Method for recycling waste lithium iron phosphate black powder to prepare battery-grade lithium carbonate and iron phosphate

Abstract

The application discloses a method for recycling and preparing battery-grade lithium carbonate and iron phosphate from waste lithium iron phosphate black powder, and belongs to the field of recycling of waste power lithium ion batteries. The lithium iron phosphate black powder is uniformly mixed with NaClO.5H2O by ball milling, and is completely calcined at 500-700 DEG C under an inert atmosphere; water is added to the calcined product, and after the soluble substance is completely dissolved, filtration is performed to obtain a lithium-containing water leaching solution and a leaching residue; the leaching residue is washed with water and is completely calcined at 700-850 DEG C under an oxygen-containing atmosphere to obtain battery-grade iron phosphate; sodium carbonate is added to the lithium-containing water leaching solution to form a mixed solution, and after reaction, filtration is performed to obtain lithium carbonate precipitate and filtrate; the lithium carbonate precipitate is washed with water and dried to obtain battery-grade lithium carbonate. In the application, sodium hypochlorite pentahydrate is used instead of acid as a lithium ion leaching agent, and battery-grade lithium carbonate and iron phosphate are directly obtained, and the subsequent iron removal step is omitted, the process conditions are mild, the process operation is simple, and the requirement for equipment is low.

Description

Technical Field

[0001] This invention belongs to the field of waste power lithium-ion battery recycling, specifically involving a method for recycling waste lithium iron phosphate black powder to prepare battery-grade lithium carbonate and iron phosphate. Background Technology

[0002] In recent years, with the rapid development of new energy vehicles and large-scale energy storage systems, lithium iron phosphate (LiFePO4) power batteries have been widely used due to their advantages such as good safety, long cycle life, and low cost. As the first batch of batteries installed in vehicles reaches the end of their service life, a large number of used lithium iron phosphate batteries are gradually entering the retirement stage. Improper disposal of retired batteries will not only cause a serious waste of resources such as lithium, iron, and phosphorus, but also pose potential hazards to the ecological environment and human health due to electrolyte leakage and metal ion leaching.

[0003] Traditional recycling processes for spent lithium iron phosphate batteries mostly employ hydrometallurgical techniques, using acid or alkaline leaching to dissolve elements such as lithium, iron, and phosphorus from the cathode material, followed by extraction, precipitation, or electrochemical methods for separation and recovery. Acid processes, using strong acids such as sulfuric acid and hydrochloric acid as leaching agents, offer advantages such as high leaching rates and fast reaction speeds, but generally suffer from high acid consumption, high wastewater treatment costs, and the tendency for iron ions to co-dissolve, leading to complex and highly corrosive subsequent iron removal processes. Alkaline processes, on the other hand, often require higher temperatures and longer processing times, and similarly face challenges in wastewater treatment and lithium loss control.

[0004] To overcome the shortcomings of traditional acid leaching processes, some studies have proposed selective lithium extraction processes assisted by oxidants. For example, a high-purity lithium carbonate and its preparation method are disclosed in the prior art (CN117163978A). This method involves reacting lithium iron phosphate with sodium hypochlorite at a relatively high temperature to convert lithium iron phosphate into ferrous phosphate precipitate and a lithium-ion-containing solution. Then, combined with cyclic heating for concentration and cooling, the lithium carbonate product is obtained.

[0005] While this method avoids the addition of inorganic acids, it still has the following drawbacks: It employs a wet reaction route, where sodium hypochlorite oxidizes lithium iron phosphate in the aqueous phase. The reaction system is limited by solid-liquid mass transfer, thus restricting lithium leaching efficiency. The lithium precipitation process relies on repeated heating-concentration and cooling cycles, resulting in high energy consumption, a long process flow, and increased equipment investment and operating costs.

