Tailings, also called mine dumps, slimes, tails, refuse, leach residue, or slickens, are the materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are distinct fromoverburden, which is the waste rock or materials overlying an ore or mineral body that are displaced during mining without being processed.
The extraction of minerals from ore can be done two ways: placer mining, which uses water and gravity to extract the valuable minerals, or hard rock mining, which uses pulverization of rock, then chemicals. In the latter, the extraction of minerals from ore requires that the ore be ground into fine particles, so tailings are typically small and range from the size of a grain of sand to a few micrometres. Mine tailings are usually produced from the mill in slurry form (a mixture of fine mineral particles and water).
Tailings represent an external cost of mining, particularly true of early mining operations which did not take adequate steps to make tailings areas environmentally safe after closure. Modern day mines, particularly in jurisdictions with well-developed mining regulations or operated by responsible mining companies, incorporate the rehabilitation and proper closure of tailings areas in the mining costs and activities. For example, the province of Quebec, Canada, requires not only submission of closure plan before the start of mining activity, but also the deposit of a financial guarantee equal to 100% of the estimated rehabilitation costs. Tailings dams are often the most significant environmental liability for a mining project.