ADC ASPHALT PLANT INFORMATION
Introduction to Asphalt Plants
This section of the Aggregate Designs Corporation web
page is intended to inform the reader about different aspects of asphalt plants,
from the different kinds that exist to the different subsystems in a typical
asphalt plant.
Over one-half of the Interstate System and 70 percent of all highways are paved with hot-mix asphalt concrete. Asphalt concrete is probably the largest single highway program investment today and there is no evidence that this will change in the near future.
Hence the importance of asphalt plant productivity.
Part 1 — Types of Asphalt Plants
Drum Asphalt Plants
In a drum asphalt plant crushed aggregate of different
sizes that is stored in cold feed bins is metered out from these hoppers onto a
conveyer belt where it enters a rotary dryer. This dryer can be either gas or
oil fired. This hot aggregate is then dropped onto a set of vibrating screens
where it can be classified into as many as 4 different sizes and is placed into
individual hot bins based on these grades. These grades are then combined in the
desired formulation by an operator weighing the aggregate out into a weigh
hopper. Reclaimed Asphalt Product (RAP) can then also be added to the mix in
some percentage. Bituminous asphalt cement, heated in a hot oil tank, is then
also weighed out for the correct final mix ratio. All ingredients travel through
the drum where it is mixed
for a short time, typically less than a minute. This Hot Mix Asphalt (HMA) is then conveyed to a hot
storage silo (often using a slat conveyer) or directly into delivery trucks and hauled to the job site.
Batch Asphalt
Plants
Batch Asphalt Plants literally make Hot Mix Asphalt (HMA) one
batch at a time. They have the advantage of being able to taylor each
truckload of HMA to exact specifications, whereas a drum asphalt plant generally
produces high volumes of the same "recipe". Aggregate, RAP and asphalt
cement are mixed in a pugmill mixer rather than a drum.
Continuous Mix
Asphalt Plants
Continuous Mix Asphalt Plants make up less than 1/2 percent of plants in the US and are most similar to drum plants.
Part 2 — Asphalt Plant Components,
Typical Drum Plant
Aggregate Feed Bins to the Asphalt Plant
Depending on the formulation of the asphalt mix up to four different
sizes of aggregate might be combined to create the final product. These
raw ingredients are stored in bins known as cold feed bins which would each
typically have a conveyor scale leading to another conveyor
called a collecting
conveyor.
Collecting Conveyor
The Collecting Conveyor takes measured amounts of
aggregate from the cold feed bins and transfers it to the drum mixer, optionally
through a scalping screen.
Scalping Screen
A Scalping screen can be added between the cold feed
bins and a conveyor going to the drum mixer. This is used to rescreen the
crushed aggregate to remove any oversize materials that might be in the rock.
Often times the rock is crushed and stockpiled on the ground and then later
reclaimed by loader and put in the cold feed bins to be blended to make the hot
mix asphalt. It would be possible for there to be some rock in the piles that
didn’t belong there and you can’t have oversize in the ac mix it would ruin the
mat when you go to lay it out and roll it. This can also aid in blending the
different sizes of aggregate prior to entering the drum.
Drum Feed Conveyor
If a scalping screen is used after the collecting
conveyor then it must send the screened product to another conveyor leading to
the drum.
Asphalt Plant Burner
A large burner with a substantial fan attached to it blows
into one end of the drum to dry and heat the aggregate mix. Typically near
the center of the drum the hot asphalt cement is injected into the drum to mix
with the hot aggregate.
Asphalt Plant Drum Mixer
Aggregate from the drum conveyor is dumped into a large
hollow cylinder known as the asphalt drum where it dries, mixes and is mixed
with the bituminous asphalt cement. Any Recycled Asphalt Product (RAP) can
also be added in measured quantities to the drum. There are paddles and
agitators in the drum to insure that thorough mixing occurs before the
conglomerate leaves the drum.
Air Filtration Baghouse
The heated air that is blown into the drum extracts
large amounts of dust and sand from the heating aggregate mix and can't simply
be exhausted into the atmosphere without being cleaned. This is the job of
the baghouse. It is a large volume enclosure with lots of filter bags that
remove and collect the fine particles coming out of the drum. These
particles are commonly fed back into the drum near the asphalt cement input to
become part of the mix. A wet venturi scrubber can also serve the function
of a baghouse.
Asphalt Plant Slat/Bucket Conveyor
Once the completed asphalt hot mix product exits the drum it
goes to a conveyor belt that is either implemented with slats or buckets.
This highly-inclined device carries the HMA to one or more silos. Slats
are the preferred implementation of this conveyor now.
Hot Mix Storage Silos
These silos are often heated so finished product
can be stored there for a number of days. From here, it is dispensed into waiting trucks to be hauled to the job.
Part 3 — Asphalt Plant Illustrations
Asphalt Plant Illustrations
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Typical drum asphalt plant. |
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* Trademarks and photos are
property of their respective companies
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Crushers
