Method for extracting lithium and recycling iron by pre-oxidation-low-temperature reduction roasting of waste lithium iron phosphate battery positive electrode material

Abstract

The invention belongs to the field of waste resource recycling, and discloses a method for extracting lithium from waste lithium iron phosphate through pre-oxidation-low-temperature reduction roasting, efficient separation of lithium and iron is realized through combination of damage of a LiFePO4 crystal structure through pre-oxidation and low-temperature reduction, and the method comprises the following steps: oxidizing roasting is performed in an oxygen atmosphere at 500-600 DEG C, Fe < 2 + > is converted into Fe < 3 + >, and Li3Fe2 (PO4) 3 and Fe2O3 are generated; reducing Fe < 3 + > into elemental iron and converting lithium into Li3PO4 by using a carbonaceous reducing agent in an inert atmosphere at 450-650 DEG C; iron powder is recovered through magnetic separation (the recovery rate is larger than or equal to 93%), tailings are leached through phosphoric acid to obtain lithium-rich liquid, and a lithium phosphate product (the lithium recovery rate is larger than or equal to 96%) is obtained after impurity removal, lithium supplementation and crystallization. Compared with a traditional process, the method has the advantages that the energy consumption is reduced by 20-30%, the resource recovery efficiency is high, iron is recovered in a high-added-value simple substance form, the reagent consumption is low, the mother liquor can be recycled, and the method has the advantages of high efficiency, environmental protection and economy and is suitable for industrial application.

Description

Technical Field

[0001] This invention belongs to the field of waste resource recycling, and in particular relates to a method for pre-oxidation-low temperature reduction roasting of waste lithium iron phosphate battery cathode materials to extract lithium and recover iron. Background Technology

[0002] Lithium-ion batteries are widely used in the electric vehicle industry due to their high energy density and good cycle performance. With the continuous expansion of the electric vehicle industry, lithium-ion battery production is growing rapidly. Their lifespan is 5-8 years, leading to an explosive increase in the number of spent lithium-ion batteries. Among them, lithium iron phosphate batteries are characterized by high safety and low production cost. Spent lithium iron phosphate batteries contain abundant valuable metal resources; recycling them can achieve comprehensive resource utilization and reduce environmental pollution. Spent lithium iron phosphate batteries can be divided into three parts: positive electrode material, negative electrode material, and electrolyte. The positive electrode material is mainly composed of LiFePO4 and Al. Recovering lithium and iron from spent lithium iron phosphate batteries can help alleviate my country's lithium resource shortage problem.

