U.S. patent application number 11/162341 was filed with the patent office on 2006-02-02 for method and apparatus for electrowinning copper using the ferrous/ferric anode reaction and a flow-through anode.
Invention is credited to Paul R. Cook, Wesley P. Hoffman, Timothy G. Robinson, Scot P. Sandoval.
Application Number | 20060021880 11/162341 |
Document ID | / |
Family ID | 39112337 |
Filed Date | 2006-02-02 |
United States Patent
Application |
20060021880 |
Kind Code |
A1 |
Sandoval; Scot P. ; et
al. |
February 2, 2006 |
METHOD AND APPARATUS FOR ELECTROWINNING COPPER USING THE
FERROUS/FERRIC ANODE REACTION AND A FLOW-THROUGH ANODE
Abstract
The present invention relates, generally, to a method and
apparatus for electrowinning metals, and more particularly to a
method and apparatus for copper electrowinning using the
ferrous/ferric anode reaction and a flow-through anode, such as,
for example, a dimensionally stable carbon, carbon composite,
metal-graphite, or stainless steel anode. In general, the use of a
flow-through anode--coupled with an effective electrolyte
circulation system--enables the efficient and cost-effective
operation of a copper electrowinning system employing the
ferrous/ferric anode reaction at a total cell voltage of less than
about 1.5 V and at current densities of greater than about 26 Amps
per square foot (about 280 A/m2), and reduces acid mist generation.
Furthermore, the use of such a system permits the use of low
ferrous iron concentrations and optimized electrolyte flow rates as
compared to prior art systems while producing high quality,
commercially saleable product (i.e., LME Grade A copper cathode or
equivalent), which is advantageous.
Inventors: |
Sandoval; Scot P.; (Morenci,
AZ) ; Cook; Paul R.; (Morenci, AZ) ; Hoffman;
Wesley P.; (Palmdale, CA) ; Robinson; Timothy G.;
(Scottsdale, AZ) |
Correspondence
Address: |
SNELL & WILMER;ONE ARIZONA CENTER
400 EAST VAN BUREN
PHOENIX
AZ
850040001
US
|
Family ID: |
39112337 |
Appl. No.: |
11/162341 |
Filed: |
September 7, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10907638 |
Apr 8, 2005 |
|
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|
11162341 |
Sep 7, 2005 |
|
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60561224 |
Jun 22, 2004 |
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Current U.S.
Class: |
205/576 |
Current CPC
Class: |
C25C 7/02 20130101; C25C
1/12 20130101 |
Class at
Publication: |
205/576 |
International
Class: |
C25C 1/12 20060101
C25C001/12 |
Claims
1. A method of electrowinning copper comprising: providing an
electrochemical cell comprising at least one flow-through anode and
at least one cathode, wherein said cathode has an active surface
area; providing a flow of electrolyte through said electrochemical
cell, said electrolyte comprising copper and solubilized ferrous
iron; oxidizing at least a portion of said solubilized ferrous iron
in said electrolyte at the at least one flow-through anode from
ferrous iron to ferric iron; removing at least a portion of said
copper from said electrolyte at the at least one cathode; and
operating said electrochemical cell at a cell voltage and at a
current density, wherein said cell voltage is less than about 1.5
Volts and wherein said current density is greater than about 26
amperes per square foot of active cathode.
2. The method according to claim 1, wherein said step of oxidizing
comprises oxidizing at least a portion of said solubilized ferrous
iron in said electrolyte at an anode comprising at least one of a
metal mesh, a porous metal structure, a wool or fabric, a plurality
of metal strips, a plurality of metal wires, a woven wire cloth, a
plurality of metal rods, a perforated metal sheet, or a combination
thereof.
3. The method according to claim 2, wherein said step of oxidizing
comprises oxidizing at least a portion of said solubilized ferrous
iron in said electrolyte at an anode comprising at least one of
titanium, tantalum, zirconium, niobium, nickel, a stainless steel,
a metal alloy, an intermetallic mixture, or a ceramic or cermet
containing one or more metals.
4. The method according to claim 1, wherein said step of oxidizing
comprises oxidizing at least a portion of said solubilized ferrous
iron in said electrolyte at an anode having an electrochemically
active coating.
5. The method according to claim 4, wherein said step of oxidizing
comprises oxidizing at least a portion of said solubilized ferrous
iron in said electrolyte at an anode having an electrochemically
active coating comprising at least one of: platinum; ruthenium;
iridium; a Group VIII metal; a Group VIII metal oxide; a compound
comprising a Group VIII metal; an oxide of titanium, molybdenum,
tantalum, or a mixture thereof; or a compound comprising titanium,
molybdenum, tantalum, or a mixture thereof.
6. The method according to claim 1, said step of oxidizing
comprises oxidizing at least a portion of said solubilized ferrous
iron in said electrolyte at an anode comprising at least one of
carbon, graphite, or a mixture of carbon and graphite.
7. The method according to claim 6, wherein said step of oxidizing
comprises oxidizing at least a portion of said solubilized ferrous
iron in said electrolyte at an anode comprising at least one of a
carbon foam, a graphite foam, a metal-graphite sintered material, a
polycrystalline graphite, a polycrystalline graphite coated or
densified with a graphitizable pitch material, a graphite foam
coated or densified with graphitizable pitch material, a fiber
reinforced carbon-carbon composite, a fiber reinforced
carbon-carbon composite incorporating graphitizable particulates, a
graphite coated metal mesh, or a carbon coated metal mesh.
8. The method according to claim 1, wherein said step of oxidizing
comprises oxidizing at least a portion of said solubilized ferrous
iron in said electrolyte at an anode having a coating comprising at
least one of carbon, graphite, a mixture of carbon and graphite, a
precious metal oxide, or a spinel.
9. The method according to claim 1, said step of oxidizing
comprises oxidizing at least a portion of said solubilized ferrous
iron in said electrolyte at an anode comprising stainless
steel-clad copper rods.
10. The method according to claim 1, said step of oxidizing
comprises oxidizing at least a portion of said solubilized ferrous
iron in said electrolyte at an anode comprising a titanium mesh
having a coating comprising a mixture of carbon black powder and
graphite powder in a graphitizable pitch binder.
11. A system for electrowinning copper from a copper-containing
electrolyte, comprising: an electrolyte stream, wherein said
electrolyte stream comprises copper and solubilized ferrous iron,
and wherein the concentration of solubilized ferrous iron in said
electrolyte stream is from about 10 to about 60 grams per liter; an
electrochemical cell, wherein said electrochemical cell comprises
at least one flow-through anode, at least one cathode, and an
electrolyte flow manifold.
12. The system according to claim 11, wherein said at least one
flow-through anode comprises at least one of a metal mesh, a porous
metal structure, a wool or fabric, a plurality of metal strips, a
plurality of metal wires, a woven wire cloth, a plurality of metal
rods, a perforated metal sheet, or a combination thereof.
13. The system according to claim 11, wherein said at least one
flow-through anode comprises at least one of titanium, tantalum,
zirconium, niobium, nickel, a stainless steel, a metal alloy, an
intermetallic mixture, or a ceramic or cermet containing one or
more metals.
14. The system according to claim 11, wherein said at least one
flow-through anode comprises an electrochemically active
coating.
15. The system according to claim 14, wherein said at least one
flow-through anode comprises an electrochemically active coating
comprising at least one of: platinum; ruthenium; iridium; a Group
VIII metal; a Group VIII metal oxide; a compound comprising a Group
VIII metal; an oxide of titanium, molybdenum, tantalum, or a
mixture thereof; or a compound comprising titanium, molybdenum,
tantalum, or a mixture thereof.
16. The system according to claim 11, wherein said at least one
flow-through anode comprises at least one of carbon, graphite, or a
mixture of carbon and graphite.
17. The system according to claim 16, wherein said at least one
flow-through anode comprises at least one of a carbon foam, a
graphite foam, a metal-graphite sintered material, a
polycrystalline graphite, a polycrystalline graphite coated or
densified with a graphitizable pitch material, a graphite foam
coated or densified with graphitizable pitch material, a fiber
reinforced carbon-carbon composite, a fiber reinforced
carbon-carbon composite incorporating graphitizable particulates, a
graphite coated metal mesh, or a carbon coated metal mesh.
18. The system according to claim 11, wherein said at least one
flow-through anode comprises at least one of carbon, graphite, a
mixture of carbon and graphite, a precious metal oxide, or a
spinel.
19. The system according to claim 11, wherein said at least one
flow-through anode comprises stainless steel-clad copper rods.
20. The system according to claim 11, wherein said at least one
flow-through anode comprises a titanium mesh having a coating
comprising a mixture of carbon black powder and graphite powder in
a graphitizable pitch binder.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 10/907,683 filed Apr. 8, 2005 and entitled
"Method & Apparatus for Electrowinning Copper Using the
Ferrous/Ferric Anode Reaction and a Flow-Through Anode," which
claims priority to U.S. Provisional Application No. 60/561,224,
filed Apr. 8, 2004, which applications, in their entirety, are
hereby incorporated by reference.
FIELD OF INVENTION
[0002] The present invention relates, generally, to a method and
apparatus for electrowinning metals, and more particularly to a
method and apparatus for copper electrowinning using the
ferrous/ferric anode reaction and a flow-through anode.
