Recycling method of invalid lithium iron phosphate black powder

Abstract

The invention discloses a recycling method of invalid lithium iron phosphate black powder. The recycling method comprises the following steps: mixing invalid lithium iron phosphate black powder with water to prepare slurry, and adding an acidic material and hydrogen peroxide for acid leaching to obtain a leachate; adjusting the pH value of the leachate to 2-3, carrying out solid-liquid separation to obtain a copper-removed solution, adding a complexing agent into the copper-removed solution, and carrying out solid-liquid separation to obtain an aluminum-removed solution; adjusting the Fe / P molar ratio of the aluminum-removed liquid to 1: (1-1.05), adjusting the pH value to 2.2, carrying out solid-liquid separation, washing, slurrying, recrystallizing and drying the obtained solid phase to obtain anhydrous iron phosphate, and taking the filtrate as a lithium-rich liquid; and mixing anhydrous iron phosphate, the lithium-rich liquid and an organic matter, carrying out spray drying to obtain powder, and calcining to obtain the carbon-coated lithium iron phosphate. According to the invention, efficient recovery of lithium, iron and phosphorus elements is realized; the prepared carbon-coated lithium iron phosphate material shows excellent electrochemical performance in a soft package battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery waste resource recovery, and in particular relates to a method for recovering spent lithium iron phosphate black powder. Background Art

[0002] Lithium iron phosphate batteries have an average service life of 5-8 years and are expected to face widespread retirement in the future. Currently, their cathode materials are primarily recovered through hydrometallurgical processes, employing either full-component leaching or selective leaching to extract valuable metals. However, due to the low intrinsic value of phosphorus and iron (P) in the cathode materials and the difficulty in removing impurities such as Al and Cu from lithium extraction slag, hydrometallurgical processes prioritize the extraction of lithium (Li), which has higher economic value. This results in the storage of large quantities of phosphorus-containing iron phosphate slag as waste. This waste not only occupies land and pollutes the environment, but also results in waste due to the unrecycled P and Fe resources.

[0003] Current methods for recycling used batteries primarily include pyrometallurgy, hydrometallurgy, high-temperature solid-phase remediation, bioleaching, and mechanical treatment. The wet method is the most widely used. The pyrometallurgical method removes carbon, organic matter, and adhesives through high-temperature calcination to recover metals and their oxides. This method offers advantages such as high recovery rates and strong applicability, but suffers from issues such as product sintering, high energy consumption, and toxic gas emissions. The wet method utilizes solvents to selectively dissolve the target metal element, then recovers the metal or its salts through precipitation and other steps. Its advantages include a simple process, low energy consumption, and moderate equipment requirements, making it suitable for large-scale production. Currently, the wet method is considered a relatively mature lithium extraction process within the industry, but the acid and alkali solutions and reducing agents used in the leaching system vary, each with its own advantages and disadvantages. For example, the most mature sulfuric acid-hydrogen peroxide system, while widely used, suffers from issues such as low economic efficiency and residual sulfate impurities. Patent application number 202310182897.3 discloses a method for regenerating spent lithium iron phosphate battery cathode materials. This method combines physical repair with hydrometallurgical processes, but the multi-step process results in a cumbersome process and a low lithium recovery rate. Therefore, the development of efficient, short-cycle regeneration processes is urgent. Patent application number 202210256273.7 proposes using heteropolyacids as electrolyte redox media to extract lithium from lithium iron phosphate. While this method produces a lithium-rich solution, the iron phosphate (FePO4) accumulated on the anode side is difficult to directly recycle due to the lack of impurity removal. Furthermore, patent application number 202210528476.7 uses alkaline leaching to remove impurities to produce lithium carbonate, but does not clearly define a resource recovery path for the residual iron phosphate in the leached residue.

[0004] In summary, developing a green and environmentally friendly lithium extraction process that can improve economic benefits without introducing additional impurities (such as sulfate and aluminum ions) is crucial for the efficient recovery of battery black powder. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a resource recovery method for spent lithium iron phosphate black powder to solve the problems of low economic benefits, complicated process and high impurity content in the existing lithium iron phosphate black powder recovery.

