Chapter-2Definition and Classification of Metallurgy, Extractive Metallurgy Classification, & Composition of Pig Iron, Manufacturing of Pig Iron, Construction and Operation of Blast Furnace.

MetallurgyMetallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their compounds, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use. Metallurgy is commonly used in the craft of metalworking.

Extractive metallurgyExtractive metallurgy is the practice of extracting metal from ore, purifying it, and recycling it.Most metals found in the Earth's crust exist as oxide and sulfide minerals. These compounds must be reduced to liberate the desired metal. There are two methods of reduction: electrolytic and chemical.Chemical reduction can be carried out in a variety of processes, including reductive smelting - the process of heating an ore with reducing agent (often, coke or charcoal) and purifying agents to separate the pure molten metal from the waste products. Some other processes for chemical reduction include autoclave hydrogen reduction and converting. The latter though does not produce the pure metal, therefore requiring further treatment of its product.Electrolytic reduction involves passing a large current through a molten metal oxide or an aqueous solution of the metal's salt. For example, aluminium is electrolyzed from bauxite dissolved in molten cryolite via the Hall-Hroult process.

Extractive metallurgy Extractive metallurgy is the practice of removing valuable metals from an ore and refining the extracted raw metals into a purer form. In order to convert a metal oxide or sulfide to a purer metal, the ore must be reduced either physically, chemically, or electrolytically.Extractive metallurgists are interested in three primary streams: feed, concentrate (valuable metal oxide/sulfide), and tailings (waste). After mining, large pieces of the ore feed are broken through crushing and/or grinding in order to obtain particles small enough where each particle is either mostly valuable or mostly waste. Concentrating the particles of a value in a form supporting separation enables the desired metal to be removed from waste products.

Prior to reduction, it is often necessary to separate metal compounds to exclude co-reduction of different metals and contamination of the product. There is a great variety of separation processes: roasting, oxidative smelting, converting, amalgamation, leaching and many others.Extractive metallurgical technologies are divided into mineral processing, hydrometallurgy and pyrometallurgy areas. Extractive metallurgical and mineral dressing operations are divided into:Ferrous metallurgy, which includes reduction of iron ore into iron, and further refinement and alloying with other metals to make steel. Non Ferrous metallurgy, which includes all other metals. This can be further broken down into: Precious metals. The recovery of gold and silver and the platinum group metals. Base metals. The recovery of lead, zinc, copper, and nickel. Light metals. The recovery of magnesium, aluminium, tin, and titanium. Minor elements. The recovery of arsenic, selenium, bismuth, tellurium, and antimony. Industrial minerals. Major examples include clays, sands, silicates, and heavy mineral sands Coal. The benficiation and treatment of coal. Gems and precious minerals. The recovery of emeralds, diamonds, sapphires, rubies, and others.

IronIron is a chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element. Iron is a lustrous, silvery soft metal. It is one of the few ferromagnetic elements.Iron and iron alloys are also the most common source of ferromagnetic materials in everyday use.

In magnetised iron, the electronic spins of all the domains are all aligned, so that the magnetic effects of neighboring domains reinforce each other. Although each domain contains billions of atoms, they are very small, about one thousandth of a centimeter across.Iron, of course, is of most importance when mixed with certain other metals and with carbon to form steels. There are many types of steels, all with different properties; and an understanding of the properties of the allotropes of iron is key to the manufacture of good quality steels.

Alpha iron, also known as ferrite, is the most stable form of iron at normal temperatures. It is a fairly soft metal that can dissolve only a small concentration of carbon (no more than 0.021% by mass at 910 C).Above 912C and up to 1401C alpha iron undergoes a phase transition from body-centered cubic to the face-centered cubic configuration of gamma iron, also called austenite. This is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.04% by mass at 1146C). This form of iron is used in the type of stainless steel used for making cutlery, and hospital and food-service equipment.

Allotropes Iron represents perhaps the best-known example for allotropy in a metal. There are four allotropic forms of iron, known as alpha, beta, gamma, and delta.As molten iron cools down it crystallizes at 1535C into its delta allotrope, which has a body-centered cubic (BCC) crystal structure.

As it cools further its crystal structure changes to face-centered cubic (FCC) at 1401C, when it is known as gamma-iron, or austenite. At 912C the crystal structure again becomes BCC as beta-iron4 is formed, and at 770C (the Curie point, Tc ) the iron becomes magnetic as alpha-iron, also known as ferrite, which is also BCC, is formed. Thus there is no change in crystalline structure, but there is a change in 'domain structure', where each domain contains iron atoms with a particular . In unmagnetised iron, all the electronic spins of the atoms within one domain are in the same direction.

