Abstract:
Integrated first stage and second stage reduction mechanisms reduce coal, minerals, biomass, and other materials. A portion of the reductive work is done in the first stage by passing centrally fed feed material centrifugally from rotating rings to counter-rotating rings with destructive effects. The resulting material, significantly reduced in size, subsequently is stripped of oversize in the second stage by passing through a closely spaced and specially contoured final pair of annular rings or crushing elements between which particles larger than the limited space are crushed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present patent application is based on, and claims priority from, U.S. provisional application Ser. No. 60/097,813, filed Aug. 25, 1998, which is incorporated herein by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to an apparatus and method for reducing the size of coal, minerals (including ores, compounds, and elements), biomass, waste, and other material. More specifically, the invention relates to a two-stage micronizing miff for reducing the size of such materials.  
           [0004]    2. Related Art  
           [0005]    Efficient size reduction technology through application of dual counter-rotating rotary mill design is undertaken in numerous patents. U.S. Pat. Nos. 5,275,631 and 5,575,824 to Brown et al. disclose combining such mean with refuse separation means. U.S. Pat. No. 5,597,127 to Brown provides for finer milling. The present invention provides an improved method and apparatus for finer milling, and with particular regard to the problem of efficient reduction of oversize particles, provides for second stage selective milling integral to the mill itself, without classification and recirculation.  
           [0006]    Classification and recirculation requires additional mechanical means which adds to capital costs. Operating costs increase also since typically discrimination is not precise and more material than necessary is returned for re-milling, including particles milled to within size specification as well as oversize particles.  
           [0007]    The term “second-stage milling” refers to size-reduction by means of a separate type than that employed in the first stage. Here, primary milling is accomplished by attrition and impacting, while second stage reduction is accomplished by crushing, or—in the case of unfriable, fibrous materials-the crushing action results in pinching and rolling which separates fibers.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of this invention is to improve the size reduction technology for coal, minerals (including ores, compounds and elements), biomass, waste and other materials.  
           [0009]    A further object of this invention is to provide more efficient and high rate means for pulverizing coal through initial milling means followed immediately by second stage milling means for reducing oversize, in order to supply a fineness grade of 99 percent smaller than mesh and 80 percent smaller than 325 mesh, or similar grade also conducive to combustion with reduced nitrous oxides formation rates.  
           [0010]    A still further object of this invention is to provide more efficient and high rate size reduction of various forms of biomass, such as wood chips, pecan shells or hybrid willow potentially useable as boiler fuel.  
           [0011]    A further object of this invention is to provide autogenous wear resistance in milling structures.  
           [0012]    Efficient high capacity size reduction of coal, minerals, biomass and other materials is accomplished through integrated first stage and second stage reduction means, in which a portion of the reductive work is done by passing centrally fed feed material centrifugally from rotating rings to counter-rotating rings with destructive effects, and the resulting material, significantly reduced in size, subsequently is stripped of oversize by passing through a closely spaced and specially contoured final pair of annular rings or crushing elements between which particles larger than the limited space are crushed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The invention is better understood by reading the following Detailed Description of the Preferred Embodiments with reference to the accompanying drawing figures, in which like reference numerals refer to like elements throughout, and in which:  
         [0014]    [0014]FIG. 1 is a diagrammatic view, partially in cross-section, of a general two-stage mill configuration in accordance with the present invention, wherein first stage reduction occurs within a system of annular, concentric, counter-rotating rings, and second stage reduction proceeds by crushing oversize particles between the outermost of the rings.  
