Roasting method for reducing hydroxyl content of quartz sand and controlling content fluctuation

Abstract

The invention belongs to the technical field of quartz sand, and discloses a roasting method for reducing the hydroxyl content of quartz sand and controlling content fluctuation, which comprises the following steps: 1) drying silicon dioxide gel prepared by a sol-gel method into powder, putting the powder into a roasting furnace, and heating to a preheating temperature through electric heating for preheating; (2) microwave heating is started after pre-vacuumizing, a high-concentration oxygen-containing atmosphere, a low-concentration oxygen-containing atmosphere and a vacuum environment are sequentially switched and circulated, and different atmospheres are matched with different microwave powers until the roasting temperature is reached; and 3) carrying out microwave heat preservation in an inert atmosphere, and then recovering to normal pressure and cooling to obtain the finished product quartz sand. The quartz sand obtained through roasting is free of cracks, the hydroxyl content is smaller than or equal to 50 ppm, and the fluctuation range of the hydroxyl content is within + / -5 ppm.

Description

Technical Field

[0001] This invention belongs to the field of quartz sand technology, specifically relating to a calcination method for reducing the hydroxyl content of quartz sand and controlling its fluctuations. Background Technology

[0002] The sol-gel method is a conventional method for synthesizing silica sand. However, due to inherent process limitations, incomplete hydrolysis of the precursor and condensation reactions during synthesis can lead to a large amount of residual hydroxyl groups. The presence of these residual hydroxyl groups not only significantly affects the physical, chemical, and optical properties of the silica sand itself, but also causes considerable problems in subsequent applications, such as reduced glass thermal stability and infrared transmittance after sintering, greatly impacting the quality and application range of the silica sand.

[0003] Generally, subsequent drying and heat treatment processes can remove some hydroxyl groups. High temperatures can drive the condensation reaction and diffusion of hydroxyl groups, thereby reducing or completely removing residual hydroxyl groups in quartz sand. However, existing traditional calcination methods often rely on heat conduction for heating, inevitably resulting in a temperature difference between the bulk phases—a temperature gradient with high surface temperature and low core temperature. Furthermore, due to limitations in the heating rate, the internal and external temperatures of the quartz sand must remain consistent over a certain period. Therefore, traditionally calcined quartz sand often exhibits high hydroxyl content with significant fluctuations. With the increasing demand for synthetic quartz sand, there is an urgent need in the market for a new method and process to solve the problem of residual hydroxyl groups. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to address the shortcomings of the existing technology by providing a calcination method for reducing the hydroxyl content of quartz sand and controlling its fluctuation. The calcination method yields quartz sand with a hydroxyl content ≤50ppm and good uniformity, with a content fluctuation range within ±5ppm.

[0005] To address the technical problem proposed in this invention, this invention provides a calcination method for reducing the hydroxyl content of quartz sand and controlling its fluctuations, comprising the following steps:

[0006] 1) The silica gel prepared by the sol-gel method was dried into powder and placed in a calcination furnace, and preheated to the preheating temperature by electric heating;

[0007] 2) After pre-vacuuming, microwave heating begins, switching and cycling through high-concentration oxygen atmosphere, low-concentration oxygen atmosphere, and vacuum environment in sequence, with different microwave powers for different atmospheres, until the calcination temperature is reached.

[0008] 3) Microwave heat preservation is carried out under an inert atmosphere, then restored to normal pressure and cooled to obtain finished quartz sand.

[0009] In the above scheme, the roasting furnace is a rotary microwave roasting furnace. The microwave frequency of the roasting furnace is 2.45GHz±25MHz, and the microwave power is adjustable in the range of 1 to 50kW. The roasting furnace rotates throughout the entire process, with a rotation speed of 10 to 50 rpm and a rotation tilt angle of 0 to 30°.

[0010] In the above scheme, the heating rate of the electric heating is 100-400℃ / h.

[0011] In the above scheme, the preheating temperature is 500-700℃ and the preheating time is 0.5-4h.

