CA1071142A - Cathode - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A cathode and hanger bar assembly comprising a hanger bar of aluminium or copper having a core of a film-forming metal metallurgically bonded thereto, the aluminium or copper being relieved along the length of the hanger bar to reveal the film-forming metal, and a continuous sheet of a film-forming metal welded along one edge only to at least part of the film-forming metal core. The assembly is particularly useful in copper electrorefining and copper electrowinning.

Description

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Thi~ invention relate~ to cathodes and ha~ particular but not e~clusive reference to cathodes for use in copper electrorefini~ and copper electro-winning.
Copper refining by electrolytic methods haQ been ~nown for many years in which pure copper i~ electrodepositea at the cathode of an electrolytic cell in which the anode i9 a sacrificial impure copper anode and which i~

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consumed during the electrolysi~. It has been generally the practice in a first stage to electrodeposit a thin layer of pure copper onto a specially prepared mother plate, in a second stage to strip off the freshly deposited pure copper from the mother plate in the form of a thin sheet or starter ~heet, and in a third stage to use this starter sheet as a cathode in another cell in which a further thick layer of pure copper i8 electrodeposited on the cathode. More recently, titanium has been used as the material for the mother plate in this process. A secona process i8 to build up a thick deposit of pure copper directly onto a titanium cathode from which it is subsequently stripped as a thick plate, thereby elim nating the first and second stages of the previous process.
When titsnium is used as a material for either the mother plate in the first process or the cathode in the second process, each mother plate or cathode is connected to the current carrying busbar by means of a hanger bar which stretches across the electrolytic cell and contacts the busbar located on one side (or both sides) of the cell. Previously, these hanger bars have been formed of copper and the connection between the copper and the titanium mother plate or cathode was by mean~ of bolts or rivets. The -, 25 electrical contact between the mother plate cathode (hereafter referred to simply as the cathode) and the hanger bar wa~ found to be inconsistent.
The cathode snd haneer bar are tightly held in the vicinity of the bolts or rivets but elsewhere the surfaces became slightly separated. The separa-tion occurred as a result of mechanical deformation, or differential thermal , ' '' ~':

' z expansion. When a separation between han~er bar and ~he~t had been formed, splashes of electrolyte were forced into the gaps, and on drying out, left crystals of, for esample, copper ~ulphate in the gap. When the gap is closed by other deformation~, the crystals prevent full closure. Further movement allo~s more material to build up in the gap and as a result, the gap i8 widened out by aratcheting type of action. Clearly, the reduction of surface contact area resulted in an increase in resistance of the ~oint.
The old copper to copper joints ~ere of high quality and the electrolyte splashe~ had a cleaning action on the copper. However, the electrolyte has no cleaning action on the titanium. AdditionQlly, the surface oxide film formed on the titanium interface interferes with the electrical contact.
~he surface film tends to grow if the titanium i3 heated as a result of the resistance acro~s the titanium copper interface. With the currents which have been used to date, the electrical contact problem has been solved by utilising a greater number of bolts or rivets to increase the electrical contact. Over the past five years or 90, however, the use of higher currents in electrolytic refining has meant that serious problems of cont~qct resistance ha~e developed.
As mPny cathode~ are used in parallel, and the current supplied iB
constant, if the resistance of one Or them increa~es, it receives less current. ~ot only does this result in a lower rate of deposition on that cathode, it also increases the current passing through the remainder of the cathodes. This can cause the nest higher resistance cathode to become over-loaded and to overheat, distort and incresse in resistance. This result~ in a further increase in current through the remainder of the cathodes and a cascade of failures can then occur.
The heating of a cathode can, in addition to increasing the load upon the remainder of the cathodes, distort the cathode. Any small amount of distortion i8 compounded by estra local growth ~here the cathode approaches , 107~142 the anode. This can then result in nodular growth of depo~it on the cqtbode with a rapid build-up of a deposit on the cathode, and a short between the cathode and anode.
Also, since the current loss in heating the joint between the cathode ~nd the hanger bar i9 a complete waste of energy and consequently money, this factor has an im?ortant bearing on the economics of electrolytic .
refining. The heat generated also distorts the joint and the sheet.
A very elegant solution to the problems associated with the~e earlier cathodes has been proposed in which the hanger bar i8 in the form of a titanium-clad copper bar in which the copper is metallurgicAlly bonded to the titanium. The main sheet of titanium is then spot-welded along one edge to the outer sheath of the hanger bar and the cathode then suffers from none of the problems mentioned above. The solution is clearly ~ery elegant since it solves in one go the great majority of the previous problems. ~owever, the product is relati~ely e~pensive to manuPacture.
The product may be m2de by placing a copper billet in a titanium cylinder and placing a further copper sheath on the outside. Copper end lids are then ~elded to the copper outer sheath and ~he product is then extruded at a high temperature to metallurgically bond the copper and titanium and the outer copper l~yer is then pickled off. The round starting billet is extruded straightaway into a substantially rectangular shape.
~ol~ever, this results in an excegs of titanium at the ends of the rectangle when seen in cross-section. The titanium is mainly needed at those points uhere spot-welding occurs and escess titanium at the ends is a ~aste. To enhance the streDeth of the hanger bar, it is necessary to cold work the product to barden the copper. It is not possible to cold dra~ the product s~nce there i8 a danger of galling of the titanium on the die and special surface treatments would be needed. Since the product ca~not be cold dra~n, it is difficult to control the final dimensions.

