Recycling method of waste lithium iron phosphate and lithium ion secondary battery

Abstract

The invention relates to the technical field of battery recovery, and particularly discloses a recovery method of waste lithium iron phosphate and a lithium ion secondary battery. The recovery method of the waste lithium iron phosphate comprises the following steps: leaching acid liquor, and carrying out solid-liquid separation to obtain acid leaching liquor and a regenerated negative electrode material; cooling and crystallizing, and carrying out solid-liquid separation to obtain first iron-removed slag and first iron-removed liquid; oxidizing, adjusting pH, removing iron, and carrying out solid-liquid separation to obtain second iron-removed slag and second iron-removed liquid; adjusting the pH to precipitate lithium phosphate, and carrying out solid-liquid separation to obtain first lithium phosphate slag and lithium precipitation mother liquor; adding a lithium source into the lithium precipitation mother solution, and carrying out solid-liquid separation after reaction to obtain second lithium phosphate slag and a phosphorus removal solution; and evaporating and concentrating the dephosphorized solution to obtain a by-product salt. By implementing the invention, the recovery of each useful component can be realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery recycling, in particular to a recycling method of waste lithium iron phosphate and a lithium ion secondary battery. BACKGROUND

[0002] Lithium iron phosphate batteries are widely used due to their low price advantage. The service life of conventional lithium iron phosphate batteries is 5-8 years, so at present, lithium iron phosphate batteries have faced large-scale retirement. Retired lithium iron phosphate batteries contain a large amount of valuable metal elements such as lithium, iron and phosphorus. If discarded at will, not only resources will be wasted, but also serious environmental pollution may be caused. Therefore, it is urgent to develop an efficient and environmentally friendly recycling process.

[0003] The existing lithium iron phosphate recycling technology mainly includes two types. One is a high-temperature solid-phase method, which realizes lithium extraction through high-temperature roasting, but has problems such as high energy consumption and serious pollution. The other is a hydrometallurgical method, which realizes the separation and recovery of metal elements through acid and alkali leaching, and has advantages such as mild reaction conditions and high recovery rate. The commonly used wet process generally uses sulfuric acid and hydrogen peroxide to co- leach to realize selective leaching of lithium, iron and phosphorus, and then remove impurity ions by adjusting pH, and then recover valuable metals through steps such as precipitation and crystallization. Although this process can effectively separate target elements, it needs to use a large amount of hydrogen peroxide in the leaching process, and a large amount of difficult-to-treat iron-containing waste slag (dihydrated iron phosphate) is easily produced in the process of adjusting pH and precipitation, which often contains impurities such as Al, Cu and Mg, and is difficult to reach the battery grade standard, so it is often piled up and treated by recycling manufacturers, causing great environmental pressure. In the process of precipitating iron, phosphorus-containing reagents often need to be added, resulting in a complex process and high process cost. SUMMARY

[0004] The technical problem to be solved by the present application is to provide a recycling method of waste lithium iron phosphate, which can realize efficient separation and recovery of lithium, iron and phosphorus, avoid adding additional phosphorus source, reduce reagent consumption and reduce process cost.

[0005] In order to solve the above technical problems, the present application provides a recycling method of waste lithium iron phosphate, which comprises the following steps: (1) The lithium iron phosphate black powder is leached with an acid solution, and solid-liquid separation is performed to obtain an acid leaching solution and a regenerated negative electrode material; (2) The acid leaching solution is cooled to-30℃-10℃ and maintained for a first preset time, and solid-liquid separation is performed to obtain a first iron removal slag and a first iron removal liquid; (3) An oxidizing agent and a first pH adjusting agent are added to the first iron removal liquid, the pH is adjusted to 2-5, then reacted for a second preset time, and solid-liquid separation is performed to obtain a second iron removal slag and a second iron removal liquid; (4) a second pH regulator is used to adjust the pH of the second iron-removing solution to 8-12, and after reacting for a third preset time, solid-liquid separation is performed to obtain first lithium phosphate residue and lithium precipitation mother liquor; (5) a lithium source is added to the lithium precipitation mother liquor, and after reaction, solid-liquid separation is performed to obtain second lithium phosphate residue and phosphorus-removing solution; (6) the phosphorus-removing solution is evaporated and concentrated to obtain a byproduct salt.

[0006] As an improvement of the above technical solution, step (3) comprises: (3.1) a third pH regulator is used to adjust the pH of the first iron-removing solution to 2-5, and after reacting for a fourth preset time, solid-liquid separation is performed to obtain third iron-removing residue and third iron-removing solution; (3.2) an oxidizing agent is added to the third iron-removing solution, and after reacting for a second preset time, solid-liquid separation is performed to obtain second iron-removing residue and second iron-removing solution.

[0007] As an improvement of the above technical solution, in step (2), seed crystals with a particle size of 0.1 mm-1 mm are added to the acid leaching solution; and / or In step (3), a filtration aid is added to the first iron-removing solution. The filtration aid is selected from one or more of graphite, polyethylene glycol, iron phosphate seed crystals, lignin sulfonate and sodium dodecyl sulfonate.

[0008] As an improvement of the above technical solution, the filtration aid is selected from graphite and / or PEG6000. The particle size of the graphite is ≥0.2 mm, and the amount of the graphite is 5%-90% of the weight of the second iron-removing residue. The amount of the PEG6000 is 0.2%-5% of the weight of the first iron-removing solution.

[0009] As an improvement of the above technical solution, in step (1), the acid solution is selected from one or more of sulfuric acid, nitric acid and hydrochloric acid; the molar concentration of H + in the acid solution is 2 mol / L-14 mol / L; the ratio of the weight of the lithium iron phosphate black powder to the volume of the acid solution is 0.2 g-0.75 g:0.8 mL-1.2 mL; the leaching temperature is 20℃-80℃, and the leaching time is 3 h-15 h; and / or In step (2), the first preset time is 2 h-6 h.

[0010] As an improvement of the above technical solution, in step (2), the acid leaching solution is cooled at a cooling rate of 0.1℃ / min-3℃ / min to -10℃-0℃. The seed crystals are ferrous sulfate, and the particle size of the ferrous sulfate is 0.5 mm-1 mm.

[0011] As an improvement to the above technical solution, in step (3), the oxidant is selected from one or more of hydrogen peroxide, air, oxygen, a mixture of SO2 and O2, and a mixture of SO2 and air; the amount of oxidant used is equal to the amount of Fe in the first iron removal liquid. 2+ 1 to 3 times the molar amount; The first pH adjuster is selected from one or more of the following: sodium hydroxide aqueous solution, potassium hydroxide aqueous solution, ammonia water, and liquid ammonia; The second preset time is 1h to 3h, and the reaction temperature is 30℃ to 70℃.

