GB2519320A

GB2519320A - Electrowinning apparatus

Info

Publication number: GB2519320A
Application number: GB1318319.9A
Authority: GB
Inventors: Robert Mills; Ian Ewart
Original assignee: Electrometals Technologies Ltd
Current assignee: Electrometals Technologies Ltd
Priority date: 2013-10-16
Filing date: 2013-10-16
Publication date: 2015-04-22
Also published as: GB201318319D0; WO2015056217A2; CA2927888A1; WO2015056217A3; AU2014335721A1

Abstract

An anode for a cell of an electrowinning metal recovery apparatus comprises a tubular anode member 20 supported on a current distributing frame extending within the member to distribute electrical current to points on an inner surface of the member. The frame preferably comprises a plurality of conductive supports 13 extending longitudinally within the member and linked by conductive cross-members 14, which together provide a plurality of current supply points to the inner surface of the member. The anode member may comprise a mesh that is preferably an expanded metal mesh. A further disclosed anode comprises a tubular anode member comprising a plurality of holes extending through a face of the member. A cell (40, figure 7) comprises a tubular cathode (46) having an internal diameter of at least 175 mm and the anode (45) of the invention provided internally of the cathode. A module (80, figure 12) has a group of electrowinning cells (70) each comprising a tubular cathode and the anode of the invention provided internally of the cathode.

Description

ELECTROWINNING APPARATUS

The present invention relates to electrowinning apparatus, in particular to an anode for a cylindrical electrowinning cell. It also relates to a cylindrical cell for an electrowinning plant and to a module for an electrowinning plant comprising a group of cylindrical cells. There is also a new method ot operating an electrowinning cell.

Electrowinning is a metal recovery process in which a solution containing metal ions (the electrolyte or liquor', which is usually the product of a leaching process), is fed into a cell comprising an anode and a cathode. An electric current is then passed between the electrodes through the liquor to deposit the metal on the cathode.

There are, broadly, two types of cell that are used in electrowinning processes.

The first, which is sometimes referred to as conventional electrowinning, comprises a large bath into which the liquor is placed. A set of plates are lowered into the liquor and an electric current is passed across the cell, causing the metal to deposit on to the cathode. The liquor is usually stagnant or circulated slowly. The set of plates typically comprise a large number of anode-cathode plate pairs arranged side-by-side in the bath. After a time, the plates can be lifted from the bath and the deposited metal harvested.

The second type of cell is generally cylindrical in shape ("cylindrical cell") and relies on much higher flow rates to improve the mass transfer of the metal. These comprise a tubular cathode, within which is housed a cylindrical anode. The cylindrical cell is mounted in an upright position and the liquor is passed up through the cell while a current is driven across it. Such cells tend to be more suited to extracting high purity metal from low quality, low concentration, mixed metal solutions than the first type.

For these cylindrical cell arrangements, the metal may be harvested in two ways.

In the first, the metal is deposited onto the inner surface of the cathode as plate (hence these are sometimes referred to as a "plate cell"). A spiral flow is usually induced in the liquor as it rises up the cell, as this helps to improve the mass transfer compared to axial flow. A starter sheet may be fitted inside the body of the cell to form the cathode, on to which the metal is plated. This allows the metal to be harvested more easily by withdrawing the starter sheet and plated metal from the top of the cell as a tube.

In the second arrangement, the metal is deposited as a powder (hence these are sometimes referred to as a "powder cell"). To form the powder the liquor is subjected to a turbulent flow as it rises up the cell. After a time, the cell is then flushed to remove the powder and recover the metal. In the powder cell, the anode tends to have a larger relative diameter and a smaller electrode gap compared to the plate cell.

Cylindrical cells for electrowinning processes are described in WO-A-92/14865, WQ-A-94/02663, WO-A-96/38602 and WO-A-01/07684. The first three are concerned with developments of plate cells and the fourth relates to powder cells.

The entire contents of these earlier patent specifications are explicitly incorporated by reference.

While these known arrangements have offered many advantages over the "conventional" electrowinning systems described above, new developments that could lead to further reductions in the capital and operational costs and offer improvements in efficiency are desired.

Viewed from a first aspect there is provided an anode for a cell of an electrowinning metal recovery apparatus, the anode comprising a tubular anode member providing a face of the anode, the anode member being supported on a current distributing frame that extends within the tubular anode member to distribute electrical current to points on an inner surface of the anode member.

Preferably the current distributing frame is of a generally cylindrical shape. The anode member may be in the form of a sleeve that is arranged to encircle the frame and to be fixed to it to receive electrical current. Thus the frame may provide a central support having a plurality of current supply points that contact the inner surface of the anode member, to distribute electrical current to the anode member.

An advantage of this design is the more even distribution of the current over the anode face and corresponding cathode surface.

By way of example, the frame may comprise a plurality of conductive supports that extend longitudinally within the anode member, the conductive supports being linked by conductive cross-members which provide a plurality of current supply points to an inner surface of the anode member, to distribute electrical current to the anode member.

Preferably the conductive supports comprise a first metal that is clad in a second metal. The first metal may be chosen for its conductive properties (e.g., copper or aluminium) and the second metal chosen for its protective properties (e.g., titanium). In one example, the conductive supports comprise titanium clad copper electrical conductors, since these can have improved current carrying capabilities over solid titanium electrical conductors.

The face of the anode comprises a surface coating of mixed metal oxides to provide an oxygen evolving mechanism during operation of the cell. The known cylindrical cells use either a solid cylindrical member or a tubular member, e.g., a pipe, having a solid face for the anode, onto which the coating is applied. After a period of use, it becomes necessary to replace the coating. Currently this necessitates the removal of the complete anode from the plant and transporting it to a facility capable of removing and subsequently recoating the anode.

