But as more waste and garbage was produced and land-fills reached their limits, solutions were needed and recycling presented one answer. Now the recycling bin on the front lawn is a familiar sight across America. But newspaper, aluminum and plastic are not the only items that can be recycled.
Motivated by crude oil shortages and the subsequent high prices of the mid 1970s, methods were devised to recycle asphalt pavements for reuse in new road construction.
At that time highway departments and contractors were doubtful of the quality of reclaimed asphalt pavement (RAP). The concept was brand new and its long-term performance was unknown, but recycled hot-mix asphalt, used properly, proved to have the same performance characteristics as hot mix made with all virgin materials.
Most importantly, RAP offered substantial savings over virgin mixes especially in areas where high-quality aggregates are scarce. Attracted to the savings and quality of RAP, more agencies began specifying its use in highway construction projects and today there are some transportation departments that allow up to 50% of RAP in highway pavements.
The increasing use of RAP posed some new problems in the processing and production of hot-mix asphalt. Some of the processes and methods that needed to be considered included reclaiming through milling and full-depth removal; processing through milling, grinding and crushing; storage; mix designs; and processing in a hot-mix facility. As solutions to these problems were found, and perfected, new technologies have come into existence. This article will cover some of the developments that took place concerning how asphalt is mixed.
Processing RAP
Since the early use of RAP in the '70s, a number of methods have been developed for mixing it in a hot-mix facility. Each method involves heating the virgin aggregate to extreme degrees. The transfer of heat through either conductive or convective methods also is a factor in the mixing process. Some involve mixing it with the virgin aggregate in a batching tower while others mix it in a drum mixer.
The batching tower methods use conductive heat transfer to heat the RAP. The virgin aggregate is superheated then added to the weigh bucket along with the RAP. Conductive heat transfer takes place in the weigh bucket and the pugmill throughout the dry-mix cycle. As the water evaporates from the wet RAP a steam explosion results causing a potential emission problem.
The more moisture in the RAP the more steam that will be produced and the higher the emissions. By enclosing the pugmill and weighbox areas, the steam can be vented to an air pollution control system. Known as the weigh bucket recycling technique, this method also is referred to as the Minnesota Method because the first project to use it took place in Maplewood, Minn., for the Minnesota DOT.
One variation on this method uses a separate weigh hopper for the RAP. The advantage of this is realized during long production runs of RAP because the batch cycle is shortened resulting in an increase in the production rate per hour.
Another batching tower method, known as the bucket elevator recycling technique, prevents the steam explosion. In this method, the cold, wet RAP is mixed with the superheated virgin aggregate as the aggregate exits the dryer and enters the bucket elevator. But there is a drawback to this method. RAP must be dry before it passes over the screens or is stored in the bin, and because the trip up the elevator is relatively short, large percentages of RAP can rarely be used because it will not be completely dried by the time it reaches the screens. This method generally runs with less than 20% RAP. There is, however, a way to avoid this problem. The addition of a heat transfer chamber on the aggregate dryer allows RAP transfer to begin in the dryer shell, and higher percentages of RAP can be used.
Drum mixers
Drum mixers have proven to be very adaptable for use with RAP. All that was needed was an opening in the drum where the RAP could be added into a cooler zone, downstream from the hot gases of the burner. Because of this adaptability, and the rising use of RAP, drum mixers have become a popular and effective way of making hot-mix on a continuous basis.
In a traditional parallel-flow drum mixer the aggregate is fed into the drum at the end where the burner is located. The aggregate is then dried and heated convectively, traveling through the dryer in the same direction as the exhaust gases. The liquid asphalt is added at the discharge end of the dryer, where the air is cooler. The RAP is added into the mid-section of the drum. This is done to keep the high temperatures in the combustion and drying end from damaging the hydrocarbons in the RAP.
Used effectively in the '70s and the '80s, the use of parallel-flow drum mixers with a central inlet for RAP, declined as emissions standards tightened. The high level of visible emissions was due to the steam distilling light oil from the virgin liquid asphalt cement and the RAP. One way to reduce the emissions was to reduce the percentage of RAP used. However, variations were made on the design of the drum, which reduced the emissions and eventually allowed for an increase in the amount of RAP.
Much of the hydrocarbon content in the emissions is caused when the virgin liquid asphalt cement comes into contact with the hot gas stream in the drum. To avoid this and cut back on the emissions, a continuous mixer can be attached to the parallel-flow dryer and used to add asphalt cement into the drum. This isolates the virgin liquid asphalt from the gas stream and lowers the hydrocarbons in the emissions.
Emissions can be reduced further by altering the design of the drier to a counter-flow design in which the virgin aggregates travel against the gas flow. This lowers emissions because the gas temperatures are reduced by cooler, moisture-laden aggregate as the gases evacuate the dryer.
An early design for a counter-flow consisted of a single drum. This design allowed a plant to meet the emission standards but it had the disadvantages of a short mixing time, and the drum itself became very hot when high percentages of RAP were used. The short melting and mixing area in the drum did not always allow sufficient melting of the RAP when high percentages were added.
Some alterations to the single-drum counter-flow design help eliminate these drawbacks. These changes resulted in joining a counter-flow dryer with a continuous mixer to create a unitized dryer and mixing device. Examples that incorporate this design include, Astec's Double Barrel mixer and CMI's Triple-Drum mixer. A unitized dryer and mixer has a distinctive profile--it looks like a traditional counter-flow drier with a sleeve slipped over one end.
The Double Drum mixer features a drum that rotates inside a stationary outer shell. The interior of the drum serves as a counter-flow dryer. The outside surface of the drum has mixing paddles and functions as the rotating shaft of a large pugmill. The bottom half of the stationary outer shell functions as the pugmill housing.
Having a large mixer allows for a longer mixing time, thus allowing the RAP to completely melt after it is mixed with the superheated virgin material. There is time to achieve a homogeneous mixture before new liquid is injected and there is time for the combined materials to cool down to normal mix temperature after RAP is added.
The Triple-Drum mixer also provides for a longer blending and mixing time, which is important when using large percentages of RAP as well as RAP with a high level of moisture. This can be accomplished because of an inner drum that rotates with the outer drum around the burner. The RAP is added into the drum sooner, providing a longer mixing time. Mixing paddles, or lifting flights are affixed to the inner surface of the outer drum so the material can be mixed and rotated along the outer surface of the inner drum. Heat is transferred through this surface from a burner within the inner drum. This design allows the burner to run at full combustion.
Both designs allow the user to run higher percentages of RAP while reducing emissions.
