WO2013132464A1

WO2013132464A1 - Electro winning and electro refining

Info

Publication number: WO2013132464A1
Application number: PCT/IB2013/051847
Authority: WO
Inventors: Jan Petrus Human
Original assignee: Jan Petrus Human
Priority date: 2012-03-09
Filing date: 2013-03-08
Publication date: 2013-09-12

Abstract

Cathode constructions are disclosed for reducing the electrical resistance to current flow through the cathode as compared to conventional cathodes. The constructions include bridges for bypassing the starter plate of the cathode and connecting the hanger bar directly to the deposited metal layers. In another construction an insert (130) of a material which is more electrically conductive than that of the starter sheet (116) is provided which is mechanically connected to the hanger bar (114) but not to the starter sheet (116) so that the sheet and insert can move relatively to one another during stripping. The insert (130) is in electrical contact with the hanger bar and the starter sheet.

Description

ELECTRO WINNING AND ELECTRO REFINING FIELD OF THE INVENTION

THIS INVENTION relates to electro winning and electro refining. BACKGROUND TO THE INVENTION In the electro winning process the metal to be recovered is dissolved in electrolyte and a current is passed through the solution between an anode and a cathode. The metal ions in the solution deposit onto the cathode and, once a sufficient thickness of metal has been deposited, the cathode is removed from the tank containing the electrolyte and stripped of its metal coating. This is usually achieved by flexing the cathode and scraping it using blades so that the recovered metal, which is not strongly bonded to the cathode, peels off.

In the electro refining process the anode comprises an impure form of the metal to be recovered. The metal is impure in that it has contaminants such as particles of the crushed ore in it. Where copper is the metal to be recovered, the anode is obtained by smelting the copper and casting it into the form of the anode. The anode and a cathode are then immersed in electrolyte and current caused to flow between the anode and the cathode. The anode is eroded by the flowing current and the metal ions are deposited onto the cathode. Ore particles which are in the anode do not form ions and

consequently are not deposited on the cathode. They fall to the bottom of the tank containing the electrolyte. These methods are extensively used in the production of copper where an acidic solution of copper sulphate is used as the electrolyte. The cathode comprises a hanger bar and a starter sheet which is usually of stainless steel or titanium. Both stainless steel and titanium have far higher electrical resistances (some 40 times higher) than copper and copper alloys and losses occur when current flows through the starter sheet. Copper and copper alloys are less resistive materials. However, the relative mechanical weakness of copper and copper alloys in relation to stainless steel and titanium make these materials unable to withstand the rigors of the cathode stripping process without being permanently bent or otherwise damaged.

In an electro winning installation or an electro refining installation the electrical energy used constitutes a significant part of the cost per kilogram of recovered metal. The high resistance of the stainless steel or titanium has the effect of reducing the current that flows between the anode and cathode when a specific voltage is applied. This leads immediately to power losses. Any reduction in the power consumed per kilogram of recovered metal improves the profitability of the plant and also reduces carbon emissions

In another method of copper production by electro winning a steel starter sheet is initially used as the cathode. After a period of, for example, two days, there is on each side of the steel starter sheet a layer of deposited copper. These layers are stripped and are trimmed to remove any irregularities along their edges thereby to provide rectangular sheets. The sheets have copper straps fitted to them. The straps enable the copper sheets to be suspended from copper hanger bars to form cathodes. The cathodes are then returned to the electrolyte filled tanks. Current flow deposits further copper on these sheets.

An advantage of this method is that during the greater part of the deposition period current flow is through highly conductive copper. A disadvantage is that additional work is involved in the procedure which results in the production of copper sheets ready for return to the tank. United States Patent Publication US 2010/021980 discloses electroplating the cathode starter sheet to provide an electrical flow path of decreased resistance. A difficulty with this is that the electroplating on the cathode is eventually stripped away with the deposited metal to expose the titanium or stainless steel starter sheet.

The present invention provides a cathode for use in an electro winning or electro refining installation in which current flow is predominantly through a low resistance metal such as copper instead of through the high resistance stainless steel or titanium which constitutes the starter sheet of the cathode.

