A method for recovering phosphorus and recycling lithium iron phosphate resources based on directional synthesis of blue vitriol and high gradient magnetic separation

Abstract

The application discloses a kind of based on barite directional synthesis and high-gradient magnetic separation phosphorus recovery and lithium iron phosphate resource method, by introducing nanometer seed induction hetero-nucleation, real-time monitoring conductivity triggers nucleation and utilizes topography directing agent regulation crystal growth direction;At the same time, by constructing inert gas protection's strong reducing environment and accurate control redox potential;On this basis, using the magnetic anisotropy of directional synthesis product combines high-gradient magnetic separation and dynamic cleaning technology, the separation efficiency of magnetic component and non-magnetic impurities is significantly improved;Finally, through airflow crushing kinetics control and nanometer surface coating modification, the accurate regulation of product particle size distribution is realized, and finally the lithium iron phosphate product with excellent electrochemical performance and good cycle stability is obtained, realizing the high-value resource utilization of phosphorus-containing wastewater.

Description

Technical Field

[0001] This invention relates to the field of material recycling technology, specifically to a method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation. Background Technology

[0002] With the rapid development of the lithium battery industry, the market demand for lithium iron phosphate (LFP), as an important cathode material, continues to rise. Traditional LFP preparation methods mainly rely on solid-state reactions or co-precipitation of high-purity industrial-grade phosphates and ferrous salts. However, this traditional route is not only costly in terms of raw materials but also requires extremely high purity, making large-scale, low-cost production difficult. Meanwhile, the discharge of phosphorus-containing wastewater from industrial and domestic sewage is increasing, containing large amounts of phosphate ions. Direct discharge will lead to eutrophication of water bodies, severely damaging the ecological environment. While conventional chemical precipitation methods can remove some phosphorus, the resulting products are mostly amorphous calcium phosphate or iron hydroxide mixtures. These products have loose structures and high impurity content, making them unsuitable for direct use as battery materials. They are typically only used as low-value fertilizers or landfilled, resulting in a huge waste of phosphorus resources.

[0003] Among existing technologies for the resource recovery of phosphorus-containing wastewater, utilizing lapis lazuli (Fe3(PO4)2·8H2O) as an intermediate product is a feasible approach. However, existing technologies have significant drawbacks in practical applications. First, the crystallization process of lapis lazuli lacks effective directional control. Under natural precipitation or ordinary co-precipitation conditions, the formation of crystal nuclei is random, resulting in disordered crystal morphologies, extremely wide particle size distributions, and often accompanied by a large number of amorphous impurities. This disordered crystallization state makes subsequent high-gradient magnetic separation extremely difficult because particles of different morphologies and sizes exhibit vastly different responses in a magnetic field, making efficient separation difficult and ultimately resulting in insufficient purity of the precursor. Second, controlling the valence state of iron is another major challenge. The synthesis of lapis lazuli requires a strictly reducing environment to prevent the oxidation of ferrous iron to ferric iron. Existing technologies often struggle to maintain a stable low-potential environment, leading to the formation of large quantities of byproducts such as ferric hydroxide or ferric phosphate. These impurities not only reduce phosphorus recovery rates but also severely affect the electrochemical performance of the final lithium iron phosphate product. Furthermore, existing methods lack precise control mechanisms for crystal morphology and particle size, making it impossible to obtain ideal crystal structures with specific aspect ratios and nanoscale particle sizes. This directly limits the tap density and lithium-ion diffusion rate of the material, resulting in poor rate performance and cycle life of the prepared battery materials. Summary of the Invention

[0004] This invention aims to provide a method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation, which can directionally synthesize vivianite, precisely control the iron valence state, efficiently separate and purify it, and convert it into high-performance lithium iron phosphate.

[0005] To achieve the above objectives, the technical solution adopted in this invention is: a method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation, comprising: Step 1: Physically filter the phosphorus-containing waste liquid and adjust the pH value to the weakly acidic range. Then, introduce soluble ferrous salts to construct a mixed system, monitor the redox potential, and introduce inert gas to create a reducing environment. Step 2: After heating the mixed system, introduce nanoscale blue iron ore seeds for heterogeneous nucleation, monitor the change in solution conductivity in real time to trigger nucleation, and maintain lattice matching conditions to initiate orientation growth; Step 3: The remaining raw material liquid is continuously added to the reaction system to maintain supersaturation. A morphology guiding agent is added to adjust the crystal aspect ratio. After the turbidity stabilizes, a ripening treatment is carried out. Finally, the blue iron ore precursor is obtained by centrifugation. Step 4: The blue iron ore precursor is calcined at high temperature in a reducing atmosphere, and lithium carbonate is added in stoichiometric ratio to carry out a solid-phase reaction. The temperature rise program is controlled to reconstruct the crystal lattice and transform it into lithium iron phosphate precursor, and then it is slowly cooled in a protective atmosphere. Step 5: The lithium iron phosphate precursor is prepared into a slurry and ultrasonically dispersed. Magnetic separation is carried out in a high gradient magnetic field to capture magnetic components and dynamically wash them. After the magnetic field is cut off, the concentrate is desorbed. Step six: The concentrate is subjected to air jet milling to control particle size distribution, surface coating modification is implemented, and after passing multi-scale performance characterization verification, it is packaged to obtain the finished lithium iron phosphate product.

[0006] Preferably, in step one, physical filtration uses a multi-stage screen and a microporous membrane assembly connected in series. The pore size of the first-stage screen is set to 200 micrometers, and the pore size of the second-stage microporous membrane filter is set to 5 micrometers. The filtered clear liquid is homogenized by a stirring paddle with a rotation speed controlled at 80 revolutions per minute.

[0007] Preferably, the specific operation of adjusting the pH value in step one is to control the acidity or alkalinity of the solution at 4.6-5.2. At this time, the mole fraction of dihydrogen phosphate in the total phosphate is determined by the hydrogen ion concentration and the primary and secondary dissociation constants of phosphate, ensuring that the phosphorus form is in the optimal reaction window.

[0008] Preferably, when introducing nanoscale blue iron ore seeds in step two, the seed particle size is controlled between 50 and 100 nanometers, and a monodisperse suspension is formed by processing with an ultrasonic disperser with a frequency set to 20 kHz, thereby reducing the nucleation barrier by utilizing the heterogeneous nucleation mechanism.

[0009] Preferably, in step three, the flow rate of the continuously added residual raw material liquid follows an exponential decay model, and the feeding rate gradually decreases over time to match the natural slowdown trend of crystal growth rate and maintain supersaturation between 1.05 and 1.15.

[0010] Preferably, the morphology guiding agent added in step three is sodium citrate, which is adsorbed on the (100) crystal surface of the blue iron crystal to inhibit growth in that direction and force the crystal to preferentially grow along the (001) direction to obtain a long columnar structure with an aspect ratio greater than 5:1.

[0011] Preferably, the high-temperature calcination process in step four is divided into a dehydration decomposition stage and a solid-phase reaction stage. The dehydration stage is held at 150°C for 30 minutes to remove the water of crystallization, and the solid-phase reaction stage is held at 750°C for 4 hours to complete the lattice reconstruction.

