Lunar Aluminum Production

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Since Luna lacks any known deposits of bauxite, the ore most commonly used on earth for aluminum production, anorthite (CaAl2Si2O8) is most commonly proposed as a lunar substitute.[1] Anorthite could be separated from the lunar highland material Anorthosite with mechanical methods. It could then be reduced through various chemical and electrochemical methods to produce aluminum.

Anorthite Production

The Anorthosite which makes up the Lunar highlands is a mix of Plagioclases, Olivines, and Pyroxenes. To separate the anorthite, anorthosite must be ground. Then, magnetic separation could leave the non-magnetic anorthite.

The magnetic materials (Ilmenite and iron oxide) could be stored for production of titanium, iron, and oxygen.

Anorthite Refinement

Direct Reduction

Main Article: FFC Cambridge Process

Anorthite could be directly reduced to its component metals using the FFC Cambridge Process. The Anorthite is pressed/sintered into a cathode, which is placed in a bath of molten calcium chloride and electrolyzed. The oxygen is stripped out, leaving behind Aluminum, Calcium, and Silicon.

This process has the advantage of inherent simplicity, as well as having only one component to recycle, the calcium chloride, which does not react chemically with the inputs, making recovery much simpler. In addition, this process runs at lower temperatures (900º-1100º C) than many other electrolysis procedures, and inert(non-consumable) anodes have been successfully demonstrated with it[2]. On the downside, energy must be expended to split all the components of the Anorthite, not just the aluminum. Splitting the silicon and calcium adds a significant amount of extra energy to the process, as both of them are strong reducers. However, if the silicon and calcium byproducts were needed for other purposes (calcium is a good electrical conductor, and silicon could be used for solar panels or rocket fuel), this extra energy cost may not be an issue.

Alumina Production

Many processes used on earth or proposed for Lunar use require alumina (Al2O3) as an input. On Earth, alumina is produced from bauxite through the Bayer process. As this process is not feasible using Anorthite, another method must be utilized.

Vacuum Distillation

Alumina could be produced from Anorthite by boiling out the impurities between 1500 ºC - 2000 ºC under vacuum conditions. The resulting material would be calcium aluminate (CaAlO4). Raising the temperature further could cause alumina to volatilize as well. [3]

Sulfuric Acid Leaching

Another method is to produce calcium aluminate as outlined previously, which is then leached in sulfuric acid, resulting in the following reaction:

CaAl2O4 + 4H2SO4 ==> CaSO4 + Al2(SO4)3 + 4H2O

Aluminium sulfate in hexadecahydrate form (Al2(SO4)3) is then separated from calcium sulphate (CaSO4 + Al2(SO4)3) by filtering and from water by evaporation (and then recovered).

Finally Alumina is obtained by roasting the aluminum sulfate releasing S2.[4]

Hydrochloric Acid Leaching

Another option is to react Anorthite with hydrochloric acid, which results in following reaction:

CaAl2Si2O8 + 8 HCl + 2 H2O==> CaCl2 + 2 AlCl3.6 H2O + 2 SiO2

The calcium chloride and hydrated aluminum chloride dissolve in the solution and are removed. They are then precipitated out of solution, dried, and heated under partial vacuum until the calcium chloride evaporates out of the mix. Temperatures of this range will cause the hydrated aluminum chloride to become alumina, releasing water and hydrogen chloride in the process:

2 AlCl3.6 H2O ==> Al2O3 + 6 HCl + 3 H2O

The water and hydrogen chloride are separated from the calcium chloride and fed back into the system. The calcium chloride is then electrolyzed into metallic calcium and chlorine. A portion of the recovered water is then split into hydrogen and oxygen. The hydrogen component is reacted with the evolved chlorine to produce hydrogen chloride, which is then fed back into the main system.

Direct Calcium Aluminate / Alumina Reduction

Calcium Aluminate (see above for production) could be simply melted and electrolyzed directly, producing aluminum and calcium oxide.[5] This has two advantages. First, it requires no imported reagents, and second, only the aluminum is split, reducing the amount of energy needed. The disadvantage is that temperatures of approximately 1600 ºC are required, making electrode material of prime concern. Carbon electrodes could be utilized at those temperatures, but the anode would continually wear away as oxygen was produced around it, creating carbon monoxide. The carbon would need to be recovered and new anodes made from it. This effectively means that a rare reagent is needed, negating the process's first stated advantage. Finding an anode material that would produce oxygen without wearing away at those temperatures could be quite difficult.

Alumina could also be directly melted and electrolyzed in the same fashion. However, this would require temperatures of approximately 2000 ºC, bringing with it again the issue of electrode material.

Hall-Heroult Process

In the Hall-Heroult process, alumina is dissolved in molten cryolite (Sodium hexafluoroaluminate, Na3 AlF6 ) around 1400 ºC. This mix is electrolyzed to separate two byproducts: aluminium and CO2. The carbon comes from the consumption of the carbon anode.

This procedure is used extensively on earth for aluminum production, and as such has the advantage of being a very mature technology. The biggest issue is the consumption of the anode, which would require the produced carbon dioxide to be captured, converted back into carbon, and recast into new anodes; an energy intensive process. It is not known if an inert(non-consumable) anode material can be found that would work under these conditions.[6]

Subchloride Process

In the subchloride process alumina is reacted with carbon and chlorine to yield AlCl3 and CO2. The AlCl3 is electrolyzed to produce aluminum while recovering the chlorine. This has the advantage that conventional carbon electrodes can be used continuously, as the produced chlorine does not react with them. However, the CO2 byproduct must be recycled, adding extra complexity and energy requirements to the system. This makes it similar to the Hall-Heroult process in difficulty, except for two advantages. First, the recycled carbon can be directly used in powdered form, it does not need to be recast into electrodes. Second, due to the low melting point of AlCl3 (120 ºC), the process does not require significant energy to melt, and is more easily handled. [7]

Carbothermal Reduction

Carbon reduction of Alumina is impossible under normal smelting conditions, due to aluminums high reduction potential. However, Alumina could be mixed with silica and carbon and melted near 2000 ºC, which would form an aluminium-silicon alloy, as well as CO2. This could be separated by cooling the Al-Si mixture to 700 - 1000 ºC and allowing the silicon to solidify and settle out of the melt.[8]

An alternate process involves alumina and carbon processed at high temperatures and low pressure into Al4C3 and carbon monoxide.[9] [10] This breaks down into Aluminum and Carbon between 1900 and 2000 ºC.[11].

In either case, CO2/CO would have to be recovered and and the carbon recycled.

See Also