Method for recycling lithium iron phosphate black powder

Abstract

The application discloses a recycling method of lithium iron phosphate black powder, which comprises the following steps: mixing the sintered lithium iron phosphate black powder with a first acidic reagent and filtering to obtain a first solution; adding hydrogen peroxide and a first alkaline reagent into the first solution, filtering to obtain a first solid and a second solution, and performing impurity removal on the second solution to obtain lithium dihydrogen phosphate; mixing the first solid with a second acidic reagent and a second alkaline reagent in sequence and filtering to obtain a second solid, and performing impurity removal on the second solid to obtain a third solid; mixing the third solid with a first phosphoric acid solution, performing three times of acid immersion, and performing impurity removal to obtain ferrous oxalate. The recycling method of the lithium iron phosphate black powder realizes selective lithium extraction and iron precipitation through multiple times of acid immersion and multiple times of alkaline immersion, guarantees high valuable element recovery rate and high product purity, effectively removes heavy metal impurities such as Fe, Al, Ti, Ni, Co and Mn, and has high purity of the prepared ferrous oxalate and lithium dihydrogen phosphate.

Description

Technical Field

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

[0002] The current market for lithium iron phosphate (manganese) black powder recycling mainly focuses on lithium recovery. However, the phosphate slag after lithium extraction is either discarded or sold as fertilizer raw material, resulting in its economic value not being fully realized. A few companies process the phosphate slag into iron phosphate, but because impurities such as Al, Ti, Ni, Co, and Mn cannot be effectively removed, the batch stability of iron phosphate products is poor and even the pass rate is low. Summary of the Invention

[0003] The purpose of this invention is to provide a method for recycling lithium iron phosphate black powder, which solves the problem of inadequate impurity removal during the recycling process.

[0004] To achieve the objectives of this invention, the following technical solution is provided: This invention provides a method for recycling lithium iron phosphate black powder, comprising: The sintered lithium iron phosphate black powder was mixed with the first acidic reagent and subjected to a first acid leaching followed by filtration to obtain the first solution; Hydrogen peroxide and a first alkaline reagent were added to the first solution to perform a first iron precipitation. After filtration, a first solid and a second solution were obtained. The second solution was then subjected to a purification operation to obtain lithium dihydrogen phosphate. The first solid is mixed sequentially with the second acidic reagent and the second alkaline reagent and filtered. The second solid is obtained by a second acid leaching and a second iron precipitation. The second solid is then subjected to a purification operation to obtain the third solid. The third solid was mixed with the first phosphoric acid solution and subjected to three acid leaching processes and impurity removal operations to obtain ferrous oxalate.

[0005] In one embodiment, the liquid-to-solid ratio of the first acidic reagent to the sintered lithium iron phosphate black powder is (3-6) mL:1g; and / or, the reaction temperature for adding the first acidic reagent is 60℃-95℃; and / or, the reaction time for adding the first acidic reagent is 1h-5h.

[0006] In one embodiment, the pH of the second solution is 1.8-3; and / or, the reaction temperature for adding the hydrogen peroxide and the first alkaline reagent is 30℃-90℃; and / or, the leaching time for adding the hydrogen peroxide and the first alkaline reagent is 1h-5h.

[0007] In one embodiment, a first solid is sequentially mixed with a second acidic reagent and a second basic reagent and then filtered to obtain a second solid, comprising: The first solid is mixed with the second acidic reagent and filtered to obtain a first presolution. The first presolution is mixed with the second alkaline reagent and filtered to obtain a second presolution and a second solid. Wherein, the pH of the second presol is 1.8-3; and / or, the reaction time of the first presol and the second alkaline reagent is 0.5h-3h; and / or, the reaction temperature of the first presol and the second alkaline reagent is 30℃-90℃.

[0008] In one embodiment, the second solid is subjected to a purification operation to obtain a third solid, comprising: The second solid was dissolved in pure water, and a second phosphoric acid solution was added and washed to obtain a third solid and a third pre-solution. Wherein, the liquid-solid ratio of pure water to the second solid is (3-6) ml:1 g; and / or, the concentration of the second phosphoric acid solution is 3%-8%; and / or, the pH of the third pre-solution is 1.5-2.5; and / or, the duration of the washing operation is 10 min-120 min.

[0009] In one embodiment, the third solid is mixed with a first phosphoric acid solution and subjected to three acid leaching processes followed by impurity removal to obtain ferrous oxalate, comprising: The third solid was mixed with the first phosphoric acid solution and subjected to three acid leaching processes followed by filtration to obtain the fourth pre-solution; Iron powder was added to the fourth pre-solution for reduction and then filtered to obtain the fifth pre-solution; The fifth pre-solution and the first oxalic acid solution were mixed and subjected to three iron precipitation processes and then filtered to obtain the fourth solid. The fourth solid is mixed with the second oxalic acid solution and aged to obtain the ferrous oxalate.

[0010] In one embodiment, the concentration of the first phosphoric acid solution is 20%-35%; and / or, the liquid-solid ratio of the first phosphoric acid solution to the third solid is (3-6) mL:1g; and / or, the reaction time for adding the first phosphoric acid solution is 20 min-120 min; and / or, the reaction temperature for adding the first phosphoric acid solution is 60℃-90℃.

[0011] In one embodiment, the concentration of the first oxalic acid solution is 2 mol / L-4 mol / L; and / or, the reaction time after mixing the fifth presolution and the first oxalic acid solution is 0.5 h-2 h; and / or, the reaction temperature after mixing the fifth presolution and the first oxalic acid solution is 30 °C-70 °C.

[0012] In one embodiment, the concentration of the second oxalic acid solution is 1.5 mol / L-3 mol / L; and / or, the liquid-to-solid ratio of the fourth solid to the second oxalic acid solution is (3-6) mL:1 g; and / or, the reaction time of the aging operation is 0.5 h-4 h; and / or, the reaction temperature of the aging operation is 60 °C-90 °C.

[0013] In one embodiment, the second solution is subjected to a purification operation to obtain lithium dihydrogen phosphate, comprising: A third alkaline reagent is added to the second solution to carry out a lithium precipitation reaction, and the solution is filtered to obtain a fourth solid. The fourth solid was mixed sequentially with the third phosphoric acid solution and the lithium hydroxide solution and filtered. The sixth pre-solution was obtained by acidification and impurity removal. The sixth presolution is mixed with lithium sulfide solid or hydrogen sulfide gas for deep impurity removal and then filtered to obtain the seventh presolution; The seventh presolution is acidified by adding a fourth phosphoric acid solution and then subjected to evaporation and crystallization to obtain the lithium dihydrogen phosphate.

