Title:
Dual Zone Wireless Pulp Washer
Kind Code:
A1


Abstract:
A wireless dual zone pulp washer with rotating concentric filters for creating centrifugal force against feedstock in order to expel liquid contained in the feedstock before further processing of the stock. A secondary filter surrounds the primary filter to capture viable pulp fibers expelled from the feedstock by the dewatering process.



Inventors:
Egan III, John J. (Centerville, OH, US)
Application Number:
11/578218
Publication Date:
09/13/2007
Filing Date:
04/11/2005
Assignee:
Kadant Black Clawson Inc. (Mason, OH, US)
Primary Class:
Other Classes:
210/512.1
International Classes:
D06F35/00; A47L25/00; B01D33/11; B04B3/00; B04B11/08; D21C9/06; D21D1/40
View Patent Images:



Primary Examiner:
HALPERN, MARK
Attorney, Agent or Firm:
WEGMAN, HESSLER & VANDERBURG (6055 ROCKSIDE WOODS BOULEVARD, SUITE 200, CLEVELAND, OH, 44131, US)
Claims:
1. A pulp thickening apparatus for removing fluid from pulp stock in a dewatering system comprising: a feed manifold for supplying the apparatus with pulp stock; a primary cylindrical filter for accepting said pulp stock from said feed manifold and for creating centrifugal force against said pulp stock to expel liquid therethrough, said primary cylinder filter being devoid of any associated wire member in contact therewith; and first removal means for removing said pulp stock from a surface of said primary cylindrical filter.

2. The pulp thickening apparatus according to claim 1 further comprising a secondary cylindrical filter surrounding said primary cylindrical filter for collecting pulp fibers that flow through said primary filter, said secondary cylindrical filter being devoid of any associated wire member in contact therewith.

3. The pulp thickening apparatus according to claim 2 further comprising a second removal means for removing said pulp stock from a surface of said secondary cylindrical filter.

4. The pulp thickening apparatus according to claim 2 wherein said secondary cylindrical filter has smaller openings than said primary cylindrical filter.

5. The pulp thickening apparatus according to claim 2 wherein said primary and secondary cylindrical filters are both rotatable around a common axis and create centrifugal force which expels fluid from said pulp stock.

6. The pulp thickening apparatus according to claim 5 further comprising an outlet for discharging fluid from said apparatus.

7. The pulp thickening apparatus as recited in claim 1 further comprising a compression roller adjacent said primary cylinder filter and between said feed manifold and said first removal means.

8. The pulp thickening apparatus as recited in claim 1 further comprising a water wash nozzle adjacent said primary cylinder filter and between said feed manifold and said first removal means.

9. The pulp thickening apparatus as recited in claim 1 wherein said feed manifold and first removal means define a first pair of feed manifold and removal means, said dewatering system further comprising another pair of feed manifold and removal means in operative association with said primary cylindrical filter.

10. A method of dewatering pulp stock comprising the steps of: a. delivering pulp stock to a wireless rotating primary filter; b. applying centrifugal force to said pulp stock; c. expelling fluid, small pulp fibers, and contaminants from said pulp stock through said rotating primary filter; d. collecting primary accepts including fibers and other material on said rotating primary filter; and e. removing said primary accepts from said primary filter.

11. The method according to claim 10 further comprising the step of receiving said small pulp fibers and contaminants on a rotating wireless secondary filter.

12. A wireless pulp thickening system comprising: two cylindrical filters for expelling water and contaminates from pulp stock by centrifugal force with a feed-line for supplying said filters with pulp stock.

13. Method for thickening pulp stock comprising: a. providing a first rotatable cylinder carrying a first filter medium; b. providing a second rotatable cylinder carrying a second filter medium; c. feeding a pulp stock suspension into said first rotatable cylinder; d. subjecting said pulp stock suspension to centrifugal force in said first rotatable cylinder to expel liquid, small fibers, and contaminants through said first filter medium and collect primary accepts from said first filter medium; e. forwarding said expelled liquid, small fibers, and contaminants into said second rotatable cylinder; and f. subjecting said expelled liquid, small fibers, and contaminants to centrifugal force in said second rotatable cylinder to expel liquid through said second filter medium and collect secondary accepts from said secondary filter medium.

14. Method as recited in claim 13, further comprising providing first vane means around the circumference of said first rotatable cylinder to regulate flow of said pulp suspension through said first filter medium.

