A method for preparing high-purity hafnium oxide by alkali fusion-crystallization

Abstract

The application provides a method for preparing high-purity hafnium oxide by an alkali fusion crystallization method, comprising the following steps: melting hafnium oxide raw materials with solid alkali and fluxing agent to obtain alkali fusion clinker; adding the alkali fusion clinker into deionized water to perform leaching and filtration, and obtaining hafnium hydroxide precipitate; adding the hafnium hydroxide precipitate into concentrated sulfuric acid to obtain hafnium sulfate solution; adding hydrogen peroxide into the hafnium sulfate solution to perform oxidation pretreatment; evaporating and concentrating the hafnium sulfate solution subjected to the oxidation pretreatment and crystallizing to obtain hafnium sulfate crystals; adding the hafnium sulfate crystals into concentrated hydrochloric acid and refluxing, evaporating and concentrating and crystallizing to obtain hafnium oxychloride crystals; dissolving the hafnium oxychloride crystals in water and adding oxalic acid to generate hafnium oxalate precipitate; and calcining the hafnium oxalate precipitate. According to the method, the purity of hafnium oxide is increased to more than 99.995%, the impurity content is less than 10 ppm, and the process is short, low in cost and environmentally friendly.

Description

Technical Field

[0001] This invention belongs to the field of inorganic material preparation technology, specifically relating to a method for preparing high-purity hafnium oxide. Background Technology

[0002] Hafnium oxide (HfO2), as a crucial functional material possessing a high melting point (approximately 2810℃), high hardness, high wear resistance, high thermal stability, and high optical refractive index, occupies an irreplaceable position in modern industrial systems. Among them, ultra-high purity hafnium oxide with a purity exceeding 99.99% (4N) is a core raw material in high-end fields such as semiconductor device manufacturing, advanced optical systems, and key components for aerospace engines. In the semiconductor industry, its excellent dielectric properties make it an ideal choice for the gate dielectric layer of metal-oxide-semiconductor (MOS) devices, effectively reducing leakage current and improving device integration and stability. In the aerospace field, hafnium oxide-based composite materials are used in high-temperature components such as rocket nozzles and gas turbine blades, and their performance directly determines the equipment's adaptability to extreme environments.

[0003] With the rapid iteration of high-end manufacturing, the market demand for ultra-high purity hafnium oxide continues to rise, and the purity requirements are also constantly increasing. Some high-end applications have proposed standards of 99.999% (5N) and above. However, hafnium and zirconium have very similar chemical properties and often coexist in nature. Moreover, hafnium minerals also contain a variety of impurities such as iron, aluminum, calcium, silicon, titanium, chromium, and nickel, which pose a huge challenge to the preparation of ultra-high purity hafnium oxide.

[0004] The current mainstream preparation methods have significant drawbacks: Although solvent extraction is widely used, the organic solvents used (such as tributyl phosphate TBP) are volatile, flammable and explosive, posing safety and environmental risks. In addition, it requires complex processes such as multi-stage countercurrent extraction, which is lengthy, energy-intensive and difficult to guarantee yield. Although sublimation can obtain products with high purity, the equipment is expensive, the operating temperature is extremely high, the production cycle is long and the product loss is large. It is only suitable for small-batch preparation in the laboratory, and the high-temperature chlorine treatment poses safety hazards.

[0005] Alkali fusion-crystallization, as a green and environmentally friendly process, has the advantage of simple operation, but it still has technical bottlenecks: uneven mixing of solid alkali and hafnium oxide during alkali fusion and incomplete reaction affect the yield; limited impurity removal efficiency, especially difficult to remove similar impurities such as zirconium and titanium, making it difficult to achieve a purity of 4N or higher; uneven crystal growth during crystallization, which easily carries impurities and increases the difficulty of subsequent processing; high calcination energy consumption, and high temperature easily leads to particle agglomeration, affecting dispersion performance.

[0006] Therefore, developing a simple, environmentally friendly, efficient, high-purity, and low-energy-consumption process for preparing ultra-high-purity hafnium oxide is of great significance for promoting the development of high-end manufacturing. Summary of the Invention

[0007] Based on the technical problems described above, the purpose of this invention is to provide a high-purity hafnium oxide preparation method with a short process flow, simple equipment, and high purification efficiency. The core is to solve the problems of the lengthy process and the easy introduction and residue of stubborn impurities such as alkali metals and sulfate ions in existing technologies (such as solvent extraction or traditional crystallization methods), and finally obtain hafnium oxide products with a purity greater than 99.995% to meet the stringent requirements of high-end industries such as semiconductors.

[0008] Specifically, the present invention provides a method for alkali fusion. A method for preparing high-purity hafnium oxide by crystallization, the method comprising the following steps:

[0009] (1) Hafnium oxide raw material is mixed with solid alkali and flux at a mass ratio of 1:2-5:0.05-0.3 and subjected to a melting reaction at 400-700℃ for 0.5-4 hours to obtain alkali-fused clinker. The solid alkali is one or more of sodium hydroxide and potassium hydroxide, and the flux is one or more of sodium carbonate and sodium nitrate.

[0010] (2) The alkaline fused clinker is added to deionized water, leached at 75-85℃ and filtered to obtain hafnium hydroxide precipitate;

[0011] (3) The hafnium hydroxide precipitate is added to concentrated sulfuric acid and reacted at 120-180°C for 1-3 hours to obtain a hafnium sulfate solution, wherein the weight ratio of the hafnium hydroxide precipitate to concentrated sulfuric acid is in the range of 1:3-1:5.

[0012] (4) Add hydrogen peroxide to the hafnium sulfate solution for oxidation pretreatment, wherein the concentration of the hydrogen peroxide is 25-35% by weight, and the weight of the added hydrogen peroxide is 0.5-2% of the weight of the hafnium sulfate solution;

[0013] (5) The hafnium sulfate solution that has undergone the oxidation pretreatment is evaporated, concentrated, and cooled to crystallize, thereby obtaining hafnium sulfate crystals;

[0014] (6) Add the hafnium sulfate crystals to concentrated hydrochloric acid and reflux for 2-4 hours, evaporate and concentrate, and cool to crystallize to obtain hafnium oxychloride crystals;

[0015] (7) Dissolve the hafnium oxychloride crystals in deionized water, and then add an aqueous oxalic acid solution to generate hafnium oxalate precipitate;

[0016] (8) The hafnium oxalate precipitate is calcined in an oxidizing atmosphere to obtain the high-purity hafnium oxide.

