Sintering method for preparing high-density synthetic sand

Abstract

The invention discloses a sintering method for preparing high-density synthetic sand, which comprises the following steps: by taking water glass as a precursor, carrying out ion exchange to obtain a silicic acid aqueous solution, quickly forming uniform gel with diluted electronic-grade ammonia water through a static pipeline mixer, freezing the gel at low temperature, and drying to obtain porous silicon dioxide powder; and after chlorination and impurity removal, screening different particle size grades, and carrying out temperature-controlled vacuum graded roasting to obtain the high-density synthetic sand. According to the method, the problems of non-uniform gel and poor compactness of the synthetic sand in the existing sol-gel method are solved, the synthetic sand with high purity and high compactness is prepared through a high-purity and high-compactness sintering mode for preparing the porous intermediate by the rapid sol-gel method, the production efficiency is effectively improved, internal pores of the synthetic sand are reduced, and the internal quality of quartz glass is improved.

Description

Technical Field

[0001] This invention relates to the field of high-purity quartz sand preparation technology, specifically to a sintering method for preparing high-density synthetic sand. Background Technology

[0002] In the field of high-purity quartz sand, there are currently two main types: synthetic sand and mineral sand. The two differ significantly in purity and application scenarios.

[0003] Mineral sand relies on the purification of natural quartz ore. Due to the limitations of ore quality, even after complex purification processes, the improvement of purity still faces bottlenecks and is difficult to meet the needs of fields with extremely high purity requirements.

[0004] Synthetic sand, through chemical synthesis technology, can achieve a purity of 6N or even 7N (the purity of mainstream products is 99.9999%-99.99999%). The total amount of metal ions and the content of hydroxyl groups can be controlled at extremely low levels. It has irreplaceable advantages in fields such as semiconductors and high-end optical glass, where the purity and performance of materials are extremely demanding.

[0005] The current mainstream preparation process for synthetic quartz sand is the sol-gel method (as seen in publications CN119430199A and CN119750590A). While this method is simple and has controllable pollution, it has significant drawbacks in industrial applications: firstly, the gel is prone to unevenness and long reaction times, affecting production efficiency; secondly, the resulting intermediate has a porous structure, requiring further sintering to form dense quartz sand particles. The porosity of the quartz sand particles directly determines the internal quality of the fused quartz glass (poor density can easily lead to a large number of bubbles inside the glass, even forming opaque milky glass). The quality of the quartz glass directly affects its application range. In the semiconductor and optics industries, the internal bubbles of the quartz glass must meet the Class 2 standard in JC / T185-2023 "Optical Quartz Glass".

[0006] Existing literature mainly focuses on the purity of synthetic quartz sand, with very little research on sintering density, especially on porous intermediates formed by the sol-gel method. This lack of research provides practical guidance for the industrial production and application of synthetic sand, making the preparation of high-density synthetic sand a pain point in the industry. Summary of the Invention

[0007] The purpose of this invention is to provide a sintering method for preparing high-density synthetic sand, so as to solve the technical problem that the formation of porous intermediates in the preparation of synthetic sand affects the density and purity of the synthetic sand.

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

[0009] A method for preparing high-purity, high-density synthetic sand includes the following steps:

[0010] Step (1): A silicic acid aqueous solution and an alkaline catalyst are simultaneously introduced into a static pipeline mixer at a flow ratio of 100:(1-10) to rapidly form a gel; the alkaline catalyst is electronic-grade ammonia diluted 5-10 times, the flow rate of the silicic acid aqueous solution is 100-500 g / s, the static pipeline mixer is a Y-shaped structure, the lower end of the Y-shaped structure is provided with intermittent spiral blades, the axial vertical position of the spaced spiral blades is the same, and the diameter of the lower end of the Y-shaped structure is 20-30 mm, with a length-to-diameter ratio of (2.5-5):1;

