Aluminium

Properties

Aluminium is a silvery-coloured metal that is never found on its own in nature. It is very light (about one third the weight of copper) yet strong; some alloys are even stronger than steel. Aluminium is both malleable (can be pressed into shape) and ductile (can be beaten and drawn into a wire). Aluminium is a very good heat and electrical conductor. It is also is highly rust-resistant, and is non-toxic. Aluminium can be alloyed with almost any other metal. Aluminium is inflammable, non-magnetic and non-sparking. These properties have made it an important metal in the modern world. As aluminium is such a reactive metal, you might think it would rust badly and therefore be useless. However, pure aluminium reacts very quickly with air or water to form a thin, almost invisible layer of aluminium oxide on its surface, which then acts as a protective coating preventing any further 'rusting'.

Uses

Aluminium is smelted from alumina which is refined from bauxite ore. All three products have a number of uses.

More than 90% of the world's bauxite production is used to make alumina with most of the remaining 10% used by the abrasive, refractory and chemical industries. Bauxite is also used in the production of high-alumina cement, as an absorbent or catalyst by the oil industry, in welding rod coatings and as a flux in making steel and ferroalloys.

Alumina's primary use is as feedstock for aluminium smelters, however it is also used for other industrial purposes. It is used in glass, porcelain and in metallic paints, such as those sprayed on cars. It is also used in the production of spark plug insulators, as a fuel component for solid rocket boosters, a filler for plastics, an abrasive (it is cheaper than industrial diamond) and in metal refineries where it is used to convert toxic hydrogen sulfide waste gases into elemental sulfur.

Alumina that occurs in a natural, crystalline state is referred to as the mineral corundum. Sometimes, corundum crystals are contaminated with trace amounts of chromium, iron, titanium, copper or magnesium. We call these crystals rubies and sapphires.

After iron and steel, aluminium is the most widely used metal on Earth. It is often alloyed with copper, zinc, magnesium, manganese or silicon and the addition of small amounts of zirconium, hafnium or scandium to these alloys markedly improves their strength. Aluminium itself has a wide range of uses from specialist aircraft construction to everyday items like knives and forks. Some of the uses are listed in the table below.

History

Around 5300 BC: The Persians made extra strong pots from clay containing aluminium oxide.

Around 2000 BC: The ancient Egyptians and Babylonians used potassium aluminium sulphate KAl(SO4)2 as a medicine to reduce bleeding. It was sourced from naturally occurring deposits in Greece and Turkey. The ancient Romans called this medical compound "alum", which is where we get the modern day word and symbol from. It is still used today to stop bleeding.

Middle Ages: Most alum came from the Papal territory of Tolfa but the price fell dramatically when a large deposit of alum shale was discovered in Yorkshire in the early 1600s. Over the next centuries, alum was used in two main areas: as a preservative for paper and as a fixing agent for dying cloth.

1808: Englishman Sir Humphry Davy tried to extract aluminium through electrolysis. He failed but did establish its existence and gave it its name.

1821: French geologist Pierre Berthier discovered an aluminium-rich material near the village of Les Baux in Provence, France. It was named bauxite after the village.

1825: Hans Christian Oersted in Denmark produced impure aluminium by heating aluminium chloride with potassium amalgam.

1827: German chemist Friedrich Wöhler extracted aluminium as a powder by reacting potassium with anhydrous aluminium chloride improving on Oersted's process.

1855: French chemist Henri Saint-Claire Deville succeeded in obtaining a solid bar of aluminium using sodium instead of the more expensive potassium. The aluminium bar was considered so precious that it was exhibited that same year alongside the French crown jewels.

1886: Two scientists on separate continents (Charles Hall in America and Paul Heroult in France) independently discovered an economical method for producing aluminium via electrolysis in molten cryolite (sodium aluminium fluoride). Incidentally, both were born in 1864 and both died in 1914.

1887: Austrian chemist Karl Josef Bayer working in Russia developed a method for extracting alumina from bauxite.

1888: Hall founded the Pittsburgh Reduction Company (known as Alcoa since 1907).

1890: The Hall-Heroult and Bayer discoveries had caused the price of aluminium to plummet 80%. In 1888, aluminium cost US$4.86 per pound. In 1893, it was US$0.78 per pound and by the late 1930s cost just US$0.20 per pound and had more than 2000 uses.

