Method for synthesizing lithium iron phosphate by using positive electrode material production waste liquid

Abstract

The present application relates to a positive material factory lithium iron phosphate waste recycling technical field, specifically to a kind of lithium iron phosphate of utilizing positive material production waste liquid phase synthesis method, comprising the following steps: leaching, reduction Fe 3+ , impurity removal, solution preparation, lithium iron phosphate crystallization, filter pressing and washing, pulping and drying, carbon coating, battery-grade lithium iron phosphate positive material is obtained.The method of the present application can realize full-component recovery of phosphorus, iron and lithium in positive material waste, improve recovery efficiency and economic value;Avoid the synthesis of lithium carbonate and iron phosphate to reduce the waste of a large amount of power and heat source, reduce a large amount of salt-containing wastewater and solid waste, reduce the impact of production process on the environment;And the recovery process is short, which greatly improves production efficiency.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of recycling of lithium iron phosphate waste materials of positive electrode materials, and particularly relates to a method for synthesizing lithium iron phosphate by using positive electrode material production waste materials in a liquid phase. BACKGROUND

[0002] With the popularity of portable products such as mobile phones, notebook computers and digital products, the lithium ion battery industry has developed rapidly, especially the explosion of electric vehicles and energy storage of lithium iron phosphate batteries, and the amount of scrapped batteries has also increased by several times. However, the waste batteries contain heavy metals, organic solvents, electrolyte and the like, and if they are not effectively treated and discarded at will, they will cause serious and lasting pollution to the surrounding environment such as soil and groundwater, and have great potential harm to ecology and human health. Therefore, the recycling and resource utilization of waste batteries have important value.

[0003] At present, many scholars in the industry have invested a lot of research on the recycling of waste positive electrode materials, but the main way is to take the steps of leaching, filter pressing, impurity removal and lithium extraction to obtain battery-grade lithium carbonate, but a large amount of phosphorus-containing iron solid waste is also generated at the same time; and only lithium is extracted in the process flow, which is undoubtedly a great waste of phosphorus and iron resources. Especially, some manufacturers cannot conduct environmental protection treatment on the iron phosphate slag, and the environmental damage caused by the discarded iron phosphate slag is also extremely serious, and a large amount of salt-containing wastewater is also generated in the process of producing lithium carbonate and iron phosphate. In addition, a large amount of electric power and heat source is needed in the drying and crushing process of the battery-grade lithium carbonate and the production process of anhydrous iron phosphate, which causes a large amount of resource waste. SUMMARY

[0004] To solve the above technical problems, the application provides a method for synthesizing lithium iron phosphate by using positive electrode material production waste materials in a liquid phase, which can recover all components of phosphorus, iron and lithium in the positive electrode material waste, improve the recovery efficiency and economic value, avoid the synthesis of lithium carbonate and iron phosphate, reduce the waste of a large amount of electric power and heat source, and reduce the environmental impact of the production process.

[0005] The technical scheme adopted by the application is as follows:

[0006] A method for synthesizing lithium iron phosphate by using positive electrode material production waste materials in a liquid phase, comprising the following steps:

[0007] S1, leaching: a reducing agent is added to the positive electrode material production waste, and sulfuric acid is continuously added for reaction, and the reaction is completed by controlling pH to obtain a leaching solution; the leaching solution is subjected to solid-liquid separation to obtain a filtrate A;

[0008] S2, reducing Fe 3+: adding iron powder into the filtrate A, controlling the pH and temperature of the solution in two times, and obtaining filtrate B through solid-liquid separation, and controlling the Fe3+ in the filtrate B within 30-100PPm;

[0009] S3, impurity removal: adding sodium sulfide into the filtrate B, and obtaining filtrate C through solid-liquid separation, and then passing the filtrate C through a resin column to obtain filtrate D;

[0010] S4, solution preparation: adjusting the molar ratio of phosphorus, iron and lithium and the pH of the filtrate D, and adding a reducing agent to obtain an adjusted solution;

