Sustainability
only to oxidise ferrous iron to ferric iron, but also to see if they could oxidise sulphides just to the point that elemental sulphur is left over as a by-product – rather than oxidising all the way to produce sulphates. This would help limit the amount of lime – calcium carbonate – required for neutralisation. “In our initial techno-economic analysis is that with the nickel prices we were having, maybe a year and a half ago, the cost of [the bioleaching] process becomes viable only if you’re minimising the amount of neutralising agents that’s required,” says Mahadevan. “We just wanted to prevent the oxidation of sulphur to sulphate – that way you can control the process, get a valuable by-product, as well as try to see if we can leach the pyrrhotite tailings and then get nickel out.” As with BacTech, the reasons for doing so are
1-3pH
The potential average levels of acidity that can be seen in legacy tailings pools.
International Water Power & Dam Construction
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clear. There remains a significant amount of nickel in many tailings sites around the world, so being able to recover these extracts in a cost-effective manner has a clear economic benefit. Likewise, on the environmental side, legacy tailings present potential ramifications should the storage measures in place fail, and freshly generated tailings are still being created and placed in tailings ponds. Mahadevan’s team’s initial goal was to run these fresh tailings, rather than legacy tailings, through bioleaching processes with its microbes to demonstrate that they could isolate the valuable nickel and create an inert stream that doesn’t need to be stored in a tailings pond. As new tailings typically possess less nickel than their older counterparts – about 0.5% compared with approximately 1%, respectively, this was a considerable challenge. “These inert tailings would be more environmentally friendly,” Mahadevan notes. “If you’re able to prove the concept on freshly generated tailings, then you can go back to these legacy tailings and then, hopefully, have an aspect of environmental clean-up.” Eventually, if the bioleaching process can be optimised both in terms of cost-effectiveness and sustainability, he adds, not only would it be more appealing in the eyes of the mining industry, but some parts of the sector might start considering whether there are other high-energy consuming processes that could theoretically be replaced with biology.
“At this point, the mining industry hasn’t traditionally used biology in a big way, so maybe this is just the first opportunity,” says Mahadevan. “Eventually, there could be many other avenues where biology will be helpful.” For example, he notes that the current microbes his term are working with also have some potential to capture and store carbon, while improved methods of cleaning up legacy tailings and remediating tailings ponds could offer benefits from a sustainable water angle.
Delve into the DNA
Beyond their work evolving microbes through adaptation, Mahadevan and his team are also pursuing a genetic engineering approach by using the emerging gene editing technique known as CRISPR – an acronym for ‘clustered regularly interspaced short palindromic repeats’.
“These CRISPR-based tools can actually go in and specifically modify certain sections of the microbe’s DNA,” explains Mahadevan. “In a way, it’s basically accelerating the natural evolution.” With his team’s work on adaptive evolution, they started with solutions containing 1% pyrrhotite solids, which the microbes from the tailings ponds could grow in. If they had been put immediately into a 20% solids solution, they would not survive. Slowly adapting the bacteria to a 5% solution over a six-to-eight-month period, however, was much more achievable, and over this time through evolution the bacteria accrued some sets of genes in larger numbers – the genes that helped them survive in more hostile environments, essentially. “The question becomes, then, can we accelerate that process by which these things are naturally occurring by spurring them on in a more directed way?” Mahadevan asks. Using CRISPR, the research team intend to see if they can manipulate these genes to increase the ability of these microbes to be more resistant to environments with high percentages of tailings solids or boosting their pH tolerance. At the same time, Mahadevan also highlights the potential for targeted modifications on the genes responsible for the oxidation of sulphur to sulphate.
While only having just started that process, the researchers have already released a publication demonstrating that, through modifying certain genes, they’ve been able to decrease the amount of sulphate produced through their bioleaching processes and instead form elemental sulphur. “It’s not just about CRISPR,” he’s quick to note.
“We’re still using the older methods that we’ve done before in the past. We want to do both. And we’ll see which ones lead to more interesting microbes, [which] we might potentially use in the future.” Regardless of where the bacteria come from or how these microbes are created, they offer a truly fascinating potential to clean up some of the worst after-effects of the mining industry, removing harmful and difficult-to-store waste products and or to boost their safety and sustainability in areas surrounding a mine site. The mining sector might be a conservative one when it comes to embracing new developments, but if bioleaching solutions can boost their cost- effectiveness and their efficiency when it comes to reclaiming metals and minerals, then they might well play a crucial role in reducing the industry’s environmental impact in the years ahead. ●
World Mining Frontiers /
www.nsenergybusiness.com
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