A front-end process method for preparing iron phosphate

Abstract

This application discloses a front-end processing method for preparing ferric phosphate. The method includes: pulping wet slag, reacting it with sulfuric acid, and dynamically replenishing water to obtain an acid-leaching slurry; obtaining ferric phosphate liquid through a first-stage filtration; mixing the liquid with a crystal inducing agent and a curing agent for a synthesis reaction and adjusting the pH, followed by a second-stage filtration to obtain a synthetic filter cake; further pulping and crystallizing the synthetic filter cake to obtain a slurry containing the precursor; and finally, obtaining a precursor filter cake through a third-stage filtration. This application significantly eliminates reaction concentration fluctuations and impurity residues through dynamic closed-loop management of the acid-leaching water volume, refined timing control of feeding washing and side-blowing during each stage of the three-stage filtration, and thermodynamic synergistic regulation of the synthesis and aging process, thereby significantly improving the purity and crystal integrity of the ferric phosphate precursor.

Description

Technical Field

[0001] This application relates to the field of lithium iron phosphate battery cathode material precursor preparation technology, and in particular to a front-end processing method for preparing lithium iron phosphate. Background Technology

[0002] As the core precursor of lithium iron phosphate (LFP) batteries, the physicochemical properties (such as purity, particle size morphology, and crystallinity) of LFP batteries directly determine the electrochemical performance of the final lithium battery. In recent years, with the explosive growth of the new energy vehicle and energy storage industries, the market demand for high-quality LFP precursors has surged, while also placing more stringent requirements on cost control, product consistency, and environmental compliance in the manufacturing process.

[0003] In existing ferric phosphate production processes, a wet synthesis route is typically employed. This involves using iron and phosphorus sources as raw materials, where precipitation or crystallization reactions occur in the liquid phase to generate ferric phosphate slurry. The final product is then obtained through solid-liquid separation, washing, and drying. However, existing technologies generally suffer from lengthy processes, low material conversion rates, difficulties in impurity separation, and significant water and energy consumption in the pretreatment processes before the synthesis reaction.

[0004] Specifically, existing technologies face the following significant technical bottlenecks in front-end processing: First, in the acid leaching process, traditional pulping and acid addition methods lack a precise dynamic water volume control mechanism, often employing a one-time fixed water addition, resulting in large fluctuations in the solid-liquid ratio within the reaction system, incomplete acid leaching reaction, and low leaching rate. Simultaneously, the cleaning water for pulping equipment is not included in the overall system planning, not only wasting water resources but also making it difficult to accurately control the concentration of the substrate, directly affecting the stoichiometric ratio of subsequent synthesis reactions. Second, in the solid-liquid separation (pressure filtration) process, the pressure filtration operations at each stage lack comprehensive planning tailored to the characteristics of different materials, often employing a crude, single filtration mode. This results in the inability to completely remove residual free acid and impurity ions from the filter cake, and the filter cake retains a high moisture content, affecting not only the purity of the ferric phosphate solution but also increasing the energy consumption and cost of subsequent drying processes. Third, in the synthesis and crystallization aging process, the pH control, temperature control and timing of the addition of crystal inducing agents in the reaction system lack systematic optimization, resulting in the formation of iron phosphate precursor crystals with different sizes and irregular morphologies, and even agglomeration or impurities, which seriously reduces the electrochemical performance of the material. Summary of the Invention

[0005] This application provides a front-end processing method for preparing ferric phosphate, in order to solve the problem that the purity of the ferric phosphate precursor obtained by the current front-end process for preparing ferric phosphate is not high enough.

[0006] To solve the above-mentioned technical problems, one technical solution adopted in this application is: to provide a front-end processing method for preparing ferric phosphate. The front-end processing method for preparing ferric phosphate includes: S1: After pulping the wet slag, it is put into an acid leaching kettle, sulfuric acid is added to carry out the acid leaching reaction, and water is dynamically added according to the test results during the reaction to obtain acid leaching slurry; S2: The acid leaching slurry is subjected to a single pressure filtration to obtain ferric phosphate solution; S3: The ferric phosphate solution is mixed with a crystal inducing agent and a curing agent to carry out a synthesis reaction. During the reaction, the pH value of the material is adjusted. The material after the synthesis reaction is completed is subjected to secondary pressure filtration to obtain a synthetic filter cake. S4: The synthetic filter cake is subjected to a synthetic pulping reaction and crystallization aging treatment in sequence to obtain a slurry containing ferric phosphate precursor; S5: The slurry containing the ferric phosphate precursor is subjected to three pressure filters to obtain the ferric phosphate precursor filter cake.

[0007] In some embodiments, the S1 acid leaching process includes the following multi-step operations: S11: Add the predetermined amount of water to the pulping pot. The actual amount of water added should be 5% to 15% less than the theoretical total amount of water added, to reserve water for cleaning. S12: Add the wet residue to the pulping pot, stir at high speed to form a uniform pulp, and collect multiple batches of pulped material for later use; S13: After the high-speed stirring is started and the preset time is continued, the pulped material is transported to the acid leaching tank, and the pulping pot is cleaned with the reserved cleaning water. The cleaning wastewater is also transported to the acid leaching tank. S14: Add the predetermined amount of sulfuric acid to the acid leaching kettle; S15: After all the sulfuric acid has been added, stir for the preset time, and then perform the water replenishment operation; S16: Add crystallization mother liquor or pure water to the acid leaching kettle to make the total weight of the kettle material reach the set value, and then take a sample to test the phosphorus content; S17: Calculate and replenish the corresponding amount of water based on the detected phosphorus content to bring the material to the target reaction concentration.

[0008] In some embodiments, in step S3, the synthesis process includes: Add a measured amount of the ferric phosphate solution to the synthesis reactor, add an appropriate amount of filtered brine or acid leaching washing water, add crystal inducing agent and curing agent respectively, and turn on the stirring equipment to stir continuously for 3 to 10 minutes; Before adding ammonia, the circulation system and ultrasonic equipment are turned on. Then, ammonia water is slowly added to the reactor to adjust the pH value of the material. Real-time sampling and testing are carried out. After confirming that the pH value reaches the target value, the material is subjected to secondary pressure filtration to obtain the synthetic filter cake.

[0009] In some embodiments, step S4, the synthetic pulping reaction includes: Add crystallization replenishing liquid to the synthesis pulping kettle, then add a measured amount of the synthesis filter cake, and turn on the steam heating system to control the material temperature; Phosphoric acid and curing agent are added in sequence, and the pH value of the reaction system is tested. The pH value is adjusted to the target range using sulfuric acid. After passing the retest, the material is fed into the crystallization reactor.