[0011] S5, lithium iron phosphate crystallization: stirring and heating the adjusted solution in a crystallization kettle, and adding lithium carbonate suspension or lithium hydroxide solution dropwise to react, and controlling the pH and reaction time in the process, and then filtering to obtain lithium iron phosphate crude product;

[0012] S6, pressure filtration and washing: performing pressure filtration and washing on the lithium iron phosphate crude product to obtain lithium iron phosphate filter cake;

[0013] S7, pulping and drying: preparing a slurry by preparing and pulping the lithium iron phosphate filter cake, a reducing organic compound and water; and performing spray drying on the slurry to obtain a mixture of lithium iron phosphate powder and reducing organic compound;

[0014] S8, carbon coating: performing calcination on the obtained mixture in a protective atmosphere to obtain lithium iron phosphate.

[0015] Preferably, the amount of the reducing agent added in the S1 step accounts for 0.1-2% of the weight of the waste material.

[0016] Preferably, the reducing agent in the S1 step includes iron powder, ascorbic acid, sodium thiosulfate, sodium pyrosulfite and sodium sulfide.

[0017] Preferably, the pH is controlled to be 1-3 in the S1 step.

[0018] Preferably, the controlling the pH and temperature of the solution in two times in the S2 step includes: controlling the temperature to be 30-70℃ and the pH to be 1.0-1.8 for the first time, and the reaction time is 4h; and controlling the temperature to be 40-70℃ and the pH to be 1.8-2.6 for the second time, and the reaction time is 4h.

[0019] Preferably, the Cu 2+ ≤10PPM, Zn 2+ ≤10PPM in the filtrate C; and the Ca 2+ ≤50PPM, Mg 2+ ≤50PPM in the filtrate D.

[0020] Preferably, the molar ratio of phosphorus, iron, lithium in the filtrate D in the S4 step is 1:(0.99-1.0):(1.1-1.5).

[0021] Preferably, the pH in the S4 step is 4-7.

[0022] Preferably, the temperature of heating in the S5 step is 95-110°C, the pH is controlled to be 5-7, the reaction time is 3-5h, and the crystallization time is 0.5-1.5h.

[0023] Preferably, the final conductivity of the washing water in the S6 step is 50-200μS / M.

[0024] The beneficial effects of the present application are:

[0025] (1) The method of the present application can realize full-component recovery of phosphorus, iron, and lithium in the positive material waste liquid, improving the recovery efficiency and economic value;

[0026] (2) The method of the present application avoids the synthesis of lithium carbonate and iron phosphate, reducing the waste of a large amount of power and heat source, reducing a large amount of salt-containing wastewater and solid waste, and reducing the environmental impact of the production process; and the recovery process is short, greatly improving the production efficiency;

[0027] (3) The present application adds a specific proportion of reducing agent to the positive material production waste, and controls the pH in the range of 1-3 to complete the reaction, which improves the leaching rate of Fe 2+ , PO4 3- , Li + , reduces the entrainment in the filter residue, and is conducive to realizing full-component recovery of phosphorus, iron, and lithium;

[0028] (4) The present application reduces trivalent iron in two different conditions, making the reduction of trivalent iron more complete and improving the recovery rate of the waste liquid;

[0029] (5) The present application can maintain the stability of divalent iron ions in the filtrate D by adjusting the composition ratio of phosphorus, iron, and lithium and pH, and adding a reducing agent, avoiding the generation of subsequent impurities, so that the synthesized lithium iron phosphate has high purity and stability, and also has good high / low temperature resistance and environmental pressure resistance;

[0030] (6) The present application can form a sequenced olivine structure of the synthesized lithium iron phosphate by controlling multiple crystallization conditions in the S5 step, improving its stability, and making it have high quality and electrochemical performance. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 The flow chart for synthesizing lithium iron phosphate from positive material production waste liquid provided by the present application. Detailed Implementation

[0032] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0033] This invention provides a method for synthesizing lithium iron phosphate from waste materials using the positive electrode production process, comprising the following steps:

[0034] S1. Leaching: A reducing agent is added to the waste material produced by the positive electrode material, and sulfuric acid is continuously added to carry out the reaction. The pH is controlled to complete the reaction and obtain the leachate. The leachate is then subjected to solid-liquid separation to obtain filtrate A.

