WO2012051446A2

WO2012051446A2 - Improved electrowinning process

Info

Publication number: WO2012051446A2
Application number: PCT/US2011/056200
Authority: WO
Inventors: Scot P. Sandoval; Brett Ashford; Casey J. Clayton
Original assignee: Freeport-Mcmoran Corporation
Priority date: 2010-10-14
Filing date: 2011-10-13
Publication date: 2012-04-19
Also published as: WO2012051446A3

Abstract

The present disclosure provides systems and methods useful in suppressing the formation of metal oxides on an anode during electrowinning of a metal value. For example, a method is disclosed comprising electrowinning a metal value from a metal value-bearing electrolyte solution using an electrowinning system, the electrowinning system comprising an anode, and adding at least one of an acrylamide and a hydrocarbon with an acrylamide functional group to the metal value-bearing electrolyte solution for the suppression of metal oxide deposition on the surface of the anode.

Description

IMPROVED ELECTROWINNING PROCESS

FIELD OF INVENTION

The present invention relates, generally, to an improved electrowinning process, and more specifically, to an electrowinning process in which a reagent is used to suppress unwanted species deposition on anode surfaces.

BACKGROUND OF THE INVENTION

Electrolytic recovery of metals is well established as an effective means of recovering metal values from a number of different sources. Typically, electrolytic recovery of metals is accomplished by providing a metal-containing solution to an electrowinning cell. An electrowinning cell may comprise at least one anode and at least one cathode immersed in the metal-containing solution. When a current is passed through the solution from anode to cathode, a metal value is plated onto the cathode. A commercial electrowinning cell may utilize a number of anodes and cathodes.

In conventional copper electrowinning processes the following reactions occur:

Cathode reaction:

Cu2+ + S042- + 2e-→ CuO + S042- (E0 = +0.345 V)

Anode reaction:

H20→ ½ 02 + 2H+ + 2e- (E0 = -1.230 V)

Overall cell reaction:

Cu2+ + S042- + H20→ CuO + 2H+ + S042- + ½ 02 (E0 = -0.885 V)

One way that has been found to potentially reduce the energy requirement for copper electrowinning is to use alternative anode reaction electrowinning (i.e., oxidation of ferrous ion to ferric ion at the anode), which occurs by the following reactions:

Cathode reaction:

Cu2+ + S042- + 2e-→ CuO + S042- (E0 = +0.345 V)

Anode reaction:

2Fe2+→ 2Fe3+ + 2e- (E0 = -0.770 V)

Overall cell reaction:

Cu2+ + S042- + 2Fe2+→ CuO + 2Fe3+ + S042- (E0 = -0.425 V) The ferric iron generated at the anode as a result of this overall cell reaction can be reduced back to ferrous iron (i.e., "regenerated") using sulfur dioxide, as follows:

Solution reaction:

2Fe3+ + S02 + 2H20→ 2Fe2+ + 4H+ + S042-

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 (Fe2+) to ferric iron (Fe3+) occurs at a lower voltage than does the decomposition of water.

Recently, improved anode chemistry and design, for example, metal oxide coated anodes, has decreased energy usage and increased metal purity in electrowinning processes. These improvements directly increase the cost effectiveness of electrowinning processes for the recovery of metal value from metal-bearing materials.

One problem associated with the use of metal oxide coated anodes is deposition of unwanted species on the anode surfaces. A number of species present in the metal bearing solution, including manganese, may oxidize at the anode. Manganese ions, for example, may oxidize to Mn3+ and Mn4+ at the anode. The oxidized manganese may precipitate out of solution and deposit on the surface of the anode in the form of various manganese oxides, including MnO(OH) and Mn02. This deposition may cause voltage escalation, which decreases the efficiency and effectiveness of the anode. Additionally, the active lifespan of the anode may be reduced. As a result, the cost effectiveness of the electrowinning process is decreased. Thus, an improved electrowinning process in which unwanted species deposition on anode surfaces is suppressed is advantageous.

