A method and system for selectively extracting lithium from lithium iron phosphate waste based on in-situ electrogenerated free radicals

Abstract

The present application belongs to the field of resource recycling of waste lithium batteries, and provides a method and system for selectively extracting lithium from lithium iron phosphate waste based on in-situ electrogenerated free radicals. The present application pretreats black powder containing lithium iron phosphate and conductive carbon material, constructs a water phase sulfate electrolyte slurry system, generates sulfate free radicals in-situ at the anode interface of the carbon-based electrode, controls pH, oxidation-reduction potential, current density, temperature and mother liquor reuse conditions, selectively releases lithium and retains solid phase of iron phosphate and conductive carbon, realizes efficient leaching of lithium, low dissolution of iron and phosphorus, and precipitation and recovery of lithium carbonate, solves the problem that slurry dispersion, free radical action, leaching efficiency and solid phase integrity are difficult to balance, and is suitable for resource recycling of waste lithium iron phosphate batteries.

Description

Technical Field

[0001] This invention belongs to the field of waste lithium battery resource recycling, specifically involving a method and system for selective lithium extraction from waste lithium iron phosphate based on in-situ electrogenerated free radicals. Background Technology

[0002] With the widespread application of lithium iron phosphate (LFP) batteries in new energy vehicles, energy storage power stations, communication backup power, and distributed energy systems, the amount of retired LFP batteries and the resulting black powder from their dismantling continues to increase. LFP black powder typically contains LFP active materials, conductive carbon materials, binder residues, and small amounts of metallic impurities. Lithium resources have high recycling value, and the iron-phosphorus solid phase also has potential for further resource utilization. Compared to nickel-cobalt-manganese cathode waste, LFP waste has a more iron-phosphorus-lithium composition, placing demands on recycling processes that are low-cost, low-pollution, highly selective, and maintain the solid phase structure as much as possible. In actual resource recovery, the process needs to effectively release lithium from the LFP crystals, suppress excessive iron and phosphorus entry into the liquid phase, and adapt to the complex effects of black powder particle dispersion, conductive carbon residue, and uneven mass transfer in the slurry. Therefore, developing lithium extraction technologies that can balance reaction efficiency, selectivity, slurry stability, and mother liquor recycling in an aqueous system is of great significance for improving the recycling value of spent LFP batteries and reducing the burden of secondary processing.

[0003] Lithium iron phosphate waste recycling technologies typically employ pathways such as acid leaching, oxidative leaching, mechanochemical oxidation, roasting, or electrochemical treatment. Acid leaching or oxidative leaching routes facilitate lithium entry into the liquid phase, but the addition of strong acids, peroxidants, or subsequent neutralization steps can easily increase salt load and impurity control pressure. Mechanochemical oxidation can promote selective lithium release, but it places high demands on equipment wear, solid-phase mixing uniformity, and continuous scale-up. Conventional electrochemical systems can reduce some of the input chemical oxidants, but are easily affected by the dispersion state of black powder, electrode interface mass transfer, and redox potential control. For example, Chinese patent CN115784267A discloses a process for recovering lithium iron phosphate from waste lithium iron phosphate cathode materials. It uses sulfuric acid and hydrogen peroxide for oxidative leaching and lithium precipitation to obtain lithium carbonate, but still relies on external oxidants and multi-step pH adjustment. Another literature reports the use of sodium persulfate for mechanochemical solid-phase oxidation to selectively release lithium, but its core lies in the solid-phase oxidation grinding process, and it does not solve the problems of in-situ generation of free radicals in aqueous electrolytic slurry, selective action on particle surface, and synergistic control of mother liquor recycling. Summary of the Invention

[0004] The purpose of this invention is to provide a selective lithium extraction method and system for lithium iron phosphate waste based on in-situ electrogenerated free radicals, which solves the problem that it is difficult to simultaneously achieve slurry dispersion, effective free radical action, lithium leaching efficiency and the integrity of the delithiation solid phase structure in current aqueous electrolysis treatment.

[0005] This invention coordinates mass transfer in the slurry and oxidation on the surface of particles by using sulfate electrolyte, retention of conductive carbon, generation of free radicals at the carbon-based electrode interface, linkage control of pH and redox potential, and reuse of lithium precipitation mother liquor. This reduces the side effects of local over-dissolution caused by the enhanced reaction and lowers the risk of insufficient lithium release caused by mild treatment, thus achieving both selective lithium extraction and solid-phase retention.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for selective lithium extraction from lithium iron phosphate waste based on in-situ electrogenerated free radicals includes the following steps:

[0008] S1. Provide lithium iron phosphate waste black powder containing lithium iron phosphate and conductive carbon material, and pretreat the lithium iron phosphate waste black powder to obtain pretreated black powder;

[0009] S2. Prepare an aqueous sulfate electrolyte using deionized water and sulfate electrolyte, and adjust the pH of the aqueous sulfate electrolyte;

[0010] S3. Disperse the pretreated black powder in the aqueous sulfate electrolyte to form a lithium extraction slurry;

[0011] S4. The lithium extraction slurry is placed in an electrochemical reactor and electrolyzed using carbon-based electrodes as the anode and cathode. The pH is controlled at 3.0-6.0 during the electrolysis process so that sulfate ions generate free radicals in situ at the anode interface and act on the surface of lithium iron phosphate particles.

[0012] S5. The lithium extraction slurry after solid-liquid separation and electrolysis yields a lithium-rich sulfate solution and a delithiated solid phase containing iron phosphate and conductive carbon materials;

[0013] S6. Treat the lithium-rich sulfate solution with a lithium carbonate precipitant to precipitate lithium in the form of lithium carbonate, and return the mother liquor after lithium precipitation as the lithium precipitation mother liquor to step S2 and / or step S3.

[0014] Furthermore, the method satisfies the following process conditions:

[0015] H1. In step S1, the pretreatment includes sieving, demagnetization and drying.

[0016] H2. In step S2, the sulfate electrolyte is anhydrous lithium sulfate, anhydrous sodium sulfate, or a combination thereof, and the pH of the aqueous sulfate electrolyte is adjusted using an acidity regulator or an alkalinity regulator.

[0017] H3. In step S3, the solid content of the lithium extraction slurry is 10.0-35.0 wt% based on the dry basis of the pretreated black powder as a percentage of the total mass of the lithium extraction slurry, and the total sulfate concentration in the aqueous sulfate electrolyte is 0.20-1.50 mol / L, and the pH is 3.0-6.0.

[0018] H4. In step S4, the electrolysis conditions include a current density of 30-180 A / m², a temperature of 25-60℃, an electrolysis time of 0.5-3.0 h, and a redox potential of the liquid phase of the electrolysis system of 0.75-1.25 V measured by a standard hydrogen electrode.

[0019] H5. In step S6, the carbonate lithium precipitant is anhydrous sodium carbonate;

[0020] H6. The conductive carbon material is graphite, the carbon-based electrode is a graphite electrode, and the in-situ electrogenerated free radicals include sulfate free radicals.

[0021] Furthermore, the aqueous sulfate electrolyte is prepared through the following steps:

[0022] A1. Add deionized water to the solution preparation tank;

[0023] A2. Add anhydrous lithium sulfate, anhydrous sodium sulfate, or a combination thereof to make the total sulfate concentration 0.20-1.50 mol / L;

[0024] A3. Adjust the pH to 3.0-6.0;

[0025] A4. After stirring, filter to remove insoluble matter to obtain the aqueous sulfate electrolyte;

[0026] A5. The conductivity of the aqueous sulfate electrolyte is 10-150 mS / cm.

[0027] Furthermore, the pretreated black powder is obtained through the following steps:

[0028] B1. Discharge, dismantle, crush and screen lithium iron phosphate waste to separate metal fragments containing aluminum and copper.

[0029] B2. Wash the sieved black powder with deionized water;

[0030] B3. Dry the washed black powder to a moisture content of 0.10-5.00 wt%.

[0031] B4. Sift the dried black powder to obtain the pretreated black powder;

[0032] B5. The pretreated black powder retains conductive carbon material from the positive electrode material, and the highest processing temperature during the pretreatment process does not exceed 300℃.

[0033] Furthermore, the in-situ electrogenic free radical generation process in step S4 is controlled by the following conditions:

[0034] C1. The distance between the anode and the cathode is 5-80 mm;

[0035] C2. The lithium extraction slurry is in a circulating state;

[0036] C3. During electrolysis, the redox potential of the liquid phase in the electrolysis system is maintained at 0.75-1.25V using a standard hydrogen electrode.

[0037] C4. Electrolysis shall be stopped when the increase in lithium concentration in the liquid phase obtained by sampling and separation during the electrolysis process, formed by the leaching of the pretreated black powder, reaches 2.0-30.0 g / L, or when the lithium leaching rate calculated based on the initial lithium content of the pretreated black powder reaches 92.0-99.9%.

[0038] C5. After stopping electrolysis, continue to keep the lithium extraction slurry in a circulating or stirred state, and then perform solid-liquid separation.

[0039] Furthermore, the precipitation of lithium carbonate in step S6 and the reuse of the lithium precipitation mother liquor are carried out through the following steps:

[0040] D1. Filter the lithium-rich sulfate solution to obtain a clear lithium-rich solution;

[0041] D2. Add anhydrous sodium carbonate to the clarified lithium-rich solution so that the molar ratio of carbonate to lithium in the clarified lithium-rich solution is 0.50-0.80:1;

[0042] D3. React at 50-95℃ and control the pH at 8.5-11.5;

[0043] D4. Solid-liquid separation yields lithium carbonate precipitate and lithium precipitation mother liquor;

[0044] D5. After replenishing the lithium precipitation mother liquor with anhydrous lithium sulfate, anhydrous sodium sulfate or a combination thereof to restore the total sulfate concentration to 0.20-1.50 mol / L, return to step S2 and / or step S3.

[0045] Furthermore, in the lithium extraction slurry, based on the mass of the dry-based pretreated black powder, the liquid-to-solid ratio of the aqueous sulfate electrolyte to the pretreated black powder is 1.8-9.0 L / kg.

[0046] Furthermore, the electrolysis in step S4 adopts a constant current mode, a constant voltage mode, or a constant current-constant voltage segmented mode, wherein the current density in the constant current mode is 30-180A / m², and the cell voltage in the constant voltage mode is 2.0-6.0V.

[0047] Furthermore, in step S5, solid-liquid separation is performed using sedimentation, centrifugation, pressure filtration, or cross-flow filtration.

[0048] Furthermore, the method satisfies at least two of the following conditions, and includes at least one of E1, E2, and E3:

[0049] E1. The lithium leaching rate in the pretreated black powder is 92.0-99.9%;

[0050] E2. The liquid phase dissolution rate of iron is 0.01-2.00%;

[0051] E3. The liquid phase dissolution rate of phosphorus is 0.05-5.00%;

[0052] E4. The residual lithium content in the delithiated solid phase is 0.05-0.70 wt% on a dry basis;

[0053] E5. The increase in lithium concentration in the lithium-rich sulfate solution formed by the leaching of the pretreated black powder is 2.0-30.0 g / L.

