- Created: Saturday, 01 November 2008 08:00
- Published: Saturday, 01 November 2008 08:00
Today's economy has proven challenging for most operations; an Aggregate Industries operation in Brandywine, Md., tells a similar story. There also is an onslaught of the usual challenges including strict permitting regulations and an abundance of material fines to deal with. But with its experienced crew, leadership and technology, this operation continues to find innovative solutions to these complex problems as market conditions change.
Aggregate Industries is an international construction and building materials company with leading regional market positions in the United Kingdom and United States. It recently was acquired by the Holcim group of companies. The operation in Brandywine, Md., serves the southern side of Washington, D.C.
Like most operations, the company has taken a hit from the housing slump and the rising costs of production. ìThe market right now is tough,î says Plant Manager Robert Wease. ìBut you still do have some construction going on out there.î
Aggregate Industries recently prospered from construction on the Woodrow Wilson Bridge. According to the Virginia Department of Transportation, this four-lane bridge was built in 1961, originally designed to carry 75,000 vehicles over the Potomac River on a daily basis. Decades later, the bridge carries 200,000 vehicles each day. Now six lanes, the bridge remains one of the worst bottlenecks in the country as traffic from the eight-lane Capital Beltway (I-495 /I-95) trickles through.
SHORT ON SAND, BURIED BY GRAVEL
This concrete-intensive project heightened the demand for concrete sand produced at the Brandywine operation, which continues to this day. The demand for gravel, however, is much smaller. So as the Brandywine operation continues fighting to produce enough sand, the gravel continues to pile up. A mountain of one-inch stone sits across a narrow path from another mountain of pea gravel. Wease says each pile threatens the other with contamination, and it's a battle keeping the two separated. This especially is true on the small footprint required by permit regulations.
Wease hopes that the markets will change to relieve some of this strain and buy up some of the gravel surplus. But in the meantime, something had to be done. The company recently installed a Terex Cedarapids MVP 450 cone crusher. It was installed right in line with the rest of the plant and is being fed a one-inch minus stone. This is crushed to a ?-inch minus manufactured sand, which currently has a higher demand than the gravel. Wease says this is a good product for asphalt, but it also can be blended with the concrete sand to help maintain a proper Fineness Modulus.
ìIt has been a pretty good crusher,î Wease says. It serves as a secondary after an HP300 crusher makes the initial hit, making a one-inch product from a four-inch material.
Wease, who has worked in the industry for about 25 years, has experience with nearly every brand and variety of crusher. He says the MVP 450 has significant advantages and cost savings.
ìWe tried them all,î Wease says. But none of them matched the wear life of this cone. The advantage is that there are no high-speed rotary shelves such as those on Vertical Shaft Impactors. Other crushers needed their wear parts replaced in as little as eight hours. ìIt is a very abrasive material.î
ELJAY ROLLERCONE MVP
A cone crusher relies on compression crushing, explains Terex Cedarapids Applications Development Manager John Vendelin. There is a moving mantal inside of a stationary liner. As the mantal moves, it pinches the rock, which breaks along its natural fault lines. This fractionated rock often is elongated or slabby, but the crusher can be set to produce a more cubical product, he says.
Compression crushing also results in longer wear life because much of the rock is crushing against itself rather than crusher parts. The hard manganese liner is the main wear component. A hard granite will wear the liner after 50,000 to 150,000 tons, Vandelin says. Other customers might be changing liners every seven to eight hours. Nonabrasive limestone would take as long as three to four years before fully wearing a liner. Vandelin says an average sand and gravel operation would likely change liners every six months.
Wease says his operation changes them three or four times a year at about $7,000 per liner. When working with VSIs, he changed out wear parts every week for $3,000.
The extended wear life is partly attributed to the hydro-pneumatic tramp-iron-relief system, Wease says. It employs five-gallon accumulators to protect the crusher when encountering an uncrushable object. All accumulators are connected to equally share the stresses generated and provide redundant protection of crusher components.
Most typically, the Eljay Rollercone MVPs are powered by electric motors through a V-belt drive system. But they also can be set up with a power takeoff shaft of a diesel engine. The MVP 450 is rated at 400 hp, so the manufacturer recommends using a 400-hp motor or two 200-hp motors. Other sizes also are available including an MVP 550 that would require 500 hp.
The model numbers are derived from their estimated throughputs. The MVP 450 can produce 450 tons per hour at a 1º-inch setting, Vandelin says. The MVP 550 produces 550 tons per hour at a 1º-inch setting. Of course, settings easily can be changed to produce a variety of materials.
ìThere are basically three different parameters that they can adjust in the field to change how the machine performs,î Vandelin says. The first is the closed sight settings, which determine the topsize of the output gradation. The second is the speed of the machine, which controls how much rock will fall through the chambers. Also, operators can choose from 10 crushing chamber configurations to compensate for the topsize. What separates the MVP from its predecessors is a thread adjustment that allows the machine to have its closed sight setting adjusted while it is crushing rock.
