5N9-grade silicon dioxide micro powder with high purity and high specific surface area and preparation method of 5N9-grade silicon dioxide micro powder

Abstract

The invention discloses 5N9-grade silicon dioxide micro powder with high purity and high specific surface area and a preparation method of the 5N9-grade silicon dioxide micro powder. The method comprises the following steps: by taking rice hulls as a raw material, carrying out acid pickling pretreatment, oxygen-limited carbonization, glucose impregnation coating, boiling fluidized drying and rapid carbonization, and vacuum high-temperature dealkalization, and then carrying out carbon thermal reduction in an argon atmosphere at 1400-1550 DEG C to generate gaseous SiO; by accurately regulating and controlling the temperature (500-900 DEG C) and the carrier gas flow of a deposition area, homogeneous nucleation and rapid cooling of SiO are achieved, sintering and growth of particles are inhibited, and the nano silicon dioxide micro powder with the specific surface area being 60-650 m / g and the purity being larger than or equal to 99.999% (the total amount of impurities is smaller than 10 ppm) is obtained. The primary particle size of the product is 5-20 nm, the aggregate D50 can be controlled to be 0.2-2 microns, and the product is amorphous spherical particles. According to the invention, high-valued utilization of the rice husks is realized, the blank of products with both high purity and ultrahigh specific surface area is filled, and the requirements of high-end fields such as aerospace, semiconductor polishing, high-performance catalyst carriers and the like can be met.

Description

Technical Field

[0001] This invention belongs to the field of inorganic non-metallic high-tech material preparation technology, specifically involving a method for preparing high-purity silica micro powder with controllable specific surface area and purity ≥99.999% using rice husks. Background Technology

[0002] High-purity silica micropowder has wide applications in chip packaging, aerospace, medical carriers, optical fibers, photovoltaics, CMP, filling, chromatographic column packing, and lithium battery separator coatings. Among them, silica with a high specific surface area (≥300 m² / g) is highly favored due to its excellent adsorption properties, thixotropy, and dispersion stability. However, although traditional precipitation, combustion, and sol-gel methods can obtain products with high specific surface area, the purity is difficult to reach the 3N9 level, let alone the 5N9 level (total metal impurities <10ppm). Traditional fumed silica produced by gas phase can achieve a purity >99.95%, but it is expensive and the specific surface area is usually concentrated between 200-400 m² / g, making it difficult to consistently exceed 400 m² / g.

[0003] In wet and conventional gas-phase processes, a high specific surface area usually means small particles and a large surface area, which easily leads to the adsorption of impurities on the surface, making it difficult to maintain purity. This invention creates an impurity-free zone in the deposition area, successfully solving this problem.

[0004] This invention uses rice husks, an agricultural waste, as a silicon source. By controlling the supersaturation and nucleation rate during the vapor deposition process, it achieves 5N9-level deep purification. The specific surface area of ​​the product is stably and accurately controlled in any range between 60-650 m² / g, filling the technological gap of achieving both high purity and ultra-high specific surface area. Summary of the Invention

[0005] The purpose of this invention is to provide a 5N9 grade high-purity silica micropowder with high specific surface area and its preparation method.

[0006] The technical solution adopted in this invention is as follows:

[0007] A method for preparing high-purity silica micropowder with high specific surface area (5N9 grade) using rice husks is described below, including the specific steps, detailed control conditions, and principle:

[0008] (1) Pickling pretreatment: The rice husks are immersed in an acidic solution to remove some of the metal impurities; deionized water is used to remove the salts, rice husk impurities, and surface dust dissolved by the pickling, so that the texture of the rice husk structure can be clearly seen. The appearance after rinsing is shown in the attached figure. Figure 1 .

[0009] (2) Carbonization treatment: The acid-washed rice husks are partially carbonized in an oxygen-deficient atmosphere to obtain rice husk carbon with a carbon content of 18-22%. When observed under 10X magnification, the rice husk carbon formed at this time is clearly black and shiny with a bright silvery-white luster. (See attached image) Figure 2 This indicates that some silica particles with a diameter of 40-65 nm are exposed. Maintaining a carbon content of 18-22% is very important for the following reasons: A) It provides a good foundation for adding an appropriate amount of glucose as a carbon source and ensuring that glucose uniformly coats the silica. Experiments show that if the carbon content exceeds 25%, the glucose liquid has difficulty penetrating the carbon layer and uniformly coating it; B) It ensures that non-silicate alkali metals fully volatilize and completely overflow through the carbon skeleton; C) It provides a good gas channel during high-temperature reduction; D) It is conducive to forming a uniform reduction reaction temperature.

