WO2014068410A1

WO2014068410A1 - Anode for use in the recovery of metals by electrolysis

Info

Publication number: WO2014068410A1
Application number: PCT/IB2013/051379
Authority: WO
Inventors: Wouter VERWEY; Jan Petrus Human
Original assignee: Zimco Group (Pty) Ltd
Priority date: 2012-10-29
Filing date: 2013-02-20
Publication date: 2014-05-08
Also published as: CL2013001563A1; PE20141276A1

Abstract

An anode particularly for use in electro winning is disclosed which comprises a hanger bar (56) and a lead anode plate (54) hanging down from the hanger bar. The plate (54) comprises two plate components (58, 60). A perforated sheet (64) of copper or a copper alloy is embedded in the lead plate between the two plate components and is in electrical contact with the hanger bar (56). A moulding method and a mould for producing the anode are also disclosed.

Description

ANODE FOR USE IN THE RECOVERY OF METALS BY ELECTROLYSIS

FIELD OF THE INVENTION

THIS INVENTION relates to an anode for use in the recovery of metals by electrolysis. BACKGROUND TO THE INVENTION

Copper and other metals are recovered by the process known as electro winning. The ore is ground and dissolved in acid which is contained in a tank. The acid forms the electrolyte of a cell which comprises a lead anode and a sheet of stainless steel or titanium which constitutes the cathode. Current flowing through the electrolyte deposits the metal ions on the cathode. When the deposited metal is of sufficient thickness, the cathode is removed from the electrolyte and stripped of its deposited metal layers. It is then returned to the electrolyte in the tank. The anodes remain in the electrolyte through a large number of deposition and stripping cycles. They are only removed when they are in need of cleaning or in need of refurbishment.

The anode comprises a hanger bar and a rectangular lead plate. Power supply to the anode is by way of the hanger bar which supports the anode from the power rails running along the top surfaces of the walls of the tank. The bar can be of copper or of another material which is electrically conductive and which can be coated to protect it from the corrosive atmosphere above the tank.

Because of the poor electrical conductivity of lead there is a voltage drop from the hanger bar to the lead plate. This reduces current flow and causes power losses.

In electro refining the anode is in the form of a cast metal block which is eroded during electrolysis. Lead anodes are in this process only used as scavenging anodes in a part of the process which removes contaminants from the electrolyte.

The present invention provides a lead anode which is constructed to reduce the electrical resistance of the anode. Lead anodes according to the invention also distribute the flow of current evenly on both sides of the anode.

The term "lead" as used herein includes both lead and lead alloys.

BRIEF DESCRIPTION OF THE INVENTION

According to a first aspect of the present invention there is provided an anode for use in the recovery of metals by electrolysis and which comprises a hanger bar and a lead plate extending downwardly from the hanger bar, there being an element of a metal which is more electrically conductive than lead embedded in the lead and extending downwardly through the plate from the hanger bar, said element being in electrical contact with the hanger bar.

In the preferred embodiment the element is of copper or a copper alloy. The element can include protrusions and / or holes which interlock with the lead of said plate. The element can be a rod, a tube or a flat strip. Preferably there are side by side elements extending downwardly through the lead plate from the hanger bar. The element can also be in the form of a mesh or can be a slit metal sheet so that, when pulled, the slits open up and the sheet elongates.

According to a second aspect of the present invention there is provided a method of manufacturing lead anode plates for use in the recovery of metals by electrolysis which comprises casting a block with at least one element embedded in it which is more electrically conductive than lead, rolling the block to stretch it and said element, and cutting the stretched block in the direction transverse to said element thereby to form at least two anode plates each with a portion of the stretched element in it.

Said element can be a tube, a rod or a flat bar and can have holes and / or protrusions for interlocking with the cast lead to minimise slippage between the lead and the element during rolling.

The element is preferably of copper or a copper alloy.

According to a third aspect of the present invention there is provided a method of manufacturing an anode which comprises placing a metallic element in a mould cavity before closing it, said element being between the surfaces which define the mould cavity and being of greater electrical conductivity than lead, feeding molten lead into the mould cavity to cast an anode plate which has a thickness which is determined by the spacing between the surfaces of the mould which bound the cavity in which the anode plate is cast and which has said element embedded in it.

The method can include the subsequent step of decreasing the distance between said surfaces of the mould to decrease the thickness of the cast anode plate and reduce its porosity.

