Patent Publication Number: US-7900860-B2

Title: Conical-shaped impact mill

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention is directed to a device for comminution of solids. More particularly, the present invention relates to a conically-shaped impact mill. 
     2. Description of the Prior Art 
     Devices for providing comminution of particulate solids are well known in the art. Amongst the many different milling devices known in the art grinding mills, ball mills, rod mills, impact mills and jet mills are most often employed. Of these, only jet mills do not rely on the interaction between the particulate solid and another surface to effectuate particle disintegration. 
     Jet mills effectuate comminution by utilization of a working fluid which is accelerated to high speed using fluid pressure and accelerated venturi nozzles. The particles collide with a target, such as a deflecting surface, or with other moving particles in the chamber, resulting in size reduction. Operating speeds of jet milled particles are generally in the 150 and 300 meters per second range. Jet mills, although effective, cannot control the extent of comminution. This oftentimes results in the production of an excess percentage of undersized particles. 
     Impact mills, on the other hand, rely on centrifugal force, wherein particle comminution is effected by impact between the circularly accelerated particles, which are constrained to a peripheral space, and a stationary outer circumferential wall. Again, although control of particle size distribution is improved and can be manipulated compared to jet mills, the particle size range of the comminuted product of an impact mill is fixed by the dimensions of the device and other operating parameters. 
     A major advance in impact mill design is provided by a design of the type disclosed in German Patent Publication 2353907. That impact mill includes a base portion which carries a rotor, mounted in a bearing housing having an upwardly aligned cylindrical wall portion coaxial with the rotational axis, and a mill casing which surrounds the rotor, defining a conical grinding path. The mill of this design includes a downwardly aligned cylindrical collar which may be displaced axially in the cylindrical wall portion and may be adjusted axially to set the grinding gap between the rotor and the grinding path. 
     An example of such a design is set forth in European Patent 0 787 528. The invention of that patent resides in the capability of dismantling the mill casing from the base portion in a simple manner. 
     Although impact mills having conical shapes, permitting a downwardly aligned cylindrical collar to be displaced axially so that the grinding gap may be adjusted, represents a major advance in the art, still those designs can be improved by further design improvements that have not heretofore been addressed. 
     Impact mills, when utilized in the comminution of elastic particles, such as rubber, are usually operated at cryogenic temperatures, utilizing cryogenic fluids, in order to make feasible effective comminution of the otherwise elastic particles. Commonly, cryogenic fluids, such as liquid nitrogen, are utilized to make brittle such elastic solid particles. In view of the fact that the cryogenic temperatures attained by the frozen particles are much lower than the ambient surrounding temperature of the mill, this temperature gradient results in a rapid temperature rise of the particles. As a result, it is apparent that maximum comminution in an impact mill, or any other mill, should begin immediately after particles freezing. However, impact mills, including the conically shaped design discussed supra, initially require the particles to move outwardly toward the periphery before comminution begins. During that period the temperature of the particles is increased, reducing comminution effectiveness. 
     Another problem associated with comminution mills in general and conical mills of the type described above in particular is the inability to alter the physical configuration of the impact mill to adjust for specific particle size requirements of the various materials. 
     Three expedients are generally utilized to change the particle size of an elastic solid whose initial size is fixed. 
     The first expedient employed in changing particle size is changing the feedstock temperature by contact with a cryogenic fluid, e.g. liquid nitrogen, to freeze the elastic solid particles to a crystalline state. The coldest temperature achievable by the particles is limited to the temperature of the cryogenic fluid. A means of controlling particle temperature is to adjust the quantity of cryogenic fluid delivered to the elastic solid particles. 
     A second expedient of changing product particle size is to alter the peripheral velocity of the rotor. This is usually difficult or impractical given the physical limits of the impact mill design. 
     A third expedient of altering particle size is to change the grinding gap between the impact elements. Generally, this step requires a revised rotor configuration. 
     An associated problem, related to alteration of rotor configuration in order to effect changes in desired product particle size, is ease of replacement of worn or damaged portions of the impact mill. As in the case of replacement of parts of any mechanical device, problems are magnified in proportion to the size and complexity of the part being replaced. 