BRIEF DESCRIPTION OF THE INVENTION

According to a first aspect of the present invention there is provided a cathode construction for use in electro winning and electro refining and comprising a horizontal hanger bar, a starter sheet attached to the hanger bar and extending downwardly from the hanger bar, an insert secured to the hanger bar and located in a recess in the upper edge of said sheet, the insert being of a metal of greater electrical conductivity than the sheet and being in electrical contact with the hanger bar, the sheet, when bent during stripping, moving with respect to said insert.

The insert can be mechanically secured to the hanger bar but not to the starter sheet. In one embodiment said insert is secured in a slot formed in the hanger bar. In another embodiment said insert is shaped so that it fits around the hanger bar and is secured to a side surface of the hanger bar.

Preferably said recess has a lower edge which is formed with ribs and grooves and the lower edge of the insert has corresponding ribs and grooves which mesh with the ribs and grooves of said lower edge. According to a second aspect of the present invention there is provided a cathode construction for use in electro winning and electro refining which comprises a horizontal hanger bar, a starter sheet extending downwardly from the hanger bar and an electrically conductive bridge in electrical contact with the hanger bar and with the starter sheet below the hanger bar, the bridge having a upper position with respect to the hanger bar and a lower position with respect to the hanger bar, the bridge contacting the starter sheet at a first zone remote from the hanger bar when the bridge is in its lower position and at a second zone intermediate the first zone and the hanger bar when in its upper position.

According to a third aspect of the present invention there is provided, in an electro winning or electro refining installation, a method of operating a cathode construction which comprise a hanger bar, a starter sheet extending downwardly from the hanger, bar, and an electrically conductive bridge in electrical contact with the hanger bar and with the starter sheet below the hanger bar, the bridge having a upper position with respect to the hanger bar and a lower position with respect to the hanger bar, the bridge contacting the starter sheet at a first zone remote from the hanger bar when the bridge is in the lower position and at a second zone intermediate the first zone and the hanger bar when in its upper position, the method comprising commencing current flow through the cathode to deposit metal on the starter sheet with the bridge in its upper position, and thereafter moving the bridge down to its second position so that it contacts the deposited metal and provides a current flow path to the deposited metal through the hanger bar and the bridge. According to a fourth aspect of the present invention there is provided, in an electro winning or electro refining installation, a method of operating a cathode construction which comprise a hanger bar, a starter sheet extending downwardly from the hanger bar, and an electrically conductive bridge in electrical contact with the hanger bar and with the starter sheet below the hanger bar, the bridge having a upper position with respect to the hanger bar and a lower position with respect to the hanger bar, the bridge contacting the starter sheet at a first zone remote from the hanger bar when the bridge is in the lower position and at a second zone intermediate the first zone and the hanger bar when in its upper position, the method comprising coating the bridge with a material which erodes away in the electrolyte and which is electrically non-conductive and commencing current flow through the cathode to deposit metal onto the starter sheet with the bridge in its lowermost position.

According to fifth aspect of the present invention there is provided a bridge for fitting to an electro winning or electro refining cathode comprising a hanger bar and a starter sheet extending downwardly from the hanger bar, the bridge including an inner component of electrically conductive material which defines a cavity for receiving the hanger bar, a slot extending downwardly from the cavity and through which, in use, the starter sheet passes, and a pair of spaced apart wall portions below said cavity, and the outer component being of resilient metal and constituting a spring clip which forces said wall portions towards one another so that, in use, they grip the starter sheet between them.

According to a sixth aspect of the present invention there is provided an electro winning or electro refining installation comprising walls defining a tank, electrolyte in the tank, power supply rails extending along the top surfaces of said walls, an anode including a hanger bar supported by said rails, the anode being partly immersed in said electrolyte, a cathode including a hanger bar supported by said rails, the cathode further including a starter sheet which extends down from the hanger bar and is partly immersed in said electrolyte, and an electrically conductive bridge in electrical contact with the hanger bar and with the starter sheet below said hanger bar, the bridge having lower position in which it is partly immersed in the electrolyte and an upper position in which it is above the surface of the electrolyte in the tank. According to a seventh aspect of the present invention there is provided a cathode for use in the electrolyte containing tank of an electro refining or electro winning plant, the cathode comprising an electrically conductive hanger bar and a metal plate, the hanger bar, in use, being horizontal and forming the electrical connection to the cathode, and the metal plate, in use, hanging down from the underside of the hanger bar and being partly immersed in the electrolyte in the tank, there being an insert in the metal of the plate and there being a bridge both of which are of a metal of greater conductivity than that of the plate, the bridge being electrically connected to the hanger bar and having a first retracted position and a second operative condition in which it extends downwardly below the hanger bar and below the upper edge of the insert, the arrangement being such that, in use, with the bridge retracted current initially flows from the hanger bar through the plate to the electrolyte, and after metal being recovered or refined is deposited on the insert and the bridge is in its operative condition, flows through the bridge and the insert to the deposited metal layer and then to the electrolyte.

