Preparation method of high-purity crystalline tridymite quartz sand

Abstract

The application provides a preparation method of low-cost high-purity crystalline cristobalite quartz sand, comprising the following steps: mixing a silicon source, high-purity alkali and deionized water in a proportion, and heating to prepare high-purity water glass; mixing the high-purity water glass with high-purity acid in a proportion, stirring to form a precipitate, sealing, standing and using the growth process of the gel to perform segregation and impurity removal, and then performing aging treatment; using deionized water to filter and rinse the precipitate, and drying to obtain amorphous high-purity silica gel powder; performing crystallization transformation and chlorination impurity removal on the amorphous high-purity silica gel powder; using deionized water and dilute acid to clean the cristobalite after crystallization and impurity removal, and drying to obtain high-purity crystalline cristobalite quartz sand. The purity of the quartz sand prepared by the preparation method can reach 5N-7N grade, the preparation cost is low, large-scale continuous production can be realized, the synthesized quartz sand is in a crystalline state, and the produced quartz glass product has lower bubble and flocculent content and better mechanical, heat-resistant and optical properties.

Description

A method for preparing high-purity crystalline cristobalite sand Technical Field

[0001] This invention relates to the field of quartz material preparation technology, and in particular to a low-cost method for preparing high-purity crystalline cristobalite sand. Background Technology

[0002] High-purity quartz is an important basic inorganic material. Due to its excellent physicochemical properties, such as excellent chemical stability, extremely low coefficient of volume expansion, extremely high heat resistance, high insulation and voltage resistance, high mechanical properties, and high light transmittance, it is widely used in many industries such as photovoltaics, semiconductors, communications, optics, aerospace, and military.

[0003] With the development of various high-tech industries, higher requirements have been placed on high-purity quartz sand. Currently, the main technologies for preparing high-purity quartz are natural quartz purification and artificial synthesis. Since the purity of naturally purified quartz sand is limited by the availability of natural quartz minerals and purification techniques, it can no longer meet the needs of various high-tech industries. Therefore, artificial synthesis of quartz sand is the only way to obtain ultra-high-purity quartz sand.

[0004] The main methods for artificially synthesizing quartz sand currently include gas-phase synthesis, chemical precipitation, and sol-gel methods. Among them, gas-phase synthesis (e.g., Chinese patent application CN116375315A discloses a method for preparing titanium-doped synthetic quartz sand) requires high-temperature conditions and complex chemical equipment, and its products contain a large amount of HCl (hydrogen chloride) vapor, thus placing high demands on reaction equipment and waste gas treatment. Chemical precipitation (e.g., Chinese patent application CN115676842A discloses a method for preparing high-purity quartz sand using ultrafine silica powder) has a lower cost but requires methods such as ion exchange to reduce the impurity content in the product. The amount of ion exchange resin used is large, and the resin cleaning and regeneration processes require a large amount of acid and water resources. Sol-gel methods (e.g., Chinese patent application CN114105150A discloses a method for synthesizing silica powder using organosilanes) use a large amount of high-purity silicon organic matter, resulting in high cost and long production cycle.

[0005] In addition to the above, the three methods share a common problem: the products are all amorphous quartz sand particles. Compared to crystalline quartz, which has a defined melting temperature, amorphous quartz sand has only a wide melting / softening temperature range. During the melting process of amorphous quartz sand into quartz glass products, a wide melting / softening zone is generated. Gases within this zone (including gases between quartz sand particles and gases generated inside the quartz sand particles during melting) are easily trapped within the melting / softening zone, leading to defects such as bubbles and flocculent matter in the prepared quartz glass, reducing the mechanical properties, thermal stability, and optical properties of the quartz product. Conversely, crystalline quartz sand, because it has a defined melting temperature, generates a narrow melting / softening zone during melting, allowing gases to escape more easily. As a result, quartz glass prepared using crystalline quartz sand has a lower content of defects such as bubbles and flocculent matter, resulting in higher product quality. Therefore, the preparation of crystalline synthetic quartz sand is the development direction of the high-purity quartz sand industry.

[0006] Currently, the main method for synthesizing crystalline quartz sand is to produce crystalline α-quartz (low-temperature quartz) quartz sand using a hydrothermal growth method similar to that used in the preparation of artificial crystals. For example, Chinese patent application CN118183758A discloses a method for synthesizing high-purity silica crystals through sequential online replenishment. This method requires the use of a high-pressure reactor for long-term growth under high temperature and pressure conditions. Not only is the production cycle long and the output low, but the use of a hydrothermal reactor during the production process may also lead to an explosion, posing a considerable danger. Therefore, it is costly, has poor product consistency, and is difficult to widely apply. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies and propose a low-cost method for preparing high-purity crystalline cristobalite sand. This method uses artificial synthesis to prepare high-purity crystalline cristobalite sand and has the advantages of high yield, low cost, high safety and industrialization.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A low-cost method for preparing high-purity crystalline cristobalite sand, the method comprising:

[0010] Step S1: Mix silicon source, high-purity alkali and deionized water in a mass ratio of 1:(1~5):(10~50) and heat to 60℃~100℃ to obtain high-purity water glass;

[0011] Step S2: Mix high-purity water glass and high-purity acid in a mass ratio of (1:0.1) to (1:10), stir to form a precipitate, seal the precipitate and let it stand for 1 to 5 days to ensure complete precipitation, and use the gel growth process to perform segregation and impurity removal. Finally, heat the precipitate and mixed acid together to 40°C to 80°C for aging treatment for 1 to 5 days.

