A method for recovering iron phosphate from lithium iron phosphate waste and its application

Abstract

The present invention relates to a method for recovering iron phosphate from lithium iron phosphate waste and its application, belonging to the technical field of recovering iron phosphate. The method comprises the following steps: firstly, mixing lithium iron phosphate waste, a reducing agent and sodium carbonate and roasting at 820-890°C, wherein the reducing agent reacts with oxygen in the lithium iron phosphate waste to generate Fe(II) which is more easily leached by acid. Subsequently, the roasted product is immersed in a mixed acid and ferrous salt solution, and the mixed acid is used to remove Fe(II). 2+ Dissolved into the solution system, promote the formation of insoluble iron phosphate precipitates, which can increase the Li + Leaching rate. Furthermore, the pH of the acid leaching system is limited to 1.0 to 5.0, so that impurities such as copper, manganese, aluminum, and nickel in the calcined product can all be dissolved in the mixed acid solution. Finally, the iron phosphate-containing solid obtained after solid-liquid separation is calcined. The present invention realizes the comprehensive utilization of lithium and iron phosphate recovered from lithium iron phosphate waste.

Description

Technical Field

[0001] The present invention relates to the technical field of recovering iron phosphate, and in particular to a method for recovering iron phosphate from lithium iron phosphate waste and application thereof. Background Art

[0002] With the rapid development of the new energy industry, the number of retired lithium iron phosphate batteries on the market continues to increase. Existing recycling of lithium iron phosphate battery positive electrode materials typically utilizes hydrometallurgical processes, which selectively leach valuable metals from the crushed positive electrode materials by adding organic or inorganic acids as leaching agents, combined with oxidants such as hydrogen peroxide, ammonium persulfate, and sodium persulfate. Due to the relatively low intrinsic value of P and Fe in positive electrode recycling, and the difficulty in removing impurities such as Al and Cu from lithium extraction slag, hydrometallurgical recycling of positive electrode materials often involves the recovery of Li, which has a high economic value. This results in the production of a large amount of unrecoverable lithium iron phosphate extraction slag in existing lithium iron phosphate recovery systems.

[0003] Existing methods for recovering iron salts from lithium-extraction slag require excessive amounts of acid and alkali, resulting in significant wastewater treatment costs. Furthermore, the treatment of phosphates in the wastewater also requires additional costs. These factors significantly limit the industrial recovery of lithium-extraction slag, which has a serious impact on the upcoming wave of lithium iron phosphate retirements. To address the shortcomings of lithium iron phosphate cathode materials, such as low electronic conductivity and low ion diffusion coefficient, lithium iron phosphate production lines typically dope metal elements such as Ni, Cu, and Mn to improve their electrochemical performance. Therefore, the introduction of these impurity metals into the recycled lithium iron phosphate cathode material is inevitable. Furthermore, since some ternary nickel-cobalt-manganese batteries may be included in the recycled lithium iron phosphate batteries, the lithium iron phosphate battery lithium-extraction slag contains impurity metals such as Ni, Cu, and Mn, and their content is far higher than the 50ppm content requirement of the Chinese battery-grade FePO4 chemical industry standard (HG / T4701-2014). Summary of the Invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for recovering iron phosphate from lithium iron phosphate waste and its application, thereby realizing the comprehensive utilization of lithium and iron phosphate recovered from lithium iron phosphate waste at the same time, and without producing lithium extraction slag. The method has the advantages of mild leaching conditions, high iron and phosphorus leaching rate, low acid and alkali consumption and high product purity.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] In a first aspect, the present invention provides a method for recovering iron phosphate from lithium iron phosphate waste, comprising the following steps:

[0007] S1: mixing lithium iron phosphate waste, a reducing agent and sodium carbonate, and then calcining at 820-890° C. to obtain a calcined product;

[0008] S2: adding the roasted product to a mixed acid and ferrous salt solution, and leaching at a pH of 1.0 to 5.0 to obtain a mixed slurry, and then performing solid-liquid separation on the mixed slurry to obtain a solid containing iron phosphate and a filtrate containing lithium; the mixed acid solution is a mixed solution of sulfuric acid and phosphoric acid;

[0009] S3 calcines the iron phosphate-containing solid to obtain battery-grade iron phosphate.

