CA1248800A - Pumped flow attrition disk zone - Google Patents

Abstract

TITLE
REVERSE FLOW DISK ATTRITION DEVICE
ABSTRACT
A disk attrition device 10 or 50 for pulping or refining slurried material has a centrifugal rotor pump that reverses the flow of material through the attrition zone 13 between rotor and stator working bars. Rotor 12 has centrifugal pump vanes 20 axially near attrition zone 13 and disposed around a peripheral pumping region 17, and rotor 12 has a hollow interior 30 extending from the inner periphery of the attrition zone to the pumping region.
Pump vanes 20 provide sufficient centrifugal pumping force so that slurried material enters the outer periphery of the attrition zone, passes radially inward through the attrition zone against the outward pumping force of the working bars, and flows through the hollow rotor interior to the pumping region, where it is pumped radially outward.

Description

TITLE
PUMPED FLOW ATTRITION DISK ZONE
BACKGROUND
I have discovered a be-tter way of operating an annular, disk-shaped a-ttrition zone ~or working slurried material in pulpers and refiners. The turning rotor bars t~at confront stator bars in suc~ attrition zones centrif-ugally pump tbe slurry radially outward from the inside to the outside of the attrition zone as t~e bar crossings work t~e material flowing through.
Such centrifugally pumping, annular attrition zones experience many problems. In pulpers, t~ey need a cleaner to keep large clumps of fiber from plugging the inside entrance to t~e attrition zone, and cleaners find it difficult to exclude contaminants and overly large fiber clumps while admitting suitable slurry to t~e attrition zone. The pumping force of t~e attrition zone, especially when combined with a restrictive cleaning bonnet, creates a hig~ vacuum under t~e rotor. Resisting t~e substantial force of this vacuum requires a strong rotor, a large thrust bearing, an elaborate packing gland around t~e rotor s~aft, and a sturdy mechanism for moving the rotor axially to adjust tbe bar clearance gap in t~e attrition zone.
SUM~ARY OF THE INVENTION
I ~ave discovered that reversing t~e fluw t~roug~ t~e attrition zone by means of a pump on the rotor creates several surprising advantages. The dynamic move-ment of the working bars in t~e attrition zone can keep out contaminants and prevent fiber clogging, thus elimina-ting any separate cleaning bonnet. The self-cleaning ability of the attrition zone also protects the rotor pump, w~ich receives worked slurry from the attrition zone as an input. T~e vacuum under t~e rotor can be greatly reduced or eliminated, simplifying construction of the rotor and its adjusting mec~anism. T~e muc~ larger entrance area to the attrition zone and the inward flow of slurry t~rough -t~e attrition zone make my device surpris-ingly aggressive and effective -- both shortening the time required to work a slurry and allowing de~ibering of previously unworkable materials.
DRAWINGS
Figure 1 is a partially schematic cross-sec-tional view of a preferred embodiment oF a disk attrition device made according to my invention;
Figure 2 is a partially cutaway plan view of tbe device of FI~. l;
Figure 3 is a plan view of a working bar element for the device of FIGS. 1 and 2;
Figure 4 is a cross-sectional view of tbe working bar element of FIG. 3, taken along the line 4-4 t~ereof;
Figure 5 is an enlarged~ fragmentary elevational view of tbe outer periphery of a preferred configuration of working bars in an attrition zone for the device of FIGS. 1 and 2;
` Figures 6 and 7 are schematic pro~iles of preferred working bar configurations for a stator and rotor respectively; and Figure 8 is a partially schematic, cross-sectiona]. view of another preferred embodiment of a pumped attrition disk device according to my invention.
DETAILED DESCRIPTION
GENERAL STRUCTURE
Attrition device 10 as s~own in the drawings is simplified for illustration hy eliminating some generally known mechanical parts. Tbis unclutters the drawings and makes t~e essential operating components more clear.
Device 10 includes a stator 11 and a rotor 1~, each bearing an annular, disk-shaped array of working bars described more fully below. Stator and rotor bars confront across a narrow gap 13 forming a disk-shaped attrition zone t~at extends around a radially outer region of the rotor and stator in a plane perpendicular to t~e rotor axis.

