CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is based upon and hereby claims priority to U.S. Provisional Patent Application No. 61/310,682, filed Mar. 4, 2010 and the content of said Provisional patent application is hereby incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to methods and equipment for grinding (comminution) of industrial minerals and other related materials to fine powders with equivalent median diameters, and, more particularly, to a method and system for achieving enhanced efficiency in such grinding operations. 
       BACKGROUND OF THE INVENTION 
       [0003]    There is currently used today a system of dry grinding of industrial rocks and minerals that utilizes a grinding mill known as a vertical stirred media mill and wherein the feedstock is introduced into a bed of a media material in a vertical container, known as a drum, and stirred with a vertical impeller or agitator. The mill can be operated in a continuous or batch process mode. 
         [0004]    Typical industrial rocks and minerals suited to the process are: limestone, clays, sand, gravel, diatomite, kaolin, bentonite, gypsum, silica, barite, gypsum, talc, nepheline syenite, mica, pumice, carbon, graphite, fluorspar, shales, inorganic pigments (various metal oxides and compounds), etc. Also cementitious materials, including but not limited to Portland cement, high alumina cement, coal ash (e.g. fly ash, bottom ash, boiler lag, fluidized bed boiler ash, spray drier ash, etc.), blast furnace slag, non-ferrous slag, natural pozzolans, matakaolin, silicate and aluminosilicate glasses, etc. may be utilized. 
         [0005]    As described in U.S. Pat. No. 6,802,898 of Liskowitz et al, with a vertical stirred media mill, the media bed expands upwards, increasing its volume, up to 50% or more, depending on the rotational speed of the impeller drive mechanism. This increase in void space significantly reduces the efficiency of energy transfer from the drive system to the grinding media to the feedstock. This operation condition can be described as an “expanded bed” mode and is the mode typically employed by commercial vertical stirred media mills. 
         [0006]    As stated in the Liskowitz patent, therefore, if the media bed in the above example is prevented from expanding, by whatever means, the mill is said to be operating in a “compressed bed” mode, sometimes referred to as a “confined bed” mode and the compressed bed mode is preferable as it results in enhanced efficiency (reduced energy consumption, increased production rate). 
         [0007]    The Liskowitz patent teaches that the compressed bed mode can be accomplished by operating the impeller at a low speed to minimize the expansion of the media bed. That reduction in speed, however, also has the consequence that there is a decrease in the grinding process efficiency and the ability to grind the feedstock into minute particles, particularly when a size of 1 micron is desired. 
         [0008]    Accordingly, it would be advantageous to have a system and method for carrying out vertical stirred bed grinding where the advantage of a confined bed mode of operation is obtained while operating the impeller at a high speed to gain the advantages of both features so as to enhance the efficiency of the grinding process. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a method and an apparatus for efficient dry grinding of industrial minerals and other related feedstocks to a material having an ultra-fine particle size (e.g., 1-10 μM median particle size) as a lower cost and more environmentally sound alternative to wet grinding. 
         [0010]    The present invention combines the features of a “confined bed&#39; mode of operation with the higher speeds of the agitator to improve efficiency by providing a physical confinement of the bed of grinding media and preventing its normal upward expansion as the agitator rotates to stir the grinding media. In an exemplary embodiment, the confinement of the grinding media is accomplished by introducing a retaining plate into the grinding chamber and which is positioned atop of the grinding media to physically prevent the upward movement of that grinding media during stirring. 
         [0011]    This approach enables the mill to be operated in the desirable “compressed bed” mode, but with the added advantage that a range of higher rotational speeds can be employed to further increase the energy input and throughput of the processing. 
         [0012]    As such, the present invention includes a system for grinding materials to produce an ultra fine material by utilizing a drum having a grinding chamber lid, thereby forming a grinding chamber within which there is a grinding media and into which the feedstock is introduced. An agitator is vertically oriented in the grinding chamber and is rotated to carry out the stirring of the combined feedstock and grinding media to create the grinding effect and to reduce the feedstock to a finely ground product. 
