CN116253302A

CN116253302A - Method and system for recovering lithium iron phosphate

Info

Publication number: CN116253302A
Application number: CN202211691030.2A
Authority: CN
Inventors: 姜海凤; 刘腾; 刘义云; 江寿良
Original assignee: Quzhou Huayou Cobalt New Material Co ltd; Zhejiang Huayou Cobalt Co Ltd
Current assignee: Quzhou Huayou Cobalt New Material Co ltd; Zhejiang Huayou Cobalt Co Ltd
Priority date: 2022-12-27
Filing date: 2022-12-27
Publication date: 2023-06-13

Abstract

The application relates to the technical field of lithium iron phosphate, in particular to a method and a system for recycling lithium iron phosphate. The application provides a method and a system for recycling lithium iron phosphate, wherein the method for recycling lithium iron phosphate comprises the following steps: performing first separation treatment on the first solution containing at least phosphate radicals and iron ions to obtain a second solution containing the iron ions and a third solution containing lithium ions; adding iron simple substance into the second solution containing iron ions to perform reduction reaction to obtain a fourth solution containing ferrous ions; adding oxalic acid into the fourth solution containing ferrous ions to perform solid phase reaction and second separation treatment to obtain a precipitate containing ferrous oxalate and a fifth solution. Unlike the traditional method for recovering lithium carbonate and ferric phosphate from lithium iron phosphate, the method creatively provides the method for recovering lithium dihydrogen phosphate and ferrous oxalate from lithium iron phosphate, thereby avoiding the risk of excessive products and widening the channel for preparing the lithium iron phosphate raw material.

Description

Method and system for recovering lithium iron phosphate

Technical Field

The application belongs to the technical field of lithium iron phosphate, and particularly relates to a method and a system for recycling lithium iron phosphate.

Background

Along with the development of new energy automobiles and the use time of power batteries, a large number of retired lithium iron phosphate batteries exist, the number of the retired lithium iron phosphate batteries is in a rapid growth state, and the pressure for processing the retired lithium iron phosphate batteries is increased. In the first aspect, metal resources in the retired lithium iron phosphate battery are recycled, so that the current situation of resource exhaustion can be relieved; in the second aspect, the ecological system is potentially polluted due to the fact that a large amount of retired lithium iron phosphate batteries are accumulated, and further negative influences on the environment can be greatly reduced due to recycling of waste batteries; in the third aspect, the potential safety hazards of fire, explosion and the like brought by retired lithium iron phosphate batteries are not quite small. Therefore, there is an urgent need to recover and reuse the resources in the retired lithium iron phosphate battery.

At present, the recovery of the lithium iron phosphate battery mainly recovers lithium resources and ferrophosphorus resources through wet treatment. For example, the literature discloses a novel method for oxidizing and leaching lithium from waste lithium iron phosphate, and lithium carbonate and ferric phosphate are prepared from waste lithium iron phosphate powder by using a novel oxidant in a green, economical and environment-friendly way; sulfuric acid is dissolved to realize leaching of lithium iron phosphate; then adding oxidant manganese oxyhydroxide to oxidize ferrous iron into ferric iron, and filtering to remove impurities; adding sodium hydroxide to regulate pH to precipitate and recover ferric phosphate, and continuously adding sodium hydroxide to perform aeration oxidation precipitation to obtain manganese oxyhydroxide so as to realize the recycling of the oxidant; and finally evaporating, concentrating, adding sodium carbonate, precipitating and recycling to obtain the battery grade lithium carbonate. The method realizes green recovery of the battery, but the final prepared substances are lithium carbonate and ferric phosphate as in other preparation methods, the types of the final products are not changed, and the recovered products are the same as the existing products in types, so that the requirements of the fields of lithium carbonate and ferric phosphate are not required, and the application range of the recovered products can be limited.

Disclosure of Invention

The application aims to provide a method and a system for recycling lithium iron phosphate, which aim to solve the technical problems of single product and single product of the existing recycled lithium carbonate and ferric phosphate.

In order to achieve the purposes of the application, the technical scheme adopted by the application is as follows:

the first aspect of the present application provides a method for recovering lithium iron phosphate, comprising the steps of:

performing first separation treatment on the first solution containing at least phosphate radicals and iron ions to obtain a second solution containing the iron ions and a third solution containing lithium ions;

adding iron simple substance into the second solution containing iron ions to perform reduction reaction to obtain a fourth solution containing ferrous ions;

adding oxalic acid into the fourth solution containing ferrous ions to perform solid phase reaction and second separation treatment to obtain a precipitate containing ferrous oxalate and a fifth solution.

The method for recycling lithium iron phosphate provided by the application comprises the steps of adding an iron simple substance into a second solution containing iron ions to perform a reduction reaction, generating +2 valent iron from +3 valent iron ions, precipitating +2 valent iron by oxalic acid to generate ferrous oxalate, other multivalent cations and Cu ²⁺ 、Ti ²⁺ 、Al ³⁺ The ferrous ions can be selectively separated from the fourth solution without forming precipitates, and further, the precipitates of ferrous oxalate are separated, so that the recovery of iron element is realized. In addition, oxalic acid is adopted as a precipitator, so that the method has the characteristics of being relatively mild, nontoxic and harmless, and can not cause harm to human bodies. Unlike the traditional method for recovering lithium carbonate and ferric phosphate from lithium iron phosphate, the method creatively provides the method for recovering ferrous oxalate from lithium iron phosphate, so that the risk of excessive products is avoided, and the channel for preparing the lithium iron phosphate raw material is widened. Further, the ferrous oxalate is a raw material for preparing high-compaction lithium iron phosphate and high-performance lithium iron manganese phosphate, and can provide raw material support for subsequent processes.

In a second aspect, the present application provides a system for recovering lithium iron phosphate comprising

A first separation treatment device for separating iron ions to obtain a second solution containing iron ions and a third solution containing lithium ions;

and a device for preparing ferrous oxalate, which is used for recovering iron ions of the second solution for preparing ferrous oxalate and the fifth solution.

The application provides a system matched with the method for recycling lithium iron phosphate in the above text, and through the cooperation between the first separation treatment device and the device for preparing ferrous oxalate, the recycling of iron element in the lithium iron phosphate can be realized, and the recycled product comprises ferrous oxalate, so that the subsequent preparation of high-compaction lithium iron phosphate and high-performance lithium iron manganese phosphate is convenient, and the degree of automation is high.

Drawings

FIG. 1 is a flow chart of a method for recovering lithium iron phosphate according to an embodiment of the present invention;

fig. 2 is a flow chart of another method for recovering lithium iron phosphate according to an embodiment of the present invention.

Detailed Description

In order to make the technical problems, technical schemes and beneficial effects to be solved by the present application more clear, the present application is further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the present application.

