A process for efficiently recovering iron phosphate from waste lithium batteries

Abstract

The invention relates to a process for efficiently recovering iron phosphate in waste lithium batteries, comprising the steps of discharging, separation of positive electrode materials, heat treatment, acid leaching, electrolysis, dealumination, precipitation and the like, so as to realize efficient recovery and utilization of waste lithium batteries.

Description

Technical Field

[0001] The invention belongs to the technical field of waste lithium battery recycling, and in particular relates to a process for efficiently recycling iron phosphate in waste lithium batteries. Background Art

[0002] LiFePO4, the positive electrode material of lithium batteries, is widely used in the fields of mobile phones, electric vehicles, etc. because of its advantages such as high discharge specific capacity, relatively stable discharge platform, good cycle stability, thermal stability and low price. In recent years, the output of lithium iron phosphate batteries has gradually increased, resulting in a gradual increase in the scrap volume of lithium iron phosphate batteries. Batteries contain toxic chemicals and high-value metals and must be recycled to promote environmental protection and sustainable development. The pretreatment process of waste lithium batteries mainly includes classification, discharge, disassembly, crushing, separation and removal of organic binders, and finally obtains black powder rich in positive electrode materials. Therefore, the present invention improves the regeneration of iron phosphate by conducting pretreatment and impregnation of waste lithium batteries.

[0003] Conventionally, the principle of similar dissolution of organic reagents is used to select suitable organic substances to dissolve the PVDF and PTFE binders commonly used in batteries to achieve the purpose of separating battery materials. Organic solvents such as DMF and NMP are commonly used for dissolution. By dissolving the binder PVDF in the positive electrode material using the principle of similar dissolution, the active material can be efficiently peeled off from the aluminum foil. However, for batteries bonded with PTFE, the above solvents fail to effectively dissolve and separate, and the organic solvent has a high viscosity. The subsequent separation of powdered materials in industrial applications is difficult to filter, and the actual operation process is cumbersome compared to other methods. Therefore, how to achieve complete separation of positive electrode materials and efficiently complete the process is the research focus of the present invention.

[0004] The battery after heat treatment can improve the subsequent sorting effect. Some studies have used Ca0-assisted pyrolysis to achieve low-temperature desorption of positive electrode powder, which greatly reduces the energy consumption during pyrolysis and can achieve harmless treatment. However, this method requires high temperature and consumes a lot of energy. The present invention focuses on studying a compound that replaces Ca0 to increase the desorption effect and reduce energy consumption.

[0005] At present, the system of sulfuric acid and hydrogen peroxide is widely used to leach the positive electrode materials in waste lithium batteries, but the acid concentration used is high, which will cause corrosion to the equipment and emit toxic gases. The waste acid after leaching will also pose a potential hazard to the human body and the environment. In recent years, there are more and more application studies on the use of easily degradable and environmentally friendly organic acids to leach waste lithium-ion batteries, and the leaching rate of metals such as cobalt, lithium, nickel and manganese in the positive electrode active materials of waste batteries is also high.

[0006] Some studies have used vitamin C as an organic acid reducing agent to leach and reduce valuable metals in waste lithium batteries. This is relatively safe and efficient, and provides a new idea for the resource utilization of waste lithium battery positive electrode materials. Summary of the invention

[0007] The purpose of the present invention is to provide a process for efficiently recovering iron phosphate from waste lithium batteries.

[0008] In order to solve the above technical problems, the specific process of the present invention is as follows:

[0009] A process for efficiently recovering iron phosphate from waste lithium batteries, comprising discharge, positive electrode material separation, heat treatment, acid leaching, electrolysis, dealumination, and precipitation, characterized in that: the positive electrode material separation comprises: adding a separation solution with a mass fraction of 15-20% to the positive electrode material, reacting for 180 minutes, at a temperature of 40°C, and stirring at a speed of 500-600 rpm until the positive electrode material is completely separated.

[0010] Wherein, the separation solution is a sodium 2,2,3,3-tetrafluoropropionate solution.

