A method for processing lithium iron phosphate materials

Abstract

This invention relates to a method for processing lithium iron phosphate material, comprising the following steps: acid leaching the lithium iron phosphate material to be processed to obtain a leaching system; adding metallic iron to the leaching system under a protective atmosphere or vacuum conditions, reacting to adjust the pH value of the leaching system to 2.5-4.0, followed by solid-liquid separation to obtain a slag phase and a liquid phase; and performing nanofiltration on the liquid phase to obtain a permeate containing LiH2PO4 and a permeate containing Fe. 2+ The concentrate. This invention uses a low-consumption, simple, and efficient process to produce high-purity lithium hydrogen phosphate and ferrous salt products. The process has high industrialization feasibility and significant recycling value.

Description

Technical Field

[0001] This invention relates to a method for processing lithium iron phosphate materials, belonging to the field of waste lithium battery recycling. Background Technology

[0002] Global demand and consumption of lithium-ion batteries are continuously growing. Due to the trend towards a circular economy and the limited supply of lithium, a key metal used in cathode production, the resource and environmental value of recycling valuable elements such as lithium and iron from spent lithium-ion batteries is becoming increasingly prominent. Currently, the mainstream technology for industrial-scale recycling of spent lithium iron phosphate batteries is hydrometallurgy. Hydrometallurgy can effectively recover lithium iron phosphate materials and obtain high-purity lithium salts and iron salts through processes such as leaching, impurity removal, and separation. However, it suffers from problems such as high reagent consumption, lengthy process steps, loss of valuable elements, and the generation of large amounts of wastewater and waste residue, leading to secondary pollution.

[0003] Chinese invention patent application CN117819576A discloses a method for recycling waste lithium iron phosphate powder. This process involves oxidative acid leaching, followed by two-stage nanofiltration to recover the lithium iron phosphate material, ultimately producing lithium hydroxide. However, this patent implements nanofiltration in a high-valence salt system, which results in poor separation efficiency. Furthermore, the recovery of lithium as lithium hydroxide requires the addition of alkaline solution to the permeate for dephosphorization and deironization, consuming large amounts of alkaline reagents. This not only makes the separation and purification process lengthy but also leads to problems such as low product purity and ineffective recovery of iron and phosphorus elements.

[0004] Chinese invention patent application CN118223041A discloses a method for preparing lithium hydroxide monohydrate from lithium iron phosphate waste powder. This patent also involves first oxidizing and acid leaching, followed by two-stage nanofiltration, and then using a bipolar membrane electrodialysis device to obtain a lithium hydroxide solution in the alkali chamber and hydrochloric acid and phosphoric acid in the acid chamber. This combines nanofiltration and electrodialysis processes to rapidly separate lithium from lithium iron phosphate and directionally prepare a high-value lithium hydroxide product. However, the equipment used in this patent, such as the bipolar membrane electrodialysis device, is costly and difficult to operate. Furthermore, other valuable elements such as iron and phosphorus in the nanofiltration concentrate are not recovered, leading to secondary pollution from the waste liquid.

[0005] Chinese invention patent application CN118221082A discloses a method for recovering iron and phosphorus from spent lithium iron phosphate batteries. The method involves selectively extracting lithium from lithium iron phosphate powder through heat treatment, then roasting the phosphorus-iron slag with carbon and metal carbonates, followed by water leaching to separate the phosphorus and iron. The leachate is filtered using a modified ceramic membrane to obtain phosphorus-containing compounds, and the water-leached slag is magnetically separated to recover iron. Although this method achieves the recovery of all elements from lithium iron phosphate, it involves pyrometallurgical, hydrometallurgical, magnetic separation, and nanofiltration processes, making the operation cumbersome. Alternating between wet and dry processes results in high energy consumption and low product purity.

[0006] Chinese invention patent application CN114506834A discloses a method for treating waste lithium iron phosphate powder and lithium iron phosphate, comprising the following steps: soaking the waste lithium iron phosphate powder in an alkaline solution, filtering to obtain alkaline-leached black powder, then adding acid for soaking, adjusting the pH of the leachate to 1.5-3.5, then adding iron powder for reaction, separating the solid and liquid to obtain sponge copper and leachate, continuing to remove heavy metals and aluminum, then adding a specific ratio of phosphorus source, iron source and lithium source to react to obtain a slurry, drying to obtain lithium iron phosphate precursor powder, adding a carbon source, and sintering under a high-temperature inert atmosphere to prepare lithium iron phosphate product. This process can simultaneously recover lithium, iron and phosphorus elements from the waste lithium iron phosphate powder; however, the impurity removal process is lengthy and requires alkaline leaching, resulting in high processing costs.

