It is estimated that nickel, lithium and cobalt production needs to increase by 150% to supply the electric vehicle and clean energy sectors in the coming years. While some of that supply will be met by recycling, most of that increase will be satisfied with new mined supply.

As much as 8 billion tonnes of tailings are produced annually, adding to the 223 billion tonnes of tailings generated through mining processes since 1771. That number is expected to increase as metals production climbs.

For Ljiljana Josic, manager of mine and waste management at Canadian engineering firm SNC Lavalin, aside from sheer volume, three other factors contribute to the rising amount of tailings.

"We have deeper deposits, declining ore grades and the expansion of mining operations," she said.

Although most global tailings are currently stored using dams, Josic noted the rising use of dewatering and filtering techniques, often called dry stacking.

"In Canada right now, 80% of the tailings are managed conventionally, conventionally means by tailings dams," said Josic. "[Dry stacking] is becoming popular because it provides tailings dewatering."

Although stressing that there is no "perfect solution," Josic did note that dewatering tailings has several benefits, including water management.

"Dewatering of the tailings creates a more solid material which consolidates the tailings," she said. "So, instead of having a huge dam, you still have some water to manage, but it is smaller."

In Canada right now, 80% of the tailings are managed conventionally

Another tailings management method that is gaining traction is co-disposal, which combines the fine material in tailings with waste rock to create a tailings facility.

"Basically, you put that fine material into walls," said Josic. "Because it's almost dry material, it provides space optimisation, reduces overall footprint because they are combined, and gives greater stability."

 By incorporating dewatered tailings, co-disposal also supports water management. 

"Both are considered suitable alternatives to traditional tailings management. Both focus on minimising water usage and offer stability," she said.

Water conservation

International mining consultancy firm O'Kane is also a supporter of dewatering and co-disposal. The firm, with offices in North America and Australia, is especially focused on the intersection of water management, waste management and mine closure.

One of the services O'Kane offers that touches all three areas is waste rock stockpile design, or "managing tomorrow's risks today," as Mike O'Kane, founder, and senior technical advisor at O'Kane, describes it.

For O'Kane designing the right tailings management solutions incorporates water management with term source control and is often kickstarted with discussions about how acidic leaching and acidic water are produced at a mine site.

"Well, it's that sulphide, or that very common pyrite, or fool's gold, that oxidises when it's brought up out of the ground and exposed to water and oxygen," said O'Kane. "Water leeches through it, and it rusts just like the bumper on your car."

However, identifying the cause of the acidic leaching is easier than preventing it.

Co-disposal offers the benefit of dewatered tailings and the stability of waste rock

"Source term control for us is saying, I want to limit the resupply of oxygen because the mine rock stockpile typically will have upwards of 75%- 80% of the acidity or acid load produced at a mine," said O'Kane. "Very often [when] tailings storage facilities are leaching, acidic drainage, often, it's [coming] from the mined rock that was used to build the dams."

Co-disposal offers the benefit of dewatered tailings and the stability of waste rock.

"You put the mine rock stockpile and the dry stack tailings material together into an integrated landform," he said.  "You're reducing the volumes, any pore space created by the larger pore size of the mine rock is integrated with the dry stack, and now you've got this oxygen limiting thing that also has geotechnical stability."

The challenges of tailings filtering

Although dewatering is often referred to as dry stacking, the name can be misleading because the filtered tailings can still retain 10%-20% water content.

While the residual water itself isn't a problem, the inability to regulate how much water each filtered tailing stores is.

"The problem with dry stacking arises when you said you would do 15%, and on average, you do. But because it comes out off-spec, some will come out at 17%- 18%," said O'Kane. "So, what is the consequence of that."

The variation can lead to instability in the stack, which can compromise the structural integrity or allow for more airflow and oxidation.

Additionally, not all tailings can be dewatered safely, a factor that Charles Dumaresq, vice president of science and environmental management at the Mining Association of Canada (MAC), pointed out.

"Not all tailings are amenable to filtering," said Dumaresq. "Even if the technology is there at scale."

This is due to the composition of the tailings, specifically the grain size.

"The finer it gets, the harder it is to filter," he said. "And sometimes you just can't do anything about the amount of fine material that's there, you can't remove it, and you can't filter it."

Dry stacking the wrong tailings can also pose a risk.

 "If we have sulphide minerals like pyrite and a risk that those are going to oxidise, that oxidation is more likely to happen and release acid and metals into the environment as the tailings get drier," Dumaresq added.

The cost of tailings management

Another deterrent of dry stacking and co-disposal is cost. The newer innovations require more capital expenditure than conventional dams; however, all three experts agree that the long-term benefits justify the expense.

"We're going to see it more and more, particularly as the technologies to be able to remove water become more accessible, more cost-effective, we'll be able to see scaling up of those dewatering technologies to larger and larger production rates of mine," said MAC's Dumaresq, noting that filtering isn't happening at projects that mine over 100,000 tpd.

"But I think we will see that in the future and that that's going to be a significant change in how tailings are managed and how those various risks associated with tailings get balanced."

For Josic, these new tailings solutions may be the answer to de-risking historical tailings dams while generating new revenue.

"There are billions of tonnes of tailings stored in legacy dams," she said.

If these decades-old tailings are reprocessed and dewatered, "mining companies can transform the tailings ponds from a liability into an asset and find value in the waste," said Josic.

Some of the costs associated with dewatering or co-disposal can also be mitigated through a design strategy that envisions mine closure as part of the original plans. An integrated mine and closure plan, where the desire to bolster resource production and balance reclamation are thought of in unison.

"You have one plan governing the entire lifecycle of the land,' said Miriam Clark, global vice president of strategy and business growth at O'Kane.

This idea of a holistic approach can also be economical.

 "When you're deciding where that truckload goes, you consider not just the perspective of what's the best outcome from a resource extraction [view], but also, where does this truckload need to go to build the landform that we're looking for post-closure," said Clark.

 

Want to be part of the tailings discussion? 

Join us at the Future of Mining, on 25-26 September 2023 in Denver, USA.

Be sure to attend the Technological solutions for improved tailing management in mining panel, where topics will include: 

• Is it possible to achieve zero water waste in tailings?  
• Where will technology make the difference? Which are the future alternative approaches to conventional tailing management? 
• Which are the main barriers to the implementation of innovative tailing management?  
• Is there a single ‘silver bullet' solution for tailings management, or do technologies such as dry-stack filtering, geotechnical instruments and satellite monitoring all need to work in sync?

Session panelists include: 

Kim Morrison, Sr. Director, Global Tailings Management, Technical Services, Newmont Corporation

Dennis Rugg, Chief Geotechnical Engineer, Freeport-McMoRan

Priscilla Nelson, Professor, Department of Mining Engineering, Colorado School of Mines