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``` 107~142 It is necessary to remove the titanium sheath at the ends to enable proper electrical contact to be made with the busbars and if the hanger bar has to fit into a Baltimore groove, the starting size has to be sufficiently large to enable all of the titanium to be removed and still leave an inner core of sufficient size and shape to fit properly into the groove.
The majority of these problems are overcome by the present invention.
By "film-forming metal" as used herein is meant a metal chosen from the group titanium, niobium, zirconium, tantalum, hafnium, or alloys of these metals.
By the present invention there is provided a cathode and hanger bar assembly comprising a hanger bar of aluminium or copper having a core of a film-forming metal metallurgically bonded thereto, the aluminium or copper being relieved along the length of the hanger bar to reveal the film-forming metal, and a continuous sheet of a film-forming metal welded along one edge only to at least part of the film-forming metal core.
The continuous sheet may be welded to an intermediate block or blocks, the block or blocks being welded to the core; preferably the block or blocks are of a film-forming metal.
The aluminium or copper may be relieved at discrete locations along the length of the hanger bar, or alternatively the aluminium or copper may be relieved along the entire length of one or both sides.
In a further embodiment, the hanger bar is formed by manufacturing a film-forming metal cored copper or aluminium bar and cutting it longitudinally to reveal a surface of the film-forming metal along one edge. The continuous sheet is directly welded to the free edge of the film-forming metal. The con-tinuous sheet may be cranked so as to overlie the free edge of the film-forming metal and to have a portion dependent below the hanger bar. Alternatively, ~` ~07114Z
the continuous ~7heet m~y be welded directly to the free edge of the film-forming metal an~7 may depend directly from it.
The block or blocks are preferably of a smaller size than the relieved portions 80 th~t the block or blocks may more easily be welded to the core.
The hanger bar may be formed by co-e~truding the core in a container of copper. The container is preferably 3ealed, and preferably the estrusion takês place at 8 temperature in the range 600-800C. The hanger bar may be cold drawn to a final shape subsequent to the extrusion step.
The present invention also provides an electrolytic cell incorporating a cathode and hanger bar assembly a8 hereinabove described. 17he cell m4y be an electrowinning cell with a non-consumable or semi-consumable anode, or an electrorefining cell with a consumable anode. There may be a plurality of qno~7es and cathodes in the cell.
The present invention still further provides a method of carrying out an electrolytic proce~s which co~prises the steps of locsting an anode and a cathode of the type hereinabove described in a solution containing ions of an electrodepositable metal connecting the cathode negatively with respect to the anode snd passing a current through the anode and cathode to deposit the metal on the cathode and removing the deposited metal from the cathode.

B~ way of example, emboaiments of the present i~ention will now be described with reference to the accompar~ing drawings of which:
Figure 1 i~ a psrt-perspective view of a prior art cathode;
Figure 2 i~ a cross-section of an extrusion billet;
Figure 3 is a cross-~ection of an e~truaed hanger bar;
Figure 4 is a part perspective view of a cathode and hanger bsr of one embodiment of the invention;
Figure~ 5 snd 6 are part perspective views of hanger l~ro~of further embodiments Or the invention;

~' Figure 7 is a side ele~ation of a hanger bar of the invention;
Fi~ure 8 i3 n cross-~ection of a Baltimore groove;
F$gure 9 i8 a perspective part-sectional ~iew of an electrolytic cell containing a cathode onlJ;
Figure 10 i3 an end view of an alternati~e form of han8er bar prior to final formation;
Figure 11 i9 a cross-~ectional part-perspect~ve view of a hanger bar ~hich has been longitudinally cut;
Figure 12 is a part-perspective ~ie~ of a hanger bar and cathode sheet; and Figure 13 is an end ele~ational view of an alternative form of hanger bar.

