GB2177115A - Anode system for the electrolytic production of manganese dioxide - Google Patents

Abstract

An anode system comprising sintered titanium strips 1 disposed in an electrolytic cell for the anodic deposition of electrolytic manganese dioxide; the anode system being supported between one flat cathode 2 and another; the longitudinal axes of the sintered titanium strips lying in the plane of the anode system; and the sintered titanium strips being turned, about their longitudinal axes, out of the plane of the anode system, and including with this plane an angle alpha between 10 and 90 DEG . The sintered titanium strips can be disposed in a zigzag arrangement (Figure 1) or parallel with one another (Figure 2). The anode system of the invention compares favourably with an anode system in plane arrangement as it permits up to 100% and more additional anode surface to be accommodated in the cell and ensures a good adhesiveness of the electrolytic manganese dioxide to the anode system. <IMAGE>

Description

SPECIFICATION Anode system for the electrolyte production of manganese dioxide This invention relates to an anode system of sintered titanium strips hooked in between two flat cathodes each in an electrolytic cell for the anodic deposition of electrolytic manganese dioxide, the longitudinal axes of the sintered titanium strips lying in the plane of the anode system.

For some years, titanium has been gaining increasing interest as a material for making anodes for the production of electrolytic manganese dioxide (briefly termed EMD hereinafter). The reason for this resides in the fact that unlike frequently used graphite it is not subject to wear and unlike lead which is also used remains practically free from corrosion and can therefore be used again and again.

A certain disadvantage of titanium resides in its tendency to become passivated under anodic load, i.e.

at a constant current density to effect an increase of the terminal voltage by the formation of an ill-conducting oxide layer on its surface. In a manganese-ion containing electrolyte however, titanium becomes covered with a well electron-conducting layer of EMD permitting an increase of the terminal voltage to be obviated even at higher current densities as normally allowable for an electrolyte free from manganese, i.e. in dilute sulfuric acid. The EMD-layer does not however protect the titanium anode so reliably that it is impossible for it to become passivated under certain conditions. This phenomenon has been described repeatedly and it has been shown that the sulfuric acid concentration and also the temperature are parameters which critically determine the electrolysis, apart from current density (cf. Chemie-lngenieur-Technik 49, 347 (1977) and GB-PS No. 977-569.

Numerous attempts to overcome these factors handicapping electrolysis have already been made. Typical is e.g. the expensive application of an activating layer on to the titanium surface in order to avoid passivation and in this way to ensure higher current densities and improved economy for an existing facility.

Another interesting method comprises increasing the effective electrode surface and thereby reducing the real current density for a given cell voltage. Thus, it has been suggested that the titanium surface should be sandblasted, i.e. roughened and thereby enlarged. Associated with this is an improved adhesiveness of deposited EMD to the anode (cf. US-PS No. 3 436 323). It has also been attempted-to achieve this by the use of expanded metal (US-PS No. 3 654 102).

Ever since 1952 and 1953, it has been known (US-PS Nos. 2 608 531 and 2 631115) that anodes made up of sintered titanium permit higher current densities to be established in a manganese sulfate bath than anodes made up of titanium plate. Despite this - perhaps for technological reasons - sintered titanium has long failed to gain practical interest in the production of EMD. Only as late as 1976 have industrial electrodes based on sintered titanium been described (DE-A-26 45 414). They are substantially more rigid than the thin titanium plates, for identical production costs.

Currently, the use of lower current densities tends to be favored in the production of EMD so that the activation of titanium is of lesser interest than the use of anodes having a large surface. The mechanical and electric properties of sintered titanium (described in Journal of Metals 34 (1982), pages 37-41) which for procedural reasons is produced in the form of plates or sheets, make sintered titanium an anode material more interesting than rolled titanium plate or massive titanium sheet as anodes thicker than heretofore with a surface roughness favorably influencing the adhesiveness of EMD which is to be deposited can be made at unchanged production costs.

The anodes used heretofore in the electrolytic production of EMD have been given the form of plates of graphite, titanium, titanium expanded metal or sintered titanium, round rods or tubes of lead or graphite (DE-A-28 53 820). An anode formed of two corrugated titanium plates welded together so as to form a tubular structure interconnected by means of small link members has also been described in the art. This is an anode of good rigidity.

An anode for use in the production of EMD has always to comply with two practical requirements, namely to ensure a) good adhesiveness during electrolysis and b) good detachability for the EMD after electrolysis.