By providing a new anode construction where the anode member is supported on a curient distributing fiame, for example, as a replaceable sleeve, it makes it possible to replace just the anode member for a new one, e.g., on site, minimising disruption to the operation of the electrowinning plant.

Thus preferably the anode member, providing the anode face, is adapted to be a replaceable part of the anode, e.g., for the purposes of repair or renewal.

This modification in the construction of the anode has also enabled further developments to be realised. Whilst these developments are preferably taken advantage of in conjunction with the new current distributing frame and anode member construction, they are inventive in their own right and can be seen as stand alone inventions.

Preferably the tubular anode member providing a face of the anode comprises a plurality of holes that extend through the face of the anode.

By providing a plurality of holes in the face of the anode, it allows the amount of the coating that provides the oxygen evolving mechanism to be reduced. As the proportion of holes to anode surface of the face is increased, there will be a direct reduction in the amount of coating required, leading to reduced anode costs.

The face of the anode member may comprise holes which make up more than 15% of the face area, for example, 30% or more of the face area, more preferably 40% or more.

Preferably the anode member is formed from a sheet of material that has been wrapped into a tubular member, providing an expanded metal sleeve that can be fitted over the frame. Supporting the anode member on a frame means that it can be of lighter weight and be made of thinner material. Any reductions that can be achieved in the weight of the anode, particular in view of the special materials for the anode (usually titanium) can lead to significant cost savings.

The anode member may comprise a sheet that is punched with holes. More preferably it comprises an expanded metal mesh. In this way the anode member can be fabricated in a cost effective way. Furthermore various different hole and shape patterns in the mesh will offer additional operating benefits in the form of electrolyte spiral flow improvement that improve mass transfer and reduction in gas formation on the surface of the anode to improve flow and reduce dendrite formation (see below).

It is more cost efficient to form the coating on an anode member in sheet form and roll it than it is to form the coating in situ on a cylindrical anode, as was the case with the existing systems. The plurality of holes, particularly when the anode member is in the form of a mesh, may also make the anode member easier to roll into a tubular shape.

Employing such an anode member, especially where it is supported on an internal current distributing frame, can shift the economies of scale towards larger cylindrical cells.

Previously, the solid cylindrical or solid pipe section of the known anodes limited the diameter and length that these could be made to due to considerations of the anode's weight and expense. This in turn set the diameter and length of the cathode, because factors such as anode-cathode separation and length of time that the liquor is in contact with the electrodes must all be taken into consideration when designing the cell.

The new anode structure allows it to be made larger than before, allowing the cell size to be increased. A larger cell diameter can achieve higher productivity per unit footprint. This can result in an associated reduction in operating costs.

Preferably the cathode diameter of a cylindrical plate cell is at least seven inches (175 mm or more), more preferably eight inches or more (200 mm or more). Due to the lower fabrication costs compared to solid anodes, anode diameters can be increased accordingly while maintaining anode-cathode spacing; preferably, however, the anode-cathode spacing is optimised further as will be described below.

During the electrowinning process, the formation of dendrites which bridge across the anode-cathode gap can be a significant problem. These can create short circuits in the electrowinning cell that not only harm the operation of the cell but usually result in damage to the anode. It has been found that providing an array of holes in the anode face, particularly when the anode member is supported on a current distributing frame as described above, allows a network of paths to be created for the distribution of current. This results in isolation of the dendrite burn which leads to smaller breaks and so makes the anode more resilient to damage from short circuiting by dendrites.

Greater resilience to the problems of dendrite formation enables further advantages to be realised, namely the size of the anode-cathode gap can be reduced. The electrical potential across the cell is a function of the separation of the electrodes.

A larger anode-cathode separation will need a higher voltage to be applied across the cell to generate a given electrical current. By reducing the gap, prefeiably through using a larger anode than previously, it allows the voltage to be reduced too, this leading to power savings and lower operating costs of the cell.

The lower anode costs also allow different sets of anodes to be supplied for given cylindrical cells, each of the anodes being of a different diameter. Hence anode size diameters can be varied within the cell and so the anode-cathode gap can be varied to tailor the cell for optimal performance given differing operating circumstances -such as differing electrolyte conditions, harvest interval targets, current efficiencies and current densities, dendrite management circumstance, desired cathode quality and targeted energy efficiency.

Thus there can also be seen to provide a new method of operating an electrowinning cell having a tubular anode where, depending on operating targets and circumstance, the tubular anode is replaced with one of greater or lesser diameter to optimise performance and vary the anode cathode gap between differing production batches.

A larger anode diameter can also allow a reduction in the anode current density (relative to the cathode current density). This in turn can lead to increases in anode coating life.

Preferably the anode diameter (i.e., the external diameter of the anode member) of a plate cell is at least four inches (100 mm), more pieferably at least five inches (125 mm), and more preferably still at least six inches or more (>150 mm) in diameter.

During operation of the cell, gases will be released, and in particular bubbles of oxygen will form on the anode. The bubbles tend to reduce the efficiency of the cell through reducing the effective surface area of the anode, e.g., where they cling to the anode face, and also through reducing the effective volume fraction of liquor where there are many gas bubbles present in the anode-cathode gap. It is therefore desirable to remove the gas from the cells.

When the anode member comprises a plurality of holes, for example, when it is in the form of a mesh! the bubbles tend to form naturally in the holes rather than occluding the coating on the anode face.

