Silica sol exists in the form of liquid, which is a dispersion system of silica particles (dispersed phase) with a diameter of 3-150nm in water or organic liquid (dispersion medium), and the morphology of the particles is mostly spherical.
The processes for preparing silica sol include: the method of treating a dilute sodium silicate solution with ion exchange resin; the method of hydrolyzing elemental silicon; the method of neutralizing a dilute water glass solution with sulfuric acid; the method of hydrolyzing silicate esters, etc.
Fumed silica exists in the form of powder. Fumed silica is a loose aggregate of silica particles with a primary particle size of 10-20nm (the aggregate particle size is mostly above 100nm, and the morphology of the aggregate is dendritic or snowflake-like).
The main method for preparing fumed silica is the combustion method. Its raw materials are generally silicon tetrachloride, oxygen (or air) and hydrogen, which react at high temperature.
The air and hydrogen are respectively pressurized, separated, cooled and dehydrated, dried on silica gel, and filtered for dust removal before being sent to the synthetic hydrolysis furnace. After the silicon tetrachloride raw material is sent to the distillation tower for distillation, it is heated and evaporated in the evaporator, and sent to the synthetic hydrolysis furnace with dry and filtered air as the carrier. After silicon tetrachloride is gasified at high temperature (flame temperature 1000~1800℃), it is hydrolyzed in the gas phase with a certain amount of hydrogen and oxygen (or air) at a high temperature of about 1800℃; the gas phase silica particles generated at this time are extremely fine, forming aerosols with the gas, which are not easy to capture, so it is first aggregated into larger particles in the aggregator, and then collected by the cyclone separator, and then sent to the deacidification furnace, and the gas phase silica is purged with nitrogen-containing air to a pH value of 4-6 to obtain the finished product.
Precipitated silica, commonly known as white carbon black, also exists in the form of powder. The particle size of precipitated silica is a solid powder of submicron to micron level, and the morphology of the particles is irregular solid particles.
The general preparation process of precipitated silica is to use industrial water glass as raw material, prepare a dilute solution of a certain concentration with water, then add a certain acid (sulfuric acid or hydrochloric acid) under certain conditions to precipitate the silica, and then wash, filter, dry, and crush to obtain the product white carbon black. White carbon black can be used as a reinforcing agent for rubber, a filler for plastic products, a matting agent for coatings, a friction agent for toothpaste, etc.