A compound regeneration method for lithium extraction slag using iron phosphate

Abstract

This invention discloses a method for the compound regeneration of iron phosphate in lithium extraction slag, belonging to the field of waste lithium-ion battery resource recycling technology. The method first pre-treats the black powder from waste lithium iron phosphate batteries by defluorination, followed by selective chlorination with chlorine and aqueous solution leaching and filtration to obtain lithium extraction slag that retains the orthorhombic crystal structure of olivine and the initial physical morphology of the raw material. The slag is then subjected to a combined surface acid etching-decarburization and crystallization treatment to obtain battery-grade prototype iron phosphate. Simultaneously, the ferric ions and phosphate ions dissolved during the surface acid etching process are crystallized and precipitated by controlling pH, temperature, and concentration conditions to obtain crystalline iron phosphate. Finally, the prototype iron phosphate and crystalline iron phosphate are mixed in a specific ratio to obtain compound regenerated iron phosphate. This invention achieves efficient regeneration of iron phosphate in lithium extraction slag through a compounding method, exhibiting significant advantages such as high iron phosphate yield, low acid and alkali consumption, and low production cost.

Description

Technical Field

[0001] This application belongs to the field of waste lithium-ion battery resource recycling technology, specifically involving a compound regeneration method for iron phosphate in lithium extraction slag. Background Technology

[0002] Lithium iron phosphate (LFP) batteries have advantages such as long lifespan, high safety, and low cost, and are widely used in new energy vehicle power batteries and large-scale electrochemical energy storage. After the end of their lifespan, LFP batteries require proper disposal; direct disposal in the natural environment will pollute water and soil, seriously harming animal and plant health and human health. Meanwhile, LFP batteries contain resources such as lithium, copper, aluminum, iron, and phosphorus, giving them certain economic value for recycling. This raises the issue of recycling and reusing spent LFP batteries.

[0003] Currently, the recycling of spent lithium iron phosphate batteries mainly involves wet processing to recover lithium. The residue formed after lithium recovery is generally called lithium extraction residue, and its main components are phosphorus, iron, and carbon. The recycling and reuse of lithium extraction residue faces the following problems and shortcomings: First, the process is extremely lengthy. Existing methods for recovering lithium extraction slag often begin by using strong acid to dissolve valuable elements like phosphorus and iron in an aqueous solution. This is followed by filtration to separate these elements from carbon, yielding a filtrate. The filtrate is then treated to remove impurities by adding a strong alkali to adjust the pH and adding a precipitant. After impurity removal, iron phosphate precipitate is obtained, which is then recrystallized. This method involves repeated dissolution, filtration, precipitation or crystallization, and impurity removal, resulting in a very lengthy process.

[0004] Secondly, the large amount of acid and alkali used generates a significant amount of wastewater that requires treatment. Firstly, separating phosphorus and iron from carbon materials in lithium extraction slag requires a highly acidic environment to obtain a phosphorus-iron-containing filtrate with a high leaching rate. Secondly, removing impurities from the phosphorus-iron filtrate requires adjusting the pH value with alkali, consuming a large amount of alkali. Simultaneously, the entire liquid-phase treatment process generates a large amount of wastewater.

[0005] Third, the cost is high and the economic efficiency is poor. The lengthy process involves repeated impurity removal, washing, and filtration, which leads to increased loss of phosphorus iron and generates a large amount of wastewater and energy consumption. In addition, the large consumption of strong acids and alkalis significantly increases the cost of recycling.

[0006] Therefore, there is an urgent need to develop a high-yield, low-acid-base-consumption, and low-cost lithium extraction slag recycling technology, especially to efficiently recover phosphorus and iron from the lithium extraction slag, thereby maximizing the recycling value of waste lithium iron phosphate batteries. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing technologies by providing a method for recovering and regenerating iron phosphate resources from lithium extraction residue obtained after selective chlorination-leaching of lithium iron phosphate battery black powder (including injected and uninjected powder). The core of this method is to mix the original iron phosphate and crystalline iron phosphate to obtain compound regenerated iron phosphate, which features high yield, low acid and alkali consumption, and low cost.

