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Iron and aluminum are the two most common metals in the Earth's crust. While we can't know for sure what we'll find for raw resources when we start building factories in space, if we assume that iron and aluminum are both readily available, would one of these be easier to work then the other?

We know from this answer that both iron and aluminum have similar longevity and oxidization concerns in the vacuum of space. Despite aluminum being much more common in the Earth's crust, it was the second of the two to be widely used due to the comparative difficulty in smelting.

It seems like a solar furnace could be used, but I am not sure where to begin looking for how, or for what raw materials would be best suited as our first space-sourced building material.

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    $\begingroup$ Heat rejection and oxygen supply. What's the ore's composition? $\endgroup$ – Deer Hunter Apr 18 '14 at 13:28
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    $\begingroup$ @DeerHunter I am assuming that space/vacuum/0Gee production would use ore in asteroids/comets. As Jack mentions in his answer current Earth bound process focus a lot on oxygen relationships. I am not sure if we know or can know what the common oxygen bonding is to iron/aluminum in space. You may assume which ever seems most likely to you. $\endgroup$ – James Jenkins Apr 18 '14 at 14:44
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    $\begingroup$ @JamesJenkins I'd think that elemental iron is quite rare and since it's produced during the final stages of massive stars, iron isotopes would be introduced to already oxygen rich environment. For example, massive Wolf–Rayet stars can emit iron in their strong solar winds, but they also tend to be oxygen rich. Similar goes for supernova nucleosynthesis with explosive oxygen and silicon burning. So I'd wager that most of iron isotopes had the chance to bond with oxygen isotopes into iron oxides with at least 1:1 atomic fraction. So that's then a source of the oxygen needed. It's in the rust ;) $\endgroup$ – TildalWave Apr 18 '14 at 15:17
  • $\begingroup$ @JamesJenkins Interesting point, if the iron/aluminum isn't bound to the other elements we find it bound to here on Earth that may change my answer significantly. $\endgroup$ – Jack Apr 18 '14 at 15:17
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    $\begingroup$ If you have metal oxides, you need not only heat for smelting, you need something to reduce the oxides to metal and separate the oxygen. On earth, iron ore is reduced with carbon monoxide. But aluminium is reduced by electrolysis. $\endgroup$ – Uwe May 16 '17 at 9:41
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Iron is generally used for steel and I will address iron in the question as referring to steel.

You need a phenomenal amount of electricity to smelt aluminum in the most commonly used method here on Earth.

Steel on the other hand requires careful management of additives to produce the correct strength and other characteristics. Steel with too much carbon is brittle and too little is weak. This is most commonly managed by using a special kind of coal called coking coal. Steel production requires vast amounts of Oxygen, as what you are doing is burning the coke to heat up the ore and infuse oxygen, then burning the oxygen out of the steel to burn out impurities which produces an immense amount of heat that needs to be rejected to the environment.

Neither of these would be easy to work with in a spacecraft due to heat and oxygen concerns. On the surface of a planet/moon/something it would be easier to work with aluminum, due to steels reliance on other minerals and massive oxygen/fuel need. Aluminum is most commonly smelted with the only major inputs being Bauxite and electricity. Steel requires coal, oxygen, iron, and other trace elements like Chromium for stainless steel.

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    $\begingroup$ +1 good overview of Earth based approach. If you used a solar furnace you would not need to consume oxygen to create heat. Imagine pointing a magnifying lens at a chunk of comet, melting and forming it in a near perfect vacuum and zero gravity. $\endgroup$ – James Jenkins Apr 18 '14 at 14:51
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    $\begingroup$ @JamesJenkins But the Oxygen is actually part of the process for steel, it is used to burn off the carbon. I agree that you could create the required heat by other means, but a process for burning off carbon without oxygen would need to be developed. (It may exist, I just am not familiar with it) $\endgroup$ – Jack Apr 18 '14 at 15:15
  • $\begingroup$ You could just go with pig iron, dump that to a planet surface and make it into steel there; it's a poor choice of material for spacecraft anyway, but valuable on the surface. Aluminum would be good for a solar forge for making spacecraft. $\endgroup$ – SF. Jun 29 '16 at 17:01
  • $\begingroup$ @JamesJenkins Look up Basic Oxygen Steelmaking $\endgroup$ – WetSavannaAnimal Jul 17 '16 at 13:21
  • $\begingroup$ You can use hydrogen instead of coke as the reducing agent. Iron ore mainly consists of iron oxides, thus you need to reduce the iron ore, i.e. take away the oxygen. This is done by using carbon from mainly coke. There is research going on to use hydrogen instead of carbon for the reduction of iron ore. So, if you find water (as your source of hydrogen) near your extraterrestrial iron ore deposit, you could do without any coal deposit nearby. $\endgroup$ – Dohn Joe Mar 8 '18 at 2:52
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Current iron and aluminium processes rely on gravity:

  • Impurities (which are lighter than the metal being processed) are skimmed off the top of the molten metal.
  • Molten metal can be relied on to stay at the bottom of the crucible, and not e.g. cling to the lid.
  • Pouring under gravity is much easier than having to rely on pressure or somesuch to get the molten metal out.
  • I suspect that for e.g. steel production, convection currents play a role in making a homogenous mass of steel (spreading the oxygen and other additives evenly through the metal)
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    $\begingroup$ "..rely on gravity:" ..or centripetal acceleration. The latter obviously requires more engineering, but offers the advantage that we can use whatever force is best for the process at hand. $\endgroup$ – Andrew Thompson Apr 23 '14 at 2:33
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Iron and aluminium are very common on Earth, but what makes them mineable is that at certain locations each metal is concentrated in sufficient quantities to make mining viable. The difference between a mineral deposit and an orebody is economics. A deposit containing 15 percent iron is uneconomic, but one containing 66 percent is potentially economic, depending on the quantities of any associated undesirable elements (gangue minerals) such as aluminium silicates and phosphorus.

