U.S. patent application number 11/660295 was filed with the patent office on 2008-05-22 for metal extraction in liquid or supercritical-fluid solvents.
Invention is credited to Chien M. Wai, Joanna S. Wang.
Application Number | 20080115627 11/660295 |
Document ID | / |
Family ID | 35968234 |
Filed Date | 2008-05-22 |
United States Patent
Application |
20080115627 |
Kind Code |
A1 |
Wang; Joanna S. ; et
al. |
May 22, 2008 |
Metal Extraction In Liquid Or Supercritical-Fluid Solvents
Abstract
A method for separating metals from metal-containing materials
by extraction without generating large quantities of liquid waste
is disclosed. Also disclosed is an extractant composition for use
with this method. The method comprises exposing a metal-containing
material to a solvent, such as supercritical carbon dioxide, an
acid-base complex, and a chelating agent that is not a component of
the acid-base complex. The metal is released into the solvent by a
combination of oxidation by an oxidizing agent in the acid-base
complex and chelation by the chelating agent. The oxidizing agent
in the acid-base complex is solubilized by a solubilizing agent.
The disclosed method and composition have many applications and are
particularly well suited for the extraction of transition metals,
including, but not limited to, platinum group metals, nom a metals
and coinage metals. Applications include the recovery of metals
from scrap materials and the planarization of semiconductor
structures.
Inventors: |
Wang; Joanna S.; (Moscow,
ID) ; Wai; Chien M.; (Moscow, ID) |
Correspondence
Address: |
KLARQUIST SPARKMAN, LLP
121 SW SALMON STREET, SUITE 1600
PORTLAND
OR
97204
US
|
Family ID: |
35968234 |
Appl. No.: |
11/660295 |
Filed: |
August 18, 2005 |
PCT Filed: |
August 18, 2005 |
PCT NO: |
PCT/US05/29635 |
371 Date: |
February 13, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60603350 |
Aug 20, 2004 |
|
|
|
60658331 |
Mar 2, 2005 |
|
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Current U.S.
Class: |
75/718 ; 75/300;
75/717; 75/722 |
Current CPC
Class: |
C22B 59/00 20130101;
C22B 11/048 20130101; C22B 23/0461 20130101; Y02P 10/20 20151101;
Y02P 10/214 20151101; C22B 3/0095 20130101; B01D 11/0288 20130101;
C22B 11/046 20130101; Y02P 10/234 20151101; B01D 11/0203 20130101;
C22B 3/0005 20130101; B01D 11/0203 20130101; B01D 11/0288
20130101 |
Class at
Publication: |
75/718 ; 75/300;
75/717; 75/722 |
International
Class: |
C21B 15/00 20060101
C21B015/00; C22B 15/00 20060101 C22B015/00 |
Claims
1. A method for extracting a metal, comprising: exposing a
metal-containing material comprising the metal to a liquid or
supercritical-fluid solvent, an acid-base complex comprising an
oxidizing agent and a solubilizing agent, and a chelating agent
that is not a component of the acid-base complex; and extracting
for an extraction time sufficient for the chelating agent to form
metal-containing complexes with at least a portion of the
metal.
2. The method of claim 1, where the metal is not a lanthanide or
actinide.
3. The method of claim 1, where the metal is selected from
transition metals, transition metal oxides, transition metal
sulfides, zero-valent transition metals, noble metals, platinum
group metals, coinage metals, or combinations thereof.
4-9. (canceled)
10. The method of claim 1, where the solvent is a gas at room
temperature and atmospheric pressure.
11. The method of claim 1, where the solvent is a
supercritical-fluid solvent.
12. The method of claim 1, where the solvent is carbon dioxide.
13. The method of claim 1, where the solvent is supercritical
carbon dioxide.
14. The method of claim 1, where the oxidizing agent is selected
from mineral acids, and combinations thereof.
15. The method of claim 1, where the oxidizing agent is selected
from the group consisting of nitric acid, sulfuric acid, and
combinations thereof.
16. (canceled)
17. The method of claim 1, where the oxidizing agent is selected to
break down into volatile and/or soluble products after oxidizing
the metal.
18. The method of claim 1, where the oxidizing agent is selected to
break down into compounds that are gases at room temperature and
atmospheric pressure and/or water after oxidizing the metal.
19. The method of claim 1, where the solubilizing agent is an alkyl
phosphate.
20. The method of claim 1, where the solubilizing agent is selected
from the group consisting of tri-alkylphosphates,
tri-alkylphosphine oxides, and combinations thereof.
21-22. (canceled)
23. The method of claim 1, where the solubilizing agent is soluble
in supercritical carbon dioxide.
24. The method of claim 1, where the chelating agent is a
.beta.-diketone, a fluorinated .beta.-diketone, or combinations
thereof.
25. The method of claim 1, where the chelating agent is
fluorinated.
26-27. (canceled)
28. The method of claim 1, where the oxidizing agent is nitric acid
and the solubilizing agent is tributylphosphate.
29. The method of claim 1, where the acid-base complex has the
formula TBP(HNO.sub.3).sub.x(H.sub.2O).sub.y, in which x is greater
than or equal to about 0.7 and y is less than or equal to about
0.7.
30. The method of claim 1, where the acid-base complex has the
formula TBP(HNO.sub.3).sub.x(H.sub.2O).sub.y, in which x is about
1.0 and y is about 0.4.
31. The method of claim 1, where the metal-containing material is
exposed to a mixture comprising the solvent, the acid-base complex,
and the chelating agent, the mixture being substantially
non-aqueous, with the exception of coordinated water molecules, if
present, on the acid-base complex, the chelating agent, or the
metal-containing complexes, or any combination thereof
32. The method of claim 1, where the acid-base complex is a first
acid-base complex, the oxidizing agent is a first oxidizing agent,
the solubilizing agent is a first solubilizing agent, and further
comprising exposing the metal-containing material to a second
acid-base complex comprising a second oxidizing agent and a second
solubilizing agent, where the first oxidizing agent is nitric acid
and the second oxidizing agent is hydrochloric acid.
33. The method of claim 1, where the solvent is supercritical
carbon dioxide, the solubility of the oxidizing agent in
supercritical carbon dioxide is less than about 0.1 moles per liter
at 50.degree. C. and 100 atm, and the solubility of the acid-base
complex in supercritical carbon dioxide is greater than about 0.5
moles per liter at 50.degree. C. and 100 atm.
34. The method of claim 1, further comprising separating the
metal-containing complexes from the solvent by reducing the
pressure of the solvent, increasing the temperature of the solvent,
or both.
35. (canceled)
6. The method of claim 1, where the metal-containing material
comprises a copper film and the copper film is dissolved at a rate
greater than about 2 nmoles per second.
37. The method of claim 1, where the metal-containing material
comprises a palladium film and the amount of palladium in the
metal-containing material is reduced by 99% in less than about 30
seconds.
38. The method of claim 1, where the metal-containing material, the
solvent, the acid-base complex and the chelating agent form a
reaction mixture, and further comprising exposing the reaction
mixture to ultrasonic energy during at least a portion of the
extraction time.
39. (canceled)
40. The method of claim 1, further comprising recycling the
solvent, solubilizing agent, chelating agent, or any combination
thereof.
41. The method of claim 1, where the chelating agent is a first
chelating agent, and further comprising exposing the
metal-containing material to a second chelating agent that is not a
component of the acid-base complex for a period of time sufficient
for the first chelating agent, the second chelating agent, and the
metal to form adducts.
42. The method of claim 41, where the first chelating agent and the
second chelating agent are different.
43. The method of claim 1, where the chelating agent is a first
chelating agent, and further comprising exposing the
metal-containing material to a second chelating agent that is not a
component of the acid-base complex for a period of time sufficient
for the second chelating agent to displace one or more coordinated
water molecules on the metal-containing complexes.
44. The method of claim 43, where the first chelating agent and the
second chelating agent are different.
45. An extraction process, comprising: exposing a metal-containing
material comprising a metal other than a lanthanide or an actinide
to a liquid or supercritical-fluid solvent that is a gas at room
temperature and atmospheric pressure, an acid-base complex
comprising an a mineral acid oxidizing agent, an alkyl phosphate
solubilizing agent, and a .beta.-diketone chelating agent that is
not a component of the acid-base complex; and extracting the metal
from the metal-containing material for an extraction time effective
for the chelating agent to form metal-containing complexes with at
least a portion of the metal.
46. The method of claim 45, where the metal is a noble metal,
platinum group metal or coinage metal.
47. The method of claim 45, where the solvent is carbon dioxide,
the solubilizing agent is tributylphosphate, the oxidizing agent is
nitric acid, and the chelating agent is
hexafluoroacetylacetone.
48. (canceled)
49. The method according to claim 45 where the metal-containing
material is a semiconductor structure.
50. The method of claim 49, where the metal is copper.
51. (canceled)
52. The method of claim 49, where the solvent is carbon dioxide,
the solubilizing agent is tributylphosphate, and the oxidizing
agent is nitric acid.
53. The method of claim 49, further comprising contacting the
surface of the semiconductor structure with a porous pad.
54. The method according to claim 45 where the metal-containing
material comprises platinum, and the method further comprises
exposing a the platinum-containing material to the liquid or
supercritical-fluid solvent, a first acid-base complex comprising
nitric acid and a first solubilizing agent, a second acid-base
complex comprising hydrochloric acid and a second solubilizing
agent, and a chelating agent that is not a component of the first
acid-base complex or the second acid-base complex.
55. (canceled)
56. The method of claim 54, where the first solubilizing agent, the
second solubilizing agent, or both is/are tributylphosphate.
57. The method of claim 54, where the chelating agent is a
.beta.-diketone.
58. The method according to claim 1 where the metal-containing
material is a nanostructure.
59. The method of claim 58, where the metal is iron.
60. The method of claim 58, where the nanostructure comprises at
least one carbon nanotube.
61. (canceled)
62. The method of claim 58, where the solvent is carbon dioxide,
the solubilizing agent is tributylphosphate, and the oxidizing
agent is nitric acid.
63. An extractant composition, comprising: a liquid or
supercritical-fluid solvent; an acid-base complex comprising an
oxidizing agent and a solubilizing agent; and a chelating agent
that is not a component of the acid-base complex.
64. The extractant composition of claim 63, where the solvent is a
gas at room temperature and atmospheric pressure.
65. The extractant composition of claim 63, where the solvent is a
supercritical-fluid solvent.
66. The extractant composition of claim 63, where the solvent is
carbon dioxide.
67. The extractant composition of claim 63, where the solvent is
supercritical carbon dioxide.
68. The extractant composition of claim 63, where the oxidizing
agent is selected from mineral acids, and combinations thereof.
69-70. (canceled)
71. The extractant composition of claim 63, where the oxidizing
agent is selected to break down into volatile and/or soluble
products after oxidizing a metal.
72. The extractant composition of claim 63, where the oxidizing
agent is selected to break down into compounds that are gases at
room temperature and atmospheric pressure and/or water after
oxidizing a metal.