The holes in the anode member also mean that the liquor is now able to flow within the anode member too. Thus a central column of liquor will be present within the tubular anode member during use. The bubbles that form on the anode face can break free and rise up within this central column instead of rising up between the anode-cathode gap. A spiral flow within the liquor will also encourage the bubbles to break free from the anode face and pass into the central column by centrifugal action. Moreover, where the anode member comprises an expanded metal mesh, the inclined formations of the resultant lattice structure may also help to guide the spiral flow over the surface of the anode member.

Previously, the bubbles tended to be swept through the cells by the flow of the liquor, and gas extraction was performed after the liquor had passed through several cells in succession. When the anode face is porous (i.e., provided with a plurality of holes), this has the effect of removing the gas bubbles, in particular the gas that is evolved at the anode, from the flow of liquor passing through the cell.

This can be collected in the head space of each individual cell.

Thus an end cap, which is positioned at the top of the anode member, is preferably arranged with a collection port to remove gas that has risen up through a central column of liquor within the anode member. This allows the gas to be extracted more easily, improving the cell efficiency. In one example the end cap comprises a gas collection manifold that corresponds to the diameter of the anode member, preferably the internal diameter.

The presence of holes in the face of the anode member (e.g., as a way to reduce coating and anode costs, to improve dendrite resilience, to improve cell efficiency, to improve gas extraction or increase the cell size) is inventive in its own right independently of the current distributing frame, and accordingly, viewed from a second aspect there is also provided an anode for a cell of an electrowinning metal recovery apparatus, the anode comprising a tubular anode member providing a face of the anode, wherein the anode member comprises a plurality of holes that extend through the face of the anode.

The present disclosure also provides an electrowinning cell having an anode as described above.

In one aspect it may be seen to provide a cell for an electrowinning metal recovery apparatus, the cell comprising a tubular cathode having an internal surface defining a cathode face and an anode provided internally of the cathode, the anode comprising a tubular anode member having an outer surface defining an anode face, the anode member being provided with holes extending through the anode face to an internal region of the anode, to allow gas released from electrowinning fluid within the cell to escape through the internal region of the anode.

In another aspect the present disclosure can also be seen to provide a cell for an electrowinning metal recovery apparatus, the cell comprising a tubular expanded metal mesh anode.

The cells are preferably of a cylindrical cell type as discussed above, preferably a plate cell. The cells may have a constant diameter along their length and be truly cylindrical (e.g., allowing fabrication using standard pipe sizes). Alternatively they may include a slight variation between the top and bottom diameter measurements, e.g., there might be a lengthwise taper to promote certain flow characteristics or to facilitate removal of the cathode from the cell body. Accordingly any references made herein to "cylindrical" in relation to the cell should also be seen to encompass "generally cylindrical", even if it is not explicitly expressed as such.

The present disclosure also provides a method of operating an electrowinning cell wherein, for an electrowinning cell comprising an anode and a cylindrical cathode, the anode having a tubular anode member which provides a face of the anode and is arranged within the cylindrical cathode, the cell also comprising an inlet for introducing liquor to a lower region of the cell and an outlet for removing the liquor from an upper region of the cell, the method includes the steps of: flushing electrowinning liquor from the inlet to the outlet through the electrowinning cell, to generate a flow of liquor; driving an electrical current across the anode and cathode through the flow of liquor; forming gas bubbles on a surface of the anode member as a result of electrowinning reactions; allowing the gas bubbles to pass through holes in the anode member and into a column of liquor present within the anode member; collecting gas from the gas bubbles that rise up the column of liquor; and extracting the gas from a vent provided above the outlet of the electrowinning cell.

Preferably the method also includes introducing a swirl into the flow of liquor as it passes through the electrowinning cell. Preferably the electrowinning cell is connected to one or more further cells arranged in series, preferably five or more cells, and the method includes pumping the liquor through the cells from the outlet of a preceding cell directly to the inlet of a following cell, wherein gas is extracted from each cell along the series of cells.

The present disclosure also provides a module for an electrowinning plant, the module having a plurality of electrowinning cells arranged into groups of cells, each group being arranged as a pair of cell lines and comprising one or more sets of cells, wherein the cells within a set are connected together in series, each cell comprising an anode as described above, and in particular where each cell comprises an expanded metal mesh anode. Each set of cells within a group is preferably coupled in parallel to a common feed pipe and a common return pipe.

Certain preferred embodiments will now be described in greater detail by way of example only and with reference to the accompanying drawings, in which: Figures 1 a and lb show two examples of current anodes for cylindrical electrowinning cells with Figure lc showing an enlargement of part of the anode of Figure ib; Figure 2 shows an example of a current distributing frame for an anode made in accordance with a preferred embodiment; Figure 3a is a view of a preferred anode member that can be fitted on to the current distributing frame of Figure 2 and Figure 3b is an enlarged view of a preferred expanded metal mesh used foi the anode member; Figure 4a is a view of a preferred anode and Figure 4b is an enlargement of a central region of the anode in use; Figure 5 is a perspective view of a further preferred embodiment of an anode frame; Figure 6 is a cross-section through an existing cylindrical electrowinning cell where the anode has a solid anode face; Figure 7 is a cross-section through a cylindrical electrowinning cell that uses a preferred anode; Figure 8 is a view of a preferred anode stabiliser and vent; Figure 9 is an end view of an existing cell arrangement in an electrowinning plant; Figure 10 is an end view of an improved arrangement for supporting the cylindrical electrowinning cells within an electrowinning plant; Figure 11 is a perspective view of a stand supporting a plurality of electrowinning cells; and Figure 12 is a perspective view of a module comprising an array of electrowinning cells for an electrowinning plant.

