Method for recovering iron phosphate from lithium iron phosphate waste and application thereof

Abstract

The invention relates to a method for recycling iron phosphate from lithium iron phosphate waste and application of the method, and belongs to the technical field of iron phosphate recycling. Comprising the following steps that firstly, the lithium iron phosphate waste, a reducing agent and sodium carbonate are mixed and roasted at the temperature of 820-890 DEG C, the reducing agent reacts with oxygen in the lithium iron phosphate waste, and Fe (II) easier to leach by acid is generated; subsequently, the roasted product is immersed in a mixed acid and ferrite solution, Fe < 2 + > is dissolved in a solution system through the mixed acid, insoluble iron phosphate precipitates are promoted to be formed, and therefore the Li < + > leaching rate of insoluble phosphate minerals in the roasted product can be increased. According to the method, a mixed acid solution is added, the pH value in an acid leaching system is limited to be 1.0-5.0, so that impurity components containing copper, manganese, aluminum, nickel and the like in a roasted product can be dissolved in the mixed acid solution, finally, an iron phosphate-containing solid obtained after solid-liquid separation is calcined, and comprehensive utilization of recovering lithium and iron phosphate from the lithium iron phosphate waste at the same time is achieved.

Description

Technical Field

[0001] The 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. The existing recycling of lithium iron phosphate battery positive electrode materials usually adopts hydrometallurgical processes, by adding organic acid or inorganic acid as a leaching agent, and then using oxidants such as hydrogen peroxide, ammonium persulfate and sodium persulfate to selectively leach valuable metals from the crushed positive electrode materials. Since the inherent value of P and Fe in positive electrode recycling is relatively low, and it is difficult to remove impurities such as Al and Cu in lithium extraction slag, when hydrometallurgical recycling of positive electrode materials is performed, Li with high economic value is often recovered, which results in a large amount of unrecoverable lithium iron phosphate lithium extraction slag produced in the existing lithium iron phosphate recycling system.

[0003] The existing method of recovering iron salts from lithium slag uses too much acid and alkali, resulting in a huge cost for wastewater treatment. At the same time, the treatment of phosphate in wastewater also requires additional costs. These factors greatly limit the industrial recovery of lithium slag, which has a serious impact on the upcoming lithium iron phosphate retirement wave. In view of the shortcomings of lithium iron phosphate positive electrode materials such as low electronic conductivity and low ion diffusion coefficient, the current lithium iron phosphate preparation process line usually dopes Ni, Cu, Mn and other metal elements to improve its electrochemical performance. Therefore, these impurity metals will inevitably be introduced into the recycling of lithium iron phosphate positive electrode materials. At the same time, since some ternary nickel-cobalt-manganese batteries may be entrained in the recycling of lithium iron phosphate batteries, the lithium iron phosphate battery lithium slag contains impurity metals such as Ni, Cu, Mn, and their content is far higher than the 50ppm content requirement of China's 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 no lithium extraction slag is generated. 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 pH = 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: firstly, lithium iron phosphate waste, a reducing agent and sodium carbonate are mixed and roasted, and the roasting is mainly to remove the organic electrolyte and the binder remaining in the electrode material. The reducing agent reacts with the oxygen in the lithium iron phosphate waste to generate FeO which is more easily leached by acid. Moreover, the carbon dioxide decomposed by sodium carbonate during the calcination process will destroy the structure of the lithium iron phosphate waste, increase its porosity and specific surface area, thereby improving the metal leaching efficiency in the subsequent acid leaching process. The sodium carbonate plays a role in promoting the reduction reaction, thereby improving the recovery rate of iron.

[0011] If the roasting temperature is lower than 820°C, Fe and Li are difficult to separate, 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 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 roasted 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- The formation of insoluble iron phosphate precipitates 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, and when the pH rises to the range of 1.0 to 5.0, iron ions will exist in the form of iron phosphate precipitation, and 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, so that the impurities are 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 and enhance its crystallinity, thereby improving the electrochemical performance of the battery. Calcination can also improve the particle size and morphology: Calcination can control the particle size and morphology of iron phosphate, making it have a more uniform particle size distribution and suitable morphology, which helps to improve the charge and discharge performance and cycle stability of the battery.

