US 3857774 A
A cell structure in which the cathode working surface is formed of titanium and the hanger bar is formed of copper-cored titanium, the working surface being welded to the titanium sheath of the hanger bar.
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Description (OCR text may contain errors)
United States Patent 1191 Field of Search 204/242, 286-288 Morton et a1. Dec. 31, 1974 CATHODES FOR ELECTROLYTIC CELL  References Cited  Inventors: Peter Harlow Morton, Solihull; John UNITED STATES PATENTS Philip Atkinson Wortley, 1,515,348 11/ 1924 Levin 204/286 Birmingham; Alan Woolcock, 2,848,411 8/1958 Hartzell 204/286 X Li hfi ld all of England 3,761,385 9/1973 Ruthel et a1 204/290 F  Assignee: Imperial Metal Industries (Kynoch) FOREIGN PATENTS OR APPLICATIONS Limited, Birmingham, 2,019,806 11/1970 Germany 204/286 Warwickshire, England I Primary ExaminerJohn H. Mack  Ffled' June 1973 Assistant Examiner-D. R. Valentine  Appl. No.: 373,313 Attorney, Agent, or Firm-Cushman, Darby &
Cushman  J lggrellgrli3Aprgicatrgn Priority Data 4132 7  ABSTRACT reat l 3 A cell structure in which the cathode working surface  U S CI 204/242 204088 is formed of titanium and the hanger bar is formed of copper-cored titanium, the working surface being Int. Cl B01k 3/04, C23b 5/68 Welded to the titanium Sheath of the hanger bar.
17 Claims, 5 Drawing Figures SHEET 2 OF 2 PATENTED DEBS] I974 CATHODES FOR ELECTROLYTIC CELL BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to cathodes and has particular, but not exclusive reference to cathodes for use in copper refining.
2. Description of the Prior Art Copper refining by electrolytic methods has been known for many years in which pure copper is electrodeposited at the cathode of an electrolytic cell of which normally the anode is a sacrificial impure copper an ode, which is consumed during the electrolysis. 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 sheet, 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 is electrodeposited on the cathode. More recently tita-' nium has been used as material for the mother plate in this process. A further development is 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 eliminating the first and second stages of the previous process.
When titanium is used as 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 bus-bar by means of a hanger'bar which stretches across the electrolytic cell and contacts the bus-bar located on one side (or both sides) of the cell. Up to now these hanger bars have been formed of copper and the connection between the copper and the titanium mother plate or cathode has been by means of bolts or rivets. The electrical contact between the mother plate or cathode (hereinafter referred to simply as the cathode) and the hanger bar has been found to be inconsistent.
. The cathode and hanger bar are tightly held in the vicinity of the bolts or rivets but elsewhere the surfaces can become slightly separated. This separation can occur as a result of mechanical deformation, or differential thermal expansion. When a separation between hanger bar and sheet has been formed, splashes of electrolyte can be forced into the gaps, and on drying out leave crystals of, for example, copper sulphate in the gap. When the gap is closed by other deformations, the crystals prevent full closure. Further movement allows more material to build up in the gap, and as a result the gap is widened out by a ratcheting type of action. Clearly the reduction of surface contact area results in an increase in resistance of the joint.
The old copper to copper joints were of high quality, and the electrolyte splashes had a cleaning action on the copper, however, the electrolyte has no cleaning action on the titanium. Additionally, the surface oxide film formed on the titanium interface interferes with the electrical contact. The surface film tends to grow if the titanium is heated as a result of the resistance across 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 so, however, the use of higher currents in electrolytic refining has meant that serious problems of contact resistance have developed.
As many cathodes are used in parallel, and the current supplied is constant, if the resistance of one of them increases it receives less current. Not 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 next highest resistance cathode to become overloaded and to overheat, distort and increase in resistance. This results 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 on the remainder of the cathodes. distort the cathode. Any small amount of distortion is compounded by extra local growth where the cathode approaches the anode. This can then result in nodular growth of deposit on the cathode, with a rapid build up of deposit on the cathode, and a short between the cathode and anode. Y
Also since the current loss in heating the joint between the cathode and the hanger is a complete waste of energy and consequently money, this factor has an important bearing on the economies of electrolytic refining.
By valve metal as used herein is meant an oxide film forming metal chosen from the group titanium, niobium, zirconium, tantalum, hafnium or an alloy of these metals.
SUMMARY OF THE INVENTION It is an object of the present invention to provide an electrolytic cell wherein the previous disadvantages mentioned above are overcome. It is also an object of the invention to provide an electrolytic cell including an anode and a cathode, the cathode comprising a hanger bar of aluminium or copper at least partly sheathed with a valve metal, and a continuous sheet of valve metal welded along one edge only to at least part of the length of the hanger bar.
