Preparation method of high-efficiency green high-purity hafnium oxide

Abstract

The application provides a high-efficiency green high-purity hafnium oxide preparation method, which comprises the following steps: melting hafnium oxide raw materials, solid alkali and fluxing agent to obtain alkali melting clinker; adding the alkali melting clinker into deionized water to perform leaching and filtration to obtain hafnium hydroxide precipitation; adding the hafnium hydroxide precipitation into concentrated sulfuric acid to obtain hafnium sulfate solution; adding a composite chelating agent into the hafnium sulfate solution to remove the precipitation to obtain low-zirconium hafnium sulfate solution; adding activated carbon into the low-zirconium hafnium sulfate solution to filter to obtain hafnium sulfate solution; evaporating and concentrating the hafnium sulfate solution and crystallizing to obtain hafnium sulfate crystal; adding the hafnium sulfate crystal into concentrated hydrochloric acid and refluxing, evaporating and cooling and crystallizing to obtain hafnium oxychloride crystal; dissolving the hafnium oxychloride crystal in water and adding oxalic acid to generate hafnium oxalate precipitation; and calcining the hafnium oxalate precipitation. The hafnium oxide obtained by the method has a purity of 99.995% or more and the zirconium impurity is reduced to 5 ppm or less.

Description

Technical Field

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

[0002] Hafnium oxide (HfO2), as a high-performance ceramic material, possesses a high melting point, high dielectric constant, excellent chemical stability, and optical properties, occupying an irreplaceable position in high-tech fields such as semiconductors, optical coatings, and the nuclear industry. In the semiconductor industry, it is a high-dielectric-constant (High-k) gate dielectric material that can replace traditional silicon dioxide, significantly improving chip integration and performance. In the optics field, high-purity hafnium oxide can be used to prepare high-precision optical coatings, enhancing device transmittance, wear resistance, and stability. In the nuclear industry, its high thermal neutron absorption cross-section makes it an ideal material for reactor control rods.

[0003] Different application areas have significantly different purity requirements for hafnium oxide. Electronic and optical grade products need to achieve a purity of over 99.995%, and the zirconium impurity content must be controlled below 5 ppm. This is because zirconium and hafnium belong to Group IVB elements, have similar atomic radii and extremely similar chemical properties, and coexist in nature in an isomorphic form. Zirconium typically contains 1%-3% (by weight) of hafnium, and trace amounts of zirconium impurities in hafnium can seriously affect the dielectric, optical, and nuclear properties of hafnium oxide, leading to a decrease in device performance or failure.

[0004] Currently, the industrial processes for preparing high-purity hafnium oxide are mainly divided into two categories: wet processes and pyrometallurgical processes. Wet processes have become the mainstream due to their simple operation, low cost, and ease of large-scale production. These include solvent extraction, ion exchange, and alkali fusion-crystallization methods. However, existing wet processes all have technical defects and cannot simultaneously achieve purity, efficiency, environmental friendliness, and safety.

[0005] Solvent extraction is currently the most widely used process, utilizing the difference in partition coefficients between zirconium and hafnium using organic extractants to achieve separation. Commonly used extractants include methyl isobutyl ketone (MIBK) and tributyl phosphate (TBP). However, this method has many problems: organic extractants are volatile, flammable, and toxic, easily generating organic wastewater and waste gas during production, resulting in significant environmental pressure and safety risks; the extraction efficiency is limited, requiring 15-20 or even more stages of multi-stage extraction, leading to complex processes, large equipment investment, high energy consumption, poor batch stability, and difficulty in reducing zirconium impurities to below 5 ppm; the organic phase is prone to emulsification and significant loss, and the back-extraction and regeneration processes are complex, increasing costs and environmental burden.

[0006] Ion exchange method achieves separation by selective adsorption of zirconium and hafnium ions by ion exchange resin. It has certain advantages in controlling ultra-high purity impurities, but it has problems such as small processing capacity, frequent resin regeneration, long process cycle and high cost, making it difficult to adapt to large-scale industrial production and easy to introduce new impurities.