[0014] The method for recycling lithium iron phosphate black powder of this invention uses lithium iron phosphate black powder to prepare battery-grade ferrous oxalate and lithium dihydrogen phosphate. It employs multiple acid leaching and multiple alkaline leaching to achieve selective lithium extraction and iron precipitation, ensuring high recovery rates of valuable elements and high product purity. It effectively removes heavy metal impurities such as Fe, Al, Ti, Ni, Co, and Mn, with the contents of Fe, Al, Ti, Ni, Co, and Mn in the impurity removal solution all less than 10 ppm. For the phosphate slag obtained from selective lithium extraction, phosphoric acid leaching is used, and the leachate is used to prepare ferrous oxalate dihydrate. This invention enables the complete recovery of all elements from lithium iron phosphate (LFP) black powder. The products, ferrous oxalate dihydrate and lithium dihydrogen phosphate, can be used as raw materials for LFP production, achieving recycling. Furthermore, the impurity content in the ferrous oxalate dihydrate prepared by this invention is superior to that of competing battery-grade products on the market. Specifically, the content of Ni, Co, and Mn is less than 5 ppm, the content of Cu is less than 1 ppm, and the content of sulfate is less than 100 ppm. The impurity content in the lithium dihydrogen phosphate is also superior to that of competing battery-grade products on the market. Specifically, the content of Ni, Co, and Mn is less than 5 ppm, the content of Cu is less than 1 ppm, the content of sulfate is less than 50 ppm, and the content of Na and K is less than 30 ppm. Moreover, this invention has a high recovery rate, with a comprehensive recovery rate of lithium, iron, phosphorus, and other elements >95%. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0016] Figure 1 This is a flowchart of a method for recycling lithium iron phosphate black powder according to one embodiment; Figure 2 This is a scanning electron microscope (SEM) image of ferrous oxalate from one embodiment. Figure 3 This is a scanning electron microscope (SEM) image of lithium dihydrogen phosphate according to one embodiment; Figure 4 This is a flowchart of one step in a method for recycling lithium iron phosphate black powder according to one embodiment; Figure 5 This is a flowchart of another step in a method for recycling lithium iron phosphate black powder according to one embodiment; Figure 6 This is a flowchart of a method for recycling lithium iron phosphate black powder according to another embodiment. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] It should be noted that when a component is said to be "fixed" to another component, it can be directly on the other component or it can be in a middle component. When a component is said to be "connected" to another component, it can be directly connected to the other component or it may be in a middle component.

[0019] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0020] The following detailed description of some embodiments of the present invention is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0021] Please refer to Figure 1 This invention provides a method for recycling lithium iron phosphate black powder, comprising: Step S10: The sintered lithium iron phosphate black powder is mixed with the first acidic reagent for a first acid leaching and then filtered to obtain the first solution.

[0022] In step S20, hydrogen peroxide and a first alkaline reagent are added to the first solution to perform a first iron precipitation. After filtration, a first solid and a second solution are obtained. The second solution is then subjected to a purification operation to obtain lithium dihydrogen phosphate.

[0023] In step S30, the first solid is mixed with the second acidic reagent and the second alkaline reagent in sequence and filtered. The second solid is obtained by a second acid leaching and a second iron precipitation. The second solid is then subjected to a purification operation to obtain the third solid.

[0024] Step S40: The third solid is mixed with the first phosphoric acid solution and subjected to three acid leaching processes and impurity removal operations to obtain ferrous oxalate.

[0025] Optionally, in step S10, the sintering conditions for lithium iron phosphate black powder are as follows: sintering is carried out in a reducing atmosphere of air, N2, or N2+H2 / CO, wherein the volume percentage of hydrogen and carbon monoxide does not exceed 5%, the sintering temperature is 350℃-600℃, and the sintering time is 0.5h-2h. The sintering temperature can be 350℃, 400℃, 450℃, 500℃, 550℃, 600℃, etc., without limitation. The sintering time can be 0.5h, 1h, 1.5h, 2h, etc., without limitation. Lithium iron phosphate black powder is rich in metallic elements such as lithium, iron, and phosphorus. These elements need to exist in specific valence states during the recovery process to ensure the purity of subsequent extraction. In an oxidizing atmosphere, iron (Fe²⁺)... + It is easily oxidized to Fe³ + This leads to the formation of impurity phases (such as LiFeO2, Li3Fe2(PO4)3, etc.), reducing the electrochemical performance of the recycled materials. Sintering in a reducing atmosphere provides protection, preventing iron oxidation and ensuring that iron in the product is in the form of Fe²⁺. + The sintering temperature is too high, which can cause the lithium source particles in the lithium iron phosphate black powder to agglomerate, destroying the material structure. Trace impurities introduced from the raw materials or environment (such as Na, K, Al, Si, etc.) may form a eutectic mixture with lithium iron phosphate at high temperatures. If the sintering temperature is too low, the solid-phase reaction will be incomplete, resulting in impurity residues.

[0026] Optionally, in step S10, mixing and filtering can also yield toner, which can be washed and dried as a byproduct.

[0027] Optionally, in step S10, the first acidic reagent can be one or more of concentrated sulfuric acid, concentrated hydrochloric acid, concentrated phosphoric acid, and concentrated nitric acid, where n(H+) = (2-4)·n(M), and M represents a metal ion, such as Fe, Al, Ti, Ni, Co, Mn, etc.

[0028] Optionally, the first alkaline reagent in step S20 may be one or more of ammonia water, ammonia gas, sodium hydroxide, potassium hydroxide, ammonium bicarbonate, etc.

[0029] Optionally, in step S20, the theoretical value of the first solution is n(H2O2) = 1 / 2 · n(Fe2+), and the actual amount added is 1.0-1.5 times the theoretical value. Hydrogen peroxide performs an oxidation reaction. As a strong oxidant, hydrogen peroxide oxidizes ferrous ions into ferric ions. Subsequently, the first alkaline reagent is used to increase the pH of the solution, which promotes the combination of ferric ions and phosphate ions to form ferric phosphate precipitate.

