GB2100288A - Electrode for electrolytic production of manganese dioxide - Google Patents

Abstract

In titanium electrodes for the electrolytic production of manganese dioxide in which the electrodes are of corrugated tubular form e.g., as in Fig. 3, the tubes are provided with a filling which may provide rigidity to the electrode and/or increase the corrosion resistance of the interior of the tubular portion of the electrodes. The filling may be complete or partial e.g. mesh, coatings, porous members, strips and may be of electrolyte resistant materials such as polymers, polypropylene rod in a rubber matrix, materials providing corrosion resistance to the interior of the tubes e.g. Pd, Ti-Pd alloy, manganese dioxide. <IMAGE>

Description

SPECIFICATION Electrode This invention relates to electrodes and has particular reference to electrodes for use in the electrolytic production of manganese dioxide.

The use of permanent titanium electrodes in electrometallurgy is increasing. The permanent titanium electrodes show advantages over other materials in that their durability and their ability to produce good electrodeposits on a long term basis is high.

In US Patent No. 4 319 977 there is described an anode which is in the form of opposing plates, so arranged as to provide a series of vertically extending tubes which are located in the electrolyte. This anode has proved to be a very effective anode for use in manganese dioxide production and the use of a corrugated crosssection has unexpectedly given improved properties in the manganese dioxide manufactured on the electrode. The design illustrated in Figures 6 to 8 of the abovementioned US patent has a number of attractive features in terms of rigidity, ease of handling and general strength of the electrode. Basically the design incorporates a series of vertically extending, open ended tubes which dip into the electrolyte. The electrodes perform very satisfactorily in terms of their efficiency in producing high quality manganese dioxide.The present invention is concerned with an electrolytic manganese dioxide anode which is an improvement in that illustrated in US Patent No.

4319977.

By the present invention there is provided an electrode for use in the electrolytic production of manganese dioxide, the electrode including two continuous sheets of metal joined in face-to-face relationship, one at least of the sheets being corrugated so as to provide rigidity to the electrode, the two sheets being joined to a hanger bar wherein the improvement comprises a filling in the tubes formed between the corrugations of the at least one sheet and the other sheet.

The sheets are preferably formed of titanium.

The hanger bar is preferably formed of a copper cored titanium clad bar. The hanger bar is preferably welded to the sheets.

The titanium may be a titanium alloy.

The filling may provide additional rigidity to the electrode and may include one or more materials chosen from a group resistant to the electrolyte, such as the group polypropylene, polyethylene, silicone rubber, furan resin or a hot melt adhesive.

The filling may be a combination of such materials such as a combination of a polypropylene rod in a silicone rubber matrix. The silicone rubber or other material may be provided with an inert extender such as silicon dioxide.

The filling may provide a corrosion resistance to the interior of the tubes and may be either a material which has in use a lower hydrogen overpotential than titanium or a material which can undergo a cathodic reaction other than hydrogen evolution at a potential more anodic than the corrosion potential of the titanium and is in electrical contact with the titanium.

The material may be palladium or a titaniumpalladium alloy, a typical alloy being titanium plus 0.2% palladium. The titanium-palladium alloy may be in the form of a wire or mesh inserted into the tubular portion of the electrode.

Alternatively the material may be manganese dioxide or a mixture of manganese dioxide and graphite. The manganese dioxide may be provided by filling the tubes with manganese dioxide before use of the electrode, or may be provided in situ by chemical oxidation of the Mn" ions in the electrolyte to Mniv. A most suitable oxidising agent is the permanganate ion. Potassium permanganate crystals may be inserted into the interior of the tubes before use. Alternatively a concentrated solution may be dispensed into the tubes.

The filling may be a combination of a material providing rigidity and a material enhancing the corrosion resistance of the electrode. Thus there may be provided manganese dioxide or a titaniumpalladium alloy on the interior of the tubular portion together with a rigid filling.

The tubes in the electrode may be so arranged as to be substantially vertically disposed in use.

The lower ends of the tube or tubes may be sealed or partially obturated, for example by crimping.

