A method for recovering lithium from spent lithium iron phosphate cathode materials based on integrated processes.

Abstract

This invention relates to the field of battery resource recycling technology and discloses a method for recovering lithium from waste lithium iron phosphate cathode materials based on an integrated process. The method includes: crushing, screening, and calcining the waste lithium iron phosphate cathode material; leaching it in a mixture of sulfuric acid and hydrogen peroxide to obtain a lithium-containing leachate; pressurizing the lithium-containing leachate through monovalent and multivalent separation nanofiltration membranes to obtain a lithium-rich permeate and collecting its outlet characteristic conductivity; introducing the lithium-rich permeate into a bipolar membrane electrodialysis unit, where it reacts under a DC electric field to generate lithium hydroxide monohydrate solid and a regenerated sulfuric acid solution; adjusting the DC operating voltage according to the outlet characteristic conductivity gradient; and returning the regenerated sulfuric acid solution to the leaching process. This invention establishes an electrochemical control mechanism across the separation interface, eliminating contamination of the bipolar membrane interface by impurity ions and ensuring continuous operational stability.

Description

Technical Field

[0001] This invention belongs to the field of battery resource recycling technology, and in particular relates to a method for recovering lithium from waste lithium iron phosphate cathode materials based on integrated processes. Background Technology

[0002] Recycling waste lithium iron phosphate cathode materials forms the basis for building a closed-loop life cycle of energy storage devices. The industry typically uses acid leaching processes to extract lithium from the electrode materials. By introducing acidic media and oxidants into the reaction system, the lithium phase in the cathode framework is converted into liquid free lithium ions, so as to obtain the lithium salt products required for precursor manufacturing. Analysis of the solid phase degradation mechanism of retired cathode materials shows that after long-term charge-discharge cycles, the material undergoes internal lattice collapse and distortion. The free lithium ions that were originally in the insertion / extraction channels are deeply pinned into the gaps in the distorted iron-phosphorus polyhedral framework due to increased lattice steric hindrance and phase structure isomerism, and are transformed into inactive solid dead lithium phase. This phase pinning effect generated at the bottom layer of the material gives the retired material a high solid phase dissociation resistance.

[0003] Traditional wet leaching processes reveal compromises in their structural design when faced with this strongly pinned dead lithium structure. In production practice, to improve the extraction efficiency of dead lithium pinned due to lattice distortion, the common practice is to increase the acidity or oxidant concentration of the leaching system. However, a highly corrosive environment exacerbates the disordered collapse of the crystal framework, leading to large-scale depolymerization of iron and phosphorus and the release of a large amount of polynuclear iron complex impurities. These impurity ions, carried by the liquid phase material, poison the downstream physical membrane channels and electrochemical conversion interfaces, causing concentration polarization and irreversible pore blockage, reducing the service life of separation components. Faced with these interception and separation challenges, existing technologies not only struggle to provide long-term anti-clogging support for the flow channels due to limitations in the hardware roller shape, but also have shortcomings in control methods. For example… For example, Chinese invention patent application CN107546436A discloses a method for recovering lithium from lithium iron phosphate and a lithium-rich solution. It selectively extracts lithium by introducing sodium persulfate solution at a specific temperature, without dissolving the iron-phosphate main material to obtain a lithium-rich solution. However, the one-way consumption process of the external strong oxidant introduces a large amount of monovalent sodium ions into the system. When recovering lithium element through precipitation, alkali must be added to the lithium-rich solution in a cascade to adjust the alkalinity and sodium carbonate precipitant must be added dropwise, which leads to the enrichment of background impurity ions in the system. This control method, which lacks a closed-loop regeneration mechanism, results in the one-way consumption of leaching and precipitation reagents and the inability to achieve in-situ circulation, resulting in the problem of high-salt wastewater discharge. This is a fundamental mismatch with the objective constraints of low polarization and low scaling required for continuous high-purity conversion in industrial applications.

[0004] Therefore, the technical problem to be solved by this invention is to address the phase characteristics of waste lithium iron phosphate cathode materials after charge-discharge cycles, to achieve the directional deconstruction and release of pinned dead lithium while maintaining the integrity of the iron-phosphorus skeleton, and to establish an integrated method for precise sieving of material charge and acid-base regeneration, so as to eliminate the limitations of high-salt wastewater discharge and one-way consumption of reagents in traditional wet recycling. Summary of the Invention

[0005] This invention aims to solve the problems of the difficulty in the directional deconstruction and release of pinned dead lithium in the failed phase of secondary batteries and the high-salt wastewater generated by the unidirectional consumption of reagents in wet process.

[0006] In this technical solution, a method for recovering lithium from waste lithium iron phosphate cathode materials based on integrated processes includes the following steps:

[0007] Step S1: The waste lithium iron phosphate cathode material is crushed and sieved and calcined in air atmosphere to remove organic binders and conductive agents, and pretreated cathode material is obtained.

[0008] Step S2: The pretreated cathode material is mixed and reacted in a reactor with a mixed solution containing sulfuric acid with a concentration of 1.5 mol / L to 2.0 mol / L and hydrogen peroxide with a concentration of 0.3 mol / L to 0.5 mol / L. Lithium ions are dissolved at 40°C to 60°C, and a lithium-containing leachate containing lithium ions and iron complexes is obtained by separation.

[0009] Step S3: The lithium-containing leachate is introduced into the nanofiltration membrane separation unit and separated under pressure at 0.4 MPa to 0.6 MPa and 40°C to 50°C on the monovalent and multivalent separation nanofiltration membrane. Iron complexes are retained to obtain lithium-rich permeate, and the outlet characteristic conductivity of the lithium-rich permeate is collected.

[0010] In step S4, the lithium-rich permeate is introduced into the bipolar membrane electrodialysis unit. Under a DC electric field, the hydroxide ions generated by the hydrolysis of the bipolar membrane react with lithium ions to generate lithium hydroxide monohydrate solid. The protons generated react with anions to generate a regenerated sulfuric acid solution. The regenerated sulfuric acid solution is returned to step S2, and the DC operating voltage is reduced from the initial value of 18V to 15V or 12V according to the outlet characteristic conductivity to suppress impurity precipitation.

