Patent Publication Number: US-7905358-B2

Title: Apparatus and methods for filtering granular solid material

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Contract No. NAS8-97238 awarded by the National Aeronautics and Space Administration (NASA). The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to apparatuses for filtering or screening granular solid materials, and to methods of filtering or screening solid material. 
     BACKGROUND OF THE INVENTION 
     There are innumerable applications in a wide range of industries in which it is necessary or desirable to filter or screen granular solid material. For example, in the agriculture industry, it is necessary to filter grain (for example, wheat, barley, and oats) to remove contaminant material prior to refining and processing the grain for human consumption. As another example, in the oil drilling industry, it is often necessary to filter formation cuttings and debris from drilling fluid prior to pumping the drilling fluid to the bottom of a well borehole being drilled. As yet another example, in the mining industry, it is often necessary or desirable to filter or screen ores from formation cuttings prior to further processing. 
     One common structure for such filters or screens includes an interwoven fabric or mesh of wires. Each of a first plurality of wires extends in a first direction generally parallel to one another, while each of a second plurality of wires extends generally perpendicular to the wires of the first plurality. Each wire extends through the mesh structure weaving over and under (in an alternating pattern) the wires extending perpendicular thereto. The resulting screen includes a plurality of apertures extending therethrough that have a generally square or rectangular cross-sectional shape. Such filters or screens are discussed in, for example, U.S. Pat. No. 1,078,380 to Reynolds, U.S. Pat. No. 2,926,785 to Sander, U.S. Pat. No. 5,626,234 to Cook et al., and U.S. Pat. No. 6,161,700 to Bakula. 
     In another common structure for such filters or screens, a plurality of apertures or holes is formed in a substantially planar sheet of material. Such filters or screens are discussed in, for example, U.S. Pat. No. 719,942 to Hermann, U.S. Pat. No. 832,012 to Custard, U.S. Pat. No. 2,496,077 to Wehner, U.S. Pat. No. 3,018,891 to Bergstrom, and U.S. Pat. No. 3,843,476 to Kramer. 
     Filters and screens are often vibrated while passing material therethrough to prevent agglomeration of the material, clogging of the screen, and to increase the overall rate at which the material passes through the screen. 
     The ability of solid particles of material to pass through a screen is at least partially a function of the size and shape of the granular material and the size and shape of the apertures of the screen. One problem that may be encountered with such filters or screens relates to contaminant matter in the form of elongated particles. For example, if a particular solid granular material comprises generally spherical particles having an average particle size (e.g., diameter), elongated particles of contaminant matter having an average length greater than the average particle size of the granular material, but cross-sectional dimensions that are smaller than the average particle size of the granular material, may be difficult to entirely remove, screen, or filter from the granular material. 
     A screen as described above may be used in an attempt to remove the elongated particles of contaminant matter from the granular material. The apertures extending through the screen may have a size and shape selected to allow the granular material to pass through the apertures, while preventing as many of the elongated particles of contaminant matter as possible from passing through the apertures. In other words, the apertures in the screen may have cross-sectional dimensions that are greater than the average particle size of the granular material, but less than the length of the elongated particles of contaminant matter. If, however, an elongated particle of contaminant matter has cross-sectional dimensions that are less than the average particle size of the granular material (and the cross-sectional dimensions of the apertures in the screen), and the elongated particle happens to be oriented such that a longitudinal axis of the elongated particle is oriented generally perpendicular to the screen, the elongated particle of contaminant matter may be capable of passing through an aperture in the screen. As a result, such filters or screens may be incapable of removing all elongated particles of contaminant matter from granular solid material. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, the present invention includes an apparatus for screening solid material. The apparatus includes a first screen and a second screen disposed adjacent the first screen. The first screen may be generally planar and may include a plurality of apertures extending therethrough. The second screen includes at least one region that is disposed at an angle relative to the first screen and at least one perforated region that includes a plurality of apertures extending therethrough. In some embodiments of the present invention, the second screen may further include at least one non-perforated region configured to prevent at least some particles of solid material from passing through the second screen. Furthermore, in some embodiments of the present invention, at least a portion of the second screen may be pleated. Such a pleated second screen may include a plurality of substantially planar regions, each of which may be oriented at an angle relative to the first screen. For example, each substantially planar region may be oriented at an acute angle of between about 20 degrees and about 70 degrees relative to the first screen. Each substantially planar region may include at least one non-perforated region configured to prevent at least some granular solid material from passing through the pleated second screen. 