Referring to Figure 1, this shows a titanium cathode sheet but which is spot-welded at 2 to an outer titanium sheet 3 of a copper oored hanger bar, indicated generally at 4. The copper core 5 i9 metallurgicall~ bonded to the titanium sheath ~. This structure pro~ides A very good and durable cathode but it does have cert;ain problems as are e~plained above. Fir~tly, the end of the hanger bar h~s to be machined as at 6 to remo~e the titanium sheath to permit contact between the copper core 5 of the hanger bar 4 and the electrical ~upply bu~bar on which the hanger bar 4 rests.
To manufacture the hanger bar, a tit nium tube 7 is placed ~round a copper billet 9 and is put inside a copper can which is then sealed. The circular cross-3ection billet is then estruded to form the rectangular hanger bar sh ped as shown in Figure 3. The e~trusion i9 carried out at an elevatea temperature to form a metallurgical bond between the copper and the titanium. Because of the temper~ture at which estrusion occurs, the copper core 10 is in the fully annealed condition after e~trusion. The titanium sheqth is required mainIy at the side~ 11 and 12 of the hanger bar where 107119L;~
spot-welding i3 to occur. Only a relati~ely thin amount of titanium or none at all is nceded at the ends of the hanger bar shape. ~owe~sr, as can b8 seen, the amount of titanium at the ends 13 and 14 i9 greal;er than at the po3itions 11 and 12 a~ a natural con3equence of the e~trusion process. ~hl~
e~cess titanium is effectively wa~ted. The rolling of the thic~ titanium tube and the ~ubsequent canning extru~ion is of course a relstively e~pensive ... .
method of producing tube but is the only one feasible for the thicknesses required.
As e~plained sbovs, the titanium has to be machined away at the ends of the han~er bar to permit contact with the electrical busbars of the electrolytic cell in which the cathode is e~entually used. Not all electro-lytlc cells use busbars, however, and an alternative form of electrical sunply i the so-called Baltimore groo~e which is sho~n in cross-section in Figure 8.
The groove is a t~pered groove 15 formed in a block of copper 16 and the hanger bar rests in the groove as shown at 17 ~o that contact is made between both sides of the groove and the corners of the hanger bar. For efficient contact, the contact has to be a copper to copper contact and hence the titanium has to be removed over both surfaces of the copper ~hich make contact with the sides of the Baltimore groove. Because the copper has to be of ~ sufficient wldth to contact the edges of the groove rather ths~
rest in the bottom, the core 10 of the hanger bar has to be of a width greater than the minimum width of the groove ~hich means that the o~srall size Or the han~er bar has to be larger than might otherwise be necessary.
An additional problem ~ith the prior art cathode is that the hanBer bar itself is not sufficiently strong when the copper is in the completely annealad condition. The hanger bar has to be able to support the weight of the cathode and the materisl deposited on it and also the weight of cell operators walking on top of the hanger bars.-To incresse the strength of the han~er bar, it i~
necessary to cold work the copper. Ideally, thi~ would be done by cold drawing :

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the hanger bar since an accurate final dimension can thereby be gi~en to the hanger bar ~nd also the copper core can be cold worked. However, the outer titanium sheath cannot be readily cold drawn because of the galling which occurs when titanium ia dra~n through a die, Although the hanger bar could be cold dra~n ~ith its outer copper layer acting as a lubricant straight after the e~trusion stage, it would be necessary to form the bar with some point to start the drawing operation and this can only be conveniently done by machining the outer l~yers of the bar which remove the lubricating copper layer. Thus, once drawing ~tarts, galling ~ill occur.
These problems are overcome by the han~er bar and cathode of the invention ~hich adopts the une~pected solution of placing the film-formine metal inside the copper hanger bar. To produce the product, a copper billet ha~ inserted into it a rod of titanium and the copper billet is then sealed.
The se~ling may be done by welding a copper plate across the ends of the copper billet. Because of the difficulties of welding coppe- to copper, ~here one copper item is large, an annular grooYe may be machined in the end of the copper billet to leave a small web to which the copper is welded.
Because the titanium bar can be machined to an accurate diameter and because the copper billet can be furnished with an accurately machined hole, the titanium bar can be made to be a close fit 1nside the copper billet 80 that evacuation or argon filling i~ unnecessary. The billet can then be e~truded as though it were a normal copper billet at sn elevated temperature to form the hanger bar shown in Figure 4. The hanger bar comprise3 a central rectangul~r core 18 of titanium metallureically bonded inside a rectangular body 19 of copper. To permit the titanium cathode working ~urfaces 20 to be welded at one edga to the hanger bar, the copper i~ remo~ed at a series of hole~ along its length as at 20 and small blocks of titanium 21 are ~pot-weldea onto the core 18. The titanium cathode working surface is then split as at 22 and 23. The tongues left by the ~plit~ are then bent and staggered ,' 9 , 10~ 2 as shown 90 that the free ends of the tongues may be spot-Nelded as at 24 to the blocks 21.
An alternative method of forming the ~oint is to machine a groove 25 along the entire length of the hanger bar and to spot-~reld a strip 26 inside the groove to the titanium core 18.- The sheet of titanium forming the cathode ma~ then be welded to the strip 26.
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A further altern tive form of manufacturing the cathode is to co-extrude 8 titanium slab ~0 in a copper block 31. The block i8 then cut longitudinally along the line 32 to form two halves and to e~ pose a free edge 33 of titanium.
10 - The product after the cutting staBe is sho-m in Figure 11. It can be seen that it is then a simple matter to weld cranked staggered legs 34 of a continuous sheet of titanium 35 directly to the edge ~3 by spot-wélds such - ~ as at 36. Alternatively, the titanium cathode sheet may ~imply ha~e a right-angled bend and be spot-welded directly to the surface 33 90 that in use it hangs directly from the surface.
In a further form of the embodiment as shown in Figure 13, the product shown in Figure 11 is machined to form a roof-shaped surface 37 which eases the forming problems associated with the staggerea legs 38. ~he legs 38 are spot-welded as at 39 to the titanium 30.
q~he product may require less titanium and may be a cheaper form of cathode to manufacture, requires le89 complicated assembly and e~trusion technology and i9 therefore cheaper to make. Additionally, the hsnger bar can be cola drawn to gi~e any required final shape and can thus be used for any particular requirement. Also, since the copper i8 on the outside, it is readily usable with the Baltimore groove 16 ~Fi~ure 8) or with a con-ventional busbar 27 as shown in Figure 9. In Figure 9, cathode 28 is sho~
in situ in an electrorefining cell 29. The cell will also contain consuhlable anodes (not shown for reason of clarity).