73. The extractant composition of claim 63, where the solubilizing
agent is an alkyl phosphate.
74. The extractant composition of claim 63, where the solubilizing
agent is selected from the group consisting of tri-alkylphosphates,
tri-alkylphosphine oxides, and combinations thereof.
75-76. (canceled)
77. The extractant composition of claim 63, where the solubilizing
agent is soluble in supercritical carbon dioxide.
78. The extractant composition of claim 63, where the chelating
agent is a .beta.-diketone a fluorinated .beta.-diketone, or
combinations thereof.
79. The extractant composition of claim 63, where the chelating
agent is fluorinated.
80-81. (canceled)
82. The extractant composition of claim 63, where the oxidizing
agent is nitric acid and the solubilizing agent is
tributylphosphate.
83. The extractant composition of claim 63, where the acid-base
complex has the formula TBP(HNO.sub.3).sub.x(H.sub.2O).sub.y, in
which x is greater than or equal to about 0.7 and y is less than or
equal to about 0.7.
84. The extractant composition of claim 63, where the acid-base
complex has the formula TBP(HNO.sub.3).sub.x(H.sub.2O).sub.y, in
which x is about 1.0 and y is about 0.4.
85. The extractant composition of claim 63, where the acid-base
complex is a first acid-base complex, the oxidizing agent is a
first oxidizing agent, the solubilizing agent is a first
solubilizing agent, and further comprising a second acid-base
complex comprising a second oxidizing agent and a second
solubilizing agent, where the first oxidizing agent is nitric acid
and the second oxidizing agent is hydrochloric acid.
86. The extractant composition of claim 63, where the chelating
agent is a first chelating agent and further comprising a second
chelating agent that is not a component of the acid-base complex.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 60/603,350, filed Aug. 20, 2004, and U.S.
Provisional Application Ser. No. 60/658,331, filed Mar. 2, 2005,
which are incorporated herein by reference.
FIELD
[0002] This disclosure relates to the extraction of metals with
chelating agents and acid-base complexes in liquid or supercritical
fluid solvents.
BACKGROUND
[0003] The separation of metals from metal-containing materials has
substantial practical significance. For example, the separation of
metals from industrial waste helps to detoxify the waste and can
recoup valuable metals for reuse. Some conventional separation
processes involve two primary steps. In a first step, the metal is
extracted from the metal-containing material by an initial
extraction process. In a second step, the metal is separated from
the materials used for or generated by the initial extraction
process. Dissolution of solids with an acid followed by solvent
extraction is an example of a widely used technique for extracting
metals from solid materials. This technique, however, typically
generates large amounts of hazardous waste, including spent
solvent. Disposal of this waste can be expensive and can have
adverse environmental consequences.
[0004] A more environmentally benign approach to metal separation
uses supercritical fluids comprising one or more chelating agents.
Methods based on this approach have been shown to separate metals
without the use of either organic solvents or aqueous solutions.
Furthermore, the chelating agents and supercritical fluids often
can be regenerated and reused multiple times. Various features of
supercritical fluid extraction of metals are disclosed in U.S. Pat.
Nos. 5,356,538, 5,606,724, 5,730,874, 5,770,085, 5,792,357,
5,840,193, 5,965,025, 6,132,491, 6,187,911, and U.S. Published
Patent Application No. 2003/0183043 ("the Wai patent documents"),
each of which is incorporated herein by reference.
[0005] The Wai patent documents reflect the development of
supercritical fluid extraction technology since 1991. U.S. Pat. No.
5,356,538 ("the '538 patent") discloses "exposing [a
metal-containing] solid or liquid to a supercritical fluid solvent
containing a fluorinated chelating agent, for a sufficient period
of time to form [metal-containing complexes] between the agent and
species that are solubilized in the supercritical fluid solvent."
See the '538 patent at column 15, lines 43-46. The '538 patent
explains that "the chelating agent is fluorinated to enhance [the]
solubility of the [metal-containing complexes] in supercritical
carbon dioxide." See the '538 patent at column 3, lines 31-33. The
'538 patent also states that "a modifier may be added to the
supercritical fluid to improve the solvent characteristics of the
supercritical fluid." See the '538 patent at column 7, lines
23-25.
[0006] A variety of chelating agents are proposed by the Wai patent
documents. For example, U.S. Pat. No. 5,606,724 ("the '724 patent")
states that the chelating agent can be "selected from the group
consisting of .beta.-diketones, halogenated .beta.-diketones,
phosphinic acids, halogenated phosphinic acids, carboxylic acids,
halogenated carboxylic acids, and mixtures thereof." See the '724
patent at column 2, lines 54-58. The '724 patent also states that,
at least for the extraction of metal from metal oxides, the
"chelating agents generally should be sufficiently acidic to donate
a proton to the metal oxide, thereby rendering the metal available
to form [metal-containing complexes] with the chelating agent." See
the '724 patent at column 2, lines 43-46.
[0007] Treating copper-containing substrates with "a `dry`
homogeneous solution in supercritical carbon dioxide that
contain[s] an oxidant, ethyl peroxydicarbonate (EPDC), and a
commercially available .beta.-diketone chelating agent" has been
disclosed. Bessel, C. A.; Denison, G. M.; Desimone, J. M.; DeYoung,
J.; Gross, S.; Schauer C. K.; Visintin, P. M. Etchant Solutions for
the Removal of Cu(0) in Supercritical CO.sub.2-Based "Dry" Chemical
Mechanical Planarization Process for Device Fabrication. J. Am.
Chem. Soc. 2003, 125(17), 4980 ("Bessel"). The method disclosed by
Bessel is not well suited for most commercial applications. In
part, this is because the method uses the oxidant EPDC, which is
highly explosive. On a large-scale, this method is too dangerous to
be of any practical value.
[0008] Extraction of uranium using an acid-base complex comprising
tributylphosphate (TBP) and nitric acid is disclosed in Samsonov,
M. D.; Wai, C. M.; Lee, S. C.; Kulyako, Y.; Smart, N. G.
Dissolution of Uranium Dioxide in Supercritical Fluid Carbon
Dioxide. Chem. Commun. 2001, 1868 ("Samsonov"), which is
incorporated herein by reference. Samsonov discloses that
"[u]ranium dioxide can be dissolved in supercritical CO.sub.2 with
a CO.sub.2-philic TBP-HNO.sub.3 [pair] to form a highly soluble
UO.sub.2(NO.sub.3).sub.22TBP complex" in a method that "requires no
water or organic solvents." See Samsonov at 1868.
[0009] The inventors of the present disclosure have discovered that
an acid-base complex, such as TBP-HNO.sub.3, while suitable for the
extraction of lanthanides and actinides, is limited in its ability
to extract other metals. For example, an acid-base complex alone is
not well suited for the separation of most transition metals, such
as platinum group metals, noble metals and coinage metals.
SUMMARY
[0010] Disclosed herein are embodiments of a method for separating
metals from metal-containing materials by extraction processes.
Also disclosed are embodiments of an extractant composition that
can be used with the disclosed method. The method can comprise, for
example, treating a metal-containing material with a solvent, a
chelating agent and an acid-base complex. The acid-base complex can
comprise an oxidizing agent that is capable of oxidizing the metal
and a solubilizing agent that is capable of solubilizing the
oxidizing agent in a solvent, such as liquid or supercritical fluid
carbon dioxide.
[0011] Some embodiments of the disclosed method are performed at
high pressures and/or low temperatures to maintain the solvent in
liquid or supercritical fluid form. The solvents, for example, can
be gases at room temperature and atmospheric pressure. Using these
solvents to replace the liquid solvents used in many conventional
metal extraction processes can make the disclosed method more
environmentally benign than conventional metal extraction
processes. Liquid or supercritical carbon dioxide is an example of
a suitable solvent for use with the disclosed method.
[0012] The oxidizing agent typically is the acid component of the
acid-base complex, while the solubilizing agent typically is the
base component of the acid-base complex. In some embodiments, the
solubility of the oxidizing agent in supercritical carbon dioxide
is less than about 0.1 moles per liter at 50.degree. C. and 100 atm
and the solubility of the acid-base complex in supercritical carbon
dioxide is greater than about 0.5 moles per 30 liter at 50.degree.
C. and 100 atm. One example of a suitable acid-base complex
comprises nitric acid as the oxidizing agent and TBP as the
solubilizing agent.
[0013] In some embodiments of the disclosed method, the metal in
the metal-containing material is first oxidized by the acid-base
complex. After being oxidized, the metal can form metal-containing
complexes with the chelating agent. This solubilizes the metal and
allows it to be separated from the metal-containing material. The
chelating agent can be, for example, a .beta.-diketone. To improve
its solubility in non-polar solvents, such as supercritical carbon
dioxide, the chelating agent can be fluorinated. In some
embodiments, the chelating agent is a fluorinated .beta.-diketone,
such as hexafluoroacetylacetone.
[0014] If present, coordinated water molecules on the
metal-containing complexes can reduce the solubility of the
metal-containing complexes in non-polar solvents, such as
supercritical carbon dioxide. In some embodiments of the disclosed
method, this effect is mitigated by exposing the metal-containing
complexes to a second, different chelating agent for a period of
time sufficient for the second chelating agent to displace the
coordinated water molecules. This can be, for example, a period
sufficient for the two chelating agents to form adducts with the
metal. The solubilizing agent originally paired with the oxidizing
agent also can serve to displace coordinated water molecules from
the metal-containing complexes. For example, TBP released from
TBP-HNO.sub.3 after oxidation of the metal can displace coordinated
water molecules on the metal-containing complexes.
[0015] The extracted metal can be recovered while the
metal-containing complexes are still within the solvent or after
the metal-containing complexes have been separated from the
solvent. The metal-containing complexes can be separated from the
solvent, for example, by converting the solvent into its gas form
while the metal-containing complexes remain in liquid form. This
can be done by reducing the pressure of the solvent and/or by
increasing the temperature of the solvent.
[0016] Embodiments of the disclosed method are particularly
effective at extracting transition metals, including, but not
limited to, platinum group metals, noble metals, coinage metals and
oxides and sulfides of these metals. Most transition metals cannot
be effectively extracted by an acid-base complex, such as
TBP-HNO.sub.3, without the addition of a separate chelating agent.
Moreover, even metals that can be adequately extracted by an
acid-base complex without the addition of a separate chelating
agent, such as lanthanides and actinides, often can be extracted
more efficiently by the addition of a separate chelating agent.
[0017] The disclosed metal separation method has many practical
applications. Some embodiments can be used to remove metals, such
as copper, from the surface of semiconductor structures. These
embodiments can be used, for example, in chemical mechanical
planarization processes in conjunction with mechanical polishing by
a porous pad. In addition, some disclosed embodiments are useful
for removing metals, such as iron, from nanostructures, such as
nanostructures comprising at least one carbon nanotube.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a phase diagram for carbon dioxide.
[0019] FIG. 2 is a schematic illustration of one embodiment of an
apparatus used to perform disclosed extractions.
