Mechanical fragmentation of precipitated silica aggregates

Abstract

A common inorganic process for the production of amorphous silica involves acidifying a sodium silicate solution in a semi-batch reaction. Even after gelation, stirring and addition of reactants are continued. The gel is fragmented and the flocs densify until a suspension is obtained. Control of size and structure of the resulting aggregates is desirable as these strongly influence product properties. The aim of this study is to understand the destruction of the gel by mechanical energy input and to thus contribute to controlling the size of the fragments. Beside the energy input, the size of the gel fragments strongly depends on the gel's mechanical properties, which in turn depend on its age, solids content, pH, salt content and temperature. The influence of these parameters was investigated for a batch process in a continuously stirred tank and a Taylor–Couette reactor and by ultrasound dispersion. It can be shown that, in terms of breakage mechanics, the gel observes the same laws as a highly viscous liquid.