Method for preparing lithium phosphate and sodium pyrophosphate from waste lithium iron phosphate material

Abstract

The application provides a method for preparing lithium phosphate and sodium pyrophosphate from waste lithium iron phosphate material. The method comprises the following steps: mixing waste lithium iron phosphate positive electrode powder with sodium dihydrogen phosphate, and performing roasting under an oxygen atmosphere, so that lithium is converted into a soluble lithium salt, and iron is converted into water-insoluble sodium pyrophosphate iron; performing water immersion and filtration on the obtained roasting product, to obtain a lithium-containing leaching solution and a leaching residue; performing concentration on the obtained lithium-containing leaching solution, then adding a phosphorus source (sodium phosphate), and adjusting the pH to 9.5-10.5, to precipitate a lithium phosphate product. Through the sodium dihydrogen phosphate auxiliary roasting-water immersion process, the application realizes efficient and selective extraction of lithium and high-purity enrichment of iron, and the whole process does not need to add additional acid and alkali, is short in process, and is environmentally friendly.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery resource recycling technology, specifically to a method for preparing lithium phosphate and sodium iron pyrophosphate from waste lithium iron phosphate materials. This method can selectively extract lithium and simultaneously enrich iron from waste lithium iron phosphate (LiFePO4, LFP) cathode materials. Background Technology

[0002] With the rapid development of the new energy vehicle industry, a large number of lithium iron phosphate (LFP) power batteries are reaching the end of their service life. The cathode materials of these spent LFP batteries contain valuable elements such as lithium, iron, and phosphorus; therefore, their efficient recycling is of great significance for resource recycling and environmental protection.

[0003] Currently, the main methods for recycling waste LFP materials are divided into hydrometallurgical and pyrometallurgical / direct regeneration methods. Hydrometallurgical recycling typically uses inorganic acids (such as sulfuric acid and hydrochloric acid) to leach valuable metal elements from the cathode powder, followed by separation and purification through chemical precipitation, solvent extraction, and other methods. While this method is technically mature, it involves a long process, high reagent consumption, and generates large amounts of saline wastewater, resulting in high treatment costs and a heavy environmental burden. Pyrometallurgical recycling or direct regeneration usually requires additional lithium and iron sources for high-temperature regeneration, but it demands high consistency of raw materials and struggles to handle waste with high impurity content.

[0004] In existing technologies, the recovery of lithium from LFP often faces a key challenge: lithium in LiFePO4 is extremely difficult to dissolve during traditional water leaching processes, while strong acid leaching leads to the co-dissolution of iron and phosphorus, increasing the difficulty and cost of subsequent lithium-iron separation. Therefore, developing a short-process, low-cost, and environmentally friendly recovery method that can selectively extract lithium and enrich iron in a high-value form in the slag has become an urgent problem to be solved in this field. Summary of the Invention

[0005] In view of this, the purpose of this invention is to overcome at least one of the aforementioned shortcomings of the prior art. For example, one objective of this invention is to achieve efficient and selective extraction of lithium and high-purity enrichment of iron; another objective is to achieve recycling without adding additional acids or alkalis, with a short process and environmental friendliness.

[0006] To achieve the above objectives, the present invention provides a method for preparing lithium phosphate and sodium iron pyrophosphate from waste lithium iron phosphate materials.

[0007] The method includes the following steps:

[0008] S1, roasting and conversion.

[0009] Waste lithium iron phosphate cathode powder is mixed with sodium dihydrogen phosphate in a uniform ratio and then calcined in an oxygen atmosphere to convert lithium in LiFePO4 into soluble lithium salt, while iron is converted into water-insoluble sodium iron pyrophosphate (NaFeP2O7).