[0012] As an improvement to the above technical solution, in step (4), the second pH adjuster is selected from one or more of sodium hydroxide aqueous solution, potassium hydroxide aqueous solution, ammonia water, and liquid ammonia. The third preset time is 0.5h~5h, and the reaction temperature is 50℃~90℃; and / or In step (5), the lithium source is selected from one or more of lithium sulfate, lithium hydroxide, lithium chloride, lithium perchlorate, lithium nitrate, and lithium bromide.

[0013] Accordingly, the present invention also discloses a method for recycling waste lithium iron phosphate, which includes the following steps: (1) Lithium iron phosphate black powder is leached with acid and the solid-liquid separation is performed to obtain acid leaching solution and regenerated negative electrode material; (2) Cool the acid leaching solution to -30℃~10℃ and maintain it for a first preset time to separate the solid and liquid to obtain the first iron removal slag and the first iron removal liquid; (3) The pH of the first iron removal liquid is adjusted to 2-5 using a third pH adjuster, and after a fourth preset reaction time, solid-liquid separation is performed to obtain the third iron removal slag and the third iron removal liquid; (4) An oxidant is added to the third iron removal liquid, and the reaction is carried out for a second preset time. Solid-liquid separation is performed to obtain the second iron removal slag and the second iron removal liquid. (5) The pH of the second iron removal liquid is adjusted to 8-12 using a second pH adjuster. After a third preset reaction time, solid-liquid separation is performed to obtain the first lithium phosphate residue and lithium precipitation mother liquor. (6) Add a lithium source to the lithium precipitation mother liquor, and after the reaction, separate the solid and liquid to obtain the second lithium phosphate residue and the phosphorus removal solution; (7) The phosphorus removal solution is evaporated and concentrated to obtain byproduct salt.

[0014] Accordingly, the present invention also discloses a lithium-ion secondary battery, which comprises lithium iron phosphate prepared from the first lithium phosphate slag, the second lithium phosphate slag, and the second iron removal slag obtained by the above-mentioned waste lithium iron phosphate recycling method as raw materials.

[0015] Implementing this invention has the following beneficial effects: The recycling method for waste lithium iron phosphate in one embodiment of the present invention includes acid leaching, cooling crystallization for primary iron removal, oxidation precipitation for secondary iron removal, lithium precipitation, and concentration of the lithium precipitation mother liquor. Based on this recycling method, firstly, regenerated negative electrode material, a first iron removal slag with ferrous salt as the core component, a second iron removal slag with iron phosphate as the core component, and a first and second lithium phosphate slag with lithium phosphate as the main component are obtained, realizing the recovery of useful components from lithium iron phosphate black powder. The core products obtained in this embodiment are lithium phosphate and iron phosphate, which can be used to prepare lithium iron phosphate positive electrode material, achieving high-value-added recycling. Secondly, the recovery rates of lithium, iron, and phosphorus in this embodiment are all high; specifically, the lithium recovery rate can reach over 92%, and the iron and phosphorus recovery rates are both above 85%. Thirdly, the recycling method in this embodiment removes iron through a process of cooling crystallization for primary iron removal and oxidation precipitation for secondary iron removal, which significantly reduces the content of various impurities in the subsequently obtained second iron removal slag (hydrated iron phosphate salt), improving the quality of the iron phosphate product. Fourthly, in the recycling method of this embodiment, iron removal is carried out by cooling crystallization, which can remove more than 50% of Fe. This eliminates the need for an external phosphorus source in the subsequent secondary iron removal process, simplifying the process and reducing costs. Attached Figure Description

[0016] Figure 1 This is a flowchart of a method for recycling waste lithium iron phosphate in one embodiment of the present invention; Figure 2 This is a flowchart of a method for recycling waste lithium iron phosphate in another embodiment of the present invention; Figure 3 These are the XRD patterns of the lithium iron phosphate cathode black powder used in various embodiments of the present invention; Figure 4 This is the XRD pattern of the regenerated negative electrode material obtained in Example 1 of this invention; Figure 5 This is the XRD pattern of the first iron-removing slag obtained in Example 1 of the present invention; Figure 6 This is the XRD pattern of the first lithium phosphate slag obtained in Example 1 of the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in further detail below. It should be noted that, unless otherwise specified, all reagents used in the examples are commercially available.

[0018] See Figure 1 This invention discloses a method for recycling waste lithium iron phosphate, which includes the following steps: S11: Lithium iron phosphate black powder is leached with acid, and solid-liquid separation is performed to obtain acid leaching solution and regenerated negative electrode material; S12: Cool the acid leaching solution to -30℃~10℃ and maintain it for a first preset time to separate the solid and liquid to obtain the first iron removal slag and the first iron removal liquid; S13: Add oxidant and pH adjuster to the first iron removal liquid, adjust the pH to 2~5, then react for a second preset time, and separate the solid and liquid to obtain the second iron removal slag and the second iron removal liquid. S14: The pH of the second iron removal liquid is adjusted to 8-12 using a second pH adjuster. After a third preset reaction time, solid-liquid separation is performed to obtain the first lithium phosphate residue and lithium precipitation mother liquor. S15: Add lithium source to lithium precipitation mother liquor, and after reaction, solid-liquid separation is performed to obtain lithium phosphate residue and phosphorus removal solution; S16: Evaporate and concentrate the phosphorus removal solution to obtain the byproduct salt.

[0019] In the above-described recycling method, firstly, through a process of acid leaching, cooling crystallization for primary iron removal, oxidation precipitation for secondary iron removal, lithium precipitation, and concentration of the lithium precipitation mother liquor, a first iron removal slag with regenerated negative electrode material and ferrous salt as core components, a second iron removal slag with phosphate as core components, and a first and second lithium phosphate slag with lithium phosphate as the main components are obtained. This achieves the recovery of useful components from lithium iron phosphate black powder. The core products obtained in this embodiment are lithium phosphate and iron phosphate, which can be used to prepare lithium iron phosphate positive electrode materials, achieving high-value-added recycling. Secondly, the recovery rates of lithium, iron, and phosphorus in this embodiment are all high. Specifically, the lithium recovery rate can reach over 92%, and the iron and phosphorus recovery rates are both over 85%. Thirdly, the recycling method in this embodiment removes iron through a process of cooling crystallization for primary iron removal and oxidation precipitation for secondary iron removal. This significantly reduces the content of various impurities in the subsequently obtained second iron removal slag (hydrated iron phosphate salt), improving the quality of the iron phosphate product. Fourthly, in the recycling method of this embodiment, iron removal is carried out by cooling crystallization, which can remove more than 50% of Fe. This eliminates the need for an external phosphorus source in the subsequent secondary iron removal process, simplifying the process and reducing costs.

