Asphalt Plant Road Construction

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.

Asphalt Plant Road Construction

Dust Extraction in Asphalt Plant

 Dust extraction  of aggregate is the process to remove dust after drying cold aggregate. A certain amount of dust is carried away in the hot gases from the dryer and it is usual for this to be collected in a dust collector.

The type of extractor normally used consists of a set of high energy cyclones and a low energy water scrubbing system. In isolated areas the scrubber may be eliminated.

The fan in the dust extractor creates suction from the dryer, hot bins etc. and delivers the dust to the cyclone type collector, where it is precipitated by centrifugal force. If the dust is found to be suitably graded, it can be returned to the plant for inclusion in the mix

Aggregate  from  the  dryer  is  fed  to  screens  mounted  over  the  hot  bins.  The screens separate the aggregate into selected sizes for proportioning, and reject oversize particles. Vibrating screens are preferred. This screening and recombining of aggregate  is carried out to correct segregation resulting from passing the combined aggregate through the dryer.

It will also help to correct variations in relative proportions of sand and coarse aggregate. However, as the smallest hot screen size is approximately 4 mm, hot screening will not correct any grading variations in material below this size.

For efficient screening the screens should be:-

• Of Adequate effective area

• Of the correct size opening

• Clean and in good condition

• Property arranged and at the correct slope

• Vibrating in the correct manner.

Hot storage Bins and Screens

For a general purpose plant there should be at least four bins to hold the hot aggregates, but  the  number  in  use  will  depend  on  the  type  and  size  of  mix  being  produced.  Bin  capacity should be adequate for continuous operation at maximum output. Each bin should be fitted with a  contents  indicator  in  full  view  of  the  operator,  and  with  an  overflow  outlet  to  ensure  that should  a  bin  become  overfull,  the  excess  will  not  spill  into  an  adjoining  bin.  A  thermometer should  be  installed  in  each  hot  bin.  The  hot  bins  function  as  small  reservoirs  to  dampen variations in supply rate and to improve batching.

Dust extraction is one step of the asphalt batch and continuous  plants. It is recommended to see that post.

Asphalt Plant Road Construction

Batch and Continuous Plants


The flow of material through both batch and continuous plants is similar up to the point of measuring and mixing. The difference in  the  types  is  that  in  the  batch  plant,  aggregate  proportioning  is  by  weight  whilst  in  the continuous plant, aggregate proportioning is by calibrated gate openings which allow a constant flow of hot aggregate and filler into the pugmill mixer.

The essential features of these plants are:-

• Cold aggregate bins fitted with feeders to meter the cold aggregate onto a belt conveyor.

• A  dryer which  is  a  rotating  steel  drum  where  flames  and  hot  air  are  forced through the cold aggregate.

• Screens to separate the hot aggregate which is then remixed in the correct proportions.

• Bins to hold hot aggregate and filler.

• Binder storage tanks

• Weight batching or continuous feeding mechanism.

• A pugmill mixer.

• Dust collectors.

• A manual, semi-automatic or automatic control system.

Surge bins  may  be provided to overcome the disruption due to dump truck delays whilst hot storage silos for the storage of hot mixed material are often provided at batch and continuous plants.


Aggregate is fed into the plant from one of the following sources:-

(a) Small open-topped bins fed by a front end loader.

(b) Large bins fed by trucks either directly or via a conveyor transfer system.

(c) Large open stockpiles (separated by bulk heads) and situated over a conveyor tunnel. Materials are stockpiled over the tunnel by belt conveyor, truck etc.

The number of bins necessary will depends on the type of mix being produced, and may vary from one, (for a patching premix plant), to four or more (for a plant producing dense graded mixes).

When charging the cold bins, care is required to minimize aggregate segregation and degradation. Enough materials must be maintained in all bins to provide a positive and uniform flow.

When a front-end loader is used to feed the cold bins, the operator should not pick up material from the storage stockpile at ground level. The bucket should be held high enough above the ground to prevent contamination when filling.

If trucks are used to charge the bin, they should deposit their loads directly above the feeder.


Each size of aggregate must be metered out by the cold feeders in correct proportions and at the correct rate otherwise variation in the final mix, or overflow of the hot bins, may occur.

The feeder units are located beneath the storage bins or stockpiles, and have controls that can be set to produce a uniform flow of aggregate to the cold elevator.

The four types of feeders in common use are:-

• Tilting Chute. These are definitely not suitable for sand and are not particularly good for aggregates as they do not provide a uniform flow. The rate of flow is very much a matter of trail and error as they cannot be calibrated accurately.

• Continuous Belt.  These  are  the  most  popular  for  handling  sands.  They  generally  have  an infinitely variable speed control which is easily adjustable and calibrated. Continuous Belt Feeder

• Reciprocating  Plate.  These  Deliver  a  uniform  flow  and  are  capable  of  handling  most materials. However, flow rates cannot be varied as readily as with the continuous belt and conveyor types.

Reciprocating Plate Feeder

• Electromagnetic  Vibrating  Chutes.  They  are  generally  used  for  feeding  coarse  aggregates.

Electromagnetic Vibratory Feeder

Close control over the feeding of cold aggregates into the dryer is necessary in order to avoid  variations  in grading; temperature and  moisture content of dried aggregate; to carryover on screens.

To ensure that the correct quantity of each aggregate is  fed to the dryer, the cold  bins should be supplied with materials if uniform grading and moisture content, and the levels of the materials in the bins should be kept constant. The feeder gates should be accurately calibrated, securely held in the set position, and kept free from obstructions.