Note: Descriptions are shown in the official language in which they were submitted.
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SELECTIVE REDUCTIVE ELECTROWINNING APPARATUS AND METHODS
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional Patent
Application Serial No. 61/675,994, entitled SELECTIVE REDUCTIVE
ELECTROWINNING APPARATUS AND METHODS, filed on July 26, 2012, the
disclosure of which is incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
[0002] The invention relates to the recovery of metal ions. In particular,
the
invention relates to an efficient electrolytic method for recovering metal
ions from
solutions utilizing a sacrificial reductant.
BACKGROUND
[0003] Metals such as Ni, Co, Cr, Ag, Au, Fe, Cu, Zn, and V are widely
used as base catalysts for many industrial applications and processes
including, oil
making and refinery, batteries, chemical processes, air emissions control, and
the
like. During many of the processes, the catalysts will lose their catalytic
functions
eventually and become wastes. In addition, in the electronics industry, these
metals represent a significant waste from board circuits.
[0004] The production of metal-containing wastes (e.g., spent catalysts,
batteries, and board circuits) has become one of the major environmental
concerns in these industries, mainly due to their toxicity and emissions to
the
environment in transport, post-processing and disposal stages. On the other
hand,
these metal wastes contain high portion of metals that have a commercial
value.
[0005] Current processes for the recovery and reclaiming of such metals
are expensive, primarily due to the high energy expended during such recovery
processes. For example, electro-winning is the typical process that is used
for the
recovery of metals from metal-containing wastes. In this process, a current or
voltage is applied to an electrochemical cell where an anode and a cathode are
submersed in an aqueous solution of the metal-containing waste. Under the
applied
electrical energy, metals in the aqueous solution are reduced at the cathode,
while water is oxidized at the anode of the electrochemical cell. The process
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operates at high cell voltages due to the high over-potential of the water
oxidation
reaction. Consequently, the high over-potential of the water oxidation
reaction
also causes the evolution of hydrogen in the cathodic compartment of the cell,
which further reduces the efficiency of the metal recovery process and affects
the
purity and the quality of the material recovered.
[0006] Therefore, a need still exists for an efficient metal recovery
process.
SUMMARY OF THE INVENTION
[0007] The present invention is premised on the realization that the
metals,
such as spent metal catalysts, can be efficiently recovered from aqueous
solutions.
More particularly, the present invention is premised on the realization that
metal ions
can be efficiently removed from aqueous solutions via electrolysis using a
divided
electrolytic cell having a basic pH anodic chamber environment containing a
sacrificial reductant, an acidic pH cathodic chamber environment containing
the
desired metal to be recoverd, and ion-conducting separator that physically
separates
the anodic and cathodic chambers.
[0008] In accordance with the present invention, a method of recovering
metals comprising applying a voltage or an electrical current to an
electrolytic cell,
comprising an anode disposed in an anodic chamber; a cathode disposed in a
cathodic chamber; a separator disposed between the anode and the cathode to
physically separate the anodic and cathodic chambers, the separator allowing
the
transport of ions between the anodic and cathodic chambers; an anolyte
disposed
within the anodic chamber, comprising a sacrificial reductant, wherein the
anolyte
has a basic pH; a catholyte disposed within the cathodic chamber, comprising
at
least one or more metallic ions dissolved therein, wherein the catholyte has
an acidic
PH; and an electrical connection between the anode and the cathode. The
voltage
or the electrical current is applied to the electrolytic cell across the
cathode and the
anode via the electrical connection, wherein the voltage or the electrical
current is
sufficient to reduce the at least one or more metallic ions to form at least
one or more
elemental metal species at the cathode, and to oxidize the sacrificial
reductant at the
anode.