15. Method as recited in claim 13, further comprising providing second vane means around the circumference of said second rotatable cylinder to regulate flow of said pulp suspension through said second filter medium.

16. Method as recited in claim 13, wherein said second filter medium has a finer mesh than said first filter medium.

17. Method as recited in claim 13, wherein said step (c) comprises feeding a stratified feed of said pulp suspension on an inside surface of said first rotatable cylinder.

18. Method as recited in claim 17 wherein said stratified feed comprises a plural layer arrangement of said pulp suspension having a first layer of short fibers and heavy fines laid contiguous to said inside surface.

19. Method as recited in claim 18 wherein said plural layer arrangement comprises a top layer of long fibers and light fines superposed on said first layer and remote from said inside surface.

20. Method as recited in claim 13, wherein said second rotatable cylinder is concentrically disposed with and surrounds said first rotatable cylinder.

21. Apparatus for thickening a pulp stock suspension comprising: a. first rotatable cylinder carrying a first filter medium; b. a second rotatable cylinder carrying a second filter medium; c. means for feeding a pulp stock suspension into said first rotatable cylinder; d. means for rotating said first and second rotatable cylinder for creating centrifugal force therein whereby pulp stock suspension in said first rotatable cylinder will impinge upon said first filter medium with primary accepts forming on one said of said first filter medium and with liquid and other material from said pulp stock suspension being expelled through said first filter medium and into contact with said second filter medium, whereby secondary accepts will gather on said second filter medium; e. means for collecting accepts material gathered on said first filter medium; f. means for collecting accepts material formed on said second filter medium; and g. means for transporting materials passing through said first and second filter media away from said apparatus.

22. Apparatus as recited in claim 21, wherein said second filter medium is of a finer mesh than said first filter medium.

23. Apparatus as recited in claim 21, wherein said second rotatable cylinder is concentric with and surrounds said first rotatable cylinder.

24. Apparatus as recited in claim 21, wherein said first rotatable cylinder comprises a first set of adjustable vane means around the circumference of said first rotatable cylinder for selectively adjusting gaps between individual ones of said vanes to regulate flow of said pulp stock suspension through said first rotatable cylinder.

25. Apparatus as recited in claim 24, wherein said second rotatable cylinder comprises a second set of adjustable vane means around the circumference of said second rotatable cylinder for selectively adjusting gaps between individual ones of said second set of vanes to regulate flow of said pulp suspension through said second filter medium.

26. Headbox feeder adapted for feeding pulp suspension to a filter medium surface, said feeder apparatus comprising a cyclonic separator in the form of an enclosed truncated cone having a central axis, a base end, and a closed apex end, an inlet opening at said base end and laterally offset from said axis to impart a rotational vector to said pulp suspension upon entry into said cone, a feed slot manifold connected to said cone between said base end and said closed apex end providing an exit for said pulp suspension.

27. Headbox feeder apparatus as recited in claim 26 wherein said feed slot manifold comprises a housing portion extending away from truncated cone.

28. Headbox feed apparatus as recited in claim 27 wherein said feed slot manifold is laterally offset from said axis.

29. Headbox feed apparatus as recited in claim 27 wherein said housing has a first leg extending away from said truncated cone and a radiused section terminating in a slot.

30. Method of feeding a pulp suspension to a filter surface comprising: a. providing a separator; b. admitting said pulp suspension to said separator; c. forming a stratified arrangement of said suspension in said separator, including a first layer of heavier fines, short fibers, and contaminants and a second layer of longer fibers and lighter fines; and d. feeding said stratified arrangement to said filter surface.

31. Method as described in claim 30 wherein said step (d) comprises placing said first layer contiguous with said filter surface with said second layer superposed over said first layer.

32. Method as described in claim 30 wherein said step (d) comprises placing said second layer of longer fibers and lighter fines contiguous with said filter surface and said first layer superposed over said second layer.

33. Method of feeding a pulp suspension to a filter surface as recited in claim 30 further comprising forming an intermediate layer of fines and fibers, said intermediate layer located intermediate said first and second layers.

34. Method of feeding a pulp suspension to a filter surface as recited in claim 28 wherein said separator is a headbox feeder apparatus.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of U.S. Provisional Patent Application 60/563,066 filed Apr. 16, 2004.

FIELD OF THE INVENTION

The present invention is related to the washing and/or thickening of pulp and more particularly to a washer with a two stage dewatering zone without a wire belt.

