Types of Asphalt Plants
Batch Mix Asphalt Plant
This type of plant adopts the production mode of preparing asphalt mixtures according to batches, i.e., the production of the next batch starts only after the mixing of each batch of mixtures is completed, so it is also known as intermittent or discontinuous mixing plant.
Its core advantage lies in the accurate control of the raw material proportion of each batch, through the weighing and metering system of sand, asphalt, mineral powder and other raw materials for accurate weighing, to ensure stable quality of the mixture.
Drum Mix Asphalt Plant(Continuous Asphalt Mixing Plant)
The biggest characteristic of drum mix asphalt plant is that it can realize continuous supply of asphalt mixture, after continuous measurement of raw materials, it will mix the raw materials while drying and heating in the drying drum, and produce finished products uninterruptedly.
Compared with batch mix plant, drum mix asphalt plant has higher production efficiency, relatively low equipment investment and operating costs, and is suitable for large-scale production of a single specification of the mixture.
Mobile Asphalt Plant
Mobile asphalt mixing plants are characterized by a high degree of portability, modular design, components can be quickly disassembled, transported and reassembled, and can be easily adapted to the needs of different construction sites, especially for the scene of the mixture to be transported across the region, or a short period of time, the need for frequent transfer of the project.
Asphalt Recycling Plants
Asphalt recycling mixing plant through advanced thermal recycling technology, the old asphalt pavement milling of waste material for heating, screening, adding regeneration agents and new aggregates, and other processing, and re-produced in line with the quality standards of the fresh asphalt mixture, used for road construction.
How Does an Asphalt Plant Work?
The production of an asphalt mixing plant follows a rigorous and systematic process, starting with the feeding of cold aggregate and ending with the unloading of the finished mix:
Cold Aggregate Feeding
Cold aggregates of different sizes are stored in separate silos, which are usually equipped with level sensors to monitor the aggregate stock in real time. In order to ensure the homogeneity of the mix, the aggregates are subjected to a strict grading test prior to feeding, and only aggregates that meet the criteria are allowed to enter the production process, thus laying a solid foundation for subsequent production.
Conveyance of raw materials to the drying drum
The belt conveyor under the silo runs at a constant speed and conveys the aggregate to the inclined conveyor. The speed and angle of inclination of the conveyor belt are precisely calculated in order to prevent segregation of the aggregate during transportation. Finally, the aggregate is fed via the inclined conveyor into the drying drum, completing the initial transfer of the raw material.
Conveyance to the screening system
The high temperature environment in the drying drum (usually 160°C – 180°C) heats and dries the material sufficiently to remove the moisture. After drying, the material is fed by an elevator at a steady rate into the screening system, ready for the next step in the particle size separation process.
Separation by particle size
The screening system is equipped with a multi-layer vibrating screen, which vibrates at different frequencies and amplitudes to finely screen the material. The screen is made of high-strength and wear-resistant materials, which can effectively separate the aggregates according to the particle size, and the separated aggregates enter into different silos through the chute to realize the accurate grading of aggregates.
Stored in independent bins
The separated materials are stored in the independent bins under the vibrating screen according to the particle size, and each bin is equipped with a weighing device, which can accurately measure the weight of the materials in the bin. At the same time, the bins are also equipped with anti-clogging devices to ensure safe storage and smooth access to the material while waiting to enter the mixer.
Release to mixer
The control system accurately weighs the aggregate in each bin through the high-precision weighing device according to the preset mixing ratio program. When the dosage weight set by the control panel is reached, the pneumatic valve opens automatically and releases the aggregate into the mixer accurately, and the error of the whole process is controlled within a very small range.
Additives are added to the mixer
At the same time as the aggregate enters the mixer, the asphalt is injected into the mixer through a pipeline delivery system at a precise flow rate. Other mineral fillers, such as ore powder, are dosed into the mixer by a screw conveyor. All materials are added synchronously according to the specified weight, and the precise proportioning is realized through the automated control system to ensure the accurate composition of the mix.
Unloading of finished mix
The mixer adopts double-shaft forced mixing structure, fully mixing the materials through the high-speed rotating blades. The whole mixing process lasts about 20 minutes to ensure that the asphalt, aggregates and additives are fully mixed. After the mixing is completed, the unloading door opens automatically, and the finished mixture is unloaded into trucks or storage silos, thus completing the whole production process.
How An Asphalt Plant Works – Drum Asphalt Plant
Aggregate feeding
Aggregates of different grain sizes enter into their respective feeding bins in an orderly manner through loaders or belt conveyors, and each bin is equipped with independent variable frequency gears and variable speed drive motors at the bottom of each bin, which can accurately regulate the feeding speed and flow rate through the PLC control system. The system adopts dynamic weighing technology to monitor the weight of aggregates in real time, with the error controlled within ±0.5%, to ensure that different specifications of crushed stone and sand are accurately conveyed according to the preset ratios, laying a quality foundation for subsequent mixing.
