PNNL

Highly magnetic, black magnetite nanoparticles are functionalized with a brownish colored metal-organic framework (MOF) shell. Rare earth elements attach themselves to the MOF shell when the functionalized nanoparticles are introduced to geothermal brine. Then, a magnetic separator is used to pull the elements from the solution. For more details, see this video. Enlarge Image

Geothermal energy plants have the potential to generate enough electricity to meet the nation’s power needs many times over without emitting any greenhouse gases. Generating revenue additional to power production may accelerate geothermal deployment in the United States and worldwide.

The DOE’s Geothermal Technologies Office is exploring unique ways to extract valuable resources that are sometimes dissolved in the large fluid volumes brought to the surface by geothermal plants. Brine pumped up from underground naturally contains minerals and other metals, such as rare earth elements, that are found within the hot rock below the Earth’s surface. Extracting these valuable elements could provide geothermal energy producers with another revenue source—the sale of rare earth elements that are used in solar panels, wind turbines, batteries, and many other clean energy technologies. However, because the concentration of these elements is often very small, conventional extraction methods are simply too large, too expensive, and would degrade the efficiency of geothermal energy plants.

Looking at a Nanoscale Level

PNNL Laboratory Fellow, Pete McGrail, and his team developed a new process to extract rare earth elements from geothermal brine using magnets and a unique nanomaterial. The new method introduces magnetic nanoparticles that have a shell made of metal organic frameworks, or MOFs, into the geothermal brine. The MOF outer layer carries molecules that are attractive to rare earth elements, causing the elements to stick to the nanoparticle. The brine is then passed through a magnetic separator to remove the nanoparticles, which are then treated to extract the attached rare earth elements. The now nanoparticle-free brine is then re-injected back underground as normal in a geothermal plant. Using this method, PNNL test results showed a nearly perfect rare earth element extraction rate—99.99 percent, and attractive economics overall.

PNNL began developing and testing the magnetic partitioning method in 2014, during the first phase of the project. In 2016 and beyond, McGrail and his team will advance the technology with the help of an additional $1.65 million recently awarded by DOE. PNNL’s industrial partners for the continued project are S.G Frantz, which will design and provide a lab-scale magnetic separator for system testing, sorbent manufacturer InnaVenture, and Global Seawater Extraction Technologies, which brings practical experience in commercial extraction of minerals from seawater. For the second phase of the project, PNNL and industry partners will create a working prototype of the complete magnetic partitioning system and test it to determine the lifetime of the nanoparticles and overall efficiency of the technology. The system will be laboratory scale, but will be operated over conditions similar to those in a typical geothermal plant.

PNNL Research Team: Jian Liu, Pete McGrail, Radha Motkuri, Satish Nune, and Praveen Thallapally.