[0011] Step (2): The gel obtained in step (1) is rapidly cooled at low temperature to obtain wet porous silica powder and then dried;

[0012] Step (3): Pass the porous silica powder obtained in step (2) into a chlorination furnace for chlorination and impurity removal. The chlorination temperature is 800-1100℃. The chlorination gas is selected from one of hydrogen chloride, chlorine, a mixture of hydrogen chloride and nitrogen, or a mixture of chlorine and nitrogen. The total flow rate of a single chlorination gas is 0.16-0.5 m³ / h. For mixed chlorination gases, the flow ratio of chloride to nitrogen is 1:(1-5).

[0013] Step (4): The silica powder obtained in step (3) is sieved and classified into nine grades: >400μm, 400-350μm, 350-300μm, 300-250μm, 250-200μm, 200-150μm, 150-100μm, 100-50μm, and <50μm.

[0014] Step (5): Place the nine grades of silica powder obtained in step (4) into a vacuum sintering furnace and calcine them under vacuum. The calcine temperature is 1200-1350℃. The calcine time for each grade is as follows: >400μm grade 10-10.5h, 400-350μm grade 9-9.5h, 350-300μm grade 8-8.5h, 300-250μm grade 7-7.5h, 250-200μm grade 6-6.5h, 200-150μm grade 5-5.5h, 150-100μm grade 4-4.4h, 100-50μm grade 3-3.5h, <50μm grade 2-2.5h. After calcine, high-purity and high-density synthetic sand is obtained.

[0015] The above technical solution achieves multi-dimensional technical optimization: First, by using a static pipeline mixer and controlling the flow ratio of silica and alkaline catalyst, gel is formed rapidly, avoiding the problem of uneven gel formation in traditional mixing methods. Simultaneously, the gelation time is shortened, reducing energy consumption in the gelation process and providing feasible conditions for continuous industrial production. Second, the dried porous silica powder undergoes chlorination treatment. Utilizing its loose, porous, amorphous structure, the chlorination process significantly improves impurity removal efficiency, laying the foundation for subsequent high-purity synthetic sand preparation. Third, the porous silica powder is divided into nine grades according to particle size and matched with different calcination times. This avoids insufficient densification due to insufficient energy for larger particles or agglomeration due to over-calcination of smaller particles, ensuring uniform density across all grades. Finally, calcination in a vacuum environment reduces external impurity contamination, further improving the purity of the synthetic sand.

[0016] The above scheme optimizes the material flow path inside the mixer by limiting the Y-shaped structure of the static pipe mixer and the diameter and length-to-diameter ratio of the lower end of the Y-shaped structure. This ensures that the material has sufficient residence time in the mixer and avoids excessive material flow rate and insufficient mixing due to too small a pipe diameter, or reduced mixing efficiency due to too large a pipe diameter. The optimized length-to-diameter ratio allows the material to undergo more thorough shearing and stirring during the flow process, further improving the uniformity of the gel and reducing the situation of excessively fast or slow local gelation. This ensures stable gel quality and provides a prerequisite for the subsequent preparation of uniform porous silica powder.

[0017] As a preferred embodiment of the present invention, the silica aqueous solution in step (1) is obtained by removing metal ions through ion exchange using water glass as a precursor; the gelation time is 1-20 min.

[0018] As a preferred embodiment of the present invention, in step (2), the gel is rapidly cooled at a low temperature of -50℃ to -30℃, and the drying temperature is 600℃.

[0019] As a preferred embodiment of the present invention, in step (3), the particle size of the porous silica is <600μm, the internal pores are mesopores, and the packing density should be <1g / cm³.

[0020] As a preferred embodiment of the present invention, the temperature of the low-temperature rapid cooling in step (2) is -50℃, -40℃ or -30℃; the calcination temperature in step (5) is 1300-1350℃, and the calcination time of each grade of silica powder is: >400μm grade 10h, 400-350μm grade 9h, 350-300μm grade 8h, 300-250μm grade 7h, 250-200μm grade 6h, 200-150μm grade 5h, 150-100μm grade 4h, 100-50μm grade 3h, <50μm grade 2h.

