Recycling method of lithium iron phosphate lithium extraction slag
By adjusting the concentration and pH of the sulfuric acid solution to generate aluminum phosphate precipitate, and then treating it with oxalic acid dihydrate and surfactants, the problem of low recovery rate of iron and phosphorus elements in lithium iron phosphate extraction residue was solved, realizing an efficient and low-cost recovery method, and producing small-particle-size ferrous oxalate dihydrate.
Patent Information
- Application Number
- CN202511573351.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-10-30
- Publication Date
- 2026-01-09
AI Technical Summary
In existing methods for recovering lithium iron phosphate residue, the recovery rates of iron and phosphorus are low, and the recovered products have large particle sizes.
The first heating and stirring reaction was carried out by adjusting the concentration and amount of sulfuric acid solution. The pH value of the second filtrate was adjusted and aluminum salt was added to generate aluminum phosphate precipitate. The third filtrate was then aged by adding oxalic acid dihydrate and surfactant to prepare high-purity ferrous oxalate dihydrate.
It improves the recovery rate of iron and phosphorus, reduces the particle size of ferrous oxalate dihydrate, simplifies the process, and reduces costs.
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Abstract
Description
Technical Field
[0001] This application relates to the field of lithium battery material recycling technology, and in particular to a method for recycling lithium iron phosphate extraction residue. Background Technology
[0002] Lithium iron phosphate is a safe and environmentally friendly cathode material for lithium-ion power batteries. It has high specific capacity, high cycle stability, and good cycle performance, and can be widely used in energy storage equipment, new energy vehicles and other fields.
[0003] Currently, the main methods for recovering lithium residue from lithium iron phosphate are wet recovery and pyrometallurgical recovery. Wet recovery primarily involves leaching phosphorus and iron elements from the residue with sulfuric acid, adding iron powder and sodium sulfide to remove copper ions, passing the acid leaching solution through an adsorbent resin to remove magnesium and calcium ions, and finally oxidizing ferrous iron with hydrogen peroxide to obtain iron phosphate. However, existing recovery methods have low recovery rates for iron and phosphorus, typically below 90%, and the resulting products tend to have large particle sizes. Summary of the Invention
[0004] To address the aforementioned technical problems, this application discloses a method for recovering lithium iron phosphate extraction slag, which improves the recovery rate of iron and phosphorus elements in the lithium iron phosphate extraction slag and reduces the particle size of the recovered products.
[0005] In the first aspect, this application provides a method for recovering lithium iron phosphate extraction residue, comprising the following steps: Concentrated sulfuric acid is added to the slurry containing lithium iron phosphate residue to prepare a sulfuric acid solution with a molar concentration of 1.6 mol / L to 2.2 mol / L. The solution is then subjected to a first heating and stirring reaction. After solid-liquid separation, a first filtrate containing ferric ions is obtained. The volume-to-mass ratio of the sulfuric acid solution to the lithium iron phosphate residue is (4~8):1. Iron powder is added to the first filtrate, and a second heating and stirring reaction is carried out. After solid-liquid separation, a second filtrate containing ferrous ions is obtained. The amount of iron powder is n1, and the amount of ferric ions in the first filtrate is n2, where 0.6 ≤ n1 / n2 ≤ 0.8. After adjusting the pH of the second filtrate to 2-5 using an alkaline substance, aluminum salt is added, and after stirring and reacting, solid-liquid separation is performed to obtain a third filtrate. The amount of aluminum salt is n3, the amount of phosphate ions in the second filtrate is n4, and 1≤n3 / n4≤1.3. Oxalic acid dihydrate and a surfactant are added to the third filtrate and reacted, followed by aging. Ferrous oxalate dihydrate is obtained by filtration, wherein the amount of oxalic acid dihydrate is n5, the amount of ferrous ions in the third filtrate is n6, and 1≤n5 / n6≤1.2.
