In the mining industry the term tailings, or tails, refers to waste material that remains after most of the desired minerals have been extracted from the mined ore in a mine processing plant. Tailings, sometimes also called slimes, slickens or leach residue, usually consist of very fine particles of ground or crushed rock and minerals mixed with water and the other chemicals added in the metallurgical processes of mineral extraction. This waste material can be handled in different ways, depending on its composition, water content, and on the regulations for mining in the specific jurisdiction. For example, the material can be mixed with water and pumped into the ocean or waterways, stored in specially built surface ponds or underground facilities, dried and stockpiled, or treated in various ways to make it safer to use and handle. Disposing of tailings is a major part of any mining operation, and this material can cause health and environmental problems if it is not handled correctly.
Tailings often contain some of the same minerals that the ore was mined for, and are sometimes reprocessed to extract this material. Mining waste also commonly contains arsenic, sulfur, various naturally radioactive materials, and heavy metals such as mercury and cadmium. The additives used when chemically processing the ore are also often part of the tailings. This can include oil, grease, cyanide, sulfuric acid, and calcium in the form of lime.
A common way to store tailings is in ponds and dams. Coal mining, for example, produces waste material called coal slurry, consisting of very fine coal particles mixed with water, that is often stored in a type of dam called a coal slurry impoundment. Tailings can also be stored underground, or dewatered and stacked in storage facilities. The often toxic content of the waste makes any storage difficult and costly to maintain, and any leaks or seepage can cause environmental damage and affect the health of nearby residents.
Worldwide, the mining industry produces hundreds of thousands of metric tons of tailings every day. Many mining companies are trying to find methods of treating their waste material so it does not require expensive long-term storage. Often the aim is to turn the waste into a soil-like material that can be used for mine reclamation projects and replanted with vegetation. Other waste material from mine processing includes overburden, which is waste rock that is removed during mining but not processed. Such material is sometimes dumped in a so-called spoil tip and left on site.