Iron (Fe)

Where Found

Iron is one of the most abundant metals in the world, constituting 35 percent of the entire planet and 5 percent of the Earth’s crust. It combines with other elements in hundreds of minerals, the most important of which are hematite and magnetite. Top producers of iron ore include Australia, Brazil, China, India, and Russia.

Primary Uses

Iron and its principal alloy, steel, are widely used in tools, machines, and structures. Historically, discoveries and inventions involving the many uses of iron have been crucially important. Iron is also essential to biological metabolism.

Technical Definition

Iron is a chemical (symbol Fe, from the Latin ferrum) and a metal of the transition Group VIII on the periodic table. Its atomic number is 26 and its atomic weight 55.487. Iron’s melting point is 1,535° Celsius, its boiling point 3,000° Celsius, and its 7.86 grams per cubic centimeter.

Description, Distribution, and Forms

Iron is the cheapest and most widely used metal in the world. It is used in three main products: wrought iron, steel, and cast iron. Although each is approximately 95 percent iron and is produced with the same fuel, each has vastly different properties, arising from different production methods. Wrought iron, containing negligible amounts of carbon, has a melting point so high that it was not achieved by humans until the nineteenth century. When hot, wrought iron can be forged and welded, and even when it is cold it is ductile—capable of being shaped and hammered. Steel contains 0.25 to 1.25 percent carbon, with a lower melting point than wrought iron. It can be forged when hot and is extremely hard when quenched (cooled quickly by plunging into water or another cooling medium). Cast iron, with approximately 2 to 4.5 percent carbon, is easily melted and poured into molds. When cool, it is soft and easily machined, but it is brittle and does not withstand tension forces well.

The principal iron ores are hematite, magnetite, limonite, pyrite, siderite, and taconite. Hematite and magnetite are the richest and most common ores. They are known as iron oxides because they are compounds of iron and oxygen.

Hematite (Fe₂O₃) can contain as much as 70 percent iron but usually contains closer to 25 percent. Significant deposits are found near Lake Superior, and in Alabama, Australia, Belgium, and Sweden. It may appear in colors ranging from black to dark red and may occur as shiny crystals, grains of rock, or loose particles.

Magnetite (Fe₃O₄) is a black magnetic material often called black sand. Limonite (2Fe₂O₃3H₂O), or brown hematite, is a hydrated variety of hematite; it is also called bog-iron ore. It can contain as much as 60 percent iron and is yellowish to brown. It is found in Australia, France, Germany, the former Soviet Union, Spain, and the United States.

Pyrite (FeS₂), also called fool’s gold because of its shiny yellowish surface, is about half sulfur. Siderite (FeCO₃) is a gray-brown carbonate ore that was once found in large deposits in Great Britain and Germany. Taconite is a hard that contains specks or bands of either hematite or magnetite.

History

Iron was probably discovered accidentally in the late Bronze Age when it was found in the ashes of fires that had been built on top of red iron ore. Artifacts of iron weapons and tools have been found in Egypt (including the Great Pyramid of Giza) dating to 2900 B.C.E. Iron has probably been made regularly since at least 1000 B.C.E. The Chinese had independently developed their own furnaces and techniques for producing cast iron by the sixth century B.C.E. The Romans acquired ironworking technology from the Greeks and spread it throughout northern Europe. Because iron ore was readily available throughout the Near East and Europe, iron was less expensive than copper and bronze, the “metals of aristocracy.” As a result, it was used to make many everyday tools and utensils, earning its later nickname, “the democratic metal.”

Through the Middle Ages, the common method of producing iron was the bloomery method. A bloomery may have been as simple as a circular hollow in the ground, several meters deep and several meters across. The iron ore was heated in a bed of burning charcoal within this hollow, often with the use of bellows to increase the fire’s temperature. As the heat reached about 800° Celsius (normally the highest temperature attainable in early bloomeries), the oxygen in the ore separated from the iron and combined with carbon to form slag. The iron changed to a pasty mass called the “bloom.” The operator removed the bloom when he judged it was ready and alternately hammered and reheated it to remove the slag and to consolidate the iron. The final product was wrought iron, produced at temperatures below iron’s melting point, a process referred to as the “direct” method. Sometimes the iron would accidentally melt in the bloomery; this was undesirable because prolonged exposure allowed the iron to absorb carbon from the charcoal, creating cast iron. Because of its lack of ductility and low resistance to abrasion, cast iron was unsuitable for working into tools and weapons and was therefore considered worthless.

