AU2014252764A1

AU2014252764A1 - Coated composite anodes

Info

Publication number: AU2014252764A1
Application number: AU2014252764A
Authority: AU
Inventors: Maryam Jozegholami BARMI; Aleksander NIKOLOSKI
Original assignee: Murdoch University
Current assignee: Murdoch University
Priority date: 2013-04-10
Filing date: 2014-04-09
Publication date: 2015-10-08
Also published as: CL2015002973A1; CA2907805A1; WO2014165912A1

Abstract

A coated composite anode for electrowinning metals and a method of preparing same are described. The coated composite anode comprises an anode core, a composite coating containing oxides of lead and cobalt, and an intermediate layer interposed between the anode core and said coating. The anode core may be a 'film forming' metal such as Ti, Al, Ta, Nb, Zr, Hf, V, Mo and W. The intermediate layer may be one or more metal oxides such as Sn0

Description

WO 2014/165912 PCT/AU2014/000386 1 COATED COMPOSITE ANODES Field The present invention relates to a coated composite anode, in particular coated 5 composite anodes for use in electrowinning of base metals. The present invention also relates to a method of preparing said anodes. Background 10 Lead based anodes have been used in the electrowinning of base metals for more than a century due to their low cost and availability. However, the conventional types of lead based anodes, such as PbCaSn, operate at relatively high oxygen overpotential, which results in significant energy consumption, and suffer from corrosion which takes place at a slow rate during the electrowinning process. 15 It has long been recognized that the presence of cobalt ions in the electrolyte, even at very low concentrations, improves the performance of lead based anodes by lowering the oxygen overpotential of the anode, decreasing the corrosion rate of the anode, and aiding oxygen evolution at the anode-electrolyte interface. There is a need, however, 2C to continuously add cobalt salt to the electrolyte because of gradual depletion of cobalt from the electrolyte with bleed streams. General research has also been directed towards incorporating cobalt in lead based anodes. A PbCaSnCo bulk alloy has been reported. However, it is quite difficult to 25 utilise the positive effects of cobalt in an anode produced from this alloy as cobalt has low solubility in lead and is prone to segregation to Sn-rich areas in the alloy. Thus a homogenous distribution of cobalt at the anode surface is not guaranteed. Additionally, it has been reported that an increase in the cobalt content altered the morphology of the PbO 2 layer and produced a thinner layer that was less dense. In 30 designing an anode containing lead and cobalt it is therefore important that the influence of the cobalt content is optimized but not exaggerated such that its net effect is negative.

WO 2014/165912 PCT/AU2014/000386 2 Research to produce catalytically active composite coated anodes which can operate as anodes to generate oxygen at a low overpotential and maintain good resistance to corrosion continues. In coated anodes containing a PbO 2 layer deposited on top of an inert substrate, titanium is the most widely used substrate. Titanium's resistance to 5 corrosion is due to the formation of a very stable passive TiO 2 layer on the surface of the metal. However, the gradual thickening of this oxide layer can also lead to deactivation of the dimensionally stable anodes. The method in which the anode coating is applied and how the substrate is prepared, determine the subsequent stability and electrochemical properties of the coated anode. An anode with an iridium 10 dioxide coating demonstrates long service life but it is more expensive than anodes with ruthenium oxide coatings (another common type of anode) and its electrocatalytic activity is slightly lower. Nevertheless, both anodes involve as key ingredient the platinum group of elements which are relatively expensive and in limited supply. 11 There is therefore a need for alternative or improved anodes for electrowinning of base metals. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the 2C common general knowledge in the art, in Australia or any other country. Summary According to a first aspect, there is provided a coated composite anode for 25 electrowinning metals comprising an anode core, a composite coating containing oxides of lead and cobalt, and an intermediate layer interposed between the anode core and said coating. According to a second aspect, there is provided a method of preparing a coated 30 composite anode for electrowinning metals comprising forming an intermediate layer on an anode core, and forming a composite coating containing oxides of lead and cobalt on the intermediate layer. The coated composite anode may be used for oxygen evolution in a process for WO 2014/165912 PCT/AU2014/000386 3 electrowinning of metals. According to a third aspect, there is provided a cell for electrowinning of metals comprising a coated composite anode as defined in the first aspect, a cathode and an 5 electrolyte. According to a fourth aspect, there is provided a method of recovering a metal in an electrowinning cell, comprising electrowinning the metal using the coated composite anode defined in the first aspect. 10 In one embodiment, the electrolyte for electrowinning metals may be a sulphuric acid electrolyte. In another embodiment, the recovered metal is selected from a group consisting of is copper, nickel, zinc and manganese. According to a fifth aspect, there is provided a lead-acid battery comprising a cathode, a coated composite anode as defined in the first aspect, and an electrolytic solution. 2C) Brief Description of Fiures Notwithstanding any other forms which may fall within the scope of the coated composite anodes and methods as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to the accompanying 25 Figures in which: Figure 1 is a cyclic voltammogram of Til SnO 2 -Sb 2

O

3 l PbO 2 -CoO, coated anode in 180 g dm~ 3

H

2

SO

4 ; 30 Figure 2 is a potential-time transient plot of Til SnO 2 -Sb 2

O

3 | PbO 2 -CoOx and PbCaSn anode; Figure 3 is a cyclic voltammogram of Ti SnO 2 -Sb 2