Drying and mixing drum
After the aggregate is screened by the primary vibrating screen, it is fed into the 2-4 meter diameter drying and mixing drum through the inclined belt conveyor. The drum rotates continuously at a speed of 3-5 revolutions per minute, and its inclination angle of 3-5°, together with the design of the internal spiral blades, enables the aggregate to form a continuous and stable curtain trajectory in the drum. The burner adopts natural gas or heavy oil as fuel, and maintains the internal temperature of the drum at 160-180℃ through the intelligent temperature control system, which not only evaporates the moisture on the surface of the aggregate quickly, but also forms a special mixing zone in the back half of the drum to ensure that the asphalt, filler and aggregate can be evenly coated in the dynamic tumbling.
Deducting system
The dusty air enters the wet deducting system of the mixing plant through the negative pressure pipeline, and firstly, it is fully contacted with the high-speed sprayed atomized water in the venturi scrubber, which captures the dusts with a particle size larger than 5 microns by using the principle of inertial collision; and then it enters into the cyclone separator, which separates the remaining fine particles through the centrifugal force, and the dusty sludge is finally discharged through the sludge discharge pump, and then recycled through the treatment of the filter press. The dust removal efficiency of the whole system is up to 99.5%, and the emission concentration is lower than 20mg/m³, which meets the stringent environmental protection standards.
Filler conveying
Mineral powder, cement and other fillers are stored in the closed filler silo with material level sensors. When the system detects a drop in the material level, the Roots blower is started, and the fillers are pushed to the drum through the pneumatic conveying pipeline at a flow rate of 15-25m/s. The system is equipped with multi-stage butterfly valves, which can be used to convey the fillers. The conveying pipe is equipped with multi-stage butterfly valves and pulse ash cleaning devices to prevent the filler from clumping and blocking, and at the same time, it is equipped with loss-in-weight scales to realize dynamic metering and ensure that the filler is added with an accuracy of ±1%.
Asphalt conveying
The base asphalt or modified asphalt stored in the asphalt storage tank with insulation layer and heating coil is heated up to 140-160℃ by thermal oil system, so that its kinematic viscosity is reduced to 0.1-0.3 Pa・s. The asphalt is conveyed to the metering tank by gear pump with constant pressure, and then accurately metered by Coriolis mass flow meter, and then sprayed into the drum through the high-temperature resistant pipeline and spraying nozzles. The pipes are heated throughout and equipped with automatic cleaning devices to avoid asphalt condensation residues affecting the conveying efficiency.
Mixing and control
The PLC control system in the central control room integrates industrial Internet of Things technology and collects and analyzes the real-time flow and temperature data of aggregate, asphalt, and filler in milliseconds through the pressure, temperature, and flow sensors distributed in each link.
What Are The Advantages Of Asphalt Plant?
High Production Efficiency
The asphalt mixing plant is equipped with a modular production line and an intelligent scheduling system, which realizes the seamless integration of aggregate conveying, heating and mixing through PLC control technology. Taking the Model 240 asphalt mixing plant as an example, it can produce 240 tons of finished materials per hour, which is equivalent to the daily workload of 150 skilled workers. The intelligent control system monitors the running status of the equipment in real time and adjusts the production rhythm automatically, which can shorten the road construction period by more than 40% under the continuous operation mode, effectively relieving the pressure on the schedule of large-scale projects.
Stabilizing production quality
Adopting three-stage dynamic weighing system, the plant controls the quantitative control of sand, asphalt, powder and other raw materials with an accuracy of ±0.5%, and cooperates with high-precision temperature sensors (temperature control accuracy of ±2℃) and double horizontal axle forced mixer to ensure that the oil-rock ratio, gradation curve and other key indexes of each batch of finished materials remain highly consistent. The German TÜV-approved data traceability system can completely record the whole process data from raw materials to finished products, and through the SPC statistical process control technology, the failure rate can be controlled within 0.3%, which provides quality assurance for the long-term stability of the road surface.
Strong Adaptability
The modularized design enables the asphalt mixing plant to switch production modes quickly. By adjusting the screen specification of the screening device, mixing parameters and batching ratio, the plant can produce 12 different types of asphalt mixtures, such as AC, SMA, OGFC, etc. The modularized design enables the plant to switch production modes quickly. In highland construction projects, the specially designed negative pressure dust removal system can adapt to the low air pressure environment; for the high humidity working conditions in coastal areas, the raw material pre-heating and dehumidification module is equipped. Whether it is -30℃ in the cold region or 45℃ in the high temperature environment, stable production can be guaranteed through customized solutions.