SUMMARY OF THE INVENTION

The present disclosure provides systems and methods useful in suppressing the formation of metal oxides on an anode during electrowinning of a metal value. For example, a method is disclosed comprising electrowinning a metal value from a metal value- bearing electrolyte solution using an electrowinning system, the electrowinning system comprising an anode, and adding at least one of an acrylamide and a hydrocarbon with an acrylamide functional group to the metal value-bearing electrolyte solution for the suppression of metal oxide deposition on the surface of the anode. In accordance with various exemplary embodiments of the present invention, an electrowinning process is improved by means of suppressing unwanted species deposition on electrode surfaces. A reagent is added to the electrolyte solution of the electrowinning cell to suppress deposition of unwanted species on anode surfaces.

In accordance with an exemplary embodiment, the unwanted species which may deposit on anode surfaces consist of metal oxides, such as, for example, manganese oxides. These manganese oxides may include, for example, MnO(OH) and Mn02.

In accordance with various exemplary embodiments of the invention the reagent may be a hydrocarbon with an acrylamide functional group. For example, the reagent may be prop-2-enamide, commonly known as acrylamide. Alternatively, the reagent may consist of a hydrocarbon chain with an acrylamide functional group.

In various exemplary embodiments of the invention, the reagent may be a polymeric reagent. In one exemplary embodiment, the reagent may consist of a polyacrylamide reagent. In a preferred aspect of the exemplary embodiment, the reagent is a nonionic polyacrylamide reagent, commercially available as Cyquest N-900.

The present invention is designed to address, among other things, various deficiencies in the prior art. The methods disclosed herein achieve advancement by reducing unwanted species deposition on anode surfaces in an electrowinning process, which enables significant enhancement in anode active life span and energy efficiency. Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. The present invention will become more fully understood from the detailed description and the accompanying drawings herein:

FIG. 1 is a cross sectional view illustrating an electrowinning cell configured to operate in accordance with an exemplary embodiment of the present invention.

FIG. 2 is a graph illustrating the results of Experiment 1.

FIG. 3 is a graph illustrating the results of Experiment 2. DETAILED DESCRIPTION

The detailed description of exemplary embodiments of the invention herein shows various exemplary embodiments and the best modes known to the inventors at this time. These exemplary embodiments and modes are described in sufficient detail to enable those skilled in the art to practice the invention and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following disclosure is intended to teach both the implementation of the exemplary embodiments and modes and any equivalent modes or embodiments that are known or obvious to those of reasonable skill in the art. All included figures are non-limiting illustrations of the exemplary embodiments and modes, which similarly avail themselves to any equivalent modes or embodiments that are known or obvious to those of reasonable skill in the art.

Various embodiments of the present invention provide improved electrowinning processes in which unwanted species deposited on electrode surfaces are suppressed. Suppression of unwanted species deposition on anode surfaces can decrease voltage escalation in the electrowinning process. By reducing voltage escalation, the electrowinning cell may require a lower voltage to operate, thereby decreasing the cost of recovering metal value. In addition, the active lifespan of the anode may be extended. Existing electrowinning process sequences may be modified to incorporate the method, and receive the benefits, provided by the present invention.

In the present invention, a reagent is used to suppress unwanted species deposition on anode surfaces in an electrowinning process. In an aspect of various embodiments, the unwanted species deposition consists of metal oxides. Often these species are created when various metal species, present in the metal-containing solution which is processed by the electrowinning cell, oxidize at the anode.

With initial reference to Figure 1, in various exemplary embodiments of the present invention, electrowinning circuit 118 may comprise one or more electrowinning cells 100, which may comprise a vessel 110 used for electrolysis containing electrolyte 115, at least one cathode 120, and at least one anode 160. Electrolyte 115 may flow through vessel 110 and exit at electrolyte out 130. Electrolyte 115 may fill vessel 110 to a height 118.