[0012] Step S3: Filter and rinse the precipitate with deionized water to remove ions from the surface of the precipitate until the pH of the filtrate reaches neutral; dry the precipitate at a temperature of 120℃~200℃ to obtain dry amorphous high-purity silica gel powder.

[0013] Step S4: Crystallization transformation and chlorination purification of amorphous high-purity silica gel powder: Place the amorphous high-purity silica gel powder in a high-temperature furnace and heat it to 1100℃~1300℃ at a heating rate of 2℃ / min~10℃ / min and hold it for 1 hour~30 hours to transform it into crystalline cristobalite; In the later stage of the holding period, chlorination gas is introduced into the high-temperature furnace for chlorination purification, and the purification process takes 0.5 hours~10 hours.

[0014] Step S5: Clean the crystallized and impurity-removed cristobalite with deionized water and dilute acid to remove chlorides from the surface of the cristobalite. After drying, high-purity crystalline cristobalite sand is obtained.

[0015] Preferably, in step S1, the silicon source is a high-purity silicon material or a high-purity silicon ore, and the purity of the silicon source is higher than 4N. The high-purity silicon material can be solar-grade silicon particles or waste, edge material, or cutting waste from solar-grade / semiconductor-grade silicon crystal pulling materials; or, the silicon source is a silicon ore with a purity higher than 4N.

[0016] Preferably, in step S1, the high-purity alkali is electronic-grade high-purity sodium hydroxide or potassium hydroxide, and the purity of the high-purity alkali is not less than 99.99%.

[0017] Preferably, in step S1, the resistivity of the deionized water is greater than 16.5 MΩ·cm and the total organic carbon is less than 20 ppmw.

[0018] Preferably, the heating method in step S1 is water bath heating or resistance heating.

[0019] Preferably, in step S2, the concentration of high-purity water glass is 5% to 35%.

[0020] Preferably, in step S2, the high-purity acid is one or more of inorganic acids and organic acids, and the concentration of the high-purity acid is 0.5 mol / L to 5.0 mol / L.

[0021] Preferably, the chlorine gas introduced in step S4 is hydrogen chloride gas, chlorine gas, or a mixture of the two.

[0022] Preferably, the purity of the high-purity crystalline quartz sand obtained in step S5 is 5N to 7N.

[0023] Preferably, the high-purity crystalline cristobalite sand obtained in step S5 has a particle size of 70μm to 300μm and a total impurity content of 0.5ppmw to 10ppmw.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows: The preparation method of the present invention can synthesize high-purity crystalline quartz sand at low cost, with a purity of 5N to 7N, which is higher than the purity of 4N to 5N of naturally purified quartz sand. At the same time, compared with the synthetic quartz sand produced by other methods, the preparation cost of the present invention is lower and the output is higher, which can realize large-scale continuous production. In addition, the synthesized quartz sand is in a crystalline state, and the quartz glass products produced using crystalline quartz sand have lower bubble and flocculent content and better mechanical, heat resistance and optical properties compared with amorphous synthetic quartz sand glass products.

[0025] The advantages of this invention also include:

[0026] 1. After the precipitation process, a longer static aging process is adopted. The precipitate in the liquid dissolves and regenerates on its own. At the solid-liquid interface, the segregation effect causes most of the impurities to separate into the liquid, so that the precipitate particles can be increased from small to large. The precipitate particle size can be made up to meet the requirements without consuming additional energy and extra time.

[0027] 2. By adopting a crystallization transformation + chlorination impurity removal process, the residual alkali metal elements in the amorphous high-purity silica gel powder are utilized to lower the temperature and energy barrier of the crystallization transformation of quartz at high temperatures. This allows the amorphous high-purity silica gel powder to be completely transformed into crystalline cristobalite in a shorter time. At the same time, chlorination gas is introduced in the later stage of heating to efficiently remove the alkali metal elements in the quartz lattice, and finally synthesize high-purity crystalline cristobalite sand, which greatly reduces the impurity content. Attached Figure Description

[0028] Figure 1 is a flowchart of a method for preparing high-purity crystalline cristobalite sand according to some embodiments of the present invention.

[0029] Figure 2 shows the X-ray diffraction pattern of high-purity crystalline cristobalite sand prepared in some embodiments of the present invention.