[0020] Specifically, in step S11, lithium iron phosphate black powder refers to waste material obtained after crushing, sorting (gravity separation, magnetic separation, etc.), and pretreatment. Its main components are lithium iron phosphate and graphite, and it may also contain small amounts of aluminum, copper, etc., depending on the specific processing technology. For example, some manufacturers separate and crush the cathode substrate separately, in which case the content of graphite and copper in the lithium iron phosphate black powder is relatively low. Some manufacturers directly mix and crush the powder, in which case the graphite content in the lithium iron phosphate black powder is high, and it may also contain a small amount of copper. In addition, it should be noted that in some implementation methods, the copper and aluminum content in the lithium iron phosphate black powder can be reduced to below 1 wt% through a gravity separation process.

[0021] Specifically, in step S11, the acid solution is one or more of sulfuric acid, nitric acid, and hydrochloric acid, but is not limited to these. Sulfuric acid is preferred, as the leaching solution obtained after sulfuric acid leaching achieves higher iron removal efficiency using a cooling crystallization process. It should be noted that the acid solution refers to a mixture of the aforementioned acids and water.

[0022] Specifically, H in the acid solution + The molar concentration is 2 mol / L to 14 mol / L, exemplarily 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, 4.5 mol / L, 5 mol / L, 5.5 mol / L, 6 mol / L, 6.5 mol / L, 7 mol / L, 7.5 mol / L, 8 mol / L, 8.5 mol / L, 9 mol / L, 9.5 mol / L, 10 mol / L, 10.5 mol / L, 11 mol / L, 11.5 mol / L, 12 mol / L, 12.5 mol / L, 13 mol / L, or 13.5 mol / L, but not limited to these. Preferably, it is 2 mol / L to 10 mol / L, more preferably 2 mol / L to 5 mol / L. Lower concentrations of acid can ensure high leaching rates of Li, Fe, and P while reducing the leaching rate of Al, minimizing impurity introduction, and facilitating subsequent treatment.

[0023] Specifically, in step S11, the weight ratio of lithium iron phosphate black powder to the volume of acid solution is 0.2g~0.75g:0.8mL~1.2mL; exemplary ratios are 0.2g:1mL, 0.3g:0.85mL, 0.4g:1.05mL, 0.5g:1.1mL, 0.6g:1.1mL, or 0.7g:1.2mL, but not limited thereto. Preferably, it is 0.2g~0.5g:1mL, more preferably 0.2g~0.4g:1mL.

[0024] Specifically, in step S11, the leaching temperature is 20℃~80℃, exemplarily 25℃, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃ or 75℃, but not limited thereto. Preferably it is 50℃~80℃, more preferably 50℃~60℃.

[0025] Specifically, in step S11, the leaching time is 3h to 20h, exemplarily 3.5h, 5h, 5.5h, 7h, 8.5h, 10h, 11.5h, 13h, 14.5h, 16h, 17.5h or 19h, but not limited to these. Preferably, it is 3h to 15h.

[0026] Preferably, in some embodiments, a two-stage leaching process can be used in step S11. Specifically, in the first stage of leaching, acid or the leachate obtained from the second stage of leaching can be used as the leaching agent; in the second stage of leaching, acid is used as the leaching agent. Based on this two-stage leaching process, the content of various metal impurities in the regenerated negative electrode powder can be effectively reduced, improving its quality; moreover, the leaching rate of Li, Fe, P, etc. in lithium iron phosphate black powder can be increased. It should be noted that when using the two-stage leaching process, the leaching temperature, liquid-solid ratio, and leaching time of the first stage of leaching can be the same as or different from those of the second stage of leaching. Specifically, the leaching temperature for the first stage is 20℃~80℃, the leaching time is 2h~10h, and the liquid-to-solid ratio is 0.8mL~1.2mL:0.2g~0.75g; the leaching temperature for the second stage is 20℃~80℃, the leaching time is 2h~10h, and the liquid-to-solid ratio is 0.8mL~1.2mL:0.2g~0.75g.

[0027] Specifically, in step S11, solid-liquid separation is performed by methods such as atmospheric pressure filtration, pressure filtration, and centrifugal filtration, but is not limited to these methods. Preferably, pressure filtration is used for solid-liquid separation.

[0028] Specifically, in step S11, after acid leaching, solid-liquid separation is performed to obtain the acid leaching solution and the regenerated negative electrode material. Further, the regenerated negative electrode material can be washed with a dilute alkaline solution to partially remove Cu, Al, etc. Alternatively, the regenerated negative electrode material can be subjected to calcination activation or other treatments.

[0029] Preferably, in some embodiments, elemental iron can be added to the acid leaching solution to remove copper ions through a reduction reaction, thereby further improving the purity of the ferric phosphate product.

[0030] Preferably, in some embodiments, the lithium iron phosphate black powder is leached with an alkaline solution before acid leaching to remove aluminum. Specifically, the leaching temperature is 20°C to 40°C, the leaching time is 3 hours to 5 hours, the liquid-to-solid ratio is 1 g to 5 g: 1 L of alkaline solution, the concentration of the alkaline solution is 30 to 50 g / L, and the alkaline solution is an aqueous solution of sodium hydroxide or potassium hydroxide, preferably an aqueous solution of sodium hydroxide.

[0031] Specifically, in step S12, by cooling the acid leaching solution to -30℃ to 10℃ and maintaining it at this low temperature for a first preset time, ferrous salts in the solution can precipitate, achieving an iron removal rate of over 50%. When sulfuric acid leaching is used, the iron removal rate can reach over 60%. After cooling and crystallization, the first iron-removed slag obtained is a hydrated ferrous salt, such as ferrous sulfate heptahydrate, but not limited to this. It should be noted that if the temperature of the acid leaching solution is too low after cooling, it may freeze, making it difficult to achieve the precipitation of the first iron-removed slag and consuming a large amount of energy; if the temperature of the acid leaching solution is too high after cooling, the crystal precipitation efficiency will be low, and the iron removal rate will decrease.

[0032] For example, after cooling, the temperature of the acid leaching solution is -27.5℃, -25℃, -15.5℃, -10℃, -7.5℃, -5℃, -2.5℃, 0℃, 2.5℃, 5℃, or 7.5℃. Preferably, it is -25℃ to 5℃, and more preferably, it is -10℃ to 0℃.

[0033] Specifically, in step S12, the first preset time is 2h to 6h, exemplarily 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, or 5.5h, but not limited to these. Preferably, it is 2h to 5h. By controlling the temperature of the acid leaching solution after cooling and the time maintained at low temperature, the iron removal rate can be effectively improved.

