I was wondering what the best ways for turning minerals mined in the asteroid belt into useable materials would be, so far I came up with three methods, utilizing nuclear, electromagnets, and finally using light from the sun to heat whatever it is I wanted up. Mainly, are those three acceptable choices, or are there options I'm overlooking?
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$\begingroup$ Good first post Daikeal. Please take the tour and when you have some spare moments browse the help center to figure this place out. Welcome to worldbuilding. $\endgroup$– Escaped dental patient.Commented Mar 28, 2020 at 16:46
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$\begingroup$ background reading: worldbuilding.stackexchange.com/questions/102963/… $\endgroup$– WillkCommented Mar 28, 2020 at 16:58
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1$\begingroup$ Thanks for accepting my answer. However here on worldbuilding we recommend waiting at least 24 hours before accepting one answer. Worldbuilding problems are usually not so urgent, and non solved question attract more answers. Therefore, allowing all the users in different time zones to give a shot gives you better chances of receiving interesting answers. $\endgroup$– L.Dutch ♦Commented Mar 28, 2020 at 17:11
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$\begingroup$ Ah, okay. Thanks for letting me know about that, I'll try to keep it in mind for future questions! $\endgroup$– DaikaelCommented Mar 28, 2020 at 17:19
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1$\begingroup$ Aluminum will be used as much as steel, no? This will be some variation of Bayer and Herault processes, with nuclear for electric power and maybe harvesting the waste heat off the nuclear. Steel will be much more difficult. Pressure vessels are easier than open crucibles, obviously. $\endgroup$– John OCommented Mar 28, 2020 at 19:49
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4 Answers
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The point of smelting is not to make the stone liquid, the point is to have a chemical reaction take place: The metal content of any ore is chemically bound to other elements, most importantly oxygen, and it's useless in that oxidized form. When you smelt an ore to get the pure metal, you are providing something to those other elements that they would much rather bind with than with the metal you want to get out.
In the case of smelting iron, you may start with an iron oxide and provide carbon monoxide to it. The carbon monoxide reacts with the oxygen to which the iron was bound, forming carbon dioxide, and leaves the pure iron behind. The carbon monoxide is formed by burning coal within the smelting oven, making sure that you don't add too much oxigen. You cannot have this reaction take place, unless you are supplying the carbon monoxide.
As such, smelting iron, for example, must always be done by burning coal. You may have unorthodox ways to procure the coal and oxygen, but in the end you must have the carbon monoxide together with the ore in a chemical reactor, which we typically call a furnace.
There is only one possible alternative: electric reduction. In this case, you heat the ore to a temperature where it becomes liquid, you bring it into contact with electrodes, and you put a current through the ore. This is the way that aluminum is produced. Unfortunately, it takes a lot of electric energy (energy that would be provided by the carbon monoxide + oxygen reaction in the smelting process), and works well only if you can find an electrode material that won't react with the oxygen / other elements that you want to remove from the ore. For instance, if you use a graphite electrode with an oxide ore, the produced oxygen will quickly eat away your electrode. Since the oxygen / other elements that you want to remove are rather reactive, and since the temperature must be quite high, it's rather difficult to find a suitable electrode material.
Edit
As Adrian Colomitchi correctly pointed out in a comment, there is actually a second alternative: Turning the ore into a plasma and doing mass spectroscopy on the ions. Very energy intensive, with a massive waste heat problem, and I believe too hard to scale up to significant throughputs, but it's a possibility that could be considered.
answered Mar 28, 2020 at 20:11
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1$\begingroup$ You can bring the ores into an ionized state - plasma - and use mass spectrometry separation. This way, you get to collect all the elements in the ores, oxygen included. Yes, it is going to take lots of energy, but that's the nature of the beast - the same happens on Earth. $\endgroup$ Commented Mar 28, 2020 at 21:37
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2$\begingroup$ @adrianColomitchi Theoretically, yes. However, that approach is totally unpracticable: 1) You need much higher temperatures than for electric reduction, and 2) you'll be operating at really low densities. Since loss of heat due to radiation grows with the fourth power of temperature and linearly with time and surface area, you'll basically have a giant heat bulb that happens to produce minuscule amounts of pure elements. If you place this on a spaceship, the entire spaceship will be glowing red before you have produced any significant amount. I wouldn't want to be a worker on that ship... $\endgroup$ Commented Mar 29, 2020 at 8:37
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$\begingroup$ 1. theoretically, yes. Practically, electron bombardment will heat the material locally, vaporize it more efficiently and let it ionized. Femtosecond gigawatt laser pulses will do the same 2. low densities high efficiency. This is why this is suited for automatic mining probe. $\endgroup$ Commented Mar 29, 2020 at 9:10
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$\begingroup$ @AdrianColomitchi Ok, I've added it as a second alternative now :-) $\endgroup$ Commented Mar 29, 2020 at 9:37
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$\begingroup$ 'Waste heat'? All I see is input for the next reaction ;). In space it might make more sense to use specialised heat pumps as heat has great difficulty to bleed out. You'll not want to create more heat than needed, making it also much more efficient to reuse the previous heat. $\endgroup$ Commented Dec 9 at 14:02
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Mind that electromagnets are not a primary source of energy, they require something to produce the electricity to feed them.
