Process for the preparation of precipitated silicas having morphology similar to pyrogenic silicas

Claim 1

A process for the preparation of a precipitated silica having an ignition weight loss of at least 3% between 120° C and 1000° C and a strength such that a sample of the silica having a weight mean particle size on a substantially unimodal distribution in the size range 10-20 microns requires less than 6500 joules of ultrasonic energy to reduce the weight mean particle size of the sample to below 3 microns and which silica, after fluid energy milling, has a performance index of at least 7, said process comprising the steps of: (a) reacting an aqueous alkali metal silicate and an acid to form primary silica particles by simultaneously adding to water, under mixing conditions, at a temperature between 85° C and the boiling point of the reaction mixture, the silicate and the acid to a degree of silicate neutralization and a concentration of SiO 2 as indicated in Area A in the drawing, (b) flocculating the primary silica particles produced by the addition of an effective amount of an alkali metal salt flocculant, (c) precipitating secondary silica in the presence of the salt by the addition of more acid or more silicate and acid to obtain a final mixture, (d) reducing the pH of the final mixture to below 7, and (e) isolating the silica so produced and spray drying it at a temperature such that the temperature of the silica never exceeds 450° C.