[0054] As a concept of this invention, it employs an in-situ electrogenization of free radicals in an aqueous sulfate electrolyte, combined with conductive carbon retention and mother liquor recycling, primarily to achieve a synergistic balance between slurry dispersion stability and selective lithium extraction performance. In existing technologies, to enhance slurry flow and dispersion, the degree of circulation disturbance or dilution is typically increased; however, this approach may reduce the effective effect of free radicals on the surface of lithium iron phosphate particles. Furthermore, increasing the free radical oxidation intensity may lead to uneven local reactions and increased iron and phosphorus dissolution. This invention, through in-situ conversion of sulfate ions at the anodic interface, pH and redox potential linkage control, the conductive interface of the carbon-based electrode, and the return of the lithium precipitation mother liquor, matches the mass transfer process with the particle surface oxidation process. This avoids excessive acid leaching and dependence on external strong oxidants, while unifying lithium release, iron and phosphorus retention, and liquid-phase circulation.

[0055] This invention also discloses an in-situ electrogenerated free radical system for selective lithium extraction from lithium iron phosphate waste, comprising:

[0056] F1. Black powder slurry preparation unit, used to mix lithium iron phosphate waste black powder containing lithium iron phosphate and conductive carbon materials with aqueous sulfate electrolyte;

[0057] F2. An electrochemical reactor, wherein the electrochemical reactor is provided with a carbon-based anode and a carbon-based cathode;

[0058] F3. A DC power supply configured to apply current to the carbon-based anode and the carbon-based cathode, and to provide a power output in constant current mode, constant voltage mode or constant current-constant voltage segmented mode, so that sulfate ions in the aqueous sulfate electrolyte generate free radicals in situ at the carbon-based anode interface;

[0059] F4. pH and redox potential control unit, wherein the pH and redox potential control unit is connected to an acidity regulator storage tank and an alkalinity regulator storage tank;

[0060] F5. Solid-liquid separation unit, used to separate lithium-rich sulfate solution and delithiated solid phase containing iron phosphate and conductive carbon materials;

[0061] F6. Lithium precipitation unit, wherein the lithium precipitation unit is connected to a carbonate lithium precipitation agent storage tank for preparing lithium carbonate;

[0062] F7. Mother liquor recycling unit, used to return lithium precipitation mother liquor to the black powder slurry preparation unit and / or its upstream aqueous sulfate electrolyte preparation location.

[0063] Furthermore, the system satisfies G1, G3, G4, and G9, and satisfies at least one of G2, G5, G6, G7, and G8:

[0064] G1. The electrochemical reactor is a single-chamber stirred electrolytic cell, a flow-through electrolytic cell, or a circulating slurry electrolytic cell, and the distance between the carbon-based anode and the carbon-based cathode is 5-80 mm;

[0065] G2. The black powder slurry preparation unit includes a stirring tank and a circulating pump. The stirring tank is used to form a lithium extraction slurry with a solid content of 10.0-35.0 wt% based on the dry basis of the pretreated black powder as a percentage of the total mass of the lithium extraction slurry. The circulating pump is used to circulate the lithium extraction slurry.

[0066] G3. The pH and redox potential control unit is configured to control the pH in the electrochemical reactor at 3.0-6.0 and the redox potential at 0.75-1.25V using a standard hydrogen electrode meter;

[0067] G4. The DC power supply is configured to provide a current density of 30-180 A / m² or a tank voltage of 2.0-6.0 V;

[0068] G5. The solid-liquid separation unit includes one or more of a sedimentation tank, centrifuge, filter press, or cross-flow filtration assembly;

[0069] G6. The lithium precipitation unit includes a heating kettle, a carbonate lithium precipitation agent dosing device, a solid-liquid separator, and a mother liquor storage tank;

[0070] G7. The system further includes a quality control unit configured to control the increase in lithium concentration in the lithium-rich sulfate solution formed by leaching the lithium iron phosphate waste black powder to be 2.0-30.0 g / L, and to control the residual lithium content in the delithiated solid phase to be 0.05-0.70 wt% on a dry basis.

[0071] G8. The mother liquor recycling unit is equipped with a sulfate replenishment port, which is used to replenish anhydrous lithium sulfate, anhydrous sodium sulfate or a combination thereof, so that the total sulfate concentration in the lithium precipitation mother liquor is restored to 0.20-1.50 mol / L;

[0072] G9. The conductive carbon material is graphite, the carbon-based anode is a graphite anode, the carbon-based cathode is a graphite cathode, and the in-situ electrogenerated free radicals include sulfate free radicals.

[0073] Furthermore, in the preparation process of the aqueous sulfate electrolyte, the total sulfate concentration in the prepared aqueous sulfate electrolyte is controlled to be 0.20-1.50 mol / L. 100 parts by mass of deionized water are added to the mixing tank, and 2.2-18.0 parts by mass of anhydrous lithium sulfate, or 2.9-22.0 parts by mass of anhydrous sodium sulfate, or both anhydrous lithium sulfate and anhydrous sodium sulfate are added. After preparation, the total sulfate concentration is calculated based on the final volume of the prepared aqueous sulfate electrolyte and the purity of the sulfate electrolyte used, and the aqueous sulfate electrolyte entering the slurry preparation step is obtained.

[0074] Furthermore, during the pH adjustment process of the aqueous sulfate electrolyte, the pH of the electrolyte after adding sulfate electrolyte is first measured. When the pH is higher than 6.0, commercially available analytical grade sulfuric acid or a diluted sulfuric acid solution obtained by diluting it is added to adjust it to 3.0-6.0. When the pH is lower than 3.0, anhydrous lithium hydroxide or lithium hydroxide monohydrate is added to adjust it back to 3.0-6.0. The acidity regulator storage tank in the system stores commercially available analytical grade sulfuric acid or a diluted sulfuric acid solution obtained by diluting it, and the alkalinity regulator storage tank stores anhydrous lithium hydroxide or lithium hydroxide monohydrate. The adjusted aqueous sulfate electrolyte enters the black powder slurry preparation step.

[0075] Furthermore, the aqueous sulfate electrolyte is stirred at 20-45℃ for 10-60 min and then filtered to remove insoluble matter with a particle size greater than 50 μm. The conductivity of the filtered aqueous sulfate electrolyte is measured, and the aqueous sulfate electrolyte with a conductivity of 10-150 mS / cm is then used in the lithium extraction slurry preparation step.

[0076] Furthermore, the carbon-based electrode uses carbon materials as the conductive substrate, selected from one or more of graphite, activated carbon, carbon felt, carbon fiber, carbon cloth, carbon paper, and glassy carbon. When using graphite electrodes, the graphite anode and cathode are arranged with an electrode spacing of 5-80 mm in a single-chamber stirred electrolytic cell, flow-through electrolytic cell, or circulating slurry electrolytic cell, and connected to a DC power supply to receive power output in constant current mode, constant voltage mode, or constant current-constant voltage segmented mode. When using carbon materials other than graphite as the anode, liquid phase samples near the anode are collected according to the free radical detection process, and electron paramagnetic resonance spectral data are recorded. The liquid phase lithium concentration, iron liquid phase dissolution rate, and phosphorus liquid phase dissolution rate are used as evaluation criteria for the suitability of the carbon-based anode.

[0077] Furthermore, during the washing of lithium iron phosphate waste black powder, the sieved black powder is mixed with deionized water at a washing liquid-to-solid ratio of 2.0-5.0 L / kg, the washing temperature is 15-45℃, and the washing time is 10-60 min. After solid-liquid separation, the washed black powder enters the drying step.

[0078] Furthermore, the washed black powder is dried at 80-150℃ for 2.0-6.0h, and the dried black powder is sieved to a D50 of 5-80μm to obtain pretreated black powder with a particle size suitable for electrolytic slurry preparation; the pretreated black powder retains conductive carbon material from the positive electrode material, and the highest processing temperature during the pretreatment process does not exceed 300℃.

[0079] Furthermore, in the lithium extraction slurry preparation process, based on the mass of the dry-based pretreated black powder, the pretreated black powder and the aqueous sulfate electrolyte are mixed at a liquid-to-solid ratio of 1.8-9.0 L / kg to form a lithium extraction slurry with a solid content of 10.0-35.0 wt% based on the mass of the dry-based pretreated black powder as a percentage of the total mass of the lithium extraction slurry. This lithium extraction slurry is then fed into the electrochemical reactor via a black powder conditioning unit. The solid content is calculated as the proportion of the dry-based pretreated black powder mass in the lithium extraction slurry sample to the total mass of the sample.

[0080] Furthermore, during the in-situ electrolytic generation of free radicals, the lithium extraction slurry circulates between the electrochemical reactor and the circulation pipeline at a linear velocity of 0.10-2.00 m / s, an electrolysis temperature of 25-60℃, an electrolysis time of 0.5-3.0 h, a current density of 30-180 A / m², and the pH of the liquid phase in the electrolysis system is maintained at 3.0-6.0.

[0081] Furthermore, the redox potential of the liquid phase in the electrolysis system was measured using a platinum redox potential electrode and a reference electrode. The reference electrode was selected from either a saturated calomel electrode or an Ag / AgCl reference electrode. During the measurement, the reference electrode type, measurement temperature, measurement location, and measured potential were recorded. The measured potential was converted to the standard hydrogen electrode reference potential based on the potential of the reference electrode relative to the standard hydrogen electrode at the measurement temperature. The potential of the reference electrode relative to the standard hydrogen electrode was determined according to the temperature correction value given in the reference electrode calibration document or electrochemical handbook. The measurement location of the redox potential was the main liquid phase region of the electrochemical reactor. During electrolysis, the converted redox potential was maintained between 0.75 and 1.25 V by adjusting the current density or cell voltage.

[0082] Furthermore, the pH and redox potential control unit includes a pH detection module, a redox potential detection module, a controller, an acidity regulator dosing pump, an alkalinity regulator dosing pump, and a DC power supply adjustment interface. The controller receives detection signals from the pH detection module and the redox potential detection module. When the pH is below 3.0 or above 6.0, it outputs a dosing command to the alkalinity regulator dosing pump or the acidity regulator dosing pump. When the redox potential converted to the standard hydrogen electrode is below 0.75V or above 1.25V, it outputs a current density or cell voltage adjustment command to the DC power supply adjustment interface.

[0083] Furthermore, during the free radical detection process, a spin trapping agent DMPO is added to the electrolysis system containing sulfate electrolyte, and a liquid phase sample is collected near the anode during the electrolysis process. Electron paramagnetic resonance detection is performed on the liquid phase sample, and the detection record includes the magnetic field scanning range, microwave frequency, microwave power, modulation amplitude, test temperature, and spectral data. The obtained spectral data is used as the data source for determining the types of in-situ electrogenerated free radicals.

[0084] Furthermore, in the free radical quenching control process, under the same pretreatment conditions of black powder, total sulfate concentration, pH, solid content, electrode material, current density, temperature, and electrolysis time, electrolysis systems without quencher, with methanol, and with tert-butanol were set up respectively. After electrolysis, the liquid phase lithium concentration, liquid phase dissolution rate of iron, and liquid phase dissolution rate of phosphorus were measured respectively. The measurement results of different systems were used to distinguish the contribution of free radical pathway and non-free radical electrochemical oxidation pathway to the selective lithium extraction process.

