Abstract:
A pulp lifter assembly for a rotary grinding mill includes an outer pulp lifter having walls defining a pulp lifter chamber and an outlet opening for radially inward discharge of slurry from the pulp lifter chamber, and an inner discharger disposed radially inward of the outer pulp lifter and circumferentially offset from the outer pulp lifter. The inner discharger defines a passage for conveying slurry substantially radially inward. A transition discharger is disposed radially between the outer pulp lifter and the inner discharger. The transition discharger has a first wall bounding an interior space and a second wall dividing the interior space into first and second regions. The second wall includes a guide that bounds a channel connecting the outlet opening of the outer pulp lifter to the passage defined by the inner discharger.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter of this application relates to apparatus for discharging material from a rotary mill that is used for grinding or comminution. 
         [0002]      FIGS. 1 and 2  show a rotary grinding mill  1  that contains material  2  to be ground therein with the aid of grinding media. The mill  1  is arranged to rotate around a rotation axis  3 . The mill has a feed trunnion  4  and a discharge trunnion  5  by which the mill is supported on bearings (not shown) to a mechanical ground. The material  2  to be ground in the mill is fed into a grinding chamber of the mill  1  through the feed trunnion  4 . Water is advantageously also fed into the mill  1  in order to create a wet grinding in the mill  1 . Between the grinding chamber and the discharge trunnion  5  of the mill  1 , a framework  6  is installed inside the mill  1  and supported to the body  7  of the mill  1 . The framework  6  supports a pulp lifter assembly that comprises guide members  8 ,  9  and a discharge cone  10 . The pulp lifter assembly directs the ground material from the grinding chamber to the discharge trunnion  5  of the mill  1 . As illustrated in  FIG. 2 , the pulp lifter assembly comprises several sequential pulp lifters  11 . Each pulp lifter  11  is attached to a grate or screen  12  having holes  13  through which the ground material  2  passes and enters a slurry pocket of the pulp lifter. As illustrated in  FIG. 1 , at least one pulp lifter  11  is at least partly immersed into the material  2  at a time during the operation of the mill  1 . The pulp lifter  11  has a substantially rectangular or trapezoidal external shape so that two external sides or edges  21  of the pulp lifter  11  are essentially parallel and two other external sides or edges  22  are convergent to each other. The pulp lifter  11  is installed in the mill  1  so that the longer external side of the two parallel sides  21  is radially outward of the shorter of the two parallel sides and is close to the body  7  of the mill  1 . 
         [0003]      FIGS. 3 to 5  illustrate two pulp lifters  11 A,  11 B partially connected to each other. Each pulp lifter  11  has a first section  15  and a second section  16  separated by a wall  23 . The grate or screen  12  with screening holes  13  is installed in front of the first section  15  of the pulp lifter  11  in the proceeding direction  19  of the material. Between the first section  15  of the pulp lifter  11 B and the second section  16  of the pulp lifter  11 A there is an opening  17 . The second section  16  of each pulp lifter  11  is provided with a guide member  18 , which extends from a point in the vicinity of the radially outer end of the leading edge  22  of the pulp lifter (with respect to the direction of rotation  24  of the mill) to a point in the vicinity of the radially inner end of the trailing edge  22  of the pulp lifter. As shown in the drawings, the guide member is constructed so that at least the part starting from the inlet of the second section is curved over at least 25% of the total length of the guide member. The outer end of the guide member (or the leading end in the direction of rotation of the mill) is directed tangentially of the mill whereas the inner or trailing end is directed essentially towards the rotating axis  3  of the mill  1 . 
         [0004]    During the operation of the mill  1 , the mill  1  is rotated around its rotation axis  3  and the pulp lifters  11  are one after another immersed into the ground or comminuted material  2 . While a given pulp lifter (such as the pulp lifter  11 A) is immersed, some of the material  2  flows through the sieve or screen  12  into the first section  15  of the pulp lifter  11 A. As the mill  1  continues to rotate, the first section  15  is step by step lifted from its immersed state, and the material in the first section  15  of the pulp lifter  11 A flows downward into the second section  16  of the pulp lifter  11 B through the opening  17 . Owing to the guide member  18  in the second section  16  of the pulp lifter  11 B the material flow is directed towards the center of the mill  1  and further by means of the guide members  8 ,  9  and  10  into the discharge trunnion  5  of the mill  1  and to the further processing of the material  2 . 
