Aluminum (Al)

Where Found

Aluminum is the most abundant metallic element in the Earth’s crust, comprising 8.3 percent of the mass in the crust by weight. As an element it is exceeded in crustal abundance only by oxygen and silicon. Commercially, the most important aluminum ore is bauxite, a mixed aluminum oxide hydroxide with a composition that varies with climate. Large reserves of this mineral exist, typically found in thick layers with little topsoil or overburden so that it can be easily mined. Worldwide reserves are tremendously large, notably in Australia, Africa, Brazil, and countries in Central America.

Primary Uses

By far the main use of aluminum in its metallic form is as a structural material in the construction and transportation (particularly aircraft) industries. Another major use is as a container material, of which the soft drink can is the most widely recognizable example. Aluminum is a more effective conductor per unit of mass than copper, so it is a more versatile material for power lines. Electrical transmission lines thus account for a sizable fraction of total world production as well.

Technical Definition

Aluminum (atomic number 13) is a member of the boron group (Group III) of the periodic table of the elements. In terms of chemical and physical properties, the metallic element aluminum is more like boron than the other elements in the group. There is only one stable, naturally occurring isotope of aluminum, with an atomic weight of 26.98154. The pure solid exists in a single crystalline form in which every aluminum atom in the solid-state lattice is surrounded by twelve others at equal distances. Aluminum has a density of 2.699 grams per cubic centimeter. It has a melting point of 660.37° Celsius and a boiling point of 2,467° Celsius.

Description, Distribution, and Forms

Aluminum is the most abundant metallic element accessible in the Earth’s crust. Of the other metallic elements, only iron and copper display abundances approaching that of aluminum. Aluminum is a constituent of igneousminerals such as feldspar and mica. When these ores weather, they tend to generate clays such as kaolinite and vermiculite. These materials are widespread in the Earth’s crust. In addition, aluminum may be found in rarer minerals such as cryolite, spinel, beryl, turquoise, and corundum. Aluminum compounds are important as precious minerals as well. The presence of a slight trace of a transition metal impurity in the aluminum oxide crystalline lattice typically imparts color to the solid. The ore of central commercial importance in the primary extraction of aluminum is bauxite, a mixed aluminum oxide and hydroxide first discovered in 1821. It is generated when silica and other materials are leached by weathering from silicates of aluminum.

An aluminum compound of some importance is lithium aluminum hydride. This compound is an effective reducing agent and functions as a hydrogenating agent. It was a mainstay in organic synthesis until supplanted by organometallic hydrides that are less expensive to produce and easier to manipulate. Commercial production of lithium aluminum hydride dates from around 1950. Within twenty years the compound had displayed reactivity with more than sixty types of organic functional groups (the part of an organic molecule that gives it a characteristic reactivity, such as a hydroxyl group).

Total annual worldwide production of aluminum from its ores is almost 38 million metric tons, and there are large-scale production plants in many locations around the world. The largest producer of primary aluminum is China with approximately 13 million metric tons, which accounts for 33 percent of the world total. Russia and Canada account for another 20 percent of the world’s production. The United States, Australia, and Brazil are also major producers of primary aluminum.

The need for primary extraction of aluminum from its ores is somewhat alleviated because of the ease with which metallic aluminum can be recycled. Recycling requires a fraction of the energy cost of primary extraction. Primary extraction consumes almost two metric tons of ore for every metric ton of aluminum produced; it also consumes approximately one-half of a metric ton of carbon, one-tenth of a metric ton of cryolite, and fifteen thousand kilowatt-hours of electrical energy. The air pollution one might expect from such a large energy output is moderated by the fact that aluminum production plants typically are run by hydroelectric power. This means that often the location in which the ore is mined and the location in which the metal is extracted are widely separated. (For most metals, the refining plant is situated as close as possible to the material source to minimize transportation costs.)

Aluminum and most aluminum compounds are relatively benign in the environment. Primary environmental impact results from the direct electrochemical extraction of aluminum from its ores. The process requires a tremendous amount of electrical energy, an applied potential of about 4 volts and a current of 105 amperes, and a reaction vessel heated to about 900° Celsius. These power requirements place tremendous demand on resources, but the availability of hydroelectric power has minimized the air and thermal pollution produced by these plants.

Aluminum also provides a means of determining acid rain effects. When lakes become acidic, aluminum can be leached from the soil and dissolved in the water. A water sample can be treated with appropriate agents that bind with the metal to form brightly colored species. The concentration of aluminum can be determined by measuring the amount of light these materials absorb. This in turn provides information about the extent of acid contamination by determining the amount of aluminum leached into the water.

