CaCO3

The most important consideration is the chemical composition of the product. The power plants supplied by Robertson Crushed Stone are looking for a stone that has high levels of calcium carbonate and very low levels of magnesium.

The primary crusher is a 34- x 44-inch Nordberg jaw crusher.

“Last year we invested in a testing machine so that we could test our own stone,” says Vice President of Operations Will Robertson. “That helps us as much as it does the customer because we know exactly what we are dealing with and what we have to do to get our numbers where they need to be.”

Samples now can be analyzed in hours versus weeks. So if there is a problem, it can be corrected immediately. Historical data on the chemical compositions also is available on each ledge, dating back 50 years. Will and Katie often joke about their dad who saves everything, but it apparently pays off because they know the calcium content of each quarry ledge. If excavations lead to a seam with high magnesium content, that material will likely be used in agricultural lime and driveways or parking lots. Charlie says blending product is an option, but the company prefers to maintain the highest quality for its customers. Also, those seams are few and far between.

A STANDARD OPERATION

The production process at Robertson Crushed Stone is typical of any aggregates operation that produces a DOT stone, explains Quarry Manager Bud Bischoff. The overburden runs an average of 50 feet deep. On average Robertson Crushed Stone invests four months removing topsoil, clay, sandstone and a small layer of poor limestone. It is all landfilled on site with the exception of some topsoil that is sold. However, Charlie points out that there is a small market for dirt in this rural setting.

A 54- x 60-inch Stedman impact crusher provides secondary crushing.

As useable material is exposed, the company turns to Impact Drilling & Blasting, an independent blasting company. Charlie says Robertson has been purchasing explosives from the company for more than 30 years. Three years ago, they contracted for its drilling and blasting services, as an in-house driller and blaster retired.

Impact Drilling & Blasting uses electric caps; a typical pattern is 12 × 14 feet. Charlie says secondary breaking at the muck pile rarely is needed. Recently, however, the company has been experimenting with various ledges, which has left a few boulders behind. In this case, a hydraulic hammer would be used for secondary breaking.

The shot rock is loaded onto two Euclid R35 haul trucks by a Volvo 330C wheel loader. Each truck makes a round trip in about 10 minutes, moving about 400 tons per hour. The material is dumped into the hopper of a vibratory feeder with a grizzly. The scalped material, which is larger than 5 inches, falls into a 34- × 44-inch Nordberg jaw crusher. The throughs bypass the crusher and are blended with crushed material on the primary conveyor.

This material is conveyed up to a 6- × 16-foot Seco screen. It is a triple-deck, but only two screens are utilized for the scrubber stone, a ¾-inch- to 0-sized material. Robertson, however, does have the ability to produce up to seven products at once with the help of a flop-gate system. The screen also is equipped with spray bars and nozzles to provide dust control. The screen also is enclosed for additional dust suppression and noise control.

The oversize of this screen is transferred to the secondary crusher; a Stedman 54- × 60-inch impact crusher. Crushed material that falls through the finishing screen meets a second stacker and falls off as finished product. Everything else is transferred to a 54 El Jay cone crusher, which is on a closed circuit and discharges oversize back onto the primary conveyor until it is sized to leave.