BACKGROUND OF INVENTION
[0003] Efficiency and cost-effectiveness of copper electrowinning
is and for a long time has been important to the competitiveness of
the copper industry. Past research and development efforts in this
area have thus focused-at least in part-on mechanisms for
decreasing the total energy requirement for copper electrowinning,
which directly impacts the cost-effectiveness of the electrowinning
process.
[0004] Conventional copper electrowinning, wherein copper is plated
from an impure anode to a substantially pure cathode with an
aqueous electrolyte, occurs by the following reactions:
[0005] Cathode reaction:
[0006]
Cu.sup.2++SO.sub.4.sup.2-+2e.sup.-.fwdarw.Cu.sup.0+SO.sub.4.sup.2
(E.sup.0=+0.345 V)
[0007] Anode reaction:
[0008] H.sub.2O.fwdarw.1/2 O.sub.2+2H.sup.++2e.sup.-
(E.sup.0=-1.230 V)
[0009] Overall cell reaction:
[0010]
Cu.sup.2++SO.sub.4.sup.2-+H.sub.2O.fwdarw.Cu.sup.0+2H.sup.++SO.sub-
.4.sup.2-+1/2O.sub.2 (E.sup.0=-0.855 V)
[0011] Conventional copper electrowinning according to the above
reactions, however, exhibits several areas of potential improvement
for, among other things, improved economics, increased efficiency,
and reduced acid mist generation. First, in conventional copper
electrowinning, the decomposition of water reaction at the anode
produces oxygen (O.sub.2) gas. When the liberated oxygen gas
bubbles break the surface of the electrolyte bath, they create an
acid mist. Reduction or elimination of acid mist is desirable.
Second, the decomposition of water anode reaction used in
conventional electrowinning contributes significantly to the
overall cell voltage via the anode reaction equilibrium potential
and the overpotential. The decomposition of water anode reaction
exhibits a standard potential of 1.23 Volts (V), which contributes
significantly to the total voltage required for conventional copper
electrowinning. The typical overall cell voltage is approximately
2.0 V. A decrease in the anode reaction equilibrium potential
and/or overpotential would reduce cell voltage, and thus conserve
energy and decrease the total operating costs of the electrowinning
operation.
[0012] One way that has been found to potentially reduce the energy
requirement for copper electrowinning is to use the ferrous/ferric
anode reaction, which occurs by the following reactions:
[0013] Cathode reaction:
[0014]
Cu.sup.2++SO.sub.4.sup.2-+2e.sup.-.fwdarw.Cu.sup.0+SO.sub.4.sup.2--
(E.sup.0=+0.345 V)
[0015] Anode reaction:
[0016] 2Fe.sup.2+.fwdarw.2Fe.sup.3++2e.sup.-(E.sup.0=-0.770 V)
[0017] Overall cell reaction:
[0018]
Cu.sup.2++SO.sub.4.sup.2-+2Fe.sup.2+.fwdarw.Cu.sup.0+2Fe.sup.3++SO-
.sub.4.sup.2-(E.sup.0=-0.425 V)
[0019] The ferric iron generated at the anode as a result of this
overall cell reaction can be reduced back to ferrous iron using
sulfur dioxide, as follows:
[0020] Solution reaction:
[0021]
2Fe.sup.3+.fwdarw.SO.sub.2.sup.-+2H.sub.2O.fwdarw.2Fe.sup.2++4H.su-
p.++SO.sub.4.sup.2-
[0022] The use of the ferrous/ferric anode reaction in copper
electrowinning cells lowers the energy consumption of those cells
as compared to conventional copper electrowinning cells that employ
the decomposition of water anode reaction, since the oxidation of
ferrous iron (Fe.sup.2+) to ferric iron (Fe.sup.3+) occurs at a
lower voltage than does the decomposition of water. However,
maximum voltage reduction--and thus maximum energy reduction-cannot
occur using the ferrous/ferric anode reaction unless effective
transport of ferrous iron and ferric iron to and from,
respectively, the cell anode(s) is achieved. This is because the
oxidation of ferrous iron to ferric iron in a copper electrolyte is
a diffusion-controlled reaction. This principle has been recognized
and applied by, among others, the U.S. Bureau of Mines Reno
Research Center and Sandoval et al. in U.S. Pat. No. 5,492,608,
entitled "Electrolyte Circulation Manifold for Copper
Electrowinning Cells Which Use the Ferrous/Ferric Anode
Reaction."
[0023] Although, in general, the use of the ferrous/ferric anode
reaction in connection with copper electrowinning is known, a
number of deficiencies are apparent in the prior art regarding to
the practical implementation of the ferrous/ferric anode reaction
in copper electrowinning processes. For example, prior embodiments
of the ferrous/ferric anode reaction in copper electrowinning
operations generally have been characterized by operating current
density limitations, largely because of the inability to obtain a
sufficiently high rate of diffusion of ferrous iron to the anode
and ferric iron from the anode. Stated another way, because these
prior applications have been unable to achieve optimum transport of
ferrous and ferric ions to and from the anode(s) in the
electrochemical cell, prior applications of the ferrous/ferric
anode reaction have been unable to cost effectively produce copper
cathode in electrochemical cells employing largely conventional
structural features.
[0024] Another aspect of the prior art that would benefit from
additional innovation relates to the configuration and composition
of anode design to help optimize the ferrous/ferric anode reaction.
For example, dimensionally stable electrodes for use in
electrowinning of metals generally consist of a base or substrate
of a valve metal, typically titanium, carrying an electrocatalytic
coating such as a mixed oxide of platinum group metal and a valve
metal forming a mixed crystal or solid solution. Many different
coating formulations have been proposed. The state of the art with
titanium mesh anodes is to place a precious metal oxide or a valve
metal oxide coating on the mesh to serve as the anode conductive
coating. These coatings typically are very expensive. An anode
coating that achieves the benefits of prior art electrocatalytic
coatings, but that also offers cost savings, would be advantageous.
Alternatively, anodes for use in connection with the ferrous/ferric
anode reaction comprised of less expensive materials--such as, for
example, carbon composite materials and stainless steels--that
perform similarly to traditional anodes but at reduced cost, would
be advantageous.
SUMMARY OF INVENTION
[0025] The present invention relates to an improved copper
electrowinning process and apparatus designed to address, among
other things, the aforementioned deficiencies in prior art
electrowinning systems. The improved process and apparatus
disclosed herein achieves an advancement in the art by providing a
copper electrowinning system that, by utilizing the ferrous/ferric
anode reaction in combination with other aspects of the invention,
enables significant enhancement in electrowinning efficiency,
energy consumption, and reduction of acid mist generation as
compared to conventional copper electrowinning processes and
previous attempts to apply the ferrous/ferric anode reaction to
copper electrowinning operations. As used herein, the term
"alternative anode reaction" refers to the ferrous/ferric anode
reaction, and the term "alternative anode reaction process" refers
to any electrowinning process in which the ferrous/ferric anode
reaction is employed.
[0026] While the way in which the present invention addresses these
deficiencies and provides these advantages will be discussed in
greater detail, below, in general, the use of a flow-through
anode--coupled with an effective electrolyte circulation
system--enables the efficient and cost-effective operation of a
copper electrowinning system employing the ferrous/ferric anode
reaction at a total cell voltage of less than about 1.5 V and at
current densities of greater than about 26 Amps per square foot
(about 280 A/m.sup.2), and reduces acid mist generation.
Furthermore, the use of such a system permits the use of low
ferrous iron concentrations and optimized electrolyte flow rates as
compared to prior art systems while producing high quality,
commercially saleable product (i.e., LME Grade A copper cathode or
equivalent), which is advantageous.
[0027] In accordance with one exemplary aspect of an embodiment of
the invention, a novel anode is utilized to address, among other
things, the aforementioned deficiencies in prior art anodes. In one
embodiment, the improved anode disclosed herein achieves an
advancement in the art by using a carbon composite type anode or a
stainless steel anode, which offer significant economic benefits as
compared to prior art anodes without sacrificing functionality. In
accordance with another embodiment, an anode comprising a carbon
and/or graphite coating is employed.
[0028] Various aspects of this invention offer the potential to
significantly decrease the cost from current anode technology.
Certain aspects will have application to ferrous/ferric
electrowinning and possibly to conventional electrowinning as
well.
[0029] These and other features and advantages of the present
invention will become apparent to those skilled in the art upon a
reading of the following detailed description when taken in
conjunction with the drawing figures, wherein there is shown and
described various illustrative embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The subject matter of the present invention is particularly
pointed out and distinctly claimed in the concluding portion of the
specification. A more complete understanding of the present
invention, however, may best be obtained by referring to the
detailed description and to the claims when considered in
connection with the drawing figures, wherein like numerals denote
like elements and wherein:
[0031] FIG. 1 is a flow diagram for an electrowinning process in
accordance with one embodiment of the present invention;
[0032] FIG. 2 illustrates an electrochemical cell configured to
operate in accordance with one exemplary embodiment of the present
invention;
[0033] FIG. 3 illustrates an exemplary anode configured in
accordance with one aspect of an exemplary embodiment of the
present invention; and
[0034] FIG. 4 illustrates another exemplary anode configured in
accordance with an aspect of another exemplary embodiment of the
present invention.
