Hot metal (blast-furnace iron)

Most blast furnaces are linked to a basic oxygen steel plant, for which the hot metal typically contains 4 to 4.5 percent carbon, 0.6 to 0.8 percent silicon, 0.03 percent sulfur, 0.7 to 0.8 percent manganese, and 0.15 percent phosphorus. Tapping temperatures are in the range 1,400° to 1,500° C (2,550° to 2,700° F); to save energy, the hot metal is transferred directly to the steel plant with a temperature loss of about 100° C (200° F).

The major determinants of the composition of basic iron are the hearth temperature and the choice of iron ores. For instance, carbon content is fixed both by the temperature and by the amounts of other elements present in the iron. Sulfur and silicon are both temperature-dependent and generally vary in opposite directions, a high temperature producing low sulfur and high silicon levels. Furnace size also influences silicon, so that large furnaces yield low-silicon iron. Phosphorus, on the other hand, is determined entirely by the amount present in the original charge. Like silica, manganous oxide is partially reduced by carbon, and its final concentration depends on the hearth temperature and slag composition.

Cast iron

Iron production is relatively unsophisticated. It mostly involves remelting charges consisting of pig iron, steel scrap, foundry scrap, and ferroalloys to give the appropriate composition. The cupola, which resembles a small blast furnace, is the most common melting unit. Cold pig iron and scrap are charged from the top onto a bed of hot coke through which air is blown. Alternatively, a metallic charge is melted in a coreless induction furnace or in a small electric-arc furnace.

There are two basic types of cast iron—namely, white and gray.

White iron

White cast irons are usually made by limiting the silicon content to a maximum of 1.3 percent, so that no graphite is present and all of the carbon exists as cementite (Fe3C). The name white refers to the bright appearance of the fracture surfaces when a piece of the iron is broken in two. White irons are too hard to be machined and must be ground to shape. Brittleness limits their range of applications, but they are sometimes used when wear resistance is required, as in brake linings.

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The main use for white irons is as the starting material for malleable cast irons, in which the cementite formed during casting is decomposed by heat treatment. Such irons contain about 0.6 to 1.3 percent silicon, which is enough to promote cementite decomposition during the heat treatment but not enough to produce graphite flakes during casting. Whiteheart malleable iron is made by using an oxidizing atmosphere to remove carbon from the surface of white iron castings heated to a temperature of 900° C (1,650° F). Blackheart malleable iron, on the other hand, is made by annealing white iron in a neutral atmosphere, again at a temperature of 900° C. In this process, cementite is decomposed to form rosette-shaped graphite nodules, which are less embrittling than flakes. Blackheart iron is an important material that is widely used in agricultural and engineering machinery. Even better mechanical properties can be obtained by the addition of small amounts of magnesium or cerium to molten iron, since these elements have the effect of transforming the graphite into spherical nodules. These SG (spheroidal graphite) irons, which are also called ductile irons, are strong and malleable; they are also easy to cast and are sometimes preferred to steel castings and forgings.

Gray iron

Gray cast irons generally contain more than 2 percent silicon, and carbon exists as flakes of graphite embedded in a combination of ferrite and pearlite. The name arises because graphite imparts a dull gray appearance to fracture surfaces. Phosphorus is present in most cast irons, lowering the freezing point and lengthening the solidification period so that gray irons can be cast into intricate shapes. Unfortunately, graphite formation is enhanced by slow solidification, and the crack-inducing effect of graphite flakes reduces the metal’s strength and malleability. Gray cast irons are therefore unsuitable when shock resistance is required, but they are ideal for such purposes as engine cylinder blocks, domestic stoves, and manhole covers. They are easy to machine because the graphite causes the metal to break off in small chips, and they also have a high damping capacity (i.e., they are able to absorb vibration). As a result, gray cast irons are used as frames for rotating machinery such as lathes.

High-alloy iron

The properties of both white and gray cast irons can be enhanced by the inclusion of alloying elements such as nickel (Ni), chromium (Cr), and molybdenum (Mo). For example, Ni-Hard, a white iron containing 4 to 5 percent nickel and up to 1.5 percent chromium, is used to make metalworking rolls. Irons in the Ni-Resist range, which contain 14 to 25 percent nickel, are nonmagnetic and have good heat and corrosion resistance.

Casting methods

Iron castings can be made in many ways, but sand-casting is the most common. First, a pattern of the required shape (slightly enlarged to allow for shrinkage) is made in wood, metal, or plastic. It is then placed in a two-piece molding box and firmly packed in sand that is held together by a bonding agent. After the sand has hardened, the molding box is split open to allow the pattern to be removed and used again, and then the box is reassembled and molten metal poured into the cavity to create the casting.

