GB1029001A - A method and an apparatus for wet and dry screening - Google Patents

Abstract

1,029,001. Wet and dry screening. INSINOORITOIMISTO-ENGINEERING BUREAU R. T. KUKKI. Jan. 24,1964, No. 3185/64. Heading B2H. Material is separated into fine and coarse fractions in a drum screen which is rotated at such a supercritical speed that the outer layer of material next to the inner surface of the screen is caused to move in the same direction as such surface but at a subcritical speed, the resulting speed difference determining the maximum size of particles which can pass through a given screen opening; the fine particles pass through the screen by the combined effect of centrifugal and gravitational forces and the retained particles are discharged separately from the screen. The maximum size of the fines can be selected by choosing the correct screen speed within the speed range between the theoretical critical speed and the speed at which the solids will centrifuge. The material may be fed as a pulp from a hopper 5 and classified by partitions 21, 22 before being directed by an adjustable baffle 7 on to the screen 1 which is driven by a variable speed drive and formed of rubber or chromium plated metal bars of rectangular or V-shaped cross-section; the inside surface of the screen should be sufficiently smooth to give a sliding coefficient of friction of less than 1.0 between the screen and material. Particles progress upwardly and fall, as shown by the arrows in Fig. 1,on to a grooved plate 15 which directs the larger particles, falling on the outer grooves, out of the screening area and into a collecting funnel 19 for the coarse fraction; the fines are returned to the screening zone for re-screening and eventually pass through the screen into a launder 8 which may be adjusted by rods 9 and shaken by a vibrator 11 to direct the fines into a collecting funnel 17. Water is sprayed from pipes 12,13,14 so as to further reduce the sliding friction between the screen and material, re-pulp and wash particles remaining on the screen, to assist removal of the coarse fraction over the rim of the screen and to clean the openings. If desired, the screen may be rotated eccentrically and for improving the discharge, rotated about an inclined axis or be conically formed. For dry separating, the feed is introduced by a vibrating feeder or by pneumatic means; air may be sprayed from the pipes 12,13,14 instead of water and a dust extractor and separator connected to the casing 16.