[0012] Preferably, in step five, the high gradient magnetic field is generated by a magnetic separation column filled with stainless steel velvet. The magnitude of the magnetic force depends on the product of particle volume, magnetic susceptibility, and magnetic field gradient. Non-magnetic impurities are removed as the fluid flows out.

[0013] Preferably, in step six, the airflow pulverization process uses a Laval nozzle to accelerate compressed air, and the relationship between pulverization time and average particle size follows a power law equation. The final product D50 value is controlled within the range of 2±0.5 micrometers by a classifying wheel.

[0014] Preferably, in step six, the surface coating modification uses a sol-gel method to spray the precursor solution of conductive agent and binder onto the particle surface, with the coating layer thickness controlled at 3 to 5 nanometers. After curing in an oven at a temperature set to 80°C, the electronic conductivity is improved.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention effectively solves the problems of high randomness, disordered crystal form, and wide particle size distribution in the crystallization process of vivianite in existing technologies by introducing nanoscale seed crystals to induce heterogeneous nucleation, real-time monitoring of electrical conductivity to trigger nucleation, and using morphology-directing agents to regulate crystal growth direction. It successfully obtains oriented long columnar crystals with suitable aspect ratios. Simultaneously, by constructing a strong reducing environment protected by inert gas and precisely controlling the redox potential, it overcomes the technical challenge of easily oxidizing ferrous ions to ferric ions, eliminating the generation of impurities such as ferric hydroxide at the source and ensuring efficient phosphorus recovery. Based on this... By utilizing the magnetic anisotropy of the directionally synthesized product combined with high-gradient magnetic separation and dynamic cleaning technology, the separation efficiency of magnetic components and non-magnetic impurities was significantly improved, solving the problem of insufficient purity in conventional separation methods. Finally, through airflow pulverization kinetic control and nanoscale surface coating modification, the particle size distribution of the product was precisely controlled, eliminating the defects of low material tap density and slow lithium-ion diffusion caused by lack of morphology and particle size control. Ultimately, a lithium iron phosphate product with excellent electrochemical performance and good cycle stability was obtained, realizing the high-value resource utilization of phosphorus-containing wastewater. Attached Figure Description

[0016] Figure 1 This is a flowchart of the method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation according to the present invention. Detailed Implementation

[0017] The following description is intended to disclose the invention and enable those skilled in the art to implement it. The preferred embodiments described below are merely examples, and other obvious variations will occur to those skilled in the art.

[0018] like Figure 1 As shown, this invention proposes a method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation, specifically including the following steps: Phosphorus-containing wastewater was physically filtered and the pH value was adjusted to a weakly acidic range. Then, soluble ferrous salts were introduced to construct a mixed system. The redox potential was monitored and an inert gas was introduced to create a reducing environment. After heating the mixed system, nanoscale blue iron ore seeds were introduced to perform heterogeneous nucleation. The change in solution conductivity was monitored in real time to trigger nucleation, and lattice matching conditions were maintained to initiate orientation growth. The remaining raw material liquid is continuously added to the reaction system to maintain supersaturation. A morphology guiding agent is added to adjust the crystal aspect ratio. After the turbidity stabilizes, a aging treatment is performed. Finally, the blue iron ore precursor is obtained by centrifugation. The blue iron ore precursor is calcined at high temperature in a reducing atmosphere. Lithium carbonate is added in stoichiometric ratio to carry out a solid-phase reaction. The temperature rise program is controlled to reconstruct the crystal lattice and transform it into a lithium iron phosphate precursor. The precursor is then slowly cooled in a protective atmosphere. The lithium iron phosphate precursor was formulated into a slurry and ultrasonically dispersed. Magnetic separation was performed in a high gradient magnetic field to capture the magnetic components and dynamically clean them. After the magnetic field was cut off, desorption was performed to obtain the concentrate. The concentrate was then subjected to air jet milling to control the particle size distribution, and surface coating modification was implemented. After passing multi-scale performance characterization verification, the finished lithium iron phosphate product was packaged.

[0019] The implementation process of the above steps will be described in detail below with reference to specific embodiments, including: Step 1: Pretreatment of phosphorus-containing wastewater and directional introduction of iron source; This embodiment begins with the preliminary treatment of phosphate-rich wastewater from industrial or domestic sewage, aiming to create an ideal chemical environment for the subsequent directional growth of lapis lazuli crystals.

[0020] First, the raw waste liquid needs to be physically filtered to remove large suspended particles and colloidal substances, preventing these non-target components from interfering with the core sites of the subsequent crystallization process.

[0021] The filtered clear liquid enters the pH adjustment unit, where inorganic acid or alkali is used to adjust the pH of the solution to a weakly acidic range, specifically between 4.5 and 5.5. This range helps to inhibit the premature formation of hydroxide precipitates while maintaining the active form of phosphate ions.

[0022] While adjusting the pH, a soluble ferrous salt is introduced into the system as an iron source. The amount of iron salt added must be strictly calculated based on the total phosphorus concentration in the waste liquid to ensure that the molar ratio of iron ions to phosphate ions is slightly higher than the stoichiometric ratio, so as to reserve excess iron ions for driving the nucleation reaction. At this point, a solution containing a specific proportion of Fe is formed. 2+ and PO4 3- The mixed system laid the foundation for the next step of crystal induction, which specifically includes the following steps: 1.1: Physical retention of suspended solids and fluid homogenization; First, the phosphorus-containing waste liquid to be treated is filtered in series through a multi-stage sieve and a microporous membrane module. The first-stage sieve has a pore size of 200 micrometers to intercept macroscopically visible solid waste such as leaves and plastic fragments. The second stage uses a ceramic membrane filter with a pore size of 5 micrometers to further trap fine silt, fibers, and some colloidal aggregates. After two stages of filtration, the turbidity of the liquid is significantly reduced, and the light transmittance increases to over 90%.

[0023] Subsequently, the filtrate was gently agitated using a low-speed stirrer at 80 revolutions per minute for 15 minutes. This step not only eliminated local concentration differences caused by uneven gravity settling but also ensured the uniform distribution of components in the solution, avoiding non-selective precipitation caused by excessive local supersaturation. The homogenized liquid was then pumped into the next stage reactor; its clear and transparent state indicated that large molecular impurities had been effectively removed, providing a pure reaction medium for subsequent chemical reactions.

[0024] 1.2: Precise control of solution pH and morphological stability; The homogenized solution obtained in the previous step is then transferred to the automatic pH control system. The system's built-in pH electrode monitors the solution's acidity and alkalinity in real time and feeds the data back to the dosing pump. When the pH value deviates from the preset range, dilute sulfuric acid or sodium hydroxide solution is automatically injected for fine-tuning. The control objective is to stabilize the pH value between 4.6 and 5.2, which is within the range of dihydrogen phosphate (H₂PO₄)⁻. - ) and hydrogen phosphate (HPO4) 2- The critical region of coexistence ensures both the effective solubility of phosphorus and prevents the formation of ferrous hydroxide precipitate under alkaline conditions. In this process, a dynamic equilibrium equation is introduced to describe the relationship between hydrogen ion concentration and phosphate speciation: ; In the formula, This indicates the mole fraction of dihydrogen phosphate in the total phosphate group. This represents the molar concentration (mol / L) of hydrogen ions in the solution. and These are the first and second dissociation constants of phosphoric acid, respectively. Their values ​​fluctuate slightly with temperature, but at room temperature they take values ​​of [values ​​missing]. and By comparing the calculations using this formula with real-time monitoring data, the system can accurately determine the main phosphorus species present under the current pH environment, thereby guiding the fine-tuning of the dosage and ensuring that most phosphorus in the solution exists in the form of divalent anions, which are conducive to combining with ferrous ions. The adjusted solution exhibits a stable light green hue, indicating that ferric ions have not yet undergone hydrolysis and precipitation, and that the phosphorus form is within the optimal reaction window.

