How Aluminum Works | HowStuffWorks

If there were ever an element that could have been voted "least likely to succeed," it would be alumin­um. Although ancient Persian potters added aluminum to their clay to strengthen their pottery, pure aluminum wasn't discovered until 1825. By then, humans had been using several metals and metal alloys (or mixtures of metal such as bronze) for thousands of years.­

Even after its discovery, aluminum seemed destined for obscurity. Chemists could only isolate a few milligrams at a time, and it was so rare that it sat beside gold and silver as a semiprecious metal. Indeed, in 1884, the total U.S. production of aluminum was just 125 pounds (57 kilograms) [source: Alcoa].

­Then, in 1886, American Charles Martin Hall and Frenchman Paul L. T. Heroult, working independently, devised a method to extract aluminum from aluminum oxide. The process, a type of electrolytic reduction, required an enor­mous amount of electrical power, but it produced the silvery-white metal in large quantities. By 1891, production of aluminum had reached well over 300 tons (272 metric tons) [source: Alcoa]. And it was finding its way into a vast array of products, from pots and pans to light bulbs and power lines to cars and motorcycles.

Today, more than a century later, aluminum is the very symbol of ubiquity. Every year, the United States produces more than 5.6 million tons (5.1 million metric tons) [source: International Aluminum Institute]. Much of that aluminum goes into beer and soda cans -- to the tune of 300 million aluminum beverage cans a day, 100 billion a year [source: Can Manufacturers Institute]. Not bad for an element that went undiscovered for such a long time.

In this article, w­e'll take a closer look at aluminum -- its properties, occurrence and behavior. We'll also examine the life cycle of aluminum, from its production using the Hall-Heroult process to its reincarnation after recycling. And, finally, we'll explore all of the uses for aluminum, including some future uses that may surprise you.

Aluminum 101

Like dozens of other elements on the periodic table, aluminum is naturally occurring. As with all elements, aluminum is a pure chemical substance that can't be broken down into something simpler. All elements are arranged in the periodic table by their atomic number -- the number of protons in their nucleus. Aluminum's lucky number is 13, so an aluminum atom has 13 protons. It also has 13 electrons.

The elements located above and below aluminum on the periodic table form a family, or group, that shares similar properties. Aluminum belongs to group 13, which also includes boron (B), gallium (Ga), indium (In) and thallium (Tl). The table to the right shows how these elements would be arranged on the periodic table. Notice that each element is represented by a symbol and that the symbol for aluminum is Al. The number above each symbol is the element's atomic weight, measured in atomic mass units (amu). Atomic weight is the average mass of an element determined by considering the contribution of each natural isotope. Aluminum's atomic weight is 26.98 amu. The number below aluminum's symbol is its atomic number.

Chemists classify the elements in group 13 as metals, except for boron, which isn't a full-fledged metal. Metals are generally shiny elements that conduct heat and electricity well. They're also malleable -- able to be hammered into various shapes -- and ductile -- able to be drawn into wires. These characteristics certainly apply to aluminum. In fact, aluminum is often used in cookware because it conducts heat so efficiently. And only copper conducts electricity better, which makes aluminum an ideal material for electrical material, including light bulbs, power lines and telephone wires. Other important properties of aluminum are listed below:

• Melting point: 660 degrees C (933 K; 1,220 degrees F)
• Boiling point: 2,519 degrees C (2,792 K; 4,566 degrees F)
• Density: 2.7 g/cm3
• High reflectivity
• Nonmagnetic
• Nonsparking
• Resistant to corrosion

These final two properties make aluminum particularly useful. Its corrosion resistance is due to chemical reactions that take place between the metal and oxygen. When aluminum reacts with oxygen, a layer of aluminum oxide forms on the outside of the metal. This thin layer shields the underlying aluminum from the corrosive effects of oxygen, water and other chemicals. As a result, aluminum is especially valuable for use outdoors. It also doesn't produce sparks when struck, which means you can use it near flammable or explosive materials.

Aluminum exists in nature in various compounds. To take advantage of its properties, it must be separated from the other elements that combine with it -- a long, complex process that starts with a rock-hard material known as bauxite.