[0017] According to certain preferred embodiments of the present invention, the hafnium oxide raw material comprises, by weight 100%, more than 85% hafnium oxide, less than 12% zirconium oxide, less than 0.5% iron, and less than 0.2% aluminum, calcium, silicon and titanium.

[0018] According to certain preferred embodiments of the present invention, in step (1), the solid alkali is sodium hydroxide and the flux is anhydrous sodium carbonate.

[0019] According to certain preferred embodiments of the present invention, in step (1), a melting reaction is carried out at 550-650°C for 1.5-2.5 hours to obtain the alkali-fused clinker.

[0020] According to certain preferred embodiments of the present invention, in step (2), the alkali-fused clinker is added to deionized water at a weight ratio of 1:4 to 1:6, leached at 75-80°C for 2-4 hours and filtered to obtain the hafnium hydroxide precipitate.

[0021] According to certain preferred embodiments of the present invention, in step (2), the leaching time of the stirring leaching is 0.5-2 hours.

[0022] According to certain preferred embodiments of the present invention, the sulfuric acid used to adjust the pH in step (2) is an aqueous solution of sulfuric acid with a concentration in the range of 20-30% by weight.

[0023] According to certain preferred embodiments of the present invention, the concentrated sulfuric acid used in step (3) is an aqueous sulfuric acid solution with a concentration in the range of 65-85% by weight.

[0024] According to certain preferred embodiments of the present invention, in step (4), the temperature of the hafnium sulfate solution is reduced to 20-35°C, and the hydrogen peroxide is added dropwise to perform the oxidation pretreatment.

[0025] According to certain preferred embodiments of the present invention, in step (5), the cooling rate is 0.5-2°C / hour.

[0026] According to certain preferred embodiments of the present invention, in step (5), the crystallization time is 8-16 hours.

[0027] According to certain preferred embodiments of the present invention, in step (6), the concentration of concentrated hydrochloric acid is 32-37% by weight.

[0028] According to certain preferred embodiments of the present invention, in step (6), the weight ratio of the hafnium sulfate crystals to the concentrated hydrochloric acid is in the range of 1:6 to 1:12.

[0029] According to certain preferred embodiments of the present invention, in step (6), the reflux temperature of the reflux reaction is 80-110°C.

[0030] According to certain preferred embodiments of the present invention, in step (6), the cooling rate is 1-3°C / hour.

[0031] According to certain preferred embodiments of the present invention, in step (6), the crystallization time is 12-24 hours.

[0032] According to certain preferred embodiments of the present invention, in step (7), the concentration of the oxalic acid aqueous solution is 10-15 by weight.

[0033] According to certain preferred embodiments of the present invention, in step (8), the calcination includes: heating to 200-400°C at a rate of 1-3°C / min, holding at that temperature for 1-2 hours, and then heating to 750-950°C at a rate of 2-5°C / min, holding at that temperature for 2-4 hours.

[0034] According to certain preferred embodiments of the present invention, the hafnium oxide prepared by the method has a purity greater than or equal to 99.995%, and the Na, K, Fe, and Cl content is [not specified]. - SO4 2- The content of all of them is less than 10 ppm.

[0035] Compared with existing technologies, the beneficial effects of this invention are as follows: Compared with solvent extraction methods, which involve lengthy processes, use large amounts of organic solvents, and pose environmental risks, and traditional double salt crystallization methods, which have poor impurity removal and low yields, this invention significantly simplifies the process flow, reduces equipment and reagent costs, and achieves the crystallization of alkali metals (Na+). + ) and sulfate (SO4) 2- The deep removal of key impurities such as hafnium oxide (content below 10 ppm) ensures that the purity of hafnium oxide can be stably maintained at over 99.995%, with high yield and environmental friendliness, meeting the stringent requirements of high-end electronic materials for ultra-high purity and impurity control. Attached Figure Description

[0036] The accompanying drawings are provided in this specification to more clearly explain the technical solutions of the present invention; however, the art is not limited thereto.

[0037] Figure 1 This illustrates the technical solution of the present invention through alkali fusion. Process flow diagram for preparing high-purity hafnium oxide by crystallization. Detailed Implementation

[0038] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. It will be understood that other embodiments may be implemented without departing from the scope or spirit of the invention. Therefore, the following detailed description is non-limiting.

[0039] Unless otherwise specified, all figures used in this specification to represent characteristic dimensions, quantities, and physical properties should be understood to be modified by the term "about" in all cases. Therefore, unless otherwise stated, the numerical parameters listed in the foregoing specification are approximations, and those skilled in the art can appropriately modify these approximations to obtain the desired properties using the teachings disclosed herein.

[0040] As mentioned above, existing high-purity hafnium oxide preparation processes (such as solvent extraction and traditional double salt crystallization) generally suffer from lengthy and complex processes, large equipment investment, and the tendency to introduce and deeply remove alkali metal ions (such as Na+). + K + ) and sulfate (SO4) 2- The presence of stubborn impurities such as [unspecified impurities] in hafnium oxide significantly affects its dielectric and high-temperature properties, limiting its application in high-end fields such as semiconductor gate dielectrics. Therefore, there is an urgent need to develop a new, efficient, short-process method capable of deep purification. This invention aims to solve these problems.

[0041] Specifically, the present invention provides a method for alkali fusion. A method for preparing high-purity hafnium oxide by crystallization, the method comprising the following steps:

[0042] (1) Hafnium oxide raw material is mixed with solid alkali and flux at a mass ratio of 1:2-5:0.05-0.3 and subjected to a melting reaction at 400-700℃ for 0.5-4 hours to obtain alkali-fused clinker. The solid alkali is one or more of sodium hydroxide and potassium hydroxide, and the flux is one or more of sodium carbonate and sodium nitrate.

[0043] (2) The alkaline fused clinker is added to deionized water for leaching and filtration to obtain hafnium hydroxide precipitate;

[0044] (3) The hafnium hydroxide precipitate is added to concentrated sulfuric acid and reacted at 120-180°C for 1-3 hours to obtain a hafnium sulfate solution, wherein the weight ratio of the hafnium hydroxide precipitate to concentrated sulfuric acid is in the range of 1:3-1:5.