Rotor 12 is mounted on axial shaft 15 that rotates and is movable axially to adjust the width of attrition gap 13. A radial plate 16 is mounted on s~aF-t 15 and extends to a slurry discbarge region 17 at tbe outer peripbery of rotor 12. An annular bar holding plate 18 is axially spaced from radial plate 16 and radially spaced from sbaft 15. Centrifugal pump vanes 20 are arranged between, and preferably welded to, radial plate 16 and bar bolding plate 18 so tbat pump vanes 20 interconnect plates 16 and 18. A replaceable annular filling 25 ~aving working bars as described below is secured to bar holding plate 1~.
Stator 11 includes a base plate 22 to w~icb is clamped or bolted an annular filling 26 wit~ working bars as explained below. 8esides bolts 23, tbere are many possible arrangements of clamps and bolding devices that can secure bar fillings 25 and 26 in place and allow their replacement w~en worn.
Mounting rotor bar filling 25 on radial plate 16 via pump vanes 20 and annular bar plate 18 makes a sturdy and simple rotor 12 witb an open annular interior 30. No radial spokes are necessary, and bollow interior 30 is undivided by any radially extending elements. T~is, toget~er witb rounded or beveled edges within interior 30 ensures t~at fibers, wood splinters, or otber materials being processed ~ave no surfaces or edges on w~ic~ to lodge. Rotor interior 30 extends in a generally annular U-turn from tbe inner perimeter 33 of attrition zone 13 to disc~arge region 17 swept by pump vanes 20~
Rotor 12 can bave a cutting edge 32 extending beyond t~e outer perimeter of radial plate 16 for cutting large objects or groups of fibers into smaller pieces that are more easily defibered. I have found tbat a flat cutter plate 31 having a polygonal shape suc~ as a ~exagon ~5 and ~aving a s~arpened outer edge 32 makes a simple and effective cutter.

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Structural details not illustrated include packing glands, bearings, means for rotating and axially adjusting s~aft 15, and means for mounting device 10 wit~in a tank or cbamber. Structures for these purposes are generally known.
Attrition device 10 can be mounted in a tank to operate as a pulper or refiner for defibering fibrous material. It can also be arranged in a smaller c~amber throug~ whic~ a slurry flows so t~at device 10 can serve as a refiner. Altbough I made device 10 witb cellulose fibers in mind, I ~ave found that it can defiber synt~etic and ot~er organic fibers. It may also be usable as a ~rinder or attrition device for reducing particle size or refining materials other t~an fibers.
T~e simple construction of device 10 makes i-t convenient for a small sized laboratory model that can operate in small containers to experiment with different materials. The same basic construction can also be scaled up to fullsized pulpers and refiners operating in large tanks or c~ambers at substantial throughput rates.
I prefer t~at pump vanes 20 be approximately semi-cylindrical as best sbown in FIG. 2. Suc~ vanes are simple to fabricate and afford excellent connectors between plates 16 and 18 for a simple and sturdy rotor construction. T~e semi-cylindrical s~ape of pumping vanes also makes t~eir radially inner surfaces rounded so t~at fibers cannot lodge on them. The illustrated version of device 10 ~as four pumping vanes 20, but ot~er numbers can be used.

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Tbe centrifugal pumping force of annular pumping zone 17 swept by vanes 20 is sufficient to draw slurried material radially inward through attrition zone 13 against t~e outward pumping force of the working bars. To accomplish t~is, pump vanes 20 are preferably positioned radially outward far enoug~ so t~at the outside diameter of pumping zone 17 is larger t~an t~e outside diameter of attrition zone 13. ~lso, t~e inside diameter of vanes 20 and pumping zone 17 is preferably larger t~an the inside diameter of attrition zone 13. T~e radially inward flow from t~e outer perimeter 34 to the inner perimeter 33 of attrition zone 13 leads to an annular U-s~aped flow through hollow rotor interior 30 and a radially outward flow t~roug~ pumping region 17~ T~is reverse flow through attrition zone 13 is also rapid enoug~ so t~at slurried material makes quickly repeated passes throug~ t~e attrition zone to expedite material processing.
Pump vanes 20 also produce a swirling flow so that breaker or cleaner vanes on rotor 12 are no longer necessary to swirl slurried material around in a circu-lational flow. Eliminating hreaker or circulation vanes on t~e rotor also eliminates t~e problem of large ob~ects wrapping or lodging on such vanes so t~at device 10 can operate cleanly with ot~erwise troublesome materials.
Another embodiment of attrition device 50 is arranged as shown in FIG. 8 for using my invention. Many components of attrition device 50 are similar to t~ose already explained including rotor 10 and sta-tor 11.
Instead of rotor s~aft 15 extending tbroug~ stator 11, rotor s~aft 15 extends t~roug~ a mount on wall 62 of a container 60, and stator 11 is mounted on wall 61 opposite wall 62. T~is makes t~e confronting working bars on rotor filling 25 and stator filling 26 meet between t~e rotor and stator wall mounts in a disk-shaped, annular attrition zone 13 positioned wit~in container 60. It also removes rotor s~aft 15 from hollow rotor interior 30 and disposes centrifugal pumping zone 17 between attrition zone 13 and s~aft mount wall 62.

