August 1, 2011
New research may yield commercial viability for this lower quality ore.
By Bill Langer
tak-uh-nahyt (noun) – A type of chert used as iron ore in parts of North America. “In the Mesabi district the local name ‘taconite’ is applied to the ferruginous chert.” (Economic Geology, 1905, Vol. I, page 48.)
Mesabi is an Ojibwa name meaning giant, and although the Ojibwa were not referring to iron ore, the Mesabi Range of northeastern Minnesota is a giant among the world’s iron deposits.
The rocks of the Mesabi Range are 1.85-billion-year-old banded iron formations composed of interbedded layers (bands) of iron, chert, siliceous shale, slate, and carbonate rocks formed. No one knows quite how these rocks were formed, and there are no modern analogs. One theory is that, at this early time in geologic history, the earth’s atmosphere was deficient in oxygen. Iron, in the presence of oxygen, oxidizes (rusts) very rapidly. But without oxygen, iron and silica could be weathered from existing rocks, transported to ocean basins, and precipitated out of solution to form the silica-rich iron deposits — all without turning into iron oxide. After the earth’s atmosphere became oxygen-rich, weathering of banded iron formations in areas such as the Mesabi Range resulted in zones of concentrated hematite (iron oxide or Fe2O3), which is high-grade iron ore containing up to 70 percent iron.
Hematite in the Mesabi Range was discovered in 1887. The ore was so rich that it was profitable to ship the iron to states along the Great Lakes and to the northeastern United States for smelting. A great deal of iron mining took place in the Mesabi Range to support the building of the infrastructure of the United States, and by the 1950s, the high-grade hematite ore bodies had been depleted. Layers of lower quality ore, called taconite, remained, however.
The problem was that taconite contains only up to 30 percent magnetite and hematite, occurring as tiny particles scattered throughout a very tough chert matrix. The low-grade taconite ore was technologically impossible to use and had no economic value, but a process of beneficiation was developed at the University of Minnesota whereby magnetite and hematite are separated from crushed and pulverized taconite ore. The iron powder is moistened, combined with a binder (primarily bentonite clay) and a flux (limestone and dolomite), rolled into marble-sized spheres, and then fired into hard, round pellets suitable for use in a blast furnace. Today, approximately 75 percent of the U.S. iron ore production comes from the Mesabi Range in Minnesota. Iron pellets are shipped to midwestern and eastern steel mills aboard large Great Lakes freighters and by rail to steel mills as far away as Alabama and Utah.
Mining and processing of taconite generates about 125 million tons of crushed byproducts every year. Since 2000, the Natural Resources Research Institute (NRRI), University of Minnesota Duluth, has been conducting research to identify new environmentally responsible and economically viable uses for taconite as aggregate in road construction, road repair, and other applications where crushed stone aggregate is normally used. Research activities include cold-temperature testing of taconite-based hot-mix asphalt designs; using microwave technology for chemical-free deicing and all-season pothole repair; the development of asphalt-free and cement-free pothole repair compounds; and use of taconite fine aggregates in high-friction/anti-skid bridge deck surfacing treatments.
The NRRI research is indeed a giant undertaking, and, when I say giant, I mean Mesabi!
Bill Langer is a geologist with the Mineral Resources Team of the U.S. Geological Survey and can be reached at 303-236-1249 or via e-mail at email@example.com.