U.S. patent application number 17/187834 was filed with the patent office on 2021-06-24 for ligand assisted chromatography for metal ion separation.
The applicant listed for this patent is Purdue Research Foundation. Invention is credited to Lei Ling, Nien-Hwa Linda Wang.
Application Number | 20210189519 17/187834 |
Document ID | / |
Family ID | 1000005429981 |
Filed Date | 2021-06-24 |
United States Patent
Application |
20210189519 |
Kind Code |
A1 |
Wang; Nien-Hwa Linda ; et
al. |
June 24, 2021 |
LIGAND ASSISTED CHROMATOGRAPHY FOR METAL ION SEPARATION
Abstract
A method of producing substantially pure rare earth elements
(REEs) from a mixture, including the steps of dissolving a mixture
containing REEs in a strong acid to result in a dissolved mixture
of metal ions, including that of REEs, capturing metal ions of REEs
in a first set of chromatographic columns comprising strong acid
cation exchange resins, washing said first set of chromatographic
columns with a salt solution to remove non-adsorbing metal ions,
eluting metal ions of REES from said first set of chromatographic
columns with a first ligand solution to result in a solution of
enriched metal ions of REEs, loading said solution of enriched
metal ions of REEs onto a second set of chromatographic columns,
and eluting bound metal ions of REEs stepwise from said second set
of chromatographic columns using a second ligand solution to afford
a substantially pure REE. The second set of chromatographic columns
comprises hydrous polyvalent metal oxide selected from the group
consisting of TiO.sub.2, ZrO.sub.2, or SnO.sub.2. The ligand of the
second ligand solution coordinates with said hydrous polyvalent
metal oxide.
Inventors: |
Wang; Nien-Hwa Linda; (West
Lafayette, IN) ; Ling; Lei; (Devens, MA) |
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Applicant: |
Name |
City |
State |
Country |
Type |
Purdue Research Foundation |
West Lafayette |
IN |
US |
|
|
Family ID: |
1000005429981 |
Appl. No.: |
17/187834 |
Filed: |
February 28, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16709973 |
Dec 11, 2019 |
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17187834 |
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15327041 |
Jan 18, 2017 |
10597751 |
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PCT/US2015/040975 |
Jul 17, 2015 |
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16709973 |
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62026487 |
Jul 18, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 15/42 20130101;
G01N 30/463 20130101; B01D 15/3828 20130101; C22B 3/06 20130101;
B01D 15/166 20130101; G01N 30/461 20130101; B01D 15/422 20130101;
B01D 15/362 20130101; B01D 2015/3838 20130101; C22B 3/24 20130101;
B01D 15/1871 20130101; C22B 59/00 20130101; Y02P 10/20
20151101 |
International
Class: |
C22B 3/20 20060101
C22B003/20; B01D 15/38 20060101 B01D015/38; B01D 15/16 20060101
B01D015/16; B01D 15/18 20060101 B01D015/18; B01D 15/36 20060101
B01D015/36; B01D 15/42 20060101 B01D015/42; C22B 3/06 20060101
C22B003/06; C22B 3/24 20060101 C22B003/24; C22B 59/00 20060101
C22B059/00; G01N 30/46 20060101 G01N030/46 |
Claims
1. A method of producing substantially pure rare earth elements
(REEs) from a mixture comprising: a. dissolving a mixture
containing REEs in a strong acid to result in a dissolved mixture
of metal ions, including that of REEs; b. capturing metal ions of
REEs in a first set of chromatographic columns comprising strong
acid cation exchange resins; c. washing said first set of
chromatographic columns with a salt solution to remove
non-adsorbing metal ions; d. eluting metal ions of REES from said
first set of chromatographic columns with a first ligand solution
to result in a solution of enriched metal ions of REEs; e. loading
said solution of enriched metal ions of REEs onto a second set of
chromatographic columns; and f. eluting bound metal ions of REEs
stepwise from said second set of chromatographic columns using a
second ligand solution to afford a substantially pure REE, wherein
said second set of chromatographic columns comprising hydrous
polyvalent metal oxide selected from the group consisting of
TiO.sub.2, ZrO.sub.2, or SnO.sub.2 and wherein ligand of said
second ligand solution coordinates with said hydrous polyvalent
metal oxide.
2. The method of claim 1, wherein said salt solution is a sodium or
ammonium salt solution with a counter ion selected from the group
consisting of chloride (Cl.sup.-), sulfate (SO.sub.4.sup.2-),
bisulfate (HSO.sub.4.sup.-), and nitrate (NO.sub.3.sup.-).
3. The method of claim 1, wherein said first ligand is
ethylenediaminetetraacetic acid (EDTA), pentetic acid (DTPA),
1,2-diaminocyclohexanetetraacetic acid (DCTA), N-(2-Hydroxyethyl)
ethylenediamine-N,N',N'-triacetic acid (HEDTA), iminodiacetic acid
(IDA), citric acid, or any combination thereof.
4. The method of claim 1, wherein said metal ions of REEs are
eluted separately by using said first ligand solution with a linear
or stepwise concentration gradient of said ligand.
5. The method of claim 1, wherein said metal ions of REEs are
eluted separately by using said first ligand solution with a linear
or stepwise gradient of pH.
6. The method of claim 1, wherein said second ligand solution is a
solution of ethylenediaminetetraacetic acid (EDTA), pentetic acid
(DTPA), 1,2-diaminocyclohexanetetraacetic acid (DCTA),
N-(2-Hydroxyethyl) ethylenediamine-N,N',N'-triacetic acid (HEDTA),
iminodiacetic acid (IDA), citric acid, or any combination
thereof.
7. The method of claim 1, wherein metal ions of REEs are eluted
separately by using said second ligand solution with a linear or
stepwise concentration gradient of said ligand.
8. The method of claim 1, wherein metal ions of REEs are eluted
separately by using said second ligand solution with a linear or
stepwise gradient of pH.
9. The method of claim 1, wherein said strong acid compromises one
or more acids selected from the group consisting of hydrochloric
acid (HCl), sulfuric acid (H.sub.2SO.sub.4), and nitric acid
(HNO.sub.3).
10. A method of substantially pure praseodymium (Pr), neodymium
(Nd), or samarium (Sm) of claim 1.
11. A method for separating a mixture of metals, comprising: a)
dissolving a mixture of metals in a strong acid to result in a
dissolved mixture; b) capturing a desired group of metal ions in a
first set of chromatography columns, the first set of
chromatography columns is washed in a salt solution to remove
non-adsorbing species, resulting in a desired group of metal ions;
c) co-eluting the desired group of metal ions with a ligand
solution to result in a further washed solution; and d) loading the
further washed solution onto a second set of chromatography
columns.
12. The method of claim 11, wherein the metal ions comprise rare
earth element ions.
13. The method of claim 12, wherein the rare earth element ions
comprise lanthanide ions.
14. The method of claim 11, wherein the salt solution is a sodium
salt solution.
15. The method of claim 11, wherein the salt solution is an
ammonium salt solution.
16. The method of claim 11, wherein the metal ions comprise at
least one lanthanide ion.
17. The method of claim 11, wherein the metal ions adsorb in the
second set of chromatography columns onto a solid phase, react with
the ligand in a solution phase, and are eluted separately.
18. The method of claim 11, wherein the metal ions are eluted
separately by using the ligand solution with a linear gradient of
ligand concentration.
19. The method of claim 11 wherein the metal ions are eluted
separately by using the ligand solution with a linear gradient of
pH.
20. The method of claim 11 wherein the metal ions are eluted
separately by using the ligand solution with stepwise changes in
ligand concentration.
