That's insane. These spill heaps always end up killing people, all so the mining company didn't have to pay to dispose of it.
I mean, you could sell salt ffs why just dump it? And what happens when it rains, surely it's absolutely fucking that soil for years to come
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It is apparently contaminated/not pure enough and refining it isn't financially viable compared to mining it from more pure deposits (according to the german wikipedia page)