Abstract:
A method of mechanically refining lignocellulosic material in a refiner having opposing refiner plates including: introducing the material to an inlet in one of the opposing refiner plates; rotating at least one of the plates with respect to the other plate, wherein the material moves radially outward through a gap between the plates due to centrifugal forces created by the rotation; as the material moves through the gap, passing the material over bars in a refiner zone of a first one the plates, each bar in the refiner zone having a leading face and an upper ridge, wherein the leading face includes a sidewall of the bar facing a direction of rotation of the opposing plate and the leading edge has an interior angle of between 150 degrees to 175 degrees, and discharging the material from the gap at a periphery of the refiner plates.

Description:
BACKGROUND OF THE INVENTION 
     This application is a divisional of U.S. application Ser. No. 12/329,245 filed Dec. 5, 2008 which claims the benefit of U.S. Provisional Patent Application 61/019,354, filed Jan. 7, 2008, the entirety of which is incorporated by reference 
    
    
     This invention relates to the comminution of lignocellulosic materials (referred to herein as “fibrous material” or “wood fibrous material”) and, particularly, to comminution using refiner plates having bars and grooves to separate fibers from lignocellulosic materials. 
     The invention is applicable to bar and groove designs for various types of refiner plates, including but not limited to disk refiners, counter-rotating disk refiners, twin and twin-flow refiners, cylindrical refiners, conical refiners and conical-disk refiners. 
     Refiner plates typically are arranged in a refiner to have facing surface separated by a gap. The plates rotate relative to each other. The fibrous material is introduced into the gap between the plates, typically, by flowing through a center inlet in one of the plates. The fibrous material flows in the gap between the plates and, in doing so, moves across the bars on the facing surfaces of the plates. As the fibrous material moves over the bars, the bars apply forces, such as compression pulses and impact forces, to the material. These forces tend to be greatest when the bars on the opposite plates cross over each other. The forces applied to the fibrous material act on the network of fibers in the material to separate individual fibers from the network and further develop these fibers. The separation of individual fibers and repeated compression of the fibrous mass results in the refining of the fibrous material. 
     Conventional refiner plates have refining bars separated by grooves arranged on a surface of the plate. The fibrous material, steam, water and other material flow through the grooves and over the bars as the material moves radially outward between the plates. Refining of the fibrous material tends not to occur in the groves. Refining occurs primarily as the fibrous material moves over the top ridges of the bars. The groves may include dams or other obstructions to prevent or restrict the flow of fibers and fluid through the grooves. 
     The bars typically include a sharp leading edge along a forward facing top edge of the bar. The conventional sharp leading edge angles of the bars are believed to promote shearing of the fibrous material passing over the bars. As bars on opposing plates pass each other, they impact and shear the fibrous material caught between the bars. The shear impacts of the fibrous material against the bar are a biproduct of the crossing of the bars. The shearing of fibrous material is undesirable. 
     Conventional wisdom views sharp leading edge angles as desirable to provide grooves with steep slopes such that the cross-sectional volume of the grooves provides sufficient flow capacity to move the fibrous material between the plates. A dull leading edge and its corresponding sloped leading face, i.e., leading sidewall, would result in conventional grooves having relatively narrow cross-sectional areas that may be insufficient to accommodate the flow of fibrous materials and the accompanying steam and water that should pass through the grooves. Examples of refiner plates with various types of leading edges on bars are shown in U.S. Pat. No. 5,039,022 entitled “Refiner Element Pattern Achieving Successive Compression Before Impact” and U.S. Pat. No. 4,678,127 entitled “Pumped Flow Attrition Disk Zone.” 
     The crossing of opposite bars creates compressive pressure pulses that impact the fibrous material between the bars. The compression pulses apply mechanical force to the fibrous material that promote the refining of the fibrous material. The compression pulses are believed to provide desirable refining action by producing high strength fibrous material. 
     There is a long felt need for refiner plates that minimize the impact forces and resulting shearing of fibrous material and maximize compression pulses to refine the material. 
