63
Rare Earth Elements Separation Principles Applied Through
Innovative Methods
Michael L. Free, Prashant K. Sarswat, Easton Sadler, and Benjamin Schroeder
University of Utah
ABSTRACT: Rare earth elements are critical to many modern devices. Much of their low market availability
and relatively high cost are related to their similarities in properties, which make them difficult to separate. This
paper provides a brief overview of various fundamental principles that can be applied to separate rare earth
elements. The possibilities within the realms of extraction, separation, and recovery of rare earth elements and
critical elements are fundamentally rooted in the atomic-level properties of these elements. Additionally, the
processing methods, incorporating factors such as time, temperature, reaction dynamics, and mass transport
parameters, play a crucial role in shaping these possibilities. To elaborate, distinct characteristics such as chemical
affinity, kinetics, mass transport, magnetic properties, density, and electrochemical and electrical properties
become pivotal in effecting the separation of these elements, as will be explored in detail within this paper.
Leveraging these differences opens avenues for innovative applications of these principles through alternative
processing methods, showcasing the potential for groundbreaking advancements in the field.
CHEMICAL AFFINITY
Chemical affinity is the main property used to separate
most elements. In general, it involves either exchanging or
combining elements, often in the form of ions, in reactions
such as an ion exchange reaction:
RH +M+ ↔ RM +H+ (1)
In which R represents a chemical compound or element, M
represents a metal atom, and H represents hydrogen. The
resulting equilibrium constant is:
K =
+
+
6RH
6RM
@6M
@6H @
@(2)
Thus, the equilibrium concentration of M+ in solution is
proportional to 1/K, and the concentration of M in the
organic ligand is proportional to K. Note that the hydrogen
ion concentration is inversely related to the M+ concen-
tration at equilibrium. The ability of chemical reactions to
separate elements from solution is based on thermodynam-
ics and is often characterized by the equilibrium constant.
The key to separating M+ from other elements is to find
compounds such as R that have a different K than other
undesired elements. In addition, R must be in a form that
can be physically separated from other constituents. In such
a scenario M+ can be separated by either collecting its rem-
nant in aqueous solution after it combines or complexes
with R or by separately removing it from the complexed
state after transferring the medium with the complexing
ligand R to another solution. In many applications R is
physically bound to a substrate such as an ion exchange
resin, or it is in a solvent that is immiscible with the phase
containing M+.
Figure 1 shows the relationship between rare earth
elements and the associated logarithm of the equilibrium
constant (Log K) for di-2-ethylhexylphosphoric acid
(D2EHPA) equilibration with metal ions as noted, data is
Rare Earth Elements Separation Principles Applied Through
Innovative Methods
Michael L. Free, Prashant K. Sarswat, Easton Sadler, and Benjamin Schroeder
University of Utah
ABSTRACT: Rare earth elements are critical to many modern devices. Much of their low market availability
and relatively high cost are related to their similarities in properties, which make them difficult to separate. This
paper provides a brief overview of various fundamental principles that can be applied to separate rare earth
elements. The possibilities within the realms of extraction, separation, and recovery of rare earth elements and
critical elements are fundamentally rooted in the atomic-level properties of these elements. Additionally, the
processing methods, incorporating factors such as time, temperature, reaction dynamics, and mass transport
parameters, play a crucial role in shaping these possibilities. To elaborate, distinct characteristics such as chemical
affinity, kinetics, mass transport, magnetic properties, density, and electrochemical and electrical properties
become pivotal in effecting the separation of these elements, as will be explored in detail within this paper.
Leveraging these differences opens avenues for innovative applications of these principles through alternative
processing methods, showcasing the potential for groundbreaking advancements in the field.
CHEMICAL AFFINITY
Chemical affinity is the main property used to separate
most elements. In general, it involves either exchanging or
combining elements, often in the form of ions, in reactions
such as an ion exchange reaction:
RH +M+ ↔ RM +H+ (1)
In which R represents a chemical compound or element, M
represents a metal atom, and H represents hydrogen. The
resulting equilibrium constant is:
K =
+
+
6RH
6RM
@6M
@6H @
@(2)
Thus, the equilibrium concentration of M+ in solution is
proportional to 1/K, and the concentration of M in the
organic ligand is proportional to K. Note that the hydrogen
ion concentration is inversely related to the M+ concen-
tration at equilibrium. The ability of chemical reactions to
separate elements from solution is based on thermodynam-
ics and is often characterized by the equilibrium constant.
The key to separating M+ from other elements is to find
compounds such as R that have a different K than other
undesired elements. In addition, R must be in a form that
can be physically separated from other constituents. In such
a scenario M+ can be separated by either collecting its rem-
nant in aqueous solution after it combines or complexes
with R or by separately removing it from the complexed
state after transferring the medium with the complexing
ligand R to another solution. In many applications R is
physically bound to a substrate such as an ion exchange
resin, or it is in a solvent that is immiscible with the phase
containing M+.
Figure 1 shows the relationship between rare earth
elements and the associated logarithm of the equilibrium
constant (Log K) for di-2-ethylhexylphosphoric acid
(D2EHPA) equilibration with metal ions as noted, data is
