New technology promises to reduce water, create a smaller environmental footprint and reduce long-term risks for mines.
By Andrew Watson
Likely, you’re reading this on your computer, smartphone, or tablet. The creation of these modern marvels requires resources extracted from the earth. Mining touches all our lives daily in small and large ways, and the industry works hard to make sure the net impact is positive, even though some of the potentially negative impacts are very long-lasting.
Despite technological advances in mineral processing, mining companies still face a major challenge: What is the best way to manage tailing materials?
In recent years, alternative tailing disposal (ATD) has promised to be the ‘silver bullet’ that will address all tailing management issues.
Finding alternative methods of tailings disposal is a topic I’ve been passionate about for years. It’s become a major focus of my work and I’ve written and presented on the topic multiple times. I’m looking forward to a further discussion at the upcoming SME Annual Conference & Expo in Denver on Tuesday, February 21.
Tailing disposal options
When considering tailings—what’s left behind after the ore is extracted—there are four major types of tailing disposal. Each has advantages and disadvantages.
Filtered tailing disposal: Tailing materials are dewatered to 85% or greater solids by weight ratio using filers aided by a vacuum or confining pressure. The dewatered tailing materials are transported by conveyor or trucked to a disposal area.
Paste disposal: Produced in specialized paste thickeners, or ultra-high-density thickeners and transported by positive displacement pumps, paste is generally used for backfill in underground workings or in other situations where gravity helps the flow.
Thickened tailing disposal: Tailing materials are “thickened” through the use of high-density or deep-cone thickeners to about 60-70% solids so the material does not segregate upon deposition. This can have some benefits with respect to permeability and lower seepage, along with the higher initial water recovery, allowing immediate reuse of a greater fraction of the process water.
Conventional tailing disposal: Tailing materials are dewatered to about 30-55% solids and transported as a slurry. This requires little energy, is favored by operators because it is simple, but a lot of water is circulated to the storage facility. Conventional disposal involves the use of dams, embankments or surface impoundments.
Finding the ‘Silver Bullet?’
Mining lower grades of ore has resulted in increased water use per unit of production. In some cases, the availability of water is one of the greatest constraints on mine development. By optimizing water recovery, ATD technologies promise to reduce water use at mines. This is of great advantage in some settings, and in others ATD technologies promise a smaller environmental footprint and reduced long-term risks.
It sounds like a win-win opportunity, but it is not always easy. There are three major considerations that limit ATD’s “silver bullet” status.
One size does not fit all
Each mine is unique. The mine’s physical setting, the ore body, and the area’s climate all greatly impact how the tailings can be handled. Considering that the social setting is ever more important, it’s a huge mistake to simply apply the same formula on different sites. While recent failures have sparked a cry for filtering everywhere, it remains important that mine owners consider the issues at each mine site and establish what would be best there.
Some considerations impacting the use of ATD:
Topography: Some technologies lend themselves toward flat topographies and sometimes are not even feasible at sites with moderately steep terrain.
Production rates: Operators seek reliable equipment and stable operations. Removing water using filters or paste thickeners from some tailings can be a challenge at high-production rates. Currently, conventional tailing disposal remains the default technology at high-production mines.
Climate: Although ATD methods have been implemented in wet climates, their successful implementation is more attractive in, and aided by, a dry climate.
The issue of water
Every mine needs water. When it comes to tailings and water, here is the philosophical question: Do we want to remove some water from the solids, or do we want to remove the solids from the water? In some settings we want to improve the performance of the tailings after disposal, while in other settings we really want to recover all the water because it is so scarce. In some cases, the residual water might pose a risk and removing it results in a more attractive post-mining outcome for the owner. Knowing what the question is leads one to the right answer for that situation. Of course, the answer isn’t always the same.
Considerations impacting the use of ATD:
Water and energy: Removing water prior to disposal makes significant amounts of water available for immediate re-use but requires significantly more energy than conventional (and more water dependent) methods. Depending upon the relief and other criteria, the benefit might still be realized. In general though, removing more water means higher operating costs.
Today’s tailings = Tomorrow’s troubles?
Nobody wants the tailings material. It’s simply the residue of mining. But it hangs out a lot longer than the products we consume. How we handle tailings now may have a significant impact on the environment and health of the mining company in the very long term. We need to know how this new landform will behave in the future.
Mine closure, post-closure liability management and the mitigation of risk need to be considered when selecting the tailings disposal method. Often this is not given as much attention as the extraction and beneficiation process, although the risks and liabilities endure far longer. Mining companies are taking a much longer-term view and exploring ATD options to improve their performance.
The next steps
This is an exciting time to work in the mining industry. Technology is changing the way we work.
And now, as MWH merges with Stantec, our teams bring the whole toolbox with us when we come to a mine and we’re in an even better position to contribute to better tailings management.
Andrew Watson and other mining experts from MWH, now part of Stantec, are presenting at the SME Annual Conference & Expo in Denver, February 19-22.
Andrew Watson is the Vice President of Mining at MWH, now part of Stantec. He has more than 20 years of experience in the design and construction of infrastructure projects around the world, focusing on managing tailings storage facility risks, mine closure and sustainable mining.