Precipitated silica is an amorphous form of silica (silicon dioxide, SiO2); it is a white, powdery material. Precipitated silica is produced by precipitation from a solution containing silicate salts.
The three main classes of amorphous silica are pyrogenic silica, precipitated silica and silica gel. Among them, precipitated silica has the greatest commercial significance. In 1999, more than one million tons were produced, half of it is used in tires and shoe soles.[1]
Like pyrogenic silica, precipitated silica is essentially not microporous (unless prepared by the Stöber process).
The production of precipitated silica starts with the reaction of a basic silicate solution with a mineral acid. Sulfuric acid and sodium silicate solutions are added simultaneously with agitation to water. Precipitation is carried out under acidic or basic conditions. The choice of agitation, duration of precipitation, the addition rate of reactants, their temperature and concentration and pH can vary the properties of the resulting silica. The formation of a gel stage is avoided by stirring at elevated temperatures. The resulting white precipitate is filtered, washed and dried in the manufacturing process in order to wash out the produced salts.[2]
Na2(SiO2)7 + H2SO4 + O → 7 SiO2 + Na2SO4 + H2O
Na2SiO3 + H2SO4 → SiO2 + Na2SO4 + H2O
The particles are porous. Primary structures typically have a diameter of 5 - 100 nm, and specific surface area 5–100 m2/g. Agglomerate size is 1 - 40 μm with average pore size is > 30 nm. Density: 1.9 - 2.1 g/cm3.