Iron ore

Hematite: the main iron ore in Brazilian mines

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 to rusty red. The iron itself is usually found in the form of magnetite (Fe₃O₄), hematite (Fe₂O₃), 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 iron making 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.

Mining

Estimated iron ore production in million tons for 2006 according to U.S. Geological Survey Template:Standard table	Country	Production
China	520
Australia	570
Brazil	300
India	150
Russia	105
Ukraine	73
United States	54
South Africa	40
Canada	33
Sweden	24
Venezuela	20
Kazakhstan	15
Iran	20
Mauritania	11
Other countries	43
Total world	1690

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World consumption of iron ore grows 10% per annum on average with the main consumers being China, Japan, Korea, the United States and the European Union.

Iron ore mining methods vary by the type of ore being mined. There are four main types of iron ore deposits worked currently, depending on the mineralogy and geology of the ore deposits. These are magnetite, titanomagnetite, massive hematite and pisolitic ironstone deposits.

Magnetite banded iron deposits

Banded iron formations (BIF) are fine grained metamorphosed sedimentary rocks composed predominantly of magnetite and silica (as quartz). Banded Iron formations are locally known as taconite within North America.

Mining of BIF formations involves coarse crushing and screening, followed by rough crushing and fine grinding to comminute the ore to the point where the crystallised magnetite and quartz are fine enough that the quartz is left behind when the resultant powder is passed under a magnetic separator.

The key economic parameters for magnetite ore being economic are the crystallinity of the magnetite, the grade of the iron within the BIF host rock, and the contaminant elements which exist within the mangetite concentrate. The size and strip ratio of most magnetite resources is irrelevant as BIF formations can be hundreds of metres thick, with hundreds of kilometres of strike, and can easily come to more than 2,500 million tonnes of contained ore.

The typical grade of iron at which a magnetite-bearing banded iron formation becomes economic 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% aluminium.

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 comminuted 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 BIF deposits must be ground to between 32 and 45 micrometres 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 aluminium, 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 BIF is currently mined extensively in Brazil, which exports significant quantities to Asia, and there is a nascent and large magnetite iron ore industry in Australia.

Magmatic magnetite ore deposits

Occasionally granite and ultrapotassic igneous rocks segregate magnetite crystals and form masses of magnetite suitable for economic concentration. Some iron ore deposits, notably in Chile, are formed from volcanic flows containing significant accumulations of magnetite phenocrysts. Chilean magnetite iron ore deposits within the Atacama Desert have also formed alluvial accumulations of magnetite in streams leading from these volcanic formations.

Some magnetite skarn and hydrothermal deposits have been worked in the past as high-grade iron ore deposits requiring little beneficiation. There are several granite-associated deposits of this nature in Malaysia and Indonesia.

Other sources of magnetite iron ore include metamorphic accumulations of massive magnetite ore such as at Savage River, Tasmania, formed by shearing of ophiolite ultramafics.

Another, minor, source of iron ores are magmatic accumulations in ultramafic to mafic layered intrusions which contain a typically titanium-bearing magnetite crystal rock (magnetitite) often with vanadium. These ores form a niche market, with specialty smelters used to recover the iron, titanium and vanadium. These ores are beneficiated essentially similar to banded iron formation ores, but usually are more easily upgraded via crushing and screening. The typical titanomagnetite concentrate grades 57% Fe, 12% Ti and 0.5% V2O5.

Hematite ore

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 BIF 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).

In Australia iron ore is won from three main sources: pisolite "channel iron deposit" ore derived by mechanical erosion of primary banded-iron formations and accumulated in alluvial channels such as at Pannawonica, Western Australia; and the dominant metasomatically-altered banded iron formation related ores such as at Newman, the Hamersley Range and Koolyanobbing, Western Australia. Other types of ore are coming to the fore recently, such as oxidised ferruginous hardcaps, for instance laterite iron ore deposits near Lake Argyle in Western Australia.

The total recoverable reserves of iron ore in India are about 9,602 million tones of hematite and 3,408 million tones of magnetite. Madhya Pradesh, Karnataka, Bihar, Orissa, Goa, Maharashtra, Andhra Pradesh, Kerala, Rajasthan and Tamil Nadu are the principal Indian producers of iron ore.

Consumption and economics

Iron is the world's most commonly used metal. It is used primarily in structural engineering applications and in maritime purposes, automobiles, and general industrial applications (machinery).

Iron-rich rocks are common worldwide, but ore-grade commercial mining operations are dominated by the countries listed in the table aside. The major constraint to economics for iron ore deposits is not necessarily the grade or size of the deposits, because it is not particularly hard to geologically prove enough tonnage of the rocks exist. The main constraint is the position of the iron ore relative to market, the cost of rail infrastructure to get it to market and the energy cost required to do so.

World production averages one billion metric tons of raw ore annually. The world's largest producer of iron ore is the Brazilian mining corporation CVRD, followed by Australian company BHP Billiton and the Anglo-Australian Rio Tinto Group. A further Australian supplier, Fortescue Metals Group Ltd, is currently entering the development stage and may eventually bring Australia's production to second in the world.

