discs about an axis of rotation u201c l u201d 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 a sector of the refining plate 22 is shown broken out in fig7 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 fig8 and 9 where it is referenced as 20 a a face 21 of the sector 20 a is seen in plan in fig8 and faces upwardly in fig9 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 moreover while the bars 24 are essentially linear as viewed in fig8 bars can have other configurations particularly curved configurations as can grooves and dams the sector 20 a as shown spins in the.