Characteristics Iron is a metal extracted mainly from the iron ore hematite. It oxidizes readily in air and water and is rarely found as a free element. In order to obtain elemental iron, oxygen and other impurities must be removed by chemical reduction. Iron is the main constituent of steel, and it is used in the production of alloys or solid solutions of various metals, as well as some non-metals, particularly carbon. The many iron alloys, which have very different properties, are discussed in the article on steel.

Applications Iron is the most used of all the metals, comprising 95% of all the metal tonnage produced worldwide. Its combination of low cost and high strength make it indispensable, especially in applications like automobiles, the hulls of large ships, and structural components for buildings. Steel is the best known alloy of iron, and some of the forms that iron can take include:Pig iron has 3.5 - 4.5% carbon and contains varying amounts of contaminants such as sulphur, silicon and phosphorus. Its only significance is that of an intermediate step on the way from iron ore to cast iron and steel.

Cast iron contains 2% 4.0% carbon, 1% 6% silicon, and small amounts of manganese. Contaminants present in pig iron that negatively affect material properties, such as sulfur and phosphorus, have been reduced to an acceptable level. It has a melting point in the range of 14201470 K, which is lower than either of its two main components, and makes it the first product to be melted when carbon and iron are heated together. Its mechanical properties vary greatly, dependent upon the form carbon takes in the alloy. 'White' cast irons contain their carbon in the form of cementite, or iron carbide. This hard, brittle compound dominates the mechanical properties of white cast irons, rendering them hard, but unresistant to shock. The broken surface of a white cast iron is full of fine facets of the broken carbide, a very pale, silvery, shiny material, hence the appellation.

In grey iron the carbon exists free as fine flakes of graphite, and also renders the material brittle due to the stress-raising nature of the sharp edged specially treated with trace amounts of magnesium to alter the shape of graphite to spheroids, or nodules, vastly increasing the toughness and strength of the material. flakes of graphite. A newer variant of grey iron, referred to as ductile iron is Carbon steel contains 2.0% carbon or less, with small amounts of manganese, sulphur, phosphorus, and silicon. Wrought iron contains less than 0.25% carbon. It is a tough, malleable product, but not as fusible as pig iron. If honed to an edge, it loses it quickly. Wrought iron is characterized by the presence of fine fibers of slag entrapped in the metal. Wrought iron is more corrosion resistant than steel. It has been almost completely replaced by mild steel for traditional "wrought iron" products and blacksmithing. Mild steel does not have the same corrosion resistance but is cheaper and more widely available.

Alloy steels contain varying amounts of carbon as well as other metals, such as chromium, vanadium, molybdenum, nickel, tungsten, etc.They are used for structural purposes, as their alloy content raises their cost and necessitates justification of their use. Recent developments in ferrous metallurgy have produced a growing range of micro alloyed steels, also termed 'HSLA' or high-strength, low alloy steels, containing tiny additions to produce high strengths and often spectacular toughness at minimal cost. Iron oxides are used in the production of magnetic storage media in computers. They are often mixed with other compounds, and retain their magnetic properties in solution. The main drawback to iron and steel is that pure iron, and most of its alloys, suffer badly from rust if not protected in some way. Painting, galvanization, plastic coating and bluing are some techniques used to protect iron from rust by excluding water and oxygen or by sacrificial protection.

Blast furnace A blast furnace is a type of metallurgical furnace used for smelting to produce metals, generally iron.In a blast furnace, fuel and ore are continuously supplied through the top of the furnace, while air (sometimes with oxygen enrichment) is blown into the bottom of the chamber, so that the chemical reactions take place throughout the furnace as the material moves downward. The end products are usually molten metal and slag phases tapped from the bottom, and flue gases exiting from the top of the furnace.Blast furnaces are to be contrasted with air furnaces (such as reverberatory furnaces), which were naturally aspirated, usually by the convection of hot gases in a chimney flue. According to this broad definition, bloomeries for iron, blowing houses for tin and smelt mills for lead would be classified as blast furnaces. However, the term has usually been limited to those used for smelting iron ore to produce pig iron, an intermediate material used in the production of commercial iron and steel.Certain modern furnaces used for non-ferrous smelting processes are known as blast furnaces, and are particularly in the production of lead and copper. However this article (except its final section) will concentrate on furnaces for the production of pig iron.