[0030] Figure 3 shows the particle size distribution of high-purity crystalline cristobalite sand prepared in some embodiments of the present invention. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the described embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. It is understood that, without conflict, some technical means of the various embodiments described herein can be substituted for or combined with each other.

[0032] In the description of this invention, the terms "first," "second," etc., are used only to distinguish the described objects and have no sequential or technical meaning. Therefore, objects specified with "first," "second," etc., may explicitly or implicitly include one or more of those objects. Furthermore, the words "one" or "a" do not indicate a quantity limitation, but rather indicate the presence of at least one, while "multiple" indicates at least two.

[0033] In the description of this invention, references to "one embodiment" or "some embodiments" mean that one or more embodiments of the invention include a particular feature, structure, or characteristic described in connection with that embodiment. Therefore, the phrases "one embodiment," "some embodiments," "other embodiments," "and other embodiments," etc., appearing in different parts of this specification do not necessarily refer to the same embodiment, but rather mean "one or more, but not all, embodiments," unless otherwise specifically emphasized.

[0034] This invention provides a low-cost method for preparing high-purity crystalline cristobalite sand. The method uses artificial synthesis to prepare high-purity crystalline cristobalite sand, and has the advantages of high yield, low cost, high safety and industrialization.

[0035] Referring to Figure 1, in some embodiments, the method for preparing this low-cost, high-purity crystalline cristobalite sand includes:

[0036] Step S1: Mix silicon source, high-purity alkali and deionized water in a mass ratio of 1:(1~5):(10~50) and heat to 60℃~100℃ to obtain high-purity water glass.

[0037] The silicon source can be high-purity silicon material or high-purity silicon ore, and the purity of the silicon source must be higher than 4N. For example, the silicon source can be solar-grade silicon granules or waste, edge material, or cutting waste from solar-grade / semiconductor-grade silicon crystal pulling materials; or, the silicon source can be silicon ore with a purity higher than 4N. It should be noted that N in the purity grade here is an abbreviation for "nine," representing the number of "9"s in the purity grade. For example, 3N represents a purity of 99.9%, 4N represents a purity of 99.99%, and so on. The purity requirements for the finished quartz sand may differ in different application scenarios. It has been verified that the purity of quartz sand prepared using silicon ore as the silicon source may be slightly lower than that prepared using high-purity silicon. However, the cost of silicon ore is lower than that of solar-grade silicon. Therefore, in some applications where the purity requirements for quartz sand are not particularly high, natural silicon ore can be used as the silicon source. This reduces manufacturing costs while still achieving a relatively high purity quartz sand to meet application needs. Conversely, in applications requiring higher purity quartz sand, solar-grade silicon can be used as the silicon source to obtain even higher purity crystalline cristobalite quartz sand, thus meeting application requirements.

[0038] High-purity alkali can be electronic-grade high-purity sodium hydroxide or potassium hydroxide, and the purity of the high-purity alkali should not be less than 99.99%. The resistivity of the deionized water is greater than 16.5 MΩ·cm, and the total organic carbon (TOC) is less than 20 ppmw. Heating methods can be water bath heating or resistance heating.

[0039] Step S2: Mix high-purity water glass and high-purity acid in a mass ratio of (1:0.1) to (1:10), stir to form a precipitate, seal the precipitate and let it stand for 1 to 5 days to ensure complete precipitation, and use the gel growth process to perform segregation and impurity removal. Finally, heat the precipitate and mixed acid together to 40°C to 80°C for aging treatment for 1 to 5 days.

[0040] The concentration of high-purity water glass ranges from 5% to 35%. High-purity acid can be one or more inorganic or organic acids. High-purity inorganic acids can include hydrochloric acid (HCl), nitric acid (HNO3), or sulfuric acid (H2SO4), while high-purity organic acids can include acetic acid (CH3COOH), oxalic acid (H2C2O4), and citric acid (C6H8O7). The concentration of high-purity acid ranges from 0.5 mol / L to 5.0 mol / L.

[0041] In step S2 of this invention, precipitation and segregation are used to increase the particle size of the precursor silica precipitate and reduce its impurity content. A longer static aging process is employed after precipitation, utilizing the self-dissolution and regeneration of the precipitate in the liquid. Segregation at the solid-liquid interface causes most impurities to segregate into the liquid, resulting in larger precipitate particles. In contrast, other existing methods for synthesizing quartz sand use ion exchange columns to replace metal ions in the solution, which is costly. Furthermore, regenerating the ion exchange column requires significant time and deionized water. Other synthesis methods also require additional granulation processes, including freezing and calcination, consuming additional energy. The preparation method of this invention only requires sealed static aging at 40°C–80°C, achieving the required particle size (70 μm–300 μm) for quartz sand without consuming additional energy or time.