[0010] (3) The key step in preparing high-purity carbon is glucose solution impregnation: The rice husk carbon is impregnated in glucose solution, so that glucose is uniformly loaded into the pores of the rice husk ash. After uniform drying and carbonization, a uniform and dense carbon film is formed on the surface of silica, which is conducive to accurately controlling the silicon-carbon ratio and rapidly generating SiO. Because the silica in rice husk carbon exists as native particles with a particle size of 40-65nm, the carbon layer after carbonization is loose. Impregnation with glucose solution of appropriate concentration can fully contact and coat the nano-silica, and under the action of the loose carbon layer, achieve dense encapsulation and supplement the carbon source. Only when the silica and the carbon layer are densely encapsulated in contact can the carbon source be replenished. Only through contact can SiO(G) be generated at high temperatures. The carbon skeleton structure in rice husk carbon provides a step-by-step carbon source and forms channels for SiO(G) and CO(G), allowing the mixed gas to be smoothly discharged. Multiple experiments have shown that without impregnation in glucose solution, SiO(G) cannot be completely generated, and the final product is Si, SiO(G), SiO2, C, CO(G), and CO2(G). The chemical equation for the carbothermic reduction reaction is SiO2 + C → SiO(G) + CO(G). This reaction requires strict control of reaction conditions to achieve complete reaction. For this invention, incomplete reaction would result in waste but would not affect the invention results.

[0011] (4) Carbonization after boiling fluidized bed drying: The impregnated material is dried quickly and uniformly using a boiling fluidized bed; then carbonization is carried out under inert gas and with accurate oxygen supply control to ensure that the carbon content in the product rice husk carbon is between 33-38%; boiling fluidized bed drying ensures that glucose uniformly and densely coats the nano-silica particles. The drying process is determined by the carbon content measured for each batch, whether it is bubbling, boiling or high-volume fluidization; carbonization is carried out under inert gas protection, and the carbon content in the product rice husk carbon is ensured to be between 33-38% through controllable oxygen-free and on-demand oxygen supply methods.

[0012] (5) Bypass vacuum dealkali treatment: Place the dried material in a vacuum furnace and remove alkali metals at 1000-1200℃ and vacuum degree ≤10Pa; it must be an independent bypass. If it shares a channel with the carrier gas silica mixture, it will cause secondary pollution.

[0013] (6) High-temperature carbothermal reduction and vapor deposition control:

[0014] A. Gasification Zone: The material is placed in the high-temperature zone and uniformly heated to 1400-1550℃ under an inert atmosphere to generate SiO gas. The peak temperature at the thermal center here should not exceed 1550℃ for a long time, otherwise SiC will be generated. An inert gas must be used as the carrier gas here, preferably high-purity argon, otherwise a mixture of Si, SiO gas and silicon dioxide will be generated.

[0015] B. Deposition Zone: SiO gas is introduced into a deposition chamber with a controllable temperature gradient along with the carrier gas. The deposition temperature is adjusted to 500-900℃. Different temperatures here result in different specific surface areas of the final product. At the same time, the gas supersaturation and residence time are different, which induces homogeneous nucleation of SiO gas to generate nanoscale primary particles of different sizes. This also prevents the particles from sintering and growing together, thus obtaining silica micro powder with high specific surface area.

[0016] (7) Decarbonization treatment (optional): If trace amounts of carbon remain in the product, low-temperature decarbonization treatment can be carried out at 500-700℃ in an oxygen-rich atmosphere. This treatment can promote the opening of pores and increase the specific surface area.

[0017] Further, the acidic solution mentioned in step (1) is hydrochloric acid, oxalic acid or a mixture of the two, with a concentration of 1-5 mol / L, a treatment temperature of 60-90℃, and a treatment time of 2-6 h.