The method can further include placing a copper or copper alloy hanger bar in the mould, the ends of the bar resting on supporting surfaces of the mould, and encasing the bar, apart from its ends, in lead during moulding of the anode plate, the bar being in electrical contact with said element.

The metallic element can be a mesh, a perforated metal plate or a solid metal plate.

According to a fourth aspect of the present invention there is provided a method of fabricating a lead anode plate which comprises manufacturing first and second lead anode plate components, placing a metal element, the electrical conductivity of which is greater than that of lead, between the plate components, said metal element extending beyond one of the edges of the anode plate constituted by the two anode plate components, compressing the composite anode plate, and lead burning the joint extending around the plate components to prevent ingress of electrolyte between the plate components.

The method can further comprise casting said anode plate components and providing recess in at least one of said components for receiving said metal element.

Said electrically conductive element can be attached to a hanger bar before said element is placed between said anode plate components.

Said element can be in the form of a perforated strip, a solid strip, a wire mesh, a perforated sheet or a non-perforated sheet.

According to a fifth aspect of the present invention there is provided a method of fabricating an anode plate which comprises manufacturing first and second anode plate components, depositing a layer of metal by electroplating one face of one of the components, the deposited metal having an electrical conductivity which is greater than that of lead, and extending to one of the edges of said one plate component, juxtaposing the components with the deposited metal layer between them, compressing the composite anode plate, lead burning the joint extending around the plate

components to prevent ingress of electrolyte between the plate components, and securing the anode plate to an electrically conductive hanger bar with said electroplating in electrical contact with the hanger bar.

One face of each plate component can be electroplated, the electroplated layers of the two components being placed in face-to-face contact and the juxtaposed anode plate components thereafter being compressed.

According to a sixth aspect of the invention there is provided an anode comprising a metal hanger bar, a metal plate secured to the hanger bar and extending downwardly from the hanger bar, the plate and the hanger bar being electroplated with a metal which has an electrical conductivity greater than that of the metal of the plate and hanger bar, and the plate and hanger bar, apart from the ends of the hanger bar, being encased in lead.

To secure the element to the hanger bar, the hanger bar can be formed with a slot and the upper end of the element can be in the slot, the hanger bar having been crimped to close-up the slot and secure the element to the hanger bar.

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 strip which is to be embedded in the lead plate of an anode;

Figure 2 is a pictorial view of a lead block immediately after casting;

Figure 3 is a pictorial view of the lead plate of an anode;

Figures 4 and 5 are pictorial views of the components of an anode;

Figure 6 is an elevation of an anode assembled from the components of Figure 5; Figures 7 and 8 are pictorial views of a mould for use in casting an anode plate;

Figure 9 is a pictorial view similar to Figure 7 and also showing an electrically conductive anode insert;

Figure 10 is a pictorial view showing the mould closed, one of the mould parts being broken away;

Figure 1 1 is an elevation of the closed mould, one mould part being shown broken away;

Figure 12 is a section on the line XII - XII in Figure 11 ;

Figure 13 is a view similar to that of Figure 10 and also illustrates the moulded anode;

Figure 14 is a pictorial view of an anode including anode plate components;

Figures 15 and 16 are pictorial views similar to Figure 14 with an anode plate component broken away to show electrically conductive material between the two components;

Figure 17 is a pictorial view showing the components of a further form of anode; and Figure 18 is a pictorial view illustrating a still further form of anode. DETAILED DESCRIPTION OF THE DRAWINGS

The strip 10 shown in Figure 1 is of copper, a copper alloy or another electrically conductive metal which has lower electrical resistivity than lead. The strip is elongate and has a plurality of holes 12 in it. It can alternatively or additionally have protrusions.

The numeral 14 in Figure 2 designates a cast lead block. The block is produced by hanging three strips 10 side-by-side in a mould which is connected at its lower end to a source of molten lead. The strips are coated by placing them in a tin solution before they are placed in the mould. Molten lead is run into the mould to form a cast block 14 and embed the strips 10 in the cast block. The lead fills the holes 12. The tin solution on the strips 10 promotes adhesion between the strips and the lead. The lead runs into the lower end of the mould and fills it from the lower end up.

The upper ends of the strips 10 can protrude from the cast block 14 as shown in Figure 2 or can be flush with the top of the cast block. The strips 10 terminate short of the lower edge of the block 14.

Whilst the block 14 is still hot it is rolled as the next stage in the manufacture of anodes in accordance with this aspect of the present invention. This stretches the block 14 to a length such that it constitutes, for example, four anode plates. The strips 10 stretch with the lead of the block 14.

