Using HCl on metals, by looking at the reactivity series of metals we see that reactive metals above hydrogen in the reactivity series, displace hydrogen from the acid as gas when the more reactive metal reacts with the acid, to make a chloride salt of the metal. without the hydrogen you no longer have HCl acid to reuse.
Metals below hydrogen the less reactive metals the hydrogen in the acid forms water in the reaction with metals as we make the metal chloride salt solution in the reaction. so again we no longer have HCl to reuse.
So without free HCl in solution, if all of the acid is used in the reaction we no longer have HCl acid.
Again by looking at the reactive series of metals, we see we can displace a less reactive metal from solution using another metal that is more reactive (in a displacement reaction, like we do with using copper to displace gold from solution). In this reaction we are changing the chloride salt in solution (or which metal chloride is involved).
Then there there are other places where we can use a reaction so that we can reuse a solution, like where we make a copper chloride solution, and rejuvenate the solution changing the CuCl back into a CuCl2solution so we are able to dissolve more copper.
We can also change one metal salt to another metal salt, example say we have a copper chloride solution, we can add sulfuric acid and distill out HCl gas (which can be collected if we wished for reuse) and make copper sulfate from that reaction in the process.
If you have a soup of reactive metals in solution (several different metals dissolved as chloride salts) in one soup of a solution you can use the reactivity series to displace the ones below the metal your using in the series of reaction.
Basically we cannot reuse HCl acid once we use it on metals, but we can convert it once used to another chemical, that can possibly be reused, or reform HCl from a salt.
Gaining an understanding of the chemical reactions through educating yourself in the chemical reactions, and what happens is really the only way you can learn how to reuse the solution or make another solution from your used HCl.