[0011] Among them, the heat treatment is to subject the separated positive electrode sheets to high-temperature treatment, add a pyrolysis auxiliary agent, the mass ratio of the pyrolysis auxiliary agent to the positive electrode material is 8:1, and the temperature is increased to 450-600°C at a heating rate of 10°C / min. The pyrolysis time is 120min and the temperature is kept warm for 30min. Nitrogen is introduced into the whole process to create an oxygen-free atmosphere to prevent oxidation of the battery materials during the pyrolysis process.

[0012] Among them, the pyrolysis auxiliary agent is WO3.

[0013] A process for efficiently recovering iron phosphate from waste lithium batteries is carried out in the following steps:

[0014] Battery discharge: Use a 5% by mass NaCl solution to discharge the used batteries for 8 hours, mechanically separate the battery cells to obtain the positive electrode, negative electrode and separator; separate the positive electrode material and perform heat treatment on it;

[0015] Acid leaching treatment: ball mill the heat-treated cathode material at a ball-to-liquid ratio of 60:1, a ball milling speed of 650 rpm, and a ball milling time of 120 min. Add the cathode powder to the acid leaching solution at a material-liquid ratio of 1:2 for 60 min at a temperature of 40±2°C and a stirring speed of 400 rpm. After leaching, filter to obtain a filtrate.

[0016] Electrolytic treatment: Add 1% methanol to the filtrate, heat to 50°C and connect a DC power supply to perform electrolytic reduction reaction at this constant temperature with a current density of 5A / cm 2 , electrolysis time 30min;

[0017] Dealumination treatment: add 0.2% hydrofluoric acid to the electrolytic solution to react and form aluminum fluoride precipitate, filter to obtain the filtrate and separate the aluminum fluoride precipitate, then centrifuge to separate the precipitant and the solution to remove aluminum ions;

[0018] Ferric phosphate precipitation is generated: after dealuminization, 1% H202 is added to the filtrate to oxidize the ferrous ions in the solution to ferric ions, 0.8% formaldehyde is added to the solution, the solution is heated to 80°C and stirred for reaction, and the solution is allowed to stand and age for 30 minutes after no red-brown gas is produced. After aging, solid ferric phosphate dihydrate is obtained by filtration, washed with clean water for 2-3 times, and then dried in a constant temperature drying oven for 24 hours to recover the battery-grade ferric phosphate product.

[0019] The acidic immersion liquid has a mass fraction of 40-50% of 5,6-O-isopropylidene-L-ascorbic acid solution.

[0020] The beneficial effects of the present invention are:

[0021] 1. The present invention realizes efficient recovery and utilization of waste lithium batteries through the steps of discharge, separation of positive electrode materials, heat treatment, acid leaching, electrolysis, dealuminization, precipitation and the like. The low energy consumption and efficient pretreatment of WO3 pyrolysis combined with separation of fluorine-containing organic liquid can improve the subsequent extraction efficiency of iron phosphate, reduce pollution, and improve the purity of the recovered material. A derivative of vitamin C is used as an organic acid to carry out acid leaching treatment on the positive electrode material, so as to stably improve the leaching rate of iron phosphate.

[0022] The fluorine atoms in the 2.2,2,3,3-sodium tetrafluoropropionate molecule have very strong electronegativity, which makes its molecular structure have strong hydrophilicity and hydrophobicity adjustment ability. The introduction of fluorine atoms can not only enhance the stability of the compound, but also adjust its solubility, so that it can interact with the non-polar parts in certain battery materials. The fluorine element in the 2,2,3,3-sodium tetrafluoropropionate molecule can interact with the fluorine atoms in PTFE, which is conducive to the dissolution and separation of PTFE. The 2,2,3,3-sodium tetrafluoropropionate solution can effectively dissolve the PTFE binder in the positive electrode material of the battery without corroding or damaging the electrode material itself. This is because the molecular structure of the compound is relatively mild and will not corrode the metal substrate like some strong acid or strong base solvents, so it will not damage copper foil and aluminum foil. The solution viscosity of 2,2,3,3-sodium tetrafluoropropionate is relatively low, which makes it have good fluidity in industrial applications and can reduce the difficulty of handling materials during the separation process.

