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Pure iron is a soft, grayish-white metal. Although iron is a common element, pure iron is almost never found in nature. The only pure iron known to exist naturally comes from fallen meteorites. Most iron is found in minerals formed by the combination of iron with other elements. Iron oxides are the most common. Those minerals near the surface of the earth that have the highest iron content are known as iron ores and are mined commercially.
Iron ore is converted into various types of iron through several processes. The most common process is the use of a blast furnace to produce pig iron which is about 92-94% iron and 3-5% carbon with smaller amounts of other elements. Pig iron has only limited uses, and most of this iron goes on to a steel mill where it is converted into various steel alloys by further reducing the carbon content and adding other elements such as manganese and nickel to give the steel specific properties.
Historians believe that the Egyptians were the first people to work with small amounts of iron, some five or six thousand years ago. The metal they used was apparently extracted from meteorites. Evidence of what is believed to be the first example of iron mining and smelting points to the ancient Hittite culture in what is now Turkey. Because iron was a far superior material for the manufacture of weapons and tools than any other known metal, its production was a closely guarded secret. However, the basic technique was simple, and the use of iron gradually spread. As useful as it was compared to other materials, iron had disadvantages. The quality of the tools made from it was highly variable, depending on the region from which the iron ore was taken and the method used to extract the iron. The chemical nature of the changes taking place during the extraction were not understood; in particular, the importance of carbon to the metal's hardness. Practices varied widely in different parts of the world. There is evidence, for example, that the Chinese were able to melt and cast iron implements very early, and that the Japanese produced amazing results with steel in small amounts, as evidenced by heirloom swords dating back centuries. Similar breakthroughs were made in the Middle East and India, but the processes never emerged into the rest of the world. For centuries the Europeans lacked methods for heating iron to the melting point at all. To produce iron, they slowly burned iron ore with wood in a clay-lined oven. The iron separated from the surrounding rock but never quite melted. Instead, it formed a crusty slag which was removed by hammering. This repeated heating and hammering process mixed oxygen with the iron oxide to produce iron, and removed the carbon from the metal. The result was nearly pure iron, easily shaped with hammers and tongs but too soft to take and keep a good edge. Because the metal was shaped, or wrought, by hammering, it came to be called wrought iron.
Tools and weapons brought back to Europe from the East were made of an iron that had been melted and cast into shape. Retaining more carbon, cast iron is harder than wrought iron and will hold a cutting edge. However, it is also more brittle than wrought iron. The European iron workers knew the Easterners had better iron, but not the processes involved in fashioning stronger iron products. Entire nations launched efforts to discover the process.
The first known European breakthrough in the production of cast iron, which led quickly to the first practical steel, did not come until 1740. In that year, Benjamin Huntsman took out a patent for the melting of material for the production of steel springs to be used in clockmaking. Over the next 20 years or so, the procedure became more widely adopted. Huntsman used a blast furnace to melt wrought iron in a clay crucible. He then added carefully measured amounts of pure charcoal to the melted metal. The resulting alloy was both strong and flexible when cast into springs. Since Huntsman was originally interested only in making better clocks, his crucible steel led directly to the development of nautical chronometers, which, in turn, made global navigation possible by allowing mariners to precisely determine their east/west position. The fact that he had also invented modern metallurgy was a side-effect which he apparently failed to notice.
The production of iron from its ore involves an oxidation-reduction reaction carried out in a blast furnace. Iron ore is usually a mixture of iron and vast quantities of impurities such as sand and clay referred to as gangue. The iron found in iron ores are found in the form of iron oxides. As a result of these impurities, iron must be first separated from the gangue and then converted to pure iron. This is accomplished by the method of pyrometallurgy, a high temperature process. The high temperatures are needed for the reduction of iron and the oxidation of the limestone which will be seen below
The production of iron from its ore involves a redox reaction carried out in a blast furnace. The furnace is filled at the top with the iron ore oxide most commonly hematite (Fe2O3Fe2O3) but can also magnetite (Fe3O4Fe3O4), carbon called coke and limestone (CaCO3CaCO3). For the purpose of this discussion the iron ore oxide hematite (Fe2O3Fe2O3) will be shown. On a side note, Hematite gets its name from the Greek word meaning blood like because of the color of one form of its powder. The Ancient Greeks believed that large deposits of hematite were formed from battles that were fought and the blood from these battles flowed into the ground
To begin the process a blast of hot air is forced in at the bottom of the furnace that helps create a large temperature variation with the bottom being 2273 K and the top 473 K. The amount of oxygen is strictly controlled so that carbon monoxide is the main product as shown:
One of the most interesting part of this redox reaction is that the majority of the carbon dioxide formed is itself reduced when it comes to contact with the unburned coke and produce more reducing agent. As the process continue the molten iron flow down through the furnace and collects at the bottom, where it is removed through an opening in the side. When it cools the impure iron is brittle and some cases soft due to the presence of the small impurities, such as sulfur and phosphorus
Thus the impure iron coming from the bottom of the furnace is further purified. The most common method is the basic oxygen furnace. In the furnace, oxygen is blown into the impure iron. This is vital because the oxygen oxidizes the phosphorus and sulfur shown in the following redox reactions:
The oxides either escapes as gases or react with basic oxides that are added or used to line the furnace. This final purification step removes much of the impurities and the result is ordinary carbon steel. Thus iron is obtained through the process of oxidation-reduction
Iron is extracted from iron ore in a huge container called a blast furnace. Iron ores such as haematite contain iron(III) oxide, Fe2O3. The oxygen must be removed from the iron(III) oxide in order to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions
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