The major limitation of the bloomery was its low volume of output per unit of labor. Even when bloomery technology had fully matured, a large bloom might weigh only 90 kilograms, and the annual output of that bloomery would probably have been less than 20 metric tons of wrought iron. To increase output, the blast furnace was developed (by building up the walls of the bloomery, according to some sources). This technology was so successful that by the middle of the sixteenth century, the blast furnace had replaced the bloomery as the prevalent method of iron production.

Early blast furnaces stood about 4.5 meters high, later reaching 10 meters or more. The use of coke—made by heating coal in an airtight container to drive out gases and tar—as a fuel, beginning in the early 1700s, allowed taller furnaces, since it did not crush as easily as charcoal and could be stacked higher. The interior cavity widened as it descended from the top opening for about two-thirds of the furnace’s height. At that point, the cavity began to narrow, culminating in a chamber at the very bottom of the furnace, called the crucible.

The structure of the furnace created a chimney effect, drafting air through it to accelerate combustion; waterwheel-powered bellows usually supplemented the draft. The ore, charcoal, and (a flux) were dumped into the blast furnace from above. As the ore melted and the level of raw materials dropped, more would be added on top of them. In this way, it was possible to keep a furnace in continuous operation for months at a time. As the ore slowly worked its way toward the crucible, it was exposed for a prolonged period to heat, which melted it (at about 1,400° Celsius), and carbon, which it absorbed. The molten iron collected in the crucible, and the slag, floating on top of the iron, was pulled off through side openings. The end product was a large volume of molten iron with a high carbon content—cast iron.

The molten iron could be tapped directly from the crucible. Some of it would be poured into oblong molds pressed into damp sand. These molds were usually laid out with several smaller molds attached at right angles to the largest mold, reminding the ironworkers of a sow and suckling pigs—hence the term “pig iron.” The pig iron would later be converted to wrought iron at a forge. The molten iron might also have been cast directly into molds for stove and fireplace parts, pots and pans, cannons, cannonballs, and many other products. In the nineteenth century, cast iron was also used for machine parts, railroad tracks, and structural elements. By that time, cast iron had found many uses, and the demand for iron products increased dramatically.

A blast furnace could produce, typically, 180 metric tons of iron per year—a tenfold increase over the bloomeries. In producing a larger output for less labor, however, a trade-off was necessary: the addition of another step in the process. To create wrought iron—the most desirable iron product until the late nineteenth century—from the cast iron coming from the blast furnace, the carbon had to be removed. This was done in a refinery hearth in which the bloom was heated indirectly without coming in contact with the fuel. In this way, the carbon already present burned off, and no additional carbon was absorbed from the fuel. Despite this added step, the blast furnace produced a much larger volume of iron, and for less labor, than previous methods had. As a result, the development of the blast furnace was the key to making iron products much more common beginning in the fifteenth century.

Even with the blast furnace, the production of good wrought iron was limited by the use of coke. Coke introduced more impurities to the cast iron than charcoal had, making it more difficult to produce high-quality wrought iron. In 1784, an Englishman, Henry Cort, devised a new process to address this problem. Known as the “puddling process,” it began by heating the pig iron in a coke-fired reverberatory furnace (one in which the heat was reflected off the roof of the furnace to keep the iron from coming in contact with the coke). Workers stirred the molten metal to expose more of it to the air, thus burning off carbon. As the carbon content decreased, the melting temperature increased, and the metal gradually stiffened, separating it from the more liquid slag. When the process was complete, workers gathered the low-carbon iron in a “puddle ball” and shaped it in a rolling mill. Thanks to Cort’s puddling process, wrought iron became an important factor in the Industrial Revolution. Its dominance of the iron market lasted until the 1860s, when steel production began on a large scale via the Bessemer process.

