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SeaDragon

Banned

From: Merry Old England

Registered: 2020-07-25

Posts: 32

Chloride Processes - the Better Way to Process Regolith

TL;DR:

Digesting Martian regolith with hydrochloric acid made from electrolysis of salts leached out from that same regolith with water provides a game changing approach to separating out regolith metals according to the following advantages:

- Chloride salts produced from HCl digestion are easily separated from one another on account of their different solubilities in water.
- They have much lower melting points than oxides like alumina and so can be operated at temperatures like 700oC instead of 1,000 oC or higher for oxides.
- They conduct electricity better, thus requiring lower voltages and hence offering greater efficiencies over molten oxide electrolysis routes.

Full Explanation:

On Mars, as on Earth, there exist many deposits of pure ores (we've seen gypsum so far but there's no reason not to suspect a cornucopia of others) that can be mined and smelted/otherwise processed. In order to make full use of these, however, large numbers of independent mining camps may be required and their outputs moved over hundreds if not thousands of kilometres of rough terrain. At the beginning of a colonisation effort this may be impractical for all but the most valuable materials - I don't care how far you'd have to go to bring me the Li, Th, F or Be you've been mining, even trucking them 5,000 km is probably worth it to get good quality nuclear reactors up and running. Iron oxide, though, not so much. It may even have to be a majority of useful materials that are produced on site. In the case of the Moon or asteroids, each thoroughly undifferentiated, there aren't any pure ores and there is no grey area even for the expensive stuff: you have to pull apart locally available trash rock into usable substances. This is, of course, much easier said than done. Typically trash rock (and with it the majority of all available rocky material in the entire solar system) is composed of mixed magnesium, iron and aluminium silicates, the most common element to which all of these are chemically bound being oxygen. For Mars in particular we have something like:

Which has a few small quantity extras (chlorine being the key to this proposal) but mostly it's as above: Fe, Mg, Al, Si and O.
Currently the state of the art for production of metals in space from undifferentiated sources like Martian regolith seems to be something along the lines of:
- "Melt the oxide, then electrolyse it like alumina" as per Molten Regolith Electrolysis ( https://isru.nasa.gov/Molten_Regolith_E … n%20oxides ).

- "React it with hot carbon/carbon monoxide/hydrogen to liberate the oxygen as CO2 or water, precipitating out the rest as base metals" which is what is most often done on Earth whenever it is physically possible.

The second approach works well provided separation into relatively pure oxides has already been achieved but this is entirely the problem: while some easy tricks permit this in general it's really very difficult to get metal oxides by themselves especially when they've formed complex minerals as has happened on Mars. The first approach sidesteps this by melting the regolith and electrolysing the whole thing but at the cost of terrible electrical inefficiency (molten oxides are not great conductors of electricity!) and temperatures as high as 1,600 oC, making it inefficient and the equipment capable of doing it very difficult to operate/maintain.

I think there's a much better way in practice.

Looking back at the soil composition for Mars you can leach out chlorine (in the form of perchlorates), sodium and potassium oxides just by soaking it in a little water. It's probably best to roast the soil first to turn those perchlorates into oxygen gas and chloride salts (probably sodium chloride), then you'd be leaching table salt instead of sodium hydroxide. In any case, once you have your chlorine compounds you can electrolyse them to get chlorine by itself as is done on Earth all the time at room temperature in aqueous solution. Burning the chlorine with hydrogen in the presence of water gives, at last, the main substance I propose we make use of:

Hydrochloric acid
If you feed Martian regolith to an aqueous mixture containing concentrated hydrochloric acid at elevated temperatures most of the metallic elements present will be torn from their oxides and form chloride salts, eg:

MgO + 2HCl(aq) -> MgCl2 + H2O
CaO + 2HCl(aq) -> CaCl2 + H2O
Al2O3 + 6HCl(aq) -> 2AlCl3(H2O)6*
etc.