[0007] The alkaline fusion-crystallization method is relatively environmentally friendly. Its core steps are alkaline fusion of raw materials, water leaching, acid dissolution, crystallization purification, precipitation and calcination, which avoids the use of organic extractants. However, it lacks targeted zirconium separation methods. It is difficult to remove zirconium impurities deeply through multi-stage crystallization alone. The product purity can usually only reach about 99.99%, and the zirconium impurity content is 20-50 ppm, which cannot meet the requirements of electronic grade and optical grade. Moreover, the process steps are cumbersome, energy consumption is high, and product yield is low.

[0008] In addition, processes such as sublimation, electrochemical separation, and supercritical fluid extraction have not yet been industrialized due to high equipment costs, high operational risks, or because they are still in the laboratory research stage.

[0009] With the rapid development of high-tech fields such as electronics and optics, the demand for high-purity hafnium oxide is constantly increasing, and the requirements for product purity, environmental friendliness, and safety are becoming increasingly stringent. Therefore, developing a simple, efficient, environmentally friendly, and highly accurate method for preparing high-purity hafnium oxide without the use of organic extractants has become an urgent technical problem to be solved in this field. Summary of the Invention

[0010] Based on the technical problems described above, this invention aims to overcome the shortcomings of existing high-purity hafnium oxide preparation processes, such as poor environmental friendliness, low safety, incomplete removal of zirconium impurities, and complex processes. This invention employs a composite chelating agent with high selectivity for zirconium to achieve deep removal of zirconium impurities, and this chelating agent can completely remove them without residue. No organic extractants are used throughout the process, avoiding the generation of organic wastewater and flammability risks. The final product obtained has a purity of over 99.995% and a zirconium impurity content ≤5ppm, meeting the requirements of electronic and optical applications. Furthermore, the process is simple, energy-efficient, and suitable for industrial production.

[0011] Specifically, the present invention provides an efficient and green method for preparing high-purity hafnium oxide, the method comprising the following steps:

[0012] (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.

[0013] (2) The alkaline fused clinker is added to deionized water for leaching and filtration to obtain hafnium hydroxide precipitate;

[0014] (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.

[0015] (4) Add a composite chelating agent to the hafnium sulfate solution, stir thoroughly and react at 50-60°C for 1.5-3 hours, then let it stand and settle for 2-4 hours, and then filter to remove the zirconium-composite chelating agent precipitate to obtain a low zirconium hafnium sulfate solution. The composite chelating agent is a mixture of sodium diethyldithiocarbamate and 8-hydroxyquinoline-5-sulfonic acid in a weight ratio of 3:1-4:1.

[0016] (5) Add activated carbon to the low-zirconium hafnium sulfate solution for adsorption, and filter to obtain hafnium sulfate solution;

[0017] (6) The hafnium sulfate solution is evaporated, concentrated, and cooled to crystallize, thereby obtaining hafnium sulfate crystals;

[0018] (7) The hafnium sulfate crystals were added to concentrated hydrochloric acid and refluxed for 2-4 hours. The mixture was then evaporated, concentrated, and cooled to crystallize, yielding hafnium oxychloride crystals.

[0019] (8) Dissolve the hafnium oxychloride crystals in deionized water, and then add an aqueous oxalic acid solution to generate hafnium oxalate precipitate;

[0020] (9) The hafnium oxalate precipitate is calcined in an oxidizing atmosphere to obtain the high-purity hafnium oxide.

[0021] 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.

[0022] According to certain preferred embodiments of the present invention, in step (1), the solid alkali is sodium hydroxide, the flux is anhydrous sodium carbonate, and a melting reaction is carried out at 550-650°C for 1.5-2.5 hours to obtain the alkali-fused clinker.

[0023] 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 75-85°C for 2-4 hours and filtered to obtain the hafnium hydroxide precipitate.

[0024] 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.

[0025] According to certain preferred embodiments of the present invention, in step (4), the weight of the added composite chelating agent is 1-3 times the weight of the hafnium sulfate solution.

[0026] According to certain preferred embodiments of the present invention, in step (5), the amount of activated carbon added is 0.5-1.0% by weight of the low zirconium hafnium sulfate solution, and the adsorption temperature of activated carbon adsorption is 50-60°C, and the adsorption time is 1-1.5 hours.

[0027] According to certain preferred embodiments of the present invention, in step (6), the cooling rate is 0.5-2°C / hour, and the crystallization time is 8-16 hours.

