Mine Tailings Storage: Safety Is No Accident
Dry stacking of tailings
Riverine and offshore disposal
The majority of the world’s large-scale surface mines store tailings in impoundments. These complex, engineered structures are designed to contain wet mine waste, which, as reported in this report, can fail with catastrophic consequences. An alternative to this method of storage is dry stacking, which involves filtering the tailings to produce a stackable material (Davies 2011). The filtered tailings are stacked in “cakes” – unlike slurries, which can be pumped to the disposal site, the filtered tailings must be transported, usually either by conveyor or truck. Dry stacking has been identified as an option in areas where water is scarce or conventional impoundments are not feasible. One advantage of dry stacking is that it can make land reclamation and rehabilitation easier. Dry-stacking systems have generally been restricted to smaller mining operations (less than 50 000 tons/day throughput), although recently the technology has been scaled up to work successfully at bigger mines (120 000 tons/day; FLSmidth pers. comms. 2017). However, the potential pitfalls must be acknowledged and thoroughly evaluated. These include (but are not limited to) high initial costs, the need for redundant machinery due to the inevitable downtime required to clean filters, impacts of climate (e.g. erosion of the stack) and potential geotechnical instability, particularly when placing filtered tailings on an existing footprint
In some areas where tailings dam construction is difficult (e.g. mountainous areas), mining companies have been allowed to dispose of tailings in river systems. In some coastal areas, submarine disposal has also been permitted. These methods are generally considered to be an undesirable option for waste management and are not permitted in many countries, including Canada and Australia. Although initially cost-effective, the documented environmental impacts and the potential for long-term effects have prompted some companies to develop internal standards that prevent riverine or marine disposal. Figure 15 shows the locations of mines carrying out marine and riverine tailings discharge in 2012. All of these mines have government permits to discharge into the ocean or rivers. One emerging issue relates to deep-sea mineral extraction, where marine waste disposal of fine material – but not strictly tailings unless processing takes place at sea, which is unlikely – is the only option. Deep-ocean ores are often much higher grade than their on-land counterparts, making them attractive in terms of the value of the ore, however, they remain unproven with concerns over financial viability. Scientists and communities have voiced concerns over the mining process and the on-site disposal of waste material, which may cause significant environmental impact due to increased turbidity and the blanketing of sea-floor organisms with fine material.
Dry stacking at the currently closed La Coipa gold and silver mine in Chile (installed 1990) with a capacity of 28 800 tonnes per day
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