Mine tailings are the ore waste of mines, and are typically a mud-like material.
Worldwide, the storage and handling of tailings is a major environmental issue. Many tailings are toxic and must be kept perpetualy isolated from the environment. Scale of tailings production is immense, since metal extraction is usually only ounces or pounds, for every ton of ore. Tailings containment facilities are regarded as the world’s largest man-made objects.
Mine tailings’ size and composition depends on the mining method. For hardrock metal mines, tailings are usually a very fine mud or powder, which is left over after ore is crushed and valuable minerals are extracted from it. Tailings may also contain chemicals used for mineral extraction.
Tailings are distinct from “waste rock”, which is the non-ore rock which miners move and discard as they dig down to access the underlying ore. They are also distinct from soil and organic matter (collectively known as “overburden”), which is removed from the surface above the ore deposit.
Oil sands production also produces tailings, which are substantially different. Oil sands tailings are the residue which remains after bitumen is separated from virgin oil sands. Although this article focuses on hardrock tailings, oil sands tailings can contain a similarly complex and toxic brew of minerals and chemicals, and pose a similar handling and storage challenge.
Disposal of mine tailings is usually the single biggest environmental concern facing a hardrock metal mine, and creates very long-term environmental liabilities which future generations must manage. Many mine tailings do not become appreciably safer over time, if stored properly, and therefore must stored for an indefinite period using current technology.
The historically-used alternative to storage was to dispose of tailings in the most convenient way possible (such as river dumping), which led to widespread environmental contamination in mining areas. This was nominally viable in earlier eras, but human production of mine tailings has increased by several orders of magnitude in the modern age, making such methods unacceptable to many societies.