Rotor plate 16 is preferably clamped to the end of rotor sbaft 15 to bold pumping vanes 20 between end plate 16 and aonular plate 18 as illustrated. Cut-ter plate 31, attac~ed to plate 18 by bolts 51, holds rotor filling 25 in place and also provides cutting edges 32.
Clamp 52 held by bolt 53 bolds stator filling 26 in place. Guard ring 40 for setting t~e dept~ of space available between stator bars is formed as an axially adjustable sleeve surrounding stator filling 26. A
cylindrical tube 22a ~olding stator base plate 22 ~as longitudinal adjusting slots 40a througb w~,icb screws 41a extend to t~read tbroug~ nuts 42a welded to stator guard sleeve 40 for adjustment axially of the attrition zone.
Adjustment scales, screws, and more elaborate mec~anisms can also be used for positioning guard ring 40 axially to set the depth of space available betweer, stator bars.
Tbe arrangement of FIG. 8 opens up bollow rotor interior 30 by eliminating shaft 15 and assuring full input flow to pumping zone 17 so tbat no cavitation occurs. It also eliminates any partial vacuum between rotor 10 and rotor wall 62. Tbis simplifies packing glands, bearings7 and axial adjustment mechanisms to set t~e widt~ of t~e gap in t~e disk plane of attrition zone 13.
~ORKING BARS
Attrition zone 13 should ~ave as many working bars as practical to produce tbe largest number of bar crossings per revolution and t~ereby accomplis~ maximum work as t~e rotor turns. This has led in the past to working bars witb rectangular cross-sectional s~apes so t~e bars can be as stout and sturdy as possible and as closely spaced as practical. Also, workers in tbe disk attrition art bave believed t~at leading bar surfaces nearly perpendicular to tbe plane of t~e attrition zone ~5 provide the most effective working surfaces possible.

I have discovered that inclining t~e leading surfaces of the working bars by an acute angle to t~e plane of the attrition zone produces surprising advan-tages, even tbough it reduces tbe number of bars tbat can be arranged wit~in a given space. I found that fibers entering tbe outside of attrition zone 13 can wrap around outer ends of bars that have a rectangular cross-sectional s~ape and, by lodging on t~e bar ends, can clog tbe entry to t~e attrition zone. For center feeding attrition zones, t~is can be avoided by giving tbe entry ends of tbe tops of the bars long, t~in tapers leading to t~e plane of t~e attrition zone gap. T~at is not practical, ~owever, for slurried fibers entering an outer perimeter 34 of attrition zone 13. Sloping t~e leading rotational surfaces of the radially outer regions of t~e working bars as s~own in FIGS. 3-7 not only solves t~e problem of fibers wrapping around bar ends, but offers ot~er advantages as well.
Alt~ough t~e sloped stator bar surfaces 38 and rotor bar surfaces 48 can extend for the fuIl lengt~ of eac~ bar, the fiber wrapping problem can be solved by sloping only t~e radially outer regions of the rotor and stator bars. T~is can be done by grinding a bevel on t~e leading edge of the outer end of eac~ stator bar 35 and rotor bar 45.
Extending the sloped leading surfaces 38 and 48 for t~e full length of respective stator bars 35 and rotor bars 45 as illustrated leaves the bars separated by grooves 36 and 46 t~at preferably deepen as they extend from inner perimeter 33 to outer perimeter 34. Also, the working edges 37 and 47 of t~e stator and rotor bars are preferably angled slig~tly from radial so t~at t~ey scissor toge-t~er, rather t~an clash. Trailing bar edges 39 and 49 are preferably approximately perpendicular to t~e plane of attrition zone 13 althoug~ a small angle for draft is required if fillings 25 and 26 are formed as annular castings as preferred.