[0042] Step S3: Filter and rinse the precipitate with deionized water to remove ions from the surface of the precipitate until the filtrate reaches neutral pH (pH 7). Dry the precipitate at 120℃~200℃ to obtain dry amorphous high-purity silica gel powder, which can be used as a quartz precursor.

[0043] Step S4: Crystallization transformation and chlorination removal of amorphous high-purity silica gel powder: Place the amorphous high-purity silica gel powder in a high-temperature furnace and heat it to 1100℃~1300℃ at a heating rate of 2℃ / min~10℃ / min and hold it for 1 hour~30 hours to transform it into crystalline cristobalite; In the later stage of the holding period, chlorine gas is introduced into the high-temperature furnace for chlorination removal, and the removal process takes 0.5 hours~10 hours, and the removal time does not exceed half of the holding time.

[0044] The chlorinating gas can be hydrogen chloride (HCl), chlorine (Cl2), or a mixture of both. The high-temperature furnace can be any existing high-temperature furnace equipment, such as a high-temperature tube furnace. The flow rate of the chlorinating gas can be appropriately set based on the furnace tube size and the mass of the amorphous high-purity silica gel powder. For example, in some embodiments, a commonly used tube furnace is used, and the flow rate of the chlorinating gas is 5 L / h to 8 L / h.

[0045] In step S4 of this invention, a crystallization transformation + chlorination impurity removal process is adopted. The alkali metal elements (with a content range of 1 to 2000 ppmw) that inevitably remain in the amorphous high-purity silica gel powder are used to reduce the temperature and energy barrier of the crystallization transformation of quartz at high temperature. This allows the amorphous high-purity silica gel powder to be completely transformed into crystalline cristobalite in a shorter time. At the same time, chlorination gas is introduced in the later stage of heating to efficiently remove the alkali metal elements in the quartz lattice. Finally, high-purity crystalline cristobalite sand is synthesized, which greatly reduces the impurity content.

[0046] Step S5: Clean the crystallized and impurity-removed cristobalite with deionized water and dilute acid to remove chlorides from the surface of the cristobalite. After drying, high-purity crystalline cristobalite sand is obtained.

[0047] The deionized water has a resistivity greater than 16.5 MΩ·cm and a total organic carbon (TOC) content less than 20 ppmw. The dilute acid can be an organic or inorganic acid. Inorganic acids can include hydrochloric acid (HCl), nitric acid (HNO3), or sulfuric acid (H2SO4), while organic acids can include acetic acid (CH3COOH), oxalic acid (H2C2O4), and citric acid (C6H8O7). The concentration of the dilute acid can be set appropriately according to the type of acid used. Preferably, the dilute acid used in step S5 can be the same type of acid at a different concentration as the high-purity acid used in step S2, for ease of management.

[0048] In the preparation method of this invention, high-purity water glass is first prepared using a silicon source, high-purity alkali, and deionized water. Then, precipitation and segregation are used to increase the particle size of the precursor silica precipitate and reduce its impurity content. Next, amorphous high-purity silica powder is prepared by washing with deionized water and slow drying. Then, the crystallization temperature is lowered by utilizing the appropriate amount of alkali metal impurities (with a content range of 1-2000 ppmw) present in the amorphous high-purity silica powder. In the subsequent crystallization transformation and chlorination impurity removal process, the amorphous high-purity silica powder is first heated to transform into crystalline cristobalite. Then, hydrogen chloride (HCl) gas, chlorine (Cl2) and other chlorination gases are introduced to remove alkali metal impurities, and finally, synthetic high-purity crystalline cristobalite sand is obtained.

[0049] In one embodiment, the high-purity crystalline cristobalite sand synthesized by the above preparation method was tested by an X-ray diffractometer, and the X-ray diffraction pattern (XRD pattern) shown in Figure 2 was obtained. It can be seen that the pattern has relatively sharp diffraction peaks, indicating that there is a crystalline structure with good crystallinity in the quartz sand.

[0050] In some embodiments, the high-purity crystalline cristobalite sand synthesized by the above preparation method was tested by a laser particle size analyzer, and the particle size distribution diagram shown in Figure 3 was obtained. It can be seen that more than 95% of the overall particle size distribution of the synthesized high-purity crystalline cristobalite sand is between 70 μm and 300 μm.

[0051] Generally, the purity of naturally purified quartz sand is 4N to 5N, while the purity of high-purity crystalline cristobalite sand synthesized using the above preparation method can reach 5N to 7N, with a total impurity content of 0.5 ppmw to 10 ppmw. Therefore, compared with naturally purified quartz sand, the high-purity crystalline cristobalite sand synthesized using the preparation method of this invention has higher purity and lower impurity content.

[0052] Meanwhile, existing methods for synthesizing quartz sand require additional energy and time, while the preparation method of this invention employs processes such as precipitation + segregation, crystallization transformation + chlorination to remove impurities, which can reduce manufacturing costs, increase output, and enable large-scale continuous production. Furthermore, the synthesized quartz sand is in a crystalline state, and quartz glass products produced using crystalline quartz sand have lower bubble and flocculent content and better mechanical, heat-resistant, and optical properties.

