Technical Field

[0001] The present disclosure relates to an electrolytic cell, in particular an electrolytic cell for electrowinning a metal from a metal-bearing solution.

Background

[0002] The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.

[0003] Electrowinning is an electrolytic process in which a current is passed between an anode and a cathode in an electrolytic cell containing a metal-bearing solution, whereby the metal is reduced and deposited at the cathode. In many electrolytic reactions, oxygen will also be generated at the anode. The process has broad application in metal extraction and recovery because several metals including copper, lead, molybdenum, gold, silver, zinc, cobalt and manganese may be suitably extracted from a metal-bearing solution by electrowinning.

[0004] In some electrowinning processes it is necessary to a maintain a different pH in the catholyte relative to the anolyte, particularly where hydrogen ions or hydroxide ions are generated at the anode. Consequently, the electrolytic cell may be configured to have separate anode and cathode compartments which are separated by a diaphragm. The diaphragm must be sufficiently impermeable to prevent diffusion of the anolyte into the cathode compartment of the cell, while at the same time minimising the electrical resistance of the diaphragm such that the electrolytic cell can be operated at an appropriate cell voltage (approx. 4.5 V-5.5 V) and not become overheated.

[0005] Several electrolytic cells have also been configured to provide a slightly higher catholyte level in the cathode compartment to maintain a hydrostatic head of pressure to bias flow of catholyte from the cathode compartment through the diaphragm to the anode compartment and ensure that the pH conditions in the cathode compartment are maintained. [0006] In a continuous mode of operation, the catholyte in the cathode compartment must be continuously replenished. This may lead to inhomogeneity in the cathode compartment in relation to temperature and catholyte composition, which in turn may affect the electrodeposition of metal on the cathode. For example, the cathode may be subject to dendritic growth of metal particularly on its edges due to increased current density, the formation of metal nodules, or the purity of deposited metal may vary. Additionally, the formation of nodules may affect the current efficiency of the electrolytic cell.

[0007] The generation of oxygen at the anode, particularly for manganese metal production, may be accompanied by production of a metal oxide sludge (“anode sludge”). It is an important electrolytic design configuration to ensure sufficient space is provided in the anode compartment to accommodate the anode sludge so that the electrolytic cell can be operated for a reasonable duration before the accumulated anode sludge needs to be removed.

[0008] Various embodiments of the electrolytic cell disclosed herein seek to overcome or improve at least some of the above mentioned disadvantages.

Summary

[0009] The present disclosure provides an electrolytic cell, in particular an electrolytic cell for electrowinning a metal from a metal-bearing solution.

[0010] In a first aspect there is provided an electrolytic cell for electrowinning a metal from a metal-bearing solution, the electrolytic cell comprising: a vessel comprising a cathodic compartment and an anodic compartment separated by a panel of diaphragm fabric; the cathodic compartment containing a catholyte comprising the metal-bearing solution and a cathode at least partially immersed therein, the anodic compartment containing an anolyte comprising a spent solution derived from electrolysis of the catholyte in the cathodic compartment and an anode at least partially immersed therein; wherein the vessel comprises:

(i) an inlet to receive a feedstream of the metal-bearing solution into the cathodic compartment,

(ii) a means to maintain the catholyte in the cathodic compartment at a predetermined temperature, (iii) a conduit to receive a gas stream to sparge the catholyte; and,

(iv) an outlet for egress of the spent solution.

[0011] In one embodiment, said means to maintain the catholyte at a predetermined temperature comprises an outlet and an inlet of the cathodic compartment in fluid communication with a pump to recirculate the catholyte through an external heat exchanger.

[0012] In one embodiment, the conduit may extend to a lower portion of the cathodic compartment. In one form of the embodiment, the conduit may be a pipe element inserted into the cathodic compartment. In some alternative embodiments, the pipe element may be provided with a bubbler head.

[0013] In one embodiment, the cathode and anode are arranged in parallel alignment with one another.

[0014] In one embodiment, the vessel is provided with an anolyte overflow outlet.

[0015] In one embodiment, the vessel has a floor provided with a drain to collect anode sludge produced by the anode. In one form, the floor may be inclined towards the drain.

The vessel may be provided with a flushing valve to discharge anode sludge from the vessel floor.