O

3 | PbO 2 -Co 3

O

4 anode in 180 g dm ' H 2 S0 4

;

WO 2014/165912 PCT/AU2014/000386 4 Figure 4 is a potential-time plot of Til SnO 2 -Sb 2

O

3 | PbO 2 -Co 3 0 4 anode compared with a conventional PbCaSn anode; 5 Figure 5 is a graphic representation of the release of cobalt from the composite coated anodes as described herein during anodic polarization; Figure 6 is a Tafel plot for oxygen evolution at composite coated anodes as described herein; 10 Figures 7a and 7b show potential-time transients of a Til SnO 2 -Sb 2

O

3 | PbO 2 -Co304 anode compared with a conventional PbCaSn anode during short- and long-term electrolysis, respectively; and, is Figures 8a and 8b are two scanning electron microscope (SEM) images of cross sections of a Til SnO 2 -Sb 2

O

3 | PbO 2 -Co 3 0 4 anode. Detailed Description 2C In one aspect, the present application relates to a coated composite anode for electrowinning metals. The reference to 'electrowinning metals' refers to an electrolytic process for depositing metals from a metal containing material that has been dissolved in an aqueous 25 electrolyte. The metal containing material may be an ore, a concentrate, tailings, or compounds containing the metal of interest and, optionally, other metals. In electrowinning, a current is passed from an anode, where oxygen is evolved, through the aqueous electrolyte containing the metal value so that the metal is extracted as it is deposited in an electroplating process onto the cathode. Examples of metals that can 30 be recovered by electrowinning include, but are not limited to, copper, nickel, zinc and manganese. A cell for electrowinning metals may comprise a coated composite anode as described herein, a cathode and an electrolyte.