[0085] Furthermore, the lithium leaching rate is calculated as the ratio of the lithium mass formed by leaching from the pretreated black powder to the initial lithium mass of the pretreated black powder. The initial lithium mass of the pretreated black powder is determined by ICP-OES or ICP-MS after acid digestion of the pretreated black powder sample before electrolysis. After electrolysis, the liquid phase sample is filtered through a 0.45 μm filter membrane, and the total lithium concentration in the liquid phase is determined. The mass of lithium formed by leaching from the pretreated black powder is obtained by deducting the initial aqueous sulfate electrolyte, the initial background lithium of the recycled mother liquor, and the exogenous lithium introduced by the pH adjuster.

[0086] Furthermore, the liquid phase dissolution rate of iron was calculated as the ratio of the mass of iron in the liquid phase after electrolysis to the initial mass of iron in the pretreated black powder, and the liquid phase dissolution rate of phosphorus was calculated as the ratio of the mass of total phosphorus in the liquid phase after electrolysis to the initial mass of phosphorus in the pretreated black powder. The liquid phase sample after electrolysis was filtered through a 0.45 μm filter membrane for elemental analysis. The initial mass of iron and initial mass of phosphorus in the pretreated black powder were determined by ICP-OES or ICP-MS after acid digestion of the pretreated black powder sample before electrolysis.

[0087] Furthermore, the residual lithium content in the delithiated solid phase is calculated based on the dry-based mass of the delithiated solid phase; the delithiated solid phase obtained from solid-liquid separation is sampled after washing and drying, and the lithium content is determined by ICP-OES or ICP-MS after acid digestion. The obtained residual lithium content is used to evaluate the degree of delithiation of the delithiated solid phase after solid-liquid separation.

[0088] Furthermore, during the electrolysis cessation determination process, samples are taken at fixed time intervals during electrolysis and solid-liquid separation is performed to measure the increase in lithium concentration formed by the leaching of pretreated black powder in the obtained liquid phase; when the increase in lithium concentration reaches 2.0-30.0 g / L, or the lithium leaching rate calculated based on the initial lithium content of pretreated black powder reaches 92.0-99.9%, electrolysis is stopped. After stopping electrolysis, the lithium extraction slurry is kept circulating or stirred for 5-30 minutes, and then solid-liquid separation is performed on the lithium extraction slurry.

[0089] Furthermore, in step S5, the water content of the delithiated solid phase after solid-liquid separation is 5.0-45.0 wt%. The delithiated solid phase enters the residual lithium content detection step or the iron phosphate solid phase recovery step, and the liquid phase obtained from solid-liquid separation enters the lithium-rich sulfate solution filtration step.

[0090] Furthermore, during the lithium carbonate precipitation process, the lithium-rich sulfate solution is first filtered to obtain a clarified lithium-rich solution. The amount of anhydrous sodium carbonate added is calculated based on the measured lithium concentration in the clarified lithium-rich solution, so that the molar ratio of carbonate to lithium is 0.50-0.80:1. After adding anhydrous sodium carbonate, the reaction is carried out at 50-95℃ for 0.5-3.0 h, and the pH is controlled at 8.5-11.5. After the reaction, solid-liquid separation is performed to obtain lithium carbonate precipitate and lithium precipitation mother liquor.

[0091] Furthermore, before reusing the lithium precipitation mother liquor, the contents of lithium ions, sodium ions, sulfate, carbonate, iron, phosphorus, aluminum, and copper in the mother liquor are detected. Anhydrous lithium sulfate, anhydrous sodium sulfate, or a combination thereof are added according to the detected total sulfate concentration to restore the total sulfate concentration in the lithium precipitation mother liquor to 0.20-1.50 mol / L. The lithium precipitation mother liquor after restoring the total sulfate concentration is returned to the aqueous sulfate electrolyte preparation step or the lithium extraction slurry preparation step.

[0092] Furthermore, when the lithium precipitation mother liquor is recycled for 2-30 batches, the lithium ion, sodium ion, sulfate, carbonate, pH, conductivity, and redox potential data are recorded for each batch before and after reuse. The lithium precipitation mother liquor after sulfate replenishment is used as the liquid phase source for the next batch of aqueous sulfate electrolyte or lithium extraction slurry. When using the lithium precipitation mother liquor as the liquid phase source, the increase in lithium concentration is calculated based on the difference in lithium concentration in the liquid phase before and after entering the electrolysis step, after deducting the external lithium.

[0093] Furthermore, when the lithium precipitation unit in the system uses a heating kettle, the operating temperature of the heating kettle is 50-95℃; the carbonate lithium precipitation agent dosing device in the lithium precipitation unit is an anhydrous sodium carbonate dosing device, and the carbonate lithium precipitation agent storage tank is an anhydrous sodium carbonate storage tank. The anhydrous sodium carbonate dosing device adds anhydrous sodium carbonate to the heating kettle according to the measured lithium concentration in the clarified lithium-rich liquid.

[0094] As another concept of this invention, an in-situ electrogenerated free radical system is designed, primarily to achieve, fix, or amplify the aforementioned synergistic effects. This system forms a recyclable lithium extraction slurry through a black powder slurry preparation unit, provides a free radical generation interface through an electrochemical reactor equipped with a carbon-based anode and a carbon-based cathode, provides power output in constant current, constant voltage, or a segmented constant current-constant voltage mode via a DC power supply, and links the acidity regulator tank, alkalinity regulator tank, and power regulation interface through a pH and redox potential control unit, ensuring that the intensity of free radical activity is coordinated with the slurry state. The solid-liquid separation unit, lithium precipitation unit, and mother liquor recycling unit further connect lithium release, lithium carbonate precipitation, and sulfate liquid phase recovery, avoiding process fragmentation caused by individually enhancing electrolysis or lithium precipitation, thus maintaining a balance between selective lithium extraction and solid phase integrity in the system structure.

[0095] The aqueous sulfate electrolyte and the pretreated black powder retaining conductive carbon material are two key technical units that demonstrate synergistic effects in this invention. The aqueous sulfate electrolyte mainly provides an ion source and conductive environment for the in-situ generation of sulfate free radicals, which can promote the release of lithium from the surface of lithium iron phosphate particles; however, if only the electrolyte concentration or redox potential is increased, it is easy to cause excessive local oxidation and increase the risk of iron and phosphorus entering the liquid phase. The pretreated black powder retaining conductive carbon material helps to improve the conductivity and particle dispersion of the slurry and reduce the mass transfer unevenness of black powder in the aqueous system; however, if only conductive carbon and circulating dispersion are relied upon, it may be difficult to provide sufficient oxidation effect on the particle surface. This invention achieves mutual correction between electrolyte interface reaction and black powder particle dispersion by matching the total sulfate concentration, pH, redox potential, current density and circulating flow ratio, ultimately taking into account dispersion stability, selective lithium extraction efficiency and solid phase retention after lithium removal.

[0096] Beneficial technical effects

[0097] 1. This invention generates sulfate radicals through electrolysis at the interface of an aqueous sulfate electrolyte and a carbon-based electrode, which concentrates the oxidation on the surface of lithium iron phosphate particles, reduces dependence on external oxidants, and reduces the possibility of excessive iron and phosphorus dissolution by controlling pH and redox potential, thereby improving the controllability of the selective lithium extraction process.

[0098] 2. The present invention retains conductive carbon material from the cathode material in the pretreated black powder and maintains the dispersion of lithium extraction slurry through circulation or stirring, which helps to improve the contact state between black powder particles and electrolyte and electrode interface, so that free radical action and slurry mass transfer are matched, reducing the risk of solid phase damage caused by uneven local reaction.

[0099] 3. This invention combines the lithium precipitation in lithium-rich sulfate solution and the return of the lithium precipitation mother liquor to the aqueous phase sulfate electrolyte preparation step or the lithium extraction slurry preparation step, so that the sulfate system is cyclically connected between lithium extraction and lithium precipitation, which helps to reduce the burden of liquid phase treatment and improve the process stability when continuously processing lithium iron phosphate waste black powder.

[0100] 4. The system of the present invention is equipped with a black powder slurry preparation unit, an electrochemical reactor, a DC power supply, a pH and redox potential control unit, a solid-liquid separation unit, a lithium precipitation unit, and a mother liquor recycling unit, so that slurry preparation, electrolysis, separation, lithium precipitation and liquid phase recovery form a closed process, which is suitable for the resource utilization treatment of lithium iron phosphate waste. Attached Figure Description

[0101] Figure 1 The graph shows the effect of total sulfate concentration on lithium leaching rate and overall selectivity score.

[0102] Figure 2 The graph shows the effect of pH on lithium leaching rate and overall selectivity score.

[0103] Figure 3 The graph shows the effect of current density on lithium leaching rate and overall selectivity score.

[0104] Figure 4 The graph shows the effect of electrolysis time on lithium leaching rate and overall selectivity score.

[0105] Figure 5 The images show the EPR-DMPO radical capture spectra of Example 1, Comparative Example 1, and Comparative Example 10.

[0106] Figure 6 The graphs show the free radical quenching-ICP combined test results for Example 1 and Comparative Example 10.

[0107] Figure 7 The pH time series curves of the electrolysis process of Example 1, Comparative Example 9 and Comparative Example 11 are shown.

[0108] Figure 8 The ORP time series curves for the electrolysis process of Example 1, Comparative Example 10, and Comparative Example 11 are shown.

[0109] Figure 9 The scatter plot and mean SD of the slurry settling index for Example 1, Comparative Example 9, and Comparative Example 11 are shown.

[0110] Figure 10 The images show the D / G spectra of Raman carbon materials from Examples 1, 9, and 11.

[0111] Figure 11 The diagram shows the main ion balance of the lithium precipitation mother liquor in Example 1 and Comparative Example 8.

[0112] Figure 12 The scatter plot and mean SD plot show the lithium recovery rates of Example 1 and Comparative Example 8.

[0113] Figure 13 The images show a comparison of macroscopic optical photographs of the final product of Example 1 and the final product of Comparative Example 11; where a is a macroscopic optical photograph of the final lithium carbonate precipitation of Example 1 and b is a macroscopic optical photograph of the final lithium carbonate precipitation of Comparative Example 11.

[0114] Figure 14 The images show a comparison of the SEM morphology of the final product of Example 1 and the final product of Comparative Example 11; where a is a low-magnification SEM image of the final lithium carbonate precipitate of Example 1, b is a low-magnification SEM image of the final lithium carbonate precipitate of Comparative Example 11, c is a medium-magnification SEM image of the final lithium carbonate precipitate of Example 1, d is a medium-magnification SEM image of the final lithium carbonate precipitate of Comparative Example 11, e is a high-magnification SEM image of the final lithium carbonate precipitate of Example 1, and f is a high-magnification SEM image of the final lithium carbonate precipitate of Comparative Example 11.

[0115] Figure 15 The images show a comparison of the TEM characterization of the final product of Example 1 and the final product of Comparative Example 11; where a is a bright-field TEM image of the final lithium carbonate precipitate of Example 1, b is a bright-field TEM image of the final lithium carbonate precipitate of Comparative Example 11, c is an HRTEM image of the final lithium carbonate precipitate of Example 1, and d is an HRTEM image of the final lithium carbonate precipitate of Comparative Example 11.

