Tailings often contain salts and metals that may be environmentally harmful if allowed to leach into the environment. The tailings slurry is disposed of in a tailings storage facility (TSF) where the solids are allowed to settle and separate from the supernatant water, sometimes over periods of decades. The performance and regulatory requirements for these TSFs are becoming increasingly stringent.
Problems can arise when a mine starts to run out of room in its TSF and there is still ore to be profitably extracted. While it is possible to expand the capacity or build a new TSF, these solutions demand considerable capital investment. There is an opportunity for innovative and environmentally protective low-capital solutions to get better use from existing TSF space. With both regulatory and community attention focused on the risk of TSFs, and competitive market forces on mineral extraction, mining companies around the world are searching for solutions to improve the cost/benefit ratio (i.e., lower capital cost/environmental protection) of their TSFs and extend the life of existing mining operations.
Over the past decade, WSP has been involved in research, testing and real-world application of polymer technology to extend the life of TSFs. Polymers are chemicals comprising complex molecules, used in a wide range of applications including water purification. When the right polymers are added to water, particles in suspension in the water are drawn by static electrical charges to the polymers to agglomerate and form larger particles, which grow in size to the point that they settle out of suspension. This leaves water that has a much lower level of suspended material (i.e., slurry), typically the source of leachable by-products.
Applying polymer water purification technology to mine waste
So how does polymer-based water purification technology help a mining company store tailings more efficiently?
In most mining operations, tailings are transported to the TSF as a slurry – in which the granular tailings material is pulled through a pipeline by the turbulence in the water. At the outfall spigot, this slurry pours out into the contained TSF so that the tailings form a “beach”. Over the mine’s life, the beach gradually fills up the TSF.
Problems can occur if the tailings are deposited as a shallow beach into the TSF, covering the available storage area without efficiently using the storage volume available in the facility. Expanding the area of the TSF is often impractical and costly and may also have negative environmental consequences from land clearing and potential groundwater contamination. Because of this, the priority is on finding ways to most efficiently use the available space.
Adding a polymer solution to the slurry can help. If injected into the pipeline a short distance from the tailings discharge location, turbulence in the pipeline will mix the polymer into the slurry stream.
As the polymer-treated tailings are discharged from the pipeline, the large and small suspended particles in the tailings segregate quickly from the water, and the water drains away. This results in a steeper beach that allows tailings to be stored in increased thickness within the available storage area.
Another benefit of polymer technology is that the water that was in the slurry contains fewer solids, meaning that it requires less treatment when it is removed from the TSF and recycled for mining operations. This allows operation of a smaller pond on the TSF, reducing evaporation losses and improving stability of the dams containing the tailings. It also helps to reduce the amount of fresh water that the mine consumes in its operations, which is particularly important in water-constrained regions.
Applying polymer technology to real-world mining situations
WSP experts from around the globe have been collaborating to understand how polymer technology can be applied to a range of minerals and environments. Every application is different – the type of polymer, amount to be used, and other aspects of the process are specific to each mine. This means that site-specific testing is required to determine if polymer technology can help improve tailings management. There is no one-size-fits-all solution here.
There is a growing understanding that, while polymer technology has its limitations, there are many situations where it can improve tailings deposition and water utilization with considerably less capital expenditure than required using conventional solutions, such as expanding the TSF.
When a mine has two- or three-years’ worth of recoverable ore left, it is not worthwhile to expand the existing TSF or build a new one. Instead, polymer technology may be enough to boost the storage efficiency of the current TSF so that the mine can continue to operate profitably until the ore body is exhausted, with no need for an expensive capital investment.
As cited in a paper prepared for the Australian Centre for Geomechanics, polymer technology has been helpful at a magnetite mine in South Australia. During the first five years of operation, water recovery was around 60% and the volume utilisation was in line with the deposition model. The high percentage of water recovered enabled the processing plant to reach its new design capability, reduce significant downtime due to water availability and provide the mining operations with sufficient water for dust suppression. The second five‐year plan is currently being finalized and progress is consistent with the tailings deposition and the dewatering model.
WSP now includes polymer technology in its toolbox when assessing the range of solutions available to solve our clients’ tailings challenges. Laboratory testing for new projects considers polymer addition alongside more conventional technologies such as thickened tailings, paste and filtration. Multi-criteria analysis of the options, that considers the capital and operating costs and environmental outcomes, allows us to select the solution that provides the most favorable outcome.
Polymer technology is a fast-moving area of science. Many of the polymers that WSP has tested in its laboratories do not even have a commercial name yet. But with testing and experience, a knowledge base is growing to gain the maximum benefit from polymer technology in mining.
Editor’s Note: Further information on this and similar technologies can be found in Patents CA2515581A1 / WO 2004/060819 A1 (Ciba Specialty Chemicals Water Treatment Limited) and WO 2014/111887 A1 (BASF (China) Company Limited).