         [0015]    [0015]FIG. 2 is a cross-sectional view of a first embodiment of the configuration of the first stage counter-rotating rotors with annular concentric ring modifications for resisting wear by retaining barriers of process material;  
         [0016]    [0016]FIG. 3 is an enlarged perspective view, partially in cross-section, of the area designated by dashed lines in FIG. 2;  
         [0017]    [0017]FIG. 4 is a partial perspective view, partially in cross-section, of a second embodiment of the first reduction stage rotor configuration including structural elements providing both shear and impact reduction;  
         [0018]    [0018]FIG. 4A is a partial perspective view, partially in cross-section, of a third embodiment of the first reduction stage rotor configuration, which is a-variant of the second embodiment shown in FIG. 4;  
         [0019]    [0019]FIG. 5 is a partial perspective view, partially in cross-section, of a fourth embodiment of the first reductions stage rotor configuration, which is effective in reducing coal or coal combined with some forms of biomass;  
         [0020]    [0020]FIG. 6 is a partial perspective view, partially in cross-section, of a fifth embodiment of the first reduction stage rotor configuration including elements for shear and impact;  
         [0021]    [0021]FIG. 7 is a cross-sectional view of a sixth embodiment of the first-stage rotor ring configuration useful in varying shear clearance between rotors;  
         [0022]    [0022]FIG. 8 is a cross-sectional view of a configuration similar to that shown in FIG. 7, but in a cast form and also showing the location of the second-stage means;  
         [0023]    [0023]FIG. 9A is a perspective view of a first embodiment of a second-stage top-size control ring;  
         [0024]    [0024]FIG. 9B is an enlarged view of the area designated by dashed lines in FIG. 9A;  
         [0025]    [0025]FIG. 9C is a cross-sectional view taken along line  9 C- 9 C of FIG. 9B;  
         [0026]    [0026]FIG. 10 is a perspective view showing the second-stage top-size control ring of FIG. 9A combined with a primary zone of the type shown in FIG. 8;  
         [0027]    [0027]FIG. 11A shows a partial, perspective view of a second embodiment of the second-stage top-size control ring;  
         [0028]    [0028]FIG. 11B is a cross-sectional view taken along line  11 B- 11 B of FIG. 11A;  
         [0029]    [0029]FIG. 12 is a partial perspective view of a third embodiment of the lower ring segment of the embodiment of second-stage top-size control ring;  
         [0030]    [0030]FIG. 13A is a partial cross-sectional view of a fourth embodiment of the lower ring segment of the embodiment of second-stage top-size control ring;  
         [0031]    [0031]FIG. 13B is a partial perspective view of the lower surface of the upper milling ring of FIG. 13A;  
         [0032]    [0032]FIG. 14A is a cross-sectional illustration of a fifth embodiment of the lower ring segment of the embodiment of second-stage top-size control ring;  
         [0033]    [0033]FIG. 14B is a partial perspective view of the lower surface of the upper milling ring of FIG. 14A;  
         [0034]    [0034]FIG. 15 is a cross-sectional view of an embodiment of a second-stage top-size control using a static upper ring;  
         [0035]    [0035]FIG. 16 is a cross-sectional view showing an embodiment of a second-stage top-size control ring for reducing wood-chip splinters or other elongated material;  
         [0036]    [0036]FIG. 17 is an inverted perspective view showing the upper rotor of FIG. 16; and  
         [0037]    [0037]FIG. 18 is a cross-sectional view of the upper rotor, taken along fine  18 - 18  of FIG. 17. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    In describing preferred embodiments of the present invention illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner to accomplish a similar purpose.  
         [0039]    [0039]FIG. 1 illustrates the general two-stage configuration, FIGS. 2 through 6 illustrate alternative first stage configurations, and FIGS. 7 through 10 are alternative second stage configurations.  
         [0040]    Referring now to FIG. 1, there is shown a general configuration of a two-stage micronizer unit  2  employed for integrated, two-stage micronizing in accordance with the present invention. The micronizer unit  2  includes a mill housing  4 , co-axial upper (or first) and lower (or second) rotors  10   a  and  10   b  housed within the mill housing  4 , and a center feed pipe  12  for passing material to the upper and lower rotors  10   a  and  10   b . The upper rotor is carried on a rotatable, hollow, vertical, first shaft  14   a  that surrounds the central feed pipe  12 . The first shaft  14   a  is rotated by a first motor  20   a . The lower rotor is mounted on a vertical second shaft  14   b  substantially coaxial with the first shaft  14   a , and is rotated by a second motor  20   b.    
