Recovering REEs from Coal Fly Ash Using Acid Digestion and Electrowinning Processes

Battelle Memorial Institute (BMI) and Rare Earth Salts (RES) demonstrated that developing an environmentally benign and economically sustainable process for recovering rare earth element (REE) products from domestic coal ash sources is possible. A part of a National Energy Technology Laboratory (NETL)-funded cooperative agreement, these results mark a significant step forward in developing domestic sources of supply for REEs, which remains a priority for the U.S. Department of Energy (DOE).

BMI and RES worked together to advance the development and validation of BMI’s acid digestion process, along with RES’ 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 a 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 the RES facility. Zinc and aluminum were in the solution with the REEs. While it was easy to remove aluminum from the solution, it was difficult to remove the large quantity of zinc during the separation and purification processes. 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’ electrowinning process recovered a concentrated rare earth oxide (REO) product. BMI and RES delivered a sample of lanthanum oxide with a purity of approximately 90 percent to NETL; the sample met both the objectives of the project and the goals of NETL’s REE program. For reference, an REO purity of 90 percent or greater 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,” explained NETL’s Project Manager Jason Hissam. “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 that the economics of the process show promise and that the technologies investigated demonstrated merit for further investigation and improvement.

As a DOE National Laboratory, NETL produces technological solutions for America’s energy challenges. NETL research is providing breakthroughs and discoveries that support domestic energy initiatives, stimulate a growing economy, and improve the health, safety, and security of all Americans. Highly skilled men and women at NETL’s sites in Albany, Oregon; Anchorage, Alaska; Houston, Texas; Morgantown, West Virginia; and Pittsburgh, Pennsylvania conduct a broad range of research activities that support DOE’s mission to advance the national, economic, and energy security of the United States.