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Iron oresRamanaidou and Wells, 2014 are rocks and from which iron can be economically extracted. The are usually rich in 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, and can be fed directly into iron-making . 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 Petroleum".
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 utilized 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 pisolite 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 . The Earth's core is thought to consist mainly of iron, but this is inaccessible from the surface. Some are thought to have originated from M-type asteroid in diameter or larger.
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 Overburden, 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.
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.
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 (Clay mineral within pisolites). Export-grade DSO ores are generally in the 62–64% Fe range.
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
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 Comminution 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.
| +Usable iron ore production in metric tons for 2015 |
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.
World production averages of raw ore annually. The world's largest producer of iron ore is the Brazilian mining corporation Vale, followed by Australian companies Rio Tinto and BHP. A further Australian supplier, Fortescue, has helped bring Australia's production to first in the world.
The seaborne trade in iron ore—that is, iron ore to be shipped to other countries—was in 2004. Australia and Brazil dominate the seaborne trade, with 72% of the market. BHP, Rio and Vale control 66% of this market between them.
In Australia, iron ore is won from three primary sources: pisolite "channel iron deposit" ore derived by mechanical erosion of primary banded-iron formations and accumulated in alluvial channels such as at Pannawonica; and the dominant metasomatically altered banded iron formation-related ores such as at Newman, the Chichester Range, 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 of hematite and of magnetite. Chhattisgarh, Madhya Pradesh, Karnataka, Jharkhand, Odisha, Goa, Maharashtra, Andhra Pradesh, Kerala, Rajasthan, and Tamil Nadu are the principal Indian producers of iron ore. World consumption of iron ore grows 10% per year on average, with the main consumers being China, Japan, Korea, the United States, and the European Union.
China is currently the largest consumer of iron ore, which translates to being the world's largest steel-producing country. It is also the largest importer, buying 52% of the seaborne trade in iron ore in 2004. China is followed by Japan and Korea, which consume a significant amount of raw iron ore and metallurgical coal. In 2006, China produced of iron ore, with an annual growth of 38%.
]] Over the last 40 years, iron ore prices have been decided in closed-door negotiations between the small handful of miners and Steelmaking, which dominate both spot and contract markets. Until 2006, prices were determined in annual benchmark negotiations between the leading iron ore producers (BHP, Rio Tinto, and Vale) and Japanese importers. In 2006, Chinese company Baosteel began handling negotiations for the importer side. The Chinese government replaced Baosteel with China Iron and Steel Association as lead negotiator in 2009. Traditionally, the first deal reached between the major producers and the major importers sets a benchmark to be followed by the rest of the industry.
Singapore Mercantile Exchange (SMX) has launched the world's first global iron ore futures contract, based on the Metal Bulletin Iron Ore Index (MBIOI) which uses daily price data from a broad spectrum of industry participants and independent Chinese steel consultancy and data provider Shanghai Steelhome's widespread contact base of steel producers and iron ore traders across China. The futures contract has seen monthly volumes over after eight months of trading.
This move follows a switch to index-based quarterly pricing by the world's three largest iron ore miners—Vale, Rio Tinto, and BHP—in early 2010, breaking a 40-year tradition of benchmark annual pricing.
The Brazilian production is expected to rise by a CAGR of 2% between 2023 and 2027, and industry analyst Fitch Solutions forecasted in 2021 that Brazil's annual production will reach by 2030.
India's iron ore production in 2023 was 285,000,000 metric tonnes and was the fourth largest producer in the world.
Carbon monoxide is the primary ingredient of chemically stripping oxygen from iron. Thus, the iron and carbon smelting must be kept in an oxygen-deficient (reducing) state to promote the burning of carbon to produce and not .
The significant effect of silicon is to promote the formation of grey iron. Grey iron is less brittle and easier to finish than white iron. It is preferred for casting purposes for this reason. British metallurgist Thomas Turner reported that silicon also reduces shrinkage and the formation of blowholes, lowering the number of bad castings. However, too much silicon present in the iron leads to increased brittleness and moderate hardness.
The strength and hardness of iron increase with the concentration of phosphorus. 0.05% phosphorus in wrought iron makes it as hard as medium-carbon steel. High-phosphorus iron can also be hardened by cold hammering. The hardening effect is true for any concentration of phosphorus. The more phosphorus, the harder the iron becomes, and the more it can be hardened by hammering. Modern steel makers can increase hardness by as much as 30%, without sacrificing shock resistance by maintaining phosphorus levels between 0.07 and 0.12%. It also increases the depth of hardening due to quenching, but at the same time, it also decreases the solubility of carbon in iron at high temperatures. This would reduce its usefulness in making blister steel (cementation), where the speed and amount of carbon absorption are the overriding considerations.
The addition of phosphorus has a downside. At concentrations higher than 0.2%, iron becomes increasingly cold short or brittle at low temperatures. Cold short is especially important for bar iron. Although bar iron is usually worked hot, its usesThis is an example. often require it to be tough, bendable, and resistant to shock at room temperature. A nail that shatters when hit with a hammer or a carriage wheel that breaks when it hits a rock would not sell well. High enough concentrations of phosphorus render any iron unusable. The effects of cold shortness are magnified by temperature. Thus, a piece of iron that is perfectly serviceable in summer might become extremely brittle in winter. There is some evidence that during the Middle Ages the very wealthy may have had a high-phosphorus sword for summer and a low-phosphorus sword for winter.