According to an eighth aspect of the present invention there is provided a method of operating a cathode in an electro winning or electro refining plant which comprises providing a metal starter plate which forms part of the cathode with an insert of a metal of greater electrical conductivity than the plate, electrolytically depositing metal on the metal plate of the cathode, the layer covering the lower portion of the insert, and thereafter electrically connecting a hanger bar of the cathode to said insert and hence to said deposited layers to bypass said metal plate. According to a ninth aspect of the present invention there is provided a composite cathode for use in electro winning and electro refining, the cathode comprising a horizontal hanger bar of copper or copper alloy and a starter sheet, the bar, in use, being horizontal and forming an electrical connection to said sheet which hangs down from the bar, there being a bridge of copper or copper alloy which is electrically insulated from said bar and electrically connected to said plate.

In one form said bar has a slot in it with an electrically insulating sleeve in said slot, said bridge being in said sleeve and depending from the bar.

Said bridge can protrude upwardly above said bar and extend in both directions parallel to said bar, end portions of the bridge forming the electrical connections to the bridge. According to a tenth aspect of the present invention there is provided a cathode construction comprising a hanger bar, a starter sheet hanging down from the hanger bar, a bridge of copper or copper alloy secured to the hanger bar and having parts which extend down adjacent both sides of the starter sheet but spaced therefrom, and holes in said parts of the bridge placing the spaces between the bridge and the sheet in communication with the exterior of the bridge.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the

accompanying drawings in which;-

Figure 1 is a pictorial view of a cathode bridge; Figure 2 is a side elevation of the bridge of Figure 1 ,

Figure 3 is an end elevation of the bridge of Figure 1 , Figure 4 is a pictorial view of a cathode construction including a cathode and the bridge of Figures 1 to 3 fitted to the cathode;

Figure 5 is a view similar to that of Figure 4 but with the bridge in a different position with respect to the cathode;

Figures 6, 7 and 8 illustrate one method of operating an electro winning or electro refining installation; Figures 9 and 10 illustrate another method of operating an electro winning or electro refining installation;

Figures 1 1 and 12 are pictorial views illustrating a further form of cathode construction including a cathode and a bridge;

Figure 13 is an elevation of a cathode construction including a cathode and two bridges; Figure 14 is a pictorial view of the cathode and bridges of Figure 13; Figure 15 is a section on the line XV - XV of Figure 13; Figure 16 is an elevation of another form of cathode; Figure 17 is a section, to a larger scale, on the line XVII - XVII of Figure 16;

Figure 18 is a pictorial view of the components of a further cathode construction before assembly;

Figure 19 is a pictorial view of a cathode construction assembled from the components of Figure 18;

Figure 20 is an elevation similar to Figure 16 and illustrating a modified form of the cathode construction;

Figure 21 is a section on the line XXI - XXI of Figure 20; and

Figure 22 is a section similar to that of Figure 21 and illustrating a different form of the connection between an insert and a hanger bar.

DETAILED DESCRIPTION OF THE DRAWINGS

The cathode bridge illustrated in Figures 1 to 3 is designated 10 and comprises an inner component 12 of copper or a copper alloy and an outer component 14 of stainless steel. The component 12 can be formed by bending a sheet of copper or copper alloy and the component 14 can be formed by bending a sheet of stainless steel.

The inner component 12 comprises two side walls 16 joined along their upper edges by a horizontal top wall 18. The parts of the side walls 16, designated 20, immediately below the wall 18, define a cavity designated 22. The cavity 22 is vertically and horizontally elongate. Its lower end is defined by two sloping flanks 24 which form downward and inward continuations of the wall parts 20.

