If the idea of horizontally feeding 6- × 6-ft chunks of limestone under a single rotor with impact bars sounds foreign that's because it is. The Schlagwalzenbrecher, which translates “impact roll crusher,” was first developed by DBT Mineral Processing for crushing at German long-wall coal mines.

For 25 years, the same technology has been adopted for low-silica limestone operations. Since these crushers were introduced to the United States five years ago, 14 have been sold. DBT Mineral Processing Vice President for North America Ed Kelleher, says there are about 250 impact roll crushers processing aggregates in other parts of the world.

The smallest of the crushers has a 40- × 40-in. opening and can produce 400 tph. The largest has a 6- × 6-ft opening and can produce 1,000 to 1,500 tph. Tonnage is rated by run-of-mine feed material.

Kelleher says if the rock can fit into the opening the machine will crush it, which greatly reduces the need for secondary breaking.

The feed system also eliminates the need for steel hoppers. Because the plant's low-profile design, the hopper is near ground level. With only a slight ramp, 100-ton trucks can dump from either side. It's low profile provides a more compact design for mobility. When the machine is moved, a new ramp and makeshift hopper can be built in about a half day.

Instead of a vibratory feeder, an impact roll crusher relies on a chain conveyor. Kelleher says 100-ton trucks can dump 6- × 6-ft pieces of limestone directly onto the chain conveyor. It is comparable to a bicycle chain, consisting of steel flight bars that join two parallel chains rotating around a head and tail sprocket. If a flight bar breaks, it can be removed and feeding resumed. Kelleher says flight bars generally withstand between 3 million and 5 million tons. The conveyor operates on four automatically lubricated bearings. If the lubrication system is not refilled, the unit's programmable logic controller will stop the machine to prevent bearings from running dry.

The flight bars drag material into the crushing chamber horizontally in a straight, continuous flow from ground level. This maintains a low profile of 14- to 18-ft total height. Kelleher says it easily fits and maneuvers in underground mines.

Feed material is crushed under a single rotor that has five impact bars for the smallest machine and up to 9 impact bars for the largest unit. “Our impact bars are like a mechanical pencil. We actually have 26 in. of wear per impact bar,” Kelleher says. Releasing a locking wedge on the front of the bar allows additional bar to be screwed out from the rear. Then the wedge is relocked, and production may resume. Total time to adjust all the elements takes about two hours, Kelleher says.

“Material hardness is not a primary issue, nor is wet and sticky material,” Kelleher says. The main factor that determines wear cost per ton is silica content. High-silica material grinds the impact bars. An Indianapolis quarry reports 8 to 9 in. of wear after crushing 1.4 million tons, Kelleher says. Warranties are based on the company's analysis of the product's silica content and abrasion indexes.

The system is regulated by a programmable logic controller. If the rotor falls below preset limits, the conveyor stops. And it automatically starts again when the rotor returns to the normal operating speed. If something is plugged, the blockage can be cleared by reversing the feed direction and dropping the oversize rocks off the end of the feeder.

The machine's major selling point is its compact and highly mobile design. As the distance to a quarry face increases, operators move the crusher closer and extend the transfer conveyors to the secondary crushers. This cuts the costs of additional haul trucks and wheel loaders, Kelleher says. The machine can be track-mounted (powered or non-powered) or put on skids.

“It's a real simple machine,” he says. There's no need for a concrete pad or ramp, and Kelleher says, it doesn't even need to be 100% level.

“One thing you have to remember, this is not a crusher. This is a plant.” It includes a feeder, crusher, superstructure, discharge and a motor control center. And it can be set up in a matter of days and moved in a matter of hours.