[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 0.2-0.5MPa, and the leaching time is 2-4h. More preferably, the leaching temperature is 85°C, the pressure is 0.5MPa, and the time is 3h. As the pressure increases, the selective recovery rate of Li gradually increases. When the pressure increases to 0.5MPa, the leaching efficiency of Li is the highest. This is related to the increase in air pressure, which 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-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%. 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 and the roasted 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 in the range of 1.0 to 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 a sodium hydroxide solution is added to the lithium-containing filtrate to obtain a precipitate and a 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, and the reducing agent reacts with oxygen in the lithium iron phosphate waste to reduce the iron element to generate Fe(II) which is easier to be 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 a ferrous salt solution, and the Fe is leached by the mixed acid. 2+ Dissolve into the solution system to 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 first and then acid leaching to reduce the generation of waste liquid. Compared with the treatment method of directly using a large amount of strong acid for acid leaching to recover lithium, it reduces the impact on the environment. 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 It is the 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 in FIG.

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

[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: discharge, disassemble, crush and other pre-treatments of retired lithium iron phosphate battery packs 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, reagents, etc. 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 at 850°C for 5 hours to obtain a calcined product; wherein the reducing agent is glucose, and the mass ratio of lithium iron phosphate waste to the reducing agent is 1:0.5. The mass ratio of lithium iron phosphate waste to sodium carbonate is 1:0.6.

[0058] S2 adds the calcined product to a mixed acid and ferrous salt solution, and uses ammonia water to adjust the pH of the system to 3.0 and leach for 3 hours at 90°C and 0.3MPa to obtain a mixed slurry, and then performs solid-liquid separation on the mixed slurry to obtain a phosphate-containing iron solid and a lithium-containing filtrate; the mixed acid solution is a mixed solution of sulfuric acid and phosphoric acid with a molar ratio of 5:1; the ferrous salt is ferrous sulfate, and the solid-liquid ratio of the ferrous salt solution to the calcined product is 7mL / g; the mass concentration of the ferrous salt solution is 55%. The solid-liquid ratio of the mixed acid solution to the calcined product is 45mL / g. The molar concentration of phosphoric acid in the mixed acid solution is 1.6mol / 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 roasting at 820°C for 6 hours to obtain a roasted product; wherein the reducing agent is charcoal powder, and the mass ratio of lithium iron phosphate waste to the reducing agent is 1:0.3. The mass ratio of lithium iron phosphate waste to sodium carbonate is 1:0.5.

[0064] S2 adds the calcined product to a mixed acid and ferrous salt solution, and uses ammonia water to adjust the pH of the system to 1.0 and leach for 4 hours at 80°C and 0.2MPa to obtain a mixed slurry, and then performs solid-liquid separation on the mixed slurry to obtain a solid containing iron phosphate and a lithium-containing filtrate; the mixed acid solution is a mixed solution of sulfuric acid and phosphoric acid with 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 5mL / 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 10mL / g. The molar concentration of phosphoric acid in the mixed acid solution is 1.5mol / 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 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 lithium iron phosphate waste to the reducing agent is 1:0.7. The mass ratio of lithium iron phosphate waste to sodium carbonate is 1:1.

[0070] S2 adds the calcined product to a mixed acid and ferrous salt solution, and uses ammonia water to adjust the pH of the system to 5.0 and leach for 2 hours at 100°C and 0.5MPa to obtain a mixed slurry, and then performs solid-liquid separation on the mixed slurry to obtain a phosphate-containing solid and a lithium-containing filtrate; the mixed acid solution is a mixed solution of sulfuric acid and phosphoric acid with 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 10mL / 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 50mL / g. The molar concentration of phosphoric acid in the mixed acid solution is 1.5mol / 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 gas 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 gas 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 in step S1 of Comparative Example 5, no sodium carbonate is added.

[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 an 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 iron 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 by ammonium phosphomolybdate volumetric method.

[0096] Among them, 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 ICP-OES method was used to determine the Al, Cu, Ni and Mn element contents of the iron phosphate samples, as shown in Table 1.