         [0041]    Although the first and second shafts  14   a  and  14   b  are described and shown as being coaxial along a vertical axis, the present invention also contemplates a configuration wherein the first and second shafts  14   a  and  14   b  are coaxial along a horizontal axis or along a sloping axis. The upper and lower rotor would then be oriented side-by-side and the remaining components of the invention hereinafter described would be similarly re-oriented.  
         [0042]    Separate upper and lower drive transmissions  22   a  and  22   b  provide counter-rotation of the rotors  10   a  and  10   b  with respect to each other. The upper and lower drive transmissions  22   a  and  22   b  can be any of various types, including right angle gears  23 , or otherwise as discussed in more detail hereinafter.  
         [0043]    The upper and lower rotors  10   a  and  10   b  comprise, respectively, a plurality, of concentric rings  24   a  and  24   b , with diameters of successive magnitudes, such that the rings  24   a  of the first rotor  10   a  interpose between the rings  24   b  of the second rotor  10   b.  All of the concentric rings  24   a  and  24   b  include a first stage or primary milling zone  30  of the general configuration.  
         [0044]    In the primary milling zone  30 , much of the size reduction work is performed as feed material F banks up centrifugally within each of the concentric rings  24   a  and  24   b , at an angle of repose of about 60°, and subsequent process material is thrown from ring to counter-rotating ring, colliding destructively with other process material or with resident reposed material. Added velocities of opposite rotations assist efficient particle size reduction, especially when ring configurations are improved as described below.  
         [0045]    A second stage or secondary milling zone  40  includes close-running, counter-rotating upper and lower rings  42   a  and  42   b  having respective facing surfaces  44   a  and  44   b . These facing surfaces  44   a  and  44   b  can be planar and uninterrupted as shown in FIG. 1, or can have other configurations as described hereinafter. Close-running clearance between the facing surfaces  44   a  and  44   b  permits on-size or under-size material to pass without further energy expenditure. However, oversize particles are broken down on these facing surfaces  44   a  and  44   b , which are configured for that purpose, as described hereinafter. In addition, the oversize crushing surfaces move air through the entire mill, improving particle-to-particle turbulent destruction in the primary milling zone  30 . A stationary annular impact ring  46  concentric with the upper and lower rings  24   a ,  24   b ,  42   a , and  42   b  can be provided on the inner wall of the mill housing  4 . The impact ring  46  provides further size reduction upon impact, and wear-resistant protection to the inner wall of the mill housing  4 .  
         [0046]    Preferably, the primary zone  30  includes three to five sets of annular rings  24   a  or  24   b  and the secondary zone  40  includes one or two sets of annular rings  42   a  or  42   b.    
         [0047]    Referring now to FIGS. 2 and 3, there is shown a first embodiment of a first reduction stage  130  of a two-stage micronizer in accordance with the present invention. In the first embodiment, the upper and lower rotors  110   a  and  110   b  comprise, respectively, upper and lower plates  150   a  and  150   b  and a plurality of concentric rings  124   a  and  124   b  mounted respectively on the upper and lower plates  150   a  and  150   b , with diameters of successive magnitudes, such that the rings  124   a  of the first rotor  110   a  interpose between the rings  124   b  of the second rotor  110   b . Each ring  124   a  or  124   b  has an inner peripheral wall  152  facing the rotor axis, an outer peripheral wall  154  facing away from the rotor axis, and an unmounted edge  156  joining the inner and outer peripheral walls  152  and  154  and facing away from its respective upper or lower plate  150   a  or  150   b.    
