WO2013058975A1

WO2013058975A1 - Detrasher and detrashing system

Info

Publication number: WO2013058975A1
Application number: PCT/US2012/058394
Authority: WO
Inventors: Christopher L. Demler; Leo LAZIMBAT; Wayne SOUTH; Iii John J. Egan
Original assignee: Kadant Black Clawson Inc.
Priority date: 2011-10-18
Filing date: 2012-10-02
Publication date: 2013-04-25

Abstract

Methods and apparatus for continuously detrashing paper stock components. The method pertains to detrashing paper stock components from a pulper wherein contaminants from the pulper are fed to a detrashing apparatus with the components separated into accepts, light contaminant and heavy contaminant fractions in the detrasher. The light contaminant fractions are continuously removed from the detrashing apparatus. The continuous detrasher comprises a top wall, a bottom wall and opposing sidewalls connected to each other and to the top and bottom walls to define an enclosed chamber. The chamber has a generally tapered cross section with a central axis extending longitudinally through the chamber from the bottom wall to the top wall. A tangential inlet is provided to the chamber to provide vortical force to the paper stock. A light contaminant outlet is provided along with an accepts outlet the latter of which is formed in a perforated bedplate in the bottom wall of the chamber. A heavy fraction outlet is also positioned in the chamber. The chamber geometry is such that the height (h) of the chamber divided by the bottom wall diameter is greater than or equal to 0.7 and wherein the diameter of the top portion of the chamber divided by the diameter of the bottom portion of chamber is greater than or equal to 0.9.

Description

DETRASHER AND DETRASHING SYSTEM

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority benefit of U.S. Provisional Patent Application Serial No. 61/548,489 filed October 18, 2011 and U.S. Provisional Patent Application Serial no. 61/588,805 filed January 20, 2012.

FIELD OF THE INVENTION

[0002] The field of the invention generally relates to repulping paper recycling systems. More specifically, the invention relates to detrashers and detrashing systems used in the paper repulping process.

BACKGROUND OF THE INVENTION

[0003] In typical batch detrashers, recovered paper pulp enters a chamber with a perforated bedplate. As the pulp is fed into the chamber, the perforated bedplate strains out any contaminants larger than the perforations and allows usable pulp material to pass through an accept valve. A rotor sweeps contaminants off the bedplate, keeping the perforations clear. As more pulp is fed into the chamber, the contamination builds up. Once the chamber becomes full with contaminants, the rotor can no longer keep the bedplate clear, and the contents of the batch detrashers must be dumped. At this point, the contaminants still may have a high quantity of usable pulp in them.

[0004] Typically, the washing and defibering processes of conventional batch detrashing operations include several steps. First, the inlet valve to the chamber is closed to stop the flow of recovered paper pulp into the chamber. Then, wash water is injected into the chamber and allowed to circulate with the contaminated material. Dilute usable pulp passes through the bedplate and out the accept valve. Once as much of the usable pulp as possible is extracted, the accept valve closes, and the low density discharge valve is opened. Dilution water is used to push the low density contamination out of the chamber. Once the low density contamination is removed, the low density discharge valve is closed. The high density discharge valve is then opened to dump any high density contamination. The high density contaminant load is generally lower than the low density contaminant load. Thus, low density contamination dumps typically occur more frequently than high density contamination dumps.

[0005] Once the high density dump is completed, the high density discharge valve is closed and the inlet valve from the pulper to the chamber is opened once again and the entire detrashing process above is repeated. Note that the detrasher must be full of water when the inlet valve is opened; otherwise material rushes into the detrasher, overwhelming the rotor and plugging the bedplate. The extra water, however, results in a low solids content (or low consistency) of the accepted pulp at the start of the batch sequence and again during the wash sequence. Additionally, the extra wash water increases energy requirements and costs. This low solids content affects all downstream processes.