WO 2014/165912 PCT/AU2014/000386 5 The coated composite anode may also be suitable for use in a lead-acid battery comprising a cathode, the coated composite anode described herein, and an electrolytic solution. The electrolytic solution may be sulphuric acid. By using the 5 anode described herein, a lead-acid battery can be obtained which is applicable as an industrial battery requiring a high input characteristic and a high output characteristic, used for petrol and diesel-fuelled vehicles, electric car, parallel hybrid electric car, simple hybrid car, power storage system, elevator, electric tools, uninterruptible power source, distributed power source, etc. 10 Coated composite anode The coated composite anode for electrowinning metals comprises an anode core, a composite coating containing oxides of lead and cobalt, and an intermediate layer is interposed between the anode core and said coating. Anode core The anode core comprises an electroconductive material. 2C0 The term 'electroconductive material' broadly refers to any substance which allows transmission of an electric current therethrough. Suitable electroconductive materials include, but are not limited to, metals such as nickel and manganese; metal alloys such as PbCaSn or stainless steel; silicon; carbon doped polymer composites; and carbon 25 doped ceramics. It will be appreciated that, in use, the electroconductive material of the anode core is shielded from the electrolyte by the intermediate layer and the composite coating and is not consumed when the anode core is employed for electrowinning metals. 30 The metals for the anode core may be any coatable metal. In particular the metal for the anode core may be a 'film forming' metal. The term 'film forming' refers to metals that readily oxidise and form a thin oxide layer on the surface of the metal. The thin oxide layers are characterised by having strongly adherent metal to metal oxide bonds WO 2014/165912 PCT/AU2014/000386 6 on the surface. The thin oxide films are passivating - in other words they protect the underlying metal from corrosion by electrolyte. Examples of suitable film forming metals include, but are not limited to, Ti, Al, Ta, Nb, Zr, Hf, V, Mo and W, with Ti being preferred. The alloys for the anode core may also comprise one or more film forming metals as described above. The anode core may be in the form of plates, rods, discs, tubes, wires or knitted wires 10 and expanded meshes of the electroconductive material. Regardless of the type and form of the anode core, the anode core is preferably pre treated to provide a clean surface before applying the intermediate layer. Any suitable techniques for cleaning the surface of electrodes, as will be well known to those skilled is in the art, may be employed. For example, the anode core may be pre-treated by mechanical cleaning techniques such as by wet polishing the surface of the anode core with successively finer grit grinding media (e.g. sand, aluminium oxide, steel and SiC). Alternatively, or in addition to mechanical cleaning techniques, the anode core may be pre-treated by one or more chemical cleaning techniques. Such chemical 2C cleaning techniques may include degreasing with solvents, electrolytic treatments, and/or etching the surface, preferably with acid solutions. Examples of acid solutions suitable for etching the surface include, but are not limited to, hydrochloric, sulphuric, perchloric, nitric, oxalic, tartaric, and phosphoric acids as well as mixtures thereof, e.g. aqua regia. It will be appreciated that caustic solutions may also be employed to etch 25 the surface of the anode core. Advantageously, the techniques employed for pre-treating the surface of the anode core may be selected to develop a desired surface roughness and/or surface morphology. 30 Following the cleaning techniques, the anode core may be subjected to rinsing and drying steps as will be well understood to those skilled in the art. Intermediate layer WO 2014/165912 PCT/AU2014/000386 7 In one embodiment, the intermediate layer comprises one or more metal oxides. In particular, the intermediate layer may comprise SnO 2 , Sb 2 0 5 , Ta 2 05, MnO 2 , CeO 2 , TiO 2 , B 2 0 3 , MnWO 4 , ZrO 2 or a combination thereof. The intermediate layer may further 5 comprise an oxide of a platinum group metal including platinum, palladium, rhodium, iridium and ruthenium or mixtures thereof. In another embodiment, the intermediate layer comprises one or more metals. The one or more metals may be capable of oxidising and forming a metal oxide film 10 thereon. For example, the intermediate layer may comprise lead. Lead is capable of forming lead oxide. In further embodiments, the intermediate layer may comprise one or more film forming metals or oxides of the film forming metals described above. 15 The inventors have observed that the intermediate layer contributes to the overall morphology of the anode, in particular to the morphology of the composite coating. In previous trials, anodes, in which composite coatings containing oxides of lead and cobalt have been applied directly to an anode core, failed to operate well over 2C prolonged periods. In particular, the composite coatings demonstrated a brittle and cracked surface structure, leading to detachment of the composite coating from the anode core and performance degradation within short periods. The inventors opine that the intermediate layer may be capable of forming strong 25 bonds with the anode core and strong bonds with the composite coating, thereby providing good dimensional stability to the composite anode. Strong metal oxide bonds may be achieved between the passivating film of the film forming metal oxides on the anode core and one or more metal oxides of the intermediate layer. Alternatively, the one or more metals in the anode core or the intermediate layer may 30 behave as the matrix for one or more metals (or metal oxides) of its respective counterpart. The intermediate layer may be formed on the anode core by applying the intermediate layer or precursors thereof to the anode core by any suitable technique, and optionally WO 2014/165912 PCT/AU2014/000386 8 converting said precursors to produce the intermediate layer. Preferably, the anode core will have been pre-treated as described above before applying the intermediate layer or precursors thereof to the anode core. 5 In some embodiments, in particular when the intermediate layer is a metal, such as lead, the intermediate layer may be applied to the anode core by electrodepositing the metal on the anode core. For example, the pre-treated anode core may be immersed in a precursor solution comprising metal cations of the desired metal. Subsequently, the desired metal may be cathodically deposited on the anode core by applying 10 potentiostatic polarisation of the anode core at a predetermined voltage, the predetermined voltage being sufficient to electrolytically deposit the desired metal. The period in which the predetermined voltage may be applied will vary depending on the concentration and nature of the metal cations in the precursor solution, pH of the precursor solution, temperature, other additives, the current density, and the desired is thickness of the intermediate layer. In other embodiments, in particular when the intermediate layer is one or more metal oxides, the intermediate layer may be applied to the anode core by thermal deposition techniques. For example, the pre-treated anode core may be immersed in a precursor 2C solution comprising one or more precursors of the one or more metal oxides. Alternatively, said solution may be applied to the pre-treated anode core by spraying. The 'wetted' anode core may then be dried at a first temperature, the first temperature being sufficient to evaporate the solution solvent, thereby producing a coating of said one or more precursors on the anode core. It will be appreciated that the first 25 temperature may vary depending on the type of solvent in the solution. For example, the first temperature may be about 110 *C or more for lower alcohol solutions. Further coatings of said one or more precursors may be applied in a similar manner, depending on the desired thickness of the intermediate layer. 30 The coating(s) of the one or more precursors may then be converted into the intermediate layer of one or more metal oxides by heating the coated anode core at a second temperature, the second temperature being sufficient to convert the one or more precursors to the one or more metal oxides. The second temperature may be about 500 *C or more.

WO 2014/165912 PCT/AU2014/000386 9 Generally, the thickness of the intermediate layer may be less than 10 micron. Composite coating containing oxides of lead and cobalt In one embodiment the composite coating containing oxides of lead and cobalt may be a homogeneous mixture of lead oxide and one or more cobalt oxides, In this particular embodiment the composite coating takes the form of a film of PbO 2 -CoO. It will be appreciated that the structural formula of CoO, may be difficult to define because it 10 may comprise a plurality of cobalt(II) and cobalt(Ill) oxides of varying structural formulae in varying relative molar amounts. The composite coating may contain from 0.1 at% to 30 at% Co, preferably 2 at% to 25 at% Co. 15 The composite coating may contain from 0.1 at% to 40 at% Pb, preferably 7 at% to 30 at% Pb. Generally, the thickness of the composite coating may be from about 10 micron to 2' about 200 micron. The film of PbO 2 -CoO, may be formed on the intermediate layer by electrodepositing said film from a precursor solution containing lead and cobalt cations. For example, the precursor solution may comprise suitable lead and cobalt salts. It will be 25 appreciated that the pH of the precursor solution may be selected to be suitable for anodic deposition of PbO-CoO. on the intermediate layer. The PbO 2 CoO, coating may be anodically deposited on the intermediate layer by applying potentiostatic polarisation of the intermediate layer at a predetermined 30 voltage, the predetermined voltage being sufficient to electrolytically deposit the PbO 2 CoO, coating. The period in which the predetermined voltage may be applied will vary depending on the concentration and nature of the metal cations in the precursor solution, the pH of the precursor solution, temperature, additives, counter ions, the current density, and the desired thickness of the intermediate layer and so forth.