[0021] The above technical solution further improves the density and quality stability of synthetic sand by optimizing the low-temperature rapid cooling temperature, calcination temperature, and calcination time: freezing temperatures of -50℃, -40℃, or -30℃ allow the gel to quickly solidify, avoiding slow moisture loss that leads to uneven gel shrinkage and collapse of the porous structure, thus ensuring the structural integrity of the wet porous silica powder; calcination temperatures of 1300-1350℃ provide sufficient energy for silica powders of different particle sizes, promoting the shrinkage of internal pores and achieving densification; calcination time (e.g., 10h for >400μm grade, 2h for <50μm grade) precisely matches the densification requirements of powders of different particle sizes. Larger particle sizes require more time to absorb energy to ensure sufficient shrinkage of internal pores, while smaller particle sizes can be densified in a shorter time, avoiding over-calcination, so that silica powders of different grades achieve uniform density, reduce internal pores, and improve the quality of synthetic sand.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. Improve production efficiency and stability, and adapt to industrialization. Achieve rapid gelation through a static pipeline mixer, shortening reaction time and reducing energy consumption. Control key indicators such as flow ratio, mixer structural parameters, and gelation time to avoid problems such as uneven gelation and large batch differences in traditional processes, and achieve continuous and stable production.

[0024] 2. Deep impurity removal ensures high purity. Through chlorination, the high reactivity of the porous structure is fully utilized to significantly reduce the content of metal impurities in the synthetic sand, meeting the stringent requirements for material purity in fields such as semiconductors and high-end optics. At the same time, the chlorination process does not damage the basic structure of silicon dioxide, preserving good conditions for subsequent densification.

[0025] 3. Achieve uniform densification: By sieving and grading and differentiating calcination times, the problem of different densification requirements for powders of different particle sizes is solved, avoiding residual pores or sintering adhesion, and ensuring uniform density of all grades of synthetic sand. After reducing internal pores, the generation of bubbles can be greatly reduced during subsequent melting of quartz glass, so that the internal quality of the glass meets the Class 2 standard in JC / T185-2023 "Optical Quartz Glass", breaking through the limitation of low glass quality caused by poor densification of existing synthetic sand. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only examples of embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1This is a particle size distribution diagram of silica obtained in Example 1 of the present invention;

[0028] Figure 2 The pore size diagram of the quartz sand obtained in Example 1 of the present invention;

[0029] Figure 3 This is a particle size distribution diagram of silica obtained in Example 2 of the present invention;

[0030] Figure 4 The pore size diagram of the quartz sand obtained in Example 2 of the present invention;

[0031] Figure 5 This is a particle size distribution diagram of silica obtained in Example 3 of the present invention;

[0032] Figure 6 This is a pore size distribution diagram of the quartz sand obtained in Example 4 of the present invention.

[0033] Figure 7 This is a particle size distribution diagram of silica obtained in Example 4 of the present invention;

[0034] Figure 8 This is a pore size distribution diagram of the quartz sand obtained in Example 4 of the present invention;

[0035] Figure 9 This is a schematic diagram of the structure of the static pipe mixer in an embodiment of the present invention;

[0036] Figure 10 for Figure 9 A schematic diagram of the structure of a broken spiral blade. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0038] Example 1:

[0039] The specific steps of the sintering method for high-density synthetic sand in Embodiment 1 of the present invention are as follows:

[0040] Step 1: Gel preparation:

[0041] Preparation of high-purity silicic acid aqueous solution:

[0042] Using water glass as a precursor, metal ions are removed through ion exchange to obtain a high-purity silica solution.