[0020] FIG. 3A is a scanning electron microscope cross section of a
silicon wafer coated with a layer of copper before extraction.
[0021] FIG. 3B is a scanning electron microscope cross section of a
silicon wafer coated with a layer of copper after extraction at
40.degree. C. and 200 atm for 30 seconds.
[0022] FIG. 4A is a photograph through the quartz window of a view
cell in which copper is being extracted with supercritical carbon
dioxide, one minute into the extraction process.
[0023] FIG. 4B is a photograph through the quartz window of a view
cell in which copper is being extracted with supercritical carbon
dioxide, 30 minutes into the extraction process.
[0024] FIG. 5A is a photograph through the quartz window of a view
cell in which gold is being extracted with supercritical carbon
dioxide, one minute into the extraction process.
[0025] FIG. 5B is a photograph through the quartz window of a view
cell in which gold is being extracted with supercritical carbon
dioxide, 35 minutes into the extraction process.
[0026] FIG. 6 is a neutron activation analysis gamma spectrum of a
trap solution after supercritical fluid extraction of gold.
[0027] FIG. 7A is an energy dispersive X-ray spectrum of a gold pin
surface before supercritical fluid extraction.
[0028] FIG. 7B is an energy dispersive X-ray spectrum of a gold pin
surface after supercritical fluid extraction.
[0029] FIG. 8A is a .sup.19F nuclear magnetic resonance spectra of
a Pd(hfa).sub.2 standard.
[0030] FIG. 8B is a .sup.19F nuclear magnetic resonance spectra of
a Hhfa standard.
[0031] FIG. 8C is a .sup.19F nuclear magnetic resonance spectra of
the trap solution after supercritical fluid extraction of palladium
shot.
[0032] FIG. 9A is a .sup.19F nuclear magnetic resonance spectra of
the trap solution after supercritical fluid extraction of palladium
shot using TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7.
[0033] FIG. 9B is a .sup.19F nuclear magnetic resonance spectra of
the trap solution after supercritical fluid extraction of palladium
shot using TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4 with an
extraction time of 2 hours.
[0034] FIG. 9C is a .sup.19F nuclear magnetic resonance spectra of
the trap solution after supercritical fluid extraction of palladium
shot using TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4 with an
extraction time of 4 hours.
[0035] FIG. 10 is a neutron activation analysis gamma spectrum of a
trap solution after supercritical fluid extraction of
palladium.
[0036] FIG. 11 is a graph showing extraction efficiency for the
extraction of lanthanum, europium and praseodymium from soil
samples spiked with La.sub.2O.sub.3, Pr.sub.2O.sub.3, and
Eu.sub.2O.sub.3 using TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4.
[0037] FIG. 12 is a graph showing extraction efficiency for the
extraction of lanthanum, europium and praseodymium from soil
samples spiked with La.sub.2O.sub.3, Pr.sub.2O.sub.3, and
Eu.sub.2O.sub.3 using TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4 and
thenoyltrifluoroacetone.
DETAILED DESCRIPTION
[0038] Throughout this disclosure, the singular terms "a," "an,"
and "the" include plural referents unless the context clearly
indicates otherwise. Similarly, the word "or" is intended to
include "and" unless the context clearly indicates otherwise.
[0039] The following terms may be abbreviated in this disclosure as
follows: acetylacetone(AA), atmosphere (atm), boiling point (BP),
centimeter (cm), central processing unit (CPU), chemical mechanical
planarization (CMP), critical pressure (P.sub.C), critical
temperature (T.sub.C), energy dispersive X-ray spectrometer (EDS),
ethyl peroxydicarbonate (EPDC), half life (t1/2),
hexafluoroacetylacetone (Hhfa), inductively coupled plasma-atomic
emission spectrometry (ICP-AES), inner diameter (ID),
kiloelectronvolt (keV), megahertz (MHz), microgram (.mu.g),
microliter (.mu.L), milliliter (nL), millimeter (mm), molar (M),
nanometers (nm), neutron activation analysis (NAA), newton (N),
nuclear magnetic resonance (NMR), scanning electron microscope
(SEM), supercritical (sc), supercritical fluid extraction (SFE),
thenoyltrifluoroacetone (TTA), trifluoroacetylacetone (TFA),
tributylphosphate (TBP), trioctylphosphine oxide (TOPO),
tributylphosphine oxide (TBPO) and triple point (T.sub.P).
[0040] Described herein are embodiments of a method for extracting
metals in a solvent, such as supercritical carbon dioxide, and
embodiments of a composition for use with the disclosed method. The
disclosed method is a substantial improvement over conventional
methods. Some conventional methods, including those disclosed in
Samsonov and the '043 application, disclose the oxidation of metals
in conjunction with the chelation of metals. These methods
primarily are directed to the extraction of uranium. Oxidation in
this context was motivated by the fact that uranium dioxide in the
+4 oxidation state does not form stable complexes with commonly
known chelating agents. Thus, Samsonov and the '043 application
disclose the use of an oxidizing agent to convert uranium in
uranium dioxide to the +6 oxidation state, which does form stable
complexes with a number of chelating agents that are soluble in
supercritical carbon dioxide. As with uranium, many metals,
including most transition metals, typically must be oxidized before
they can be complexed with a proper chelating agent to form stable
metal-containing complexes that are sufficiently soluble in
non-polar solvents, such as supercritical carbon dioxide.
[0041] Samsonov and the '043 application disclose oxidation and
chelation by a single acid-base complex. See Samsonov at 1868 and
the '043 application at paragraph 20. This method is adequate for
oxidizing and complexing certain metals, including many lanthanides
and actinides, such as uranium, but it is not adequate for
oxidizing many other metals. The metals that can be oxidized and
chelated by a single acid-base complex are metals that can bond to
large numbers of ligands. For example, uranium and lanthanum can
form stable complexes with TBP-HNO.sub.3, such as
UO.sub.2(NO.sub.3).sub.22TBP and La(NO.sub.3).sub.3TBP,
respectively. Many other metals, however, cannot bond to large
numbers of ligands and will not form stable complexes with
TBP-HNO.sub.3. Among these metals are most transition metals, such
as platinum group metals, noble metals and coinage metals.
[0042] The efficiency of extraction processes directed to metals
that cannot bond to large numbers of ligands can be improved by the
addition of a chelating agent that is not a component of the
acid-base complex. By way of theory, the metal first is oxidized by
the oxidizing agent in the acid-base complex and then is chelated
by the separate chelating agent. Embodiments of the disclosed
method that use an acid-base complex and a chelating agent are
capable of efficiently extracting many metals that cannot be
efficiently extracted by conventional methods.
[0043] Bessel discloses using a .beta.-diketone chelating agent and
EPDC for extracting copper. Bessel, however, does not disclose an
oxidizing agent that is bound within an acid-base complex, such as
TBP-HNO.sub.3. Binding the oxidizing agent within an acid-base
complex is a significant improvement over Bessel. Because
supercritical carbon dioxide is non-polar and most effective
oxidizing agents are polar, very few effective oxidizing agents are
soluble in supercritical carbon dioxide without being associated
with another compound. The oxidizing agent disclosed by Bessel is
one of the few oxidizing agents that is soluble in supercritical
carbon dioxide without being associated with another compound, but
it also is highly explosive and not well suited for commercial
applications.
[0044] After oxidizing the metal, organic oxidizing agents, such as
EPDC, break up into organic fragments that can be difficult to
separate from the metal being recovered. In contrast, oxidizing
agents paired in acid-base complexes typically break down into
water, gases and other easily separable products. For example,
after reducing copper, nitric acid typically is converted into
water and nitrogen dioxide.
[0045] Acid-base complexes, such as TBP-HNO.sub.3, are superior
oxidizing agents for extracting most transition metals, such as
platinum group metals, noble metals, and coinage metals. Before
oxidation, these metals can be in a variety of forms, such as
zero-valent, oxide or sulfide form. Nitric acid is conventionally
used for metal dissolution and extraction, but alone it is not
soluble in supercritical carbon dioxide. In contrast, when nitric
acid is bound to a CO.sub.2-philic solubilizing agent such as TBP,
the resulting acid-base complex is highly soluble in supercritical
carbon dioxide. TBP therefore serves as a carrier for introducing
the acid into the supercritical carbon dioxide phase during metal
extractions.
[0046] As discussed above, embodiments of the disclosed method use
an oxidizing agent that is solubilized by incorporation into an
acid-base complex and at least one chelating agent that is not a
component of the acid-base complex. Surprisingly, it has been
discovered that the acid-base complex and the chelating agent have
a synergistic effect on the extraction process. In some
embodiments, such as in the extraction of lanthanides and
actinides, the oxidized metal can be bound by the acid-base complex
or by the separate chelating agent. With most metals, however, only
the chelating agent is capable of binding to the metal to form
stable complexes in the solvent.
[0047] Extraction processes incorporating both an acid-base complex
and at least one separate chelating agent potentially can be many
times faster than conventional extraction processes. The increased
extraction rate is especially pronounced with the extraction of
transition metals such as platinum group metals, noble metals and
coinage metals. For example, high extraction rates were observed
with TBP-HNO.sub.3 as the acid-base complex and a fluorinated
.beta.-diketone as the chelating agent. The examples below indicate
that in some implementations, the extraction rate is very fast
initially and then slows once the bulk of the metal has been
extracted. For example, in one trial about 50% of the copper in a
copper film was extracted after 30 seconds, about 98% was extracted
after 2 minutes and substantially all of the copper was extracted
after 4 minutes. Some embodiments of the disclosed method can
dissolve copper at a rate greater than about 2 nm per second,
typically greater than about 2.2 nm per second, and even more
typically greater than about 2.4 nm per second.
[0048] By way of theory, it is possible that in some embodiments of
the disclosed method, the solubilizing agent in the acid-base
complex serves a duel role. In addition to solubilizing the acid so
that the acid can oxidize the metal and thereby promote chelation,
certain solubilizing agents are capable of increasing the
solubility of the metal-containing complexes after formation of the
metal-containing complexes. For example, the solubilizing agent in
an acid-base complex may replace coordinated water molecules on the
metal-containing complexes, which increases the solubility of the
metal-containing complexes in non-polar solvents, such as
supercritical carbon dioxide. TBP is an effective solubilizing
agent for solubilizing both the acid and the metal-containing
complexes.
A. Metals
[0049] Embodiments of the present method are suitable for
extracting or purifying many different types of metals from
metal-containing materials. Metal-containing materials include
mixtures of metal and extraneous materials as well as
metal-containing compounds, such as metal oxides and metal
sulfides. In general, metals are elements that form positive ions
in solution and produce oxides that form hydroxides rather than
acids with water. More specifically, metals include all elements
other than metalloids and non-metals. The metalloids are boron,
silicon, germanium, arsenic, antimony, tellurium, and polonium. The
non-metals are hydrogen, carbon, nitrogen, oxygen, fluorine,
phosphorus, sulfur, chlorine, selenium, bromine, iodine, and
astatine. Throughout this disclosure, the term "metals" shall refer
to both metals and metalloids.
