High-purity quartz sand preparation method based on low-temperature solvent freezing method

Abstract

The invention belongs to the technical field of quartz sand preparation, and discloses a high-purity quartz sand preparation method based on a low-temperature solvent freezing method. According to the preparation method provided by the invention, the silicic acid is taken as a raw material, the silicic acid sol is added to accelerate gelation, 10-20nm silicic acid mass points formed in the silicic acid sol are taken as seed crystals to be mixed with the silicic acid, the gelation time can be shortened to 1-3h from the traditional 12-24h, and the size uniformity of gel particles is improved by 50%. According to the preparation method of the high-purity quartz sand, low-temperature solvent freezing regulation is taken as a core, integrated treatment of a rotary quartz tube furnace and static calcination of a vacuum aluminum oxide ceramic inner chamber high-temperature furnace are integrated, and the quartz sand with the purity larger than or equal to 99.9999%, the sphericity larger than or equal to 0.88, the particle size distribution CV smaller than or equal to 5%, the hydroxyl content smaller than or equal to 20 ppm and the total metal impurity content smaller than or equal to 1 ppm is obtained. The method is suitable for the fields with strict requirements on purity, morphology, particle size uniformity, hydroxyl content and impurity content of quartz sand.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of quartz sand preparation, and relates to a high-purity quartz sand preparation method based on a low-temperature solvent freezing method. BACKGROUND

[0002] High-purity quartz sand is quartz sand with a SiO2 purity of ≥99.995% (4N5 grade) and Fe2O3≤0.001%, and has the characteristics of small thermal expansion coefficient, high temperature resistance (melting point about 1750℃), high insulation (resistivity >10 16 Ω·cm), high hardness (Mohs 7 grade), strong acid and alkali corrosion resistance, and light transmittance >92%, refractive index 1.54~1.55, and is a key material for manufacturing optical devices, and is widely used in high-tech fields such as semiconductors, photovoltaics, optical fiber communication, aerospace, etc.

[0003] At present, high-purity quartz sand on the market is mainly obtained by natural mineral purification method, however, high-purity natural quartz ore bed needs to rely on import, and domestic lacks 4N5 grade or above mineral resources, resulting in high cost, and gas-liquid inclusions in natural ore are difficult to completely remove in conventional process, affecting the purity of quartz sand. Chemical synthesis of quartz sand is also one of the main technical paths, for example, based on liquid phase synthesis method, sodium silicate is reacted with acid to generate precipitate, and quartz sand is prepared by washing, drying and calcining, but the preparation process has the problems of easy agglomeration and limited product purity (usually <4N).

[0004] At the same time, the quartz sand prepared by the existing process also has the problems of large particle size distribution variation coefficient and low sphericity, which is difficult to meet the application scenarios with high requirements for particle morphology and distribution consistency in high-end packaging, precision casting and other fields. The particle size distribution variation coefficient (CV) of the quartz sand prepared by the existing process is generally between 10~30%, and the existence of inclusions in natural quartz ore and the uniformity of the crystal structure of quartz ore will directly affect the particle size distribution of quartz sand. The air flow classification technology can significantly reduce the CV to about 15%, but the equipment investment is large. The quartz sand prepared by this process is mostly irregular angular, and the sphericity is generally between 0.6~0.7, even if it is shaped by ball milling, it can only be improved to about 0.8.

[0005] Therefore, it is of great significance to explore a high-purity quartz sand preparation method to solve the problems of easy agglomeration, difficulty in removing inclusions and impurities, uneven particle size distribution and low sphericity existing in the existing process. SUMMARY

[0006] To solve the above technical problems, the application provides a high-purity quartz sand preparation method based on a low-temperature solvent freezing method.

[0007] In one aspect, the application provides a high-purity quartz sand preparation method based on a low-temperature solvent freezing method, which specifically comprises the following steps: S1: Silica sol preparation: Dissolve silica with a purity of ≥99.9999% in ultrapure water to prepare a 0.3-0.8 wt% silica solution; transfer the silica solution into a 99.99% quartz material stirring tank, stir at a speed of 1000-1500 rpm, and drop 0.5-2 mol / L electronic grade hydrochloric acid at a rate of 1-3 mL / min to adjust the pH to 1.5-2.5, and continuously stir for 30-60 min to obtain uniformly dispersed silica sol.

[0008] S2: Gel preparation: Take silica with a water content of 90-95 wt%, mix it with the silica sol of S1 at a volume ratio of 10:1-3, and place it in a 99.99% quartz material reaction kettle, stir at 25-40℃ and 500-800 rpm for 1-3 h to form a uniform block-shaped gel with a water content of 85-90 wt%.

