Iron ore
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Iron ores are rocks and minerals from which metallic iron can be 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.[1] Iron ore cargoes may affect magnetic compasses.
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[edit] Mining
| Estimated iron ore production in million tons for 2004 according to U.S. Geological Survey[2]
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Iron ore types and mining methods vary geographically.
Modern mines, in Minnesota and Michigan in the U.S., 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.
In Australia iron ore is won from three main sources: massive magnetite ore such as at Savage River, Tasmania; pisolite "channel-iron" ore derived by mechanical erosion of primary banded-iron formations and accumulated in alluvial channels such as at Robe River, Western Australia; and the dominant 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 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 producers of iron ore in the country.
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. In 2006, China produced 588 million ton of iron ore,with an anual growth of 38%. But China is still the world biggest iron ore imported country. Iron-rich rocks are common worldwide, but ore-grade commercial mining operations are dominated by the countries listed in the table aside.
Iron is the world's most commonly used metal.
Pure 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 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.
[edit] 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
2CO. - Carbon monoxide (CO) is the principal reduction agent.
- Stage One: 3Fe2 O3 + CO 2Fe3 O4 + CO2
- Stage Two: Fe3 O4 + CO 3Fe O + CO2
- Stage Three: FeO + CO Fe + CO2
- Stage One: 3Fe2 O3 + CO
- Limestone fluxing chemistry: CaCO3 CaO + CO2
[edit] 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.
[edit] 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
[edit] Steel
About 98% of iron ore is used to make steel, which is iron that has had the carbon pounded out of it or excluded from it in another way. Iron is not as strong as steel, so that is why so much of it is turned into steel. Steel is very useful. Before steel was used to make railroads, train wheels would get snagged on broken Iron that led to horrible accidents. Steel was also used in constroction of the Sears tower, the Gateway Arch, and the Empire State Building.
[edit] References
- ^ IRON ORE - Hematite, Magnetite & Taconite. Mineral Information Institute. Retrieved on April 7, 2006.
- ^ U.S. Geological Survey. Retrieved on March 7, 2006.
