GB1228215A - Screening device - Google Patents

Abstract

1,228,215. Sifting apparatus. R. SPONZA. 18 April, 1968 [19 April, 1967; 18 Sept., 1967], No. 18331/68. Heading B2H. An apparatus for screening pulverulent material, Fig. 1 comprises a screen 1 of surface of revolution configuration, at least one pneumatic brush 22 located outside the screen 1 to blow air therethrough and an impeller 12 arranged to rotate inside the screen, the brush and the impeller each exerting a force component in a direction parallel to the axis of the impeller to urge the material being screened along the screen as well as a radial force component. The screen 1 is rotatably mounted at the end of a hollow driving shaft 2 through which the impeller driving shaft 8 extends. The screen is enclosed in a chamber 13 which has a lower hopper portion 15. The material to be screened is fed at 19 and together with air sucked through an inlet 18 by the impeller 12 enters through slots 16 into the interior of the shaft 2 whence it passes into the interior of the screen 1 to be centrifugally flung by the impeller 12 with the air against the screen 1. Air is also sucked in by the impeller 12 through an inlet 30. At the same time, the material is caused to travel along the screen by axial force components 21 obtained by giving to the angle # of the outlet edge of the blade, Fig. 3, a value which is not constant over the entire outlet edge of the blade. A similar result may be obtained by modifying the angle of the root of the blade instead of the angle of the outlet edge. Alternatively, the outlet edges or roots of the impeller blades may be inclined relative to the impeller axis, Fig. 2. Alternatively again, the axial force component is produced by the outlet edges or roots of the blades being of variable radius along the length of the impeller. An axial force component is also provided by jets of gaseous fluid issuing from slots 23 in a tube 22 connected at 22a to a pressure-fluid, source, the jets, depending on their orientation, also serving to clear the screen apertures and to cause the material to be raised inwardly of the screen to facilitate its movement axially relative to the screen. The jets of fluid may, alternatively, be arranged to act internally of the screen or between the screen and the impeller or may pass inside the inner periphery of a hollow impeller. The axial force component may further be provided by giving to the sieve a frusto-conical form or by causing a pressure difference across the chamber 13. To facilitate movement of the material the screen may also be inclined to the horizontal. The coarse unscreened fraction is discharged from the screen through a pipe 20 while the fine screened fraction falls into the hopper 15. The impeller may, as shown, be formed of two sections welded to a transverse disc 25 which prevents entrainment of the fine fraction by the coarse fraction. The screen 1, Fig. 4, comprises a frame 1a-1d on which are releasably mounted sifting panels 1e to which the screen mesh 1f is fixed. One of the panels may, instead of the screen mesh, be provided with a grinding surface to impart grinding action on to the material being sifted. The portion of the screen surface against which the entering material is flung by centrifugal force may be non-perforated and may consist of sections spaced apart by gaps for the passage of the fine fraction. In another embodiment, the screen is non- rotatable so that the hollow driving shaft 2 and the seal 29 between the screen 1 and a wall of the chamber 13 Fig. 1 are omitted. The screen in this case may be vibrated. In a further embodiment, Fig. 5, the material and the air are sucked by the impeller 60 through ducts 61, 62 and impinge on a disc 25 whereby they are caused to reverse their axial direction and are centrifuged as in Fig. 1. The pneumatic brush is omitted from Fig. 5 for convenience. In a still further embodiment, Fig. 6 (not shown), instead of a mechanical seal 29 as in Fig. 1, a pneumatic seal is provided between an annular member secured to the screen and a fixed member secured to the walls of the chamber 13. Another type of pneumatic seal is described with reference to Figs. 7 and 8 (not shown).