[0034] Specifically, in step S12, solid-liquid separation is performed by methods such as atmospheric pressure filtration, pressure filtration, and centrifugal filtration, but is not limited to these methods. Preferably, centrifugal filtration or pressure filtration is used for solid-liquid separation. Since the crystals obtained after cooling and crystallization have relatively small particle sizes, pressure filtration is often required to improve filtration efficiency.

[0035] Preferably, in some embodiments, in step S12, the cooling rate during the cooling process is controlled to be 0.1℃ / min to 3℃ / min. By controlling the cooling rate, spontaneous crystal nucleation can be suppressed, the particle size of the first iron removal slag can be increased, facilitating the solid-liquid separation of the first iron removal slag and the first iron removal liquid in the later stage, and the loss of Li and P can also be reduced, that is, the recovery rate of Li and P can be improved. Exemplarily, the cooling rate is 0.3℃ / min, 0.6℃ / min, 0.9℃ / min, 1.2℃ / min, 1.5℃ / min, 1.8℃ / min, 2.1℃ / min, 2.4℃ / min or 2.7℃ / min, but is not limited thereto. Preferably, the cooling rate is 0.1℃ / min to 1℃ / min.

[0036] Preferably, in some embodiments, in step S12, seed crystals with a particle size of 0.1 mm to 1 mm are added to the acid leaching solution before cooling. Specifically, the seed crystals can be ferrous sulfate, ferrous chloride, or ferrous nitrate, but are not limited to these. By adding seed crystals, spontaneous nucleation of crystals can be suppressed, allowing supersaturated solute to grow on the seed crystals, thereby increasing the particle size of the first iron removal slag and reducing the amount of first iron removal liquid entrained in it, i.e., reducing the loss of Li, P, etc., during the process. For example, the particle size of the seed crystals is 0.1 mm to 0.5 mm, 0.2 mm to 0.7 mm, 0.3 mm to 0.8 mm, or 0.5 mm to 1 mm, but is not limited to these. Preferably, the particle size of the seed crystals is 0.5 mm to 1 mm.

[0037] Specifically, the amount of seed crystals added is 0.5% to 1.5% of the total mass of ferrous salts in the acid leaching solution, for example, 0.6 wt%, 0.8 wt%, 1.0 wt%, 1.2 wt%, or 1.4 wt%. It should be noted that the total mass of ferrous salts in the acid leaching solution is calculated based on the acid anions in the acid solution. For example, if the acid solution is sulfuric acid, the total mass of ferrous sulfate in the acid leaching solution is calculated, and the seed crystals are added based on this mass.

[0038] More preferably, in some embodiments, in step S12, the acid leaching solution is cooled to -10°C to 0°C at a cooling rate of 0.1°C / min to 3°C / min; and before cooling, ferrous sulfate with a particle size of 0.5 mm to 1 mm is added to the acid leaching solution as seed crystals. Based on this embodiment, not only is the filtration rate higher, but the loss of Li and P during the filtration process can also be significantly reduced.

[0039] Preferably, in some embodiments, step S13 includes: Specifically, in step S13, by adding an oxidant to the first iron removal solution, the remaining Fe can be removed. 2+ Oxidized to Fe 3+ The pH is then adjusted using a first pH adjuster, and the precipitate forms ferric phosphate dihydrate, resulting in the second iron removal slag, which is mainly composed of ferric phosphate dihydrate. Based on the above process, the phosphorus source required for the ferric phosphate dihydrate precipitation in this step and the lithium phosphate precipitation in step S14 is sufficient, eliminating the need for an external phosphorus source and simplifying the preparation process.

[0040] Specifically, in step S13, the oxidant is selected from one or more of hydrogen peroxide, air, oxygen, a mixture of SO2 and O2, and a mixture of SO2 and air, but is not limited thereto. Preferably, the oxidant is oxygen or hydrogen peroxide. Specifically, the concentration of hydrogen peroxide is 25wt%~35wt%, preferably 25wt%~30wt%, and more preferably 27.5wt%~30wt%.

[0041] Specifically, the amount of oxidant used is equal to the amount of Fe in the first iron removal solution. 2+ Complete oxidation requires 1 to 3 times the amount of oxidant needed. That is, the amount of oxidant needed is equal to the amount of Fe in the first iron removal solution. 2+ The molar amount is 1 to 3 times; exemplary values ​​are 1, 1.3, 1.6, 1.9, 2.2, 2.5, or 2.8 times, but not limited thereto. Preferably, it is 1.1 to 1.5 times, to completely oxidize the Fe in the first iron removal solution. 2 + It should be noted that in the technical solution of this invention, some of the iron is separated by cooling and crystallization, which also reduces the amount of oxidant used.

[0042] Specifically, in step S13, the first pH adjuster is selected from one or more of the following: sodium hydroxide aqueous solution, potassium hydroxide aqueous solution, ammonia, and liquid ammonia, but is not limited to these. The concentration of the sodium hydroxide solution and potassium hydroxide solution is 10wt%~60wt%. It should be noted that directly adding solid sodium hydroxide or potassium hydroxide, or using aqueous solutions with excessively high concentrations, can lead to significant Li loss. Preferably, the first pH adjuster is a sodium hydroxide aqueous solution or ammonia; more preferably, a sodium hydroxide aqueous solution with a concentration of 10wt%~50wt% is selected.

[0043] Specifically, in step S13, the pH of the first iron removal solution is adjusted to 2-5 using a first pH adjuster. If the pH is too high, lithium phosphate will precipitate, resulting in a loss of lithium source. Preferably, the pH of the first iron removal solution is adjusted to 3-5 using a first pH adjuster.

[0044] Specifically, in step S13, the reaction time (i.e., the second preset time) is 1h to 3h, exemplarily 1.3h, 1.6h, 1.9h, 2.2h, 2.5h or 2.8h, but not limited thereto. Preferably, it is 1.5h to 3h.

[0045] Specifically, in step S13, the reaction temperature is 20℃~70℃, exemplarily 25℃, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃ or 65℃, but not limited thereto. Preferably, it is 30℃~70℃, more preferably 30℃~50℃.

[0046] Specifically, in step S13, solid-liquid separation is performed by methods such as atmospheric pressure filtration, pressure filtration, and centrifugal filtration, but is not limited to these methods. Preferably, centrifugal filtration or pressure filtration is used for solid-liquid separation. It should be noted that the second iron removal slag obtained in step S13, which mainly consists of ferric phosphate dihydrate, has a small particle size, and pressure filtration is often required to ensure a high filtration rate.