[0016] In a second aspect there is provided an assembly of electrolytic cells for electrowinning a metal from a metal-bearing solution, the assembly comprising: a vessel configured to define an electrolysis zone for electrowinning the metal from the metal-bearing solution and a spent solution zone in fluid communication with the electrolysis zone to facilitate removal of spent solution derived from electrolysis of the metal-bearing solution from the vessel, an arrangement of a plurality of alternating cathodic compartments and anodic compartments disposed in the electrolysis zone, wherein adjacent cathodic and anodic compartments are separated by a panel of diaphragm fabric, each cathodic compartment containing a catholyte comprising the metal-bearing solution and a cathode at least partially immersed therein, each anodic compartment containing an anolyte comprising the spent solution and an anode at least partially immersed therein; wherein the vessel comprises: (i) an inlet to receive a feedstream of the metal-bearing solution into the electrolysis zone, (ii) a means to maintain the metal-bearing solution in the electrolysis zone at a predetermined temperature,

(iii) a conduit to receive a gas stream to sparge the metal-bearing solution; and

(iv) an outlet in the spent solution zone for egress of the spent solution.

[0017] In one embodiment, the vessel comprises a frame configured to support the arrangement of alternating cathodic compartments and anodic compartments in the electrolysis zone,

[0018] In one form, the frame may facilitate flow of spent solution from the anodic compartments in the electrolysis zone to the spent solution zone.

[0019] In one embodiment, the spent solution zone has a floor provided with a drain to collect anode sludge produced by the anode. In one form, the floor may be inclined towards the drain. The spent solution zone may be provided with a flushing valve to discharge anode sludge from the floor thereof.

[0020] In one embodiment, said means to maintain the metal-bearing solution at a predetermined temperature comprises a heat exchange coil configured to be immersed in the electrolysis zone of the vessel in heat exchange communication with the metal-bearing solution, wherein the heat exchange coil comprises an inlet and an outlet for recirculation of a heat exchange medium.

[0021] In one embodiment, the conduit to sparge the metal-bearing solution may correspond to an internal perimeter of the electrolysis zone and be provided with a plurality of apertures along its length.

[0022] In one embodiment, the outlet for egress of the spent solution is configured to maintain a lower level of anolyte than catholyte in the vessel. For example, said outlet may comprise a weir having an effective height that may be operably varied.

[0023] In a third aspect there is provided a system for electrowinning a metal from a metal bearing solution, the system comprising: an assembly of electrolytic cells as defined above; a source of the metal-bearing solution in fluid communication with the input port of the electrolysis zone of the vessel; and, an electric current supply associated with the anodes and the cathodes whereby supply of electric current causes metal to be reduced and deposited on a surface of the cathodes and oxygen to be generated on the anodes.

[0024] In a further aspect there is provided a cathodic compartment for an electrowinning cell, the cathodic compartment comprising a box having at least one panel of diaphragm fabric, the cathodic compartment containing a catholyte comprising a metal-bearing solution and a cathode at least partially immersed therein, wherein the cathodic compartment is capable of being suspended in the electrowinning cell; the box comprising:

(i) an input port to receive a feedstream of the metal-bearing solution into the cathodic compartment,

(ii) an outlet and an inlet in fluid communication with a pump to recirculate the catholyte through a heat exchanger to maintain the catholyte in the cathodic compartment at a predetermined temperature, and

(iii) a conduit to receive a gas stream to sparge the catholyte.

[0025] In one embodiment, the input port is provided with a Y-shaped element to receive a combined stream of the feedstream and the recirculated catholyte.

[0026] In one embodiment, the at least one panel of diaphragm fabric may be configured to remove catholyte as an overflow into an anodic compartment of said cell and maintain a predetermined level of catholyte in the cathodic compartment. For example the at least one panel of diaphragm fabric may be provided with an catholyte overflow outlet therein. The catholyte overflow outlet may include a plurality of channels in a lower rim thereof. Alternatively, a lower rim of the catholyte overflow outlet may be bevelled, in particular in a direction biased towards the anodic compartment.

Brief Description of Drawings

[0027] Preferred embodiments will now be further described and illustrated, by way of example only, with reference to the accompanying drawings in which:

[0028] Figure 1 is a schematic representation of one embodiment of an electrolytic cell for electrowinning a metal from a metal-bearing solution;

[0029] Figure 2 is a schematic representation of one embodiment of a cathodic compartment for the electrolytic cell shown in Figure 1 ; [0030] Figure 3 is schematic representation of an alternative embodiment of a cathodic compartment for the electrolytic cell shown in Figure 1 ;

[0031 ] Figures 4a and 4b are respective schematic representations of alternative embodiments of catholyte overflow outlets formed in the panel of diaphragm fabric of the cathodic compartments;

[0032] Figure 5 is a partial side view of one embodiment of an electrolytic cell assembly for electrowinning a metal from a metal-bearing solution;

[0033] Figure 6 is a partial end view of the electrolytic cell assembly as shown in Figure 5;

[0034] Figure 7 is a cross-section along line B-B of Figure 6 showing an alternating arrangement of anodes and cathodes;

