Best Practices for Crushing, Screening RAP for HMA Production
BY Jay Giltz
Asphalt producers know that many factors go into making the highest quality hot-mix asphalt (HMA) end product from reclaimed asphalt pavement (RAP). Important considerations include the need for correct size and shape of material, structural soundness, and a minimum of moisture, fines and white rock going into the mix.
Additionally, production capacity is a key factor in assuring asphalt drum mixers can operate at the sometimes 24/7 demand that is generated in peak asphalt season. A slip in production capacity at the front end leads to disruption in the whole asphalt system. This article will discuss what producers can do to assure crushing and screening of RAP meets best practices for achieving highest product quality and production capacity goals.
There are many choices in crushing and screening equipment to get the job done. When it comes to crushing, jaw crushers, cone crushers and impact crushers all have specific applications. Jaw and cone crushers are popularly used equipment types, but there are key reasons why an impact crusher may be the best choice for achieving the end quality product producers desire, along with providing the necessary production capacity to keep high-quality hot mix flowing.
Crushing and screening RAP is a recycling application, which requires very different end product needs than virgin aggregate and even concrete crushing applications. Jaw and cone crushers are highly effective in crushing hard rock. RAP, however, is not hard rock, and the unique characteristics of RAP crushing can actually cause cone crushers to negatively impact product output.
As cone crushers crush and re-crush the RAP to break it apart, the asphalt can be literally stripped from the RAP, creating white rock, which now requires the extra step and cost of recoating.
A jaw crusher in a RAP circuit is used for primary reduction, prepping it for the cone and requiring at least two crushers, and in some instances, a vertical shaft impactor (VSI) to make a spec product. An impactor in most RAP applications typically requires only one crusher. Two may be necessary where very high capacity or special products are required.
Cone crushers can work fairly well until heat is added into the equation, but as heat and pressure build, the RAP can become a hardened material, so compressed that it can become uncrushable, resulting in what is called “Bonne Float,” almost metal-to-metal compaction. The forces that can be created are directed down through the bearings and can destroy them, requiring extensive and expensive down time.
Impact crushers, on the other hand, use a simpler design comprised of a rotor and two bearings that spin to launch the RAP against a curtain. In my 45 years in the crushing and screening business, 26 of those selling the UltraMax® Impactor for Eagle Crusher Company, I have sold jaws, cones, impactors, hammermills, VSIs, roll crushers, and even Wood Hogs. They all have a specific application in which each one excels. The simplicity of the Eagle Crusher impactor’s three-bar, solid-steel rotor works like this for RAP:
The action of exploding the material with the force of the blow bars and launching it into the primary curtain at the proper angle, then whacking it again with the secondary curtain liners to prepare it for its final reduction, creates a very consistent mix to be sent on to the drum. The RAP impacting into the curtain breaks the interstitial lattice of the RAP’s binding matrix and creates a beneficiation of the original virgin aggregate—simply put, knocking off the arrowheads and cubing up the product. The result is a minimum of white rock and fines produced by the impactor, and a more structurally sound cubical product. Cone crushers tend to “pancake” the RAP and do little to reduce the elongated aggregate, rather than produce the cubical spec product required for highest quality recycling of RAP.
As the impactor separates the binding matrix, the material is aired out and moisture is released, making for a drier material going into the hot mix. Drier material saves energy as it eliminates the need for more gas in the burner.
Why, then, are jaw and cone crushers so popular for RAP? Many producers have grown up in quarries where they have seen, firsthand, the ability of jaw and cone crushers to effectively crush hard rock with wear parts that withstand the crushing process with minimal replacement. This effectiveness and wear-part longevity mindset carries over into the purchase of equipment for recycling of RAP, when recycling RAP is actually a very different application.
In my experience, an impactor works best in RAP with an open setting rather than a closed one. The open setting controls the size and production with speed. With that in mind, an impactor can retain its product gradation as well as capacity throughout its wear life.
Too many times, only the secondary curtain setting is adjusted because it is easier to do than adjusting the primary curtain. However, for best practice, both should be adjusted proportionately to maintain a correct reduction ratio and highest product quality.
When it comes to production capacities of impact crushers compared to jaw and cone crushers for the crushing and screening of RAP, Ryan Freeman, the general manager at Rason Materials, shared his experience.
“Rason operates five asphalt plants and four crushing operations, all on Long Island, New York,” Freeman said. “Working with our Rason company management team, I was able to make the switch from an original configuration of three crushers—a jaw, impactor, and cone with two screen decks, the impactor was not designed for the unique properties of asphalt crushing—to a new configuration of two Eagle Crusher impactors with two screen decks swapped out. We were able to double production capacity based on tons per hour with one less crusher and half the labor.”
Freeman continued, “There’s a lot less skilled labor out there for cone crushers, so finding good cone crusher help is more difficult. Plus, cone crushers are harder to maintain for the products Rason produces.”
One other and often overlooked part of any spec RAP system is the screen. A properly sized and type of screen is critical to any RAP operation. Producers are typically making finer sizes than other types of aggregate and recycling production. For RAP, the screen is the determining factor for net production.
Producers can have all the crushers in the world but if RAP can’t be screened, it probably can’t be used.