[0007] Chinese invention patent application CN116018710A discloses a method for treating a leachate solution obtained from black material from spent lithium-ion batteries. The method includes setting the pH of the leachate solution to approximately pH 1.2 to 2.5, adding iron powder to induce copper replacement, adding lime after copper replacement, and then adjusting the pH of the leachate solution to approximately pH 6 after adding lime to extract calcium fluoride, titanium hydroxide, aluminum hydroxide, ferric hydroxide, and ferric phosphate. This method also requires adjusting the pH of the leachate solution with alkali to facilitate subsequent iron powder replacement to remove copper. Further pH adjustment is needed to remove impurities such as aluminum, making the impurity removal process lengthy and demanding on operational skills, thus increasing the difficulty of industrial application. Summary of the Invention

[0008] In view of the shortcomings of the prior art, the purpose of this invention is to provide a simple and effective method for processing lithium iron phosphate materials.

[0009] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0010] A method for processing lithium iron phosphate material includes the following steps:

[0011] S1. Acid leaching is performed on the lithium iron phosphate material to be treated to obtain a leaching system;

[0012] S2. Under a protective atmosphere or vacuum, metallic iron is added to the leaching system to react, and the pH value of the leaching system is adjusted to 2.5-4.0. Then, solid-liquid separation is performed to obtain slag phase and liquid phase.

[0013] Alternatively, after solid-liquid separation of the leaching system to obtain a leachate, metallic iron is added to the leachate under a protective atmosphere or vacuum to react and adjust the pH of the leachate to 2.5-4.0, for example, pH 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.7, or 4.0. Solid-liquid separation is then performed to obtain a slag phase and a liquid phase.

[0014] S3. Perform nanofiltration on the liquid phase to obtain a permeate containing LiH2PO4 and a permeate containing Fe. 2+ The concentrated liquid.

[0015] Further, in S1, the lithium iron phosphate material to be treated, acid, and water are mixed to form a slurry, so that the initial liquid-solid ratio of the system is 3-5 ml:1 g, for example, 3 ml:1 g, 3.5 ml:1 g, 4 ml:1 g, 4.5 ml:1 g, or 5 ml:1 g. After leaching, a slurry is obtained.

[0016] Among them, the H provided by the acid + The total amount of acid used is 1-1.5 times the theoretical acid amount, for example, 1.0 times, 1.05 times, 1.1 times, 1.15 times, 1.2 times, 1.25 times, 1.3 times, 1.35 times, 1.4 times, 1.45 times, or 1.5 times. The theoretical acid amount is the sum of twice the molar amount of Li and Fe in the lithium iron phosphate material to be treated (i.e., n(Li) + 2n(Fe)). This ensures sufficient leaching of the lithium iron phosphate material while avoiding excessive acid, which would reduce the efficiency of subsequent impurity removal stages and cause additional acid loss.

[0017] Preferably, the acid is one or more of H2SO4, HCl, and HNO3, with H2SO4 being the most preferred.

[0018] Furthermore, in S1, the acid leaching temperature is 10-100℃, preferably 20-80℃, and even more preferably 25-75℃, for example 10℃, 15℃, 20℃, 25℃, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃, 85℃, 90℃, 95℃, or 99℃;

[0019] Preferably, the acid leaching time is 10-180 min, more preferably 20-150 min, and even more preferably 30-140 min, for example 10 min, 15 min, 20 min, 25 min, 30 min, 40 min, 45 min, 60 min, 75 min, 90 min, 100 min, 120 min, 130 min, 140 min, or 150 min.

[0020] Furthermore, in S2, the metallic iron is at least one of metallic iron powder and metallic iron filings;

[0021] Furthermore, excess metallic iron is recovered from the leaching system or slag phase after pH adjustment by magnetic separation.

