A process for recovering iron and lithium from spent lithium iron phosphate batteries

Abstract

The present application relates to the technical field of battery recycling, and particularly relates to a process for recovering iron and lithium from waste lithium iron phosphate batteries. The process comprises the following steps: pretreating the waste lithium iron phosphate batteries, recovering lithium carbonate and recovering iron oxide. The present application simultaneously introduces ultrasonic waves, a surfactant and gentle heating in a sodium hydroxide solution, and significantly enhances the alkaline dealumination process through the synergistic effect of the three. The surfactant can reduce the surface tension of the solution, making the alkali solution more easily penetrate the surface of the aluminum foil sheet wrapped in hydrophobic carbon, eliminating the problem of reaction interface obstruction, shortening the dealumination time, and greatly improving the treatment efficiency; the alkali consumption is significantly reduced, saving reagent cost; the residual aluminum is reduced to a very low level, reducing the interference of residual aluminum on the subsequent recovery of lithium and iron, and improving the recovery rate of lithium and iron.

Description

Technical Field

[0001] This invention relates to the field of battery recycling technology, specifically to a process for recovering iron and lithium from spent lithium iron phosphate batteries. Background Technology

[0002] With the rapid development of electric vehicles and energy storage systems, the number of spent lithium iron phosphate (LFP) batteries is increasing year by year. These batteries have advantages such as long cycle life, good safety, and low cost; however, improper disposal after retirement not only wastes valuable resources but also creates environmental pressure. LFP batteries contain approximately 1%–2% lithium and 20%–30% iron, both important strategic resources. Lithium is a key element in the new energy industry, while iron, although relatively less valuable, can be efficiently recycled, reducing primary mineral mining and solid waste emissions. Therefore, recovering lithium and iron from spent LFP batteries has significant economic and environmental benefits.

[0003] Currently, the processes for recovering valuable metals from spent lithium iron phosphate batteries are mainly divided into two categories: pyrometallurgical and hydrometallurgical methods. Pyrometallurgical processes typically involve high-temperature roasting or smelting to induce phase transformations or volatilization and enrichment of the metal elements. For example, mixing the cathode material with a reducing agent and then reducing and roasting it at temperatures above 1000°C can yield an alloy or slag containing lithium and iron, which can then be further processed to extract lithium. While these methods are relatively simple, they suffer from problems such as extremely high energy consumption, stringent temperature requirements for equipment, and difficulties in handling phosphorus-containing fumes. Furthermore, lithium is easily lost through volatilization at high temperatures, resulting in a low recovery rate. Hydrometallurgical processes mainly rely on strong acid or strong alkali leaching. While the mild leaching conditions and low operating temperatures allow for high leaching rates, they often face several common challenges: First, poor selectivity, with impurities such as iron, aluminum, and copper entering the solution simultaneously with lithium, requiring subsequent multi-stage extraction or precipitation separation, resulting in a lengthy process and high reagent consumption; second, the neutralization and impurity removal process of the leaching solution generates a large amount of phosphorus- and iron-containing waste residue, increasing processing costs; and third, significant deficiencies exist in the pretreatment and dealuminization stage.

[0004] In reality, the black powder obtained from the crushing and sorting of waste batteries often contains a small amount of aluminum shavings derived from aluminum foil. Since aluminum is selectively soluble under alkaline conditions, the industrial practice currently uses sodium hydroxide solution for room-temperature stirring dealuminization in the pretreatment stage. This process is time-consuming and requires high alkali consumption. Furthermore, because the residual carbon in the black powder is hydrophobic, the alkaline solution cannot fully penetrate the aluminum foil surface, resulting in incomplete dealuminization. The residual aluminum then interferes with the extraction of lithium and iron in subsequent processes, reducing product purity. However, the residual graphite carbon in the black powder is highly hydrophobic, making it difficult for the alkaline solution to fully wet and penetrate the carbon-coated aluminum foil surface. This hinders the reaction interface, making the traditional dealuminization process time-consuming and requiring high alkali consumption. Moreover, if dealuminization is not efficient, the residual aluminum enters subsequent lithium chloride extraction or acid leaching processes, competing with lithium for reaction and forming products such as aluminates or aluminum chloride, affecting lithium extraction efficiency. It also interferes with reduction roasting and magnetic separation in the iron and phosphorus recovery process, reducing the purity of the final product. In the recovery of iron and phosphorus, existing technologies mostly employ reduction roasting to convert ferric phosphate into elemental iron or iron oxides, followed by dissolution with dilute acid to remove phosphorus, and finally recovery of iron. This process is complex and generates a large amount of phosphorus-containing waste acid. Another method involves directly smelting iron-rich slag at high temperatures to obtain pig iron or ferroalloys, but this method is extremely energy-intensive, and phosphorus is difficult to separate, resulting in excessive phosphorus impurities in the iron products. These shortcomings severely restrict the economic feasibility and environmental friendliness of the recovery process.

