Method for continuously preparing battery-grade iron phosphate by taking raffinate acid as raw material

Abstract

The invention relates to the technical field of iron phosphate production, and discloses a method for continuously preparing battery-grade iron phosphate by taking raffinate acid as a raw material, which comprises the following steps: 1) dissolving ferrous sulfate heptahydrate, and adding ammonia water to remove impurities to obtain a ferrous sulfate solution A; 2) diluting the raffinate acid, adding ammonia water and a fluorination auxiliary agent, and removing impurities to obtain a mixed solution B; 3) adding an oxidant and a pH regulator into the mixed solution B to obtain a mixed solution C; 4) adding the ferrous sulfate solution A into the first-stage reaction kettle at a constant speed, continuously adding the solution C into the first-stage reaction kettle, the second-stage reaction kettle and the third-stage reaction kettle which are connected in series in a split-flow manner for synthesis, and filtering and washing obtained slurry to obtain a filter cake; and 5) mixing the filter cake, adding phosphoric acid, heating for crystal transformation, discoloring to obtain aged slurry, and filtering, washing, drying, calcining and crushing to obtain the battery-grade ferric phosphate. According to the method, the low-cost raffinate acid is used for replacing high-cost monoammonium phosphate or phosphoric acid and other raw materials, and a continuous process is combined, so that the cost is remarkably reduced, and the batch stability and quality of the product are improved.

Description

Technical Field

[0001] This invention relates to the field of iron phosphate production technology, and in particular to a method for the continuous preparation of battery-grade iron phosphate using residual raffinate as raw material. Background Technology

[0002] In recent years, the new energy industry has experienced explosive growth, leading to overcapacity in the sector. Iron phosphate producers face multiple challenges, resulting in a tense overall situation. Among these challenges are rising raw material prices, increasing energy costs, and stricter environmental regulations. Intense market competition has led to continuously declining product prices, severely squeezing profit margins. The industry urgently needs to reduce costs and enhance market competitiveness through technological innovation and process optimization. Currently, the mainstream process for iron phosphate production is the ammonium process, which typically uses battery-grade or high-quality monoammonium phosphate as the phosphorus source. Since the phosphorus source accounts for over 30% of the production cost, the high production costs, coupled with overcapacity and continuously declining product prices, contribute to persistently high production costs.

[0003] Raffinate is a byproduct of the production of refined phosphoric acid, mainly derived from the residual liquid after solvent extraction for purifying wet-process phosphoric acid. Producing 1 ton of refined phosphoric acid with a P2O5 content of 46%–54% will generate 1 ton of raffinate. Because impurities from crude phosphoric acid (such as iron, aluminum, and magnesium ions) are enriched in the raffinate during extraction, it has a high content of metal impurities. In the liquid-phase precipitation synthesis of ferric phosphate, precise control of the reaction system's pH value has a significant impact on ferric phosphate crystal formation and product quality. On the one hand, ensuring the pH value of the reaction system is within the optimal precipitation range for ferric phosphate is crucial for efficient precipitation of the target product. On the other hand, improper pH control must be avoided to prevent the simultaneous precipitation of most impurity ions, which would adversely affect product purity and make it difficult for battery-grade ferric phosphate products to meet the needs of downstream customers. To further improve product quality and production efficiency, continuous production processes for ferric phosphate have been actively researched and developed. Continuous production avoids the frequent start-up and shutdown operations of traditional intermittent production, reducing energy consumption and time losses, significantly improving production efficiency, lowering labor costs, and facilitating strict control over reaction conditions such as temperature, time, and flow rate. This makes the production process more stable and helps improve the consistency of ferric phosphate product quality.

[0004] Existing methods for reducing the production cost of ferric phosphate often employ the substitution of raw materials to lower raw material costs. For example, the method disclosed in CN117585653A uses phosphate rock, pyrite, phosphide slag, low-quality acid, and agricultural monoammonium phosphate as phosphorus sources after acid dissolution, with phosphorus content below 20%; and uses acid-dissolved pyrite, pyrite slag, and waste iron blocks as iron sources. Crude ferric phosphate is prepared through mixing, oxidation, and precipitation steps. This method directly uses substances with excessively high impurity content as raw materials, resulting in impurity elements being incorporated into the ferric phosphate product while generating ferric phosphate in a complex environment with many impurities. When it is necessary to further improve the purity of the product to meet the requirements of downstream customers, the difficulty of impurity removal increases significantly. Furthermore, this method requires multiple precipitation and washing processes throughout the entire process, resulting in a large water consumption.