     In another aspect, the present invention includes methods of screening solid material. According to the methods, particles of solid material are passed through a composite screen. In particular, particles of solid material may be passed through a first plurality of apertures in a generally planar first screen. At least some of the particles of solid material also may be passed through a second plurality of apertures in a perforated region of a second screen. The perforated region of the second screen may be disposed adjacent the first screen and oriented at an angle relative to the first screen. Some of the particles may be retained on a non-perforated region of the second screen to prevent those particles from passing through the second screen. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings in which: 
         FIG. 1  is a perspective view of a composite screen assembly that embodies teachings of the present invention; 
         FIG. 2  is a plan view of a first screen of the composite screen assembly shown in  FIG. 1 ; 
         FIG. 3  is a plan view of a second screen of the composite screen assembly shown in  FIG. 1 ; 
         FIG. 4  is a perspective view of a portion of the second screen shown in  FIG. 3 ; 
         FIG. 5  is a cross-sectional view of a portion of the composite screen assembly shown in  FIG. 1 ; 
         FIG. 6  is an enlarged view of a portion of  FIG. 5  illustrating the orientation of an aperture extending through the second screen of the composite screen assembly; 
         FIG. 7  is an enlarged view like that of  FIG. 6  illustrating an additional embodiment of a second screen that may be used with the composite screen assembly of  FIG. 1 , in which the apertures extending through the second screen are oriented at an angle relative to a surface of the screen; 
         FIG. 8  is a side view of the second screen shown in  FIG. 3 ; 
         FIG. 9  is a side view like that of  FIG. 8  illustrating an additional embodiment of a second screen that may be used with the composite screen assembly of  FIG. 1 , in which an edge or surface of the second screen extends at an angle relative to the gravitational field when material is passed through the second screen; 
         FIG. 10  is a perspective view of another composite screen assembly that embodies teachings of the present invention; 
         FIG. 11  is a cross-sectional view of the composite screen assembly shown in  FIG. 10 ; and 
         FIG. 12  is a top plan view of an additional embodiment of a screen that may be used with the composite screen assembly shown in  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A composite screen assembly  10  that embodies teachings of the present invention is shown in  FIG. 1 . The composite screen assembly  10  may be used for screening or filtering contaminant matter (such as, for example, particles of a foreign material) from solid granular material. The composite screen assembly  10  includes a first screen  12  and a second screen  14 . As shown in  FIG. 1 , the first screen  12  may be disposed adjacent the second screen  14  such that matter passing through the first screen  12  encounters the second screen  14 . In the configuration shown in  FIG. 1 , the first screen  12  is positioned over the second screen  14 . The composite screen assembly  10  optionally may include a frame assembly  18 . Furthermore, one or more handles  26  may be provided on the composite screen assembly  10  to facilitate handling thereof. 
       FIG. 2  is a plan view of a first screen of the composite screen assembly shown in  FIG. 1 . Referring to  FIG. 2 , the first screen  12  may be generally planar. The first screen  12  may include a plurality of apertures  30  formed through a substantially planar layer of material  32  of the first screen  12 . By way of example and not limitation, the substantially planar layer of material  32  may be a layer of sheet metal. In additional embodiments, the substantially planar layer of material  32  may include a polymer material (such as, for example, polyurethane or polyethylene), a ceramic material (such as, for example, alumina, silica, zirconia, or silicon nitride), or any other solid material. The apertures  30  may be disposed in a selected, ordered array across the first screen  12 . By way of example and not limitation, the apertures  30  may be disposed in a plurality of rows and columns. As an example, the apertures  30  may be disposed in a hexagonal pattern (often referred to as a triangular pattern), as shown in  FIG. 2 . In additional embodiments, it is contemplated that the apertures  30  may be disposed in a square pattern, a rectangular pattern, or any other pattern. Furthermore, the first screen  12  may include a fabric or mesh of interwoven wires, thread, fibers, etc. 