[0005] To address the shortcomings of existing technologies, this invention provides a process for recovering iron and lithium from spent lithium iron phosphate batteries. In the pretreatment dealuminization step, this process introduces a triple synergistic enhancement of ultrasound, surfactants, and gentle heating, which significantly shortens the dealuminization time, reduces alkali consumption, and controls the aluminum residue to an extremely low level. Furthermore, it improves the recovery of lithium and iron while reducing aluminum residue. Summary of the Invention

[0006] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a process for recovering iron and lithium from waste lithium iron phosphate batteries.

[0007] A process for recovering iron and lithium from spent lithium iron phosphate batteries specifically includes the following steps:

[0008] S1: Pre-treatment of spent lithium iron phosphate batteries

[0009] Waste lithium iron phosphate batteries are immersed in sodium hydroxide solution and discharged for 24-26 hours. Then, they are crushed to a particle size of ≤20mm using a shear crusher. The iron shell is then separated by 0.2T magnetic separation, and the copper-aluminum mixture is separated by eddy current separation. The remaining material is passed through a 200-mesh sieve to obtain black powder. 10-20 parts by weight of the black powder are added to 40-50 parts by weight of sodium hydroxide solution for alkaline dealuminization. The product is then filtered, and the filter cake is washed with deionized water until neutral. After drying at 100-105℃ for 12-14 hours, purified black powder is obtained.

[0010] S2: Recycling lithium carbonate

[0011] Mix 45-50 parts by weight of purified black powder with 22-24 parts by weight of ammonium chloride and place the mixture in a closed reactor. Under nitrogen protection, heat the mixture to 200-210℃ and hold for 15-20 minutes. After cooling, add 200 parts by weight of deionized water and stir at room temperature for 30-40 minutes. Filter the mixture to obtain a lithium-containing filtrate and an iron-rich filter residue. Recover lithium carbonate from the lithium-containing filtrate by evaporation concentration and a sodium carbonate solution displacement reaction.

[0012] S3: Recycling iron oxides

[0013] The iron-rich filter residue was dried to constant weight, mixed evenly with Na2CO3, and then calcined under CO2 atmosphere. After cooling, deionized water was added, and the mixture was filtered to obtain sodium phosphate solution and water-leached solid residue. Sodium phosphate was recovered by evaporation and crystallization of the sodium phosphate solution. The water-leached solid residue was dried, ground, and then magnetically separated using a weak magnetic separator to obtain iron oxide containing Fe3O4.

[0014] Furthermore, the alkaline dealumination in step S1 specifically includes the following steps:

[0015] Add the black powder to the sodium hydroxide solution and stir at 200-300 r / min for 30-40 minutes at room temperature (22-24℃). While stirring, add 0.2% (by weight) of Tween-80 to the sodium hydroxide solution. Place the reaction system in an ultrasonic cleaner or probe-type ultrasonic device and set the ultrasonic power to 100W. Then heat the system to maintain the temperature at 48-52℃ and react for 10-15 minutes under the above conditions.

[0016] Furthermore, the recovery of lithium carbonate from the lithium-containing filtrate in step S2 through evaporation concentration and sodium carbonate solution displacement reaction specifically includes the following steps:

[0017] The lithium-containing filtrate is evaporated and concentrated to 1 / 3 of its original volume, then heated to 90-100℃, and 25-30 parts by mass of a 20% saturated Na2CO3 solution are added. The mixture is stirred and reacted for 1-1.5 hours. The crude product is obtained by hot filtration. Finally, the crude lithium carbonate is washed twice with hot water at 80-85℃ and dried at 100-105℃ for 12-14 hours to obtain lithium carbonate.