[0043] Rapid gel preparation via pipe mixing:

[0044] Prepare an alkaline catalyst: dilute electronic-grade ammonia water 5 times;

[0045] Feed parameters: The flow rate of the silica aqueous solution is controlled at 200 g / s, and the flow rate of the alkaline catalyst is 5 g / s. The two are simultaneously introduced into the static pipeline mixer at this flow rate ratio (100:2.5) to quickly form a gel.

[0046] Static pipe mixer parameters: The static pipe mixer is... Figure 9-10 The Y-shaped structure shown has a lower diameter of 30mm and a length of 100mm (ensuring a length-to-diameter ratio of 3:1). The lower end of the Y-shaped structure is equipped with intermittent helical blades, and the spaced helical blades are positioned vertically and axially in the same position.

[0047] Gel time: 2 min.

[0048] Step 2: Low-temperature rapid cooling and drying to obtain porous silica powder:

[0049] The obtained gel was frozen at -50°C to obtain wet porous silica powder; then the wet porous silica powder was dried at 600°C to obtain porous silica powder.

[0050] Step 3: Chlorination for impurity removal:

[0051] Porous silica powder is passed into a chlorination furnace for chlorination to remove impurities.

[0052] Chlorination temperature: 1000℃;

[0053] Chlorine gas: A mixture of hydrogen chloride and nitrogen gas, wherein the flow rate of hydrogen chloride is 0.2 m³ / h and the flow rate of nitrogen is 0.4 m³ / h (chloride to nitrogen flow rate ratio 1:2).

[0054] Characteristics of porous powder: The bulk density of silica powder at this time is 0.5 g / cm³, the average pore size is 12.1 nm, and the particle size distribution ranges from 23 to 478 μm.

[0055] Step 4: Screening and grading:

[0056] The silica powder after chlorination and impurity removal was sieved according to particle size to obtain nine grades, specifically: 478-400μm, 400-350μm, 350-300μm, 300-250μm, 250-200μm, 200-150μm, 150-100μm, 100-50μm, and 50-23μm.

[0057] Step 5: Graded roasting:

[0058] The nine grades of silica powder were placed into a vacuum sintering furnace and calcined under vacuum conditions.

[0059] Firing temperature: 1330℃;

[0060] Calcination time: The corresponding times for each grade are as follows: 478-400μm particle size 10h, 400-350μm particle size 9h, 350-300μm particle size 8h, 300-250μm particle size 7h, 250-200μm particle size 6h, 200-150μm particle size 5h, 150-100μm particle size 4h, 100-50μm particle size 3h, and 50-23μm particle size 2h. After calcination, high-purity and high-density synthetic quartz sand is obtained.

[0061] Laser particle size distribution analysis was performed on the porous silica obtained in Example 1 of this invention. The results are as follows: Figure 1 As shown, the particle size distribution approximates a normal distribution, indicating good gel uniformity. The pore size distribution of the synthesized sand obtained after BET testing is shown below. Figure 2 As shown, the pore size distribution is relatively concentrated, with a very small number of large pore structures and an average pore size of 5.1 nm, indicating that the sintering process achieves effective shrinkage.

[0062] The true density of the synthetic sand obtained in Example 1 of this invention was determined by the immersion method to be 2.2 g / cm³, and the total amount of metal impurities was 0.84 ppm (the content of each impurity is detailed in Table 1).

[0063] Referring to Table 2, the synthetic sand obtained in Example 1 of this invention was used to prepare quartz glass through plasma melting process. In 100g of glass, there were 2 bubbles with a diameter of 0.03-0.3mm, and 0 bubbles with a diameter of 0.31-0.7mm and >0.7mm, which meets the requirements of Class 2 in JC / T185-2023 "Optical Quartz Glass".

[0064] Table 1: Metal Impurity Content of Synthetic Sand in Example 1

[0065]

[0066] Table 2: Bubble characteristics of the quartz glass obtained in Example 1:

[0067]

[0068] Example 2:

[0069] The sintering method for high-density synthetic sand in Embodiment 2 of this invention has the same specific steps as in Embodiment 1, with the following differences in configuration:

[0070] The alkaline catalyst is diluted 5 times with electronic-grade ammonia.