     The second screen  14  of the composite screen assembly  10  ( FIG. 1 ) is shown in  FIG. 3 . The second screen  14  includes a plurality of apertures  34  formed through a layer of material  36 . The layer of material  36  of the second screen  14  may be formed from or include the same material used to form the layer of material  32  of the first screen  12 . In additional embodiments, the layer of material  36  of the second screen  14  and the layer of material  32  of the first screen  12  may be formed from or include different materials. 
     In the embodiment shown in  FIG. 3 , the second screen  14  is not substantially planar. In contrast to the first screen  12 , the layer of material  36  of the second screen  14  may have an accordion or pleated structure in which a plurality of alternating folds define a plurality of substantially planar regions  40 - 1 ,  40 - 2  . . .  40 - i , each of which may be disposed at an angle relative to adjacent substantially planar regions  40 - 1 ,  40 - 2  . . .  40 - i  and the first screen  12 . 
     The first screen  12  may include a first frame member  20 , as shown in  FIG. 2 , and the second screen  14  may include a second frame member  22 , as shown in  FIG. 3 . When the first screen  12  is positioned over and adjacent the second screen  14  to form the composite screen assembly  10  shown in  FIG. 1 , the first frame member  20  and the second frame member  22  together may form the frame assembly  18  shown in  FIG. 1 . Optionally, the first frame member  20  may be welded, bolted, or otherwise secured to the second frame member  22 . In additional embodiments, the first frame member  20  may simply rest upon the second frame member  22 , or a snap-fit may be provided between the first frame member  20  and the second frame member  22 , when the composite screen assembly  10  is being used to filter a particular solid material. Furthermore, complementary features may be formed on the first frame member  20  and the second frame member  22  to facilitate alignment of the first frame member  20  with the second frame member  22 . By way of example, a plurality of pins (not shown) may be provided that extend from a surface of the first frame member  20 , and a plurality of complementary holes configured to receive the pins may be provided in an opposing surface of the second frame member  22 , or vice versa. Complementary ridges and grooves, or any other complementary alignment features, may be used in place of, or in addition to, pins and holes. 
       FIG. 4  is an enlarged perspective view of a portion of the layer of material  36  of the second screen  14 . As shown therein, the apertures  34  may be located in a perforated region  42 - 1 ,  42 - 2  . . .  42 - l  of each substantially planar region  40 - 1 ,  40 - 2  . . .  40 - i  of the second screen  14 . Furthermore, each substantially planar region  40 - 1 ,  40 - 2  . . .  40 - i  of the second screen may include at least one substantially non-perforated region  44 - 1 ,  44 - 2  . . .  44 - m , in which no apertures  34  are provided. 
     The apertures  34  may be disposed in a selected, ordered array across the second screen  14  in each of the perforated regions  42 - 1 ,  42 - 2  . . .  42 - l  thereof. By way of example and not limitation, the apertures  34  may be disposed in a plurality of rows and columns. As an example, the apertures  34  may be disposed in a hexagonal pattern (often referred to as a triangular pattern), as shown in  FIG. 4 . In additional embodiments, the apertures  34  may be disposed in a square pattern, a rectangular pattern, or any other pattern or substantially ordered array. 
     In some embodiments of the invention, each of the perforated regions  42 - 1 ,  42 - 2  . . .  42 - l  may have a width, measured as the width of the smallest rectangle capable of encompassing each of the apertures  34  extending therethrough, that is between about 35% and about 65% of a width of each of the substantially planar regions  40 - 1 ,  40 - 2  . . .  40 - i  of the second screen  14 . In one particular embodiment of the invention, set forth merely as an example, each of the perforated regions  42 - 1 ,  42 - 2  . . .  42 - l  may have a width, measured as the width of the smallest rectangle capable of encompassing each of the apertures  34  extending therethrough, that is about 12.7 millimeters (½ of an inch), and each of the substantially planar regions  40 - 1 ,  40 - 2  . . .  40 - i  of the second screen  14  may have a width that is about 25.4 millimeters (about 1 inch). Furthermore, in some embodiments of the present invention, each of the perforated regions  42 - 1 ,  42 - 2  . . .  42 - l  may be generally centered within each of the respective substantially planar regions  40 - 1 ,  40 - 2  . . .  40 - i  of the second screen  14 . 
