CA1306378C - Method for decreasing energy consumption during refining of fiber material while maintaining capacity - Google Patents

Abstract

ABSTRACT

The energy consumption of a cellulosic fibrous material refiner is significantly reduced, while the capacity is maintained by reducing grinding frequency while increasing the retention time and power amplitude (edge bar load). The grinding frequency is maintained between about 200-2,000 Hz, preferably between about 300-900 Hz. The retention time is more than a second, being increased at least about 100 times compared to conventional disk refiners.
The power amplitude is at least doubled. Retention time is increased by greatly increasing the retention volume by removing the majority (at least about 90%) of the steam at approximately the area that it is generated, minimizing the number of cutting elements, and disposing the grinding surfaces of the refiner so that they define a volume of revolution (e.g. a frusto-conical or cylindrical volume).

Description

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MET~OD _OR DECREASING ENERGY CONSU~PTION_D~RING
R~EI_ NG O~ Fl ER MATERIAL W~ILE

BACKGROUND AND SUM~ARY OF THE NVENTION

In the production mechanical paper pulps, it has long been recognized that a higher yield can be obtained from a given amount of raw cellulosic fibrous material compared to chemical pulping processes. Mechanical pulping refers to refiner.
mechanical pulping (RMP); thermomechanical pulping (TMP), chemimechanical pulping (CMP), and chemithermomechanical pulping (CTMP) and other methods of producing high-yield pulps. In mechanical pulping, the chips are broken down into progressively smaller chips or pulp using a refiner or the like.
Typically, a refiner includes relatively movable grinding surfaces defining a grinding zone therebetween wherein chips are reduced to form pulp.
These grinding surfaces, for example oppositely disposed discs or conical surfaces, are relatively rotated by an electric motor. In areas where electrical co~t~ are high, the cost of operating the ' ~, .

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refiner can be prohibltl~Je~ For example, approximatel~ 1,000 ~cW~ per ton o pulp may be u~ed por refininq ~taSIe with approximately 2, 00~ kl~ pe~
ton produced pulp or the conven'cionAl two~ ta~e re i ner .
It has been theorlzed that tha ~ower consumption of a ref.t~er c~n be ~igni~ic~ntly red~ced by reduc1nq the grindinq fre~u~ncy o ~h~ refin~r. It ha~ been sug~ested that thi6 be done by red~lcing the refiner speed. Ho~ever when the refiner ~peed i~ reduced, ao is the capac~ty o the refin~r to pro~uce p~l~, and the ~ed~od pulp production in almo~t ~ery in~tance be con~ldered u~cceptable from ~e commercial ~andpoint.
Accor~iJ~g to the pre~en~ invention, it l~
po~ible to signi~icantly reduce ener~y con~ump~ion o the refiner while not ~lgnific~ntly ~dver~ely af~ectlng re~lner capaci~y Ip~llp production). This is accomplished accordin~ to the pr~ent invention by siqni.fican~ly reducing the grindin~ freq~lency of the refiner, w11ile at the ~ame ti~e ~ignificantly lncren~ing retention ti~e a~d power amplitude.
As used in the pre~ent ~peci ica~ion ~nd claims, "~r~ndinq fre~uet-cy" ~eans the number of relative revolutions per second (rps) of the grinding surfaces (the n~mber of rotor revolutlon~/~ec, where there i~
a ro~or ~nd a ~tator) ~ultiplied by ~he number of ~`OOVe8 ~cutting element~) at ~h~ pulp dischArge en~
of the rotor grindit~ ~u~ce. In conventional com~ercial di~kre~iner~, the rotor i~ rot~ed At between ~bout 1,000-1,800 rpm, with the cuttin~
element6 commonly numbering betwe~l about 400-600, ~o that a grinding frequency of at lea~t abou~ 6,000 ~3~3~

Hz is provided, and the grinding frequency can be 30,000 Hz, or even more. According to the present invention, the grinding frequency is reduced by an order of magnitude or two. Typically, according to the present invention the number of cutting elements and the rpms of the grinding surfaces are provided so that a major portion of the power dissipation of the refiner takes place at a grinding frequency of between about 200-2,000 Hz, preferably between about 300-900 Hz (e.g. between about 300-800 Hz).
Pulp production is maintained according to the present invention, while power consumption is greatly reduced, by significantly increasing both the retention time and the power amplitude. The retention time is the average amount of time fibrous cellulosic material is within the grinding zone, and the power amplitude is the edge bar load.
According to the present invention, the retention time is increased by significantly increasing the retention volume. The retention volume is increased by removing the majority of the steam (e.g. at least about 90% of the steam) in generally the area that it is generated within the grinding zone. The st~am takes up a significant amount of space in the grinding zone, and if removed then the retention volume is increased greakly.
Retention volume is further increased because a minimum number of cutting elements are utilized, and the cutting elements themselves take up volume within the grinding zone. For example, the number of cutting element~ can be limited to between about 12-67 (e.g. 20-60) compared to abouk 400-600 in .