One of the issues of smelting Fe and Al in Space is what minerals of each metal will be smelted. Different mineralogies dictate different smelting methods. Most open cut mines in Brazil and Australia produce an iron ore blended product of about 60 percent iron. Sweden mines iron ore from underground which contains 35 percent iron. This is because they are mining different iron minerals: mainly haematite rich ores in Brazil and Australia and predominantly magnetite in Sweden.

In Space, mineralogies may be discovered that are unknown on Earth.

The general process to get smelted metal is: Mine minerals containing the metal Concentrate the minerals required and remove as much of the gangue minerals as possible Smelt the concentrated minerals to produce metal Each step produces its own quantities of waste and product. If smelting is undertaken in Space, what is to be done with the waste products of smelting?

It is most likely that any concentrated forms of metals, available for smelting, will be found in clumps similar to the asteroids of our solar system or on planets or moons. For any space based smelter, transporting the raw materials will require a lot of effort, equipment and energy. Having a planet/moon based smelter for planet/moon minerals would be the most feasible option.

The other things to consider are why do you want to establish a Space based iron or aluminium smelter? What and where will its product be used? Will the smelter be operated on a continuous basis or a campaign basis during its production life and what will be the production life of the smelter? Do you expect the organization owing the smelter to operate it at a profit? Or will it be a government owned item of infrastructure that will be solely used for the advancement of science?

One issue specifically for smelting aluminium is that with our current Earth based smelting techniques a lot of electricity is required and it wasn't until electricity was produced on an industrial scale that aluminium became more widely available.

Maybe 3D printing could be an option for iron and aluminium as it is for titanium now: http://www.smh.com.au/technology/sci-tech/csiro-builds-sophie-a-dragon-20140110-30lpf.html

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Iron in space can be obtained from suitable asteroids and so does not really require "smelting" (reduction from an oxide) at all. Elemental aluminium is not commonly found.

Asteroidal iron would probably need some processing to make a high-quality structural material. Many ideas have been proposed for mining and processing this material, but I am not aware of any really detailed studies or experiments. One could imagine melting a suitable sized asteroid floating in space using a solar concentrator, but it would undoubtedly not melt smoothly due to the presence of pockets of other materials, so you'd need a way to catch red-hot lumps of iron that flew off when something shattered. Alternatively one could grind the rock up first, magnetically separate the iron from everything else and hope to be able to melt the powder more tidily.

For most purposes, it might be better to just slice the material up with a laser or a saw and use it "as is" adding thickness to compensate for it's poor mechanical qualities.

Aluminium is much more difficult. This web page discusses a number of approaches to extracting aluminium metal on the moon, most of which require some additional material as inputs that would be tricky to recycle. Other metals such as magnesium might be easier to extract.

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It's important to understand the special challenges imposed by Earth's atmosphere, which would not be present to such a degree on other bodies.

In the beginning, Earth's atmosphere had very little free oxygen. After a while organisms capable of photosynthesis arrived and started generating free oxygen. Dissolved iron reacted with this oxygen and precipitated out of early oceans, becoming today's veins of iron ore which are exploited by the steel industry. In other words, most of the iron ore we use today is a result of interactions with an oxygen rich atmosphere.

The first kind of iron used by men was actually meteoric iron. Not having been around for long enough to be oxidized by the atmosphere, it could be fashioned directly into tools. It should be clear now, that one of the advantages of being in space is the possibility of finding iron and other metals that have not been exposed to oxidization at all, and it might be possible to fashion it into parts simply by melting it. The quality may not be as high, but if it is cheap enough it may not matter for some purposes such as space stations, where a thicker metal wall (to compensate for weaker metal) would provide additional shielding.

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  • $\begingroup$ This answer could be improved by including some references that support the primary premise that meteoric iron is not oxidized, and that it need only be reshaped to be useful. $\endgroup$ – James Jenkins Nov 26 '14 at 11:24
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It occurs to me that two of the major obstacles to vacuum smelting presented here are the postulated power requirements to smelt Fe and/or Al as well as the gravitational environment of that smelting.

Taking them in order, it is pretty much a foregone conclusion that the sun generates and emits several orders of magnitude more energy than we could ever use in several lifetimes. The harnessing of that solar energy via a solar collector or mirror refraction would solve that issue.

Second, centripetal and/or centrifugal force via rotation of the smelter would solve the issue of gravitational separation of wastes from product.

In the case of iron or, rather steel, smelting, why wouldn't taking a page from the electronics industry work? Use an analogous technique to the doping of Silicon wafers to achieve desired electrical pathways, except instead inject carefully measured amounts of elements and minerals into the molten iron to obtain the grade/quality of steel needed or required.
Another idea/suggestion would be to have the crucible made from extremely high temperature resistant ceramic blends to prevent carbon absorption inherent in using a graphite crucible.

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  • $\begingroup$ doping is't such great way, trough adding materials is't big problem, can be done usual way, or same way as coating with plasma made. Extreme-super crucible also not very much needed - is't possible to smelt metal in metal crucible, as forces acting on that crucible may be close to 0 and structural strength of that crucible is't big issue, and mass of it also not issue. Cylinder with 3-5m tick walls, slight rotation to allow it to be heated from inside(that's important) 5-10 meters of material to smelt, and external structure of that tube will be below melting point of steel. $\endgroup$ – MolbOrg Jul 16 '16 at 0:49

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