WO2003091497A1

WO2003091497A1 - A screen section, screen cylinder, screening device, and method of screening fibrous material

Info

Publication number: WO2003091497A1
Application number: PCT/US2003/012624
Authority: WO
Inventors: Frey A. Frejborg
Original assignee: Frejborg Frey A
Priority date: 2002-04-24
Filing date: 2003-04-24
Publication date: 2003-11-06
Also published as: AU2003221774A1

Abstract

A screen section, a cylindrical screen basket, a screening device, and a method for treating comminuted cellulosic fibrous material to provide improved fractionation or debris removal and improved efficiency of operation. The screen section, screen basket, screening device, and method for treating employ a screen configuration that, among other things, minimizes reject or fractionation thickening. One aspect of the invention provides a screen element (51) having a first surface (60) having a plurality of first apertures (62) for passing fibrous material and liquid when the first surface (60) is exposed to pressure pulses generally directed in a first direction (72); a second surface (61), opposite the first surface (60), having a plurality of second apertures (68) in fluid communication with the first apertures (62) for passing liquid when the second surface (61) is exposed to pressure pulses generally directed in a second direction (76), opposite the first direction; and means for minimizing the passage of fibrous material through the first apertures (62) in the second direction (76).

Description

A Screen Section, Screen Cylinder, Screening Device, and Method of Screening Fibrous Material

Cross Reference to Related Applications

This application claims priority to pending U.S. Provisional Application 60/375,112 filed on April 24, 2002, the disclosure of which is incorporated by reference herein in its entirety.

Background of the Invention

In the processing of cellulosic fibrous material, for example, synthetic fibers, virgin wood fibers, and recycled paper fibers, among others, to produce paper and related products, it is often necessary to isolate fibrous material from slurries of fibrous material and other components. This process is known as "screening" and typically comprises passing a fibrous slurry through a device, typically called a "screen" or "pressure screen" which isolates the fibrous material from, for example, non-fibrous materials, such as undesirable dirt, over-sized wood pieces, fiber bundles, or other forms of debris.

A related process of treating fibrous material is "fractionation". By a mechanism similar to screening, fractionation is a process of isolating one fiber fraction from another fiber fraction, for example, long wood fibers from short wood fibers. Fractionation is typically performed in devices similar to the devices used for screening, for example, circular cylinders having apertures for allowing one size of fiber to pass while minimizing the passage of other fibers. Fractionation is typically performed with screen baskets having smaller apertures, for example, smaller apertures than are typically used for "screening". Fractionation may also be performed at typically higher fiber slurry consistencies than screening, for example, at 2 to 3 % consistency instead of 0.5 to 1.5 % consistency.

U.S. patents 4,529,520; 4,880,540; 4,950,402; and 5,000,842 and PCT publication WO 00/65151 generally discuss pressure screens and pressure-screen screen-basket assemblies used to treat fibrous slurries in the Pulp and Paper Industry. Such screens are used to isolate fibrous material from undesirably material, that is, debris, in several areas of fiber or paper processing. For example, in the chemical processing of wood chips to make cellulose pulp (for example, by the kraft or sulfite processes), screens are used to remove fibers from spent cooking liquors so that the liquors can be re-used and the fibers salvaged. Screens are also used to remove debris and undesirably large cellulose material (that is, shives and slivers), collectively referred to as "rejects", from acceptable fibers, collectively referred to as "accepts", prior to further processing of the accepts. These screens are also used for fractionation, as discussed above. In the processing of recycled fibers, screens are used to remove debris, including ink, dirt, and plastics, glues, and wax substances (collectively known as "stickies") during the recovery and processing of recycled papers from, for example, old news print (ONP), old corrugated containers (OCC), or mixed office waste (MOW), among other recycled paper sources. Screens are also used in the area of the paper machine (that is, as "head-box screens") in order to remove dirt or other debris from the slurry of pulp fibers prior to or during the paper making process.

Ideally, in the art of fiber slurry screening and fractionation, a slurry of fiber, liquid (essentially water), and debris is separated into a slurry of acceptable fiber and liquid and a slurry of debris and liquid at the lowest cost. However, due to the mechanics of screen operation and the behavior of fibers in solution, the operation of conventional screening devices is a balance between screen efficiency and screen cost. One behavior of fibers that interferes with the ideal separation of fiber from fiber slurries is the phenomenon known as "fiber flocculation". Fibers in solution, for example, cellulose fibers in solution, have a natural tendency to form fiber aggregates known as "flocks". The longer the individual fibers and the rougher the fiber surfaces, the stronger will be the tendency to form flocks and the stronger will be the internal bond forces between fibers in a flock. The formation of fiber flocks in a fiber suspension can be minimized by agitating the fiber suspension to prevent the fibers from agglomerating. However, this agitation (typically continuous agitation) requires the input of energy to maintain a turbulent kinetic energy field to prevent fiber agglomeration. This aggressive agitation of fibers in suspension to, among other things, prevent fiber flocculation is sometimes referred to as "fluidization", that is, making the suspension behave more like a fluid than a solid. Depending on the feed-side consistency of a screen, the characteristics of the fiber being handled, and the amount of turbulent kinetic energy introduced, fiber flocks may form in any given localized area, and typically form rapidly. Fiber flocks can form within a few seconds or even within a few fractions of a second.

In fiber screening and fiber fractionation, one way to minimize fiber flocculation and promote fluidization is to reduce the consistency of the fiber slurry introduced to the screening device. Another way to minimize fiber flocculation is to continuously agitate the fiber slurry in the vicinity of the screening surface.

Screening or fractionating devices, or simply screens, typically include at least one cylindrical, perforated screening basket and some means of repeatedly applying pressure pulses to the screening basket to aid in the isolation of the accepts from the rejects. These pressure pulses are typically applied by hydrofoils, or simply "foils", which are either rotated relative to a stationary screening basket or fixed relative to a rotating screening basket. Foils can be mounted on rotating arms (known as "spiders") or mounted upon the surface of rotating cylinders, both of which are known as "rotors". Foils, either stationary or moving, travel within a few millimeters of the surface of the screen basket, either the outside surface or the inside surface, and propel the liquid between the foil and the screen surface against the surface as, for example, the rotor turns. This hydraulic pressure, or "positive pulse", against the screen surface provides a localized pulse of fluid pressure to the fiber mat that is built on the screen surface and aids in propelling acceptable fiber through the screen, while the unacceptable fiber and rejects remain on the near side, or "rejects side" or "feed side", of the screen basket.

The shape of conventional foils also provides what is called the "negative pulse". After the leading edge of the foil imparts the positive pulse, the shape of the foil behind the leading edge and the speed of the foil across the surface of the screen provides a localized region of lower pressure that draws fluid from the accept side of the screen to the feed side. This negative pulse helps to dislodge fibers and debris from the apertures on the feed side of the screen and clears the way for the flow of acceptable fiber through the screen. This positive-pulse-negative-pulse process is repeated continuously as the slurry of treated fibers advances along the screen basket from the inlet to the outlet, typically in a vertical flow path, for example, a helical, vertical flow path. The faster the rotor is turned the more radially oriented the helical flow path will become.

A similar mechanism is effected in fractionation, though in fractionation the "reject" stream comprises fiber typically of a different characteristic, for example, longer length, than the "accept" stream. Though aspects of the present invention can be used to effect both screening and fractionation, the terms "screen" or "screen element", or "screening method" are used throughout this discussion. It is to be understood that the use of these terms does not limit the aspects discussed to screening alone, all aspects of the present invention are typically also applicable to fractionation.

One common feature of screen baskets and screening and fractionating devices of the prior art is what is referred to as "reject thickening". Typically, the hydraulic pressure of the fluid introduced to the screening device at the inlet is greater than the hydraulic pressure of the slurry at the accept outlet. As a result, as the fiber slurry proceeds along the screen basket from the inlet to the outlet, the difference in pressure across the screen basket typically causes liquid in the feed side (also referred to as the "reject side") of the screen basket to flow through the screen to the accept side of the screen basket, thus reducing the relative fluid content of the slurry on the feed side of the screen as the slurry proceeds toward the reject outlet. For example, where the fiber slurry introduced at the inlet of the screening device is typically between about 0.8% and 3.0% consistency (that is, the weight percent fiber to total weight of the slurry), the slurry on the reject side may thickened by as a much as 50 % or even 100% depending upon the screening device used. For example, for a feed consistency of about 3.0%, the slurry on the feed side of the screen may be thickened to about 4.5% consistency or more. However, this reject thickening interferes with proper operation of the screening or fractionating device, for example, the thicker slurry on the feed side of the screen basket is more likely to promote the agglomeration of the fibers into undesirable flocks. Among other things, the fiber flocks interfere with the flow of acceptable fiber through the screen and thus negatively impact the capacity of the screen basket at later stages of the screening. In extreme cases, reject thickening may cause the entire screen surface to be plugged with fiber and render the screen useless unless unplugged. In order to minimize reject thickening, some screen suppliers have chosen to limit the length of their screen baskets in order to limit the potential for reject thickening. Reject thickening also negatively impacts power usage and can cause stress upon screen components, for example, on foils, screens, and drive components. Thus, there is a need in the art to provide screening elements or screen baskets having screening elements that reduce or minimize reject thickening. Aspects of the present invention overcome limitations due to reject thickening and, among other things, provide improved debris removal, better utilization of screen area, and lower localized passing velocities through the screens and screen elements.

