Slag

From old copper smelter at Clarkdale, Arizona, alongside the Verde Canyon scenic railroad. The rusty corrugated iron is the remains of improvised dams to keep the molten slag off the railroad! Slag is esssentially an iron-rich glass that (in theory) contains most of the impurities extracted from the copper, but which still contains significant Cu values, as well as other metals. At the time we were there a pilot project to reprocess the slag to extract metals was in the works, but I don't know its current status.