Rotary impact mill

In one aspect of the invention, a rotary impact mill has a milling chamber defined by housing with an inlet, an outlet, and at least one wall. A plurality of impact hammers located within the milling chamber are fastened to and longitudinally disposed along a rotor assembly connected a rotary driving mechanism. At least one of the impact hammers has a body with a first hardness. The impact hammer also has a wear resistant insert bonded to the body, wherein the wear resistant insert comprises a hard surface with a second hardness greater than the first hardness.

BACKGROUND OF THE INVENTION

Hammermills are often used to reduce the size of solid material. Materials often used in mills include coal, asphalt, cement, limestone, chemical fertilizer, barks, rocks, mineral, and food products. The materials are often feed into an inlet where the material falls into a milling chamber. The milling chamber typically comprises a plurality of impact hammers and may comprise a screen. The impact hammers are typically fastened at a proximal end to a rotary assembly; they are either rigidly fixed to the rotor assembly or the impact hammers may be free-swinging. As the material is feed into the chamber, the rotary assembly rotates bringing the impact hammers into contact with the material. The size reduction on each impact depends on the differential speed between the hammers and material, size of the material, and hardness of the material. If a screen is present, the screen may allow only the desired material particle size to pass to the outside of the chamber to an outlet where the particles can be collected or funneled to another machine where the material may be further processed.

Due to the impact and/or abrasive nature of the material, the impact hammers may wear requiring continual maintenance and down time of the hammermill.

In the prior art, U.S. Pat. Nos. 6,405,950; 5,938,131; 4,638,747; and U.S. Patent Publication 2004/0129808, all of which are herein incorporated by reference for all that they contain, disclose hammermills which may be compatible with the present invention.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the invention, a rotary impact mill has a milling chamber defined by a housing with an inlet, an outlet, and at least one wall. A plurality of impact hammers located within the milling chamber are fastened to and longitudinally disposed along a rotor assembly connected a rotary driving mechanism. At least one of the impact hammers has a body with a first hardness. The impact hammer also has a wear resistant insert bonded to the body, wherein the wear resistant insert comprises a hard surface with a second hardness greater than the first hardness.

In some embodiments of the present invention, the body is made of steel, stainless steel, a cemented metal carbide, manganese, hardened steel, metal or combinations thereof. The hard surface may be made of a material selected from the group consisting of diamond, natural diamond, vapor deposited diamond, polycrystalline diamond, cubic boron nitride, a cemented metal carbide, or combinations thereof. The hard surface may comprise a hardness of at least twice the first hardness and in some cases at least five times the hardness.

FIG. 1is a perspective diagram of a rotary impact mill100. A milling chamber101is defined by at least one wall102of a housing103which supports an internal screen104, which is typically cylindrical or polygonal. Within the screen104a rotary assembly105comprises a plurality of shafts106connected to a central shaft107which is in turn connected to a rotary driving mechanism (not shown). The rotary driving mechanism may be a motor typically used in the art to rotate the rotor assembly of other hammermills. Although there are four shafts106shown, two, one, or any desired number of shafts may be used. A plurality of impact hammers108are longitudinally spaced and connected to each of the shafts106at the hammer's proximal end109. The hammers108may be rigidly attached to the shafts106or the hammers108may be free-swinging. In some embodiments, the rotor assembly105comprises just the central shaft107and the impact hammers108are connected to it.

The housing103also comprises an inlet110and an outlet111. Typically the inlet110is positioned above the rotor assembly107so that gravity directs the material towards it through an opening112in the screen104, although the inlet110may instead be disposed in one of the sides113of the housing103. When in the milling chamber101, a material may be reduced upon contact with the impact hammers108. The screen104may comprise apertures (not shown) only large enough to allow the desired maximum sized particle through. Upon impact however, a distribution of particle sizes may be formed, some capable of falling through the apertures of the screen104and others too large to pass through. Since the larger particle sizes may not be able pass through the apertures, they may be forced to remain within the screen104and come into contact again with one of the impact hammers108. The hammers108may repeatably contact the material until they are sized to pass through the apertures of the screen104.