[0030] The method for recycling lithium iron phosphate black powder of this invention uses lithium iron phosphate black powder to prepare battery-grade ferrous oxalate and lithium dihydrogen phosphate. It employs multiple acid leaching and multiple alkaline leaching to achieve selective lithium extraction and iron precipitation, ensuring high recovery rates of valuable elements and high product purity. It effectively removes heavy metal impurities such as Fe, Al, Ti, Ni, Co, and Mn, with the contents of Fe, Al, Ti, Ni, Co, and Mn in the impurity removal solution all less than 10 ppm. For the phosphate slag obtained from selective lithium extraction, phosphoric acid leaching is used, and the leachate is used to prepare ferrous oxalate dihydrate. This invention enables the complete recovery of all elements from lithium iron phosphate (LiFePO4) black powder. The products, ferrous oxalate dihydrate and lithium dihydrogen phosphate, can be used as raw materials for LiFePO4 production, achieving recycling. Furthermore, the impurity content in the ferrous oxalate dihydrate prepared by this invention is superior to that of competing battery-grade products on the market. Specifically, the content of Ni, Co, and Mn is less than 5 ppm, the content of Cu is less than 1 ppm, and the content of sulfate is less than 100 ppm. The impurity content in the lithium dihydrogen phosphate is also superior to that of competing battery-grade products on the market. Specifically, the content of Ni, Co, and Mn is less than 5 ppm, the content of Cu is less than 1 ppm, the content of sulfate is less than 50 ppm, and the content of Na and K is less than 30 ppm. Moreover, this invention boasts a high recovery rate, with a comprehensive recovery rate of lithium, iron, and phosphorus exceeding 95%. Specifically, the lithium recovery rate is calculated as follows: lithium recovery rate = (lithium element in lithium dihydrogen phosphate - lithium element in added lithium hydroxide) / lithium in the raw material; iron recovery rate = (iron element in ferrous oxalate - reduced iron) / iron in the raw material; phosphorus recovery rate = (phosphorus element in lithium dihydrogen phosphate - added phosphorus) / phosphorus in the raw material.

[0031] Electron micrograph of ferrous oxalate prepared using the lithium iron phosphate black powder recycling method of the present invention is shown below. Figure 2 As shown, the electron microscope image of the prepared lithium dihydrogen phosphate is as follows. Figure 3As shown in the figure, ferrous oxalate has different morphological structures and particle sizes. At the same time, the prepared ferrous oxalate particles and lithium dihydrogen phosphate have regular morphologies and no dust on the particle surface, indicating that the ferrous oxalate crystals and lithium dihydrogen phosphate crystals have good crystallinity.

[0032] In one embodiment, the liquid-to-solid ratio of the first acidic reagent to the sintered lithium iron phosphate black powder is (3-6) mL:1g. Optionally, the liquid-to-solid ratio can be 3mL:1g, 3.5mL:1g, 4mL:1g, 4.5mL:1g, 5mL:1g, 5.5mL:1g, 6mL:1g, etc., without limitation. Specifically, the sintered lithium iron phosphate black powder can be dissolved in pure water first, and then the acid solution can be added for mixing. The liquid-to-solid ratio directly affects the leaching efficiency and acid consumption. If the liquid-to-solid ratio is too small, the first acidic reagent decomposes faster, the effective concentration decreases, and the lithium-ion leaching rate decreases. While a large liquid-to-solid ratio can reduce the viscosity of the slurry, it increases the equipment load and acid consumption, reducing economic efficiency. A moderate liquid-to-solid ratio can balance lithium-ion leaching efficiency and cost.

[0033] And / or, the reaction temperature for adding the first acidic reagent is 60℃-95℃. Optionally, the reaction temperature can be 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, etc., without limitation. If the reaction temperature is too low, the reaction will be incomplete, and the lithium leaching rate will be low; if the reaction temperature is too high, the reaction time can be shortened, but excessive evaporation or side reactions may occur. A moderate reaction temperature can ensure the lithium leaching rate while reducing the dissolution loss of iron.

[0034] And / or, the reaction time for adding the first acidic reagent is 1-5 hours. Optionally, the reaction time can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, etc., without limitation. Too short a reaction time may result in incomplete lithium leaching; too long a reaction time may trigger side reactions (such as the formation of LiFe(PO4)(OH) impurities). A moderate reaction time corresponds to a higher lithium leaching rate and fewer impurities.

[0035] In one embodiment, the pH of the second solution is 1.8-3. Optionally, the pH of the second solution can be 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, etc., without limitation. If the pH is too high, impurity elements such as aluminum and magnesium will also precipitate, leading to a decrease in the purity of the first solid. If the pH is too low, the precipitation of iron phosphate will be insufficient. A suitable pH can ensure sufficient precipitation of iron ions and minimize the introduction of other impurity elements.

[0036] And / or, the reaction temperature for adding hydrogen peroxide and the first alkaline reagent is 30℃-90℃. Optionally, the reaction temperature can be 30℃, 40℃, 50℃, 60℃, 70℃, 80℃, 90℃, etc., without limitation. When the reaction temperature is too low, the precipitation rate is slow, crystal growth is insufficient, and it may lead to amorphous precipitates or uneven particle size. When the reaction temperature is too high, the decomposition of hydrogen peroxide is accelerated. When the reaction temperature is moderate, the concentration of hydrogen peroxide as a reactant can be ensured without reducing the precipitation rate, thereby improving production efficiency.

[0037] And / or, the leaching time with hydrogen peroxide and the first alkaline reagent is 1-5 hours. Optionally, the leaching time can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, etc., without limitation. Insufficient leaching time may lead to incomplete oxidation, affecting the subsequent precipitation effect; if the time is too long, it will affect the production cycle, increase energy consumption and equipment requirements; controlling the leaching time ensures the full conversion and precipitation of iron ions.

[0038] In one embodiment, step S30, mixing the first solid with a second acidic reagent and a second alkaline reagent sequentially and filtering to obtain a second solid, includes: mixing the first solid with the second acidic reagent and filtering to obtain a first pre-solution, and mixing the first pre-solution with the second alkaline reagent and filtering to obtain a second pre-solution and a second solid.