[0022] Metallic iron can, on the one hand, consume the acid in the leaching system, adjust the pH value to prepare for nanofiltration, and also precipitate and remove impurities such as aluminum and displace and remove impurities such as copper. On the other hand, it can reduce the small amount of ferric ions generated during the leaching process, allowing iron to be converted into Fe. 2+ It exists stably in the liquid phase, facilitating the subsequent separation of lithium and iron salts in the nanofiltration process to obtain high-purity, Fe-rich lithium salts. 2+ Prepare materials for the concentrate (Fe) 2+ Lithium dihydrogen phosphate (LiH2PO4) is easier to separate via nanofiltration, which also helps optimize the subsequent cooling and crystallization separation of iron and lithium in the concentrate. Furthermore, excess metallic iron in the leaching system can be easily separated and recovered through methods such as magnetic separation. In summary, the single reagent (metallic iron) used in this invention has multiple functions, including precipitation impurity removal, displacement impurity removal, pH adjustment, and reduction. It does not introduce additional impurities and is inexpensive. The impurity removal steps are simple, and the overall process is convenient, which helps to convert iron, phosphorus, and lithium elements into high-purity products and improve the recovery rate of valuable elements.

[0023] Optionally, an electromagnet is installed in the container where metallic iron is added to the leaching system or leachate for reaction. During the reaction, the electromagnet is turned off (i.e., not energized); after the pH value of the leaching system is adjusted to 2.5-4.0, the electromagnet is turned on (i.e., energized), and the reaction system is discharged to the downstream process. Thus, during the impurity removal reaction, the electromagnet is turned off, allowing the iron powder to be fully mixed and reacted with the leaching system or leachate; after the impurity removal reaction is completed, the electromagnet is turned on. At this time, the electromagnet is magnetic and adsorbs the residual iron powder in the reaction system, allowing the material other than the iron powder to be smoothly discharged to the downstream process; when the container is used to remove impurities from the next batch of leaching system or leachate, the electromagnet can be turned off. At this time, the adsorbed iron powder can be reused for impurity removal and pH adjustment, effectively increasing the utilization rate of iron powder and reducing the solid content in the material entering the downstream side, thus reducing the pressure on subsequent processing.

[0024] Furthermore, in S2, the protective atmosphere is one or more of nitrogen and argon.

[0025] Further, in S3, the liquid phase is subjected to a first nanofiltration treatment to obtain concentrated solution I (containing Fe). 2+ The concentrate (containing partially intercepted LiH2PO4) and permeate I (rich in LiH2PO4) are then subjected to a second nanofiltration process to obtain permeate II (rich in LiH2PO4) and concentrate (rich in Fe).2+ (and a very small amount of intercepted LiH2PO4);

[0026] Permeate I and permeate II are mixed to form the permeate;

[0027] Preferably, the liquid phase is diluted to a Fe concentration of <100 g / L, for example, 50 g / L, 60 g / L, 70 g / L, or 80 g / L, and then subjected to a first nanofiltration treatment;

[0028] Preferably, the concentrate I is mixed with water at a mass ratio of 2:3-8, and then subjected to a second nanofiltration treatment. More preferably, the concentrate I is mixed with water at a mass ratio of 2:4-7; specifically, for example, 2:3, 2:4, 2:5, 2:6, 2:7, or 2:8.

[0029] Furthermore, during the first nanofiltration process, the preset pressure of the nanofiltration membrane is controlled to be 0.1-6 MPa, or even more specifically, 0.5-5 MPa; for example, 0.1 MPa, 0.2 MPa, 0.5 MPa, 1.0 MPa, 1.5 MPa, 2.0 MPa, 2.5 MPa, 3.0 MPa, 3.5 MPa, 4.0 MPa, 4.5 MPa, 5.0 MPa, 5.5 MPa, or 6.0 MPa.

[0030] During the second nanofiltration process, the preset pressure of the nanofiltration membrane is controlled to be 0.1-8 MPa, or more specifically, 0.2-6 MPa; for example, 0.1 MPa, 0.2 MPa, 0.5 MPa, 1.0 MPa, 1.5 MPa, 2.0 MPa, 2.5 MPa, 3.0 MPa, 3.5 MPa, 4.0 MPa, 4.5 MPa, 5.0 MPa, 5.5 MPa, 6.0 MPa, 6.5 MPa, 7.0 MPa, 7.5 MPa, or 8.0 MPa.

[0031] Further, after step S3, the concentrated solution is sequentially cooled and crystallized, followed by solid-liquid separation, washing, and drying to obtain a ferrous salt product. Optionally, the crystallization mother liquor is returned to the liquid phase. The iron in the concentrated solution exists in the form of ferrous iron, which facilitates the cooling and crystallization separation of iron and lithium, resulting in a ferrous salt product with higher purity.

