GB2050870A

GB2050870A - Screening solids suspended in a liquid

Info

Publication number: GB2050870A
Application number: GB8015585A
Authority: GB
Original assignee: WHITEFIELD SYSTEMS HOLDINGS LT
Current assignee: WHITEFIELD SYSTEMS HOLDINGS LT
Priority date: 1979-05-12
Filing date: 1980-05-12
Publication date: 1981-01-14

Abstract

A separator and a method of segregating coarser particulate material from finer particulate material suspended in a liquid. The separator comprises a vessel (11) having an inlet chamber (24) receiving the liquid under superatmospheric pressure and first and second outlet chambers (28 and 29). A cylindrical screen (25) extends between the inlet and second outlet chambers and contains an auger (30) in contact therewith to wipe the screen and carry the larger particulate material to the second outlet chamber (29). The flow of liquid and finer particulate material from the chamber (22) is restricted by a weir or valve to maintain a superatmospheric pressure within the chamber (28). <IMAGE>

Description

SPECIFICATION Improvements in separators The present invention relates to separators for segregating larger particulate material from smaller particulate material suspended in a liquid, and to separating methods. The invention has application to paper manufacture and is useful for removing from the pulp or stuff being fed to the head box of the paper-making machine all coarser fibres and other particles which in particular may have found their way into the feed stock from waste paper being re-used or re-cycled.

It is known to use a cylindrical screen for filtering the stock in order to remove therefrom material which is too coarse for the paper being manufactured and foreign materials whose presence is attributable to the use of reclaimed waste products.

In order to prevent the screen from becoming clogged, a rotor is arranged coaxially of the screen and is provided with foils which co-operate with the inlet side or the outlet side of the screen or both in order to wipe off the coarser particles which are too large to pass through the screen. Generally, the cylindrical screen is arranged with its axis more or less vertical so that the coarser particles fall to the bottom of the vessel in which the screen is disposed and can be removed from the vessel. The foils are in the form of one or more straight rods regularly spaced around the screen and extending the length of the screen.

A disadvantage of this known apparatus is that the foils, as they move over the screen in the vicinity of the pump outlet from the vessel, produce pressure pulses which are transmitted to the head box and which cause non-uniformity in the paper being laid.

According to one aspect of the present invention, a separator for segregating larger particulate material from smaller particulate material suspended in a liquid comprises a pressure-tight vessel having therein a cylindrical screen and a rotatable auger provided with at least one scroll which co-operates with an inlet side or an outlet side of the screen, the vessel having a first outlet chamber at the outlet side of the screen and the auger extending between an inlet chamber and a second outlet chamber, the vessel having an inlet to the inlet chamber and first and second outlets from the first and second outlet chambers respectively;; meansforfeeding under superatmospheric pressure to said inlet liquid having particulate material suspended therein so that such liquid flows through the screen to the outlet chamber and takes with it smaller particulate material, rotation of the auger causing the scroll to wipe the inlet or outlet side of the screen and so dislodge therefrom larger particulate material which does not pass through the screen and to transport the larger particulate material to the outlet chamber; and means for restricting discharge of the liquid and smaller particulate material suspended therein through the first outlet, provision being made for restricting discharge of the larger particulate material through the second outlet, whereby superatmospheric pressure is maintained at substantially the whole of both sides of the screen.

Preferably the auger extends axially within the screen and the inlet side is on the inside of the screen. A converse arrangement in which the scroll envelopes the screen is possible.

Conveniently the scroll comprises one or more preferably continuous helically formed strips of material secured to the auger. The scroll may be single or multi-start. In an alternative the scroll comprises one or more helically formed ribbons secured to the auger but spaced therefrom by mounting struts.

In the case of a multi-séar. scroll one or more of the scrolls may be discontinuous.

The scroll is advantageously provided with wiper means, such as a brush strip, a flexible wiper strip, or foils which engages the surFace of the screen at its inlet side.

In an alternative embodiment the auger extends axially within the screen on the outlet side thereof and the scroll is preferably provided with wiper means in the form of foils comprising a plurality of longitudinal blades secured to the edge of the scroll and aligned with their longitudinal axis parallel to the wall of the screen.