conventional refiners. Still further, the retention volume can be increased by the configuration of the grinding zone. The grinding zone can be configured in the shape of a volume of revolution (a cone or cylinder) by disposing the grinding surfaces so that they are frusto-conical or cylindrical. Retention time according to the invention is at least about a second, and typically is on the order of greater than three seconds, and with 90% steam removal could tvpically be expected to be in the range of five-six seconds. This compares with a retention time of about five milliseconds in conventional disk refiners. Thus the retention time is at least about 100 times greater according to the invention than in conventional disk refiners.
Power amplitude is inherently increased according to the invention when the number of bars are minimized, and the grinding zone is de~ined so that it is "longer", increasing the effective length of the cutting elements. The power amplitude according to the invention could typically be at least double that of a conventional disk refiner, and can be expected to be on the order of about five times greater.
~ he cellulose pulp produced utilizing the methods according to the invention will have different properties than conventional mechanical pulps since the net afect of the new procedures according to the invention ~ill be to cause structural changes to the fibrous material in dif~erent ways than they have typically occurred in conventional disk refiner~.
The de91gn of the reflner accordlng to the '`` '` ' .
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present invention, having a minimum number of cutting elements (bars) results in much more area of the grinding elements being available for steam removal so that the steam can be removed in an effective manner to achieve the desired increase in retention time. According to the invention, also, this additional area for steam removal is effectively utilized, and the steam velocity is controlled, so as to remove the majority (e.g. at least about 90%) of the generated steam while minimizing the amount of fiber withdrawn with the steam.
According to one aspect of the present invention, there is provided a method of refining cellulosic fibrous material utilizing juxtaposed relatively movable grinding surfaces defining a grinding zone between them, with a material inlet to the grinding zone and a material outlet from the grinding zone, comprising the steps of: (a) Grinding the material between the grinding surfaces so that the m~jority (if not all) of power dissipation of the refiner takes place at a grinding frequency of about 200-2,000 Hz (preferably 300-900 Hz, e.g.
300-800). And (b) retaining the material within the grinding zone at a retention time of at least about one second (preferabIy greater than three seconds, e.g. on the order of about five-six seconds).
According to another aspect of the present invention there is provided a method of refining cellulosic fibrous material utilizing juxtaposed grinding surfaces movable relative to each other with an inlet and outlet for the fibrous material, comprising the steps of: Grinding material between the surfaces at a grinding fre~uency of about .~

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~ ~ ~3402-106 200-2,000 Hz; and removing the majority ~e.y. at least about 90%) of the steam generated between ~he material inlet and outlet, approxima~ely at the area of steam generation. Preferahly there is also provided the step of forming the grinding surfaces so khat a grinding zone between the yrinding surfaces defines a volume of revolution (e.g. cone or cylinder) about an axis with the inlet and outlet adjacent opposite ends of the volume of revolution.
The invention also contemplates a method of refining cellulosic fibrous material having jux~aposed grinding surfaces capable of relative rotation, with an inlet and an outlet for the fibrous material, comprising the steps of significantly reducing energy consumption during refining, while maintaining substantially the same capacity, compared to a conventional commercial disk refiner having cutting elements numbering between about 400-600 and a grinding frequency of at least about 6,0Q0.
The method steps are accomplished by significantly reducing refiner fre~uency while significantly increasing (e.y. increasing 100 times or more) the retention time, and significantly increasing (e.g. at least doubling) the power amplitude.
The new cellulose pulp produced according to the present invention is difficult to define in terms of properties, but is cleaxly distinct from conventional mechanical pulps. Cellulose pulp according to the invention is best defined as pulp produced by practising the method steps set forth above.
According to another aspect of the present invention there is provlded a refiner comprising a housing; first and second grinding surfaces defining a grinding zone therebetween; means for effecting relative rotation between said grinding surfaces to -~r:~f.~B
-6a 23402-106 effect grinding of fihrous ma~erial in the grinding zone; and inlet to said grinding zone, and an outlet from said grinding zone; at least one of said grindiny surfaces comprisiny a plurali~y of cutting elements dispose~ therealong and having a leading edge and a trailing edge in the direction of relative rotation thereof; and means for removing steam generated in the grinding zone at approximately the area that it is generated, said steam removal means comprising:
means deflning a plurality of elongated slots in said first grinding surface adjacent the trailing edges of said bars;
and a screen plate having a plurality of openings therein, each significantly smaller than said elongated slot, on the opposite side of said first grinding surface from said cutting bars one in operative association with each of said elongated slots.
According to still another aspect of the present inventlon, a refiner is provided wlth structure for optimum removal of steam generated in the grinding .

