Method for recycling lithium dihydrogen phosphate from waste lithium iron phosphate battery

Abstract

The invention discloses a method for recovering lithium dihydrogen phosphate from waste lithium iron phosphate batteries, which comprises the following steps: S1, under the condition of oxygen isolation, adding water into lithium iron phosphate black powder to prepare slurry, adding a transforming agent into the slurry, and after the reaction is completed, carrying out solid-liquid separation to obtain filtrate A; s2, under the condition of oxygen isolation, adding iron powder into the filtrate A, so that ferric ions in the filtrate A are converted into ferrous ions; then adjusting the pH value of the filtrate A to 4-5, and carrying out solid-liquid separation to obtain filtrate B; s3, crystallizing the filtrate B to obtain lithium dihydrogen phosphate; the transforming agent comprises oxalic acid, the amount of substance of the oxalic acid is n1, and the amount of substance of iron in the lithium iron phosphate black powder is n2, n1gt; n1, n2. The method for recycling lithium dihydrogen phosphate from the waste lithium iron phosphate battery has the effect of effectively removing copper, aluminum and iron impurity elements without heating, strong acid, strong alkali, oxidizing agent and other conditions.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery material recycling. Specifically, it relates to a method for recycling lithium dihydrogen phosphate from waste lithium iron phosphate batteries. Background Art

[0002] As one of the most widely used cathode materials for lithium-ion batteries at present, lithium iron phosphate has been widely used in the field of power batteries for new energy vehicles due to its excellent high-temperature stability, high specific capacity, wide raw material sources and low cost. With the continuous expansion of the new energy vehicle market scale and the rapid growth of the vehicle ownership, the amount of scrapped power batteries shows an increasing trend year by year, and the problem of treating waste lithium iron phosphate batteries is becoming increasingly prominent.

[0003] In the recycling process of waste lithium iron phosphate batteries, the valuable metals in the cathode material are mainly recycled through hydrometallurgy technology. The existing process usually includes the following steps: First, the waste batteries are subjected to crushing and sorting to obtain lithium iron phosphate black powder. Subsequently, chemical reagents such as acids, alkalis, oxidants, and reductants are used to cause hydrolysis, neutralization, oxidation, complexation, or reduction reactions of the valuable metals in the batteries, and then leach them from the battery materials into the solution. Finally, various impurity removal processes are used to separate and purify the leachate. However, during the battery crushing process, components such as current collector aluminum foil, copper foil, plastic, graphite, and carbon black inevitably mix into the waste, resulting in the acid leachate containing Fe 3+ , Cu 2+ , Al 3+ and other metal ions. Since the pH value ranges of the salts formed by these three ions are similar, it is technically difficult to achieve their efficient separation industrially.

[0004] In the prior art, Patent CN114934177A discloses a method for deeply removing aluminum and copper during the recycling of waste lithium iron phosphate. This method thermally acid-dissolves the waste lithium iron phosphate in a nitrogen atmosphere, filters to remove insoluble conductive carbon black and copper foil, and then uses iron powder, ferrous hydroxide or ferrous oxide to adjust the solution pH value to 3.5 - 4.0, so that aluminum phosphate is completely precipitated while ferrous phosphate remains dissolved, thereby obtaining a lithium and iron enriched solution. However, this method has the following technical defects: First, this method requires a large amount of acid solution, which will cause a large amount of ferrous ions to dissolve into the liquid phase during the pH adjustment process, resulting in an imbalance in the proportions of lithium, iron, and phosphorus elements in the solution, bringing difficulties to the subsequent recycling processes of lithium and iron elements.

[0005] Secondly, in actual industrial production, it is difficult to accurately control the pH value of the enriched solution in the range of 3.5 to 4.0 using iron powder or iron salt, and the reaction equilibrium point is difficult to accurately determine. When the pH value is lower than 3.5, aluminum phosphate precipitation is incomplete; and once the pH value exceeds 4.0, a large amount of ferrous phosphate will precipitate, which makes this method difficult to implement stably in actual applications.

[0006] In addition, the aluminum phosphate generated by this method is in the form of suspended flocculent precipitate, which will adsorb some lithium-containing substances during the filtration process, causing additional loss of lithium elements.