The hanger bar may be completely sheathed with a valve metal. The edge may be in the form of cranked legs, said legs being welded to said hanger bar. Said legs may be staggered and may be spaced apart. Said edge may be spot welded to said hanger bar. The cell may be an electro-winning cell and the anode may be a nonconsumable anode. The cell may alternatively be an electrorefining cell, and the anode may be a consumable anode. The cell may have said hanger bar located with at least one end on an electrical bus-bar, the hanger bar having the valve metal sheath relieved at the point of contact with the busbar. There may be a plurality of cathodes. The continuous sheet of valve metal may be free from precious metal. The hanger bar may be produced by co-extruding at an elevated temperature, the copper or aluminium core and a valve metal sheath or part sheath. The valve metal is preferably ti tanium. During the extrustion stage, the valve metal may be surrounded by a lubricant metal such as copper, the lubricant metal being subsequently removed. for instance, by pickling or machining. The hanger bar may include strengthening means.
BRIEF DESCRIPTION OF THE DRAWING By way of example, embodiments of the present invention will now be described with reference to the accompanying drawings, of which:
FIG. 1 is a scrap sectional view of a rivetted joint;
FIG. 2 is a scrap sectional view of a bolted joint;
FIG. 3 is a scrap sectional view of a copper/titanium interface end;
FIG. 4 is a perspective view of a hanger bar and a portion of a cathode.
FIG. 5 is a view along the arrow V of FIG. 4; FIG. 6 is an enlarged view along the arrow VI of FIG.
FIG. 7 is a scrap perspective view of an electrolytic cell and cathode assembly;
FIG. 8 is a graph of milivolts v time;
FIG. 9 is a cross-section of an extrusion assembly, and
FIG. 10 is a crosssection of an alternative hanger bar.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, this shows two plates of copper 1, connected to a plate of titanium 2, by means of a rivet 3. The rivet 3 is placed in a hole passing through the plates 1 and 2 and is compressed to hold it in position. The compression tends to fatten the centre of the rivet so that the centre of the rivet is forced into intimate contact with the bore of the hole through the plate 2. The heads of the rivet are also forced into intimate contact with the faces 5 on the plates 1. The current path for current passing from the titanium plate 2 to the copper plates 1 tends to pass along the direction of the arrow 6, through the interface between the bore in the plate 2 and the surface of the rivet 3, along the rivet, and through the surfaces 5 into the copper plates 2. Thermal cycling of the rivet tends to cause it to relax and the contact resistance consequently increases.
The bolted joint shown in FIG. 2 has a different configuration. The plates 1 and 2 are shown bolted together by means of bolt 7 and nut 8. The act of tightening the bolts tends to stretch it, tending to thin the bolt, resulting in a gap 9 along its length between the bolt and the bore. The current path between the titanium 2 and the copper l is largely along the lines of the arrows 6a. It can be seen that the current largely passes between the copper and the titanium in the compressed regions beneath the head of the bolt 7 and the nut 8. Thermal cycling of the joint results in the tension in the bolt gradually becoming less because of creep in the bolt and in the copper. There is consequently a decrystals, for example, copper sulphate. During one part ofa thermal cycle, the differential expansion of copper and titanium may cause the copper to move away from the titanium and the deposit builds up as shown as 10 in FIG. 3. When the joint cools down, the copper cannot contract fully onto the titanium as it is held by the wedge of salt 10. Thus the next cycle. forces the copper further away from the titanium and is prevented again from returning by a further deposit of electrolyte salt which builds up the wedge 10. There is, therefore, a ratcheting action occurring which tends to separate the copper from the titanium and hence to increase the contact resistance of the joints.
Also, of course, the thermal cycling affects the oxide layer on the titanium tending to increase it and again increasing the surface contact resistance of the joints. Although the copper layers 1 have been shown on both sides of the titanium core 2, it will be appreciated that there could be only a single layer of copper 2 utilising a rivetted or bolted joint and the same principles would apply.
Referring to FIG. 4, a hanger bar 11 has welded to it a titanium sheet l2.'The titanium sheet has at its upper end, cranked, staggered legs l3, l4 and 15, two of which 13 and 15 are on one side of the hanger bar 11 and the other 14 is on the other side of the hanger bar 11. The staggered legs 13,14 and 15 are spaced apart to leave gaps 16 and 17, which ease handling of the cathode assembly in use. The legs 13,14 and 15 are spot welded as at 18 to the hanger bar.
The hanger bar 11 has a central copper core 19 as is seen most clearly in FIG. 5. Surrounding the copper core 19 is a titanium sheath 10. Thus the legs 13,14 and 15 of titanium are welded themselves to a titanium surface to provide a good electrical contact. The end 21 or 22 of the hanger bar 11 is machined to remove the titanium sheath to reveal a face 23 of copper.
FIG. 7 illustrates an electrolytic cell 25 containing a series of anodes and cathodes spaced alternately. The anodes have hanger bars which connect to bus-bar 24a on the left-hand side of the cell as seen in the drawing and the cathodes have hanger bars which connect to the bus-bar 24b on the right-hand side of the cell as seen in the drawing. In the case of electrorefining, the anodes are consumable, whereas in the case of electrowinning, the anodes are non-consumable and may be of lead, graphite, platinised titanium or any other suitable material.
