WO2004042119A1

WO2004042119A1 - A method for obtaining a good contact surface on an electrode support bar and a support bar

Info

Publication number: WO2004042119A1
Application number: PCT/FI2003/000827
Authority: WO
Inventors: Karri Osara; Veikko Polvi
Original assignee: Outokumpu Oyj
Priority date: 2002-11-07
Filing date: 2003-11-06
Publication date: 2004-05-21
Also published as: AU2003276297A1; AR041906A1; PE20040438A1; FI114924B; FI20021991A; FI20021991A0

Abstract

The invention relates to a method for forming a good contact surface on an electrode support bar used in electrolysis, where at least part of the bar is made of copper. In the method a highly electroconductive layer is formed on at least one end of the copper section. The highly electroconductive layer forms a metallic bond with the copper of the support bar, which can be achieved preferably with either thermal spray coating or soldering technique. The invention also relates to an electrode support bar either partially or wholly made of copper, of which at least one end is coated with a highly electroconductive material.

Description

A METHOD FOR OBTAINING A GOOD CONTACT SURFACE ON AN ELECTRODE SUPPORT BAR AND A SUPPORT BAR

The invention relates to a method for forming a good contact surface on an electrode support bar used in electrolysis, where at least part of the bar is made of copper. In the method a highly electro-conductive layer is formed on at least one end of the copper section. The highly electro-conductive layer forms a metallic bond with the copper of the support bar, which can be achieved preferably with either thermal spray coating or soldering technique. The invention also relates to an electrode support bar either partially or wholly made of copper, of which at least one end is coated with a highly electro-conductive material.

The fabrication of many metals such as copper, zinc and nickel, includes an electrolytic stage when the pure metal to be produced is deposited onto a cathode using an electric current, leaving the impurities in the solution. Electrolytic recovery is carried out for instance with an electrolyte containing sulphuric acid in a full electrolysis cell and electrodes (anodes and cathodes) made of electro-conductive material that are immersed in turn in the electrolyte. Anodes and cathodes are generally composed of a plate section, which is immersed in the cell, and a support bar or lug attached to the upper edge of the plate section. The electrode is suspended in the electrolysis cell by means of the support bars or lugs, which are supported on the edges of the cell. The bars or lugs connect the electrodes to the electrical circuit via busbars located on the edges of the cell. One end of the support bar is generally located on top of an insulating rail. In one method the other end of the support bar is also placed on top of a conducting and compensating rail. The metal to be produced is brought to the process either as a soluble anode, termed an active anode (electrorefining), or the metal is dissolved in the electrolyte, in which case the anodes are insoluble or passive anodes (electrowinning). The core of the bar in electrode support bars used in electrolysis may be made of steel for instance and the casing section made of copper. This kind of support bar is described for example in US patent 4,871 ,436. The end of the support bar acts as the contact surface against the cell busbar.

Support bars made of totally copper may be used in for instance copper electrolysis (electrorefining), where copper seed plates are used as cathodes. In these the end of the support bar is automatically the contact surface.

A copper core can also be used in electrode support bars, whereby the protective casing or jacket of the bar is composed of another metal or metal alloys. Depending on the situation, a casing of steel, titanium or lead can be used around the copper core and the electrode plate itself is attached to the casing. Steel and titanium casing is used particularly in cathodes and lead casing in anodes e.g. in zinc electrolysis, which works on the electrowinning principle. The contact surface of a support bar equipped with a protective casing is formed for example by machining the casing around the copper open at the end of the bar or by using casting technique solution, so that the surface of the copper core is revealed. Thus the copper core is used as the contact surface, which is placed on top of the electrolysis cell busbar.