21. The method of claim 11 wherein the metal ions are eluted
separately by using the ligand solution with stepwise changes in
pH.
22. The method of claim 11, wherein the mixture of metals is first
dissolved in a 0.1 M-2 M strong acid solution, the strong acid has
at least one of hydrochloride acid (HCl), sulfuric acid
(H.sub.2SO.sub.4) and nitric acid (HNO.sub.3); wherein the salt
solution has a concentration of about 0.01 M to about 1 M; wherein
the salt solution comprises co-ions including at least one of the
following chloride (Cl.sup.-), sulfate (SO.sub.4.sup.2-), bisulfate
(HSO.sub.4.sup.-), and nitrate (NO.sub.3.sup.-); wherein the first
set of chromatography columns used to capture the metal ions is
packed with strong-acid cation exchange resins; wherein the ligand
is configured to elute the metal ions and form at least one complex
with metal ions with different equilibrium constants or stability
constants; wherein the ligand comprises at least one of or a
combination of ethylenediaminetetraacetic acid (EDTA), pentetic
acid (DTPA), 1,2-Diaminocyclohexanetetraacetic acid (DCTA),
N-(2-Hydroxyethyl) ethylenediamine-N,N',N'-triacetic acid (HEDTA),
iminodiacetic acid (IDA), or citric acid.
23. The method of claim 22, wherein the second set of
chromatography columns used to separate the lanthanides is packed
with an adsorbent with a ligand immobilized by covalent
attachment.
24. The method of claim 22, wherein the second set of
chromatography columns used to separate the lanthanides is packed
with an adsorbent with a ligand immobilized by physical adsorption.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present patent application is a continuation of
co-pending U.S. patent application Ser. No. 16/709,973, filed on
Dec. 11, 2019, which was related to and claims the priority benefit
of then co-pending U.S. Provisional Patent Application Ser. No.
62/026,487, filed Jul. 18, 2014, the contents of which is hereby
incorporated by reference in its entirety into this disclosure.
TECHNICAL FIELD
[0002] The present disclosure generally relates to metal ion
separation, and in particular to a metal separation process using
ligand-assisted chromatography.
BACKGROUND
[0003] This section introduces aspects that may help facilitate a
better understanding of the disclosure. Accordingly, these
statements are to be read in this light and are not to be
understood as admissions about what is or is not prior art.
[0004] Metals in general, and in particular for example, rare earth
elements (REE's), are critical components of many high-valued
products, such as petroleum refining catalysts, phosphors in color
television and flat panel displays (cell phones, portable DVDs, and
laptops), permanent magnets, and rechargeable batteries for hybrid
and electric vehicles. Rare earth elements consists of 15
lanthanides (Ln's), scandium and yttrium. Currently, the REE's used
in the U.S. are primarily imported from China, which produces more
than 90% of the Ln's used globally. Since China has reduced the
export quota almost by half since 2010, it is highly desirable to
develop efficient and cost-effective processes to produce and
recover REE's domestically.
[0005] As an example, a typical production process for the rare
earth elements can include the following steps: (1) physical
separations (gravity concentration, flotation, magnetic, or
electrostatic separation) which are used to separate rare earth
minerals from sands and rocks in the ore; (2) dissolution of rare
earth minerals in acidic or caustic solutions; (3) separation of
each REE element from the mixture solutions; (4) precipitation of
each REE element using oxalic acid to obtain solid REE oxalate,
which is then decomposed under heat to form REE oxide of a single
element. Among these steps, Step (3) is most challenging and costly
because many of the REE's are present in the solution, and they
have very similar chemical properties, ionic sizes, and
charges.
[0006] The current large-scale production of REE's is mainly based
on solvent extraction. Almost 20 sequential and parallel extraction
steps using organic solvents (naphthenic acid or phosphorous-based
extractants) and strong acids (hydrochoric acid or sulfuric acid)
are needed to separate the REE's into eight or ten major fractions.
Such a method requires large amounts of organic extractants and
highly acidic or caustic aqueous solutions. The numerous unit
operations generate a lot of environmentally-hazardous wastes and
result in a large footprint and high costs.
[0007] An alternative method to separate REE's is ligand-assisted
displacement chromatography using an ion exchanger. In this method,
the REE's are loaded onto a strong-acid cation exchange resin, and
then displaced by sodium or ammonium ions in the presence of a
ligand. In order to increase the purity and yield up to 90%, a
large column (0.45 L), a large amount of ligand solution (>130
column volumes), and a long displacement time (>3 weeks) are
required to separate a small amount of REE's (<2 g), resulting
in low productivity and poor ligand efficiency. Worse still, after
each run, the column needs to be regenerated by a concentrated
solution of acid or transition metal salt, which increases the
operation cost significantly. As a result, this method is estimated
to have a production cost of $40/kg, which is not economical for
large-scale productions.
[0008] Another method to achieve REE's separation is extraction
chromatography, in which a chelating agent is immobilized onto a
resin to increase the selectivity of the sorbent for the REE's. The
resins were developed by Argonne National Laboratory in the 1970's,
and have been tested in analytical chromatography. Column test data
showed that two small columns (with 0.3 g resin) can be used in
tandem to capture and purify six REE's using two pH elution steps.
However, the resin supply is limited at present, and the resin life
is not well known. Most importantly, the resin cost is over
$16,000/kg, which is highly uneconomical for large-scale REE's
separation.
[0009] There is therefore an unmet need for an efficient, cost
effective method and system for achieving rare earth metal ion
separation.
SUMMARY
[0010] In one aspect, a method for separating a mixture of ions, in
particular, rare earth ions, is presented. The method comprises
dissolving a mixture of ions in a strong acid to result in a
dissolved mixture, capturing the desired metal ions in a first set
of chromatography columns, the columns are washed in a salt
solution to remove non-adsorbing or weakly-adsorbing species,
co-eluting the washed solution with a ligand solution to result in
a further washed solution, and loading the further washed solution
onto a second set of chromatography columns. The mixture of ions
comprises metal ions. In another aspect, the metal ions comprise
rare earth element ions. In yet another aspect, the metal ions
comprise lanthanide ions. In yet another aspect, the metal ions
comprise at least one lanthanide ion. In another aspect, the salt
solution is a sodium salt solution. In another aspect, the salt
solution is an ammonium salt solution.
[0011] In another aspect, the metal ions adsorb in the second set
of chromatography columns onto a solid phase, react with the ligand
in a solution phase, and are eluted separately. The metal ions can
be eluted separately by using the ligand solution with a linear
gradient of ligand concentration. The metal ions can be eluted
separately by using the ligand solution with a linear gradient of
pH. The metal ions can also be eluted separately by using the
ligand solution with stepwise changes in ligand concentration. The
metal ions can also be eluted separately by using the ligand
solution with stepwise changes in pH.
[0012] In yet another aspect, the metal ions are first dissolved in
a 0.1 M-2 M strong acid solution. The strong acid can be one or a
combination of hydrochloride acid (HCl), sulfuric acid
(H.sub.2SO.sub.4), or nitric acid (HNO.sub.3). The salt solution
has a concentration of about 0.01 M to about 2 M. In another
aspect, the salt solution comprises co-ions, including one of the
following chloride (Cl.sup.-), sulfate (SO.sub.4.sup.2-), bisulfate
(HSO.sub.4.sup.-), and nitrate (NO.sub.3.sup.-). The first set of
chromatography columns used to capture the desired metal ions can
be packed with strong-acid cation exchange resins or other
exchangers. The ligand used to elute the metal ions can form
complexes with metal ions with different equilibrium constants (or
stability constants). The ligand comprises, for example,
ethylenediaminetetraacetic acid (EDTA), pentetic acid (DTPA),
1,2-Diaminocyclohexanetetraacetic acid (DCTA), N-(2-Hydroxyethyl)
ethylenediamine-N,N',N'-triacetic acid (HEDTA), iminodiacetic acid
(IDA), or citric acid. In one aspect, the ligand is EDTA.