[0009] S3: Gel crushing and centrifugal dewatering: Crush the block-shaped gel into 0.5-2 mm particles using a 99.99% quartz material crusher, transfer the particles into a polytetrafluoroethylene centrifugal bag, and centrifuge at 8000-10000 rpm for 10-15 min to remove free water, obtaining gel particles with a water content of 70-75 wt%.

[0010] S4: Low-temperature solvent freezing treatment: Add the gel particles to a low-temperature solvent pre-cooled to -5 to -30℃ at a solid-liquid ratio of 1:8-12 (g:mL), stir at 400-600 rpm for 20-120 min in a constant temperature environment of -5 to -30℃, and maintain a temperature fluctuation of ≤±2℃. The low-temperature solvent is one of methanol, n-hexane or cyclohexane, and the viscosity of the above low-temperature solvent is 0.8-1.8 mPa·s at a temperature of -5 to -30℃.

[0011] S5: filtration, washing and thawing: the frozen particles are filtered and separated by using a 0.1 μm quartz fiber filter membrane, and washed with ultrapure water at 0-10 ℃ for 3-5 times, and centrifuged at 8000 rpm for 5-8 min after each washing; the particles are slowly thawed at 25 ℃ for 3-4 h after washing, to obtain silica wet particles.

[0012] S6: integrated drying and calcination of rotary quartz tube furnace: the wet particles are transferred into a rotary quartz tube furnace (the inner wall roughness Ra is ≤0.1 μm), the temperature in the furnace is controlled at 120-180 ℃, and the rotation rate is 15-20 rpm, and the surface adsorbed water and residual solvent are removed by drying for 30 min, to obtain dry particles with a water content of ≤1%. The rotation state is maintained, pure oxygen with a purity of ≥99.999% is introduced, the oxygen flow is 30-50 mL / min, the temperature is increased to 400 ℃ at a rate of 5 ℃ / min, and the temperature is maintained for 2 h, the temperature is increased to 600 ℃, and the temperature is maintained for 3 h, and the temperature is increased to 800 ℃, and the temperature is maintained for 2 h, to complete the gradient calcination, and remove the carbon impurities and part of the low-boiling-point inclusions, and the carbon impurity content of the removed particles is ≤0.001 wt%, and the low-boiling-point inclusion content is ≤0.5 vol%.

[0013] S7: static calcination in a vacuum alumina ceramic inner bore high-temperature furnace: the calcined particles are transferred into a vacuum alumina ceramic inner bore high-temperature furnace (the inner bore material is 99.9% high-purity alumina ceramic), the door of the furnace is closed, and the vacuum is pumped to ≤10 -4 Pa (preferably ≤5×10 -5 Pa), the temperature is increased to 1000-1300 ℃ at a rate of 2 ℃ / min, and the temperature is maintained statically for 4-6 h, during which pure argon with a purity of ≥99.999% is introduced, the argon flow is 20-30 mL / min (the argon flow is 20 mL / min at 1000 ℃, and the argon flow is 30 mL / min at 1300 ℃), the hydroxyl groups and the remaining inclusions are removed, and dense silica particles are obtained.

[0014] S8: grading treatment: the dense silica particles of S7 are slightly crushed by using a 99.99% quartz material roll mill, the roll gap of the roll mill is 300 μm, the roll speed is 100-200 r / min, and the roll pressure is 50-200 MPa; the particles are classified by using an air flow classifier, the air flow speed is 20-25 m / s, the classification wheel speed is 4000-5000 rpm, and the particles with a particle size distribution CV of ≤5% are collected, to obtain high-purity quartz sand.

[0015] The high-purity quartz sand prepared by the preparation method has a purity of ≥99.9999%, a sphericity of ≥0.88, a particle size distribution CV of ≤5%, a hydroxyl group content of ≤20 ppm, an inclusion content of ≤0.1 vol%, a tap density of ≥1.70 g / cm 3 , and a bulk density of ≥1.45 g / cm 3The total content of metal impurities is less than or equal to 1 ppm (Fe, Al, Na each less than or equal to 0.3 ppm).

[0016] In another aspect, the application claims a high-purity quartz sand prepared by the above method. The purity of the high-purity quartz sand is greater than or equal to 99.9999%, the sphericity is greater than or equal to 0.88, the particle size distribution CV is less than or equal to 5%, the hydroxyl content is less than or equal to 20 ppm, and the total content of metal impurities is less than or equal to 1 ppm.

[0017] The high-purity quartz sand prepared by the method has good application prospects in high-end fields such as semiconductor chip manufacturing quartz crucibles, high-end optical lens substrates, or radio frequency plasma etching equipment quartz components, which have strict requirements on purity, morphology, and hydroxyl content.

