Operations Focus: Hard Rock to RAP

For decades asphalt has been one of the most commonly recycled materials, with reclamation rates as high as 95 percent. The Hoover Inc. paving division of LaVergne, Tenn., likes to keep plenty of reclaimed asphalt pavement (RAP) on hand and will stockpile several years’ worth in advance to keep it readily available.

A paving company’s investment cost in processing RAP from waste to a salable commodity assures regulators that it is destined for reuse. Therefore, Tennessee regulations will allow an asphalt company to stockpile 10 to 20 percent more RAP material, depending on the plant’s mix ratio, when the asphalt is first consistently crushed and screened to a maximum acceptable reduction, which in this case was three-eighths-inch minus.

Ephraim Hoover Jr., a freight transportation provider, established Hoover Inc. in 1955, when President Dwight D. Eisenhower’s vision of an interstate highway system was just getting started. The company had all the orders it could fill. In the beginning, these were crushed stone orders for concrete, asphalt paving and grading. The crushed stone operations continue today but are joined by a paving division. The paving division and its 300 employees provide municipal, state and federal construction services, such as grading, drainage, asphalt paving, base and surface construction, pavement marking and striping, from five locations in three states. Hoover also serves its commercial and industrial customer needs with building, mechanical, electrical, demolition, sewer and water line installation services.

Three of Hoover’s hot-mix asphalt plants are located in Tennessee and have their own quarries. Two more plants are located in Mississippi and Alabama.

Hoover has its own crushers at its Lebanon, Tenn., plant and quarry, but it wanted to expand its capabilities with a portable crusher to help tackle a scheduled 14-month surge in RAP production for its stockpiles. Salesman Stacy Lynn of the Atlas Copco Nashville store hooked Hoover up with a portable Atlas Copco Powercrusher PC 3 impact crusher and Atlas Copco HCS 5515 screen in late 2012. In the first four months, the setup had already contributed 120,000 tons to the company’s RAP reserves without a hitch, according to Hoover.

A Big Appetite
Hoover Maintenance Technician Jessie Carney was impressed that the crusher could handle so much material. “I’m surprised it could go through all that we’ve fed it. That’s a pretty good machine,” he says.

Eric Amberson, Atlas Copco product line manager for Powercrushers in the U.S., says the reason the PC 3 does so well with such an abrasive material as RAP is that it was designed as a primary crusher.

However, its high throughput rate begins before the box, at the vibratory hopper and conveyor feed. Fines are sifted down to the bottom and through its grizzly onto the main conveyor, significantly decreasing the volume of abrasive material entering the chamber, which bypasses the crusher box altogether, according to Amberson.

The geometry of the box itself, Amberson explains, is that of a primary crusher. The PC 3 crushing chamber’s capacity is created by its swing beams’ relationship to its rotor, which allows for larger feed sizes, he adds. The feed inlet is 4 feet by 2 feet, 4 inches.

These factors combine to make the PC 3 an effective crusher of highly abrasive RAP, says Amberson. “It has a 250 [metric ton] per hour throughput rating, even at the reduction ratio an asphalt plant needs, for instance, three-eighth-inch minus.” The rating is based on actual 100-percent-reduction throughput, not just an input rate or free-flow output rate, according to Atlas Copco.

A Varied Input
At the Lebanon site, Hoover is keeping RAP to less than three-fourths of an inch by running the PC 3 with a five-eighths-inch top screen and nine-sixteenths-inch bottom screen. Thomas Hoover Jr., grandson of the Ephraim Hoover Jr. and paving division operations manager, says the company was seeing a production average of 200 tons an hour, though he adds, “At times we’ve seen 350 tons an hour. That’s the difference between wet and dry. Moisture slows it down.”

Asphalt also has an inherent tendency to cake or blind on the screens, which worsens with heat and moisture. The HCS 5515 screen is adjustable to accommodate these material characteristics, however, and it was during its dialing-in period that Thomas Lovvorn, Hoover’s paving superintendent, noted a secondary benefit to renting the PC 3.

He had, at times, run a batch of rock from the quarry through it. “[The crusher] cleans it right up,” he says. He spread his hands about 2 feet apart to indicate the size of stone that was dropped in the hopper. “It will take big rocks. Whatever you put in it is going to get ground,” he says, which incidentally gave Hoover its desired versatility to produce aggregate as well as RAP.

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The PC 3 is powered by a 385-horsepower engine. Though diesel fuel had been in the $4 range at the time the project began, Hoover and Lovvorn say they believe the cost of operating the crusher is reasonable. The PC 3 consumed about 12 gallons per hour, totaling 75 gallons per 10-hour shift. The screener ran on 20 to 30 gallons of fuel per shift.

Hoover plans to have the Powercrusher PC 3 for about 14 months total, during which time Atlas Copco will provide preventive maintenance and be on call if a problem should arise. Hoover says the stockpile at that point is expected to last the company up to five years, depending on future project demands.

This article was submitted on behalf of Atlas Copco, based in Commerce City, Colo. More information is available at www.atlascopco.us.