Silica, also known as hydrated silicon dioxide, is a fine powder or ultra-fine particulate silicon dioxide. High-purity silica has a SiO₂ content of up to 99.8%, light weight, original particle size <0.0003mm, specific gravity 2.319～2.653, and melting point 1750℃. The classic manufacturing methods of silica are divided into three categories: fumed process, precipitation process, and dissociation process. Industrial preparation methods are all combinations or improvements of these three methods.
1 Fumed Process
1.1 Production of Fumed Silica
Fumed silica is an amorphous powder produced by high-temperature hydrolysis of silicon chlorides such as silicon tetrachloride or trichloromethylsilane in a mixed air-hydrogen flow. It is usually spherical particles with hydroxyl groups and adsorbed water on the surface, particle size between 7～40nm, large specific surface area, high chemical purity, SiO₂>99.8%. Fumed silica can be divided into hydrophilic and hydrophobic types according to surface treatment, and different models according to specific surface area. The domestic fumed silica market is almost monopolized by several large foreign companies such as Evonik, Cabot, and Wacker. China only has a few small-scale production units. For example, Guangzhou Guangzhou Jebees Technology & Industry Co., Ltd. has carried out bold technological innovation on the basis of absorbing and digesting foreign advanced technologies, and took the lead in building a production line in China using silicone by-products to produce fumed silica, with an annual output of 500 tons per unit and 4 hydrophilic product models currently available; Jilin Chemical Research Institute produces fumed silica using by-products of silicone monomers. The capacity of a single set of foreign fumed silica plants is more than 4,000 tons per year, and the largest single set of Cabot Corporation in the United States can reach 9,000 tons per year. Foreign fumed silica has a complete range of varieties and grades. Cabot Corporation in the United States has more than ten varieties of untreated fumed silica (including 3 compressed varieties), and the main treated silica grades are TS-720, TS-610, TS-530. In contrast, China has few grades of hydrophilic silica series, the development of modified silica series has just started, and there is a gap between the capacity and varieties of single fumed silica plants and the world's advanced level. However, China has conquered a world-class chemical project - nano-silica technology, breaking the monopoly of a few developed countries such as Germany, the United States, and Japan in this field. The successful development and promotion of "Nano Silicon Dioxide Gas Phase Combustion Preparation Technology and Equipment Development" undertaken by the Key Laboratory of Ultra-fine Materials Preparation and Application of East China University of Science and Technology and Shanghai Chlor-Alkali Co., Ltd. will form an output value of 1 billion yuan, effectively promoting the development of China's silicone and other industries. In February 2004, China National BlueStar (Group) Corporation and Cabot Corporation of the United States signed a contract. The two parties will invest about 30 million US dollars to build Cabot BlueStar (Jiangxi) Chemical Co., Ltd., the largest and world-class fumed silica production plant in China. It is expected to be completed in 2005 with an annual capacity of 5,000 tons.
1.2 Application of Fumed Silica
Fumed silica is widely used in silicone rubber, cable compounds, unsaturated polyester resins, adhesives, paints, inks, copier toners, food and cosmetics, playing roles in reinforcement, thickening, anti-caking, and controlling rheology and thixotropy of the system.
1.2.1 Application in Silicone Rubber Fumed silica is widely used in room temperature vulcanized silicone rubber and high temperature vulcanized silicone rubber. They are usually dispersed in the matrix in the form of agglomerates to form a three-dimensional network structure, with a large contact area with the silicone rubber base material and many cross-linking points formed during vulcanization, thus thickening and reinforcing the silicone rubber. The three-dimensional network structure formed by fumed silica is relatively stable and in an "elastic" state. Under external force, it will be temporarily damaged, reducing the viscosity of the system and showing good fluidity; when the external shear force is removed, the three-dimensional network structure will quickly return to the state before stress, giving the system good thixotropy. In addition, due to the small particle size and spherical shape of fumed silica, it disperses in the base material to form a uniform system, often with good optical properties, and can be used to manufacture white transparent silicone rubber products. Room temperature vulcanized acidic silicone rubber produced with fumed silica as filler has a wide consistency range and effective vulcanization performance, excellent adhesion to various substrates with or without primer, and excellent storage stability at room temperature, widely used in the construction industry.
