Patent Publication Number: US-6340240-B1

Title: Drum mixer having isolated aggregate transport channels

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. Ser. No. 09/324,248, filed Jun. 2, 1999, now U.S. Pat. No. 6,267,493 entire content of which is hereby expressly incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Drum mixers for manufacturing asphaltic compositions out of an aggregate material are known in the art. An aggregate material known as “recycled asphaltic pavement” (RAP) is an inexpensive and plentiful aggregate material which can be used to manufacture an asphaltic composition. The RAP material is formed from a mixture of an asphaltic material, aggregates and mineral binder or “fines”. 
     Virgin aggregates can also be used in manufacturing asphaltic compositions. As the virgin aggregate flows through the drum mixer, it is combined with liquid asphalt and fines to produce the asphalt composition. However, producing an asphaltic composition from virgin aggregate is more expensive than producing the asphaltic composition from RAP material because the virgin aggregate is more costly than the RAP material, and more asphaltic material must be added to the virgin aggregate. 
     When RAP material has been used in previous drum mixers, the RAP material was introduced into the drum mixer in a different location separate from the virgin aggregate to minimize what is known in the art as “blue smoke” and also to not degrade the RAP material. And, as a practical matter, the ratio of RAP material which could be used relative to virgin aggregate was about 25% with maximums up to 50% in some cases. Thus, it has been necessary to use a substantial amount of expensive virgin aggregate in producing the asphaltic composition. 
     By increasing the ratio of RAP material to virgin aggregate, the costs of manufacturing the asphaltic composition can be significantly reduced. It is to such a drum mixer for manufacturing an asphaltic composition out of a high ratio of RAP material to virgin aggregate material that the present invention is directed. 
     SUMMARY OF THE INVENTION 
     The present invention is a drum mixer for heating, mixing and drying an aggregate material, such as a mixture of recycled asphaltic pavement and virgin aggregate. The drum mixer includes an inclined drum having a first end and a second end. The drum forms a pre-heating/blending section adjacent the first end, a heating/mixing section adjacent the second end, a heating/drying/mixing section therebetween, and a discharge outlet adjacent the second end. 
     A feed assembly for feeding aggregate material is provided. The feed assembly feeds the aggregate material into the pre-heating/blending section of the drum for movement of the aggregate material sequentially through the pre-heating/blending section, the heating/drying/mixing section, the heating/mixing section, and the discharge outlet. A burner assembly extends from the second end of the drum into the heating/drying/mixing section of the drum. The burner assembly creates a high temperature gas stream which flows through the heating/drying/mixing and preheating/blending sections of the drum. 
     Tubular compartments are positioned in the heating/drying/mixing section of the drum so as to form a plurality of aggregate transporting channels. The aggregate transporting channels within the tubular compartments are out of direct contact with the high temperature gas stream while the tubular compartments are exposed to the high temperature gas stream. Thus, the aggregate material is heated, and dried indirectly via conduction of heat through the tubular compartments as the aggregate material passes through the aggregate transporting channels. 
     In some aspects of the present invention, the drum mixer may also include one or more fluid injectors and fines injectors, both of which communicate with the drum for injecting an asphaltic fluid, fines or other additives, such as fibers or anti-strip agents into the drum so that the asphaltic fluid, fines or other additives are mixed with the aggregate material to form the asphaltic composition. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a schematic view of a drum mixer constructed in accordance with the present invention. 
     FIG. 2 is a cross-sectional view of the drum mixer taken along lines  2 — 2  in FIG.  1 . 
     FIG. 3 is a cross-sectional view of the drum mixer taken along lines  3 — 3  of FIG.  1 . 
     FIG. 4 is a cross-sectional view of the drum mixer taken along lines  4 — 4  of FIG.  1 . 
     FIG. 5 is a partial, perspective view of a mixing/heating/drying section of the drum mixer depicted in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     Referring now to the drawings and in particular to FIG. 1, shown therein is a drum mixer  10  constructed in accordance with the present invention, for heating, drying and mixing of aggregate material (not shown) in high-temperature desorption applications. One use of the present invention is for manufacturing asphaltic compositions out of a variable ratio of a mixture of recycled asphaltic pavement (RAP material) and virgin aggregate. When used for manufacturing asphaltic compositions, the ratio of RAP material and virgin aggregate in the mixture can be varied between 100% RAP material to 0% virgin aggregate, and 0% RAP material to 100% virgin aggregate without any modification to the drum mixer  10 . 
