BACKGROUND TO THE INVENTION

In the recovery of minerals such as copper, electrowinning and electrorefining methods are used. In the first method the copper ore is crushed and then treated with acid so that the metal dissolves in the acid. The metal ions are recovered by using the acid as the electrolyte in a tank in which there are alternate anodes and cathodes that are immersed in the electrolyte. Current flowing through the electrolyte deposits the metal ions as layers on the surfaces of the cathodes.

In electrorefining the copper or other metal to be recovered is cast into the form of impure anodes, and these are immersed in the electrolyte. Current flowing through the electrolyte in the tank between the anodes and the cathodes erodes away the anodes and deposits the metal ions as layers on both surfaces of the cathodes.

Gasing is an inevitable consequence of these processes. The bubbles evolved rise to the surface of the electrolyte and then burst. The result is a mist of electrolyte which rises into the air above the tank. It is known to place hollow, neutral bouyancy balls in the tanks. The balls are generally in two or more layers with the upper balls protruding just above the upper surface of the electrolyte and the lower balls submerged. It is known that the parts of the electrodes which are just below the liquid level, that is, the parts with the balls between them, play little part in the electrodeposition of the metal that is being recovered. The deposited layer is thinner than it is lower down the cathode.

In Figures 1 and 2 the reference numerals 10.1 and 1 0.2 designate two anodes and the numeral 12 designates a cathode which is between the two anodes 10.1 , 10.2. Also shown in Figure 1 are balls 14.1 which are intended to inhibit mist production as will be described below. The plate 16 of the cathode 12 has acid resistant synthetic plastics material edge strips 18 along all three edges thereof. The edge strips 18 are not shown in Figure 1 . These strips prevent plating occurring on the edges of the cathode plate 16. An edge layer of deposited metal inhibits stripping of the cathode plate of its deposited layer of metal.

The cathode plate 16 extends down from a hanger bar 20. The hanger bars of the anodes 10.1 , 10.2 are designated 22 and their plates are designated 24. If they are for electrowinning the plates 24 are of lead. If they are for electrorefining they are of the metal to be recovered. The end portions of the bars 20, 22 which protrude laterally beyond the plates rest on the top surfaces of the walls of the electrolyte tank in which the anodes and cathodes are immersed. Conductive rails are provided on the top surfaces of the tank walls, and these constitute the bus bars which connect the anodes and cathodes to a d.c. power supply. The electrolyte level is shown at L in Figure 1 .

The hollow neutral bouyancy balls 14.1 conventionally occupy the gaps between the anodes and the cathodes and are intend to reduce the mist that is generated when bubbles of the gas evolved burst at the surface. The balls 14.1 in the upper layer each float with a minor portion of its surface above the level of the electrolyte. It is found that in commercial use of anodes and cathodes described above, there is reduced metal deposition in the zone designated Z in Figure 1 . The balls 14 occupy the spaces between the cathodes and the anodes on each side thereof and there is consequently less electrolyte in this zone. It is an object of the present invention to increase the efficiency of deposition onto the cathodes with respect to the efficiency of deposition onto cathodes in the presently used type of tank which has submerged mist inhibiting balls in it. Stainless steel, the material generally used in manufacturing cathode plates, and lead used in the manufacture of anodes for electrowinning, both offer high resistance to the flow of electrical current. They are only used because they are resistant to the acid in which they are immersed. Copper and copper alloys on the other hand offer little resistance to the flow of current but are rapidly eroded by the acid used as electrolyte in the cell.

Another object of the present invention is to provide an electrode which combines high and low resistivity metals so that the resultant anode offers lower resistance to current flow than does an anode with a plate composed only of stainless steel or only of lead.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the present invention there is provided an electrode for use in electrowinning or electrorefining which comprises a hanger bar and a metal plate, the hanger bar extending along the top edge of the plate and the plate having a bottom edge and opposed side edges, there being an edge strip of electrically insulating material fitted to each of the side edges of the plate and within each edge strip an elongate insert of a material which is of greater conductivity than the metal of the electrode plate, the inserts being in electrical contact with said electrode plate.

The edge strip can be of channel shape and include horizontally spaced vertically extending flanges and a vertically extending web, said insert being within said channel adjacent the web. To hold the insert in place there can be recesses extending along the opposed inner faces of the flanges adjacent the web, each insert spanning between two flanges with opposite ends of the insert in said recesses.

In another form the strip can have a groove in each of the inside faces of the flanges, the grooves being spaced from the web and there being one of said elongate inserts in each groove. In the preferred form the inserts are in contract with the hanger bar so that a current flow path exists from the hanger bar to the inserts. The ends of the inserts can be in sockets of the hanger bar.

According to another aspect of the present invention there is provided a cell tank comprising alternate anodes and cathodes along the length thereof and, between the anodes and cathodes, balls which float in the electrolyte with the greater part of their volume above the electrolyte level.

The balls can be of foamed synthetic plastics material such as acid resistant polystyrene. The balls are preferably solid in the sense that they do not have hollow interiors.