1.2.2 Application in Plastics and Unsaturated Polyester Resins Fumed silica is also often used in plastics, elastomers and unsaturated polyester resins. Adding a small amount of fumed silica to plastic mixing in addition to traditional fillers will produce a significant reinforcing effect, greatly improving the hardness and mechanical properties of the material, thereby improving the processing technology and product performance. Adding a small amount of fumed silica to unsaturated polyester resin can give the resin excellent transparency and physical properties, which help to improve the quality of downstream products. Fumed silica plays an irreplaceable role in industrial development, but its wide application is limited due to its high price. For example, the rubber industry still uses a large amount of precipitated silica.
2 Precipitation Process
The precipitation process is to react water glass with sulfuric acid or hydrochloric acid to generate silicic acid, which is then decomposed to produce silica. The SiO₂ content is about 90%, with large market demand, mainly used as rubber reinforcing filler. Most silica manufacturers in China adopt this method. The production technology and equipment of precipitated silica are simple, with low product activity, difficult to control particles, poor affinity, low reinforcement performance, and serious bonding of hydrophilic groups on the particle surface, weakening the bonding force of the product. Secondary crystallization to produce ultra-fine silica is an improved technology based on the precipitation process, with secondary seed crystal treatment. The new secondary crystallization process can realize fully automatic industrial production. Its SiO₂ content is more than 94%, specific surface area reaches 269～320m²/g, maximum particle size is 1000 mesh, and the finest can reach nano-scale. The mass production of highly dispersed gel-free silica by Shanghai Jiuchen Fine Chemical Co., Ltd. marks the completion of China's largest production base of highly dispersed gel-free silica. The total investment of the project reaches 170 million yuan, with an initial annual production target of 45,000 tons, and an annual output of 100,000 tons after full completion, exceeding the output of Japan's largest silica manufacturer, becoming a large-scale fine chemical production base with leading technology and the largest scale in Asia. The new product has all independent intellectual property rights. Precipitated silica is widely used as filling and reinforcing agent for rubber and plastics, additive for synthetic resins (polyester resin, elastic polyurethane), anti-blocking agent for polypropylene and non-toxic PVC plastic films, insulating and heat-insulating filler for electronic and electrical industry, etc.
3 Dissociation Process
3.1 Non-metallic Mineral Method Raw materials used to produce silica from non-metallic minerals include diatomite, opal, serpentine, bentonite, kaolin, wollastonite, quartz sand, sepiolite, attapulgite, fly ash, zircon, coal gangue, yellow phosphorus ore, etc. It is technically feasible and economically beneficial to produce silica using non-metallic minerals, providing a new way for the deep processing and comprehensive utilization of non-metallic minerals. Silica can also be prepared by calcination and transformation of clay minerals. The Third Military Medical University has successfully developed a pollution-free advanced technology for producing silica and polyaluminum from diatomite. Zhejiang Guangke Chemical Co., Ltd. and Yezhu Chemical Co., Ltd. in Linjiang City, Jilin Province produce silica from diatomite. The technology of first producing water glass from non-metallic minerals and then producing silica from water glass is still the precipitation process.
3.2 Gramineous Plant Method Silica produced from rice husk and rice husk ash is between the precipitation process and the fumed process. Its cost is not only far lower than the fumed process, but also lower than the precipitation process; its quality is much higher than the precipitation process, close to the fumed process. Yibin Wuliangye Group Fine Chemical Co., Ltd. is the only manufacturer in China that produces silica (silicon dioxide) from plants, with an annual output of 4,000 tons. If rice husk and rice husk ash are used as raw materials, water glass is obtained after alkali soaking, and the water glass reacts with acid to obtain a precipitate, which is filtered, washed and dried to obtain silica. This technology is still the precipitation process.
3.3 By-product Recovery Method
Methods include: producing silica from yellow phosphorus furnace slag; producing silica from sodium fluorosilicate; producing silica from coal ash; producing silica by one-step hydrolysis of silicon tetrafluoride, a by-product of phosphate fertilizer plants; preparing silica from SiO₂, a by-product of NaF production; preparing silica from waste residue of water purifier production; preparing silica from waste silica sol.