[0006] Batch detrashers and systems also allow contamination to build-up between batches, which wears system components. Because the contamination remains in the batch detrashers for long periods of time, the contamination begins to break-down, creating smaller particulates of contamination that are even harder to remove. Batch detrashers must also be isolated from the rest of the paper repulping system. Isolation is typically accomplished through a series of valves on the inlet and outlet streams of the detrasher. These valves frequently have high cycle rates, increasing the amount of maintenance required to keep the system running.

BRIEF DESCRIPTION OF THE INVENTION

[0007] One aspect of the invention pertains to a method of detrashing paper stock contaminants from a pulper. In one exemplary embodiment, the method comprises feeding contaminants from the pulper to the detrashing apparatus. In the detrashing apparatus the paper stock contaminants are separated into an accepts, light contaminant and heavy contaminant fractions. In accordance with one aspect of the invention, the light contaminants are continuously removed from the detrashing apparatus. In further embodiments, the paper stock is repulped paper stock and the paper stock contaminants from the pulper are processed in a junk removal located downstream from the pulper with rejects from the junk remover fed to the detrashing apparatus.

[0008] In other aspects of the invention, the contaminants from the pulper are fed to the detrashing apparatus by a variable speed pump and, in some exemplary embodiments, the variable speed pump provides the motive force needed to convey the pulp stock contaminants to the detrashing apparatus and to provide the force required to separate the light contaminant and heavy contaminant fractions in the detrashing apparatus.

[0009] In other exemplary embodiments, the detrashing apparatus is a centrifical separation device and, in certain embodiments, the heavy contaminant fraction formed in the detrashing apparatus is also continuously removed from the detrashing apparatus.

[0010] In accordance with other aspects of the invention, the light contaminant fraction formed in the detrashing apparatus is fed continuously to a dewatering apparatus wherein rejects from the dewatering apparatus are removed continuously therefrom. Further, the flow of pulp stock contaminants from the variable speed pump to the detrashing apparatus and the flow of the light contaminant fraction from the detrashing apparatus to the dewatering apparatus may be continuous and are not interrupted by any valves or the like.

[0011] In other exemplary embodiments, a detrashing apparatus is provided of the type that separates paper stock into light contaminant, heavy contaminant and accepts fractions. The apparatus may, for example, comprise a top wall, a bottom wall, and opposing sidewalls connected to each other and to the top and bottom walls to thereby define an enclosed chamber. The chamber may have a generally tapered cross section with a central longitudinal axis extending from the bottom wall to the top wall.

[0012] The apparatus may further comprise a tangential inlet to the chamber configured to provide vortical force to the paper stock as it enters the chamber. In some embodiments, a light contaminant outlet communicating with the top wall proximate the central axis is provided for removal of the light contaminant fraction from the chamber. Further, the chamber may comprise a perforated bedplate formed along the bottom wall in combination with a rotatable rotor member superposed adjacent the bedplate. The rotatable rotor member is configured to sweep over the bedplate with the accept fraction of the paper stock exiting the chamber through the perforated bedplate.

[0013] In further exemplary embodiments, a heavy fraction outlet is provided in the chamber and the chamber may be provided with a height (h) measured from the bottom wall to the top wall parallel to the central axis. The chamber has a top diameter (dt) defining the diameter between the opposing sidewalls at the top wall juncture and a bottom diameter (db) defining the diameter of the opposing sidewalls at their juncture with the bottom wall. In one preferred embodiment, h/db J> 0.7 and

[0014] In another exemplary embodiment, the top wall of the chamber comprises a concavity section formed therein adjacent the central axis of the chamber and the top wall also has a generally annular raised collar section surrounding the concavity. The raised collar has a height (hd) measured from the pinnacle of the collar to the nadir of the concavity in a direction that is parallel to the central axis. The apparatus further comprises a vortex finder disposed along the central axis and communicating with the light contaminant outlet. The vortex finder has a top and bottom with the bottom of the vortex finder extending into the chamber. The vortex finder has a height (Vf) measured from the nadir of the concavity to the bottom of the vortex finder. In one particular embodiment 0 _<_ hd _<_ 1/2 h. In a further embodiment 0 _< Vf_< 2/3 h.

