FIELD OF THE INVENTION

The invention relates to the electrolytic processing of metals, specifically to the electrowinning of metal from an electrolyte solution bearing the metal in ionic form. The invention relates to an improvement in the way in which the anodes in an electrolytic cell are physically supported in the cell and supplied with current.

BACKGROUND OF THE INVENTION

In the electrowinning of metals a cell usually contains n cathodes and (n+1) anodes arranged in parallel in the order anode, cathode, anode, cathode, anode etc.

Traditionally, the recovery of metal by electrowinning utilises lead based anodes. A common anode design is a cast or rolled sheet or "blade" of lead alloy consisting mostly of lead at around 99% purity, the remainder usually consists of minor alloying or doping elements including calcium, tin, antimony and silver, which combined form around 1 % of the alloy. The combination of alloying elements in the lead anode depends on the metal which is to be electrolytically recovered. In the case of copper electrowinning a lead-calcium-tin (Pb-Ca-Sn) alloy is commonly used, in the case of zinc electrowinning, the alloy is usually lead doped with silver (Pb-Ag). As shown in Figure 1a, a lead sheet 101 is suspended from a copper hanger bar 102. The lead sheet 101 is usually of a thickness in the range between 6 mm and 10 mm.

The permanent cathodes 109 used in copper electrowinning usually use stainless steel as the cathode blade material. The cathode blades are sized such that the exposed part of the cathode blade in the electrolyte allows for a deposit of copper which is in the order of 1 m high and 1 m wide. The lead sheet of the anode is usually sized to be slightly shorter and slightly narrower (by 1 to 10 cm) than the cathodes in the same cell. For example - if the immersed surface of the cathode in the electrolyte solution is 1 m high by 1 m wide, then the immersed surface of the anodes might be sized at 0.95 m high and 0.9 m wide.

The total mass of a lead based anode for electrowinning is usually in the range of 70 to 100kg. The anode hanger bar 102 rests on a conducting inter-cell busbar 103 located in a capping board 105 which rests on the top of the side-wall of the tank (not shown).

The inter-cell busbars are typically made of copper and usually run along the length of the side walls of the tanks perpendicular to the electrode hanger bars. In some electrowinning plants the inter-cell busbar 103 is referred to as a "dog-bone" due to the characteristic cross-sectional profile.

The anode derives a current supply from an electrical contact with the busbar 103 seen on the left side of figure 1a and figure 1 b.

Another inter-cell busbar 103 is seen on the right side of figure 1 a and figure 1 b, this also rests on the wall of the next tank in the circuit, and conducts current from the cathodes of the first cell to the anodes of the next cell in electrical series.

In the conventional practice the inter-cell busbar conducts current from one cell to the next cell in electrical series. These inter-cell busbars contain a considerable amount of copper. A single inter- cell bus bar can contain several hundred kilos of copper. One of the benefits of this invention is that the inter-cell busbars are no longer required, giving a cost saving in materials. The anode hanger bar may, in some arrangements, also derive current from or deliver current to a second conductive busbar 104, referred to as an "equalizer-bar" on the opposite side of the tank. This arrangement is referred to as "double-contact" and allows for improved distribution of current in the cell, e.g. by combating cold-contacts.

The equalizer bar is electrically insulated from the main busbar typically by use of nonconducting capping boards 105, or in the case of the cathodic equaliser bar 107 by use of a non-conducting spacer 106.

In some electrowinning plants an alternative curved hanger bar design known as a "steerhorn" may be employed in place of 102. The steerhorn concept allows the hanger bar and consequently the anode to sit lower in the cell and closer to the air-electrolyte interface than a straight hanger bar, decreasing the amount of unused lead of sheet 101 sitting above the electrolyte surface. The present invention eliminates the need for a steerhorn hanger bar arrangement. The copper hanger bar 102 of a lead based anode is usually covered in lead which provides protection for the copper bar that would otherwise corrode when held at an anodic potential in the aggressive acid mist environment typically found above the electrolyte surface.