High resource utilization
Adopting multi-stage vibrating screening and crushing technology, the 0-5mm stone chips and dust are utilized twice, and combined with the RAP (Recycled Asphalt Material) recycling system, up to 50% of the old material can be blended. Intelligent batching system automatically adjusts the proportion of new aggregate and asphalt according to the performance parameters of the old material, realizing the recycling of resources. According to estimation, every 10,000 tons of recycled asphalt mixture can save 6,000 tons of sand and gravel resources, reduce asphalt consumption by 400 tons, significantly reducing production costs and environmental load.
Environmental protection and energy saving
The integrated pulse bag filter with activated carbon adsorption device controls the dust emission concentration below 10mg/m³, which is 50% lower than the national standard. Adopting thermal storage combustion technology, the thermal efficiency of fuel oil is increased to more than 92%. Meanwhile, equipped with the waste heat recovery system, the waste heat of the exhaust gas from the drying cylinder is used for asphalt heat preservation and heat-conducting oil heating, which reduces the comprehensive energy consumption by 18%. Fully enclosed silos and noise reduction devices effectively control dust and noise pollution, realizing the goal of green production.
Short construction period
By adding warm mixing agent and special modified asphalt, the paving temperature of asphalt mixture is reduced by 30℃, which shortens the cooling and curing time. In urban road repair projects, the use of fast-setting asphalt mixture, paving can be opened to traffic in 2 hours. Combined with mechanized paving equipment, the average daily working capacity of a single paver can reach 5000 square meters, comparing with the traditional process, the working period is compressed by 60%, which greatly reduces the impact of construction on traffic.
Long service life
By optimizing the design of asphalt colloid structure and adopting SBS, rubber powder and other high-performance modifiers, the rutting resistance factor of asphalt mixture is increased by more than 3 times, and the low-temperature bending strain is up to 2,500 με. In the heavy-duty traffic section, the specially-designed skeleton compact grading structure can withstand the action of standard axle load for more than 10 million times. According to the third-party test, the designed service life of the pavement produced by this process can be more than 15 years, and the maintenance cycle is extended to twice that of the traditional pavement.
Key Components of an Asphalt Plant
Cold Feed Bins
Cold feed bins are the starting unit of asphalt production, usually consisting of several independent bins for categorizing and storing aggregates of different grain sizes (e.g. coarse gravel, fine sand, etc.). Each silo is equipped with an adjustable feeder at the bottom, which ensures that the gradation of the subsequent mixture meets the design requirements by accurately controlling the speed and proportion of the aggregate discharged. This fine storage and distribution mechanism is the basis for ensuring the stability of asphalt mixture quality.
Conveyor Belts
The conveyor belt system is like the “material artery” of the asphalt mixing plant, which continuously conveys the aggregates from the cold feed silo to the various processing stages through the multi-segment articulated high-strength belts. Its inclined and horizontal combination of layout design, can adapt to the complex station space, while with the frequency conversion speed control device, according to the production demand can be flexibly adjusted conveying efficiency, to ensure the continuity and accuracy of the material flow.
Drying Drum
The drum dryer is a cylindrical rotating equipment with spiral blades installed inside, which continuously lifts and sprinkles the aggregate through low-speed rotation to form a uniform material curtain. The high temperature hot air flows from one end of the drum in the counterflow direction, which is in full contact with the material curtain, evaporates the moisture on the surface of the aggregate quickly and heats it up to the working temperature of 160-180°C. This design not only improves the drying efficiency, but also improves the drying efficiency. This counter-current heating design not only improves drying efficiency, but also effectively reduces energy consumption.
Burner
As the core heat source of the drum dryer, the burner usually adopts natural gas, heavy oil or coal dust as fuel. By precisely controlling the mixing ratio of fuel and air, the burner can produce a stable high temperature flame (about 1000℃) to provide continuous heat energy for drying aggregates in the drum. The advanced burner system is also equipped with automatic ignition, flame monitoring and temperature feedback devices to ensure a safe and efficient combustion process.
Dust Collector
The dust collector utilizes pulsed bag filtration technology to capture dust particles generated during the drying and conveying process, effectively reducing the emission concentration to below environmental standards. When the dust-containing airflow enters the box, the fine particles are intercepted by the filter bag, and the regular pulse blowing system cleans up the dust adhering to the surface of the filter bag, and the separated dust can be recycled for filler mixing, realizing the recycling of resources and green production.
