Method for removing aluminum from lithium iron phosphate black powder

Abstract

The present application belongs to the technical field of lithium iron phosphate battery recycling, and particularly relates to a method for removing aluminum from lithium iron phosphate black powder, comprising the following steps: (1) mixing lithium iron phosphate black powder with water to prepare a slurry; (2) mixing the slurry prepared in step (1) with an aluminum removal liquid, heating and reacting under stirring, and then performing solid-liquid separation to obtain a first filtrate and a filter residue, and washing the filter residue to obtain aluminum-removed lithium iron phosphate; the aluminum removal liquid is prepared by mixing a solute with a solvent, and the solute comprises sodium chloride, sodium sulfide, sodium phosphate and sodium oxalate.

Description

Technical Field

[0001] This invention belongs to the field of lithium iron phosphate battery recycling technology, and specifically relates to a method for removing aluminum from lithium iron phosphate black powder. Background Technology

[0002] Lithium iron phosphate (LFP), as one of the earliest commercially available cathode materials for electric vehicles, faces an increasingly urgent need for battery recycling as new energy electric vehicles are being phased out. Unlike other cathode materials (ternary lithium, lithium cobalt oxide, etc.), LFP does not contain precious metals such as Co and Ni, and its inherent lithium content, which has high recycling value, is very low, approximately 2-4 wt%, resulting in low economic benefits from recycling and insufficient incentive for companies to recycle it. It is worth noting that LFP has a stable olive-shaped structure, which it retains after cycling. Theoretically, LFP can be repaired and regenerated simply by replenishing the lithium lost during cycling.

[0003] Waste lithium iron phosphate (LFP) black powder is obtained from LFP batteries through crushing, sorting, and sieving. During the pretreatment process, impurities such as graphite, Cu, and Al are inevitably introduced. For Al, a small amount of doping can improve the specific capacity and rate performance of LFP batteries to some extent; however, excessive doping not only reduces the amount of active material Li... + The amount of aluminum in LFP also leads to interfacial reactions with LiFePO4 / C, generating inert substances AlPO4 and Li3PO4, which reduces the conductivity and specific capacity of LFP. HG / T4701-2021 clearly states that the Al content in LFP should be below 0.05%. Therefore, reducing the aluminum content in recycled LFP black powder is crucial for the remediation and regeneration of lithium iron phosphate.

[0004] Currently reported methods for aluminum removal include alkaline leaching, selective leaching, ion exchange resin leaching, neutralization, and chemical precipitation. Chemical precipitation and ion exchange resin leaching target the removal of aluminum ions from dissolved cathode powder solutions, while aluminum in black powder exists as aluminum particles with oxidized surfaces. Neutralization and selective leaching methods mostly use acid to dissolve aluminum, which inevitably leads to the loss of phosphorus iron. Based on the characteristics of whether lithium iron phosphate and aluminum dissolve in alkaline solutions, alkaline leaching is currently the most widely used aluminum removal method. However, in practical applications, the presence of sodium hydroxide can lead to the loss of Fe. 2+ The lithium iron phosphate is oxidized to iron phosphate and Fe3O4, which simultaneously destroys the structure of the iron phosphate, thereby leaching out lithium phosphate. Other reported aluminum removal agents, such as lithium hydroxide, lithium carbonate, ammonia, and sodium carbonate, essentially remove aluminum by reacting the hydroxide ions generated from hydrolysis or ionization with aluminum or aluminum hydroxide. This process often results in the destruction of the lithium iron phosphate structure, leading to the leaching of phosphorus, iron, and lithium. This significantly increases the difficulty and cost of directly remediating waste lithium iron phosphate.

