A method for low-temperature and high-efficiency deconstruction and recovery of lithium based on local strengthening effect of sodium peroxide

Abstract

The application relates to the technical field of waste lithium ion battery recycling, in particular to a method for lithium recovery by low-temperature efficient deconstruction of lithium iron phosphate based on the local strengthening effect of sodium peroxide. The method comprises the following steps: S1, uniformly mixing lithium iron phosphate material and sodium peroxide according to a preset mass ratio, grinding and refining to obtain a dry mixture containing sodium peroxide; S2, placing the mixture in a calcination device to obtain a calcination product; S3, water immersion separation: stirring and leaching the calcination product in deionized water, and performing solid-liquid separation to obtain a lithium-containing aqueous solution and an iron-based leaching residue; S4, lithium recovery and purification: removing impurities from the lithium-containing aqueous solution by separation and purification to obtain a lithium salt product; and S5, iron resource recovery: performing acid leaching treatment on the iron-based leaching residue to obtain an iron-containing leaching solution, so that the iron resource recovery is realized. The application solves multiple technical bottlenecks such as large acid consumption in the existing hydrometallurgy, high energy consumption in the pyrometallurgy and insufficient activity of conventional alkaline salt calcination.

Description

Technical Field

[0001] This application relates to the field of waste lithium-ion battery recycling technology, and in particular to a method for low-temperature and efficient deconstruction and recovery of lithium from lithium iron phosphate based on the local enhancement effect of sodium peroxide. Background Technology

[0002] With the rapid development of new energy vehicles and energy storage industries, lithium iron phosphate batteries have been widely used due to their advantages such as high safety, long cycle life, and low cost. As a large number of lithium iron phosphate batteries enter their retirement period, their recycling and regeneration can not only alleviate the shortage pressure of key resources such as lithium and iron, but also reduce environmental pollution, thus having significant resource value and environmental significance.

[0003] Currently, lithium iron phosphate (LFP) recovery technologies mainly include hydrometallurgy, pyrometallurgy, and salt-assisted roasting. Hydrometallurgy dissolves the metal through strong acid leaching, but suffers from high acid and alkali consumption, high wastewater treatment costs, and difficulty in separating lithium from iron. Pyrometallurgy, while suitable for large-scale processing, requires traditional processes at temperatures of 800–1200°C, resulting in extremely high energy consumption and potential metal volatilization losses. Salt-assisted roasting lowers the roasting temperature by adding inorganic salts. Alkaline salt roasting has attracted attention due to its ability to generate soluble lithium salts; however, conventional alkaline salts (such as sodium hydroxide) lack sufficient activity at low temperatures, requiring temperatures above 500°C for effective decomposition, and still suffer from high energy consumption and incomplete reactions.

[0004] In existing technologies, sodium peroxide, as a strong oxidizing alkaline reagent, is used for the decomposition of some minerals. However, its application in the field of lithium iron phosphate (LFP) recovery faces significant technical bottlenecks: Conventionally, at temperatures below 300°C, the oxidizing and alkali-fusing activities of sodium peroxide are difficult to activate, failing to effectively destroy the stable olivine structure of LFP; while excessive use of sodium peroxide can lead to runaway reactions and product encapsulation. Therefore, developing a method for low-temperature, efficient decomposition of LFP using sodium peroxide to address the high energy consumption and low efficiency of existing technologies has become an urgent need in the LFP recovery field. Summary of the Invention

