Coincidentally, a Facebook friend posted this video of the place: Baotou toxic lake.
As i suspected, it's a standard - although huge - tailings dam. Anywhere there's a metalliferous mine, you'll find one (or more) of these. I've only worked in one mine (in Australia) and their tailings dam had been incompetently built and managed - and it leaks into the surrounding soil and water table. I suspect they're like that everywhere, as mining companies only care about money, not the environment, and governments turn a blind eye.
The dust masks worn by the people in this video are rather melodramatic - and useless. It doesn't look dusty and the masks will have no effect on the sulfur dioxide and other gases given off by the tailings.