[0010] The reaction principle equation for the roasting process in this application is:

[0011]

[0012] Furthermore, the optimal mass ratio of waste lithium iron phosphate cathode powder to sodium dihydrogen phosphate is 1:2.5. This ratio is crucial to ensure the complete conversion of lithium into soluble lithium salts (LiNa5(PO4)2 and Li2NaPO4). Sodium dihydrogen phosphate serves as both a phosphorus and sodium source, participating in the formation of sodium iron pyrophosphate (NaFeP2O7). When the ratio is below 1:1, insufficient phosphate ions lead to a decrease in lithium extraction rate; when the ratio is in the range of 1:1.5 to 2.5, the lithium conversion efficiency is high and the byproducts are few; a ratio exceeding 2.5 will result in excessive sodium salt, increasing the sodium ion concentration in the subsequent water leaching solution, interfering with subsequent lithium precipitation, and reducing economic efficiency.

[0013] Furthermore, the roasting temperature is 300~700℃, and the time is 2~5 hours. Even further, the roasting temperature is 400~700℃.

[0014] The roasting temperature directly affects the decomposition and conversion kinetics of LiFePO4. Below 400℃, the conversion is incomplete; preferably, the roasting temperature can be 600℃. At around 600℃, LiFePO4 can be efficiently converted into soluble lithium salts (LiNa5(PO4)2 and Li2NaPO4) and insoluble residue NaFeP2O7, and NaFeP2O7 crystallizes well, facilitating subsequent water leaching separation. When the roasting temperature exceeds 700℃, it may lead to sintering and particle agglomeration of the material, affecting the product structure and leaching effect.

[0015] The calcination time can be 2 to 5 hours to ensure that the reaction proceeds fully. If the time is too short, the conversion will be incomplete; if the time is too long, energy consumption will increase and may cause excessive grain growth, affecting leaching kinetics.

[0016] Furthermore, the calcination conditions are a pure oxygen atmosphere with a flow rate of 50–200 mL / min. This flow rate is recommended based on an oxygen supply ratio of approximately 0.1–0.3 L / min per gram of raw material and can be adjusted appropriately according to the reactor size.

[0017] S2, Lithium extraction by water leaching.

[0018] The calcined product obtained in step S1 is subjected to water leaching treatment, and after filtration, a lithium-ion-rich leachate and an iron-containing leachate residue are obtained.

[0019] Furthermore, the solid-liquid ratio for water leaching is 10–250 g / L. While a lower solid-liquid ratio is beneficial for mass transfer, it results in lower lithium concentration and higher energy consumption for subsequent concentration. Conversely, a higher solid-liquid ratio leads to higher viscosity, making stirring difficult and causing uneven leaching. The optimal solid-liquid ratio for water leaching is 50 g / L.

[0020] Furthermore, the water immersion temperature is between 30 and 70°C, as temperature affects dissolution kinetics and solubility. Too low a temperature results in a slow reaction; too high a temperature increases energy consumption and may cause excessively rapid water evaporation. The optimal water immersion temperature is 40°C, at which point the lithium salt dissolves rapidly and the leaching time is short.

[0021] The immersion time should be 10–60 minutes to ensure complete lithium dissolution; excessively long immersion times do not improve efficiency. The optimal immersion time is 15 minutes, at which point leaching essentially reaches equilibrium.

[0022] S3, lithium precipitation.

[0023] The lithium-containing leachate obtained from S2 was concentrated, and then a phosphorus source was added and the pH was adjusted to 9.5–10.5 to obtain the Li3PO4 product.

[0024] Because the lithium concentration in the leachate is usually low (especially when coexisting with iron, phosphate, etc.), direct lithium precipitation is inefficient and consumes a lot of reagents. The lithium-containing leachate obtained in step S2 is concentrated. After concentration, Li... + Increasing the concentration improves subsequent precipitation. Adding a phosphorus source and adjusting the solution pH to 9.5-10.5 is optimal for lithium phosphate precipitation. Below pH 9.5, precipitation is incomplete; above pH 10.5, other impurities may co-precipitate, affecting product purity. The precipitated lithium phosphate product is then obtained.