[0005] It is worth noting that lithium iron phosphate (LFP) reacts much more reliably with alkaline solutions than with Al and alkali. This causes a rapid drop in pH at the initial feeding stage of conventional aluminum removal solutions, preventing Al dissolution. Simultaneously, the oxide film on the Al surface also contributes to the slow dissolution of Al. Furthermore, because the aluminum removal system is exposed to air, Fe in LFP... 2+ Inevitably, it will be oxidized to ferric iron, generating impurity phases. After the aluminum removal reaction, the decrease in temperature and pH during solid-liquid separation leads to the formation of Al(OH)3 crystal nuclei, causing filtration difficulties and Al residue. Furthermore, the large amount of high-salt wastewater generated by traditional alkaline leaching requires enormous energy consumption if treated by evaporation, which is detrimental to sustainable development. Non-destructive aluminum removal from waste lithium iron phosphate black powder is a challenging problem for the direct recycling of lithium iron phosphate, urgently requiring the development of an economical, efficient, and environmentally friendly method for aluminum removal from lithium iron phosphate black powder that does not damage the lithium iron phosphate structure. Summary of the Invention

[0006] This invention aims to at least solve one of the technical problems existing in related technologies. To this end, this invention proposes a method for removing aluminum from lithium iron phosphate black powder. This method efficiently removes aluminum from lithium iron phosphate black powder, ensuring the performance of the repaired lithium iron phosphate.

[0007] The above-mentioned technical objective of the present invention is achieved through the following technical solution:

[0008] A method for removing aluminum from lithium iron phosphate black powder includes the following steps:

[0009] (1) Mix lithium iron phosphate black powder with water to make a slurry;

[0010] (2) The slurry obtained in step (1) is mixed with the aluminum removal liquid and heated and reacted under stirring. The solid and liquid are separated to obtain a primary filtrate and a filter residue. The filter residue is washed to obtain lithium iron phosphate after aluminum removal. The aluminum removal liquid is composed of a solute and a solvent. The solute includes sodium chloride, sodium sulfide, sodium phosphate and sodium oxalate.

[0011] In one embodiment, in step (1), the aluminum content in the lithium iron phosphate black powder is greater than 0.5%.

[0012] In one embodiment, in step (1), the liquid-to-solid ratio of the water to the lithium iron phosphate black powder is (1-2) mL: 1 g.

[0013] In one embodiment, in step (1), the liquid-to-solid ratio of the water to the lithium iron phosphate black powder is (1-1.5) mL: 1 g.

[0014] In one embodiment, in step (1), the mixing is carried out in a water bath at a temperature of 50-90°C.

[0015] In one embodiment, in step (1), the mixing is carried out in a water bath at a temperature of 50-80°C.

[0016] In one embodiment, in step (1), the mixing is carried out under stirring at a speed of 200-800 rpm.

[0017] In one embodiment, in step (1), the mixing is carried out under stirring at a speed of 450-800 rpm.

[0018] In one embodiment, in step (2), the concentration of sodium chloride in the aluminum removal solution is 10-50 g / L, and the mass ratio of sodium chloride, sodium sulfide, sodium phosphate and sodium oxalate in the aluminum removal solution is 1:(0.5-20):(2.5-20):(0.5-10).

[0019] In one embodiment, in step (2), the concentration of sodium chloride in the aluminum removal solution is 10-50 g / L, and the mass ratio of sodium chloride, sodium sulfide, sodium phosphate and sodium oxalate in the aluminum removal solution is 1:(5-20):(5-10):(1-5).

[0020] In one embodiment, in step (2), the mass ratio of the aluminum removal liquid to the slurry is (1-10):1.

[0021] In one embodiment, in step (2), the heating temperature is 50-90°C.

[0022] In one embodiment, in step (2), the heating temperature is 50-80°C.

[0023] In one embodiment, the method further includes a step (3) of circulating the primary filtrate obtained in step (2) to remove aluminum. The specific content of step (3) is as follows: the primary filtrate obtained in step (2) is mixed with waste lithium iron phosphate black powder, sodium sulfide is added, the mixture is heated and reacted under stirring, and the solid and liquid are separated to obtain filtrate and filter residue. The obtained filter residue is washed to obtain lithium iron phosphate after aluminum removal. The obtained filtrate is used for the next circulating aluminum removal step. The circulating aluminum removal step is repeated 1-5 times, and aluminum removal waste liquid is obtained after the step is completed.

[0024] In one embodiment, in step (3), the mass ratio of the added sodium sulfide to the volume of the primary filtrate is 10-50 g / L.