[0011] Preferably, step S1 consists of the following sequential sub-steps: Step S11, placing the waste lithium iron phosphate cathode material in a shear crusher for mechanical shearing to a particle size of 0.5 mm to 2.0 mm; Step S12, physically classifying the crushed product through a standard vibrating screen of 100 mesh to 200 mesh and collecting the undersize; Step S13, feeding the undersize into a rotary kiln and introducing air, controlling the calcination temperature at 500°C to 600°C, controlling the calcination time at 2 h to 4 h, and removing the polyvinylidene fluoride binder and conductive carbon black.

[0012] Preferably, in step S2, the pretreated cathode material and the mixed solution are mixed in a heated and stirred reactor with a polytetrafluoroethylene liner, the stirring speed is controlled at 200 rpm to 400 rpm, and the mixing reaction time is controlled at 1.5 h to 3.0 h.

[0013] Preferably, in step S2, the addition of hydrogen peroxide to the reaction vessel consists of the following sub-steps: step S21, adding 60% of the total mass of hydrogen peroxide within the first 30 minutes of the mixing reaction; step S22, continuously adding the remaining mass of hydrogen peroxide dropwise during the remaining time of the mixing reaction to maintain the local concentration of hydrogen peroxide in the reaction system below 0.1 mol / L.

[0014] Preferably, in step S3, the monovalent and multivalent separation nanofiltration membrane has a magnesium sulfate rejection rate of greater than or equal to 98.2% and a lithium chloride rejection rate of less than or equal to 12.5%; when the lithium-containing leachate is separated under pressure, the cross-flow velocity on the membrane surface within the membrane module is controlled to be between 1.5 m / s and 2.5 m / s.

[0015] Preferably, in step S4, the bipolar membrane electrodialysis unit is a three-chamber structure composed of alternating bipolar membranes, cation exchange membranes, and anion exchange membranes, with an acid chamber, a salt chamber, and an alkali chamber arranged sequentially inside. The lithium-rich permeate is continuously passed into the salt chamber, and under a constant operating current density of 400 A / m² to 600 A / m², monovalent lithium ions pass through the cation exchange membrane into the alkali chamber and react with hydroxide ions generated by hydrolysis at the bipolar membrane interface, thus enriching and generating a lithium hydroxide solution in the alkali chamber.

[0016] Preferably, step S4 consists of the following sub-steps with a causal feedback relationship: Step S41, using an online conductivity sensor to detect the fluid conductivity of the brine drain in the bipolar membrane electrodialysis unit; Step S42, when the fluid conductivity decreases to 10 mS / cm to 15 mS / cm, the brine drain is discharged as residual liquid, and lithium-rich permeate is introduced into the brine; Step S43, the discharged residual liquid is introduced into a mixing vessel to adjust the initial conductivity of the mixed solution in step S2.

[0017] Preferably, step S3 consists of the following sub-steps with a causal feedback relationship: Step S31, using a pressure sensor to detect the transmembrane pressure difference between the inlet and outlet of the membrane module in the nanofiltration membrane separation unit, and calculating the rate of increase of the transmembrane pressure difference; Step S32, when the rate of increase of the transmembrane pressure difference exceeds 1.5 kPa / h for 20 minutes, the regenerated sulfuric acid solution generated in the bipolar membrane electrodialysis unit is introduced into the permeate side of the nanofiltration membrane separation unit to perform reverse acid washing on the monovalent and multivalent separation nanofiltration membranes.

[0018] Preferably, after enriching and generating a lithium hydroxide solution in the alkali chamber, the following product purification steps are further included: feeding the lithium hydroxide solution into an evaporator crystallizer, performing vacuum evaporation crystallization at a temperature of 60°C to 80°C, cooling to 20°C to 25°C and then cooling to crystallize, obtaining the precipitated crystals by centrifugation, washing the crystals 2 to 3 times with anhydrous ethanol, and drying them in a vacuum environment of 40°C to 50°C for 10 to 12 hours to obtain lithium hydroxide monohydrate solid; wherein, the purity of the lithium hydroxide monohydrate solid is greater than or equal to 99.9%.

[0019] Compared with existing technologies, the method for recovering lithium from waste lithium iron phosphate cathode materials based on integrated processes in this invention has the following advantages:

[0020] 1. In the integrated process for recovering lithium from waste lithium iron phosphate cathode materials, a specific temperature-controlled calcination method is used to treat the degraded lithium iron phosphate material, which directionally transforms the degraded solid phase structure into a lithium-rich phase and a high-valence iron phase. This adjustment of the solid phase state and the control of the acidity of the leaching solution work synergistically. The specific proportion of hydrogen peroxide is added to selectively act on the weak sites of the lithium-rich phase, driving the preferential release of free lithium ions. At the same time, the iron element is locked in the high-valence solid residue, preventing low-valence polynuclear iron complexes from entering the liquid phase. This effectively controls the ion background complexity of the lithium-containing leaching solution, freeing the subsequent physical separation system from the interference of cumbersome chemical precipitation and impurity removal processes, and establishing a clear ion separation channel from the source.

[0021] 2. The specific charge state in the lithium-containing leachate precisely corresponds to the surface charge characteristics of the selected specific nanofiltration membrane. Under the set pressure window and pH environment adjustment, the spontaneous sieving mechanism of the Donnan effect and steric hindrance at the flow channel interface takes effect. This physical interception action specifically restricts the passage of high-valence iron ions, allowing monovalent lithium ions to pass smoothly through the membrane pores to form a lithium-rich permeate. This eliminates the entrainment loss caused by the forced precipitation of a large amount of chemical additives in the traditional process, achieving high-rate in-situ retention of high-valence impurities and maintaining a high proportion of monovalent ion permeation. This ensures that the downstream electrochemical conversion unit stably obtains a high-purity liquid-phase material reaction source that meets the requirements for battery-grade precursor manufacturing.