[0010] In some embodiments, step S4, the crystallization aging includes: Heat the material in the crystallization reactor to 95~98℃, turn on the heat preservation system and start timing; After keeping the material warm for 1-2 hours, take a sample to test its condition, and add phosphoric acid according to the set ratio based on the test results. Continue to keep warm for 1-2 hours, with the total holding time controlled within the range of 2-4 hours, to complete the crystallization process; The material is fed into an aging reactor for aging to obtain the slurry containing the iron phosphate precursor.

[0011] In some embodiments, step S2, the initial pressure filtration of the acid leaching slurry, includes: Feeding and filtration stage: Start the filter press circulation valve, and close the circulation valve after the filtrate is clear and transparent, and collect the corresponding acid leaching solution; Washing stage: Filtered brine is used as the washing solution first. If the water volume is insufficient, pure water is added. The filtrate after washing flows into the corresponding multiple washing tanks in sequence. Pressing stage: Maintain the pressing state for 20-40 minutes; Side-blowing phase: Maintain the side-blowing state for 20-40 minutes, and open the right-side dark flow valve 1-3 minutes before the end of the side-blowing process.

[0012] In some embodiments, step S3, the secondary pressure filtration of the material after the synthesis reaction is completed, includes: Feeding and filtration stage: Start the filter press circulation valve, and close the circulation valve after the filtrate is clear and transparent, and collect the corresponding mother liquor; Washing stage: Pure water is used as the washing medium for multiple washes, and the filtrate after washing flows into the corresponding multiple washing tanks in sequence. Pressing stage: Maintain the pressing state for 20-40 minutes; Side-blowing phase: Maintain the side-blowing state for 20-40 minutes, and open the right-side dark flow valve 1-3 minutes before the end of the side-blowing process; During periodic operation, after each preset batch of filter press and washing is completed, the washing tank is replenished with water to maintain a stable liquid level.

[0013] In some embodiments, step S5 involves subjecting the slurry containing the iron phosphate precursor to three-stage pressure filtration, including: Feeding and filtration stage: Start the filter press circulation valve, and close the circulation valve after the filtrate is clear and transparent, and collect the corresponding mother liquor; Washing stage: Multiple washes are performed using a washing medium, and the filtrate after washing flows into the corresponding washing tanks in sequence. Pressing stage: Maintain the pressing state for 20-40 minutes; Side-blowing phase: Maintain side-blowing for 20-40 minutes, and open the right-side dark flow valve 1-3 minutes before the side-blowing ends.

[0014] In some embodiments, after step S5, a high salinity treatment process is further included: The high-salinity water produced during the three-stage pressure filtration and washing stage is adjusted to pH value with ammonia water before being subjected to pressure filtration to achieve resource recovery and solid-liquid separation.

[0015] In some embodiments, the step of adding ammonia to adjust the pH value of the high-salt water generated during the three-stage pressure filtration washing stage before performing pressure filtration includes: Ammonia solution was gradually added dropwise to the high-salt water, and the pH value of the solution was monitored in real time. The pH value was then precisely adjusted to between 8.5 and 9.5. Pump the high-salt water with the adjusted pH value into the filter press, start the filter press circulation valve, and when the circulating filtrate becomes clear and transparent, close the circulation valve and open the feed valve to perform filter press.

[0016] The beneficial effects of this application are as follows: Unlike existing technologies, this application discloses a front-end processing method for preparing ferric phosphate. In the embodiment of this application, by dynamically replenishing water based on detection results during the acid leaching process in step S1, the concentration fluctuations caused by the blind introduction of cleaning water from the pulping equipment are completely eliminated, ensuring that the acid leaching reaction is always within the optimal liquid-to-solid ratio range, significantly improving the leaching rate and the stability of the reaction system. By rationally arranging primary, secondary, and tertiary pressure filtration in steps S2, S3, and S5, timely and efficient solid-liquid separation is achieved on the materials after each stage of the reaction, effectively removing free acid and impurity ions from the acid leaching residue, synthesis mother liquor, and precursor slurry, ensuring the purity of the intermediate materials. Furthermore, by adding a crystal inducing agent and a curing agent and adjusting the pH value of the material in the synthesis process in step S3, and sequentially performing the synthesis pulping reaction and crystallization aging treatment in the pulping and aging process in step S4, a systematic control over the crystal growth and morphology of the precursor is achieved, significantly improving the purity and crystal integrity of the ferric phosphate precursor. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort, wherein: Figure 1 This is a schematic flow chart of an embodiment of the front-end processing method for preparing ferric phosphate provided in this application; Figure 2 Is it like this? Figure 1 The flowchart of step S1 in the method shown is a schematic diagram of an embodiment. Detailed Implementation

[0018] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0019] The terms "first," "second," and "third" used in the embodiments of this application are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first," "second," or "third" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or devices.

[0020] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0021] To address the problems of uncontrollable acid leaching concentration, low pressure filtration and washing efficiency with high impurity residue, poor quality of synthesized crystals, and lack of resource utilization of high-salt water in existing ferric phosphate pretreatment processes, this application provides a pretreatment process for preparing ferric phosphate. This process constructs a high-purity, high-efficiency, and low-energy-consumption pretreatment system for ferric phosphate precursor preparation by dynamically managing the amount of acid leaching water, precisely controlling the timing of each stage of the three-stage pressure filtration, using physical field assistance and precise thermodynamic regulation of the synthesis and aging processes, and implementing closed-loop recovery treatment of high-salt water.

[0022] Specifically, see Figure 1 , Figure 1 This is a schematic flow chart of an embodiment of the pretreatment process for preparing ferric phosphate provided in this application. The pretreatment process for preparing ferric phosphate includes: Step S1: After pulping the wet slag, put it into an acid leaching kettle, add sulfuric acid to carry out the acid leaching reaction, and dynamically replenish the water according to the test results during the reaction to obtain acid leaching slurry.

[0023] In this embodiment, step S1 is the starting point for materials and the basis for stoichiometry in the entire front-end processing process. Wet slag typically originates from iron-phosphate slag or industrial waste generated in upstream processes. It has a high moisture content; if directly added to the acid leaching reactor, it will not only cause dust generation and environmental corrosion, but also drastically reduce the surface area of ​​the acid leaching reaction due to material agglomeration, creating dead zones and ultimately resulting in a low leaching rate. Therefore, pulping is necessary beforehand.

[0024] However, traditional pulping processes typically involve filling the pulping pot with a fixed amount of water all at once, pumping the pulp directly into the acid leaching tank after pulping, and then rinsing the remaining pulp inside the pulping pot with additional fresh water. This high-concentration wash water is then treated directly as wastewater or discharged as overflow. This approach has two fatal flaws: First, the water used to rinse the pulping pot is not included in the total water volume budget for the acid leaching reaction, causing drastic fluctuations in the liquid-to-solid ratio within the acid leaching tank, resulting in accidental dilution of the sulfuric acid concentration and disruption of the acid leaching reaction kinetics. Second, discharging the wash water as wastewater not only wastes a large amount of water resources but also removes dissolved effective iron and phosphorus components, leading to material loss.

