Method for synthesizing lithium iron phosphate by using lithium extraction residue of lithium iron phosphate

Abstract

The application discloses a method for synthesizing lithium iron phosphate by using lithium extraction residue of lithium iron phosphate, and the lithium extraction residue is subjected to acid dissolution to obtain a solution containing iron and phosphorus; and the obtained solution containing iron and phosphorus is subjected to hydrothermal treatment to obtain lithium iron phosphate products. The preparation method directly prepares the lithium iron phosphate products from the lithium extraction residue, and does not need to synthesize iron phosphate products first, so that the lithium extraction residue can be used in a high value way, and the method has the advantages of short flow and high resource utilization rate.

Description

Technical Field

[0001] This invention relates to an efficient resource-based treatment method for the residual phosphorus iron slag after lithium extraction from waste lithium iron phosphate black powder, belonging to the field of power battery recycling and resource utilization. Background Technology

[0002] With the explosive growth of the new energy vehicle industry and energy storage demand, the recycling and disposal of spent lithium-ion batteries has become a global environmental and resource concern. Lithium iron phosphate (LiFePO4) batteries are widely used due to their high safety and long cycle life. However, as time goes on, my country will face a wave of retired lithium iron phosphate batteries, making the development of economical and efficient recycling methods for these batteries particularly necessary. Since the added value of iron and phosphorus in lithium iron phosphate batteries is relatively low, current recycling of spent lithium iron phosphate mainly focuses on the "lithium extraction" stage, which involves extracting the higher-value lithium element through pyrometallurgical or hydrometallurgical processes. Selective lithium extraction hydrometallurgical leaching was the mainstream technology for treating spent lithium iron phosphate batteries in the past. This technology creates a "weak acid + strong oxidation" reaction condition, where lithium iron phosphate reacts in solution to obtain a lithium-containing solution and lithium-extraction residue, achieving selective lithium leaching. The lithium-containing solution is then purified, concentrated, and further processed into products such as lithium carbonate.

[0003] The residue after lithium extraction mainly contains iron, phosphorus, graphite, and some impurities. Iron and phosphorus primarily exist in the form of ferric phosphate (FePO4), ferric hydroxyphosphate (Fe5(PO4)4(OH)3·2H2O), and hydrated ferric phosphate. Currently, ferric phosphate slag is typically treated as low-value solid waste. Some articles also suggest using it as a heavy metal adsorbent in water or preparing products such as ferric phosphate (FePO4). However, the overall utilization rate of ferric phosphate slag is low, and its resource utilization currently suffers from problems such as long processes, high energy consumption, and insufficient high-value utilization of iron and phosphorus resources. Summary of the Invention

[0004] The main objective of this invention is to provide a method for directly synthesizing lithium iron phosphate from waste lithium iron phosphate extraction slag, aiming to solve the technical problems of long processing flow, low added value, and resource waste of iron-phosphate slag after lithium extraction in the prior art.

[0005] To achieve the above objectives, the present invention provides a method for synthesizing lithium iron phosphate using lithium iron phosphate extraction residue, comprising: The residual phosphorus-iron slag after selective lithium iron phosphate black powder is treated by selective lithium extraction leaching process. The slag is then acid-leached to obtain a leachate containing iron and phosphorus, thus separating iron and phosphorus from graphite. The phosphorus-iron slag contains 20%~50% Fe, 10%~30% P, 1%~10% C, 0.01%~0.3% Li, <0.1% Cu and <0.1% Al by mass percentage. The main chemical components of the phosphorus-iron slag include iron phosphate, hydroxy iron phosphate, iron phosphate hydrate and graphite. The pH of the leachate is adjusted to 4-7. The pH-adjusted leachate is mixed with lithium-containing substances. The molar ratio of lithium, iron, and phosphorus in the mixed solution is adjusted. Additives are added for hydrothermal synthesis to obtain lithium iron phosphate powder. The reaction temperature range during the hydrothermal synthesis process is 160℃-250℃. The additives are iron powder and ascorbic acid.

[0006] Preferably, the molar ratio of lithium, iron, and phosphorus in the mixed solution is adjusted to a range of 6:1:1 to 1:1:1.

[0007] Preferably, the molar ratio of lithium, iron, and phosphorus in the mixed solution is adjusted to a range of 2:1:1.

[0008] Preferably, when adjusting the lithium, iron, and phosphorus components in the mixed solution, the lithium-containing substance added is one or more of lithium sulfate, lithium carbonate, and lithium hydroxide, and the phosphorus-containing substance added is one or more of ammonium phosphate, hydrogen phosphate, and dihydrogen phosphate.

[0009] Preferably, the reaction time in the hydrothermal synthesis process is 2 to 20 hours.