[0018] Further, step S3, which involves recovering iron oxides, includes the following steps:

[0019] The iron-rich filter residue was dried to constant weight at 100-110℃. The dried iron-rich filter residue was mixed with Na2CO3 at a mass ratio of 1:(0.3-0.5) and placed in a corundum boat in a tube furnace. Nitrogen gas was first used to replace the air, and then the atmosphere was switched to CO2. The temperature was increased to 600-650℃ at 10℃ / min and held for 1-1.5 hours. After cooling, the product was added to deionized water at a liquid-solid ratio of (3-4) mL:1g and stirred at 80-85℃ for 30-40 minutes. The mixture was filtered to obtain sodium phosphate solution and water-leached solid residue. Sodium phosphate was recovered by evaporating and crystallizing the sodium phosphate solution.

[0020] After drying and grinding, the water-immersed solid residue is magnetically separated using a 0.2T weak magnetic separator for 2-3 times to obtain iron oxide containing Fe3O4.

[0021] Furthermore, the ratio of waste lithium iron phosphate batteries to sodium hydroxide solution is 1g:(150-200)mL.

[0022] Furthermore, the concentration of the sodium hydroxide solution is 5 wt%.

[0023] Furthermore, the flow rate of the CO2 atmosphere is 0.3-0.5 L / min.

[0024] Furthermore, the water-immersed solid residue is ground to a particle size ≤0.5mm.

[0025] Furthermore, the non-magnetic slag remaining after magnetic separation of water-immersed solid residue can be used as recycled building filler or disposed of in a harmless landfill.

[0026] The present invention has the following advantages:

[0027] 1. This invention introduces ultrasound, surfactant, and gentle heating into a sodium hydroxide solution simultaneously, significantly enhancing the alkaline dealumination process through their synergistic effect. Specifically, the ultrasonic cavitation effect generates numerous microbubbles in the liquid phase. These bubbles, upon their sudden and violent collapse, release localized high temperatures, high pressures, and microjets, physically stripping away the dense oxide film adhering to the aluminum foil surface and the resulting sodium aluminate product layer, continuously exposing fresh aluminum metal surfaces and thus significantly increasing the reaction rate between aluminum and the alkaline solution. The addition of the surfactant effectively reduces the surface tension of the solution, making it easier for the alkaline solution to wet and penetrate the aluminum foil sheet surface coated with hydrophobic carbon particles from the black powder. This eliminates the reaction interface obstruction caused by carbonaceous components, ensuring sufficient contact between the alkaline solution and aluminum. Simultaneously, gentle heating maintains the reaction system temperature within a suitable range, further accelerating the aluminum dissolution kinetics. The beneficial effects of the synergistic effect of the above three aspects include: shortened dealuminization time and significantly improved processing efficiency; reduced residual aluminum content in purified black powder, effectively avoiding problems such as aluminum ions competing with lithium for reaction during subsequent lithium chloride extraction, forming insoluble aluminates to encapsulate valuable metals, and interfering with iron-phosphorus reduction roasting and magnetic separation. Ultimately, the recovery rates of both lithium and iron are significantly improved.

[0028] 2. This invention mixes iron-rich filter residue, mainly composed of ferric phosphate and residual graphite carbon obtained after lithium extraction, with sodium carbonate and roasts it under a carbon dioxide atmosphere. Subsequently, a sodium phosphate solution is obtained simply by water leaching. The remaining magnetic residue is then magnetically separated to obtain high-grade iron concentrate. The innovation of this process lies in the fact that sodium carbonate can efficiently catalyze the gasification reaction of residual carbon in the filter residue with carbon dioxide, generating carbon monoxide with strong reducing power. This catalytic effect allows the carbon gasification reaction, which originally required temperatures above 800℃, to occur at 600-650℃, significantly reducing the roasting temperature and thus reducing energy consumption and equipment temperature resistance requirements. The generated carbon monoxide further reduces the ferric phosphate in the filter residue, converting trivalent iron into strongly magnetic magnetite (Fe3O4). Simultaneously, phosphate ions combine with sodium ions to form water-soluble sodium phosphate. After roasting, the sodium phosphate quickly dissolves into the solution through a simple water leaching process, and the sodium phosphate product can be recovered by evaporation and crystallization. The magnetite and unreacted carbon remain in the solid residue, and Fe3O4-containing iron oxides can be efficiently separated using weak magnetic separation. This step enables a reduction process at a lower temperature, significantly reducing energy consumption and lowering equipment investment and operating costs. It also fully utilizes the graphite carbon naturally present in waste batteries as a reducing agent, eliminating the need to add coke or coal powder, thus saving material costs. At the same time, it achieves the simultaneous separation of iron and phosphorus, avoiding the complex steps of acid leaching to remove phosphorus after reduction roasting in traditional processes. Furthermore, the entire water leaching process does not generate phosphorus-containing wastewater, eliminating the risk of secondary pollution and significantly improving environmental friendliness. Attached Figure Description

[0029] Figure 1 This is a flowchart of the process for recovering iron and lithium from spent lithium iron phosphate batteries according to the present invention.