[0025] To address the aforementioned issues, the S1 acid leaching process in this embodiment employs a mechanism for pre-reserving cleaning water and dynamically calculating water replenishment. (See attached document.) Figure 2 , Figure 2 This is a schematic flow diagram of an embodiment of the acid leaching process in step S1 provided in this application. Step S1 includes: Step S11: Add the predetermined amount of water to the mixing pot. The actual amount of water added is 5% to 15% less than the theoretical total amount of water added, reserving water for washing.

[0026] In this step, the theoretical total water volume refers to the total water volume required for the entire acid leaching system, calculated based on the moisture content of the wet residue, the target acid leaching liquor solid ratio, and the sulfuric acid concentration required for the chemical reaction. In this embodiment, when adding water to the pulping pot, not the entire theoretical total water volume is added, but rather 5% to 15% less is intentionally added. This 5% to 15% reduction is used as a budgeted amount reserved for cleaning water.

[0027] In a specific implementation scenario, if the theoretical total water addition is 1000 liters, only 850 to 950 liters of water are actually added to the pulping pot. The remaining 50 to 150 liters of water are used to clean the pulping pot in subsequent step S13. By pre-reserving this water volume, it is ensured from the source that even if water from the cleaning process is introduced, the total amount of water entering the pickling tank will still strictly equal the theoretical total water addition, thus ensuring the absolute stability of the liquid-solid ratio in the pickling reaction. The percentage difference between the actual water addition and the theoretical total water addition is strictly limited to 5% to 15%. If it is less than 5%, the reserved cleaning water is insufficient to thoroughly rinse the residual wet residue on the pulping pot wall and the stirring blades, resulting in material loss and incomplete equipment cleaning. If it is more than 15%, the reserved water is too large, which may cause cavitation or conveying difficulties in the transfer pump when the pulp in the pulping pot is transported to the pickling tank due to the low liquid level of the pulp itself. At the same time, the excessive concentrated introduction of cleaning water may also dilute the sulfuric acid concentration in the pickling tank locally and momentarily. Preferably, the reserved ratio can be 5%, 8%, 10%, 12% or 15%, depending on the ratio of the volume to the internal surface area of ​​the mixing pot.

[0028] Step S12: Add the wet residue to the pulping pot, stir at high speed to form a uniform pulp, and collect multiple batches of pulped material for later use.

[0029] In this step, the high-speed agitator of the pulping tank is activated, with the rotation speed typically set at 200-400 rpm to provide sufficient shear force to break up the agglomerated structure of the wet sludge. The wet sludge is rapidly dispersed and separated in the water, forming a homogeneous slurry with good flowability. Since the volume of wet sludge processed at the front end is usually large, a single pulping operation may not be sufficient to meet the material requirements for a single batch of acid leaching. Therefore, a "multiple batches of pulped material for backup" approach is adopted. That is, the slurry prepared in the pulping tank is transferred in batches to a buffer tank or directly pumped to the acid leaching tank, and this process is repeated multiple times until the total amount of wet sludge required for a single batch of acid leaching is met. This multi-batch feeding not only improves the equipment's turnover rate but also avoids uneven mixing caused by excessive single-batch pulping. In a more specific implementation, to ensure consistency between batches, the volume of each batch of pulped material is precisely measured using a flow meter.

[0030] Step S13: After starting the high-speed stirring for the preset time, the pulped material is transported to the acid leaching tank, and the pulping pot is cleaned with the reserved cleaning water. The cleaning wastewater is also transported to the acid leaching tank.

[0031] In this step, after multiple batches of pulped material have been discharged, high-speed stirring continues for a preset time (e.g., 5-15 minutes) to ensure that the concentrated slurry remaining at the bottom of the pulping pot is fully suspended, preventing material accumulation in the dead zone. Subsequently, the pulped material is thoroughly transported to the acid leaching tank using a centrifugal pump or diaphragm pump. After the conveying pipeline is emptied, the cleaning water reserved in step S11 (i.e., the 5%-15% of the unadded water) is immediately used to perform high-pressure or circulating flushing of the inside of the pulping pot, the stirring blades, and the discharge pipe. The cleaning wastewater containing trace amounts of slurry generated during flushing is no longer waste but is also introduced into the acid leaching tank. This operation achieves zero discharge and full utilization of cleaning water, thoroughly cleaning the equipment and recovering 100% of the effective iron and phosphorus material adhering to the equipment walls back into the reaction system, significantly improving raw material utilization and the accuracy of material balance.

[0032] Step S14: Add a predetermined amount of sulfuric acid to the acid leaching kettle.

[0033] In this step, after the material transfer is complete, the stirring system of the acid leaching kettle is activated, and sulfuric acid of a predetermined concentration, such as 93% or 98% concentrated sulfuric acid, or pre-prepared dilute sulfuric acid, is slowly added to the kettle via an acid-resistant metering pump. The predetermined amount of sulfuric acid is precisely calculated based on the iron content in the wet slag and the stoichiometric ratio of the acid leaching reaction. During the acid addition process, a violent exothermic reaction occurs in the material inside the acid leaching kettle. Therefore, it is usually necessary to activate the cooling water system of the kettle jacket to remove the heat of reaction and control the temperature inside the kettle within a suitable acid leaching temperature range, such as 60℃~85℃, to prevent excessively high temperatures from causing phosphoric acid decomposition or localized overboiling.

[0034] Step S15: After all the sulfuric acid has been added, stir for a preset time, and then add water.

[0035] In this step, the addition of concentrated sulfuric acid releases a large amount of heat of dilution upon contact with water in the slurry. Immediately adding water could lead to extremely uneven concentrations in certain areas. Therefore, after all the sulfuric acid has been added, continuous stirring for a predetermined time, such as 10-20 minutes, is necessary to allow the sulfuric acid to fully diffuse in the slurry, making full contact with the surface of the wet slag particles and initiating a preliminary reaction. After stirring for the predetermined time, the system temperature and initial concentration distribution tend to stabilize. Subsequent water addition at this point avoids iron ion hydrolysis and precipitation due to excessively low local acidity, or phosphoric acid volatilization due to excessively high local acidity.

[0036] Step S16: Add crystallization mother liquor or pure water to the acid leaching kettle to make the total weight of the kettle material reach the set value, and then take a sample to test the phosphorus content.

[0037] In this step, water replenishment is not only to achieve the required liquid-to-solid ratio for the reaction, but also to provide a suitable concentration of ferric phosphate solution for subsequent synthesis processes. The preferred water replenishment medium is circulating water from the system, such as crystallization mother liquor or acid leaching washing water, to maximize water resource utilization and recover residual available phosphorus. When the circulating water is insufficient or the impurity content exceeds the standard, pure water is used for replenishment. Water replenishment is stopped once the weighing sensor in the acid leaching reactor reaches the set total weight. At this point, there may still be incompletely dissolved iron slag or localized concentration differences in the system, therefore, it is necessary to sample and test the phosphorus content in the slurry. Testing is typically performed using chemical titration or X-ray fluorescence spectrometry. By measuring the phosphorus content, the actual phosphoric acid concentration and liquid-to-solid ratio in the current slurry can be calculated, providing data support for precise water replenishment in the next step.