ìIn the original Eljay cone, it was a shim-adjust machine, and you had to shut down the feed to make a setting change,î Vendelin says. ìThese newer machines have the thread adjustment on them, so that they can be adjusted while it is crushing rock to compensate for manganese wear and that type of thing.î
The final feature that Terex boasts is the frictionless roller bearing drive train; it allows more of the applied power to go into crushing rock while less power is applied to the internal frictions. A large oil cooler minimizes the amount of heat generated that could result in downtime.
While the Brandywine operation continues looking for creative ways to decrease its surplus of gravel, there is another material of abundance that must be dealt with.
ìThe biggest problem with sand and gravel operations is fines and how to handle them and where to put them all,î Wease says. The company currently is experimenting with Geotubes, a dewatering solution. These industrial-fabric bags are filled with slurries from a 75-foot-wide and 20-foot-deep fines-recovery tank; the water runs out while the fines remain inside the bags. Wease says that the bags are stackable, but can only be used once. As the bags fill, the fines are removed and hauled away. For the Brandywine operation, the Geotube bags may prove cost prohibitive because the material has to be moved again after it dries. But they do work, he says.
ìThese would be real good in a situation where you were working around housing,î Wease adds, ìwhere they are making a pond, and you couldn't have any discharge at all.î
A LOT OF STRIPPING, A DIRTY RESERVE
The Brandywine strip mine spans nearly 1,800 acres and should yield material for up to 20 years, Wease estimates. But the operation is constantly on the move, clearing the overburden and digging material out of the ground. There is an average of eight feet of overburden, which consists of some thick timber, topsoil and clays. The useable material underneath runs anywhere from six to 20 feet. As the operation follows the seams of material, it is transported to the plant with diesel-thirsty haul trucks. Conveyors were ruled out as an option. The topsoil and clay are used for reclamation purposes.
A 30-ton haul truck can make four or five trips in one hour from the farthest point. It's a long haul, but it is considered a privilege. Aggregate Industries maintains two other operations in the area that must rely on material trucked in from off-site. These plants started in the 1930s but all of the material has been mined out. The costs are tremendous, for both the company and the consumer.
The feed pile for the plant has three different feed points above one tunnel that leads to a McLanahan plant, Wease says. Because of the clays in the material, it has to be helped along with an excavator. The three openings allow a constant flow of materials (about 650 tph) if one is breached.
From the feed pile, material is conveyed to a 8- ◊ 20-foot triple-deck shaker, where spray bars begin the washing process. The waters and fines fall straight through and are pumped to the fines-recovery tank. The larger material is taken by two log washers and is dispersed into various locations; the sands, starting at ? minus, are processed by twin dewatering screws.
Everything larger than one inch is fed into an HP300 crusher, which crushes material to one-inch minus. It is sized by a second shaker screen into æ-inch gravel, pea gravel and a little extra concrete sand from the first shaker screen. The æ-inch gravel can be stockpiled or diverted to the 450 MVP cone crusher for manufactured sand.
WEIGHING IN ON EFFICIENCY
There are two 980 loaders working the stockpiles. Both are equipped with Loadrite bucket scales. Wease says this prevents over- and under-loading, which saves fuel because no one is returning to the pile to top off or take off. In addition, the technology also has improved significantly and loader operators are more willing to try it out.
ìThe equipment today, it's like sitting in a lounge chair in your living room,î Wease says. ìMost of our equipment has air conditioning, Ö and radio.î
As trucks are loaded, they weigh out on a Mettler Toledo scale. Drivers communicate with the scale house through a speaker. The loader operators also are in constant communication with the scale house via CB radio.
BOUND BY RED TAPE
One of the largest challenges to any operation is conforming to the sanctions established by the county. The Brandywine operation is limited to 200 loads per day, which is about 4,000 tons per day, yet the plant is capable of producing 800 tons per hour. It is restricted to 16 hours per day ó 6 a.m. to 10 p.m. The operation is not permitted to operate on Sundays or holidays, and is restricted to 26 Saturdays a year. There also is a specific footprint that the plant is confined to as well as a height restriction. As a result, managing stockpiles is much more difficult as gravel piles up.
ìThis will probably be the last sand and gravel operation in Prince George's County,î Wease explains. ìIt was a war getting the permits. The neighbors don't want you in here.î
This anti-mining trend continues to move across the country, despite efforts by the industry as a whole. The Brandywine operation is about a half mile off the road and is surrounded by thick timber; it is only marked by a small sign, ìAggregate Industriesî with the address. The operation is nearly silent and conforms to strict environmental guidelines set forth by the parent company Holcim. Even a simple oil leak must be reported, contained and dealt with appropriately, Wease explains.
Aggregate Industries often extends itself to the community. Wease says that an annual ìconstruction dayî is growing in popularity, and it also hosts Earth Day events. ìWe try to let (the community) know what we are doing.î The company tries to explain the importance of its product, as well as the costs associated with covering up resources or banning the practice of mining. Trucking product in from beyond county lines will only increase the costs. The company also emphasizes that it is not harming the environment. And when the resource is fully mined out, the land will be restored to a pristine condition through continued mitigation efforts.