An apparatus for transporting particles, comprising: an endless conveyor belt having a plurality of perforations along its length, the conveyor belt being configured to transport the particles along a transport direction while they are aspirated to said perforations, wherein the transport device further comprises: an elongate vacuum box extending along the transport direction, the vacuum box being open along one longitudinal side and being connectable to a source of negative pressure so as to apply a negative pressure to the vacuum box; and an elongate slider that covers said longitudinal side of the vacuum box, the slider being provided with a plurality of suction openings (351, 352), the suction openings (351, 352) defining a free cross section that varies along the transport direction, wherein the first conveyor belt is slidably guided on said slider along the transport direction, so that the negative pressure in the vacuum box causes a negative pressure at said perforations of the conveyor belt, the negative pressure at the perforations being modulated along the transport direction according to the free cross section defined by the suction openings (351, 352) of the slider, wherein the slider defines a longitudinal groove that extends along the transport direction, the longitudinal groove creating a clearance space between the conveyor belt and a slider portion in which the suction openings (351, 352) are provided characterized in that the apparatus further comprises a barrier member that is arranged on the slider to cover selected suction openings (351, 352) of the slider, and wherein the barrier member is arranged in the longitudinal groove of the slider. an endless conveyor belt having a plurality of perforations along its length, the conveyor belt being configured to transport the particles along a transport direction while they are aspirated to said perforations, wherein the transport device further comprises: an elongate vacuum box extending along the transport direction, the vacuum box being open along one longitudinal side and being connectable to a source of negative pressure so as to apply a negative pressure to the vacuum box; and an elongate slider that covers said longitudinal side of the vacuum box, the slider being provided with a plurality of suction openings (351, 352), the suction openings (351, 352) defining a free cross section that varies along the transport direction, wherein the first conveyor belt is slidably guided on said slider along the transport direction, so that the negative pressure in the vacuum box causes a negative pressure at said perforations of the conveyor belt, the negative pressure at the perforations being modulated along the transport direction according to the free cross section defined by the suction openings (351, 352) of the slider, wherein the slider defines a longitudinal groove that extends along the transport direction, the longitudinal groove creating a clearance space between the conveyor belt and a slider portion in which the suction openings (351, 352) are provided characterized in that the apparatus further comprises a barrier member that is arranged on the slider to cover selected suction openings (351, 352) of the slider, and wherein the barrier member is arranged in the longitudinal groove of the slider. an elongate vacuum box extending along the transport direction, the vacuum box being open along one longitudinal side and being connectable to a source of negative pressure so as to apply a negative pressure to the vacuum box; and an elongate slider that covers said longitudinal side of the vacuum box, the slider being provided with a plurality of suction openings (351, 352), the suction openings (351, 352) defining a free cross section that varies along the transport direction, wherein the first conveyor belt is slidably guided on said slider along the transport direction, so that the negative pressure in the vacuum box causes a negative pressure at said perforations of the conveyor belt, the negative pressure at the perforations being modulated along the transport direction according to the free cross section defined by the suction openings (351, 352) of the slider, wherein the slider defines a longitudinal groove that extends along the transport direction, the longitudinal groove creating a clearance space between the conveyor belt and a slider portion in which the suction openings (351, 352) are provided characterized in that the apparatus further comprises a barrier member that is arranged on the slider to cover selected suction openings (351, 352) of the slider, and wherein the barrier member is arranged in the longitudinal groove of the slider.