Injecting carbon dioxide into rocks deep underground can turn the planet-warming gas into mineral form, permanently keeping it out of the atmosphere. Now, researchers say it may be possible to use the same process to simultaneously extract key metals used in clean electricity technology.
鈥淏y doing this surgical mining and doing things deep down-hole, we鈥檙e able to keep what we don鈥檛 want down there, and bring up the stuff we want,鈥� says at Pacific Northwest National Laboratory (PNNL) in Washington state.
Miller and his colleagues became interested in this idea after finding that injecting CO2 into some types of igneous rocks released nickel, cobalt and other metals, while also mineralising the CO2. They reasoned it may be possible to then extract those metals by injecting chemical compounds that bind selectively to them. The resulting metal-rich brine could聽be pumped out via a separate well, and purified on the surface.
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They have since used laboratory experiments and some field tests to investigate how this reaction could work in actual rock formations around 800 metres underground, by the US Department of Energy鈥檚 Advanced Research Projects Agency-Energy (ARPA-E) on carbon-negative mining. , also at PNNL, says these tests show the basic chemistry works as expected, and they are now moving on to test the extraction process in actual wells. 鈥淲e are starting to do some geophysical monitoring and do some basic injections,鈥� he says.
Most of their work has focused on ultramafic igneous rocks, which are rich in olivine 鈥� a mineral containing nickel and cobalt 鈥� as well as the magnesium and calcium that mineralises CO2. These ultramafic rocks are the main component of Earth鈥檚 mantle and occur near the surface in numerous regions around the world.
Based on their lab tests, the researchers estimate the approach could yield huge volumes of metal. According to what they say are conservative estimates, a single cubic kilometre of ultramafic rock could produce half a million tonnes of nickel and 21,000 tonnes of cobalt, while permanently storing 100 million tonnes of CO2. When accounting for the emissions generated during the process, that would amount to more than 200 kilograms of net carbon removal for every kilogram of nickel produced, they estimate 鈥� making the nickel mining carbon-negative.
Both nickel and cobalt are key ingredients in current battery technologies, and rising demand for the metals has driven a surge in environmentally destructive mining in many parts of the world. Restricted supplies have also spurred research on new ways of procuring these minerals, including surprising ideas like harvesting them from metal-accumulating plants and seaweed.
at Laval University in Canada and his colleagues have demonstrated that a similar approach can use left over from previous mining operations, and he thinks the new idea to聽try it out underground is promising. However, he says it will be challenging to get enough of the olivine to react with CO2 in a deposit of solid rock underground. Extracting low concentrations of metal from the brine once it is pumped back aboveground could also prove difficult, he says.
Another difficulty could be figuring out how to extract the metal brine without pumping out the CO2 they are trying to store along with it, says at the University of Guelph in Canada. And while he says their general estimates of the potential check out, real rocks may prove more varied聽in quality. 鈥淚 doubt that this is technology that you can use everywhere,鈥� he says.
EarthArxiv