In this application, the term "and/or" describes an association relationship of an association object, which means that there may be three relationships, for example, a and/or B may mean: a alone, a and B together, and B alone. Wherein A, B may be singular or plural. The character "/" generally indicates that the context-dependent object is an "or" relationship.

In the present application, "at least one" means one or more, and "a plurality" means two or more. "at least one of" or the like means any combination of these items, including any combination of single item(s) or plural items(s). For example, "at least one (a), b, or c)", or "at least one (a, b, and c)", may each represent: a, b, c, a-b (i.e., a and b), a-c, b-c, or a-b-c, wherein a, b, c may be single or multiple, respectively.

It should be understood that, in various embodiments of the present application, the sequence number of each process does not mean that the sequence of execution is sequential, and some or all of the steps may be executed in parallel or sequentially, where the execution sequence of each process should be determined by its functions and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

The terminology used in the embodiments of the application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used in the specification of the present application and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

The weights of the relevant components mentioned in the embodiments of the present application may refer not only to specific contents of the components, but also to the proportional relationship between the weights of the components, and thus, any ratio of the contents of the relevant components according to the embodiments of the present application may be enlarged or reduced within the scope disclosed in the embodiments of the present application. Specifically, the mass described in the specification of the examples of the present application may be a mass unit known in the chemical industry such as μ g, mg, g, kg.

The terms first and second are used for descriptive purposes only and are not to be construed as indicating or implying any relative importance or number of features indicated in order to distinguish one object from another. For example, a first XX may also be referred to as a second XX, and similarly, a second XX may also be referred to as a first XX, without departing from the scope of the embodiments of the present application. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include one or more such feature.

In a first aspect, a method for recovering lithium iron phosphate is provided, please refer to fig. 1, which includes the following steps:

step S10: performing first separation treatment on the first solution containing at least phosphate radicals and iron ions to obtain a second solution containing the iron ions and a third solution containing lithium ions;

step S20: adding iron simple substance into the second solution containing iron ions to perform reduction reaction to obtain a fourth solution containing ferrous ions;

step S30: adding oxalic acid into the fourth solution containing ferrous ions to perform solid phase reaction and second separation treatment to obtain a precipitate containing ferrous oxalate and a fifth solution.

The method for recycling the lithium iron phosphate provided by the embodiment of the application can recycle the iron element in the lithium iron phosphate. In the first aspect, unlike the traditional method for recycling lithium carbonate and ferric phosphate from lithium iron phosphate, the method creatively provides for recycling ferrous oxalate from lithium iron phosphate, so that the risk of excessive products is avoided, and the channel for preparing the lithium iron phosphate raw material is widened. Further, the ferrous oxalate is a raw material for preparing high-compaction lithium iron phosphate and high-performance lithium iron manganese phosphate, and provides raw material support for subsequent processes. In addition, oxalic acid is adopted as a precipitator in the embodiment of the application, and the method has the advantages of being relatively mild, nontoxic, harmless and the like. In a second aspect, the method for recovering lithium iron phosphate provided in the embodiments of the present application first performs a first separation treatment on a first solution containing at least phosphate and iron ions, so as to facilitate subsequent recovery of iron ions. Then, elemental iron is added to the second solution containing iron ions for reduction reaction, and the elemental iron reduces +3 valent iron to +2 valent iron, so that ferrous ions are selectively separated from the fourth solution. Finally, adding oxalic acid into the fourth solution containing ferrous ions for solid phase reaction and second separation treatment, and selectively precipitating ferrous iron by adopting oxalic acid to generate ferrous oxalate, other multivalent cations and Cu ²⁺ 、Ti ²⁺ 、Al ³⁺ No precipitate is formed, and further the precipitate of ferrous oxalate is separated, so that the recovery of iron element is realized.

In some embodiments, the methods of recovering lithium iron phosphate of the present application are applicable to the recovery of spent lithium iron phosphate batteries. Specifically, in step S10, in order to dissolve each element of the electrode material in the solution for subsequent wet recovery of each element, the method further includes the following steps of the first solution preparation method:

step S101: leaching lithium iron phosphate in the battery by phosphoric acid to obtain a first solution, wherein the first solution further comprises at least one of aluminum ions, copper ions or titanium ions.

In the embodiment of the application, the phosphoric acid is used for leaching the lithium iron phosphate in the battery, so that iron, lithium and other elements can be dissolved in the solution, and new impurities can not be introduced by adding the phosphoric acid. In addition, the moderate acid-phosphoric acid is adopted as the leaching agent, so that the corrosion to equipment is light, and the investment on the equipment can be reduced.

In some embodiments, leaching uses a mixture of phosphoric acid and water to leach the recovered lithium iron phosphate positive electrode powder, specifically 85% phosphoric acid at a concentration of V _{Phosphoric acid} ：V _{Water and its preparation method} Is 1: 2-1: 10, as 1: 2. 1: 3. 1: 4. 1: 5. 1: 6. 1: 7. 1: 8. 1: 9. 1:10, etc. Under the system condition, each element in the battery can be fully leached. Specifically, the acid concentration is too high, side reactions are easy to occur, and the reaction rate is too high, so that explosion is easy to occur; the acid concentration is too low, which is unfavorable for improving the reaction rate and cannot meet the actual production requirement.

In some embodiments, the solid to liquid ratio of the lithium iron phosphate positive electrode powder and the mixture in the leaching system is 1g:2 mL-1 g:20mL, such as 1g:2mL, 1g:3mL, 1g:5mL, 1g:7mL, 1g:9mL, 1g:11mL, 1g:13mL, 1g:15mL, 1g:17mL, 1g:20mL, etc. Under the system condition, each element in the battery can be fully leached so as to carry out subsequent wet recovery on each element.

In some embodiments, the temperature of leaching is 20-60 ℃, such as 20 ℃, 30 ℃, 40 ℃, 50 ℃, 60 ℃, and the like. Under the system condition, a proper reaction environment can be provided for leaching, and the recovery rate of each element is improved. Specifically, the temperature is too high, side reactions are easy to occur, and the reaction rate is too high, so that explosion is easy to occur; the temperature is too low, which is not beneficial to improving the reaction rate and can not meet the actual production requirement.

In some embodiments, leaching may be performed under agitation, specifically at a rate of 100 to 600r/min, such as 100r/min, 200r/min, 300r/min, 400r/min, 500r/min, 600r/min, etc. Under the condition of the stirring speed, each element in the battery can be completely dissolved in the first solution. Specifically, the stirring speed is too high, the reaction rate is too high, and explosion is easy to occur; too low stirring speed can reduce the reaction rate and cannot meet the actual production requirements.

