the milled material is obtained.

[0095] S2. Transfer the ball-milled material to a reaction vessel, add 540 mL of deionized water, stir and leach at 20°C for 2 hours. After leaching, filter and separate to obtain lithium-containing leachate and iron-rich phosphorus slag. The lithium leaching rate is 97.8%.

[0096] S3. Adjust the pH of the lithium-containing leachate to 7 with ammonia, and filter to remove trace amounts of hydroxide precipitate. Slowly add phosphoric acid to the filtrate to adjust the Li / P molar ratio to 1:1. Then concentrate the filtrate to supersaturation at 80°C, cool to crystallize (solid content 70 wt%), filter and dry to obtain lithium dihydrogen phosphate product. Testing showed that the purity of the obtained lithium dihydrogen phosphate was 99.7%, and the sodium content was 5 ppm. The mother liquor obtained from filtration was returned and combined with the next batch of lithium-containing leachate for recycling in the production of lithium dihydrogen phosphate.

[0097] S4. Iron-rich phosphorus slag and 1 mol / L oxalic acid solution were mixed at a mass-to-volume ratio of 1 g / 12 mL and stirred until no more solids dissolved. The mixture was then filtered to obtain a filtrate. Ascorbic acid was added to the filtrate at a mass ratio of 100:70, and the mixture was stirred to allow precipitation to occur for 0.5 h. During the precipitation reaction, ammonia was added to maintain the pH of the reaction solution at 2-2.5. After the reaction was complete, the mixture was filtered to obtain ferrous oxalate dihydrate precipitate. The precipitate was washed and dried to obtain ferrous oxalate dihydrate. The purity of the product was 99.3%, and the iron recovery rate was 96.8%.

[0098] Example 8 S1. Mix 100g of waste lithium iron phosphate black powder, 120g of ammonium chloride, 5g of tartaric acid, 5g of urea, and 7.9g of acetone, and add them to a ball mill jar. Add zirconia grinding balls at a ball-to-material ratio of 20:1. Then, install the ball mill jar on a planetary high-energy ball mill, set the ball mill speed to 600r / min, and ball mill for 6 hours. During the ball milling process, the temperature inside the ball mill jar is controlled by intermittent operation and air cooling to ensure that it does not exceed 60℃. The ball milling is carried out in an air atmosphere. After ball milling, the ball-milled material is obtained.

[0099] S2. Transfer the ball-milled material to a reaction vessel, add 1L of deionized water, stir and leach at 60℃ for 0.5h. After leaching, filter and separate to obtain lithium-containing leachate and iron-rich phosphorus slag. The lithium leaching rate is 96.8%.

[0100] S3. Adjust the pH of the lithium-containing leachate to 7 with NaOH solution, and filter to remove trace amounts of hydroxide precipitate. Slowly add phosphoric acid to the filtrate to adjust the Li / P molar ratio to 1.02:1. Then concentrate the filtrate at 80°C until supersaturated, cool to crystallize (solid content 65 wt%), filter and dry to obtain lithium dihydrogen phosphate product. Testing showed that the purity of the obtained lithium dihydrogen phosphate was 99.4%, and the sodium content was 7 ppm. The mother liquor obtained from filtration was returned and combined with the next batch of lithium-containing leachate for recycling in the preparation of lithium dihydrogen phosphate.

[0101] S4. Iron-rich phosphorus slag and 1.5 mol / L oxalic acid solution were mixed at a mass-to-volume ratio of 1 g / 6 mL. After stirring until no more solids dissolved, the mixture was filtered to obtain the filtrate. Then, ascorbic acid was added at a mass ratio of 100:55 (iron-rich phosphorus slag:ascorbic acid), and the mixture was stirred for 2 hours to induce precipitation. During the precipitation reaction, ammonia was added to maintain the pH of the reaction solution at 2-2.5. After the reaction was completed, the mixture was filtered to obtain ferrous oxalate dihydrate precipitate. After washing and drying, ferrous oxalate dihydrate was obtained. The purity of the product was 99.2%, and the iron recovery rate was 95.9%.

[0102] The recovery efficiency of lithium iron phosphate in Examples 1-8 and Comparative Examples 1-5 of this invention is shown in Table 1.

