Method for converting 6N-grade quartz sand into high-purity alpha-cristobalite by doping and coating

Abstract

The invention discloses a method for converting 6N-grade quartz sand into high-purity alpha-cristobalite by doping and coating, and belongs to the technical field of inorganic non-metallic material preparation. The method comprises the following steps: with 6N-grade quartz sand as a raw material, coating the surface of the 6N-grade quartz sand with a silicic acid precursor containing alkali metal elements to form a coating layer; after drying, carrying out temperature-controlled crystallization at a relatively low temperature, inducing the quartz sand to generate lattice reconstruction from outside to inside by utilizing alkali metal in a coating layer, and converting the quartz sand into alpha-cristobalite; after crystallization, a surface coating layer and residual impurities are removed through an acid pickling process, the high-purity characteristic is recovered, and a high-purity alpha-cristobalite product is obtained. According to the method, efficient crystallization of the 6N-grade quartz sand at the relatively low temperature is achieved, the purity of the obtained product can reach the 6N grade, the crystal phase is pure, the light transmittance is excellent, the method can be widely applied to the fields of optical lenses, electronic packaging, aerospace high-temperature-resistant parts and the like, and the method has the advantages of being simple in process, low in energy consumption and suitable for large-scale production.

Description

Technical Field

[0001] This invention belongs to the fields of inorganic non-metallic materials and materials preparation technology, and specifically relates to a method for converting 6N grade quartz sand into high-purity α-cristobalite by doping and coating. Background Technology

[0002] High-purity quartz sand is widely used in semiconductors, optical communications, aerospace, and high-end optical devices due to its excellent chemical stability, low coefficient of thermal expansion, and high light transmittance. As these industries move towards higher precision, more stringent requirements are being placed on the purity and crystal form of quartz sand. High-purity α-cristobalite, in particular, with its specific crystal structure and defined melting point, can significantly reduce defects such as bubbles and flocculents when melting quartz glass products, thereby improving the mechanical strength and optical uniformity of the products. However, existing methods for preparing high-purity cristobalite still have many limitations.

[0003] Currently, the main methods for preparing high-purity cristobalite are high-temperature calcination and exogenous doping. High-temperature calcination requires extreme temperatures of 1400–1600℃, which not only consumes a huge amount of energy but also easily leads to particle agglomeration, impurity volatilization causing secondary pollution, and reducing product purity and transmittance. Exogenous doping often uses the direct addition of solid dopants to drive crystallization, but this suffers from uneven dopant dispersion, easily causing crystal structure disorder and introducing new impurities, resulting in a decrease in product purity to below 3N grade. Meanwhile, 6N grade quartz sand, due to its dense crystal structure and lack of impurities as mineralization centers, cannot achieve effective crystallization even at 1300℃, limiting its high-value application in the preparation of high-purity α-cristobalite.

[0004] To address the aforementioned problems, researchers have attempted to prepare high-purity crystalline quartz through artificial synthesis or modification. For example, Chinese patent application CN119911913A discloses a method for preparing high-purity crystalline cristobalite sand. This method uses silicon sources and high-purity alkali as raw materials, obtaining amorphous silica gel powder through dissolution, precipitation, and aging, followed by high-temperature crystallization and chlorination to remove impurities, thus obtaining crystalline cristobalite. This method is essentially a "de novo synthesis" route, where silicon compounds are completely dissolved and then re-precipitated and crystallized. While this can achieve high purity, the raw materials must be liquid precursors, making it impossible to directly process existing high-purity quartz sand particles. Furthermore, the overall crystallization process relies on residual alkali metals in the silica gel powder itself, resulting in uniform impurity distribution, making it difficult to completely remove residual metal ions from the bulk phase during subsequent chlorination. Chinese patent CN117567023B discloses a production process for processing quartz sand into ultra-white sand. This method involves preparing a reagent of sodium silicate, sodium bicarbonate, and sodium fluoride, hydrolyzing it, and then mixing it with quartz sand. After preheating and high-temperature phase transformation treatment, the aim is to improve the surface smoothness and whiteness of the quartz sand. However, this process essentially falls under the category of surface physical modification and impurity removal. The reagent components mainly act on the particle surface at high temperatures, promoting surface melting and smoothing, and partially transforming iron oxides, without involving the reconstruction of the internal crystal lattice of the quartz sand. Although the product obtained by this method has improved whiteness, it still retains the crystalline or amorphous characteristics of the original ore, and cannot achieve a directional transformation to high-purity α-cristobalite. Furthermore, the alkali metals and fluorides in the reagent are difficult to completely remove after the reaction, and the residual impurities limit its application in ultra-high purity environments.

