They are the leftover tailings from making fertilizer. They are made from gypsum, silica and acid. Several years ago a company developed a process to extract yellow cake uranium for reactor pellets from the stacks
If you're talking about that mountain sized duke that had 2 excavators on top of it, that's what they call a gypsum stack. Basically, it's a gargantuan pool of acid, that is a so far useless byproduct of phosphate mining. We have many of them in central Florida. I recently read an article about a scientist trying to figure out a use for it.
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