The copper hanger bar 102 in a lead based EW anode has two main functions:

• it provides a mechanical support for the heavy lead sheet 101 suspended below,

• it carries electrical current from the busbar 103 running along the side of the cell to the active lead sheet of the anode.

Other considerations that the hanger bar 102 ideally fulfils:

• it should allow for the accurate positioning of the anode in the cell, • it should keep the anode in a stable position in the cell during normal operation, preventing the anode from moving or tilting,

• it should be able to withstand high temperatures and not be easily combustible - due to possible ohmic heating of components inside the cell e.g. in the case of short circuiting.

The operating cell voltage for copper electrowinning using the state-of-the-art lead anodes is in the order of 2.0 volts when operating at a current density in the order of 300 A/m ² .

Copper EW processes utilising lead anodes typically operate with some cobalt sulfate added to the electrolyte solution. Cobalt in the electrolyte at a concentration in the order of low hundreds of parts per million (ppm) is believed to decrease the spalling of lead or lead oxide from the lead anode surface and also to slightly depolarize the voltage of the anode reaction - electrolytic splitting of water into oxygen and protons otherwise known as the oxygen evolution reaction (OER) - lowering the overall cell voltage and thus lowering the total energy

requirement to electrowin copper.

In the last decade, some copper electrowinning tankhouses have replaced their lead anodes with mixed metal oxides (MMO) coated titanium anodes. Such anodes are usually a single titanium plate or a pair of titanium meshes, in both cases the titanium substrate surface is coated with a mixture of electrocatalytic metal oxides. An example coating can be a mixture of iridium oxide in a tantalum oxide matrix. Mesh electrodes are generally preferred because of their higher active surface area and the improved electrolyte circulation they allow.

These MMO coated anodes are more electrochemically active for oxygen evolution than lead anodes, giving a decrease in the cell voltage in the order of 300 mV, which represents an approximate 15% saving in electrical power compared to lead anodes. Coated titanium anodes do not require cobalt sulfate in the electrolyte and this gives a direct cost saving compared with systems which employ lead anodes.

Coated titanium anodes, particularly where a mesh electrode is used, have a high sheet resistance compared with the sheet resistance of a solid lead sheet. If the mesh electrodes are only connected to the hanger bars along their top edge, there is an unacceptable voltage difference between the top and bottom of the mesh electrodes due to the high resistance of the mesh.

The approach to date for the typical structure of an MMO coated titanium anode is illustrated in the sketch Figure 2. Two titanium meshes 210 are attached by direct welding or by welding to interconnecting titanium pieces, onto vertically-arranged titanium-clad copper bars 21 1 which carry current between the copper hanger bar 102 and the titanium meshes 210.

In an electrowinning tankhouse it is not desirable to use a very thick anode; the thicker the anode, the fewer anodes can be fitted into each cell. The more vertical bars 211 used, the narrower the diameter needs to be and the thinner overall anode structure. In practice the vertical titanium-clad copper bars 211 number from a minimum of one vertical titanium-clad copper per anode up to eight or more per anode.

The titanium clad copper bars are welded, screwed, soldered or otherwise connected to the copper hanger bar 102, so that there is an electrical connection between the copper of the horizontal hanger bar and the copper core of the vertical titanium-clad copper bars. In some plants a steerhorn design of hanger bar (not shown) is employed to allow the hanger bar to sit lower in the cell.

The electrons generated from oxygen evolution pass from the electrocatalytic MMO coating into the titanium mesh 210, up through the titanium-clad copper bars 207, into the copper hanger bar 102, into the busbar 103, and via the external circuit to the permanent cathodes 108 where they reduce metal ions in solution which deposit as a plate metal on the permanent cathode surface 109.

MMO coated titanium anode meshes are typically much lighter than a comparable lead-alloy blade of a lead based anode. The whole MMO anode assembly has a mass in the order of 30 kg, of which the straight copper hanger bar 102 or steerhorn hanger bar (not shown) forms approximately 10 kg.