     BRIEF DESCRIPTION OF THE INVENTION 
     To reduce the shear impacts of energy transfer into the fibrous material, at least one of a pair of opposite refining elements includes bars having a dull bar edge. To reduce the tendency of sharp edges on the leading edge of bars to shear fibrous material, the leading edge angle of a bar should preferably be dull, e.g., between 150 degrees and 175 degrees. A dull leading edge on a bar should reduce the impacts between the bars and fibrous material that are caused by the sharp leading bar edges of conventional refiner plates. Minimizing the impacts should reduce shearing of fibrous materials and thereby maximize the strength of the fibers separated through repeated compression refining. 
     One embodiment of the invention is a refiner plate, such as a stator plate or a rotor plate, for a mechanical refining system, the plate comprising: a refining surface including bars and grooves, wherein the bars have a leading edge defined by an interior angle of between 150 degrees to 175 degrees. The bars may each include a leading face extending from the leading edge to a trailing face of an adjacent bar. The may include leading face having an upper sidewall section forming an angle of between 150 degrees to 175 degrees with respect to an upper ridge of the bar and a lower sidewall section substantially perpendicular to a substrate of the bar. Further, the leading face of the bars may be concave or convex. In addition, the trailing edge of the bars may have an interior angle of between 80 degrees to 140 degrees. The grooves between the bars may each have a groove bottom formed by an intersection of the leading face and a trailing face of a bar. 
     Another embodiment of the invention is a refiner plate for a mechanical refining system, the plate comprising: a refining surface including bars and grooves; each of the grooves has a width extending between the upper ridges of adjacent bars; the bars each have a leading face, an upper ridge surface and a leading edge formed by an intersection of the leading face and the upper ridge surface, wherein the leading edge has an interior angle between the leading face and the upper ridge surface of between 150 to 175 degrees, and wherein a width of the upper ridge surface of each bar is in a range of 30 percent to 75 percent of a total width of the ridge surface and the width of a groove. 
     A further embodiment of the invention is a method of mechanically refining lignocellulosic material in a refiner having opposing refiner plates, the method comprising: introducing the material to an inlet in one of the opposing refiner plates; rotating at least one of the plates with respect to the other plate, wherein the material moves radially outward through a gap between the plates due to centrifugal forces created by the rotation; as the material moves through the gap, passing the material over bars in a refiner section of a first one the plates, wherein the bars on at least one of the plates has a leading edge defined by an interior angle of between 150 degrees to 175 degrees, and discharging the material from the gap at a periphery of the refiner plates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a portion of a conventional refiner plate, e.g., a rotor and stator plate, showing a conventional geometric cross-sectional shape of bars and grooves. 
         FIG. 2  shows a crossing of conventional bars of opposing plates, where the bars are shown in cross-section. 
         FIG. 3  is a chart of the force applied to fibrous material between the crossing bars shown in  FIG. 2 . 
         FIG. 4  is a cross-sectional view of a portion of a refiner plate, e.g., a stator plate, showing a novel geometric cross-sectional shape of bars and grooves. 
         FIG. 5  shows a crossing of conventional bar of one refiner plate with a novel bar of an opposing refiner plate, opposing plates, wherein the bars are shown in cross-section. 
         FIG. 6  is a chart of the force (solid line) applied to fibrous material between the crossing bars shown in  FIG. 5 , as compared to the force (dotted line) applied to fibrous material between the crossing bars shown in  FIGS. 2 and 3 . 
         FIG. 7  shows the crossing of bars both of which have novel profiles, of opposing plates, where the bars are shown in cross-section. 
         FIGS. 8   a  and  8   b  show in a cross-section bars having a flat leading sidewall ( 8   a ) and a curved leading sidewall ( 8   b ). 
         FIG. 9  is an enlarged cross-sectional view of a portion of a refiner plate, e.g., a stator plate, showing a novel geometric cross-sectional shape of bars and grooves. 
