Production Process

Production process of Synthetic Amorphous Silica (or SAS manufacture)

There are two main production methods for the manufacturing of silica. Based on the method of production, different forms of SAS products with diverse functions and applications are produced:

  • The thermal route: producing the pyrogenic type of silica, commonly known also as fumed silica.
  • The wet route: producing precipitated silica, silica gel and colloidal.

Thermal route

The manufacturing process for pyrogenic SAS is based on the hydrolysis of volatile silanes, especially silicon tetrachloride (SiCl4), in the flame of an oxygen-hydrogen burner. SiCl4 is continuously vaporised, mixed with dry air and then with hydrogen, fed to the reactor and hydrolysed. Hydrolysis is followed by the growth (nucleation, condensation, coagulation) and aggregation of pyrogenic SAS particles.

The properties of pyrogenic silica products can be controlled by varying process parameters, such as feedstock, flame composition and flame temperature.

Wet route

Precipitated SAS is produced via a wet production route, which is based on a reaction of alkali metal silicate solutions with acids in water. Raw materials for the production of precipitated silica and silica gels are:

  • aqueous alkali metal silicate solutions (e.g. waterglass)
  • aqueous acids, (e.g. sulphuric acid).

An alkali metal silicate solution reacts with acid to generate a silica sol (very small particles suspended in water) that can be stabilised as such (colloidal silica), forced to precipitate (precipitated silica), or allowed to form a gel (silica gel) – depending on the reaction and process conditions. In all cases, the main by-product, an alkali salt of the acid used, is removed. The level of salt retained in the SAS depends on the intended final application.

In the case of both precipitated and silica gel, the sol particles form strongly bound aggregates. By its own nature, gel forms bigger agglomerates, and depending on the process, form agglomerates through weaker interactions. As highly porous materials, the water content is very high, and depending on how the water is dried (or not) makes different product families and modifies the agglomerates further.

Surface chemistry:

Synthetic Amorphous Silica, produced by wet or thermal route, has silanol (Si-OH) groups on the surface that render untreated SAS hydrophilic. SAS may be rendered hydrophobic by surface treatment. The two main categories of surface-treated SAS are:

  • Alkylsilyl treated silica: SAS is treated with silanes and siloxanes, resulting in unreactive alkyl groups at the surface to bring hydrophobic properties.
  • Functionalised alkylsilyl treated silica: functional organic groups are grafted onto SAS surface to improve the cross-linking between the inorganic filler and the organic matrix in a polymer, rubber or coating.