[0015] In other embodiments, the detrashing apparatus is a device that operates using centrifical separation principles. In one embodiment the heavy fraction outlet is located along the bottom wall and proximate the circumferential perimeter of the chamber. [0016] In further embodiments, the detrashing apparatus is configured to provide continuous outlet of the light contaminants through the light contaminant outlet and, in certain aspects, a variable speed pump may be provided in communication with the tangential inlet to the chamber and is configured to provide the motive g-force to separate the paper stock into the light contaminant, heavy contaminant and accepts fractions.

[0017] In yet another embodiment, the detrashing apparatus is in communication with a downstream dewatering apparatus. Here, the dewatering apparatus communicates with the light contaminant outlet of the continuous detrasher and the dewatering apparatus is configured such that the variable speed pump provides the motive force to convey the light contaminant fraction through the light contaminant outlet of the continuous detrasher and to the downstream dewatering apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a cut-a-way side view of one embodiment of a continuous detrasher apparatus in accordance with the invention;

[0019] FIG. 2 is a schematic cross-sectional view of the continuous detrasher shown in FIG. 1 ; and

[0020] FIG. 3 is a schematic process diagram showing one embodiment of a continuous detrashing system.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0021] A continuous detrasher and continuous pulping system have been developed, eliminating the need for extra wash water and multiple valves to isolate the detrasher. The continuous detrasher takes advantage of the centrifugal forces created by the inlet pulp's angular momentum in combination with the rotor's induced angular momentum to continuously concentrate the lower density contaminants (or "lights") into the "core" or vortex of the pulp in the continuous detrasher. The strength of the flow in the vortex, and therefore the reject rate ratio, is adjustable using the feed flow rate, pressure, and solids content, and by the continuous detrasher rotor speed.

[0022] The same separation forces that concentrate the lights into the vortex also create an outer zone of high density contaminants, or "heavies" that may also be removed continuously. Since in one embodiment of the invention both the lights and heavies leave the detrasher continuously, the chamber does not completely fill with contamination. The contamination generally experiences less residence time in the apparatus than in typical detrasher processes. The reduced residence time results in contaminates at near original size. Thus, the quality of the usable pulp and wear life of the components may be improved.

[0023] The usable pulp or "accepts" may also be removed continuously from the detrasher through the bedplate without dilution, resulting in a relatively constant solids level and fewer downstream process disturbances. Maintenance and operating costs may be significantly lower due to the reduction of valves around the detrasher and associated process upsets.

[0024] FIG. l shows one exemplary embodiment of a continuous detrasher configuration. The continuous detrasher comprises a chamber (101), with a perforated bedplate (103) and a rotating wiping rotor (105). As shown, the chamber, in cross section, has a tapered cylinder or frusto-conical shape with the larger diameter portion of the chamber positioned at the "top" of the apparatus as shown and with the smaller diameter section of the chamber located at the bottom of the apparatus adjacent a motor housing (not shown). Contaminated pulp is fed into the detrasher chamber (101) via tangential inlet (107) that is positioned tangent to the circumference of the generally cylindrically shaped chamber so as to impart a vortical or circumferential flow vector to the incoming pulp flow. As shown, the inlet (107) is spaced slightly above the bottom of the chamber (101). As the pulp is fed into and flows in the chamber (101), the perforated bedplate (103) strains out any contaminants larger than the perforations and allows usable pulp material ("accepts") to pass through outlet (109). A wiping rotor (105) sweeps contaminants off the bedplate (103), keeping the perforations clear. Low density contaminants (or "lights") become concentrated in an area (111) of the chamber near the vortex created by the flow of the pulp. Also near the pulp vortex is a "vortex finder" (113). The lights travel up the vortex finder (113) and out the top of the chamber (101) through the low density contaminant outlet (115). The high density contaminants (or "heavies") exit the chamber (101) via the high density contaminant outlet (117).