[0005] Therefore, there is an urgent need for a preparation method that can maintain the high purity of raw materials and achieve the directional conversion of 6N grade quartz sand into high-purity α-cristobalite at relatively low temperatures. Summary of the Invention

[0006] The purpose of this invention is to solve the technical problem that high-purity quartz sand, due to its extremely low impurity content (total impurities <1ppm) and lack of mineralization centers required for crystallization, is difficult to crystallize below 1300℃ using conventional processes. The invention provides a method for converting 6N grade quartz sand into high-purity α-cristobalite by doping and coating.

[0007] To achieve the above objectives, the technical solution adopted by the present invention includes the following steps:

[0008] Step 1: Add alkali metal hydroxide to the silicic acid solution, stir, ultrasonically disperse, and degas to obtain a silicic acid precursor containing alkali metal elements; the concentration of the silicic acid solution is 100-200 mg / L based on silicon dioxide; in the silicic acid precursor containing alkali metal elements, based on the total mass of silicon dioxide and alkali metal elements as 100%, silicon dioxide is 99.9%-99.99%, and alkali metal elements are 0.01%-0.1%.

[0009] Step 2: Add 6N grade quartz sand to the silica precursor containing alkali metal elements for coating treatment, so that a precursor coating layer is formed on the surface of the quartz sand. After filtration and drying, the coated quartz sand is obtained.

[0010] Step 3: Place the coated quartz sand in a nitrogen atmosphere for crystallization treatment to transform the quartz sand crystal form into α-cristobalite; the crystallization treatment conditions are: first heat to 500-700℃ and hold for 20-40 minutes, then heat to 900-1100℃ and hold for 3-5 hours.

[0011] Step 4: After washing the generated α-cristobalite with deionized water and anhydrous ethanol, it is placed in an acid solution for treatment, then washed with deionized water until neutral, and dried to obtain high-purity α-cristobalite product.

[0012] Furthermore, in step 1, the alkali metal hydroxide is one or more of sodium hydroxide, lithium hydroxide, and potassium hydroxide.

[0013] Furthermore, in step 1, the stirring is performed at 200-400 r / min for 20-40 min; the dispersion is performed by ultrasonic dispersion for 15-25 min; and the degassing is performed by vacuum degassing for 15-25 min.

[0014] Furthermore, in step 2, the purity of the 6N grade quartz sand is ≥99.9999%, and the particle size is 50-200μm. Before use, the 6N grade quartz sand is first ultrasonically cleaned with ethanol for 20-30 minutes, then ultrasonically cleaned with deionized water for 15-25 minutes, dried with nitrogen, and then dried at 100-150℃.

[0015] Furthermore, in step 2, the mass ratio of the 6N grade quartz sand to the silica precursor containing alkali metal elements is 1:3 to 5.

[0016] Furthermore, in step 2, the conditions for the coating treatment are as follows: first, stir at 200-400 r / min for 40-80 min, then sonicate at 150-250 W for 30-50 min, stirring for 5 min every 10 min during the process.

[0017] Furthermore, in steps 2 and 4, the drying temperature is 100–150°C.