Hot Elevator
In intermittent mixing plant, the Hot Elevator is responsible for vertically transporting the drum-dried high-temperature aggregates to the top of the mixing tower. Its closed chain hopper structure prevents heat loss and dust spillage, and is equipped with temperature sensors to monitor the material temperature in real time. The precise dosing function of the hot elevator can synchronize with the mixing rhythm of the mixing tower to ensure the timeliness and accuracy of aggregate supply.
Mixing Tower
The mixing tower is the core production unit of asphalt mixture, which integrates the precise measurement and full mixing function of aggregate, hot asphalt and mineral powder. High-speed rotating mixing blades in a short period of time (about 45-60 seconds) will be preheated aggregate and heated to 150-170 ℃ liquid asphalt, filler uniformly mixed to form a specific viscosity and gradation of hot mix asphalt. The temperature and mixing time in the tower are precisely controlled by a PLC system to ensure consistent quality from batch to batch.
Storage Silos
The storage silos are used for temporary storage of finished hot mix asphalt, and are equipped with insulation and heating devices to maintain the storage temperature of the asphalt mixture at 140-160°C to prevent hardening and caking. The bottom of the silo is equipped with a screw conveyor and discharge valves, through an automated control system to achieve on-demand discharging, to ensure efficient loading of transportation vehicles, and is equipped with a material level monitoring system to provide real-time feedback on the amount of material stored in the silo.
Common Problems and Solutions Affecting Asphalt Mixing Plants Efficiency
The cold feed unit is faulty
Cold feed unit failure is usually triggered by multiple factors. At the raw material supply level, insufficient raw materials in the cold feed bin or blockage of the bin outlet will lead to interruption or uneven feeding, the solution includes real-time monitoring of the material level, timely replenishment of raw materials, and regular cleaning of the bin outlet. Equipment failure, abnormal inverter parameters or hardware damage, will cause belt speed instability, need to calibrate the inverter parameters and replace the damaged parts; belt aging, running or fracture problems can be solved by adjusting the tensioning device, calibrate the position of the pulley and replace the new belt.
Asphalt concrete mixer malfunction
The mixer failure involves a number of key components. Fixed bearings are easy to wear out due to long-term high-load operation and insufficient lubrication, so it is necessary to add high-temperature grease regularly and replace the bearings when they are seriously worn out. Uneven mixing can be caused by worn mixing blades and improperly set mixing times. Replace the blades and optimize the mixing parameters. Temperature sensor failure will lead to temperature control inaccuracy, need to calibrate the sensor regularly, replace it in time when damaged, and combined with infrared thermometer to assist in monitoring.
Problems with the screen
Screen mesh is too large is a common failure, which will lead to inaccurate grading of aggregates, ultimately resulting in an abnormal mixture of oil and stone ratio, affecting the quality of asphalt concrete. Detection, can be compared before and after the screening of aggregate particle size distribution, the use of standard screen mesh sampling inspection. The solution is to replace the screen in line with the specifications, and regularly check the screen wear, establish a screen replacement cycle system, while optimizing the operating parameters of the screening equipment to ensure stable screening results.
Vibrating screen system on mobile asphalt plant
Vibrating screen system failure is mainly reflected in the exciter failure, screen damage and vibration motor abnormalities. Shaker eccentric block wear or loose connection, will weaken the screening effect, need to fasten the connection parts, timely replacement of worn eccentric block; screen broken will lead to aggregate screening does not meet the standard, need to immediately replace the screen, and strengthen the daily inspection. Vibrating motor overheating, phase loss operation, will affect the stability of the equipment, need to check the motor heat dissipation, repair circuit failure, to ensure the normal operation of the vibrating screen system.
Asphalt concrete unloading temperature is uneven
Uneven unloading temperature is easy to produce “paste” and “white material” phenomenon. Too high a temperature causes the asphalt to age and form “paste”, which reduces the strength and durability of the concrete, while too low a temperature results in “white material”, which fails to ensure that the asphalt is adequately coated with the aggregates. The solution is to optimize the heating system, calibrate the temperature control device of the thermal oil furnace to ensure uniform heating; strengthen the temperature monitoring in the mixing process, and reasonably adjust the mixing time and rotational speed; at the same time, check the heat preservation measures at the unloading port to reduce the heat loss in the unloading process.
Conclusion
Through the comprehensive analysis of “How Does an Asphalt Plant Work” in this article, we have gained an in-depth understanding of the types of asphalt mixing plants, their working principles, significant advantages, key components, and common problems and solutions. With the continuous progress of science and technology, asphalt mixing plant technology is also continuing to innovate and develop, the future will be towards a more intelligent, environmentally friendly and efficient direction.
