A hot mix plant is a combination of several components both electrical and mechanical working in sync to make a product which we call as HMA (Hot mix asphalt). The combination of these components allow cold aggregates to heat and then mix the same with liquid bitumen and minerals in varying proportions to produce asphalt mix as per requirements.
These plants can be categorized as stationary and portable. Usually the stationary plant components are fixed and portable plant components are on wheels facilitating movement from place to place.
Hot mix asphalt plants can also be categorized as continuous type and batch type. Continuous plants produce asphalt mixture in continuous operation and batch type plants produce HMA in batch.
In continuous mixing plants, the ingredients are added, dried and mixed continuously in a drum. In batching plant, the ingredients are pre-weighed as per the required amount and then added into a separate pug mill mixer to form a batch.
The functioning of both the plants is different but the main purpose is to make hot mix asphalt as per the requirement.
Counter flow asphalt plant is also a continuous plant but featuring a different technology compared to the traditional drum plant. In counter flow plant, the flow of aggregate and flame direction is opposite. Dust suction is done at the point where the aggregates enter the drum. Counter-flow plants can feature a single drum for drying and mixing or two drums (separate for drying and mixing) or drum for heating and a mixer for mixing.
Usually drum plants are smaller than the asphalt batch plant and require less space, less energy and less efforts to run. The components are also less in the drum asphalt plants which make them easy to transport and ship.
Here is what we are going to discuss in the article below:
A. Common operations and components to both the batch and drum type.
B. Operation of parallel flow asphalt mixing plant.
C. Process of counter flow asphalt mixer.
D. Working of asphalt batch mix plants.
There are certain common operations common to both the types and the components are also similar. They are as below:
Component: Aggregate feeder
Operation: Cold aggregate storage and feeding
Component: Drum and burner
Operation: Drying and mixing (parallel flow) | Drying of aggregates (Batch type)
Component: Fuel tank for dryer burner
Operation: Storage and supply of fuel to the burner.
Component: Primary scalping screen and charging conveyor
Operation: Removal of oversized material before the aggregates enter the drum.
Component: Primary and secondary pollution control units
Operation: Collection of heavy and fine particles.
Component: Asphalt tanks
Operation: Storage and heating of bitumen.
Component: Filler bins
Operation: Storage and transfer to filler material.
Component: RAP feeder
Operation: Feeding of recycled material.
Component: Control panel
Operation: Controlling of the entire plant.
Component: Storage silo
Operation: Storage of hot mix asphalt before dispatch.
Operation of parallel flow asphalt mixing plant:
This process is a continuous process where the ingredients are added by proportions to the cold feed bins. The feeder bins store the cold aggregates in separate bins and transfer the same to the drying drum. The main difference between this process and the batch plants starts here. Here there is a single drum which heats and mixes in a single chamber (drum). Aggregates are introduced to the burner end of the drum and the flow of aggregates and the burner flame is parallel to each other and hence the name parallel flow. Before introduction into the drum, the aggregates have to pass through a primary scalping screen. This will help to remove the oversized aggregates and prevent them from entering the drum. Once the cold aggregates enter the drum, they are treated by the flame which makes them hot and ready for mixing with the hot bitumen and other additive. As the drum rotates, the aggregates move towards the other end of the drum. Liquid bitumen is added to the aggregates on the other half of the drum. This liquid bitumen flows by a pump which is controlled by a variable speed drive. So the flow of aggregates determine the addition of bitumen and minerals into the drum. If the aggregate flow is more, the control panel sends message to the bitumen drive which sends the bitumen in set proportion to the aggregates. If the aggregate flow drops, the control panel will send signal to the bitumen drive thereby decreasing the inflow of the bitumen into the drum. Minerals and RAP are also added into the second half of the drum where the temperature is much lower. The introduction of minerals and recycled asphalt is also controlled by a variable speed drive just like the bitumen.
The exhaust gases are sucked towards the discharge end of the drum and are treated first by primary and then by secondary dust collector. Primary dust collector is a cyclone device which traps most of the heavy dust and settles them not allowing them to enter the secondary dust collector. Secondary dust collector is usually venture scrubber type wet dust collector or a bag house filter which settles the lighter particles and prevent them from escaping. Wet dust collectors use water jets to trap and treat them to cyclonic effect to settle the dust. Bag filters do not require water to clean and it cleans the dust by trapping them to bags made of special material designed for this purpose.