WO publication 00/17419 describes a method for the fabrication of a permanent cathode suspension bar, where the suspension bar is fabricated of a rigid metal outer casing with a highly electro-conductive core connected inside it. According to the publication the outer casing is made of refined steel and the core of copper or aluminium. The parts of the support bar are connected to each other either by drawing with an arbor, upsetting or casting. The end of the steel casing is removed from the finished bar so that the exposed copper core is visible and can be used as a contact surface. WO publication 02/40749 describes another method for the fabrication of a suspension bar for electrodes. According to this method, in addition to the conductive core, joint material is placed inside the casing, and the suspension bar is heat-treated. During heat treatment a metallurgical joint is formed between the core and casing with the aid of the joint material. The joint material most often used is tin. The publication does not mention in more detail how the contact surface on the end of the suspension bar is formed.

Rapid wear of the contact surface is a problem for support bars with a copper contact surface. This is mainly due to the oxidation of copper into oxide and the corrosion of oxide into copper sulphate under the effect of the electrolyte. Copper sulphate formed on the contact surface further weakens the electrical conductivity of the support bar. Oxidation brings about an increase in voltage drop, because the electrical conductivity of copper oxide is significantly weaker than that of pure copper. Problems caused by oxidation are particularly intensified in lead anodes equipped with copper-containing support bars, which otherwise have a fairly long lifetime.

Now a method has been developed, which relates to the attainment of a good contact surface on an electrode support bar used in the electrolysis of metals, where at least part of the bar is made of copper. Part of the support bar may be made of some other material such as steel, titanium or lead, or the support bar may be totally made of copper. According to the method now developed, the area on the lower surface of at least one end of the support bar, the contact surface, which is to touch the electrolysis cell busbar, is coated with a highly electro-conductive metal such as silver or silver alloy. The copper and silver are attached to each other by means of a transmission layer. When a metallic joint is formed between the copper part of the support bar and the transmission layer and coating material formed on its lower surface, the problems caused by wear or oxidation of the lower surface of the contact surface are avoided. The invention also relates to the electrode support bar used in electrolysis manufactured with this method, where a highly electro-conductive layer is formed on the copper contact surface on at least one end of the said bar.

The features presented in the claims are characteristic of the invention.

It is important that the contact surface in the electrode support bar conducts electricity well. The use of a highly electro-conductive metal such as silver or silver alloy as coating material ensures an effective feed of current to the electrode. The metallurgical principle for the use of silver is that although it forms oxides on its surface, at relatively low temperatures the oxides are no longer stable and decompose back to the metallic form. For the above reasons oxide films do not form on the silver plating made on contact surfaces of a support bar in the same way as they do for example on a copper surface. Coating helps ensure that the electrical quality level of electrolysis also remains high in a long run.

Silver does not form a metallurgical, good adhesive joint directly on top of copper, so instead a thin transmission layer has to be formed on the copper first, preferably one of tin or a tin-dominant alloy. Hereafter in the text for the sake of simplicity we shall refer only to tin, but the term also covers tin- dominant alloys. A tin layer can be formed in many ways as by beforehand tin plating through heating, electrolytic coating or by thermal spraying directly on the surface point before the actual coating. After this, the tin surface can be coated with silver or silver alloy. The coating with silver of the copper contact surface of the support bar can be carried out advantageously for instance with thermal spraying or soldering technique.

Oxidations are removed from the copper section acting as contact surface at the end of the support bar before the coating is formed. It is advantageous to carry out the procedure on new bars too, but particularly when the method is applied to improve the electrical conductivity of used bars, the removal of oxidation is necessary. Removal can be done for instance by sandblasting. When the contact surface is the straight lower edge of the copper part, coating is performed as described above. The contact surface can also be a formed by a notch on the lower surface of the copper part of the bar. In this case joint pieces, coatings, are attached to the sides of the notch on the support bar. The service life of a support bar can be extended according to this method in a simple way, in that the coating of the contact surface or surfaces at the ends of the bar can be renewed as required. When the coating is renewed, it is worth straightening out the contact surfaces, because the contact surfaces may have worn so that the geometry of the contact is no longer as desired. Obviously this impairs the feed of current to the cathode.

