NO830562L - ELECTRODE, SPECIAL ANODE FOR THE DEVELOPMENT OF OXYGEN IN ACID ELECTROLYTES, AND PROCEDURES IN THE PREPARATION OF IT - Google Patents

Description

The technical area

The present invention relates to dimensionally stable electrodes, and more particularly anodes for oxygen development in an acidic electrolyte, of the type used e.g. by methods for the electroextraction of metals from acidic electrolytes.

State of the art

Lead or lead alloy anodes have been widely used for methods of electrorecovery of metals from sulfate solutions. However, they are nevertheless subject to less important limitations, such as a high oxygen overvoltage and loss of anode material leading to contamination of the electrolyte, and also of the metal product formed on the cathode.

Anodes of lead-silver alloy provide some reduction of the oxygen overvoltage and an improvement of the current yield, but they are still subject to the above-mentioned limitations as a whole.

It has been proposed to use dimensionally stable titanium anodes with a platinum metal oxide coating for anodic evolution of oxygen, but such anodes are generally exposed to a more or less rapid passivation and oxidation of the titanium base.

It has also been proposed to give the titanium base a protective undercoating comprising a platinum group metal under the outer coating, but such coatings generally do not provide sufficient protection of the titanium base to justify the high price when using precious metals.

Cells for the electroextraction of metal generally require a large anode surface and work at a low current density to ensure a uniform electrolytic deposition of metal on the cathode, so that the costs of using a titanium base become relatively important and must also be taken into account.

Dimensionally stable anodes with mixed oxide coatings comprising platinum group metals and valve metals are described in US patent document 3632498. An example in this patent document relates to the production of a finely divided mixed Ti-Pd oxide powder which is then applied by rolling or hammering into a rod of soft titanium. However, the incorporated amount of noble metal in the mixed oxide powder and applied to the electrode in this way could be completely prohibitive for various industrial applications. Thus, when the surface of the electrode is to be covered essentially completely with the mixed oxide powder, and more particularly if the electrode is intended for use at a relatively low current density as used in electromining of metals, the costs of the precious metal applied in this way in the form of a mixed oxyd pose a particular obstacle.

Description of the invention

The invention aims to provide an improved anode for oxygen development in an acidic electrolyte.

The invention also aims to provide

an anode with a base of lead or lead alloy, with improved electrochemical properties for the anodic evolution of oxygen in an acidic electrolyte to thereby substantially completely avoid the loss of the anode material and thereby avoid the limitations of ordinary lead or lead alloy anodes encumbered with.

It is a further object of the invention to provide a simple method for producing such an electrode with improved usage properties.

The electrochemical performance of the anode is improved according to the invention by providing the anode with titanium particles that are catalytically activated with ruthenium in oxide form and are partially embedded in the surface of the anode base of lead or lead alloy so that they are firmly anchored and electrically connected to the base. The other non-embedded part of

the said catalytic particles thus protrude from the mentioned surface of the anode base and can thereby provide a surface for oxygen evolution which can be considerably larger than the underlying surface of the anode base of lead or lead alloy.

According to the invention, the partially embedded catalytic particles are preferably arranged so that they essentially cover the entire surface of the lead or lead alloy base and provide a maximum surface for oxygen evolution and thereby more precisely provide an essentially even distribution of the anode current density.

The use of ruthenium to catalytically activate titanium particles for an anode according to the invention is particularly advantageous as ruthenium can provide an excellent electrocatalyst for oxygen evolution at a relatively low price compared to the use of other metals from the platinum group.

The catalytic particles applied to the anode according to the invention advantageously consist of titanium sponge and can have a size within the range 150-1250 µm, preferably 300-1000 µm.

The quantity of the catalytic particles applied per area unit of the anode base for the anode according to the invention, should generally be sufficient to essentially cover the entire anode base.

It has now been shown that relatively large amounts of particles corresponding to over 400 g/m are generally necessary for the production of electrodes with satisfactory performance properties. Larger amounts of up to 1000 g/m 2 or even greater have also been found to be beneficial.

The catalytic particles advantageously comprise a minimum amount of ruthenium corresponding to no more than 6% by weight of the titanium in the particles and evenly distributed over a very large surface.

The large amounts of catalytic particles that have been indicated above, e.g. 500-1000 g/m 2 can still lead to quite high costs for the ruthenium. It is therefore particularly important to reduce the loss of ruthenium as far as possible when using the anode.

It has now been established by experiment that when the titanium particles are activated with manganese as well as with ruthenium in oxide form, the stability of the catalyst increases with respect to ruthenium dioxide used alone or in other combinations.