[0025] In one embodiment, in step (3), the mass ratio of the added sodium sulfide to the volume of the primary filtrate is 10-25 g / L.

[0026] In one embodiment, in step (3), the heating temperature is 50-90°C.

[0027] In one embodiment, in step (3), the heating temperature is 50-80°C.

[0028] In one embodiment, the method further includes the following steps: after the aluminum removal waste liquid is allowed to stand at low temperature, the supernatant is centrifuged and returned to step (3) as filtrate for aluminum removal.

[0029] In one embodiment, the temperature for the low-temperature settling is 0-20°C, and the settling time is 6-24 hours.

[0030] In one embodiment, the temperature for the low-temperature settling is 0-5°C, and the settling time is 6-8 hours.

[0031] In one embodiment, the centrifugation speed is 3500-10000 rpm, and the centrifugation time is 5-30 min.

[0032] In one embodiment, the centrifugation speed is 3500-6000 rpm, and the centrifugation time is 5-15 min.

[0033] The beneficial effects of this invention are:

[0034] (1) The method for removing aluminum from lithium iron phosphate black powder of the present invention uses a specific aluminum removal solution, which includes sodium chloride, sodium sulfide, sodium phosphate and sodium oxalate. Sodium chloride promotes the dissolution of the alumina film; sodium sulfide provides reducing properties to prevent Fe from being absorbed into the lithium iron phosphate. 2+ Oxidation prevents the structure of lithium iron phosphate from being destroyed while providing alkaline aluminum dissolution; sodium phosphate, in addition to inhibiting the dissolution of phosphate ions in lithium iron phosphate, can also construct an alkaline buffer system to prevent structural damage and Al(OH)3 formation caused by excessive pH changes; sodium oxalate inhibits the formation of Al(OH)3 crystal nuclei while promoting solid-liquid separation. The lithium iron phosphate treated by the method of this invention can be directly repaired by lithium replenishment without additional Fe / P calculation for additional replenishment and control, and can ensure the performance of lithium iron phosphate after repair.

[0035] (2) The method for removing aluminum from lithium iron phosphate black powder in this invention first prepares a slurry and then removes aluminum, avoiding the safety hazard of serious overflow caused by directly adding lithium iron phosphate black powder to the aluminum removal liquid; the aluminum removal liquid after use can be reused through simple operation, avoiding the problem of high alkali consumption in the traditional alkali leaching method; the aluminum removal waste liquid that has been recycled multiple times can be regenerated by simply cooling, settling and centrifuging and then adding sodium sulfide, avoiding the generation and treatment of high-salt wastewater, which has great economic and environmental value;

[0036] (3) In the method for removing aluminum from lithium iron phosphate black powder of the present invention, a trace amount of lithium iron phosphate is dissolved in the aluminum removal liquid and accumulates continuously during circulation, which can further inhibit the dissolution of subsequent lithium iron phosphate components and the destruction of the structure, thereby further ensuring the non-destructive removal of aluminum. Attached Figure Description

[0037] Figure 1 The XRD comparison diagrams of lithium iron phosphate powder obtained after initial aluminum removal using aluminum removal agents with different ratios in Examples 1-5 of the present invention, pure lithium iron phosphate, and waste lithium iron phosphate black powder are shown.

[0038] Figure 2 The above are XRD comparison images of lithium iron phosphate powder obtained by aluminum removal in Examples 1-5 of the present invention, pure lithium iron phosphate, and waste lithium iron phosphate black powder.

[0039] Figure 3 Electron micrographs of the waste LFP black powder used in Examples 1-5 of this invention;

[0040] Figure 4 This is an electron microscope image of lithium iron phosphate powder after aluminum removal in Example 1 of the present invention. Detailed Implementation

[0041] The present invention will be further described below with reference to specific embodiments.

[0042] Example 1:

[0043] A method for removing aluminum from lithium iron phosphate black powder includes the following steps:

[0044] (1) Add 50g of waste lithium iron phosphate black powder to 50ml of pure water, heat in a 60℃ water bath and stir at a stirring speed of 450rpm until the waste lithium iron phosphate black powder and water are mixed into a uniform slurry, wherein the aluminum content in the waste lithium iron phosphate black powder is 1.01%.

