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Smelting is for extracting metals from oxides or sulfides. It can be used on the Moon or on Mars. It does not work for asteroid mining. There you have just pi

Smelting is for extracting metals from oxides or sulfides. It can be used on the Moon or on Mars. It does not work for asteroid mining. There you have just pieces of iron with a low content of alloying elements. Among the alloying elements, nickel is the most abundant (17.3 times less than iron), then cobalt (20.9 times less than nickel), then germanium (20.4 times less than cobalt). The precious metals that would justify the mining operation are present as a few grams each for a ton of iron. Melting the alloy will never separate the metals by itself. However perhaps some kind of floating zone melting could enrich the proportion of precious metals in a part of the iron, but it is very unlikely that a high enough enrichment could be achieved by a reasonable number of zone melting passes. On earth the cheaper metals could be dissolved by an acid solution, but on metallic asteroids you have neither water nor acids. The SciFi solution would be to vaporize and ionize the metal alloy and separate the metal ions by their specific charge, like in mass spectrometry. This would need a lot of energy, but at least it does not need reactants and it is the only method that achieves almost perfect separation. However for now, the throughput of such a ionic separator is extremely small, too small for industrial production. Perhaps it will become possible to scale such ionic separators to acceptable productivities.

fnordpiglet 2y ago
Yes I was actually thinking of a gaseous centrifuge process. It would require turning the metals into a gas yet not ionizing them and venting the iron gas into space leaving an ever enriched gold. Realistically you don’t need to enrich to total purity.
- adrian_b 2y ago
  Unfortunately that does not work, because these metals are among those with the highest boiling points, even at very low pressures (which is why the oldest solid objects that have formed in the Solar System, at its very beginning, when it was still very hot, have been refractory grains of platinum-group metals with tungsten and rhenium; their condensation has been followed by that of refractory minerals with high content of oxides of aluminum, calcium, titanium and zirconium; and only after additional cooling by the mainstream condensation of silicates and iron alloy). There are no materials from which you could make a centrifuge for gaseous osmium and iridium. Vaporizing the input metal with an electron beam and ionizing the vapors allows after that contactless interaction with the ions, using electric fields and magnetic fields, guiding them into separate condensation chambers (which need strong cooling). The ionic current of such separators must be increased several orders of magnitude over those currently existing, for this to become a viable separation technology.
  - fnordpiglet 2y ago
    Except we are talk about iron and gold not osmium or iridium. Iirc carbon ceramics like hafnium carbide have melting points significantly beyond the vapor point of iron?
    - adrian_b 2y ago
      The main valuable content of the metal alloy from which metallic asteroids are made is given by the platinum-group metals, including osmium and iridium. Of all metals that could be extracted, osmium and iridium are those for which there is the greatest difference in abundance between the surface of the Earth and the metallic asteroids. If the temperature is not high enough to vaporize the platinum-group metals, they will remain as solid grains, which are likely to damage any centrifuge. Making a centrifuge from a ceramic material like hafnium carbide is unlikely to work, due to its fragility and low tensile strength, especially at very high temperatures. A ceramic coating of the parts in contact with the hot gas might work, but even if there is a lot of experience in making such things nobody has made parts working at temperatures so high as needed for this application. The problems for making such a gaseous centrifuge are similar to those for making a high temperature gas turbine. During the last century, tremendous resources have been dedicated for increasing the maximum temperature of gas turbines. The working temperatures have been slowly increased, but more and more slowly in recent years and there is very little hope that it is possible to increase the working temperatures much beyond what has been already achieved. A metal separation centrifuge would require working temperatures not higher by 10% or by 50%, but temperatures at least 3 to 6 times higher than for the existing gas turbines. Based on the existing experience in improving gas turbines, I believe that this separation method can be safely dismissed. While a centrifuge is not feasible, there are chances to use a part of your proposal. Heating the metal alloy at a temperature high enough so that iron will sublimate quickly (together with nickel, cobalt, gold etc.) while platinum-group metals will sublimate very slowly could produce an alloy highly enriched in platinum-group metals with a mass many times lower than the starting mass. This method cannot separate any individual metals and it would lose the gold and other possibly useful components, like germanium, gallium or cobalt, but it could reduce the mass enough so it may make sense to take the concentrated alloy with platinum-group metals away from the asteroid, to a place where it could be processed with more selective methods. While such a method has some small chances of being profitable, it is very wasteful. Real asteroid mining must separate the metal alloy in all its components, because all can be very valuable, less for being brought back on Earth, but for building any kind of structures in space or on other planets/asteroids.

