IE46061B1 - Manufacture of titanium anodes suitable for use in the electrolytic production of manganese dioxide - Google Patents

Description

This invention relates to a process for making titanium anodes which are suitable for use in the electrolytic production of manganese dioxide, the anodes comprising a titanium cbre whose surface is at least partially coated with an activated covering layer which is formed thereon by applying a noble metal thereto.

It is known that activated metal anodes can be used in electrolytic processes. More particularly, titanium anodes which have been activated by means of a noble metal are employed in the electrolysis of alkali metal chlorides. Use has also been made of these titanium anodes in the electrolytic production of manganese dioxide on a laboratory scale.

These metal anodes, e.g, titanium anodes, have heretofore been activated either by the thermal decomposition of a noble metal salt on the surface of the anode core metal in contact with air temperatures approximately of 55O°C, or by electro-depositing the noble metal on the core metal.

Anodes which are activated in the manner just described are suitable for use in all electrolytic operations producing gaseous or electrolyte-soluble products.

Difficulties have, however, been encountered in the electrolytic production of manganese dioxide, which in a known process, is deposited in the form of a solid material on the anode, from which, after termination of the electrolysis, it has to be removed, and inasmuch as the noble metal layer adheres only moderately strongly to the surface of the core metal it is therefore very liable to be removed, at least partially, together with the manganese dioxide. When replaced in the electrolytic bath, such an anode naturally cannot regain its full initial activity, so that, after a relatively short operation period, manganese dioxide is liable to be irregularly deposited on the anode, and the average potential difference between the anode and cathode rapidly increases and exceeds the maximum acceptable value.

Apart from these difficulties, it is an important requirement for noble metal coated anodes that they should remain serviceable over long periods, for reasons of economy.

Xt is therefore an object of the present invention to provide a process for making a titanium anode activated by means of a noble metal coating which has improved adhesion to the core metal, and improved resistance to mechanical stress, in comparison with the prior anodes of which we are aware.

The present invention now unexpectedly provides, in a process for making titanium anodes activated by means of a noble metal by applying the noble metal to the surface of a titanium anode core by cathodic deposition from a solution of a salt of the noble metal, or by thermal decomposition of a solution of a salt of the noble metal, the improvement which comprises heating the anode so treated at a temperature of 800 to 1000°C for 3 to 5 hours with the resultant formation of a coating on the anode surface consisting of an alloy of titanium and the noble metal, this improvement providing an improved adhesion between the surface layer and the core and an improved resistance to mechanical stress, whereby the 061 - 4 anode is made suitable for use in electrolytic manganese VVmhh't t«'n.

Although an act- ί val-lmj ooatlug can he ptuiluoeul on a titanium anode core at temperatures lower than 800°C or higher than 1000°C, the fact remains that these coating operations take place at reaction velocities which are not as favourable as might be desired, so that the resulting coatings are not recommended for technical use.

It is preferable for the heating of the treated anodes to be carried out in an atmosphere of an inert gas, or under reduced pressure, e.g. a pressure lower than —6 -7 —8 bar, preferably 10 to 10 bar. Argon may more preferably be used as the inert gas, and the noble metal employed is preferably gold or a metal belonging to the group of platinum metals, e.g. platinum, iridium or ruthenium. If desired, use may be made of more than one noble metal? for instance, platinum and iridjim may be used together (cf. Example 1 below). The group of platinum metals referred to herein comprises Ru, Rh, Pd, Os, Ir and Pt. In order that the manganese dioxide may be effectively held by the anode, it is good practice to use, as the titanium anode core, a titanium sheet provided at regular or other intervals with apertures permitting deposits of manganese dioxide precipitated on the two sides Of the sheet to grow towards one another. Use is more preferably made of a titanium anode core in the form of expanded metal, double nosed sheet metal, or slotted sheet metal.

In the accompanying drawings, Figures la, lb and lc show specimens of the abovementioned expanded metal, double nosed sheet metal, and slotted sheet metal, respectively, and Figures 2, 3 and 4 are graphs which are explained below.

An anode core comprising a structure made from a - 5 titanium sponge which is compressed and subsequentlysintered under vacuum can also be used.

It is also preferable to activate by means of the noble metal only that portion of the anode which is to be immersed in the electrolytic bath, as hydrogen evolved during electrolysis would be liable to be adsorbed by a noble-metal-coated portion of the anode disposed above the bath, and could soon cause embrittlement and breakage of the anode.