[0045] (2) The above slurry was added to 200 ml of aluminum removal solution, kept at 60℃ in a water bath, and stirred continuously at 400 rpm for 3 hours. The aluminum removal solution was composed of sodium chloride, sodium sulfide, sodium phosphate, sodium oxalate, and water. The concentration of sodium chloride in the aluminum removal solution was 10 g / L, and the mass ratio of sodium chloride, sodium sulfide, sodium phosphate, and sodium oxalate in the aluminum removal solution was 1:14:10:2.5. After the reaction was completed, the solid and liquid were separated by filter paper to obtain filtrate and filter residue. The obtained filter residue was washed with pure water until neutral to obtain lithium iron phosphate powder after aluminum removal. The XRD of the lithium iron phosphate powder after aluminum removal is as follows: Figure 1 As shown, the aluminum content in the waste lithium iron phosphate black powder is 1.01%, and the aluminum content in the lithium iron phosphate after aluminum removal is 0.0163%; the electron micrograph of the waste lithium iron phosphate black powder is shown below. Figure 3 As shown, the electron microscope image of lithium iron phosphate powder after aluminum removal is as follows: Figure 4 As shown. Comparison Figure 3 and Figure 4 It can be seen that the morphology of lithium iron phosphate did not change significantly after aluminum removal, and no other impurity peaks were observed in the XRD, indicating that the aluminum removal process did not damage the structure and morphology of lithium iron phosphate.

[0046] Example 2:

[0047] A method for removing aluminum from lithium iron phosphate black powder includes the following steps:

[0048] (1) Add 60g of waste lithium iron phosphate black powder to 90ml of pure water, heat in an 80℃ water bath and stir at a stirring speed of 500rpm until the waste lithium iron phosphate black powder and water are mixed into a uniform slurry, wherein the aluminum content in the waste lithium iron phosphate black powder is 1.01%.

[0049] (2) The above slurry was added to 210 ml of aluminum removal solution, kept in an 80°C water bath, and stirred continuously at 400 rpm for 3 hours. The aluminum removal solution was composed of sodium chloride, sodium sulfide, sodium phosphate, sodium oxalate, and water. The concentration of sodium chloride in the aluminum removal solution was 20 g / L, and the mass ratio of sodium chloride, sodium sulfide, sodium phosphate, and sodium oxalate in the aluminum removal solution was 1:6.7:6.7:2. After the reaction was completed, the solid and liquid were separated by filter paper to obtain filtrate and filter residue. The obtained filter residue was washed with pure water until neutral to obtain lithium iron phosphate powder after aluminum removal. The XRD of the obtained lithium iron phosphate powder is as follows. Figure 1 As shown, the aluminum content in lithium iron phosphate after aluminum removal is 0.0267%.

[0050] (3) Add 60g of waste lithium iron phosphate black powder to the filtrate obtained in step (2), stir until homogeneous, and then add sodium sulfide at a rate of 20g / L according to the volume of the filtrate. Keep the mixture in an 80℃ water bath and stir continuously at 400rpm for 3 hours. Then, filter the mixture through filter paper to separate the solid and liquid, obtaining the filtrate and filter residue. Rinse the obtained filter residue with pure water until neutral to obtain lithium iron phosphate powder after aluminum removal. The XRD of the lithium iron phosphate powder obtained after further aluminum removal is shown in the figure. Figure 2 As shown, the aluminum content in lithium iron phosphate after aluminum removal is 0.0334%.

[0051] Example 3:

[0052] A method for removing aluminum from lithium iron phosphate black powder includes the following steps:

[0053] (1) Add 80g of waste lithium iron phosphate black powder to 80ml of pure water, heat in a 50℃ water bath and stir at a stirring speed of 500rpm until the waste lithium iron phosphate black powder and water are mixed into a uniform slurry, wherein the aluminum content in the waste lithium iron phosphate black powder is 1.01%.