     Yet another problem associated with impact mills resides in power transmission to effectuate rotation of the rotor. Present designs employ multiple belt or gear power transmission means which are oftentimes accompanied by unacceptable noise levels. A corollary of this problem is that if power transmission speeds are reduced to abate excessive noise, rotor speed is reduced so that comminution results are unacceptable. It is thus apparent that a method of improved power transmission, unaccompanied by unacceptable loud noise, is essential to improved operation of impact mills. 
     BRIEF SUMMARY OF THE INVENTION 
     A new impact mill has now been developed which addresses problems associated with conically-shaped impact, adjustable gap comminution mills of the prior art. 
     The impact mill of the present invention provides means for initiation of comminution of solid particles therein at a lower cryogenic temperature than heretofore obtainable. That is, comminution in the impact mill of the present invention is initiated at the point of introduction of the solid particles into the impact mill even before the particles reach the grinding path formed between the rotor and the stationary mill casing utilizing the lowest particle temperature. Therefore, comminution efficiency is maximized. 
     In accordance with the present invention, an impact mill is provided which includes a base portion upon which is disposed a rotor rotatably mounted in a bearing housing. The conical shaped rotor has an upwardly aligned conical surface portion coaxial with the rotational axis. A plurality of impact knives are mounted on the conical surface. The impact mill is provided with an outer mill casing within which is located a conical track assembly which surrounds the rotor. The mill casing has a downwardly aligned cylindrical collar which may be axially adjusted to set a grinding gap between the rotor and the grinding track assembly. The top surface of the rotor is provided with a plurality of impact knives complimentary with a plurality of stationary impact knives disposed on the top inside surface of the mill casing. 
     The impact mill of the present invention also addresses the issue of adjustability of comminution of different sizes and grades of selected solids. This problem is addressed by providing segmented internal conical grinding track sections which are provided with variable impact knive configurations. This solution also addresses maintenance and replacement issues. 
     In accordance with this embodiment of the present invention an impact mill is provided in which a base portion disposed beneath a rotor rotatably mounted in a bearing housing. The conical shaped rotor has an upwardly aligned conical surface portion coaxial with a rotational axis. A plurality of impact knives are mounted on the conical surface. The impact mill is provided with an outer mill casing which supports a conical grinding track assembly which surrounds the rotor. The mill casing has a downwardly aligned cylindrical collar which may be axially adjusted to set a grinding gap between the rotor and the grinding track assembly wherein the mill casing is formed of separate conical sections. 
     The internal grinding track assembly composed of separate conical sections offers the selection of alternate tooth configurations through a series of interlocking frustum cones. Each cone assembly configuration is selected to match a particular feedstock characteristic or desired comminuted end product. Each section of the grinding track assembly can increase or decrease the number of impacts with any peripheral velocity of rotary knives thus providing a matrix of operating parameters. The changing of the shape and angle of the conical grinding track assembly alters particle directions and provide additional particle-to-particle collisions. An ergonomic feature of this invention allows the replacement of worn or damaged frustum conical cones without the necessity of replacing the entire grinding track assembly. 
     The impact mill of the present invention also addresses the issue of effective power transmission without accompanying noise pollution. 
     In accordance with a further embodiment of the present invention an impact mill is provided with a base portion upon which is disposed a rotor rotably mounted in a bearing assembly. The conical shaped rotor has an upwardly aligned conical surface portion coaxial with the rotational axis. A plurality of impact knives are mounted on the conical surface. The impact mill is provided with an outer mill casing which supports a conical grinding track assembly which surrounds the rotor. The mill casing has a downwardly aligned cylindrical collar which may be axially adjusted to set a grinding gap between the rotor and the grinding track assembly. To mitigate belt slippage and excessive noise when operating at high speeds, the rotor shaft of the impact mill is provided with a sprocketed drive sheave wherein the rotor is rotated by a synchronous sprocketed belt, in communication with a power source, accommodated on the sprocketed drive sheave. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be better understood by reference to the accompanying drawings of which: 
         FIG. 1  is an axial sectional view of the impact mill of the present invention; 
         FIG. 2  is an axial sectional view of a portion of the impact mill demonstrating feedstock introduction therein; 
         FIG. 3  is a plan view of impact knives disposed on the top of the upper housing section of the impact mill and on the top of the rotor; 
         FIGS. 4A ,  4 B and  4 C are plan views of rotating and stationary impact knife arrays of alternate configurations shown in  FIG. 3 ; 
         FIGS. 5A ,  5 B and  5 C are cross sectional views, taken along plane A-A of  FIGS. 4A ,  4 B and  4 C, respectively, demonstrating three impact knife designs; 
         FIG. 6  is a sectional view of an embodiment of a rotor of an outer concentric grinding track of the impact mill; 
         FIG. 7  is a sectional view showing alignment of a typical interconnected grinding track; 
         FIG. 8  is a schematic representation of a transmission means for rotating the rotor of the impact mill; and 
         FIG. 9  is an isometric view of a synchronous belt and a sprocketed drive sheave in communication with said belt utilized in the transmission of power to the impact mill. 