[0035] S2, reduced Fe 3+ Iron powder is added to filtrate A, and the pH and temperature of the solution are controlled in two stages. After solid-liquid separation, filtrate B is obtained, which reduces the Fe content of filtrate B. 3+ Keep it within 30-100 ppm;

[0036] S3. Impurity removal: Sodium sulfide is added to filtrate B, and after solid-liquid separation, filtrate C is obtained. Then, filtrate C is passed through a resin column to obtain filtrate D.

[0037] S4. Solution preparation: Adjust the molar ratio of phosphorus, iron, and lithium and the pH of filtrate D, and add a reducing agent to obtain the adjusted solution.

[0038] S5. Lithium iron phosphate crystallization: Stir the adjusted solution evenly in the crystallization vessel, heat it, and add lithium carbonate suspension or lithium hydroxide solution dropwise to react. Control the pH and reaction time during the process. After crystallization, filter to obtain crude lithium iron phosphate.

[0039] S6. Filtration and washing: The crude lithium iron phosphate is subjected to filtration and washing to obtain lithium iron phosphate filter cake;

[0040] S7. Slurry preparation and drying: The lithium iron phosphate filter cake is mixed with reducing organic compounds and water to prepare a slurry; the slurry is then spray-dried to obtain a mixture of lithium iron phosphate powder and reducing organic compounds.

[0041] S8, Carbon Coating: The resulting mixture is calcined in a protective atmosphere to obtain lithium iron phosphate.

[0042] In some embodiments, the amount of reducing agent added in step S1 is 0.1-2% of the weight of the waste.

[0043] In some embodiments, the reducing agent in the S1 step comprises iron powder, ascorbic acid, sodium thiosulfate, sodium metabisulfite, sodium sulfide.

[0044] In some embodiments, the pH in the S1 step is controlled to be 1-3.

[0045] Applicants found in research that the conditions in the leaching stage of the positive electrode material production waste affect the leaching effect of Fe 2+ , PO4 3- , Li + , and if the leaching rate is low, it cannot achieve good recovery purposes, causing resource waste. The present application can improve the leaching rate of Fe 2+ , PO4 3- , Li + by adding a specific proportion of reducing agent to the positive electrode material production waste while controlling the pH to be in the range of 1-3 to complete the reaction, reducing the entrainment in the filter residue, thereby facilitating the full-component recovery of phosphorus, iron, and lithium.

[0046] In some embodiments, the pH and temperature of the solution in the S2 step are controlled twice, including: the first time, the temperature is controlled to be 30℃-70℃, the pH is controlled to be 1.0-1.8, and the reaction time is 4h; the second time, the temperature is controlled to be 40℃-70℃, the pH is controlled to be 1.8-2.6, and the reaction time is 4h.

[0047] There is a certain amount of Fe 3+ in the positive electrode material production waste in the leaching stage. In order to improve the recovery rate of the waste, the Fe 3+ in the filtrate A is reduced. However, in the reduction process of trivalent iron, it is often not completely reduced due to improper environment, which not only causes resource waste, but also affects the purity of the lithium iron phosphate synthesized subsequently. Applicants found after research that the reduction of trivalent iron is more complete when it is reduced under two different conditions, specifically including: the first time, the temperature is controlled to be 30℃-70℃, the pH is controlled to be 1.0-1.8, and the reaction time is 4h; the second time, the temperature is controlled to be 40℃-70℃, the pH is controlled to be 1.8-2.6, and the reaction time is 4h. This method can make the reduction of trivalent iron more complete. Applicants believe that on the one hand, the acidity and temperature under this concentration effectively prevent the hydrolysis of trivalent iron, which is conducive to the reduction reaction, and on the other hand, it can maintain the stability of divalent iron, so that the Fe 3+ in the filtrate A is reduced to Fe 2+ to the greatest extent, improving the recovery rate of the waste.

[0048] In some embodiments, the Cu 2+ in the filtrate C is ≤10PPM, and the Zn 2+ is ≤10PPM; the Ca 2+≤50PPM, Mg 2+ ≤50PPM.

