I'm honestly having a bit of trouble putting my misgiving into thoughts (which if anything demonstrates I haven't thought this all through), but if you throw together say 10 materials, how likely are you to be able to separate out those 10 materials again? Or will you only be able to separate a couple metals with certain convenient melting points?

I do see what you're saying though. Maybe the easy question is, what actually happens when electronics are recycled today? How much of the original material is retrieved? Maybe I'm just entirely wrong even given the state of the art today?

I only know a bit about this but from what I understand this is similar to how most modern mining works. Many minerals are mined in very low concentrations, which means they then need to be extracted out of the host material. Part of that process often involves multiple steps where the ore is soaked in various chemical solutions, which separate certain materials. It can be a very complicated process involving a lot of toxic chemicals.

The rare earth metals are kinda irritating to separate because they have very similar physical properties, but it obviously can be done. They frequently co-occur in ore deposits for exactly that reason.