US3843063A

US3843063A - Shredding and defiberizing machine

Info

Publication number: US3843063A
Application number: US00330081A
Authority: US
Inventors: R Honeyman
Original assignee: Morden Machine Co
Current assignee: Voith Morden Inc
Priority date: 1973-02-06
Filing date: 1973-02-06
Publication date: 1974-10-22
Anticipated expiration: 1991-10-22
Also published as: FR2216391B1; FR2216391A1; DE2405702A1; JPS49116303A; BR7400638D0; GB1449067A; CA1000983A; JPS548762B2; DE2405702C3; SE405992B; DE2405702B2

Abstract

A machine for shredding and defiberizing papermaking material and extracting refined pulp therefrom, the machine including a substantially dome-shaped rotor having outwardly extending blades moving over a cooperating stationary bedplate and method of construction thereof, the blades being organized into groups having varying heights so as to achieve optimum circulation or horsepower utilization or both as a function of consistency.

Description

United States Patent [191.

Honeyman 1 Oct. 22, 1974 1 SHREDDlNG AND DEFIBERIZING MACHllNE [75] Inventor: Robert Blakeley Honeyman, Carmel,

Calif.

[73] Assignee: Morden Machines Company,

Portland, Oreg. [22] Filed: Feb. 6, 1973 [21] Appl. No.: 330,081

3,339,851 9/1967 Felton et a1 241/4617 3,486,702 12/ l 969 Kmeco 3,713,594 l/1973 Biakley ct al. 241/4617 Primary ExaminerGranville Y. Custer, Jr.

Attorney, Agent, or Firm-Dawson, Tilton, Fallon &

Lungmus [5 4 ABSTRACT A machine for shredding and defiberizing papermaking material and extracting refined pulp therefrom, the

machine including a substantially dome-shaped rotor. having outwardly extending blades moving over a cooperating stationary bedplate and method of construction thereof, the blades being organized into groups having varying heights so as to achieve optimum circulation or horsepower utilization or both as a function of consistency.

9 Claims, 12 Drawing Figures 520m wmmo: 33mm 0% com 0mm 08 m? w {I WdU PAIENTEBBM 22 m4 Omw wwm

was

I SHREDDING AND DEFIBERIZING MACHINE BACKGROUND AND SUMMARY OF THE INVENTION For example, a paper manufacturer might operate the pulper for a continuous machine with a pulp con sisting of 3-4 percent. However, on other occassions (as for batch operations or for other considerations) the consistency may be 6 percent or higher. Also, the capacity (cubic contents of the tank) often had determined the design, particularly of the rotor so that for different sized installations, the easy way had been to extrapolate as contrasted to tailoring the machine for the intended usage. Still further, even when optimal values of power utilization for a given consistency and capacity are approached, there is lacking the cireulation necessary to achieve efficient pulping.

The inefficiencies (and inequities when viewed from the standpoint of an ecology-ninded public) are avoided through the practice of the invention where a procedure and structure are provided utilizing varying height blades according to power, consistency, capacity and circulation requirements.

Further, a specific preferred form of machine is provided wherein the rotor assembly, which moves over the face of the stationary cooperating bedplate, has a plurality of blades extending the same radial distance from the rotor hub. The bottom faces of the blades have the same shape and surface area, decreasing in width from the hub to the blade tip. The top surfaces of the blades are flat and are of identical width, but, although they leave the hub at approximately the same radial distance from the axis of rotation of the rotor, they do not all have the same slope with respect to the bedplate and consequently the blades vary in height being arranged in more or less symmetrical order around the hub, the height and number of the corresponding blades depending on the general nature of the material being treated and the operating requirements. I have found it advantageous to provide from about 2 to about 4 times as many lower height blades as compared with greater height blades. The leading face of each blade is substantially perpendicular to the plane of the bedplate and extends obliquely away from the direction of rotation of the rotor. The trailing face of each blade has a concave downward slope from the rear edge of the top face. The bottom face of the blades are identically grooved to form bars parallel to the leading face of the blade and the grooves and bars extend in under the periphery of the hub preventing the possibility of any material on the bedplate escaping treatment by getting in under the hub. This feature also will prevent extraneous trash material from collecting inwardly beneath the rotor hub. The stationary bedplate for batch operation usually will have radiallyextending bars. For continuous operation the bedplate will have suitable perforations, or radial bars with perforations in the grooves, all of which are well-known.