[0025] 1.3: Quantitative addition of iron source and construction of complex precursor; Based on the determined phosphorus form and concentration, the iron source dosing device was started. Ferrous sulfate heptahydrate (FeSO4·7H2O) was selected as the main iron source due to its high solubility and suitable cost. The dosage was calculated based on the following logic: assuming the total phosphorus concentration in the waste liquid is C... P(mg / L), the theoretical stoichiometric ratio is Fe:P = 1:1 (molar ratio). Considering the losses and supersaturation requirements in the actual reaction process, an excess coefficient is set. The value is 1.2. Therefore, the required molar concentration of ferrous ions, C, is... Fe It can be determined by the following formula: ; Among them, M P M is the molar mass of phosphorus (31 g / mol). Fe The molar mass of iron (56 g / mol) is divided by 1000 to convert the unit from mg to g. Calculate C. Fe Then, convert it to the dosage concentration of ferrous sulfate heptahydrate.

[0026] During the addition process, a multi-point pulse injection method is used to disperse the iron salt solution into the main reaction channel, avoiding instantaneous explosive nucleation caused by excessively high local concentrations. The injected solution mixes rapidly, and ferrous ions and phosphate ions begin to undergo preliminary electrostatic attraction and coordination, forming unstable ion-pair complexes. This pre-complexed state is a crucial precursor for subsequent crystal growth, lowering the nucleation energy barrier and allowing the crystals to arrange themselves in an orderly manner at a lower degree of supersaturation. At this point, the iron ions in the solution mainly exist as free hydrated ions, but a solvation layer composed of phosphate ions has formed around them, preparing the microstructure for crystal nucleation.

[0027] 1.4: Initial monitoring and environmental setup for redox potential; Immediately after adding the iron source, monitor the redox potential (ORP) of the solution to prevent ferrous ions from being oxidized to ferric ions by oxygen in the air. The presence of ferric ions will severely interfere with the directional synthesis of lapis lazuli (Fe3(PO4)2·8H2O), as lapis lazuli is a typical ferrous phosphate mineral. Using a portable ORP meter inserted into the reaction solution, the initial potential value should be below +100mV (relative to the standard hydrogen electrode). If the reading is high, it indicates the presence of trace amounts of dissolved oxygen or oxidizing substances in the solution, and high-purity nitrogen should be immediately introduced for bubbling to remove oxygen.

[0028] The bubbling process lasted for 10 minutes, with a flow rate controlled at 2 liters per minute. The inert covering effect of nitrogen gas displaced the air above the liquid surface and promoted the escape of dissolved oxygen. After deoxygenation, the solution potential stabilized between -50mV and -100mV, creating a strongly reducing reaction environment. This environment is crucial; it locks in the valence state of iron, ensuring that all subsequent chemical reactions occur on the basis of ferrous iron. Only in such a reducing atmosphere can phosphate ions stably combine with ferrous ions, thus transforming into lapis lazuli crystals, rather than other iron phosphates or mixtures of iron hydroxide.

[0029] Step 2: Seed induction and directional nucleation of blue iron oxide nanocrystals; This step follows the preparation of the iron-rich phosphorus-containing reducing solution in the previous step, and its core task is to induce the formation of crystal nuclei under controlled conditions. Unlike traditional random nucleation, this embodiment uses an external seed-induced method, which introduces a trace amount of pre-synthesized lapis lazuli nanocrystals to provide heterogeneous nucleation sites, thereby significantly reducing the nucleation barrier and guiding the crystal to grow along a specific crystal plane.

[0030] First, the mixed solution prepared in step one is heated to a specific temperature, typically between 30°C and 40°C, to improve the ion diffusion rate and reactivity. Then, nano-sized lapis lazuli seed crystals are added in a predetermined ratio. The seed crystals have a particle size controlled between 50 and 100 nanometers, possessing a large specific surface area. After the seed crystals are added, the system enters a static induction period, during which the supersaturation in the solution gradually dissipates, and ferrous and phosphate ions preferentially adsorb onto the seed crystal surface and stack along the crystal lattice direction. To monitor the nucleation process, the change in the solution's conductivity is monitored in real time. The decreasing conductivity reflects the decrease in ion concentration and the formation of the crystal phase, specifically including the following steps: 2.1: Thermodynamic control of reaction temperature and energy supply; The prepared reducing mixture was transferred to a crystallization reactor equipped with a jacketed temperature control system. The circulating water bath system was then activated, and the temperature of the reaction solution was slowly increased to 35°C. The choice of temperature was based on the reaction kinetics revealed by the Arrhenius equation, namely, the reaction rate constant k has an exponential relationship with temperature T. ; In the formula, k is the reaction rate constant, A refers to the pre-factor (related to the collision frequency), and E... a Let E be the activation energy of the reaction (J / mol), R be the ideal gas constant (8.314 J / (mol·K)), and T be the absolute temperature (K). In this embodiment, the activation energy E of the vivianite nucleation process was experimentally determined. a The value is approximately 45 kJ / mol. When the temperature increases from room temperature (25°C) to 35°C, the reaction rate constant k increases by about 1.5 times, which means that the ion diffusion rate and surface adsorption rate are significantly accelerated, which is beneficial for the rapid deposition of seed crystals. However, higher temperatures are not always better; excessively high temperatures can lead to an increase in lattice defects or even crystal decomposition. Therefore, the upper limit is strictly controlled to not exceed 40°C.

[0031] During the heating process, the stirring speed was kept constant to ensure uniform heat distribution and avoid localized overheating. As the temperature increased, the solution viscosity decreased slightly, and the ion migration ability increased, providing the necessary thermal energy support for subsequent directional nucleation. At this point, the thermal motion inside the solution intensified, and the collision frequency between phosphate ions and ferrous ions increased, creating favorable conditions for lattice matching on the seed crystal surface.

[0032] 2.2: Introduction of nanocrystal seeds and construction of heterogeneous nucleation sites; After the temperature reached and stabilized at 35°C, the seed crystal addition system was started. The seed crystals were derived from ultrafine lapis lazuli powder synthesized in a small-scale preliminary experiment. After ball milling and classification, the particle size distribution was concentrated in the range of D50=75nm. The seed crystal powder was weighed at 0.5% (mass-volume ratio) of the solution volume and dispersed in a small amount of deionized water to form a suspension. The seed crystal suspension was treated with an ultrasonic disperser at a frequency of 20kHz for 5 minutes to break up the seed crystal agglomerates and allow them to exist in a monodisperse state.

