Asphalt Production - the secret is in the mixing
A thermal mixing process is required to produce asphalt from aggregate and bitumen for road construction and an asphalt mixing plant is essential here. Whether road-mobile, transportable or stationary – every mixture is a winner with tailor-made solutions by BENNINGHOVEN, the specialist for asphalt mixing plants.
In batch production, the components of the asphalt mixture are first weighed before being mixed in batches in an asphalt mixer according to a specific recipe. This process is more flexible as the recipe for the mixture can be changed for each batch. What is more, a greater mixing quality can be achieved due to the more precise quantities added and adapted mixing cycles.
(Please click on the blue numbers to get more information about the individual components of the basic functions of an asphalt mixing plant.)
Cold feed system
At an asphalt mixing plant, the aggregate material is stockpiled according to grain size and type. This material is usually loaded into the individual hoppers of the cold feed system by a wheel loader. The minerals are dispensed via dosing belts (normally frequency-controlled) onto the collecting belt below, specific to the recipe. In this way, a corresponding cold feed recipe is generated on the collecting belt. The material is then transported to the dryer drum via this collecting belt.
The dryer drum is known as a rotary dryer. Here, the material is dried and heated for further processing. For the manufacture of asphalt, it is essential to remove the moisture from the base material to ensure bonding with the bitumen. Designed at an appropriate inclination, it transports the material through the entire drum via rotary movements. Lifter plates in the drum provide a thick material curtain in the drum, enabling efficient heat exchange. In addition to the dusts produced, water vapour is also generated during this drying or heating of the aggregate. In order to keep heat losses to a minimum during the drying process, the drum tube is insulated against heat radiation.
The burner used for drying and heating can be operated with fuel oil, fat oil, natural gas, fluid gas, carbon dust or a combination of these combustibles. During operation, the operator can change the combustible with the help of his controls. Benninghoven is the only manufacturer of asphalt mixing machines worldwide developing and manufacturing its own burners. Here you can learn more about our combustion technology.
Dust collection system
The flue gas produced in the dryer drum and other exhaust air from the production process are extracted under vacuum and cleaned in the fabric filter. The flue gas is fed to the pre-separator of the filter together with the water vapour. The coarse particles of exhaust gas are separated in this pre-separator via deflection and by slowing down the flow speed. This coarse filler is fed directly back into the mixing process via screw conveyor. The fine fraction of exhaust gas, known as fine filler, is cleaned in the actual filter housing. The filter cloth gradually becomes clogged with dust particles, so it is periodically dedusted by a scavenging air fan. The fine filler generated is collected In the dust collecting trough beneath the filter housing and forwarded via a screw conveyor installed in the trough and subsequently deposited in the reclaimed filler silo.
Screening and mixing tower
The actual mixing process takes place in the screening and mixing tower. The materials described in the upstream process arrive at the mixing tower screen via a hot stone elevator. The hot aggregate material is uniformly supplied to the screen's screening decks. The screen is activated via two unbalance motors, stimulating the screen unit, and thus the screens, to vibrate vertically. The input material is thus divided into the individual fractions and separated according to grain size, stocked in the respective compartments of the hot bin section below.
Mixing and weighing section
The mixing and weighing section is one of the core elements of an asphalt mixing plant. This is where the actual mixing process takes place. From the hot bin section the hot material, stocked according to grain size, is dispensed into the aggregate weigh hopper below corresponding to the mixing recipe. The aggregate weigh hopper is mounted on loadcells to facilitate the weighing. The individual fractions are weighed sequentially, i.e. the discharge doors open consecutively and dispense the preset quantity of the respective grain into the aggregate weigh hopper. The weighing of the bitumen and the filler is carried out in parallel to the weighing of the aggregate material in the aggregate weigh hopper. The bitumen is weighed in the bitumen weigh hopper and dispensed into the mixer. The required filler is weighed in the filler weigh hopper, which is mounted on loadcells, and delivered into the mixer via an inlet screw conveyor.
The actual mixing process takes place in the twin-shaft pugmill. Once the weighing processes have been completed, the discharge doors of the aggregate weigh hopper open initially and the mineral falls into the mixer below. The mixer consists of two counterrotating shafts with mixing arms and mixing blades. In the mixer, the mineral is mixed intensively with the binder and the filler, and is worked into a homogeneous mass. After a mixing time of 40 seconds, the mixer is emptied by opening the mixer discharge door. The entire mixing cycle takes 45 seconds.
Mixed material storage silo
The material is either discharged directly onto a lorry or into a mixed material storage silo via a skip. The skip is installed beneath the mixer and travels, by means of an electric drive, to the corresponding position of the required silo chamber where it discharges the asphalt mixture. For a certain duration, production quantities can be stored in the mixed material storage silo before being loaded. Each silo chamber features an electrically heated, pneumatically-operated discharge door, which discharges the asphalt mixture onto the lorry below. The silos are insulated to prevent loss of temperature in the asphalt mixture.
Both the filler reclaimed from the plant and the imported filler are stored in a filler silo. Only the reclaimed filler is necessary for the basic function of an asphalt mixing plant. There is also the option of storing and using imported filler (foreign filler). The filler then arrives at the mixing tower for further processing via a filler elevator and a screw conveyor.
The bitumen binder is a core component of the asphalt. The different types of bitumen, tailored to its relevant intended use, are stored separately in appropriate thermally-insulated bitumen tanks. These are heated in order to maintain the required temperature of the bitumen, approx. 160 - 180 °C. These days bitumen tanks are heated almost exclusively via electricity, with a feasible alternative being tanks also heated by thermal oil. This means that the energy from the hot thermal oil is transferred to the bitumen via heat exchangers. The hot bitumen is removed from the tanks via a pump and conducted to the bitumen weigh hopper via heated and insulated pipelines. The tanks are filled by tanker lorry via a fill pump, through pipelines into the respective tank.
A command cabin for operating personnel is available to control the processes of an asphalt mixing plant. The operating menu for the entire mixing plant is installed in this cabin. From here, all drives can be started in manual mode. Here, all data required for the mixing process is collated and displayed. As well as managing the required recipes and monitoring or implementing the mixing process, operating personnel can also control the entire plant process.
A quick change of recipe - necessary to react to changing demands - is only possible in discontinuous or batch production. This makes asphalt mixing plants flexible for any purpose.