METHOD AND DEVICE FOR PREPARING ASPHALT

Abstract: The invention relates to a device for preparing asphalt, comprising a cylindrical drum (3). The drum is rotatable about its longitudinal axis which is inclined downwards the horizontal plane. The drum has a feeding means (9) arranged at the intake end of the drum and adapted to feed material, in the form of suitable fractions of stone material and/or used asphalt, to the drum; feeding means (15, 17) for supplying a binder; a discharge means (13) arranged at the discharge end of the drum and adapted to discharge prepared asphalt paving material from the drum; a drive means (19) for rotating the drum; a burner (11) arranged at the intake end; and a plurality of carrying means (33) arranged in the drum. The carrying means which extend from the intake end of the drum to the discharge end thereof, receive the material from the feeding means and convey it through the drum. Each carrying means comprises a wall portion (35) which is connected to the wall (41) of the drum and which extends inwards from the wall of the drum, and a groove-shaped portion (37) which is connected to the wall portion and which is terminated with a longitudinal free edge (55), which is arranged at a distance from the wall and in front of the wall portion seen in the direction of rotation of the drum. The carrying means are so designed and mutually arranged that the material in a carrying means (57) during one revolution falls over the edge, is caught by the anteriorly situated carrying means (59), seen in the direction of rotation, and is returned to the carrying means which the material initially left via the drum wall portion (61) of the two neighbouring means. The invention also concerns a corresponding method.

Description: METHOD AND DEVICE FOR PREPARING ASPHALT Field of the Invention The present invention relates to a device for preparing asphalt according to the preamble to claim 1 and a corresponding method according to the preamble to claim 8 of the appended claims. Background Art A prior art device for preparing asphalt is disclosed in CH-628,381. This device is a stationary asphalt plant which comprises a rotary drum having three differ- ent sections through which passes the stone material that is supplied to the drum. At the intake end of the drum a burner is arranged which leads into the drum. In the first section of the drum, which is a burning section, the supplied material is transported in closed ducts. Then follow a second and a third section, where the material is transported in open grooves, which are fixed to the inside of the drum wall. The grooves serve to mix the material, thereby exposing it evenly to the heat. The grooves are of different shape and placed in the various sections. The drum rotates slowly, the material falling out of the grooves and down through the hot exhaust gases, which begin where the flame ends, to the bottom of the drum. From the bottom, the material is again moved upwards by the grooves. A binder is added at the begin- ning of the third section and mixed therein with the stone material for forming asphalt. The ducts in the first section of the asphalt plant in the above-mentioned publication are closed to prevent the stone material from falling through the flame of the burner and interfering with the combustion. However, this gives the drawback that the material in the closed ducts is not heated to any considerable extent. Therefore the first section has extremely low efficiency. On the other hand, the efficiency is good in section 2, in which the material falls through the hot exhaust gases. However, at the same time this involves another drawback. The material, which consists of crushed stone having a certain average particle size, also contains fine fractions, such as stone dust and gravel. Owing to the high velocity of the gases, the fine fractions are blown through the drum and out through the outlet. There they must be processed by filtration, which results in additional cost, additional handling and a reduced gas flow through the drum. Moreover, the device is not usable for asphalt recovery since the supply of material containing a binder leads to undesired emission of noxious gases. The noxious gases are generated as the binder contacts the hot exhaust gases when the material is falling through the drum, above all in the second section, but also in the third section. Immediately after the flame, the gases have too high a temperature. In the third section, above all at the beginning thereof, there is also a risk that noxious gases form in new production of asphalt since a binder is added and then exposed to the hot exhaust gases as the material falls through them. US-4,738,539 discloses another prior art device which is made up in essentially the same fashion as the device according to CH-628,381. In this case, the ate- rial will, however, not come into direct contact with the jet of hot exhaust gases after the adding of the binder, which takes place at the end of the drum. The two known devices are ineffectively constructed, which makes them unnecessarily long. Summary of the Invention One object of the invention is to provide a method and a device for preparing asphalt, which are as applicable to asphalt recovery as to new production. A further object of the invention is to provide a method and a device for preparing asphalt, in which the material is heated more rapidly and more effectively compared with prior art, with no risk of emission of noxious gases which is caused by too high a temperature of the binder and without having to take care of fine particles which otherwise would pollute the environment. The objects are achieved