         FIG. 10  is an enlarged cross-sectional view of a portion of a refiner plate, e.g., a stator plate, showing another novel geometric cross-sectional shape of bars and grooves. 
         FIG. 11  is a cross-sectional diagram showing a refiner having a refiner housing for an annular rotor disc and plate assembly and an annular stator disc and plate assembly. 
         FIG. 12  is a front view of the annular stator disc shown in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a cross-sectional view of a portion of a conventional refiner plate  10 , e.g., a rotor or stator plate, showing a conventional geometric cross-sectional shape of bars  14  and grooves  12 . The bars have a relatively sharp leading edge  16  formed by the intersection of the leading face  18  of the bar and the ridge  20  at the upper surface of the bar. The leading face  18  is a sidewall of the bar facing the direction of rotation if on a rotor plate and facing the approaching rotor bars if on a stator plate. 
     The angle of the leading edge is defined as the interior angle  21  between the leading face and ridge  20  of the bar. A conventional leading edge angle is sharp, such as in a range of 90 degrees to 100 degrees and may include leading edge angles as small as 75 degrees. The sharp leading edges on bars, e.g., having a leading edge angle of 75 to 100 degrees, tend to shear fibrous material caught between opposite bars as the bars on opposite refiner plates cross during rotation of one or both of the refiner plates. 
     The sharp leading edge of the conventional bar provides a steep leading face  18  that is nearly perpendicular with respect to the substrate  22  of the refiner plate. The trailing face  24  of a bar is on the opposite side of the bar to the leading face. The trailing face  24  is steep and typically forms an interior angle with the ridge  20  of between 90 to 100 degrees. The steep leading and trailing faces of the bar results in grooves  12  that are relatively wide from the top to the bottom  25  of the groove at the level of the substrate  22 . The grooves typically have a generally flat surface bottom  25  between the lower corners of the leading and trailing faces of adjacent bars. The wide grooves  12  have large cross-sectional areas that allow for relatively large volumes of material flow, e.g., steam and water, through the grooves. The capacity of the wide grooves to pass large volumes of material enhances the capacity of the refiner plate apparatus to handle a large flow of fibrous material moving between the plates. 
       FIG. 2  shows a crossing of conventional bars  26 ,  30  of opposing plates, where the bars are shown in cross-section. The plates may be a rotor plate  26  moving in a rotational direction (arrow  28 ) with respect to a stationary stator plate  30 . The rotor and stator plates are opposite to each other, such that the ridges  20  of the bars on opposing plates pass each other with a relatively small refining gap  32 , e.g., 0.5 to 4 millimeters, between the ridges. The refining gap  32  between the crossing bars tends to be the region where much of the refining action occurs to separate fibers from the fibrous material. The pressures and forces applied to the fibrous material in the refining gap are greater than the pressures and forces in regions between a groove and a bar, or between opposing grooves. The higher pressures and forces in the refining gap  32  cause the fibers to separate from the network of fibers in the fibrous material. 
     Fibrous material  34  being refined by the plates may be sheared in the gap  32  between the plates. The sharp leading edges  16  of the conventional bars can directly impact and shear the fibrous material  34 . The shearing of wood fibrous material is not desired. Shearing may break fibers, reduce the length of the fibers in the pulp produced by refining and reduce the potential strength of fiber based products produced with the pulp. Shearing the fibrous material is believed to be most acute in the gap  32  as the sharp leading edges  16  cross of opposing bars. The sharp leading edge and the steep slope of the leading face of the bar tend to impact fibrous material between the plates. The impacts shear the fibrous material. 
       FIG. 3  is a chart  36  depicting the forces (F), as understood by the inventor, applied to fibrous material between the crossing bars shown in  FIG. 2 . The horizontal axis  40  of the chart  36  depicts movement of a bar moving through a distance (d) in the direction of the arrow  28 . The trace  38  represents the force applied to the material between the refiner plates. As the ridge of a bar on one plate moves over the groove of an opposite plate (represented by distance d 1 ), a very low force  40  is applied to the fibrous material between the bar and groove. 
