Section 2.13.2.
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Aluminum Production on the Moon with Existing Silicon Production

Geoffrey A. Landis

Aluminum is a byproduct of step 1F of a silicon extraction process, but not in pure form. If the titanium fluoride is not returned to the reduction after separation in vapor form at the initial fluoridation stage, then the product from step 1F will be an aluminum/iron mixture. There are roughly 2 atoms of iron for every three atoms of aluminum in the lunar soil. Since the lowest temperature liquid phase of metallic aluminum and iron mixtures is liquid aluminum plus iron aluminide (Al3Fe), the aluminum/iron resultant phase will be iron aluminides, primarily Al3Fe. If the feed material is a high-aluminum/low-iron soil such as the Apollo 16 soil (27% Al2O3, 5% FeO), metallic aluminum will be formed.

Purifying the aluminum can be done by a number of processes, either by separating the aluminum fluoride by chemical means, by making an aluminum-enriched alloy and removing aluminum by melting, or by other means of removing the aluminum at the alloy stage.

A simple physical method for separating out the aluminum is by vacuum distillation. This process exploits the higher vapor pressure of aluminum over iron; it is a convenient method for lunar application since it requires only heat and vacuum. This requires heating the alloy mixture to a temperature at which aluminum vapor is evolved, and then exposing a cooled surface for the vapor to condense upon. The relatively pure film of aluminum produced is then removed for use by heating the condensing surface to melt the aluminum.

Vacuum distillation could also be used to separate the aluminum fluoride from the fluoride salt mixture in the same way, first subliming aluminum salt AlF3 away from the iron salt at 1291 °C. This has the disadvantage that, since the fluorine must be recovered to be reused as reactant, the distillation cannot allow any wasted material. Loss of aluminum metal, on the other hand, is of little concern, since aluminum is abundant on the moon. Thus, if high-aluminum soil cannot be easily found to use as feedstock, vacuum distillation will be used to separate the aluminum from the alloy mix.

An alternative material for electrical conduction is metallic calcium. This is not used on the Earth because of the extremely high reactivity of calcium in an oxygen atmosphere, but this may not be a problem on the moon.

Calcium has the property of having one of the highest conductivity to density ratios of any easily-available metal, that is, metallic calcium makes the lightest wires. However, aluminum is adequate for wire use, and aluminum technology is well developed. Therefore I have not baselined calcium for use in the production sequence discussed.


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