Electrodes 150, which include multiple anodes 160 and cathodes 120, are active when submerged in electrolyte 115 to the point of height 118. As those skilled in the art will appreciate, height 118 is most efficient when essentially all of the area of the plurality of electrodes 150 is submerged in electrolyte 115. Plurality of electrodes 150 may be any number of anodes 160 and cathodes 120, generally placed in an alternating pattern. It should be appreciated in accordance with the present invention that any number and any configuration of anodes 160 and/or cathodes 120 may be utilized.

For purposes of this description of various embodiments of the present invention, the term "cathode" refers to a complete electrode assembly to which negative polarity is applied and is typically connected to a power source (not shown). As used herein, the term "flow- through cathode" refers to any cathode 120 configured to enable electrolyte 115 solution to pass through cathode 120 during the electro winning process.

In various embodiments of conventional and alternate anode reaction chemistry electrowinning operations, such as, for example, those used in copper purification, a copper starter sheet, stainless steel "blank", or titanium "blank" may be used as cathode 120 in the electrowinning cell. In an exemplary embodiment, cathode 120 may be configured as a flow-through cathode.

In general, any anode configuration now known or hereafter devised suitable to achieve the processing parameters and objectives described herein may be used in accordance with various embodiments of the present invention. Various embodiments of the present invention may utilize conventional "plate"-type (i.e., non-flow-through) anodes, other flow-through or non-flow-through anodes of various geometries (e.g., cylindrical anodes), flow-through anodes, or a combination of types within one or more electrowinning cells. Any anode, however, that enables electrowinning of metal value from a metal value containing solution may be employed in connection with the present invention.

In accordance with one exemplary embodiment of the invention, however, at least one flow-through anode is utilized in connection with the electrowinning cell. As used herein, the term "flow-through anode" refers to any anode configured to enable electrolyte to pass through it.

When a flow-through anode is utilized, any now known or hereafter devised flow- through anode may be utilized in accordance with various exemplary embodiments of the present invention. Possible configurations include, but are not limited to, metal, metal wool, metal fabric, other suitable conductive nonmetallic materials (e.g., carbon materials), an expanded porous metal structure, metal mesh, expanded metal mesh, corrugated 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.

In accordance with 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 accordance with a preferred aspect of the exemplary embodiment, the anode comprises titanium, because, among other things, titanium is rugged and corrosion-resistant. Titanium anodes potentially have useful lives of up to fifteen years or more.

With continued reference to Figure 1, in various embodiments of the present invention, anode 160 may comprise a combination of conductive materials in which an outer surface of a first conductive material is coated or clad by a second conductive material. In various embodiments, the conductive material may comprise, for example, copper, copper alloy, aluminum, copper aluminum alloys, stainless steel, titanium, gold, combinations thereof, or any other electrically conductive material. In an exemplary embodiment, anode 160 may comprise a copper or copper alloy core and a titanium cladding clad over the core.

In various exemplary embodiments, anode 160 may also optionally comprise any 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.

In various exemplary embodiments, anode 160 may also comprise multiple layers of electrochemically active coatings. In various embodiments, the present invention provides an anode comprising a conductive substrate, a first layer comprising a compound in crystalline phase on an outer surface of the conductive substrate, and a second layer comprising a compound having at least a portion thereof in an amorphous phase on surface of the first layer.

In various embodiments, the compound can be Ir02. However, those skilled in the art will appreciate that the invention can be practiced using any compound or mixture which has a glass transition temperature and/or can be in either a crystalline or an amorphous phase. In an exemplary embodiment, the crystalline Ir02 containing layer and the amorphous Ir02 containing layer may contain a mixture of the Ir02 and tantalum oxide such as Ta205.

In accordance with an exemplary embodiment, the unwanted species which may deposit on anode surfaces consist of metal oxides, such as, for example, manganese oxides. These manganese oxides may include MnO(OH) and Mn02. As a means of providing an exemplary illustration, Mn3+ ions present in the electrolyte solution may precipitate as MnO(OH) and deposit on the surface of anodes. In this example, manganese ions present in the electrolyte solution as Mn2+ may remain in solution instead forming manganese oxides and depositing on the surface of anodes.