[0008] This invention provides a method for the compound regeneration of iron phosphate in lithium extraction slag, the method comprising: First, the waste lithium iron phosphate battery black powder undergoes defluorination pretreatment. The waste lithium iron phosphate battery black powder is loaded into a defluorination reactor, oxygen is removed from the reactor, the temperature is raised to 500℃-600℃, and a defluorination carrier gas is continuously introduced and kept at this temperature for 20 minutes to 2 hours. The defluorination tail gas is then quickly discharged from the defluorination reactor. The defluorination carrier gas is a non-oxidizing gas containing crackable H-containing gases, and is mixed with one or more of nitrogen, argon, carbon dioxide, and helium. The crackable H-containing gases are one or more of the following: manganese acid gas, methyl carbonate gas, ethyl methyl carbonate gas, ethyl carbonate gas, and dimethyl carbonate gas; or a mixture of hydrogen and methane; or a mixture of methanol and methane; or a mixture of ethyl carbonate gas, ethane, and water vapor. After defluorination pretreatment, the residual fluorine content in the waste lithium iron phosphate battery black powder is <0.1%.

[0009] Secondly, selective chlorination of waste lithium iron phosphate battery black powder after defluorination pretreatment was carried out. Chlorine was used as an oxidant to react chemically with lithium iron phosphate in the battery black powder to generate iron phosphate and lithium chloride. The temperature was controlled at 0℃-200℃ and the reaction time was 5 minutes-120 minutes. The molar ratio of chlorine to iron in the battery black powder was 0.50-0.60 to obtain chlorinated clinker. The chlorinated clinker was leached with an aqueous solution and filtered to obtain lithium extraction slag and lithium chloride-containing brine. The iron phosphate in the lithium extraction slag retained the olivine-type orthorhombic crystal structure and the basic physical morphology of the initial raw material lithium iron phosphate cathode material.

[0010] Third, the lithium extraction slag is subjected to a combined surface acid etching and decarburization-crystallization treatment to obtain battery-grade iron phosphate; this prototype iron phosphate (i.e., battery-grade iron phosphate) retains the Fe / P molar ratio of the initial raw material lithium iron phosphate cathode material. The preparation of the prototype iron phosphate from the lithium extraction slag through the combined surface acid etching and decarburization-crystallization treatment includes two steps, wherein: The first step is the surface acid etching process. The lithium extraction slag is immersed in the acid etching solution, with the pH controlled at 0-1.0, the liquid-to-solid ratio at 5-20, the etching temperature at 25℃-95℃, and the etching time at 2-10 hours. Then, it is filtered, washed, and dried to obtain impurity-free lithium extraction slag with copper and aluminum impurities removed. The aluminum content in the impurity-free lithium extraction slag is <0.030%, and the copper content is <0.0010%. The solubility rate of iron phosphate in the lithium extraction slag during the surface acid etching process is controlled at 5%-30%. In this step, some iron phosphate and impurities such as copper and aluminum dissolve together in the acid etching solution. First, the copper and aluminum impurities in the acid etching solution are removed. Then, the crystallization conditions are controlled to recrystallize and precipitate the iron phosphate dissolved in the acid etching solution. After filtration, washing, drying, and calcination, crystalline iron phosphate can be obtained. The second step is the decarbonization and crystal transformation process. The lithium residue after impurity removal is loaded into the reactor, heated to 650℃-800℃, and air or oxygen is continuously introduced for oxidation and decarbonization. The temperature is maintained for 2-5 hours to transform the residue into crystal, thus obtaining the prototype iron phosphate. The prototype iron phosphate is hexagonal and has a carbon content of <0.10%.

[0011] Finally, the original ferric phosphate and crystalline ferric phosphate are mixed to obtain a compound regenerated ferric phosphate, wherein the compound regenerated ferric phosphate satisfies the following: and , ;in, , These represent the masses of the original ferric phosphate and the crystalline ferric phosphate, respectively. , , These represent the Fe / P molar ratios of the original ferric phosphate, crystalline ferric phosphate, and compound ferric phosphate, respectively. The tap density represents the compound ferric phosphate; the Fe / P ratio of the crystalline ferric phosphate satisfies: Median particle size of crystalline iron phosphate Median particle size compared to the prototype iron phosphate satisfy: .