Obtaining Iron Ore

An ore’s quality for commercial purposes depends on several factors. While a pure ore may contain as much as 70 percent iron, ores are seldom found in their pure state. It is more realistic to expect a 50 to 60 percent iron content. At less than 30 percent, an ore is probably uneconomical. Other factors in determining an ore’s quality include the amount of constituents such as silicon and phosphorus in the ore, the geographical location of the ore, and the ease with which it can be extracted and processed.

In prehistoric times iron ore was probably gathered from meteorites, high-grade outcroppings, and other sources that required little or no work to extract. As the demand increased and those sources were exhausted, mining techniques had to be developed to extract iron ore from the Earth.

Most iron ore is obtained either by the process or by hard-rock shaft mining. Open-pit mining is employed when the ore is lying near the surface. Large machinery removes the overlying soil and rocks (called overburden) to expose the ore. It is then broken up with explosives and loaded onto a transportation system (usually large earth-moving trucks) by huge power shovels. As the process continues, the equipment digs deep into the earth, creating a large pit often several square kilometers in area and 150 meters or more deep. Most of the world’s iron ore is mined in this way.

Ore that lies deep below the surface is removed via the more traditional hard-rock shaft mining. A shaft is sunk near the deposit from which tunnels and additional shafts branch out into the deposit. Shaft mining is much more expensive and dangerous than open-pit mining and is normally used only for very high-grade ore that cannot be reached in any other way.

All ores must be processed before being sent to the blast furnace; the ore’s quality and iron content determine the degree and type of processing needed. At a minimum, ore must be crushed, screened, and washed prior to reducing in a blast furnace.

In the screening process, ore is separated into lumps that are large enough to be put into the blast furnace (7 to 25 millimeters across) and smaller particles called fines. Fines are not suitable for use in a blast furnace because the particles will pack together and hinder the efficient flow of hot gases. To correct this, a process called sintering is used to make larger particles out of the fines. Sintering begins by moistening the fines to make particles stick together. Coke is then added to the mixture. After passing under burners, the coke ignites, heating the fines until they fuse into larger particles suitable for use in the blast furnace.

As the best ore deposits become exhausted (or become uneconomical to mine because of their inaccessibility), methods of upgrading low-quality ore become necessary. Collectively, these processes are known as beneficiation. The first step in beneficiation is to concentrate the ore by one of several techniques. The general objective is to concentrate the iron and remove the silica. Most techniques rely on the difference between the density of iron and that of the surrounding rock to separate the two materials. Ore might be leached and dried, pulverized and floated in a mixture of oil, agglomerated into larger particles, or separated magnetically. Concentrating the ore by these techniques reduces both the shipping costs and the amount of waste at the blast furnace plant.

After beneficiation, the concentrated ore is a very fine powder that would not work properly in a blast furnace. Since the concentrate is too small even for sintering, the pelletizing process is used. In pelletizing, the concentrate is moistened and tumbled in a drum or on an inclined disk, and the resulting balls of ore are fired to a temperature of about 1,300° Celsius to dry and harden them. These pellets are usually about 10 to 15 millimeters across and are then ready for the blast furnace.

Although the exact chemical processes have been fully understood beginning only during the twentieth century, the goal of iron making has always been to release oxygen from its chemical bond with iron. The blast furnace is the most efficient and common way to do this. Modern blast furnaces work on the same principles as those developed in the fifteenth century, but they are larger and have benefited from centuries of refinement to the design, materials, and process. A modern blast furnace may be as much as 30 meters tall and 10 meters in diameter. Because of improvements in materials, a blast furnace may stay in continuous operation for two years, requiring maintenance only when its brick lining wears out. Some of the most important advances involve the use of mathematical modeling and supercomputers to provide more accurate and timely control over the process. The output of a modern furnace may exceed 10 million kilograms per day.

