Equipment upgrade improves safety, product quality

Oklahoma’s oldest and largest crushed stone producer, Dolese Bros. Co., recently completed a $14 million upgrade of its limestone quarry in Hartshorne that included construction of a new, completely automated crushing plant and the upgrade and replacement of most of the quarry’s trucks, crushers and screens.

Typically, the Hartshorne quarry produces about 1 million tons of product per year. With the quarry upgrade, production per hour has doubled while operation costs have been significantly reduced.

Dolese has seven additional quarries, four additional sand-and-gravel locations in Oklahoma, and one sand-and-gravel facility in Baton Rouge, La. The company also operates about 50 ready-mix concrete plants.

Mark Helm, Dolese president and COO, says improved efficiency and increased production resulting from the upgrade made the investment worthwhile.

“Our company has owned the Hartshorne operation since the early 1960s,” Helm says. “Our market for that quarry is fairly rural and covers a large area partially impacted by the growth of the domestic oil and gas industry. The plant was remodeled and added to numerous times over the years. Achieving the quarry’s production goals required considerable overtime and we had growing concerns for potential safety issues.”

Upgrade planning

Hartshorne quarry’s full mine-reserve evaluation, including engineering work completed by an outside geologist and drilling to confirm the presence of limestone, was completed before any upgrade decisions were made. Since it’s projected that the quarry’s reserves will last about four more decades, the upgrade was economically feasible.

As plans for the upgrade got underway, identification of the quarry’s limestone reserves aided in the new plant layout and allowed for efficient configuration of the new plant and haul roads.

“The Hartshorne pit sits along a long narrow ridge,” says Walter Colijn, Dolese aggregate engineer. “In designing the new plant, we decided to place it at about the center of the reserve. That way we can work in different reserve areas without the need for a long haul route.”

Effectively equipped

Among the quarry’s equipment issues was the fact that the old style crushers and screens were too small to process the quantity of product necessary to meet production goals. In general, outdated design of quarry equipment hampered production and maintenance efficiency. An outdated electrical system access also reduced production effectiveness.

The primary crusher, a portable jaw in the mining pit, was not included as part of the new plant project, although it was later “parked” outside of the pit.

“Using loaders and haul trucks to feed the plant opens up new areas of the pit where we can work,” Helm says. “Our engineering department played a major role in designing the new plant to facilitate product flow. In selecting trucks, we matched truck size to the amount of product our primary jaw crusher produces.”

A rebuilt Symons 7-ft. cone crusher was installed as the secondary crusher with the purpose of reducing the size of the rock from the main surge pile. The initial tertiary cone and vertical shaft impact crushers were replaced with Telsmith high-speed cones.

“These cone crushers helped produce a more cubical finished product that’s preferred by many customers,” Colijn says. “Additionally, these new Telsmith [crushers] are equipped with a newer-style hydraulic system that allows the crusher-setting adjustments to be done on the fly.”

Natural resources

Using some of the pit’s natural topography, Dolese located the new plant at a higher elevation to aid product flow through the process. In redesigning their conveyor system, Dolese was able to maintain low angles on a number of conveyors, reducing the horsepower needed to transport materials.

Under the quarry’s primary and secondary surge piles, using 2-ft. x 2-ft. x 4-ft. concrete blocks as the walls and concrete slabs as the roof, Dolese built reclaim tunnels with access on both ends for safety, service and maintenance. The main surge pile is set on a naturally occurring slope to take advantage of the slope’s elevation and allow use of multiple drawn-down chutes to feed material from the piles.

Electrifying improvements

To improve both efficiency and safety, Dolese used both capacitors and high-resistance grounding (HRG) electrical systems, which make it possible for one operator to automate the entire crushing plant. HRG systems offer many advantages to quarrying operations, including maintaining service continuity and assisting with locating the source of any ground fault.

HRG systems also limit phase-to-ground currents to 5 to 10 amps, reducing potential arcing current and essentially eliminating arc-flash hazards associated with phase-to-ground faults.

“The HRG system is much safer for quarry employees,” Colijn says. “In the case of a fault, the systems may minimize or eliminate mechanical and thermal damage to shorted transformer and rotating machinery windings.”

Capacitors that are part of the electrical system make electric usage more efficient. Capacitors work in a way similar to the backup on an alarm clock, which keeps the clock running for a limited time, using stored energy and releasing it when the plant draws a temporary electrical surge.

Guarding safety

In its upgrade, Dolese addressed safety issues surrounding quarry operations by reviewing its guarding equipment. The Mine Safety and Health Administration requires that moving or rotating equipment or equipment parts be equipped with guards when in use. Moving parts included in the regulation include belts, gears, shafts, pulleys, sprockets, spindles, drums, flywheels, chains and other reciprocating, rotating or moving equipment parts exposed to employee contact.

One of Dolese’s considerations was how to design effective guards that were mobile enough to avoid injury to employees who needed to move/remove or replace them.

“We wanted to keep guard weights down to 50 lbs. or less,” Colijn says. “That way, they can easily be lifted and moved for servicing without risking injury. We weren’t able to keep 100 percent of our guards under that weight, but we contracted with the company that constructed our plant to fabricate the guard structures according to our specific needs.”

Most of the guards were made using expanded metal with a thin metal frame and lifting handles. The guards are typically hung from brackets attached to the structures.
Ongoing success

In addition to greatly improving product processing, the Dolese Hartshorne upgrade has increased flexibility of the mining process there.

Dolese, which originated in Chicago, was developed by brothers who owned a road-paving operation. The brothers anticipated finding better work opportunities in the aggregate business, which brought them to Oklahoma in 1902. In the 1950s, Dolese moved its main office to Oklahoma.

“Our company has been pretty successful at the quarrying business since 1902,” Helm says. “We have great employment stability in the Hartshorne area, serving as one of the biggest employers there. We expect to supply rock and stone products to customers in that region for many decades to come.”

Take note

Using some of the pit’s natural topography, Dolese placed the new plant at a higher elevation to aid product flow through the process.

Loretta Sorensen is a freelance writer in Yankton, S.D. She produces material on a variety of topics, serves as a ghostwriter, and has authored her own books.