Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in colour from dark grey, bright yellow, deep purple, to rusty red. The iron itself is usually found in the form of magnetite (Fe3O4), hematite (Fe2O3), goethite, limonite or siderite. Hematite is also known as "natural ore". The name refers to the early years of mining, when certain hematite ores contained 66% iron and could be fed directly into blast furnaces. Iron ore is the raw material used to make pig iron, which is one of the main raw materials to make steel. 98% of the mined iron ore is used to make steel.
Minnesota's iron ore was actually discovered while prospectors were searching for gold. Since the object of their search was gold, the iron was ignored. It turned out the iron would become much more valuable to northern Minnesota than the small amount of gold found. Iron ore was discovered on the three iron ranges at different times. The first ore shipped from the Vermilion Range was in 1884, the Mesabi Range in 1892, and the Cuyuna Range in 1911.
Hematite iron ore deposits are currently exploited on all continents, with the largest intensity of exploitation in South America, Australia and Asia. Most large hematite iron ore deposits are sourced from metasomatically altered banded iron formations and rarely igneous accumulations. Hematite iron is typically rarer than magnetite bearing banded iron formations or other rocks which form its main source or protolith rock, but it is considerably cheaper and easier to beneficiate the hematite ores and requires considerably less energy to crush and grind. Hematite ores however can contain significantly higher concentrations of penalty elements, typically being higher in phosphorus, water content (especially pisolite sedimentary accumulations) and aluminium (clays within pisolites).
Magnetite banded iron deposits
Banded iron formations are fine grained metamorphosed sedimentary rocks composed predominantly of magnetite and silica (quartz). Banded iron formations are locally known as taconite within North America.
Mining and processing of banded iron formations involves coarse crushing and screening, followed by rough crushing and fine grinding to reduce the ore to the point where the crystallized magnetite and quartz are fine enough that the quartz is left behind when the resultant powder is passed under a magnetic separator.
The key parameters for magnetite ore being economic are the crystallinity of the magnetite, the grade of the iron within the banded iron formation host rock, and the contaminant elements which exist within the magnetite concentrate. The size and strip ratio of most magnetite resources is irrelevant as banded iron formations can be hundreds of meters thick, with hundreds of kilometers of strike, and can easily come to more than 2,500 million tons of contained ore.
The typical grade of iron at which a magnetite-bearing banded iron formation becomes economically feasible to mine is roughly 25% Fe, which can generally yield a 33% to 40% recovery of magnetite by weight, to produce a concentrate grading in excess of 64% Fe by weight. The typical magnetite iron ore concentrate has less than 0.1% phosphorus, 3-7% silica and less than 3% aluminum.
The grain size of the magnetite and its degree of comingling with the silica groundmass determine the grind size to which the rock must be reduced to enable efficient magnetic separation to provide a high purity magnetite concentrate. This determines the energy inputs required to run a milling operation. Generally most magnetite banded iron formation deposits must be ground to between 32 and 45 micrometers in order to provide a low-silica magnetite concentrate. Magnetite concentrate grades are generally in excess of 63% Fe by weight and usually are low phosphorus, low aluminum, low titanium and low silica and demand a premium price.
Currently magnetite iron ore is mined in Minnesota and Michigan in the U.S., and Eastern Canada mine taconite. Magnetite bearing banded iron formation is currently mined extensively in Brazil, which exports significant quantities to Asia, and there is a large and developing magnetite iron ore industry in Australia.
Taconite is an iron-bearing, high-silica, flint-like rock. It is a Precambrian sedimentary rock referred to as a banded iron formation due to the typical alternating iron-rich layers and shale or chert layers. The very finely dispersed iron content, present as magnetite, is generally 25 to 30%. In the late 19th and early 20th centuries, iron ore was of such high quality that taconite was considered an uneconomic waste product. After World War II, most of the high grade ore in the United States had been mined out, and so taconite was turned to as a new source of iron. To process taconite, the ore is ground into a fine powder, the iron is separated from the waste rock by using strong magnets, and then the powdered iron concentrate is combined with bentonite clay and limestone as a flux and rolled into pellets about one centimeter in diameter that are approximately 65% iron. The pellets are heated to very high temperatures to oxidize the magnetite (Fe3O4) to hematite (Fe2O3) for further processing.
The Mesabi Iron Range region of Minnesota is a major production area. The taconite iron concentrate is hauled by rail to loading docks at Silver Bay, Two Harbors and the Twin Ports of Duluth, Minnesota and Superior, Wisconsin, all on Lake Superior. The ore is generally shipped by lake freighters to other locations on the Great Lakes. Many steelmaking centers are located near Lake Erie and lower Lake Michigan, around Gary Indiana.