The screen is conveniently made from sheet metal having a multitude of holes, which holes preferably widen conically from the inlet side to the outlet side.

Another aspect of the present invention is a method of segregating coarser particulate material from finer particulate material suspended in a liquid, in which the suspension is fed to an inlet chamber of a vessel, substantially the whole of whose interior is maintained under superatmospheric pressure and which contains a cylindrical screen through which liquid and the finer particulate material pass to a first outlet chamber, and in which an auger co-operating with the screen is rotated so that a scroll of the auger wipes the screen and conveys coarser particulate material to a second outlet chamber.

Preferably the auger co-operates with the side of the screen remote | fmm the first outlet chamber to wipe the inlet side of the screen.

The inlet liquid with the particulate material sus pended therein may be fed under pressure either by means of a pump or under gravity from a header tank. Pressure can be maintained with the vessel by restricting the flow of material from at least the first outlet chamber by means of a suitably adjusted valve or by means of a fixed or adjustable weir.

Whilst the invention has particular application to paper-making it does have wider application and may be used in many environments in which it is desired to remove larger particulate material from a suspension.

The invention is further described, by way of example, with reference to the drawings, in which Figure 1 is a somewhat diagrammatic side elevation of a separator according to a preferred embodiment of the invention, and Figure 2 is an elevation partially sectioned of the separator as viewed in the direction of the arrow 2 in Figure 1.

A separator 10 for removing coarser fibres and other particles from pulp stock being fed to the head box (not shown) of a paper-making machine comprises a generally cylindrical pressure4ightvessel 11. The vessel is supported on a frame 12 so that the axis of the vessel is at approximately 30 to the vertical. An elevated header tank 13 is shown as feeding the stock to an inlet 14 at the lower end of the vessel 11 through a feed line 15 and a control valve 16. The vessel 11 is thereby maintained under pressure. In practice it may be more convenient to feed the stock under pressure by means of a pump, such as a variable speed pump.

The vessel 11 has near its lower end a first outlet 17 which is connected by a line 18 and a valve 19 to an inlet 50 at the lower end of an upright cylindrical vessel 51 whose outlet 52 is also at its lower end and is connected to the head box of the paper-making machine. The vessel 51 is partitioned by a weir 53 whose upper portion 54 is in the form of an adjustable weir blade. The vessel 11 has a second outlet 20 at its upper end which leads via a chute 21 to a receptacle 23 for the separated coarse material or rejects.

As can be seen from Figure 2, the inlet 14 communicates with an inlet chamber 24 in the lower end of the vessel 11. A cylindrical screen 25 is secured to and extends between lower and upper annular partition walls 26 and 27. A first outlet or "accepts" chamber 28 with which the first or "accepts" outlet 17 communicates is formed by the annular chamber defined between the screen 25 and the cylindrical outerwall ofthevessel 11 between the partition walls 26 and 27. Above the upper partition wall 27 is a second outlet or "rejects" chamber 29 with which the second or "rejects" outlet 21 communicates.

An auger 30 extends axially through the screen 25 from the inlet chamber 24 to the rejects chamber 29 and comprises a hollow shaft 31 journalled to a lower bearing 32. The upper end of the shaft 31 is attached by coupling bolts 33 to a stub shaft 34 extending downwardly from a reduction gear box 35 which is mounted on top of the vessel 11 and which is preferably a variable speed gear box. A seal assembly 39 seals the stub shaft 34 to the upper end of the vessel 11. A drive motor 36 is connected by a belt drive 37 to the input shaft 38 of the gear box 35 as shown in Figure 1.

The auger 30 has helical flights 40 disposed about the shaft 31 and co-operating with the inner surface of the cylindrical screen 25. The lower end of the interior of the cylindrical screen 25 communicates with the inlet chamber 24 and its upper end communicates with the rejects chamber 29. Thus, the stock introduced into the inlet chamber 24 passes upwardly within the screen 25 whose inner surface, with which the flights 39 co-operate, serves as the inlet side of the screen. The helical flights constitute a multi-start scroll. The screen has a multitude of small holes which are preferably tapered so that they widen towards the outlet side of the screen. The holes are preferably at least 11/2 mm diameter but a diameter of up to 5 mm may be suitably employed.