     The drum mixer  10  includes an inclined drum  12  which has a first end  14  and a second end  16 . The drum  12  forms a preheating/blending section  18 , a mixing/heating section  20  and a mixing/heating/drying section  22 . The drum  12  is inclined such that the section  18  is raised above the section  20  so that the aggregate material moving through the drum  12  flows toward the section  20 . The angle of the incline can be adjusted via any suitable mechanical assembly, such as adjustable hydraulic supports (not shown). The aggregate material passing through the drum  12  is indicated in FIG. 1 by the arrows  23 . 
     The section  18  is disposed adjacent the first end  14  of the drum  12 . The section  20  is disposed adjacent the second end  16  of the drum  12 . The section  22  is disposed between the section  18  and the section  20 . 
     The drum mixer  10  is also provided with a feeder assembly  24 . The feeder assembly  24  serves to feed aggregate material into the first end  14  of the inclined drum  12  for movement of the aggregate material sequentially through the section  18 , the section  22 , and the section  20 . The feeder assembly  24  can be any suitable feeder assembly, such as a screw auger, a chute, or a fast-moving conveyor belt which projects the aggregate material through the first end  14  of the inclined drum  12  and thereby into the section  18 . 
     The drum mixer  10  is also provided with a burner assembly  26 . The burner assembly  26  is shown in dashed lines in FIG.  1 . The burner assembly  26  extends from the second end  16  of the inclined drum  12  and into the section  22  of the inclined drum  12 . When ignited, the burner assembly  26  creates a high temperature gas stream to radiantly, convectively and conductively heat the interior of the drum  10 . The high temperature gas stream flows through the section  22  and the section  18 , so as to indirectly heat the aggregate material as the aggregate material passes through the section  22  and to come into contact with and thereby directly heat the aggregate material as the material travels through the section  18 . 
     As best shown in the cross-sectional views of FIGS. 2,  3  and  4 , a plurality of tubular compartments  28 , which are constructed of a heat conductive material such as stainless steel, is supported in the section  22  of the inclined drum  12 . Although twelve tubular compartments  28  are depicted in FIGS. 2,  3  and  4 , for purposes of clarity, only three tubular compartments  28  are labeled as  28   a ,  28   b , and  28   c . It will be understood that twelve tubular compartments  28  are shown in FIGS. 2,  3  and  4  by way of example only, and more or fewer tubular compartments  28  can be used in practicing the present invention. The tubular compartments  28  define a plurality of aggregate transporting channels  32 , only three of which are labeled for purposes of clarity in FIGS. 2,  3 , and  4  by the reference numerals  32   a ,  32   b  and  32   c.    
     The tubular compartments  28  extend longitudinally through at least a portion of the section  22  of the drum  12 . The tubular compartments  28  are exposed to the high temperature gas stream and are thereby directly heated by the high temperatures gas stream. Each of the tubular compartments  28  defines an aggregate transporting channel  32 , within the tubular compartments  28 , such that at least a portion of each of the aggregate transporting channels  32  is isolated from the other aggregate transporting channels  32 . The aggregate transporting channels  32 , within the tubular compartments, are out of direct contact with the high temperature gas stream produced by the burner assembly  26 . Thus, the aggregate material passing through the aggregate transporting channels  32  is heated indirectly by the high temperature gas stream via conduction through the heat conductive tubular compartments  28  as the aggregate material passes through the aggregate transporting channels  32 . A plurality of flights  33  may be supported within the aggregate transporting channels  32  so as to move the aggregate material through the aggregate transporting channels  32  and into the section  20  of the drum  12  as the drum  12  rotates. 
     The tubular compartments  28  are circumferentially positioned so that interior surfaces  34  (FIG. 4) on the tubular compartments  28  surround and define an internal combustion chamber  35  (FIGS. 1 and 4) within the drum  12 . The combustion chamber  35  is positioned in the section  22  to receive the high temperature gas stream produced by the burner assembly  26 . The drum mixer  10  also includes an outer shell  36  which surrounds the tubular compartments  28  in the section  22 . The outer shell  36  is constructed of a thermally insulating material such as a ceramic material encompassed by an outer metal hull (not shown). 
     The outer shell  36  defines a longitudinally extending cavity  38  therein. The outer shell  36  includes a first inwardly extending annular shoulder portion  40  (FIG. 1) and a second inwardly extending annular shoulder portion  42  (FIG.  1 ). 