[0050] The genus of metals includes many species, including,
without limitation, alkali metals, alkali-earth metals, transition
metals, noble metals, coinage metals, rare metals, rare-earth
metals, transuranic metals, light metals, heavy metals and
radioactive metals. The extraction rates of metals that form stable
complexes with acid-base complexes, such as TBP-HNO.sub.3, can be
improved by some embodiments of the disclosed method. But disclosed
embodiments are particularly well suited for the extraction of
metals that do not form stable complexes with acid-base complexes,
such as TBP-HNO.sub.3. For example, some embodiments of the
disclosed method are well suited for the extraction of metals other
than lanthanides and actinides. Some embodiments of the disclosed
method are especially well suited for extracting transition metals,
including the sub-genuses of noble metals, platinum group metals
and coinage metals. Before oxidation by the oxidizing agent, these
metals can be in a variety of forms, such as zero-valent, oxide and
sulfide form.
[0051] There is some overlap among the sub-genuses of transition
metals. Noble metals, in general, are metals that are resistant to
oxidation. The noble metals are gold, silver, palladium, platinum,
rhodium, iridium, and osmium. The platinum group metals are
platinum, palladium, iridium, rhodium, ruthenium and osmium. The
coinage metals are copper, gold, nickel, silver and platinum.
B. Solvents
[0052] In embodiments of the disclosed method, the separation of
metals occurs in a solvent that can be a fluid and/or a
supercritical fluid. In some embodiments the solvent is a gas at
room temperature and atmospheric pressure. These solvents are
useful, in part, because they can be separated easily from the
metal-containing complexes by decreasing their pressure and/or
increasing their temperature.
[0053] A compound exists as a supercritical fluid when it is at a
temperature and pressure above a critical temperature and pressure
characteristic of the compound. Materials in a supercritical state
exhibit properties of both a gas and a liquid. Supercritical fluids
typically are able to act as solvents, like subcritical liquids,
while also exhibiting the improved penetration power of gases. This
makes supercritical fluids a preferred class of solvents for the
extraction of metals.
[0054] Suitable solvents include, but are not limited to, carbon
dioxide, nitrogen, ethylene, propane, and propylene. Carbon dioxide
is a preferred solvent for both subcritical and supercritical fluid
extractions because of its moderate chemical constants and its
inertness. Carbon dioxide has a critical temperature (T.sub.C) of
31.degree. C. and a critical pressure (P.sub.C) of 73 atm.
Supercritical carbon dioxide is non-explosive and thoroughly safe
for extractions. Carbon dioxide also is a preferred solvent because
it is abundantly available and relatively inexpensive.
[0055] FIG. 1 is a phase diagram for carbon dioxide, which shows
the conditions necessary to produce either subcritical liquid
carbon dioxide or supercritical carbon dioxide. Certain conditions
above the critical point produce a supercritical carbon dioxide
fluid solvent useful for metal extraction processes. Representative
conditions can be found in the examples below.
[0056] As an alternative to supercritical carbon dioxide, liquid
carbon dioxide is suitable for some embodiments of the disclosed
method. At room temperature carbon dioxide becomes a liquid above
5.1 atm. Depending on the pressure, liquid carbon dioxide has a
density comparable to or slightly greater than the density of
supercritical carbon dioxide. Thus, the solvation power of liquid
carbon dioxide is comparable to or slightly greater than that of
supercritical carbon dioxide. Liquid carbon dioxide is able to
dissolve metal-containing complexes, but liquid carbon dioxide does
not have the "gas-like" properties of supercritical carbon dioxide.
Liquid carbon dioxide has a high viscosity, a low diffusivity, and
consequently a poor penetration power compared to supercritical
carbon dioxide. The extraction efficiency of liquid carbon dioxide
may depend on the applied pressure. In addition, it may be possible
to improve the extraction efficiency of liquid carbon dioxide by
applying agitation.
[0057] The liquid and supercritical fluid solvents used in
embodiments of the disclosed method may be used individually or in
combination. Examples of suitable solvents, and their critical
temperatures and pressures, are shown in Table 1.
TABLE-US-00001 TABLE 1 Physical Properties of Selected
Supercritical Fluids Molecular Fluid Formula T.sub.C (.degree. C.)
P.sub.C (atm) Carbon dioxide CO.sub.2 31.1 72.9 Ammonia NH.sub.3
132.5 112.5 n-Pentane C.sub.5H.sub.12 196.6 33.3 n-Butane
C.sub.4H.sub.10 152.0 37.5 n-Propane C.sub.3H.sub.6 96.8 42.0
Sulfur hexafluoride SF.sub.6 45.5 37.1 Trifluoromethane CHF.sub.3
25.9 46.9 Methanol CH.sub.3OH 240.5 78.9 Ethanol C.sub.2H.sub.5OH
243.4 63.0 Isopropanol C.sub.3H.sub.7OH 235.3 47.0 Diethyl ether
(C.sub.2H.sub.25).sub.2O 193.6 36.3 Water H.sub.2O 374.1 218.3
C. Solubility Modifiers
[0058] In some embodiments of the disclosed method, a modifier can
be added to the solvent to vary the characteristics thereof. For
example, a modifier can be added to the solvent to enhance the
solubility of a particular complexed metal. Some useful modifiers
are low-to-medium boiling point alcohols and esters, such as lower
alkyl alcohols and esters. As used herein, the term "lower alkyl"
refers to compounds having ten or fewer carbon atoms, and includes
both straight-chain and branched-chain compounds and all
stereoisomers. Typical modifiers can be selected from the group
consisting of methanol, ethanol, ethyl acetate, and combinations
thereof. The modifiers are added to the solvent in an amount
sufficient to vary the characteristics thereof. This can be an
amount, for example, between about 0.1% and about 20.0% by weight.
The modifiers contemplated for use with embodiments of the
disclosed method most typically are not supercritical fluids at the
disclosed operating conditions. Rather, the modifiers simply are
dissolved in the liquid and/or supercritical fluid solvents to
improve their solvent properties.
[0059] In one embodiment of the disclosed method, a modifier is
combined with the solvent prior to introducing the solvent into an
extraction vessel. Alternatively, the solvent and the modifier can
be added to the extraction vessel separately.
D. Chelating Agents
[0060] A partial list of chelating agents useful for solubilizing
metals in non-polar solvents is provided in Table 2. The list is
for illustration only. Other chelating agents, whether now known or
hereafter discovered, useful for forming metal-containing complexes
also may be used to practice embodiments of the disclosed method.
Beneficial factors to consider in the selection of chelating agents
include, but are not limited to, high stability constants of the
metal-containing complexes formed, fast complexation kinetics, good
solubility in the solvent for both the chelating agent and the
metal-containing complexes formed, and sufficient specificity to
allow selective extraction of a metal or a group of metal ions.
TABLE-US-00002 TABLE 2 Metal Chelating Agents Oxygen Donating
Chelating Agents cupferron chloranilic acid and related reagents
.beta.-diketones and related reagents
N-benzoyl-N-phenylhydroxylamine and related reagents crown ethers
and calixarenes dibutylcellosolve
(C.sub.4H.sub.9OC.sub.2H.sub.4OC.sub.4H.sub.9) octanol-2 and
related reagents methyl isobutyl ketone and related reagents
Nitrogen Donating Chelating Agents .alpha.-dioximes
diaminobenzidine and related reagents porphyrins and related
reagents Oxygen and Nitrogen Donating Chelating Agents
8-hydroxyquinoline nitrosonaphthols and nitrosophenols
diphenylcarbazide and diphenylcarbazone tri-alkyl amines, such as
(C.sub.nH.sub.2n+1).sub.3N and related reagents Phosphate and
Phosphine Oxide Chelating Agents tri-n-alkylphosphine oxide, alkyl
groups = C.sub.4H.sub.9, C.sub.8H.sub.17, etc. n-tributyl phosphate
and related reagents bis(2,4,4-trimethylpentyl)phosphinic acid
(Cyanex 272) and related reagents Sulfur Donating Chelating Agents
dithizone and related reagents bismuthiol II thioxine
(C.sub.9H.sub.7NS.cndot.2H.sub.2O)
bis(2,4,4-trimethylpentyl)-monothiophosphinic acid (Cyanex 302) and
related reagents Bis(2,4,4-trimethylpentyl)-dithiophosphinic acid
(Cyanex 301) and related reagents dioctyl sulfide
[(C.sub.8H.sub.17).sub.2S] dioctyl sulfoxide
[(C.sub.8H.sub.17).sub.2SO] thiourea derivatives and related
reagents
[0061] For extracting metals directly from metal oxides or
sulfides, the chelating agent preferably is a protic chelating
agent with enough acidity to break the chemical bonds in the metal
oxides or sulfides. Once these bonds are broken, the metal can, in
some embodiments, form metal-containing complexes with the
chelating agent. Without limiting the scope of the disclosure,
chelating agents that are particularly useful for forming
metal-containing complexes with the metal of metal oxides and
sulfides include .beta.-diketones and related reagents, phosphinic
acids and related reagents, and carboxylic acids and related
reagents.
[0062] Mixtures of chelating agents can be useful in some disclosed
embodiments. Without limiting the present disclosure, the
initially-formed, metal-containing complexes can include at least
one water molecule complexed with the metal and chelating agent.
The presence of at least one water molecule increases the polarity
of the metal-containing complexes. In some embodiments, the at
least one water molecule can be displaced by exposing the
initially-formed, metal-containing complexes to a separate,
different chelating agent. Removing the at least one water molecule
can substantially increase the solubility of the metal-containing
complexes in the extraction medium. Metals having at least two
different classes of chelating agents coupled thereto are referred
to herein as "adducts."
[0063] A person of ordinary skill in the art will realize that the
selection of a chelating agent, or mixture of chelating agents,
will depend on a number of factors, including the ability of the
chelating agent(s) to form metal-containing complexes with a
particular metal, the availability of the chelating agent(s), the
toxicity of the chelating agent(s), etc. A variety of chelating
agents also can be selected for use as the second, different
chelating agent for displacing metal-bound substances, such as
water. In some embodiments, basic chelating agents are preferred
for use as the second, different chelating agent for displacing
polar compounds, such as water. For example, phosphorous-containing
chelating agents, such as phosphates and phosphinic acids, are
highly effective at displacing water molecules from the
initially-formed, metal-containing complexes.
[0064] Without limitation, chelating agents that may be useful for
practicing embodiments of the disclosed method include
.beta.-diketones, phosphine oxides (such as trialkylphosphine
oxides, triarylphosphine oxides, and alkylarylphosphine oxides),
phosphinic acids, carboxylic acids, phosphates (such as
trialkylphosphates, triarylphosphates, and alkylarylphosphates),
crown ethers, phosphine sulfides, phosphorothioic acids,
thiophosphinic acids, halogenated analogs of these chelating
agents, and mixtures of these chelating agents. Some of the useful
chelating agents have lower alkyl functional groups.