[0012] Compared with the prior art, the present invention has the following beneficial technical effects: (1) High yield: In the traditional wet process, the loss of iron and phosphoric acid is mainly in two aspects. First, some iron and phosphorus will enter the lithium leaching solution and be removed as impurities in the subsequent purification process. Second, the liquid phase impurity removal process after all the lithium slag is dissolved in acid will also inevitably lead to the loss of iron and phosphorus. The yield of traditional wet iron phosphate is generally 90%. This invention selectively extracts lithium through gas-solid chlorination. Iron and phosphorus do not participate in the chlorination reaction. During the leaching of the chlorinated clinker with aqueous solution, the loss of phosphorus and iron due to dissolution into the aqueous solution is less than 0.2%. The dissolution rate of ferric phosphate in the lithium extraction residue during the surface acid etching process is controlled at 5%-30%, which means the original ferric phosphate yield is 70%-95%, equivalent to directly recovering 70%-95% of the phosphorus and iron resources. The approximately 5%-30% of phosphorus and iron dissolved in the aqueous solution can be further recovered using a traditional wet recrystallization process, resulting in a crystalline ferric phosphate yield of up to 90%, which is equivalent to recovering approximately 4.5%-27% of the total. Therefore, the overall phosphorus and iron yield of this invention is approximately 97%-99.5%.

[0013] (2) Low acid and alkali usage: More than 70% of the iron phosphate in this invention is prepared by a combined process of surface acid etching and decarburization to achieve the recovery of the prototype iron phosphate. The acid concentration and usage in the acid etching process are much lower than the acid concentration and usage required by the traditional wet process to completely dissolve the iron phosphate. This invention greatly reduces the consumption of materials.

[0014] (3) Low cost: Compared with traditional wet processes, this invention has a significant low cost advantage. Since the recovery cost of prototype ferric phosphate is only about 30% of that of crystalline ferric phosphate, the compound ferric phosphate of this invention is a mixture of prototype ferric phosphate and crystalline ferric phosphate. The higher the proportion of prototype ferric phosphate, the lower the cost of compound ferric phosphate. According to the mixing ratio of prototype ferric phosphate and crystalline ferric phosphate of this invention, under optimal conditions, the cost per ton of compound ferric phosphate is only about 44% of that of crystalline ferric phosphate using traditional methods. Attached Figure Description

[0015] Figure 1 This is a process flow diagram of a compound regeneration method for lithium extraction slag according to the present invention. Detailed Implementation

[0016] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way.

[0017] Example 1 This embodiment provides a method for the compound regeneration of iron phosphate in lithium extraction slag. The process flow of this method is shown in the attached figure. Figure 1 As shown, the specific steps are as follows: (1) Defluorination pretreatment. First, 100 kg of waste lithium iron phosphate battery black powder was subjected to defluorination pretreatment. The waste lithium iron phosphate battery black powder was loaded into the defluorination reactor, the oxygen in the defluorination reactor was removed, the temperature was raised to 550°C, the defluorination carrier gas was continuously introduced and kept at the temperature for 60 minutes, and the defluorination tail gas was quickly discharged from the defluorination reactor. The defluorination carrier gas is a non-oxidizing gas, which contains crackable H-containing gas and nitrogen with a volume content of 20%. It is a mixture of crackable H-containing ethane and water vapor. After the defluorination pretreatment of waste lithium iron phosphate battery black powder, the defluorinated battery black powder was obtained with a residual fluorine content of 0.045%.

[0018] (2) Lithium extraction by chlorination. The product obtained in the first step, namely the waste lithium iron phosphate battery black powder after defluorination pretreatment, is subjected to selective chlorination for lithium extraction. Chlorine is used as an oxidant to react chemically with lithium iron phosphate in the battery black powder to generate iron phosphate and lithium chloride. The reaction time is controlled at 80°C for 45 minutes. The molar ratio of chlorine to iron in the battery black powder is 0.525. Finally, chlorinated clinker is obtained. The chlorinated clinker is leached with aqueous solution and filtered to obtain lithium extraction slag and lithium chloride brine. The iron phosphate in the lithium extraction slag retains the olivine-type orthorhombic crystal structure and retains the basic physical morphology of the initial raw material lithium iron phosphate cathode material.