[0012] In the above scheme, the pre-evacuation to a vacuum level of 0.1–20 Pa, preferably 5–10 Pa. In this invention, vacuum level = atmospheric pressure - absolute pressure.

[0013] In the above scheme, the volume fraction of oxygen in the high-concentration oxygen-containing atmosphere is 40-80%, preferably 60-80%, and the remainder is inert gas.

[0014] In the above scheme, the flow rate of the high-concentration oxygen-containing atmosphere is 1-20 L / min, and the pressure inside the furnace after introduction is 0.2-0.8 MPa, preferably 0.3-0.6 MPa.

[0015] In the above scheme, the high-concentration oxygen-containing atmosphere is combined with high-power microwave heating, with a power of 20-50kW, preferably 30-50kW.

[0016] In the above scheme, the single maintenance time of the high-concentration oxygen-containing atmosphere is 1 to 30 minutes, preferably 10 to 15 minutes.

[0017] In the above scheme, the volume fraction of oxygen in the low-concentration oxygen-containing atmosphere is 1-10%, preferably 1-6%, and the remainder is inert gas.

[0018] In the above scheme, the flow rate of the low-concentration oxygen-containing atmosphere is 1-20 L / min, and the pressure inside the furnace after introduction is 0.2-0.8 MPa, preferably 0.3-0.6 MPa.

[0019] In the above scheme, the low-concentration oxygen-containing atmosphere is combined with medium-power microwave heating, and the microwave power is 5 to 20 kW (excluding 20 kW), preferably 10 to 20 kW (excluding 20 kW).

[0020] In the above scheme, the single maintenance time of the low-concentration oxygen-containing atmosphere is 1 to 30 minutes, preferably 10 to 15 minutes.

[0021] In the above scheme, the vacuum degree of the vacuum environment is 0.1 to 20 Pa, preferably 1 to 10 Pa.

[0022] In the above scheme, the vacuum environment is combined with low-power microwave heating, with a power of 0.1 to 5 kW (excluding 5 kW), preferably 0.1 to 3 kW. The purpose of using low power in this stage is microwave heat preservation, and this stage does not involve temperature increase.

[0023] In the above scheme, the single maintenance time of the vacuum environment is 1 to 30 minutes, preferably 10 to 15 minutes.

[0024] In the above scheme, the sintering temperature is 1100-1400℃. After reaching the sintering temperature, step 2) ends immediately, and step 3) begins.

[0025] In the above scheme, the inert gas and inert atmosphere are one or more of Ar, He, and N2.

[0026] In the above scheme, the inert atmosphere is introduced at a flow rate of 1-20 L / min, and the pressure inside the furnace after introduction is 0.2-0.8 MPa.

[0027] In the above scheme, in step 3), the power of microwave heat preservation is 0.1 to 5 kW, and the heat preservation time is 4 to 10 hours.

[0028] In the above scheme, the cooling rate is ≤10℃ / min.

[0029] The sintered quartz sand obtained by this invention is free of cracks, has a hydroxyl content of ≤50ppm and good uniformity with a content fluctuation range within ±5ppm.

[0030] The main technical concept of this invention is as follows:

[0031] To address the issues of high hydroxyl content and large fluctuations in hydroxyl content frequently encountered during the synthesis of quartz sand, previous researchers have often explored solutions by using different gas atmospheres or setting temperature gradients. However, the applicant's research has revealed that the root cause lies in problems such as uneven heating distribution and excessive temperature gradients in the traditional calcination method, necessitating a heating method more suitable for quartz sand calcination. Microwave heating is a technology that uses microwave energy to directly heat the interior of materials, perfectly solving the problems of uneven temperature distribution and excessive temperature gradients in traditional heating processes. However, because the dielectric loss of quartz sand is directly proportional to temperature, the dielectric loss of quartz sand is low at low temperatures, resulting in a weak microwave thermal effect and low microwave heating efficiency. Furthermore, different wavelengths significantly affect the penetration power of microwaves into the interior of quartz sand. These technical shortcomings of microwave heating greatly limit its application in quartz sand calcination.