The inner edges 26 of the flanks 24, which are also the lower edges of the flanks 24, are spaced apart. The flanks 24 are extended downwardly by two vertical wall portions 28 which bound a slot 30. The upper edges of the portions 28 join the flanks along the edges 26. At their lower ends the wall portions 28 are bent outwardly at right angles to form two horizontal oppositely directed flanges 32. The bends designated 34 between the wall portions 28 and the flanges 32 are shaped so as to define the entrance 36 to the slot 30. The outer component 14 comprises two vertical walls 38 which are joined along their upper edges by a horizontal wall 40. A lifting eye 42 is welded or otherwise secured to the top surface of the wall 40.

The outer component 14 also comprises two box structures 44 (see particularly Figure 3). To create each of these structures the sheet of stainless steel is bent along the three parallel lines designated 46, 48 and 50. From Figure 3 it will also be seen that the box structures each comprise the lowermost part of each wall 38, a horizontal bottom wall 52, an inner vertical wall 54 parallel to the wall 38, and an upper horizontal wall 56. The box structures 44 fit between the flanks 24 and the flanges 32. The walls 38 are immediately adjacent the wall parts 20 and outwardly thereof. The wall 40 is above the wall 18. The length of the component 12 is greater than that of the component 14 as can best be seen in Figure 2.

Turning now to Figures 4 and 5, the cathode illustrated is designated 58 and comprises a rectangular starter sheet 60 to the upper edge of which a hanger bar 62 is welded or otherwise secured. The end portions 64 of the hanger bar 62 protrude beyond the vertical edges of the starter sheet 60. The hanger bar 62 is of copper or a copper alloy and the starter sheet is usually of stainless steel or titanium. Copper and copper alloys are less frequently used as starter sheet materials. The bridge 10 has an upper position with respect to the cathode (Figure 4) and a lower position with respect to the cathode (Figure 5).

The starter sheet 60, immediately below the hanger bar 62, has two horizontally spaced openings 66. Strips 68 of an electrolyte resistant, electrically insulating synthetic plastics material are fitted to the vertical edges of the sheet 60 to prevent deposition of metal on the vertical edges of the sheet 60. The strips 68 are shown in Figures 4 and 5 but not in Figures 6 to 10.

The operating sequence illustrated in Figures 6 to 8 will now be described. The cathode is returned to the electrolyte tank after stripping with the bridge 10 in the upper position as shown in Figures 4 and 6. The hanger bar 62 is at the lower end of the cavity 22 and in close electrical contact with the parts 20 of the walls 16. The starter sheet 60 emerges from the bridge 10 through the slot 30. The electrolyte level is as shown at EL in Figures 4 and 5 and the bridge 10 is consequently not in contact with the electrolyte when in the position shown in Figure 4.

The cathode is, in use, positioned between two anodes which are also immersed in the electrolyte in the tank.

The current flow, whilst the bridge 10 is in its upper position, is from an external circuit to the electrolyte through the anodes, through the electrolyte, the starter sheet 60 and the hanger bar 62 to back to the external circuit. Current flow in the electrolyte causes metal ions to migrate in the electrolyte to the starter sheet 60 where they are deposited on both sides of the starter sheet.

After a number of hours, the deposited metal forms layers on each side of the starter sheet 60 up to the level EL. Once the starter sheet is covered with deposited layers on each side, the bridge 10 is pressed down to the position shown in Figures 5 and 7. The hanger bar 62 is now at the upper end of the cavity 22 and the starter sheet 60 extends down through the cavity 22 and emerges through the slot 30. The bridge 10 now extends to a level below the electrolyte level EL. A substantial portion of the current consequently flows from the electrolyte to the deposited metal layers and from these layers through the low resistance component 12 to the hanger bar 62. The electrical resistance offered by the cathode installation comprising the bridge 10 and the cathode 58 is lower than that offered by the cathode 58 in the absence of the bridge 10. Deposition continues until the deposited layers are of sufficient thickness to require cathode stripping. Using the eye 42, the bridge 10 is lifted to the position shown in Figure 8. The bridge 10 is now above that part of the sheet 60 which is bent and scraped during stripping. The cathode construction comprising the cathode and the bridge is raised by a gantry having lifting elements which enter the openings 66 and transported to the stripping machine.