[0007] At present, although polyvinyl pyrrolidone (PVP) has been used to separate Fe 3+ , Cu 2+ 、Al 3+ PVP can effectively precipitate aluminum ions without precipitating iron salts, but due to its high price, it is difficult to meet the economic requirements of large-scale industrial production.

[0008] In summary, developing a simple, environmentally friendly and economically feasible recycling process for efficiently recovering valuable metals in waste lithium iron phosphate has become a key technical issue that needs to be urgently solved in the current industry. Summary of the invention

[0009] In view of the deficiencies in the prior art, the object of the present invention is to provide a method for recovering lithium dihydrogen phosphate from discarded lithium iron phosphate batteries.

[0010] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention includes: A method for recovering lithium dihydrogen phosphate from discarded lithium iron phosphate batteries comprises the following steps: S1. In the absence of oxygen, lithium iron phosphate black powder is added with water to prepare a slurry, a conversion agent is added to the slurry, and after the reaction is completed, solid-liquid separation is performed to obtain filtrate A; S2. Add iron powder to filtrate A under oxygen-free conditions to convert trivalent iron ions in filtrate A into divalent iron ions; then adjust the pH of filtrate A to 4-5, and perform solid-liquid separation to obtain filtrate B; S3, crystallizing the filtrate B to obtain lithium dihydrogen phosphate; The conversion agent includes oxalic acid, and the amount of the oxalic acid is n 1 The amount of iron in the lithium iron phosphate black powder is n 2 , n 1 >n 2 .

[0011] In step S1 of the present invention, ①Lithium iron phosphate (LiFePO 4 ) reacts with oxalic acid as follows: , The generated FeC 2 O 4 belongs to a poorly soluble electrolyte and will form ferrous oxalate precipitate. The generated LiH 2 PO 4 will form Li + , H + , PO 4 3- ; ② Plastics, graphite, carbon black, etc. mixed in the lithium iron phosphate black powder can neither dissolve nor react with oxalic acid and exist in the form of precipitate; ③ The aluminum element mixed in the lithium iron phosphate black powder will be converted into Al 3+ under acidic conditions, and the solubility product Ksp of aluminum phosphate (AlPO 4 ) is very low. Al -21 will combine with PO 3+ to form AlPO 4 3- precipitate; 4 precipitate; ④ After solid-liquid separation, the main ions in filtrate A are Li + , PO 4 3- , H + and a small amount of Fe 3+ .

[0012] ⑤ After solid-liquid separation, the main components of the filter residue include aluminum phosphate, ferrous oxalate, copper powder, graphite, carbon black and plastics. The ferrous oxalate in the filter residue only needs to redissolve the filter residue in an acidic solvent and then introduce oxygen or ozone to convert ferrous oxalate into soluble ferric oxalate. After filtration, the filtrate is crystallized to separate and recover iron elements, which is conducive to reducing the difficulty of recovering iron elements.

[0013] In the present invention, the conversion agent includes oxalic acid and does not use oxalates. Common oxalates include sodium oxalate, potassium oxalate, and ammonium oxalate. Sodium oxalate, potassium oxalate, and ammonium oxalate will introduce soluble ions into filtrate A, such as sodium ions, potassium ions, and ammonium ions. The salts formed by soluble ions and phosphate are not easily separated from lithium dihydrogen phosphate, which affects the subsequent purification of lithium dihydrogen phosphate. In addition, sodium oxalate, potassium oxalate, and ammonium oxalate cannot provide a large amount of hydrogen ions after dissolving in water, and additional acid still needs to be added to provide sufficient hydrogen ions; moreover, introducing additional acid will also bring the problem of new impurity acid radicals that need to be removed.

[0014] In step S2 of the present invention, ① Iron powder can react with a small amount of ferric iron in the solution, and the reaction equation is as follows: , , Ferric iron is reduced to ferrous iron, which then combines with oxalate ions and precipitates in the form of ferrous oxalate, thus facilitating the further removal of impurities in filtrate A and improving the purity of the obtained potassium dihydrogen phosphate; ② When the pH of filtrate B is less than 4, the hydrogen ion concentration in the solution is too high, resulting in the formation of LiH 2 PO 4 containing more H 3 PO 4 , When the pH of filtrate B is greater than 5, the hydrogen ion concentration in the solution is too low, resulting in the formation of LiH 2 PO 4 containing more Li 2 HPO 4 , Li 3 PO 4 . When the pH of filtrate B is 4 - 5, H 2 PO 4 3- is the main existing form, which is conducive to the crystallization of LiH 2 PO 4 .