[0010] The method for recovering iron phosphate of the present invention comprises the following steps: first, mixing lithium iron phosphate waste, a reducing agent, and sodium carbonate and calcining the mixture. The calcination primarily removes residual organic electrolyte and binder from the electrode material. The reducing agent reacts with oxygen in the lithium iron phosphate waste to produce FeO, which is more easily acid-leached. Furthermore, the carbon dioxide decomposed by the sodium carbonate during the calcination process destroys the structure of the lithium iron phosphate waste, increasing its porosity and specific surface area, thereby improving the metal leaching efficiency during the subsequent acid leaching process. The sodium carbonate also promotes the reduction reaction, thereby increasing the iron recovery rate.

[0011] If the calcination temperature is lower than 820°C, Fe and Li are difficult to separate, resulting in the loss of Fe element; if the calcination temperature is higher than 890°C, Fe(II) is reduced to Fe, and the lack of ferrous salt promotes the formation of ferric phosphate, resulting in a significant loss of iron and phosphorus content.

[0012] In addition, when sodium carbonate is calcined at high temperature, it can act as an activator to help decompose the chemical bonds of lithium iron phosphate and achieve carbon thermal reduction. In the specific process, LiFePO4 is reduced to FeO, NaLi2PO4 (insoluble lithium) and LiNa5(PO4)2 (soluble lithium). Among them, the proportion of insoluble lithium is more than 70%, so in order to dissolve the lithium element as much as possible, the calcined product is subsequently immersed in a mixed acid and ferrous salt solution, and the mixed acid is used to reduce Fe 2+ Dissolved into the solution system, in an acidic environment, Fe 2+ Will react with NaLi2PO4 and PO4 in NaLi2PO4 3- Formation of insoluble iron phosphate precipitates, which can increase the Li + Leaching rate. Phosphoric acid is added to the mixed acid, which reacts with oxygen in the air to remove Fe 2+ Oxidized to Fe 3+ The above reaction principle is:

[0013] FeO+2H + →Fe 2+ +H2O;

[0014] Fe 2+ +NaLi2PO4→FePO4↓+2Na ++2Li + ;

[0015] Fe 2+ +LiNa5(PO4)2→FePO4↓+5Na + +2Li + ;

[0016] 4Fe 2+ +4H3PO4+O2→4FePO4↓+2H2O+4H + .

[0017] After roasting, impurities such as Ni, Mn and Cu in LiFePO4 waste exist in the solid form of Ni3(PO4)2 (soluble nickel), Mn3(PO4)2 (insoluble manganese), Cu3(PO4)2, CuO, AlPO4 and NiO. If the pH in the system is less than 1.0, iron generally exists in the phosphoric acid solution in the form of iron ions. When the pH rises to the range of 1.0 to 5.0, iron ions will exist in the form of iron phosphate precipitates. Ferrous phosphate impurities will also exist, but the presence of ferrous ions will also promote the dissolution of Li+ and the formation of ferrous phosphate. When the pH of the system exceeds 5.0, aluminum generally exists in the form of aluminum hydroxide, and the three components of copper, manganese and nickel will also precipitate, which will reduce the purity of the subsequent iron phosphate precipitation. Therefore, setting the pH to 1.0 to 5.0 can keep all impurities such as Al, Cu, Ni, and Mn in the solution. At the same time, the above impurities have good solubility in the mixed acid solution, which allows the impurities to be separated from the iron phosphate solid. The above reaction principle is:

[0018] CuO+2H + →Cu 2+ +H2O;

[0019] NiO+2H + →Ni 2+ +H2O;

[0020] Mn3(PO4)2+H3PO4→3Mn 2+ +2H2PO4 - ;

[0021] Ni3(PO4)2+H3PO4→3Ni 2+ +2H2PO4 - ;

[0022] Cu3(PO4)2+H3PO4→3Cu 2+ +2H2PO4 - ;

[0023] AlPO4+H3PO4→Al 3+ +2H2PO4- .

[0024] Finally, the iron phosphate solid obtained after solid-liquid separation is calcined. The high temperature during the calcination process helps the iron phosphate form a better crystal structure, enhancing its crystallinity, thereby improving the electrochemical performance of the battery. Calcination also improves the particle size and morphology: Calcination can control the particle size and morphology of the iron phosphate, giving it a more uniform particle size distribution and suitable morphology, which helps improve the battery's charge and discharge performance and cycle stability.