Blast furnace diagram 1. Hot blast from Cowper stoves 2. Melting zone 3. Reduction zone of ferrous oxide 4. Reduction zone of ferric oxide 5. Pre-heating zone 6. Feed of ore, limestone and coke 7. Exhaust gases 8. Column of ore, coke and limestone 9. Removal of slag 10. Tapping of molten pig iron 11. Collection of waste gases

Chemistry The main chemical reaction producing the molten iron is:Fe2O3 + 3CO 2Fe + 3CO2Preheated blast air blown into the furnace reacts with the carbon in the form of coke to produce carbon monoxide and heat. The carbon monoxide then reacts with the iron oxide to produce molten iron and carbon dioxide. Hot carbon dioxide, unreacted carbon monoxide, and nitrogen from the air pass up through the furnace as fresh feed material travels down into the reaction zone. As the material travels downward, the counter-current gases both preheat the feed charge, decompose the limestone to calcium oxide and carbon dioxide, and begin to reduce the iron oxides in the solid state. The main reaction controlling the gas atmosphere in the furnace is called the Boudouard reaction:C + O2 CO2 CO2 + C 2CO

The decomposition of limestone in the middle zones of the furnace proceeds according to the following reaction:CaCO3 CaO + CO2The calcium oxide formed by decomposition reacts with various acidic impurities in the iron (notably silica), to form the slag which is essentially calcium silicate, CaSiO3.[24]The "pig" iron produced by the blast furnace has a relatively high carbon content of around 4-5%, making it very brittle, and of little commercial use. Some pig iron is used to make cast iron. The majority of pig iron produced by blast furnaces undergoes further processing to reduce the carbon content and produce various grades of steel used for tools and construction materials.Although the efficiency of blast furnaces is constantly evolving, the chemical process inside the blast furnace remains the same. According to the American Iron and Steel Institute; "Blast furnaces will survive into the next millennium because the larger, efficient furnaces can produce hot metal at costs competitive with other iron making technologies. One of the biggest drawbacks of the blast furnaces is the inevitable carbon dioxide production as iron is reduced from iron oxides by carbon and there is no economical substitute - steelmaking is one of the unavoidable industrial contributors of the CO2 emissions in the world (see Greenhouse gases).

HOW A BLAST FURNACE WORKS?

The purpose of a blast furnace is to chemically reduce and physically convert iron oxides into liquid iron called "hot metal". The blast furnace is a huge, steel stack lined with refractory brick, where iron ore, coke and limestone are dumped into the top, and preheated air is blown into the bottom. The raw materials require 6 to 8 hours to descend to the bottom of the furnace where they become the final product of liquid slag and liquid iron. These liquid products are drained from the furnace at regular intervals. The hot air that was blown into the bottom of the furnace ascends to the top in 6 to 8 seconds after going through numerous chemical reactions. Once a blast furnace is started it will continuously run for four to ten years with only short stops to perform planned maintenance.

processIron oxides can come to the blast furnace plant in the form of raw ore, pellets or sinter. The raw ore is removed from the earth and sized into pieces that range from 0.5 to 1.5 inches. This ore is either Haematite (Fe2O3) or Magnetite (Fe3O4) and the iron content ranges from 50% to 70%. This iron rich ore can be charged directly into a blast furnace without any further processing. Iron ore that contains a lower iron content must be processed or beneficiated to increase its iron content. Pellets are produced from this lower iron content ore. This ore is crushed and ground into a powder so the waste material called gangue can be removed. The remaining iron-rich powder is rolled into balls and fired in a furnace to produce strong, marble-sized pellets that contain 60% to 65% iron. Sinter is produced from fine raw ore, small coke, sand-sized limestone and numerous other steel plant waste materials that contain some iron.

These fine materials are proportioned to obtain a desired product chemistry then mixed together. This raw material mix is then placed on a sintering strand, which is similar to a steel conveyor belt, where it is ignited by gas fired furnace and fused by the heat from the coke fines into larger size pieces that are from 0.5 to 2.0 inches. The iron ore, pellets and sinter then become the liquid iron produced in the blast furnace with any of their remaining impurities going to the liquid slag.The coke is produced from a mixture of coals. The coal is crushed and ground into a powder and then charged into

into a powder and then charged into an oven. As the oven is heated the coal is cooked so most of the volatile matter such as oil and tar are removed. The cooked coal, called coke, is removed from the oven after 18 to 24 hours of reaction time. The coke is cooled and screened into pieces ranging from one inch to four inches. The coke contains 90 to 93% carbon, some ash and sulfur but compared to raw coal is very strong. The strong pieces of coke with a high energy value provide permeability, heat and gases which are required to reduce and melt the iron ore, pellets and sinter.The final raw material in the iron making process in limestone. The limestone is removed from the earth by blasting with explosives. It is then crushed and screened to a size that ranges from 0.5 inch to 1.5 inch to become blast furnace flux . This flux can be pure high calcium limestone, dolomitic limestone containing magnesia or a blend of the two types of limestone.Since the limestone is melted to become the slag which removes sulfur and other impurities, the blast furnace operator may blend the different stones to produce the desired slag chemistry and create optimum slag properties such as a low melting point and a high fluidity.All of the raw materials are stored in an ore field and transferred to the stockhouse before charging. Once these materials are charged into the furnace top, they go through numerous chemical and physical reactions while descending to the bottom of the furnace.