Iron ore

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 steel-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. Iron ore cargoes may affect magnetic compasses. Normally, loading rates are very high, preplanning of ballasting operation is essential.

Mining

Modern mines, in Minnesota and Michigan in the US, and Eastern Canada mine taconite. Taconite is a term to describe magnetite ore that is very hard and fine-grained. Taconite must be first processed into iron oxide pellets by crushing and magnetic separation of the magnetite to concentrate the iron to about 66% before it can be used in steel-making.

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.

In Australia iron ore is won from three main sources; massive magnetite ore as at Savage River, Tasmania; pisolite "channel-iron" ore derived by mechanical erosion of primary banded-iron formations and accumulated in alluvial channels such at Robe River, Western Australia; and banded iron formation related ores such as at Newman, Hamersley and Koolyanobbing, Western Australia. Other types of ore are coming to the fore recently, such as oxidised ferruginous hardcaps, for instance iron ore deposits near Argyle, Western Australia.

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 companies BHP Billiton and Rio Tinto Group. China is currently the largest consumer of iron ore, which translates to be the world's largest steel producing country. Iron ore is common worldwide, but commercial mining operations are dominated by the countries listed below:

Estimated iron ore production in million tons for 2004, according to U.S. Geological Survey

{| border="1" cellpadding="2" cellspacing="0" align="center"

- Country Production

- China 280

- Australia 220

- Brazil 220

- India 110

- Russia 95

- Ukraine 66

- United States 54

- South Africa 40

- Canada 31

- Sweden 22

- Venezuela 18

- Kazakhstan 17

- Iran 16

- Mauritania 10

- Other countries 40

- Total world 1250

Refining

Pure elemental iron is virtually unknown on the surface of the Earth except as wüstite 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 Revolutionary war 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.

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.

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 principle reduction agent.

Stage One: 3Fe₂ O₃ + CO

\to

2Fe₃ O₄ + CO₂

Stage Two: 2Fe₃ 2(O₂) + 2CO

\to

6Fe O + 2CO₂

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 metal 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, however, 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.

Steel is produced by precise addition of carbon to iron. Steel is thus an alloy.

External links

References

Economic geology | Iron | Resource extraction