[0054] (2) The above slurry was added to 520 ml of aluminum removal solution, kept in a 50°C water bath, and stirred continuously at 600 rpm for 4 hours. The aluminum removal solution was composed of sodium chloride, sodium sulfide, sodium phosphate, sodium oxalate, and water. The concentration of sodium chloride in the aluminum removal solution was 50 g / L, and the mass ratio of sodium chloride, sodium sulfide, sodium phosphate, and sodium oxalate in the aluminum removal solution was 1:5:5:2. After the reaction was completed, the solid and liquid were separated by filter paper to obtain filtrate and filter residue. The obtained filter residue was washed with pure water until neutral to obtain lithium iron phosphate powder after aluminum removal. The XRD of the obtained lithium iron phosphate powder is as follows. Figure 1 As shown, the aluminum content in lithium iron phosphate after aluminum removal is 0.0216%.

[0055] (3) Add 70g of waste lithium iron phosphate black powder to the filtrate obtained in step (2), stir until homogeneous, and then add sodium sulfide at a rate of 10g / L according to the volume of the filtrate. Keep the mixture in a 60℃ water bath and stir continuously at 600rpm for 4 hours. Then, filter the mixture through filter paper to separate the solid and liquid, obtaining the filtrate and filter residue. Rinse the obtained filter residue with pure water until neutral to obtain lithium iron phosphate powder after aluminum removal. The XRD of the lithium iron phosphate powder obtained after further aluminum removal is shown in the figure. Figure 2 As shown, the aluminum content in lithium iron phosphate after aluminum removal is 0.0235%.

[0056] Example 4:

[0057] A method for removing aluminum from lithium iron phosphate black powder includes the following steps:

[0058] (1) Add 70g of waste lithium iron phosphate black powder to 100ml of pure water, heat in an 80℃ water bath and stir at a stirring speed of 500rpm until the waste lithium iron phosphate black powder and water are mixed into a uniform slurry, wherein the aluminum content in the waste lithium iron phosphate black powder is 1.01%.

[0059] (2) The above slurry was added to 700 ml of aluminum removal solution, kept in a 50°C water bath, and stirred continuously at 500 rpm for 4 hours. The aluminum removal solution was composed of sodium chloride, sodium sulfide, sodium phosphate, sodium oxalate, and water. The concentration of sodium chloride in the aluminum removal solution was 30 g / L, and the mass ratio of sodium chloride, sodium sulfide, sodium phosphate, and sodium oxalate in the aluminum removal solution was 1:8:10:2. After the reaction was completed, the solid and liquid were separated by filter paper to obtain filtrate and filter residue. The obtained filter residue was washed with pure water until neutral to obtain lithium iron phosphate powder after aluminum removal. The XRD of the obtained lithium iron phosphate powder is as follows. Figure 1 As shown, the aluminum content in lithium iron phosphate after aluminum removal is 0.0312%.

[0060] (3) Add 120g of waste lithium iron phosphate black powder to the filtrate obtained in step (2), stir until homogeneous, and then add sodium sulfide at a rate of 10g / L according to the volume of the filtrate. Keep the mixture in an 80℃ water bath and stir continuously at 800rpm for 3 hours. Then, filter the mixture through filter paper to separate the solid and liquid, obtaining the filtrate and filter residue. Rinse the obtained filter residue with pure water until neutral to obtain lithium iron phosphate powder after aluminum removal. The XRD of the lithium iron phosphate powder obtained after further aluminum removal is shown in the figure. Figure 2 As shown, the aluminum content in lithium iron phosphate after aluminum removal is 0.0431%.

[0061] Example 5:

[0062] A method for removing aluminum from lithium iron phosphate black powder includes the following steps:

[0063] (1) Add 100g of waste lithium iron phosphate black powder to 100ml of pure water, heat in an 80℃ water bath and stir at a stirring speed of 800rpm until the waste lithium iron phosphate black powder and water are mixed into a uniform slurry, wherein the aluminum content in the waste lithium iron phosphate black powder is 1.01%.

