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.

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Asphalt Batch Plants Controls

Asphalt batch plants controls fall into three categories, depending upon the degree of automation used in the operation of the plant:-

• Manual
• Semi-automatic
• Automatic

It is normal in all these systems for the power control of the weighing and mixing process to be automated. Even in the so called manual plants, air or hydraulic cylinders activated by electric switches have replaced hand lever systems. They operate supply bin gates, feeders bitumen valve and the weigh batch and pugmill discharge gates.

In semi-automatic plants, all operations from the weigh batch to pugmill discharge are under automatic cycle control. The entire measuring and mixing phase of the plant is handled automatically.

In a fully automatic system an operator would:-

(a) Select the total weight required to be carried by the truck.

(b) Select the mix required either by pressing a button for say one of the six most common mixes or by inserting the appropriate punch card into the control panel.

(c) Wait for results.

The automatic mechanisms do the rest including monitoring the mix temperature, hot bin levels, weighing materials, controlling mixing times and mixing the correct number of batches and part batches to fill the truck.

(b) Continuous Plant Controls

The inherently automatic operation of the continuous mix plant can be extended by the addition of several additional automatic controls, viz:-
(a) Automatic burner controls
(b) Automatic mix discharge
(c) Automatic mixer and gradation cut-offs in case of hot bin shortage or improper feed
(d) Electric interlocks which shut down the plant in case of failure anywhere in the

This Asphalt Batch plants Controls can be change with technology.

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Proportioning Materials of Asphalt Batch and Continuous Plants

proportioning materials of asphalt batch and continuous plants, there  is  little  difference  in  principle between batch and continuous plants.

However beyond this point the two types of plant differ considerably and therefore each will be discussed separately.

(a) Batch Plants – Weigh Batching

The  fine  and  coarse  aggregates  are  dropped  from  the  hot  storage  bins  through  gates, which are either manually or mechanically operated , onto the weigh batch hopper. The gate  of  each  aggregate  bin  is  closed  when  the  correct  weight  of  material  has  been discharged into the weigh batch hopper.

The  filler  should  be  weighed  separately  because  of  the  relatively  small  amounts involved.  Weighing  may  be  done  manually,  but  modern  plants  provide  for  automatic

The weigh batch hopper empties directly into the mixing chamber or pugmill.

Bitumen may be weighed in a special bucket or it may be measured by a meter for each batch.  With  either  system  the  bitumen  should  be  distributed  uniformly  over  the  full
length of the pugmill.

(b) Continuous Plants – Proportioning Devices

Accurate proportioning materials of the mix components is achieved volumetrically by means of calibrated gates on the bins for aggregate and filler and a metering pump for the binder. The binder is sprayed onto the stream of aggregate as it enters the pugmill.

A  positive  interlock  between  the  controls  of  the  binder,  aggregate  and  filler  feeders  is essential  for  a  uniform  mix.  The  positive  interlock  ensures  that  aggregate,  filler  and binder are supplied in constant proportions even if the total output varies.

In both the batch and the continuous plant the mixing unit consists of a pugmill (paddle )The  function  of  the  pugmill  is  to  mix  the  separate  constituents,  (coarse  and  fine
aggregate,  filler  and  binder)  to  achieve  complete  coating  of  all  particles  and  homogeneity throughout the mix. This should be achieved as quickly as possible, and hence the mixing unit should be of adequate capacity and mixing efficiency.

The main parts of the pugmill are paddles, paddle tips, liner, shafts, discharge gate and heated jacket.

(a)     Pugmills for Batching Plants

Pugmills  may  be  of  the  single  or double  shaft  type  but   double  shaft  pugmills,  in which the blades or paddles rotate in opposite     directions,     are     more efficient  and  will  be  the  only  ones followed.

The  capacity  of  the  pugmill  should  be  such  that  the  maximum  batch  does  not  fill  the chamber  to  a  greater  depth  than  the  maximum  height  of  the  blades.  The  zone  between  the maximum height of the blades and the bottom of the pugmill is termed the live zone.

In order to mix efficiently, a pugmill should be neither over-filled nor under-filled.

Overfilled Pugmill

The  mix  is  discharged  from  the pugmill  by  opening  a  rapid  operating, sliding  gate  in  the  bottom  of  the  pugmill, This  allows  the  material  to  fall into a truck (or alternatively  into a skip  for delivery  to  a  surge  bin  or  hot  storage  silo). The  discharge  gate  must  have  a  positive action,  opening  quickly  to  allow  the  mixed batch to drop into the truck as a mass. If the batch is discharged as a flow, there is a risk of segregation.

(b) Pugmills for Continuous Plants

The pugmill in a continuous flow plant is similar to that in a batch plant except that it is usually longer and narrower. In order to move the mix forward, most of the paddles are
turned in one direction, except the last few which are usually reversed to prevent the mix from being discharged too rapidly.

The mix is discharged over a level-control gate at one end of the pugmill. Mixing time is controlled by this gate which, when raised or lowered, increases or decreases the amount
of material in the pugmill and consequently the time taken for the mix to pass through. You can see more details of  proportioning of materials in batch and continuous plants .

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Filler Feeding in asphalt Plant

 Filler feeding  is separate storage for filler and the means for its controlled introduction into the pugmill should be provided. Measurement may be by weight or volume.

Filler is not passed through the dryer, as the flow of air to the dust extractor would carry most of the fine particles to the dust collector, and into the wet scrubber (if installed) or into the atmosphere.

Feeding of the filler is usually by mechanical means, which should be checked regularly to ensure that no obstructions have occurred to affect uniform feeding.

In  batch  plants  the  filler  is  proportioned  by  weight.  In  continuous  mix  plants  the proportioning  is  achieved  by  varying  the  filler  gate  opening  and  interlocking  the  filler  feed system  with  the  aggregate  feed  system.  The  average  proportions  of  filler  in  the  mix  can  be checked by comparing the total quantity of filler used and the quantity of mix produced. Do you need to more details about filler feeding and see batch and continuous plants.