Aluminum

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Aluminum History Aluminum is the most abundant (8,13%) metallic element in the earth's crust and after oxygen and silicon, the third most abundant of all elements in the crust. Because of its strong affinity to oxygen, it is not found in the elemental state but only in combined forms such as oxides or silicates. The metal derives its name from alumen, the Latin name for alum. In 1761 L. B. G de Morveau proposed the name alumine for the base in alum, and in 1787Lavoisier definitely identified it as the oxide of a still undiscovered metal. In 1807 Sir Humphrey Davy proposed the name aluminum for this metal and later agreed to change it to aluminum. Shortly thereafter, the name aluminium was adopted to conform to the "ium" ending of most elements, and this spelling is now in general use throughout the world. Aluminum was also the accepted spelling in the United States until 1925 when the American Chemical Society officially reverted to aluminum. Hans Christian Oersted is now generally credited with having been the first to prepare metallic aluminum. He accomplished this in 1825 by heating anhydrous aluminum chloride with potassium amalgam and distilling off the mercury. Frederick Wöhler improved the process between 1827 and 1845 by substituting potassium for the amalgam and by developing a better method for dehydrating aluminum. In 1854 Henri SainteClaire Deville substituted sodium for the relatively expensive potassiumand, by using sodium aluminum chloride instead of aluminum chloride, produced the first commercial quantities of aluminum in a pilot plant near Paris. Several plants using essentially this process were subsequently built in Great Britain, but none survived for long the advent in 1886 of the electrolytic process, which has dominated the industry ever since. The development of the electrolytic process dates back to Sir Humphrey Davy who in 1807 attempted unsuccessfully to electrolyze a mixture of alumina and potash. Later, in 1854 Robert Wilhelm Von Bunsen and Sainte-Claire Deville independently prepared aluminum by electrolysis of fused sodium aluminum chloride, but this process was not exploited for lack of an economic source of electricity. Gramme's invention of the dynamo (in 1886) changed this and paved the way for the invention of the modern process. In 1866, Charles Martin Hall of Oberlin (Ohio) and Paul L. T. Héroult of France, both of them 22 years old at the time, discovered and patented almost simultaneously the process in which alumina is dissolved in molten cryolite and decomposed electrolytically. This reduction process, generally known as the Hall-Héroult process, has successfully withstood many attempts to supplant it; it remains the only method to produce aluminum in commercial quantities. Etymology The metal originally obtained its name from the Latin word for alum, alumen. The name alumina was proposed by L. B. G. de Moreveau, in 1761 for the base in alum, which was positively shown in 1787 to be the oxide of a yet to be discovered metal. Finally, in 1807, Sir Humphrey Davy proposed that this still unknown metal be referred to as aluminum. This was then altered further to that of aluminium so to agree with the "ium" spelling that ended most of the elements. This is the spelling that is generally used throughout the world. That is, until the American Chemical Society in 1925 officially reverted the spelling back to aluminum, which is how it is normally spelled in the United States. Production Process Stage 1 Converting Bauxite to Alumina STEP 1- Crushing and Grinding: Alumina recovery begins by passing the bauxite through screens to sort it by size. It is then crushed to produce relatively uniformly sized material. The ore is then fed into large grinding mills and mixed with a caustic soda solution (sodium hydroxide) at high temperature and pressure. The grinding mill rotates like a huge drum while steel rods - rolling around loose inside the mill - grind the ore to an even finer consistency. The process is a lot like a kitchen blender only much slower and much larger. The material finally discharged from the mill is called slurry. The resulting liquor contains a solution of sodium aluminate and undissolved bauxite residues containing iron,silicon, and titanium. These residues - commonly referred to as "red mud" - gradually sink to the bottom of the tank and are removed. STEP 2-Digesting: The slurry is pumped to a digester where the chemical reaction to dissolve the alumina takes place. In the digester the slurry under 50 pounds per square inch pressure - is heated to 300 °Fahrenheit (145 °Celsius). It remains in the digester under those conditions from 30 minutes to several hours. More caustic soda is added to dissolve aluminum containing compounds in the slurry. Undesirable compounds either don't dissolve in the caustic soda, or combine with other compounds to create a scale on equipment which must be periodically cleaned. The digestion process produces a sodium aluminate solution. Because all of this takes place in a pressure cooker, the slurry is pumped into a series of "flash tanks" to reduce the pressure and heat before it is transferred into "settling tanks." STEP 3-Settling: Settling is achieved primarily by using gravity, although some chemicals are added to aid the process. Just as a glass of sugar water with fine sand suspended in it will separate out over time, the impurities in the slurry - things like sand and iron and other trace elements that do not dissolve - will eventually settle to the bottom. The liquor