A greensand casting is made in a sand mold bonded with clay, the name referring not to the colour of the sand but to the fact that the mold is uncured. Dry-sand molds are similar, except that the sand is baked before receiving any metal. Alternatively, hardening can be effected by mixing sodium silicate into the sand to create chemical bonds that make baking unnecessary. For heavy castings, molds made of coarse loam sand backed up with brick and faced with highly refractory material are used.

Sand-casting produces rough surfaces, and a much better finish can be achieved by shell molding. This process involves bringing a mixture of sand and a thermosetting resin into contact with a heated metal pattern to form an envelope or shell of hardened sand. Two half-shells are then assembled to make a mold. Wax patterns also can be used to make one-piece shell molds, the wax being removed by melting before the resin is cured in an oven.

For some high-precision applications, iron is cast into permanent molds made of either cast iron or graphite. It is important, however, to ensure that the molds are warmed before use and that their internal surfaces are given a coating to release the casting after solidification.

Most castings are static in that they rely on gravity to cause the liquid metal to fill the mold. Centrifugal casting, however, uses a rotating mold to produce hollow cylindrical castings, such as cast-iron drainpipes.

Wrought iron

Although it is no longer manufactured, the wrought iron that survives contains less than 0.035 percent carbon. It therefore consists essentially of ferrite, but its strength and malleability are reduced by entrained puddling slag, which is elongated into stringers by rolling. As a result, breaking a bar of wrought iron reveals a fibrous fracture not unlike that of wood. The other elements present are silicon (0.075 to 0.15 percent), sulfur (0.01 to 0.2 percent), phosphorus (0.1 to 0.25 percent), and manganese (0.05 to 0.1 percent). This relative purity is the reason why wrought iron has a reputation for good corrosion resistance.

Iron powder

Iron powders produced by crushing and grinding or by atomizing a stream of molten metal are made into small components by pressing or rolling them into compacts, which are then sintered. The density of the compacts depends on the pressure used, but porous compacts suitable for self-lubricating bearings or filters can be given accurate dimensions by using this technique.

Chemical compounds

Apart from being a source of iron, hematite is used for its reddish colour in cosmetics and as a pigment in paints and roof tiles. Also, when cobalt and nickel oxides are added to hematite, a group of ceramic materials closely related to magnetite, known as ferrites, are formed. These are ferromagnetic (i.e., highly magnetic) and are widely used in computers and in electronic transmission and receiving equipment.

Iron is a constituent of human blood, and various iron compounds have medical uses. Ferric ammonium citrate is an appetite stimulator, and ferrous gluconate, ferrous sulfate, and ferric pyrophosphate are among compounds used to treat anemia. Ferric salts act as coagulants and are applied to wounds to promote healing.

Iron compounds are also widely used in agriculture. For example, ferrous sulfate is applied as a spray to acid-loving plants, and other compounds are used as fungicides.

Robert Donald Walker

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Mining giant puts new spin on China resource push May 27, 2025, 6:30 AM ET (Reuters)

mining, process of extracting useful minerals from the surface of the Earth, including the seas. A mineral, with a few exceptions, is an inorganic substance occurring in nature that has a definite chemical composition and distinctive physical properties or molecular structure. (One organic substance, coal, is often discussed as a mineral as well.) Ore is a metalliferous mineral, or an aggregate of metalliferous minerals and gangue (associated rock of no economic value), that can be mined at a profit. Mineral deposit designates a natural occurrence of a useful mineral, while ore deposit denotes a mineral deposit of sufficient extent and concentration to invite exploitation.

When evaluating mineral deposits, it is extremely important to keep profit in mind. The total quantity of mineral in a given deposit is referred to as the mineral inventory, but only that quantity which can be mined at a profit is termed the ore reserve. As the selling price of the mineral rises or the extraction costs fall, the proportion of the mineral inventory classified as ore increases. Obviously, the opposite is also true, and a mine may cease production because (1) the mineral is exhausted or (2) the prices have dropped or costs risen so much that what was once ore is now only mineral.

William Andrew Hustrulid

History

Archaeological discoveries indicate that mining was conducted in prehistoric times. Apparently, the first mineral used was flint, which, because of its conchoidal fracturing pattern, could be broken into sharp-edged pieces that were useful as scrapers, knives, and arrowheads. During the Neolithic Period, or New Stone Age (about 8000–2000 bce), shafts up to 100 metres (330 feet) deep were sunk in soft chalk deposits in France and Britain in order to extract the flint pebbles found there. Other minerals, such as red ochre and the copper mineral malachite, were used as pigments. The oldest known underground mine in the world was sunk more than 40,000 years ago at Bomvu Ridge in the Ngwenya mountains, Swaziland, to mine ochre used in burial ceremonies and as body colouring.