[0032] This invention addresses the issue of low dielectric loss in quartz sand under low-temperature conditions during microwave heating. In a dynamic frequency-modulated composite calcination process, conventional electric heating is first employed to activate and improve the dielectric loss of the quartz sand. Then, microwave heating is used to concentrate microwave energy within the quartz sand, accelerating the pyrolysis of organic matter (such as residual silanes or solvents in synthetic raw materials) and rapidly removing impurities. Simultaneously, by adjusting different microwave powers and a stepped oxygen atmosphere, a high-power microwave electric field causes dipole orientation polarization of the polar Si-OH bonds, weakening bond energy and promoting the migration of numerous hydroxyl groups to the surface. Combined with a high-concentration oxygen atmosphere, this accelerates the hydroxyl removal rate. A medium-power microwave electric field matches the dielectric loss of the quartz sand, achieving uniform bulk heating. Combined with a low-concentration oxygen atmosphere, this avoids excessive silicon free radical generation under prolonged high-oxygen conditions, preventing lattice defects. Low-power microwaves prevent overheating that could lead to SiO2 network reconstruction and block hydroxyl escape channels. Combined with a vacuum environment, this accelerates the diffusion and removal of desorption products, preventing re-adsorption. Precise control of microwave power and oxygen atmosphere can ensure uniform heating of polar molecules (water, hydroxyl groups) within quartz sand of different particle sizes, efficiently stimulating hydroxyl molecular vibrations and promoting the rapid escape of residual hydroxyl groups from both inside and outside the quartz sand as water molecules. This reduces the hydroxyl content within the quartz sand while maintaining good hydroxyl content uniformity. Since microwave energy directly acts on the quartz sand lattice, it can also promote the transformation of amorphous SiO2 into α-quartz and the transformation of α-quartz into β-quartz, ensuring uniform release of thermal stress and reducing hot cracking. Simultaneously, using a rotary kiln not only ensures continuous production but also guarantees sufficient contact between the quartz sand and oxygen during rotation, allowing for more uniform dehydroxylation. Therefore, this invention can simultaneously solve problems such as hot cracking, high hydroxyl content, and poor uniformity that occur during quartz sand kiln firing.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention addresses the problem of high and fluctuating hydroxyl content in synthetic quartz. It first enhances the dielectric loss of the quartz sand through preheating with electric heating, increasing the microwave thermal effect, followed by microwave heating. By dynamically adjusting the microwave power, high-temperature uniform sintering is achieved, promoting lattice transformation and reducing hot crack formation. During this process, alternating treatment with an oxygen-containing atmosphere and vacuum ensures sufficient powder shrinkage and sintering, accelerating the rapid escape of internal hydroxyl molecules as water molecules, thus reducing the hydroxyl content. The resulting quartz sand is crack-free, with a hydroxyl content ≤50ppm and a fluctuation range within ±5ppm. Furthermore, this method is more energy-efficient than traditional direct high-temperature calcination, demonstrating promising industrial application prospects. Attached Figure Description

[0034] Figure 1The graph shows the relationship between the hydroxyl content and the number of samplings for each example and comparative test. Detailed Implementation

[0035] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0036] In the following examples, the silica gel used was prepared by the sol-gel method via the hydrolysis of tetraethoxysilane. The specific steps are as follows:

[0037] 1) Add tetraethoxysilane and 7 times its equivalent of high-purity water to a jacketed vertical stirred tank;

[0038] 2) Then, 50°C warm water was introduced into the jacket, and the hydrolysis reaction was carried out for 1 hour under the condition of stirring speed of 50 ppm to obtain a uniform milky white sol.

[0039] 3) After stopping stirring, let stand for 30 minutes, then heat with a jacket at 150°C to remove methanol and water until a white gel is obtained. Then let it cool and age naturally to obtain silica gel.