[0028] According to certain preferred embodiments of the present invention, in step (7), the concentration of concentrated hydrochloric acid is 32-37% by weight.

[0029] According to certain preferred embodiments of the present invention, in step (7), the weight ratio of the hafnium sulfate crystals to the concentrated hydrochloric acid is in the range of 1:6 to 1:12.

[0030] According to certain preferred embodiments of the present invention, in step (7), the reflux temperature of the reflux reaction is 80-110°C.

[0031] According to certain preferred embodiments of the present invention, in step (7), the cooling rate is 1-3°C / hour.

[0032] According to certain preferred embodiments of the present invention, in step (7), the crystallization time is 12-24 hours.

[0033] According to certain preferred embodiments of the present invention, in step (8), the concentration of the oxalic acid aqueous solution is 10-15 by weight.

[0034] According to certain preferred embodiments of the present invention, in step (9), 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.

[0035] 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%, a zirconium impurity content ≤ 5 ppm, and Na, K, Fe, and Cl content are relatively low. - SO4 2- The content of all of them is less than 10 ppm.

[0036] Compared with existing technologies, the beneficial effects of this invention are as follows: First, zirconium impurities are thoroughly removed, resulting in high product purity. Using a targeted composite chelating agent combined with a multi-step purification process, the product purity reaches over 99.995%, with zirconium impurities ≤5ppm and the total content of other metal impurities ≤10ppm, meeting electronic and optical grade requirements. Second, it is green and safe, with no organic extractants throughout the process. The chelating agent can be completely decomposed without residue, and the zirconium-containing alkaline solution and washing liquid can be recycled, achieving zero wastewater discharge. Third, the process is simple and efficient, requiring no complex multi-stage extraction equipment. Fourth, it has low energy consumption and controllable costs. Fifth, the product performance is stable and adaptable to various high-tech fields. Attached Figure Description

[0037] 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.

[0038] Figure 1 A process flow diagram for preparing high-purity hafnium oxide according to the technical solution of the present invention is shown. Detailed Implementation

[0039] 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.

[0040] 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.

[0041] As mentioned above, firstly, existing mainstream processes struggle to achieve deep removal of zirconium impurities. Solvent extraction can only reduce zirconium impurities to above 200 ppm, while the alkali fusion-crystallization method results in zirconium impurity content of 20-50 ppm, failing to meet the stringent requirements of ≤5 ppm for electronic and optical grades. Secondly, the organic extractants used in solvent extraction are volatile, flammable, and toxic, easily generating organic wastewater and waste gas, posing significant environmental pressure and safety risks, and resulting in high carbon emission intensity. Furthermore, ion exchange methods have low throughput, require frequent resin regeneration, and are costly, hindering large-scale production; the alkali fusion-crystallization method is cumbersome, energy-intensive, and has low yield. In addition, other processes or equipment, such as sublimation and electrochemical separation, are costly, risky, or still in the laboratory stage and not yet industrialized. With the increasing demand for high-purity hafnium oxide in high-tech fields, there is an urgent need for a preparation method that can balance zirconium removal precision, environmental friendliness, safety, efficiency, and large-scale production. This invention aims to solve these problems.

[0042] Specifically, the present invention provides an efficient and green method for preparing high-purity hafnium oxide, comprising the following steps:

[0043] (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.

[0044] (2) The alkaline fused clinker is added to deionized water, leached at 75-85℃ and filtered to obtain hafnium hydroxide precipitate;

[0045] (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.

[0046] (4) Add a composite chelating agent to the hafnium sulfate solution, stir thoroughly and react at 50-60°C for 1.5-3 hours, then let it stand and settle for 2-4 hours, and then filter to remove the zirconium-composite chelating agent precipitate to obtain a low zirconium hafnium sulfate solution. The composite chelating agent is a mixture of sodium diethyldithiocarbamate and 8-hydroxyquinoline-5-sulfonic acid in a weight ratio of 3:1-4:1.

[0047] (5) Add activated carbon to the low-zirconium hafnium sulfate solution for adsorption, and filter to obtain hafnium sulfate solution;

[0048] (6) The hafnium sulfate solution is evaporated, concentrated, and cooled to crystallize, thereby obtaining hafnium sulfate crystals;

[0049] (7) The hafnium sulfate crystals were added to concentrated hydrochloric acid and refluxed for 2-4 hours. The mixture was then evaporated, concentrated, and cooled to crystallize, yielding hafnium oxychloride crystals.