[0005] This application provides a method for low-temperature, high-efficiency deconstruction and recovery of lithium from lithium iron phosphate based on the local enhancement effect of sodium peroxide, the method comprising: S1. Pretreatment of mixture: Lithium iron phosphate material and sodium peroxide are mixed evenly according to a preset mass ratio, ground and refined to make the particle size of the mixture ≤50μm, and dried at 100~140℃ for 1~3h to obtain a dry mixture containing sodium peroxide. S2. Low-temperature roasting and deconstruction: The mixture is placed in a roasting device and roasted at 250-320°C for 1.5-3 hours under ventilation conditions. Sodium peroxide undergoes an oxidation-alkali fusion synergistic reaction during roasting and forms local exothermic micro-hot spots to produce a local strengthening effect, thereby destroying the lithium iron phosphate lattice structure and obtaining the roasted product. The roasted product contains lithium sodium mixed phosphate and sodium ferrite NaFeO2. S3. Water leaching separation: The roasted product is added to deionized water at a solid-liquid ratio of 1:10 to 1:30 g / mL and stirred for leaching. Solid-liquid separation is then performed to obtain a lithium-containing aqueous solution and an iron-based leaching residue. S4. Lithium recovery and purification: The lithium-containing aqueous solution is separated and purified to remove impurities and obtain lithium salt products; S5. Iron Resource Recovery: The iron-based leaching residue is subjected to acid leaching treatment to obtain iron-containing leachate, thereby realizing iron resource recovery.

[0006] The above technical solution involves mixing lithium iron phosphate and sodium peroxide in a specific ratio, drying at 100–140°C, and then calcining at 250–320°C under ventilation. This process utilizes the thermal decomposition of sodium peroxide to release oxygen and simultaneously undergo a strongly alkaline melting reaction, oxidizing Fe... 2+ For Fe 3+ and with PO4 3- Li + Soluble lithium-sodium mixed phosphate (LiNa5(PO4)2) and insoluble sodium ferrite (NaFeO2) are generated in situ. The process generates micro-hot spots due to localized intense exothermic reactions, which significantly enhances the reaction kinetics. This allows the olivine lattice, which normally requires temperatures above 500°C to be destroyed, to be completely deconstructed at temperatures far below that. Subsequently, selective leaching of lithium and enrichment of iron residues can be achieved simply by water immersion. The lithium-containing liquid and iron slag are then purified and recovered in a targeted manner. This achieves efficient and synergistic separation and resource utilization of lithium and iron under the premise of low temperature, low energy consumption, and low acid and alkali consumption, solving multiple technical bottlenecks such as high acid consumption in existing hydrometallurgical processes, high energy consumption in pyrometallurgical processes, and insufficient activity in conventional alkaline salt roasting.

[0007] Optionally, the lithium iron phosphate material has a purity of ≥95%, a particle size of <100μm, and a total content of copper and aluminum impurities of ≤3%.

[0008] By limiting the purity, particle size, and impurity content of lithium iron phosphate raw materials, the above technical solutions ensure that the main component is LiFePO4, preventing non-target components (such as conductive carbon, binder residues, or transition metal oxides) from interfering with the oxidation-alkali fusion path of sodium peroxide and preventing the formation of insoluble impurity phases by side reactions. On the other hand, controlling the particle size to <100μm increases the specific surface area, enhances the interfacial contact efficiency with sodium peroxide, and promotes the uniform triggering of local strengthening effects. At the same time, limiting the total amount of copper and aluminum impurities to ≤3% can effectively inhibit the formation of complex sodium aluminate or sodium cuprate coatings with sodium and phosphorus during roasting, ensuring the crystal phase integrity and water solubility of lithium-sodium mixed phosphate, thereby supporting the technical effect of lithium leaching rate ≥90% and subsequent lithium carbonate product purity ≥98%.

[0009] Optionally, in step S1, the mass ratio of sodium peroxide to lithium iron phosphate material is 1.4:1 to 2.0:1, preferably 1.6:1 to 1.8:1.