[0022] 3. The pure, high-conductivity monovalent fluid obtained through the specific separation barrier in the upstream stage directly serves as the reaction matrix for the downstream DC electric field, effectively eliminating the polarization and scaling contamination of the heterogeneous ion exchange membrane interface by multivalent transition metal ions. This drives the bipolar membrane water dissociation interface to operate in a low-polarization state. Under the drive of this DC electric field, the directional and selective migration of protons and hydroxide ions occurs simultaneously, enabling monovalent lithium ions to be efficiently converted into high-purity lithium hydroxide monohydrate solid products. At the same time, the regenerated sulfuric acid solution recovered in situ by the system has high solid-phase reaction activity. It is directly returned to the upstream acid leaching process in the closed-loop material pipeline flow to serve as a circulating leaching reagent, thereby establishing a self-sustaining closed material flow system for the process system. Attached Figure Description

[0023] Figure 1 This is a flowchart of the mixing reaction and membrane separation of waste lithium iron phosphate cathode materials according to the present invention;

[0024] Figure 2 This is a diagram of the nanofiltration separation and electrodialysis unit for waste lithium iron phosphate cathode materials of this invention. Detailed Implementation

[0025] The technical solutions of the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application are within the scope of protection of this application.

[0026] A method for recovering lithium from waste lithium iron phosphate cathode materials based on integrated processes includes the following steps:

[0027] Step S1: The waste lithium iron phosphate cathode material is crushed and sieved and calcined in air atmosphere to remove organic binders and conductive agents, and pretreated cathode material is obtained.

[0028] Step S2: The pretreated cathode material is mixed and reacted in a reactor with a mixed solution containing sulfuric acid with a concentration of 1.5 mol / L to 2.0 mol / L and hydrogen peroxide with a concentration of 0.3 mol / L to 0.5 mol / L. Lithium ions are dissolved at 40°C to 60°C, and a lithium-containing leachate containing lithium ions and iron complexes is obtained by separation.

[0029] Step S3: The lithium-containing leachate is introduced into the nanofiltration membrane separation unit and separated under pressure at 0.4 MPa to 0.6 MPa and 40°C to 50°C on the monovalent and multivalent separation nanofiltration membrane. Iron complexes are retained to obtain lithium-rich permeate, and the outlet characteristic conductivity of the lithium-rich permeate is collected.

[0030] In step S4, the lithium-rich permeate is introduced into the bipolar membrane electrodialysis unit. Under a DC electric field, the hydroxide ions generated by the hydrolysis of the bipolar membrane react with lithium ions to generate lithium hydroxide monohydrate solid. The protons generated react with anions to generate a regenerated sulfuric acid solution. The regenerated sulfuric acid solution is returned to step S2, and the DC operating voltage is reduced from the initial value of 18V to 15V or 12V according to the outlet characteristic conductivity to suppress impurity precipitation.

[0031] Preferably, step S1 consists of the following sequential sub-steps: Step S11, placing the waste lithium iron phosphate cathode material in a shear crusher for mechanical shearing to a particle size of 0.5 mm to 2.0 mm; Step S12, physically classifying the crushed product through a standard vibrating screen of 100 mesh to 200 mesh and collecting the undersize; Step S13, feeding the undersize into a rotary kiln and introducing air, controlling the calcination temperature at 500°C to 600°C, controlling the calcination time at 2 h to 4 h, and removing the polyvinylidene fluoride binder and conductive carbon black.

[0032] Preferably, in step S2, the pretreated cathode material and the mixed solution are mixed in a heated and stirred reactor with a polytetrafluoroethylene liner, the stirring speed is controlled at 200 rpm to 400 rpm, and the mixing reaction time is controlled at 1.5 h to 3.0 h.

[0033] Preferably, in step S2, the addition of hydrogen peroxide to the reaction vessel consists of the following sub-steps: step S21, adding 60% of the total mass of hydrogen peroxide within the first 30 minutes of the mixing reaction; step S22, continuously adding the remaining mass of hydrogen peroxide dropwise during the remaining time of the mixing reaction to maintain the local concentration of hydrogen peroxide in the reaction system below 0.1 mol / L.

[0034] Preferably, in step S3, the monovalent and multivalent separation nanofiltration membrane has a magnesium sulfate rejection rate of greater than or equal to 98.2% and a lithium chloride rejection rate of less than or equal to 12.5%; when the lithium-containing leachate is separated under pressure, the cross-flow velocity on the membrane surface within the membrane module is controlled to be between 1.5 m / s and 2.5 m / s.

[0035] Preferably, in step S4, the bipolar membrane electrodialysis unit is a three-chamber structure composed of alternating bipolar membranes, cation exchange membranes, and anion exchange membranes, with an acid chamber, a salt chamber, and an alkali chamber arranged sequentially inside. The lithium-rich permeate is continuously passed into the salt chamber, and under a constant operating current density of 400 A / m² to 600 A / m², monovalent lithium ions pass through the cation exchange membrane into the alkali chamber and react with hydroxide ions generated by hydrolysis at the bipolar membrane interface, thus enriching and generating a lithium hydroxide solution in the alkali chamber.

[0036] Preferably, step S4 consists of the following sub-steps with a causal feedback relationship: Step S41, using an online conductivity sensor to detect the fluid conductivity of the brine drain in the bipolar membrane electrodialysis unit; Step S42, when the fluid conductivity decreases to 10 mS / cm to 15 mS / cm, the brine drain is discharged as residual liquid, and lithium-rich permeate is introduced into the brine; Step S43, the discharged residual liquid is introduced into a mixing vessel to adjust the initial conductivity of the mixed solution in step S2.

[0037] Preferably, step S3 consists of the following sub-steps with a causal feedback relationship: Step S31, using a pressure sensor to detect the transmembrane pressure difference between the inlet and outlet of the membrane module in the nanofiltration membrane separation unit, and calculating the rate of increase of the transmembrane pressure difference; Step S32, when the rate of increase of the transmembrane pressure difference exceeds 1.5 kPa / h for 20 minutes, the regenerated sulfuric acid solution generated in the bipolar membrane electrodialysis unit is introduced into the permeate side of the nanofiltration membrane separation unit to perform reverse acid washing on the monovalent and multivalent separation nanofiltration membranes.

[0038] Preferably, after enriching and generating a lithium hydroxide solution in the alkali chamber, the following product purification steps are further included: feeding the lithium hydroxide solution into an evaporator crystallizer, performing vacuum evaporation crystallization at a temperature of 60°C to 80°C, cooling to 20°C to 25°C and then cooling to crystallize, obtaining the precipitated crystals by centrifugation, washing the crystals 2 to 3 times with anhydrous ethanol, and drying them in a vacuum environment of 40°C to 50°C for 10 to 12 hours to obtain lithium hydroxide monohydrate solid; wherein, the purity of the lithium hydroxide monohydrate solid is greater than or equal to 99.9%.