[0047] Preferably, in some embodiments, in step S13, a filter aid is added to the first iron removal liquid to increase the particle size of the second iron removal slag, reduce solid-liquid separation resistance, improve solid-liquid separation efficiency, and also reduce the entrainment of Li and P by the second iron removal slag, thereby increasing the recovery rate. Specifically, the filter aid may be one or more of graphite, polyethylene glycol, iron phosphate seed crystals, lignin sulfonate, and sodium dodecyl sulfonate, but is not limited to these. The amount of filter aid used is 5% to 90% of the weight of the second iron removal slag.

[0048] More preferably, in some embodiments, in step S13, the filter aid is selected from graphite and / or PEG6000, and the particle size of the graphite is ≥0.2mm. Graphite can increase the porosity between particles, thereby reducing filtration resistance and significantly reducing the entrainment of lithium-containing mother liquor, thus significantly reducing lithium loss. PEG6000 can provide physical adsorption and bridging, promoting particle aggregation and increasing particle size, which also helps to reduce filtration resistance and lithium loss. Specifically, the particle size of the graphite is 0.2mm~2.0mm, preferably 0.2mm~1.5mm.

[0049] Specifically, when graphite is used as a filter aid, the amount of graphite used is 5% to 90% of the weight of the second iron-removing slag, exemplarily 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, or 85%, but not limited thereto. Preferably, the amount of graphite used is 10% to 60% of the weight of the second iron-removing slag, more preferably 10% to 30%.

[0050] Specifically, when PEG6000 is used as a filter aid, its dosage is 0.2% to 5% of the weight of the first iron removal liquid, exemplarily 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, or 4.5%, but not limited thereto. Preferably, it is 0.5% to 3%. More preferably, it is 0.5% to 2%. Within this dosage range, PEG6000 can effectively promote particle agglomeration, significantly improve the filtration rate, and at the same time avoid affecting the separation effect due to an increase in solution viscosity caused by excessive addition.

[0051] More preferably, in some embodiments, graphite and PEG6000 are used simultaneously as filter aids in step S13. Their synergistic effect not only reduces the amount of both used but also significantly enhances particle aggregation and filtration efficiency, further reducing the adsorption and entrainment of lithium and phosphorus by the second iron removal slag. Specifically, based on this embodiment, the amount of graphite used is 5% to 10% of the weight of the second iron removal slag, and the amount of PEG6000 used is 0.2% to 1.0% of the weight of the first iron removal liquid.

[0052] S131: The pH of the first iron removal liquid is adjusted to 2-5 using a third pH adjuster, and after a fourth preset reaction time, solid-liquid separation is performed to obtain the third iron removal slag and the third iron removal liquid. S132: An oxidant is added to the third iron removal liquid, and the reaction is carried out for a second preset time. Solid-liquid separation is performed to obtain the second iron removal slag and the second iron removal liquid.

[0053] Based on the above implementation method, a third iron removal slag with hydrated ferrous phosphate as the main component can be obtained by adjusting the pH of the first iron removal liquid. This step S31 can also effectively remove residual Al impurities. Then, after subsequent oxidation, a high-purity second iron removal slag is obtained, which can fully meet the preparation requirements of battery-grade iron phosphate.

[0054] Specifically, in step S131, the third pH adjuster is selected from one or more of the following: sodium hydroxide aqueous solution, potassium hydroxide aqueous solution, ammonia, and liquid ammonia, but is not limited to these. The concentration of the sodium hydroxide solution and potassium hydroxide solution is 10wt%~60wt%. It should be noted that directly adding solid sodium hydroxide or potassium hydroxide, or using aqueous solutions with excessively high concentrations, can lead to significant Li loss. Preferably, the third pH adjuster is a sodium hydroxide aqueous solution or ammonia; more preferably, a sodium hydroxide aqueous solution with a concentration of 10wt%~50wt% is selected.

[0055] Specifically, in step S131, the reaction time (i.e., the fourth preset time) is 0.5 h to 2 h, preferably 0.5 h to 1 h. The reaction temperature is 10 °C to 60 °C, preferably 20 °C to 40 °C. Within this temperature range, the stable formation of hydrated ferrous phosphate is beneficial, while also promoting the deep removal of impurity elements. It should be noted that if the temperature is too high, it will cause Fe... 2+ Oxidized Fe 3+ .

[0056] Specifically, in step S131, solid-liquid separation is carried out by means of atmospheric pressure filtration, pressure filtration, centrifugal filtration, etc., but not limited to these methods.

[0057] Preferably, in some embodiments, the third iron removal slag is recycled to S11 and combined with lithium iron phosphate black powder for leaching. Based on this method, the recovery efficiency of P and Fe can be further improved, and the generation of by-products can be reduced. However, it should be noted that after 2 to 3 cycles, the Al content in the slag will be high, and further recycling should be abandoned at this point.

[0058] Specifically, the process conditions in step S132 are the same as those in step S13 above, except that the third iron removal liquid replaces the first iron removal liquid, and the first pH adjuster is not added. The conditions for generating the second iron removal slag are the same as those in the aforementioned steps.

[0059] Specifically, in step S14, the second pH adjuster is selected from one or more of sodium hydroxide aqueous solution, potassium hydroxide aqueous solution, ammonia, and liquid ammonia, but is not limited thereto. Preferably, the first pH adjuster is selected from sodium hydroxide aqueous solution or ammonia; more preferably, sodium hydroxide aqueous solution is selected, with a concentration of 10wt% to 50wt%.

[0060] Specifically, in step S14, the pH of the second iron removal liquid is adjusted to 8-12 using a second pH adjuster, preferably 9-11. Under these conditions, a first lithium phosphate slag with lithium phosphate as the main component can be obtained.

[0061] Specifically, in step S14, the reaction temperature is 50℃~90℃, exemplarily 55℃, 60℃, 75℃ or 85℃. Within this temperature range, it is beneficial for the full precipitation and growth of lithium phosphate crystals, thereby improving the purity and crystallinity of the product. Preferably, the reaction temperature is 70℃~90℃.

[0062] Specifically, in step S14, the reaction time (i.e., the third preset time) is 0.5h to 5h, exemplarily 1h, 2h, 3h or 4h, but not limited to these. Preferably, the reaction time is 1h to 3h, which ensures that the reaction proceeds fully within this time period, while avoiding the formation of by-products, which is beneficial for obtaining high-purity first lithium phosphate slag.

[0063] Specifically, in step S14, solid-liquid separation is performed by methods such as atmospheric pressure filtration, pressure filtration, and centrifugal filtration, but is not limited to these methods. Preferably, centrifugal filtration or pressure filtration is used for solid-liquid separation.