[0010] Through the above technical solution, by precisely controlling the mass ratio of sodium peroxide to lithium iron phosphate, sufficient Na can be provided during the roasting process. + It participates in the formation of the LiNa5(PO4)2 structure and has sufficient O2. - / ·O2 - The oxidizing power completely converts Fe²⁺ into Fe. 3+ To ensure stable entry into the NaFeO2 lattice while avoiding excessive sodium peroxide causing violent exothermic reactions that could lead to localized melting and agglomeration of the material or an increase in side reactions; when the mass ratio is 1.4:1, the oxidation and alkali melting capabilities are critically sufficient, and lattice destructive processes can still be achieved, but the lithium leaching rate is slightly lower; when the mass ratio reaches 1.8:1, the local strengthening effect is strongest, the density and intensity of micro-hot spots are optimal, and the lithium leaching rate reaches over 97%; however, when the mass ratio exceeds 2.0:1, although the reaction is more vigorous, it is easy to cause sodium salt supersaturation precipitation covering the unreacted interface, which reduces the lithium release efficiency. This synergistic mechanism has been verified by the complete disappearance of the LiFePO4 characteristic peak in the XRD spectrum and the change curve of lithium leaching rate in ICP detection.

[0011] Optionally, in step S2, the preferred roasting temperature is 300±10℃, and the preferred roasting time is 2~2.5h.

[0012] By limiting the calcination temperature to the range of 300±10℃ and combining it with a holding time of 2 to 2.5 hours, sodium peroxide begins to decompose controllably at this temperature and continuously releases active oxygen species, simultaneously stimulating alkali fusion behavior. This ensures the full progress of the oxidation-alkali fusion synergistic reaction while avoiding excessively high temperatures that would approach the traditional high-energy-consumption range and lose the "low-temperature" advantage. At this temperature, the activation energy of the reaction is significantly reduced, and the formation rate and stability of micro-hot spots reach an optimal balance. The 2.5-hour holding time ensures complete crystal phase transformation, and XRD detection confirms the absence of LiFePO4 residue. If the time is shortened to 2 hours, the reaction in some areas is insufficient, and the lithium leaching rate decreases by about 3%. If the time is extended to 3 hours, there is no significant gain and energy consumption increases. This demonstrates that the parameter combination provides a dual guarantee for both reaction sufficiency and energy saving.

[0013] Optionally, in step S3, the preferred solid-liquid ratio of the water immersion separation process is 1:20 g / mL.

[0014] The above technical solution, by setting the solid-liquid ratio to 1:20 g / mL, ensures complete dissolution of the lithium-sodium mixed phosphate (LiNa5(PO4)2), avoids excessive water usage leading to excessive load on subsequent evaporation and concentration, and also prevents insufficient dissolution due to insufficient water usage. Experimental verification shows that this ratio can achieve a lithium leaching rate of ≥96.5% within a leaching time of 45 min at room temperature to 40℃, while maintaining an iron content of <1.0 mg / g in the leachate, ensuring efficient separation of lithium and iron. If the solid-liquid ratio is lower than 1:15, lithium dissolution is limited; if it is higher than 1:25, although lithium is completely dissolved, the solution concentration is too low, significantly increasing the evaporation energy consumption in step S4. Therefore, this preferred value achieves a balance between reaction sufficiency and energy economy.

[0015] Optionally, in step S3, the water immersion temperature of the water immersion separation process is room temperature to 40°C, and the immersion time is 30 to 50 minutes, preferably 45 minutes.

[0016] Through the above technical solution, in this invention, by controlling the water immersion temperature within the range of room temperature to 40°C, no additional heating is required, saving energy. Moreover, this temperature is sufficient to maintain the high solubility and diffusion rate of lithium-sodium mixed phosphate. The leaching time is set to 30-50 minutes, which is the minimum sufficient time window determined based on the dissolution kinetics of lithium-sodium mixed phosphate at this temperature. Preferably, 45 minutes can ensure that dissolution equilibrium is achieved. ICP test shows that the lithium leaching rate has reached its peak at this time, and further extending the time will increase the leaching rate by less than 0.5%, while the iron dissolution remains below the detection limit. This indicates that the time parameter accurately matches the dissolution rate of the target substance and the impurity suppression requirements, demonstrating the high efficiency separation characteristics under mild conditions.