The stretched block is then cut transversely to separate the stretched block into four plates (designated 16 in Figures such as Figure 3) each forming the lead component of an anode.

As it is necessary to make electrical connections to the strips 10 they must accessible from the upper edge of each plate. This can be achieved as shown in Figure 3 by cutting away a section from the top of the plate so that the strips 10 protrude. The protruding upper ends are secured to a hanger bar, preferably by inserting them into a groove in the hanger bar, and crimping the bar onto the strips 10. In the alternative construction of Figure 4 to 6 the upper ends of the strips 10 are flush with the upper edge of the plate 16. The hanger bar 18 has a groove 20 (Figures 5 and 6) in the lower face thereof to receive the upper edge of the plate 16. The hanger bar and lead plate are welded to one another with the strips 10 in electrical contact with the hanger bar 18 thereby to form the anode.

The electrical and mechanical connections between the copper of the strips 10 and that of the bar 18 can be enhanced by providing a tin coating between the two.

It will be understood that the strips 10 will be visible at the lower end of three out of the four anode plates cut from a stretched block. The visible ends are coated with lead, for example by lead burning, to conceal them and prevent them corroding.

The mould illustrated in Figures 7 to 13 comprises a fixed mould part 24 and a further mould part 26 which is mounted for movement towards and away from the mould part 24. The mould part 26 comprises two mould pieces 26.1 , 26.2 (Figure 1 1 ) which are capable of movement relative to one another. Reference numeral 28 indicates the diagrammatically illustrated hydraulic cylinder which moves the part 26.

Guide pins 30 protrude from the mould part 26 and the mould part 24 has sockets 32 which receive the pins 30 as the mould is closed.

Part of the mould cavity is in the form of a recess 34 in the mould part 24. The remainder of the mould cavity is in the form of a recess 36 in the mould part 26.

The recess 34 has a main rectangular portion 38 and a horizontal portion 40. The portion 38 is shallower than the portion 40. The portion 40 is extended in both directions beyond the vertical boundaries 42 of the portion 38 by rectangular mould cavity extensions 44.

The recess 36, see Figure 8, matches the recess 34 in shape. Where applicable like parts have been designated with like references to which the suffix .1 has been added.

Apart from the gate 45, the flow path which leads to the lower end of the mould from the lead melting pot, and through which molten lead flows into the mould, has not been illustrated. The cooling channels through which chilled water flows have also been omitted.

In Figure 9 a mould insert 46 is shown between the mould parts 24 and 26. The insert 46, which is of copper, a copper alloy or any other metal which has good electrical conductivity, comprises a cross bar 48 and three strips 50 which extend downwardly from the cross bar 48. The strips 50 are welded to the cross bar 48 and are each formed with a multitude of through holes. The bar 48 eventually becomes the hanger bar of the moulded composite anode.

As shown in Figures 10, 11 and 12 the ends of the cross bar 48 are close fits in the extensions 44. The insert 46 hangs in the mould cavity. The position the insert 46 adopts when the mould is closed is best seen in Figure 12.

When the mould part 26 is moved to the closed position, the mould pieces 26.1 , 26.2 are as shown in Figure 12. The piece 26.2 abuts the mould part 24.

When the valve (not shown) in the flow path leading to the gate 45 at the bottom of the cavity is opened, lead runs in filling the cavity from the bottom upwards. As shown in Figure 13 the lead which has run in fills the mould cavity. The insert 46 is entirely embedded in the lead apart from the ends of the cross bar 48 which are in the cavity extensions 44, 44.1. It will be understood that the composite anode comprises the insert 46 and a cast anode plate 47 (Figure 13). The inlet valve is then closed.

Whilst the lead is solidifying, the part 26.1 is moved towards the part 24 relatively to the part 26.2 by the cylinder 28. This has the effect of exerting crushing pressure on the cast plate, thereby decreasing the thickness dimension of the cast plate. As a consequence its porosity is decreased.

The anode 52 of Figures 14 to 16 comprises a composite anode plate 54 and a hanger bar 56. As illustrated in Figure 15, the anode plate 54 comprises a first cast anode plate component 58 and a second cast anode plate component 60. The length and width of one component can be smaller than that of the other but they can also be of the same size.

As shown in Figure 15 there are, between the components 58 and 60, strips 62 of a material such as copper which has a greater electrical conductivity than lead. The strips 62 are shown as being perforated but they can be solid.

