Abstract: Contaminated portions of bulk material are removed from a flow of bulk material with the aid of a flow or drop pipe in a casing that is movable back and forth between a discharge position and a normal flow position. The flow or drop pipe is equipped with baffle plates or bends for sufficiently retarding the free flow of the flowable bulk material to enable a detector and control mechanism to respond to the presence of contaminations and to then move the casing for the discharge of a contaminated portion of bulk material. The movement of the casing of the flow pipe assures that a slide gate or a portion of the casing closes the upper inlet to prevent the further flow of bulk material when the casing is in a discharge position. Due to the retardation of the free flow of the bulk material, it is possible to construct the apparatus with a substantially shorter structural height than was possible heretofore.