Slots may be used instead of holes. A sufficient number of holes or slots is provided to give the screen a 12% open area but an open area up to 23% may be desirable in some cases.

In operation, stock is fed to the inlet 14 under pressure determined by the setting of the valve 16. If a pump is used instead of a header tank 13 then the inlet pressure may be adjusted by adjusting the pump speed or the pump displacement, if the pump is a variable displacement pump. The stock passes upwardly into the cylindrical screen 25 and most of the liquid passes through the holes of the screen into the accepts chamber 28 and takes with it the finer fibres which can pass through the holes. This liquid, together with the suspended fibres flows through the outlet line 18 to the box and the valve 19 andlor the weir blade 54 are adjusted so as to provide sufficient restriction to the fluid flow to maintain a positive or superatmospheric pressure within the "accepts" chamber 28.

The auger 30 is continuously rotated by the motor 36 at an adjusted speed of up to 100 rpm, although higher speeds may be appropriate for some applications, and the flights 40 perform a wiping action on the inlet side of the cylindrical screen 25 to dislodge the coarser fibres and other particles which do not pass through the screen and to transport these coarser fibres and particles upwardly into the rejects chamber 29. This wiping action is preferably enhanced by fixing a brush strip to the outer helical periphery of the flights 40 so that the brush bristles actually brush the lodged fibres from the screen. In an embodiment preferred for some applications, the major lower portions 41 of the flights is of relatively coarse pitch and are in the form of ribbons supported clear of the shaft 31, the brush strip being provided on this lower portion 41.The minor upper portion 42 is of not such coarse pitch and is mounted directly on the shaft 31 and the brush strip can extend to the minor flight portion 42. Alternatively, wipers can be provided on the outer periphery of the flights to perform a positive wiping action on the inlet side of the screen to dislodge fibres etc. from the screen. Other designs of auger to suit conditions are feasible.

The coarser fibres and other particles are discharged from the rejects chamber 29 in the form of a cake or slurry through the chute 21. In the case of a cake, the latter, as it is formed by the upper end portion of the auger, restricts the discharge of the cake through the outlet 20 and thereby maintains the.

superatmospheric pressure within the screen 25.

The formation of the cake in the rejects chamber 29 can be achieved by keeping the top of the weir blade 54 slightly below the top of the auger so that just the top end of the screen 25 is no longer at superatmospheric pressure. In the case of a slurry, a valve in the outlet 20 is adjusted to restrict the flow of slurry and so maintain a positive or superatmospheric pressure in the rejects chamber 29.

Heavier particles such as stones, sand and metal, may fall directly to the bottom of the inlet chamber 24 and may be removed from time to time through an auxiliary outlet 60 and a valve 61.

The separator and separation method of the invention are suitable for low consistency and high consistency stock. Satisfactory separation can be ensured by appropriate adjustment of the valves 16 and 19 and the weir blade 54 and, if necessary, by adjusting the speed of the auger 30 by adjusting the gear box 35 when the latter is made adjustable.

Whilst the vessel 11 is illustrated as being somewhat inclined to the vertical, other attitudes may be used, e.g. substantially horizontal or substantially vertical. Also the inlets and outlets may be interchanged so that, e.g. the chamber 29 becomes the inlet chamber and the chamber 24 becomes the rejects chamber. It is also possible for the accepts outlet 17 to be positioned elsewhere than illustrated, e.g. at or towards the top of the vessel 10. Also the apparatus may be designed so that the various chambers and connections can be used optionally as inlet or outlet.

In an alternative construction (not illustrated) the separator is constructed as an inlet flow screen with the auger is positioned axially within the cylindrical screen. The inlet to the screen being from the outside and the outlet being on the inside. The holes in the screen taper from the inlet side to the outlet side. In this form of construction the auger carries a plurality of longitudinal foil blades which contact the inside surface of the screen. The foil blades are aligned parallel to the screen walls and are spaced at intervals along the scroll. The length of the blades and the spring is so chosen that the entire surface of the screen is wiped. The pulsations generated by the foils are directed toward the inlet side of the screen and therefore the pressure pulses are unlikely to be transmitted to the outlet.

