In work for DoE's Critical Materials Institute, scientists at ORNL are working to refine a sorbent that can more effectively recover lithium salts from concentrated brines at geothermal plants, which pump hot water from geothermal deposits and use it to generate electricity. Concentrated brines left over from the operation are then pumped back into the ground.

Those brines can contain as much as 250 to 300 parts-per-million lithium. By some estimates, as much as 15,000t per year of lithium carbonate could be recovered from a single geothermal power plant in the Salton Sea area of California—one of the most mineral-rich brine sources in the US. There are currently 13 geothermal plants in the region and more are planned.

ORNL and its research partners are working to improve the capacity and selectivity of a sorbent that could extract the lithium from these brines. The lithium-aluminium-layered double hydroxide chloride (LDH) sorbent they are developing is a low-cost, reusable option for large-scale industrial plants.

The LDH sorbent is made up of layers of the materials, separated by water molecules and hydroxide ions that create space, allowing lithium chloride to enter more readily than other ions such as sodium and potassium. After the sorbent loads with lithium chloride, it is selectively washed to remove unwanted ions, and then to unload the remaining lithium chloride.

In a bench-scale demonstration, the LDH sorbent recovered more than 91 per cent of lithium from a simulated brine.

ORNL scientists recently used inelastic neutron scattering to explore the structure of different variants of the sorbent. It allowed researchers to probe deep into the material and explore the ordering of water molecules between the sorbent's layers, providing information on the material's stability and its lithium recovery efficiency. The work was performed on the VISION instrument at the Spallation Neutron Source—a DoE Office of Science User Facility at ORNL.

The sorbent's thermochemical properties were also characterised using differential scanning calorimetry and thermogravimetry at the University of California-Davis. The tests explored how the ordering of water molecules in the sorbent has a direct impact on the material's stability and effectiveness. The scientists confirmed that by replacing some of the aluminium with iron in the sorbent, the material is made more thermodynamically stable and can be used as an alternative sorbent for extracting lithium.

By putting the sorbent through these paces, "we get valuable information on how water is accommodated in the sorbent's layers and how that dictates separation and stability," said Parans Paranthaman, principal investigator for the project and leader of the Materials Chemistry Group at ORNL.

"With a better understanding of the molecular structure and behaviour of the material, we can create sorbents with greater throughput that could reduce the size and cost of plant construction, for instance, or develop variants that would work with lower-temperature brines," added Bruce Moyer, a project team member and leader of the Chemical Separations Group at ORNL. "The more versatile the sorbent is, the more options there are for industry to supply the lithium we're going to need for energy storage."

ORNL scientists are also working on a membrane to concentrate the geothermal brines before they are exposed to the sorbent, which increases the efficiency of the process. The next steps are to scale up the process and run tests that simulate real-world conditions, the researchers noted.

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