         [0048]    The first shaft  114   a  is rotated by a first motor  120   a , by means of a belt drive  160 . The second rotor  110   b  is mounted on a second shaft  114   b , and is rotated by a second motor  120   b , by means of direct drive. Direct drive is the most efficient of the drive transmission types as disclosed herein.  
         [0049]    As best shown in FIG. 3, the rings  124   a  and  124   b  of the upper and lower rotors  110   a  and  110   b  are provided with cut-outs  170  spaced along their unmounted surfaces. The spacing of the cut-outs  170  is mass-balanced, that is, the cut-outs  170  are equidistant from each other, or if not equidistant, then spaced with respect to diametral lines in such a way that the mass of the rings  124   a  and  124   b  is balanced about their axis of rotation.  
         [0050]    Preferably, the cut-outs are cut to a depth measured from the unmounted surfaces of the rings  124   a  and  124   b  of between about ⅜ inch to about 1 inch in rings  124   a  and  124   b  less than about 6 inches deep overall, or about ⅛ to about ⅙ of overall ring depth in larger rings  124   a  and  124   b.    
         [0051]    Vertical bars  172  are affixed to the rings  124   a  and  124   b  adjacent each of the cut-outs  170 , to the trailing side of the cut-outs  170 , which is downstream relative to the direction of rotor rotation, and at an angle relative to radial lines extending from the center of the rings  124   a  and  124   b . In this embodiment, the rings  124   a  and  124   b  are near, but not abutting each of the cut-outs  170 . Pockets  174  are defined at the conjunctions of the rings  124   a  and  124   b  and their respective bars  172  and the spaces between their respective bars  172  and the edge of each cut-out. The bars  172  retain process material in the pockets  174 , for a purpose to be discussed hereinafter.  
         [0052]    Each of the bars  172  has an interior face  172   a  facing the rotor axis, a pair of opposed side faces  172   b , and an unmounted face  172   c  which extends from the unmounted edge of the ring. In this embodiment, the unmounted faces  172   c  are perpendicular to the side faces  172   b , while the interior faces  172   a  of the bars  172  are sloped, as shown in FIG. 3, in order to vary the proximity with the next ring on the opposed rotor by axial displacement.  
         [0053]    In addition, horizontal caps  176  extend inwardly from the unmounted edge  156  of the rings  124   a  and  124   b  over the unmounted faces  172   c  of the vertical bars  172  so as to crown the vertical bars  172 . The horizontal caps  176  enhance retention of the process material and provide a protective barrier against wear to the vertical bars  172 . The side faces  176   a  of the horizontal caps  176  are not parallel, but diverge from the interior to the exterior of the ring, to accord with the angular displacement of the bars  172  as described above. The angle at which the sides diverge can be selected according to the process material. Some materials will require deeper pockets  174  to retain protective resident process material.  
         [0054]    In a second embodiment of the first reduction stage rotor  230 , as shown in FIG. 4, the interior faces  272   a  of the bars  272  are perpendicular to the surface of the rotor  230 , and the horizontal caps  176  are omitted. The primary zone ring configuration illustrated in FIG. 4 also places the bars  272  adjacent the cut-outs  270 ; however in this embodiment, the bars  272  abut the cut-outs  270 . Improved shearing can be achieved by selecting radial clearances between the bars  272  of successive rings  224   a  and  224   b , based on the process material particle sizes. Closer radial clearance between successive rings  224   a  and  224   b  promotes shearing of material, such as some forms of biomass, passing through the cut-outs  270  of any one ring and striking the bars  272  on the succeeding ring.  
         [0055]    Retention of process material improves the wear resistance of the bars  272 . The bars  272  can be sloped on their interior faces  272   a  (nearest the axis), as shown and described with reference to FIGS. 2 and 3, or they can be perpendicular to the surface of the rotor on their interior sides, as shown and described with reference to FIG. 4.  