DETAILED DESCRIPTION
[0035] The present invention exhibits significant advancements over
prior art processes, particularly with regard to process
efficiency, process economics, and reduction of acid mist
generation. Moreover, existing copper recovery processes that
utilize conventional electrowinning process sequences may, in many
instances, easily be retrofitted to exploit the many commercial
benefits the present invention provides.
[0036] With initial reference to FIG. 1, an electrowinning process
100 illustrating various aspects of an exemplary embodiment of the
invention is provided. Electrowinning process 100 generally
comprises an electrowinning stage 101, a ferrous iron regeneration
stage 103, and an acid removal stage 105. Copper-rich commercial
electrolyte 11 is introduced to electrowinning stage 101 for
recovery of the copper therein. Electrowinning stage 101 produces
cathode copper (stream not shown) and a ferric-rich electrolyte
stream 13. At least a portion of ferric-rich electrolyte stream 13
is introduced into ferrous iron regeneration stage 103 as
electrolyte regeneration stream 15. Manifold circulation stream 16
comprises the portion of ferric-rich electrolyte stream 13 not sent
to ferrous iron regeneration stage 103, as well as recycle streams
12 and 14 from ferrous iron regeneration stage 103 and acid removal
stage 105, respectively, and serves as a flow control and fluid
agitation mechanism in accordance with one aspect of the invention
discussed herein below.
[0037] Generally speaking, increasing the operating current density
in an electrowinning cell increases the cell voltage. This
increased voltage demand translates into increased energy costs for
producing copper, which affects the profitability of the
electrowinning operation. On the other hand, certain other
parameters in alternative anode reaction processes-such as, for
example, temperature and iron concentration in the electrolyte-may
be controlled in a manner that mitigates the effect of increased
current density on cell voltage. For instance, as the temperature
of the electrolyte is increased, cell voltage tends to decrease.
Similarly, as the concentration of iron in the electrolyte
increases, voltage tends to decrease in electrowinning cells
employing the alternative anode reaction. Nevertheless, the
mitigating effect of increased temperature and increased iron
concentration on high cell voltage is limited.
[0038] In general, processes and systems configured according to
various embodiments of the present invention enable the efficient
and cost-effective utilization of the alternative anode reaction in
copper electrowinning at a cell voltage of less than about 1.5 V
and at current densities of greater than about 26 A/ft.sup.2 (about
280 A/m.sup.2). Furthermore, the use of such processes and/or
systems reduces generation of acid mist and permits the use of low
ferrous iron concentrations in the electrolyte and optimized
electrolyte flow rates, as compared to prior art systems, while
producing high quality, commercially saleable product.
[0039] While various configurations and combinations of anodes and
cathodes in the electrochemical cell may be used effectively in
connection with various embodiments of the invention, preferably, a
flow-through anode is used and electrolyte circulation is provided
using an electrolyte flow manifold capable of maintaining
satisfactory flow and circulation of electrolyte within the
electrowinning cell.
[0040] In accordance with other exemplary embodiments of the
invention, a system for operating an alternative anode reaction
process includes an electrochemical cell equipped with at least one
flow-through anode and at least one cathode, wherein the cell is
configured such that the flow and circulation of electrolyte within
the cell enables the cell to be advantageously operated at a cell
voltage of less than about 1.5 V and at a current density of
greater than about 26 A/ft.sup.2. Various mechanisms may be used in
accordance with the present invention to enhance electrolyte flow,
as detailed herein. For example, an electrolyte flow manifold
configured to inject electrolyte into the anode may be used, as
well as exposed "floor mat" type manifold configurations and other
forced-flow circulation means. In accordance with various
embodiments of the invention, any flow mechanism that provides an
electrolyte flow effective to transport ferrous iron to the anode,
to transport ferric iron from the anode, and to transport copper
ions to the cathode such that the electrowinning cell may be
operated at a cell voltage of less than about 1.5 V and at a
current density of greater than about 26 A/ft.sup.2, is
suitable.
[0041] These and other exemplary aspects of the present invention
are discussed in greater detail herein below.
[0042] In accordance with one aspect of the invention, ferrous
iron, for example, in the form of ferrous sulfate (FeSO.sub.4), is
added to the copper-rich electrolyte to be subjected to
electrowinning, to cause the ferrous/ferric (Fe.sup.2+/Fe.sup.3+)
couple to become the anode reaction. In so doing, the
ferrous/ferric anode reaction replaces the decomposition of water
anode reaction. As discussed above, because there is no oxygen gas
produced in the ferrous/ferric anode reaction, generation of "acid
mist" as a result of the reactions in the electrochemical cell is
eliminated. In addition, because the equilibrium potential of the
Fe.sup.2+/Fe.sup.3+couple (i.e., E.sup.0=-0.770 V) is less than
that for the decomposition of water (i.e., E.sup.0=-1.230 V); the
cell voltage is decreased, thereby decreasing cell energy
consumption.
[0043] Moreover, as is discussed in greater detail herein below,
enhanced circulation of electrolyte between the electrodes
increases the diffusion rate of ferrous iron to the anode. As the
diffusion rate of ferrous iron to the anode increases, the overall
cell voltage generally decreases, resulting in a decrease in the
power required for electrowinning the copper.
[0044] In accordance with one exemplary embodiment of the present
invention, a flow-through anode with an electrolyte injection
manifold is incorporated into the cell as shown in FIG. 2. As used
herein, the term "flow-through anode" refers to any anode
configured to enable electrolyte to pass through it. While fluid
flow from the manifold provides electrolyte movement, a
flow-through anode allows the electrolyte in the electrochemical
cell to flow through the anode during the electrowinning process.
The use of a flow-through anode with manifold electrolyte injection
decreases cell voltage at lower electrolyte flow rates, as compared
to the prior art, and at lower electrolyte iron concentrations as
compared to the prior art, through enhanced diffusion of ferrous
iron to the anode. Prior art systems, for example, relied upon a
"brute force" approach to increasing current density in
electrowinning operations, elevating electrolyte flow rate,
electrolyte temperature, and electrolyte iron concentration in
their attempts. Prior art attempts, however, achieved maximum
current densities of only up to 26 A/ft.sup.2, and even then,
average cell voltages were well above 1.0 V. Utilizing a
flow-through anode in combination with effective electrolyte
injection, however, the present inventors are able to operate
electrowinning processes at current densities of 26 A/ft.sup.2 and
cell voltages of well below 1.0 V, while also dramatically
decreasing the electrolyte flow rate and electrolyte iron
concentration. Decreasing iron concentration without adversely
affecting the efficiency or quality of the electrowinning operation
is economically desirable, because doing so decreases iron make-up
requirements and decreases the electrolyte sulfate saturation
temperature, and thus decreases the cost of operating the
electrowinning cell.
[0045] In accordance with various aspects of exemplary embodiments
of the invention, electrolyte injection manifolds with bottom
injection, side injection, and/or in-anode injection are
incorporated into the cell to enhance ferrous iron diffusion.
EXAMPLE 1 herein demonstrates the effectiveness of an in-anode
electrolyte injection manifold for decreasing cell voltage.
[0046] In accordance with an exemplary embodiment of the invention,
an overall cell voltage of less than about 1.5 V is achieved,
preferably less than about 1.20 V or about 1.25 V, and more
preferably less than about 0.9 V or about 1.0 V.
[0047] Generally speaking, as the operating current density in the
electrochemical cell increases, the copper plating rate increases.
Stated another way, as the operating current density increases,
more cathode copper is produced for a given time period and cathode
active surface area than when a lower operating current density is
achieved. Alternatively, by increasing the operating current
density, the same amount of copper may be produced in a given time
period, but with less active cathode surface area (i.e., fewer or
smaller cathodes, which corresponds to lower capital equipment
costs and lower operating costs).
[0048] As current density increases using the ferrous/ferric anode
reaction, cell voltage tends to increase due in part to the
depletion of ferrous ions at the anode surface. This can be
compensated for by increasing transport of ferrous ions to the
anode as current density increases in order to maintain a low cell
voltage. The prior art was limited to current densities of 26
A/ft.sup.2 (280 A/m.sup.2) and below for copper electrowinning
using the ferrous/ferric anode reaction in large part because of
ferrous iron transport limitations. Stated another way, previous
attempts that increased flow rates and increased iron concentration
in the electrolyte to achieve high current densities were
unsuccessful in decreasing overall cell voltage. Various
embodiments of the present invention allow for operation at current
densities above-and significantly above-26 A/ft.sup.2 while
maintaining cell voltages of less than about 1.5 V.
[0049] As will be described in greater detail herein below,
exemplary embodiments of the present invention permit operation of
electrochemical cells using the ferrous/ferric anode reaction at
current densities of from about 26 to about 35 A/ft.sup.2 at cell
voltages of less than about 1.0 V; up to about 40 A/ft.sup.2 at
cell voltages of less than about 1.25 V; and up to about 50
A/ft.sup.2 or greater at cell voltages of less than about 1.5
V.
[0050] In accordance with an exemplary embodiment of the invention,
a current density of from about 20 to about 50 amps per square foot
of active cathode (about 215 A/m.sup.2 to about 538 A/m.sup.2) is
maintained, preferably greater than about 26 A/ft.sup.2 (280
A/m.sup.2), and more preferably greater than about 30 A/ft.sup.2
(323 A/m.sup.2) of active cathode. It should be recognized,
however, that the maximum operable current density achievable in
accordance with various embodiments of the present invention will
depend upon the specific configuration of the process apparatus,
and thus an operating current density in excess of 50 A/ft.sup.2
(538 A/m.sup.2) of active cathode may be achievable in accordance
with the present invention.
