Abstract:
A cost efficient single-drive particulate material pulverizing and classifying mill capable of operating at high efficiencies and able to conveniently produce selectively sized particles from a wide variety of materials during milling operation without restructuring the mill configuration.

Description:
TECHNICAL FIELD 
     This invention relates generally to improvements in mills for pulverizing particulate material and classifying the resulting particles. 
     BACKGROUND 
     Pulverizing and classification of dry materials is practiced today using hammer-screen mills, impact attrition mills, ball mills, pin mills and others outfitted with internal classifiers that separate the coarse and the fine particle fractions. The air classifying mill is popular equipment for pulverizing and separating particulate material (e.g., chemicals, minerals, fertilizers, foodstuffs, and powder coatings) into selectively sized particles. In general, air classifying mills can be classified into two types; single- and dual-drive. 
     Dual-drive mills can pulverize a wide variety of materials and allow the operator to control the particle size simply by drive speed adjustment. Typically, dual-drive air classifying mills comprise a housing containing a pulverizing disk and a particle classifier, wherein each of the pulverizing disk and the classifier are controlled by a separate drive. One drive adjusts the classifier speed to control the particle size while the second drive controls the pulverizing disk speed, which is adjusted depending on the hardness and toughness of the particulate material. Because the speeds of the classifier and the pulverizing disk are each controlled by a separate drive, they can be adjusted independently. A disadvantages of dual-drive mills is that they are mechanically complicated and thus more expensive and more prone to mechanical failure than mills of simpler construction. 
     Single-drive mills are of more simple construction, less expensive, and more rugged because both the pulverizing disk and particle classifier are driven by the same drive. As such, the operator has limited control over particle size, and versatility with respect to material hardness is very limited. Consequently, there is less control of particle size and the type of particulate material that can be pulverized is relatively limited. 
     A need exists for an economically priced and rugged single-drive particulate material pulverizing and classifying mill capable of conveniently producing selectively sized particles from a wide variety of materials. 
     SUMMARY OF THE INVENTION 
     The present invention provides a mill that satisfies this need. The invention provides a cost efficient and rugged single-drive air classifying mill of simple design capable of operating at high efficiency and able to produce a wide range of particle sized for a wide range of materials. 
     In one embodiment, a mill is provided for pulverizing and classifying particulate material comprising: a housing having an air intake, a particulate-material inlet, a particle outlet, and a particulate material pulverizing and classifying assembly for receiving the particulate material from the inlet, pulverizing the particulate material into pulverized particles, and separating the pulverized particles by a size of the pulverized particles. The pulverizing and classifying assembly is rotatably mounted on a drive shaft and positioned within the housing. Advantageously, the mill has a particle-removal conduit connected to the particle outlet and extending into the assembly for removal of the pulverized particles. Preferably, the housing is provided with an inner liner having a selectively shaped pattern to assist in pulverizing the particulate material. 
     In another embodiment, the mill of the invention has a particulate material pulverizing and classifying assembly comprising a pulverizing disk rotatably supported by the drive shaft, a classifier supported by the pulverizing disk and having first and second ends. The second end is adjacent to the housing while the first end adjacent to the pulverizing disk. A pulverizing domain exists between the pulverizing disk and the housing and the particle-removal conduit extends into the second end of the classifier for separating the pulverized particles according to size. 
     Also, it is preferred that the extension of the particle-removal conduit is adjustable to control the size of the pulverized particles accepted by the classifier and thereafter directed to the particle outlet for collection. 
     Preferably, the second end of the classifier fits at least partially within a groove in the housing to prevent the pulverized particles from passing around the second end of the classifier. It is also preferable that the mill further comprise a shroud oriented parallel to the drive shaft and surrounding at least a portion of the assembly to separate the pulverizing domain from the classifying zone and to assist in directing the pulverized particles to the classifier. 
