A Rock Bottom Coal Cleaning Process

Alot of what makes coal dirty isn't coal at all, but the rock mixed in with it. The rock contains sulfur, a factor in acid rain, as well as toxic heavy metals such as lead. To rid coal of rock, miners often dump it into a slurry of water and an iron ore called magnetite. The clean chunks float on the slurry, while the rock-laden ones sink. So as not to waste the chunks that sink, they're broken into pieces, and the process is repeated. But if the pieces get too tiny, they all sink.

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