WO 2014/165912 PCT/AU2014/000386 10 In an alternative embodiment the composite coating containing oxides of lead and cobalt may comprise a heterogeneous mixture of lead oxide and C030 4 particles. In this particular embodiment, the composite coating takes the form of a film of lead oxide 5 with Co 3

O

4 particles dispersed and embedded therein. Preferably, the CoO 4 particles may have a mass-median-diameter (MMD) in the range of about 30 micron to about 0.1 micron. The inventor has noted that the particle size of the CoO 4 particles may decrease with use over the operating life of the coated 10 composite electrode and it is possible that some enrichment of the film of lead oxide with CoO may occur. The composite coating may contain from 0.1 at% to 30 at% Co, preferably 2 at% to 15 at% Co. 15 The composite coating may contain from 0.1 at% to 40 at% Pb, preferably 7 at% to 30 at% Pb. Generally, the thickness of the composite coating may be from about 10 micron to 2C about 200 micron. The film of PbO 2 -Co 3

O

4 may be formed on the intermediate layer by electrodepositing said film from a precursor solution containing lead and C030 4 particles dispersed in the precursor solution. For example, the precursor solution may comprise suitable lead 25 salts. It will be appreciated that the pH of the precursor solution may be selected to be suitable for anodic deposition of PbO-Co 3

O

4 on the intermediate layer. The PbO 2 -Co 3

O

4 coating may be anodically deposited on the intermediate layer by applying potentiostatic polarisation of the intermediate layer at a predetermined 30 voltage, the predetermined voltage being sufficient to electrolytically deposit the PbO 2 CoO 4 coating. The period in which the predetermined voltage may be applied will vary depending on the concentration and nature of the metal cations and CoO 4 particles dispersed in the precursor solution, pH of the precursor solution, temperature, counter ions in the precursor solution, additives, properties of the suspended solids, the current WO 2014/165912 PCT/AU2014/000386 11 density, the desired thickness of the intermediate layer, and so forth. The coated composite anodes described herein display a highly catalytic character for oxygen evolution which results in lower energy consumption. They are inexpensive 5 and easy to prepare. Further, it may be possible to recondition said coated composite anodes by re-applying the composite coating by techniques as described in the preceding paragraphs. Non-limiting examples of coated composite anodes, methods of preparation thereof, 10 and electrolytic behavior will now be described. Example I Preparation of coated composite anodes Preparation and testing of the coated composite anodes was carried out using a is standard three-electrode electrochemical system. The reference electrode was a mercury mercurous sulphate electrode (MSE). Potentials are reported in the normal hydrogen electrode (NHE) scale. The ohmic drop between the reference and working electrode was not compensated. A glass cell with a plastic lid was used to hold the anode, the cathode and the reference electrode. The temperature was kept constant 2C during the experiments by using a thermostated water bath. The generated data was recorded on a computer using LabVIEW* 7.1 software. Potentiostat EG&G Princeton Applied Research, model 173, was used in all experiments. The anode core comprised a rod with a diameter of 5.00 mm of Ti grade 2 supplied by 25 Titanium International or Ni supplied by Good Fellows with 99.99 % purity. The anode cores were subjected to pre-treatment to clean the surface and ensure uniform coverage by the coatings over the entire surface area. The pre-treatment of the anode core consisted of several steps, involving wet polishing the anode core with successively finer (600 and 1200 grit) SiC papers, followed by 10 minutes of cleaning 30 in an ultrasonic bath containing deionised water. In the case of the Ti anode core, the Ti was then subjected to an etching procedure of boiling in 10% oxalic acid for 2 h. Next, substrates were rinsed with deionised water and dried in a stream of air.

WO 2014/165912 PCT/AU2014/000386 12 Til Pb| PbO 2 -CoO. A metallic Pb intermediate layer was cathodically deposited onto the pre-treated Ti rod by potentiostatic polarisation of the Ti rod at -0.625 V immersed in a 0.5 M lead sulphamate and 0.5 M cobalt nitrate solution (pH adjusted to 1.6) for 1 h. A composite coating of PbO 2 -CoO, was subsequently electrodeposited onto the 5 metallic Pb intermediate layer by 30 minutes of anodic deposition of PbO 2 -CoO, at 1.6 V. Ti| SnO 2 -Sb 2 0 3 l PbO 2 -CoO, and Ni| SnO 2 -Sb 2

O

3 PbO 2 -CoO, A SnO 2 -Sb 2

O

3 intermediate layer was thermally deposited onto the anode core (Ni or Ti) prior to 10 anodic deposition of the PbO 2 -CoOx composite layer. To apply the Sn0 2 -Sb 2 0 3 intermediate layer, a precursor solution was prepared by dissolving 20 g SnC 4 . 5H 2 0, 2 g SbCI 3 and 13.2 mL HCI into 100 mL of isopropanol. The pre-treated anode core was dipped in the precursor solution for 5 minutes followed by 15 minutes of drying in an oven at 1 10 C. This procedure was repeated 3 times. Subsequently, the coated is anode core was annealed in a muffle furnace at 500*C for 1 h. Fresh precursor solution was used for each treatment. A PbO 2 -CoO, composite layer was then electrodeposited from the solution containing 0.5 M lead sulphamate + 0.5 M cobalt nitrate. The electrodeposition was carried out potentiostatically at 1.75 V over a period of 2 h. 2C) Ti| SnO 2 -Sb 2

