Shiny outlook for greener aluminum production

Aluminum production uses a lot of energy, from mining bauxite ore, as shown here in Huntley, Western Australia, to refining and smelting the metal. A new smelting material may reduce the energy used in the process.Photograph is courtesy of Alumina Limited.

Aluminum is crucial to global transportation. The metal makes up 80 percent of an airplane’s weight, up to 90 percent of a tractor-trailer’s weight, is responsible for fuel-efficient engines in light cars and trucks, and is an important part of high- speed rails. It is also a key part of the food and beverage, construction and electrical industries, and its use is increasing in the medical field. Yet, producing aluminum from ore uses an incredible amount of energy, leaving researchers trying to figure out new, more energy-efficient ways to produce the metal. Now, a promising new technology offers to reduce the energy need by up to 30 percent.

Since the current basic aluminum production process was created in 1886, the industry has been working to decrease the amount of energy it requires, says Patricia Plunkert, a mineral commodity specialist at the U.S. Geological Survey in Reston, Va. And they’ve made great strides, she says. In the 1950s, producing 1 kilogram of aluminum used 21 kilowatt-hours of electricity, according to the International Aluminum Institute. Today, the worldwide average is about 15.7 kilowatt-hours per kilogram of aluminum.

The process is still too reliant on electricity, however. For example, Australia is the world’s fifth largest producer of the metal, and about 15 percent of its electricity use is in aluminum smelting. And although the process to create recycled aluminum only uses about 5 percent of the electricity that primary production uses, according to the Aluminum Institute, most of the world uses much more new aluminum than recycled aluminum, Plunkert says.

Producing new aluminum begins with mining and transporting bauxite ore to a plant where it is refined into alumina. It then goes to a smelting plant, where the alumina is dissolved in a bath of “molten cryolite,” a high melting point material that acts as a solvent for the alumina and behaves as an electrically conductive medium, says Theo Rodopoulos, a researcher at CSIRO in Australia, who is leading the Light Metals Flagship team in developing the new aluminum production method. An electrical current is then applied through the solution, which causes the aluminum metal to separate out and sink to the bottom of the solution, where it can be siphoned off, he says.

The new production method is somewhat similar to the old, with the real difference being the melting point of the liquid bath in which the aluminum is generated, Rodopoulos says. In the traditional process, that bath is around 1,000 degrees Celsius — requiring a lot of electricity to maintain the temperature. The new method replaces the high melting point material with an “ionic liquid,” which has a significantly lower melting point than cryolite, “thereby reducing the energy input required to keep the bath molten,” he says. Ionic liquids are salts that are liquid at close to room temperature, rather than being dissolved in another liquid. Indeed, this process will “bring about huge energy savings,” says Robin Rogers, a chemist at the University of Alabama, who is working on new applications for ionic liquids.

People are not only looking at the use of ionic liquids in aluminum production, but also the production of other metals, such as titanium, Rogers says. Ionic liquids could also have applications in reprocessing nuclear waste and other industrial processes. Really, the possibilities are endless, he says.

Using ionic liquids to produce aluminum and other metals is “scientifically, pretty solid already,” Rogers says. However, the process has only been investigated in small-scale experiments in the lab so far, and though the scientific feasibility of depositing aluminum in ionic liquids has been demonstrated, “it is still the early days,” Rodopoulos says. In developing any commercial process, he says, several steps need to occur “before we get to a demonstration or commercial plant.” Large-scale production is probably still 10 or more years away, even if the necessary production process criteria are met, he says.

Nonetheless, “a low-temperature process [that] provides significant energy and environmental benefits could have a major impact on the aluminum industry,” Rodopoulos says. Indeed, “it’s quite exciting,” Rogers adds.

Megan Sever

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U.S. Geological Survey mineral commodity specialist Michael J. Potter has compiled the following information about kyanite, a critical component in refractory and ceramic products.

Kyanite and related minerals are used in making refractories, which are materials that can withstand high-temperature environments, generally in excess of 1,100 degrees Celsius. Refractories form an inner lining to furnaces, kilns and other containers with which molten metals and glass come into contact. Fifty to 70 percent of global refractory consumption is related to the steel industry.

Kyanite, andalusite and sillimanite are minerals with the same chemical formula, but with different crystal structures and chemical properties. The theoretical composition is about 63 percent alumina and 37 percent silica. The three minerals are sometimes referred to as the “sillimanite group.” All of these minerals convert to mullite (about 88 percent) and silica (about 12 percent) upon “calcining,” or heating from about 1,250 degrees Celsius to 1,500 degrees Celsius.

Examples of refractories that contain andalusite, kyanite and/or mullite include firebrick, insulating brick, kiln furniture, refractory shapes and monolithic refractories (made of a single piece or as a continuous structure), such as castables (refractory concrete), gunning mixes, mortars and plastics. Monolithic refractories are used in many of the same consuming industries as refractory brick, including iron and steelmaking and nonferrous metal smelting. Iron and steel production is the leading use of refractories worldwide.

The interlocking grain structure of andalusite, kyanite and mullite gives added mechanical strength to refractories and other nonrefractory ceramic materials. Other end uses of kyanite and related materials include brake shoes and pads, electrical porcelain, foundry use, precision casting molds and sanitaryware.

Kyanite is mined only in a few countries. The United States is the largest producer, with all of the kyanite coming from central Virginia. Kyanite Mining Corp-oration’s ore deposits are kyanite quartzites containing 15 to 40 percent kyanite and usually about 5 percent of other minerals such as mica, pyrite and rutile. The remainder is quartz. Company data are proprietary, but a nongovernmental estimate puts U.S. output at 90,000 metric tons per year. This includes both raw, marketable kyanite product and calcined kyanite (mullite). An estimated one-third of the production is exported to countries worldwide.

South Africa is the leading producing country of andalusite, with an estimated 235,000 metric tons of marketable output of the mineral in 2005. France produces an estimated 65,000 tons per year of andalusite. China reportedly has some production of andalusite, but official data have not been received by the U.S. Geological Survey (USGS). A company in North Carolina mines a deposit containing andalusite, pyrophyllite and sericite (mica), and sells blends of the minerals to producers of ceramics and refactories.

Outside of China and India, sillimanite has had limited production. The mineral can occur in metamorphic rocks but is not often in a form that can be easily extracted. According to a nongovernmental source, China may produce an estimated 20,000 tons per year of sillimanite, but USGS has not obtained official data. India’s long coastline has large deposits of beach sands that contain heavy minerals, including sillimanite and zircon. The country’s output of sillimanite is an estimated 15,000 tons per year. Extensive research is being carried out to utilize the beach-sand-derived sillimanite and zircon. For example, mullite aggregate was developed by reaction sintering of sillimanite sand and calcined alumina. Also, high-alumina bricks and high-temperature insulating bricks have been developed using sillimanite sand obtained from beach sands.

For more information on kyanite, visit minerals.usgs.gov/minerals.

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