     As the sharp leading edge and steep leading face of one conventional bar approaches the sharp leading edge and steep leading face of an opposite conventional bar, the force applied to the fibrous material between the bars increases dramatically, as indicated by the rapidly rising portion  42  of the force trace  38 . As the leading edges of the opposing bars cross, the force spikes  46  because the leading bar edges violently impact the fibrous material. The force spike  46  is at an excessive level  48  that can shear the fibrous material, break fibers in the material and otherwise harm the material. 
     The ridges of the opposing bars cross during a distance d 2  in  FIG. 2 . After the leading edges  16  of opposing bars cross and the bar ridges are opposite to each other, the force quickly reduces to a force level  50  which is relatively high. This high force level  50  results from a compressive pressure pulse applied by the crossing of the bar ridges  20 . The high level of forces  50  is sufficient to refine the fibrous material, such as to cause fibers to be separated from the fiber network of a wood material. The high level of forces  50  is believed to not substantially shear the fibrous material or otherwise damage the material to the same extent that occurs by application of the excessive force level  48  during a force spike  46 . The force spike  46  is an undesirable and unnecessary trait of many conventional refiner plates. 
       FIG. 4  is a cross-sectional diagram of a refiner plate  52  having bars  54  and grooves  56 . The bars have a leading face  58  having a slope of approximately 5 to 40 degrees with respect to a plane of the ridges of the bars. The slope may be applied to the entire leading face from the ridge to the substrate. Alternatively, the slope may be applied to an upper section of the leading face adjacent the ridge, while a lower section of the leading face is steeper, such as having a slope of 45 to 90 degrees. 
     The leading edge  60  is formed at the intersection of the leading face  58  and the ridge  62  of the bar. The interior angle  61  of the leading edge is dull and may be in a range of 140 degrees to 175 degrees, and preferably in a range of 155 degrees to 175 degrees, and most preferably at 160 degrees. 
     The leading face  58  has a shallow slope resulting from the dull leading edge angle. Because of its shallow slope, the leading face of each bar extends substantially the entire width of the groove  56 . Due to its shallow slope and dull leading edge, the leading face  58  gradually applies an increasing compressive pressure to the fibrous material between the plates, as the leading face approaches a bar on an opposing plate. The trailing face  64  of the bars  54  may be substantially parallel, e.g., an interior angle of 90 degrees to 100 degrees, with respect to an axis  66  of the plate. The bar  54  and groove  56  shapes provide a compressive bars and groove pattern. 
     The grooves  56  between the bars are formed by the leading face and trailing face of adjacent bars. The slope of the leading face  58  of the bar gradually reduces the depth of the groove in a direction approaching the leading edge  60  of the bar. Due to the slope of the leading face  58 , the groove may have a cross sectional shape of a triangle in which the leading face  58  and trailing face  64  intersect at the bottom  62  of the groove. The cross-sectional area of the groove should be sufficient to allow water, steam and other fluids in the fibrous material to flow through the grooves of the refiner plate without inhibiting the flow of the fibrous material between the opposing plates. 
     The grooves  56  are shallow, especially near the leading edge  60  of the bar. The shallow groove promotes smooth movement of the fibrous material through the refining gap between crossing bars. The shallow groove tends to move fibrous material into the refining gap between crossing bars. The dull leading edges and sloped leading faces of the bars shown in  FIG. 4  tend to increase the concentration of fibrous material in the compression sites of the refining gap between the ridges of bars and thereby increase the energy applicable in compression refining. In contrast, conventional grooves tend to impact against fibrous material, do not provide a smooth transition over the leading edge and into the gap between opposing ridges of bars and tend to allow fibrous material to gather in the groove. 
