Stefanie Tompkins

Transcript

Priscilla Nelson: 

The mining industry, for the most part has just thought of tailings as waste. It’s something that you just want to get rid of. But if now we can say, “Well, yeah, you can get the primary ore, but you can also get all of these other secondary streams of materials from tailings that have value downstream,” now we can change the whole economics of mining. 

Priscilla Nelson: 

My name is Priscilla P. Nelson, and I am a professor in the department of mining engineering at the Colorado School of Mines. 

The Conveyor: 

You’re listening to The Conveyor, the podcast that brings you the latest research, new discoveries and world-changing ideas from Colorado School of Mines. 

The Conveyor: 

A lot of considerations have to be made when operating and managing a mine, but many people may not know what goes into managing a mine after it’s closed, including waste materials. Can you explain what mine tailings are and why they’re important? 

Priscilla Nelson: 

Sure. Mine tailings are the waste solid materials after separation of the valuable minerals from the economically worthless minerals, so after So after mining and extraction and concentration, all of these residual materials are generally discharged or transported as a slurry, as a hydraulic slurry, and we’ve got a problem with the tailings has both the environmental impact on what are you going to do with this slurry, but it’s also the sheer magnitude of the volume. 

The Conveyor: 

How large are we talking here? 

Priscilla Nelson: 

Annually, the world demands about 20 billion tons of minerals and that includes 40 different elements required to make an average mobile phone. It’s been estimated, seven to 10 tons of tailings for each phone and overall, we have five cubic meters of tailings produced every second globally from gold production, and about three cubic meters from copper production. So in many ways, mining is actually in the business of producing tailings in the metal that you’re trying to separate as a byproduct in terms of volume. 

The Conveyor: 

Wow, so we’re talking a lot of waste then. 

Priscilla Nelson: 

Well, yeah. This is part of the worldwide waste problem, which is even more critical than the domestic waste and industrial waste broadly, because we just have ground up rock being produced in huge volumes. So we have to figure out what to do with these materials in a very safe way. And then you have to do something with it. We use the tailings to actually build a dam and embankment that retains that slurry and the consequences of environmental impacts and social impacts derive from that, because if the tailings embankment fails, then we have a huge wave of slurry mud moving downstream. And if it doesn’t fail, you still have water slowly moving through the embankment causing some chemical changes in the material and resulting in many locations in what is called acid mine drainage that is an environmental problem for our waterways. So the water that seeps out of the embankment has very high chemical contents, in many cases, a lot of sulfides. It can be very low pH, very acidic and cause problems for fish and other wildlife and plants downstream. These tailings dams are some of the largest manmade features in the world. 

The Conveyor: 

For context, how does a mine tailings dam compare to, say, a water dam? 

Priscilla Nelson: 

A typical water embankment dam might have a total height of 50 meters, maybe up to 100 meters in total height, but the mining industry is making tailings dams now that are 300, 500 and some have even talked about 1,000 meters high tailings dams. So as they get taller, the potential for really significant problems to occur, with any kind of failure become very immediate. 

The Conveyor: 

Yeah, and say a dam were to break, there could be some serious consequences. 

Priscilla Nelson: 

Right. Perhaps most notably in the news was one in Brazil, an iron ore mine dam that resulted in over 200 people being killed downstream because of this wave of mud that comes out from behind the tailings dam. This has resulted in a whole new generation of regulations being proposed that require all sorts of work to be done on tailings dams that before were never forefront and something that was being done overtly right now while the tailings are being produced. Eventually when you get into the closure phase, when you’re planning on shutting down the mine, that’s when you pay attention to really the long-term stability of the tailings, but now, investors, NGOs, all sorts of interested stakeholders are making demands on the mining industry to increase the level of security with which those tailings are disposed of. 

The Conveyor: 

So how are companies trying to mitigate tailings or at least refocusing their efforts towards their production? 

Priscilla Nelson: 

There’s two things there, yeah. Tailings have certainly and recently caused a major refocusing. People are thinking about remining some of the old tailings that are at legacy mines, because the processing techniques have improved, because we now find that there’s additional minerals, some of them critical minerals that we could pull out of those old tailings. So in many parts of the world, there are projects being proposed now to remine tailings and reprocess them. 

And the second thing is we have to think about how the materials that are in the tailings are changing throughout the process, from the time you mine it through the mineral processing, through the transportation and then the placement and interaction with the environment. Each one of those steps causes a change in the tailings and in the tailings mineralogy and that affects what you can or cannot do with the tailings at any point along that flow path. 

People at mines try to do as much as they can with the tailings in what I call direct use on site. So they may be able to take more of the water out of the tailings and make something that is much drier, much easier to work with and actually use that as a backfill. If they’re underground, they can fill up underground void spaces and enhance the stability of the mining operation. By and large, most of the tailings come through in a slurry at this point. And in some cases, the slurry can be used for direct use, in some cases for fertilizers, for soil amendments if the mineralogy is right and if the chemistry is right. 

The Conveyor: 

That’s great. Then yeah, I’m assuming the research into recycling tailings is looking quite promising. 

Priscilla Nelson: 

Very much so. There has been a huge interest in the research community about what are the possibilities for tailings reuse. And I think a lot of the focus has been on trying to develop these mineral polymers that can be more than just a substitution for cement. These mineral polymers, these are three dimensional networks of silica and alumina chains that actually form a polymer quite different from concrete and cement. These materials can actually be made to be even stronger than concrete. So we may find all sorts of additional substitutions down the road. 

And we can also take the tailings, and we can melt the tailings and produce a glass fiber, which can make carbon fiber or fiberglass fibers, and that includes reinforcement for concrete, which is 100 percent recyclable. One of the difficulties of recycling concrete right now is that you have to pull the steel out, and if you use rebar which is made from these tailings fibers, you don’t have to pull it out. What we have to do is some fundamental work to really understand the character of the tailings themselves, because tailings are a major earth resource that have not been considered to be a resource. So we’re rewriting the page for tailings and saying, “No, they are earth resources. You just haven’t been thinking about them that way.” 

Priscilla Nelson: 

Thanks for listening to The Conveyor. To learn more about how Colorado School of Mines is solving some of the world’s biggest engineering and scientific challenges, visit mines.edu and then remember to join us back here for our next episode.

This episode of The Conveyor was produced by Ashley Spurgeon and was hosted and edited by Dannon Cox. 

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About the Podcast

The Conveyor brings listeners insights into the latest research, new discoveries and world-changing ideas from Colorado School of Mines.

The viewpoints and opinions expressed by featured guests do not necessarily represent those of Colorado School of Mines.