[0025] Preferably, in step S1, the calcination time is 4 to 6 hours. More preferably, the calcination temperature is 850° C. and the calcination time is 5 hours.

[0026] Preferably, the reducing agent is at least one of glucose, graphite powder, coke powder, and charcoal powder, more preferably glucose.

[0027] Preferably, the mass ratio of lithium iron phosphate waste to reducing agent is 1:(0.3-0.7).

[0028] Preferably, the mass ratio of lithium iron phosphate waste to sodium carbonate is 1:(0.5-1).

[0029] Preferably, in step S2, the leaching temperature is 80-100°C, the leaching pressure is 0.2-0.5 MPa, and the leaching time is 2-4 hours. More preferably, the leaching temperature is 85°C, the pressure is 0.5 MPa, and the time is 3 hours. As the pressure increases, the selective recovery rate of Li gradually increases. When the pressure increases to 0.5 MPa, the leaching efficiency of Li is the highest. This is because the increase in air pressure accelerates the reaction of O2 and Li in the solution. + However, further increasing the gas pressure had a slight effect on the selective recovery of Li.

[0030] Preferably, the molar ratio of sulfuric acid to phosphoric acid in the mixed acid solution is (3-6):1.

[0031] Preferably, the ferrous salt is ferrous sulfate.

[0032] Preferably, the solid-liquid ratio of the ferrous salt solution to the calcined product is 5-10 mL / g; the mass concentration of the ferrous salt solution is 50-60%. 2+ Therefore, the amount of ferrous salt added later should be as small as possible, which is used to promote the formation of iron phosphate.

[0033] Preferably, the solid-liquid ratio of the mixed acid solution to the calcined product is 10 to 50 mL / g. More preferably, the molar concentration of phosphoric acid in the mixed acid solution is 1.5 to 2 mol / L.

[0034] Preferably, in step S2, ammonia water is used to control the pH value to be between 1.0 and 5.0.

[0035] Preferably, in step S3, the calcination temperature is 500-600° C. and the calcination time is 4-8 hours.

[0036] Preferably, the method further comprises step S4, wherein sodium hydroxide solution is added to the lithium-containing filtrate to obtain a precipitate and lithium hydroxide mother liquor, thereby completing the recycling of lithium. The specific reaction principle is:

[0037] Ni 2+ +2NaOH→Ni(OH)2↓+2Na + ;

[0038] Cu 2+ +2NaOH→Cu(OH)2↓+2Na + ;

[0039] Mn 2+ +2NaOH→Mn(OH)2↓+2Na + ;

[0040] Al 3+ +3NaOH→Al(OH)3↓+3Na + .

[0041] In a second aspect, the present invention provides a battery-grade iron phosphate prepared by the method for recovering iron phosphate from lithium iron phosphate waste.

[0042] In a third aspect, the present invention provides an application of battery-grade iron phosphate in the preparation of positive electrode materials for lithium-ion batteries.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] The method for recovering iron phosphate from lithium iron phosphate waste of the present invention comprises the following steps: firstly, the lithium iron phosphate waste, a reducing agent and sodium carbonate are mixed and roasted at 820-890°C, the reducing agent reacts with oxygen in the lithium iron phosphate waste to reduce the iron element to generate Fe(II) which is more easily leached by acid. In order to dissolve the lithium element as much as possible, the roasted product is subsequently immersed in a mixed acid and ferrous salt solution, and the mixed acid is used to reduce the Fe(II) to 0.05% by weight. 2+ Dissolved into the solution system, promote the formation of insoluble iron phosphate precipitates, which can increase the Li +Leaching rate. Moreover, phosphoric acid is added to the mixed acid, which can further promote the formation of iron phosphate precipitation in the presence of air. Moreover, the pH in the acid leaching system is limited to 1.0-5.0, so that the impurity components such as copper, manganese, aluminum, nickel, etc. in the roasted product can be dissolved in the mixed acid solution. Finally, the iron phosphate solid obtained after solid-liquid separation is calcined. The high temperature during the calcination process helps the iron phosphate to form a better crystal structure and enhance its crystallinity, thereby improving the electrochemical performance of the battery. The present invention adopts a method of calcination followed by acid leaching to reduce the generation of waste liquid, and reduces the impact on the environment compared to the treatment method of directly using a large amount of strong acid for acid leaching to recover lithium. The present invention realizes the comprehensive utilization of lithium and iron phosphate recovered from lithium iron phosphate waste at the same time, and has the advantages of mild leaching conditions, high iron and phosphorus leaching rate (more than 90%), low acid and alkali consumption, and high product purity (more than 90%). BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is a process flow chart of Example 1;