     The grooves  56  shown in  FIG. 4  have a reduced cross-sectional area as compared to conventional grooves, such as shown in  FIG. 1 . Due to the limited volume available in the grooves  56 , the refiner plates with the reduced cross-sectional area grooves are most suited to be (but not necessarily) one of the following: (1) a compression bar edge design on one of the refining plates and a conventional bar edge design on the opposite refining plate; (2) a compression bar edge design and a conventional bar edge design alternating between the refining annular zones on opposite refining plates; (3) a compression bar edge design on both refining plates in conjunction, with flow-enhancing design features, such as steam pockets (as shown in U.S. Pat. No. 5,863,000), steam grooves (U.S. Pat. No. 4,676,440), pumping/feeding grooves, or (4) other modifications that enhance the capacity of the refiner plates to fibrous material water and steam. 
       FIG. 5  shows, in cross-section, the crossing of bars  54 ,  12 , where one of the bars  54  has the dull leading edge shown in  FIG. 4  and the opposite bar has a conventional sharp leading edged such as shown in  FIG. 1 . In this example, the bar crossing is shown with a rotor plate  26  having bars  12  having a leading face  18  with a sharp leading edge  16 . The bars of the stator plate  52  have a sloped leading face  58  with a dull leading edge  60 . The rotor plate moves in a rotational direction shown by the arrow  68 . 
     The fibrous material  70  is refined in the gap between the opposing bars on the rotor and stator plates and, particularly, by the compressive pressure applied to the material as the opposing bars cross. The pressure applied to the fibrous material results from the crossing of the bars  12 ,  54  which reduces the gap between the refiner plates and thereby increases the pressure in the gap and applied to the fibrous material  70  in the gap. 
     The shallow slope of the leading face  58  of the stator bar  54  gradually increases the pressure applied to the fibrous material  70  as the bar  12  of the rotor passes over the groove  56  in the stator plate and approaches a leading edge  60  of the stator bar  54 . The shallow slope of the leading face  58  of the stator bar reduces the tendency of the fibrous material to be violently impacted by the leading edges of the crossing bars. The gradual pressure increase resulting from the sloped leading face  58  and dull leading edge  60  of the stator bar is less prone to impacting and shearing of the material due to the profile of that bar. The sharp leading edge  16  of the rotor bar  12  in  FIG. 5  is believed to be less prone to impacting and shearing the chip material because the fibrous material are not pinched between an opposing sharp leading edges of opposite bars. 
       FIG. 6  is a chart  72  depicting the forces (F), as understood by the inventor, applied to fibrous material between a crossing of the opposing bars shown in  FIG. 5  and  FIG. 2 . The solid line force trace  74  depicts the perceived forces applied to fibrous material  70 , e.g., wood chips, between the rotor and stator plates  26 ,  52  shown in  FIG. 5 . The dotted line trace  76  shows the perceived forces applied to the fibrous material  34  between the rotor and stator plates  26 ,  30  shown in  FIG. 2 . 
     The dotted line trace  76  is similar to the trace  38  shown in the chart  36  of  FIG. 3 . The dotted line trace  76  is presented in  FIG. 6  by way of comparison to illustrate the pressure spike resulting from the crossing of bars with conventional sharp leading edges as compared to the pressures (shown by solid line trace  74 ) that result from bar crossings, wherein at least one of the bars has a sloped leading face and dull leading edge, (a “compression bar design.”) 
     The solid line force trace  74  shows the gradual increase  78  in forces applied to the fibrous material as the leading edge  16  of the rotor bar  12  passes over the groove  56  of the stator bar  54 . The gradual increase in force is in contrast to the rapid rise in force (see trace portion  42  in  FIG. 3 ) that is believed to occur when conventional bars having sharp leading edges approach, as shown by the dotted line trace  76  in  FIG. 6 . The shallow slope of the leading face  58  of the stator compression bar  54  is believed to cause the forces to increase gradually to a maximum force, indicated by the crest  90  of the force trace  74 . 
     The solid line force trace  74  shows substantially no spike in impact forces being applied to the fibrous material by the crossing of a the dull leading edge of a compression bar and a sharp leading edge of the rotor bar. The spike of impact forces (see spike in dotted line  76 ) as opposing sharp leading edges crossed in conventional bar profiles are believed to be avoided when at least one refiner plate has compression bars, such as bar  54  shown in  FIG. 5 . 