[0018] Compared with the prior art, the technical solution provided by the application at least has the following beneficial effects or advantages: (1) The application provides a high-purity quartz sand preparation method based on a low-temperature solvent freezing method. Ultra-high-purity silicic acid with a purity of greater than or equal to 99.9999% and a water content of 90-95% is used as a raw material, and silicic acid sol is added to accelerate gelation. In the application, 0.3-0.8wt% of silicic acid solution is adjusted to a pH of 1.5-2.5 by hydrochloric acid, and 10-20nm nanoscale silicic acid particles are formed in the silicic acid sol, which act as "seeds" and are mixed with ultra-high-purity silicic acid. The gelation time can be shortened from 12-24h to 1-3h, and the uniformity of gel particle size is improved by 50%. The application controls the drop rate of the hydrochloric acid solution to be 1-3mL / min, avoiding the rapid polymerization of silicic acid molecules caused by local pH drop, preventing gel agglomeration, and thus reducing the content of impurities and inclusions in the quartz sand, and improving the purity and bulk density of the quartz sand.

[0019] (2) The quartz sand regulating particle morphology is prepared by low-temperature solvent freezing method, and methanol, n-hexane and cyclohexane are used as low-temperature solvents. The viscosity of the low-temperature solvents is 0.8-1.8 mPa·s under the condition of-5 to-30 DEG C, and the viscosity of traditional solvent ethanol is 1.5-2.0 mPa·s under the condition of-5 to-30 DEG C. The viscosity of the low-temperature solvents selected in the application is lower, which can fully penetrate the gap between the gel particles, destroy the hydrogen bond between the particles and inhibit agglomeration. The surface tension of cyclohexane is 21-23 mN / m, which can guide the particles to form a spherical structure spontaneously during stirring, and improve the sphericity of the quartz sand. The sphericity of the quartz sand prepared by the application is greater than or equal to 0.88, which can reduce mechanical wear during use, reduce material porosity, improve material density and prolong service life. The freezing temperature of the application is controlled at-5 to-30 DEG C, and the freezing time is 20-120 min, which provides a slow freezing environment, avoids the formation of large ice crystals by rapid freezing and destroys the particle morphology. The particle size can be adjusted and controlled by stirring time. The quartz sand with a particle size of 5-8 um can be prepared by freezing for 20 min, and the quartz sand with a particle size of 10-15 um can be prepared by freezing for 120 min, which meets the requirements of different application scenarios.

[0020] (3) The application integrates drying and calcination to ensure the purity of quartz sand by using a rotary quartz tube furnace. The particles are evenly heated under the condition of 120-180 DEG C and 15-20 rpm, and the water content can be reduced to less than or equal to 1% within 30 min. A gradient calcination scheme is provided, residual solvents are removed at 400 DEG C, carbon impurities are oxidized at 600 DEG C, and low-boiling inclusions are decomposed at 800 DEG C. Oxygen is introduced in a rotating state, and the oxygen flow is 30-50 mL / min to ensure sufficient oxidation, reducing the carbon impurity content from 0.005 wt% to less than 0.001 wt%. The integrated process provided by the application does not transfer materials during drying and calcination, which reduces the risk of impurity pollution by 60% compared with the traditional process which needs to transfer materials between the drying oven and the tube furnace. The inner wall of the quartz tube is polished to Ra≤0.1 um, which can reduce particle adhesion and avoid morphology damage.

[0021] (4) The application realizes deep removal of impurities and morphology protection by using a vacuum alumina ceramic inner bore high-temperature furnace for static calcination. The inner bore of the high-temperature furnace is a 99.9% high-purity alumina ceramic inner bore, which is resistant to high temperature (≥1600 DEG C) and does not dissolve metal ions, solving the pollution problem of traditional metal inner bore furnaces. During the static calcination process, the particles do not displace, which ensures that the spherical morphology formed in the early stage is not damaged, and the sphericity is maintained at greater than or equal to 0.88.

[0022] (5) The quartz sand prepared by the application has a purity of greater than or equal to 99.9999%, a sphericity of greater than or equal to 0.88, a particle size distribution CV of less than or equal to 5%, a hydroxyl content of less than or equal to 20 ppm, an inclusion content of less than or equal to 0.1 vol%, and a tap density of greater than or equal to 2.20 g / cm 3Bulk density ≥ 1.45 g / cm 3 The total content of metal impurities is ≤1 ppm (each of Fe, Al and Na is ≤0.3 ppm), and the problems of traditional process particle agglomeration, impurity pollution, equipment compatibility and low sphericity are solved, and the high purity quartz sand is suitable for the fields of semiconductor chip manufacturing, high-end optical lens and other fields with strict requirements on purity, morphology and hydroxyl content. BRIEF DESCRIPTION OF DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0024] Figure 1 The scanning electron microscope image and the optical microscope image of the high-purity quartz sand. Figure 1 The A in the scanning electron microscope image of the high-purity quartz sand; Figure 1 The B in the optical microscope image of the high-purity quartz sand. DETAILED DESCRIPTION

[0025] In the following, the technical solutions of the present application will be described in conjunction with the embodiments, but the present application is not limited to the following embodiments. The experimental methods and detection methods described in each embodiment are conventional methods unless otherwise specified; the reagents and materials described are commercially available unless otherwise specified; and the % in the following examples is the mass percentage unless otherwise specified.