PRIOR ART

The following prior art is disclosed.

(1) Apparatus for use in eiectrorefining and electrowinning. WO 2012020243 A1 , publication date February 18, 2012.

(2) System for power control in cells for electrolytic recovery of a metal. WO2013117805 A1 , publication date August 15, 2013.

(3) Self protected anodes and cathodes in electrolytic cell arrangements. Fl 124587 (B), publication date October 31 , 2014.

PROBLEMS WITH CONVENTIONAL PRACTICE

The copper hanger bars 102 used in conjunction with MMO coated titanium anodes (Figure 2) are not usually coated or clad in a protective metal - compared with lead-based anodes (Figure 1) where lead is typically used to clad the copper hanger bar, thereby protecting it from corrosion by acid mist.

The underside of the copper hanger bar 102 of a MMO anode is thus typically exposed to the acid mist evolved at the air-electrolyte interface during electrowinning operation, and the copper hanger bar is susceptible to corrosion when held at an anodic potential. There is a risk of crevice corrosion at the junction between the copper hanger bar and the titanium clad copper bars suspended below the hanger bar.

The electrocatalytic coatings on the titanium mesh of the anode lose their effectiveness or activity over time and the coatings need to be renewed at intervals (typically after a few years), thus the anodes must be serviced. Present anode designs (Figure 2) in which the vertical titanium-coated copper vertical bars are welded into the hanger bar does not facilitate this servicing.

Conventional hanger bars rely on an electrical contact between a common busbar 103 and the hanger bar 102 which is open to corrosion by acid mist and deterioration by electrolyte penetration between the two contact surfaces. Electrical contact may therefore be lost, leading to loss of production and requiring operator intervention to correct the problem.

Some tankhouses use a "double-contact" arrangement in which both ends of the hanger bar are electrically active with the second hanger bar end resting on a second common busbar 104 referred to as an "equalizer bar". The main function of the equalizer bar is to combat "cold contacts"; if the connection between the main-busbar 103 and the hanger-bar 102 is not good or not successful, then the anode has a second chance to draw current into the hanger bar from its neighbouring anodes via the equalizer bar 104 at the opposite end of the hanger bar.

In conventional tankhouse designs, the anodes are positioned and spaced in the cell by

• the use of busbars or capping boards with fixed positions, or

• by the crane-bale equipment used to raise and lower the electrodes into the cell.

In conventional anode designs, the measurement of total anode current is difficult. A current transducer 212 can be fitted around the hanger bar end but this is an unsatisfactory location due to the large cross section of the hanger bar end, the small space afforded for the location of the sensor and the high exposure to acid mist at that location, as well as exposure to stray magnetic fields from adjacent hanger bars which can compromise accuracy. Current sensors 213 may also be located around the narrower vertical conductor bars 21 1. The total anode current is the sum of all the current sensors 213. Note that those sensors 213 are positioned close to the acid electrolyte and therefore at a high risk of deterioration by acid attack.

OBJECTIVE OF THE INVENTION

An objective of the invention is to solve the above problems, and to introduce additional advantages, by separating the two main functions of the conventional hanger bar. These functions are:

(i) mechanical support of the anode electrode

(ii) supply of electrical current to the anode electrode.

With only one function to perform (mechanical support) the hanger bar can be optimized to perform that function. It can be made lighter for the same load bearing capacity and can be constructed from a material (or coated with a material) which does not corrode in the acid-mist laden environment encountered above electrowinning cells.

The function of current supply to the vertical current distribution bars can now be undertaken by dedicated electrical conductors which have no load-bearing function. The most appropriate conductors to fulfil this function are insulated cables. The insulation around the cables will permit the core of the cables to be made from high-conductivity copper since the insulation will protect the copper from the acid mist. The connection to the vertical current distribution bars to these cables can be distinct from the mechanical coupling of these vertical bars to the nonconducting hanger bar. The electrical and mechanical connections of the vertical current distribution bars are thus separated, affording better opportunities to protect these connections than if they were combined in one connection.

