WO2025054848A1 - Method for preparing iron phosphate by means of full-chain integrated recycling of waste lithium iron phosphate battery - Google Patents

Abstract

Provided in the present disclosure is a method for preparing iron phosphate by means of full-chain integrated recycling of a waste lithium iron phosphate battery. The method comprises the following steps: (1) mixing a waste lithium iron phosphate material with a mixed chloride, and heating and melting the mixture, so as to obtain a mixed melt; (2) mixing the mixed melt with sodium stearate, then subjecting the mixture to a blowing reaction, and separating same to obtain carbon-containing lithium iron phosphate scum; (3) mixing the scum with an acid solution, adding hydrogen peroxide to perform a leaching reaction, and carrying out solid-liquid separation to obtain a lithium-containing solution and carbon-containing phosphorus iron slag; and (4) mixing the carbon-containing phosphorus iron slag with an acid solution, leaching the mixture to obtain a phosphorus iron solution, adjusting the phosphorus iron ratio, and adding a complexing agent to perform a coprecipitation reaction, so as to obtain iron phosphate. The method of the present disclosure can improve the purity of the recycled lithium, also avoid a multi-step impurity-removal process required to be carried out during the process of preparing iron phosphate from the phosphorus iron slag after lithium extraction, and further improve the purity and phosphorus iron recovery rate of the iron phosphate prepared from the phosphorus iron slag.

Description

A method for preparing iron phosphate by recycling waste lithium iron phosphate batteries through full-chain integration Technical Field

The present invention belongs to the technical field of resource recovery and relates to a method for preparing iron phosphate by recycling waste lithium iron phosphate batteries through full-chain integration.

Background Art

Lithium-ion batteries are widely used in electric vehicles, large-scale energy storage, and electronic equipment due to their high capacity and energy density. With the increasing use of electric vehicles in recent years, a large number of waste lithium-ion batteries will accumulate over time. Recycling these waste lithium-ion batteries can increase resource utilization and avoid environmental pollution. Among lithium-ion batteries, lithium iron phosphate batteries are widely used in power batteries due to their good stability and low cost.

At present, the main method for recovering lithium iron phosphate from waste lithium-ion batteries is to extract and recover lithium through selective leaching, and finally leave the ferrophosphorus slag after lithium extraction. However, the lithium solution obtained by directly leaching waste lithium iron phosphate contains more impurities. And because the ferrophosphorus slag after lithium extraction contains iron and phosphorus elements, it needs to be recycled and reused.

CN109250696A discloses a method for recovering nano iron phosphate from lithium iron phosphate batteries: the lithium iron phosphate battery is discharged to below 2.0V, crushed and disassembled, and lithium iron phosphate powder, aluminum powder and copper powder in the battery are separated; the lithium iron phosphate powder is placed in a vacuum tube furnace; the lithium iron phosphate powder is added to a dilute hydrochloric acid solution while stirring, and hydrogen peroxide and a suspension are simultaneously added dropwise to a three-necked flask reactor through a peristaltic pump, and stirred and ultrasonicated at the same time; the mixed solution is filtered and the filtrate is taken to obtain a leaching solution; the leaching mixed solution and the alkaline solution are synchronously added to a mixing reactor through a peristaltic pump, and the two liquids are mixed, stirred and ultrasonicated to obtain a light yellow powdered iron phosphate.

CN115611250A discloses a method for recovering high-purity iron phosphate from waste lithium iron phosphate positive electrode powder. The method comprises the following steps: performing preliminary impurity removal by primary acid leaching and first heat preservation self-precipitation to obtain crude iron phosphate, and then performing secondary acid leaching re-dissolution and second heat preservation self-precipitation to obtain iron phosphate.

The above scheme adopts a wet treatment, that is, adding ferrophosphorus slag into an acidic solution or an alkaline solution for multiple leaching to achieve impurity removal and purification to obtain ferric phosphate. If only acid leaching or alkaline leaching is used for impurity removal, the impurity content of the prepared ferric phosphate is still high and the process flow is long. Alternatively, resin adsorption is used for impurity removal, but the deep impurity removal effect is poor.

Summary of the invention

The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

The purpose of the present disclosure is to provide a method for preparing iron phosphate by recycling waste lithium iron phosphate batteries in a full-chain integrated manner. The method described in the present disclosure can improve the purity of recovered lithium, while avoiding the multi-step impurity removal process required in the process of preparing iron phosphate from ferrophosphorus slag after lithium extraction, thereby further improving the purity and recovery rate of iron phosphate prepared from ferrophosphorus slag.

