coal waste

Coal refuse, also known as coal waste, rock, slag, coal tailings, waste material, rock bank, culm, boney, or gob, is the material left over from coal mining, usually as tailings piles or spoil tips. For every tonne of hard coal generated by mining, 400 kg (880 lb) of waste material remains, which includes some lost coal that is partially economically recoverable. Coal refuse is distinct from the byproducts of burning coal, such as fly ash.