[0025] FIG. 2 shows a cross-section view of the embodiment shown in FIG. 1 highlighting various geometric aspects of the apparatus. Although the continuous detrasher is depicted vertically in this embodiment, other orientations are possible including, a horizontal orientation or orientations at an angle between horizontal and vertical. For simplicity, the in- flow and out- flow streams are not shown in FIG 2. The continuous detrasher comprises a chamber (201) with a top end chamber section (202) and a bottom end chamber section (204) and a central longitudinal axis (206). The top end and bottom end of the chamber are separated by a distance, or height h. The bottom end has a diameter, db which, as shown, in less than the diameter dt thus providing a generally tapered geometry to the cylinder. The perforated bedplate (203) and a wiping rotor (205) are located at the bottom end of the chamber. As shown, rotor (205) is adapted for rotation around axis (206). The top end of the chamber has a diameter dt.

[0026] The top end or "dished head" of the chamber includes a central located dish or concavity (221) surrounded by a generally annular raised collar (222). The concavity extends into the chamber by a distance, hd. The vortex finder (213) extends though the concavity (221) along axis (206) and into the chamber (201) by a distance, Vf. The collar has a pinnacle (227) with the lowermost extremity of the concavity or nadir shown at (229). It was surprisingly discovered that some chamber geometries are more efficient at separating low density contaminants from high density contaminants.

[0027] In one embodiment, the chamber has a geometry wherein h/db is greater to or equal to 0.7 (i.e. , h/db > 0.7). In other embodiments, 0 _< hd _< 1/2 h. Further geometrical embodiments can be dt/db J> 0.9 and 0 _< Vf <_^ 2/3 h. [0028] Turning back again to FIG. 1 , it can be seen that, in this embodiment, the density contaminant outlet (117) is provided adjacent to the circumferential periphery off the bedplate (103) at the bottom end of the chamber and the light contaminants exit through the vortex finder (113) through outlet (115) that is positioned along the central axis of the chamber. The accepts exit through outlet (109) positioned underneath and in communication with the bedplate (103).

[0029] The geometry of the machine allows it to use inlet pulp angular momentum in combination with the rotor induced angular momentum to continuously concentrate via centrifugal forces, lower density, often referred to as lights, contaminants into a 'core' flow that is removed continuously from the machine. The strength of the core flow, and therefore the reject rate ratio, is adjustable using the feed flow rate, pressure, and solids content and by the continuous detrasher rotor speed. Similarly, these separation forces create an outer diameter zone of concentrated higher density, or heavy's, contamination that can, in some embodiments, be removed from the machine continuously. With the rejects leaving the machine continuously, the inlet chamber never fills with contamination and accept flow can be taken continuously through the bedplate at constant solids level so there are no downstream process disturbances.

[0030] Also, the pulping system can operate at a uniform, and ideally lower, contaminant level so that performance of this system is improved. Maintenance and operating costs can be significantly lower due to the elimination of all the valves around the detrasher and associated process upsets. The high shear of the in-chamber separation reduces flake content and achieves the washing effect of the current batch process on a continuous basis. Initial results also show that the reduced residence time of contaminants in the detrasher produces contaminants at near original size. (Batch detrashing typically operates with a 10-20 minute collection period before dumping, whereas average residence time in the continuous detrasher is less than 2 minutes.) This means that less breakdown of the contaminants occurs and therefore accept quality and wear life of the components may be improved. [0031] FIG. 3 shows one embodiment of a continuous detrashing system. In this embodiment, pulp stock such as recovered paper (2) enters a pulper (4). The pulper may, for example, comprise a pulping device such as the type shown in U.S. Patent 4,725,007, incorporated by reference herein. Pulper accepts (6) exit the pulper (4) and are sent elsewhere for further processing (not shown). Contaminated pulp enters a variable speed recessed impeller pump, or similar fluid transfer device (8). Pulp flow rates through the pump may be controlled with a dilution water stream (10). The variable speed pump (8) transfers the contaminated material to the continuous detrasher (12) which separates the material into lights (14), continuous detrasher accepts (16), and heavies (20). The continuous detrasher may for example be of the type shown for example in FIGS. 1 and 2.