A further objective of the invention is to reduce the weight of the hanger bar. The traditional dual-duty hanger bars are of continuous cross section, anode hanger bar dimensions are typically of a length of between 1200 and 1300 mm, a height of between 30 and 45 mm and a breadth of between 15 and 30 mm. The mass of an individual hanger bar can be in the order of 10 kg. The hanger bars include a large amount of copper. In a medium sized electrowinning plant using 5000 anodes, this represents the use of 50 tonnes of copper in hanger bars alone - a considerable cost. The non-conducting, single-duty hanger bars can instead be made of high mechanical strength, low-density material and use an optimal load-bearing structure such as a lattice-beam. The cables which carry the current can be made only as long as necessary to reach the designated vertical current distribution bar. As an alternative to copper, and as a means of further reducing the overall anode weight, the cable cores may be of aluminium which has a higher specific conductivity than copper. Weight reduction can be of significant advantage since this facilitates the manual handling of the electrodes rather than requiring the use of a crane.

A further objective of the invention is to replace the exposed and unreliable contact between the hanger bar and the common busbar with a more secure cable connection with greater pressure between conducting surfaces and to afford the option of sealing that joint against electrolyte ingress. If required, the connection can be made at a location which is sheltered from exposure to acid mist - an option not available with the conventional, dual-duty hanger bar design. For rapid anode changing, the connection between the cables and the common busbar can be a plug-and-socket arrangement.

A further objective of the invention is to facilitate the use of current measuring arrangements to permit the total anode current to be measured. A small current sensor can be located around each of the current carrying cables at a convenient location and be sealed against acid mist. The total anode current can be obtained by summing all the cable currents.

A further objective of the invention is to facilitate the deployment of the techniques for the use of point of use power supplies as disclosed in Prior art (1) WO 2012020243 A1. The use of cables to carry the electrode current facilitates the use of point-of-use power converters with the associated benefits and advantages. The converters can be current controlled, allowing exact control of the total current delivered to the individual anode. The converters can be deployed in such a manner as to control the current density on each side of the anode so that current density is accurately determined on both sides of the anode, irrespective of inaccuracy in the placement of the anode or the associated cathodes. The use of controlled current converters provides protection against damage to the anode electrode by excess current due to uneven current distribution in the cell, or produced by incipient or actual short circuits. The current converters incorporate current sensing for the purpose of current control and current data feedback to a central control room.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:

Figure 1a (a side view) and Figure 1 b (a plan view) - a conventional lead-alloy anode with a solid sheet of lead and a conventional copper hanger flanked by two permanent cathodes, resting on a double contact busbar arrangement.

Figure 2 (a side view) - a conventional titanium mesh anode coated with mixed metal oxide (MMO) catalyst and a conventional copper hanger bar, resting on a double contact busbar arrangement. Figures 3a (a side view) and Figure 3b (a plan view) - a titanium mesh anode with a nonconducting hanger bar and a single power electronics unit located on one end of the hanger bar.

Figure 4a (a side view) and Figure 4b (a plan view) - a titanium mesh anode with a nonconducting hanger bar and power electronics units located on both ends of the hanger bar, each powering half of the vertical titanium clad bars.

Figure 5a (a plan view) and Figure 5b (a plan view) - variations on figure 4 with a different way of connecting the power electronics units, where one unit powers one mesh or face of the anode.

Figure 6 (a plan view) - an arrangement in which the magnitudes of the current drawn by the power supplies from adjacent cathodes is independently controlled.

DETAILED DESCRIPTION OF THE INVENTION

The invention is illustrated by way of the following embodiment but is not limited to the system described hereafter. Figure 3a (side view) and Figure 3b (plan view) illustrate the first embodiment. The hanger bar 314 is used only as a mechanical support structure - a beam, not a conductor of electricity. The hanger bar 314 is omitted from the plan view of figure 3b for clarity of the electrical connections.