[0015] Preferably, in step S1, when making the lithium iron phosphate black powder into a slurry by adding water, the amount of water added is such that the pH of the solution after the slurry reacts with the conversion agent is less than 3.

[0016] In the present invention, LiFePO 4 usually needs to be completely dissolved under the condition of pH less than 4. The amount of water added is such that the pH of the solution after the slurry reacts with the conversion agent is less than 3, ① There is enough oxalic acid to react with lithium iron phosphate, which is conducive to improving the conversion rate of metallic lithium, ② LiFePO4 can be completely dissolved, which is conducive to improving the conversion rate of metallic lithium, ③ There is enough H + in the solution, which is conducive to obtaining the crystallization of LiH 2 PO 4 .

[0017] Preferably, in step S1, after the reaction is completed, water is added to make the pH of the obtained filtrate A 2.7 - 2.9.

[0018] In the present invention, when the H + concentration in the solution is relatively high, during the subsequent addition of iron powder, in the displacement reaction between the acid and iron, and the redox reaction between ferric iron ions and iron, the displacement reaction and the redox reaction will compete, resulting in an increased difficulty in eliminating ferric iron ions. Adding water in step S1 makes the pH of the obtained filtrate A 2.7 - 2.9, and the rate of the displacement reaction is greatly reduced.

[0019] In the present invention, in the step S1, when the black powder of lithium iron phosphate is made into a slurry by adding water, the mass ratio of the black powder of lithium iron phosphate to water is 1:(1.5 - 2.5).

[0020] Preferably, n 1 =(1.5 - 2)n 2 .

[0021] Preferably, the conversion agent further includes phosphoric acid, and the amount of substance of the phosphoric acid is n 3 , and the amount of substance of aluminum in the black powder of lithium iron phosphate is n 4 , n 3 >n 4 .

[0022] Preferably, n 3 =(1.0 - 1.2)n 4 .

[0023] Preferably, 1.4(n 2 +n 4 ) ≤ (n 1 +n 3 ) ≤ 2(n 2 +n 4 ).

[0024] Preferably, in the step S1, the conversion agent and the slurry react at 15°C - 30°C, and the reaction duration is 0.5 - 3 h.

[0025] Preferably, in the step S2, the reaction duration of the iron powder and the filtrate A is 0.5 - 1 h.

[0026] Preferably, in the step S2, the pH of the filtrate A is adjusted by the iron powder or water.

[0027] In the present invention, when the pH of the filtrate A is adjusted by the iron powder in the step S2, the iron powder can undergo a displacement reaction with the acid, thereby consuming excessive H + , increasing the pH of the filtrate A, and at the same time, the divalent iron obtained combines with the oxalate ion and precipitates in the form of ferrous oxalate.

[0028] Compared with the prior art, the advantages of the present invention include: (1) A method for recycling lithium dihydrogen phosphate from waste lithium iron phosphate batteries provided by the present invention can perform acid leaching treatment without heating, which is beneficial to reducing energy consumption; (2) A method for recycling lithium dihydrogen phosphate from waste lithium iron phosphate batteries provided by the present invention can separate metallic lithium without strong acids, strong bases, or oxidants, thus eliminating the steps of removing sulfate ions, nitrate ions, chloride ions, etc. The impurity removal method is simple, the operation difficulty is small, and the operation cost is low; (3)The method for recycling lithium dihydrogen phosphate from waste lithium iron phosphate batteries provided by the present invention can effectively remove impurity elements such as copper, aluminum, and iron, with a high yield of metallic lithium and high purity of the obtained lithium dihydrogen phosphate. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings described below are only some embodiments recorded in the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0030] Figure 1 It is a schematic flow chart of a method for recycling lithium dihydrogen phosphate from waste lithium iron phosphate batteries in Embodiment 1 of the present invention; Figure 2 It is an XRD characterization pattern of the lithium dihydrogen phosphate obtained in Embodiment 1 of the present invention; Figure 3 It is a SEM scanning electron micrograph of the lithium dihydrogen phosphate obtained in Embodiment 1 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] To enable those skilled in the art to understand the features and effects of the present application, the following will generally explain and define the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art for the present application. When there are conflicts, the definition in this specification shall prevail.