The corrugated sheets may be formed so as to extend over the entire length of the hanger bar and may be cranked to provide additional strength in the region of the hanger bar. The upper and/or lower ends of the corrugations may be rounded and terminated.

The sheets of metal may be formed of zirconium and the electrode may be used as a cathode for hydrogen evolution in an electrolyte such as a manganese sulphate solution.

It will be appreciated that the term "filling" as used herein is not intended to mean only a complete filling but includes within its meaning partial fillings such as mesh, coatings, porous members, strips and other items located within the tubular members.

By way of example embodiments of the present invention will now be described with reference to the accompanying drawings, of which: Figure 1 is a perspective view of an electrode in accordance with the present invention; Figure 2 is a side elevational view of an electrode in accordance with the present invention; Figure 3 is a view along the line Ill-Ill of Figure 1; Figure 4 is a plan view of an electrolytic cell incorporating electrodes in accordance with the present invention; Figure 5 is a side elevational view of a second embodiment of the present invention; Figure 6 is a sectional view along the line VI--VI of Figure 5; Figure 7 is a sectional view of one strip; Figure 8 is a side elevational view of the strip of Figure 7 at one end; Figure 9 is a view of the strip illustrated in Figure 8 after subsequent forming;; Figure 10 is a schematic cross-sectional view similar to Figure 3; and Figure 11 is a diagram of corrosion current against titanium electrode potential v in a typical manganese sulphate plus sulphuric acid electrolyte.

Referring to Figures 1 and 2 these show an electrode for use in a manganese dioxide production cell. The electrode comprises a hanger bar 1 having a titanium sheath 2 and a copper core 3. The titanium sheath is partially removed at a position 4 so as to reveal the copper core 5. The portion 5 of the copper core makes electrical contact with an electrical current feed located alongside the cell in a normal manner.

Dependent from the hanger bar 1 is the working portion of the electrode indicated generally by 6 on to which manganese dioxide is deposited during operation of the electrode. The portion 6 is formed of a series of corrugated sheets which are welded together in face-to-face relationship. A series of ears 7 extends upwardly from the corrugated sheets and are integral with the sheets. The ears 7 are spot welded at 8 to the titanium sheath 2 of the hanger bar 1.

As is most clearly illustrated in Figure 3 the corrugated sheets making up the portion 6 form a series of parallel tubes 9, 10 which are effectively interconnected by fianges 11, 12. The design illustrated in Figure 3 is made of a series of strips, each strip having two corrugated portions and the strips being staggered to produce the design illustrated. The end strips 13, 14 have only a single corrugation so as to form a uniform design.

Located within the tubes thus formed is a filler material indicated generally by 15, the function and composition of which will be described in more detail below.

With the design illustrated in US Patent No.

4 319 977 a problem can arise from blockages in the ends of the tubes. The manganese dioxide produced on the electrodes is, after harvesting, passed through a series of processing stages by handling equipment. The handling equipment can become corroded by the strongly acid electrolyte liquor if the manganese dioxide contains excessive quantities of the electrolyte liquor. With the design illustrated in US Patent No. 4 319 977 the electrolyte liquor can theoretically pour out of the open ends of the tubes as the tubes are lifted from the electrolyte. However, occasional blockages of the ends of the tubes can arise, resulting from the formation of growths of manganese dioxide over the lower ends of the tubes. This can result in sudden surges of electrolyte falling out of the tubes when the blockages are loosened during the harvesting procedure.Even if the bottom ends of the tubes are sealed permanently by, for example, crimping it is possible to get acid leaking out of the tubes.

The use of solid fillings physically prevents acid from getting into the inside of the tubes in large quantities and when manganese dioxide is present on the inside of the tubes there is inhibition of the corrosion which might otherwise occur, particularly with those designs in which the fillings do not completely exclude the electrolyte.

It can be appreciated, therefore, that a resilient filling will reduce the quantity of liquor which can find its way into the tubes thereby reducing the quantity of liquor which can be carried over into the harvested manganese dioxide and hence corrode the subsequent handling equipment.