[0032] Preferably, the cooling crystallization temperature is -10-10°C, and more preferably -8-8°C; specifically, for example, -10°C, -9°C, -8°C, -7°C, -6°C, -5°C, -4°C, -3°C, -2°C, -1°C, 0°C, 1°C, 2°C, 3°C, 4°C, 5°C, 6°C, 7°C, or 8°C.

[0033] Preferably, the ferrous salt product is FeSO4·7H2O.

[0034] Further, after S3, the permeate is oxidized and then separated into solid and liquid components to obtain a lithium dihydrogen phosphate solution.

[0035] Preferably, an oxidizing gas and / or an oxidant are introduced into the permeate; more preferably, the oxidizing gas includes one or more of air, oxygen, and ozone; even more preferably, the oxidant is hydrogen peroxide.

[0036] Alternatively, the solid-liquid separation method may be filtration separation.

[0037] Further, the lithium dihydrogen phosphate solution is evaporated and crystallized to obtain the lithium dihydrogen phosphate product;

[0038] Preferably, the evaporation and crystallization temperature is 110-180°C, further 130-170°C, and even further 140-160°C; specifically, for example, 110°C, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, or 180°C.

[0039] Preferably, after evaporation and crystallization, the product is washed, dried, and lithium dihydrogen phosphate is obtained.

[0040] In this invention, metallic iron is first used to simplify and remove impurities from the acid leaching reaction system under a protective atmosphere or vacuum. At the same time, the valence state of elements such as iron in the reaction system is controlled. Then, nanofiltration separation is used to separate lithium, phosphorus, and iron, which are enriched in the form of lithium dihydrogen phosphate and ferrous salt solutions, respectively. This can significantly shorten the separation and recovery process, optimize the separation effect and efficiency, effectively reduce the consumption of reagents such as alkali, avoid the generation of high-salt wastewater, reduce the cost of separation and purification, and help to achieve efficient resource recovery.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0042] (1) In this invention, excess acid is neutralized with metallic iron after acid leaching, avoiding the loss of acid-base leaching reagents. At the same time, impurities such as aluminum and copper can be removed, and ferric iron is reduced to ferrous iron, preparing the valence state for subsequent nanofiltration and cooling crystallization separation of lithium phosphorus and iron. In some embodiments, this invention uses only a slightly excess acid solution during acid leaching, and neutralizes a small amount of excess acid with metallic iron after leaching, which more effectively avoids the loss of acid-base reagents and overcomes the process defects of existing oxidative leaching recovery of lithium iron phosphate, which requires the addition of oxidant and alkaline reagents after acid leaching to precipitate iron phosphate, resulting in the consumption of acid-base neutralization and oxidant.

[0043] (2) The present invention uses metallic iron to regulate the pH of the system under a protective atmosphere or vacuum environment, which can not only remove aluminum impurities in the leaching solution, but also remove copper impurities through displacement reaction, achieving multiple effects. At the same time, due to the increase of pH of the leaching solution, non-acidic nanofiltration membrane can be used for nanofiltration treatment, which helps to reduce the lithium interception rate and is more conducive to the separation of lithium, phosphorus and iron. The subsequent simple and easy-to-operate cooling crystallization process can achieve the separation of iron, phosphorus and lithium. The concentrate can recover most of the iron through one crystallization, and the purity of the obtained ferrous salt is above 97%.

[0044] (3) This invention achieves efficient and simple separation and purification of lithium, phosphorus and iron in ferrous salt system through nanofiltration-cooling crystallization. It not only avoids the use of oxidants and alkaline solutions, but also has better lithium-phosphorus and iron separation effect and simple process operation. In addition, lithium and phosphorus are directly recovered as lithium dihydrogen phosphate, which overcomes the defects of existing technologies that recover lithium, iron and phosphorus elements separately, such as lengthy process steps, low product added value and large material flow.

[0045] (4) This invention can produce high-purity lithium hydrogen phosphate and ferrous salt products using a low-consumption, simple, and efficient process route. The process has high industrialization feasibility and significant recycling value. The lithium hydrogen phosphate product obtained by this invention has high purity, reaching battery grade. Attached Figure Description

[0046] Figure 1 This is the XRD pattern of the lithium dihydrogen phosphate product of Example 1 in this invention.

