Abstract:
A refiner plate defines an axis of rotation that further defines variations in azimuth. The refiner plate includes an annular body portion and a plurality of azimuthally spaced-apart elongate bars projecting from the body portion. The bars have top surfaces having elevations that vary as functions of azimuth. A disc refiner further defines a direction of rotation of the refiner plate. Preferably, each top surface slopes downwardly in the direction opposite the direction of rotation, to provide for relief. Preferably in addition, each bar has a side intersecting the respective top surface that leans forwardly in the direction of rotation, to provide for attack.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a refiner plate, which is typically used in a type of milling machine known as an attrition mill or disc refiner.  
       BACKGROUND  
       [0002]     Different types of “engineered” wood particles are used to produce a corresponding variety of engineered wood products. In the production of highly refined wood products such as fiberboard and paper, chips or other comminuted wood or wood refuse is milled or ground to produce small “particles” or bundles of fibers. Attrition mills or “disc refiners” are commonly used for this purpose. As a class, these produce a fine defibration and fibers with a high degree of slenderness.  
         [0003]     Two general types of disc refiners are the “single-revolving-disc” and the “double-revolving-disc.” Both types rely on relative spinning motion between two coaxially disposed discs defining a small gap between opposed, grinding faces of the discs. In the single-revolving-disc design, one of the discs is stationary, while in the double-revolving-disc design, the two discs counter-rotate.  
         [0004]     Raw material, typically chips, is input to the disc refiner substantially along the axis of rotation of the disc(s). The material is flung radially outwardly through the gap as a result of centrifugal force imparted to the material as a result of contact with the grinding faces of the spinning disc(s).  
         [0005]     Two such discs  2   a ,  2   b , are shown in cross-section in  FIG. 1 . There is a gap “G” between grinding faces  3  of the two discs through which the material being worked travels as it is refined into particles. Due to elasticity and therefore flexure of the discs, the spacing of the gap “G” changes as a result of the forces encountered when working the material. Particularly, the presence of the material tends to spread the discs apart.  
         [0006]     This is typically compensated for by providing a slight “face taper” on the discs, shown highly exaggerated in  FIG. 1 , so that the grinding faces  3  are not absolutely perpendicular to the axis of rotation L when the discs are not processing any material. The face taper is small, so that the grinding face is flat to within about 0.0025″ even when unloaded.  
         [0007]      FIG. 2  shows an annular sector of one of the discs  2  looking down the axis of rotation L. The axis of rotation L is an axis of azimuthal symmetry of the disc. Visible on the disc  2  is the grinding face  3 . With additional reference to  FIG. 3  showing a cross-section of the disc, the grinding face  3  is defined by top surfaces  4   a  of protruding structures  4  known and referred to in the art as “bars.” The bars  4  project above an annular body portion  9  of the disc.  
         [0008]     The top surfaces  4   a  of all of the bars are typically at the same elevation “h bar ” with respect to a reference plane “P REF ” ( FIG. 3 ) that is perpendicular to the axis of rotation L. The top surfaces h bar  of two opposed discs provide the desired grinding action.  
         [0009]     Referring back to  FIGS. 2 and 3 , raw material flows across the grinding face  3  in the directions indicated as “D FLOW ,” i.e., radial directions r with respect to the axis L. Due to the azimuthal symmetry of the bars shown in this example, the disc  2  may equally well spin in either of the directions indicated as “D SPIN .” 
         [0010]     The bars  4  are spaced apart by depressions known and referred to in the art as “grooves”  5 , the top surfaces  5   a  of which are at a lower elevation “h groove ,” than the top surfaces  4   a  of the bars.  
         [0011]     The grooves  5  are typically provided with a radially spaced apart series of structures known and referred to in the art as “dams”  6  that extend cross-wise across the grooves to join adjacent bars. The dams  6  have top surfaces  6   a  that are at an elevation “h dam ” that is, at least for the most part, lower in elevation than the top surfaces of the bars; however the elevation of a given dam increases with the dam&#39;s radial distance from the axis L, and the top surface  6   a  of the radially outermost dam is often at the same elevation as the top surfaces  4   a  of the connected, adjacent bars.  