In order to achieve this disclosure purpose, the present disclosure adopts the following technical solutions:

In a first aspect, the present disclosure provides a method for preparing iron phosphate by recycling waste lithium iron phosphate batteries in an integrated manner, the method comprising the following steps:

(1) mixing waste lithium iron phosphate material with mixed chloride, and performing heating and melting treatment to obtain a mixed melt;

(2) mixing the mixed melt with sodium stearate, and then performing an air blowing reaction, and obtaining a carbon-containing lithium iron phosphate slag on the upper layer after separation;

(3) mixing the carbon-containing lithium iron phosphate slag with the first acid solution to obtain a mixed solution, adding hydrogen peroxide to carry out a leaching reaction, and performing solid-liquid separation to obtain a lithium-containing solution and carbon-containing phosphorus iron slag;

(4) mixing the carbon-containing ferrophosphorus slag with a second acid solution to obtain a ferrophosphorus solution by leaching. An iron source and/or a phosphorus source is added to the solution to adjust the phosphorus-iron ratio, and a complexing agent is added to carry out a coprecipitation reaction to obtain iron phosphate.

In the process of recycling waste lithium iron phosphate, the present invention advances the copper and aluminum impurity removal step to before lithium extraction, and adds sodium stearate after mixing and melting waste lithium iron phosphate powder and mixed chloride. Since the density of aluminum and copper impurities contained in waste lithium iron phosphate is greater than the density of lithium iron phosphate and the density of molten mixed chloride, they are deposited at the bottom of the molten potassium chloride and sodium chloride mixture. Since the density of lithium iron phosphate is less than the density of the sodium chloride and potassium chloride mixture, sodium stearate is added as a surfactant and a foaming agent, and the melt can be stirred after gas is blown in. Since the carbon particles are fine and have many adsorption active sites on their surfaces, they are easy to aggregate with lithium iron phosphate into larger slag groups, and can have strong adsorption with bubbles, so that they can migrate to the surface of the melt together with the bubbles. After the carbon particles entrain the lithium iron phosphate and rise to the surface of the melt, they further form larger scum through collision. The large scum is difficult to be brought back into the melt by gas stirring, and finally the lithium iron phosphate and aluminum and copper impurities are separated. The impurity content in the subsequent ferrophosphorus slag after lithium extraction is low, and only contains carbon particles, avoiding the subsequent aluminum and copper removal steps, and high-purity iron phosphate can be directly obtained through one-step acid leaching. The present invention deeply removes copper and aluminum in waste lithium iron phosphate through a one-step reaction, which can improve the purity of recovered lithium and avoid the subsequent multi-step impurity removal steps of ferrophosphorus slag, so that the purity and recovery rate of the recovered iron phosphate are further improved.

In one embodiment, the mixed chloride in step (1) includes potassium chloride and sodium chloride.

In one embodiment, the mass ratio of potassium chloride to sodium chloride is (0.8-1.2):1, for example: 0.8:1, 0.9:1, 1:1, 1.1:1 or 1.2:1, etc.

The present invention selects potassium chloride and sodium chloride, and controlling the mass ratio of the two can reduce the melting point of the mixed salt, so that the mixed material can be molten at a relatively low temperature. The melt is used as a medium, and the density of the melt is appropriate. While saving energy, the separation of aluminum-copper impurities and carbon-containing lithium iron phosphate is achieved.

In one embodiment, the mass ratio of the waste lithium iron phosphate material to the mixed chloride is 1:(5-10), for example: 1:5, 1:6, 1:7, 1:8 or 1:10, etc.

In one embodiment, the temperature of the heating and melting treatment in step (1) is 300-400°C, for example, 300°C, 320°C, 350°C, 380°C or 400°C.

In one embodiment, the heating and melting treatment takes 10 to 30 minutes, for example, 10 minutes, 15 minutes, 20 minutes, 25 minutes or 30 minutes.

In one embodiment, the mass ratio of the sodium stearate and the waste lithium iron phosphate material in step (2) is (2-10):100, for example: 2:100, 4:100, 6:100, 8:100 or 10:100, etc.

In one embodiment, the gas for blowing reaction includes argon and/or nitrogen.