BACKGROUND OF THE INVENTION

The pulp and paper making industry has for many years made regular use of apparatus for washing and/or thickening pulp and paper stock. One apparatus commonly used in the prior art practice, described in U.S. Pat. No. 5,382,327 by Seifert et al., is known as a double nip thickener (DNT) or simply a washer and is relatively closely comparable in structure and mode of operation with a cylinder type paper forming machine in that its main components are a pair of wire-covered cylinders and a vat in which the cylinders rotate.

The DNT may be used as an extraction device, to thicken the stock, or may be used primarily for washing of the pulp fibers. In operation, the pulp stock is fed to the nip between a rotating breast cylinder and wire entrained thereover. The pulp is dewatered by centrifugal force and travels on the wire to a second or couch roller for further dewatering. The pulp is removed from the couch roller by gravity and a doctor blade and collected on a conveyor which moves the dewatered pulp downstream from the DNT.

In many of these prior art devices, an endless belt or loop of foraminous “wire” is trained around the rolls and defines therewith a space in which the other operating parts of the washer are located. The wire may consist preferably of a plastic material, i.e., polyester. Since the product of the machine is pulp, rather than a sheet on which wire marking may be undesirable, the wire may be of the pin seam type wherein the ends of the wire belt include overlapping loops which are releasably fastened together by a metal “pin” inserted through these loops. Use of pin seam wires makes possible the changing of wires after wear without the cantilevering of the rolls which is required if the wire is an endless loop without a seam. In addition, a wire thickener or washer has limited rotational speed due to wire belt and other component wear concerns, thus the hydraulic capacity of the machine is directly related to the rotational speed.

During the dewatering process of the pulp, pulp stock is fed on to the rotating wiring. Centrifugal force throws fluid with contaminants, and usually good fibers from the pulp, outward through the filtered wire.

Accordingly, it would be beneficial to have a pulp thickening or washer device that would eliminate a wire belt as the filtering medium, increase the rotational speed of the thickener to create additional centrifugal force therefore removing more fluid from the pulp, and prevent the waste of good pulp fibers.

SUMMARY OF THE INVENTION

The present invention provides for a pulp washing or thickening apparatus for removing fluid from pulp stock in a dewatering system. The apparatus includes a feed manifold for supplying the apparatus with pulp stock, a primary cylindrical rotating filter for accepting the pulp stock from the feed manifold and for creating centrifugal force against the pulp stock to expel liquid, and a first doctor blade for assisting the removal of the pulp stock from an inner surface of the primary cylindrical filter after the pulp stock is dewatered.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a front view of one embodiment of the washer;

FIG. 2 shows a side view of the washer of FIG. 1;

FIG. 3 shows a perspective front view of the internal components of an apparatus in accordance with the present invention;

FIG. 4 shows a perspective back view of the apparatus shown in FIG. 3;

FIG. 5 is a diagrammatical view of a preferred pulp feeder in accordance with the invention, positioned adjacent the inner or primary cylinder in the apparatus shown in FIG. 3;

FIG. 6 is a simplified cross sectional view of a stratified pulp feed fed adjacent the filter surface of the inner cylinder shown in FIG. 5.

FIG. 7 is a diagrammatical view of another pulp feeder positioned adjacent the inner or primary cylinder in the apparatus shown in FIG. 3;

FIG. 8 is a simplified cross sectional view of an inverted pulp feed nozzle taken along the plane shown by the arrows 8-8 in FIG. 7;

FIG. 9 is a simplified schematic cross section of another embodiment of one of the rotatable filter cylinders in accordance with the invention highlighting positioning of a compression roller between the pulp feed and doctor removal;

FIG. 10 is a simplified schematic cross section of another embodiment of one of the rotatable filter cylinders in accordance with the invention showing disposition of a wash water feed nozzle in relation to the traveling mat; and

FIG. 11 is a simplified schematic cross section of yet another embodiment showing plural sets of feed headers and doctor removal means.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described with reference to the drawings. FIG. 1 illustrates a structure of a pulp thickening or dewatering apparatus 5. The structure operates with a motor 10 and includes a headbox 20 for delivering feedstock and a primary cylindrical filter 30 for thickening the pulp.