In the anode treatment in accordance with this invention, it is possible for the noble metal atoms to diffuse into the substrate, i.e. the titanium anode core, so that a layer comprising an alloy of the noble metal and the titanium is formed on the anode surface. This alloy layer forms an integral part of the substrate in that it is substantially no longer liable to be removed from the anode under mechanical stress. X-ray investigations made on surfaces treated in the manner described have given evidence of the presence of inter—metallic phases, e.g. of the type MeTi or MeTi^, where Me stands for Pt, Ru or Ir. As already indicated hereinabove, the noble metal employed is preferably gold or a motal belonging to the platinum group, a mixture of two or more of the latter being used if desired.

The thermal treatment comprising heating to 800 to 1000°C should desirably be taken to the stage at which diffusion begins, but it is not advisable to take it to the stage where the bulk of the noble metal applied to the titanium substrate would be liable to penetrate deeply into the interior of this substrate. If this were the case, the anode surface would be liable to have too small a content of noble metal, with the resultant formation of an alloy layer whose electro-chemical behaviour would increasingly approach that of the core metal, i.e. titanium, itself. 6 0 61 It is also preferable for the noble metal layer applied to the anode surface to have a thickness of at least 10 mm and to contain at least 1-2 mol % of noble metal. Factors v/hich critically determine the optimum diffusion conditions are the type of furnace used, the temperature selected, the length of the treatment period, and the nature of the noble metal used. The conditions employed should desirably be arrived at empirically in each particular case, e.g. with the use of a microprobe.

Prior to applying the noble metal layer to the anode surface, the latter should preferably be subjected to a preparatory treatment, e.g. to treatment with a degreasing or fat-dissolving egent (e.g. an alcohol, halogenated hydro-carbon or surfactant), followed by sand-blasting, if necessary or desirable.

In addition to the various forms of titanium anode core metal receiving the active noble metal layer described hereinabove, it is possible to use this core metal in the form of (e.g.) a tube, rod or net, according to the requirements in respect of cell design which apply in any given case. It is, however, particularly advantageous to use slotted sheet metal (cf. Figure lc) or double nosed sheet metal (cf. Figure lb), these forms of titanium core metal permitting the precipitating manganese dioxide to be effectively held by the anode.

The quality of titanium anodes made by the present process was tested during electrolysis. To this end, the anodes were freed after a predetermined period from manganese dioxide which had been precipitated thereon. The manganese dioxide was mechanically removed from the anode by bending or more generally by a knocking treatment, and the electrode was used again in a similar electrolytic operation. This procedure, herein termed an operating cycle, was repeated until the anode-cathode potential difference during the electrolysis reached or exceeded a - 7 value at which operation under commercially attractive conditions was no longer possible, or until the manganese dioxide began precipitating irregularly so that economic utilization of the cell capacity was no longer possible.

The anodes of the present invention were, more particularly, tested under the following conditions: Temperature of electrolytic bath: 95°C Manganese concentration in electrolytic bath: 0.7 mol/litre Sulphuric acid concentration in electrolytic bath: 0.7 mol/litre Current density: 1.2 amperes/dm , calculated on the formal surface.

The term formal surfcce means twice the area calculated from the overall dimensions (usually length and breadth) of the anode which is immersed in the bath.

The efficiency of the anodes made by the present process was tested in a series of experiments. The test results are indicated in the graphs comprising Figures 2 to 4 of the accompanying drawings. Figure 2 shows more specifically the efficiency (as represented by the anodecathode potential difference for up to 200 days) of a noble metal coated (actually ruthenium-coated) anode made from titanium sponge which was compressed and subsequently sintered. In this connection, it is interesting to note that by virtue of the temperatures used for the sintering treatment under vacuum, the ruthenium was not in the form of ruthenium dioxide, no blue-black coloration being observable such as is typical of commercial grade dimensionally-stable titanium anodes intended for use in the production of chlorine. The anodes prepared in accordance with this invention (curves 2, 3 and 4) were silvercoloured in all those cases in which they were coated with platinum metals. Figures 2 also shows the influence, on - 8 the service life of the anode, of the quantity of noble metal (Ru) deposited on the anode core metal. The service life of the anode is approximately proportional to the quantity of ruthenium deposited (as described in the following Example 2, but with various quantities of ruthenium).

