the axis 32 of drum 18 is inclined the angle of inclination of the axis 32 is in the range of 5 to 15 degrees and ascends from the inlet opening 28 to the outlet opening 30 a feeding hopper 34 is mounted for vibration on swinging means 36 supported by vertical supports 38 a vibrator 40 is in engagement with the feeding hopper 34 a feeding tube 42 is associated with the feeding hopper 34 and extends with downward inclination through the inlet opening 28 into the interior of the drum 18 the drum 18 has a diameter of about 2 3 meter and a length of 4 to 5 meters it is important that the outlet end 44 of the feeding tube 42 has a substantial axial distance from the inlet opening 28 this distance is in the range of 15 to 40 of the overall length of the drum 18 and should be not less than 1 0 meter in the prefered embodiment this distance is 1 5 meters at the inside surface of the peripheral cylindrical wall 46 of the drum 18 a pair of helical webs 48 50 are fastened forming a conveyor the webs have a constant height of 5 at most of.