[0032] Optionally, contaminated pulp from pulper (4) may enter a junk remover (18) before it is forwarded to the pump (8) and then the continuous detrasher (12). The junk remover may be of any type and is well known to those of ordinary skill in the art. One exemplary junk removal device is shown in U.S. Patent 4, 129,259, incorporated herein by reference. The flow of contaminated pulp through the junk remover (18) may be controlled with a dilution water stream (10). As shown, heavies (20) from detrasher (12) may be cycled back to the junk remover.

[0033] The concentrated lights (14) leave the detrasher (12) continuously and enter a dewatering drum screen (22) where the water and pulp are separated and recovered. The accepts (16) from the continuous detrasher, in one embodiment, may also be removed on a continuous basis and can, for example, be combined with the accepts flow from the pulper and forwarded to a dump chest or the like for further processing.

[0034] Optionally, the drum screen (22) may have multiple sections. The first section may comprise a washing zone (24) where water is added (26) and any carryover fiber is freed from the contamination prior to entering a perforated zone (28) where the water and fiber are separated and recovered. The thickened low density rejects (30) leave the system and may be disposed of using traditional methods. The filtrate (32) leaves the drum screen (22) and may be returned either intermittently or continuously to the pulper (4).

[0035] In one embodiment, there are no control valves installed between the main system components, including pump (8), continuous detrasher (12), and washing drum screen (24). In another embodiment, the motive force conveying the fluid flow to and through components (12), and (22) is supplied by the pressure created by pump (8). In another embodiment, the entirety of the motive force conveying the fluid flow through (12) and (22) may be gravity. In other aspects of the invention, flow control valves and logic programmable controllers may be used to control the continuous detrasher accepts (16) quantity and the dilution water stream (10). In yet another embodiment, the drum screen dilution (26) may be controlled by the flow of lights (14) exiting the continuous detrasher (12). In another embodiment, lights and heavies ratios may be controlled by pump and detrasher speeds, operating pressures, flows, and consistency of the continuous detrasher accepts (16).

[0036] Generally, the continuous detrashing system and methods of the invention, in various aspects, may be viewed to have one or more of the following characteristics:

1. The system is stable and all downstream systems can be run at their optimum point.

2. Contaminant loading in the pulper never goes above acceptable levels so wear is minimized and peak capacity is achieved.

3. Contaminant residence time in the detrasher is minimized so wear levels are reduced and contaminant breakdown is negligible.

4. Accept product quality is improved due to reduced contaminant breakdown.

5. System uptime and operating cost is improved by the elimination of all the valves required to operate a conventional batch detrashing system and the reduced wear and tear on the equipment as noted above.

6. Equipment size can be reduced because the system no longer needs to process peak flows that occur at given points in a batch system process. For example, the drum screen only needs to process a steady flow instead of a short surge that is much greater.

[0037] In one exemplary embodiment, our process utilizes a variable speed recessed impeller pump to feed the current continuous detrasher with enough pressure to drive centripetal forces required to effectively separate the lower and higher density contaminants. This motive force can be achieved by other means including elevated gravity feed if practical. The pulp may be diluted ahead of the pump to allow it to act as a mixer and if lower consistency is desired to improve contaminant mobility in the detrasher (depending on the base process conditions).

[0038] The Detrashing System requires no valves at its maximum implementation - control of reject rates and washing efficiency can be done with variable speed of the pump and detrasher along with dilution rates. The result is a system with very low maintenance and no process upsets due to batch cycles and wash water changes. Additionally, wash water requirements are reduced saving energy and cost.