[0019] (3) Surface etching-decarburization and crystal transformation combined treatment. Battery-grade iron phosphate is obtained by surface etching and decarburization and crystal transformation combined treatment of lithium extraction slag; the prototype iron phosphate retains the Fe / P molar ratio of the initial raw material lithium iron phosphate cathode material. The preparation of prototype iron phosphate from lithium extraction slag through surface etching and decarburization and crystal transformation combined process includes two steps: The first step, the surface acid etching process, involves immersing the lithium extraction slag in an acid etching solution. The pH of the etching solution is controlled at 0.5, the liquid-to-solid ratio at 10, the etching temperature at 65℃, and the etching time at 4 hours. After filtration, washing, and drying, impurities-free lithium extraction slag is obtained. The aluminum content in the impurity-free lithium extraction slag is 0.024%, the copper content is 0.0005%, and the solubility of iron phosphate in the lithium extraction slag during the surface acid etching process is 15.2%. The second step, the decarbonization and crystal transformation process, involves loading the lithium-extracting slag after impurity removal into a reactor, heating it to 700°C, continuously introducing air for oxidation and decarbonization, and holding it at that temperature for 3 hours to perform crystal transformation, thereby obtaining the prototype iron phosphate. The prototype iron phosphate is hexagonal in shape and has a carbon content of 0.03%.

[0020] (4) Recrystallization of ferric phosphate in acid etching solution. In the acid etching process of lithium slag, 15.2% of ferric phosphate is dissolved in the acid etching solution, and impurities such as copper and aluminum are also dissolved in the acid etching solution. First, the copper and aluminum impurities in the acid etching solution are removed. Then, the crystallization conditions are controlled, the pH value of the solution is controlled at 1.0, and the crystallization temperature is 75℃. The ferric phosphate dissolved in the acid etching solution is recrystallized and precipitated, filtered, washed, and dried. Then, it is calcined at 650℃ for 3 hours in an aerobic environment to obtain crystalline ferric phosphate.

[0021] (5) Mixing and compounding. The original ferric phosphate and crystalline ferric phosphate are mixed to obtain compounded regenerated ferric phosphate. The compounded regenerated ferric phosphate satisfies: and , ;in, , These represent the masses of the original ferric phosphate and the crystalline ferric phosphate, respectively. , , These represent the Fe / P molar ratios of the original ferric phosphate, crystalline ferric phosphate, and compound ferric phosphate, respectively. The tap density represents the compound ferric phosphate; the Fe / P ratio of the crystalline ferric phosphate satisfies: Median particle size of crystalline iron phosphate Median particle size compared to the prototype iron phosphate satisfy .

[0022] After testing, in this embodiment, the Fe / P ratio of the prototype iron phosphate was found to be... =0.96, Fe / P ratio of crystalline iron phosphate =0.99, and the total yield of phosphorus and iron was 98.5%.

[0023] Example 2 This embodiment provides a method for the compound regeneration of iron phosphate in lithium extraction slag. The process flow of this method is shown in the attached figure. Figure 1 As shown, the specific steps are as follows: (1) Defluorination pretreatment. First, 100 kg of waste lithium iron phosphate battery black powder was subjected to defluorination pretreatment. The waste lithium iron phosphate battery black powder was loaded into the defluorination reactor, the oxygen in the defluorination reactor was removed, the temperature was raised to 600℃, the defluorination carrier gas was continuously introduced and kept at the temperature for 20 minutes, and the defluorination tail gas was quickly discharged from the defluorination reactor. The defluorination carrier gas is a non-oxidizing gas, which contains crackable H-containing gas and nitrogen with a volume content of 30%. It is a mixture of crackable H-containing ethane and water vapor. After the defluorination pretreatment of waste lithium iron phosphate battery black powder, the defluorinated battery black powder was obtained with a residual fluorine content of 0.027%.

[0024] (2) Lithium extraction by chlorination. The product obtained in the first step, namely the waste lithium iron phosphate battery black powder after defluorination pretreatment, was subjected to selective chlorination with chlorine. Chlorine was used as an oxidant to react chemically with the lithium iron phosphate in the battery black powder to generate iron phosphate and lithium chloride. The reaction time was controlled at 0°C for 120 minutes, and the molar ratio of chlorine to iron in the battery black powder was 0.55, finally yielding chlorinated clinker. The chlorinated clinker was leached with aqueous solution and filtered to obtain lithium extraction slag and lithium chloride-containing brine. The iron phosphate in the lithium extraction slag retained the olivine-type orthorhombic crystal structure and the basic physical morphology of the initial raw material lithium iron phosphate cathode material.