[0023] 3. Tungsten trioxide WO3 is a transition metal oxide. The higher oxidation state of tungsten in its chemical structure gives it strong catalytic activity. During the pyrolysis process, WO3 can promote the decomposition of organic matter, such as binders and solvents in batteries, and also contribute to the conversion of harmful gas HF. Tungsten, as a transition metal, has rich redox properties and can effectively participate in the reaction under high temperature conditions to promote the decomposition or conversion of reactants. There are oxygen vacancies and high surface activity in the crystal structure of WO3, which provides more reactive sites for the decomposition reaction. WO3 can remain stable at higher temperatures during the pyrolysis process and is not as susceptible to corrosion or damage as other oxides, such as CaO. Therefore, WO3, as a pyrolysis auxiliary agent, helps to protect pyrolysis equipment, extend the service life of the equipment, and reduce the maintenance cost of the equipment due to corrosion. When WO3 is used as an auxiliary agent, a higher desorption rate can be achieved at a lower temperature, further reducing energy consumption.

[0024] 4. Vitamin C can increase the leaching rate, but the reaction temperature cannot be too high. When the temperature exceeds 80°C, the leaching rate tends to decrease, because vitamin C is unstable at higher temperatures and is easy to decompose, thus becoming ineffective. 5,6-O-isopropylidene-L-ascorbic acid is an organic acid derivative, and its structure and properties are milder and more environmentally friendly than strong acids or strong oxidants. It is not only easily degraded after the reaction, but also has a smaller burden on the environment, which helps to achieve the goal of greening the resource utilization of waste lithium batteries. The use of 5,6-O-isopropylidene-L-ascorbic acid can reduce corrosive damage, reduce equipment losses, reduce the cost of waste acid treatment, and due to its higher stability, it may reduce the demand for reducing agents, thereby further reducing production costs.

[0025] 5.5,6-O-isopropylidene-L-ascorbic acid has better thermal stability due to the introduction of isopropylidene protecting groups in its molecular structure. At higher temperatures, the stability of this derivative is much higher than that of vitamin C. The isopropylidene protecting group is chemically modified to make it less likely to be decomposed, so that it can maintain its reduction activity for a longer time, ensuring that the reduction reaction during the leaching process proceeds stably.

[0026] The isopropylidene group introduced into 6.5,6-O-isopropylidene-L-ascorbic acid can effectively protect its redox activity and avoid its premature decomposition or oxidation in an acidic environment. Therefore, 5,6-O-isopropylidene-L-ascorbic acid can better maintain its reducing agent effect in the leaching process of high acid concentration and long time, ensuring the efficiency and sustainability of the leaching reaction. DETAILED DESCRIPTION

[0027] The present invention is further described in detail below in conjunction with the examples, and the ball mills used below are all DECO-PBM-V-0.4L. 18650 type waste lithium cobalt oxide batteries are recovered, and the waste batteries are discharged for 8 hours using a 5% by mass NaCl solution, and the battery cores are mechanically separated to obtain positive electrodes, negative electrodes and separators, and the recovered positive electrode materials of the same batch are divided into 6 groups and the following implementation methods are respectively carried out.

[0028] Example 1

[0029] Positive electrode material separation: add 18% by mass of 2,2,3,3-tetrafluoropropionic acid sodium solution to the positive electrode material, react for 180 min, at a temperature of 40° C., and at a stirring speed of 550 rpm until the positive electrode material is completely separated;

[0030] Example 2

[0031] Positive electrode material separation: add 15% by mass of sodium 2,2,3,3-tetrafluoropropionate solution to the positive electrode material, react for 180 min at 40° C. and stirring speed of 600 rpm until the positive electrode material is completely separated;

[0032] Example 3

[0033] Positive electrode material separation: add 20% by mass of 2,2,3,3-tetrafluoropropionic acid sodium solution to the positive electrode material, react for 180 min at 40° C. and stirring speed of 500 rpm until the positive electrode material is completely separated;

[0034] Comparative Example 1

[0035] The difference between this comparative example and Example 1 is that the 2,2,3,3-tetrafluoropropionic acid sodium solution in this comparative example is a DNF solution, and the rest is the same as Example 1.