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zone, the structure according to the invention taking advantage of the minimum number of cutting bars, and thus the relatively large area on the grinding surfaces between the cutting bars.
It is the primary object of the present invention to provide for a significant reduction in power consumption of a refiner producing mechanical pulp, while maintaining refiner capacity (pulp production). This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended c 1 aims .

BRIEE DESCRIPTI~N.OF T~E DRAWINGS

FIGURE 1 is a schematic side view, partly in cross-section and partly in elevation, of a portion of a refiner utilizable for the practice of the method according to the present invention;
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FIGURE 2 is a cross-sectional view taken along lines 2-2 of FIGURE l;

FIGURE 3 is a view similar to that of FIGURE 1 illustrating a refiner having a cylindrical grinding zone, for the practice of the method of the present invention;

FIGURE 4 is a top view of a portion of the rotating grinding surface of the refiner of FIGURE 1, showing steam removal means associated therewith; and FIGURE 5 i~ a cro~s-sectional view taken 3 ~3~3'7~3 along lines 5-5 of FIGURE 4.

DETAIL~D DESCRIPTION OF THE DRAWINGS

The details of an exemplary refiner for the practice of the method according to the present invention will be seen clearly in said co-pending application serial number 37,005, filed April 10, 1987, to which attention is directed. For ease of illustration the refiner illustrated in the drawings of the subject application is simplified.
FIGURES 1 and 2 illustrate a refiner indicated generally by reference numeral 10, constructed for the practice of a method according to the present invention. The refiner includes an outer housing 12 defining a fibrous materia] inlet 14 at one end of the refiner and an outer housing 16 at the opposite end defining a fibrous material annular outlet 18.
Between axially aligned outer housings 12 and 16, there is provided an axially aligned frusto-conical stator 20 having a plurality of elongated grinding or cutting elements 22 projecting inwardly along its inner frusto-conical surface 24. A frusto-conically shaped rotor 26 is mounte~ in juxtaposition with stator 20 and has a plurality of elongated grinding or cutting elements (or bars) 28 spaced circumferentially thereabout one from the other along the outer frusto-conical surface 30 thereof. The number of elements 2i3 may vary from portion to portion of the surface 30, as may their configuration~. Conse~uently, it will be appreciated that the frusto-conically shaped stator and rotor, 20 and 26, re~pectively, together with their cutting .