[0053] In some other preferred embodiments, the method for preparing high-purity crystalline cristobalite sand includes:

[0054] Step S1: Using high-purity silicon as the silicon source, high-purity silicon, high-purity alkali (electronic-grade high-purity sodium hydroxide or potassium hydroxide) and deionized water (resistivity greater than 16.5 MΩ·cm, TOC less than 20 ppmw) are mixed in a mass ratio of 1:(2~4):(20~40) and heated to 60℃~85℃ to obtain high-purity water glass.

[0055] Step S2: Mix high-purity water glass and high-purity acid in a mass ratio of (1:0.5) to (1:8.5), stir to form a precipitate, and seal the precipitate and let it stand for 2 to 4 days to ensure complete precipitation. Segregation and impurity removal are carried out by the growth process of the gel. Finally, heat the precipitate and the mixed acid together to 45°C to 75°C for aging treatment for 2 to 4 days.

[0056] Step S3: Filter and rinse the precipitate with deionized water to remove ions from the surface of the precipitate until the filtrate reaches neutral pH (pH value 7). Dry the precipitate at 140℃~190℃ to obtain dry amorphous high-purity silica gel powder.

[0057] Step S4: Crystallization transformation and chlorination removal of amorphous high-purity silica gel powder: Place the amorphous high-purity silica gel powder in a high-temperature furnace and heat it to 1100℃~1300℃ at a heating rate of 3℃ / min~8℃ / min and hold it for 5 hours~25 hours to transform it into crystalline cristobalite; In the later stage of the holding period, chlorine gas is introduced into the high-temperature furnace for chlorination removal, and the removal process takes 1 hour~5 hours.

[0058] Step S5: Clean the crystallized and impurity-removed cristobalite with deionized water and dilute acid to remove chlorides from the surface of the cristobalite. After drying, high-purity crystalline cristobalite sand is obtained.

[0059] The high-purity crystalline cristobalite sand synthesized using the above preparation method has higher purity and lower impurity content. Simultaneously, it can reduce manufacturing costs, increase output, and enable large-scale continuous production. The synthesized quartz sand is in a crystalline state, and quartz glass products produced using this crystalline quartz sand have lower bubble and flocculent content and better mechanical, heat-resistant, and optical properties. Example 1

[0060] Methods for preparing high-purity crystalline cristobalite sand include:

[0061] Step S1: Mix solar-grade silicon, electronic-grade high-purity sodium hydroxide, and deionized water (resistivity greater than 16.5 MΩ·cm, TOC less than 20 ppmw) in a mass ratio of 1:3:30 and heat to 60°C to obtain high-purity sodium water glass.

[0062] Step S2: Slowly add 2 mol / L electronic grade high-purity nitric acid to 13% high-purity sodium silicate solution. The mass ratio of high-purity sodium silicate solution to high-purity nitric acid solution is 1:2. Stir the solution until a precipitate is completely formed. Let the precipitate and solution stand together for 24 hours to allow for segregation and impurity removal. Then, place the precipitate and mixed acid on a 50°C heating plate for aging treatment for 24 hours.

[0063] Step S3: Filter and rinse the precipitate with deionized water to remove ions from the surface of the precipitate until the filtrate reaches neutral pH (pH 7). Place the filter cake on a heating plate at 150°C to dry and dehydrate, obtaining dry amorphous high-purity silica gel powder.

[0064] Step S4: Crystallization transformation and chlorination removal of amorphous high-purity silica gel powder: The amorphous high-purity silica gel powder is placed in a high-temperature tube furnace and heated to 1100℃ at a heating rate of 3℃ / min and held for 3 hours to transform it into crystalline cristobalite; during the last 1.5 hours of holding, dry HCl gas is introduced into the high-temperature tube furnace for chlorination removal, and the flow rate of HCl gas is 6L / h.

[0065] Step S5: Clean the crystallized and impurity-removed cristobalite with deionized water and dilute nitric acid to remove chlorides from the surface of the cristobalite. After drying, high-purity crystalline cristobalite sand is obtained.

[0066] Subsequently, the obtained high-purity crystalline cristobalite sand was subjected to inductively coupled plasma (ICP) testing. The test results are shown in Table 1 (where "~" indicates that the content of the element is below the detection limit):

[0067] Table 1. ICP test results of quartz sand in Example 1

[0068] Impurity element content (ppmw): Li, Na, K, Al, Fe, Ti, Ca: ~1.1~0.2 0.4 0.1 0.2 Total impurity element content (ppmw): Mg, Cr, Ni, Mn, Cu, B: ~~~~~~2.0 surface

[0069] It can be seen that the total impurity content in this high-purity crystalline cristobalite sand is 2.0 ppmw, and the purity reaches grade 5N8. Therefore, the crystalline cristobalite sand synthesized by the above preparation method has higher purity and lower impurity content. Quartz glass products produced using this high-purity crystalline cristobalite sand have lower bubble and flocculent content and better mechanical, heat resistance, and optical properties. Example 2

[0070] Methods for preparing high-purity crystalline cristobalite sand include:

[0071] Step S1: Mix solar-grade silicon, electronic-grade potassium hydroxide, and deionized water (resistivity greater than 16.5 MΩ·cm, TOC less than 20 ppmw) in a mass ratio of 1:4:50 and heat to 70°C to obtain high-purity potassium water glass.