[0006] The purpose of the present invention is achieved through the following technical solutions: A method for recycling spent lithium iron phosphate black powder comprises the following steps: (1) Mixing spent lithium iron phosphate black powder with water to form a slurry, adding acidic substances and hydrogen peroxide for acid leaching to obtain a leachate; (2) Adjusting the pH of the leaching solution to 2-3, performing solid-liquid separation to obtain a copper-removed solution, adding a complexing agent to the copper-removed solution, and performing solid-liquid separation to obtain an aluminum-removed solution; (3) adjusting the Fe / P molar ratio of the liquid after aluminum removal to 1:1~1.05, adjusting the pH to 2~3, and then separating the solid and liquid. The obtained solid phase is washed, slurried, recrystallized and dried to obtain anhydrous ferric phosphate, and the filtrate is a lithium-rich liquid; (4) Anhydrous iron phosphate, lithium-rich liquid and organic matter are mixed, spray-dried into powder and then calcined to obtain carbon-coated lithium iron phosphate.

[0007] Preferably, in step (1), the solid-to-liquid ratio of spent lithium iron phosphate black powder to water is 50-200 g / L.

[0008] Preferably, in step (1), the acidic substance is phosphoric acid.

[0009] Preferably, in step (1), the amount of acidic substance added should be sufficient to maintain the pH of the slurry at 1-1.5.

[0010] Preferably, in step (1), the amount of hydrogen peroxide added is 5-10 wt % of the spent lithium iron phosphate black powder.

[0011] Preferably, in step (1), the acid leaching time is 1 to 2 hours.

[0012] Preferably, in step (1), the concentration of hydrogen peroxide is 30%.

[0013] Preferably, in step (2), the pH of the leachate is adjusted to 2-3 by adding an alkaline substance to the leachate to adjust the pH to 2-3.

[0014] Preferably, the alkaline substance is at least one of ammonia water, sodium hydroxide and sodium carbonate.

[0015] Preferably, in step (2), the molar ratio of the complexing agent to the aluminum ions in the copper-removed solution is 1 to 6:1.

[0016] Preferably, in step (2), the complexing agent is at least one of sodium tetraphenylborate, hydrofluoric acid, sodium fluoride and ammonium fluoride.

[0017] Preferably, in step (3), the Fe / P molar ratio of the dealuminized solution is adjusted to 1:1-1.05 by adding phosphoric acid solution.

[0018] Preferably, in step (3), the pH is adjusted to 2-3 by adding ammonia water.

[0019] Preferably, in step (3), the specific operation of slurrying is: stirring the washed wet solid uniformly to obtain a slurry.

[0020] Preferably, in step (3), the specific operation of recrystallization is: dissolving the slurried material in water, and then purifying the crystals through a heating-cooling cycle.

[0021] Preferably, in step (4), the mass ratio of anhydrous ferric phosphate, lithium-rich solution and organic matter is 10-15:50-100:0.8-1.5.

[0022] Preferably, in step (4), the organic matter is at least one of polyvinyl pyrrolidone, dopamine, glucose and sucrose.

[0023] Preferably, in step (4), the spray drying parameters are: inlet temperature of 200-230°C, and feed rate of 2-7 mL / min.

[0024] Preferably, in step (4), the specific operation of calcination is: in a reducing atmosphere, heating to 400-500°C at 5-9°C / min and holding at this temperature for 1-3 hours, then continuing to heat to 650-750°C and holding at this temperature for 1-3 hours. The purpose of staged calcination is to produce lithium iron phosphate with a uniform and complete crystal structure, avoiding incomplete particle structure and performance defects caused by a single high-temperature treatment.

[0025] Preferably, the reducing atmosphere is a hydrogen-nitrogen mixture with a volume fraction of 3% to 7% H2. The main function of the reducing atmosphere is to prevent the divalent iron in the material from being oxidized to trivalent iron.

[0026] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses phosphoric acid to acid-leach waste lithium iron phosphate black powder, avoiding the introduction of difficult-to-remove impurities such as sulfate ions and chloride ions, thereby achieving efficient resource recovery of the three elements lithium, iron and phosphorus.