         [0056]    [0056]FIG. 4A shows a third embodiment  230 ′ of a first reduction stage rotor configuration, which is a variant of the second embodiment  230  shown in FIG. 4. The third embodiment  230 ′ is identical to the second embodiment  230 , except that pairs of cut-outs  270 ′ are formed in the rings  224   a ′ and  224   b ′ abutting both sides of the bars  272 ′. The pairs of cut-outs  270 ′ are placed on both sides of the bars  272 ′ so that by switching the direction of rotation of the rings  224   a ′ and  224   b ′ (by switching the direction of their respective drive motors), new surfaces will be brought into service against which process material will impact when thrown from the preceding ring  224   a ′ or  224   b ′. The process material is then re-accelerated to rim speed in the opposite direction and thrown through the cut-out  270 ′ upstream of the bar  272 ′. The advantage of this embodiment is that, rather than losing machine service time for repairs to the worn surfaces, the motors can be reversed to present new surfaces.  
         [0057]    [0057]FIG. 5 shows a fourth embodiment  330  of a first reduction stage rotor configuration, used in reducing coal or coal combined with some forms of biomass. When used for this purpose, the leading side face  372   b , of the bar  372  forms an angle with a tangent T to the ring  374   a  or  374   b  (that is, the leading side face  372   b   1  is positioned at an angle of about 3° to about 30° relative to a normal N to a tangent T at the trailing edge of the cut-out  370 ). Angling the leading side face improves the size distribution of the product, producing more superfine particles. This is believed to be due to increased air movement within the mill, promoting particle-to-particle impacts and improving size reduction by adding velocities to the process material. The interior faces  372   a  of the bars  372  are planar and beveled to make them approximately parallel to the tangent T.  
         [0058]    Referring now to FIG. 6, there is shown a fifth embodiment of the primary zone rotor ring  430 . This embodiment is characterized by the omission of cut-outs. Instead of cut-outs, the rings  424   a  and  424   b  are provided with vertical bars  472  positioned at equidistant points around the inner peripheral walls  480   a  of the rings  424   a  and  424   b . The bars  472  are higher than the rings  424   a  and  424   b , so that the unmounted faces  472   c  of the bars  472  are offset from the unmounted edges  482  of the rings  424   a  and  424   b , and in the portions which extend beyond the unmounted edges  482  of the rings  424   a  and  424   b , the bars  472  have exterior faces  472   d  that are even with the outer peripheral walls  480   b  of the rings  424   a  and  424   b . Shearing action is promoted by providing a close clearance C between the bars  472 .  
         [0059]    [0059]FIG. 7 illustrates a sixth embodiment of a first reduction stage rotor configuration  530 , for use in a mill in which it is useful to be able to vary the shear clearance between the rotor rings  524   a  and  524   b . In this embodiment, the rings  524   a  and  524   b  are provided with both cut-outs  570  and bars  572  either closely or immediately adjacent the cut-outs  570 , the inner and outer peripheral ring walls  580   a  and  580   b  are angled such that they form obtuse angles with the ring plates  550   a  and  550   b , respectively, and the bar interior faces  572   a  are parallel to the ring inner peripheral walls  580   a , such that the slope of any ring outer peripheral wall  580   b  is parallel with the slope of bar interior faces  572   b  on the opposed rotor. The angle formed by the inner and outer peripheral ring walls  580   a  and  580   b  and the ring plates  550   a  and  550   b , respectively, is less than about 120°, since the angle of repose of the retained material is about 60°, as measured on the acute side of the angle.  
         [0060]    Due to the angles of the facing surfaces of the bars  572  and the rings  524   a  and  524   b , as one rotor is displaced axially relative to the other rotor, the shear clearance between rotor rings  524   a  and  524   b  varies. By raising or lowering either of the rotors, the shear clearance can be increased or decreased. In close-running-clearance, the bars  572  and the adjacent cut-outs  570  can shear material against the edges of cut-outs  570 .  