[0035] Figure 8 is a perspective view of the embodiment shown in Figure 7;

[0036] Figure 9 is a perspective view of one embodiment of a heat exchanger coil arranged, in use, to be associated with the electrolytic cell assembly shown in Figures 5-8;

[0037] Figure 10 is a perspective view of one embodiment of a frame to facilitate fluid communication between an electrolysis zone and a spent solution zone in the electrolytic cell assembly shown in Figures 5-8;

[0038] Figure 11 is another perspective view of the frame shown in Figure 10;

[0039] Figure 12 is a graphic representation of cell potential and cathodic current density measured over a 24 h period of operation of the electrolytic cell as described herein with reference to the Example;

[0040] Figure 13 is a graphic representation of pH of the catholyte and anolyte measured over a 24 h period of operation of the electrolytic cell as described herein with reference to the Example; and

[0041 ] Figure 14 is a graphic representation of catholyte temperature measured over a 24 h period of operation of the electrolytic cell as described herein with reference to the Example. Description of Embodiments

[0042] The present disclosure relates to an electrolytic cell, in particular to an electrolytic cell for electrowinning metal from a metal-bearing solution.

GENERAL TERMS

[0043] Throughout this specification, unless specifically stated otherwise or the context requires otherwise, reference to a single step, composition of matter, group of steps or group of compositions of matter shall be taken to encompass one and a plurality (i.e. one or more) of those steps, compositions of matter, groups of steps or groups of compositions of matter. Thus, as used herein, the singular forms "a", "an" and "the" include plural aspects unless the context clearly dictates otherwise. For example, reference to "a" includes a single as well as two or more; reference to "an" includes a single as well as two or more; reference to "the" includes a single as well as two or more and so forth.

[0044] Each example of the present disclosure described herein is to be applied mutatis mutandis to each and every other example unless specifically stated otherwise. The present disclosure is not to be limited in scope by the specific examples described herein, which are intended for the purpose of exemplification only. Functionally-equivalent products, compositions and methods are clearly within the scope of the disclosure as described herein.

[0045] The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

[0046] When an element or layer is referred to as being “on”, “engaged to”, “connected to” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to”, “directly connected to” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). [0047] Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

[0048] Reference to positional descriptions, such as lower and upper, are to be taken in context of the embodiments depicted in the figures, and are not to be taken as limiting the invention to the literal interpretation of the term but rather as would be understood by the skilled addressee.

[0049] Spatially relative terms, such as “inner,” “outer,” “beneath”, “below”, “lower”,

“above”, “upper” and the like, may be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[0050] The term "and/or", e.g., "X and/or Y" shall be understood to mean either "X and Y" or "X or Y" and shall be taken to provide explicit support for both meanings or for either meaning.

[0051 ] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

[0052] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

[0053] The term “about” as used herein means within 5%, and more preferably within 1%, of a given value or range. For example, “about 3.7%” means from 3.5 to 3.9%, preferably from 3.66 to 3.74%. When the term “about” is associated with a range of values, e.g., “about X% to Y%”, the term “about” is intended to modify both the lower (X) and upper (Y) values of the recited range. For example, “about 20% to 40%” is equivalent to “about 20% to about 40%”.

ELECTROLYTIC CELL, ASSEMBLY AND SYSTEM FOR ELECTROWINNING METAL

[0054] Referring to the figures, where like numerals refer to like parts throughout, Figures 1-3 show one embodiment of an electrolytic cell 10 suitable for electrowinning a metal from a metal-bearing solution. It will be appreciated that the metal-bearing solution may comprise a plurality of different metal ions including impurities in varying concentrations. In use, the electrolysis conditions (e.g. cell potential, applied current, pH, temperature) of the electrowinning cell 10 may be selected to reduce and recover a desired target metal from the metal-bearing solution. The term “electrolysis” as used herein refers to an electrochemical process in which an electric current is used to generate an otherwise non-spontaneous electrochemical reaction.

[0055] The electrolytic cell 10 includes a tank 12 having opposing side walls 14, front and rear walls 16 and floor 18. The tank 12 may be fabricated from any suitable rigid material that is chemically inert under the operating conditions of the electrolytic cell 12. Suitable rigid materials include, but are not limited to, metals and alloys or polymeric materials.

[0056] The tank 12 defines an anodic compartment 20 containing an anolyte and at least one anode 22 arranged to be at least partially immersed in the anolyte. As used herein, the term “anolyte” refers to an aqueous salt solution capable of allowing an electric current to flow to a positively-charged anode. Generally, the anolyte may comprise a metal-depleted metal-bearing solution (i.e. spent catholyte), catholyte overflow (see below), an electrolyte having a composition that is compatible with the catholyte or a mixture of two or more of the foregoing. In some embodiments, at start up or commencement of electrolysis, the anolyte may be the same composition as the catholyte.