Figures la and lb illustrate two types of anode that are used currently in cylindrical electrowinning cells.

In the first design of Figure la, the anode 1 comprises a length of pipe, e.g., a two inch diameter pipe of titanium. A lower region 2 of the anode 1 is adapted for mounting the electrode in the body of the cell and comprises a support flange 3 (the cell body is not shown). A positive potential is applied to the anode via a connection to a power supply (also not shown) on the lower region 2 of the anode 1, below the support flange 3 and externally of the cell.

The anode 1 comprises a coated region 4 extending along a majority of its length, that in use is enclosed within the body of the cell. This coated region 4 provides a solid face of the anode which transfers the current from the power supply to the liquor within the cell. The coating comprises mixed metal oxides, these providing a catalyst for the evolution of oxygen as pad of the electrowinning reactions that take place within the cell.

When mounted in the cell, the top end of the anode 1 would also be supported by an end cap (not shown in the figure but visible as a conical support in Figure 6).

An alternative anode design is shown in Figure lb. The anode 1 is shown the other way up in the figure, though in use it would be mounted in the same orientation as anode 1 of Figure la. It comprises a lower region 5 that is formed from a titanium rod for feeding current into the anode 1'. The lower region 5 includes a support flange 6. Attached to the titanium rod 5 is a titanium tube (e.g., which may be around two inches (50 cm) in diameter) that forms the upper region 7 of the anode 1'. The tube of this upper region? is provided with an oxygen evolving coating on its outer surface. As with the first example, the anode 1' has a solid anode face.

The oxygen evolving coating is key to the process of electrowinning and will deteriorate over time and require replacement. The replacement of the coating can be done by either removing the residual coating and re-applying a new coating, or by replacing the entire anode, a more costly option.

Figure 2 is a perspective view of a new anode frame 10 that is intended to be used as a support for a new anode member (the anode member is not shown in Figure 2 to allow the frame 10 to be seen but can be seen in Figure 4a). This version can provide a replacement for the anode 1 shown in Figure la or anode 1' of Figure lb with a suitable choice of stem 11.

The anode frame 10 is of a generally cylindrical configuration, in as much as it provides an elongate sub-structure that a cylindrical sleeve can fit over. The sub-structure is arranged so that it can contact an inner surface of the cylindrical sleeve at a set of predetermined positions that are spaced along and preferably around the inner cylindrical surface of the sleeve. These positions are spaced so as to distribute the current to the cathode as evenly as possible, preferably both longitudinally and circumterentially. By spreading the current supply points in this way, regions of high current density can be avoided (such regions can lead to uneven deposits and dendrites). -12-

The purpose of the anode frame 10 is two-told; namely to provide a structural support for the anode member which is in the form of a sleeve and to distribute electrical current to the inner surface of the anode member evenly.

To achieve this purpose, the frame comprises one or more conductive supports that extend longitudinally within the anode member. The one or more conductive supports may contact the anode member themselves, and could be provided with formations that act as current supply points to the anode member. Alternatively, as shown by the example in Figure 2, the one or more conductive supports 13 are conductively linked to current transmission members 14 that also help to stabilise the conductive supports 13. These current transmission members 14 may extend transversely to the one or more conductive supports 13, and each current transmission member 14 may be provided with a set of formations 17 that act as current supply points to supply current to the anode member.

Thus in Figure 2, the current distributing frame (the anode frame 10) is mounted onto a power connection section in the form of a stem 11 (the lower region of the anode). The stem 11 is configured to support the anode within the electrowinning cell, and in use, is arranged to project beyond the body of the cell to provide an external connection to a power supply. The stem 11 could, of course, comprise other shapes depending on the configuration of the cell.

The current distributing frame part of the anode of Figure 2 comprises, connected to the power connection section (stem 11), a current distribution plate 12, a plurality of current distribution rods 13, a set of current transmission members 14 and a stabilising hub 15 (together making the current distributing frame 10). This region of the anode extending between the current distribution plate 12 and the stabilising hub 15 is configured to be positioned within the electrowinning cell and generally submerged within the liquor in use.

The current distribution plate 12 is shown as a disc-shaped member. In practice, alternative shapes are possible. Its function is to provide support to one end of the current distribution rods 13 and to distribute the current that it receives from the power connection to each of those rods 13. The pad may be machined from titanium, and its circumferential surface can provide a region of attachment for a lower end of the anode member.

In the figure, the frame 10 is provided with four current distribution rods 13 (one of these is hidden from view by the current distribution rod 13 at the front of the image). The frame 10 may, of course, comprise other numbers of rods 13. For example, there may be more than one rod 13 and up to twelve such rods 13 in the anode, more preferably between three and nine rods 13. more preferably tour, five, six, seven or eight such rods 13.

The conductive supports 13 preferably comprise a core of a first metal and a cladding of a second metal, the first metal having a greater electrical conductivity than the second.

As an example, the current distribution rods 13 may comprise a copper core (or other highly conductive metal such as aluminium) that is clad with titanium (or other suitable protective metal). The copper core enhances the current carrying capabilities compared to rods of solid titanium. Copper is approximately seven times more conductive than titanium, allowing the cross-section of the longitudinally extending conductive supports to be made smaller than if they were made of solid titanium. This not only substantially reduces the weight of the conductive supports, it also keeps more space available within the anode. The choice of titanium for the outer surface of the rods is in order to provide sufficient corrosion resistance for most electrowinning situations. Depending on the chemical and physical environment within the electrowinning cell, alternative metals may be used as appropriate.