[0005] For example, the method disclosed in CN118405676A involves three-stage countercurrent extraction of raffinate, followed by back-extraction, concentration and defluorination, chemical defluorination, re-extraction, and re-back-extraction to obtain an NH4H2PO4 solution. The resulting raw material reacts with ferrous sulfate to produce ferric phosphate. However, this method requires repeated extraction of the raffinate, which cannot be used directly and requires pre-extraction and purification to meet the requirements of ferric phosphate synthesis. The expensive extractant and the cumbersome process of multiple extractions and back-extractions increase production costs, making it unsuitable for large-scale industrial production. The ferric phosphate produced by this method has a high magnesium impurity content and residual extractant. The presence of these impurities affects the electrical performance of the subsequently prepared battery materials.

[0006] Existing continuous methods for preparing ferric phosphate have the following problems: For example, in the method disclosed in CN118619227A, iron and phosphate salts are mixed in a 1:1 ratio in the reactor, leading to rapid reaction of the materials within the reactor. The high ion concentration results in severe impurity coating during the formation of ferric phosphate crystals, poor material uniformity, and difficulty in controlling the reaction. Although the method disclosed in CN117263154A also uses a continuous reaction method with the raw materials for ferric phosphate synthesis added simultaneously, the excessively high pH value during the synthesis process easily produces a large amount of the byproduct ferric hydroxyphosphate, affecting the quality of the ferric phosphate product. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for the continuous preparation of battery-grade iron phosphate using residual phosphate raffinate as a raw material. This method uses residual phosphate raffinate as the phosphorus source and ferrous sulfate as the iron source, continuously processing the two raw materials to obtain a stable and consistent iron phosphate synthesis raw material. Battery-grade iron phosphate is then obtained through a continuous iron phosphate preparation process. This method is suitable for replacing monoammonium phosphate with residual phosphate while fully ensuring the performance stability of each batch of products in continuous production.

[0008] The solution of the present invention is: A method for the continuous preparation of battery-grade iron phosphate using residual raffinate as a raw material includes the following steps: 1) Ferrous sulfate heptahydrate, ammonia and pure water are continuously fed into a multi-stage series ferrous reaction tank for impurity removal. The reaction slurry is filtered to obtain ferrous sulfate solution A. 2) The residual acid, ammonia, fluorination aid and pure water are continuously fed into the phosphate salt reaction tank for impurity removal reaction. The reaction slurry is filtered to obtain phosphate salt solution B, which is mixed with oxidant and ammonia to obtain mixed solution C. 3) Ferrous sulfate solution A is continuously added to the primary reactor at a constant flow rate. Mixed solution C is split and continuously and simultaneously added to the primary reactor, secondary reactor, and tertiary reactor for synthesis reaction. The resulting synthesis slurry is filtered and washed to obtain a synthesis filter cake. The primary reactor, secondary reactor, and tertiary reactor are set in series. 4) The synthesized filter cake is continuously fed to the slurry preparation tank. After the pure water and phosphoric acid are added to complete the dispersion, the resulting slurry is continuously fed into the first-stage aging tank and the second-stage aging tank connected in series for crystallization aging. After aging, the slurry is filtered, washed, dried, calcined and crushed to obtain battery-grade iron phosphate.

[0009] As a preferred technical solution, the impurity removal reaction temperature in both step 1) and step 2) is 55-65℃; the pH value of the impurity removal reaction in step 2) is 3-7.

[0010] As a preferred technical solution, the multi-stage series ferrous reactor consists of ferrous reactor 1, ferrous reactor 2, and ferrous reactor 3 connected in sequence, with the residence time of the material in each ferrous reactor being 20 to 30 minutes.

[0011] As a preferred technical solution, the residence time of materials in the primary reactor, secondary reactor, and tertiary reactor is 10-20 min, respectively; and the residence time of materials in the primary aging reactor and secondary aging reactor is 40-60 min, respectively.