Feedstock is delivered to the apparatus 5 similarly to the prior art thickeners or washers. The headbox 20, shown in FIG. 3, delivers a jet of feedstock onto an inner surface 32 of the primary cylindrical filter 30. Feedstock to be thickened or dewatered is fed to the headbox 20 by a feedline (not shown) from a typical stock supply pump (not shown).

The motor 10, shown in FIGS. 1 and 2, drives axle 2 housed in bearing journal 12, shown in FIG. 4. The motor 10 rotates the primary cylindrical filter 30 through belt 14. Headbox 20 delivers the feedstock to the inner surface 32. Centrifugal force is created by the rotation of the cylindrical filter 30 and controlled by the set speed of the motor 10. As the primary cylindrical filter 30 rotates, the force pulls fluid, and other contaminants from the feedstock. In addition to the liquid, the centrifugal force is strong enough to pull viable, small pulp fibers through the wire meshed body of the primary filter. The fluid, contaminants, and viable fibers are referred to as primary discharge from the primary cylindrical filter 30. The openings of the meshed body are of sufficiently large size to assure full flow therethrough of the primary discharge which may be included in the feedstock.

The jet of feedstock from the headbox 20 is directed at a high point onto the inner surface 32 of the primary cylinder 30. Some of the stock is immediately partially dewatered because the force of the jet causes liquid to travel through the meshed body. Accepts material may be scraped from the inside of the cylinder 30 by doctor blade 50 with the accepts collected by the doctor assembly transported away through outlet pipe 51.

Turning to FIGS. 3 and 4, the primary discharge is filtered through a meshed body of the primary cylindrical filter 30 and caught or received by an inner surface 42 of a secondary cylindrical filter 40. The secondary cylindrical filter 40 surrounds and is concentric with the primary cylindrical filter 30 and preferably has a more finely meshed body than the primary filter 30. The motor 10 also rotates the secondary cylindrical filter 40 too, since the cylinders 32 and 40 are commonly mounted to surface 62 which is fixed to axle 2 through hub 16. Centrifugal force is again created on the pulp around the secondary filter 40. In the embodiment shown in the drawings, the meshed body of the secondary filter 40 has smaller openings than the primary filter in order to predominantly discharge liquid, or secondary discharge, and not pulp fibers. In one aspect of the invention, the purpose of the secondary cylindrical filter 40 is to “catch” and save the acceptable fibers discharged from the primary filter 30 by centrifugal force and continue the dewatering process. Optionally, additional cylindrical filters may be added to the apparatus to further filter and “catch” the good fibers that pass through the meshed body of the secondary filter 40.

As shown, the primary and secondary cylindrical filters are attached to a common drive. Alternatively, the two filters could be separately driven for g-force optimization. The primary cylindrical filter is driven with about 30-120 g-forces and the secondary cylindrical filter is driven with about 60-300 g-forces to provide the desired filtrate clarity and capacity. Variable speed drives can also be provided for either or both of the cylindrical filters so that g-loads and washing efficiency can be controlled within a wide range of operating parameters. As stated above, in contradistinction to many of the prior art devices, wires or similar rotating meshes are not trained or disposed around the cylinders in the present invention in order to form a nip with the cylinder to express liquid from the pulp suspension located in the nip region. Accordingly, the apparatus 5 is subjected to lower amounts of loading, can achieve higher rotation speeds, and can result in increased hydraulic capacity and fines separation efficiency.

The continuous application of centrifugal force causes the liquid component of the feedstock to be expressed through the wire meshing of the primary and secondary filters while the pulp materials suspended therein are held by the meshed body and form a layer on the inner surfaces 32, 42 of the primary and secondary cylindrical filters 30, 40. The liquid expressed through the secondary discharge is expelled from the apparatus 5 through port 60. As known to those skilled in the art, the liquid often carries very fine solid particles such as ink, PSA, etc. These solids are desirably discharged through the port also along with the liquid carrier. As shown, secondary accepts material traveling on an inner surface 42 of the cylinder 40 is removed via doctor 52 and travels through egress piping 55.

The function of the foraminous bodies on the cylinders is to serve as a filter medium that holds the fiber from the feedstock and other desired solid constituents of the feedstock on its inner surface against the action of centrifugal force, which is the major factor causing dewatering of the retained pulp. Vanes 35, 33 shown in FIGS. 3 and 4, are located along the circumference of the primary and secondary cylindrical filters 30, 40. These vanes create openings that may be controlled by a solenoid switch or other conventional controlling means to make the openings therebetween smaller or larger. By adjusting the openings between the vanes, the flow rate and pressure through the filter media are capable of being varied depending on the criteria and conditions of the thickening process. Mat thickness is controlled by the feed flow, feed consistency, and relative rotation speed, in addition to the filter open area and the amount of material that passes through the filter.