[0046] Figure 2 The XRD spectra of the calcined products of Example 1 and Comparative Example 2 are shown;

[0047] Figure 3 It is the potential-pH diagram of the Fe-P-H2O system;

[0048] Figure 4 This is the system potential-pH diagram of Al-P-H2O.

[0049] Figure 5 This is the XRD spectrum of the iron phosphate of Example 1. DETAILED DESCRIPTION

[0050] In order to better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0051] The sources of ingredients used in the following examples and comparative examples are as follows:

[0052] Lithium iron phosphate waste: The raw material is retired lithium iron phosphate batteries, and its manufacturer is Guangdong Yuexing Renewable Resources Recycling Company. Its processing method is: the retired lithium iron phosphate battery packs are discharged, disassembled, crushed and other pre-processed to obtain diaphragms, copper and aluminum metals, iron lithium powder, graphite powder and shells, which are lithium iron phosphate waste.

[0053] Unless otherwise specified, other materials and reagents used in the examples can be obtained from commercial sources.

[0054] Figures 3-4 The potential-pH diagrams of Fe-P-H2O system and Al-P-H2O system were drawn using HSC6.0 software.

[0055] Example 1

[0056] A method for recovering iron phosphate from lithium iron phosphate waste, such as Figure 1 As shown, the following steps are included:

[0057] S1: mixing lithium iron phosphate waste, a reducing agent, and sodium carbonate, and then calcining the mixture at 850° C. for 5 hours to obtain a calcined product; wherein the reducing agent is glucose, and the mass ratio of the lithium iron phosphate waste to the reducing agent is 1:0.5. The mass ratio of the lithium iron phosphate waste to the sodium carbonate is 1:0.6.

[0058] S2: The calcined product is added to a mixed acid and ferrous salt solution, and the pH of the system is adjusted to 3.0 with aqueous ammonia. The mixture is leached at 90°C and 0.3 MPa for 3 hours to obtain a mixed slurry. The mixed slurry is then subjected to solid-liquid separation to obtain a ferrous phosphate-containing solid and a lithium-containing filtrate. The mixed acid solution is a mixed solution of sulfuric acid and phosphoric acid in a molar ratio of 5:1. The ferrous salt is ferrous sulfate, and the solid-to-liquid ratio of the ferrous salt solution to the calcined product is 7 mL / g. The mass concentration of the ferrous salt solution is 55%. The solid-to-liquid ratio of the mixed acid solution to the calcined product is 45 mL / g. The molar concentration of phosphoric acid in the mixed acid solution is 1.6 mol / L.

[0059] S3 calcines the iron phosphate-containing solid at 550° C. for 7 hours to obtain battery-grade iron phosphate.

[0060] S4: adding a 2 mol / L sodium hydroxide solution with a mass four times that of the filtrate to the lithium-containing filtrate to obtain a precipitate and a lithium hydroxide mother liquor, thereby completing the recovery and utilization of lithium.

[0061] Example 2

[0062] A method for recovering iron phosphate from lithium iron phosphate waste comprises the following steps:

[0063] S1: mixing lithium iron phosphate waste, a reducing agent, and sodium carbonate, and then calcining the mixture at 820° C. for 6 hours to obtain a calcined product; wherein the reducing agent is charcoal powder, and the mass ratio of the lithium iron phosphate waste to the reducing agent is 1:0.3. The mass ratio of the lithium iron phosphate waste to the sodium carbonate is 1:0.5.