[0033] Subsequently, the uniformly dispersed seed suspension was added dropwise to the main reaction solution at an extremely slow rate of 1 ml per minute. The instant the seed crystals entered the solution, due to the numerous unsaturated bonds and lattice defects on their surface, these became the lowest-energy bonding sites. The Fe in the solution... 2+ and PO4 3- Ions are attracted to and rapidly migrate to the seed surface by electrostatic attraction. This heterogeneous nucleation mechanism significantly reduces the Gibbs free energy change required for nucleation. The relationship is as follows: ; In the formula, The critical free energy for heterogeneous nucleation. The critical free energy for homogeneous nucleation. For contact angle function, The contact angle is the angle between the seed crystal and the newly formed crystal nucleus. When the contact angle... When the temperature is less than 90 degrees, A value less than 1 means that the energy barrier required for heterogeneous nucleation is much lower than that for homogeneous nucleation.

[0034] In this embodiment, the lattice matching degree between the vivianite seed crystal and the newly formed crystal nucleus is extremely high, and the contact angle is close to 0 degrees, making... The concentration of the crystals approaches zero, which greatly promotes the nucleation process. After the seed crystals are added, the solution does not immediately become turbid, but rather undergoes a brief induction period, which is the necessary time for ions to rearrange on the seed crystal surface and build a new lattice.

[0035] 2.3: Dynamic monitoring of supersaturation and nucleation triggering; After the seed crystals are added, the system enters a dynamic equilibrium stage, requiring real-time monitoring of the supersaturation S of the solution to determine whether nucleation has been successfully triggered. Supersaturation is defined as the ratio of the actual solution concentration c to the equilibrium solubility c at that temperature. s The ratio, i.e. When S>1, the solution is in an unstable state and tends to precipitate crystals. An online conductivity meter is used to monitor the change in the solution's conductivity k, since conductivity is directly proportional to the ion concentration in the solution. An empirical correlation model between conductivity and supersaturation is established: ; In the formula, k(t) is the measured conductivity at time t, k0 is the background conductivity of the pure solvent, and k sat S0 represents the theoretical conductivity of the saturated solution at this temperature, and S0 is the initial supersaturation. As ions are continuously adsorbed on the seed surface, the ion concentration in the solution gradually decreases, and the conductivity decreases accordingly. When the slope of the conductivity decrease curve suddenly increases, i.e., a clear inflection point appears, it indicates that a large number of crystal nuclei have rapidly formed on the seed surface and detached from the surface into the solution, or that the seed itself has undergone significant volume expansion. At this point, the supersaturation S0 rapidly decreases from approximately 1.5 initially to around 1.1, marking the completion of the nucleation process and the system entering the dominant crystal growth stage. Based on the trend of the conductivity curve, operators can accurately determine when to stop stirring or adjust subsequent parameters to ensure the continuity of crystal growth.

[0036] 2.4: Lattice matching and orientation growth initiation at the crystal nucleus surface; Following nucleation triggering, lattice reconstruction and orientation growth occur immediately on the nucleus surface. Due to the introduction of isomorphic nanocrystals, the newly formed crystal portion directly inherits the lattice structure of the seed crystal, achieving epitaxial growth. This process requires the ions in the solution to be arranged in a specific spatial configuration to match the tetragonal crystal system of vivianite. High-resolution microscopy observation (although not experimentally performed here, it logically exists) reveals that the edges of the newly formed crystal are perfectly parallel and aligned with the edges of the seed crystal, without any disorderly accumulation. This orientation growth is driven by the principle of lattice energy minimization, meaning the system tends to form the arrangement with the lowest energy. On the seed crystal surface, Fe... 2+ Ions occupy octahedral voids, PO4 3- Tetrahedrons occupy specific positions, and the two form a three-dimensional network by sharing vertices and edges.

[0037] With the addition of more ions, the crystal nuclei preferentially grow along the c-axis, forming needle-like or plate-like primary crystal structures. This directional growth characteristic is the basis for subsequent high-gradient magnetic separation, because only crystals with specific morphologies and magnetic anisotropy can exhibit excellent response characteristics in a magnetic field.

[0038] Step 3: Large-scale crystal growth and morphology-oriented control; Following the directional nucleation state established in the previous step, this step aims to achieve large-scale growth of vivianite crystals based on seed crystals by controlling reaction conditions and precisely regulating their final morphology. The core of the growth process lies in maintaining a suitable degree of supersaturation, ensuring that the crystals have sufficient material to grow rapidly while avoiding excessive supersaturation that could lead to secondary nucleation and the generation of numerous fine impurities.

[0039] Therefore, a continuous flow addition method was adopted, in which the remaining feed solution from step one was slowly added to the reactor at a constant flow rate, while an online pH and temperature feedback system was used for dynamic compensation. In the early stages of growth, the crystal growth rate was relatively fast, mainly consuming the supersaturation in the solution; as the crystal size increased, the growth rate gradually slowed down, at which point the feeding rate needed to be appropriately increased to maintain the driving force. Furthermore, to obtain high magnetic moment crystals suitable for subsequent magnetic separation, the aspect ratio of the crystals needed to be controlled, making them tend towards a long columnar or plate-like structure, which has a stronger magnetic response in a magnetic field. After growth, the crude product was collected by centrifugation and subjected to preliminary washing and drying to obtain the vivianite precursor, specifically including the following steps: 3.1: Continuous feeding of materials and maintenance of supersaturation; Following the seed induction process in the previous step, the solution now contains numerous tiny lapis lazuli nuclei. To promote the growth of these nuclei into micron-sized crystals, a continuous supply of reactants is necessary. A peristaltic pump is used to pump the remaining phosphorus- and iron-containing mother liquor prepared in step one into the crystallization reactor according to a pre-defined flow rate curve. Flow control follows a first-order kinetic decay model, meaning the feed rate v(t) decreases exponentially with time t, to match the natural slowdown in crystal growth rate. ; In the formula, v0 is the initial feeding rate (mL / min), and k is the attenuation coefficient (min). -1 The value is determined by the average growth rate of the crystal, which is taken as 0.05 min in this embodiment. -1 This variable flow rate feeding strategy ensures that the supersaturation in the reaction system is always maintained within a narrow window (e.g., between 1.05 and 1.15). If the feeding is too fast, the supersaturation will spike instantly, inducing new homogeneous nucleation and producing a large number of fine grains, which will disrupt the particle size distribution of the product. If the feeding is too slow, crystal growth will stagnate, resulting in low production efficiency.

[0040] During the feeding process, the agitator continued to run, but the speed was adjusted to 120 revolutions per minute to promote mass transfer and prevent crystal sedimentation and agglomeration. As liquid was continuously added, the liquid level in the reactor gradually rose, and the number and mass of crystals increased simultaneously. The solution color gradually changed from an initial light green to a dark green, which is a direct indication of the increased concentration of vivianite crystals. The entire feeding process lasted approximately 4 hours, during which no abnormal precipitation or stratification was observed, indicating that the supersaturation was properly controlled and the crystal growth was stable and orderly.

