Down in Milltown, Ind., a family operation is steadily producing more than 700,000 tons per year of high-quality limestone. Robertson Crushed Stone has been hard at work for 29 years, and reserves are projected to last another 60 years at current production rates.

From left to right: Bud Bischoff, and Charlie, Will and Katie Robertson.

There's not much else in this small town, maybe a thousand people and the scenic Blue River. It's located about 30 minutes northwest of Louisville, probably too far for a competitive advantage in the construction materials market. Still, the Robertsons have been able to maintain a solid operation and a good life selling material to coal-fired power plants.

The ledges of the quarry face at Robertson Crushed Stone yield a limestone that is rich in calcium carbonate and low in magnesium. This chemical composition is ideal for scrubber stone used to remove sulfur dioxide from power plant emissions. President Charlie Robertson says the company's two biggest customers are Duke Energy's Gibson Station in Princeton, Ind., and Indianapolis Power & Light in Petersburg, Ind.

GROWING BY DEMAND

The competition among aggregate operations producing this stone is tremendous, explains Vice President of Sales Katie Robertson. She says Duke Energy's Gibson Station alone is projected to consume up to 1 million tons of scrubber stone this year. This demand keeps the Robertsons busy with potential for growth.

Charlie says demand for scrubber stone increased in the early 1980s and really took off in the early 2000s as environmental regulations became more stringent. Acid rain, which is largely caused by sulfur dioxide, was a growing concern. Before the regulations went into effect, a power plant would only consume an average of 50,000 tons per year. The power plants operate in accordance with the law, Charlie says.

“The federal mandates made our markets grow because they would not have installed the scrubbers otherwise,” Charlie says. “They are very expensive to build and very expensive to operate.”

A FINE OPERATION

The main product is as large as 3/4 inch but also contains fines as well. At the power plant it will be ground to 200 mesh.

The process is called wet scrubbing. The stone that the plant receives is ground by ball mills to a 200 mesh or finer. It is mixed with water to create a slurry, which will absorb the sulfur dioxide from the coal emissions. As the material dries from the heat, it falls to the bottom of the scrubber as a cake. Often it is just disposed of in the landfills, but it also can be transformed into a synthetic gypsum and sold to the wallboard industry, Charlie says.

Most aggregate operations that focus on the construction materials market are concerned by the level of fines entering their product. This is less of an issue for the scrubber stone market because the end product is a fine dust. Charlie says the company avoids filling the stockpiles with too many fines because that becomes a handling issue. The stone the power plants receive is no larger than ¾ inch.