[0050] (8) Dissolve the hafnium oxychloride crystals in deionized water, and then add an aqueous oxalic acid solution to generate hafnium oxalate precipitate;

[0051] (9) The hafnium oxalate precipitate is calcined in an oxidizing atmosphere to obtain the high-purity hafnium oxide.

[0052] Figure 1 The process flow diagram for preparing high-purity hafnium oxide according to the technical solution of the present invention is shown, specifically including the following steps:

[0053] (1) The hafnium oxide raw material is melted with solid alkali and flux to obtain alkali-fused clinker;

[0054] (2) Add the alkali-fused clinker to deionized water for leaching and filtration to obtain hafnium hydroxide precipitate;

[0055] (3) Add hafnium hydroxide precipitate to concentrated sulfuric acid to obtain hafnium sulfate solution;

[0056] (4) Add a composite chelating agent to the hafnium sulfate solution to remove the precipitate and obtain a low-zirconium hafnium sulfate solution;

[0057] (5) Add activated carbon to the low-zirconium hafnium sulfate solution and filter to obtain hafnium sulfate solution;

[0058] (6) The hafnium sulfate solution was evaporated, concentrated, and crystallized to obtain hafnium sulfate crystals;

[0059] (7) Add hafnium sulfate crystals to concentrated hydrochloric acid and reflux, evaporate and cool to crystallize, and obtain hafnium oxychloride crystals;

[0060] (8) Dissolve hafnium oxychloride crystals in water and add oxalic acid to form hafnium oxalate precipitate;

[0061] (9) Calcining the hafnium oxalate precipitate.

[0062] Specifically, 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.

[0063] The following details the specific steps of the method for preparing high-purity hafnium oxide according to the present invention.

[0064] Step (1): Raw material mixing and melting reaction

[0065] The goal of step (1) is to convert solid hafnium oxide feedstock into hafnium salts, while creating conditions for subsequent impurity removal.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] Step (2): Leaching and preparation of hafnium hydroxide precipitation

[0070] 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.

[0071] 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.

[0072] Step (3): Preparation of hafnium sulfate solution

[0073] The purpose of step (3) is to convert hafnium hydroxide precipitate into hafnium sulfate, which is easily soluble in water, in order to prepare for subsequent zirconium removal and crystallization purification using a composite chelating agent.

[0074] 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.

[0075] 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.

[0076] Step (4): Zirconium removal using composite chelating agent

[0077] One of the innovations of this invention lies in the use of sodium diethyldithiocarbamate (DDTC) and 8-hydroxyquinoline-5-sulfonic acid (8HQS) in a weight ratio of 3:1 to 4:1 as a composite chelating agent. This combination exhibits high selectivity for zirconium ions, enabling deep removal of zirconium impurities. The complexation constant of DDTC with zirconium ions is much higher than that with hafnium ions, allowing it to preferentially form stable, insoluble complexes with zirconium ions. 8HQS, on the other hand, has excellent selective recognition ability for zirconium ions; the hydroxyl and quinoline nitrogen atoms in its molecular structure can form multidentate complexes with zirconium ions, further enhancing its ability to capture zirconium ions.

[0078] When used in a weight ratio of 3:1 to 4:1, the two exhibit a significant synergistic effect. DDTC is highly hydrophobic, and the resulting zirconium-DDTC complex is also highly hydrophobic and easily settles, but its ability to capture low concentrations of zirconium ions is limited when used alone. 8HQS is highly hydrophilic, allowing it to diffuse rapidly in aqueous solutions and form complexes with low concentrations of zirconium ions. Furthermore, its complexes can interact with the DDTC-zirconium complexes to form larger flocculent precipitates, improving settling rate and filtration efficiency. If the ratio deviates from 3:1 to 4:1, the synergistic effect weakens. For example, an excessively high DDTC ratio can lead to overly hydrophobic precipitates that float on the surface and are difficult to filter; an excessively high 8HQS ratio will reduce the settling performance of the precipitate and prolong the settling time.