[0009] In accordance with another embodiment of the present invention, an
electrochemical cell comprising an anode in an anodic chamber; a cathode in a
cathodic chamber; a separator disposed between the anode and the cathode to
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physically separate the anodic and cathodic chambers, the separator allowing
the
transport of ions between the anodic and cathodic chambers; an anolyte
disposed
within the anodic chamber, comprising a sacrificial reductant, wherein the
anolyte
has a basic pH; a catholyte disposed within the cathodic chamber, comprising
one or
more metallic ions dissolved therein, wherein the catholyte has an acidic pH;
and an
electrical connection between the anode and the cathode.
[0010] The objects and advantages of the present invention will be further
appreciated in light of the following detailed description and example in
which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a diagrammatical view of a simplified electrolytic cell,
in
accordance with an embodiment of the present invention;
[0012] FIG. 2 is a cyclic voltammagram showing a comparison of recovering
nickel via a traditional electrowinning (TE) process, and a selective
reductive
electrowinning (SRE) process in accordance with an embodiment of the present
invention; and
[0013] FIG. 3 is a graph of current (mA) versus time (seconds) comparing
the
electrochemical performance of a TE process versus a SRE process in accordance
with an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0014] FIG. 1 is a diagrammatic depiction of a simplified electrolytic cell
10
configured for flow cell processing to achieve the recovery of metals from an
aqueous solution. The simplified electrolytic cell 10 comprises a cathodic
chamber
15 containing a cathode 20, an anodic chamber 25 containing an anode 30,
wherein
the cathodic chamber 15 and the anodic chamber 25 are physically separated
from
each other by a separator 35. However, while also serving as a physical
barrier
between the cathode 20 and the anode 30, the separator 35 allows the transport
of
ions between the cathodic chamber 15 and the anodic chamber 25. The cathode 20
and the anode 30 are configured with an electrical connection 40 therebetween
along with a voltage source 45, which supplies a voltage or an electrical
current to
the electrochemical cell 10.
[0015] According to embodiments of the present invention, a feedstock
solution 50 containing one or more metal ions intended for recovery becomes at
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least one component of a catholyte 52 having an acidic pH, which is flowed
through
the cathodic chamber 15, and thereby contacting the cathode 15, through a
cathodic
chamber inlet 53 and a cathodic chamber outlet 55. A sacrificial reductant 60
becomes at least one component in an anolyte 62 having a basic pH, which is
flowed
through the anodic chamber 25, and thereby contacting the anode 30, through an
anodic chamber inlet 63 and an anodic chamber outlet 63. Optionally, the
effluents
of the cathodic chamber 15 and the anodic chamber 25 may be recirculated
through
their respective recirculation pathways 70, 80.
[0016] According to the present invention, the metals are present in the
feedstock 50 in the form of cations, (i.e., oxidized forms of a metal). By way
of
example, but without limitation, metals amenable to the present method of SRE
processing include, but are not limited to, zinc, chromium, tantalum, gallium,
iron,
cadmium, indium, thallium, cobalt, nickel, tin, lead, copper, bismuth, silver,
mercury,
chromium, niobium, vanadium, manganese, aluminum, and combinations thereof.
Accordingly, in one embodiment, a metal suitably recovered from an aqueous
sample include nickel. Accordingly, in one embodiment, a metal suitably
recovered
from an aqueous sample include cobalt. The respective reduction reactions are
shown below:
Equation 1: Ni+2(aq) + 2 e- ¨> Ni (-0.26V vs. SHE)
Equation 2: Co+2(aq) + 2 e- ¨> Ni (-0.28V vs. SHE)
[0017] According to embodiments of the present invention, the feedstock 50
is
not particularly limited in the concentration of its metal(s). Exemplary metal
concentrations include, but are not limited to from about 500 pm and lower,
from
about 250 ppm and lower, from about 100 ppm and lower, or from about 50 ppm
and
lower. Moreover, the de-metalized water obtained from the above feedstock may
have metal concentrations sufficiently low to permit direct discharge to the
environment without further processing. For more concentrated feedstock
solutions,
the catholyte 52 may be recirculated until the desired metal reduction is
achieved.