[0003] Currently, there are two main methods for disposing of spent lithium iron phosphate batteries: one is to recover valuable metals, and the other is to regenerate lithium iron phosphate cathode materials. The most common method for recovering valuable metals is the wet recycling process. This process uses chemical reagents such as inorganic acids (HCl, H2SO4, HNO3, H3PO4) and organic acids (citric acid, oxalic acid, ascorbic acid) to dissolve the valuable metals, achieving metal enrichment, separation, and extraction. For example, patent document CN112331949A discloses a method for recovering phosphorus, iron, and lithium from spent lithium iron phosphate batteries. Specifically, it discloses leaching spent lithium iron phosphate powder after alkaline leaching with a mixed solution of sulfuric acid and hydrogen peroxide; adjusting the pH of the leachate to obtain crude iron phosphate; obtaining battery-grade iron phosphate after acid dissolution, precipitation, and calcination; evaporating and concentrating the lithium-containing filtrate and adding an alkaline solution to obtain lithium carbonate precipitate, thus producing battery-grade lithium carbonate. This method has a short process flow and a simple reaction system, but it results in a large amount of slag, high lithium loss rate, high acid and alkali consumption, and the generation of a large amount of wastewater during the leaching process, putting pressure on the environment. For example, patent document CN120288725A discloses a method for recovering lithium and iron sources from waste lithium iron phosphate battery cathode sheets through a combination of pyrometallurgical and hydrometallurgical processes, and finally regenerating new lithium iron phosphate materials through solid-state sintering. Specifically, it discloses a method of mixing and roasting fine lithium iron phosphate cathode powder with sodium persulfate, washing the roasted mixture with deionized water, and obtaining a filtrate by stirring and filtration. After oxidation, pH adjustment to 6-7, precipitation, and filtration, the filtrate yields a reddish-brown Fe(OH)3 precipitate and LiOH solution. The Fe(OH)3 and LiOH are converted into iron phosphate and lithium carbonate, respectively, and finally, the recycled lithium iron phosphate material is obtained through solid-state sintering. This method has a high raw material utilization rate, directly obtaining lithium iron phosphate material and reducing industrialization steps, but its reaction system is complex, requires precise control of raw material composition, and has a higher technical threshold, which is not conducive to large-scale promotion and application. A patent document with publication number CN111118294A discloses a method for stepwise recovery of valuable metals from waste lithium-ion battery materials. This method involves mixing waste lithium-ion battery cathode materials with a carbon-containing solid reducing agent for reduction roasting. The roasted residue is then subjected to neutral leaching to extract lithium, yielding a lithium-rich solution and neutral leaching residue. The neutral leaching residue can be separated into magnetic and non-magnetic substances using weak magnetic separation. This method achieves efficient and preferential recovery of lithium and stepwise extraction of nickel, cobalt, and manganese, simplifying the process and reducing costs. However, the leaching solution after the reaction is difficult to recycle, increasing wastewater treatment costs. Furthermore, the direct reduction using solid carbon requires no pretreatment, and the reduction temperature for some raw materials remains as high as 800-900℃, resulting in poor energy consumption. Therefore, there is an urgent need to develop a method for recovering waste lithium iron phosphate battery cathode materials that uses less reagent, has a high metal recovery rate, good separation effect, and a simple process. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the background art above, and to provide a method for pre-oxidation-low temperature reduction roasting of waste lithium iron phosphate battery cathode materials to extract lithium and recover iron. The method achieves efficient separation of lithium and iron by pre-oxidation to destroy the lithium iron phosphate crystal structure and low temperature reduction, while recovering iron resources in the form of iron powder, thereby reducing energy consumption and pollution.

[0005] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows: A method for pre-oxidation-low-temperature reduction roasting of spent lithium iron phosphate battery cathode materials to extract lithium and recover iron includes the following steps: (1) Place the waste lithium iron phosphate battery cathode material powder under an oxidizing atmosphere for oxidative roasting to obtain a pre-oxidized product; (2) The pre-oxidized product is mixed with a reducing agent and reduced at low temperature under an inert atmosphere to obtain a reduced product; (3) The reduction product is subjected to magnetic separation to recover elemental iron, and the tailings of the magnetic separation are subjected to phosphoric acid leaching to obtain lithium-rich leachate.

[0006] The above-mentioned method for pre-oxidation-low-temperature reduction roasting of waste lithium iron phosphate battery cathode materials to extract lithium and recover iron is preferably further modified by removing impurities from the lithium-rich leachate, replenishing the lithium source, crystallizing, filtering, and drying to obtain lithium phosphate products.

[0007] The above-mentioned method for pre-oxidation-low-temperature reduction roasting of waste lithium iron phosphate battery cathode material to extract lithium and recover iron is preferably described in step (1), wherein the main components of the lithium iron phosphate battery cathode material powder include: Li 3.65-4.25 wt%, Fe 28.62-33.28 wt%, Al 0.80-2.29 wt%, P 16.50-18.60 wt%, and the sieve particle size is ≤100 mesh.

[0008] In the above-mentioned method for pre-oxidation-low-temperature reduction roasting of waste lithium iron phosphate battery cathode material to extract lithium and recover iron, preferably, in step (1), the oxidation roasting temperature is 500-600℃ and the oxidation time is 120-240 min; the oxygen atmosphere refers to oxygen and the gas flow rate is 10-50 mL / min.

