Silica is the general name of white powdery X-ray amorphous silicic acid and silicate products, which mainly refers to precipitated silica, fumed silica, ultrafine silica gel and aerogel, and also includes powdery synthetic aluminum silicate and calcium silicate. Silicon dioxide is a porous substance, and its composition can be composed of SiO2? NH2O means that NH2O exists in the form of surface hydroxyl groups. Soluble in caustic soda and hydrofluoric acid, insoluble in water, solvents and acids (except hydrofluoric acid). High temperature resistance, non-flammable, tasteless and odorless, with good electrical insulation.
According to the production method, silica can be roughly divided into precipitated silica and fumed silica. Under normal circumstances, fumed silica is white amorphous flocculent translucent solid colloidal nanoparticles (particle size less than 100nm), which is non-toxic and has a huge specific surface area (100 ~ 400 m2/g). The fumed silica is nano-silica, the purity of the product can reach 99%, and the particle size can reach 10~20nm, but the preparation process is complex and expensive. Precipitated silica can be divided into traditional precipitated silica and special precipitated silica. The former refers to silicon dioxide produced with sulfuric acid, hydrochloric acid, CO2 and water glass as basic raw materials, while the latter refers to silicon dioxide produced by special methods such as high gravity technology, sol-gel method, chemical crystallization method, secondary crystallization method or reverse micelle microemulsion method.
Precipitated silica is mainly used as a reinforcing agent for natural rubber and synthetic rubber, a toothpaste friction agent, etc. Fumed silica is mainly used as reinforcing agent of silicone rubber, coating and thickening agent of unsaturated resin, while ultrafine silica gel and aerogel are mainly used as coating matting agent, thickening agent and opening agent of plastic film.
2. Production method
The traditional method of preparing silica is to use sodium silicate, silicon tetrachloride and ethyl orthosilicate as silicon sources. Except for sodium silicate, other costs are high. The new method uses cheap nonmetallic ore as silicon source, which greatly reduces the production cost of silicon dioxide.
2. 1 traditional method
(1) gas phase method
Mainly chemical vapor deposition (CAV) method, also known as pyrolysis, dry method or combustion method. Its raw materials are generally silicon tetrachloride, oxygen (or air) and hydrogen, which react at high temperature. The reaction formula is:
Sicl4+2h2+O2-> silica+hydrochloric acid
Air and hydrogen are respectively pressurized, separated, cooled, dehydrated, dried by silica gel, dedusted and filtered, and then sent to a synthetic hydrolysis furnace. The raw material silicon tetrachloride is sent to a rectification tower for rectification, then heated and evaporated in an evaporator, and then sent to a synthetic hydrolysis furnace with dried and filtered air as a carrier. Silicon tetrachloride is gasified at a high temperature (flame temperature 1000~ 1800℃), and then hydrolyzed with a certain amount of hydrogen and oxygen (or air) at a high temperature of 1800℃. At this time, the generated fumed silica particles are extremely fine, and form aerosol with gas, which is difficult to capture. Therefore, they are first aggregated into larger particles in the concentrator, then collected by the cyclone separator, and then sent to the deacidification furnace, and the gas-phase silica is purged with nitrogen-containing air until the PH value is 4 ~ 6, which is the finished product.
Degussa and cabot are leading the world in gas phase production technology. Their production facilities are large in scale, highly automated, with low product cost, many brands (especially functional special products used in special fields) and good quality, such as uniform surface area distribution and low water content. China's Shenyang Chemical Co., Ltd. and Shanghai Chlor-Alkali Chemical Co., Ltd. also use gas phase production, but they are far behind foreign big companies in terms of production scale, production technology, automation degree and product brand.
(2) precipitation method
Precipitation method, also known as sodium silicate acidification method, uses sodium silicate solution to react with acid, and after precipitation, filtration, washing, drying and calcination, white carbon black is obtained. The reaction formula is:
Sodium silicate+2h+-> Silica+2na+H2O
Most domestic production enterprises adopt precipitation method.