This improved electrocatalytic property and stability of the Ru-Mn-oxide system under conditions that lead to oxygen evolution in acidic media constitutes a particularly advantageous feature of the catalytically activated titanium particles used on a lead base for an anode according to

the invention.

It has also been shown that when titanium oxide is formed by

thermal splitting of the activated particles, a further improved stability of the particles is obtained.

It has also been shown that a more effective application

of the ruthenium is achieved if larger activated particles are first pressed into the lead anode base, after which smaller particles are pressed in which can advantageously contain a higher proportion of ruthenium than the larger particles. This two-step method has been shown to improve contact with the lead base and also the long-term stability of the catalytically activated particles.

It has also been found that a further pressing step to apply unactivated particles of a valve metal or a valve metal oxide, more particularly zirconium dioxide, can further increase the stability of the activated particles. This is of particular importance for methods of electro-extraction of metals from electrolytes containing Mn 2+ ions where the deposition of poorly electrically conductive MnC 2 can be detrimental to the performance results obtained with the anode.

In the examples below, various embodiments of the invention are described.

Example 1

An activating solution was prepared by dissolving 0.57 g of RuCl3.aq. and 1.33 g of Mn(NO 3 ) 2 -aq in 4 ml of 1-butyl alcohol. The solution was then diluted with six times its weight of 1-butyl alcohol.

3.25 g of Ti sponge (particle size above 630 µm) was degreased with trichlorethylene, dried and impregnated with the activation solution. After each impregnation, the titanium sponge was dried at 100°C for approximately 1 hour. A heat treatment was then carried out for 10 minutes at 200°C and finally for approx. 10 minutes at 400°C under an external air flow. This activation method was performed 5 times. The amounts of Ru and Mn obtained in this way corresponded to 28.4 mg Ru/g Ti and 36.0 mg Mn/g Ti.

The same activation solution was also used for 4.9 g

Ti-sponge (particle size 315-630yUm). The temperatures

for drying and heating and furthermore the number of impregnations were identical to those used for the larger particles. However, the duration of the heat treatment at 400°C was 12 minutes. The amounts of Ru and Mn in this case corresponded to 27 mg Ru/g Ti sponge and 34 mg Mn/g Ti sponge.

The activated titanium sponge particles were then pressed

on a lead plate test piece. The larger particle sizes (over 630 µm) were first pressed at 290 kg/cm<2> while obtaining Ti, Mn and Ru quantities per area unit of the lead sheet of respectively 322, 11.5 and 9.1 g/m 2. Then smaller activated titanium particles (315-630 µm) were pressed at 360 kg/cm<2>, obtaining Ti, Mn and Ru quantities of respectively. 400,

13.7 and 10.8 g/m<2>.

An electrode test piece (l 62) was thereby produced with

a lead base that was uniformly covered with titanium sponge particles that were activated with Ru-Mn oxide in an amount equal to 722

g/m<2>Ti sponge, 19.9 g/m<2>Ru and 25.2 g/m<2>Mn.

This electrode sample was examined as an oxygen-evolving anode in r^SO^ (150 g/litre). The electrode potential (oxygen half-cell potential) at a current density of 500

A/m 2 was 1.57 V measured in relation to NHE after 68 days, 1.59 V after 194 days and 1.75 V after 210 days used as anode.

For the sake of comparison, it should be mentioned that another anode test piece (L 61) which was produced by directly pressing smaller particles of activated Ti sponge onto lead and with higher Ru and Mn amounts corresponding to, respectively. 27.9 and 35.4 g/m<2>, showed an anode potential of 1.62 V after 69 days of operation under identical conditions and a potential of 1.63 V when anode operation was stopped after 194 days.

A further anode sample (L 76) was prepared in a similar manner to sample L 62, but the larger particles were only activated 4 times instead of 5 times. The total Ru and Mn amounts were high in this case. 22.1 and 28.0 g/m 2. The anode was examined under identical conditions and showed a potential of 1.5 V measured in relation to NHE after 22 days and 1.8 V after 140 days used as anode.

Example 2

An anode test piece (L 64) was prepared in a similar manner

way as the test piece L 62 according to example 1, but with higher Ru and Mn amounts of resp. 23.1 and 29.3 g/m<2>. The anode was examined in a solution for electroextraction of

2+

Zn and containing Mn as a main contaminant.

Its voltage after operation for 60 h and 120 h as an oxygen-evolving anode in this solution was respectively 1.68 V and 1.73 V measured relative to NHE (normal hydrogen electrode). The current density was 400 A/m 2. Mn oxide was not deposited in

during this period.