After it undergoes that process, aluminum is very soft and lightweight in its pure form. Sometimes it's desirable to change these properties -- to make aluminum stronger and harder, for instance. To accomplish this, metallurgists will combine aluminum with other metallic elements, forming what are known as alloys. Aluminum is commonly alloyed with copper, magnesium and manganese. Copper and magnesium increase the strength of aluminum, while manganese enhances aluminum's corrosion resistance.

Mining and Refining Aluminum

Aluminum isn't found in nature as a pure element. It exhibits relatively high chemical reactivity, which means it tends to bond with other elements to form compounds. More than 270 minerals in Earth's rocks and soils contain aluminum compounds. This makes aluminum the most abundant metal and the third most abundant element in Earth's crust. Only silicon and oxygen are more common than aluminum. The next most common metal after aluminum is iron, followed by magnesium, titanium an­d manganese.­

The primary s­ource of aluminum is an ore known as bauxite. An ore is any naturally occurring solid m­aterial from which a metal or valuable mineral can be obtained. In this case, the solid material is a mixture of hydrated aluminum oxide and hydrated iron oxide. Hydrated refers to water molecules that are chemically bound to the two compounds. The chemical formula for aluminum oxide is Al2O3. The formula for iron oxide is Fe2O3­.

Deposits of bauxite occur as flat layers lying near the Earth's surface and may cover many miles. Geologists locate these deposits by prospecting -- taking core samples or drilling in soils suspected of containing the ore. By analyzing the cores, scientists are able to determine the quantity and quality of the bauxite.

­After th­e ore is discovered, open-pit mines typically provide the bauxite that will eventually become aluminum. First bulldozers clear land above a depos­it. Then workers loosen the soil with explosives, which bring the ore to the surface. Giant shovels then scoop up the bauxite-rich soil and dump it into trucks, which carry the ore to a processing plant. ­France was the first site of large-scale bauxite mining. In the United States, Arkansas was a major supplier of bauxite before, during and after World War II. But today, the material is predominantly mined in Australia, Africa, South America and the Caribbean.

The first step in the commercial production of aluminum is the separation of aluminum oxide from the iron oxide in bauxite. This is accomplished using a technique developed by Karl Joseph Bayer, an Austrian chemist, in 1888. In the Bayer process, bauxite is mixed with caustic soda, or sodium hydroxide, and heated under pressure. The sodium hydroxide dissolves the aluminum oxide, forming sodium aluminate. The iron oxide remains solid and is separated by filtration. Finally, aluminum hydroxide introduced to the liquid sodium aluminate causes aluminum oxide to precipitate, or come out of solution as a solid. These crystals are washed and heated to get rid of the water. The result is pure aluminum oxide, a fine white powder also known as alumina.

­Alumina is a handy material in its own right. Its hardness makes it useful as an abrasive and as a component in cutting tools. It can also be used to purify water and to make ceramics and other building materials. But its primary use is to act as a starting point to extract pure aluminum. In the next section, we'll see look at the steps required to transform alumina into aluminum.

Aluminum Smelting

­Transforming alumina -- aluminum oxide -- into aluminum represented a major milestone in the industrial revolution. Until modern smelting techniques evolved, only small quantities of aluminum could be obtained. Most ea­rly processes relied on displacing aluminum with more reactive metals, but the metal remained expensive and relatively elusive. That all changed in 1886 -- the year two aspiring chemists and industrialists developed a smelting process based on electrolysis.

Electrolysis literally means "breaking down by electricity," and it can be used to decompose one chemical into component chemicals. The traditional setup for electrolysis requires two metal electrodes being submerged in a liquid or molten sample of a material containing positive and negative ions. When the electrodes are connected to a battery, one electrode becomes a positive terminal, or anode. The other electrode becomes a negative terminal, or cathode. Because the electrodes are electrically charged, they attract or repel charged particles dissolved in the solution. The positive anode attracts negatively charged ions, while the negative cathode attracts positively charged ions.

