The decision between stationary and mobile primary crushing is typically one based on the individual circumstances of the mine site. Type of material, haul distance, mine plan, number of benches and existing equipment investments will weigh heavy in determining which primary to select.

In September 1999, Rock Products reported that Camas Minnesota's Larson Quarry, part of Aggregate Industries, commissioned the first track-mounted primary and movable conveyors in the United States. And Graniterock has been crushing at the face for 20 years.

But this method has been slow to win the heart of the industry. The decision of where to crush is very situational, says Johnny Arellano, Rinker Materials' operations manager-southern region. “The decision has to be made on a situation-by-situation basis,” he says. “You have to construct a model and look at all the numbers on paper.”

And Arellano knows something about constructing such a model. In 1999, the distance from the face to the crusher at Rinker's FEC Quarry near Miami had outgrown the quarry's nine 85-ton haul trucks' capabilities. The bottom line for FEC at that time: $13 million to move the stationary primary crusher, $11 million to add new haul trucks, or $6 million to buy used haul trucks.

“Then it made sense to go with low-hour good, used trucks,” Arellano says. But adding haul trucks is never cheap. The number of employees, the operating costs and maintenance costs factor into the decision, he adds.

And that decision is one FEC is now revisiting. In about six years, the face again will be too far from the primary to maintain production levels. This time, Arellano is looking to move the primary closer to the face (two to three miles) and conveying to the rest of the plant.

For South Florida, it is difficult to make crushing at the face work. “We have to blend material off many faces, so it makes sense to haul,” he says.

However, some operations in that area are using a portable unit to crush and stockpile DOT base (3½-in. minus) at the face and sell material from that pile, Arellano says. Rock for other products are trucked to a stationary primary.

In spite of its higher cost, hauling is the unanimous choice at Vulcan Materials' operations. “We don't have a single (quarry) operation where we crush at the face,” says Ronnie Walker, Vulcan's vice president of operations for the construction materials group. However, Vulcan did seriously consider it in North Carolina and Virginia. But in the end, hauling won out.

The reason, Walker says, is that Vulcan often shoots multiple benches, blends from those benches and does not have the space for conveyor systems. “It all comes down to your mine plan,” he says. If a mine has a long, deep single face and the area for a conveyor system, crushing at the face makes sense, Walker says.

Like Arellano, Walker says Vulcan will move the primary closer to the face when it is no longer feasible to increase the haul truck fleet. Vulcan will then use conveyors to make up the difference between the primary and the rest of the processing plant.

Walker says there is no arguing the cost savings conveying offers compared with using haul trucks. However, the lack of production capabilities and the amount of moving can detract from portable crushers. Moving a portable unit more than once every 10 days would be a detriment to production, he says.

An application where crushing at the face does make sense is when a company has several smaller operations. The unit could be moved from site to site, using in-place conveyors for hauling to the plant, Walker says. Such an application would mean an operation could crush four or five months worth of material at each site without having a stationary primary sitting idle, he adds.

In 1999, Camas' Larson quarry was faced with replacing all four of its 50-ton haul trucks. Instead, the company bought a Nordberg Lokotrack system. The tracked-system averages 700 tph and peaks at 850 tph. The unit can be moved several hundred feet from the face during blasts and back again without disconnecting the conveyors. The entire move takes about one hour.

Key to Camas' crushing choice for Larson was that the tracked system was consistent with the existing mine plan. Unlike Vulcan and Rinker, the Larson Quarry mines one bench at a time. Add to that, all four trucks had to be replaced at the same time. Had it only been one or two trucks, the operation may not have gone with a portable system, says Norm Jagger, vice president and general manager of aggregate operations for Camas Minnesota.

Watsonville, Calif.-based Graniterock has been crushing at the face with a portable primary since the early 1980s. “It was very sensible because of the deep, highwall granite deposit,” says A.R. Wilson Quarry Plant Manager Frank Williams.

Graniterock blasts about once every 10 days and pulls the wheel-mounted Nordberg gyratory primary away from the face. It only takes two to three hours to re-level the crusher after a move, Williams says. From the primary, the material is conveyed 1.1 miles to the plant.

“We have a lot less vehicular traffic because of the conveyors,” Williams says. And that, he adds, has improved safety at the quarry.

One reason a producer may choose a wheeled in-pit primary is the difference in feeder size.

“Tracked units have short feeders to maintain a center of gravity for stability,” says Mark Krause, director of aggregates for Terex. “Most tracked machines are designed in Europe where they are fed by an excavator or shovel. A wheeled unit that allows for a 20- to 24-ft-long feeder can better accommodate 8- to 12-cu-yd wheel loaders.”

Wheeled in-pit primaries make more sense, due to their lower initial cost compared with equivalent-sized tracked units. However, this is contingent on an operation being able to drop a lot of material per blast, which allows for less frequent moves.

The typical move time between a wheeled primary and an equivalent tracked primary can be considerable.

It takes about 30 minutes to set up a track-mounted primary; a wheeled unit takes as much as seven hours. On track units, there is no need to level the frames or build ramps, says Tommi Lehtonen, sales support manager/specialist in track-mounted crushers for Metso Minerals.

But that gap may be narrowing. According to Paul Smith, product manager for Johnson Crushers International, its newly developed Fast Pack, may redefine the limitations of wheeled portable systems.

“The Fast Pack system uses hydraulically engaged foundations that eliminate the need for cribbing,” Smith says. “Basically, the system has a hydraulic lever that brings down the foundations, lifting the plant off its wheels and in a ready-to-go operating position.” Fast Pack can consist of a whole plant, including primary and secondary screens and crushers, he says. “The whole system can be moved in four hours.”

Metso Minerals has taken a different route to full-system portability. In Europe, Metso has built road-portable, in-pit plant systems on tracks. Although these plants can produce as much as 400 tph, none have been delivered in North America. Metso uses a system of wheels that are directly connected to the wheel-mounted frame, which eliminates the extra weight and height of a trailer.

Next Step, Modularity?

Recycle and road contractors first caught on to the efficiencies of portable crushing plants, and in certain areas, are processing virgin aggregate. But it makes more sense for aggregate producers — far more knowledgeable in material processing, especially in making product to spec — to move from processing virgin aggregate to other processing services such as concrete and asphalt recycling.

The move to more portability might be the beginning of an important paradigm shift in the aggregate industry. It may prove not only a cost-saving measure in-pit, but also a way to be more flexible to handle shifts in market demand.

Modularity may be the catchword to best describe this paradigm shift. Astec has launched its system of modular components for static plants. The company realized unprecedented delivery and set-up times for customers. And these operations will be able to modify and upgrade a plant by moving, adding, and/or replacing modular units.

Terex's Krause offers this possible scenario for an aggregate company:

A company secures three to five operations around a metropolitan area. Its equipment consists of modular pieces designed to meet the needs of different deposits.

Instead of having fixed overhead plants, certain modular pieces can be used to build portable plants in more than one deposit. Depending on demand cycles, a company may be able to move parts and pieces to operate two plants, then move the pieces in a different configuration to handle the processing at other locations. It may enable the company to get away with two crews rather than five.

“If you have three to five operations in a metropolitan area, and mobilize your assets that way, you can be more competitive,” Krause says. If business booms, a company can rent a plant for six months and build the cost into the bid. “Pay as you go, and keep overhead low” may be a saving strategy, especially for smaller companies unable to permit land for big operations.

Companies that figure out the best way to use assets — whether using a static or a mobile primary — will be the companies of the future.

Bill Welgoss is a freelance writer with more than 10 years covering the aggregate industry.