Surprisingly it has been found that deliberate manganese dioxide addition to the interior of the tubes provides a very effective method of reducing corrosion within the tubes. Given the fact that manganese dioxide is formed on the anode in use, the fact that the manganese dioxide added deliberately at the beginning can significantly reduce corrosion is extremely surprising. It is believed that the titanium of the anode surface can exist effectively in two states. in one state corrosion continues and the interior of the tubes can be destroyed by the corrosion. In the second state the titanium can be inert.When the anode is used without the filling it is possible that a small quantity of manganese dioxide can be deposited on the interior of the tube gradually during operation of the tube but at such a slow rate, particularly at the upper ends of the tubes, that the corrosion-free state is never reached. By deliberatately providing manganese dioxide in the tubes the state of no corrosion is readily attained.

In an alternative form of the invention the filling 1 5 is in the form of a solid semi-rigid material such as a low melting point polyethylene. Such a material would be poured hot into the tubular portions and would be permitted to solidify on cooling. An alternative filler material would be solid polypropylene or a silicone rubber rather than polyethylene. However, in view of the expense of silicone rubbers it may be desirable to partially fill the tubular portions with the rubber and to insert a polypropylene rod such as rod 1 7. This will result in the silicone rubber being squeezed into all the interstices of the tubes and an adequate filling can thus be provided.In addition to the advantage of reducing the spillage of partially retained liquid such a filling has now been found to have the advantage that it assists in the removal of the electrolytically deposited manganese dioxide and reduces damage to the electrodes. Because of the natural resilience of the material it has been found that when the electrodes are tapped to remove the manganese dioxide the resilience enhances removal significantly. The filling has also been found to reduce damage to the electrodes which can otherwise occur during the removal process.

Referring to Figure 4 this shows an electrolytic cell 1 8 in which a series of cathodes 19, 20, 21 are separated from one another by a series of anodes 22, 23, 24. The operation of such a cell is from thereon conventional.

Referring to Figures 5 to 9 these show an alternative form of electrode which has further improved rigidity than that illustrated in Figures 1 to 3 and when combined with a rigidising interior filling produces a very advantageous electrode structure. Essentially the main difference between the embodiment illustrated in Figures 5 to 9 and the embodiment illustrated in Figures 1 and 2 is that the corrugated sheets are so formed as to overlap with the hanger bar over their entire width. The hanger bar 25, again formed of an outer sheath 26 of titanium with a copper core 27, has spot welded to it as at 28, 29 the upper edges of a corrugated face of the anode work sheet 30. It will be seen that the corrugations, such as corrugation 31, extend virtually to the upper edge 32 of the sheet and certainly over the cranked or transition region 33.In the transition region the corrugations are indented as at 34 to give added strength to the transitional region. The ends of the corrugations as at 35 are rounded and terminated to produce a sturdy and attractive design.

The electrode is manufactured by preforming stripes, such as the strip illustrated in Figure 8, to produce a pair of corrugations 36, 37 which, as illustrated, extend longitudinally out from the plane of the paper. The strips are then pressed by suitable tools to the shape illustrated in Figure 9.

Essentially the pressing provides a crank 38 together with the indentations 39, 40 in each of the corrugations to reinforce the cranked zone.

The corrugations are flattened and rounded at the ends 41, 42. This results in the formation of ears of titanium 43, 44 which are cropped off along the dotted lines 45, 46 to produce straight sides again. These strips are then joined together by spot welding as at position 47 in Figure 5 and the assembled anode face is then spot welded to the copper cored titanium hanger bar.

It will be appreciated that wider strips may be used having a greater number of corrugations.

Thus strips with 5 or more corrugations may be used. Although such designs are more easily fabricable they are found to require greater corrosion protection to prevent corrosion occurring in the interior of the tubes. Frequently the strip designs of electrodes would be manufactured around the filling members such as the polypropylene rods.