[0045] (4) Add hydrogen peroxide to the hafnium sulfate solution for oxidation pretreatment, wherein the concentration of the hydrogen peroxide is 25-35% by weight, and the weight of the added hydrogen peroxide is 0.5-2% of the weight of the hafnium sulfate solution;

[0046] (5) The hafnium sulfate solution that has undergone the oxidation pretreatment is evaporated, concentrated, and cooled to crystallize, thereby obtaining hafnium sulfate crystals;

[0047] (6) Add the hafnium sulfate crystals to concentrated hydrochloric acid and reflux for 2-4 hours, evaporate and concentrate, and cool to crystallize to obtain hafnium oxychloride crystals;

[0048] (7) Dissolve the hafnium oxychloride crystals in deionized water, and then add an aqueous oxalic acid solution to generate hafnium oxalate precipitate;

[0049] (8) The hafnium oxalate precipitate is calcined in an oxidizing atmosphere to obtain the high-purity hafnium oxide.

[0050] Figure 1 This illustrates the technical solution of the present invention through alkali fusion. The process flow diagram for preparing high-purity hafnium oxide by crystallization includes the following steps:

[0051] (1) The hafnium oxide raw material is melted with solid alkali and flux to obtain alkali-fused clinker;

[0052] (2) Add the alkali-fused clinker to water, leach and filter to obtain hafnium hydroxide precipitate;

[0053] (3) Add hafnium hydroxide precipitate to concentrated sulfuric acid to obtain hafnium sulfate solution;

[0054] (4) Add hydrogen peroxide to the hafnium sulfate solution for oxidation pretreatment;

[0055] (5) The hafnium sulfate solution that has undergone oxidation pretreatment is evaporated, concentrated and crystallized to obtain hafnium sulfate crystals;

[0056] (6) Add hafnium sulfate crystals to concentrated hydrochloric acid and reflux, evaporate and concentrate and crystallize to obtain hafnium oxychloride crystals;

[0057] (7) Dissolve hafnium oxychloride crystals in water, add oxalic acid, and hafnium oxalate precipitate is formed;

[0058] (8) Hafnium oxalate precipitate was calcined to obtain hafnium oxide.

[0059] Compared to the limitations of existing technologies where some processes have stringent requirements for raw material purity (such as requiring a hafnium oxide content ≥90% by weight), the present invention has greater raw material adaptability and can directly use industrial-grade crude hafnium oxide as raw material. Preferably, the hafnium oxide raw material, based on its total weight of 100%, comprises: more than 85% by weight of hafnium oxide, less than 12% by weight of zirconium oxide, less than 0.5% by weight of iron, and less than 0.2% by weight of one or more of aluminum, calcium, silicon, and titanium. The hafnium oxide raw material that can be used in the present invention can be related hafnium oxide raw material products from Jiangxi Zhonghafnium New Materials Co., Ltd. From the perspective of elemental characteristics, zirconium and hafnium have similar atomic radii and extremely similar chemical properties, making zirconium one of the most difficult impurities to remove during hafnium oxide purification. The present invention effectively solves the technical problem of zirconium-hafnium separation through subsequent multi-stage crystallization and precipitation conversion processes. Even if the zirconium oxide content in the raw material is as high as 12% by weight, the zirconium impurity in the final product can still be controlled at an extremely low level, ensuring product purity.

[0060] The following details the specific steps of the method for preparing high-purity hafnium oxide according to the present invention.

[0061] Step (1): Raw material mixing and melting reaction

[0062] The goal of step (1) is to convert solid hafnium oxide feedstock into hafnium salts, while creating conditions for subsequent impurity removal.

[0063] According to the technical solution of this invention, the mass ratio of hafnium oxide raw material, solid alkali, and flux is 1:2-5:0.05-0.3. The solid alkali provides a strongly alkaline environment to decompose hafnium oxide into hafnium salts (such as Na2HfO3). Its dosage affects the reaction conversion rate: if the amount of solid alkali is too small, the hafnium oxide will not decompose completely, resulting in raw material residue and a decreased yield; if the amount is too large, it will increase the sulfuric acid consumption in the subsequent neutralization step and introduce too many alkali metal ions, increasing the difficulty of impurity removal. The flux is selected from one or more of sodium carbonate and sodium nitrate, and its function is to lower the melting point of the molten system and promote the uniform mixing and reaction of the solid alkali and hafnium oxide. According to the technical solution of the present invention, the amount of flux is controlled at 0.05-0.3 (mass ratio). If the amount is insufficient, the fluxing effect is not obvious, the system is difficult to melt, and the reaction efficiency is low. If the amount is too high, it will increase the impurity content in the subsequent leachate and may form insoluble compounds with other impurities, affecting the subsequent separation effect.

[0064] Preferably, sodium hydroxide is selected as the solid alkali, and anhydrous sodium carbonate is selected as the flux. Sodium hydroxide has advantages such as strong alkalinity, relatively low melting point (318℃), and low cost. Compared with potassium hydroxide, it has higher solubility in water and better subsequent leaching effect. When anhydrous sodium carbonate is used as a flux and is compounded with sodium hydroxide, it can lower the eutectic point of the molten system to below 300℃, significantly reducing the melting reaction temperature. At the same time, its chemical properties are stable and will not introduce new harmful impurities.

[0065] Regarding the melting reaction conditions, according to the technical solution of the present invention, the temperature is 400-700℃, and the reaction time is 0.5-4 hours. When the temperature is too low (below 400℃), the reaction rate between the solid alkali and hafnium oxide is slow, or even difficult to achieve a substantial reaction, resulting in a low raw material conversion rate. When the temperature is too high (above 700℃), insoluble complex compounds (such as solid solutions of zirconate and hafnium salt) may be formed, increasing the difficulty of subsequent leaching and separation. The reaction time needs to be matched with the temperature. Under low-temperature conditions, the reaction time needs to be extended to ensure a complete reaction, while under high-temperature conditions, the reaction time can be appropriately shortened to avoid over-reaction. Preferably, the reaction is carried out at 550-650℃ for 1.5-2.5 hours. Under these conditions, complete decomposition of hafnium oxide can be ensured while avoiding energy waste and side reactions.