[0006] Therefore, it is necessary to propose a new process route based on the existing oxidant-assisted lithium extraction system, which can directly produce battery-grade lithium carbonate through a simpler process without using strong acids or relying on complex equipment, while simultaneously producing high-quality battery-grade iron phosphate. Furthermore, by using an optimized filtrate circulation control strategy to suppress impurity accumulation, the system can achieve efficient lithium recovery and long-term stable product quality, thereby shortening the recovery process and reducing overall energy consumption and costs. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a method for recycling waste lithium iron phosphate black powder to prepare battery-grade lithium carbonate and iron phosphate, which achieves the purpose of recycling battery-grade lithium carbonate and iron phosphate through an optimized method based on solid-phase calcination.

[0008] To solve the above technical problems, the technical solution adopted by this invention is: a method for recycling waste lithium iron phosphate black powder to prepare battery-grade lithium carbonate and iron phosphate, comprising: Step 1: The lithium iron phosphate black powder and NaClO·5H2O are ball-milled and mixed evenly, and then calcined completely at 500~700℃ under an inert atmosphere. Step 2: Add water to the calcined product, and filter after the soluble substances have fully dissolved to obtain lithium-containing aqueous leaching solution and leaching residue; Step 3: After washing the leaching residue with water, it is calcined completely at 700~850℃ in an oxygen-containing atmosphere to obtain battery-grade iron phosphate; Step four: Add sodium carbonate to the lithium-containing aqueous solution to form a mixed solution. After the reaction is complete, filter to obtain lithium carbonate precipitate and filtrate. The lithium carbonate precipitate is washed with water and dried to obtain battery-grade lithium carbonate.

[0009] In a preferred embodiment, in step one, the mass ratio of lithium iron phosphate black powder to NaClO·5H2O is 2:(1~1.2).

[0010] In a preferred embodiment, the heating rate in step one is 5–10 °C / min.

[0011] In a preferred embodiment, the calcination time in step one is 6 to 8 hours.

[0012] In a preferred embodiment, the calcination time in step three is 3 to 5 hours.

[0013] In a preferred embodiment, in step four, the pH of the mixed solution is ≥11.

[0014] In a preferred embodiment, in step four, Na in the mixed solution + With Li + The molar ratio of the substances is controlled at (1.0~1.5):1.

[0015] As a preferred embodiment, hydrochloric acid is added to the filtrate obtained in step four to adjust the pH value to neutral, and then evaporated and crystallized to obtain solid sodium chloride.

[0016] In a preferred embodiment, the temperature of the evaporation crystallization process is 80–100°C.

[0017] As a preferred embodiment, the mother liquor obtained from evaporation and crystallization is returned to step two and added to the calcined product for recycling.

[0018] Compared with the prior art, the present invention has the following beneficial effects: This invention uses sodium hypochlorite pentahydrate (NaClO·5H2O) instead of acid as a lithium-ion leaching agent. During solid-phase calcination, lithium enters the water leaching solution in the form of lithium hydroxide, while iron remains in the leaching residue in the form of FePO4. This avoids the problem of large-scale co-dissolution of iron ions in the traditional acid leaching process, eliminates the need for subsequent iron removal steps, and the obtained FePO4 only needs to be calcined to remove residual carbon to meet battery-grade standards, further simplifying the process and reducing costs.

[0019] This invention involves directly adding sodium carbonate to a lithium-containing aqueous solution and preparing battery-grade lithium carbonate in one step through filtration and washing, thereby reducing energy consumption and production costs. It can be used to directly produce lithium iron phosphate materials.

[0020] This invention employs a combination of evaporation and crystallization with filtrate recycling to achieve filtrate reuse, thereby maintaining the high purity and stability of lithium carbonate products during multiple batches of continuous operation.

