Aluminum deposits

Aluminum is a ubiquitous metal that is critical to many industries. Because of the characteristics of aluminum ore, it was not practical to produce until the late nineteenth century. Aluminum production remains power-intensive, so most aluminum is recycled.

Aluminum Ore

Aluminum is an important metal because of its high strength, low density, and high corrosion resistance. It is used in items ranging from cans to airframes. However, extensive use of aluminum by humans is a new phenomenon.

Historically, aluminum production has required more energy than could be provided. Because of the chemical characteristics of aluminum, the metal could not be produced on a large scale until the development of the Hall-Heroult process in 1886, in conjunction with dependable and inexpensive electricity.

Aluminum has become an important metal that is used to make electric vehicles, solar panels, and wind turbines. However, a 2023 report by the Environmental Integrity Project found that aluminum manufacturing throughout the world emits more than one billion tons of carbon dioxide into the atmosphere annually. This is equivalent to the energy use of 150 million homes. While aluminum plays an important role in the shift to clean energy and transportation, the report recommends that aluminum producers take measures to reduce pollution so there is less harm to people and the environment.

Bauxite Ore

Aluminum is highly reactive and is seldom found in its elemental state. As such it is found as ore, with bauxite (discovered in 1821) as the primary source of aluminum. Bauxite is a rock composed of several aluminum-rich minerals, such as gibbsite, boehmite, and diaspore, in a heterogeneous mixture with various iron oxides and the clay mineral kaolin. Kaolin itself is rich in aluminum, and it has been considered as a potential source. However, given the abundance of bauxite, kaolin processing is not of economic interest.

There are two primary forms of bauxite: lateritic and karst. Lateritic bauxite is silicate based, whereas karst bauxite tends to be carbonate. Karst bauxite forms above carbonate rocks, such as limestone and dolomite, in a karst region. A karst region is a large area of carbonate rock formed during periods in Earth’s history called marine incursions, when a shallow ocean covers a region that was formerly dry land. For the karst region to have formed, the ocean must have been shallow.

Lateritic bauxites are formed atop silicates. Laterite weathering is intense tropical weathering that produces a soil called laterite. Weathering, the process by which soil is formed, requires the bedrock to be broken down by the conjunction of natural forces such as wind, rain, and seasonal temperature fluctuations.

Equally, chemistry plays a large role in the process known as chemical weathering. Organisms play a role as well, with many plants secreting chemicals to break down rocks. Weathering is distinct from erosion, which moves materials; weathering occurs in situ.

Soil is produced in areas of wet weather and vigorous plant life—namely, in the tropics. Tropical soil is rich in many important minerals and, particularly in the Amazon, is poor in nutrients. This type of soil occurs throughout the tropics or areas that once were tropics. Thus, many tropical countries are good sources for bauxite mining. Bauxite is often found in large deposits near the surface that covers large areas; it also can be found in deposits deeper underground.

In addition to bauxite, other potential sources of aluminum are kaolin clay, oil shale, anorthosite, and coal. However, these sources are not economically viable, and given the widespread availability of bauxite, there is no economic incentive to make use of them.

Mining

Because most bauxite deposits are on the surface, bauxite mining often takes the form of strip mining. First, the land above the deposit is cleared of vegetation. Explosives are then used to loosen the soil before heavy equipment digs up the bauxite-rich soils. This method often makes use of bucket-wheel excavators and chain-bucket excavators.

Bucket-wheel and chain-bucket excavators are some of the largest machines made by humans. While the specific mechanisms are different, each has a series of excavating buckets on a loop that pick up the dirt and place it on a belt that leads to a central hub. Once there, the soil is placed into trucks or onto a belt that takes the bauxite to a refinery. These machines are effective but can be expensive to repair. Often, bulldozers and backhoes are used to the same effect. Although efficient, such mining practices disrupt the environment, particularly in tropical regions. In the process, land must be cleared over the active mine area. The mined area must then be restored, or the land will face erosion. Even with soil restoration, the environment is damaged.

Bayer Process

Bauxite is only 30 to 54 percent alumina. Once mined, bauxite ore is heated to about 150 to 200 degrees Celsius (302 to 392 degrees Fahrenheit) in a pressure vessel with a solution of sodium hydroxide. The sodium hydroxide solution converts the bauxite into aluminum hydroxide, which then dissolves in the hydroxide solution. The rest of the bauxite does not dissolve, forming a waste product called red mud. It is formed mostly of oxidized iron.

Little can be done with red mud, as it cannot be built upon and is highly basic. It must be dried in settlement ponds and stored in landfills. One of the main problems associated with bauxite refining is the management of this red mud. Additionally, should a settlement pond fail, the resultant slide of red mud can destroy towns and pollute the environment.

