Abstract:
A refiner plate for the face of a refiner disc comprising a plurality of refiner segments arranged side-by-side on the face of the disc to form a substantially annular refining region. Each refiner segment has a plurality bars and grooves for refining a lignocellulosic material and forming a path for receiving and transmitting steam generated during the refining process. A plurality of dams are disposed within the grooves to retard the movement of the lignocellulosic material. A partial height portion of each dam adjacent the trailing face of the bar provides a flow path for the steam.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is the national state of International Application No. PCT/US98/17184 filed Aug. 19, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to disc refiners for lignocellulosic material. More particularly, the present invention relates to refiner plate segments for such an apparatus. 
     In high consistency mechanical pulp refiners, the wood fibers are worked between two relatively rotating discs on which refiner plates are mounted. The plates usually have radial bars and grooves. A large volume of steam is produced between the plates as a result of this retained between the plates on the bar surfaces despite the high velocity of the flowing steam, and the enormous centrifugal forces. Typically, the steam is exhausted via the grooves. Dams which extend to the bar surface are provided in the grooves to interrupt material flow and thus improve the retention time of the material in the refining region. 
     In a typical refiner plate with radial bars and grooves, the bars provide impacts or pressure pulses which separate and fibrillate the fibers. The grooves enable radially directed feeding of the fibers and steam extraction. Near the perimeter of the plates, high radial steam flow and high centrifugal force both act to sweep the fibers outwardly from between the plates prematurely, thus reducing the refining effectiveness. The flow restrictions due to a small gap between opposed plates and fiber-filled grooves result in a steam pressure peak between the plates, located radially inward from the perimeter. This pressure peak is a major source of the refining thrust load, and can induce control instability at high motor loads. It is thus desirable that the steam generated during refining be discharged from the refining region as quickly as possible, while retaining the pulp within the region as long as possible. 
     Although the surface dams promote better and more homogeneous refining and help develop the pulp pad between the segments, they restrict the flow of steam in the grooves. Reduced steam flow results in higher pressure peaks, creating a greater thrust load and more instability. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the invention is a refiner plate segment for refining lignocellulosic material. The refiner plate segment has a refining zone comprising a plurality of alternating, substantially radially disposed bars and grooves. Each of the bars has a top grinding surface, a leading surface and a trailing surface. At least one dam extends radially across a groove intermediate the leading bar and the trailing bar of a bar pair. The dam comprises a first portion extending from the trailing surface of the leading bar and a second portion extending from the leading surface of the trailing bar. Each of the portions of the dam have a top surface, where the top surface of the second portion is substantially co-planar with the top surface of the bars and the top surface of the first portion is disposed intermediate the top surface of the bars and the base of the groove. 
     Preferably, a plurality of the dams are disposed in each of the grooves. In one embodiment, the trailing surface and the top surface of the leading bar define a cavity disposed adjacent the first portion of the dam. The cavity creates an arcuate shaped segment in the trailing face of the leading bar and has a bottom which is substantially co-planar with the top surface of the first portion of the dam to reduce the possibility of inducing turbulence in the flow of material. 
     It is an object of the present invention to provide a refiner plate for the face of a refiner disc, which facilitates the removal of steam while retaining the pulp in the refiner region to achieve satisfactory pulp quality. 
     It is also an object of the present invention to provide a refiner plate for the face of a refiner disc, which directs the material to be refined onto the refining surfaces of the disc. 
     Other objects and advantages of the invention will become apparent from the drawings and specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings in which: 
     FIG. 1 is an elevation view of a refiner plate segment having a steam management system in accordance with the invention; 
     FIG. 2 is an enlarged elevation view of Area  2  of FIG. 1 
     FIG. 3 is an enlarged section view along line  3 — 3  of FIG. 1; 
     FIG. 4 is an enlarged section view along line  4 — 4  of FIG. 1; 
     FIG. 5 is an enlarged elevation view of an alternate embodiment of the surface dam and bar of FIG. 3; 
     FIG. 6 is an elevation view of an alternate embodiment of the refiner plate segment of FIG. 1; and 
     FIG. 7 is an enlarged section view along line  7 — 7  of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to the drawings wherein like numerals represent like parts throughout the several figures, a refiner plate having a steam management system in accordance with the present invention comprises a plurality of refiner plate segments  10  which are securable to the front face of a substantially circular refiner disc  12  (FIG.  3 ). Although in the illustrated embodiment each segment  10  has two zones  14 ,  16  each having a differently oriented set of patterns, each segment  10  could alternatively have a single, three or more zones having respective sets of patterns (FIG.  1 ). 
     The plate segments  10  are attached to the disc face, in any convenient or conventional manner, such as by bolts (not shown) passing through bores  18 . One end of the bolt engages the disc and at the other end has head structure bearing against a countersunk surface  20 . The disc  12 , only a portion of which is shown, has a center about which the disc rotates, and a substantially circular periphery. The refiner plate segments  10  are arranged side-by-side on the face of the disc  12 , to form a substantially annular refiner face  22 , shown generally at (FIG.  3 ). The refiner face  22  forms a portion of a refiner region, when confronting another refiner plate (not shown) carried by another disc. 
     With reference to FIG. 1, each refiner plate segment  10  has an inner edge  24  near the center of the disc, an outer edge  26  near the periphery of the disc, and trailing and leading side edges  28 ,  30  which abut the leading and trailing side edges of adjacent refiner plate segments, respectively. The remainder of this description will refer to a single plate segment  10 , but it should be understood that all the segments  10  which define the annular plate, are preferably substantially similar. The plate segment  10  has, on its face, at least one, and preferably two or three, distinct patterns of bars  32  and grooves  34  between the bars, whereby material to be refined can flow in the grooves  34  in the general direction from the inner edge  24  to the outer edge  26  of the plate segment  10 . The bars  32  and grooves  34  extend substantially radially, i.e., radially, or parallel to a radius of the disc  12 , or obliquely at an acute angle to such a radius. 