by a device and a method according to the invention as defined in claims 1 and 8, respectively. The preambles to claims 1 and 8 are based on CH-628,381. Brief Description of the Drawings The invention and further advantages thereof will now be described in more detail by way of embodiments and with reference to the accompanying drawings, in which Fig. 1 is a schematic side view of a device for preparing asphalt according to an embodiment of the invention; Fig. 2 is a schematic cross-section of a drum included in the device in Fig. 1; and Fig. 3 is a schematic view on a larger scale of a detail of Fig. 2 and shows an alternative embodiment. Description of Embodiments As shown in Fig. 1, the device 1 comprises, according to an embodiment of the invention, a cylindrical drum 3 having an intake end 5 and a discharge end 7; a first feeding means 9; a burner 11; a discharging means 13; a second feeding means 15; a third feeding means 17; a drive means 19 and a frame 21. The drum 3 has a front end wall 29 and a rear end wall 30. The end walls 29, 30 are fixed to the frame 21. The frame 21 comprises an adjustable supporting leg 63. The supporting leg 63 is set such that the drum 3 inclines in relation to the horizontal plane. The first feeding means 9, which below is referred to as a feeder 9, comprises a material container 23 and a conveyor 25 which preferably is a belt conveyor. The feeder 9 feeds material, which either is one or more fractions of crushed stone, such as crushed aggregate and/or stone dust, or wholly or partly is old asphalt which is to be reused, to the drum 3 in the intake end 5 thereof through a hole in the front end wall 29 of the drum 3. The material is heated in the drum. This process will be described in more detail below. The second feeding means 15, which below is referred to as a subinjector 15, and the third feeding means 17, which below is referred to as a final injector 17, add a binder to the material in the intake end 5 and the discharge end 7, respectively. The subinjector 15 is used to add, when necessary, a small amount of binder so as to bind dust in the material. The final injector 17 supplies a suitable amount of binder to the discharging means 13, thereby obtaining asphalt with properties as desired. In the recovery of used asphalt, any supply of stone material and binder depends on the age of the asphalt and other properties thereof, and on what properties are desired for the prepared asphalt. The burner 11, which is a burner conventional for the purpose, is arranged in the intake end 5 of the drum 3, such that the opening of the burner 11 is conventionally arranged inside the end wall 29 of the drum 3 or approximately in the plane of the end wall 29, for instance, as shown in the above-mentioned publications. The burner 11 comprises a fan that drives a controllable flow of exhaust gases through the drum 3. The drum 3 is supported by four supporting rolls 19 included in the drive means 19, two on each side of the drum 3. The drive means drives two of the supporting rolls for rotation of the drum 3 about its longitudinal axis. The ends 5, 7 of the drum 3 are rotatably fitted into the end walls 29, 30, which thus assist in holding the drum 3 in place. Each supporting roll 19 has a friction lining engaging the outside of the drum. In the discharge end 7 of the drum 3, the dis- charging means 13, which in this embodiment consists of an essentially conventional mixing feeder, is adapted to discharge the material and the added binder during mixing. The mixing feeder 13 projects a distance into the drum 3 through the rear end wall 30 and catches the material. The mixing feeder 13 has a semicylindrical lower part and a discharge means in the form of a longitudinal shaft with inclined vanes which are attached thereto and which, during rotation of the shaft, project and simultaneously mix the material. The prepared asphalt is discharged by the mixing feeder 13 to a standby container, such as a lorry platform 31. A chimney 12 is arranged in the end wall 29. It serves as outlet for the gases, essentially water vapour and hot air, which are entrained through the drum 3. It is important that the water vapour does not remain in the drum 3 to any great extent. The chimney 12, which is a pipe, is arranged at the end wall top, which, owing to the inclination of the drum 3, at the same time is the highest point of the drum 3. The hot gases which are entrained through the drum 3 return through the upper regions of the drum 3 and escape through the chimney 12. This results in the further advantage that additional thermal energy is recovered from the gases for heating the material. A certain part of the gases is entrained through the mixing feeder 13. The above-described parts of the device are arranged on a frame transportable by means of a lorry, such as the frame 21, which is a demountable load body. This makes the device 1 mobile and easily transportable by means of a lorry to an optional place of production. An important factor for permitting such a compact device as to be accommodated on, for instance, such a demountable load body 21 is the internal construction of the drum 3, which will now be described in more detail. As shown in Fig. 2, carrying means, or grooves, 33 are arranged inside the drum 3. The grooves 33 are elongate and extend like internal flanges along the inside of the drum 3, essentially all the way from the intake end 5 of the drum 3 to the discharge end 7 there- of. Each groove 33 comprises a radial wall portion 35 and a groove-shaped or curved portion 37 which is connected to the wall portion 35. The wall portion 35 is essentially planar and is, in a longitudinal fixing