In an exemplary electrowinning cell which utilizes the oxidation of ferrous ions reaction, the ratio of ferrous ions to Mn3+ ions may affect the deposition rate of manganese oxides on the surface of anodes. For example, a ratio of ferrous ions to Mn3+ ions of 8: 1 or more may shift the equilibrium balance in the electrolyte solution between Mn2+ and Mn3+ ions, reducing the number of Mn3+ ions in solution. This reduction may decrease the deposition of oxides, such as MnO(OH), on the surface of anodes. These mechanisms are intended to serve as examples of possible manners in which unwanted species may form and deposit on the surface of anodes. Therefore, they represent only a possible embodiment and are non-limiting.

In accordance with various exemplary embodiments of the invention, a reagent is added to the electrolyte solution of the electrowinning cell to suppress deposition of unwanted species on anode surfaces.

In accordance with an exemplary embodiment of the invention, in reference back to

Figure 1, reagent stream 140 may be added to the electrowinning cell to suppress unwanted species deposition on the surface of anode 160. In an exemplary embodiment of the present invention, reagent stream 140 may be added to the electrowinning cell prior to the start of the electrowinning process. In another exemplary embodiment of the present invention, reagent stream 140 may be added continually to the electrowinning cell throughout the operation of the electrowinning process. In an aspect of the exemplary embodiment, the amount of reagent added may be a function of the rate of deposition of unwanted species on anode surfaces in the absence of the reagent. In another aspect of the exemplary embodiment, reagent may be added in a quantity determined to maintain a level of deposition of unwanted species on anode surfaces less than or equal to a predetermined level.

In yet another exemplary embodiment of the present invention, reagent stream 140 may be added to the electrowinning cell at various specific time-points during the operation of the electrowinning cell. In an aspect of the exemplary embodiment, reagent may be added at specific time points that correspond with the amount of time required for a predetermined quantity or layer thickness of unwanted species to deposit on anode surfaces.

In addition to the exemplary embodiments previously disclosed, it should be appreciated that any method of adding reagent to the electrowinning cell which is effective in suppressing unwanted species deposition on anode surfaces may be used in the context of the present invention.

In accordance with an exemplary embodiment of the present invention, the reagent may be added to the electrolyte solution at a concentration in the range of about 2 to about 100 ppm. In accordance with a preferred aspect of the exemplary embodiment, the reagent may be added to the electrolyte solution at a concentration in the range of about 5 to about 50 ppm, and more preferably in the range of about 10 to 15 ppm. However, any concentration of reagent which successfully suppresses unwanted species deposition on anode surfaces is in accordance with the present invention.

In an exemplary embodiment, the reagent may be provided in a number of different forms. For example, the reagent may comprise a dry powder. Alternatively, the reagent may be in an emulsion. The reagent may be provided in any state in which it can be added to the electrowinning cell and dispersed sufficiently.

In accordance with an exemplary embodiment of the invention, the reagent may be a hydrocarbon with an acryl amide functional group. For example, the reagent may be prop- 2-enamide, commonly known as acrylamide. Alternatively, the reagent may consist of a hydrocarbon chain with an acrylamide functional group. However, any such reagent that effectively inhibits or reduces unwanted species deposition on electrodes in accordance with the present invention may be used.

In an exemplary embodiment of the invention, the reagent may be a polymeric reagent. In various exemplary embodiments, the polymeric reagent may consist of a polyacrylamide reagent. Such a polyacrylamide reagent may be composed of acrylamide monomers, the monomer consisting of an acrylamide group attached to a methyl group. In another exemplary embodiment, the polymeric reagent may be composed of (alk)acrylamide monomers, which consist of an alkyl chain with an acrylamide group attached.

An exemplary polymeric reagent may be substantially linear, with minimal crosslinking. In another configuration, the polymeric reagent may have a varied degree of crosslinking, depending on the desired solubility of the reagent. For example, the polymeric reagent may be exposed to crosslinking agents to generate a sufficient degree of crosslinking. However, any such polymer that effectively inhibits or reduces unwanted species deposition on electrodes in accordance with the present invention, regardless of the degree of linearity, may be used.