[0026] The resistivity of ultrapure water is ≥18.2 MΩ·cm; the silicic acid is purchased from Xi'an Blue Bridge New Energy Technology Co., Ltd., and the purity is ≥99.9999%; the methanol is purchased from Sinopharm Chemical Reagent Co., Ltd., and the purity is ≥99.999%; the n-hexane is purchased from Sinopharm Chemical Reagent Co., Ltd., and the purity is ≥99.99%; and the cyclohexane is purchased from Sinopharm Chemical Reagent Co., Ltd., and the purity is ≥99.99%.

[0027] The stirring tank, the reaction kettle and the crusher are made of 99.99% quartz material or polytetrafluoroethylene material, and the quartz material is preferably used; the stirring tank, the reaction kettle, the crusher and the roller mill made of quartz material are purchased from Wuxi Noya Machinery Co., Ltd.

[0028] The quartz fiber filter membrane is purchased from Tianjin Kuansai New Material Technology Co., Ltd.

[0029] The rotary quartz tube furnace and the vacuum alumina ceramic inner bore high-temperature furnace are purchased from Luoyang Juxing Kiln Co., Ltd.

[0030] The application is based on a high-purity quartz sand preparation method by low-temperature solvent freezing method, and the whole operation is carried out in a Class 100 clean room, and specifically comprises the following steps: S1: Silicic acid sol preparation: Dissolve silicic acid with a purity of ≥99.9999% in ultrapure water to prepare a 0.3-0.8 wt% silicic acid solution; transfer the silicic acid solution into a 99.99% quartz material stirring tank, stir at a speed of 1000-1500 rpm, and add 0.5-2 mol / L electronic grade hydrochloric acid at a rate of 1-3 mL / min, adjust the pH to 1.5-2.5, and continue stirring for 30-60 min to obtain uniformly dispersed silicic acid sol.

[0031] S2: Gel preparation: Take silicic acid with a water content of 90-95 wt%, mix it with the silicic acid sol of S1 at a volume ratio of 10:1-3, and place it in a 99.99% quartz material reaction kettle, and stir at 25-40℃ and 500-800 rpm for 1-3 h to form a uniform block-shaped gel with a water content of 85-90 wt%.

[0032] S3: Gel crushing and centrifugal dewatering: Crush the block-shaped gel into 0.5-2 mm particles with a 99.99% quartz material crusher, transfer the particles into a polytetrafluoroethylene centrifugal bag, and centrifuge at 8000-10000 rpm for 10-15 min to remove free water, and obtain gel particles with a water content of 70-75 wt%.

[0033] S4: Low-temperature solvent freezing treatment: Add the gel particles to the low-temperature solvent pre-cooled to -5 to -30℃ at a solid-liquid ratio of 1:8-12 (g:mL), stir at 400-600 rpm for 20-120 min in a constant temperature environment of -5 to -30℃, and maintain a temperature fluctuation of ≤±2℃. The low-temperature solvent is one of methanol, n-hexane or cyclohexane, and the viscosity of the above low-temperature solvent is 0.8-1.8 mPa·s at a temperature of -5 to -30℃.

[0034] S5: Filtration, washing and thawing: Filter and separate the frozen particles using a 0.1 μm quartz fiber filter membrane, wash them 3-5 times with 0-10℃ ultrapure water, and centrifuge them at 8000 rpm for 5-8 min after each washing; and slowly thaw the particles at 25℃ for 3-4 h after washing to obtain silica wet particles.

[0035] S6: Integrated drying and calcination in a rotating quartz tube furnace: The wet particles are transferred into a rotating quartz tube furnace (inner wall roughness Ra≤0.1 μm), the temperature in the furnace is controlled at 120-180℃, the rotating speed is 15-20 rpm, and the particles are dried for 30 min to remove the surface adsorbed water and residual solvent, obtaining dry particles with a water content of ≤1%. The particles are kept in a rotating state, and pure oxygen with a purity of ≥99.999% is introduced at a flow rate of 30-50 mL / min. The temperature is raised to 400℃ at a rate of 5℃ / min, and the particles are kept at this temperature for 2 h. The temperature is raised to 600℃ at a rate of 5℃ / min, and the particles are kept at this temperature for 3 h. The temperature is raised to 800℃ at a rate of 5℃ / min, and the particles are kept at this temperature for 2 h. The gradient calcination is completed, and the carbon impurities and part of the low-boiling inclusions are removed. The carbon impurity content of the particles after removal is ≤10 ppm, and the low-boiling inclusion content is ≤0.1 vol%.