[0021] This invention achieves effective recovery of lithium and iron elements from waste lithium iron phosphate cathode materials. The entire process does not use strong acids and has no waste liquid discharge. The sodium hypochlorite pentahydrate, sodium carbonate, and hydrochloric acid used are all common industrial chemicals. The process conditions are mild, the operation is simple, and the equipment requirements are low, making it suitable for industrial application. Attached Figure Description

[0022] Figure 1 This is a process flow diagram of the recycling of waste lithium iron phosphate black powder to prepare battery-grade lithium carbonate and iron phosphate according to the present invention; Figure 2 The image shows the XRD pattern of battery-grade lithium carbonate obtained in Example 1. Figure 3 The image shows the XRD pattern of battery-grade lithium carbonate obtained in Example 2. Figure 4 The XRD pattern of the lithium carbonate product obtained in Comparative Example 1; Figure 5 The image shows the XRD pattern of the battery-grade iron phosphate obtained in Example 1. Detailed Implementation

[0023] The basic concept of this invention is based on the solid-phase calcination, water leaching, filtration, and lithium precipitation with sodium carbonate using sodium hypochlorite pentahydrate (NaClO·5H2O). By optimizing the process parameters of solid-phase reaction, water leaching separation, lithium precipitation, and crystallization, this invention achieves efficient separation and co-production recovery of lithium, iron, and phosphorus elements in waste lithium iron phosphate cathodes without the use of strong acids and complex equipment, thereby obtaining battery-grade lithium carbonate and iron phosphate.

[0024] Based on the above inventive concept, a typical embodiment of the present invention provides a method for recycling waste lithium iron phosphate black powder to prepare battery-grade lithium carbonate and iron phosphate, comprising the following steps.

[0025] Step 1: Solid-phase calcination.

[0026] The lithium iron phosphate black powder obtained from waste lithium iron phosphate batteries was ball-milled and mixed evenly with sodium hypochlorite pentahydrate (NaClO·5H2O), and then calcined at high temperature under an inert atmosphere. The main reaction was as follows: 2LiFePO4+ NaClO·5H2O → 2FePO4+ Li2O + NaCl + 5H2O↑; FePO4 is insoluble in water and is filtered and separated as leaching residue along with residual carbon and other components in step two; NaClO·5H2O removes the water of crystallization as water vapor during calcination, and excess sodium hypochlorite is completely decomposed into sodium chloride and oxygen at high temperature. 2NaClO → 2NaCl + O2↑.

[0027] The preferred mass ratio of lithium iron phosphate black powder to NaClO·5H2O is 2:(1 to 1.2). For example, the mass ratio can be 2:1, 2:1.1, 2:1.2, etc.

[0028] The high-temperature calcination temperature is set to 500–700℃, and the calcination time is 6–8 hours. For example, the calcination temperature can be selected from 500℃, 520℃, 550℃, 600℃, 650℃, 680℃, 700℃, etc.; the calcination time can be selected from 6 hours, 7 hours, 8 hours, etc.

[0029] The heating rate to the calcination temperature is 5 to 10 °C / min. For example, 5 °C / min, 6 °C / min, 7 °C / min, 8 °C / min, 9 °C / min, 10 °C / min, etc. can be selected.

[0030] Step 2: Water immersion separation.

[0031] Water is added to the calcined product, and after the soluble substances are fully dissolved, the mixture is filtered to obtain a lithium-containing aqueous leaching solution and leaching residue.

[0032] When water is added to the calcined product, Li2O reacts with water to generate LiOH: Li2O + H2O = 2LiOH. LiOH enters the aqueous phase to form a lithium-containing aqueous solution.

[0033] Step 3: Prepare ferric phosphate.

[0034] After washing the leaching residue with water, it is calcined at high temperature in an oxygen-containing atmosphere to obtain battery-grade iron phosphate. The main reaction that occurs is C + O2 = CO2. After removing carbon impurities, battery-grade FePO4 is obtained.

[0035] The high-temperature calcination temperature is set at 700–850℃, and the calcination time is 3–5 hours. For example, the calcination temperature can be selected from 700℃, 750℃, 780℃, 800℃, 820℃, 850℃, etc.; the calcination time can be selected from 3 hours, 4 hours, 5 hours, etc.

[0036] Step 4: Prepare battery-grade lithium carbonate.

[0037] Sodium carbonate (Na₂CO₃) is added to a lithium-containing aqueous leaching solution to form a mixed solution. After the reaction is complete, the solution is filtered to obtain lithium carbonate precipitate and filtrate. The lithium carbonate precipitate is washed with water and dried to obtain battery-grade lithium carbonate. The mass fraction of Li₂CO₃ in battery-grade lithium carbonate is ≥99.5%.