         [0013]    As a further feature of the present inventive system, the retaining plate has at least one hole formed therein that is dimensioned to allow the finely ground product to pass upwardly through the retaining plate while preventing the grinding media from passing though the retaining plate. In an exemplary embodiment, the at least one hole can be a plurality of curved slots. 
         [0014]    As a still further feature of the present invention, the retaining plate may be located at a desired position retaining the grinding media by the use of at least one, and preferable two, support rods that extend downwardly through a grinding chamber lid and the retaining plate is suspended by those support rod or rods. 
         [0015]    In one exemplary embodiment, the support rods may be hollow and communicate with openings that pass through the retaining plate so as to introduce feedstock as well as grinding media into the grinding chamber through one or both of the support rods. A set of elongated plugs can inserted into the hollow support rods to seal the openings in the retaining plate. With this feature, there is also a locking system to lock the support rods to the grinding chamber lid when the retaining plate has been positioned at its desired location. 
         [0016]    The selection of the diameter and nature of the grinding media used in the mill is based on the desired product size of the material to be ground in order to reduce the void space between the grinding media particles. Steel grinding media with diameters in the range 1 mm to 6 mm are typically used. Ceramic grinding media are also often desirable where it is important to maximize the brightness of the ground final mineral product. 
         [0017]    The finely ground product produced by this system can be substantially less than 20 μm (microns), and more preferably in the 1 to 10 μm range. The method can achieve products with a median particle size as fine as 1 μm, not otherwise feasible at realistic rates with an “expanded bed” dry mill. 
         [0018]    Neglecting edge effects, the void fraction for a packed bed of monodispersed (same size) spherical grinding media is about 40% by volume. The minimum theoretical void fraction is 36% for monodispersed spheres. This leads to an optimal ratio of grinding media to feed of about 60:40 by volume. 
         [0019]    When the media bed is confined and the voids between the grinding media are substantially populated with the mineral feedstock, improvements in the efficiency of the system are observed which can be attributed to “autogenous grinding.” In essence, the confined mineral is grinding itself in the void space, thereby reducing wear on the grinding media and ancillary equipment and, further, improving the energy transfer in the mill. The yield of the process is 100%, meaning that all of the feedstock is converted to final product. 
         [0020]    While the present invention is particularly adapted to dry grinding and will be hereinafter described as such, it will be seen that it is also suitable for and adaptable to, wet grinding applications with a savings in energy and an increased efficiency. 
         [0021]    Other features of the present stirred media grinding mill will become more apparent in light of the following detailed description of a preferred embodiment thereof and as illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a perspective view of a vertical compressed bed, stirred medium mill grinding system; 
           [0023]      FIG. 2  is a cross sectional view of the internal components of the stirred medium grinding system of  FIG. 1 ; 
           [0024]      FIG. 2A  is a top view of a retaining plate used with the present invention; and 
           [0025]      FIG. 3  is an exploded view illustrating components of the stirred medium grinding system as shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Turning now to  FIG. 1 , there is shown a stirred bed grinding system  10  constructed in accordance with the present invention. As can be seen, the system  10  is mounted to a frame  12  having a horizontal section  14  and a vertical, raised section  16 . A motor  18  is mounted onto the horizontal section  14  and a coupling  20  connects the motor  18  to a gearbox  22  where the rotating power of the motor  20  is converted to a vertical rotating shaft  24  at a predetermined speed. 
         [0027]    Mounted to the vertical, raised section  16  is the stirred medium grinding mill  26  and which has an outer, cylindrical enclosure  28  having ports  30  and  32  that are used to circulate a cooling medium, such as water, through the stirred medium grinding mill  26  as will later be explained. 
