Asphalt Plant Road Construction

Drum Asphalt Mixing Plants

The drum asphalt mixing plants  process uses the dryer drum to simultaneously heat, dry and mix the aggregate with bitumen. A cold feed unit proportions the aggregate from stockpiles and feeds it into the rotary drum where the bitumen is added. Hot screens , hot bins and pugmills are dispensed with. Output from these asphalt mixing plants varies from 150-800 tonnes per hour.

You can see the components of a typical drum asphalt mixing plant and following pictures illustrates the flow of material through such a asphalt plant.

Material Flow Diagram of drum asphalt mixing plants

Drum Asphalt Mixing Plant Details

Cold Feed Unit

The cold feed unit is normally a self contained unit with three to five bins and mechanical feeder for each unit. The aggregate gradation of drum asphalt mixing plants is controlled by a combination of variable speed belts and variable gate opening on each bin. An alternative system is individual weigh belts for each bin.

Aggregate Conveyor

The combined aggregate then moves to the drum of asphalt plant on a conveyor belt which is fitted with a load  cell type of weighing  system. The  belt  scales are  interlocked with the  bitumen pump which adjusts the bitumen content to match the amount of combined aggregate passing over the  belt  scales.  Moisture  is  added  to  the  aggregate  to  produce  steam  when  the  aggregate enters the dryer thereby providing an inert atmosphere in which asphalt mixing takes place.

Binder Feeding System of asphalt mixing plants

The  bitumen  binder can  be  introduced  in two ways. It  may  be sprayed onto the stream of cold wet aggregate in the heat chute or the bitumen spray line may extend some distance into the drum and the bitumen introduced at this point.


The predetermined bitumen-aggregate material drops off the conveyor and enters a hopper at the upper end of the drum in drum asphalt mixing plants. The lifting flights are designed such that the bitumen aggregate blend is directed away from the hottest part of the flame.

The drum is fired by a conventional type burner using either diesel or propane-type fuel. A combination of heating, drying and mixing produces a uniform asphalt paving material in about 3-5 minutes in the drum of  asphalt plants .

The low-oxygen atmosphere resulting from the steam given off by the wet aggregate reduces hardening  of  the  bitumen.  The  discharge  temperature  of  the  asphalt mix  is  usually  maintained between 850 – 1050 C depending upon conditions. This is lower than that for the other two types of  asphalt plant such as batch and continuous asphalt plants and the mixture may contain between 1% and 2% of water, which improves the handling and compact ability at lower temperatures.

Dust Extraction

One  of  the  primary  objectives  in  the  development  of  the  drum-asphalt mixing plant  process  was  the elimination  of  dust  particles  in  the  exhaust  gases.  This  objective  has  not  been  completely realized and in built up areas a scrubbing system is required.