[0072] Step S2: Slowly add 4.5 mol / L electronic-grade high-purity hydrochloric acid to 10% high-purity potassium silicate solution. The mass ratio of high-purity potassium silicate to high-purity hydrochloric acid solution is 1:0.75. Stir the solution until a precipitate is completely formed. Let the precipitate and solution stand together for 3 days to allow the gel to grow and remove impurities through segregation. Finally, place the precipitate and mixed acid together on a 60°C heating plate for aging treatment for 2 days.

[0073] Step S3: Filter and rinse the precipitate with deionized water to remove ions from the surface of the precipitate until the filtrate reaches neutral pH (pH 7). Place the filter cake on a heating plate at 170°C to dry and dehydrate, obtaining dry amorphous high-purity silica gel powder.

[0074] Step S4: Crystallization transformation and chlorination removal of amorphous high-purity silica gel powder: The amorphous high-purity silica gel powder is placed in a high-temperature tube furnace and heated to 1250°C at a heating rate of 5°C / min and held for 10 hours to transform it into crystalline cristobalite; During the last 3 hours of holding, a dry HCl and Cl2 mixed gas is introduced into the high-temperature tube furnace at a flow rate of 8L / h.

[0075] Step S5: Clean the crystallized and impurity-removed cristobalite with deionized water and dilute hydrochloric acid to remove chlorides from the surface of the cristobalite. After drying, high-purity crystalline cristobalite sand is obtained.

[0076] Subsequently, the obtained high-purity crystalline cristobalite sand was subjected to inductively coupled plasma (ICP) testing, and the test results are shown in Table 2 (where "~" indicates that the content of the element is below the detection limit):

[0077] Table 2. ICP test results of quartz sand in Example 2

[0078] Impurity element content (ppmw): Li, Na, K, Al, Fe, Ti, Ca ~~~0.1 0.1 ~0.2; Total impurity element content (ppmw): Mg, Cr, Ni, Mn, Cu, B ~~~~0.1 0.5 surface

[0079] It can be seen that the total impurity content in this high-purity crystalline cristobalite sand is 0.5 ppmw, and the purity reaches grade 6N5. Therefore, the crystalline cristobalite sand synthesized by the above preparation method has higher purity and lower impurity content. Quartz glass products produced using this high-purity crystalline cristobalite sand have lower bubble and flocculent content and better mechanical, heat resistance, and optical properties. Example 3

[0080] Methods for preparing high-purity crystalline cristobalite sand include:

[0081] Step S1: Mix solar-grade silicon, electronic-grade potassium hydroxide, and deionized water (resistivity greater than 16.5 MΩ·cm, TOC less than 20 ppmw) in a mass ratio of 1:2.5:20 and heat to 65°C to obtain high-purity potassium water glass.

[0082] Step S2: Slowly add 0.5 mol / L electronic-grade high-purity sulfuric acid to 25% high-purity potassium silicate solution. The mass ratio of high-purity potassium silicate solution to high-purity sulfuric acid solution is 1:5. Stir the solution until a precipitate is completely formed. Let the precipitate and solution stand together for 3 days to allow the gel to grow and remove impurities through segregation. Finally, place the precipitate and mixed acid together on a 75°C heating plate for aging treatment for 3 days.

[0083] Step S3: Filter and rinse the precipitate with deionized water to remove ions from the surface of the precipitate until the filtrate reaches neutral pH (pH 7). Place the filter cake on a heating plate at 200°C to dry and dehydrate, obtaining dry amorphous high-purity silica gel powder.

[0084] Step S4: Crystallization transformation and chlorination removal of amorphous high-purity silica gel powder: The amorphous high-purity silica gel powder is placed in a high-temperature tube furnace and heated to 1100℃ at a heating rate of 5℃ / min and held for 5 hours to transform it into crystalline cristobalite; During the last 2 hours of holding, a dry HCl and Cl2 mixed gas is introduced into the high-temperature tube furnace for chlorination removal, with a chlorination gas flow rate of 8L / h.

[0085] Step S5: Clean the crystallized and impurity-removed cristobalite with deionized water and dilute acid to remove chlorides from the surface of the cristobalite. After drying, high-purity crystalline cristobalite sand is obtained.

