Precipitated Silica

Precipitated silica consists of a three-dimensional network of coagulated primary silica particles. The latter grow to sizes higher than 4-5 nm before they coagulate to form the aggregated clusters.

Precipitated silicas are synthesized by acidifying sodium silicate. In nearly all commercial processes, sulfuric acid is used as the acid source. Under standard conditions, the sodium silicate solution and the acid are fed simultaneously in a stirred vessel containing water. Precipitation is carried out under alkaline 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 silica. The formation of a gel stage is avoided by stirring at elevated temperatures.

In the next stage, the precipitated silica slurry is washed to remove soluble salts. The washing conditions, although important, have a negligible effect on the final product properties. Different filter types such as filter presses, rotary, or belt filters can be used. The resultant filter cake with typical solid content between 15-25% is then dried. The most common drying techniques are spray drying and rotary drying, which give rise to different particle shapes, degrees of agglomeration, and to a lesser extent porosity. The dried silica may be subjected to milling and classifying steps to obtain a specific particle size distribution.

The precipitated silicas distinguish themselves from silica gels on the basis of pore structure. Precipitates typically have a broad meso/macroporous pore structure reflected in the pore size distribution, whereas gels generally have a more narrow microporous or mesoporous structure.

All Grace Davison precipitated silicas are synthetic amorphous silica products. In contrast to naturally occurring silicas, the synthetic origin of these materials eliminates the risk of contamination with crystalline quartz particles as it may well occur in mined products. Amorphous silica, in contrast to crystalline silica such as cristobalite or quartz, does not present a potential health hazard to workers with respect to irreversible lung injuries such as lung tumors or silicosis. For further details regarding single products and environmental, health and safety (EHS) information please contact us.