[0103] Table 1 The data in Table 1 show that: Comparative Example 1, using sodium chloride instead of ammonium chloride, achieved a lithium leaching rate of 96.8%, comparable to 97.3% in Example 1, indicating that sodium chloride can replace ammonium chloride in terms of leaching efficiency; however, the sodium content in the obtained lithium dihydrogen phosphate was as high as 185 ppm, far exceeding the 6 ppm in Example 1, and the product purity also decreased from 99.6% to 99.3%. This may be because the introduction of sodium ions would severely contaminate the lithium dihydrogen phosphate product during subsequent crystallization, while using ammonium chloride avoids this problem at the source. Comparative Example 2, without the addition of citric acid and ammonium chloride, had a lithium leaching rate of less than 1%. Without an activator, ball milling alone could not effectively decompose the waste lithium iron phosphate, thus releasing almost no lithium under water leaching conditions. In Comparative Example 3, without the addition of citric acid, the lithium leaching rate increased from nearly zero in Comparative Example 2 to 82%, but it was still significantly lower than the 97.3% in Example 1. Furthermore, the iron recovery rate was only 91%, lower than the 96.5% in Example 1. This may be because, although ammonium chloride is the main leaching agent, the lack of citric acid's synergistic grinding effect resulted in insufficient mechanochemical activation efficiency, leading to incomplete leaching. In Comparative Example 4, only citric acid was added, without ammonium chloride, and the lithium leaching rate was only 8%. This may be because citric acid alone is insufficient for the dissociation of waste lithium iron phosphate black powder. In Comparative Example 5, after replacing citric acid with hydrogen peroxide, the lithium leaching rate further improved compared to Example 1, but the purity of lithium dihydrogen phosphate and the iron recovery rate decreased. The reason may be that while hydrogen peroxide promotes lithium dissolution, it also significantly enhances iron dissociation, leading to an increased iron dissolution rate, thereby reducing the purity of lithium dihydrogen phosphate and the iron recovery rate. In Example 2, without the addition of polyethylene glycol, the lithium leaching rate decreased slightly from 97.3% to 96.5% compared to Example 1, and the iron recovery rate decreased from 96.5% to 94.8%. This may be because polyethylene glycol helps improve the wetting contact between the components, thereby enhancing the mechanochemical activation effect. In Example 3, without temperature control, the lithium leaching rate decreased significantly from 97.3% to 89.2% compared to Example 1, and the iron recovery rate decreased from 96.5% to 94.1%. This may be because the local temperature was too high during ball milling, which may have caused ammonium chloride to decompose before reacting with lithium iron phosphate and then escape, thus weakening the destructive effect of mechanochemical activation on the lithium iron phosphate lattice and ultimately affecting the lithium leaching efficiency. In Example 4, urea was used to replace citric acid in equal amounts, and the lithium leaching rate and iron recovery rate decreased slightly compared to Example 1. In Examples 5 and 6, a combination of urea and citric acid was used, and the lithium leaching rate and iron recovery rate were further improved compared to Example 1.

[0104] Examples 7 and 8 adjusted the process parameters, resulting in some fluctuations in the lithium leaching rate and iron recovery rate, but both showed good results.

[0105] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for recycling waste lithium iron phosphate cathode materials, characterized in that, Includes the following steps: S1. Lithium iron phosphate black powder, ammonium chloride and grinding aid are mixed and then ball-milled to obtain ball milling material and exhaust gas; wherein: the grinding aid is selected from one or more of urea, citric acid, tartaric acid, malic acid, ascorbic acid and ammonium acetate; S2. After mixing the ball milling material with water, stir and leach to obtain a lithium-containing leachate and a phosphorus-iron leachate residue. S3. After adjusting the pH of the lithium-containing leaching solution to 6-8, filter out the impurities; add phosphoric acid to the filtrate, and then concentrate, evaporate, crystallize, filter and dry in sequence to obtain lithium dihydrogen phosphate. S4. Add oxalic acid solution to the ferric phosphate leaching residue, stir until the solids no longer dissolve, filter to obtain filtrate, add reducing agent to filtrate, stir to react, control the pH of the solution to 2~2.5 during the reaction, filter a second time to obtain ferrous oxalate.

2. The method for recycling waste lithium iron phosphate cathode material according to claim 1, characterized in that, In step S1, the mass ratio of lithium iron phosphate black powder to ammonium chloride is 1:(0.3~1.2), and the mass ratio of lithium iron phosphate black powder to grinding aid is 1:(0.1~0.5). And / or, the grinding aid is selected from one or more of citric acid, tartaric acid, malic acid, ascorbic acid and a mixture of urea.

3. The method for recycling waste lithium iron phosphate cathode materials according to claim 1 or 2, characterized in that, In step S1, a dispersant is added before ball milling; wherein the dispersant is selected from one or more of polyethylene glycol, ethanol, and acetone; the mass of the dispersant is 0.5 to 3% of the total mass of lithium iron phosphate black powder, ammonium chloride, and grinding aid.

4. The method for recycling waste lithium iron phosphate cathode material according to claim 1, characterized in that, In step S1, the ball mill is a high-energy ball mill with a rotation speed of 600~1000 r / min and a milling time of 1~6 h; the grinding balls are zirconia balls with a ball-to-material ratio of (5~20):

1.

5. The method for recycling waste lithium iron phosphate cathode materials according to claim 1 or 4, characterized in that, During the ball milling process, intermittent ball milling and / or forced cooling methods are used to control the temperature during the ball milling process to not exceed 60℃.

6. The method for recycling waste lithium iron phosphate cathode materials according to claim 1 or 4, characterized in that, In step S1, the exhaust gas generated during the ball milling process is absorbed by hydrochloric acid solution, and the ammonium chloride generated after absorption is returned to step S1 for use.

7. The method for recycling waste lithium iron phosphate cathode materials according to claim 1, characterized in that, In step S2, the mass ratio of ball milling material to water is 1:(1~8); the stirring and leaching temperature is 20~60℃, and the stirring and leaching time is 0.5~2h.

8. The method for recycling waste lithium iron phosphate cathode material according to claim 1, characterized in that, In step S3, phosphoric acid is added to adjust the molar ratio of Li to P in the filtrate to (0.98~1.02):1; And / or: evaporate, concentrate, and crystallize to a solid content of 60-75 wt%; And / or: After filtration, the mother liquor is mixed with the next batch of lithium-containing leachate and recycled for the preparation of lithium dihydrogen phosphate.

9. The method for recycling waste lithium iron phosphate cathode material according to claim 1, characterized in that, In step S4, the concentration of the oxalic acid solution is 1~1.5mol / L, and the mass-to-volume ratio of the ferrophosphorus leaching residue to the oxalic acid solution is 1g:(6~12)mL; And / or: The reducing agent is one or both of ascorbic acid and sodium sulfite; the mass ratio of phosphorus-containing iron leaching residue to reducing agent is 100:(50~70).

10. The method for recycling waste lithium iron phosphate cathode material according to claim 1, characterized in that, In step S4, the stirring reaction time is 0.5~2h.

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