These two requirements are contradictory and imply making a compromise. Large-surfaced plane anodes are easy to free from an EMD-layer by hammering, the adhesiveness of the EMD-layers being rather low. Tubular structures on which EMD can freely grow on all sides provide a good basis for EMD-adhesiveness which is the better the smaller the diameter of the tubular structures. They are however spaced apart at small separations so that it is possible for EMD to also grow into the interstices from which they can be removed laboriously only. This is not very critical in the event of deformable lead anodes being used but EMD-removal is a laborious procedure in all those cases in which rigid and fixed anode structures are concerned.

It is therefore an object of the present invention to provide a titanium anode based on sintered titanium combining the advantages of a good EMD-adhesiveness during the electrolysis with a good detachability permitting a large-surfaced anode to be placed in a given cell volume, and full use to be made of the advantages in which sintered titanium compares so favorably with massive titanium.

This object has been achieved by replacing broad sintered metal plates by relatively narrow sintered titanium strips with the dimensions a/b/l (a = width, b = thickness and I = length) which are arranged so that the longitudinal axes of the strips lie in the longitudinal direction of the principal plane of the anode and the strips are turned away around their longitudinal axes from the principal plane of the electrode at a certain angle a of between 10 and 90". The strips may be in parallel with each other or arranged in angled relationship; in this latter case, the angle alternately takes a value of a" and 1800-a. The strips should preferably not be contacting each other, but should be spaced apart.It is preferable for the strips to be fixed hanging from above to below, although the invention also provides for them to be turned at an angle of 90 , i.e. for the strips to be secured horizontally in the cell.

The present invention relates more particularly to an anode system of sintered titanium strips hooked in between two flat cathodes each in an electrolytic cell for the anodic deposition of electrolytic manganese dioxide, the longitudinal axes of the sintered titanium strips lying in the plane of the anode system, characterized in that the sintered titanium strips have their longitudinal axes turned away from, and include with, the plane of the anode system an angle a between 10 and 90".

Further preferred and optional features of the anode system of this invention provide: a) for the sintered titanium strips to be in parallel with each other and for the distance d between the sintered titanium strips, measured in the projection of the strips on to the plane of the anode system, to be < O or O or > 0; b) for the sintered titanium strips and the plane of the anode system alternately to include angle a and l800-a in zigzag arrangement, and for the distance d between the sintered titanium strips, measured in the projection of the strips on to the plane of the anode system, to be 0; c) for the angle a to be between 30 and 70 ;; d) for the sintered titanium strips to have a width which is more than twice their thickness but less than half the distance of one cathode to the next.

The anode system of this invention is shown diagrammatically and in top plan view in Figures 1 - 4 of the accompanying drawing.

Figure 7 shows the sintered titanium strips (1) of the anode system in zigzag arrangement between two flat cathodes (2), the strips being alternately turned away from plane (3) of the anode system at angle a or 180 a.

Figure 2 shows the sintered titanium strips of the anode system in parallel arrangement between two flat cathodes (2), the strips being turned away from plane (3) of the anode system at angle a.

Figure 3 shows two sintered titanium strips as an enlarged representation of a portion of Figure 1, where a stands for the width of the strip, b stands for the thickness of the strip, d stands for the strip separation distance in the projection of the strips on to the plane of the anode system, and D stands for the thickness of the anode system. The angle ,8 between the strip edge a and strip diagonal is of importance to the following calculation formulae.

Figure 3 shows the case where d > 0.

Figure 4 shows two sintered titanium strips as an enlarged representation of a portion of Figure 2 and illustrates the case where d < 0.

The width selected for the strips, the angles a and number of strips selected per anode system depend on the dimensions of an existing cell and the total current load the anode system is exposed to. The formulae indicated below permit the thickness (D) and the effective surface of the anode system (Ooff) as well as the ratio (Q) of the effective (Off) to formal surface (0,,,) of the anode system to be calculated for strips of given dimensions, conditional upon the size selected for angle a. It is inversely possible on the evidence of diagrams made from these formulae inferentially to determine the minimum size to be selected for angle a so as to provide an anode system having a certain effective surface.

00ff = n . (a + b). L. 2 (III) Q = toff (IV); form = 2 B . L (V) Oform In the above formula, n stands for the number of strips per anode system; a stands for the width of the strips; b stands for the thickness of the strips; L stands for the strip length; d stands for the strip separation distance in the projection of the strips on to the plane of the anode system; B stands for the width of the anode system.

The separation distance from cathode to cathode available within the electrolytic cell determines the maximum thickness to be selected for the anode system (D) to which must be added the thickness of the deposited EMD and the minimum distance of the covered anode system from the flat cathodes. Incidentally D is a function of the strip width (a) and angle a but also of strip thickness (b) and angle ss (cf.

equation (II) and Figure 3).