[0071] The flow rate of the silicic acid aqueous solution is 300 g / s, and the flow rate of the alkaline catalyst is 10 g / s, with a flow ratio of 100:3.3.

[0072] The static pipe mixer has a diameter of 30mm and a length of 100mm (length-to-diameter ratio of approximately 3.3:1).

[0073] The gelation time was 1 minute.

[0074] The gel is rapidly cooled at -30℃ and dried at 600℃.

[0075] The chlorination purification temperature is 1100℃.

[0076] The chlorination gas is pure hydrogen chloride gas with a flow rate of 0.5 m³ / h;

[0077] Characteristics of porous powder: The bulk density of silica powder at this time is 0.63 g / cm³, the average pore size is 15.6 nm, and the particle size distribution ranges from 20 to 573 μm.

[0078] The nine grades of silica powder are: 573-400μm, 400-350μm, 350-300μm, 300-250μm, 250-200μm, 200-150μm, 150-100μm, 100-50μm, and 50-20μm.

[0079] Vacuum calcination temperature: 1300℃.

[0080] The calcination times for each grade are as follows: 573-400μm grade 10h, 400-350μm grade 9h, 350-300μm grade 8h, 300-250μm grade 7h, 250-200μm grade 6h, 200-150μm grade 5h, 150-100μm grade 4h, 100-50μm grade 3h, and 50-20μm grade 2h.

[0081] Laser particle size distribution analysis was performed on the porous silica obtained in Example 2 of this invention. The results are as follows: Figure 3 As shown, the particle size distribution approximates a normal distribution, indicating good gel uniformity. The pore size distribution of the synthesized sand obtained after BET testing is shown below. Figure 4 As shown, the pore size distribution is relatively concentrated, with a very small number of large pore structures and an average pore size of 6.2 nm, indicating that the sintering process achieves effective shrinkage.

[0082] The true density of the synthetic sand obtained in Example 1 of this invention was determined by the immersion method to be 2.19 g / cm³, and the total amount of metal impurities was 0.47 ppm (the content of each impurity is detailed in Table 3).

[0083] As shown in Table 4, when the synthetic sand obtained in Example 2 is used to prepare quartz glass through plasma melting, there are 3 bubbles with a diameter of 0.03-0.3 mm in 100g of glass, and 0 bubbles with a diameter of 0.31-0.7 mm and >0.7 mm. This meets the Class 2 requirements in JC / T185-2023 "Optical Quartz Glass".

[0084] Table 3: Metal Impurity Content of Quartz Sand Obtained in Example 2

[0085]

[0086] Table 4: Bubble Characteristics of Quartz Glass Obtained in Example 2

[0087]

[0088] Example 3:

[0089] The sintering method for high-density synthetic sand in Embodiment 2 of this invention has the same specific steps as in Embodiment 1, but with the following different configurations:

[0090] Alkaline catalyst: dilute electronic grade ammonia water 10 times.

[0091] The flow rate of the silicic acid aqueous solution is 100 g / s, and the flow rate of the alkaline catalyst is 1 g / s, with a flow ratio of 100:1.

[0092] The static pipe mixer has a diameter of 20mm and a length of 50mm (length-to-diameter ratio 2.5:1).

[0093] The gelation time was 20 minutes.

[0094] The gel is rapidly cooled at -50°C and dried at 600°C.

[0095] The chlorination purification temperature is 900℃.

[0096] The chlorination gas is a mixture of chlorine and nitrogen, with a chlorine flow rate of 0.16 m³ / h and a nitrogen flow rate of 0.8 m³ / h (chloride to nitrogen flow rate ratio 1:5).