[0039] Specifically, the first solid can be slurried with pure water first, and then acid can be added for further adjustment. The liquid-to-solid ratio of pure water to the first solid is (3-6) ml:1g. Optional liquid-to-solid ratios include 3mL:1g, 3.5mL:1g, 4mL:1g, 4.5mL:1g, 5mL:1g, 5.5mL:1g, 6mL:1g, etc., without limitation. A liquid-to-solid ratio that is too high may cause hydrolysis, reducing the iron recovery rate. A liquid-to-solid ratio that is too low will result in poor cleaning of the first solid, and impurities that may be attached to the surface of the first solid cannot be removed. A suitable liquid-to-solid ratio can prevent crystal agglomeration and reduce dissolution losses caused by excessively high local concentrations. The second acidic reagent can be phosphoric acid, sulfuric acid, hydrochloric acid, nitric acid, etc., without limitation.

[0040] And / or, the pH of the second presolution is 1.8-3. Optionally, the pH of the second presolution can be 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, etc., without limitation. If the pH is too high, impurities such as aluminum and magnesium will also precipitate, leading to a decrease in the purity of the second solid. If the pH is too low, the precipitation of ferric phosphate will be insufficient. A suitable pH can ensure sufficient precipitation of iron ions and minimize the introduction of other impurities.

[0041] And / or, the reaction time of the first pre-solution and the second alkaline reagent is 0.5h-3h. Optionally, the reaction time can be 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, etc., without limitation. Too short a reaction time may result in incomplete reaction, low iron precipitation rate, and fine precipitate particles, making filtration difficult; too long a reaction time may cause precipitate particle agglomeration or co-precipitation of impurities, reducing product purity. A moderate reaction time corresponds to a higher iron precipitation rate and fewer impurities.

[0042] And / or, the reaction temperature of the first pre-solution and the second alkaline reagent is 30℃-90℃. Optionally, the reaction temperature can be 30℃, 40℃, 50℃, 60℃, 70℃, 80℃, 90℃, etc., without limitation. If the reaction temperature is too low, the reaction will be incomplete, and the lithium leaching rate will be low; if the reaction temperature is too high, the reaction time can be shortened, but excessive evaporation or side reactions may occur. A moderate reaction temperature can ensure the lithium leaching rate while reducing the dissolution loss of iron.

[0043] Step S30 is used to achieve secondary iron precipitation, which can further precipitate residual iron ions and remove divalent metal impurities such as calcium and magnesium. The stepwise precipitation can avoid the co-precipitation of impurities (such as aluminum and copper) with iron, improve the purity of iron phosphate products, and meet the requirements of battery-grade materials.

[0044] In one embodiment, step S30 involves removing impurities from the second solid to obtain a third solid, which includes dissolving the second solid in pure water, adding a second phosphoric acid solution, and washing to obtain a third solid and a third pre-solution.

[0045] The liquid-to-solid ratio of pure water to the second solid is (3-6) ml:1g. Optional ratios include 3mL:1g, 3.5mL:1g, 4mL:1g, 4.5mL:1g, 5mL:1g, 5.5mL:1g, 6mL:1g, etc., without limitation. A too-high liquid-to-solid ratio may lead to hydrolysis, reducing the iron recovery rate. A too-low ratio results in poor cleaning of the second solid, making it difficult to remove impurities that may be attached to its surface. A suitable liquid-to-solid ratio prevents crystal agglomeration, facilitates subsequent dissolution operations, and reduces dissolution losses due to excessively high local concentrations. Pure water is neutral, providing a neutral starting point for subsequent dilute phosphoric acid washing and avoiding direct contact with strong acid, which could lead to excessively high local acidity on the crystal surface.

[0046] And / or, the concentration of the second phosphoric acid solution is 3%-8%. Optionally, the concentration of the second phosphoric acid solution is 3%, 4%, 5%, 6%, 7%, 8%, etc., without limitation. Dilute phosphoric acid provides hydrogen ions, which reduces the degree of hydrolysis of iron through the common ion effect, reduces the formation of Fe(OH)2, and avoids product loss. The second phosphoric acid solution can dissolve and wash away impurities such as sulfates and chlorides that are not completely removed by pure water, while avoiding the introduction of new impurities. If the concentration of the second phosphoric acid solution is too low, it may not be able to adequately prevent the hydrolysis of iron; if the concentration of the second phosphoric acid solution is too high, it may excessively corrode the crystal surface during the washing process. A second phosphoric acid solution of moderate concentration can dissolve and remove impurities on the crystal surface without corroding the crystal surface.

[0047] And / or, the pH of the third pre-solution is 1.5-2.5. Optionally, the pH of the third pre-solution is 1.5, 1.7, 1.9, 2.1, 2.3, 2.5, etc., without restriction. At this pH, the third pre-solution inhibits the hydrolysis and oxidation of iron, and the concentration of oxalate ions is moderate, promoting precipitation and avoiding side reactions. If the pH of the third pre-solution is too high, iron precipitation will occur, reducing the yield of the final product.

[0048] And / or, the washing operation duration is 10 min–120 min. Optionally, the washing operation duration is 10 min, 30 min, 50 min, 70 min, 90 min, 110 min, 120 min, etc., without limitation. A washing operation duration that is too short may not adequately dissolve and remove impurities from the crystal surface; a washing operation duration that is too long will prolong the production cycle. A moderate washing duration can efficiently dissolve and remove impurities from the crystal surface.

[0049] Please refer to Figure 4 In one embodiment, step S40 includes: Step S41: Mix the third solid with the first phosphoric acid solution, perform three acid leachings, and filter to obtain the fourth pre-solution.

[0050] Step S42: Add iron powder to the fourth presol solution for reduction and filter to obtain the fifth presol solution.

[0051] Step S43: Mix the fifth pre-solution and the first oxalic acid solution, perform three iron precipitation tests and filter to obtain the fourth solid.

[0052] Step S44: The fourth solid is mixed with the second oxalic acid solution and aged to obtain ferrous oxalate.

[0053] Optionally, in step S42, the reduction principle is: n(Fe) = n(copper) + 1 / 2·n(Fe3+). The actual amount of iron powder added is 1-1.5 times the theoretical molar value to ensure the full removal of copper and the full reduction of iron, so that the fifth pre-solution is an iron-phosphorus solution.