Careful control of phosphorus can be of great benefit in casting operations. Phosphorus depresses the liquidus, allowing the iron to remain molten for longer and increasing fluidity. The addition of 1% can double the distance molten iron will flow. The maximum effect, about , is achieved at a concentration of 10.2%. For foundry work, Turner felt the ideal iron had 0.2–0.55% phosphorus. The resulting iron-filled molds with fewer voids also shrank less. In the 19th century, some producers of decorative cast iron used iron with up to 5% phosphorus. The extreme fluidity allowed them to make very complex and delicate castings, but they could not be weight-bearing, as they had no strength.
There are two remedies for high-phosphorus iron. The oldest, easiest, and cheapest is avoidance. If the iron that the ore produced was cold short, one would search for a new source of iron ore. The second method involves oxidizing the phosphorus during the fining process by adding iron oxide. This technique is usually associated with puddling in the 19th century, and may not have been understood earlier. For instance, Isaac Zane, owner of Marlboro Iron Works, did not appear to know about it in 1772. Given Zane's reputation for keeping abreast of the latest developments, the technique was probably unknown to the ironmasters of Virginia and Pennsylvania.
Phosphorus is generally considered to be a deleterious contaminant because it makes steel brittle, even at concentrations of as little as 0.6%. When the Gilchrist–Thomas process allowed the removal of bulk amounts of the element from cast iron in the 1870s, it was a significant development because most of the iron ores mined in continental Europe at the time were phosphorus. However, removing all the contaminants by fluxing or smelting is complicated, and so desirable iron ores must generally be low in phosphorus to begin with.
Aluminium is difficult to reduce. As a result, aluminium contamination of the iron is not a problem. However, it does increase the viscosity of the slag. This will have several adverse effects on furnace operation. The thicker slag will slow the descent of the charge, prolonging the process. High aluminium will also make it more difficult to tap off the liquid slag. At the extreme, this could lead to a frozen furnace.
There are a number of solutions to a high-aluminium slag. The first is avoidance; do not use ore or a lime source with a high aluminium content. Increasing the ratio of lime flux will decrease the viscosity.
Hot short iron is brittle when hot. This was a serious problem, as most iron used during the 17th and 18th centuries was bar or wrought iron. Wrought iron is shaped by repeated blows with a hammer while hot. A piece of hot, short iron will crack if worked with a hammer. When a piece of hot iron or steel cracks, the exposed surface immediately oxidizes. This layer of oxide prevents the mending of the crack by welding. Large cracks cause the iron or steel to break up. Smaller cracks can cause the object to fail during use. The degree of hot shortness is in direct proportion to the amount of sulfur present. Today, iron with over 0.03% sulfur is avoided.
Hot short iron can be worked, but it must be worked at low temperatures. Working at lower temperatures requires more physical effort from the smith or forgeman. The metal must be struck more often and harder to achieve the same result. A mildly sulfur-contaminated bar can be worked, but it requires a great deal more time and effort.
In cast iron, sulfur promotes the formation of white iron. As little as 0.5% can counteract the effects of slow cooling and a high silicon content. White cast iron is more brittle, but also harder. It is generally avoided because it is difficult to work, except in China, where high-sulfur cast iron, some as high as 0.57%, made with coal and coke, was used to make bells and chimes. According to , good foundry iron should have less than 0.15% sulfur. In the rest of the world, a high-sulfur cast iron can be used for making castings, but it will make poor wrought iron.
There are a number of remedies for sulfur contamination. The first, and the one most used in historic and prehistoric operations, is avoidance. Coal was not used in Europe (unlike China) as a fuel for smelting because it contains sulfur and therefore causes hot short iron. If an ore resulted in hot short metal, looked for another ore. When mineral coal was first used in European blast furnaces in 1709 (or perhaps earlier), it was coked. Only with the introduction of hot blast from 1829 was raw coal used.
The importance attached to low-sulfur iron is demonstrated by the consistently higher prices paid for the iron of Sweden, Russia, and Spain from the 16th to 18th centuries. Today, sulfur is no longer a problem. The modern remedy is the addition of manganese, but the operator must know how much sulfur is in the iron because at least five times as much manganese must be added to neutralize it. Some historic irons display manganese levels, but most are well below the level needed to neutralize sulfur.
Sulfide inclusion as manganese sulfide (MnS) can also be the cause of severe pitting corrosion problems in low-grade stainless steel such as AISI 304 steel. Under oxidizing conditions and in the presence of moisture, when sulfide oxidizes, it produces thiosulfate anions as intermediate species, and because the thiosulfate anion has a higher equivalent electromobility than the chloride anion due to its double negative electrical charge, it promotes pit growth. Indeed, the positive electrical charges born by Fe2+ cations released in solution by Fe oxidation on the Anode zone inside the pit must be quickly compensated/neutralized by negative charges brought by the electrokinetic migration of anions in the capillary pit. Some of the Electrochemistry processes occurring in a capillary pit are the same as those encountered in capillary electrophoresis. The higher the anion electrokinetic migration rate, the higher the rate of pitting corrosion. Electrokinetic transport of ions inside the pit can be the rate-limiting step in the pit growth rate.
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