Aluminium

Aluminium is a relatively new material, compared with iron, but it is finding increased use in the building industry. The process now used for extracting aluminium from its ore, bauxite, requires tremendous amounts of electricity which about 10kWh for each pound of metal. As a consequence, any large aluminium-extraction industry must be located near the source of abundant low cost electric power at present hydroelectric power.

The ore is strip-mined in various parts of the world and shipped to smelters located near large hydroelectric power developments. Other essentials ingredients in the manufacture of aluminium are petroleum coke, cryolite and fluorspar.

Properties of aluminium

Aluminium is highly resistant to weather and to corrosive industrial and seacoast atmospheres. Although exposure causes surface oxidation and dulling, the metal is not weakened structurally. The oxide form an added protective coating, so that the weathering rate soon tends to level off. The average penetration of weathering on architectural alloys is not much deeper after half a century than after 2 years. Exposure in a typical industrial atmosphere produces pitting to an average depth of only 0.003 in [ 0.08 mm] after 52 years.

Pure aluminum (99.996 % pure) is the most corrosion-resistant form of aluminum or its alloys, but it is extremely soft and weak. Alloys of aluminum with chromium, manganese and silicon are still highly resistant to corrosion, alloy containing appreciable amounts of copper are more susceptible to corrosion. The addition of Zinc improves the machining qualities of aluminum. Nickel increases it hardness.

Production of aluminum

Aluminium is a strongly reactive metal that forms a high-energy chemical bond with oxygen. Compared to most other metals, it is difficult to extract from ore, such as bauxite, due to the energy required to reduce aluminium oxide (Al₂O₃). For example, direct reduction with carbon, as is used to produce iron, is not chemically possible, since aluminium is a stronger reducing agent than carbon. Aluminium oxide has a melting point of about 2,000 °C. Therefore, it must be extracted by electrolysis. In this process, the aluminium oxide is dissolved in molten cryolite and then reduced to the pure metal. The operational temperature of the reduction cells is around 950 to 980 °C. Cryolite is found as a mineral in Greenland, but in industrial use it has been replaced by a synthetic substance. Cryolite is a chemical compound of aluminium, sodium, and calcium fluorides: (Na₃AlF₆). The aluminium oxide (a white powder) is obtained by refining bauxite in the Bayer process of Karl Bayer. (Previously, the Deville process was the predominant refining technology.)

The electrolytic process replaced the Wöhler process, which involved the reduction of anhydrous aluminium chloride with potassium. Both of the electrodes used in the electrolysis of aluminium oxide are carbon. Once the ore is in the molten state, its ions are free to move around. The reaction at the cathode (negative electrode) is

Al³⁺ + 3 e⁻ → Al

Here the aluminium ion is being reduced (electrons are added). The aluminium metal then sinks to the bottom and is tapped off.

At the anode (positive electrode), oxygen is formed:

2 O²⁻ → O₂ + 4 e⁻

This carbon anode is then oxidized by the oxygen, releasing carbon dioxide.

O₂ + C → CO₂

The anodes in a reduction cell must therefore be replaced regularly, since they are consumed in the process.

Unlike the anodes, the cathodes are not oxidized because there is no oxygen present, as the carbon cathodes are protected by the liquid aluminium inside the cells. Nevertheless, cathodes do erode, mainly due to electrochemical processes. After five to ten years, depending on the current used in the electrolysis, a cell has to be rebuilt because of cathode wear.

Aluminium electrolysis with the Hall-Héroult process consumes a lot of energy, but alternative processes were always found to be less viable economically and/or ecologically. The worldwide average specific energy consumption is approximately 15±0.5 kilowatt-hours per kilogram of aluminium produced (52 to 56 MJ/kg). The most modern smelters achieve approximately 12.8 kW·h/kg (46.1 MJ/kg). (Compare this to the heat of reaction, 31 MJ/kg, and the Gibbs free energy of reaction, 29 MJ/kg.) Reduction line currents for older technologies are typically 100 to 200 kA; state-of-the-art smelters,operate at about 350 kA. Trials have been reported with 500 kA cells.

Used of aluminum

• Transportation (automobiles, aircraft, trucks, railway cars, marine vessels, bicycles etc.)
• Packaging (cans, foil, etc.)
• Water treatment
• Treatment against fish parasites such as Gyrodactylus salaris.
• Construction (windows, doors, siding, building wire, etc.)
• Cooking utensils
• Electrical transmission lines for power distribution
• MKM steel and Alnico magnets
• Super purity aluminium (SPA, 99.980% to 99.999% Al), used in electronics and CDs.
• Heat sinks for electronic appliances such as transistors and CPUs.
• Substrate material of metal-core copper clad laminates used in high brightness LED lighting.
• Powdered aluminium is used in paint, and in pyrotechnics such as solid rocket fuels and thermite.