[0036] Comparative Example 2

[0037] The difference between this comparative example and Example 1 is that the content of the sodium 2,2,3,3-tetrafluoropropionate solution is different. Specifically, the positive electrode material is separated as follows: a 30% sodium 2,2,3,3-tetrafluoropropionate solution is added to the positive electrode material, and the reaction is carried out for 180 minutes at a temperature of 40° C. and a stirring speed of 550 rpm until the positive electrode material is completely separated; the rest is the same as Example 1.

[0038] Comparative Example 3

[0039] The difference between this comparative example and Example 1 is that the content of the sodium 2,2,3,3-tetrafluoropropionate solution is different. Specifically, the positive electrode material is separated as follows: a 10% sodium 2,2,3,3-tetrafluoropropionate solution is added to the positive electrode material, and the reaction is carried out for 180 minutes at a temperature of 40° C. and a stirring speed of 550 rpm until the positive electrode material is completely separated; the rest is the same as Example 1.

[0040] Test 1: Cathode material stripping rate

[0041] The positive electrode sheet material (M) before discharge separation and the positive electrode sheet material (m) after organic solution treatment in Examples 1-3 and Comparative Examples 1-3 were weighed, and each test was repeated 3 times. Three groups of samples were weighed every 60 minutes and the average value was taken.

[0042]

[0043] m is the mass of the positive electrode material after treatment with the organic solution, in g; M is the total mass of the positive electrode material before stripping, in g.

[0044] The results are shown in Table 1 below.

[0045] Table 1

[0046]

[0047]

[0048] The following implementation modes are all carried out on the basis of Example 1.

[0049] Example 4

[0050] Heat treatment: The separated positive electrode sheet is subjected to high temperature treatment, and WO3, a pyrolysis auxiliary agent, is added. The mass ratio of the pyrolysis auxiliary agent to the positive electrode material is 8:1. The temperature is raised to 350°C at a heating rate of 10°C / min. The pyrolysis time is 120min and the temperature is kept for 30min. Nitrogen is introduced into the whole process to create an oxygen-free atmosphere to prevent oxidation of the battery material during the pyrolysis process.

[0051] Acid leaching treatment: ball mill the heat-treated positive electrode material at a ball-to-liquid ratio of 60:1, a ball milling speed of 650 rpm, and a ball milling time of 120 min. Add 45% by mass fraction of 5,6-O-isopropylidene-L-ascorbic acid solution to the positive electrode powder at a material-liquid ratio of 1:2 for 60 min at a temperature of 40°C and a stirring speed of 400 rpm. After leaching, filter to obtain a filtrate.

[0052] Electrolytic treatment: Add 1% methanol to the filtrate, heat to 50°C and connect a DC power supply to perform electrolytic reduction reaction at this constant temperature with a current density of 5A / cm 2 , electrolysis time 30min;

[0053] Dealumination treatment: add 0.2% hydrofluoric acid to the electrolytic solution to react and form aluminum fluoride precipitate, filter to obtain the filtrate and separate the aluminum fluoride precipitate, then centrifuge to separate the precipitant and the solution to remove aluminum ions;

[0054] Ferric phosphate precipitation is generated: after dealuminization, 1% H202 is added to the filtrate to oxidize the ferrous ions in the solution to ferric ions, 0.8% formaldehyde is added to the solution, the solution is heated to 80°C and stirred for reaction, and the solution is allowed to stand and age for 30 minutes after no red-brown gas is produced. After aging, solid ferric phosphate dihydrate is obtained by filtration, washed 3 times with clean water, and then dried in a constant temperature drying oven for 24 hours to recover the battery-grade ferric phosphate product.