[0012] As a preferred technical solution, the P2O5 concentration of the residual acid is 20% to 50%.

[0013] As a preferred technical solution, the fluorination aid is ammonium fluoride or sodium fluorosilicate, and the added mass of the fluorination aid is 0.5% to 3% of the mass of the residual acid.

[0014] As a preferred technical solution, the oxidant is hydrogen peroxide, and the amount of oxidant used is related to the amount of Fe in the ferrous sulfate solution A. 2+ The molar ratio is 1.1 to 1.4.

[0015] As a preferred technical solution, the pH value of the mixed solution C is 6.5-7.5; the ratio of the added flow rate of ferrous sulfate solution A to that of the mixed solution C is 1.5-3:1; the ratio of the added flow rate of the mixed solution C in the primary reactor, secondary reactor, and tertiary reactor is 4-6:3-2:3-2; the constant temperature of the primary reactor, secondary reactor, and tertiary reactor is 50-60℃; and the pH value of the reaction system is 1.5-2.5.

[0016] As a preferred technical solution, the constant temperature of the slurry preparation tank is 40-60℃, and the pH value of the resulting slurry is controlled to be 1.0-2.5 by adding phosphoric acid; the constant temperature of the primary aging tank and the secondary aging tank is 85-95℃.

[0017] Compared with the prior art, the advantages of the present invention are: (1) Low cost and environmentally friendly: This invention uses residual acid to replace monoammonium phosphate in the preparation of iron phosphate, which reduces production costs. Residual acid is widely available and has a stable supply, which can effectively ensure the sustainability of raw materials. Secondly, this alternative makes full use of chemical by-products, improves the resource recycling rate, and meets the requirements of green and sustainable development. In addition, the use of residual acid also reduces the dependence on traditional chemical raw materials. This method eliminates the need for any pretreatment of the residual acid, allowing for direct reaction and reducing pretreatment steps. It is suitable for continuous production using residual acid as raw material, yielding a product with a particle size of D50 = 1.65 μm and low impurity content (each impurity ion content is only S 73.81 mg / Kg, Na 28.41 mg / Kg, Mg 32.47 mg / Kg, Ca 18.81 mg / Kg, Al 15.88 mg / Kg, Zn 15.43 mg / Kg, Ni 2.73 mg / Kg, Cr 1.64 mg / Kg, Mn 31.62 mg / Kg), thus producing ferric phosphate with the required impurity content.

[0018] (2) High product quality and high production efficiency: The continuous synthesis process of this invention can effectively improve the consistency and purity of the product, meeting the downstream customers' demand for high-quality materials. Secondly, production efficiency is significantly improved, reducing start-up and shutdown time and energy consumption in intermittent processes, and lowering overall production costs. In addition, it can reduce manual operation, achieve a high degree of automation, and reduce quality fluctuations and safety risks caused by human intervention.

[0019] (3) Stable and easily scalable process control: This invention achieves continuous operation of the entire process from raw material processing to synthesis. This method enables precise flow control, ensuring stable delivery and uniform mixing of raw materials, avoiding product performance fluctuations caused by batch-to-batch instability, and further improving production efficiency and product quality. Simultaneously, this method reduces material loss and time waste, lowers production costs, and facilitates the automation and digitization of the entire production process, enhancing process controllability and safety, and meeting the demands of large-scale and high-quality production. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention 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 embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the continuous preparation process of battery-grade iron phosphate using residual acid as raw material according to the present invention. Figure 2 This is a schematic diagram of the process of materials passing through each stage of the reaction vessel in Embodiment 1 of the present invention. Detailed Implementation

[0022] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to specific embodiments.

[0023] Example 1: Ferrous sulfate heptahydrate, ammonia, and hot pure water are continuously fed into ferrous sulfate reaction tank 1 and then sequentially through ferrous sulfate reaction tanks 2 and 3 to complete the impurity removal reaction. Ferrous sulfate reaction tanks 1, 2, and 3 are connected by pumps and pipelines and maintained at a constant temperature of 60°C for the reaction. The resulting slurry is filtered by plate and frame filter press A to obtain ferrous sulfate solution A. The iron content of ferrous sulfate solution A is measured to be 4.50%. The solution is preheated to 45°C and stored in a ferrous sulfate storage tank for later use.