[0039] Example 1: In the recycling and reuse of spent lithium iron phosphate battery cathode materials after long-term charge-discharge cycle fatigue, the inactive dead lithium phase is deeply pinned inside the gaps of the iron-phosphorus polyhedral framework due to internal lattice collapse and distortion. The solid phase dissociation resistance is high. The traditional wet process uses strong acid to dissolve the entire volume indiscriminately, causing disordered large-area collapse of the iron-phosphorus framework, thereby releasing polynuclear iron complex impurities into the liquid phase. When these hidden impurities carrying electrochemical fatigue characteristics enter the downstream material screening and electrochemical conversion process with the material flow, they will cause concentration polarization and irreversible pore blockage at the interface between the separation membrane channel and the bipolar membrane water dissociation catalyst, thereby reducing the service life of the core separation components and causing the purity of the final precursor product to deteriorate.

[0040] The method claimed in this invention, in a specific battery resource recycling process, involves feeding retired waste lithium iron phosphate cathode material into a shearing and crushing machine for mechanical shearing, controlling the product particle size to be 0.5 mm to 2.0 mm. The material is then physically classified using a 100-200 mesh standard vibrating screen, and the undersize material is collected. This undersize material is then fed into a rotary kiln and purged with air, with the calcination temperature controlled at 500°C to 600°C and the calcination time controlled at 2 hours to 4 hours. This process removes polyvinylidene fluoride binder and conductive carbon black, thereby obtaining a product containing a high-valence iron phase. The pretreated cathode material, containing lithium-rich phase, was then introduced into a heated and stirred reactor lined with polytetrafluoroethylene (PTFE). It was mixed with a mixed solution containing sulfuric acid with an initial concentration of 1.5 mol / L to 2.0 mol / L and hydrogen peroxide with an initial concentration of 0.3 mol / L to 0.5 mol / L. The stirring speed was controlled at 200 rpm to 400 rpm, the reaction temperature at 40°C to 60°C, and the overall mixing reaction time at 1.5 h to 3.0 h. The reaction was carried out within the first 30 m... 60% of the total mass of hydrogen peroxide is added to the system at a time, and the remaining mass is added dropwise over the remaining time to maintain a local concentration of hydrogen peroxide below 0.1 mol / L. This stepwise addition and continuous slow dropwise flow rate control is based on matching the consumption kinetics of hydrogen peroxide in an acidic reaction system at 40°C to 60°C as measured in previous experiments. This is achieved by consuming most of the easily releasing lithium phase in a single reaction within the first 30 minutes, and then increasing the flow rate from 0.5 L / min to 0.8 L / min over the subsequent 120 minutes. The remaining 40% by mass of hydrogen peroxide is added dropwise at a constant low rate, allowing the hydrogen peroxide to react immediately upon addition. This spontaneously ensures that the instantaneous local concentration of hydrogen peroxide in any micro-region within the reactor remains below the failure threshold of 0.1 mol / L without the need for complex online monitoring instruments. This drives the selective oxidation of hydrogen peroxide to preferentially act on weak sites in the lattice fatigue distortion framework, causing the deeply pinned free lithium ions to dissolve and release in a directional manner, while simultaneously locking iron elements in the high-valence solid residue.

[0041] The reaction products were subjected to solid-liquid separation to obtain a lithium-containing leachate containing lithium ions and polynuclear iron complexes. This lithium-containing leachate was continuously fed into a nanofiltration membrane separation unit using a material transfer pump. The cross-flow velocity at the membrane surface within the membrane module was controlled at 1.5 m / s to 2.5 m / s, the operating pressure at 0.4 MPa to 0.6 MPa, the pH of the feed solution (hydrogen ion concentration index) at 4 to 6, and the temperature at 40°C to 50°C. The system utilized a surface rejection rate of ≥98.2% for magnesium sulfate and ≤12% for lithium chloride. A 0.5% monovalent and multivalent separation nanofiltration membrane uses the Donnan effect and steric hindrance at the flow channel interface to trap multivalent iron impurities, obtaining a lithium-rich permeate through which monovalent lithium ions pass. This ensures that the lithium ion permeability is greater than or equal to 85% and the separation factor between lithium ions and trivalent iron ions is greater than or equal to 600. At the same time, an online conductivity sensor installed on the outlet pipeline of the nanofiltration membrane separation unit is used to collect the outlet characteristic conductivity of the lithium-rich permeate in real time. This parameter characterizes the transient leakage state of trace characteristic impurity ions breaking through the nanofiltration interception interface.

[0042] Lithium-rich permeate is continuously introduced as a monovalent liquid phase material flow into a bipolar membrane electrodialysis unit with a three-chamber structure consisting of alternating bipolar membranes, cation exchange membranes, and anion exchange membranes. It is then introduced into the internal salt chamber. A constant operating current density of 400 A / m² to 600 A / m² is maintained, along with a circulation flow rate of 60 L / h to 100 L / h. The sodium sulfate concentration in the electrode chambers is controlled at 0.4 mol / L to 0.6 mol / L. Meanwhile, the central control system adjusts the DC operating current applied to the bipolar membrane electrodialysis unit based on the characteristic conductivity of the preceding outlet. The pressure is controlled by gradient limiting, using the baseline conductivity established under standard solution as a reference. In the specific calculation, the central control system continuously collects characteristic conductivity data from the outlet pipeline of the nanofiltration membrane separation unit with a 5-second sliding time window. The current collected value is subtracted from the historical value at the beginning of the window, and then divided by the 5-second time interval to calculate the real-time rate of change of characteristic conductivity, which is the outlet characteristic conductivity gradient. This gradient data serves as the input to the closed-loop controller, used to monitor the rate fluctuation of trace impurity ions penetrating the membrane pores in real time. When the outlet characteristic conductivity... When the voltage continuously exceeds the first preset deviation threshold of the reference standard, the first-stage voltage regulation command is automatically triggered, and the DC operating voltage is reduced from the initially set 18V to 15V. The standard solution refers to a clear solution with a concentration of 0.15 mol / L, prepared using high-purity lithium chloride solute and deionized water during system startup, deployment, or calibration. This pure solution is circulated through a pipeline equipped with the same online conductivity sensor at a standard material temperature of 25 degrees Celsius to obtain a constant conductivity baseline value free from impurities. This value is... As the basic conductivity, when the outlet characteristic conductivity reaches a higher second preset deviation threshold, a second-stage voltage regulation command is triggered and the DC operating voltage is stepped down to 12V. In response to the discrete voltage reduction action in the front-end interception state, the transmembrane electric field traction force on the surface of the bipolar membrane is weakened, the polarized ion flux density at the membrane interface is reduced, and the trace impurities mixed with the permeate are prevented from polarized adsorption and scaling deposition at the bipolar membrane catalytic desorption layer. This allows monovalent lithium ions to pass through the cation exchange membrane into the alkali chamber and react with hydroxide ions generated by hydrolysis at the bipolar membrane interface to enrich and generate lithium hydroxide solution.

