A world where Mother Nature extracts in-demand metals from the ground in cost-effective and sustainable ways is being touted as a major value adder to conventional mining methods.
In the last decade, researchers worldwide have discovered a number of “super plants” capable of absorbing metals from land previously used for mining operations, through a process called phytomining.
Of the planet’s 320,000 recognised plant species, around 700 have been dubbed “hyperaccumulators” for their ability to thrive in metal-rich soils without suffering phytotoxic (or poisonous) effects.
These species feed on mining waste normally stored in tailings facilities and often contain remnants of valuable metals such as cobalt, nickel, zinc and gold.
They draw in and accumulate high concentrations of these metals and other elements from the ground in their biomass as they grow.
Some species of hyperaccumulators can retain up to 1% cobalt or 4% nickel in their shoots, which translates to more than 25% metal when the shoots are harvested and incinerated to ash (known as “bio-ore”).
Researchers are now on a quest to discover whether widespread farming of hyperaccumulators could provide an alternative to conventional and environmentally destructive mining methods while helping to rehabilitate former mine sites.
If proven, the concept has potential to remedy the mining industry’s biggest problem of polluted waterways from abandoned mines.
Studies have shown hyperaccumulators planted on a closed mine site are capable of salvaging the remaining metals deep beneath the ground’s surface over time, generating additional revenue.
The idea could be enough incentive to persuade mining companies to invest in rehabilitation or mine waste cleanup.
At the University of Queensland’s Sustainable Minerals Institute in Australia, Associate Professor Peter Erskine and his research team have been involved in the development of phytomining technology at the institute’s Centre for Mined Land Rehabilitation.
The team has discovered that a field of one type of hyperaccumulators planted across a nickel-rich site like a mine tailings dam could potentially yield up to 300kg of nickel per hectare every year, meaning the critical metal used in rechargeable batteries and stainless steel manufacturing could be harvested rather than mined.
Professor Erskine said the process – when implemented at a large scale – could become a sustainable option for the mining of rare metals and the transition from carbon-fuelled mining.
“We already know Queensland is home to native plants that have this ability to absorb metal [and] we are currently growing plants using metal-rich soil and tailings from around [the state],” he said.
“Our work has the potential to unlock a sustainable stream of critical metals, including from mine wastes and tailings, that still hold residual metals of interest.”
Advancing the potential for plants to pull metals out of the ground received approval from the Queensland government earlier this month, when it announced it would invest $1 million into a joint phytomining study with the Sustainable Minerals Institute.
Funded under the state’s $23 million New Economy Minerals Initiative, the “world-first” study will assess the ability of native plants such as selenium weed and a variety of macadamia trees to become hyperaccumulators.
If successful, it could supply sustainably-sourced rare earth metals with low environmental impact and potentially change how some mines operate in the future.
Professor Erskine believes this will be “looked upon favourably around the world” and enhance Queensland’s position in the critical metals market.
“Phytomining could be used to access unconventional resources that are not viable using existing mineral processing techniques,” he said.
“This includes mine wastes such as tailings which still hold residual metals of interest; in effect, phytomining could turn waste into new resources.”
The joint study will run for four and a half years and include testing of rare earth element-rich material from the Phosphate Hill mine and closed Mary Kathleen mine near Cloncurry, and the Peak Range in central Queensland.
A series of experiments will follow to establish optimal conditions, the range of application and limitations for the identified plant species and different rare earth-rich material types.
Professor Erskine said he is confident the phytomining of nickel could quickly proceed to full-scale production and phytomining of cobalt, thallium and selenium is on the horizon.
Malaysian “metals farm”
That forecast is supported by Professor Erskine’s colleague and plant specialist Antony van der Ent, whose thesis work in 2015 spurred the establishment of a proof-of-concept “metals farm” in Malaysia’s Kinabalu Park.
The farm comprises four acres of 20-foot-tall, leafy-green shrub, tended to by local villagers with the aim of demonstrating that hyperaccumulator trees can be used to mine metals.
Every few months, the villagers shave off about a foot of growth from the plants and burn the crop to produce the ashy bio-ore.
“We can now demonstrate that metal farms can produce between 150kg to 250kg of nickel per hectare [170 to 280 pounds per acre] annually,” Mr van der Ent said.
“At the midpoint of that range, a farmer would net a cool US$3,800 per acre of nickel at today’s prices which is on par with some of the best-performing agricultural crops on fertile soils, while the operating costs are similar.”
Mr van der Ent said he was planning to scale-up the Malaysia trial to nearly 50 acres, which would call for the application of an industrial-scale hydrometallurgical plant to separate the target metal from its ore via a water-based medium.
“They won’t have to manually burn the crop as they are doing now, meaning the process will be carbon negative, as opposed to carbon neutral,” he said.
Supporters of phytomining see the greatest potential in Indonesia and the Philippines, which are two of the world’s biggest nickel ore producers.