     In another embodiment, the mill of the invention comprises a classifier wherein the first end has a diameter that is different than that of the second end to create a tangential air velocity differential between the first end and the second end to assist in the separation of the pulverized particles. In one embodiment, the diameter of the first end of the classifier is larger than the diameter of the second end to provide a higher tangential air velocity at the first end. In another embodiment, the diameter of the first end of the classifier is smaller than the diameter of the second end to provide a higher tangential air velocity at the second end. Preferably, the classifier is in the shape of a cone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the invention will become better understood with reference to the following detailed description, examples, appended claims, and accompanying drawings where; 
     FIG. 1 is a side view, in cross section, of a particulate material pulverizing and classifying mill according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of the invention comprises a single-drive particulate material pulverizing and classifying mill with a particle-removal conduit adjustably extending into the classifier to control the size of the pulverized particles accepted by the classifier (i.e., allowed to pass through the classifier) and thus directed to the particle outlet. During mill operation, the classifier rotates at the same rpm as the pulverizing rotor. The particle size accepted by the classifier can easily be adjusted by varying the air flow rate, rotor rpm, and the extension of the particle-removal conduit into the classifier. The extension of the particle-removal conduit into the classifier changes the effective cut size of the classification. The particle-removal conduit extension effect in conjunction with classifier design provides a mechanism to continuously adjust the particle fineness without stopping the mill operation, changing the air flow rate, or adjusting the drive shaft rpm. In addition, variation of the particle-removal conduit diameter or the number of blades on the classifier can provide further adjustability to the fineness of grinds. Advantageously, because only a single drive is necessary, the cost is much less and the mill is less complicated than dual-drive pulverizing and classifying assemblies. 
     The particulate material pulverizing and classifying mill of the invention is understood more fully with reference to FIG.  1 . In one embodiment, the particulate material pulverizing and classifying mill of the invention comprises vertically oriented cylindrical hollow housing  4  having inner liner  8 . Preferably, liner  8  comprises a selectively shaped pattern, preferably a ridge pattern, to assist in pulverizing the particulate material. Housing  4  is provided with particulate-material inlet  12 , particle outlet  14 , air intake  16 , drive shaft  20 , supported by suitable bearing member  24 , and particle-removal conduit  28 . Drive shaft  20  is oriented along the vertical axis of housing  4 . Within housing  4 , drive shaft  20  rotatably supports a particulate material pulverizing and classifying assembly comprising pulverizing disk  36  that supports classifier  32 . The classifier has first end  32   a  and a second end  32   b , and is supported such that the first end  32   a  is adjacent to the pulverizing disk  36  and second end  32   b  is adjacent to the upper section of housing  4 . 
     Pulverizing disk  36  is of a diameter such that an effective pulverizing domain  40  exist between inner liner  8  and the periphery of pulverizing disk  36 . Preferably, pulverizing disk  36  supports a plurality of beaters  44  the outer edges of which are oriented parallel to drive shaft  20 . 
     In a preferred embodiment, classifier  32  is positioned on drive shaft  20  such that the classifier&#39;s second end  32   b  fits at least partially within groove  46  to ensure particle flow through classifier  32  before exiting the mill through particle-removal conduit  28 . Preferably, classifier  32  comprises a plurality of angularly disposed and selectively spaced vane members  48 . The dimensions and shape of classifier  32  and vane members  48  are variable and one of skill in the art can readily make adjustments depending on the desired fineness of the ground product. Advantageously, because particle-removal conduit  28 &#39;s extension into classifier  32  is adjustable, the fineness of ground product can be varied during milling operation with a relatively inexpensive classifier. For example, a 6-vane classifier, when used for classification in the present single-drive mill, gives comparable fineness of ground product to that of a 24-vane classifier used in a conventional dual-drive mill. The most preferred classifier shape is a cone shape. In the preferred classifier design, the diameter of the first end is different than that of the second end. In one embodiment, when a fine grind of particles is desired, it is preferable that the diameter of the second end  32   b  is smaller than the diameter of the first end  32   a . In an alternative embodiment, when a coarse grind of particles is desired, it is more preferable that the diameter of the second end  32   b  is larger than the diameter of the first end  32   a.    
     Particle-removal conduit  28  is oriented along the vertical axis of housing  4  and extends through particle outlet  14  and into housing  4  into the classifier&#39;s second end  32   b . Particle-removal conduit  28  can be of any dimensions and one of skill in the art can readily determine the conduit dimensions for the particular application. The particle outlet  14  is provided with a suction source (not shown). 
     The extension of particle-removal conduit  28  into classifier  32  can be adjusted. As will be explained more fully, this extension determines the fineness of the particles accepted by the classifier. As used herein “extension” means the length of the portion of particle-removal conduit  28  located within classifier  32 . 