[0066] Step (2): Leaching and preparation of hafnium hydroxide precipitation

[0067] According to certain preferred embodiments of the present invention, the weight ratio (solid-liquid ratio) of alkali-fused clinker to deionized water is 1:4-1:6. The leaching temperature is 75-85℃, preferably 75-80℃. The temperature range of 75-85℃ ensures leaching efficiency and allows the leaching time to be controlled within 2-4 hours. Water leaching utilizes the hydrolysis reaction of hafnium salts and the selective dissolution of impurities to achieve precipitation and preliminary impurity removal of the target product (hafnium hydroxide). Soluble impurities such as Na2SiO3 enter the aqueous phase, while the hafnium-rich substances generated by the hydrolysis of hafnium salts enter the precipitate phase (mainly hafnium hydroxide solid precipitate), achieving preliminary separation. The formation mechanism of hafnium hydroxide precipitate is as follows: hafnium salts in alkali-fused clinker undergo a hydrolysis reaction in hot water (75-85℃) to generate hafnium hydroxide precipitate, with the reaction formula being Na2HfO3 + 2H2O → Hf(OH)4 + 2NaOH. The reaction is preferably carried out at a leaching temperature of 75-85°C.

[0068] According to the technical solution of the present invention, the leaching time is controlled at 2-4 hours. After leaching, a vacuum filtration device is used for filtration. Preferably, the hafnium hydroxide precipitate obtained by filtration is washed with deionized water more than 3 times. The purpose of washing is to remove impurities such as sodium ions and potassium ions adsorbed on the surface of the precipitate. During the washing process, the amount of water used for each wash is 2-3 times the mass of the precipitate. The washing method is decantation, that is, first add deionized water to the precipitate, stir evenly, let it stand to settle, and then pour out the supernatant. This process is repeated 3 times.

[0069] Step (3): Preparation of hafnium sulfate solution

[0070] The purpose of step (3) is to convert hafnium hydroxide precipitate into hafnium sulfate, which is easily soluble in water, in preparation for subsequent oxidation pretreatment and crystallization purification.

[0071] Specifically, according to the technical solution of the present invention, the weight ratio of hafnium hydroxide precipitate to concentrated sulfuric acid is 1:3-1:5, the reaction temperature is 120-180℃, and the reaction time is 1-3 hours. The role of concentrated sulfuric acid is to provide an acidic environment so that hafnium hydroxide undergoes a neutralization reaction to form hafnium sulfate (Hf(SO4)2). The reaction temperature is controlled at 120-180℃. Within this temperature range, the reaction rate can be accelerated, promoting the dissolution of hafnium hydroxide. When the temperature is too low (below 120℃), the reaction rate is slow, requiring a longer reaction time, and may lead to incomplete dissolution. When the temperature is too high (above 180℃), the oxidizing power of concentrated sulfuric acid is enhanced, which may cause some impurity ions to be oxidized to form insoluble compounds, while also increasing energy consumption and the risk of equipment corrosion.

[0072] Preferably, the concentration of concentrated sulfuric acid is 65-85% by weight. This concentration range ensures reaction efficiency, reduces the risk of equipment corrosion, and facilitates the control of subsequent processes.

[0073] Step (4): Hydrogen peroxide oxidation pretreatment

[0074] In step (4), the low-valence impurity ions (such as Fe) in the solution are removed through the oxidation effect of hydrogen peroxide. 2+ Ti 3 + Hydrogen peroxide oxidizes hafnium sulfate to a higher oxidation state, preventing it from mixing into hafnium sulfate crystals during subsequent crystallization, thus improving crystallization purification efficiency. Furthermore, the addition of hydrogen peroxide can remove trace organic impurities and some non-metallic impurities remaining in the raw material through a synergistic oxidation-decomposition effect. These impurities are difficult to remove using conventional crystallization or precipitation processes, but hydrogen peroxide, under acidic conditions, can oxidize and decompose them into volatile or water-soluble products such as carbon dioxide and water, which are then separated and removed through subsequent evaporation, concentration, and filtration steps. For trace amounts of non-metallic impurities such as sulfur and phosphorus that may be present in the raw material, hydrogen peroxide can oxidize them to higher oxidation state oxyacid anions (such as SO42-). 2- PO43- Some of the oxyacid anions can form insoluble salts with a small amount of metal ions in the solution, remaining in the mother liquor during crystallization or being excluded from hafnium oxalate precipitation during the subsequent oxalic acid precipitation step, thus achieving deep purification. Simultaneously, the reactive oxygen species generated during hydrogen peroxide oxidation can disrupt the adsorption or complexation between some impurity ions and hafnium ions, reducing the probability of impurity adsorption on the crystal surface and further reducing the impurity content entrained in the crystal.

[0075] According to the technical solution of the present invention, the hydrogen peroxide concentration is 25-35% by weight, and the amount added is 0.5-2% by weight of the hafnium sulfate solution. Hydrogen peroxide, as a mild oxidant, can oxidize Fe under acidic conditions. 2+ Oxidized to Fe 3+ Ti 3+ Oxidation to Ti 4+ The hydrolysis characteristics of high-valence iron and titanium ions in aqueous solution differ more significantly from those of hafnium ions, making them more likely to remain in the mother liquor during subsequent crystallization, thus achieving separation from hafnium. When the hydrogen peroxide concentration is too low (below 25 wt%), its oxidizing power is insufficient, making it difficult to completely oxidize low-valence impurity ions, resulting in poor impurity removal. When the concentration is too high (above 35 wt%), it not only increases costs but may also cause hydrogen peroxide to decompose and generate oxygen, leading to a large number of bubbles in the solution, affecting subsequent operations. Excessive hydrogen peroxide may also decompose during subsequent crystallization, introducing trace impurities. When the amount added is too low (below 0.5%), the oxidant dosage is insufficient, resulting in incomplete oxidation. When the amount added is too high (above 2%), the excess hydrogen peroxide cannot react completely, potentially causing side effects in subsequent processes (such as oxidation of oxalate ions).

[0076] Preferably, in step (4), the temperature of the hafnium sulfate solution is first lowered to 20-35°C, and hydrogen peroxide is added dropwise. Low temperature conditions can reduce the decomposition rate of hydrogen peroxide, improve its utilization rate, and avoid rapid decomposition of hydrogen peroxide due to excessively high temperature, which would affect the oxidation effect; the dropwise addition method can make hydrogen peroxide evenly dispersed in the hafnium sulfate solution, avoid side reactions caused by excessively high local concentration, and ensure that the oxidation reaction is stable and complete.

[0077] Step (5): Hafnium sulfate cooling crystallization

[0078] Step (5) involves separating hafnium sulfate from some impurities through crystallization, which is a crucial step in the purification process. This invention optimizes crystal growth conditions by controlling the cooling rate and crystallization time, thereby reducing impurity entrainment and improving crystal purity.