[0055] Example 5

[0056] Heat treatment: The separated positive electrode sheet is subjected to high-temperature treatment, and WO3, a pyrolysis auxiliary agent, is added. The mass ratio of the pyrolysis auxiliary agent to the positive electrode material is 8:1. The temperature is raised to 400°C at a heating rate of 10°C / min. The pyrolysis time is 120min and the temperature is kept at 30min. Nitrogen is introduced into the whole process to create an oxygen-free atmosphere to prevent oxidation of the battery material during the pyrolysis process.

[0057] Acid leaching treatment: ball mill the heat-treated positive electrode material at a ball-to-liquid ratio of 60:1, a ball milling speed of 650 rpm, and a ball milling time of 120 min. Add 40% by mass fraction of 5,6-O-isopropylidene-L-ascorbic acid solution to the positive electrode powder at a material-liquid ratio of 1:2 for 60 min at a temperature of 42°C and a stirring speed of 400 rpm. After leaching, filter to obtain a filtrate.

[0058] Electrolytic treatment: Add 1% methanol to the filtrate, heat to 50°C and connect a DC power supply to perform electrolytic reduction reaction at this constant temperature with a current density of 5A / cm 2 , electrolysis time 30min;

[0059] Dealumination treatment: add 0.2% hydrofluoric acid to the electrolytic solution to react and form aluminum fluoride precipitate, filter to obtain the filtrate and separate the aluminum fluoride precipitate, then centrifuge to separate the precipitant and the solution to remove aluminum ions;

[0060] Ferric phosphate precipitation is generated: after dealumination treatment, 1% H202 is added to the filtrate to oxidize the ferrous ions in the solution to ferric ions, 0.8% formaldehyde is added to the solution, the solution is heated to 80°C and stirred for reaction, and the solution is allowed to stand and age for 30 minutes after no red-brown gas is produced. After aging, solid ferric phosphate dihydrate is obtained by filtration, washed twice with clean water, and then dried in a constant temperature drying oven for 24 hours to recover the battery-grade ferric phosphate product.

[0061] Example 6

[0062] Heat treatment: The separated positive electrode sheet is subjected to high-temperature treatment, and WO3, a pyrolysis auxiliary agent, is added. The mass ratio of the pyrolysis auxiliary agent to the positive electrode material is 8:1. The temperature is raised to 400°C at a heating rate of 10°C / min. The pyrolysis time is 120min and the temperature is kept at 30min. Nitrogen is introduced into the whole process to create an oxygen-free atmosphere to prevent oxidation of the battery material during the pyrolysis process.

[0063] Acid leaching treatment: ball mill the heat-treated cathode material at a ball-to-liquid ratio of 60:1, a ball milling speed of 650 rpm, and a ball milling time of 120 min. Add 50% by mass 5,6-O-isopropylidene-L-ascorbic acid solution to the cathode powder at a material-liquid ratio of 1:2 for 60 min at a temperature of 38°C and a stirring speed of 400 rpm. After leaching, filter to obtain a filtrate.

[0064] Electrolytic treatment: Add 1% methanol to the filtrate, heat to 50°C and connect a DC power supply to perform electrolytic reduction reaction at this constant temperature with a current density of 5A / cm 2 , electrolysis time 30min;

[0065] Dealumination treatment: add 0.2% hydrofluoric acid to the electrolytic solution to react and form aluminum fluoride precipitate, filter to obtain the filtrate and separate the aluminum fluoride precipitate, then centrifuge to separate the precipitant and the solution to remove aluminum ions;

[0066] Ferric phosphate precipitation is generated: after dealuminization, 1% H202 is added to the filtrate to oxidize the ferrous ions in the solution to ferric ions, 0.8% formaldehyde is added to the solution, the solution is heated to 80°C and stirred for reaction, and the solution is allowed to stand and age for 30 minutes after no red-brown gas is produced. After aging, solid ferric phosphate dihydrate is obtained by filtration, washed 3 times with clean water, and then dried in a constant temperature drying oven for 24 hours to recover the battery-grade ferric phosphate product.