[0017] Optionally, in step S4, the separation and purification process includes: The lithium-containing aqueous solution is subjected to ion exchange or evaporation concentration to remove excess sodium ions; After adding Na2CO3 to the lithium-containing aqueous solution after removing excess sodium ions, Li2CO3 is precipitated and obtained, with the purity of Li2CO3 being ≥98%.

[0018] The above technical solution, by first removing excess sodium ions and then adding sodium carbonate for precipitation, avoids the formation of sodium ions. + With CO3 2- Co-precipitation can form Na₂CO₃ inclusions or affect the crystal morphology of Li₂CO₃, ensuring the selectivity of the precipitation reaction; ion exchange or evaporation concentration can remove Na₂CO₃ inclusions. + / Li + The molar ratio was reduced from >10 to <1.2, significantly improving the purity of lithium carbonate crystals. Subsequently, Na2CO3 was added at 60℃ and reacted for 30 min, which not only accelerated the nucleation and growth rate but also avoided the aggregation of Li2CO3 or the introduction of new impurities due to high temperature. The XRD pattern of the final product was in perfect agreement with the standard card JCPDS No. 00-018-1049, with a purity of 99.2%, meeting the requirements of battery-grade lithium salts.

[0019] Optionally, in step S4, the reaction conditions for the lithium-containing aqueous solution and Na2CO3 are: stirring at 60°C for 30 minutes.

[0020] Through the above technical solution, by setting the reaction temperature to 60℃ and maintaining stirring for 30 minutes, the Li... + With CO3 2- Collision frequency and reaction rate promote uniform generation and moderate growth of Li2CO3 crystal nuclei, resulting in a concentrated particle size distribution (D). 50 The crystals are approximately 8.2 μm in size and are easy to filter and wash. On the other hand, it avoids problems such as local boiling, rapid evaporation of water and local supersaturation precipitation of Na2CO3 that may be caused by higher temperatures (such as above 80°C), thus ensuring the stoichiometry and phase purity of the precipitate. This condition works synergistically with the aforementioned sodium removal step to jointly support the technical effect of Li2CO3 purity ≥98%, and has been consistently verified by the XRD and ICP-OES data of consecutive batches of products in Examples 1 to 3.

[0021] Optionally, in step S5, the acid solution used in the acid leaching process is either hydrochloric acid or sulfuric acid, and the concentration of the acid solution is 2 to 6 mol / L, preferably 3 to 4 mol / L.

[0022] The above technical solution, by selecting hydrochloric acid or sulfuric acid as the leaching agent and controlling the concentration within the range of 2-6 mol / L, can effectively dissolve the iron-based components (mainly amorphous Fe(OH)3 and a small amount of incompletely converted NaFeO2) in the roasted product, generating soluble FeCl3 or Fe2(SO4)3. When the concentration is below 2 mol / L, the acid strength is insufficient, and the iron leaching rate is <90%. When the concentration is above 6 mol / L, it will aggravate equipment corrosion and increase the difficulty of waste acid treatment. The preferred concentration of 3-4 mol / L can achieve an iron leaching rate of ≥95.6% under the conditions of 50℃ and 60 min, with reasonable acid consumption and safe operation, reflecting the dual suitability of this parameter for iron resource recovery efficiency and industrial applicability.

[0023] Optionally, in step S5, the acid leaching temperature is 30–70°C, the leaching time is 30–90 min, and the iron leaching rate is ≥95%.