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elements 22 and 28, define a grinding zone ~2 constituting a volume of revolution, in this embodiment a frusto-conical volume of revolution, about the elongated axis A-A of refiner lO.
The rotor 26 is suitably mounted in conventional bearings, not shown, and driven, for example, by a conventional electric motor, also not shown. It will also he appreciated that while stator 20 is shown in a fixed position and rotor 26 rotates within stator 20, an opposite configuration may be provided with the rotatable rotor external to the fixed stator.
Alternativèly, both the grinding surfaces may be rotated, e.g. in opposite directions.
Disposed within the inner surface of rotor 26 and spaced therefrom is a frusto-conical housing 34 fixed and forming a part of the outer housing 16. A
plurality of openings 36 are disposed through rotor 26 at longitudinaIly and circumferentially spaced positions thereabout for purposes of venting steam from the grinding zone 32 between stator 20 and rotor 26. A steam vent passage 37 is disposed between the fixed housing 34 and rotatable rotor 26 for venting the steam from the grinding zone 32 through a steam outlet 38.
In using the refiner illustrated in Figures 1 and 2 hereof, fibrous material (e.g. wood chips) is ed into inlet 14 and into the grinding zone 32, by conventional feeding means (not ~hown). The grinding action in the grinding zone 32 reduces the chips, and the fibrous material flows out the refiner through outlet 18. In contra t to conventional disc-type grinders for this purpose Which are normally rotated at between 1,000-1,800 rpm With cuttlng elements commonly numbering 400 through 600, giving a grinding frequency of 6,000-20,000 Hz, or more, the present invention reduces energy consumption by decreasing the grinding frequency; while maintaining pulp production ~e.g. maintaining the rotary speed at between about 1,000-1,800 rpm). To accomplish this, the number of cutting elements is substantially reduced (minimized), e.g. to approximately 1~-67 (e.g. 20-60). Thu~, the refiner is operated so that the vast majority, if not all, of the power dissipation of the refiner takes place at low grinding frequency, e.g. about 200-2,000 Hz (preferably about 300-800/900 E~z). With this reduction in grinding frequency, the energy consumption may be reduced significantly. A 50%
reduction is conceivable.
It will be further appreciated that the volume and retention time of the fibrous material in the grinding zone is increased using a frusto-conical grinding zone as compared, for example, with a disc-type grinder of the prior art. Consequently, the length of the grinding zone can be increased while maintaining the diameter of the refiner within reasonable limits and on the order o the diameter of disc-type refining units.
Furthermore, the grinding action is considerably enhanced (and the retention volume signiflcantly effectively increased) by the removal or discharge of the steam generated by the grinding action or as a result of any liquid added to the grinding zone which turns into steam. The wide ~pacing resulting from the minimum number of cutting elements (bars) 28 allow~ area for effoctive removal of the majority ' , - . . , . ~ , i3 ~'~

(e.g. at least about 90%) of the st~am where generated. Thus the steam is discharged from the grinding zone substantially continuously along its length through the openings 36 for removal from the refiner by passage 37 and outlet 38. Removal of the steam decreases the compressibility of the chip-fiber-water-steam mixture.
The steam ~uantity is also regulated by keeping a suitable pressure difference between the steam outlet and the fiber material outlet.
; In the embodiment of the invention illustrated in Figure 3, like elements are referred to by like numerals, succeeded by the letter notation a. In this form, the stator housing 20a and rotor 26a are cylindrical in shape, defining a grinding zone 32a therebetween, constituting a volume of revolution abo~t the axis A-A. As in the prior embodiment, cutting elements 22a and 28a are formed on the stator and rotor, respectively, for the full length of the grinding zone 32a. The rotor has steam outlet openings 36a spaced axially and circumferentially throughout its surface for directing steam from the grinding zone 32a into the passage 37a or discharge through outlet 38a.
The capacity of the refiner compared to conventional refiners i~ maintained, while power consumption is significantly reduced, by significantly increa~ing retention time and power amplitude (edge bar load). The retention time is increased by removal of steam, removal of metal (the - number of bars are minimized, which bars take up volume), and providing a larger area by increasing the length of the grinding zone by providing it as a ;

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volume of revolution (e.g. cone). Typically the retention time would be measured in seconds, as opposed to milliseconds in conventional disk refiners, the retentioh time according to the invention being at least about one second, preferably greater than three seconds, and can be expected to be in the range of about five-six seconds. The power amplitude is inherently increased by minimizing the number of bars and by lengthening the bars (by providing a conical grinding zone, for example~. The power amplitude would typically be at least doubled compared to conventional disk refiners, and could be expected to be on the order of about five times greater.
An effective manner of steam removal to achieve the.objectives according to the invention may be seen from an inspection of FIGURES 4 and 5. In this embodiment, a rotor 226 has a plurality of cutting bars 228 having leading and trailing edges 50, 51, respectively in the direction of rotation 52.
Adjacent the trailing edges 51 of the bars 228 are provided elongated slots 230. The slots typically would be more than an inch long and have substantial width. Disposed in operative association with.each slot 230, on the opposite side of the rotor 226 from the bars 228, is a screen plate 55 which may be attached by welds 56 or the like to the rotor 226, each screen plate 55 having a plurality of holes 58 formed therein, the holea 58 each being of significantly less dimension than the slot 230. Such a construction takes advantage of the large available area between bars 228 on the rotor 226, and the structure illustrated in FIGURES ~ and 5 control3 the .~