         [0012]     The bars  4 , grooves  5 , and dams  6  can be recognized to form a pattern that is typically repeated in some fashion over the entire grinding face, similar to a tread pattern on a shoe or a tire. An extreme variability in such patterns has been provided in the prior art as would be expected by the analogy to tires and shoes.  
         [0013]     The top surfaces of the grooves in conjunction with the elevation of the top surfaces of the dams provide for flinging the material up onto the top surfaces of the bars where the material is ground. Because refinement results from grinding, it is generally desirable that the top surfaces of the bars that perform this grinding lie in a single plane and are as wide as possible consistent with providing the beneficial effects of the grooves and dams.  
         [0014]     U.S. Pat. No. 5,704,559 to Fröberg et al. represents a different strategy and model than that described above, one which relies on a certain cooperation between the patterns of the two discs.  
         [0015]     A single “bar” as described above in the context of the &#39;559 patent has both high and low bar portions, the terms “high” and “low” being used to describe the overall elevation of the bar portions with respect to a reference plane “P REF ” that is perpendicular to the axis of rotation L.  
         [0016]     Referring to  FIGS. 4-6  showing two refining elements  10  and  11  according to the &#39;559 patent, a refining element  10  has a bar  12   a  and an opposed refining element  11  has a bar  12   b . The refining element  10  is stationary and the refining element  11  rotates.  
         [0017]     The bar  12   a  includes high bar portions  13  that are disposed directly opposite corresponding low bar portions  16  of the bar  12   b ; and the bar  12   b  includes high bar portions  15  that are disposed directly opposite corresponding low bar portions  14  of the bar  12   a . The bars  12   a  and  12   b  are spaced apart to provide a gap  12  through which raw material flows in the direction D FLOW  ( FIG. 4 ). The refining element  11  spins in the direction D SPIN  ( FIGS. 5-6 ).  
         [0018]     The top surfaces of both the high and low bar portions,  13 - 16  are angled with respect to the reference plane P REF  ( FIG. 4 ). Particularly, the surfaces are inclined in the direction of increasing radial distance from the axis of rotation L.  
         [0019]     In addition, transition surfaces connecting the high and low bar portions of the same refining element are also angled from the perpendicular to the reference plane P REF . This feature is particularly asserted to “knead” more softly the material being worked, here referred to as “pulp,” as well as force the pulp to move between the two discs. It is further asserted that this working of the pulp is rendered even more effective due to the inclined top surfaces of the bar portions. It is asserted more generally that the configuration effectively disperses impurities without reducing the “freeness” of the pulp and improves the strength of the pulp.  
         [0020]     Further, the high bar portions  15  on the rotating element  11  have a greater length, in the direction D FLOW , than the high bar portions  13  on the stationary element  10 , and this is asserted to provide a “pump effect” which increases throughput (capacity).  
         [0021]     Whether or not an improvement in pulp strength, impurity distribution or throughput can be realized from the configuration of the &#39;559 patent, it is a disadvantage that opposed bars of the respective refiner elements must be closely toleranced to align with each other. It is also an inherent disadvantage of “kneading” the pulp as taught in the &#39;559 patent that this action breaks down the fibers and requires that a large amount of power be provided to the rotating element. Accordingly, there is a need for a refiner plate that provides for further improvements over the prior art.  
       SUMMARY  
       [0022]     A refiner plate defines an axis of rotation that further defines variations in azimuth. The refiner plate includes an annular body portion and a plurality of azimuthally spaced-apart elongate bars projecting from the body portion. The bars have top surfaces, each top surface having an elevation that varies, with respect to a reference plane perpendicular to the axis of rotation, as a function of azimuth.  