       FIG. 5  is a partial cross sectional view of the composite screen assembly  10  ( FIG. 1 ) illustrating the first screen  12  and the second screen  14 . As shown therein, the layer of material  32  of the first screen  12  includes a first major surface  46  and an opposing second major surface  48 . Similarly, the layer of material  36  of the second screen  14  includes a first major surface  52  and an opposing second major surface  54 . The first major surface  52  of the layer of material  36  is on a side of the second screen  14  generally facing the first screen  12 . The alternating folds of the accordion or pleated second screen  14  may define a plurality of concave edges, each of which defines a valley  58 - 1 ,  58 - 2  . . .  58 - k , and a plurality of convex edges, each of which defines a peak  60 - 1 ,  60 - 2  . . .  60 - j . The concave edges defining the valleys  58 - 1 ,  58 - 2  . . .  58 - k  and the convex edges defining the peaks  60 - 1 ,  60 - 2  . . .  60 - j  each extend along the first major surface  52  of the layer of material  36  of the second screen  14 . As used herein, the term “concave edge” means any edge defined at the intersection between two intersecting surfaces wherein the angle between the intersecting surfaces adjacent the edge is less than 180 degrees. As used herein, the term “convex edge” means any edge defined between two intersecting surfaces wherein the angle between the surfaces adjacent the edge is greater than 180 degrees. Such intersecting surfaces may be planar, curved, or may have any shape. In this manner, the plurality of concave edges form a plurality of valleys  58 - 1 ,  58 - 2  . . .  58 - k  on the first major surface  52  of the second screen  14 , while the plurality of convex edges form a plurality of peaks  60 - 1 ,  60 - 2  . . .  60 - j  on the first major surface  52  of the second screen  14 . 
     As shown in  FIG. 5 , the non-perforated regions  44 - 1 ,  44 - 2  . . .  44 - m  of each substantially planar region  40 - 1 ,  40 - 2  . . .  40 - i  of the second screen  14  may be disposed adjacent the valleys  58 - 1 ,  58 - 2  . . .  58 - k . As illustrated in  FIG. 3 , the plurality of valleys  58 - 1 ,  58 - 2  . . .  58 - k  and the plurality of peaks  60 - 1 ,  60 - 2  . . .  60 - j  may be substantially linear (i.e., extending in a substantially straight direction), and may extend substantially parallel to one another across the second screen  14 . In this configuration, each substantially planar region  40 - 1 ,  40 - 2  . . .  40 - i  may have a substantially identical rectangular shape. In additional embodiments, the plurality of valleys  58 - 1 ,  58 - 2  . . .  58 - k  and the plurality of peaks  60 - 1 ,  60 - 2  . . .  60 - j  may be non-linear and may not extend in a parallel manner across the second screen  14 . In such a configuration, the substantially planar regions  40 - 1 ,  40 - 2  . . .  40 - i  may have different shapes. Furthermore, each of the valleys  58 - 1 ,  58 - 2  . . .  58 - k  may be substantially disposed in a single plane, and each of the peaks  60 - 1 ,  60 - 2  . . .  60 - j  may be substantially disposed in a single plane. In additional embodiments, the valleys  58 - 1 ,  58 - 2  . . .  58 - k  may not be disposed in a single plane, and the peaks  60 - 1 ,  60 - 2  . . .  60 - j  may not be disposed in a single plane. 
