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Mining: A Closer Look at Aluminium

© Savithri Shimada

Aluminium is commonly associated with cans, but with only a few small additives, the metal can become as strong as steel, but with the advantage of being able to resist corrosion in air and water. Aluminium’s corrosion resistance is due to the way it oxidises in the presence of oxygen, to form a thin, non-porous layer of aluminium oxide on its surface. This itself is aluminium rust, but it prevents further corrosion by blocking the passage of oxygen to the metal underneath. In effect, the aluminium provides itself with a protective coating, unlike steel.

Aluminium is commercially extracted in several steps:

1. Digestion – Crushed bauxite is fed into a caustic soda solution, called slurry. The bauxite slurry is mixed in digesters, and maintained at medium-high temperatures and pressures for thirty minutes. The aluminium oxide and caustic soda react to produce a solution called 'green liquor'.

2. Clarification – The alumina is separated from residual sand (usually containing undissolved iron oxides, silica and other trace materials) and mud (mostly iron oxide and fine quartz) by settling the 'green liquor' and then passing it through filters.

3. Precipitation – The 'green liquor' is cooled by passing through a heat exchanger and then through tall vessels where alumina hydrate crystals are precipitated.

4. Calcination – The alumina hydrate crystals as a slurry into large classifier vessels, in which large crystals are pulled out and smaller ones are returned to the digesters for further processing in ‘green liquor’. In the calcination process the heated crystals are desiccated to form pure alumina.

5. Hall-Heroult smelting process – Aluminium is extracted from the alumina at 960C in an electrolytic process. An electric current is passed through a molten solution of alumina and cryolite (NaAlF) in a series of reduction cells which are lined at the bottom with cathodic carbon. Anodic carbon rods are partially immersed into the top of each pot. The oxygen from the alumina to reacts with the anodic carbon to form carbon dioxide gas. Liquid aluminium settles at the bottom of the cell and is drawn off by a vacuum crucible.

6. The molten aluminium is siphoned from the furnaces for use, as in aluminium alloys.

Recycling aluminium to produce secondary aluminium requires 5% of the energy needed to produce molten aluminium from bauxite, not to mention considerable savings in bauxite. Over 20% of aluminium produced in Australia is from recycled material.

Aluminium is an extremely versatile material and can be used in most applications, given the right processing methods and additives. Its abundance and hence its low cost should ensure the metal and its alloys a firm place in future technologies in the construction, automotive, machine, aerospace and food processing industries, among others. Its low density makes it an ideal automobile material in view of global efforts to limit greenhouse gas emissions, as aluminium-alloy body cars are light but strong enough to conform to safety standards. Australian companies such as Austal in Perth are employing aluminium alloys in the construction of ships and boats.