Method for high-value recovery of lithium from lithium iron phosphate lithium extraction residue to prepare battery-grade ferrous oxalate

Abstract

The application discloses a method for preparing battery-grade ferrous oxalate from lithium extraction residue of lithium iron phosphate, and belongs to the technical field of lithium iron phosphate recycling. The method comprises the following steps: acid leaching of lithium extraction residue of lithium iron phosphate, impurity removal and purification of iron-phosphorus solution, pre-regulation of phosphorus form, directional coordination synthesis of ferrous oxalate, graded washing and phosphorus solidification, crystal form stabilization and drying. The application realizes pre-regulation of the phosphorus form, adds a complexing agent into the purified iron-phosphorus solution, forms stable phosphorus complexes with the complexing agent and free HPO4 2‑ , reduces the reactivity of the complexes with Fe 2+ , and inhibits the generation of FeHPO4 impurities from the source. The application realizes preferential coordination of oxalate and Fe 2+ through directional coordination synthesis, realizes deep removal of residual phosphorus through graded washing, and prevents adsorption of phosphorus elements in the process of crystal form transformation through crystal form stabilization.

Description

Technical Field

[0001] This invention belongs to the field of lithium iron phosphate recycling technology, specifically relating to a high-value recovery method for lithium iron phosphate extraction slag, and particularly a method for preparing battery-grade ferrous oxalate using lithium iron phosphate extraction slag as raw material. Background Technology

[0002] Preparing battery-grade ferrous oxalate (FeC2O4·2H2O) from lithium iron phosphate extraction slag is a core pathway for achieving closed-loop recycling of lithium iron phosphate cathode materials. Currently, mainstream technologies employ a wet process: acid leaching → reduction and impurity removal → oxalic acid precipitation → crystal form control → washing and drying. However, this process requires significant solutions to address the deep removal of impurities and the presence of Fe... 3+ Key technical challenges include efficient reduction, directional separation of phosphorus and iron, and controllable product morphology and particle size.

[0003] The main drawback of existing technology is that the product purity is substandard, making it difficult to meet battery-grade requirements. Specifically, this manifests as: excessive impurity content: SO4 in the product. 2- and Cl - The content of impurities such as residual phosphorus (100~5000ppm), Ca / Mg / Ni (50~200ppm), and heavy metals (10~50ppm) is generally too high. The product has a wide particle size distribution (D50=5~30μm), irregular morphology, and serious particle agglomeration.

[0004] Furthermore, lithium extraction slag contains a variety of impurities, which leach out simultaneously with iron after acid dissolution, and the conventional neutralization and replacement method is not precise enough in removing impurities. Secondly, phosphorus and iron have a strong chemical bond, and co-precipitation or entrainment easily occurs during oxalic acid precipitation, making it difficult to completely separate phosphorus. Thirdly, the existing process involves fewer washing cycles and uses water with low purity, resulting in high levels of soluble salt residue, which further affects product purity. Therefore, this application provides a method for high-value recovery of lithium iron phosphate residue to prepare battery-grade ferrous oxalate to meet the demand. Summary of the Invention

[0005] This invention aims to provide a method for efficiently separating iron and phosphorus elements from lithium iron phosphate residue and directionally preparing battery-grade ferrous oxalate. Through the synergistic effect of multiple technologies, the formation of impurities such as FeHPO4 is significantly suppressed, ensuring that the purity of ferrous oxalate meets battery-grade standards and satisfies the production requirements of lithium iron phosphate cathode materials for lithium-ion batteries.

[0006] To solve the above-mentioned technical problems, the core logic of this invention is: to regulate the phosphorus speciation of free HPO4. 2- It is converted into a stable phosphorus complex, reducing its affinity for Fe. 2+ The reactivity of oxalate and Fe; through precise process control, the reaction of oxalate and Fe... 2+The preferential coordination of ferrous oxalate is directed to form ferrous oxalate; residual phosphorus impurities are removed by graded washing; and finally, crystal stabilization treatment is performed to prevent ferrous oxalate from adsorbing phosphorus during crystal transformation.