     Referring again to  FIG. 5 , each substantially planar region  40 - 1 ,  40 - 2  . . .  40 - i  may be oriented at an angle  43  relative to adjacent substantially planar regions  40 - 1 ,  40 - 2  . . .  40 - i . By way of example and not limitation, each angle  43  may be between about 20 degrees and about 70 degrees. More particularly, each angle  43  may be between about 40 degrees and about 50 degrees. In one particular embodiment, set forth merely as an example, each angle  43  may be approximately 45 degrees. Furthermore, each substantially planar region  40 - 1 ,  40 - 2  . . .  40 - i  may be oriented at an angle relative to the first screen  12 . By way of example and not limitation, each substantially planar region  40 - 1 ,  40 - 2  . . .  40 - i  may be oriented at an acute angle between about 20 degrees and about 80 degrees relative to the first screen  12 . More particularly, each substantially planar region  40 - 1 ,  40 - 2  . . .  40 - i  may be oriented at an acute angle between about 40 degrees and about 80 degrees relative to the first screen  12 . In one particular embodiment, set forth merely as an example, each substantially planar region  40 - 1 ,  40 - 2  . . .  40 - i  may be oriented at an acute angle of about 67.5 degrees relative to the first screen  12 . 
     The non-perforated regions  44 - 1 ,  44 - 2  . . .  44 - m  of each substantially planar region  40 - 1 ,  40 - 2  . . .  40 - i  of the second screen  14  may be disposed adjacent the valleys  58 - 1 ,  58 - 2  . . .  58 - k . In this configuration, the non-perforated regions  44 - 1 ,  44 - 2  . . .  44 - m  may be configured to prevent at least some material from passing through the second screen  14  when the material is being screened or filtered using the composite screen assembly  10 . As shown in  FIG. 5 , to filter material (not shown) using the composite screen assembly  10 , the composite screen assembly  10  may be oriented substantially horizontally (relative to the gravitational field), and particulate material may be poured, dumped, or otherwise provided on the first major surface  46  of the first screen  12 . At least some of the material may pass through the apertures  30  of the first screen  12 , as indicated by the directional arrows. As material passes through the apertures  30  of the first screen, the material falls onto the first major surface  52  of the second screen  14 . At least some of the material may fall onto the non-perforated regions  44 - 1 ,  44 - 2  . . .  44 - m  of the second screen  14 . This material may be collected in the valleys  58 - 1 ,  58 - 2  . . .  58 - k  adjacent the non-perforated regions  44 - 1 ,  44 - 2  . . .  44 - m . At least some of the material falling onto the first major surface  52  of the second screen  14  may pass through the apertures  34  of the second screen  14 , as indicated by the directional arrows. As shown in  FIG. 5 , the directional arrows passing through the apertures  34  of the second screen  14  are oriented at an angle with respect to the directional arrows passing through the apertures  30  of the first screen  12 . 
     In this configuration, as particles or granules of material pass through the composite screen assembly  10 , the particles must change direction at least one time as the particles pass through the first screen  12  and the second screen  14 . This change in direction may hinder or prevent elongated contaminant particles from passing through the composite screen assembly. For example, elongated particles of contaminant matter may have cross-sectional dimensions that allow the elongated particles to pass through the apertures  30  of the first screen  12  (and the apertures  34  of the second screen) when the longitudinal axes of the elongated particles are appropriately oriented relative to the apertures  30  of the first screen  12 . The elongated particles of contaminant matter may have longitudinal dimensions that prevent the elongated particles from passing through the apertures  30  of the first screen  12  (and/or the apertures  34  of the second screen  14 ) when the longitudinal axes of the elongated particles are oriented generally transverse to the apertures  30  of the first screen  12  (and/or the apertures  34  of the second screen  14 ). If elongated particles of contaminant matter happen to be aligned with and pass through an aperture  30  of the first screen  12 , such elongated particles are likely to be oriented generally transverse relative to the apertures  34  of the second screen  14 , and therefore, may be unlikely to pass through the apertures  34  of the second screen  14  and collected in the valleys  58 - 1 ,  58 - 2  . . .  58 - k  adjacent the non-perforated regions  44 - 1 ,  44 - 2  . . .  44 - m  of the second screen  14 . 