In one embodiment, the gas flow rate of the blowing reaction is 10-100 mL/s, for example, 10 mL/s, 20 mL/s, 50 mL/s, 80 mL/s or 100 mL/s.

In one embodiment, the blowing reaction time is 10 to 30 minutes, for example, 10 minutes, 15 minutes, 20 minutes, 25 minutes or 30 minutes.

In one embodiment, the separation in step (2) includes removing the solid on the upper layer of the melt with a filter to obtain carbon-containing lithium iron phosphate slag.

In one embodiment, the first acid solution in step (3) comprises sulfuric acid.

In one embodiment, the mass ratio of the carbon-containing lithium iron phosphate slag to the first acid solution is 1:(1.2-1.8), for example: 1:1.2, 1:1.4, 1:1.5, 1:1.6 or 1:1.8.

In one embodiment, the pH of the mixed solution is 1.5-2, for example, 1.5, 1.6, 1.7, 1.8 or 2.

In one embodiment, the molar ratio of the hydrogen peroxide and lithium iron phosphate in step (3) is (0.3-0.8):1, for example: 0.3:1, 0.4:1, 0.5:1, 0.6:1 or 0.8:1, etc.

In one embodiment, the leaching reaction time is 2 to 3 hours, for example, 2 hours, 2.2 hours, 2.5 hours, 2.8 hours or 3 hours.

In one embodiment, after the leaching reaction, alkali is added to adjust the pH to carry out the reaction.

In one embodiment, the pH is 4 to 6, for example, 4, 4.5, 5, 5.5 or 6.

In one embodiment, the reaction time is 0.5 to 1 h, for example, 0.5 h, 0.6 h, 0.8 h, 0.9 h or 1 h.

In one embodiment, the second acid solution in step (4) comprises any one of sulfuric acid, nitric acid or phosphoric acid, or a combination of at least two thereof.

In one embodiment, the concentration of the second acid solution is 0.8-1.2 mol/L, for example, 0.8 mol/L, 0.9 mol/L, 1 mol/L, 1.1 mol/L or 1.2 mol/L.

In one embodiment, during the leaching process, the concentration of hydrogen ions in the system is ≥0.8 mol/L.

In one embodiment, the leaching time is 1 to 2 hours, for example, 1 hour, 1.2 hours, 1.5 hours, 1.8 hours or 2 hours.

In one embodiment, the iron source in step (4) comprises ferrous sulfate.

In one embodiment, the phosphorus source comprises phosphoric acid.

In one embodiment, the phosphorus-iron ratio is adjusted to P:Fe=1.0-1.1, for example: 1, 1.02, 1.05, 1.08 or 1.1.

In one embodiment, the complexing agent includes aqueous ammonia.

In one embodiment, the pH of the coprecipitation reaction is 1.8 to 2.2, for example, 1.8, 1.9, 2, 2.1 or 2.2.

As an optional solution of the present disclosure, the method comprises the following steps:

(1) mixing waste lithium iron phosphate material with mixed chloride in a mass ratio of 1:(5-10), heating and melting at 300-400° C. for 10-30 min to obtain a mixed melt, wherein the mixed chloride includes potassium chloride and sodium chloride in a mass ratio of (0.8-1.2):1;

(2) mixing the mixed melt with sodium stearate in a mass ratio of 100:(2-10), blowing air at a speed of 10-100 mL/s for reaction for 10-30 min, and separating the upper layer to obtain carbon-containing lithium iron phosphate slag;

(3) mixing the carbon-containing lithium iron phosphate slag with the first acid solution in a mass ratio of 1:(1.2-1.8) to obtain a mixed solution with a pH of 1.5-2, adding hydrogen peroxide to carry out a leaching reaction for 2-3 hours, adding alkali to adjust the pH to 4-6 and reacting for 0.5-1 hour, and performing solid-liquid separation to obtain a lithium-containing solution and carbon-containing phosphorus iron slag;

(4) mixing the carbon-containing ferrophosphorus slag with a second acid solution having a concentration of 0.8 to 1.2 mol/L, controlling the concentration of hydrogen ions in the system to be ≥ 0.8 mol/L and leaching for 1 to 2 hours to obtain a ferrophosphorus solution, adding an iron source and/or a phosphorus source to the ferrophosphorus solution to adjust the phosphorus-iron ratio, adding ammonia water, adjusting the pH to 1.8 to 2.2, performing a coprecipitation reaction, and obtaining ferric phosphate.