[0038] Step S17: Calculate and add the corresponding amount of water based on the detected phosphorus content to bring the material to the target reaction concentration.

[0039] In this step, based on the phosphorus content data obtained from sampling and testing in step S16, the difference between the current system and the target reaction concentration is calculated by substituting it into the pre-established reaction concentration model or water replenishment calculation formula. Due to fluctuations in the moisture content of the wet residue and minor errors in the preceding pulping operation, the actual concentration after the initial water replenishment often does not perfectly match the precise concentration required for the synthesis process.

[0040] This embodiment employs a closed-loop dynamic compensation mechanism of "detection-calculation-replenishment" to precisely replenish the corresponding amount of water into the acid leaching reactor, ensuring that the phosphorus content, acidity, and other indicators of the material accurately reach the target reaction concentration. This dynamic fine-tuning ensures extremely high consistency in the concentration of the ferric phosphate solution input to the downstream synthesis process, laying the most solid material foundation for the synthesis reaction to generate high-quality ferric phosphate precursors. After water replenishment and final stirring, a uniformly concentrated and fully reacted acid leaching slurry is obtained.

[0041] Step S2: Perform a single pressure filtration on the acid leaching slurry to obtain ferric phosphate solution.

[0042] In this embodiment, the acid leaching slurry obtained in step S1 contains dissolved ferric phosphate solution and unreacted residues such as silica and insoluble minerals. To obtain pure synthetic raw materials, solid-liquid separation is necessary. Single-stage filtration processes typically employ chamber filter presses or diaphragm filter presses. However, existing filtration operations often focus only on whether the final filter cake is compacted, neglecting the clarity of the filtrate at the initial feeding stage and the displacement efficiency of the washing process. This results in a large loss of soluble iron and phosphorus with the residue, and the filter cloth is easily clogged.

[0043] To improve the separation efficiency and product purity of primary pressure filtration, step S2 involves the following stages of primary pressure filtration of the acid leaching slurry: Feeding and filtration stage: Start the filter press circulation valve, and close the circulation valve after the filtrate is clear and transparent, and collect the corresponding acid leaching solution.

[0044] At this stage, the acid leaching slurry is pumped into the various filter chambers of the filter press. Initially, because the filter cloth capillaries have not yet established an effective filter cake layer, the filtrate passing through the filter cloth will contain a large amount of extremely fine suspended solids, appearing turbid. In this embodiment, the filter press's circulation valve is activated to return the initially turbid filtrate to the acid leaching slurry tank for further circulation and filtration. As the filter cake gradually thickens and forms a dense filter layer within the filter chambers, the filtrate gradually becomes clear and transparent. Once the operator or the automatic control system detects that the filtrate is clear and transparent using a turbidimeter, the circulation valve is closed, and the clear filtrate (i.e., the qualified ferric phosphate acid leaching solution) is directed into the designated acid leaching solution collection tank. This operation prevents suspended solids from entering subsequent synthesis processes and contaminating the product, ensuring extremely high clarity of the ferric phosphate solution.

[0045] Washing stage: Filtered brine is used as the washing solution first. If the water volume is insufficient, pure water is added. The filtrate after washing flows into the corresponding multiple washing tanks in sequence.

[0046] At this stage, in order to completely displace the acid leaching solution (i.e., available iron and phosphorus) remaining in the micropores of the filter cake, the filter cake needs to be washed with water. This embodiment employs a countercurrent washing or multi-stage washing strategy. The washing medium is not fresh pure water, but rather the brine produced in the downstream crystallization process. The brine mainly consists of dilute ammonium sulfate or dilute sodium sulfate solution containing a small amount of impurities, which is chemically inert to the acid leaching residue and can dissolve the remaining available iron and phosphorus. Fresh pure water is only added when the amount of brine is insufficient to complete the washing task. This strategy significantly reduces fresh water consumption and achieves cascaded utilization of wastewater within the system. The washed filtrate flows sequentially into multiple corresponding washing tanks, such as the first washing tank, the second washing tank, etc., according to its concentration gradient and impurity content. High-concentration washing water can be used for pulping and replenishment in the next batch of acid leaching, while low-concentration washing water is used for the initial washing in the next stage, achieving optimal utilization of water resources.

[0047] Pressing stage: Maintain the pressing state for 20-40 minutes.

[0048] At this stage, after washing, to further reduce the free water in the filter cake and recover the entrained acid leachate, the diaphragm pressing system of the filter press is activated. High-pressure water or compressed air is injected into the diaphragm plate, causing the diaphragm to inflate and compress the filter cake. In this embodiment, the pressing time is strictly controlled between 20 and 40 minutes. If the pressing time is less than 20 minutes, the capillary water inside the filter cake cannot be fully squeezed out, resulting in an excessively high moisture content in the filter cake. This not only increases the difficulty of subsequent waste residue treatment but also wastes the effective components entrained in the water. If the pressing time exceeds 40 minutes, the filter cake has been compacted to its limit, and further pressing cannot significantly reduce the moisture content. Instead, it wastes energy and accelerates the mechanical fatigue damage of the diaphragm plate and filter cloth. In a preferred embodiment, the pressing time can be 25 minutes, 30 minutes, or 35 minutes.

[0049] Side-blowing phase: Maintain the side-blowing state for 20-40 minutes, and open the right-side dark flow valve 1-3 minutes before the end of the side-blowing process.

[0050] At this stage, after pressing, a certain amount of moisture remains deep inside the filter cake and in the water outlet channels of the filter plates. Compressed air is turned on for side blowing, using high-pressure airflow to penetrate the filter cake channels and blow out the residual moisture. The side blowing time is also precisely controlled between 20 and 40 minutes to ensure that the moisture is completely removed, while avoiding over-drying that could cause the filter cake to crack and pulverize. More importantly, in this embodiment, the concealed flow valve on the right side of the filter press is opened in advance, for example, 1 to 3 minutes before the side blowing process is about to end. During the long side blowing process, the fine particles carried by the airflow are prone to crystallize and clump at the outlet of the concealed flow channel (i.e., the hidden drainage channel inside the filter plates) due to the drying of moisture. If the valve is closed completely to discharge material the moment the side blowing stops, it is very easy to cause blockage of the concealed flow channel. By opening the right-side concealed flow valve in advance, the final high-pressure airflow can forcefully blow out the fine particles and residual droplets that are about to crystallize from the channel, effectively cleaning and unblocking it. This thoroughly prevents channel blockage caused by crystallization, significantly extends the service life of the filter cloth and filter plate, and ensures smooth liquid flow for the next batch of filter press. After completing the side blowing, pull open the filter plate to remove the acid leaching residue and collect the pure ferric phosphate solution.