In some embodiments, the temperature of the leaching and m _{Fixing device} :V _{Liquid and its preparation method} Compared with the prior art, the leaching time is 0.2 to 5 hours, such as 0.2 hours, 1 hour, 2 hours, 3 hours, 3.5 hours, 5 hours and the like, so as to meet the actual production requirements.

In some embodiments, to achieve separation of the second solution containing iron ions and the third solution containing lithium ions, the first separation process comprises the steps of:

step S102: performing second ultrafiltration treatment on the first solution to remove solid particles to obtain an eighth solution;

step S103: and performing second nanofiltration treatment on the eighth solution to obtain a second solution containing iron ions and a third solution containing lithium ions.

In the embodiment of the application, the second ultrafiltration treatment and the second nanofiltration treatment have the characteristics of no phase change, low energy consumption and high automation degree, the first solution can be purified, the removal rate of divalent and more than divalent ions is about 98%, and the first solution is separated by utilizing the selective permeability and screening action of the nanofiltration membrane on the ions to obtain a third solution containing lithium ions (containing lithium ions) and a second solution containing iron ions (containing a large amount of iron ions, copper ions, aluminum ions, titanium ions and the like). Specifically, the method of the first separation treatment in the embodiment of the present application mainly includes two steps. And in the first step, performing second ultrafiltration treatment on the first solution to obtain an eighth solution without solid particles so as not to interfere with the subsequent recovery process, wherein the solid particles comprise carbon powder. A second step of subjecting the eighth solution to a second nanofiltration treatment for separating monovalent ions from multivalent ions, wherein the second solution contains not only iron ions but also other multivalent ions, such as Cu ²⁺ 、Ti ²⁺ 、Al ³⁺ Plasma of multivalent ion, third solutionThe liquid mainly contains lithium ions.

In addition, the membrane separation process has the characteristics of low energy consumption, high separation efficiency, simple equipment, environmental friendliness and the like. The method relies on the sieving and selective permeability of the membrane to realize the purposes of separating, removing impurities and purifying materials under the drive of pressure. In the embodiment of the application, the ultrafiltration membrane is used for removing impurities in the first solution, wherein the impurities comprise carbon powder, and the solution subjected to ultrafiltration impurity removal enters a nanofiltration system.

In some embodiments, the pressure of the second ultrafiltration treatment is-0.05 to-0.5 bar, such as-0.2 bar, -0.3bar, -0.4bar, -0.5bar, and the like. The pressure of the system is favorable for separating carbon powder. Specifically, the pressure is too high, the water permeability of the first solution is too large, the corresponding trapped substances accumulate too much on the surface of the ultrafiltration membrane, the resistance is too large, the water permeability rate is too small, and the particles entering the micropores of the membrane are easy to block the channels; too low a pressure is detrimental to the water permeability of the first solution and can reduce the production rate.

In some embodiments, the temperature of the second ultrafiltration treatment is 20 to 40 ℃, such as 20 ℃, 35 ℃, 40 ℃, and the like. The system is favorable for separating solid particles at the temperature. Specifically, the temperature is too low, and the viscosity of the first solution increases along with the temperature, so that the flowing resistance is improved, and the water permeability rate is correspondingly reduced; too high a temperature will affect the performance of the membrane and reduce the separation effect on the solid particles. In addition, in general, the working temperature of the hollow fiber ultrafiltration membrane should be 25±5 ℃, and a high temperature resistant membrane material and a shell material may be selected when working at a higher temperature, for example, the hollow fiber ultrafiltration membrane, and in the actual production process, the material of the ultrafiltration membrane performing the second ultrafiltration treatment and the temperature value of the second ultrafiltration treatment may be adjusted according to the actual situation, and are not limited to the above temperature values.

In some embodiments, the ultrafiltration membrane subjected to the second ultrafiltration treatment has a pore size of 10 to 100nm, such as 10nm, 20nm, 80nm, 100nm, etc., which facilitates solid-liquid separation. Specifically, the pore diameter is too large, large-particle substances can pass through the ultrafiltration membrane, the interception amount can be reduced, and the separation effect can be reduced; the pore diameter is too small, and substances with small particles cannot pass through the ultrafiltration membrane, so that the interception amount is increased, and the recovery rate of each element is reduced.

In some embodiments, the concentration factor is 1-10 times, such as 1-time, 15-time, 20-time, etc., under the pressure and temperature conditions of the second ultrafiltration treatment of the system, and the system has better recovery rate of iron.

In some embodiments, the pressure of the second nanofiltration process is 5 to 50bar, such as 20bar, 30bar, 40bar, 50bar, etc. The pressure of the system is favorable for separating the second solution containing the iron ions from the third solution containing the lithium ions. Specifically, the excessive pressure can cause the deterioration of the water permeability of the membrane and the ageing of the membrane due to severe compaction and deformation of the membrane, and can reduce the separation effect; too low a pressure is detrimental to the water penetration of the eighth solution and can reduce the production rate. The proper operating pressure should be selected according to the pressure resistance of the actual treatment liquid and the selected nanofiltration membrane, and thus, the pressure of the second nanofiltration treatment in the embodiment of the present application is not limited to the pressure values described above.

In some embodiments, the temperature of the second nanofiltration process is 20-40 ℃, such as 20 ℃, 30 ℃, 40 ℃, and the like. The separation of the second solution containing iron ions and the third solution containing lithium ions is facilitated at the temperature of the system. Specifically, the temperature is too low, and the viscosity of the eighth solution increases along with the temperature, so that the flowing resistance is improved, and the water permeability rate is correspondingly reduced; too high a temperature will affect the performance of the membrane, which will lead to deterioration of the water permeability of the membrane and ageing of the membrane, thereby reducing the separation effect of the second solution containing iron ions and the third solution containing lithium ions.

In some embodiments, the nanofiltration membrane subjected to the second nanofiltration treatment has a pore size of 1 to 10nm, such as 1nm, 5nm, 10nm, etc. Is favorable for realizing solid-liquid separation. Specifically, substances with overlarge pore diameters and high valence ions can pass through the nanofiltration membrane, so that the retention amount of iron ions can be reduced, and the recovery rate of iron elements can be reduced; the pore diameter is too small, and substances with low valence ions cannot pass through the nanofiltration membrane, so that the interception amount is increased, and the separation effect is reduced.

In some embodiments, under the pressure and temperature conditions of the second nanofiltration treatment of the system, the concentration multiple is 2-10 times, such as 5 times, 7 times, 10 times, and the like, and the washing water amount is 1-10 times, such as 1 time, 5 times, 10 times, and the like, so that the method has better recovery rate of iron.

In some embodiments, after the step of the first separation process described above, the method further comprises the step of:

step S104: and adding lithium salt into the third solution containing lithium ions to adjust the molar ratio of phosphoric acid to lithium to be 1:1, and performing crystallization treatment to obtain lithium dihydrogen phosphate.