         [0005]    As the pulp lifter  11 A rises, material that is in the radially outer region of the first section  15  flows downwards (see the arrow  19  in  FIG. 4 ) into the second section  16  of the pulp lifter  11 B through the opening  17  and is directed towards the central axis of the mill by the guide member. As the pulp lifters continue to rise, the material in the section  16  of the pulp lifter  11 B is further directed towards the central axis and is discharged from the pulp lifter onto the guide members  8  and  9 , which direct the material onto the cone  10 . The material is unable to accumulate or collect in the outer lower corner region of the section  16 . 
         [0006]    The mill shown in  FIGS. 1-5  rotates in the counter clockwise direction as seen in  FIG. 2 . Let us consider the situation where the pulp lifter  11 A is at the 6 o&#39;clock position (directly below the axis of rotation of the mill). In this case, several holes  13  in the grate  12  are immersed in the slurry and slurry enters the first section  15  of the pulp lifter  11 A. Slurry also flows through the opening  17  into the second section  16  of the pulp lifter  11 B, but cannot enter the lower rear (outer trailing) corner region of the second section because that region is blocked by the guide member  18 . As the mill rotates from the 6 o&#39;clock position towards the 3 o&#39;clock position, the orientation of the pulp lifter  11 A changes and some of the holes in the forward rows are exposed above the slurry while at least the radially outermost hole of the trailing row remains immersed. Since the slurry on the upstream side of the grate and the slurry in the first section  15  are in communication, pressure equilibrium between the upstream side of the grate and the first section is attained if the slurry in the first section of the pulp lifter flows downwards as the pulp lifter  11 A rises, so that the free surface of the slurry in the pulp lifter tends to remain always lower than the free surface of the slurry on the upstream side of the grate keeping the flow gradient across the grate. In case the mill is fed more material  2  than the designed capacity of the pulp lifters, there is a possibility that some slurry will flow back out of the first section to the upstream side of the grate, but because the opening  17  is much larger than the holes  13  the major effect will be that the equilibrating flow will pass through the opening  17  into the second section  16  of the pulp lifter  11 B. Further, because of the curved shape of the guide member, the lowest point in the available space in the second section  16  of the pulp lifter  11 B, i.e. the space that is not blocked by the guide member  18 , will move radially inwards, towards the central axis of the mill, as the mill rotates from the 6 o&#39;clock position towards the 3 o&#39;clock position instead of remaining in the lower outer corner of the second section. Depending on the depth of the slurry on the upstream side of the grate, some of the slurry in the second section may overflow the radially inner end of the guide member  18  and move towards the guide cone  10 . In any event, when the pulp lifter  11 A reaches the 3 o&#39;clock position substantially all the slurry will have passed into the second section of the pulp lifter  11 B and much of the slurry will have moved from the pulp lifter  11 B towards the guide cone and as the pulp lifter reaches the 12 o&#39;clock position, slurry will fall downward from the pulp lifter onto the guide cone  10 . 
         [0007]      FIG. 6  illustrates a practical implementation of the pulp lifter that is shown more schematically in  FIGS. 3-5 . Viewing the pulp lifter along the axis of rotation of the mill, the pulp lifter has a continuous back wall  24 , an inner edge wall  25  formed with a discharge opening (not shown), and a leading edge wall  26 . The pulp lifter is open at its front side. An intermediate wall  23  is spaced from the back wall  24  and is connected to the back wall by the guide  18 . The guide  18  and the intermediate wall  23  separate the first section  15  of the pulp lifter from the second section  16 . The leading edge wall  26  is formed with transfer openings  17 . The grate (not shown) is attached to the pulp lifter using fasteners that engage holes  27  in the leading edge wall. When multiple pulp lifters are installed in a grinding mill, the first section  15  of the leading pulp lifter communicates with the second section  16  of the following pulp lifter through the transfer openings  17  in the leading edge wall  26  of the following pulp lifter. In operation, slurry enters the first section  15  of a pulp lifter through the holes in the grate as the lifter passes through the 6 o&#39;clock position. As the pulp lifter rotates towards the 3 o&#39;clock position, the pulp lifter rises relative to the following pulp lifter and slurry in the first section  15  of the leading pulp lifter flows through the transfer openings  17  into the second section  16  of the following pulp lifter. As the pulp lifters continue to rotate, the slurry in the second section of the following pulp lifter flows along the guide  18  and flows through the opening in the inner edge wall  25  towards the cone  10 , as explained above. The configuration of the guide  18  is somewhat different in  FIG. 6  from  FIGS. 3-5 , in that the radially outer end of the guide is not tangential to the periphery of the mill, but the essential function of the guide, preventing comminuted material from remaining against the peripheral wall of the mill as the pulp lifter rotates from the 6 o&#39;clock position towards the 3 o&#39;clock position, is the same. 
