AU709099B2 - Means and method for screening suspensions - Google Patents

Description

WO 97/25124 PCT/AU97/00003 MyEANS AND METHOD FOR SCREENING SUSPENSIONS Field of the Invention The present invention relates to means and methods for the screening of suspensions of at least two types of particulate materials in a fluid to achieve separation of the two types of particles one from the other.

Background Art It is known from Australian Patent Specification No AU-B-50521/85 to screen suspensions in a device comprising a screening drum that has a substantially horizontal shaft about which it is rotatable. In this device feed means are provided inside the device that have an outlet gap for discharging a flow of suspension substantially tangentially in the direction of rotation of the screening drum. The feed means is adapted to impart to the flow of the suspension a speed substantially equal to the peripheral speed of the drum.

Undersize particles and the suspending fluid pass radially outwardly through the screening drum while the oversize particles are retained within the screen and can be removed laterally by external jets or sprays of water, air, or any other suitable fluid and discharged through an open end of the screen.

Disclosure of the Invention The present inventor has now realised that substantial advantages can be achieved if the screening drum is mounted substantially vertically, or at an angle to the horizontal, rather than horizontally as previously proposed.

Thus, in a first aspect, the present invention consists in a method for screening a suspension of at least two types of particles dispersed in a fluid, comprising the steps of: rotating a substantially cylindrical sieve means, through which the fluid and only one of the said two types of particles may readily pass, about a substantially vertical axis of symmetry; introducing the suspension into the sieve means substantially tangentially to, and in the direction of rotation of, the sieve means at a speed substantially equal to the peripheral speed of the sieve means; collecting the one type of particle from radially outside the sieve means; and collecting at least the other type of particle.

The other type of particles are preferably washed to below the sieve means by lifting off the other type of particles collected on an inner surface WO 97/25124 PCT/AU97/00003 2 of the sieve means using, for example, a jet or spray of water, air or other suitable fluid.

In a further aspect, the present invention consists in a screening means for a suspension of at least two types of particles dispersed in a fluid, comprising: a substantially cylindrical sieve means, through which the fluid and only one of the said two types of particles may readily pass; means to cause the sieve means to rotate about a substantially vertical axis of symmetry; and means to collect at least the other type of particles.

In one embodiment of this aspect, the screening means further comprises a means of spraying an outer surface of the sieve means with a jet of fluid. The jet of fluid, in part, passes through the sieve means and dislodges the other type of particles that have accumulated on an inner surface of the sieve means. The dislodged particles will preferably be washed into a launder disposed below the sieve means for the collection of the other type of particles.

The screening means can comprise more than one sieve means. In one embodiment, the screening means can be operated where only one or some of a number of sieve means are employed at one time.

The advantages of using a substantially cylindrical sieve means designed to rotate about a substantially vertical axis, as opposed to the horizontal axis as previously proposed, are substantial. They include: the bearings may be at the top of the sieve means well out of the range of falling or draining water; heavier concentrations of solids, including fibrous materials, have a substantially vertical fall into a launder or other suitable collection device thereby avoiding "hangup" of sticky or fibrous materials; the sieve means are substantially self-cleaning and self-draining when the flow of suspension stops; the sieve means may be easily accessed by lifting a lightweight moulded or fabricated cover; the drive for the sieve means may be disposed above or below the sieve means and a single drive may run a plurality of sieve means; WO 97/25124 PCT/AU97/00003 3 the sieve means may be held in place with its own weight and substantially without retention bolts thereby facilitating quick and easy screen replacement; the arrangement is readily adapted to multiple unit installations with common headers, launders and the like; a substantially vertical arrangement allows unlimited length/diameter ratios as there is less tendency for the screen to accumulate a load of solids during shutdown, there is also a space advantage in having the screen longer and the cost of launders, pipes, and the like, is less; and a substantially vertical sieve axis is better disposed for flexible mounting of the sieve means as changes in load do not deflect the axis direction. A single point flexible mounting will allow an out-of-balance screen to rotate about its centre of gravity, placing less stress on the relatively fragile screen medium, and transmitting less vibration to the support structure.

As used in this specification the term substantially vertical axis means an axis that is no more than about 450 inclined to the vertical, preferably no more than 30' and most preferably no more than 150.

The sieve means may comprise a frame covered with any suitable sieve medium which will allow through one type of particle but inhibit the passage of the other type. Suitable sieve media include wedgewire screens rolled into cylinders; laser cut perforated metal screens; polyurethane wedgewire screen panels on a cylindrical frame; or conventional cloth screens of woven wires of metal or plastic.