[0097] Characteristics of porous powder: The bulk density of silica powder at this time is 0.41 g / cm³, the average pore size is 9.3 nm, and the particle size distribution ranges from 20 to 523 μm.

[0098] The nine grades of silica powder are: 523-400μm, 400-350μm, 350-300μm, 300-250μm, 250-200μm, 200-150μm, 150-100μm, 100-50μm, and 50-20μm.

[0099] The roasting temperature is 1330℃.

[0100] The corresponding roasting times for each grade are as follows: 523-400μm grade 10h, 400-350μm grade 9h, 350-300μm grade 8h, 300-250μm grade 7h, 250-200μm grade 6h, 200-150μm grade 5h, 150-100μm grade 4h, 100-50μm grade 3h, and 50-20μm grade 2h.

[0101] Laser particle size distribution analysis was performed on the porous silica obtained in Example 3 of this invention. The results are as follows: Figure 5 As shown, the particle size distribution approximates a normal distribution, indicating good gel uniformity. The pore size distribution of the synthesized sand obtained after BET testing is shown below. Figure 6 As shown, the pore size distribution is relatively concentrated, with a very small number of large pore structures and an average pore size of 4.4 nm, indicating that the sintering process achieves effective shrinkage.

[0102] The true density of the synthetic sand obtained in Example 1 of this invention was determined by the immersion method to be 2.22 g / cm³, and the total amount of metallic impurities was 1.41 ppm (the content of each impurity is detailed in Table 5).

[0103] As shown in Table 6, when the synthetic sand obtained in Example 3 is used to prepare quartz glass through plasma melting, there is one bubble with a diameter of 0.03-0.3 mm in 100g of glass, and no bubbles with a diameter of 0.31-0.7 mm or >0.7 mm. This meets the Class 2 requirements in JC / T185-2023 "Optical Quartz Glass".

[0104] Table 5: Metal Impurity Content of Quartz Sand Obtained in Example 3

[0105]

[0106] Table 6: Bubble Characteristics of Quartz Glass Obtained in Example 3

[0107]

[0108] Example 4:

[0109] The sintering method for high-density synthetic sand in Embodiment 4 of this invention has the same specific steps as in Embodiment 1, with the following differences in configuration:

[0110] The alkaline catalyst is prepared by diluting electronic-grade ammonia water 10 times.

[0111] The flow rate of the silicic acid aqueous solution is 400 g / s, and the flow rate of the alkaline catalyst is 40 g / s, with a flow ratio of 100:10.

[0112] The static pipe mixer has a diameter of 30mm and a length of 150mm (length-to-diameter ratio 5:1).

[0113] The gelation time was 1 minute.

[0114] The gel is rapidly cooled at -40℃ and dried at 600℃.

[0115] Chlorination temperature: 800℃.

[0116] The chlorine gas is pure chlorine gas, and the flow rate is 0.5 m³ / h.

[0117] Characteristics of porous powder: The bulk density of silica powder at this time is 0.77 g / cm³, the average pore size is 17.8 nm, and the particle size distribution range is 20-589 μm.

[0118] The nine grades of silica powder are: 589-400μm, 400-350μm, 350-300μm, 300-250μm, 250-200μm, 200-150μm, 150-100μm, 100-50μm, and 50-20μm.

[0119] The vacuum calcination temperature is 1200℃;

[0120] The corresponding roasting times for each grade are as follows: 589-400μm grade 10h, 400-350μm grade 9h, 350-300μm grade 8h, 300-250μm grade 7h, 250-200μm grade 6h, 200-150μm grade 5h, 150-100μm grade 4h, 100-50μm grade 3h, and 50-20μm grade 2h.

[0121] Laser particle size distribution analysis was performed on the porous silica obtained in Example 4 of this invention. The results are as follows: Figure 7 As shown, the particle size distribution approximates a normal distribution, indicating good gel uniformity. The pore size distribution of the synthesized sand obtained after BET testing is shown below. Figure 8 As shown, the pore size distribution is relatively concentrated, there is no large pore structure, and the average pore size is 7.2 nm, indicating that the sintering process achieves effective shrinkage.