[0027] (2) The present invention adopts a spray drying method to prepare iron phosphate precursor particles with uniform particle size and high sphericity. After a high-temperature solid-phase lithiation reaction, the resulting lithium iron phosphate positive electrode material has a capacity retention rate of 95.3% to 96.3% after 100 cycles at a 1C rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is the XRD diffraction spectrum of the carbon-coated lithium iron phosphate powder prepared in Example 1. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0030] Example 1 A method for recycling spent lithium iron phosphate black powder, comprising the following steps: (1) Pre-processing the waste lithium iron phosphate batteries, including but not limited to one or more of discharge, heat treatment (high temperature oxygen-free roasting), crushing, and screening, to obtain spent lithium iron phosphate black powder with a particle size of less than 160 mesh; (2) Weigh 80g of spent lithium iron phosphate battery black powder, add deionized water to adjust the initial solid-liquid ratio to 180g / L, homogenize for 30min, add 82g of 85% phosphoric acid solution to adjust the system pH to 1.5, then add hydrogen peroxide (concentration 30%) accounting for 7% of the weight of spent lithium iron phosphate battery black powder, and react for 2h to obtain the leachate; the main elements of the leachate are shown in Table 1; Table 1 Statistics of main element contents in leachate

[0031] It can be seen from Table 1 that the lithium content of the leachate is 0.61%, the iron content is 4.85%, the phosphorus content is 2.49%, the aluminum content is 0.20%, and the copper content is 0.25%. Subsequent impurity removal experiments are carried out with the black powder leachate of this quality.

[0032] (3) Sodium carbonate solution was added to the leachate to adjust the pH of the system to 2.5. The mixture was stirred for 20 min to separate the copper from the solid and liquid to obtain a copper-free solution. 4.67 g of sodium tetraphenylborate was added to the copper-free solution to remove aluminum from the solid and liquid to obtain an aluminum-free solution. The main elements of the aluminum-free solution are shown in Table 2. Table 2 Statistics of main element contents in the solution after aluminum removal

[0033] As can be seen from Table 2, the lithium content of the leachate is 0.60%, the iron content is 4.55%, the phosphorus content is 3.29%, the aluminum content is 0.001%, and the copper content is 0.001%. The dealuminized liquid of this quality is used to perform a hydrothermal reaction to synthesize anhydrous ferric phosphate.

[0034] (4) 200 g of the dealuminized solution was placed in a reactor, and an 85% phosphoric acid solution was added to adjust the Fe:P molar ratio in the solution to 1:1.02. Ammonia was added to slowly adjust the pH to 2.2. After solid-liquid separation, the obtained solid phase was washed, slurried, recrystallized and dried to obtain high-purity anhydrous iron phosphate. The filtrate was a lithium-rich solution. (5) 76.7 g of lithium-rich solution, 10 g of anhydrous ferric phosphate, and 1 g of sucrose were mixed evenly, and then spray-dried in a spray dryer at an inlet temperature of 220 °C and a feed rate of 6 mL / min to obtain a grass-green powder; (6) The obtained grass green powder was heat treated in a hydrogen-nitrogen mixed gas with a H2 volume fraction of 5%, firstly heated to 400°C and kept warm for calcination for 2 h, and then continued to heat to 700°C and kept warm for calcination for 2 h to obtain black carbon-coated lithium iron phosphate powder.

[0035] Figure 1 The XRD diffraction spectrum of the carbon-coated lithium iron phosphate powder prepared in Example 1 is shown in FIG. Figure 1 We can know that Example 1 successfully prepared pure phase lithium iron phosphate.

[0036] Example 2 A method for recycling spent lithium iron phosphate black powder, comprising the following steps: (1) Pre-processing the waste lithium iron phosphate batteries, including but not limited to one or more of discharge, heat treatment (high temperature oxygen-free roasting), crushing, and screening, to obtain spent lithium iron phosphate black powder with a particle size of less than 160 mesh; (2) Weigh 80g of spent lithium iron phosphate battery black powder, add deionized water to adjust the initial solid-liquid ratio to 150g / L, homogenize for 30min, add 82g of 85% phosphoric acid solution to adjust the system pH to 1.3, then add hydrogen peroxide (30%) accounting for 5% of the weight of spent lithium iron phosphate battery black powder, and react for 2h to obtain the leachate; the main elements of the leachate are shown in Table 3; Table 3 Statistics of main element contents in leachate

[0037] It can be seen from Table 3 that the lithium content of the leachate is 0.62%, the iron content is 4.95%, the phosphorus content is 3.39%, the aluminum content is 0.21%, and the copper content is 0.35%. Subsequent impurity removal experiments are carried out with the black powder leachate of this quality.

[0038] (3) Sodium carbonate solution was added to the leachate to adjust the pH of the system to 2.5. The mixture was stirred for 20 min to separate the copper from the solid and liquid to obtain a copper-free solution. 4.17 g of sodium fluoride was added to the copper-free solution to remove aluminum from the solid and liquid to obtain an aluminum-free solution. The main elements of the aluminum-free solution are shown in Table 4. Table 4 Statistics of main element contents in the solution after aluminum removal

[0039] As can be seen from Table 4, the lithium content of the leachate is 0.60%, the iron content is 4.65%, the phosphorus content is 3.39%, the aluminum content is 0.001%, and the copper content is 0.001%. The dealuminized liquid of this quality is used to perform a hydrothermal reaction to synthesize anhydrous ferric phosphate.

