FIELD OF THE INVENTION THIS INVENTION relates to electrowinning.

BACKGROUND TO THE INVENTION

Electrowinning is a method used to recover metals such as copper and manganese. Ore containing the metal is ground and then dissolved in an electrolyte. An electrowinning installation comprises a tank which contains the electrolyte and which has in it electrodes in the form of cathodes and anodes. The tank with anodes and cathodes in it is referred to as an electrowinning cell. Each cathode comprises an electrolyte resistant metal plate. Suitable metals are stainless steel and titanium. Each anode comprises a plate of lead and the anodes and cathodes alternate along the length of the tank. Hanger bars support the cathode and anode plates and connect the plates to the electrical supply. Current flows though the cell causing the ions of the metal which is to be recovered to be deposited on the cathodes. The cathodes are periodically stripped of their deposited metal coatings. The cathodes deteriorate over time but at a slow rate. The anodes deteriorate at a faster rate than the cathodes and have to be replaced more frequently. Factors which influence the efficiency of the cell are current density in the electrolyte and the variation in current density from the upper parts of the plates to their lower parts. Current density decreases with increasing depth from the surface of the electrolyte. The present invention provides cathodes and anodes which, when used in a cell, decrease the variation of current density from the top of the cell to the bottom of the cell and provide a significantly higher current density. BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the present invention there is provided an electrode for an electrowinning cell which comprises a hanger bar for supporting the electrode and constituting the electrical connection to the electrical supply and a plate depending from the hanger bar, there being a slot in the electrode extending from its top edge, through the hanger bar and into said plate, the slot terminating adjacent the lower edge of the plate and dividing the plate into two plate sections, the sections being joined by a part of the plate below the slot which constitutes a bridge. The slot can have therein electrically insulating material which prevents the slot from closing up.

Said material can be a settable compound or a strip of synthetic plastics material. Said plate can be of a material such as stainless steel which resists attack by electrolyte in which form the electrode is a cathode. If the plate is of lead the electrode is an anode.

According to a further aspect of the present invention there is provided an electrode for an electrowinning cell which comprises a hanger bar for supporting the electrode from the walls of the tank of the cell and constituting the electrical connection to the electrical supply and a plate depending from the hanger bar, there being a slot in the electrode extending from its top edge, through the hanger bar and into said plate, the slot terminating adjacent the lower edge of the plate and dividing the plate into two plate sections, the sections being joined by a part of the plate below the slot which constitutes a bridge, there further being a first channel shaped element of electrically insulating material fitted to the top edge of said electrode and bridging said slot and a second channel shaped element of metal which is fitted over the first element and clamped to it to secure it in place.

When the plate is of lead, and the electrode is consequently an anode, an element of greater electrical conductivity than lead can be embedded in the lead which constitutes the bridge.

The first and second elements can each comprise a web and two flanges, there being a slit through each of said webs, said slits being in register with one another.

A metallic bridging element is provided which can be inserted said slits so as electrically to connect the parts of the hanger bar which are on opposite sides of said slot.

The invention also provides a method of installing electrodes as defined in the preceding paragraph which comprises placing them in a cell, making the electrical connections to the electrodes and thereafter removing said bridging elements.

According to another aspect of the present invention there is provided a comb-type anode which comprises a hanger bar, a bottom bar and a plurality of columns extending from the hanger bar to the bottom bar, there being a slot in said hanger bar which divides the hanger bar into two separate parts, first and second groups of columns respectively depending from said parts of the hanger bar and being connected to one another by way of said bottom bar.

An element of greater conductivity than lead can be embedded in said bottom bar. A first channel shaped element of electrically insulating material can be fitted to the hanger bar so as to bridge said slot and a second channel shaped metallic element can be fitted over the first element and clamped to it.

The first and second elements can each comprise a web and two flanges, there being a slit through each of said webs, said slits being in register with one another.

A metallic bridging element is provided which can be inserted into said slits so as electrically to connect the parts of the hanger bar which are on opposite sides of said slot.