fnordpiglet 2y ago

Yes I was actually thinking of a gaseous centrifuge process. It would require turning the metals into a gas yet not ionizing them and venting the iron gas into space leaving an ever enriched gold. Realistically you don’t need to enrich to total purity.

adrian_b 2y ago
Unfortunately that does not work, because these metals are among those with the highest boiling points, even at very low pressures (which is why the oldest solid objects that have formed in the Solar System, at its very beginning, when it was still very hot, have been refractory grains of platinum-group metals with tungsten and rhenium; their condensation has been followed by that of refractory minerals with high content of oxides of aluminum, calcium, titanium and zirconium; and only after additional cooling by the mainstream condensation of silicates and iron alloy). There are no materials from which you could make a centrifuge for gaseous osmium and iridium. Vaporizing the input metal with an electron beam and ionizing the vapors allows after that contactless interaction with the ions, using electric fields and magnetic fields, guiding them into separate condensation chambers (which need strong cooling). The ionic current of such separators must be increased several orders of magnitude over those currently existing, for this to become a viable separation technology.
- fnordpiglet 2y ago
  Except we are talk about iron and gold not osmium or iridium. Iirc carbon ceramics like hafnium carbide have melting points significantly beyond the vapor point of iron?
  - adrian_b 2y ago
    The main valuable content of the metal alloy from which metallic asteroids are made is given by the platinum-group metals, including osmium and iridium. Of all metals that could be extracted, osmium and iridium are those for which there is the greatest difference in abundance between the surface of the Earth and the metallic asteroids. If the temperature is not high enough to vaporize the platinum-group metals, they will remain as solid grains, which are likely to damage any centrifuge. Making a centrifuge from a ceramic material like hafnium carbide is unlikely to work, due to its fragility and low tensile strength, especially at very high temperatures. A ceramic coating of the parts in contact with the hot gas might work, but even if there is a lot of experience in making such things nobody has made parts working at temperatures so high as needed for this application. The problems for making such a gaseous centrifuge are similar to those for making a high temperature gas turbine. During the last century, tremendous resources have been dedicated for increasing the maximum temperature of gas turbines. The working temperatures have been slowly increased, but more and more slowly in recent years and there is very little hope that it is possible to increase the working temperatures much beyond what has been already achieved. A metal separation centrifuge would require working temperatures not higher by 10% or by 50%, but temperatures at least 3 to 6 times higher than for the existing gas turbines. Based on the existing experience in improving gas turbines, I believe that this separation method can be safely dismissed. While a centrifuge is not feasible, there are chances to use a part of your proposal. Heating the metal alloy at a temperature high enough so that iron will sublimate quickly (together with nickel, cobalt, gold etc.) while platinum-group metals will sublimate very slowly could produce an alloy highly enriched in platinum-group metals with a mass many times lower than the starting mass. This method cannot separate any individual metals and it would lose the gold and other possibly useful components, like germanium, gallium or cobalt, but it could reduce the mass enough so it may make sense to take the concentrated alloy with platinum-group metals away from the asteroid, to a place where it could be processed with more selective methods. While such a method has some small chances of being profitable, it is very wasteful. Real asteroid mining must separate the metal alloy in all its components, because all can be very valuable, less for being brought back on Earth, but for building any kind of structures in space or on other planets/asteroids.