[0057] Generally, the front and rear walls 16 of the tank 12 are configured to support the at least one anode 22 to extend in parallel alignment with the side walls 14 of the tank 12. In this regard, the front and rear walls 16 may be provided with respective brackets (not shown) to support the anode 22 in a desired orientation in the tank 12. The brackets may be integral with the inner surface of the front and rear walls 16 of the tank 12. For example, the brackets may be grooves or protrusions on the inner surface of the front and rear walls 16 in which respective ends of the anode 22 are received or supported thereon, respectively.

[0058] The tank 12 is configured to maintain a predetermined level of anolyte in the tank 12. In this particular embodiment, a side wall 14 of the tank 12 includes an overflow pipe 24. The overflow pipe 24 facilitates egress of the anolyte from the tank 12 to maintain the level of anolyte in the tank 12 to marginally below the overflow pipe 24. It will be appreciated that the position of the overflow pipe 24 in the side wall 14 of the tank 12 will be selected to maintain a predetermined level of anolyte in the tank 12. Generally, the overflow pipe 24 is disposed in an upper portion 14a of the side wall 14 of the tank 12.

[0059] The floor 18 of the tank 12 may be tapered or inclined towards a drain 26 that is arranged, in use, to collect anode sludge generated at the anode 22 during electrolysis. The term “anode sludge” as used herein refers to insoluble compounds, for example metal oxides, generated at the anode 22 during electrolysis which tend to sink under gravity to the floor 18 of the tank 12. The tank 12 may be further provided with a flushing valve 28 proximal the drain 26 to facilitate removal of the anode sludge from the tank 12. The flushing valve 28 may be operated on an intermittent basis, as desired, in particular when the amount of anode sludge is above acceptable levels in the tank 12.

[0060] The electrolytic cell 10 also includes at least one cathodic compartment 30 containing a catholyte and a cathode 32 adapted to be at least partially immersed in the catholyte. As used herein, the term “catholyte” refers to an aqueous salt solution capable of allowing an electric current to flow from a negatively-charged cathode. Generally, the catholyte comprises the metal-bearing solution resident in the cathodic compartment 30. It will be appreciated that in a continuous process, the catholyte may comprise a mixture of feedstream comprising the metal-bearing solution and metal-depleted metal-bearing solution from which the target metal has been progressively reduced and deposited on the cathode 32. [0061 ] The cathodic compartment 30 is defined by a box 34 that is adapted to be suspended in the tank 12 and at least partially immersed in the anolyte. The box 34 has opposing side walls 36, front and rear walls 38, base 40 and a top 42 having a cut-out portion 44 therein configured to receive and support the cathode 32. The cut-out portion 44 may be configured in parallel alignment with the side walls 36 so that the cathode 32 extends between the front and rear walls 38 in parallel alignment with the side walls 36 of the box 34.

[0062] In use, the box 34 is disposed in the tank 12 in an arrangement in which the cathode 32 and the anode 22 are in parallel alignment with one another. To that end, front and rear walls 38 of the box 38 may be configured to be engaged with associated brackets on respective front and rear walls 16 of the tank 12 so that the side walls 36 of the cathodic compartment 30 are in parallel alignment with the side walls 14 of the tank 12.

[0063] The box 34 may be fabricated from any suitable rigid plastic material that is chemically inert under the operating conditions of the electrolytic cell 12. Suitable examples of rigid plastic materials include, but are not limited to, polymeric materials such as polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, and so forth.

[0064] The box 34 is provided with at least one panel 46 of diaphragm fabric. The diaphragm fabric is sufficiently permeable to facilitate fluid flow of catholyte from the cathodic compartment 30 to the anodic compartment 20 and flow of electric current between the anodic and cathodic compartments 20, 30 of the electrowinning cell 10. The diaphragm fabric may be a woven or non-woven polymeric material. Suitable examples of woven or non-woven polymeric materials include, but are not limited to, polyester and polypropylene filter cloth, polyvinyl chloride, acrylonitrile.