         [0061]    [0061]FIG. 8 shows a seventh embodiment of a first reduction stage rotor configuration  630 , which is similar to the sixth embodiment shown in FIG. 7, but in the form of castings  610   a ,  610   b , and to which bars  672  of hardened material have been affixed. A “top-size control ring set,” or second-stage milling zone, can be provided radially outwardly of the upper and lower rotors of the first stage milling zone, at a position indicated by reference numeral  684 , as discussed in greater detail below.  
         [0062]    Referring now to FIGS. 9A, 9B, and  9 C, there is shown a first embodiment of a second-stage crushing ring  786   a  that forms a part of a second-stage milling zone, and which can be installed in association with the lower rotor of FIG. 8. The crushing ring  786   a  has a planar upper face into which a plurality of spaced bevels or grooves  790  are incised. The bevels or grooves  790  can extend either radially or at an angle to radii of the ring. These bevels form acute angles relative to the planar upper surface, and have a feed depth of less than ⅛ inch.  
         [0063]    A flat, hardened ring (not shown) is installed opposite on the upper rotor. Second-stage crushing of oversize particles occurs as particles and air are moved centrifugally and mechanically through the control ring set. Oversize particle reduction is accomplished as particles are caught in the sweep of the bevels on the ring.  
         [0064]    Referring now to FIG. 10, there is shown a second-stage crushing ring  786   a  identical to that shown in FIGS. 9A and 9B, instead in association with a cast upper rotor of the type shown in FIGS. 7 and 8.  
         [0065]    [0065]FIGS. 11A and 11B show a second embodiment of a second-stage milling zone  840 . This embodiment includes an uninterrupted, planar upper ring  886   a  and an opposed planar crushing lower ring  886   b , the surface of which is interrupted with radial or radially-angled bevels or grooves  890  that taper radially to a flat minimum clearance land. The uninterrupted upper ring  886   a  is mounted either independently of its associated rotor so as to be static, or dependently with its associated rotor so as to rotate therewith; whereas the interrupted lower ring  886   b  is mounted dependently with its associated rotor so as to rotate therewith, whereupon oversize particles are crushed between the land and an opposed, uninterrupted planar ring. Oversize particles and gases are moved centrifugally outward to the periphery of the ring set. In so doing particles move up the slope until they are crushed in the restricted gap which is sized to allow passage of only 100 mesh particles or smaller, in milling coal for suppressing nitrous oxides emissions in combustion. This embodiment differs from that shown in FIGS. 9 and 10 in that the radially-angled bevels or grooves  890  taper radially outwardly to a land  890   a , and is preferred due to certainty it provides that only particles within specification will pass.  
         [0066]    [0066]FIG. 12 shows a third embodiment of a secondary milling zone ring  986 . Manufacture of the lower secondary milling zone ring  986  is simplified by constructing it of two spaced annular sections, ring section A and ring section B. In this embodiment, the lower secondary milling ring  986  includes means for channeling flows of particles and gases such that particles are separated from gas-flow paths and impelled into a plurality of crushing zones. In particular, a plurality of equidistant or mass-balanced V-shaped cuts  988  are formed traversing the entire width of the ring section A and extending into a portion of the ring section B, one face  988   a  of the cuts  988  being either substantially perpendicular or acutely angled relative to the crushing surface and the other face  988   b  being angled relative to the crushing surface to define an inclined surface. The edges of the cuts  988  are substantially co-extensive with radii of the secondary milling ring.  
         [0067]    A plurality of equidistant or mass-balanced cuts  990  are formed in the ring section B, each cut  990  being circumferentially offset from a respective cut A One face  990   a  of the cuts  990  is either substantially perpendicular or acutely angled relative to the crushing surface, the other face  990   b  being angled relative to the crushing surface to define an inclined surface. The faces of the cuts  990  are formed at an angle either substantially radial to the secondary milling ring  986  or acutely angled relative to radii, with the outermost ends of the cuts  990  being away from the direction of rotation so that acute angled cuts  990  force more air draft, yet engage and crush lesser percentages of oversize particles, each feature being preferred where increased fan action and decreased fineness are desired.  