[0079] Preferably, the amount of the composite chelating agent added is 1-3% by weight of the hafnium sulfate solution. This dosage range is based on the optimized result of the zirconium ion concentration in the solution. In actual production, the dosage of the chelating agent can be adjusted according to the zirconium oxide content in the raw material. When the zirconium content in the raw material is high, the dosage can be appropriately increased to 2-3% by weight; when the zirconium content is low, the dosage can be controlled at 1-2% by weight.

[0080] The reaction temperature is controlled between 50-60℃. This temperature range ensures the chelation reaction rate while preventing the chelating agent from decomposing due to excessive heat. Below 50℃, the reaction rate is slow, requiring a longer reaction time; above 60℃, DDTC is easily decomposed, and the complexation stability of 8HQS decreases, leading to poorer zirconium removal. The reaction time is set to 1.5-3 hours to ensure sufficient reaction between zirconium ions and the chelating agent to form a stable complex.

[0081] After the reaction is complete, allow the solution to settle for 2-4 hours to allow the zirconium-composite chelating agent to fully settle to the bottom of the container, facilitating filtration and separation. During the settling process, avoid stirring or vibration to prevent the precipitate from becoming suspended. Use a vacuum filtration device to remove the precipitate, obtaining a low-zirconium hafnium sulfate solution. At this point, the zirconium ion content in the solution can be reduced to below 5 ppm, achieving deep removal of zirconium impurities.

[0082] Step (5): Activated carbon adsorption

[0083] Activated carbon is added in step (5) to achieve efficient adsorption and removal of residual composite chelating agent (DDTC; 8HQS).

[0084] From the perspective of adsorption mechanism, activated carbon possesses a porous structure and a large specific surface area, with its surface rich in oxygen-containing functional groups such as hydroxyl and carboxyl groups, providing ample active sites for adsorption. For DDTC, the dithiocarbamate group in its molecular structure exhibits strong polarity and coordination characteristics, readily dissociating into negatively charged ions in aqueous solution. These ions can be adsorbed through hydrogen bonding and electrostatic attraction with the oxygen-containing functional groups on the activated carbon surface. Simultaneously, the hydrophobic hydrocarbon group (ethyl) in the DDTC molecule can interact hydrophobically with the graphitized surface of activated carbon, further enhancing the adsorption effect. For 8-hydroxyquinoline-5-sulfonic acid, its molecule possesses functional structures including hydroxyl (-OH), a quinoline ring, and a sulfonic acid group (-SO3H). The strong polarity of the hydroxyl and sulfonic acid groups enables them to form hydrogen bonds and electrostatic adsorption with the functional groups on the activated carbon surface. Furthermore, the aromatic structure of the quinoline ring can bind to the graphite layer structure of activated carbon through π-π stacking interactions. This synergistic effect of multiple forces significantly improves adsorption selectivity and adsorption capacity.

[0085] Another purpose of activated carbon adsorption is to remove residual trace impurity ions and colloidal particles that may form in the solution.

[0086] Preferably, the amount of activated carbon added is 0.5-1.0% by weight of the low-zirconium hafnium sulfate solution. During adsorption, continuous stirring at a rate of approximately 200 r / min is preferred to promote solution flow, ensure uniform dispersion of activated carbon particles, and prevent local adsorption saturation. After adsorption, the activated carbon particles are removed by filtration to obtain a purified hafnium sulfate solution. At this point, the content of organic impurities and trace metal impurities in the solution can be reduced to below 1 ppm, further improving the purity of the solution.

[0087] Step (6): Hafnium sulfate cooling crystallization

[0088] Step (6) involves the separation of 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.

[0089] 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.

[0090] Step (7): Hafnium oxychloride crystallization purification

[0091] In step (7), 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] Step (8): Preparation of hafnium oxalate precipitate

[0096] Step (8) 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 + The separation of ).

[0097] 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.

[0098] 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.

[0099] Step (9): Calcination of hafnium oxalate to prepare high-purity hafnium oxide

[0100] Step (9) 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.

[0101] 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).

[0102] 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.

[0103] The preparation process of this invention consists of multiple steps, including melting, leaching, acid dissolution, chelation to remove zirconium, oxidative decomposition, crystallization purification, precipitation, and calcination. These steps form a close synergistic effect, jointly achieving high purity and high yield of the product.