portion 39 thereof, connected to the wall 41 of the drum. The fixing portion 39 comprises two interconnected, elongate supporting portions 43, 45. The supporting portions 43, 45 are V-shaped in cross-section and pass in the tip of the V into the rest of the wall portion 35, while the leg ends of the V, i.e. the longitudinal side edges 47, 49 of the supporting portions 43, 45, engage the inside of the drum wall 41. The fixing portion 39 also comprises fixing means 51, such as threaded pins, which are fixed to the base of the groove which can be said to be formed by the supporting portions 43, 45 and which are arranged at suitable distances from each other along the supporting portions 43, 45. The fixing means 51 extend through holes in the wall 41 of the drum 3 and are detachably anchored, for instance, by means of nuts 53, to the outside of the drum 3. As a result, the grooves 33 are detachably connected to the drum 3. The supporting portions 43, 45, arranged in the shape of a V, of the fixing means distribute the forces to which the fixing portion is subjected by means of the material present in the grooves 33. This design differs from the design of the prior art devices. The groove-shaped portion 37 forms a curved extension of the wall portion 35 and is terminated with a free longitudinal side edge 55 at a distance from the wall 41. The cross-sectional view in Fig. 2 illustrates the inte- rior of the drum 3 seen from the intake end 5 thereof. The grooves 33 are hook-shaped in cross-section. The drum 3 rotates clockwise in Fig. 2, i.e. the free edges of the grooves 33 are positioned in front of the wall portions 35 seen in the direction of rotation. The belt conveyor 25 is arranged adjacent to the circumference of the drum 3 and throws the material into the grooves 33 through the hole in the front end wall 29. The grooves 33 are so designed and arranged in relation to each other that the material, which is shown as hatched areas in the grooves 33, during the rotation moves around in a space which is defined by two neigh- bouring grooves, for instance, the grooves 57 and 59, that portion 61 of the wall of the drum 3 which is positioned between the two grooves 57, 59 and an opening 60 which is formed between the grooves 57, 59, by being arranged at a distance from each other, seen in the circumferential direction of the drum 3. During a revolution, beginning when the groove 57 is at the top, the material in the groove 57 first falls out of the groove, past the opening between the grooves 57 and 59 and down onto the anteriorly positioned groove 59, seen in the direction of rotation, or more specifically onto the rear side of the wall portion 35 of the groove 59. Subsequently, the material slides/rolls down onto the wall portion 61 of the drum and spreads over this wall portion. The groove 57 has now reached its bottom position. On its way up the material slides/rolls once more into the groove 57. In this procedure, the material moves simultaneously towards the discharge end 7 thanks to the inclination of the drum 3 in relation to the horizontal plane. In an end portion at the discharge end 7, the grooves 33 are slit in the groove-shaped portion 37. As a result, the material falls out of the grooves 33 and down into the mixing feeder 13. There are several variables affecting the possibility of providing a device which according to the in- vention retains the material in the space such that it does not fall out through the opening 60. The variables are: the speed of rotation of the drum 3; the design and location of the grooves 33 in relation to each other and the amount of material in the grooves 33. It is desirable to have a fairly low speed of rotation such that the force mainly acting on the material is the force of gravity. This results in good mixing of the material. To high a speed of rotation implies that the material is pressed towards the wall 41 and retained in approximately the same position during a revolution. This also interferes with the transport of material through the drum 3. The design of the grooves 57, 59 and the placing thereof in relation to each other are such that the grooves can support a considerable amount of material without the material falling out through the opening 60. Therefore the grooves should be arranged closer to each other than in the above described prior art devices, and the free edge 55 should be extended upwards so far that the groove-shaped portion 37 can receive a suitable amount of material. In the embodiment shown, the groove-shaped portion 37 has a radius of curvature which is 0.15 m; the height of the wall portion 35 is 0.3 m; the diameter of the drum 3 is 2.2 m and the number of grooves 33 is 12. The groove-shaped portion 37 is in this embodiment a semicylinder . It may here be noted that in this embodiment the length of the drum is 4 m. These dimensions should only be considered an example which satisfies the condition that the material does not fall through the interior of the drum 3 but essentially rotates in the space between two neighbouring grooves 33 in the way as described above. It should be noted that, of course, the amount of material has an influence. If an extremely large amount of material is fed into the grooves 33, it will fall over the edge 55 even if the space between the edge 55 and the wall 41 is small. In the prior art devices, however, the condition is inverted, i. e. the material will fall out between the grooves independently of how small an amount of material that is fed. The design and relative position of the grooves 33 are thus decisive factors for the achieving of the objects of the invention. If the grooves thus are suitably designed