[0079] The crystallization process employs an evaporation-concentration followed by cooling crystallization method. First, the hafnium sulfate solution is evaporated and concentrated to 1 / 3-1 / 2 of its original volume, increasing the concentration of hafnium sulfate in the solution and creating conditions for crystallization. According to a preferred embodiment of the present invention, the cooling rate is 0.5-2°C / hour, and the crystallization time is 8-16 hours. Through this crystallization purification step, the purity of hafnium in the hafnium sulfate crystals can be increased to over 99.9%, while most impurities such as iron, aluminum, and calcium remain in the mother liquor, achieving secondary impurity removal.

[0080] Step (6): Hafnium oxychloride crystallization purification

[0081] In step (6), hafnium sulfate is converted into hafnium oxychloride. By taking advantage of the difference in crystallization characteristics of the two compounds, the sulfate ions and hafnium are separated, and other impurities are further removed.

[0082] According to a preferred embodiment of the present invention, the concentration of concentrated hydrochloric acid is 32-37% by weight, the weight ratio of hafnium sulfate crystals to concentrated hydrochloric acid is 1:6-1:12, the reflux temperature is 80-110°C, the reflux reaction time is 2-4 hours, the cooling rate is 1-3°C / hour, and the crystallization time is 12-24 hours. Hafnium sulfate and concentrated hydrochloric acid undergo a metathesis reaction under reflux conditions to produce hafnium oxychloride (HfOCl2) precipitate and sulfuric acid. The reaction equation is: Hf(SO4)2 + 2HCl → HfOCl2 + H2SO4.

[0083] Preferably, the reflux temperature is 80-110℃. Within this temperature range, the volatility of concentrated hydrochloric acid is moderate, which ensures the reaction rate while preventing excessive volatilization of hydrochloric acid that would lead to a decrease in the concentration of hydrochloric acid in the reaction system. The reflux reaction time is 2-4 hours to ensure that hafnium sulfate is completely converted into hafnium oxychloride.

[0084] This step, through compound transformation and secondary crystallization, achieves deep removal of sulfate impurities, while further removing impurities such as iron, aluminum, and silicon, and SO4 from hafnium oxychloride crystals. 2- The content can be reduced to below 10 ppm.

[0085] Step (7): Preparation of hafnium oxalate precipitate

[0086] Step (7) involves reacting oxalic acid with hafnium oxychloride to generate a poorly soluble hafnium oxalate precipitate, thereby achieving the reaction of hafnium with residual impurity ions (such as Cl-) in the solution. - Na + ) separation.

[0087] Preferably, the concentration of the oxalic acid aqueous solution is 10-15% by weight. The amount of oxalic acid added is approximately 1.2 times the theoretical stoichiometric amount. Oxalic acid reacts with hafnium oxychloride: HfOCl2 + H2C2O4 → Hf(C2O4)2 + 2HCl. The resulting hafnium oxalate precipitate is insoluble in water, while the Cl in the solution...- Na + Impurity ions remain in the aqueous solution, achieving separation. Preferably, the concentration of the oxalic acid aqueous solution is 10-15% by weight. The amount of oxalic acid added is 1.2 times the theoretical stoichiometric amount to ensure that hafnium oxychloride reacts completely to form hafnium oxalate precipitate, avoiding the loss of hafnium due to insufficient oxalic acid.

[0088] Preferably, the temperature should be controlled at around 40°C during the reaction. The oxalic acid aqueous solution should be added slowly with stirring to avoid excessively high local concentrations that could cause hafnium oxalate precipitate agglomeration, affecting filtration and washing efficiency. After precipitate formation, allow it to stand for at least 4 hours to ensure complete precipitation, then wash with deionized water until no Cl- is present in the washing solution. - (Silver nitrate solution test) to thoroughly remove residual impurity ions.

[0089] Step (8): Calcination of hafnium oxalate to prepare high-purity hafnium oxide

[0090] Step (8) involves converting hafnium oxalate precipitate into hafnium oxide while removing residual carbon, hydrogen, and other elements to ultimately obtain a high-purity hafnium oxide product. This invention optimizes the calcination process to ensure the complete decomposition of hafnium oxalate and avoids impurity residues and particle agglomeration.

[0091] According to the technical solution of the present invention, preferably, the calcination process is divided into two stages: the first stage involves heating to 200-400℃ at a rate of 1-3℃ / min and holding for 1-2 hours; the second stage involves heating to 750-950℃ at a rate of 2-5℃ / min and holding for 2-4 hours. The first stage is a low-temperature dehydration and oxalate decomposition stage, where hafnium oxalate first loses its water of crystallization at 200-400℃, and then the oxalate decomposes into gases such as carbon dioxide and carbon monoxide. Slow heating can prevent the rapid escape of moisture and gases from causing the precipitate particles to break, while ensuring that the decomposition products are fully discharged; holding for 1-2 hours can ensure that dehydration and preliminary decomposition are complete, avoiding residual moisture or undecomposed oxalate from affecting the subsequent high-temperature calcination effect. The second stage is a high-temperature oxidation stage, where hafnium oxalate is completely decomposed at 750-950℃ to generate hafnium oxide (Hf(C2O4)2→HfO2+ 2CO2+ 2CO).

[0092] The calcination process is carried out in an oxidizing atmosphere (such as oxygen), which can promote the oxidation of reducing gases such as carbon monoxide, avoid contaminating the hafnium oxide product, and prevent carbon residues produced by the decomposition of oxalate, thus ensuring product purity.

[0093] Through the synergistic effect of the above eight-step process, the hafnium oxide product prepared by this invention achieves a stable purity of over 99.995%, with Na, K, Fe, and Cl content significantly reduced. - SO4 2-The impurity content is all below 10ppm, which fully meets the requirements of high-end fields such as semiconductor device manufacturing and key components of aerospace engines.

[0094] This invention represents a significant advancement over existing technologies: Compared to solvent extraction, it eliminates the need for flammable and explosive organic solvents, avoiding safety and environmental risks. Simultaneously, the process flow is shortened by over 30%, and energy consumption is reduced by over 25%. Compared to the traditional alkali fusion-crystallization method, this invention significantly improves impurity removal efficiency through innovative steps such as oxidation pretreatment, secondary crystallization, and oxalic acid precipitation. In particular, it effectively addresses the deep removal of stubborn impurities such as zirconium, iron, and sulfate, increasing product purity from 99.9% in traditional processes to over 99.995%. Compared to sublimation, this invention operates at a lower temperature (maximum calcination temperature of 950℃, far lower than the sublimation method's 1800℃), requires simpler equipment, shortens the production cycle by over 60%, and increases product yield to over 89%, reaching a maximum of 93.5%, making it suitable for large-scale industrial production.

