[trhway]

>concentrating one gram of platinum from one ton of iron would be tremendously difficult, requiring a huge amount of energy.

melting one ton of iron requires 500KWh, 12 gallons of gasoline, less than $100 on Earth. Or 5 Tesla car batteries fully charged by say 30x30 m solar array in 2.5 hours - cost nothing in space once you got the hardware there. This is why mining in space is going to be a pretty big thing once/if we get cheap launch capability.

[vkou]

> melting one ton of iron requires 500KWh, 12 gallons of gasoline, less than $100 on Earth

The spot price for platinum today is $68, so you'd be losing money doing it.

[nerdralph]

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.

[adrian_b]

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.

[card_zero]

> 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.

[cjs2]

You’d have to deal with the Curie Point of the metals.

[card_zero]

Oh bugger. Things get disordered when vaporized? Who'd have thought.

[trhway]

>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]

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.

[DoctorOetker]

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)

[adrian_b]

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]

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

[adrian_b]

Chemical methods would be much more efficient, but they would need huge amounts of chemicals that do not exist on asteroids, so they need to be brought from elsewhere.

It would be impossible to bring millions of tons of acid and of water, so if an acid would be used it would have to be regenerated, e.g. by the electrolysis of the iron-nickel-cobalt salts, which would also need a lot of energy.

Designing a process that could regenerate and purify the acid in a closed cycle, with no losses of any fluids, due to the difficulty of replacing them, would be a very difficult task. Nothing remotely similar has ever been achieved. On Earth, any such methods use at least vast amounts of water and air that are not recycled.

Also, any chemical methods would need to be performed inside a perfectly sealed installation.

Vaporizing iron and the other more volatile metals with an electron beam could be made in a partially open vessel, in the vacuum from the surface of the asteroid. The main difficulty would be to ensure that the metal vapor goes in a certain direction and not omnidirectionally, to avoid its condensation all over the installation.

When vaporizing metals in vacuum with an electron beam, you do not pass through a liquid phase, but the metal is vaporized directly from solid pellets. This method ensures a high efficiency of conversion between electrical energy and heat that is actually used for vaporizing the iron, instead of being lost in the environment.

Thus there would be no molten iron to be preheated, even supposing that there would be materials suitable for a heat exchanger working at such temperatures.

Moreover, even if one would first melt the iron in a closed vessel, heat exchangers transfer heat well only between dense fluids, i.e. liquids, supercritical fluids or at least gases at high pressures. The liquid iron qualifies, but not the iron vapor, from which transferring the heat would be bad. Better heat transfer could be achieved if the iron vapor would condense inside the heat exchanger, but for that a means to ensure a high enough pressure for the vapor would be needed, but that may be difficult to ensure without preventing its advance in the pipes. Liquid iron can be pumped with magnetohydrodynamic pumps, but for pumping iron vapor there is no easy method. Perhaps one could ionize the vapor, to be able to move it with electric fields.

A heat exchanger working at a temperature so high would tend to have a very high heat loss, due to radiation. It may be difficult to ensure that you recover more heat than the extra heat that is lost.

In any case both the attempt to use chemical methods or the attempt to make a heat exchanger for iron vapor would be engineering challenges that require solutions far beyond everything that has ever been done on Earth.

By contrast, vaporizing metals in vacuum with an electron beam is a routine technology on Earth. The only big challenge is that in normal vaporization installations the vapors go in all directions. On Earth this is not a problem, because everything around is covered with some thin metallic foil, on which the vapors condense. After that the metal foil is dumped if the evaporated metal is cheap, or it is sent to metal extraction by chemical methods if the metal is precious and it must be recycled.

On an asteroid, in order to avoid the deterioration of the installation, one needs either a method to move the useless vapor in a certain directon, e.g. by ionization and then moving with an electric field, or perhaps by making iron foil and covering the installation, and then working in batches, where one vaporizes an iron pellet, so-that the platinum-group metals remain in the pellet holder and the volatile metals are deposited on the iron foil, which is then dumped and the cycle repeats.

This is the only method that could be done with existing technologies, with minimal improvements over them.

However, it would not be worthwhile, as the cost of extracting thus platinum-group metals from an asteroid would be many times greater than on Earth.

[Figs]

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

[adrian_b]

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]

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...

[adrian_b]

Different melting points are easy to exploit only when metals do not mix in liquid state.

Even when metals do not mix in solid state, but they mix in liquid state, that usually cannot be used for separation, because the liquid solution will become solid at a temperature different from the melting temperatures of the components and lower than them, and the solid alloy will consist of the component metals intimately mixed at the level of microscopic crystals, so you cannot separate them (this is called an eutectic alloy, like the lead-tin alloy used for soldering, where by solidifying it you do not obtain separate lead and tin, but just a non-separable alloy, and by remelting the solid alloy you obtain a liquid solution, where again, the metals cannot be separated).

If the metals also mix when solid, the solid metal is a solid solution that does not melt at any of the melting temperatures of its components, but at an intermediate temperature, and the metals cannot be separated regardless whether the alloy is solid or liquid.

Here, in asteroid cores, the precious metals are present in a very small proportion, so they form either a liquid solution when molten or a solid solution when solidified.

The melting temperatures of platinum et al. do not matter, the melting temperature of the alloy is slightly lower than that of iron, corresponding to that of an iron-nickel alloy. The other alloying elements are in quantities small enough that they have negligible influence on the melting temperature.

In conclusion, differences in melting points can only very seldom be exploited for metal separation and they cannot be used for the iron alloys of planetary or asteroid cores.

You can exploit only either the difference in boiling points or the differences in chemical reactivity with acids or oxidizing agents.