Patent Application
20080245913
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.
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.
The present invention may be better understood by reference to the accompanying drawings of which:
a, 4b and 4c are plan views of rotating and stationary impact knife arrays of alternate configurations shown in
a, 5b and 5c are cross sectional views, taken along plane A-A of
An impact mill 100 includes three housing sections: a lower base portion section 1a, a center housing section 1b and a top housing section 1c. The lower base portion section 1a carries a bearing housing 2 in which a rotor 3 is rotatably mounted. The center housing section 1b is concentrically nested 7 in the lower housing section 1a and provides concentric vertical alignment for the upper housing section 1c. A plurality of bolts 8 is provided for the detachable connection of the two housing sections. The top housing section 1c 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 1c 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 1c 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 3a fastened to rotor 3 and the grinding track assembly 5. The top section 1c also includes a downwardly aligned cylindrical collar 11 which may be displaced axially within the center housing section 1b. The cylindrical collar 11 forms an integral component of the top section 1c. 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 1b. 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 1c is securely fastened to the center section 1b by means of a plurality of bolts 15. The upper section 1c 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 1c, which takes the feedstock 18 to a labyrinth horizontal space 40 between the upper section 1c 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 1c 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 1c, 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 1c. 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 1c 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 1c 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 21b or 19b, a curved tee-shape 21a or 19a or a square edge 21c or 19c 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 1c.
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 1c. 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 3a 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
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.