[0095] The present invention will now be described in more detail with reference to embodiments. It should be noted that these descriptions and embodiments are intended to facilitate understanding of the present invention and are not intended to limit the invention.

[0096] Example

[0097] In this invention, unless otherwise specified, all reagents used are commercially available products and are used directly without further purification. Furthermore, "%" refers to "weight %" and "parts" refers to "parts by weight".

[0098] Table 1 below lists specific information about the raw materials used in the embodiments and comparative examples of the present invention.

[0099] Table 1 List of Experimental Materials

[0100]

[0101] Table 2 below lists specific information about the experimental equipment used in the embodiments and comparative examples of the present invention.

[0102] Table 2 List of Experimental Equipment

[0103]

[0104] Test methods

[0105] Based on the method described in detail below, the hafnium oxide samples prepared in the following examples and comparative examples are compared regarding hafnium oxide purity, impurity content, and chloride ion content (Cl). - ) content, sulfate (SO4) 2- The content and product yield were evaluated.

[0106] (I) Hafnium oxide purity test (refer to GB / T 34500.2-2017 "High-purity hafnium chemical analysis methods - Part 2: Determination of impurity element content - Inductively coupled plasma mass spectrometry")

[0107] Weigh 0.1 g (accurate to 0.0001 g) of sample into a polytetrafluoroethylene digestion vessel, add 5 mL of hydrofluoric acid and 2 mL of nitric acid, seal, and place in a microwave digester. Digest according to the set program (heat to 180℃, hold for 30 min). After digestion, cool to room temperature, transfer the solution to a 100 mL volumetric flask, dilute to the mark with deionized water, and mix well. Determine the concentration of Hf and each impurity element in the solution using inductively coupled plasma mass spectrometry (ICP-MS). The purity of hafnium oxide is obtained by calculating the mass fraction of Hf. The standard addition method is used for calibration during the test. The blank test uses the same digestion procedure but without sample addition; the result is calculated after subtracting the blank value.

[0108] (II) Testing of alkali metal (Na, K) and impurities such as iron, aluminum, calcium, silicon and titanium (refer to GB / T 34500.2-2017)

[0109] The same sample pretreatment method as used for hafnium oxide purity testing was employed. The concentrations of Na, K, Fe, Al, Ca, Si, and Ti in the sample solution were determined by ICP-MS. Instrument parameters were set as follows: RF power 1550W, sampling depth 8mm, carrier gas flow rate 1.05L / min, auxiliary gas flow rate 0.8L / min, and nebulizer gas flow rate 0.45L / min. Appropriate isotopes were selected (…). 23 Na、 39 K, 56 Fe、 27 Al、 40 Ca, 28 Si、 48 i) Perform the determination, plot the calibration curve with multi-element standard solution, the correlation coefficient is ≥0.9995, perform 3 parallel determinations, take the average value as the final result, and the detection limit is 0.1ppm.

[0110] (III) Chloride ions (Cl) - Content test (refer to GB / T 15453-2018 "Determination of Chloride Ions in Industrial Circulating Cooling Water - Silver Nitrate Titration Method")

[0111] Weigh 1 g (accurate to 0.0001 g) of sample into a beaker, add 50 mL of deionized water, heat to boiling for 10 min, cool to room temperature, transfer to a 100 mL volumetric flask, dilute to the mark, shake well, and filter dry. Take 25 mL of the filtrate into an Erlenmeyer flask, add 2 drops of phenolphthalein indicator, adjust with nitric acid solution until the red color fades, then add 1 mL of potassium chromate indicator, and titrate with 0.01 mol / L silver nitrate standard titration solution until a brick-red precipitate appears and does not fade for 30 seconds. Perform a blank test simultaneously, and calculate Cl based on the amount of silver nitrate standard solution consumed. - The content was determined in triplicate, with a relative deviation of ≤0.5%.

[0112] (iv) Sulfate (SO4) 2- Content test (refer to GB / T 11899-1989 "Determination of Sulfate in Water - Gravimetric Method")

[0113] Weigh 2g (accurate to 0.0001g) of sample into a beaker, add 100mL of deionized water, heat to dissolve, cool to room temperature, transfer to a 200mL volumetric flask, dilute to volume, shake well, and filter dry. Take 100mL of the filtrate into a beaker, add 2mL of hydrochloric acid solution to acidify, heat to near boiling, slowly add 10mL of hot 0.1mol / L barium chloride solution while stirring, continue boiling for 10min, and keep warm in a boiling water bath for 1h. After cooling to room temperature, filter through a glass frit crucible dried to constant weight at 105℃, wash the precipitate with warm deionized water until no chloride ions are present in the washings (test with silver nitrate solution). Dry the crucible in a 105℃ oven to constant weight, and calculate SO4 based on the mass of the precipitate. 2- The content was determined in triplicate, with a relative deviation of ≤0.8%.

[0114] (v) Product yield calculation (refer to HG / T 3696-2002 "Methods for Calculating Yield of Chemical Products")

[0115] Calculate the product yield using the following formula:

[0116] Yield = (Actual mass of high-purity hafnium oxide product obtained × Hafnium oxide purity in the product) / (Mass of hafnium oxide in the raw material) × 100%

[0117] The formula is: Hafnium oxide mass in raw material = Hafnium oxide raw material mass × Hafnium oxide purity in raw material. The actual product mass is measured by electronic balance (accurate to 0.0001g), and the product purity is calculated by ICP-MS test results. The calculation is performed in triplicate, and the average value is taken as the final yield.

[0118] Example 1

[0119] Hafnium oxide sample 1 was prepared in Example 1 by the following steps.

[0120] (1) Raw material mixing and melting reaction: Weigh 100g hafnium oxide raw material, 200g sodium hydroxide (solid alkali), and 5g anhydrous sodium carbonate (flux), place them in an agate mortar and grind for 30 min until they are evenly mixed, then transfer them to a high-temperature resistant ceramic crucible. Place the crucible in a muffle furnace, set the heating rate to 5℃ / min, heat to 400℃, and hold for 0.5h to carry out the melting reaction. After the reaction is completed, allow it to cool naturally to room temperature to obtain a grayish-white alkali-fused clinker.

