Mine tailings soak up greenhouse gas
Researchers have shown for the first time that tailings, the mountains of rubble left after mining for nickel and asbestos, absorb significant amounts of CO2. The discovery, made at mines in Québec, British Columbia, the Yukon Territory, and Australia, could help mines earn credit for what is considered the most permanent form of greenhouse gas sequestration.
“We were examining tailings to characterize them as a feedstock for an industrial reactor for CO2 sequestration but found that the tailings were already reacting directly with the atmosphere,” says Greg Dipple, a geologist at the University of British Columbia. The surprising discovery, presented November 23 at the Yukon Geoscience Forum in Whitehorse, came after scientists used isotopic tracers to show that atmospheric CO2 was the precursor of the carbon-containing compounds in the rocks.
The sequestration reaction requires water, which comes from rainfall and ponds in which tailings are sometimes stored, Dipple says. Atmospheric CO2 forms carbonic acid in water, and as the water evaporates it reacts with metallic cations, such as magnesium(II), to precipitate stable carbonate minerals, he explains. Serpentine-group minerals, found in tailings from white asbestos (chrysotile) and nickel mines, are the most abundant and widespread source of magnesium cations, adds Grant Abbott, chief geologist at the Yukon Geological Survey.
Because mineral carbonation was so prevalent in the tailings of one mine, Dipple estimates that the mine may be fixing tens of thousands of tons of CO2 per year. Mining companies could boost carbon fixation at the mine to roughly 1 million tons per year by introducing more water, lowering the pH, and increasing the surface area of the tailings, he speculates.
If costs can be lowered to about $8–10 per ton, mining companies could set up reactor sites that could absorb nearly 1 billion tons of CO2 per year, which is one-third of the global carbon sequestration target set by policy makers, Dipple says. Work is under way to determine how to accelerate carbon sequestration for the lowest cost, but field experiments are still a couple of years away, he adds. However, Abbot warns, “Mineral carbonation sounds promising as a form of carbon sequestration, but it all depends on the CO2 credit market.”