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steam velocity. The velocity of the steam flowing through the slots 230 is relatively small, meaning that a relatively small amount of fiber will be entrained therewith~ However because of the large size of the slots 230, the volume of stea~ flowing will be great. The holes 58 are small so that any fiber entrained in the steam will have a tendency to be prevented from flowing there-through, and the steam velocity will increase flowing through the holes 58 so that they will have a tendency to be kept unclogged, and then the velocity will again be reduced after passing through the openings 58 into a large interior volume of the rotor tas in steam vent passage 37). Preferably a two stage separation of fibers and steam takes place as in U.S. Patent No. 4,754,935 issued July 5, 1988.
Theoretical calculations verify the ability to achieve the desired results according to the invention. The following calcuIations are based on a comparison of a lOOOmm diameter disc refiner with a low frequency refiner having a maximum diameter of lOOOmm with a minimum diameter of 400mm and a total rotor length of 850mm. The comparison is as follows:

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Standard Disc Low Frequency Refiner _ Re iner Wood Temp. of feed C 20 20 Water content of feed t/t 2-3 2-3 Energy input kWh/bdt (assumed) 1000 500 S~eam release m /bdt 600-47S 155-30 Active volume in refiner (200 tbdp/d)liter 6 38 Average retention time (no steam separated) milliseconds 4.4-5.5 106-53 Average retention time (all steam not feasible separated) Seconds 6-7 Relative reten-tion time Low Frequency/
Standard Disc 1 1600-1100 Power amplitude can also be calculated to compare the power amplitude of a standard commercial disk refiner and one according to the invention.
Applying the "edge bar" theory, and assuming 400 bars each 333 millimeters long for a conventional disk refiner, and 12 bars 425 millimeters long and another 12 twice that long, in a refiner according to the invention, the relative power amplitude~ w$11 be as ~ollows:

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8000 kW (assumed) B = = 0.002 kWs/mm;
25 rps ~ 400 ~ 333 mm 4000 kW (assumed) B= _ = 0.010 kWs/mm.
25 rps * ~12+24) * 425 mm Thus it will be seen that according to the invention the retention time is increased compared to a conventional disk refiner even if there is no steam separation, and is on the order of seconds as compared to mllliseconds if there is complete steam separation ~with 90% steam separation a retention time of about five-six seconds could be expected in the practice of the invention), and the relative retention time is more than 1,000 times greater according to the invention than in a conventional disk refiner. Further the power amplitude is on the order of five times greater.
It will thus be seen that according to the present invention, there is provided a method for reducing the energy consumption of a refiner or defibrator of pulp material, while maintaining refiner production. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modiications may be made thereo within the scope of the invention, which scope is to be accorded the broadest interpretation of the appended claims so as to encompass all eguivalent pulps, and structures.

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Claims (32)

1. A method of refining cellulosic fibrous material utilizing juxtaposed relatively movable grinding surfaces defining a grinding zone between them, with a material inlet to the grinding zone and a material outlet from the grinding zone, comprising the steps of:
(a) grinding the material between the grinding surfaces so that the majority of power dissipation of the refiner takes place at a grinding frequency of about 200-2,000 Hz, and (b) retaining the material within the grinding zone a retention time of at least about one second.

2. A method as recited in claim 1 wherein step (a) is practiced so that the grinding frequency is between about 300-900 Hz.

3. A method as recited in claim 2 wherein step (a) is practiced so that the grinding frequency is between about 300-800 Hz.

4. A method as recited in claim 2 wherein step (b) is practiced so that the retention time is greater than three seconds.

5. A method as recited in claim 1 wherein step (b) is practiced by removing the majority of the steam generated in the refiner, between the material inlet and outlet, at about the area of steam generation.

6. A method as recited in claim 5 wherein step (b) is practiced by removing at least about 90 percent of the steam generated in the refiner, between the material inlet and outlet, at about the area of steam generation.

7. A method as recited in claim 5 wherein step (b) is practiced by disposing the grinding surfaces so that the grinding zone therebetween defines a volume of revolution about an axis with the material inlet and outlet adjacent opposite ends of the volume of revolution.

8. A method as recited in claim 7 wherein step (b) is further practiced by defining a frusto-conical volume of revolution.

9. A method as recited in claim 8 wherein step (b) is further practiced by minimizing the number of cutting elements on the grinding surfaces.

10. A method as recited in claim 9 wherein step (b) is further practiced by providing about 20-60 cutting elements on the grinding surfaces.

11. A method as recited in claim l wherein one of the grinding surfaces is a stator and the other is a rotor, and wherein about 12-67 cutting elements are provided on the rotor.

12. A method as recited in claim 9 wherein step (a) is practiced so that the grinding frequency is between about 300-900 Hz.

13. A method as recited in claim 1 wherein one grinding surface is rotated relative to the other at about 1,000-1,800 rpm.