[0064] S2: The calcined product is added to a mixed acid and ferrous salt solution, and the pH of the system is adjusted to 1.0 with aqueous ammonia. The mixture is leached at 80°C and 0.2 MPa for 4 hours to obtain a mixed slurry. The mixed slurry is then subjected to solid-liquid separation to obtain a ferrous phosphate-containing solid and a lithium-containing filtrate. The mixed acid solution is a mixed solution of sulfuric acid and phosphoric acid in a molar ratio of 3:1. The ferrous salt is ferrous sulfate, and the solid-liquid ratio of the ferrous salt solution to the calcined product is 5 mL / g. The mass concentration of the ferrous salt solution is 60%. The solid-liquid ratio of the mixed acid solution to the calcined product is 10 mL / g. The molar concentration of phosphoric acid in the mixed acid solution is 1.5 mol / L.

[0065] S3 calcines the iron phosphate-containing solid at 500° C. for 8 hours to obtain battery-grade iron phosphate.

[0066] S4: adding a 2 mol / L sodium hydroxide solution with a mass four times that of the filtrate to the lithium-containing filtrate to obtain a precipitate and a lithium hydroxide mother liquor, thereby completing the recovery and utilization of lithium.

[0067] Example 3

[0068] A method for recovering iron phosphate from lithium iron phosphate waste comprises the following steps:

[0069] S1: mixing lithium iron phosphate waste, a reducing agent, and sodium carbonate, and then calcining the mixture at 890° C. for 4 to 6 hours to obtain a calcined product; wherein the reducing agent is graphite powder, and the mass ratio of the lithium iron phosphate waste to the reducing agent is 1:0.7. The mass ratio of the lithium iron phosphate waste to the sodium carbonate is 1:1.

[0070] S2: The calcined product is added to a mixed acid and ferrous salt solution, and the pH of the system is adjusted to 5.0 with aqueous ammonia. The mixture is leached at 100°C and 0.5 MPa for 2 hours to obtain a mixed slurry. The mixed slurry is then subjected to solid-liquid separation to obtain a ferrous phosphate-containing solid and a lithium-containing filtrate. The mixed acid solution is a mixed solution of sulfuric acid and phosphoric acid in a molar ratio of 6:1. The ferrous salt is ferrous sulfate, and the solid-liquid ratio of the ferrous salt solution to the calcined product is 10 mL / g. The mass concentration of the ferrous salt solution is 50%. The solid-liquid ratio of the mixed acid solution to the calcined product is 50 mL / g. The molar concentration of phosphoric acid in the mixed acid solution is 1.5 mol / L.

[0071] S3 then calcined the iron phosphate-containing solid at 600° C. for 4 h to obtain battery-grade iron phosphate.

[0072] S4: adding a 2 mol / L sodium hydroxide solution with a mass four times that of the filtrate to the lithium-containing filtrate to obtain a precipitate and a lithium hydroxide mother liquor, thereby completing the recovery and utilization of lithium.

[0073] Example 4

[0074] The difference between Example 4 and Example 1 is that the leaching pressure in step S2 of Example 4 is 0.1 MPa.

[0075] Example 5

[0076] The difference between Example 5 and Example 1 is that the leaching pressure in step S2 of Example 5 is 0.6 MPa.

[0077] Comparative Example 1

[0078] The difference between Comparative Example 1 and Example 1 is that the calcination temperature in step S1 of Comparative Example 1 is 810°C.

[0079] Comparative Example 2

[0080] The difference between Comparative Example 2 and Example 1 is that the calcination temperature in step S2 of Comparative Example 2 is 900°C.

[0081] Comparative Example 3

[0082] The difference between Comparative Example 3 and Example 1 is that the pH of the system in step S2 of Comparative Example 3 is 0.5.

[0083] Comparative Example 4

[0084] The difference between Comparative Example 4 and Example 1 is that the pH of the system in step S2 of Comparative Example 4 is 5.5.

[0085] Comparative Example 5

[0086] The difference between Comparative Example 5 and Example 1 is that sodium carbonate is not added in step S1 of Comparative Example 5.

[0087] Comparative Example 6

[0088] The difference between Comparative Example 6 and Example 1 is that potassium carbonate is used instead of sodium carbonate in step S1 of Comparative Example 6.

[0089] Comparative Example 7

[0090] The difference between Comparative Example 7 and Example 1 is that the phosphoric acid in the mixed acid solution is replaced by hydrochloric acid.

