Lunar Aluminum Production

Our satellite lacks of bauxite, therefore, anorthite will be used instead. Anorthite (CaAl₂Si₂O₈) could be separed from Anorthosite with mechanical and chemical methods to produce Alumina (aluminium oxide, Al₂O₃).

There are other several proposed methods to obtain aluminium (even those that doesn't require electrolysis). However, almost all include an importation of catalysts and/or reactants. It is true that reactants can be recycled but they limit the total output to the total amount of reactant present and to the speed that the process can recycle them. Another process would be Ion-sputtering to obtain other material and as a residue Aluminium.

Proposed Anorthite Production Process

Anorthosite is a mix of Plagioclases, Olivines, and Pyroxenes. To separate the anorthite, anorthosite a must be grind . Then, magnets could be put in contact with the powder to separate everything from the non-magnetic anorthite.

The magnetic materials (even Ilmenite, iron oxide, silica and magnesia) could be stored for production of Titanium and other metals.

Proposed Alumina Production Process

On Earth aluminium is subjected to the Hall-Heroult process where bauxite undergoes the Bayer process to become alumina.

Alumina could be produced from Anorthite by boiling the impurities (between 1500 ºC - 2000 ºC ). The resulting material would be calcium aluminate (CaAlO₄). That can be leached in sulfuric acid. The following reaction would be:

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 aluminium sulfate releasing S₂.

Future Hall-Heroult Adaptation

Alumina is dissolved in molten cryolite (sodium hexafluoroaluminate, Na₃ AlF₆ ) around 1400 ºC. This mix is electrolyzed to separate two byproducts: aluminium and CO₂.

Pure alumina can be electrolyzed, but, it melts around the impractical 2000 ºC without the addition of cryolite. It is not known how refractory containers are going to be made on the Moon, neither if the Hall-Heroult process can be adapted with or without the importation of fluorine for the cryolite.

Subchloride Process

Chlorine and carbon that would be imported from Earth. The alumina is then carbochlorinated (carbonated and chlorinated) to yeild AlCl₃ which is electrolyzed. Electrolysis of AlCl₃ does not consume the precious lunar carbon electrodes as does conventional Hall-Heroult electrolysis of alumina. The carbochlorination byproduct CO₂ must be recycled.

Proposed Carbothermal Reduction

Carbon would be imported from Earth. Then the alumina would be mixed with silica and carbon and melted near 2000 ºC. An aluminium-silicon alloy will form. This could be separated by cooling the Al-Si mixture to 700 ºC-1000 ºC. and the silicon will solidify and settle out of the melt. CO₂ must be recovered and carbon recycled.

Other Electrolysis Methods

There are two process that doesn't require importation on Earth.

The first one is just boil the anothite and to obtain gases at different temperatures, first silica glass then calcium aluminate. With more temperature we obtain alumina and calcium oxide the temperature is reduced and the gases condensed and the liquid is electrolyzed to obtain aluminium, calcium and oxygen (all of them pure). This would require temperatures over 2560 ºC, very efficient super-refractories and a tremendous input of energy.

The second is not going that far with just electrolyzing anorthite at about 1600 ºC. The other byproduct would be calcium oxide.

See Also

Aluminium