SAS manufacture

SAS is produced by thermal route (pyrogenic/fumed) or wet route (precipitated, gel, colloidal) processes. In the initial particle formation step, primary particles with dimensions less than 100 nm are formed by nucleation, coagulation and coalescence. By covalent bonding, these particles form indivisible units, the aggregates, with external dimensions typically above 100 nm; they are fused together with no clear physical boundaries among them. These aggregates combine to form agglomerates in the micron size range by physical attraction forces (van der Waals) and hydrogen bridges. An identifying feature of SAS powder is that it is put on the market as micron-sized agglomerates with an internal structure in the nanoscale. This fact is true for all currently known SAS products in powder form, independent of manufacturer, process and trade name. Colloidal silica is placed on the market as aqueous preparations of nanoparticles.

Wet route

Precipitated SAS is produced via a wet production route, which is based on a reaction of alkali metal silicate solutions with mineral acids.

This process description includes synthetic amorphous silica, both precipitated and gel. Raw materials for the production of precipitated silica and silica gels are aqueous alkali metal silicate solutions (e.g. waterglass) and mineral acids, (e.g. sulphuric acid). An alkali metal silicate solution reacts with mineral acids to generate precipitates or gels, depending on the reaction and process conditions. The suspension generated from precipitation is filtered. Precipitated silica is washed and dried, and can be subsequently milled and granulated.

Silica gel is produced in a comparable process but not in a water suspension. The process is ending in a solid form which is grinded before washed and processed further, as described above.

Thermal route

Pyrogenic SAS is made by hydrolysis of volatile chlorosilanes or methylchlorosilanes in an air-hydrogen flame at temperature between 1200-1600 °C in a continuous operating process.

Precursor particles grow by nucleation, condensation and coagulation to yield primary particles, under the reaction conditions primary particles collide and stable non-dispersible aggregates are formed. By physical attraction forces between aggregates larger agglomerates are formed.

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