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.
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.
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.