[0040] In the following embodiments, the roasting furnace used is a rotary 2.45GHz industrial microwave roasting furnace with an adjustable microwave frequency in the range of 0.1 to 50kW. It contains a rotating tray that rotates throughout the production process at a speed of 10 to 50 rpm and a tilt angle of 0 to 30°. It is also equipped with temperature measuring instruments such as an infrared thermometer to ensure that the temperature control accuracy is within ±5℃.

[0041] Example 1

[0042] A calcination method for reducing the hydroxyl content of quartz sand and controlling its fluctuations includes the following steps:

[0043] 1) The silica gel prepared by the sol-gel method was dried to obtain silica powder with a particle size of 50-1000 μm. The powder was put into a quartz glass crucible and then placed in a calcining furnace. The temperature was raised to 600℃ at a rate of 300℃ / h by electric heating and held for 0.5h. The calcining furnace was rotated at a speed of 10 rpm and an inclination angle of 10° throughout the process until the quartz sand was finally calcined.

[0044] 2) After pre-evacuating to a vacuum level of 2 Pa, microwave heating is first performed at a high output power, while a high-concentration oxygen-containing atmosphere is introduced into the furnace at a rate of 5 L / min to a pressure of 0.3 MPa, and maintained for 10 min. Then, microwave heating is performed at a medium output power, while a low-concentration oxygen-containing atmosphere is introduced into the furnace at a rate of 5 L / min to a pressure of 0.3 MPa, and maintained for 10 min. Next, microwave heat preservation is performed at a low output power, while vacuuming is performed to a vacuum level of 2 Pa, and maintained for 10 min. Subsequently, the switching and cycling are performed in the order of high-concentration oxygen-containing atmosphere + high-power microwave heating, low-concentration oxygen-containing atmosphere + medium-power microwave heating, and vacuum environment + low-power microwave heat preservation, until the roasting furnace reaches 1200℃.

[0045] Among them, the power of high-power microwave heating is 40kW, the power of medium-power microwave heating is 10kW, and the power of low-power microwave heat preservation is 1kW; the volume fraction of oxygen in the high-concentration oxygen-containing atmosphere is 60%, and the remainder is N2; the volume fraction of oxygen in the low-concentration oxygen-containing atmosphere is 5%, and the remainder is N2.

[0046] 3) Charge N2 into the furnace at a rate of 5L / min until the pressure reaches 0.4MPa, keep warm with low-power microwave for 6h, then stop the microwave and restore to normal pressure, cool to room temperature at a rate of 10℃ / min to obtain the finished quartz sand.

[0047] The finished quartz sand obtained in this embodiment is visually free of cracks. Tested using the liquid bath method, the hydroxyl content of the quartz sand prepared in this embodiment is 40 ppm. Hydroxyl content testing at different locations showed good uniformity, with fluctuations within ±2 ppm. The bulk density is 1.33 g / cm³. 3 .

[0048] Comparative Example 1

[0049] The only difference between Comparative Example 1 and Example 1 is that:

[0050] In step 2), during the switching and cycling of atmosphere and microwave frequency, the operation of low-concentration oxygen atmosphere + medium-power microwave heating is not performed. Instead, the switching and cycling are performed in the order of high-concentration oxygen atmosphere + high-power microwave heating and vacuum environment + low-power microwave heat preservation.

[0051] The finished quartz sand obtained in this comparative example showed visible cracks. Through liquid bath testing, the hydroxyl content of the quartz sand prepared in this example was 130 ppm. Hydroxyl content testing at different locations showed only moderate uniformity, with a fluctuation range of ±20 ppm. The bulk density was 1.22 g / cm³. 3 .

[0052] Comparative Example 2

[0053] The only difference between Comparative Example 2 and Example 1 is that:

[0054] In step 2), electric heating is still used, but the high-power microwave heating is replaced with electric heating at a rate of 400℃ / h, the medium-power microwave heating is replaced with electric heating at a rate of 200℃ / h, and the low-power microwave heat preservation is replaced with electric heating heat preservation.