[0043] The alkaline chamber maintains a high flow rate of 1.5 to 2.5 meters per second for circulation. This utilizes a strong shear flow field to suppress local supersaturation in the micro-regions of the membrane surface, preventing solid-phase nucleation and deposition of lithium ions and hydroxide ions on the bipolar membrane surface or in the flow channel mesh. This high-flow-rate circulation rapidly removes the enriched liquid reactants from the membrane stack flow channel and introduces them into an external circulation tank, resulting in a stable enrichment of a high-purity solution on a macroscopic scale. This ensures that no solid particles precipitate and clog the flow channel. The lithium hydroxide solution is then fed into an evaporator crystallizer for vacuum evaporation and crystallization at 60°C to 80°C, followed by cooling and crystallization at 20°C to 25°C. After centrifugation and washing with anhydrous ethanol two to three times, the solution is dried in a vacuum environment at 40°C to 50°C for 10 to 12 hours, ultimately obtaining a battery-grade lithium hydroxide monohydrate solid product with a purity greater than or equal to 99.9%. The protons generated by the bipolar membrane dissociation react in acid... The regenerated sulfuric acid solution generated by combining with anions in the chamber is returned to the upstream acid leaching process via a closed-loop pipeline to serve as a circulating leaching reagent. Under the premise of completely avoiding the mixing of exogenous impurity sodium ions and the discharge of high-salt wastewater, the regeneration closed-loop material flow is completed. During continuous operation, when the rate of increase of the transmembrane pressure difference between the inlet and outlet of the nanofiltration membrane module exceeds 1.5 kPa per hour for 20 minutes, the central control system switches the online automatic three-way valve to temporarily switch the system to backwash mode. At this time, a portion of the regenerated sulfuric acid solution is introduced into the permeate side of the nanofiltration membrane separation unit at a backwash pressure of 0.15 MPa to 0.25 MPa and a membrane surface flow rate of 1.0 m / s to 1.5 m / s to perform online reverse acid washing for 15 minutes to dissolve and strip the trace amounts of polynuclear iron complexes trapped on the membrane surface in situ. After the backwash is completed, the valve is reset and the system resumes normal filtration.

[0044] Example 2: When the system faces the high-value reuse of retired lithium iron phosphate battery cathode materials with high ion background complexity, this experiment is conducted on an integrated power battery resource recycling test platform equipped with a shear crusher, a standard vibrating screen, a rotary kiln, a polytetrafluoroethylene-lined heated and stirred reactor, a nanofiltration membrane separation unit, and a three-chamber bipolar membrane electrodialysis unit. The temperature control accuracy of the test platform is ±0.5℃, the pressure transmitter resolution is 0.01MPa, and the sampling frequency of the online conductivity sensor is 10Hz. Among them, the setting of the leaching temperature, as the core control variable, balances the lithium-ion dissolution of the degraded solid phase structure. The release rate and the inhibition rate of disordered large-area collapse of the iron-phosphorus framework are mutually restrictive causal relationships. When the dead lithium pinning rate inside the decommissioned lithium iron phosphate cathode material increases, in order to drive free lithium ions to escape the lattice steric hindrance, the parameter setting tends to the upper limit of the preset leaching temperature range to enhance the solid-liquid mass transfer driving force. Conversely, it tends to the lower limit of the leaching temperature range to reduce the dissipation and release rate of polynuclear iron complex impurities into the liquid phase. In response to this control relationship, the process parameters of the experimental group and the control group using the method claimed in this invention are set at preset numerical boundary endpoints and their medians, respectively, to construct a quantitative verification basis for parameter coverage.

[0045] Multiple reaction groups were configured on the experimental platform. The process parameters for Experimental Group 1 were set as follows: calcination temperature 500℃, leaching temperature 40℃, initial sulfuric acid concentration 1.5 mol / L, initial hydrogen peroxide concentration 0.3 mol / L, nanofiltration operating pressure 0.4 MPa, and DC current density 400 A / m². The process parameters for Experimental Group 2 were set as follows: calcination temperature 550℃, leaching temperature 50℃, initial sulfuric acid concentration 1.75 mol / L, initial hydrogen peroxide concentration 0.4 mol / L, nanofiltration operating pressure 0.5 MPa, and DC current density 500 A / m². The process parameters for Experimental Group 3 were set as follows: calcination temperature 600℃, leaching temperature 60℃, initial sulfuric acid concentration 2.0 mol / L, initial hydrogen peroxide concentration 0.5 mol / L, nanofiltration operating pressure 0.6 MPa, and DC current density 400 A / m². The flow density was 600 A / m². To verify the synergistic effect of the components and the exclusivity of the control logic, five control groups were set up: control group 1 (selective removal of hydrogen peroxide), control group 2 (disabling the working voltage gradient limiting control), control group 3 (lowering the leaching temperature to 35°C), control group 4 (raising the leaching temperature to 65°C), and control group 5 (raising the initial hydrogen peroxide concentration to 0.6 mol / L). High-frequency and low-frequency composite conductivity background noise with a signal-to-noise ratio of 22 dB, generated by bubble pulsation and transient flow rate shear, was actively introduced into the material circulation channel of the bipolar membrane electrodialysis unit. Simultaneously, three types of cyclically decaying and decommissioned lithium iron phosphate materials with initial dead lithium pinning rates of 15.4%, 28.2%, and 41.7% were selected to construct different initial dead lithium pinning rate operating systems, and the system's operating status under non-ideal conditions was tested.

