[0001] This invention relates to the manufacture of asphalt and more particularly to a process
and apparatus for manufacturing asphalt mixes in which at least a portion of the starting
material comprises salvaged or reclaimed asphalt.
[0002] The use of reclaimed asphalt provides economic advantages not only in that it reduces
the quantity of virgin or new material required to produce a given batch of asphalt
but also because it reduces the problem of disposing of old asphalt which, for example,
may have been removed from the surface of a roadway.
[0003] Salvaged or reclaimed asphalt which contains bitumen is not suitable for incorporation
into conventional asphalt making processes. When recycling in these processes the
materials are raised to a relatively high temperature to produce an adequate transfer
of heat. At these temperatures bitumen may degrade and it may create a pollution problem.
Also, at raised temperatures the reclaimed material which contains bitumen may become
tacky so that it is particularly difficult to handle and to assimilate in the asphalt
making process particularly in relation to batch plant operations. Moreover, higher
operating temperatures inevitably lead to increased energy consumption and maintenance
costs.
[0004] The present invention seeks to provide a process for producing asphalt which utilizes
at least a proportion of reclaimed asphalt as a starting material and which alleviates
or minimises these difficulties, and an apparatus for carrying out such a process.
[0005] According to the invention there is provided a process for producing asphalt mixes
in which at least a portion of the starting material is reclaimed asphalt, the process
comprising the steps of:
providing at least one grade of aggregate material;
transferring a metered quantity of the aggregate to a dryer for drying and heating
the aggregate;
screening the dried and heated aggregate into separate holding compartment according
to grade;
providing a supply of reclaimed asphalt of suitable consistency;
transferring the reclaimed asphalt to a reclaim holding compartment located in heat
transfer relationship with at least one of the aggregate holding compartments whilst
being physically separated therefrom, to permit heat transfer to take place between
the dried and heated aggregate and the reclaimed asphalt;
discharging the aggregate and the reclaimed asphalt from the holding compartments
to effect at least partial blending of the aggregate and the reclaimed asphalt; and
adding bitumen to the at least partially blended aggregate and reclaimed asphalt.
[0006] Filler material may be added as required.
[0007] A proportion of the screened aggregate may be diverted into the reclaim holding compartment
to effect a degree of preblending and preferably heat transfer of the aggregate and
asphalt in the reclaim holding compartment. The aggregate preferably is diverted in
the same proportions in which it is fed to the screening step.
[0008] According to the invention there is also provided apparatus for use in such a process,
the apparatus comprising a batch tower including a plurality of aggregate holding
compartments for holding preheated aggregate and at least one reclaim holding compartment
for holding reclaimed asphalt, the reclaim holding compartment being in heat-exchange
relationship with at least one of the aggregate holding compartments so that, in use,
heat may pass from the preheated aggregate to heat the reclaimed asphalt.
[0009] The apparatus may also include at least one cold feed hopper for storing starting
material aggregate. Preferably the apparatus includes a plurality of cold feed hoppers
for storing aggregate of various sizes to be used in producing the final mix. In one
form the apparatus in accordance with the invention may include six cold feed hoppers.
[0010] The apparatus preferably includes a dryer for aggregate. A belt conveyor or the like
may be provided to transfer aggregate from the cold feed hoppers to the dryer to preheat
and dry the aggrregate. The belt feeding conveyor may be controlled by means of an
automatic control system to ensure that the correct proportion of aggregate is fed
from each cold feed hopper.
[0011] The hot and dried aggregate may be transported to the batch tower by means of a transfer
belt, elevator or other suitable means.
[0012] The apparatus may include a sizing screen arrangement for separating the preheated
and dried aggregate into various grades and a holding system for holding the graded
material. The sizing screen arrangement may be provided in any suitable form to separate
the aggregate into any suitable number of size ranges as may be desired.
[0013] The holding system may include one or more separate compartments for holding sized
and graded aggregate. Each holding compartment is preferably provided by an aggregate
holding bin. The aggregate holding bins may be arranged in any convenient manner and
preferably are arranged in line beneath the sizing screen arrangement. In an alternative,
the aggregate holding compartments may be provided as separate aggregate compartments
within a holding bin. Preferably the aggregate holding bins or compartments are arranged
in mutual heat transfer relationship thereby to provide heat transfer therebetween
and to minimise heat loss and temperature variation.
[0014] The reclaim holding compartment may be provided by means of a reclaim holding bin
or a reclaim holding compartment of an aggregate holding bin. The reclaim holding
compartment is located in heat transfer relationship with at least one aggregate holding
bin, preferably such that it receives optimum heat transfer from the aggregate holding
bin or bins. This arrangement permits transfer of heat between the dried and heated
aggregate held in the holding system and the reclaimed material.
[0015] Each holding bin may include one or more outlet gates for the discharge of held material.