[0018] Furthermore, in step 3, the crystallization treatment conditions are as follows: first, heat to 600℃ at a heating rate of 4-8℃ / min and hold for 20-40 min, then heat to 1000-1100℃ at a heating rate of 3-5℃ / min and hold for 3-5 h.

[0019] Furthermore, in step 4, the acid solution is a mixed solution of 5%–7% hydrofluoric acid aqueous solution and 15%–20% HCl aqueous solution in a volume ratio of 1:1–3, and the acid solution is ultrasonically treated for 15–25 minutes.

[0020] Furthermore, the total impurity content of the high-purity α-cristobalite prepared by the above method is 0.5 to 1 ppm, and the X-ray diffraction pattern shows the characteristic diffraction peak of α-cristobalite at 2θ=21.9°.

[0021] The beneficial effects of this invention are as follows:

[0022] 1. This invention uses a silicate precursor containing alkali metal elements such as sodium and lithium to surface-coat 6N grade quartz sand, precisely positioning the mineralizer on the particle surface through surface-confined doping. The alkali metal elements (sodium, lithium, potassium, etc.) act as mineralizers, playing a triple role at high temperatures: lowering the activation energy barrier for the transformation of SiO2 from amorphous or low-temperature quartz to α-cristobalite; promoting lattice reconstruction, reducing the crystallization temperature from the conventional above 1300℃ to 900-1100℃, successfully lowering the crystallization temperature by 200-400℃; and achieving directional crystallization from the outside in. After crystallization, the coating layer is completely peeled off as a "sacrificial layer" through acid washing, achieving both efficient crystallization and complete restoration of the high-purity characteristics of the core, breaking through the technical bottleneck of the difficulty in low-temperature crystallization of high-purity materials.

[0023] 2. This invention employs a staged purification process: first, deionized water is used to wash away physically adsorbed impurities; then, a mixture of hydrofluoric acid and hydrochloric acid of a specific concentration is used to precisely etch the residual coating layer, resulting in a more thorough removal of alkali metals and surface contaminants. Compared to traditional chlorination or single acid washing methods, this process can completely remove impurities introduced during coating and crystallization, restoring the total impurity content of the final product to 0.5–1 ppm, stably achieving the 6N grade high-purity standard, perfectly balancing the dual objectives of "low-temperature induced crystallization" and "ultra-high purity maintenance."

[0024] 3. The α-cristobalite product obtained by this invention has a pure crystal phase and is free of impurities; the product has excellent light transmittance and can maintain good light transmittance up to 1300℃. This high-purity, high-transmittance, and high-thermal-stability α-cristobalite material can be widely used in scenarios with extremely stringent requirements for material purity and thermal stability, such as lithography machine lenses, high-temperature observation windows, electronic packaging substrates, aerospace high-temperature resistant components, and high-temperature industrial kiln windows, effectively expanding the application fields of high-purity quartz materials.

[0025] 4. This invention does not require high-temperature and high-pressure equipment; the main processes can be completed in conventional reactors and tube furnaces. It features low crystallization temperature and short production cycle. Using commercially available 6N grade quartz sand as the direct raw material avoids complex precursor synthesis processes, significantly reducing production costs. The overall process boasts advantages such as simple operation, uniform crystallization, high product consistency, and promising prospects for industrial scale-up, achieving low-cost, large-scale preparation of high-purity α-cristobalite. Attached Figure Description

[0026] Figure 1 It is the XRD pattern of untreated quartz sand.

[0027] Figure 2 This is the XRD pattern of α-cristobalite prepared in Example 1.

[0028] Figure 3 The image shows the XRD pattern of α-cristobalite prepared in Example 2.

[0029] Figure 4 This is the XRD pattern of α-cristobalite prepared in Example 3.

[0030] Figure 5 This is the XRD pattern of α-cristobalite prepared in Example 4.