[0064] Specifically, the first lithium phosphate residue obtained in step S14, after simple washing and drying, can be directly used as a lithium source to prepare lithium iron phosphate, battery-grade lithium dihydrogen phosphate, or battery-grade lithium carbonate. The preliminary process of this technical solution can significantly reduce the content of various impurities and optimize the purity of lithium phosphate products.

[0065] Specifically, in step S15, the lithium source is selected from one or more of lithium sulfate, lithium hydroxide, lithium chloride, lithium perchlorate, lithium nitrate, and lithium bromide, but is not limited thereto. Preferably, lithium sulfate is selected as the lithium source.

[0066] Specifically, in step S15, the reaction temperature is 50℃~90℃, for example 60℃, 70℃, 80℃ or 85℃. Within this temperature range, it is beneficial for the full dissolution of lithium salt and the stability of the reaction system, and promotes the efficient generation of the target product.

[0067] Specifically, in step S15, the reaction time is 0.5h to 3h, exemplarily 1h, 1.5h or 2.5h, preferably 1h to 2h.

[0068] Specifically, in step S15, solid-liquid separation is performed by methods such as atmospheric pressure filtration, pressure filtration, and centrifugal filtration, but is not limited to these methods. Preferably, centrifugal filtration or pressure filtration is used for solid-liquid separation.

[0069] Specifically, in step S16, the phosphorus removal solution is concentrated by evaporation to obtain a byproduct salt including one or more of sodium sulfate, potassium sulfate, ammonium sulfate, sodium chloride, potassium chloride, and sodium nitrate. Preferably, the byproduct salt is a sulfate.

[0070] Accordingly, refer to Figure 2 The present invention also discloses a method for recycling waste lithium iron phosphate, comprising the following steps: S21: Lithium iron phosphate black powder is leached with acid, and solid-liquid separation is performed to obtain acid leaching solution and regenerated negative electrode material; S22: Cool the acid leaching solution to -30℃~10℃ and maintain it for a first preset time to separate the solid and liquid to obtain the first iron removal slag and the first iron removal liquid; S23: The pH of the first iron removal solution is adjusted to 2-5 using a third pH adjuster. After a fourth preset reaction time, solid-liquid separation is performed to obtain the third iron removal slag and the third iron removal solution. The third iron removal slag is recycled to step S21 and leached together with the lithium iron phosphate black powder. S24: Add an oxidant to the third iron removal liquid, react for a second preset time, and then separate the solid and liquid to obtain the second iron removal slag and the second iron removal liquid; S25: The pH of the second iron removal liquid is adjusted to 8-12 using a second pH adjuster. After a third preset reaction time, solid-liquid separation is performed to obtain the first lithium phosphate residue and lithium precipitation mother liquor. S26: Add a lithium source to the lithium precipitation mother liquor, and after the reaction, separate the solid and liquid to obtain lithium phosphate residue and phosphorus removal solution; S27: Evaporate and concentrate the phosphorus removal solution to obtain the byproduct salt.

[0071] In the recycling method of the above embodiments, one process involves acid leaching, cooling crystallization for primary iron removal, pH adjustment for secondary iron removal, oxidation precipitation for tertiary iron removal, lithium precipitation, and concentration of the lithium precipitation mother liquor to obtain a first iron removal slag with regenerated negative electrode material and ferrous salt as the core components, a second iron removal slag with phosphate as the core component, and a first and second lithium phosphate slag with lithium phosphate as the main component. This achieves the recovery of useful components from lithium iron phosphate black powder. The core products obtained in this embodiment are lithium phosphate and iron phosphate, which can be used to prepare lithium iron phosphate positive electrode material, achieving high-value-added recycling. Secondly, the recovery rates of lithium, iron, and phosphorus in this recycling method are all high, and the recovery rate is further improved through the recycling of the third iron removal slag. Specifically, the lithium recovery rate can reach over 95%, and the recovery rates of iron and phosphorus are both above 90%. Thirdly, the recycling method in this embodiment removes iron through a three-stage process: cooling crystallization for primary iron removal, pH adjustment for secondary iron removal, and oxidation precipitation for tertiary iron removal. This significantly reduces the content of various impurities in the subsequent second iron removal slag (hydrated iron phosphate), improving the quality of the iron phosphate product and making it directly usable in the production of battery-grade lithium iron phosphate. Fourthly, the primary iron removal process using cooling crystallization in this embodiment can remove more than 50% of Fe, eliminating the need for an external phosphorus source in the subsequent secondary iron removal process, simplifying the process and reducing costs. It should be noted that traditional processes generally use sulfuric acid and hydrogen peroxide for oxidative leaching, followed by the addition of a phosphorus source and pH adjustment to precipitate iron as iron phosphate dihydrate. This process not only requires an external phosphorus source but also results in iron phosphate dihydrate containing a significant amount of impurities such as aluminum, copper, and calcium, failing to meet battery-grade standards and making it difficult to use directly. Currently, recycling plants mostly handle it by stockpiling. In contrast, the iron phosphate product obtained from the recycling process in this embodiment is directly battery-grade.

[0072] Accordingly, the present invention also discloses a lithium-ion secondary battery, which comprises lithium iron phosphate prepared from the first lithium phosphate slag, the second lithium phosphate slag, and the second iron removal slag obtained by the above-mentioned recycling method as raw materials. Specifically, the first lithium phosphate slag and the second lithium phosphate slag can be washed and dried, and the second iron removal slag can be washed, dried, dehydrated at high temperature, and then mixed with a carbon source (such as glucose) and sintered to obtain lithium iron phosphate, but is not limited thereto.

[0073] The present invention will be further described below with reference to specific embodiments: The chemical composition of the lithium iron phosphate cathode black powder used in the following embodiments is shown in Table 1, and its X-ray diffraction pattern (XRD pattern) is shown in Table 1. Figure 3 As shown.

[0074] Table 1 Chemical Composition of Lithium Iron Phosphate Black Powder

[0075] Example 1 The recycling method for waste lithium iron phosphate in this embodiment includes the following steps: (1) Lithium iron phosphate black powder was leached with acid, and solid-liquid separation was performed to obtain acid leaching solution and regenerated negative electrode material. Specifically, the acid solution was 3.0 mol / L sulfuric acid, the solid-liquid ratio was 0.3 g / mL, the leaching temperature was 60℃, and the leaching time was 8 h. After leaching, the solution was filtered, and the filtrate was the acid leaching solution. The filter residue was first washed with 2 mol / L sodium hydroxide solution at 60℃ with stirring for 2 h at a solid-liquid ratio of 200 g / L, and then washed twice with water at 60℃ with a solid-liquid ratio of 400 g / L to obtain the regenerated negative electrode material. The XRD pattern of the regenerated negative electrode material is shown below. Figure 4 As shown in Table 2, the main components of the acid leaching solution are the main components of the solution.

