Iron oresRamanaidou and Wells, 2014 are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the form of magnetite (, 72.4% Fe), hematite (, 69.9% Fe), goethite (, 62.9% Fe), limonite (, 55% Fe), or siderite (, 48.2% Fe).
Ores containing very high quantities of hematite or magnetite (typically greater than about 60% iron) are known as natural ore or direct shipping ore, and can be fed directly into iron-making blast furnaces. Iron ore is the raw material used to make pig iron, which is one of the primary raw materials to make steel — 98% of the mined iron ore is used to make steel. In 2011 the Financial Times quoted Christopher LaFemina, mining analyst at Barclays Capital, saying that iron ore is "more integral to the global economy than any other commodity, except perhaps oil."
Sources
Elemental iron is virtually absent on the Earth's surface except as iron-nickel alloys from meteorites and sporadic forms of deep mantle xenoliths. Although iron is the fourth most abundant element in Earth's crust, composing about 5% by weight, the vast majority is bound in silicate or, more rarely, carbonate minerals, and smelting pure iron from these minerals would require a prohibitive amount of energy. Therefore, all sources of iron used by human industry exploit comparatively rarer iron oxide minerals, primarily hematite.
Prehistoric societies used laterite as a source of iron ore. Before the industrial revolution, most iron was obtained from widely available goethite or bog ore, for example, during the American Revolution and the Napoleonic Wars. Historically, much of the iron ore used by industrialized societies has been mined from predominantly hematite deposits with grades of around 70% Fe. These deposits are commonly referred to as "direct shipping ores" or "natural ores". Increasing iron ore demand, coupled with the depletion of high-grade hematite ores in the United States, led after World War II to the development of lower-grade iron ore sources, principally the use of magnetite and taconite.
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, hematite, and pisolitic ironstone deposits.
The origin of iron can be ultimately traced to its formation through nuclear fusion in stars. Most of the iron is thought to have originated in dying stars that are large enough to explode as supernovae. The Earth's core is thought to consist mainly of iron, but this is inaccessible from the surface. Some iron meteorites are thought to have originated from asteroids in diameter or larger.
Banded iron formations
Banded iron formations (BIFs) are sedimentary rocks containing more than 15% iron composed predominantly of thinly-bedded iron minerals and silica (as quartz). Banded iron formations occur exclusively in Precambrian rocks, and are commonly weakly-to-intensely metamorphosed. Banded iron formations may contain iron in carbonates (siderite or ankerite) or silicates (minnesotaite, greenalite, or grunerite), but in those mined as iron ores, oxides (magnetite or hematite) are the principal iron mineral.Harry Klemic, Harold L. James, and G. Donald Eberlein, (1973) "Iron," in United States Mineral Resources, US Geological Survey, Professional Paper 820, p.298-299. Banded iron formations are known as taconite within North America.
The mining involves moving tremendous amounts of ore and waste. The waste comes in two forms: non-ore bedrock in the mine (overburden or interburden locally known as mullock), 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 ores The key parameters for magnetite ore being economic are the crystallinity of the magnetite, the grade of the iron within the banded iron formation host rock, and the contaminant elements which exist within the magnetite concentrate. The size and strip ratio of most magnetite resources are irrelevant, as a banded iron formation can be hundreds of meters thick, extend hundreds of kilometers along strike, and can easily come to more than three billion or more tonnes of contained ore.
The typical grade of iron at which a magnetite-bearing banded iron formation becomes economic is roughly 25% iron, which can generally yield a 33% to 40% recovery of magnetite by weight, to produce a concentrate grading over 64% iron by weight. The typical magnetite iron ore concentrate has less than 0.1% phosphorus, 3–7% silica, and less than 3% aluminium.
As of 2019, magnetite iron ore is mined in Minnesota and Michigan in the United States, eastern Canada, and northern Sweden. Magnetite-bearing banded iron formation is mined extensively in Brazil as of 2019, which exports significant quantities to Asia, and there is a nascent and large magnetite iron ore industry in Australia.
Direct-shipping (hematite) ores Direct-shipping iron ore (DSO) deposits (typically composed of hematite) are currently exploited on all continents except Antarctica, with the largest intensity in South America, Australia, and Asia. Most large hematite iron ore deposits are sourced from altered banded iron formations and (rarely) igneous accumulations.
DSO deposits are typically rarer than the magnetite-bearing BIF or other rocks which form their primary source, or protolith rock, but are considerably cheaper to mine and process as they require less beneficiation due to the higher iron content. However, DSO ores can contain significantly higher concentrations of penalty elements, typically being higher in phosphorus, water content (especially pisolite sedimentary accumulations), and aluminium (clays within pisolites). Export-grade DSO ores are generally in the 62–64% Fe range.