[0030] Figure 2 This is a table showing the change in carbon gasification conversion rate at 600-650℃ according to the present invention.

[0031] Figure 3 This is a curve showing the fitting of the Arrhenius equation in this invention. Detailed Implementation

[0032] Example 1:

[0033] A process for recovering iron and lithium from spent lithium iron phosphate batteries, such as... Figure 1 As shown, it includes the following steps:

[0034] S1: Pre-treatment of spent lithium iron phosphate batteries

[0035] Waste lithium iron phosphate batteries were immersed in a 5 wt% sodium hydroxide solution and discharged for 24 hours. The ratio of waste lithium iron phosphate batteries to sodium hydroxide solution was 1 g: 150 mL. The batteries were then crushed to a particle size ≤20 mm using a shear crusher. The iron shells were then separated by 0.2T magnetic separation, followed by eddy current separation to separate the copper-aluminum mixture. The remaining material was passed through a 200-mesh sieve to obtain black powder. Ten parts by weight of the black powder were added to 40 parts by weight of the sodium hydroxide solution and stirred at 200 r / min for 30 minutes at room temperature (22℃). During stirring, 0.2% by weight of Tween-80 (by weight of the sodium hydroxide solution) was added. The reaction system was ultrasonically treated with an ultrasonic power of 100 W, and then heated to maintain the system temperature at 48℃. The alkaline dealuminization was completed under the above conditions for 10 minutes. The product was then filtered, and the filter cake was washed with deionized water until neutral. After drying at 100℃ for 12 hours, purified black powder was obtained.

[0036] S2: Recycling lithium carbonate

[0037] 50 parts by mass of purified black powder and 24 parts by mass of ammonium chloride were mixed and placed in a closed reactor. Nitrogen gas was purged and the temperature was raised to 210°C and held for 15 minutes. After cooling, 200 parts by mass of deionized water were added and stirred at room temperature for 30 minutes. The mixture was then filtered to obtain lithium-containing filtrate and iron-rich filter residue.

[0038] The lithium-containing filtrate was evaporated and concentrated to 1 / 3 of its original volume, then heated to 90°C, and 30 parts by mass of a 20% saturated Na2CO3 solution were added. The mixture was stirred and reacted for 1 hour. The crude product was obtained by hot filtration. Finally, the crude lithium carbonate was washed twice with hot water at 80°C and dried at 100°C for 12 hours to obtain lithium carbonate.

[0039] S3: Recycling iron oxides

[0040] The iron-rich filter residue was dried to constant weight at 100℃. The dried iron-rich filter residue was mixed with Na2CO3 at a mass ratio of 1:0.4 and placed in a corundum boat in a tube furnace. Nitrogen gas was first used to replace the air, and then the atmosphere was switched to CO2 at a flow rate of 0.3 L / min. The temperature was increased to 600℃ at a rate of 10℃ / min and held for 1 hour. After cooling, the product was added to deionized water at a liquid-to-solid ratio of 3.5 mL:1 g and stirred at 80℃ for 30 minutes. The mixture was filtered to obtain a sodium phosphate solution and a water-leached solid residue. The sodium phosphate solution was evaporated and crystallized to recover sodium phosphate.

[0041] After the water-immersed solid residue is dried and ground to a particle size ≤0.5mm, it is then magnetically separated twice using a 0.2T weak magnetic separator to obtain iron oxide containing Fe3O4.

[0042] Example 2:

[0043] A process for recovering iron and lithium from spent lithium iron phosphate batteries, such as... Figure 1 As shown, it includes the following steps:

[0044] S1: Pre-treatment of spent lithium iron phosphate batteries

[0045] Waste lithium iron phosphate batteries were immersed in a 5wt% sodium hydroxide solution and discharged for 26 hours. The ratio of waste lithium iron phosphate batteries to sodium hydroxide solution was 1g:200mL. The batteries were then crushed to a particle size ≤20mm using a shear crusher. The iron shells were then separated by 0.2T magnetic separation, followed by eddy current separation to separate the copper-aluminum mixture. The remaining material was passed through a 200-mesh sieve to obtain black powder. 20 parts by weight of the black powder were added to 50 parts by weight of the sodium hydroxide solution and stirred at 300r / min for 40 minutes at room temperature (24℃). During stirring, 0.2% by weight of Tween-80 (by weight of the sodium hydroxide solution) was added. The reaction system was ultrasonically treated with an ultrasonic power of 100W, and then heated to maintain the system temperature at 52℃. The reaction was carried out under the above conditions for 15 minutes to complete alkaline dealuminization. The product was then filtered, and the filter cake was washed with deionized water until neutral. After drying at 105℃ for 14 hours, purified black powder was obtained.

