CA2203562A1 - A size reduction apparatus for comminuting materials - Google Patents

Abstract

A size reduction apparatus which comminutes material by impact and attrition.
The apparatus features are impeller that rotates in a frusto-conical shaped comminution chamber disposed in a comminution housing. A propeller rotates with the impeller to expel comminuted material from the housing. A comminuted material outlet is tangentially oriented with respect to the housing. The advantage is an energy efficient size reduction apparatus that is capable of producing fine particles at low temperatures.

Description

CA 02203~62 1997-04-23 A SIZE REDUCTION APPARATUS FOR COMMINUTING MATERIALS

Field Of The Invention The present invention relates to apparatus for reducing the particle size of free flowing m~t~ri~l~ and, in particular, to apparatus for reducing the particle size of free flowing m~teri~l~ by impact and attrition.

Back~round Of The Invention Size reduction is among the oldest of the arts, yet it is still widely practiced and e~enti~l to a broad sector of modern industry. In general mechanical comminlltion is accomplished by one of two processes, rupturing by co,l,pression or bursting andattrition using kinetic energy. The first process is practiced using roller mills, jaw crushers, pendulum mills or the like. The second process is practiced using h~mmer mills, pin mills, jet mills or the like.
Modern methods for producing fine-grained materials are generally very energy intensive and, consequently, tend to be costly. However, the demand for fine-grained materials is quite high, so that any reduction in energy consumption that can berealized is very important. Many different types of mills have been invented forreducing particle size by impact and attrition. Among the most relevant to the present invention are those described in British Patent 1 601 129, and United States Patents 2,750,120 and 4,697,743.
British Patent 1 601 129 to British Steam Specialities Limited was published on October 28, 1981. It describes an apparatus for reducing the size of fatty and moist materials and agglomerates which would normally block conventional size reducingapparatus. The apparatus comprises a grinding chamber having a frusto-conical shaped, apertured side wall, and a blade element which is turned about an axis in the grinding chamber to force material through the apertures in the sidewall. The blade element is of an open planar frame construction and preferably made from round rod that is shaped in an open frusto-conical profile. While this machine is eff1cacious for its intended purpose, experimentation has shown that it is not well adapted for the size reduction of hard materials.

CA 02203~62 1997-04-23 United States Patent 2,750,120 which issued June 12, 1956 to P~llm~nn describes an apparatus for mechanically ~ integrating materials into small particles, including materials that defy processing by conventional grinding devices. The apparatus comprises a pair of impact shells respectively having facing, concave 5 configuration to form an impact chamber with a peripheral discharge slot. One of the shells is mounted in a stationary position and the other is mounted to rotate with respect to the first. A multi-bladed impact rotor is mounted for rotation inside the impact chamber, the impact rotor having tips which provide limited clearance in a restricted region adjacent the discharge slot to break the material to be comminuted 10 into small particles before it is discharged through the slot. The rotatable shell and the impact rotor are preferably counter rotated to enhance the comminution of material.
While this m~.hine is apparently effective in reducing the size of a wide range of materials, it is quite bulky, expensive to construct and not particularly energy efficient because it requires two drive motors, one for impact rotor and one for the impact shell.
United States Patent 4,697,743 which issued October 6, 1987 to Bjorck et al.
describes a method and al)p~dlus for finely crushing particles of material in an impact mill. The particles to be colllmillul~d are passed through a feeding tube or channel to a crushing chamber cont~ining a rotor having its axle parallel to the feeding tube and having first impact surfaces on the rotor impact the particles and drive them into 20 stationary impact surfaces located about a periphery of the crushing chamber and positioned so that the particles impact them at substantially right angles to their line of travel. In order to facilitate the efficient use of the kinetic energy, the crushing chamber and the feed tube are evacuated so that the rotor rotates in a vacuum. Adischarge orifice is located on one side of the crushing chamber to permit crushed 25 particles to discharge by gravity from the chamber. A recirculation system may be provided to recirculate oversized particles back through the crushing chamber. This apparatus is particularly adapted to reducing hard materials to very small particle sizes of 1011 or less. It is not adapted to the high volume, energy efficient size reduction of food materials such as seeds or grains for the production of flours, pastes or granules.