Gold was one of the first metals utilized, being mined from streambeds of sand and gravel where it occurred as a pure metal because of its chemical stability. Although chemically less stable, copper occurs in native form and was probably the second metal discovered and used. Silver was also found in a pure state and at one time was valued more highly than gold.

According to historians, the Egyptians were mining copper on the Sinai Peninsula as long ago as 3000 bce, although some bronze (copper alloyed with tin) is dated as early as 3700 bce. Iron is dated as early as 2800 bce; Egyptian records of iron ore smelting date from 1300 bce. Found in the ancient ruins of Troy, lead was produced as early as 2500 bce.

One of the earliest evidences of building with quarried stone was the construction (2600 bce) of the great pyramids in Egypt, the largest of which (Khufu) is 236 metres (775 feet) along the base sides and contains approximately 2.3 million blocks of two types of limestone and red granite. The limestone is believed to have been quarried from across the Nile. Blocks weighing as much as 15,000 kg (33,000 pounds) were transported long distances and elevated into place, and they show precise cutting that resulted in fine-fitting masonry.

One of the most complete early treatments of mining methods in Europe is by the German scholar Georgius Agricola in his De re metallica (1556). He describes detailed methods of driving shafts and tunnels. Soft ore and rock were laboriously mined with a pick and harder ore with a pick and hammer, wedges, or heat (fire setting). Fire setting involved piling a heap of logs at the rock face and burning them. The heat weakened or fractured the rock because of thermal expansion or other processes, depending on the type of rock and ore. Crude ventilation and pumping systems were utilized where necessary. Hoisting up shafts and inclines was done with a windlass; haulage was in “trucks” and wheelbarrows. Timber support systems were employed in tunnels.

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Great progress in mining was made when the secret of black powder reached the West, probably from China in the late Middle Ages. This was replaced as an explosive in the mid-19th century with dynamite, and since 1956 both ammonium nitrate fuel-blasting agents and slurries (mixtures of water, fuels, and oxidizers) have come into extensive use. A steel drill with a wedge point and a hammer were first used to drill holes for placement of explosives, which were then loaded into the holes and detonated to break the rock. Experience showed that proper placement of holes and firing order are important in obtaining maximum rock breakage in mines.

The invention of mechanical drills powered by compressed air (pneumatic hammers) increased markedly the capability to mine hard rock, decreasing the cost and time for excavation severalfold. It is reported that the Englishman Richard Trevithick invented a rotary steam-driven drill in 1813. Mechanical piston drills utilizing attached bits on drill rods and moving up and down like a piston in a cylinder date from 1843. In Germany in 1853 a drill that resembled modern air drills was invented. Piston drills were superseded by hammer drills run by compressed air, and their performance improved with better design and the availability of quality steel.

Developments in drilling were accompanied by improvements in loading methods, from handloading with shovels to various types of mechanical loaders. Haulage likewise evolved from human and animal portage to mine cars drawn by electric locomotives and conveyers and to rubber-tired vehicles of large capacity. Similar developments took place in surface mining, increasing the volume of production and lowering the cost of metallic and nonmetallic products drastically. Large stripping machines with excavating wheels used in surface coal mining are employed in other types of open-pit mines.

Water inflow was a very important problem in underground mining until James Watt invented the steam engine in the 18th century. After that, steam-driven pumps could be used to remove water from the deep mines of the day. Early lighting systems were of the open-flame type, consisting of candles or oil-wick lamps. In the latter type, coal oil, whale oil, or kerosene was burned. Beginning in the 1890s, flammable acetylene gas was generated by adding water to calcium carbide in the base of a lamp and then released through a jet in the centre of a bright metal reflector. A flint sparker made these so-called carbide lamps easy to light. In the 1930s battery-powered cap lamps began entering mines, and since then various improvements have been made in light intensity, battery life, and weight.

Although a great deal of mythic lore and romance has accumulated around miners and mining, in modern mining it is machines that provide the strength and trained miners who provide the brains needed to prevail in this highly competitive industry. Technology has developed to the point where gold is now mined underground at depths of 4,000 metres (about 13,100 feet), and the deepest surface mines have been excavated to more than 700 metres (about 2,300 feet).

George B. Clark William Andrew Hustrulid