[0040] (4) 200 g of the dealuminized solution was placed in a reactor, and an 85% phosphoric acid solution was added to adjust the Fe:P molar ratio in the solution to 1:1.02. Ammonia was added to slowly adjust the pH to 2.2. After solid-liquid separation, the obtained solid phase was washed, slurried, recrystallized and dried to obtain high-purity anhydrous iron phosphate. The filtrate was a lithium-rich solution. (5) 76.7 g of lithium-rich solution, 10 g of anhydrous ferric phosphate, and 1 g of sucrose were mixed evenly, and then spray-dried in a spray dryer at an inlet temperature of 220 °C and a feed rate of 6 mL / min to obtain a grass-green powder; (6) The obtained grass green powder was heat treated in a hydrogen-nitrogen mixed gas with a H2 volume fraction of 5%, firstly heated to 400°C and kept warm for calcination for 2 h, and then continued to heat to 700°C and kept warm for calcination for 2 h to obtain black carbon-coated lithium iron phosphate powder.

[0041] Example 3 A method for recycling spent lithium iron phosphate black powder, comprising the following steps: (1) Pre-processing the waste lithium iron phosphate batteries, including but not limited to one or more of discharge, heat treatment (high temperature oxygen-free roasting), crushing, and screening, to obtain spent lithium iron phosphate black powder with a particle size of less than 160 mesh; (2) Weigh 80g of spent lithium iron phosphate battery black powder, add deionized water to adjust the initial solid-liquid ratio to 150g / L, homogenize for 30min, add 82g of 85% phosphoric acid solution to adjust the system pH to 1.3, then add hydrogen peroxide (30%) accounting for 8% of the mass of spent lithium iron phosphate battery black powder, and react for 2h to obtain the leachate; the main elements of the leachate are shown in Table 5; Table 5 Statistics of main element contents in leachate

[0042] It can be seen from Table 5 that the lithium content of the leachate is 0.58%, the iron content is 4.95%, the phosphorus content is 3.59%, the aluminum content is 0.24%, and the copper content is 0.28%. Subsequent impurity removal experiments are carried out with the black powder leachate of this quality.

[0043] (3) Sodium carbonate solution was added to the leachate to adjust the pH of the system to 2.5. The mixture was stirred for 20 min to separate the copper from the solid and liquid to obtain a copper-free solution. 4.27 g of hydrofluoric acid was added to the copper-free solution to separate the aluminum from the solid and liquid to obtain an aluminum-free solution. The main elements of the aluminum-free solution are shown in Table 6. Table 6 Statistics of main element contents in the solution after aluminum removal

[0044] It can be seen from Table 6 that the lithium content of the leachate is 0.55%, the iron content is 4.35%, the phosphorus content is 3.39%, the aluminum content is 0.001%, and the copper content is 0.001%. The dealuminized liquid of this quality is used to perform a hydrothermal reaction to synthesize anhydrous ferric phosphate.

[0045] (4) 200 g of the dealuminized solution was placed in a reactor, and an 85% phosphoric acid solution was added to adjust the Fe:P molar ratio in the solution to 1:1.02. Ammonia was added to slowly adjust the pH to 2.2. After solid-liquid separation, the obtained solid phase was washed, slurried, recrystallized and dried to obtain high-purity anhydrous iron phosphate. The filtrate was a lithium-rich solution. (5) 76.7 g of lithium-rich solution, 10 g of anhydrous ferric phosphate, and 1 g of dopamine were mixed uniformly, and then spray-dried in a spray dryer at an inlet temperature of 220 °C and a feed rate of 2 mL / min to obtain a grass-green powder; (6) The obtained grass green powder was heat treated in a hydrogen-nitrogen mixed gas with a H2 volume fraction of 3%, firstly heated to 400°C and kept warm for calcination for 2 h, and then continued to heat to 700°C and kept warm for calcination for 2 h to obtain black carbon-coated lithium iron phosphate powder.

