The invention relates to a grader which employs an air separation principle. The grader includes a rotor rotatably supported in a housing. The rotor includes a pair of rotor disks on its outer boundaries extending radially substantially to the housing. A plurality of blades are disposed on the rotor between the rotor disks. A circumferential channel is formed by the rotor disks, the housing and the plurality of blades. The bladed rotor functions as a vertical sink in order to grade a solid which is fed into the circumferential channel via a tangential passage.