An exemplary polymeric reagent may be substantially homogenous. A homogenous polymeric reagent may be composed of repeating monomer units without any copolymer constituents. For example, a homogenous polymeric reagent may be comprised of polyacrylamide, which may be formed from the homopolymerization of acrylamide monomer units. Alternatively, the polymeric reagent may be comprised of a number of copolymers. For example, such a reagent may comprise acrylamide monomer units copolymerized with other monomers. However, any such polymer that effectively inhibits or reduces unwanted species deposition on electrodes in accordance with the present invention, regardless of the degree of copolymerization, may be used.

An exemplary polymeric reagent may be nonionic. For example, polymeric reagents, such as polyacrylamide, may comprise acrylamide monomers copolymerized with cationic, anionic, or nonionic monomers to create a cationic, anionic or nonionic polymeric reagent. In an exemplary embodiment, the polymeric reagent is a nonionic polymer, preferably a nonionic polyacrylamide. Such a reagent may consist of a homopolymerization of acrylamide monomers. In another exemplary embodiment, the reagent consists of copolymerization of acrylamide monomers with nonionic monomers, such as alkyl acrylates or acrylonitriles. However, any such polymer that effectively inhibits or reduces unwanted species deposition on electrodes in accordance with the present invention, regardless of its ionic state, may be used.

Depending on the conditions of the electrowinning cell, an exemplary reagent may consist of polymers with an average formula weight within a particular range. For example, in one embodiment, the reagent may be a polymer with formula weight in the range of about 5.0x104 to about 5.0x106 g mol. fn accordance with a preferred aspect of the exemplary embodiment, the formula weight of the reagent may be in the range of about 9.0x105 to about 4.0x106 g/mol, and more preferably, in the range of about 1.0 xl06 to about 3.0 xl06 g/mol. However, any reagent that effectively inhibits or reduces unwanted species deposition on electrodes in accordance with the present invention, regardless of its formula weight, may be used.

Similarly, an exemplary reagent may consist of polymers with an average specific gravity within a particular range. For example, in one embodiment, the reagent may be a polymer with a specific gravity in the range of about 0.5 to about 2.0. In accordance with a preferred aspect of the exemplary embodiment, the specific gravity of the reagent may be in the range of about 0.65 to about 1.50, and more preferably, in the range of about 0.75 to about 1.25. However, any such reagent that effectively inhibits or reduces unwanted species deposition on electrodes in accordance with the present invention, regardless of its specific gravity, may be used.

The conditions of the electrowinning cell may further dictate the basic or acidic nature of the reagent. In an exemplary embodiment, the reagent is a polymeric reagent which is added to the electrowinning cell as an aqueous solution. The exemplary reagent solution may have a pH in the range of about 4.0 to about 8.0. In accordance with a preferred aspect of the exemplary embodiment, the reagent, when in aqueous solution, may have a pH in the range of about 4.5 to about 7.5, and more preferably, in the range of about 5.0 to about 7.0. However, any such polymer that effectively inhibits or reduces unwanted species deposition on electrodes in accordance with the present invention, regardless its pH in solution, may be used.

It should be appreciated that any reagent which can be used to suppress unwanted species deposition on anode surfaces may be used in the context of the present invention. These reagents may include, for example, monomers or polymers that are commercially available. For example, a commercially available polyacrylamide reaction that performs in accordance with the present invention may be used. In an exemplary embodiment, Cyquest N-900, which is a commercially available processing aid for electrowinning processes, may be used.

EXAMPLES

A number of experiments were conducted in accordance with various exemplary embodiments of the present invention using metal oxide coated titanium anodes. In particular, these experiments sought to evaluate the effectiveness of a reagent to reduce manganese oxide deposition on the surface of anodes in an electrowinning cell. The examples described herein are exemplary and not intended to be limiting in describing the full scope of compositions and methods of this invention.