[0054] Please refer to Figure 6 Optionally, in step S43, filtration yields a fourth solid and a second phosphoric acid mother liquor, with the second phosphoric acid mother liquor containing 15%-30% phosphoric acid. Approximately two-thirds of the second phosphoric acid mother liquor is recycled for secondary acid leaching in the next batch in step S30, and the remaining approximately one-third is used for lithium precipitation in subsequent step S22 and impurity removal in step S24. Optionally, the third solid is iron oxalate.

[0055] Optionally, step S44 also includes a centrifugal drying operation, specifically centrifuging and drying the solid-liquid mixture after aging, with a drying temperature of 80℃-110℃ and a drying time of 1h-6h, and the centrifuged solution can be used for lithium precipitation in step S22 and impurity removal in step S24.

[0056] Optionally, steps S42 and S44 are both carried out in a nitrogen atmosphere to ensure that the content of ferric iron in ferrous oxalate dihydrate is less than 0.1%.

[0057] For the ferrophosphate slag obtained after selective lithium extraction, this invention uses a first phosphoric acid solution for leaching to obtain a first solution and prepare ferrous oxalate dihydrate. The mother liquor from subsequent preparation processes is a phosphoric acid solution. A portion of the phosphoric acid solution is returned to the phosphoric acid leaching process, and the remainder is used to prepare lithium dihydrogen phosphate, enabling the recycling of the phosphoric acid solution with only a small amount of additional replenishment required. Compared to traditional wet recycling processes, this invention generates less wastewater, achieves phosphoric acid recycling, and has lower production costs.

[0058] In one embodiment, the concentration of the primary phosphoric acid solution is 20%-35%. Optionally, the concentration of the primary phosphoric acid solution can be 20%, 25%, 30%, 35%, etc., without limitation. If the concentration of the primary phosphoric acid solution is too low, the decomposition of the primary phosphoric acid solution is accelerated, the effective concentration is reduced, and the iron ion leaching rate decreases. If the concentration of the primary phosphoric acid solution is too high, it will promote the formation of iron-phosphate complexes, reducing the iron ion leaching efficiency. When the concentration of the primary phosphoric acid solution is moderate, the iron ion leaching rate is stable and efficient.

[0059] And / or, the liquid-to-solid ratio of the first phosphoric acid solution to the third solid is (3-6) mL:1g. Optionally, the liquid-to-solid ratio can be 3mL:1g, 4mL:1g, 5mL:1g, 6mL:1g, etc., without limitation. When the liquid-to-solid ratio is too low, the viscosity of the slurry increases, the fluidity deteriorates, and the contact area and diffusion rate between phosphoric acid and black powder particles are limited. When the liquid-to-solid ratio is too high, larger reaction equipment and more phosphoric acid are required, leading to increased production costs. When the liquid-to-solid ratio is moderate, the leaching efficiency of iron ions is high and the production cost is moderate.

[0060] And / or, the reaction time for adding the primary phosphoric acid solution is 20-120 min. Optionally, the reaction time can be 20 min, 40 min, 60 min, 80 min, 100 min, 120 min, etc., without limitation. Too short a reaction time may lead to incomplete iron leaching; too long a reaction time does not significantly improve the iron leaching rate, but only prolongs the experimental period. A moderate reaction time can balance the leaching rate and the experimental period.

[0061] And / or, the reaction temperature for adding the primary phosphoric acid solution is 60℃-90℃. Optionally, the reaction temperature can be 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, etc., without limitation. At lower reaction temperatures, the dissolution rate of lithium iron phosphate and the migration rate of iron ions are slower, resulting in a lower leaching rate. Excessively high reaction temperatures may cause phosphoric acid decomposition or volatilization, reducing leaching efficiency. Controlling the reaction temperature between 60℃ and 90℃ ensures both the normal use of the primary phosphoric acid solution and leaching efficiency.

[0062] In one embodiment, the concentration of the first oxalic acid solution is 2 mol / L to 4 mol / L. Optionally, the concentration of the first oxalic acid solution can be 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, etc., without limitation. If the concentration of the first oxalic acid solution is too low, it will prolong the reaction time required to form ferrous oxalate. If the concentration of the first oxalic acid solution is too high, nucleation may occur, resulting in excessively small particle size of the generated ferrous oxalate. A moderate concentration of the first oxalic acid solution ensures that the particle size of the generated ferrous oxalate is not too fine, while also improving the reaction cycle.

[0063] And / or, the reaction time after mixing the fifth pre-solution and the first oxalic acid solution is 0.5 h to 2 h. Optionally, the reaction time can be 0.5 h, 1 h, 1.5 h, 2 h, etc., without limitation. Excessive reaction time will affect the reaction cycle. Insufficient reaction time is not conducive to the complete reaction of oxalic acid and ferrous ions. An appropriate reaction time ensures the complete reaction of oxalic acid and ferrous ions without excessively prolonging the experimental cycle.

[0064] And / or, the reaction temperature for mixing the fifth pre-solution and the first oxalic acid solution is 30℃-70℃. Optionally, the reaction temperature can be 30℃, 40℃, 50℃, 60℃, 70℃, etc., without limitation. When the reaction temperature is too low, molecular motion weakens, the collision frequency of reactants decreases, resulting in a significant decrease in the reaction rate. When the reaction temperature is too high, oxalic acid easily decomposes into carbon dioxide and water at high temperatures, leading to a decrease in reactant concentration, a decrease in reaction rate, or even failure to fully react with impurity particles. When the reaction temperature is moderate, the concentration of oxalic acid as a reactant is maintained without reducing the reaction rate, thereby improving the precipitation efficiency of impurities.

[0065] In one embodiment, the concentration of the second oxalic acid solution is 1.5 mol / L to 3 mol / L. The concentration of the second oxalic acid solution can be 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, etc., and is not limited. If the concentration of the second oxalic acid solution is too low, incomplete reaction may result in residual ferrous ions in the product, which also affects purity. If the concentration of the second oxalic acid solution is too high, impurities (such as unreacted oxalate ions or byproducts) may be introduced, reducing product purity. A moderate concentration of the second oxalic acid solution ensures product purity.

[0066] And / or, the liquid-to-solid ratio of the fourth solid and the second oxalic acid solution is (3-6) mL:1g. Optionally, the liquid-to-solid ratio can be 3 mL:1g, 3.5 mL:1g, 4 mL:1g, 4.5 mL:1g, 5 mL:1g, 5.5 mL:1g, 6 mL:1g, etc., without limitation. Due to hindered crystal growth and secondary nucleation, a low liquid-to-solid ratio can lead to irregular morphology, rough surface, and easy agglomeration of aged ferrous oxalate crystals. A high liquid-to-solid ratio will affect equipment and production costs. An appropriate liquid-to-solid ratio helps to save costs while producing stable crystal forms.