[0006] In some embodiments of this application, the reaction temperature of the first heating and stirring reaction is 60°C to 100°C, and the reaction time is 2h to 6h.
[0007] In some embodiments of this application, the reaction temperature of the second heating and stirring reaction is 40℃~100℃, and the reaction time is 1h~4h.
[0008] In some embodiments of this application, the aging treatment temperature is 75℃~85℃, and the holding time is 2h~4h.
[0009] In some embodiments of this application, the mass ratio of the surfactant to the dihydrate oxalic acid is (1~2):(98~99).
[0010] In some embodiments of this application, the concentration of the sulfuric acid solution is 1.5 mol / L to 1.7 mol / L.
[0011] In some embodiments of this application, the surfactant is selected from at least one of hexadecyltrimethylammonium bromide, sodium dodecyl sulfonate, and polyvinylpyrrolidone.
[0012] In some embodiments of this application, the alkaline substance is selected from at least one of sodium hydroxide, potassium hydroxide, and sodium carbonate.
[0013] Secondly, this application provides a method for recovering ferrous oxalate dihydrate from lithium iron phosphate lithium extraction slag as described in the first aspect.
[0014] In some embodiments of this application, the Dv50 of the ferrous oxalate dihydrate is 1.2 μm to 1.3 μm.
[0015] Compared with the prior art, this application has at least the following beneficial effects: This application provides a method for recovering lithium iron phosphate residue, preparing ferrous oxalate dihydrate. By controlling the concentration and amount of sulfuric acid solution within the range specified in this application, the mass and heat transfer effect of the first heating and stirring reaction can be accelerated, significantly increasing the leaching rate of iron phosphate and thus improving the iron recovery rate. Furthermore, by controlling the pH value of the second filtrate within the range specified in this application, aluminum phosphate precipitate can be generated after the addition of aluminum salt, thereby more thoroughly removing phosphate ions from the second filtrate. This not only avoids a decrease in the purity of ferrous oxalate dihydrate due to the presence of phosphate ions in subsequent steps but also improves the phosphorus recovery rate. By adding oxalic acid dihydrate and a surfactant to the third filtrate and reacting followed by aging, high-purity ferrous oxalate dihydrate can be generated using oxalic acid dihydrate, and the particle size of ferrous oxalate dihydrate can be reduced using the surfactant. In summary, this application improves the recovery rate of iron and phosphorus elements from lithium iron phosphate residue and reduces the particle size of ferrous oxalate dihydrate. Furthermore, the process flow of this application is simple and can avoid the use of expensive chemicals such as hydrogen peroxide and ion exchange resins, achieving efficient recovery of iron and phosphorus elements from lithium extraction slag at low cost. Detailed Implementation
[0016] The technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.
[0017] In this application, the terms "first," "second," etc., are primarily used to distinguish different devices, elements, or components (the specific types and structures may be the same or different), and are not intended to indicate or imply the relative importance or quantity of the indicated devices, elements, or components. Unless otherwise stated, "a plurality of" means two or more.
[0018] The first aspect is a method for recovering lithium iron phosphate extraction residue, comprising the following steps: Step A: Add concentrated sulfuric acid to the slurry containing lithium iron phosphate residue, and prepare a sulfuric acid solution with a molar concentration of 1.6 mol / L to 2.2 mol / L by mixing the water and concentrated sulfuric acid in the slurry. Perform the first heating and stirring reaction, and obtain the first filtrate containing ferric ions after solid-liquid separation. The volume-to-mass ratio of sulfuric acid solution to lithium iron phosphate residue is (4~8):1. Step B: Add iron powder to the first filtrate, carry out a second heating and stirring reaction, and obtain a second filtrate containing ferrous ions after solid-liquid separation. The amount of iron powder is n1, the amount of ferric ions in the first filtrate is n2, and 0.6≤n1 / n2≤0.8. Step C: Adjust the pH of the second filtrate to 2-5 using an alkaline substance, add aluminum salt, stir and react, then perform solid-liquid separation to obtain the third filtrate. The amount of aluminum salt is n3, and the amount of phosphate ions in the second filtrate is n4, where 1≤n3 / n4≤1.3. Step D: Add oxalic acid dihydrate and surfactant to the third filtrate and react. Then, age the mixture and filter it to obtain ferrous oxalate dihydrate. The amount of oxalic acid dihydrate is n5, and the amount of ferrous ions in the third filtrate is n6. 1≤n5 / n6≤1.2.