To electrically isolate the balls from the anodes and the cathodes, bars of synthetic plastics material can be provided at a level at, or just above, the electrolyte level. Preferably part of each synthetic plastics material bar is immersed in the electrolyte and part is above the level of the electrolyte. The cathode plates can have edge strips to prevent deposition on the edges of the cathode plates, the upper ends of the vertical edge strips being connected to said synthetic plastic material bars. BRIEF DESCRIPTION OF THE DRAWING

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 3 is an edge view of a cathode and two anodes in accordance with the present invention;

Figure 4 is a pictorial view of the cathode and anodes of Figure 1 ;

Figure 5 is a pictorial view of one of the anodes of Figures 3 and 4 with its component parts separate;

Figure 6 a pictorial view of the cathode of Figures 3 and 4 with its component parts separated;

Figure 7 is a pictorial view of another cathode;

Figures 8 and 9 are pictorial views illustrating edge strips with conductive inserts; and

Figure 10 illustrates the connect of a conductive insert to a hanger bar.

DETAILED DESCRIPTION OF THE DRAWINGS In Figure 3 balls 14.2 are shown which are of an acid resistant material such as lightweight polystyrene and which can be solid as opposed to hollow as are the balls 14.1. The lower layer of balls 14.2 floats with the greater parts of their volumes above the electrolyte. Upper layers of balls 14.2 are above the electrolyte level. It will be understood that the zone Z is, when the balls float in this way, filled with electrolyte and consequently plays a full part in the electrodeposition of the metal ions.

However, it is found that the surfaces of the balls 14.2 are wetted by the acid mist. A low value current flows from ball to ball through the mist layers on the balls, and this current plays no part in the electrodeposition of metal.

Reference numerals 26 in Figures 3 and 4 designate one embodiment of a structure in accordance with the present invention which inhibits current flow through the mist on the surfaces of the balls.

Each structure 26 comprises two bars 28 of acid resistant synthetic plastics material and two connectors 30. Both end parts of both bars 28 are formed with one or more transverse teeth 32. The teeth 32 are shown in Figures 5 and 6. Each connector 30 has two passageways 34 through it which receive the ends of the bars 28. Each passageway is formed with one or more ratchet-like teeth. Interengagement between the teeth of the bars and teeth in the passageway prevents the bars 28 being withdrawn from the passageways 34 after they have been pushed through to the position shown in Figure 4.

Structures 26 are fitted are fitted around the anode plates and around the cathode plates at the level L so that part of each bar 28 is immersed in the electrolyte and part is above the level L. In Figure 7 the vertical edge strips 18.1 are each formed, at their upper and lower ends, with passageways 34.1 which receive the ends of the bars 28. In this form the connectors 26 are not required.

As will be seen from Figure 3, the bars 28 prevent the balls from touching the surfaces of the anodes and the cathodes. The balls are electrically isolated from the anodes and cathode. The only electrical flow path from the anodes to the cathodes is through the acid mist layers on the top surfaces of the bars 28. This, however, is a flow path of high resistance. Experimental work has shown that what lost current there is due to flow from the anodes to the cathodes by way of the balls, it is insignificant compared to the increased current flow, and increased deposition, in zone Z.

The edge strip 36 shown in Figure 8 is extruded in synthetic plastics material and is relatively stiff. It comprises two parallel flanges 38 which are joined by a web 40. In use the flanges and the web extend vertically and the vertically extending edge of a cathode plate CP fits between the flanges 38. The extruded strip 36 has opposed recesses 42 in the inside faces of the flanges 38.

An elongate electrically conductive insert in the form of an element 44 of a metal which has a conductivity greater than that of the metal of the plate CP is fitted against the inner face of the web 40 with its edge in the recesses 42. The plate CP is usually of stainless steel but in some instances is of titanium. The element 44 can be of copper or a copper alloy. The recesses 42 hold the element 44 in place and the edge of plate CP is, in use, in contact with the element 44.

The strip 46 shown in Figure 9 is of rubber or of a rubber like compound and is consequently flexible. Elongate electrically conductive inserts constitutes by elements 50 extend along the strip in the side grooves 52 of the strip 46 and in contact with opposed zones extending along the edges of the cathode plate CP. To increase the electrical conductivity a further element can be provided which occupies the same position in the strip as the element 44 in Figure 8.

In Figure 8 the plate CP is shown prior to being inserted into the strip 36. In Figure 9 the plate CP is shown fully inserted into the strip 46 and with edge zones thereof in contract with the elements.

After the cathode plate has been removed from the cell, but before it is stripped, relatively inflexible strips of the type shown in Figure 8 have to be removed from the cathode. If, however, the flexible strip of Figure 9 is used, then the cathode plate can be flexed to dislodge the deposited metal without the necessity of removing the strip.

It is possible for the strip which is fitted to the bottom edge of the cathode plate to have the form illustrated in Figures 8 and 9.

In Figure 10 part of the plate 16 and part of the hanger bar 20 of a cathode 1 2 are illustrated. Also illustrated is a strip 46. Elements 50 are shown protruding from the strip 46. The elements 50 taper and are inserted into a socket 52 in the underside of the hanger bar 20.

To protect the elements 50 from corrosion, the strip 46 extends, in use, to the underside of the hanger bar.

Edge strips with conductive inserts can be secured to the edges of the cathode plates as described. They can also be secured to the edges of anode plates.