[0024] By setting the acid leaching temperature to 30–70℃ and controlling the leaching time to 30–90 min, the reaction kinetics are significantly improved by using moderate heating to accelerate the diffusion of H⁺ into the iron-based slag and the breaking rate of Fe-O bonds. At the same time, the reaction kinetics are not aggravated by acid volatilization or re-aggregation of ferric hydroxide colloids caused by high temperature (>70℃). At 30℃, it takes 90 min to achieve a leaching rate of 95%, while at 50℃, a stable leaching rate of 95.6% can be achieved in 60 min. This shows that the temperature-time window can ensure efficient iron leaching while taking into account energy consumption control and process robustness. This effect can be directly verified by the complete disappearance of the characteristic peak of Fe(OH)₃ in the iron-based slag in the XRD pattern and the iron leaching rate data in ICP detection. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application 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 this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 A flowchart of a method for low-temperature and high-efficiency decomposition and recovery of lithium from lithium iron phosphate based on the local enhancement effect of sodium peroxide is provided as an embodiment of this application; Figure 2 An X-ray diffraction (XRD) pattern of lithium iron phosphate, a raw material, provided in an embodiment of this application; Figure 3 An X-ray diffraction (XRD) pattern of a calcined product provided in an embodiment of this application; Figure 4ICP test results of the calcined product aqua regia digest and water extract provided in an embodiment of this application; Figure 5 The XRD pattern of a lithium salt product (lithium carbonate) provided in an embodiment of this application; Figure 6 X-ray diffraction (XRD) pattern of water leaching residue provided in an embodiment of this application Figure 7 This is a curve showing the change in lithium leaching rate corresponding to different mass ratios of sodium peroxide to lithium iron phosphate, provided in one embodiment of this application. Figure 8 The lithium leaching rate variation curves corresponding to different calcination temperatures are provided in one embodiment of this application. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0028] Furthermore, the term "and / or" in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this article, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship.

[0029] The embodiments of this application will now be described in further detail with reference to the accompanying drawings.

[0030] This invention provides a method for the low-temperature, high-efficiency decomposition and recovery of lithium from lithium iron phosphate based on the local enhancement effect of sodium peroxide. The overall technical solution is as described in this application, mainly including the following core technical elements: pretreatment of mixed materials, low-temperature roasting and decomposition, water leaching separation, lithium recovery and purification, and iron resource recovery. These technical elements work together to constitute the overall technical solution of this invention.

[0031] Example 1 Weigh 1.0g of lithium iron phosphate material and 1.8g of sodium peroxide, mix them evenly in an agate mortar, and grind for 15min until the particle size is <50μm; transfer to an oven and dry at 120°C for 2h to obtain a dry mixture containing sodium peroxide; place the mixture in a corundum crucible, place it in a muffle furnace, place it in a ventilated calcination apparatus, keep the system ventilated, raise the temperature to 300°C at a rate of 5°C / min, hold for calcination for 2.5h, and cool naturally to room temperature to obtain a gray-black calcined product; transfer the calcined product to a beaker, add 20mL of deionized water at a solid-liquid ratio of 1:20g / mL, magnetically stir and leach for 45min at room temperature, vacuum filter to obtain a colorless transparent lithium-containing aqueous solution and a reddish-brown iron-based leaching residue; concentrate the lithium-containing aqueous solution to 1 / 3 of its original volume in a rotary evaporator, and pass it through a cation exchange resin (Dowex 50WX8, H + (Type) column treatment to remove excess Na + The effluent was collected, and 1.2 times the theoretical amount of Na₂CO₃ solution was added. The mixture was stirred in a 60°C water bath for 30 min, allowed to stand for 30 min, filtered, washed three times with deionized water, and dried under vacuum at 60°C for 12 h to obtain white lithium carbonate powder. The iron-based leaching residue was added to 10 mL of 4 mol / L hydrochloric acid solution, stirred and leached in a 50°C water bath for 60 min, and filtered to obtain a yellow iron-containing leaching solution. XRD test ( Figure 2 , Figure 3 The results showed that the characteristic diffraction peaks of the raw material lithium iron phosphate at 20.6°, 26.2°, and 30.4° completely disappeared in the calcined product, and characteristic peaks of LiNa5(PO4)2 (PDF#86-1527) and NaFeO2 (PDF#75-1601) appeared in the calcined product; ICP-OES test ( Figure 4 The results showed that the Li content in the aqua regia digest was 35.2 mg / g, the Li content in the water extract was 34.1 mg / g, and the lithium leaching rate was 96.9%; the iron content in the water extract was only 0.8 mg / g; the obtained lithium carbonate XRD spectrum ( Figure 5 The XRD pattern of the iron-based leaching residue is completely consistent with the standard card JCPDS No. 00-018-1049, with a purity of 99.2%; Figure 6 The sample mainly consisted of amorphous Fe(OH)3, and after leaching with 4 mol / L hydrochloric acid, the iron leaching rate was 95.6%.