[0038] After adding sodium carbonate, the main reaction that occurs is: 2LiOH + Na2CO3 = Li2CO3↓ + 2NaOH; After filtration, it is washed with water and dried to obtain battery-grade lithium carbonate.

[0039] The sodium carbonate is added to the mixed solution to a pH ≥ 11, for example, the pH is controlled at 11, 12, 13 or 14; and the Na in the mixed solution... + With Li + The molar ratio of Na to Na is controlled at (1.0–1.5):1. + With Li + The molar ratio of substances is controlled at 1.0:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, etc.

[0040] Based on the above steps, the filtrate from step four can be acidified, evaporated and crystallized to obtain sodium chloride, and the filtrate obtained after evaporation and crystallization can be recycled, i.e., step five.

[0041] Step 5: Acidify the filtrate and evaporate it for crystallization. Add hydrochloric acid to the filtrate obtained in step four to adjust the pH to neutral, and a neutralization reaction occurs: NaOH + HCl = NaCl + H2O. Then, evaporate and crystallize to precipitate NaCl crystals to obtain solid sodium chloride.

[0042] The evaporation crystallization process is controlled at a temperature of 80–100°C until all sodium chloride crystals precipitate. For example, the evaporation crystallization process temperature can be controlled at 80°C, 85°C, 90°C, 95°C, 100°C, etc.

[0043] The mother liquor obtained from evaporation and crystallization can be returned to step two and added to the calcined product for recycling.

[0044] The technical solutions claimed in this invention will be further described below through some embodiments. However, the embodiments and comparative examples are for explaining the implementation schemes of this invention and do not exceed the scope of the subject matter of this invention. The scope of protection of this invention is not limited by the embodiments. Unless otherwise specified, the materials and reagents used in this invention can be obtained from commercially available products in the art.

[0045] Example 1 S1: Solid-phase calcination Take 500 g of positive electrode lithium iron phosphate black powder separated from waste lithium iron phosphate batteries, and mix it evenly with 275 g of NaClO·5H2O (mass ratio of the two is 2:1.1) by ball milling. Under nitrogen protection, the temperature is raised to 600℃ at a heating rate of 10℃ / min, held for 7 h, and then naturally cooled to room temperature to obtain the calcined product.

[0046] S2: Water leaching separation and preparation of ferric phosphate Add 3000 mL of deionized water to the calcined product obtained in step S1, stir and leach for 1 h, filter and separate to obtain lithium-containing water leaching solution and leaching residue; wash the leaching residue with water and place it in a muffle furnace, raise the temperature to 750°C at a heating rate of 10°C / min under air atmosphere, hold for 4 h, and after natural cooling, obtain battery-grade iron phosphate product.

[0047] The obtained iron phosphate product was analyzed by ICP-OES and chemical titration. The results showed that the product contained 29.1% iron (Fe) and 16.2% phosphorus (P), with a molar ratio of Fe to P of approximately 1.00:1.01. The main content (calculated as FePO4) was 99.7%. The total content of metallic impurities was ≤100 ppm, of which Na≤50 ppm, K≤10 ppm, Ca≤20 ppm, and Mg≤10 ppm. All indicators met the requirements of the chemical industry standard HG / T 4701-2021 "Iron Phosphate for Batteries".

[0048] S3: Preparation of battery-grade lithium carbonate Sodium carbonate was added to the aqueous extract to adjust the pH of the solution to 11, controlling the Na+ content. + With Li + The molar ratio was 1.2:1, and the reaction was stirred for 0.5 h. The mixture was filtered to obtain lithium carbonate precipitate and filtrate. The lithium carbonate precipitate was washed with hot water until the conductivity of the washing solution was ≤100 μS / cm, and then dried at 120℃ for 4 h to obtain battery-grade lithium carbonate product.

[0049] S4: Filtrate acidification, evaporation crystallization, and recycling Add hydrochloric acid to the filtrate obtained in step S3 to adjust the pH to 7.0, and then evaporate and crystallize at 90°C until sodium chloride crystals precipitate. Filter to separate the sodium chloride solid, and return the remaining mother liquor to step S2 for recycling after treatment.