     
    
    
     DETAILED DESCRIPTION 
     An impact mill  100  includes three housing sections: a lower base portion section  1   a , a center housing section  1   b  and a top housing section  1   c . The lower base portion section  1   a  carries a bearing housing  2  in which a rotor  3  is rotatably mounted. The center housing section  1   b  is concentrically nested  7  in the lower housing section  1   a  and provides concentric vertical alignment for the upper housing section  1   c . A plurality of bolts  8  is provided for the detachable connection of the two housing sections. The top housing section  1   c  provides a concentric tapered nest for a conical grinding track assembly  5 . The conical grinding track assembly  5  is securely connected to the top housing section  1   c  at its lower end  6 . The rotor  3  is driven by a motor  34  by means of a belt  32  and a sheave  4  provided at the lower end of the rotor shaft. 
     The top section  1   c  includes the conical grinding track assembly  5 . The grinding track assembly  5  has the shape of a truncated cone. Grinding track assembly  5  surrounds rotor  3  such that a grinding gap S is formed between grinding knives  3   a  fastened to rotor  3  and the grinding track assembly  5 . The top section  1   c  also includes a downwardly aligned cylindrical collar  11  which may be displaced axially within the center housing section  1   b . The cylindrical collar  11  forms an integral component of the top section  1   c . An outwardly aligned flange  12  is provided at the upper end of the cylindrical collar  11 . A plurality of spacer blocks  14  is disposed between flange  12  and a further flange  13  which is disposed at the upper end of center section  1   b . Thus, spacer blocks  14  define the axial setting between flanges  12  and  13 . Therefore, spacer blocks  14  define the width of the grinding gap S. As such, this width is adjustable. Once the desired grinding gap S is set, the top section  1   c  is securely fastened to the center section  1   b  by means of a plurality of bolts  15 . The upper section  1   c  and the grinding track assembly  5  are disposed coaxially with the rotor axis A. 
     Cryogenically frozen feedstock  18  enters the impact mill  100  through entrance  20  by means of a path, defined by top  16  of upper housing section  1   c , which takes the feedstock  18  to a labyrinth horizontal space  40  between the upper section  1   c  and rotor  3 . Feedstock  18  moves to the peripheral space defined by gap S by means of centrifugal force through a path defined by the inner housing surface of the top  16  of the upper housing section  1   c  and the top portion  17  of rotor  3 . The feedstock  18  is at its minimum temperature as it enters horizontal space  40 . Thus, impact knives  19 , connected to the top portion  17  of rotor  3 , as well as the stationary impact knives  21 , disposed on the inner housing surface of the top  16  of upper housing section  1   c , provide immediate comminution of the feedstock  18 , which in prior art embodiments were subject to later initial comminution in the absence of the plurality of impact knives  19  and  21 . 
     In a preferred embodiment, illustrated by the drawings, impact knives  19  and  21  are disposed in a radial direction outwardly from axial axis A to the circumferential edge on the top portion  17  of rotor  3  and the inner housing surface of top  16  of top housing section  1   c . It is preferred that three to seven knife radii be provided. In one particularly preferred embodiment, impact knives  21  are radially positioned on the inner housing surface of top  16  of the top housing section  1   c  and impact knives  19  are positioned on top portion  17  of rotor  3  in five equiangular radii, 72° apart from each other. However, greater numbers of impact knives, such as six knive radii, 60° apart or seven knive radii, 51.43° apart, may also be utilized. In addition, a lesser number of impact knives, such as three knife radii, 120° apart, may similarly be utilized. 