[0067] Comparative Example 4

[0068] The difference between this comparative example and Example 4 is that no pyrolysis auxiliary agent WO3 is added in this comparative example. The specific heat treatment is as follows: the separated positive electrode sheet is subjected to high-temperature treatment, the temperature is increased to 350°C at a heating rate of 10°C / min, the pyrolysis time is 120min, and the temperature is kept for 30min. Nitrogen is introduced into the whole process to create an oxygen-free atmosphere to prevent oxidation of the battery material during the pyrolysis process; the rest is the same as Example 4.

[0069] Comparative Example 5

[0070] The difference between this comparative example and Example 4 is that the pyrolysis auxiliary agent for heat treatment in this comparative example is CaO; the rest is the same as Example 4.

[0071] Comparative Example 6

[0072] The difference between this comparative example and Example 4 is that the pyrolysis auxiliary agent used in the heat treatment in this comparative example is CaO, and the temperature is raised to 600° C. at a heating rate of 10° C. / min. The rest is the same as Example 4.

[0073] Comparative Example 7

[0074] The difference between this comparative example and Example 4 is that the amount of WO3 added in this comparative example is different. The specific heat treatment is: the separated positive electrode sheet is subjected to high-temperature treatment, and a pyrolysis auxiliary agent WO3 is added. The mass ratio of the pyrolysis auxiliary agent to the positive electrode material is 12:1. The temperature is increased to 350°C at a heating rate of 10°C / min. The pyrolysis time is 120min and the temperature is kept warm for 30min. Nitrogen is introduced into the whole process to create an oxygen-free atmosphere to prevent oxidation of the battery material during the pyrolysis process. The rest is the same as Example 4.

[0075] Comparative Example 8

[0076] The difference between this comparative example and Example 4 is that the amount of WO3 added in this comparative example is different. The specific heat treatment is: the separated positive electrode sheet is subjected to high-temperature treatment, and a pyrolysis auxiliary agent WO3 is added. The mass ratio of the pyrolysis auxiliary agent to the positive electrode material is 5:1. The temperature is increased to 350°C at a heating rate of 10°C / min. The pyrolysis time is 120min and the temperature is kept warm for 30min. Nitrogen is introduced into the whole process to create an oxygen-free atmosphere to prevent oxidation of the battery material during the pyrolysis process. The rest is the same as Example 4.

[0077] Comparative Example 9

[0078] The difference between this comparative example and Example 4 is that the 5,6-O-isopropylidene-L-ascorbic acid solution in the acid leaching treatment of this comparative example is a vitamin solution; the rest is the same as Example 4.

[0079] Comparative Example 10

[0080] The difference between this comparative example and Example 4 is that the 5,6-O-isopropylidene-L-ascorbic acid solution in the acid leaching treatment of this comparative example is a sulfuric acid solution; the rest is the same as Example 4.

[0081] Comparative Example 11

[0082] The difference between this comparative example and Example 4 is that the amount of 5,6-O-isopropylidene-L-ascorbic acid solution added in this comparative example is different. The specific acid leaching treatment is as follows: the positive electrode material after heat treatment is ball-milled, the ball-to-material ratio is 60:1, the ball milling speed is 650rpm, and the ball milling time is 120min. The positive electrode powder is added with a mass fraction of 45% 5,6-O-isopropylidene-L-ascorbic acid solution according to a material-liquid ratio of 1:5, the time is 60min, the temperature is 40±2°C, the stirring speed is controlled to 400rpm, and the leaching is filtered to obtain a filtrate; the rest is the same as Example 4.

[0083] Comparative Example 12

[0084] The difference between this comparative example and Example 4 is that the amount of 5,6-O-isopropylidene-L-ascorbic acid solution added in this comparative example is different. The specific acid leaching treatment is as follows: the positive electrode material after heat treatment is ball milled, the ball-to-material ratio is 60:1, the ball milling speed is 650rpm, and the ball milling time is 120min. The positive electrode powder is added with a mass fraction of 45% 5,6-O-isopropylidene-L-ascorbic acid solution according to a material-liquid ratio of 1:1, the time is 60min, the temperature is 40±2°C, the stirring speed is controlled to 400rpm, and the leaching is filtered to obtain a filtrate; the rest is the same as Example 4.

