Metal Profile: Iron Ore

What is Iron Ore?

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Iron ore pellets. Courtesy: Vale

Iron ore is a vital input material in the production of primary steel.

Relative abundance of the minerals hematite (Fe2O3) and magnetite (Fe3O4) make these the most sought after and economical sources of iron ore, although other source minerals include goethite (FeOOH), limonite (FeOOH*H2O) and siderite (FeCO3).

These minerals are most often extracted from geological formations known as banded iron formations (BIFs).

Once mined the ores are processed for preparation for delivery to steel smelters.

Banded Iron Formations:

BIFs are iron-rich sedimentary deposits that are almost exclusively found in Precambrian rocks. That is, those more than 600 million years old.

The iron oxides that provide sources of iron ore contained within these formations are generally classified as either magnetite of hematite.

BIFs are made up of layers of iron-bearing minerals alternate with layers of shales or cherts. In all but the most highly enriched hematite ore, these waste products (gangue) must be separated from the iron in order to produce ore concentrate suitable for blast furnaces.

Hematite:

High-grade hematite deposits are naturally enriched iron deposits found within BIFs. These are believed to have formed as a result of geo- and hydrothermal processes oxidizing magnetite. Over millennia, such oxygen-rich environments would have naturally leached and concentrated the contained iron, while eliminating its magnetic properties.

From the time of Bessamer's invention of the modern steelmaking through much of the 20th century, only high-grade hematite - known as direct shipping ore (DSO) - was mined and used for steel production. DSO refers to iron ore that naturally contains between 58 and 64 percent iron, which can be used directly in blast furnaces.

However, the depletion of high-grade deposits along with growth in demand for steel and improved beneficiation techniques for processing magnetite has led iron ore miners to exploit more low=quality deposits.

The Pilbara region of Western Australia possesses the world's largest reserves of high-grade hematite. The Hamersley Range, which sits upon a massive iron-rich BIF in the region, provides the world with over 100 million tonnes of iron ore each year. BHP Billiton, Fortescue Metals and Rio Tinto all have significant iron ore mining assets in the region.

While Australia is blessed with massive amounts of the rusty red hematite, which accounts for over 90 percent of the country's iron ore exports, Brazil also has vast deposits. The world's largest iron ore mine, the Carajas Mine, operated by Vale in Brazil's Para state, is also characterized by high-grade hematite.

Magnetite:

Magnetite is a naturally occurring igneous mineral. Ores containing magnetite generally contain between 25 and 40 percent iron, much less than hematite, meaning that they must be treated prior to sale and delivery to steel makers.

Although costly and complex, upgrading magnetite ore to magnetite concentrate and pellets results in a more pure and desirable product than DSO.

As a result, such products can be sold at a premium to offset additional processing costs.

The term taconite is commonly used in North America to describe mineral sources of iron ore that are characterized by fine-grained magnetite.

Growing demand from North American steel makers post-World War II, led to the development of the Labrador Trough in eastern Canada, as well as taconite deposits in the Mesabi and Marquette iron ranges in Minnesota and Michigan.

Cliffs Natural Resources, the Iron Ore Company of Canada, ArcelorMittal and US Steel all have stakes in the continent's taconite market.

In China, the world's largest producer (and consumer) of iron ore, major deposits are primarily defined by low-grade magnetite.

The Anshan-Benxi iron ore zone in the northeast of the country, as well as the Qianluan and Daye iron zones in Hebei and Hubei provinces, are all dominated by magnetite, with iron contents ranging between 25 and 35 percent.

Beneficiation:

Beneficiation refers to the process of concentrating and upgrading iron ore in order to make a product suitable for use in blast furnaces for steelmaking.

While DSO containing between 54 and 60 percent iron content and crushed to a size of between 7 to 25 millimeters (0.28 to 1.0 inches) can generally be used to directly charge furnaces after crushing, screening and blending, finer particles (those less than 7 millimeters, or 0.28 inches, in size) resulting from the crushing process must be sintered to provide a more desirable product.

Beneficiation of magnetite ores, on the other hand, requires crushing, grinding and a multi-stage magnetic separation process, whereby the iron ore fines are passed over drum magnets to separate the magnetic iron oxide from waste material - known as gangue. 

The resulting magnetite concentrate, which contains roughly 68 to 70 percent iron, is a fine powder that cannot be directly used by blast furnaces and is too small to be sintered. Therefore, it must be pelletized for use in basic oxide furnaces.

Pelletizing requires moistening the concentrate with water and additives. The mixture is then tumbled in a rotating drum or pelletizer. The balls formed are consequently heated to between 1250 and 1340°C (2300-2440°F) before being cooled.

The finished product is pellets, or balls, that are 10 to 15 millimeters in diameter and contain between 67 and 72 percent iron.

Sources:
Chim, Sandy. Iron Ore - from hematite to magnetite. Resource World. April 2012.
URL: http://www.proactiveinvestors.com/genera/files/sponsor_files/sandy_chim__april_2012.pdf
Ironinvestingnews.com. Types of Iron Ore: Hematite vs. Magnatite. Sept. 5, 2013.
URL: http://ironinvestingnews.com/5978-types-of-iron-ore-hematite-vs-magnetite.html
Iron Fact Sheet. Australian Atlas of Minerals Resources, Mines & Processing Centres. Geoscience Australia.
URL: http://www.australianminesatlas.gov.au/education/fact_sheets/iron.html
Magnetite Facts and Figures. Magnetite Network.
URL: http://www.magnetitenetwork.com.au/who-we-are/magnetite/

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