The EPA vs. American Mining Congress case in the early 90s resulted in the "remanded smelter slag" rule. The mining industry convinced the court, and even EPA, that treating slag piles as "waste" with 365 day storage ("speculative accumulation") under RCRA statute, did more harm than good. Today, it's an industrial mineral, and can be kept under basically the same conditions as mined angelsite or other leaded silicate, because treating it otherwise is anti-recycling.
Anti-recycling means that the identical chemical solid is governed more strictly if it's recovered from waste instead of mined from the ground.