     The high level of forces  80  applied to the fibrous material in the compression stage of the bar crossing are sufficient to refine the material. The shallow slope of the leading face of the stator bar is believed to avoid a force spike as the leading edges cross of opposing bars. Avoiding the spikes in the forces applied to the fibrous material reduces the shearing of fibrous materials as the leading edges of opposite bars cross. The maximum force level  80  occurs as the ridges of the opposite bars cross. After the bars cross, the forces on the chip material are reduced as the bars pass over an opposing groove. The forces shown in  FIG. 6  are repeatedly applied to the fibrous material as the rotor bars cross the stator bars. 
       FIG. 7  shows in cross-section a rotor plate  82  and a stator plate  84  which both have bars  86  having leading faces  88  with shallow slopes and dull leading edges. The fibrous material  90  is subjected to repeated compression pulses as the bars cross as the rotor plate moves in the rotation direction indicated by the arrow. The forces applied to the fibrous material by the crossing bars  86  tend to be entirely or at least primarily due to compression forces applied to the material. The crossing bars have a cross-sectional profile, e.g., sloped leading face and dull leading edge, that minimize impact forces applied when the bars cross. The minimization of impact forces should reduce or eliminate the shearing of fibers due to the crossing of the leading edges of opposing bars. 
     As shown in  FIGS. 4 and 7 , compression bars with a dull leading edge and a leading face having a shallow slope may be arranged on one or both of a pair of opposing plates. Preferably, these bars are arranged on at least the stator plate (see  FIG. 5 ), but may be arranged solely on a rotor plate or on both opposing plates, e.g., a rotor-rotor pair of plates and a rotor-stator pair of plates ( FIG. 7 ). 
       FIGS. 8A and 8B  each show in cross-section a portion of a refiner plate having bars  54 ,  92  with dull leading edges and leading faces having a shallow slope. The bar  54  shown in  FIG. 8A  is substantially the same as the bar  54  shown in  FIG. 4 . Particularly, the leading face  58  of the bar  54  is substantially planar and forms a straight line in cross-section. The bar  92  shown in  FIG. 8B  has a convex leading face  94  that merges into the ridge  98  of the bar without any creases or other abrupt changes at the leading edge  96  of the bar  92 . The planar leading face  58  shown in  FIG. 8   a  may facilitate fabrication, e.g., molding, of the plate. The convex leading face  94  and curved leading edge  96  section of bar  92  shown in  FIG. 8   b  may minimize impacts and spikes in the forces applied to the fibrous material due to the crossing of the leading edges of bars in opposite plates. 
       FIG. 9  is an enlarged cross-sectional view of a portion of a refiner plate  100 , e.g., a stator plate, showing a novel geometric cross-sectional shape of bars  102  and grooves  104 . The bars have a sloped leading face  106  and a dull leading edge  108 . It is preferable that the width (c) of the bar ridge  110  be substantially equal to the width (b) of the groove  104 . For example, the widths of the grooves and bars may be each in a range of two to eight millimeters (mm) and, preferably, in a range of two to four millimeters. The ratio of bar width to the combined widths (d) of bar and groove should be in a range of 30 percent to 75 percent, and preferably in a range of 40 percent to 60 percent. 
     The angle (a) of the leading edge  108  of the bar  102  should be in a range of 150 degrees to 175 degrees. The angle (e) of the trailing bar edge  112  should preferably in approximately 90 degrees, such as between 80 degrees to 100 degrees. A sharp angle on the trailing edge provides a trailing face with a steep slope and allows for deep grooves having a relatively large cross-sectional area. Alternatively, the trailing edge angle (e) may be wide, e.g., 150 degrees to 175 degrees, especially if the refiner plate is to operate in either rotational directions. 
     The groove cross-sectional area should be sufficient to allow the fibrous material, steam and water to pass between the refiner plates. In addition, the groove should have a depth sufficient to allow compression relief after the bars have crossed. A groove that is too shallow may be inadequate to provide compression relief after the bars cross. Without sufficient compression relief, the efficiency of the energy transfer to the fibrous may be reduced. 