[0018] Furthermore, the carbonization temperature in step (2) is 500-750℃, and the carbonization time is 1-3h. When the temperature exceeds 750 degrees, especially 800 degrees, the amorphous silica in the rice husk gradually crystallizes.

[0019] Furthermore, step (2) can also employ oxygen-deficient combustion with precise control, which can maximize the use of energy and quickly obtain rice husk carbon with a carbon content that can be controlled between 12-25%. In the later stage, the carbon source can be supplemented by adjusting the concentration of glucose solution, performing a slight carbonization and then soaking.

[0020] Further, the concentration of the glucose solution in steps (3) and (4) is 10-30 wt%, the immersion time is 1-2 h, fluidized bed drying is used to ensure uniform coating, the drying temperature is 100-150 ℃, the carbonization temperature is 350-500 ℃, and the carbonization time is 20-30 minutes.

[0021] Furthermore, the bypass vacuum dealkali treatment described in step (5) can also be performed without using a separate vacuum furnace, but in the same high-temperature furnace as in step (6). However, a separate vacuum channel must be used to ensure that the extracted alkali metal does not contaminate the gaseous SiO channel.

[0022] Furthermore, the temperature of the low-temperature deposition zone in step (6) is 800-900℃, while the comparative example uses 900-1200℃. The flow rate of the carrier gas argon is 1-5L / min, and the specific surface area is between 60-300 m² / g. At this time, the original particle size of silica varies greatly, and the particle size is generally large, concentrated between 20-100nm.

[0023] Furthermore, the prepared silica micro powder has a purity of ≥99.999%, a total impurity content of <10ppm, and a single alkali metal or transition metal content of <0.5ppm.

[0024] Furthermore, in step (6), the temperature of the deposition zone is controlled at 500-800℃, preferably 600-800℃, and the argon flow rate is 2-8 L / min. Rapid cooling inhibits particle growth, so that the original particle size is controlled at 5-20 nm, thereby obtaining a high specific surface area of ​​300-650 m² / g.

[0025] After multiple experiments, it was found that the relationship between specific surface area and temperature is not linear. In the specific implementation process, multiple experiments were conducted step by step according to the requirements to finally determine the technical parameters of the process.

[0026] The CO generated in this invention, after being oxygenated, produces high-purity CO2 that can be collected and used separately. Attached Figure Description

[0027] Figure 1 This is a magnified 3X image of the appearance of rice husks after acid washing in Example 1.

[0028] Figure 2 This is a 10X magnified image of the appearance of rice husk charcoal with a carbon content of 20% in Example 1. Detailed Implementation

[0029] Example 1 (Target specific surface area ≥ 300 m² / g)

[0030] (1) Take rice husks and soak them in 5% hydrochloric acid at 80°C for 4 hours. Rinse them with deionized water in a countercurrent manner until they are neutral.

[0031] (2) Carbonize at 600℃ with limited oxygen for 2 hours to obtain rice husk charcoal with a carbon content of 20%.

[0032] (3) Immerse in 20% glucose solution for 1.5 h and then fluidize and dry at 120 °C.

[0033] (4) Dealkali removal at 1050℃ and 5Pa for 3 hours.

[0034] (5) Place the material in a high-temperature furnace with an argon flow rate of 5 L / min, a vaporization zone temperature of 1500℃, and a deposition zone temperature of 750℃, and collect the product.

[0035] Test results: Purity 99.9992% (tested by a third party using ICP-MS), specific surface area 369 m² / g (tested by 33H-2000BET), native particle size 8-15 nm (tested by JEM 1200-EX TEM), D50=0.8um (LS-pop6 laser particle size analyzer).

[0036] Example 2 (Target specific surface area ≥ 500 m² / g)

[0037] Adjust the deposition zone temperature to 650℃ and the argon flow rate to 8 L / min to increase the gas supersaturation; the remaining operations are the same as in Example 1.

[0038] Test results: Purity 99.9991%, specific surface area 586 m² / g, original particle size 5-10 nm, D50=0.5 μm.

[0039] Example 3 (Increasing specific surface area and decarbonization treatment)

[0040] Based on Example 2, the sample was further treated in an air atmosphere at 550°C for 1 hour.