O

3 PbO 2 -Co 3

O

4 and Ni| SnO 2 -Sb 2 Os| PbO 2 -Co 3

O

4 Thermal deposition of a Sn0 2 -Sb 2

O

3 intermediate layer on the anode cores (Ni and Ti) was performed as described above. A subsequent composite coating of PbO 2 -Co 3

O

4 25 was then applied to the intermediate layer. A precursor solution containing lead ions and suspended CoO 4 particles was prepared by combining a solution of 0.13 M lead acetate and 0.9 M lead nitrate with 5% w/v of C030 4 (powder < 10 pm) supplied by Sigma Aldrich. The solid particles were suspended by mild agitation, The pH of the electrolyte was adjusted to 4.4. The coated anode core was immersed in a bath of the 30 precursor solution maintained at 250C and the electrodeposition was carried out potentiostatically at 1.75 V for 2h. Immediately after the preparation of the composite coating, the composite coated anode was rinsed with deionised water and dried in air under ambient conditions.

WO 2014/165912 PCT/AU2014/000386 13 Details of the composition of the different composite coated anodes are summarised in Table 1. Table 1 § Composition of PbO 2 -CoO, and PbO 2 -Co 3

O

4 composite coated anodes with different interlayers Composite Coated Anode Co (at. %) Ph (at. %) Xc, (ColCo+Pb) Ti l Pb 1 PbO 2 CoO, 9.0 24.0 0.27 Ti l SnOrSb2O3| PbOI-CoO, 5.4 15.3 0.26 Ti SnO 2 rSb2O 3 |PbO 2 -CoO, 2.8 8.0 0.26 Til SnO2-Sb 2

O

3 | PbO 2 -Co 3 0 4 3.2 17.0 0.16 Ni SnOr-Sb2O 3 | PbO 2 -Co,0 4 20.5 7.2 0.74 Example 2 Electrochemical characterisation 10 Anodisation tests Short term (16 h) and long term (168 h) anodisation tests were conducted to evaluate the anode potential of each composite coated anode under typical copper 15 electrowinning conditions. The composite coated anodes were tested by anodisation in an electrolyte containing 180 g dm- 3

H

2

SO

4 prepared using Analytical Reagent (AR) grade sulphuric acid in deionised water, by conducting galvanostatic polarisation at a current density of 300 A m 2 . The temperature of the electrolyte was maintained at 40±2 'C by circulating water from a thermostated water bath and the electrolyte was 20 agitated by a magnetic stirrer. During the anodisation, electrolyte solution samples were taken periodically to determine the amount of cobalt released to the electrolyte. The samples were analysed by Atomic Absorption Spectrophotometer (AAS) model GBC 933 PLUS. This was one indication of service life, as the amount of cobalt in the anode is critical to the effectiveness of the anode for copper electrowinning. 2 5e Cyclic Voltammetry WO 2014/165912 PCT/AU2014/000386 14 Cyclic voltammetry was used to estimate the starting potential for oxygen evolution on the fresh composite coated anodes in 180 g dm- 3

H

2

SO

4 and compare this with the oxygen evolution potential on a conventional (uncoated) PbCaSn anode. Overpotential for the start of the oxygen evolution reaction was thus estimated as the 5 difference between the observed potential for oxygen evolution during the anodic polarisation scan and the standard potential calculated for the system, which under the conditions involving 180 g dm- 3

H

2

SO

4 and 40 *C is 1.236 V. The potential of the working electrode (anode) was cycled from the initial potential, which was the rest potential observed on a freshly prepared composite coated anode during open circuit 10 potential measurement, to +2 V and then swept back to -1 V at a scan rate of 2 mV sec 1 . Overpotential for oxygen evolution was also determined during steady operation at 300 A m 2 , on both the fresh anode surface and after a 16 h period of conditioning. These measurements were taken from potential-time transients recorded at the start and end of each test. 15 Figure 1 shows a cyclic voltammogram of the Ti SnOrSb 2

O

3 | PbO 2 -CoO, coated anode in 180 g dm- 3

H

2

SO

4 . Scanning in the anodic direction, a sharp increase was observed at about 1.67 V which can be related to the simultaneous nucleation of PbO 2 and evolution of oxygen. The overpotential for the start of the oxygen evolution (- 440 2C mV) is significantly lower on the surface of this composite coated anode than on a conventional PbCaSn anode surface, which was observed at overpotential of approximately 640 mV. In the return sweep, reduction of PbO 2 to PbSO 4 takes place at about 1.5 V. There is an insignificant peak at potential more negative than -0.3 V, which is linked to discharge of PbO 2 and PbSO 4 to Pb. The composite coated anode 25 was virtually passive between 1.5 V to -0.3 V. The Til SnO 2 -Sb 2

O

3 | PbO 2 -CoO, coated anode operated initially at an anode potential more than 300 mV lower than that observed with the conventional PbCaSn anode. However, as is apparent from Figure 2, this difference in potential decreased after 15 30 minutes of polarisation to approximately 180 mV, and thereafter the potential of this composite coated anode gradually increased further over 16 h of anodisation until its operating potential eventually become stable at 2.1 V, which is slightly higher although still relatively close to the stable anode potential observed for the conventional PbCaSn anode.