[0036] S7: Static calcination in a vacuum alumina ceramic inner bore high-temperature furnace: The calcined particles are transferred into a vacuum alumina ceramic inner bore high-temperature furnace (the inner bore material is 99.9% high-purity alumina ceramic). After the furnace door is closed, the furnace is evacuated to ≤10 -4 Pa (preferably ≤5×10 -5 Pa), the temperature is raised to 1000-1300℃ at a rate of 2℃ / min, and the particles are kept at this temperature for 4-6 h. During this period, pure argon with a purity of ≥99.999% is introduced at a flow rate of 20-30 mL / min (20 mL / min at 1000℃, and 30 mL / min at 1300℃). The hydroxyl groups and remaining inclusions are removed, and dense silica particles are obtained.

[0037] S8: Fractionation: The dense silica particles of S7 are slightly crushed using a 99.99% quartz roll mill, with a roll gap of 300 μm, a roll speed of 100-200 r / min, and a roll pressure of 50-200 MPa. The particles are fractionated using an air classifier, with an air flow speed of 20-25 m / s and a classification wheel speed of 4000-5000 rpm. Particles with a particle size distribution CV of ≤5% are collected, and high-purity quartz sand (recovery rate ≥80%) is obtained. The high-purity quartz sand prepared by the method of the present application has a purity of ≥99.9999%, a sphericity of ≥0.88, a particle size distribution CV of ≤5%, a hydroxyl group content of ≤20 ppm, an inclusion content of ≤0.1 vol%, a tap density of ≥2.20 g / cm 3 , a bulk density of ≥1.45 g / cm 3 , and a total metal impurity content of ≤1 ppm (Fe, Al, and Na each ≤0.3 ppm).

[0038] Example 1 This example provides a method for preparing high-purity quartz sand based on a low-temperature solvent freezing method, which specifically includes the following steps: S1: Dissolve silicic acid in ultrapure water to prepare a 0.3wt% silicic acid solution, transfer the silicic acid solution into a 99.99% quartz material stirring tank, stir at 1200 rpm, add 2 mol / L electronic grade hydrochloric acid at a rate of 1 mL / min, adjust the pH to 1.5-2.5, continue stirring for 60 min, and obtain a uniformly dispersed silicic acid sol.

[0039] S2: Take the silicic acid with a water content of 90wt%, mix it with the silicic acid sol of S1 according to the volume ratio of 10:3, and place it in a 99.99% quartz material reaction kettle, stir at 25℃ and 800 rpm for 2h, and form a uniform block-shaped gel with a water content of 90wt%.

[0040] S3: Gel crushing and centrifugal dewatering: crush the block-shaped gel into 0.5-2mm particles with a 99.99% quartz material crusher, transfer the particles into a polytetrafluoroethylene centrifugal bag, and centrifuge at 8000 rpm for 15 min to remove free water, obtaining gel particles with a water content of 75wt%.

[0041] S4: Low-temperature solvent freezing treatment: add the gel particles to the previously cooled to -20℃ low-temperature solvent methanol according to the solid-liquid ratio of 1:8 (g:mL), the viscosity of methanol at -20℃ is 1.2 mPa·s, stir at 500 rpm for 80 min in a constant temperature environment at -20℃, and maintain the temperature fluctuation ≤±2℃.

[0042] S5: Filter and separate the frozen particles using a 0.1 μm quartz fiber filter membrane, wash with 0-10℃ ultrapure water for 3-5 times, centrifuge at 8000 rpm for 5-8 min after each washing; after washing, the particles are slowly thawed at 25℃ for 3-4h, and obtain silica wet particles.

[0043] S6: Transfer the wet particles into a rotating quartz tube furnace, control the furnace temperature to 120℃ and the rotation rate to 20 rpm, dry for 30 min to remove surface adsorbed water and residual solvent, and obtain dry particles with a water content of ≤1%. Keep the rotating state, introduce pure oxygen with a purity of ≥99.999%, oxygen flow rate is 50 mL / min, increase the temperature to 400℃ at a rate of 5℃ / min, and keep the temperature for 2h, increase the temperature to 600℃ for 3h, and increase the temperature to 800℃ for 2h, complete the gradient calcination, remove carbon impurities and part of low boiling point inclusions.