[0031] Figure 6 This is the XRD pattern of α-cristobalite prepared in Example 5.

[0032] Figure 7 This is the XRD pattern of α-cristobalite prepared in Example 6. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to these embodiments.

[0034] Example 1

[0035] Step 1: Add 0.2 mg of sodium hydroxide to 1000 mL of a silicic acid solution with a concentration of 120 mg / L (based on silicon dioxide), stir at 300 r / min for 30 min, then ultrasonically disperse at 180 W for 20 min, and vacuum degas for 20 min to obtain a stable and homogeneous sodium-containing silicic acid precursor. In the sodium-containing silicic acid precursor, based on a total mass of 100% silicon dioxide and sodium, silicon dioxide accounts for 99.9% and sodium accounts for 0.1%.

[0036] Step 2: Select 6N grade quartz sand with a purity of 99.9999% and a particle size distribution of 50–200 μm. First, ultrasonically clean it with ethanol for 25 min, then ultrasonically clean it with deionized water for 20 min. After drying with nitrogen, place it in an oven at 120℃ for 30 min to obtain clean quartz sand raw material. Add 10 g of clean quartz sand raw material to 40 g of sodium-containing silica precursor, stir at 300 r / min for 60 min, then ultrasonically clean it at 180 W for 40 min, stirring for 5 min every 10 min during the process, so that a precursor coating layer forms on the surface of the quartz sand. After filtration, spread the filter cake evenly on a quartz tray and place it in an electrically heated constant temperature forced-air drying oven at 135℃ for 18 h, gently turning it once every 1 h during the process, finally obtaining coated quartz sand.

[0037] Step 3: Spread the coated quartz sand evenly in a high-purity quartz boat, place it in a tube furnace, and introduce nitrogen gas at a rate of 5 L / min as a protective gas for crystallization treatment to transform the quartz sand crystal form into α-cristobalite. The crystallization treatment conditions are as follows: first, heat the temperature to 600℃ at a rate of 6℃ / min and hold for 30 min, then heat the temperature to 900℃ at a rate of 4℃ / min and hold for 4 h, and then cool the temperature to room temperature with the furnace at a rate of 2.5℃ / min.

[0038] Step 4: The generated α-cristobalite was ultrasonically cleaned with deionized water for 30 min, filtered, rinsed 4 times with deionized water, rinsed once with anhydrous ethanol, dried with nitrogen, and then placed in a 1:1 mixture of 200 mL of 6% hydrofluoric acid aqueous solution and 18% HCl aqueous solution. The mixture was ultrasonically treated for 20 min to remove the surface coating and residual alkali metal salts. It was then ultrasonically cleaned with deionized water until the pH of the cleaning solution reached neutral. The mixture was then spread flat on a quartz tray and dried at 135℃ for 18 h. After natural cooling, high-purity α-cristobalite product was obtained.

[0039] Example 2

[0040] Step 1: Add 0.41 mg of lithium hydroxide to 1000 mL of a silicic acid solution with a concentration of 120 mg / L (based on silicon dioxide), stir at 300 r / min for 30 min, then ultrasonically disperse at 180 W for 20 min, and vacuum degas for 20 min to obtain a stable and homogeneous lithium-containing silicic acid precursor. In the lithium-containing silicic acid precursor, based on a total mass of 100% silicon dioxide and lithium, silicon dioxide accounts for 99.9% and lithium accounts for 0.1%.

[0041] Step 2: Replace the sodium-containing silica precursor in Step 2 of Example 1 with an equal mass of lithium-containing silica precursor. The other steps are the same as in Step 2 of Example 1.

[0042] Step 3: This step is the same as step 3 in Example 1.

[0043] Step 4: This step is the same as step 4 in Example 1, to obtain a high-purity α-cristobalite product.

