GB2110962A - Powder classifier - Google Patents

Abstract

Powder suspended in an air stream may be classified into coarse and fine fractions by subjecting it simultaneously to centrifugal and centripetal forces. In a classifier having rotating discs, with no guide vanes between the disc, and an axial outlet, some nibs or larger particles may be carried over by the increasing velocity of the air stream as approaching the axis of rotation. In the construction shown, powder fed from a hopper 3 is suspended in an air stream and carried to a zone peripheral to rotating discs 7, 8 and 9. The air is drawn off, with a fine powder fraction, via a hollow shaft 4. In order to avoid the carry over of larger particles by the accelerating air stream, the discs 7 and 9 are tapered at 14 to make the cross-section of the air flow passages between the pair of discs 7, 8 and 8, 9 substantially constant over the inlet zone. <IMAGE>

Description

SPECIFICATION Powder processing equipment This invention relates to equipment for processing of powdèt.

It is knownthat particles of powders can be separated into two fractions of different sizes by subjecting a powder to centrifugal and centripetal forces at the same time. This is achieved in several commercial machines by pulling air to the centre of a rotating system.

In commercially available machines of this type the rotary system incorporates a rotor including vanes located between spaced discs which normally converge towards the periphery. Such arrangements are complex to manufacture and not susceptible to easy repair. While such machines have found acceptance for large scale operations, they are difficult to clean so are normally only usable for long runs. If manufactured to a small scale for runs on smaller amounts of material there would also be a tendency to clogging in use.

A simple form of mechanism was proposed some 20 years ago which would have utilized two or three flat discs spaced apart. The powder was to have been blown in and the mechanism would operate under a slight positive pressure. However, this mechanism did not find commercial acceptance.

Many classifiers give fine products which contain occasional "nibs" (larger particles) caused by larger particles being bounced across the classifying zone into the fine powder outlet airstream. As this airstream is usually moving in towards the centre of the rotor, the area available is continuously decreasing and thus the air velocity increases. Larger particles, which would not normally go to the fines, are thus accelerated in this airstream and cannot escape back to the coarse discharge point.

In accordance with a first aspect of the invention, there is provided a powder classifier comprising a plurality of discs mounted on a hollow shaft for rotation therewith to define one or more clear flow passages between discs without guide vanes, means for feeding powder entrained in an air stream to a circumferential zone of the discs to pass into the flow passage(s) formed between adjacent discs while lead from the circumferential zone to the interior of the hollow shaft, in which, at least an entrance zone to the flow passage(s), the discs are formed to provide a flow passage of cross-section tapering outwards so as to reduce the acceleration of the air as it travels radially inwardly towards the hollow shaft.

Preferably, the tapered portion of the flow passage isdesigned to maintain a constant cross-section area of the air passage formed between two adjacent discs.

By reducing the acceleration of the air as it moves radially inwards, the chance of entrainment of nibs is substantially decreased, since any particles which do not jump the classifying zone and enter the inward travelling stream are likely to be thrust out by the centrifugal force in view of the slower movement of the airsteam.

The classifier may include an inlet shroud to guide the incoming feed material to a predetermined limited zone of the peripheral zone located behind a position of a discharge duct for a coarser fraction.

The invention will be further described with reference to the accompanying diagrammatic drawing, in which: Figure lisa diagrammatic sectional view of one embodiment of the invention; Figure 2 is a view similar to part of Figure 1 and showing a modification; and Figure 3 is an end elevation of the modified version.

Turning first to Figure 1, a stator housing 1 is solid with an air inlet duct 2 and with a feed hopper 3 for powdered material. A hollow rotor shaft4 acts as an outlet for air and suspended fine powder. A fan (not shown) is connected downstream of the shaft 4 to provide the required suction and the powder itself may be collected via a small bag filter or cyclone.

