Mill Tailings are, by definition, fine-particle residues of milling operations that are devoid of metal values. Particle-size distribution is one of the most important ways of characterizing tailings. The mining industry distinguishes “sands” and “slimes” as components of tailings.
Tailings are basically the materials left over after separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are distinct from overburden: the waste rock or other material that overlies an ore or mineral body and is displaced during mining without being processed.
ECONOMICS of Mill Tailings
Losses to tailings are the most important parameter in deciding whether a deposit or milling process is economically viable or not. Early milling operations often did not take adequate steps to make tailings areas environmentally safe after closure. Modern mines, particularly those in jurisdictions with well-developed mining regulations and those operated by responsible mining companies, often include the rehabilitation and proper closure of tailings areas in their costs and activities.
Site selection for tailings disposal has to be based on economic and environmental considerations. The tailings impoundment site has to be close to the mill for economic reasons and to conform with the following three requirements:
- Be mineralogically barren
- Have strong structural geology to bear the weight of the impoundment
- Have geomorphology that allows surface waters to bypass the dam or drain through it
Factors influencing tailing impoundment design include site characteristics, tailing characteristics, effluent characteristics, and mine/mill characteristics.
Tailings may be discarded on land, into a watercourse, or in a sizable water body (a lake or sea). In the case of underground mining, at least part of the tailings may have to be pumped back into the mine to backfill excavated space. Still, usually, a significant proportion of the tailings remains to be discarded. If possible, tailings should not be stored underground; it is prudent to dispose of milling tailings on the surface and have them easily available when more efficient extraction processes exist. Storing tailings inactive open-pit mines is obviously impossible, and abandoned open-pit quarries are, as a rule, far from active mills.
Historically, tailings were disposed of in the most convenient manner, such as downstream running water or down drains. Because of concerns about these sediments in the water and other issues, tailings ponds came into use. The sustainability challenge in the management of tailings and waste rock is to dispose of the material. It is inert or, if not, stable and contained, to minimize water and energy inputs and the surface footprint of wastes and move toward finding alternate uses.