         [0023]     A disc refiner further defines a direction of rotation of the refiner plate. Preferably, each top surface slopes downwardly in the direction opposite the direction of rotation, to provide for relief. Preferably in addition, each bar has a side intersecting the respective top surface that leans forwardly in the direction of rotation, to provide for attack.  
         [0024]     It is to be understood that this summary is provided as a means of generally determining what follows in the drawings and detailed description and is not intended to limit the scope of the invention. Objects, features and advantages of the invention will be readily understood upon consideration of the following detailed description taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1  is a cross-sectional view of a pair of opposed refiner plates in a typical disc refiner.  
         [0026]      FIG. 2  is a plan view of a broken sector of a typical prior art a refiner plate.  
         [0027]      FIG. 3  is a cross-sectional view of the sector of  FIG. 2 , taken along a line  3 - 3  thereof.  
         [0028]      FIG. 4  is a cross-sectional view of a pair of opposed refining elements according to U.S. Pat. No. 5,704,559.  
         [0029]      FIG. 5  is a plan view of a broken sector of one of the refining elements of  FIG. 4 .  
         [0030]      FIG. 6  is a plan view of a broken sector of the other refining element of  FIG. 4 .  
         [0031]      FIG. 7  is a pictorial view of a pair of refiner plates according to the present invention, with a sector of one of the refiner plates broken to reveal a cutting face.  
         [0032]      FIG. 8  is a plan view of the broken sector of  FIG. 7 .  
         [0033]      FIG. 9  is a pictorial view of the broken sector of  FIG. 8 .  
         [0034]      FIG. 10  is a cross-sectional view of the sector of  FIG. 8 , taken along a line  10 - 10  thereof.  
         [0035]      FIG. 11  is a cross-sectional view like that of  FIG. 10  showing two sectors in cooperation.  
         [0036]      FIG. 12  is a cross-sectional view of a sector like that shown in  FIG. 10  having a jointed top surface according to a first alternative embodiment.  
         [0037]      FIG. 13  is a cross-sectional view of a sector like that shown in  FIG. 10  having a jointed top surface according to a second alternative embodiment.  
         [0038]      FIG. 14  is a side elevation of a generic combination of refiner plates providing for relief according to the invention.  
         [0039]      FIG. 15  is a side elevation of another combination of refiner plates providing for relief according to the invention.  
         [0040]      FIG. 16  is a plan view of a broken sector of one of the refiner plates of  FIG. 15  showing a relief in schematic form.  
         [0041]      FIG. 17  is a side elevation of still another combination of refiner plates providing for relief according to the invention.  
         [0042]      FIG. 18  is a plan view of a broken sector of one of the refiner plates of  FIG. 17  showing a relief in schematic form.  
         [0043]      FIG. 19  is a side elevation of yet another combination of refiner plates providing for relief according to the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0044]     Reference will now be made in detail to specific preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings.  
         [0045]     Referring to  FIG. 7 , a pair of refiner plates  20  and  22  are shown in opposition, as they would be confronting one another in an attrition mill (not shown).  
         [0046]     Relative spinning of the discs about an axis of rotation “L” is provided as known in the art. That is, either one of the discs can be made to spin while the other disc is held stationary, or both of the discs can be made to spin in counter-rotation.  
         [0047]     A sector of the refining plate  22  is shown broken out in  FIG. 7 . The removal of this sector reveals a corresponding sector of the plate  20  that is shown in plan, i.e., looking down the axis of rotation L, in  FIGS. 8 and 9  where it is referenced as  20   a . A face  21  of the sector  20   a  is seen in plan in  FIG. 8  and faces upwardly in  FIG. 9 . The face  21  exhibits an exemplary pattern of elongate bars  24 , corresponding grooves  26 , and dams  28 . It should be understood that the pattern shown, while being preferred, is not essential to the invention, and that the shape of the grooves and the presence of the dams, while preferably being provided substantially as shown, are also not essential to the invention.  
