In preparation for this document, several things struck me. These extraction (smelting) techniques have been developed over thousands of years, and rarely change significantly. They have been developed for minerals found in an oxygen- and water-rich atmosphere. They seem to fall into two broad categories: electrolytic methods and chemical methods. Chemical methods typically use large amounts of materials other than ore for processing (for example, the use of lime and coal in the processing of iron). These methods would require importation of these materials, unless an appropriate exo-LEO substitute can be found. Electrolytic methods, on the other hand, use large amounts of electricity to force a chemical reaction. They typically require large water baths, filled with noxious chemicals. But once the initial water is transported to the moon, these techniques will likely be the best choice.
For each metal, I have summarized the important Terran methods, and suggested modifications for the Lunar environment. In my suggestions, I have tried to limit the use of non-Lunar resources, most notably, hydrogen/water and carbon. I have also assumed an abundance of energy (what do you expect? The moon has to have something to offer). I have further assumed no special mining techniques and no motherlode veins. Just scoop up some regolith, and dump it in.
As an aside, as I studied the composition of the Lunar regolith, I discovered it was some nasty stuff. As table 1 shows, the moon is made of a very basic mixture of materials. This brings to mind a problem I have not run across before. As these materials absorb water (from small leaks in habitats, as is inevitable, or if brought into a habitat by construction design or accident) they will absorb it at a ratio of one molecule of water per molecule of oxide. The subsequent reactions will make a caustic solution, posing a danger to the long-term survival of some metals, plastics and fabrics. Imagine a slow drip from a not-quite perfect weld, just below a condenser. Over the years, regolith that has been piled upon the shelter as shielding absorbs the water, and starts an electrolytic reaction (this is how salt causes cars to rust). This will definitely attack steel, and will eventually corrode aluminum.
Table 1: Estimated Composition of Moon Compared with Other Objects
|Oxide||Bulk Moon||Terra crust||Mare basalts||Cl Meteorite|
I suspect that glass-blowing will become a big hobby on the moon. All you need is sand (a.k.a. SiO2), heat and spare time. There are some important uses for silicon (grease, fiberglass), but I can't think of any that require large amounts of pure silicon, so I will ignore this element. Dumping of waste SiO2 from smelting processes will be a problem on a Lunar colony.
On Earth, iron is without question the most important metal. It holds up our buildings and transports oxygen to our brains. But, unfortunately, translating iron smelting to the moon will be difficult. The Terran process uses carbon monoxide to reduce iron oxides (mostly hydrated Fe2O3 and Fe3O4) to the metal and carbon dioxide. Unfortunately, blast furnaces use coal as their CO source. Carbon dioxide can be converted to CO by reaction with hydrogen gas (water is the other product), but humans will not produce enough CO2 to support a large-scale smelting process (this may be a way to recycle waste vegetable matter).
The next step in the process is the removal of silicon. The Terran method uses lime (CaO). While CaO is available in the regolith, it is about 10% by mass of the amount needed. Additionally, the resultant "calcium silicate slag" will be difficult to recycle. Did I mention that the smelting process occurs at 500-1000° F? Anyway, the result of this process is pig iron.
While I intend for this treatise to cover only the metal extraction, not processing, I must point out that steel contains 0.05 to 2.0% carbon. This means about 20 kg per metric ton of steel produced. We may want to take a step back to wrought iron. All oxygen produced by this method (about 300 kg per metric ton of iron) ends up as carbon dioxide. I have yet to find reference to any electrolytic methods for the production of iron.
Where iron is disappointing, aluminum is promising. The standard method for production of aluminum is electrolysis, and it may be directly convertible to a Lunar environment. First the bauxite (Al2O3) is dissolved in sodium hydroxide. Pure Al(OH)3 is precipitated by bubbling CO2 through the solution. Purification at this step is considered easier than purification of the metal.
The Hall process is carried out in large iron pots, lined with carbon (used as cathodes). In these are placed carbon anodes. NaAlF6 is added and melted. To this is added Al2O3. The current reduces the oxide to the metal, liberating oxygen. The procedure is carried out at 1000°C, and since Al melts at 660°C, it collects as a liquid on the bottom of the crucible.
I suspect that the direct addition of aluminum hydroxide would be fine, aside from the large plume of steam (and corresponding loss of energy) you would get. I am not sure why cryolite (NaAlF6) is used along with the oxide. It may improve the conductivity of the solution.
The basic nature of the oxides of these metals, delineated above, suggests a method for removing these less useful metals from the regolith. Suspending the regolith in water will readily dissolve Na2O, K2O, and to a lesser degree MgO and CaO (lime) via the equations below:
|CaO + H2O = Ca(OH)2||Na2O + H2O = 2 NaOH|
|K2O + H2O = 2 KOH||MgO + H2O = Mg(OH)2|
The bound hydrogen could then be released by electrolysis of the melted hydroxide. This was the commercial preparation for sodium for many years and should be applicable to the other hydroxides with varying degrees of difficulty.
The metals themselves are nearly useless in an oxygen- and water-filled environment, and are extremely soft, as well. However, in the near-vacuum of the moon, new alloys or other uses may present themselves.
While it would be useful to have some regolith to experiment on, an approximation can be made. I would volunteer, but I do not have access to the refractory equipment required for these experiments. The chemicals are relatively cheap.
I am still trying to track down some NASA documents that may provide other suggestions for separation techniques. This is a work very much in progress.