Engineering news
In China, and particularly in the ‘lithium triangle’ in Bolivia, Argentina and Chile, the extraction of the reactive metal is having a big environmental impact.
Vast evaporation ponds, where lithium-rich brine sits after it’s been pumped from the ground, can leak toxic chemicals into local waterways. A similar process can occur in other industries, from copper mining to oil and gas – which also seek to use the power of the sun to reduce large volumes of dirty water into small amounts of solids.
Now, American researchers have developed a system they believe could speed up the evaporation process, and reduce the amount of time that dirty materials have to impact on the environment and local wildlife.
“This is a big societal problem we're trying to solve,” says Ravi Prasher, a scientist at the Department of Energy’s Lawrence Berkeley National Laboratory. “To either dispose of the wastewater or to extract a valuable salt like lithium, you would like to increase the evaporation rate dramatically and in a scalable manner. If we could do so, that could reduce the environmental impact by reducing the amount of land required.”
Previous pushes to speed up the evaporation rate have involved structures that float on the surface of the water to localise the heat. But these structures are porous, and tend to get clogged up with the contaminants that they’re trying to separate. “”Over time, the performance of the floating absorbers drops dramatically," says Akanksha Menon, a co-author on the paper which was published yesterday in the journal
Nature Sustainability. “Sometimes the salts will get stuck on the surface and will reflect sunlight rather than absorbing it.”
The Berkeley Lab researchers set out to find a solution that could avoid such issues. “We realised if you look at the properties of water, it has very strong absorption in the mid-infrared wavelength range,” Menon says. “If you shine mid-infrared light on water, it'll absorb it so strongly it retains all of that heat in a very thin layer.”
The team built a device they liken to a "radiation transformer," which takes energy from sunlight in the range of 400 to 1,500 nanometres and converts it to 3,000 nanometres or greater, which is in the mid-infrared range.
Their photo-thermal device - a flat sheet that selectively absorbs solar energy on one side and emits mid-infrared energy on the other - sits above the water in an evaporation pond like an umbrella. "A site may have an array of these solar umbrellas, likely sitting on tent posts, about a foot or so above the water," says Menon.
In experiments in the lab, their prototype device enhanced the evaporation rate by more than 100 per cent over natural evaporation. The researchers noted that such solar umbrellas could also play a role in desalination plants, which are emerging as a solution for growing water demand around the world, but disposal of the by-product - concentrated brine - remains a problem. “If you're going to do large-scale desalination, one of the biggest challenges is how to come up with scalable technologies," Prasher says. "This is potentially is a highly scalable zero-liquid discharge technology, which doesn't require any energy because it's based on solar energy.”
Want the best engineering stories delivered straight to your inbox? The Professional Engineering newsletter gives you vital updates on the most cutting-edge engineering and exciting new job opportunities. To sign up, click here.
Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.