[0051] One clear advantage of processes configured in accordance
with various embodiments of the present invention is that a higher
current density as compared to the prior art is achievable at the
same cell voltage when using a flow-through anode with forced-flow
manifold electrolyte injection. For example, the U.S. Bureau of
Mines, as reported in S. P. Sandoval, et al., "A Substituted Anode
Reaction for Electrowinning Copper," Proceedings of Copper 95-COBRE
95 International Conference, v. III, pp. 423-435 (1995), achieved a
current density of only about 258 A/m.sup.2 (about 24.0 A/ft.sup.2)
in an experimental test wherein the electrowinning cell was
operated continuously for five days with two conventional cathodes
and three conventional anodes (i.e., non-flow-through anodes) and
with a side-injection circulation manifold. The electrolyte flow
rate was about 0.24 gpm/ft.sup.2 and the electrolyte temperature
was approximately 104.degree. F. The iron concentration in the
electrolyte measured approximately 28 g/L and the average cell
voltage over the five-day test period was 0.94 V.
[0052] Results of experimental testing performed in accordance with
an exemplary embodiment of the present invention, however, clearly
demonstrate the benefits of the present invention over the prior
art. In such testing, a current density of about 30
A/ft.sup.2--twenty-five percent greater than the current density
achieved in the U.S. Bureau of Mines testing--was achieved using an
electrowinning cell with three conventional cathodes and four
flow-through anodes (in this instance, titanium mesh anodes with an
iridium oxide-based coating), and with a bottom-injection "floor
mat" circulation manifold. Electrolyte iron concentration,
electrolyte flow rate, temperature, and cell voltage were similar
to those employed in the U.S. Bureau of Mines test.
[0053] Further illustrating the benefits of the present invention,
EXAMPLE 1 herein demonstrates that cell voltages of about 1.0 V and
about 1.25 V are achievable at current densities of about 35
A/ft.sup.2 (377 A/m.sup.2) and about 40 A/ft.sup.2 (430 A/m.sup.2),
respectively.
[0054] In conventional electrowinning processes utilizing the
decomposition of water anode reaction, electrolyte mixing and
electrolyte flow through the electrochemical cell are achieved by
circulating the electrolyte through the electrochemical cell and by
the generation of oxygen bubbles at the anode, which cause
agitation of the electrolyte solution as the oxygen bubbles rise to
the surface of the electrolyte in the cell. However, because the
ferrous/ferric anode reaction does not generate oxygen bubbles at
the anode, electrolyte circulation is the primary source of mixing
in the electrochemical cell. The present inventors have achieved an
advancement in the art by recognizing that an electrochemical cell
configured to allow a significant increase in mass transport of
relevant species between the anode (e.g., ferrous/ferric ions) and
the cathode (e.g., copper ions) by enhancing electrolyte flow and
circulation characteristics when utilizing the alternative anode
reaction would be advantageous.
[0055] Enhanced circulation of the electrolyte between the
electrodes increases the rate of transport of ions to and from the
electrode surfaces (for example, copper ions to the cathode,
ferrous ions to the anode, and ferric ions away from the anode)
and, as a result, generally decreases the overall cell voltage.
Decreasing the cell voltage results in a decrease in the power
demand for electrowinning. Enhancing circulation of the
electrolyte, however, generally requires an increase in the power
demand of the electrolyte pumping system. Thus, the objectives of
decreasing cell voltage and increasing electrolyte circulation are
preferably balanced. In accordance with one aspect of an exemplary
embodiment of the invention, the total power requirement of the
electrochemical cell may be optimized by minimizing the sum of the
power required to circulate the electrolyte through the
electrochemical cell and the power used to electrowin the copper at
the cathode.
[0056] Referring now to FIG. 2, an electrochemical cell 200 in
accordance with various aspects of an exemplary embodiment of the
invention is provided. Electrochemical cell 200 generally comprises
a cell 21, at least one anode 23, at least one cathode 25, and an
electrolyte flow manifold 27 comprising a plurality of injection
holes 29 distributed throughout at least a portion of the cell 21.
In accordance with one aspect of an embodiment of the invention,
electrochemical cell 200 comprises an exemplary apparatus for
implementation of electrowinning step 101 of electrowinning process
100 illustrated in FIG. 1. These and other exemplary aspects are
discussed in greater detail hereinbelow.
[0057] In accordance with one aspect of an exemplary embodiment of
the present invention, anode 23 is configured to enable the
electrolyte to flow through it. As used herein, the term
"flow-through anode" refers to an anode so configured, in
accordance with one aspect of an exemplary embodiment of the
invention.
[0058] Any now known or hereafter devised flow-through anode may be
utilized in accordance with various aspects of the present
invention. Possible configurations include, but are not limited to,
metal, wool or fabric, an expanded porous metal structure, metal
mesh, expanded metal mesh, multiple metal strips, multiple metal
wires or rods, woven wire cloth, perforated metal sheets, and the
like, or combinations thereof. Moreover, suitable anode
configurations are not limited to planar configurations, but may
include any suitable multiplanar geometric configuration.
[0059] While not wishing to be bound by any particular theory of
operation, anodes so configured allow better transport of ferrous
iron to the anode surface for oxidation, and better transport of
ferric iron away from the anode surface. Accordingly, any
configuration permitting such transport is within the scope of the
present invention.
[0060] Anodes employed in conventional electrowinning operations
typically comprise lead or a lead alloy, such as, for example,
Pb--Sn--Ca. One disadvantage of such anodes is that, during the
electrowinning operation, small amounts of lead are released from
the surface of the anode and ultimately cause the generation of
undesirable sediments, "sludges," particulates suspended in the
electrolyte, or other corrosion products in the electrochemical
cell and contamination of the copper cathode product. For example,
copper cathode produced in operations employing a lead-containing
anode typically comprises lead contaminant at a level of from about
1 ppm to about 4 ppm. Moreover, lead-containing anodes have a
typical useful life limited to approximately four to seven years.
In accordance with one aspect of a preferred embodiment of the
present invention, the anode is substantially lead-free. Thus,
generation of lead-containing sediments, "sludges," particulates
suspended in the electrolyte, or other corrosion products and
resultant contamination of the copper cathode with lead from the
anode is avoided.
[0061] In accordance with one aspect of an exemplary embodiment of
the invention, the anode is formed of one of the so-called "valve"
metals, including titanium (Ti), tantalum (Ta), zirconium (Zr), or
niobium (Nb). The anode may also be formed of other metals, such as
nickel (Ni), stainless steel (e.g., Type 316, Type 316L, Type 317,
Type 310, etc.), or a metal alloy (e.g., a nickel-chrome alloy),
intermetallic mixture, or a ceramic or cermet containing one or
more valve metals. For example, titanium may be alloyed with
nickel, cobalt (Co), iron (Fe), manganese (Mn), or copper (Cu) to
form a suitable anode. In another example, stainless steel may be
clad upon copper to form a suitable anode. Preferably, in
accordance with one exemplary embodiment, the anode comprises
titanium, because, among other things, titanium is rugged and
corrosion-resistant. Titanium anodes, for example, when used in
accordance with various embodiments of the present invention,
potentially have useful lives of up to fifteen years or more.
Moreover, when a metal anode is utilized, such an anode may also
comprise an electrochemically active coating. Exemplary coatings
include those provided from platinum, ruthenium, iridium, or other
Group VIII metals, Group VIII metal oxides, or compounds comprising
Group VIII metals, and oxides and compounds of titanium,
molybdenum, tantalum, and/or mixtures and combinations thereof.
Ruthenium oxide and iridium oxide are two preferred compounds for
use as an electrochemically active coating on titanium anodes.
[0062] In accordance with another aspect of an exemplary embodiment
of the invention, the anode comprises a titanium mesh (or other
metal, metal alloy, intermetallic mixture, or ceramic or cermet as
set forth above) upon which a coating comprising carbon, graphite,
a mixture of carbon and graphite, a precious metal oxide, or a
spinel-type coating is applied. Preferably, in accordance with one
exemplary embodiment, the anode comprises a titanium mesh with a
coating comprised of a mixture of carbon black powder and graphite
powder.
[0063] In accordance with an exemplary embodiment of the invention,
the anode comprises a carbon foam, graphite foam, or a
metal-graphite sintered material wherein the exemplary metal
described is titanium. In accordance with other embodiments of the
invention, the anode may be formed of a carbon composite material.
Examples of such composite materials include polycrystalline
graphite, polycrystalline graphite coated or densified with a
graphitizable pitch material or pyrolytic carbon, graphite foam
coated or densified with graphitizable pitch or pyrolytic carbon, a
metal-graphite sintered material coated or densified with
graphitizable pitch or pyrolytic carbon, fiber-reinforced
carbon-carbon composites, graphite and/or carbon coated metallic
mesh, and the like. In these applications, the coating or
densification seals the structure preventing particle removal,
while increasing strength, toughness, and conductivity. The coating
may contain fiber reinforcement in order to further enhance
performance characteristics.
