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.
An aerial view of a bauxite mine and alumina processing plant in Australia
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.
Bauxite Ore Processing:
Aluminum is found in varying amounts in nature as aluminosilicates (contains aluminum, silicon, and oxygen) in various types of clay. As the minerals are weathered they gradually breakdown into various forms of hydrated aluminum oxide, Al2O3.xH2O, known as bauxite.
The bauxite is purified by the Bayer Process. First the ore is mixed with a hot concentrated solution of sodium hydroxide. The NaOH will dissolve the oxides of aluminum and silicon but not other impurities such as iron oxides, which remains insoluble. The insoluble materials are removed by filtration.
The solution which now contains the oxides of aluminum and silicon are next treated by bubbling carbon dioxide gas through the solution. Carbon dioxide forms a weak acid solution of carbonic acid which neutralizes the sodium hydroxide from the first treatment. This neutralization selectively precipitates the aluminum oxide, but leaves the silicates in solution. Again filtration is used for the separation. After this stage the purified aluminum oxide is heated to evaporate the water.
Aluminum in the metal form is very difficult to obtain by using some of the traditional chemical methods involving carbon or carbon monoxide as reducing agents to reduce the aluminum ions to aluminum metal. One of the earliest and costly methods in 1850 was to reduce aluminum chloride with sodium metal to obtain aluminum metal and sodium chloride. (Sodium metal is not easy to obtain either). As a result some of the earliest aluminum metal was made into jewelry.
In 1886, Charles Hall, an American (23 yrs. old), and Paul Heroult, a Frenchmen (23 yrs old), simultaneously and independently developed the process still in use today to make aluminum metal. The purified aluminum oxide is mixed with cryolite, a mixture of sodium fluoride and aluminum fluoride, and heated to about 980 degrees Celsius to melt the solids. The mixture melts at a much lower temperature than aluminum oxide would by itself.
The hot molten mixture is electrolyzed at a low voltage of 4-5 volts, but a high current of 50,000-150,000 amps. Aluminum ions are reduced to aluminum metal at the cathode (the sides and bottom of the electrolysis cell). At the anode, oxygen is produced from the oxide ions. The anode material is carbon in the form of graphite, which also is oxidized and must be replaced quite frequently.
The electricity used to produce aluminum is relatively high. One pound of aluminum requires 6-8 kilowatt-hours of electrical energy. This amount of aluminum can be used to make 23 pop cans or one 300 watt light bulb burning for one hour is required to make one pop can.
Live chat with our professional customer service! Get the quotation list.Chat Now