Another aspect of prior art screening basket design and operation is the impact of the negative pulse flow upon the performance or capacity of the screen. As discussed above, conventional foil and screen operation typically provide a negative pulse of fluid pressure which dislodges fibers and debris from screen apertures as the foil passes the apertures. However, this negative flow of slurry from the accept side to the feed side of the screen typically includes liquid and at least some fiber, in particular, some acceptable fiber. That is, in typical prior art screen design and operation, at least some good, acceptable fiber that has already been screened once is transferred from the accept side of the screen to the reject or feed side of the screen. Of course, if this fiber is to be recovered it must again pass through the screen during the positive pulse cycle. This "recirculation" of acceptable fibers impacts the efficiency and capacity of the screening device by, among other things, interfering with the passage of fibers that have not already passed through the screen to the accept side of the screen and, for a given production rate, requires that the instantaneous flow through screen apertures be greater than the nominal flow (indicative of the production rate) to account for the recirculation of acceptable fibers. Thus, there is thus also a need in the art to provide screen elements, screen baskets, and methods of screening which minimize the undesirable recirculation of acceptable fiber to the reject side of the screen.

A further undesirable feature of the negative pulse that characterizes conventional screen design and operation is the effect the negative pulse injection or flow has on the fiber slurry on the feed side of the screen. The negative pulse not only can undesirably carry acceptable fiber back to the feed side of the screen, as discussed above, but furthermore the negative pulse, and the volume of liquid it comprises, can introduce localized regions of higher fluid pressure or pressure fields on the feed side of the screen. These localized regions of high-pressure liquid on the feed side of the screen, that is, typically having a pressure greater than the ambient fluid pressure on the feed side of the screen, and also substantially greater than the localized pressure in the adjacent accept side of the screen basket, will naturally cause the flow of slurry to the regions of lower pressure. Due to the dynamic rotation of the foil and the agitated state of the slurry on the feed side of the screen, it is likely that this high-pressure fluid introduced to the feed side from the accept side will likely flow through the same or adjacent apertures toward the accept side of the screen. This initial injection-type flow, which is, typically, a rapid, relatively higher volume flow compared to nominal feed-side-to-accept-side flow, can carry undesirable debris to the accept side of the screen. Thus, the rapid, relatively higher volume injection of fluid from the accept side to the feed side due to the "negative pulse" can negatively impact the debris removal efficiency of screening devices. Thus, there is also a need in the art to provide screen elements, screen baskets, foil elements, and methods of screening which minimizes the undesirable flow of relatively high-pressure, high-volume liquid from the accept side of the screen to the feed side of the screen.

However, according to the existing art, the perforations through the screen basket are typically optimized to enhance the flow of acceptable fiber and liquid from the feed side of the screen to the accept side of the screen, with little or no consideration of the impact of the flow of the slurry, its content, pressure, or volume passing from the accept side to the feed side during the negative pulse. In fact, in some prior art screen designs (for example, the screen design described by Weckroth, et al. in the article "Recent developments in papermachine headbox screening", Paper and Timber, Vol. 83, No. 6, 2001), the open area of the screen is maximized to promote passage of accept flow with little or no consideration of the impact of such a design has on the rate at which acceptable fiber is recirculated from the accept side of the screen back to the feed side of the screen. In addition, typical prior art screen design also disregards the impact of the creation of localized high-pressure regions on the feed side of the screen on the flow of debris from the feed side to the accept side. Thus, there is also a need in the art to provide a screening element and a screening device and method which minimizes the recirculation of acceptable fiber from the accept side of the screen to the reject side of the screen and minimizes the pressure or volume of liquid injected into the feed side of the screen by the negative pulse which characterize screen design and operation.

During fractionation, for example, when separating long fibers from short fibers in a fiber slurry, the fiber in the slurry being treated typically builds up on the feed side of the fractionating element, or screen. This fiber build up, or fiber mat, on the surface of the screening element typically creates a further barrier to the passage of fiber, especially, long fibers, from the feed side of the screen to the accept side of the screen. Though the screen itself and the fiber mat limit the passage of fiber, the water in the slurry is relatively unhindered and typically passes through the fiber mat and the screen cylinder to the accept side of the screen. This passage of water, typically radially, may be assisted by the centrifugal effect of the rotating rotor or rotating cylinder. This passage of water from the feed side to the accept side of the screen inherently promotes dewatering or thickening of the slurry on the feed side of the screen during fractionation. The thickening of the slurry on the feed side of the screen limits the capacity and reduces the efficiency of the fractionation process. Due to presence of thickening, fractionation is typically performed at higher than normally desired consistencies which creates unstable operating conditions and limits the capacity of the fractionating process. In one aspect of the present invention, the performance of a fractionating screen can be improved by reducing the degree of thickening that occurs, for example, by selectively diluting the slurry being fractionated.

One important operational factor in screening or fractionation is the "absolute passing velocity". Absolute passing velocity is the velocity of slurry through the apertures during the positive pulse or flow cycle from the feed side toward the accept side of the screen cylinder. In fractionation, since the fibers being separated are much smaller than conventional "debris", the apertures used are typically considerably smaller than conventional apertures used for screening or debris removal. Effective separation of longer fibers from short fibers not only requires smaller apertures, but also requires considerably lower absolute passing velocity. The lower the absolute passing velocity, the lower the pushing / pulling forces on the fibers. In the conventional fractionation art, the desire to have lower passing velocities and smaller apertures is addressed by using screens having the highest possible percentage of open area. As a result, in the conventional art, fractionation is typically capacity limited, that is, the amount of fiber that can be processed in any one device is limited. According to one aspect of the present invention, these limitations on capacity and fractionation efficiency are overcome.

Prior art screen designs such as those shown in U.S. patent 4,471 ,298 of Holz and U.S. Patent 4,529, 519 of Holz do not recognize or address these problems, nor do the devices disclosed in these patents provide the same improvements as the present invention. The present invention and many of its aspects address these and other limitations of prior art screening and fractionation.

Summary of the Invention

Aspects of the present invention address many of the problems associated with the operation of screens, in particular, pressure screens, in the prior art. One aspect of the invention is a screen element for treating slurries of fibrous material and liquid, the devices having means for exposing the screen element to fluid pressure pulses, the screen element comprising: a first surface having a plurality of first apertures for passing some fibrous material and liquid when the first surface is exposed to pressure pulses generally directed in a first direction; a second surface, opposite the first surface, having a plurality of second apertures in fluid communication with the first apertures for passing liquid when the second surface is exposed to pressure pulses generally directed in a second direction, opposite the first direction; and means for minimizing the passage of fibrous material through the first apertures in the second direction. In one aspect of the invention, the means for minimizing the passage of fibrous material comprises obstructions extending across the second apertures. In another aspect of the invention, the first apertures in the first surface comprise slotted apertures separated by ribs and wherein the obstructions extending across the second apertures comprise the ribs.

Another aspect of the invention is a device for separating at least some fibrous material from a slurry of fibrous material, debris, and liquid, the device comprising: an inlet for introducing the slurry; at least one annular screen element for separating fibrous material from the slurry to produce an accept slurry having little or no debris and a reject slurry having debris, the screen element comprising: a first surface having a plurality of first apertures for passing fibrous material and liquid when the first surface is exposed to pressure pulses generally directed in a first direction; a second surface, opposite the first surface, having a plurality of second apertures in fluid communication with the first apertures for passing liquid when the second surface is exposed to pressure pulses generally directed in a second direction, opposite the first direction; and means for minimizing the passage of fibrous material through the first apertures in the second direction; and at least one foil for providing the pressure pulses, the at least one foil mounted for relative movement with respect to the annular screen element; means for providing relative movement between the at least one annular screen element and the at least one foil; at least one reject slurry discharge; and at least one accept slurry discharge. Again, in one aspect of the invention, little or no fibrous material passes though the first apertures in the first direction, however, typically at least some fibrous material passes through the first apertures in the first direction. In one aspect of the invention, the means for minimizing the passage of fibrous material comprises obstructions extending across the second apertures. In another aspect of the invention, the at least one annular screen element comprises a plurality of spaced annular screen elements.

Another aspect of the invention is a method for separating fibrous material from a slurry of fibrous material, debris, and liquid, using a screen element comprising: a first surface having a plurality of first apertures for passing at least some fibrous material and liquid when the first surface is exposed to pressure pulses generally directed in a first direction; a second surface, opposite the first surface, having a plurality of second apertures for passing liquid when the second surface is exposed to pressure pulses generally directed in a second direction, opposite the first direction; and means for minimizing the passage of fibrous material through the first apertures in the second direction; the method comprising: introducing the slurry to the first surface of the screen element; exposing the slurry on the first surface of the screen element to pressure pulses generally in the first direction, wherein some of the fibrous material and some of the liquid passes through the first apertures in the first direction to produce some accept slurry containing little or no debris; exposing the accept slurry to pressure pulses generally in the second direction wherein liquid from the accept slurry passes through the second apertures and through the first apertures in the second direction; providing means to minimize the flow of fibrous material from the accept slurry through the first apertures in the second direction; discharging the accept slurry containing little or no debris; and discharging the treated slurry containing debris. Again, in one aspect of the invention, little or no fibrous material passes though the first apertures in the first direction, however, typically at least some fibrous material passes through the first apertures in the first direction.

A further aspect of the present invention is a cylindrical screen assembly for devices for treating slurries of fibrous material and liquid, the devices having means for exposing the screen element to fluid pressure pulses, the cylindrical screen assembly comprising at least one cylindrical screen section, the at least one cylindrical screen section comprising: a first surface having a plurality of first apertures for passing fibrous material and liquid when the first surface is exposed to pressure pulses generally directed in a first direction; a second surface, opposite the first surface, having a plurality of second apertures in fluid communication with the first apertures for passing liquid when the second surface is exposed to pressure pulses generally directed in a second direction, opposite the first direction; and means for minimizing the passage of fibrous material through the first apertures in the second direction. Again, in one aspect of the invention, little or no fibrous material passes though the first apertures in the first direction, however, typically at least some fibrous material passes through the first apertures in the first direction. The treatment may be debris removal or fiber fractionation. In one aspect of the invention, the at least one cylindrical screen section comprises a plurality of cylindrical screen sections.