CA 02203~62 1997-04-23 Summary Of The Invention It is an object of the present invention to provide a simple, energy efficient a~p~dlus for reducing the particle size of free flowing materials, including materials that are tough, soft, hard, dry or oily.
It is a further object of the invention to provide an dpp~LldlUS for reducing the particle size of free flowing materials which is inexpensive to construct and m~in1~in.
It is yet a further object of the invention to provide an apparatus for reducingthe particle size of free flowing materials which can produce particles of from about 25 mm to about lOIl in size in a single pass, with a short residence time in the com~ lulion chamber.
In accordance with the invention, there is provided a size reduction apparatus for comminuting m~teri~1~, comprising in combination: a housing surrounding a commin11tion chamber, an impeller rotatable in the commin11tion chamber, a size reduction screen disposed about a periphery of the comminution chamber, a propeller l 5 disposed within the housing beneath the comminution chamber, the propeller being disposed in a plane that is parallel to a plane of the impeller; a material inlet opening in the housing for introducing material to be comminuted, the inlet opening being adapted to permit the introduction of material into the comminution chamber; a material outlet opening in the housing for ejecting comminuted material from thehousing; and, at least one shaft for turning the impeller to commin11te materialintroduced in to the comminution chamber and for turning the propeller to eject comminuted material from the housing.
The size reduction dppaldlus for cor~ ;tlg materials in accordance with the invention therefore provides a very simple machine which is inexpensively constructed but is very efficient in comminuting a wide variety of materials useful in the food, chemical, and pharmaceutical industries, to name but a few. The principal components of the apparatus are an impeller which rotates inside a comminution chamber and a propeller which rotates in a plane parallel with the impeller beneath the comminution chamber and is disposed within a housing that surrounds the comminution chamber.The comminution chamber preferably has a solid top and bottom wall and a frusto-conical sidewall that is perforated to form a cornminution screen. The comminution screen is preferably constructed of a plurality of screen segments which fit together CA 02203~62 1997-04-23 end-to-end to form the frusto-conical screen. The screen segments are preferably made from a thick steel stock that can be more than 200% thicker than the diameter of the apertures through which comminuted material passes.
The segmented comminution screen has several advantages. First, it can be 5 made of heavier material than is possible if the screen is made in a single piece.
Second, if a segment is accidentally damaged, only the damaged segments need be replaced. Third, segments of different aperture size or surface texture can be interspersed to improve efficiency or to provide a comminuted material with a desired range of particle sizes without multiple passes and post mixing.
The impeller can be made in any desired configuration, but preferably has an outer cutting edge that rotates in close proximity to the comminution screen. Ingeneral, the larger a side surface area of the impeller, the more air flow is induced through the machine and, consequently, the larger the particle size of the finished product, regardless of other factors. Conversely, the smaller the side surface area of 15 the impeller, the less air is induced to flow through the comminution chamber. This increases the residence time of material in the comminution chamber and therefore reduces particle size, regardless of other factors.
The propeller serves the function of counterbalancing the air flow induced by the impeller, serves as a beater to further reduce particle size, and ejects comminuted 20 material from the machine. The propeller is preferably driven by the same shaft as the impeller. The orientation of the propeller with respect to the impeller is preferably adjustable. The orientation of the propeller with respect to the impeller has an affect on the residence time of material in the comminution chamber, and therefore has an affect of particle size. An optimum angle between the propeller and the impeller25 depends on the material being comminuted and the desired particle size. In general, an angle of about 45~ to 67~ provides the smallest particle size without affecting throughput.
Comminuted material is preferably ejected from the machine through a discharge tube which may be oriented in the direction of rotation of the 30 impeller/propeller, or in a direction counter to their direction of rotation. The orientation of the discharge tube also has an affect on the residence time of material in the machine and, therefore, an affect on the particle size of the comminuted material.