[0044] Example 3

[0045] Step 1: Add 0.17 mg of potassium hydroxide to 1000 mL of a silicic acid solution with a concentration of 120 mg / L (based on silicon dioxide), stir at 300 rpm for 30 min, then ultrasonically disperse at 180 W for 20 min, and degas under vacuum for 20 min to obtain a stable and homogeneous potassium-containing silicic acid precursor. In the potassium-containing silicic acid precursor, based on a total mass of 100% silicon dioxide and potassium, silicon dioxide accounts for 99.9% and potassium accounts for 0.1%.

[0046] Step 2: Replace the sodium-containing silica precursor in Step 2 of Example 1 with an equal mass of potassium-containing silica precursor. The other steps are the same as in Step 2 of Example 1.

[0047] Step 3: This step is the same as step 3 in Example 1.

[0048] Step 4: This step is the same as step 4 in Example 1, to obtain a high-purity α-cristobalite product.

[0049] Example 4

[0050] Step 1: Add 0.02 mg of sodium hydroxide to 1000 mL of a silicic acid solution with a concentration of 120 mg / L (based on silicon dioxide), stir at 300 r / min for 30 min, then ultrasonically disperse at 180 W for 20 min, and vacuum degas for 20 min to obtain a stable and homogeneous sodium-containing silicic acid precursor. In the sodium-containing silicic acid precursor, based on a total mass of 100% silicon dioxide and sodium, silicon dioxide accounts for 99.99% and sodium accounts for 0.01%.

[0051] Step 2: This step is the same as step 2 in Example 1.

[0052] Step 3: Spread the coated quartz sand evenly in a high-purity quartz boat, place it in a tube furnace, and introduce nitrogen gas at a rate of 5 L / min as a protective gas for crystallization treatment to transform the quartz sand crystal form into α-cristobalite. The crystallization treatment conditions are as follows: first, heat the temperature to 600℃ at a rate of 6℃ / min and hold for 30 min, then heat the temperature to 1100℃ at a rate of 4℃ / min and hold for 4 h, and then cool the temperature to room temperature with the furnace at a rate of 2.5℃ / min.

[0053] Step 4: This step is the same as step 4 in Example 1, to obtain a high-purity α-cristobalite product.

[0054] Example 5

[0055] Step 1: Add 0.04 mg of lithium hydroxide to 1000 mL of a 120 mg / L (based on silica) silica solution, stir at 300 rpm for 30 min, then ultrasonically disperse at 180 W for 20 min, and vacuum degas for 20 min to obtain a stable and homogeneous lithium-containing silica precursor. In the lithium-containing silica precursor, based on a total mass of 100% silica and lithium, silica comprises 99.99% and lithium comprises 0.01%.

[0056] Step 2: Replace the sodium-containing silica precursor in Step 2 of Example 1 with an equal mass of lithium-containing silica precursor. The other steps are the same as in Step 2 of Example 1.

[0057] Step 3: This step is the same as step 3 in Example 4.

[0058] Step 4: This step is the same as step 4 in Example 1, to obtain a high-purity α-cristobalite product.

[0059] Example 6

[0060] Step 1: Add 0.017 mg of potassium hydroxide to 1000 mL of a silicic acid solution with a concentration of 120 mg / L (based on silicon dioxide), stir at 300 rpm for 30 min, then ultrasonically disperse at 180 W for 20 min, and vacuum degas for 20 min to obtain a stable and homogeneous potassium-containing silicic acid precursor. In the potassium-containing silicic acid precursor, based on a total mass of 100% silicon dioxide and potassium, silicon dioxide accounts for 99.99% and potassium accounts for 0.01%.

[0061] Step 2: Replace the sodium-containing silica precursor in Step 2 of Example 1 with an equal mass of potassium-containing silica precursor. The other steps are the same as in Step 2 of Example 1.

[0062] Step 3: This step is the same as step 3 in Example 4.

[0063] Step 4: This step is the same as step 4 in Example 1, to obtain a high-purity α-cristobalite product.