[0065] The panel(s) 46 may be disposed in a correspondingly shaped cut-out portion 48 in one or both of the opposing side walls 36. In this way, the panel 46 may be in spaced parallel alignment with the cathode 32. The panel 46 may be fixed to a margin surrounding the cut-out portion 48 by any suitable technique. For example, outer edges of the panel 46 may be fixed to the margin surrounding the cut-out portion 48 by a suitable adhesive, such as silicone or by plastic welding. Alternatively, the panel 46 may be clamped to the side wall 36 by interposing the panel 46 between the side wall 36 and a gasket (not shown) which is mounted thereon. In some embodiments, the panel 46 may be additionally rigidified by interposing the panel 46 between the side wall 36 and a mesh spacer (not shown) which is mounted thereon. [0066] The panel 46 of diaphragm fabric may be configured to remove catholyte as an overflow into the anodic compartment 20. For example the at least one panel 46 of diaphragm fabric may be provided with an catholyte overflow outlet 50 therein. Generally, the catholyte overflow outlet 50 is configured as a slit in the diaphragm fabric. The catholyte overflow outlet 50 may include a plurality of channels 52 in a lower rim 54 thereof. Alternatively, the lower rim 54 of the catholyte overflow outlet 50 may be bevelled, in particular in a direction biased towards the anodic compartment 20.

[0067] Alternatively, or additionally, the side wall 36 of the box 34 may include one or more catholyte overflow outlets 50’ to allow catholyte to egress to the anodic compartment 20. Generally, the catholyte overflow outlets 50’ will be disposed above the panel 46 of diaphragm fabric. It will be appreciated that the catholyte overflow outlets 50’ in the side walls 36 of the box 34 may be slits, circular holes or other suitably-shaped apertures.

[0068] The catholyte overflow outlet 50 may be disposed in an upper portion of the panel 46 at an effective height sufficient to maintain a predetermined level of catholyte in the cathodic compartment 30. The predetermined level of catholyte in the cathodic compartment 30 is selected to establish and maintain an effective hydrostatic head between the cathodic and anodic compartments 30, 20 to ensure catholyte flows from the cathodic compartment 30 to the anodic compartment 20.

[0069] The box 34 is provided with an input port 56 to receive a feedstream of the metal bearing solution into the cathodic compartment 30. The input port 56 may be disposed in the top 42 proximal to the cut-out portion 44 so that feedstream is fed to an upper portion of the cathodic compartment 30. Alternatively, the input port 56 may be disposed in the top 42 in fluid communication with a passageway (56’) extending downwardly through one of the front, rear or side walls 38, 36 and into a lower portion of the cathodic compartment 30.

[0070] It will be appreciated that the flow of feedstream into the cathodic compartment 30 may result in temperature and pH differentials between incoming feedstream and catholyte resident in the cathodic compartment 30, particularly if the incoming feedstream is not mixed with the catholyte. While not bound by theory, the inventors have opined that maintaining the catholyte at a constant temperature and pH within the cathodic compartment 30 during electrolysis improves homogeneous deposition of metal on the cathode 32.

[0071] To that end, the electrowinning cell 10 may be configured to recirculate the catholyte through a heat exchanger 100. In one embodiment, the top 42 of the box 34 may be provided with an outlet 58 and an inlet 60 in fluid communication with a pump 200 to recirculate the catholyte through the heat exchanger 100. The pump 200 may be any suitable pump including, but not limited to, a peristaltic pump. In this way, the catholyte in the cathodic compartment 30 may be maintained at a predetermined temperature, generally from about 35 °C to about 45 °C, in particular from about 38 °C to about 42 °C, or about 40 °C.

[0072] Recirculating the catholyte through the heat exchanger 100 also maintains the catholyte in the cathodic compartment 30 at a relatively constant pH, in particular a pH that favours target metal reduction at the cathode 32.

[0073] In some embodiments, the input port 56 may be provided with a Y-shaped element 62 to receive a combined stream of the feedstream and the recirculated catholyte.

[0074] Alternatively, or additionally, the electrowinning cell 10 may be configured to mix the feedstream and the catholyte in the cathodic compartment 30. The box 34 may be provided with a conduit 64 to receive a gas stream to sparge the catholyte. The conduit 64 may extend from a further inlet 66 in the box 34 to a lower portion of the cathodic compartment 30. In one form of the embodiment, the conduit 34 may be a pipe element inserted into the cathodic compartment. The pipe element may be provided with a bubbler head to improve gas distribution in the catholyte and therefore enhance mixing. The gas stream may comprise air or an inert gas such as nitrogen or argon.

[0075] The anode 22 and the cathode 32 may be fabricated from an electrically conductive material that is inert or insoluble under the electrolysis conditions. In particular, the anode 22 may be fabricated from an electrically conductive material having an acceptable oxygen overvoltage and minimum side reactions. Suitable examples of electrically conductive materials from which the anode may be fabricated include, but are not limited to, lead, a lead alloy (e.g. Pb-Ag, 99:1 ) or a mixed metal oxide, such as a dimensionally stable anode. Dimensionally stable anodes comprise a substrate, such as a titanium plate or mesh, with a plurality of metal oxides coated thereon.