[0032] The theories or mechanisms described and disclosed herein, whether correct or incorrect, shall not limit the scope of the present application in any way, that is, the content of the present application can be implemented without being limited by any specific theory or mechanism.

[0033] In this article, "the present application" is the same as "the present invention" and "the present disclosure".

[0034] In this article, the components and technical features of the present application are described using "a", "one", "a kind of" or similar expressions. Such descriptions are only for convenience of expression and give a general meaning to the scope of the present application. Therefore, such descriptions should be understood to include one or at least one, and the singular also includes the plural, unless clearly referring to other meanings.

[0035] In this article, "or any combination thereof" is the same as "or any one combination thereof", and "any one", "any kind", "any one" are the same as "any one", "any kind", "any one".

[0036] In this document, terms such as "comprising", "including", "having", "containing", or any other similar terms are open-ended transitional phrases that are intended to cover non-exclusive inclusions. For example, a composition or article thereof containing multiple elements is not limited to only the elements listed herein, but may also include other elements that are not explicitly listed but are ordinarily inherent in the composition or article. In addition, unless otherwise clearly stated to the contrary, the term "or" refers to an inclusive "or", rather than an exclusive "or". For example, any of the following situations satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist), A is false (or does not exist) and B is true (or exists), A and B are both true (or exist). Further, in this document, the terms "comprising", "including", "having", "containing" should be construed as having specifically disclosed and simultaneously covering closed transitional terms such as "consisting of", "composed of", "the balance being", etc., as well as transitional terms such as "substantially consisting of", "mainly composed of", "mainly consisting of", "essentially containing", "essentially composed of", "essentially consisting of", "inherently containing", etc.

[0037] In this document, all features or conditions defined in the form of numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are only for the sake of brevity and convenience. Accordingly, the description of a numerical range or percentage range should be regarded as having covered and specifically disclosed all possible sub-ranges and individual numerical values within the range (including integers and fractions), especially integer values. For example, a range description such as "1.0 to 8.0" or "between 1.0 and 8.0" or "between 1.0 and 8.0" should be regarded as having specifically disclosed all sub-ranges such as 1.0 to 8.0, 1.0 to 7.0, 2.0 to 8.0, 2.0 to 6.0, 3.0 to 6.0, 4.0 to 8.0, 3.0 to 8.0, etc., and should be regarded as covering the endpoint values, especially sub-ranges defined by integer values, and should be regarded as having specifically disclosed individual numerical values such as 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, etc. within the range. Unless otherwise specified, the foregoing method of interpretation applies to all contents of this application, regardless of the breadth of the range.

[0038] If a quantity, concentration, or other numerical value or parameter is expressed as a range, a preferred range (or a better range), or a series of upper and lower limits, it should be understood that all ranges formed by any pair of the upper limit or preferred value (or better value) of the range and the lower limit or preferred value (or better value) of the range have been specifically disclosed herein, regardless of whether these ranges are separately disclosed. In addition, when a numerical range is mentioned in this document, unless otherwise stated, the range should include its endpoints and all integers and fractions within the range.

[0039] In this text, on the premise of achieving the object of the invention, a numerical value should be understood to have the precision of the significant digits of that numerical value. For example, the number 40.0 should be understood to cover the range from 39.50 to 40.49.

[0040] It should be understood that the features disclosed in each embodiment of this text can be combined arbitrarily to form the technical solutions of this application, as long as there is no contradiction in the combination of these features.

[0041] The following will describe this application with specific implementation manners and examples. It should be understood that these specific implementation manners and examples are merely illustrative and are not intended to limit the scope and use of this application.

[0042] Unless otherwise specified, the methods, reagents, and conditions used in the preparation examples, comparative examples, and examples below are conventional methods, reagents, and conditions in this field.

[0043] In the following descriptions: Commercially available lithium iron phosphate recycled powder: by mass percentage, containing 17.02% phosphorus, 31.33% iron, 3.62% lithium, and 2.22% aluminum.

[0044] Oxalic acid: the content of the active ingredient ≥ 99.6%.

[0045] Polyphosphoric acid (H 6 P 4 O 13 ): the content of the active ingredient ≥ 99%.

[0046] Preparation Example Preparation Example 1 The raw materials used in this preparation example include: commercially available lithium iron phosphate recycled powder.