Lithium may be recovered in embodiments of the present application. The third solution containing lithium ions comprises main components of lithium dihydrogen phosphate and phosphoric acid, lithium salt is added into the third solution, the molar ratio of the phosphoric acid to the lithium is adjusted to be 1:1 for crystallization treatment, and then the lithium dihydrogen phosphate is finally converted into the lithium dihydrogen phosphate, so that an alkali neutralization process is not needed, and the high auxiliary material cost and the wastewater treatment cost are omitted. Unlike the traditional recovery of lithium carbonate and ferric phosphate from lithium iron phosphate, the method provides recovery of lithium dihydrogen phosphate from lithium iron phosphate, so that the risk of excessive products is avoided, and the source channel of lithium dihydrogen phosphate is widened. Furthermore, the method provides raw material support for the subsequent preparation of high-compaction lithium iron phosphate and high-performance lithium manganese iron phosphate.

In some embodiments, the method of recovering lithium iron phosphate includes the step of adding elemental iron to the second solution containing iron ions for a reduction reaction. Specifically, in step S20, the molar ratio of iron ions to elemental iron in the second solution is 1 (0.3-1), such as 1:0.3, 1:0.8, 1:1, etc. By adjusting the ratio, the iron ions in the first solution can be completely reduced to ferrous ions.

In some embodiments, the temperature of the reduction reaction is 20-40 ℃, such as 20 ℃, 25 ℃, 30 ℃, 35 ℃, 40 ℃, and the like. The system can provide a proper reaction environment for the reduction reaction at the temperature, and is favorable for improving the recovery rate of iron element. Specifically, the temperature is too high, which is easy to cause side reaction; the temperature is too low, which is not beneficial to improving the reaction rate and can not meet the actual production requirement.

In some embodiments, the reduction reaction may be carried out under stirring conditions, specifically at a stirring speed of 100 to 600r/min, such as 100r/min, 200r/min, 300r/min, 400r/min, 500r/min, 600r/min, etc. Under the condition of the stirring speed, the iron simple substance can be highly dispersed in the first solution. Specifically, as the stirring speed increases to a certain value, the reaction rate does not increase, and the stirring speed is too high, so that the loss of the instrument is increased; too low stirring speed can reduce the reaction rate and cannot meet the actual production requirements.

In some embodiments, the reaction time is 0.2 to 5 hours, such as 0.2 hours, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, etc. under the conditions of the temperature and stirring speed of the reduction reaction, so as to meet the actual production requirements.

In some embodiments, the method of recovering lithium iron phosphate includes the step of adding oxalic acid to a fourth solution containing ferrous ions for a solid phase reaction, a second separation treatment. Specifically, in step S30, the molar ratio of ferrous ions to oxalic acid is 1:0.8-1.5, such as 1:1. By adjusting the ratio, ferrous ions in the fourth solution can be completely precipitated, so that the recovery rate of the ferrous ions is improved.

In some embodiments, the apparatus for performing the second separation process comprises a plate and frame filter press. After the fourth solution is subjected to a solid phase reaction, the obtained mixture can be separated into a ferrous oxalate filter cake and a ferrous oxalate mother liquor by a plate-and-frame filter press.

In some embodiments, to improve the recovery rate of phosphoric acid, after step S30, the method further includes the steps of:

step S401: performing first ultrafiltration treatment on the fifth solution to remove ferrous oxalate particles contained in the fifth solution to obtain a sixth solution;

step S402: and performing first nanofiltration treatment on the sixth solution to remove multivalent ions in the sixth solution, so as to obtain a seventh solution containing phosphoric acid.

In the embodiment of the application, the first ultrafiltration treatment and the first nanofiltration treatment have the characteristics of no phase change, low energy consumption and high automation degree, the first ultrafiltration treatment and the first nanofiltration treatment are adopted to purify the fifth solution containing phosphoric acid, and the selective permeability of the nanofiltration membrane to ions and the screening effect are utilized to separate and purify the fifth solution containing phosphoric acid to obtain the seventh solution containing phosphoric acid with higher purity. Specifically, the method of the second separation treatment in the embodiment of the present application mainly includes two steps. And in the first step, performing first ultrafiltration treatment on the first solution to obtain a sixth solution without ferrous oxalate particles so as not to interfere with the subsequent recovery process. And secondly, carrying out first nanofiltration treatment on the sixth solution, and separating monovalent ions from multivalent ions to obtain a seventh solution containing phosphoric acid.

In some embodiments, the first nanofiltration process further comprises returning phosphoric acid in the seventh solution to the first solution. After the iron is recovered from the iron phosphate leaching solution in the embodiment of the application, phosphoric acid is returned to the system, and the phosphoric acid is used for further leaching the iron phosphate slag, so that the recovery rate of phosphorus element in the whole process is high.

In some embodiments, the pressure of the first ultrafiltration treatment is 0.1 to 0.5bar, such as 0.1bar. The pressure of the system is favorable for separating ferrous oxalate particles. Specifically, the pressure is too high, the water permeability of the fifth solution is too large, the corresponding trapped substances accumulate too much on the surface of the ultrafiltration membrane, the resistance is too large, the water permeability rate is too small, and the particles entering the micropores of the membrane are easy to block the channels; too low a pressure is detrimental to the water penetration of the fifth solution and can reduce the production rate.

In some embodiments, the temperature of the first ultrafiltration treatment is 20 to 40 ℃, such as 20 ℃, 35 ℃, 40 ℃, and the like. The system is favorable for separating ferrous oxalate particles at the temperature. Specifically, the temperature is too high, and the viscosity of the fifth solution increases along with the temperature, so that the flowing resistance is improved, and the water permeability rate is correspondingly reduced; too high a temperature will affect the performance of the membrane and reduce the separation effect on the solid particles. In addition, in general, the working temperature of the hollow fiber ultrafiltration membrane should be 25±5 ℃, and a high temperature resistant membrane material and a shell material may be selected when working at a higher temperature, for example, the hollow fiber ultrafiltration membrane, and in the actual production process, the material of the ultrafiltration membrane performing the first ultrafiltration treatment and the temperature value of the first ultrafiltration treatment may be adjusted according to the actual situation, and are not limited to the above temperature values.

In some embodiments, the ultrafiltration membrane subjected to the first ultrafiltration treatment has a pore size of 10 to 100nm, such as 10nm, 20nm, 80nm, 100nm, etc., which facilitates separation of ferrous oxalate particles. Specifically, the pore diameter is too large, large-particle substances can pass through the ultrafiltration membrane, the interception amount can be reduced, and the separation effect can be reduced; the pore diameter is too small, and substances with small particles cannot pass through the ultrafiltration membrane, so that the interception amount is increased, and the recovery rate of each element is reduced.