That apart, you are forgetting:
- RF smelting. It is used on Earth to purify silicon wafers, for example.
- Impact. If two bodies impact each other, most of the kinetic energy they have will be converted in heat. That's how Earth was born as a sphere of molten rocks.
- Tidal heating might be another option, if you are in the range of some massive body. Swing the clump of materials close enough to it, so that the tidal forces, deforming it and ending up increasing its temperature.
- Aerobraking might be another option. Dump the mineral into the atmosphere of a planet, and let the atmosphere convert the kinetic energy into heat. Be sure to plan well where the molten ore will land.
answered Mar 28, 2020 at 16:57
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1$\begingroup$ Good job Dutch! I could not think of any other methods besides those OP laid out and here you have 4. I like yours better than OP because none are routinely used own Earth and so they would make more exciting fiction. $\endgroup$– WillkCommented Mar 28, 2020 at 17:01
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2$\begingroup$ Feeling of guts: nope for tidal heating - if you get to the point of melting everything into a lava (and you still didn't separate the minerals), your thermal radiation is going to cool the thing faster. The temperatures you can achieve may be enough to melt the ice, but not enough to allow you mining asteroids. Aerobraking is awfully inefficient - not only most of your asteroid is going to end in atmosphere, through ablation, but you have another problem: getting you surviving core out of from the grav well (I hope that you don't want to use Earth's atmosphere to mine asteroid iron) $\endgroup$ Commented Mar 28, 2020 at 21:45
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$\begingroup$ Two other methods for melting metals under vacuum are electron beam and vacuum arc furnaces. $\endgroup$ Commented Mar 29, 2020 at 4:36
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This only works for metals that melt below 1084°C (1984°F):
A copper pot filled with the unmelted metal and closed with latches could be placed inside a shut ceramic box. Surrounding the copper pot are high-wattage incandescent lightbulbs. The lightbulbs are then powered all at once for a long period of time to generate heat to power the lightbulbs. Afterward, the copper pot can be taken outside the box and opened to reveal the melted metal.
This is actually how Easy-Bake Ovens work: https://entertainment.howstuffworks.com/easy-bake-oven2.htm
Copper heat conduction: https://www.metalsupermarkets.com/which-metals-conduct-heat-best/
Metal melting points: https://en.wikipedia.org/wiki/Metals_of_antiquity
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1$\begingroup$ That may work for melting some metals. Extracting metal from ores/minerals... that's another story. E.g. you know how aluminium melts at 660C? Well, that's not gonna help for reducing the aluminium oxide to aluminium - at 2072C aluminium oxide will melt but still refuse to decompose. $\endgroup$ Commented Mar 29, 2020 at 12:02
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Fluxes and Alloys
Most metal refinement is about a lot more than just making things hot. In the case of every ore, you need to remove the elements that you don't want in the ore and add the elements that you do. Historical smelting worked specifically because charcoal contains lime which is a good flux for things like iron and copper smelting and burning it in direct contact with iron ore releases enough ash to alloy iron into carbon steel. So, while the melting of the ore was important, there was a much more complex chemistry happening specifically because they were smelting the ore with charcoal.
Every ore on Earth is smelted with some kind of flux, which are the chemicals designed to bind to and remove the unwanted impurities from the metal you are smelting. Common fluxes include potassium chloride, sodium carbonate, potash, charcoal, coke, borax, lime, and lead sulfide. Here on Earth, these things are plentiful because they are all either water-soluble salts that naturally accumulate due to Earth's weather patterns or they are the result of biological processes, but in space, these things will all be quite rare, and never naturally pure.
Furthermore, if you are refining metals for a space based civilization, you are not going to be making any pure metals, but rather some alloy of metals. Most modern metals are alloys of 4-7 elements. While historical steel was just iron and carbon, many modern steels require small amounts of magnesium, tungsten, chromium, nickel, copper, aluminum, and/or titanium in order to achieve the performance required for any given steal's use. While all of these metals are prevalent in space, they mean that you need not just one, but many different kinds of fluxes to be able to isolate and refine all of the metals you will need to be able to make modern alloys. This means that you will either need to transport large amounts of flux from Earth to your space refineries or you will need to transport your raw or incompletely refined ores down to Earth.
