Process for the production of crystalline sodium disilicate in an externally heated rotary kiln having temperature zones

A process for the preparation of crystalline sodium disilicates having a laminar structure, a molar ratio of SiO.sub.2 to Na.sub.2 O of (1.9 to 2.1) :

• 1 and a water content of less than 0.3% by weight, which comprises:
• reacting sand with sodium hydroxide solution in a molar ratio of SiO.sub.2 to Na.sub.2 O of (2.0 to 2.3):
• 1 at temperature of 180.degree. to 240.degree. C. and pressures of 10 to 30 bar with formation of a water glass solution having at least 20% by weight of solids;
• spray-drying the water glass solution with hot air at 200.degree. to 300.degree. C. with the formation of a pulverulent amorphous sodium disilicate having a water content (determined at loss on heating at 700.degree. C.) of 15 to 23% by weight and a bulk density of more than 300 g/l;
• grinding the spray-dried pulverulent amorphous sodium disilicate;
• introducing the ground sodium disilicate into a rotary kiln equipped with devices for moving solids, externally heated via the wall and having at least four different temperature zones in its interior, wherein in the first temperature zone adjacent to the entry opening 200.degree. to 500.degree. C. is maintained, in the second temperature zone 400.degree. to 600.degree. C. is maintained, in the third temperature zone 550.degree. to 700.degree. C. is maintained and in the fourth temperature zone adjacent to the discharge opening 650.degree. to 800.degree. C. is maintained;
• and treating the ground sodium disilicate in said externally heated rotary kiln for 1 to 60 minutes with formation of crystalline sodium disilicate.