Alkyl-substituted chelating agents with chain lengths of about
eight carbons, especially branched-chain alkyl groups, are
characterized by high solubilities in supercritical carbon dioxide.
Some of the chelating agents useful for practicing embodiments of
the present method are discussed in detail below.
[0065] 1. .beta.-diketones
[0066] The carbon atoms of a ketone are assigned Greek letters to
designate positions along the carbon chain relative to the carbonyl
carbon. The first carbon adjacent the carbonyl carbon is designated
.alpha., the second such carbon is designated .beta., and so on. A
.beta.-diketone has at least two ketone carbonyls wherein one
ketone carbonyl is located on a carbon .beta. to the other ketone
carbonyl. The .beta.-diketones that can be used as chelating agents
in embodiments of the present method generally satisfy the
following Formula 1:
##STR00001##
wherein R.sub.1 and R.sub.2 typically are selected independently
from the group consisting of aliphatic groups, including
functionalized aliphatic groups, particularly alkyl groups and even
more particularly lower alkyl groups, such as halogenated lower
alkyl groups, aryl groups, halogenated aryl groups, thenoyl groups,
and mixtures thereof. As used herein, a "halogenated lower alkyl
group," such as a fluorinated ethyl group, has at least one of the
hydrogen atoms present on the alkyl group replaced with a halogen
atom, such as a fluorine atom. The term "halogenated lower alkyl
group" also can refer to compounds wherein all or any number of the
hydrogen atoms have been replaced with halogen atoms, such as
fluorine atoms.
[0067] In some suitable .beta.-diketones, R.sub.1 and R.sub.2 of
Formula 1 are selected independently from the group consisting of
methyl groups, fluorinated methyl groups, trifluoromethyl groups,
ethyl groups, fluorinated ethyl groups, pentafluoroethyl groups,
propyl groups, fluorinated propyl groups, heptafluoropropyl groups,
butyl groups, fluorinated butyl groups, and nonafluorobutyl groups.
Specific examples of suitable .beta.-diketones include, without
limitation, acetylacetone, dibutyldiacetate,
trifluoroacetylacetone, hexafluoroacetylacetone,
thenoyltrifluoroacetylacetone, and
heptafluoro-butanoylpivaroylmethane. For some embodiments, the
preferred .beta.-diketones are hexafluoroacetylacetone and
dibutyldiacetate.
[0068] The chelating agent can be halogenated to enhance its
solubility and/or the solubility of the metal-containing complexes
in supercritical carbon dioxide. For some embodiments, the
preferred chelating agents are halogenated. Some halogenated,
metal-containing complexes, particularly fluorinated,
metal-containing complexes, are characterized by solubilities in
supercritical carbon dioxide that are enhanced by two to three
orders of magnitude relative to the solubilities of the
corresponding non-halogenated, metal-containing complexes in
supercritical carbon dioxide. For illustrative purposes only, and
without limiting the present disclosure, a suitable fluorinated
chelating agent is shown below:
##STR00002##
[0069] Most types of .beta.-diketones are commercially available.
They can be purchased, for example, from Sigma-Aldrich (St. Louis,
Mo.).
[0070] In general, .beta.-diketones form stable complexes with
metals and hence are useful complexing agents for extracting metals
from metal-containing materials. For example, the fluorinated
.beta.-diketones shown in Table 3 can be used with supercritical
carbon dioxide solvents for the extraction of metal ions. All of
the .beta.-diketones shown in Table 3, except
thenoyltrifluoroacetone, are liquids at room temperature and
atmospheric pressure.
TABLE-US-00003 TABLE 3 Fluorinated .beta.-diketones Used for the
Extraction of Metal Ions Using Supercritical Carbon Dioxide
.beta.-diketone R1 R2 MW BP Acetylacetone (760 Torr) CH.sub.3
CH.sub.3 100.12 139.degree. Trifluoroacetylacetone CH.sub.3
CF.sub.3 154.09 107.degree. Hexafluoroacetylacetone CF.sub.3
CF.sub.3 208.06 70.degree.-71.degree. Thenoyltrifluoroacetone (9
Torr) Thenoyl CF.sub.3 222.18 103.degree.-104.degree.
Heptafluorobutanoylpivaroyl- C(CH.sub.3).sub.3 C.sub.3F.sub.7
296.18 33.degree. methane (2.7 Torr)
[0071] .beta.-diketones exist in at least two tautomeric forms, the
"keto" tautomer and the "enol" tautomer. Tautomerism is a type of
isomerism in which migration of a hydrogen atom results in two or
more different structures called tautomers. By way of theory,
.beta.-diketones react with metal ions to form metal-containing
complexes either through the enol tautomer or through an enolate
anion, which is a negatively charged enol tautomer. The following
equilibrium illustrates the tautomeric forms of a
.beta.-diketone:
##STR00003##
[0072] In some applications, such as the extraction of metals from
complex matrixes, the presence of a small amount of water can
significantly increase the extraction efficiency. Without limiting
the present disclosure to one theory of operation, water molecules
may facilitate the release of the metal from the metal-containing
material. One skilled in the art will realize that the amount of
water used during the extraction process may vary. In some
embodiments, extraction efficiency may be sufficiently increased by
adding about 1 .mu.L of water per 1 .mu.g of metal ions.
[0073] 2. Phosphinic Acids
[0074] As used herein, "phosphinic acid" refers to an organic
derivative of hypophosphorous acid [HP(OH).sub.2]. The phosphinic
acid chelating agents that are useful for practicing embodiments of
the disclosed method generally satisfy the following Formula 2:
##STR00004##
wherein R.sub.3 and R.sub.4 are selected independently from the
group consisting of aliphatic groups, including functionalized
aliphatic groups, particularly alkyl groups and even more
particularly lower alkyl groups, such as halogenated lower alkyl
groups, aryl groups, halogenated aryl groups, thenoyl groups, and
mixtures thereof. In some embodiments, R.sub.3 and R.sub.4
preferably are lower alkyl or fluorinated lower alkyl groups. One
example, without limitation, of a suitable phosphinic acid
chelating agent is bis(2,4,4-trimethylpentyl)phosphinic acid
(CYANEX 272)as shown below:
##STR00005##
[0075] Thiophosphinic acids also are useful for performing
extractions. Examples, without limitation, of thiophosphinic acids
include bis(2,4,4-trimethylpentyl)-dithiophosphinic acid (CYANEX
301), bis(2,4,4-trimethylpentyl)-monothiophosphinic acid (CYANEX
302), and analogs thereof. The structures of CYANEX 301 and CYANEX
302 are shown as follows:
##STR00006##
[0076] 3. Carboxylic Acids
[0077] The carboxylic acids generally useful for practicing
embodiments of the disclosed method typically satisfy the following
Formula 3:
##STR00007##
wherein R.sub.5 generally is selected from the group consisting of
aliphatic groups, including functionalized aliphatic groups,
particularly alkyl groups and even more particularly lower alkyl
groups, such as halogenated lower alkyl groups, aryl groups,
halogenated aryl groups, thenoyl groups, and mixtures thereof.
R.sub.5 also can be functionalized, such as with functional groups
including without limitation, hydroxyl, carbonyl and amine
groups.
[0078] Examples, without limitation, of carboxylic acids that
satisfy Formula 3 include methanoic acid (also referred to as
formic acid), ethanoic acid, propanoic acid, butanoic acid,
pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid,
nonionic acid, decanoic acid, and branched analogs of these
compounds. Some unsaturated carboxylic acids, such as acrylic acid
and methacrylic acid, as well as cyclic carboxylic acids, such as
cyclohexanecarboxylic acid, also can be used as suitable chelating
agents for the extraction of some metals. In addition, some
halogenated forms of alkyl, unsaturated alkyl, and cyclic
carboxylic acids can be used as suitable chelating agents for the
extraction of some metals. Examples, without limitation, of aryl
carboxylic acids suitable as chelating agents include benzoic acid
and phenylacetic acid, as well as halogenated derivatives
thereof.
[0079] 4. Phosphates
[0080] The phosphates generally useful for practicing embodiments
of the disclosed method typically satisfy the following Formula
4:
##STR00008##
wherein R.sub.6-R.sub.8 are selected independently from the group
consisting of aliphatic groups, including functionalized aliphatic
groups, particularly alkyl groups and even more particularly lower
alkyl groups, such as halogenated lower alkyl groups, aryl groups,
halogenated aryl groups, thenoyl groups, and mixtures thereof.
[0081] 5. Crown Ethers
[0082] Crown ethers generally useful for practicing embodiments of
the disclosed method typically satisfy the following Formula 5:
##STR00009##
wherein x is OH or NHOH and (X) is a dibenzo crown ether of the
formula dibenzo [13+3 m ]-crown-[4+m] ether, m is an integer from 0
to about 5, n is an integer from 0 to about 6, and R.sub.9 is
hydrogen or a lipophilic hydrocarbyl group having from 1 to about
18 carbon atoms. For example, (X) can be represented as:
##STR00010##
In some embodiments, R.sub.9 can be selected from the group
consisting of alkyl groups, cycloalkyl groups, alkenyl groups, and
aryl groups.
[0083] In some embodiments, the crown ether is represented by the
following Formula 6:
##STR00011##
wherein X is OH or NHOH; R.sub.10 and R.sub.11 are selected
independently from the group consisting of aliphatic groups,
including functionalized aliphatic groups, particularly alkyl
groups and even more particularly lower alkyl groups, such as
halogenated lower alkyl groups, aryl groups, halogenated aryl
groups, thenoyl groups, and mixtures thereof; and R.sub.12 and
R.sub.13 are selected independently from the group consisting of
hydrogen and halogens.
[0084] 6. Calixarene Crown Ethers
[0085] The fluorinated calixarene crown ethers, also known as
molecular baskets, generally useful for practicing embodiments of
the disclosed method typically satisfy the following Formula 7:
##STR00012##
In Formula 7, R, R' and n are defined by any row in Table 4.