[0007] The overall process route is as follows: acid leaching of lithium iron phosphate residue → purification of iron and phosphorus solution → pre-regulation of phosphorus speciation → directional coordination synthesis of ferrous oxalate → graded washing and phosphorus fixation → crystal stabilization → drying → battery-grade ferrous oxalate product.

[0008] The specific technical solution is as follows: Step 1: Mix the lithium iron phosphate residue with sulfuric acid and a certain amount of water, stir and leach to obtain a phosphorus iron solution.

[0009] Step 2: Add a purifying agent to the phosphorus-iron solution to remove impurity ions that leach out along with the iron and phosphorus, thereby purifying the solution.

[0010] Step 3: Add a complexing agent to the purified ferric phosphate solution and adjust the pH of the system. The complexing agent reacts with the free HPO4 in the system. 2- It combines to form a stable phosphorus complex, reducing its affinity for Fe. 2+ The reactivity of the phosphorus can be improved to inhibit the formation of FeHPO4 impurities at the source and achieve targeted regulation of phosphorus speciation.

[0011] Step 4: Slowly add oxalic acid solution dropwise to the system after phosphorus speciation has been controlled. Utilizing the reducing properties of oxalic acid, the Fe... 3+ Reduced to Fe 2+ Subsequently, by precisely controlling the amount of oxalic acid, the dropping rate, and the reaction temperature, the reaction of oxalate ions with Fe was achieved. 2+ Preferred coordination of the precipitate leads to the directional formation of ferrous oxalate precipitate, further reducing the formation of FeHPO4 impurities.

[0012] Step 5: The synthesized ferrous oxalate precipitate is subjected to solid-liquid separation and washed sequentially with deionized water, dilute oxalic acid solution, and then deionized water. The liquid-to-solid ratio for each wash is 5-8:1, and the washing time is 15-20 minutes. The first stage of washing with deionized water removes soluble salts and attached ions from the surface, the dilute oxalic acid solution further removes residual phosphorus impurities, and the second stage of washing with deionized water removes residual oxalic acid. This staged washing process achieves deep removal of residual phosphorus while reducing the dissolution loss of ferrous oxalate.

[0013] Step 6: Place the washed ferrous oxalate precipitate into a constant temperature oven and dry it at 60~70℃ for 30~40 minutes, then raise the temperature to 100~110℃ and dry it for 120~150 minutes. During the drying process, an inert gas (nitrogen or argon) is introduced for protection to prevent ferrous oxalate from adsorbing phosphorus during crystal transformation and to avoid its oxidation.

[0014] Step 7: Cool the product after crystal stabilization to room temperature and pulverize it to a particle size of D50=1~5μm to obtain the battery-grade ferrous oxalate product. Compared with the prior art, the present invention has at least the following beneficial effects:

[0015] This invention effectively solves the problem of high impurity content in existing products through the synergistic effects of phosphorus speciation pre-regulation, directional coordination synthesis, graded washing and phosphorus fixation, and crystal stabilization. The prepared ferrous oxalate product has a purity of ≥99.5%, with SO4 content significantly reduced. 2- and Cl - The content is ≤150ppm, the residual phosphorus content is ≤50ppm, the total content of metal impurities such as Ca / Mg / Ni is ≤50ppm, and the total heavy metal content is ≤30ppm. All indicators meet the industry standard for battery-grade ferrous oxalate.

[0016] The ferrous oxalate product prepared by this invention has a particle size D50 of 1~5μm, uniform particle size distribution (span ≤1.2), regular flaky morphology, and effectively suppressed agglomeration. It has good dispersibility and can be directly used in the preparation of lithium iron phosphate cathode material for lithium-ion batteries without additional grinding and grading treatment.