[0046] S2: Recycling lithium carbonate

[0047] 48 parts by mass of purified black powder and 23 parts by mass of ammonium chloride were mixed and placed in a closed reactor. Nitrogen gas was purged and the temperature was raised to 210°C and held for 20 minutes. After cooling, 200 parts by mass of deionized water were added and stirred at room temperature for 40 minutes. The mixture was then filtered to obtain lithium-containing filtrate and iron-rich filter residue.

[0048] The lithium-containing filtrate was evaporated and concentrated to 1 / 3 of its original volume, then heated to 100°C, and 26 parts by mass of a 20% saturated Na2CO3 solution were added. The mixture was stirred and reacted for 1.5 hours. The crude product was obtained by hot filtration. Finally, the crude lithium carbonate was washed twice with hot water at 85°C and dried at 105°C for 14 hours to obtain lithium carbonate.

[0049] S3: Recycling iron oxides

[0050] The iron-rich filter residue was dried to constant weight at 110℃. The dried iron-rich filter residue was mixed with Na2CO3 at a mass ratio of 1:0.5 and placed in a corundum boat in a tube furnace. Nitrogen gas was first used to replace the air, and then the atmosphere was switched to CO2 at a flow rate of 0.5 L / min. The temperature was increased to 650℃ at 10℃ / min and held for 1.5 hours. After cooling, the product was added to deionized water at a liquid-solid ratio of 4 mL:1 g and stirred at 85℃ for 40 minutes. The mixture was filtered to obtain a sodium phosphate solution and a water-leached solid residue. The sodium phosphate solution was evaporated and crystallized to recover sodium phosphate.

[0051] After the water-immersed solid residue is dried and ground to a particle size ≤0.5mm, it is then magnetically separated three times using a 0.2T weak magnetic separator to obtain iron oxide containing Fe3O4.

[0052] Example 3:

[0053] A process for recovering iron and lithium from spent lithium iron phosphate batteries, such as... Figure 1 As shown, it includes the following steps:

[0054] S1: Pre-treatment of spent lithium iron phosphate batteries

[0055] Waste lithium iron phosphate batteries were immersed in a 5 wt% sodium hydroxide solution and discharged for 25 hours. The ratio of waste lithium iron phosphate batteries to sodium hydroxide solution was 1 g: 180 mL. The batteries were then crushed to a particle size ≤ 20 mm using a shear crusher. The iron shells were then separated by 0.2T magnetic separation, followed by eddy current separation to separate the copper-aluminum mixture. The remaining material was passed through a 200-mesh sieve to obtain black powder. 15 parts by weight of the black powder were added to 45 parts by weight of sodium hydroxide solution and stirred at 250 r / min for 35 minutes at room temperature (23℃). During stirring, 0.2% by weight of Tween-80 (by mass of sodium hydroxide solution) was added. The reaction system was ultrasonically treated with an ultrasonic power of 100 W, and then heated to maintain the system temperature at 50℃. The reaction was carried out for 12 minutes under the above conditions to complete alkaline dealuminization. The product was then filtered, and the filter cake was washed with deionized water until neutral. After drying at 102℃ for 13 hours, purified black powder was obtained.

[0056] S2: Recycling lithium carbonate

[0057] 50 parts by mass of purified black powder and 24 parts by mass of ammonium chloride were mixed and placed in a closed reactor. Nitrogen gas was purged and the temperature was raised to 205°C and held for 18 minutes. After cooling, 200 parts by mass of deionized water were added and stirred at room temperature for 35 minutes. The mixture was then filtered to obtain lithium-containing filtrate and iron-rich filter residue.

[0058] The lithium-containing filtrate was evaporated and concentrated to 1 / 3 of its original volume, then heated to 100°C, and 30 parts by mass of a 20% saturated Na2CO3 solution were added. The mixture was stirred and reacted for 1.5 hours. The crude product was obtained by hot filtration. Finally, the crude lithium carbonate was washed twice with hot water at 85°C and dried at 105°C for 13 hours to obtain lithium carbonate.

