with a conveyor duct 23 with the intermediary of a fiber tuft driving fan 22 with the conveyor duct 23 there is connected a tubular switch deflector tube 24 which is connected with the electric switching device 20 arranged as shown in fig1 when the evaluating device 19 determines the presence of a foreign body 39 in the fiber material 13 the switching device 20 emits a signal which is applied to the non illustrated driving device for the tubular switch 24 which in response pivots in the direction of the arrow a into the position 24a shown in phantom lines in this manner the fiber material containing the foreign bodies 39 is caused to follow a detour subsequently the tubular switch 24 is pivoted back into its normal solid line position in the direction of the arrow b to ensure that the fiber material now free from foreign bodies may follow its normal path to further processing turning now to fig3 above the downstream end of the upper reach 10a of the conveyor belt 10 there is provided a suction device 26 which by means of the blower 28 lifts the fiber tufts off the conveyor belt 10 through the suction nozzle 26a and normally drives the fiber material into the duct 26b for further processing for this operation a pivotal gate 29 is in its closed position shown in phantom lines at 29a when.