In a modification of the machine for batch operations the bedplate is perforated and extends over an annular recirculation chamber which has inwardly-located discharge ports leading upwardly to the underside of the rotor hub with the result that a portion of the semidefiberized material which is forced down through the perforations in the bedplatewill be drawn up into the low pressure zone beneath the hub and subjected to further defiberizing action without again traveling the complete path of the material being circulated throughout the tank.

DESCRIPTION OF THE DRAWINGS FIGS. 3, 4 and 5 are an end elevation and crossseetional elevations of one of the higher blades on the rotor, taken on lines 3-3, 4-4 and 55, respectively of FIG. 1 and drawn to a considerably enlarged scale;

FIG-6 is a fragmentary bottom view of the rotor taken on the line indicated at 6-6 in FIG. 2, but drawn to the same scale as FIG. 1;

FIG. 7 is a fragmentary top plan view of a modified form of rotor, drawn to the same scale as FIG. 1, but showing a portion of a typical cooperating stationary bedplate employing small perforations;

FIG. 8 is sectional elevation of the rotor with the bedplate of FIG. 7 taken along the axis of the rotor and thus along the line indicated at 8-8 in'FIG. 7, drawn to the same scale as FIG. 2, and showing also the discharging chamber beneath the perforated bedplate when the machine is mounted in the tank for continuous operation as opposed to batch operation;

FIG. 9 is a sectional elevation of the rotor and bedplate showing the machine installed in the tank for a modified operation embodying recirculation of portions of the semi-treated material for batch operations;

FIG. 10 is a top plan view of the bedplate used in the modified operation illustrated in FIG. 9 and taken on line 10-10 of FIG. 9;

FIG. 11 is a chart of performance characteristics (speed versus power) of different kinds of rotors; and

FIG. 12 is a chart similar to FIG. 11 but relating speed to circulation.

Inasmuch as the invention in its broader aspects can be better appreciated by relating it to specific mechanical embodiment, reference is first made to FIGS. 1-6. The machine shown there includes a rotor, indicated in general by R, which operates in conjunction with a cooperating, stationary bedplate. The rotor has a hollow, round, substantially dome-shaped hub 10 which is sewhich rests on the peripheral portion of the mounting member flange 11 and is firmly secured thereto by a series of recessed screws 14. The hub may be adjusted with respect to the mounting member when desired by the interposition of shims 15 between the hub 10 and the flange 11' of the mounting member for a reason later apparent.

The hub 10 of the rotor R is formed with plurality of integral blades 16 starting on the hub at approximately the same radial distance from the axis of rotor rotation and extending the same distance beyond the periphery of the rotor hub. The bottom faces 16 of these blades all extend in the same plane perpendicular to the axis of rotation and in a plane parallel to the plane of the stationary bedplate later mentioned. While the blades 16 are of various heights, as presently explained, the bottom faces 16' are all identical in size and shape (see FIG. 6) and decrease in width from the hub to their outer tips to form a web behind the leading face of each blade. It will be appreciated however, that the bottom faces 16' and the defibering surface of the bedplate may be a flat surface or a surface of revolution as the frustro-conical surface described in US. Pat. No. 2,858,990. However, it has been found more advantageous to provide the flat surface herein defined.

As shown by FIGS. 1 and 6 in which the arrows X indicate the direction of rotation of the rotor, the leading face 17 of each blade extends obliquely away from the direction of rotation of the rotor. Also, the leading face of each blade is substantially perpendicular to the common plane of the bedplate. The bottom faces 16' of the rotor blades extend in under the periphery of the rotor hub designated 18 in FIGS. 2 and 6. It is understood that the rotor can be manufactured for rotation in the opposite direction of that shown by reversing the design of rotor blades.