[0064] In accordance with one aspect of an exemplary embodiment,
the graphite foam is produced from a mesophase pitch material such
as described by Klett fetal. in U.S. Pat. No. 6,261,485. Unlike
polymeric resin-based precursors such as, for example, phenolic
resin, polyvinyledene fluoride resin, polyacrylonitrile resin,
phenol/formaldehyde resin, urethane resin, and the like, mesophase
pitch foam is graphitizable and is highly conductive electrically,
when graphitized, forming an electrically-continuous-low-resistance
structure. To increase the mechanical properties and conductivity
of this foam material, it can be coated or densified with a
graphitizable material such as that formed from the pyrolysis of a
hydrocarbon gas, such as methane or a pitch-based material in the
form of petroleum pitch, coal tar pitch, or mesophase pitch formed
by, for example, a heat-soak, solvation, or catalytic
polymerization process.
[0065] In accordance with another aspect of an exemplary
embodiment, a fiber-reinforced carbon-carbon composite may comprise
fibers in one or more dimensions that may be positioned in an
oriented fashion by any technique now known or hereafter devised,
such as, for example, weaving, braiding, and filament winding, or
in random fashion (such as in the case of a felt). These fibers
form a preform which is densified with a matrix material, that
rigidizes the preform, fills in the porosity thereby increasing the
density, increases the toughness, and increases the conductivity if
it is graphitizable. It should be noted, however, that different
materials can be employed to rigidize and densify the preform. The
carbon fibers employed in the fabrication of fiber-reinforced
composites preferably comprise pitch-based, mesophase pitch-based,
and catalytically-produced carbon and graphite fibers, such as, for
example, those produced by Applied Sciences of Cedarville, Ohio.
However, polyacrilonitrile and rayon-based carbon fibers may also
be employed. The matrix employed in the fabrication of fiber
reinforced composite preferably may comprise a petroleum
pitch-based precursor, a coal tar pitch-based precursor, a
polymeric resin-based polymer precursor, a gas phase
hydrocarbon-based precursor, or a mesophase pitch-based precursor.
In accordance with one exemplary embodiment, the matrix is a
mesophase pitch-based precursor impregnated into the composite.
[0066] It should be noted that to further increase the conductivity
of these composite materials, graphitizable particulates such as
graphitized carbon black, carbon nanotubes, and/or graphite flakes
or particles may also be incorporated in the composite. In
addition, all these carbon-based materials can be heat-treated to
temperatures between about 1500.degree. C. and about 3000.degree.
C., and preferably between about 2500.degree. C. and about
3000.degree. C., to graphitize these materials in order to further
increase their conductivity.
[0067] Moreover, a metal in the metallic mesh or metal-graphite
sintered exemplary embodiment is described herein and shown by
example using titanium; however, any metal may be used without
detracting from the scope of the present invention. Exemplary
embodiments of such anodes are set forth in various of the Examples
herein.
[0068] In accordance with one exemplary embodiment, a wire mesh may
be welded to the conductor rods, wherein the wire mesh and
conductor rods may comprise materials as described above for
anodes. In one exemplary embodiment, the wire mesh comprises of a
woven wire screen with 80 by 80 strands per square inch, however
various mesh configurations may be used, such as, for example, 30
by 30 strands per square inch. Moreover, various regular and
irregular geometric mesh configurations may be used. In accordance
with yet another exemplary embodiment, a flow-through anode may
comprise a plurality of vertically-suspended stainless steel rods,
or stainless steel rods fitted with graphite tubes or rings. In
accordance with another aspect of an exemplary embodiment, the
hanger bar to which the anode body is attached comprises
copper.
[0069] Conventional copper electrowinning operations use either a
copper starter sheet or a stainless steel or titanium "blank" as
the cathode. In accordance with one aspect of an exemplary
embodiment of the invention, the cathode is configured as a metal
sheet. The cathode may be formed of copper, copper alloy, stainless
steel, titanium, or another metal or combination of metals and/or
other materials. As illustrated in FIG. 2 and as is generally well
known in the art, the cathode 25 is typically suspended from the
top of the electrochemical cell such that a portion of the cathode
is immersed in the electrolyte within the cell and a portion
(generally a relatively small portion, less than about twenty
percent (20%) of the total surface area of the cathode) remains
outside the electrolyte bath. The total surface area of the portion
of the cathode that is immersed in the electrolyte during operation
of the electrochemical cell is referred to herein, and generally in
the literature, as the "active" surface area of the cathode. This
is the portion of the cathode onto which copper is plated during
electrowinning.
[0070] In accordance with various embodiments of the present
invention, the cathode may be configured in any manner now known or
hereafter devised by the skilled artisan.
[0071] In certain embodiments of the present invention, the effect
of enhanced electrolyte circulation on the cathode reaction is to
promote effective transfer of copper ions. In order to promote a
cathode deposit that is of high quality, the electrolyte
circulation system should promote effective diffusion of copper
ions to the cathode surface. When the copper diffusion rate is
sufficiently hindered, the crystal growth pattern can change to an
unfavorable structure that may result in a rough cathode surface.
Excessive cathode roughness can cause an increase in porosity that
can entrain electrolyte, and thus impurities, in the cathode
surface. An effective diffusion rate of copper is one that promotes
favorable crystal growth for smooth, high quality cathodes. Higher
current density requires a higher rate of copper transfer to the
cathode surface. For production of high quality, commercially
acceptable cathodes, the maximum practical current density is
limited in part by the copper diffusion rate that promotes
favorable crystal growth patterns. In the present invention, the
electrolyte circulation system utilized in the electrochemical cell
to facilitate the ionic transfer to or from the anode is also
effective at promoting effective diffusion of copper ions to the
cathode. For example, use of the flow through anode enhances the
copper ion transfer to the cathode in a similar manner to the
ferrous and ferric ion transfer to and from the anode.
[0072] In accordance with an exemplary embodiment of the present
invention, the copper concentration in the electrolyte for
electrowinning is advantageously maintained at a level of about 20
to about 60 grams of copper per liter of electrolyte. Preferably,
the copper concentration is maintained at a level of from about 30
to about 50 g/L, and more preferably, from about 40 to about 45
g/L. However, various aspects of the present invention may be
beneficially applied to processes employing copper concentrations
above and/or below these levels.
[0073] Generally speaking, any electrolyte pumping, circulation, or
agitation system capable of maintaining satisfactory flow and
circulation of electrolyte between the electrodes in an
electrochemical cell such that the process specifications described
herein are practicable may be used in accordance with various
embodiments of the invention.
[0074] Injection velocity of the electrolyte into the
electrochemical cell may be varied by changing the size and/or
geometry of the holes through which electrolyte enters the
electrochemical cell. For example, with reference to FIG. 2 wherein
electrolyte flow manifold 27 is configured as tubing or piping
inside cell 21 having injection holes 29, if the diameter of
injection holes 29 is decreased, the injection velocity of the
electrolyte is increased, resulting in, among other things,
increased agitation of the electrolyte. Moreover, the angle of
injection of electrolyte into the electrochemical cell relative to
the cell walls and the electrodes may be configured in any way
desired. Although an approximately vertical electrolyte injection
configuration is illustrated in FIG. 2 for purposes of reference,
any number of configurations of differently directed and spaced
injection holes are possible. For example, although the injection
holes represented in FIG. 2 are approximately parallel to one
another and similarly directed, configurations comprising a
plurality of opposing injection streams or intersecting injection
streams may be beneficial in accordance with various embodiments of
the invention.
[0075] In accordance with one embodiment of the invention, the
electrolyte flow manifold comprises tubing or piping suitably
integrated with, attached to, or inside the anode structure, such
as, for example, inserted between the mesh sides of an exemplary
flow-through anode. Such an embodiment is illustrated, for example,
in FIG. 3, wherein manifold 31 is configured to inject electrolyte
between mesh sides 33 and 34 of anode 32. Yet another exemplary
embodiment is illustrated in FIG. 4, wherein manifold 41 is
configured to inject electrolyte between mesh sides 43 and 44 of
anode 42. Manifold 41 includes a plurality of interconnected pipes
or tubes 45 extending approximately parallel to the mesh sides 43
and 44 of anode 42 and each having a number of holes 47 formed
therein for purposes of injecting electrolyte into anode 42,
preferably in streams flowing approximately parallel to mesh sides
43 and 44, as indicated in FIG. 4.
[0076] In accordance with another embodiment of the invention, the
electrolyte flow manifold comprises an exposed "floor mat" type
manifold, generally comprising a group of parallel pipes situated
lengthwise along the bottom of the cell. Details of an exemplary
manifold of such configuration are disclosed in the Examples
herein.
[0077] In accordance with yet another embodiment of the invention,
the high flow rate and forced-flow electrolyte flow manifold is
integrated into or attached to opposite side walls and/or the
bottom of the electrochemical cell, such that, for example, the
electrolyte injection streams are oppositely directed and parallel
to the electrodes. Other configurations are, of course,
possible.
[0078] In accordance with various embodiments of the present
invention, any electrolyte flow manifold configuration that
provides an electrolyte flow effective to transport ferrous iron to
the anode, to transport ferric iron from the anode, and to
transport copper ions to the cathode such that the electrowinning
cell may be operated at a cell voltage of less than about 1.5 V and
at a current density of greater than about 26 A/ft.sup.2, is
suitable.