After passage through the screen104the sized reduced particles may be funneled through the outlet111for collection. In other embodiments the particles may be directed towards another machine for further processing, such as when coal is the material being reduced and fine coal particles are directed towards a furnace for producing power. It may be necessary to provide low pressure in the vicinity of the outlet111to remove the particles, especially the fines, through the outlet111. The low pressure may be provided by a vacuum.

As shown inFIG. 1, the rotor assembly105is positioned such it is substantially perpendicular to the flow of material feed into the inlet110. In other embodiments, the rotor assembly105may be positioned such that it is substantially parallel or diagonally disposed with respect to the flow of feed material. In some embodiments, there are multiple rotor assemblies.

The impact hammers108comprises a wear resistant insert114bonded to the body115of the impact hammer108. The wear resistant insert114may reduce wear of the hammer body115, which is typically more extreme at the body's distal end116.

FIG. 2is a perspective diagram of a preferred embodiment of an impact hammer108. Four wear resistant inserts114are bonded to a distal end116of the impact hammer's body115. Preferably cavities200are formed near the edge201of the body115on the impact side202of the body115. The inserts114may be brazed within the cavities200or press fit. In some embodiments, the inserts114don't protrude from body202, but are flush or retracted with in the cavity200. The inserts114may protrude out of the body 0.100 to 3.00 inches depending on the material to be reduced. In some embodiments, the inserts are simply bonded to a flat surface of the body115. The diameter203of the inserts may range from three mm to the entire width204of the body115. Preferably 13-19 mm diameter inserts are used. Preferable a longitudinal edge insert205is as close to its longitudinal edge206as possible. To achieve this, the insert205may be bonded to the body115such that a small portion of the insert205hangs over the edge206, which overhang is then removed by grinding. The overhang may be allowable, depending on the spacing of the impact hammers108along the rotor assembly105. If the overhang doesn't interfere with adjacent longitudinally spaced hammers, the grinding step may not be necessary.

The body115of the hammers114may be made of steel, stainless steel, a cemented metal carbide, manganese, hardened steel, metal, or combinations thereof; each of these materials may exhibit a first hardness of the body115. Typically hardened steel is used. The wear resistant inserts114may be of a solid material or a combination of materials. Preferably the insert114comprises the combination of a cemented metal carbide substrate208with a superhard material bonded to it, such as polycrystalline diamond, to form the hard surface207. However, a superhard material may also comprise natural diamond, vapor deposited diamond, cubic boron nitride, or combinations thereof. A hard material such as a cemented metal carbide may also be sufficient to form a hard surface207for the wear resistant insert114. Solid inserts of hard materials such as cemented metal carbides, diamond, natural diamond, vapor deposited diamond, polycrystalline diamond, or cubic boron nitride may also be used which already have an inherent hard surfaces207. The surfaces of solid hard materials, in some cases, may be made harder by doping or infiltrating the materials with higher or lower concentrations of metals and/or hard materials to achieve a desired hardness. The hardness of the hard surface207may be at least twice as hard as the first hardness of the hammer body115. In other embodiments, the hard surface207is at least five times as hard. In the preferred embodiment, a hardened steel body is used with the preferred insert.

The hard surface207may be bonded to the substrate208with a non-planar interface to increase the strength of the bond. Also the superhard material may be a sintered body, such as in embodiments where a polycrystalline diamond is used, and may be made thermally stable by removing a thin layer of metal binders (which may have a high coefficient of thermal expansion than the grains of the superhard material) in the hard surface by leaching. In other embodiments, the hard surface may comprises a metal binder concentration less 40 weight percent. In embodiments, where polycrystalline diamond is used a higher concentration of cobalt typically reduces the brittleness of the polycrystalline diamond but as a tradeoff increases it susceptibility to wear. Preferably the polycrystalline diamond has a cobalt concentration of four to ten weight percent. Adjusting the metal binder concentration in the cemented metal carbide may also have the same effect. Preferably the carbide is a tungsten carbide comprising a cobalt concentration of 6 to 14 weight percent. Polycrystalline diamond grain size distribution can also play an important role in the strength of the diamond and also in its failure mode. Preferably, the grain sizes are within 0.5 to 300 microns. Preferably, the hard surface207is also polished to reduce crack initiation starting points that may be created during manufacturing. Although several preferred characteristics have been identified, any concentrations and characteristics of hard surfaces207are encompassed within the claims.

