GB953690A - Improvements in dust classifiers - Google Patents

Abstract

953,690. Centrifugal classifiers; electrostatic precipitators; pneumatic separators. S. MASUDA, and T. AOKI. Jan. 14, 1963, No. 1628/63. Headings B2H, B2J and B2P. In a classifier in which dust particles dispersed in air are separated into coarse and fine fractions under the action of centrifugal or other force, means utilizing either a unidirectional or alternating electric field are provided for charging the dust particles electrostatically to the same polarity before they reach the separating zone so as to improve the dispersion of the particles as they enter the zone. In Fig. 1, the dust to be classified is fed into an inner casing 3 from a hopper 12 and through a hollow shaft 10 on to a rotary distributer disc 4 from which it is emitted under centrifugal force through an ion curtain 27 and into the separating zone 11 of the classifier where it is completely dispersed due to the repulsive forces acting between the charged particles. In the zone 11 the particles are subjected to centrifugal force and to a lifting air current produced respectively by vortex fan blades 9 and circulator fan blades 8 supported by the shaft 10 which is fixed to the rotating disc shaft 7. The coarser particles flung against the wall of the casing 3 are prevented from flowing into the outer casing 2 by baffles 13 adjustable by a handle 14 and fall into the inner case 15 to be removed through a duct 16. The finer particles are carried by the upward air current into the outer casing from which they are removed at 19. The flow of air is regulated by varying the opening vanes 20 with a handle 21. The ion curtain 27 is produced between earthed, needletype corona discharge electrodes 22 at the lower edge of a cylinder 23 held by the fan blades 9 and a circular, tubular ion collecting electrode 24 supported by an insulator 25 and fed from a high voltage source 26. Dust accumulations are shaken off continuously by means of electromagnetic vibrators 28-31. The finer particles are prevented from being recirculated with the air into the casing 3 by means of an electrostatic precipitator comprising corona discharge electrodes 34 and a dust collecting annular conical electrode 32 supported by insulators 33. The disc 4 may be of insulating material such as plastics or ceramics. In Fig. 2, the corona electrode 41 is constituted by the sharp outer circumference of a thin annular plate fixed to a circular plate 42 carrying the rotating disc 4. The ion collecting electrode 24 is fixed to the disc 4 by an insulator 43 and is fed through a metal brush 44. Alternatively, the plate 42 may carry vertical or horizontal needle electrodes fixed on its circumference or a circular wire electrode supported from its circumference or have its periphery sharpened to constitute the discharge electrode. In modifications of Fig. 1, (a) the rotating disc 4 has a sharpened circumference and the ion collecting electrode comprises a circular ring supported by an insulator above the disc 4 ; (b) an electrostatic, inductiontype charging device is constituted by an annular electrode supported by an insulator above a rotating metal disc 4 and fed with high voltage whereby the dust is charged on the disc 4 to the same polarity as the disc. In Fig. 6, dust is introduced through a duct 65 actuated by a vibrator 66 into air fed through an inlet 61 to a cyclone separator 63 and passes through an ion curtain 70 formed between an earthed needle electrode 67 and a collecting electrode 68. The fine particles leave through the tube 64 with the air. In a further modification of the dust charging device, for use with dust having electrical resistivity of the order of 10<SP>13</SP> ohmcm., metal contact surfaces, such as vibrating or non-vibrating screen meshes or finned plates, are fixed within or at the entrance of the dust emitting device so that the streaming dust is brought into contact with the metal surfaces and is unipolarly electrified owing to the difference in contact potentials. For treating large quantities of dust use may be made of multiple electrode pairs for successive charging stages or of multiple dust emitting devices each having its own electrode pair. The accumulated dust may be shaken off by any form of vibrating or hammering device which may be mechanical, electromechanical or pneumatic and may be driven intermittently or continuously. The classifier may be combined with a crushing mill, a closed circuit mill or an air-swept mill. Examples of materials treated are given. Agglomeration or adhesion of the dust particles with each other or to metal surfaces, which is caused by surface activity of the dust or by contact electrification, may be reduced by preprocessing the dust in an apparatus using surface activating agents such as triethanolamine or diethylene glycol or water or by injecting steam, water or moist air into the classifier or, when the latter is used with a crushing mill, by adding the processing agent in the mill itself.