[0013] In another aspect, the second set of chromatography columns
used to separate the metal ions is packed with a robust adsorbent,
which can have adsorption sites for the metal ions, or a ligand
immobilized either covalently or via strong physical adsorption. In
another aspect, the adsorbent is a ligand-preloaded adsorbent
having a similar affinity but small or opposite selectivity for the
metal ions compared to the ligand. In one embodiment, the adsorbent
is a hydrous polyvalent metal oxide. In yet another aspect, the
hydrous polyvalent metal oxide can be TiO.sub.2. In yet another
aspect, the hydrous polyvalent metal oxide can be ZrO.sub.2. In yet
another aspect, the hydrous polyvalent metal oxide can be
SnO.sub.2. In yet another aspect, the adsorbent comprises chelating
resins with functional groups of iminodiacetic acid. In yet another
aspect, the adsorbent comprises other ligands or adsorption
sites.
[0014] In one aspect, the metal ions comprise at least one of
praseodymium (Pr), neodymium (Nd), and samarium (Sm). In yet
another aspect, the capture and separation of metal ions,
specifically lanthanides, are carried out at temperatures in the
range of about 0.degree. C. to about 100.degree. C. In another
aspect, the capture and separation of lanthanides are carried out
at pressures between about 0.5 atmospheres and about 400
atmospheres. In yet another aspect, the temperatures are in the
range of 15.degree. C. to 25.degree. C., and the pressure is 1 atm.
In yet another aspect, the separation is performed at pH in the
range of about 3 to about 11 and ligand concentration between about
0.001 M and about 1 M. In yet another aspect, the separation is
performed at pH 9 and the ligand concentrations are in the range of
0.1 M to 0.4 M. In yet another aspect, the separation of metal ions
is performed in at least one of a batch mode with linear gradient
elution, batch mode with stepwise gradient elution, and continuous
mode with stepwise gradient elution.
BRIEF DESCRIPTION OF DRAWINGS
[0015] FIG. 1 is an illustration depicting the adsorption and
complexation mechanisms of lanthanides (Ln's) in the
ligand-assisted elution chromaography.
[0016] FIG. 2a shows the properties of Brosted acid sites on the
titania adsorbent.
[0017] FIG. 2b shows the properties of Brosted base sites on the
titania adsorbent.
[0018] FIG. 2c shows the properties of Lewis acid sites, on the
titania adsorbent.
[0019] FIG. 3 shows effluent history of Pr, Nd, and Sm in a
displacement test using titania.
[0020] FIG. 4a shows the effluent history of Pr, Nd, and Sm for the
ligand-assisted elution using 0.04 M DTPA.
[0021] FIG. 4b shows the effluent history of Pr, Nd, and Sm for the
ligand-assisted elution using 0.2 M EDTA.
[0022] FIG. 5a shows the adsorption isotherms of Nd on EDTA-free
titania adsorbent.
[0023] FIG. 5b shows the adsorption isotherms of Sm on EDTA-free
titania adsorbent.
[0024] FIG. 5c shows the adsorption isotherms for Nd EDTA-preloaded
titania adsorbent.
[0025] FIG. 5d shows the adsorption isotherms for Sm on
EDTA-preloaded titania adsorbent.
[0026] FIG. 5e shows the Bi-Langmuir model as tested for Nd.
[0027] FIG. 5f shows the Bi-Langmuir model as tested for Sm.
[0028] FIG. 6a shows results from isocratic elution tests at the
EDTA concentration of 0.1 M for the separation of Pr, Nd, and Sm
using EDTA (pH 9) as the ligand.
[0029] FIG. 6b shows results from isocratic elution tests at the
EDTA concentration of 0.2 M for the separation of Pr, Nd, and Sm
using EDTA (pH 9) as the ligand.
[0030] FIG. 6c shows results from isocratic elution tests at the
EDTA concentration of 0.35 M for the separation of Pr, Nd, and Sm
using EDTA (pH 9) as the ligand.
[0031] FIG. 6d shows results from isocratic elution tests at the
EDTA concentration of 0.4 M for the separation of Pr, Nd, and Sm
using EDTA (pH 9) as the ligand.
[0032] FIG. 7 shows the linear gradient elution for the separation
of Pr, Nd, and Sm using EDTA (pH 9) as the ligand.
[0033] FIG. 8 shows the simulated step-wise elution process for the
separation of Pr, Nd, and Sm.
[0034] FIG. 9 shows an embodiment of large-scale production of
Ln's.
[0035] FIG. 10a shows column profiles during the feed loading (t=2
min).
[0036] FIG. 10b shows column profiles during the feed loading
(t=100 min).
[0037] FIG. 10c shows column profiles during the Na.sup.+
displacement step (t=103 min).
[0038] FIG. 10d shows column profiles during the Na.sup.+
displacement step (t=110 min).
[0039] FIG. 10e shows the effluent history of the entire process
including Ln capture and Na.sup.+ displacement.
[0040] FIG. 11a shows the continuous counter-current chromatography
processes for the separation of 3 Ln elements.
[0041] FIG. 11b shows the continuous counter-current chromatography
processes for the separation of 15 lanthanides or metal ions.
DETAILED DESCRIPTION
[0042] For the purposes of promoting an understanding of the
principles of the present disclosure, reference will now be made to
the embodiments illustrated in the drawings, and specific language
will be used to describe the same. It will nevertheless be
understood that no limitation of the scope of this disclosure is
thereby intended. In addition, it should be appreciated that
although the separation of lanthanides is presented in this
disclosure, this is only for demonstrative purposes and is not
intended to be limiting of the scope of this disclosure, and the
processes described herein can thus be applied to metal ions,
including but not limited to rare earth element ions. Rare earth
element ions can include cerium, dysprosium, erbium, europium,
gadolinium, holmium, lanthanum, lutetium, neodymium, praseodymium,
promethium, samarium, scandium, terbium, thulium, ytterbium, and
yttrium.
[0043] Presented herein is a novel ligand-assisted elution
chromatography process and system for the separation of metal ions,
rare earth element ions, and lanthanides (Ln) using a robust and
low-cost sorbent. The sorbent can be organic or inorganic. In one
embodiment, the inorganic sorbent is titania. The adsorbent is a
ligand-preloaded adsorbent having a similar affinity but small or
opposite selectivity for the metal ions compared to the ligand. In
an embodiment, the titania column is first preloaded with a ligand
solution. The separation of Ln's is used as an example here. After
the Ln's mixture is loaded onto the ligand-immobilized column, the
ligand solution is used to elute the adsorbed Ln's. The element
that can form a more stable complex with the ligand elutes earlier
in the effluent. Analysis showed that the overall selectivity
equals the ratio of the ligand selectivity to the sorbent
selectivity. In addition, the Ln's can be well separated only if
the adsorption isotherm parameters and complexation equilibrium
constants are in the same order of magnitude.
[0044] Based on the results, several ligands were screened, among
which ethylenediaminetetraacetic acid (EDTA) was found to have the
best complexation equilibrium constants for separating the Ln's on
a titania column. A ternary separation of Pr, Nd, and Sm was tested
using EDTA. Pure products of each element were obtained with high
purity under well-designed ligand concentrations. Linear-gradient
elution was used to concentrate the products and shorten the cycle
time. The recovery yields for high-purity Ln's (>95%) exceed 95%
for all three products. Rate model simulations taking into account
adsorption, mass transfer, and reactions were developed to verify
the mechanism of ligand-assisted elution and separation. The
simulation results agree closely with the experimental data.
