Trends in the Rare Earth Element Content of U.S.-Based Coal Combustion Fly Ashes

Rare earth elements (REEs) are critical
and strategic materials
in the defense, energy, electronics, and automotive industries. The
reclamation of REEs from coal combustion fly ash has been proposed
as a way to supplement REE mining. However, the typical REE contents
in coal fly ash, particularly in the United States, have not been
comprehensively documented or compared among the major types of coal
feedstocks that determine fly ash composition. The objective of this
study was to characterize a broad selection of U.S. fly ashes of varied
geological origin in order to rank their potential for REE recovery.
The total and nitric acid-extractable REE content for more than 100
ash samples were correlated with characteristics such as the major
element content and coal basin to elucidate trends in REE enrichment.
Average total REE content (defined as the sum of the lanthanides,
yttrium, and scandium) for ashes derived from Appalachian sources
was 591 mg kg–1 and significantly greater than in
ashes from Illinois and Powder River basin coals (403 and 337 mg kg–1, respectively). The fraction of critical REEs (Nd,
Eu, Tb, Dy, Y, and Er) in the fly ashes was 34–38% of the total
and considerably higher than in conventional ores (typically less
than 15%). Powder River Basin ashes had the highest extractable REE
content, with 70% of the total REE recovered by heated nitric acid
digestion. This is likely due to the higher calcium content of Powder
River Basin ashes, which enhances their solubility in nitric acid.
Sc, Nd, and Dy were the major contributors to the total REE value
in fly ash, based on their contents and recent market prices. Overall,
this study shows that coal fly ash production could provide a substantial
domestic supply of REEs, but the feasibility of recovery depends on
the development of extraction technologies that could be tailored
to the major mineral content and origins of the feed coal for the
ash.