[0039] In the preferred embodiment, a recessed impeller pump transfers the contaminated material to the continuous detrasher which separates the material into accept pulp and low and high density rejects. The concentrated low density rejects leave the detrasher and enter a dewatering drum screen. In the preferred embodiment, the drum screen has two sections; a first washing zone where water is added and any carryover fiber is freed from the contamination prior to entering the perforated zone where the water and fiber is recovered. The thickened low density rejects leave the dewatering drum screen and can be disposed of using traditional methods. The concentrated high density rejects leave the continuous detrasher and in the preferred method are returned either intermittently or continuously to the pulper for removal with other high density contaminants.

[0040] There are no control valves installed between the main system components: pulping system, pump, continuous detrasher, and washing drum screen. Flow control may be used to control the continuous detrasher accepts quantity, and system consistency control is ratio driven based on total detrashing system flow sum of continuous detrasher accepts and continuous detrasher "lights" rejects for stable operation. Drum screen dilution may be ratio controlled by detrasher low density reject flow. Baseline low and high density reject ratios are established by the design of the equipment and once in operation are controlled by pump and detrasher speeds, operating pressures, flows, and consistency.

[0041] Generally, the continuous detrashing system and process may be viewed to apply to processes that remove contaminants from a continuous pulping system continuously using a detrasher that does not require any cycling valves. The process, in one respect, uses a feed pump to transfer the contaminated pulp from the repulper to the continuous detrasher. Additionally, gravity or other motive force may be used to transfer the contaminated material from the pulper to the continuous detrasher. A washing and dewatering drum may be used to reclaim the water from the concentrated low density rejects from the detrasher. Alternatively, a vibrating screen or other conventional valve recovering device may be used to reclaim this water.

[0042] In another aspect of the invention, the concentrated heavy rejects from the continuous detrasher may be sent back to the pulper or junk tower or the like operatively associated with the pulper. Alternatively, these heavy rejects may be sent to a separate dewatering device (for example, an incline screw extractor or perforated bin). The water may be reclaimed.

[0043] Generally, the continuous detrasher may be, in some exemplary embodiments, viewed as a continuous tub-type pulper as commonly used in the paper recycling industry using centrifugal force to continuously separate lower and higher density contaminants from an accept fiber flow.

[0044] The centrifugal separating energy for the continuous detrasher may be created using a combination of inlet stock angular momentum and an internal rotor. The accept fiber separation may be accomplished by using a perforated extraction plate, a rotor keeping the extraction plate from binding, and the creation of centrifugal force that continuously separates low and high density contamination. The rotor that keeps the bedplate clear also serves to provide centrifugal energy for separating low and high density contamination. In some instances, it may be advantageous to remove the low density contaminants on a continuous basis while the higher density contaminants may be intermittently purged.

[0045] The detrasher may also utilize a casing design having an optimized flow pattern created by favoring the development of low density contaminant separation forces by reducing toroidal swirl deflaking forces while increasing those developing centrifugal swirl around the machine centerline, as defined by a casing height (h) greater than or equal to 70% of the root diameter (db).

[0046] Further, in other exemplary embodiments, the body has tapered side walls (such as dt/db J> 0.9) encouraging rotational circulation and development of a low pressure zone concentrating low and high density contaminants in different areas for removal. In other embodiments, the detrasher apparatus may be provided with a "dished" head to encourage vortical toroidal circulation within the inlet and separation chamber. Exemplary limits are defined as 0

1/2 h.

[0047] In still additional exemplary embodiments, the continuous detrasher is provided with a vortex finder to target removal of the concentrated low density, light, contaminants. Exemplary limits are defined as 0 _<_ Vf <_ 2/3 h. The detrasher may also utilize an optimized design having an inlet separation chamber with a volume as required to produce an average residence time of 15 to 120 seconds based on feed flow.

[0048] This written description uses examples to disclose the invention, and to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

[0049] What is claimed is:

Claims

1. A method of detrashing paper stock contaminants from a pulper, said method comprising feeding contaminants from said pulper to a detrashing apparatus and separating said contaminants into accepts, light contaminant and heavy contaminant fractions in said detrashing apparatus and continuously removing said light contaminants from said detrashing apparatus.

