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?
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.
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.
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.
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.
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