[0025] (3) Surface etching-decarburization and crystal transformation combined treatment. Battery-grade iron phosphate was obtained by subjecting lithium extraction slag to a combined surface etching-decarburization and crystal transformation treatment; this prototype iron phosphate retained the Fe / P molar ratio of the initial raw material lithium iron phosphate cathode material. The preparation of prototype iron phosphate from lithium extraction slag through a combined surface etching-decarburization and crystal transformation process involves two steps, wherein: The first step, the surface acid etching process, involves immersing the lithium extraction slag in an acid etching solution. The pH of the etching solution is controlled at 1.0, the liquid-to-solid ratio at 20, the etching temperature at 95℃, and the etching time at 2 hours. After filtration, washing, and drying, a cleaned lithium extraction slag with copper and aluminum impurities removed is obtained. The aluminum content in the cleaned lithium extraction slag is 0.019%, the copper content is 0.0007%, and the solubility of iron phosphate in the lithium extraction slag during the surface acid etching process is 20.5%. The second step, the decarbonization and crystal transformation process, involves loading the lithium-extracting slag after impurity removal into a reactor, heating it to 800°C, continuously introducing air for oxidation and decarbonization, and holding it at that temperature for 2 hours to perform crystal transformation, thereby obtaining the prototype iron phosphate. The prototype iron phosphate is hexagonal in shape and has a carbon content of 0.02%.

[0026] (4) Recrystallization of ferric phosphate in acid etching solution. In the acid etching process of lithium slag, 20.5% of ferric phosphate is dissolved in the acid etching solution, and impurities such as copper and aluminum are also dissolved in the acid etching solution. First, the copper and aluminum impurities in the acid etching solution are removed. Then, the crystallization conditions are controlled, the pH value of the solution is controlled at 0.5, and the crystallization temperature is 80℃. The ferric phosphate dissolved in the acid etching solution is recrystallized and precipitated, filtered, washed, and dried. Then, it is calcined at 700℃ for 2 hours in an aerobic environment to obtain crystalline ferric phosphate.

[0027] (5) Mixing and compounding. The original ferric phosphate and crystalline ferric phosphate are mixed to obtain compounded regenerated ferric phosphate. The compounded regenerated ferric phosphate satisfies: and , ;in , These represent the masses of the original ferric phosphate and the crystalline ferric phosphate, respectively. , , These represent the Fe / P molar ratios of the original ferric phosphate, crystalline ferric phosphate, and compound ferric phosphate, respectively. The tap density represents the compound ferric phosphate; the Fe / P ratio of the crystalline ferric phosphate satisfies: Median particle size of crystalline iron phosphate Median particle size compared to the prototype iron phosphate satisfy: .

[0028] After testing, the Fe / P ratio of the prototype iron phosphate in this embodiment was found to be... =0.97, Fe / P ratio of crystalline iron phosphate =0.99, and the total yield of phosphorus and iron was 98.0%.

[0029] Example 3 This embodiment provides a method for the compound regeneration of iron phosphate in lithium extraction slag. The process flow of this method is shown in the attached figure. Figure 1 As shown, the specific steps are as follows: (1) Defluorination pretreatment. First, 100 kg of waste lithium iron phosphate battery black powder was subjected to defluorination pretreatment. The waste lithium iron phosphate battery black powder was loaded into the defluorination reactor, the oxygen in the defluorination reactor was removed, the temperature was raised to 500℃, the defluorination carrier gas was continuously introduced and kept at the temperature for 120 minutes, and the defluorination tail gas was quickly discharged from the defluorination reactor. The defluorination carrier gas is a non-oxidizing gas, which contains crackable H-containing gas and nitrogen with a volume content of 40%. It is a mixture of crackable H-containing ethane and water vapor. After the defluorination pretreatment of waste lithium iron phosphate battery black powder, the defluorinated battery black powder was obtained with a residual fluorine content of 0.036%.

[0030] (2) Lithium extraction by chlorination. The product obtained in the first step, namely the waste lithium iron phosphate battery black powder after defluorination pretreatment, is subjected to selective chlorination with chlorine. Chlorine is used as an oxidant to react chemically with lithium iron phosphate in the battery black powder to generate iron phosphate and lithium chloride. The reaction time is controlled at 200℃ for 5 minutes, and the molar ratio of chlorine to iron in the battery black powder is 0.58, finally obtaining chlorinated clinker. The chlorinated clinker is leached with aqueous solution and filtered to obtain lithium extraction slag and lithium chloride-containing brine. The iron phosphate in the lithium extraction slag retains the olivine-type orthorhombic crystal structure and retains the basic physical morphology of the initial raw material lithium iron phosphate cathode material.