Magmatic magnetite ore deposits Granite and ultrapotassic igneous rocks were sometimes used to segregate magnetite crystals and form masses of magnetite suitable for economic concentration. A few iron ore deposits, notably in Chile, are formed from volcanic flows containing significant accumulations of magnetite phenocrysts.
Mine tailings For every one ton of iron ore concentrate produced, approximately 2.5–3.0 tons of iron ore tailings will be discharged. Statistics show that there are 130 million tons of iron ore tailings discharged every year. If, for example, the mine tailings contain an average of approximately 11% iron, there would be approximately 1.41 million tons of iron wasted annually. These tailings are also high in other useful metals such as copper, nickel, and cobalt, and they can be used for road-building materials like pavement and filler and building materials such as cement, low-grade glass, and wall materials. While tailings are a relatively low-grade ore, they are also inexpensive to collect, as they do not have to be mined. Because of this, companies such as Magnetation have started reclamation projects where they use iron ore tailings as a source of metallic iron.
The two main methods of recycling iron from iron ore tailings are magnetizing roasting and direct reduction. Magnetizing roasting uses temperatures between for a time of under 1 hour to produce an iron concentrate (Fe3O4) to be used for iron smelting. For magnetizing roasting, it is important to have a reducing atmosphere to prevent oxidization and the formation of Fe2O3 because it is harder to separate as it is less magnetic. Direct reduction uses hotter temperatures of over and longer times of 2–5 hours. Direct reduction is used to produce sponge iron (Fe) to be used for steel-making. Direct reduction requires more energy, as the temperatures are higher and the time is longer, and it requires more reducing agent than magnetizing roasting.H.T. Shen, B. Zhou, et al. "Roasting-magnetic separation and direct reduction of a refractory oolitic-hematite ore" Min. Met. Eng., 28 (2008), pp. 30-43
Extraction
Lower-grade sources of iron ore generally require beneficiation, using techniques like crushing, milling, gravity or heavy media separation, screening, and silica froth flotation to improve the concentration of the ore and remove impurities. The results, high-quality fine ore powders, are known as fines.
Magnetite Magnetite is magnetic, and hence easily separated from the gangue minerals and capable of producing a high-grade concentrate with very low levels of impurities.
The grain size of the magnetite and its degree of commingling 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.
Mining of banded iron formations involves coarse crushing and screening, followed by rough crushing and fine grinding to comminute the ore to the point where the crystallized magnetite and quartz are fine enough that the quartz is left behind when the resultant powder is passed under a magnetic separator.
Generally, most magnetite banded iron formation deposits must be ground to between to produce a low-silica magnetite concentrate. Magnetite concentrate grades are generally more than 70% iron by weight and usually are low in phosphorus, aluminium, titanium, and silica, and demand a premium price.
Hematite Due to the high density of hematite relative to associated silicate gangue, hematite beneficiation usually involves a combination of beneficiation techniques. One method relies on passing the finely-crushed ore over a slurry containing magnetite or other agent such as ferrosilicon, which increases its density. When the density of the slurry is calibrated correctly, the hematite will sink and the silicate mineral fragments will float and can be removed.Gaudin, A.M, Principles of Mineral Dressing, 1937
Production and consumption
+Usable iron ore production in metric tons for 2015 Country Production Australia 817,000,000 Brazil 397,000,000 China 375,000,000 India 156,000,000 Russia 101,000,000 South Africa 73,000,000 Ukraine 67,000,000 United States 46,000,000 Canada 46,000,000 Iran 27,000,000 Sweden 25,000,000 Kazakhstan 21,000,000 Other countries 132,000,000 Total world 2,280,000,000
Iron ore represents 93% of metals mined worldwide in 2021. Steel, of which iron is the key ingredient, represents almost 95% of all metal used per year.Iron ore pricing emerges from stone age, Financial Times, October 26, 2009
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 exists. The primary constraint is the position of the iron ore relative to the market, the cost of rail infrastructure to get it to market, and the energy cost required to do so.
Mining iron ore is a high-volume, low-margin business, as the value of iron is significantly lower than that of base metals.Iron ore pricing war, Financial Times, October 14, 2009 It is highly capital-intensive and requires significant investment in infrastructure, such as rail, to transport the ore from the mine to a freight ship. For these reasons, iron ore production is concentrated in the hands of a few major players.