Tailings are the materials remaining after valuable minerals have been extracted from ore during mining operations. Generated in vast quantities annually, they differ from overburden, which is the waste rock removed to access the ore body but not processed for mineral extraction. The management of these leftover materials is an ongoing consideration within the mining sector.
How Tailings Are Produced and What They Contain
Ore, which is rock containing valuable minerals, is initially crushed and ground into a fine powder, often forming a slurry when mixed with water. This comminution, or grinding, increases the surface area of the mineral particles, making the valuable components more accessible for extraction. Various methods, such as froth flotation, gravity separation, or chemical leaching, are then employed to concentrate and extract the target minerals.
Tailings typically consist of finely ground rock particles, residual water from the processing, and various chemicals or reagents used during the separation stages. For example, cyanide is commonly used in gold processing, and sulfuric acid or petroleum byproducts can also be present depending on the ore type and extraction method. The specific composition of tailings varies widely, influenced by the original ore’s mineralogy, the degree of grinding, and the particular processing techniques applied. Tailings can contain elements like silica, iron, aluminum, and sulfur, along with trace amounts of heavy metals such as arsenic, lead, copper, and mercury, which were naturally present in the ore or introduced during processing.
Methods for Tailings Storage
A common method involves storing tailings in impoundments, often buttressed by large earthen dams known as tailings dams. These dams are constructed in various ways, including upstream, downstream, or centerline methods, with the choice influenced by factors like site geology, climate, and the characteristics of the tailings themselves. Tailings are typically pumped into these facilities as a slurry, a mixture of fine mineral particles and water, where solids settle over time.
Other storage techniques aim to reduce the water content in tailings. Dry stacking, also known as filtered tailings, involves dewatering the tailings to a moist, soil-like consistency, allowing them to be stacked in piles. This method reduces the need for large impoundments and minimizes water content.
Paste backfill is another approach, where tailings are dewatered to a paste-like consistency, often mixed with a binder like cement, and then pumped back into underground mine voids. This technique provides structural support for underground mines and reduces the volume of tailings requiring surface storage. The selection of a storage method depends on a balance of technical feasibility, environmental considerations, and economic factors.
Understanding Environmental Impacts of Tailings
Tailings can pose several environmental concerns if not managed properly. One significant issue is acid mine drainage (AMD), which occurs when sulfide minerals within the tailings, such as pyrite, react with oxygen and water. This reaction generates sulfuric acid, which can then leach heavy metals like arsenic, lead, and mercury from the tailings into surrounding soil and water bodies. This contamination can harm aquatic life, impact drinking water sources, and affect agricultural land.
Dust generation is another concern, particularly from dry tailings surfaces, as wind can carry fine particulate matter and toxic substances over wide areas. Inhaling this dust can cause respiratory issues and other health problems for nearby communities. Tailings dam failures represent a catastrophic risk, with historical incidents releasing millions of tons of waste into the environment. Such failures can lead to widespread contamination, destruction of infrastructure, and long-term ecological damage, including smothering habitats and altering water chemistry.
Reclaiming and Repurposing Tailings
Managing tailings sites often involves reclamation to restore the landscape and reduce environmental impact. Reclamation typically includes stabilizing the tailings, covering them with uncontaminated material, and revegetating the area to prevent erosion and integrate the site back into the natural environment. This process helps to minimize dust generation and control the leaching of contaminants.
There is also a growing focus on repurposing or reprocessing existing tailings, transforming them from a waste product into a potential resource. Advancements in mineral processing technologies allow for the re-extraction of residual valuable minerals, such as copper, gold, or rare earth elements, that were not efficiently recovered during initial processing. Beyond mineral recovery, tailings can be repurposed for use in construction materials, such as bricks, cement, or as aggregates in road construction, thereby reducing the need for virgin materials and potentially mitigating environmental hazards.