In some embodiments, the concentration factor is 2-10 times, such as 2-10 times, 20 times, 40 times, etc., under the pressure and temperature conditions of the first ultrafiltration treatment of the above system, with better recovery of phosphoric acid.

In some embodiments, the pressure of the first nanofiltration process may be 5bar, 20bar, 35bar, but is not limited thereto. And under the pressure of the system, multivalent ions in the sixth solution are further removed, so that the purpose of purifying phosphoric acid is achieved. Specifically, the pressure is too high, the water permeability of the sixth solution is too high, the corresponding trapped substances accumulate too much on the surface of the ultrafiltration membrane, the resistance is too high, the water permeability rate is too low, and the particles entering the micropores of the membrane are easy to block the channels; too low a pressure is detrimental to increasing the water permeability of the sixth solution and can reduce the production rate.

In some embodiments, the temperature of the first nanofiltration process is 20-40 ℃, such as 20 ℃, 30 ℃, 35 ℃, 40 ℃, and the like. And at the temperature of the system, multivalent ions in the sixth solution are further removed, so that the purpose of purifying phosphoric acid is achieved. Specifically, the temperature is too low, and the viscosity of the sixth solution increases along with the temperature, so that the flowing resistance is improved, and the water permeability rate is correspondingly reduced; too high a temperature will affect the performance of the membrane, which may lead to deterioration of the water permeability of the membrane and degradation of the membrane, thereby reducing the separation effect of multivalent ions.

In some embodiments, the pore size of the nanofiltration membrane subjected to the first nanofiltration treatment is 1-10 nm, such as 1nm, 5nm, 10nm, etc., so as to further remove multivalent ions in the sixth solution, thereby achieving the purpose of purifying phosphoric acid. Specifically, substances with overlarge pore diameters and high valence ions can pass through the nanofiltration membrane, so that the retention amount of iron ions can be reduced, and the recovery rate of iron elements can be reduced; the pore diameter is too small, and substances with low valence ions cannot pass through the nanofiltration membrane, so that the interception amount is increased, and the separation effect is reduced.

In some embodiments, under the pressure and temperature conditions of the first nanofiltration treatment of the system, the concentration multiple is 2-10 times, such as 2-10 times, 20 times, and the like, and the washing water amount is 1-10 times, such as 1-5 times, 10 times, and the like, so that the phosphoric acid recovery rate is better.

In some embodiments, the method of recovering lithium iron phosphate further comprises the step of preparing high compaction lithium iron phosphate or high performance lithium manganese iron phosphate from lithium dihydrogen phosphate and ferrous oxalate. For example, lithium dihydrogen phosphate and ferrous oxalate are mixed and then sintered to obtain lithium iron phosphate. For another example, lithium dihydrogen phosphate, ferrous oxalate and a manganese source are mixed and then sintered to obtain lithium iron manganese phosphate. Therefore, the lithium dihydrogen phosphate and the ferrous oxalate recovered in the above text can be used as raw materials for preparing high-compaction lithium iron phosphate and high-performance lithium manganese iron phosphate.

In a second aspect, embodiments of the present application provide a system for recovering lithium iron phosphate, comprising

The embodiment of the application provides a system matched with the method for recycling lithium iron phosphate in the application, and through the matching between the first separation treatment device and the device for preparing ferrous oxalate, the recycling of iron element in the lithium iron phosphate can be realized, and the recycled product comprises ferrous oxalate, so that the subsequent preparation of high-compaction lithium iron phosphate and high-performance lithium iron manganese phosphate is convenient, and the degree of automation is high.

In order to further recover the lithium element and the phosphorus element, the system in the above document further comprises a lithium dihydrogen phosphate preparation device and a second separation treatment device. Specifically, the system for recovering lithium iron phosphate comprises a first separation treatment device for separating iron ions to obtain a second solution containing the iron ions and a third solution containing the lithium ions;

the lithium dihydrogen phosphate preparation device is used for recovering lithium ions in the third solution to prepare lithium dihydrogen phosphate;

a device for preparing ferrous oxalate, which is used for recovering iron ions of the second solution for preparing ferrous oxalate and a fifth solution;

and the second separation treatment device is used for recovering the phosphoric acid in the fifth solution.

The embodiment of the application provides a system matched with the method for recycling lithium iron phosphate in the above text, and through the cooperation among a separation treatment device, a lithium dihydrogen phosphate preparation device, a ferrous oxalate preparation device and a purification treatment device, the recycling of iron element, lithium element and phosphorus element in the lithium iron phosphate can be realized, and the recycled products comprise lithium dihydrogen phosphate and ferrous oxalate, so that the subsequent preparation of high-compaction lithium iron phosphate and high-performance lithium iron manganese phosphate is convenient, and the degree of automation is high. Specifically, a first separation treatment device is used for carrying out first separation treatment on the first solution to obtain a second solution containing iron ions and a third solution containing lithium ions; recovering lithium ions in the third solution by using a device for preparing lithium dihydrogen phosphate, and preparing lithium dihydrogen phosphate; recovering iron ions in the second solution by using a device for preparing ferrous oxalate, and preparing ferrous oxalate and a fifth solution; and purifying the fifth solution by using a second separation treatment device to obtain a seventh solution containing phosphoric acid, and circularly adding the seventh solution into the first separation treatment device to leach the lithium iron phosphate in the battery.

In some embodiments, the first separation treatment device and the second separation treatment device may be an ultrafiltration-nanofiltration-reverse osmosis combined device for separating multivalent ions from solid particles. Specifically, the first separation processing device is used for separating lithium ions and multivalent ions. According to the characteristics that nanofiltration membranes have different selective permeabilities to solutes and the rejection rate of multivalent ions is higher than that of monovalent ions, the nanofiltration membrane technology is utilized to separate the leaching solution, so that the separation and concentration of lithium and iron elements are realized. The process greatly reduces the use amount of acid and alkali and avoids the technical processes of extraction with organic solvents and the like.

In some embodiments, the system may specifically include, but is not limited to, a pulverizer, an acidification tank, an ultrafiltration-nanofiltration-reverse osmosis combined device, a crystallizer, a screen, a plate frame, a stirrer, etc., wherein the pulverizer, the screen are used for preparing lithium iron phosphate black powder, the acidification tank and the stirrer are used for leaching the lithium iron phosphate black powder, the ultrafiltration-nanofiltration-reverse osmosis combined device may be used for performing the processes of the first ultrafiltration treatment, the first nanofiltration treatment, the second ultrafiltration treatment and the second nanofiltration treatment, the crystallizer is mainly used for preparing lithium dihydrogen phosphate, and the plate frame is mainly used for solid-liquid separation.

The present application is further illustrated by way of examples, and it should be noted that the examples of the present invention are merely illustrative of the contents of the present application and are not to be construed as limiting the technical solutions of the present application.

