64 XXXI International Mineral Processing Congress 2024 Proceedings/Washington, DC/Sep 29–Oct 3
based on references1–2. The greater the difference between
Log K values for different elements, the more easily the ele-
ments are separated. Figure 2 shows how the differences in
Log K values results in different solvent extractant loading
levels for six rare earth elements as a function of pH.
The separation between the elements is based on the
relative differences between equilibrium concentrations of
the desired metal in the solution and in the ion exchanging
medium as illustrated in Figure 3. At a pH of 1.0 on the
abscissa (x-axis), the fraction of metal associated with the
extractant phase is about 0.34 for M2 in the aqueous phase,
whereas for M1 it is only about 0.1. Thus, the ratio of M2
to M1 in the extractant phase is about 3.4. However, at the
same pH the liquid phases have very similar compositions.
Thus, the separation would only be effective using the
extractant phase. Conversely, at a pH of 2.5, the organic
and solid phases have similar compositions of M2 and M1,
whereas the aqueous liquid phase has a M2 to M1 ratio
of about 0.3 or a M1 to M2 ratio of about 3.3. Thus, it is
critical to look at appropriate conditions and phases when
determining the most effective separation.
Another approach to separation based on chemical
affinity is a precipitation reaction such as:
X– +M+ « MX (3)
In which X represents a chemical species or element. The
resulting compound MX that forms in a solution is insol-
uble and precipitates out as a solid precipitate that can be
Figure 1. Comparison of Log K values for selected elements. (Data based 0.75 M di-2-ethylhexylphosphoric
acid (D2EHPA) in toluene and 0.5 M HCl Tracer concentrations of rare earths Peppard et al., 1953, and
Peppard, et al 1957.)
Figure 2. Comparison of organic solution species concentrations in a typical aqueous solution that is
mixed with D2EPHA as a function of pH based on data from Figure 1
based on references1–2. The greater the difference between
Log K values for different elements, the more easily the ele-
ments are separated. Figure 2 shows how the differences in
Log K values results in different solvent extractant loading
levels for six rare earth elements as a function of pH.
The separation between the elements is based on the
relative differences between equilibrium concentrations of
the desired metal in the solution and in the ion exchanging
medium as illustrated in Figure 3. At a pH of 1.0 on the
abscissa (x-axis), the fraction of metal associated with the
extractant phase is about 0.34 for M2 in the aqueous phase,
whereas for M1 it is only about 0.1. Thus, the ratio of M2
to M1 in the extractant phase is about 3.4. However, at the
same pH the liquid phases have very similar compositions.
Thus, the separation would only be effective using the
extractant phase. Conversely, at a pH of 2.5, the organic
and solid phases have similar compositions of M2 and M1,
whereas the aqueous liquid phase has a M2 to M1 ratio
of about 0.3 or a M1 to M2 ratio of about 3.3. Thus, it is
critical to look at appropriate conditions and phases when
determining the most effective separation.
Another approach to separation based on chemical
affinity is a precipitation reaction such as:
X– +M+ « MX (3)
In which X represents a chemical species or element. The
resulting compound MX that forms in a solution is insol-
uble and precipitates out as a solid precipitate that can be
Figure 1. Comparison of Log K values for selected elements. (Data based 0.75 M di-2-ethylhexylphosphoric
acid (D2EHPA) in toluene and 0.5 M HCl Tracer concentrations of rare earths Peppard et al., 1953, and
Peppard, et al 1957.)
Figure 2. Comparison of organic solution species concentrations in a typical aqueous solution that is
mixed with D2EPHA as a function of pH based on data from Figure 1