*(some of them do form hydrates like aluminium, but typically you can either boil out the excess water taken up or else (in aluminium's peculiar case) get the anhydrous chloride in some other way)

The great thing about having a mixture of chlorides over a mixture of oxides is their solubility: chlorides tend to dissolve in water easily but to differing extents depending on which chloride you have. In this way, you can precipitate out different chlorides one by one simply by slowly boiling off water in which the salts are dissolved. Running your collected salts through this sort of process several times could provide very pure separated piles of different metal chlorides. This is absolutely key to successfully processing these materials.

Once you have your chlorides separated you can electrolyse them in separate electrolysis chambers to collect up individual metal products. In this too chloride salts have a massive advantage: unlike oxides, chlorides tend to have much lower melting points and much higher electrical conductivities so that electrolysis can be more electricity efficient and work with less exotic materials. Of course, they're highly corrosive to many material choices but not immensely so - an excellent paper on the topic of molten aluminium chloride electrolysis ( http://cecri.csircentral.net/2112/1/21-1985.pdf ) claims that any common refractory nitride as cladding works just fine. A silicon nitride coating around conventional steel has been demonstrated in numerous pilot plants (which never took off on Earth because there's no shortage of high quality bauxite ore and trivially cheap shipping/carbon for electrolysis).

It actually gets better than this: in the case of lowish reactivity metals there's even a patent from the mid 1950s ( https://patents.google.com/patent/US2810685A/en ) which suggests that you can use aqueous solutions of chloride salts to still get pure metal as a product at least in the case of manganese by using ammonium chloride and membrane separation - but hey, if it works for manganese it should still be possible for chromium! In the patent it describes a system that delivers metallic manganese at room temperature for half the electricity per unit mass required for aluminium electrolysis. These trace elements in Martian regolith can thus be easily concentrated and produced, providing essential feedstock for batteries and steels and other uses. The chlorine gas liberated by electrolysis of metal chlorides can then be reacted with hydrogen to produce hydrochloric acid again and thus digest more regolith.

After digestion by hydrochloric acid (and assuming you leached or baked out all the volatiles, sulphur included) all that's left is likely silica and trace elements. Relatively pure silica is essential for production of lubricant oils, seals and synthetic rubber (organosilanes) as well as alloys and even semiconductor applications like solar cells after a great deal more effort.

Overall, I think this is a much better way to do things if all you have to work with is undifferentiated rock.

SpaceNut

Administrator

From: New Hampshire

Registered: 2004-07-22

Posts: 29,433

Re: Chloride Processes - the Better Way to Process Regolith

Perchlorates and Perchlorate (ClO 4 –) are reactive chemicals first detected in arctic Martian soil by NASA's Phoenix lander that plopped down on Mars over a decade ago in May 2008. The research emphasizes that perchlorate is widespread in Martian soils at concentrations of between 0.5 to 1 percent. There are dual implications of calcium perchlorate on Mars. On one hand, at such concentrations, perchlorate could be an important source of oxygen.

https://en.wikipedia.org/wiki/Martian_soil
https://en.wikipedia.org/wiki/Perchlorate

Most perchlorates are colorless solids that are soluble in water. Four perchlorates are of primary commercial interest: ammonium perchlorate (NH4(ClO4)), perchloric acid (HClO4), potassium perchlorate (K(ClO4)), and sodium perchlorate (Na(ClO4)). Perchlorate is the anion resulting from the dissociation of perchloric acid and its salts upon their dissolution in water. Many perchlorate salts are soluble in non-aqueous solutions

https://www.nature.com/news/2008/080806 … .1016.html

calcium perchlorate (Ca (ClO4)2) Martian soils at concentrations between 0.5 and 1%
magnesium perchlorate (Mg (ClO 4) 2) temperature for the liquid phase of magnesium perchlorate – 206 degrees Kelvin – is a temperature found on Mars at the Phoenix landing site.

http://faculty.washington.edu/dcatling/ … lorate.pdf
Atmospheric origins of perchlorate on Mars

https://www.epa.gov/sites/production/fi … _final.pdf

https://agupubs.onlinelibrary.wiley.com … 10GL045269
Concentrated perchlorate at the Mars Phoenix landing site