INNOVATION-May-June-2020

F E A T U R E

I n central British Columbia this summer, researchers will begin field trials at what could become Canada’s first carbon-neutral mine. Tonnes of mine tailings (waste rock) is generated as a by- product of extracting valuable metals and minerals from the Earth. Moving, storing, and rehabilitating this waste material is a major challenge for mine planners and operators: it is costly and requires careful management to reduce any impact on the environment. But new research out of BC is developing a method to put this material to good use. Researchers working on the Carbon Capture, Utilization, and Storage in Mine Tailings project are testing how a certain type of ultramafic rock found in tailings, when crushed and exposed to air, reacts naturally with carbon dioxide to form stable, inert carbonate minerals, providing safe, long-term storage for excess CO 2 from the atmosphere. A UBC student tests a CO 2 sensor on a field pilot demonstration of CO 2 injection into tailings at the De Beers Gahcho Kué Mine in Northwest Territories, in July 2019. The six-metre-long pipeline was filled with tailings and used as a flow-through reactor, to assess the effectiveness of tailings for capture and mineralization of CO 2 . P hoto courtesy of D e B eers G roup .

“We estimate that reacting just 10 percent of a mine’s waste stream could be more than enough to offset the annual carbon emissions produced by a mining operation,” said UBC Professor Gregory Dipple, director of the Bradshaw Research Initiative for Minerals and Mining (BRIMM), who is leading the research. ULTRAMAFIC ROCK: THE ULTIMATE SOLUTION For more than a decade, researchers have been examining how a certain rock type—serpentinized ultramafic rocks—can sequester carbon in mine tailings. These magnesium-rich rocks originate in the Earth’s mantle, tens of kilometres below the surface. Over millions of years, the rock moves up through the crust, undergoing physical and chemical alterations. Under specific conditions, when magnesium is present and carbon dioxide is absent, the magnesium hydroxide mineral brucite forms in these ultramafic rocks. This mineral plays a key role in sequestering carbon when the rock eventually makes it to the surface: brucite naturally consumes carbon dioxide from rainwater, groundwater, and air to form a solid, stable magnesium carbonate mineral. Normally, this natural weathering reaction happens very slowly, but in mine waste where certain ultramafic rocks are crushed and exposed to the air, it becomes highly reactive. “Reactions that normally take tens to hundreds of thousands of years can happen very quickly,” said Dipple. Ultramafic rocks are not all the same. When they first take shape, they contain only trace amounts of water and CO 2 . As they move through the crust, they can be deformed and sheared. Water can flow through them and they get hydrated and altered. “Water gets added to the rock in a process called serpentinization,” said Dipple. “The serpentinization generates the minerals that are highly reactive to CO 2 . We need that serpentinization to have a good prospect.” However, a second stage of alteration, called carbonate alteration, can destroy CO 2 reactivity. If the rock encounters fluids that contain carbon dioxide during deformation in the crust, the hunger for CO 2 is satisfied before they reach the surface. “So, we have the little Goldilocks zone that needs to be serpentinized but not carbonated,” said Dipple. In BC, the goals of the project are twofold: to find ways to maximize the carbon-consuming reaction in this type of ultramafic rock in a real-world setting, and to map the locations of serpentinized ultramafic rocks

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