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How do you separate metal alloys into their constituent elements, especially industrially? I don't think you can always use melting points, because alloys, depending on the composition (looking at phase curves) can have one common melting point.

Here's a phase diagram for a copper-nickel alloy, which I think is a system where both components are completely soluble in one another in all ratios.

enter image description here

This composition cannot be separated by cooling it down, as I understand it (even though, as mentioned here, the composition would not be uniform if the molten alloy is not cooled down slowly enough). As described here, the solid that would start to separate (the alpha phase) is itself an alloy.

EDIT: What about electrolysis? Could that work here?

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  • $\begingroup$ Separate one alloy from another, i.e. distinguish them apart, or separate out the various elements? $\endgroup$
    – Jon Custer
    Nov 16, 2016 at 20:54
  • $\begingroup$ @JonCuster separate them into the consituting elements $\endgroup$
    – Zubo
    Nov 16, 2016 at 20:56
  • $\begingroup$ Maybe it can be interesting I don't know. en.wikipedia.org/wiki/Zone_melting $\endgroup$
    – ParaH2
    Nov 16, 2016 at 23:02
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    $\begingroup$ I think this depends heavily on the metal. For example, purifying iron for steel via the Bessemer process. Gold can be purified in part by cyanide leaching, but there are other methods of parting gold. $\endgroup$
    – Zhe
    Nov 17, 2016 at 1:43
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    $\begingroup$ Essentially, the phases formed from molten metals and molten salts (given that they mix at all, which is not always so) may be either "metal-like" or "salt-like". In the first case it would conduct electric current using electrons, much like electric wires in your house, and would be able to do so for years without any change. In the second case it would be able to undergo electrolysis, like any molten salt, and you might be able to tune the conditions so that the metals separate. Then it is just another recipe starting with "Dissolve both metals..." (dissolve in a molten salt, that is). $\endgroup$
    – Ivan Neretin
    Nov 18, 2016 at 6:12

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You seem to approach the separation of alloys much like the separation of water-soluble salts, probably based on the fact that both involve the liquid/solid phase diagrams, which may even look alike. This is wrong; these systems are fundamentally different.

Say, you have a water solution of some well-soluble salt which you need to recrystallize. You cool it down, so the salt precipitates. You filter the crystals out of the solution, then you probably rinse them with something which would remove the traces of solution without damaging the crystals, or just let them dry.

Now suppose you have a metal alloy, maybe not even of the kind you mentioned above; let it have a simple eutectic diagram, like Bi-Cd. Bi-Cd phase diagram

You melt it, then carefully cool it down to a certain point, and end up with a mixture of some (supposedly) pure metal crystals with a still-liquid alloy. What good does it do? You can't really filter it. You can't rinse the crystals. You can't let them "dry". It is useless.

With metal alloys, your best bet is to dissolve both metals and separate them chemically, based on their different properties. In some relatively rare cases, you might be able to melt the alloy and perform chemical conversion of one component without affecting the other; examples include cupellation or Bessemer process referenced earlier in the comments. In yet more exotic cases, you might be lucky enough to chemically etch one component out of solid alloy; this is how Raney nickel is made. Unless you are that lucky, go back to the start of this paragraph.

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  • $\begingroup$ Thanks for the information on cupellation and Raney nickel, that is helpful. So, you're saying that it generally very much depends on the case? Also, could you elaborate in your answer on how metal composition phase diagrams are different from non-metal ones? As in the sources I linked to, the behaviour of the alloy when heated is shown. The recrystallizing you bring up is valid, but it doesn't have much to do with the issue. Also, where's the problem in decanting molten metal and let it drain unter its weight, separating the residue, using something like a modified Bessemer container? $\endgroup$
    – Zubo
    Nov 17, 2016 at 12:47
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    $\begingroup$ Generally you have to dissolve both metals and do some wet chemistry. As for the phase diagrams, they are not very different, but the systems are. Molten metal is not quite like water. You decant it, and a good deal of it just remains there (thanks to higher viscosity and surface tension), and you can't rinse it off, nor would it evaporate like water (since its vapor pressure is way lower than that of water). $\endgroup$
    – Ivan Neretin
    Nov 17, 2016 at 13:13

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