[0067] And / or, the aging process can last from 0.5 h to 4 h. Optionally, the aging time can be 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, etc., without limitation. Excessive aging time may lead to crystal aggregation or secondary nucleation, resulting in increased particle size and crystal form transformation. Insufficient reaction time leads to unstable crystal form of the aged product. An appropriate reaction time can form ferrous oxalate with a stable crystal structure.

[0068] And / or, the aging reaction temperature is 60℃-90℃. Optionally, the aging reaction temperature can be 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, etc., without limitation. When the reaction temperature is too low, the system has a high degree of supersaturation, high resistance to particle movement, slow crystal growth, and is prone to forming β-type ferrous oxalate. When the reaction temperature is too high, particle movement intensifies, the crystal growth rate accelerates, and it tends to form more stable α-type ferrous oxalate. When the reaction temperature is moderate, it helps to form mixed-crystal ferrous oxalate, thereby improving the stress concentration problem of single-crystal ferrous oxalate during sintering.

[0069] Please refer to Figure 5 In one embodiment, step S20 involves removing impurities from the second solution to obtain lithium dihydrogen phosphate, including: Step S21: Add a third alkaline reagent to the second solution to carry out a lithium precipitation reaction and filter to obtain a fourth solid.

[0070] In step S22, the fourth solid is mixed sequentially with the third phosphoric acid solution and the lithium hydroxide solution and filtered. The sixth presolution is obtained by acidification and impurity removal.

[0071] Step S23: Mix the sixth presolution with lithium sulfide solid or hydrogen sulfide gas for deep impurity removal and filtration to obtain the seventh presolution.

[0072] Step S24: Add the fourth phosphoric acid solution to the seventh presolution for acidification and perform evaporation and crystallization to obtain lithium dihydrogen phosphate.

[0073] Optionally, in step S21, the third alkaline reagent is one or more of ammonia water, ammonia gas, sodium hydroxide, potassium hydroxide, lithium hydroxide, and ammonium bicarbonate. The third alkaline reagent is used to precipitate impurity ions such as aluminum ions, magnesium ions, and calcium ions in the second solution. After adding the third alkaline reagent to the second solution, solid-liquid separation is performed, and the corresponding solution contains byproducts such as ammonium salts and sodium salts. These byproducts can be obtained by evaporation, crystallization, and drying. Furthermore, the reaction temperature in step S21 is 60℃-90℃, the reaction time is greater than 5 minutes, and the pH of the solution after adding the third alkaline reagent is 9-12.

[0074] Optionally, step S22 may also include washing the fourth solid by using hot water at 60℃-90℃ for multi-stage countercurrent washing until the conductivity of the washing solution is less than 1.5 mS / cm, and then dissolving it with a second phosphoric acid solution after solid-liquid separation.

[0075] Optionally, in step S22, the pH of the fourth solid dissolved in the third phosphoric acid solution is less than 3, and the pH of the sixth pre-solution after adding lithium hydroxide solution is 3.5-5. The lithium hydroxide solution can be saturated or near-saturated lithium hydroxide solution, the reaction time is greater than 10 min, and the sixth pre-solution is obtained after solid-liquid separation. This step can be used to remove impurity elements such as iron, aluminum, and titanium. The pH of the sixth pre-solution is not limited to 3.5, 4, 4.5, or 5.

[0076] Optionally, in step S23, when hydrogen sulfide gas is introduced, the reaction pressure is controlled at 0.1 MPa - 0.5 MPa, and the reaction time is 0.5 h - 2 h. After the reaction, solid-liquid separation is performed to obtain the seventh pre-solution. This step mainly removes heavy metal impurities such as Ni, Co, Mn, Zn, Fe, and Cu.

[0077] Please refer to Figure 6 Optionally, in step S24, the fourth phosphoric acid solution is industrial-grade or battery-grade phosphoric acid, resulting in a pH of 1.8-3 for the seventh pre-solution. The pH of the seventh pre-solution can be 1.8, 2, 2.5, 3, 3.5, etc., without limitation. The evaporation and crystallization operation includes: evaporation at 110℃-130℃ until obvious solid precipitation occurs in the seventh pre-solution; after no obvious solid precipitation occurs, heating is stopped and the temperature is lowered to 30℃-60℃, with a cooling time of 0.5h-3h; centrifugation is performed, controlling the water content of the solid to be less than 5%; the centrifuged solid is dried at 100℃-120℃ until the water content is <0.35%, yielding the lithium dihydrogen phosphate product. The first phosphoric acid mother liquor obtained after centrifugation can be recycled for the next batch of lithium dihydrogen phosphate impurity removal process.

[0078] A specific method for recycling lithium iron phosphate black powder can be found by referring to... Figure 6 The first phosphoric acid mother liquor obtained after evaporation and crystallization can be reused in the impurity removal process of lithium dihydrogen phosphate preparation; the second phosphoric acid mother liquor obtained after reaction with the first oxalic acid solution and filtration can be used in the impurity removal process of lithium dihydrogen phosphate preparation and the impurity removal process of ferrous oxalate preparation.

[0079] The technical solution of the present invention will be described in detail below through specific embodiments.

[0080] Example 1 Please refer to Figure 6 The method for recycling lithium iron phosphate black powder in this embodiment is as follows: Step S10: The sintered lithium iron phosphate black powder in nitrogen is mixed with a first acidic reagent and filtered to obtain a first solution. The sintering temperature is 500℃, and the sintering time is 2h. The first acidic reagent is sulfuric acid, and the liquid-to-solid ratio of the first acidic reagent to the sintered lithium iron phosphate black powder is 4ml:1g. The reaction temperature for mixing the first acidic reagent is 90℃, and the reaction time for adding the first acidic reagent is 4h.

[0081] In step S20, hydrogen peroxide and a first alkaline reagent are added to the first solution. After filtration, a first solid and a second solution are obtained. The second solution is then subjected to a purification process to obtain lithium dihydrogen phosphate. The first alkaline reagent is ammonia; the pH of the second solution is 2.0; the reaction temperature for adding hydrogen peroxide and the first alkaline reagent is 60°C; and the leaching time for adding hydrogen peroxide and the first alkaline reagent is 2 hours.