WO 2014/165912 PCT/AU2014/000386 15 A freshly prepared Til SnO 2 -Sb 2

O

3 | PbO 2 -Co 3

O

4 composite coated anode containing about 3 at% cobalt was used for voltammetric measurements to assess its anodic properties. Figure 3 shows a single scan cyclic voltammogram using the Ti l SnO 2 5 Sb 2

O

3 | PbO 2 -Co 3

O

4 coated anode in 180 g dm 3

H

2

SO

4 . Scanning in the anodic direction, a sharp increase was observed at about 1.56 V which can be related to the simultaneous nucleation of PbO 2 and evolution of oxygen. The overpotential for the start of the oxygen evolution reaction (- 330 mV) is again significantly lower on the surface of this composite coated anode than on a conventional PbCaSn anode (- 640 10 mV). In the return sweep, reduction of PbO 2 to PbSO 4 takes place at about 1.4 V. There is an insignificant peak at potential more negative than -0.3 V, which is attributed to discharge of PbO 2 and PbSO 4 to Pb. Figure 4 and 5 show that the Ti SnO 2 -Sb 2

O

3 | PbO 2 -Co 3

O

4 coated anode had very 15 good stability and maintained a relatively stable anode potential of 1.73 V over 16 and 168 h of polarisation. The amount of cobalt released from the Til SnO 2 -Sb 2

O

3 | PbO 2 -Co 3

O

4 anode surface to the electrolyte was monitored over a period of 168 h of polarisation. It was observed 20 that the release of cobalt from the anode is insignificant during the first 50 h (Figure 5). In the presence of 10 mg dm 3 thiourea, a greater amount of cobalt was released initially but the concentration become stable after 100 h. This composite coated anode showed significant depolarisation compared to the PbCaSn and great stability compared to the other coated anodes tested during the 168 h of anodisation in the 25 sulfuric acid electrolyte. The Til SnO 2 -Sb 2

O

3 | PbO 2 -Co 3

O

4 coated anode operated at an anode potential initially about 400 mV lower than that observed for the conventional PbCaSn anode. As the results in Figure 4 show, the operating potential of the Til SnO2-Sb2O31 PbO2-Co3O4 30 coated anode stabilises relatively quickly attaining a steady potential of 1.70 V after 10 minutes. This compares favourably to the PbCaSn anode which stabilises at 2.07 V after 20 minutes. Although the difference in potential between the two anodes gradually decreases with time to approximately 380 mV, partly due to the slow depolarisation of the PbCaSn anode, after 16 h of anodisation the composite coated WO 2014/165912 PCT/AU2014/000386 16 anode is still operating at a potential of 1.73 V, which is a significant improvement compared to the conventional PbCaSn anode, Tafel plot measurements Tafel slopes of the anodic reaction were also measured to examine the electrochemical reaction rate characteristics on the different anodes, The data for the Tafel plots were generated from the same set of result recorded during the cyclic polarisation measurements, which the anodic Tafel slope for each anode estimated 10 from the linear section of the log current density versus potential where the oxygen evolution reaction was observed to be activation controlled, which varied from one anode to another. The overpotential (ri) for the oxygen evolution reaction at the operating current density is and the Tafel slopes for each of the titanium based composite coated anodes and PbCaSn are summarised in Table 2. All of the fresh composite coated anodes showed lower oxygen evolution overpotentials than a conventional PbCaSn anode (864 mV). However, a variation in the surface roughness was observed between the different anodes, and also, only the two anodes with a SnO 2 Sb 2

O

3 interlayer showed promising 2C stability over 16 h of anodisation. Another indicator of electrochemical reaction mechanisms is the Tafel slope under activation controlled conditions. Table 2 Overpotential for oxygen evolution reaction on the composite coated anodes 25 Composite coated anode 11, mV lj. nV Tafel slope, (fresh anode) (after 16 h) mV dec& Ti PbI PbO-CoO, 634 Ti SnO 2 -Sb2O 3 1 PbO2-CoO, 494 864 88 Ti l SnO 2 -Sb 2

O

3 PbOI-Co 3 0 4 464 464 47 PbCaSn 864 834 122 WO 2014/165912 PCT/AU2014/000386 17 The Tafel plot for oxygen evolution reaction on Til SnO 2 -Sb 2

O

3 j PbO 2 -CoO.anode showed a slope of 88 mV dec-, which is much lower than the slope of 122 mV dec- 1 observed for a conventional PbCaSn anode. In addition, incorporation of Co 3

O

4 particles into the coating rather than CoO, further reduced the Tafel slope to 47 mV 5 dec 1 (see Figure 6). The Tafel slope is an intensive parameter which does not depend on the electrode surface area so it can be used to compare the kinetics and mechanism of a reaction on different surfaces. Therefore, the observed decrease in the Tafel slope for the 10 Til SnO 2 -Sb 2