[0046] Example 4 A method for recycling spent lithium iron phosphate black powder, comprising the following steps: (1) Pre-processing the waste lithium iron phosphate batteries, including but not limited to one or more of discharge, heat treatment (high temperature oxygen-free roasting), crushing, and screening, to obtain spent lithium iron phosphate black powder with a particle size of less than 160 mesh; (2) Weigh 80g of spent lithium iron phosphate battery black powder, add deionized water to adjust the initial solid-liquid ratio to 150g / L, homogenize for 30min, add 82g of 85% phosphoric acid solution to adjust the system pH to 1.7, then add hydrogen peroxide (concentration 30%) accounting for 7% of the weight of spent lithium iron phosphate battery black powder, and react for 2h to obtain the leachate; the main elements of the leachate are shown in Table 7; Table 7 Statistics of main element contents in leachate

[0047] It can be seen from Table 7 that the lithium content of the leachate is 0.60%, the iron content is 5.05%, the phosphorus content is 3.79%, the aluminum content is 0.25%, and the copper content is 0.33%. Subsequent impurity removal experiments are carried out with the black powder leachate of this quality.

[0048] (3) Sodium carbonate solution was added to the leachate to adjust the pH of the system to 2.5. The mixture was stirred for 20 min to separate the copper from the solid and liquid to obtain a copper-free solution. 4.27 g of hydrofluoric acid was added to the copper-free solution to separate the aluminum from the solid and liquid to obtain an aluminum-free solution. The main elements of the aluminum-free solution are shown in Table 8. Table 8 Statistics of main element contents in the liquid after aluminum removal

[0049] It can be seen from Table 8 that the lithium content of the leachate is 0.58%, the iron content is 4.95%, the phosphorus content is 3.69%, the aluminum content is 0.001%, and the copper content is 0.001%. The dealuminized liquid of this quality is used to perform a hydrothermal reaction to synthesize anhydrous ferric phosphate.

[0050] (4) 200 g of the dealuminized solution was placed in a reactor, and an 85% phosphoric acid solution was added to adjust the Fe:P molar ratio in the solution to 1:1.02. Ammonia was added to slowly adjust the pH to 2.2. After solid-liquid separation, the obtained solid phase was washed, slurried, recrystallized and dried to obtain high-purity anhydrous iron phosphate. The filtrate was a lithium-rich solution. (5) 76.7 g of lithium-rich solution, 10 g of anhydrous ferric phosphate, and 1 g of glucose were mixed evenly, and then spray-dried in a spray dryer at an inlet temperature of 220 °C and a feed rate of 2 mL / min to obtain a grass-green powder; (6) The obtained grass green powder was heat treated in a hydrogen-nitrogen mixed gas with a H2 volume fraction of 2%, firstly heated to 400°C and kept warm for calcination for 2 h, and then continued to heat to 700°C and kept warm for calcination for 2 h to obtain black carbon-coated lithium iron phosphate powder.

[0051] Example 5 A method for recycling spent lithium iron phosphate black powder, comprising the following steps: (1) Pre-processing the waste lithium iron phosphate batteries, including but not limited to one or more of discharge, heat treatment (high temperature oxygen-free roasting), crushing, and screening, to obtain spent lithium iron phosphate black powder with a particle size of less than 160 mesh; (2) Weigh 80g of spent lithium iron phosphate battery black powder, add deionized water to adjust the initial solid-liquid ratio to 150g / L, homogenize for 30min, add 82g of 85% phosphoric acid solution to adjust the system pH to 1.7, then add hydrogen peroxide (30%) accounting for 8% of the weight of spent lithium iron phosphate battery black powder, and react for 2h to obtain the leachate; the main elements of the leachate are shown in Table 9; Table 9 Statistics of main element contents in leachate

[0052] It can be seen from Table 9 that the lithium content of the leachate is 0.60%, the iron content is 4.95%, the phosphorus content is 3.69%, the aluminum content is 0.26%, and the copper content is 0.30%. Subsequent impurity removal experiments are carried out with black powder leachate of this quality.

[0053] (3) Sodium carbonate solution was added to the leachate to adjust the pH of the system to 2.5. The mixture was stirred for 20 min to separate the copper from the solid and liquid to obtain a copper-free solution. 4.27 g of ammonium fluoride was added to the copper-free solution to separate the aluminum from the solid and liquid to obtain an aluminum-free solution. The main elements of the aluminum-free solution are shown in Table 10. Table 10 Statistics of main element contents in the solution after aluminum removal

[0054] As can be seen from Table 10, the lithium content of the leachate is 0.57%, the iron content is 4.65%, the phosphorus content is 3.59%, the aluminum content is 0.001%, and the copper content is 0.001%. The dealuminized liquid of this quality is used to perform a hydrothermal reaction to synthesize anhydrous ferric phosphate.