[0076] Table 2. Concentrations of phosphorus, iron, and lithium in the acid leaching solution

[0077] (2) Cool the acid leaching solution to allow crystallization, and separate the solid and liquid to obtain the first iron removal slag and the first iron removal liquid; Specifically, ferrous sulfate seed crystals with an average particle size of 0.3 mm, representing 1% of the total ferrous sulfate content in the solution, were added to the acid leaching solution. The solution was then cooled to 0°C at a rate of 0.5°C / min and held at that temperature for 3 hours. The solution was filtered to obtain the first iron removal slag and the first iron removal liquid. The main components of the first iron removal liquid are shown in Table 3. The first iron removal slag was dried under vacuum at 90°C (partial separation of bound water to form FeSO4·H2O) to obtain a dried solid. The contents of iron, lithium, phosphorus, and aluminum in the dried sample were tested to be 29.03 wt%, 0.05 wt%, 0.85 wt%, and 0.08 wt%, respectively. Its XRD pattern is shown in the figure below. Figure 5 As shown.

[0078] Table 3 Concentrations of phosphorus, iron, lithium, and aluminum in the first iron removal solution

[0079] (3) Add the third pH adjuster to the first iron removal liquid, and then react for a fourth preset time. The solid and liquid are separated to obtain the third iron removal slag and the third iron removal liquid. Specifically, the first iron removal liquid was diluted with an equal volume of water, and a 50wt% sodium hydroxide aqueous solution was added to raise the pH to 3.5. The reaction was carried out at room temperature for 2 hours, and the third iron removal wet slag and the third iron removal liquid were obtained by filtration. The lithium, iron, phosphorus and aluminum contents in the sample of the third iron removal wet slag after washing and drying were measured to be 0.11wt%, 15.07wt%, 9.18wt% and 0.46wt%, respectively. The specific composition of the second iron removal liquid is shown in Table 4. It can be seen from the table that the aluminum ion concentration is 0.06g / L.

[0080] Table 4. Concentrations of phosphorus, iron, lithium, and aluminum in the third iron removal solution.

[0081] (4) Add an oxidant to the third iron removal liquid, react for a second preset time, and then separate the solid and liquid to obtain the second iron removal slag and the second iron removal liquid; Specifically, 8 wt% of graphite with an average particle size of 0.5 mm from the second iron removal slag and 0.5 wt% of PEG6000 from the third iron removal liquid were added to the third iron removal liquid. The mixture was stirred evenly, and then hydrogen peroxide was added to oxidize all the ferrous iron to ferric iron. The reaction was carried out at 50°C for 2 hours. The mixture was then filtered to obtain the second iron removal slag and the second iron removal liquid. The contents of lithium, iron, phosphorus and aluminum in the sample of the third iron removal slag after washing and drying were measured to be 0.07 wt%, 27.37 wt%, 14.92% wt%, and 0.11 wt%, respectively. The specific composition of the second iron removal liquid is shown in Table 5. It can be seen from the table that the aluminum ion concentration is 0.005 g / L.

[0082] Table 5. Concentrations of phosphorus, iron, lithium, and aluminum in the second iron removal solution.

[0083] (5) The pH of the second iron removal liquid is adjusted by the second pH adjuster, and after a third preset reaction time, solid-liquid separation is performed to obtain the first lithium phosphate residue and lithium precipitation mother liquor; Specifically, sodium hydroxide solution was added to the second iron removal liquid to raise the pH to 10, and the mixture was stirred at 80°C for 1 hour. The mixture was then filtered to obtain the first lithium phosphate residue and the lithium precipitation mother liquor. The composition of the lithium precipitation mother liquor is shown in Table 6. The purity of the first lithium phosphate residue, after washing and drying, was determined to be 99.2%, and its XRD pattern is shown in [Table 6]. Figure 6 .

[0084] Table 6. Concentrations of phosphorus, iron, lithium, and aluminum in lithium precipitation mother liquor

[0085] (5) Add lithium sulfate to the lithium precipitation mother liquor, react at 80°C for 0.5 h, and then separate the solid and liquid to obtain the second lithium phosphate residue and the phosphorus removal solution. The phosphate concentration in the phosphorus removal solution is less than 0.1 g / L.

[0086] (6) The phosphorus removal solution is evaporated and concentrated to obtain sodium sulfate.

[0087] Example 2 This embodiment provides a method for recycling waste lithium iron phosphate, which differs from Embodiment 1 in that: In step (2), ferrous sulfate seed crystals are not added.

[0088] Everything else is the same as in Example 1.

[0089] Example 3 This embodiment provides a method for recycling waste lithium iron phosphate, which differs from Embodiment 1 in that: In step (2), the cooling rate is 3.5℃ / min.

[0090] Everything else is the same as in Example 1.

[0091] Example 4 This embodiment provides a method for recycling waste lithium iron phosphate, which differs from Embodiment 1 in that: In step (2), the cooling rate is 3.5℃ / min, and no ferrous sulfate seed crystals are added.

[0092] Everything else is the same as in Example 1.

[0093] Example 5 This embodiment provides a method for recycling waste lithium iron phosphate, which differs from Embodiment 1 in that: In step (4), only 2% by weight of PEG6000 of the first iron removal liquid is added, and no graphite is added.

[0094] Everything else is the same as in Example 1.

[0095] Example 6 This embodiment provides a method for recycling waste lithium iron phosphate, which differs from Embodiment 1 in that: In step (4), only 20 wt% of the weight of the second iron removal slag is added.

[0096] Everything else is the same as in Example 1.

[0097] Example 7 This embodiment provides a method for recycling waste lithium iron phosphate, which differs from Embodiment 1 in that: In step (4), PEG6000 and graphite are not added.

[0098] Everything else is the same as in Example 1.

[0099] Example 8 This embodiment provides a method for recycling waste lithium iron phosphate, which differs from Embodiment 4 in that: In step (4), PEG6000 and graphite are not added.

[0100] Everything else is the same as in Example 4.

[0101] Example 9 This embodiment discloses a method for recycling waste lithium iron phosphate, which differs from Embodiment 1 in that: Excluding step (3), in step (4), the pH of the first iron removal solution is raised to 3.5 using NaOH aqueous solution, and then the reaction is carried out.

[0102] The recovery rates of each element in each embodiment, and the Fe / P ratio of the second iron removal slag, are calculated using the following methods: The Li recovery rate is calculated as follows: (Lithium in the first lithium phosphate residue + lithium in the second lithium phosphate residue + lithium in the first iron removal residue + lithium in the second iron removal residue) / Lithium in lithium iron phosphate cathode black powder The Fe recovery rate is calculated as follows: (Iron from the first iron removal slag + iron from the second iron removal slag) / Iron from lithium iron phosphate cathode black powder.