[0018] While the pH of the feedstock solution 50 is not limited, according
to
embodiments of the present invention the pH of the catholyte 52 is acidic,
(i.e. pH is
less than 7). According to an embodiment, the pH of the catholyte 52 is about
3 to
about 6. Accordingly, to lower pH, one or more acidic electrolytes may be
combined
with the feedstock. Exemplary acidic electrolytes include, but are not limited
to, boric
acid, sulfuric acid, hydrochloric acid, phosphoric acid, or combinations
thereof.
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[0019] According to embodiments of the present invention, the anolyte 62
includes a sacrificial reductant, which effectively lowers the electrochemical
potential
of the electrolytic cell. Exemplary sacrificial reductants include, but are
not limited to,
urea; ammonia; ammonium salts; alcohols, such as ethanol or methanol, or
combinations thereof. For example, in one embodiment, the sacrificial
reductant 60
may comprise ammonium hydroxide. The sacrificial reductant is provided to the
anode 30 in an amount that exceeds the stoichiometric amount required by
metals in
the catholyte 52. Advantageously, the sacrificial reductant may be present in
the
anolyte 62 in a large excess and the excess sacrificial reductant is recycled
in the
process.
[0020] According to embodiments of the present invention the pH of the
anolyte 62 is basic, (i.e. pH is greater than 7). According to an embodiment,
the pH
of the anolyte 62 is about 9 or greater. Accordingly, to raise pH of the
anolyte, one
or more alkaline electrolytes may be combined with the sacrificial reductant
60.
Alkaline electrolytes may be liquids and/or gels. In one embodiment, the
alkaline
electrolyte comprises an alkali metal hydroxide or an alkali earth metal
hydroxide
salt, such as lithium hydroxide, rubidium hydroxide, cesium hydroxide, barium
hydroxide, strontium hydroxide, potassium hydroxide, sodium hydroxide,
magnesium
hydroxide, calcium hydroxide, sodium carbonate, potassium carbonate, and
mixtures
thereof may be used. For example, in one embodiment, the anolyte 62 may
comprise an alkaline electrolyte such as potassium hydroxide.
[0021] In an alternative embodiment, the anolyte 62 may comprise as a gel,
such as a solid polymer electrolyte. Suitable alkaline electrolytic gels
include, for
example, those gels containing polyacrylic acid, polyacrylates,
polymethacrylates,
polyacrylamides, sulfonated-polymers and similar polymers and copolymers. The
alkaline electrolytic gel may be prepared using any suitable method. One
method
includes forming a polymer and then injecting a hydroxide salt electrolyte
into the
polymer to form an alkaline electrolyte gel or polymeric mixture. In another
method,
the monomer may be polymerized in the presence of a hydroxide salt
electrolyte.
[0022] The electrodes, (i.e., cathode 20 and anode 30) may each comprise a
conductor or a support that can be coated with a more active conductor. With
respect to the cathode 20, the conducting component is not particularly
limited to any
species of conductor, but the conducting component should be comprised of a
substrate whereon the metal can deposit. For example, the conducting component
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of the cathode 20 may comprise carbon, such as carbon fibers, carbon paper,
glassy
carbon, carbon nanofibers, carbon nanotubes, and the like; or conducting
metals,
such as cobalt, copper, iridium, iron, nickel, platinum, palladium, ruthenium,
rhodium
and mixtures and alloys thereof. The support material and/or conducting
component
of the cathode 20 should be selected so as to be compatible with the acidic
electrolyte of the catholyte 52.
[0023] According to a principle of the present invention, metal ions are
reduced at the cathode 20 and are deposited thereon. Moreover, metal
deposition
rates are related to the available surface area. As such, large surface area
substrates are generally preferred.