While the rods 13 are shown as being of circular cross-section, clearly other cross-sections such as oval, triangular, square, rectangular, hexagonal or polygonal cross-section, are also possible.

The plurality of current distribution rods 13 extend longitudinally within the frame 10, preferably for the entire length between the current distribution plate 12 and the stabilising hub 15. In the figure they are shown as straight rods that are parallel to one another. However other configurations are possible. Their function is to -14-provide support to the current transmission members 14 arranged along the length of the frame 10 and to convey electrical current to them evenly. Accordingly, it is possible that the conductive supports 13 may take other forms in order to achieve this purpose.

For example, the conductive supports 13 may extend longitudinally with a spiral form. They could also comprise members having a more bar like section, for example, they could be present as longitudinally extending plates or struts. In one example a longitudinally extending member having a cross-shaped or star-shaped cross section could be used as a central strut that either provides its own current supply points or is provided with current transmission members in a similar fashion to the embodiment shown. However, cladding such members with a protective metal could be more difficult which might be more undesirable from the point of view of current conduction.

The current transmission members 14 in this embodiment each comprise a set of holes 16 that the current distribution rods 13 pass through. Each current transmission member 14 may therefore receive current from a plurality of the conductive supports 13. The main function of each current transmission member 14 is to connect with and transmit current to the anode member. In the case of the cross-shaped current transmission members 14, it does this at a plurality of current supply points 17 where the current transmission member 14 contacts the anode member. Each current transmission member 14 also serves to align and stabilise the current distribution rods 13.

Thus the transversely extending current transmission members 14 and the longitudinally extending conductive supports 13 together form a scaffold or sub-structure for the anode member.

The current transmission member 14 could also take many forms. For example, it could have a more ring-like form or it comprise arms that extend from a central region to support the current distribution rods. In other embodiments, the functions of the current distribution and the current transmission through the current supply points 17 may be split between different components. For example, there could be an alignment member that supports and positions the current distribution rods 13 and there could be a further component or components that provides the current supply points 17 to the anode member. The current transmission members 14 could also have a different shape depending on where they are positioned within the frame 10, but preferably they are all the same for ease of production.

Each current transmission member 14 may also include a central void 18 and recesses 19 arranged around its perimeter as shown. These allow gas bubbles to rise up through the central region 28 of the frame 10.

In the figure, four current transmission members 14 are shown. In practice, the current distribution frame 10 may comprise more or fewer such transversely extending members.

The spacing of the current supply points 17 is preferably chosen so that the current density received by the cathode is as even as possible, particularly in the longitudinal direction but preferably also in the circumferential direction. Thus the spacing of the current supply points 17 on the anode may be larger in the longitudinal direction than in the circumferential direction, so that when these points are projected across the anode-cathode gap to the cathode, a substantially even distribution is achieved.

The stabilising hub 15 serves to stabilise the other ends of the current distribution rods 13. In the disc-shaped example shown, the circumference of the stabilising hub 15 provides a region for attaching the anode member to. The stabilising hub 15 may also comprise one or more holes or recesses (not shown in this figure but can be seen in Figure 4a) to allow gas to escape from the top of the anode during operation. As with the current transmission members 14, the stabilising hub 15 may be machined from titanium.

The stabilising hub 15 could also comprise a non-planar member. For example, it could comprise a conical or dome shape member, e.g., to encourage the movement of the gas bubbles within the central region 28 towards a point of collection.

Figure 3a is a perspective view of the anode member 20 which is to be mounted on the current distributing frame 10. It comprises an inner surface 21 and an outer surface 22 that provides the anode face. The anode member 20 is tubular and of circular cross section. The diameter of the anode member is preferably constant along its length. However, there may be situations where, for example, the cathode is not perfectly cylindrical, e.g., where it has a slight taper to facilitate harvest of the plate metal or to assist the liquor flow, while a constant anode-cathode gap is desired along the length of the cell. In such circumstances it may be desirable to provide a slight taper on the anode member 20 too.

The anode member 20 preferably is of a length that extends from the current distribution plate 12 all the way up to the stabilising hub 15. It would also be possible for the anode member 20 to comprise a plurality of anode member sections, for example, 2, 3, 4 or 5 anode member sections of shorter length that are mounted end to end along the current distributing frame 10. In some embodiments it may also be desirable to provide the anode member 20 in two parts (or more) which are split longitudinally, to aid fitting of the anode member 20 to the current distributing frame 10.

Thus, the anode member 20 comprises a sleeve that fits over and is supported by the current distributing frame 10, the frame 10 contacting the inner surface 21 of the anode member at a plurality of current supply points 17, to distribute current to the anode member 20.

Figure 3b shows a close up of the surface of a preferred anode member 20, which is in the form of an expanded metal mesh. The mesh structure provides the anode face 22 with an array of holes 23 that may make up between 10-70%, more preferably between 20-50% of the surface area ot the anode face 22. The remainder of the anode face 22 is provided by the metal lattice structure 24 of the mesh, which includes the oxygen evolving coating on at least one side thereof.

Visible in Figure 3b is how the pattern of the expanded metal mesh can provide a surface where the lattice structure 24 extends at an angle a to the longitudinal direction. This profiling of the anode member surface can assist the spiral flow of the liquor across the anode member 20 and, by angling the surface of the mesh in towards the centre, can also assist gas bubble entry into the central tube for extraction.

