Science - USA (2020-09-04)

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1158 4 SEPTEMBER 2020 • VOL 369 ISSUE 6508 sciencemag.org SCIENCE


2014, Wilson and colleagues analyzed mine
tailings from the Mount Keith nickel mine
in Western Australia and found that the
mine’s 11 million tons of tailings produced
each year spontaneously react with CO 2 ,
locking up about 40,000 tons of the gas.
That’s equivalent to about 11% of the CO 2
output from the mine’s operations.
Still, weathering is slow, and most al-
kaline wastes wind up either buried or
submerged in ponds, and thus aren’t ex-
posed to air. “It’s a matter of getting those
reactions to happen at a faster rate,” says
Alison Shaw, a geochemist with Lorax En-
vironmental Services who heads De Beers’s
research on mineralizing CO 2.
At Gahcho Kué, Dipple and his students
tested a way to speed up the process. The
mine’s tailings include a wet, siltlike slurry
and dry, sandlike grains. Dipple and his
students packed a 6-meter-tall column
with the greenish slurry and sprayed water
on 1 cubic meter of the sand. With both
their slurry and dry wastes, they bubbled
in a mix of gases—10% CO 2 and 90%
nitrogen—that matched the exhaust from
the local diesel power plant that powers
the mine.
The waste soaked up CO 2 for as long as
44 hours, they found, converting it into
minerals. The newly made magnesium car-
bonate minerals acted like glue, solidifying
the previously free-flowing tailings, much
like sand turned to sandstone. Most im-


portant, the waste took up CO 2 200 times
faster than it did through natural weather-
ing, Dipple says.
This summer, he was set to return to
Gahcho Kué to scale up the tests and use
actual diesel exhaust. But those tests are on
hold because of the coronavirus pandemic.
De Beers has funded other projects
around the world, Shaw says. For example,
Wilson’s team is exploring whether dilute
acids speed up weathering. Lab studies sug-
gest the acids could leach magnesium out
of mine waste, making it available to react

with CO 2. Another project, led by Gordon
Southam of the University of Queensland,
St. Lucia, is adding cyanobacteria to the
mix. These photosynthetic bacteria capture
CO 2 from the atmosphere, and lab studies
have shown they speed carbon mineraliza-
tion. If these efforts work, Shaw says, they
could repair mines’ reputation as environ-
mental blights, making them part of a solu-
tion to climate change. Anglo American, De
Beers’s parent company, has announced it
wants to harness alkaline wastes to create
the first carbon-neutral mine by 2040.

DIAMOND MINES aren’t the only places
where such studies are underway; another
is the Woodsreef chrysotile mine in New
South Wales in Australia. (Chrysotile is a
form of asbestos that is still widely used
in building materials in some parts of the
world.) Wilson and her colleagues sprayed
the mine’s ultramafic rock tailings with
dilute sulfuric acid, causing magnesium
to leach out. The alkaline tailings then
neutralized the acid and locked up CO 2
that was bubbled through, as much as
100 times faster than normal weathering.
Jennifer Wilcox, a chemical engineer at
Worcester Polytechnic Institute, and her
colleagues are pursuing a related strategy
at the Stillwater nickel mine in Montana.
“The tailings are not particularly reactive,”
she says. But CO 2 is mildly acidic; bub-
bling it through the tailings helps release PHOTO: CHIP CHIPMAN/BLOOMBERG/GETTY IMAGES

Bricks and mortar
Small companies in a nascent industry are using
carbon mineralization to capture carbon dioxide (CO 2 )
in construction materials.

COMPANY PRODUCT
CarbonCure Technologies Concrete
Solidia Technologies Concrete
CO 2 Concrete Concrete
Carbicrete Concrete
Cambridge Carbon Capture Fire-retardant materials
Mineral Carbonation
International

Cement and plasterboard

O.C.O. Technology Construction aggregate
Blue Planet Construction aggregate
Orbix Construction aggregate

At a platinum and palladium mine in Montana, a technician stirs a slurry of waste rock, some of the 2 billion tons of alkaline waste generated each year.


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