[0044] S7: Transfer the calcined particles into a vacuum alumina ceramic inner bore high temperature furnace, vacuum to ≤5×10 -5 Pa after closing the furnace door, increase the temperature to 1000℃ at a rate of 2℃ / min, and keep the temperature static for 6h, during which pure argon with a purity of ≥99.999% is introduced, argon flow rate is 20 mL / min, remove hydroxyl and residual inclusions, and obtain dense silica particles.

[0045] S8: Classification treatment: slightly crush the dense silica particles of S7 with a 99.99% quartz material roll mill, the roll gap is 300 pm, the roll speed is 100 r / min, and the roll pressure is 200 MPa; classify by an air classifier, the air flow speed is 20 m / s, the classification wheel speed is 4000 rpm, and collect the particles with a particle size distribution CV≤5%, i.e., high-purity quartz sand Figure 1 , the performance parameters are shown in Table 1.

[0046] Example 2 S1: Dissolve silicic acid in ultrapure water to prepare a 0.5wt% silicic acid solution, transfer the silicic acid solution to a 99.99% quartz material stirring tank, stir at 1000 rpm, add 1 mol / L electronic grade hydrochloric acid at a rate of 2 mL / min, adjust the pH to 1.5-2.5, and continue stirring for 50 min to obtain a uniformly dispersed silicic acid sol.

[0047] S2: Take the silicic acid with a water content of 93wt%, mix it with the silicic acid sol of S1 according to a volume ratio of 10:2, and place it in a 99.99% quartz material reaction kettle, stir at 30°C and 500 rpm for 3h, and form a uniform block-shaped gel with a water content of 86wt%.

[0048] S3: Gel crushing and centrifugal dewatering: crush the block-shaped gel into 0.5-2mm particles with a 99.99% quartz material crusher, transfer the particles into a polytetrafluoroethylene centrifugal bag, and centrifuge at 9000 rpm for 12 min to remove free water, obtaining gel particles with a water content of 73wt%.

[0049] S4: Low-temperature solvent freezing treatment: add the gel particles to the low-temperature solvent n-hexane pre-cooled to -5°C according to a solid-liquid ratio of 1:10 (g:mL), the viscosity of n-hexane at -5°C is 0.421 mPa·s, stir at 400 rpm for 120 min in a constant temperature environment of -5°C, and maintain the temperature fluctuation≤±2°C.

[0050] S5: Filter and separate the frozen particles using a 0.1 pm quartz fiber filter membrane, wash the particles with ultrapure water at 0-10°C for 3-5 times, centrifuge at 8000 rpm for 5-8 min after each washing; after washing, slowly thaw the particles at 25°C for 3-4h, and obtain silica wet particles.

[0051] S6: The wet particles were transferred into a rotating quartz tube furnace, the temperature in the furnace was controlled at 150°C, the rotating speed was 15 rpm, and the particles were dried for 30 min to remove the surface adsorbed water and residual solvent, thereby obtaining dry particles with a water content of ≤1%. The rotating state was maintained, and pure oxygen with a purity of ≥99.999% was introduced at a flow rate of 30 mL / min, and the temperature was increased to 400°C at a rate of 5°C / min, and the temperature was maintained for 2 h, and the temperature was increased to 600°C and maintained for 3 h, and the temperature was increased to 800°C and maintained for 2 h, thereby completing the gradient calcination and removing carbon impurities and part of the low-boiling-point inclusions.

[0052] S7: The calcined particles were transferred into a vacuum alumina ceramic inner bore high-temperature furnace, and after the furnace door was closed, the vacuum was pumped to ≤10 - 4 Pa, the temperature was increased to 1200°C at a rate of 2°C / min, and the temperature was maintained statically for 5 h, during which pure argon with a purity of ≥99.999% was introduced at a flow rate of 25 mL / min, thereby removing the hydroxyl groups and the remaining inclusions, and obtaining dense silica particles.

[0053] S8: Fractionation: The dense silica particles of S7 were slightly crushed with a 99.99% quartz roll mill, the roll gap was 300 μm, the roll speed was 150 r / min, and the roll pressure was 100 MPa; fractionation was performed by an air flow classifier, the air flow speed was 20 m / s, the fractionation wheel speed was 4500 rpm, and particles with a particle size distribution CV of ≤5% were collected, thereby obtaining high-purity quartz sand, and the performance parameters are shown in Table 1.

[0054] Example 3 S1: Silicon acid was dissolved in ultrapure water to prepare a 0.8wt% silicon acid solution, the silicon acid solution was transferred into a 99.99% quartz stirring tank, and stirring was performed at a speed of 1500 rpm, 0.5 mol / L electronic grade hydrochloric acid was added at a rate of 3 mL / min, the pH was adjusted to 1.5-2.5, and stirring was continued for 30 min, thereby obtaining uniformly dispersed silicon acid sol.