In order that the welded joints may be provided with extra rigidity, and to enable the spot welds to be located with greater tolerances, the overlap between adjacent sections may be modified from that illustrated in Figure 3. Referring to Figure 10 this shows one corrugated member 48 which is opposed by two corrugated sheets 49 and 50. The corrugated sheets are spot welded together at 51, 52. Where the edges of the sheets 49 and 50 meet, however, the ends 53 and 54 overlap. A single spot weld 55 is then used to join together layer 56 of sheet 48 and portions 53 and 54. By the use of such a three-layer overlap enhanced rigidity is provided to the electrode and the tolerances associated with location of the spot welds are increased.With the design illustrated in Figure 3 it is necessary to provide either multiple spot welds or, if a single spot weld is used, as in Figure 5, spot weld 47, the weld must be accurately located so as to join together all three sheets.

The manganese dioxide or titanium-palladium alloy corrosion inhibiting filling may be combined with one or more of the other types of fillings. A particularly advantageous method of providing manganese dioxide in the tubes is to chemically oxidise the manganese sulphate solution which forms the working electrolyte of the manganese dioxide cell. Oxidation of the manganese sulphate causes the Mn" ions to be oxidised to Mn'V as manganese dioxide which is deposited out of solution. Any suitable oxidising agent or mechanism may be used. A particularly suitable material is a soluble permanganate such as potassium permanganate. Crystals of potassium permanganate can be inserted into the tubes before the electrodes are inserted into the electrolyte.The crystals, on contacting the electrolyte, dissolve and the oxidising solution thus formed oxidises the manganese sulphate solution to form manganese dioxide which occurs preferentially on the surface of the metal forming the electrode. A particularly advantageous feature of the use of a permanganate is that manganese dioxide is produced by the reduction of the permanganate ion itself as well as by the oxidation of the manganese sulphate.

In order that the mechanism of the protection of the interior of the tubular portions may be understood, although the explanation given below is given without any intention of restricting the scope of the invention and without prejudice to the novelty or inventiveness of the invention, reference will be made to Figure 11 of the accompanying drawings. In Figure 11 there is shown a graph of corrosion of titanium as measured by the corrosion current i against the applied potential v with respect to a saturated mercurous sulphate electrode. Effectively the corrosion rate of titanium follows the solid line 57 and it can be seen that the corrosion rate increases to a maximum as the titanium becomes more cathodic, i.e. as the titanium goes in the negative potential direction. The normal hydrogen evolution potential/current line for titanium is shown dotted at 58.It can be seen that the crossover point is at 59 and in a free solution the crossover point indicates the rate of corrosion of titanium. It can be seen that at that point the corrosion rate is almost at its maximum.

By comparison, the potential for hydrogen evolution on a titanium-palladium alloy is shown by the dotted line 60. From this it can be seen that the crossover point at 61 corresponds to a lower rate of corrosion of the titanium. Thus the presence of titanium-palladium alloys in the tube will lead to hydrogen evolution on the titaniumpalladium alloy, will render the titanium more anodic than it would otherwise be and hence reduce the rate of corrosion. The dotted line 62 corresponds to the potential/current graph for the reaction MnO2 going to a lower valent manganese oxide believed to be MnOOH. It can be seen that the crossover point in this case is at 63 and again corresponds to a reduction in the corrosion rate of the titanium. The operating potential for the anode normally is approximately +1 v on this scale.It will be seen, therefore, that for maximum corrosion protection the material selected for use inside the tube should render the titanium of the tube more anodic than it would otherwise be, thus protecting the interior of the tube.

Although it might be thought that rendering the material more cathodic would also improve the corrosion rate, this is not possible as the titanium will simpiy evolve hydrogen over its own surface and thus it would not be practical to move down the curve to the right beyond the position 59.

It will be appreciated that any suitable material could be used which complies with the requirements set out above, provided it did not interfere with the effective portion of the cell. The materials proposed in this specification are by reason of example only.

The two state aspect of titanium appears to account for the fact that the provision of manganese dioxide on the interior of the tubes can significantly restrict corrosion of the tubes.

However, if the manganese dioxide is not provided initially the reaction inside the tubes may, in adverse circumstances, be corrosion of the titanium.

Although the description above has made reference to sulphate electrolytes other anions such as chloride, nitrate or acetate may be used.