Although the impact hammer108comprises a generally rectangular shape, the impact hammer108may comprise any general shape including, but not limited to generally cylindrical, generally triangular, tapers, beveled, generally conical, generally stepped, or combinations thereof.

In some embodiments of the present invention, the hammer is a bar hammer, a T-shaped hammer, a ring-type hammer, a toothed type-ring hammer or combinations thereof.

FIG. 3discloses a single flat insert300bonded to a distal most edge201of the hammer body115. This insert may be made of a solid material such as tungsten carbide or polycrystalline diamond, or it may also comprise a carbide substrate with a hard or superhard material bonded to it. The edge201is recessed slightly such that the hard surface207is flush with the body115. The insert300may be bonded to body115with a braze material braze material comprising silver, gold, copper, nickel, palladium, boron, chromium, silicon, germanium, aluminum, iron, cobalt, manganese, titanium, tin, gallium, vanadium, indium, phosphorus, molybdenum, platinum, or combinations thereof.FIG. 4discloses an insert similar to the embodiment disclosed inFIG. 3except that its surface207forms a positive angle400with the surface of the body115. This may be advantageous in embodiments where it is desired to have the hard surface207be more aggressive in cutting the material instead of mostly impacting the material.FIG. 5discloses a plurality of smaller inserts500bonded to the hammer108. This may be advantageous in that large polycrystalline diamond inserts may be more expensive to fabricate than smaller inserts.

FIG. 6discloses a plurality of domed inserts600bonded proximate the distal edge201of the hammer body115. Contacting the material with a domed insert600may generate a more explosive impact than a sharper insert. The desired balance of blunt inserts to sharp inserts would depend on the type of material being reduce, the rate that material is feed into the milling chamber, and the differential speed being the material and insert.FIG. 7discloses a triangular inserts700which an axial length701disposed along the width204of the hammer body115.FIG. 8discloses multiple inserts800bonded to the distal most edge201of the hammer body115which form a negative angle801with the hammer body surface. The negative angle801may reduce the forces involved with the impact between the material and the insert, but it may also reduce the inserts susceptibility to wear. Again, depending on the type of material being reduced, inserts positioned in a negative or positive rake angle desired.

FIG. 9discloses a hammer body115with domed inserts600bonded proximate the distal edge201. A distal surface900substantially normal to the axis901of the hammer body115also comprises a plurality of inserts600. This may be advantageous for reducing wear of the distal end116of the hammer108in situations where the distal end116of the hammer body108comes into contact with the screen104(seeFIG. 1) or if a material particle braces itself between the screen104and the hammer108.FIG. 10discloses a signal flat insert1000bonded directly to the distal normal surface900.FIG. 11discloses inserts1100positioned such that their axes1101form an angle1102with a line normal1103the axial length1104of the hammer body115. Again, positive or negative angles may be desirable depending on the type of material being reduced. It is believed that the harder and/or more abrasive of a material being reduced, the more negative an angle ought to be, since this would reduce the amount of wear the hard surface would be exposed to.FIG. 12discloses inserts1200bonded to longitudinal edges206of the hammer body115. Material particles may pass over the longitudinal edges206and also be susceptible to wear. The distal end116of the hammer body115is typically more susceptible to wear because it travels the farthest distance per rotation of the rotor assembly105causing the distal end116to travel at a higher velocity than the rest of the hammer body115and causing it to be more susceptible to wear. Although other regions of the hammer body may be less susceptible to wear, they may still come into contact with the material being reduced and may benefit from having a wear resistant insert bonded to it. Although the embodiment ofFIG. 12discloses a single solid long insert1200bonded to the longitudinal edge206, in other embodiments the smaller inserts may be positioned longitudinally and adjacent one another along the edge. Further any geometry of insert may be used.

FIGS. 13-32all disclose various embodiments of geometries of the inserts114. Each geometry may be advantageous depending on the material and application of the rotary impact mill. These inserts may be bonded or otherwise attached anywhere on the hammer body, although they are preferably attached proximate its distal end. In embodiments, where the rotation of the rotor assembly is revisable, it may be beneficial to have the wear resistant inserts bonded to the side of the body opposite of the impact side.