       FIG. 6  is an enlarged view of a portion of the second screen  14  illustrating an aperture  34  that has been formed through the layer of material  36  of the second screen  14  from the first major surface  52  to the second major surface  54  thereof. As shown in  FIG. 6 , in some embodiments of the present invention, the apertures  34  may be defined by a substantially cylindrical surface  67  of the layer of material  36  of the second screen  14 . In this configuration, each aperture  34  may have a generally circular cross-sectional shape and a longitudinal axis  35 . In some embodiments, the longitudinal axis  35  may be oriented substantially perpendicular to the first major surface  52  of the second screen  14 . As illustrated in  FIG. 7 , in additional embodiments of the present invention, the longitudinal axis  35  of each aperture  34  may be oriented at an angle  68  relative to the first major surface  52  of the second screen  14 . In such a configuration, any elongated particles of contaminant matter that have passed through the first screen  12  may be more likely to be oriented generally transverse to the apertures  34  of the second screen  14 , and therefore, unlikely to pass through the apertures  34  of the second screen  14 . By way of example and not limitation, the angle  68  between the longitudinal axis  35  of each aperture  34  and the first major surface  52  of the second screen  14  may be between about 20 degrees and about 80 degrees. More particularly, the angle  68  between the longitudinal axis  35  of each aperture  34  and the first major surface  52  of the second screen  14  may be between about 40 degrees and about 80 degrees. In one particular embodiment of the present invention, set forth merely as an example, the angle  68  between the longitudinal axis  35  of each aperture  34  and the first major surface  52  of the second screen  14  may be about 67.5 degrees. 
     Referring again to  FIG. 5 , the apertures  30  of the first screen  12  may have a size and shape that is substantially identical to the size and shape of the apertures  34  of the second screen  14 . In other embodiments, the apertures  30  of the first screen  12  may have a size that differs from a size of the apertures  34  of the second screen  14 , a shape that differs from a shape of the apertures  34  of the second screen  14 , or both a size and shape that differs from a size and shape of the apertures  34  of the second screen  14 . By way of example and not limitation, each of the apertures  30  of the first screen  12  and the apertures  34  of the second screen  14  may have a substantially circular cross-sectional shape. 
     In some embodiments of the present invention, the substantially uniform diameter of the apertures  30  of the first screen  12  may be between about 1.1 times and about 15 times an average particle size of particles of solid material to be screened using the composite screen assembly  10 . More particularly, the substantially uniform diameter of the apertures  30  of the first screen  12  may be between about 5 times and about 10 times an average particle size of the particles of solid material to be screened using the composite screen assembly  10 . Furthermore, in some embodiments of the present invention, the apertures  34  of the second screen  14  may have a substantially uniform diameter that is between about 1.3 and about 1.7 times the substantially uniform diameter of the apertures  30  of the first screen  12 . 
     In one particular embodiment, set forth merely as an example, a solid particulate material may have an average particle size of about 0.20 millimeter, the apertures  30  of the first screen  12  may have a substantially uniform diameter of between about 0.22 millimeter and about 3.00 millimeters, and the apertures  34  of the second screen  14  may have a substantially uniform diameter between about 2.85 millimeters and about 5.10 millimeters. For example, the apertures  30  of the first screen  12  may have a substantially uniform diameter of about 2.40 millimeters and the apertures  34  of the second screen  14  may have a substantially uniform diameter of about 3.20 millimeters. 
     In some embodiments of the present invention, the apertures  30  of the first screen  12  may comprise between about 20% and about 50% of the area of the first screen  12 , and the layer of material  32  may comprise between about 50% and about 80% of the area of the first screen  12 . Similarly, the apertures  34  of the second screen  14  may comprise between about 10% and about 30% of the area of the second screen  14 , and the layer of material  36  may comprise between about 70% and about 90% of the area of the second screen. In one particular embodiment, set forth merely as an example, the apertures  30  of the first screen  12  may comprise about 33% of the area of the first screen  12 , and the layer of material  32  may comprise the remainder of the area of the first screen  12 . Similarly, the apertures  34  of the second screen  14  may comprise about 20% of the area of the second screen  14 , and the layer of material  36  may comprise the remainder of the area of the second screen  14 . 