With initial reference to Figure 2, Experiment 1 demonstrates that in electrowinning processes, unwanted species (in this case, manganese oxides) may deposit on the surface of anodes in the absence of a reagent.

In this experiment, the concentration of manganese ions and the percentage of manganese oxide deposited on the surface of anodes in three different electrowinning cells were recorded. As the electrowinning processes advanced, the percentage of manganese oxide deposited on the surface of the anodes continually increased. No means were taken to reduce or inhibit the deposition of manganese oxides on the surface of the anodes. Over the course of three months, the manganese oxide deposition levels increased from between about 0% - 2.5% to about 6% to 15%.

With initial reference to Figure 3, Experiment 2 demonstrates that the addition of a reagent (in this case, Cyquest N-900) may both inhibit and remove unwanted species deposition on the surface of anodes in electrowinning processes.

This experiment was conducted in four full size electrowinning cells using metal oxide coated titanium anodes and stainless steel cathodes. During the experiment, a nonionic polyacrylamide reagent (Cyquest N-900) was added, and the level of reagent was varied over time. Initially, 220 g/ton copper of polyacrylamide reagent was added to the electrolyte and the cell began operation. Next, the polyacrylamide reagent was discontinued for a period. Finally, polyacrylamide reagent was reintroduced at a concentration of 220 g/ton copper.

The concentration of manganese in solution and percentage of manganese deposited on the anode surface were measured as the experiment progressed. During the first phase of the experiment, in the presence of the polyacrylamide reagent, the concentration of manganese in the electrolyte varied from about 0.000 g L to 0.020 g/L, and the percentage of manganese deposited on the anode varied from about 0% to 1%. After the polyacrylamide reagent was discontinued, the percentage of manganese deposited on the anode increased to about 16% at approximately the same concentration of manganese in solution. Manganese concentration was then increased to approximately 0.100 g/L and the addition of polyacrylamide resumed. After the polyacrylamide reagent was reintroduced, the percentage of manganese deposited on the anode decreased to between about 0% and 2%, in the presence of significant, for example, on the order of about 0.100 g/L, manganese in the electrolyte.

The present invention has been described above with reference to a number of exemplary embodiments. 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. 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. For example, various aspects and embodiments of this invention may be applied to electrowinning of metals other than copper, such as nickel, zinc, cobalt, and others. Although certain preferred aspects of the invention are described herein in terms of exemplary embodiments, such aspects of the invention may be achieved through any number of suitable means now known or hereafter devised. Accordingly, these and other changes or modifications are intended to be included within the scope of the present invention.

Claims

CLAIMS What is claimed is:

1. A method comprising:

electrowinning a metal value from a metal value-bearing electrolyte solution using an electrowinning system, the electrowinning system comprising an anode; and adding at least one of an acrylamide and a hydrocarbon with an acrylamide functional group to the metal value-bearing electrolyte solution for the suppression of metal oxide deposition on the surface of the anode.

2. The method of claim 1, wherein the metal oxide comprises manganese oxide.

3. The method of claim 1, wherein the acrylamide comprises polymeric acrylamide.

4. The method of claim 1, wherein the acrylamide comprises monomelic acrylamide.

5. The method of claim 3, wherein the formula weight of the polymeric acrylamide is between about 5x104 g mol to about 5x106 g/mol.

6. The method of claim 3, wherein the concentration of the polymeric acrylamide is between about 2 parts per million and about 100 parts per million.

7. The method of claim 1, wherein, during the adding, the at least one of an acrylamide and a hydrocarbon with an acrylamide functional group is present in an aqueous solution having a pH of between about 4 and about 8.

8. The method of claim 1, wherein, during the adding, the at least one of an acrylamide and a hydrocarbon with an acrylamide functional group comprises a dry powder.

9. The method of claim 1, wherein the adding is performed prior to the electrowinning.

10. The method of claim 1, wherein the adding is performed at least one of after or during the electrowinning.

11. The method of claim 1 , wherein the anode comprises a titanium surface at least partially coated with a compound comprising Ir02 and Ta205.