[0025] Furthermore, the concentration is achieved through heating and evaporation. After concentration, the lithium ion concentration in the leachate is not less than 5 g / L. Increasing the lithium concentration promotes the precipitation reaction towards the formation of lithium phosphate, reduces reagent consumption, and improves precipitation efficiency. If the concentration is too low, precipitation is slow, and the product particles are small and difficult to filter.

[0026] Furthermore, the phosphorus source includes sodium phosphate, and the molar ratio of sodium phosphate to lithium ions in the concentrated leachate is (0.32-0.35):1, which is close to the stoichiometric ratio of lithium phosphate (Li3PO4) (P:Li ≈ 1:3), slightly in excess to ensure complete precipitation of lithium, while avoiding the introduction of excessive impurities by phosphate ions.

[0027] Furthermore, the precipitation is achieved through a precipitation reaction under stirring conditions. The reaction temperature is 40–80°C, and the reaction time is 0.1–2 hours. 40–80°C promotes the precipitation reaction rate and crystal growth, facilitating the formation of easily filterable lithium phosphate particles. Too low a temperature results in a slow reaction; too high a temperature may cause localized overheating or moisture evaporation. A stirring time of 0.1–2 hours ensures the precipitation reaction is fully completed. Too short a time results in incomplete precipitation; too long a time does not improve efficiency. The optimal stirring time is 1 hour.

[0028] S4, Phosphorus iron recovery.

[0029] The leaching residue obtained in step S2 is mainly sodium iron pyrophosphate (NaFeP2O7), which can be directly used as an iron and phosphorus source for recycling.

[0030] Compared with the prior art, the beneficial effects of the present invention include:

[0031] (1) This invention achieves efficient conversion and leaching of lithium (leaching rate greater than 99%) through unique NaH2PO4-assisted oxidation roasting, while iron is almost completely not leached (leaching rate less than 0.3%), thus solving the problem of lithium-iron separation from the source.

[0032] (2) The present invention can obtain high-purity Li3PO4 and NaFeP2O7 products in only three steps: roasting-water leaching-precipitation, eliminating complex separation steps such as solvent extraction and ion exchange, greatly shortening the process flow and reducing investment and operating costs.

[0033] (3) The entire recycling process of this invention does not require the use of any strong acid, strong alkali or other harmful chemical reagents, thus avoiding the generation of a large amount of waste acid and wastewater and reducing the environmental burden.

[0034] (4) This invention achieves the complete recovery of the three main elements, lithium, iron and phosphorus. The leaching residue has a single composition (NaFeP2O7) and high purity, and can be directly used as a raw material for preparing other iron-based cathode materials, thus realizing a closed-loop cycle of elements. Attached Figure Description

[0035] The above and other objects and / or features of the present invention will become clearer from the following description taken in conjunction with the accompanying drawings, in which:

[0036] Figure 1 This is a process flow diagram of the recovery of waste lithium iron phosphate by sodium dihydrogen phosphate-assisted roasting in this invention.

[0037] Figure 2 The effect of the mass ratio of sodium dihydrogen phosphate to waste lithium iron phosphate on the leaching rates of lithium and iron.

[0038] Figure 3The X-ray diffraction (XRD) spectra of the calcined product before and after water immersion under optimal conditions are shown.

[0039] Figure 4 The effect of calcination temperature on the leaching rates of lithium and iron.

[0040] Figure 5 The effect of different roasting times on lithium and iron leaching rates.

[0041] Figure 6 XRD patterns of calcined products at different calcination times.

[0042] Figure 7 The effect of the solid-liquid ratio in water immersion on the leaching rates of lithium and iron.

[0043] Figure 8 The effect of immersion time on the leaching rates of lithium and iron.

[0044] Figure 9 The effect of water immersion temperature on the leaching rates of lithium and iron.

[0045] Figure 10 The XRD pattern of the recovered lithium phosphate (Li3PO4).

[0046] Figure 11 SEM image of the recovered lithium phosphate (Li3PO4). Detailed Implementation

[0047] In the following, the method for preparing lithium phosphate and sodium iron pyrophosphate from waste lithium iron phosphate materials according to the present invention will be described in detail with reference to exemplary embodiments.