[0064] Comparative Example 1

[0065] Step 1: Select 6N grade quartz sand with a purity of 99.9999% and a particle size distribution of 50-200μm. First, ultrasonically clean it with ethanol for 25 min, then ultrasonically clean it with deionized water for 20 min. After drying with nitrogen, place it in an oven at 120℃ for 30 min to obtain clean quartz sand raw material. Spread 10g of clean quartz sand raw material evenly in a high-purity quartz boat, place it in a tube furnace, and purge with nitrogen at 5L / min as a protective gas for crystallization treatment. The crystallization treatment conditions are as follows: first, heat to 600℃ at a heating rate of 6℃ / min and hold for 30 min, then heat to 1100℃ at a heating rate of 4℃ / min and hold for 4 h, and then cool to room temperature in the furnace at a rate of 2.5℃ / min.

[0066] Step 2: The crystallized product was ultrasonically cleaned with deionized water for 30 min, filtered, rinsed 4 times with deionized water, rinsed once with anhydrous ethanol, dried with nitrogen, and then placed in a 1:1 mixture of 200 mL of 6% hydrofluoric acid aqueous solution and 18% HCl aqueous solution. The mixture was ultrasonically treated for 20 min to remove the surface coating and residual alkali metal salts. The product was then ultrasonically cleaned with deionized water until the pH of the cleaning solution reached neutral. The product was then spread flat on a quartz tray and dried at 135℃ for 18 h, followed by natural cooling.

[0067] The samples obtained in each step of Examples 1 to 6 above were analyzed by ICP-MS, and the results are shown in Table 1.

[0068] Table 1

[0069]

[0070] ICP-MS analysis showed that the total impurity content of the α-cristobalite product prepared in Example 1 was 0.8 ppm, the total impurity content of the α-cristobalite product prepared in Example 2 was 0.6 ppm, the total impurity content of the α-cristobalite product prepared in Example 3 was 0.5 ppm, the total impurity content of the α-cristobalite product prepared in Example 4 was 0.7 ppm, the total impurity content of the α-cristobalite product prepared in Example 5 was 0.9 ppm, and the total impurity content of the α-cristobalite product prepared in Example 6 was 0.8 ppm. Furthermore, the contents of Fe, Al, Ti, and other elements were all ≤0.1 ppm, and the residual alkali metal elements were ≤0.1 ppm.

[0071] Depend on Figure 1 As can be seen, the XRD pattern of the untreated quartz sand shows no obvious sharp diffraction peaks, indicating that the sample is completely amorphous and no crystalline phase is formed. The XRD patterns of the samples treated by the methods in Examples 1-6 show (…). Figures 2-7The samples obtained by the method in Comparative Example 1 all exhibited sharp diffraction peaks characteristic of α-cristobalite at 2θ=21.9°, with no impurity phase diffraction peaks, proving that the quartz sand was successfully transformed into α-cristobalite with high purity. However, XRD analysis showed that the samples obtained after treatment by the method in Comparative Example 1 did not have obvious diffraction peaks at 2θ=21.9°, and were still amorphous or non-crystalline, indicating that 6N grade quartz sand could not achieve effective crystallization at 1100℃ without coating induction.

[0072] The results of the above embodiments show that, through doping and coating, the present invention can achieve efficient conversion of 6N grade quartz sand to high-purity α-cristobalite at a relatively low temperature of 900-1100℃, successfully solving the technical problem of the difficulty in crystallizing 6N grade quartz sand at low temperature. Moreover, the product purity is stable at 6N grade, with pure crystal phase, and has the dual advantages of simple process and high product performance, and has broad prospects for industrial application.

