A SMALL-ANGLE X-RAY-SCATTERING STUDY ON HIGH PH SILICA PRECIPITATIONS

Silica precipitations were carried out at high pH (7-10) and elevated temperatures (60-90 degrees C) by means of simultaneous dosing of dilu ted water glass and sulphuric acid into a stirred thermostatted reacti on vessel. In order to investigate the development of the vulnerable s ilica structures during the processes, every 5-10 min small samples we re taken from the reaction mixture and analysed using small angle X-ra y scattering. It was found that a narrow particle size distribution wa s maintained throughout the entire preparation procedure (or at least from about 1 h until the end of the process), despite the fact that th e primary silica particles were growing continuously. In addition, a c ontinuous decrease in the total number of primary particles was observ ed, indicating that the primary particles were involved in clustering and aging processes, and that (except for the initial reaction stages) newly dosed silica does not form new particles but is deposited onto the already existing particles. Ostwald ripening, which is regarded as the most important aging mechanism, probably proceeds relatively rapi dly under the applied process conditions, leading to a persistent high degree of monodispersity.