[0048] To address the problems in the prior art, this invention provides a method for selectively recovering lithium and iron from spent lithium iron phosphate batteries, specifically a method for preparing lithium phosphate and sodium iron pyrophosphate from spent lithium iron phosphate materials. Figure 1 A process flow diagram of the present invention is shown, as follows: Figure 1The method, as shown, includes: uniformly mixing the recovered lithium iron phosphate cathode powder with sodium dihydrogen phosphate, and calcining it in a tube furnace under an oxygen atmosphere to convert the insoluble lithium in LiFePO4 into soluble lithium salts (LiNa5(PO4)2 and Li2NaPO4); subsequently, water leaching is performed to transfer lithium to the filtrate, while iron is entirely enriched in the leaching residue as sodium iron pyrophosphate (NaFeP2O7), achieving efficient separation of iron and lithium. After heating and concentrating the lithium-containing filtrate, a phosphorus source (Na3PO4) and a pH adjuster are added to precipitate Li3PO4. Specifically, as an example, the method includes: first, mixing the cathode powder and sodium dihydrogen phosphate at a mass ratio of 1:2.5, and calcining it at 600°C under an oxygen atmosphere for 4 hours to convert lithium into soluble lithium salts (Li2NaPO4 and LiNa5(PO4)2), while iron is converted into water-insoluble sodium iron pyrophosphate (NaFeP2O7) solid. Subsequently, the calcined product was leached in water (solid-liquid ratio 50 g / L, leaching at 40°C for 15 minutes), and after filtration, a lithium-containing leachate and an iron-containing leachate residue were obtained. The leachate was then concentrated to a lithium concentration of not less than 5 g / L, sodium phosphate was added as a phosphorus source, and the pH was adjusted to 9.5–10.5. The mixture was stirred at 80°C for 1 hour to precipitate a high-purity lithium phosphate product. Finally, the NaFeP₂O₇ in the water-leached residue could be directly recovered as an iron and phosphorus source.

[0049] Clearly, this invention achieves highly efficient lithium leaching (leaching rate > 99%) through a sodium dihydrogen phosphate-assisted roasting-water leaching process, while iron is almost completely retained in the slag (leaching rate < 0.3%). The leaching slag is pure, containing only NaFeP2O7. Furthermore, the entire process requires no addition of acid or alkali reagents, has a short process path, produces few byproducts, is environmentally friendly, and can achieve full recovery of lithium, iron, and phosphorus, demonstrating promising application prospects.

[0050] Example 1

[0051] (1) Weigh out 6 portions of 1g waste lithium iron phosphate cathode powder and mix them evenly with analytical grade sodium dihydrogen phosphate (NaH2PO4) in the following different mass ratios (1:1, 1:1.5, 1:2, 1:2.5, 1:3, 1:3.5).

[0052] (2) Place the mixed materials into a corundum boat, put it into a tube furnace, heat it to 600°C at a rate of 5°C / min under an oxygen atmosphere, and keep it at that temperature for 4 hours. After completion, cool it to room temperature with the furnace.

[0053] (3) Add each portion of roasted product to deionized water at a solid-liquid ratio of 50:1 (g / L), stir and leach at 40°C for 15 minutes, and then filter to separate the leachate and leach residue.

[0054] (4) The lithium and iron content in each leachate was analyzed using an atomic absorption spectrophotometer (AAS), and the leaching rate was calculated. The results are as follows: Figure 2 As shown in the figure, the amount of sodium dihydrogen phosphate added significantly affects the conversion and leaching efficiency of lithium, as well as the selective retention of iron. When the mass ratio is low (1:1), insufficient sodium source leads to insufficient conversion of lithium in LiFePO4 into soluble lithium salts, resulting in a low lithium leaching rate (82.5%). The lithium leaching rate is highest (99.6%) when the mass ratio reaches 1:2.5. Figure 3 The XRD patterns of the calcined products before and after water immersion under optimal conditions demonstrate that the insoluble lithium in lithium iron phosphate is converted into soluble lithium salts (Li2NaPO4 and LiNa5(PO4)2) through calcination with the assistance of sodium dihydrogen phosphate (NaH2PO4), while the iron remains in the filter residue as NaFeP2O7.