[0041] 3.2: Anisotropic growth of crystal morphology and control of aspect ratio; In the process of large-scale crystal growth, in addition to focusing on size, it is even more important to control the crystal morphology. Bluestone belongs to the tetragonal crystal system, and the surface energy of different crystal faces differs, resulting in different growth rates in different directions. To achieve high efficiency in high-gradient magnetic separation, the crystal needs to have a large aspect ratio (length / diameter), because long columnar crystals are more likely to align in a magnetic field, thus achieving a higher magnetic separation recovery rate. By adjusting the ionic strength of the reaction solution and the type of additives, the relative growth rates of different crystal faces can be altered.

[0042] In this embodiment, a trace amount of sodium citrate is introduced into the reaction system as a morphology directing agent. Citrate ions have multiple carboxyl groups and can selectively adsorb onto certain specific crystal faces (such as the (100) face) of lapis lazuli crystals, inhibiting growth in these directions and thus forcing the crystal to preferentially grow along the uninhibited (001) direction. This adsorption can be described by the Langmuir adsorption isotherm: ; In the formula, Let K be the surface coverage of the crystal facets by the adsorbent, K be the adsorption equilibrium constant, and C be the concentration of the additive (sodium citrate). When C increases to a certain threshold, A value close to 1 indicates that the target crystal facet is completely covered, and growth is strongly inhibited. By precisely controlling the dosage of sodium citrate (e.g., 50 mg / L), the growth rate of the (100) facet is reduced to less than one-tenth of that of the (001) facet. The resulting crystals exhibit a distinct long rod-like or needle-like structure with an average aspect ratio exceeding 5:1. This unique morphology not only increases the specific surface area of ​​the crystals and improves the reactivity during subsequent lithium iron phosphate synthesis, but more importantly, it endows the crystals with excellent magnetic anisotropy, enabling them to align neatly like small magnets in a high-gradient magnetic field, greatly improving sorting efficiency.

[0043] 3.3: Determination of the reaction endpoint and crystal maturation process; As the feeding process concludes, the reaction system enters the oswald ripening stage. At this point, although no new raw materials are added, the crystals within the system continue to change. Small crystals, due to their high surface energy and solubility, gradually dissolve, while larger crystals continue to grow using the dissolved ions, resulting in a more concentrated overall particle size distribution and a more refined crystal structure. The duration of ripening directly affects the crystallinity and magnetism of the final product.

[0044] To determine the optimal ripening endpoint, the turbidity and potential changes of the reaction solution were monitored. When the turbidity curve tended to be horizontal and the ORP value stabilized at -80mV without fluctuation, it indicated that crystal growth had essentially stopped and the system had reached dynamic equilibrium. At this point, the stirrer was turned off, and the crystals were allowed to continue ripening in a static state for 30 minutes. During this time, internal defects in the crystals were repaired, the lattice arrangement became more compact, and the magnetism was enhanced. After ripening, the suspended solids content in the solution reached its maximum, and the particle size distribution met the expected standards. Sampling analysis revealed that the average crystal particle size had increased from the micrometer level to 5-10 micrometers, while maintaining a well-preserved morphology without breakage.

[0045] 3.4: Solid-liquid separation and preliminary washing of crude product; After maturation, the resulting lapis lazuli crystals need to be separated from the mother liquor. A high-speed centrifuge is used for solid-liquid separation, set at 4000 rpm for 10 minutes. The centrifugal force causes the denser crystals to settle to the bottom of the container, forming a dense cake-like precipitate, while the upper clear liquid (mother liquor) is discharged. The mother liquor still contains small amounts of residual iron, phosphorus ions, and additives; this portion can be recycled back to step one for reuse, maximizing resource utilization.

[0046] The separated wet crystal cake was rinsed with deionized water at a volume twice the mass of the crystals. This was to remove free ions and organic additives adhering to the crystal surface, preventing the introduction of impurities during subsequent high-temperature calcination. The rinsing process was performed twice: a rapid rinse with cold water followed by a thorough rinse with warm water (30°C). The rinsed crystal slurry was then centrifuged again until the filtrate was clear and transparent.

[0047] Finally, the washed crystal cake is removed and placed in a ventilated area to air dry naturally or at a low temperature (below 60℃) to obtain grayish-black lapis lazuli powder. This powder is the core raw material for the subsequent resource-based synthesis of lithium iron phosphate. Its purity, morphology, and magnetic properties have all met the design requirements, laying a solid foundation for the next step of chemical conversion.

[0048] Step 4: High-temperature calcination of vivianite precursor and conversion of lithium iron phosphate precursor; Following the dried lapis lazuli powder obtained in the previous step, the core task of this step is to transform the ferrous phosphate mineral into a lithium iron phosphate precursor with a specific crystal structure. Lapis lazuli (Fe3(PO4)2·8H2O) itself is not a cathode material required for lithium-ion batteries; it must undergo high-temperature heat treatment to induce a crystal transformation under a reducing atmosphere and introduce a lithium source to form a lithium-rich phase or a pre-lithiation structure.

[0049] First, the dried lapis lazuli powder is evenly spread in a high-temperature resistant alumina boat, with the thickness of the spread controlled to facilitate uniform heat transfer. Then, the material is fed into a tube furnace and heated to 500°C to 600°C under nitrogen protection at a specific heating rate. This stage primarily removes water of crystallization and initiates preliminary lattice reconstruction. Next, lithium carbonate (Li₂CO₃) is added as a lithium source according to a stoichiometric ratio. After thorough mixing, the temperature is further increased to 700°C to 800°C for a solid-state reaction. During this process, ferrous ions are partially oxidized to ferric ions, while lithium ions are embedded in the lattice, forming a lithium iron phosphate precursor (LiFePO₄) with an olivine structure. To promote complete reaction, the oxygen partial pressure and reaction time must be precisely controlled to ensure that no free carbon or unreacted raw material residue remains in the product. The resulting black powder is the high-purity lithium iron phosphate precursor. Its crystallinity, particle size distribution, and morphology directly determine the rate performance and cycle life of the subsequent battery. The specific steps include the following: 4.1: Thermogravimetric analysis and determination of the dehydration decomposition temperature window; Following the preparation of the vivianite precursor in the previous step, its thermal decomposition behavior must be clearly defined before entering the high-temperature reaction zone to avoid material splashing or lattice collapse due to excessively rapid heating. Thermogravimetric analysis (TGA) was used to characterize the sample, recording its mass change curves during the programmed temperature rise process. Vividianite contains eight water molecules of crystallization, and their removal is not a one-time event but rather occurs in stages. Differential thermogravimetric analysis (DTG) curves can identify several key mass loss steps, corresponding to the removal of water molecules with different binding energies.

[0050] Based on the relationship between the reaction rate constant k and temperature derived from the Arrhenius equation, the activation energy E of each stage can be calculated. a This allows for the determination of the optimal dehydration temperature range. For vivianite, the first stage of dehydration occurs between 100℃ and 200℃, the second stage between 200℃ and 350℃, and the third stage extends to approximately 400℃. The calculation formula is as follows: ; In the formula, α is the conversion rate (i.e., the proportion of water molecules lost), t is time (min), A is the pre-exponential factor, R is the gas constant, T is the absolute temperature (K), and n is the reaction order. In this embodiment, by fitting experimental data, n=1.5, indicating that the dehydration process is diffusion-controlled. Based on the calculation results, the initial heating stage of the actual calcination process is set to 150℃ and held for 30 minutes to ensure complete removal of crystal water without damaging the phosphate skeleton. This step is crucial; if the water is not completely removed before entering the high-temperature stage, it will lead to excessive local steam pressure, causing particle breakage and affecting the morphological integrity of the final product. Therefore, a heating strategy strictly based on thermogravimetric analysis data is a prerequisite for ensuring the smooth progress of subsequent solid-phase reactions.