[0047] The preparation method of this preparation example includes: grinding the commercially available lithium iron phosphate recycled powder to 400 mesh, and then drying it at 80 °C for 5 h to obtain black lithium iron phosphate powder.

[0048] Preparation Example 2 The raw materials used in this preparation example include: polyphosphoric acid.

[0049] The preparation method of this preparation example includes: dissolving polyphosphoric acid in an equal mass of water at room temperature, standing for 48 h, and then adding water to prepare a phosphoric acid aqueous solution with a mass concentration of 50%.

[0050] Example Example 1 The raw materials used in this example include: 1 kg of black lithium iron phosphate powder obtained in Preparation Example 1 and 755.3 g of oxalic acid.

[0051] Referring to Appendix Figure 1 , the preparation method of this example includes: S1. Under the atmosphere of nitrogen as the protective gas at 15°C, stir the black lithium iron phosphate powder evenly with 1.5 kg of water to make a slurry. Then add oxalic acid to the slurry and stir for 3 h. Next, filter to obtain an intermediate filtrate. After measuring the pH of the obtained intermediate filtrate, add water to adjust the pH of the filtrate to 2.7 to obtain filtrate A. Measure the Fe content of filtrate A by a spectrophotometer. 3+ Content; S2. Under the atmosphere of nitrogen as the protective gas at 15°C, add iron powder to filtrate A. The mass of the iron powder is calculated according to the Fe content measured in step S1 and the volume of filtrate A. The result is 1.725 g of iron powder. After reacting for 1 h, add water to adjust the pH of filtrate A to 4, and then filter to obtain filtrate B. 3+ Content, filtrate A volume for calculation, the result is 1.725 g of iron powder. After reacting for 1 h, add water to adjust the pH of filtrate A to 4, and then filter to obtain filtrate B; S3. Concentrate and evaporate filtrate B at -0.15 MPa and 80°C to 0.1 times of its volume, then crystallize at room temperature, and then after centrifugal separation and filtration, wash with high-purity water to obtain lithium dihydrogen phosphate.

[0052] Example 2 The difference between this example and Example 1 is that in the raw materials used in this example, the amount of oxalic acid used is 1007 g, and the iron powder in step S2 is 2.025 g.

[0053] Example 3 The difference between this example and Example 1 is that the raw materials used in this example also include 161.2 ml of the phosphoric acid aqueous solution prepared in Preparation Example 2. In step S1, when adding oxalic acid to the slurry, add the phosphoric acid aqueous solution at the same time, and the iron powder in step S2 is 1.950 g.

[0054] Example 4 The difference between this example and Example 1 is that the raw materials used in this example also include 193.4 ml of the phosphoric acid aqueous solution prepared in Preparation Example 2. In step S1, when adding oxalic acid to the slurry, add the phosphoric acid aqueous solution at the same time, and the iron powder in step S2 is 1.800 g.

[0055] Example 5 The raw materials used in this example include: 1 kg of black lithium iron phosphate powder obtained in Preparation Example 1 and 906.3 g of oxalic acid.

[0056] The preparation method of this example includes: S1. Under the atmosphere of nitrogen as the protective gas at 30°C, stir the black lithium iron phosphate powder evenly with 2 kg of water to make a slurry. Then add oxalic acid to the slurry and stir for 0.5 h. Next, filter. After measuring the pH of the obtained filtrate, add water to adjust the pH of the filtrate to 2.8 to obtain filtrate A. Measure the Fe content of filtrate A by a spectrophotometer. 3+ Content; S2. Under the atmosphere of 30 °C with nitrogen as the protective gas, gradually add iron powder to filtrate A within 0.5 h to adjust the pH of filtrate A to 4.5, and then filter to obtain filtrate B; S3. Concentrate and evaporate filtrate B at -0.15 MPa and 80 °C to 0.1 times of its volume, then crystallize at room temperature, and after centrifugal separation and filtration, wash with high-purity water to obtain lithium dihydrogen phosphate.

[0057] Example 6 The raw materials used in this example include: 1 kg of black lithium iron phosphate powder obtained in Preparation Example 1, 1007 g of oxalic acid, and 193.4 ml of phosphoric acid aqueous solution prepared in Preparation Example 2.