The present invention can be used advantageously in place of a vibratory type screen which dips in a pulp vat and through which coarser rejects cannot pass. The apparatus of the invention can remove finer rejects which would pass through the vibratory type screen.

Claims

1. A separator for segragating larger particulate material from smaller particulate material suspended in a liquid comprising a pressure-tight vessel having therein a cylindrical screen and a rotatable auger provided with at least one scroll which co-operates with an inlet or outlet side of the screen, the vessel having a first outlet chamber at the outlet side of the screen, and the auger extending between an inlet chamber and a second outlet chamber, the vessel having an inlet to the inlet chamber and first and second outlets from the first and second outlet chambers respectively; means for feeding under superatmospheric pressure to said inlet liquid having particulate material suspended therein so that such liquid flows through the screen to the outlet chamber and takes with it smaller particulate material, rotation of the auger causing the scroll to wipe the inlet or outlet side of the screen and so dislodge therefrom larger particulate material which does not pass through the screen and to transport the larger particulate material to the second outlet chamber, and means for restricting discharge of the liquid and smaller particulate material suspended therein through the first outlet, provision being made for restricting discharge of the larger particulate material through the second outlet, whereby superatmospheric pressure is maintained at substantially the whole of both sides of the screen.

2. A separator as claimed in claim 1, in which the scroll co-operates with the inlet side of the screen and the first outlet chamber is at the outlet side of the screen remote from the auger.

3. A separator as claimed in claim 1 or 2, in which the auger extends axially within the screen so that the inlet side of the screen is its inside.

4. A separator as claimed in claim 1 or 2, in which the auger extends axially outside the screen and the inlet side of the screen is the outside.

5. A separator as claimed in any preceding claim, in which the scroll is provided with wiper means.

6. A separator as claimed in claim 5, in which the wiper means is a brush strip or a flexible strip secured to the edge of the scroll.

7. A separator as claimed in any preceding claim, in which the screen is sheet metal and has a multitude of holes therein which widen from the inlet side to the outlet side.

8. A separator as claimed in claim 7, in which the holes are circular and widen conically.

9. A separator as claimed in any preceding claim in which the means for feeding liquid having particulate material suspended therein at superatmospheric pressure to said inlet comprises an elevated header tank connected to the inlet by a feed line and a control valve, and the means for restricting discharge of the liquid and smaller particulate material suspended therein through the first outlet comprises an upright cylindrical vessel connected to the first outlet from the vessel and having an inlet and outlet at the lower end thereof which are separated by a weir which partitions the cylindrical vessel and which has an adjustable weir blade at the upper portion thereof, the adjustable weir blade also serving to restrict the discharge of larger particulate material from the second outlet.

10. A separator as claimed in any preceding claim, in which the scroll is helical and comprises one or more flights.

11. A separator as claimed in claim 10, in which at least one flight is a discontinuous helical ribbon supported clear of the auger shaft.

12. A separator as claimed in claim 5, in which the wiper means comprise a plurality of longitudinal blades secured to the edge of the scroll with their longitudinal axes parallel with the screen wall.

13. A method of segregating coarser particulate material from finer particulate material suspended in a liquid, in which the suspension is fed to an inlet chamber of a vessel, substantially the whole of whose interior is maintained under superatmospheric pressure and which contains a cylindrical screen through which liquid and the finer particulate material pass to a first outlet chamber, and in which an auger co-operating with the screen is rotated so that a scroll of the auger wipes the screen and conveys coarser particulate material to a second outlet chamber.

14. A method as claimed in claim 13 in which the auger co-operates with the screen at the side thereof remote from the first outlet chamber so that the auger wipes the inlet side of the screen.

15. A method as claimed in claim 13 or 14, in which the inlet liquid with the particulate material suspended therein is fed under pressure either by means of a pump or under gravity from a header tank and in which the vessel is maintained under superatmospheric pressure by restricting the flow of material from at least the first outlet chamber by means of a suitably adjusted valve or by means of a weir.

16. Aseparatorconstructed and arranged and adapted to operate substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.

17. A method of segregating coarser particulate material from finer particulate material suspended in a liquid substantially as hereinbefore described with reference to the accompanying drawings.