The first doctor blade 50 and associated outlet pipe 51 remove the thickened/dewatered pulp from the inner surface 32 of the primary filter. The thickened/dewatered pulp is transferred from the inner surface 32 of the primary cylindrical filter 30 to a downstream process in the papermaking system. A screw like conveyor (not shown) or other transport means may be operatively associated with the pipe 51 to aid in transferring the pulp. When the mat touches the doctor blades, the mat may become airborne. Directional steering jets of air may be employed to keep the mat airborne and blow the mat out of the washer 5 into the doctor collection assembly. Alternatively, a vacuum may be used to pull the pulp from the washer 5. It is noted that the primary accepts may be subjected to an additional thickening operation before removal from the washer 5.

For the dewatered pulp on the inner surface 42 of the secondary cylindrical filter 40, a second doctor blade assembly 52 and associated collection pipe 55 are provided, and the removal process is the same as above. The purpose of the second doctor blade 52 is to clean the filter media and recover any fiber that was plugged on the filter media surface of the secondary filter 42.

To release the mat of pulp from the filter media, a backflush of water at a pressure great enough to exceed the opposing g-force may be used to impinge the cylinder surface just upstream from or at the leading edge of the first doctor blade assembly 50. The pulp not collected by the first doctor blade assembly 50 will be filtered out again on the primary cylindrical filter 30. A second backflush of water at the leading edge of the second doctor blade 52 may be used to collect the pulp that is retained on the secondary cylindrical 40 filter in order that the filter media does not become clogged over time.

In an alternative embodiment of the present invention, non-contacting doctor blades may be used to remove the thickened mat from the filter media of the primary and secondary cylindrical filters 30, 40. For example, a very fine, high pressure jet or fan of water or air that blows from the inside to get under the fiber mat and lift the mat off the filter media or blows from the backside to blow the mat off may be mentioned as exemplary.

In an alternative method of operation, a pulsed system may be used to increase the dewatering levels and thicken the pulp more. This could be achieved by selective, intermittent contact of the first and second doctor blades 50, 52 with their associated rotating cylinders to remove the thickened pulp after every two or more revolutions of the primary and secondary cylindrical filters 30, 40. This would allow a longer period of time for centrifugal force to be applied to the pulp forcing liquid to expel from the pulp.

In the washer 5, the filter media for the primary and secondary cylindrical filters 30, 40 may be replaceable. In one embodiment, the filter media is in the form of a cartridge that inserts into the primary and secondary cylindrical filters 30, 40. In another embodiment, the filter media may be installed and tensioned in place to the primary or secondary cylindrical filters 30, 40.

FIG. 5 depicts a preferred pulp feed device or headbox 20 in accordance with the invention. Here, the headbox is in the form of a cyclonic separator having a tangential or swirl inlet 90 through which pulp suspension is fed under high pressure conditions as is well known in the art to impart a rotational pattern to the pulp suspension as it travels through the cyclone. The cyclonic separator includes a conical section 72 with the diameter of the cone decreasing from the inlet to the solid apex 74 end of the cyclone. The headbox overall is in the form of a truncated cone having an axis 95, base 93, and enclosed apex end 74. The inlet opening 90 is laterally offset from the axis 95.

An elongated nozzle in the form of a slotted opening 78 comprising a generally planar housing surface 76 extends tangentially from the conical body in laterally offset position from axis 95. Pulp suspension exits the cyclone through this slotted opening 78 with the opening positioned adjacent the inner surface 32 of the primary filter 30.

The slotted opening presents an elongated opening with its elongated dimension oriented parallel to the surface of the inner cylinder 32 as this surface is positioned adjacent the opening 78 during its rotation.

As shown in FIG. 6, the suspension exiting the opening 78 and thus being fed to the inner surface 32 of the primary filter is in the form of a stratified feed 100 comprising a laminar, layered arrangement of suspended solids and entrained moisture from the pulp suspension. The heavier fines and short filters are predominantly located in a first layer 80 contiguous to the inner surface 32 of the primary filter. The top layer 84 most remote from the inner surface consists primarily of long fibers and light fines with an intermediate layer 82 composed of a mixture of fines and fibers.