A still further aspect of the present invention is a method for screening a slurry of fibrous material and liquid using a cylindrical screen assembly having a surface, the method comprising: introducing the slurry of fibrous material to the first end of the cylindrical screen assembly at a first consistency wherein the slurry flows along the surface of the cylindrical screen assembly; exposing the slurry to pressure pulses as the slurry flows along the surface of the cylindrical screen assembly; at a first elevation of the screen assembly, removing at least some liquid and some fibrous material from the slurry to produce a slurry having a second consistency higher than the first consistency; at a second elevation of the screen assembly, removing at least some liquid and some fibrous material from the slurry while reintroducing at least some of the liquid removed at the second elevation to produce a slurry having a third consistency, lower than the second consistency; discharging the slurry from the cylindrical screen assembly. Though in the process of the present invention, typically, at least some liquid is removed from the slurry, in one aspect of the invention, removing at least some liquid comprises removing little or no liquid from the slurry. In one aspect of the invention, exposing the slurry to pressure pulses comprises providing at least one hydrofoil adjacent the surface of the cylindrical screen assembly and causing the at least one hydrofoil and the surface of the screen assembly to move relative to each other. In another aspect of the invention, the method further comprises, at a third elevation of the screen assembly, removing at least some liquid and some fibrous material from the slurry to produce a slurry having a fourth consistency higher than the third consistency. A further aspect of this invention is a method that further comprises, at a fourth elevation of the screen assembly, removing at least some liquid and some fibrous material from the slurry while reintroducing at least some of the liquid removed at the fourth elevation to produce a slurry having a fifth consistency, lower than the third consistency.

A still further aspect of the invention is a rotor for use in a slurry treating device, the rotor comprising: at least one foil comprising: a leading edge having a first surface; a trailing edge; and an arc length of at least about 25 degrees; a foil support; and means for mounting the foil support to a to a drive means. In one aspect of the invention, the foil support comprises one of a cylindrical drum and a spider. In another aspect of the invention, the foil support comprises a cylindrical drum having an external surface, and wherein the first surface makes an angle between about 60 degrees and about 120 degrees with the external surface of the drum.

A still further aspect of the invention is a method for fractionating a slurry of fibrous material containing a short fiber fraction and a long fiber fraction, the method using a screen element including a first surface having a plurality of first apertures for passing fibrous material and liquid when the first surface is exposed to pressure pulses generally directed in a first direction; a second surface, opposite the first surface, having a plurality of second apertures for passing liquid when the second surface is exposed to pressure pulses generally directed in a second direction, opposite the first direction; and means for minimizing the passage of fibrous material through the first apertures in the second direction; the method comprising: introducing the slurry to the first surface of the screen element; exposing the slurry on the first surface of the screen element to pressure pulses generally in the first direction, wherein at least some of the short fiber fraction and some of the liquid passes through the first apertures in the first direction to produce an accept slurry containing little or no long fiber fraction; exposing the accept slurry to pressure pulses generally in the second direction wherein liquid from the accept slurry passes through the second apertures and through the first apertures in the second direction; providing means to minimize the flow of short fiber fraction from the accept slurry through the first apertures in the second direction; discharging the accept slurry containing mostly the short fiber fraction; and discharging the treated slurry containing mostly the long fiber fraction.

These and other aspects of the present invention will be become more apparent upon review of the following description of the attached figures.

Brief Description of Figures

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following detailed descriptions of aspects of the invention and the accompanying drawings in which:

FIGURE 1 is a cross-sectional view of a conventional screening device having a screen cylinder according to one aspect of the present invention.

FIGURE 2 is a perspective view of the rotor cylinder shown in FIGURE 1.

FIGURE 3 is an elevation view, partially in cross-section of a screen cylinder according to one aspect of the present invention.

FIGURE 4 is a detailed view of a section of the screen cylinder shown in FIGURE 3. FIGURE 5 is a detailed cross-sectional view of a screening device illustrating screening device operation according to one aspect of the present invention.

FIGURE 5A is a detailed cross-sectional view similar to FIGURE 5 of a screening device illustrating prior art screening device operation.

FIGURE 6 is a comparative illustration of the thickening aspects of one aspect of the present invention compared to the prior art.

FIGURES 7A and 7B illustrate one method of fabricating one aspect of the present invention.

FIGURES 8A and 8B illustrate another method of fabricating one aspect of the present invention.

FIGURE 9 is a perspective view of a rotor according to one aspect of the present invention.

FIGURE 10 is a detailed cross-sectional view of a foil shown in FIGURE 9 according to one aspect of the present invention.

FIGURE 11 is a detailed cross-sectional view of an alternative foil to the foil shown in FIGURE 10 according to one aspect of the present invention.

FIGURE 12 is a side elevation view of the foil shown in FIGURE 11 as viewed along lines 12-12 in FIGURE 11.

FIGURE 13 is a partial perspective view of screen cylinder according to another aspect of the present invention. FIGURES 14A and 14B are front and rear views, respectively, of a screen section according to one aspect of the present invention.

FIGURES 15A and15B are front and rear views, respectively, of a screen section according to the prior art.

Detailed Description of Figures

FIGURE 1 illustrates a cross-section of a typical rotary pressure screen 10 which embodies one aspect of the present invention. As is conventional, pressure screen 10 includes a cylindrical housing 12 having a domed top head 14 and a bottom head 16. Housing 12 is typically a steel housing, for example, a carbon or stainless steel housing. The slurry to be treated by pressure screen 10 is introduced via inlet 18 and the treated slurry is discharged either through accepts outlet 20 or rejects outlet 22. As is also conventional, pressure screen 10 includes a stationary screen cylinder, or "basket", 24 and rotor 26, for example, a drum-type rotor, mounted for rotation in housing 12. Screen basket 24 typically includes one or more mounting flanges 27 which are mounted to housing 12 by means of one or more housing flanges 25, for example, by means of threaded fasteners (not shown). Rotor 26 typically includes a plurality of hydrofoils (or "foils") 29 mounted at various predetermined elevations about the periphery of drum 26 (see FIGURE 2). As is conventional, rotor 26 is coupled to an appropriate drive means 28 which rotates rotor 26 at the speed desired, typically, a speed providing a foil tip speed of between about 20 meters per second [m/s] and about 30 m/s. For example, for a typical rotor having a diameter of about 600 mm, drive means 28 may rotate rotor 26 at a speed of between about 630 rpm and about 950 rpm. Of course, the speed of rotation will vary depending upon the diameter of the rotor in order to adhere to the preferred tip speed. As shown in FIGURE 1 , drive means 28 may be a drive shaft 30 coupled to rotor 26 and driven by a plurality of belt sheaves 32 coupled to a drive motor (not shown) via plurality of drive belts, for example, v-type drive belts. It is understood that, according to aspects of the present invention, other drive means may be used, including direct drive, gear-box drive, and chain drive, among others. Drive means 28 typically will include conventional bearings and seals as appropriate for the mounting of rotor 26 in housing 12.

According to conventional screen operation, in one aspect of the invention, a slurry of fibrous material 34, for example, a slurry of comminuted cellulosic fibrous material, liquid, and undesirable debris is introduced to inlet 18 as indicated by arrow 34. When the slurry of fibrous material 34 comprises cellulosic fibrous material, slurry 34 may comprise virgin fibers, for example, hardwood or softwood fibers, or recycled fibers (including pre- and post-consumer recycled fibers), such as old news print (ONP), old corrugated containers (OCC), or mixed office waste (MOW), among other recycled fiber sources. Slurry 34 typically comprises a slurry having consistency (that is, a percent of fiber weight to total slurry weight) of between about 1.0 % and about 3.0%, depending upon the application in which screen 10 is used. The liquid in the slurry may, typically, be water, but other liquids may be treated with aspects of the present invention. If the slurry is one encountered in the Pulp and Paper Industry, the liquid may contain miscellaneous compounds, such as dissolved organic material, spent cooking chemical, bleaching chemical, paper machine additives, and the like. When used in the Pulp and Paper Industry, slurry 34 may be forwarded from various sources in a Pulp Mill or Paper Mill, for example, from a digester, from a brownstock washer, from a bleach plant, or a paper machine or paper machine approach system. Though shown in FIGURE 1 as being introduced in a radial direction for simplicity of illustration, slurry 34 may be introduced in a tangential direction, for example, in a direction that coincides with the rotation of drum rotor 26 (see FIGURE 2). The radial or tangential feed of the slurry may be directed to either the inside or the outside of the screen cylinder 24 depending upon whether the screen operates in an out-flow or an in-flow mode of operation.

Though the aspect of the invention shown in FIGURE 1 is illustrated as a stationary basket 24 mounted about a drum rotor 26 having foils 29, it is understood that in one aspect of the invention, basket 24 may be mounted for rotation and the foils 29 may be stationary. In addition, instead of a drum-type rotor, in one aspect of the invention, foils 29 may be mounted on one or more radial arms, that is, on a "spider". In addition, in one aspect of the invention, inlet 18 may be located on the bottom of the pressure screen 10 and the slurry 34 flows from the bottom upward toward the outlets 20 and 22. Furthermore, though in FIGURE 1 the flow of slurry from the feed side of basket 24 to the reject side of basket 24 is radially outward, the flow may also be radially inward, as is conventional in the art. In contrast to the aspect of the invention shown in FIGURE 1 , in another aspect of the invention, foils 29 travel along the outside surface of screen 24 and the feed side of screen 24 is the outside surface of screen 24 and the accept side of screen 24 is the inside surface of screen 24. However, such radially inward operation typically provides lower capacity per unit screen area because, among other things, the radially inward pressure pulses of foil 29 are not aided by the force of centrifugal acceleration. In contrast, the radially outward operation as shown in FIGURE 1 is aided by the force of centrifugal acceleration.