[0009] In the pre-oxidation reaction of step (1), the Fe of LiFePO4 2+ Oxidized to Fe 3+ The pre-oxidation products Li3Fe2(PO4)3 and Fe2O3 are generated, which destroy the original olivine crystal structure and provide favorable conditions for the subsequent reduction process. The reaction equation is as follows: LiFePO4+ O2 Li3Fe2(PO4)3+ Fe2O 3。

[0010] In the above-mentioned method for pre-oxidation-low temperature reduction roasting of waste lithium iron phosphate battery cathode material to extract lithium and recover iron, preferably, in step (2), the reducing agent is at least one of coke, graphite, activated carbon, and biomass, and the mass ratio of the reducing agent to the pre-oxidation product is 1:(4-6).

[0011] In the above-mentioned method for pre-oxidation-low-temperature reduction roasting of waste lithium iron phosphate battery cathode material to extract lithium and recover iron, preferably, in step (2), the low-temperature reduction roasting temperature is 450-650℃ and the reduction time is 180-300 min; the inert atmosphere is nitrogen and the gas flow rate is 10-50 mL / min.

[0012] During the low-temperature reduction calcination reaction in step (2), Fe in Li3Fe2(PO4)3 and Fe2O3 3+ The Fe is reduced to elemental Fe, while Li is converted to Li3PO4. The reaction equation is as follows: Li3Fe2(PO4)3+ Fe2O3+ C Li3PO4 + Fe + CO2 .

[0013] In the above-mentioned method for pre-oxidation-low-temperature reduction roasting of waste lithium iron phosphate battery cathode materials to extract lithium and recover iron, preferably, in step (3), the magnetic field strength of the magnetic separation is 150-250 kA / m. During the magnetic separation process, elemental Fe is adsorbed by the magnetic field to obtain iron powder product (recovery rate ≥93%); the main component of the magnetic separation tailings is Li3PO4, and it also contains a small amount of unreacted reducing agent and impurities.

[0014] In the above-mentioned method for pre-oxidation-low-temperature reduction roasting of waste lithium iron phosphate battery cathode material to extract lithium and recover iron, preferably, in step (3), the phosphoric acid concentration during the phosphoric acid leaching process is 1.0-5.0 mol / L, the leaching temperature is 30-70℃, the leaching liquid-to-solid ratio is 3-8 mL:1 g, and the leaching time is 30-120 min, so that Li3PO4 is completely dissolved to obtain a lithium-rich leaching solution. The reaction equation is as follows: Li3PO4+ H3PO4 LiH2PO 4。

[0015] The preferred method for pre-oxidation-low-temperature reduction roasting of waste lithium iron phosphate battery cathode materials to extract lithium and recover iron includes the following steps for removing impurities from the lithium-rich leachate: adjusting the pH to 3.8-4.0 with ammonia (NH3·H2O), holding at 50-70℃ for 30-90 min to ensure complete precipitation of FePO4 and AlPO4, filtering to remove precipitates of iron phosphate and aluminum phosphate, and obtaining a purified leachate.

[0016] The above-mentioned method for pre-oxidation-low-temperature reduction roasting of spent lithium iron phosphate battery cathode materials to extract lithium and recover iron is preferably characterized by supplementing the lithium source by adding LiOH solution dropwise to the purified leachate to adjust the pH to 8.0-8.5, maintaining the temperature at 50-70℃ and stirring for 30-120 min. At this point, the Li in the solution... + It precipitates as Li3PO4. After vacuum filtration, the filter residue is dried at 80℃ for 3-5 h to obtain lithium phosphate product (lithium recovery rate ≥96%). The main component of the crystallization mother liquor after filtration is diammonium hydrogen phosphate, which can be recycled to the lithium-rich leachate to adjust the pH and remove impurities, reducing the amount of ammonia used.