     In a preferred embodiment, impact knives  21  and  19 , disposed on the inner housing surface of top  16  of upper housing section  1   c  and the top portion  17  of rotor  3 , respectively, are identical. Their shape may be any convenient form known in the art. For example, a tee-shape  21   b  or  19   b , a curved tee-shape  21   a  or  19   a  or a square edge  21   c  or  19   c  may be utilized. The impact knives  21  and  19  may also have tapered tips to maximize impact efficiency. The taper may be any acute angle  23 . An angle of 30°, for example, is illustrated in the drawings. Impact knives  19  are fastened to the top portion  17  of rotor  3  and impact knives  21  are fastened to the inner housing surface of top  16  of upper housing section  1   c.    
     Frozen feedstock  18  is charged into mill  100  by means of a stationary funnel  24 , which is provided at the center of inner housing surface of top  16  of upper housing section  1   c . Feedstock  18  immediately encounters the top portion  17  of rotor  3  and is accelerated radially and tangentially. In this radial and tangential movement feedstock  18  encounters the plurality of stationary and rotating impact knives  21  and  19 . This impact, effected by the rotating knives, shatters some of the radially accelerated feedstock  18  as it disturbs the flow pattern so that turbulent radial and tangential solid particle flow toward the stationary knives results. After impact in the aforementioned space, denoted by reference numeral  40 , feedstock  18  continues its turbulent radial and tangential movement toward the series of rotating knives  3   a  mounted on the outer rim of the rotor  3 . These impacts increase the tangential release velocity as feedstock  18  undergoes its final particle size reduction within conical grinding path  10  whose volume is controlled by gap S. 
     The conically shaped impact mill  100 , in a preferred embodiment, utilizes a conical grinding track assembly formed of separate conical sections. This design advance permits a series of mating interlocking frustum cones to alter the grinding track pattern within mill  100 . In this embodiment, each conical grinding track assembly section  5  is selected to match a particular feedstock or desired end product. Each section of the assembly  5  is provided with alternate impact knife configurations which provides capability of either increasing or decreasing the number of impacts to which feedstock  18  is subjected. In addition, the adjustment of the shape and angle of the impact surfaces of the conical assembly sections  5  also permit alteration of the direction of the feedstock particles. 
     Another advantage of this preferred embodiment of mill  100  is economic. The replacement of worn or damaged conical sections, without the requirement of replacing the entire conical assembly, reduces maintenance costs. 
     Interconnection of the conical grinding track assembly sections  5  may be provided by any connecting means known in the art. One such preferred design utilizes key interlocks, as illustrated in  FIG. 7 . Therein, complementary shapes of sections  26  and  27  result in an interlocking assembly. Specifically, sections  26  and  27  are interlocking mating frustum cones. 
     In this preferred embodiment impact mill  100  is divided into a plurality of sections. The drawings illustrate a typical design, a plurality of three sections: a top section  26 , a middle section  27  and a bottom section  28  with the grinding track assembly secured in place at its lower end  6 . This configuration allows for the external adjustment of the grinding gap by adding or subtracting spacer blocks  14 . 
     In another embodiment of the present invention impact mill  100  includes a power transmission means which provides direct power transmission at lower noise levels than heretofore obtainable. In a typical design of the power transmission means to the mill  100  of the present invention, noise associated therewith is reduced by up to about 20 dbA. To provide this reduced noise level, without adverse effect on power transmission, a synchronous sprocketed belt  32 , accommodated on a sprocketed drive sheave  4  on rotor  3 , effectuates rotation of rotor  3 . The belt  32  is in communication with a power source, such as engine  34 , which rotates a shaft  35  that terminates at a sheave  30 , identical to sheave  4 . In a preferred embodiment, belt  32  is provided with a plurality of helical indentations  33  which engage helical teeth  31  on sheaves  4  and  30 . The chevron-like design allows for the helical teeth  31  to gradually engage the sprocket instead of slapping the entire tooth all at once. Moreover, this design results in self-tracking of the drive belt and, as such, flanged sheaves are not required. 
     In operation, a power source, which may be engine  34 , turns shaft  35  connected thereto. Shaft  35  is fitted with sheave  30 , identical to sheave  4 . The belt  32  communicates between sheaves  4  and  30 , effecting rotation of rotor  3 . Substantially all contact between belt  32  and sheaves  4  and  30  occurs by engagement of teeth  31  of the sheaves with grooves  33  of belt  32  which significantly reduces noise generation. 
     The above embodiments are given to illustrate the scope and spirit of the present invention. These embodiments will make apparent to those skilled in the art other embodiments. These other embodiments are within the contemplation of the present invention. Therefore, the present invention should be limited only by the appended claims.