The shaft 4 is mounted in bearings 5 and carries at one end a drive pulley 6 and at the opposite end, within the stator housing 1, a plurality of discs. In the example shown, three discs 7,3 and 9 are illustrated.

These are mounted in spaced relationship on a flange 11 of the shaft 4 by means of screws 12 and spacers 13.

In operation of the embodiment shown in Figure 1, air entering through the duct 2 entrains the powder fed in through the hopper 3 and spreads outwardly over the outer or right hand face of disc 7 towards a peripheral zone la within the stator housing 1.

During this phase, the particles are given some circumferential acceleration by contact with the rotary disc 7. The air is then reversed in direction and has to flow inwardly between the adjacent pairs of discs 7 and 8, and 8 and 9 and then outwardly through the open centres of the discs 8 and 9 into the hollow shaft 4. The circumferential movement of the powder ensures centrifugal force and the inward movement of the air forms a centripetal force, so that the coarse particles in the powder tend to remain at the periphery of the discs 7,8 and 9, while the fine particles in the powder are drawn inwards to the hollow shaft 4, through which they are removed.

The coarse powder is retained in the peripheral zone and is removed periodically through a flap valve (not shown) operated by a solenoid mechanism.

In a classifier with flat discs, the air velocity increases as it progresses towards the centre in view of the reducing cross-section, and in order to overcome this, the inner faces of the discs 7 and 9 are tapered as shown at 14 so as to provide a tapering cross-section to the flow passages and a substantially constant cross-section of area over the radially outer portion of the flow passages. By this means, the acceleration of the air is deferred and therefore larger particles or nibs which are carried out of the peripheral zone of the discs may be thrown out by centrifugal force away from contact with the rapidly rotating discs and are not carried to the interior of the hollow shaft 4 near the centre of the rotating discs.

In the arrangement described above, removal of the coarse powder fraction through a flap valve cannot be achieved during operation without a substantial entrainment of unclassified powder from the peripheral zone.

In order to overcome this, there is provided a feed shroud 21 (see Figures 2 and 3) fixedly mounted in the housing 1 at the exit from the inlet duct 2 and extending close to the rotating disc 7 to limit spreading of the powder entering the peripheral zone la so that it all enters a portion 1 b of the zone 1a immediately downstream, in the direction of rotation, indicated by the arrow 22 of Figure 3, of a coarse fraction discharge duct 23. In this way, the powder will have completed almost one revolution in the peripheral zone 1 a before reaching the coarse discharge duct 23, and so will have been substantially freed of fine powder. The duct 23 may thus be opened during operation to withdraw the coarser fractions without entraining significant quantities of the fine powder fractions Figure 3 also shows the whole classifier mounted on a base plate 24, and including a motor mounting 25 and other components. A clamp for a coarse collection bottle is shown at 26 and a solenoid for operating a coarse discharge flap valve is shown at 27.

Various modifications may be made within the scope of the invention. Thus, the number of discs may be greater than three. Also, two or more units as described may be operated in a tandem or series arrangement.

Claims (4)

1. A powder classifier comprising a plurality of discs mounted on a hollow shaft for rotation therewith to define one or more clear flow passages between the discs without guide vanes, the said flow passage(s) leading from a circumferential zone of the discs to the interior of the hollow shaft, means for feeding powder entrained in an air stream to the circumferential zone of the discs, in which, at least at an entrance zone to the flow passages, the discs are formed to provide a flow passage having a cross -section tapering outwards so as to reduce the acceleration of the air as it travels radially inwardly towards the hollow shaft.

2. A powder classifier as claimed in ctaim 1, in which the tapered portion of the flow passage is designed to maintain a constant cross-sectional area of the air passage formed between two adjacent discs.

3. A powder classifier as claimed in claim 1 or 2, including an inlet shroud to guide the incoming feed material to a predetermined limited zone of the peripheral zone located behind a position of a discharge duct for a coarser fraction.

4. A powder classifier substantially as hereinbefore described with reference to the accompanying drawings.