and arranged relative to each other and the amount of material is adapted thereto, the speed of rotation has no effect on the retaining of the material in the space. Also at the lowest possible speed of rotation, the material is retained in said space. At the same time it is still possible to keep the distance between two neighbouring grooves 33 sufficiently great for hot gases to enter the space and heat the material. If one wants to have the possibility of supplying more material without the material falling out through the opening 60, this can be achieved by increasing the speed of rotation. An upper limit for compensating for a larger amount of material by means of the speed of rotation is obtained when the speed of rotation has a detrimental effect on the moving around of the material in the space. There are, of course, many combinations of the above-mentioned variables that result in an inventive device. It is, however, within the scope of the capacity of a person skilled in the art to try out a suitable combination in a given situation on the basis of the description above. An advantage of the moving around of the material in a limited space is that the above problems of the prior art technique are obviated. This means that the material does not fall through the drum and come into direct contact with the hottest exhaust gases. At the same time the defined space is not closed, as is the case with closed ducts, but hot gases may enter the space via the openings between the grooves 33. As a result, the problem of noxious gases from recovered asphalt or from the smaller amount of binder that is added by means of the subinjec- tor 15 is obviated while at the same time efficient heating is still achieved. Moreover, the fine fractions are not ejected through the discharge end 7. The continuous movement of the material results in efficient admixing of hot air in the space for efficient heating of the material. In the lower half of the drum 3, the material is spread in a thin layer over a large surface, which results in good exposure to the hot gases. Since the jet of hot gases is not interfered with by material falling therethrough, a considerably more even temperature is also achieved in the entire drum 3 compared with the prior art devices, which makes the process still more efficient. Thanks to the above-described improvements in relation to the prior art devices, the inventive device has considerably higher efficiency. This means, among other things, that it can be made considerably shorter than the prior art devices with the same production capacity. The amount of material that is supplied, i.e. the amount of material per unit of length that each groove 33 carries, is controlled not only by a suitable filling degree in the grooves 33 for the material not to fall out through the opening 60, but also by, for instance, the type of material. Both the feeding rate and the speed of rotation are in the embodiment shown adjustable to achieve the optimum result in each case. The embodiment described above is but a non-limiting example of how the inventive device can be designed. As will be appreciated by one skilled in the art, several modifications are feasible within the scope of the invention as defined in the appended claims. Below follow some examples of modifications. The grooves are in one modification provided with one or more flanges extending along the groove-shaped portion. Such a flange is advantageously designed in a simple manner as a piece of flat iron, which at its one side edge surface is connected to the groove. The flange or flanges increases/increase the supply of heat to the groove and, thus, the material thereof, which accelerates the heating. Consequently the feeding rate can be increased. The shape of the grooves 33 is variable. An example of a variant is shown in Fig. 3 by dash-dot-dot lines starting from the fixing portion 39 and coinciding with the groove-shaped portions 37. In this variant, the wall portion 35 does not constitute an extension of the groove-shaped portion 37 in the tangent direction thereof, as in the embodiment described above, but makes an angle with this tangent direction. This means that the portion of the groove-shaped portion 37 which is closest to the wall portion 35 forms a bead which efficiently prevents the material from falling off the wall portion of a groove 33 when it falls down from the groove closest above. There is a small risk of this occurring in the above embodiment, which, however, is more cost-effective to manufacture. The groove-shaped portion can, for instance, have a cross-sectional shape other than a semicylinder, such as a polygonal or asymmetric shape. The fixing portion can be designed in an optional manner, although in the shown embodiment the angled supporting portions 43, 45 of the fixing portion 39 facilitate the moving around of the material past the transition between the wall 41 of the drum 3 and the carrying means 33, and the design of the fixing portion results in easily detachable carrying means. The size of the device is conditioned by the desired capacity. As described above, the device can owing to its unique design be formed as a mobile unit, which is an ad- vantage that does not exist in the prior art devices described by way of introduction. It is, however, also possible to design the device as a stationary unit, thereby rendering it possible to make it considerably larger than the mobile variant. For mobile embodiments, the above-mentioned demountable load body is only one of many applicable frames. A further variant is to design the device as a unit which is fixedly anchored to a vehicle, such as a tractor or a trailer.