[0017] This invention reduces the reactivity of phosphate and ferrous ions by regulating phosphorus speciation, and achieves efficient separation of iron and phosphorus by combining directional coordination synthesis and staged washing. Iron recovery rate is ≥90%, and phosphorus removal rate is ≥97%, effectively solving the problem of low resource utilization rate of lithium iron phosphate extraction residue. Attached Figure Description

[0018] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate embodiments of the present disclosure and, together with the specification, further serve to explain the principles of the present disclosure and enable those skilled in the art to implement and use the present disclosure.

[0019] Figure 1 This is a process flow diagram of the high-value recovery of lithium iron phosphate residue to prepare ferrous oxalate in this invention; Figure 2 This is the XRD pattern of the ferrous oxalate sample in this invention. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0022] like Figure 1 and Figure 2 This invention illustrates a method for high-value recovery of lithium iron phosphate residue to prepare battery-grade ferrous oxalate. Example

[0023] A method for high-value recovery of lithium iron phosphate residue to prepare battery-grade ferrous oxalate, the specific steps of which are as follows: Acid leaching of lithium iron phosphate extraction residue: Select lithium iron phosphate extraction residue crushed to powder (containing 23.6% iron, 11.2% phosphorus, and SO42-). 2- 1.3%, containing Cl - Using 0.8% (Ca / Mg / Ni total content 0.3%, heavy metal total content 0.05%) as raw material, add 98% sulfuric acid and deionized water, heat and stir for 2 hours, filter to remove insoluble impurities, and obtain acid leaching iron phosphorus solution.

[0024] Iron-phosphorus solution purification: Two impurity removal agents are added to the acid-leached iron-phosphorus solution to remove heavy metals and impurity ions such as Ca and Mg.

[0025] Phosphorus speciation pre-regulation: Add 50g of citric acid (complexing agent, the amount added is 5% of the raw material mass) to the purified iron-phosphorus solution, adjust the pH value of the system to 1.5 with dilute alkali, control the system temperature at 45℃, stir at 350r / min, and react for 40 minutes to obtain the phosphorus speciation pre-regulated slurry.

[0026] Directed coordination synthesis of ferrous oxalate: A 1.0 mol / L oxalic acid solution was slowly added dropwise to the phosphorus speciation-controlled slurry at a dropping rate of 6 mL / min. Oxalic acid reacts with Fe... 2+ The molar ratio was 1.08:1. After the addition was complete, the system temperature was maintained at 55℃, the stirring speed was 350 r / min, and the reaction was continued for 75 minutes to form ferrous oxalate precipitate.

[0027] Phosphorus fixation by fractional washing: The reaction slurry was filtered to obtain ferrous oxalate precipitate, which was then washed sequentially with deionized water, 0.15 mol / L dilute oxalic acid solution, and deionized water. The liquid-to-solid ratio for each wash was 6:1, and the washing time was 18 minutes. After washing, the precipitate was filtered until no water dripped.

[0028] Crystal stabilization: The washed ferrous oxalate precipitate was placed in a constant temperature oven and dried at 65°C for 35 minutes, then the temperature was raised to 105°C and dried for 135 minutes. Nitrogen gas was introduced for protection during the drying process, with a nitrogen flow rate of 0.5 L / min.

[0029] Drying and pulverizing: After stabilizing the crystal form, the product is cooled to room temperature and pulverized with a universal pulverizer to a particle size of D50=2.3μm to obtain battery-grade ferrous oxalate product.

[0030] Product testing: The battery-grade ferrous oxalate product prepared in this embodiment was tested, and the results are as follows: purity 99.85%, SO42- ⇌ 100%. 2- Content 82ppm, Cl - The content is 75ppm, phosphorus content is 42ppm, total Ca / Mg / Ni content is 37ppm, total heavy metal content is 20ppm, D50=2.3μm, particle size distribution range is 1.1, iron recovery rate is 95.6%, and phosphorus removal rate is 99.3%. All indicators meet the industry standard for battery-grade ferrous oxalate and can be directly used in the preparation of lithium iron phosphate cathode materials for lithium-ion batteries.