The bottom face 16' of each of the blades is formed preferably with equally spaced bars 19 (FIG. 6) of identical width, extending parallel to the leading face of the blade. In theillustration given, the bars extend from the outer end of the blade inwardly under the hub. As illustrated, these may have a common inner termination. The bottom face of each blade has four such bars, the bars being of varying length. The inner end of the last and shortest bar on the bottom face of each blade terminates close to the inner end of the first and longest bar on the bottom of the next succeeding blade. The stationary bedplate 20, in the machine of FIGS. 1 and 2, extends in a plane perpendicular to the axis of rotation of the rotor and has central opening to accommodate the upper portion of the rotor shaft 12 with a suitable sealing ring around the shaft. This bedplate is secured by screws to a base plate 21 of similar size, which in turn is secured in a corresponding recess in the wall of the tank 23 in which the treatment of the pulp material takes place. The peripheral portion of the bedplate 20, starting from inside the hub periphery, is grooved so as to form a series of bars 22 extending radially out to the periphery of the bedplate and thus cooperating with the bars on the bottom faces of the rotor blades to produce the desired defiberizing action on the material passing between the opposed sets of bars. The amount of clearance between the opposed sets of bars can be adjusted as desired, by means of the interposed shims 15 (FIG. 2) between the rotor hub 10 and the flange 11' of the rotor mounting member 11 previously mentioned. It is appreciated that bedplate bars may assume other patterns than radial.

The fact that the bars 19 on the bottom faces 16 of the blades 16 extend in under the periphery of the rotor hub is an advantageous feature since this renders less likely the collecting and building up of damaging extraneous material, between the rotor and the bedplate, in the area within the rotor hub periphery. The bottom face of the bottom flange 11' of the mounting member for the rotor hub also is preferably formed with obliquely-extending bars 11a (FIG. 6) to engage and throw out any material which might possibly enter into this space.

Although the bottom faces of the rotor blades are exactly the same size and shape, the blades are not all the same height. As in US. Pat. No. 2,858,990, the higher blades act to facilitate and hasten the breaking up of large pieces or slabs of material encountered by the retor. However, too large a number of higher blades would negate the destructive tearing effect of the proportionately fewer blades, on large masses of material and, also, result in uneconomical power consumption. Further, too many blades in total will tend to obstruct the flow of partially treated material into the defiberingzone at the surface of the bedplate and also can restrict materially the pumping or circulating capacity of the rotor. In the particular example illustrated, the rotor is shown with a total of twelve blades which are of three different heights, as indicated somewhat diagrammatically in FIG. 2, the highest blades being indicated by the reference a, intermediate blades by the reference b, and the low blades by the reference c. When the rotor is made with twelve blades, it is possible to provide four blades of the three heights a, b, and c symmetrically arranged, or, depending upon the nature of the material being treated, it is possible, as another example, to have three of the high blades 0, with each followed by two of the intermediate blades b, and one low blade c symmetrically arranged. The selection of the proper arrangement and height of the blades with respect to the particular nature of the material being treated and the proper rotor speed make possible the most efficient and economical power consumption.

As previously mentioned, the leading face of each rotor blade is preferably flat and substantially perpendicular to the plane of the bedplate, extending out obliquely in a direction away from the direction of rotation. The top face of each blade is flat and narrow with parallel edges, the slope of the top face with respect to the plane of the bedplate depending upon the height of the blade at its tip. The trailing face of each blade slopes downwardly toward the succeeding blade, which downward slope becomes more and more concave inwardly along the blade, and this concave trailing face aids in feeding the material down to the bedplate and to the leading face of the succeeding blade. FIGS. 3, 4 and 5 indicate an end elevation, an intermediate crosssection, and a cross-section taken further inwardly respectively of one of the high blades of the rotor.

In FIGS. 7 and 8 of the rotor R is shown in combination with a bedplate 24 which, instead of having radial bars to cooperate with the bars on the bottom faces of the rotor blades, is perforated in the annular area over which the rotor blades pass, and extending beyond this area when additional capacity is required. An annular discharging passageway 25 (FIG. 8), provided in the tank housing beneath the annular perforated area of the bedplate, leads down into an annular chamber 26 which preferably has a sloping wall leading to a controlled outlet pipe 27. The use of perforated bedplates for continuous operation in machines for treating paper stock is in itself well-known and need not be further described. However, the rotor R of the present invention with the novel blades, when used in combination with such a perforated bedplate, also makes possible more efficient and economical performance in cases Where continuous operation is desired as opposed to batch operations.

FIGS. 9 and 10 illustrate a modification of the machine for batch operations in which semi-defiberized material receives additional defiberizing action without having again to travel the entire circulation pattern of the material in the tank surrounding the rotor. In FIGS. 9 and 10 of the bedplate 28 is formed with perforations, or with radial bars 29 with perforations 30 in the grooves between the bars, (in the annular path over which the rotor blades pass). An outer annular chamber 31 beneath the annular perforated portion of the bedplate is connected with an inner annular chamber 32. The bedplate 28 is also provided with relatively large openings 33 above the inner chamber 32, which openings lead upward to the under side of the mounting member flange 11' of the rotor. The material being treated is forced downwardly through the perforated bedplate by the pressure waves preceding the advancing rotor blades and into chamber 31 thence flowing into chamber 32 and up into the low pressure zone beneath the rotor through ports 33. There it re-enters the defibering zone and is carried outwardly again by centrifugal action of the rotor blades, being subject in this way to additional defiberizing action beneath the rotor blades.