The invention also provides a method of installing electrodes as defined in the preceding paragraph which comprises placing them in a cell, making the electrical connections to the electrodes and thereafter removing said bridging elements.

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 diagrammatic pictorial view of an electrowinning electrode in the form of an anode;

Figure 2 is a pictorial view of an electrode comprising a plate and hanger bar; Figure 3 is an "exploded" pictorial view of the components of a cathode; Figure 4 illustrates two cathodes and an anode forming an electrowinning cell; Figure 5 is a pictorial view of a further form of anode; Figure 6 is a pictorial view of the lower part of an anode; and

Figure 7 is a pictorial view of a rolled anode. DETAILED DESCRIPTION OF THE DRAWINGS

The anode 10 shown in Figure 1 comprises a hanger bar 12, a bottom bar 14 and vertically extending parallel cylindrical columns 16 spanning between the hanger bar 12 and the bottom bar 14. The bottom bar 14 and the columns 16 are of lead or a lead alloy. The hanger bar 12 comprises a core of a metal such as copper which is coated with lead to protect it from the electrolyte in the electrowinning tank. The level of the electrolyte is as shown at L. An exposed portion 18 of the copper core at one end of the hanger bar 12 rests on the rail (not shown) which constitutes the positive of the two supply rails which run along the walls of the electrowinning tank. The term "lead" as used herein includes a lead alloy.

The hanger bar 12 has a slot 20 through it so that it is constituted by two hanger bar parts 12.1 and 12.2 which are electrically isolated from one another. The anode 10 illustrated has ten columns 16 spanning between the bars 12 and 14.

The slot 20 divides the columns 16 into two groups of five columns. The first column group is connected directly to the positive supply rail by way of the hanger bar part 12.2. The second group of five columns 16 is connected to the supply rail by way of the bottom bar 14 and the columns 16 of the first group of five columns. The slot 20 is filled with an electrically insulating compound such as that sold under the trade mark SIKAFLEX. Alternatively, an element of synthetic plastics material can be inserted into the slot. This is to prevent the slot closing up due to the weight of the anode whilst the anode is in use.

An inverted channel 24 of electrically insulating material is pressed onto the hanger bar 12 and spans across the slot 20. A metal clamp 26, also of inverted channel shape, is forced over the channel 24 and closed-up so that it tightly grips the channel 24 and prevents relative movement between it and the hanger bar parts 12.1 and 12.2.

The insulating channel 24 and the clamp 26 each have two flanges and a web which joins the flanges. The webs of the channel 24 and clamp 26 are formed with registering slits 28 and 30 respectively.

A conductive element 32 is provided which can be pushed through the slits 28 and 30 and into upper part of the slot 20 to form a temporary electrical connection between the hanger bar parts 12.1 and 12.2. The reason for this will be described below. As stated above the columns 16 and the bottom bar 14 are of lead. To reduce the electrical resistance between the two groups of columns, a bar of a metal which has better conductivity than lead, such as copper, can be embedded in the lead of the bottom bar 14. It is also possible to construct the core of the hanger bar part 12.1 using a material which is less expensive than copper even if it has a high electrical resistance. It will be understood that there is minimal current flow through the part 12.1 . The part 12.2 remains as lead encased copper. The electrode 34 shown in Figure 2 comprises a plate 36 which has its upper edge secured to a hanger bar 38. The plate 36 and hanger bar 38 are formed with a slot 40. The slot 40 passes through the hanger bar 38 and divides it into two hanger bar parts 38.1 and 38.2.

The slot 40 terminates close to the lower edge of the plate 36 and divides the plate 36 into a first plate part 36.1 and a second plate part 36.2 which are joined by a bridge 42 below the slot 40. If the plate 36 is of stainless steel or titanium then the electrode 34 is a cathode. If the plate 36 is a lead plate then Figure 2 illustrates an anode.

Turning now to Figure 3, this illustrates the electrode 34 of Figure 2 and components designated 24.1 , 26.1 and 32.1 which are identical to the components 24, 26 and 32 of Figure 2. The plate 36 is a stainless steel plate and hence a cathode is illustrated in

Figure 3.