         [0048]     Moreover, while the bars  24  are essentially linear as viewed in  FIG. 8 , bars can have other configurations, particularly curved configurations, as can grooves and dams.  
         [0049]     The sector  20   a  as shown spins in the direction “D SPIN .” Material to be refined flows radially over the face  21  from the axis of rotation L in directions “D FLOW .” It should be understood that this material may be any material, but it is typically and preferably wood, and more particularly and preferably, wood chips.  
         [0050]     The direction D SPIN  lies along an azimuthal direction “AD,” and the direction D FLOW  lies along a perpendicular, radial direction “r.” Both the azimuthal and radial directions are therefore defined with respect to the axis of rotation L, an azimuthal direction being a direction of constant radius, and a radial direction being a direction of constant azimuth.  
         [0051]     While the elongate bars  24  as shown in  FIG. 8  are oriented precisely along radial lines r, they may deviate somewhat from such lines, and they will necessarily so deviate if they are curved. In any case, bars are oriented generally in radial directions, meaning that they extend in radial directions at least more so than they extend in azimuthal directions.  
         [0052]      FIG. 10  shows the cross-section indicated in  FIG. 8 . The bars  24  project above an annular body portion  19  of the refiner plate. Space between the bars  24  defines grooves  26 . Preferably, adjacent bars viewed in radial cross-sections taken at the same radial distance have the same profile, such as shown.  
         [0053]     A reference plane “P REF ” is shown that lies in the plane of  FIG. 8  and is perpendicular to the axis of rotation L (see  FIG. 7 ). This reference plane is used to reference elevation. It is to be understood that elevation can vary in radial directions and that there will in general be a lack of perfect perpendicularity of the reference plane and the perpendicular to the axis of rotation, as a result of the face taper discussed above, and that this variation can be ignored for all practical purposes herein.  
         [0054]     The refiner plates  20  and  22  are preferably annular according to standard practice, but would not need to be to function. In any case, the axis of rotation L is an axis of symmetry of the refiner plates.  
         [0055]     According to the invention, it is desired to provide for increased cutting action, decreased grinding action, or both, as compared to prior art disc refiners and refiner plates. To the extent the material to be refined is cut rather than ground, the resulting particles will be exposed to a minimum of damage and therefore have superior mechanical characteristics such as strength. At the same time, the power required to produce particles is dramatically reduced, providing important practical cost savings.  
         [0056]     Continuing with reference to  FIG. 10  and as also seen in  FIG. 9 , in accord with this intention the bars  24  have top surfaces  24   a  that are angled with respect to the reference plane by a relief angle θ, sloping downwardly away from the direction of rotation D SPIN  and therefore varying as a function of azimuth. More particularly, in this example, the top surfaces  24   a  are planar; they have a maximum elevation “h MAX ” at a leading or “upstream” side  25  of the bar that faces in the direction of spin D SPIN ; and a minimum elevation h MIN  at a trailing or “downstream” side  27  of the bar, the elevation decreasing in proportion to the relief angle θ. The relief angle θ is positive in the direction shown in  FIG. 10 , indicating the direction of slope.  
         [0057]      FIG. 11  shows a cross-section like that of  FIG. 10  of the bars corresponding to the disc  20  and an opposed disc  28 , showing a manner of cooperation between two discs. The opposed disc  28  may have bars with top surfaces that are parallel to the reference plane P REF , i.e., a zero relief angle θ, as in the prior art.  
         [0058]     The disc  28  is assumed to be stationary. An instance of material “M” to be refined is shown that is also, for simplicity, assumed to be stationary. Because the disc  20  spins in the direction D SPIN , the bar  24 , will first impact the material M at a sharp cutting edge “SE” (referenced also in  FIG. 9 ) defined by the intersection of the top surface  24   a   1  of the bar  24   1  and the leading side  25   1  of the bar. This edge will be made sharper as the relief angle θ ( FIG. 10 ) is increased and the attack angle α is decreased.  
