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Acid Digestion and Electrowinning Processes Demonstrate Recovery of Rare Earth Elements from Coal Fly Ash
Battelle Memorial Institute and Rare Earth Salts recovered a Lanthanum rare earth oxide at a purity of about 90% from domestic coal ash resources.

An NETL-supported collaboration demonstrated favorable results that showed potential toward developing an environmentally benign and economically sustainable process for generating rare earth element (REE) products from domestic coal ash sources, marking a step forward in enabling a domestic supply of these critical materials.

As part of an NETL-funded cooperative agreement, Battelle Memorial Institute (BMI) and Rare Earth Salts (RES) worked together to advance the development and validation of BMI’s acid digestion process, along with RES’s novel electrowinning separation and purification process.

Acid digestion is a method of making metals easier to separate by first dissolving a coal ash sample into solution by adding acids and heating it until the metals break away from the other undesired materials.  Electrowinning is a process in which metal ions present in a solution are separated using a direct current.

BMI successfully scaled up their acid digestion process to increase the concentration of mixed REE materials and provide enough material for RES’s facility.  In solution with the REEs were zinc and aluminum. Aluminum was easily removed whereas the high quantity of zinc presented a challenge in the separation and purification process.
To overcome this challenge, BMI selected a new extractant composition selective for zinc over the REEs prior to the traditional solvent extraction steps as a pretreatment to remove zinc. 

With the zinc interference minimized, RES’s electrowinning process can recover a concentrated rare earth oxide (REO) product. BMI and RES delivered a sample of Lanthanum Oxide with a purity of approximately 90% to NETL, successfully meeting the objectives of the project and the goals of NETL’s Rare Earth Element program.  A purity of 90% or greater REO renders the material suitable for further processing into a pure metal form for subsequent incorporation into commodity or national defense products.

“As we work to develop a domestic source of REEs immune to global market disruptions, several methods of extracting these elements have been investigated,” NETL Project Manager Jason Hissam said. “The methods might differ, but the overall challenge remains the same: Find a way to do it in an economically feasible and environmentally benign manner. While there’s still a lot of work ahead, this project is a step in the right direction.”

A subsequent techno-economic assessment indicated the economics of the process show promise and the technologies investigated demonstrated merit for further investigation and improvement.

The U.S. Department of Energy’s National Energy Technology Laboratory develops and commercializes advanced technologies that provide reliable and affordable solutions to America's energy challenges. NETL’s work supports DOE’s mission to advance the national, economic, and energy security of the United States.