[0047] Figure 2 This is the XRD pattern of the ferrous salt product of Example 1 in this invention.

[0048] Figure 3 This is a digital photograph of the leaching system obtained after leaching in Comparative Example 2 of this invention. Detailed Implementation

[0049] The present invention will be described in detail below with reference to embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of the present invention can be combined with each other. Unless otherwise specified, the relevant percentages refer to mass percentages.

[0050] The main components of the iron phosphate material (raw material) used in the embodiments and comparative examples of this invention are as follows: Li 3.499%, Fe 29.007%, Al 0.195%, P 17.285%, Cu 0.02%, Ca 0.008%, Mg 0.019%, Mn 0.007%.

[0051] Total leaching rate of iron, phosphorus and lithium = Total number of moles of iron, phosphorus and lithium in the leachate / Total number of moles of iron, phosphorus and lithium in the raw material * 100%;

[0052] Iron yield from primary crystallization = (mass of Fe in ferrous sulfate obtained from primary crystallization / mass of Fe in raw material) * 100%;

[0053] Lithium yield = mass of Li in lithium dihydrogen phosphate product / mass of Li in raw material * 100%.

[0054] Example 1

[0055] The method for processing lithium iron phosphate material in this embodiment includes the following steps:

[0056] S1. Mix the lithium iron phosphate material to be treated with acid and water to make the initial liquid-solid ratio of the system 4ml:1g. After leaching at 60℃ for 120min, the leaching system is obtained.

[0057] The acid is H₂SO₄; the H₂ provided by the acid + The total amount is 1.2 times the theoretical amount of acid used;

[0058] S2. Under a nitrogen atmosphere, iron powder is added to the leaching system to react and adjust the pH value of the leaching system to 3.1. Then, solid-liquid separation is performed to obtain slag phase and liquid phase.

[0059] S3. Perform a first nanofiltration treatment on the liquid phase to obtain concentrate I and permeate I; then perform a second nanofiltration treatment on the concentrate I to obtain permeate II and concentrate; mix permeate I and permeate II to obtain permeate;

[0060] The liquid phase is diluted to a Fe concentration of 58.7 g / L before undergoing a first nanofiltration process.

[0061] The concentrated solution I was mixed with water at a mass ratio of 2:5 and then subjected to a second nanofiltration process.

[0062] During the first nanofiltration process, the preset pressure of the nanofiltration membrane is controlled at 3.5 MPa; during the second nanofiltration process, the preset pressure of the nanofiltration membrane is controlled at 3.0 MPa.

[0063] S4. The concentrated solution is cooled and crystallized at 0℃, then filtered, washed, and dried sequentially to obtain the ferrous salt (FeSO4·7H2O) product, the XRD pattern of which is shown below. Figure 2 As shown;

[0064] S5. Oxygen is introduced into the permeate for oxidation treatment to completely oxidize the ferrous iron. Solid-liquid separation is then performed to obtain a lithium dihydrogen phosphate solution. The lithium dihydrogen phosphate solution is then evaporated and crystallized at 150°C, washed, and dried to obtain the lithium dihydrogen phosphate product. Its XRD pattern is shown below. Figure 1 As shown.

[0065] Tests showed that the total leaching rate of phosphorus, iron, and lithium in S1 was 99.47%; the purity of FeSO4·7H2O in the ferrous salt product was 97.71%, and the recovery rate of primary crystallized iron was 60.14%; the purity of the lithium dihydrogen phosphate product was 99.51%, reaching battery grade, and the lithium recovery rate was 96.06%.

[0066] Example 2

[0067] The method for processing lithium iron phosphate material in this embodiment includes the following steps:

[0068] S1. Mix the lithium iron phosphate material to be treated with acid and water to make the initial liquid-solid ratio of the system 5ml:1g, and leach at 50℃ for 180min to obtain the leaching system.

[0069] The acid is H₂SO₄; the H₂ provided by the acid + The total amount is 1.1 times the theoretical amount of acid used;

[0070] S2. Under an argon atmosphere, iron powder is added to the leaching system to react and adjust the pH value of the leaching system to 3.3. Then, solid-liquid separation is performed to obtain slag phase and liquid phase.

[0071] S3. Perform a first nanofiltration treatment on the liquid phase to obtain concentrate I and permeate I; then perform a second nanofiltration treatment on the concentrate I to obtain permeate II and concentrate; mix permeate I and permeate II to obtain permeate;

[0072] The liquid phase is diluted to a Fe concentration of 68.8 g / L before undergoing a first nanofiltration process.