Reference is now made to FIG. 11. As indicated, this is a graph or chart of performance characteristics of certain rotors. More specifically, the numeral 363 designates the plot relating speed in rpm as a function of brake horsepower for a rotor having 12 blades of differ ent sizes, provided in three groups. This is illustrated in part in FIG. 7 where it will be noted that there are three high blades 0, six intermediate height blades b and three low blades c. In FIG. 12, the performance characteristic of the surface circulation rate in fpm is related to the speed of the rotor and the plot relating these for the embodiment of the invention seen in FIG. 7 is also designaged 363.

In FIGS. 11 and 12, there is seen two other plots relating performance characteristics. These plots are designated 306 and 309 and relate, respectively, to a 9 blade rotor and a 12 blade rotor, the 9 blade rotor, for example having three high blades a and six low blades 0, arranged with two low blades following each high blade. The 12 blade rotor has 3 low blades following each high blade. In some instances the 9 blade rotor may be developed from the same pattern as the 12 blade rotor but eliminating one of the low blades after each high blade. In such a case the blade spacing is not equal. However the rotor is still balanced.

Still further, in FIGS. 11 and 12, the plots 114 relate to the performance characteristics of the commercial embodiment of US. Pat. No. 2,858,990, the series FGVl 14 machine of the assignee Modern Machines Company of Portland, Oregon of the aforesaid patent.

In the Series FGV-l 14 machine, there are three high blades a and twenty-one low blades 0 arranged with seven low blades c following each high blade a. The data for the plots of FIGS. 11 and 12 were obtained with the other variables, viz., size or capacity and stock consistency being the same.

Referring now to FIG. 12, it will be noted that to achieve a circulation rate of fpm in the Series FGV- I 14 machine (see point 34 on plot 114), it is necessary to employ in excess of 250 brake horsepower that corresponding to the point 35 on plot 114 in FIG. 11. On the other hand, to achieve the same circulation rate utilizing the blade arrangement of FIG. 7, see point 36 in FIG. 12, it is necessary to use only about I25 brake horsepower as determined by the abscissa corresponding to the point 37 in FIG. 11. In like fashion, the power requirementsare substantially less for the rotor configurations responsible for the

performance characteristics

306 and 309 when compared with the Series FGV-l 14 machine rotor responsible for the plots 114-.

Alternatively, it will be appreciated that the invention provides an advantageous method by which circulation rates can be increased without increasing power.

The plot in FIGS. 11 and 12 are not single line curves but rather are ranges or bands based upon a number of tests. The speed and power figures are correlated to a 27 inch diameter rotor as extrapolated from laboratory models this being considered a proven approach in this field. The tests were correlated to a given size commercial pulping unit installed in a maximum size tank. All tests treated stock of identical specifications at 6 percent A.D. consistency. Data was obtained using a dynomatic variable speed drive with power corrected by formula to actual brake horsepower output and correlated to commercial experience. The data is typical of an extensive program numbering over tests. In the maximum size tank (in proportion to rotor diameter) as used in the series of tests referred to, it was determined that the allowable minimum surface velocity of the stock at a given location in the tank would be 50 fpm to provide adequate mixing and submergence and therefore, is used as a point of comparison.

The plots were bracketed to show the range or band that the respective rotors fall into. The circulation curves will showa greater range, primarily due to air entrainment at higher speeds, and also to minor uncontrollable variables in stock viscosity. In the comparisons shown, the averages or mid-points of the range of curves was selected.

The 306 rotor is designed for medium or large tanks at low consistency, or small tanks at high consistency. The 309 rotor is for general purpose, or intermediate applications. The 363 rotor is for high consistency, and, or large tank applications. Atequal or better defibering efficiencies, as rated by horsepower per day, per ton of stock, the new series rotors show clear advantages over the FGV-1l4 series.