One preferred method of coating the support bar contact surfaces is coating with silver using thermal spraying technique, since the melting point of silver is 960°C. An AgCu alloy can also be used as coating material e.g. in the form of wire or powder. The melting point of eutectic AgCu alloy is even lower than that of silver and therefore is suitable for contact surface coating with the technique in question.

Out of the thermal spraying techniques available, in practice at least the techniques based on gas combustion have proved practicable. Of these, High Velocity Oxy-Fuel (HVOF) spraying is based on the continuous combustion at high pressure of fuel gas or liquid and oxygen occurring in the combustion chamber of the spray gun and the generation of a fast gas flow with the spray gun. The coating material is fed into the gun nozzle most often axially in powder form using a carrier gas. The powder particles heat up in the nozzle and attain a very high kinetic speed (several hundreds of metres per second) and are directed onto the piece to be coated.

In ordinary flame spraying, as the mixture of fuel gas and oxygen bums it melts the coating material, which is in wire or powder form. Acetylene is generally used as fuel gas due to its extremely hot flame. The coating material wire is fed through the wire nozzle with a feed device using a compressed air turbine or electric motor. The gas flame burning in front of the wire nozzle melts the end of the wire and the melt is blown using compressed air as a metallic mist onto the piece to be coated. The particle speed is in the order of 100 m/s.

Thermal spraying technique melts the surface material and since the molten droplets of the silver-bearing coating have a high temperature, a metallurgical bond is generated between the copper, tin and coating material in the coating of the contact surface of the copper end of the support bar. Thus the electrical conductivity of the joint is good. The metal joining method gives rise to a eutectic of the ternary alloy of silver, tin and copper in the joint area e.g. in a temperature range of 380 - 600°C. If necessary, separate heat treatment can be carried out after spraying, which promotes the formation of a metallurgical joint.

When soldering technique is used to form a coating on the contact surface of the support bar, the surface to be treated is cleaned and a tin layer is formed on it, preferably less than 50 μm thick. Then the silver coating is carried out with some suitable burner. The tin layer melts and when the coating sheet is placed on top of the molten tin, it is easy to position in the correct place.

The method also relates to an electrode support bar containing copper used in electrolysis. A highly electro-conductive layer is formed particularly on an area of the lower of the copper surfaces on the ends of the support bar, the contact surface, which comes into contact with the electrolysis cell busbar. For a highly electro-conductive metal or metal alloy, silver is used, or a silver alloy such as silver copper. The coating of the contact surface is preferably carried out e.g. by soldering or thermal spraying technique, where a metallurgical joint is formed between the contact surface and the coating. W

The method according to this invention is described further in the attached drawings and the following examples, where

Figure 1 shows the relative voltage drop of new cathodes as a function of relative compressive force, and 5 Figure 2 shows the relative voltage drop of used cathodes as a function of relative compressive force.

Example

Permanent cathodes in copper electrolysis (electrorefining), which were of

10 commercial scale dimensions, were kept in a test cell for the duration of a normal production period. Some of the cathode support bars were conventional i.e. they had an ordinary copper contact surface. On the other hand, some of the support bars had a silver coating on the contact surface formed by spray coating. The contact surfaces of the electrolysis cell busbars

15 were copper.

Figure 1 shows a graphical presentation of the results from electrolysis of the new cathodes. The relative voltage drop of the ordinary copper contact surfaces is much greater than the relative voltage drop of the contact

20 surfaces coated with silver as a function of compressive force. Compressive force means the force with which the cathode stresses the busbar. The increase in weight of the cathode as the electrorefining period proceeds has been taken into account in the relative compressive force and the value of 100 corresponds to the average weight of the cathode at the end of the

25 period.