[0055] (4) 200 g of the dealuminized solution was placed in a reactor, and an 85% phosphoric acid solution was added to adjust the Fe:P molar ratio in the solution to 1:1.02. Ammonia was added to slowly adjust the pH to 2.2. After solid-liquid separation, the obtained solid phase was washed, slurried, recrystallized and dried to obtain high-purity anhydrous iron phosphate. The filtrate was a lithium-rich solution. (5) 76.7 g of lithium-rich solution, 10 g of anhydrous ferric phosphate, and 1 g of polyvinyl pyrrolidone were mixed uniformly, and then spray-dried in a spray dryer at an inlet temperature of 220 °C and a feed rate of 2 mL / min to obtain a grass-green powder; (6) The obtained grass green powder was heat treated in a hydrogen-nitrogen mixed gas with a H2 volume fraction of 7%, firstly heated to 400°C and kept warm for calcination for 2 h, and then continued to heat to 700°C and kept warm for calcination for 2 h to obtain black carbon-coated lithium iron phosphate powder.

[0056] Comparative Example 1 A method for recycling spent lithium iron phosphate black powder, comprising the following steps: (1) Pre-processing the waste lithium iron phosphate batteries, including but not limited to one or more of discharge, heat treatment (high temperature oxygen-free roasting), crushing, and screening, to obtain spent lithium iron phosphate black powder with a particle size of less than 160 mesh; (2) Weigh 80g of spent lithium iron phosphate battery black powder, add deionized water to adjust the initial solid-liquid ratio to 150g / L, homogenize for 30min, add 82g of 85% phosphoric acid solution to adjust the system pH to 1.5, then add hydrogen peroxide (concentration 30%) accounting for 7% of the weight of spent lithium iron phosphate battery black powder, and react for 2h to obtain the leachate; the main elements of the leachate are shown in Table 11; Table 11 Statistics of main element contents in leachate

[0057] It can be seen from Table 11 that the lithium content of the leachate is 0.60%, the iron content is 4.95%, the phosphorus content is 2.69%, the aluminum content is 0.26%, and the copper content is 0.30%. Subsequent impurity removal experiments are carried out using black powder leachate of this quality.

[0058] (3) Sodium carbonate solution was added to the leachate to adjust the pH of the system to 2.5. The mixture was stirred for 20 min to separate the copper from the solid and liquid to obtain a copper-free solution. 4.27 g of ammonium fluoride was added to the copper-free solution to separate the aluminum from the solid and liquid to obtain an aluminum-free solution. The main elements of the aluminum-free solution are shown in Table 12. Table 12 Statistics of main element contents in the solution after aluminum removal

[0059] It can be seen from Table 12 that the lithium content of the leachate is 0.57%, the iron content is 4.65%, the phosphorus content is 2.59%, the aluminum content is 0.001%, and the copper content is 0.001%. The dealuminized liquid of this quality is used to perform a hydrothermal reaction to synthesize anhydrous ferric phosphate.

[0060] (4) 200 g of the dealuminized solution was placed in a reactor, and an 85% phosphoric acid solution was added to adjust the Fe:P molar ratio in the solution to 1:1.02. Ammonia was added to slowly adjust the pH to 2.2. After solid-liquid separation, the obtained solid phase was washed, slurried, recrystallized and dried to obtain high-purity anhydrous iron phosphate. The filtrate was a lithium-rich solution. (5) Take 76.7g of lithium-rich liquid, 10g of anhydrous iron phosphate and 1g of polyvinyl pyrrolidone and mix them evenly, then heat treat them under nitrogen atmosphere, first heat to 400℃ and keep warm for 2h, then continue to heat to 700℃ and keep warm for 2h to obtain black carbon-coated lithium iron phosphate powder.