Through the employment of a number of high blades in combination with low and/or intermediate height blades, I am able to obtain improved circulation rates without the requirement of additional power or substantial savings in power without sacrifice or optimum rates of circulation. For this purpose, it is advantageous to provide the sum of the intermediate and lowheight blades to be about twice or more the number of the high blades a. Optimally, the ratio of interediate and low height blades to the high blades is between 2 and I the b blades 2 /2 inches and the blades 1 /z inches.

The heights, size and configurations of the rotor blades, in combination with any of the bedplates mentioned, provide several advantages. They enable the material or slurry in the tank to be circulated at an improved level of efficiency relative to power usage. The high blades perform the necessary, primary breakdown of the larger pieces or slabs of material enhanced by the leading faces of all the blades providing an abrupt oblique angle of attack (relative to rotation) on the material encountered. The hazard of having any tramp metal or other foreign material reaching the defibering zone is practically eliminated when incorporating rotor blades that are substantially perpendicular to a fiat bedplate. By substantially perpendicular, I include faces that slope slightly rearwardly but not significantly forwardly; The trailing face of each blade acts to feed the material into the space ahead of the succeeding blade, but the blade contours are also so designed as to prevent any blinding or plugging of the rotor by the material encountered, and the bottom faces of the blades provide proportionately large, working or defiberizing surfaces acting in conjunction with any of the stationary bedplates illustrated to accomplish the desired defiberizing results efficiently and with an economical consumption of power.

I claim:

1. A machine for shredding and defiberizingpapermaking material including a rotor, means for rotating said rotor about an axis, a stationary bedplate formed into a surface of revolution about said axis of rotation and defining a plurality of shear edges providing a generally planar defibering zone, a substantially domeshaped hub on said rotor, a plurality of blades extending from said hub, each blade having a predetermined height and including a substantially flat forward work surface extending perpendicular to said bedplate and forming an oblique angle with a radius of said rotor away from the directionof rotation, the trailing surface of each blade further defining a concave downward slope along a portion thereof from the rear edge of top face for drawing material downinto an intermediate location on the working face of a succeeding blade, the bottom faces of said blades extending in a surface parallel to and spaced slightly from said bedplate, and defining a plurality of bars each providing a straight shearing edge, said blades including a first set of blades of a first predetermined height spaced symmetrically about said axis of rotation and a second set of blades of a second predetermined height less than said first height and spaced symmetrically about said axis, the ratio of the number of blades in said second set to the number of blades in said first set being in the range of 2-4 to 1.

.2. The structure of claim 1 wherein each blade includes a trailing web portion for carrying said bars, the concavity of said trailing surface of said blades increasing in a direction inwardly along each blade, the curvature of the trailing surface of each blade extending out along its associated web portion.

3. The structure of claim 2 wherein said bars beneath said blades are parallel to said work surface and extend inwardly beneath the periphery of said hub to force material from beneath said hub outwardly under centrifugal force.

4. The structure of claim 3 wherein said second set of blades includes blades of at least two different heights other than the height of said first set of blades.

5. A machine for shredding and defiberizing papermaking material including a rotor, means for rotating said rotor about an axis, a stationary bedplate formed into surface of revolution about said axis of rotation and defining a plurality of shear edges providing a generally planar defigering zone, a substantially domeshaped hub on said rotor, a plurality of blades extending from said hub, each bladehaving a predetermined height and including a substantially flat forward work surface extending perpendicular to said bedplate and forming an oblique angle with a radius of said rotor away from the direction of rotation, the trailing surface of each blade further defining a concave downward slope along a portion thereof from the rear edge of the top for drawing material down into an intermediate location on the working face of a succeeding blade, the bottom faces of said blades extending in a surface parallel to and spaced slightly from said bedplate and defining a plurality of bars each providing a straight shearing edge, said blades including a first set of blades of a first predetermined height spaced symmetrically about said axis of rotation, a second set of blades of a second predetermined height less than said first height and spaced symmetrically about said axis, one on either side of an associated blade of said first set, and a third set of blades of a third predetermined height spaced symmetrically about said axis of rotation and equidistant between said blades of said first set, the number of blades in said first and third sets being equal, and the number of blades in said second set being twice the number of blades in said first set.

6. The structure of claim 5 wherein the relationship of the height of each of said blades of said first set, said second set and said third set is approximately respectively 3.5 to 2.5 to 1.5.

7. The structure of claim 5 wherein each blade includes a trailing web portion for carrying said bars, the concavity of said trailing surfaces of said blades increasing in a direction inwardly along each blade and extending out along an associated web portion.