         [0028]    Turning now to  FIG. 2 , taken along with  FIG. 1 , there is shown a cross sectional view of a portion of the stirred medium grinding mill  26  of the present invention. As can be seen, there is a spiral flange  34  that traverses the exterior of a drum  36  and which forms the path for the water or other medium that circulates through the grinding mill  26  for cooling purposes. In  FIG. 2 , the cylindrical enclosure  28  has been removed, however, it is sealed to the outer edges of the spiral flange  34  in order to form the passage for the cooling medium. 
         [0029]    Thus, in  FIG. 2 , the drum  36  has an inner surface  38  that forms a grinding chamber  40  and which contain a grinding media comprised of spherical shaped elements to carry out the grinding of the feedstock. As previously described, the grinding media is selected based upon the desired product size of the material to be ground in order to reduce the void space between the grinding media particles. The composition of the grinding media may vary depending upon the characteristics of the ground final mineral product and may include steel grinding media with diameters in the range 1 mm to 6 mm. Ceramic grinding media are also often desirable where it is important to maximize the brightness of the ground final mineral product. 
         [0030]    Within the grinding chamber  40  is an agitator shaft  42  that is connected to, and, therefore, rotated by, the vertical rotating shaft  24  and there are a plurality of paddles  44  that are affixed to the agitator shaft  42  to carry out that grinding process. Basically, as the agitator shaft  42  is rotated, the paddles  44  stir up and agitate the grinding media mixed with the feedstock to grind the feedstock into the desired fineness. 
         [0031]    The upper opening of the grinding chamber  40  is closed by a grinding chamber lid  46  that may be secured to the drum  36  by means such as bolts  48 . Mounted to the grinding chamber lid  46  is a bearing housing  50  to contain the upper bearing  52  for the agitator shaft  42 . 
         [0032]    A retaining plate  54  is located within the grinding chamber  40  and the retaining plate  54  is a circular plate, fabricated and machined from wear-resistant material, that slides over the agitator shaft  42  with tight tolerance between the agitator shaft  42  and a shaft hole  53  formed in the center of the retaining plate  54  and between the outside diameter of retaining plate  54  and the inner surface  38  of the drum  36 . The wear resistant retaining plate  54  can be constructed of a variety of materials, including but not limited to hardened steel, ceramic coated steel, ceramic, or other suitable materials. 
         [0033]    As shown in  FIG. 2A , taken along with  FIG. 2 , the retaining plate  54  has a plurality of arcuate, spaced slots  56  that are cut into the retaining plate  54  at intervals and, in the exemplary embodiment, the slots  56  are arcs of circles having different radii and having a common center point. The width of the curved slots  56  is such that microfine dust-laden, air may pass through the slots  56  but grinding media is constrained below the retaining plate  54  and cannot pass through the slots  56  in the retaining plate  54 . 
         [0034]    In the exemplary embodiment, the slots  56  are curved, however, it can be seen that the slots  56  may take other configurations or may be a drilled hole or holes, it being of importance that size of the slots  56  be such that the grinding median cannot pass therethrough such that the retaining plate  54  acts to constrain the upward movement of the filter media as it is stirred and acts to create a compressed mode of the stirred bed grinding system  10 . In such a mode, the dust within the air is the product thereby produced and the grinding media is used to produce that product. Alternate embodiments include different aperture sizes and shapes of the slots within the retaining plate, as well as different materials of construction for all components. 
         [0035]    The retaining plate  54  is shown in  FIG. 2  as resting on an optional hard stop  58  within the drum  36 , and the retaining plate  54  can be located at that position relative to the grinding chamber lid  46  lid of the drum  36  by means of two support rods  60 , which are hollow in nature. These support rods  60  can be threadedly engaged to, or may be welded to the retaining plate  54  at the distal ends of the support rods  60  and are free to slide axially through stanchions  62  that are rigidly affixed to the upper surface of the grinding chamber lid  46 . 
         [0036]    In an exemplary embodiment, the support rods  60  are locked into place retaining the retaining plate  54  at a desired distance or depth from the grinding chamber lid  46  (or “constraining depth”) by means set screws  64 , and preferably three set screws  64  per support rod  60  (only one of which is shown) and the set screws  66  are threaded into the stanchions  62 . 