[0051] Step S3: Mix ferric phosphate solution with crystal inducing agent and curing agent to carry out synthesis reaction, and adjust the pH value of the material during the reaction. After the synthesis reaction is completed, perform secondary pressure filtration on the material to obtain synthetic filter cake.

[0052] In this embodiment, the synthesis of ferric phosphate solution is a complex precipitation and crystallization process that requires specific pH, temperature, and crystal form control conditions. Existing technologies often employ simple mechanical stirring and rapid ammonia addition, leading to sudden changes in local pH values ​​and the formation of amorphous precipitates or impurity particles.

[0053] To solve this problem, step S3 of this embodiment specifically includes: Add a measured amount of the ferric phosphate solution to the synthesis reactor, add an appropriate amount of filtered brine or acid leaching washing water, add crystal inducing agent and curing agent respectively, and turn on the stirring equipment to stir continuously for 3 to 10 minutes.

[0054] In this step, the pure ferric phosphate solution collected in S2 is pumped into the synthesis reactor. To adjust the initial concentration and ionic strength of the reaction system, an appropriate amount of filtered brine or acid leaching wash water is added. The circulating water within the system is utilized again, demonstrating the closed-loop design of the water system. Subsequently, a crystal inducing agent and a curing agent are added. The crystal inducing agent is, for example, small organic molecules or nanocrystals preferentially adsorbed on specific crystal faces. The curing agent is, for example, a cross-linking polymer or complexing agent that controls the growth rate and structure of the precipitate. After addition, the stirring equipment is turned on and stirred continuously for 3-10 minutes. This ammonia-free stirring time is extremely critical, as it ensures that the crystal inducing agent and curing agent are completely and uniformly dispersed in the system, providing an isotropic and uniform reaction field for subsequent ammonia precipitation and avoiding localized abnormal nucleation caused by uneven distribution of the inducing agent.

[0055] Before adding ammonia, the circulation system and ultrasonic equipment are turned on. Then, ammonia water is slowly added to the reactor to adjust the pH value of the material. Real-time sampling and testing are carried out. After confirming that the pH value reaches the target value, the material is subjected to secondary pressure filtration to obtain the synthetic filter cake.

[0056] In this step, the addition of ammonia is crucial for initiating the precipitation of ferric phosphate. In this embodiment, before adding ammonia, not only is a conventional material circulation pump activated to enhance macroscopic convection, but an ultrasonic device is also activated. The ultrasonic device, through an ultrasonic transducer installed on the outer wall or inside the reactor, emits high-frequency ultrasonic waves into the liquid phase, generating a strong cavitation effect. Under the action of the cavitation microjets, the ammonia water is instantly pulverized into micron- or even nano-sized droplets upon addition, achieving rapid mixing with the feed liquid at the microscopic level, completely eliminating the precipitation of ferric hydroxide impurities caused by excessively high local pH values. Subsequently, ammonia water is slowly added, and the pH is monitored in real-time using an online pH meter or by offline sampling. When the pH value reaches the target value, ammonia addition is stopped. The precipitate generated by the reaction grows into well-structured, uniformly sized ferric phosphate particles under the synergistic effect of the ultrasonic field and the crystal inducing agent. After the synthesis reaction is completed, a secondary pressure filtration process similar to step S2 (but with a different washing medium) is used to separate the solid and liquid components of the slurry.

[0057] Specifically, step S3, which involves secondary pressure filtration of the material after the synthesis reaction, includes: Feeding and filtration stage: Start the filter press circulation valve, and close the circulation valve after the filtrate is clear and transparent, and collect the corresponding mother liquor; Washing stage: Pure water is used as the washing medium for multiple washes, and the filtrate after washing flows into the corresponding multiple washing tanks in sequence. Pressing stage: Maintain the pressing state for 20-40 minutes; Side-blowing phase: Maintain the side-blowing state for 20-40 minutes, and open the right-side dark flow valve 1-3 minutes before the end of the side-blowing process; During periodic operation, after each preset batch of filter press and washing is completed, the washing tank is replenished with water to maintain a stable liquid level.

[0058] In this secondary filtration process, the mechanisms of feed filtration, pressing, and side-blowing stages are similar to those of the primary filtration process, but the focus differs. Since the synthesized product is ferric phosphate precipitate, its filter cake has high compressibility and density. Therefore, pure water must be used as the washing medium for multiple washes during the washing stage. If brine is used at this stage, impurity ions (such as sodium and sulfate ions) in the salt will be adsorbed on the surface of the ferric phosphate lattice or trapped in the micro-cracks of the filter cake, severely contaminating the precursor. Multiple pure water washes utilize the concentration gradient to completely displace free ammonium and sulfate ions from the mother liquor. The washed filtrate flows sequentially into multiple washing tanks. The clearer portion can be used for acid leaching and pulping, while the more turbid portion serves as high-salt water for subsequent processing. During periodic operation, to ensure the liquid level balance and stable washing pressure difference in the multi-stage washing tanks, the washing tanks are automatically replenished after each preset batch (e.g., 3-5 batches) of filtration and washing to maintain a stable liquid level and prevent disruption of the washing flow field due to water evacuation.

[0059] After the secondary filtration, the resulting synthetic filter cake, while possessing the preliminary chemical composition of iron phosphate, is still far from optimal in terms of microstructure, crystal integrity, and impurity arrangement as a precursor for lithium-ion battery cathode materials. The particles inside the filter cake often exhibit severe agglomeration and compaction due to the high-pressure compression during filtration, resulting in insufficient fusion of grain boundaries. Direct drying or sintering would lead to a sharp decrease in the specific surface area and a reduction in electrochemical active sites in the final product. Therefore, step S4, a pulping and aging process, is necessary to reshape the crystal growth environment and improve the crystal structure.

[0060] Step S4: The synthetic filter cake is subjected to synthetic pulping reaction and crystallization aging treatment in sequence to obtain a slurry containing ferric phosphate precursor.

[0061] In this embodiment, step S4 is a key process for deeply reconstructing the physicochemical properties of the material after secondary pressure filtration. This process is specifically divided into two sub-stages with different thermodynamic and kinetic objectives: synthesis pulping reaction and crystallization aging treatment.

[0062] First, regarding the synthetic pulping reaction, it specifically includes: Add crystallization replenishing liquid to the synthesis pulping kettle, then add a measured amount of the synthesis filter cake, and turn on the steam heating system to control the material temperature; Phosphoric acid and curing agent are added in sequence, and the pH value of the reaction system is tested. The pH value is adjusted to the target range using sulfuric acid. After passing the retest, the material is fed into the crystallization reactor.