[0031] This invention encompasses any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of this invention. To provide the public with a thorough understanding of this invention, specific details are described in detail in the following preferred embodiments; however, those skilled in the art will fully understand the invention even without these details. Furthermore, to avoid unnecessary misunderstanding of the essence of this invention, well-known methods, processes, procedures, components, and circuits are not described in detail.

[0032] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for high-value recovery and preparation of battery-grade ferrous oxalate from lithium iron phosphate extraction slag, characterized in that, Includes the following steps: Step 1: Acid leaching of lithium iron phosphate residue. The lithium iron phosphate residue is mixed with sulfuric acid and water and stirred to leach out, resulting in a phosphorus iron solution. Step 2: Purification of iron-phosphorus solution. Add an impurity removal agent to the iron-phosphorus solution to remove impurity ions and obtain a purified iron-phosphorus solution. Step 3: Phosphorus speciation pre-regulation. A complexing agent is added to the purified phosphorus-iron liquid to adjust the pH of the system, allowing the complexing agent to react with the free HPO4. 2- This combination forms a stable phosphorus complex, reducing its affinity for Fe. 2+ The reactivity of the reaction is reduced, and the formation of FeHPO4 impurities is inhibited. Step 4: Directional coordination synthesis of ferrous oxalate. Oxalic acid solution is slowly added dropwise to the phosphorus speciation-controlled system, first to... 3 + Reduced to Fe 2+ Then, by controlling the amount of oxalic acid, the dropping rate, and the reaction temperature, the oxalate ions react with Fe... 2+ Preferential coordination leads to the directional formation of ferrous oxalate precipitate; Step 5: Graded washing to solidify phosphorus. After separating the solid and liquid of the ferrous oxalate precipitate, it is washed in sequence with deionized water, dilute oxalic acid solution, and deionized water to remove soluble salts, residual phosphorus, and residual oxalic acid. Step 6: Crystal stabilization. The washed ferrous oxalate precipitate is dried at 60-70℃ for 30-40 minutes under inert gas protection, and then the temperature is raised to 100-110℃ for 120-150 minutes. Step 7: Drying. Cool and pulverize the product after crystal stabilization to a particle size of D50 = 1~5μm to obtain battery-grade ferrous oxalate.

2. The method according to claim 1, characterized in that, The complexing agent in step three is citric acid, and the amount added is 5% of the mass of the lithium iron phosphate residue.

3. The method according to claim 1, characterized in that, In step three, the pH of the system is adjusted to 1.5, the system temperature is 45℃, the stirring speed is 350r / min, and the reaction time is 40 minutes.

4. The method according to claim 1, characterized in that, In step four, the dropping rate of the oxalic acid solution is 6 mL / min, and the oxalic acid reacts with Fe... 2+ The molar ratio was 1.08:1, the reaction temperature was 55℃, and the reaction time was 75 minutes.

5. The method according to claim 1, characterized in that, In step five, the liquid-to-solid ratio for each wash is 5-8:1, and the washing time is 15-20 minutes.

6. The method according to claim 5, characterized in that, In step five, the concentration of the dilute oxalic acid solution is 0.15 mol / L.

7. The method according to claim 1, characterized in that, The inert gas in step six is ​​nitrogen or argon.

8. The method according to claim 1, characterized in that, In step seven, the final battery-grade ferrous oxalate product obtained has a purity of ≥99.5%, of which SO4 2- and Cl - Content ≤150ppm, phosphorus content ≤50ppm, total Ca / Mg / Ni content ≤50ppm, total heavy metal content ≤30ppm, D50=1~5μm.

9. The method according to claim 1, characterized in that, The method achieves an iron recovery rate of ≥90% and a phosphorus removal rate of ≥97%.

10. The method according to claim 1, characterized in that, The battery-grade ferrous oxalate product is used to prepare lithium iron phosphate cathode materials for lithium-ion batteries.

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