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Al~bough stator and rotor working bars can have different configurations, it is often convenient to make them identical as shown in FIG. 5. Then rotor filling 25 can be tbe same as t~e stator filling 26 illustrated in FIGS. 3 and 4.
The preferred slope angles a and b for tbe leading surfaces of stator and rotor bars are each acu-te angles to the plane of attrition zone 13 and are eac~
preferably witbin the range of 30 to 60. Material encountering tbe sloped leading surface of the outer extremity of a working bar is t~en urged along the slope toward the working edge 37 or 47 in tbe plane of the attrition zone gap w~ere it encounters a confronting bar.
T~is action can nibble away bits of a large fiber clump, eacb time it encounters the entry to t~e attrition zone.
Since tbe sloped leading edges prevent fiber lodgmen-t, tbe bars eit~er draw fibers up tbe slope into the attrition zone plane for working or cast off fiber clumps or foreign objects too large to enter t~e attrition zone.
Radially outer end surfaces 35E and 45E of t~e stator and rotor bars preferably lie in a cylindrical locus that is interrupted by grooves 36 and 46 formed between bar ends 35E and 45E. The dynamic effect is a periodically interrupted cylindrical barrier to entry of contaminants into the attrition zone. Especially larger objects experience t~e bar ends as a cylindrical wall t~rough w~ic~ small entrance openings in t~e form of ~ixed stator grooves 46 and rotating rotor grooves 36 afford little opportunity for entry into the attrition zone.
This, combined with the ability of bar ends 35E and 45E to nibble small bits from large fiber clumps and tbe ability of sloped leading surfaces 38 and 48 to avoid Fiber lodgment, keeps large fiber clumps and contamlnants out of the attrition zone and yet enables the device to defiber objects and materials that have prevlously been recalcitrant.

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Tbe discovery that material entering t~e attrition zone passes mostly between stator bars 35 allows the material being processed to be controlled with a simple guard 40 encircling stator element 26 as s~own in FIGS. 5 and 6. Guard ring or strip 40 encircles s-tator element 26 and can be axially adjusted by screws 41 to open or restrict t~e space available in grooves 36 between bars 35. Moving guard 40 to a position near working edges 37 restricts entry into t~e attrition zone to finer mate-rial and excludes unwanted materials such as plastics orot~er contaminants t~at s~ould not be broken into small pieces. Positioning guard 40 to open t~e access to grooves 36 admits larger pieces of material or groups of fibers into t~e attrition zone for processing and makes device 10 more aggressive. Since stator 11 is not moving, guard 40 can be axially adjusted during operation w~ile results are observed. Guard 40 is also simpler t~an a cleaning bonnet and more easy to adjust for adapting device 10 to work on different materials. It also allows device 10 to work on con-taminated Furnish wit~out s~red-ding plastics and other contaminants into the pulp.
OPERATION
My attrition device 10 or 50 can be operated with sha~t 15 vertical, horizonkal, or at some ot~er angle; and it can be mounted in tanks or c~ambers tllat vary in size or shape, depending on objectives. As a small laboratory model, my attrition mac~ine can process small amounts of materials to determine wbat can be pulped, refined, or processed; what power, time, attrition zone gap, and ot~er parameters will be required; and w~at settings for guard ~0 are workable witb different mate-rials. Device 10 or 50 can operate as a refiner in a chamber t~roug~ w~ich pulped material flows; and it can operate to pulp, defiber, and process a wide variety of ma-terials.
A working prototype has ac~ieved unique results in defibered cotton shirts, rags, surgical gowns, flakes, s~ives, and wood chips. It works rapidly and aggressively with these materials, completing the defibering in less time and with less energy than previous devices. My device can also pulp nonwoven fibers, many different varieties of organic fibers, and some synt~etic fibers.
Cutting edge 32 is useful for cutting large fibrous objects into smaller swatches t~at can be defibered more rapidly. Working bars at the outer perimeter 34 of t~e attrition zone nibble and tear at fiber clumps and quickly pick larger pieces into bits that can pass t~roug~ tne attrition zone. Once materials are reduced to fragments small enougb to flow t~roug~ tbe attrition gap, complete defibering occurs quickly because of the rapid and repeated flow of material between the working bars.
The sloped leading edges 38 and 48 of tbe outer ends of t~e working bars combined wit~ an unobstructed, ~ollow interior and rounded pump vanes 20 avoid all fiber lodgment and keep t~e rotor and stator clean during opera-tion. T~e adjustable guard around t~e grooves between t~e stator bars allows the mac~ine to be adjusted during operation to exclude contaminants from t~e attrition zone.
Experimental work t~us far has s~own my attri-tion device to be more aggressive, versatile, and effi-cient t~an previous pulpers. It can defiber large objects and some types o~ fibrous materials that conventional pulpers cannot work. It may also be usable for processing materials not yet tried.