Example 1

The embodiment provides a method for recovering lithium iron phosphate, please refer to fig. 2, comprising the following steps:

step S1 (leaching): 5L 85wt% phosphoric acid and 50L pure water are taken and added into a reaction tank, the temperature is raised to 60 ℃, 13.75kg of lithium iron phosphate black powder is added, stirring and leaching are carried out for 5h, the stirring speed is 400r/min, and a first solution is obtained, wherein the first solution contains lithium ions, iron ions and phosphate ions. The leaching rate of lithium iron phosphate is 97.3 percent.

Step S2 (second ultrafiltration): adding 50L of leaching solution into ultrafiltration small test equipment, wherein the operating condition is that the temperature is 40 ℃, the pressure is-0.2 bar, the concentration multiple is 20 times, 45L of impurity removing solution is obtained, 5L of ultrafiltration A residual acid is subjected to plate frame filter pressing, 4L of plate frame clear solution is subjected to ultrafiltration, and carbon powder is removed, so that eighth solution is obtained;

step S3 (second nanofiltration): adding 45L of ultrafiltration eighth solution into a nanofiltration small test device, wherein the operating condition pressure is 50bar, the temperature is 30 ℃, the concentration multiple is 10 times, and 9L of washing water is added after concentration to obtain a second solution containing iron ions and a third solution containing lithium ions, wherein the non-iron ions comprise lithium ions.

Step S4 (preparation of lithium dihydrogen phosphate): and adding lithium salt into the third solution containing lithium ions to adjust the molar ratio of phosphoric acid to lithium to be 1:1, and performing crystallization treatment to obtain lithium dihydrogen phosphate. The lithium recovery was measured to be 96.7%.

Step S5 (iron reduction): and (3) testing the content of iron ions in the second solution by adopting a potassium dichromate redox titration method, adding iron powder with the molar quantity which is 0.8 times that of the tested iron into the second solution obtained in the step (S3), wherein the reaction temperature is 40 ℃, the reaction time is 1h, and the stirring speed is 300r/min, so as to obtain a fourth solution containing ferrous ions.

Step S6 (iron precipitation): adding oxalic acid with the molar weight of 1 time of ferrous ions into the fourth solution obtained in the step 5, wherein the reaction temperature is 40 ℃, the reaction time is 3 hours, and the stirring speed is 300r/min. And (3) the reacted solution passes through a plate-and-frame filter press to obtain a precipitate containing ferrous oxalate and a fifth solution. The iron recovery was measured to be 98.5%.

Step S7 (first ultrafiltration); adding 8L of fifth solution into ultrafiltration small test equipment, wherein the operating condition is that the temperature is 40 ℃, the pressure is 0.1bar, the concentration multiple is 10 times, removing ferrous oxalate particles contained in the fifth solution to obtain 7.8L of sixth solution, and combining 0.2L of ultrafiltration B residual acid into a plate frame system.

Step S8 (first nanofiltration): adding 7.5L of sixth solution into nanofiltration small test equipment, wherein the operating condition pressure is 20bar, the temperature is 30 ℃, the concentration multiple is 10 times, and 1.5L of pure water is added after concentration to obtain seventh solution containing phosphoric acid. The phosphorus recovery was measured to be 98.2%.

Step S9 (recycling phosphoric acid): the phosphoric acid in the seventh solution is returned to the first solution.

The recovery effect in example 1 is shown in Table 1.

TABLE 1 recovery effect data

Example 2

step S1 (leaching): 10L 85wt% phosphoric acid and 50L pure water are taken and added into a reaction tank, the temperature is raised to 60 ℃, 12kg of lithium iron phosphate black powder is added, stirring and leaching are carried out for 5 hours, the stirring speed is 400r/min, and a first solution is obtained, wherein the first solution contains lithium ions, iron ions and phosphate ions. The leaching rate of the lithium iron phosphate is 97.3 percent;

step S2 (second ultrafiltration): adding 50L of leaching solution into ultrafiltration small test equipment, wherein the operating condition is that the temperature is 40 ℃, the pressure is-0.2 bar, the concentration multiple is 20 times, 47.5L of impurity removing solution is obtained, 2.5L of ultrafiltration A residual acid is fed into a plate frame for filter pressing, 4L of plate frame clear liquid is subjected to ultrafiltration, and carbon powder is removed, so that eighth solution is obtained;

Step S3 (second nanofiltration): adding 45L of ultrafiltration eighth solution into a nanofiltration small test device, wherein the operating condition pressure is 50bar, the temperature is 30 ℃, the concentration multiple is 5 times, and 20L of washing water is added after concentration to obtain a second solution containing iron ions and a third solution containing lithium ions, wherein the non-iron ions comprise lithium ions.

Step S4 (preparation of lithium dihydrogen phosphate): and adding lithium salt into the third solution containing lithium ions to adjust the molar ratio of phosphoric acid to lithium to be 1:1, and performing crystallization treatment to obtain lithium dihydrogen phosphate. The lithium recovery rate is 96.7% by measurement;

step S5 (iron reduction): testing the content of iron ions in the second solution by adopting a potassium dichromate redox titration method, adding iron powder with the molar quantity 1 times that of the tested iron into the second solution obtained in the step S3, wherein the reaction temperature is 40 ℃, the reaction time is 1h, and the stirring speed is 300r/min, so as to obtain a fourth solution containing ferrous ions;

step S6 (iron precipitation): and (3) adding oxalic acid with the molar weight of 1 time of ferrous ions into the fourth solution obtained in the step (S5), wherein the reaction temperature is 40 ℃, the reaction time is 3 hours, and the stirring speed is 300r/min. And (3) the reacted solution passes through a plate-and-frame filter press to obtain a precipitate containing ferrous oxalate and a fifth solution. The iron recovery rate is 97.5% after measurement;

Step S7 (first ultrafiltration); adding 8L of ferrous oxalate mother liquor into ultrafiltration small test equipment, wherein the operating condition is that the temperature is 40 ℃, the pressure is 0.3bar, the concentration multiple is 10 times, removing ferrous oxalate particles contained in the fifth solution to obtain 7.2L of sixth solution, and combining 0.8L of ultrafiltration B residual acid into a plate frame system;

step S8 (first nanofiltration): adding 7L of sixth solution into nanofiltration small test equipment, wherein the operating condition pressure is 20bar, the temperature is 30 ℃, the concentration multiple is 10 times, and 1.5L of pure water is added after concentration to obtain seventh solution containing phosphoric acid. The phosphorus recovery was measured to be 98.2%.

The recovery effect in example 2 is shown in Table 2.