[0032] Example 2 Under the same preparation conditions as in Example 1, only the mass ratio of sodium peroxide to lithium iron phosphate in step S1 was adjusted from 1.8:1 to 1.6:1 (i.e., 1.0g of lithium iron phosphate corresponds to 1.6g of sodium peroxide). The calcined product was then subjected to water leaching, lithium purification, and iron acid leaching in sequence. The results showed that the product still achieved the technical effects of the present invention, with a lithium leaching rate of 92.3%, a lithium carbonate purity of 98.8%, and an iron leaching rate of 95.1%. This demonstrates that the technical solution of the present invention has good feasibility and stability within a mass ratio range of 1.4:1 to 2.0:1.

[0033] Example 3 Under the same preparation conditions as in Example 1, only the mass ratio of sodium peroxide to lithium iron phosphate in step S1 was adjusted from 1.8:1 to 2.0:1 (i.e., 1.0g of lithium iron phosphate corresponds to 2.0g of sodium peroxide). The calcined product was then subjected to water leaching, lithium purification, and iron acid leaching in sequence. The results showed that the product still achieved the technical effects of the present invention, with a lithium leaching rate of 84.4%, a lithium carbonate purity of 98.5%, and an iron leaching rate of 95.3%. This demonstrates that the technical solution of the present invention has good feasibility and stability within a mass ratio range of 1.4:1 to 2.0:1.

[0034] Example 4 With all other preparation conditions the same as in Example 1, only the calcination temperature in step S2 was adjusted from 300°C to 280°C. The calcined product was then subjected to water leaching, lithium purification, and iron leaching in sequence. The results showed that the product still achieved the technical effects of the present invention, with a lithium leaching rate of 90.5%, a lithium carbonate purity of 98.6%, and an iron leaching rate of 94.8%. This demonstrates that the technical solution of the present invention has good feasibility and stability within the calcination temperature range of 250–320°C.

[0035] Example 5 With all other preparation conditions the same as in Example 1, only the calcination time in step S2 was adjusted from 2.5 h to 2.0 h. The calcined product was then subjected to water leaching, lithium purification, and iron acid leaching in sequence. The results showed that the product still achieved the technical effects of the present invention, with a lithium leaching rate of 93.1%, a lithium carbonate purity of 99.0%, and an iron leaching rate of 95.2%. This demonstrates that the technical solution of the present invention has good feasibility and stability within a calcination time range of 1.5–3.0 h.

[0036] Example 6 With all other preparation conditions the same as in Example 1, only the water leaching temperature in step S3 was adjusted from room temperature to 40°C, while the other conditions remained unchanged, to obtain a lithium-containing aqueous solution and an iron-based leaching residue. The results showed that the product still achieved the technical effects of the present invention, with a lithium leaching rate of 97.1%, a lithium carbonate purity of 99.3%, and an iron leaching rate of 95.0%, thus demonstrating that the technical solution of the present invention has good feasibility and stability within the water leaching temperature range of room temperature to 40°C.

