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(11) | EP 0 119 328 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Drying-mixing drum for preparing bituminous mix, particularly by use of recovered material |
| (57) The drum is provided with a burner (7), which is made operating with controlled amount
of air so as to avoid that its flame causes the combustion of the bitumen contained
in the recovered material. Bituminous mixes of very good quality with high percent
of recovered material can thus be obtained. |
[0001] The present invention relates to a drying-mixing drum for preparing bituminous mix, particularly by use of recovered material.
[0002] It is known the present trend to reuse the material recovered from asphalt pavements to prepare new bituminous mix ready for the use {E.S.D.-A. System).
[0003] There are already available in the market several recycling plants, the main member of which is constituted by the drying-mixing drum, that is by a rotating cylinder provided with a burner, in which a suitable percent of recovered material and virgin material is subject to drying and mixing together with bitumen, asphalt or other fluid cementing material.
[0004] In connection with this drying-mixing drum, in all the known plants there is the problem of avoiding that a too high overheat ing of the bitumen contained in the recovered material causes the combustion or the degradation of the same bitumen, thereby damaging the characteristics thereof and giving rise to a product of bad quality.
[0005] In a drum of known type the above said problem has been solved by introducing the recovered material into the middle section of the drum, far from the end in which the burner is arranged and the virgin material is introduced. The latter is brought to high temperature by the flame of the burner and then largely transmits the heat to the recovered material by contact when, from the middle section of the drum on, the mixing of the two materials begins. There results from this the main drawback of this known drum, which is to exploit only a half part of the drum for the mixing, with consequent low efficiency of the plant. Moreover, it is absolutely necessary to employ a not-negligible quantity of virgin material with consequent low percent of recovered material in the finished product. The working of the plant with only recovered material is then quite inconceivable.
[0006] In another drum of known type the same problem has been solved, on the contrary, by providing the burner with a suitably bored flame shielding cone, which has the function of shielding the bitumen of the recovered material from the flame radiation and distributing the flow of hot gases evenly inside the drum, thereby preventing the formation of localized peaks of temperature, which would cause the combustion of the bitumen. All the material (reco vered and virgin) may thus be introduced into the drum near the burner and the whole extension of the drum may be exploited to obtain a product of good quality and with high percent of recovered material; since the heating of the recovered material is direct, between gas and material, the absence of virgin material may even be conceived. On the other hand, as a consequence of the use of the flame shield, there are the drawbacks of an increased constructional complexity, a low thermal efficiency and the neces sity of a troubled work of removal of the flame shield in case of use of virgin material only.
[0007] In view of such a state of the art, it is object of the present invention to realize a drying mixing drum, particularly suitable for the treatment of recovered material, which drum combines the advantages of the constructional simplicity, the high efficiency with high percent of recovered material and the large and total variability of the composition from all recovered material to all virgin material with the most various intermediate percents.
[0008] According to the invention, this object has been reached by a drying-mixing drum, comprising a rotating cylinder provided with a charging end with axial burner and with a discharging end, cha racterized in that it comprises means for controlling the amount of air fed to the burner at such a value as to allow drying and mixing of the introduced material, but not the combustion and the degradation of the bitumen contained in the recovered material possibly making part of said material.
[0009] That is, air is fed to the burner at an almost stoichiometric ratio with the fuel to be burnt, so that a subitant-ially inert at mosphere, containing oxygen in negligible quantity, is produced inside the drum.
[0010] More precisely, according to a preferred embodiment of the present invention, the above said means for controlling the amount of air consist in that the only feeding path for the entire necessary air passes through the burner, which is provided in its turn with means for forcing the air into the drum, adjusting the amount of the same according to the different requirements.
[0011] There are further provided means for maintaining inside the cylinder a selected positive pressure able to avoid secundary and uncontrolled infiltrations of air, which would occur if a pressure lower than that existing outside set up inside the drum.
[0012] In this way, without any complicated mechanical device, the flame of the burner is put in condition to avoid damage of the bitumen of the recovered material, at the same time having the helpful effect of ensuring a suitabl.e temperature for the drying and mixing of the material and, thereof, for the formation of the desired bituminous mix. The whole material, both inert and recovered, may thus be introduced near the burner, with consequent possibility of reaching high levels of efficiency with high percents of recovered material and very good quality of the finished product. At the same time, without any structural change but mere Iy through suitable adjustment of the burner flame, the same drum may be enabled to work with virgin material only.