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Claims (29)

I claim:

1. In an attrition device having an attrition zone formed between a stator and rotor having annularly arrayed confronting bars contiguously separated by grooves, said attrition device being arranged within a tank of wetted and unpumpable fiber material that may contain contaminants, an improvement making said attrition zone self-cleaning while said attrition zone defibers said unpumpable material, said improvement comprising:
a. said rotor having a hollow interior that is enclosed between an inner periphery of said attrition zone and an annular discharge region;
b. impeller vanes arranged within said annular discharge region to pump slurry from said hollow rotor interior radially outward through said discharge region and thereby to pull fiber clumps of said material into an outer periphery of said attrition zone, flow said fiber clumps radially inward through said attrition zone against the outward pumping force of said confronting bars, and draw said slurry from said inner periphery of said attrition zone through said hollow rotor to said discharge region;
c. radially outer ends of said bars on said rotor and stator at said outer periphery of said attrition zone lying in a cylindrical locus and presenting an interrupted cylindrical barrier to entry of said contaminants into said attrition zone; and d. leading surfaces of said bars at said outer periphery being sloped to prevent fibers from wrapping around outer ends of said bars.

2. The improvement of claim 1 wherein said rotor interior is unobstructed by any radially extending elements, and radially inner extremities of said impeller vanes are sufficiently rounded to avoid fiber lodgment.

3. The improvement of claim 1 wherein the outside diameter of said impeller vanes is larger than the outside diameter of said attrition zone.

4. The improvement of claim 1 wherein said slope of said leading surfaces of said bars is from 30° to 60° from the plane of a gap between said confronting bars of said rotor and stator.

5. The improvement of claim 1 including a guard ring around an outer periphery of said stator and means for axially adjusting said guard ring relative to said stator bars to set a depth of space available for said fiber clumps to enter between said stator bars.

6. The improvement of claim 1 wherein said closed interior of said rotor is shaped so that said slurry makes an annular U-turn between said inner periphery of said attrition zone and said annular discharge region.

7. The improvement of claim 1 including means for axially adjusting the width of a gap between said confronting bars of said rotor and stator in said attrition zone.

8. A self-cleaning attrition device immersed in a container of wetted and unpumpable fiber material for defibering said material into a slurry, said attrition device comprising:
a. a stator and rotor having bars contiguously separated by grooves to confront in an annular, disk-shaped attrition zone arranged within said container;
b. a wall mount through which a shaft for said rotor extends into said container;
c. another wall mount supporting said stator within said container;
d. said wall mounts being arranged on opposite wall regions of said container so that said attrition zone is between said opposite wall mounts;
e. said rotor having a hollow interior that is enclosed between an inner periphery of said attrition zone and an annular peripheral discharge region;
f. impeller vanes arranged within said annular discharge region for moving said slurry radially outward through said discharge region and thereby drawing fiber clumps of said material radially inward through said attrition zone and through said hollow rotor interior; and g. leading surfaces of said bars at said outer periphery of said attrition zone being sloped to prevent fibers from wrapping around outer ends of said bars.

9. The attrition device of claim 8 wherein said discharge region is axially spaced from said attrition zone so that said slurry flows in an annular U-turn through said hollow interior of said rotor.

10. The attrition device of claim 8 wherein the outside diameter of said impeller vanes is larger than the outside diameter of said attrition zone.

11. The attrition device of claim 8 wherein said slope of said bars at said outer periphery is 30° to 60°
from the plane of a gap between said confronting bars of said rotor and stator.

12. The attrition device of claim 8 including a guard ring around an outer periphery of said stator and means for axially adjusting said guard ring relative to said stator bars to set a depth of space available for said fiber clumps to enter between said stator bars.

13. The attrition device of claim 8 including means for axially adjusting the width of a gap between said confronting bars of said rotor and stator in said attrition zone.

14. A self-cleaning attrition zone immersed in a container of wetted and unpumpable fiber material that may contain contaminants so that said attrition zone can defiber said material into a slurry, said self-cleaning attrition zone comprising:

a. a rotor and stator having bars continguously separated by grooves to confront in an annular array:
b. impeller vanes arranged within an annular discharge from said rotor, which has an enclosed and hollow interior so that said impeller vanes force slurry from said hollow interior radially outward through said annular discharge and thereby draw fiber clumps of said material radially inward through said attrition zone from an outer periphery of said attrition zone and through said hollow interior to said discharge;
c. said rotor interior being unobstructed by any radially extending elements, and radially inner extremities of said impeller vanes being sufficiently rounded to avoid fiber lodgment;
d. radially outer ends of said bars on said rotor and stator at said outer periphery of said attrition zone lying in a cylindrical locus and presenting an interrupted cylindrical barrier to entry of said contaminants into said attrition zone; and e. leading surfaces of said bars at said outer periphery of said attrition zone being sloped from the plane of a gap between said rotor and stator bars so as to prevent fibers from wrapping around outer ends of said bars.

