Sustainability
$7bn in compensation to those affected. Recently, there have been attempts to reduce reliance on water- based tailings storage, as elements of the industry turn to dry-stacking, which can reduce a mine’s water consumption and removes any chance of a catastrophic flood or other long-term storage issues. Yet, up until recently, this method wasn’t seen as cost-effective at scale, as the act of removing water from tailings for storage could be quite expensive.
Even if the industry stops requiring new water-based tailings storage facilities, the legacy tailings already in existence would continue to pose a problem. However, advances in bioleaching technology – where valuable metals are extracted from low-grade ore through the use of microorganisms – offer new ways to treat and remove the harmful material in fresh and legacy tailings, and to aid the reclamation of leftover metals from a mine’s waste products, boosting profitability and efficiency.
270
The number of deaths as a result of the
Brumadinho tailings dam disaster in January 2019.
BBC 36
Speeding up natural processes On the ground floor of this burgeoning space in mining is BacTech, an environmental technology company that specialises in bioleaching and remediation solutions. Its focus is on the processing and recovery of valuable metals like gold, silver, cobalt and copper, while transforming harmful contaminants like arsenic into benign products that can then be safely disposed of. BacTech makes use of naturally occurring bacteria that is harmless to both humans and the environment to neutralise resource-rich mining sites – boosting both the environmental and economic situation through comprehensive metal recovery. These bacteria are typically microbes that are found to thrive naturally in tailings ponds – the resource-rich sites in question – capable of surviving in those hostile environments. “Our tagline for the company is ‘Our bugs eat rocks’ because that was the easiest way to explain it to people,” notes Ross Orr, president and CEO of BacTech, with a laugh. “At the conferences we go to, that’s all we have on our booth – and sure enough, it does attract people.” To help explain how BacTech’s solutions function, Orr compares the process to a brick wall, where the bacteria target the mortar and separate metals and minerals into their core components. In the company’s bioleaching process, ore concentrates are continually fed into the process, and, over a period of six days, the concentrate moves through a series of tanks, agitation and fertilisers to ensure that the bacteria function at a very high level. The concentrate is then broken down from what is referred to as an ‘arsenopyrite mineral’, with the iron and arsenic dissolving in the solution while the gold and silver remain as a solid, though are not yet amenable to conventional gold recovery. “The sulphur would be the mortar I was talking about,” Orr explains, hearkening back to his brick wall
comparison. “The acidic environment in the tanks is between 1.5–1.7pH, which ensure that any base metals or other metals that dissolve in acid, like arsenic, do so, and anything that doesn’t, like gold and silver, go in a separate direction at the end of the process.” The next step is a solid/liquid separation where the gold and silver that have been liberated, go for conventional recovery to create a gold or silver doré bar for sale. The liquid now contains diluted base metals and other elements such as arsenic and iron, and is treated with limestone to raise the pH level. As the pH rises, ferric arsenate is precipitated from the liquid as gypsum – a benign form of arsenic that is dewatered and dry-stacked. “I like to say we’re the only nuts that go looking for arsenic, but it’s a big market,” adds Orr. In Sudbury, BacTech is involved with a consortium to investigate the reprocessing of up to 100 million tonnes of pyrrhotite tailings that have been deposited in lakes around the area. Pyrrhotite is a sulphide mineral that is made up iron (60%), nickel, cobalt and elemental sulphur. BacTech aims to sell the elemental sulphur removed from the process to the sulphuric acid industry, just as the iron extracted is sold to the steel industry. Beyond simply reclaiming valuable metals from waste products, however, applying bioleaching processes to pyrrhotite minerals also has the added benefit of destroying harmful sulphides. “You’re eliminating potential acid mine drainage issue or acid rock issues, because you’re oxidising all of the sulphides,” Orr explains. “That’s why tailings are of interest to us. Because the stuff that was missed in the first go-around, through flotation, is the stuff in tailings that’s causing all the problems – eventually oxidising on their own just through nature and producing acid water [that can then leak] into the rivers.”
While a number of large companies have invested in bioleaching, such as Goldfields and Glencore, Orr has found that the mining industry as a whole has been slow to embrace the technology. “I’m constantly asked, ‘Why isn’t everybody using bioleaching?’” he notes. There are a couple of factors for this, however – the first being that bioleaching is still in its nascency in some ways, and can offer a slightly less effective recovery rate when it comes to gold, say, than traditional methods in roasting and smelting. For operations that might process half a million or so ounces of gold a year, a few percentages less in your gold recovery rate could mean tens of millions of dollars in lost revenue.
“Miners, if nothing else, are very good engineers and very cost-effective,” says Orr. “So, they tend to sort of plug their nose and stick to the conventional.” The advantage of bioleaching compared to traditional methods of recovery, however, is that it has a much lower environmental impact, and as the world continues to drive the importance of environmental,
World Mining Frontiers /
www.nsenergybusiness.com
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