         [0037]    Alternatively, the outside diameter of the support rods  60  can be externally threaded and threadedly engage internal threads formed in the inner diameter of the corresponding stanchions  62 . This arrangement of support rods  60  is thus infinitely adjustable so that the location of the retaining plate  54  and thus the volume of the grinding media being constrained can be readily adjustable by the user. 
         [0038]    Since the support rods  60  are hollow, and can communicate with the grinding chamber  40  through holes  61  in the retaining plate  54 , feedstock as well as grinding media can be introduced into the grinding chamber  40  through one or both of the support rods  60 . While there are various systems that can be used to recover the finely ground powder from the grinding chamber  40 , one means illustrated in the exemplary embodiment is by the use of an outlet conduit  65  that communicates with an opening  67  into the interior of the grinding chamber  40  above the retaining plate  54  and a pneumatic system can be used to withdraw the finely ground product therefrom. 
         [0039]    Within each hollow support rod  60 , there is shown an elongated plug  66 , which consists of a hollow rod  68  of smaller diameter than the inner area of the support rods  60  such that the hollow rod has a plug shaped piece  70  at the distal end, and threads formed at the proximal end. This elongated plugs  66  slide down into the support rods  60  to plug the holes  61  that have been precisely match machined into the retaining plate  54  corresponding to the centerline of each support rod  60 . 
         [0040]    Turning finally to  FIG. 3 , taken along with  FIGS. 1 and 2 , it can be seen that the assembly of the stirred bed grinding system  10  can be carried out by inserting the retaining plate  54  and support rod  60  subassembly into the stanchions  62  and locking the retaining plate  54  in its desired location with respect to the grinding chamber lid  46 . The agitator shaft  42  can then be slid through the opening  53  in the retaining plate  54  and installed through the grinding chamber lid  46  and upper bearing  52 . Accordingly, the entire assembly may then be lowered into the grinding chamber  40  and the grinding chamber lid  46  secured to the drum  36  by the bolts  48 . 
         [0041]    Grinding media  76  may then be poured into the one or both of the hollow support rods  60  to fill the grinding chamber  40 . Once the grinding media  76  has filled the grinding chamber  40  completely, the elongated plugs  66  can be inserted into the hollow support rods  60  and secured in place by means of threaded fasteners  78  which lock on the outer diameter of the elongated plugs  66  and the outer diameter of the top end of the support rods  60 . This is a significant advantage over all other designs in that grinding media  76  can easily added to the grinding chamber  40  after the entire assembly is installed, rather than required the grinding media to be drained and the entire assembly be removed and disassembled, should one desire to change the amount of grinding media within the mill. 
         [0042]    Another advantage of the design is that, due to the hollow nature of the support rods and the ability of the elongated plugs to protrude into the grinding zone, temperature and force measurements may easily be obtained, whereas, in previous designs such was impossible, due to the violent nature of the grinding within the chamber. 
         [0043]    While the present invention has been set forth in terms of a specific embodiment of embodiments, it will be understood that the present stirred bed grinding system herein disclosed may be modified or altered by those skilled in the art to other configurations. Accordingly, the invention is to be broadly construed and limited only by the scope and spirit of the claims appended hereto.
A system and method are disclosed for the grinding of industrial minerals to fine powders. The system includes a grinding chamber having a generally vertically oriented agitator that is rotated to stir the grinding media intermixed with a feedstock. As the agitator stirs the grinding media and feedstock, there is a retaining plate that is also located within the grinding chamber and positioned atop of the grinding media to prevent the expansion of the grinding media when it is being stirred. The retaining plate has one or more openings to allow the finely ground feedstock to pass upwardly therethrough but to prevent any grinding media from passing through the retaining plate. Introduction of the feedstock may be through one or more hollow support rods that locate the retaining plate in the desired location within the grinding chamber. The support rods are locked into position when the retaining plate is properly located.