A method for recovering iron phosphate from waste lithium iron phosphate batteries

Abstract

The present invention discloses a method for recovering iron phosphate from waste lithium iron phosphate batteries, belonging to the technical field of waste lithium iron phosphate battery recycling, and comprising the following steps: treating the electrode active material powder with a sodium hydroxide solution to obtain an aluminum-removed material; leaching the aluminum-removed material with a sulfuric acid + hydrogen peroxide solution to obtain a lithium-rich leaching solution and an iron-containing filter residue; removing aluminum and iron from the lithium-rich leaching solution, adjusting the pH value, adding a diatomite-based purifying agent, stirring and adsorbing for 1-2 h, and filtering to obtain a purified lithium-rich solution; adding sodium hydroxide to the purified lithium-rich solution to adjust the pH to 11-13, and adding a carbonate to obtain battery-grade lithium carbonate; treating the iron-containing filter residue with sulfuric acid and iron powder, removing aluminum by using a pyridinecarboxylic acid compound, and then treating with hydrogen peroxide, followed by solid-liquid separation to obtain battery-grade iron phosphate. The recycling method of the invention has a simple process, can simultaneously realize the recycling of carbon black, iron phosphate and lithium carbonate, has a high recovery rate, and the obtained products have high purity.

Description

Technical Field

[0001] The present invention belongs to the technical field of recycling of waste lithium iron phosphate batteries, and particularly relates to a method for recovering iron phosphate from waste lithium iron phosphate batteries. Background Art

[0002] In recent years, lithium iron phosphate (LiFePO4) has become one of the most promising cathode materials in power batteries. According to statistics, the market share of the installed capacity of lithium iron phosphate batteries has increased from 34% in 2019 to 51% in 2021. Since the average service life of lithium iron phosphate batteries is 5 to 8 years, and lithium iron phosphate batteries are rich in strategic valuable resources such as lithium, phosphorus, iron, and graphite. Therefore, in the face of the large-scale retirement of lithium iron phosphate batteries, recycling them can not only relieve the environmental pressure brought by battery waste, but also bring certain social and economic benefits, which is conducive to the green and sustainable development of the entire industry.

[0003] At present, the recycling processes of waste lithium iron phosphate batteries are divided into two categories. One is the full leaching recycling process, and the other is the selective lithium extraction technology. In the full leaching recycling process, it is necessary to precipitate elements such as iron and phosphorus before recovering lithium. During the precipitation process of iron and phosphorus, the precipitation product inevitably entrains some lithium elements, resulting in the loss of lithium elements and the decrease in the purity of iron phosphate. The selective lithium extraction technology is a more economical and effective method for recovering lithium based on the wet full leaching method, but the obtained lithium carbonate and iron phosphate have low purity and low recovery rate. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for recovering iron phosphate from waste lithium iron phosphate batteries, so as to solve the problems of low recovery rate and low purity of the recycled metal products of existing waste lithium iron phosphate batteries.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A method for recovering iron phosphate from waste lithium iron phosphate batteries includes the following steps:

[0007] (1) Making the waste lithium iron phosphate batteries into electrode active material powder, placing it in a sodium hydroxide solution, stirring and reacting at room temperature for 1 - 2 h. After filtration, the filter residue is rinsed with deionized water 1 - 2 times to obtain aluminum-removed material;

[0008] The impurity aluminum in the electrode active material powder exists in the form of metal. The alkali solution is used to make it enter the solution in the form of aluminate ion, and the aluminum in the electrode active material powder is removed by filtration and solid-liquid separation.

[0009] (2) Placing the aluminum-removed material in a sulfuric acid solution, adding hydrogen peroxide, and leaching at 50 - 60 °C for 2 h to obtain a lithium-rich leaching solution and an iron-containing filter residue;

[0010] Under high redox potential and weak acidic conditions, lithium iron phosphate will be in the form of Li + It exists in the form of solid iron phosphate, and the main reaction formula is:

[0011] LiFePO4+H2SO4+H2O2→LiSO4+FePO4+H2O, through dilute acid + oxidant leaching, the lithium in the aluminum removal material enters the solution in the form of lithium sulfate and is separated from components such as iron phosphate and carbon black;

[0012] (3) The pH value of the lithium-rich leaching solution is adjusted to 3.5-5.5 with sodium hydroxide, and then the solid-liquid separation is performed to obtain an iron-removing aluminum liquid. The pH value of the iron-removing aluminum liquid is then adjusted to 3±0.5 with a sulfuric acid solution, and a diatomaceous earth-based purifier is added. The mixture is stirred and adsorbed for 1-2 hours, and filtered. The filtrate is the purified lithium-rich liquid. The filter residue is placed in a 1 mol / L sodium hydroxide solution for desorption to obtain a regenerated diatomaceous earth-based purifier.