A perspective view of a typical prior art drum rotor 26 is shown in FIGURE 2. The relative rotation of rotor 26 is illustrated by arrow 39. Rotor 26, is typically a metallic rotor, for example, carbon steel, stainless steel, or aluminum rotor, but rotor 26 may also be non-metallic, for example, high- strength plastic, polymer, or composite material. Again, rotor 26 is typically circular cylindrical rotor having a cylindrical section 38 and a top section 40. As described earlier, rotor 26 typically includes a plurality of foils 29 mounted on the surface of cylindrical section 38. Foils 29 may be integrally molded into or welded onto rotor 26 or may be removably mounted, for example, by means of fasteners. As shown in FIGURE 2, top section 40 may include a mounting hub 42, for example, for coupling drive shaft 30 (see FIGURE 1) to rotor 26, for example, by means of threaded fasteners (not shown). Though various cylindrical drum geometries may be used according to the present invention, drum 26 in FIGURES 1 and 2, as shown in FIGURE 1 , is a hollow drum with an open bottom. Also, though rotor 26 in FIGURE 2 includes rectangular-shaped foils 29, the present invention is not limited to rectangular-shaped foils, for example, circular foils may also be used in one aspect of the invention, for example, circular, "bump"-type foils, may be used.

The debris or undesirable material content of slurry 34 may comprise, dirt, rocks, stones, tramp material (for example, nuts and bolts or other metal fragments), uncooked fibers (that is, slivers and shives), and, in the case of recycled fibers, ink, stickies (that is, glues, adhesives and the like), and staples, among other undesirable materials.

After entering the pressure screen 10 via inlet 18, the pressure at which slurry 34 is introduced to pressure screen 10, typically between about 20 psi and about 35 psi, forces slurry 34 to pass through the annular gap 35 between the outside diameter of rotor 26 and the inside surface of basket 24 as indicated by arrow 36. As the slurry passes through the gap 35, the rotation of drum 26 with foils 29 promote a helical flow of slurry 34 as the slurry passes through gap 35. In addition, as slurry 34 passes through gap 35 slurry 34 is repeatedly exposed to pressure pulsations as the foils 29 pass over the inner surface of basket 24. As is known in the art, the pressure pulsations created by foils 29 promote the passage of fiber through the apertures in screen 24, for example, radially outward, and into the accept annulus 31 between screen 24 and housing 12. The accept slurry is discharged via outlet 20. However, as is also known in the art, foils 29 produce a "negative pulse" from accept side of screen 24 to the feed side of screen 24 (described in further detail below with respect to FIGURE 5). Among other things, the negative pulse aids in back purging fiber from the apertures on the feed side of screen 24 wherein acceptable fiber is more likely to subsequently pass from the feed side of screen 24 to the accept side screen 24. However, as will be discussed below, the negative pulse of slurry in the conventional art provides certain disadvantages, some of which are overcome by one or more aspects of the present invention.

FIGURE 3 illustrates an elevation view, partially in cross-section, of screen basket 24 shown in FIGURE 1 according to one aspect of the present invention. According to this aspect of the present invention, basket 24 includes a perforated cylindrical section 44, an upper mounting flange 27 and a lower mounting flange 46. According to one aspect of the invention, cylindrical section 44 comprises a series of perforated screen subsections 48, 49, 50, 51 , 52, 53, 54, and 55. Sections 48-55 may be separated by unperforated sections 56, 57, and 58. According to one aspect of the invention, screen sections 48, 49, 50, 52, 53, and 55 may comprise any form of conventional perforated screen section, including smooth bars, wires, machined slots, or machined holes and the like. Screen sections 48, 49, 50, 52, 52, and 55 may also be non-smooth screen sections, for example, these screens may be one or more contoured screen sections. However, according to one aspect of the invention, screen basket 24 comprises at least one section 51 or 54 having perforations that promote dilution of the slurry on the feed side of the screen basket, for example, compared to the thickening that characterizes screen sections 48-50 and the like.

A detailed cross section screen section 51 according to one aspect of the invention is shown in FIGURE 4. As shown in FIGURE 4, according to one aspect of the invention, screen section 51 comprises a first surface 60, for example, a surface on the feed side of basket 24, having a plurality of apertures 62 each having a width or diameter 64. Apertures 62 may comprise circular holes, horizontal or vertical slots, among other types of apertures. According to one aspect of the present invention, screen section 51 also comprises a second surface 61 having a plurality of apertures 68 which are in fluid communication with at least some of apertures 64 on surface 60 of screen section 51. Apertures 68, according to one aspect of the invention, typically have a width 70, for example, a diameter 70, which is greater than width 64 of apertures 62. According to one aspect of the invention, apertures 68 include some form of obstruction which limits the flow of fiber though apertures 68. For example, when apertures 62 in surface 60 comprise slots, the ribs between slots 62 provide obstructions to the flow of fiber though apertures 68. The obstructions in apertures 68 may be horizontally-oriented, vertically-oriented, or both. In other words, the obstructions may be axially or circumferentially oriented with respect to screen cylinder 24. According to one aspect of the present invention screen section 51 having apertures 62 and 68 promotes dilution of slurry 34 as slurry 34 passes through annular gap 35 between rotor 26 and screen basket 24. As shown in FIGURE 4, screen section 51 may have a thickness 63 of between about 4 mm and about 10 mm, and is preferably about 6 mm to about 8 mm.

The function of screen section 51 is illustrated in the sectional view shown in FIGURE 5. FIGURE 5 illustrates a partial sectional view of pressure screen 10 as viewed through section lines 5-5 in FIGURE 1. FIGURE 5 includes a cross-section of the cylindrical section of pressure screen 10, including a cross-section of cylindrical housing 12, a cross- section of screen section 51 shown in FIGURES 3 and 4, and a section of rotor 26 with representative foil 29. The rotation of rotor 26 is illustrated by arrow 39. As shown in FIGURE 5, according to one aspect of the present invention, as foil 29 of rotor 26 passes along screen section 51 , the foil generates a pressure pulse, that is, a "positive pressure pulse", as indicated by arrows 72, which forces a slurry of fiber and liquid (preferably with little or no debris) through apertures 62 and apertures 68 into the annulus 75 between the housing 12 and screen section 51. In consequence, the rapid flow of slurry as indicated by arrows 72 generates a pressure pulse in the annulus 75 which contributes to the rapid "negative pulse" of slurry through apertures 68 and apertures 62 toward the feed side 77 of screen section 51 induced by foil 29, as indicated by arrows 76. However, according to one aspect of the present invention, and contrary to the prior art, the fiber content of the negative pulse of slurry represented by arrows 76 is minimized wherein at least some of the fiber that would pass to feed side 77 (according to the conventional art) is prevented from passing to feed side 77. In one aspect of the invention, little or no fiber passes through apertures 62 from accept side 75 to feed side 77 of screen section 51. As a result, according to one aspect of the invention, the relative liquid content of the slurry on the feed side 77 of screen section 51 is increased, that is, the fiber consistency is reduced, effecting a localized dilution of the slurry on the feed side of the screen basket 24.

The minimization of the flow of fiber carried by the "negative pulse" represented by arrows 76, according to the present invention, provides at least three beneficial consequences. First, the minimization of the flow of "acceptable" fiber from the "accept" side of screen 24 to the feed side of the screen reduces the need to "recycle" acceptable fiber through the screen. This can increase the capacity of the screen by, among other things, eliminating the need to screen this fiber again. In addition, this reduces the instantaneous nominal aperture "passing flow" to process slurry at a given capacity. Second, preventing at least some fiber from passing from the accept side to the feed side of screen 24 may reduce the pressure and volume of flow to the feed side 77 due to the negative pulse. Specifically, as the fibers in the negative pulse flow build up on the accept side 75 of screen 24, the fibers form a restriction to the flow of further fibers and liquid to the feed side 77 of screen 24.

The disadvantages of the conventional art in this regard are illustrated in FIGURE 5A. FIGURE 5A is a cross sectional view similar to FIGURE 5 and illustrates a pressure screen 210 which operates according to the prior art. This prior art screen 210 included a cylindrical housing 212, a screen 251 , and a rotor 226 with representative foil 229. The rotation of rotor 226 is illustrated by arrow 239. As is typical of prior art screens, the section of screen 251 shown in FIGURE 5A represents the cross section of screening medium that can extend essentially over the entire height of the screen cylinder, for example, as illustrated by the multiple screen sections 82 in screen cylinder 78 shown in FIGURE 6. Contrary to the present invention, screen section 251 in FIGURE 5A comprises discrete bar elements or wires 252, for example, wedge-shaped wires. Screen section 251 has a feed side annulus 277 and an accept side annulus 275. As in FIGURE 5, the positive pulse due to foil 229 is indicated by arrows 272 and the flow due to the negative pulse is indicated by arrows 276. As shown in FIGURE 5A, according to the conventional art, there is typically very, if any, restriction to the reverse flow of slurry, as indicated by arrows 276, from accept side 275 to feed side 277.

It is helpful to recognize that this rapid flow of slurry from the accept side 275 to the feed side 277 due to the negative pulse is indeed very rapid. A more accurate description of this rapid flow would be to call this flow an "injection" of liquid from the accept side 275 to the feed side 277. Calculation of the flows and speeds of conventional pressure screen operation suggest that the duration of such injections into feed side 277 occurs in only about 1 to about 5 milliseconds, again, very rapid. As a result, the injection of slurry, as indicated in FIGURE 5A as arrows 276, and the pressure of the injection creates regions of localized higher pressure on feed side 277 of screen 251. When this higher pressure volume of flow is introduced to feed side 277, the natural tendency of the higher pressure fluid is to seek areas of lower pressure. Due to the dynamic rotation of the slurry in the vicinity of, for example, foil 229 in FIGURE 5A, this higher pressure liquid naturally attempts to vent through the same or next available aperture, for example, as shown by arrow 279 in FIGURE 5A. Since arrow 279 represents a higher pressure, higher volume flow of liquid, this flow can transfer undesirable debris from feed side 277 to accept side 275 of screen 251. The flow represented by arrow 279 is further encouraged due to the relatively lower pressure that is developed in accept annulus 275 of the screen due to the flow represented by arrows 276. These localized pressure differentials create undesirably high localized passing velocities which push debris or fiber from feed side 277 to accept side 275. Thus, these pressure differentials can also negatively effect the efficiency of debris removal or fiber fractionation. Analysis also suggests that in some prior art screen cylinder designs, for example, in wedge-wire- type screens, this passage of debris from the feed side to the accept side under the influence of the high-pressure negative pulse may be exacerbated by the so-called "Coanda effect", as indicated by arrows 280. In addition, analysis suggests that these rapid injections of negative pulse liquid contribute to the higher power consumption of some conventional screens.