[0017] This invention reveals that the olivine-type crystal structure of LiFePO4 can be destroyed at only 500-600℃ under a pure oxygen atmosphere, oxidizing ferrous iron to ferric iron. The resulting pre-oxidation products are mainly Li3Fe2(PO4)3 and Fe2O3, creating favorable thermodynamic and kinetic conditions for subsequent low-temperature reduction. Since the phases are destroyed, the reduction reaction can proceed smoothly without the high temperatures (>900℃) required by traditional pyrometallurgical processes. Therefore, during the reduction process, under an inert atmosphere at 450-650℃, iron can be reduced to elemental iron, and lithium can be converted to lithium phosphate, achieving fundamental phase separation. The reduction slag is then subjected to magnetic separation to recover iron, and the tailings from the magnetic separation can be efficiently recycled to recover lithium through a wet acid leaching-purification process. The mother liquor from crystallization can be recycled to a lithium-rich leaching solution for purification and impurity removal.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Significantly reduced energy consumption and high resource recovery efficiency: By destroying the crystal structure through pre-oxidation (500-600℃), the subsequent reduction roasting temperature is greatly reduced to 450-650℃, which reduces energy consumption by 20%-30% compared with the traditional pyrometallurgical process (usually requiring 900-1100℃). Moreover, the low-temperature operation effectively reduces the high-temperature volatilization loss of lithium, ensuring that the lithium recovery rate is as high as 96% or more; at the same time, iron is efficiently recovered in elemental form, with a recovery rate of not less than 93%.

[0019] (2) Environmentally friendly, with low reagent consumption and pollution emissions: The reduction process uses only solid carbonaceous reducing agents, and almost no difficult-to-treat toxic gases are generated; the wet lithium extraction stage uses only phosphoric acid as a leaching agent, and the types of reagents are simple; and the crystallization mother liquor can be recycled for the front-end impurity removal process, reducing the amount of waste liquid discharged by about 50%, and reducing wastewater treatment costs and environmental pressure.

[0020] (3) Thorough separation of lithium and iron, high added value of products: The innovative “pre-oxidation-reduction” process of this invention realizes the directional conversion of iron to elemental iron and lithium to lithium phosphate. The efficient and thorough separation of iron and lithium can be achieved through simple magnetic separation, avoiding the interference of iron ions in the subsequent lithium purification process and ensuring the purity of lithium phosphate products. Moreover, the iron resources are recovered in the form of high-value iron powder, rather than traditional low-value iron slag or intermediate chemicals, which improves the economic efficiency of the overall recycling process.

[0021] (4) The process is simple, the operating conditions are mild, and it is easy to industrialize: The entire process of this invention only includes four main steps: "pre-oxidation-reduction roasting-magnetic separation-lithium purification". The process is short, the equipment requirements are simple, and there is no need for complex high-temperature or high-pressure devices. All reactions are carried out at atmospheric pressure and medium temperature (≤650℃). The reaction conditions are mild, the operation is strong, the safety risk is low, and it is very conducive to large-scale industrial application.

[0022] In summary, this invention effectively overcomes the technical shortcomings of existing wet processes, such as high acid consumption and large amounts of waste liquid, as well as traditional pyrometallurgical processes, such as high energy consumption, large lithium loss, and difficulty in high-value recovery of iron resources. It provides a new method for recycling waste lithium iron phosphate batteries that is efficient, environmentally friendly, economical, and easy to industrialize. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 The image shows the XRD pattern of the product of the waste lithium iron phosphate cathode material after pre-oxidation at 550°C and calcination in Example 1 of this invention.

[0025] Figure 2 The image shows the XRD pattern of the reduced product after the pre-oxidized product in Example 1 of this invention was reduced at 600°C.

[0026] Figure 3 The image shows the XRD pattern of the elemental iron product obtained after magnetic separation in Embodiment 1 of the present invention.

[0027] Figure 4 The image shows the XRD pattern of the lithium phosphate product obtained by acid leaching lithium from the magnetic separation tailings in Example 1 of this invention.

[0028] Figure 5 This is a process flow diagram of the pre-oxidation-low-temperature reduction roasting lithium extraction process of waste lithium iron phosphate battery cathode material according to the present invention. Detailed Implementation

[0029] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0030] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0031] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0032] The main components of the waste lithium iron phosphate battery cathode material powder processed in the following examples and comparative examples include Li 4.15 wt%, Fe 30.28 wt%, Al 1.69 wt%, P 17.34 wt%, and the sieve particle size is ≤100 mesh.