[0019] In step A, the lithium iron phosphate residue is the solid waste residue from crushed waste lithium iron phosphate batteries after lithium extraction. It mainly contains elements such as iron, phosphorus, copper, aluminum, titanium, and sulfur. The slurry containing the lithium iron phosphate residue is a mixture of the residue and water. This application allows for the initial addition of the residue to water to obtain a slurry, followed by mixing the slurry with concentrated sulfuric acid. This process prepares a sulfuric acid solution by combining the water in the slurry with the concentrated sulfuric acid, resulting in a more uniform dispersion of the residue. The concentrated sulfuric acid used in this application can refer to 98% sulfuric acid, i.e., a molar concentration of 18.4 mol / L. The first filtrate obtained after the initial heating and stirring reaction and solid-liquid separation contains ferric ions, and the main component of the filter residue is graphite. The inventors discovered that when the concentration of the sulfuric acid solution is too low, or the volume-to-mass ratio of the sulfuric acid solution to the lithium iron phosphate residue is too low, the mass and heat transfer effect during the first heating and stirring reaction is slow, leading to a decrease in the leaching rate of iron phosphate and affecting the iron recovery rate. Conversely, when the concentration of the sulfuric acid solution is too high, or the volume-to-mass ratio of the sulfuric acid solution to the lithium iron phosphate residue is too high, the leaching rate of metallic impurities such as copper and aluminum in the residue increases, thereby increasing the difficulty of subsequent impurity removal in the acid leaching solution and increasing costs. This application, by controlling the concentration of the sulfuric acid solution and the volume-to-mass ratio of the sulfuric acid solution to the lithium iron phosphate residue within the above-mentioned range, can accelerate the mass and heat transfer effect during the first heating and stirring reaction, significantly improve the leaching rate of iron phosphate, thereby increasing the iron recovery rate, and avoid the increased difficulty of subsequent impurity removal caused by excessively concentrated sulfuric acid solution. The water used in this application can be deionized water. In this application, the volume-to-mass ratio of the sulfuric acid solution to the lithium iron phosphate residue refers to the ratio of the volume of the sulfuric acid solution to the mass of the lithium iron phosphate residue, abbreviated as liquid-solid ratio.
[0020] In step B, the main purpose of adding iron powder is to reduce the ferric ions in the first filtrate to ferrous ions. By controlling the amount of iron powder (n1, i.e., the amount of iron powder added) and the amount of ferric ions (n2) in the first filtrate within the above-mentioned range, the ferric ions can be effectively reduced to ferrous ions while avoiding excessive iron powder addition that would increase costs. After the second heating and stirring reaction, the ferric ions are reduced to ferrous ions, and the ferrous copper ions are reduced to elemental copper. The second filtrate obtained after solid-liquid separation contains ferrous ions, and the main component of the filter residue is sponge copper byproduct. This application can quantitatively analyze the first filtrate using inductively coupled plasma atomic emission spectrometry (ICP) to determine the amount of ferric ions in the first filtrate.
[0021] In step C, adjusting the pH of the second filtrate to 2-5 is to make it acidic, causing aluminum phosphate precipitate to form after the addition of aluminum salt. This more thoroughly removes phosphate ions from the second filtrate, preventing a decrease in the purity of ferrous oxalate dihydrate due to the presence of phosphate ions in subsequent steps and improving phosphorus recovery. Controlling the amount of aluminum salt (n3, i.e., the amount added) and the amount of phosphate ions in the second filtrate (n4) within the above range facilitates the thorough removal of phosphate ions from the second filtrate. After stirring and solid-liquid separation, the resulting third filtrate contains ferrous ions. At this point, impurities have been removed, and the main component of the filter residue is aluminum phosphate byproduct. This application can quantitatively analyze the second filtrate using inductively coupled plasma atomic emission spectrometry (ICP-AES) to determine the amount of phosphate ions in the second filtrate.