[0050] The main content and impurity content of the lithium carbonate product obtained in Example 1 were determined by ICP-OES. The results showed that the mass fraction of Li2CO3 in the product was 99.6%, the total content of metal impurities was 95 ppm, and the content of each individual impurity (such as Na≤250 ppm, K≤10 ppm, Fe≤10 ppm, Ca≤50 ppm, etc.) all met the requirements of the battery-grade lithium carbonate industry standard YS / T 582-2023.

[0051] Example 2 S1: Solid-phase calcination Take 1000 g of positive electrode lithium iron phosphate black powder separated from waste lithium iron phosphate batteries, mix it with 550 g of NaClO·5H2O (mass ratio of the two is 2:1.1), and ball mill for 4 h to make it uniformly mixed; under argon protection, heat to 650℃ at a heating rate of 10℃ / min, hold for 7 h, and cool naturally to obtain the calcined product.

[0052] S2: Water leaching separation and preparation of ferric phosphate Add 6000 mL of deionized water to the calcined product obtained in step S1, stir and leach for 1.5 h, and separate by filtration to obtain lithium-containing water leaching solution and leaching residue; wash the leaching residue with water until the conductivity of the filtrate is ≤200 μS / cm, dry it, and then heat it to 800℃ in air at a heating rate of 10℃ / min, keep it at that temperature for 4 h, and then pulverize it after natural cooling to obtain battery-grade iron phosphate product.

[0053] The obtained iron phosphate product was analyzed by ICP-OES and chemical titration. The results showed that the product contained 29.2% iron (Fe) and 16.1% phosphorus (P), with a molar ratio of Fe to P of approximately 1.00:0.99. The main content (calculated as FePO4) was 99.6%. The total content of metallic impurities was ≤120 ppm, of which Na≤60 ppm, K≤10 ppm, Ca≤30 ppm, and Mg≤15 ppm. All indicators met the requirements of the chemical industry standard HG / T 4701-2021 "Iron Phosphate for Batteries".

[0054] S3: Preparation of battery-grade lithium carbonate Sodium carbonate was added to the aqueous extract to adjust the pH of the solution to 12, controlling the Na+ content. + With Li +The molar ratio was 1.3:1, and the mixture was stirred for 1 hour. Filtration yielded lithium carbonate precipitate and filtrate. The lithium carbonate precipitate was washed with hot water until the conductivity of the washing solution was ≤100 μS / cm, and then dried at 120℃ for 4 hours to obtain battery-grade lithium carbonate product.

[0055] S4: Filtrate acidification, evaporation crystallization, and recycling Add hydrochloric acid to the filtrate obtained in step S3 to adjust the pH to 7.0, and then evaporate and crystallize at 85°C until sodium chloride crystals precipitate. Filter to separate the sodium chloride solid, and return the remaining mother liquor to step S2 for recycling after treatment.

[0056] The main content and impurity content of the lithium carbonate product obtained in Example 2 were determined by ICP-OES. The results showed that the mass fraction of Li2CO3 in the product was 99.7% and the total content of metal impurities was 88 ppm, which met the requirements of battery-grade lithium carbonate.

[0057] Comparative Example 1 The only difference between this comparative example and Example 1 is that in step S4, the filtrate is not subjected to acidification, evaporation, or crystallization treatment; it is simply recycled. That is, the filtrate obtained in step S3 is directly returned to step S2 for recycling. No hydrochloric acid is added, and no evaporation or crystallization is performed. Sodium carbonate is only added to maintain pH=11. This process is repeated for 6 batches.

[0058] ICP-OES test results showed that as the number of circulating batches increased, the Na content in the filtrate decreased. + Cl - As the content of metal impurities gradually increased, the mass fraction of Li2CO3 in the sixth batch of lithium carbonate products dropped to 98.5%, and the total content of metal impurities reached 360 ppm, which no longer met the requirements for battery grade.