at the top of the tank (which looks like coffee) is now directed through a series of filters. After washing to recover alumina and caustic soda, the remaining red mud is pumped into large storage ponds where it is dried by evaporation. The alumina in the still warm liquor consists of tiny, suspended crystals. However there are still some very fine, solid impurities that must be removed. Just as coffee filters keep the grounds out of your cup, the filters here work the same way. The giant-sized filters consist of a series of "leaves" - big cloth filters over steel frames - and remove much of the remaining solids in the liquor. The material caught by the filters is known as a "filter cake" and is washed to remove alumina and caustic soda. The filtered liquor - a sodium aluminate solution - is then cooled and pumped to the "precipitators." STEP 4-Precipitation: Imagine a tank as tall as a six-story building. Now imagine row after row of those tanks called precipitators. The clear sodium aluminate from the settling and filtering operation is pumped into these precipitators. Fine particles of alumina - called "seed crystals" (alumina hydrate) - are added to start the precipitation of pure alumina particles as the liquor cools. Alumina crystals begin to grow around the seeds, then settle to the bottom of the tank where they are removed and transferred to "thickening tanks." Finally, it is filtered again then transferred by conveyor to the "calcination kilns." STEP 5-Calcination: Calcination is a heating process to remove the chemically combined water from the alumina hydrate. That's why, once the hydrated alumina is calcined, it is referred to as anhydrous alumina. "Anhydrous" means "without water." From precipitation, the hydrate is filtered and washed to rinse away impurities and remove moisture. A continuous conveyor system delivers the hydrate into the calcining kiln. The calcining kiln is brick-lined inside and gas-fired to a temperature of 2,000 °F or 1,100 °C. It slowly rotates

(to make sure the alumina dries evenly) and is mounted on a tilted foundation which allows the alumina to move through it to cooling eqipment. (Newer plants use a method called fluid bed calcining where alumina particles are suspended above a screen by hot air and calcined.) The result is a white powder like that shown below: pure alumina. The caustic soda is returned to the beginning of the process and used again. At this point, the alumina is ready for conversion into aluminum at a smelter. Alumina is also used in making chemical and ceramics. Stage 2 Converting Alumina to Aluminum Smelting: In 1886, two 22-year-old scientists on opposite sides of the Atlantic, Charles Hall of the USA and Paul L.T. Heroult of France, made the same discovery - molten cryolite (a sodium aluminum fluoride mineral) could be used to dissolve alumina and the resulting chemical reaction would produce metallic aluminum. The Hall-Heroult process remains in use today. The Hall-Heroult process takes place in a large carbon or graphite lined steel container called a " reduction pot". In most plants, the pots are lined up in long rows, called potlines. The key to the chemical reaction necessary to convert the alumina to metallic aluminum is the running of an electrical current through the cryolite/alumina mixture. The process requires the use of direct current (DC) - not the alternating current (AC) used in homes. The immense amounts of power required to produce aluminum is the reason why aluminum plants are almost always located in areas where affordable electrical power is readily available. Some experts maintain that one percent of all the energy used in the United States is used in the making of aluminum. The electrical voltage used in a typical reduction pot is only 5.25 volts, but the amperage is VERY high - generally in the range of 100,000 to 150,000 amperes or more. The current flows between a carbon anode (positively charged), made of petroleum coke and pitch, and a cathode (negatively charged), formed by the thick carbon or graphite lining of the pot. When the electric current passes through the mixture, the carbon of the anode combines with the oxygen in the alumina. The chemical reaction produces metallic aluminum and carbon dioxide. The molten aluminum settles to the bottom of the pot where it is periodically syphoned off into crucibles while the carbon dioxide - a gas - escapes. Very little cryolite is lost in the process, and the alumina is constantly replenished from storage containers above the reduction pots. The metal is now ready to be forged, turned into alloys, or extruded into the shapes and forms necessary to make appliances, electronics, automobiles, airplanes cans and hundreds of other familiar, useful items. Aluminum is formed at about 900 °C, but once formed has a melting point of only 660 °C. In some smelters this spare heat is used to melt recycled metal, which is then blended with the new metal. Recycled metal requires only 5 per cent of the energy required to make new metal. Blending recycled metal with new metal allows considerable energy savings, as well as the efficient use of the extra heat available. When it comes to quality, there is no difference between primary metal and recycled metal. The smelting process required to produce aluminum from the alumina is continuous the potline is usually kept in production 24 hours a day year-round. A smelter cannot easily be stopped and restarted. If production is interrupted by a power supply failure of more than four hours, the metal in the pots will solidify, often requiring an expensive