TABLE-US-00004 [0086] TABLE 4 R-Groups in Calixarene Crown Ethers R
R' n t-butyl H 1
(CH.sub.2).sub.3S(CH.sub.2).sub.2(CF.sub.2).sub.7CF.sub.3 H 1
t-butyl H 3 t-butyl CH.sub.2(C.dbd.O)OCH.sub.2CH.sub.3 1
(CH.sub.2).sub.3S(CH.sub.2).sub.2(CF.sub.2).sub.7CF.sub.3
CH.sub.2(C.dbd.O)OCH.sub.2CH.sub.3 1 t-butyl CH.sub.2(C.dbd.O)NHOH
1 (CH.sub.2).sub.3S(CH.sub.2).sub.2(CF.sub.2).sub.7CF.sub.3
CH.sub.2(C.dbd.O)NHOH 1 t-butyl CH.sub.2COOH 1
(CH.sub.2).sub.3S(CH.sub.2).sub.2(CF.sub.2).sub.7CF.sub.3
CH.sub.2COOH 1
(CH.sub.2).sub.3S(CH.sub.2).sub.2(CF.sub.2).sub.7CF.sub.3
CH.sub.2(C.dbd.S)N(C.sub.2H.sub.5).sub.2 1
(CH.sub.2).sub.3S(CH.sub.2).sub.2(CF.sub.2).sub.7CF.sub.3;
CH.sub.2(C.dbd.O)N(CH.sub.2CH.sub.2SH)(CHCH.sub.3CH.sub.2CH.sub.3)
1 t-butyl
CH.sub.2(C.dbd.O)O(CH.sub.2).sub.2NH(C.dbd.S)NH(CH.sub.2).sub.3S(C-
H.sub.2).sub.2(CF.sub.2).sub.7CF.sub.3 1
[0087] 7. Thiourea and Derivatives
[0088] The thiourea derivatives generally useful for practicing
embodiments of the disclosed method typically satisfy the following
Formula 8:
##STR00013##
In Formula 8, when R.dbd.R'.dbd.H, the chemical structure is
thiourea. In some thiourea derivatives, R and R' are selected from
alkyl groups, phenyl groups and their fluorinated derivatives.
E. Acid-Base Complexes
[0089] Some disclosed acid-base complexes comprise an oxidizing
agent and a solubilizing agent. In general, the oxidizing agent is
capable of oxidizing the metal to be extracted and the solubilizing
agent is capable of solubilizing the oxidizing agent in a solvent,
such as a non-polar solvent (e.g. supercritical carbon dioxide). As
described in the examples below, an oxidizing agent, such as
HNO.sub.3, can be combined with a CO.sub.2-philic solubilizing
agent, such as TBP, to form an acid-base complex that is soluble in
non-polar solvents, such as supercritical carbon dioxide. Without
the presence of a solubilizing agent, many oxidizing agents, such
as HNO.sub.3, are insoluble in supercritical carbon dioxide.
[0090] The oxidizing agent typically is the acid component of the
acid-base complex. Suitable oxidizing agents include Lewis acids,
Bronsted-Lowry acids, mineral acids, and combinations thereof.
Specific examples include, but are not limited to, nitric acid and
sulfuric acid. In some embodiments, the oxidizing agent is a
compound that, after oxidizing the metal, is converted into
products that are easily separable from the metal being extracted.
For example, in some embodiments, the oxidizing agent is a
non-organic acid.
[0091] The solubilizing agent typically is the base component of
the acid-base complex. Suitable solubilizing agents to be paired
with the oxidizing agents include Lewis bases, Bronsted-Lowry
bases, and combinations thereof. Solubilizing agents that are well
suited for use with the disclosed method include phosphates, such
as R.sub.3PO, (RO)R.sub.2PO and (RO).sub.2RPO, in which R is an
alkyl group, such as a butyl group or an octyl group. Examples of
useful solubilizing agents include, but are not limited to,
tri-alkylphosphates, such as tri-butylphosphate (TBP),
tri-alkylphosphine oxides, such as tri-octylphosphine oxide (TOPO)
and tri-butylphosphine oxide (TBPO), other Lewis bases soluble in
supercritical carbon dioxide and combinations thereof.
[0092] TBP-HNO.sub.3 can be prepared, for example, by mixing TBP
with a concentrated nitric acid solution. Nitric acid dissolves in
the TBP phase forming a Lewis acid-base complex of the general
formula TBP(HNO.sub.3).sub.x(H.sub.2O).sub.y, which is separable
from the remaining aqueous phase. The x and y values depend on the
relative amount of TBP and nitric acid used in the preparation.
TBP-HNO.sub.3 complexes of different x and y values have been
characterized by conventional titration methods as well as by
proton nuclear magnetic resonance (NMR) spectroscopy. Data for
different complexes are shown in the examples below. The complex is
soluble in super critical carbon dioxide and becomes miscible with
super critical carbon dioxide at high pressures.
F. Extraction Procedures
[0093] In some embodiments of the disclosed method, the acid-base
complex is formed prior to the extraction process. Once formed, the
acid-base complex is placed in a reaction vessel along with the
metal-containing material and a chelating agent. In some
embodiments, a separate, different chelating agent also is added at
this time. The solvent is introduced into the vessel and the vessel
then is pressurized to, such as to a pressure greater than the
critical pressure of the solvent. As described in the '043
application, the extraction efficiency of at least some extractions
can be improved by subjecting the contents of the reaction vessel
to ultrasonic vibrations.
[0094] After chelation, the metal-containing complexes then are
separated from the solvent. This can be done, for example, by
reducing the pressure of the solvent thereby converting the solvent
from a liquid or supercritical fluid into a gas. Any reduction of
the pressure of the solvent can facilitate precipitation of metals
and/or metal-containing complexes suspended in the solvent. In some
embodiments, the pressure can be reduced to approximately
atmospheric pressure, and the solvent expanded into a collection
container. The gaseous solvent can be reused, such as by recycling
it back through the extraction process.
[0095] Before or after separation from the solvent, the elemental
metal can be separated from the metal-containing complexes by any
number of known methods, including treatment with concentrated
nitric acid. The reduction of .beta.-diketone complexes of
palladium, copper, silver and other metals dissolved in
supercritical carbon dioxide to their elemental state in the
presence of hydrogen fluoride has been reported. Ye, X. R.; Wai, C.
M.; Zhang, D. Q.; Kranov, Y.; McIlroy, D. N.; Lin, Y.; Engelhard,
M. Immersion Deposition of Metal Films on Silicon and Germanium
Substrates in Supercritical Carbon Dioxide. Chem. Mater. 2003, 15,
83 ("the Wai and Zhang article"). In the Wai and Zhang article, the
metals formed thin films on silicon surfaces after separation from
.beta.-diketone complexes. The reduction of the .beta.-diketone
complexes of these metals was attributed to the following reaction
(M=metal):
4HF+Si.degree.+2M(hfa).sub.2.rarw.SiF.sub.4+4Hhfa+2 M.degree.
[0096] It also may be possible to reduce metal .beta.-diketone
complexes in solvents, such as supercritical carbon dioxide, by
adding a reducing agent such as H.sub.2 or NaBH.sub.3CN to the
fluid phase. This was reported in Blackburn, J. M.; Long, D. P.;
Cabanas, A.; Watkins, J. J. Deposition of Conformal Copper and
Nickel Films from Supercritical Carbon Dioxide. Science 2001,
294(5), 141 and Blackburn, J. M.; Long, D. P.; Watkins, J. J.
Reactive Deposition of Conformal Palladium Films from Supercritical
Carbon Dioxide Solution. Chem. Mater. 2000, 12, 2625. Copper and
palladium films can be formed by hydrogen reduction of
Cu(hfa).sub.2 or Pd(hfa).sub.2 at moderate temperatures in
supercritical carbon dioxide. Similar processes for the separation
of other metals from metal-containing complexes are known in the
art.
G. Applications
[0097] The disclosed metal separation method has many practical
applications. For example, some embodiments of the disclosed method
can be used to recover valuable metals from waste materials, such
as gold from abandoned electronics and platinum and palladium from
used catalysts. In addition, some embodiments of the disclosed
method can be used to remove metal from the surfaces of
semiconductor structures, such as in CMP processes in conjunction
with mechanical polishing by a porous pad. The removal of metal,
particularly copper, is an important part of chemical mechanical
planarization (CMP) processes used in the manufacture of
semiconductor devices.
[0098] Some embodiments of the disclosed metal separation method
also can be used to remove contaminants from nanostructures, such
as carbon nanotubes. For example, some embodiments of the disclosed
method can be used to remove metal catalysts from nanostructures.
Metal catalysts are commonly used in the synthesis of
nanostructures. These catalysts typically become contaminants after
the synthesis processes are complete. Iron, for example, often is
used in the synthesis of carbon nanotubes. Experiments have
demonstrated that some embodiments of the disclosed method are well
suited for removing iron from carbon nanotubes. These and other
embodiments also can be used in connection with other
nanostructures and/or other metal contaminants.
EXAMPLES
[0099] The following examples are provided to illustrate certain
particular embodiments of the disclosure. Additional embodiments
not limited to the particular features described are consistent
with the following examples.
[0100] Among other features of the present disclosure, the
following examples illustrate that metals (such as copper, gold,
palladium and platinum) in different structures (such as films,
coatings, and solid granules) can be dissolved in solvents, such as
supercritical carbon dioxide, comprising an oxidizing agent, such
as HNO.sub.3, and a chelating agent, such as a fluorinated
.beta.-diketone. The HNO.sub.3, for example, can be introduced via
an acid-base complex comprising a CO.sub.2-philic solubilizing
agent, such as TBP. Other CO.sub.2-insoluble oxidizing agents also
can be introduced into non-polar solvents, such as supercritical
carbon dioxide, by the same principle. It will be evident that the
rapid supercritical fluid dissolution process described in the
following examples can be applied to the extraction of a variety of
metals for a variety of useful purposes.
Example 1
Preparing the Materials and Equipment
[0101] TBP was purchased from Alfa Aesar (Ward Hill, Mass.). Nitric
acid [69.5% (w/w)] was obtained from Fisher Chemical (Fair Lawn,
N.J.). The complexes of TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7
and TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4 were prepared by the
methods described in Enokida, Y.; El-Fatah, S. A.; Wai, C. M.
Ultrasound Enhanced Dissolution of UO.sub.2 in Supercritical
CO.sub.2 Containing a CO.sub.2-Philic TBP-HNO.sub.3 Complexant.
Ind. Eng. Chem. Res. 2002, 41, 2282 and Enokida, Y.; Tomika, O.;
Lee, S. C.; Rustenholtz, A.; Wai; C. M. Characterization of a
Tri-n-butyl Phosphate-Nitric Acid Complex: a CO.sub.2-soluble
Extractant for Dissolution of Uranium Dioxide. Ind. Eng. Chem. Res.
2003, 42, 5037 ("the Enokida references"), which are incorporated
herein by reference. For example, in some trials the
TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 was prepared by mixing 5
mL of TBP with 0.82 mL of concentrated HNO.sub.3 (15.5 M). The
solubility of the TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 was
about 1.7 mole % in supercritical carbon dioxide at 40.degree. C.
and 110 atm.
[0102] Instrument grade carbon dioxide (purity 99.99%) was obtained
from Oxarc (Spokane, Wash.). Palladium shot (1 mm diameter), copper
shot (3 mm diameter), Hhfa, Pd(hfa).sub.2 and Cu(hfa).sub.2 were
purchased from Sigma-Aldrich (St. Louis, Mo.). De-ionized water
(Millipore Milli-Q system, Bedford, Mass.) was used for the
preparation of all aqueous solutions. The surface morphology of the
metal films was examined by an Amray 1830 scanning electron
microscope (SEM). An energy dispersive X-ray spectrometer (EDS) was
used to measure the metal film composition. A Brucker 300 MHz
nuclear magnetic resonance (NMR) spectrometer was used to identify
metal-containing complexes dissolved in the carbon dioxide
phase.