China is currently the largest consumer of iron ore, which translates to be the world's largest steel producing country. China is followed by Japan and Korea, which consume a significant amount of raw iron ore and metallurgical coal. In 2006, China produced 588 million tons of iron ore, with an annual growth of 38%.

This heap of iron ore pellets will be used in steel production.

Pure iron is virtually unknown on the surface of the Earth except as Fe-Ni alloys from meteorites and very rare forms of deep mantle xenoliths. Therefore, all sources of iron used by human industry exploit iron oxide minerals, the primary form which is used in industry being hematite.

However, in some situations, more inferior iron ore sources have been used by industrialized societies when access to high-grade hematite ore was not available. This has included utilisation of taconite in the United States, particularly during World War II, and goethite or bog ore used during the American Revolution and the Napoleonic wars. Magnetite is often used because it is magnetic and hence easily liberated from the gangue minerals.

Inferior sources of iron ore generally required beneficiation. Due to the high density of hematite relative to silicates, beneficiation usually involves a combination of crushing and milling as well as heavy liquid separation. This is achieved by passing the finely crushed ore over a bath of solution containing bentonite or other agent which increases the density of the solution. When the density of the solution is properly calibrated, the hematite will sink and the silicate mineral fragments will float and can be removed.

Taconite mining involves moving tremendous amounts of ore and waste. The waste comes in two forms, bedrock in the mine (mullock) that isn't ore, and unwanted minerals which are an intrinsic part of the ore rock itself (gangue). The mullock is mined and piled in waste dumps, and the gangue is separated during the beneficiation process and is removed as tailings. Taconite tailings are mostly the mineral quartz, which is chemically inert. This material is stored in large, regulated water settling ponds.

Magnetite is beneficiated by crushing and then separating the magnetite from the gangue minerals with a magnet. This is usually so efficient that lower grade ore can be treated when it is magnetite than a comparable grade of hematite ore, especially when the magnetite is quite coarse.

To convert an oxide of iron to metallic iron it must be smelted or sent through a direct reduction process.

Smelting

Iron ore consists of oxygen and iron atoms bonded together into molecules. To create pure iron, the ore must be smelted to remove the oxygen. Oxygen-iron bonds are strong, and to remove the iron from the oxygen, a stronger elemental bond must be presented to attach to the oxygen. Carbon is used because the strength of a carbon-oxygen bond is greater than that of the iron-oxygen bond, at high temperatures. Thus, the iron ore must be powdered and mixed with coke, to be burnt in the smelting process.

However, this is not entirely as simple as that; carbon monoxide is the primary ingredient of chemically stripping oxygen from iron. Thus, the iron and carbon smelting must be kept at an oxygen deficient reduced state to promote burning of carbon to produce CO not CO₂.

Air blast and charcoal (coke): 2C + O₂ $\to$ 2CO.

Carbon monoxide (CO) is the principal reduction agent.

Stage One: 3Fe₂ O₃ + CO $\to$ 2Fe₃ O₄ + CO₂

Stage Two: Fe₃ O₄ + CO $\to$ 3Fe O + CO₂

Stage Three: FeO + CO $\to$ Fe + CO₂

Limestone fluxing chemistry: CaCO₃ $\to$ CaO + CO₂

Contaminants

Ideally iron ore contains only iron and oxygen. In nature this is rarely the case. Typically, iron ore contains a host of deleterious elements which are unwanted in modern steel.

Silica

Iron ore typically contains silicates, usually in the form of quartz. Silica is undesirable because silicon does not bond with carbon during the smelting process and can remain in the iron after it is refined. Historically, siliceous iron ore created wrought iron, a malleable and strong form of iron used by blacksmiths throughout history.
Modern steelmaking techniques generally use lime and other fluxes to help remove the silica from the molten iron ore, and form a slag on the surface of the molten metal. This slag can then be removed.

Phosphorus

Phosphorus is a deleterious contaminant because it makes steel brittle, even at concentrations of as little as 0.5%. Phosphorus cannot be easily removed by fluxing or smelting, and so iron ores must generally be low in phosphorus to begin with. The iron pillar of India which does not rust is protected by a phosphoric composition. Phosphoric acid is used at a rust converter because phosphoric iron is less susceptible to oxidation.

Aluminium

Aluminium is generally present in iron ores as clay. This is usually removed by washing the iron ore, and by fluxing. However, again, iron oxide deposits must be relatively low in aluminium in order to be considered ore.

Sulfur

Sulfur is unwanted because it produces undesirable sulfur dioxide gases in the flue emissions from a smelter and interferes with the smelting process.

External links

• Abandoned Iron Mine Photos and History
• History of the Iron Ore Trade on the Great Lakes
• United States Colonial Iron Ore Industry
• Australia's Largest Iron Ore Tenement Holder
• Karara Magnetite Deposit, Western Australia, Gindalbie Metals Australia Website
• Southdown Magnetite Deposit, Western Australia, Resource Overview - Grange Resources Ltd Website
• Scunthorpe grew after the rediscovery of large ironstone deposits