The invention relates to a process and the respective installation for thermal treatment of granular solids, in particular for producing aluminum oxide from aluminum hydroxide, wherein the solids are heated in at least one preheating stage and then reacted in a reactor at 700 to 1400 C. In at least one preheating stage, the average temperature gradient of the solids amounts to <15K/s and the dwell time of the solids amounts to 15 s.