Process for the preparation of a supported catalyst for the polymerization of .alpha.-olefins

A process for the preparation of a supported catalyst for the polymerization of .alpha.-olefins, in which

• (1) a silica-containing carrier gel is first prepared by a method in which
• (1.1) a sodium or potassium waterglass solution is introduced into a stream of an aqueous mineral acid which is subjected to angular momentum, longitudinally and tangentially to the stream, the resulting silica hydrosol is sprayed in the form of drops into a gaseous medium and is allowed to solidify to a hydrogel, and the resulting hydrogel is freed from salts by washing without prior aging,
• (1.2) if necessary, not more than 30% by weight of the water present in the hydrogel is extracted from the hydrogel (1.1) with the aid of at least one C.sub.1 -C.sub.4 -alcohol or C.sub.3 -C.sub.5 -ketone,
• (1.3) the resulting hydrogel (1.1) or (1.2) is dried, carrier gel formation taking place, and
• (1.4) the resulting carrier gel (1.3) is milled and is fractionated according to particle size,
• (2) the carrier gel (1) is laden with chromium trioxide or with a chromium compound which can be converted into chromium trioxide under the conditions of process step (3), resulting in a chromium-containing carrier gel, and
• (3) the chromium-containing carrier gel (2) is heated at from 400.degree. to 1100.degree. C. for from 10 to 1,000 minutes in an anhydrous gas stream containing oxygen in a concentration of more than 10% by volume,
• wherein the hydrogel resulting from process step (1.1) or (1.2) is dried in process step (1.3) in the course of not more than 300 seconds in a fast dryer at an inlet temperature of from 200.degree. to 600.degree. C.