However, according to one aspect of the present invention shown in FIGURE 5, the restriction to flow provided by the build-up of fibers in apertures 68 on the accept side of the screen limits the volume of flow to the feed side and limits the subsequent pressure pulse to the feed side annulus 77 in FIGURE 5. As a result, the volumetric flow and pressure pulse due to the negative pulse (arrows 76 in FIGURE 5) are lower than those found in conventional screen designs and operation and result in lower potential to pass undesirable debris or fibers in the flow represented by arrow 79 in FIGURE 5.

The third beneficial consequence of aspects of the present invention is the dilution of the slurry on the feed side of the screen. The reduction or minimization of the flow of fiber from the accept side of the screen to the feed side of the screen with the "negative pulse" effects a localized dilution of the feed side slurry. According to one aspect of the invention, this dilution of the slurry on the feed side of the screen can help to minimize reject thickening during screening and can improve fiber passage during fractionation and other negative effects feed side thickening can cause. The impact of this localized dilution according to one aspect of the present invention is illustrated in FIGURE 6.

FIGURE 6 illustrates two partial screen baskets 78 and 80, similar to screen basket 24. Screen basket 78 is a typical prior art screen basket having typical prior art screen sections 82. Screen basket 80 comprises screen sections 84 according to one aspect of the present invention. Screen basket 80 is essentially identical to screen basket 24 and includes conventional screen sections 82, as in basket 78, but also includes screen sections 84 according to one aspect of the present invention. Curves 85 and 86 in FIGURE 6 represent the relative variation of the consistency of the slurry as the slurry proceeds along the feed side of the respective screen baskets, 78 and 80. Abscissas 79 and 81 of FIGURE 6 represent the relative consistency of the slurry in weight percent. Screen sections 82 in screen 78, as is typical of the prior art (and for which an example is shown by wire-type screen 251 in FIGURE 5A), comprise apertures that effectively filter liquid and fiber from the slurry on the feed side of screen basket 78 as the slurry passes along screen basket 78 (for example, in the downward direction of FIGURE 6.) As a result, the slurry contacts the feed side of screen basket 78 at an initial consistency C₀ and, as shown by curve 85, as the slurry on the feed side of screen basket 78 progresses along screen basket 78, the liquid content of the slurry decreases, that is, the consistency of the slurry increases (or the slurry thickens) resulting in a final consistency "A" in FIGURE 6. As discussed above, this phenomenon is recognized in the art as "reject thickening" which impacts the capacity and operation of the screen. For example, when reducing the aperture size to improve debris removal efficiency, reject thickening can result in pressure screen failure due to screen plugging. Some attempts have been made in the art to minimize or avoid reject thickening by, for example, introducing a separate source of dilution liquid to the screen or by limiting the length of the screen cylinder. These and other modifications to screens to address reject thickening are obviated by employing one aspect of the present invention.

According to one aspect of the present invention, as illustrated by curve 86 of FIGURE 6, as the slurry progresses along screen 80 the slurry is thickened from initial consistency C₀, (for example, a consistency ranging from about 0.5% to about 3.0%), as is conventional, while passing the initial conventional screen sections 82. However, according to one aspect of the invention, upon reaching the first screen section 84, the diluting effect of screen section 84 (as described above with respect to FIGURE 5) dilutes the slurry on the feed side wherein the consistency of the slurry is reduced as shown at 87 on curve 86. After the consistency is reduced at screen section 84, in one aspect of the invention, the slurry encounters one or more thickening screen sections 82 and the consistency of the slurry increases. Screen basket 80 may include one or more diluting screen sections 84 where further dilution occurs, for example, at 88 on curve 86 of screen 80. Again, after passing screen section 84 the slurry may encounter one or more thickening screen sections 82 wherein the consistency of the reject slurry becomes "B" in FIGURE 6. However, consistency B, according to one aspect of the present invention consistency, is typically less than a consistency that will interfere with the operation of screen 80 or cause pluggage of screen basket 80. Consistency B will typically be less than consistency A of prior art screen basket 78. For example, consistency B may be at least 10 % less than consistency A, and can be as much as 20 % or even 50% less than consistency A.

Apertures 62 and 68 (as shown in FIGURE 4), according to the present invention, may comprise assorted configurations and still effect the desired results. Apertures 62 may be circular or non-circular holes, for example, square or rectangular holes. When apertures 62 comprise holes, the holes may be arranged in any pattern, for example, apertures 62 may comprise a plurality of holes arranged in a plurality of lines of holes. These holes or lines of holes may be either uniformly or non-uniformly positioned or spaced, for example, staggered or in-line, and still effect the present invention. Apertures 62 may also be slots, for example, slots machined in solid plate, or slots positioned between horizontal rods, bars or wires, for example, between wedge wires. The slots that comprise apertures 62 may be horizontal or vertical slots, and in one aspect of the invention, the slots may be oriented at an angle to the vertical, for example, at angle between about 30 and about 60 degrees to the vertical. Though the surface of the screen having apertures 62 may take any form or shape, for example, a contoured shape, in one aspect of the invention, the surface of the screen bearing apertures 62 is smooth, that is, the surface has little or no contours. In one aspect of the invention, the surface facing the accept side of screen section 51 includes one or more contours, projections (for example, axial bars or rods 132, see FIGURE 13), or recesses, that promote turbulence of the fiber slurry, for example, to disrupt any fiber flocks or fiber mats that may be created, or for mixing, or for flow interruption for improved fiber orientation. These projections (for example, bars or rods) may also act as flow interrupters. These flow interrupters may be used to hinder or prevent the flow, for example, the tangential flow, of fiber slurry. In some instances, when the flow of slurry passes screen section 51 too quickly, the intensity of the hydraulic pulse impinging upon the screen surface, and thus also the negative pulse, can be undesirably decreased.

According to one aspect of the invention, when apertures 62 in FIGURE 4 are circular holes, the diameter 64 of the holes may be between about 0.5 mm and about 1.5 mm, for example, between about 0.8 mm and about 1.2 mm. When apertures 62 are slots, the width 64 of the slots may be between about 0.10 mm and about 0.50 mm, for example, between about 0.15 mm and about 0.25 mm. The width, diameter or other width dimension 64 of apertures 62 may also vary.

Similarly, apertures 68 in FIGURE 4 may be circular or non-circular holes, for example, square or rectangular holes. When apertures 68 comprise holes, the holes may be arranged in any pattern, for example, apertures 68 may comprise a plurality of holes arranged in a plurality of lines of holes. These holes 68 or lines of holes may be either uniformly or non-uniformly positioned or spaced, for example, staggered or in-line, and still effect the present invention. Apertures 68 may also be slots, for example, slots machined in solid plate, or slots positioned between horizontal rods, bars or wires, for example, between wedge wires. The slots that comprise apertures 68 may be horizontal or vertical slots, and in one aspect of the invention, the slots that comprise apertures 68 may be oriented at an angle to the vertical, for example, at an angle between about 30 and about 60 degrees to the vertical. According to one aspect of the invention, apertures 68 have a width dimension 70. When apertures 68 are circular holes, the diameter 70 of these holes may be between about 3 mm and about 25 mm, for example, between about 5 mm and about 10 mm. When apertures 68 are slots, the width 70 of the slots may be between about 3 mm and about 12 mm, for example, between about 5 mm and about 10 mm. In one aspect of the invention, the width dimension 70 of apertures 68 is greater than the width dimension 64 of apertures 62. The width or other dimension 70 of apertures 68 may also vary.

FIGURES 7A, 7B, 8A, and 8B illustrate two representative methods for manufacturing screen section 51 according to the present invention. In FIGURES 7A and 7B, the present invention is provided as drilled or milled holes, or voids in a plate. As shown in FIGURE 7A, screen section 51 may be fabricated by first drilling or milling blind holes or slots 68 at least about 50%, preferably at least about 75%, through a plate 90. For example, for a plate thickness of about 8 mm, the depth of holes 68 may be about 6 mm to about 7 mm. Again, plate 90 is preferably a steel plate of thickness between about 4 mm and about 8 mm. The diameter of blind holes 68 may typically be between about 5 mm and about 10 mm. As shown in FIGURE 7B, after drilling or milling blind holes 68 in plate 90, apertures 62 may be drilled in plate 90 to provide the present invention. According to one aspect of the present invention, at least some holes 62 communicate with holes 68. After completion of drilling holes 62 and 68, and after appropriate deburring, abrasive blasting, electrolytic polishing, and the like, plate 90 with holes 62 and 68 is typically rolled to its desired diameter to provide screen section 51.

FIGURES 8A and 8B, illustrate another method of fabricating the present invention. As shown in FIGURES 8A and 8B, the present invention may also be fabricated by holes and continuous slots in a plate 92. As shown in FIGURE 8A, similar to FIGURE 8B, screen section 51 may be fabricated by first drilling or milling blind holes 94 at least about 50%, preferably at least about 75% through a plate 92. For example, for a plate thickness of about 8 mm, the depth of holes 94 may be about 6 mm to about 7 mm. Again, plate 92 is preferably a steel plate of thickness between about 4 mm and about 8 mm. The width dimension or diameter of blind holes 94 is typically between about 5 and about 10 mm. As shown in FIGURE 8B, after drilling or milling blind holes 94 in plate 92, apertures may be machined in plate 92 as continuous slots 96 to provide the present

invention. According to one aspect of the present invention, at least some slots 68 communicate with holes 94. After completion of drilling or milling holes 94 and slots 96, and after appropriate deburring and the like, plate 92 may be typically rolled to its desired diameter to provide screen section 51.