History

Although aluminum is the most abundant metallic element accessible on Earth, it was not isolated in its elemental form until the early 1800s. Even then, for most of the nineteenth century, its rarity made it a precious metal with an expensive price tag and uses centering on decorative rather than practical applications. Chemical thermodynamics, the study of energy relationships in chemical reactions, provides a basis for understanding why isolating aluminum was so difficult and why a pyrometallurgical technique could not be used.

Most metallic elements, with the exception of gold, silver, and a few others, tend to react with oxygen in the atmosphere or with other elements and form compounds rather than remain in their elemental, metallic state. To isolate the metal from the compound in its elemental form, the metal must receive electrons in a process called reduction. Typically this process requires heat energy, so the reduction (or smelting) must be done at elevated temperatures. Some elements, such as copper and lead, can be reduced at a relatively low, easily attainable temperature. Because of their ease of extraction, these elements have been known for a long time. Other elements, such as iron, require a much higher temperature for the reduction reaction to proceed at an appreciable rate. These higher temperatures are much more difficult to obtain, requiring a higher level of technology, especially in terms of furnace development. Elemental iron could not be produced until this higher technological level was reached. Still other elements, such as aluminum, can be made to undergo chemical reduction only at temperatures so high that they are not commercially attainable or physically containable. Rather than being reduced at high temperatures, they are reduced by utilizing an electrochemical reaction that requires application of an electrical potential high enough to overcome the energy barrier to the reaction. This procedure eliminates the need for the extremely high temperatures of a pyrometallurgical reduction, but it requires the existence of a large, reliable source of electricity for industrial-scale production to be feasible. Such an electrical source did not exist until the nineteenth century, when the dynamo was invented. Availability of the dynamo and hydroelectric power provided the massive amounts of electrical energy needed to produce large quantities of metallic aluminum.

While the elemental form of aluminum was rare and even unknown prior to the technological advances of the 1800s, compounds containing aluminum have been known since the days of ancient Greece and Rome. The very name “aluminum” is derived from “alum,” the common name for the hydrate of aluminum potassium sulfate. In the Greek and Roman civilizations, alum was known as an astringent (a substance that causes muscles to contract) and a mordant (a substance that causes dye molecules to adhere to cloth). Sir Humphry Davy, a pioneer of electrochemical methods, attempted to isolate the metal but was unsuccessful. He proposed the name alumium, a name consistent in form with such names as sodium and potassium, elements that Davy had discovered in 1807 and named in terms of their historical sources (soda ash and potash). This was changed soon after to aluminum and was still further modified to aluminium. The United States is one of the few countries still preferring the use of the name aluminum.

The discovery of aluminum was facilitated by Davy’s isolation of sodium and potassium in their elemental forms. These elements are effective reducing agents, meaning they have a strong tendency to force electrons onto other materials, thereby reducing their charge. In the early 1800s, Hans Christian rsted used sodium amalgamated with mercury to chemically reduce aluminum from aluminum chloride. While he was able to isolate and identify aluminum in this process, the reaction was inefficient, yielding small amounts of impure aluminum. In 1845, Friedrich Wöhler prepared samples of aluminum that were large enough to allow preliminary determination of the elusive element’s chemical and physical properties. The results of these studies stimulated a great deal of academic interest in the metal, although little interest in commercial utilization of the metal arose.

The first commercial production method was developed in 1854 by Henri-Étienne Sainte-Claire DeVille. This method used sodium as a reducing agent. Its introduction caused the price of aluminum to drop dramatically. Aluminum had cost about twelve hundred dollars per kilogram; with the introduction of the improved synthetic method, the price dropped to less than five hundred dollars per kilogram in the mid-nineteenth century. Even then, the element was perceived more as a curiosity than as a useful natural resource. At the time of the Paris exposition in 1855, the samples of aluminum displayed were billed as “silver from clay,” a term evocative of both the perceived value of the element and the typical source from which it was derived.

By the year 1880, construction of a plant capable of producing almost fifty thousand kilograms of aluminum per year using the Sainte-Claire DeVille process was begun. By the time the plant had been built and gone into full-scale production, the cost of aluminum had dropped to about twenty dollars per kilogram. During this same time period, the first serious attempt to produce aluminum in the United States was made byWilliam Frishmuth. Among the more notable items produced was a three-kilogram pyramid of aluminum that was used to cap the Washington Monument as a functional part of its lightning rod system. Prior to its installation, this oddity was placed on public display.