A modern blast furnace has five readily identifiable sections; from the top down they are: throat, stack, barrel, bosh, and hearth (or crucible). The ore, coke, and limestone (collectively called the charge) enter the furnace through the throat. The distribution and timing of the charge is carefully monitored at all times to ensure proper operation. The throat opens onto the stack, which resembles a cone with the top cut off. The stack widens as it descends because the temperature of the charge increases as it works its way down the furnace, causing the charge to expand. The next section, the barrel, is a short, straight section that connects the stack to the bosh, a shorter, upside-down version of the stack. The bosh narrows as it descends because the iron is beginning to liquefy and compact by the time the charge reaches the bosh. At the bottom of the bosh are nozzles called tuyeres through which blast air is blown into the furnace. The air coming through the tuyeres has been preheated to about 1,000° Celsius or higher, and oxygen is sometimes added to it. This hot air causes the coke in the charge to burn. The oxygen in the air combines with carbon from the coke to create carbon monoxide gas, which in turn removes the oxygen from the ore. The burning coke also produces temperatures up to 3,000° Celsius to melt the iron. The liquid metal collects in the bottom section, called the hearth or crucible. Just as in earlier furnaces, the slag floats on the molten iron, and workers periodically pull it off through openings in the side of the furnace.

Several direct reduction processes (in which the temperature never exceeds iron’s melting point) were developed in the twentieth century but are used only in special circumstances. The basic process relies on hot gases to reduce the iron ore in a way roughly analogous to the process of the earlier bloomeries. Since the iron is never completely melted, slag never forms, and the final product contains impurities that must be removed during the steelmaking process. Direct reduction furnaces can be built more quickly and cheaply than blast furnaces, and they produce less pollution. The disadvantages are that they require a supply of cheap and the iron ore must be processed to a very high grade.

Uses of Iron Ore

The vast majority of iron produced in blast furnaces is converted to steel. The remainder is cast as pig iron and later converted to either cast iron or wrought iron. At a foundry, the pig iron is melted to a liquid state in a cupola (a small version of a blast furnace) and then cast in molds (some of them are still made with damp sand) to make machine parts, pipes, engine blocks, and thousands of other items. Wrought iron is now made in limited quantities. Its production begins by melting pig iron and removing impurities. The molten iron is then poured over a slag and formed into blooms that can then be shaped into products.

Iron is used in a vast range of special-purpose alloys developed for commercial applications. The major classifications of these alloys are discussed below only in broad outline; within each grouping there remains an enormous variety because of the wide range of special needs.

Magnetic alloys are either retentive (hard) or nonretentive (soft) of magnetism. The hard alloys remain magnetized after the application of a magnetic field, thus creating a permanent magnet. One family of hard alloys contains cobalt and molybdenum (less than 20 percent of each), while another contains aluminum, nickel, cobalt, copper, and titanium. Once magnetized they are used in such applications as speaker magnets, electrical meters, and switchboard instruments because of the constancy of their magnetic field and their resistance to demagnetization. The soft alloys also fall into two families: those with nickel and those with aluminum. The nickel alloys are used in communications and electric power equipment, while those containing aluminum are used to carry alternating current.

High-temperature alloys, used in high-temperature environments such as turbine blades in gas turbines and superchargers, are generally referred to as either iron-based, cobalt-based, or nickel-based. They are formulated to retain their chemical identity, physical identity, and the strength required to perform their intended function, all at extreme, high temperatures.

The most common electrical-resistance alloys are best known as heating elements in toasters, radiant heaters, water heaters, and so on. They usually contain nickel (as much as 60 percent), chromium (approximately 20 percent), and sometimes aluminum (approximately 5 percent). Alloys without the nickel have higher resistivity and lower density and are used in potentiometers, rheostats, and similar applications.

Corrosion-resistant alloys are designed to resist corrosion from liquids and gases other than air or oxygen and usually contain varying amounts of nickel and chromium along with combinations of molybdenum, copper, cobalt, tungsten, and silicon. No one is capable of resisting the effects of all corrosive agents, so each is tailored to its intended purpose.

The powdered iron technique employs iron that has been finely ground and mixed with metals or nonmetals to form the desired alloys. After a binder is added, the mixture is pressed to the desired shape in a mold. This process has the advantages of precise control over the makeup of the alloy and the ability to form iron pieces to precise dimensions with little or no working required afterward.

Iron is important to almost every organism and is used in a variety of ways. It is involved in oxygen transport, electron transfer, reactions, and reduction reactions. Iron is a constituent of human blood. Some iron compounds have medical uses, such as stimulating the appetite, treating anemia, coagulating blood, and stimulating healing.

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