         [0068]    A plurality of equidistant or mass-balanced cuts  992   a re formed at the junction of ring sections A and B (that is, at the junction of the outer circumference of ring section A and the inner circumference of ring section B), joining cuts  988  and  990 . Cuts  992  extend in an approximately circumferentially orientation, one face  992   a  of the cuts  992  being angled relative to the crushing surface to define an inclined surface. The angle of face  992   a  can be varied as indicated at  992   a ′ to force a sharper change of direction of the air flow.  
         [0069]    Thus, each crushing zone comprises a plane on the surface of the rotor inclining toward a flat surface of the counter-rotating rotor so that oversize particles wedge between the flat and inclined surfaces and are crushed. The inclined surfaces occur in a plurality of grouped sequences of at least two inclined planes per sequence, with their inclines in alternating orientation, so that the first surface generally inclines chordally, and the second surface, located progressively outwardly beyond the radial location of the first inclined surface, generally faces the axis. Any third inclined surface—if applied—is located progressively outwardly beyond the radial location of the second inclined surface, generally facing chordally. All inclined surfaces are proximal to each other so that together they form a continuous and zig-zag channel to the outer periphery of the rotor device, the plurality of grouped sequences being spaced equidistantly around the rotor periphery.  
         [0070]    Particles of process material are moved centrifugally out of the primary milling zone and into the cuts  988 , where some move up the inclined surface of the cuts  988  until they are crushed within the close running clearance of the lower ring  986  and a flat surfaced counter-rotating upper ring in a manner similar to that previously described in connection with FIG. 11.  
         [0071]    Other particles are crushed in a similar manner on or at the top of the surfaces of the cuts  992  and  990 . As the movement of gases through and out of the rotating system must in large measure be completed by changes of direction at cuts- 992  and then again at cuts  990 , oversize particles are ejected from the gas flows at the direction changes and impelled by their masses up the surfaces of cuts  992  and  990  to be crushed.  
         [0072]    The configuration of FIG. 12 provides high likelihood that all oversize particles will be reduced to specification, while also providing higher rates of air movement through the rotor set, thus improving particle to particle impact rates through turbulence within the primary reduction zone.  
         [0073]    Referring now to FIGS. 13A and 13B, there is shown a fourth embodiment of a second stage milling zone  1040 , in which the upper surface of the lower ring  1086   b  is configured as an uninterrupted conical surface and the lower surface of the upper ring  1086   a  is configured as a conical surface interrupted by a plurality of spaced, radially-extending grooves  1094  defining grinding teeth. Each tooth comprises a crushing slope  1094   a  and a flattened apex  1094   b , adjacent teeth being separated by planar lands  1094   c . The upper and lower rotors can be provided with annular rings as disclosed in connection with FIGS.  2 - 7 .  
         [0074]    The upper and lower milling rings are integral with the upper and lower rotors, respectively, so as to rotate respectively with the upper and lower rotors. The uninterrupted conical surface of the lower ring  1086   b  resists radial centrifugal movement of particles emanating from the primary reduction zone. The amount of resistance is proportional to the angle of the conical surface; thus, the greater the slope of the conical surface, the greater the amount of resistance. Oversize particles are swept by centrifugal force into the grooves  1094  as the milling rings rotate relative to each other. Secondary crushing of oversize particles takes place between the multiple grinding teeth rotating in close clearance near the counter-rotating conical surface of the lower ring  1086   b .  
         [0075]    [0075]FIGS. 14A and 14B illustrate a fifth embodiment similar to the embodiment of FIGS. 13A and 13B, but in which upper milling ring  1186   a  is separate from the upper rotor  1110   a , and remains stationary while the upper rotor  1110   a , the lower rotor  1110   b , and the lower milling ring  1186   b  rotate.  