[0116] Figure 16 The images show the evolution of macroscopic optical photographs of the pretreated black powder, aqueous sulfate electrolyte, electrolysis product, and final lithium carbonate precipitate in Example 1; where a is a macroscopic optical photograph of the pretreated black powder, b is a macroscopic optical photograph of the aqueous sulfate electrolyte, c is a macroscopic optical photograph of the electrolysis product, and d is a macroscopic optical photograph of the final lithium carbonate precipitate. Detailed Implementation

[0117] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0118] Example 1

[0119] S1: Overall processing scale and raw material status. 10.0 kg of lithium iron phosphate waste black powder (dry basis) was used as the processing target. The black powder contained lithium iron phosphate, graphite-type conductive carbon material from the cathode material, and a small amount of sieveable separable metal fragments. After discharge, disassembly, crushing, and sieving, metal fragments containing aluminum and copper were separated. The sieved black powder was mixed with deionized water at a washing liquid-to-solid ratio of 2.0 L / kg, mechanically stirred at 15°C for 10 min, and after solid-liquid separation, dried at 80°C for 2.0 h to achieve a black powder moisture content of 0.10 wt%. It was then sieved to a D50 of 5 μm to obtain pretreated black powder. The highest processing temperature during this pretreatment process was 80°C, and the conductive carbon material from the cathode material was retained in the pretreated black powder.

[0120] S2: Preparation of aqueous sulfate electrolyte. Add 100 parts by mass of deionized water and 2.2 parts by mass of commercially available anhydrous lithium sulfate (purity ≥99.0%) to the mixing tank. Stir at 20°C for 10 min, and filter to remove insoluble matter with a particle size greater than 50 μm. After preparation, calculate the total sulfate concentration as 0.20 mol / L based on the final volume and the purity of the anhydrous lithium sulfate. After measuring the pH of the electrolyte after adding the sulfate electrolyte, adjust the pH to 3.0 using commercially available analytical grade sulfuric acid or a diluted sulfuric acid solution obtained by dilution. The conductivity of the resulting aqueous sulfate electrolyte is 10 mS / cm.

[0121] S3: Slurry Preparation and Electrolysis. Pretreated black powder and aqueous sulfate electrolyte were fed into the black powder slurry preparation unit at a liquid-to-solid ratio of 9.0 L / kg to form a lithium extraction slurry with a solid content of 10.0 wt% (dry basis, based on the dry weight of the pretreated black powder). The lithium extraction slurry entered a circulating slurry electrolysis cell. The distance between the graphite anode and cathode was 5 mm. The lithium extraction slurry circulated between the electrochemical reactor and the circulation pipeline at a linear velocity of 0.10 m / s. Electrolysis was performed in constant current mode with a current density of 30 A / m², a stable cell voltage of 2.0 V, an electrolysis temperature of 25 °C, and an electrolysis time of 0.5 h. The pH of the liquid phase in the electrolysis system was maintained at 3.0. The redox potential was measured using a platinum redox potential electrode and an Ag / AgCl reference electrode and converted to a standard hydrogen electrode reference. The redox potential was maintained at 0.75 V. After the set processing time is reached, the circulation continues for 5 minutes, and then sedimentation and solid-liquid separation are performed to obtain lithium-rich sulfate solution and delithiated solid phase. The delithiated solid phase has a water content of 5.0 wt%.

[0122] S4: Lithium-rich solution precipitation and mother liquor return. The lithium-rich sulfate solution was filtered through a 0.45 μm filter to obtain a clarified lithium-rich solution. Based on the measured lithium concentration in the clarified lithium-rich solution, commercially available anhydrous sodium carbonate (≥99.0%) was added to achieve a carbonate to lithium molar ratio of 0.50:1. The reaction was carried out at 50℃ for 0.5 h, with the pH controlled at 8.5. After the reaction, solid-liquid separation yielded lithium carbonate precipitate and a lithium precipitation mother liquor. The lithium precipitation mother liquor was tested for lithium ion, sodium ion, sulfate, carbonate, iron, phosphorus, aluminum, and copper content. Anhydrous lithium sulfate was then added to restore the total sulfate concentration to 0.20 mol / L, and this mother liquor was used as the liquid phase source for the subsequent two batches of aqueous sulfate electrolyte.

[0123] Quality testing methods and results: Before electrolysis, the pretreated black powder samples were digested with acid, and the initial lithium, iron, and phosphorus contents were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). After electrolysis, the liquid phase samples were filtered through a 0.45 μm filter membrane, and the liquid phase lithium, iron, and phosphorus contents were determined, after deducting the initial background lithium from the aqueous sulfate electrolyte, the recycled mother liquor, and the exogenous lithium introduced by the pH adjuster. Based on the leached lithium mass and the initial lithium mass, the lithium leaching rate was 92.0% ± 0.4%, the lithium concentration increment from the leaching of the pretreated black powder was 2.0 g / L ± 0.1 g / L, the liquid phase iron dissolution rate was 0.01% ± 0.00%, the liquid phase phosphorus dissolution rate was 0.05% ± 0.01%, the residual lithium content in the delithiated solid phase was 0.70 wt% ± 0.03 wt%, and the number of tests was 3. After adding the spin trapping agent DMPO to the liquid phase sample near the anode, electron paramagnetic resonance detection was performed. The spectral data matched the sulfate radical response and were used to determine the in-situ electrogenerated radical process in this embodiment.

[0124] Features and application scenarios of this embodiment: This embodiment employs milder electrolysis conditions, a lower sulfate concentration, and a higher liquid phase ratio, making it suitable for processing scenarios where the black powder particle size is fine, conductive carbon is sufficiently retained, and the risk of iron and phosphorus entering the liquid phase needs to be reduced. The energy input of this scheme is low, facilitating the coordination of slurry flow and electrode interface reactions during small-batch or low-load continuous operation.

[0125] Example 2

[0126] Raw materials and solution preparation conditions: 100.0 kg of lithium iron phosphate waste black powder (dry basis) was used as the treatment target. The black powder underwent discharge, dismantling, crushing, sieving, and demagnetization to separate aluminum and copper fragments. The sieved black powder was mixed with deionized water at a washing liquid-to-solid ratio of 5.0 L / kg and stirred at 45°C for 60 min. After solid-liquid separation, it was dried at 150°C for 6.0 h. The resulting dried black powder had a moisture content of 5.00 wt% and was sieved to a D50 of 80 μm, yielding pretreated black powder. The maximum processing temperature during pretreatment was 150°C, and the pretreated black powder retained graphite-type conductive carbon material.

[0127] Electrolyte and high-solids-content slurry preparation process: 100 parts by weight of deionized water and 18.0 parts by weight of commercially available anhydrous lithium sulfate (purity ≥99.0%) were added to the mixing tank. The mixture was stirred at 45℃ for 60 min. After filtration to remove insoluble matter with a particle size greater than 50 μm, the total sulfate concentration was calculated to be 1.50 mol / L based on the final volume and the purity of the sulfate electrolyte. Commercially available anhydrous lithium hydroxide (purity ≥98.0%) was used to adjust the pH of the electrolyte to 6.0, resulting in an aqueous sulfate electrolyte with a conductivity of 150 mS / cm. The pretreated black powder was mixed with the aqueous sulfate electrolyte at a liquid-to-solid ratio of 1.9 L / kg to form a lithium extraction slurry with a solid content of 35.0 wt% (based on dry basis, the mass of the pretreated black powder) as a percentage of the total mass of the lithium extraction slurry. The lithium extraction slurry was then fed into a single-chamber stirred electrolytic cell via the black powder preparation unit.

[0128] Electrochemical reaction and solid-liquid separation process: In this embodiment, the distance between the graphite anode and the graphite cathode is 80 mm. During electrolysis, the lithium extraction slurry is in a circulating flow state with a circulation linear velocity of 2.00 m / s. The DC power supply is output in constant voltage mode, with a cell voltage of 6.0 V, a current density controlled at 180 A / m², an electrolysis temperature of 60 °C, and an electrolysis time of 3.0 h. The pH and redox potential control unit maintains the system pH at 6.0 and the redox potential converted to a standard hydrogen electrode at 1.25 V. After electrolysis, stirring is continued for 30 min, followed by solid-liquid separation by pressure filtration to obtain a lithium-rich sulfate solution and a delithiated solid phase. The delithiated solid phase has a water content of 45.0 wt%.

[0129] Lithium precipitation and mother liquor recycling process: After filtration, the lithium-rich sulfate solution becomes a clear lithium-rich solution. Commercially available anhydrous sodium carbonate (≥99.0% purity) is added based on the measured lithium concentration to achieve a carbonate to lithium molar ratio of 0.80:1. The reaction is carried out at 95℃ for 3.0 h, with the pH controlled at 11.5. Solid-liquid separation is performed after the reaction to obtain lithium carbonate precipitate and lithium precipitation mother liquor. Anhydrous lithium sulfate is added to the lithium precipitation mother liquor after testing to restore the total sulfate concentration to 1.50 mol / L. The restored lithium precipitation mother liquor is then returned to the lithium extraction slurry preparation step, and data on lithium ions, sodium ions, sulfate, carbonate, pH, conductivity, and redox potential are continuously recorded for 30 batches before and after reuse.

[0130] Quality testing methods and results: The initial lithium, iron, and phosphorus contents of the pretreated black powder were determined by inductively coupled plasma mass spectrometry after acid digestion. The liquid phase after electrolysis was filtered through a 0.45 μm filter before determining the lithium, iron, and phosphorus content. After deducting exogenous lithium, the calculated lithium leaching rate was 99.9% ± 0.1%, the lithium concentration increment from the pretreated black powder leaching was 30.0 g / L ± 0.5 g / L, the liquid phase dissolution rate of iron was 2.00% ± 0.06%, and the liquid phase dissolution rate of phosphorus was 5.00% ± 0.10%. The residual lithium content in the delithiated solid phase was 0.05 wt% ± 0.01 wt%, and the number of tests was 3. Electron paramagnetic resonance (EPR) was used to detect the liquid phase sample near the anode. The magnetic field scanning range, microwave frequency, microwave power, modulation amplitude, and test temperature were recorded. The obtained spectra were used to determine the formation of sulfate free radicals.

[0131] The features of this embodiment are: It employs a combination of high solids content, high sulfate concentration, strong power output, and long electrolysis time, making it suitable for scenarios with high lithium release requirements, sufficient equipment circulation capacity, and a need to increase the throughput per unit volume. This scheme reduces the impact of uneven local reactions on the stability of the delithiated solid phase during high-load operation through pH and redox potential linkage control.

[0132] Example 3

[0133] I. Processing Object and Pretreatment Conditions: This embodiment uses 50.0 kg of dry-based lithium iron phosphate waste black powder as the processing object. After discharge, disassembly, crushing, sieving, and demagnetization, aluminum and copper fragments are separated from the black powder. The sieved black powder is mixed with deionized water at a washing liquid-to-solid ratio of 3.5 L / kg, stirred at 30°C for 30 min, and after solid-liquid separation, dried at 110°C for 4.0 h to achieve a water content of 2.50 wt%, and then sieved to a D50 of 40 μm. The highest processing temperature during pretreatment is 110°C, and the resulting pretreated black powder retains the graphite-type conductive carbon material from the cathode material.

