Process for producing crystalline sodium silicates having a layered structure

A process for producing crystalline sodium silicates having a layered structure, an SiO.sub.2 /Na.sub.2 O molar ratio of (1.9 to 2.1):1 and a water content of less than 0.3% by weight from a waterglass solution containing at least 20% by weight of solids, which comprises

• a) obtaining the waterglass solution by reacting quartz sand with sodium hydroxide solution in an SiO.sub.2 /Na.sub.2 O molar ratio of (2.0 to 2.3):1 at temperatures from 180.degree. to 240.degree. C. and pressures from 10 to 30 bar,
• b) treating the waterglass solution in a spray-drying zone with hot air at 200.degree. to 300.degree. C. for a residence time of 10 to 25 seconds and at a temperature of the exit gas leaving the spray-drying zone of 90.degree. to 130.degree. C., to form a pulverulent amorphous sodium silicate having a water content (determined as the loss of ignition at 700.degree. C.) of 15 to 23% by weight and a bulk density of more than 300 g/l,
• c) introducing the pulverulent, amorphous, water-containing sodium silicate into an obliquely arranged rotary kiln fitted with devices for moving solids and being insulated in such a way that its outside wall temperature is less than 60.degree. C., and treating said water-containing sodium silicate in said rotary kiln with flue gas in counter-current at temperatures from more than 500.degree. to 850.degree. C. for 1 to 60 minutes to form crystalline sodium silicate,
• d) exhausting the exit gas from the rotary kiln in the central region thereof and in the region of the end where the pulverulent amorphous sodium silicate having a water content of 15 to 25% by weight is introduced, purifying said exit gas by means of a dry dust filter, and admixing the sodium silicate taken from the dry dust filter to the pulverulent, amorphous, water-containing sodium silicate destined to be introduced into the rotary kiln with the resultant increase of the throughput of sodium silicate through the rotary kiln per unit of time,
• e) comminuting the crystalline sodium silicate emerging from the rotary kiln by means of a mechanical crusher to grain sizes of 0.1 to 12 mm, and
• f) grinding the comminuted crystalline sodium silicate by means of a mill to grain sizes of 2 to 400 .mu.m with the resultant formation of crystalline sodium silicates having a lime-combining capacity of more than 80 mg Ca/g Na.sub.2 Si.sub.2 O.sub.5 at 20.degree. C.