Sir Humphry Davy, the British chemist credited with giving aluminum its name, tried unsuccessfully to produce aluminum by electrolysis in the early 1800s. The French schoolteacher and amateur chemist Henri Saint-Claire Deville also came up empty-handed. Then, in February 1886, after several years of experimentation, American Charles Martin Hall came across just the right formula: passing a direct current through a solution of alumina dissolved in molten cryolite, or sodium aluminum fluoride (Na3AlF6). Until 1987, cryolite was mined from deposits found on the west coast of Greenland. Today, chemists synthesize the compound from the mineral fluorite, which is much more common.

The steps in aluminum smelting are described below:

2Al2­O3 + 3C -> 4Al + 3CO2­

The alumin­um smelting process developed by Hall resulted in large amounts of pure aluminum. Suddenly, the metal was no longer rare. The idea of producing aluminum via electrolytic reduction in cryolite wasn't rare, either. A Frenchman by the name of Paul L.T. Heroult came up with the same idea just a few months later. Hall, however, received a patent for the process in 1889, one year after he founded the Pittsburgh Reduction Company, which would later become the Aluminum Company of America, or Alcoa. By 1891, aluminum production reached well over 300 tons (272 metric tons) [source: Alcoa].

Aluminum Fabricating

The vats used in the Hall-Heroult process are known as pots. A large pot can produce more than 2 tons of aluminum each day. But companies can and do multiply that output by connecting several pots togethe­r in potlines. One smelting plant may contain one or more potlines, each with 200 to 300 pots. Inside these pots, aluminum production continues day and night to make sure the metal remains in its liquid form.

Once a day, workers siphon aluminum from the potlines. Much of the metal is set aside to become fabricating ingots. To make a fabricating ingot, molten aluminum proceeds to large furnaces where it can be mixed with other metals to form alloys. From there, the metal undergoes a cleaning process known as fluxing. Fluxing uses gases such as nitrogen or argon to separate impurities and bring them to the surface so they can be skimmed away. The purified aluminum is then poured into molds and cooled rapidly by spraying cold water over the metal.

Some of the aluminum siphoned from the potlines isn't alloyed or cleaned. Instead, it's poured directly into molds, where it cools slowly and hardens to form foundry (or remelt) ingots. Primary aluminum plants sell remelt ingots to foundries. Foundries return the aluminum to its liquid state and proceed with the alloying and fluxing themselves. They then turn the aluminum into various parts -- for appliances, automobiles and other applications -- by using the following fabricating techniques.­

• Casting: Aluminum can be cast into an infinite variety of shapes by pouring the molten metal into a mold. As the aluminum cools and hardens, it takes the shape of the mold. Casting is used to make solid, uniquely shaped objects, such as parts for car engines, aluminum hammers and the bottoms of electric irons.
• Rolling: By repeatedly passing heated aluminum ingots through heavy rollers, the metal can be flattened into thin sheets or even wafer-thin foils. It takes about 10 to 12 passes to make the thinnest foils, which can be a mere 0.15-millimeter thick.
• Extruding: Extrusion involves forcing softened aluminum through a die. The shape of the die opening determines the shape of the extruded aluminum.
• Forging: Forging, a process whereby aluminum is hammered or pressed, results in superstrong metal. This method makes forged aluminum ideal for stress-bearing parts of aircraft and automobiles.

­A Beverage Can Is BornA beverage can starts with a circular piece of metal punched from an aluminum sheet. This circle, which is 5.5 inches (14.0 cm) in diameter, is called a blank. One machine draws the blank into a cup with a diameter of 3.5 inches (8.9 cm). A second machine redraws the cup, elongating it, ironing it and thinning out the sides. Finally, the can is cleaned, decorated and "necked" to accommodate the lid.

­ ­­
• Drawing: To make wire, an aluminum rod is pulled through a series of successively smaller dies, a process known as drawing. Drawing aluminum can produce wire that is less than 10 millimeters in diameter.
• Machining: Traditional machining operations, such as turning, milling, boring, tapping and sawing, are easily performed on aluminum and its alloys. Machining is often used to produce bolts, screws and other small pieces of hardware.­

­Aluminum is an attractive metal and often requires no finish. But it can be polished, painted and electroplated. For example, beer and soda makers use a printing process to affix their labels on aluminum cans (see sidebar). Typical printing formulations are often lacquer coatings that both adhere well to the aluminum and provide aesthetic appeal. Of course, such finishes are a concern when it comes to recycling because they must be removed. In the next section, we'll explore how aluminum is recycled in detail.

