[gst]

> This is different with metals where you can "just" melt everything and separate the metals.

How would you separate the metals after melting them? My understanding is that you can extract some high valuable metals via different processes, but otherwise it's technologically and economically not feasible to separate molten metals.

[nmehner]

Well, yes, you are right.

These are actually two steps: Melt everything into the "Black Mass" and use Hydrometallurgy to extract the metals afterwards. I think https://www.youtube.com/watch?v=XylDkcrJgTw gives a good overview (although the voice is somewhat hard to listen to).

[missedthecue]

Different metals have different melting points, don't they?

[Groxx]

To maybe make the other answers a bit more explicit / layman friendly:

Yes, and if you have a bunch of small pieces of different metals all mixed together, you can very carefully melt one, extract it, then the next, etc by gradually raising the temperature.

But if you take an alloy, e.g. bronze, you're combining ~2 metals of different melting points (copper ~1100C, and tin ~230C) to get essentially a new metal with a usually-middling melting point (bronze ~920-970C). Raising bronze to ~300C will not melt the tin out for you to collect. It's not just a surface area issue either, chopping it up into many tiny pieces / a powder won't melt more out and give you something to collect. None will. It'll all melt at bronze's temperature, and until then it all remains solid.

Similarly, if you melt bronze and then cool it down a bit, the copper won't solidify on its own. The whole solution will solidify around 950C. It's kinda like how salty water solidifies at a lower temperature - it's not that the water is freezing at -1C but the salt is not, that'd be a slush (and wouldn't make much sense anyway, salt is solid at room temperature). The solution behaves as a new "whole".

If you want to separate two alloyed metals, you generally have to chemically bind some to something else so it comes out of solution in a way you can capture. Then you can skim it off / extract it from the bottom / etc. One less-visually-obvious strategy is to just keep the alloy hot and exposed to the air, so some of it oxidizes and comes out as slag - that's chemical separation using oxygen. It works pretty well for gold, because unlike many metals gold does not oxidize.

You can also sometimes let the metals separate themselves by density by just letting it sit while melted..... but you'll be fighting against any internal currents set up by temperature differences, so it's difficult, imprecise, and slow to do in practice. But for some combinations and equipment it's feasible.

[gst]

Yes, but if a metal with a lower melting point is in fluid state inside another solid metal that's not better than having both in a fluid (or solid) state.

Here's a related Quora discussion: https://www.quora.com/If-you-smelted-a-varied-mix-of-scrap-m...

[sdenton4]

Seems like you can choose during manufacturing to make things easier to disassemble to enable easier recycling... So the answer to low-melting metals inside high-melting metals is just "Don't do that" because otherwise you'll miss the 95% recyclable policy.