2.2 new methods
The new method mainly uses nonmetallic minerals and their extensions as silicon sources, and uses precipitation method to prepare white carbon black. The key technology is to convert crystalline silica and silicate into amorphous silica. The main raw materials are wollastonite, opal, halloysite, olivine, ophiolite, kaolin, hard kaolin, coal gangue and fly ash.
This paper focuses on the preparation technology of white carbon black from kaolin or hard kaolin, coal gangue or fly ash.
(1) takes kaolin or hard kaolin as raw materials.
Firstly, kaolin or hard kaolin is crushed to 50-60 meshes, then calcined at 500-600℃ for 2 hours, then the calcined soil is mixed with industrial hydrochloric acid with a concentration of 30% at a ratio of 1: 2.5 (by weight), then acid-leached at about 90℃ for 7 hours, and then neutralized, filtered, washed and dried to obtain white carbon black. The product quality conforms to GB 65438+. At the same time, the efficient water purifying agent polyaluminum chloride is obtained. The reaction formula of roasting and acid leaching is as follows:
Roasting: Al2O3? 2Si02？ 2h 20-->Al2O3？ Silicon dioxide +2h2o
Acid leaching: Al2O3? 2sio 2+6h cl+9h2o-->2AlCl3？ 6H20 + 2SiO2
(2) taking coal gangue or fly ash as raw materials
Firstly, coal gangue or fly ash is crushed to a particle size less than 120 mesh, and then it is divided into two steps:
The first step is to produce sodium silicate: the pulverized coal gangue or fly ash and soda ash are evenly mixed according to the weight ratio of 1:50, then melted at high temperature (1, 400 ~ 1, 500℃, 1 hour), and extracted and dissolved with water (1, 000).
The second step is to produce white carbon black: firstly, sodium silicate is prepared into a water glass solution (modulus 2.4 ~ 3.6, SiO2 content 4~ 10%), then it is leached in 5~20% sulfuric acid (28~32℃, 8~ 16 hours), then the temperature is raised to 80℃, and the PH value is adjusted by stirring. Silica has high activity and purity.
3. Capacity/output
In China, precipitated silica is the main product, and 1958 has realized the industrial production of precipitated silica, but the output is small and the development is slow. The output of 1987 is only 2687 tons; 1.990 exceeded 10,000 tons, reaching 1.600 tons; 1994 increased to 28 1000 tons, 9.4 times higher than 1987; From 65438 to 0997, the output of silica in China reached about 80,000 tons. In 2004, it rose to nearly 300,000 tons. It can be seen that the output of white carbon black in China has advanced by leaps and bounds in recent ten years.
The production and sales of silica in China are relatively extensive. There are more than a dozen provinces and cities that produce white carbon black, among which Fujian, Shandong, Jiangsu, Jiangxi and other provinces have large output. At present, there are dozens of silica production enterprises in China with a total production capacity of about 400,000 tons/year. Among them, large precipitated silica producers include Fujian Nanping Jia Lian Chemical Co., Ltd., Shanghai Chen Jiu Fine Chemical Co., Ltd., Zhuzhou Xinglong Chemical Co., Ltd., Rhodia Silica (Qingdao) Co., Ltd., Wuxi Hengheng Silica Co., Ltd., Tengzhou Xinxu Chemical Co., Ltd. and so on. Large fumed silica producers include Shenyang Chemical Industry Co., Ltd. and Guangzhou Jibishi Technology Industry Co., Ltd. ..
Due to the optimistic market prospect of silica, some domestic enterprises are currently building or planning to build silica projects, including reconstruction and expansion, as well as new construction. Such as Shanghai Chen Jiu Fine Chemical Co., Ltd., Puyang Guangpu Petrochemical Company, Hubei Xingfa Group, etc.
It is estimated that the output of silica in China will reach about 350,000 tons in 2005, and the output of silica in China is expected to exceed 500,000 tons/year in 2006.