[0059] S3: Recycling iron oxides

[0060] The iron-rich filter residue was dried to constant weight at 100℃. The dried iron-rich filter residue was mixed with Na2CO3 at a mass ratio of 1:0.3 and placed in a corundum boat in a tube furnace. Nitrogen gas was first used to replace the air, and then the atmosphere was switched to CO2 at a flow rate of 0.4 L / min. The temperature was increased to 625℃ at 10℃ / min and held for 1 hour. After cooling, the product was added to deionized water at a liquid-solid ratio of 3 mL:1 g and stirred at 80℃ for 35 minutes. The mixture was filtered to obtain a sodium phosphate solution and a water-leached solid residue. The sodium phosphate solution was evaporated and crystallized to recover sodium phosphate.

[0061] After the water-immersed solid residue is dried and ground to a particle size ≤0.5mm, it is then magnetically separated three times using a 0.2T weak magnetic separator to obtain iron oxide containing Fe3O4.

[0062] Comparative Example 1:

[0063] Compared with Example 1, Comparative Example 1 differs in that ultrasonic treatment is not performed in step S1, while the other steps remain unchanged, and is referred to as Comparative Example 1.

[0064] Comparative Example 2:

[0065] Compared with Example 1, Comparative Example 2 differs in that Tween-80 is not added in step S1, while the other steps remain unchanged, and is referred to as Comparative Example 2.

[0066] Comparative Example 3:

[0067] Compared with Example 1, Comparative Example 3 differs in that no heating is performed in step S1, but the system temperature is kept at room temperature of 24°C after ultrasonic treatment, while the other steps remain unchanged. This is referred to as Comparative Example 3.

[0068] The following tests were performed on Examples 1-3 and Comparative Examples 1-3:

[0069] The purified black powder obtained in step S1 was used to determine its aluminum content by inductively coupled plasma atomic emission spectrometry.

[0070] Take the purified black powder obtained in step S1 and determine its total lithium content by inductively coupled plasma atomic emission spectrometry. Then take the lithium carbonate obtained in step S2 and determine its lithium content. Calculate the lithium recovery rate.

[0071] Take the iron-rich filter residue from step S2 and determine its iron content by inductively coupled plasma atomic emission spectrometry. Then take the iron oxide containing Fe3O4 obtained in step S3 and determine its iron content. Calculate the iron recovery rate, as shown in Table 1.

[0072] Table 1

[0073] Example 1 0.21 96.7 95.9 Example 2 0.19 97.0 96.3 Example 3 0.23 96.8 96.1 Comparative Example 1 0.84 89.5 87.8 Comparative Example 2 0.91 88.9 87.2 Comparative Example 3 0.62 91.3 90.5

[0074] As can be seen from Table 1, Examples 1-3 all adopted a three-in-one enhanced dealuminization method of "ultrasound + surfactant + heating", reducing the aluminum content to about 0.2 wt%, with a lithium recovery rate of ≥96.5% and an iron recovery rate of ≥95.8%.

[0075] In Comparative Example 1, ultrasonic treatment was omitted in step S1, and dealuminization was carried out solely by stirring, surfactant, and heating. The results showed that the aluminum content in the purified black powder significantly increased to 0.84%. Due to the high aluminum residue, during the subsequent lithium chlorination process, some aluminum competed with lithium for the chlorination reaction or formed insoluble aluminates to encapsulate lithium, resulting in a lithium recovery rate of 89.5%. Simultaneously, the aluminum residue in the iron-rich filter residue interfered with the sodium carbonate catalytic reduction roasting and magnetic separation processes, reducing the iron recovery rate to 87.8%. This comparison demonstrates that the physical stripping effect generated by ultrasonic cavitation is crucial for breaking the passivation film on the aluminum foil surface and exposing the fresh aluminum surface; without ultrasound, both the dealuminization reaction rate and depth decreased significantly.

[0076] Comparative Example 2 did not add Tween-80 surfactant in step S1, but retained ultrasound and heating. The purified black powder showed the highest aluminum content, reaching 0.91%. This is because the residual graphite carbon in the black powder is highly hydrophobic. Without surfactant, the alkaline solution could not adequately wet and penetrate the aluminum foil surface, resulting in a limited reaction interface. Even with ultrasound and heating, aluminum could not be effectively dissolved. This caused the most severe aluminum contamination, with lithium and iron recovery rates dropping to 88.9% and 87.2%, respectively, the lowest among all comparative examples. This verifies the important role of surfactants in reducing surface tension and improving wettability, making them an indispensable synergistic factor.

