Tailings (also known as slimes, tailings pile, tails, leach residue, or slickens ) are the materials left over after the process of separating the valuable fraction from the worthless fraction of an ore.
Tailings represent external costs of mining. As mining techniques and the price of minerals improve, it is not unusual for tailings to be reprocessed using new methods, or more thoroughly with old methods, to recover additional minerals. Yesterday's tails can be tomorrow's resource, as seen during the 1990s when the extensive tailings dumps of Kalgoorlie / Boulder in Western Australia were re-processed profitably by KalTails Mining.
In coal and oil sands mining, the word 'tailings' refers specifically to fine waste suspended in water.