[0041] Test results: Specific surface area further increased to 652 m² / g, carbon content <0.01%.

[0042] Example 4 (Target specific surface area ≤ 100m² / g)

[0043] In Example 1, the temperature of the deposition zone was adjusted to 890℃; the remaining operations were the same as in Example 1.

[0044] Test results: Purity 99.9994%, specific surface area 75 m² / g, original particle size 78-81 nm, D50=1.6 μm.

[0045] Comparative Example 1

[0046] The deposition zone temperature was 1000℃. The collected product had a specific surface area of ​​55 m² / g and the particles were clearly sintered.

[0047] The deposition zone temperature was 1100℃, the specific surface area was only 16 m² / g, the particles were obviously sintered, and the generated silica was clearly crystallized.

[0048] Beneficial effects of the invention

[0049] 1. Possesses both ultra-high purity and ultra-high specific surface area: Product purity ≥ 99.999% (5N9), total impurities < 10ppm; specific surface area adjustable from 60-650 m² / g, solving multiple technical problems such as the low specific surface area of ​​traditional high-purity products. This specification of product is unmatched by 5N9 grade crystalline silica, and can be transformed into perfectly spherical crystalline silica microspheres through simple high-temperature treatment.

[0050] 2. Fine particle size and good dispersibility: The original particle size is 5-20 nm, and the aggregate particle size D50 can be controlled within 0.2-2 μm, meeting the needs of CMP, high-end science, aerospace, chip and other fields for nano-dispersed silica.

[0051] 3. Pure crystal form: The product is amorphous silicon dioxide, which appears as a bun shape in XRD. It has no crystal diffraction peaks and has excellent thermal stability.

[0052] 4. Green Economy: It has realized the ultra-high value utilization of rice husks, agricultural waste.

Claims

1. A high-purity, high-specific-surface-area silica micro-powder of grade 5N9 made from rice husk and its preparation method, characterized in that, Includes the following steps: a. Pickling pretreatment; b. Temperature-limited quantitative carbonization treatment; c. Uniform glucose impregnation and coating followed by recarbonization treatment; d. Vacuum removal of alkali metals; heat treatment at a vacuum degree ≤10Pa and a temperature of 1000-1200℃ to remove alkali metals; e. High-temperature reduction and vapor deposition control: The material is heated to 1400-1550℃ in an inert atmosphere to generate SiO gas; the SiO gas is introduced into the deposition zone, and by controlling the deposition temperature of 500-900℃ and the carrier gas flow rate, SiO homogeneous nucleation and deposition are achieved to obtain silica micro powder with high specific surface area.

2. The method according to claim 1, characterized in that: The acidic solution mentioned in step a is hydrochloric acid, oxalic acid, or a mixture of both, with a concentration of 1-5 mol / L, a treatment temperature of 60-90℃, and a treatment time of 2-5 h.

3. The method according to claim 1, characterized in that: The carbonization process described in step b involves carbonizing the acid-washed rice husks under anaerobic conditions and at a maximum temperature not exceeding 750 degrees Celsius to ensure that the carbon content of the rice husks is 18-22%.

4. The method according to claim 1, characterized in that: In step c, the glucose is uniformly coated and then re-carbonized. The concentration of the glucose solution is 20-25%. Fluidized bed drying is used to ensure uniform coating. Carbonization is carried out under inert gas and with precise oxygen supply control to ensure that the carbon content in the rice husk carbon of the product is between 33-38%.

5. The method according to claim 1, characterized in that: In step d, the alkali metal removal process involves setting up an independent vacuum channel and heat-treating the metal at a vacuum level ≤10Pa and a temperature of 1000-1200℃.

6. The method according to claim 1, characterized in that: In step e, high-temperature reduction and vapor deposition are controlled by heating the material to 1400-1550℃ in an inert atmosphere to generate SiO gas. The SiO gas is then introduced into the deposition zone, and the deposition temperature is controlled at 500-900℃ and the argon flow rate is 1-8 L / min, so that SiO can be homogeneously nucleated and deposited to obtain silica micropowder with high specific surface area.

7. The method according to claim 1, characterized in that: The silica micro powder is in an amorphous state, with a controllable specific surface area of ​​60-650 m² / g and a purity ≥99.999%.

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