         [0008]      FIGS. 7 and 8  illustrate another pulp lifter. The pulp lifter shown in  FIGS. 7 and 8  is similar to that shown in  FIG. 6  except that the intermediate wall  23  is not coextensive with the back wall  24  but extends only over the second section  16  of the pulp lifter. Thus, the space between the back wall and the intermediate wall that is not available to slurry in the lifter shown in  FIG. 6  because of the guide  18  is part of the first section in the lifter shown in  FIGS. 7 and 8 . 
         [0009]    Consequently, the area available for transfer of slurry from the first section  15  to the second section  16  via the transfer opening  17  is greater in the case of  FIGS. 7 and 8  than in the case of  FIG. 6 . In addition, it will be appreciated that when multiple pulp lifters as shown in  FIG. 6  are installed, the trailing edge wall  28  of the leading pulp lifter partially blocks the transfer openings  17  of the following pulp lifter, and only the portion forward of the dashed line  29  shown in  FIG. 6  is available for flow of slurry. In the case of  FIGS. 7 and 8 , for a pulp lifter of similar size the transfer openings  17  of the following pulp lifter are of greater effective area because they are not partially blocked by the leading pulp lifter. 
         [0010]    The use of the guide  18  in the pulp lifters shown in the drawings is advantageous for several reasons. First, the transfer of slurry from the first section  15  to the second section  16  through the transfer opening prevents flowback through the grate from the second section as the pulp lifter rises from the 6 o&#39;clock position to the 3 o&#39;clock position. Second, by preventing accumulation of material in the outer trailing area of the pulp lifter, the guide  18  ensures that there is minimal carryover of pebbles and slurry as the mill rotates. 
         [0011]    The pulp lifter assembly described in U.S. Pat. No. 7,566,017 includes a pulp lifter structure that comprises an outer pulp lifter, an inner pulp lifter, and a discharger. Referring to  FIGS. 9-13  of the drawings, in which the pulp lifter structure is oriented so that it rotates in the clockwise direction when viewed along the axis of rotation of the mill from the feed trunnion, the outer pulp lifter has a leading wall  102 , a radially outer wall  104 , a radially inner wall  106 , an axially downstream wall  108 , and an intermediate wall  110  that is generally parallel to and spaced from the axially downstream wall  108  and is connected to the axially downstream wall by a curved guide  112 . The walls  102 - 110  and the guide  112  define an inlet chamber  115  that is open towards the viewer and to the right of the figure. The leading wall  102  is formed with a transfer opening  117  ( FIG. 9A ) that provides access to an outlet chamber  116  defined between the intermediate wall  110  and the axially downstream wall  108  and bounded by the guide  112 . The radially inner wall is formed with an outlet opening  119 . Multiple outer pulp lifters as shown in  FIGS. 9 and 9A  are attached to the axially downstream wall of the mill in an annular array. The inlet chamber  115  of a leading pulp lifter communicates with the outlet chamber  116  of a following pulp lifter via the transfer opening  117  in the wall  102  of the following pulp lifter. 
         [0012]    Referring to  FIG. 10 , inner pulp lifters  120  are attached to the axially downstream wall of the body of the mill in an annular array inward of the outer pulp lifters  100 . There is one inner pulp lifter  120  for each two adjacent outer pulp lifters  100 . Each inner pulp lifter  120  comprises an axially downstream wall  122  and two radial walls  124 , the radial walls  124  being aligned respectively with the leading walls  102  of two adjacent outer pulp lifters  100 . Each two adjacent radial walls  124  of an inner pulp lifter define a channel  126  into which the outlet opening of an outer pulp lifter debouches. Similarly, the following radial wall  124  of a leading inner pulp lifter and the leading radial wall of a following inner pulp lifter define a channel into which the outlet opening  119  of an outer pulp lifter debouches. 
