Progress in research and application of wollastonite synthetic porous silica

Wollastonite is a naturally occurring metasilicate fiber mineral with many excellent industrial applications ^[1] . Wollastonite is a finely divided ceramic raw material quality, metallurgical additives; high aspect ratio wollastonite is an ideal substitute for asbestos and chopped glass fibers used as rubber, plastic and the filler in paint, from increment The dual role of reinforcement. However, the effect of application as a filler in paints, rubbers, and plastics is not satisfactory. The wollastonite concentrate has high content of CaSiO ₃ , less impurities and easy reaction with inorganic acid, and is an inexpensive natural silicon source for preparing amorphous silica. Amorphous SiO _{2 is} widely used as a reinforcing agent in high-grade rubber, sealant, paper, plastic, cable, filler, covering agent and coating, thickener in ink, anti-settling agent, honeycomb porous structure, high The specific surface SiO ₂ can be used as a new catalyst carrier, selective adsorbent, aerospace insulation material, etc. in the high-tech field ^[2] . The use of wollastonite to react with acid to prepare high specific surface area SiO _{2 has} become a major development direction for the comprehensive development and utilization of wollastonite.

I. Research overview

The preparation of high specific surface area SiO _{2 by} reacting wollastonite with acid is an important part of the domestic research in the field of inorganic materials. Some domestic scholars have published relevant research results. Peng Renyong ^[3] prepared a porous high specific surface area SiO _{2 by} reacting wollastonite suspension with hydrochloric acid. The results show that when pH value is ≤1.0, wollastonite reacts with hydrochloric acid to form a large amount of stable silicic acid sol; the final reaction (pH Value = 4.0) Hydrolysis and polycondensation of silicic acid and hydrogen bond formation of Si-O and OH groups at a suitable rate to form a weakly crosslinked, networked, low density silicic acid gel, the final product SiO ₂ ratio The surface area is increased.

Chen Qingchun et al ^[4] used porous wollastonite suspension to react with hydrochloric acid to synthesize porous silica. In the process of reaction, inorganic auxiliaries and organic auxiliaries were added, and the pH of the reaction system was used to control the acid addition rate. The results show that the wollastonite synthetic porous silica, the specific surface area of ​​the product is mainly related to the polymerization rate K, but the dissolution rate K in the first step will limit the polymerization rate K. When the reaction system stabilizes the pH value>2, the specific surface area of ​​the product is obvious. When the pH is less than 1.5, V1 is large enough without affecting V2, and the specific surface area of ​​the product is greatly increased. Meanwhile, when the reaction stabilizes the pH value and the reaction time is constant, the determinant of the specific surface area of ​​the product is neutralization. The final pH. Within the scope of its study, the final pH value of neutralization was 4, and the specific surface area decreased sharply with increasing pH.

Wang Yanji et al ^[5] synthesized high specific surface area porous SiO ₂ by adding additives to the reaction solution. The synthesis condition is pH value≤1.5, the additive is polyethylene glycol and NH4CI, and the system is neutralized to pH value=4.0 after reaction, and the product is obtained by solid-liquid separation, drying, and burning at 650-750 °C for 2 hours. The specific surface area of ​​the product obtained by the test was 479±45 m ² /g, the apparent density was 0.34±0.03 g/cm ³ , the DBP sorption rate was 173±16 mL/100 g, and the average median diameter was 6.9±0.5 gm. Focus on l ~ 2nm.

Chen et al. ^[6] used five-factor four-level orthogonal experiment to investigate the factors affecting the yield of wollastonite synthetic multi-space SiO ₂ powder. The study concluded that when the reaction time is uniform for 80 min, the aging time is 20 min, the reaction temperature is 50-55 ° C, the acid used is 12 mol/L HCl, and the wollastonite dosage is 80 g, the suspension mass fraction has the greatest influence on the powder yield, NH ₄ C1 addition has the least impact. The maximum yield was 20% of the suspension mass fraction, PEC20000 for the surfactant, 6.0 for the final pH of the system, 5% for the NH4C1, and 40 min for the surfactant.

Second, the preparation method of porous silica

There are many reports on the preparation of high specific surface area SiO ₂ ^[7] , but mostly using tetraethyl orthosilicate (TEOS) or methyl orthosilicate (TMOS) as raw materials, using sol-gel method, by adding reagents or supercritical Dry method to achieve.

There are relatively few studies on the preparation of porous silica from wollastonite. In the related literature ^{[3, 4]} , a two-step reaction in the process of preparing porous silica in the presence of CaSiO3 dissolution and silicic acid polymerization into a solid state is proposed, namely:

CaSiO ₃ +2HCl + H ₂ O = H4SiO ₄ + CaCl ₂

The above formula is a dissolution reaction. The reaction rate is determined by the acid concentration, that is, the lower the pH of the system, the faster the reaction rate.

