Dewatering tailings waste is typically given a cursory glance and eliminated as too ‘expensive’ because of its upfront costs. In fact, there’s often a compelling business case for constructing a dry tailings storage facility. Sue Aitken looks at the cost/benefit analysis mines could be conducting before dismissing dewatering out of hand.
It’s common practice for mines to store tailings waste as a fluid slurry behind earthen embankments or in depleted mine pits. Looked at purely from an upfront cost perspective, this is generally the most economical option. But look at the same problem long-term, and it’s a different story. Unless mines view tailings management from a total cost/benefit perspective, the analysis could be way off the mark.
Different perspective – different conclusion
At the project development stage, when mines consider tailings management, the emphasis is on keeping start-up costs to a minimum. At this point, a slurry approach with its low opex and capex is almost always the preferred – and arguably the better – option. However, at closure, tailings storage facilities (TSFs) represent a significant liability.
Slurry tailings often contain up to 70% water by volume stored in the pore spaces between soil particles and occupy large disturbed footprints. Rehabilitating these huge, soft ponds of yoghurt-like material is time consuming and expensive. Accessing the surface to cap the tailings and shape desirable landforms for revegetation from the resulting swamp is often difficult. Waiting for the sediment to slowly settle and consolidate on its own can take decades.
Dry-stack tailings change the equation
A dewatering approach that extracts most of the liquid and changes tailings from a fluid to a soil-like material prior to placement, offers many mine sites substantial long-term benefits, including:
- Faster remediation – Dewatered tailings can be ‘dry-stacked’ into earth mounds or waste dumps and easily shaped into final landforms. This material is amenable to normal earthworks construction practices, widening the pool of engineers and contractors who are able to provide design and construction services followed by hand-off to operations.
- Smaller disturbed area – Useful for projects with restricted real estate for the combined operations of mining, waste management and tailings management, dry tailings are more compact and can be stacked in a smaller disturbed footprint than a slurry pool.
- Increased flexibility – If needed, the storage facilities can be located in several discrete dumps, used as backfill and capable of supporting vehicle traffic.
- Smoother cash flow – Rehabilitation can be carried out progressively, spreading the cost over the life of the mine, rather than hitting in a lump sum at the end.
- Better water management – Dewatering simplifies water management with immediate efficient water recovery, and results in negligible seepage or leachate losses from the base of the dry stack TSF. Mines can often immediately reuse the recovered water for processing purposes, reducing raw water demand. In areas where ground water is scarce this can represent a substantial cost benefit. In wetter climates, dry stack tailings management can also reduce the overall demand for contact water treatment.
- Safety – Removing most of the water from the tailings stream prior to storage safeguards its physical stability, eliminating the catastrophic risk of a tailings dam failure with its consequent environmental and social impacts. Safer, self-supporting stacks are also particularly attractive in seismic areas.
- No need for retention embankments – Putting in equipment to dewater tailings can help to avoid the cost of building and possibly lining an earth dam to contain tailings.
- Value from existing tailings retreatment – Some existing tailings contain economic quantities of ore which can be recovered through reprocessing. Dewatering the resulting tailings from the reprocessing exercise offers an opportunity to selectively restore the soil-like material back into the existing TSF potentially eliminating the need for further tailings storage facilities while the reclaim operation occurs.
Keep your options open
This is not to suggest that mines should set up dewatering facilities from the get go – or that dewatering is always a good solution for every project.
A balanced approach may be to start with a slurry approach to get a lower cost start-up, and then convert to dewatering while stepping out over the soft slurry is not too difficult to implement while operations continue. Once operating, a mine should be generating cash flow and in a better position to consider the investment in dry stack tailings infrastructure. This approach can provide all the benefits of dewatering at an opex and capex price point the operation can more readily afford.
At Beca, we suggest that our mining clients at least give themselves this option. This would require designing the water collection system and locating the starter-dam in a way that it can be easily converted to dewatered tailings management facilities if desired.
As public expectations for standards of remediation and mine closure get higher, this is an appropriate time for mining executives to consider the potential for using dry stack tailings to reduce their risk exposure to this growing issue. Notably, following the 2014 Mount Polley dam breach, new regulations in British Columbia, Canada now require any new mine project application to consider dewatering tailings.
Whether or not other regulators follow suit, mining executives would be remiss if the option of dewatering tailings is not on the table when considering tailings management and closure strategies.
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