[0010] Preferably, the amount of iron powder added is equal to the Fe content in the solution. 3+ The amount of ascorbic acid added is 10% to 100% of the molar amount of Fe in the solution. 3+ 5% to 100% of the molar amount.

[0011] Preferably, the amount of iron powder added is equal to the Fe content in the solution. 3+ 50% of the molar amount, the amount of ascorbic acid added is equal to the Fe in the solution. 3+ 15% of the molar weight.

[0012] The basic idea and technical principle of this invention are as follows: This invention directly utilizes the residue from lithium extraction of waste lithium iron phosphate as raw material. Although the lithium content in the residue is reduced, the crystal structure or iron-phosphorus framework of lithium iron phosphate is still retained. Iron and phosphorus are transferred into the solution through acid dissolution, and then lithium iron phosphate is directly crystallized in situ through a hydrothermal reaction. This method skips the complex steps of "preparing iron phosphate - mixed lithium source - high-temperature solid-state sintering" in traditional processes, achieving a leap from "waste residue" to "high-end cathode material".

[0013] Compared with the prior art, the present invention has the following beneficial effects: (1) Short process and low cost: There is no need to convert lithium extraction slag into high-purity iron phosphate intermediate. It can be synthesized directly by hydrothermal method, which saves the iron phosphate preparation process, greatly reduces energy consumption and production cost. At the same time, the solution after hydrothermal reaction can be recycled.

[0014] (2) High resource utilization rate: It realizes the full recycling and utilization of iron and phosphorus resources in waste batteries, avoids the generation of secondary solid waste, and conforms to the development concept of circular economy.

[0015] (3) Excellent product performance: By controlling the pH value, molar ratio and additives of hydrothermal synthesis, lithium iron phosphate powder with high crystallinity can be obtained. Attached Figure Description

[0016] Figure 1 The XRD pattern of product A obtained in Example 1; Figure 2 The XRD pattern of product B obtained in Comparative Example 1 is shown. Figure 3 The image shows the XRD pattern of product C obtained in Comparative Example 2. Detailed Implementation

[0017] The present invention will be described in detail with reference to the following embodiments.

[0018] Example 1 Take 20g of the phosphorus iron slag remaining after selective lithium extraction from waste lithium iron phosphate black powder. The elemental content of the phosphorus iron slag is shown in Table 1: Table 1 Elemental content in the residue Add 200 mL of 2 mol / L sulfuric acid solution to the phosphorus-iron slag for acid leaching to obtain a leachate mainly containing iron and phosphorus.

[0019] Add ammonia to the leachate to adjust the pH of the solution to 6.0.

[0020] Weigh out lithium sulfate solid and add it to the above solution, adjusting the molar ratio of Li:Fe:P in the solution to 2:1:1.

[0021] Add iron powder to the mixed solution (the amount added is equal to the amount of Fe in the solution). 3+ 50% of the molar amount) and ascorbic acid (added in the amount of Fe in the solution) 3+ 15% (molar amount) as a compound additive.

[0022] The mixture was transferred to a hydrothermal reactor and reacted at 200°C for 6 hours.

[0023] After the reaction was completed, the product was cooled, filtered, washed, and dried to obtain product A.

[0024] Comparative Example 1 The additive in Example 1 was replaced with iron powder (the amount added was equal to the Fe in the solution). 3+ (65% of the molar amount), with other conditions remaining unchanged, product B is obtained.

[0025] Comparative Example 2 The additive in Example 1 was replaced with ascorbic acid (the amount added was equal to the Fe in the solution). 3+ (65% of the molar amount), with other conditions remaining unchanged, product C is obtained.

[0026] X-ray diffraction (XRD) analysis and chemical composition analysis were performed on the products obtained in Example 1 and Comparative Examples 1-2 to analyze the phase composition and the molar ratios of Li:Fe and Li:P in the products. A molar ratio closer to 1 indicates better material properties. Results are as follows: Figure 1-3 As shown.

[0027] from Figure 1 It can be observed that product A, obtained by adding a combination of iron powder and ascorbic acid, shows a clear and sharp characteristic peak of LiFePO4, with no other impurity peaks, indicating that the product is mainly LiFePO4. The molar ratios of Li:Fe and Li:P in the product are 0.98 and 1.01, respectively. In contrast, products B and C, obtained by adding iron powder alone and ascorbic acid alone, contain characteristic peaks of LiFe(PO4)(OH) and Fe3(PO4)2(OH)2, respectively, in addition to the characteristic peak of LiFePO4, indicating that the products contain more impurity phases. Furthermore, the molar ratios of Li:Fe and Li:P in product B are 0.84 and 0.93, respectively; and the molar ratios of Li:Fe and Li:P in product C are 0.92 and 0.85, respectively.