     The tubular compartments  28  are provided with exterior surfaces  43 . Only two of the exterior surfaces  43  are labeled as  43   a  and  43   b  for purposes of clarity in FIGS. 2,  3  and  4 . The first and second shoulder portions  40  and  42  matingly engage the exterior surfaces  43  of the tubular compartments  28  so as to form a seal therebetween, whereby the outer shell  36  rotates with the drum and a longitudinally extending annular flue gas exhaust passageway  44  is formed in between the exterior surfaces  43  of the tubular compartments  28  and the outer shell  36 . The annular flue gas exhaust passageway  44  extends in between the first shoulder portion  40  and the second shoulder portion  42  of the outer shell  36 . The outer shell  36  rotates or moves with the inclined drum  12  as the inclined drum  12  is rotated. 
     As best shown in FIGS. 4 and 5, the tubular compartments  28  are suspended in a spaced-apart relationship to form a plurality of flue gas exhaust channels  48  extending therebetween. Although twelve exhaust channels  48  are depicted in FIG. 4, only three of the flue gas exhaust channels  48  have been labeled in FIG. 4 with the reference numerals  48   a ,  48   b  and  48   c  for purposes of clarity. In general, each flue gas exhaust channel  48  is disposed between a pair of adjacently disposed tubular compartments  28 . The flue gas exhaust channels  48  are arranged to receive the high temperature gas stream from the combustion chamber  35  and to discharge the high temperature gas stream into the annular flue gas exhaust passageway  44 . The flue gas exhaust channels  48  extend substantially the entire longitudinal length of the tubular compartments  28 . The flue gas exhaust channels  48  extend radially outwardly from the combustion chamber  35 . 
     As best shown in FIG. 1, the burner assembly  26  of the drum mixer  10  includes a burner head  50  positioned adjacent to or within the combustion chamber  35  to inject the high temperature gas stream into the combustion chamber  35  as indicated by the arrow  52 . The high temperature gas stream travels from the combustion chamber  35  radially outwardly into the flue gas exhaust channels  48  as indicated by the arrows  54  and  56 , and into the annular passageway  44  so that the high temperature gas stream surrounds the tubular compartments  28  thereby heating all sides of the tubular compartments  28 . 
     The drum mixer  10  also includes a flue gas diverter  58  (FIGS. 1,  3  and  5 ) which is positioned adjacent the combustion chamber  35  and the flue gas exhaust channels  48  so as to substantially partition the cavity formed by the interior surfaces  34  of the tubular compartments  28  into the combustion chamber  35  and an exhaust passageway  60  (FIGS.  1  and  2 ). The combustion chamber  35  is positioned to receive the high temperature gas stream directly from the burner head  50  of the burner assembly  26 . The flue gas diverter  58  diverts the high temperature gas stream radially outwardly into the annular flue gas exhaust passageway  44  (as indicated by the arrows  54  and  56 ) so that the diverted high temperature gas stream travels past the flue gas diverter  58  as indicated by the arrows  64 ,  66 ,  68  and  70 . The diverted high temperature gas stream then travels radially inwardly through the flue gas exhaust channels  48  (as indicated by the arrows  72  and  74 ) and into the exhaust passageway  60 . The diverted high temperature gas stream is then discharged into the section  18  of the drum  12  as indicated by the arrows  76  and  78 , so as to directly heat the aggregate material passing through the section  18 . The high temperature gas stream is then discharged out of the first end  14  of the drum  12  and into a filtration system as indicated by the arrows  80  and  82 . 
     As shown in FIG. 3, a plurality of spatially disposed slots  84  is formed through the flue gas diverter  58  so that a portion of the high temperature gas stream passes directly from the combustion chamber  35  to the exhaust passageway  60 , so that the high temperature gas stream heats those portions of the drum  12  disposed adjacent the flue gas diverter  58 . Although twelve slots  84  are shown in FIG. 3, only two of the slots  84  have been labeled in FIG. 3 for purposes of clarity and more or fewer slots  84  can be utilized in practicing the present invention. 
     Referring again to FIG. 1, the section  18  of the drum  12  will be described in more detail. A first set of flights  86 , a second set of flights  88 , and a third set of flights  90  are supported within the section  18  of the drum  12 . Only one flight in each of the first and third sets  86  and  90  is shown and only two flights in the second set  88  are shown, for purposes of clarity. It will be understood that a plurality of flights is disposed in each of the first, second and third sets  86 ,  88  and  90  such that the flights in each set extend circumferentially about the interior of the section  18 . 