[0059] Comparative Example 2 The only difference between this comparative example and Example 1 is that the calcination temperature in step S1 is 700℃ and the holding time is shortened to 5h. That is, take 500 g of positive electrode lithium iron phosphate black powder separated from waste lithium iron phosphate batteries, mix it with 275 g of NaClO·5H2O (the mass ratio of the two is 2:1.1) by ball milling, raise the temperature to 700℃ at a heating rate of 10℃ / min under nitrogen protection, hold for 5 h, and cool naturally to room temperature to obtain the calcined product.

[0060] Experimental results show that the lithium carbonate product obtained in this comparative example has a Li2CO3 mass fraction of 99.1%, a total metal impurity content of 165 ppm, and a lithium extraction rate of 93%, all of which are lower than those in Example 1. The product purity barely meets the battery-grade requirements, but the stability is poor.

[0061] The lithium carbonate products obtained from Examples 1, 2, Comparative Example 1, and 2 were collected, and the main Li₂CO₃ content and total metal impurity content were measured using ICP-OES. The battery-grade requirements were evaluated according to industry standard YS / T 582-2023, "Battery-Grade Lithium Carbonate." The purity analysis results of the iron phosphate products obtained from each example and comparative example are given in the corresponding examples. Their main content and impurity levels meet the requirements of chemical industry standard HG / T 4701-2021, "Iron Phosphate for Batteries."

[0062] Battery-grade lithium carbonate, according to YS / T 582-2023, has a Li₂CO₃ mass fraction ≥ 99.5%, and the content of each metal impurity should meet the corresponding sub-limits (e.g., Na ≤ 250 ppm, K ≤ 10 ppm, Fe ≤ 10 ppm, Ca ≤ 50 ppm, etc.). The total metal impurity content of the products obtained in the embodiments of this invention is all below 120 ppm, and the content of each sub-impurity meets the battery-grade requirements. The purity of iron phosphate refers to the requirements of the chemical industry standard HG / T 4701-2021 "Iron Phosphate for Batteries," generally requiring ≥ 99.5%.

[0063] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A method for recycling waste lithium iron phosphate black powder to prepare battery-grade lithium carbonate and iron phosphate, characterized in that, include: Step 1: The lithium iron phosphate black powder and NaClO·5H2O are ball-milled and mixed evenly, and then calcined completely at 500~700℃ under an inert atmosphere. Step 2: Add water to the calcined product, and filter after the soluble substances have fully dissolved to obtain lithium-containing aqueous leaching solution and leaching residue; Step 3: After washing the leaching residue with water, it is calcined completely at 700~850℃ in an oxygen-containing atmosphere to obtain battery-grade iron phosphate; Step four: Add sodium carbonate to the lithium-containing aqueous solution to form a mixed solution. After the reaction is complete, filter to obtain lithium carbonate precipitate and filtrate. The lithium carbonate precipitate is washed with water and dried to obtain battery-grade lithium carbonate.

2. The method according to claim 1, characterized in that: In step one, the mass ratio of lithium iron phosphate black powder to NaClO·5H2O is 2:(1~1.2).

3. The method according to claim 1, characterized in that: In step one, the heating rate is 5–10 °C / min.

4. The method according to claim 1 or 2, characterized in that: In step one, the calcination time is 6 to 8 hours.

5. The method according to claim 4, characterized in that: In step three, the calcination time is 3 to 5 hours.

6. The method according to claim 1 or 5, characterized in that: In step four, the pH of the mixed solution is ≥11.

7. The method according to claim 6, characterized in that: In step four, the Na in the mixed solution + With Li + The molar ratio of the substances is controlled at (1.0~1.5):

1.

8. The method according to claim 7, characterized in that: Add hydrochloric acid to the filtrate obtained in step four, adjust the pH to neutral, and then evaporate and crystallize to obtain solid sodium chloride.

9. The method according to claim 8, characterized in that: The temperature for the evaporation crystallization process is 80–100°C.

10. The method according to claim 9, characterized in that: The mother liquor obtained from evaporation and crystallization is returned to step two and added to the calcined product for recycling.

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