         [0013]    The pulp lifter structure further comprises dischargers  130  ( FIGS. 11 and 12 ) that are attached to the axially downstream wall of the mill in an annular array inward of the inner pulp lifters  120 . Each discharger has an axially downstream wall  132  and two radial walls  134  and  136  projecting from the wall  132 . Each discharger defines a discharge channel between its two radial walls  134 ,  136 , and each two adjacent dischargers define a discharge channel between the following wall  136  of the leading discharger and the leading wall  134  of the following discharger. It will be noted from  FIG. 11  that the leading wall  134  is radially shorter than the following wall  136 . The channel defined between the two walls  134 ,  136  of the discharger, and the channel defined between the wall  134  of the leading discharger and the wall  136  of the following discharger, open into a discharge space defined between the wall  136  of the leading discharger and the wall  136  of the following discharger. The axially downstream wall  132  of the following discharger is formed with an opening  138  that communicates with the discharge space defined between the following wall  136  of the following discharger and the wall  136  of the leading discharger. 
         [0014]    Referring to  FIG. 12 , a center liner  140  is attached to the inner pulp lifter  120  and a grate plate  150  is attached to the outer pulp lifter  100 . The grate plates  150  collectively form the grate of the grinding mill. 
         [0015]    In operation, as the mill rotates and an outer pulp lifter approaches the 6 o&#39;clock position, slurry (which may include pebbles) enters the inlet chamber through the openings  152  in the grate plate. As the outer pulp lifter moves towards the 9 o&#39;clock position, the outer pulp lifter rises relative to the following pulp lifter and slurry in the inlet chamber  115  of the leading pulp lifter flows through the transfer opening  117  in the leading wall of the following outer pulp lifter and enters the outlet chamber  116  of that pulp lifter. As the mill continues to rotate, the slurry in the outlet chamber of the outer pulp lifter flows along the guide  112  and flows through the opening  119  in the radially inner wall  106  into the channel  126  of the inner pulp lifter, and ultimately into the discharger  130 . Most of the slurry leaves the discharger through the opening  138  and moves towards the guide cone (not shown). 
         [0016]    The speed with which particles in the pulp lifter move towards the dischargers  130  influences the efficiency of the pulp lifter structure, in that higher velocity particles are likely to reach the discharge space by the time that the discharger attains the 12 o&#39;clock position, whereas lower velocity particles are more likely to be impeded by friction against the trailing wall that bounds the discharge channel of the inner pulp lifter or discharger  130 , so that the particles do not reach the discharge space by the time the discharger attains the 12 o&#39;clock position, and are more likely to be carried over and remain in the pulp lifter structure during the next revolution of the mill. 
         [0017]    The velocity that is attained by particles moving towards the discharger  130  depends on the curvature of the guide  112  and the angular extent of the guide about the axis of rotation of the pulp lifter structure. For larger values of the curvature of the guide, a particle moves with greater velocity radially inward along the guide as the pulp lifter rises. Similarly, for larger values of the angular extent of the guide about the axis of rotation of the pulp lifter, the particle is subject to the influence of the guide over a greater proportion of the revolution of the pulp lifter. However, ease of fabrication of the components of the pulp lifter structure, and ease of assembly, are facilitated if the pulp lifter has a smaller angular extent about the axis of rotation. The pulp lifter structure described with reference to  FIGS. 9-12  is designed such that there are 32 individual pulp lifters distributed about the axis of rotation of the mill. Consequently the guide  112  of each pulp lifter has an angular extent of 11.25°. It would be desirable to increase the angular extent of the guide if this could be achieved without adversely affecting the manufacturability of the pulp lifter structure. 