     It may be necessary or desirable when screening particulate material using the composite screen assembly  10  to determine whether any particles of contaminant matter are present in the particular material being screened. Optionally, the first screen  12  may be periodically removed during a screening process, and material that has been collected in the valleys  58 - 1 ,  58 - 2  . . .  58 - k  of the second screen  14  adjacent the non-perforated regions  44 - 1 ,  44 - 2  . . .  44 - m  may be tested or otherwise inspected to detect the presence of any contaminant particles contained therein. 
       FIG. 8  is a side view of a portion of the second screen  14 . As shown therein, the valleys  58 - 1 ,  58 - 2  . . .  58 - k  may extend substantially parallel across the second screen  14  relative to the peaks  60 - 1 ,  60 - 2  . . .  60 - j . An additional embodiment of a second screen  14 ′ that may be used with the composite screen assembly  10  ( FIG. 1 ) is shown in  FIG. 9 . As shown therein, the valleys  58 - 1 ,  58 - 2  . . .  58 - k  may extend at an angle  70  across the second screen  14 ′ relative to the peaks  60 - 1 ,  60 - 2  . . .  60 - j . In this configuration, as particles of material being screened pass through the composite screen assembly  10  ( FIG. 1 ) in the direction illustrated by the directional arrows, at least some of the particles may fall onto the non-perforated regions  44 - 1 ,  44 - 2  . . .  44 - m  ( FIG. 5 ) of the second screen  14  and may be collected in the valleys  58 - 1 ,  58 - 2  . . .  58 - k  of the second screen  14  adjacent non-perforated regions  44 - 1 ,  44 - 2  . . .  44 - m . These particles of material that are collected in the valleys  58 - 1 ,  58 - 2  . . .  58 - k  adjacent non-perforated regions  44 - 1 ,  44 - 2  . . .  44 - m  may migrate (at least partially due to gravity) down the slope that results from the angle  70  between the valleys  58 - 1 ,  58 - 2  . . .  58 - k  and the peaks  60 - 1 ,  60 - 2  . . .  60 - j  in the direction indicated by directional arrow  74 . 
     A funnel, chute, vacuum source or other collection device  76  configured to collect particles of material may be provided and used to collect the particles of material that migrate across the second screen  14 ′ down the slope. In this configuration, the material that is collected by the collection device  76  may be inspected to detect the presence of contaminant matter. In the configuration shown in  FIG. 9 , the material that is collected by the collection device  76  may be inspected without interrupting the screening process to remove the first screen  12 , as previously described herein. Furthermore, in this configuration, the material that is collected by the collection device  76  may be continuously inspected without interrupting the screening process. As a result, the efficiency of a screening process may be improved by using the second screen  14 ′ as part of the composite screen assembly  10  previously described herein. 
     The composite screen assembly  10  previously described herein is illustrated as having a generally rectangular shape. Other embodiments of the present invention may have other shapes and configurations. 
     Another composite screen assembly  90  that embodies teachings of the present invention is shown in  FIGS. 10 and 11 . The composite screen assembly  90  includes a first screen  92  and a second screen  94 . Optionally, the composite screen assembly  90  also may include a housing  98 . As shown in  FIGS. 10 and 11 , the housing  98  may have a frustoconical shape. In additional embodiments, the housing  98  may have a generally cylindrical shape or any other shape. 
     Referring to  FIG. 11 , the first screen  92  may include a plurality of apertures  100  each extending through a layer of material  102 . The second screen  94  may include a layer of material  106  that has a generally conical shape. The layer of material  106  of the second screen  94  may include a perforated region  110  in which a plurality of apertures  104  extend through the layer of material  106 , and a non-perforated region  112  that is substantially free of apertures  104 . As shown in  FIG. 11 , the non-perforated region  112  may be located below the perforated region  110  (when the composite screen assembly  90  is oriented generally horizontally with respect to gravity) and may include the bottom-most point  116  formed by the conical second screen  94 . In this configuration, the non-perforated region  112  of the second screen  94  is configured to prevent at least some particles of material from passing through the second screen  94  during a screening process. 