[0091] Performance Testing

[0092] 1. The iron phosphate product obtained in Example 1 was subjected to X-ray diffraction test, and the results were as follows: Figure 5 As shown, Figure 5 This is the XRD spectrum of the ferric phosphate product obtained in Example 1. It can be seen that the characteristic peaks of the ferric phosphate product obtained correspond to those of the FePO4 standard, proving that the obtained product is ferric phosphate.

[0093] 2. Determination method of main element content of products

[0094] The main elements of the ferric phosphate samples obtained in Examples 1, 4 to 5 and Comparative Examples 1 to 7 were determined as follows:

[0095] The iron content was determined by potassium dichromate titration and the iron content was determined by ammonium phosphomolybdate volumetric method.

[0096] The iron-phosphorus ratio is calculated according to the following formula:

[0097]

[0098] Where: ω1 is the mass fraction of the iron element determined above, expressed in %;

[0099] ω2——is the mass fraction of phosphorus element determined above, expressed in %.

[0100] The Al, Cu, Ni and Mn element contents of the ferric phosphate samples were determined by ICP-OES method, as shown in Table 1.

[0101] 3. Determination of iron yield, phosphorus yield and lithium yield

[0102] ① The iron content of the calcined product and the leached solid in step S2 of Examples 1, 4-5, and Comparative Examples 1-7 was determined by potassium dichromate titration and ammonium phosphomolybdate volumetric method, respectively. The total iron yield and total phosphorus yield are shown in the following formula:

[0103] Total iron yield:

[0104] Total phosphorus yield:

[0105] ② The calcined product and leaching filtrate in step S2 and the lithium hydroxide mother liquor in step S4 in Examples 1, 4-5, and Comparative Examples 1-7 were each analyzed for Li using ICP-OES. The total lithium yield is shown in the following formula:

[0106] C Li =C Li1 ×C Li2 ×100%. Specific

[0107] See Table 2 for data.

[0108] Table 1 Contents of main elements and iron-phosphorus ratio in each group of ferric phosphate samples

[0109]

[0110]

[0111] Table 2 Element yield data of each group of iron phosphate samples and lithium hydroxide samples

[0112]

[0113] As shown in Tables 1 and 2, the iron phosphate sample of Example 1 complies with the requirements of HG / T 4701-2014 for iron phosphate products for batteries. Furthermore, the yields of phosphorus and iron in the iron phosphate sample are both as high as 99%, and the total lithium yield in the lithium hydroxide sample is as high as 93%. This demonstrates that the present method for recycling lithium hydrogen phosphate waste can effectively recover both iron phosphate and lithium resources.

[0114] The air pressures in the leaching process of Examples 4 and 5 were 0.1 MPa and 0.6 MPa, respectively. As the pressure increased, the selective recovery rate of Li gradually increased. When the air pressure increased to 0.5 MPa, the leaching efficiency of Li was the highest. This is because the increase in air pressure accelerated the reaction of O2 and Li in the solution. + However, further increasing the gas pressure has a slight effect on the selective recovery of Li. Therefore, the preferred leaching pressure of the present invention is 0.2-0.5 MPa, within which a good Li yield can be obtained.

[0115] The calcination temperature of Comparative Example 1 is 810°C, and the calcination temperature of Comparative Example 2 is 900°C. Figure 2 It can be seen that if the roasting temperature is lower than 820 ° C, some LiFePO4 will still exist in the roasted product, making it difficult to separate Fe and Li, resulting in the loss of Fe element; if the roasting temperature is higher than 890 ° C, Fe(II) is reduced to Fe, and the lack of ferrous iron promotes NaLi2PO4 and LiNa5(PO4)2 to form ferric phosphate, resulting in a significant loss of iron and phosphorus content. Therefore, the present invention sets the roasting temperature to 820 ° C to 890 ° C. Within this temperature range, Fe and Li can be completely separated, facilitating subsequent acid leaching and ferrous reduction to form ferric phosphate.