[0024] Residual raffinate (P2O5 concentration of 30%), ammonia, fluorination aid (ammonium fluoride), and hot pure water are continuously fed into phosphate reaction tank A, and then sequentially passed through phosphate reaction tanks B and C to complete the impurity removal reaction. Phosphate reaction tanks A, B, and C are connected in sequence by pumps and pipelines. The residence time of the reaction slurry in each of the three tanks is 20 minutes, the pH value is 5.0±0.2, and the temperature of each phosphate reaction tank is kept constant at 60℃. After the reaction, the slurry enters plate and frame filter press B and is filtered to obtain phosphate solution B. The fluorination aid is ammonium fluoride, and the added mass is 2% of the mass of residual raffinate. The phosphorus content of phosphate solution B is 5.50%. After being fed into the mixed liquid storage tank, it is mixed with hydrogen peroxide and ammonia to obtain mixed solution C, which is stored in the mixed liquid storage tank.

[0025] Ferrous sulfate solution A and mixed solution C are simultaneously added to the primary reactor at flow rates of 60 g / min and 18 g / min, respectively. When the liquid level in the reactor reaches 80% of its volume, the discharge valve is opened, and the slurry is transported from the primary reactor to the secondary reactor. At the same time, mixed solution C enters the secondary reactor at a flow rate of 6 g / min. When the liquid level in the secondary reactor reaches 80%, the discharge valve is opened, and the slurry is transported from the secondary reactor to the tertiary reactor. Simultaneously, mixed solution C enters the tertiary reactor at a flow rate of 6 g / min. When the liquid level in the tertiary reactor reaches more than 80%, the discharge valve is opened, and the synthesized slurry is discharged from the outlet of the tertiary reactor and fed into the plate and frame filter press C for filtration to obtain a filter cake. After washing, the synthesized filter cake is obtained.

[0026] Synthetic filter cake, pure water, and phosphoric acid are continuously fed into the synthetic filter cake mixing tank to complete the slurry dispersion. The temperature is then raised, and the pH of the discharged slurry is 1.6. The temperature of the mixing tank is kept constant at 85℃. The discharged slurry is continuously fed into the primary aging tank and the secondary aging tank in sequence. Each aging tank is kept at a constant temperature of 92℃ for aging and color change reactions. The slurry stays in each aging tank for 40 minutes. After the stay is completed, it enters the next aging tank. The aged slurry is filtered by a plate and frame filter press D, and the resulting filter cake is washed, dried, calcined, and crushed to obtain battery-grade iron phosphate.

[0027] The reaction tanks, reactors, and aging tanks are connected in series. Power is supplied by pumps (such as slurry pumps and metering pumps) installed on the connecting pipelines to ensure that materials are continuously and sequentially pumped from the previous stage to the next. Solid raw material ferrous sulfate heptahydrate is fed using a screw feeder, while other liquid raw materials and materials are transported through pipelines. Pumps and mass flow meters are linked to precisely control the feed flow rate.

[0028] Comparative Example 1: (Phosphate salts were added in a 1:1:1 ratio in the primary, secondary, and tertiary reactors) The remaining unspecified operations shall be performed in accordance with Example 1.

[0029] Ferrous sulfate solution A and mixed solution C are added to the primary reactor at flow rates of 60 g / min and 10 g / min, respectively. When the liquid level in the reactor reaches 80% of its volume, the discharge valve is opened, and the slurry is conveyed from the primary reactor to the secondary reactor. Simultaneously, mixed solution C enters the secondary reactor at a flow rate of 10 g / min. When the liquid level in the secondary reactor reaches 80%, the discharge valve is opened, and the slurry is conveyed from the secondary reactor to the tertiary reactor. At the same time, mixed solution C enters the tertiary reactor at a flow rate of 10 g / min. Comparative Example 2: (Phosphate salts were added only from the primary reactor) The remaining unspecified operations shall be performed in accordance with Example 1.

[0030] Ferrous sulfate solution A and mixed solution C are added to the primary reactor at flow rates of 60 g / min and 30 g / min, respectively. The slurry is continuously transported to the secondary and tertiary reactors to complete the heat preservation reaction.