[0064] (2) The above slurry was added to 500 ml of aluminum removal solution, kept at 80°C in a water bath, and stirred continuously at 600 rpm for 6 hours. The aluminum removal solution was composed of sodium chloride, sodium sulfide, sodium phosphate, sodium oxalate, and water. The concentration of sodium chloride in the aluminum removal solution was 40 g / L, and the mass ratio of sodium chloride, sodium sulfide, sodium phosphate, and sodium oxalate in the aluminum removal solution was 1:5:8:1. After the reaction was completed, the solid and liquid were separated by filter paper to obtain filtrate and filter residue. The obtained filter residue was washed with pure water until neutral to obtain lithium iron phosphate powder after aluminum removal. The XRD of the obtained lithium iron phosphate powder is as follows. Figure 1 As shown; after removing aluminum, the aluminum content in lithium iron phosphate is 0.0198%.

[0065] (3) Add 150g of waste lithium iron phosphate black powder to the filtrate obtained in step (2), stir until homogeneous, and then add sodium sulfide at a rate of 25g / L according to the volume of the filtrate. Keep the mixture in an 80℃ water bath and stir continuously at 600rpm for 6 hours. Then, filter the mixture through filter paper to separate the solid and liquid, obtaining the filtrate and filter residue. Rinse the obtained filter residue with pure water until neutral to obtain lithium iron phosphate powder after aluminum removal. The XRD of the lithium iron phosphate powder obtained after further aluminum removal is shown in the figure. Figure 2 As shown, the aluminum content in lithium iron phosphate after aluminum removal is 0.0211%.

[0066] (4) The filtrate (aluminum-removed waste liquid) obtained in step (3) was placed in a 5℃ environment and allowed to stand for 8 hours to allow some aluminum ions in the filtrate to be converted into Al(OH)3 precipitate. Then, it was introduced into a centrifuge tube and centrifuged at 5000 rpm for 5 minutes. After centrifugation, the supernatant was taken, and 80g of waste lithium iron phosphate black powder was added. After stirring evenly, sodium sulfide was added at a rate of 10g / L according to the volume of the filtrate. The mixture was heated to 80℃ and stirred continuously at 500 rpm for 6 hours. After the reaction was completed, the solid and liquid were separated by filtering with filter paper to obtain the filtrate and the filter residue. The filter residue was washed with pure water until neutral to obtain the aluminum-removed lithium iron phosphate powder. The XRD of the aluminum-removed lithium iron phosphate powder is as follows. Figure 2 As shown, the aluminum content in lithium iron phosphate after aluminum removal is 0.0287%.

[0067] (5) Add 140g of waste lithium iron phosphate black powder to the filtrate obtained in step (4), stir until homogeneous, and then add sodium sulfide at a rate of 12.5g / L according to the volume of the filtrate. Keep the mixture in an 80℃ water bath and stir continuously at 800rpm for 3 hours. Then, filter the mixture through filter paper to separate the solid and liquid, obtaining the filtrate and filter residue. Rinse the obtained filter residue with pure water until neutral to obtain aluminum-removed lithium iron phosphate powder. The XRD of the aluminum-removed lithium iron phosphate powder is shown in Figure 1. Figure 2 As shown, the aluminum content in lithium iron phosphate after aluminum removal is 0.0311%.

[0068] Experimental example:

[0069] The content of each element in lithium iron phosphate after aluminum removal in Examples 1-5 was tested using inductively coupled plasma spectrometry (ICP). The test results are shown in Table 1 below.

[0070] Table 1: Elemental Content Test Results

[0071]

[0072] As shown in Table 1, the aluminum content in the lithium iron phosphate powder after the first aluminum removal process using the method of the present invention is less than 0.0312%, the aluminum content after one cycle of filtrate removal is less than 0.0431%, and the aluminum content after the first aluminum removal process after filtrate regeneration is only 0.0287%, and the aluminum content after one cycle of filtrate regeneration is only 0.0311%. This indicates that the method of the present invention for removing aluminum from lithium iron phosphate powder has excellent aluminum removal effect. Regardless of whether it is used for the first time, in a cycle, or during regeneration, the aluminum removal solution, at different concentration ratios, can reduce the aluminum content in waste lithium iron phosphate powder with an original aluminum content of 1.01% to below 0.05%. Furthermore, simultaneously... Figure 1 and Figure 2XRD analysis showed that the structure of lithium iron phosphate was not damaged. Electrochemical testing: Lithium iron phosphate powder after aluminum removal from Examples 1 to 5 was used as the cathode material, and batteries were fabricated using the cathode materials. The electrochemical performance of the batteries was then tested, and specific data were obtained using equipment such as an electrochemical workstation. The initial discharge capacity and initial efficiency were tested at a constant temperature of 45°C, a charge / discharge voltage of 2.0-3.75V, and a charge / discharge rate of 0.1C. The test results are shown in Table 2.

