Selective Recovery of Rare Earth Elements from Coal Fly Ash Leachates Using Liquid Membrane Processes

Coal combustion residues and other
geological waste materials have
been proposed as a resource for rare earth elements (REEs, herein
defined as the 14 stable lanthanides, yttrium, and scandium). The
extraction of REEs from residues often generate acidified leachates
that require highly selective separation methods to recover the REEs
from other major soluble ions in the leachates. Here, we studied two
liquid membrane processes (liquid emulsion membranes, LEM, and supported
liquid membranes, SLM) and compared them to standard solvent extraction
techniques for selective recovery and concentration of REEs from a
leachate of coal fly ash. All separation methods involved an organic
solution of di­(2-ethylhexyl)­phosphoric acid dissolved in kerosene
or mineral oil and an acid strippant solution of 5 M nitric acid for
the liquid-based separations. The LEM configuration, which separated
REEs by immersing an acid-in-oil emulsion in the ash leachate, resulted
in similar recovery percentages of individual REEs as the conventional
solvent extraction approach. The recovery of REEs in the SLM configuration,
which involved the impregnation of the solvent in a hydrophobic membrane,
was slower than the LEM process. However, the SLM process was notably
more selective for the heavy (and higher value) REEs, while the conventional
extraction and LEM processes were more selective for the light REEs.
A flux-based model of the extraction processes suggested that recovery
rates were limited by REE affinity for the solvent chelator in the
SLM, while the rates of REEs separation via LEM were limited by diffusive
mass transfer across the liquid membrane. Altogether, these results
help to identify specific steps in the recovery process that future
work should target in the development of scalable liquid membrane
separations for REE recovery.