The present invention may also be provided by fabricating the apertures on separate plates, or other perforated constructions, and then joining the plates to provide the present invention. For example, apertures 62 may be provided in one plate and apertures 68 provided in another plate and then the two plates joined, for example, by welding, heat shrinking, or other fabrication method. Apertures 62 may also be provided by a wires or wire mesh. Apertures 62, either holes or slots, in the feed side of the screen section may be provided in one plate, for example, a plate having a thickness between about 0.5 mm and about 2 mm, by means of milling, drilling, water-jet cutting, punching, laser cutting, electro-discharge machining (EDM), chemical or photo-etching and the like. Apertures 68 may be provided by similar processes in a second plate. After the apertures are provided in each plate, the plates may be joined for example, by welding, adhesives, or high-temperature fusion. The plates having apertures 62 and 68 may also be joined by first rolling the plate and welding the plates into appropriate cylinders and shrink-fitting one of the plates inside the other plate.

FIGURES 9, 10, and 11 illustrate cylindrical rotor 100 according to another aspect of the invention. Similar to drum rotor 26 shown in FIGURE 2, the relative rotation of rotor 100 is illustrated by arrow 102. Rotor 100, is also typically a metallic rotor, for example, carbon steel, stainless steel, or aluminum rotor, but rotor 100 may also be non-metallic, for example, high- strength plastic, polymer, or composite material. Rotor 100 according to this aspect of the invention is typically a circular cylindrical rotor having a cylindrical section 104 and a top section 106. As described earlier with respect to rotor 26, rotor 100 typically includes a plurality of foils 108 mounted on the surface of cylindrical section 104. Foils 108 may be integrally molded into or welded onto rotor 100 or may be removably mounted, for example, by means of fasteners. As shown in FIGURE 9, top section 106 may include a mounting hub 110, for example, for coupling drive shaft 30 (see FIGURE 1) to rotor 100, for example, by means of threaded fasteners (not shown). Though various cylindrical drum geometries may be used for rotor 100, according to the present invention, rotor 100 in FIGURE 9 comprises a hollow drum with an open bottom.

According to this aspect of the invention, rotor 100 includes at least one foil 112. Foil 112 according to this aspect of the invention is longer in arc length than, for example, foils 108 of rotor 100. For example, where conventional foils like foils 108 may have an arc length of less than 20 degrees, the arc length of foil 112 according to this aspect of the invention may be at least 25 degrees, or even at least 30 degrees, possibly as much as 60 degrees. These angular arc lengths may typically apply to a standard 600 mm diameter rotor cylinder, but may also apply to cylinders of larger or smaller diameter. According to one aspect of the invention, the longer foil 112 increases the percentage of screen areas that is exposed to the reverse (that is, negative) flow while exposing a smaller percentage of the screen area to the higher-pressure short duration "positive" flow that characterizes screen operation. A cross section of foil 112 according to one aspect of the invention is illustrated in FIGURE 10. As shown in FIGURE 10, foil 112 includes a leading edge 111 and a trailing end 113. According to one aspect of the invention, leading edge 111 includes an abrupt step or transition 115 from the surface of, for example, drum 104. Transition 115 may be planar or curved as shown in FIGURE 10. When planar, the surface of transition 115 may make an angle of between about 30 degrees and about 150 degrees with the surface of drum 104. In one aspect of the invention, the surface of transition 115 may make an angle between about 60 degrees and 120 degrees with the surface of drum 104. In one aspect of the invention, the angle is about 45 degrees. The sharp or abrupt transition 115 in foil 112 provides for an a relatively high pressure pulse upon the surface of the screen basket, for example basket 24 of FIGURE 3, adjacent to which foil 112 travels. Trailing end 113 may also include a planar or curved surface 117. When planar, surface 117 typically comprises an angle between about 6 degrees and about 12 degrees with the surface of drum 104. Foil 112 typically will have a maximum height above the surface of drum 104 of about 15 mm.

FIGURES 11 and 12 illustrate an alterative foil 112A according to another aspect of the present invention. FIGURE 11 illustrates a side view of foil 112A as viewed along lines 12-12 in FIGURE 11. Foil 112A is similar in geometry and operation to foil 112 but includes at least one, preferably two, side baffles 114. One or more side baffles 114 may have a height that is greater to, equal to or less than the height of foil 112A.

Though rotor 100 with one or more foils 112 or 112A or a combination thereof provides certain advantages over prior art rotors and foils, according to one aspect of the invention, rotor 100 with foils 112 or 112A is used in conjunction with screen section 51 , 54 or screen basket 24 illustrated in FIGURES 3 and 4. According to this aspect of the invention, foils 112 or 112A work in conjunction with the diluting screen section 51 to provide improved screen performance and debris removal efficiency. According to one aspect of the invention, the abrupt leading surface 115 of foil 112 provides a relatively high pressure pulse to the surface of screen basket 24 and the relatively longer arc length of foil 112 provides for a relatively long negative pulse or extended suction pulse, for example, a suction pulse, preferably with little or no fiber present, to effect a more uniform dilution. The relatively longer, slower dilution flow and the fiber build-up on the accept side of the screen 24 that such a longer, slower dilution flow can effect, is then followed by an abrupt purge pulse as surface 115 of the leading edge of the subsequent foil 112 is encountered. That is, one aspect of the present invention is the combination of one or more screen sections 51 , as shown in FIGURES 3 and 4, and one or more foils 112/112A shown in FIGURES 9 through 12. According to this aspect of the invention, the flow of liquid into the accept side of the screen, for example, screen 24, is minimized and the flow of liquid to the feed side from the accept side is maximized, for example, to provide the desired dilution in the vicinity of these screens and foils.

Though FIGURE 9 illustrates a drum-type rotor, aspects of the present invention may comprise foils mounted on open rotors, for example, rotors having radial arms upon which foils, for example, foils 112, 112A shown in FIGURES 10-12, are mounted. These foils are sometimes referred to as "spider-type" foils or simply "spiders".

FIGURE 13 illustrates a partial view of another aspect of the invention. FIGURE 13 illustrates a section 120 of a cylindrical screen assembly, for example, similar to screen assembly 24, according to another aspect to the invention. In the aspect shown in FIGURE 13, diluting screen section 122 is positioned between thickening screen sections 122 and 124. Screen sections 122 may be essentially the same as screen section 51 of FIGURES 3 and 4; screen sections 122 and 124 may be essentially the same as screen sections 50 and 52 of FIGURES 3 and 4. In this aspect of the invention, feed side apertures 128 comprise vertical slots and accept side apertures 130 comprise slots or round holes. According to this aspect of the invention, screen section 122 is subdivided into screen subsections 123 and subsections 123 are separated by bars 132, for example vertical bars. As discussed above, bars 132 can provide various functions, including disrupting fiber flocks or fiber mats, mixing, flow interruption or fiber re-orientation. The orientation of bars 132 may vary depending upon the application of the invention, for example, depending upon the speed, type, or orientation of the rotor used or the speed of rotation of the screen cylinder in applications where the cylinder rotates. Bars 132 may be aligned with the axis of the screen cylinder or may be angled with respect to the axis of the screen cylinder. In one aspect of the invention, bars 132 may be oriented to form an angle between about 1 degree and about 45 degrees with the axis of the screen cylinder. In another aspect of the invention, bars 132 may be oriented to form an angle between about 15 degrees and about 30 degrees with the axis of the screen cylinder.

FIGURE 14A is front view and FIGURE 14B is a rear view of a screen section 310 according to one aspect of the invention. The aspects illustrated in FIGURES 14A and 14B are similar to screen section 51 shown in FIGURES 3 and 4 and also similar to screen sections 122 and 123 shown in FIGURE 13.

Screen section 310 may be fabricated from metal plate, for example, AISI 304 or AISI 316 stainless steel plate. Screen section 310 includes a first surface 312 having a first set of apertures 314. In the aspect of the invention shown in FIGURE 14A, apertures 314 are slotted apertures, though circular, square, or rectangular holes may also be used. In FIGURE 14A, apertures 314, for example, vertical slots, may have a width of about 0.10 mm to about 0.50 mm. As shown in FIGURE 14B, screen section 310 also includes a second surface 316 having second apertures 318. In the aspect of the invention shown in FIGURE 14B, apertures 318 are evenly distributed circular holes, though, slots, square, or rectangular holes may also be used. Apertures 318, for example, the circular holes shown in FIGURE 14B may have a diameter of about 3 mm to about 25 mm, depending upon the application and the method of manufacture. According to one aspect of the invention, apertures 314 are in fluid communication with apertures 318.

According to one aspect of the invention, for example, for debris removal, surface 312 may comprise the feed side of plate 310. According to another aspect of the invention, for example, for fractionation, surface 316 may comprise the feed side of plate 310. The aspect illustrated in FIGURE 14A and 14B are is similar to screen section 51 shown in FIGURES 3 and 4 and also similar to screen sections 122 and 123 shown in FIGURE 13.

FIGURE 15A is an exit side view showing relief grooves and FIGURE 15B is a feed side view of a contoured screen section 410 having milled or machined slots according to the prior art, for example, as used for cylinder 78 in FIGURE 6, over which the present invention is an improvement. Screen section 410 may be used, for example, in the screen sections 48, 49, and 50 shown in FIGURE 3. Screen section 410 includes a first surface 416 having slots at the bottom of contoured grooves 418 and a second surface 412 having relief grooves 414 connected via the slots to contour grooves 418 in FIGURE 15B. Slot location and the geometry of the contour groove 418 in screen section 410 are designed, like many screens in the prior art, to promote passage of acceptable fiber from the feed side 416 to the accept side 412. However, contrary to the present invention, the screen plate shown in FIGURES 15A and 15B, and others like it, is designed with little or no consideration of the magnitude or content of the negative-pulse flow.