Using and Recycling Aluminum

Because of its versatility, aluminum lends itself to numerous applications. In fact, it's the second-most used metal after steel, with annual primary production reaching 24.8 million tons (22.5 million metric tons) in 2007 [source: International Aluminum Institute]. Much of that output goes to the 187 billion aluminum cans produced worldwide [source: Novelis]. The automotive industry is aluminum's fastest-growing market. Making car parts from aluminum -- everything from wheel rims to cylinder heads, pistons and radiators -- makes a car lighter, reducing fuel consumption and pollution levels. By some estimates, a car incorporating 331 pounds (150 kg) of aluminum should see fuel consumption reduced by 0.43 gallons per 100 miles [source: Autoparts Report].

Here are some other important uses of aluminum.

• Automotive and transportation: car and motorcycle parts, airplane bodies and parts, license plates
• Building and construction: siding and roofing, gutters, window frames, interior and exterior paint, hardware
• Cans and closures: beverage and food cans, bottle closures
• Packaging: aluminum foil, foil wraps, aluminum trays, candy and gum wrappers
• Electrical: power and telephone lines, light bulbs
• Health and hygiene: antacids, astringents, buffered aspirin, food additives
• Cooking: utensils, pots and pans
• Sporting goods and recreation: golf clubs and baseball bats, lawn furniture
  

­A­mazingly, most of the aluminum ever made is still in use today. That's because it can be recycled over and over again without losing its quality. Most aluminum that gets recycled comes from one of three sources: used beverage cans, parts from old automobiles and scrap collected during the manufacture of aluminum products [source: World Book]. Aluminum can recycling is one of the great successes of the modern sustainability movement (If you're a big recycler, be sure to read What one thing should I recycle?). The first national can-recycling program began in 1968, and today, about 66 billion cans are recycled each year in the United States alone [source: Alcoa].

Aluminum can recycling is a closed-loop process, which means the new product made after the recycling process is the same as the one before. There are six steps to closed-loop can recycling:

­Muc­h of the innovation in the aluminum industry is related to improving the efficiency of production and recycling. But, as we'll see in the next section, the demand for aluminum will only grow as new and exciting applications emerge.

The Future of Aluminum

­Primary production of aluminum requires tremendous energy. It also produces greenhouse gases that affect global warming. According to the International Aluminum Institute, manufacturing new stocks of aluminum releases 1 percent of the global human-induced greenhouse gas emissions. A top industry priority is to decrease these emissions through reduction measures, increased recycling and the use of aluminum in vehicles, aircraft, watercraft and trains. In fact, using lightweight aluminum components in vehicles is one of the most significant advances in automotive design and manufacturing. Every kilogram (2.2 pounds) of heavier material that is replaced by aluminum results in the elimination of 22 kilograms (44 pounds) of carbon dioxide over the lifetime of the vehicle [source: International Aluminum Institute].

Another promising application is the use of aluminum in fuel-cell-powered cars. Researchers at Purdue University recently discovered that aluminum could be used to produce hydrogen fuel efficiently. The process begins with aluminum pellets, which are mixed into liquid gallium to produce liquid aluminum-gallium. When ­water is added, the aluminum reacts with the oxygen to form a gel. Hydrogen gas, which can be collected and used to power a fuel cell, is also produced.

­Innovations such as these will increase the demand for aluminum. And even though the metal is relatively young, it is one of the most important in the history of human civilization. When the archaeologists and anthropologists of tomorrow reflect on the society of the 19th, 20th and 21st centuries, they could very likely label it the Aluminum Age, placing it next to the Stone, Bronze and Iron ages as one of the most significant periods in human cultural development.

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More Great Links

• International Aluminum Institute Web Site
• Alcoa Web Site
• Novelis Web Site
• Can Manufacturers Institute
  

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