In Appalachia and the Illinois Basin, coal companies use a process called "wet washing" to reduce the amount of non-combustible material. There are other methods of separating coal and non-coal used in other places, primarily where mining occurs in arid areas with limited water supplies.
In a wet washing plant, or coal preparation plant, the raw coal is crushed and mixed with a large amount of water, magnetite and organic chemicals. The chemicals are primarily patented surfactants, designed to separate clays from the coal, and flocculants, designed to make small particle clump together.
The huge volume of waste water left over is coal slurry. The slurry is composed of particles of rock, clay and coal too small to float or sink as well as all the chemicals used to wash the coal. While the coal industry likes to claim that the particles of "natural rock strata" and chemicals are perfectly safe, testing has shown coal slurry to be highly toxic.
This is what happens when pipes rupture or containment basins leak. To read more about recent incidents in West Virginia and North Carolina, see the article, "Coal Slurry Catastrophes Continue: West Virginia Hit With Another Environmental Emergency."
(All photos used with permission from Appalachian Voices, an award-winning, environmental non-profit committed to protecting the land, air and water of the central and southern Appalachian region, focusing on reducing coal’s impact on the region and advancing a vision for a cleaner energy future.)