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

[0102] ① The iron content of the roasted product and the leached solid in step S2 in Examples 1, 4-5 and Comparative Examples 1-7 was determined by potassium dichromate titration and ammonium phosphomolybdate volumetric method. 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 in step S2, the leaching filtrate and the lithium hydroxide mother liquor in step S4 in Examples 1, 4 to 5 and Comparative Examples 1 to 7 were respectively measured for Li element using the ICP-OES method. 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 Content 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 meets the requirements of HG / T 4701-2014 for iron phosphate products for batteries. Moreover, the yields of phosphorus and iron in the iron phosphate sample are as high as 99%, and the total lithium yield in the lithium hydroxide sample is as high as 93%, indicating that the method for recycling lithium hydrogen phosphate waste of the present invention can effectively recycle 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 related to the fact that the increase in air pressure accelerated the recovery 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 gas pressure of the present invention is 0.2-0.5 MPa, within which a better 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 still exists in the roasted product, and Fe and Li are difficult to separate, 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 generate ferric phosphate, and the iron and phosphorus content are significantly lost. Therefore, the present invention sets the roasting temperature to 820°C to 890°C, within which Fe and Li can be completely separated, which is convenient for subsequent acid leaching and ferrous iron 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, so the performance and yield of iron phosphate are significantly reduced. 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, in order to allow copper, manganese, nickel and aluminum to remain in the solution during the acid leaching process, the iron phosphate is separated in the form of a precipitate, so the present invention limits the system pH to 1.0 to 5.0 during the mixed acid immersion in step S2.

[0117] In comparative example 5, no sodium carbonate is added during the roasting process. Due to the lack of sodium carbonate activation, it is difficult to separate iron and lithium in lithium iron phosphate by carbon thermal reduction, resulting in a significant reduction in the yield of iron and lithium. In comparative example 6, potassium carbonate is used instead of sodium carbonate, and the performance and related yield of the iron phosphate sample are reduced, indicating that the activation performance of sodium carbonate is better. In comparative example 7, phosphoric acid is used instead of hydrochloric acid, and the content of other metal impurities in the iron phosphate sample is significantly increased, indicating that the mixed acid of phosphoric acid and sulfuric acid can improve the solubility of Al, Mn, Cu and Ni, so that they are completely separated from iron phosphate, thereby improving the performance, purity and yield of the iron phosphate sample.

[0118] In summary, the present invention realizes the comprehensive utilization of lithium and iron phosphate recovered from lithium iron phosphate waste, 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%). The iron phosphate recovered by the present invention meets 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 solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution 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; S2: adding the roasted product to a mixed acid and ferrous salt solution, and leaching at pH = 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; S3 calcines the iron phosphate-containing solid to obtain battery-grade iron phosphate.

2. The method for recovering iron phosphate from lithium iron phosphate waste according to claim 1, characterized in that: In the 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 or 2, characterized in that: At least one of the following (I) to (III): (I) the reducing agent is at least one of glucose, graphite powder, coke powder, and charcoal powder; (II) The mass ratio of lithium iron phosphate waste to reducing agent is 1:(0.3-0.7); (III) The mass ratio of lithium iron phosphate waste to sodium carbonate is 1: (0.5-1).

4. The method for recovering iron phosphate from lithium iron phosphate waste according to claim 1, characterized in that: In the 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.

5. The method for recovering iron phosphate from lithium iron phosphate waste according to claim 1 or 4, characterized in that: At least one of the following (I) to (II): (I) the molar ratio of sulfuric acid to phosphoric acid in the mixed acid solution is (3-6):1; (II) the ferrous salt is ferrous sulfate; (III) The solid-to-liquid ratio of the ferrous salt solution and the calcined product is 5-10 mL / g; and the mass concentration of the ferrous salt solution is 50-60%. (IV) The solid-to-liquid ratio of the mixed acid solution to the calcined product is 10 to 50 mL / g.

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

0.

7. The method for recovering iron phosphate from lithium iron phosphate waste according to claim 1, characterized in that: In step S3, the calcination temperature is 500-600° C. and the calcination time is 4-8 hours.

8. The method for recovering iron phosphate from lithium iron phosphate waste according to claim 1, characterized in that: 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.

9. A battery-grade iron phosphate, characterized in that: The method for recovering iron phosphate from lithium iron phosphate waste according to any one of claims 1 to 8 is used to prepare the iron phosphate.

10. Application of battery-grade iron phosphate in the preparation of positive electrode materials for lithium-ion batteries.

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