       SUMMARY OF THE INVENTION 
       [0018]    In accordance with a first aspect of the disclosed subject matter there is provided a pulp lifter assembly for a rotary grinding mill, the pulp lifter assembly comprising an outer pulp lifter including walls defining a pulp lifter chamber and an outlet opening for radially inward discharge of slurry from the pulp lifter chamber, an inner discharger disposed radially inward of the outer pulp lifter and circumferentially offset from the outer pulp lifter, the inner discharger defining a passage for conveying slurry substantially radially inward, and a transition discharger disposed radially between the outer pulp lifter and the inner discharger, wherein the transition discharger comprises a first wall bounding an interior space, and a second wall dividing the interior space into first and second regions, wherein the second wall includes a guide that bounds a channel connecting the outlet opening of the outer pulp lifter to the passage defined by the inner discharger. 
         [0019]    In accordance with a second aspect of the disclosed subject matter there is provided a pulp lifter assembly for a rotary grinding mill, the pulp lifter assembly comprising: at least first and second outer pulp lifters each including walls defining walls defining an outer pulp lifter chamber and defining an outlet opening for radially inward discharge of slurry from the pulp lifter chamber, an inner discharger disposed radially inward of the outer pulp lifters and circumferentially offset from the first outer pulp lifter, the inner discharger defining a passage for conveying slurry substantially radially inward, and a transition discharger disposed radially between the outer pulp lifters and the inner discharger, wherein the transition discharger comprises: a first wall bounding an interior space, and a second wall dividing the interior space into first and second regions, wherein the second wall includes a first guide that bounds a first channel connecting the outlet opening of the first outer pulp lifter to the passage defined by the inner discharger and also bounds a channel connecting the outlet opening of the second outer pulp lifter to a second passage bounded by the inner discharger. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
           [0021]      FIG. 1  shows a sectional side view of a rotary grinding mill in accordance with the prior art, 
           [0022]      FIG. 2  is a sectional view of the grinding mill taken on the line A-A of  FIG. 1 , 
           [0023]      FIG. 3  shows a schematic front view of two pulp lifter units of the grinding mill shown in  FIG. 1 , 
           [0024]      FIG. 4  shows the structure of  FIG. 3  in section taken on the line B-B, 
           [0025]      FIG. 5  shows the structure of  FIG. 3  as a schematic side view, 
           [0026]      FIG. 6  is a perspective view of a second pulp lifter in accordance with the prior art, 
           [0027]      FIG. 7  is a perspective view of a third pulp lifter in accordance with the prior art, 
           [0028]      FIG. 8  is a perspective view illustrating the manner in which the pulp lifter shown in  FIG. 7  cooperates with other pulp lifters of similar structure, 
           [0029]      FIG. 9  is a perspective view of a component of a fourth pulp lifter structure in accordance with the prior art, 
           [0030]      FIG. 9A  is a view of the component shown in  FIG. 8  taken on the line  9 A- 9 A of  FIG. 9 , 
           [0031]      FIGS. 10-12  are perspective views of the fourth pulp lifter structure at different stages of assembly, 
           [0032]      FIG. 13  is a view similar to  FIG. 2  of a pulp lifter assembly embodying the subject matter disclosed in this application, 
           [0033]      FIGS. 14-16  are enlarged perspective views of the pulp lifter assembly shown in  FIG. 13  at different stages of assembly, and 
           [0034]      FIG. 17  is an enlarged partial view of a further pulp lifter assembly embodying the subject matter disclosed in this application. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]      FIGS. 13-16  illustrate a pulp lifter assembly that comprises an annular array of outer pulp lifters  200 , similar to the pulp lifters  100  shown in  FIGS. 11 and 12 , and a circular arrangement of inner dischargers  230 , similar to the dischargers  130  shown in  FIGS. 11 and 12 . Each inner discharger  230  defines a discharge channel between its two radial walls  234 ,  236 , and each leading discharger and the adjacent following discharger define a discharge channel between the wall  236  of the leading discharger and the wall  234  of the following discharger. As in the case of  FIG. 11 , the wall  234  of the following discharger is radially shorter than the wall  236  of the leading discharger. The channel defined between the two walls  234 ,  236  of a following discharger  230 , and the channel defined between the wall  234  of the following discharger and the wall  236  of the adjacent leading discharger, open into a discharge space defined between the wall  236  of the leading discharger and the wall  236  of the following discharger. The axially downstream wall (or back wall)  232  of the following discharger is formed with an opening (not shown in  FIGS. 13-16  but similar to the opening  138  shown in  FIG. 11 ) that communicates with the discharge space defined between the wall  236  of the following discharger and the wall  236  of the leading discharger. The two radial walls  234 ,  236  of each inner discharger  230  thus define a first discharge channel, and the wall  234  of a following discharger and the wall  236  of the adjacent leading discharger define a second discharge channel, which meets the discharge channel defined by the two radial walls of the following discharger at the inner end of the radial wall  234 . 