[0022] In step D, by controlling the amount of oxalic acid dihydrate (n5, i.e., the amount of oxalic acid dihydrate added) and the amount of ferrous ions (n6) in the second filtrate within the aforementioned range, the ferrous ions in the second filtrate can be fully converted into ferrous oxalate dihydrate. Through aging treatment, the purity and crystallinity of the product can be improved, as well as its chemical stability and batch consistency. Furthermore, oxalic acid dihydrate and the surfactant exert a synergistic effect, not only utilizing oxalic acid dihydrate to generate high-purity ferrous oxalate dihydrate, but also utilizing the surfactant to reduce the particle size of ferrous oxalate dihydrate.
[0023] This application provides a method for recovering lithium iron phosphate residue. Ferrous oxalate dihydrate is prepared. By controlling the concentration and amount of sulfuric acid solution within the range specified in this application, the mass and heat transfer effect of the first heating and stirring reaction can be accelerated, significantly increasing the leaching rate of iron phosphate and thus improving the iron recovery rate. Furthermore, by controlling the pH value of the second filtrate within the range specified in this application, aluminum phosphate precipitate can be formed after the addition of aluminum salt, thereby more thoroughly removing phosphate ions from the second filtrate. This not only avoids a decrease in the purity of ferrous oxalate dihydrate due to the presence of phosphate ions in subsequent steps but also improves the phosphorus recovery rate. By adding oxalic acid dihydrate and a surfactant to the third filtrate and reacting followed by aging, high-purity ferrous oxalate dihydrate can be generated using oxalic acid dihydrate, and the particle size of ferrous oxalate dihydrate can be reduced using the surfactant. In summary, this application not only improves the recovery rate of iron and phosphorus elements from lithium iron phosphate residue. Furthermore, the process flow of this application is simple and can avoid the use of expensive chemicals such as hydrogen peroxide and ion exchange resins, achieving efficient recovery of iron and phosphorus elements from lithium extraction slag at low cost.
[0024] In one embodiment of this application, the reaction temperature of the first heating and stirring reaction is 60℃~100℃, preferably 77℃~83℃, and the reaction time is 2h~6h. By controlling the reaction temperature and reaction time of the first heating and stirring reaction within the above range, the iron phosphate in the lithium iron phosphate extraction residue can be completely dissolved to form trivalent iron ions, which is beneficial to improving the iron recovery rate.
[0025] In one embodiment of this application, the reaction temperature of the second heating and stirring reaction is 40℃~100℃, preferably 57℃~63℃, and the reaction time is 1h~4h. By controlling the reaction temperature and reaction time of the second heating and stirring reaction within the above range, the ferric ions in the first filtrate can be more thoroughly reduced to ferrous ions and the ferrous copper ions can be reduced to elemental copper, which is beneficial to improving the iron recovery rate.
[0026] In one embodiment of this application, the aging treatment temperature is 75℃~85℃, and the holding time is 2h~4h. By controlling the aging treatment temperature and time within the above range, the purity and crystallinity of the product can be improved, as well as the chemical stability and batch consistency of the product.
[0027] In one embodiment of this application, the mass ratio of surfactant to oxalic acid dihydrate is (1~2):(98~99). By controlling the mass ratio of surfactant to oxalic acid dihydrate within the above range, due to the strong binding force between oxalic acid dihydrate and ferrous ions, ferrous oxalate dihydrate crystals are first formed. Then, the surfactant is adsorbed onto the surface of the formed ferrous oxalate dihydrate crystals, giving the ferrous oxalate dihydrate crystals a charge. The electrostatic repulsion counteracts the van der Waals forces between the ferrous oxalate dihydrate crystals, thereby keeping the ferrous oxalate dihydrate crystals dispersed to reduce agglomeration, thus generating ferrous oxalate dihydrate with a smaller particle size.