[0055] S2: The silicon acid with a water content of 95wt% was mixed with the silicon acid sol of S1 at a volume ratio of 10:1, and was placed in a 99.99% quartz reaction kettle, and was stirred at 40°C and 800 rpm for 1 h, thereby forming a uniform block-shaped gel with a water content of 86wt%.

[0056] S3: Gel crushing and centrifugal dewatering: The block-shaped gel was crushed into 0.5-2 mm particles using a 99.99% quartz crusher, the particles were transferred into a polytetrafluoroethylene centrifugal bag, and were centrifuged at 10000 rpm for 10 min to remove the free water, thereby obtaining gel particles with a water content of 70wt%.

[0057] S4: low-temperature solvent freeze treatment: the gel particles were added to the low-temperature solvent pre-cooled to -25°C at a solid-liquid ratio of 1:12 (g:mL), the viscosity of n-hexane was 0.467 mPa-s at -25°C, and the temperature fluctuation was maintained at ≤±2°C in a constant temperature environment at -25°C, and stirring was performed at 600 rpm for 20 min.

[0058] S5: the frozen particles were separated by filtration using a 0.1 μm quartz fiber filter membrane, and were washed with ultrapure water at 0-10°C for 3-5 times, and centrifuged at 8000 rpm for 5-8 min after each washing; the particles were slowly thawed at 25°C for 3-4 h after washing, and silica wet particles were obtained.

[0059] S6: the wet particles were transferred into a rotating quartz tube furnace, the temperature in the furnace was controlled at 180°C, and the rotation rate was 15 rpm, and the particles were dried for 30 min to remove the surface adsorbed water and residual solvent, and dry particles with a water content of ≤1% were obtained. The rotation state was maintained, and pure oxygen with a purity of ≥99.999% was introduced, the oxygen flow was 40 mL / min, the temperature was increased to 400°C at a rate of 5°C / min, and was kept for 2 h, the temperature was increased to 600°C and kept for 3 h, and the temperature was increased to 800°C and kept for 2 h, and the gradient calcination was completed to remove carbon impurities and part of the low-boiling-point inclusions.

[0060] S7: the calcined particles were transferred into a vacuum alumina ceramic inner bore high-temperature furnace, vacuum was drawn to ≤5×10 -5 Pa after the furnace door was closed, the temperature was increased to 1300°C at a rate of 2°C / min, and was kept for 4 h in a static state, during which pure argon with a purity of ≥99.999% was introduced, and the argon flow was 30 mL / min, and the hydroxyl and remaining inclusions were removed, and dense silica particles were obtained.

[0061] S8: grading treatment: the dense silica particles of S7 were slightly crushed by a 99.99% quartz material roll mill, the roll gap was 300 μm, the roll speed was 200 r / min, and the roll pressure was 50 MPa; grading was performed by an air flow classifier, the air flow speed was 25 m / s, and the grading wheel rotation speed was 5000 rpm, and particles with a particle size distribution CV of ≤5% were collected, and high-purity quartz sand was obtained, and the performance parameters are shown in Table 1.

[0062] Comparative Example 1 The preparation method of the quartz sand of the present comparative example was the same as that of Example 1, except that the dropping rate in S1 was 5 mL / min, and the dropping rate was too fast to cause a sudden pH drop in the solution system, resulting in too fast polymerization of silicic acid molecules and gel aggregation, and the performance parameters of the prepared quartz sand are shown in Table 1.

[0063] Comparative Example 2 The preparation method of the quartz sand of the present comparative example was the same as that of Example 1, except that ethanol was used as the low-temperature solvent in S4, and the viscosity of ethanol was 2.0 mPa-s at -20°C, and the performance parameters of the prepared quartz sand are shown in Table 1.

[0064] Comparative Example 3 The preparation method of the quartz sand in the present comparative example is the same as that in Example 1, except that the low-temperature solvent in S4 is pre-cooled to -40℃, and the quartz sand is prepared under the constant temperature environment of -40℃ and stirring at 400 rpm for 10 min. The performance parameters of the prepared quartz sand are shown in Table 1.

[0065] Comparative Example 4 The preparation method of the quartz sand in the present comparative example is the same as that in Example 1, except that the drying in S6 is performed in a drying oven and the calcination in S7 is performed in a vacuum high-temperature sintering furnace (purchased from Luoyang Juxing Kiln Co., Ltd., with a metal inner bore). The performance parameters of the prepared quartz sand are shown in Table 1.

[0066] Table 1 Performance parameters of quartz sand

[0067] Note: Values outside the performance range of the quartz sand of the present application are indicated in bold.