[0104] 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.

[0105] Example

[0106] 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".

[0107] Table 1 below lists specific information about the raw materials used in the embodiments and comparative examples of the present invention.

[0108] Table 1 List of Experimental Materials

[0109]

[0110] Table 2 below lists specific information about the experimental equipment used in the embodiments and comparative examples of the present invention.

[0111] Table 2 List of Experimental Equipment

[0112]

[0113] Test methods

[0114] According to the method described in detail below, the hafnium oxide samples prepared in the following examples and comparative examples are compared regarding hafnium oxide purity, zirconium and other impurities, and chloride ion content (Cl). - ) content, sulfate (SO4) 2- The content and product yield were evaluated.

[0115] (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")

[0116] 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.

[0117] (II) Test for zirconium and other impurities (Na, K, Fe, Al, Ca, Si, Ti) content (refer to GB / T 34500.2-2017)

[0118] The same sample pretreatment method as used for hafnium oxide purity testing was employed. The concentrations of Zr, 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 (…). 90 Zr、 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.

[0119] (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")

[0120] 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%.

[0121] (iv) Sulfate (SO4) 2- Content test (refer to GB / T 11899-1989 "Determination of Sulfate in Water - Gravimetric Method")

[0122] 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%.

[0123] (v) Product yield calculation (refer to HG / T 3696-2002 "Methods for Calculating Yield of Chemical Products")

[0124] Calculate the product yield using the following formula:

[0125] 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%

[0126] 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.

[0127] Example 1

[0128] Hafnium oxide sample 1 was prepared in Example 1 by the following steps.

[0129] (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 30min 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.

[0130] (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.

[0131] (3) Preparation of hafnium sulfate solution: The washed hafnium hydroxide precipitate was transferred to a corrosion-resistant reactor, and 65% concentrated sulfuric acid was added at a weight ratio of 1:3. After sealing the reactor, it was placed in an oil bath and heated to 120°C. The reaction was kept at this temperature for 1 hour, and the mixture was stirred once 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.

[0132] (4) Removal of zirconium by composite chelating agent: Add 1% by weight of composite chelating agent (sodium diethyldithiocarbamate and 8-hydroxyquinoline-5-sulfonic acid in a weight ratio of 3:1) to hafnium sulfate solution, stir thoroughly, react at 50°C for 1.5 h, then let stand and settle for 2 h, and filter with a vacuum filtration device to remove the zirconium-composite chelating agent precipitate to obtain a low-zirconium hafnium sulfate solution.

[0133] (5) Activated carbon adsorption: Add 0.5% of the weight of the low zirconium hafnium sulfate solution to the low zirconium hafnium sulfate solution, adsorb at 50°C for 1 h, and filter to obtain purified hafnium sulfate solution.

[0134] (6) Hafnium sulfate crystallization: The purified hafnium sulfate solution was transferred to a rotary evaporator, and the vacuum degree was set to -0.08 MPa and the temperature to 80 °C. 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 °C / h. The solution was allowed to stand for crystallization for 8 h. The crystals were collected by filtration, washed twice with a small amount of deionized water, and dried under vacuum (60 °C, 2 h) to obtain hafnium sulfate crystals.

[0135] (7) 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 a vacuum of -0.09 MPa and a temperature of 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 stand and crystallize for 12 hours. Filter and collect the crystals, and vacuum dry (50°C, 3 hours) to obtain hafnium oxychloride crystals.

[0136] (8) 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 with 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.

[0137] (9) 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 1℃ / min and held for 1 h. Then the temperature was increased to 750℃ at a rate of 2℃ / min and held for 2 h. After calcination, the mixture was naturally cooled to room temperature to obtain white powdered hafnium oxide sample 1.

[0138] Based on the detailed description above regarding hafnium oxide purity, impurity content (zirconium, alkali metals (Na, K) and iron, aluminum, calcium, silicon, titanium), and chloride ion content (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.

[0139] Example 2-10

[0140] Hafnium oxide samples were prepared in a manner similar to that of Example 1, except that the formulation and preparation conditions were changed as shown in Table 3 below.