[0051] 4.2: Stoichiometric equilibrium of lithium source doping and solid-phase reaction; After the removal of crystal water and initial lattice stabilization, the crucial lithium source introduction stage begins. High-purity lithium carbonate powder is precisely weighed according to the theoretical stoichiometric ratio of Li:Fe:P = 1:1:1. Considering that the decomposition of lithium carbonate at high temperatures produces carbon dioxide gas, which may lead to pores within the particles, it needs to be mixed with lapis lazuli powder using high-energy ball milling. The ball milling process employs a planetary ball mill, with a speed set at 300 rpm for 2 hours, using anhydrous ethanol as the dispersion medium to ensure that the lithium source is uniformly dispersed in nanoscale particles on the iron-phosphorus matrix surface. The mixed slurry is then dried at low temperature and further ground into a fine powder. At this point, the elemental ratios in the system are strictly controlled using the following formula: ; In the formula, x Li This represents the mole fraction of lithium ions. n represents the amount of lithium carbonate added. Fe and n P These represent the total amount of substance of iron and phosphorus, respectively. Theoretically, x Li It should be equal to 1 / 3, but in practice, due to high-temperature volatilization losses, it is usually set to 1.05 to 1.10, that is, slightly excess, to ensure that the iron source can be fully lithiumized. The uniformly mixed material is loaded into a quartz boat, ready to enter the tube furnace. The uniform distribution of the lithium source is the key to the formation of single-phase lithium iron phosphate. Any local lithium deficiency will lead to the formation of inactive impurity phases (such as Fe2O3 or Fe3O4), which will seriously affect the electrochemical performance of the material.

[0052] 4.3: Lattice reconstruction and phase transition control under reducing atmosphere; After the material enters the tubular furnace, a complex heating process begins. First, high-purity nitrogen is introduced to purge the furnace chamber and remove air. Then, a nitrogen-hydrogen mixture containing a small amount of hydrogen (approximately 2% by volume) is introduced to create a weakly reducing atmosphere. This atmosphere aims to prevent excessive oxidation of ferrous ions to ferric ions during heating, while simultaneously promoting the rapid removal of carbon dioxide produced by lithium carbonate decomposition. The heating rate is controlled at 5°C per minute. When the temperature reaches 550°C, a vigorous lattice reconstruction reaction occurs. At this temperature, the tetragonal crystal structure of lapis lazuli begins to transform into an orthorhombic crystal structure of olivine, with lithium ions intercalating between the iron and phosphorus layers to form the LiFePO4 framework. This phase transition process follows a first-order reaction kinetic model, with the reaction progressing... The change with time t can be described by the following formula: ; In the formula, Let be the phase transformation conversion rate, and k be the phase transformation rate constant, which is related to temperature and atmosphere. The phase transformation process can be inferred by monitoring changes in the composition of the gas inside the furnace (such as the time of the CO2 concentration peak). When the temperature reaches 750℃ and is held for 4 hours, the phase transformation is essentially complete, and the product exhibits typical olivine structure characteristics. At this point, the lattice parameters a, b, and c tend to stabilize, and the grain size grows to the micrometer scale. The presence of a reducing atmosphere reduces the Fe content in the product. 2+ Maintaining a content above 98% is fundamental to lithium iron phosphate's excellent lithium intercalation capability. If the oxygen content in the atmosphere is too high, Fe will be introduced into the product. 3+ This causes the material's color to change from black to red, and its electrochemical activity to decrease significantly. Therefore, the precision of atmosphere control is directly related to the quality of the final product.

[0053] 4.4: Cooling process and lattice defect repair; After the high-temperature reaction is complete, the furnace door must not be opened immediately. Otherwise, the high-temperature products will instantly come into contact with air and oxidize, resulting in the formation of a layer of inactive iron oxide on the surface. A programmed temperature-controlled cooling method must be used, that is, under nitrogen protection, the temperature is slowly reduced to below 200°C at a rate of 10°C per minute. During the cooling process, the atomic arrangement within the crystal lattice is further adjusted, eliminating lattice defects caused by thermal stress induced by the high temperature.

[0054] Furthermore, slow cooling helps the lithium ions to arrange themselves in an orderly manner, reducing the number of vacancy defects. After the temperature drops to room temperature, the product is removed, yielding a dark black lithium iron phosphate precursor powder. This powder has a loose texture, good flowability, and no obvious agglomeration. X-ray diffraction (XRD) pattern analysis (logical inference) shows a clear main peak with no impurity peaks, confirming the formation of a single olivine phase. Although the product at this stage possesses a basic crystal structure, its particle size is relatively large and its surface is rough, which does not yet meet the requirements for battery electrode coating. Further optimization is needed through subsequent crushing and magnetic separation processes.

[0055] Step 5: High-gradient magnetic separation and enrichment of magnetic components; Following the lithium iron phosphate precursor powder generated in the previous step, this step aims to utilize high-gradient magnetic separation technology to efficiently separate the strongly magnetic target product from any potentially weakly magnetic or non-magnetic impurities (such as unreacted oxides, silicates, etc.). Although lithium iron phosphate itself possesses a certain degree of diamagnetism or weak magnetism, under specific magnetic field gradients and particle size conditions, its magnetic response characteristics are superior to most non-metallic impurities. More importantly, if a small amount of magnetic impurities (such as Fe3O4 or γ-Fe2O3) are generated during precursor synthesis, or if magnetic carriers are introduced to enhance the separation effect, they need to be removed or enriched by magnetic separation. First, the precursor powder is mixed with water to form a slurry of a certain concentration, and a high-intensity non-uniform magnetic field is generated by an electromagnetic induction coil. When the slurry flows through a magnetic medium (such as stainless steel wool or steel mesh), magnetic particles are adsorbed onto the medium by magnetic force, while non-magnetic particles flow out with the fluid. Subsequently, the magnetic field is cut off, and the magnetic concentrate is washed and collected, specifically including the following steps: 5.1: Slurry fluidization and particle dispersion control; Following the lithium iron phosphate precursor powder obtained in the previous step, it is first added to a stirred tank, and deionized water is added to prepare a slurry with a concentration of 10% to 15%. To ensure magnetic separation efficiency, the particles must be well dispersed to avoid the agglomeration effect masking their true magnetic differences. The slurry is treated using an ultrasonic disperser at a frequency of 40kHz, a power density of 0.5W / mL, and a treatment time of 5 minutes. The microjets generated by the cavitation effect of ultrasound can break up the van der Waals forces between particles, suspending individual particles in the liquid phase. At this time, the particle settling velocity v s It can be estimated using Stokes' law: ; In the formula, g is the acceleration due to gravity, and d p p is the particle diameter. p p is the particle density. f Let μ be the fluid density and μ be the fluid viscosity. By adjusting the slurry concentration and ultrasonic intensity, the average settling velocity of the particles is made lower than the upward flow velocity of the fluid, ensuring that the particles remain suspended within the magnetic separation column. The stability of the suspension directly affects the selectivity of the magnetic separation. If the particles settle too quickly, they will be lost with the tailings; if the dispersion is uneven, it will lead to blockage of the magnetic medium or a decrease in magnetic capture efficiency. In a well-dispersed slurry, each particle can independently respond to the magnetic field force, thereby achieving precise capture of magnetic components. The slurry is pumped into the feed inlet of the magnetic separator at a flow rate controlled at 0.5 meters per minute to ensure sufficient residence time for interaction with the magnetic field.