[0055] The finished quartz sand obtained in this comparative example showed visible cracks. Through liquid bath testing, the hydroxyl content of the quartz sand prepared in this example was 140 ppm. Hydroxyl content testing at different locations showed only moderate uniformity, with a fluctuation range of ±25 ppm. The bulk density was 1.1 g / cm³. 3 .

[0056] Comparative Example 3

[0057] The only difference between Comparative Example 3 and Example 1 is that:

[0058] In step 2), no oxygen-containing atmosphere is introduced, and the vacuum degree is controlled at 5 Pa throughout the process. The process is switched and cycled in the order of high-power microwave heating, medium-power microwave heating, and low-power microwave heat preservation.

[0059] The finished quartz sand obtained in this comparative example showed visible cracks. Through liquid bath testing, the hydroxyl content of the quartz sand prepared in this example was 130 ppm. Hydroxyl content testing at different locations showed only moderate uniformity, with a fluctuation range of ±20 ppm. The bulk density was 1.15 g / cm³. 3 .

[0060] Comparative Example 4

[0061] The only difference between Comparative Example 4 and Example 1 is that:

[0062] The rotating tray of the roasting oven does not rotate and remains stationary throughout the production process.

[0063] The finished quartz sand obtained in this comparative example showed visible cracks. Through liquid bath testing, the hydroxyl content of the quartz sand prepared in this example was 120 ppm. Hydroxyl content testing at different locations showed only moderate uniformity, with a fluctuation range of ±15 ppm. The bulk density was 1.25 g / cm³. 3 .

[0064] Example 2

[0065] A calcination method for improving internal cracks and reducing hydroxyl content in quartz sand includes the following steps:

[0066] 1) The silica gel prepared by the sol-gel method was dried to obtain silica powder with a particle size of 50-1000 μm. The powder was put into a quartz glass crucible and then placed in a calcining furnace. The temperature was raised to 500℃ at a rate of 200℃ / h by electric heating and held for 0.5h. The calcining furnace was rotated at a rotation speed of 20 rpm and a tilt angle of 20° throughout the process until the quartz sand was finally calcined.

[0067] 2) After pre-evacuating to a vacuum level of 10 Pa, microwave heating is first performed at a high output power, while a high-concentration oxygen-containing atmosphere is introduced into the furnace at a rate of 10 L / min to a pressure of 0.5 MPa, and maintained for 15 min. Then, microwave heating is performed at a medium output power, while a low-concentration oxygen-containing atmosphere is introduced into the furnace at a rate of 10 L / min to a pressure of 0.5 MPa, and maintained for 15 min. Next, microwave heat preservation is performed at a low output power, while vacuuming is performed to a vacuum level of 10 Pa, and maintained for 15 min. Subsequently, the switching and cycling are performed in the order of high-concentration oxygen-containing atmosphere + high-power microwave heating, low-concentration oxygen-containing atmosphere + medium-power microwave heating, and vacuum environment + low-power microwave heat preservation, until the roasting furnace reaches 1300℃.

[0068] Among them, the power of high-power microwave heating is 30kW, the power of medium-power microwave heating is 15kW, and the power of low-power microwave heat preservation is 2kW; the volume fraction of oxygen in the high-concentration oxygen-containing atmosphere is 70%, and the remainder is N2; the volume fraction of oxygen in the low-concentration oxygen-containing atmosphere is 2%, and the remainder is N2.

[0069] 3) Charge N2 into the furnace at a rate of 10L / min until the pressure reaches 0.6MPa, keep warm with low-power microwave for 8 hours, then stop the microwave and restore to normal pressure, cool to room temperature at a rate of 5℃ / min to obtain the finished quartz sand.

[0070] The finished quartz sand obtained in this embodiment is visually free of cracks. Tested using the liquid bath method, the hydroxyl content of the quartz sand prepared in this embodiment is 50 ppm. Hydroxyl content testing at different locations showed good uniformity, with fluctuations within ±2 ppm. The bulk density is 1.30 g / cm³. 3 .