     The composite screen assembly  90  may be used to filter or screen particulate material in a manner substantially similar to that previously described in relation to the composite screen assembly  10 . In particular, particulate material may be poured, dumped, or otherwise provided onto the first screen  92 . At least some of the particles of material may pass through the apertures  100  of the first screen  92 , in the direction generally represented by the directional arrows. As particles of material pass through the apertures  100  of the first screen  92 , the particles fall onto the second screen  94 . At least some of the particles of material may fall onto the non-perforated region  112  of the second screen  94 . These particles of material may be collected in the non-perforated region  112  of the second screen  94  and prevented from passing through the second screen  94 . At least some of the particles of material may fall onto perforated region  110  of the second screen  94  and may pass through the apertures  104  of the second screen  94 , in the direction generally represented by the directional arrows. As shown in  FIG. 11 , the directional arrows passing through the apertures  104  of the second screen  94  are oriented at an angle with respect to the directional arrows passing through the apertures  100  of the first screen  92 . 
     In this configuration, as particles or granules of material pass through the composite screen assembly  90 , the particles must change direction at least one time as the particles pass through the first screen  92  and the second screen  94 . This change in direction may hinder or prevent elongated particles of foreign material from passing through the composite screen assembly in the same manner previously described in relation to the composite screen assembly  10 . 
     An additional embodiment of a second screen  94 ′ that may be used with the composite screen assembly  90  ( FIGS. 10 and 11 ) is shown in  FIG. 12 . The second screen  94 ′ may include a plurality of concentric concave edges each defining a valley  126 - 1 ,  126 - 2  . . .  126 - n  and a plurality of concentric convex edges each defining a peak  128 - 1 ,  128 - 2  . . .  128 - o . A plurality of regions  130 - 1 ,  130 - 2  . . .  130 - p , each having a generally frustoconical shape, may be defined between adjacent valley  126 - 1 ,  126 - 2  . . .  126 - n  and peak  128 - 1 ,  128 - 2  . . .  128 - o . Each frustoconical region  130 - 1 ,  130 - 2  . . .  130 - p  may include a perforated region and a non-perforated region (not shown) similar to those previously described in relation to the second screen  14  ( FIG. 3 ). The non-perforated regions may be disposed adjacent the valleys  126 - 1 ,  126 - 2  . . .  126 - n  in the second screen  94 ′, in which particles of material may be collected and prevented from passing through the second screen  94 ′. In such a configuration, a cross-section of the second screen  94 ′ extending through the center  132  of the second screen may appear substantially similar to the cross-sectional view of the second screen  14  shown in  FIG. 5 . 
     During a screening or filtering process using a screen assembly that embodies teachings of the present invention (such as, for example, the composite screen assembly  10  shown in  FIG. 1  and the composite screen assembly  90  shown in  FIG. 10 ), a device configured to transmit mechanical vibrations to the screen assembly may be used to enhance the flow of particulate material through the screen assembly. Furthermore, referring again to  FIG. 9 , when using a second screen such as the second screen  14 ′, mechanical vibrations transmitted to the composite screen assembly  10 , and in particular the second screen  14 ′, may facilitate migration of particulate material in the valleys of the second screen  14 ′ down the slope that results from the angle  70  between the valleys  58 - 1 ,  58 - 2  . . .  58 - k  and the peaks  60 - 1 ,  60 - 2  . . .  60 - j  in the direction indicated by directional arrow  74  and towards the collection device  76 . 
     There are certain applications in which the present invention may be particularly useful. Such applications include the screening of materials that are likely to include elongated particles of contaminant matter. By way of example and not limitation, certain methods of manufacturing granular ammonium perchlorate may result in the inadvertent inclusion of elongated particles of metal with the granular ammonium perchlorate. As a result, the present invention may find particular utility in screening particles of solid ammonium perchlorate to remove elongate particles of foreign material. Furthermore, it is contemplated that screening apparatuses that embody teachings of the present invention may be used to filter or screen solid material from a liquid material. For example, a slurry or a suspension may be passed through a screening apparatus that embodies teachings of the present invention to remove at least some solid matter from the slurry or suspension. 
     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.