         [0059]     The sharp edge SE will tend to cut the material M into smaller pieces. As these pieces are transmitted toward the trailing side  27   1  of the bar, the greater spacing between the top surface  24   a   1  and the top surface  28   a   2  of the opposing bar  28   2  of the disc  28  reduces the amount of grinding that would otherwise occur. In effect, to a substantial extent, grinding has been replaced with cutting.  
         [0060]     In that regard, the top surfaces  24   a  define a face “F” of the refiner plate that corresponds to the “grinding face” described above in connection with the prior art. The term “grinding face” will be used herein to describe the face “F” and the like herein according to the present invention for consistency with prior art usage and definition of terms, but it should be understood that grinding action provided by the face “F” can be greatly reduced, or essentially eliminated according to the invention and to this extent the term is a misnomer.  
         [0061]     The relief angle is preferably in the range 1&lt;θ&lt;30 degrees measured with respect to the reference plane, is more preferably in the range 2&lt;θ&lt;10 degrees, and is most preferably 6 +/−1 degrees, or about 6 degrees.  
         [0062]     A non-zero relief angle both increases cutting action and decreases grinding action, the more so with increased relief angle θ. However, there is a limit to the amount of relief that is desirable for two reasons. First, the strength of the cutting edge SE is reduced with greater relief. Second, the top surface if sloped too much allows the material M to fall from the trailing side  27  to a lower elevation where it is not well positioned to be cut by the cutting edge SE of the next bar.  
         [0063]     Returning to  FIG. 10 , the leading side  25  of the bars is also preferably angled from the perpendicular to the reference plane, leaning forwardly into the direction of rotation, to define an attack angle α. The attack angle α is preferably in the range 45&lt;α&lt;90 degrees measured with respect to the reference plane, and is most preferably in the range 85+0/−10 degrees.  
         [0064]     The attack angle provides for attack as known in the art, though it should be noted that a smaller attack angle provides for a greater amount of attack. Greater attack contributes to increasing cutting action, by further sharpening the cutting edge SE.  
         [0065]      FIGS. 12 and 13  show two illustrative alternative embodiments of bars according to the present invention that employ jointed top surfaces.  FIG. 12  shows a jointed top surface  34   a  for a bar  34  projecting from a body portion  39  of a refiner plate  38 . The top surface  34   a  has two planer portions  34   a   1  and  34   a   2 . The portion  34   a   1  is leading or upstream with respect to the direction of rotation D SPIN , relative to the portion  34   a   2 , which is trailing or downstream. The upstream portion  34   a   1  is provided with a non-zero relief angle θ and the downstream portion  34   a   2  is provided with a zero relief angle. The relief angle of the upstream portion  34   a   1  can be substantially greater than that described above and still provide for the essentially the same overall elevation of the bar. This configuration maximizes the cutting action while minimizing the effect on the grooves and dams.  
         [0066]     The relief angle can be made larger than in the bars  24  as a consequence of adjusting widths “W” of the portions, namely an upstream width W 1  and downstream width W 2  of the upstream and downstream portions  34   a   1  and  34   a   2 , as will be readily appreciated by persons of ordinary mechanical skill.  
         [0067]      FIG. 13  shows another jointed top surface  44   a  for a bar  44  projecting from a body portion  49  of a refiner plate  48 . The bar  44  has an upstream portion  44   a , and a downstream portion  44   a   2 . In this case, which is inverse to that described immediately above, the downstream portion  44   a   2  is provided with a non-zero relief angle and the upstream portion  44   a   1  has a zero relief angle. This provides for some additional grinding and less cutting; however, it may be desirable to maximize the life of the refiner plate. That is, the refiner plate may be renewed by the process known as “jointing” by grinding or facing the upstream portion  44   a   1 . Widths W of the portions, namely an upstream width W 1  and downstream width W 2  of the upstream and downstream portions  44   a   1  and  44   a   2  respectively, may be adjusted to provide a desired trade-off.  