[0079] In accordance with an exemplary embodiment of the invention,
electrolyte flow rate is maintained at a level of from about 0.1
gallons per minute per square foot of active cathode (about 4.0
L/min/m.sup.2) to about 1.0 gallons per minute per square foot of
active cathode (about 40.0 L/min/m.sup.2). Preferably, electrolyte
flow rate is maintained at a level of from about 0.1 gallons per
minute per square foot of active cathode (about 4.0 L/min/m.sup.2)
to about 0.25 gallons per minute per square foot of active cathode
(about 10.0 L/min/m.sup.2). It should be recognized, however, that
the optimal operable electrolyte flow rate useful in accordance
with the present invention will depend upon the specific
configuration of the process apparatus, and thus flow rates in
excess of about 1.0 gallons per minute per square foot of active
cathode (in excess of about 40.0 L/min/m.sup.2) or less than about
0.1 gallons per minute per square foot of active cathode (less than
about 4.0 L/min/m.sup.2) may be optimal in accordance with various
embodiments of the present invention.
[0080] Generally, as the operating temperature of the
electrochemical cell (e.g., the electrolyte) increases, better
plating at the cathode is achievable. While not wishing to be bound
by any particular theory, it is believed that elevated electrolyte
temperatures provide additional reaction energy and may provide a
thermodynamic reaction enhancement that, at constant cell voltage,
results in enhanced copper diffusion in the electrolyte as
temperature is increased. Moreover, increased temperature also may
enhance ferrous diffusion, and can result in overall reduction of
the cell voltage, which in turn results in greater economic
efficiency. EXAMPLE 2 demonstrates a decrease in cell voltage with
increasing electrolyte temperature. Conventional copper
electrowinning cells typically operate at temperature from about
115.degree. F. to about 125.degree. F. (from about 46.degree. C. to
about 52.degree. C.).
[0081] In accordance with one aspect of an exemplary embodiment of
the present invention, the electrochemical cell is operated at a
temperature of from about 110.degree. F. to about 180.degree. F.
(from about 43.degree. C. to about 83.degree. C.). Preferably, the
electrochemical cell is operated at a temperature above about
115.degree. F. (about 46.degree. C.) or about 120.degree. F. (about
48.degree. C.), and preferably at a temperature below about
140.degree. F. (about 60.degree. C.) or about 150.degree. F. (about
65.degree. C.). However, in certain applications, temperatures in
the range of about 155.degree. F. (about 68.degree. C.) to about
165.degree. F. (about 74.degree. C.) may be advantageous.
[0082] It should be recognized, however, that while higher
operating temperatures may be beneficial for the reasons outlined
above, operation at such higher temperatures may require the use of
materials of construction designed and selected to satisfactorily
withstand the more rigorous operating conditions. In addition,
operation at higher temperatures may require increased energy
demands.
[0083] The operating temperature of the electrochemical cell may be
controlled through any one or more of a variety of means well known
in the art, including, for example, an immersion heating element,
an in-line heating device (e.g., a heat exchanger), or the like,
preferably coupled with one or more feedback temperature control
means for efficient process control.
[0084] A smooth plating surface is optimal for cathode quality and
purity, because a smooth cathode surface is denser and has fewer
cavities in which electrolyte can become entrained, thus
introducing impurities to the surface. Although it is preferable
that the current density and electrolyte flow rate parameters be
controlled such that a smooth cathode plating surface is
achievable, operating the electrochemical cell at a high current
density may nonetheless tend to result in a rough cathode surface.
Thus, in accordance with one aspect of an exemplary embodiment of
the present invention, an effective amount of a plating reagent is
added to the electrolyte stream to enhance the plating
characteristics, and thus the surface characteristics of the
cathode, resulting in improved cathode purity. Any plating reagent
effective in improving the plating surface characteristics, namely,
smoothness and porosity, of the cathode may be used. For example,
suitable plating reagents (sometimes called "smoothing agents") may
include thiourea, guar gums, modified starches, polyacrylic acid,
polyacrylate, chloride ion, and/or combinations thereof may be
effective for this purpose. When used, an effective concentration
of the plating reagent in the electrolyte--or, stated another way,
the effective amount of plating reagent required--invariably will
depend upon the nature of the particular plating reagent employed;
however, the plating reagent concentration generally will be in the
range of from about 20 grams of plating reagent per tonne of copper
plated to about 1000 g/tonne.
[0085] As ferrous iron is oxidized at the anode in the
electrochemical cell, the concentration of ferrous iron in the
electrolyte is naturally depleted, while the concentration of
ferric iron in the electrolyte is naturally increased. In
accordance with one aspect of an exemplary embodiment of the
invention, the concentration of ferrous iron in the electrolyte is
controlled by addition of ferrous sulfate to the electrolyte. In
accordance with another embodiment of the invention, the
concentration of ferrous iron in the electrolyte is controlled by
solution extraction (SX) of iron from copper leaching
solutions.
[0086] In order for the ferrous/ferric couple to maintain a
continuous anode reaction, the ferric iron generated at the anode
preferably is reduced back to ferrous iron to maintain a
satisfactory ferrous concentration in the electrolyte.
Additionally, the ferric iron concentration preferably is
controlled to achieve satisfactory current efficiency in the
electrochemical cell.
[0087] In accordance with an exemplary embodiment of the present
invention, the total iron concentration in the electrolyte is
maintained at a level of from about 10 to about 60 grams of iron
per liter of electrolyte. Preferably, the total iron concentration
in the electrolyte is maintained at a level of from about 20 g/L to
about 40 g/L, and more preferably, from about 25 g/L to about 35
g/L. It is noted, however, that the total iron concentration in the
electrolyte may vary in accordance with various embodiments of the
invention, as total iron concentration is a function of iron
solubility in the electrolyte. Iron solubility in the electrolyte
varies with other process parameters, such as, for example, acid
concentration, copper concentration, and temperature. As explained
hereinabove, decreasing iron concentration in the electrolyte is
generally economically desirable, because doing so decreases iron
make-up requirements and decreases the electrolyte sulfate
saturation temperature, and thus decreases the cost of operating
the electrowinning cell.
[0088] In accordance with an exemplary embodiment of the present
invention, the ferric iron concentration in the electrolyte is
maintained at a level of from about 0.001 to about 10 grams of
ferric iron per liter of electrolyte. Preferably, the ferric iron
concentration in the electrolyte is maintained at a level of from
about 1 g/L to about 6 g/L, and more preferably, from about 2 g/L
to about 4 g/L.
[0089] Referring again to FIG. 1, in accordance with another aspect
of an exemplary embodiment of the invention, the concentration of
ferric iron in the electrolyte within the electrochemical cell is
controlled by removing at least a portion of the electrolyte from
the electrochemical cell, for example, as illustrated in FIG. 1 as
electrolyte regeneration stream 15 of process 100.
[0090] In accordance with one aspect of an exemplary embodiment of
the invention, sulfur dioxide 17 may be used to reduce the ferric
iron in electrolyte regeneration stream 15. Although reduction of
Fe.sup.3+ to Fe.sup.2+ in electrolyte regeneration stream 15 in
ferrous regeneration stage 103 may be accomplished using any
suitable reducing reagent or method, sulfur dioxide is particularly
attractive as a reducing agent for Fe.sup.3+ because it is
generally available from other copper processing operations, and
because sulfuric acid is generated as a byproduct. Upon reacting
with ferric iron in a copper-containing electrolyte, the sulfur
dioxide is oxidized, forming sulfuric acid. The reaction of sulfur
dioxide with ferric iron produces two moles of sulfuric acid for
each mole of copper produced in the electrochemical cell, which is
one mole more of acid than is typically required to maintain the
acid balance within the overall copper extraction process, when
solution extraction (SX) is used in conjunction with
electrowinning. The excess sulfuric acid may be extracted from the
acid-rich electrolyte (illustrated in FIG. 1 as stream 18)
generated in the ferrous regeneration stage for use in other
operations, such as, for example, leaching operations.
[0091] With reference to FIG. 1, the acid-rich electrolyte stream
18 from ferrous regeneration stage 1.03 may be returned to
electrowinning stage 101 via electrolyte recycle streams 12 and 16,
may be introduced to acid removal stage 105 for further processing,
or may be split (as shown in FIG. 1) such that a portion of
acid-rich electrolyte stream 18 returns to electrowinning stage 101
and a portion continues to acid removal stage 105. In acid removal
stage 105, excess sulfuric acid is extracted from the acid-rich
electrolyte and leaves the process via acid stream 19, to be
neutralized or, preferably, used in other operations, such as, for
example a heap leach operation. The acid-reduced electrolyte stream
14 may then be returned to electrowinning stage 101 via electrolyte
recycle stream 16, as shown in FIG. 1.
[0092] In sum, copper electrowinning using the ferrous/ferric anode
reaction in accordance with one embodiment of the present invention
produces two products--cathode copper and sulfuric acid.
[0093] In accordance with another aspect of an exemplary embodiment
of the invention, the ferric-rich electrolyte is contacted with
sulfur dioxide in the presence of a catalyst, such as, for example,
activated carbon manufactured from bituminous coal, or other types
of carbon with a suitable active surface and suitable structure.