[0121] (2) Leaching and preparation of hafnium hydroxide precipitate: Deionized water was slowly added to the cooled alkali-fused clinker at a solid-liquid ratio of 1:4 (by weight). The mixture was placed in a constant-temperature water bath at 75°C and leached with stirring for 2 hours at a stirring rate of 300 r / min. After leaching, the mixture was filtered using a vacuum filtration device to obtain hafnium hydroxide precipitate, which was then washed three times with deionized water.

[0122] (3) Preparation of hafnium sulfate solution: The washed hafnium hydroxide precipitate was transferred to a corrosion-resistant reactor, and concentrated sulfuric acid (65% by weight) was added at a weight ratio of 1:3. The reactor was sealed and placed in an oil bath, heated to 120°C, and kept at that temperature for 1 hour. During the reaction, the mixture was stirred every 30 minutes for 10 minutes each time. After the reaction was completed, the mixture was cooled to room temperature to obtain a clear and transparent hafnium sulfate solution.

[0123] (4) Oxidation pretreatment: Transfer the hafnium sulfate solution to a constant temperature reaction tank, adjust the temperature to 45℃, and slowly add 35% hydrogen peroxide at a rate of 1 mL / min. The amount added is 0.5% of the weight of the hafnium sulfate solution. After the addition is completed, continue stirring and reacting for 1 hour to complete the oxidation pretreatment.

[0124] (5) Hafnium sulfate crystallization: The pre-treated hafnium sulfate solution was transferred to a rotary evaporator, and the vacuum degree was set to -0.08 MPa and the temperature to 80℃. The solution was evaporated and concentrated to 1 / 3 of its original volume. The concentrate was then transferred to a crystallizer and cooled to room temperature at a cooling rate of 2℃ / h. The solution was allowed to stand for crystallization for 8h. The crystals were collected by filtration, washed twice with a small amount of deionized water, and dried under vacuum (60℃, 2h) to obtain hafnium sulfate crystals.

[0125] (6) Hafnium oxychloride crystallization: Weigh hafnium sulfate crystals and add concentrated hydrochloric acid (32% by weight) at a weight ratio of 1:6. Transfer the solution to a reflux apparatus, heat to 80°C, and reflux for 2 hours. After the reaction is complete, transfer the reaction solution to a rotary evaporator and evaporate and concentrate it under vacuum of -0.09 MPa and 70°C until a crystalline film appears. Then transfer the solution to a crystallizer and cool it to room temperature at a cooling rate of 1°C / h. Allow it to crystallize for 12 hours, filter and collect the crystals, and vacuum dry (50°C, 3 hours) to obtain hafnium oxychloride crystals.

[0126] (7) Preparation of hafnium oxalate precipitate: Hafnium oxychloride crystals were dissolved in deionized water to prepare a 0.5 mol / L hafnium oxychloride solution. The solution was placed in a constant temperature water bath at 40°C. A 10% by weight oxalic acid aqueous solution was slowly added while stirring. The amount of oxalic acid added was 1.2 times the theoretical stoichiometric amount. After the addition was complete, stirring was continued for 1 hour. The solution was allowed to stand for 4 hours to precipitate. The precipitate was collected by filtration and washed with deionized water until no Cl- was found in the washing solution. - (Test with silver nitrate solution) Hafnium oxalate precipitate was obtained.

[0127] (8) Preparation of high-purity hafnium oxide by calcination: Hafnium oxalate precipitate was placed in a ceramic crucible and placed in a muffle furnace. The temperature was increased to 200℃ at a rate of 2℃ / min and held for 1 h. Then the temperature was increased to 750℃ at a rate of 4℃ / min and held for 2 h. After calcination, the mixture was naturally cooled to room temperature to obtain a white powder, which is hafnium oxide sample 1.

[0128] Based on the detailed description above regarding hafnium oxide purity, impurity content (alkali metals (Na, K) and iron, aluminum, calcium, silicon, titanium), and chloride ions (Cl... - ) content, sulfate (SO4) 2- The specific test method for the content of hafnium oxide was used to test sample 1, and the results are shown in Table 4 below. Furthermore, the product yield of Example 1 was calculated according to the product yield test method described in detail above, and the results are shown in Table 4 below.

[0129] Examples 2-10 and Comparative Examples 1-5

[0130] Hafnium oxide samples were prepared in a manner similar to that of Example 1, with the only difference being that the formulation and preparation conditions were changed as shown in Tables 3 and 4 below.

[0131] Based on the test methods described in detail above, the purity of hafnium oxide, impurity content (alkali metals (Na, K) and iron, aluminum, calcium, silicon, titanium), and chloride ion content (Cl) of Examples 2-10 and Comparative Examples 1-5 were tested. - ) content, sulfate (SO4) 2-The content and product yield were analyzed, and the results are shown in Table 5 below.

[0132] Table 3 Formulations and process conditions for Examples 1-10 (E1-E10)

[0133]

[0134] Table 4. Formulations and process conditions for Comparative Examples 1-5 (CE1-CE5)

[0135]

[0136] Table 5. Test results of Examples 1-10 (E1-E10) and Comparative Examples 1-5 (CE1-CE5)

[0137]

[0138] The results above show that the hafnium oxide products prepared in Examples 1-10 of this invention all achieve a purity of over 99.995%, with Examples 9 and 10 reaching 99.998% and 99.999% respectively, fully meeting the purity requirements of ultra-high purity hafnium oxide (4N5 and above) in high-end fields such as semiconductors and aerospace. In contrast, the products in Comparative Examples 1-5 have a maximum purity of only 99.982%, failing to reach the target purity of 99.995%, and their impurity content is significantly higher than that of the products in the Examples, demonstrating that the process of this invention has a significant effect on improving product purity and reducing impurity content.