15. The attrition zone of claim 14 wherein said slope of said leading surfaces of said bars is from 30° to 60°
from the plane of said gap.

16. The attrition zone of claim 14 including means for axially adjusting the width of said gap in said attrition zone.

17. A self-cleaning attrition device immersed in a tank of wetted and unpumpable fiber material that may contain contaminants, said self-cleaning attrition device comprising:
a. a rotor and stator having bars contiguously separted by grooves to confront in an annular array in an attrition zone:
b. radially outer ends of said bars on said rotor and stator at an outer periphery of said attrition zone lying in a cylindrical locus and presenting an interrupted cylindrical barrier to entry of contaminants into said attrition zone;
c. leading surfaces of said bars at said outer periphery being sloped at 30° to 60° from the plane of an attrition zone gap between said rotor and stator bars so that fiber material does not wrap around said outer ends of said bars;
d. said rotor having a hollow interior that is enclosed between an inner periphery of said attrition zone and a centrifugal annular discharge region; and e. impeller vanes arranged around said rotor within said annular discharge region for moving small clumps of said fiber material radially inward through said attrition zone, through said hollow rotor, and radially outward through said discharge region.

18. The attrition device of claim 17 arranged for flowing said clumps of said material in an annular U-turn between said inner periphery of said attrition zone and said annular discharge region.

19.The attrition device of claim 17 including means for axially adjusting the width of said attrition zone gap between said rotor and stator bars.

20. The attrition device of claim 17 wherein a guard ring around an outer periphery of said stator is axially adjustable to set a depth of space available for said clumps to enter between said stator bars.

21. A pulper for pulping unpumpable and wetted fiber material that may contain contaminants, said pulper comprising:
a. a rotor and stator arranged within a tank for said unpumpable material so that bars on said rotor and stator are contiguously separated by grooves and confront in an annular, disk-shaped attrition zone:
b. said rotor having a hollow interior that is enclosed between an inner periphery of said attrition zone and an annular peripheral discharge region axially spaced from said attrition zone;
c. impeller vanes arranged within said discharge region of said rotor for forcing defibered material radially outward through said discharge region to draw fiber clumps from said material radially inward through said attrition zone to become defibered and pass through said hollow rotor; and d. the leading surfaces of outer regions of said bars being sloped so that fibers cannot wrap around outer ends of said bars.

22. The pulper of claim 21 wherein said leading surfaces of said bars are sloped by 30° to 60° from the plane of said attrition zone.

23. The pulper of claim 21 including means for axially adjusting the width of a gap between said rotor and stator bars in said attrition zone.

24. The pulper of claim 21 wherein a guard ring around the outer periphery of said stator bars is axially adjustable to set a depth of space available for said material to enter between said stator bars.

25. The pulper of claim 21 wherein said rotor and said stator are mounted on opposite walls of said tank.

26. A self-cleaning attrition device immersed in a tank for defibering wetted and unpumpable fiber material filling said tank and possibly containing contaminants, said self-cleaning attrition device comprising:

a. a rotor and stator having bars ccntiguously separated by grooves to confront in an annular array forming an attrition zone;
b. impeller vanes arranged within an annular discharge region of said rotor, said rotor being hollow and closed between said annular discharge region and an inner periphery of said attrition zone axially spaced from said annular discharge region so that said impeller vanes draw fibers, but not said contaminants, radially inward through said attrition zone; and c. leading surfaces of outer regions of said bars being sloped 30° to 60° relative to the plane of said attrition zone so that fibers cannot wrap around outer ends of said bars.

27. The self-cleaning attrition device of claim 26 including means for axially adjusting the width of a gap between said rotor and stator bars in said attrition zone.

28. The self-cleaning attrition device of claim 26 wherein an outside diameter of said impeller vanes is larger than an outside diameter of said attrition zone.

29. The self-cleaning attrition device of claim 26 including a guard ring around an outer periphery of said stator and means for axially adjusting said guard ring relative to said stator bars to set a depth of space available for said fibers to enter between said stator bars.