[0141] Based on the test methods described in detail above, Examples 2-10 were tested regarding hafnium oxide purity, impurity content (zirconium, alkali metals (Na, K), and iron, aluminum, calcium, silicon, titanium), and chloride ion content (Cl). - ) content, sulfate (SO4) 2- The content and product yield were analyzed, and the results are shown in Table 4 below.

[0142] Comparative Example 1

[0143] Comparative hafnium oxide samples were prepared in a manner similar to that of Example 1, except that step (4) of removing zirconium with the composite chelating agent was not performed.

[0144] Based on the test methods described above, Comparative Example 1 was tested for hafnium oxide purity, impurity content (zirconium, alkali metals (Na, K) and iron, aluminum, calcium, silicon, titanium), and chloride ions (Cl). - ) content, sulfate (SO4) 2- The content and product yield were analyzed, and the results are shown in Table 4 below.

[0145] Comparative Example 2

[0146] Comparative Example 2 was prepared in a manner similar to that of Example 1, except that in step (4), a chelating agent (sodium diethyldithiocarbamate) at a weight of 1% of the solution was added to the hafnium sulfate solution, stirred thoroughly, reacted at 50°C for 1.5 h, and then allowed to stand and settle for 2 h. The zirconium-chelating agent precipitate was removed by filtration using a vacuum filtration device.

[0147] Based on the test methods described above, Comparative Example 2 was tested for hafnium oxide purity, impurity content (zirconium, alkali metals (Na, K) and iron, aluminum, calcium, silicon, titanium), and chloride ions (Cl). - ) content, sulfate (SO4) 2- The content and product yield were analyzed, and the results are shown in Table 4 below.

[0148] Comparative Example 3

[0149] Comparative Example 3 was prepared in a manner similar to that of Example 1, except that in step (4), a chelating agent (8-hydroxyquinoline-5-sulfonic acid) at a weight of 1% of the solution was added to the hafnium sulfate solution, stirred thoroughly, reacted at 50°C for 1.5 h, and then allowed to stand and settle for 2 h. The zirconium-chelating agent precipitate was removed by filtration using a vacuum filtration device.

[0150] Based on the test methods described above, Comparative Example 3 was tested for hafnium oxide purity, impurity content (zirconium, alkali metals (Na, K) and iron, aluminum, calcium, silicon, titanium), and chloride ions (Cl). - ) content, sulfate (SO4) 2- The content and product yield were analyzed, and the results are shown in Table 4 below.

[0151] Table 3 Formulations and process conditions for Examples 1-10 (E1-E10)

[0152]

[0153] Table 4. Test results of Examples 1-10 (E1-E10) and Comparative Examples 1-3 (CE1-CE3)

[0154]

[0155] The results above show that the hafnium oxide products prepared in Examples 1-10 of this invention exhibit excellent purity and impurity control performance, meeting the stringent requirements of electronic and optical grade materials. Regarding hafnium oxide purity, the purity of all products in these examples reaches 99.995% or higher, with Examples 2, 4, 6, and 9 achieving a purity as high as 99.996%, superior to the purity of Comparative Examples 1-3 (99.978%-99.985%). This demonstrates that the multi-step purification process of this invention can effectively remove various impurities, achieving deep purification of hafnium oxide.

[0156] The present invention demonstrates a particularly significant technical advantage in the removal of zirconium impurities. The zirconium impurity content in Examples 1-10 was controlled within the range of 4.0-4.9 ppm, meeting the target requirement of ≤5 ppm. In contrast, Comparative Example 1, which did not employ the composite chelating agent removal step, had a zirconium impurity content as high as 35.6 ppm. Comparative Examples 2 and 3, which used a single chelating agent, had zirconium impurity contents of 18.3 ppm and 22.5 ppm, respectively. This result confirms that the composite chelating agent of sodium diethyldithiocarbamate and 8-hydroxyquinoline-5-sulfonic acid (weight ratio 3:1-4:1) selected in this invention has extremely strong selective chelating ability for zirconium ions, achieving deep removal of zirconium impurities and solving the technical problem of incomplete zirconium-hafnium separation in existing processes.