TABLE 2 recovery effect data

Example 3

step S1 (leaching): 5L 85wt% phosphoric acid and 50L pure water are taken and added into a reaction tank, the temperature is raised to 40 ℃, 7.5kg of lithium iron phosphate black powder is added, stirring and leaching are carried out for 1h, the stirring speed is 200r/min, and a first solution is obtained, wherein the first solution contains lithium ions, iron ions and phosphate ions. The leaching rate of the lithium iron phosphate is 95.4 percent;

Step S2 (second ultrafiltration): adding 50L of leaching solution into ultrafiltration small test equipment, wherein the operating condition is that the temperature is 35 ℃, the pressure is-0.3 bar, the concentration multiple is 10 times, 45L of impurity removing solution is obtained, 5L of ultrafiltration A residual acid is subjected to plate frame filter pressing, 4L of plate frame clear solution is subjected to ultrafiltration, and carbon powder is removed, so that eighth solution is obtained;

step S3 (second nanofiltration): adding 45L of ultra-filtered impurity solution into a nanofiltration small test device, wherein the operating condition pressure is 20bar, the temperature is 30 ℃, the concentration multiple is 5 times, and 30L of washing water is added after concentration to obtain a second solution containing iron ions and a third solution containing lithium ions, wherein the non-iron ions comprise lithium ions.

Step S4 (preparation of lithium dihydrogen phosphate): and adding lithium salt into the third solution containing lithium ions to adjust the molar ratio of phosphoric acid to lithium to be 1:1, and performing crystallization treatment to obtain lithium dihydrogen phosphate. The lithium recovery rate is 95.6% after measurement;

step S5 (iron reduction): testing the content of iron ions in the second solution by adopting a potassium dichromate redox titration method, adding iron powder with the molar quantity which is 0.8 times that of the tested iron into the second solution obtained in the step S3, wherein the reaction temperature is 40 ℃, the reaction time is 5 hours, and the stirring speed is 400r/min, so as to obtain a fourth solution containing ferrous ions;

Step S6 (iron precipitation): and (3) adding oxalic acid with the molar weight of 0.8 of ferrous ions into the fourth solution obtained in the step (S5), wherein the reaction temperature is 35 ℃, the reaction time is 2 hours, and the stirring speed is 400r/min. And (3) the reacted solution passes through a plate-and-frame filter press to obtain a precipitate containing ferrous oxalate and a fifth solution. The iron recovery rate is 90.1% by measurement;

step S7 (first ultrafiltration); adding 8L of ferrous oxalate mother liquor into ultrafiltration small test equipment, wherein the operating condition is that the temperature is 35 ℃, the pressure is 0.5bar, the concentration multiple is 10 times, removing ferrous oxalate particles contained in the fifth solution to obtain 7.8L of sixth solution, and combining 0.2L of ultrafiltration B residual acid into a plate frame system;

step S8 (first nanofiltration): adding 7.5L of sixth solution into nanofiltration small test equipment, wherein the operating condition pressure is 35bar, the temperature is 35 ℃, the concentration multiple is 10 times, and 10L of pure water is added after concentration to obtain seventh solution containing phosphoric acid. The phosphorus recovery was measured to be 90.7%.

The recovery effect in example 3 is shown in Table 3.

TABLE 3 recovery effect data

Comparative example 1

This comparative example provides a method of recovering lithium iron phosphate comprising the steps of:

Step S1: 5L of 98% sulfuric acid and 50L of pure water are taken and added into a reaction tank, the temperature is raised to 40 ℃, 13.75kg of lithium iron phosphate black powder is added, 5L of hydrogen peroxide is added, stirring and leaching are carried out for 1h, and the stirring speed is 200r/min. The lithium leaching rate is 92.1 percent;

step S2: carrying out solid-liquid separation on the lithium leaching solution containing the iron phosphate slag by adopting a filter press to obtain iron phosphate slag and lithium leaching solution;

step S3: adjusting the pH value of the lithium leaching solution to 5.5 by adopting 30% aqueous alkali, removing iron and aluminum in the lithium leaching solution, and carrying out solid-liquid separation to obtain the lithium leaching solution with iron and aluminum removed;

step S4: adjusting the pH value of the lithium leaching solution to 12 by adopting 30% aqueous alkali, removing impurities such as magnesium, calcium, manganese and the like in the lithium leaching solution, and carrying out solid-liquid separation to obtain refined lithium leaching solution;

step S5: concentrating the refined lithium-containing leaching solution by adopting triple effect evaporation, and controlling the lithium content to be 28g/L;

step S6: adding the lithium leaching solution into a sodium sulfate solution, and controlling the molar ratio of lithium to carbonate to be 2:1.2, the reaction temperature is 85 ℃, and the heat preservation time is 1h;

step S7: and (3) carrying out solid-liquid separation to obtain a lithium carbonate filter cake and a lithium carbonate mother solution.

Step S8: and (3) washing the lithium carbonate filter cake by adopting a saturated lithium carbonate solution at the temperature of 85 ℃, wherein the washing end point is the conductivity less than or equal to 500us/cm. The lithium recovery rate of steps 3 to 8 was 63.9%. Lithium recovery = lithium leaching rate (step 1): phosphorus recovery (step 3 to step 8), phosphorus recovery 58.9%;

Step S9: iron phosphate slag is used as a raw material, and the solid-to-liquid ratio is 1: adding 30% sulfuric acid, wherein the leaching temperature is 45 ℃, the leaching time is 2 hours, and the iron leaching rate is 92.1%;

step S10: adding sodium fluoride into the ferric phosphate leaching solution to remove aluminum, wherein the adding amount is n (F: al) =5:1, the reaction time is 30min, and solid-liquid separation is carried out to obtain a ferric phosphate solution after aluminum removal;

step S11: adjusting the pH value of the ferric phosphate leaching solution to 4 by adopting 30% caustic soda flakes, and carrying out solid-liquid separation to obtain a ferric phosphate dihydrate filter cake and ferric phosphate mother liquor;

step S12: and (3) cleaning the iron phosphate filter cake by adopting washing water of which the temperature is 45 ℃ and 50 times, and drying the cleaned iron phosphate dihydrate in an oven at the temperature of 350 ℃ for 10 hours. The recovery rate of the ferric phosphate from the step 10 to the step 12 is 80.4 percent. Iron phosphate recovery = iron leaching rate (step 9) ×phosphorus recovery (steps 10 to 12), 74%. The recovery effect in comparative example 1 is shown in Table 4.