[0055] Example 2

[0056] (1) Weigh out 4 portions of 1g waste lithium iron phosphate cathode powder, and mix each portion with analytical grade sodium dihydrogen phosphate (NaH2PO4) at a mass ratio of 1:2.5.

[0057] (2) Place the four mixed materials into a corundum boat, put them into a tube furnace, and heat them to 400℃, 500℃, 600℃ and 700℃ respectively at a rate of 5℃ / min under an oxygen atmosphere. Then, keep them at each target temperature for 4 hours and cool them to room temperature after completion.

[0058] (3) Add each portion of roasted product to deionized water at a solid-liquid ratio of 50:1 (g / L), stir and leach at 40°C for 15 minutes, and then filter to separate the leachate and leach residue.

[0059] (4) The lithium and iron content in each leachate was analyzed using an atomic absorption spectrophotometer (AAS), and the leaching rate was calculated. The results are as follows: Figure 4 As shown in the figure, the results indicate that the calcination temperature significantly affects the lithium conversion efficiency.

[0060] Example 3

[0061] (1) Weigh out 4 portions of 1g waste lithium iron phosphate cathode powder, and mix each portion with analytical grade sodium dihydrogen phosphate (NaH2PO4) at a mass ratio of 1:2.5.

[0062] (2) Place the four mixed materials into a corundum boat, put them into a tube furnace, heat them to 600°C at a rate of 5°C / min in an oxygen atmosphere, and keep them heated for 1 hour, 2 hours, 3 hours and 4 hours respectively. After completion, cool them to room temperature with the furnace.

[0063] (3) Add each portion of roasted product to deionized water at a solid-liquid ratio of 50:1 (g / L), stir and leach at 40°C for 15 minutes, and then filter to separate the leachate and leach residue.

[0064] (4) The lithium and iron content in each leachate was analyzed using an atomic absorption spectrophotometer (AAS), and the leaching rate was calculated. The results are as follows: Figure 5 As shown in the figure. The results indicate that the lithium leaching rate gradually increases with increasing calcination time. From Figure 6 XRD patterns obtained at different calcination times showed that a calcination time of less than 2 hours resulted in insufficient reaction time, while 4 hours ensured complete reaction and yielded the highest and most stable lithium leaching rate. Figure 6 The four XRD patterns from top to bottom correspond to 4h, 3h, 2h, and 1h, respectively.

[0065] Example 4

[0066] (1) Weigh 1g of waste lithium iron phosphate cathode powder and mix it with analytical grade sodium dihydrogen phosphate (NaH2PO4) at a mass ratio of 1:2.5.

[0067] (2) Place the mixed material in a corundum boat, put it into a tube furnace, heat it to 600°C at a rate of 5°C / min under an oxygen atmosphere, and keep it at that temperature for 4 hours. After completion, cool it to room temperature with the furnace.

[0068] (3) The roasted products were added to deionized water at solid-liquid ratios of 10:1, 25:1, 50:1, 100:1, and 250:1 (g / L), respectively. The mixture was stirred at 40°C for 15 minutes and then filtered to separate the leachate and leach residue.

[0069] (4) The lithium and iron content in each leachate was analyzed using an atomic absorption spectrophotometer (AAS), and the leaching rate was calculated. The results are as follows: Figure 7 As shown in the figure. The results indicate that the solid-liquid ratio has a significant impact on the lithium leaching rate. When the solid-liquid ratio is too low, the solution may become supersaturated, potentially leading to lithium salt recrystallization; when the solid-liquid ratio is too high, although it is beneficial for diffusion, it increases the cost of subsequent concentration.

[0070] Example 5

[0071] (1) Weigh 1g of waste lithium iron phosphate cathode powder and mix it with analytical grade sodium dihydrogen phosphate (NaH2PO4) at a mass ratio of 1:2.5.