         [0036]    Referring to  FIG. 16 , a grate plate  250  is attached to the outer pulp lifter  200 . The grate plates  250  collectively form the grate of the grinding mill. 
         [0037]    Between the annular array of outer pulp lifters  200  and the circular arrangement of inner dischargers  230  is an annular array of transition dischargers  220 . For each inner discharger  230  there is a corresponding transition discharger  220 , and each transition discharger  220  is positioned between the two radii that bound the corresponding inner discharger  230 . 
         [0038]    As shown in  FIG. 13 , the pulp lifter assembly comprises sixteen inner dischargers and sixteen transition dischargers, and each transition discharger is associated with three angularly adjacent pulp lifters. One of the three pulp lifters (referred to as a center pulp lifter) is associated exclusively with the transition discharger whereas each of the other two pulp lifters (referred to as leading and trailing pulp lifters) is associated with two angularly adjacent transition dischargers. 
         [0039]    Referring to  FIG. 14 , each transition discharger  220  includes a back wall  221  lying substantially parallel and coplanar with the back wall  232  of the inner discharger module and three walls  222 - 224  projecting substantially perpendicularly to the back wall  221 . The back wall  221  includes attachment structures  221 A for receiving fasteners for attaching the transition discharger to the frame of the body of the mill. The back wall has two radial edges and inner and outer peripheral edges. 
         [0040]    The projecting wall  222  extends the entire distance from the outer peripheral edge of the back wall to the inner peripheral edge of the back wall and includes attachment structures  222 A at each end for receiving fasteners that attach a liner  240  ( FIG. 16 ) to the back wall of the transition discharger. The projecting wall  222  is curved, its leading side being concave and its trailing side being convex. The radially outer end of the leading side of the wall  222  is adjacent the leading side of the outlet opening  219  in the leading pulp lifter, whereas the leading side of the inner end of the wall is substantially flush with the leading side of the wall  236  of the inner discharger  230 . 
         [0041]    The projecting wall  222  may be considered to be composed of inner and outer segments that meet at a radius that is midway between the radial edges of the back wall  221 . The projecting wall  223 , including the attachment structure  223 A, corresponds in configuration to the inner segment of the wall  222  and extends from the leading radial edge of the back wall to the inner peripheral edge of the back wall. The projecting wall  224 , including the attachment structure  224 A, corresponds in configuration to the outer segment of the wall  222  and extends from the outer peripheral edge of the back wall to the trailing radial edge of the back wall. Thus, as shown in the drawings, the projecting walls  223  and  224  of a following transition discharger and a leading transition discharger respectively together have substantially the configuration of the projecting wall  222  of a transition discharger. The walls  222  and  223  of a center transition discharger and the wall  224  of the leading transition discharger form a first channel and the walls  222  and  224  of the center transition discharger and the wall  223  of a following transition discharger form a second channel. The two channels extend from the outer peripheral edge of the annular array of transition dischargers to the inner peripheral edge of the annular array of transition dischargers and the trailing walls defining the respective channels are curved such that the inner end of the trailing wall trails the outer end of that wall. 
         [0042]    The liner  240  of the transition discharger covers the channels defined between the wall  222  and the walls  223  and  224 . The liner is formed with holes for receiving fasteners that attach the liner to the attachment structures  222 A,  223 A and  224 A and with attachment eyes for facilitating handling of the transition discharger. 