[0024] According to another principle of the present invention, the
oxidation of
a sacrificial reductant occurs at the anode 30 in the alkaline electrolyte
composition
or medium of the anodic chamber 25. Exemplary sacrificial reductants urea and
ammonia are oxidized at the anode 30 in an alkaline electrolyte medium
according to
the following equations:
Equation 3: 2 NH3+ 6 OH- ¨> N2 + 6 H20 + 6 e- (-0.77 V vs. SHE)
Equation 4: CO(N H2)2 + 6 OH- ¨> N2 + 5 H20 + CO2 + 6 e- (-0.034 V vs. SHE)
[0025] Therefore, the conducting component of the anode 30 may be one or
more metals active toward adsorbing and oxidizing the sacrificial reductants
urea
and/or ammonia. For example, one or more metals active toward the oxidation of
ammonia include metals disclosed in commonly-assigned U.S. Patent No.
7,485,211, which is incorporated herein by reference in its entirety. By way
of further
example, the oxidation of ammonia may be performed with a conducting component
comprising platinum, iridium, ruthenium, rhodium and their combinations. The
conducting component may be co-deposited as alloys and/or by layers.
[0026] Additionally, metals active toward the oxidation of urea include
metals
disclosed in commonly-assigned U.S. Patent Application Publication No.
2009/0095636, which is incorporated herein by reference in its entirety. For
example, the oxidation of urea may be performed with a conducting component
comprising transition metals, such as nickel; or precious metals such as
platinum,
iridium, ruthenium, rhodium; and their combinations. Especially effective
metals for
the oxidation of urea include nickel and other transition metals. The metals
may be
co-deposited as alloys and/or by layers. Moreover, the active metals may be in
an
oxidized form, such as nickel oxyhydroxide.
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[0027] Further, metals active toward the oxidation of ethanol and methanol
include those metals disclosed in commonly-assigned U.S. Patent Application
Publication No. 2008/0318097, which is incorporated herein by reference in its
entirety.
[0028] By way of example and without limitation, the anode 30 may comprise
nickel electrodeposited on a carbon support, such as carbon fibers, carbon
paper,
glassy carbon, carbon nanofibers, or carbon nanotubes, or nickel formed into
beads
and suspended in a nickel gauze.
[0029] One electrode found to be favorable to the oxidation of urea is an
activated nickel oxyhydroxide modified nickel electrode (NOMN). For example,
the
NOMN electrode may be comprised of metallic substrates (Ni foil, Ni gauze, Ti
foil
and Ti gauze) that have been electroplated with Ni using a Watts bath.
Specifically,
the plated nickel electrode may be activated by being immersed in a solution
containing nickel sulfate, sodium acetate, and sodium hydroxide at 33 C.
Stainless
steel may be used as a counter electrode. The plated nickel electrode may be
used
as the anode and cathode by manual polarity switching at 6.25 A/m2 for four 1
minute cycles and 2 two minute cycles. Finally, the electrode may be kept as
the
anode at the same current and maintained thereat for two hours. The activated
electrodes yield higher current densities than those of M/Ni, where M
represents a
metallic substrate.
[0030] While anodes having large surface areas are favorable, the
structure of
the anode 30 is not limited to any specific shape or form. For example, the
conducting component may be formed as foil, wire, gauze, bead, or coated onto
a
support. Suitable anode 30 support materials may be chosen from many known
supports, such as foils, meshes and sponges, for example. The support material
may include, but is not limited to, Ni foils, Ti foils, carbon fibers, carbon
paper, glassy
carbon, carbon nanofibers, and carbon nanotubes. Aside from these specific
support materials listed, other suitable supports will be recognized by those
of
ordinary skill in the art. The selection of the conducting component and/or
the
support materials of the anode 30 should be selected so as to be compatible
with the
basic electrolyte of the anolyte 62.
[0031] The separator is used to compartmentalize the cathodic chamber 15
and the anodic chamber 25. Separators should be constructed from materials
chemically resistant to the electrolyte compositions of the catholyte 52 and
the
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anolyte 62. According to an embodiment, the separator comprises a cation
conducting polymer including a polymeric backbone comprising
polyetheretherketones, polyetherketones, polyethersulfones, polyphenylene
sulfide,
polyphenylene ethers, polyparaphenylene, polyethylene, polypropylene,
polystyrene,
a fluoropolymer, or combinations thereof; and a plurality of protonic acid
groups
covalently bonded to the polymeric backbone. Exemplary protonic acid groups
include, but are not limited to, sulfonic acids, carbonic acids, phosphoric
acids,
boronic acids, or combinations thereof. According to one embodiment, the
cation
conducting polymer comprises a sulfonated tetrafluoroethylene-based
fluoropolymer-
copolymer; a sulfonated poly(ether ether ketone); or a sulfonated polyimide.