[0033] Example 1: A method for pre-oxidation-low-temperature reduction roasting to extract lithium from waste lithium iron phosphate battery cathode materials according to the present invention is shown in the process flow diagram below. Figure 5 As shown, it includes the following steps: (1) Accurately weigh 20 g of waste lithium iron phosphate battery cathode material powder and spread it evenly in the corundum crucible. To ensure full oxidation, the thickness of the powder should not exceed 5 mm. (2) The corundum crucible was placed in a tube furnace for pre-oxidation roasting. During the roasting process, oxygen was introduced at a flow rate of 20 mL / min as an oxidizing atmosphere. The roasting temperature was controlled at 550℃ and roasted for 180 min. After the roasting reaction was completed, the furnace was cooled to room temperature to obtain the pre-oxidized product. Its XRD pattern is shown in the figure. Figure 1 As shown; (3) Grind the pre-oxidized product, add activated carbon at a mass ratio of 5:1 and mix evenly. After mixing, place the mixture back into a tube furnace for reduction calcination. During the calcination process, nitrogen gas with a flow rate of 20 mL / min is introduced as a protective gas. The reduction calcination temperature is controlled at 600℃ and calcined for 240 min. After the calcination reaction is completed, cool the furnace to room temperature to obtain the reduced product. Its XRD pattern is shown below. Figure 2 As shown; (4) The reduction roasting product was ground and subjected to magnetic separation using an XCGS-73 magnetic separator. The magnetic field strength was controlled at 200 kA / m to obtain 5.87 g of magnetic product elemental iron (purity 96.8%) and magnetic separation tailings (non-magnetic product: lithium phosphate and a small amount of impurities). The XRD pattern of the magnetic product elemental iron is shown in the figure. Figure 3 As shown; (5) Add the magnetic separation tailings to the reaction beaker, add 3 mol / L phosphoric acid at a liquid-to-solid ratio of 6 mL:1 g, stir at 60℃ for 60 min to completely dissolve Li3PO4 and obtain a lithium-rich leachate; then add ammonia (NH3·H2O) to the lithium-rich leachate to adjust the pH to 3.95 to remove impurities, keep at 50℃ for 60 min to ensure complete precipitation of FePO4 and AlPO4; then vacuum filter to remove precipitates of iron phosphate and aluminum phosphate to obtain a purified leachate; transfer the purified leachate to a crystallization reactor, add LiOH solution dropwise at a uniform rate to replenish the lithium source and adjust the pH to 8.0, keep at 60℃ and stir for 90 min, at which point the Li in the solution will be reduced. + The lithium phosphate precipitates as Li3PO4, and the mother liquor from crystallization can be recycled to the lithium-rich leaching solution for purification and impurity removal. After vacuum filtration, the filter residue is dried at 80℃ for 3 h to obtain the lithium phosphate product, the XRD pattern of which is shown below. Figure 4 As shown.

[0034] The test results showed that the total Fe yield after magnetic separation was 93%, and the total lithium yield after acid leaching was 96.3%.