[0077] Comparative Example 3 omitted the heating step in step S1; that is, ultrasound and surfactant were present, but the reaction was carried out at room temperature (24°C). At this point, the purified black powder aluminum content was 0.62%, between that of the Example and Comparative Examples 1 and 2; the lithium recovery rate was 91.3%, and the iron recovery rate was 90.5%, significantly lower than the Example but better than the other two Comparative Examples. This indicates that gentle heating significantly accelerates aluminum dissolution; without heating, even with the retention of ultrasound and surfactant, the aluminum removal efficiency would still decrease by approximately 0.4 percentage points, resulting in a reduction of approximately 5-6 percentage points in lithium and iron recovery rates. This result confirms the indispensability of gentle heating in the entire three-in-one strengthening system.

[0078] In Examples 1-3, the carbon gasification conversion rate of the samples with added Na2CO3 was calculated at three temperature points: 600℃, 625℃, and 650℃. Then, the carbon gasification conversion rate of the products obtained in Examples 1-3 without added Na2CO3 was calculated. The carbon gasification conversion rate and CO yield were based on iron-rich filter residue. The results are shown in Table 2. The trend of carbon gasification conversion rate at 600-650℃ is shown below. Figure 2 .

[0079] Table 2

[0080] Example 1 Add to 70.4±1.1 17.58±0.24 1.760±0.028 No additions 14.2±08 3.55±0.12 0.355±0.018 Example 2 Add to 85.6±1.1 21.38±0.24 2.140±0.028 No additions 37.2±0.8 9.29±0.12 0.930±0.018 Example 3 Add to 78.5±1.1 19.61±0.24 1.962±0.028 No additions 23.3±0.8 5.82±0.12 0.583±0.018

[0081] The results showed that at three temperature points of 600℃, 625℃ and 650℃, the carbon gasification conversion rate of the sample with added Na2CO3 was significantly higher than that of the blank group without addition; at 600℃, the carbon conversion rate increased from 14.2% to 70.4% and the CO yield increased from 3.55 mmol / g to 17.59 mmol / g, indicating that effective CO generation capacity can be formed at low temperatures.

[0082] The above samples were kinetically fitted using the apparent first-order kinetic method within the isothermal range: k = -ln(1-Xc) / t, and the apparent activation energy was calculated using the Arrhenius equation lnk = lnA-Ea / (R·T). The results are shown in Tables 3 and 4, and the Arrhenius equation fitting curves are shown below. Figure 3 As shown.

[0083] Table 3

[0084] Example 1 Add to 0.02029 -3.898 No additions 0.00255 -5.971 Example 2 Add to 0.03230 -3.433 No additions 0.00775 -4.860 Example 3 Add to 0.02562 -3.664 No additions 0.00442 -5.421

[0085] Table 4

[0086] <![CDATA[No Na2CO3 added]]> lnk = -17.905·1000 / T + 14.529 0.9995 148.9 <![CDATA[Add Na2CO3]]> lnk = -7.493·1000 / T + 4.682 0.9998 62.3

[0087] The fitting results show that the apparent activation energy decreased from approximately 148.9 kJ / mol to approximately 62.3 kJ / mol after the addition of Na2CO3, a decrease of approximately 58.2%. This is consistent with the experimental trend of a significant increase in CO yield at 600-650℃, and can serve as kinetic support for the effectiveness of the "low-temperature catalytic gasification" technology.