CA 02203~62 1997-04-23 If the discharge tube is oriented in the direction of rotation, the throughput increases and the particle size increases, other factors being unchanged. On the other hand, when the discharge tube is oriented in the opposite direction, the particle size is decreased and the throughput decreases.
As a general rule, for any given material to be comminuted by the apparatus in accordance with the invention, a given surface area per unit of time can be throughput.
Thus, for instance, if a coarse meal is to be produced from a grain, the weight per unit of time throughput will be much greater than the weight per unit of time when a fine flour is made from the same grain, but the surface area of the cornminuted material per unit of time will remain about constant.

Brief Description Of The Drawin~s The present invention will now be explained by way of example only and with reference to the following drawings wherein:
FIG. 1 is an elevational view of a size reduction appaldl~ls installed in an operational condition on an installation base;
FIG. 2 is an axial cross-section of the size reduction appalalus taken along lines II-II of FIG. 1;
FIG. 3 is a radial cross-sectional view of the size reduction apparatus taken along lines III-III of FIG. 2;
FIG. 4, which appears on sheet one of the drawings, is an elevational view of one configuration for a comminution screen segment in accordance with the invention;
and FIG. 5, which also appears on sheet one of the drawings, is an elevational view of three preferred configurations for impellers for use in the size reduction apparatus in accordance with the invention.

Detailed Description Of The Preferred Embodiment FIG. 1 shows an elevational view of a size reduction apparatus, generally indicated by the reference 10, in accordance with the invention installed on a support base 12 and connected by a plurality of v-belts 14 to an electric motor 17 whichpowers the size reduction apparatus 10. The size reduction apparatus includes a CA 02203~62 1997-04-23 tr~n~mi.~ion stand 16 which supports a comminution housing 18, the structure andfunction of which will be explained in detail with reference to FIGS. 2 and 3.
Attached to a top of the comminution housing 18 is a material inlet opening 20. The electric motor 17 preferably has a torque rating of 15 to 50 horsepower.
FIG. 2 shows an axial cross-sectional view of the size reduction al~palaLus 10 taken along lines II-II of FIG. 1. As explained above, the si~ reduction al)palatus includes a tr~n~mi~ion stand 16 which supports a comminution housing 18. The comminution housing 18 is bolted to the tr~n~mi~ion stand 16 by bolts 22. The tr~n~mi~sion stand 16 rotatably supports a drive shaft 24 which is equipped on its lower end with a multi-groove pulley 26 to which the v-belts 14 (see FIG. 1) areconnected. Located within the comminution housing 18 is a comminution chamber, generally indicated by the reference 28. The co~ inution chamber includes a top plate 30, a bottom plate 32, and a peripheral comminution screen 34, the structure of which will be explained in more detail with reference to FIGS. 3 and 4. Attached to the drive shaft 24 and rotatable therewith is an impeller 36 located in the collllllinu~ion chamber 34. The impeller 36 is preferably provided with two opposing blades affixed to an impeller hub 38 which is in turn removably affixed to the drive shaft 24 using a key, or the like. Positioned beneath the collllllillution chamber 28 and also attached to the drive shaft 24 and rotatable therewith is a propeller 38. The propeller is likewise keyed, or otherwise removably attached to the drive shaft 24.
The comminution chamber 28 is supported in the comminution housing by a plurality of comminution chamber support brackets 33 which are secured to a top wall 56 of the comminl-tion housing by a pair of bolts 58. The comminlltion chamber top plate is removable to permit the impeller 36 and the comminution screen 34 to beserviced. The collllllhlution chamber top plate 30 is clamped in place by a plurality of brackets 60. Each bracket 60 is secured by a bolt which passes through a slot 64. The bolt 62 is threaded into the comrninution housing top wall 56 and the comminution chamber support brackets 33. When the bolt 62 is tightened with the bracket 60 in the locking position (as illustrated) the comminution chamber top plate 30 is lockedsecurely in place. If all bolts 62 are loosened and the brackets 60 slid outwardly to a released position, the comminution chamber top plate can be removed so that the impeller 36 and the comminution screen 34 can be accessed for maintenance.