[0046] After the material transfer and solid-liquid separation steps were completed, the continuous operation characteristics of each sample group were measured. When the initial dead lithium pinning rate was 28.2%, sample group two showed a total lithium recovery rate of 95.42%, and the transmembrane pressure difference after 100 hours of continuous nanofiltration membrane operation increased by only 0.01 MPa. Furthermore, the lithium hydroxide solution enriched in the alkali chamber, after vacuum evaporation crystallization and vacuum drying for 12 hours, yielded a lithium hydroxide monohydrate solid product with a purity of 99.95%. Meanwhile, sample groups one and three, used as parameter boundary endpoints, also showed total lithium recovery rates of 93.14% and 96.18%, respectively, with prepared lithium hydroxide monohydrate solid products achieving purities of 99.91% and 99.92%, respectively. The transmembrane pressure difference increases were 0.02 MPa and 0.03 MPa, respectively. In contrast, control group one, lacking hydrogen peroxide, experienced a total lithium recovery rate of only 41.32% due to the lack of directional oxidation at weak solid phase sites. In contrast, control group two, which did not employ gradient voltage regulation, experienced localized polarization and scaling of the bipolar membrane catalyst layer after 34.5 hours of operation due to background noise interference and trace impurity ion leakage, caused by the continuous operating electric field. The membrane module tank voltage increased by 3.42V after 100 hours of operation, and the purity of the alkaline chamber product was 98.42%. Furthermore, control group three, operating at 35°C, suffered from insufficient solid-phase reaction kinetics, resulting in a total lithium recovery rate of only 62.81%. The control group operating at 65°C... In Group 4, the concentration of iron impurity ions in the lithium-containing leachate increased from less than 0.05 g / L in Group 2 to 2.45 g / L due to the disordered depolymerization of the iron-phosphorus skeleton. This caused the adsorption and blockage of polynuclear complexes on the nanofiltration membrane surface. After 100 hours, the transmembrane pressure difference was 0.28 MPa and the product purity was 99.14%. In Control Group 5, which operated under an initial concentration of 0.6 mol / L hydrogen peroxide, the total lithium recovery rate was 95.45% due to the self-decomposition side reaction and the gas expansion of the reactor.

[0047] X-ray diffraction spectroscopy was used to confirm the phase structure of the final products obtained from each sample group. The data showed that the products of experimental group 1, experimental group 2 and experimental group 3 all corresponded to the standard diffraction peak positions of monoclinic lithium hydroxide monohydrate. The particle surface morphology showed a regular geometric crystal state and no lattice inclusions of exogenous sodium phase and impurity iron phase. There is an electrochemical correlation between process parameters and separation efficiency. When the initial concentration of hydrogen peroxide or the leaching temperature exceeds the preset working range, the chemical stability and reaction selectivity of the framework structure change. By controlling the stepwise release of hydrogen peroxide to maintain its local concentration below 0.1 mol / L, and by implementing gradient amplitude control of DC operating voltage based on the outlet characteristic conductivity, a mechanism for eliminating concentration polarization and secondary deposition resistance was established at the level of physical parameter flow. The directional dissolution and dissociation of pinned dead lithium in decommissioned lithium iron phosphate materials and the low polarization and high charge sieving are causally related. The entire integrated process maintains the same closed-loop technical indicators under different decommissioned material conditions.

[0048] Example 3: This example combines Figures 1 to 2 The method for recovering lithium from waste lithium iron phosphate cathode materials based on integrated processes is described, such as... Figure 1 As shown, step S1 involves crushing and sieving the waste lithium iron phosphate cathode material and calcining it in air to remove organic binders and conductive agents, obtaining pretreated cathode material. Step S2 involves mixing the pretreated cathode material with a mixed solution containing sulfuric acid at a concentration of 1.5 mol / L to 2.0 mol / L and hydrogen peroxide at a concentration of 0.3 mol / L to 0.5 mol / L in a reactor, dissolving lithium ions at 40°C to 60°C, and separating to obtain a lithium-containing leachate containing lithium ions and iron complexes. Step S3 involves introducing the lithium-containing leachate into a nanofiltration membrane separation unit for the separation of monovalent and polyvalent ions. The lithium-rich permeate is separated by pressurization at 0.4 MPa to 0.6 MPa and 40°C to 50°C on a nanofiltration membrane, retaining iron complexes to obtain lithium-rich permeate. The outlet characteristic conductivity of the lithium-rich permeate is collected. In step S4, the lithium-rich permeate is introduced into a bipolar membrane electrodialysis unit. Under a DC electric field, the hydroxide ions generated by the hydrolysis of the bipolar membrane react with lithium ions to generate lithium hydroxide monohydrate solid. The generated protons react with anions to generate a regenerated sulfuric acid solution. The regenerated sulfuric acid solution is returned to step S2, and the DC operating voltage is reduced from the initial value of 18V to 15V or 12V according to the outlet characteristic conductivity to suppress impurity precipitation.

[0049] like Figure 2 As shown, waste lithium iron phosphate cathode material is input into the pretreatment unit, the pretreatment unit outputs pretreated cathode material, the pretreated cathode material is input into the leaching reaction unit, the leaching reaction unit outputs lithium-containing leachate, the lithium-containing leachate is input into the nanofiltration separation unit, the nanofiltration separation unit outputs lithium-rich permeate and outlet characteristic conductivity, the lithium-rich permeate is input into the bipolar membrane electrodialysis unit, the outlet characteristic conductivity is input into the central control system, the central control system outputs DC operating voltage, the DC operating voltage is input into the bipolar membrane electrodialysis unit, the bipolar membrane electrodialysis unit outputs lithium hydroxide monohydrate solid and regenerated sulfuric acid solution, and the regenerated sulfuric acid solution is input into the leaching reaction unit.

[0050] Example 4: When multiple bipolar membrane electrodialysis units are deployed in a cluster in parallel to cope with the recycling of high-throughput industrial-grade retired battery materials, the system faces local dynamic impedance fluctuations caused by multi-electrode stacking and polarization interference caused by trace iron leakage during long-term operation. This deviation from the operating conditions leads to the failure of fixed empirical thresholds and the risk of rapid fouling of membrane chromatography. To address this, the control system runs a set of parameter calibration procedures when the system starts up or detects baseline drift to determine the first preset deviation threshold and the second preset deviation threshold for adaptive adjustment of DC operating voltage.