[0013] Since the electrode active material powder is often mixed with electrolytes, the iron-aluminum removal liquid inevitably contains more anionic impurities such as fluoride ions and phosphates. The lithium carbonate formed by direct lithium precipitation will enrich impurities. Therefore, diatomaceous earth-based purifiers are used to purify the iron-aluminum removal liquid. However, in the environment of low pH value, there is a risk of lanthanum hydroxide in the diatomaceous earth-based purifier falling off. In the environment of high pH value, the diatomaceous earth-based purifier will be protonated to form La-O - The negatively charged surface generates electrostatic repulsion with fluoride ions and phosphate ions, resulting in a decrease in the adsorption effect. After research, it was determined that the purification effect was best when the pH was controlled at 3±0.5;

[0014] (4) adjusting the pH of the purified lithium-rich solution to 11-13, adding carbonate at 90-95° C., stirring the reaction, and separating the solid and liquid to obtain battery-grade lithium carbonate;

[0015] (5) Mix the iron-containing filter residue, iron powder, water and concentrated sulfuric acid, react at 65-80° C. for 4-6 hours, filter to obtain an acidic slurry, remove aluminum from the acidic slurry to obtain a dealuminized ferrophosphorus liquid, add hydrogen peroxide to the dealuminized ferrophosphorus liquid, adjust the pH, separate the solid and the liquid, wash, and obtain battery-grade iron phosphate.

[0016] Further, the diatomaceous earth-based purifier is obtained by the following steps:

[0017] S1. Mix anhydrous ethanol and deionized water, adjust the pH to 3-4 with formic acid, add coupling agent KH-570, stir for 0.5-1h, add acidified diatomaceous earth, stir ultrasonically at 40-50°C for 1h, filter, and dry at 50°C to constant weight to obtain grafted unsaturated double bond diatomaceous earth;

[0018] S2. Place the grafted unsaturated double bond diatomite, 2,2 - dimethoxy - 2 - phenylacetophenone, and THF in a flask. After stirring for 10 - 30 min, add an aqueous solution of cysteine dropwise. After the addition is complete, stir and react under ultraviolet lamp irradiation for 20 min, then perform suction filtration. Wash and dry the filter cake to obtain bifunctional group - modified diatomite;

[0019] S3. Ultrasonically disperse the bifunctional group - modified diatomite in deionized water, then add La(NO3)3·6H2O. While stirring at a rotation speed of 500 - 1000 r / min, add a 1 mol / L sodium hydroxide solution dropwise to adjust the pH to 9 - 10. Let it stand at 60 °C for 18 h, then centrifuge. Wash the precipitate twice with deionized water and absolute ethanol respectively, and finally dry it to constant weight at 60 °C to obtain the diatomite - based purifying agent.

[0020] Diatomite has the advantages of large specific surface area, wide source, and low price, and is very suitable as an adsorbent. However, diatomite is negatively charged on the surface in water (due to the action of silicon hydroxyl groups), and unmodified diatomite cannot directly adsorb fluoride ions and phosphate ions that are also negatively charged. Rare earth compounds represented by lanthanum have strong affinity for anions such as fluoride ions, but have small particle size, are prone to agglomeration, and have poor adsorption effect. Therefore, in the present invention, acidified diatomite is modified. First, it is subjected to coupling treatment with the coupling agent KH - 570 to introduce unsaturated double bonds on its surface. Then, under the action of a photoinitiator, the unsaturated double bonds react with the sulfhydryl groups of cysteine through click reaction to introduce abundant amino and carboxyl groups on the surface of diatomite. Finally, through the coordination of carboxyl groups with lanthanum, lanthanum hydroxide is introduced on the surface of the bifunctional group - modified diatomite. On the one hand, it overcomes the problems of small particle size and poor adsorption of lanthanum hydroxide, and on the other hand, enables the active groups, diatomite, and lanthanum hydroxide to play an adsorption role synergistically.

[0021] Further, in step S1, the dosage ratio of absolute ethanol, deionized water, coupling agent KH - 570, and acidified diatomite is 80 - 100 mL: 20 - 30 mL: 1 - 2 g: 5 g.

[0022] Further, in step S2, the dosage ratio of the grafted unsaturated double bond diatomite, 2,2 - dimethoxy - 2 - phenylacetophenone, THF, and cysteine is 6 - 7 g: 0.01 - 0.02 g: 70 - 100 mL: 0.3 - 0.6 g. The aqueous solution of cysteine is composed of cysteine and deionized water according to 1 g: 10 mL.

[0023] Further, in step S3, the dosage ratio of the bifunctional group - modified diatomite, deionized water, and LaNO3·6H2O is 2 g: 100 mL: 3.5 - 4.3 g.