Figure 3 illustrates the variation in respect of time (again up to 200 days) of the anode-cathode potential difference for a platinum-coated anode whose Rt coating was produced by the decomposition of hexachloroplatinic acid at 55O°C in contact with air (curve 1), the abovementioned variation being compared with the variation of the anode-cathode potential difference with a group of anodes made in accordance with this invention and employing expanded titanium sheet (curve 2).

The various noble metals naturally differ in efficiency, and different quantities of noble metal have to be used to obtain anodes of a given service life but employing different noble metals. This is exemplified in Figure 4 for anodes whose substrates were of double nosed sheet metal coated with ruthenium in four cases (curves 1, 2, 3 and 5) and platinum in one case (curve 4).

EXAMPLE 1 An anode was coated with 8 g/m of ruthenium.

For this purpose, a plate of sintered titanium sponge was degreased and painted with a solution com5 posed of: 146 g of RuC13 . 4 H20 430 g of concentrated hydrochloric acid and 430 g of ethanol.

Next, the plate was dried at 120°C and subsequently heated for 45 minutes at 55O°C in contact with air. The procedure of painting and heat-treating was carried out three more times, and the plate was thereafter heated for 4 hours at 900°C under vacuum of 10 ? bar.

EXAMPLE 2 An anode was coated with 10 g/m of platinum.

For this purpose, an expanded titanium sheet was degreased, sand-blasted and wetted by dipping it in a solution composed of: 34-6061 - 10 220 g of H2PtClg . 4.3 H20 445 g of concentrated hydrochloric acid and 445 g of ethanol.

The expanded sheet was dried at 120°C and heated for 30 minutes at 550° C in contact with air. The procedure of dipping the electrode in the solution, drying and heating it in contact with air was carried out two more times, and the expanded metal was then heated for 5 hours at 800°C under argon.

EXAMPLE 3 An anode was coated with 20 g/m of platinum by electrodeposition.

Eor this purpose, sintered titanium plates prepared from titanium sponge were degreased and immersed in an electrolytic solution composed of: 200 g of H2PtCl6 1000 g of Na2HPO4 200 g of (NH4)2HPO4 50 g of NHdCl and litres of water.

The solution was used at a temperature of 68°C, and 2 the electrolytic current density was 0.5 ampere/dm (the anode-cathode potential difference being 1.5 volts).

After 120 minutes, the plates were taken out, dried, and heated for 3 hours at 1000°C

Claims (12)

1. CLAIM Si1. In a process for making titanium anodes activated by means of a noble metal by applying the noble metal to the surface of a titanium anode core by

2. Process as claimed in claim 1, wherein the heating of the treated anodes is carried out under reduced pressure or in an atmosphere of an inert gas.

3. Process as claimed in claim 2, wherein the said 20 heating of the anodes is carried out under a pressure lower than 10 -6 bar.

4. Process as claimed in claim 3, wherein the said heating of the anodes is carried out under a pressure of 10 ? to 10 -8 bar. 25 5. Process as claimed in any of claims 1 to 4, wherein the noble metal employed is gold or a metal belonging to the group of platinum metals.

5. Double nosed sheet metal, or slotted sheet metal. 5 cathodic deposition from a solution of a salt of the noble metal, or by thermal decomposition of a solution of a salt of the noble metal, the improvement which comprises heating the anode so treated at a temperature of 800 to 1000°C for 3 to 5 hours with the resultant

6. Process as claimed in claim 5, wherein the noble metal employed is platinum or ruthenium.

7. Process as claimed in any of claims 1 to 6, wherein the anode core is a titanium sheet provided at intervals with apertures permitting deposits of - 12 manganese dioxide precipitated on the two sides of the sheet to grow towards one another.

8. Process as claimed in claim 7, wherein the titanium anode core is in the form of expanded metal,

9. Process as claimed in any of claims 1 to 6, wherein the anode core used comprises a structure made from a titanium sponge which is compressed and subsequently sintered under vacuum.

10. 10. Process as claimed in any of claims 1 to 9, wherein only that portion of the anode which is to be immersed in the electrolytic bath is activated by means of the noble metal. 10 formation of a coating on the anode surface consisting of an alloy of titanium and the noble metal, this improvement providing an improved adhesion between the surface layer and the core and an improved resistance to mechanical stress, whereby the anode is made suitable 15 for use in electrolytic manganese dioxide production.

11. Process for making titanium anodes conducted 15 substantially as described in any of the Examples herein

12. Titanium anodes whenever obtained by a process as claimed in any of claims 1 to 11.