Silica Production Processes and Applications

Silica, also known as hydrated silica, is silicon dioxide in the form of fine powder or ultrafine particles. High-purity silica has a SiO₂ content of up to 99.8%; it is lightweight, with an initial particle size of <0.0003 mm, a specific gravity of 2.319–2.653, and a melting point of 1750 °C. The classic methods for manufacturing white silica are divided into three categories: the gas-phase method, the precipitation method, and the dissociation method. Industrial preparation methods are combinations or modifications of these three approaches.

1 Gas-Phase Method

1.1 Production of Gas-Phase White Silica

Silica produced by the gas-phase method is an amorphous powder formed by the high-temperature hydrolysis of silicon chlorides—silicon tetrachloride or trichloromethylsilane—in a mixed stream of air and hydrogen. It typically consists of spherical particles with hydroxyl groups and adsorbed water on their surfaces, with particle sizes ranging from 7 to 40 nm and a large specific surface area. high chemical purity (SiO₂ > 99.8%). Depending on whether surface treatment has been performed, fumed silica can be classified as hydrophilic or hydrophobic; it can also be categorized into different grades based on its specific surface area.

The domestic market for fumed silica is almost entirely monopolized by several major foreign companies, such as Degussa, Cabot, and Wacker, while China has only a few small-scale production facilities. For example, Guangzhou Jibishi Technology Industrial Co., Ltd., building on the absorption and assimilation of advanced foreign technologies, has undertaken bold technological innovations and was the first in China to establish a production line for manufacturing fumed silica using silicone by-products, with an annual output per unit reaching 500 metric tons. Currently, the company offers four grades of hydrophilic products; the Jilin Chemical Research Institute also produces fumed silica using silicone monomer by-products, among other initiatives. Overseas single-unit capacity for fumed silica generally exceeds 4,000 metric tons per year; the largest single unit operated by Cabot Corporation in the United States can reach an annual output of 9,000 metric tons. Overseas fumed silica products feature a comprehensive range of varieties and grades. Cabot Corporation in the United States offers more than ten varieties of untreated fumed silica (including three compressed varieties), while its treated fumed silica primarily includes TS-720, TS-610, and TS-530. In contrast, China’s range of hydrophilic fumed silica grades is limited, and the development of modified fumed silica is still in its infancy; the capacity and variety of single-unit fumed silica production facilities in China lag behind world-class standards. However, China has successfully tackled a world-class chemical engineering challenge—nano-silica technology—breaking the monopoly held by a few developed countries, such as Germany, the United States, and Japan, in this field. The successful development and promotion of the “Technology and Equipment for the Gas-Phase Combustion Production of Nano-Silica,” jointly undertaken and completed by the Key Laboratory of Ultrafine Materials Preparation and Application at East China University of Science and Technology and Shanghai Chlor-Alkali Co., Ltd., is expected to generate 1 billion yuan in output value and effectively drive the development of China’s organosilicon and other industries. In February 2004, China BlueStar (Group) Corporation and Cabot Corporation of the United States signed an agreement to jointly invest approximately $30 million in the construction of China’s largest and world-class fumed silica production plant—Cabot BlueStar (Jiangxi) Chemical Co., Ltd. Completed in 2005, the plant has an annual production capacity of 5,000 metric tons.

1.2 Applications of Aerosil

Aerosil is widely used in silicone rubber, cable compounds, unsaturated polyester resins, adhesives, paints and coatings, inks and copier toner, food, and cosmetics, where it serves to reinforce, thicken, prevent caking, and control the rheology and thixotropy of the system.

1.2.1 Applications in Silicone Rubber

Aerogel silica is extensively used in room-temperature vulcanizing (RTV) and high-temperature vulcanizing (HTV) silicone rubbers. It is typically dispersed in the matrix in the form of agglomerates, forming a three-dimensional network structure. This structure provides a large surface area in contact with the silicone rubber matrix and creates numerous cross-linking points during vulcanization, thereby thickening and reinforcing the silicone rubber. The three-dimensional network structure formed by fumed silica is relatively stable and exists in an “elastic” state. Under external force, it is temporarily disrupted, reducing the system’s viscosity and imparting good flowability; when the external shear force is removed, the three-dimensional network structure rapidly returns to its pre-stress state, giving the system good thixotropic properties. Furthermore, because fumed silica has a very small particle size and spherical shape, it disperses uniformly in the binder to form a homogeneous system, often exhibiting good optical properties, which enables the production of white, transparent silicone rubber products. Room-temperature-curing acid silicone rubber produced using fumed silica as a filler has a wide viscosity range and effective curing properties. It exhibits excellent adhesion to various primed or unprimed substrates and possesses outstanding storage stability at room temperature, making it widely used in the construction industry.