The means to drive the screen may comprise a conventional mechanical or electrical motor coupled directly or indirectly to the sieve means. An alternative arrangement is to use a hydrodynamic or hydrostatic drive to drive the sieve means. In its simplest form a hydrodynamic drive may comprise an impeller formed integrally with a hub on which a cylindrical sieve screen is hung. A stream of drive water is directed to impinge against the impeller. This same stream may be used as a cleaning stream for the sieve screen or may be merely recycled to drive the sieve screen. In this way a simple valve may be used to control the rotation of each of a group of devices driven from a common hydrodynamic fluid source.

In a still further embodiment, the means to rotate the sieve screen can comprise the jet of fluid which is directed at the outer surface of the sieve WO 97/25124 PCT/AU97/00003 4 means to remove the particles collected on the inner surface. The rotational speed of the sieve means can be controlled by adjusting the pressure, flow rate and angle of impingement of the jet of fluid.

The present invention can be used to separate coarser material from finer material in slurries. For example, the present invention can be used to separate coal fines in slurry stream based on the size of the fines in the slurry. Other applications can be envisaged including use in chrysotile and asbestos fibre particle separation.

Brief Description of the Drawings Hereinafter by way of example only, preferred embodiments of the present invention are described with reference to the accompanying drawings, in which: Fig. 1 is a partly cut-away side elevational view of one embodiment of a screening device according to the present invention; Fig. 2 is a vertical sectional view through the screening device of Fig.

1; Fig. 3 is a plan view of the screening device of Fig. 1; Fig. 4 is a partly cut away side elevational view of another embodiment of a screening device according to the present invention; Fig. 5 and Fig. 6 are respectively an underneath plan view and a cutaway side elevational view of a hydrodynamic drive for a screening device according to the present invention; Fig. 7 is a partially cut-away side elevational view of a still further embodiment of the screening device according to the present invention; Fig. 8 is a cross-sectional view of one of the sieve means of the screening device depicted in Fig. 7 showing details of the header and cleaning apparatus; Fig. 9 is a plan view of the header of Fig. 8, showing the deflector plate, cleaning fingers and cleaning bar; and Fig. 10 is a further cross-sectional view of the sieve means of Fig. 7.

Best Mode of Carrying out the Invention A screening device for a coal slurry suspension containing at least two types of particles dispersed in a fluid according to the present invention is generally depicted as 10 in Figs. 1 to 6. The screening device 10 could be readily adapted to screen coarser material from finer material in other slurries.

WO 97/25124 PCT/AU97/00003 The screening device 10 comprises a frame 11 (see Fig. 3) and a pair of cylindrical laser cut stainless steel screens 12 having slots of 150 microns width. In the arrangement shown in Figs. 1 to 3, the screens 12 are each mounted vertically with a closed top 13 and an open bottom 14. The top 13 is connected to a centrally located spindle 15 mounted in a bearing 16. The upper end of each spindle 15 carries a pulley wheel 17 connected by a drive belt 18 to a common motor through a common drive shaft 19. Each of the screens 12 is covered by a light weight cylindrical shield 21 that drops over the drive shaft 15 when the pulley 17 has been removed. Each shield 21 directs material that passes through the associated screen 12 into a launder 22 below the respective screen 12.

Material to be screened is introduced through a horizontal duct 23 from whence it rises upwardly and then downwardly into a vertical discharge header 24. Each such header 24 has an elongate discharge nozzle arranged to project the suspension to be screened onto the inside surface of the screen 12 and substantially tangentially thereto (as depicted by arrow B in Fig. 3).

The suspension on the inside surface of the screen 12 is subject to strong centrifugal force that projects the fluid, usually a liquid and normally water, in which the particulate matter is suspended, and the undersize particles radially out through the screen and into launder 22. The oversize particles are held by centrifugal force against the inside surface of the screen 12 until they are physically removed. This removal is via air, water, or another fluid being discharged forcefully substantially radially inwardly, against the outside of the screen 12 (as depicted by arrow A in Fig. This fluid is projected through elongate, vertically oriented, nozzles 26 (see Fig.

The oversize particles dislodged by the fluid discharged through nozzles 26 are captured by vertical launder 27. The launders 27 are supported on either side by plates 28 connecting the launders to a respective one of the tubular support members 29. The tubular support member 29 supports, through a bearing 31, the spindle The arrangement shown in Fig. 4 differs from that shown in Figs. 1 to 3 only in that valve 32 is provided to control the flow of suspension individually to each screen 12.

In the arrangement shown in Figs. 5 and 6, the screen 12 is driven by a pelton wheel integral with the screening device 10. Each support member WO 97/25124 PCT/AU97/00003 6 29 is connected internally to a source of high pressure water. This water is directed, through arm 33, against a series of vanes 34 on the underside of the top 13 of the associated screen 12. The screen 12 is thus driven by thile water pressure. The rate may be determined by controlling the flow of water through tap 35. The water discharged against the vanes 34 may be collected and recycled as clear water or it may be used to flush oversize product down the vertical launder 27.

