White carbon black is hydrated silica, which is fine powder or ultrafine particle silica. The SiO2 content of high purity is 99.8%, light weight, original particle size < 0.0003mm, specific gravity 2.319 ~ 2.653, melting point 1750 ℃. The classical manufacturing methods of silica can be divided into three categories: gas phase method, precipitation method and dissociation method. The industrial preparation methods are the combination or improvement of these three methods.
1 gas phase method
1.1 production of fumed silica fumed silica is an amorphous powder formed by high-temperature hydrolysis of silicon chloride, silicon tetrachloride or trichloromethylsilane in the mixed gas flow of air and hydrogen. It is often spherical particles with hydroxyl groups and adsorbed water on the surface. The particle size is between 7 ~ 40nm, large specific surface area, high chemical purity, and SiO2 > 99.8%. Fumed silica can be divided into hydrophilic and hydrophobic types according to whether it has been surface treated or not, and can be divided into different types according to the size of specific surface. The domestic fumed silica market is almost monopolized by several large foreign companies such as Degussa, Cabot and Wacker, and there are only a few small-scale units in China. For example, on the basis of absorbing and digesting foreign advanced technology, Guangzhou jibishi Technology Industry Co., Ltd. has carried out bold technological innovation and took the lead in building a production line for the production of fumed silica by using organic silicon by-products in China. The annual output of a single machine has reached 500t. At present, there are four types of hydrophilic products; Jilin Chemical Research Institute produces fumed silica from by-products of silicone monomers. The capacity of a single set of gas-phase silica units abroad is more than 4000t per year, and the maximum scale of a single set of units of Cabot company in the United States can reach 9000t per year. There are many kinds and brands of fumed silica abroad. There are more than ten varieties of non treated fumed silica (including three compressed varieties) of Cabot company in the United States. The treated silica mainly includes ts-720, ts-610 and ts-530. In contrast, there are few brands of hydrophilic silica Series in China, and the modified silica series has just started to develop. There is a gap in the capacity and variety of single set of gas-phase silica devices compared with the world advanced level. However, China has overcome the world-class chemical subject – nano silica technology, breaking the monopoly position of a few developed countries such as Germany, the United States and Japan in this field. The Key Laboratory of ultra-fine material preparation and application of East China University of technology and Shanghai chlor alkali Co., Ltd. jointly undertook the successful development and promotion of “nano silica gas phase combustion preparation technology and equipment development”, which will form an output value of 1 billion yuan and effectively promote the development of organic silicon and other industries in China. In February 2004, China Bluestar (Group) Corporation and Cabot Corporation of the United States signed a contract. The two sides will invest about US $30 million to build Cabot Bluestar (Jiangxi) Chemical Co., Ltd., China’s largest and world-class gas-phase silica production plant. It is expected to be completed in 2005 with an annual capacity of 5000t. 1.2 application of fumed silica fumed silica is widely used in silicone rubber, cable materials and unsaturated polyester resin, adhesives, paint coatings, inks and copier toner, food and cosmetics. It can strengthen, thicken, resist caking, and control the rheology and thixotropy of the system.
1.2.1 application of fumed silica in silicone rubber is widely used in room temperature vulcanized silicone rubber and high temperature vulcanized silicone rubber. They are often dispersed in the matrix in the form of agglomerates to form a three-dimensional network structure, with large contact surface with silicone rubber base material and many cross-linking points formed during vulcanization, so as to thicken and strengthen silicone rubber. The three-dimensional network structure formed by fumed silica is relatively stable and in an “elastic” state. Under the action of external force, it will be temporarily damaged, reduce the viscosity of the system and show good fluidity; When the external shear force is removed, the three-dimensional network structure will quickly return to the state before the force, so that the system has good thixotropy. In addition, because the particle size of fumed silica is very small and the particles are spherical, they are dispersed in the base material to form a uniform system, which often has good optical properties and can produce white and transparent silicone rubber products. The room temperature vulcanized acid silicone adhesive produced with fumed silica as filler has a wide consistency range and effective vulcanization performance. It has excellent bonding ability to all kinds of substrates coated with primer or not, and has excellent storage stability at room temperature. It is 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. In addition to adding traditional fillers, a small amount of fumed silica will produce obvious reinforcing effect during plastic mixing, greatly improve the hardness and mechanical properties of materials, so as to improve the processing technology and the performance of products. Adding a small amount of fumed silica to unsaturated polyester resin can give the resin excellent transparency and excellent physical properties, which are helpful to improve the quality of downstream products. Fumed silica plays an irreplaceable role in industrial development, but its wider application is often limited due to its high price. For example, precipitated silica is still widely used in rubber industry.