An
exemplary sulfonated tetrafluoroethylene-based fluoropolymer-copolymer is
ethanesulfonyl fluoride, 241-[difluoro-[(trifluoroethenyl)oxy]methyl]-1,2,2,2-
tetrafluoroethoxy]-1,1,2,2,-tetrafluoro-, with tetrafluoroethylene.
[0032] The electrolytic cell may operate over varying ranges of temperature
and pressure. The operating pressure may be about atmospheric pressure or
ambient pressure with no upper pressure limit other than the physical limits
of the
reaction vessel. The operating temperature range may be from about the
freezing
point of the waste water to about 100 C and may be related to the operating
pressure of the electrolytic cell. At one atmosphere of pressure, it is
practical to
keep the operating temperature to about 80 C or less, because at higher
temperatures it is difficult to maintain ammonia in solution. For example, an
acceptable operating temperature may be within a range from about 0 C to
about
80 C; or from about 20 C to about 65 C. More specifically, an operating
temperature within a range from about 20 C to about 30 C is particularly
useful.
[0033] The present invention is not limited to any particular source of
electricity. That is, electricity can be provided from renewable energy
sources: wind,
solar, etc., storage sources (batteries), and conventional grid power
generation.
[0034] But according to embodiments of the present invention, the voltage
difference applied across the cathode 20 and the anode 30 of the
electrochemical
cell 10 is maintained at a value that provides for the reduction of the metal
ions while
avoiding substantial hydrogen generation at the cathode or substantial oxygen
generation at the anode. As used herein, "substantial" hydrogen evolution and
"substantial" oxygen evolution means that less than about 20% of the
electrical
energy is spent generating hydrogen and/or oxygen. In other words, about 80%
or
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more of the applied voltage is spent removing the waste metal ions. For
example, in
one embodiment, less than about 10% of the electrical energy is spent
generating
hydrogen and/or oxygen. In yet another embodiment, less than about 5% of the
electrical energy is spent generating hydrogen and/or oxygen. In yet another
embodiment, less than about 3% of the electrical energy is spent generating
hydrogen and/or oxygen. In one exemplary embodiment, the voltage applied
across
the cathode 20 and the anode 30 does not generate any hydrogen at the cathode.
[0035] The voltage difference applied across cathode 20 and the anode 30
can vary depending on the sacrificial reductant and the metal to be recovered.
For
example, for the recovery of nickel using ammonia as the sacrificial
reductant,
voltages between about 0.14 V and 0.9 V are sufficient, whereas voltages
between
about 0.66 V and about 1.1 V are sufficient when urea is used as the
sacrificial
reductant. According to an embodiment of the present invention, the voltage
difference applied across the cathode 20 and the anode 30 of a single
electrolytic
cell for the recovery of nickel may be maintained at a voltage of about 1.1
volts or
lower. In another exemplary embodiment, the single cell voltage difference may
be
at a value between about 0.01 volts to about 1.1 volts. In yet another
embodiment,
the single cell voltage may be at a value of about 0.2 volts to about 0.9
volts. For
example, metals such as zinc, chromium, tantalum, gallium, iron, cadmium,
indium,
thallium, cobalt, nickel, tin, lead, chromium, niobium, vanadium, manganese,
aluminum, and combinations thereof can be recovered using a cell voltage that
is
sustained no higher than about 1.5 V. For example, suitable cell voltages
include,
but are not limited to, 1.4 V, 1.3 V, 1.2 V, or 1.1 V, for example.