An important advantage of using an anode member 20 in the form of a mesh is that gas bubbles formed on the anode will tend naturally to form in and cling to the holes 23 of the mesh rather than occupy regions of the anode face 22 (as would be the case with a solid anode). This can improve the efficiency of the cell.

The anode member 20 may be fabricated from a sheet of expanded metal mesh, preferably of titanium with an oxygen evolving coating, that is rolled or otherwise formed into a tubular form for attachment to the current distributing frame 10. It may be connected or welded in place against the current supply points 17 to provide a good electrical connection to the power connection section 11.

Figure 4a shows a perspective view of the assembled anode 25 with the anode member 20 in place on the current distributing frame 10. In practice the current distribution plate 12, the current distribution rods 13 and the current transmission members 14 would be generally obscured from view by the anode member 20 though may be visible at certain angles through the mesh.

Figure 4b shows a close up of a section of the anode 25. Gas bubbles 26 forming within the liquor 27 on the anode member 20 will tend to become drawn into the space 28 within the anode member 20. There they will want to rise within the confines of the anode member 20, passing through the gaps in the current transmission members 14 and between the current distribution rods 13, and finally through holes 29 provided in the stabilising hub 15.

Figure 5 shows a further preferred embodiment ot a current distributing frame 10. It is mounted on a stem 11 at the base of the frame 10, which provides a power connection section 11 that receives a connection to an external power supply (not shown). A support flange 6, similar to the support flange 6 of Figure 1 b, is provided on the power connection section 11 for supporting the anode within the cell.

The power connection section 11 conveys the electrical current to a current distribution plate 12, which in turn distributes itto a plurality of current distribution rods 13. In the arrangement shown there are six rods 13, but the frame 10 could comprise fewer or more rods 13 as desired.

The current distribution rods 13 extend from the current distribution plate 12 to a stabilising hub 15. both of which are in the form ot machined discs provided with holes to receive the ends of the rods 13.

Arranged along the length of the rods 13 are a series of current transmission members 14, which are in the form of rings that encircle the rods 13. The ring shaped members of this embodiment extend in the longitudinal direction to form collars which support a section of each rod 13. The current transmission members 14 are welded or otherwise attached to the rods 13 where they touch, in order to transmit the current from the rods 13 to the inner surface 21 of the anode member 20.

In the embodiment of Figures, the current transmission members 14 provide a continuous, circular, circumferential surface 30 that is arranged to contact and transmit electrical current to the inner surface 21 of the anode member 20. A plurality of current supply points 17 are, however, provided along the length of the frame 10 by the series of current transmission members 14, each spaced equally from the next to distribute the current along the anode member 20 as evenly as possible in the longitudinal direction, as well as around the anode member 20 as evenly as possible.

The ring-shaped current transmission member 14 could also be provided with a fluted or knurled circumferential surface, e.g., either as a machined surface or a corrugated profile that forms the cross-section of the member 14. In this way, each current transmission member 14 (or a selection ot the current transmission members 14) could be provided with a plurality of current supply points 17 around their circumference as desired.

In the embodiment of Figures, the current distribution plate 12 is provided with a circumferential cut-out 31, and similarly the stabilising hub 15 is provided with a circumferential cut-out 32 facing in the opposite direction. These cut-outs 31, 32 offer a seat for the bottom and top edges of the sleeve-like anode member 20 respectively.

During the operation of the electrowinning cell, bubbles that form on the anode member 20 can rise up through the structure of the anode frame 10 un-obstructed by the current transmission members 14 and escape up through the hole 29 provided in the stabilising hub 15.

Figure 6 shows a cross section through a known electrowinning cell 35 in which the anode 1 has a solid anode face. Bubbles of gas 36, which form on the anode face during the electrowinning operation, are swept through the cell by the liquor 37 as it flows from one cell 35 to the next. A spiral flow may be induced in the liquor 37 as it rises from an inlet 38 at the bottom of the cell 35 to the outlet 39 at the top of the cell 35. The spiral flow causes bubbles to congregate towards the centre of the flow through centrifugal forces pushing the liquid to the outer cathode wall of the cell.

The liquor 37 with the gas bubbles 36 may pass through five or more cells 35 before the gas is removed using a special end cap (not shown).

Figure 7 shows a cross section through a cell 40 made in accordance with a preferred embodiment. The cell 40 includes an inlet 43 and an outlet 44, arranged to pass the liquor 42 between a tubular anode 45 that is positioned within a tubular cathode 46. The anode 45 preferably comprises a current distributing frame 10 and anode member 20 as described in Figures 2 to 3b. In this arrangement, when gas bubbles 41 form at the anode 45, they will tend to become drawn in to a central column 47 of liquor 42 present within an inner region 28 of the anode 45, particularly when there is a spiral flow. Here the gas bubbles 41 will rise within the central column 47 and the gas can be extracted via a vent 48 above the liquor 42 (the vent 48 will be described in more detail below).

Table 1 illustrates the dimension and weight considerations for the previous solid face anodes. As the nominal pipe size is increased, the weight increases dramatically and there are significant weight to radius ratio increases. These increases in weight represent not only additional costs in relation to the bulk material, but also in terms of the structure required to support the anodes. In all cases the weight-to-radius ratio increases are of the order of greater than 1.