[0061] Comparative Example 2 A method for recycling spent lithium iron phosphate black powder, comprising the following steps: (1) Pre-processing the waste lithium iron phosphate batteries, including but not limited to one or more of discharge, heat treatment (high temperature oxygen-free roasting), crushing, and screening, to obtain spent lithium iron phosphate black powder with a particle size of less than 160 mesh; (2) Weigh 80g of spent lithium iron phosphate battery black powder, add deionized water to adjust the initial solid-liquid ratio to 180g / L, homogenize for 30min, add 82g of 85% phosphoric acid solution to adjust the system pH to 1.5, then add hydrogen peroxide (30%) accounting for 5% of the weight of spent lithium iron phosphate battery black powder, and react for 2h to obtain the leachate; the main elements of the leachate are shown in Table 13; Table 13 Statistics of main element contents in leachate

[0062] It can be seen from Table 13 that the lithium content of the leachate is 0.60%, the iron content is 4.75%, the phosphorus content is 2.29%, the aluminum content is 0.32%, and the copper content is 0.65%. Subsequent impurity removal experiments are carried out using black powder leachate of this quality.

[0063] (3) Sodium carbonate solution was added to the leachate to adjust the pH of the system to 2.5, and the mixture was stirred for 20 minutes to separate the copper from the solid and liquid to obtain a copper-removed solution. The pH of the copper-removed solution was adjusted to 4.5, and the mixture was stirred for 20 minutes to remove the aluminum from the solid and liquid to obtain an aluminum-removed solution. The main elements of the aluminum-removed solution are shown in Table 14. Table 14 Statistics of main element contents in the solution after aluminum removal

[0064] It can be seen from Table 14 that the lithium content of the leachate is 0.55%, the iron content is 4.15%, the phosphorus content is 2.09%, the aluminum content is 0.11%, and the copper content is 0.002%. The dealuminized liquid of this quality is used to perform a hydrothermal reaction to synthesize anhydrous ferric phosphate.

[0065] (4) 200 g of the dealuminized solution was placed in a reactor, and an 85% phosphoric acid solution was added to adjust the Fe:P molar ratio in the solution to 1:1.02. Ammonia was added to slowly adjust the pH to 2.2. After solid-liquid separation, the obtained solid phase was washed, slurried, recrystallized and dried to obtain high-purity anhydrous iron phosphate. The filtrate was a lithium-rich solution. (5) 76.7 g of lithium-rich solution, 10 g of anhydrous ferric phosphate, and 1 g of sucrose were mixed evenly, and then spray-dried in a spray dryer at an inlet temperature of 220 °C and a feed rate of 6 mL / min to obtain a grass-green powder; (6) The obtained grass green powder was heat treated in a hydrogen-nitrogen mixed gas with a H2 volume fraction of 5%, firstly heated to 400°C and kept warm for calcination for 2 h, and then continued to heat to 700°C and kept warm for calcination for 2 h to obtain black carbon-coated lithium iron phosphate powder.

[0066] The carbon-coated lithium iron phosphate powders prepared in Examples 1 to 5 and Comparative Examples 1 to 2 were used as positive electrode active materials to prepare soft-pack batteries for electrochemical performance testing.

[0067] Among them, the preparation steps of the soft pack battery are: (1) Cathode preparation: Carbon-coated lithium iron phosphate powder (mass fraction 95%), acetylene black (first conductive agent, 2.5%), carbon black (second conductive agent, 0.5%), and polyvinylidene fluoride (PVDF, binder, 2%) were placed in N-methylpyrrolidone (NMP) solvent and stirred to form a homogeneous slurry. The slurry was coated on an aluminum foil current collector with an area density of 280 mg / cm 2 , and obtain the positive electrode.

[0068] (2) Battery assembly: A soft-pack battery was assembled using graphite as the negative electrode, a polypropylene microporous membrane as the separator, and 1 mol / L LiPF6 / EC+DEC (mass ratio of 1:1) as the electrolyte.

[0069] Electrochemical performance test items are as follows: (1) First charge and discharge specific capacity test The voltage range is 2.5~3.5V, the current density is 0.1C, and the test results are shown in Table 15. Among them, 1C is the current required for the battery to discharge the rated capacity of 170mAh / g in 1h.

[0070] (2) Cyclic performance test Under the conditions of a voltage range of 2.5~3.5V and a current density of 1C, after a certain number of charge and discharge cycles, the charge specific capacity and discharge specific capacity were tested respectively, and the capacity retention rate was calculated. The calculation results are also shown in Table 15.