[0076] The cathode 32 may be fabricated from an electrically conductive material having a high hydrogen overvoltage and surface properties that subsequently allow the deposited metal to be readily removed. Suitable examples of electrically conductive materials from which the cathode 32 may be fabricated include, but are not limited to, metals such as titanium, copper, aluminium or alloys such as stainless steel, in particular 316 stainless steel, and Hastelloy. The term THastelloy’ refers to a group of corrosion-resistant nickel alloys, namely the Ni-Mo and the Ni-Cr-Mo alloys. Hastelloy electrodes are characterized by high resistance to hydrochloric, sulfuric, phosphoric, acetic and formic acids, media containing chloride and fluoride ions. The cathode 32 may also be a composite of the aforementioned materials. For example, the cathode 32 may comprise a busbar fabricated from copper having a stainless steel outer layer. The stainless steel outer layer may be applied to the copper by wrapping and welding a stainless steel sheet thereto.

[0077] Generally, the anode 22 will have a smaller surface area than the cathode 32. In some embodiments, the anode 22 has a surface area less than half the surface area of the cathode 32.

[0078] A suitable power supply may be configured in electrical communication with the anode 22 and the cathode 32 to supply a cell potential of from about 4.0 V to about 5.0 V to the electrowinning cell 10 to maintain a current density of about 200 A/m ² to about 600 A/m ² , in particular a current density of about 240 A/m ² to about 500 A/m ² .

[0079] In some embodiments, an assembly of electrolytic cells may be provided by arranging a plurality of anodes 22 and a plurality of cathodes 32 in spaced alternating parallel alignment with one another. The anodes 22 and cathodes 32 may be as described above.

[0080] One embodiment of the assembly 100 of electrolytic cells for electrowinning a metal from the metal-bearing solution will now be described with reference to Figures 5-11 .

[0081] Referring to Figures 5-8, there is shown a vessel 110 configured to define an electrolysis zone 112 and a spent solution zone 114 in fluid communication with the electrolysis zone 112. In use, the electrolysis zone 112 houses an arrangement of alternating cathodic compartments 116 and anodic compartments 118. Each cathodic compartment 116 contains the metal-bearing solution and a cathode 32 at least partially immersed therein and each anodic compartment 118 contains an anolyte (i.e. spent solution) and an anode 22 at least partially immersed therein.

[0082] In this particular embodiment, each anodic compartment 118 is defined by a an open-ended bag 120 fabricated from four interconnecting panels of diaphragm fabric. The diaphragm fabric may be a woven or non-woven polymeric material. Suitable examples of woven or non-woven polymeric materials include, but are not limited to, polyester and polypropylene filter cloth, polyvinyl chloride, acrylonitrile.

[0083] The bag 120 may be suspended from rigid rods 122 supported by a frame 124, as shown in Figure 11 , as will be described later. In use the anodic compartments 118 are spaced in parallel alignment with one another in the electrolysis zone 112, whereby respective spaces therebetween define the cathodic compartments 116 in the electrolysis zone 112.

[0084] The vessel 110 may include opposing side walls 126, front and rear walls 128 and a floor 130. The floor 130 of the vessel 110 may be supported by a pedestal 132 and a base 134. The front and rear walls 128 may each be provided with a laterally extending flange 136 that may be conveniently used as lifting points for the vessel 110 and its contents. The vessel 110 may be fabricated from any suitable rigid material that is chemically inert to the metal-bearing solution, the resulting spent solution and the operating conditions of the electrolysis. Suitable rigid materials include, but are not limited to, metals and alloys or polymeric materials, such as polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, and so forth.

[0085] In use, the electrolysis zone 112 of the vessel is arranged to facilitate recovery of metal from the metal-bearing solution by means of electrolysis, in particular by electroplating the metal on the cathodes 32 in the cathodic compartments 116. In this particular embodiment, the electrolysis zone 112 may take the form of an inner chamber 138 generally spaced apart from and disposed above the floor 130 of the vessel 110.

[0086] The inner chamber 138 is defined by an inner wall 140 extending between the front and rear walls 128 and spaced apart from one of the side walls 126a in parallel alignment therewith. A lower end 142 of the inner wall 140 is spaced apart from and disposed above the floor 130 of the vessel 110. The lower end 142 is provided with a first lip 144 laterally extending therefrom. A second lip 146 laterally extends from the other of the side walls 126b spaced apart from and above the floor 130 of the vessel 110 so that the first and second lips 144, 146 are in lateral facing alignment with one another.