[0045] The separation process disclosed herein is much more
efficient than the conventional sequential and parallel solvent
extraction processes. All the REE's can be separated within one set
of chromatography columns under room temperature and relatively
mild pH conditions. Both the sorbent and the ligand are inexpensive
and readily available. The ligand is generally recognized as safe
and most of the ligand can be recycled after each run. No harsh or
expensive chemicals are needed for column regeneration. To increase
sorbent productivity and to reduce the amount of ligand required
and the production cost, a continuous counter-current
chromatography process with step-wise elution can be used for
large-scale production.
[0046] The separation of metal ions in a ligand-assisted
chromatography system is controlled by both adsorption and
complexation reactions in the mobile phase (FIG. 1). FIG. 1
illustrates the adsorption and complexation of Ln's in the
ligand-assisted elution chromaography. Still referring to FIG. 1, L
is the ligand, Ln is the lanthanide, LLn is the complex formed by
the ligand and the lanthanide. K.sub.C is the complexation
equilibrium constant, a is the linear Langmuir isotherm parameter.
The sorbent is presaturated with the ligand, which adsorbs on the
sorbent. The adsorbed ligand is a part of the stationary phase, and
not shown explicitly in FIG. 1. The counter-ion of the ligand
NH.sub.4.sup.+ can compete with Ln's for the Brosted acid site and
the ligand-preloaded Lewis acid site in titania. NH4.sup.+, which
is the co-ion of the ligand, may weakly adsorb onto the adsorbent
and thus affect the retention of Ln peaks. The mechanism of Ln
adsorption onto the ligand-immobilized titania is explained further
below, and in addition, the key controlling factors on Ln's elution
and separation in ligand-assisted chromatography are described. The
rate model and simulations are described as well.
[0047] Adsorption Mechanism:
[0048] Titania is a complex sorbent with three types of adsorption
sites (FIGS. 2a-2c): Brosted acid (BA), Brosted base (BB), and
Lewis acid (LA) sites. FIG. 2a shows the properties of Brosted acid
sites, FIG. 2b shows the properties of Brosted base sites, and FIG.
2c shows the properties of Lewis acid sites, on the titania
adsorbent. At a high pH, the protons on the BA sites (TiOH) can
react with the OH.sup.- in the solution, and the resulting
TiO.sup.- groups have a high affinity for cations (FIG. 2a). At a
low pH, the protons in the solution can adsorb on the BB sites
(Ti--O--Ti), which in turn can bind anions (FIG. 2b). The LA sites
(Ti) are coordinatively unsaturated titanium atoms, which have
vacant orbitals for electrons (FIG. 2c). Many Lewis bases with
extra electrons, such as OH.sup.-, PO.sub.4.sup.3-,
SO.sub.4.sup.2-, and COO.sup.-, can adsorb strongly onto the LA
sites. If the adsorbed Lewis bases have more charges than needed
for adsorption, they can serve as additional adsorption sites for
cations.
[0049] If a ligand with multiple COO.sup.- groups is preloaded onto
a titania sorbent, some of the COO.sup.- groups can bind strongly
with the LA sites. Under this condition, the Ln's can adsorb on
both the BA sites and the free COO.sup.- groups of the ligand
adsorbed on the LA sites. The adsorption data can be correlated
using a Bi-Langmuir isotherm model according to Eq. (1):
q = a 1 C 1 + b 1 C + a 2 C 1 + b 2 C ( 1 ) ##EQU00001##
where q and C are the solid-phase and the liquid-phase
concentrations in local equilibrium; a and b are the linear and
nonlinear Langmuir isotherm parameters; subscripts "1" and "2"
represent Sites 1 (BA sites) and Sites 2 (LA sites),
respectively.
[0050] However, if a ligand is present in the mobile phase, the
adsorption sites which have much weaker affinity for the Ln's than
the ligand will not be able to retain Ln's. Retention is needed to
allow the complexation reactions in the mobile phase to accelerate
the migration of the Ln's that have higher affinity for the ligand.
The effects of adsorption and complexation on Ln separation are
discussed below.
Ligand-Assisted Elution Chromatography:
[0051] In conventional elution chromatography, the migration of
solutes along the column results from repetitive adsorption and
desorption. In ligand-assisted elution chromatography, the
adsorption is strong, and the desorption is driven by a reversible
complexation of Ln or metal ions and the ligand in the liquid phase
(FIG. 1). Since different metal ions can form complexes with the
ligand with different complexation equilibrium constants, they can
migrate at different velocities in the column, resulting in the
separation.
[0052] For a linear isotherm system, the retention factor of a
solute peak in the presence of a ligand has the following
expression according to Eq. (2):
k ' = 1 t a 1 + K C [ L ] ( 2 ) ##EQU00002##
where .epsilon..sub.t is the total void fraction in the column,
K.sub.C is the complexation equilibrium constant, and [L] is the
ligand concentration. The product K.sub.C[L] can be considered as a
dimensionless complexation equilibrium constant. It is noteworthy
that "a" must be in the same order of magnitude as K.sub.C[L] to
guarantee a reasonable time scale for elution. If a
<<K.sub.C[L], the complexation reaction is much stronger than
the adsorption, resulting in elution of the solutes at the void
volume. If a >>K.sub.C[L], the complexation is too weak
compared to the adsorption; the solute is likely to be trapped in
the column and cannot be eluted.
[0053] The ratio of the retention factors of two solutes gives the
overall selectivity in the system, as shown in Eq. (3).
.alpha. = k 2 ' k 1 ' = ( a 2 a 1 ) ( 1 + K C 1 [ L ] 1 + K C 2 [ L
] ) ( 3 ) ##EQU00003##
[0054] In most cases, the complexation is strong and
K.sub.C[L]>>1, so that Eq. (3) can be reduced to Eq. (4).
.alpha. = ( a 2 a 1 ) ( K C 1 K C 2 ) = .alpha. A d s o r b e n t
.alpha. Ligand ( 4 ) ##EQU00004##
where a.sub.Sorbent=a.sub.2/a.sub.1 is the sorbent selectivity, and
a.sub.Ligand=K.sub.C2/K.sub.C1 is the ligand selectivity. If the
sorbent has little selectivity for the solutes, the overall
selectivity is dominated by the ligand selectivity.
[0055] For a nonlinear isotherm system, the retention factor does
not have a simple analytical expression. In addition, the co-ion of
the ligand, NH.sub.4.sup.+, can also adsorb weakly onto the
ligand-loaded LA sites and affect the retention of Ln peaks.
Nevertheless, the results obtained from the linear isotherm system
can still serve as the guidelines for designing nonlinear isotherm
systems. To achieve efficient and high-purity separation, one has
to select the ligand such that K.sub.C[L] has the same order of
magnitude as a, and the ratio a.sub.Ligand/a.sub.Sorbent should be
1.2 or larger.
[0056] VErsatile Reaction and SEparation (VERSE) Model and
Simulation:
[0057] The VERSE model and simulations take into account multiple
mass-transfer effects (axial dispersion, film mass transfer,
intra-particle pore and surface diffusion) in chromatography, and
incorporates a variety of models for adsorption (including
Langmuir, Bi-Langmuir, Freundlich, and Mass action) and reactions
(aggregation, decomposition, isomerization, etc.). A simulation
program can numerically solve the partial differential mass balance
equations in the bulk phase and the particle phase. The effluent
histories and dynamic column profiles can be displayed and exported
after the simulations are completed. The figures and animations
generated by the simulations are important for verifying the models
and the separation mechanisms.
