This is similar to the "rare earth tailings" phenomenon, isn't it. A mine is built to extract one metal, the most profitable at that time, and everything else u

This is similar to the "rare earth tailings" phenomenon, isn't it. A mine is built to extract one metal, the most profitable at that time, and everything else unextracted ends up in mine tailings or refinery slag. But, in addition to truly worthless silicates, that "waste" includes a bunch of other metals at low purity which may eventually be economic to extract.