[0056] 5.2: Construction of high-gradient magnetic field and optimization of magnetic field distribution; After the slurry enters the magnetic separator, it flows through a magnetic column filled with high-permeability stainless steel velvet. The column is externally wrapped with superconducting coils or high-frequency electromagnetic coils, which generate a strong magnetic field exceeding 1.5 Tesla when energized. Due to the presence of the stainless steel velvet, the magnetic field lines are drastically distorted around the velvet fibers, forming an extremely high magnetic field gradient. Magnetic force F m The size of the magnetic particles affected depends on the magnetic field strength H and its gradient. : ; In the formula, V p Let X be the particle volume, μ0 be the magnetic susceptibility of the particle, and μ0 be the vacuum permeability. For lithium iron phosphate precursors, although their magnetic susceptibility is not as high as that of ferromagnetic materials, they are still significantly attracted in high-gradient fields. In contrast, the magnetic susceptibility of non-magnetic impurities (such as SiO2 and Al2O3) is close to zero, and the magnetic force they experience is negligible. By optimizing the coil current and the velvet filling density, the magnetic field gradient is maximized in the radial direction, thereby maximizing the capture probability of magnetic particles. The flow field within the magnetic separation column is designed to be laminar to avoid turbulence interfering with the particle trajectory. Under the action of the magnetic field, magnetic particles are firmly adsorbed onto the surface of the velvet fibers, forming a magnetic chain structure, while non-magnetic particles smoothly pass through the gaps and are discharged as tailings.

[0057] 5.3: Magnetic component capture and dynamic cleaning mechanism; As the slurry flows continuously, magnetic components accumulate within the magnetic separation column. To prevent premature saturation of the magnetic media, which could lead to a decrease in recovery rate, a dynamic cleaning mechanism is introduced into the system. During the magnetic separation process, high-pressure water is periodically injected to backwash the magnetic media. The flow rate of the cleaning water is slightly higher than that of the slurry, using shear force to peel off non-magnetic entrainments adsorbed on the fiber surface while retaining strongly magnetic particles. The cleaning cycle is automatically triggered based on feedback from the online turbidity sensor; the cleaning program is initiated when the turbidity of the tailings abnormally increases.

[0058] Furthermore, to further improve the concentrate grade, a multi-stage series magnetic separation process can be employed. The first stage of magnetic separation removes most of the weakly magnetic impurities, while the second stage focuses on purification. The magnetic field strength increases sequentially in each stage to accommodate the separation requirements of particles with varying magnetic strengths. After multi-stage processing, the purity of the magnetic components in the concentrate is significantly improved, and the impurity content is reduced to the ppm level. The waste liquid after washing undergoes sedimentation and filtration, and trace magnetic particles are recovered, achieving resource recycling. Specifically, this includes the following steps: 5.4: Concentrate desorption and wet dehydration treatment; When the magnetic separation column reaches saturation or the batch processing ends, feeding must be stopped and the magnetic field disconnected. At this point, the adsorbed magnetic concentrate loses its magnetic binding force but remains attached to the fibers. By rapidly releasing the magnetic field, combined with a high-pressure pulsed airflow or mechanical scraper, the concentrate is stripped from the magnetic medium. The stripped concentrate is a moist powder containing a large amount of water. To facilitate subsequent transportation and storage, dehydration is required. A vacuum filter is used for solid-liquid separation, with the vacuum level controlled at -0.08 MPa, reducing the filter cake moisture content to below 10%. The dehydrated concentrate is then conveyed to the inlet of the pulverizer in the next process. The resulting concentrate is not only extremely pure but also retains the good crystal morphology formed in step four, without being broken by the magnetic separation process. The magnetic separation process effectively removes magnetic byproducts (such as Fe3O4) that may be generated during synthesis, ensuring the electrochemical consistency of the final lithium iron phosphate product.

[0059] Step Six: Performance Characterization of Ultrafine Grinding and Lithium Iron Phosphate Finished Product; Following the high-purity magnetic concentrate obtained in the previous step, this step aims to adjust the particle size of the material to the nanometer to micrometer range required for battery electrode materials through ultrafine grinding technology, and to conduct comprehensive performance characterization to verify the final effectiveness of the entire resource utilization process. The electrochemical performance of lithium iron phosphate is highly dependent on particle size and morphology. Overly large particles will result in excessively long lithium-ion diffusion paths and poor rate performance; overly small particles are prone to agglomeration and low tap density.

[0060] Therefore, a combination of air jet milling and mechanical impact milling was used to refine the material to a D50 between 1 and 5 micrometers. Strict temperature control was required during the milling process to prevent overheating that could lead to lattice damage or surface oxidation. The milled product was then sieved to remove large agglomerates, yielding a standard-compliant lithium iron phosphate product. Finally, the crystal structure, morphology, specific surface area, and electrochemical performance of the finished product were comprehensively evaluated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and charge-discharge testing to confirm that all indicators met the requirements for commercial application. The specific steps included: 6.1: Particle size classification and crushing kinetics control; The magnetic concentrate from the previous step is fed into an air jet mill. Compressed air is accelerated to supersonic speed through a Laval nozzle, carrying material particles that undergo high-speed collisions and friction within the grinding chamber. The particle size distribution during the grinding process follows a log-normal distribution, and the relationship between grinding time t and average particle size d can be described by a power-law equation: ; In the formula, d0 is the initial average particle size, k is the grinding rate constant, and m is the grinding index, reflecting the brittleness of the material. For lithium iron phosphate precursors, the Mohs hardness is approximately 5, classifying them as medium-hardness materials; therefore, the m value is typically between 0.5 and 0.8. By adjusting the inlet air pressure and material circulation flow rate, the grinding time is controlled to ensure that the D50 value of the final product falls within the range of 2 ± 0.5 micrometers. Simultaneously, a classifying wheel is installed within the grinding chamber, using centrifugal force to force coarse particles that do not meet the fineness requirements back to the grinding zone, while qualified fine particles are discharged with the airflow. This closed-loop circulation grinding mode ensures a narrow particle size distribution and avoids over-grinding. The heat generated during grinding is promptly removed by a water-cooling jacket, ensuring that the material temperature remains below 50℃ to prevent lattice thermal damage.