[0087] The electrolysis zone 112 is provided with an inlet 148 to deliver metal-bearing solution thereto. In this particular embodiment, the inlet 148 is disposed in a box-like projection 150 disposed in the side wall 126b in fluid communication with the inner chamber 138. In use, the metal-bearing solution flows into the electrolysis zone 112 and the cathodic compartments 116. It will be appreciated that at start up or commencement of electrolysis, the anodic compartments 118 may also contain the metal-bearing solution. However, as electrolysis progresses (or in a steady state continuous process), spent solution seeps from the cathodic compartments 116 into the anodic compartments 118 and thence to the spent solution zone 114 as will be described later.

[0088] It will be appreciated that the flow of metal-bearing solution into the electrolysis zone 112 may result in temperature and pH differentials between incoming feedstream and catholyte resident in the cathodic compartment 116, particularly if the incoming feedstream is not mixed with the catholyte. While not bound by theory, the inventors have opined that maintaining the catholyte at a constant temperature and pH within the cathodic compartment 116 during electrolysis improves homogeneous deposition of metal on the cathode 32.

[0089] To that end, the vessel 110 is provided with a heat exchange coil 152 configured to be immersed in the electrolysis zone 112 of the vessel 110 in heat exchange communication with the metal-bearing solution. The heat exchange coil 152 includes an inlet 154a and an outlet 154b for recirculation of a heat exchange medium. Any suitable heat exchange medium, such as water or glycol may be circulated through the heat exchange coil 152. As shown in Figure 8, the heat exchange coil 152 may conform to an internal perimeter of the inner chamber 138 of the vessel 110, wherein a base thereof may be supported by the first and second lips 144, 146.

[0090] The vessel 110 also includes a conduit 156 configured to be immersed in the electrolysis zone 112 of the vessel 110 to sparge the metal-bearing solution and enhance mixing. Preferably said conduit 156 is perforated along its length. The gas stream may comprise air or an inert gas such as nitrogen or argon.

[0091 ] The frame 124 is configured to support the arrangement of alternating cathodic compartments 116 and anodic compartmentsl 18 in the electrolysis zone 112, Referring to Figures 10 and 11 , the frame 124 may include an open upper rectangular section 158 interconnected to a generally closed lower rectangular section 160 by a plurality of generally vertical elongate members 162. In use, the lower rectangular section 160 of the frame 124 is supported by the first and second lips 144, 146 in the electrolysis zone 112. In this way, the frame 124 may be readily inserted into the vessel 110 or removed therefrom.

[0092] Respective front and rear sides 164 of the upper rectangular section 158 are provided with correspondingly aligned grooves 166. The grooves 166 are arranged, in use, to support the rigid rods 122 from which the open-ended bags 120 of diaphragm fabric are suspended. The bags 120 may be additionally supported on the lower rectangular section 160 by means of ties or weights.

[0093] The lower rectangular section 160 is provided with a plurality of openings 168 which are configured to align with respective overlying anodic compartments 118 so that the respective lower openings of the open-ended bags 120 align with the openings 168. In this way, the electrolysis zone 112 is in fluid communication with the spent solution zone 114, and spent solution may flow from the anodic compartments 118 in the electrolysis zone 112 to the spent solution zone 114.

[0094] As shown in Figure 10, the lower rectangular section 160 of the frame 124 may be provided with a rebate to conveniently house the conduit 156 to sparge the electrolysis zone 112.

[0095] The frame 124 may be fabricated from any suitable rigid plastic material that is chemically inert under the operating conditions of the assembly of electrolytic cells 100. Suitable examples of rigid plastic materials include, but are not limited to, polymeric materials such as polypropylene, polyvinylidene fluoride, polytetrafluoroethylene, and so forth.

[0096] In use, the anodes 22 and cathodes 32 may be lowered into and immersed in the respective anodic and cathodic compartments 118, 116. As shown in Figures 7 and 8, respective upper edges 170 of the front and rear walls 128 of the vessel 110 are provided with regularly spaced and correspondingly aligned grooves or protrusions 172 to support the anodes and cathodes 22, 32 in spaced alternating parallel alignment. In use, the anodes and the cathodes 22, 32 may be respectively electrically connected to an electric current supply source via an intercell busbar. Banana plugs or similar may be used to allow the cathodes 32 to be readily removed without having to risk significant dissolution of metal on the cathodes 32 at termination of the electrolysis.

[0097] A suitable power supply may be configured in electrical communication with the anodes 22 and the cathodes 32 to supply a cell potential of from about 4.0 V to about 5.0 V to the assembly 100 of electrolytic cells to maintain a current density of about 200 A/m ² to about 600 A/m ² , in particular a current density of about 240 A/m ² to about 500 A/m ² . [0098] The floor 130 of the vessel 110 may be tapered or inclined towards a drain 174 that is arranged, in use, to collect anode sludge generated at the anodes 22 during electrolysis. The vessel 110 may be further provided with a flushing valve 176 proximal the drain 174 to facilitate removal of the anode sludge from the spent solution zone 114 of the vessel 110. The flushing valve 176 may be operated on an intermittent basis, as desired, in particular when the amount of anode sludge is above acceptable levels in the vessel 110.