     In a preferred embodiment, the mill of the invention includes stationary shroud  66 , preferably in the form of a hollow cone that surrounds at least part of classifier  32 . Shroud  66  may be supported by a plurality of baffles  52  that are secured to inner liner  8  of housing  4 . 
     In operation, particle-removal conduit  28  is connected to particle outlet  14 , which, in turn, is connected to a suction source (not shown) to draw air though classifier  32 . Drive shaft  20  is rotated at high speed by a motor of any suitable construction, preferably, a variable speed motor. The material to be ground is fed at a preselected rate into housing  4  through particulate-material inlet  12  to be ground in pulverizing domain  40  by the action of the rapidly rotating pulverizing disk  36  and stationary inner liner  8 . Particulate-material inlet  12  can be at any location on housing  4 . Preferably, for hard-to-grind material, particulate-material inlet  12  is positioned at a point adjacent to the periphery of the pulverizing disk  36 . The fan action generated by pulverizing disk  36  effects a swirling air current. The ground material is entrained in the swirling air current and conveyed over the top of shroud  66  thereby presented to classifier  32 &#39;s periphery, whereon the coarse particles are selectively rejected by the classifier&#39;s centrifugal action and the smaller particles are directed through classifier  32  by the radial-air current to particle-removal conduit  28 . The extension of particle-removal conduit  28  into classifier  32  provides a mechanism to continuously adjust the fineness of the ground product without stopping mill operation to restructure the pulverizer/classifier assembly, changing the air flow rate, or changing the drive shaft rpm. 
     For example, when classifier  32  is in the shape of a cone, as it rotates, a tip speed differential is created between the classifier&#39;s first end  32   a  and second end  32   b . At a constant total air flow rate, the apparent radial-air velocity through classifier cone  32  is affected by the extension of particle-removal conduit  28  into the classifier cone. The extension of particle-removal conduit  28  can be adjusted manually or otherwise prior to or during operation from the outside of the mill. The collected particle size (i.e., the size of the particles allowed to pass through the classifier) is determined by the local-tip speed and the apparent radial-air velocity through classifier cone  32 , according to the extension of particle-removal conduit  28 . 
     The classifier&#39;s smaller diameter end is characterized by lower tip speed and lower centrifugal force, whereas the classifier&#39;s larger diameter end is characterized by higher tip speed and higher centrifugal force. This creates a significant pressure differential gradient along the axial direction of the classifier. For example—without wishing to be bound by any particular theory—when the classifier is in the shape of a cone, and the first end  32   a  is the larger diameter end (i.e., the arrangement in FIG.  1 ), at constant total air flow, the effective classification surface is near the entrance end of particle-removal conduit  28  (i.e., the end of the conduit extending into the classifier). As the extension of particleremoval conduit  28  into classifier  32  increases, particles have to pass through a higher classifier tip speed region, potentially with a lower radial-air velocity. This provides increased propensity for extraction of fine particles. But as the particle-removal conduit  28  is extended very close to larger diameter end  32   a  (i.e., past a radial-air velocity minimum) the radial-air velocity will began to increase, consequently, extraction of coarser particles will be increasingly favored. 
     Thus, if coarser particles are desired, the extension of particle-removal conduit  28  can be adjusted to a lesser extension into classifier  32  In this way, the particles are presented to the classifier at the zone of lower-classification tip speed and higher radialair velocity. But if finer particles are desired, the extension of particle-removal conduit  28  can be increased such that the particle-removal conduit  28  extends further into classifier  32 . As to the case in which the second end  32   b  of the cone classifier  32  is the larger diameter end, the effective classification surface is between the entrance end of particle-removal conduit  28  and first end  32   a  of classifier  32 . As the extension of particle-removal conduit  28  into classifier  32  increases, the particles will be separated through a lower tip-speed region of the classifier, which region has a higher radial-air velocity, and the extracted particles get coarser. This situation is in favor of producing coarse-ground product. 
     In any case, the rejected coarse particles are conveyed around the bottom of shroud  66  to the pulverizing domain  40  for further pulverizing, while the fine particles are conveyed through the particle-removal conduit for collection. 
     Although the present invention has been described in considerable detail with reference to certain preferred embodiments, other embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiments contained herein.