[0024] Further, the acidified diatomite is obtained through the following steps:

[0025] The diatomite is sieved through a 100-mesh sieve and then placed in a 30 wt% sulfuric acid solution. At room temperature, it is stirred at a rotation speed of 200 - 300 r / min for 5 - 10 h, then centrifuged. The precipitate is washed with deionized water until the washing liquid is neutral and then dried. Finally, it is calcined in a muffle furnace at 400 °C for 2 h. The dosage ratio of diatomite to sulfuric acid solution is 10 - 20 g : 50 - 200 mL.

[0026] Further, the electrode active material powder is prepared from waste lithium iron phosphate batteries through discharging, disassembling, crushing, roasting, and screening.

[0027] Further, in step (1), the mass ratio of the electrode active material powder to the sodium hydroxide solution is 1 : 3, and the mass fraction of the sodium hydroxide solution is 3%.

[0028] Further, in step (2), the pH of the sulfuric acid solution is 2.5, the mass ratio of the aluminum-removing material to the sulfuric acid solution is 1 : 4, and the dosage of hydrogen peroxide is 150% of the theoretical dosage.

[0029] Further, in step (3), the dosage of the diatomite-based purifying agent is 5 - 10 g / L.

[0030] Further, the specific operation of removing aluminum from the acidic slurry in step (5) is as follows:

[0031] Under nitrogen protection, a pyridinecarboxylic acid compound is added to the acidic slurry, and after reacting at 100 °C for 2 h, solid-liquid separation is carried out to obtain the aluminum-removed iron phosphate solution.

[0032] Further, in step (5), the mass ratio of the iron-containing filter residue, iron powder, water, concentrated sulfuric acid, pyridinecarboxylic acid compound, and hydrogen peroxide is 300 - 400 : 45 - 80 : 2600 - 6100 : 600 - 822 : 4 - 15 : 400 - 600. The mass fraction of the concentrated sulfuric acid is 98%, and the mass fraction of the hydrogen peroxide is 30%.

[0033] Further, the pyridinecarboxylic acid compound in step (5) is 2-pyridinecarboxylic acid and / or 3-phenyl-2-pyridinecarboxylic acid.

[0034] The beneficial effects of the present invention:

[0035] 1. The present invention provides a method for recovering iron phosphate from waste lithium iron phosphate batteries. Firstly, the electrode active material powder is treated with an alkali solution to remove impurity aluminum from the source. Then, a combination of dilute acid and hydrogen peroxide is used for selective lithium extraction to obtain a lithium-rich leachate and an iron-containing filter residue. Aiming at the problem of high contents of fluoride ions and phosphate ions in the lithium-rich leachate after removing aluminum and iron, the self-made diatomite-based purifying agent is added to the liquid after removing iron and aluminum, and a reasonable adsorption pH is controlled. While removing the anion impurities in the liquid after removing iron and aluminum, new impurities are not introduced, and battery-grade lithium carbonate is obtained. Moreover, the diatomite-based purifying agent has the renewable characteristic and can be recycled, which is beneficial to reducing the production cost. Finally, sulfuric acid and iron powder are used to dissolve and treat the iron-containing filter residue, carbon black is recovered, aluminum is removed by pyridinecarboxylic acid compounds, and finally the iron-phosphorus ratio is adjusted to obtain battery-grade iron phosphate. In summary, the recovery method of the present invention has a simple process, can realize the recovery of carbon black, iron phosphate, and lithium carbonate, has a high recovery rate, and the obtained products have high purity.

[0036] 2. The present invention uses natural diatomite with a hierarchical pore structure and high stability as the carrier, and obtains bifunctional group-modified diatomite, namely active amino groups and carboxyl groups, after being modified by coupling agent KH-570 and cysteine in sequence. The carboxyl group serves as the active site of lanthanum, and lanthanum hydroxide is stably loaded in the pores of the diatomite to obtain the diatomite-based purifying agent, overcoming the problems of easy agglomeration, poor adsorption performance, and difficult recovery of lanthanum hydroxide. Moreover, the active amino groups of the diatomite-based purifying agent are protonated under acidic conditions to form NH3 + which can combine with anions such as fluoride ions through the action of electrostatic attraction, greatly improving the removal ability of fluoride ions and phosphate ions. In summary, the diatomite-based purifying agent of the present invention has a high adsorption capacity and selectivity for fluoride ions and phosphate ions, and can achieve the purpose of deep fluoride and phosphorus removal. Specific Embodiments

[0037] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0038] Example 1

[0039] The diatomite-based purifying agent is obtained through the following steps:

[0040] S1. Mix 80 mL of absolute ethanol and 20 mL of deionized water, adjust the pH to 3 with formic acid, add 1 g of coupling agent KH-570, stir for 0.5 h, then add 5 g of acidified diatomite, perform ultrasonic stirring at 40 °C for 1 h, filter, and dry at 50 °C to constant weight to obtain diatomite grafted with unsaturated double bonds;

[0041] S2. Place 6 g of grafted unsaturated double bond diatomaceous earth, 0.01 g of 2,2-dimethoxy-2-phenylacetophenone, and 70 mL of THF into a flask. After stirring for 10 min, add dropwise an aqueous solution of cysteine containing 0.3 g of cysteine. After the addition is complete, stir and react under ultraviolet light irradiation for 20 min. Then, perform suction filtration, wash the filter cake, and dry it to obtain bifunctional group modified diatomaceous earth. The aqueous solution of cysteine is composed of cysteine and deionized water in a ratio of 1 g:10 mL.

[0042] S3. Ultrasonically disperse 2 g of bifunctional group modified diatomaceous earth into 100 mL of deionized water. Then, add 3.5 g of La(NO3)3·6H2O. While stirring at a rotation speed of 500 r / min, add dropwise a 1 mol / L sodium hydroxide solution to adjust the pH to 9. Let it stand at 60 °C for 18 h, then centrifuge. Wash the precipitate twice with deionized water and absolute ethanol respectively, and finally dry it at 60 °C to a constant weight to obtain the diatomaceous earth-based purifying agent.

[0043] The acidified diatomaceous earth is obtained through the following steps:

[0044] Pass the diatomaceous earth through a 100-mesh sieve and place it in a 30 wt% sulfuric acid solution. Stir at room temperature at a rotation speed of 200 r / min for 5 h. Then, centrifuge. Wash the precipitate with deionized water until the washing liquid is neutral and then dry it. Finally, calcine it in a muffle furnace at 400 °C for 2 h. The dosage ratio of diatomaceous earth to sulfuric acid solution is 10 g:50 mL.

[0045] Example 2

[0046] The diatomaceous earth-based purifying agent is obtained through the following steps:

[0047] S1. Mix 100 mL of absolute ethanol and 30 mL of deionized water, and adjust the pH to 4 with formic acid. After adding 2 g of coupling agent KH-570, stir for 1 h and then add 5 g of acidified diatomaceous earth. Ultrasonically stir at 50 °C for 1 h, then filter and dry at 50 °C to a constant weight to obtain grafted unsaturated double bond diatomaceous earth.

[0048] S2. Place 7 g of grafted unsaturated double bond diatomaceous earth, 0.02 g of 2,2-dimethoxy-2-phenylacetophenone, and 100 mL of THF into a flask. After stirring for 30 min, add dropwise an aqueous solution of cysteine containing 0.6 g of cysteine. After the addition is complete, stir and react under ultraviolet light irradiation for 20 min. Then, perform suction filtration, wash the filter cake, and dry it to obtain bifunctional group modified diatomaceous earth. The aqueous solution of cysteine is composed of cysteine and deionized water in a ratio of 1 g:10 mL.

[0049] S3. Ultrasonically disperse 2 g of bifunctional modified diatomite into 100 mL of deionized water, then add 4.3 g of La(NO₃)₃·6H₂O, and dropwise add 1 mol / L sodium hydroxide solution under stirring at a speed of 1000 r / min. Adjust the pH to 10, let it stand at 60 °C for 18 h, centrifuge, wash the precipitate twice with deionized water and anhydrous ethanol respectively, and finally dry it at 60 °C to constant weight to obtain the diatomite-based purifying agent.

[0050] The acidified diatomite is obtained through the following steps:

[0051] Pass the diatomite through a 100-mesh sieve and place it in a 30 wt% sulfuric acid solution. Stir at a speed of 300 r / min at room temperature for 10 h, then centrifuge. Wash the precipitate with deionized water until the washing liquid is neutral and then dry it. Finally, calcine it in a muffle furnace at 400 °C for 2 h. The dosage ratio of diatomite to sulfuric acid solution is 20 g:200 mL.

[0052] Comparative Example 1

[0053] Mix the bifunctional modified diatomite and lanthanum hydroxide in Example 1 according to a mass ratio of 2:1.6 to obtain the diatomite-based purifying agent.

[0054] Comparative Example 2

[0055] Mix the acidified diatomite and lanthanum hydroxide in Example 1 according to a mass ratio of 2:1.6 to obtain the diatomite-based purifying agent.

[0056] Comparative Example 3

[0057] This comparative example is lanthanum hydroxide.