[0058] In simulating the Ln separation processes, we used the pore
diffusion model, the assumptions and equations of which have been
reported elsewhere. The axial dispersion coefficient was estimated
from Chung and Wen correlation, and the film mass-transfer
coefficient was obtained from the Wilson and Geankoplis
correlation. Although the titania has two types of sites (BA and
LA) for Ln adsorption, the BA sites were found to have much weaker
affinity for the Ln's than the ligand, and thus have negligible
effect on the retention of Ln peaks. Therefore, we considered only
the high-affinity sites, or the ligand-loaded LA sites, in the
simulations, and used the Langmuir adsorption isotherm model
instead of the Bi-Langmuir model.
[0059] The actual values of K.sub.C, a, and b are large
(>10.sup.7). If they are used in the simulations, the time
required for convergence would be extremely long. In fact, as long
as K.sub.C[L] is much greater than 1, the retention of peaks
depends primarily on a dimensionless ratio a/K.sub.C[L], Eq. (2),
rather than the individual values of a, b, and K.sub.C. It has been
verified in the simulations that when the value of a/K.sub.C[L] is
fixed, increasing both a and K.sub.C[L] does not affect the peak
shape or retention time. In order to simulate the separation
processes more efficiently without affecting the peak retention, we
scaled down the values of a, b, and K.sub.C, while satisfying the
following: (1) The ratio K.sub.C(Sm): K.sub.C(Nd): K.sub.C(Pr) is
the same as that reported by the literature; (2) The ratio a(Sm):
a(Nd): a(Pr) is the same as the experimental data; (3) The
adsorption capacity, or the value of a/b, is consistent with the
experimental data; (4) The values of K.sub.C[L] are much greater
than 1, and they are similar to the Langmuir a values.
[0060] Materials and Methods:
[0061] The materials and experiments described below were used to
separate the Ln's and to understand the mechanisms of Ln's
separation. Solution preparation, pH measurement, column packing,
and column tests were all performed at room temperature,
20.+-.1.degree. C.
[0062] Materials:
[0063] Praseodymium (III) nitrate hexahydrate
(Pr(NO.sub.3).sub.3.6H.sub.2O), neodymium (III) nitrate hexahydrate
(Nd(NO.sub.3).sub.3.6H.sub.2O), and samarium (III) nitrate
hexahydrate (Sm(NO.sub.3).sub.3.6H.sub.2O) were ordered from
Sigma-Aldrich, Co. (St. Louis, Mo.). The ligands
ethylenediaminetetraacetic acid (EDTA) and
diethylenetriaminepentaacetic acid (DTPA) were also purchased from
Sigma-Aldrich, Co. (St. Louis, Mo.), whereas citric acid was
purchased from J. T. Baker (Phillipburg, N.J.). The sorbent
Sachtopore 80 (TiO.sub.2, 80 .mu.m, 60 .ANG.) was manufactured by
ZirChrom Separations, Inc. (Anoka, Minn.). Sodium hydroxide (NaOH),
nitric acid (HNO.sub.3), and ammonium hydroxide (NH.sub.4OH) were
purchased from Mallinckrodt Baker, Inc. (Paris, Ky.). Distilled
Deionized Water (DDW) was obtained from a Millipore (Bedford,
Mass.) four stage cartridge system.
[0064] Millipore glass columns (60 cm L.times.1.1 cm ID) and
Omnifit glass columns (15 cm L.times.1.5 cm ID) were ordered from
VWR International (West Chester, Pa.) for sorbent packing. An AKTA
explorer 100 unit (GE Healthcare, Piscataway, N.J.), which consists
of a P-901 binary pump, an M-925 mixer, a UV-900 UV-absorption
monitor (able to simultaneously monitor at three wavelengths), a
pH/C-900 online pH, a conductance monitor, and a Frac-950 fraction
collector, was used for chromatography experiments. A Dell-PC with
Unicorn 5.01 software was connected to the AKTA unit for data
storage and processing.
[0065] Displacement Test:
[0066] The displacement test was used to check if the sorbent
Sachtopore 80 (S80) has sufficiently high selectivity to separate
the Ln's. The column size was 49 cm L.times.1.16 cm ID. After the
column was packed, it was washed with 0.2 M NaOH, 0.2 M HNO.sub.3,
and DDW, to remove any impurities in the sorbent. A 30 mL solution
of Pr, Nd and Sm (0.02 N for each element) was then fed into the
column. After the feed loading, a solution of 0.05 M HNO.sub.3 was
pumped into the column to displace the adsorbed Ln's. The linear
velocities for loading and displacement were both 0.2 cm/min. Pr,
Nd, and Sm were detected using an online UV-vis detector at 444 nm,
575 nm, and 401 nm, respectively. HNO.sub.3 was monitored using an
online pH sensor. After the bands of Pr, Nd, and Sm were displaced
by the HNO.sub.3 front, the displacement was stopped and the column
was washed with DDW for 50-100 column volumes until the pH returned
to 6 and the conductivity dropped below 0.003 mS/cm.
[0067] Ligand-Assisted Elution Tests:
[0068] In ligand-assisted elution tests, the S80 column (49 cm
L.times.1.16 cm ID) was first preloaded with a ligand solution, the
pH of which was adjusted to a target value by titrating with
NH.sub.4OH. The Ln's (Pr, Nd, and Sm) were dissolved in the same
ligand solution, and the concentrations were 0.02 N for each
element. The column was then fed with 30 mL of the Ln solution, and
subsequently eluted by the ligand solution. The linear velocities
for loading and elution were both 0.2 cm/min. Pr, Nd, and Sm were
detected at 444 nm, 575 nm, and 404 nm, respectively.
[0069] DTPA (pH 9), EDTA (pH 9), and citric acid (pH 7) were tested
for isocratic elution, whereas EDTA (pH 9) was also tested for
linear gradient elution. In isocratic elution tests, the eluant was
the same as the ligand solution used for preloading. In gradient
elution, the ratio of the two pumps was programmed as a function of
time, so that the ligand concentration could increase linearly from
the preloading concentration to a target value. The experimental
conditions for each ligand tested are summarized in Table 1. Before
switching to a different ligand system, the column was washed with
0.2 M NaOH, 0.2 M HNO.sub.3, and then DDW, until the pH returned to
6 and the conductivity dropped below 0.003 mS/cm.