[0134] II. Electrolyte Preparation and Slurry Adjustment: Add 100 parts by weight of deionized water and 12.8 parts by weight of commercially available anhydrous sodium sulfate (purity ≥99.0%) to the preparation tank. Stir at 30℃ for 35 minutes, and filter to remove insoluble matter with a particle size greater than 50 μm. Based on the final volume and the purity of the sulfate electrolyte, the total sulfate concentration is calculated to be 0.90 mol / L. Adjust the pH to 4.5 using commercially available analytical grade sulfuric acid or a diluted sulfuric acid solution obtained from it, and commercially available anhydrous lithium hydroxide. The conductivity of the resulting aqueous sulfate electrolyte is 80 mS / cm. Mix the pretreated black powder with the aqueous sulfate electrolyte at a liquid-to-solid ratio of 3.5 L / kg to form a lithium extraction slurry with a solid content of 22.0 wt% (dry basis) of pretreated black powder as a percentage of the total mass of the lithium extraction slurry.

[0135] III. Electrolysis Operation Process: The lithium extraction slurry enters a flow-through electrolytic cell with a 40mm gap between the graphite anode and cathode. Electrolysis employs a constant current-constant voltage segmented mode, with the initial stage operating at a current density of 100A / m², and the subsequent stage maintaining the cell voltage at 4.0V. The electrolysis temperature is 45℃, and the electrolysis time is 1.5h. During electrolysis, the lithium extraction slurry circulates between the electrochemical reactor and the circulation pipeline at a linear velocity of 1.00m / s; the pH is maintained at 4.5, and the redox potential, converted to a standard hydrogen electrode, is maintained at 1.00V. After electrolysis stops, circulation continues for 15 minutes, followed by solid-liquid separation by centrifugation to obtain a lithium-rich sulfate solution and a delithiated solid phase. The delithiated solid phase contains 20.0wt% water.

[0136] IV. Lithium Precipitation and Liquid Phase Return: After filtering the lithium-rich sulfate solution to obtain a clarified lithium-rich solution, commercially available anhydrous sodium carbonate (≥99.0%) was added according to the measured lithium concentration, making the molar ratio of carbonate to lithium 0.65:1. The reaction was carried out at 75°C for 1.5 hours, and the pH was controlled at 10.0. After the reaction, lithium carbonate precipitate and lithium precipitation mother liquor were obtained by centrifugation. Before reuse, the lithium precipitation mother liquor was tested for lithium ion, sodium ion, sulfate, carbonate, iron, phosphorus, aluminum, and copper content, and anhydrous sodium sulfate was added to restore the total sulfate concentration to 0.90 mol / L. The lithium precipitation mother liquor after salt replenishment was returned to the aqueous sulfate electrolyte preparation step. This example recorded data for 10 batches of recycling.

[0137] V. Quality Testing Methods and Results: Elemental analysis was performed on both the pretreated black powder before electrolysis and the liquid phase samples after electrolysis, following acid digestion and prefiltration treatment. The lithium leaching rate was calculated as the ratio of the lithium mass leached from the pretreated black powder to the initial lithium mass of the pretreated black powder, and the result was 96.5% ± 0.3%. The lithium concentration increment from the pretreated black powder leaching was 15.0 g / L ± 0.3 g / L. The liquid phase dissolution rate of iron was 0.80% ± 0.04%, and the liquid phase dissolution rate of phosphorus was 2.00% ± 0.08%. The residual lithium content in the delithiated solid phase was 0.30 wt% ± 0.02 wt%, and the number of tests was 3. Electron paramagnetic resonance (EPR) was used to detect the liquid phase samples near the anode, and the spectral data were recorded and used for free radical identification.

[0138] The process features and application directions of this embodiment: This embodiment uses anhydrous sodium sulfate as the sulfate electrolyte, combined with a flow-through electrolyzer and a constant current-constant voltage segmented operation mode, which is suitable for scenarios that require reducing the amount of lithium salt replenishment while maintaining a moderate processing intensity. This solution takes into account electrolyte cost, slurry circulation status, and stable connection of the lithium-rich liquid precipitation process.

[0139] Example 4

[0140] In this embodiment, 20.0 kg of lithium iron phosphate waste black powder (dry basis) was subjected to discharge, disassembly, crushing, sieving, and demagnetization in sequence to separate metal fragments containing aluminum and copper. The sieved black powder was mixed with deionized water at a washing liquid-to-solid ratio of 4.0 L / kg, stirred at 25°C for 45 min, and then subjected to solid-liquid separation. The washed black powder was dried at 130°C for 5.0 h, resulting in a moisture content of 1.20 wt%. It was then sieved to a D50 of 60 μm to obtain pretreated black powder. The highest processing temperature during pretreatment was 130°C, and the pretreated black powder retained graphite-type conductive carbon material.

[0141] 100 parts by weight of deionized water, 10.0 parts by weight of commercially available anhydrous lithium sulfate, and 5.0 parts by weight of commercially available anhydrous sodium sulfate were added to the mixing tank. The purity of both sulfate electrolytes was ≥99.0%. The mixture was stirred at 35°C for 45 min. After filtration to remove insoluble matter with a particle size greater than 50 μm, the total sulfate concentration was calculated to be 1.20 mol / L based on the final volume and the purity of the sulfate electrolyte. The pH was measured after adding the sulfate electrolyte. The pH was adjusted to 5.5 using commercially available analytical grade sulfuric acid or a diluted sulfuric acid solution obtained from it, and commercially available lithium hydroxide monohydrate. The conductivity of the resulting aqueous sulfate electrolyte was 120 mS / cm.

[0142] Pretreated black powder was mixed with aqueous sulfate electrolyte at a liquid-to-solid ratio of 2.6 L / kg to form a lithium extraction slurry with a solid content of 28.0 wt% (dry basis, based on the dry weight of the pretreated black powder). The lithium extraction slurry was fed into a circulating slurry electrolytic cell via a black powder conditioning unit. The distance between the graphite anode and cathode was 60 mm. A constant current-constant voltage segmented DC power supply was used, with a current density controlled at 150 A / m² and a cell voltage controlled at 5.0 V. The electrolysis temperature was 55 °C, and the electrolysis time was 2.2 h. The circulating linear velocity of the lithium extraction slurry was 1.50 m / s, and the system pH was maintained at 5.5. The redox potential was measured using a platinum redox electrode and a saturated calomel electrode and converted to a standard hydrogen electrode reference, maintaining a redox potential of 1.15 V. After electrolysis, stirring continued for 20 min, followed by cross-flow filtration to separate the lithium-rich sulfate solution and the delithiated solid phase. The delithiated solid phase contained 30.0 wt% water.

[0143] After filtration to form a clear lithium-rich solution, commercially available anhydrous sodium carbonate (≥99.0% purity) was added according to the measured lithium concentration, resulting in a carbonate to lithium molar ratio of 0.70:1. The reaction was carried out at 85°C for 2.0 h, with the pH controlled at 10.8. Solid-liquid separation was performed after the reaction to obtain lithium carbonate precipitate and lithium precipitation mother liquor. Before reuse, the lithium precipitation mother liquor was tested for lithium ion, sodium ion, sulfate, carbonate, iron, phosphorus, aluminum, and copper content. Anhydrous lithium sulfate and anhydrous sodium sulfate were added to restore the total sulfate concentration to 1.20 mol / L. The restored lithium precipitation mother liquor was returned to the black powder slurry preparation unit. Data from 20 batches of this recycling process were recorded in this example.

[0144] Quality testing methods and results: Pre-treated black powder samples were acid-digested before electrolysis, and the initial lithium, iron, and phosphorus contents were determined. Liquid phase samples after electrolysis were filtered through a 0.45 μm filter membrane, and the total lithium, iron, and phosphorus contents in the liquid phase were determined. The mass of lithium formed from the leaching of pre-treated black powder was calculated after deducting the initial background lithium from the initial electrolyte, the initial background lithium from the recycled mother liquor, and the exogenous lithium introduced by the pH adjuster. Three tests were conducted. The lithium leaching rate was 98.0% ± 0.2%, the lithium concentration increment from the leaching of pre-treated black powder was 24.0 g / L ± 0.4 g / L, the liquid phase dissolution rate of iron was 1.20% ± 0.05%, the liquid phase dissolution rate of phosphorus was 3.00% ± 0.09%, and the residual lithium content in the delithiated solid phase was 0.18 wt% ± 0.01 wt%. Electron paramagnetic resonance (EPR) was used to record the spectral data of the liquid phase sample near the anode, and the obtained response matched the sulfate radical generation process.

[0145] The applicable scenario for this embodiment: This embodiment uses a combination of anhydrous lithium sulfate and anhydrous sodium sulfate for salt replenishment, a circulating slurry electrolyzer, and a cross-flow filtration connection method, which is suitable for continuous processing scenarios where multiple batches of mother liquor are reused, lithium-rich liquid is stabilized for lithium precipitation, and delithiated solid phase is subsequently recovered. The solution preparation, slurry conditioning, electrolysis, lithium precipitation, and liquid phase return steps of this scheme are tightly linked, making it easy to implement within a wide process window.

[0146] Comparative Example 1: Basically the same as Example 1, except that in step S2, 2.2 parts by mass of commercially available anhydrous lithium sulfate was replaced with 2.8 parts by mass of commercially available anhydrous lithium nitrate, so that the molar concentration of lithium salt was the same as the molar concentration of lithium ions provided by anhydrous lithium sulfate in Example 1, and the total concentration of sulfate in the resulting electrolyte was 0.00 mol / L, with other conditions remaining unchanged.

[0147] Comparative Example 2: It is basically the same as Example 1, except that in step S2, commercially available analytical grade sulfuric acid is used to adjust the pH of the aqueous electrolyte to 2.5, and in steps S3 and S4, the pH of the lithium extraction slurry and the liquid phase of the electrolysis system is kept at 2.5, while other conditions remain unchanged.

[0148] Comparative Example 3: It is basically the same as Example 1, except that in step S3, the solid content of the lithium extraction slurry is adjusted from 10.0wt% to 8.0wt%, and the liquid-solid ratio of the aqueous electrolyte to the pretreated black powder is adjusted to 11.5L / kg based on the mass of the dry pretreated black powder. Other conditions remain unchanged.

[0149] Comparative Example 4: Basically the same as Example 1, except that the current density in constant current mode during the electrolysis process in step S3 is adjusted from 30A / m² to 20A / m², while other conditions remain unchanged.

[0150] Comparative Example 5: It is basically the same as Example 1, except that the electrolysis time in step S3 is adjusted from 0.5h to 0.3h. After the set processing time is reached, the circulation continues for 5 minutes before sedimentation and solid-liquid separation. Other conditions remain unchanged.

[0151] Comparative Example 6: It is basically the same as Example 1, except that in step S3, the graphite anode and graphite cathode are replaced with 316L stainless steel anode and 316L stainless steel cathode. The size of the two electrodes, the effective area, the electrode spacing of 5mm and the connection method are the same as in Example 1, and other conditions remain unchanged.

[0152] Comparative Example 7: Basically the same as Example 1, except that the linear velocity of the lithium extraction slurry circulating between the electrochemical reactor and the circulation pipeline in step S3 is adjusted from 0.10 m / s to 0.05 m / s, while other conditions remain unchanged.

[0153] Comparative Example 8: It is basically the same as Example 1, except that in step S4, commercially available anhydrous sodium carbonate is replaced with commercially available anhydrous potassium carbonate, and anhydrous potassium carbonate is added according to the measured lithium concentration in the clarified lithium-rich solution so that the molar ratio of carbonate to lithium is still 0.50:1. The lithium precipitation temperature of 50°C, the reaction time of 0.5h and the pH of 8.5 remain unchanged, and other conditions remain unchanged.