[0073] Table 2: Electrochemical Performance Test Results

[0074]

[0075] As shown in Table 2, the lithium iron phosphate obtained after aluminum removal using the method of the present invention still has good electrochemical performance. Regardless of the concentration ratio of the aluminum removal solution or the number of aluminum removal cycles, the first efficiency of the aluminum-removed lithium iron phosphate is higher than 98%, and the first discharge specific capacity is greater than 154 mAh / g.

Claims

1. A method for removing aluminum from lithium iron phosphate black powder, characterized in that: Includes the following steps: (1) Mix lithium iron phosphate black powder with water to make a slurry; (2) The slurry obtained in step (1) is mixed with the aluminum removal liquid and heated and reacted under stirring. The solid and liquid are separated to obtain a primary filtrate and a filter residue. The filter residue is washed to obtain lithium iron phosphate after aluminum removal. The aluminum removal liquid is composed of a solute and a solvent. The solute includes sodium chloride, sodium sulfide, sodium phosphate and sodium oxalate.

2. The method for removing aluminum from lithium iron phosphate black powder according to claim 1, characterized in that: In step (1), the liquid-to-solid ratio of the water to the lithium iron phosphate black powder is (1-2) mL: 1 g.

3. The method for removing aluminum from lithium iron phosphate black powder according to claim 1, characterized in that: In step (1), the mixing is carried out in a water bath at a temperature of 50-90°C.

4. The method for removing aluminum from lithium iron phosphate black powder according to claim 1, characterized in that: In step (2), the concentration of sodium chloride in the aluminum removal solution is 10-50 g / L, and the mass ratio of sodium chloride, sodium sulfide, sodium phosphate and sodium oxalate in the aluminum removal solution is 1:(0.5-20):(2.5-20):(0.5-10).

5. The method for removing aluminum from lithium iron phosphate black powder according to claim 1, characterized in that: In step (2), the mass ratio of the aluminum removal liquid to the slurry is (1-10):

1.

6. The method for removing aluminum from lithium iron phosphate black powder according to claim 1, characterized in that: In step (2), the heating temperature is 50-90℃.

7. The method for removing aluminum from lithium iron phosphate black powder according to claim 1, characterized in that: It also includes a step (3) of circulating the primary filtrate obtained in step (2) to remove aluminum. The specific content of step (3) is as follows: the primary filtrate obtained in step (2) is mixed with waste lithium iron phosphate black powder, sodium sulfide is added, and the mixture is heated and reacted under stirring. The solid and liquid are separated to obtain filtrate and filter residue. The obtained filter residue is washed to obtain lithium iron phosphate after aluminum removal. The obtained filtrate is used for the next circulating aluminum removal step. This circulating aluminum removal step is repeated 1-5 times. After the step is completed, aluminum removal waste liquid is obtained.

8. The method for removing aluminum from lithium iron phosphate black powder according to claim 7, characterized in that: In step (3), the mass ratio of the added sodium sulfide to the volume of the first filtrate is 10-50 g / L.

9. The method for removing aluminum from lithium iron phosphate black powder according to claim 7, characterized in that: It also includes the following steps: After the aluminum removal waste liquid is allowed to stand at low temperature, it is centrifuged and the supernatant is taken as filtrate and returned to step (3) for aluminum removal.

10. The method for removing aluminum from lithium iron phosphate black powder according to claim 9, characterized in that: The temperature for the low-temperature settling is 0-20℃, and the settling time is 6-24h.

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