|Sedimentary iron formation rock|
|Primary||Magnetite, hematite and chert|
|Secondary||Siderite, greenalite, minnesotaite and stilpnomelane|
Taconite (IPA: ['tækənaɪt]) is a variety of iron formation, an iron-bearing (over 15% iron) sedimentary rock, in which the iron minerals are interlayered with quartz, chert, or carbonate. The name "taconyte" was coined by Horace Vaughn Winchell (1865–1923) – son of Newton Horace Winchell, the Minnesota State Geologist – during their pioneering investigations of the Precambrian Biwabik Iron Formation of northeastern Minnesota. He noted the rock had a superficial resemblance to iron-bearing rocks from the Taconic Mountains of New York state.
The iron content of taconite, commonly present as finely dispersed magnetite, is generally 25% to 30%.
In the late 19th and early 20th centuries, the United States was mining such an abundance of iron ore of high quality that taconite was considered an uneconomic waste product. By the end of World War II, however, much of the high-grade iron ore in the United States had been exhausted. Taconite became valued as a new source of the metal.
To process taconite, the ore is ground into a fine powder, the magnetite is separated from the gangue by strong magnets, and the powdered iron concentrate is combined with a binder such as bentonite clay and limestone as a flux. As a last step, it is rolled into pellets about 10 millimeters in diameter that contain about 65% iron. The pellets are fired at a very high temperature to harden them and make them durable. This is to ensure that the blast furnace charge remains porous enough to allow heated gas to pass through and react with the pelletized ore. Firing the pellet oxidizes the magnetite (Fe3O4) to hematite (Fe2O3), an exothermic reaction that reduces the cost of pelletizing the concentrate. E. W. Davis of the University of Minnesota Mines Experiment Station is credited with developing the pelletizing process. Since the commercial development of this process in the Lake Superior region in the 1950s, the term "taconite" has been used globally to refer to iron ores amenable to upgrading by similar processes.
Major producers of iron ore pellets from taconite in North America include Iron Ore Company of Canada, Cliffs Natural Resources, Inc., U.S. Steel, and ArcelorMittal. These processed taconite-ore pellets are also referred to as "taconite". Because this is the form that is typically transported by rail and ship, and cargo of these is often discussed, this usage of the term is very common.
The Mesabi Iron Range region of the American state of Minnesota is a major production area. The taconite iron ore pellets are hauled by railroad to the ports of Silver Bay, Two Harbors and the Twin Ports of Duluth, Minnesota, and Superior, Wisconsin, all on Lake Superior. The docks at Escanaba, Michigan, on Lake Michigan, also ship taconite from the Marquette iron range in Michigan, and occasionally ore from Minnesota is hauled by rail there. Marquette, Michigan, also has a taconite dock that loads bulk freighters with ore from the Marquette iron range. The ore is generally shipped by lake freighters to locations on the lower Great Lakes. Many steelmaking centers are near Lake Erie. Due to increased international demand, taconite is shipped to Mexico and China.
Taconite and human health
Beginning in 1955, the crushed waste rock (tailings) from processing taconite of the Reserve Mining Company in Minnesota were discharged in a slurry into Lake Superior. The tailings contained 40% of the mineral series cummingtonite-grunerite, which, being in the amphibole group, form particles similar to those of asbestos minerals. The wastes were shown to affect Lake Superior, the source of drinking water for many cities; for example, tests of Duluth, Minnesota's water supply showed 100 billion fibers per liter of water. There was no epidemiological proof whether these particles caused cancer or were safe. On April 20, 1974, the US District Court judge ruled that the drinking water and Lake Superior must be protected from the asbestos-like particles. The Reserve Mine was forced to begin disposing of tailing wastes on the land, and to implement air pollution control equipment, instead of discharging them directly to Lake Superior. This became one of the costliest pollution prevention cases in U.S. history. Studies have found no adverse health effects from drinking Lake Superior water.
A 2003 study of taconite miners concluded that the most likely cause of 14 of the 17 cases of mesothelioma among miners on the iron range was contact with asbestos. Since that study was concluded, 35 more Iron Range miners have been diagnosed with the disease. Mesothelioma occurs at twice the expected rate among the population of the northeastern region of Minnesota, including the Iron Range. Spurred by the 2003 study, the Minnesota Department of Health (MDH) studied the relationship of fibrous minerals in taconite and taconite dust, and lung conditions similar to asbestosis, pleural mesothelioma and other pleural conditions which occur following asbestos exposure. Because these conditions can be triggered by industrial asbestos, which was also used in taconite mining and processing, the study attempted to determine what, if any, influence naturally occurring fibrous minerals in taconite may have played. The lengthy epidemiological study of Minnesota iron miners concluded in December 2014 that those working 30 years in the iron mines had a lifetime chance of having a mesothelioma of 3.33 cases per thousand people, more than double the background rate of 1.44 cases per thousand people.
- Winchell, Horace V. (1891) "The Mesabi iron range," in: Winchell, Newton H., ed., The Geological and Natural History Survey of Minnesota (Minneapolis, Minnesota, USA: Harrison & Smith), vol. 20, p. 124. From p. 124: "This rock is widely spread over the whole length of the Mesabi, and being different from anything found elsewhere and peculiar to this horizon of the Taconic, has been called taconyte by the writer."
- "Taconite", Department of Natural Resources
- , Future Work
- National Transportation Safety Board (NTSB) (May 4, 1978). "Marine Accident Report: SS Edmund Fitzgerald Sinking in Lake Superior on November 10, 1975" (PDF). NTSB. p. 9. Retrieved 2017-03-29.
- Thomas R. Huffman, "Enemies of the People, Asbestos and the Reserve Mining Trial" http://collections.mnhs.org/MNHistoryMagazine/articles/59/v59i07p292-306.pdf
- E. E. Sigurson, “Observations of cancer incidence surveillance in Duluth, Minnesota,” Environmental Health Perspectives, Nov. 1983, v.53 p61-67.
- Hilding and others, “Biological effects of ingested amosite asbestor, taconite tailings, diatomaceous earth and Lake Superior water in rats,” Archives of Environmental Health, Nov.-Dec. 1981, v.36 n.6 p.298-303.
- Hemphill, Stephanie (2007-06-08). "Researchers look for links between taconite and mesothelioma". Minnesota Public Radio. Retrieved 2007-07-18.
- "Final findings". University of Minnesota Foundation, 14 Jan. 2015.
- Manuel, Jeffrey T., “Mr. Taconite: Edward W. Davis and the Promotion of Low-Grade Iron Ore, 1913–1955,” Technology and Culture, 54 (April 2013), 317–45.
- Manuel, Jeffrey T., Taconite Dreams: The Struggle to Sustain Mining on Minnesota's Iron Range, 1915-2000. Minneapolis: University of Minnesota Press, 2015.