[0154] Comparative Example 9: This example is essentially the same as Example 1, except that during electrolysis in step S3, the initial pH of the electrolyte is adjusted to 3.0 only in step S2. After electrolysis begins, the pH detection module, acidity regulator addition pump, and alkalinity regulator addition pump are not activated for closed-loop regulation; all other conditions remain unchanged. This comparative example is used to verify the effect of closed-loop pH control during electrolysis on the selective lithium extraction process.

[0155] Comparative Example 10: This example is essentially the same as Example 1, except that the redox potential detection module and DC power supply adjustment interface are not used for closed-loop control of the redox potential during electrolysis in step S3. It still operates at a constant current of 30 A / m² and records the measured potential, with other conditions remaining unchanged. This comparative example is used to verify the effect of redox potential closed-loop control on the selective lithium extraction process.

[0156] Comparative Example 11: This example is essentially the same as Example 1, except that the circulation pump is shut off during electrolysis in step S3, preventing the lithium extraction slurry from circulating between the electrochemical reactor and the circulation pipeline. Instead, mechanical stirring is used within the electrolytic cell to maintain solid suspension. After electrolysis, mechanical stirring continues for 5 minutes before sedimentation and solid-liquid separation. Other conditions remain unchanged. This comparative example is used to verify the effect of the circulating flow state on the selective lithium extraction process.

[0157] Characterization and performance testing:

[0158] Lithium leaching rate and lithium concentration increment tests were used to evaluate the degree of lithium transfer from pretreated black powder to lithium-rich sulfate solution. Samples of pretreated black powder digested before electrolysis and liquid phase samples filtered through a 0.45 μm membrane after electrolysis were taken. Lithium content was determined using ICP-OES or ICP-MS, and exogenous lithium, including anhydrous lithium sulfate, lithium hydroxide, initial background lithium from recycled mother liquor, or lithium introduced by pH adjusters, was deducted. The lithium leaching rate was calculated based on the mass of leached lithium compared to the initial lithium mass, and the lithium concentration increment was calculated based on the difference in lithium concentration in the liquid phase before and after the electrolysis step, after deducting exogenous lithium. Each group was tested in triplicate, and the mean and standard deviation were recorded. The average lithium leaching rate, standard deviation of lithium leaching rate, average lithium concentration increment, and standard deviation of lithium concentration increment were obtained, in % and g / L, respectively. Higher values ​​indicate a higher degree of lithium extraction.

[0159] Phosphorus leaching rate and comprehensive selectivity score were used to evaluate the degree of selective lithium extraction. Initial iron and phosphorus contents were determined from pretreated black powder acid digestion samples before electrolysis. Iron and total phosphorus contents were determined from the post-electrolysis liquid phase after filtration through a 0.45 μm filter membrane. The liquid phase leaching rates of iron and phosphorus were calculated separately. The comprehensive selectivity score was calculated as "lithium leaching rate − 4 × iron leaching rate − 1.5 × phosphorus leaching rate − 3 × residual lithium content". This score served as a relative evaluation index between different process conditions. Iron leaching rate, phosphorus leaching rate, and residual lithium content were used to characterize the effects of liquid phase impurity load, phosphorus infiltration into the liquid phase, and solid phase delithiation degree on selectivity, respectively. Each group was repeated in triplicate. The experiments yielded iron leaching rate, phosphorus leaching rate, and comprehensive selectivity score, expressed as %, %, and points. Lower iron and phosphorus leaching rates and higher scores indicated better selectivity.

[0160] The residual lithium content and structure retention index tests in the lithium solid phase were used to evaluate the retention state of the solid phase after lithium removal. The delithiated solid phase obtained from solid-liquid separation was washed with deionized water, dried at 80℃ until constant mass, and then sampled for acid digestion. The residual lithium content was determined using ICP-OES or ICP-MS. A separate dried solid phase was subjected to XRD or Raman spectroscopy. The structure retention index was calculated based on the integrity of the characteristic peaks of the iron phosphate phase, the peak area retention rate, and the change in the characteristic peaks of graphite carbon. Each group was repeated in triplicate. The average residual lithium content, standard deviation of residual lithium content, average structure retention index, and standard deviation were obtained, in units of wt% and %, respectively. Lower residual lithium and higher structure retention index are preferred.

[0161] The slurry dispersion stability test was used to evaluate the suspension and circulation status of black powder particles in the aqueous sulfate electrolyte during electrolysis. Samples of the slurry before electrolysis and the circulating samples after electrolysis were placed in a transparent graduated cylinder and allowed to stand at 25°C for 30 minutes. The proportion of the sedimentation layer to the total volume was recorded; simultaneously, the pressure difference fluctuation in the circulation pipeline and the deviation of the sampled solid content were recorded. The experiment was conducted with the same volume of slurry from Example 1 and each comparative example, in triplicate. The average sedimentation index over 30 minutes, the standard deviation of the sedimentation index over 30 minutes, the fluctuation of the circulating pressure difference, and the deviation of the solid content were obtained, in units of %, kPa, and wt%. Lower sedimentation index and fluctuation indicate better dispersion stability.

[0162] Electrogenerated free radical characterization was used to evaluate the free radical generation state of sulfate electrolyte at the anolyte interface. DMPO spin trapping agent was added to the electrolysis system, and liquid phase samples were collected near the anode 10 minutes after the start of electrolysis. Electron paramagnetic resonance (EPR) testing was immediately performed, and the magnetic field scanning range, microwave frequency, microwave power, modulation amplitude, test temperature, and spectral data were recorded. A methanol and tert-butanol quenching system was also set up to measure the change in lithium leaching rate before and after quenching. The relative intensity of sulfate free radical EPR and the change in lithium leaching rate after quenching were obtained, in units of au and percentage points. The relative intensity of EPR was used to characterize the response intensity of free radical-related active species and is included in the experimental data table.

[0163] The lithium carbonate precipitation and mother liquor reuse test was used to evaluate the feasibility of post-treatment and liquid-phase recycling of lithium-rich sulfate solutions. After filtration, the lithium concentration of the lithium-rich sulfate solution was measured. A carbonate lithium precipitant was added according to a predetermined carbonate to lithium molar ratio. The reaction was carried out at a specified temperature and pH, followed by solid-liquid separation. The lithium content in the precipitate, the lithium content in the mother liquor, the total sulfate concentration, and the contents of sodium ions, carbonate, iron, phosphorus, aluminum, and copper were measured. The lithium recovery rate and the sulfate recovery deviation after salt replenishment were calculated. Each group was repeated in triplicate. The lithium recovery rate and sulfate recovery deviation were obtained from the experiment, expressed as % and %, respectively. Higher lithium recovery rates and lower recovery deviations were preferred.

[0164] To verify the comprehensive improvement effect of the aqueous sulfate electrochemical selective lithium extraction scheme of the present invention on the efficient release of lithium, suppression of impurity components and adaptability to subsequent lithium precipitation in lithium iron phosphate waste black powder, a single-factor investigation was first conducted on the key process parameters affecting the electrolytic leaching process. Figures 1 to 4 The effects of total sulfate concentration, system pH, current density, and electrolysis time on lithium leaching rate and overall selectivity score are presented sequentially. The overall selectivity score can be used to simultaneously characterize lithium leaching efficiency, the inhibition effect of non-target element leaching such as iron and phosphorus, and system operational stability, thereby avoiding the bias caused by evaluating the quality of the process solely based on lithium leaching rate.

[0165] like Figure 1 As shown, under the basic conditions of pH approximately 3.0, current density 30 A / m², and electrolysis time 0.5 h, only the total sulfate concentration was varied. The results showed that as the total sulfate concentration gradually increased from a low value to a moderate range, the system's conductivity and ion transport capacity improved, leading to an increase in lithium leaching rate and a corresponding rise in the overall selectivity score. However, when the total sulfate concentration continued to increase, although the ionic strength in the system further increased, it may cause non-target ion migration, local side reactions, or changes in the solid-liquid interface state, resulting in a decrease in the overall score. This result indicates that a higher sulfate concentration is not necessarily more beneficial; rather, there exists a suitable concentration window that balances lithium release and impurity suppression. Based on the scope of the claims and the conditions of the examples, a stable process window can be formed between the lithium leaching rate and the overall selectivity score when the total sulfate concentration is controlled between 0.20-1.50 mol / L. Below 0.20 mol / L, the system's conductivity and free radical-related response are insufficient, while above 1.50 mol / L, the overall selectivity score decreases.

[0166] Based on determining the appropriate range of total sulfate concentration, the effect of the system's pH on selective leaching was further investigated. Figure 2The results show that, under conditions of a total sulfate concentration of 0.20 mol / L, a current density of 30 A / m², and an electrolysis time of 0.5 h, only the pH of the system was changed. When the pH was too low, acid dissolution was enhanced, easily causing the leaching of non-target components such as iron and phosphorus, thus reducing the overall selectivity. When the pH was too high, the oxidative leaching kinetics of the system were insufficient or the interfacial reaction was limited, resulting in a decrease in lithium release efficiency. In contrast, when the pH was in a moderately acidic range, especially at approximately pH 3.0, the lithium leaching rate remained at a high level, while the overall selectivity score was also superior. These results demonstrate that reasonable pH control can promote the extraction of lithium from the lithium iron phosphate lattice while avoiding non-selective dissolution under strong acid conditions, thus addressing the technical problem of "difficulty in separating lithium leaching from iron and phosphorus dissolution" in traditional acid leaching methods.

[0167] Figure 3 The effect of electrochemical oxidation intensity on the reaction results was further investigated. Under conditions of a total sulfate concentration of 0.20 mol / L, pH of approximately 3.0, and an electrolysis time of 0.5 h, only the current density was varied. The results showed that when the current density was too low, insufficient formation of active oxide species in the system resulted in weak electrochemical driving force, making it difficult to achieve optimal lithium leaching rate and overall score. When the current density was too high, although the reaction rate might increase, side reactions were enhanced, and local pH and redox state fluctuations intensified, thus affecting selectivity and process stability. Considering the scope of the claims and the conditions of the examples, a balance between lithium leaching rate and overall selectivity score could be achieved when the current density was controlled between 30-180 A / m², indicating that the method described in this invention requires controlling the electrochemical oxidation intensity within a suitable range to achieve the effect of "sufficient delithiation without excessive oxidation or dissolution."

[0168] Figure 4 The effect of limiting the electrolysis process was verified from the perspective of reaction time. Under conditions of a total sulfate concentration of 0.20 mol / L, pH of approximately 3.0, and a current density of 30 A / m², only the electrolysis time was varied. The results showed that when the electrolysis time was too short, lithium migration from the solid phase to the liquid phase was insufficient, resulting in a low leaching rate. When the electrolysis time was extended to a moderate range, the lithium leaching rate significantly improved, and the overall selectivity score was also at a relatively good level. Further extending the electrolysis time may lead to over-reaction, accumulation and dissolution of impurities, or deterioration of the slurry interface, resulting in a decrease in the overall score. Therefore, controlling the electrolysis time within the range of 0.5-3.0 h can improve the selective leaching efficiency of lithium and avoid the release of non-target components and increased energy consumption due to over-electrolysis.