CA 02203~62 1997-04-23 Material to be comminuted is introduced through the material inlet opening 20.
The rate of introduction is dependent upon the power rating of the motor 17 (see FIG.
1) as well as the friability of the material to be comminuted. In general, it ispreferable to control the inflow of material to be comminuted through the material 5 inlet opening 20 using a computer controlled feed mech~ni~m such as described in United States Patent 5,240,324 which is incorporated herein by reference. The material is introduced through the material inlet opening 20 to the center of the comminution chamber where it is forced outwardly by centrifugal force created by the rotating impeller 36. The impact of the impeller with the material causes the material 10 to rupture. In addition, attrition of the material occurs between an outer end of the impeller and the comminution screen 34, as will be explained in more detail withreference to FIG. 3. Once the material has been reduced in particle size, it is ejected into the comminution housing where it may impact the propeller 38 and/or the walls of the comminution housing 18. Because the apertures in the comminution screen 34 are 15 oriented at right angles to the direction of travel the impeller 36, the particles ejected from the comminution chamber are an average considerably smaller than the aperture diameter. Eventually, the comminuted material is ejected through the material outlet 21 which is preferably rotatable to be oriented in a direction that is aligned with the direction of rotation of the impeller and the propeller or counter to the direction of 20 rotation of the impeller and the propeller as will be explained in more detail below. In general, the propeller 38 serves three functions. First, it counterbalances air flow through the size reduction apparatus 10 induced by the impeller 36. The extent of this counterbalancing action is dependent on an angular relationship between the impeller 36 and the propeller 38 as will be explained in more detail with relation to FIGS. 3 25 and Table II. Second, the propeller acts as a beater which can be effective in further reducing particle size. Finally, the impeller 38 creates a centrifugal force to eject comminuted material from the size reduction apparatus 10 through the material outlet 21.
FIG. 3 shows a radial cross-sectional view of the size reduction apparatus 10 30 taken along lines III-III of FIG. 2. The impeller 36 rotates in the comminution chamber 28, and the propeller 38 rotates in the comminution housing 18 beneath comminution chamber 28. The side wall of the comminution chamber 28 is the CA 02203~62 1997-04-23 , comminution screen 34 which is preferably constructed from a plurality of comminution screen segments 40. Each segment is locked in place in a groove 42 (see FI(~. 2) located between a screen segment support bracket 44 and a ridge machined in the respective top plate 30 and the bottom plate 32 of the comminution chamber 28.
The advantages of a segmented comminution screen are several. First, if a screensegment is damaged by foreign material entering the comminution chamber, that segment can be replaced without the expense of replacing the entire screen. Second, segments having di~elellt aperture sizes can be inserted to accommodate different requirements in terms of particle size so that specific proportions of given particle sizes can be produced in a single run without post mixing by properly adjusting the apertures in the various segments of the comminution screen. Third, because the segments are easier to machine than a single piece screen, the segments can be much thicker than otherwise possible. The screen therefore lasts much longer and permits the comminution of very hard materials, which would otherwise be impossible to grind.
As shown in FIG. 3, the direction of rotation of the impeller 36 and the propeller 38 are usually clockwise, though a counterclockwise rotation is equally effective. Normally, the propeller 38 is preferably offset from the impeller 36 so that they are not aligned. The offset is indicated in FIG. 3 by an angle A. The offset of the propeller 38 from the impeller 36 affects to a certain degree the particle size of the comminuted material because the propeller counterbalances air flow through the size reduction aLp~alus 10 and therefore affects the residence time of material in the comminution chamber 28. An ideal angle between the propeller 38 and the impeller36 depends upon the desired particle size and the material to be processed. In general, testing indicates that an angle of about 45~ to about 67~ provides the greatest reduction in particle size without affecting the throughput of the apparatus. This relationship is discussed in more detail below in reference to Table II. As briefly described above, the material outlet 21 is preferably rotatably attached to the comminution housing 18 so that it may be oriented in a direction counter to the direction of rotation of the impeller 36 and the propeller 38 as shown in FIG. 3, or in alignment with that direction of rotation (not illustrated). The orientation of the material outlet 21 also has an affect on the particle size of materials comminuted by the size reduction apparatus CA 02203~62 1997-04-23 . .

