So Firstly, what IS Aluminum?
Aluminum is a chemical element in the boron group with symbol Al and atomic number 13. It is a silvery-white, soft, nonmagnetic, ductile metal. Aluminium is the third most abundant element (after oxygen and silicon), and the most abundant metal in the Earth’s crust. It makes up about 8% by weight of the Earth’s solid surface. Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals. The chief ore of aluminium is bauxite.
Aluminium is remarkable for the metal’s low density and for its ability to resist corrosion due to the phenomenon of passivation. Structural components made from aluminium and its alloys are vital to the aerospace industry and are important in other areas of transportation and structural materials. The most useful compounds of aluminium, at least on a weight basis, are the oxides and sulfates.
Despite its prevalence in the environment, no known form of life uses aluminium salts metabolically. In keeping with its pervasiveness, aluminium is well tolerated by plants and animals. Owing to their prevalence, potential beneficial (or otherwise) biological roles of aluminium compounds are of continuing interest.
Aluminium is a relatively soft, durable, lightweight, ductile and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness. It is nonmagnetic and does not easily ignite. A fresh film of aluminium serves as a good reflector (approximately 92%) of visible light and an excellent reflector (as much as 98%) of medium and far infrared radiation. The yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa. Aluminium has about one-third the density and stiffness of steel. It is easily machined, cast, drawn and extruded.
Aluminium atoms are arranged in a face-centered cubic (fcc) structure. Aluminium has a stacking-fault energy of approximately 200 mJ/m2.
Aluminium is a good thermal and electrical conductor, having 59% the conductivity of copper, both thermal and electrical, while having only 30% of copper’s density. Aluminium is capable of being a superconductor, with a superconducting critical temperature of 1.2 Kelvin and a critical magnetic field of about 100 gauss (10 milliteslas).
Corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the metal is exposed to air, effectively preventing further oxidation. The strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper. This corrosion resistance is also often greatly reduced by aqueous salts, particularly in the presence of dissimilar metals.
In highly acidic solutions aluminium reacts with water to form hydrogen, and in highly alkaline ones to form aluminates— protective passivation under these conditions is negligible. Also, chlorides such as common sodium chloride are well-known sources of corrosion of aluminium and are among the chief reasons that household plumbing is never made from this metal.
However, owing to its resistance to corrosion generally, aluminium is one of the few metals that retain silvery reflectance in finely powdered form, making it an important component of silver-colored paints. Aluminium mirror finish has the highest reflectance of any metal in the 200–400 nm (UV) and the 3,000–10,000 nm (far IR) regions; in the 400–700 nm visible range it is slightly outperformed by tin and silver and in the 700–3000 (near IR) by silver, gold, and copper.
Aluminium is oxidized by water at temperatures below 280 °C to produce hydrogen, aluminium hydroxide and heat:
2 Al + 6 H2O → 2 Al(OH)3 + 3 H2
This conversion is of interest for the production of hydrogen. Challenges include circumventing the formed oxide layer, which inhibits the reaction and the expenses associated with the storage of energy by regeneration of the Al metal.
Natural Occurrence of Aluminum
Stable aluminium is created when hydrogen fuses with magnesium either in large stars or in supernovae.
In the Earth’s crust, aluminium is the most abundant (8.3% by weight) metallic element and the third most abundant of all elements (after oxygen and silicon). Because of its strong affinity to oxygen, it is almost never found in the elemental state; instead it is found in oxides or silicates. Feldspars, the most common group of minerals in the Earth’s crust, are aluminosilicates. Native aluminium metal can only be found as a minor phase in low oxygen fugacity environments, such as the interiors of certain volcanoes. Native aluminium has been reported in cold seeps in the northeastern continental slope of the South China Sea and Chen et al. (2011) have proposed a theory of its origin as resulting by reduction from tetrahydroxoaluminate Al(OH)4− to metallic aluminium by bacteria.
It also occurs in the minerals beryl, cryolite, garnet, spinel and turquoise. Impurities in Al2O3, such as chromium or iron yield the gemstones ruby and sapphire, respectively.
Although aluminium is an extremely common and widespread element, the common aluminium minerals are not economic sources of the metal. Almost all metallic aluminium is produced from the ore bauxite (AlOx(OH)3–2x). Bauxite occurs as a weathering product of low iron and silica bedrock in tropical climatic conditions. Large deposits of bauxite occur in Australia, Brazil, Guinea and Jamaica and the primary mining areas for the ore are in Australia, Brazil, China, India, Guinea, Indonesia, Jamaica, Russia and Suriname.
How Aluminum Is Made
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.
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.
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Rock and Minerals