In 1886, two 22-year-old scientists on opposite sides of the Atlantic, Charles Hall
of the USA and Paul L.T. Heroult
of France, made the same discovery – molten cryolite
(a sodium aluminum fluoride mineral) could be used to dissolve alumina and the resulting chemical reaction would produce metallic aluminum. The Hall-Heroult process
remains in use today.
The Hall-Heroult process takes place in a large carbon
lined steel container called a reduction pot
. In most plants, the pots are lined up in long rows called potlines
The key to the chemical reaction necessary to convert the alumina to metallic aluminum is the running of an electrical current through the cryolite/alumina mixture. The process requires the use of direct current (DC) – not the alternating current (AC) used in homes. The immense amounts of power required to produce aluminum is the reason why aluminum plants are almost always located in areas where affordable electrical power is readily available. Some experts maintain that one percent of all the energy used in the United States is used in the making of aluminum.
The electrical voltage used in a typical reduction pot is only 5.25 volts, but the amperage is VERY high – generally in the range of 100,000 to 150,000 amperes or more. The current flows between a carbon anode (positively charged), made of petroleum coke and pitch, and a cathode (negatively charged), formed by the thick carbon or graphite lining of the pot.
When the electric current passes through the mixture, the carbon of the anode combines with the oxygen in the alumina. The chemical reaction produces metallic aluminum and carbon dioxide. The molten aluminum settles to the bottom of the pot where it is periodically syphoned off into crucibles while the carbon dioxide – a gas – escapes. Very little cryolite is lost in the process, and the alumina is constantly replenished from storage containers above the reduction pots.
The metal is now ready to be forged, turned into alloys, or extruded into the shapes and forms necessary to make appliances, electronics, automobiles, airplanes cans and hundreds of other familiar, useful items.
Aluminum is formed at about 900°C, but once formed has a melting point of only 660°C. In some smelters this spare heat is used to melt recycled metal, which is then blended with the new metal.
Recycled metal requires only 5 per cent of the energy required to make new metal. Blending recycled metal with new metal allows considerable energy savings, as well as the efficient use of the extra heat available. The quality of recycled metal is good and great for most all aluminum uses, except in those industries such as aerospace where only very high quality primary aluminum is warranted.
The smelting process required to produce aluminum from the alumina is continuous the potline is usually kept in production 24 hours a day year-round. A smelter cannot easily be stopped and restarted. If production is interrupted by a power supply failure of more than four hours, the metal in the pots will solidify, often requiring an expensive rebuilding process. The cost of building a typical, modern smelter is about $1.6 billion.
Most smelters produce aluminum that is 99.7% pure – acceptable for most applications. However, super pure aluminum (99.99%) is required for some special applications, typically those where high ductility or conductivity is required. It should be noted that what may appear to be marginal differences in the purities of smelter grade aluminum and super purity aluminum can result in significant changes in the properties of the metal.