It is also possible for the bridge to be in the lower position shown in Figures 5 and 9 when the cathode is returned to the tank after stripping. The bridge 10 consequently dips into the electrolyte. To prevent current flow preferentially being through the bridge 10 and not through the starter sheet 60, the bridge is coated with an electrically semi- conductive or electrically non-conductive material such as silicone which erodes away during the initial stages of electro wining or electro refining. The coating initially prevents preferential current flow through the bridge directly to the electrolyte. Once the deposited metal layers are in place and the coating on the bridge has been eroded, current flow is predominately through the bridge bypassing the highly resistive metal of the starter sheet. Once metal layers of sufficient thickness to require stripping have been deposited, the bridge 10 is raised using the lifting eye 42 to the position shown in Figure 10. The cathode construction comprising the bridge 10 and cathode 58 is then removed from the tank for stripping of the deposited metal.

After stripping of the cathode starter sheet, the bridge 10 is pressed back to the position shown in Figure 9, re-coated and the cathode construction is returned to the electrolyte tank. The bridge illustrated in Figures 1 to 3 is slid onto the hanger bar of the cathode from one end. If lifting eyes protrude upwardly from the top surface of the hanger bar, these prevent the bridge being attached by sliding it on. To enable bridges to be fitted to such hanger bars, the component 12 is fabricated in two parts. Each part includes a side wall 16 and a top wall 18. The component 14 is also in two parts. Each part includes a side wall 38, a top wall 40 and a box structure 44. The two parts of the components 12 and 14 are placed around the hanger bar with the top walls of the two parts of the inner component overlapping and the top walls of the two parts of the outer component overlapping. The four stacked top walls are then secured together by screws, studs or nuts and bolts.

Referring now to Figures 1 1 and 12, the cathode illustrated is designated 70 and comprises a hanger bar 72 and a metal starter sheet 74 which, in use, hangs down from the hanger bar 72. The protruding ends of the bar 72 rest on the electrical conductor rails which extend along the top edges of the walls of the tank.

A horizontally elongate insert 76 of copper or a copper alloy is welded or riveted into a horizontal slot in the sheet 74. Openings 78 in the upper part of the sheet 74 receive the lifting hooks (not shown) of the overhead gantry used to raise the cathode construction and carry it to the stripping machine.

A bridge 80 is shown in Figures 1 1 and 12 and comprises two copper or copper alloy strips 82 which are connected together by a clip 84. The clip 84 is of a resilient material such as spring steel or stainless steel and is of inverted U-shape in cross section.

The upper surface of each strip 82 is also formed with a slot 86 for receiving one of the limbs of the clip 84. Each strip 82 is also formed with two horizontal grooves 88 and 90 separated by a protrusion 92. During assembly the strips 82 with the clip 84 attached are forced down on the hanger bar 72 and plate starter sheet 74. The hanger bar 72 is initially in the lower groove 88 as shown in Figure 1 1.

The cathode is placed in the tank in the condition shown in Figure 1 1 . Current flows through the cell constituted by the illustrated cathode construction, two lead anodes and the electrolyte in the tank. The electrolyte level is shown by the line EL in Figures 1 1 and 12. After a period of operation a layer of copper is deposited onto each side of the sheet 74 and covers the lower part of the insert 76. The bridge is then pressed down so that the bar 72 encounters the protrusions 92 and pushes the strips 82 apart against the action of the clip 84. The bar 72 enters the channel constituted by the facing grooves 90. The faces of the strips 82 are pressed towards one another by the clip 84. The bridge 80 forms an electrical connection between the hanger bar 72 and the insert 76, and hence between the hanger bar 72 and the deposited metal layers on the sheet 74. The insert 76 can initially be coated with silicon to limit deposition of copper onto the insert. The coating erodes away before the bridge is pressed down to its lower position.