1900: Just 8000 tonnes of aluminium were produced but 100 years later 24.5 million tonnes were produced and in 2016 an estimated 57.6 million tonnes were produced.

1911: German chemist Alfred Wilm developed important aluminium alloys that were solid enough for the manufacture of items such as aircraft. There was great demand for aluminium during the World War II and the Korean War as planes had previously been made of wood and fabric.

1922: Aluminium foil was manufactured.

1955: The first aluminium smelter in Australia was opened at Bell Bay, Tasmania.

1958: Aluminium soft drink cans were produced for the first time.

1961: Alcoa forms Alcoa of Australia.

1963: Alcoa of Australia opens the first alumina refinery in Australia at Kwinana in Western Australia followed by Pinjarra in 1972 and Wagerup in 1984.

Today: In 2016 an estimated 57.6 million tonnes of Aluminium were produced. This is more than all other non-ferrous metals combined. Australia mines bauxite in Queensland, Western Australia, the Northern Territory and Tasmania and is the world's largest bauxite producer. Australia is also a world leader in the production of alumina and aluminium. The three Western Australian refineries supply 45% of Australia's alumina and 11% of the global total, making them the largest single source of alumina in the world.

Formation

Bauxite is the most common aluminium ore. Bauxite occurs as a weathered cover or blanket, known as laterite or duricrust, over a variety of alumina-bearing rocks. It forms when large amounts of rainfall leach away the more mobile elements in the host rock leaving the relatively immobile aluminium with some silicon, iron and titanium. Because of the way it forms, bauxite deposits can be very extensive and are found on almost every continent.

The main aluminium minerals in bauxite are gibbsite [Al(OH)₃, also written as Al₂O₃.H₂O in oxide notation], boehmite [AlO(OH), written as Al₂O₃.H₂O in oxide notation] and diaspore, which is a polymorph (alternate form) of boehmite but is denser and harder. Pure alumina (Al₂O₃) contains 52.9% aluminium and 47.1% oxygen. Bauxite may be very hard or as soft as mud and may occur as compacted earth (both friable and re-cemented), small balls (pisolites), or hollow, twig-like material (tubules). Its colours may be buff, pink, yellow, red, or white, or any combination of these. Aluminium is also present in many precious gemstones such as turquoise, rubies, sapphires, emeralds, topaz, jade and aquamarines.

Resources

Bauxite ore contains sufficiently high levels of aluminium oxides and suitably low levels of iron oxide (Fe₂O₃) and silica (SiO₂) to be economically mineable. The amount of reactive silica is particularly important as this form of silica consumes the caustic soda needed to make alumina, thus low reactive silica is desirable. Other potential sources of aluminium include a variety of rocks and minerals such as aluminous shale and slate, aluminium phosphate rock and high-alumina clays.

The world's largest economic bauxite resources occur in Guinea, Australia, Brazil, Vietnam and Jamaica. In Australia, bauxite is mined from established open-cut operations at Weipa in Queensland, Gove in the Northern Territory and the Darling Range in Western Australia. In addition, new mines have recently begun operation in the Cape York region of Queensland and in central Tasmania. Other bauxite deposits occur in northern Western Australia, New South Wales and eastern Queensland but are currently uneconomic to mine.

Australia's alumina refineries are located in Western Australia (Kwinana, Wagerup, Pinjarra and Worsely) and Queensland (QAL and Yarwun) and its aluminium smelters are located in Tasmania (Bell Bay), Queensland (Boyne Island), Victoria (Portland) and New South Wales (Tomago). China is the world's largest consumer of aluminium and, despite strong domestic production, imports large amounts of alumina and raw bauxite, accounting for more than 40% of global consumption. Other large aluminium markets are the United States of America, Japan and Europe but these regions possess few economic bauxite deposits and also rely on bauxite and alumina imports for their alumina refineries and aluminium smelters.