Claims: CLAIMS 1. A device for preparing asphalt, comprising a cylindrical drum (3) which is arranged for rotation about its longitudinal axis, the longitudinal axis inclining towards the horizontal plane, and which has an intake end (5) and a discharge end (7); a feeding means (9) arranged at the intake end of the drum and adapted to feed mate- rial, in the form of suitable fractions of stone material and/or used asphalt, to said drum; a further feeding means (15, 17) for supplying a binder to the material; a discharge means (13) arranged at the discharge end of the drum and adapted to discharge prepared asphalt paving material from the drum; a drive means (19) for rotating the drum; a burner (11) which is arranged at the intake end and the exhaust gases of which are supplied to the interior of the drum; and a plurality of carrying means (33) which are arranged in the drum and connected thereto and which extend along the drum, for receiving the material from the feeding means and transporting the material through the drum, each carrying means comprising a wall portion (35) which is connected to the wall (41) of the drum and extends inwards from the wall of the drum, and a groove-shaped portion (37) which is connected to the wall portion and which is terminated with a longitudinal free edge (55), which is arranged at a distance from the wall and in front of the wall portion seen in the direction of rotation of the drum, c h a r a c t e r i s e d in that the carrying means extend from the intake end of the drum to the discharge end thereof, and that the carrying means are so designed and mutually arranged that the material in a carrying means (57) during one revolution falls over said free edge, is caught by the anteriorly situated carrying means (59) , seen in the direction of rotation, and via the drum wall portion (61) positioned between the two neighbouring carrying means is returned to the carrying means which the material initially left. 2. A device as claimed in claim 1, c h a r a c t e r i s e d in that each carrying means is hook-shaped in cross-section, the wall portion being essentially planar. 3. A device as claimed in claim 1 or 2, c h a r a c t e r i s e d in that said further feeding means (15) for supplying a binder to the material is arranged at the intake end of the drum. 4. A device as claimed in claim 3, c h a r a c t e r i s e d in that a third feeding means (17) for supplying a binder to the material is arranged at the discharge end of the drum, and that the discharge means is arranged for discharging during mixing. 5. A device as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the wall portion comprises a fixing portion (39) with two interconnected elongate supporting portions (43, 45), which are V-shaped in cross-section and, in the tip of the V, pass into the rest of the wall portion (35) while, in the leg ends (47, 49) of the V, they engage the inside of the drum wall (41) . 6. A device as claimed in claim 5, c h a r a c - t e r i s e d in that the fixing portion (39) comprises fixing means which extend through the wall of the drum to the outside thereof and are detachably fixed to the outside. 7. A device as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that it comprises a demountable load body (21) , which constitutes a frame on which the other components that are included are arranged, thereby achieving mobility of the device. 8. A method of preparing asphalt by means of a device comprising a cylindrical drum (3) , which is arranged for rotation about its longitudinal axis and which has an intake end (5) and a discharge end (7), the longitudinal axis inclining in relation to the horizontal plane; the method comprising the steps of rotating the drum; feeding material, in the form of suitable fractions of stone material and/or used asphalt, to the drum at the intake end thereof to a plurality of carrying means (33) which are arranged in the drum and which are connected to and extend along the drum and which contribute to mixing and transporting of the material through the drum; supplying hot exhaust gases to the interior of the drum for heating the material; adding a binder to the material; at the discharge end of the drum, discharging prepared asphalt paving material from the drum; forming each carrying means to comprise a wall portion (35) which is connected to the wall (41) of the drum and which extends inwards from the wall of the drum, and a groove-shaped portion (37), which is connected to the wall portion and which is terminated with a longitudinal free edge (55) , which is positioned at a distance from the wall and in front of the wall portion seen in the direction of rota- tion of the drum, c h a r a c t e r i s e d by the steps of forming the carrying means so as to extend from the intake end of the drum to the discharge end thereof; forming the carrying means and arranging them relative to each other such that the material in a carrying means (57) during one revolution is made to move around in a predetermined loop, the material falling over said free edge, being caught by the anteriorly situated carrying means (59) , seen in the direction of rotation, and being returned via the drum wall portion (61) positioned between two neighbouring carrying means to the carrying means which the material initially left. 9. A method as claimed in claim 8, c h a r a c t e r i s e d by the step of adjusting the feeding of the material to the drum such that the amount of material in the carrying means is maximised on the condition that it is to be retained in said loop. 10. A method as claimed in claim 8 or 9, c h a r a c t e r i s e d by the step of setting the speed of rotation such that the rotation of the drum contributes to retaining the material in the loop.

Inventors: BENGTSSON KARL ARNE, BENGTSSON, KARL, ARNE
Applicants: ASFALTSTEKNIK I GAELLOE AB, BENGTSSON KARL ARNE, BENGTSSON, KARL, ARNE
Application year: 1998
Main classification: E01C19
IPC codes: E01C19, F26B11