[0157] Regarding other impurity control, the contents of metallic impurities such as Na, Fe, Al, Ca, and Si+Ti in the products of the examples were all at low levels. Specifically, the Na content was 7.5-8.5 ppm, the Fe content was 6.1-7.1 ppm, the Al content was 3.4-4.0 ppm, the Ca content was 2.8-3.4 ppm, the total Si+Ti content was 2.1-2.7 ppm, and K impurities were not detected. Meanwhile, Cl... - The content should be controlled at 5.6-6.7 ppm, SO4 2- The content was controlled between 7.1 and 8.2 ppm, and the content of all impurities was below 10 ppm. In contrast, the contents of various impurities in Comparative Examples 1-3 were all relatively high, especially the Fe content in Comparative Example 1, which reached 18.5 ppm, and the Cl content was also high. - The content reaches 17.6 ppm, indicating that the activated carbon adsorption and multi-step crystallization process of the present invention can effectively remove residual chelating agents and other trace impurities, ensuring product purity.

[0158] Regarding product yield, the yields of Examples 1-10 were consistently between 89.1% and 90.6%, with an average yield of 89.9%, while the yields of Comparative Examples 1-3 were only 82.5%-85.7%. This indicates that the process design of the present invention not only achieves efficient purification but also effectively reduces material loss during the intermediate process, balancing purification effect and production efficiency.

[0159] In summary, this invention successfully prepared high-purity hafnium oxide with a purity ≥99.995%, zirconium impurities ≤5ppm, and other impurities all below 10ppm through targeted zirconium removal using composite chelating agents, multi-step purification, and optimized process parameters. Furthermore, the product yield is high, and the process is green and safe, overcoming the shortcomings of existing solvent extraction methods (high environmental risks), ion exchange methods (high costs), and traditional alkali fusion-crystallization methods (poor purification effects). It possesses significant technical advantages and promising prospects for industrial application.

[0160] 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 highly efficient and environmentally friendly method for preparing high-purity hafnium oxide, characterized in that, 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) Add a composite chelating agent to the hafnium sulfate solution, stir and react at 50-60℃ for 1.5-3 hours, then let it stand and settle for 2-4 hours, and then filter to remove the zirconium-composite chelating agent precipitate to obtain a low zirconium hafnium sulfate solution. The composite chelating agent is a mixture of sodium diethyldithiocarbamate and 8-hydroxyquinoline-5-sulfonic acid in a weight ratio of 3:1-4:

1. (5) Add activated carbon to the low zirconium hafnium sulfate solution for adsorption, and filter to obtain hafnium sulfate solution; (6) The hafnium sulfate solution was evaporated, concentrated, and cooled to crystallize, yielding hafnium sulfate crystals; (7) Add hafnium sulfate crystals to concentrated hydrochloric acid and reflux for 2-4 hours. Then evaporate, concentrate, and cool to crystallize to obtain hafnium oxychloride crystals. (8) Dissolve hafnium oxychloride crystals in deionized water, and then add oxalic acid aqueous solution to form hafnium oxalate precipitate; (9) The hafnium oxalate precipitate was calcined in an oxidizing atmosphere to obtain high-purity hafnium oxide.

2. The preparation 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 preparation method according to claim 1, characterized in that, The solid alkali is sodium hydroxide, the flux is anhydrous sodium carbonate, and the mixture is subjected to a melting reaction at 550-650°C for 1.5-2.5 hours to obtain the alkali-fused clinker.

4. The preparation 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 preparation 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 preparation method according to claim 1, characterized in that, In step (4), the weight of the added composite chelating agent is 1-3 times the weight of the hafnium sulfate solution.

7. The preparation method according to claim 1, characterized in that, In step (5), the amount of activated carbon added is 0.5-1.0% by weight of the low zirconium hafnium sulfate solution, and the adsorption temperature of activated carbon adsorption is 50-60℃, and the adsorption time is 1-1.5 hours.

8. The preparation method according to claim 1, characterized in that, In step (6), the cooling rate is 0.5-2 °C / hour, and the crystallization time is 8-16 hours; and / or In step (7), 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 to 1:12, the reflux temperature of the reflux reaction is 80-110°C, the cooling rate is 1-3°C / hour, and the crystallization time is 12-24 hours.

9. The preparation method according to claim 1, characterized in that, In step (8), the concentration of the oxalic acid aqueous solution is 10-15% by weight.

10. The preparation method according to claim 1, characterized in that, In step (9), 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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