Of all the cargos moved across the Great Lakes, limestone has the most diverse customer base. The steel industry uses "flux-stone" as a purifying agent in the steelmaking process. The flux-stone is either added directly to the blast furnace or mixed in with the iron ore at the mine to produce "fluxed pellets."
The construction industry uses "aggregate" as a base for highways, parking lots, sewer systems... The chemical and paper industries also use limestone. When all the applications for limestone are combined, it is estimated that each American uses 8,000 pounds every year.
The annual stone float for U.S.-Flag lakers is approximately 23 million tons. The development of fluxed pellets has actually increased the stone trade for U.S.-Flag lakers above pre-recession levels.
The Great Lakes region is blessed with an almost inexhaustible supply of limestone and the quarry at Rogers City, Michigan, is the largest in the world.
Since stone is somewhat high in moisture content and is often "washed" before loading into vessels, the trade is a bit more weather-sensitive than other cargos. The stone trade generally resumes in early April and finishes by late December.
A pair of U.S.-Flag lakers deliver limestone to the port of Duluth/Superior. The stone will then be railed to the ore mines on the Mesabi Range to produce fluxed pellets. Once the ships have discharged their stone cargos, they will load iron ore.
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