[0103] The calculation method for P recovery rate is as follows: (Phosphorus in the second iron removal slag + phosphorus in the first lithium phosphate slag + phosphorus in the second lithium phosphate slag) / phosphorus in lithium iron phosphate cathode black powder.

[0104] It should be noted that the above calculations of Li, Fe, and P recovery rates were all performed under the condition that the third iron removal slag was not recycled.

[0105] The Fe / P ratio is the molar ratio of Fe to P in the second iron removal slag.

[0106] The test data is shown in the table below: Table 7 Test Results

[0107] The above description is a preferred embodiment of the invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the invention, and these improvements and modifications are also considered to be within the scope of protection of the invention.

Claims

1. A method for recycling waste lithium iron phosphate, characterized in that, Includes the following steps: (1) Lithium iron phosphate black powder is leached with acid and the solid-liquid separation is performed to obtain acid leaching solution and regenerated negative electrode material; (2) Cool the acid leaching solution to -30℃~10℃ and maintain it for a first preset time to separate the solid and liquid to obtain the first iron removal slag and the first iron removal liquid; (3) Add an oxidant and a first pH adjuster to the first iron removal liquid, adjust the pH to 2-5, and then react for a second preset time to obtain the second iron removal slag and the second iron removal liquid through solid-liquid separation. (4) The pH of the second iron removal liquid is adjusted to 8-12 using a second pH adjuster. After a third preset reaction time, solid-liquid separation is performed to obtain the first lithium phosphate residue and lithium precipitation mother liquor. (5) Add a lithium source to the lithium precipitation mother liquor, and after the reaction, separate the solid and liquid to obtain the second lithium phosphate residue and the phosphorus removal solution; (6) The phosphorus removal solution is evaporated and concentrated to obtain byproduct salt.

2. The method for recycling waste lithium iron phosphate as described in claim 1, characterized in that, Step (3) includes: (3.1) The pH of the first iron removal liquid is adjusted to 2-5 using a third pH adjuster, and after a fourth preset reaction time, solid-liquid separation is performed to obtain the third iron removal slag and the third iron removal liquid; (3.2) An oxidant is added to the third iron removal liquid, and the reaction is carried out for a second preset time. Solid-liquid separation is performed to obtain the second iron removal slag and the second iron removal liquid.

3. The method for recycling waste lithium iron phosphate as described in claim 1, characterized in that, In step (2), seed crystals with a particle size of 0.1 mm to 1 mm are added to the acid leaching solution; and / or In step (3), a filter aid is added to the first iron removal liquid; The filter aid is selected from one or more of graphite, polyethylene glycol, ferric phosphate seed crystals, lignin sulfonate, and sodium dodecyl sulfonate.

4. The method for recycling waste lithium iron phosphate as described in claim 3, characterized in that, The filter aid is selected from graphite and / or PEG6000; The graphite has a particle size ≥0.2mm and is used in an amount of 5% to 90% of the weight of the second iron removal slag. The amount of PEG6000 used is 0.2% to 5% of the weight of the first iron removal liquid.

5. The method for recycling waste lithium iron phosphate as described in claim 1, characterized in that, In step (1), the acid solution is selected from one or more of sulfuric acid, nitric acid, and hydrochloric acid; the acid solution contains H... + The molar concentration is 2 mol / L to 14 mol / L; the weight ratio of the lithium iron phosphate black powder to the volume of the acid solution is 0.2 g to 0.75 g : 0.8 mL to 1.2 mL; the leaching temperature is 20 °C to 80 °C, and the leaching time is 3 h to 15 h; and / or In step (2), the first preset time is 2h~6h.

6. The method for recycling waste lithium iron phosphate as described in claim 3, characterized in that, In step (2), the acid leaching solution is cooled to -10℃ to 0℃ at a cooling rate of 0.1℃ / min to 3℃ / min; The seed crystals are ferrous sulfate with a particle size of 0.5 mm to 1 mm.

7. The method for recycling waste lithium iron phosphate as described in claim 1, characterized in that, In step (3), the oxidant is selected from one or more of hydrogen peroxide, air, oxygen, a mixture of SO2 and O2, and a mixture of SO2 and air; the amount of oxidant used is equal to the amount of Fe in the first iron removal liquid. 2+ 1 to 3 times the molar amount; The first pH adjuster is selected from one or more of the following: sodium hydroxide aqueous solution, potassium hydroxide aqueous solution, ammonia water, and liquid ammonia; The second preset time is 1h to 3h, and the reaction temperature is 30℃ to 70℃.

8. The method for recycling waste lithium iron phosphate as described in claim 1, characterized in that, In step (4), the second pH adjuster is selected from one or more of the following: sodium hydroxide aqueous solution, potassium hydroxide aqueous solution, ammonia water, and liquid ammonia; The third preset time is 0.5h~5h, and the reaction temperature is 50℃~90℃; and / or In step (5), the lithium source is selected from one or more of lithium sulfate, lithium hydroxide, lithium chloride, lithium perchlorate, lithium nitrate, and lithium bromide.

9. A method for recycling waste lithium iron phosphate, characterized in that, Includes the following steps: (1) Lithium iron phosphate black powder is leached with acid and the solid-liquid separation is performed to obtain acid leaching solution and regenerated negative electrode material; (2) Cool the acid leaching solution to -30℃~10℃ and maintain it for a first preset time to separate the solid and liquid to obtain the first iron removal slag and the first iron removal liquid; (3) The pH of the first iron removal liquid is adjusted to 2-5 using a third pH adjuster, and after a fourth preset reaction time, solid-liquid separation is performed to obtain the third iron removal slag and the third iron removal liquid; (4) An oxidant is added to the third iron removal liquid, and the reaction is carried out for a second preset time. Solid-liquid separation is performed to obtain the second iron removal slag and the second iron removal liquid. (5) The pH of the second iron removal liquid is adjusted to 8-12 using a second pH adjuster. After a third preset reaction time, solid-liquid separation is performed to obtain the first lithium phosphate residue and lithium precipitation mother liquor. (6) Add a lithium source to the lithium precipitation mother liquor, and after the reaction, separate the solid and liquid to obtain the second lithium phosphate residue and the phosphorus removal solution; (7) The phosphorus removal solution is evaporated and concentrated to obtain byproduct salt.

10. A lithium-ion secondary battery, characterized in that, The lithium iron phosphate includes lithium iron phosphate prepared from the first lithium phosphate residue, the second lithium phosphate residue, and the second iron removal residue obtained by the recycling method of waste lithium iron phosphate as described in any one of claims 1 to 9.

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