Aluminum

Al	13
Aluminium

Abundance: 8,1% crust

Aluminum, periodic table Al, is the third most common element in the Martian crust, after oxygen and Silicon. It has 13 protons, it's most common isotope is 27Al, with 14 neutrons.

Aluminum oxides are abundant on Mars as on Earth. Most aluminum is incorporated into alumina-silicate minerals, such as feldspar ((KAlSi₃O₈ – NaAlSi₃O₈ – CaAl₂Si₂O₈). Erosion, either by wind or water, can separate out the alumina (Al₂O₃), possibly as fine dusts that would have turned into compact silts and clays over time. This might be a useful source of aluminum ore.

Aluminum is widely used in applications where strong and light materials are needed. On Mars, iron and steel weigh almost exactly the same as aluminum does on Earth, and could thus serve in many of those situations.

Production of aluminum

For in-situ production, a source of aluminum ore such as alumina deposits would be very helpful in reducing the processing requirements.

In the current production methods, aluminum requires high electric power to reduce it from its oxide form using electrolysis. Carbon anodes and cathodes are used for the process, with the carbon from the anodes combining with the oxygen in alumina (Al₂O₃) to reduce the alumina to aluminum. Therefore, the present production methods require a constant supply of carbon for the process.

Before the aluminum can be reduced, it needs to be separated from other elements is the original ore. On Earth the main ore used in the mineral bauxite, that is unlikely to be available on Mars. Bauxite is generally the result of intense weathering of base rock, and is usually found in tropical countries. Alumina is separated from the bauxite minerals by the Bayer process.

Work has been going on for several decades on the carbothermic process, which uses carbon and thermal power, to try to make it as economical on Earth as electrolytic reduction. If thermal power is cheaper than electric power on Mars relative to Earth, due for example to being more suitable for an import-minimizing economy, the carbothermic process will be relatively more attractive. Of course carbon itself would need to be produced on Mars, which has no handy coal reserves as a carbon source. Methane (CH₄) might be a practical alternative, if methane clathrates, for example, are found on Mars. Carbon can also be extracted from the martian atmosphere, as it is mostly carbon dioxide.

Alcoa has announced in 2018 the production of aluminum using a new electrolytic process that does not produce CO₂ or require carbon anodes. This might be applicable on Mars.

Once it has been produced, aluminum is relatively easy to recycle and less prone to corrosion than iron and steel.

The embodied energy of aluminum is 155-220 MJ/kg. (To be confirmed, may include significant recycled aluminum at the lower value).

Greenhouse Gases Produced by Aluminum Smelting

On Earth, molten aluminum must be kept away from oxygen or it will oxidize. They used to use a super greenhouse gas sulphur hexafluoride (SF6) over pools of molten aluminum ore. Sulphur hexafluoride is extremely chemically inert, even at high temperatures. However, this is a super greenhouse gas, and current regulations require that great care be made to prevent it from leaking into the atmosphere, or require the use other, more expensive, chemicals. Either of these solutions, increase the cost of of the industrial process.

On Mars, the cheaper form of aluminum smelting can be used, because leaking Super Greenhouse Gases into the atmosphere is something that people want. This is typical for other industries. Where ever super greenhouse gases are regulated, (to prevent warming the Earth), on Mars this regulation is not needed.

Uses

• Electrical conductors will be a significant use case for aluminum. In particular if copper is difficult to find on Mars.
• Aluminum is a good material for extrusions used in construction materials such as window and door frames, in particular due to its corrosion resistance. Its use as a structural materials is usually not favorable, as it is more sensitive to fatigue than steel, and more expensive to produce.
• Mobile equipment may benefit from aluminum alloy parts.
• Lithium - aluminum alloys are very strong and light, and are used in the space industry.
• Batteries using aluminum exist and have high energy density, but for the moment are used as single use applications as they are not easily rechargeable.
• Transparent aluminum (Aluminium oxynitride, (AlN)_x·(Al₂O₃)_1−x,) can serve as super strong windows. The materials is a transparent ceramic rather than a metal. However, the productions cost are likely to be high. A competing material might be alumina (Al2O3), again an expensive material to produce as a transparent ceramic.
• Aluminum cans are a common aluminum product.