[0058] Test the samples obtained in Examples 1 - 2 and Comparative Examples 1 - 3. The specific method is as follows:

[0059] Weigh 100 g of LiFePO₄ cathode waste powder, place it in 400 g of sulfuric acid solution with a pH of 2.5, add hydrogen peroxide, and leach at 50 °C for 2 h. The dosage of hydrogen peroxide is 150% of the theoretical dosage to obtain the LiSO₄ leaching solution and leaching residue. The contents of various elements in the LiSO₄ leaching solution are shown in Table 1. Add each group of samples to the LiSO₄ leaching solution, with a dosage of 3.0 g / L, at a temperature of 25 °C. After adsorbing for 60 min, take the supernatant to test the contents of fluoride ions and phosphate ions, and calculate the initial adsorption rates of fluoride ions and phosphate ions. Then filter, place the filter residue in 1 mol / L sodium hydroxide solution for desorption for 12 h, separate the solid and liquid, dry at 60 °C to obtain the regenerated sample, and test the adsorption rates of fluoride ions and phosphate ions again according to the above dosage ratio, temperature, and test method, which are recorded as the regenerated adsorption rates of fluoride ions and phosphate ions. The test results are shown in Table 2:

[0060]

[0061]

[0062] It can be seen from the data recorded in Table 2 that, compared with Comparative Example 1, Comparative Example 2 and Comparative Example 3, the diatomite-based purifying agent obtained in Example 1 and Example 2 has higher removal rates of fluoride ions and phosphate radicals, and has good regeneration performance.

[0063] Example 3

[0064] A method for recovering iron phosphate from waste lithium iron phosphate batteries, comprising the following steps:

[0065] (1) Immerse the waste lithium iron phosphate battery in a 5wt% sodium chloride solution until the discharge termination voltage is 1V, disassemble to obtain the battery cell, then perform mechanical force overall crushing, roasting, screen out the electrode active material powder with a particle size less than 0.1mm, and then place it in a 3wt% sodium hydroxide solution 3 times its mass, stir and react at room temperature for 1h, after filtration, the filter residue is rinsed once with deionized water to obtain an aluminum-removed material;

[0066] (2) Place the aluminum-removed material in a sulfuric acid solution with a pH of 2.5, add hydrogen peroxide, leach at 50°C for 2h to obtain a lithium-rich leaching solution and an iron-containing filter residue, the mass ratio of the aluminum-removed material to the sulfuric acid solution is 1:4, and the dosage of hydrogen peroxide is 150% of the theoretical dosage;

[0067] (3) Adjust the pH value of the lithium-rich leaching solution to 3.5 with sodium hydroxide and then perform solid-liquid separation to obtain an iron- and aluminum-removed liquid, then adjust the pH value of the iron- and aluminum-removed liquid to 2.5 with a sulfuric acid solution with a molar concentration of 0.05mol / L, add the diatomite-based purifying agent of Example 1, stir and adsorb at 25°C for 1h, filter, the filtrate is a purified lithium-rich solution, and the filter residue is placed in a 1mol / L sodium hydroxide solution for desorption to obtain a regenerated diatomite-based purifying agent;

[0068] (4) Add sodium hydroxide to the purified lithium-rich solution to adjust the pH to 11, at a temperature of 90°C, add sodium carbonate in an amount 2 times the theoretical amount in excess, stir and react for 1h, after precipitation, centrifugation and filtration, wash with hot water 2 times to obtain battery-grade lithium carbonate;

[0069] (5) Mix the iron-containing filter residue, iron powder, water, and concentrated sulfuric acid, react at 65 °C for 4 h, filter to obtain an acidic slurry. Under nitrogen protection, add 2-pyridinecarboxylic acid to the acidic slurry, react at 100 °C for 2 h, then separate the solid and liquid to obtain an iron-aluminum-removed phosphorus-iron solution. Add hydrogen peroxide to the iron-aluminum-removed phosphorus-iron solution, stir, adjust the pH to 1.8 with ammonia water, react at 100 °C for 4 h, then separate the solid and liquid, wash to obtain battery-grade iron phosphate. Control the mass ratio of the iron-containing filter residue, iron powder, water, concentrated sulfuric acid, 2-pyridinecarboxylic acid, and hydrogen peroxide to be 300:45:2600:600:4:400, the mass fraction of concentrated sulfuric acid is 98%, and the mass fraction of hydrogen peroxide is 30%.

[0070] Through calculation and detection, the lithium recovery rate in the positive electrode sheet of this example is 95.81%, the purity of the obtained lithium carbonate is 99.97%, the iron-phosphorus recovery rate is 97.21%, and the purity of the obtained iron phosphate is 99.55%.