lO. In general, the orientation shown in FIG. 2 provides the greatest reduction in particle size, though some increase in output temperature and some decrease in throughput is experienced. As noted above, as a general rule, for any given material to be comminuted by the size reduction al)p~dlus 10, a given surface area per unit of time can be throughput using a motor 16 of a given power rating. Ch~nging the orientation of the material outlet 21 is a further parameter for controlling particle size.
FIG. 4 shows a potential configuration for a comminution screen segment 40.
As noted above, the comminution of certain tough, dry or hard materials may be facilitated by providing a textured surface on an inside surface of the comminution screen segments 40. For instance, ridges or grooves 46 may be interspersed with comminution apertures 48. Many other grooved or ridged configurations are also possible, as will be al~pa~ t to those skilled in the art. It is well known that textured surfaces on commimltion screens can facilitate the comminution of certain materials.
The screen segments 40 may be constructed from durable materials such as steel or stainless steel. They are preferably tempered and may be case hardened or surface treated with a wear resistant agent such as carbide steel, or the like.
FIG. S shows three preferred configurations for the impeller 36. Each impeller has a shaft attachment portion 49 and a hammer portion 54. The first configuration is the impeller shown in FIG. 2 wherein a substantially rectangular impeller fits closely within the comll~hlulion chamber 28. This configuration induces the greatest air flow through the size reduction apparatus 10. Air is drawn in along with material to be comminuted through the material input opening 20. In general, the greater the air flow through the size reduction apparatus 10, the greater the throughput and the greater the particle size of the comminuted material, all other factors being constant.
The second configuration of the impeller 36 includes a rectangular or a trapezoidal window 50 which reduces air flow through the size reduction apparatus 10 and consequently increase residence time in the comminution chamber, thereby generally reducing particle size of the comminuted material. The third preferredconfiguration for the impeller 36 includes a narrow shank 52 which connects the inner and outer ends of the impeller. This configuration induces the least air flow through the chamber and consequently generally produces comminuted material of the finest particle size. If very fine particles are to be output, this impeller is the preferred ' '' 10 configuration Each of the impeller configurations is also preferably provided with a hardened cutting insert which is welded, bolted or otherwise affixed to the hammer portion 54 in a manner well known in the art to an outer end of the impeller 36. The hardened insert may be constructed of a hardened steel, solid carbide or a metal coated 5 or penetrated with hard chrome, ceramic or carbide. The hardened cutting insert 54 helps extend the effective life of the impeller 36 arld consequently reduces maintenance.
Table I shows test results of a variety of food materials cornminuted using a size reduction a~aldl~ls 10 in accordance with ~e invention. Column 1 of the table 10 shows the material comminuted. Column 2 shows the product produced as a result of the cornrninution, colurnn 3 shows the output in pounds per hour, column 4 shows the particle size of the cornminuted material and colurnn 5 shows the power rating of the motor that powered ~e size reduction a~p~lus 10 used for cornminution.

