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ASPHALT PLANT OPERATION

1  Introduction to Asphalt Plants

Part 1 — Types of Asphalt Plants

2  Drum Asphalt Plants

3  Batch Asphalt Plants3.1  Continuous Mix Asphalt Plants

Part 2 — Asphalt Plant Components

4  Aggregate Feed Bins to the Asphalt Plant

5  Collecting Conveyor

6  Scalping Screen (with respect to Asphalt Plants)

7  Drum Feed Conveyor

8  Asphalt Plant Burner

9  Asphalt Plant Drum Mixer

10  Air Filtration Baghouse (environmental)11  Asphalt Plant Slat/Bucket Conveyor

12  Hot Mix Storage Silos

13  Asphalt Cement Storage Tank

Part 3 — Asphalt Plant Illustrations

14  Asphalt Plant Illustrations

Introduction to Asphalt Plants

This section of the Aggregate Designs Corporation web page is intended to informthe reader about different aspects of asphalt plants, from the different kinds thatexist to the different subsystems in a typical asphalt plant. Over one-half of theInterstate System and 70 percent of all highways are paved with hot-mix asphaltconcrete. Asphalt concrete is probably the largest single highway programinvestment 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 PlantsIn a drum asphalt plant crushed aggregate of different sizes that is stored in coldfeed bins is metered out from these hoppers onto a conveyer belt where it enters arotary dryer. This dryer can be either gas or oil fired. This hot aggregate is thendropped onto a set of vibrating screens where it can be classified into as many as 4different sizes and is placed into individual hot bins based on these grades. Thesegrades are then combined in the desired formulation by an operator weighing theaggregate out into a weigh hopper. Reclaimed Asphalt Product (RAP) can then also

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be added to the mix in some percentage. Bituminous asphalt cement, heated in ahot oil tank, is then also weighed out for the correct final mix ratio. All ingredientstravel through the drum where it is mixed for a short time, typically less than aminute. This Hot Mix Asphalt (HMA) is then conveyed to a hot storage silo (oftenusing 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 USand 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 aconveyor scale leading to another conveyor called a collecting conveyor.

Collecting Conveyor 

The Collecting Conveyor takes measured amounts of aggregate from the cold feedbins 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 goingto the drum mixer. This is used to rescreen the crushed aggregate to remove anyoversize materials that might be in the rock. Often times the rock is crushed andstockpiled on the ground and then later reclaimed by loader and put in the coldfeed 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 inthe 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.

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Drum Feed Conveyor 

If a scalping screen is used after the collecting conveyor then it must send thescreened 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 drumto dry and heat the aggregate mix. Typically near the center of the drum the hotasphalt 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 asphaltcement. Any Recycled Asphalt Product (RAP) can also be added in measuredquantities 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 sandfrom 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 largevolume 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 drumnear the asphalt cement input to become part of the mix. A wet venturi scrubbercan 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 conveyorbelt that is either implemented with slats or buckets. This highly-inclined devicecarries 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 numberof days. From here, it is dispensed into waiting trucks to be hauled to the job.

Part 3 — Asphalt Plant Illustrations

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Asphalt Plant Illustrations

Typical drum asphalt plant