The cathode 94 of Figures 13 to 15 comprises a hanger bar 96 and a starter sheet 98. The hanger bar 96 is welded or riveted to the top edge of the sheet 98. Openings 100 in the sheet 98 immediately below the bar 96 enable the cathode to be lifted by the gantry.

Strips 102 of synthetic plastic materials prevent copper being deposited along the edges of the sheet 98.

Two bridges 104 are shown attached to the hanger bar 96.

Each bridge 104, see particularly Figure 15, comprises two copper or copper alloy strips 106 which lie one on each side of the hanger bar 96. A resilient clip 108 pressed down on the strips 106 forces the strips 106 together and holds them in place on the cathode. The clip 108 can be of metal or synthetic plastics material.

Each strip 106 has a hole 1 10 in it close to its lower edge. The holes place the spaces inside the bridge and between the bridge and the sheet in communication with the exterior of the bridge.

The electrolyte level is as shown at EL in Figure 13. The level is above the holes 110 and below the openings 100.

During the initial period for which electrolysis is taking place, deposition occurs in the holes 1 10 as well as onto the plate 98. Eventually there is a direct connection between the strips 106 and the layers of metal deposited on the plate 98 by way of the copper deposited in the holes 110.

During stripping the clips 108 are broken and new bridges are fitted before the cathodes are returned to the tank.

Turning now to Figures 16 and 17, these show a cathode 1 12 which comprises a hanger bar 1 14 and a metal starter sheet 1 16. Lifting openings in the starter sheet are designated 1 18.

The upper edge of the sheet 1 16 is configured to provide an upwardly open recess 120 which has two parallel, vertical, horizontally spaced edges 122 and a lower edge 124 which is castellated. This provides alternating grooves 126 and ribs 128 along the lower edge 124 of the recess 120.

An insert 130 of copper or copper alloy is secured into the recess 120. The shape of the insert 130 matches that of the recess 120. Mechanical means such as rivets and studs can be used to attach the upper part of the insert 130 to the sheet 1 16. Flat metal strips can be secured to the sheet 1 16 and to the insert 130 to secure the insert to the sheet. Alternatively, the insert 130 can be welded into the recess 120. In a further embodiment the grooves 126 and ribs 128 are omitted and the insert 130 has four straight edges and is rectangular.

The electrolyte level crosses the grooves 126 and ribs 128 so that the ribs are at least partly immersed in the electrolyte.

During stripping the sheet 1 16 is bent. Because the lower edge of the insert 130 is not attached to the sheet, the lower part of the insert does not bend with the sheet and there is relative movement between them. This causes deposited metal to break away from the insert. The bond between the copper alloy insert and the deposited copper is weak enough to permit this to happen.

In the cathode construction of Figures 18 and 19 the cathode is designated 132, the hanger bar 134, the bridge 136 and the starter sheet 138. The bar 134 has a rectangular slot 140 cut in it and a rectangular sleeve 142 of electrically insulating material is pressed into the slot 140. The upper end of the bridge 136 fits in the sleeve 142 and is consequently electrically isolated from the bar 134.

The upper edge of the bridge 136 is slightly above the upper edge of the sleeve 142 as shown in Figure 19. In a modified form that has not been illustrated the bridge 136 is T- shaped. The cross bar of the T extends parallel to, and above, the bar 134, to positions beyond both ends of the bar 134.

The lower edge of the bridge 136 is formed with a rectangular section groove 144. The upper edge of the sheet 138 has a recess 146 in it which conforms in shape to that of the bridge 136. The horizontal edge 148 of the recess 146 fits in the groove 144 and the edges 150 of the sheet 138 are welded to the underside of the bar 134. The underside of the bar 134 can have a groove in it for receiving the edges 150 of the sheet 138 or can be flat as shown in Figure 18.

In conventional cathodes the stainless steel sheet 138 is secured directly to the copper hanger bar 134. Current consequently has to flow through the steel plate to the electrolyte. The resistivity of stainless steel is about thirty four times greater than that of copper and this represents a significant loss of power.

It is necessary, in the period immediately after the cathode is immersed in the electrolyte, to prevent current flow being preferentially through the copper bridge. If this is allowed it has the effect of depositing the copper almost entirely onto the bridge and not onto the steel sheet. Once layers of copper are deposited on both sides of the starter sheet, current flow bypasses the starter sheet and this leads to efficient operation.