[0071] The above embodiments are merely examples for clear illustration and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations, and any obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A calcination method for reducing the hydroxyl content of quartz sand and controlling its fluctuations, characterized in that, The following steps are involved: 1) The silica gel prepared by the sol-gel method was dried into powder and placed in a calcination furnace, and preheated to the preheating temperature by electric heating; 2) After pre-vacuuming, microwave heating begins, switching and cycling through high-concentration oxygen atmosphere, low-concentration oxygen atmosphere, and vacuum environment in sequence, with different microwave powers for different atmospheres, until the calcination temperature is reached. 3) Microwave heat preservation is carried out under an inert atmosphere, then restored to normal pressure and cooled to obtain finished quartz sand.

2. The calcination method for reducing the hydroxyl content of quartz sand and controlling its fluctuation according to claim 1, characterized in that, The high-concentration oxygen atmosphere contains 40-80% oxygen by volume, with the remainder being inert gases; the low-concentration oxygen atmosphere contains 1-10% oxygen by volume, with the remainder being inert gases.

3. The calcination method for reducing the hydroxyl content of quartz sand and controlling its fluctuation according to claim 1, characterized in that, The high-concentration oxygen atmosphere contains 60-80% oxygen by volume, with the remainder being inert gases; the low-concentration oxygen atmosphere contains 1-6% oxygen by volume, with the remainder being inert gases.

4. The calcination method for reducing the hydroxyl content of quartz sand and controlling its fluctuation according to claim 1, characterized in that, The high-concentration oxygen-containing atmosphere is combined with high-power microwave heating, with a power of 20-50kW; the low-concentration oxygen-containing atmosphere is combined with medium-power microwave heating, with a microwave power of 5-20kW; and the vacuum environment is combined with low-power microwave heating, with a power of 0.1-5kW.

5. The calcination method for reducing the hydroxyl content of quartz sand and controlling its fluctuation according to claim 1, characterized in that, The high-concentration oxygen-containing atmosphere is combined with high-power microwave heating, with a power of 30-50kW; the low-concentration oxygen-containing atmosphere is combined with medium-power microwave heating, with a microwave power of 10-20kW; and the vacuum environment is combined with low-power microwave heating, with a power of 0.1-3kW.

6. The calcination method for reducing the hydroxyl content and controlling its fluctuation in quartz sand according to claim 1, characterized in that, The pressure of the high-concentration oxygen-containing atmosphere and the low-concentration oxygen-containing atmosphere is 0.2-0.8 MPa; the vacuum degree of the vacuum environment is 0.1-20 Pa; and the single maintenance time of the high-concentration oxygen-containing atmosphere, the low-concentration oxygen-containing atmosphere, and the vacuum environment is 1-30 min.

7. The calcination method for reducing the hydroxyl content of quartz sand and controlling its fluctuation according to claim 1, characterized in that, The sintering temperature is 1100–1400℃; in step 3), the pressure of the inert atmosphere is 0.2–0.8 MPa, the power of the microwave heat preservation is 0.1–5 kW, the heat preservation time is 4–10 h, and the cooling rate is ≤10℃ / min.

8. The calcination method for reducing the hydroxyl content of quartz sand and controlling its fluctuation according to claim 1, characterized in that, The heating rate of the electric heating is 100-400℃ / h; the preheating temperature is 500-700℃ and the preheating time is 0.5-4h; the pre-vacuuming is performed to a vacuum degree of 0.1-20Pa.

9. The calcination method for reducing the hydroxyl content of quartz sand and controlling its fluctuation according to claim 1, characterized in that, The roasting furnace is a rotary microwave roasting furnace with a microwave frequency of 2.45GHz±25MHz and an adjustable microwave power in the range of 1 to 50kW. The roasting furnace rotates throughout the entire process with a rotation speed of 10 to 50rpm and a rotation tilt angle of 0 to 30°.

10. A type of quartz sand obtained by calcination using the method described in claim 1, characterized in that, The hydroxyl content of the quartz sand is ≤50ppm and the content fluctuation range is within ±5ppm.

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