     The flights in the first set  86  can be conventional “kicker” flights. The kicker flights in the first set  86  serve to move the aggregate material into the section  18  and generally toward the second set of flights  88 . 
     The flights in the second set  88  can be conventional “lift” flights, which serve to lift the aggregate material from the bottom of the drum  12  to the top thereof as the drum  12  rotates so that the aggregate material will fall from the top of the drum  12  in a veil in a manner well known in the art to more effectively heat the aggregate material by direct contact with the high temperature gas stream. 
     The flights in the third set  90  may be conventional “kicker” flights which serve to guide the aggregate material into the aggregate transporting channels  32 . 
     One or more injectors  92  can be disposed through the first end  14  of the drum  12  for injecting additives into the drum  12 . Three injectors  92  are shown in FIG. 1 by way of example. The additives can be fines, dust, fibers, asphaltic fluids, or other additives known in the art. 
     The section  20  of the drum mixer  10  will now be described in more detail. The drum mixer  10  may also include one or more injectors  94  extending into the drum  12  from the second end  16  for injecting additives into the drum  12 . Three injectors  94  are shown in FIG. 1 by way of example. For example, one of the injectors  94  can be a fluid injector for injecting asphaltic fluid, and two of the injectors can be fines injectors for injecting mineral filler and/or dust into the drum  12 . 
     A plurality of rotatable dams  96  is supported within the section  20  of the drum  12  for advancing or retarding the flow of the aggregate material and/or any additives through the section  20  for a shorter or longer retention time within the section  20 . As shown in FIG. 1, the injectors  94  may inject the additives into the section  20  near the section  22  so that the additives will be thoroughly mixed and heated with the aggregate material as the aggregate material moves through the section  20 . 
     A discharge chute  98  is provided on the second end  16  of the drum  12  for discharging the composition formed from the aggregate materials and additives. 
     The high temperature gas stream flows in an opposite direction with respect to the aggregate material as the aggregate material passes through the drum  12 . Thus, the temperature of the high temperature gas stream is decreased as the high temperature gas stream imparts energy into the drum  12  as the high temperature gas stream moves toward the first end  14  of the drum  12 . When the high temperature gas stream enters into the section  18  of the drum  12 , the temperature should be sufficiently low so that the high temperature gas stream will not cause blue smoke or serious product degradation to the aggregate material and/or any additives added thereto. As the high temperature gas stream exits the section  18 , the high temperature gas stream is directed into a filtering system, such as a conventional baghouse, for filtering out any particulate material traveling in the high temperature gas stream. To prevent the high temperature gas stream from overheating the filtering system, it is typical for the section  18  to be sized so that the temperature of the high temperature gas stream exiting the section  18  is in a range of about 240° F. to about 400° F. 
     The inclined drum  12  may be supported on roller supports or “trunions” which may be disposed on either side of the section  22  of the drum  12 . The trunions  100  may be mounted onto a trailer (not shown) so that the drum mixer  10  is portable. The drum  12  may be rotated on the trunions  100  by a conventional motor (not shown). It should be noted that the aggregate transporting channels  32 , which may be disposed circumferentially in the drum  12 , tend to distribute the weight of the aggregate material in the drum  12  evenly about the drum  12  to balance the drum  12 . Thus, one skilled in the art will appreciate that the load on the motor or motors rotating the drum  12  is about 50% of the load placed on the motors rotating the conventional drums which lift the aggregate material from the bottom of the drum to the top of the drum with lift flights so that the aggregate material falls from the top of the drum to the bottom in a veil. 
     The unique design of the inclined drum  12  permits the burner assembly  26  to radiantly, conductively and convectively heat the interior of the drum  12  above a predetermined temperature of about 300° F. so that asphaltic material included in the aggregate material will not adhere to any of the surfaces within the drum  12 . Thus, the drum mixer  10  of the present invention is capable of processing aggregate materials including any ratio including a high ratio (above about 50% ) of RAP material to virgin aggregate without modification. In fact, the drum mixer  10  of the present invention is capable of processing one hundred percent RAP material thereby substantially reducing the cost of producing the asphaltic composition. In addition, substantially any blue smoke which forms in the drum  12  is incinerated so that the blue smoke does not cause environmental problems. 
     Changes may be made in the construction and the operation of the various components, elements, and assemblies described herein and changes may be made in the steps or the sequence of steps of the methods described herein without departing from the spirit and the scope of the invention as defined in the following claims.