[0063] During this stage, the synthesized filter cake, due to high-pressure pressing, has an extremely dense internal structure. If it is directly added to pure water for pulping, it is very easy to form a large number of pseudo-particles or clumps with an outer water film, preventing the internal dry powder from being exposed. Therefore, this embodiment does not use pure water, but adds a crystallization replenishing solution. This crystallization replenishing solution is usually a buffer solution containing a specific concentration of phosphate ions or a specific pH value. Its ionic strength matches the Zeta potential of the iron phosphate particle surface, and it can quickly penetrate into the microcracks inside the filter cake through osmotic pressure in the early stage of pulping, achieving rapid wetting and deagglomeration of the particles.

[0064] After the synthetic filter cake is added, the steam heating system is turned on. Steam exchanges heat through coils installed in the jacket or inside the pulping vessel, precisely controlling the material temperature within a suitable range, typically 50℃~60℃. This temperature range provides the thermal kinetic energy needed to break the hydrogen bonds between particles without being too high and triggering premature irreversible crystallization phase transitions.

[0065] During the stirring and dispersion process, phosphoric acid and a curing agent are added sequentially. The purpose of adding phosphoric acid at this stage is to replenish any trace amounts of phosphorus that may have been lost during the secondary pressure filtration and washing process, ensuring a precise stoichiometric Fe / P molar ratio. Simultaneously, the multidentate coordination of phosphate ions etches and trims the particle surface, eliminating sharp angles and defects generated in the previous process. The addition of the curing agent further enhances the steric hindrance effect on the particle surface, preventing secondary agglomeration during subsequent heating. After the addition of materials, the pH value of the reaction system is monitored in real time using an online pH meter. Since the addition of acid and the dissolution of materials alter the system's pH, high-purity sulfuric acid must be used to adjust the pH value to the target range, such as 1.5–2.5. The introduction of sulfuric acid not only adjusts the pH value but also provides the specific anion background intensity required to maintain stable crystal growth. After adjustment, a closed-loop verification process with successful retesting is mandatory to prevent misjudgments caused by pH meter probe delays or localized unevenness, ensuring that the material fed into the crystallization reactor is in an absolutely perfect initial reaction state.

[0066] Secondly, regarding crystallization aging, it specifically includes: Heat the material in the crystallization reactor to 95~98℃, turn on the heat preservation system and start timing; After keeping the material warm for 1-2 hours, take a sample to test its condition, and add phosphoric acid according to the set ratio based on the test results. Continue to keep warm for 1-2 hours, with the total holding time controlled within the range of 2-4 hours, to complete the crystallization process; The material is fed into an aging reactor for aging to obtain the slurry containing the iron phosphate precursor.

[0067] During this stage, the material is rapidly heated to 95-98°C, a temperature very close to the boiling point of aqueous solutions at normal pressure. At such high temperatures, the solubility of the ferric phosphate precursor particles increases significantly, and the system enters the Ostwald ripening stage of "dissolution-recrystallization." Small particles dissolve preferentially due to their high surface energy, and the solute, driven by the concentration gradient, deposits onto the surface of larger particles, promoting further growth and spheroidization of the larger particles. Simultaneously, the high temperature accelerates the ion rearrangement within the crystal lattice, causing an irreversible transformation from amorphous or weakly crystalline phases to thermodynamically more stable phases (such as quasi-ferric phosphate or the target crystalline phase of specific hydrates).

[0068] Turn on the heat preservation system and strictly time it. After heat preservation for 1-2 hours, the particle growth reaches a critical plateau. At this time, sample and test the material state. The D50 particle size and distribution span can be monitored by a laser particle size analyzer, or the diffraction peak intensity can be monitored by XRD. Based on the test results, accurately replenish phosphoric acid according to the set ratio. In the early stage of high-temperature ripening, some phosphate ions may escape in gaseous form or segregate at the particle growth interface, resulting in phosphorus depletion at the interface. Replenishing phosphoric acid at this time can accurately repair oxygen and phosphorus vacancies in the crystal lattice, further improving the integrity of the crystal lattice.

[0069] After adding phosphoric acid, continue heating for 1-2 hours to allow the newly added solute to fully integrate into the crystal lattice and complete interface fusion. The total heating time is strictly controlled within the range of 2-4 hours. If the total time is less than 2 hours, curing is incomplete, and the crystal form still has a large number of defects, which will require more energy to achieve phase transition during subsequent sintering, and the electrochemical kinetics of the product will be sluggish. If the total time exceeds 4 hours, excessive particle growth will occur, the particle size will be too large, or even severe hard agglomeration will occur, and the specific surface area will decrease sharply. This will not only affect the rate performance of the battery, but also cause the slurry viscosity to increase dramatically, placing a great mechanical load on subsequent pumping and pressure filtration.

[0070] After the crystallization process is complete, the material is fed into an aging reactor for cooling and aging. Aging is usually carried out with slow stirring, and the temperature is gradually reduced from high temperature to room temperature. This cooling process stabilizes the lattice contraction, eliminates internal stress, and ultimately yields a slurry containing iron phosphate precursor with good dispersibility, perfect crystal form, and uniform particle size distribution.

[0071] Step S5: The slurry containing the ferric phosphate precursor is subjected to three pressure filters to obtain the ferric phosphate precursor filter cake.

[0072] After preparing a high-quality slurry, it is essential to thoroughly separate the solid precursors from the mother liquor, which contains a large amount of ammonium sulfate, free acid, and organic impurities. Because the slurry, after high-temperature aging, has extremely high viscosity and the particles exhibit certain colloidal characteristics, conventional pressure filtration easily leads to instantaneous blinding of the filter cloth, resulting not only in a sharp increase in filtration resistance but also in the extreme difficulty of washing away the high-salt impurities entrained in the filter cake. In this embodiment, a three-stage pressure filtration strategy optimized for the precursor characteristics is employed in step S5.

[0073] Specifically, the slurry containing the ferric phosphate precursor is subjected to three pressure filtration processes, including: Feeding and filtration stage: Start the filter press circulation valve, and close the circulation valve after the filtrate is clear and transparent, and collect the corresponding mother liquor; Washing stage: Multiple washes are performed using a washing medium, and the filtrate after washing flows into the corresponding washing tanks in sequence. Pressing stage: Maintain the pressing state for 20-40 minutes; Side-blowing phase: Maintain side-blowing for 20-40 minutes, and open the right-side dark flow valve 1-3 minutes before the side-blowing ends.

[0074] The feeding filtration stage is similar to the aforementioned pressure filtration. The initial turbid liquid is returned to the crystallization kettle through the circulation valve to ensure that only the clear mother liquor after a stable filter cake layer is established enters the collection tank. This mother liquor is rich in high-concentration ammonium sulfate and can be recycled as a by-product.

