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. Guangzhou Jibisheng Technology Industry Co., Ltd. is the supplier of fumed silica with the largest output, the most complete brand and the most advanced technology in China, and is the drafting unit of the national standard GB20020-2005 for fumed silica.
(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. 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 mainly introduces the technology of preparing 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 2H2O-> Al2O3 2Si02+2H2O
Acid leaching: Al2O3 2si02+6HCl+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.