Since you didn't show your math, I did a quick calculation. .45J/g/C specific heat of iron means .45MJ/tonne. 1811K to melt iron means 815MJ/tonne. 3.6kWh/MJ, so 226.4 kWh should melt 1t of iron.

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adrian_b 7 hours ago | parent [-]

Yes, but melting is just the beginning of the process. Even your computation is incomplete, because it is not enough to heat iron until the melting temperature, you must also provide the additional latent heat of melting. Similarly for boiling iron, after heating to the boiling temperature there is an additional latent heat of vaporization.

There is still no easy way to separate platinum-group metals from liquid iron, so you must vaporize the iron, to exploit the fact that platinum-group metals have higher boiling temperatures. It is true however that at the low pressures easily achievable in vacuum, vaporization is easier than on Earth.

Otherwise than by vaporization, you could dissolve iron with an acid, but on such asteroids you do not have with what to make an acid, so you would have to transport it from some other asteroid, or more likely from a satellite of Jupiter. You would also need a chemical plant to make the acid and also to recycle the iron salts into regenerated acid. This is so much more complicated, that vaporization of the iron might be simpler.

Finally, you must account for the fact that the energy required to vaporize one ton of iron produces less than a gram of platinum and of each other platinum-group metals. It is unlikely that you could build there a solar array big enough to provide energy for vaporizing a million of tons of iron, to make a ton of platinum, so you would need a nuclear reactor.

While platinum-group metals might be obtained as a minuscule residue after vaporizing the iron, gold has about the same vapor pressure as the much more abundant iron, nickel, cobalt and germanium, so it would be impossible to extract it from iron by vaporization. It could be extracted only with a chemical method, e.g. with an acid or with oxygen, which need to be brought from elsewhere.

Taking all these into account, it seems that there is no chance of being able to mine precious metals at a cost less than on Earth any time soon, e.g. in the current century. Extraordinary reductions in the cost of interplanetary transport would be needed and in the cost of building a metallurgic plant on an asteroid.

Mining asteroids would make sense only if some people would decide to live in huge space stations with artificial gravity, instead of on Earth, and then some asteroids would be mined for making steel and other construction materials, to be used in the interplanetary space, not on Earth.

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card_zero 23 minutes ago | parent | next [-]

> gold has about the same vapor pressure as the much more abundant iron, nickel, cobalt and germanium, so it would be impossible to extract it from iron by vaporization

Magnets!

Will fill in the details of this idea later.

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DoctorOetker 5 hours ago | parent | prev | next [-]

Your calculation assumes the heat must be considered wasted, but what prevents a counter-current heat exchange configuration from attaining ridiculously higher efficiencies? not to speak of just using saner approaches like chemical separation (gold and iron are very different chemically)

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adrian_b 4 hours ago | parent [-]

Heat exchangers for metal vapors at temperatures of a few thousand kelvin would be a significant technical challenge.

A heat exchanger needs fluids between which heat can be exchanged. Besides the fact that it would be very difficult to have pipes for fluids at such temperatures, it would not be so easy to efficiently heat the fluid more than it was heated by the recovered heat and then control somehow a fluid jet to transfer efficiently heat to the iron that must be vaporized.

Even if some heat would be recovered from the vapors, the losses due to imperfect heat transfer from fluid to iron might be greater than the recovered heat. Moreover, it is not clear what could be used as the working fluid, because those asteroids are depleted in volatile elements, so any fluid must be brought from elsewhere and any fluid losses would be irreplaceable.

Probably the easiest and most efficient way to heat iron until vaporization would be with an electron beam, but it would not be easy to ensure that the iron vapors do not destroy the installation and they condense in a safe place, from which the iron can be somehow evacuated.

	▲	DoctorOetker 4 hours ago \| parent [-]
		working fluid? the same hot iron vapour is used to heat the incoming molten iron, no heat exchanger is perfect so the preheat would inevitably be a few percent short of the target temperature, the remainder is just the energy you supply to negate any heat lost through insulation (space is large, so one could use a ridiculously large insulation) not that any of this matters, since chemical methods would be much more efficient

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Figs 6 hours ago | parent | prev | next [-]

Could you use a centrifuge to separate the elements instead of vaporizing it?

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adrian_b 4 hours ago | parent [-]

You cannot use a centrifuge to separate solid iron.

Using a centrifuge with liquid iron would create a gradient of concentration of the heavier elements dissolved in it, but that would not be enough to separate them.

All that could be done with a centrifuge with liquid iron would be to obtain an iron alloy enriched in heavy elements. However, I doubt that it would be possible to make a centrifuge for liquid iron that would have a lifetime sufficient to process quantities of the order of one million tons of iron. I do not think that until now anyone has ever tried to make a centrifuge that could work with a liquid metal at such a temperature. Most materials lose their strength at such temperatures, so the risk of breakage for the centrifuge would be extremely high, a risk that is increased by how heavy iron is.

It is also not clear if such an enrichment of the heavy elements would bring a sufficient simplification to further processing steps to make it worthwhile.

	▲	Figs 3 hours ago \| parent [-]
		Iron and platinum have different melting points. If you melt the alloy, then spin it to concentrate the platinum, couldn't you coax the platinum to separate out as solid clumps by adjusting the temperature? Alternatively, there are differences in magnetic properties that could be exploited... This isn't my field, so I'm just spitballing. I bet if you can get the cost of launch and interplanetary transit to be low enough for people to really start tinkering with asteroid mining though, someone will crack the metallurgy issues...

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trhway 5 hours ago | parent | prev [-]

>energy required to vaporize one ton of iron produces .... so you would need a nuclear reactor.

it is less than 2500KWh - under $250 of nuclear generated power on Earth. The best - fastest and efficient - way to travel outside planet's LEO that is available today is solar or nuclear powering ion thruster, with only nuclear really beyond Mars. So anyway you come into the asteroid belt with a reactor. A submarine or icebreaker like reactor - 70MW - would power vaporizing of almost 30 tons/hour of iron. Note, that nuclear reactor in space is tremendously cheaper than on Earth as all the regulation, safety, etc. costs either disappear completely or reduced a lot.

	▲	adrian_b 4 hours ago \| parent [-]
		You have forgotten many zeros. If you produce a few grams of a precious metal, that cannot justify the trip until there. To produce something of the order of one ton, which still seems too low to cover the expenses, you need to process something of the order of one million tons of iron. With your estimation that could take several years. In reality the energy consumption would be much greater, because one must cut chunks of iron and transport them to the vaporization installation, then also transport elsewhere the condensed iron. So you would need a decent number of submarine like reactors in order to achieve an acceptable productivity. There is no doubt that it would be feasible, but the problem is that at the current prices there would be no way to recover the expenses.