The ASTEC Double Barrel® drum mixer is designed to move aggregate through the mix process with maximum efficiency. Drying of the virgin aggregate is the first step in the process and takes place in the inner drum. The drying process begins when the virgin aggregate enters the inner drum through a sealed gravity chute. The flop gate restricts air entry into the drum while allowing the aggregate to pass through. The angle of the ceramic-lined inlet chute keeps material flowing freely into the drum. State-of-the-art flights move the material through the drying zones. Once dried and heated, the aggregate leaves the drum through exit openings and enters the outer mixing chamber.
Adding materials in the right order and at the right time is the key to making good mix on a consistent basis. That’s where the ASTEC Double Barrel® dryer/drum mixer’s exclusive sequential mixing process excels. Ingredients are added to the hot mixture in an order that allows better temperature equalization and even distribution of all particles throughout the mix.
First in the mixing sequence, recycle enters into the mixing chamber through the recycle inlet. Recycle, which may be RAP, roofing shingles, crumb rubber, or a mixture thereof, is heated by contact with the hot virgin aggregate.
At the point where virgin and recycled materials are at the proper temperature and thoroughly mixed, liquid asphalt cement (AC) is injected into the mixing chamber. As the mix moves through the mixing chamber, it is continually stirred by mixing paddles.
Finally, baghouse fines and other additives enter the mixing chamber and become embedded in the thick layer of asphalt coating the rock. ASTEC’s sequential mixing keeps fines from soaking up more than their share of AC because the AC has been well distributed before fines are added.
Hydrocarbons and Steam
There is a continuous release of steam as the RAP is introduced and heated. This is one of the keys to the success of the Double Barrel as a processor of RAP – steam blankets the superheated virgin aggregate and RAP displacing the oxygen. As a result there is less oxidation of the mix. As the RAP dries, blue smoke emissions can also be produced along with steam.
Steam and blue smoke are pulled into the burner flame by the baghouse fan. The hydrocarbons in the blue smoke are incinerated while the steam simply passes to the baghouse and out of the exhaust stack.