Another one is the gas-phase method, chemical vapor deposition (CAV) method, also known as pyrolysis method, dry method or combustion method. Its raw materials are generally silicon tetrachloride, oxygen (or air) and hydrogen, which react at high temperatures.
The reaction equation is: SiCl4+2H2+O2 - >SiO2+4HCl. Air and hydrogen are respectively subjected to pressurization, separation, cooling and dehydration, silica gel drying, dust removal and filtration before being sent into the synthetic hydrolysis furnace. After the raw material of silicon tetrachloride is sent to the distillation column for distillation, it is heated and evaporated in the evaporator, and then sent to the synthetic hydrolysis furnace with dry and filtered air as the carrier. Silicon tetrachloride is vaporized at high temperatures (flame temperature 1000-1800℃) and then undergoes gas-phase hydrolysis with a certain amount of hydrogen and oxygen (or air) at around 1800℃. At this point, the gas-phase silica particles generated are extremely fine and form aerosols with gases, which are difficult to capture.
Therefore, they are first aggregated into larger particles in an aggregator, then collected by a cyclone separator, and sent to a deacidification furnace. The gas-phase silica is washed with nitrogen-containing air until the PH value is 4 to 6, which is the finished product