The reaction of sulfur dioxide and ferric iron is preferably
monitored such that the concentration of ferric iron and ferrous
iron in the acid-rich electrolyte stream produced in the ferrous
regeneration stage can be controlled. In accordance with an aspect
of another embodiment of the invention, two or more
oxidation-reduction potential (ORP) sensors are used--at least one
ORP sensor in the ferric-rich electrolyte line upstream from the
injection point of sulfur dioxide, and at least one ORP sensor
downstream from the catalytic reaction point in the ferric-lean
electrolyte. The ORP measurements provide an indication of the
ferric/ferrous ratio in the solution; however, the exact
measurements depend on overall solution conditions that may be
unique to any particular application. Those skilled in the art will
recognize that any number of methods and/or apparatus may be
utilized to monitor and control the ferric/ferrous ratio in the
solution. The ferric-rich electrolyte will contain from about 0.001
to about 10 grams per liter ferric iron, and the ferric-lean
electrolyte will contain up to about 6 grams per liter ferric
iron.
[0094] The following examples illustrate, but do not limit, the
present invention.
EXAMPLE 1
[0095] TABLE 1 demonstrates the advantages of a flow-through anode
with in-anode electrolyte injection for achieving low cell voltage.
An in-anode manifold produces a lower cell voltage at the same flow
or decreases flow requirements at the same current density versus
bottom injection. TABLE 1 also demonstrates that a cell voltage
below 1.10 V is achievable at a current density of about 35
A/ft.sup.2 (377 A/m.sup.2) and a cell voltage below 1.25 V is
achievable at a current density of about 40 A/ft.sup.2 (430
A/m.sup.2).
[0096] Test runs A-F were performed using an electrowinning cell of
generally standard design, comprising three full-size conventional
cathodes and four full-size flow-through anodes. The cathodes were
constructed of 316 stainless steel and each had an active depth of
41.5 inches and an active width of 37.5 inches (total active
surface area of 21.6 ft.sup.2 per cathode). Each anode had an
active width of 35.5 inches and an active depth of 39.5 inches and
was constructed of titanium mesh with an iridium oxide-based
coating. The anodes used in accordance with this EXAMPLE 1 were
obtained from Republic Anode Fabricators of Strongsville, Ohio,
USA.
[0097] Test duration was five days (except test runs C, D, E and F,
which were 60-minute tests designed to measure voltage only, at
constant conditions), with continuous 24-hour operation of the
electrowinning cell at approximately constant conditions. Voltage
measurements were taken once per day using a handheld voltage meter
and voltages were measured bus-to-bus. The stated values for
average cell voltage in TABLE 1 represent the average voltage
values over the six-day test period. Electrolyte flow measurements
were performed by a continuous electronic flow meter (Magmeter),
and all electrolyte flow rates in TABLE 1 are shown as gallons per
minute of electrolyte per square foot of cathode plating area. The
plating reagent utilized in all test runs was PD 4201 modified
starch, obtained from Chemstar from Minneapolis, Minn. The
concentration of plating reagent in the electrolyte was maintained
in the range of 250-450 grams per plated ton of copper.
[0098] Electrolyte temperature was controlled using an automatic
electric heater (Chromalox). Iron addition to the electrolyte was
performed using ferrous sulfate crystals (18% iron). Copper and
iron concentration assays were performed using standard atomic
absorption tests. Copper concentration in the electrolyte was
maintained at a level of about 41-46 g/L using solution
extraction.
[0099] The concentration of sulfuric acid in the electrolyte was
maintained at a level of about 150-160 g/L using an Eco-Tec
sulfuric acid extraction unit (acid retardation process).
[0100] The current to each electrowinning cell was set using a
standard rectifier. The operating current density for each test run
was calculated by dividing the total Amps on the rectifier setting
by the total cathode plating area (i.e., 64.8 ft.sup.2).
[0101] Ferrous regeneration was accomplished using sulfur dioxide
gas, which was injected into an electrolyte recycle stream, then
passed through an activated carbon bed in order to catalyze the
ferric reduction reaction. The reaction was controlled using ORP
sensors, which measured ORP in the range of 390 to 410 mV (versus
standard silver chloride reference junction). Sufficient sulfur
dioxide was injected into the electrolyte recycle stream such that
the ORP was maintained within the range of 390 to 410 mV.
[0102] Average copper production rate for test runs A and B, which
were operated at a current density of 30 A/ft.sup.2, was 112 lbs.
per day. The copper cathode produced for test runs A and B measured
less than 0.3 ppm. lead and less than 5 ppm sulfur. Copper purity
did not vary overall according to the specific test conditions
employed. Copper assays on test runs C-F were not performed because
of the relatively short test duration.
[0103] Test runs A, C, and E were performed using a
bottom-injection "floor mat" injection manifold configuration. The
bottom-injection manifold included eleven 1'' diameter PVC pipes
configured to run the length of the electrowinning cell (i.e.,
approximately perpendicular to the active surfaces of the
electrodes). Each of the eleven pipes positioned one 3/16''
diameter hole in each electrode gap (i.e., there were eleven holes
approximately evenly spaced within each electrode gap).
[0104] Test runs A, D, and F were performed using an in-anode
injection manifold configuration. The in-anode injection manifold
was configured using a distribution supply line adjacent to the
electrodes, with direct electrolyte supply line comprising 3/8''
ID.times.1/2'' OD or 1/4'' ID.times.3/8'' OD polypropylene tubing
branching from the distribution supply line and leading to each
anode. Each electrolyte supply line included five equally-spaced
dropper tubes that branched from the electrolyte supply line and
were positioned to inject electrolyte directly into the anode,
between the mesh surfaces of the anode. No electrolyte injection
occurred directly adjacent to the cathodes. TABLE-US-00001 TABLE 1
Electro- Cathode Electrolyte lyte Electro- Average Current Manifold
Iron lyte Electrolyte Cell Density, Distributor Conc., Flow,
Temperature, Voltage, Test A/ft.sup.2 Design g/L gpm/ft.sup.2
.degree. F. V A 30 Bottom 25.5 0.41 125 0.95 Injection B 30
In-Anode 25.5 0.41 125 0.90 Injection C 35 Bottom 28 0.66 125 1.02
Injection D 35 In-Anode 28 0.24 125 1.10 Injection E 40 Bottom 28
0.66 125 1.12 Injection F 40 In-Anode 28 0.24 125 1.25
Injection
EXAMPLE 2
[0105] TABLE 2 demonstrates that increasing temperature decreases
cell voltage.
[0106] Test runs A-C were performed using an electrowinning cell of
generally standard design, comprising three full-size conventional
cathodes and four full-size flow-through anodes. The cathodes were
constructed of 316 stainless steel and each had an active depth of
41.5 inches and an active width of 37.5 inches (total active
surface area of 21.6 ft.sup.2 per cathode). Each anode had an
active width of 35.5 inches and an active depth of 39.5 inches and
was constructed of titanium mesh with an iridium oxide-based
coating. The anodes used in accordance with this EXAMPLE 2 were
obtained from Republic Anode Fabricators of Strongsville, Ohio,
USA.
[0107] Test duration was six days, with continuous 24-hour
operation of the electrowinning cell at approximately constant
conditions. Voltage measurements were taken once per day using a
handheld voltage meter and voltages were measured bus-to-bus. The
stated values for average cell voltage in TABLE 2 represent the
average voltage values over the six-day test period. Electrolyte
flow measurements were performed by a continuous electronic flow
meter (Magmeter), and all electrolyte flow rates in TABLE 2 are
shown as gallons per minute of electrolyte per square foot of
cathode plating area. The plating reagent utilized in all test runs
was PD 4201 modified starch, obtained from Chemstar from
Minneapolis, Minn. The concentration of plating reagent in the
electrolyte was maintained in the range of 250-450 grams per plated
ton of copper.
[0108] Electrolyte temperature was controlled using an automatic
electric heater (Chromalox). Iron addition to the electrolyte was
performed using ferrous sulfate crystals (18% iron). Copper and
iron concentration assays were performed using standard atomic
absorption tests. Copper concentration in the electrolyte was
maintained at a level of about 41-46 g/L using solution
extraction.
[0109] The concentration of sulfuric acid in the electrolyte was
maintained at a level of about 150-160 g/L using an Eco-Tec
sulfuric acid extraction unit (acid retardation process).
[0110] The current to each electrowinning cell was set using a
standard rectifier. The operating current density for each test run
was calculated by dividing the total Amps on the rectifier setting
by the total cathode plating area (i.e., 64.8 ft.sup.2).
[0111] Ferrous regeneration was accomplished using sulfur dioxide
gas, which was injected into an electrolyte recycle stream, then
passed through an activated carbon bed in order to catalyze the
ferric reduction reaction. The reaction was controlled using ORP
sensors, which measured ORP in the range of 390 to 410 mV (versus
standard silver chloride reference junction). Sufficient sulfur
dioxide was injected into the electrolyte recycle stream such that
the ORP was maintained within the range of 390 to 410 mV.
[0112] Average copper production rate for all test runs, which were
operated at a current density of 30 A/ft.sup.2, was 112 lbs. per
day. The copper cathode produced for all test runs generally
measured less than 0.3 ppm lead and less than 5 ppm sulfur. Copper
purity did not vary overall according to the specific test
conditions employed.
[0113] Test runs were performed using a bottom-injection "floor
mat" injection manifold configuration. The bottom-injection
manifold included eleven one-inch (1'') diameter PVC pipes
configured to run the length of the electrowinning cell (i.e.,
approximately perpendicular to the active surfaces of the
electrodes). Each of the eleven pipes positioned one 3/16''
diameter hole in each electrode gap (i.e., there were eleven holes
approximately. evenly spaced within each electrode gap).