TABLE 4 recovery effect data

Comparative example 2

Step S2: adjusting the pH value of the leaching solution to about 3.5 by using alkali, and carrying out suction filtration to obtain 10.2g of filter residues and 45.2L of filtrate;

step S3: adding 45L of the filtrate obtained in the step 2 into ultrafiltration small test equipment, wherein the operating condition is that the temperature is 35 ℃, the pressure is-0.3 bar, and the concentration multiple is 10 times, so as to obtain 40.5L of impurity-removing liquid and 4.5L of ultrafiltration A residual acid;

step S4: adding 40L of impurity removing liquid into nanofiltration small test equipment, wherein the operating condition pressure is 30bar, the temperature is 30 ℃, the concentration multiple is 5 times, and 32L of nanofiltration purified liquid and 8L of nanofiltration residual acid are obtained;

step S5: adding 32L of nanofiltration purified liquid into reverse osmosis pilot plant, wherein the operating condition pressure is 40bar, the temperature is 30 ℃, and the concentration multiple is 10 times, so as to obtain 28.8L of nanofiltration purified liquid and 3.2L of reverse osmosis residual acid;

step S6: further evaporating the nanofiltration purified solution to enable the concentration of lithium to reach 30g/L, adding saturated sodium carbonate at the temperature of 85 ℃ to precipitate lithium carbonate, and performing treatments such as washing, recrystallization, washing, drying and the like to obtain lithium phosphate and lithium carbonate, so that a lithium carbonate product cannot be obtained;

step S7: the solution containing iron ions, aluminum ions, titanium ions, copper ions, phosphate radicals and sulfuric acid (ultrafiltration A residual acid, nanofiltration residual acid and reverse osmosis residual acid) is subjected to copper removal by iron powder replacement, the pH value is adjusted to 4.5 to remove aluminum, and the pH value is adjusted again to precipitate phosphate radicals to recover ferric phosphate. Iron forms a large amount of iron slag in step S2, and the iron phosphate recovery rate is only 0.45%, and iron phosphate cannot be effectively recovered.

TABLE 5 recovery effect data

In summary, the products of the recovery methods in comparative examples 1 and 2 were lithium carbonate and iron phosphate. The recovery methods in examples 1 to 3 were carried out with battery grade lithium dihydrogen phosphate and ferrous oxalate, and the phosphoric acid in the lithium leaching solution was finally converted to lithium dihydrogen phosphate entirely, without the need for an alkali neutralization process, and high auxiliary material costs and wastewater treatment costs were omitted. The recovery rates of lithium, iron and phosphorus resources of examples 1 to 3 are all higher than those of comparative examples 1 and 2. The final products of examples 1 to 3 were less sodium, potassium, calcium, iron, aluminum, sulfate, chloride and hydrochloric acid insoluble than those of comparative examples 1 and 2, and the products were purer using the recovery methods provided in examples 1 to 3.

Compared with the lengthy treatment process for recovering the lithium iron phosphate in the comparative example 1, the recovery method in the examples 1 to 3 has the characteristics of short-cut and low cost, the recovery method in the examples 1 to 3 can efficiently remove the impurity ions in the lithium leaching solution by nanofiltration, and has less other impurity ions coprecipitated with ferrous phosphate, and the products of lithium dihydrogen phosphate and ferrous oxalate are stable and have high quality. The recovery rate of iron phosphate in examples 1 to 3 was higher than that in comparative example 2 in which the filter residue was not subjected to recovery treatment.

The foregoing description of the preferred embodiments of the present application is not intended to be limiting, but is intended to cover any and all modifications, equivalents, and alternatives falling within the spirit and principles of the present application.

Claims

1. A method of recovering lithium iron phosphate, comprising:

adding an iron simple substance into the second solution containing the iron ions to perform a reduction reaction to obtain a fourth solution containing the ferrous ions;

2. The method of recovering lithium iron phosphate of claim 1, wherein the molar ratio of iron ions to elemental iron in the first solution is 1 (0.3 to 1);

or/and the temperature of the reduction reaction is 20-40 ℃, the time is 0.2-5 h, and the stirring speed is 100-600 r/min;

or/and the molar ratio of the ferrous ions to the oxalic acid is 1 (0.8-1.5).

3. The method of recovering lithium iron phosphate of claim 1, further comprising a method of preparing the first solution:

leaching lithium iron phosphate in the lithium iron phosphate positive electrode powder by adopting a mixture of phosphoric acid and water to obtain the first solution, wherein the first solution further comprises at least one of aluminum ions, copper ions or titanium ions.

4. A method of recovering lithium iron phosphate according to claim 3, wherein the leached conditional phosphoric acid has a concentration of 85%, V _{Phosphoric acid} ：V _{Water and its preparation method} Is 1: 2-1: 10, the solid-to-liquid ratio of the lithium iron phosphate positive electrode powder to the mixture is 1g:2 mL-1 g:20mL;

or/and, the leaching temperature is 20-60 ℃, the time is 0.2-5 h, and the stirring speed is 100-600 r/min.

5. The method for recovering lithium iron phosphate of claim 1, further comprising:

performing first ultrafiltration treatment on the fifth solution to remove ferrous oxalate particles contained in the fifth solution, so as to obtain a sixth solution;

and performing first nanofiltration treatment on the sixth solution to remove multivalent ions in the sixth solution, so as to obtain a seventh solution containing phosphoric acid.

6. The method of recovering lithium iron phosphate of claim 5, further comprising returning phosphoric acid in the seventh solution to the first solution after the first nanofiltration process;

Or/and, the pressure of the first ultrafiltration treatment is 0.1-0.5 bar, the temperature is 20-40 ℃, and the concentration multiple is 2-10 times;

or/and, the pressure of the first nanofiltration treatment is 5-50 bar, the temperature is 20-40 ℃, the concentration multiple is 2-10 times, and the washing water amount is 1-10 times.

7. The method of recovering lithium iron phosphate of claim 1, wherein the first separation process comprises:

performing second ultrafiltration treatment on the first solution to remove solid particles, so as to obtain an eighth solution;

and performing second nanofiltration treatment on the eighth solution to obtain the second solution and the third solution.

8. The method for recovering lithium iron phosphate of claim 7, further comprising:

adding lithium salt into the third solution containing lithium ions to adjust the molar ratio of phosphoric acid to lithium to be 1:1 for crystallization treatment, so as to obtain lithium dihydrogen phosphate;

or/and the pressure of the second ultrafiltration treatment is-0.05 to-0.5 bar, the temperature is 20-40 ℃, and the concentration multiple is 5-20 times;

or/and, the pressure of the second nanofiltration treatment is 5-50 bar, the temperature is 20-40 ℃, the concentration multiple is 2-10 times, and the washing water amount is 1-10 times.

9. The method of recovering lithium iron phosphate of claim 8, further comprising the step of preparing a high-compaction lithium iron phosphate or a high-performance lithium iron manganese phosphate from the lithium dihydrogen phosphate and ferrous oxalate.

10. A system for recovering lithium iron phosphate, comprising:

and the device is used for recovering the iron ions of the second solution to prepare ferrous oxalate and a fifth solution.