[0051] The parameter calibration procedure uses the initial conductivity data of the pure lithium-rich permeate collected by an online conductivity sensor as the technical characteristic of the input object. It controls the material delivery pump to gradually reduce the circulation flux of the nanofiltration membrane separation unit in step increments of 10 L / h under a constant current density, thereby artificially inducing the graded leakage of trace polynuclear iron complex impurity ions. Simultaneously, the change in the characteristic conductivity of the lithium-rich permeate outlet is continuously collected by the online conductivity sensor at a sampling frequency of 10 Hz. To quantify the local polarization evolution state on the bipolar membrane surface caused by the leakage of characteristic impurity ions, the method divides the difference between the real-time collected outlet characteristic conductivity and the baseline conductivity established under the standard solution by the baseline conductivity to achieve dimensionless dimensionality reduction, thereby determining a conductivity relative deviation index as a voltage adjustment trigger criterion. Simultaneously, the cell voltage response of the bipolar membrane electrodialysis unit is recorded by a voltage acquisition sensor on the experimental platform. When the cell voltage is detected to change with conductivity... When the relative deviation increases and shows a monotonically increasing trend proportionally, and its slope reaches the first critical inflection point, the relative deviation of conductivity measured at this time is locked as the first preset deviation threshold. Specifically, the monotonically increasing trend proportionally refers to the overall evolution trend of the tank voltage in the macroscopic unpolarized range. However, in the microscopic response, the leakage of impurities caused by the step reduction of the flow rate presents a discrete pulse-like accumulation, resulting in a local step perturbation superimposed on the tank voltage curve. This perturbation causes the rate of change of local voltage with respect to the relative deviation to change abruptly when it reaches a specific leakage amount, thus forming the first critical inflection point. This point marks the transition of the system from the completely stable ohmic region to local polarization. The circulation flux continues to decrease until the tank voltage shows an exponentially sharp increase in polarization inflection point. The relative deviation of conductivity corresponding to this polarization inflection point is locked as the second preset deviation threshold. Through this purely textual quantitative deduction logic, the dynamic evolution resistance is transformed into a definite operating boundary.

[0052] Under the adaptive boundary constraints established by the calibration procedure, the central control system, in subsequent continuous operation, can trigger the gradient limiting regulation response of the DC operating voltage in situ without delay by accurately comparing the relative deviation of conductivity obtained by real-time acquisition and calculation with the first and second preset deviation thresholds. Under the condition of high external background interference noise and an initial dead lithium pinning rate of 41.7%, the first-stage voltage regulation command lowers the DC operating voltage from the initially set 18V stepwise to 15V, and the second-stage voltage regulation command further lowers it stepwise to 12V. The polarization electric field intensity at the bipolar film interface is discretely convergently controlled, thereby avoiding the influence of material flow. The polarization adsorption of the mixed characteristic impurity ions simultaneously improves the flux selectivity of monovalent lithium ion cross-membrane separation and the separation stability of bipolar membrane catalysis. The lithium hydroxide solution enriched in the alkali chamber, after subsequent vacuum evaporation crystallization and vacuum drying, stably produces battery-grade lithium hydroxide monohydrate solid product with a purity of ≥99.9%. The sulfuric acid solution regenerated in the acid chamber is flowed in a closed loop through pipeline to the front end as a co-leaching reagent. This eliminates the technical problems of rapid membrane fouling and system shutdown caused by the rigid range of fixed empirical threshold settings in traditional wet recovery systems. The entire integrated process has complete industrial applicability and legal robustness for long-term reproducible operation in multi-level complex environments.

[0053] Example 5: When the system faces the condition of continuous cyclic processing of multiple batches of lithium-containing leachate for a long time, the bipolar membrane electrodialysis unit experiences local dynamic impedance migration deviation due to trace adsorption of impurities on the membrane surface and electrode degradation. To offset the reduction in mass transfer efficiency caused by the deviation, the central control system retrieves the feedback adjustment loop before outputting the pulse excitation, controls the material delivery pump to fill the chamber with deionized water, and uses a potential sensor to collect the initial residual voltage under zero current. The difference between this voltage and the preset standard zero potential voltage is calculated to determine the hardware residual potential deviation. The preset standard zero potential voltage refers to the voltage at which the bipolar membrane electrodialysis unit... In the initial state of a newly assembled membrane electrodialysis unit without any material contamination, the acid, salt, and alkali chambers are completely filled with deionized water with a conductivity of less than 2 microsiemens per centimeter. Under static conditions with an externally applied DC operating current of 0 amperes, the intrinsic static contact potential difference is measured across the positive and negative main electrodes using a high-precision potential sensor. This reference voltage value is calibrated to 0.01 volts. When the DC operating voltage is input, it is automatically superimposed onto the initial operating voltage to correct the electric field loss caused by increased impedance and maintain the normal operation of the proton dissociation rate at the bipolar membrane interface.

[0054] During the 150th hour of actual operation, due to electrode interface wear, the initial residual voltage inside the chamber was detected to have increased from 0.02V under normal conditions to 0.38V. The aforementioned feedback adjustment loop calculated the difference between the residual voltage and the standard zero potential voltage of 0.01V, determining that the hardware residual potential deviation was 0.37V. At this point, the central control system adjusted the initially set DC operating voltage of 15.0V upward by 0.37V, driving the electrodialysis unit to operate stably at 15.37V. This action prevented the fluctuation of hydroxide yield caused by the weakening of the actual transmembrane voltage drop. The lithium hydroxide solution generated in the alkali chamber was subjected to vacuum evaporation crystallization and drying treatment to produce a battery-grade lithium hydroxide monohydrate solid product with a purity of 99.93%. Meanwhile, the sulfuric acid solution regenerated in situ in the acid chamber maintained a concentration of 1.8mol / L and was returned to the upstream process in a closed loop, ensuring that the entire integrated process maintained constant product indicators under hardware wear conditions.

[0055] The embodiments of this application have been described above with reference to the accompanying drawings. Unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other. This application is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of this application without departing from the spirit of this application and the scope of protection of this invention, and all of these forms are within the protection scope of this application.

