At first blush, tailings ponds are fairly simple structures that hold tailings and enough water to cover them completely, which makes ponds one of the most cost-effective and widely used storage options. Since tailings storage brings no current financial benefit to mining operators, use of these ponds is an attractive solution whenever local regulations and site conditions permit them.
Conventional impoundments are generally preferred for relatively high-grade ores where further processing is expected to be justifiable in the short term. They are constructed embankments that resemble process ponds and are lined with a geosynthetic material to prevent leaks. The embankments for conventional impoundments are generally more robust than other options since they’re expected to hold both tailings and water. These impoundments are used for small volumes of ore and are not considered cost-effective for the vast volume of post-processing tailings.
Dry stacks are built with tailings containing only about 15% water. They’re placed in large piles, often with a liner underneath to prevent passage of toxic liquids into the soil. The mound is then tightly compacted, rendering the stack relatively impervious to precipitation and weathering. Dry stacks are relatively simple to construct since there is no expectation of exposure to significant amounts of water. Instead, surrounding embankments may be designed merely to retain any potential runoff, bleed water and fine silt. This makes them well suited for dry, flat areas with low annual rainfall since re-wetting the stack could activate the tailings and generate acid rock drainage.
In-pit storage describes refilling mined out, open pits with tailings, overburden, and other mine waste. Generally, these pits are unlined, so tailings are placed above the level of local groundwater. In some areas, this method is preferred by local populations to the alternative of leaving vast pits in the ground, but a host of complications can attend this type of disposal and it does not generally produce the best results.
So why is exposure to water so important?
When sulfide compounds in tailings are exposed to air, they undergo chemical reactions that produce sulfuric acids. Since many ores (e.g., copper, lead and zinc) are rich in sulfides, the volume of sulfuric acid produced in mining waste piles can be significant. In fact, the various processes involved with beneficiation (extraction) of ore expose these compounds to accelerated weathering, which in turn hastens their conversion to acid. As the acid drains through stacks of exposed tailings, it dissolves and releases other dangerous elements like cobalt, cadmium, and mercury. However, the practice of completely submerging tailings in water significantly limits this kind of interaction with the air, so underwater storage is frequently employed to reduce or eliminate this type of acid drainage.