[0037] Example 7 With all other preparation conditions the same as in Example 1, only the leaching time in step S3 was adjusted from 45 min to 30 min to obtain a lithium-containing aqueous solution and an iron-based leaching residue. The results show that the product still achieves the technical effects of the present invention, with a lithium leaching rate of 91.7%, a lithium carbonate purity of 98.7%, and an iron leaching rate of 94.9%, thus demonstrating that the technical solution of the present invention has good feasibility and stability within a leaching time range of 30–50 min.

[0038] Example 8 With all other preparation conditions the same as in Example 1, only the sodium carbonate precipitation reaction temperature in step S4 was adjusted from 60°C to 50°C, while the other conditions remained unchanged, to obtain lithium carbonate. The results showed that the product still achieved the technical effects of the present invention, with a lithium carbonate purity of 98.4% and a lithium recovery rate of 96.5%, thus demonstrating that the technical solution of the present invention has good feasibility and stability within the precipitation reaction temperature range of 50–70°C.

[0039] Example 9 With all other preparation conditions the same as in Example 1, only the acid solution in step S5 was changed from 4 mol / L hydrochloric acid to 3 mol / L sulfuric acid, while the other conditions remained unchanged, to obtain an iron-containing leachate. The results showed that the product still achieved the technical effects of the present invention, with an iron leaching rate of 95.4%. This demonstrates that hydrochloric acid or sulfuric acid in the concentration range of 2–6 mol / L are suitable for the iron resource recovery step of the present invention, and the technical solution has good raw material adaptability and process robustness.

[0040] Example 10 With all other preparation conditions the same as in Example 1, only the acid leaching temperature in step S5 was adjusted from 50°C to 30°C, and the leaching time was extended from 60 min to 90 min, to obtain an iron-containing leachate. The results showed that the product still achieved the technical effect of the present invention, with an iron leaching rate of 95.2%, thus demonstrating that the technical solution of the present invention has good feasibility and stability under the acid leaching parameter combination of 30–70°C and 30–90 min.

[0041] To verify the substantial progress of the method of the present invention compared with the prior art, the following comparative experiments were conducted. The test items covered the degree of crystal structure decomposition (XRD), lithium leaching rate (ICP-OES), iron leaching rate (ICP-OES), and lithium product purity (XRD). The results are shown in Table 1.

[0042] Table 1 Results of the effect test

[0043] Comparative Example 1: Sodium peroxide was replaced with an equal mass of sodium hydroxide, and all other conditions were the same as in Example 1. The XRD pattern still showed obvious characteristic peaks of LiFePO4 (20.6°, 26.2°), indicating that sodium hydroxide could not effectively stimulate the oxidation-alkali fusion synergistic reaction at 300°C, and the crystal lattice was not destroyed; ICP test showed that the lithium leaching rate was only 42.7%, far lower than 96.9% in Example 1, confirming that conventional alkaline salts have severely insufficient activity at low temperatures.

[0044] Comparative Example 2: Without the addition of sodium peroxide, lithium iron phosphate was directly calcined at 600°C for 2.5 h, with other conditions the same as in Example 1. The XRD pattern showed that the characteristic peaks of LiFePO4 did not decrease, indicating that simple high-temperature heat treatment could not destroy its stable olivine structure; the lithium leaching rate was only 8.3%, indicating that without the participation of an oxidant, Fe... 2+ The inability to be oxidized to a higher valence state to drive lattice reconstruction further confirms that the oxidation-alkali melting synergy and local strengthening effect of sodium peroxide are irreplaceable.

[0045] The results show that by introducing sodium peroxide and precisely controlling its dosage and calcination process, the present invention can achieve complete deconstruction of the lithium iron phosphate lattice at a low temperature of 250–320°C. The lithium leaching rate is increased by 126% compared with Comparative Example 1 and by 1070% compared with Comparative Example 2. Furthermore, lithium carbonate products with a purity of ≥98% and iron-containing leachate with an iron leaching rate of ≥95% are obtained, demonstrating significant and unexpected technical effects.