[0028] In one embodiment of this application, the surfactant is selected from at least one of hexadecyltrimethylammonium bromide (CTAB), sodium dodecyl sulfonate, and polyvinylpyrrolidone. The surfactant can adsorb onto the surface of the generated ferrous oxalate dihydrate crystals, reducing the agglomeration of ferrous oxalate dihydrate crystals and facilitating the formation of smaller particle sizes of ferrous oxalate dihydrate.
[0029] In one embodiment of this application, the alkaline substance is selected from at least one of sodium hydroxide, potassium hydroxide, and sodium carbonate, and is capable of adjusting the pH value of the second filtrate within the range of this application.
[0030] In one embodiment of this application, the aluminum salt is selected from at least one of aluminum sulfate, aluminum chloride, and aluminum nitrate.
[0031] Secondly, this application provides a method for recovering lithium iron phosphate residue as described in any of the above embodiments to obtain ferrous oxalate dihydrate.
[0032] In one embodiment of this application, ferrous oxalate dihydrate has a Dv50 of 1.2 μm to 1.3 μm, which is a relatively small particle size.
[0033] In this application, Dv50 represents the particle size that, in the volumetric particle size distribution, reaches 50% of the total volumetric size, starting from the smallest particle size.
[0034] Example The following examples, embodiments, and comparative examples illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below.
[0035] Example 1 <Acid leaching> 1 kg of lithium iron phosphate residue was first added to 5.48 L of deionized water and mixed to form a slurry containing the residue. Then, 522 mL of 98% concentrated sulfuric acid (molar concentration 18.4 mol / L) was added to prepare a sulfuric acid solution with a molar concentration of 1.6 mol / L. The mixture was heated to 80 °C for the first heating and stirring reaction. After stirring for 4 hours, the solid and liquid were separated using a filter press to obtain the first filtrate and graphite residue. The volume-to-mass ratio of sulfuric acid solution to lithium iron phosphate residue was 6:1. The residue was found to contain 20% iron, 11% phosphorus, and 0.2% copper and aluminum. Therefore, the amount of ferric ions (n2) in the first filtrate was 3.57 mol. <Removing copper> 120g of iron powder (molar amount n1 = 2.14mol) was added to the first filtrate, and a second heating and stirring reaction was carried out, i.e., stirring at 60℃ for 2 hours. Then, solid-liquid separation was performed using a filter press to obtain a second filtrate containing ferrous ions and a sponge copper byproduct; where n1 / n2 = 0.6; the molar amount n4 of phosphate ions in the second filtrate was 3.53mol. <Aluminum Removal> The pH of the second filtrate was adjusted to 4.0 using sodium hydroxide, and then 1208g of aluminum sulfate (the amount of substance n3 is 3.53mol) was added. After the reaction was complete, the solid and liquid were separated by a filter press to obtain a third filtrate containing ferrous sulfate and aluminum phosphate byproduct. The amount of substance n6 of ferrous ions in the third filtrate was determined to be 5.42mol. <Preparation of Ferrous Oxalate Dihydrate> 758g of oxalic acid dihydrate (molar amount n5 = 6mol) and 10.3g of CTAB were added to the third filtrate. After the reaction was complete, the mixture was aged at 80℃ for 3 hours, then washed with deionized water and dried to obtain the ferrous oxalate dihydrate product. The ratio of n5 to n6 was 1.11.
[0036] Example 2 Except for adjusting the pH of the second filtrate to 2.0 with sodium hydroxide in the aluminum removal step, the rest is the same as in Example 1.
[0037] Example 3 Except for adjusting the pH of the second filtrate to 3.0 with sodium hydroxide in the aluminum removal process, the process is the same as in Example 1.
