Ore Processing
Section 2.2.3.
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Melt-Quench-Leech Aluminum Extraction

David Burkhead

The main drivers of the mass of a melt-quench-leech aluminum extraction cycle are how much you have to process how fast (it takes more mass to process more material) and how concentrated the regolith is in desired minerals. Assuming you need to produce 1 tonne of aluminum a day and the regolith runs 3% aluminum by weight, that means you need to process 30 tonnes of regolith every 24 hours. A group of robots capable of moving 1.25 tonnes of regolith per hour might mass 2 tonnes.

To melt the rock, you'll need about (to a fairly loose approximation) 100 kJ per kg. That's 830 kW hours of energy for the entire day's load (or 35 kW source operating for the entire day). A solar furnace (solar energy to heat) is on the order of 70% efficient, and during the lunar day the solar energy available is 1360 watts per square meter of collection area. That leads to a collector area of 37 square meters, or a 7-meter diameter solar reflector. The mass of such an inflatable reflector would be negligible.

The next step is the quench. If the melt can be poured in a thin stream (high surface to volume ratio) you can get the "quench" from radiation alone--no need for special equipment. That requires a vacuum environment though, to prevent contamination and oxidation of the melt (which is exactly the opposite of the effect you want). The moon, of course, has plenty of vacuum, so that's to the good. Another advantage of this method is that it leaves the glass produced in a nice, handleable form, with the high surface area-to-volume ratio you want for the next phase.

Next is the leech process, which requires vats of hot acid. A lot depends on the leech time and how much acid is required for a given amount of lunar glass. Let's say the leech time is six hours and the total volume is twice the glass volume. 30 metric tons of glass is about 10 cubic meters. You'll need the same amount of acid but you can use the volume 4 times a day, so that's about 4 metric tons. Of the H2SO4, 65% can be lunar oxygen, leading to 1.4 tonnes of sulphuric acid components which must come from Earth. The vats themselves are pretty big, probably another tonne, but it would probably be possible to use solar heat to melt vats out of lunar rock or glass, saving the shipping weight.

From leech, you have to recover the alumina from the acid. That will require more vats to carry cooling acid--cooling the acid should be sufficient since the solubility of alumina is highly temperature-dependent. For this we double acid and vat weights. This gives us a total to this point of about 12 metric tons, 5 of which must come from Earth.

The next step is standard aluminum production. The alumina is dissolved in molten cryolyte and electrolyzed. A facility to do this might mass 3 tonnes.

This gives us a total of 8 metric tons on the moon. Much of the mass, perhaps 7 metric tons, can be avoided by the use of lunar materials. The 8 tonnes will have to come from Earth. It will probably require a crew of 5 to operate.

The result will be a plant capable of producing a metric ton of aluminum every day, but only during the lunar daylight periods (the solar furnace). This means about 180 tonnes per year, and 5300 tonnes of tailings enriched in other elements, ready for oxygen extraction (which further enriches ilmenite to TiO2 and FeO). Processing that quantity of regolith would also free several tonnes of volatiles per year, which would be essential for life support resupply.

Content by David Burkhead <>

Ore Processing

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