Preferably the outlet gate of the preferred reclaim holding bin is arranged such that
it enhances mixing of the aggregate and reclaimed material. In one form the outlet
gate of the reclaim holding bin may be located between the outlet gates of the aggregate
holding bins.
[0016] The apparatus of the invention preferably includes also a separate recycle feed hopper
for storing reclaimed asphalt material to be recycled prior to it passing to the reclaim
holding bin. The reclaimed material which is stored in the recycle feed hopper may
be metered to the reclaim holding bin through a separate recycle belt feeder as desired.
The recycle belt feeder preferably is controlled by the same automatic control system
as that preferably used for controlling the preferred belt feeding arrangement associated
with the aggregate feed hoppers. This permits the proportions of aggregate and reclaimed
material to be relatively adjusted as required.
[0017] The reclaimed material may be transported to the batch tower by means of a transfer
belt or elevator. Preferably a conveyor belt is used to transfer the reclaimed material
from the recycle hopper to the appropriate holding compartment. The reclaimed material
preferably is arranged to enter the reclaim holding compartment or bin approximately
at the centre thereof.
[0018] The apparatus may include a weighing hopper for receiving aggregate and/or reclaimed
asphalt from the batch tower. The proportion of aggregate and reclaimed asphalt discharged
from the holding bins into the weighing hopper may be controlled according to the
grade of mix required in the final batch.
[0019] Bitumen may be added to the mix from a suitable bitumen storage tank or other facility.
The bitumen preferably is added to the mix in a suitable mixing chamber. Filler materials,
such as fine material or powder may also be added to the mixing chamber or elsewhere.
An associated filler tank or silo may be provided for this purpose. In a preferred
arrangement of the invention fines may be collected by extraction from exhaust of
the preferred aggregate dryer and used to provide at least part of the filler. Extraction
may be by means of a bag filter for example.
[0020] According to a further aspect of the invention, a diverter gate may be associated
with the reclaim holding compartment or bin. In one form the diverter gate may be
arranged to run along a dividing wall between the aggregate holding bins and the reclaim
holding bin. Preferably the diverter gate runs the full length along the dividing
wall.
[0021] In one form the diverter gate may be actuated to draw at least a proportion of aggregate
into the reclaim holding bin. One advantage of allowing aggregate to enter the reclaim
holding bin is that it permits preblending and heat transfer between the reclaimed
and aggregate materials as the blend travels towards the discharge section of the
reclaim holding bin.
[0022] The degree of blend and heat transfer may be controlled to some extent by adjusting
the speed of the belt conveyor supplying the reclaimed material and/or the position
of the diverter gate.
[0023] In one form the diverter gate may be located on the underside of the sizing screen.
Preferably the diverter gate is positioned across the full length of the sizing screen.
The diverter gate may be adapted to draw aggregate into the reclaim holding bin preferably
in the proportions in which they are fed to the screen by the hot elevator.
[0024] The quantity of aggregate material which is diverted into the reclaim holding bin
preferably is a function of the position of the diverter gate. In one form the diverter
gate may be arranged to divert for example from 0 to 30% or more of the incoming aggregate
into the reclaim holding bin and the amount will depend upon the position of the gate.
The position of the diverter gate may be set by adjusting a manual stop. The diverter
gate preferably is actuated pneumatically from a control station.
[0025] The proportion of aggregate which enters the reclaim holding bin thus may be adjusted
by selecting the corresponding gate stop position. The proportion of reclaimed material
which enters the reclaim holding bin preferably is selected by the belt feeder speed
and discharge gate opening of the recycle feed hopper. Typically in a recycling process,
the aggregate material feeders are driven in the correct proportions to the reclaimed
material feeder so that the blend in the reclaim holding bin is as designed to suit
parameters of the mix and the materials. The belt feeders preferably operate on a
pre-calibrated drive volumetric basis which does not fluctuate with moisture.
[0026] A load cell preferably is provided on a recycle transfer belt to indicate presence
of reclaimed material. This may provide an operator with information as to when the
diverter gate should be operated to commence the simultaneous flow of reclaimed and
aggregate materials into the reclaim holding compartment.
[0027] When recycling is not required a diverter chute may be provided at the end of the
sizing screen to permit material passing over the top deck to be diverted into the
reclaim holding bin so that for coarse mixes using all new material the reclaim bin
can be used as the largest stone bin.
[0028] The process and apparatus described above provides versatility in the proportion
of reclaimed material able to be fed through the asphalt making system. The process
allows time for heat transfer between reclaimed material and raw aggregate and may
be readily set and adjusted in accordance with the appropriate mix design.
[0029] Because the reclaimed material is introduced into the asphalt making process subsequent
to grading taking place, clogging of the grading screen with tacky reclaimed material
is effectively avoided.