The strips 62 are welded, spot welded or crimped to the hanger bar 56. Preferably the upper ends of the strips are inserted into a groove in the hanger bar and then the bar is crushed to close up the groove. Once the strips 62 have been placed between the components 58, 60 the composite anode plate is compressed. Whilst manual clamping of the anode plate components 58, 60 can provide sufficient compression, this method is too slow for use in production. Clamping of the anode plate components between a fixed plate and a plate which is moved by an hydraulic cylinder is required to enable an acceptable production rate to be achieved. Compression improves the grain structure of the lead and ensures that the strips 62 are in intimate contact with the components 58, 60. The lead is forced into the perforations of the strips if the strips have perforations. The joint between the two components is lead burned to prevent ingress of electrolyte between the components 58, 60.

If desired the perforations in the strips 62, if provided, are filled using molten lead before the components 58 and 60 are juxtaposed.

The anode plate components 58 and 60 can be rolled after being cast but before being secured to one another. Alternatively the components 58 and 60 can be extruded. If the components are extruded it is also possible simultaneously to extrude the layer of electrically conductive material between the components.

The strips 62 can be replaced by a mesh comprising intersecting copper wires.

Alternatively, as shown in Figure 16, the strips 62 can be replaced by a perforated metal sheet 64. The size and configuration of the sheet 64 is such that it covers substantially the entire area of the composite anode plate, or substantially the entire area of the smaller component 58, 60 if they are not of the same size.

As shown in Figure 17, the anode plate components 58, 60 can be cast and can be provided with elongate recesses 66 which extend almost the full height of the anode plate components. The recesses 66 terminate short of the lower edges of the

components 58, 60 and extend to the upper edges of the components 58, 60.

Both components 58, 60, or only one of them, can be formed with the elongate recesses. The strips 62 fit in the recesses 66.

To ensure that the components 58, 60 are properly secured together, holes can be drilled through the composite anode and the holes filled with molten lead. This is done whilst the components 58, 60 are under clamping pressure. Close contact between the electrically conductive material and the lead of the anode plate components is necessary to dissipate the heat generated by current flowing through the conductive material.

In the embodiment of Figure 18 the conductive material is in the form of a solid sheet 68. The dimensions of the sheet are such that it covers substantially the entire area of the composite anode plate (or the area of the smaller one of the components 58, 60 if they are not of the same size) and protrudes beyond the upper edge of the composite anode plate. The hanger bar 56 has a slot 70 in it to receive the protruding upper edge of the sheet 68. The sheet 68 and bar 56 are welded or crimped together so that they are in electrical contact with one another.

In accordance with a further embodiment of the invention one face of each component 58, 60 is provided with an electroplated copper layer. The upper edge of each component is also electroplated. The hanger bar has a slot for receiving the upper edges of the components 58, 60 which have their copper layers in face to face contact.

The deposited metal layers along the upper edges of the components 58, 60 are in contact with the hanger bar, and current flows through the deposited layers. The deposited layers along the upper edges of the components 58, 60 can be welded to the hanger bar.

It will be understood that one face of one of the components can be plated provided a sufficient thickness of deposited metal is achieved.

Whilst, in all the relevant figure of the drawings, the electrically conductive element is shown as extending either to the lower end of the anode plate or close to it, it is possible for the element to extend a lesser distance into the plate from the upper end thereof. Some conventional anodes are manufactured with holes through them to permit electrolyte circulation. If anodes as described above require electrolyte circulation holes, corresponding openings are punched in the metal element. Pins are provided in the mould which pass through the openings in the metal element. The pins are smaller than the openings. When lead flows into the mould it enters the spaces which are around the pins and embed the parts of the element in which the openings are formed whilst still leaving the requisite holes in the lead after the pins have been removed. In an experiment two anodes immersed in electrolyte were connected in parallel across a single d.c. power source. The first anode was of conventional construction and comprised a lead plate. The second anode had a copper element embedded in it as described above. Current flow through the second anode exceeded that through the first anode, this being indicative of the lower resistance to current flow of the second anode.

In addition to reducing the voltage drop from the part of the anode plate adjacent he hanger bar to the lower edge of the plate, the element which is embedded in the lead forms a direct electrical flow path which bypasses the lead to copper weld where the plate is secured to the hanger bar.

It is also possible for the anode, in accordance with another aspect of the present invention, to comprise a steel hanger bar and a steel plate. The upper end of the plate is welded or otherwise secured to the underside of the hanger bar. The plate and hanger bar are electroplated using a metal, preferably copper, which has an electrical conductivity greater than that of steel. The plate and the hanger bar, apart from its free ends, are encased in lead. The mould of Figures 7 to 13 can be used for the purpose of encasing the electroplated steel of the anode in lead.