TABLE I

MATERIAL PRODUCT OUTPUT PARTICLE SIZE POWER
RATnNG
Oat Husk Powder 300 Ib/hr 50% <100 USS 40 amps 35% <40 USS
Wheat Bran Powder 350 Ib/hr 85% <100 USS 40 amps Wheat Bran Increase Bulk 2000 IbAIr 34% <16 USS 40 amps Density Durum Wheat Flour 420 Ib/hr 77% <80 USS 4Q amps Brown Rice Coarse Flour 900 Ib/hr 52% <100 11 40 amps Brown Rice Fine Flour 500 Ib/hr 71% <100 ~1 40 amps Cinnamon Bark Powder 400 Ib/hr 66% <100 !1 40 amps 60% <120 ~

CA 02203~62 1997-04-23 - ' 11 Mushroom and Powder 100 Ib/hr 60% <120 11 30 amps Stems-Dried -Shitake Whole Hard Bread Flour 500 Ib/hr 52% <80 USS 40 amps S Wheat Flour Cake and High Ratio 1500 Ib/hr 99.5% <270 USS 40 amps Pastry- Soft, Flour Wheat Chlorinated Wheat Germ Flour 500 Ib/hr 75% <100 USS 35 amps Whole Soybean Fine Flour 1150 Ib/hr 81% <100 USS 40 amps Whole Soybean Coarse Flour 2000 Ib/hr 65% <100 USS 40 amps Whole Soybean Meal 5200 Ib/hr 20% <100 USS 40 amps 80% ~20 USS

Depending on the particle size and the material to be comminuted, a size reduction ~3al~lus 10 in accordance with the invention can output from 100 to 5200 lbs/hr at temperatures which range generally from 120~F to about 180~F. Most materials process at about 120~F. Tough, dry materials are heat generating but if throughput is controlled, the temperature at the material outlet 21 generally will not 20 exceed 180~F.
As discussed above, the angle A of separation between the propeller and the impeller also has an affect on particle size without noticeably affecting output of the size reduction apparatus 10 in accordance with the invention. Table II shows theresults of a test to determine the affect of angle A on the particle size of durum wheat 25 tailings processed by the size reduction apparatus 10. Column 2 of Table II shows the angle A (see FIG. 2). Column 3 shows the output in pounds per hour of flour produced during the test. Column 4 shows the particle size of the flour produced and column 5 shows the power consumption.

CA 02203~62 1997-04-23 MATERIAL ANGLE A OUTPUT PARTICLE SIZE POWER
CONSUMPTION
Durum Wheat 90~ 750 lb/hr32%~80 USS Mesh 20 amps Tailings 120~F
Durum Wheat 22.5 750 Ib/hr32%<80 USS Mesh 22 amps Tailings 120~F
Durum Wheat 45.0 750 Ib/hr42%<80 USS Mesh 22 amps Tailings 120~F
Durum Wheat 67.0 750 Ib/hr39%<80 USS Mesh 22 amps Tailings As is aLpa~ , the least power was consumed while producing about 750 lbs of flour per hour when angle A was set at 90~. A fine flour was produced with about32% being less than 80 USS Mesh with an output temperature of about 120~F. An angle A of 22.5 for the production of durum wheat flour consumed 22 arnps to produce 750 lbs of flour per hour with no change in particle size, the particle size being 32% less than 80 USS Mesh. The output temperature was also 120~. When angle A was increased to 45~, the output remained constant at 750 lbs/hr and the power consumption remained constant at 22 amps but the particle size of the flour was 20 reduced so that 42% was less than 80 USS Mesh while the output temperature remained constant at 120~F. At 67~ for angle A, the output remained constant as did the power consumption while the particle size increased slightly so that 39% was less than 80 USS Mesh. In general, an angle A of 45~ to 67~ is preferred when the greatest reduction in particle size is desired. If energy consumption is more important than 25 particle size then angle A should be set at about 90~.
While the examples documented above relate to the comminution of food stuffs, it will be appreciated by those in the art that the size reduction apparatus 10 in accordance with the invention is equally adapted to the comminution of chemicals, CA 02203~62 1997-04-23 ! ~
; ~ 13 pharmaceuticals, and other industrial products such as plastics, fibrous plant materials, etc.
Changes and modifications to the illustrative embodiment will be apparent to those skilled in the art. The scope of the invention is therefore intended to be limited S solely by the scope of the appended claims.