[0169] Based on the above parameter optimization, the invention was further verified through free radical detection and quenching experiments that it does not rely on simple acid dissolution, but achieves selective delithiation by electrochemically inducing the generation of active oxide species. Figure 5The images show the EPR-DMPO radical capture spectra of Examples 1, 1 (Comparative Example), and 10. The results show that the radical capture signal is more pronounced in Example 1, while the signal in the comparative example samples is weaker or attenuated. This indicates that under the sulfate concentration, pH, current density, and electrolysis conditions specified in this invention, the system can effectively generate and maintain active oxide species participating in the delithiation reaction. This active oxidative environment preferentially acts on lithium sites in the lithium iron phosphate structure, promoting the release of lithium from the solid lattice and providing a reaction basis for selective leaching.

[0170] Figure 6 The results of free radical quenching-ICP combined tests for Example 1 and Comparative Example 10 under conditions of no quencher, addition of methanol, and addition of tert-butanol are shown. The results indicate that the lithium leaching rate significantly decreased after the addition of free radical quenchers such as methanol or tert-butanol, suggesting that free radicals or related active oxides play a crucial role in the lithium release process. This result further demonstrates that the electrochemical leaching process described in this invention differs from the traditional acid leaching method, which primarily relies on hydrogen ion dissolution. Instead, it achieves targeted lithium release through controlled electrochemical oxidation and the participation of free radicals. Therefore, this invention can achieve a high lithium leaching rate under relatively mild acidic conditions while simultaneously reducing the non-selective dissolution of components such as iron and phosphorus.

[0171] To verify the stability of this reaction mechanism during actual operation, Figure 7 and Figure 8 pH and ORP during the electrolysis process were monitored over time. Figure 7 The results show that Example 1 exhibits less pH fluctuation during electrolysis, while Comparative Examples 9 and 11 show more significant pH decreases. This indicates that the sulfate system, moderate pH, and circulating flow conditions used in Example 1 can buffer or suppress local acidification, maintaining a relatively stable acid-base environment. pH stability is crucial for inhibiting the excessive dissolution of non-target components such as iron and phosphorus, and directly affects the purification burden and lithium precipitation quality of the subsequent lithium-rich solution.

[0172] Figure 8 The results show that Example 1 maintained a relatively stable redox potential during the reaction, while the ORP of Comparative Examples 10 and 11 decreased more significantly. Stable ORP indicates that the system has the ability to continuously generate or maintain active oxide species, thus ensuring the continuous delithiation process. Conversely, when the ORP decreases significantly, the system's oxidation capacity is insufficient or diminished, easily leading to reduced delithiation efficiency, unclear reaction endpoints, and poor batch stability. Figure 7 and Figure 8 It can be seen that the present invention not only improves the instantaneous leaching effect through parameter combination, but also maintains a stable acid-base state and redox environment during electrolysis, thereby improving the controllability of the reaction process.

[0173] Furthermore, Figure 9 and Figure 10 The impact of this invention on subsequent separation and material stability was verified from the perspective of slurry properties and solid-phase structure retention. Figure 9 The scatter plots and mean SD plots of the sedimentation index of the slurries in Examples 1, 9, and 11 are shown. The results indicate that Example 1 has a lower 30-minute sedimentation index and less dispersion among parallel samples, suggesting good dispersion stability of the slurry after the reaction, making it less prone to abnormal agglomeration or rapid sedimentation. This effect is related to the 0.10 m / s circulating flow conditions and mild electrolysis environment in Example 1, which helps maintain the uniformity of mass transfer at the solid-liquid interface, reduces local over-reaction and solid entrainment, and facilitates subsequent filtration, washing, and continuous processing.

[0174] Figure 10 The images show the Raman D / G spectra of the carbon materials in Examples 1, 9, and 11. The results show that the relative relationship between the D and G peaks in Example 1 is relatively stable, with no obvious abnormal changes indicating carbon structure degradation. This suggests that the electrolytic delithiation process described in this invention effectively preserves the conductive carbon network and surface carbon structure in the waste lithium iron phosphate material. These results demonstrate that this invention does not destroy the overall solid-phase system through strong corrosion or severe oxidation, but rather achieves selective lithium migration while maintaining a relatively stable carbon structure. This is beneficial in reducing the risks of carbon particle breakage, refinement, and subsequent entrainment.

[0175] Based on the stability of selective leaching at the front end Figure 11 and Figure 12 Further verification of the adaptability of lithium-rich solution to the subsequent lithium precipitation process. Figure 11 The diagram shows the main ion balance of the mother liquor after lithium precipitation in Example 1 and Comparative Example 8. The results show that the residual lithium ions in the mother liquor after lithium precipitation in Example 1 are low, while the levels of accompanying ions such as sodium, potassium, and sulfate are relatively controllable. This indicates that the content of interfering ions in the upstream leachate and the ion balance of the system are more conducive to the lithium carbonate precipitation reaction. This result proves that the present invention, through selective control at the upstream stage, can reduce ion interference and mother liquor load in the subsequent lithium precipitation stage.

[0176] Figure 12 The image shows a scatter plot and mean SD graph of the lithium precipitation recovery rates for Example 1 and Comparative Example 8. The results show that the lithium precipitation recovery rate of Example 1 is higher than that of Comparative Example 8, and the parallel samples exhibit smaller fluctuations. This indicates that the lithium-containing solution obtained by this invention not only has a suitable lithium concentration but also a more stable composition, making it more suitable for reacting with sodium carbonate to form lithium carbonate precipitate. Combined with... Figure 11 It is understood that the present invention can form a good connection between front-end electrolytic leaching and back-end lithium deposition and recovery, thereby improving the overall lithium resource recovery rate and batch stability.

[0177] Figures 13 to 15The effects of circulating flow conditions and overall process stability were further verified from the perspectives of the macroscopic appearance, microscopic morphology, and crystal structure of the final lithium carbonate product. Figure 13 This is a comparison of macroscopic optical photographs of the final product of Example 1 and the final product of Comparative Example 11. Figure 13 Example 1 (shown in Figure a) shows that the final lithium carbonate precipitate was obtained from 10.0 kg of pretreated black powder with a dry basis weight of 5 μm, through constant current electrolysis at pH 3.0, a total sulfate concentration of 0.20 mol / L, a constant current of 30 A / m², a circulating flow at 0.10 m / s, and lithium precipitation with sodium carbonate at 50 °C. The product was white to off-white in color and relatively uniformly distributed, indicating that the process can obtain a relatively clean precipitate product under high lithium leaching conditions. Figure 13 Comparative Example 11 (shown in b) had its circulation pump shut off during electrolysis, with solids kept suspended only by mechanical stirring. The final precipitate exhibited a deepening of the grayish-white color, localized agglomeration, or entrainment of dark fine particles. This comparison demonstrates that circulating flow helps improve slurry uniformity, reduces the amount of carbon particles, iron-phosphorus particles, or other non-target solids entering the lithium precipitation stage with the lithium-rich solution, thereby improving the appearance stability of the final product.

[0178] Figure 14 The image shows a comparison of the SEM morphology of the final product of Example 1 and the final product of Comparative Example 11. Figure 14 As shown in Example 1, the final lithium carbonate precipitate is relatively uniformly distributed under low magnification, with fewer coarse agglomerates, indicating that the composition of the lithium-rich solution and the lithium precipitation environment are relatively stable under the combined effects of pH 3.0, ORP of approximately 0.75V, and a circulating flow of 0.10m / s. Figure 14 As shown in Comparative Example 11 (b), after the circulation pump was shut off, the precipitated powder was more prone to agglomeration and uneven local accumulation, reflecting a decrease in slurry mass transfer and solid-liquid separation stability under non-circulation conditions. Further observation of the medium-magnification SEM images... Figure 14 In Example c, the lithium carbonate crystal particles or clusters obtained in Example 1 have clearer morphology and a more concentrated particle size distribution, indicating that mild electrolysis, effective filtration, and stable lithium deposition conditions help coordinate crystal nucleation and growth. In contrast, Figure 14 In Comparative Example 11 (d), the crystal clusters are more unevenly packed, accompanied by a wider particle size distribution, indicating that eliminating the circulating flow weakens the consistency of the liquid phase composition and the suspended solid state before lithium deposition. In the high-magnification SEM images, Figure 14 The crystal surface of Example 1 shown in Figure e is relatively clean and the edges are relatively clear, indicating that low iron and phosphorus leaching and 0.45μm filtration can reduce the adhesion of impurities; Figure 14 The crystal surface of Comparative Example 11 shown in f has fine attached particles or local rough structures, indicating that insufficient circulation will increase the risk of carbon particles, iron-phosphorus fine particles or other non-target solids being entrained.

[0179] Figure 15 The structure of the final product was further characterized using TEM and HRTEM. Figure 15 In the bright-field TEM image of the final lithium carbonate precipitate in Example 1 shown in Figure a, the thin edges of the dispersed crystals or the local outlines of the broken crystals are relatively clear, indicating that it has a clear crystal morphology basis. Figure 15 In Comparative Example 11 shown in b, irregular attachments or areas of contrast difference are present at the crystal edges, indicating that impurity entrainment or localized uneven deposition is more likely to occur after the circulation flow is shut off.

[0180] In high-resolution characterization, Figure 15 c. The lattice fringes of the final lithium carbonate precipitate in Example 1 can be observed, which, combined with the approximately 0.248 nm interplanar spacing characteristic in the lithium carbonate crystal structure, are consistent with the target lithium carbonate crystal structure. Figure 15 As shown in Comparative Example 11, sample d exhibits decreased lattice fringe continuity, an amorphous deposited layer, or localized disordered regions, indicating a reduction in the local structural integrity of the precipitated crystal under non-cyclic conditions.

[0181] at last, Figure 16 The macroscopic optical photographs show the evolution of the pretreated black powder, aqueous sulfate electrolyte, electrolytic products, and final lithium carbonate precipitation in Example 1, illustrating the continuity and traceability of the process route of the present invention from black powder pretreatment, electrolytic lithium extraction to lithium carbonate precipitation. Figure 16 Image a is a macroscopic optical photograph of the pretreated black powder. This sample was obtained from lithium iron phosphate waste black powder through discharge, dismantling, crushing, sieving, water washing, drying at 80℃ for 2.0h, and sieving again. The D50 is 5μm, and it is a black or dark gray-black powder, indicating that the raw material has reached a particle size state that can be stably slurryed and electrolyzed. Figure 16 b is a macroscopic optical photograph of the aqueous sulfate electrolyte, with a total sulfate concentration of 0.20 mol / L, a pH of 3.0, and a conductivity of 10 mS / cm. The electrolyte has been filtered to remove insoluble matter with a particle size greater than 50 μm, indicating that the electrolyte has stable conductivity and a weakly acidic reaction environment, which can meet the requirements of subsequent electrochemical delithiation.

[0182] Figure 16 c is a macroscopic optical photograph of the products after electrolysis, including a lithium-rich sulfate solution and a delithiated solid phase with a water content of 5.0 wt%. The lithium concentration increase in the lithium-rich solution resulting from the leaching of pretreated black powder is 2.0 g / L ± 0.1 g / L, indicating that under the optimized conditions described above, lithium has been effectively transferred from the lithium iron phosphate solid phase to the liquid phase. Figure 16 Image d is a macroscopic optical photograph of the final lithium carbonate precipitate. This product was obtained by reacting anhydrous sodium carbonate with a lithium-rich solution filtered through a 0.45 μm filter membrane at 50 °C and pH 8.5 for 0.5 h, indicating that the lithium-rich solution obtained at the front end can be further converted into solid lithium carbonate product.