Claims

1. A method for recovering lithium from waste lithium iron phosphate cathode materials based on integrated processes, characterized in that, Includes the following steps: Step S1: The waste lithium iron phosphate cathode material is crushed and sieved and calcined in air atmosphere to remove organic binders and conductive agents, and pretreated cathode material is obtained. Step S2: The pretreated cathode material is mixed and reacted in a reactor with a mixed solution containing sulfuric acid with a concentration of 1.5 mol / L to 2.0 mol / L and hydrogen peroxide with a concentration of 0.3 mol / L to 0.5 mol / L. Lithium ions are dissolved at 40°C to 60°C, and a lithium-containing leachate containing lithium ions and iron complexes is obtained by separation. Step S3: The lithium-containing leachate is introduced into the nanofiltration membrane separation unit and separated under pressure at 0.4 MPa to 0.6 MPa and 40°C to 50°C on the monovalent and multivalent separation nanofiltration membrane. Iron complexes are retained to obtain lithium-rich permeate, and the outlet characteristic conductivity of the lithium-rich permeate is collected. In step S4, the lithium-rich permeate is introduced into the bipolar membrane electrodialysis unit. Under a DC electric field, the hydroxide ions generated by the hydrolysis of the bipolar membrane react with lithium ions to generate lithium hydroxide monohydrate solid. The protons generated react with anions to generate a regenerated sulfuric acid solution. The regenerated sulfuric acid solution is returned to step S2, and the DC operating voltage is reduced from the initial value of 18V to 15V or 12V according to the outlet characteristic conductivity to suppress impurity precipitation.

2. The method for recovering lithium from waste lithium iron phosphate cathode materials based on integrated processes according to claim 1, characterized in that, Step S1 consists of the following sequential sub-steps: Step S11, placing the waste lithium iron phosphate cathode material in a shear crusher for mechanical shearing to a particle size of 0.5 mm to 2.0 mm; Step S12, physically classifying the crushed product through a standard vibrating screen of 100 to 200 mesh and collecting the undersize material; Step S13, feeding the undersize material into a rotary kiln, introducing air, controlling the calcination temperature at 500°C to 600°C, controlling the calcination time at 2 h to 4 h, and removing the polyvinylidene fluoride binder and conductive carbon black.

3. The method for recovering lithium from waste lithium iron phosphate cathode materials based on integrated processes according to claim 1, characterized in that, In step S2, the pretreated cathode material and the mixed solution are mixed in a heated and stirred reactor with a polytetrafluoroethylene liner, the stirring speed is controlled at 200 rpm to 400 rpm, and the mixing reaction time is controlled at 1.5 h to 3.0 h.

4. The method for recovering lithium from waste lithium iron phosphate cathode materials based on integrated processes according to claim 1, characterized in that, In step S2, the addition of hydrogen peroxide to the reaction vessel consists of the following sub-steps: Step S21, 60% of the total mass of hydrogen peroxide is added within the first 30 minutes of the mixing reaction; Step S22, the remaining mass of hydrogen peroxide is continuously added dropwise during the remaining time of the mixing reaction to maintain the local concentration of hydrogen peroxide in the reaction system below 0.1 mol / L.

5. The method for recovering lithium from waste lithium iron phosphate cathode materials based on an integrated process according to claim 1, characterized in that, In step S3, the monovalent and multivalent separation nanofiltration membrane has a magnesium sulfate rejection rate of greater than or equal to 98.2% and a lithium chloride rejection rate of less than or equal to 12.5%; when the lithium-containing leachate is separated under pressure, the cross-flow velocity on the membrane surface within the membrane module is controlled to be between 1.5 m / s and 2.5 m / s.

6. The method for recovering lithium from waste lithium iron phosphate cathode materials based on an integrated process according to claim 1, characterized in that, In step S4, the bipolar membrane electrodialysis unit is a three-chamber structure composed of alternating bipolar membranes, cation exchange membranes, and anion exchange membranes, with an acid chamber, a salt chamber, and an alkali chamber arranged sequentially inside. The lithium-rich permeate is continuously passed into the salt chamber, and under a constant operating current density of 400 A / m² to 600 A / m², monovalent lithium ions pass through the cation exchange membrane into the alkali chamber and react with hydroxide ions generated by water dissociation at the bipolar membrane interface, thus enriching and generating a lithium hydroxide solution in the alkali chamber.

7. The method for recovering lithium from waste lithium iron phosphate cathode materials based on an integrated process according to claim 1, characterized in that, Step S4 consists of the following sub-steps with a causal feedback relationship: Step S41, using an online conductivity sensor to detect the fluid conductivity of the brine drain in the bipolar membrane electrodialysis unit; Step S42, when the fluid conductivity decreases to 10 mS / cm to 15 mS / cm, the brine drain is discharged as residual liquid, and lithium-rich permeate is introduced into the brine; Step S43, the discharged residual liquid is introduced into the mixing vessel to adjust the initial conductivity of the mixed solution in step S2.

8. The method for recovering lithium from waste lithium iron phosphate cathode materials based on an integrated process according to claim 1, characterized in that, Step S3 consists of the following sub-steps with a causal feedback relationship: Step S31, using a pressure sensor to detect the transmembrane pressure difference between the inlet and outlet of the membrane module in the nanofiltration membrane separation unit, and calculating the rate of increase of the transmembrane pressure difference; Step S32, when the rate of increase of the transmembrane pressure difference exceeds 1.5 kPa / h for 20 minutes, the regenerated sulfuric acid solution generated in the bipolar membrane electrodialysis unit is introduced into the permeate side of the nanofiltration membrane separation unit to perform reverse acid washing on the monovalent and multivalent separation nanofiltration membranes.

9. A method for recovering lithium from waste lithium iron phosphate cathode materials based on an integrated process, as described in claim 6, characterized in that, After enriching and generating a lithium hydroxide solution in the alkali chamber, the following product purification steps are also included: the lithium hydroxide solution is fed into an evaporator crystallizer and subjected to vacuum evaporation crystallization at a temperature of 60°C to 80°C. After cooling to 20°C to 25°C, the crystals are cooled and crystallized. The precipitated crystals are obtained by centrifugation, and the crystals are washed 2 to 3 times with anhydrous ethanol. The crystals are then dried in a vacuum environment at 40°C to 50°C for 10 to 12 hours to obtain lithium hydroxide monohydrate solid. The purity of the lithium hydroxide monohydrate solid is greater than or equal to 99.9%.

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