RECOVERY OF RARE EARTH ELEMENTS FROM COAL FLY ASH USING IONIC LIQUIDS
Laura M. Stoy
Dr. Ching-Hua Huang
Dr. Susan Burns, Dr. Sotira Yiacoumi, Dr. Yuanzhi Tang, Dr. Arthur Ragauskas
August 19, 2021, 1:00 pm - 4:00 pm Eastern
Virtual- Blue Jeans link to be provided
Rare earth elements (REEs), the 15 lanthanides and Sc and Y, have played an invaluable role in the progress of clean energy technology
and high-tech manufacturing in past decades. Their high demand and global scarcity have led to disruptions in supply, exacerbated by the
fact that there are no adequate replacements. Thus, it is crucial to develop alternative sources to secure a steady supply of REEs. Coal fly ash
(CFA), a byproduct of burning coal for electricity, may be one such source. Conventional REE-CFA recovery methods are energy and
material intensive and leach elements indiscriminately, generating impure mixtures of REEs. Ionic liquids (ILs) may be one solution, but to
date, they have not been applied to CFA.
This dissertation focuses on the IL betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]) for preferential extraction of REEs from
different CFAs. Efficient extraction relies on [Hbet][Tf2N]'s thermomorphic behavior with water: upon heating, water and the IL form a
single liquid phase, and REEs are leached from CFA via a proton-exchange mechanism. Upon cooling, the water and IL separate, and
leached elements partition between the IL and aqueous (AQ) phases. REEs were preferentially extracted over bulk elements from CFAs into
the IL phase then recovered in a subsequent mild acid stripping step, regenerating the IL. Two optimizations, alkaline pretreatment and
adding supplement betaine, significantly improved REE leaching efficiency and separation between REEs and bulk elements. Significantly,
this method consistently exhibits a particularly high extraction efficiency for scandium.
The IL extraction process yields a mildly acidic REE-rich solution contaminated with Fe. To address this, three strategies for limiting Fe
coextraction into the IL phase were investigated: magnetic separation, alternative salts, and ascorbic acid reduction. The latter two methods
proved successful and should be used to generate an REE-rich acidic solution with very low concentrations of Fe.
To gain a better understanding of CFA leaching behavior with [Hbet][Tf2N], eighteen additional elements were studied (29 total). It was
found that in the IL phase, bulk elements were found in low concentrations, trace elements were not found, and of the actinides, Th was
extracted into the IL phase and U was not leached at all. Other important optimizations were also studied, including pH, temperature, and
duration of leaching. The process is also compared to several published CFA solid extraction methods and CFA leacheate separation methods
to place this dissertation in context with existing literature. Finally, a number of process sustainability improvements are recommended,
including the use of microwave heating, water and IL recovery strategies, and beneficial uses of residual solids.
Finally, two other ILs were studied along with [Hbet][Tf2N] to investigate the effect of IL's cation functional group modifications. The two
ILs possess the same anion [Tf2N], but one with a less acidic cation having an alcohol group, choline [Chol], and one with a more acidic
cation having an alkyl sulfonic acid group, trimethylammoniumethane hydrogen sulfate ([N111C2OSO3H]), in comparison with [Hbet], which
has a carboxyl group. [Chol][Tf2N] was broadly unsuccessful at leaching almost all elements from all CFA samples tested. [N111C2OSO3H]
[Tf2N] was more successful, achieving greater or comparable leaching efficiencies but was not able to separate REEs from bulk and trace
constituents.
Overall, the research outcome of this dissertation filled several knowledge gaps in REE recovery. The method presented is novel and is
among the first to demonstrate direct application of an IL to solid CFA for efficient recovery of REEs. The recyclability of IL and mild
extraction conditions offer significant advantages for environmental sustainability. Altogether, this thesis builds a foundation for new
IL-based strategies for future extractions from CFA and other REE-rich wastes.