[0183] In conclusion, Figures 1 to 16The effectiveness of this invention has been demonstrated from multiple perspectives, including process parameters, reaction mechanism, process stability, slurry separation performance, lithium precipitation compatibility, and final product quality. This method achieves efficient and selective lithium release, suppression of non-target components such as iron and phosphorus, stable reaction environment maintenance, and improved lithium carbonate product quality by controlling the total sulfate concentration at 0.20-1.50 mol / L, the system pH at 3.0-6.0, the current density at 30-180 A / m², and the electrolysis time at 0.5-3.0 h, combined with a 0.10-2.00 m / s circulating flow and subsequent filtration and lithium precipitation steps. Therefore, this invention effectively solves the problem of simultaneously achieving high lithium leaching efficiency, selectivity, process controllability, and stable lithium precipitation products in existing lithium iron phosphate waste recycling processes.

[0184] Table 1 Lithium extraction and selectivity performance

[0185]

[0186] Table 2. Structure retention, sedimentation, and lithium recovery performance

[0187]

[0188] As can be seen from the performance of the examples and comparative examples in Tables 1 and 2, Example 1 achieves a high lithium leaching rate, extremely low iron-phosphorus dissolution, and a high structure retention index under mild electrolysis conditions, demonstrating the selectivity advantage under low-intensity operation. In Examples 2-4, as the sulfate concentration, solid content, current density, and electrolysis time increase, the lithium concentration increment and lithium recovery rate further increase, but the iron-phosphorus dissolution and sedimentation index increase accordingly, indicating that the process intensity needs to be matched with pH, ​​redox potential, and cycling conditions. Comparative Examples 1, 4, 5, and 6 show that when sulfate radical precursors are lacking, current density is insufficient, electrolysis time is too short, or electrode materials are changed, the lithium leaching rate, EPR relative strength, and residual lithium index all deviate from the examples. Comparative Examples 9-11 show that when any synergistic control of pH, redox potential, and circulating flow is weakened, the selectivity score, sedimentation index, or structure retention index all show adverse changes, supporting the necessity of synergistic process control.

[0189] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that any equivalent structural transformations made under the concept of the present invention and using the contents of the specification and drawings of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for selective lithium extraction from lithium iron phosphate waste based on in-situ electrogenerated free radicals, characterized in that, Includes the following steps: S1. Provide lithium iron phosphate waste black powder containing lithium iron phosphate and conductive carbon material, and pretreat the lithium iron phosphate waste black powder to obtain pretreated black powder; S2. Prepare an aqueous sulfate electrolyte using deionized water and sulfate electrolyte, and adjust the pH of the aqueous sulfate electrolyte; S3. Disperse the pretreated black powder in the aqueous sulfate electrolyte to form a lithium extraction slurry; S4. The lithium extraction slurry is placed in an electrochemical reactor and electrolyzed using carbon-based electrodes as the anode and cathode. The pH is controlled at 3.0-6.0 during the electrolysis process so that sulfate ions generate free radicals in situ at the anode interface and act on the surface of lithium iron phosphate particles. S5. The lithium extraction slurry after solid-liquid separation and electrolysis yields a lithium-rich sulfate solution and a delithiated solid phase containing iron phosphate and conductive carbon materials; S6. Treat the lithium-rich sulfate solution with a lithium carbonate precipitant to precipitate lithium in the form of lithium carbonate, and return the mother liquor after lithium precipitation as the lithium precipitation mother liquor to step S2 and / or step S3.

2. The method according to claim 1, characterized in that, The method satisfies the following process conditions: H1. In step S1, the pretreatment includes sieving, demagnetization and drying. H2. In step S2, the sulfate electrolyte is anhydrous lithium sulfate, anhydrous sodium sulfate, or a combination thereof, and the pH of the aqueous sulfate electrolyte is adjusted using an acidity regulator or an alkalinity regulator. H3. In step S3, the solid content of the lithium extraction slurry is 10.0-35.0 wt% based on the dry basis of the pretreated black powder as a percentage of the total mass of the lithium extraction slurry, and the total sulfate concentration in the aqueous sulfate electrolyte is 0.20-1.50 mol / L, and the pH is 3.0-6.

0. H4. In step S4, the electrolysis conditions include a current density of 30-180 A / m², a temperature of 25-60℃, an electrolysis time of 0.5-3.0 h, and a redox potential of the liquid phase of the electrolysis system of 0.75-1.25 V measured by a standard hydrogen electrode. H5. In step S6, the carbonate precipitant is anhydrous sodium carbonate; H6. The conductive carbon material is graphite, the carbon-based electrode is a graphite electrode, and the in-situ electrogenerated free radicals include sulfate free radicals.

3. The method according to claim 1, characterized in that, The aqueous sulfate electrolyte is prepared by the following steps: A1. Add deionized water to the solution preparation tank; A2. Add anhydrous lithium sulfate, anhydrous sodium sulfate, or a combination thereof to make the total sulfate concentration 0.20-1.50 mol / L; A3. Adjust the pH to 3.0-6.0; A4. After stirring, filter to remove insoluble matter to obtain the aqueous sulfate electrolyte; A5. The conductivity of the aqueous sulfate electrolyte is 10-150 mS / cm.

4. The method according to claim 1, characterized in that, The pretreated black powder is obtained through the following steps: B1. Discharge, dismantle, crush and screen lithium iron phosphate waste to separate metal fragments containing aluminum and copper. B2. Wash the sieved black powder with deionized water; B3. Dry the washed black powder to a moisture content of 0.10-5.00 wt%. B4. Sift the dried black powder to obtain the pretreated black powder; B5. The pretreated black powder retains conductive carbon material from the positive electrode material, and the highest processing temperature during the pretreatment process does not exceed 300℃.

5. The method according to claim 1, characterized in that, The in-situ generation of free radicals in step S4 is controlled by the following conditions: C1. The distance between the anode and the cathode is 5-80 mm; C2. The lithium extraction slurry is in a circulating state; C3. During electrolysis, the redox potential of the liquid phase in the electrolysis system is maintained at 0.75-1.25V using a standard hydrogen electrode. C4. Electrolysis shall be stopped when the increase in lithium concentration in the liquid phase obtained by sampling and separation during the electrolysis process, formed by the leaching of the pretreated black powder, reaches 2.0-30.0 g / L, or when the lithium leaching rate calculated based on the initial lithium content of the pretreated black powder reaches 92.0-99.9%. C5. After stopping electrolysis, continue to keep the lithium extraction slurry in a circulating or stirred state, and then perform solid-liquid separation.

6. The method according to claim 1, characterized in that, The precipitation of lithium carbonate and the reuse of the lithium precipitation mother liquor in step S6 are carried out through the following steps: D1. Filter the lithium-rich sulfate solution to obtain a clear lithium-rich solution; D2. Add anhydrous sodium carbonate to the clarified lithium-rich solution so that the molar ratio of carbonate to lithium in the clarified lithium-rich solution is 0.50-0.80:1; D3. React at 50-95℃ and control the pH at 8.5-11.5; D4. Solid-liquid separation yields lithium carbonate precipitate and lithium precipitation mother liquor; D5. After replenishing the lithium precipitation mother liquor with anhydrous lithium sulfate, anhydrous sodium sulfate or a combination thereof to restore the total sulfate concentration to 0.20-1.50 mol / L, return to step S2 and / or step S3.

7. The method according to claim 1, characterized in that, The liquid-to-solid ratio of the aqueous sulfate electrolyte to the pretreated black powder in the lithium extraction slurry is 1.8-9.0 L / kg, based on the dry basis of the pretreated black powder.

8. The method according to claim 1, characterized in that, The electrolysis in step S4 adopts constant current mode, constant voltage mode or constant current-constant voltage segmented mode, wherein the current density in constant current mode is 30-180A / m², and the cell voltage in constant voltage mode is 2.0-6.0V.

9. An in-situ electrogenerated free radical system for selective lithium extraction from lithium iron phosphate waste, characterized in that, include: F1. Black powder slurry preparation unit, used to mix lithium iron phosphate waste black powder containing lithium iron phosphate and conductive carbon materials with aqueous sulfate electrolyte; F2. An electrochemical reactor, wherein the electrochemical reactor is provided with a carbon-based anode and a carbon-based cathode; F3. A DC power supply configured to apply current to the carbon-based anode and the carbon-based cathode, and to provide a power output in constant current mode, constant voltage mode or constant current-constant voltage segmented mode, so that sulfate ions in the aqueous sulfate electrolyte generate free radicals in situ at the carbon-based anode interface; F4. pH and redox potential control unit, wherein the pH and redox potential control unit is connected to an acidity regulator storage tank and an alkalinity regulator storage tank; F5. Solid-liquid separation unit, used to separate lithium-rich sulfate solution and delithiated solid phase containing iron phosphate and conductive carbon materials; F6. Lithium precipitation unit, wherein the lithium precipitation unit is connected to a carbonate lithium precipitation agent storage tank for preparing lithium carbonate; F7. Mother liquor recycling unit, used to return lithium precipitation mother liquor to the black powder slurry preparation unit and / or its upstream aqueous sulfate electrolyte preparation location.

10. The system according to claim 9, characterized in that, The system satisfies conditions G1, G3, G4, and G9, and satisfies at least one of conditions G2, G5, G6, G7, and G8: G1. The electrochemical reactor is a single-chamber stirred electrolytic cell, a flow-through electrolytic cell, or a circulating slurry electrolytic cell, and the distance between the carbon-based anode and the carbon-based cathode is 5-80 mm; G2. The black powder slurry preparation unit includes a stirring tank and a circulating pump. The stirring tank is used to form a lithium extraction slurry with a solid content of 10.0-35.0 wt% based on the dry basis of the pretreated black powder as a percentage of the total mass of the lithium extraction slurry. The circulating pump is used to circulate the lithium extraction slurry. G3. The pH and redox potential control unit is configured to control the pH in the electrochemical reactor at 3.0-6.0 and the redox potential at 0.75-1.25V using a standard hydrogen electrode meter; G4. The DC power supply is configured to provide a current density of 30-180 A / m² or a tank voltage of 2.0-6.0 V; G5. The solid-liquid separation unit includes one or more of a sedimentation tank, centrifuge, filter press, or cross-flow filtration assembly; G6. The lithium precipitation unit includes a heating kettle, a carbonate lithium precipitation agent dosing device, a solid-liquid separator, and a mother liquor storage tank; G7. The system further includes a quality control unit configured to control the increase in lithium concentration in the lithium-rich sulfate solution formed by leaching the lithium iron phosphate waste black powder to be 2.0-30.0 g / L, and to control the residual lithium content in the delithiated solid phase to be 0.05-0.70 wt% on a dry basis. G8. The mother liquor recycling unit is equipped with a sulfate replenishment port, which is used to replenish anhydrous lithium sulfate, anhydrous sodium sulfate or a combination thereof, so that the total sulfate concentration in the lithium precipitation mother liquor is restored to 0.20-1.50 mol / L; G9. The conductive carbon material is graphite, the carbon-based anode is a graphite anode, the carbon-based cathode is a graphite cathode, and the in-situ electrogenerated free radicals include sulfate free radicals.

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