When the cathode construction as shown in Figures 18 and 19 is used, current can initially flow through the bar 134 directly to the steel sheet 138. Once a layer of copper has been deposited onto each face of the sheet 138 sufficient to carry the requisite current, supply is switched from the bar 134 to the bridge 136. The sheet 138 no longer forms part of the electrical flow path and current flows through the bridge 136 and into the copper layers on the sheet 138.

It is also possible to prevent differential deposition on the bridge by coating the bridge with an electrically non- conductive material which is etched away by the electrolyte over a period of time. This time is sufficient to prevent deposition on the bridge occurring until sufficient copper has been deposited onto the steel sheet to carry the current and thereby prevent excessive deposition on the bridge.

The cathode construction of Figures 20 and 21 is similar to that of Figures 16 and 17 and like parts have been designated with like reference numerals. As best seen in Figure 21 , the hanger bar 1 14 has a slot 152 in it and the insert 130 is in this slot.

Connectors 154, only shown as a centre line CL in Figure 21 , pass through the hanger bar 1 14 and the insert 130 to secure the insert to the hanger bar. There is thus a mechanical as well as an electrical connection between the hanger bar 1 14 and the insert 130.

There is no mechanical connection between the insert 130 and the starter sheet 1 16. Hence when the sheet 1 16 is bent during stripping, the insert 130 does not bend with it and such relative movement ensures that the deposited copper which bridges between the insert 130 and the starter sheet 1 16 breaks away.

It is convenient for the insert to be a snug fit in the recess provided for it and for there consequently to be an electrical connection between the insert and the starter sheet.

In Figure 22 the insert, designated 156, is bent twice at 158, 160 so that it fits snugly around the hanger bar 1 16. The centre lines of the connectors are again shown at CL. The insert 156 is not connected to the sheet 1 16.

Claims

CLAIMS:

1. A cathode construction for use in electro winning and electro refining and

comprising a horizontal hanger bar, a starter sheet attached to the hanger bar and extending downwardly from the hanger bar, an insert secured to the hanger bar and located in a recess in the upper edge of said sheet, the insert being of a metal of greater electrical conductivity than the sheet and being in electrical contact with the hanger bar, the sheet, when bent during stripping, moving with respect to said insert.

2. A cathode construction as claimed in claim 1 , wherein the insert is mechanically secured to the hanger bar but not to the starter sheet.

3. A cathode construction as claimed in claim 2, wherein said insert is secured in a slot formed in the hanger bar.

4. A cathode construction as claimed in claim 2, wherein said insert is shaped so that it fits around the hanger bar and is secured to a side surface of the hanger bar.

5. A cathode construction as claimed in any preceding claim, wherein said recess has a lower edge which is formed with ribs and grooves and the lower edge of the insert has corresponding ribs and grooves which mesh with the ribs and grooves of said lower edge.

6. A cathode construction for use in electro winning and electro refining which

comprises a horizontal hanger bar, a starter sheet extending downwardly from the hanger bar and an electrically conductive bridge in electrical contact with the hanger bar and with the starter sheet below the hanger bar, the bridge having a upper position with respect to the hanger bar and a lower position with respect to the hanger bar, the bridge contacting the starter sheet at a first zone remote from the hanger bar when the bridge is in its lower position and at a second zone intermediate the first zone and the hanger bar when in its upper position.

7. In an electro winning or electro refining installation, a method of operating a

cathode construction which comprise a hanger bar, a starter sheet extending downwardly from the hanger bar, and an electrically conductive bridge in electrical contact with the hanger bar and with the starter sheet below the hanger bar, the bridge having a upper position with respect to the hanger bar and a lower position with respect to the hanger bar, the bridge contacting the starter sheet at a first zone remote from the hanger bar when the bridge is in the lower position and at a second zone intermediate the first zone and the hanger bar when in its upper position, the method comprising commencing current flow through the cathode to deposit metal on the starter sheet with the bridge in its upper position, and thereafter moving the bridge down to its second position so that it contacts the deposited metal and provides a current flow path to the deposited metal through the hanger bar and the bridge.

