The invention relates to a device for processing fibres at the drum of a flat card which comprises a main carding area, a pre-carding area and a post-carding area. According to the invention, a plurality of separation modules are provided in the post-carding area for the removal of impurities. The post-carding area comprises a carding element provided for processing the fibres once they have passed the separation modules and before they have reached the card doffers. The state of the removed fibres is essentially determined when the latter-mentioned carding element is being processed.