[0071] Example 4

[0072] A method for recovering iron phosphate from waste lithium iron phosphate batteries, comprising the following steps:

[0073] (1) Immerse the waste lithium iron phosphate battery in a 5 wt% sodium chloride solution until the discharge termination voltage is 1 V, disassemble to obtain the battery core, then perform mechanical force overall crushing, roasting, and screen out the electrode active material powder with a particle size less than 0.1 mm. Then place it in a 3 wt% sodium hydroxide solution three times its mass, stir and react at room temperature for 1.5 h. After filtration, the filter residue is rinsed once with deionized water to obtain an aluminum-removed material;

[0074] (2) Place the aluminum-removed material in a sulfuric acid solution with a pH of 2.5, add hydrogen peroxide, and leach at 55 °C for 2 h to obtain a lithium-rich leaching solution and an iron-containing filter residue. The mass ratio of the aluminum-removed material to the sulfuric acid solution is 1:4, and the amount of hydrogen peroxide used is 150% of the theoretical amount;

[0075] (3) Adjust the pH value of the lithium-rich leaching solution to 4.5 with sodium hydroxide, then separate the solid and liquid to obtain an iron-aluminum-removed liquid. Then adjust the pH value of the iron-aluminum-removed liquid to 3 with a sulfuric acid solution with a molar concentration of 0.08 mol / L, add the diatomite-based purifying agent of Example 3, stir and adsorb at 25 °C for 1 - 2 h, filter, the filtrate is a purified lithium-rich solution, and the filter residue is placed in a 1 mol / L sodium hydroxide solution for desorption to obtain a regenerated diatomite-based purifying agent;

[0076] (4) Add sodium hydroxide to the purified lithium-rich solution to adjust the pH to 12, at a temperature of 93 °C, add sodium carbonate in an amount 2 times the theoretical amount, stir and react for 2 h. After precipitation, centrifuge and filter, and wash twice with hot water to obtain battery-grade lithium carbonate;

[0077] (6) Mix the iron-containing filter residue, iron powder, water, and concentrated sulfuric acid, react at 70 °C for 5 h, filter to obtain an acidic slurry. Under nitrogen protection, add 2-pyridinecarboxylic acid to the acidic slurry, react at 100 °C for 2 h, then perform solid-liquid separation to obtain an iron-aluminum-removed phosphoric acid iron solution. Add hydrogen peroxide to the iron-aluminum-removed phosphoric acid iron solution, stir, adjust the pH to 1.8 with ammonia water, react at 100 °C for 4 h, then perform solid-liquid separation and wash to obtain battery-grade iron phosphate. Control the mass ratio of the iron-containing filter residue, iron powder, water, concentrated sulfuric acid, 2-pyridinecarboxylic acid, and hydrogen peroxide to be 350:60:4100:722:9:500, the mass fraction of concentrated sulfuric acid is 98%, and the mass fraction of hydrogen peroxide is 30%.

[0078] Among them, the preparation method of the electrode active material powder is the same as that in Example 3.

[0079] Through calculation and detection, the lithium recovery rate in the positive electrode sheet of this example is 96.20%, the purity of the obtained lithium carbonate is 99.98%, the iron and phosphorus recovery rate is 97.84%, and the purity of the obtained iron phosphate is 99.52%.

[0080] Example 5

[0081] A method for recovering iron phosphate from waste lithium iron phosphate batteries, comprising the following steps:

[0082] (1) Immerse the waste lithium iron phosphate battery in a 5 wt% sodium chloride solution until the discharge termination voltage is 1 V, disassemble to obtain the battery core, then perform mechanical force overall crushing, roasting, and screen out the electrode active material powder with a particle size less than 0.1 mm. Then place it in a 3 wt% sodium hydroxide solution three times its mass, stir and react at room temperature for 2 h. After filtration, the filter residue is rinsed twice with deionized water to obtain an aluminum-removed material;

[0083] (2) Place the aluminum-removed material in a sulfuric acid solution with a pH of 2.5, add hydrogen peroxide, and leach at 60 °C for 2 h to obtain a lithium-rich leaching solution and an iron-containing filter residue. The mass ratio of the aluminum-removed material to the sulfuric acid solution is 1:4, and the amount of hydrogen peroxide used is 150% of the theoretical amount;

[0084] (3) Adjust the pH value of the lithium-rich leaching solution to 5.5 with sodium hydroxide, then perform solid-liquid separation to obtain an iron-aluminum-removed liquid. Then adjust the pH value of the iron-aluminum-removed liquid to 3.5 with a sulfuric acid solution with a molar concentration of 0.1 mol / L, add the diatomite-based purifying agent in Example 3, stir and adsorb at 25 °C for 2 h, filter, the filtrate is a purified lithium-rich solution, and the filter residue is placed in a 1 mol / L sodium hydroxide solution for desorption to obtain a regenerated diatomite-based purifying agent;

[0085] (4) Add sodium hydroxide to the purified lithium-rich solution to adjust the pH to 13, at a temperature of 95 °C, add sodium carbonate in an amount 2 times the theoretical amount, stir and react for 3 h, and wash the precipitate by centrifugation and filtration three times with hot water to obtain battery-grade lithium carbonate;