[0103] Silicon wafers (approximately one millimeter thick and
coated with copper layers) were obtained from Micron Technology
(Boise, Id.). CPU processors with gold connectors and copper strips
(0.8.times.2 cm.sup.2), including IBM and Intel PENTIUM.RTM.
processors, were cut from circuit boards. The size of the gold pin
connectors from the CPU processors was about 0.4 cm
(length).times.480 .mu.m (diameter) with an average of 34 .mu.m of
pure gold coating, as determined by SEM imaging. Under the gold
coating, the connectors comprised a Co, Ni and Fe alloy. EDS data
showed that the surface layer of the copper strips also contained a
small amount of gold.
[0104] The SFE procedures described in Examples 2-5 were performed
with an apparatus similar to the apparatus illustrated in FIG. 2.
The apparatus comprised a liquid CO.sub.2 tank 10, a high-pressure
syringe pump 12, a high-pressure view cell 14 with a quartz window
16 (20-mL volume and 5-cm path length), a stirring and heating
plate 18, a thermocouple 20, a pressure transducer 22 and a
collection vial 24. Detailed descriptions of the view-cell 14 are
given in Wang, S; Koh, M.; Wai, C. M. Nuclear Laundry Using
Supercritical Fluid Solutions. Ind. Chem. Eng. Res. 2004, 43(7),
1580 (the "Wang reference"), which is incorporated herein by
reference.
[0105] All supercritical fluid extraction experiments were
performed at 40.degree. C. and a pressure of 150 or 200 atm. The
supercritical fluid extraction apparatus and general procedures
were similar to those described in the Enokida references and in
the Wang reference. Carbon dioxide was supplied with an Isco, Model
260D syringe pump (Lincoln, Nebr.). The extraction vessel was
either a 14 mL stainless-steel vessel or a 6.2 mL stainless-steel
vessel comprising a 5.2 mL porous internal cell and a 1 mL conduit
underneath the internal cell connecting it to an outlet valve.
During the extractions, the extraction vessel was placed in an oven
to maintain a desired temperature. The flow rate of the
supercritical carbon dioxide was controlled by the Isco pump. At
the oven exit, stainless steel tubing (316 SS, 1/16 inch O.D. and
0.030 inch I.D.) with a length of 20 cm was used as a pressure
restrictor.
[0106] Non-destructive instrumental neutron activation analysis
(NAA) was used to determine the concentration of certain metals in
the solid and liquid samples. Some samples and standards, including
the gold samples and standards, were irradiated for 3 minutes in a
one megawatt TRIGA nuclear reactor at a steady neutron flux of
6.times.10.sup.12 Ncm.sup.-2sec.sup.-1. After being cooled for 18
hours, the neutron-activated samples and standards were counted
individually on a Ge(Li) detector.
Example 2
Copper Dissolution
[0107] A piece of silicon wafer (1.5.times.1.5 cm) coated with
copper was placed in a small glass beaker (0.7 cm diameter). Hhfa
(0.20 mL) was placed in another beaker (2 cm diameter). Both
beakers then were placed in a stainless steel reactor (14 mL
volume, preheated to 40.degree. C.) containing 1.5 mL of
TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7. The reaction cell was
pressurized to 200 atm. The Hhfa in the beaker and the
TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 in the reaction cell each
were agitated with a mini stirring bar. A 2-liter plastic container
was used as a collection vessel at the outlet of the reaction cell.
The lid of the plastic container was perforated to allow for
CO.sub.2 expansion after the dissolution reactions. Individual
reaction times were set at 0.25, 0.5, 1, 2, and 4 minutes. After
reaction, the stainless steel cell was depressurized by fully
opening the outlet valve, and the silicon wafer then was removed
from the system, rinsed with hexane, and dried by a nitrogen gas
stream.
[0108] The SEM images of the wafer before and after 30 seconds of
supercritical carbon dioxide extraction are shown in FIG. 3. The
thickness of the copper film on the original silicon wafer was
about 128 nm (FIG. 3A). After the supercritical carbon dioxide
treatment, the thickness of the copper film was reduced to about 49
nm (FIG. 3B). FIGS. 3A and 3B show that the dissolution of copper
using TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 and Hhfa in
supercritical carbon dioxide was particularly rapid when compared
to the dissolution of copper by known supercritical fluid
extraction processes. When the same dissolution process was
conducted using liquid carbon dioxide (at 24.degree. C. and 150
atm), the speed of dissolution was slower by about one order of
magnitude.
[0109] In a separate trial, in which only pure TBP and Hhfa were
used, no copper removal was observed from the silicon wafer under
the same experimental conditions. By way of theory, the HNO.sub.3
carried by the TBP into the supercritical carbon dioxide phase
apparently is important for oxidation of the copper, and oxidation
of the copper is important for the subsequent complexation and
dissolution of the copper in the supercritical carbon dioxide
phase.
[0110] The contents of the view cell 1 and 30 minutes into the
extraction can be seen in FIG. 4A and FIG. 4B, respectively. The
color of the supercritical carbon dioxide solution became deep
green when the copper dissolution trials were conducted with copper
strips and copper shot (FIG. 4B). In contrast, the distinctive
green color of Cu(hfa).sub.2 could not be detected by the naked eye
when the trials were performed on silicon wafers because the wafers
only contained a small amount of copper. Ultra violet-visible
spectra of the supercritical fluid phase were consistent with a
Cu(hfa).sub.2 complex solution. Because of its paramagnetic
properties, Cu(hfa).sub.2 does not show a measurable NMR
spectrum.
Example 3
Gold Dissolution
[0111] Gold-coated connector pins cut from a CPU processor (6
pieces with a total weight of about 20 mg) were placed in a
high-pressure view cell preheated to 40.degree. C.
TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 (6 mL) and Hhfa (0.5 mL)
were added to the reactor and carbon dioxide was introduced into
the reactor until the pressure reached 150 atm. The supercritical
carbon dioxide solution was agitated with a magnetic stirrer. After
about one minute, the solution turned light yellow (FIG. 5A) and
then became orange red after about 35 minutes (FIG. 5B).
Fe(hfa).sub.3 in supercritical carbon dioxide is red, so the
observed color change indicated that the gold coated on the surface
of the pins was dissolved first followed by the dissolution of the
exposed iron and other metals from the interior of the pins.
[0112] Additional gold extraction trials were performed in a
specially designed stainless steel vessel. This vessel was fitted
with a porous stainless steel cup with a conduit underneath the cup
on the bottom of the vessel. The cup served as a filter to remove
particulates greater than about 20 .mu.m in size. In one trial, a
total of 80 pins from a computer processor were loaded into the cup
followed by 6 mL of TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 and
0.5 mL of Hhfa. The system was preheated to 40.degree. C. followed
by pressurization to 200 atm. A magnetic stir bar was used to
agitate the solution in the cup and then the system was left static
for 2 hours. After 2 hours, the supercritical fluid phase was
released slowly through the conduit underneath the cup and
collected in a small container. A known amount of the trap solution
was placed in a polyethylene vial ( dram) and heat-sealed for
neutron activation analysis.
[0113] After depressurizing the system, the solution collected in
the trapping vial was examined by NAA. The samples and the
standards were irradiated for 3 minutes in a one megawatt TRIGA
nuclear reactor at a steady neutron flux of 6.times.10.sup.12
Ncm.sup.-2sec.sup.-1. The irradiated samples and standards were
counted individually 3 hours post irradiation on a Ge(Li) detector
and counted again 18 hours post irradiation. The y spectrum
obtained 3 hours after irradiation showed the major .sup.198Au
(t1/2=2.69 days) peak at 411.8 keV as shown in FIG. 6. The 846.8
keV peak was assigned to .sup.56Mn (t1/2=2.58 hours). When the
sample was counted 18 hours after irradiation, the 846.8 keV peak
was significantly reduced. This decrease was consistent with the
decay of .sup.56Mn. The .sup.56Mn most likely came from the
stainless steel extraction cell system. A 511 keV positron
annihilation peak also was also observed, which could have resulted
from the decay of other high energy y rays from .sup.56Mn (1810 and
2113 keV).
[0114] The NAA indicated that 619 .mu.g of gold was extracted with
a counting error of 0.69%. Before supercritical fluid extraction,
the original connectors showed only gold peaks in the EDS spectrum
(FIG. 7A), indicating that the exteriors of the connectors were
composed primarily of pure gold. After the supercritical fluid
extraction, the gold peaks became smaller and other peaks,
including those of iron, nickel, and cobalt were observed (FIG.
7B). The interior of the connector pins apparently was composed of
these metals.
[0115] These results clearly illustrate that gold in CPU connectors
can be rapidly dissolved in supercritical carbon dioxide using a
mixture of Hhfa and TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7. When
the same dissolution process was conducted using liquid carbon
dioxide (at 24.degree. C. and 150 atm), the speed of dissolution of
gold was slower by a factor of between 5 and 10. In another trial,
in which the dissolution of gold-coated connector pins was carried
out using the same components at room temperature and ambient
pressure, the dissolution rate was one to two orders of magnitude
slower than the dissolution rate achieved with supercritical carbon
dioxide. By way of theory, the high diffusivity of supercritical
carbon dioxide probably is an important factor in facilitating the
oxidation and transport of metal species into the supercritical
carbon dioxide fluid phase.
[0116] In an additional set of trials, the quantities of extracted
metal obtained with Hhfa and
TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 were compared to the
quantities of extracted metal obtained without Hhfa, without
TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 and without HNO.sub.3. A
control trial also was performed without the gold connector pins.
In each trial except for the control, 80 gold connector pins were
extracted at 40.degree. C. and 150 atm for 2 hours. The quantities
of extracted metal were determined by ICP-AES, except for gold,
which was analyzed by NAA. The results of these trials are
summarized in Table 5. Trial 1 was performed with
TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 and Hhfa. Trial 2 was
performed with TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 and without
Hhfa. Trial 3 was the control and was performed with
TBP(HNO.sub.3).sub.0.7 (H.sub.2O).sub.0.7 and Hhfa, but without the
gold pins. Trial 4 was performed with Hhfa and without
TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7. Finally, trial 5 was
performed with TBP and Hhfa and without HNO.sub.3.
TABLE-US-00005 TABLE 5 Dissolution of Gold Pins using
TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 and Hhfa Fe Co Ni Au Ca Cu
Cd K Zn Trial (.mu.g) (.mu.g) (.mu.g) (.mu.g) (.mu.g) (.mu.g)
(.mu.g) (.mu.g) (.mu.g) 1 8052 663.6 2607 619 97.6 25.1 1.1 1.1
27.5 2 710 10 4.4 2.2 14 1.2 17 9.7 4 3 10.1 2.5 2.9 0 11.4 3.4
0.41 5.8 6.1 4 nothing extracted 5 nothing extracted
[0117] The results in Table 5 indicate that significant quantities
of gold and other metals can be extracted using
TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 and Hhfa. Significantly
less extraction was observed when Hhfa was not used. No extraction
was observed without TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7.