rebuilding process. The cost of building a typical, modern smelter is about $1.6 billion. Most smelters produce aluminum that is 99.7% pure - acceptable for most applications. However, super pure aluminum (99.99%) is required for some special applications, typically those where high ductility or conductivity is required. It should be noted that what may appear to be marginal differences in the purities of smelter grade aluminum and super purity aluminum can result in significant changes in the properties of the metal. Properties Aluminum falls into group three on the periodic table, with 13 protons and 14 neutrons, giving it a mass of 27. Throughout this website the different properties of the element aluminum are examined, and explained, this section is simply a complete list of the different characteristics of aluminum, with links to other sections of the website where they are discussed in further depth. Youngs Modulus: 70 GPa (in alloys) This is a measure of elasticity, or how easily something with stretch. It is found by dividing stress by strain. Where stress is the load applied to the material divided by the cross sectional area of the material, and where strain in the increase in length over the original length (so percentage increase of length). As stress is measured in Pascals (or force per unit area) and strain is imply a ratio without units the Youngs modulus is also measured in Pa. Aluminum's Youngs modulus is quite low, and elements like Iron for example have much higher values such as 211 GPa. Aluminum's alloys in general have higher modulus's of elasticity due to pinned dislocations in their structure. Ductility & Malleability: High These are also a property relating to how easily deformation occurs. Aluminum is both very malleable, and very ductile. In fact aluminum is the 2nd most malleable metal, and the 6th most ductile, both of these are very important for its uses. Malleability is the property of a metal to be deformed by compression without cracking or rupturing, and ductility is the ability to deform plastically without fracture under tensile force. In a practical sense if a material is malleable it means that it is possible to roll it into sheets, and if it is ductile it can be drawn into wires. Aluminum can do both like most metals can. The structure of metals as positive ions in a "sea" of negative electrons means that the positions of the ions are not as fixed as one may first imagine, and as the ions are just held in place by attraction they can slide past each other giving metals malleability and ductility. Hardness: 420 MPa Hardness is the ability to not be easily scratched, in is measured in Pascals or force per unit area. Aluminum's hardness is relatively low due to its low density (see below). This means that it is easier to scratch than other metals like steel are. Density: Low 2700 kg / cubic metre This gives aluminum its unique lightweight property and extends the uses of aluminum vastly. Really this is the key property that sets aluminum apart from so many other metallic elements. Transport vessels made of aluminum can carry more cargo with the same amount of fuel than vessels made of other materials as the aluminum itself it so much lighter. Why does it have such a low density? Because of the number of

protons and neutrons (which make up almost all of an atoms weight) in the nucleus. Aluminum has a mass of just 27, which is very low compared to most other metals. Melting Point: 660.32 °C Boiling Point: 2519 °C These relatively low thermal points help aluminum to be reshaped, and welding quite easily. For a metal a melting point of 660.32 °C is very low. Electrical Resistivity: Low 2.65 x 10-8 Ohm metres Aluminum has a very low electrical resistivity, and therefore a high electrical conductivity. This is measured in Ohms meters, as resistivity is equal to the resistance of a certain sized piece of the material multiplied by the area and divided by the length. This gives a standard measure for a material that can be used easily in comparison. In relation to other metals aluminum has quite a low resistivity, it is not the best conductor, but it is by no means the worst. Aluminum is also a very good thermal conductor as is discussed in conductivity. Reflectivity: High 71% unpolished and when polished: 97% Aluminum is extremely reflective, when polished (some sources say) it is the most reflective material, for this reason it is now frequently used in lights. It is also very heat reflective and because of this people use it in their car windscreens to reflect the heat, and to keep the car cool on sunny days. Aluminum has a unique set of properties that make it so essential to the modern world. It is ductile, strong (in alloys), lightweight, highly conductive, and extremely reflective. Application The oxide quickly formed in surface of the pure metal turns it ideal for a lot of decoration applications. Because it has high electrical conductivity, ductility and low atomic mass, it is frequently used in electrical transmission lines. The metal has also been used in the coating of telescope mirrors, as well as in the production ofaluminum foils, used in the packing of victuals. In the pure form, the metal has a small mechanical resistance, being therefore generally used in alloys with copper,manganese, silicon, magnesium and zinc, with a wide range of mechanical properties. These alloys are used in construction, airplane and automobile structures, traffic signs, heat dissipatives, storage deposits, bridges and kitchen utensils.

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