TABLE-US-00001 TABLE 1 Experimental conditions for ligand-assisted
elution tests Column size (cm Superficial Feed Feed L .times. cm
ID) velocity (cm/min) concentration (N) volume (mL) 49 .times. 1.16
0.2 0.02 for Pr, Nd, Sm 30 Isocratic elution Presaturant and Eluant
Ligand pH Concentration (M) DTPA 9 0.04 EDTA 9 0.1, 0.2, 0.35, 0.4
Citric acid 7 0.2 Linear gradient elution Ligand pH Concentration
(M) EDTA 9 0.1-0.4
[0070] Frontal Tests for Isotherm Estimation:
[0071] The adsorption isotherms for the Ln's were obtained by
multiple frontal tests using a small S80 column (4.8 cm L.times.1.5
cm ID), which was washed in sequence with 0.2 M NaOH, 0.2 M
HNO.sub.3, and DDW prior to the tests. The isotherm measurement was
first conducted in the absence of ligand. The solutions prepared
for the isotherm measurement were 0.002 N, 0.005 N, 0.01 N, 0.02 N,
0.05 N, and 0.1 N of Pr, Nd, and Sm in DDW. A more concentrated
solution was loaded to the column once the sorbent was equilibrated
with a less concentrated solution. After all the concentrations
were tested for one Ln, the column was washed with 0.2 M HNO.sub.3
and DDW, and then used for a different Ln. The Ln concentration in
the sorbent, which is in equilibrium with a solution phase
concentration, can be calculated as follows:
q i + 1 = q i + ( C i + 1 - C i ) V br , i + 1 V C ( 5 )
##EQU00005##
where C.sub.i and C.sub.i+1 are the solution phase concentrations
at the i.sup.th and (i+1).sup.th frontals; q.sub.i and q.sub.i+1
are the sorbent phase concentration in equilibrium with C.sub.i and
C.sub.i+1, respectively. When 1=0, C.sub.i and q.sub.i are both
zero. V.sub.br,i+1 is the net breakthrough volume (dead volume and
void volume were subtracted) for the (i+1).sup.th frontal; V.sub.C
is the column packing volume.
[0072] The measurement of Ln adsorption isotherm on the
ligand-immobilized sorbent was conducted on the same column (4.8 cm
L.times.1.5 cm ID), which was preloaded with 0.4 M EDTA (pH 9).
Before the Ln's were loaded, the system was washed by 1 column
volume (V.sub.C) of DDW to avoid complexation of Ln's and EDTA in
the tubing. The solutions prepared for the isotherm measurement
were 0.001N, 0.002 N, 0.005 N, 0.01 N, 0.02 N, 0.05 N, and 0.1 N of
Pr, Nd, and Sm in DDW. Unlike the ligand-free isotherm tests, the
column was regenerated by the 0.4 M EDTA solution and washed by 1
V.sub.C of DDW each time before it was loaded with a different
concentration or a different element. As a result, the sorbent
phase Ln concentration can be simply calculated using Eq. (6).
[0073] Results:
[0074] Elution Behaviors of Ln's in the Displacement Test:
[0075] In the displacement test, the adsorbed Pr, Nd, and Sm were
displaced from the titania sorbent by 0.05 M HNO.sub.3. The
chromatogram is shown in FIG. 3 (specifically, FIG. 3 shows the
effluent history of Pr, Nd, and Sm in the displacement test using
titania; the column size is 49 cm L.times.1.16 cm ID; the
superficial velocities for loading and displacement are both 0.2
cm/min; the feed concentration is 0.02 N for each element, and the
feed volume is 30 mL; the pH sensor reading is inaccurate due to
device limitations, it is able to show the breakthrough of
HNO.sub.3 behind the Sm band). The total volume shown in FIGS. 3-7
includes extra-column dead volume (0.13 V.sub.C), total void volume
(0.62 V.sub.C), and feed loading volume (0.58 V.sub.C). The pH
values monitored by the online sensor were inaccurate due to device
limitations, but the changes in pH indicate the breakthrough time
of HNO.sub.3 front. The bands of Pr and Nd overlapped, indicating
that the sorbent has no selectivity for these two elements. The
sorbent has higher affinity for Sm than for Pr and Nd, so the band
of Sm was behind those of Pr and Nd. However, the bands of Pr and
Nd had significant tailing and the band of Sm was thus
contaminated. As a result, the selectivity of titania sorbent was
found insufficient to achieve high-yield and high-purity separation
for the Ln's.
[0076] Comparison of Various Ligands in the Elution Tests:
[0077] As shown in Table 1, three ligand candidates, DTPA, EDTA,
and citric acid, were screened via the elution tests, the procedure
of which were described above. The chromatograms obtained from the
elution tests using DTPA and EDTA are shown in FIGS. 4a and 4b,
respectively. FIGS. 4a and 4b show the effluent history of Pr, Nd,
and Sm for the ligand-assisted elution using 0.04 M DTPA (FIGS. 4a)
and 0.2 M EDTA (FIG. 4b). The experimental conditions are shown in
Table 1.
[0078] When DTPA was used as the ligand, all the Ln's were
co-eluted at the void volume. The reason is that DTPA complexes too
strongly with the Ln's, which cannot adsorb onto the sorbent
(K.sub.C[L]>>a). When EDTA was used, the Ln's were eluted
separately with a reasonably small retention volume, because EDTA
has a high selectivity for the Ln's, and the K.sub.C[L] for the
complexation reaction has a similar value as the Langmuir a value
for Ln's adsorption (K.sub.C[L].about.a). When citric acid was
used, none of the Ln's were eluted after 10 column volumes (not
shown in FIGS. 4a and 4b). The complexation was apparently too weak
compared to the Ln's adsorption (K.sub.C[L]<<a), and the Ln's
thus strongly adsorbed on the column. To avoid the accumulation of
Ln's in the column, concentrated EDTA solution (0.4 M, pH 9) was
used as the eluant, and all the Ln's were completely eluted as a
single band at the void volume.
[0079] Ln's Adsorption Isotherms on the Titania Sorbent:
[0080] In the absence of a ligand, the Ln's, if dissolved in DDW,
can adsorb weakly on the BA sites of the titania. The pH values of
the Ln solutions were around 5. The adsorption isotherms of Nd and
Sm on EDTA-free titania adsorbent are shown in FIGS. 5a and 5b,
respectively. The isotherm of Pr was found to be identical to that
of Nd, and was not shown separately. The experimental data were
correlated closely using the Langmuir isotherm model, and the
parameters obtained from the data are listed in Table 2.
TABLE-US-00002 TABLE 2 Langmuir and Bi-Langmuir isotherm parameters
Isotherm Sorbent Model parameters Pr/Nd Sm EDTA-free Langmuir a 7.6
8.4 titania b (1/N) 88.3 92.4 R.sup.2 0.995 0.995 EDTA-preloaded
Langmuir a 44.8 47.9 titania b (1/N) 128.1 140.9 R.sup.2 0.920
0.866 Bi-Langmuir a.sub.1 14.7 10.3 b.sub.1 (1/N) 44.7 31.1 a.sub.2
1.2 .times. 10.sup.7 2.3 .times. 10.sup.7 b.sub.2 (1/N) 1.6 .times.
10.sup.8 2.4 .times. 10.sup.8 R.sup.2 0.976 0.980
[0081] When the sorbent was preloaded with EDTA (0.4 M, pH 9), the
slopes of the isotherm curves and the total capacities for the Ln's
increased significantly (FIGS. 5c and 5d, showing the adsorption
isotherms for Nd and Sm on EDTA-preloaded titania adsorbent), and
the data could not be fitted well by the Langmuir model (Table 2).
The results indicated that the EDTA-preloaded titania have
heterogeneous sites for Ln's adsorption. Therefore, the Bi-Langmuir
model was also tested and it was found to fit the data better than
the Langmuir model (FIGS. 5e and 5f). The parameters showed that
one type of adsorption site has high affinity but small capacity
for the Ln's, whereas a second type has low affinity but large
capacity (Table 2). It should be noted that data points in FIGS. 5c
and 5d are the same as those in FIGS. 5e and 5f, but the fittings
are based on different models. In FIGS. 5a-5d, the data are fitted
by the Langmuir model, whereas in FIGS. 5e-5f, the data are fitted
by the Bi-Langmuir model. The isotherm parameters obtained from the
fittings are listed in Table 2.
