What possibilities are there for refining aluminium on the moon?

Question

Engines burning aluminium and liquid oxygen have been investigated for use on the moon because there is plenty of both elements in the soil - about 13% and 40% of highland soil, respectively. Propellant thus has the potential to be sourced from the moon directly, but it isn't practical unless a refining process can be found that uses the moon's environment to extract pure aluminum efficiently using local resources. A viable way to extract the oxygen is known already, using a pyrolysis process.

Is there a way of taking advantage of the moon's environment to do this effectively? You have at your disposal:

• A hard vacuum
• A month-long day that is strong sunlight for two weeks then a two week night - it is estimated that a furnace using concentrated sunlight could reach 3000 K.
• 1/6 Earth gravity
• No moisture - chemicals are reduced
• Powdery soil that is loose to a depth of 10 or 15 cm
• Some soil samples from Apollo 16 were a quarter alumina by weight

A source of fuel in space is a prerequisite for a larger ongoing human presence there. A rocket launched from Earth can't manage to be more than about 5% payload. Shipping fuel from Earth to any off-world base by rocket would be prohibitively expensive (even if the rocket is reusable). This is why there has been so much interest in sources of water ice in space - it can be electrolysed into liquid hydrogen and liquid oxygen, which makes excellent propellant. But you have to go to the asteroid belt to get it (there are indications there might be significant reserves in craters at the moon's poles in permanent shade, but this is inconclusive). Once there is a way to refuel in space, all sorts of things become possible that are beyond our reach right now. An option that can compete with the price of mining asteroids has business potential. All such undertakings are going to require vast initial outlays that won't pay off for a decade or more. The process being considered here only has to meet that standard.

This question is a partner to a question at SE's Space Exploration site - How feasible is it to use aluminum and liquid oxygen as future propellant sourced from the moon?. I realized it was largely a chemistry question and so decided to ask about it here. There are two references in the other question: one from Wickman Spacecraft & Propellant, and one from the Artemis Data Book. I'm afraid neither discusses the chemistry and I haven't found anything else. The AIAA has several papers on it in their catalogue.

Answer 1

My theory was to use electromagnetic separation, aka the Calutrons used in the Manhattan Project. Fine powder feedstock (check), into an arc discharge to vaporize and ionize (lots of good designs for high volume ion sources out there), electrostatic acceleration, then electromagnets to mass separate. Power with a solar panel array. No moving parts (except for the powder feed). No vacuum systems needed. And you separate all of the elements at once. Now, it only works during sunlight, but during the lunar night you could do maintenance and collect the separated elements.

Answer 2

I don't think this is going to be a good idea.

Notice that the Al and O existing in the Moon's soil are not there are free elements, but are tightly bound in minerals.

On Earth, Al is extracted from bauxite - this is naturally occurring Al₂O₃. This extraction process is extremely energy intensive. In fact, they transport the bauxite mined elsewhere to Iceland because the electricity in there is so cheap, instead of doing it on the spot of the mine, or nearby.

The Al and O that occur on the Moon are mostly in the form of the mineral anorthite plagioclase: CaAl₂Si₂O₈. Extracting the Al from there would be even harder! You first have to extract the Al₂O₃ out of the CaAl₂Si₂O₈, and only then you can get the Al out. This mineral is not unique to the Moon. In fact, anorthite, albite (NaAlSi₃O₈) and all minerals in between are the most common minerals in Earth's crust, and we still don't use them as a source for Al.

That said, burning Al as a fuel would not be a good idea because the end result is a solid, not a gas.

Answer 3

The standard process for electrolysing $\ce{Al2O3}$ to $\ce{Al}$ should work fine on the moon. Note that on Earth, a consumable carbon anode is used to reduce the voltage required, giving the reaction (Hall–Héroult process)

$$\ce{Al2O3 + 1.5C → 2 Al + 1.5 CO2}$$

An alternative, non-consumable anode would be used on the moon, to avoid the need for anode material and to give oxygen instead of $\ce{CO2}$.

The process for purifying $\ce{Al2O3}$ would be rather more difficult. On Earth, this is done by dissolving bauxite in sodium hydroxide solution, filtering out insoluble iron salts, cooling, precipitating out $\ce{Al2O3}$, and discarding the liquor containing silicates.

This process is highly intensive in water and sodium hydroxide. Major advances would be needed to make it less intensive in these materials, and to make the electrolysis step more tolerant of impurities.

I think we are better off looking for water to use as feedstock for our propellant factory. It's far easier to purify (all you have to do is distill it).

Answer 4

Ionic liquid refining of aluminum is a promising solution which uses 3 kWh of electricity per kg instead of around 15 kWh for existing methods.