Mining

Extraction of aluminium metal takes place in three main stages: mining of bauxite ore, refining the ore to recover alumina and smelting alumina to produce aluminium. Bauxite is mined by surface methods (open-cut mining) in which the topsoil and overburden are removed by bulldozers and scrapers. The topsoil is then stored and later used for revegetating and restoring the area after mining is completed. The underlying bauxite is mined by front-end loaders, power shovels or hydraulic excavators. Some bauxite ores are merely crushed, dried and shipped. Other bauxite is treated after crushing by washing to remove some of the clay, reactive silica and sand waste; and then dried in rotary kilns. The ore is loaded into trucks, railway cars or onto conveyor belts and transported to ships or refineries.

A number of factors in the aluminium production cycle relate to the environment and considerable resources are allocated to minimise the impact of mining, refining and smelting on the surrounding environment. Mine rehabilitation is carried out, making every effort to return the area to at least its original condition. Care is taken with the handling and disposal of red mud from the refineries. This mud is usually pumped into dams which are sealed with impervious material to prevent pollution of the surrounding countryside.

Processing

In almost all commercial operations, alumina is extracted from the bauxite by the Bayer refining process. The process, discovered by Karl Josef Bayer in 1888, consists of four stages.

Digestion: the finely ground bauxite is fed into a steam-heated unit called a digester. Here it is mixed, under pressure, with a hot solution of caustic soda. The aluminium oxide of the bauxite (and the reactive silica) reacts with the caustic soda forming a solution of sodium aluminate or green liquor and a precipitate of sodium aluminium silicate.

Clarification: the green liquor or alumina-bearing solution is separated from the waste: the undissolved iron oxides and silica which were part of the original bauxite and now make up the sand and red mud waste. This stage involves three steps: firstly, the coarse sand-sized waste is removed and washed to recover caustic soda; secondly, the red mud is separated out; and, finally the remaining green liquor is pumped through filters to remove any residual impurities. The sand and mud are pumped together to residue lakes and the green liquor is pumped to heat exchangers where it is cooled from 1000°C to around 650-790°C.

Precipitation: the alumina is precipitated from the liquor as crystals of alumina hydrate. To do this, the green liquor solution is mixed in tall precipitator vessels with small amounts of fine crystalline alumina, which stimulates the precipitation of solid alumina hydrate as the solution cools. When completed the solid alumina hydrate is passed on to the next stage and the remaining liquor, which contains caustic soda and some alumina, goes back to the digesters.

Calcination: the alumina hydrate is washed to remove any remaining liquor and then dried. Finally, it is heated to about 1000°C to drive off the water of crystallisation, leaving the alumina—a dry, pure white, sandy material. A portion of the alumina may be left in the hydrate form or further processed for the chemical industry.

Alumina is turned into aluminium through a smelting process. All commercial production of aluminium is based on the Hall-Héroult smelting process in which the aluminium and oxygen in the alumina are separated by electrolysis. Electrolysis involves passing an electric current through a molten solution of alumina and natural or synthetic cryolite (sodium aluminium fluoride). The molten solution is contained in reduction cells or pots which are lined at the bottom with carbon (the cathode) and are connected in an electrical series called a potline. Inserted into the top of each pot are carbon anodes, the bottoms of which are immersed in the molten solution.

The passage of an electric current causes the oxygen from the alumina to combine with the carbon of the anode forming carbon dioxide gas. The remaining molten metallic aluminium collects at the cathode on the bottom of the pot. Periodically, it is siphoned off and transferred to large holding furnaces. Impurities are removed, alloying elements added and the molten aluminium is cast into ingots.

The smelting process is a continuous one. As the alumina content of the cryolite bath is reduced more is added. Heat generated by the passage of the electric current maintains the cryolite bath in its molten state so that it will dissolve the alumina. A great amount of energy is consumed during the smelting process; from 14,000 - 16,000 kilowatt hours of electrical energy is needed to produce one tonne of aluminium from about two tonnes of alumina. Aluminium is sometimes referred to as 'solid electricity' owing to the large amount of power used in its production. The availability of cheap electricity is therefore essential for economic production.

Aluminium ingots are produced in various shapes and sizes depending on their end use. They may be rolled into plate, sheet, foil, bars or rods. They may be drawn into wire which is stranded into cable for electrical transmission lines. Presses extrude the ingots into hundreds of different useful and decorative forms or fabricating plants may convert them into large structural shapes.

Further information

• Further resource and production information