[0072] (2) Place the mixed material in a corundum boat, put it into a tube furnace, heat it to 600°C at a rate of 5°C / min under an oxygen atmosphere, and keep it at that temperature for 4 hours. After completion, cool it to room temperature with the furnace.

[0073] (3) Add the calcined product to deionized water at a solid-liquid ratio of 50:1 (g / L), stir and leach at 40°C for 5 minutes, 10 minutes, 15 minutes, 30 minutes and 60 minutes respectively, and then filter to separate the leachate and leach residue.

[0074] (4) The lithium and iron content in each leachate was analyzed using an atomic absorption spectrophotometer (AAS), and the leaching rate was calculated. The results are as follows: Figure 8 As shown in the figure, the immersion time significantly affects the leaching kinetics of lithium. Insufficient dissolution occurs when the immersion time is too short (<15 minutes); leaching equilibrium is reached after 30 minutes, and extending the time to 60 minutes has no significant effect on improving the leaching rate.

[0075] Example 6

[0076] (1) Weigh 5 waste lithium iron phosphate cathode powders with a mass of 1g each and mix them evenly with analytical grade sodium dihydrogen phosphate (NaH2PO4) at a mass ratio of 1:2.5.

[0077] (2) Place the mixed material in a corundum boat, put it into a tube furnace, heat it to 600°C at a rate of 5°C / min under an oxygen atmosphere, and keep it at that temperature for 4 hours. After completion, cool it to room temperature with the furnace.

[0078] (3) Add the roasted product to deionized water at a solid-liquid ratio of 50:1 (g / L), and control the water immersion temperature to 30℃, 40℃, 50℃, 60℃ and 70℃ respectively. Stir and immerse for 15 minutes at each temperature, and then filter to separate the leachate and leach residue.

[0079] (4) The lithium and iron content in each leachate was analyzed using an atomic absorption spectrophotometer (AAS), and the leaching rate was calculated. The results are as follows: Figure 9 As shown in the figure. The results indicate that increasing the water immersion temperature is beneficial to improving the lithium leaching rate and the final leaching rate, but the improvement effect tends to slow down after the temperature exceeds 40℃.

[0080] Example 7

[0081] (1) Take 500 mL of the lithium-containing leachate obtained in Example 6 at a water immersion temperature of 40°C (the Li⁺ concentration was determined to be approximately 2.1 g / L by ICP-OES), place it in a 1000 mL beaker, and heat and concentrate it in an 80°C constant temperature water bath until the solution volume is reduced to 1 / 5 of the original volume (approximately 100 mL). At this point, the Li⁺ concentration will decrease. + The concentration is approximately 10.5 g / L.

[0082] (2) Slowly add stoichiometric amounts of sodium phosphate (Na3PO4, Li3PO4) to the concentrated hot leachate while continuously stirring. +The phosphorus source was a mixture of sodium hydroxide (NaOH) with a molar ratio of approximately 0.33:1. The pH of the reaction system was then slowly adjusted using a 2 mol / L sodium hydroxide (NaOH) solution, monitored with a precision pH meter, until the pH was precisely adjusted between 9.5 and 10.5.

[0083] (3) The above reaction system was kept at 60°C and stirred for 2 hours to allow the precipitate to have a complete crystal structure and particle growth. After the reaction was completed, the mixture was naturally cooled to room temperature and then vacuum filtered to obtain a white precipitate.

[0084] (4) Wash the precipitate three times with a small amount of deionized water and anhydrous ethanol to remove residual sodium ions and other soluble impurities. Place the washed filter cake in a vacuum drying oven and dry it at 110°C for 12 hours to obtain the final product.

[0085] (5) Characterize the dried product. XRD analysis (e.g.) Figure 10 As shown in the image, the obtained product is pure-phase lithium phosphate (Li3PO4), without any other impurity peaks. The SEM image of the obtained Li3PO4 is shown below. Figure 11 As shown in Table 1, the ICP-OES test results of the Li3PO4 product obtained in Example 7 show that the purity of the recovered product is greater than 99.5%, and the calculated lithium precipitation recovery rate exceeds 99%.