[0082] It appears that EDTA adsorbs on the LA sites, and some of
the free COO.sup.- groups can serve as additional adsorption sites
for the Ln's. Since the interactions between the COO.sup.- groups
and the Ln's are strong, the EDTA-loaded LA sites appear to be the
high-affinity sites for the Ln's. The BA sites have higher affinity
and capacity for the Ln's at pH 9 than at pH 5, but the affinity is
still much lower than the EDTA-loaded LA sites.
[0083] Isocratic and Gradient Elution Using EDTA for Ln's
Separation:
[0084] Since EDTA was found to be the most promising ligand for
separating the Ln's on the titania sorbent, it was tested at
different concentrations for the elution of the Ln's. The isocratic
elution tests were performed at the EDTA concentrations of 0.1 M,
0.2 M, 0.35 M, and 0.4 M, and the results are shown in FIGS. 6a-6d,
respectively. Specifically, FIGS. 6a-6d show the results from the
isocratic elution tests for the separation of Pr, Nd, and Sm using
EDTA (pH 9) as the ligand. The concentrations of EDTA are shown in
each of FIGS. 6a-6d (EDTA concentration of 0.1 M in FIG. 6a; EDTA
concentration of 0.2 M in FIG. 6b; EDTA concentration of 0.35 M in
FIG. 6c; and EDTA concentration of 0.4 M in FIG. 6d). The solid
lines were obtained from experiments and the dashed lines were
obtained from simulations. The experimental conditions and the
parameters used in the simulations are given in Table 1 and Table
4, respectively. When EDTA concentration was low (0.1 M), the Ln
peaks were well resolved, but the product concentrations were low
and the retention times were long. When EDTA concentration was high
(0.4 M), the product concentrations were high but the resolution
was poor.
[0085] In order to achieve relatively high product concentrations
without sacrificing the purities, linear gradient elution was
tested for separating the Ln's. The EDTA concentration was
increased from 0.1 M to 0.4 M linearly over 750 minutes, or from
1.9 V.sub.C to 4.8 V.sub.C in the effluent, FIG. 7 (specifically,
FIG. 7 shows the linear gradient elution for the separation of Pr,
Nd, and Sm using EDTA (pH 9) as the ligand; the concentration of
EDTA increases from 0.1 M to 0.4 M; the solid lines are obtained
from experiments and the dash lines are obtained from simulations;
the experimental conditions and the parameters used in the
simulations are given in Table 1 and Table 4, respectively; the
purities and yields for each component are listed in Table 3). The
elution time was similar to that of the isocratic elution with 0.2
M EDTA, but the product concentrations and the purities of the
slow-moving elements (Nd and Pr) were significantly higher. As
shown in Table 3, the purities and yields for all three elements
were 95% or higher.
[0086] The dashed lines in FIGS. 6a-6d and 7 were obtained from
VERSE simulations. The models and assumptions considered in the
simulations were explained above. Since the affinity of the BA
sites for the Ln's is negligible compared to the complexation of
EDTA and the Ln's in the solution phase (Table 2), only the
modified LA sites were considered in the simulations, and the
Langmuir isotherm model was used. The ratio a(Sm): a(Nd) was
lowered by 15% compared to the fitted isotherm parameters in Table
2 to match with the elution data. The ratio a(Nd): a(Pr) was kept
the same as that in Table 2. The parameters used in the simulations
are summarized in Table 4. The close agreement between the
simulations and the experimental data supports the proposed
mechanisms and the models.
TABLE-US-00003 TABLE 3 Purities and yields obtained in the linear
gradient elution Lanthanide Element Purity (%) Yield (%) Sm 99 97
Nd 95 96 Pr 97 96
[0087] A similar method to elute the Ln's with changing ligand
concentration is step-wise elution, which can be applied readily to
continuous separation processes. The step-wise elution of Pr, Nd
and Sm from titania with increasing EDTA concentrations was
simulated by VERSE as shown in FIG. 8 (specifically, FIG. 8 shows
the simulated step-wise elution process for the separation of Pr,
Nd, and Sm; the concentration of EDTA increases from 0.1 M to 0.25
M to 0.4 M; the parameters used in the simulations are the same as
those in Table 4; the feed concentration, feed volume, and
operating velocity are the same as those shown in Table 1). The
parameters are the same as those in Table 4. This method is shown
to be feasible for separating Ln's with high purity and high
yield.
TABLE-US-00004 TABLE 4 Parameters used in VERSE simulations for
EDTA-assisted elution on titania System Parameters L (cm) ID (cm) R
(.mu.m) .epsilon..sub.b .epsilon..sub.p 49 1.16 40 0.35 0.4
Reaction Parameters Reaction k.sub.+ (M.sup.-1min.sup.-1) k.sub.-
(min.sup.-1) K.sub.C (M.sup.-1) Pr + EDTAPrEDTA 250 2 125 Nd +
EDTANdEDTA 450 2 225 Sm + EDTASmEDTA 1440 2 720 Isotherm Parameters
(Langmuir) Component a b (M.sup.-1) Pr 1000 37500 Nd 1000 37500 Sm
1600 60000 NH.sub.4 0.64 8 others 0 0 Mass Transfer Parameters
D.sub.b D.sub.p E.sub.b k.sub.f Component (cm.sup.2/min)
(cm.sup.2/min) (cm.sup.2/min) (cm/min) All Species 0.0004 0.00004
From Chung From Wilson and Wen and [19] Geankoplis [20] Numerical
Parameters Axial Step Size Collocation Points Tolerance Elements
(L/u.sub.0) Axial Particle Absolute (M) Relative 50 0.1 4 1
10.sup.-5 10.sup.-4
[0088] Large-Scale Production and Cost Analysis:
[0089] In one embodiment, the ligand-assisted elution
chromatography process disclosed herein can be extended to
large-scale production of Ln's. In practice, the production should
have a capture step prior to the separation step, as shown in FIG.
9. The rare earth mineral separated from rocks and sands is first
dissolved in a strong acid, and is loaded onto a strong-acid cation
exchange column loaded with Na.sup.+. Under the strongly acidic
condition, the trivalent Ln's can be captured by the ion-exchange
resin, whereas most of the monovalent and divalent metal ions
adsorb weakly and will pass through the column. Examples of
strong-acid cation exchange resins include but are not limited to
Dowex 50WX8 and Amberlite IR120. The protons remained on the resin
can be displaced by a NaCl solution, which prevents precipitation
of Na-form EDTA by H.sup.+ during stripping of Ln's from the ion
exchange column. An example of Ln capture and NaCl displacement
process was simulated as shown in FIG. 10s 10a-10e (specifically,
FIGS. 10a-10e show the simulated Ln.sup.3+ capture and Na.sup.+
displacement process on a cation exchange column; the column is
initially Na.sup.+.sub.- loaded; the models and parameters used in
the simulations are listed in Table 5; the operating velocity for
loading and washing are both 1.1 cm/min; FIGS. 10a and 10b are
column profiles during the feed loading (0-100 min or 0-23
V.sub.C); the feed contains 0.06 N Ln.sup.3+ and 1 N H.sup.+; FIGS.
10c and 10d are column profiles during the Na.sup.+ displacement
step (100-120 min or 23-27 V.sub.C); the Na.sup.+ concentration is
0.2 N; FIG. 10e is the effluent history of the entire process
including Ln capture and Na.sup.+ displacement). The parameters
used in the simulation are shown in Table 5.