Table 1 -Titanium Dimension & Weight Consideration for Anode Materials -20 - Nominal Pipe Outside Weight Weight-to-Size Diameter inch lbs/ft (kglm) radius inch (mm) (mm) ratio increase from 1-1/2" (38.1 mm) pipe* 1-1/2" (38.1) 1.9" (48.26) 1.20 (1.79) Note I 2" (50.8) 2.375" (60.33) 1.52 (2.26) 1.06 2-1/2" (63.5) 2.875" (73.03) 2.03 (3.02) 1.35 3" (76.2) 3.5" (88.90) 2.49 (3.71) 1.28 Note2 3-1/2" (88.9) 4" (101.60) 2.86 (4.26) 1.25 Note2 4" (101.6) 4.5" (1 14.30) 3.23 (4.81) 1.24 Note2 5" (127) 5.563" (114.30) 4.47 (6.65) 1.41 Note 1: 1-1/2" (38.1 mm) Nominal pipe is used for current anodes Note 2: 3", 3-1/2", 4" & 5" (76.2, 88.9, 101.6 & 127 mm) pipe would be comparable material for use in larger diameter cell design Table 2 shows a comparison for anodes which are made using the new construction. As the size of the anode increases, so too will its weight. However, the weight-to-radius ratio increase from the nominal two inch diameter anode is now less than 1, preferably less than 0.9 and more preferably achieves weight-to-radius ratio increases of less than 0.8.

Table 2 -Dimension and Weight Proportions of open face anodes Weight-to-radius OD of Anode Approximate Weight of ratio increase inch(mm) Anode lb (kg) from 2" (50 mm) Anode 2 (50.8) 3.9 (1.8) 3 (76.2) 5.1 (2.3) 0.62 4(101.6) 6.3 (2.9) 0.62 4.5 (114.3) 7.4 (3.6) 0.72 (127) 8.5 (3.9) 0.79 5.5 (139.7) 9.3 (4.2) 0.79 -21 -Table 3 shows how anode diameter can be selected for particular cathode sizes in order to vary the anode-cathode gap. It is preferred to use an anode-cathode geometry as highlighted in bold in the table below, where the anode-cathode gap is set to be between 1.4 to 3.0 inches (35.6 to 76.2 mm), more preferably between 1.5 to 2.5 inches (38.1 to 63.5 mm), and most preferably around 1.9 inches (48.3 mm).

Table 3 -Varying Cell Geometry Anode Diameter (lnches*) Cell OD lID 2" 2.5' 3" 3.5" 4" 4.5" 5" 5.5" 6" 6.5' 7" 7.5" 8" 8.5" (Inches) 6.625 Anode- 2.2 1.9 1.7 1.4 1.2 0.9 0.7 0.4 0.2 th d 6.357 Ca 0 e gap 8.625 3.2 2.9 2.7 2.4 2.2 1.9 1.7 1.4 1.2 0.9 0.7 0.4 0.2 (inches) 8.329 10.75 4.2 4.0 3.7 3.5 3.2 3.0 2.7 2.5 2.2 2.0 1.7 1.5 1.2 1.0 10.42 12.75 5.2 4.9 4.7 4.4 4.2 3.9 3.7 3.4 3.2 2.9 2.7 2.4 2.2 1.9 12.39 * multiply inch measurement by 25.4 to convert to mm The lower anode costs (e.g., as a result of the weight savings) also allow a set of anodes to be supplied for each cylindrical cell, each of the anodes being of a different diameter.

Thus there can also be seen to be provided a new method of operating an electrowinning cell having a tubular anode where the tubular anode is replaced one or more times with a tubular anode of differing diameter, to suit changed operating circumstances or objectives.

Where the required current density needed for unit production remains steady, a larger anodic surface area will decrease the anodic current density directly in proportion to the increase in the anode surface area. Where previous electrowinning arrangements needed a cathodic current density of, say, 600 amps per square metre, the nature of the six inch (150 mm) diameter cathode with a two -22 -inch (50 mm) diameter anode providing a two inch (50 mm) anode-cathode gap requires an anodic current density of 1600 amps per square metre. Using larger anode sizes, e.g. with an eight inch (200 mm) diameter cathode, can see anodic current densities drop to less than 900 amps per square metre, representing a significant drop that compensates well for the required openings of a mesh anode member 20. Coupled with the improved gas extraction and reduced anode-cathode gap, this can offer significant improvements in the current efficiency as well as reductions in the power consumption.

Figure 8 illustrates an example of an anode stabiliser and vent 50 that could be fitted to the top of a preferred anode member 20. It comprises a tubular base portion 51 that fits into the top of the anode member 20, and a substantially conical hollow body portion 52 that leads into a venting portion 53. The venting portion 53 is provided with one or more vents 54 for extracting the gas that is evolved at the anode 45. This item therefore serves the dual purpose of stabilising the anode 45 within the cell 40 as well as venting the gases which collect within the central column 47 of the anode 45.

Figure 9 illustrates an end view of how a group of 6 inch (150 mm) cylindrical electrowinning cells 60 are arranged on a stand 61 in an existing plant arrangement. The cells 60 are held in rows of 5 or more on either side of the stand 61. The stand 61 also carries all the hydraulics 62 (the tubes carrying the liquor between the cells 60) both at the top of the frame 61 between the two rows of cells and immediately below the cells. The total width w of this existing arrangement with the cells 60 on the frame 61 is about 3 foot 4 inches (about 1015 mm).

Figure 10 illustrates an end view of how a group of 8 inch (200 mm) cylindrical electrowinning cells 70 can now be arranged on a stand 71. With the new anode arrangement as described above, the cells 70 can be connected in series because the gas can be vented from each individual cell 70. This allows the hydraulics 72 to be arranged running below the cells, allowing the cells 70 to be mounted closer together. The width w for an arrangement of 8 inch (200 mm) cells can be reduced to around 2 foot 9 inches (around 840 mm). The total height h of the cells 70 on the stand 71 is around 8 foot 2 inches (around 2490 mm). With these changes, it can lead to substantial reductions in the footprint occupied by the plant. Also -23 -connecting the cells 70 in series can help to reduce the power required to pump the liquor through the cells 70.