[0086] Subsequently, the obtained high-purity crystalline cristobalite sand was subjected to inductively coupled plasma (ICP) testing, and the test results are shown in Table 3 (where "~" indicates that the content of the element is below the detection limit):

[0087] Table 3. ICP test results of quartz sand in Example 3

[0088] Impurity element content (ppmw): Li, Na, K, Al, Fe, Ti, Ca: ~0.1, 3.8, ~0.1, 0.1, 0.2; Total impurity element content (ppmw): Mg, Cr, Ni, Mn, Cu, B: 0.1, ~~~~, 0.1, 4.5 surface

[0089] It can be seen that the total impurity content in this high-purity crystalline cristobalite sand is 4.5 ppmw, and the purity reaches grade 5N5. Therefore, the crystalline cristobalite sand synthesized by the above preparation method has higher purity and lower impurity content. Quartz glass products produced using this high-purity crystalline cristobalite sand have lower bubble and flocculent content and better mechanical, heat resistance, and optical properties. Example 4

[0090] Methods for preparing high-purity crystalline cristobalite sand include:

[0091] Step S1: Mix the cleaned natural silicon ore, electronic grade potassium hydroxide, and deionized water (resistivity greater than 16.5 MΩ·cm, TOC less than 20 ppmw) in a mass ratio of 1:3:10 and heat to 65°C to obtain high-purity potassium water glass.

[0092] Step S2: Slowly add 3 mol / L electronic-grade high-purity nitric acid to 18% high-purity potassium silicate solution. The mass ratio of high-purity potassium silicate solution to high-purity sulfuric acid solution is 1:1.5. Stir the solution until a precipitate is completely formed. Let the precipitate and solution stand together for 2 days to allow the gel to grow and remove impurities through segregation. Finally, place the precipitate and mixed acid together on a 75°C heating plate for aging treatment for 2 days.

[0093] Step S3: Filter and rinse the precipitate with deionized water to remove ions from the surface of the precipitate until the filtrate reaches neutral pH (pH 7). Place the filter cake on a heating plate at 190°C to dry and dehydrate, obtaining dry amorphous high-purity silica gel powder.

[0094] Step S4: Crystallization transformation and chlorination removal of amorphous high-purity silica gel powder: The amorphous high-purity silica gel powder is placed in a high-temperature tube furnace and heated to 1100℃ at a heating rate of 3℃ / min and held for 2 hours to transform it into crystalline cristobalite; In the last hour of holding, a dry HCl and Cl2 mixed gas is introduced into the high-temperature tube furnace for chlorination removal, and the flow rate of the chlorination gas is 6L / h.

[0095] Step S5: Clean the crystallized and impurity-removed cristobalite with deionized water and dilute acid to remove chlorides from the surface of the cristobalite. After drying, high-purity crystalline cristobalite sand is obtained.

[0096] Subsequently, the obtained high-purity crystalline cristobalite sand was subjected to inductively coupled plasma (ICP) testing, and the test results are shown in Table 4 (where "~" indicates that the content of the element is below the detection limit):

[0097] Table 4. ICP test results of quartz sand in Example 4

[0098] Impurity element content (ppmw): Li, Na, K, Al, Fe, Ti, Ca ~0.1, 5.6, 2.8, 0.6, 0.2, 0.4; Total impurity element content (ppmw): Mg, Cr, Ni, Mn, Cu, B 0.1 ~~~~~9.8 surface

[0099] It can be seen that the total impurity content in this high-purity crystalline cristobalite sand is 9.8 ppmw, and the purity reaches the 5N level. Therefore, the crystalline cristobalite sand synthesized by the above preparation method has higher purity and lower impurity content. Quartz glass products produced using this high-purity crystalline cristobalite sand have lower bubble and flocculent content and better mechanical, heat resistance, and optical properties. It should be noted that in step 1 of this embodiment, natural silicon ore is used as the silicon source. Although the purity of the obtained quartz sand is slightly lower than that of quartz sand obtained using solar-grade silicon as the silicon source, the cost of natural silicon ore is lower than that of solar-grade silicon. Therefore, in some scenarios where the purity requirements for quartz sand are not particularly high, natural silicon ore can be used as the silicon source, which can reduce manufacturing costs and achieve relatively high purity in the obtained quartz sand to meet application requirements.

[0100] In summary, this invention provides a low-cost method for preparing high-purity crystalline cristobalite sand. First, high-purity water glass is prepared using high-purity silicon or high-purity silicon ore, high-purity alkali, and deionized water. Then, precipitation and segregation are used to increase the particle size of the precursor silica precipitate and reduce its impurity content. Next, amorphous high-purity silica gel powder is prepared by washing with deionized water and slow drying. Then, by utilizing the appropriate amount of alkali metal impurities present in the amorphous high-purity silica gel powder, its crystallization temperature is lowered. In the subsequent crystallization transformation and chlorination impurity removal process, the amorphous high-purity silica gel powder is first heated to transform into crystalline cristobalite, and then chlorination gas is introduced for chlorination impurity removal to remove alkali metal impurities, ultimately obtaining synthetic high-purity crystalline cristobalite sand.