[0049] In the S3 impurity removal step, the first step is to remove Cu. 2 Zn 2+ In addition to heavy metal ions, sodium sulfide is added to filtrate B while it is being stirred. The sulfide precipitate is then separated by filtration to obtain filtrate C. To ensure the removal effect, filtrate C is tested, and the test results must meet the following requirements: Cu 2+ ≤10PPM, Zn 2+ ≤10 PPM, total heavy metal ions; then ≤50 PPM; then calcium and magnesium characteristic adsorption resin column is used to remove Ca from filtrate C. 2+ and Mg 2+ , to obtain Ca 2+ ≤50PPM, Mg 2+ The filtrate D with a purity of ≤50 PPM is purified in this step, which is beneficial for the subsequent synthesis of higher purity lithium iron phosphate.

[0050] In some embodiments, the molar ratio of phosphorus, iron, and lithium in the filtrate D is adjusted to 1:(0.99-1.0):(1.1-1.5) in step S4.

[0051] In some implementations, the pH in step S4 is 4-7.

[0052] In some embodiments, the reducing agent in step S4 is not particularly limited, but is preferably ascorbic acid.

[0053] In some embodiments, the substance used to adjust the molar ratio of phosphorus, iron, and lithium in the filtrate D includes one or more of phosphoric acid, lithium phosphate, ferrous sulfate, lithium carbonate, and lithium hydroxide.

[0054] The composition ratio of phosphorus, iron, and lithium in filtrate D affects the purity and performance of the synthesized lithium iron phosphate. Phosphorus plays a crucial role in the performance of lithium iron phosphate batteries, as it can mitigate the interaction between metal elements and prevent electrode shrinkage or deformation during charging. Secondly, iron plays an important role in lithium-ion storage and safety, inhibiting oxidation. Lithium generates a large amount of volts in the battery. This application, by adjusting the composition ratio of phosphorus, iron, and lithium and the pH, and by adding a reducing agent, particularly adjusting the molar ratio of phosphorus, iron, and lithium in filtrate D to 1:(0.99-1.0):(1.1-1.5), the pH to 4-7, and using ascorbic acid as the reducing agent, can maintain the stability of ferrous ions in filtrate D, preventing the formation of subsequent impurities. This results in synthesized lithium iron phosphate with high purity and stability, as well as good resistance to high / low temperatures and environmental pressure.

[0055] In some embodiments, the temperature of the heating in the S5 step is 95-110℃, the pH is controlled to be 5-7, the reaction time is 3-5h, and the aging time is 0.5-1.5h.

[0056] The parameters of the crystallization in the S5 step have an impact on the structure of the synthesized lithium iron phosphate, further affecting the quality and electrochemical performance of the synthesized lithium iron phosphate. The applicant found that by controlling the multiple crystallization conditions in the S5 step, especially under the condition that the heating temperature is 95-110℃, the pH is controlled to be 5-7, the reaction time is 3-5h, and the aging time is 0.5-1.5h, the synthesized lithium iron phosphate can have higher quality and electrochemical performance. It is possible that the lithium iron phosphate synthesized under this condition can form a sequenced olivine structure, improving its stability.

[0057] In some embodiments, the final conductivity of the washing water in the S6 step is 50-200μS / M.

[0058] By pressing and washing the lithium iron phosphate crude product multiple times, some impurity ions can be removed. Further detection of the conductivity of the washing water can determine the washing effect. The final conductivity of the washing water in the range of 50-200μS / M can determine that the sulfate, sodium ion, and potassium ion are controlled within the range of battery-grade lithium iron phosphate, and the washing can be completed.

[0059] In some embodiments, the weight ratio of the lithium iron phosphate filter cake, the reducing organic compound, and water in the S7 step is 1:(0.05-0.2):(2-4).

[0060] Preferably, the reducing organic compound is not specially limited and includes but is not limited to at least one of glucose, sucrose, and reduced starch.

[0061] In some embodiments, the carbon coating in the S8 step specifically includes: vacuum feeding the mixture of the dried lithium iron phosphate powder and the reducing organic compound to a calcination kiln, calcining at a temperature of 700-850℃ in a nitrogen-protected atmosphere, and the calcination time is 18-24h.