[0116] The pH value of comparative example 3 during the acid leaching process is 0.5, while that of comparative example 4 is 5.5. Figures 3-4 It can be seen that if the pH in the system is less than 1.0, iron generally exists in the phosphoric acid solution in the form of iron ions, which is not conducive to the subsequent formation of iron phosphate precipitation, resulting in a significant decrease in the performance and yield of iron phosphate. When the system pH exceeds 5.0, aluminum generally exists in the form of aluminum hydroxide, and the three components of copper, manganese and nickel will also precipitate, which will reduce the purity of the subsequent iron phosphate precipitation. Therefore, in order to allow copper, manganese, nickel and aluminum to remain in the solution during the acid leaching process and iron phosphate to be separated in the form of precipitation, the present invention limits the system pH to 1.0-5.0 during mixed acid immersion in step S2.

[0117] In Comparative Example 5, no sodium carbonate was added during the calcination process. Due to the lack of sodium carbonate activation, the iron and lithium in the lithium iron phosphate were difficult to separate via carbothermal reduction, resulting in a significant reduction in the iron and lithium yields. In Comparative Example 6, potassium carbonate was used in place of sodium carbonate, resulting in a decrease in the performance and yield of the iron phosphate sample, indicating that sodium carbonate provides better activation performance. In Comparative Example 7, phosphoric acid was used in place of hydrochloric acid, resulting in a significant increase in the content of other metal impurities in the iron phosphate sample, indicating that the mixed acid of phosphoric acid and sulfuric acid can enhance the solubility of Al, Mn, Cu, and Ni, allowing for their complete separation from the iron phosphate, thereby improving the performance, purity, and yield of the iron phosphate sample.

[0118] In summary, the present invention achieves the comprehensive utilization of lithium and iron phosphate from lithium iron phosphate waste, with the advantages of mild leaching conditions, high iron and phosphorus leaching rate (over 90%), low acid and alkali consumption, and high product purity (over 90%). The iron phosphate recovered using this invention complies with the requirements of HG / T 4701-2014 for iron phosphate products for batteries.

[0119] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for recovering iron phosphate from lithium iron phosphate waste, characterized in that: The following steps are involved: S1: mixing lithium iron phosphate waste, a reducing agent, and sodium carbonate, and then calcining at 820-890° C. to obtain a calcined product; wherein the reducing agent is at least one of glucose, graphite powder, coke powder, and charcoal powder; the mass ratio of the lithium iron phosphate waste to the reducing agent is 1:(0.3-0.7); and the mass ratio of the lithium iron phosphate waste to the sodium carbonate is 1:(0.5-1); S2: adding the roasted product to a mixed acid and ferrous salt solution, and leaching at a pH of 1.0 to 5.0 to obtain a mixed slurry, and then performing solid-liquid separation on the mixed slurry to obtain a solid containing iron phosphate and a filtrate containing lithium; wherein the mixed acid is sulfuric acid and phosphoric acid, and the molar ratio of the sulfuric acid to the phosphoric acid is (3 to 6):1; S3 calcines the iron phosphate-containing solid to obtain battery-grade iron phosphate; wherein the calcination temperature is 500-600° C. and the calcination time is 4-8 hours.

2. The method for recovering iron phosphate from lithium iron phosphate waste according to claim 1, wherein: In step S1, the calcination time is 4 to 6 hours.

3. The method for recovering iron phosphate from lithium iron phosphate waste according to claim 1, wherein: In step S2, the leaching temperature is 80-100° C., the leaching pressure is 0.2-0.5 MPa, and the leaching time is 2-4 hours.

4. The method for recovering iron phosphate from lithium iron phosphate waste according to claim 1 or 3, wherein: At least one of the following (I) to (III): (I) the ferrous salt is ferrous sulfate; (II) the solid-to-liquid ratio of the ferrous salt solution to the calcined product is 5-10 mL / g; the mass concentration of the ferrous salt solution is 50-60%; (III) The solid-to-liquid ratio of the mixed acid and the calcined product is 10-50 mL / g.

5. The method for recovering iron phosphate from lithium iron phosphate waste according to claim 1, wherein: In step S2, ammonia water is used to control the pH value to be between 1.0 and 5.

0.

6. The method for recovering iron phosphate from lithium iron phosphate waste according to claim 1, wherein: The method further comprises step S4, adding a sodium hydroxide solution to the lithium-containing filtrate to obtain a precipitate and a lithium hydroxide mother liquor.

Images