Once the solution cools, the dissolved aluminum hydroxide precipitates out. When it is heated to 980 degrees Celsius (1,800 degrees Fahrenheit), the aluminum hydroxide decomposes to alumina, releasing water vapor in the process. This process, invented in 1887 by Carl Josef Bayer and named for him, turns bauxite to aluminum oxide. Bayer had been working on supplying alumina to the textile industry, where the alumina was used to fix colors in the cotton-dying process. (The role is called mordant.) Several processes had already been invented for turning bauxite to aluminum oxide, but the Bayer process was more efficient.

Alumina has been used in many industries. Because it is chemically inert, it is often used as filler in plastics and sunscreen. It also can be used as an abrasive as a substitute for diamonds. Alumina also is used in many kinds of sandpaper. Additionally, alumina can be used as a catalyst to help convert hydrogen sulfide into sulfur. This helps clean emissions, including the emissions of factories.

Hall-Heroult Processing

Alumina must be processed to make aluminum; this is done through the Hall-Heroult process, whereby aluminum is formed through electrolysis. While most such processes can use water, the high reactivity of aluminum makes the use of water impossible. Thus, alumina must be dissolved in cryolite (sodium hexafluoroaluminate), a light-colored rare mineral in which alumina is soluble.

An interesting property of cryolite is that its refractive index is close to that of water. This means it bends light the same way water does and thus would be near invisible if put in a bowl of water. Cryolite is used as flux. A flux is a substance used to preserve purity and to clean and improve flow in the metal being worked with. When smelting, fluxes remove impurities. This is especially important with aluminum because of its high reactivity.

Both alumina and cryolite have high melting points of more than 1,000 degrees Celsius (1,832 degrees Fahrenheit). The resultant mixture is then electrolyzed, with the liquid aluminum being precipitated on the cathode. A voltage of between 3 and 5 volts is required, with the rate of precipitation proportional to the current. The liquid aluminum is then siphoned off.

The process releases carbon dioxide and hydrogen fluoride, which are vented. Hydrogen fluoride is neutralized to sodium fluoride. The process requires much electricity, both for deposition and for heating purposes. As a result, most aluminum smelters are located near sources of relatively inexpensive electricity, such as hydroelectric stations. Because of the high energy requirements for producing aluminum, the metal was treated like gold before the development of the Hall-Heroult process.

The Hall-Heroult process was discovered nearly simultaneously by two chemists, leading to the process’s name. One chemist, Charles Martin Hall, invented the process in 1886. Heroult came up with the idea a few months later. Hall, however, patented the process in 1889 and then founded the Pittsburgh Reduction Company, later to become the Aluminum Company of America, or Alcoa.

Recycling

Even after the development of the Hall-Heroult process, producing new aluminum remained energy intensive. As a result, it is far more efficient, even today, to reuse already processed aluminum. Recycling takes only 5 percent of the energy needed to make new aluminum. Because of widespread recycling, 75 percent of the aluminum produced since 1888 is still in use.

First, in recycling, cans and other aluminum components are removed from waste. This is often done by a form of electromagnetic sorting called an eddy current separator, which works by producing a powerful magnetic field and moving the metals through it. Electromagnetic induction causes the metal to fly into a collection bin.

Ferrous, or magnetic, metals can cause damage to such as system and are usually removed first. After being separated, the aluminum parts are cleaned to prevent oxidation during smelting; oxidation ruins the aluminum. Only then is the aluminum melted and recast; gases such as chlorine or nitrogen remove any hydrogen released in the process.

In general, a recycled can is made into a new can within sixty days. Because of the massive energy and financial savings, the recycling process is economically significant. An empty aluminum can is worth about 1 US cent, and the energy savings from recycling one single can is enough to run the average television set for three hours.

Places

Geologists locate bauxite by prospecting for it. They take core samples of soils around the world in regions suitable for bauxite and examine them for evidence of the ore. Through study of the samples, they can learn the quality and quantity of the bauxite at the site.

Global resources are estimated to be between 55 and 75 billion tons, sufficient for current demand for several centuries. Of these resources, 33 percent is in Africa, 24 percent in Oceania, 22 percent in South America and the Caribbean, and 15 percent in Asia. Jamaica has a particularly high concentration, and reports suggest that there could be large deposits in Vietnam (of about 11,000 megatons), which would makes these sites the largest bauxite reserves in the world. The largest producer now is Australia (producing one-third of the world’s bauxite), followed by China, Brazil, Guinea, and India. Bauxite production is a major industry in the regions where it is mined.

Principal Terms

carbonate rock: a rock composed mainly of calcium carbonate

electrolysis: process by which liquid or dissolved metals are separated by electromagnetic attraction

heterogeneous mixture: a nonuniform mixture

ion: an atom with a net charge due to the addition or loss of electrons

karst: a region formed by the weathering of underlying rock, typically limestone or dolomite

mineral: a naturally occurring solid with a specific chemical composition

ore: chemical compounds of a desired material that are economically viable to exploit

pressure vessel: a container designed to hold gases or liquids at a much higher pressure than the surrounding pressure

rock: a mixture of minerals

strip mining: a method of mining that occurs on the earth’s surface

Bibliography

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