[0038] Example 4 Except for adjusting the pH of the second filtrate to 5.0 with sodium hydroxide in the aluminum removal process, the process is the same as in Example 1.
[0039] Example 5 Except for adjusting the amount of CTAB added to 5.65g in the <Preparation of Ferrous Oxalate Dihydrate>, the rest is the same as in Example 1.
[0040] Example 6 Except for adjusting the amount of CTAB added to 16.95g in the <Preparation of Ferrous Oxalate Dihydrate>, the rest is the same as in Example 1.
[0041] Example 7 Except for the acid leaching process, in which the amount of concentrated sulfuric acid added is changed to prepare a sulfuric acid solution with a molar concentration of 1.8 mol / L by preparing the concentrated sulfuric acid and deionized water in the slurry, the process is the same as in Example 1.
[0042] Example 8 Except for the acid leaching process, in which the amount of concentrated sulfuric acid added is changed to prepare a sulfuric acid solution with a molar concentration of 2.2 mol / L by combining concentrated sulfuric acid and deionized water in the slurry, the process is the same as in Example 1.
[0043] Example 9 Except for adjusting the temperature of the first heating and stirring reaction to 70°C in the acid leaching process, the temperature of the second heating and stirring reaction to 70°C in the copper removal process, and the aging treatment temperature to 75°C in the preparation of ferrous oxalate dihydrate, the rest of the process is the same as in Example 1.
[0044] Example 10 Except for adjusting the temperature of the first heating and stirring reaction to 100°C in the acid leaching process, adjusting the temperature of the second heating and stirring reaction to 100°C in the copper removal process, and adjusting the aging treatment temperature to 85°C in the preparation of ferrous oxalate dihydrate, the rest are the same as in Example 1.
[0045] Comparative Example 1 Except for the acid leaching process, in which the amount of concentrated sulfuric acid added is changed to prepare a sulfuric acid solution with a molar concentration of 1.2 mol / L by preparing the concentrated sulfuric acid and deionized water in the slurry, the process is the same as in Example 1.
[0046] Comparative Example 2 Except for the absence of surfactant in the preparation of ferrous oxalate dihydrate, the rest is the same as in Example 1.
[0047] Test methods and equipment: Determination of iron and phosphorus leaching rates: The iron content in the lithium extraction residue and the iron content in the first filtrate were determined using inductively coupled plasma chromatography (ICP). The iron leaching rate was calculated as: (Iron content in the first filtrate / Iron content in the lithium extraction residue) × 100%. The phosphorus content in the lithium extraction residue and the phosphorus content in the second filtrate were determined using inductively coupled plasma chromatography. The phosphorus leaching rate was calculated as: (Iron content in the second filtrate / Phosphorus content in the lithium extraction residue) × 100%.
[0048] Determination of Na, Al, Mn, and Cu elemental contents in lithium extraction slag and ferrous oxalate dihydrate finished products: The contents of Na, Al, Mn and Cu in lithium extraction residue and ferrous oxalate dihydrate were determined using inductively coupled plasma chromatography.
[0049] Determination of the amount of phosphate ions in the second filtrate: The phosphorus content in the second filtrate was determined using inductively coupled plasma chromatography (ICP-C). The phosphorus content was then equated to the phosphate ion content.
[0050] Determination of the amount of ferrous ions in the third filtrate: The iron content in the third filtrate was determined using inductively coupled plasma chromatography. Since there was an excess of iron powder, the iron content was equated to the content of ferrous iron.
[0051] Particle size test of ferrous oxalate dihydrate finished product: The particle size of the finished ferrous oxalate dihydrate product was tested using a laser particle size analyzer.