[0035] Comparative Example 1: The only difference between this comparative example and Example 1 is the oxidation temperature and time in step (2). In this comparative example, the oxidation roasting temperature in step (2) is 450°C and the roasting time is 180 min. Other process parameters are the same as in Example 1. As a result, 4.33 g of magnetic product elemental iron was obtained, with a total Fe yield of 68.7% and a total lithium yield of 81.3% after acid leaching. Example 2: A method for pre-oxidation-low-temperature reduction roasting to extract lithium from waste lithium iron phosphate battery cathode materials according to the present invention is shown in the process flow diagram below. Figure 5 As shown, it includes the following steps: (1) Accurately weigh 20 g of waste lithium iron phosphate battery cathode material powder and spread it evenly in the corundum crucible. To ensure full oxidation, the thickness of the powder should not exceed 5 mm. (2) The corundum crucible was placed in a tube furnace for pre-oxidation roasting. During the roasting process, oxygen with a flow rate of 20 mL / min was introduced as an oxidizing atmosphere. The oxidation roasting temperature was controlled at 600℃ and roasted for 180 min. After the roasting reaction was completed, the furnace was cooled to room temperature to obtain the pre-oxidized product. (3) Grind the pre-oxidized product, add activated carbon at a mass ratio of 5:1 and mix evenly. After mixing, put it into a tube furnace again for reduction calcination. During the calcination process, nitrogen gas with a flow rate of 20 mL / min is introduced as a protective gas. The reduction calcination temperature is controlled at 550℃ and calcined for 240 min. After the calcination reaction is completed, cool it to room temperature with the furnace to obtain the reduced product. (4) Grind the reduction roasting product and perform magnetic separation using an XCGS-73 magnetic separator. Control the magnetic field strength to 200 k A / m to obtain 5.95g of magnetic product elemental iron (purity 97.4%) and magnetic separation tailings (non-magnetic product: lithium phosphate and a small amount of impurities). (5) Add the magnetic separation tailings to the reaction beaker, add 3 mol / L phosphoric acid at a liquid-to-solid ratio of 6 mL:1 g, stir at 60℃ for 60 min to completely dissolve Li3PO4 and obtain a lithium-rich leachate; then add ammonia (NH3·H2O) to the lithium-rich leachate to adjust the pH to 3.95 to remove impurities, keep at 50℃ for 60 min to ensure complete precipitation of FePO4 and AlPO4; then vacuum filter to remove precipitates of iron phosphate and aluminum phosphate to obtain a purified leachate; transfer the purified leachate to a crystallization reactor, add LiOH solution dropwise at a uniform rate to replenish the lithium source and adjust the pH to 8.0, keep at 60℃ and stir for 90 min, at which point the Li in the solution will be reduced. + The lithium phosphate precipitates as Li3PO4, and the mother liquor from crystallization can be recycled to the lithium-rich leaching solution for purification and impurity removal. After vacuum filtration, the filter residue is dried at 80°C for 3 hours to obtain the lithium phosphate product.

[0036] The test results showed that the total Fe yield after magnetic separation was 94.3%, and the total lithium yield after acid leaching was 96.9%.

[0037] Comparative Example 2: The only difference between this comparative example and Example 2 is the reduction temperature and time in step (3). In this comparative example, the reduction roasting temperature in step (3) is controlled at 400℃ and the roasting time is 240 min. Other process parameters are the same as in Example 2. As a result, 5.05 g of magnetic product elemental iron was obtained, with a total Fe yield of 80.1% and a total lithium yield of 93% after acid leaching. Example 3: A method for pre-oxidation-low-temperature reduction roasting to extract lithium from waste lithium iron phosphate battery cathode materials according to the present invention is shown in the process flow diagram below. Figure 5 As shown, it includes the following steps: (1) Accurately weigh 20 g of waste lithium iron phosphate battery cathode material powder and spread it evenly in the corundum crucible. To ensure full oxidation, the thickness of the powder should not exceed 5 mm. (2) The corundum crucible was placed in a tube furnace for pre-oxidation roasting. During the roasting process, oxygen with a flow rate of 20 mL / min was introduced as an oxidizing atmosphere. The oxidation roasting temperature was controlled at 600℃ and roasted for 180 min. After the roasting reaction was completed, the furnace was cooled to room temperature to obtain the pre-oxidized product. (3) Grind the pre-oxidized product, add activated carbon at a mass ratio of 4:1 and mix evenly. After mixing, put it into a tube furnace again for reduction calcination. During the calcination process, nitrogen gas with a flow rate of 20 mL / min is introduced as a protective gas. The reduction calcination temperature is controlled at 650℃ and calcined for 300 min. After the calcination reaction is completed, cool it to room temperature with the furnace to obtain the reduced product. (4) Grind the reduction roasting product and perform magnetic separation using an XCGS-73 magnetic separator. Control the magnetic field strength to 200 k A / m to obtain 5.98 g of magnetic product elemental iron (purity 97.8%) and magnetic separation tailings (non-magnetic product: lithium phosphate and a small amount of impurities). (5) Add the magnetic separation tailings to the reaction beaker, add 3 mol / L phosphoric acid at a liquid-to-solid ratio of 6 mL:1 g, stir at 30~70℃ for 60 min to completely dissolve Li3PO4 and obtain a lithium-rich leachate; then add ammonia (NH3·H2O) to the lithium-rich leachate to adjust the pH to 3.95 to remove impurities, keep at 50℃ for 60 min to ensure complete precipitation of FePO4 and AlPO4; then vacuum filter to remove precipitates of iron phosphate and aluminum phosphate to obtain a purified leachate; transfer the purified leachate to a crystallization reactor, add LiOH solution dropwise at a uniform rate to replenish the lithium source and adjust the pH to 8.0, keep at 50℃ and stir for 90 min, at which point the Li in the solution will be reduced. + The lithium phosphate precipitates as Li3PO4, and the mother liquor from crystallization can be recycled to the lithium-rich leaching solution for purification and impurity removal. After vacuum filtration, the filter residue is dried at 80°C for 3 hours to obtain the lithium phosphate product.

