Stationary and portable asphalt plants manufacture the viscous material from which roads, runways, and parking lots are made. They mix heated liquid asphalt cement (AC) with aggregate, filler, and additives to be laid down as blacktop pavement.
Read More (About Asphalt Plants)Asphalt plants manufacture the material from which roads, runways, and parking lots are made. They mix asphalt—a thick, sticky form of petroleum sometimes called bitumen—with aggregate, filler (binding material), additives, and other materials to make a heated liquid to be laid down as blacktop pavement.
In a nutshell, the aggregate is screened for size, dried, and mixed with the additives and heated liquid asphalt cement (liquid AC, or binder) in a drum or mixing tower. The finished mix is then dropped into haul trucks waiting below or elevated into storage tanks, or silos, for temporary storage.
The more detailed version is that the aggregate starts out in cold feed bins according to its coarseness or fineness. A carefully chosen mixture of various sizes of aggregate is then conveyed across a screen to sift out oversized pieces. Next, a belt conveyor transports the cold aggregate into a rotating, heated drum to dry.
In a continuous drum mix plant—designed for ongoing operation—liquid AC from heated storage tanks mixes with the dried aggregate in the drum. The finished product is then elevated into truck beds or silos. A batch mix plant differs not only in the finite amounts of product it’s designed to produce, but also in the fact that it elevates the dried aggregate into a tower to mix it with the liquid AC. Trucks drive under the tower to receive their loads of molten asphalt mix.
In both types of plants, the temperature of the asphalt is tightly controlled to match the application, such as hot mix asphalt (HMA) or the energy-saving warm mix asphalt (WMA). In most cases, a proportion of recycled asphalt pavement reclaimed from torn-up roadways (RAP) is added to the mix to lower the cost and environmental impact of the asphalt production. Some plants add recycled asphalt shingles (RAS), while some use special mixing units and reaction tanks to infuse the liquid AC with recycled, ground tire rubber (GTR).
During the production process, loose dust and fines (small particles) are collected in the baghouse by filters and a powerful fan, and are then introduced into the mix. Additives such as limestone or water may be added next, depending upon the needs of the contractor for the job at hand. For example, the project may specify a traditional Superpave (Superior Performing Asphalt Pavements) mix tailored to the climate of the area under construction, or a so-called regression mix with a higher proportion of liquid AC for potentially additional resilience over time. Plants with storage silos may keep them heated so they can store the fluid asphalt mix for several days before use.
A stationary asphalt plant is a relatively permanent facility set up for long-term production. It’s the most expensive type of plant, but also the most productive. A stationary plant can benefit from structures such as paved and possibly covered aggregate storage areas, which can give the aggregate some protection from moisture and reduce drying time.
A portable asphalt plant operates under the same general principles, but comes on a number of trailers to be used on-site. The components are mounted directly on the trailers, which deploy stabilizing feet to level their loads. Many portable plants are batch plants designed to allow haul trucks to drive under their mixing tower via ramps of packed earth or rock on either side of its trailer.
A relocatable plant is similar to a portable one, but its modular components are mounted on skids or built-in foundations. In other words, they’re transportable by trailer rather than trailer-mounted pieces. Lowboy trailers are often used to relocate components when conditions warrant.
Component manufacturers strive to improve asphalt plants to reduce their cost per ton and increase the precision of each mix. Dryer/mixer drums and baghouse components in particular have received upgrades to reduce their energy costs, inefficiencies, emissions, fumes, and smoke. Self-cleaning, adjustable drying flights in the drum have reduced maintenance requirements and lowered fuel consumption. Other drum improvements have allowed producers to increase the proportion of cheaper, recycled RAP in the mix while maintaining the quality of the product. The industry’s embrace of low-viscosity warm mix asphalt has likewise lessened its environmental impact by allowing more RAP content and reducing fuel usage, and hence emissions and costs.
Variable frequency drive (VFD) motors in conveyors, baghouse fans, and other components have reduced energy usage and improved efficiency when the plant is running at partial capacity. Advancements in weighing and metering have improved the accuracy of mix proportions, and other upgrades have allowed producers to support more types of mix and quickly switch from one to another.
Moreover, wireless monitoring over mobile devices lets managers check on product levels at a glance. It can also alert them to a drop in hot oil heater temperature, such as when a nighttime storm knocks out power to the site.
ADM, Aesco Madsen, Ammann, ASTEC, CEI, CMI, MAXAM, Reliable, Stansteel, and Systems Equipment all manufacture asphalt plant components and/or entire systems. You’ll find plenty of new and used plants and components for sale at PavingEquipment.com, including dryers, drum mixers, burners, cold feed bins, conveyors, elevators, baghouses, shears, scalping screens, meters, hot oil heaters, silos, tanks, feeders, control systems, and more.
If you are looking for specific types of asphalt plants, check out the portable asphalt plants and stationary asphalt plants categories.
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