     The shape of the groove and the sidewalls of the bars may be designed to provide sufficient cross-sectional area for the groove and compression relief to the fibrous material. Preferably, the upper portion of the leading sidewall is sloped and the leading edge is dull, as described above, to minimize the impacts by the leading edges on fibrous material as the bars cross. The lower portion of the leading sidewall my be steeply sloped or substantially perpendicular to the substrate to increase the cross-sectional area of the plate. 
       FIG. 10  is an enlarged cross-sectional view of a portion of a refiner plate  114 , e.g., a stator plate, showing another novel geometric cross-sectional shape of bars  115  and grooves  116 . The bars include a generally flat upper ridge  117  and a leading sidewall having a sloped upper sidewall section  118  with a curved leading edge  119  as the sidewall merges into the upper ridge. The leading sidewall also includes a substantially straight lower sidewall section  120  to increase the depth and cross-sectional area of the groove. 
     The lower sidewall section  120  of the leading sidewall and the trailing sidewall  64  may have draft angles, e.g., angles from a line perpendicular to the substrate  22  of the plate, of less than one or two degrees and be substantially perpendicular to the substrate  22  of the plate  114 . The transition between the upper sidewall section  118  and lower sidewall section  120  may be determined to provide a desired cross-sectional area of a groove and is preferably approximately in the middle of the bar between the upper ridge  117  and substrate  22 . 
       FIG. 11  is a cross-sectional diagram showing a refiner  121  having a refiner housing  122  that encloses an annular rotor disc  124  and an annular stator disc  126 . The discs each support, respectively, an annular rotor plates  128  (which may also be an annular assembly of plate segments) and an annular stator plate  130  (which may also be an annular assembly of plate segments). The rotor disc  124  is mounted on a shaft  132  that is rotated (see arrow on a half circle) by a motor  134 . A mechanical adjustment, e.g., a screw, moves the shaft axially (see doubled headed arrow) to move the rotor disc and plate axially relative to the stator disc and plate. The axial adjustment determines the gap  136  between the opposing surfaces of the plates. 
     Unrefined fibrous material is introduced through a center inlet  138  of the stator disc and enters the gap  136  between the plates. The material moves radially outward through the gap due to the centrifugal forces imparted by the rotation of the rotor disc. As the material moves between the plates, the material passes between crossing bars of the opposing plates and is thereby refined into a pulp having separated fibers. The refined pulp exits the gap  136  at the peripheries of the refiner plates and is discharged through outlet  140  from the refiner. Each refiner plate  141  may include multiple annular and concentric refining zones  142 ,  144 ,  146  and  148 . The refining zones each have a pattern of bars and grooves arranged on the surface of the refining plate. Generally, opposing plates have similar annular refining sections that are aligned when placed in the refiner. The stator plate  130  may, for example, include an inner annular section  142  having bars with dull leading edges and shallow leading faces and an outer annular section  144  having bars with sharp leading edges and steep sloped leading faces. The rotor plate  128  may have an inner annular section  148  having bars with sharp leading edges and steep leading faces and an outer annular refining section  146  having bars with dull leading edges and shallow leading faces. 
       FIG. 12  is a front view that generically shows a disc  131 , that may be a rotor disc or stator disc. An annular array of refiner plates  141  are arranged on the disc  131 . Refiner plates often include two or more annular refining zones  150 ,  152  and  154 . Each refining zone typically has a uniform pattern of bars and grooves. 
     It is preferable, that bars with dull leading edges and shallow sloped leading faces be on at least one plate of a pair of opposite plates for each of the annular refining sections. However, pairs of opposite plates may be arranged such that one or more of the annular refining zones  150 ,  152  have bars with sharp leading edges and steep leading faces on both plates, and at least one annular refining zone  154  has bars with dull leading edges and shallow sloped leading faces on at least one of the plates. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.