[0101] The high-purity crystalline cristobalite sand synthesized using the preparation method of this invention has higher purity and lower impurity content. Furthermore, the preparation method of this invention employs processes such as precipitation + segregation, crystallization transformation + chlorination for impurity removal, which can reduce manufacturing costs, increase yield, and enable large-scale continuous production. The synthesized quartz sand is in a crystalline state, and quartz glass products produced using crystalline quartz sand have lower bubble and flocculent content and better mechanical, heat-resistant, and optical properties.

[0102] The present invention has been described in the above-described embodiments; however, these embodiments are merely examples for implementing the present invention. It must be noted that the disclosed embodiments do not limit the scope of the present invention. Conversely, any modifications and refinements made without departing from the spirit and scope of the present invention are within the scope of patent protection of the present invention.

Claims

1. A method for preparing high-purity crystalline cristobalite sand, characterized in that: The preparation method of the high-purity crystalline cristobalite sand includes: Step S1: Mixing silicon source, high-purity alkali, and deionized water in a mass ratio of 1:(1-5):(10-50), and heating to 60℃-100℃ to obtain high-purity water glass; wherein, the silicon source is solar-grade silicon particle material or waste, edge material, or cutting waste of solar-grade / semiconductor-grade silicon crystal pulling material, or silicon ore with a purity higher than 4N; the high-purity alkali is electronic-grade high-purity sodium hydroxide or potassium hydroxide, and the purity of the high-purity alkali is not less than 99.99%. Step S2: Mix high-purity water glass and high-purity acid in a mass ratio of (1:0.1) to (1:10), stir to form a precipitate, seal the precipitate and let it stand for 1 to 5 days to ensure complete precipitation. Finally, heat the precipitate and the mixed acid together to 40℃ to 80℃ for aging treatment for 1 to 5 days. Utilize the self-dissolution and regeneration process of the precipitate in the liquid, and use segregation at the solid-liquid interface to cause most impurities to segregate into the liquid. Step S3: Use deionized water to treat the precipitate. Filter and rinse to remove ions from the surface of the precipitate until the pH of the filtrate reaches neutral; dry the precipitate at 120℃~200℃ to obtain dry amorphous high-purity silica gel powder; wherein the residual alkali metal content in the amorphous high-purity silica gel powder ranges from 1~2000ppmw; Step S4: crystallization transformation and chlorination purification of the amorphous high-purity silica gel powder: place the amorphous high-purity silica gel powder in a high-temperature furnace and heat it to 110℃ at a heating rate of 2℃ / min~10℃ / min. The amorphous high-purity silica gel powder is heated at 0℃~1300℃ for 1 hour to 30 hours. The alkali metal elements in the amorphous high-purity silica gel powder are used to reduce the temperature and energy barrier of the crystallization transformation of quartz at high temperature, and the amorphous high-purity silica gel powder is transformed into crystalline cristobalite. In the later stage of the heat preservation, chlorine gas is introduced into the high-temperature furnace for chlorination and impurity removal. The impurity removal process takes 0.5 hours to 10 hours. Step S5: The crystallized and impurity-removed cristobalite is cleaned with deionized water and dilute acid to remove chlorides on the surface of the cristobalite. After drying, high-purity crystalline cristobalite sand is obtained.

2. The method for preparing high-purity crystalline cristobalite sand as described in claim 1, characterized in that: In step S1, the resistivity of the deionized water is greater than 16.5 MΩ·cm, and the total organic carbon is less than 20 ppmw.

3. The method for preparing high-purity crystalline cristobalite sand as described in claim 1, characterized in that: In step S1, the heating method is either water bath heating or resistance heating.

4. The method for preparing high-purity crystalline cristobalite sand as described in claim 1, characterized in that: In step S2, the concentration of high-purity water glass is 5% to 35%.

5. The method for preparing high-purity crystalline cristobalite sand as described in claim 1, characterized in that: In step S2, the high-purity acid is one or more of inorganic and organic acids, and the concentration of the high-purity acid is 0.5 mol / L to 5.0 mol / L.

6. The method for preparing high-purity crystalline cristobalite sand as described in claim 1, characterized in that: The chlorination gas introduced in step S4 is hydrogen chloride gas, chlorine gas, or a mixture of the two.

7. The method for preparing high-purity crystalline cristobalite sand as described in claim 1, characterized in that: The high-purity crystalline cristobalite sand obtained in step S5 has a purity of 5N to 7N.

8. The method for preparing high-purity crystalline cristobalite sand as described in claim 1, characterized in that: The high-purity crystalline cristobalite sand obtained in step S5 has a particle size of 70μm to 300μm and a total impurity content of 0.5ppmw to 10ppmw.

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