Compared with the prior art, the present invention has the following beneficial effects:

(1) The method disclosed in the present invention can improve the purity of recovered lithium, while avoiding the multi-step impurity removal process required in the process of preparing ferric phosphate from ferrophosphorus slag after lithium extraction, thereby further improving the purity and ferrophosphorus recovery rate of ferrophosphate prepared from ferrophosphorus slag.

(2) The method disclosed in the present invention recycles waste lithium iron phosphate materials to obtain lithium purity of more than 97.8%, and iron phosphate purity of more than 99.5%, wherein the iron content can reach more than 29.85%, the P content can reach more than 16.82%, and the aluminum content can reach less than 0.001%, and the copper content can reach less than 0.004%. The iron recovery rate can reach more than 99.3%, and the phosphorus recovery rate can reach more than 99.2%, thereby achieving deep impurity removal and efficient recovery of lithium iron phosphate materials.

Still other aspects will be apparent upon reading and understanding the detailed description.

DETAILED DESCRIPTION

The technical solution of the present disclosure is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only to help understand the present disclosure and should not be regarded as specific limitations of the present disclosure.

Example 1

This embodiment provides a method for recycling waste lithium iron phosphate batteries to prepare iron phosphate in an integrated manner. The method comprises the following steps:

(1) ball-milling waste lithium iron phosphate powder and sieving it through a 50-mesh sieve to obtain waste lithium iron phosphate powder, placing the lithium iron phosphate powder in a furnace, adding a mixture of sodium chloride and potassium chloride in a mass ratio of 1:1, and keeping the temperature at 350° C. for 20 minutes, wherein the mass ratio of the lithium iron phosphate to the mixture of potassium chloride and sodium chloride is 1:7;

(2) Sodium stearate with a mass of 6% of the mass of lithium iron phosphate is added to the furnace, and argon gas is blown into the melt at the lower end of the furnace for 20 minutes. The gas flow rate is controlled to be 50 mL/s. Carbon and lithium iron phosphate form a slag phase floating on the upper layer, and copper and aluminum impurities are deposited at the bottom of the melt. Subsequently, a filter is used to remove the lithium iron phosphate and carbon slag floating on the upper layer from above to separate the aluminum and copper impurities from the carbon-containing lithium iron phosphate;

(3) washing the obtained carbon-containing lithium iron phosphate with water and then adding it to a sulfuric acid solution, controlling the pH at 1.7 and the solid-liquid ratio at 1:1.5, then slowly adding hydrogen peroxide with a molar amount of 0.5 times the molar amount of lithium iron phosphate to react for 2.5 hours to selectively leach lithium, adding alkali to adjust the pH to 5 after the reaction is completed, reacting for 0.7 hours, and then performing solid-liquid separation to obtain a lithium-containing solution and carbon-containing phosphorus iron slag;

(4) adding the carbon-containing ferrophosphorus slag to a sulfuric acid solution with a concentration of 1 mol/L to control the concentration of hydrogen ions in the solution to 0.8 mol/L, and carrying out the leaching reaction for 2 hours. After the reaction is completed, the solid-liquid separation is carried out to obtain a high-purity ferrophosphorus solution. Ferrous sulfate or phosphoric acid is added to the obtained ferrophosphorus solution to adjust the phosphorus-iron ratio to P:Fe=1.05. Ammonia water is added to adjust the pH to 2 for a coprecipitation reaction. After the reaction is completed, the solid-liquid separation is carried out to obtain a high-purity ferric phosphate.

Example 2

This embodiment provides a method for preparing iron phosphate by recycling waste lithium iron phosphate batteries in an integrated manner, the method comprising the following steps:

(1) ball-milling waste lithium iron phosphate powder and sieving it through a 40-mesh sieve to obtain waste lithium iron phosphate powder, placing the lithium iron phosphate powder in a furnace, adding a mixture of sodium chloride and potassium chloride in a mass ratio of 1:1, and keeping the temperature at 300° C. for 30 minutes, wherein the mass ratio of the lithium iron phosphate to the mixture of potassium chloride and sodium chloride is 1:5;