Typically the hanger bar 314 will be made, for example, of titanium which is a strong, lightweight and corrosion-resistant material. It may be constructed in a variety of ways with the object of providing the required strength with low weight. A lattice structure is proposed but other designs are possible - for example an I-beam or a tubular box-girder design in which any appropriate cross-sectional profile may be employed. The ends of the hanger bar 314 bar rest on capping boards on top of the walls of the electrolytic cell (or tank) and may be located in position by a mechanical system which ensures the anode is positioned accurately and maintains that position during operation. Advantageously the beam can be shaped to suppress movement of the anode during use e.g. preventing swinging, and to assist in the accurate positioning of the hanger bar on the tank edge. Advantageously the hanger bar 314 can be shaped to disengage acid mist above the electrolyte surface.

The hanger bar 314 can be made of a composite material, which can include, but is not limited to, a fire-resistant fibre reinforced plastic (FRP), or polymers such as polypropylene, and which may contain a structural element (for example an I-beam) buried inside the polymer in order to enhance the strength of the structure.

Typically the hanger bar 314 will be required to carry a mass of 20 kg or more including; the MMO coated titanium mesh sheets 210, the current delivery bars 211 and the cables 316. The weight of the power electronics unit 315 may also need to be carried if it is located on the hanger bar 314.

The titanium-clad copper current delivery bars 211 are suspended through the titanium beam hanger bar 314 and secured by a fastening means such as a thread and bolt assembly 317 and are electrically insulated from said hanger bar 314 e.g. by use of insulating sheaths 319. The MMO coated titanium mesh sheets 210 are connected to either side of the titanium clad copper bars 21 1 , either by directly welding on to the titanium clad copper bars or by first welding a titanium spacing element(s) (not shown) onto the titanium clad bar and subsequently welding the MMO coated titanium mesh 210 onto the titanium spacing element for ease of securing the titanium mesh in place and making a good electrical connection.

Current is conveyed from the power electronic unit 315 directly into the vertical current distribution bars 211 via insulated cables 316. Current enters the power electronics unit 315 through cable 318 connected to a busbar 320 which runs along the side of the tank and on which rests the ends of the conducting cathode hanger bars. Current leaving the anode, passing through the electrolyte and being collected by the cathodes, is returned to the input side of the power supply 315 via the busbar 320 and the cabling 318. The connection of wire 318 to the busbar 320 can be made permanently and securely using a terminal lug and a bolt. A plastic cover protects the connection between 318 and 320 from acid mist. Alternatively the connection between 318 and 320 may be made by pressure alone as in present practice.

To enable anodes to be changed easily, the terminal lug and bolt can be replaced by a plug and socket arrangement with a suitable plastic cover.

The power electronics unit 315 needs to be fed from an external power supply. This external power supply can be either AC or DC and will typically be delivered to the power supply electronics unit 315 by a flexible cable incorporating two wires (live and neutral for AC, plus and minus for DC). That power lead too will need to be connected via a plug and socket or similar disconnectable arrangement.

The power electronics unit 315 may be located on the non-conducting hanger bar 314 as shown in Figure 3 or alternatively it may be mounted on the side of the tank.

The power electronic unit (Prior art (1)) will typically contain a current measuring arrangement for reporting current magnitude and the detection of conditions which could result in damage to the anode. The power electronics unit 315 will typically incorporate current control so that the appropriate current density can be used in normal operations and so that the anode mesh can be protected during abnormal conditions (for example an actual or incipient short between anode and cathode).

The anode and associated power electronics unit 315 may be operated in a variety of ways according to the type of power electronics unit employed. The power electronic unit 315 may be a power supply as described in prior art (1). Prior art (1) describes a number of ways that this power electronic unit may be used and a person skilled in the art of power electronics will understand the ways in which prior art (1) may be applied to the present invention.