The slot 40 has a synthetic plastics material strip 44 in it to prevent the slot 40 closing up, due to the weight of the cathode, when the cathode is hanging from the walls of the cell. The free edges of the plate 36 are inserted into synthetic plastic material channels

46 to prevent metal being deposited around the edges of the plate.

Turning now to Figure 4 this shows two cathodes 48 of the form illustrated in Figure 2 which are on either side of an anode 50. Both cathodes 48 and the anode 50 are provided with sets of bridging components of the form shown in Figures 1 and 3.

However, only one set of such components is shown. When the anode and cathodes shown in Figure 4 are installed in the cell, the elements 32.1 are in place. Consequently the anode and cathodes do not, in respect of the current flow through them, differ from standard unslotted anodes and cathodes. Once all the anodes and cathodes of a cell have been installed, the elements 32.1 are all removed. Current flow in the cathodes and anodes is then from one hanger bar part, through one plate section, through the bridge below the slot to the other plate section. The elements 32.1 are required because if they are not present during installation an excessive current flow results.

Anodes of the form shown in Figure 1 , and the intervening cathode plates of steel or titanium, are installed in the same way as just described with reference to Figure 4. The elements 32 are then removed. The anode 52 shown in Figure 5 is of the same construction as that illustrated in Figure

2 except in that it has a mild steel block 54 between the hanger bar parts 38.1 and 38.2. The copper and mild steel cores of the hanger bar parts 38.1 and 38.2 touch opposite sides of the block 34. The block 54 protrudes from the lead which encapsulates the cores so that there is no bridge of lead between the lead encapsulating the copper core and the lead encapsulating the mild steel core. The electrical resistance of mild steel is many times greater than that of either copper or lead and this inhibits current flow directly from one hanger bar part to the other.

The anode is manufacture by first assembling the hanger bar. A mild steel dowl or a threaded pin has its ends in bores at the ends of the hanger bars and passes through the block 54. The centre line of the pin is coincident with the centre line the hanger bar, and the purpose of this is to provide a hanger bar structure which can be suspended in a mould. Once the hanger bar structure is in the mould and the mould closed, the entire hanger bar structure, apart from the edges of the block 54, is encapsulated in lead as the plate 36 is cast.

A synthetic plastics material cover 56 is shown in Figure 6. This is of channel section. The flanges of the channel are on each side of the plate 36 and cover the bridge 42.

The web of the cover 56 is below the plate 36.

An acid resistant adhesive can be used to secure the cover 56 to the anode plate 36. The cover 56 protects the bridge 42 from corrosion by the electrolyte. The excessive corrosion rate at the bridge results from the high current flow in the bridge.

To reduce the electrical resistance of the bridge, a metal insert of greater electrical conductivity than lead can be embedded in the lead which constitutes the centre part of the bottom bar 14 (Figure 1 ). Likewise a metal insert can be embedded in the bridge 42 (Figure 2) when the illustrated electrode in an anode. The insert can be embedded in the lead during casting if the anode is a cast anode. If the anode is a rolled anode then a slit can be cut in its lower edge, the insert hammered in to the slit and then the slit closed up by the procedure known as lead burning. The anode 58 of Figure 7 is a rolled anode. In this form the hanger bar comprises two metal cores 60.1 and 60.2 which are secured by pins 62 to the lead plate 64. Only the pins 62 securing the left hand core 60.1 to the plate 64 are visible in Figure 7, the right hand core being within the lead sheath designated 66. A block, which is not shown but is as illustrated at 54 in Figure 5, is provided between the cores 60.1 and 60.2.

To inhibit ingress of corrosive electrolyte between the sheath 66 and the core parts, covers 68 with openings in them for receiving the cores can be pressed onto the protruding ends of the core parts. A sealant such as SIKAFLEX can be used to seal-off any gaps between the cover, the core and the sheath.

Experimental work has shown that the current density in the electrolyte does not display the same diminishing top to bottom decrease of conventional unslotted plates. The current density decreases far less over the heights of the plates.