[0073] The concentrated solution I was mixed with water at a mass ratio of 2:7 and then subjected to a second nanofiltration process.

[0074] During the first nanofiltration process, the preset pressure of the nanofiltration membrane is controlled at 4.0 MPa; during the second nanofiltration process, the preset pressure of the nanofiltration membrane is controlled at 3.5 MPa.

[0075] S4. The concentrate is cooled and crystallized at -5℃, then filtered, washed, and dried to obtain the ferrous salt (FeSO4·7H2O) product.

[0076] S5. Oxygen is introduced into the permeate to oxidize the ferrous iron completely, followed by solid-liquid separation to obtain a lithium dihydrogen phosphate solution. The lithium dihydrogen phosphate solution is then evaporated and crystallized at 160°C, washed, and dried to obtain the lithium dihydrogen phosphate product.

[0077] Testing revealed that the total leaching rate of phosphorus, iron, and lithium in S1 was 99.32%; the purity of FeSO4·7H2O in the ferrous salt product was 98.16%, and the recovery rate of primary crystallized iron was 61.7%; the purity of the lithium dihydrogen phosphate product was 99.67%, reaching battery grade, with a lithium recovery rate of 95.21%.

[0078] Example 3

[0079] The method for processing lithium iron phosphate material in this embodiment includes the following steps:

[0080] S1. Mix the lithium iron phosphate material to be treated with acid and water to make the initial liquid-solid ratio of the system 3ml:1g, and leach at 30℃ for 60min to obtain the leaching system.

[0081] The acid is H₂SO₄; the H₂ provided by the acid + The total amount is 1.05 times the theoretical amount of acid used;

[0082] S2. Under a nitrogen atmosphere, iron powder is added to the leaching system to react and adjust the pH value of the leaching system to 3.6. Then, solid-liquid separation is performed to obtain slag phase and liquid phase.

[0083] S3. Perform a first nanofiltration treatment on the liquid phase to obtain concentrate I and permeate I; then perform a second nanofiltration treatment on the concentrate I to obtain permeate II and concentrate; mix permeate I and permeate II to obtain permeate;

[0084] The liquid phase is diluted to a Fe concentration of 73.1 g / L before undergoing a first nanofiltration process.

[0085] The concentrated solution I was mixed with water at a mass ratio of 2:8 and then subjected to a second nanofiltration process.

[0086] During the first nanofiltration process, the preset pressure of the nanofiltration membrane is controlled at 4.5 MPa; during the second nanofiltration process, the preset pressure of the nanofiltration membrane is controlled at 4.0 MPa.

[0087] S4. The concentrate is cooled and crystallized at 5°C, filtered and separated sequentially, washed, and dried to obtain ferrous salt (FeSO4·7H2O) product.

[0088] Oxygen is introduced into the permeate for oxidation treatment, so that the ferrous iron is completely oxidized. Then, solid-liquid separation is performed to obtain lithium dihydrogen phosphate.

[0089] S5. The lithium dihydrogen phosphate solution is evaporated and crystallized at 130°C, washed, and dried to obtain the lithium dihydrogen phosphate product.

[0090] Tests showed that the total leaching rate of phosphorus, iron, and lithium in S1 was 99.04%; the purity of FeSO4·7H2O in the ferrous salt product was 98.54%, and the recovery rate of primary crystallized iron was 58.97%; the purity of the lithium dihydrogen phosphate product was 99.32%, and the lithium recovery rate was 96.03%.

[0091] Example 4

[0092] The method for processing lithium iron phosphate material in this embodiment includes the following steps:

[0093] S1. Mix the lithium iron phosphate material to be treated with acid and water to make the initial liquid-solid ratio of the system 5ml:1g, and leach at 90℃ for 30min to obtain the leaching system.

[0094] The acid is H₂SO₄; the H₂ provided by the acid + The total amount is 1.25 times the theoretical amount of acid used;

[0095] S2. Under a nitrogen atmosphere, iron filings are added to the leaching system to react and adjust the pH value of the leaching system to 3.0. Then, solid-liquid separation is performed to obtain slag phase and liquid phase.

