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There is a very large amount of iron oxide on the surface of Mars within easy reach (once you are there of course).

What would be the practical challenges to collecting this and turning it into usable Iron? What would be the most straight-forward process to implement for a Martian colony, and what kind of energy per kg of product would be needed?

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    $\begingroup$ @Fred what is Mars' atmosphere made of? What are the byproducts of extracting breathable oxygen from it? $\endgroup$ – uhoh Dec 9 '20 at 7:00
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    $\begingroup$ @uhoh the byproduct is the horrible awful not good at all carbon monoxide. We'd need some extra process to get carbon. $\endgroup$ – SF. Dec 9 '20 at 7:37
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    $\begingroup$ IIRC CO is used in iron production @SF. $\endgroup$ – GdD Dec 9 '20 at 10:00
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    $\begingroup$ Building on what @GdD said, here's a relevant link: en.wikipedia.org/wiki/Carbon_monoxide#Metallurgy $\endgroup$ – Pitto Dec 9 '20 at 11:19
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    $\begingroup$ @Pitto Neat one! $\endgroup$ – SF. Dec 9 '20 at 15:42
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Useable Iron is not pure metallic iron, it is iron with the right amount of carbon. Too few carbon and the iron is too soft, too much and the iron is too hard and brittle.

A little bit of other metalls like manganese (the most common one), nickel, chromium, molybdenum, vanadium, silicon, and boron. Less common alloyants include aluminium, cobalt, copper, cerium, niobium, titanium, tungsten, tin, zinc, lead, and zirconium. To much sulfur could be bad.

It depends on the application what alloys are needed, there is no universal alloy useful for all applications. As John Custer wrote, there are some alloys covering a broad range of uses.

The same is true for aluminum, pure alumin is weak too.

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  • $\begingroup$ Are the secondary metals available and carbon in easy to mine quantities on Mars? $\endgroup$ – gwally Dec 9 '20 at 18:59
  • $\begingroup$ As here on Earth, 304 and 316 cover a broad range of uses, but you will need those pesky alloying elements indeed. All the more reason to think broadly on processing 'ores'... $\endgroup$ – Jon Custer Dec 9 '20 at 19:11
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    $\begingroup$ Just plain pure iron with 0.20% carbon is mild steel which is an extremely useful alloy for many purposes. $\endgroup$ – ikrase Dec 9 '20 at 21:00
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    $\begingroup$ Why and how is rust forming on moon? just fyi the rusting process of iron requires the presence of both oxygen and water which are going to be in very short supply, so for practical purposes the alloy need not be rust-proof. $\endgroup$ – uhoh Dec 9 '20 at 21:04
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    $\begingroup$ @ikrase No carbon steel has 0.05% up to 2.1% carbon by weight. If you are lucky, you dont need to add or to remove carbon. But do wee know something about the carbon content of martian iron oxide? Removing the oxygen may remove more carbon than needed. Several methods to remove carbon from steel used on Earth expose the iron to oxygen. $\endgroup$ – Uwe Dec 9 '20 at 22:11
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As other answers here have noted, simply splitting the oxygen from the iron doesn't produce USABLE iron - for that, you need carbon content. I'm sure there are other workable approaches to this, but here's one:

To get elemental iron:

  1. Produce water from Martian wells
  2. Split the water with Hydrolysis, and capture the H2.
  3. Mine Martian regolith and tumble separate it to get the impurities down to a workable level.
  4. React the hydrogen with it, a la this article.

To get elemental carbon:

  1. Mine CO2 from the Martian atmosphere.
  2. Split the C from the O2 however you want, including creating methane from the hydrogen in step 1, and then separating it via thermolytic catalyzation, or simply with some MOXIE-like device.

Combine the two in proper measures, and melt it. As others have noted here, you'll need nuclear power for more than one part of this process.