For the sake of comparison, it should be mentioned that lead sample pieces which included only large activated particles (size over 630 µm) or only smaller particles (size 315-630 µm) and with total Ru and Mn amounts corresponding to respectively 19-20 and 24-25 g/m 2, showed a higher anode voltage of 1.72-1.75 V measured in relation to NHE after 60 hours of operation. A thick anodic deposit of Mn oxide was observed in both cases.

Example 3

Ti sponge (particle size 315-630 µm) was activated

in a similar way as in example 1. It was then pressed on lead at 270fog/' cm 2 and gave an amount of Ti, Mn and Ru corresponding to, respectively. 4 27, 15.1 and 11.9 g/m<2>. Finally, particulate ZrO~ (particle size 150-500 µm) was pressed with a pressure of approx. 410 kg/cm 2 on top of the Ti sponge and gave a ZrC^ amount corresponding to 248 g/m 2 .

The electrode sample produced in this way

(L 82) was investigated as an oxygen-evolving anode in H2SO4

(150 g/litre). The electrode voltage at a current density of 500 A/m 2 was 1.5 V in relation to NHE after use as anode for 150 hours. It was 1.59 V after 293 days, and the electrode test piece is still in operation. This corresponds to a voltage saving of 410 mV compared to the use of pure, untreated lead.

Example 4

Ti sponge (particle size 315-630 µm) was first activated with a Ru- and Mn-containing solution as described in example 1. The activation method is also identical to the method described in example 1.

After this activation, a top coating was applied by impregnation with a solution containing Ti-butoxide and which was prepared by dissolving 1.78 g of Ti-butoxide in 3.75 ml of 1-butyl alcohol and 0.25 ml of HCl.

The impregnated sponge was dried for approx. 1 hour at 100°C. A heat treatment was then carried out at 250°C for 12 minutes and finally at 400°C for approx. 12 minutes under an external air stream.

The obtained activated titanium particles were then pressed onto lead at approx. 250 kg/cm 2. The electrode test piece (L 84)

was produced in this way with a lead base that was evenly covered with Ru-Mn oxide-activated titanium sponge particles "top-coated" with Ti oxide in amounts corresponding to 13.3 gRu/<m><2>,

2 2 2

16.9 g Mn/m , 5.8 g Ti/m and 515 g Ti sponge/m .

This electrode sample was examined as an oxygen-evolving anode in r^SO^ (150 g/liter). Its voltage at a current density of 500 A/m<2>was 1.49 V relative to NHE after being used as an anode for 130 hours. This corresponds to a voltage saving of 510 mV compared to the use of untreated lead. The anode voltage was 1.64 V after 128 days, and this corresponds to a voltage saving of 360 mV

in relation to the use of untreated lead.

Example 5

Ti sponge (particle size 315-630 µm) was first activated with a Ru-containing solution which was prepared by dissolving 134 g of RuCl-j.H^jO per liter of butyl alcohol. The Ti sponge was impregnated with the Ru 2 -containing solution, heated for 20 minutes at 120°C to evaporate the solvent and heat treated for 15 minutes at 250°C and finally for another 15 minutes at 450°C. This impregnation, drying and firing was repeated four times. The amount of ruthenium obtained in this way was 30 mg/g Ti sponge.

After this activation, a top coating of TiO 2 was applied to the activated particles by impregnation with a solution which had been prepared by mixing 1.8 g of titanium butoxide with 3.75 ml of butyl alcohol. The drying, heating and firing steps were the same as described above for the Ru-containing activating solution. These steps were repeated twice and yielded a titanium amount, applied as TiO 2 , of 5 mg/g Ti sponge.

The activated titanium particles were then pressed onto lead at 250 kg/cm 2 in the form of a lead plate test piece and gave a particle amount of 50 0 g/m 2 corresponding to 15 g/m 2 Ru and 2.5 g/m 2 Ti applied to the particles evenly distributed on the lead-over surface.

This electrode sample was examined as an oxygen-evolving anode in H2S04 (150 g/litre). The electrode voltage at a current density of 500A/m<2>was 1.66 V in relation to NHE after use as anode for 2000 hours.

Example 6

TiC^-rutile particles with a size of 315-630 µm are activated by impregnation with the following solution: 0.54 g RuCl 3 .H 2 O, 1.8 g butyl titanate, 0.25 ml HCl and 3.75 ml butyl alcohol.

After the impregnation, the particles are dried at 100°C in air and burned for 10 minutes at 4 4 0°C under an air stream. This method is repeated 4 times. The particles obtained are activated with RuC^-TiC^ •

The particles are then pressed onto a lead plate test piece at a pressure of 250 kg/cm 2 . The amount of particles is 400 g/m<2>corresponding to a Ru amount and Ti amount of respectively 15 and 16 g/m<2> (applied as Ru02-Ti02).

