Crystalline silicic acid, its salts, and processes for their preparation

Abstract

Crystalline sodium silicates having a sheet structure and an ion exchange capacity of 83-130 mmol of Na.sup.+ /mol of SiO.sub.2 are prepared by a method in which an aqueous reaction mixture which contains sodium silicate, has a molar ratio SiO.sub.2 /Na.sub.2 O of 3.9:1 to 15:1 and a molar ratio H.sub.2 O/(Na.sub.2 O+SiO.sub.2) of 3:1 to 80:1 and contains 0.01 to 30% by weight, relative to the amount of SiO.sub.2 in the reaction mixture employed, of seed crystals of the desired crystalline sodium silicate is prepared, the mixture is heated to temperatures of 160.degree. to 250.degree. C., the reaction is carried out at least until, in the X-ray diffraction pattern of a sample filtered off under suction and dried at 120.degree. C., the ratio of the intensity of the reflection at the interplanar spacing d.sub.1 =(20.+-.2).multidot.10.sup.-8 cm to the intensity of any reflection present at the interplanar spacing d.sub.2 =(15.5.+-.1.5).multidot.10.sup.-8 cm is at least 3:1, and the reaction is terminated before the intensity of the reflection at the interplanar spacing d.sub.3 =(3.34.+-.0.04).multidot.10.sup.-8 cm reaches or exceeds the intensity of the reflection at the interplanar spacing d.sub.4 =(3.44.+-.0.04).multidot.10.sup.-8 cm. A crystalline silicic acid having a sheet structure and the overall composition H.sub.2 Si.sub.x O.sub.2x+1, wherein 15<x<24, and its sodium salt are described.