As can be clearly seen in FIG, 7, the cathode working surface is a continuous surface and it is important to notice that there are no apertures, or bits of large size in the surface which would otherwise act as a key the material deposited onto the cathode and preventing its subsequent easy removal. Clearly, one would wish to remove the cathode deposited material in a single operation and it would not normally be practical to machine off or dissolve the deposited material. It may be desirable to use some form of releasing coating on the cathode surface to enhance removal. However, it is unnecessary to coat the surface with a precious metal which is, of course, extremely expensive. It is to be noted that the cathodes would not act as anodes if the polarity of the cell were reversed since the plain valve metal surface would rapidly oxidise under the anodic conditions to form a non-conducting oxide film which would stop further electro-conduction.
The hanger bar 11 is prepared by co-extruding a copper core and titanium sheath at an elevated temperature in the range of 400800C to provide a good metallurgical bond between the copper and the titanium. When the cathode is in use, therefore, the electrical current path is between the copper bus-bars 24 and the copper core 19 via the face 23, through the interface between the core 19 and the sheath 20 and through the welds 18 into the cathode l2.
The use of the welded construction illustrated above can be compared with one of the previous constructions which again used staggered legs on the end of the cathode which were in contact with a copper hanger bar and the welds 18 were substituted by bolts which passed through the titanium cathode and the hanger bar as described above.
When referring to FIG. 8 which shows expected millivolt drops across a hanger bar/cathode interface v time in use, the lines 26, 27, 28 and 29 represent millivolt drops in the bolted constructions and the line 30 represents millivolt drops in a welded arrangement. It can be seen that the scatter of voltage drops across the interface is very large in the bolted constructions and although some of the voltage drops remain constant or increase only a small amount, there are cases in which the voltage increases dramatically such as the case 29 where overheating occurs and the joint becomes useless. In such a case the cathode has to be withdrawn from service and the hanger bar and surface unbolted and the joint remade. The voltage drop at the welded interface is, however, much smaller to start with and is constant since there is no mechanical joint to deterio rate.
Although the legs 13,14 and have been shown staggered, there is no need for such an arrangement and they could all be on one side, in which case the main body of the sheet could either be cranked to lie underneath the centre-line of the hanger bar or could be dependent directly downwards from the line of the spot welds 18.
Alternative arrangements are envisaged in which the gaps 16 and 17 are omitted and the edge of the sheet is welded directly to the titanium sheath 20. Although spot welding has been described above, other forms of welding could be used as required such as seam welding or other electrical resistance welding, or fusion welding. The core 19 whichhas been described as being formed of copper could be formed of aluminium if required. v
Although the hanger bar illustrated and described above has a continuous sheath of titanium completely surrounding it, it will be appreciated that it is within the scope of the invention tohave only a partial sheath as, for example, has been illustrated in FIGS. 9 and 10. In FIG. 9, a copper billet 31 is inserted into two titanium curved sheets 32 and the billet is then extruded at a high temperature to effect a metallurgical bond between the copper and the titanium. The excess copper is then machined away from the extruded section to re veal the titanium plates 32 so that welds can be made to them.
Again in FIG. 10, a copper billet 33 is almost completely surrounded by a titanium sheet 34 and the assembly extruded to form a metallurical bond between the titanium and the copper. The hanger bar is then used in the same way as has been described above.
If required, the hanger bar may be strengthed by using a strong steel insert to support the weight of the coated cathode when used and also the weight of workmen walking over the surface of the cells using the hanger bars as a floor.
I. An electrolytic cell including an anode and a cathode, the cathode comprising a hanger bar of aluminium or copper at least partly sheathed with a valve metal, and a continuous sheet of valve metal welded along one edge only to at least part of the length of the valve metal sheath of the hanger bar.
2. The cell of claim 1 wherein said hanger bar is completely sheathed with valve metal.
3. The cell of claim 1 wherein said edge is in the form of cranked legs, said legs being welded to said valve metal sheath of the hanger bar.
4. The cell of claim 3 wherein said legs are staggered.
5. The cell of claim 1 wherein said legs are spaced apart.
6. The cell of claim 1 wherein said edge is spot welded to said valve metal'sheath of the hanger bar.
7. The cell of claim 1 wherein said cell is an electrowinning cell and the anode is a non-consumable anode.
8. The cell of claim 1 wherein said cell is an electrorefining cell and the anode is a consumable anode.
9. The cell of claim 1 wherein said hanger bar is located at at least one end on an electrical bus-bar, the hanger bar having the valve metal sheath relieved at the point of contact with the bus-bar.
10. The cell of claim 8 wherein there is a plurality of cathodes.
11. The cell of claim 9 wherein there is a plurality of cathodes.
12. The cell of claim 1 wherein the continuous sheet of valve metal is free from precious metal.
13. The cell of claim 1 wherein said hanger bar is produced by coextruding at an elevated temperature a cludes strengthening means.