[0139] Regarding impurity control, the products in the examples contained impurities such as Na, Fe, and Cl. - SO4 2- The contents of all were below 10 ppm, with the Na content in Example 10 being only 3.1 ppm, the Fe content 2.5 ppm, and the Cl content... - Content 2.8ppm, SO4 2- The Fe content was 3.2 ppm, and the contents of impurities such as Al, Ca, and Si+Ti were also at extremely low levels (1.1-1.5 ppm), effectively solving the technical problem of the difficulty in deeply removing stubborn impurities such as alkali metal ions and sulfate ions in existing technologies. In contrast, Comparative Example 1, lacking a hydrogen peroxide oxidation pretreatment step, had a Fe content as high as 18.3 ppm, a Na content of 15.7 ppm, and a Cl content of... - and SO4 2-The content of all impurities exceeded 16 ppm, indicating poor impurity removal. In Comparative Example 2, insufficient solid alkali (mass ratio 1:1, lower than the 1:2-5 range specified in this invention) led to incomplete melting of the raw materials, resulting in a product yield of only 78.3%, and significantly higher levels of impurities such as Na, Fe, and Al. In Comparative Example 3, the flux dosage exceeded the range specified in this invention (0.4, higher than the upper limit of 0.05-0.3), increasing the amount of impurities entrained in the leachate, particularly Cl. - The content reached 15.3 ppm; Comparative Example 4 had a melting temperature lower than the lower limit of 400°C (350°C) set by this invention, resulting in insufficient alkali fusion reaction, with a product purity of only 99.980% and a yield of 80.2%; Comparative Example 5 had a leaching solid-liquid ratio higher than the range of 1:4-1:6 (1:3) specified by this invention, leading to incomplete leaching of hafnium salts and SO4. 2- The content reached 13.5 ppm, with a yield of only 81.4%.

[0140] Regarding product yield, the yields of Examples 1-10 were all above 89%, with Example 10 achieving a yield as high as 93.5% and Example 9 achieving a yield of 92.3%, demonstrating that the process of this invention can ensure high purity while also exhibiting excellent yield performance. On the other hand, the yields of the comparative examples were generally between 78% and 83%, lower than those of the examples, indicating problems such as incomplete reaction of raw materials, severe impurity entrainment during crystallization, or excessive precipitation loss.

[0141] Furthermore, Example 10 exhibits superior product purity and yield, with lower impurity content. This is because the appropriate oxidation pretreatment temperature and dropping method effectively oxidize low-valence impurities in the solution, facilitating their removal during subsequent crystallization; the slow cooling rate and suitable crystallization time promote the orderly growth of hafnium sulfate and hafnium oxychloride crystals, reducing impurity entrainment; and the optimized calcination process ensures complete decomposition of hafnium oxalate, preventing residual impurities from affecting product performance.

[0142] In summary, this invention achieves efficient preparation of high-purity hafnium oxide by optimizing technical elements such as raw material ratio, melting temperature, leaching conditions, oxidation pretreatment parameters, crystallization rate, and calcination process. Compared with existing technologies, the product of this invention has higher purity, lower impurity content, and better yield. Furthermore, the process is shorter and more environmentally friendly, overcoming the technical bottlenecks of existing methods. It can meet the stringent requirements of high-end fields such as semiconductor device manufacturing and aerospace for ultra-high-purity hafnium oxide, and has significant industrial application value and market prospects.

[0143] Obviously, those skilled in the art can make various modifications and variations to this disclosure without departing from the spirit and scope of this disclosure. Therefore, if such modifications and variations fall within the scope of this invention, this disclosure is also intended to include such modifications and variations.

Claims

1. A method for alkali fusion A method for preparing high-purity hafnium oxide by crystallization, characterized in that, The method includes the following steps: (1) Mix hafnium oxide raw material with solid alkali and flux at a mass ratio of 1:2-5:0.05-0.3 and carry out a melting reaction at 400-700℃ for 0.5-4 hours to obtain alkali-fused clinker. The solid alkali is one or more of sodium hydroxide and potassium hydroxide, and the flux is one or more of sodium carbonate and sodium nitrate. (2) Add the alkali-fused clinker to deionized water for leaching and filtration to obtain hafnium hydroxide precipitate; (3) Add hafnium hydroxide precipitate to concentrated sulfuric acid and react at 120-180℃ for 1-3 hours to obtain hafnium sulfate solution, wherein the weight ratio of hafnium hydroxide precipitate to concentrated sulfuric acid is in the range of 1:3-1:5; (4) The temperature of the hafnium sulfate solution is lowered to 20-35°C, and hydrogen peroxide is added dropwise for oxidation pretreatment. The concentration of hydrogen peroxide is 25-35% by weight, and the weight of the added hydrogen peroxide is 0.5-2% of the weight of the hafnium sulfate solution. (5) The hafnium sulfate solution that has undergone oxidation pretreatment is evaporated, concentrated and cooled to crystallize, to obtain hafnium sulfate crystals, wherein the cooling rate is 0.5-2℃ / hour and the crystallization time is 8-16 hours; (6) Add hafnium sulfate crystals to concentrated hydrochloric acid and reflux for 2-4 hours, evaporate and concentrate and cool to crystallize to obtain hafnium oxychloride crystals, wherein the cooling rate is 1-3℃ / hour and the crystallization time is 12-24 hours; (7) Dissolve hafnium oxychloride crystals in deionized water, and then add oxalic acid aqueous solution to form hafnium oxalate precipitate; (8) The hafnium oxalate precipitate was calcined in an oxidizing atmosphere to obtain high-purity hafnium oxide.

2. The method according to claim 1, characterized in that, The hafnium oxide raw material comprises, by weight, more than 85% hafnium oxide, less than 12% zirconium oxide, less than 0.5% iron, and less than 0.2% aluminum, calcium, silicon and titanium.

3. The method according to claim 1, characterized in that, In step (1), the solid alkali is sodium hydroxide, the flux is anhydrous sodium carbonate, and the melting reaction is carried out at 550-650°C for 1.5-2.5 hours to obtain the alkali-fused clinker.

4. The method according to claim 1, characterized in that, In step (2), the alkali-fused clinker is added to deionized water at a weight ratio of 1:4 to 1:6, leached at 75-85°C for 2-4 hours and filtered to obtain the hafnium hydroxide precipitate.

5. The method according to claim 1, characterized in that, The concentrated sulfuric acid used in step (3) is an aqueous solution of sulfuric acid with a concentration in the range of 65-85% by weight.

6. The method according to claim 1, characterized in that, In step (6), the concentration of the concentrated hydrochloric acid is 32-37% by weight, the weight ratio of the hafnium sulfate crystals to the concentrated hydrochloric acid is in the range of 1:6-1:12, and the reflux temperature of the reflux reaction is 80-110℃.

7. The method according to claim 1, characterized in that, In step (7), the concentration of the oxalic acid aqueous solution is 10-15% by weight.

8. The method according to claim 1, characterized in that, In step (8), the calcination includes: heating to 200-400°C at a rate of 1-3°C / min, holding for 1-2 hours, and then heating to 750-950°C at a rate of 2-5°C / min, holding for 2-4 hours.

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