Pidgeon process for the Moon base ISRU?

[trurle]

I am evaluating the possibility what magnesium may be the most-easily obtainable structural material on Moon.
The following ISRU sequence is proposed:
1) Regolith melt electrolysis produce ingots made of ferro-silicon, as by-product of breathable oxygen production
2) Ferro-silicon ingots are crushed and mixed with magnesium-rich crushed rocks
3) The mix is heated to ~1150C in ceramic or steel vessel to release magnesium vapour, similar to Pidgeon process

I invite the comments and proposal on following topics:
1) Suitability of Moon rocks for the Pidgeon process (seems major constituents are all compatible with process, although i have doubts regarding minor components like sodium, titanium or chromium which may result in unwanted by-products)
2) On Earth, Pidgeon process is evolving to high-pressure, high-temperature variants. Is it different for the conditions on the Moon?
3) Plausible magnesium alloying with Moon feed-stocks to improve alloy properties?
4) General comments and suggestions on the image of the ISRU scheme attached to this post?

 

[jedishrfu]

One thing to consider is whether any process is dependent on gravity to separate quantities and of course on the moon the gravity is much less.

Also power on the moon would be limited to solar cell production after awhile so any process using electricity may suffer from lack thereof.

 

[Rive]

4) General comments and suggestions on the image of the ISRU scheme attached to this post?

The seemingly abandoned 'FeNi sand' line of products might be able to challenge the 'most-easily obtainable' claim based on energy requirement.
While the amount of soil to be moved might be more, the energy requirement might be still lower since there is just one heating and no electrolysis in the process.
But to spice up things you can try throwing some metal powder based 3D printing into the pot too.

 

[trurle]

The seemingly abandoned 'FeNi sand' line of products might be able to challenge the 'most-easily obtainable' claim based on energy requirement.
While the amount of soil to be moved might be more, the energy requirement might be still lower since there is just one heating and no electrolysis in the process.
But to spice up things you can try throwing some metal powder based 3D printing into the pot too.

I have a doubts about FeNi sand availability and energy intensity of smelting. The typical concentration of FeNi of meteorite origin in lunar soil is about 0.04%-0.2%, and most of it may be heavily agglutinated with the rock material. Separation (by hammering/forging and magnetic separation) may be energy intensive, and smelting itself is at 1500C, while 1150C required for magnesium production and 700C for magnesium smelting.
I must point out what rock melt electrolysis step in magnesium production chain is likely should be done anyway for breathable oxygen production, resulting in energy spent for electrolysis irrelevant to structural material production expenditure. Of course, rock melt electrolysis machine may be useless for unmanned base, in this case argument for the FeNi is true.

 

[Rive]

I must point out what rock melt electrolysis step in magnesium production chain is likely should be done anyway for breathable oxygen production,

The necessity of this part is often overestimated: technology to recycle CO2 (without plants) is already available. New oxygen will be needed only if new living area needs to be filled.

The typical concentration of FeNi of meteorite origin in lunar soil is about 0.04%-0.2%, and most of it may be heavily agglutinated with the rock material. Separation (by hammering/forging and magnetic separation) may be energy intensive

Too many 'maybe', on both side, no way to be sure without further input. I've just wanted to point out that this product line seems to be unjustly neglected there, especially with some potential in 3D printing
Harvesting sand on the moon for FeNi feels like a perfect task for mass produced 'cheap' autonomous vehicles, running on solar power alone.
But most of it is Sci-Fi right now, anyway.

 

[trurle]

The necessity of this part is often overestimated: technology to recycle CO2 (without plants) is already available. New oxygen will be needed only if new living area needs to be filled.

Sabatier reaction you linked do require expendable water input, which is problematic on Moon. Metabolic water can supply only half of demand for breathable oxygen even if 100% recycled, with the rest (about 0.8kg per man per day) need to be imported or transported from Moon poles, at least doubling the life-support mass budget for realistic oxygen recycling efficiency of 50%.
With food (carbohydrates) metabolizing and water electrolysis, the total Sabatier process equation is:
CH₂O+O₂+H₂O=>CH₄+2O₂
Alternative to water is expendable hydrogen input, which lighter but have unsolved storage problems, and require an 67% instead of 50% recycling efficiency to close oxygen-supply budget. 67% oxygen recycling efficiency is extremely problematic, because it either limit moonwalk (off-base) time or require to capture CO₂ produced during moonwalks.
Sabatier process can be more efficient if methane is converted to heavy hydrocarbons, recycling up to half of hydrogen atoms which are otherwise lost from cycle, of if fat-rich diet is considered, but all these options require even higher recycling efficiencies, in the 70-80% range.

On the other hand, oxygen production by rock melt electrolysis do not require to recycle carbon dioxide at all, have no expendable inputs not available at any Moon site, and place no constraint on diet. My basic assumption for Moon base is 0% recycling of CO₂ and 75% recycling of H₂O with ISRU O₂ input. This is a partially-open cycle which allows to run life support on metabolic water and require only dry food and regolith as inputs.

 

[Rive]

Sabatier reaction you linked do require expendable water input,

If you check that link again, in the middle somewhere there are some lines about recovering H₂. Accepting the H loss was an engineering decision for the ISS due the problems with handling pyrolytic graphite in zero g environment. At full swing this system is expected to just cleanly split CO2 to C and O₂.

 

[trurle]

If you check that link again, in the middle somewhere there are some lines about recovering H₂. Accepting the H loss was an engineering decision for the ISS due the problems with handling pyrolytic graphite in zero g environment. At full swing this system is expected to just cleanly split CO2 to C and O₂.

Methane splitting does help to reduce hydrogen leakage from the life support system, correct. And just bring you to the point when oxygen leakage by wasted CO₂ become dominant. How much realistically you expect to capture CO₂, especially with vacuum-suit constraints? Even 50% CO₂ capture required for Sabatier process with no methane decomposition is tough. Also, molecular O₂ leakage may be an issue.

This topic actually related to "CO2 scrubber challenge" review during which i have found no Moon-base suitable scrubber hardware last year. May be somebody can propose one?