[0096] S3. Perform a first nanofiltration treatment on the liquid phase to obtain concentrate I and permeate I; then perform a second nanofiltration treatment on the concentrate I to obtain permeate II and concentrate; mix permeate I and permeate II to obtain permeate;

[0097] The liquid phase is diluted to a Fe concentration of 54.4 g / L before undergoing a first nanofiltration process.

[0098] The concentrated solution I was mixed with water at a mass ratio of 2:6 and then subjected to a second nanofiltration process.

[0099] During the first nanofiltration process, the preset pressure of the nanofiltration membrane is controlled at 3.0 MPa; during the second nanofiltration process, the preset pressure of the nanofiltration membrane is controlled at 3.0 MPa.

[0100] S4. The concentrate is cooled and crystallized at -5℃, filtered and separated sequentially, washed, and dried to obtain ferrous salt (FeSO4·7H2O) product.

[0101] Oxygen is introduced into the permeate for oxidation treatment so that the ferrous iron is completely oxidized. After solid-liquid separation, a lithium dihydrogen phosphate solution is obtained.

[0102] S5. The lithium dihydrogen phosphate solution is evaporated and crystallized at 150°C, washed, and dried to obtain the lithium dihydrogen phosphate product.

[0103] Tests showed that the total leaching rate of phosphorus, iron, and lithium in S1 was 99.68%; the purity of FeSO4·7H2O in the ferrous salt product was 97.24%, and the recovery rate of primary crystallized iron was 57.42%; the purity of the lithium dihydrogen phosphate product was 99.74%, reaching battery grade, and the lithium recovery rate was 94.76%.

[0104] Comparative Example 1

[0105] Example 1 was repeated, except that in S2, hydrogen peroxide was used instead of metallic iron, and the final pH of the leaching system was 0.21; the amount of hydrogen peroxide added was 10 ml: 100 mL in ratio to the initial volume of the liquid phase in the leaching system of S1; and the concentration of hydrogen peroxide was 30%. In S3, an acid-resistant nanofiltration membrane was used for the first nanofiltration treatment and the second nanofiltration treatment.

[0106] As a result, the concentrate could not be separated from ferric iron by cooling and crystallization; after the permeate was treated with alkali to remove aluminum and iron, it was then evaporated and crystallized, and the purity of the resulting lithium dihydrogen phosphate product was only 95.29%, which was significantly lower than the battery grade requirement, and the lithium yield was 87.71%.

[0107] It is evident that the leaching solution obtained after acid leaching and oxidation is a highly acidic leachate. Since ferric iron and ferric aluminum have similar properties, it is difficult to separate and remove impurities from iron and aluminum, and there is no effect on removing impurities from copper. The highly acidic leachate also increases the acid resistance requirements of the nanofiltration membrane, and only acid-resistant nanofiltration membranes can be used. At the same time, the lithium interception rate increases, leading to a decrease in lithium yield.

[0108] Comparative Example 2

[0109] Example 1 is repeated, except that S2 is carried out in an atmospheric atmosphere.

[0110] See results Figure 3 The leaching system produces Fe 3+ Fe 2+ With PO4 3 The co-precipitation phenomenon, which appears grayish-green, is likely due to the oxidation of ferrous iron upon contact with air without a protective atmosphere, resulting in the significant precipitation and loss of iron and phosphorus, and an increased loss of Li inclusions. Ultimately, the leaching rate of Fe was only 41.32%, the leaching rate of phosphorus was 38.39%, and the leaching rate of Li was only 95.12%.

[0111] Comparative Example 3

[0112] Repeat Example 1, except that in S2, sodium hydroxide is used instead of iron powder, and the amount of sodium hydroxide added is such that the pH value of the leaching system is adjusted to 3.1.

[0113] Testing revealed that the purity of FeSO4·7H2O in the ferrous salt product was 97.13%, and the recovery rate of primary crystallized iron was 58.55%. The purity of the lithium dihydrogen phosphate product was 92.47%, and the lithium recovery rate was 95.88%. The possible reason is that sodium impurities were introduced, and sodium ions can pass through the nanofiltration membrane, affecting the purity of the finished lithium dihydrogen phosphate product.

[0114] Comparative Example 4

[0115] Example 1 is repeated, except that: in S2, the amount of metallic iron added is controlled so that the pH value of the leaching system is adjusted to 1.6, and then solid-liquid separation is performed to obtain slag phase and liquid phase; in S3, acid-resistant nanofiltration membrane is used for first nanofiltration treatment and second nanofiltration treatment.