(2) Add sodium stearate with a mass equivalent to 2% of the mass of lithium iron phosphate into the furnace, and Argon gas was blown into the melt for 10 minutes, and the gas flow rate was controlled to be 100 mL/s. Carbon and lithium iron phosphate formed slag phase floating on the upper layer, and copper and aluminum impurities were deposited at the bottom of the melt. Subsequently, the lithium iron phosphate and carbon slag floating on the upper layer were scooped out from above using a filter to separate aluminum and copper impurities from carbon-containing lithium iron phosphate;

(3) washing the obtained carbon-containing lithium iron phosphate with water and then adding it to a sulfuric acid solution, controlling the pH at 1.5 and the solid-liquid ratio at 1:1.5, then slowly adding hydrogen peroxide with a molar amount of 0.3 times the molar amount of lithium iron phosphate to react for 2.5 hours to selectively leach lithium, adding alkali to adjust the pH to 4 after the reaction is completed, reacting for 0.5 hours, and then performing solid-liquid separation to obtain a lithium-containing solution and carbon-containing phosphorus iron slag;

(4) adding the carbon-containing ferrophosphorus slag to a sulfuric acid solution with a concentration of 1.2 mol/L to control the concentration of hydrogen ions in the solution to be 1.2 mol/L, and performing the leaching reaction for 1.5 h. After the reaction is completed, solid-liquid separation is performed to obtain a high-purity ferrophosphorus solution. Ferrous sulfate or phosphoric acid is added to the obtained ferrophosphorus solution to adjust the phosphorus-iron ratio to P:Fe=1. Ammonia water is added to adjust the pH to 1.8 for coprecipitation reaction. After the reaction is completed, solid-liquid separation is performed to obtain high-purity ferric phosphate.

Example 3

This embodiment provides a method for preparing iron phosphate by recycling waste lithium iron phosphate batteries in an integrated manner, the method comprising the following steps:

(1) ball-milling waste lithium iron phosphate powder and sieving it through a 60-mesh sieve to obtain waste lithium iron phosphate powder, placing the lithium iron phosphate powder in a furnace, adding a mixture of sodium chloride and potassium chloride in a mass ratio of 1:1, and keeping the temperature at 400° C. for 10 minutes, wherein the mass ratio of the lithium iron phosphate to the mixture of potassium chloride and sodium chloride is 1:10;

(2) Sodium stearate with a mass of 10% of the mass of lithium iron phosphate is added to the furnace, and argon gas is blown into the melt at the lower end of the furnace for 30 minutes. The gas flow rate is controlled to be 10 mL/s. Carbon and lithium iron phosphate form a slag phase floating on the upper layer, and copper and aluminum impurities are deposited at the bottom of the melt. Subsequently, a filter is used to remove the lithium iron phosphate and carbon slag floating on the upper layer from above to separate the aluminum and copper impurities from the carbon-containing lithium iron phosphate;

(3) The obtained carbon-containing lithium iron phosphate was washed with water and then added to a sulfuric acid solution, the pH was controlled at 2, and the solid-liquid ratio was 1:1.5, then slowly add hydrogen peroxide with a molar amount of 0.8 times the molar amount of lithium iron phosphate to react for 3 hours to selectively leach lithium, add alkali after the reaction to adjust the pH to 6, react for 1 hour, and then perform solid-liquid separation to obtain a lithium-containing solution and carbon-phosphorus-iron slag;

(4) adding the carbon-containing ferrophosphorus slag to a sulfuric acid solution with a concentration of 1.5 mol/L to control the concentration of hydrogen ions in the solution to 0.8 mol/L, and carrying out the leaching reaction for 1 hour. After the reaction is completed, the solid-liquid separation is carried out to obtain a high-purity ferrophosphorus solution. Ferrous sulfate or phosphoric acid is added to the obtained ferrophosphorus solution to adjust the phosphorus-iron ratio to P:Fe=1.1, and ammonia water is added to adjust the pH to 2.2 for coprecipitation reaction. After the reaction is completed, the solid-liquid separation is carried out to obtain high-purity ferric phosphate.

Example 4

The only difference between this embodiment and embodiment 1 is that the mass of sodium stearate is 1% of the mass of lithium iron phosphate, and other conditions and parameters are exactly the same as those in embodiment 1.

Example 5

The only difference between this embodiment and embodiment 1 is that the mass of sodium stearate is 12% of the mass of lithium iron phosphate, and other conditions and parameters are exactly the same as those in embodiment 1.