Figure 11 is a perspective view of a stand 71 supporting a group of thirty six electrowinning cells 70 arranged as two lines or rows of cells 70. Each set of half a dozen cells 70 are connected in series to feed and return pipes 73, 74 respectively, with each cell 70 being connected by a linking pipe 75 that joins the top of one cell (the outlet 44) to the bottom of the next (the inlet 43). Each set of cells along a given line of cells 70 are connected to the supply of electrowinning liquor in parallel.

As seen more easily in Figure 10, the hydraulics 72 for the electrowinning cells 70 are preferably located beneath the cells 70, extending underneath the two cell lines or group of cells 70.

Figure 12 shows a module 80 for an electrowinning plant comprising an array of electrowinning cells 70, the cells being arranged in groups 75 of cells 70. Each group 75 of cells 70 is supported on a stand 71 and preferably comprises the sets of cells 70 connected to the feed and letuin pipes 73, 74 in selies and parallel as described above in relation to Figure 11. In the arrangement shown, there are thirty six electrowinning cells 70 in each group (two lines of eighteen cells 70, with three sets of six cells 70 supplied with liquor in series and connected as sets in parallel) arranged on a stand 71. There am four such groups of cells 70 enclosed within the footprint of a gantry 76, the gantry 76 providing a raised platform to allow access between the groups 75 for harvesting, maintenance and inspection of the cells 70.

Thus a module 80 of cells 70 may comprise over one hundred electiowinning cells 70, more preferably over 120 cells, with each cell comprising an anode as described in the foiegoing. For the illustrated embodiment, the module 80 includes 144 such electrowinning cells in total. Modules 80 with other airangements of cells 70 are, of course, also envisaged and are considered within the spirit and scope of the invention as defined by the appending claims.

An electrowinning plant preferably comprises a pluiality of such preferred modules 80, each module comprising a plurality of electrowinning cells having an anode as described in at least claim 1 of this specification.

Claims

-24 -Claims: 1. An anode for a cell of an electrowinning metal recovery apparatus, the anode comprising a tubular anode member providing a face of the anode, the anode member being supported on a current distributing frame that extends within the tubular anode member to distribute electrical current to points on an inner surface of the anode member.
2. An anode as claimed in claim 1, wherein the current distributing frame comprises a plurality of conductive supports that extend longitudinally within the anode member, the conductive supports being linked by conductive cross-members which together provide a plurality of current supply points to an inner surface of the anode member, to distribute electrical current to the anode member.
3. An anode as claimed in claim 2, wherein the plurality of conductive supports distribute electrical current to the anode member in the longitudinal direction.
4. An anode as claimed in claim 3, wherein the conductive supports distribute electrical current to a plurality of points on the anode member in the circumferential direction.
5. An anode as claimed in any of claims 2 to 4, wherein the conductive supports comprise titanium clad copper electrical conductors.
6. An anode as claimed in any preceding claim, wherein the anode member comprises a replaceable sleeve that is supported on the current distributing frame.
7. An anode as claimed in any preceding claim, wherein the tubular anode member providing a face of the anode comprises a plurality of holes that extend through the face of the anode.
8. An anode as claimed in claim 7, wherein the face of the anode member may comprise holes which make up more than 15% of the face area, preferably 30% or more of the face area.

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9. An anode as claimed in claim 7 orB, wherein the anode member comprises a mesh, in particular an expanded metal mesh.
10. An anode fora cell of an electrowinning metal recovery apparatus, the anode comprising a tubular anode member providing a face of the anode, wherein the anode member comprises a plurality of holes that extend through the face of the anode.
11. An anode as claimed in claim 10, wherein the anode member is supported on a current distributing frame, the current distributing frame comprising at least one conductive support that extends longitudinally within the anode member, the at least one conductive support being conductively linked to conductive cross-members which together provide a plurality of current supply points to an inner surface of the anode member, to distribute electrical current to the anode member.
12. A cell for an electrowinning metal recovery apparatus, the cell comprising a tubular cathode having an internal surface defining a cathode face and an anode provided internally of the cathode, wherein the anode is as claimed in any preceding claim and the cathode has an internal diameter of at least seven inches (at least 175 mm), and more preferably eight inches or more (200 mm or more).
13. A module for an electrowinning plant having a group of electrowinning cells, each cell comprising a tubular cathode having an internal surface defining a cathode face and an anode provided internally of the cathode, wherein the anode is as claimed in any of claims ito 11.
14. A module as claimed in claim 13 having a plurality of groups of electrowinning cells, each group being arranged as a pair of cell lines and comprising one or more sets of cells, wherein the cells within a set are connected together in series and are coupled to a common feed and return of electrowinning liquor that extends below the cell lines.
15. A module as claimed in claim 13 or 14, where the module comprises more than 100 electrowinning cells, more preferably more than 120 cells.

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16. Atubularanodefora cylindrical cell ofan electrowinning metal recovery apparatus substantially as hereinbefore described with reference to Figures 2 to 4b or Figure 5 of the accompanying drawings.
17. A cylindrical cell for an electrowinning metal recovery apparatus substantially as hereinbefore described with reference to Figure 7 of the accompanying drawings.
18. A module for an electrowinning plant comprising a group of cylindrical cells substantially as hereinbefore described with reference to Figure 12 of the accompanying drawings.