[0088] It should be understood that those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims. Parts not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A process for recovering iron and lithium from spent lithium iron phosphate batteries, characterized in that, Specifically, the following steps are included: S1: Pre-treatment of spent lithium iron phosphate batteries Waste lithium iron phosphate batteries are immersed in sodium hydroxide solution and discharged for 24-26 hours. Then, they are crushed to a particle size of ≤20mm using a shear crusher. The iron shell is then separated by 0.2T magnetic separation, and the copper-aluminum mixture is separated by eddy current separation. The remaining material is passed through a 200-mesh sieve to obtain black powder. 10-20 parts by weight of the black powder are added to 40-50 parts by weight of sodium hydroxide solution for alkaline dealuminization. The product is then filtered, and the filter cake is washed with deionized water until neutral. After drying at 100-105℃ for 12-14 hours, purified black powder is obtained. S2: Recycling lithium carbonate Mix 45-50 parts by weight of purified black powder with 22-24 parts by weight of ammonium chloride and place the mixture in a closed reactor. Under nitrogen protection, heat the mixture to 200-210℃ and hold for 15-20 minutes. After cooling, add 200 parts by weight of deionized water and stir at room temperature for 30-40 minutes. Filter the mixture to obtain a lithium-containing filtrate and an iron-rich filter residue. Recover lithium carbonate from the lithium-containing filtrate by evaporation concentration and a sodium carbonate solution displacement reaction. S3: Recycling iron oxides The iron-rich filter residue was dried to constant weight, mixed evenly with Na2CO3, and then calcined under CO2 atmosphere. After cooling, deionized water was added, and the mixture was filtered to obtain sodium phosphate solution and water-leached solid residue. Sodium phosphate was recovered by evaporation and crystallization of the sodium phosphate solution. The water-leached solid residue was dried, ground, and then magnetically separated using a weak magnetic separator to obtain iron oxide containing Fe3O4.

2. The process for recovering iron and lithium from spent lithium iron phosphate batteries according to claim 1, characterized in that, The alkaline dealumination in step S1 specifically includes the following steps: Add the black powder to the sodium hydroxide solution and stir at 200-300 r / min for 30-40 minutes at room temperature (22-24℃). While stirring, add 0.2% (by weight) of Tween-80 to the sodium hydroxide solution. Place the reaction system in an ultrasonic cleaner or probe-type ultrasonic device and set the ultrasonic power to 100W. Then heat the system to maintain the temperature at 48-52℃ and react for 10-15 minutes under the above conditions.

3. The process for recovering iron and lithium from spent lithium iron phosphate batteries according to claim 2, characterized in that, Step S2, which involves recovering lithium carbonate from the lithium-containing filtrate through evaporation concentration and a sodium carbonate solution displacement reaction, specifically includes the following steps: The lithium-containing filtrate is evaporated and concentrated to 1 / 3 of its original volume, then heated to 90-100℃, and 25-30 parts by mass of a 20% saturated Na2CO3 solution are added. The mixture is stirred and reacted for 1-1.5 hours. The crude product is obtained by hot filtration. Finally, the crude lithium carbonate is washed twice with hot water at 80-85℃ and dried at 100-105℃ for 12-14 hours to obtain lithium carbonate.

4. The process for recovering iron and lithium from spent lithium iron phosphate batteries according to claim 3, characterized in that, Step S3, which involves the recovery of iron oxides, includes the following steps: The iron-rich filter residue was dried to constant weight at 100-110℃. The dried iron-rich filter residue was mixed with Na2CO3 at a mass ratio of 1:(0.3-0.5) and placed in a corundum boat in a tube furnace. Nitrogen gas was first used to replace the air, and then the atmosphere was switched to CO2. The temperature was increased to 600-650℃ at 10℃ / min and held for 1-1.5 hours. After cooling, the product was added to deionized water at a liquid-solid ratio of (3-4) mL:1g and stirred at 80-85℃ for 30-40 minutes. The mixture was filtered to obtain sodium phosphate solution and water-leached solid residue. Sodium phosphate was recovered by evaporating and crystallizing the sodium phosphate solution. After drying and grinding, the water-immersed solid residue is magnetically separated using a 0.2T weak magnetic separator for 2-3 times to obtain iron oxide containing Fe3O4.

5. The process for recovering iron and lithium from spent lithium iron phosphate batteries according to claim 1, characterized in that, The ratio of waste lithium iron phosphate batteries to sodium hydroxide solution is 1g:(150-200)mL.

6. The process for recovering iron and lithium from spent lithium iron phosphate batteries according to claim 2, characterized in that, The concentration of the sodium hydroxide solution is 5 wt%.

7. The process for recovering iron and lithium from spent lithium iron phosphate batteries according to claim 4, characterized in that, The flow rate of the CO2 atmosphere is 0.3-0.5 L / min.

8. The process for recovering iron and lithium from spent lithium iron phosphate batteries according to claim 4, characterized in that, Water-immersed solid residues are ground to a particle size ≤0.5mm.

9. The process for recovering iron and lithium from spent lithium iron phosphate batteries according to claim 4, characterized in that, The non-magnetic slag remaining after water-immersed solid residues undergo magnetic separation can be used as recycled building filler or disposed of in a harmless landfill.

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