International PCT Publication WO 00/65151 brings forth inherent screening problems caused by two typical methods of construction of slot- type screening cylinders: one formed from discrete elements and the other by machined slots in plates. Both of these types of construction represent conventional prior art construction methods, prior art screening methods, and prior art screening devices over which aspects of the present invention are improvements. FIGURE 5A and screen 251 illustrate the typical shape of discrete elements fabricated from drawn wire that allows the undesirable excessive volumetric reverse flow 276, in FIGURE 5A. This undesirable excessive flow promotes an undesirable reduced debris removal during screening and poor efficiency during fractionation.

Typical prior art screening sections, for example, screen sections 50 and 52 in FIGURE 3 and screen section 410 illustrated in FIGURES 15A and 15B (having a contoured inlet or feed side 416) are typically produced by machining plate. The apertures, for example, slots, in these machined plates typically have substantially sharp edges produced by the machining process. These sharp edges typically impose a restriction to the volume of the reverse flow during the negative pulse, thus also restricting the volume of dilution produced by the negative pulse. This inherent reduced volume of dilution in the reverse flow, due to machined apertures, enhances debris removal efficiency, but promotes undesirable thickening of the slurry on the feed side of the plate, for example, in the annulus 77 shown in FIGURE 5, and results in decreased capacity due to thickening, as indicated by thickening curve 85 shown in FIGURE 6.

According to one aspect of the present invention, for example, the aspect illustrated in FIGURE 5 integrated into cylinder 78 in FIGURE 6, the volumetrically significant liquid from a sufficient negative pulse adds the desired dilution to the slurry on the feed side (for example, in annulus 77) and a conventional machined screen cylinder, for example, cylinder 78 in FIGURE 6, can then resemble the performance of cylinder 80 in FIGURE 6, for example, and produce a similar saw tooth curve for consistency.

According to one aspect of the present invention, the capacity and efficiency of machined screen plate sections or screen plate cylinder is improved when used for treating fibrous slurries, for example, for screening or fractionating. Machined slot-type screen plate, in contrast to wire-type screens or discrete bar-type screens, is typically fabricated from solid metal plate, for example, by milling, laser cutting, water-jet cutting, punching, drilling, electron beam cutting, and electron discharge machining (EDM), among other methods. After machining, the plate is typically rolled to the desired diameter to provide screening or fractionating screen cylinders. In one aspect of the invention, the invention comprises machined screen plate, for example, machined screen plate having contoured surfaces, for instance, as shown in FIGURE 15A and 15B.

One beneficial consequence is that the dilution mechanism of the present invention, for example, as shown in FIGURE 5, can be used to improve the performance of a prior art screen cylinder and foil assembly, for example, the assembly shown in FIGURE 5A having screen 251 and foil 229 mounted on rotor 226. That is, the prior art cylinder 78 in FIGURE 6 and its typical screening mechanism in FIGURE 5A can be used with a screen section according to the present invention to substantially decrease the relatively larger, undesirable volume of liquid introduced by the negative pulse flow, as indicted by flow 276 in FIGURE 5A, for example, the undesirable return of accept slurry to the feed side of the screen. According to one aspect of the invention, the present invention allows the screen designer to modify, for example, reduce, the volume of reverse flow, for example, by modifying the design of foil 229 and/or the contour or shape of screen elements 251 , while replenishing the reduced reverse flow in a subsequent screen section according to the present invention, for example, with the dilution mechanism of screen 84 in FIGURE 6. Moreover, the dilution liquid, as indicted by flow 76 in FIGURE 5, according to the present invention, will have a significantly lower consistency, that is, containing little or no fibers. Thus, according to this aspect of the present invention, the net liquid supply or absolute liquid supply of dilution added to the slurry in the feed side, for example, in annulus 277 in FIGURE 5A, can be modified, for example, reduced or maintained about the same, and the excessive hydrodynamic speed and pressure forces, which negatively affects debris removal efficiency, can be substantially reduced for a prior art screening apparatus, for example the prior art screen shown in FIGURE 5A. As a result, prior art screen cylinder sections, such as section 82 in FIGURE 6 or screen 251 in FIGURE 5A, and screen sections according to the present invention can be used together to provide, for example, the consistency characterized by curve 86 for cylinder 80 in FIGURE 6.

Due to the benefit of one or more aspects of the present invention's effective dilution of the feed side suspension, existing foils or protrusions can be either redesigned to provide for reduced negative-reverse flow magnitudes (pulses) and /or operated in a gentler mode. Both of these actions can result in decreased reverse flow from the accept side of the screen to the feed side of the screen, and accordingly provide lower pressure pulses from the feed side to the accept side of the screen. Again, by decreasing this pressure pulse from the accept side to the feed side of the screen, and reducing its flow volume into the feed side of the screen, the resulting flow from the feed side to the accept side with undesirable debris, during screening or during fractionation, can be decreased and a cleaner or more uniform accept slurry provided. Though aspects of the invention are most conducive for use with machined screen plate sections or screen cylinders, aspects of the present invention may also be used for wire- or bar-type screen sections or screen cylinders. For example, aspects of the present invention may be used in one or more sections of a screen cylinder and wire- or bar-type screens may be used in one or more other sections of a screen cylinder.

According to one aspect of the invention, screen plate sections may be machined according to aspects of the present invention, for example, as in plate 310 shown in FIGURES 14A and 14B, and then rolled to provide different treatment surfaces. For example, in one aspect of the invention, plate 310 may be rolled wherein first surface 312 is an inside surface, for example, a feed surface. In another aspect of the invention, plate 310 may be rolled where second surface 316 is an inside surface, or a feed surface.

In one aspect of the invention, for example, where a fractionating device is desired, a screen cylinder may comprise a first screen section having larger apertures on the feed side of the cylindrical screen, for example, the large apertures 318 on surface 316 of FIGURE 14B, and a second screen section positioned downstream of the first screen section having smaller apertures, for example, apertures 314 on surface 312, on the feed side. The sequence of screen sections may be varied as desired to provide fiber separation and slurry dilution, for example, with little or no thickening. In one aspect of the invention, the number of screen sections used is only limited by the size of the space available in the device, that is, the size of the device is not limited by slurry thickening.

According to aspects of the present invention, screening elements and screen sections are provided having apertures which treat fibrous slurries while effecting the liquid content of the slurry being treated, for example, diluting the slurry. These screen elements and screen sections take advantage of the unique flow characteristics and fluid pressure fields encountered in screening and fractionation to improve the efficiency and capacity of these processes. Aspects of the present invention also minimize the re-circulation of liquid and fiber from the accept side of the screen to the feed side of the screen. This reduces the absolute passing velocity through the screen and thereby improves the debris removal and fiber separation efficiency.

Aspects of the present invention provide improved methods of treating slurries of fibrous materials. When applied to debris removal or screening, aspects of the present invention provide improved capacity and screening efficiency, for example, increased debris removal at reduced fiber losses with rejects. When applied to fractionation, aspects of the present invention, provide increased capacity at higher segregation levels for different fiber fractions.

While the invention has been particularly shown and described with reference to certain aspects of the present invention, it will be understood by those skilled in the art that various changes in form and details may be made to the invention without departing from the spirit and scope of the invention described in the following claims.

Claims

claim:

1. A screen element for treating slurries of fibrous material and liquid, the devices having means for exposing the screen element to fluid pressure pulses, the screen element comprising: a first surface having a plurality of first apertures for passing some fibrous material and liquid when the first surface is exposed to pressure pulses generally directed in a first direction; a second surface, opposite the first surface, having a plurality of second apertures in fluid communication with the first apertures for passing liquid when the second surface is exposed to pressure pulses generally directed in a second direction, opposite the first direction; and means for minimizing the passage of fibrous material through the first apertures in the second direction.

2. The screen element as recited in claim 1 , wherein the means for minimizing the passage of fibrous material comprises obstructions extending across the second apertures.

3. The screen element as recited in claim 2, wherein the first apertures in the first surface comprise slotted apertures separated by ribs and wherein the obstructions extending across the second apertures comprise the ribs.

4. The screen element as recited in claim 2, wherein the obstructions comprise one or more horizontally-oriented obstructions and vertically-oriented obstructions.

5. The screen element as recited in any one of claims 2 to 4, wherein the obstructions comprise wires.

6. The screen element as recited in any one of claims 1 to 5, wherein the screen element is a cylindrical screen element.

7. The screen element as recited in any one of claims 1 to 6, wherein the first direction is one of radially outward and radially inward.

8. The screen element as recited in any one of claims 1 to 7, wherein the first aperture comprises a first narrowest dimension and the second aperture comprises a second narrowest dimension and wherein the second narrowest dimension is greater than the first narrowest dimension.

9. The screen element as recited in any one of claims 1 to 8, wherein the first apertures are slots.

10. The screen element as recited in any one of claims 1 to 9, wherein the second apertures are one of circular apertures, rectangular apertures, and square apertures.

11. The screen element as recited in any one of claims 1 to 10, wherein the screen element is fabricated from one of machined plate, wires, and bars.

12. The screen element as recited in any one of claims 1 to 11 , wherein the fibrous material comprises one of natural and synthetic fibrous material.

13. The screen element as recited in any one of claims 1 to 12, wherein the screen element comprises one screen element in a cylindrical screen assembly.