Obtaining Aluminum

The difficulty in extracting aluminum from its ores, coupled with the interesting properties of the metal, made the quest for an economical means of production a target of many prospective inventors. The key discoveries needed to realize large-scale production were made almost simultaneously in 1886 by two young men, Charles Martin Hall in the United States and Paul Héroult in France. Both men were in their early twenties. The key to the success of these individuals lay in the extractive technique they utilized (an electrochemical method) and critical experimental modifications that they made. Hall correctly deduced that, at the high operating temperatures of his apparatus, impurities from the clay container he was using as a reaction vessel may have been causing undesired side reactions to take place, thus preventing aluminum formation. He eliminated the clay vessel and instead used one lined with graphite, a form of elemental carbon. The graphite also served as one of the electrodes in the electrochemical cell. With these modifications, Hall was able to produce large pieces of aluminum. These pieces were kept by the company Hall founded and dubbed the “crown jewels.” Commercial production escalated rapidly, resulting in another tremendous downward shift in price, from around twelve dollars per kilogram to about seventy-five cents per kilogram only fifteen years later.

Uses of Aluminum

Aluminum is of great importance in modern society. Many of the engineering accomplishments of the twentieth century would have been impossible without the availability of aluminum. Modern airliners would be impossible to construct without it. The importance of aluminum is based on its great strength, light weight, and resistance to corrosion. This resistance occurs because typically a thin film of chemically inert aluminum oxide forms on the surface of the metal, shielding it from further corrosion. This protective layer can be artificially produced through an anodization process. Production of differing oxide layer thicknesses provides a variety of appearances and properties in the anodized materials. An oxide layer on the order of 15 micrometers in thickness gives sufficient corrosion protection for exterior structural use. Aluminum’s appearance makes it a desirable external feature. The first skyscraper to be clad with aluminum was built in the 1950’s. The limited tensile strength of aluminum means that alloys typically offer better service as structural components. There is no doubt that, after the construction industry, the aviation industry is the greatest beneficiary of aluminum’s benefits. Aluminum and aluminum alloys make possible strong, lightweight airframes.

In addition to their major applications when in the metallic state, aluminum compounds are utilized in industrial processes and to make materials ranging from cements to chemical reaction catalysts. Aluminum compounds are important in the production of portland cement, first patented in 1824. This strong cement does not dissolve in water. John Smeaton found aluminum silicates to be an important ingredient of this cement. Portland cement contains approximately 11 percent by mass tricalcium aluminate. Although it was known to be an important component, the chemical structure of this compound was not known until 1975. The aluminate ion is a cyclic ion, with a key structural feature of individual aluminum and oxygen atoms linked alternately into a twelve-member ring. The reaction of this material with water is a key step and must be properly controlled to facilitate the setting of the cement. After the production process, the finished cement contains about 5 percent aluminum oxide. The portland cement industry is an important one, with production in the United States on the order of 85 million metric tons per year. Another versatile cement, ciment fondu, is useful in marine environments and contains up to 40 percent aluminum oxide.

While aluminum itself is light and strong, its properties can typically be enhanced by an alloying process. Typically, aluminum is alloyed with copper, manganese, zinc, or silicon. Each of these alloying agents produces alloys with certain desired properties. The strength-to-weight factor is enhanced in copper alloys. Silicon alloys have low melting points and do not expand much when heated. This property makes these materials particularly useful as welding filler and for the production of castings. The corrosion resistance of aluminum is enhanced by alloying with magnesium. These alloy types are widely used in ship construction, especially as external fittings. Perhaps the most widely seen examples of aluminum alloys are the manganese alloys, used in cookware, storage tanks, furniture, and highway signs.

One of the more spectacular uses of aluminum is based on its strong affinity for oxygen and its tendency to lose electrons (to be oxidized) in chemical reactions. This is well demonstrated by the thermite reaction, in which aluminum reacts with another metal oxide to reduce the other metal to its elemental form and generate aluminum oxide. This type of reaction tends to liberate considerable thermal energy and proceed very rapidly. This rapid heat evolution causes the other metal to be generated in a molten form. Thermite reactions are used in welding operations.

While there is only one stable isotope of aluminum, a short-lived radioactive isotope, aluminum 26, it is of potential use in dating the universe. There is evidence from ancient rock samples that this isotope was common in the early solar system, and this isotope decays to a magnesium isotope. Measurement of the ratio of this decay product to the remaining aluminum 26 allows determination of the age of the object, in much the same way that the ratio of isotopes is used in carbon dating. From these measurements, estimating the length of time it took the solar system to develop becomes possible.

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