8. The structure of claim 7 wherein the bars beneath said blades are parallel to said work surface and extend inwardly under the periphery of said hub to force material from beneath said hub outwardly under centrifugal force.

9. A machine for shredding and defiberizing papermaking material including a rotor, means for rotating said rotor about an axis, a stationary bedplate extending in a plane generally perpendicular to said axis of rotation, a substantially dome-shaped hub on said rotor, a first plurality of equally spaced blades having their top edges starting from said hub at approximately raannular chamber inwardly of a the defibering zone, whereby material to be treated passing through said bedplate from said defibering zone into said first annular chamber is forced into said second annular chamber and thence upwardly through said enlarged openings to be recirculated through said defibering zone.

Claims

1. A machine for shredding and defiberizing paper-making material including a rotor, means for rotating said rotor about an axis, a stationary bedplate formed into a surface of revolution about said axis of rotation and defining a plurality of shear edges providing a generally planar defibering zone, a substantiAlly dome-shaped hub on said rotor, a plurality of blades extending from said hub, each blade having a predetermined height and including a substantially flat forward work surface extending perpendicular to said bedplate and forming an oblique angle with a radius of said rotor away from the direction of rotation, the trailing surface of each blade further defining a concave downward slope along a portion thereof from the rear edge of top face for drawing material down into an intermediate location on the working face of a succeeding blade, the bottom faces of said blades extending in a surface parallel to and spaced slightly from said bedplate, and defining a plurality of bars each providing a straight shearing edge, said blades including a first set of blades of a first predetermined height spaced symmetrically about said axis of rotation and a second set of blades of a second predetermined height less than said first height and spaced symmetrically about said axis, the ratio of the number of blades in said second set to the number of blades in said first set being in the range of 2-4 to 1.

2. The structure of claim 1 wherein each blade includes a trailing web portion for carrying said bars, the concavity of said trailing surface of said blades increasing in a direction inwardly along each blade, the curvature of the trailing surface of each blade extending out along its associated web portion.

5. A machine for shredding and defiberizing papermaking material including a rotor, means for rotating said rotor about an axis, a stationary bedplate formed into surface of revolution about said axis of rotation and defining a plurality of shear edges providing a generally planar defigering zone, a substantially dome-shaped hub on said rotor, a plurality of blades extending from said hub, each blade having a predetermined height and including a substantially flat forward work surface extending perpendicular to said bedplate and forming an oblique angle with a radius of said rotor away from the direction of rotation, the trailing surface of each blade further defining a concave downward slope along a portion thereof from the rear edge of the top for drawing material down into an intermediate location on the working face of a succeeding blade, the bottom faces of said blades extending in a surface parallel to and spaced slightly from said bedplate and defining a plurality of bars each providing a straight shearing edge, said blades including a first set of blades of a first predetermined height spaced symmetrically about said axis of rotation, a second set of blades of a second predetermined height less than said first height and spaced symmetrically about said axis, one on either side of an associated blade of said first set, and a third set of blades of a third predetermined height spaced symmetrically about said axis of rotation and equidistant between said blades of said first set, the number of blades in said first and third sets being equal, and the number of blades in said second set being twice the number of blades in said first set.

9. A machine for shredding and defiberizing papermaking material including a rotor, means for rotating said rotor about an axis, a stationary bedplate extending in a plane generally perpendicular to said axis of rotation, a substantially dome-shaped hub on said rotor, a first plurality of equally spaced blades having their top edges starting from said hub at approximately radial distance from the axis of rotation of said rotor, the bottom faces of said blades extending in a plane parallel to and spaced slightly from said bedplate to define a defibering zone, said bottom faces of said blades defining a plurality of bars providing shearing edges, a second plurality of blades extending from said hub and having bottom faces identical to the blades in the first set, and being of lesser height, the ratio of the number of blades in said second set to the number of blades in said first set being in the range of about 2-4 to 1, said stationary bedplate being perforated in an annular region beneath said defibering zone, a first annular chamber beneath said bedplate and said defibering zone, a second annular chamber spaced inwardly from and in communication with said first annular chamber, said bedplate having enlarged openings above said second annular chamber inwardly of the defibering zone, whereby material to be treated passing through said bedplate from said defibering zone into said first annular chamber is forced into said second annular chamber and thence upwardly through said enlarged openings to be recirculated through said defibering zone.