14. A method as recited in claim 2 wherein one grinding surface is rotated relative to the other at about 1,000-1,800 rpm.

15. A method as recited in claim 3 wherein one grinding surface is rotated relative to the other at about 1,000-1,500 rpm.

16. A method as recited in claim 2 wherein step (b) is practiced by removing the majority of the steam generated in the refiner, between the material inlet and outlet, at about the area of steam generation.

17. A method as recited in claim 1 wherein steps (a) and (b) are practiced by minimizing the number of cutting elements provided on the grinding surfaces, and rotating the grinding surfaces with respect to each other at about 1,000-1,800 rpm so that compared to a commercial disk refiner having cutting elements numbering between about 400-600 and a grinding frequency of about 6,000 Hz or greater, energy consumption is significantly reduced while the capacity of the refiner is maintained substantially the same.

18. A method as recited in claim 17 wherein the reduction in energy consumption is on the order of about 50%.

19 19. A method of refining cellulosic fibrous material utilizing juxtaposed grinding surfaces defining a grinding zone therebetween, and movable relative to each other with an inlet and outlet for the fibrous material, comprising the steps of:
(a) grinding material between the surfaces at a grinding frequency of about 200-2,000 Hz; and (b) removing the majority of the steam generated between the material inlets and outlets, approximately at the area of steam generation.

20. A method as recited in claim 19 wherein step (b) is practiced to remove at least about 90 percent of the generated steam approximately at the area of steam generation.

21. A method as recited in claim 19 comprising the further step of disposing the grinding surfaces so that the grinding zone between the grinding surfaces defines a volume of revolution about an axis with the inlet and outlet adjacent opposite ends of the volume of revolution.

22. A method as recited in claim 21 wherein said step of disposing the grinding surfaces is practiced so that the grinding surfaces define a frusto-conical volume of revolution.

23. A method as recited in claim 21 wherein said step of disposing the grinding surfaces is practiced so that the grinding surfaces define a cylindrical volume of revolution.

24. A method as recited in claim 20 wherein the grinding frequency is between about 300-900 Hz.

25. A method of refining cellulosic fibrous material utilizing juxtaposed grinding surfaces movable relative to each other with an inlet and outlet for the fibrous material, comprising the steps of:
grinding material between the surfaces at a grinding frequency of about 200-2,000 Hz; and disposing the grinding surfaces so that a grinding zone between the grinding surfaces defines a volume of revolution about an axis with the inlet and outlet adjacent opposite ends of the volume of revolution.

26. A method as recited in claim 25 wherein said step of disposing the grinding surfaces is practiced so that the grinding surfaces define a frusto-conical volume of revolution.

27. A method as recited in claim 26 wherein the grinding frequency is between about 300-900 Hz.

28. A method of refining cellulosic fibrous material having juxtaposed grinding surfaces defining a grinding zone therebetween, and capable of relative rotation, with an inlet and an outlet for the fibrous material, comprising the steps of grinding material between the surfaces at a grinding frequency of about 200-2,000 Hz significantly reducing energy consumption during refining, while maintaining substantially the same capacity, compared to a conven-tional commercial disk refiner having cutting elements numbering between about 400-600 and a grinding frequency of at least about 6,000.

29. A method as recited in claim 28 wherein said step of reducing energy consumption is practiced to reduce energy consumption by about 50%.

30. A method as recited in claim 28 wherein said steps are practiced by significantly reducing grinding frequency, while significantly increasing retention time of material in the grinding zone, and significantly increasing power amplitude.

31. A method as recited in claim 30 wherein the retention time is increased by about 100 times or more, and wherein the power amplitude is at least doubled.

32. A refiner for refining according to the method of claim 1 comprising, a housing; first and second grinding surfaces de-fining a grinding zone therebetween; means for effecting relative rotation between said grinding surfaces to effect grinding of fibrous material in the grinding zone; an inlet to said grinding zone, and an outlet from said grinding zone; at least one of said grinding surfaces comprising a plurality of cutting elements disposed therealong and having a leading edge and a trailing edge in the direction of relative rotation thereof; and means for removing steam generated in the grinding zone at approximately the area that it is generated, said steam removal means comprising:
means defining a plurality of elongated slots in said first grinding surface adjacent the trailing edges of said bars; and a screen plate having a plurality of openings therein, each significantly smaller than said elongated slot, on the opposite side of said first grinding surface from said cutting bars on in operative association with each of said elongated slots.