Although the greater part of the description has been concerned with titanium electrodes, and in particular titanium anodes for use in manganese dioxide production, the design of the invention could be utilised with zirconium sheets. In such an arrangement the zirconium sheets could be used as cathodes in sulphate-containing solutions such as manganese sulphate, sulphuric acid or acidified sulphate-containing solutions. In sulphatecontaining solutions the cathode reaction is frequently the evolution of hydrogen. If titanium is used as a cathode it may hydride, and this may reduce its economic life in some circumstances.

Zirconium, however, hydrides more slowly than titanium and as a result zirconium corrugated electrodes may be utilised as cathodes in manganese dioxide production cells, replacing the presently used graphite or lead electrodes.

Claims (23)

1. An electrode for use in the electrolytic production of manganese dioxide, the electrode including two continuous sheets of metal joined in face-to-face relationship, one at least of the sheets being corrugated so as to provide rigidity to the electrode, the two sheets being joined to a hanger bar wherein the improvement comprises a filling in the tubes formed between the corrugations of the at least one sheet and the other sheet.

2. An electrode as claimed in Claim 1 in which the filling provides additional rigidity to the electrode.

3. An electrode as claimed in Claim 2 in which the filling includes one or more materials chosen from the group polypropylene, polyethylene, silicone rubber, furan resin or a hot melt adhesive.

4. An electrode as claimed in Claim 3 in which the filling includes a combination of materials together with an extender such as silicon dioxide.

5. An electrode as claimed in Claim 1 in which the filling provides a corrosion resistance to the interior of the tubes.

6. An electrode as claimed in Claim 5 in which the filling is a material either having a lower hydrogen overpotential than titanium or being a material which can undergo a cathodic reaction other than hydrogen evolution at a potential more anodic than the corrosion potential of the titanium and is in electrical contact with the titanium.

7. An electrode as claimed in Claim 5 in which the material comprises palladium or a titaniumpalladium alloy.

8. An electrode as claimed in Claim 7 in which the titanium-palladium alloy is in the form of a wire or mesh inserted into the tubular portion of the electrode.

9. An electrode as claimed in Claim 5 in which the filling is selected from the group manganese dioxide or a mixture of manganese dioxide and graphite.

1 0. An electrode as claimed in Claim 9 in which the manganese dioxide is formed in situ in the electrode by the oxidation of Mn" to Mniv in the electrolyte.

1 An electrode as claimed in Claim 10 in which the manganese dioxide is formed by oxidation of the Mn" ions by soluble permanganate, preferably potassium permanganate.

12. An electrode as claimed in Claim 11 in which the potassium permanganate is provided in crystalline form in the electrode before use.

13. An electrode as claimed in Claim 1 in which the filling is a combination of material providing rigidity and material enhancing the corrosion resistance of the electrode.

14. An electrode as claimed in any one of Claims 1 to 1 3 in which the sheets are formed of titanium or zirconium.

1 5. An electrode as claimed in Claim 14 in which the hanger bar has a copper core and has metallurgically bonded to the core cladding of titanium.

16. An electrode as claimed in Claim 1 5 in which the titanium cladding covers the entire circumference of the core.

17. An electrode as claimed in Claim 16 in which the hanger bar is welded to the sheets.

1 8. An electrode as claimed in any one of Claims I to 17 in which the lower ends of the tube or tubes are sealed or partially obturated by crimping.

19. An electrode as claimed in Claim 1 in which the corrugated sheets are formed so as to extend over the entire length of the hanger bar and are cranked to provide additional strength in the region of the hanger bar.

20. An electrode as claimed in Claim 19 in which the upper ends of the corrugations are rounded and terminated.

21. An electrode substantially as herein described with reference to and as illustrated in Figures 1 to 3 or Figures 5 to 9 of the accompanying drawings.

22. An electrolytic cell for the production of manganese dioxide incorporating electrodes as claimed in any one of Claims 1 to 21.

23. An electrolytic cell as claimed in Claim 22 in which the tubes of the electrode are so arranged as to be substantially vertically disposed in use.