TABLE-US-00005 TABLE 5 Parameters used in VERSE simulations for Ln
capture and NaCl wash on ion exchange resin System Parameters L
(cm) ID (cm) R (.mu.m) .epsilon..sub.b .epsilon..sub.p 5 1.5 50
0.35 0.55 Isotherm Parameters (Langmuir) K.sub.i-Na.sup.+ (Mass
Action equilibrium Component constant for ion exchange) Na.sup.+ 1
H.sup.+ 0.5 Ln.sup.3+ 5 Mass Transfer Parameters D.sub.b D.sub.p
E.sub.b k.sub.f Component (cm.sup.2/min) (cm.sup.2/min)
(cm.sup.2/min) (cm/min) All Species 0.001 0.0001 From Chung From
Wilson and Wen and [19] Geankoplis [20] Numerical Parameters Axial
Step Size Collocation Points Tolerance Elements (L/u.sub.0) Axial
Particle Absolute (M) Relative 100 0.1 4 1 10.sup.-4 10.sup.-3
[0090] During the feed loading step (0-100 min or 0-23 V.sub.C),
the concentrated H.sup.+ displaces the pre-loaded Na.sup.+ (FIG.
10a), and the Ln.sup.3+ displaces the adsorbed H.sup.+ (FIG. 10b).
In the displacement step (100-120 min or 23-27 V.sub.C), the Ln's
adsorbed strongly on the resin so that the peak in the bulk phase
shrinks rapidly (FIG. 10c). The remaining H.sup.+ adsorbed on the
resin is displaced by the Na.sup.+ (FIG. 10d) and is eventually
cleared out from the column. No leakage of Ln's occurs over the
entire loading and displacement processes (FIG. 10e).
[0091] The captured Ln's are then eluted by Na-form EDTA, and
loaded onto a EDTA-preloaded titania column. A gradient of EDTA
concentration will be used to elute the adsorbed Ln's from the
titania column. A well-designed gradient elution can achieve
high-yield and high-purity separation for all the Ln's. Compared to
the ligand-assisted displacement chromatography, the
ligand-assisted elution chromatography process is more productive.
More importantly, the latter does not need harsh or expensive
chemicals for column regeneration, leading to a lower production
cost.
[0092] A continuous counter-current chromatography process can be
used to increase the productivity and reduce the cost of Ln's
separation. FIGS. 11a and 11b show the continuous counter-current
chromatography processes for the separation of 3 Ln elements (FIGS.
11a) and 15 Ln elements (FIG. 11b). Eluant 1-15 are EDTA solutions
with increasing concentrations, and Prod 1-15 are different Ln
elements. The effluents collected in the waste tanks are EDTA
solutions, which can be recycled and reused. An entire cycle for
ternary separation contains three major zones: feeding, elution,
and washing (FIG. 11a). In the feeding zone, the Ln mixture is
loaded onto the column. In the elution zone, different Ln's can be
eluted at different EDTA concentrations. In the washing zone, the
column is flushed by a diluted EDTA solution. The concentrated EDTA
solution in the effluent can be collected and reused. After the
washing step, the next cycle will start with the feeding step.
Since EDTA has significant selectivity for all adjacent Ln pairs
(Table 6), the separation process in FIG. 11a can also be extended
to 15 elements, as shown in FIG. 11b.
TABLE-US-00006 TABLE 6 Selectivity of EDTA for adjacent Ln's Ln
pairs .alpha..sub.EDTA Ce--La 3.7 Pr--Ce 2.5 Nd--Pr 1.8 Pm--Nd
Sm--Pm .alpha..sub.EDTA(Sm--Nd) = 3.2 Eu--Sm 1.5 Gd--Eu 1.05 Tb--Gd
4.2 Dy--Tb 2.3 Ho--Dy 2.6 Er--Ho 1.8 Tm--Er 3.1 Yb--Tm 1.8 Lu--Yb
1.9
[0093] As an example demonstrative of the method disclosed herein,
a preliminary cost analysis was conducted for the production of
Ln's based on the following assumptions: (1) The production scale
is 20,000 metric tons (m.t) per year, which is the annual capacity
claimed by MolyCorp, the major Ln production company in the United
States; (2) The production time is 320 days per year; (3) The price
for a single unit is $300,000 for batch process, and $1,000,000 for
continuous process, with a depreciation of 10 years; (4) The column
size is 3 m L.times.4.5 m ID; (5) The costs of chemicals (market
price in China, May 2014) are EDTA-$2,000/m.t, sorbent-$1,000/m.t
(life time-10 years), HCl-$200/m.t, NaCl-$100/m.t, oxalic
acid-$700/m.t, and the cost of water (market price in USA) is
$0.5/m.t; (6) The excess chemicals used in dissolution (HCl) and Ln
precipitation (oxalic acid) can be recycled; (7) The total feed
concentration of Ln's is 0.06 N, and the residence time L/u.sub.s
for the feed loading is 250 min, the same as those used in our
experimental tests (FIGS. 6a-6d and 7). The cost estimations for
batch and continuous processes are shown in Table 7. It is
noteworthy that if 99% of the EDTA can be recycled, the production
cost is estimated to be $3.4 per kilogram, which is lower than the
current production cost in China, $5.6/kg, and in Australia,
$10.1/kg.
[0094] The estimated cost for ligand-assisted elution is based on
EDTA at pH 9. Optimization of the pH of EDTA may reduce the cost to
below $3.4/kg.
TABLE-US-00007 TABLE 7 Preliminary estimations of production costs
Batch Continuous 95% 99% 95% 99% EDTA EDTA EDTA EDTA Recycle
Recycle Recycle Recycle Dissolution Cost (The excess acid 0.3 0.3
0.3 0.3 is assumed to be recycled) ($/kg) Capture and Salt Washing
0.2 0.2 0.2 0.2 Cost ($/kg) Separation Chemical ($/kg) 22.9 6.1 7.6
2 Cost ($/kg) Sorbent ($/kg) 0.3 0.3 0.1 0.1 Equip ($/kg) 1.3 1.3
0.1 0.1 Precipitation Cost ($/kg) 0.7 0.7 0.7 0.7 Total ($/kg) 25.7
8.9 9.0 3.4
[0095] Conclusions:
[0096] A ligand-assisted elution chromatography process has been
developed for the separation of Ln's. The mechanism of Ln
separation in the presence of a ligand has been studied. The Ln's
can be well separated only if the overall selectivity, which
approximates the ratio of the ligand selectivity to the sorbent
selectivity, is significantly greater than 1, and the dimensionless
complexation equilibrium constant K.sub.C[L] and the Langmuir a
value are in the same order of magnitude
(K.sub.C[L]/a.about.1).
[0097] Based on the analysis, several ligands have been tested,
among which EDTA was found to be the best ligand for separating the
Ln's on a titania column. The process was demonstrated by a ternary
separation of Pr, Nd, and Sm. Pure products of each element were
obtained under well-designed ligand concentrations. In order to
concentrate the products and shorten the cycle time,
linear-gradient elution was used, and the purities and yields for
all three elements were greater than 95%. Rate model simulations
taking into account adsorption, mass transfer, and reactions were
used to verify the proposed mechanisms and to elucidate the
dynamics of ligand-assisted separation. The effluent histories
obtained from the simulations agreed closely with the experimental
data.
[0098] As mentioned above, the processes herein disclosed can be
extended to separate other lanthanides or other species with
similar properties, including other rare earth elements or other
metal ions. For large-scale production, economical continuous
processes can be used for metal ion separation to increase the
productivity and lower the cost. A preliminary cost estimation for
rare earth element separation, for example, shows that if most of
the ligand (99%) is recycled and reused, the ligand-assisted
elution chromatography processes are environmentally benign and
less costly than the current solvent extraction processes.
[0099] Those skilled in the art will recognize that nigh-infinite
modifications can be made to the specific implementations described
above. The implementations should not be limited to the particular
limitations described. Other implementations may be possible.
* * * * *