A process for preparing alpha-alumina

A process for the production of alumina with a proportion of alpha-alumina of at least 98 wt .-%, which is present largely in the form of isolated particles with an average diameter of 1 to 50 microns, in which a granulate which - substantially comprises aggregated primary particles of one or more transition aluminas, each having an average primary particle diameter of 5 to 50 nm, - an average diameter of 500 to 5000 microns and - a tamped density of at least 250 g / l having, containing at 800 ° C to 1200 ° C in an at least 70 vol .-% of hydrogen chloride gas, or at least 70 vol .-% of chlorine gas atmosphere over a period of 0.5 to 5 hours treated.