A further embodiment of a screening device according to the present invention is generally depicted as 10a in Figs. 7 to 10. In this embodiment, where like features have been numbered as described above, each of the screens 12 are mounted on the screening device 10a at 450 to the horizontal.

In the embodiment 10a, a deflector plate 40 is disposed proximate the outlet of the nozzles 25 to deflect the cylindrical stream from the nozzles into a wide, flat stream before it reaches the screen 12. The spacing and dimensions of the nozzles 25 provides full coverage of the surface of the screen 12. In one embodiment, the nozzles 25 can constitute a single slot for discharge of the suspension.

Oversize material, agglomerating material, or fibrous material can have lumps which are larger than the orifices of the nozzles 25, or a number of lumps which are individually smaller than the orifice can combine to block a nozzle 25 in certain instances. In order to remove such blockages, nozzle cleaning fingers 41 are mounted on a cleaning bar 42, which can be rotated about its axis to push the fingers 41 through the orifice, dislodging the lumps back into the header 24. As the cleaning bar 42 is rotated about its axis, the end cover plate 43 is moved away from the header end nozzle 44 allowing the oversize material to be discharged into the launder 27. The downward flow in the header 24 also assists this action. For those applications where nozzle blockage is rare, oversize material can collect at the bottom of header 24 and, if necessary, be discharged infrequently by manual removal of a plug.

The downward flow of the suspension into the header 24 also allows sizing of the header to compensate for the change in head pressure along the length of the header 24. Where the header pressure is high, the isolating valve 45 can be closed during the cleaning process. This arrangement of cleaning equipment allows for automation of the cleaning process.

WO 97/25124 PCT/AU97/00003 7 The material which is retained on the inner surface of each sieve screen 12 is removed by a water jet directed towards the outer surface of the screen 12 from a spray header. The water passes through the slots in the screen 12 and so washes the retained material into the launder 27 where it is collected.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims (14)

1. A screening means for a suspension of at least two types of particles dispersed in a fluid, comprising: a substantially cylindrical sieve means, through which the fluid and only one of the said two types of particles may readily pass; means to cause the sieve means to rotate about a substantially vertical axis of symmetry; means for introducing a suspension into the sieve means in a direction of rotation of the sieve means; and means to collect at least the other type of particles.

2. The screening means of claim 1 wherein the axis of symmetry is inclined no more than 45° to the vertical, or no more than 30° to the vertical, or no more than 15° to the vertical.

3. The screening means of claim 1 comprising at least two substantially cylindrical sieve means each inclined at an angle of 450 to the vertical.

4. The screening means of any one of the preceding claims wherein the sieve means comprises a frame covered with a sieve medium which will allow through the one type of particle but inhibit the passage of the other type.

5. The screening means of claim 4 wherein the sieve media include wedgewire screens rolled into cylinders, laser cut perforated metal screens, polyurethane wedgewire screen panels on a cylindrical frame, or cloth screens of woven wires of metal or plastic.

6. The screening means of any one of the preceding claims wherein the means to rotate the sieve means comprises a mechanical or electrical motor coupled directly or indirectly to the sieve means.

7. The screening means of any one of claims 1 to 5 wherein the means to rotate the sieve means comprises a hydrodynamic or hydrostatic drive.

8. The screening means of claim 7 wherein the hydrodynamic drive comprises an impeller having a plurality of vanes and formed integrally with the sieve means, the impeller being driven by a stream of water that is directed to impinge against the vanes.

9. The screening means of any one of the preceding claims further comprising a means of spraying an outer surface of the sieve means with a jet of fluid. iMED SHEET 'PEA/AU )CTJA 97 00 3 RECEIVE 2 5 ir? 9 The screening means of any one of the preceding claims wherein the means for introducing a suspension comprises at least one discharge nozzle arranged to project the suspension to be screened onto an inner surface of the sieve means.

11. The screening means of claim 10 wherein a deflector plate is disposed proximate the discharge nozzle.

12. The screening means of claims 10 or 11 wherein the suspension is introduced into the sieve means substantially tangentially to the sieve means.

13. The screening means of any one of claims 10 12 further comprising a discharge nozzle cleaning means.

14. The screening means of claim 13 wherein the discharge nozzle cleaning means comprises a plurality of fingers adapted to enter the discharge nozzle and dislodge any particles caught therein.

15. A method for screening a suspension of at least two types of particles dispersed in a fluid, comprising the steps of: rotating a substantially cylindrical sieve means, through which the fluid and only one of the said two types of particles may readily pass, about a substantially vertical axis of symmetry; introducing the suspension into the sieve means substantially tangentially to, and in the direction of rotation of, the sieve means at a speed substantially equal to the peripheral speed of the sieve means; collecting the one type of particle from radially outside the sieve means; and collecting at least the other type of particle. 'AMENDED SHEET iF' d'f!