[0086] (7) Mix the iron-containing filter residue, iron powder, water, and concentrated sulfuric acid, react at 80 °C for 6 h, filter to obtain an acidic slurry. Under nitrogen protection, add 3-phenyl-2-pyridinecarboxylic acid to the acidic slurry, react at 100 °C for 2 h, then perform solid-liquid separation to obtain an iron-aluminum-phosphorus-removed liquid. Add hydrogen peroxide to the iron-aluminum-phosphorus-removed liquid, stir, adjust the pH to 1.8 with ammonia water, react at 100 °C for 4 h, then perform solid-liquid separation and wash to obtain battery-grade iron phosphate. Control the mass ratio of the iron-containing filter residue, iron powder, water, concentrated sulfuric acid, 3-phenyl-2-pyridinecarboxylic acid, and hydrogen peroxide to be 400:80:6100:822:15:600, the mass fraction of concentrated sulfuric acid is 98%, and the mass fraction of hydrogen peroxide is 30%.

[0087] Among them, the preparation method of the electrode active material powder is the same as that in Example 3.

[0088] After calculation and detection, the lithium recovery rate in the positive electrode sheet of this example is 96.34%, the purity of the obtained lithium carbonate is 99.99%, the iron-phosphorus recovery rate is 97.75%, and the purity of the obtained iron phosphate is 99.63%.

[0089] Comparative Example 4

[0090] A method for recovering iron phosphate from waste lithium iron phosphate batteries. Compared with Example 3, replace the diatomite-based purifying agent in Example 3 with the substance in Comparative Example 1, and the other raw materials and processes are the same as those in Example 3.

[0091] After calculation and detection, the lithium recovery rate in the positive electrode sheet of this example is 95.80%, the purity of the obtained lithium carbonate is 99.55%, the iron-phosphorus recovery rate is 97.25%, and the purity of the obtained iron phosphate is 99.52%.

[0092] Comparative Example 5

[0093] A method for recovering iron phosphate from waste lithium iron phosphate batteries. Compared with Example 3, replace the diatomite-based purifying agent in Example 3 with the substance in Comparative Example 2, and the other raw materials and processes are the same as those in Example 3.

[0094] After calculation and detection, the lithium recovery rate in the positive electrode sheet of this example is 95.79%, the purity of the obtained lithium carbonate is 99.32%, the iron-phosphorus recovery rate is 97.23%, and the purity of the obtained iron phosphate is 99.54%.

[0095] Comparative Example 6

[0096] A method for recovering iron phosphate from waste lithium iron phosphate batteries. Compared with Example 3, replace the diatomite-based purifying agent in Example 3 with the substance in Comparative Example 3, and the other raw materials and processes are the same as those in Example 3.

[0097] After calculation and detection, the lithium recovery rate in the positive electrode sheet of this embodiment is 95.82%, the purity of the obtained lithium carbonate is 99.27%, the iron-phosphorus recovery rate is 97.19%, and the purity of the obtained iron phosphate is 99.56%.

[0098] Comparative Example 7

[0099] A method for recovering iron phosphate from waste lithium iron phosphate batteries. Compared with Example 5, in step (3) of Example 5, "adjusting the pH value of the iron and aluminum removal liquid to 3.5 with sulfuric acid with a molar concentration of 0.1 mol / L" is changed to "adjusting the pH value of the iron and aluminum removal liquid to 2.0 with sulfuric acid with a molar concentration of 0.1 mol / L", and the other raw materials and processes are the same as those in Example 5.

[0100] After calculation and detection, the lithium recovery rate in the positive electrode sheet of this comparative example is 96.32%, the purity of the obtained lithium carbonate is 99.74%, the iron-phosphorus recovery rate is 97.72%, and the purity of the obtained iron phosphate is 99.60%.

[0101] Comparative Example 8

[0102] A method for recovering iron phosphate from waste lithium iron phosphate batteries. Compared with Example 5, in step (3) of Example 5, "adjusting the pH value of the iron and aluminum removal liquid to 3.5 with sulfuric acid with a molar concentration of 0.1 mol / L" is changed to "adjusting the pH value of the iron and aluminum removal liquid to 4.0 with sulfuric acid with a molar concentration of 0.1 mol / L", and the other raw materials and processes are the same as those in Example 5.

[0103] After calculation and detection, the lithium recovery rate in the positive electrode sheet of this comparative example is 96.31%, the purity of the obtained lithium carbonate is 99.52%, the iron-phosphorus recovery rate is 97.74%, and the purity of the obtained iron phosphate is 99.61%.

[0104] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device.