[0062] In some embodiments, after the S8 step, grinding and packaging can also be included, specifically grinding the calcined qualified lithium iron phosphate and packaging after inspection.

[0063] Example 1

[0064] Please refer to Figure 1 A method for synthesizing lithium iron phosphate by using positive electrode material production waste liquid, comprising the following steps:

[0065] S1, leaching: iron powder is added to the positive material production waste at 1% of the weight of the waste, and sulfuric acid is continuously added for reaction in a reducing atmosphere, and the reaction is completed by controlling the pH to 2 to obtain a leaching solution, and the leaching solution is subjected to solid-liquid separation to obtain a filtrate A;

[0066] S2, reducing Fe 3+ : iron powder is added to the filtrate A, and the pH and temperature of the solution are controlled twice, the first time the temperature is controlled to 50°C, pH=1.4, and the reaction time is 4h; the second time the temperature is controlled to 55°C, pH is 2.2, and the reaction time is 4h, and the filtrate B is obtained by solid-liquid separation, and the Fe 3+ is controlled within 30-100PPm;

[0067] S3, impurity removal: sodium sulfide is added to the filtrate B, and the filtrate C is obtained after solid-liquid separation, and then the filtrate C is passed through a resin column to obtain the filtrate D;

[0068] Cu 2+ in the filtrate C is ≤10PPM, Zn 2+ is ≤10PPM; Ca 2+ in the filtrate D is ≤50PPM, and Mg 2+ is ≤50PPM;

[0069] S4, solution preparation: phosphoric acid, ferrous sulfate, and lithium carbonate are used to adjust the molar ratio of phosphorus, iron, and lithium in the filtrate D to 1:0.99:1.3 and the pH to 5, and ascorbic acid is added to obtain an adjusted solution;

[0070] S5, lithium iron phosphate crystallization: the adjusted solution is stirred uniformly in a crystallization kettle at a rotation speed of 100r / min, heated to 105°C, and reacted by dropwise adding lithium carbonate suspension, and the pH is controlled to 6 and the reaction time is 4h, and then the lithium iron phosphate crude product is obtained after filtering after 1h of crystallization;

[0071] S6, pressure filtration and washing: the lithium iron phosphate crude product is subjected to pressure filtration and washing, and the final conductivity of the washing water is 50-200μS / M, and the lithium iron phosphate filter cake is obtained;

[0072] S7, pulping and drying: the lithium iron phosphate filter cake, glucose, and water are proportioned according to the weight ratio of 1:0.1:3 to obtain a slurry; and the slurry is subjected to spray drying to obtain a mixture of lithium iron phosphate powder and glucose;

[0073] S8, carbon coating: the obtained mixture is calcined at 780°C for 22h in a nitrogen atmosphere to obtain lithium iron phosphate;

[0074] S9, the calcined lithium iron phosphate is ground, and packaged after inspection.

[0075] Example 2

[0076] Referring to Figure 1 A method for synthesizing lithium iron phosphate by using positive electrode material production waste liquid, comprising the following steps:

[0077] S1, leaching: adding 0.1% iron powder by weight of the waste material in the positive electrode material production waste, continuously adding sulfuric acid in the reducing atmosphere to react, controlling the pH to 1 to complete the reaction to obtain a leaching solution, and performing solid-liquid separation on the leaching solution to obtain a filtrate A;

[0078] S2, reducing Fe 3+ : adding iron powder in the filtrate A, controlling the pH and temperature of the solution twice, the first time controlling the temperature to 30°C, pH=1, and the reaction time is 4h; the second time controlling the temperature to 40°C, pH is 1.8, and the reaction time is 4h, and the filtrate B is obtained by solid-liquid separation, and the Fe 3+ is controlled within 30-100PPm;

[0079] S3, impurity removal: adding sodium sulfide in the filtrate B, and obtaining a filtrate C after solid-liquid separation, and then passing the filtrate C through a resin column to obtain a filtrate D;