[0068] As shown in Table 1, the quartz sand prepared by the preparation method of the present application has a purity of ≥99.9999%, a sphericity of ≥0.88, a particle size distribution CV of ≤5%, a hydroxyl content of ≤20 ppm, an inclusion content of ≤0.1 vol%, a tap density of ≥1.70 g / cm 3 , a bulk density of ≥1.45 g / cm 3 , and a total content of metal impurities of ≤1 ppm (each of Fe, Al, and Na ≤0.3 ppm). The slow freezing process of -5~-30℃ and stirring for 20~120 min provided by the present application can avoid large ice crystals generated by quick freezing, and improve the sphericity. Outside the slow freezing range provided by the present application, the sphericity is poor. The traditional drying-calcination process is prone to introduce impurities during the transfer of materials between the drying oven and the tube furnace, and the calcination in the vacuum high-temperature sintering furnace can lead to the dissolution of metal ions and pollution of the quartz sand.

[0069] The above description has better described the basic principles, main features and advantages of the present application. The above examples and specification only describe the preferred embodiments of the present application, and the present application is not limited by the above examples. Various changes and improvements to the technical solutions of the present application made by those skilled in the art without departing from the spirit and scope of the present application shall fall within the scope of protection of the present application.

Claims

1. A method for preparing high-purity quartz sand based on low-temperature solvent freezing, characterized in that, include: A gel was obtained by adding silica sol and stirring with silica sol. The gel was broken into particles of 0.5-2 mm and centrifuged to obtain gel particles; The gel particles were added to a low-temperature solvent pre-cooled to -5 to -30°C and stirred at -5 to -30°C and 400 to 600 rpm for 20 to 120 minutes to obtain frozen particles. The frozen particles were filtered, washed, and thawed to obtain wet silica particles. The wet silica particles are transferred into a rotating quartz tube furnace, dried, and then subjected to gradient calcination. The high-purity quartz sand is obtained by sintering in a vacuum high-temperature furnace. The low-temperature solvent is one of methanol, n-hexane, or cyclohexane.

2. The preparation method according to claim 1, characterized in that, The method for preparing the silica sol includes: preparing a 0.3-0.8 wt% silica solution, stirring at 1000-1500 rpm, adding 0.5-2 mol / L hydrochloric acid dropwise at a rate of 1-3 mL / min, adjusting the pH to 1.5-2.5, and stirring continuously for 30-60 min to obtain the silica sol.

3. The preparation method according to claim 1, characterized in that, The water content of the silicic acid is 90-95 wt%, and the volume ratio of silicic acid to silicic acid sol is 10:1-3. The addition of silica sol and stirring are carried out at 25~40℃, with a stirring speed of 500~800rpm, for 1~3 hours.

4. The preparation method according to claim 1, characterized in that, The centrifugation speed is 8000~10000 rpm, and the centrifugation time is 10~15 min. The water content of the gel particles is 70~75 wt%. The mass-to-volume ratio of the gel particles to the low-temperature solvent is 1:8~12.

5. The preparation method according to claim 1, characterized in that, The filtration uses a 0.1μm quartz fiber filter membrane; The cleaning process uses ultrapure water at 0~10℃, followed by centrifugation at 8000rpm for 5~8min. The thawing temperature is room temperature, and the thawing time is 3-4 hours.

6. The preparation method according to claim 1, characterized in that, The drying temperature is 120~180℃, the rotation speed is 15~20rpm, and the drying time is 30min; Drying yields dry granules with a moisture content of ≤1%.

7. The preparation method according to claim 1, characterized in that, The calcination is a gradient calcination, with oxygen introduced at a flow rate of 30-50 mL / min. The gradient calcination includes: heating at 5℃ / min to 400℃ and holding for 2 hours, heating to 600℃ and holding for 3 hours, and heating to 800℃ and holding for 2 hours.

8. The preparation method according to claim 1, characterized in that, The inner chamber of the vacuum high-temperature furnace is made of alumina ceramic; The vacuum during sintering is below 10. -4 Pa; The sintering process includes: heating to 1000~1300℃ at a rate of 2℃ / min, holding at a static temperature for 4~6 hours, and introducing argon gas during the process at a flow rate of 20~30mL / min.

9. The preparation method according to claim 1, characterized in that, After sintering, the material is further pulverized and subjected to airflow classification. The crushing is achieved by a roller mill with a roller gap of 300μm, a roller speed of 100~200r / min, and a roller pressure of 50~200MPa. The airflow velocity for the airflow classification is 20~25m / s, and the classification wheel speed is 4000~5000rpm.

10. A high-purity quartz sand, characterized in that, The high-purity quartz sand is prepared by any one of the preparation methods described in claims 1 to 9, and the purity of the high-purity quartz sand is ≥99.9999%, the sphericity is ≥0.88, the particle size distribution CV is ≤5%, the hydroxyl content is ≤20ppm, and the total content of metal impurities is ≤1ppm.

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