The decrease in power losses in the anodes described above as compared with conventional solid lead means that the power consumption of an electro winning installation is reduced. This leads directly to a reduction in the production of greenhouse gases and other undesirable by-products of electrical power generation.

Conversely, for the same consumption of electricity, a greater tonnage of metal can be recovered.

When used as a scavenging anode in an electro refining installation, anodes according to invention have the same advantage in that they reduce power consumption in the scavenging section of the plant.

Claims

1. An anode for use in the recovery of metals by electrolysis and which comprises a hanger bar and a lead plate extending downwardly from the hanger bar, there being an element of a metal which is more electrically conductive than lead embedded in the lead and extending downwardly through the lead plate from the hanger bar, said element being in electrical contact with the hanger bar.

2. An anode as claimed in claim 1 , wherein said element is of copper or a copper alloy.

3. An anode as claimed in claim 1 or 2, wherein said element includes protrusions and / or holes which interlock with the lead of said plate.

4. An anode as claimed in claim 1 , 2 or 3 and including side by side elements

extending downwardly through the lead plate from the hanger bar.

5. An anode as claimed in claim 1 or 2, wherein said element is in the form of a mesh.

6. A method of manufacturing lead anode plates for use in the recovery of metals by electrolysis which comprises producing a lead block with at least one element embedded in it which is more electrically conductive than lead, rolling the block to stretch it and said element, and cutting the stretched block in the direction transverse to said element thereby to form at least two anode plates each with a portion of the stretched element in it.

7. A method of manufacturing an anode which comprises placing a metallic element in a mould cavity before closing it, said element being between the surfaces which define the mould cavity and being of greater electrical conductivity than lead, feeding molten lead into the mould cavity to cast an anode plate which has a thickness which is determined by the spacing between the surfaces of the mould which bound the cavity in which the anode plate is cast, and which has said element embedded in it.

8. A method as claimed in claim 7, and including, subsequent to casting, the step of decreasing the distance between said surfaces of the mould to decrease the thickness of the cast anode plate and decrease its porosity.

9. A method as claimed in claim 7 or 8, and further including placing a copper or copper alloy hanger bar in the mould, the ends of the bar resting on supporting surfaces of the mould, and encasing the bar, apart from its ends, in lead during moulding of the anode plate, the bar being in electrical contact with said element.

10. A method as claimed in claim 7, 8 or 9, wherein the metallic element is a mesh, a perforated metal plate or a solid metal plate.

1 1. A method of fabricating a lead anode plate which comprises manufacturing first and second lead anode plate components, placing a metal element, the electrical conductivity of which is greater than that of lead, between the plate components, said metal element extending beyond one of the edges of the anode plate constituted by the two anode plate components, compressing the composite anode plate, and lead burning the joint extending around the plate components to prevent ingress of electrolyte between the plate components.

12. A method as claimed in claim 11 , and comprising casting said anode plate

components and providing a recess in at least one of said components for receiving said metal element.

13. A method as claimed in claim 12, wherein said electrically conductive element is attached to a hanger bar before said element is placed between said anode plate components.

14. A method of fabricating an anode plate which comprises manufacturing first and second anode plate components, depositing a layer of metal by electroplating one face of one of the components, the deposited metal having an electrical conductivity which is greater than that of lead and extending to one of the edges of said one plate component, juxtaposing the components with the deposited metal layer between them, compressing the composite anode plate, lead burning the joint extending around the plate components to prevent ingress of electrolyte between the plate components and securing the anode plate to an electrically conductive hanger bar with said electroplating in electrical contact with the hanger bar.

15. A method as claimed in claim 14, wherein one face of each plate component is electroplated, the electroplated layers of the two components are placed in face- to-face contact, and the anode plate components are compressed.

16. An anode comprising a metal hanger bar, a metal plate secured to the hanger bar and extending downwardly from the hanger bar, the metal plate and the hanger bar being electroplated with a metal which has an electrical conductivity greater than that of the metal of the plate and hanger bar, and the plate and hanger bar, apart from the ends of the hanger bar, being encased in lead.

17. An anode as claimed in any one of claims 1 to 4, wherein said hanger bar has a slot in it and the upper part of said element is in the slot, said hanger bar having been crimped to close-up the slot and secure the element to the hanger bar.