[0046] Example 11 The lithium carbonate product prepared by this invention, after battery-level performance evaluation, achieved an initial discharge specific capacity of 158.2 mAh / g (0.1C, 2.7–4.3V) in an NCM523 / Li half-cell, a coulombic efficiency of 94.7%, and a capacity retention of 92.3% after 100 cycles, indicating its potential for direct use as a precursor for lithium-ion battery cathode materials. Experimental results show that the lithium carbonate prepared by this invention exhibits good electrochemical performance in the preparation of lithium-ion battery cathode materials, and therefore can be used to prepare battery-grade lithium salt products to prevent and / or address bottlenecks in industries related to lithium resource shortages.

Claims

1. A method for low-temperature, high-efficiency deconstruction and recovery of lithium from lithium iron phosphate based on the local enhancement effect of sodium peroxide, characterized in that, include: S1. Pretreatment of mixture: Lithium iron phosphate material and sodium peroxide are mixed evenly according to a preset mass ratio, ground and refined to make the particle size of the mixture ≤50μm, and dried at 100~140℃ for 1~3h to obtain a dry mixture containing sodium peroxide. S2. Low-temperature roasting and deconstruction: The mixture is placed in a roasting device and roasted at 250-320°C for 1.5-3 hours under ventilation conditions. Sodium peroxide undergoes an oxidation-alkali fusion synergistic reaction during roasting and forms local exothermic micro-hot spots to produce a local strengthening effect, thereby destroying the lithium iron phosphate lattice structure and obtaining the roasted product. The roasted product contains lithium sodium mixed phosphate and sodium ferrite NaFeO2. S3. Water leaching separation: The roasted product is added to deionized water at a solid-liquid ratio of 1:10 to 1:30 g / mL and stirred for leaching. Solid-liquid separation is then performed to obtain a lithium-containing aqueous solution and an iron-based leaching residue. S4. Lithium recovery and purification: The lithium-containing aqueous solution is separated and purified to remove impurities and obtain lithium salt products; S5. Iron Resource Recovery: The iron-based leaching residue is subjected to acid leaching treatment to obtain iron-containing leachate, thereby realizing iron resource recovery.

2. The method according to claim 1, characterized in that, The lithium iron phosphate material has a purity of ≥95%, a particle size of <100μm, and a total content of copper and aluminum impurities of ≤3%.

3. The method according to claim 1, characterized in that, In step S1, the mass ratio of sodium peroxide to lithium iron phosphate material is 1.4:1 to 2.0:1, preferably 1.6:1 to 1.8:

1.

4. The method according to claim 1, characterized in that, In step S2, the preferred roasting temperature is 300±10℃, and the preferred roasting time is 2~2.5h.

5. The method according to claim 1, characterized in that, In step S3, the preferred solid-liquid ratio for the water immersion separation process is 1:20 g / mL.

6. The method according to claim 1, characterized in that, In step S3, the water immersion temperature in the water immersion separation process is room temperature to 40°C, and the immersion time is 30 to 50 minutes, preferably 45 minutes.

7. The method according to claim 1, characterized in that, In step S4, the separation and purification process includes: The lithium-containing aqueous solution is subjected to ion exchange or evaporation concentration to remove excess sodium ions; After adding Na2CO3 to the lithium-containing aqueous solution after removing excess sodium ions, Li2CO3 is precipitated and obtained, with the purity of Li2CO3 being ≥98%.

8. The method according to claim 1, characterized in that, In step S4, the reaction conditions for the lithium-containing aqueous solution and Na2CO3 are: stirring at 60°C for 30 min.

9. The method according to claim 1, characterized in that, In step S5, the acid solution used in the acid leaching process is either hydrochloric acid or sulfuric acid, and the concentration of the acid solution is 2 to 6 mol / L, preferably 3 to 4 mol / L.

10. The method according to claim 1, characterized in that, In step S5, the acid leaching temperature is 30-70℃, the leaching time is 30-90 min, and the iron leaching rate is ≥95%.

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