Figure 3b is a plan view sketch of the anode structure. The vertical current conductors 21 1 are electrically and mechanically connected to both meshes 210. Cables 316 are connected to individual current sources within the power unit 315 so that the current delivered to each of the vertical conductors can be individually controlled. By this means an even current density can be maintained across the surface of the anode meshes 210.

109a and 109b are cathodes on each side of the anode supported by conducting hanger bars 108a and 108b respectively. The conductive cathode hanger bars 108a and 108b rest on the conducting busbar 320 to provide a source of current for the anode power units 315.

Figure 4a and Figure 4b illustrate an alternative arrangement of the power supplies. Figure 4a shows a side view sketch of an anode with two power electronics units 315a and 315b, located at either end of the non-conducting hanger bar 314.

Figure 4b shows the same sketch of the anode in plan-view, the hanger bar 314 is omitted from this view for clarity. In this arrangement current is fed into the vertical current-carrying titanium clad copper bars 211 a and 211 b from the nearest power electronics unit.

Half of the vertical current-carrying titanium clad copper bars 21 1a are connected by cables 316a to the first power electronics unit 315a. The other half of the vertical current-carrying titanium clad copper bars 211 b are connected by cables 316b to the second power electronics unit 315b. For this reason the total number of titanium clad bars should be divisible by two (2, 4, 6, 8 etc.). The operation of power electronics units 315a and 315b will need to be coordinated electronically. The advantage of using two power electronics units is that the lengths of wires 316a and 316b are minimised thereby minimising the mass of these conductors and also minimising the electrical power losses in these wires for any given mass of wire.

Figure 5a and Figure 5b also shows the use of two power electronics units 315a and 315b with a variation in arrangement to achieve discrete control of the current flowing at a single anode face. This is achieved by the arrangement of the connection of the power electronics to the vertical current-carrying titanium clad copper bars 211 a and 211 b and further how the vertical current carrying bars are then connected to the titanium meshes 210a and 210b. In this example,

• The power electronics unit 315a is only connected to titanium clad copper bars 21 1a which are in turn connected to the MMO coated titanium mesh 210a.

• The power electronics unit 315b is only connected to titanium clad copper bars 21 1 b which are in turn connected to the MMO coated titanium mesh 210b.

• There is no electrical connection for current to flow from power electronics unit 315a to titanium mesh 210b.

• There is no electrical connection for current to flow from power electronics unit 315b to titanium mesh 210a.

In Figure 5a and Figure 5b the separation between MMO coated titanium meshes 210a and 210b is exaggerated for clarity. In practice it is sufficient that mesh 210a is welded only to titanium clad copper bars 211 a and mesh 210b is welded only to titanium clad copper bars 211 d as current will pass into the anode meshes only via the welds. The purpose of this arrangement is that the current flowing to cathode 109a and cathode 109b can be controlled independently, thereby permitting the optimisation of current density at the relevant surface of each of the cathodes 109a and 109b. Figure 5b shows an alternative arrangement in which the continuous cathode busbars of 320a and 320b seen in figure 5a are replaced with shorter sections of busbar or "pads" on which the cathode hanger bars 108a and 108b rest.

Figure 6 shows an alternative arrangement for obtaining independent current density control on the relevant cathode surfaces. The power unit 315a draws current from cathode hanger bar 108a via pad 320a and cable 318a. The power unit 315b draws current from cathode hanger bar 108a via pad 320b and cable 318b. By this means, the power units 315a and 315b control the current density at the face of cathode 109a opposite the MMO coated titanium mesh 210a.

The power unit 315a also draws current from cathode hanger bar 108b via pad 320a and cable 318c. The power unit 315b also draws current from cathode hanger bar 108b via pad 320b and cable 318d. By this means, the power units 315a and 315b control the current density at the face of cathode 109b opposite the MMO coated titanium mesh 210b.