By employment of the stratified feed shown in FIG. 6, fluid throughput is enhanced since the long fibers and light materials are slightly spaced from the filter medium surface. These long fibers and light materials, if placed directly against the cylinder could tend to form a thickened mat, blocking unimpeded access of the fines to the openings in the mesh. In other situations, it may be desirable to include a nozzle feeder of the type in which the stratified feed shown in FIG. 6 is inverted. That is, adapters or the like could be associated with the nozzle to invert the feed shown in FIG. 6 so that the lighter fines and long fibers would be placed contiguous to the inner surface of the cylinder with the short fibers and heavier fibers disposed in a “top” layer remote from the surface. This type of inverted disposition of the stratified feed can also be achieved by rotating the positioning of the conical body 72 180° from the particular disposition shown in FIG. 5.

FIG. 7 depicts another embodiment in which an inverted nozzle 178 is used in lieu of the nozzle 78 shown in FIG. 5. The nozzle 178 comprises an approximate 2-3 o'clock location take off spout 200 from the clockwise rotation of the pulp suspension as it rotates within the cyclonic conical body 72. An elongated, housed, radiused channel 202 terminating in slot 204 feeds the suspension to the filter surface 32 in the inverted position.

As shown in FIG. 8, employment of this type of nozzle exit from the cyclonic separator flips the orientation of the pulp suspension from that depicted in FIG. 6 so that the light, long fibers 84 are disposed along the surface 32 with an intermediate layer 82 of fines and intermediate fibers disposed over the layer 84 and a dense layer 80 of heavy fines and short fibers extending over the layer 82.

In accordance with the embodiment shown in FIGS. 7 and 8, improved washing efficiency can be provided as the long light fibers serve as a supplement to the filter medium to help with filtering and yield loss. As indicated above, the stratified feed shown in FIGS. 5 and 6 would be chosen in those instances in which hydraulic capacity would need be enhanced.

FIG. 9 depicts another embodiment showing primary filter 30 with compression roll 188 located intermediate headbox 20 and doctor 50. In this aspect of the invention, the pulp suspension is compressed by the nip of the roller along the inner surface 32 of the primary filter upstream from mat removal via doctor 50 to enhance mat consistency. Although the arrangement shown in this figure depicts a compression roller in operative association with the primary filter, compression roller or rollers could also be used in conjunction with the secondary filter, also to improve consistency of the mat being removed.

It is also possible to add wash shower nozzles along either or both of the rotating cylinders. FIG. 10 shows one such arrangement wherein wash shower nozzle 190 is provided intermediate headbox 20 and doctor 50 to provide wash water to the traveling mat. Other arrangements such as that shown in FIG. 11 are possible wherein plural feed headers and plural doctors can be disposed along the rotating filter medium. As shown in FIG. 11, a second or downstream set of feed header and doctor removal means is provided downstream from a first set. Downstream header 20b and downstream doctor 50b provide for plural pulp feed and removal cycles along a single rotatable filter surface. This results in a very cost effective design.

Preliminary results have indicated that the dual zone washer is effective in removing ash, ink, and stickies, and fines from pulp slurries that may include, for example, copy paper, OCC, and mixed office waste. At present, it is preferred that the inner wire medium be more open than the outer wire medium. Present data suggests that ash removal is optimized with a 60 micron filter medium with fines removal optimized utilizing a 200 micron filter medium. The filter medium may, for example, include nylon, polyester, or other polymer mesh screens having opening from about 30-600 microns. Woven polymer media having square openings are presently preferred. However, a wedgewire filter cartridge may also be mentioned as an acceptable filter medium. For example, a wedgewire cartridge having slot openings of from about 0.001-0.010 inches may be provided with 0.020-0.045 inch thick smooth wires. Preliminary results indicate that the dual zone washer shows improved ash removal while exhibiting minimal fiber loss.

Exemplary operating parameters include inner and outer concentrically disposed cylinders having lengths of about 300 cm. The inner cylinder, in one embodiment, has a diameter of about 250 cm. Thus, the apparatus is compact, leading to decreased floor space requirements. G-forces exerted on the slurry typically may be on the order of about 50-350 G. Initial feed consistency to the dual zone washer may be on the order of about 0.1-10%, preferably about 0.5-5.0%.

Although the invention has been described in detail with reference to certain preferred embodiments thereof, other embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiment contained herein.