Claims (14)

1. A size reduction apparatus for comminuting materials, comprising in combination:
a housing surrounding a comminution chamber;
an impeller rotatable in the comminution chamber;
a size reduction screen disposed about a periphery of the comminution chamber;
a propeller disposed within the housing beneath the comminution chamber, the propeller being disposed in a plane that is parallel to a plane of the impeller;a material inlet opening in the housing for introducing material to be comminuted, the inlet opening being adapted to permit the introduction of material into the comminution chamber;
a material outlet opening in the housing for ejecting comminuted material from the housing; and at least one shaft for turning the impeller to comminute material introduced into the comminution chamber and for turning the propeller to eject comminuted material from the housing.

2. A size reduction apparatus for comminuting materials as claimed in claim 1 wherein the comminution chamber is an inverted frusto-conical shape.

3. A size reduction apparatus for comminuting materials as claimed in claim 1 wherein the size reduction screen is segmented, the screen including at least two segments that cooperate to form the periphery of the comminution chamber.

4. A size reduction apparatus for comminuting materials as claimed in claim 1 wherein the size reduction screen has an inside surface adjacent the impeller and the inside surface is textured to promote the comminution of materials.

5. A size reduction apparatus for comminuting materials as claimed in claim 4 wherein the texture of the inside surface of the size reduction screen comprises parallel grooves.

6. A size reduction apparatus for comminuting materials as claimed in claim 1 wherein the impeller comprises a pair of single flat blades adapted to attach to the shaft, the impeller blades extending both vertically and horizontally across thecomminution chamber so that only a functional gap exists between peripheral edges of the impeller blades and adjacent walls of the comminution chamber.

7. A size reduction apparatus for comminuting materials as claimed in claim 1 wherein the impeller comprises a pair of single flat blades adapted to attach to the shaft, the impeller blades extending both vertically and horizontally across thecomminution chamber so that only a functional gap exists between peripheral edges of each impeller blade and adjacent walls of the comminution chamber but each impeller blade includes a window to reduce air flow through the comminution chamber.

8. A size reduction apparatus for comminuting materials as claimed in claim 1 wherein the impeller comprises a pair of single blades adapted to attach to the shaft, the impeller including a shaft attachment portion, a hammer portion and a shank portion for interconnecting the shaft attachment portion and the hammer portion, the impeller extending horizontally across the comminution chamber so that only a functional gap exists between an outer peripheral edge of the hammer portion andadjacent walls of the comminution chamber.

9. A size reduction apparatus for comminuting materials as claimed in claim 3 wherein the segments of the size reduction screen segments comprise heavy steel plate pierced by a plurality of spaced-apart bores of a size adapted to produce a desired comminuted particle size.

10. A size reduction apparatus for comminuting materials as claimed in claim 1 wherein an orientation of the propeller with respect to the impeller can be adjusted to further control the particle size of comminuted materials.

11. A size reduction apparatus for comminuting materials as claimed in claim l wherein the material inlet opening for introducing material to be comminuted is concentric with a center of rotation of the impeller.

12. A size reduction apparatus for comminuting materials as claimed in claim 1 wherein the material outlet opening for ejecting comminuted material from the housing is located on a side of the housing.

13. A size reduction apparatus for comminuting materials as claimed in claim 12 wherein the material outlet opening comprises an exit tube that is tangentially oriented to a side of the housing.

14. A size reduction apparatus for comminuting materials as claimed in claim 13 wherein the exit tube may be swiveled between a position wherein the exit tube is oriented in a direction of rotation of the impeller and the propeller and a position wherein the exit tube is oriented opposite a direction of rotation of the impeller and the propeller.