14. A device for separating at least some fibrous material from a slurry of fibrous material, debris, and liquid, the device comprising: an inlet for introducing the slurry; at least one annular screen element for separating fibrous material from the slurry to produce an accept slurry having little or no debris and a reject slurry having debris, the screen element comprising: a first surface having a plurality of first apertures for passing fibrous material and liquid when the first surface is exposed to pressure pulses generally directed in a first direction; a second surface, opposite the first surface, having a plurality of second apertures in fluid communication with the first apertures for passing liquid when the second surface is exposed to pressure pulses generally directed in a second direction, opposite the first direction; and means for minimizing the passage of fibrous material through the first apertures in the second direction; and at least one foil for providing the pressure pulses, the at least one foil mounted for relative movement with respect to the annular screen element; means for providing relative movement between the at least one annular screen element and the at least one foil; at least one reject slurry discharge; and at least one accept slurry discharge.

15. The device as recited in claim 14, wherein the means for minimizing the passage of fibrous material comprises obstructions extending across the second apertures.

16. The device as recited in claim 14 or claim 15, wherein the first apertures in the first surface comprise slotted apertures separated by ribs and wherein the obstructions extending across the second apertures comprise the ribs.

17. The device as recited in claim 15 or claim 16, wherein the obstructions comprise one or more of horizontally-oriented obstructions and vertically-oriented obstructions.

18. The device as recited in any one of claims 14 to 17, wherein the at least one annular screen element comprises a plurality of spaced annular screen elements.

19. The device as recited in any one of claims 14 to 18, wherein the first direction is one of radially outward and radially inward.

20. The device as recited in any one of claims 14 to 19, wherein the first apertures are slots.

21. The device as recited in any one of claims 14 to 20, wherein the second apertures are one of circular apertures, rectangular apertures, and square apertures.

22. The device as recited in any one of claims 14 to 21 , wherein the screen element is fabricated from one of machined plate, wires, and bars.

23. The device as recited in any one of claims 14 to 22, wherein the fibrous material comprises one of natural and synthetic fibrous material.

24. The device as recited in any one of claims 14 to 23, wherein the at least one foil for providing the pressure pulses comprises at least one arcuate foil having an arcuate length of at least about 30 degrees.

25. The device as recited in any one of claims 14 to 24, wherein the at least one foil is mounted on a cylindrical drum.

26. The device as recited in claim 25, wherein the at least one foil comprises at least one radially-extending side baffle.

27. A method for separating fibrous material from a slurry of fibrous material, debris, and liquid, using a screen element comprising: a first surface having a plurality of first apertures for passing fibrous material and liquid when the first surface is exposed to pressure pulses generally directed in a first direction; a second surface, opposite the first surface, having a plurality of second apertures for passing liquid when the second surface is exposed to pressure pulses generally directed in a second direction, opposite the first direction; and means for minimizing the passage of fibrous material through the first apertures in the second direction; the method comprising: introducing the slurry to the first surface of the screen element; exposing the slurry on the first surface of the screen element to pressure pulses generally in the first direction, wherein some of the fibrous material and some of the liquid passes through the first apertures in the first direction to produce an accept slurry containing little or no debris; exposing the accept slurry to pressure pulses generally in the second direction wherein liquid from the accept slurry passes through the second apertures and through the first apertures in the second direction; providing means to minimize the flow of fibrous material from the accept slurry through the first apertures in the second direction; discharging the accept slurry containing little or no debris; and discharging the treated slurry containing debris.

28. The method as recited in claim 27, wherein providing means to minimize the flow of fibrous material from the accept slurry through the first apertures comprises providing obstructions extending across the second apertures.

29. The method as recited in claim 27 or 28, wherein the first apertures in the first surface comprise slotted apertures separated by ribs in the first surface and wherein providing obstructions extending across the second apertures comprises locating the ribs across the second apertures.

30. The method as recited in claims 28 or 29, wherein providing obstructions extending across the second apertures comprises providing one or more horizontally-oriented obstructions and vertically-oriented obstructions.

31. The method as recited in any one of claims 27 to 30, wherein exposing the slurry on the first surface of the screen element to pressure pulses generally in the first direction comprises causing the at least one foil and the first surface to pass in relative motion with each other.

32. A cylindrical screen assembly for treating slurries of fibrous material and liquid in devices having means for exposing the cylindrical screen assembly to fluid pressure pulses, the cylindrical screen assembly comprising at least one cylindrical screen section, the at least one cylindrical screen section comprising: a first surface having a plurality of first apertures for passing fibrous material and liquid when the first surface is exposed to pressure pulses generally directed in a first direction; a second surface, opposite the first surface, having a plurality of second apertures in fluid communication with the first apertures for passing liquid when the second surface is exposed to pressure pulses generally directed in a second direction, opposite the first direction; and means for minimizing the passage of fibrous material through the first apertures in the second direction.

33. The cylindrical screening assembly as recited in claim 32, wherein the means for minimizing the passage of fibrous material comprises obstructions extending across the second apertures.

34. The cylindrical screening assembly as recited in claim 32 or

33, wherein the first apertures in the first surface comprise slotted apertures separated by ribs and wherein the obstructions extending across the second apertures comprise the ribs.

35. The cylindrical screening assembly as recited in claim 33 or

34, wherein the obstructions comprise one or more horizontally-oriented obstructions and vertically-oriented obstructions.

36. The cylindrical screening assembly as recited in any one of claims 32 to 35, wherein the first direction is one of radially outward and radially inward.

37. The cylindrical screening assembly as recited in any one of claims 32 to 36, wherein the first aperture comprises a first narrowest dimension and the second aperture comprises a second narrowest dimension and wherein the second narrowest dimension is greater than the first narrowest dimension.

38. The cylindrical screening assembly as recited in any one of claims 32 to 37, wherein the first apertures are slots.

39. The cylindrical screening assembly as recited in any one of claims 32 to 38, wherein the second apertures are one of circular apertures, rectangular apertures, and square apertures.

40. The cylindrical screening assembly as recited in any one of claims 32 to 39, wherein the at least one cylindrical screen section comprises a plurality of cylindrical screen sections.

41. The cylindrical screening assembly as recited in any one of claims 32 to 40, wherein the first surface is a smooth surface.

42. The cylindrical screening assembly as recited in any one of claims 32 to 40, wherein the first surface is a non-smooth surface.

43. The cylindrical screening assembly as recited in claim 42, wherein the non-smooth first surface comprises at least one of bars, contours, recesses and projections.

44. A method for treating a slurry of fibrous material and liquid using a cylindrical screen assembly having a surface, the method comprising: introducing the slurry of fibrous material to the first end of the cylindrical screen assembly at a first consistency wherein the slurry flows along the surface of the cylindrical screen assembly; exposing the slurry to pressure pulses as the slurry flows along the surface of the cylindrical screen assembly; at a first elevation of the screen assembly, removing at least some liquid and some fibrous material from the slurry to produce a slurry having a second consistency higher than the first consistency; at a second elevation of the screen assembly, removing at least some liquid and some fibrous material from the slurry while reintroducing at least some of the liquid removed at the second elevation to produce a slurry having a third consistency, lower than the second consistency; discharging the slurry from the cylindrical screen assembly.

45. The method as recited in claim 44, wherein exposing the slurry to pressure pulses comprises providing at least one hydrofoil adjacent the surface of the cylindrical screen assembly and causing the at least one hydrofoil and the surface of the screen assembly to move relative to each other.

46. The method as recited in claims 44 or 45, further comprising, at a third elevation of the screen assembly, removing at least some liquid and some fibrous material from the slurry to produce a slurry having a fourth consistency higher than the third consistency.

47. The method as recited in claim 46, further comprising, at a fourth elevation of the screen assembly, removing at least some liquid and some fibrous material from the slurry while reintroducing at least some of the liquid removed at the fourth elevation to produce a slurry having a fifth consistency, lower than the third consistency.

48. The method as recited in any one of claims 44 to 47, wherein the first consistency comprises a consistency between about 0.5 % and about 3.0%.

49. The method as recited in any one of claims 44 to 48, wherein the surface of the screen assembly is an internal surface of the screen assembly.

50. A rotor for use in a slurry treating device, the rotor comprising: at least one foil comprising: a leading edge having a first surface; a trailing edge; and an arc length of at least about 25 degrees; a foil support; and means for mounting the foil support to a to a drive means.

51. The rotor as recited in claim 50, wherein the foil support comprises one of a cylindrical drum and a spider.

52. The rotor as recited in claim 51 , wherein the foil support comprises a cylindrical drum having an external surface, and wherein the first surface makes an angle between about 60 degrees and about 120 degrees with the external surface of the drum.

53. The rotor as recited in any one of claims 50 to 52, wherein the trailing edge comprises a surface and the surface of the trailing edge makes an angle between about 6 degrees and about 12 degrees with the external surface of the drum.

54. The rotor as recited in any one of claims 50 to 53, wherein the arc length of the foil is at least about 35 degrees.

55. The rotor as recited in any one of claims 50 to 54, wherein the foil further comprises at least one side baffle.

56. A method for fractionating a slurry of fibrous material containing a short fiber fraction and a long fiber fraction, the method using a screen element comprising: a first surface having a plurality of first apertures for passing fibrous material and liquid when the first surface is exposed to pressure pulses generally directed in a first direction; a second surface, opposite the first surface, having a plurality of second apertures for passing liquid when the second surface is exposed to pressure pulses generally directed in a second direction, opposite the first direction; and means for minimizing the passage of fibrous material through the first apertures in the second direction; the method comprising: introducing the slurry to the first surface of the screen element; exposing the slurry on the first surface of the screen element to pressure pulses generally in the first direction, wherein at least some of the short fiber fraction and some of the liquid passes through the first apertures in the first direction to produce an accept slurry containing little or no long fiber fraction; exposing the accept slurry to pressure pulses generally in the second direction wherein liquid from the accept slurry passes through the second apertures and through the first apertures in the second direction; providing means to minimize the flow of short fiber fraction from the accept slurry through the first apertures in the second direction; discharging the accept slurry containing mostly the short fiber fraction; and discharging the treated slurry containing mostly the long fiber fraction.