1.2.2 Applications in Plastics and Unsaturated Polyester Resins

Fumed silica is also commonly used in plastics, elastomers, and unsaturated polyester resins. When a small amount of fumed silica is added to plastic compounds in addition to traditional fillers during compounding, it produces a significant reinforcing effect, greatly improving the material’s hardness and mechanical properties, thereby enhancing both the processing performance and the performance of the final products. In unsaturated polyester resins, adding a small amount of fumed silica imparts excellent transparency and superior physical properties to the resin, all of which help improve the quality of downstream products. Fumed silica plays an irreplaceable role in industrial development; however, its relatively high cost often limits its wider application. For example, the rubber industry currently continues to rely heavily on precipitated silica.

2 Precipitation Method

The precipitation method involves reacting water glass with sulfuric acid or hydrochloric acid to form silicic acid, which is then decomposed to produce white silica. With a SiO₂ content of approximately 90%, it is in high demand in the market and is primarily used as a reinforcing filler in rubber. Most silica gel production plants in China primarily use this method to produce silica gel. The production technology and equipment for silica gel produced by the precipitation method are simple; however, the product has low reactivity, difficult-to-control particle size, poor affinity, and low reinforcing performance. Additionally, the particle surfaces are heavily bonded with hydrophilic groups, which weakens the product’s bonding strength. The production of ultrafine white silica via recrystallization is an improved technology that incorporates secondary seed treatment based on the precipitation method. The new recrystallization process enables fully automated control of industrial-scale production. The product has an SiO₂ content of over 94%, a specific surface area of 269–320 m²/g, a maximum particle size of 1000 mesh, and the finest particles can reach the nanoscale.

The mass production of highly dispersed, gel-free white silica marks the completion of China’s largest production base for this product. With a total investment of 170 million yuan, the project’s initial production target is an annual output of 45,000 metric tons. Upon full completion, annual production capacity will reach 100,000 metric tons, surpassing the output of Japan’s largest white silica producer and establishing itself as a large-scale fine chemicals production base that leads in technology and ranks first in Asia in terms of scale. The new product is backed by fully independent intellectual property rights. Precipitated white silica is widely used as a filler and reinforcing agent in rubber and plastics; as an additive in synthetic resins (polyester resins, elastic polyurethanes); as a sizing agent for polypropylene and non-toxic polyvinyl chloride (PVC) plastic films; and as an insulating and thermal insulation filler in the electronics and electrical industries.

3 Dissociation Method

3.1 Non-metallic Mineral Method

Raw materials used to produce white carbon black from non-metallic minerals include diatomaceous earth, protein clay, serpentine, bentonite, kaolin, wollastonite, quartz sand, sepiolite, attapulgite, fly ash, zircon, coal gangue, and yellow phosphorus ore. The production of silica from non-metallic minerals is technically feasible and economically viable, providing a new avenue for the deep processing and comprehensive utilization of non-metallic minerals. Silica can also be prepared by calcining and converting clay minerals. The Third Military Medical University has successfully developed advanced, pollution-free technologies for producing silica and polyaluminum chloride from diatomaceous earth. Zhejiang Guangke Chemical Co., Ltd. and Linjiang Yezhu Chemical Co., Ltd. in Jilin Province produce silica from diatomaceous earth. If water glass is first produced from non-metallic minerals and then used to produce silica, the process still relies on the precipitation method.

3.2 The Grass Family Method

Silica produced from rice husks and rice chaff ash falls between the precipitation method and the vapor-phase method. Not only is its cost far lower than that of the vapor-phase method, but it is also lower than that of the precipitation method; its quality is far superior to that of the precipitation method and approaches that of the vapor-phase method. Yibin Wuliangye Group Fine Chemicals Co., Ltd. is the only manufacturer in China that produces silica (silicon dioxide) from plant materials, with an annual production capacity of 4,000 metric tons. If rice husks and rice bran ash are used as raw materials, alkali leaching is performed to obtain water glass; the water glass is then reacted with acid to form a precipitate, which is filtered, washed with water, and dried to yield silica. This technology still falls under the precipitation method.

3.3 By-Product Recovery Method

Methods include: producing white carbon black from yellow phosphorus slag; producing white silica from sodium fluorosilicate; producing white silica from coal ash; producing white silica via a one-step hydrolysis process using silicon tetrafluoride, a byproduct of phosphate fertilizer plants; producing white silica from SiO₂ byproducts of NaF production; producing white silica from waste residue generated during the production of water purifying agents; and producing white silica from waste silica sol.