[0031] Comparative Example 3: (Extended stay) The remaining unspecified operations shall be performed in accordance with Example 1.

[0032] The residence time of materials in each of the ferrous reaction tanks A, B, and C, as well as the phosphate reaction tanks A, B, and C, is 30 minutes. The residence time of the slurry in the primary, secondary, and tertiary reactors is 20 minutes. The residence time of the slurry in the primary and secondary aging reactors is 60 minutes.

[0033] Comparative Example 4: (None of them are continuous) The remaining unspecified operations shall be performed in accordance with Example 1.

[0034] Both the raw material purification process and the iron phosphate synthesis were carried out using a batch method.

[0035] The ferric phosphate products obtained in Example 1 and Comparative Examples 1-4 were subjected to the following tests. The test methods were performed according to commonly used methods in the art and will not be described in detail here. The results are shown in Table 1: Table 1. Results of ferric phosphate product indicators obtained in Example 1 and Comparative Examples 1-4 Example 1 compares the effect of reaction mode on the physicochemical properties of ferric phosphate products with Comparative Examples 1-4.

[0036] The experimental results show that, in Comparative Example 1, adjusting the feeding ratio of mixed solution C in the primary and secondary reactors to 1:1 slightly increased the particle size of the reaction product and also slightly increased the impurity content. This indicates that the method in Example 1 yielded the product with the optimal particle size and the lowest impurity content.

[0037] In Comparative Example 2, after the mixture was completely added to the primary reactor, the impurity content of the ferric phosphate product increased significantly, and the particle size was larger, resulting in a lower iron-to-phosphorus ratio. This is because adding the raw materials all at once in the primary reactor leads to intense localized reactions, potentially generating a large amount of heat and accelerating the reaction. This results in uneven ferric phosphate particle formation, even leading to undesirable morphology or agglomeration, causing a large amount of impurities to be trapped. Furthermore, hydrogen peroxide easily decomposes into oxygen and water under high temperature or vigorous stirring conditions. Pouring it all at once causes localized high temperatures, triggering hydrogen peroxide decomposition, reducing its oxidation efficiency, and thus affecting the oxidation state and quality of the final product, resulting in a lower iron-to-phosphorus ratio.

[0038] In Comparative Example 3, the residence time of the slurry in each continuous reaction was extended. Prolonged stirring generates continuous mechanical shear force, which breaks down larger particles into smaller ones. Furthermore, prolonged stirring leads to localized supersaturation in the solution, inducing secondary nucleation and generating a large number of new small particles, thus reducing the overall average particle size.

[0039] In Comparative Example 4, the same batch process with the same formulation was used to prepare ferric phosphate. The ferric phosphate product data obtained were similar to those in Example 1, proving that the batch process product can be reproduced using a continuous process.

[0040] Example 2 The difference from Example 1 is that: the ferrous impurity removal reaction temperature is 55°C; the residual acid impurity removal reaction temperature is 55°C; the pH value is 3; and the material residence time in each ferrous reaction tank is 30 min. The material residence time in the primary, secondary, and tertiary reactors is 10 minutes. The material residence time in the primary aging reactor and the secondary aging reactor is 60 minutes. The concentration of P2O5 in the residual raffinate used is 20%; the fluorination aid is sodium fluorosilicate, and the added mass is 1% of the mass of the residual raffinate; the ratio of the total amount of ferrous sulfate solution A to the total amount of mixed solution C is 1.5:1; Phosphoric acid is added to the mixing tank during the mixing process to control the pH value of the resulting mixed slurry to be 1.0.

[0041] Example 3 The difference from Example 1 is that: the ferrous impurity removal reaction temperature is 65°C; the residual acid impurity removal reaction temperature is 65°C; the pH value is 7; and the material residence time in each ferrous reaction tank is 20 min. The material residence time in the primary, secondary, and tertiary reactors is 20 minutes. The material residence time in the primary aging reactor and the secondary aging reactor is 40 minutes. The concentration of P2O5 in the residual raffinate used is 50%; the fluorination aid is sodium fluorosilicate, and the added mass is 2% of the mass of the residual raffinate; the ratio of the total amount of ferrous sulfate solution A to the total amount of mixed solution C is 2:1. Phosphoric acid is added to the mixing tank during the mixing process to control the pH value of the resulting mixed slurry to 2.5.