         [0068]     A refiner plate having bars defining a particular relief angle, or in the case of the jointed surface embodiments a particular combination of relief angles, may be and according to the invention often are preferably paired with an opposed refiner plate having bars defining a different relief angle or set of relief angles, as next illustrated in connection with  FIGS. 14-16 .  
         [0069]      FIG. 14  shows a generic pair of opposed refiner plates in side elevation. One of the plates  50  has a “grinding face”  53  with bars (not shown) all having a “relief” referenced as “R 50 ,” i.e., a relief angle that defines the hidden line shown. The other plate  60  has a “grinding face”  63  that similarly has a relief “R 60 .” Tests have indicated that it is preferable to provide that the relief R 50  is not equal to the relief R 60 . For example, the relief R 50  may be 6° while the relief R 60  may be zero. Testing of this particular combination shows a very significant reduction in power consumption; on the other hand, the quality of the particles produced is not optimum in that there is a tendency to produce particles that are over-size. This trade off will be advantageous, however, where power consumption considerations are paramount and particle quality of is of lesser concern, such as in pre-processing or pre-refining operations.  
         [0070]      FIG. 15  shows the refiner plate  50  of  FIG. 14  paired with an alternative refiner plate  70  according to the invention. The refiner plate  70  has bars (not shown) having top surfaces comprising multiple planar segments at varying elevations. More particularly, the plate  70  in this example provides a set of two reliefs R 71  and R 72  that increase with radial distance r from the axis of rotation L. Referring in addition to  FIG. 16  showing schematically a sector of the refiner plate  70  in plan, the relief R 71 is applied to a radially innermost portion of the plate  70  defined between radial distances “r 1 ” and “r 2 ” referenced from the axis of rotation L. The relief R 72  is then applied to the remaining (in this case), radially outermost portion of the plate between the radial distances r 2  and “r 3 .” A single bar may extend over both the innermost and outermost portions and therefore have two reliefs, or separated bars aligned but spaced apart in the radial direction such as shown in  FIG. 8  can be provided; where such bars are disposed within the innermost region they may have one relief R 71 , and where such bars are disposed within the outermost region they may have the other relief R 72 .  
         [0071]     Test results for the two refiner plates  50  and  70 , where the relief R 50  is 6° while the reliefs R 71  and R 72  are zero and 6°, respectively, show both high quality particles and a power reduction of 10-15% over the prior art.  
         [0072]      FIGS. 17-18  illustrate another refiner plate  80  having bars (not shown) with top surfaces comprising multiple planar segments at varying elevations. Particularly, the plate  80  provides for four different reliefs R 81 , R 82 , R 83 , and R 84  that increase with radial distance r from the axis of rotation L. For example, the reliefs R 81-84  can be zero degrees, 2°, 4°, and 6°, progressing from relatively radially inner portions to relatively radially outer portions of the refiner plate.  
         [0073]      FIG. 19  shows yet another alternative refiner plate  90  defining a relief R 90  that is actually a continuum of relief angles that continuously vary with distance r, preferably increasing with radial distance as shown. The top surfaces of the bars in this example will be helical rather than planar. And helical top surfaces may be combined with planar top surfaces such as shown in  FIG. 16  in any combination.  
         [0074]     Such a manner of providing for relief may be combined with the manner shown in the embodiment of  FIG. 16 .  
         [0075]     It is to be understood that, while a specific refiner plate has been shown and described as preferred, other configurations and methods could be utilized, in addition to those already mentioned, without departing from the principles of the invention. The terms “refiner plate,” “disc refiner” and “bar” are terms art and are therefore intended to have the specific meanings ordinarily attributed to them by persons of ordinary skill.  
         [0076]     The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions to exclude equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.