An interior rear-view mirror for cars is so designed that a portable GPS device may be readily mounted on. The outer disks include a number of flat outer disks and a number of corrugated outer disks. Floating gates extend between the protruding portions of adjacent insulators, and are in contact with the protruding portions of the adjacent insulators. For influencing the dwell time, the material to be treated is supplied to at least two processing regions one after the other and flows through these regions, wherein the material transport direction is a cross flow relative to the flow of the fluidization means.