“Long term liability” are three words that form one of the more worrying expressions for many mining companies today.
Tailings pond in rural Utah.
In many cases, those liabilities come from traditional water-covered tailings storage facilities (TSF) that require perpetual maintenance, but they have also become a significant operational and financial burden for many companies.
As well, companies are increasingly concerned about their ‘social licence to operate’ – which can be quickly lost if the facilities’ dams are overtopped or give way, releasing tailings and water.
A case in point: the aftermath of the Mount Polley mine tailings disaster of August, 2014 in central British Columbia, in which some 10 million cubic metres of water, and 4.5 million cubic metres of slurry were released into downstream watercourses.
Following the recommendations of the Independent Expert Investigation and Review Panel in April, 2015, a group of Alaskan Aboriginals, B.C. First Nations, private businesses, scientists and others signed a letter asking the B.C. government to halt the permitting of wet-tailings facilities for new and proposed mines in the province .When public concern about anything increases, politicians take notice, and regulations tend to follow. This usually results in concern from financial sources, causing difficulty for companies that are perceived to be vulnerable to long-term liabilities stemming from current or past operations.
Accordingly, there is increasing interest by mining companies in ways to reduce the size of their tailings ponds, develop alternatives to maintaining large ponds of water, or even eliminate them entirely. This can help them move towards the elusive goal of “walk-away closure.”
Can the threat from tailings be neutralized?
Traditional tailings disposal includes slurrying of the tailings materials for transport by pipeline to the TSF. This transport method results in large volumes of water reporting to the TSF where this slurry water, combined with runoff from normal precipitation and extreme storm events, results in ponds of water on the surface of the TSF.
Depending on the chemistry of the tailings, water covers (or alternative cover systems) are required to prevent Acid Rock Drainage.
Alternative tailings-handling and disposal methodologies have been developed over the past few years. Here are three of the more common alternatives, which can also reduce or avoid the need for water-covered TSFs: Paste or thickened tailings on surface – One way to help reduce the amount of water going to the TSF is through thickening the tailings, so that the product moves through the pipe in a plug-flow rather than a slurry. One major benefit of this approach is that the tailings will generally not segregate, as happens in slurry deposition, so the result is a relatively homogeneous mixture.
This in itself can help maintain saturation of the tailings, and prevent oxidation and acid generation.
Dry-stack disposal on surface – This method allows tailings to be placed in a dense state (i.e. compacted). As with thickened tailings or paste, in high-precipitation environments, it won’t eliminate water from the TSF. However, tailings can be made much more stable, and with appropriate decommissioning (pending the presence of sulphides), this can essentially become a walk-away solution for many tailings deposits. However, this method is the most energy intensive discussed here.
Underground backfill using paste tailings – One method that is more promising along the road to walk-away closure is putting the tailings back underground. Backfilling supports the working of the ore body next to a worked-out stope, as well as helping manage the risk of subsidence and collapse of the workings post-closure. Groundwater rising in the mine after closure will help to keep the tailings away from air, and help prevent acid generation. However, this method too has its downside: only about half of the rock pulled out of the mine as development rock or ore can be fitted back into the worked-out stopes, leaving the rest to be disposed of on surface.
Reducing the volume and acid-generating capacity of tailings
Given the limitations on these methods, and the growing public pressure to avoid water-covered TSFs, mining companies can benefit from re-thinking their approach to ore processing.
In some cases, this could have the potential to generate additional revenue, but more importantly, there could be opportunities to reduce their potential environmental risks.
The key lies in being willing to consider broader horizons, as regards to the kind of business they are in. This has to do with what might be called “mining myopia” – that some mining company executives would declare, “We’re a gold-mining company,” or perhaps, “We focus on copper.” A gold-mining company might recognize that the ore they are working contains silver that might be extracted.
However, experience has found that many companies ignore the possibility that more metals can be pulled from the ore, which would decrease the chances of those metals leaching from the tailings, and also reduce the chances for acid generation.
But given the cost and liability issues, it is increasingly important for mining companies to render those tailings as benign as possible. Tailings that do not exceed regulatory thresholds for metals, acid-generating compounds and other hazards may be able to be disposed of without the cost and liability of a water-covered impoundment.
This can be done through processing the ore in a way that concentrates the sulphides.
For example, the ore may have 2% sulphides, and by processing you may be able to remove the sulphides in perhaps 5% of the ore. This sulphide concentrate could then be evaluated for further processing to neutralize the sulphides and to recover additional product.
But the true value of further concentration lies in being able to remove the acid-generating and metals portion of the tailings, or at least reduce them to the point that they do not need special disposal such as a water-covered tailings impoundment.
Being able to produce tailings that are environmentally benign opens up all kinds of possibilities for tailings disposal, in that there are few or no long-term liabilities attached to those tailings.
Previously, it might not have been worthwhile to pull more mineralization out of the ore. However, given the costs and liabilities associated with the disposal of tailings that contain hazards such as salts, metals and acid-generating potential, rendering those tailings benign can be well worth the effort and cost.
Squeezing value (and liability) from the waste stream
How should the sulfide-bearing concentrate be processed? It depends on a range of factors.
In an established mine camp in Sudbury, Ontario, for example, the wide range of metallurgical facilities in the area means that it is quite likely best to truck the concentrate to another facility for processing.
If there are no smelters or other processing facilities within a reasonable distance, however, it may be feasible to set up a processing facility on or near the mine’s mill. This processing facility might be owned and operated by the mine, or by a third party that buys the concentrate for processing. Whether there is additional processing of concentrate depends on what minerals there may be in the concentrate.
If removal of those minerals is not practical, other options should be considered.
In underground mines, it is best to give the sulfide-bearing waste product priority when it comes to finding space underground, and making sure that it is in a place that will be below the water-table when dewatering stops. That way, this hazardous portion of the mine’s waste stream is locked away underground, away from air and free of liability. In open-pit mines, the high-sulfide part of the waste can be encapsulated and disposed of in a part of the TSF that will be below the water table after dewatering stops.
In open-pit mines, the high-sulfide part of the waste can be encapsulated and disposed of in a part of the TSF that will be below the water table after dewatering stops.
Taking extra care with the parts of a mine’s waste stream that contain environmentally hazardous materials, separating them out for special treatment, is a cost that many mining companies might think not worthwhile, but with the fast-growing concern about the risks and liabilities stemming from conventional TSFs, this is an area of good future potential.
*Leo C. Botham, MSCE, P.Eng. is a Senior Geotechnical Engineer specializing in Mine Waste Management and the design and construction of other mine infrastructure. He is Vice-president of Clifton Associates, Saskatoon, SK.