[0082] In step S30, the first solid is mixed with the second acidic reagent and filtered to obtain a first presolution. The first presolution and the second alkaline reagent are then mixed and filtered to obtain a second presolution and a second solid. The second acidic reagent is phosphoric acid, and the second alkaline reagent is ammonia. The pH of the second presolution is 2.2. The reaction time of the first presolution and the second alkaline reagent is 2 hours. The reaction temperature of the first presolution and the second alkaline reagent is 60°C.

[0083] The second solid was purified by dissolving it in pure water, adding a second phosphoric acid solution, and washing to obtain a third solid and a third pre-solution. The liquid-to-solid ratio of pure water to the second solid was 4 ml: 1 g; the concentration of the second phosphoric acid solution was 5%; and / or, the pH of the third pre-solution was 2.5; the washing time was 1 h.

[0084] The second solution was purified, a third alkaline reagent was added, and the solution was filtered to obtain a fourth solid. The third alkaline reagent was lithium hydroxide; the pH of the solution after adding the third alkaline reagent was 10.

[0085] Multi-stage countercurrent washing was performed using hot water at 60℃-90℃ until the conductivity of the washing solution was less than 1.5 mS / cm. After solid-liquid separation, the solution was dissolved in a third phosphoric acid solution, then mixed with a saturated lithium hydroxide solution and filtered to obtain the sixth pre-solution. The pH of the sixth pre-solution was 4.5.

[0086] The sixth presolution was mixed with hydrogen sulfide gas and filtered to obtain the seventh presolution. The pH of the seventh presolution was 4.2. The reaction was controlled at a micro-positive pressure of 0.5 MPa and the reaction time was 2 h.

[0087] Lithium dihydrogen phosphate was obtained by adding the fourth phosphoric acid solution to the seventh pre-solution and then evaporating and crystallizing it. Specifically, the evaporation temperature was 120℃ until obvious solid precipitated in the seventh pre-solution; after no obvious solid precipitated, heating was stopped and the temperature was lowered to 60℃ for 3 hours; the solid was centrifuged and the water content was controlled to be less than 5%; the centrifuged solid was dried at 120℃ until the water content was <0.35%, thus obtaining the finished lithium dihydrogen phosphate product.

[0088] Step S40: The third solid is mixed with the first phosphoric acid solution and subjected to three acid leaching processes and impurity removal operations to obtain ferrous oxalate.

[0089] The third solid was mixed with the first phosphoric acid solution and filtered to obtain the fourth pre-solution. The concentration of the first phosphoric acid solution was 25%; the liquid-solid ratio of the first phosphoric acid solution to the third solid was 4 ml: 1 g; the reaction time for adding the first phosphoric acid solution was 90 min; and the reaction temperature for adding the first phosphoric acid solution was 85 °C.

[0090] Iron powder was added to the fourth presol solution for reduction and then filtered to obtain the fifth presol solution.

[0091] The fifth presolution and the first oxalic acid solution were mixed and filtered to obtain the fourth solid. The concentration of the first oxalic acid solution was 3 mol / L; and / or, the reaction time after adding the first oxalic acid solution was 1 h; and / or, the reaction temperature at which the first oxalic acid solution was added to the fifth presolution was 60 °C.

[0092] The fourth solid was mixed with the second oxalic acid solution and aged to obtain ferrous oxalate. The concentration of the second oxalic acid solution was [value missing]; the liquid-solid ratio of the fourth solid to the second oxalic acid solution was 4 ml: 1 g; the aging time was 3 h; and / or the aging temperature was 90 °C. The solid-liquid mixture after aging was centrifuged and dried at 100 °C for 3 h to obtain ferrous oxalate.

[0093] Example 2 The difference between the recycling method of lithium iron phosphate black powder in this embodiment and that in Example 1 is that the reaction time of the first pre-solution and the second alkaline reagent is 0.1 h; and the reaction temperature of the first pre-solution and the second alkaline reagent is 20 °C.

[0094] Example 3 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the reaction time of the first pre-solution and the second alkaline reagent is 4 hours; and the reaction temperature of the first pre-solution and the second alkaline reagent is 90°C.

[0095] Example 4 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the concentration of the second phosphoric acid solution is 1% and the washing operation lasts for 5 minutes.

[0096] Example 5 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the concentration of the second phosphoric acid solution is 15% and the washing operation time is 200 min.

[0097] Example 6 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the concentration of the first phosphoric acid solution is 10%; the reaction time for adding the first phosphoric acid solution is 10 min; and the reaction temperature for adding the first phosphoric acid solution is 40°C.

[0098] Example 7 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the concentration of the first phosphoric acid solution is 45%; the reaction time for adding the first phosphoric acid solution is 200 min; and the reaction temperature for adding the first phosphoric acid solution is 120°C.

[0099] Example 8 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the concentration of the first oxalic acid solution is 0.5 mol / L, and the reaction temperature for adding the first oxalic acid solution to the fifth pre-solution is 20°C.

[0100] Example 9 The difference between the recycling method of lithium iron phosphate black powder in this embodiment and that in Example 1 is that the concentration of the first oxalic acid solution is 5 mol / L, and the reaction temperature for adding the first oxalic acid solution to the fifth pre-solution is 85°C.

[0101] Example 10 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the addition time of the first oxalic acid solution is 0.1 h; and the reaction time after adding the first oxalic acid solution is 0.1 h.

[0102] Example 11 The difference between the method for recycling lithium iron phosphate black powder in this embodiment and that in Example 1 is that the addition time of the first oxalic acid solution is 2 hours; and the reaction time after adding the first oxalic acid solution is 4 hours.

[0103] Example 12 The difference between the recycling method of lithium iron phosphate black powder in this embodiment and that in Example 1 is that the reaction temperature of the aging operation is 40°C and the reaction time of the aging operation is 0.1h.

[0104] Example 13 The difference between the recycling method of lithium iron phosphate black powder in this embodiment and that in Example 1 is that the reaction temperature of the aging operation is 110°C and the reaction time of the aging operation is 6 hours.

[0105] Comparative Example 1 The difference between the recycling method of lithium iron phosphate black powder in this comparative example and that in Example 1 is that hydrogen peroxide and the first alkaline reagent are not added, and the second acidic reagent and the second alkaline reagent are not mixed.