O

3 | PbO 2 -CoO, and Ti SnO-Sb 2

O

3 | PbO 2 -Co 3

O

4 anodes indicates that the reaction is depolarised due to a change in the reaction mechanism rather than a possible variation in the surface area of the anodes. Lower Tafel slopes for oxygen evolution on composite anodes containing lead and cobalt oxides have also been reported in the literature. For the evolution of oxygen from 1 mol dm 3 NaOH solution, is PbO 2 -CoOX coated anodes have shown Tafel slopes of 60-70 mV dec 1 , whereas a PbO2-Co 3

O

4 coated anode has been reported with a Tafel slope of 59 mV dec-. Overall, variation of the Tafel slope is a sign that the mechanism of the electrochemical reaction has changed which indicates that the electrocatalytic effect is real. In other words, significantly less energy is required to generate the same amount of oxygen 2C gas from the sulfate electrolyte on the composite anodes compared to the conventional PbCaSn anodes that are commonly used in industry at the moment for copper electrowinning. Figures 7a and 7b show anodisation of the Ti-SnO 2 -Sb 2

O

3 -PbO 2 -Co 3

O

4 composite 25 coated anode compared with aconventional PbCaSn anode. As can be seen from the Figures, incorporation of cobalt into the surface coating of the composite anode resulted in reduced anode potential relative to PbCaSn. The composite coated anode operated at a very low potential of approximately 1.8 V, indicative of depolarisation of approximately 300 mV versus PbCaSn and demonstrated great stability. Analyses of 30 the electrolytes revealed that the loss of cobalt from Ti based composite coated anodes took place at a rate of 0.037 pg cm 3 h for the Ti-SnO 2 -Sb 2

O

3 -PbO 2 -Co 3

O

4 anode. Longer term tests (168 h) were carried out under the same conditions. It can be seen from Figure 7b, that the Ti-SnO 2 -Sb 2

O

3 -PbO 2 -Co 3 0 4 composite anode WO 2014/165912 PCT/AU2014/000386 18 operated at a lower potential than that of PbCaSn and remained stable over the entire anodisation period. Example 3 Corrosion rate measurements After the electrolysis in 180 g dm~ 3

H

2 SO4 for 168 h, each cathode was examined for lead deposition. There was no evidence of lead deposition on the cathodes in any of the tests using Ti-SnO 2 -Sb 2

O

3 -PbO 2 -Co 3

O

4 composite coated anode, whereas in the test using PbCaSn anode significant amounts of lead were deposited on each cathode. 10 Table 3 shows the concentration of lead released from the surface of each electrode, slime formed and lead on cathodes after electrolysis. Table 3 15 Pb in Slime Pb in Pb on Corrosion Anode electrolyte, mass, slime, cathodes, rate, mg dm g n g gkg" g gm- h 1 Ti-SnO 2 -Sb20 3 1.5 0.3 290 -- 0.1 PbO 2 -CoO 4 The rate of lead loss from the anodes and resultant incorporation of lead into cathodes can be expected to decrease with the use of the Ti-SnOr-Sb 2

O

3 -PbOr-CoO 4 composite coated anodes. It is evident that the corrosion rate of the PbCaSn anode 20 after 168 h is 6.7 g m- 2 ho 1 , whereas the composite coated anode corroded at a much slower rate of 0.1 g m 2 h 1 . These results illustrate the effectiveness of composite coating in preventing acid penetration through the surface layer to the Ti substrate and stability of the coating as 25 resulted in the low lead dissolution. As is evident from the results, incorporation of Pb on the cathodes is negligible using Ti-SnOr-Sb 2

O

3 -PbOr-Co 3 0 4 composite coated anodes.

WO 2014/165912 PCT/AU2014/000386 19 Example 4 SEM images Two SEM images of a cross section of a Til SnO 2 -Sb 2

O

3 | PbO 2 -Co 3 0 4 anode after anodisation in acid are shown in Figures 8a and 8b. Three distinct layers are indicated 5 on the images: a Ti substrate (or core), a thermally deposited SnO 2 -Sb 2

O

3 interlayer, and a composite coating of PbO 2 -Co 3 O4. The lead-cobalt oxide composite layer has a roughened surface and porous structure but a relatively dense underlayer. It can be seen that acid could penetrate through the top composite layer but the thermally deposited SnO 2 -Sb 2

O

3 interlayer has protected the Ti substrate from acid attack and 10 deactivation. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The is present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary 2C implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. 2 5

Claims

1. A coated composite anode for electrowinning metals comprising an anode core, 5 a composite coating containing oxides of lead and cobalt, and an intermediate layer interposed between the anode core and said coating.

2. The coated composite anode according to claim 1, wherein the anode core comprises an electroconductive material. 10

3. The coated composite anode according to claim 2, wherein the electroconductive material is selected from a group comprising metals, metal alloys, silicon, carbon doped polymer composites; and carbon doped ceramics. 15

4. The coated composite anode according to claim 3, wherein the metal is a coatable metal.

5. The coated composite anode according to claim 4, wherein the metal is a film forming metal. 20

6. The coated composite anode according to claim any one of the preceding claims wherein the anode core comprises Ti, Ni, Fe, Al, Ta, Nb, Zr, Hf, V, Mo or W.