[0013] An embodiment of the drum according to the invention is illustrated for better clarity, but without any limiting intention, in the enclosed drawings, in which:
Fig. I shows a drying-mixing drum according to the invention in axial section along line I-I of Fig. 2;
Fig. 2 shows said drum in transversal section along line II-II of Fig. 1;
Fig. 3 shows the burner and the corresponding air forced feed group, included in said drum, from left with respect to Fig. 1.
[0014] With reference to the drawings, there is shown a drying-mixing drum, which basically comprises a long cylinder 1 provided with a pair of rings 2 and 3 for its support by means of suitable groups of idle rollers and with a central toothed ring4 for its slow ro tation by means of suitable motor-driven pinion. Inside the cylin der 1, fixed to the internal wall thereof, there are arranged a plurality of longitudinal tongues 5, whose function is to homogenize the material during its movement caused by the inclination of the longitudinal axis of the cylinder, from one end to the other of the same cylinder.
[0015] The cylinder 1 has a charging end 20 (on the right, looking at Fig. I, which is closed by a cover 6 and to which a burner 7, a material supply hopper 8 and a feed pipe 9 for fluid cementing material, in particularly bitumen, are associated.
[0016] The burner 7 has its mouth 10 housed in a chamber 11 together with a small tube 12 for the formation of a pilot flame for the firing of the burner. Said mouth is fed in known manner with sui table fuel and further receives the necessary air for the combustion through one feed way 13, to which a prefixed flow of air is delivered in forced and controlled manner by a fan 14 with a motor 15, which fan sucks air from a variable width mouth 16 in the sense of the arrow F of Fig. 3.
[0017] The hopper 8, in its turn, is provided with a pair of feed chutes 17, which, by diverging progressively from one another, bring the material to diametrally opposed points of the charging end of the cylinder 1 (fig. 2). As shown in Fig. 1, there are arranged inclined radial tongues 18, which distribute the material along the circumference of the cylinder and on the several longitudinal tongues 5.
[0018] The pipe 9, in its turn, enters inside the cylinder 1 and deve lops parallelly to the axis thereof to such a distance that its delivery flow is not directly affected by the burner flame.
[0019] At the charging end of the cylinder 1 there is finally arranged a pressure sensor 19, which is schematically illustrated in Fig. 1.
[0020] The cylinder 1 has moreover a discharging end 21 (on the left, looking at Fig. 1), which includes a discharging mouth 22, from which a smoke sucking pipe 23 goes off. In said pipe 23 there is inserted a throttle valve 24 driven by a motor 25 controlled by the pressure sensor 19.
[0021] In the operation, a prefixed percent of recovered aggregate and virgin aggregate (for example, 70% and 30%, respectively) is introduced into the drum through the hopper 8 and distributed along the circumference of the cylinder 1 by means of the inclined tongues 18. By suitable adjustment of the amount of air for ced through the burner 7 by the fan 14 and with a suitable pressure maintained by the valve 24, adjusted by the signal detected by the pressure sensor 19 in the charging end 20, it is acted in such a way that the burner has available a suitably metered amount of oxygen and secundary suctions of air by negative pressure occur. The flame of the burner 7 is thus able to produce a suf ficient temperature to dry and mix the introduced material, to which bitumen or other fluid cementing material introduced through the pipe 9 is added later on, but is not able to cause the combustion of the bitumen contained in the recovered aggregate. The entire cylinder is therefore used for mixing the materials, thus obtaining consequently a finished product of good quali ty with high percent of recovered material. The bituminous mix thus obtained is finally discharged through the outlet mouth 22.
1. Drying-mixing drum for preparing bituminous mix, particular ly by use of recovered
material, comprising a rotating cylinder with a charging end with axial burner and
with a discharging end, characterized in that it comprises means for controlling the
amount of air fed to the burner at such a value as to allow drying and mixing of the
introduced matesrial, but not the combustion or the degradation of the bitumen contained
in the recovered material possibly making part of said material.
2. Drying-mixing drum according to claim 1, characterized in that said means comprise
one air feed way passing through the burner, which is provided with means for forced
and controlled introduction of air, and means for maintaining inside the cylinder
a selected positive pressure able to avoid secundary and in- controlled infiltrations
of air, which would occur if a pressure lower than that existing outside set up inside
the drum.
3. Drying-mixing drum according to claim 2, characterized in that said means for forced
and controlled introduction of air consist of a fan provided with driving motor and
of a variable- width suction mouth.
4. Drying-mixing drum according to claim 2, characterized in that said means for maintaining
a selected pressure comprise a pressure sensor inserted in the cylinder near the burner
and a valve system inserted in a smoke sucking pipe and controlled by said pressure
sensor.