eli5 – how does the process of purifying metals goes?
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My answer to your other post should expand on the methods already mentioned here:
Depends a lot on the metal and the impurities, as well as the level of perfection you want. General methods non-exhaustively include: different melting points, leaching/dissolving of parts or all by acid (including chemical separation and recovery), electrolysis (in solution as a salt or as molten metal), (re-)crystallization (from solutions, molten state or even vapor), chromatography, and a bunch of really exotic methods.
One of the most common impurities in gold are silver and copper. They all melt around 1000°C and mix/alloy pretty well, so separation by heat is not an option. Nitric acid dissolves the latter two, but not gold. Yet that only works of the acid can reach all of them, but if the alloy is relatively pure (say above 80% gold) to begin with, a lot of the impurities are enclosed by gold, never to be reached by acid. This is resolved by first adding even more silver/copper (melt it, throw silver in), so that now the gold is a minority and left over as fine dust by the acid.
The result of that is, say 99.9% pure (not actual numbers). To get even better, you could dissolve the gold with cyanide (acidic potassium cyanide solution) or as chloride (with aqua regia or HCl+H2O2). Then you put gold electrodes (i.e. just sticks) into that solution and apply some electricity (say 5 volts); this will make gold atoms move to the minus one (“cathode”), creating a rather pure layer. Note that the center part you started with may have lesser purity, but should at least by reasonably pure (say 99%).
Oh, and if you want to separate the silver from copper as well, you can again use nitric acid (you already dissolved them in the first step anyway). the created silver and copper nitrate are both soluble in water. Now add table salt (sodium chloride): silver chloride is insoluble, while copper chloride is soluble. Hence every silver chloride created will immediately fall out, partially to the bottom and partially as a white milky suspension. Just add enough salt, and all silver will be in that form. Then filter the solid stuff off, and do one of many options to get the silver chloride back into silver (just enough heat should already do the trick). You can also recover the copper, but that usually is not worth it.
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