[0099] The spent solution zone 114 of the vessel 110 is generally L-shaped and includes the spaces defined between the floor of the vessel 130, the first and second lips 144, 146 and the lower rectangular section 160 of the frame 124, and between the side wall 126a and the inner wall 140.

[0100] The spent solution zone 114 of the vessel 110 is configured to maintain a predetermined level of spent solution in the vessel 110. In particular, the effective level of spent solution in the spent solution zone 114 should be lower than the effective level of metal-bearing solution in the electrolysis zone 112. In this regard, the level of metal-bearing solution in the cathodic compartments 116 should be less than the level of solution in the anodic compartments 118. To that end, the spent solution zone 114 is provided with an outlet 178 for egress of the spent solution therefrom. The outlet 178 is disposed in a box-like projection 180 disposed in an upper section of the side wall 126a. The box-like projection 180 is in fluid communication with the spent solution zone 114. The outlet 178 is an upright pipe with a threaded fitting whose effective height in the box-like projection 180 can be manually adjusted to vary the level of liquid, thus behaving effectively as a weir. In use, the effective height of the outlet 178 is adjusted so that the level of spent solution in the vessel 110, and in particular in the anodic compartments 118, is less than the level of metal-bearing solution in the cathodic compartments 116.

Example

[0101 ] The following example is to be understood as illustrative only. It should therefore not be construed as limiting the embodiments of the disclosure in any way.

[0102] The electrowinning cell 10 as described with reference to Figures 1 to 4b was used to recover manganese (Mn) from a manganese-containing feed solution with a composition according to Table 1 and operated under conditions summarised in Table 2. The cell 10 was also provided with sensors to measure pH, temperature and cell potential. The electrolysis was performed for 4 , 8 and 24 h, respectively.

Table 1

Table 2

[0103] Fresh feed was fed into the cathodic compartment 30 through input port 56. The amount of fresh feed introduced into the cell 10 was weighed on a scale and automatically logged. The catholyte in the cathodic compartment 30 was circulated through a heat exchanger 100 via a peristaltic pump 200 to maintain the desired cell temperature of 40 °C. The catholyte flowed from the cathodic compartment 30 through the panel 46 of diaphragm fabric into the anodic compartment 20 and also exited the cathodic compartment 30 through catholyte overflow outlet 50 in the panel 46 of diaphragm fabric. Excess anolyte was removed from the anodic compartment 20 as anolyte overflow through anolyte overflow pipe 24. [0104] Figure 12 shows the recorded cell potential and cathodic current density over a 24 h period. An average cell potential of 4.7 V was obtained in the absence of drift across the 24 h test period. The cell 10 was consistently operated at about 32 A/m ² throughout the 24 h test period. The liquid level in the anodic and cathodic compartments 20, 30 was steadily maintained throughout the test with a fixed difference in liquid levels between the two compartments.

[0105] Figure 13 shows respective pH measurements of the catholyte in the cathodic compartment 30 and the anolyte in the anodic compartment 20 of the cell 10 during the 24 h test. The gradual increase in catholyte pH from 7.2 to 7.9 and decrease in the anolyte pH with operating time is consistent with expected behaviour.

[0106] Figure 14 shows the catholyte temperature recorded during the 24 h test. The small variation in temperature demonstrates that gas sparging in the cathodic compartment 30 ensure good mixing of recirculated catholyte and feed in the cathodic compartment 30.

[0107] Manganese metal plated on the cathode 32 with a metallic grey finish. There was no significant formation of dendrites at the edges of the cathode 32 or nodules formed on the surface of the manganese metal. The appearance and morphology of the plated manganese metal suggests that there was good mixing of the catholyte in the cathodic compartment 30. A sample of the plated manganese metal was submitted for trace metal analysis and the reported grade of 99.9% was well above the industry standard of 99.5%.

[0108] The calculated current efficiency was 72% and the energy consumption was 6.4 kWh/kg electrolytic Mn. Good mass accountability of 98.1% was obtained. Key results for the 4, 8 and 24 h tests are summarised in Table 3.

[0109] The anode 20 showed evidence of a dark black coating of MnC>2. There was a slight difficulty in draining the anode sludge from the cell 10 because flakes of MnC>2 had peeled off from the anode 20, but this did not present a major problem. The surface of the panel of diaphragm fabric proximal the anode 20 also had dark staining but, on inspection after the tests, showed no damage or tears and its permeability was remained uncompromised.

[0110] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Table 3