The lightest metal is in heavy demand, thanks to the ever-growing market for cell phones, electric cars, and other products that rely on lithium-ion batteries. Experts debate whether the supply of lithium can keep up with this demand.
A newly improved sorbent could offer an environmentally friendly way to get lithium from a relatively untapped resource in the U.S.: the brine produced by geothermal power plants (Environ. Sci. Technol. 2017, DOI: 10.1021/acs.est.7b03464). These plants pump hot water from deep geothermal deposits and use it to generate electricity, leaving behind a salty solution that can contain hundreds of parts per million lithium. Passing the brine through a sorbent that captures the lithium ions before pumping the brine back underground could collect the metal without the heavy environmental impacts of typical extraction methods.
Brines from salt lakes in South America supply much of the world’s lithium; harvesting it in evaporation ponds can take a year or more, leaving behind lots of salt waste. The element is also extracted from the mineral spodumene, which must be mined, heated at high temperatures, and treated with acid.
Sorbents are a promising alternative for collecting lithium from sources where it is less abundant, like brines. The sorbent in the new study is made by intercalating aluminum hydroxide, or gibbsite, with lithium chloride to produce layers of [LiAl2(OH)6]+ with chloride ions and water in between them. Voids in the structure can fill with additional lithium ions, but are too small to let in competing cations like sodium—which is about 100 times as abundant as lithium in brines—and potassium.
A now-defunct company called Simbol Materials developed an earlier version of the sorbent about 10 years ago and built a pilot plant based on the technology near California’s Salton Sea geothermal area, which has more than 10 geothermal power plants. Elon Musk’s company Tesla reportedly offered to buy the company for $325 million in 2014, but the deal never went through. Soon afterward, the company shut down operations. Simbol’s assets were purchased by Alger Alternative Energy (AAE) in 2016.
Meanwhile, Parans Paranthaman of Oak Ridge National Laboratory and his colleagues, as part of the U.S. Department of Energy-funded Critical Materials Institute, have refined the sorbent with Simbol’s former chief technology officer, Stephen Harrison, to improve its capacity and selectivity for lithium.
To test the improved sorbent, the researchers poured a simulated brine solution containing lithium, potassium, sodium, boron, calcium, and manganese through a column containing the sorbent. To collect the lithium trapped on the sorbent, they rinsed it with a dilute solution of lithium chloride. The researchers captured 91% of the initial lithium in the brine. They calculated that the method was 48 and 212 times as selective for lithium as for sodium and potassium, respectively, and could be done in less than two hours on an industrial scale.
Other lithium-harvesting sorbents that have been tested include delithiated manganese oxides and layered hydrogen titanates. York R. Smith, a metallurgical engineer at the University of Utah who was not involved in the study, says some of the highest performing ones adsorb 30 to 90 mg Li/g sorbent; the ORNL team’s sorbent has a capacity of about 6 mg Li/g sorbent, Paranthaman says. However, these other sorbents usually require acid treatment to release the sorbed lithium, generating waste. Smith says the researchers’ use of lithium chloride instead of acid is very attractive from an environmental perspective.
The process still needs to be refined to produce commercial-grade lithium chloride that is free of impurities, Paranthaman says. And for use in batteries, that lithium chloride must be turned into lithium hydroxide or lithium carbonate.
AAE, which plans to soon change its name to MEK Lithium, is designing a commercial lithium extraction facility that will use the sorbent near the Salton Sea, with the goal of producing 15,000 tons of lithium carbonate equivalent per year by 2020, Harrison says. Global production of lithium carbonate is approximately 186,000 tons per year, and the entire Salton Sea geothermal area could produce up to 120,000 tons per year of lithium carbonate equivalent, the researchers estimate.