[0071] Table 15 Electrochemical performance test results

[0072] As shown in Table 15, first, the carbon-coated lithium iron phosphate powders prepared in Examples 1 to 5 of the present invention exhibit good electrochemical properties when used to prepare soft-pack batteries, and meet the performance standards of commercial lithium iron phosphate materials. This shows that the recycled materials obtained by the full-element recovery and regeneration method of failed lithium iron phosphate black powder provided by the present invention have the potential for industrial application. Secondly, in Comparative Example 1, the material is directly heat-treated under a nitrogen atmosphere without undergoing a spray drying step. Since the nitrogen atmosphere cannot effectively prevent the divalent iron in the material from being oxidized to trivalent iron, the final product contains trivalent iron impurities, thereby affecting the electrochemical performance of the battery; in addition, the direct heat treatment without spray drying granulation causes the obtained lithium iron phosphate particles to have an uneven particle size distribution and irregular morphology, further affecting their electrochemical performance. Finally, Comparative Example 2 promotes aluminum precipitation by adjusting the pH value of the system to achieve aluminum removal, but its aluminum removal efficiency is low, far inferior to the method of the present invention, resulting in a high content of aluminum impurities in the obtained lithium iron phosphate product, which in turn leads to a decrease in the electrochemical performance of the battery.

[0073] The specific embodiments of the present invention described above do not limit the scope of protection of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the scope of protection of the claims of the present invention.

Claims

1. A method for recycling spent lithium iron phosphate black powder, characterized in that: The steps include: (1) Mixing spent lithium iron phosphate black powder with water to form a slurry, adding acidic substances and hydrogen peroxide for acid leaching to obtain a leachate; (2) Adjusting the pH of the leaching solution to 2-3, performing solid-liquid separation to obtain a copper-removed solution, adding a complexing agent to the copper-removed solution, and performing solid-liquid separation to obtain an aluminum-removed solution; (3) adjusting the Fe / P molar ratio of the liquid after aluminum removal to 1:1~1.05, adjusting the pH to 2~3, and then separating the solid and liquid. The obtained solid phase is washed, slurried, recrystallized and dried to obtain anhydrous ferric phosphate, and the filtrate is a lithium-rich liquid; (4) Anhydrous iron phosphate, lithium-rich liquid and organic matter are mixed, spray-dried into powder and then calcined to obtain carbon-coated lithium iron phosphate.

2. The method for recycling spent lithium iron phosphate black powder according to claim 1, characterized in that: The solid-to-liquid ratio of the spent lithium iron phosphate black powder to water in step (1) is 50-200 g / L; The amount of the acidic substance added in step (1) should be such that the pH of the slurry is maintained at 1-1.5; The amount of hydrogen peroxide added in step (1) is 5-10 wt% of the spent lithium iron phosphate black powder.

3. The method for recycling spent lithium iron phosphate black powder according to claim 2, characterized in that: The acidic substance in step (1) is phosphoric acid.

4. The method for recycling spent lithium iron phosphate black powder according to claim 1, characterized in that: The method of adjusting the pH of the leachate to 2-3 in step (2) is to adjust the pH to 2-3 by adding an alkaline substance to the leachate.

5. The method for recycling spent lithium iron phosphate black powder according to any one of claims 1 to 2, characterized in that: The molar ratio of the complexing agent in step (2) to the aluminum ions in the copper removal solution is 1 to 6:

1.

6. The method for recycling spent lithium iron phosphate black powder according to claim 4, characterized in that: The alkaline substance is at least one of ammonia water, sodium hydroxide and sodium carbonate; The complexing agent in step (2) is at least one of sodium tetraphenylborate, hydrofluoric acid, sodium fluoride and ammonium fluoride.

7. The method for recycling spent lithium iron phosphate black powder according to claim 1, characterized in that: In step (3), the Fe / P molar ratio of the dealuminized solution is adjusted to 1:1-1.05 by adding phosphoric acid solution; In step (3), the pH is adjusted to 2-3 by adding ammonia water.

8. The method for recycling spent lithium iron phosphate black powder according to claim 1, characterized in that: The mass ratio of the anhydrous ferric phosphate, lithium-rich solution and organic matter in step (4) is 10-15:50-100:0.8-1.

5.

9. The method for recycling spent lithium iron phosphate black powder according to claim 1, characterized in that: The specific operation of the calcination in step (4) is: in a reducing atmosphere, heating to 400-500°C at 5-9°C / min and keeping warm for 1-3 hours, then continuing to heat to 650-750°C and keeping warm for 1-3 hours.

10. The method for recycling spent lithium iron phosphate black powder according to claim 9, characterized in that: The organic matter in step (4) is at least one of polyvinyl pyrrolidone, dopamine, glucose and sucrose; The reducing atmosphere is a hydrogen-nitrogen mixed gas with a H2 volume fraction of 3% to 7%.

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