FIG. 13discloses a rounded insert600. A rounded insert600may include a domed insert, a semi-spherical insert, a conical insert, or combinations thereof. A layer of hard material, preferably a superhard material1300such as polycrystalline diamond is bonded to the substrate208. Preferably, the superhard layer is made of diamond and is bonded to the substrate208while still in the high pressure, high temperature press.FIG. 14discloses an insert with a flat head1400. A non-planar interface1401between the hard layer1300and substrate208is shown.FIG. 15discloses a stepped insert1500. This may be advantageous since the top plateau1501will contact the material first with a small surface area allowing a greater penetration into the material, thereby weakening the material just before the second plateau1502contacts the now weakened region of the material allowing the impact of the second plateau to affect a greater volume of the material.FIG. 16discloses an insert1600with a generally cylindrical shape1601and a conical end1602.FIG. 17discloses another embodiment of a stepped insert1500, but with more plateaus.FIG. 18discloses an insert1800with at least one peak1801and at least one recess1802.

FIG. 19discloses a rounded insert600with a spiral groove1900formed in it. Any pattern of grooves1900may be used. Grooves that substantially lie parallel to the axis of the insert600may also be beneficial.FIG. 20discloses a frustoconical insert2000with a conic section2001form on its plateau2002.FIG. 21discloses a generally rectangular insert2100with a concave inwardly sloping top2101.FIG. 22discloses a generally rectangular insert2200.FIG. 23discloses a frustoconical insert2300with a hard layer2301bonded to a substrate208.FIG. 24discloses a generally conical insert2400with a rounded tip2401. A non-planar interface1401is also disclosed.FIG. 25discloses a slightly convex top surface2501of an insert2500.FIG. 26discloses a generally pyramidal insert2600with a generally triangular top2601.

FIGS. 27-32all disclose an insert with an asymmetric geometry. In many cases the asymmetry may deflect the material particles in a various paths. Because the differential speed between the material and the impact hammers has end effect on the efficiency of the size reduction, it may be advantageous to deflect some of the particles. After impact with a symmetric hammer the particle will tend to travel in the same direction as the hammer, lowering the speed differential because both the material and the hammer are traveling in the same vector. However, it is believed if the particles are deflected such that some of the momentum of is pushing the particle in a different direction, the differential speed between the hammer and particle within the same vector is reduce per same unit of impact force. There may be different inserts with different geometries bonded to the same hammer body, some of which may deflect the particles in different paths from one another.

FIG. 33discloses a rotary impact mill100with a polygonal screen3300. As the impact hammers108travel within a circular path within the milling chamber101the corners3301of the polygonal screen3300may help to agitate the particles and help in size reduction. In some embodiments, there may be deflectors3301positioned within the corners3301or other places within the milling chamber101which help agitate the particles. These deflectors3302may also be subject to wear due to some of the high particle velocities. These deflectors3302may also comprise a wear resistant insert114with a hard surface. In some embodiments, the screen3300may be adapted to shake, oscillate, rock, or otherwise move to further help agitate the particles of the material.

FIG. 34discloses an embodiment of the rotary impact mill100with no screen. As material3400is feed into the milling chamber101the material is reduced upon impact with the impact hammers108and thrust towards a plurality of deflectors3302attached to at least one wall102of the milling chamber101. The material may be reduced again upon impact with the deflectors3302and again reduced each time the material comes into contact with the impact hammers108until the material particles fall through the outlet111at the bottom of the milling chamber101.

FIG. 35discloses an offset inlet of the milling chamber101. The impact hammers108direct the material3400upon contact over a screen3501disposed above the outlet111of the milling chamber101. In this case, the impact hammers108are rigidly fixed to the rotor assembly105. The hammers108force an intimate contact between the material3400and the screen3501, such that particles of the material3400are sheared off into the outlet111. In some embodiments, the screen may also move, causing the material to be reduced by attrition. Material particles too large to pass through the screen3501are cycled through the milling chamber101back to the screen3501until they are the appropriate size.