Ellen Phiddian
A team of US researchers has found a more efficient and environmentally acceptable way to extract lithium from mines and seawater – with string.
The method, which just uses cellulose thread and water, is 20 times faster than conventional extraction methods.
“Our approach is cheap, easy to operate, and requires very little energy,” says Professor Zhiyong Jason Ren, a civil and environmental engineer at Princeton University, US, and senior author on a paper describing the research, published in Nature Water.
“It’s an environmentally friendly solution to a critical energy challenge.”
One of the most common ways to mine lithium, particularly in South America, is to extract it from briny solutions on lithium-rich salt flats. This is done with massive evaporation ponds and can take months to produce a high enough concentration of lithium salts.
Then, additional chemical treatments are needed to separate out the lithium from other elements – particularly sodium, which is chemically similar to lithium.
Ren and colleagues’ method taps into capillary action: water’s tendency to move up in small spaces, because of adhesion and cohesion.
“We aimed to leverage the fundamental processes of evaporation and capillary action to concentrate, separate, and harvest lithium,” says Ren.
“We do not need to apply additional chemicals, as is the case with many other extraction technologies, and the process saves a lot of water compared to traditional evaporation approaches.”
It could even work on seawater, which has a lot of lithium in concentrations too low to be economically viable.
The new method uses a yarn made from cellulose. The researchers found 4 threads, twisted together and cut into 70cm strips, worked best.
After being soaked in pure water, the researchers hung 100 of the strings above solutions that mimicked either seawater (with a low concentration of sodium and lithium) or brine from Salar de Atacama in Chile (with higher concentrations of both elements).
The bottom 3cm of each string was immersed in the solutions, which allowed the string to soak up water via capillary action. After 5 days, with the solutions at the bottom being continually topped up, the strings were removed and dried.
Both the lithium and the sodium were soaked up onto the string with the water, but they stuck to the string at different points. Sodium crystallised lower on the string, while lithium crystallised higher.
Once dried, different sections of the string were washed with water, giving separate sodium- and lithium-rich solutions.
The researchers were able to yield liquids that were 39 times more concentrated in lithium than their initial mining brine, and 675 times more concentrated than their seawater. The strings could then be re-used.
“Our process is like putting an evaporation pond on a string, allowing us to obtain lithium harvests with a significantly reduced spatial footprint and with more precise control of the process,” says co-author Dr Sunxiang Zheng, previously a postdoctoral fellow at Princeton.
“If scaled, we may open up new vistas for environmentally friendly lithium extraction.”
Zheng is now launching a startup called PureLi Inc to develop the technology further.
“As a researcher, you know firsthand that many new technologies are too expensive or difficult to scale,” says Zheng.
“But we are very excited about this one, and with some additional efficiency improvements, we think it has incredible potential to make a real impact on the world.”
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