Figure 2 shows the results that correspond to those in Figure 1 , when the test was performed with used cathodes taken from production. The relative voltage drop is calculated at a value of 100 i.e. at the worst value, when the 30 electrorefining period begins with used cathodes with a copper contact surface. The relative voltage drop and relative compressive force of Figure 1 are on the same scale as those in Figure 2. The graphs also show that in all cases the voltage drop is greatest when the weight of the cathode is smallest, but that as the weight of the cathode increases the contact with the busbar improves. The graphs also show that the voltage drop of bars treated in accordance with the invention is far smaller than for conventional contact surfaces. A particularly clear difference can be seen for used cathodes. The example also indicates that a silver contact surface is not so much dependent on compressive force as a copper contact surface.

Claims

PATENT CLAIMS

1. A method for obtaining a good contact surface on the end of an electrode support bar used in electrolysis of metals, whereby the electrode is composed of a plate section, which is immersed into an electrolysis cell, and a support bar, where at least part of the bar is made of copper, and where a copper contact surface of at least one end of the support bar is placed on top of a cell busbar, characterised in that a transmission layer is formed on the contact surface, the area on the lower surface of the copper end of the support bar, after which the contact surface is coated with a highly electroconductive metal or metal alloy, so that the coating material forms a metallurgical joint with the copper and the transmission layer.

2. A method according to claim 1 , characterised in that the transmission layer is of tin or a tin-dominant alloy.

3. A method according to claim 1 or 2, characterised in that the highly electroconductive coating layer is silver.

4. A method according to claim 1 or 2, characterised in that the highly electroconductive coating layer is a silver-copper alloy.

5. A method according to any of the above claims, characterised in that the copper core of the support bar is surrounded by a casing section of refined steel, titanium or lead, which is removed from the lower surface of at least one end of the support bar.

6. A method according to any of claims 1 - 4, characterised in that the support bar is fabricated wholly of copper.

7. A method according to any of claims 1 - 4, characterised in that the casing of the support bar is made of copper.

8. A method according to any of the above claims, characterised in that the highly electroconductive coating layer is formed using soldering technique.

9. A method according to any of claims 1 - 7, characterised in that the highly electroconductive coating layer is formed using thermal spraying technique.

10. A method according to claim 9, characterised in that the thermal spraying technique is based on gas combustion.

11. A method according to claim 9 or 10, characterised in that the thermal spraying technique is high velocity oxy-fuel spraying.

12. A method according to any of the above claims, characterised in that the highly electroconductive coating material is in powder form.

13. A method according to claim 9 or 10, characterised in that the thermal spraying technique is flame spraying.

14. A method according to any of claims 1 -11 or 13, characterised in that the highly electroconductive coating material is in wire form.

15. A method according to any of the above claims, characterised in that the contact surface is subjected to heat treatment after coating.

16. A support bar for an electrode used in an electrolysis of metals, where the plate section of the electrode is meant to be immersed in an electrolysis cell and a electrode support bar to be supported at both ends on the edges of the electrolysis cell, whereby at least part of the support bar is made of copper, and where a contact surface to be located on top of a busbar is formed on at least one end of the support bar, characterised in that a transmission layer is formed on the contact surface of the support bar, after which the contact surface is coated with a highly electroconductive metal or metal alloy, so that the copper, transmission layer and coating material have formed a metallurgical joint.

17. A support bar according to claim 16, characterised in that the transmission layer is tin or a tin-dominant alloy.

18. A support bar according to claim 16 or 17, characterised in that the highly electroconductive coating layer is silver.

19. A support bar according to claim 16 or 17, characterised in that the highly electroconductive coating layer is a silver-copper alloy.

20. A support bar according to any of claims 16 - 19, characterised in that the highly electroconductive coating layer is formed using soldering technique.

21. A support bar according to any of claims 16 - 19, characterised in that the highly electroconductive coating layer is formed using thermal spraying technique.

22. A support bar according to any of claims 16 - 21 , characterised in that the copper core of the support bar is surrounded by a casing of refined steel, titanium or lead, which has been removed from the lower surface of at least one end of the support bar. A support bar according to any of claims 16 - 22, characterised in that the casing of the support bar is made of copper.

A support bar according to any of claims 16 - 22, characterised in that the support bar is made wholly of copper.