Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND DEVICE FOR PREPARING ASPHALT
Field of the Invention
The present invention relates to a device for pre-
paring 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 dis-
closed 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 mate-
rial 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
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material falls through the hot exhaust gases. However, at
the same time this involves another drawback. The mate-
rial, 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, addi-
tional 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 mate-
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 appli-
cable 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
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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 in-
cluded in the device in Fig. l; 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, accord-
ing 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
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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 dis-
charge 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 de-
sired 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 conven-
tionally arranged inside the end wall 29 of the drum 3 or
approximately in the plane of the end wall 29, for in-
stance, 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 fric- -
tion 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
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mixing. The mixing feeder 13 projects a distance into the
drum 3 through the rear end wall 30 and catches the mate-
rial. The mixing feeder 13 has a semicylindrical lower
part and a discharge means in the form of a longitudinal
5 shaft with inclined vanes which are attached thereto and
which, during rotation of the shaft, project and simul-
taneously mix the material. The prepared asphalt is dis-
charged 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 mare 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-
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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 essen-
tially 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 sup-
porting portions 43, 45. The supporting portions 43, 95
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 93, 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 con-
nected to the drum 3. The supporting portions 43, 45,
arranged in the shape of a V, of the fixing means distri-
bute 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 exten-
sion 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
seen in the direction of rotation.
35 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.
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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 revo-
lution, 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. Sub-
sequently, the material slides/rolls down onto the wall
portion 61 of the drum and spreads over this wall por-
tion. 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 possi-
bility 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 rota-
tion such that the force mainly acting on the material is
the force of gravity. This results in good mixing of the
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. 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 condi-
tion 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
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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 con-
tact 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-
for 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 conti-
nuous 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 mate-
rial is spread in a thin layer over a large surface,
which results in good exposure to the hot gases. Since
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the jet of hot gases is not interfered with by material
falling therethrough, a considerably more even tempera-
ture is also achieved in the entire drum 3 compared with
the prior art devices, which makes the process still more
5 efficient.
Thanks to the above-described improvements in rela-
tion 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
10 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 inven-
tion 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 man-
ner 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
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portion 35 does not constitute an extension of the
groove-shaped portion 37 in the tangent direction there-
of, 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 sup-
porting portions 43, 45 of the fixing portion 39 faci-
litate the moving around of the material past the tran-
sition 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 de-
scribed by way of introduction. It is, however, also pos-
sible 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 demount-
able 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.