[0031] (3) Surface etching-decarburization and crystal transformation combined treatment. Battery-grade iron phosphate was obtained by subjecting lithium extraction slag to a combined surface etching-decarburization and crystal transformation treatment; this prototype iron phosphate retained the Fe / P molar ratio of the initial raw material lithium iron phosphate cathode material. The preparation of prototype iron phosphate from lithium extraction slag through a combined surface etching-decarburization and crystal transformation process involves two steps, wherein: The first step, the surface acid etching process, involves immersing the lithium extraction slag in an acid etching solution. The pH of the etching solution is controlled at 0, the liquid-to-solid ratio at 5, the etching temperature at 25℃, and the etching time at 10 hours. After filtration, washing, and drying, impurities-free lithium extraction slag is obtained. The aluminum content in the impurity-free lithium extraction slag is 0.0210%, the copper content is 0.0004%, and the solubility rate of iron phosphate in the lithium extraction slag during the surface acid etching process is 25.7%. The second step, the decarbonization and crystal transformation process, involves loading the lithium-extracting residue after impurity removal into a reactor, heating it to 650°C, continuously introducing air for oxidation and decarbonization, and holding it at that temperature for 5 hours to achieve crystal transformation, thereby obtaining the prototype iron phosphate. The prototype iron phosphate is hexagonal in shape and has a carbon content of 0.01%.

[0032] (4) Recrystallization of ferric phosphate in acid etching solution. In the acid etching process of lithium slag, 25.7% of ferric phosphate is dissolved in the acid solution, and impurities such as copper and aluminum are also dissolved in the acid solution. First, the copper and aluminum impurities in the acid solution are removed. Then, the crystallization conditions are controlled, the pH value of the solution is controlled at 0.7, and the crystallization temperature is 90℃. The ferric phosphate dissolved in the acid solution is recrystallized and precipitated, filtered, washed, and dried. Then, it is calcined at 600℃ for 4 hours in an aerobic environment to obtain crystalline ferric phosphate.

[0033] (5) Mixing and compounding. The original ferric phosphate and crystalline ferric phosphate are mixed to obtain compounded regenerated ferric phosphate. The compounded regenerated ferric phosphate satisfies: and , ;in , These represent the masses of the original ferric phosphate and the crystalline ferric phosphate, respectively. , , These represent the Fe / P molar ratios of the original ferric phosphate, crystalline ferric phosphate, and compound ferric phosphate, respectively. The tap density represents the compound ferric phosphate; the Fe / P ratio of the crystalline ferric phosphate satisfies: Median particle size of crystalline iron phosphate Median particle size compared to the prototype iron phosphate satisfy: .

[0034] After testing, the Fe / P ratio of the prototype iron phosphate in this embodiment was found to be... =0.982, Fe / P ratio of crystalline iron phosphate =0.99, and the total yield of phosphorus and iron was 97.4%.

[0035] The above provides a detailed description of a method for the compound regeneration of iron phosphate in lithium extraction slag. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of the embodiments are merely for the purpose of helping to understand the core ideas of this application. It should be noted that those skilled in the art can make various improvements and modifications to this application without departing from its principles, and these improvements and modifications also fall within the protection scope of the claims of this application.

Claims

1. A method for the compound regeneration of iron phosphate in lithium extraction slag, characterized in that, This method involves mixing proto-ferric phosphate and crystalline ferric phosphate to obtain a compound regenerated ferric phosphate, wherein: The prototype iron phosphate is battery-grade iron phosphate obtained by surface acid etching-decarburization and crystallization combined process of lithium extraction slag. This prototype iron phosphate retains the Fe / P molar ratio of the initial raw material lithium iron phosphate cathode material. Crystallized iron phosphate is battery-grade iron phosphate obtained by controlling pH, temperature and concentration conditions to crystallize and precipitate the ferric ions and phosphate ions dissolved in the aqueous solution from the lithium extraction slag during the preparation of the prototype iron phosphate. The compounded regenerated ferric phosphate satisfies: and , ;in, , These represent the masses of the original ferric phosphate and the crystalline ferric phosphate, respectively. , , These represent the Fe / P molar ratios of the original ferric phosphate, crystalline ferric phosphate, and compound ferric phosphate, respectively. The tap density represents the compound ferric phosphate; the Fe / P ratio of the crystalline ferric phosphate satisfies: Median particle size of crystalline iron phosphate Median particle size compared to the prototype iron phosphate satisfy ; The lithium extraction slag is waste lithium iron phosphate battery black powder. It is first defluorinated and pretreated, and then selectively chlorinated with chlorine to obtain chlorinated clinker. The chlorinated clinker is leached and filtered with an aqueous solution to obtain lithium extraction slag. The iron phosphate in the lithium extraction slag retains the olivine-type orthorhombic crystal structure and the basic physical morphology of the initial raw material lithium iron phosphate cathode material.