[0036] Thus, in accordance with embodiments of the invention, the recovery
of
metals from the feedstock 50 is realized by simultaneously contacting the
catholyte
52 containing the feedstock 50 with the cathode 20 and contacting the anolyte
62
containing the sacrificial reductant 60 with the anode 30 of the
electrochemical cell
10. At the anode 30 of the electrochemical cell 10, the electro-oxidation of
the
sacrificial reductant 60, for example ammonia, in alkaline electrolyte takes
place
according to Equation 3 as discussed above, while at the cathode 20 of the
electrochemical cell 10 the reduction of the metal species, such as nickel,
takes
place according to Equation 1 to thereby deposit metallic nickel on the
cathode.
[0037] According to the foregoing, it should be readily apparent that the
electrolytic method disclosed herein provides for the efficient recovery of
metals, by
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utilizing a sacrificial reductant to lower the requisite electrical potential.
While the
embodiment of FIG. 1 is shown as a flow cell, the principles of the present
invention
are readily adaptable to other configurations, such as batch processing.
[0038] The present invention will be further appreciated in view of the
following example.
[0039] EXAMPLE
[0040] Artificial battery waste (e.g., feedstock) was mimicked using NiCl2.
Two experiments were performed at 1.3 V constant potential to deposit/recover
nickel on a Ti substrate; a traditional electrowinning (TE) process, and a
Selective
Reductive Electrowinning (SRE) process. For both the TE and the SRE processes,
the cathode used was Ti foil (8 cm2) and the anode used was Pt-Ir deposited on
carbon paper (8 cm2). The anode and cathode were separated using a
Nafion0117 membrane. For the TE process, the same solution was used as the
catholyte and the anolyte, which was a solution containing 0.25 M NiCl2, 1 M
KCI,
and 30 g/L H3B04. For the SRE process, the catholyte was a solution containing
0.25 M NiCl2, 1 M KCI, and 30 g/L H3B04, whereas the anolyte was a solution
containing 1M NH4OH and 1 M KOH. The total time for electrochemical recovery
of
Ni was fixed at 2 hours. The mass change was measured for the both the TE and
the SRE processes and compared.
[0041] As shown in FIG. 2, cyclic voltammetry experiments provide a
comparison for the recovery of nickel using the traditional electrowinning
(TE)
process versus the selective reductive electrowinning (SRE) process of the
present
invention. The cell voltage for the SRE process to recover nickel decreases
from
2.35 V to 0.54 V, which represents a 77% lower energy consumption when
compared to the TE process. As shown in Table 1, the SRE process
outperforms the TE process in all variables affecting the cost of nickel
recovery.
[0042] Table 1: Comparison of traditional electrowinning (TE) versus
selective
reductive electrowinning (SRE) processes.
Variable TE SRE Comparison
Cell Voltage (V) 2.35 0.54 SRE provide 77% lower energy
consumption
Faradaic or Current 73 99 SRE provides 36% higher current
Efficiency (%) efficiency
Power Consumption per g 2.9 0.5 SRE provides 83% lower power
of Ni recovered (W/g of Ni) consumption
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[0043] As shown in FIG. 3, at the applied voltage, a substantial current
was
observed in the SRE process, which indicates that the recovery of the Ni metal
is
feasible in the SRE process at the applied voltage. Conversely, the current is
very
low for the TE process, and thus not feasible or practical for the TE process
at the
applied voltage. After two hours of operation, 11.5 mg of nickel was recovered
in
the cathode during the SRE process, while no nickel was recovered in the TE
process.
[0044] While the present invention has been illustrated by the description
of
one or more embodiments thereof, and while the embodiments have been described
in considerable detail, they are not intended to restrict or in any way limit
the scope
of the appended claims to such detail. Additional advantages and modifications
will
readily appear to those skilled in the art. The invention in its broader
aspects is
therefore not limited to the specific details, representative product and
method and
illustrative examples shown and described. Accordingly, departures may be made
from such details without departing from the scope of the general inventive
concept.
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