Silicic acid polymerization is more complicated. Under larger acidity conditions, silicic acid is mainly present in soluble oligomers and forms, where Am represents [Si(OH) ₄ m+2] ^2- and An represents [Si(OH) ₄ n+2] ^2- , Its reaction formula is as follows:

The above formula is a polymerization reaction, no release, and the pH value of the system does not change. If the molecular weight is not sufficiently large, it is also possible to continue polymerization with the neutral oligomer molecule H2An until the polymer undergoes a phase change to precipitate and precipitate hydrated silica. The rate of polymerization is related to the H ₂ An and concentration of the system (determining the concentration of the system). The total response is:

CaSiO ₃ (s) + 2HCI = SiO ₂ (s) + CaCl ₂ + H ₂ O

Third, the performance characteristics analysis

Since porous silica is a synthetic amorphous silica ultrafine particle, it has acid resistance, alkali resistance, high temperature resistance and good electrical insulation and dispersion properties. At the same time, due to the development of amorphous silica, it has a high specific surface area, especially mesopores (pore size 2 to 50 nm). Due to the medium pore size, the distribution range is narrow and uniform, and the specific surface area is huge, which is especially suitable for large organic A catalyst carrier for molecular synthesis.

Fourth, applied research

(1) Application in the field of adsorption.

Porous silica has a large specific surface area and pore volume, and has great application prospects in the field of adsorption, especially adsorption of organic dyes.

(2) Application in silicone rubber.

Porous SiO ₂ can promote the formation of silicone rubber, which has certain reinforcing effect on silicone rubber. Among them, the mechanical properties such as tensile strength and elongation of vulcanized rubber can meet the requirements of certain rubber products. Compared with other samples with different specific surface areas and particle sizes obtained by acid-treated wollastonite, the small particle size and high specific surface area of ​​porous SiO ₂ promoted the reinforcing properties ^[8] .

(3) Preparation of a nano-silica insulating film.

As a film form of silica aerogel, nanoporous silica film inherits all its excellent properties and can be used as broadband anti-reflection film, anti-glare coating, high efficiency thermal insulation layer, acoustic impedance coupling material, low-media Electrical constant insulating layer, ultra high speed integrated circuit substrate and separation film, filter film, catalytic film and the like. Therefore, it has broad application prospects in the fields of optics, thermals, acoustics, electricity and chemistry ^[9] .

(4) Other applications.

Due to the porosity and high specific surface area of ​​the porous silica, other applications such as catalyst support, exhaust gas purification, and the like can be performed.

V. Conclusion

High specific surface SiO ₂ is a new type of lightweight porous amorphous solid material with many special properties and broad application prospects. In terms of basic research, the structure of porous SiO ₂ and the interaction between its network and adsorbed molecules have attracted great interest; in application, porous SiO ₂ has been used in catalyst carriers, gas filter materials, and high-efficiency heat-insulating materials. Wait. Due to the unique physical and chemical properties of wollastonite, the use of wollastonite to prepare SiO _{2 with} high specific surface area will be the main development direction of wollastonite development and utilization in the future.

references:

[1] Wang Guangwei, Yang Linchun. World wollastonite production, consumption and international trade [J]. China Non-Metallic Mining Industry Guide, 2005, (5): 57-59.

[2] Zhang Lingyan, Xiao Yuju, Fang Heping. The status quo and development trend of the development and utilization of wollastonite resources in China [J]. Mineral Protection and Utilization, 2003, (2): 45-47.

[3] Peng Renyong. Discussion on Mechanism of Preparation of Porous High Specific Surface Area Silica from Wollastonite[J].China Powder Science and Technology,2003,(6):12-15.

[4] Lu Qingchun et al. Synthesis of Porous Silica from Wollastonite[J].Journal of East China Geological Institute, 2001,24(1):64 66.

[5] Wang Yanji et al. Synthesis of High Specific Surface Area Porous Silica from Wollastonite and Its Characterization[J].Journal of East China Geological Institute,2002,25(2):212-215.

[6] Lu Qingchun et al. Investigation on the Factors Affecting the Yield of Porous SiO2 Powder from Wollastonite[J].化工矿物与加工,2003,(4):16-18.

[7] Wang Yanji, Xiao Xuxian, Peng Renyong. Discussion on the reaction kinetics of wollastonite and hydrochloric acid [J]. Inorganic Salt Industry, 1998, (5): 12-13.

[8] Chen Qingchun, Liu Xiaodong, Deng Huiyu. Application of wollastonite porous silica in silicone rubber [J]. Chemical Minerals and Processing, 2003, (10): 2l-23.

[9] Zhao Zongyan and so on. Research progress of nanoporous silica thermal insulation film[J].Journal of Functional Materials,2006.37(12):1859-1862.

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