[0106] The purity of the prepared lithium dihydrogen phosphate and ferrous oxalate were tested, and the test results are as follows.

[0107] Table 1-1

[0108] Table 1-2

[0109] Table 1-3

[0110] Comparing Examples 1 to 13 in Tables 1-1, 1-2, and 1-3 with Comparative Example 1, it can be seen that after reducing the impurity removal process, the number of impurity elements in the final product of Comparative Example 1 increased, and the results were all inferior to those of Examples 1-13. The overall recovery rates of Li, Fe, P, and other elements in the black powder of Comparative Example 1 were all lower than those of Examples 1-13.

[0111] In the description of the embodiments of the present invention, it should be noted that the orientation or positional relationship of the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and other indicators are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0112] The above description discloses only one preferred embodiment of the present invention, and should not be construed as limiting the scope of the present invention. Those skilled in the art will understand that all or part of the processes of the above embodiments can be implemented, and equivalent changes made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for recycling lithium iron phosphate black powder, characterized in that, include: The sintered lithium iron phosphate black powder was mixed with the first acidic reagent and subjected to a first acid leaching followed by filtration to obtain the first solution; Hydrogen peroxide and a first alkaline reagent were added to the first solution to perform a first iron precipitation. After filtration, a first solid and a second solution were obtained. The second solution was then subjected to a purification operation to obtain lithium dihydrogen phosphate. The first solid is mixed sequentially with the second acidic reagent and the second alkaline reagent and filtered. The second solid is obtained by a second acid leaching and a second iron precipitation. The second solid is then subjected to a purification operation to obtain the third solid. The third solid was mixed with the first phosphoric acid solution and subjected to three acid leaching processes and impurity removal operations to obtain ferrous oxalate.

2. The method for recycling lithium iron phosphate black powder according to claim 1, characterized in that, The liquid-to-solid ratio of the first acidic reagent to the sintered lithium iron phosphate black powder is (3-6) mL:1g; and / or, the reaction temperature for adding the first acidic reagent is 60℃-95℃; and / or, the reaction time for adding the first acidic reagent is 1h-5h.

3. The method for recycling lithium iron phosphate black powder according to claim 1, characterized in that, The pH of the second solution is 1.8-3; and / or the reaction temperature for adding the hydrogen peroxide and the first alkaline reagent is 30℃-90℃; and / or the leaching time for adding the hydrogen peroxide and the first alkaline reagent is 1h-5h.

4. The method for recycling lithium iron phosphate black powder according to any one of claims 1 to 3, characterized in that, The first solid is mixed sequentially with a second acidic reagent and a second basic reagent, and then filtered to obtain a second solid, comprising: The first solid is mixed with the second acidic reagent and filtered to obtain a first presolution. The first presolution is mixed with the second alkaline reagent and filtered to obtain a second presolution and a second solid. Wherein, the pH of the second presol is 1.8-3; and / or, the reaction time of the first presol and the second alkaline reagent is 0.5h-3h; and / or, the reaction temperature of the first presol and the second alkaline reagent is 30℃-90℃.

5. The method for recycling lithium iron phosphate black powder according to any one of claims 1 to 3, characterized in that, The second solid is subjected to a purification operation to obtain a third solid, comprising: The second solid was dissolved in pure water, and a second phosphoric acid solution was added and washed to obtain a third solid and a third pre-solution. Wherein, the liquid-solid ratio of pure water to the second solid is (3-6) ml:1 g; and / or, the concentration of the second phosphoric acid solution is 3%-8%; and / or, the pH of the third pre-solution is 1.5-2.5; and / or, the duration of the washing operation is 10 min-120 min.

6. The method for recycling lithium iron phosphate black powder according to any one of claims 1 to 3, characterized in that, The third solid is mixed with the first phosphoric acid solution and subjected to three acid leaching processes followed by impurity removal to obtain ferrous oxalate, comprising: The third solid was mixed with the first phosphoric acid solution and subjected to three acid leaching processes followed by filtration to obtain the fourth pre-solution; Iron powder was added to the fourth pre-solution for reduction and then filtered to obtain the fifth pre-solution; The fifth pre-solution and the first oxalic acid solution were mixed and subjected to three iron precipitation processes and then filtered to obtain the fourth solid. The fourth solid is mixed with the second oxalic acid solution and aged to obtain the ferrous oxalate.

7. The method for recycling lithium iron phosphate black powder according to claim 6, characterized in that, The concentration of the first phosphoric acid solution is 20%-35%; and / or, the liquid-solid ratio of the first phosphoric acid solution to the third solid is (3-6) mL:1g; and / or, the reaction time for adding the first phosphoric acid solution is 20min-120min; and / or, the reaction temperature for adding the first phosphoric acid solution is 60℃-90℃.

8. The method for recycling lithium iron phosphate black powder according to claim 6, characterized in that, The concentration of the first oxalic acid solution is 2 mol / L-4 mol / L; and / or, the reaction time after mixing the fifth presol and the first oxalic acid solution is 0.5 h-2 h; And / or, the reaction temperature for mixing the fifth presolution and the first oxalic acid solution is 30℃-70℃.

9. The method for recycling lithium iron phosphate black powder according to claim 6, characterized in that, The concentration of the second oxalic acid solution is 1.5 mol / L-3 mol / L; and / or, the liquid-solid ratio of the fourth solid to the second oxalic acid solution is (3-6) mL:1 g; and / or, the reaction time of the aging operation is 0.5 h-4 h; and / or, the reaction temperature of the aging operation is 60 °C-90 °C.

10. The method for recycling lithium iron phosphate black powder according to claim 1, characterized in that, The second solution is subjected to a purification process to obtain lithium dihydrogen phosphate, comprising: A third alkaline reagent is added to the second solution to carry out a lithium precipitation reaction, and the solution is filtered to obtain a fourth solid. The fourth solid was mixed sequentially with the third phosphoric acid solution and the lithium hydroxide solution and filtered. The sixth pre-solution was obtained by acidification and impurity removal. The sixth presolution is mixed with lithium sulfide solid or hydrogen sulfide gas for deep impurity removal and then filtered to obtain the seventh presolution; The seventh presolution is acidified by adding a fourth phosphoric acid solution and then subjected to evaporation and crystallization to obtain the lithium dihydrogen phosphate.

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