7. The coated composite anode according to any one of the preceding claims, 25 wherein the anode core takes a form selected from a group comprising plates, rods, discs, tubes, wires, knitted wires, mesh, or expanded mesh.

8. The coated composite anode according to any one of the preceding claims, wherein the intermediate layer comprises one or more metals or one or more metal 30 oxides.

9. The coated composite anode according to claim 8, wherein the one or more metal oxides are selected from a group comprising SnO 2 , Sb 2 0 5 , Ta 2 0 5 , MnO 2 , CeO 2 , TiO 2 , B 2 0 3 , MnWO 4 , ZrO 2 , oxides of a platinum group metal including platinum, WO 2014/165912 PCT/AU2014/000386 21 palladium, rhodium, iridium and ruthenium, or a combination thereof.

10. The coated composite anode according to any one of claims 1 to 7, wherein the intermediate layer comprises one or more metals capable of oxidising and forming a 5 metal oxide film.

11. The coated composite anode according to claim 10, wherein the intermediate layer is Pb. 10

12. The coated composite anode according to claim 9, wherein the intermediate layer is SnO 2 -Sb 2 O 3 .

13. The coated composite anode according to any one of the preceding claims, wherein the intermediate layer is less than about 10 micron thick. 15

14. The coated composite anode according to any one of the preceding claims, wherein the composite coating containing oxides of lead and cobalt is a homogeneous mixture of lead oxide and one or more cobalt oxides. 2'

15. The coated composite anode according to claim 14, wherein the composite coating is a film of PbO 2 -CoO,.

16. The coated composite anode according to claim 15, wherein the composite coating contains from 0.1 at% to 30 at% Co. 2 5

17. , The coated composite anode according to claim 16, wherein the composite coating contains from 2 at% to 25 at% Co.

18. The coated composite anode according to claim 15, wherein the composite 30 coating contains from 0.1 at% to 40 at% Pb.

19. The coated composite anode according to claim 18, wherein the composite coating contains 7 at% to 30 at% Pb. WO 2014/165912 PCT/AU2014/000386 22

20. The coated composite anode according to any one of claims 1 to 14, wherein the composite coating containing oxides of lead and cobalt is a heterogeneous mixture of lead oxide and CoO 4 particles. 5

21. The coated composite anode according to claim 20, wherein the composite coating is of a film of lead oxide with CoO 4 particles dispersed and embedded therein.

22. The coated composite anode according to claim 21, wherein the CoO 4 particles has a mass-median-diameter (MMD) in the range of about 30 micron to about 0.1 10 micron.

23. The coated composite anode according to claim 20, wherein the composite coating contains from 0.1 at% to 30 at% Co 1

24. The coated composite anode according to claim 23, wherein the composite coating contains from 2 at% to 15 at% Co.

25. The coated composite anode according to claim 20, wherein the composite coating contains from 0.1 at% to 40 at% Pb

26. The coated composite anode according to claim 24, wherein the composite coating contains from 7 at% to 30 at% Pb.

27. The coated composite anode according to any one of the preceding claims, 25 wherein the thickness of the composite coating is from about 10 micron to about 200 micron.

28. A method of preparing a coated composite anode for electrowinning metals comprising forming an intermediate layer on an anode core, and forming a composite 30 coating containing oxides of lead and cobalt on the intermediate layer.

29. The method according to claim 28, wherein forming the intermediate layer on the anode core comprises electrodepositing the intermediate layer from a precursor solution. WO 2014/165912 PCT/AU2014/000386 23

30. The method according to claim 28, wherein forming the intermediate layer on the anode core comprises contacting the anode core with a solution containing a precursor of the intermediate layer to the anode core and converting the precursor solution to the 5 intermediate layer.

31. The method according to claim 30, wherein converting the precursor solution comprises thermal deposition. 10

32. The method according to any one of claims 28 to 31, wherein the anode core is pretreated prior to applying the intermediate layer to the anode core.

33. The method according to any one of claims 28 to 32, wherein forming a composite coating containing oxides of lead and cobalt on the intermediate layer is comprises electrodepositing a film of PbO 2 -CoOI onto the intermediate layer.

34. The method according to any one of claims 28 to 32, wherein forming a composite coating containing oxides of lead and cobalt on the intermediate layer comprises electrodepositing a film of PbO 2 -Co 3 O 4 from a precursor solution containing 2C lead and C030 4 particles dispersed in the precursor solution.

35. A cell for electrowinning metals containing a coated composite anode as defined in any one of claims 1 to 27, a cathode and an electrolyte. 25

36. A method of recovering a metal in an electrowinning cell, comprising electrowinning the metal using the coated composite anode defined in any one of claims 1 to 27.

37. The method according to claim 36, wherein the electrolyte for electrowinning 30 metals is a sulphuric acid electrolyte.

38. The method according to claim 37, wherein the recovered metal is selected from a group consisting of copper, nickel, zinc and manganese. WO 2014/165912 PCT/AU2014/000386 24

39. A lead-acid battery comprising a cathode, a coated composite anode as defined in any one of claims 1 to 27, and an electrolytic solution.