2 precipitation method precipitation method is to react water glass with sulfuric acid or hydrochloric acid to produce silicic acid, and then decompose to prepare silica. SiO2 content is about 90% and there is a large market demand. It is mainly used as rubber reinforcing filler. China’s white carbon black production plants mainly use this method to produce white carbon black. The production technology and equipment of precipitated silica are simple, the product activity is not high, the particles are difficult to control, the affinity is poor, the reinforcement performance is low, and the hydrophilic group bonding on the particle surface is serious, which weakens the binding force of the product. The production of ultrafine silica by secondary crystallization is an improved technology of secondary seed treatment on the premise of precipitation production technology. By adopting the new secondary crystallization process, the industrial production can be fully controlled automatically. The SiO2 content is more than 94%, the specific surface area is 269 ~ 320m ^ 2 / g, the maximum particle size is 1000 mesh, and the maximum particle size can reach nanometer level. The mass production of high dispersion, non gel silica, Shanghai long Chen Fine Chemical Co., marks the completion of the largest production base of high dispersion and gel free silica in China. The total investment of the project is 170 million yuan. The production target of the first phase is to produce 45000 tons per year. After all, it can produce 100000 tons per year, exceeding the output of Japan’s largest white carbon black production enterprise. It has become a large-scale fine chemical production base with leading technology and the largest scale in Asia. New products have all independent intellectual property rights. Precipitated silica is widely used as filling and reinforcing agent of rubber and plastics, additive of synthetic resin (polyester resin and elastic polyurethane), opening agent of polypropylene and non-toxic PVC plastic film, insulation filler in electronic and electrical industry, etc.
3 dissociation method
3.1 nonmetallic ore method the raw materials used to prepare silica from nonmetallic ore include diatomite, proteinite, serpentine, bentonite, kaolin, wollastonite, quartz sand, sepiolite, attapulgite, fly ash, zircon, coal gangue, yellow phosphorus ore, etc. The preparation of white carbon black from non-metallic ore is technically feasible and has good economic benefits. It provides a new way for deep processing and comprehensive utilization of non-metallic ore. Silica can also be prepared by calcination and conversion of clay minerals. The Third Military Medical University has successfully studied the pollution-free advanced technology of producing silica and polyaluminum with diatomite. Zhejiang Guangke Chemical Co., Ltd. and Jilin Linjiang yezhu Chemical Co., Ltd. produce silica with diatomite. If water glass is made from non-metallic ore first, and then white carbon black is made from water glass, the technology is still precipitation method.
3.2 the white carbon black produced from rice husk and husk ash by gramineous plant method is between precipitation method and gas phase method. Its cost is not only much lower than that of gas phase method, but also lower than that of precipitation method; Its quality is much higher than that of precipitation method and close to that of gas phase method. Yibin Wuliangye Group Fine Chemical Co., Ltd. is the only manufacturer of white carbon black (silica) produced by household plants in China, with an annual output of 4000t of white carbon black. If rice husk and husk ash are used as raw materials, sodium silicate is obtained after alkali leaching, sodium silicate reacts with acid to obtain precipitate, and silica is obtained after filtration, water washing and drying, this technology is still precipitation method.
3.3 by product recovery methods include: preparing white carbon black from yellow phosphorus slag; Preparation of silica from sodium fluosilicate; White carbon black can be prepared from coal ash; Silica was prepared by one-step hydrolysis of silicon tetrafluoride, a by-product of phosphate fertilizer plant; Preparation of white carbon black from SiO2, a by-product of NaF production; White carbon black was prepared from the waste residue of water purifying agent; Silica was prepared from waste silica sol.