[0038] The test results showed that the total Fe yield after magnetic separation was 95.2%, and the total lithium yield after acid leaching was 97.5%.

[0039] Comparative Example 3: The only difference between this comparative example and Example 3 is the mass ratio of the pre-oxidized product to activated carbon in step (3). In this comparative example, the mass ratio of the pre-oxidized product to activated carbon in step (3) is 8:1. Other process parameters are the same as in Example 3. As a result, 4.89 g of magnetic product elemental iron was obtained, with a total iron yield of 77.5% and a total lithium yield of 90.9% after acid leaching. Comparative Example 4: The difference between this comparative example and Example 3 is that the pre-oxidation stage of steps (1) and (2) is omitted. The waste lithium iron phosphate battery cathode material powder is directly ground and added to activated carbon at a mass ratio of 4:1 and mixed evenly for reduction calcination. Other process parameters are the same as in Example 3.

[0040] The results yielded 1.58 g of elemental iron as a magnetic product, with a total iron yield of 25.16%. The total lithium yield after acid leaching was 79.1%. Due to the lack of a pre-oxidation step in this comparative example, the olivine structure of lithium iron phosphate was not destroyed, making it difficult to reduce iron during the low-temperature reduction stage. If the reduction temperature is continuously increased, lithium volatilization loss will occur.

[0041] Comparative Example 5: The comparative example, using the same raw materials as Comparative Example 3, employs a wet process—leaching with sulfuric acid and hydrogen peroxide—including the following steps; (1) Accurately weigh 20 g of waste lithium iron phosphate battery cathode material powder, add 2 mol / L sulfuric acid at a liquid-solid ratio of 6 mL:1 g, add 10% hydrogen peroxide at 1.2 times the theoretical amount, stir and react at 60℃ for 120 min, and filter to obtain lithium-rich leachate and iron phosphate slag.

[0042] (2) Add ammonia to the lithium-rich leachate to adjust the pH to 3.95 to remove impurities, keep it at 50℃ for 60 min to ensure that FePO4 and AlPO4 are completely precipitated; then filter under vacuum to remove ferric phosphate and aluminum phosphate precipitates to obtain purified leachate.

[0043] (3) After the purified leachate is concentrated, it is transferred to the crystallization reactor. CO2 gas is introduced into the leachate at a flow rate of 50 mL / min for carbonation. The reaction is stirred at 50℃ for 30 min. After filtration, waste liquid and lithium carbonate are obtained.

[0044] Testing showed that the total lithium recovery rate after acid leaching was 92.5%. Although this comparative method can also achieve a lithium recovery rate of around 92%, it produces a large amount of phosphorus-iron slag and consumes a significant amount of hydrogen peroxide. Not only does it require the treatment of large quantities of wastewater, but it also necessitates the removal of impurities from the phosphorus-iron slag for reuse, making the cost structure more complex and expensive.

