Worley mineral processing technology and expert solutions VP Mervyn Stevens states that the mining industry has taken big strides to improve its impact on both nearby communities and the world.
He explains that this has seen targets set around renewable energy, automation and zero emissions. While there has been progress in these areas, there is another critical consideration for a truly responsible mine: mine tailings.
“When you separate valuable minerals from rock, it leaves a lot of waste. You can’t simply sweep it away and hope for the best.”
While automation and digitalisation have transformed the processes inside mines, this evolution has not always extended to the management of waste processes.
“A lot of tailings dams were built in the 1960s, and [have] since been managed by several different operators,” he says. “In our experience, when we [secure] a dam project, there are often information gaps on how it was constructed and operated in the past. Often we need to investigate and understand the dam to find the safest possible way to manage it.”
As a part of the response to recent wet tailings failures that sent ripples across the industry, the International Council on Mining and Metals (ICMM) published its revised global standard for tailings facilities.
This focuses on prescriptive monitoring and policing of tailings facilities, including a need for greater disclosure of the risks and impacts for all stakeholders, which includes the public.
“Most of the current focus is on risk management, rather than on the closure of a dam,” explains Stevens. “Initially, the priority is to get a better understanding of the risk of those liabilities on mining sites, which differ depending on the specifics of the site. From there, we can find the best way to address those risks.”
He explains that no two tailings dams are the same. To assess dam health, there needs to be an understanding of what happened in the past and whether it was well managed. The company looks at the gradings of materials to give it an idea of its impact.
“If rock has been added to the dam, the bigger pieces have a high friction angle and won’t move as much. But if the particles are very small – as with gold and bauxite – then these can shift over one another quickly, changing the bearing capacity and shear properties of the underlying tailings,” says Stevens.
He adds that tailings facilities have a long life span, and are exposed to seismic events, drought, periods of high rainfall and cyclones. Worley’s hydrologists help customers understand and plan for the effects of weather events and longer-term climate change, including plans on how a facility might be affected by these events, and how to best mitigate risk through flood capability modelling and spillway compliance.
Dam embankments are not the only aspect governing tailings dam safety. Water, both on the surface and underground, has an important role in the long-term stability of the dam and its impact on the environment.
“Seepage through the acid-generating tailings can migrate through the base of the dam into groundwater, degrading water quality and affecting sensitive environmental receptors for years to come,” says Stevens.
“Our hydrogeologists and geochemists work together with our broader team to identify migration pathways, groundwater flow rates and chemical reactions that govern seepage and related impacts through dam design and construction through to capping and closure. This helps to mitigate risks and address issues before they arise.”
No Dams, No Dam Failure
“Where there is water, there is risk. However, newer methods are emerging that don’t involve dams,” Worley principal geotechnical consultant specialising in dams and tailings Johann Vos says.
“There is a movement toward dry stacking of tailings, which involves taking the water out of tailings and stacking them in an appropriate place. The dry material is shaken out and transported to a storage area, either above ground or back within the mine itself. The company can also create stable landforms integrating dry material, which has been through a process of partial dewatering, into the natural landscape.”
This process can be much safer and is more sustainable in the long term, through better erosion protection, better water management and, ultimately, a reduced risk of failure.
“When compared with a dam, this method minimises the need to manage seepage, especially when you consider that slopes [a number of] tailings dams are located near towns and environmentally sensitive areas,” adds Vos.
Mining the Mined
“With declining ore grades, we’re actually generating more waste than we did 50 years ago,” says Stevens. “We’re having to process so much material, and a lot of what’s mined still ends up as waste.”
He explains that technologies like ore sorting technology NextOre can help separate valuable minerals from waste before energy- and chemical-intensive processing, thereby reducing the amount of tailings, water consumption and greenhouse-gas generation.
“Another strategy is to reprocess old tailings,” adds Stevens.
He points out that what was considered a cut-off grade ore in 1970 is much higher than what is now considered acceptable. Material that was being thrown out 50 years ago has value today and is easier to access than material at the bottom of a deep pit.
“In one project, our customer is looking to extract low-grade material from their tailings waste. It’s not being sold, but they want to see what happens when they blend the ‘waste’ into a marketable material. And with precious metals like lithium and cobalt in demand, ‘waste’ is now becoming economical to remine,” says Stevens.
“It’s all about aiming for a sustainable model,” adds Vos. “The endgame isn’t to have nothing left over, but we want to turn as much mining waste as we can into an amenity to society. This could mean turning land over to local farming, or any other safe and beneficial outcomes for the stakeholders of the mine.
“There’s always going to be a waste product when you mine, and we need to reduce as much as possible to minimise the impact on the environment. But in our view, this doesn’t have to be a burden,” says Stevens.