[0105] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for recovering iron phosphate from waste lithium iron phosphate batteries, characterized in that, It includes the following steps: (1) Make the waste lithium iron phosphate battery into electrode active material powder, place it in sodium hydroxide solution, stir and react for 1 - 2 h, filter, wash the filter residue to obtain aluminum-removed material; (2) Place the aluminum-removed material in sulfuric acid solution, add hydrogen peroxide for leaching to obtain lithium-rich leaching solution and iron-containing filter residue; (3) Adjust the pH value of the lithium-rich leaching solution to 3.5 - 5.5 with sodium hydroxide and then perform solid-liquid separation to obtain iron- and aluminum-removed liquid. Adjust the pH value of the iron- and aluminum-removed liquid to 3 ± 0.5 with sulfuric acid solution, add diatomite-based purifying agent, stir and adsorb for 1 - 2 h, filter, and the filtrate is purified lithium-rich liquid; Adjust the pH of the purified lithium-rich liquid to 11 - 13, add carbonate at 90 - 95 °C, stir and react, then perform solid-liquid separation to obtain battery-grade lithium carbonate; Mix the iron-containing filter residue, iron powder, water and concentrated sulfuric acid, react at 65 - 80 °C for 4 - 6 h, filter to obtain acidic slurry, remove aluminum from the acidic slurry to obtain aluminum-removed phosphorus iron liquid, add hydrogen peroxide to the aluminum-removed phosphorus iron liquid, adjust the pH and then perform solid-liquid separation, wash to obtain battery-grade iron phosphate; The diatomite-based purifying agent is obtained through the following steps: S1. Mix absolute ethanol and deionized water, adjust the pH to 3 - 4 with formic acid, add coupling agent KH-570, stir for 0.5 - 1 h, then add acidified diatomite, perform ultrasonic stirring at 40 - 50 °C for 1 h, filter, and dry at 50 °C to constant weight to obtain grafted unsaturated double bond diatomite; S2. Place the grafted unsaturated double bond diatomite, 2,2-dimethoxy-2-phenylacetophenone and THF in a flask, stir for 10 - 30 min, then dropwise add an aqueous solution of cysteine. After the dropping is completed, stir and react under ultraviolet lamp irradiation for 20 min, perform suction filtration, wash and dry the filter cake to obtain bifunctional group-modified diatomite; S3. Ultrasonically disperse the bifunctional group-modified diatomite into deionized water, then add La(NO3)3·6H2O, dropwise add 1 mol / L sodium hydroxide solution under stirring conditions, adjust the pH to 9 - 10, stand at 60 °C for 18 h, centrifuge, wash and dry to obtain the diatomite-based purifying agent.

2. The method for recovering iron phosphate from waste lithium iron phosphate batteries according to claim 1, characterized in that, In step S1, the dosage ratio of absolute ethanol, deionized water, coupling agent KH-570 and acidified diatomite is 80 - 100 mL: 20 - 30 mL: 1 - 2 g: 5 g.

3. The method for recovering iron phosphate from waste lithium iron phosphate batteries according to claim 1, characterized in that, In step S2, the dosage ratio of grafted unsaturated double bond diatomite, 2,2-dimethoxy-2-phenylacetophenone, THF and cysteine is 6 - 7 g: 0.01 - 0.02 g: 70 - 100 mL: 0.3 - 0.6 g.

4. The method for recovering iron phosphate from waste lithium iron phosphate batteries according to claim 1, characterized in that, In step S3, the dosage ratio of bifunctional group-modified diatomite, deionized water and LaNO3·6H2O is 2 g: 100 mL: 3.5 - 4.3 g.

5. The method for recovering iron phosphate from waste lithium iron phosphate batteries according to claim 1, characterized in that, In step (1), the mass ratio of the electrode active material powder to the sodium hydroxide solution is 1:3, and the mass fraction of the sodium hydroxide solution is 3%.

6. The method for recovering iron phosphate from waste lithium iron phosphate batteries according to claim 1, characterized in that, In step (2), the pH of the sulfuric acid solution is 2.5, the mass ratio of the aluminum-removed material to the sulfuric acid solution is 1:4, and the dosage of hydrogen peroxide is 150% of the theoretical dosage.

7. The method for recovering iron phosphate from waste lithium iron phosphate batteries according to claim 1, characterized in that, In step (3), the dosage of the diatomite-based purifying agent is 5 - 10 g / L.

8. The method for recovering iron phosphate from waste lithium iron phosphate batteries according to claim 1, characterized in that, In step (5), the specific operation of removing aluminum from the acidic slurry is as follows: Under nitrogen protection, a pyridinecarboxylic acid compound was added to the acidic slurry, and after reacting for 2 h at 100 °C, solid-liquid separation was carried out to obtain an aluminum-removed phosphorus-iron solution.

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