Comparative Example 1

This comparative example provides a method for recycling waste lithium iron phosphate materials to prepare iron phosphate, the method comprising the following steps:

(1) Waste lithium iron phosphate powder is ball-milled and sieved through a 50-mesh sieve to obtain waste powder.

(2) The above waste lithium iron phosphate powder is added to a sulfuric acid solution, the pH is controlled at 1.7, the solid-liquid ratio is 1:1.5, and then hydrogen peroxide with a molar amount that is 0.5 times the molar amount of lithium iron phosphate is slowly added to react for 2.5 hours to selectively leach lithium. After the reaction is completed, alkali is added to adjust the pH to 5 and react for 0.7 hours to perform solid-liquid separation to obtain a lithium-containing solution and ferrophosphorus slag.

(3) The ferrophosphorus slag obtained in step (2) is added to a sulfuric acid solution with a concentration of 1 mol/L to control the concentration of hydrogen ions in the solution to 0.8 mol/L, and the leaching reaction is carried out for 2 hours. After the reaction is completed, the solid and liquid are separated to obtain a ferrophosphorus solution.

(4) adding an iron source or a phosphorus source to the ferrophosphorus solution obtained in step (3) to adjust the phosphorus-iron ratio to P:Fe=1.05, adding ammonia water to adjust the pH to 2 for coprecipitation reaction, and after the reaction is completed, solid-liquid separation to obtain high-purity ferric phosphate.

Comparative Example 2

The only difference between this comparative example and Example 1 is that sodium stearate is not added, and other conditions and parameters are exactly the same as those in Example 1.

Performance Testing:

The purity of recovered lithium, iron phosphate index and recovery rate obtained from the test examples and comparative examples are shown in Table 1:

Table 1

As can be seen from Table 1, from Examples 1-3, the method disclosed in the present invention recycles waste lithium iron phosphate materials to obtain a lithium purity of more than 97.8%, and an iron phosphate purity of more than 99.5%, wherein the iron content can reach more than 29.85%, and the P content can reach more than 16.82%. At the same time, the aluminum content can reach less than 0.001%, and the copper content can reach less than 0.004%. The iron recovery rate can reach more than 99.3%, and the phosphorus recovery rate can reach more than 99.2%, thereby achieving deep impurity removal and efficient recovery of lithium iron phosphate materials.

By comparing Example 1 with Examples 4-5, it can be seen that the waste lithium iron phosphate material of the present invention is recycled to prepare phosphoric acid In the process of iron ore separation, the addition amount of sodium stearate will affect the recovery effect. The recovery effect is better when the addition amount of sodium stearate is controlled at 2-10% of the mass of lithium iron phosphate. If the addition amount of sodium stearate is too large, a large amount of sticky and fine foam will be formed to adsorb impurity metals, which is not conducive to the separation of lithium iron phosphate and aluminum and copper impurities. If the addition amount of sodium stearate is too small, the foaming rate is too low and the number of bubbles is too small, which is not conducive to bringing lithium iron phosphate and carbon to the slag layer, thereby reducing the flotation efficiency.

By comparing Example 1 and Comparative Example 1, it can be seen that the present invention selects potassium chloride and sodium chloride, and controlling the mass ratio of the two can reduce the melting point of the mixed salt, so that the mixed material can achieve a molten state at a lower temperature, and the melt is used as a medium. The density of the melt is appropriate, and the separation of aluminum-copper impurities and carbon-containing lithium iron phosphate is achieved while saving energy.

Claims (16)