[0028] Example 2 Based on Example 1, the additive was changed to iron powder (the amount added was equal to the Fe in the solution). 3+ 30% of the molar amount) and ascorbic acid (added in the amount of Fe in the solution) 3+ The product was obtained with 35% molar amount of LiFePO4, and other conditions remained unchanged. XRD analysis showed clear and sharp characteristic peaks of LiFePO4. The molar ratios of Li:Fe and Li:P in the product were detected to be 0.94 and 1.03, respectively.

[0029] Example 3 Based on Example 1, the additive was changed to iron powder (the amount added was equal to the Fe in the solution). 3+ 15% of the molar amount) and ascorbic acid (added in the amount of Fe in the solution) 3+(50% of the molar amount), with other conditions remaining unchanged. The obtained product showed clear and sharp characteristic peaks of LiFePO4 by XRD analysis, and the molar ratios of Li:Fe and Li:P in the product were detected to be 0.95 and 0.98, respectively.

[0030] Comparative Example 3 Take 20g of phosphorus iron slag remaining after selective lithium extraction from waste lithium iron phosphate black powder. The elemental content of the phosphorus iron slag is shown in Table 1.

[0031] Add 200 mL of 2 mol / L sulfuric acid solution to the phosphorus-iron slag for acid leaching to obtain a leachate mainly containing iron and phosphorus.

[0032] Iron powder was added to the leachate, and the solution was filtered to obtain filtrate and residue. Add ammonia to the filtrate to adjust the pH of the solution to 6.0.

[0033] Weigh out lithium sulfate solid and add it to the above solution, adjusting the molar ratio of Li:Fe:P in the solution to 2:1:1.

[0034] Add ascorbic acid to the mixed solution (the amount added is equal to the Fe content in the solution). 3+ (65% of the molar weight).

[0035] The mixture was transferred to a hydrothermal reactor and reacted at 200°C for 6 hours.

[0036] After the reaction is complete, the product is obtained by cooling, filtering, washing, and drying.

[0037] XRD analysis revealed characteristic peaks of LiFePO4 and Fe3(PO4)2(OH)2 in the product, with molar ratios of Li:Fe and Li:P of 0.91 and 0.93, respectively. This indicates the presence of impurity phases in the product.

Claims

1. A method for synthesizing lithium iron phosphate from lithium iron phosphate extraction residue, characterized in that, include: The residual phosphorus-iron slag after selective lithium iron phosphate black powder is treated by selective lithium extraction leaching process. The slag is then acid-leached to obtain a leachate containing iron and phosphorus, thus separating iron and phosphorus from graphite. The phosphorus-iron slag contains 20%~50% Fe, 10%~30% P, 1%~10% C, 0.01%~0.3% Li, <0.1% Cu and <0.1% Al by mass percentage. The main chemical components of the phosphorus-iron slag include iron phosphate, hydroxy iron phosphate, iron phosphate hydrate and graphite. The pH of the leachate is adjusted to 4-7. The pH-adjusted leachate is mixed with lithium-containing substances. The molar ratio of lithium, iron, and phosphorus in the mixed solution is adjusted. Additives are added for hydrothermal synthesis to obtain lithium iron phosphate powder. The reaction temperature range during the hydrothermal synthesis process is 160℃-250℃. The additives are iron powder and ascorbic acid.

2. The method as described in claim 1, characterized in that, Adjust the molar ratio of lithium, iron, and phosphorus in the mixed solution to a range of 6:1:1 to 1:1:

1.

3. The method as described in claim 2, characterized in that, Adjust the molar ratio of lithium, iron, and phosphorus in the mixed solution to a range of 2:1:

1.

4. The method as described in claim 1, characterized in that, When adjusting the lithium, iron, and phosphorus components in the mixed solution, the lithium-containing substance added is one or more of lithium sulfate, lithium carbonate, and lithium hydroxide, and the phosphorus-containing substance added is one or more of ammonium phosphate, hydrogen phosphate, and diammonium phosphate.

5. The method as described in claim 1, characterized in that, The reaction time in the hydrothermal synthesis process is 2~20h.

6. The method as described in claim 1, characterized in that, The amount of iron powder added is equal to the Fe content in the solution. 3+ The amount of ascorbic acid added is 10% to 100% of the molar amount of Fe in the solution. 3+ 5% to 100% of the molar amount.

7. The method as described in claim 6, characterized in that, The amount of iron powder added is equal to the Fe content in the solution. 3+ 50% of the molar amount, the amount of ascorbic acid added is equal to the Fe in the solution. 3+ 15% of the molar weight.

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