| Sedimentary iron formation rock | |
 | |
| Composition | |
|---|---|
| Primary | Magnetite,hematite andchert |
| Secondary | Siderite,greenalite,minnesotaite andstilpnomelane |
Taconite (/ˈtækənaɪt/) is a variety ofbanded iron formation, aniron-bearing (over 15% iron)sedimentary rock, in which the iron minerals are interlayered withquartz,chert, orcarbonate. The nametaconyte was coined in the 19th century by Horace Vaughn Winchell – son ofNewton Horace Winchell, theMinnesota state geologist – during their pioneering investigations of thePrecambrian Biwabik Iron Formation of northeastern Minnesota within theMesabi Range. He believed the sedimentary rock sequence hosting the iron-formation was correlative with theTaconic orogeny of New England and referred to the unfamiliar and unnamed iron-bearing rock as the 'taconic rock' ortaconyte.[1] Following development of high grade direct shippingiron ore deposits on the Mesabi Range, containing up to 65% iron and as little as 1.25%silica, miners termed the unaltered iron-formation wall rock taconite.
The iron content is generally 30% to 35%, and the silica content generally around 45%. The iron is commonly present asmagnetite, ironsilicates, and iron-bearing carbonates, and locally martite (hematite) formed by oxidation of magnetite.Horizons containing magnetite as the dominant mineral have been extensively mined since 1955 to produce iron orepellets; the term 'taconite' has consequently been colloquially adapted to describe the magnetite iron-formation ores (taconite iron ore), the mining, milling, magnetic separation, and agglomerating process (taconite process), and the product iron ore pellets (taconite pellets).
In the late 19th and early 20th centuries, the United States was mining such an abundance ofiron ore of high quality that taconite was considered an uneconomic waste product. By the end ofWorld War II much of the high-grade iron ore in the United States had been exhausted, so taconite became valued as a source of the metal.[2]
To process taconite, the ore is ground into a fine powder, the magnetite is separated from thegangue by strong magnets, and the powdered iron concentrate is combined with a binder such asbentonite clay and limestone as aflux. 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 theblast furnace charge remains porous enough to allow heated gas to pass through and react with thepelletized ore. Firing the pellet oxidizes the magnetite (Fe3O4) tohematite (Fe2O3), anexothermic reaction that reduces the cost of pelletizing the concentrate.Edward Wilson Davis of theUniversity of Minnesota Mines Experiment Station is credited with developing the pelletizing process. Since the commercial development of this process in theLake Superior region in the 1950s, the term "taconite" has been used globally to refer to iron ores amenable to upgrading by similar processes.[citation needed]
Major producers of iron ore pellets from taconite in North America includeIron Ore Company of Canada,Cliffs Natural Resources, Inc.,U.S. Steel, andArcelorMittal. 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.[citation needed]
TheMesabi Range inMinnesota is a major production area. The taconite iron ore pellets are hauled by rail to the ports ofSilver Bay andTwo Harbors and theTwin Ports ofDuluth, Minnesota, andSuperior, Wisconsin, all on Lake Superior. The docks atEscanaba, Michigan, onLake Michigan also ship taconite from theMarquette Iron Range in Michigan, and occasionally ore from Minnesota is hauled by rail there.Marquette, Michigan, also has a taconite dock that loadslake freighters with ore from the Marquette Iron Range. The ore is generally shipped to locations on the lowerGreat Lakes. Many steelmaking centers are nearLake Erie. Due to increased international demand, taconite is shipped to Mexico and China.[3]
TheSSEdmund Fitzgerald, which sank in Lake Superior on November 10, 1975, was carrying about 26,116long tons of taconite pellets.[4]
Beginning in 1955, Reserve Mining Company discharged crushed waste rock (tailings) from their Silver Bay processing plant into Lake Superior. The tailings contained 40% of theamphibole group mineral seriescummingtonite-grunerite, which may formasbestiform particles. A small fraction of the fine-grained tailings were shown to widely disperse along the western shore of Lake Superior, the source of drinking water for numerous cities; for example, tests of Duluth's water supply showed 100 billion fibers per liter of water. There was no epidemiological proof whether these particles caused cancer. In 1974 U.S. District Court Judge Miles Lord ruled that the drinking water and Lake Superior must be protected from theasbestos-like particles. Reserve Mining Company was forced to begin disposing of tailing wastes on the land and to implement air pollution control equipment. This became one of the costliest pollution prevention cases in U.S. history.[5] Government-funded studies have found no adverse health effects from drinking Lake Superior water.[6][7]
A 2003 study of taconite miners concluded that the most likely cause of 14 of the 17 cases ofmesothelioma among miners on theIron 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.[8]
Spurred by the 2003 study, the Minnesota Department of Health studied the relationship of fibrous minerals in taconite and taconite dust, and lung conditions similar toasbestosis, pleural mesothelioma and otherpleural conditions which occur following asbestos exposure. Because these conditions can be triggered by industrial asbestos, which was used in taconite mining and processing, as well as other industrial facilities in northeastern Minnesota, the study attempted to determine what, if any, influence naturally occurring fibrous minerals in taconite may have played.[8] The lengthy epidemiological study of Minnesota iron miners concluded in December 2014 that those working 30 years in the iron mines and living to be 80 years old had a lifetime chance of having a mesothelioma of 3.33 cases per thousand such workers, more than double the background rate of 1.44 cases per thousand people living to 80 years old.[9]