Encyclopedia
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 ,
hematite ,
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
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.
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. Iron-rich rocks are common worldwide, but ore-grade 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
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.
Taconite mining involves moving tremendous amounts of ore and waste. The waste comes in two forms, bedrock in the mine that isn't ore, and unwanted minerals which are an intrinsic part of the ore rock itself . 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.
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 CO
2.
- Air blast and charcoal : 2C + O2 2CO.
- Carbon monoxide is the principle reduction agent.
Stage One: 3Fe
2 O
3 + CO 2Fe
3 O
4 + CO
2Stage Two: Fe
3 O
4 + CO 3Fe O + CO
2Stage Three: FeO + CO Fe + CO
2- Limestone fluxing chemistry: CaCO3 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.
SilicaIron 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.
PhosphorusPhosphorus 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.
AluminiumAluminium 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.
SulfurSulfur is unwanted because it produces undesirable
sulfur dioxide gases in the flue emissions from a smelter and interferes with the smelting process.
See also
External links
References