Similarly, no extraction was observed without HNO.sub.3. The
control trial without the gold pins showed trace quantities of
metal, possibly from the reaction system.
Example 4
Palladium Dissolution
[0118] Dissolution of palladium shot in supercritical carbon
dioxide was performed using
TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4 and Hhfa at 40.degree. C.
and 150 atm. FIG. 8 shows .sup.19F NMR spectra for Pd(hfa).sub.2
(FIG. 8A), Hhfa (FIG. 8B), and a trap solution obtained after the
palladium dissolution (FIG. 8C). The palladium dissolved in
supercritical carbon dioxide using
TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4 and Hhfa showed a light
yellow color. The .sup.19F NMR spectrum of the trapped solution
confirmed that the extracted palladium was Pd(hfa).sub.2.
[0119] For comparison, dissolution of palladium shot in
supercritical carbon dioxide also was performed using
TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 and Hhfa at 40.degree. C.
and 150 atm. The .sup.19F NMR spectra in FIG. 9 show that a greater
amount of palladium was converted to Pd(hfa).sub.2 using
TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4 (FIG. 9A) than was
converted using TBP(HNO.sub.3).sub.0.7(H.sub.2O).sub.0.7 (FIG. 9B).
FIG. 9 also shows that the amount of Pd(hfa).sub.2 increased and
the amount of Hhfa decreased when the extraction time was extended
from 2 hours to 4 hours.
[0120] For comparison to palladium shot, palladium films (100 nm
thickness) on silicon wafers also were tested.
TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4 was used as the acid-base
complex. After a 30 second extraction at 40.degree. C. and 200 atm,
EDS results showed that only 1% of the palladium metal remained on
the silicon wafer surface. No palladium was found on the silicon
wafer surface after four minutes.
[0121] TBP has a UV absorbance around 230 nm with an absorption
shoulder at 280 nm. Hhfa, as a chelating agent, has an absorption
band centered at 280 nm. Since a Pd(hfa).sub.2 standard solution in
a TBP matrix has a broad absorption band around 230-450 nm, it can
be difficult to identify individual peaks and to quantify the
Pd(hfa).sub.2 complex. The .sup.19F NMR spectrum of the trapped
solution, however, confirmed the extracted palladium was in the
chemical form of Pd(hfa).sub.2 with the chemical valence of +2,
consistent with the Pd(hfa).sub.2 standard.
[0122] Another trial was carried out under the following
conditions: 2 grains of palladium shot (3 mm diameter), a cell with
a volume of 35.3 mL, 6 mL of
TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4, 0.5 mL of Hhfa, and 6
hours of SFE at 40.degree. C. and 200 atm. After SFE, the trap
solution was back extracted using concentrated HCl (5 mL) and
H.sub.2O (2 mL). One mL of the aqueous phase was spiked into a
small plastic vial, heat sealed and prepared for neutron activation
analysis (NAA). The palladium sample and palladium standard
solutions, including an atomic absorption standard and a neutron
activation standard, were irradiated for 20 minutes in a one
megawatt TRIGA nuclear reactor. FIG. 10 is a NAA gamma spectrum for
the trap solution. .sup.109Pd was found to emit a y ray at 88 keV
with a half-life of 13.46 hours. Quantitative analysis of the
sample indicated that a total of 3.98 mg of palladium was extracted
from the palladium shot.
[0123] Dissolution of the palladium shot also was verified by
.sup.19F NMR. The .sup.19F NMR spectra of Pd(hfa).sub.2 and Hhfa
standards showed peaks at -72.99 ppm and -76.48 ppm, respectively.
The NMR spectrum of the trap solution indicated that the extracted
Pd-hfa was in the form of Pd(hfa).sub.2. The NMR spectrum for the
trap solution also showed peaks at -72.99 ppm and -76.48 ppm,
indicating Pd(hfa).sub.2 and Hhfa, respectively.
[0124] In additional trials, elemental palladium was extracted from
different media, including activated carbon and alumina. The
samples first were dried in an oven for several hours at 80.degree.
C. to remove moisture. Two samples (10.about.20 mg) were weighed,
with one to serve as a standard and the other to be the sample. The
extraction was performed with 6 mL of
TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4 and 0.3 mL of Hhfa. The
temperature of the system was maintained at 50.degree. C. and the
pressure was elevated to 200 atm. The reaction time was 3 hours.
After SFE, the palladium samples were transferred to plastic vials.
The palladium samples and standards were heat sealed in plastic
vials for neutron activation analysis. The samples and standards
were irradiated in a one megawatt nuclear reactor at a steady flux
of 6.times.10.sup.12Ncm.sup.-2sec.sup.-1 for 5 min. The samples and
standards then were cooled for 2 hours and counted individually by
a Ge(Li) detector for 5 minutes. The extraction efficiencies for
these trials are shown in Table 6. The results show that palladium
can be extracted from a variety of different matrices by the
disclosed method.
TABLE-US-00006 TABLE 6 Extraction of Palladium from Pd/Carbon and
Pd/Al.sub.2O.sub.3 Catalysts Sample Extraction Efficiency (%)
Pd/carbon (5% w/w) 85.0 Pd/carbon (10% w/w) 51.7 Pd/Al.sub.2O.sub.3
(5% w/w) 96.8
Example 5
Platinum Dissolution
[0125] Dissolution of platinum metal in supercritical carbon
dioxide was performed using a mixture of TBP-nitric acid and
TBP-hydrochloric acid as the oxidizing solution and Hhfa as the
chelating agent. Another successful trial was performed using the
same oxidizing agent, but TTA instead of Hhfa as the chelating
agent. The oxidizing solution was prepared by mixing 3 mL of TBP
with 0.7 mL of concentrated nitric acid and 2.1 mL of concentrated
hydrochloric acid. In one trial, the oxidizing solution was placed
in contact with a piece of platinum foil (about 0.8 cm radius) in
supercritical carbon dioxide at 50.degree. C. and 200 atm in the
presence of Hhfa. After 3 hours of reaction, the trap solution was
removed and treated with 5 mL of concentrated HCl to separate the
platinum. About 0.37 mg of platinum was recovered.
[0126] TBP-hydrochloric acid can be corrosive to stainless steel
reaction vessels, such as the reaction vessel used in the described
trials. A special supercritical CO.sub.2 reaction vessel that can
resist corrosion of hydrochloric acid may result in better
dissolution of platinum metal using this supercritical fluid metal
extraction method.
Example 6
Dissolution of Lanthanide Oxides
[0127] This example shows that, while lanthanides and actinides can
be oxidized and chelated by a single acid-base complex, the
addition of a separate chelating agent can improve the extraction
efficiency. In this example, La.sub.2O.sub.3, Pr.sub.2O.sub.3, and
Eu.sub.2O.sub.3 were the target compounds and
TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4 was used as the
extractant.
[0128] A TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4 complex was
prepared by mixing 5 mL of TBP with 1.3 mL of concentrated nitric
acid (15.5 M) in a Pyrex flask with stirring. After 30 minutes of
mixing, the mixture was allowed to undergo phase separation. The
organic phase containing the TBP-HNO.sub.3 complex was removed by a
pipette.
[0129] A high-pressure supercritical fluid extraction system
equipped with a porous stainless steel cup was used for the
extractions. The three lanthanide oxides (La.sub.2O.sub.3,
Pr.sub.2O.sub.3, and Eu.sub.2O.sub.3) were introduced into soil
collected from a rural location north of Moscow, Id. For each
trial, 300-500 mg of soil was spiked with 10% by weight of each of
the three lanthanide oxides. The soil samples were placed in the
porous stainless steel cup along with 5 mL of
TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4. Supercritical carbon
dioxide then was fed into the extraction cell from a port on the
top of the cell. After being introduced into the cell, the
supercritical carbon dioxide flowed through the porous stainless
steel cup and was collected from a port at the bottom of the cell.
The spiked soil samples were treated with 40 minutes of static
extraction followed by 60 minutes of dynamic flushing. Lanthanide
concentrations in the soil before and after the supercritical fluid
extraction were measured by NAA. To perform the NAA analysis, the
soil samples were placed in polyethylene vials, heat sealed, and
irradiated in a one megawatt TRIGA nuclear reactor with a steady
flux of 6.times.10.sup.12 Ncm.sup.-2sec.sup.-1. Neutron irradiation
and counting were done at the Nuclear Radiation Center, Washington
State University.
[0130] FIG. 11 and Table 7 show the percent extraction of each
lanthanide oxide by supercritical carbon dioxide at 40.degree. C.
and different pressures (100, 150, and 200 atm) with
TBP(HNO.sub.3).sub.1.0(H.sub.2O).sub.0.4 as an extractant.
TABLE-US-00007 TABLE 7 Extraction of Lanthanide Oxides with
TBP/HNO.sub.3 TBP/HNO.sub.3 P (atm) La Eu Pr 100 71 90 81 150 79 91
86 200 74 92 78
As shown in FIG. 11 and Table 7) europium, praseodymium and
lanthanum were removed from the soil at extraction efficiencies of
about 90-92%, 78-86% and 71-79%, respectively. The high extraction
efficiency for Eu.sup.3+ suggests that Am.sup.3+ also can be
extracted effectively under the same conditions.
[0131] To test the effect of the addition of a separate chelating
agent on the extraction efficiency of the lanthanides, additional
trials were performed using a mixture of TTA and TBP-HNO.sub.3. The
results are shown in FIG. 12 and Table 8.
TABLE-US-00008 TABLE 8 Extraction of Lanthanide Oxides with
TBP/HNO.sub.3/TTA TBP/HNO.sub.3/TTA P (atm) La Eu Pr 100 80 90 84
150 85 87 82 200 85 98 83
[0132] As shown in FIG. 12 and Table 8, the percent extraction of
each lanthanide oxide increased by about 5-10% and the differences
between them became smaller. For example, europium, praseodymium
and lanthanum were extracted at about 90-98%, 82-84% and 80-85%,
respectively.
[0133] To further verify the results, an additional trial was
performed with 100 mg samples of the spiked soil treated with 20
minutes of static extraction followed by 40 minutes of dynamic
flushing. The extractions were performed at 40.degree. C. and 100
atm. The percent extractions achieved with and without TTA are
shown in Table 9.
TABLE-US-00009 TABLE 9 Extraction of Lanthanide Oxides with
TBP/HNO.sub.3 or TBP/HNO.sub.3/TTA Percent Extraction Extractant La
Eu Pr TBP/HNO.sub.3 71 90 81 TBP/HNO.sub.3/TTA 80 90 84
These results and the other results described in this example show
that the addition of a .beta.-diketone, such as TTA, can enhance
the extraction efficiency of lanthanide oxides.
OTHER EMBODIMENTS
[0134] Other embodiments of the invention will be apparent to those
of ordinary skill in the art from a consideration of this
specification, or practice of the invention disclosed herein. It is
intended that the specification and examples be considered as
exemplary only, with the true scope and spirit of the invention
being indicated by the following claims.
* * * * *