The anode system of this invention compares favorably with an anode system in plane arrangement as it permits up to 100 % and more additional anode surface to be accomodated in an electrolytic cell and as the geometrical configuration selected ensures a good EMD-adhesiveness to the anode system.

Example 1 Zigzag arrangement of sintered titanium strips in anode system Strip width (a) = 4 cm Strip thickness (b) = 0.8 cm Strip separation (d) = 0.2 cm Formal width of anode system: 100 cm Formal length of anode system: 100 cm Formal bilateral surface of anode system ( form) 2 m2 = = effective surface of anode system O,ff Ofo, n = number of strips per anode system D = thickness of anode system TABLE 1 a n D O(eff) Q (degrees) (cm) (m2) 10 22 1.48 2.11 1.06 20 22 2.12 2.11 1.06 30 23 2.69 2.21 1.10 40 25 3.18 2.40 1.20 50 27 3.58 2.59 1.30 55 29 3.74 2.78 1.39 60 32 3.86 3.03 1.54 65 35 3.96 3.36 1.68 70 39 4.03 3.74 1.87 75 45 4.07 4.32 2.16 80 53 4.08 5.09 2.54 (90 83 4.00 7.97 3.94) Table 1 shows that deviations of up to a = 20 fail to produce any significant advantage over the plane arrangement (a = 0 ) of the sintered titanium strips at a strip separation (d) of 2mm, as the interstices between the strips become very rapidly bridged by EMD so that the anode system then continues to work the same way as a plane anode system, save that it is under inner stress. Deviations of more than a 700 are also unfavorabie as the free space left between the strips is continuously becoming narrower so that EMD is increasingly more difficult to remove; with a = 900, the zigzag arrangement is ultimately left and some sort of a plane but unreasonably thick anode is obtained again.As can be seen, it is optimal for the sintered titanium strips to deviate along their longitudinal axes from the plane of the anode system at an angle a of 30 - 70" or alternately of 150 - 110 . Table 1 also shows that the thickness of the anode system in an angle region of about 70" to less than 90" exceeds 4 cm as the diagonal breadth of the sintered titanium strips commences working out here.

Example 2 Inclined parallel arrangement of sintered titanium strips in anode system.

The dimensions are as in Example 1 with the following exceptions: Strip width (a) = 3 cm Strip thickness (b) = 0.6 cm Strip separation (d) = 0 TABLE 2 a n D Ofeffl Q (degrees) (cm) lem2) (0 33 0.6 2.38 1.19) 10 32 1.11 2.30 1.15 20 33 1.59 2.38 1.19 30 34 2.02 2.45 1.22 40 37 2.39 2.66 1.33 50 41 2.68 2.96 1.48 55 45 2.80 3.24 1.62 60 49 2.90 3.53 1.76 65 55 2.97 3.96 1.98 70 62 3.02 4.46 2.23 75 73 3.05 5.26 2.63 In the event of a smaller effective surface being actually needed, it is naturally also possible for the number n to be reduced, i.e. for a smaller number of strips to be placed in the anode system at larger separations d.

It will be understood that the invention has been described above purely by way of example, and that various modifications of detail can be made within the ambit of the invention.

Claims (11)

1. Anode system of sintered titanium strips hooked in between two flat cathodes each in an electrolytic cell for the anodic deposition of electrolytic manganese dioxide, the longitudinal axes of the sintered titanium strips lying in the plane of the anode system, characterized in that the sintered titanium strips have their longitudinal axes turned away from, and include with, the plane of the system an angle a between 10 and 90".

2. Anode system as claimed in claim 1, wherein the individual sintered titanium strips are in parallel with each other.

3. Anode system as claimed in claim 2, wherein the distance d between the sintered titanium strips, measured in the projection of the strips on to the plane of the anode system is < O or O or > 0.

4. Anode system as claimed in claim 1, wherein the sintered titanium strips and the plane of the anode system alternately include angles a and 1800 -a in zigzag arrangement.

5. Anode system as claimed in claim 4, wherein the distance d between the sintered titanium strips, measured in the projection of the strips on to the plane of the anode system is ~ 0.

6. Anode system as claimed in any one of the preceding claims, wherein the angle c is between 30 and 70"C.

7. Anode system as claimed in any one of the preceding claims, wherein the sintered titanium strips have a width which is more than twice their thickness but less than half the distance from one cathode to the next.

8. Anode system as claimed in any one of the preceding claims substantially as described in Example 1 or 2 herein.

9. Anode system as claimed in any one of the preceding claims substantially as described herein with reference to the accompanying drawing.

10. Process for the production of electrolyte manganese dioxide by anodic deposition, wherein an anode system as claimed in any one of the preceding claims is employed.

11. Electrolytic manganese dioxide produced by a process as claimed in claim 10.