[0080] Cu 2+ ≤10PPM, Zn 2+ ≤10PPM in the filtrate C; Ca 2+ ≤50PPM, Mg 2+ ≤50PPM in the filtrate D;

[0081] S4, solution preparation: adjusting the molar ratio of phosphorus, iron and lithium in the filtrate D to 1:1:1.1 and the pH to 4 by using lithium phosphate, ferrous sulfate and lithium hydroxide, and adding ascorbic acid to obtain an adjusted solution;

[0082] S5, lithium iron phosphate crystallization: stirring the adjusted solution uniformly in a crystallization kettle at a rotating speed of 80r / min, heating to 95°C, adding lithium carbonate suspension liquid dropwise to react, controlling the pH to 5 and the reaction time to 5h in the process, and then filtering to obtain lithium iron phosphate crude product after crystallization for 0.5h;

[0083] S6, pressure filtration and washing: performing pressure filtration and washing on the lithium iron phosphate crude product, so that the final conductivity of the washing water is 50-200μS / M, and a lithium iron phosphate filter cake is obtained;

[0084] S7, pulping and drying: pulping and drying the lithium iron phosphate filter cake, sucrose and water according to a weight ratio of 1:0.05:2 to obtain a slurry; and spray drying the slurry to obtain a mixture of lithium iron phosphate powder and sucrose;

[0085] S8, carbon-coated: the obtained mixture is calcined at 700°C for 24h under nitrogen atmosphere to obtain lithium iron phosphate;

[0086] S9, the calcined lithium iron phosphate is ground, and is packaged after inspection.

[0087] Example 3

[0088] Please refer to Figure 1 A method for synthesizing lithium iron phosphate by using positive electrode material production waste liquid, comprising the following steps:

[0089] S1, leaching: iron powder is added to the positive electrode material production waste in an amount of 2% by weight of the waste, and sulfuric acid is continuously added for reaction in a reducing atmosphere, and the reaction is completed by controlling the pH to 3 to obtain a leaching solution, and the leaching solution is subjected to solid-liquid separation to obtain a filtrate A;

[0090] S2, reducing Fe 3+ : iron powder is added to the filtrate A, and the pH and temperature of the solution are controlled twice, the first time the temperature is controlled to 70°C, pH=1.8, and the reaction time is 4h; the second time the temperature is controlled to 70°C, pH is 2.6, and the reaction time is 4h, and the filtrate B is obtained after solid-liquid separation, and the Fe 3+ of the filtrate B is controlled within 30-100PPm;

[0091] S3, impurity removal: sodium sulfide is added to the filtrate B, and the filtrate C is obtained after solid-liquid separation, and then the filtrate C is passed through a resin column to obtain a filtrate D;

[0092] Cu 2+ in the filtrate C is ≤10PPM, Zn 2+ is ≤10PPM; Ca 2+ in the filtrate D is ≤50PPM, and Mg 2+ is ≤50PPM;

[0093] S4, solution preparation: phosphoric acid, ferrous sulfate, and lithium carbonate are used to adjust the molar ratio of phosphorus, iron, and lithium in the filtrate D to 1:0.995:1.5 and the pH to 7, and ascorbic acid is added to obtain an adjusted solution;

[0094] S5, lithium iron phosphate crystallization: the adjusted solution is uniformly stirred in a crystallization kettle at a rotation speed of 120r / min, heated to 110°C, and reacted by dropwise adding lithium carbonate suspension, and the pH is controlled to 7 and the reaction time is 3h during the process, and then the lithium iron phosphate crude product is obtained after crystallization for 1.5h and filtration;

[0095] S6, pressure filtration and washing: the lithium iron phosphate crude product is subjected to pressure filtration and washing, and the final conductivity of the washing water is 50-200μS / M to obtain a lithium iron phosphate filter cake;

[0096] S7, pulping and drying: the lithium iron phosphate filter cake is dosed with reducing starch and water according to a weight ratio of 1:0.2:4 to obtain a slurry; the slurry is spray dried to obtain a mixture of lithium iron phosphate powder and reducing starch;

[0097] S8, carbon coating: the obtained mixture is calcined at 850°C for 18h in a nitrogen atmosphere to obtain lithium iron phosphate;

[0098] S9, the calcined lithium iron phosphate is ground, and packaged after inspection.