[0122] The true density of the synthetic sand obtained in Example 1 of this invention was determined by the immersion method to be 2.16 g / cm³, and the total amount of metallic impurities was 0.92 ppm (the content of each impurity is detailed in Table 7).

[0123] As shown in Table 8, when the synthetic sand obtained in Example 4 was used to prepare quartz glass through plasma melting, there were 5 bubbles with a diameter of 0.03-0.3 mm in 100g of glass, and 0 bubbles with a diameter of 0.31-0.7 mm and >0.7 mm. This meets the Class 2 requirements in JC / T185-2023 "Optical Quartz Glass".

[0124] Table 7: Metal Impurity Content of Quartz Sand Obtained in Example 4

[0125]

[0126] Table 8: Bubble Characteristics of Quartz Glass Obtained in Example 4

[0127]

[0128] The working principle of the above-mentioned multiple embodiments of the present invention through the synergistic steps of the entire process of "pretreatment-gelation-setting-impurity removal-grading-densification" is as follows:

[0129] 1. Ion exchange pretreatment: laying the foundation for high purity. Using water glass as a precursor, metal ions are removed through ion exchange, initially reducing the impurity content and obtaining a high-purity silicate aqueous solution. This eliminates initial impurity interference for the subsequent preparation of high-purity synthetic sand and avoids the accumulation of metal ions in subsequent stages.

[0130] 2. Pipeline Mixing Gel: This method addresses the issues of uniformity and efficiency by simultaneously introducing a silica acid aqueous solution and diluted electronic-grade ammonia (alkaline catalyst) into a static pipeline mixer at a set flow ratio. The internal structure of this mixer allows the two materials to undergo sufficient shearing and contact during the flow process, avoiding the problems of "localized overly rapid reaction and localized insufficient reaction" in traditional mixing methods, thus achieving rapid and uniform gelation.

[0131] 3. Freeze-drying: To preserve the porous structure and prepare for impurity removal, the uniform gel is rapidly cooled at low temperature to quickly solidify the gel structure and avoid uneven gel shrinkage caused by slow moisture loss. The powder is then dried to obtain porous silica powder, ensuring that the powder maintains a mesoporous structure (bulk density <1g / cm³, particle size <600μm) - the loose porous structure can increase the contact area of ​​the subsequent chlorination impurity removal reaction.

[0132] 4. Chlorination for Impurity Removal: Deeply remove residual impurities by passing porous silica powder into a chlorination furnace and introducing chlorination gas (hydrogen chloride, chlorine, or a mixture of hydrogen chloride and nitrogen). Utilizing the porous amorphous structure of the powder, the chlorination gas can fully penetrate into the powder and react with residual metal impurities to generate volatile metal chlorides, which are then discharged with the gas. If a mixed gas is used, the concentration of chlorination gas is adjusted by controlling the ratio of chloride to nitrogen flow rate to avoid excessively high concentrations that could damage the powder structure or excessively low concentrations that would result in incomplete impurity removal.

[0133] 5. Sieving and Classification: To adapt to different densification requirements, the chlorinated powder is sieved according to particle size (>400μm to <50μm, with each 50μm grade as one grade): Different particle sizes require different amounts of energy to shrink and densify. The larger the particle size, the more and deeper the internal pores, requiring more energy to allow the pores to shrink fully. If sintering is carried out directly without grading, problems such as "insufficient densification of large particles and excessive sintering and adhesion of small particles" are likely to occur. After grading, sintering parameters can be designed specifically.

