The unsightly mounds of rock waste surrounding most metal ore mines have long been a source of environmental concern. However, a team of UBC researchers is investigating the potential of these “mine tailings” to absorb carbon dioxide from the atmosphere, enabling individual mines to contribute to the fight against climate change.
In a natural process called carbon mineralization, rainwater and process waters carrying carbon dioxide (CO2) react with abundant silicate minerals in the tailings to form highly stable carbonate minerals, effectively trapping the CO2. This process is of particular interest to Greg Dipple, a professor in UBC Vancouver’s Department of Earth & Ocean Sciences and member of the Mineral Deposit Research Unit (MDRU).
“More than 90 per cent of the Earth’s carbon is bound in carbonate minerals,” says Dipple. “It’s where carbon wants to be, thermodynamically, but carbon mineralization occurs over geological time and is difficult to reproduce kinetically. If we can devise a way to accelerate this process, mines could use it to offset their own emissions.”
Carbon mineralization occurs most often in tailings that are rich in magnesium silicate, found at nickel, diamond, chrysotill and some gold and platinum mines. To this end, Dipple’s team is studying two active mine sites, the Diavik Diamond Mine in the Northwest Territories and Mount Keith Nickel Mine in Western Australia.
Sasha Wilson, a doctoral student in Dipple’s lab, has collected over 1,000 tailings samples from both mine sites. Working alongside postdoctoral fellow Stuart Mills, who determines the crystal structures of the minerals in each sample, Wilson quantifies the mineral content and calculates the rate of carbon uptake. Postdoc Shaun Barker then extracts and purifies the CO2 captured in each sample, and identifies its source using tracer isotopes.
“The main reservoir for modern carbon at the mine sites is the atmosphere,” Dipple explains. “Large quantities of trapped carbon in our samples are radiogenically modern, which means they didn’t come from ancient bedrock and have therefore been absorbed as a greenhouse gas from the local atmosphere.” Under a carboncounting scheme, Dipple says officials could use this fingerprinting protocol to verify the source of CO2 absorbed by a mine’s tailings, proving that it is offsetting a portion of its own emissions.
To date, Dipple’s team has shown that tailings from some mines naturally absorb up to 50,000 tonnes of CO2 a year, which is about 15 per cent of annual mine greenhouse gas emissions. The next step is to explore ways to accelerate mineral carbonation.
“We have a handful of potential acceleration strategies,” Dipple says. “Some involve scrubbing from emissions stacks and other point sources of carbon; others involve relatively straightforward aqueous geochemistry, such as changing pH or adding catalysts; and some use microbes to bind CO2. The next stage is to identify the most prospective ones and move from bench-top to field.”
Greg Dipple’s research is conducted in partnership with the Geomicrobiology Unit at the University of Western Ontario and the Research School of Earth Sciences at the Australian National University. It is funded by Diavik Diamond Mines Inc., BHP Billiton and the Natural Sciences and Engineering Research Council (NSERC).
Reprinted with permission from: Frontier, volume 6, Spring/Summer 2009