Production process and applications of silica gel

Thread Content

Silica gel, also known as hydrated silica, is silica in the form of fine powder or ultra-fine particles; those of high purity have a SiO2 content of up to 99.8%, are lightweight, and have an initial particle size of 99.8%. Vapor-phase silica can be divided into hydrophilic and hydrophobic types depending on whether surface treatment has been applied, and it can also be classified into different grades based on its specific surface area. The domestic market for gas-phase silica in China is almost monopolized by a few large foreign companies such as Degussa, Cabot, and Wacker; there are only a few small-scale production facilities in the country. For example, Guangzhou Gibisi Technology Industry Co., Ltd., by building on the absorption and adaptation of advanced foreign technologies and pursuing bold innovations, was the first in China to establish a production line for manufacturing fumed silica using silicone by-products; the annual output per unit of this facility is 500 tons. Currently, there are 4 types of hydrophilic products available ； Jilin Chemical Industry Research Institute produces gas-phase silica and similar products from by-products of silicone monomers. The annual production capacity of a single unit for vapor-phase silica in foreign countries is all above 4,000 tons; the largest capacity for such a unit produced by the American company Cabot reaches 9,000 tons per year. Foreign vapor-phase silica has a wide variety of types and grades. Cabot Corporation in the United States offers more than a dozen types of non-treated fumed silica, including 3 compressed varieties; the main treated silicas are TS-720, TS-610, and TS-530. In contrast, China has a limited range of hydrophilic silica grades; the modified silica series is still in its early stages of development, and the capacity of single-unit facilities for producing silica via the vapor phase method, as well as the variety of grades available, fall short of world-class standards. However, our country has overcome a world-class chemical industry challenge—the technology of nano-silica—and broken the monopoly held by a few developed countries such as Germany, the United States, and Japan in this field. The successful development and dissemination of the technology for preparing nanosilica via gas-phase combustion, carried out jointly by the Key Laboratory of Ultrafine Material Preparation and Applications at East China University of Science and Technology and Shanghai Chlor-Alkali Co., Ltd., will generate an economic output of 1 billion yuan and effectively promote the development of industries such as silicone in China. In February 2004, China Blue Star (Group) Corporation and the American company Cabot signed a contract; together they would invest around $30 million to build China’s largest and world-class vapor-phase silica production plant – Cabot Blue Star (Jiangxi) Chemical Co., Ltd. It is expected to be completed in 2005, with an annual production capacity of 5,000 tons. 1.2 Applications of vapor-phase silica Vapor-phase silica is widely used in silicone rubber, cable materials and unsaturated polyester resins, adhesives, paints and coatings, inks and copier toners, as well as in food and cosmetics. It serves functions such as reinforcement, thickening, anti-caking, and control of the rheology and thixotropy of systems. 1.2.1 Applications in silicone rubber: Vapor-phase silica is widely used in room-temperature vulcanized silicone rubber and high-temperature vulcanized silicone rubber. It is often dispersed in the matrix in the form of agglomerates, forming a three-dimensional network structure. With a large contact area with the silicone rubber matrix and numerous cross-linking points formed during vulcanization, it serves to thicken and reinforce the silicone rubber. The three-dimensional network structure formed by gas-phase silica has relative stability and is in an “elastic” state; under the influence of external forces, it is temporarily disrupted, which reduces the viscosity of the system and results in good fluidity ； When the external shear force is removed, the three-dimensional network structure rapidly returns to its state before being stressed, endowing the system with good thixotropy. Furthermore, since the particle size of white carbon black produced by the vapor phase method is very small and the particles are spherical, they disperse uniformly in the matrix to form a homogeneous system, often resulting in good optical properties, which enables the production of white and transparent silicone rubber products. Room-temperature vulcanizing acidic silicone adhesives produced using fumed silica as a filler via a vapor-phase method exhibit a wide range of viscosities and effective vulcanization properties; they offer excellent bonding strength to various substrates, whether primed or unprimed, and possess good storage stability at room temperature. They are widely used in the construction industry. 