[0086] Table 1. ICP-OES elemental analysis results of the recovered lithium phosphate products.

[0087]

[0088] Obviously, compared with the prior art, the present invention provides an environmentally friendly method for the comprehensive utilization of waste lithium iron phosphate cathode materials, which realizes the efficient and selective extraction of lithium and the high-purity enrichment of iron. Moreover, the entire process does not require the addition of additional acids or alkalis, and the whole process is short, achieving efficient and clean separation and resource utilization of lithium and iron.

[0089] Although the present invention has been described above in conjunction with exemplary embodiments and accompanying drawings, those skilled in the art should understand that various modifications can be made to the above embodiments without departing from the spirit and scope of the claims.

Claims

1. A method for preparing lithium phosphate and sodium iron pyrophosphate from waste lithium iron phosphate materials, characterized in that, The method includes the following steps: S1. Mix waste lithium iron phosphate battery cathode powder with sodium dihydrogen phosphate and roast it in an oxygen atmosphere to convert lithium into soluble lithium salt and iron into water-insoluble sodium pyrophosphate. S2. The roasted product obtained in S1 is subjected to water leaching and filtration to obtain lithium-containing leachate and leachate residue; S3. The lithium-containing leachate obtained in S2 is concentrated, and then a phosphorus source is added and the pH is adjusted to 9.5-10.5 to obtain the Li3PO4 product.

2. The method for preparing lithium phosphate and sodium iron pyrophosphate from waste lithium iron phosphate materials according to claim 1, characterized in that, In step S1, the mass ratio of the lithium iron phosphate cathode powder to the sodium dihydrogen phosphate is 1:1 to 3.

5.

3. The method for preparing lithium phosphate and sodium iron pyrophosphate from waste lithium iron phosphate materials according to claim 1, characterized in that, In step S1, the roasting temperature is 300~700℃ and the time is 2~5 hours.

4. The method for preparing lithium phosphate and sodium iron pyrophosphate from waste lithium iron phosphate materials according to claim 1, characterized in that, In step S1, the oxygen atmosphere is flowing oxygen with a flow rate of 50–200 mL / min.

5. The method for preparing lithium phosphate and sodium iron pyrophosphate from waste lithium iron phosphate materials according to claim 1, characterized in that, In step S2, the solid-liquid ratio of the water immersion is 10–250 g / L, the water immersion temperature is 30–70℃, and the water immersion time is 10–60 min.

6. The method for preparing lithium phosphate and sodium iron pyrophosphate from waste lithium iron phosphate materials according to claim 1, characterized in that, In step S2, the leaching residue is sodium iron pyrophosphate, which is recycled as an iron and phosphorus source.

7. The method for preparing lithium phosphate and sodium iron pyrophosphate from waste lithium iron phosphate materials according to claim 1, characterized in that, In step S3, the concentration is achieved by heating and evaporation, and the lithium ion concentration in the leachate after concentration is not less than 5 g / L.

8. The method for preparing lithium phosphate and sodium iron pyrophosphate from waste lithium iron phosphate materials according to claim 1, characterized in that, In step S3, the phosphorus source is sodium phosphate, and the molar ratio of sodium phosphate to lithium ions in the concentrated leachate is (0.32-0.35):

1.

9. The method for preparing lithium phosphate and sodium iron pyrophosphate from waste lithium iron phosphate materials according to claim 1, characterized in that, In step S3, after adding the phosphorus source and adjusting the pH, the process further includes: stirring to induce the precipitation of lithium phosphate, followed by filtering, washing and drying the precipitate to obtain the lithium phosphate product.

10. The method for preparing lithium phosphate and sodium iron pyrophosphate from waste lithium iron phosphate materials according to claim 9, characterized in that, The precipitation reaction is carried out under stirring at a temperature of 40–80°C for a time of 0.1–2 hours.

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