[0042] 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 illustrative of the 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 present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for the continuous preparation of battery-grade iron phosphate using residual raffinate as a raw material, characterized in that, Includes the following steps: 1) Ferrous sulfate heptahydrate, ammonia and pure water are continuously fed into a multi-stage series ferrous reaction tank for impurity removal. The reaction slurry is filtered to obtain ferrous sulfate solution A. 2) The residual acid, ammonia, fluorination aid and pure water are continuously fed into the phosphate salt reaction tank for impurity removal reaction. The reaction slurry is filtered to obtain phosphate salt solution B, which is mixed with oxidant and ammonia to obtain mixed solution C. 3) Ferrous sulfate solution A is continuously added to the primary reactor at a constant flow rate. Mixed solution C is split and continuously and simultaneously added to the primary reactor, secondary reactor, and tertiary reactor for synthesis reaction. The resulting synthesis slurry is filtered and washed to obtain a synthesis filter cake. The primary reactor, secondary reactor, and tertiary reactor are set in series. 4) The synthesized filter cake is continuously fed to the slurry preparation tank. After the pure water and phosphoric acid are added to complete the dispersion, the resulting slurry is continuously fed into the first-stage aging tank and the second-stage aging tank connected in series for crystallization aging. After aging, the slurry is filtered, washed, dried, calcined and crushed to obtain battery-grade iron phosphate.

2. The method for continuous preparation of battery-grade iron phosphate using residual raffinate as a raw material as described in claim 1, characterized in that: The impurity removal reaction temperature in both steps 1) and 2) is 55-65℃; the pH value of the impurity removal reaction in step 2) is 3-7.

3. The method for continuous preparation of battery-grade iron phosphate using residual raffinate as a raw material as described in claim 1, characterized in that: The multi-stage ferrous reactor consists of ferrous reactor 1, ferrous reactor 2, and ferrous reactor 3 connected in sequence, with the residence time of the material in each ferrous reactor being 20–30 min.

4. The method for continuous preparation of battery-grade iron phosphate using residual raffinate as a raw material as described in claim 1, characterized in that: The residence time of materials in the primary, secondary, and tertiary reactors is 10–20 min, respectively; the residence time of materials in the primary and secondary aging reactors is 40–60 min, respectively.

5. The method for continuous preparation of battery-grade iron phosphate using residual raffinate as a raw material as described in claim 1, characterized in that: The concentration of P2O5 in the residual acid is 20% to 50%.

6. The method for continuous preparation of battery-grade iron phosphate using residual raffinate as a raw material as described in claim 1, characterized in that: The fluorination aid is ammonium fluoride or sodium fluorosilicate, and the added mass of the fluorination aid is 0.5% to 3% of the mass of the residual acid.

7. The method for continuous preparation of battery-grade iron phosphate using residual raffinate as a raw material as described in claim 1, characterized in that: The oxidant is hydrogen peroxide, and the amount of oxidant used is related to the amount of Fe in ferrous sulfate solution A. 2+ The molar ratio is 1.1 to 1.4:

1.

8. The method for continuous preparation of battery-grade iron phosphate using residual raffinate as a raw material as described in claim 1, characterized in that: The pH value of the mixed solution C is 6.5–7.5; the ratio of the added flow rate of ferrous sulfate solution A to that of mixed solution C is 1.5–3:1; the ratio of the added flow rate of mixed solution C in the primary, secondary, and tertiary reactors is 4–6:3–2:3–2; the constant temperature of the primary, secondary, and tertiary reactors is 50–60°C; and the pH value of the reaction system is 1.5–2.

5.

9. The method for continuous preparation of battery-grade iron phosphate using residual raffinate as a raw material as described in claim 1, characterized in that: The constant temperature of the slurry preparation tank is 40-60℃, and the pH value of the resulting slurry is controlled by adding phosphoric acid to be 1.0-2.5; the constant temperature of the primary aging tank and the secondary aging tank is 85-95℃.

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