[0030] The temperature of the blend is permitted to stabilize and become uniform due to
progressive heat transfer or "heat soaking" which occurs at the holding stage and
to a further extent at the preblending and mixing stages. As a result the temperature
of the final mix is such as to avoid undesirable degradation of the bitumen in the
final mixing stage.
[0031] Because the temperature in the final mix stabilizes progressively in the process
it is possible to run the dryer at a lower temperature and obtain acceptable heat
transfer between the aggregate and reclaimed materials. This results in significantly
lower energy consumption as well as reduced maintenance costs.
[0032] A preferred form of asphalt recycling system according to the invention will now
be described in greater detail by way of example only with reference to the accompanying
drawing which is a schematic representation of the system.
[0033] Referring to the drawing, there are shown six cold feed hoppers 10 for storing virgin
aggregate material. The aggregate material is metered accurately by six corresponding
belt feeders 11 having variable speed drive adjustable by means of an automatic feedback
control system (not shown). A transfer belt 12 transfers the aggregate material to
a dryer 13.
[0034] The dried and hot aggregate material is transported to a sizing screen 15 of a batch
tower by means of a hot elevator 14. The batch tower includes four holding bins 16
for holding graded aggregate after the latter passes through the sizing screen 15.
A fifth holding bin 18 is provided alongside holding bin 16 for accepting reclaimed
material. The reclaimed material is supplied from a separate feed hopper 19 and is
metered to the system via a belt feeder 20. Recycle feeder 20 is controlled automatically
in correct proportion to the aggregate feeders. The reclaimed material is elevated
to the fifth holding bin 18 by means of a conveyor belt 21.
[0035] A diverter gate 22 is provided along a dividing wall 23 between holding bins 16 and
18. The diverter gate 22 is selectively positioned by means of a pneumatic control
system (not shown) to divert from 0 to 30% or more of incoming aggregate into holding
bin 18 as may be required.
[0036] When not recycling, the sizing screen 15 may be fitted with a diverter chute (not
shown) to divert coarse material into holding bin 18. Holding bin 18 then acts as
the largest stone bin.
[0037] Holding bins 16 and 18 are provided with a plurality of outlet gates shown generally
at 24. Material is discharged via the outlet gates to a weighing hopper 25. Finally,
bitumen, and filler if used, is added to the mix prior to it being blended and discharged
for transport.
1. A process for producing asphalt mixes in which at least a portion of the starting
material is reclaimed asphalt, the process comprising the steps of:
providing at least one grade of aggregate material;
transferring a metered quantity of the aggregate to a dryer for drying and heating
the aggregate;
screening the dried and heated aggregate into separate holding compartment according
to grade;
providing a supply of reclaimed asphalt of suitable consistency;
transferring the reclaimed asphalt to a reclaim holding compartment located in heat
transfer relationship with at least one of the aggregate holding compartments whilst
being physically separated therefrom, to permit heat transfer to take place between
the dried and heated aggregate and the reclaimed asphalt;
discharging the aggregate and the reclaimed asphalt from the holding compartments
to effect at least partial blending of the aggregate and the reclaimed asphalt; and
'
adding bitumen to the at least partially blended aggregate and reclaimed asphalt.
2. A process according to claim 1, including the further step of finally mixing the
aggregate, reclaimed asphalt and bitumen.
3. A process according to claim 1 or claim 2, including the step of diverting a portion
of the screened aggregate to the recycle holding compartment to effect a degree of
preblending and heat transfer.
4. Apparatus for use in the production of asphalt mixes wherein at least a portion
of the starting material is reclaimed asphalt, the apparatus comprising a batch tower
including a plurality of aggregate holding compartments for holding preheated aggregate
and at least one reclaim holding compartment for holding reclaimed asphalt, the reclaim
holding compartment being in heat-exchange relationship with at least one of the aggregate
holding compartments so that, in use, heat may pass from the preheated aggregate to
heat the reclaimed asphalt.
5. Apparatus according to claim 4, wherein the batch tower also includes screening
means to separate pre-heated aggregate into selected aggregate holding compartments
according to the size of the aggregate.
6. Apparatus according to claim 5, wherein the screening means comprises a sizing
screen located above the aggregate holding compartments.
7. Apparatus according to any one of claims 4 to 6, wherein each holding compartment
includes at least one outlet gate for discharging held material.
8. Apparatus according to any one of claims 4 to 7, wherein the reclaim holding compartment
incorporates a diverter gate actuable to draw at least a portion of aggregate into
the reclaim holding compartment.
9. Apparatus according to any one of claims 4 to 8, which also includes a weighing
hopper for receiving aggregate and reclaimed asphalt from the holding compartments.
10. Apparatus according to any one of claims 4 to 8 or claim 9 including a mixing
chamber for receiving aggregate and reclaimed asphalt from the holding compartments
or from the weighing hopper, respectively.
11. Apparatus according to any one of claims 4 to 10, further including means for
adjusting the supply of reclaimed asphalt as a proportion of the aggregate material.