The dark side of light metals: dealing with oxidation and dross

Aluminium loves to exist as an oxide, Al2O3. This oxide was probably the first refractory phase to condense from the early solar nebulae. Rubies and sapphires (Al2O3, corundum) are hard and stable naturally-occurring precious minerals. Bauxite (an impure hydrated form of Al2O3) needs to be coerced, at the expense of 125-210 GJ energy usage and 10-22.5 t CO2 emissions through the entire extraction process (in particular, energy production for the Hall- Héroult electrolytic refining stage), into the final form of a 1 tonne bundle of purity Al ingot ex. smelter. Aluminium is therefore quaintly (and often disparagingly) referred to as 'solidified electricity', implying the need to be extremely cautious with its use since it is one of the most energy-intensive engineering materials. Fortunately, aluminium alloys have also had good press, being highly versatile and durable in service and being readily recyclable (at about 5% of the energy required to produce the primary Al initially). However, the ground is shifting and with the growing focus on embodied energy and carbon footprints, aluminium's processing and use is being revisited