         [0043]    In operation of the pulp lifter assembly, each pulp lifter  200  in turn rotates through the 6 o&#39;clock position, in which slurry enters the pulp lifter through holes  252  in the grate plate  250 . As the pulp lifter rotates towards the 9 o&#39;clock position, the pulp lifter rises relative to the following pulp lifter and slurry in the first section  215  of the leading pulp lifter flows through the transfer openings (not shown in  FIGS. 13-16 ) into the second section  216  of the following pulp lifter, as described with reference to  FIGS. 9-12 . As the pulp lifters continue to rotate, the slurry in the second section  216  of the following pulp lifter flows along the leading side of the guide  218  and flows through the opening  219  in the inner edge wall towards the annular array of transition dischargers. Depending on the angular position of the pulp lifter relative to the transition dischargers, the slurry either enters the channel between leading side of the wall  222  of a following transition discharger and the trailing side of the wall  224  of a leading transition discharger, or enters the channel between the trailing side of the wall  222  and the leading side of the wall  224  of the same transition discharger, and flows down the leading side of the wall  222  or  224 , as the case may be. The rotation of the pulp lifter assembly provides a force that tends to fling the slurry back into the outer pulp lifter, but the slope of the wall  222  (or  223  and  224 ), particularly as the pulp lifter rotates beyond the 10 o&#39;clock position, provides a centripetal force that resists outward movement of the slurry, and the slurry falls under the force of gravity into the inner discharger and passes towards the discharge cone. 
         [0044]    It will be appreciated from inspection of  FIGS. 13-16  that a particle that enters a channel of the transition discharger, for example at the 10 o&#39;clock position, will be accelerated more strongly than would be the case in the event that the projecting walls were radial, as shown in  FIGS. 9-12 . Accordingly, the particle attains a higher velocity before it reaches the 12 o&#39;clock position, and there is a greater likelihood that the particle will be discharged from the pulp lifter instead of being carried over for a second revolution of the mill. 
         [0045]    The pulp lifter assembly described with reference to  FIGS. 13-16  includes only one annular array of transition dischargers  220 . In a modification of the pulp lifter assembly shown in  FIGS. 13-16 , there may be two (or more) arrays of transition dischargers between the annular array of outer pulp lifters and the circular arrangement of inner dischargers. Thus,  FIG. 17  illustrates a pulp lifter assembly including an array of outer transition dischargers  320  and an array of inner transition dischargers  340  between the pulp lifters  300  (which are essentially the same as the pulp lifters  200 ) and the inner dischargers  330 . 
         [0046]    As shown in  FIG. 17 , each outer transition discharger  320  is associated with three angularly adjacent pulp lifters  300 . The center pulp lifter is associated exclusively with the outer transition discharger whereas each of the other two pulp lifters is associated with two angularly adjacent outer transition dischargers. The outer transition discharger  320  includes a back wall  321  and two walls  322 ,  324  projecting substantially perpendicularly to the back wall. The back wall  321  includes attachment structures (not shown) for receiving fasteners for attaching the outer transition discharger to the frame of the body of the mill. The back wall has two radial edges and inner and outer peripheral edges. 
         [0047]    The projecting walls  322 ,  324  each extend the entire distance from the outer peripheral edge of the back wall  321  to the inner peripheral edge of the back wall and include attachment structures (not shown) for receiving fasteners that attach a liner (not shown, but similar in function to the liner  240  shown in  FIG. 16 ) to the back wall of the transition discharger. Each of the projecting walls  322 ,  324  is curved, its leading side being concave and its trailing side being convex. The radially outer end of the leading side of the wall  322  is adjacent the trailing side of the outlet opening of the leading pulp lifter whereas the radially outer end of the leading side of the wall  324  is adjacent the trailing side of the outlet opening of the center pulp lifter. The two projecting walls  322 ,  324  of an outer transition discharger define a first transition channel whereas the wall  322  of a given outer transition discharger and the wall  324  of an adjacent leading outer transition discharger define a second transition channel. 
         [0048]    The inner transition discharger  340  shown in solid lines in  FIG. 17  is associated with two adjacent outer transition dischargers  320 . One of the associated outer transition dischargers is illustrated in solid lines and is referred to as the aligned outer transition discharger. The other associated outer transition discharger is shown only partially, in dashed lines, and is referred to as the leading outer transition discharger. The inner transition discharger  340  includes a back wall  341  and two walls  342 ,  344  projecting substantially perpendicularly to the back wall. The back wall  341  includes attachment structures (not shown) for receiving fasteners for attaching the inner transition discharger to the frame of the body of the mill. The back wall has two radial edges and inner and outer peripheral edges. 