2. The method for regenerating iron phosphate in lithium extraction slag according to claim 1, characterized in that, The lithium extraction slag is used to prepare prototype iron phosphate through a combined surface acid etching-decarburization and crystal transformation process, specifically including two steps: The first step is the surface acid etching process, in which the lithium extraction slag is immersed in an acid etching solution. The pH of the acid etching solution is controlled at 0-1.0, the liquid-to-solid ratio is controlled at 5-20, the etching temperature is controlled at 25℃-95℃, and the etching time is controlled at 2-10 hours. Then, it is filtered, washed, and dried to obtain impurity-free lithium extraction slag with copper and aluminum impurities removed. The aluminum content in the impurity-free lithium extraction slag is <0.030%, the copper content is <0.0010%, and the solubility rate of iron phosphate in the lithium extraction slag during the surface acid etching process is controlled at 5%-30%. The second step is the decarbonization and crystal transformation process. The lithium residue after impurity removal is loaded into the reactor, heated to 650℃-800℃, and air or oxygen is continuously introduced for oxidation and decarbonization. The temperature is maintained for 2-5 hours to transform the residue into crystal, thus obtaining the prototype iron phosphate. The prototype iron phosphate is hexagonal and has a carbon content of <0.10%.

3. The method for regenerating ferric phosphate in lithium extraction slag according to claim 2, characterized in that, In the aforementioned acid etching process for lithium extraction slag, some iron phosphate and impurities such as copper and aluminum dissolve in the acid etching solution. First, the copper and aluminum impurities in the acid etching solution are removed. Then, the crystallization conditions are controlled at pH 0.5-1.5 and crystallization temperature 50℃-90℃ to recrystallize and precipitate the iron phosphate dissolved in the acid etching solution. The solution is then filtered, washed, dried, and calcined at 550℃-750℃ for 2-4 hours to dehydrate, yielding crystalline iron phosphate.

4. The method for regenerating ferric phosphate in lithium extraction slag according to claim 1 or 3, characterized in that, The aforementioned pre-treatment of waste lithium iron phosphate battery black powder involves loading the waste lithium iron phosphate battery black powder into a defluorination reactor, removing oxygen from the reactor, heating to 500℃-600℃, continuously introducing defluorination carrier gas and maintaining the temperature for 20 minutes to 2 hours, and quickly discharging the defluorination tail gas from the reactor. The defluorination carrier gas is a non-oxidizing gas containing crackable H-containing gases, and is mixed with one or more of nitrogen, argon, carbon dioxide, and helium. The crackable H-containing gases are one or more of the following: manganese acid gas, methyl carbonate gas, ethyl methyl carbonate gas, ethyl carbonate gas, and dimethyl carbonate gas; or a mixture of hydrogen and methane; or a mixture of methanol and methane; or a mixture of ethyl carbonate gas, ethane, and water vapor. After the pre-treatment, the residual fluorine content in the waste lithium iron phosphate battery black powder is <0.1%.

5. The method for regenerating ferric phosphate in lithium extraction slag according to claim 1, characterized in that, The defluorination pretreatment of waste lithium iron phosphate battery black powder followed by selective chlorination with chlorine gas refers to the chemical reaction between chlorine gas as an oxidant and lithium iron phosphate in the waste lithium iron phosphate battery black powder to produce iron phosphate and lithium chloride. The reaction is carried out at a temperature of 0℃-200℃ for a reaction time of 5 minutes-120 minutes, and the molar ratio of chlorine gas to iron in the battery black powder is 0.50-0.60.

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