1.2.2 Applications in plastics and unsaturated polyester resins – Vapor-phase silica is also commonly used in plastics and elastomers, as well as in unsaturated polyester resins. When mixing plastics, adding a small amount of fumed silica in addition to traditional fillers has a significant reinforcing effect; it **increases the hardness and mechanical properties of the material, thereby improving both the processing conditions and the performance of the final product. Adding a small amount of vapor-phase silica to unsaturated polyester resin can endow the resin with excellent transparency and superior physical properties, all of which contribute to improving the quality of the final products. Vapor-phase silica plays an irreplaceable role in industrial development, but its high cost often limits its more widespread use; for example, the rubber industry still makes extensive use of precipitated silica. 2 Precipitation method: The precipitation method involves reacting water glass with sulfuric acid or hydrochloric acid to produce silicic acid, which is then decomposed to yield silica gel. Its SiO2 content is around 90%, and it has high market demand, mainly used as a reinforcing filler for rubber. China’s silica gel manufacturers primarily use this method to produce silica gel. The production technology and equipment for precipitated silica involve simple processes; however, the resulting product has low activity, its particles are difficult to control, it exhibits poor affinity, weak reinforcing properties, and severe bonding of hydrophilic groups on the particle surface, which reduces the product’s cohesion. The production of ultra-fine white carbon black through secondary crystallization is an improved technique that involves secondary seed treatment, built upon the precipitation-based production method. By adopting the new secondary crystallization process, industrial production can be fully automated. Its SiO2 content is over 94%, the specific surface area ranges from 269 to 320 m^2/g, the maximum particle size is 1000 mesh, and the finest particles can reach the nanoscale. The mass production of highly dispersed, gel-free silica by Shanghai Jiuchen Fine Chemicals Co., Ltd. marks the establishment of China’s largest production facility for such silica. The total investment in this project amounts to 170 million yuan. The initial production target is 45,000 tons per year; once fully operational, the production capacity will reach 100,000 tons per year, exceeding the output of Japan’s largest silica production company. It will thus become a large-scale fine chemical manufacturing facility that leads in terms of technology and ranks first in Asia in terms of scale. The new product possesses full independent intellectual property rights. Precipitated silica is widely used as a filling and reinforcing agent in rubber and plastics, an additive in synthetic resins (polyester resins, elastic polyurethanes), an opener for polypropylene and non-toxic PVC plastic films, as well as an insulating and heat-insulating filler in the electronics and electrical industry. 3 Dissociation Method 3.1 Non-metallic ore method The raw materials used to produce silica gel from non-metallic ores include diatomite, protein clay, serpentine, bentonite, kaolin, wollastonite, quartz sand, sepiolite, attapulgite, fly ash, zircon, coal cinder, and yellow phosphorus ore. It is technically feasible to produce silica gel using non-metallic minerals, and it offers good economic benefits, providing a new pathway for the deep processing and comprehensive utilization of such minerals. White carbon black can also be prepared through the calcination and transformation of clay minerals. The Third Military Medical University has successfully developed an advanced, pollution-free technology for producing silica and polyalumina using diatomite. Zhejiang Guangke Chemical Co., Ltd. and Jilin Province Linjiang City Yezhu Chemical Co., Ltd. produce silica gel using diatomite. If water glass is first produced from non-metallic minerals and then silica is made from water glass, the technique remains the precipitation method. 3.2 Gramineous plants method: The silica produced using rice husks and rice hull ash as raw materials falls between the precipitation method and the vapor phase method. Its cost is not only much lower than that of the vapor phase method, but also lower than that of the precipitation method ； Its quality is far higher than that of the precipitation method, and is close to that of the vapor phase method. Yibin Wuliangye Group Fine Chemicals Co., Ltd. is the only manufacturer in China that produces silica gel (silicon dioxide) from plants, with an annual production capacity of 4,000 tons of silica gel. If rice husks and grain husk ash are used as raw materials, water glass is obtained through alkaline leaching; when water glass reacts with acid, a precipitate is formed, and after filtration, washing, and drying, silica gel is produced. This technique remains a precipitation method. 3.3 By-product recovery method: Producing silica gel from yellow phosphorus slag ； Production of silica gel using sodium fluosilicate ； White carbon black can be produced from coal ash ； Production of silica aerogel via one-step hydrolysis of silicon tetrafluoride, a byproduct of phosphate fertilizer plants ； Preparing silica fume using SiO2, a byproduct of NaF production ； Preparation of silica gel from waste residues from the production of water purifiers ； White carbon black is prepared using waste silica sol.