Claims

1. A method for pre-oxidation-low-temperature reduction roasting of waste lithium iron phosphate battery cathode material to extract lithium and recover iron, characterized in that, Includes the following steps: (1) Place the waste lithium iron phosphate battery cathode material powder under an oxidizing atmosphere for oxidative roasting to obtain a pre-oxidized product; (2) The pre-oxidized product is mixed with a reducing agent and reduced at low temperature under an inert atmosphere to obtain a reduced product; (3) The reduction product is subjected to magnetic separation to recover elemental iron, and the tailings of the magnetic separation are subjected to phosphoric acid leaching to obtain lithium-rich leachate.

2. The method for pre-oxidation-low-temperature reduction roasting to extract lithium and recover iron from waste lithium iron phosphate battery cathode materials as described in claim 1, characterized in that, The lithium-rich leachate is further treated by removing impurities, replenishing the lithium source, crystallizing, filtering, and drying to obtain lithium phosphate products.

3. The method for pre-oxidation-low-temperature reduction roasting to extract lithium and recover iron from waste lithium iron phosphate battery cathode materials as described in claim 1, characterized in that, In step (1), the main components of the lithium iron phosphate battery cathode material powder include: Li 3.65-4.25 wt%, Fe 28.62-33.28 wt%, Al 0.80-2.29 wt%, P 16.50-18.60 wt%, and the sieve particle size is ≤100 mesh.

4. The method for pre-oxidation-low-temperature reduction roasting to extract lithium and recover iron from waste lithium iron phosphate battery cathode materials as described in claim 1, characterized in that, In step (1), the oxidation calcination temperature is 500-600℃ and the oxidation time is 120-240 min.

5. The method for pre-oxidation-low-temperature reduction roasting to extract lithium and recover iron from waste lithium iron phosphate battery cathode materials as described in claim 1, characterized in that, In step (2), the reducing agent is at least one of coke, graphite, activated carbon, and biomass, and the mass ratio of the reducing agent to the pre-oxidized product is 1:(4-6).

6. The method for pre-oxidation-low-temperature reduction roasting to extract lithium and recover iron from waste lithium iron phosphate battery cathode materials as described in claim 1, characterized in that, In step (2), the low-temperature reduction calcination temperature is 450-650℃ and the reduction time is 180-300min; the inert atmosphere is nitrogen and the gas flow rate is 10-50 mL / min.

7. The method for pre-oxidation-low-temperature reduction roasting to extract lithium and recover iron from waste lithium iron phosphate battery cathode materials as described in claim 1, characterized in that, In step (3), the magnetic field strength of the magnetic separation is 150-250 kA / m.

8. The method for pre-oxidation-low-temperature reduction roasting to extract lithium and recover iron from waste lithium iron phosphate battery cathode materials as described in claim 1, characterized in that, In step (3), the phosphoric acid concentration during the phosphoric acid leaching process is 1.0-5.0 mol / L, the leaching temperature is 30-70℃, the leaching liquid-solid ratio is 3-8 mL:1 g, and the leaching time is 30-120 min.

9. The method for pre-oxidation-low-temperature reduction roasting to extract lithium and recover iron from waste lithium iron phosphate battery cathode materials as described in claim 2, characterized in that, The steps for removing impurities from lithium-rich leachate include adjusting the pH to 3.8-4.0 with ammonia, keeping it at 50-70℃ for 30-90 min, and filtering to obtain purified leachate.

10. The method for pre-oxidation-low-temperature reduction roasting to extract lithium and recover iron from waste lithium iron phosphate battery cathode materials as described in claim 9, characterized in that, Supplementing the lithium source refers to adding LiOH solution dropwise to the purified leachate to adjust the pH to 8.0-8.5; crystallization refers to stirring at 50-70℃ for 3-120 min, at which point the Li in the solution... + It precipitates as Li3PO4.

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