[0075] The water washing stage is the core of the three-stage filtration process. Due to the extremely strong adsorption capacity of the precursor filter cake for impurities, this embodiment employs a multi-stage countercurrent multiple water washing strategy. The washing medium (preferably pure water or low-salt recycled water) is injected into the filter chamber multiple times. After each injection, the filter is allowed to stand for a certain period to allow the high-salt mother liquor in the filter cake capillary to reach concentration diffusion equilibrium with the washing water before the next filtration replacement. The washed filtrate flows into multiple corresponding washing tanks according to concentration gradients. The high-salt washing water goes to the high-salt water treatment process, while the low-salt washing water serves as the washing source for the aforementioned secondary or primary filtration, forming a closed-loop water resource system.

[0076] During the pressing stage, the diaphragm applies high pressure to the precursor filter cake. Due to the plasticity of the precursor particles, this stage, lasting 20-40 minutes, can completely squeeze out the free water inside the filter cake, reducing the final filter cake moisture content to an extremely low level, typically below 30%. This greatly reduces the burden on subsequent drying equipment and lowers energy consumption.

[0077] During the side-blowing stage, the strategy of opening the right-side dark flow valve 1-3 minutes in advance is particularly crucial because the precursor slurry is prone to scaling in the dark flow channel. The airflow not only removes moisture but also blows the trace amounts of precursor powder adhering to the dark flow channel wall back into the filter chamber, preventing the high-value precursor from caking and being lost in the pipeline, and also eliminating the heavy manual labor of frequently disassembling the machine to unclog the pipeline.

[0078] After completing step S5, the front-end processing of the main material is complete. However, as a complete industrial closed-loop system, the waste generated during the process, if left untreated, will not only cause environmental pollution but also waste valuable resources. In particular, the water washing stage in step S5 generates a large amount of complex high-salt water to thoroughly clean the filter cake. If this water is directly fed into the MVR (Mechanical Vapor Recompression) evaporation and crystallization system, trace amounts of iron ions and heavy metal impurities will cause scaling in the evaporator and may even contaminate the quality of the final crystallized salt.

[0079] Therefore, after step S5, this application adds a high-salinity water treatment process to achieve a dual closed loop of system materials and water system: the high-salinity water generated in the three-stage pressure filtration washing stage is added to ammonia water to adjust the pH value and then subjected to pressure filtration treatment to achieve resource recovery and solid-liquid separation.

[0080] Specifically, the high salinity treatment process includes: Ammonia solution was gradually added dropwise to the high-salt water, and the pH value of the solution was monitored in real time. The pH value was then precisely adjusted to between 8.5 and 9.5. Pump the high-salt water with the adjusted pH value into the filter press, start the filter press circulation valve, and when the circulating filtrate becomes clear and transparent, close the circulation valve and open the feed valve to perform filter press.

[0081] High-salinity water typically contains large amounts of ammonium sulfate and sodium sulfate, as well as trace amounts of incompletely precipitated ferric and ferrous ions, and trace amounts of heavy metal complexes that may be introduced during the synthesis stage. Gradually adding ammonia to high-salinity water not only neutralizes the free acid in the water but also causes the trace ferric ions to undergo strong hydrolysis, forming ferric hydroxide colloidal precipitate. Simultaneously, in a weakly alkaline environment (pH 8.5–9.5), polyvalent metal ions in the water (such as calcium and magnesium ions, if present) can also be co-precipitated and removed as hydroxides or basic salts. If the pH is below 8.5, incomplete precipitation of metal ions will still threaten the safe operation of subsequent evaporation and crystallization equipment; if the pH is above 9.5, excessive ammonia will not only cause ammonia volatilization and waste, and air pollution, but will also generate a large amount of ammonia-rich secondary steam during subsequent evaporation, severely corroding the equipment.

[0082] After the hydroxide precipitates from these tiny suspended particles, the system essentially forms an extremely dilute suspension. At this point, a high-pH brine solution is pumped into a dedicated filter press. Because the suspended particles are extremely fine and present in low concentrations, they easily penetrate the filter cloth; therefore, the circulation valve must be activated to return the initial filtrate to the original water tank for further filtration. When a sufficiently thick layer of ferric hydroxide sludge is trapped on the filter cloth surface, forming a dense filtration layer that makes the circulating filtrate clear and transparent, the filtration process is complete. At this point, the circulation valve is closed, and the feed valve is opened to begin formal filtration.

[0083] The clarified filtrate separated by pressure filtration is mainly composed of pure ammonium sulfate and sodium sulfate mixed brine, which fully meets the feed standards for industrial-grade evaporation and crystallization and can be directly pumped to the subsequent evaporation and crystallization system to recover industrial salt. The retained filter cake (rich in trace metal impurities such as ferric hydroxide) can be periodically unloaded, collected, and returned to the pulping process in step S1 for re-acid leaching and purification. This design not only completely solves the problem of impurity contamination from high-salt water but also achieves the resource recovery of trace iron in the waste residue, realizing the complete utilization of materials throughout the entire process.

[0084] In summary, the dynamic water replenishment budget of acid leaching in step S1 eliminates the blind spot of concentration fluctuation; the refined pressure filtration operation in steps S2 and S3 ensures the purity and separation efficiency of the material; the thermodynamic and kinetic synergistic regulation in step S4 creates a perfect crystal structure; the efficient pressing and side-blowing anti-clogging logic in step S5 significantly reduces energy consumption and equipment failure rate; and the subsequent high-salt water addition and ammonia pressure filtration purification closes the loop of water resource and salt recovery.

[0085] Unlike existing technologies, this application discloses a front-end treatment process for preparing iron phosphate. In the S1 acid leaching process, this application employs a multi-stage precise control mechanism combining reserved cleaning water and dynamically calculated water replenishment. This completely eliminates concentration fluctuations caused by the blind introduction of cleaning water from the pulping equipment, ensuring the acid leaching reaction remains within the optimal liquid-to-solid ratio range, significantly improving the leaching rate of iron slag and the stability of the reaction system. Furthermore, by specifically designing refined and coordinated operations for feed filtration, washing, pressing, and side blowing in the three-stage pressure filtration processes (S2, S3, and S5), this not only maximizes the removal of free acid and impurity ions by utilizing differentiated washing strategies of filtered brine and pure water, but also significantly reduces [the following text is incomplete and requires further context: "by pressing for a specific duration and side blowing operation with the dark flow valve opened in advance."] The filter cake moisture content was reduced, and pipeline crystallization blockage was effectively prevented. Furthermore, the synergistic effect of the circulation system and ultrasonic equipment was introduced into the S3 synthesis process to ensure the uniformity of materials and the consistency of crystal growth during pH adjustment. In the S4 pulping and aging process, the perfect growth and morphology control of precursor crystals were achieved through stepped heating and staged heat preservation and acid supplementation, which significantly improved the purity and crystal integrity of the iron phosphate precursor. Finally, by innovatively adding a high-salt water treatment process, the high-salt water generated from precursor washing was adjusted to pH by adding ammonia and pressure filtration to achieve valuable metal precipitation and salt water resource recovery, thus completely closing the material flow of the front-end treatment process.

