A breakthrough in solar evaporation research led Monash engineers to find a better way to extract lithium from brine, using a combination of solar evaporation, selective dissolution and solvent recycling.
There is nothing terribly wrong with current lithium extraction processes.
Salty groundwater from underground salt flats is pumped into vast evaporation ponds. Over a period of 1-2 years, the sun evaporates the water and the brine is transported between ponds, eventually leaving concentrated salts.
A series of chemical and physical processes are then performed on the brine to remove unwanted minerals. The resulting lithium-rich solution is processed in a plant to produce lithium carbonate or lithium hydroxide.
The only problem is that it can be done more quickly, efficiently and sustainably, according to Professor Huanting Wang of Monash University’s Department of Chemical and Biological Engineering.
Lithium recovery rates from such ponds varies from 30-70 per cent, and the process is slow and water-intensive.
“On average, half of the lithium will be lost during the solar evaporation,” Wang told create. “The production cycle can total two years.”
In 2011, Wang and his team were working on ways to accelerate the evaporation process. By placing a film of photothermal material across the surface of the water in the evaporation pond, such as a black-coloured polymer foam or carbon, the surface level of the water was heated more quickly and evaporation was accelerated, they discovered.
If evaporation of a pond typically took 12 months, such a film could reduce that time by a third.
“To start this project, I asked a student to use photothermal material to evaporate organic solvent,” Wang said. “We didn’t really think that this can be used to extract lithium.
“Then the student got the idea to use this for other things. An obvious example was lithium extraction.”
By leveraging differences in solubility between lithium salts and coexisting salts, the team discovered, the post-evaporation separation process can be more efficient, without the need for freshwater inputs.
READ: Could this membrane technology replace traditional lithium extraction methods?
How the new process works
By evaporating water out of the brine, potentially also collecting purified water in the process, a solid, mixed salt product is created.
Then, using common solvents such as ethanal and acetone, the lithium-containing salts are selectively dissolved, leaving most of the unwanted salts behind.
The process requires very little fresh water, can run almost entirely on solar power – apart from a potential, low-energy requirement for pumps – and recovers around 95 per cent of lithium.
It removes impurities such as boron and sulphate and recycles over 99 per cent of the solvents in the process.
READ: Are the days of lithium-ion batteries over? Here are some alternatives
What might a commercial plant look like?
The process begins with solar evaporation accelerated by photothermal materials, requiring a pond which, Wang said, could be enclosed if the production of purified water was a desired output.
Then, Wang said, two other tanks are required. In the first closed tank, industrial ethanol is added to the solid salts to selectively dissolve the lithium-containing salts. The ethanol is subsequently evaporated and condensed back in the closed system for recycling.
In the other, the acetone-ethanol mixed solution dissolves the resulting batch of solid salts, producing a high-purity lithium salt that is then able to be processed into whatever form is required.
Of course, this has only been conducted so far in a controlled, laboratory environment that included a solar simulator lamp above a miniaturised pond, enclosed in glass and covered with a layer of carbon as the photothermal material.
Scaling this process up for commercialisation will involve additional challenges, Wang said, including engineering the large-scale evaporation and condensation systems required for solvent recovery.
Work is also required around the varied capability of a commercial plant during periods of poor weather, tolerance levels for remaining, unevaporated water in the salt solids and the potential impact of a greater level of impurities in real brine, as opposed to simulated brine used in the lab.
Successful laboratory results don’t necessarily guarantee commercial success, Wang said.
“A lot of processes that work very well in the lab may not be compatible at all in the real world,” he said. “That’s the reality, and that will be the big challenge for engineers.”
What is the final, engineered goal?
The end goal is not the replacement of evaporation ponds, Wang said.
Instead, it is the development of a better way of doing things, as the economy’s hunger for lithium continues to increase.
“We just want to make the process simpler and more efficient. What we’re trying to do is engineer greater sustainability into an essential process.”
The research paper Precision Dissolution of Salts for Solar-Driven Lithium Extraction from Brines, by Pan Liu, Zhikao Li and Huanting Wang, has been published in Environmental Science & Technology.
EVENT: Register for the 44th Hydrology and Water Resources Symposium