[0052] Table 1: Performance data of each comparative example in each embodiment
[0053] Table 2: Elemental content analysis results of lithium extraction slag, products from Example 1 and Example 4
[0054] Referring to Table 1, it can be seen from Examples 1 to 10 and Comparative Examples 1 to 1 that the recovery method of lithium iron phosphate slag in this application significantly improves the leaching rate of iron and phosphorus elements compared to Comparative Example 1, thereby improving the recovery rate of iron and phosphorus elements in lithium iron phosphate slag; compared to Comparative Example 2, the recovery method of this application significantly reduces the particle size of ferrous oxalate dihydrate.
[0055] Based on Table 2, the data from lithium extraction slag, Example 1, and Example 4 show that the recovery method of lithium iron phosphate slag in this application significantly reduces the content of Na, Al, Mn, and Cu elements in ferrous oxalate dihydrate, resulting in higher purity of the prepared ferrous oxalate dihydrate.
[0056] The above provides a detailed description of a method for recovering lithium iron phosphate residue disclosed in this application. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the technical solutions and core inventive points of the embodiments of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.
Claims
1. A method for recovering lithium iron phosphate extraction residue, characterized in that, Includes the following steps: Concentrated sulfuric acid is added to the slurry containing lithium iron phosphate residue to prepare a sulfuric acid solution with a molar concentration of 1.6 mol / L to 2.2 mol / L. The solution is then subjected to a first heating and stirring reaction. After solid-liquid separation, a first filtrate containing ferric ions is obtained. The volume-to-mass ratio of the sulfuric acid solution to the lithium iron phosphate residue is (4~8):
1. Iron powder is added to the first filtrate, and a second heating and stirring reaction is carried out. After solid-liquid separation, a second filtrate containing ferrous ions is obtained. The amount of iron powder is n1, and the amount of ferric ions in the first filtrate is n2, where 0.6 ≤ n1 / n2 ≤ 0.
8. After adjusting the pH of the second filtrate to 2-5 using an alkaline substance, aluminum salt is added, and after stirring and reacting, solid-liquid separation is performed to obtain a third filtrate. The amount of aluminum salt is n3, the amount of phosphate ions in the second filtrate is n4, and 1≤n3 / n4≤1.
3. Oxalic acid dihydrate and a surfactant are added to the third filtrate and reacted, followed by aging. Ferrous oxalate dihydrate is obtained by filtration, wherein the amount of oxalic acid dihydrate is n5, the amount of ferrous ions in the third filtrate is n6, and 1≤n5 / n6≤1.
2.
2. The method for recovering lithium iron phosphate residue according to claim 1, characterized in that, The reaction temperature for the first heating and stirring reaction is 60℃~100℃, and the reaction time is 2h~6h.
3. The method for recovering lithium iron phosphate residue according to claim 1, characterized in that, The reaction temperature for the second heating and stirring reaction is 40℃~100℃, and the reaction time is 1h~4h.
4. The method for recovering lithium iron phosphate residue according to claim 1, characterized in that, The aging process is carried out at a temperature of 75℃~85℃ for 2h~4h.
5. The method for recovering lithium iron phosphate residue according to claim 1, characterized in that, The mass ratio of the surfactant to the dihydrate oxalic acid is (1~2):(98~99).
6. The method for recovering lithium iron phosphate residue according to claim 1, characterized in that, The concentration of the sulfuric acid solution is 1.5 mol / L to 1.7 mol / L.
7. The method for recovering lithium iron phosphate residue according to claim 1, characterized in that, The surfactant is selected from at least one of hexadecyltrimethylammonium bromide, sodium dodecyl sulfonate, and polyvinylpyrrolidone.
8. The method for recovering lithium iron phosphate residue according to claim 1, characterized in that, The alkaline substance is selected from at least one of sodium hydroxide, potassium hydroxide, and sodium carbonate.
9. Ferrous oxalate dihydrate obtained by the method for recovering lithium iron phosphate residue according to any one of claims 1 to 8.
10. Ferrous oxalate dihydrate according to claim 9, characterized in that, The Dv50 of the ferrous oxalate dihydrate is 1.2 μm to 1.3 μm.