[0086] The above description is merely an embodiment of this application and does not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A pretreatment process for preparing ferric phosphate, characterized in that, Includes the following steps: S1: After pulping the wet slag, it is put into an acid leaching kettle, sulfuric acid is added to carry out the acid leaching reaction, and water is dynamically added according to the test results during the reaction to obtain acid leaching slurry; S2: The acid leaching slurry is subjected to a single pressure filtration to obtain ferric phosphate solution; S3: The ferric phosphate solution is mixed with a crystal inducing agent and a curing agent to carry out a synthesis reaction. During the reaction, the pH value of the material is adjusted. The material after the synthesis reaction is completed is subjected to secondary pressure filtration to obtain a synthetic filter cake. S4: The synthetic filter cake is subjected to a synthetic pulping reaction and crystallization aging treatment in sequence to obtain a slurry containing ferric phosphate precursor; S5: The slurry containing the ferric phosphate precursor is subjected to three pressure filters to obtain the ferric phosphate precursor filter cake.

2. The pretreatment process for preparing ferric phosphate according to claim 1, characterized in that, Step S1 includes: S11: Add the predetermined amount of water to the pulping pot. The actual amount of water added should be 5% to 15% less than the theoretical total amount of water added, to reserve water for cleaning. S12: Add the wet residue to the pulping pot, stir at high speed to form a uniform pulp, and collect multiple batches of pulped material for later use; S13: After the high-speed stirring is started and the preset time is continued, the pulped material is transported to the acid leaching tank, and the pulping pot is cleaned with the reserved cleaning water. The cleaning wastewater is also transported to the acid leaching tank. S14: Add the predetermined amount of sulfuric acid to the acid leaching kettle; S15: After all the sulfuric acid has been added, stir for the preset time, and then perform the water replenishment operation; S16: Add crystallization mother liquor or pure water to the acid leaching kettle to make the total weight of the kettle material reach the set value, and then take a sample to test the phosphorus content; S17: Calculate and replenish the corresponding amount of water based on the detected phosphorus content to bring the material to the target reaction concentration.

3. The pretreatment process for preparing ferric phosphate according to claim 1, characterized in that, Step S3 includes: Add a measured amount of the ferric phosphate solution to the synthesis reactor, add an appropriate amount of filtered brine or acid leaching washing water, add crystal inducing agent and curing agent respectively, and turn on the stirring equipment to stir continuously for 3 to 10 minutes; Before adding ammonia, the circulation system and ultrasonic equipment are turned on. Then, ammonia water is slowly added to the reactor to adjust the pH value of the material. Real-time sampling and testing are carried out. After confirming that the pH value reaches the target value, the material is subjected to secondary pressure filtration to obtain the synthetic filter cake.

4. The pretreatment process for preparing ferric phosphate according to claim 3, characterized in that, In step S4, the synthetic pulping reaction includes: Add crystallization replenishing liquid to the synthesis pulping kettle, then add a measured amount of the synthesis filter cake, and turn on the steam heating system to control the material temperature; Phosphoric acid and curing agent are added in sequence, and the pH value of the reaction system is tested. The pH value is adjusted to the target range using sulfuric acid. After passing the retest, the material is fed into the crystallization reactor.

5. The pretreatment process for preparing ferric phosphate according to claim 4, characterized in that, In step S4, the crystallization aging includes: Heat the material in the crystallization reactor to 95~98℃, turn on the heat preservation system and start timing; After keeping the material warm for 1-2 hours, take a sample to test its condition, and add phosphoric acid according to the set ratio based on the test results. Continue to keep warm for 1-2 hours, with the total holding time controlled within the range of 2-4 hours, to complete the crystallization process; The material is fed into an aging reactor for aging to obtain the slurry containing the iron phosphate precursor.

6. The pretreatment process for preparing ferric phosphate according to claim 1, characterized in that, In step S2, the first pressure filtration of the acid leaching slurry includes: Feeding and filtration stage: Start the filter press circulation valve, and close the circulation valve after the filtrate is clear and transparent, and collect the corresponding acid leaching solution; Washing stage: Filtered brine is used as the washing solution first. If the water volume is insufficient, pure water is added. The filtrate after washing flows into the corresponding multiple washing tanks in sequence. Pressing stage: Maintain the pressing state for 20-40 minutes; Side-blowing phase: Maintain side-blowing for 20-40 minutes, and open the right-side dark flow valve 1-3 minutes before the end of the side-blowing process.

7. The pretreatment process for preparing ferric phosphate according to claim 1, characterized in that, In step S3, the secondary pressure filtration of the material after the synthesis reaction is completed includes: Feeding and filtration stage: Start the filter press circulation valve, and close the circulation valve after the filtrate is clear and transparent, and collect the corresponding mother liquor; Washing stage: Pure water is used as the washing medium for multiple washes, and the filtrate after washing flows into the corresponding multiple washing tanks in sequence. Pressing stage: Maintain the pressing state for 20-40 minutes; Side-blowing phase: Maintain the side-blowing state for 20-40 minutes, and open the right-side dark flow valve 1-3 minutes before the end of the side-blowing process; During periodic operation, after each preset batch of filter press and washing is completed, the washing tank is replenished with water to maintain a stable liquid level.

8. The pretreatment process for preparing ferric phosphate according to claim 1, characterized in that, In step S5, the slurry containing the iron phosphate precursor is subjected to three pressure filtrations, including: Feeding and filtration stage: Start the filter press circulation valve, and close the circulation valve after the filtrate is clear and transparent, and collect the corresponding mother liquor; Washing stage: Multiple washes are performed using a washing medium, and the filtrate after washing flows into the corresponding washing tanks in sequence. Pressing stage: Maintain the pressing state for 20-40 minutes; Side-blowing phase: Maintain side-blowing for 20-40 minutes, and open the right-side dark flow valve 1-3 minutes before the side-blowing ends.

9. The pretreatment process for preparing ferric phosphate according to claim 1, characterized in that, Following step S5, a high-salinity water treatment process is also included: The high-salinity water produced during the three-stage pressure filtration and washing stage is adjusted to pH value with ammonia water before being subjected to pressure filtration to achieve resource recovery and solid-liquid separation.

10. The pretreatment process for preparing iron phosphate according to claim 9, characterized in that, The process of adding ammonia to adjust the pH value of the high-salt water generated during the three-stage pressure filtration washing stage before pressure filtration includes: Ammonia solution was gradually added dropwise to the high-salt water, and the pH value of the solution was monitored in real time. The pH value was then precisely adjusted to between 8.5 and 9.

5. Pump the high-salt water with the adjusted pH value into the filter press, start the filter press circulation valve, and when the circulating filtrate becomes clear and transparent, close the circulation valve and open the feed valve to perform filter press.

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