A method for preparing iron phosphate by recycling waste lithium iron phosphate batteries in a full-chain integrated manner, the method comprising the following steps: (1) mixing waste lithium iron phosphate material with mixed chloride, and performing heating and melting treatment to obtain a mixed melt; (2) mixing the mixed melt with sodium stearate, and then performing an air blowing reaction, and obtaining a carbon-containing lithium iron phosphate slag on the upper layer after separation; (3) mixing the carbon-containing lithium iron phosphate slag with the first acid solution to obtain a mixed solution, adding hydrogen peroxide to carry out a leaching reaction, and performing solid-liquid separation to obtain a lithium-containing solution and carbon-containing phosphorus iron slag; (4) mixing the carbon-containing ferrophosphorus slag with a second acid solution, leaching to obtain a ferrophosphorus solution, adding an iron source and/or a phosphorus source to the ferrophosphorus solution to adjust the phosphorus-iron ratio, and adding a complexing agent to perform a coprecipitation reaction to obtain ferric phosphate. The method according to claim 1, wherein the mixed chloride in step (1) comprises potassium chloride and sodium chloride. The method according to claim 1 or 2, wherein the mass ratio of potassium chloride to sodium chloride is (0.8-1.2):1. The method according to any one of claims 1 to 3, wherein the mass ratio of the waste lithium iron phosphate material to the mixed chloride is 1:(5 to 10). The method according to any one of claims 1 to 4, wherein the temperature of the heating and melting treatment in step (1) is 300 to 400°C. The method according to any one of claims 1 to 5, wherein the heating and melting treatment time is 10 to 30 minutes. The method according to any one of claims 1 to 6, wherein the mass ratio of the sodium stearate to the waste lithium iron phosphate material in step (2) is (2-10):100. The method according to any one of claims 1 to 7, wherein the gas for the blowing reaction comprises argon gas and/or nitrogen. The method according to any one of claims 1 to 8, wherein the gas flow rate of the blowing reaction is 10 to 100 mL/s. The method according to any one of claims 1 to 9, wherein the blowing reaction time is 10 to 30 minutes. The method according to any one of claims 1 to 10, wherein the separation in step (2) comprises removing the solid on the upper layer of the melt with a filter to obtain carbon-containing lithium iron phosphate slag. The method according to any one of claims 1 to 11, wherein the first acid solution in step (3) comprises sulfuric acid; Optionally, the mass ratio of the carbon-containing lithium iron phosphate slag to the first acid solution is 1:(1.2-1.8); Optionally, the pH of the mixed solution is 1.5-2. The method according to any one of claims 1 to 12, wherein the molar ratio of hydrogen peroxide to lithium iron phosphate in step (3) is (0.3 to 0.8):1; Optionally, the leaching reaction time is 2 to 3 hours; Optionally, after the leaching reaction, alkali is added to adjust the pH to carry out the reaction; Optionally, the pH is 4 to 6; Optionally, the reaction time is 0.5 to 1 h. The method according to any one of claims 1 to 13, wherein the second acid solution in step (4) comprises any one of sulfuric acid, nitric acid or phosphoric acid, or a combination of at least two thereof; Optionally, the concentration of the second acid solution is 0.8 to 1.2 mol/L; Optionally, during the leaching process, the concentration of hydrogen ions in the system is ≥0.8 mol/L; Optionally, the leaching time is 1 to 2 hours. The method according to any one of claims 1 to 14, wherein the iron source in step (4) comprises sulfur Ferrous acid; Optionally, the phosphorus source includes phosphoric acid; Optionally, the phosphorus-iron ratio is adjusted to P:Fe=1.0-1.1; Optionally, the complexing agent includes aqueous ammonia; Optionally, the pH of the coprecipitation reaction is 1.8 to 2.2. The method according to any one of claims 1 to 15, wherein the method comprises the following steps: (1) mixing waste lithium iron phosphate material with mixed chloride in a mass ratio of 1:(5-10), heating and melting at 300-400° C. for 10-30 min to obtain a mixed melt, wherein the mixed chloride includes potassium chloride and sodium chloride in a mass ratio of (0.8-1.2):1; (2) mixing the mixed melt with sodium stearate in a mass ratio of 100:(2-10), blowing air at a speed of 10-100 mL/s for reaction for 10-30 min, and separating the upper layer to obtain carbon-containing lithium iron phosphate slag; (3) mixing the carbon-containing lithium iron phosphate slag with the first acid solution in a mass ratio of 1:(1.2-1.8) to obtain a mixed solution with a pH of 1.5-2, adding hydrogen peroxide to carry out a leaching reaction for 2-3 hours, adding alkali to adjust the pH to 4-6 and reacting for 0.5-1 hour, and performing solid-liquid separation to obtain a lithium-containing solution and carbon-containing phosphorus iron slag; (4) mixing the carbon-containing ferrophosphorus slag with a second acid solution having a concentration of 0.8 to 1.2 mol/L, controlling the concentration of hydrogen ions in the system to be ≥ 0.8 mol/L and leaching for 1 to 2 hours to obtain a ferrophosphorus solution, adding an iron source and/or a phosphorus source to the ferrophosphorus solution to adjust the phosphorus-iron ratio, adding ammonia water, adjusting the pH to 1.8 to 2.2, performing a coprecipitation reaction, and obtaining ferric phosphate.