Asphalt drum mix plant and counter flow asphalt mixing plant are the two types of plants that are categorized as continuous mixing plants. Both these types of plants produce hot mix asphalt in a continuous process. Batch mix plants are also there which make hot mix asphalt in batch. The end product is to get hot mix asphalt but the process of batching plants and continuous plants is totally different from each other. The difference in the process is what makes the quality of hot mix asphalt different in all the types of plants.
Asphalt drum mix plant process starts with the feeding of cold aggregates into feed bins. The equipment is usually equipped with three or four bin feeders (or more) and aggregates are loaded into different bins as per their sizes. This is done so that different sized aggregates can be graded as per the requirement. Each bin is provided with separate adjustable gates for controlling the flow of material. There is a long conveyor belt below the bins which takes the aggregates to the scalping screen.
The next part is the screening process. Here is a single deck vibrating screen which removes the oversized aggregates thus preventing them from entering the drum.
Charging conveyor plays an important role in the asphalt plant process because it not only transfers the cold aggregates from below the screen to the drum but also does the weighing of the aggregates. This conveyor is equipped with a load cell which continuously weighs the aggregates and sends signal to the control panel.
Drying and mixing drum is responsible for two operations first drying and then mixing. This drum is continuously rotating and in the course of the rotation, aggregates are transferred from one end to the other. The aggregates are treated to heat by the burner flame to reduce the moisture content in the aggregates. When we talk about the process in a parallel flow plant, the aggregates move away from the burner flame and in the counter flow plant, the aggregates move towards the burner flame. On the other end of the drum, the heated aggregates are mixed with bitumen and minerals. The drum plays an important role in the drum mix plant process.
Fuel tank for drying drum burner stores and provides fuel to the drum burner. Apart from that, the critical component includes asphalt storage tanks which store, heat and also pump required asphalt to the drying drum for mixing with the hot aggregates. Filler silos are for addition for optional filler / binder material into the mixer.
Pollution control devices play an important part in the process. They help to eliminate the harmful gases that may have escaped the environment. Primary dust collector is a dry dust collector and it works in sync with secondary dust collector which can be a bag filter or a wet dust scrubber.
Load out conveyor collects the ready hot mix asphalt from below the drum and takes it to the waiting truck or into the storage silo. Optional storage silo stores the HMA till the truck arrives.
Control panel with today’s machines comes with modern and sophisticated controls. They allow storage of different mix recipes as per customer’s demand. Plant can be controlled from a single place from the control panel.
There are two types of continuous mixing plants: parallel flow and counter flow. The below image represents and compares both the parallelflow and counterflow asphalt plant. In parallel flow plant, the flow of aggregates is parallel to the burner flame. This also means that the aggregates are moving away from the burner flame during their journey. In the counter flow plant, the flow of aggregates is opposite (counter) to that of burner flame and hence the aggregates are moving towards the burner flame before getting mixed with bitumen and other minerals. This seems simple but it makes a huge difference to the process of both these types of asphalt mixers and even affects the quality of the HMA. It is believed that counter flow mixer is better for fuel saving and also offers better HMA compared to the other.
However tough or challenging you hot mix asphalt job may be, if you choose the right type of equipment from the right manufacturer you will be amazed with the results that you machine delivers.