         [0049]    The projecting walls  342 ,  344  each extend the entire distance from the outer peripheral edge of the back wall  341  to the inner peripheral edge of the back wall and include attachment structures (not shown) for receiving fasteners that attach a liner (not shown, but similar in function to the liner  240  shown in  FIG. 16 ) to the back wall of the transition discharger. Each of the projecting walls  342 ,  344  is curved, its leading side being concave and its trailing side being convex. The radially outer end of the wall  342  is adjacent the radially inner end of the wall  322  of the aligned outer transition discharger whereas the radially outer end of the wall  344  is adjacent the radially inner end of the wall  324  of the leading outer transition discharger. The two projecting walls  342 ,  344  of an inner transition discharger define a first transition channel, as an extension of the second transition channel defined by the wall  322  of the aligned outer transition discharger and the wall  324  of the leading outer transition discharger, whereas the wall  344  of a given inner transition discharger and the wall  342  of the adjacent leading inner transition discharger define a second transition channel, as an extension of the first transition channel defined by the walls  322 ,  324  of the leading outer transition discharger. 
         [0050]    The inner discharger  330  is associated with an aligned inner transition discharger  340  and a leading inner transition discharger and includes a back wall  331  and three walls  332 ,  334 ,  336  projecting substantially perpendicularly to the back wall. The back wall  331  includes attachment structures (not shown) for receiving fasteners for attaching the outer transition discharger to the frame of the body of the mill. The back wall has two radial edges aligned respectively with the radial edges of the back wall of the aligned inner transition discharger. 
         [0051]    The projecting wall  334  extends from a location about half way along the outer peripheral edge of the back wall  331  to a location about half way along the trailing radial edge of the back wall  331 . At its radially outer end, the wall  334  is aligned with the radially inner end of the wall  344  of the aligned inner transition discharger. The projecting wall  332  is of similar configuration to the wall  334 , but extends from a location in the region of the leading end of the outer peripheral edge of the back wall to a location about half way between the outer peripheral edge of the back wall and the radially inner edge of the wall  331  and about half way between the radial edges of the back wall. The projecting wall  336  extends from a location about half way along the leading radial edge of the back wall to a location near the radially inner region of the back wall. At its radially outer end, the wall  336  is aligned with the radially inner end of the wall  334  of the leading inner discharger. Each of the projecting walls is curved, its leading side being concave and its trailing side being convex. 
         [0052]    The two projecting walls  334 ,  332  of an inner discharger define a first discharger channel, as an extension of the second transition channel defined by the wall  344  of the aligned inner transition discharger and the wall  342  of the leading inner transition discharger, whereas the wall  332  of a given inner discharger and the wall  334  of the adjacent leading inner discharger define a second discharger channel, as an extension of the first transition channel defined by the walls  342 ,  344  of the leading inner transition discharger. It will be noted that the discharger channels cross the radial boundary between adjacent inner dischargers  330 . 
         [0053]    It will be appreciated that because the projecting walls of the transition dischargers and the inner dischargers are configured so that the inner end of each wall trails the outer end of the wall, and in particular is curved so that the leading side of the wall forming the following boundary of a channel is inclined to the radius at a greater angle at radially outward positions than at radially inward positions, a particle that enters a channel of an outer transition discharger, for example at the 10 o&#39;clock position, will continue to be accelerated by gravity as the mill rotates even when the particle enters the discharger  330 . Accordingly, the particle attains a higher velocity before it reaches the 12 o&#39;clock position than it would in the case of the pulp lifter shown in  FIGS. 9-12 , and there is a greater likelihood that the particle will be discharged from the pulp lifter instead of being carried over for a second revolution of the mill. 
         [0054]    It will be appreciated that the disclosed subject matter is not restricted to the particular embodiment(s) that has (have) been described, and that variations may be made therein without departing from the scope of the subject matter as defined in the appended claims, as interpreted in accordance with principles of prevailing law, including the doctrine of equivalents or any other principle that enlarges the enforceable scope of a claim beyond its literal scope. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated. The word “comprise” or a derivative thereof, when used in a claim, is used in a nonexclusive sense that is not intended to exclude the presence of other elements or steps in a claimed structure or method.