[0099] Although embodiments of the present application have been shown and described, it is to be understood that various modifications, substitutions, replacements and changes can be made to these embodiments without departing from the principles and spirit of the present application, the scope of the present application being defined by the appended claims and their equivalents.

Claims

1. A method for producing lithium iron phosphate by liquid phase synthesis using cathode material production waste, characterized by, The method comprises the following steps: S1, leaching: adding a reducing agent to the positive material production waste, continuously adding sulfuric acid to react, controlling pH to complete the reaction to obtain a leaching solution, and performing solid-liquid separation on the leaching solution to obtain a filtrate A; S2, reducing Fe 3+ : adding iron powder to the filtrate A, controlling the pH and temperature of the solution in two times, obtaining filtrate B through solid-liquid separation, and reducing Fe 3+ to within 30-100 PPM; S3, impurity removal: adding sodium sulfide to the filtrate B, performing solid-liquid separation to obtain a filtrate C, and then passing the filtrate C through a resin column to obtain a filtrate D; S4, solution preparation: adjusting the molar ratio of phosphorus, iron and lithium and the pH of the filtrate D, and adding a reducing agent to obtain an adjusted solution; S5, lithium iron phosphate crystallization: uniformly stirring and heating the adjusted solution in a crystallization kettle, adding lithium carbonate suspension or lithium hydroxide solution dropwise to react, controlling pH and reaction time in the process, and filtering to obtain lithium iron phosphate crude product after aging; S6, pressure filtration and washing: performing pressure filtration and washing on the lithium iron phosphate crude product to obtain a lithium iron phosphate filter cake; S7, pulping and drying: preparing a slurry by preparing and pulping the lithium iron phosphate filter cake, a reducing organic compound and water; and performing spray drying on the slurry to obtain a mixture of lithium iron phosphate powder and the reducing organic compound; S8, carbon coating: calcining the obtained mixture in a protective atmosphere to obtain lithium iron phosphate; In the S1 step, the amount of the reducing agent accounts for 0.1-2% of the weight of the waste; and in the S1 step, the pH is controlled to be 1-3. In the S2 step, the pH and temperature of the solution are controlled twice, including: first controlling the temperature to be 30-70 DEG C, the pH to be 1.0-1.8, and the reaction time to be 4 hours; and second controlling the temperature to be 40-70 DEG C, the pH to be 2.2-2.6, and the reaction time to be 4 hours.

2. The method for producing lithium iron phosphate by liquid phase synthesis using cathode material production waste according to claim 1, characterized in that, In the S1 step, the reducing agent includes ascorbic acid, sodium thiosulfate, sodium metabisulfite and sodium sulfide.

3. The method for producing lithium iron phosphate by liquid phase synthesis using cathode material production waste according to claim 1, characterized in that, Cu in the filtrate C 2+ ≤ 10 PPM, Zn 2+ ≤ 10 PPM; Ca in the filtrate D 2+ ≤ 50 PPM, Mg 2+ ≤ 50 PPM. 4.The method for producing lithium iron phosphate by liquid-phase synthesis using cathode material production waste according to claim 1, characterized in that, In the S4 step, the molar ratio of phosphorus, iron and lithium of the filtrate D is adjusted to be 1:(0.99-1.0):(1.1-1.5). 5.The method for producing lithium iron phosphate by liquid-phase synthesis using cathode material production waste according to claim 1, characterized in that, In the S4 step, the pH is 4-7. 6.The method for producing lithium iron phosphate by liquid-phase synthesis using cathode material production waste according to claim 1, characterized in that, In the S5 step, the heating temperature is 95-110 DEG C, the pH is controlled to be 5-7, the reaction time is 3-5 hours, and the aging time is 0.5-1.5 hours. 7.The method for producing lithium iron phosphate by liquid-phase synthesis using cathode material production waste according to claim 1, wherein, In the S6 step, the final conductivity of the washing water is 50-200 muS / m.

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