Claims

1. A method for converting 6N grade quartz sand into high-purity α-cristobalite using doping coating, characterized in that, Includes the following steps: Step 1: Add alkali metal hydroxide to the silicic acid solution, stir, ultrasonically disperse, and degas to obtain a silicic acid precursor containing alkali metal elements; the concentration of the silicic acid solution is 100-200 mg / L based on silicon dioxide, and the silicic acid precursor containing alkali metal elements, based on the total mass of silicon dioxide and alkali metal elements as 100%, has silicon dioxide content of 99.9%-99.99% and alkali metal elements content of 0.01%-0.1%; Step 2: Add 6N grade quartz sand to the silica precursor containing alkali metal elements for coating treatment, so that a precursor coating layer is formed on the surface of the quartz sand. After filtration and drying, the coated quartz sand is obtained. Step 3: Place the coated quartz sand under a nitrogen atmosphere for crystallization treatment to transform the quartz sand crystal form into α-cristobalite; the crystallization treatment conditions are: first heat to 500-700℃ and hold for 20-40 minutes, then heat to 900-1100℃ and hold for 3-5 hours. Step 4: After washing the generated α-cristobalite with deionized water and anhydrous ethanol, it is placed in an acid solution for treatment, then washed with deionized water until neutral, and dried to obtain high-purity α-cristobalite product.

2. The method for converting 6N grade quartz sand into high-purity α-cristobalite using doping coating according to claim 1, characterized in that, In step 1, the alkali metal hydroxide is one or more of sodium hydroxide, lithium hydroxide, and potassium hydroxide.

3. The method for converting 6N grade quartz sand into high-purity α-cristobalite using doping coating according to claim 1, characterized in that, In step 1, the stirring is performed at 200-400 r / min for 20-40 min; the dispersion is performed by ultrasonic dispersion for 15-25 min; and the degassing is performed by vacuum degassing for 15-25 min.

4. The method for converting 6N grade quartz sand into high-purity α-cristobalite using doping coating according to claim 1, characterized in that, In step 2, the purity of the 6N grade quartz sand is ≥99.9999%, and the particle size is 50-200μm. Before use, the 6N grade quartz sand is first ultrasonically cleaned with ethanol for 20-30 minutes, then ultrasonically cleaned with deionized water for 15-25 minutes, dried with nitrogen, and then dried at 100-150℃.

5. The method for converting 6N grade quartz sand into high-purity α-cristobalite using doping coating according to claim 1, characterized in that, In step 2, the mass ratio of the 6N grade quartz sand to the silica precursor containing alkali metal elements is 1:3 to 5.

6. The method for converting 6N grade quartz sand into high-purity α-cristobalite using doping coating according to claim 1, characterized in that, In step 2, the coating treatment conditions are as follows: first, stir at 200-400 r / min for 40-80 min, then sonicate at 150-250 W for 30-50 min, stirring for 5 min every 10 min during the process.

7. The method for converting 6N grade quartz sand into high-purity α-cristobalite using doping coating according to claim 1, characterized in that, In steps 2 and 4, the drying temperature is 100–150°C.

8. The method for converting 6N grade quartz sand into high-purity α-cristobalite using doping coating according to claim 1, characterized in that, In step 3, the crystallization treatment conditions are as follows: first, heat to 600℃ at a heating rate of 4-8℃ / min and hold for 20-40 min, then heat to 1000-1100℃ at a heating rate of 3-5℃ / min and hold for 3-5 h.

9. The method for converting 6N grade quartz sand into high-purity α-cristobalite using doping coating according to claim 1, characterized in that, In step 4, the acid solution is a mixture of 5%–7% hydrofluoric acid aqueous solution and 15%–20% HCl aqueous solution in a volume ratio of 1:1–3, and the acid solution is used for ultrasonic treatment for 15–25 minutes.

10. The method for converting 6N grade quartz sand into high-purity α-cristobalite using doping coating according to claim 1, characterized in that, The total impurity content of the high-purity α-cristobalite is 0.5 to 1 ppm, and the X-ray diffraction pattern shows the characteristic diffraction peak of α-cristobalite at 2θ = 21.9°.

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