This is a worst case scenario, of course - you might be able to find ore with significant carbon content in the first place. This papers over a lot of the details of the iron separation that are covered in the linked paper, but it's a broad back of the envelope approach that I think is workable.

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    $\begingroup$ Excellent answer! If I were making a reality TV show "Survivor, Mars edition" I would have them build a solar thermal furnace rather than forage for radioisotopes, but that's neither here nor there. $\endgroup$ – uhoh Dec 30 '20 at 23:16
  • $\begingroup$ Yeah, maybe something along the lines of that effort that Bill Gates has backed. $\endgroup$ – Chris B. Behrens Dec 31 '20 at 0:16
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    $\begingroup$ I would definitely transport refined nuclear fuel from Earth and run Kilopower reactors, but... You totally could run Benghazi burner / poor man's pebble bed reactors in a pinch. I think that kind of thing will happen once the Martian colony evolves beyond the carefully planned early stages. $\endgroup$ – Chris B. Behrens Dec 31 '20 at 0:39
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No known planet besides Earth has both oxygen and fossil fuels for energy, and so Mars will need either something on the order of electrical solar cells or nuclear for energy. If you are talking nuclear, you will need to find viable uranium and thorium ores, use some sort of energy reserves to purify it, and then that could be the starting point for more energy sources for building solar cells or mining and purifying more uranium ores.

At some point this energy could be used to melt part of the polar ice caps for cooling of the reactors, and the filling and warming of large insulated chambers and caverns with water and carbon dioxide for food. As for lighting for plants, it is not obvious whether artificial lighting of an area insulated from the cold Martian environment might be better than trying to create something transparent for solar input but still capable of insulating from the extreme cold of Mars. Probably some of the direct heat from the rods could be sent to an alternate high temperature chamber for metallurgy, but it is not obvious whether something like electrolysis for aluminum might be better anyway.

On Earth, the most standard way of reducing iron involves fossil fuels and oxygen.

On another planet, where those are not available, it is not impossible that reducing other metals might potentially be more favorable because of alternate methods being needed.

It is not obvious whether the Moon or an asteroid like Ceres or something like Phobos could be better than Mars because of the lower gravity wells, or Mercury because of the solar inputs. As for Venus, I have read about the possibility of floating balloons very high in the atmosphere where the atmospheric pressure is more similar to Earth, filled with oxygen as a lift gas, but I would be averse to falling from the balloon.

Mars however is very high on the list for livability behind Earth, either for robots, humans, or genetically engineered intelligent organisms designed to withstand the vacuum of space.

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Ignoring the issue of getting specific alloys, as that is a hard question common to all answers so far given, and outside my knowledge...

What you want to use is this: https://en.m.wikipedia.org/wiki/FFC_Cambridge_process

Electrolyse the iron oxide ore on a liquid salt bath. Advantages of lower working temperatures than a smelter (900c Vs 1200c), no need to deal with hydrogen (outgassing, explosion risk), avoids requirement to capture the steam produced by hydrogen reduction. No need to extract hydrogen from water.

Heck, consider - the Sabatier process for making methane from CO2 doesn't yield sufficient oxygen to burn the methane. Why waste hydrogen on producing iron, when you can turn your iron production into part of your manufacturing process to get home?

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The heat of formation of iron oxide from the elements is approximately -825kJ/mol, with each mole being about 160 g of iron (III) oxide.

https://janaf.nist.gov/tables/Fe-030.html

2 Fe + 3/2 O2 -> Fe2O3

So the reverse reaction produces 1.5 moles of oxygen gas from 160g of iron oxide, which is about 48g of oxygen.

Any process that takes iron oxide and produces elemental iron and oxygen from it, must pay at least this energy expenditure. The decomposition temperature of iron oxide is about 1539C, so thermal losses would be a big factor in any furnace that seeks to separate iron oxide into its constituent elements.

Alloying the iron you produce is a small concern, the alloying constitutents are usually of such small quantities that you can bring them from Earth if they can not be conveniently produced on site.

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