2. A method as recited in Claim 1 , wherein said paper stock is repulped paper stock and wherein said paper stock contaminants from said pulper are processed in a junk remover with rejects from said junk remover fed to said detrashing apparatus.

3. A method as recited in Claim 1 , wherein said contaminants from said pulper are fed to said detrashing apparatus by a variable speed pump, said variable speed pump providing the motive force to convey said pulp stock contaminants to said detrashing apparatus and to provide the force required to separate the light contaminant and heavy contaminant fractions in said detrashing apparatus.

4. A method as recited in Claim 3, wherein said detrashing apparatus is a centrifugal separation device.

5. A method as recited in Claim 1 , wherein said heavy contaminant fraction formed in said detrashing apparatus is continuously removed therefrom.

6. A method as recited in Claim 3, wherein said light contaminant fraction formed in said detrashing apparatus is fed continuously to a dewatering apparatus wherein rejects from said dewatering apparatus are removed continuously therefrom.

7. A method as recited in Claim 6, wherein flow of said contaminants from said variable speed pump to said detrashing apparatus and flow of said light contaminant fraction formed in said detrashing apparatus to said dewatering apparatus are continuous and not interrupted by valves.

8. Detrashing apparatus for separation of paper stock into light contaminant, heavy contaminant, and accepts fractions, said apparatus comprising

(a) a top wall

(b) a bottom wall

(c) opposing sidewalls connected to each other and to said top and bottom walls thereby defining an enclosed chamber, said chamber having a generally tapered cross section with a central axis extending from said bottom wall to said top wall

(d) a tangential inlet to said chamber to provide vortical force to said paper stock as it enters said chamber

(e) a light contaminant outlet communicating with said top wall proximate said central axis for removal of said light contaminant fraction from said chamber

(f) a perforated bedplate in said bottom wall

(g) a rotatable rotor member superposed adjacent said bedplate and configured to sweep over said bedplate with said accept fraction of said paper stock exiting said chamber through said perforated bedplate.

(h) a heavy fraction outlet in said chamber;

(i) said chamber having a height (h) measured from said bottom wall to said top wall parallel to said central axis, said chamber having a top diameter (dt) defining the diameter between said opposing side walls at said top wall, and a bottom diameter (db) defining the diameter of said opposing sidewalls at said bottom wall, wherein h/db J> 0.7 and dt/dbj> 0.9.

9. Detrashing apparatus as recited in Claim 8, wherein said top wall comprises a concavity section formed therein adjacent said central axis and said top wall also having a generally annular raised collar section surrounding said concavity, said raised collar having a height (hd) measured from the pinnacle of said collar to the nadir of said concavity in a direction parallel to said central axis; said apparatus further comprising a vortex finder disposed along said central axis and communicating with said light contaminant outlet, said vortex finder having a top and bottom with said bottom of said vortex finder extending into said chamber, said vortex finder having a height (Vf) measured from the nadir of said concavity to said bottom of said vortex finder, wherein 0 hd _<_^ 1/2 h.

10. Detrashing apparatus as recited in Claim 9, wherein 0 _< Vf <_ 2/3 h.

11. Detrashing apparatus as recited in Claim 9, wherein said apparatus is a centrifugal separation device.

12. Detrashing apparatus as recited in Claim 11 , wherein said heavy fraction outlet is located along said bottom wall proximate the perimeter of said chamber.

13. Detrashing apparatus as recited in Claim 9 configured to provide continuous outlet of said light contaminants through said light contaminant outlet.

14. Detrashing apparatus as recited in Claim 13, wherein a variable speed pump is in communication with said tangential inlet to said chamber and is configured to provide the motive force to separate said paper stock into said light contaminant, heavy contaminant and accepts fractions.

15. Detrashing apparatus as recited in Claim 14, wherein a dewatering apparatus is in communication with said light contaminant outlet, said dewatering apparatus configured such that said variable speed pump provides the motive force to convey said light contaminant fraction thru said light contaminant outlet and to said dewatering apparatus.