[0134] 6. Vacuum graded calcination: To achieve uniform densification, powders of different grades are placed separately in a vacuum sintering furnace. The calcination temperature is controlled and matched with different calcination times (the larger the particle size, the longer the time, such as >400μm requires 10-10.5h, <50μm requires 2-2.5h). At the same time, the vacuum environment can avoid contamination by external impurities and reduce residual gas during the sintering process. By matching "temperature + time", sufficient but not excessive energy is provided to each grade of powder to ensure that the internal pores of all powders shrink uniformly, and finally a high-density synthetic sand is formed.

[0135] The above description illustrates the basic principles of the present invention. The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. The above embodiments and descriptions in the specification are only for illustrating the principles of the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and scope of the present invention without departing from the scope of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing high-purity, high-density synthetic sand, characterized in that... Includes the following steps: Step (1): A silicic acid aqueous solution and an alkaline catalyst are simultaneously introduced into a static pipeline mixer at a flow ratio of 100:(1-10) to rapidly form a gel; the alkaline catalyst is electronic-grade ammonia diluted 5-10 times, and the flow rate of the silicic acid aqueous solution is 100-500 g / s; the static pipeline mixer has a Y-shaped structure, with intermittent spiral blades inside the lower end of the Y-shaped structure, the axial vertical positions of the spaced spiral blades are the same, and the diameter of the lower end of the Y-shaped structure is 20-30 mm, with a length-to-diameter ratio of (2.5-5):1; Step (2): The gel obtained in step (1) is rapidly cooled at low temperature to obtain wet porous silica powder and then dried; Step (3): Pass the porous silica powder obtained in step (2) into a chlorination furnace for chlorination to remove impurities; the chlorination temperature is 800-1100℃, and the chlorination gas is selected from one of hydrogen chloride, chlorine, a mixture of hydrogen chloride and nitrogen, or a mixture of chlorine and nitrogen. The total flow rate of a single chlorination gas is 0.16-0.5 m³ / h, and the flow ratio of chloride to nitrogen in the mixed chlorination gas is 1:(1-5). Step (4): The silica powder obtained in step (3) is sieved and classified into nine grades: >400μm, 400-350μm, 350-300μm, 300-250μm, 250-200μm, 200-150μm, 150-100μm, 100-50μm, and <50μm. Step (5): Place the nine grades of silica powder obtained in step (4) into a vacuum sintering furnace and calcine them under vacuum. The calcine temperature is 1200-1350℃. The calcine time for each grade is as follows: >400μm grade 10-10.5h, 400-350μm grade 9-9.5h, 350-300μm grade 8-8.5h, 300-250μm grade 7-7.5h, 250-200μm grade 6-6.5h, 200-150μm grade 5-5.5h, 150-100μm grade 4-4.4h, 100-50μm grade 3-3.5h, <50μm grade 2-2.5h. After calcine, high-purity and high-density synthetic sand is obtained.

2. The method for preparing high-purity, high-density synthetic sand according to claim 1, characterized in that: In step (1), the silica aqueous solution is obtained by removing metal ions through ion exchange using water glass as a precursor; the gelation time is 1-20 min.

3. The method for preparing high-purity, high-density synthetic sand according to claim 1, characterized in that: In step (2), the gel is rapidly cooled at a low temperature of -50℃ to -30℃ and dried at 600℃.

4. The method for preparing high-purity, high-density synthetic sand according to claim 1, characterized in that: The porous silica mentioned in step (3) has a particle size of <600μm, internal pores are mesoporous, and the packing density should be <1g / cm³.

5. The method for preparing high-purity, high-density synthetic sand according to claim 1, characterized in that: The temperature of the low-temperature rapid cooling in step (2) is -50℃, -40℃ or -30℃; the calcination temperature in step (5) is 1300-1350℃, and the calcination time of each grade of silica powder is: >400μm grade 10h, 400-350μm grade 9h, 350-300μm grade 8h, 300-250μm grade 7h, 250-200μm grade 6h, 200-150μm grade 5h, 150-100μm grade 4h, 100-50μm grade 3h, <50μm grade 2h.

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