Investigations on the semi-batch precipitation of silica.

The semi-batch precipitation process of silica from sodium silicate and sulphuric acid has been structured into a primary particle production stage, an aggregation process until the gel point is reached and finally a gel fragmentation and compaction process. The progress of aggregation was analyzed by photon correlation spectroscopy. The reaction limited aggregation kernel was found to be proportional to B'(t)(q(p - q))A/2, where A assumes the value of one. The quantity B'(t) varies between 3.4 x 10(-20) and 2.6 x 10(-20) 1/min and is only a weak function of time. The stability ratio W for the aggregation process decreases with increasing aggregate size. During the period after the gel point the gel fragmentates. The gel clusters have an initial fractal dimension of approximately 1.8 which rapidly increases to 2.4 and stays roughly constant until the end of the process. The specific surface area decreases from 480 to 300 m2/g. The primary particles have a uniform size of 22.7 nm.