         [0076]    Referring to FIG. 15, there is shown a sixth embodiment of the second stage milling zone  1240 , in which the lower milling ring  1286   b  is integral with the lower rotor  1210   b  so as to be rotatable therewith and is configured as described in connection with FIGS. 9, 11A and  11 B, or  12 , and in which the upper rotor second-stage size control ring (i.e., the upper milling ring)  1286   a  is separate from the upper rotor  1210   a  and is uninterrupted and static. It will be appreciated by those of skill in the art that, alternatively, the lower milling ring  1286   b  can be separate from the lower rotor  1210   b , while the upper rotor second-stage size control ring (i.e., the upper milling ring)  1286   a  is integral with the upper rotor  1210   a  so as to be rotatable therewith and is uninterrupted and static, as long as adequate air movement is provided. In another alternative, both the upper and the lower milling rings  1286   a  and  1286   b  can be integral with their respective rotors  1210   a  and  1210   b , so as both to be rotatable counter to each other.  
         [0077]    In the embodiment as shown in FIG. 15, the primary reduction zone  1230  comprises annular rings  1224   a  and  1224   b  against which process material banks up, providing impact and abrasion action to reduce incoming material. The secondary milling zone  1240  crushes the oversize particles between the static upper ring  1286   a  and the rotating lower ring  1286   b . No classification or recirculation is needed. The sized material passes through a preset gap G between the upper and lower control rings  1286   a  and  1286   b  at their outer edges, and exits to a collection bin (not shown).  
         [0078]    FIGS.  16 - 18  show a seventh embodiment in which the second stage  1340  includes at least one pair of opposing close-clearance rings  1396  configured for reducing oversize material for example, for orienting and shearing long wood-chip splinters into shorter pieces. Each of the rings  1396  has an inner peripheral wall  1396   a  and an outer peripheral wall  1396   b . Draft impeller ribs can optionally be placed at the locations designated at  1398   a  or  1398   b.    
         [0079]    As best shown in FIG. 18, in cross-section, the inner peripheral wall  1396   a  of each ring  1396  is sloped at an angle of about 45° to the vertical. The outer peripheral wall  1396   b  has a crown portion  1396   c  and a root portion  1396   d , the crown portion  1396   c  in cross-section being perpendicular to the inner wall and sloping at an angle of about 45° to the vertical (so as to be complementary to the inner peripheral wall  1396   a  of the opposing ring) and the root portion  1396   d  in cross-section forming an angle of 100° with the horizontal. The inner wall has radially-extending ribs  1396   e  formed therein. In the rings  1396  that are configured to orient the wood-chip splinters, these ribs  1396   e  are denominated alignment ribs, and they are more closely spaced to orient the wood-chip-splinters with their long dimensions in a radial direction for shearing. In the rings  1396  that are configured to shear the wood-chip splinters, these ribs  1396   e  are denominated shear ribs, and they are more widely spaced to permit passage of the splinters into the grooves for a given cut-off length. The wood-chip splinters are sheared by counter-rotation of the rings  1396 . This embodiment is preferred for very fine final stage reduction of wood chips for use in boiler firing known as reburn, in which much finer fuel is combusted in the upper regions of furnaces.  
         [0080]    Modifications and variations of the above-described embodiments of the present invention are possible, as appreciated by those skilled in the art in light of the above teachings. For example, as discussed above, although “upper” and “lower” are used herein to designate the relative positions of various elements of the invention, the configurations of these elements as described herein are applicable regardless of spatial orientation of the axis of rotation, since centrifugal force acting through the proprietary elements of the rotating system yields equivalent size reduction effects regardless of location relative to gravitation. Vertical axis orientation permits more even loading on bearings and better retention of resident banked-up material, especially on start-ups and shut downs.  
         [0081]    It is therefore to be understood that, within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described.