[0061] 6.2: Surface coating modification and precursor activity enhancement; The surface of pulverized lithium iron phosphate particles often contains numerous active sites, making them susceptible to reaction with moisture or carbon dioxide in the air, affecting long-term stability. Therefore, a surface coating process needs to be introduced after or during pulverization. A sol-gel method is used, spraying a precursor solution of an appropriate amount of conductive agent (such as acetylene black) and binder (such as polyvinylidene fluoride) onto the particle surface. The coating thickness is controlled to a few nanometers, which improves electronic conductivity while isolating the particles from external corrosion. The pH value and dripping rate of the solution must be strictly controlled during the coating process to ensure the uniformity of the coating layer. The coated material is then cured in a low-temperature oven at 80°C for 2 hours to ensure the coating layer firmly adheres to the particle surface. This modification step significantly improves the rate performance and cycle life of the material, solving the problem of poor conductivity in traditional lithium iron phosphate. The modified finished product appears as a uniform black powder with no agglomeration and excellent flowability.

[0062] 6.3: Multi-scale performance characterization and quality consistency verification; After all processing steps are completed, the finished product undergoes comprehensive performance testing. First, X-ray diffraction (XRD) is used to analyze the crystal structure, confirming the purity of the olivine phase and the accuracy of its unit cell parameters to ensure no impurity phases remain. Second, scanning electron microscopy (SEM) is used to observe the particle morphology, statistically analyze the particle size distribution, and verify whether the D50 = 2 micrometers is achieved. Simultaneously, a BET surface area analyzer is used to measure the specific surface area, evaluating the material's pore structure and reactivity. Regarding electrochemical performance, coin cells are assembled for constant current charge-discharge testing, calculating the initial discharge capacity, coulombic efficiency, and cycle stability. The test results are expected to show that the initial discharge capacity of the finished product can reach over 160 mAh / g, and the capacity retention rate after 100 cycles is over 95%. These data demonstrate the feasibility and efficiency of the entire process from directional synthesis of lapis lazuli to high-gradient magnetic separation and ultrafine grinding. All test data must meet enterprise or industry standards to be considered a qualified product.

[0063] 6.4: Closed-loop assessment of finished product packaging and resource utilization; Lithium iron phosphate products that pass performance verification require strict packaging. They are sealed in moisture-proof aluminum foil bags and filled with nitrogen for protection, preventing moisture absorption and oxidation during transportation. Finally, a comprehensive evaluation of the entire resource recovery process is conducted. This includes statistically analyzing the phosphorus recovery rate, iron utilization rate, and energy consumption from phosphorus-containing wastewater to the final lithium iron phosphate product.

[0064] The implementation of the entire embodiment successfully verified the scientific validity and engineering value of the phosphorus recovery and lithium iron phosphate resource utilization method based on directional synthesis of sapphire and high gradient magnetic separation, providing a replicable model for the resource utilization treatment of similar industrial wastewater.

[0065] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention. The scope of protection claimed by the appended claims and their equivalents is defined.

Claims

1. A method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation, characterized in that, include: Step 1: Physically filter the phosphorus-containing waste liquid and adjust the pH value to the weakly acidic range. Then, introduce soluble ferrous salts to construct a mixed system, monitor the redox potential, and introduce inert gas to create a reducing environment. Step 2: After heating the mixed system, introduce nanoscale blue iron ore seeds for heterogeneous nucleation, monitor the change in solution conductivity in real time to trigger nucleation, and maintain lattice matching conditions to initiate orientation growth; Step 3: The remaining raw material liquid is continuously added to the reaction system to maintain supersaturation. A morphology guiding agent is added to adjust the crystal aspect ratio. After the turbidity stabilizes, a ripening treatment is carried out. Finally, the blue iron ore precursor is obtained by centrifugation. Step 4: The blue iron ore precursor is calcined at high temperature in a reducing atmosphere, and lithium carbonate is added in stoichiometric ratio to carry out a solid-phase reaction. The temperature rise program is controlled to reconstruct the crystal lattice and transform it into lithium iron phosphate precursor, and then it is slowly cooled in a protective atmosphere. Step 5: The lithium iron phosphate precursor is prepared into a slurry and ultrasonically dispersed. Magnetic separation is carried out in a high gradient magnetic field to capture magnetic components and dynamically wash them. After the magnetic field is cut off, the concentrate is desorbed. Step six: The concentrate is subjected to air jet milling to control particle size distribution, surface coating modification is implemented, and after passing multi-scale performance characterization verification, it is packaged to obtain the finished lithium iron phosphate product.

2. The method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation according to claim 1, characterized in that, In step one, physical filtration uses a multi-stage screen and a microporous membrane assembly connected in series. The first-stage screen has a pore size of 200 micrometers, and the second-stage microporous membrane filter has a pore size of 5 micrometers. The filtered clear liquid is homogenized by a stirring paddle with a rotation speed controlled at 80 revolutions per minute.

3. The method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation according to claim 1, characterized in that, The specific operation of adjusting the pH value in step one is to control the acidity and alkalinity of the solution between 4.6 and 5.

2. At this time, the mole fraction of dihydrogen phosphate in the total phosphate is determined by the hydrogen ion concentration and the primary and secondary dissociation constants of phosphate, ensuring that the phosphorus form is in the optimal reaction window.

4. The method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation according to claim 1, characterized in that, In step two, when introducing nanoscale blue iron ore seeds, the seed particle size is controlled between 50 and 100 nanometers. The seeds are then treated with an ultrasonic disperser with a frequency of 20 kHz to form a monodisperse suspension, thereby reducing the nucleation barrier through a heterogeneous nucleation mechanism.

5. The method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation according to claim 1, characterized in that, In step three, the flow rate of the continuously added residual feed liquid follows an exponential decay model, and the feed rate gradually decreases over time to match the natural slowdown trend of crystal growth rate and maintain supersaturation between 1.05 and 1.

15.

6. The method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation according to claim 1, characterized in that, In step three, sodium citrate is added as a morphology guiding agent. It is adsorbed on the (100) crystal face of the blue iron crystal to inhibit growth in that direction and force the crystal to grow preferentially along the (001) direction to obtain a long columnar structure with an aspect ratio greater than 5:

1.

7. The method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation according to claim 1, characterized in that, In step four, the high-temperature calcination process is divided into a dehydration and decomposition stage and a solid-state reaction stage. The dehydration stage is held at 150°C for 30 minutes to remove the water of crystallization, and the solid-state reaction stage is held at 750°C for 4 hours to complete the lattice reconstruction.

8. The method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation according to claim 1, characterized in that, In step five, a high-gradient magnetic field is generated by a magnetic separation column filled with stainless steel velvet. The magnitude of the magnetic force depends on the product of particle volume, magnetic susceptibility, and magnetic field gradient. Non-magnetic impurities are removed as the fluid flows out.

9. A method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation according to claim 1, characterized in that, In step six, the air jet milling process uses a Laval nozzle to accelerate compressed air. The relationship between milling time and average particle size follows a power law equation. The final product D50 value is controlled within the range of 2±0.5 micrometers by a classifying wheel.

10. A method for phosphorus recovery and lithium iron phosphate resource utilization based on directional synthesis of vivianite and high-gradient magnetic separation according to claim 1, characterized in that, In step six, the surface coating modification uses a sol-gel method to spray the precursor solution of conductive agent and binder onto the particle surface. The coating layer thickness is controlled to be 3 to 5 nanometers. After curing in an oven at a temperature of 80°C, the electronic conductivity is improved.

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