8. In an electro winning or electro refining installation, a method of operating a

cathode construction which comprise a hanger bar, a starter sheet extending downwardly from the hanger bar, and an electrically conductive bridge in electrical contact with the hanger bar and with the starter sheet below the hanger bar, the bridge having a upper position with respect to the hanger bar and a lower position with respect to the hanger bar, the bridge contacting the starter sheet at a first zone remote from the hanger bar when the bridge is in the lower position and at a second zone intermediate the first zone and the hanger bar when in its upper position, the method comprising coating the bridge with a material which erodes away in the electrolyte and which is electrically non- conductive and commencing current flow through the cathode to deposit metal onto the starter sheet with the bridge in its lowermost position.

9. A bridge for fitting to an electro winning or electro refining cathode comprising a hanger bar and a starter sheet extending downwardly from the hanger bar, the bridge including an inner component of electrically conductive material which defines a cavity for receiving the hanger bar, a slot extending downwardly from the cavity and through which, in use, the starter sheet passes, and a pair of spaced apart wall portions below said cavity, and the outer component being of resilient metal and constituting a spring clip which forces said wall portions towards one another so that, in use, they grip the starter sheet between them.

10. An electro winning or electro refining installation comprising walls defining a tank, electrolyte in the tank, power supply rails extending along the top surfaces of said walls, an anode including a hanger bar supported by said rails, the anode being partly immersed in said electrolyte, a cathode including a hanger bar supported by said rails, the cathode further including a starter sheet which extends down from the hanger bar and is partly immersed in said electrolyte, and an electrically conductive bridge in electrical contact with the hanger bar and with the starter sheet below said hanger bar, the bridge having lower position in which it is partly immersed in the electrolyte and an upper position in which it is above the surface of the electrolyte in the tank.

1 1. A cathode for use in the electrolyte containing tank of an electro refining or

electro winning plant, the cathode comprising an electrically conductive hanger bar and a metal plate, the hanger bar, in use, being horizontal and forming the electrical connection to the cathode, and the metal plate, in use, hanging down from the underside of the hanger bar and being partly immersed in the electrolyte in the tank, there being an insert in the metal of the plate and there being a bridge both of which are of a metal of greater conductivity than that of the plate, the bridge being electrically connected to the hanger bar and having a first retracted position and a second operative condition in which it extends downwardly below the hanger bar and below the upper edge of the insert, the arrangement being such that, in use, with the bridge retracted current initially flows from the hanger bar through the plate to the electrolyte, and after metal being recovered or refined is deposited on the insert and the bridge is in its operative condition, flows through the bridge and the insert to the deposited metal layer and then to the electrolyte.

12. A method of operating a cathode in an electro winning or electro refining plant which comprises providing a metal starter plate which forms part of the cathode with an insert of a metal of greater electrical conductivity than the plate, electrolytically depositing metal on the metal plate of the cathode, the layer covering the lower portion of the insert, and thereafter electrically connecting a hanger bar of the cathode to said insert and hence to said deposited layers to bypass said metal plate.

13. A composite cathode for use in electro winning and electro refining, the cathode comprising a horizontal hanger bar of copper or copper alloy and a starter sheet, the bar, in use, being horizontal and forming an electrical connection to said sheet which hangs down from the bar, there being a bridge of copper or copper alloy which is electrically insulated from said bar and electrically connected to said plate.

14. A composite cathode as claimed in claim 13, wherein said bar has a slot in it with an electrically insulating sleeve in said slot, said bridge being in said sleeve and depending from the bar.

15. A composite cathode as claimed in claim 13 or 14, wherein said bridge protrudes upwardly above said bar and extends in both directions parallel to said bar, end portions of the bridge forming the electrical connections to the bridge.

16. An electro winning or electro refining installation including a tank and composite cathodes as claimed in claim 13, 14 or 15, a first current supply path to said bar, a second current supply path to said bridge, and means for switching current between said supply paths.

17. A cathode construction comprising a hanger bar, a starter sheet hanging down from the hanger bar, a bridge of copper or copper alloy secured to the hanger bar and having parts which extend down adjacent both sides of the starter sheet but spaced therefrom, and holes in said parts of the bridge placing the spaces between the bridge and the sheet in communication with the exterior of the bridge.