[0116] Testing revealed that the purity of FeSO4·7H2O in the ferrous salt product was 96.82%, and the recovery rate of primary crystallized iron was 59.94%; the purity of the lithium dihydrogen phosphate product was 96.82%, and the lithium recovery rate was 84.43%. Possible reasons include: the pH value controlled by S2 was too low, resulting in incomplete removal of elements such as aluminum, affecting product purity; and the requirement to use an acid-resistant nanofiltration membrane increased the lithium interception rate, thus affecting the lithium recovery rate.

[0117] The above embodiments should be understood as being used only to illustrate the present invention more clearly, and not to limit the scope of the present invention. After reading the present invention, any modifications of the present invention in various equivalent forms by those skilled in the art fall within the scope defined by the appended claims.

Claims

1. A method for processing lithium iron phosphate material, characterized in that, Includes the following steps: S1. Acid leaching is performed on the lithium iron phosphate material to be treated to obtain a leaching system; S2. Under a protective atmosphere or vacuum, metallic iron is added to the leaching system to react, and the pH value of the leaching system is adjusted to 2.5-4.

0. Then, solid-liquid separation is performed to obtain slag phase and liquid phase. Alternatively, after solid-liquid separation of the leaching system, a leachate is obtained; then, under a protective atmosphere or vacuum, metallic iron is added to the leachate to react and adjust the pH of the leachate to 2.5-4.0, followed by solid-liquid separation to obtain a slag phase and a liquid phase. S3. Perform nanofiltration on the liquid phase to obtain a permeate containing LiH2PO4 and a permeate containing Fe. 2+ The concentrated liquid.

2. The processing method according to claim 1, characterized in that, In S1, the lithium iron phosphate material to be treated, acid, and water are mixed to form a slurry, so that the initial liquid-to-solid ratio of the system is 3-5 ml: 1 g. After leaching, a slurry is obtained. Among them, the H provided by the acid + The total amount is 1-1.5 times the theoretical amount of acid used; Preferably, the acid is one or more of H2SO4, HCl, and HNO3.

3. The processing method according to claim 1, characterized in that, In S1, the acid leaching temperature is 10-100℃; And / or, the pickling time is 10-180 min.

4. The processing method according to claim 1, characterized in that, In S2, the metallic iron is at least one of metallic iron powder and metallic iron filings; And / or, recover excess metallic iron from the leaching system or slag phase after pH adjustment by magnetic separation.

5. The processing method according to claim 1, characterized in that, In S2, the protective atmosphere is one or more of nitrogen and argon.

6. The processing method according to any one of claims 1-5, characterized in that, In step S3, the liquid phase is subjected to a first nanofiltration treatment to obtain concentrated liquid I and permeate I; then, concentrated liquid I is subjected to a second nanofiltration treatment to obtain permeate II and concentrated liquid. Permeate I and permeate II are mixed to form the permeate; Preferably, the liquid phase is diluted to a Fe concentration of <100 g / L before undergoing the first nanofiltration treatment; Preferably, the concentrate I is mixed with water at a mass ratio of 2:3-8 and then subjected to a second nanofiltration treatment.

7. The processing method according to claim 6, characterized in that, During the first nanofiltration process, the preset pressure of the nanofiltration membrane is controlled to be 0.1-6 MPa; When performing the second nanofiltration process, the preset pressure of the nanofiltration membrane is controlled to be 0.1-8 MPa.

8. The processing method according to any one of claims 1-5, characterized in that, After S3, the concentrate is subjected to cooling crystallization, solid-liquid separation, washing, and drying to obtain the ferrous salt product. Preferably, the cooling crystallization temperature is -10 to 10°C; Preferably, the ferrous salt product is FeSO4·7H2O.

9. The processing method according to any one of claims 1-5, characterized in that, After S3, the permeate is oxidized and then separated into solid and liquid components to obtain a lithium dihydrogen phosphate solution. Preferably, an oxidizing gas and / or an oxidant are introduced into the permeate; more preferably, the oxidizing gas includes one or more of air, oxygen, and ozone; even more preferably, the oxidant is hydrogen peroxide.

10. The processing method according to claim 9, characterized in that, The lithium dihydrogen phosphate solution is evaporated and crystallized to obtain the lithium dihydrogen phosphate product; preferably, the evaporation and crystallization temperature is 110-180℃.

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