Or can’t you? Is the only way to turn 14k gold into 18k gold by adding more gold?
Alloys are mixtures of metal elements, which are the building blocks of chemistry. You can “unalloy” an alloy by reacting those metal elements with various things. For instance I just read of some mix of metals that was dissolved in sulfuric acid, and then something else was added to make one of the elemental metals precipitate out. But all of these strategies are based on some kind of chemical difference between the metals.
Alloys are solid solutions. When melted, they can be separated using techniques that would separate cooler solutions, such as distillation and electrolytic processes.
I understand that in theory distillation would work, but wouldn’t that be d–n difficult in practice given the temperatures necessary?
Depends on the metals. If you have some sort of mercury amalgam, I would think you can cook off the Hg pretty easily. Plus, you can always do this under low pressure.
I don’t know that anyone actually purifies metals this way. Hopefully someone will stop by who knows more, but I’ve read that the purification of copper (after the smelting) yields all sorts of other metals that can be sold off. I’m pretty sure it’s mostly electrolytic procedures.
For silicon, we purify it (remove the impurities) by a process called zone purification.wp. This method certainly has been used in the microelectronics industry.
You can also use freezing (like is done for applejack) to freeze out one phase from a molten state.
What about the biblical allusion to refining gold by heating it and then skimming the (presumably lighter) impurities off the top?
In this case, I think that the impurities oxidize, while the gold doesn’t. The oxides are what floats.
Alloys are basically metal solutions. Solutions are only stable in a certain range of temperature and concentration In order to figure out where a certain solution is stable you look at a phase diagram. It’s been a long time since I’ve used one of these, but they aren’t that difficult to figure out. You can stepwise remove impurities by heating and cooling. I suspect this is pretty much how zone purification works. The melt zone would be just barely above the melting point of silicon.
Wouldn’t this be the process called ‘smelting’ (or ‘refining’)?
Smelting is a chemical process. Essentially a way to remove oxides from your ore by giving them some carbon to burn and produce CO2 instead of metal oxides. It’s performed at high temperature. Not normally used to separate alloys, but see below.
Refining is a general term that applies to a lot of different processes that all have the same goal: to extract a purer form of some substance from another.
Generally with ores, the problem is that the metal is chemically bonded to oxides or other stuff and needs to be chemically unbound and then separated.
In the case of an alloy, the problem is merely one of separation - though in some cases where the melting points are similar (or the alloy is eutectic), then you might need to use some chemistry tricks to make one of the metals react with impurities so that you can more easily separate them.
There are lots of ways to do this - and it depends on exactly what you’re trying to separate - because the physical properties of the components, as well as the properties of the alloy itself may affect what processes are effective and which are not.
For example if you have lead/tin solder, this is generally a eutectic alloy, and so the alloy melts at a lower temperature than either the lead or the tin. You can’t use partial solidification here (and of course this is a desireable property for solder - you don’t want it to separate when heated and resolidified). The solution here would be to use a chemical reaction to oxidize one of the components and then extract the precipitate from the molten alloy.
On the other hand some systems (like gold/silver) are non-eutectic. Here it’s simply a matter of melting it and then cooling it enough so the one metal starts to solidify while the other remains molten. (analogous to the previous applejack freezing example).
I looked at refining my scrap gold a few months ago. The websites would have you believe using aqua regia were relatively easy. I had a hard time believing it.