[0085] Test 2: Leaching rate

[0086] The filtrate after acid leaching is taken to detect the metal ion concentration and the leaching rate (ξ).

[0087]

[0088] c is the numerical value of the ion concentration in the leachate, in g / L; V is the numerical value of the volume of the leachate, in L; m is the numerical value of the mass of the raw material, in g; w is the numerical value of the content of each valuable metal in the raw material, in %.

[0089] The results are shown in Table 2.

[0090] Table 2

[0091] sample Fe leaching rate (%) Example 4 99.35 Example 5 99.32 Example 6 99.26 Comparative Example 4 85.69 Comparative Example 5 88.57 Comparative Example 6 91.46 Comparative Example 7 98.10 Comparative Example 8 94.52 Comparative Example 9 95.84 Comparative Example 10 92.87 Comparative Example 11 97.68 Comparative Example 12 95.24

Claims

1. A process for efficiently recovering iron phosphate from waste lithium batteries, comprising discharging, separation of positive electrode materials, heat treatment, acid leaching, electrolysis, dealumination, and precipitation, characterized in that: The positive electrode material separation is as follows: adding a separation solution with a mass fraction of 15-20% to the positive electrode material, reacting for 180 minutes, at a temperature of 40° C., and stirring at a speed of 500-600 rpm until the positive electrode material is completely separated; The separation solution is a sodium 2,2,3,3-tetrafluoropropionate solution.

2. A process for efficiently recovering iron phosphate from waste lithium batteries as claimed in claim 1, characterized in that: The heat treatment is to subject the separated positive electrode sheet to high-temperature treatment, add a pyrolysis auxiliary agent, the mass ratio of the pyrolysis auxiliary agent to the positive electrode material is 8:1, heat up to 450-600°C at a heating rate of 10°C / min, pyrolysis time is 120min, and heat preservation is 30min. Nitrogen is introduced into the whole process to create an oxygen-free atmosphere to prevent oxidation of the battery material during the pyrolysis process.

3. A process for efficiently recovering iron phosphate from waste lithium batteries as claimed in claim 2, characterized in that: The pyrolysis auxiliary agent is WO3.

4. A process for efficiently recovering iron phosphate from waste lithium batteries as claimed in claim 1, characterized in that: Proceed as follows: Battery discharge: Use a 5% by mass NaCl solution to discharge the used batteries for 8 hours, mechanically separate the battery cells to obtain the positive electrode, negative electrode and separator; separate the positive electrode material and perform heat treatment on it; Acid leaching treatment: ball mill the heat-treated cathode material at a ball-to-liquid ratio of 60:1, a ball milling speed of 650 rpm, and a ball milling time of 120 min. Add the cathode powder to the acid leaching solution at a material-liquid ratio of 1:2 for 60 min at a temperature of 40±2°C and a stirring speed of 400 rpm. After leaching, filter to obtain a filtrate. Electrolytic treatment: Add 1% methanol to the filtrate, heat to 50°C and connect a DC power supply to perform electrolytic reduction reaction at this constant temperature with a current density of 5A / cm 2 , electrolysis time 30min; Dealumination treatment: add 0.2% hydrofluoric acid to the electrolytic solution to react and form aluminum fluoride precipitate, filter to obtain the filtrate and separate the aluminum fluoride precipitate, then centrifuge to separate the precipitant and the solution to remove aluminum ions; Ferric phosphate precipitation formation: After dealuminization, add 1% H202 to the filtrate to oxidize the ferrous ions in the solution to ferric ions, add 0.8% formaldehyde to the solution, heat the solution to 80°C and stir to react, let it stand and age for 30 minutes after no red-brown gas is produced, filter out solid ferric phosphate dihydrate after aging, wash with clean water 2-3 times, and then dry in a constant temperature drying oven for 24 hours to recover the battery-grade iron phosphate product.

5. A process for efficiently recovering iron phosphate from waste lithium batteries as claimed in claim 4, characterized in that: The acidic immersion liquid has a mass fraction of 40-50% of 5,6-O-isopropylidene-L-ascorbic acid solution.

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