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It will be appreciated that aluminium may be used instead of copper and it will al90 be appreciated that other film-forming metals may be used insteQd of titanium althou2h titanium i8 preferred since it ig cheaper than any other film-forming metal.
It will also be appreciated that a number of further modifications may be made to the arrangement described without falling outside the scope of thé invention. For example~ the copper sheath may be reliered by milling a transvarse strip from a part of the surface. A number of hsnger bars may be msnufactured in a single operation by gang milling a series of bars across their width. It will also be appreciated that to ease the sepsra-tion of the titanium, a pair of titanium members may be extruded within a copper sheath so as to form s break in the extruded product and the copper ma~ be remo~ed locall-; to reveal the break to fonm two hanger bars from a single extruded product.

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Claims (17)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A cathode and hanger bar assembly comprising a hanger bar of aluminium or copper having a core of a film-forming metal metallurgically bond-ed thereto, the aluminium or copper being relieved along the length of the hanger bar to reveal the film-forming metal, and a continuous sheet of a film-forming metal welded along one edge only to at least part of the film-forming metal core.

2. The assembly of claim 1 in which the continuous sheet is welded to an intermediate block or blocks, the block or blocks being welded to the core, preferably the block or blocks being of a film-forming metal.

3. The assembly of claim 1 or 2 in which the aluminium or copper is relieved at discrete locations along the length of the hanger bar.

4. The assembly of claim 1 or 2 in which the aluminium or copper is relieved along the entire length of one or both sides.

5. The assembly of claim 2 in which the block or blocks are of smaller size than the relieved portions to facilitate welding of the block or blocks to the core.

6. The assembly of claim 1 in which the hanger bar is formed by co-extruding the core in a container of copper.

7. The assembly of claim 6 in which the container is sealed and in which extrusion takes place at a temperature in the range 600 to 800°C.

8. The assembly of claim 6 or 7 in which the hanger bar is cold drawn to final shape subsequent to the extrusion step.

9. An electrolytic cell incorporating a cathode and hanger bar assembly as claimed in claim 1.

10. A cell as claimed in claim 9 which is an electrowinning cell having a non-consumable or semi-consumable anode.

11. A cell as claimed in claim 9 in which the cell is an electrorefining cell and in which there is a consumable anode.

12. A cell as claimed in claim 9 in which there are a plurality of anodes and cathodes in the cell.

13. The assembly of claim 1 in which the hanger bar is formed by manu-facturing a film-forming metal cored copper or aluminium bar and cutting it longitudinally to reveal a surface of the film-forming metal along one edge.

14. The assembly of claim 13 in which the continuous sheet is directly welded to the free edge of the film-forming metal.

15. The assembly of claim 13 or 14 in which the continuous sheet is cranked so as to overlie the free edge of the film-forming metal and to have a portion dependent below the hanger bar.

16. The assembly of claim 13 or 14 in which the continuous sheet is welded directly to the free edge of the film-forming metal and depends directly from it.

17. The assembly of claim 13 or 14 in which the continuous sheet is welded directly to the free edge of the film-forming metal and depends directly from it.