The mixed material is dispatched from the other end of the drum and is transferred to a storage silo or to a truck. If it is transferred to a hot mix silo, it can be loaded into the truck.
Process of counter flow asphalt mixer:
The process of counter flow plant is similar to the parallel flow plant and is a continuous process. Here also the entry and quantity of bitumen and minerals is controlled by the flow of aggregates just like the parallel flow plant.
The main differentiating factor lies in the flow of aggregates compared to the burner flame position which is opposite and hence the name counter flow. Even the flow of exhaust gases is from the point where aggregates enter the drum. The addition of bitumen, RAP and minerals is behind the burner flame. This addition of bitumen, RAP and minerals can be in the same drum or in a separate drum or in a separate mixer. This prevents bitumen, RAP and minerals from getting in direct contact with the flame. The main advantage of a counter-flow plant is that it will allow addition of RAP into the mix.
The addition of liquid bitumen, RAP and minerals is controlled by a variable speed drive and is synchronized with the flow of aggregates just like in the parallel-flow asphalt mixers.
Here we can see that there is only a single drum. The flow of aggregate and burner flame is opposite and the addition of RAP and liquid bitumen is just behind the burner flame.
There are two separate chambers: first is counter flow drum only for drying of virgin aggregates, second is the pug mill type mixing unit for mixing of hot aggregates with RAP and liquid bitumen.
Here there are two drums one above the other. The drum which received the virgin aggregates is fitted with the burner on the other side and only drying of aggregates takes place in this drum. Covering this drum is a drum which allows entry of liquid bitumen and RAP. Aggregates after heating are transferred to the outer drum through openings in the inner drum.
Working of asphalt batch mix plants:
This plant features the most advance technology for making HMA. The process starts when cold aggregates are fed to the appropriate aggregate feeder bins. They then are metered from the hoppers individually and move towards the drying drum. In between they have to pass though a primary scalping screen for removal of oversized materials. A conveyor takes the cold aggregates collectively into the drying drum. The drum featured here is counterflow type and the dry aggregates move from one end to the burner end by the help of the drum rotation. During this process they get fully dried. The aggregates get transferred from one end to the other and leave the burner end of the drum to fall on a elevator. This elevator is fully covered and it takes the aggregates to the top end of the tower unit where vibrating screens is placed.
Dust collection is done at the entry point of the aggregates and treated by a primary and secondary dust collector. Here the primary dust collector is cyclone type and the secondary dust collector can be wet scrubber or a bag house.
The screens are inclined horizontally and are of different sieve sizes depending on the mix material requirement. Once the aggregates pass thought the vibrating screen, they are separated and placed in different bins just below the vibrating screen. This separation is according to the size of the aggregates. This screen has provision for removal of oversize and overflow material. In certain asphalt batch plants, there is a separate bin which stores RAP material. The aggregates are stored in the bins separately and the bins open as per the batch which is set. Here the quantity of each size of aggregates can be controlled as per requirement. At the same time, the bitumen which is stored in the bitumen tanks is flowed from the tanks below by an asphalt pump into a bucket near the mixer. This bucket weighs the bitumen and then pumps the same into the mixer. Before the entry of the bitumen into the mixer, hot aggregates (and RAP) have already entered the mixing unit. The material already in the mixing unit is as per the desired ratio. The mixing time of the twin shaft mixer can be set as per requirement.
After mixing, the HMA is discharged into a storage silo or into waiting trucks below which takes them to the job sites.
View simple video explaining the difference below:
You are going to make a long term investment. It is important that you understand your primary requirement and then select a product accordingly. There are many companies offering drum and asphalt batching plant for sale. Before buying an equipment it is important to check all asphalt plant parts and ensure that it meets or exceeds your requirement. It is equally important that the equipment that you are buying is easy to use, maintain, transport and is easy on the pocket. This is one investment which can yield a lot of money, if taken care.