     A first or inlet zone  14  has a multiplicity of bars  36  and grooves  38  between adjacent bars  36 , all of which extend substantially in the radial direction. This pattern is especially adapted for receiving wood chips, wood pulp, or the like and performing an initial refining operation thereon to reduce the size of the material and funnel it radially outward into a second, refining zone  16 . The refining zone  16  has a multiplicity of bars  40  and grooves  42  between adjacent bars  40 , which also extend in parallel, substantially radially. A third, outer zone may be provided between the refining zone  16  and the outer edge  26  of the plate. As shown in FIG. 1, each zone  14 ,  16  may comprise a plurality of fields  44 ,  44 ′, where each field  44 ,  44 ′ has a uniform pattern. In the embodiment shown in FIG. 1, the segment  10  has two fields  44 ,  44 ′ in each zone  14 ,  16 . The patterns promote the flow of steam radially outward to the outer edge of the disc and radially inward to the inner edge of the disc for evacuation while retarding the flow of material to ensure that the material is fully refined. 
     Since the disc  12  and plate rotate, the partially refined material is directed, as a result of centrifugal force, radially outward. Substantial quantities of steam are also generated in the refining zone  16  producing a steam flow with high radial velocity. Especially with relatively large discs, the centrifugal forces acting on the steam and partially refined chips increase dramatically as the material moves farther and farther radially outward. Although it is highly desirable that the steam be quickly exhausted from the refining region, it is essential that the partially refined fibers not be prematurely exhausted along with the steam. This condition is influenced by the radial pressure profile along the disc face due to steam generated by the refining at high consistency. Since the pressure peak is between the inner and outer edges  24 ,  26  of the plate, the steam flows forward (radially outward) from the outer side of the pressure peak and backward (radially inward) inside the pressure peak, against the material feed. 
     With reference to FIGS. 1 to  4 , the refining zone  16  of a first embodiment of refiner plate segment  10  includes a plurality of partial dams  46  disposed in each of the grooves  42 . Each dam  46  includes a full height portion  48  having a top surface  50  which is substantially co-planar with the top grinding surface  52  of the bars  40  and a partial height portion  54  having a top surface  56  disposed intermediate the top surface  52  of the bars  40  and the base  58  of the grooves  42 . The full height portion  48  extends from the leading surface  60  of an adjacent trailing bar  62  to a point P intermediate the leading edge  64  of the trailing bar  62  and the trailing edge  66  of an adjacent leading bar  68 . The partial height portion  54  extends from point P to the trailing face  70  of the leading bar  68 . The relative width of the full height portion  48  and the partial height portion  54 , the height of the partial height portion  54 , and the shape of the partial dam  46  are dependent on the characteristics of the material to be refined and the degree of refining that is desired. For example, point P may be substantially mid way between the leading edge  64  of the trailing bar  62  and the trailing edge  66  of the leading bar  68  and the partial height portion  54  may extend substantially half way up the height of the groove  42 , as shown in FIG.  3 . 
     The fibers or material in the groove  42  has a tendency to travel along the leading surface or face  60  of the trailing bar  62 . Therefore, the full height portion  48  is positioned adjacent the leading face  60  of the trailing bar  62  to block the flow of material through the grooves  42  and to force such material up into the gap between the opposed refiner plates for further refining. As shown in FIG. 4, the dam  46  may have a radially extending ramp face  72  to guide the material out of the groove  42  and into the gap. The steam flowing through the passage  74  provided by the partial height portion  54  adjacent the trailing face  70  of the leading bar  68  carries very little material. Therefore, the amount of material that bypasses each partial dam  46  is very small. 
     With reference to FIG. 5, a trailing portion of each leading bar  68 ′ may also be removed to form a cavity  76  which extends the cross-sectional area of the passage  74 ′ defined by the partial height portion  54  of the dam  46 . Preferably, the cavity  76  forms an arcuate-shape trailing face segment  78  adjacent the partial dam  46 . Such a shape minimizes the turbulence on the inboard side of the partial dam  46  which could cause additional material to be entrained in the steam that passes through the flow passage  74 ′. Similarly, the bottom surface  80  of the cavity  76  is substantially co-planar with the top  56  of the partial height portion  54  to provide a relatively smooth surface that minimizes turbulence. The additional flow area provides for increased steam flow with little or no additional bypass flow of material. 
     U.S. patent application Ser. No. 08/886,310, filed Jul. 1, 1997, assigned to the assignee of the subject invention and hereby incorporated by reference, discloses a refiner plate having a variable pitch. One or more fields in the refining zone of such a refiner plate may have a combination of wide, deep grooves  82  and shallow, narrow mini-grooves  84 . The wider and deeper grooves  82  provide a greater cross-sectional area for the flow of steam. The blades  86  created by the mini-grooves  84  define additional grinding surfaces and cause additional pulsations during refining operations, increasing the grinding efficacy of the refiner disc. With reference to FIGS. 6 and 7, a variable pitch refiner may also include partial dams  46 ′ in accordance with the subject invention. The mini-grooves  84  do not account for a significant portion of the steam flow. Therefore, the partial dams  46 ′ would be located only in the wide grooves  82 . Each dam  46 ′ includes a full height portion  48 ′ having a top surface  50 ′ which is substantially co-planar with the top grinding surface  52 ′ of the blades  86  and a partial height portion  54 ′ having a top surface  56 ′ disposed intermediate the top surface  52 ′ of the blades  86  and the base  58 ′ of the wide grooves  82 . 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. For example, the present invention may also advantageously implemented on a three zone segment. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.