[0058] The preparation method of this example includes: S1. Under the atmosphere of 30 °C with nitrogen as the protective gas, stir the black lithium iron phosphate powder evenly with 1.5 kg of water to make a slurry, then add oxalic acid and phosphoric acid aqueous solution to the slurry and stir for 0.5 h, then filter to obtain an intermediate filtrate. After measuring the pH of the obtained intermediate filtrate, add water to adjust the pH of the filtrate to 2.9 to obtain filtrate A; S2. Under the atmosphere of 30 °C with nitrogen as the protective gas, gradually add iron powder to filtrate A within 0.5 h to adjust the pH of filtrate A to 5, and then filter to obtain filtrate B; S3. Concentrate and evaporate filtrate B at -0.15 MPa and 80 °C to 0.1 times of its volume, then crystallize at room temperature, and after centrifugal separation and filtration, wash with high-purity water to obtain lithium dihydrogen phosphate.

[0059] Result test Characterize the lithium dihydrogen phosphate obtained in Example 1 by XRD and SEM.

[0060] It should be understood that the above examples are only to illustrate the technical concept and characteristics of the present invention, and the purpose is to enable those familiar with this technology to understand the content of the present invention and implement it accordingly. It cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention belongs, without departing from the concept of the present invention, several simple deductions or substitutions can be made. Any equivalent changes or modifications made according to the spirit of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for recovering lithium dihydrogen phosphate from discarded lithium iron phosphate batteries, characterized in that: The steps include: S1. In the absence of oxygen, lithium iron phosphate black powder is added with water to prepare a slurry, a conversion agent is added to the slurry, and after the reaction is completed, solid-liquid separation is performed to obtain filtrate A; S2. Add iron powder to filtrate A under oxygen-free conditions to convert trivalent iron ions in filtrate A into divalent iron ions; then adjust the pH of filtrate A to 4-5, and perform solid-liquid separation to obtain filtrate B; S3, crystallizing the filtrate B to obtain lithium dihydrogen phosphate; The conversion agent includes oxalic acid, the amount of the oxalic acid is n1, the amount of iron in the lithium iron phosphate black powder is n2, and n1>n2.

2. The method for recovering lithium dihydrogen phosphate from discarded lithium iron phosphate batteries according to claim 1, characterized in that: In the step S1, when lithium iron phosphate black powder is added with water to prepare slurry, the amount of water added is such that the pH of the solution after the slurry reacts with the conversion agent is less than 3.

3. The method for recovering lithium dihydrogen phosphate from discarded lithium iron phosphate batteries according to claim 1, characterized in that: In step S1, water is added after the reaction is completed so that the pH of the obtained filtrate A is 2.7-2.9; And / or, in step S1, when lithium iron phosphate black powder is added with water to form slurry, the mass ratio of lithium iron phosphate black powder to water is 1:(1.5-2.5).

4. The method for recovering lithium dihydrogen phosphate from discarded lithium iron phosphate batteries according to claim 1, characterized in that: n1=(1.5~2)n2.

5. The method for recovering lithium dihydrogen phosphate from discarded lithium iron phosphate batteries according to claim 1, characterized in that: The conversion agent also includes phosphoric acid, the amount of the phosphoric acid is n3, the amount of aluminum in the lithium iron phosphate black powder is n4, and n3>n4.

6. The method for recovering lithium dihydrogen phosphate from discarded lithium iron phosphate batteries according to claim 5, characterized in that: n3=(1.0~1.2)n4.

7. The method for recovering lithium dihydrogen phosphate from discarded lithium iron phosphate batteries according to claim 6, characterized in that: 1.4(n2+n4)≤(n1+n3)≤2(n2+n4).

8. The method for recovering lithium dihydrogen phosphate from discarded lithium iron phosphate batteries according to claim 1, characterized in that: In the step S1, the conversion agent reacts with the slurry at 15° C. to 30° C., and the reaction time is 0.5 to 3 hours.

9. The method for recovering lithium dihydrogen phosphate from discarded lithium iron phosphate batteries according to claim 1, characterized in that: In the step S2, the iron powder reacts with the filtrate A at 15° C. to 30° C., and the reaction time is 0.5 to 1 h.

10. The method for recovering lithium dihydrogen phosphate from discarded lithium iron phosphate batteries according to claim 1, characterized in that: In the step S2, the pH of the filtrate A is adjusted by iron powder or water.

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