A hot mix plant is a combination of several components both electrical and mechanical working in sync to make a product which we call HMA (Hot mix asphalt). The combination of these components allows 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 the 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 two basic types of asphalt plants used in the process of making hot mix asphalt are asphalt drum mix plants and batch plants.
Batch plants vs Drum plants:
Asphalt batch plants will make hot mix asphalt in fixed batches. One by one batch is continuously made to make hot mix asphalt.
Some of the advantages of asphalt batch plant are:
It offers the best quality of hot mix asphalt.
The flexibility offered is best.
It is possible to edit batch sizes and mix material designs frequently.
The control systems are very advanced allowing users to have multiple options.
The level of pollution control is better than a drum mixer.
On the other hand, drum mixers are very much preferred in developing countries. They produce HMA in a continuous process. They are good for small to medium-sized projects. The end product quality is not as accurate as a batch plant.
Some of the advantages of continuous drum mixers are:
The initial cost is low compared to the batch plant.
The land required to set up is less compared to the batch plant.
The process here is continuous. Meaning uninterrupted production is there.
It is easy to have a higher capacity machine without an exponential increase in its price.
The functioning of both the plants is different but the main purpose is to make hot mix asphalt as per the requirement.
A counterflow asphalt plant is also a continuous plant but featuring a different technology compared to the traditional drum plant. In the counterflow 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 a 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 effort to run. The components are also less in the drum asphalt plants which makes them easy to transport and ship.
Here is what we are going to mention in the article below:
Common operations and components to both the batch and drum type.
Operation of parallel flow asphalt mixing plant.
Process of counterflow asphalt mixer.
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: 1. Component: Aggregate feeder Operation: Cold aggregate storage and feeding
2. Component: Drum and burner Operation: Drying and mixing (parallel flow) | Drying of aggregates (Batch type)
3. Component: Fuel tank for dryer burner Operation: Storage and supply of fuel to the burner.
4. Component: Primary scalping screen and charging conveyor Operation: Removal of oversized material before the aggregates enter the drum.
5. Component: Primary and secondary pollution control units Operation: Collection of heavy and fine particles.
6. Component: Asphalt tanks Operation: Storage and heating of bitumen.
7. Component: Filler bins Operation: Storage and transfer to filler material.
8. Component: RAP feeder Operation: Feeding of recycled material.
9. Component: Control panel Operation: Controlling of the entire plant.
10. Component: Storage silo Operation: Storage of hot mix asphalt before dispatch.
Operation of parallel flow asphalt mixing plant:
The operation of a parallel flow type of asphalt mixing plant 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 that 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 asphalt mixing plant. 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 another 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 determines the addition of bitumen and minerals into the drum. If the aggregate flow is more, the control panel sends a 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 a 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 the primary and then by the secondary dust collector. A primary dust collector is a cyclone device that traps most of the heavy dust and settles them not allowing them to enter the secondary dust collector. A secondary dust collector has usually ventured scrubber type wet dust collector or a baghouse 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 a 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 counterflow asphalt mixer:
The process of counterflow plant is similar to the parallel flow plant and is a continuous process. Here also the entry and quantity of bitumen and minerals are 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 counterflow. 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 the 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 the counterflow drum only for drying of virgin aggregates, second is the pug mill type mixing unit for mixing 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 that 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 asphalt batch mix plant features the most advanced technology. It is best for getting quality HMA. The process-working 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 through a primary scalping screen for the 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 with 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 an elevator. This elevator is fully covered and it takes the aggregates to the top end of the tower unit where vibrating screens are 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 a wet scrubber or a baghouse.
The screens are inclined horizontally and are of different sieve sizes depending on the mixed material requirement. Once the aggregates pass through 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 the removal of oversize and overflow material. In certain asphalt batch plants, there is a separate bin that 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 a simple video explaining the difference below:
CONCLUSION:
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 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 that can yield a lot of money if taken care of.
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