TABLE-US-00002 TABLE 2 Electro- Cathode Electrolyte lyte Electro-
Average Current Manifold Iron lyte Electrolyte Cell Density,
Distributor Conc., Flow, Temperature, Voltage, Test A/ft.sup.2
Design g/L gpm/ft.sup.2 .degree. F. V A 30 Bottom 28.6 0.28 125
0.92 Injection B 30 Bottom 27.2 0.28 135 0.88 Injection C 30 Bottom
26.9 0.28 125 0.95 Injection
EXAMPLE 3
[0114] TABLE 3 demonstrates the effectiveness of a graphite foam
and two titanium-graphite sintered anodes. The first
titanium-graphite sintered anode comprised of 12% graphite and 88%
titanium. The second titanium-graphite sintered anode comprised of
8% graphite and 92% titanium. The titanium-graphite sintered anodes
were prepared by mixing powders of the graphite and titanium and
pressing them into a hollow cylinder. The hollow cylinders were
then attached to a titanium rod or a co-extruded copper-titanium
rod. The rods with attached cylinders are then sintered to provide
mechanical strength. Finally, the rods are hung from a copper
hanger bar to form the anode.
[0115] The present inventors have demonstrated that use of such
anodes in connection with an electrowinning process using the
ferrous/ferric anode reaction enables an average cell voltage of
less than 1.0 V. Approximately 3-inch by 5-inch samples of such
anodes were immersed in copper-containing electrolyte solution and
attached to the positive pole of a DC power supply so that the
sample would function as the cell anode. A stainless steel "blank"
was also immersed in the bath and utilized as the cell cathode.
[0116] A current of 2.6 Amps was passed through the cell using the
DC power supply. This corresponded to a current density of
approximately 26.6 A/ft.sup.2 based on the immersed plating area of
the stainless steel cathode. The temperature of the electrolyte
bath was maintained at about 120.degree. F. using an immersion
heater. The copper-containing electrolyte contained about 40 g/L
copper, about 155 g/L sulfuric acid, and about 28 g/L iron.
Electrolyte was injected into the cell from the bottom using an
injection manifold at a flow rate of about 100 mL/min for the
titanium-graphite sintered anodes and about 160 mL/min for the
graphite foam anode. TABLE-US-00003 TABLE 3 Cathode Average Current
Electrolyte Electrolyte Cell Density, Anode Flow, Temperature,
Voltage, Test A/ft.sup.2 Material mL/min .degree. F. V A 30 Carbon
Foam 96 125 0.92 B 30 12% Graphite 98 124 0.89 88% Titanium C 30 8%
Graphite 96 125 0.86 92% Titanium
[0117] The titanium-graphite sintered anodes exhibited flow-through
characteristics because the vertical rods allowed electrolyte to
flow in between the rods. The graphite foam anode was first tested
as a solid slab. However, drilling holes in the slab to incorporate
flow-through characteristics lowered the electrolyte flow rate
required to about 100 mL/min in order to achieve a cell voltage
below 1 Volt.
EXAMPLE 4
[0118] TABLE 4 demonstrates the effectiveness of a carbon composite
anode configured in accordance with aspects of another exemplary
embodiment of the present invention. Three carbon composite anode
samples were tested. For each of the three carbon composite
samples, a 3-inch by 5-inch section was cut and drilled with two
holes for mounting in a bench-scale copper electrowinning cell.
Each carbon composite sample was immersed in copper-containing
electrolyte solution and attached to the positive pole of a DC
power supply so that the carbon composite sample would function as
the cell anode. A stainless steel "blank" was also immersed in the
bath and utilized as the cell cathode.
[0119] A current of 2.6 Amps was passed through the cell using the
DC power supply. This corresponded to a current density of
approximately 30 A/ft.sup.2 based on the immersed plating area of
the stainless steel cathode. The temperature of the electrolyte
bath was maintained at about 120.degree. F. using an immersion
heater. The copper-containing electrolyte contained about 40 g/L
copper, about 155 g/L sulfuric acid, and about 28 g/L iron.
Electrolyte was injected into the cell from the bottom using an
injection manifold at a rate of about 160 mL/min. Copper plating
was carried out for a period of several days for each continuous
test, wherein each test anode sample comprised a woven carbon
composite fiber sample with varying degrees of density and
rigidity. The low density sample was of lower density and rigidity
than the medium density sample, and the medium density sample was
of lower density and rigidity than the high density sample.
[0120] The present inventors have demonstrated that use of a carbon
composite anode such as the one described in this Example in
connection with an electrowinning process using the ferrous/ferric
anode reaction enables an average cell voltage of less than 1.0 V.
The carbon composite anodes were first tested as slabs. Drilling
holes into the carbon composite anode to provide flow-through
characteristics lowered the electrolyte flow rate requirement to
achieve a cell voltage less than 1 volt to about 100 mL/min.
TABLE-US-00004 TABLE 4 Anode Sample 1 2 3 (Low Density) (Medium
Density) (High Density) Average Cell 0.76 0.85 0.90 Voltage,
Volts
EXAMPLE 5
[0121] For this Example, a carbon-coated titanium mesh anode
configured in accordance with other aspects of an embodiment of the
present invention was tested. A 3-inch by 5-inch sample of bare
titanium mesh was provided, and a coating comprising a mixture of
graphite powder (obtained from Superior Graphite Company) and
carbon black powder (obtained from Columbian Chemicals Company) was
applied to the titanium mesh. The sample was cut and drilled with
two holes for mounting in a bench-scale copper electrowinning cell.
The sample was immersed in copper-containing electrolyte solution
and attached to the positive pole of a DC power supply so that the
sample would function as the cell anode. A stainless steel "blank"
was also immersed in the bath and utilized as the cell cathode.
[0122] Initially, a current of 2.3 Amps was passed through the cell
using the DC power supply. This corresponded to a current density
of approximately 26.6 A/ft.sup.2 based on the immersed plating area
of the stainless steel cathode. The temperature of the electrolyte
bath was maintained at about 120.degree. F. using an immersion
heater. The copper-containing electrolyte contained about 40 g/L
copper, about 155 g/L sulfuric acid, and about 28 g/L iron.
Electrolyte was injected into the cell from the bottom using an
injection manifold at a rate of about 105 mL/min. Under these
conditions, cell voltage measured about 1.2 Volts. Increasing the
flow rate of electrolyte into the cell to about 180 mL/min
decreased the overall cell voltage to about 1.14 Volts.
[0123] After three days of continuous operation at a current of 2.3
Amps and an electrolyte flow rate of about 105 mL/min, however,
cell voltage had decreased to approximately 0.92 Volts. This
observation suggests that the carbon-coated titanium mesh anode
sample had a brief "break in" period before optimal utility was
achievable. When cell amperage was increased to 2.6 Amps
(corresponding to a current density of about 30 A/ft.sup.2) and
electrolyte flow rate was increased to 115 mL/min, the cell voltage
measured 1.03 Volts.
EXAMPLE 6
[0124] A flow-through anode utilized in connection with this
Example comprised a woven wire cloth made of Type 316 stainless
steel. A 3-inch by 5-inch sample of woven wire cloth comprising
80.times.80 strands per square inch was immersed in a
copper-containing electrolyte solution and attached to the positive
pole of a DC power supply so that the sample would function as the
anode in the electrowinning cell. The strand diameter of the wires
in the sample was 0.007 inches, and the percent open area of the
woven structure was 19%. A stainless steel "blank" also was
immersed in the electrolyte solution and was utilized as the cell
cathode.
[0125] A current of 2.8 Amps was passed through the cell using the
DC power supply. This corresponded to a current density of
approximately 32 A/ft.sup.2 based on the immersed plating area of
the stainless steel cathode. The temperature of the electrolyte
bath was maintained at about 125.degree. F. using an immersion
heater. The copper-containing electrolyte contained about 40 g/L
copper, about 155 g/L sulfuric acid, and about 32 g/L iron.
Electrolyte was injected into the cell from the bottom using an
injection manifold at a flow rate of about 50 mL/min. The present
inventors demonstrated that use of a stainless steel woven wire
cloth anode such as the one described in this Example in connection
with an electrowinning process using the ferrous/ferric anode
reaction enables an average cell voltage of about 1.08 Volts.
[0126] An effective and efficient method of copper electrowinning
using the ferrous/ferric-sulfur dioxide anode reaction has been
presented herein. Further, the present inventors have advanced the
art of copper hydrometallurgy by recognizing the advantages of
using the ferrous/ferric anode reaction in connection with copper
electrowinning processes, and have developed an improved system for
utilizing the ferrous/ferric anode reaction to achieve greater
efficiency over conventional copper electrowinning processes.
[0127] The present invention has been described above with
reference to a number of exemplary embodiments and examples. It
should be appreciated that the particular embodiments shown and
described herein are illustrative of the invention and its best
mode and are not intended to limit in any way the scope of the
invention as set forth in the claims. Those skilled in the art
having read this disclosure will recognize that changes and
modifications may be made to the exemplary embodiments without
departing from the scope of the present invention. These and other
changes or modifications are intended to be included within the
scope of the present invention.
* * * * *