The fabricated activated lead electrode was investigated

as anode in an aqueous solution containing 150 g H2S0^per liter at room temperature. The applied anode current density was 500 A/m 2 . An oxygen half-cell voltage of 1.75 V 1. relative to NHE was obtained after operation for 300 hours. After 1000 hours, the anode voltage reached the same value as the voltage for an anode of pure, untreated lead.

Example 7

An activation solution was prepared as described in Example 1, but instead of diluting it six times its amount with n-butyl alcohol (as in Example 1) it was diluted only three times its amount.

4.11 g of Ti sponge (particle size 400-630 µm) was impregnated with the activation solution. After each impregnation, the titanium sponge was dried for approx. 1 hour at 100°C. A heat treatment was then carried out for approx. 10 minutes at 250°C and finally at 400°C for approx. 10 minutes under an external air stream. This activation method was performed 3 times. The amounts of Ru and Mn obtained in this way were 36.2 mg Ru/g Ti and 45.8 mg Mn/g Ti.

The activation method described in example 1 for the Ti sponge with a particle size greater than 630 µm was in this case also used for the larger particles (above 630 µm). However, the activation was only performed 4 times. The amounts of Ru and Mn obtained in this way were

2 3.5 mg Ru/g Ti and 29.9 mg Mn/g Ti.

The activated titanium sponge particles were then pressed and partially embedded in the surface of a lead plate specimen.

The larger particles (size above 630 µm) were first pressed at 240 kg/cm 2 and gave Ti, Mn and Ru quantities per area unit of the lead sheet of respectively 350, 10.5 and 8.3 g/m p. An electrode sample (L 95) was thus prepared with a lead base which was uniformly covered with Ru-Mn oxide-activated titanium sponge particles in an amount corresponding to 760 g/m 2Ti sponge,

23.2 g/m 2 Ru and 29.3 g/m<2>Mn. This electrode sample was examined as an oxygen-evolving anode in H„SO. (150 g/litre). The electrode voltage at a current density of 500 A/m 2 was

1.65 V compared to NHE after use as anode for 287 days.

For comparison, another anode test piece was made

(L 93) which was prepared by directly pressing smaller particles of activated Ti sponge at 280 kg/cm 2 onto lead with Ru-

and Mn amounts corresponding to respectively 15.4 and 19.5 g/m 2 , examined under identical conditions. The electrode voltage after 289 days was 1.78 V in relation to NHE.

A further anode sample (L 92) was prepared in a similar manner to sample L 95, but the smaller particles (400-630 µm) were activated as described in Example 1 (sample L 62). In this case, the total amounts of Ti, Mn and Ru were respectively 726, 22.5, and 17.7 g/m 2. The larger particles and the smaller particles were pressed at hhy. 29 0 kg/cm<2>and 410 kg/cm<2>The anode has been examined under identical conditions and showed a voltage of 1.78 V in relation to NHE after being used as anode for 289 days.

Example 8

An activation solution was prepared as described in Example 7. 4.22 g of larger particles (particle size above 630 µm) were activated twice under the conditions specified in Example 7, yielding 21.5 mg Ru/g Ti and 27, 4 mg Mn/g Ti.

Another activation solution was applied to Ti sponge with a smaller particle size of 400-630 µm. This activation solution corresponds to that described in Example 7, but with the difference that it was diluted with only twice its amount of 1-butyl alcohol. Two activations were carried out in accordance with example 7. The amounts of Ru and Mn per g Ten were respectively 25.9 and 32.9 mg.

An anode test piece (L 120) was prepared by first pressing the larger particles at 210 kg/cm 2 which gave Ti, Mn and Ru amounts of respectively. 360, 9.8 and 7.7 g/m 2. Smaller, activated titanium particles (400-630 µm) were then pressed at 320 kg/cm 2 which gave Ti, Mn and Ru amounts of respectively. 4 20, 13.9 and 10.9 g/m 2. The total amounts of Ti, Mn and Ru obtained in this way were respectively 780, 23.7 and 18.6 g/m<2>.

The electrode test piece was examined as an oxygen-releasing anode ±^ 2^ 0^ (150 g/liter). The electrode voltage at a current density of 500 A/m<2>was 1.58 V in relation to NHE after use for 218 days.

Example 9

Titanium sponge (400-630 µm) was oxidized as follows before being activated with Ru-Mn oxide.

4.74 g of titanium sponge was activated once with the activation solution described in Example 1. The heat treatment was carried out for 13 minutes at 400°C under an external air flow after the Ti sponge had been dried at 100°C. The amounts of Ru and Mn were respectively 5.2 and 6.6 mg/g Ti sponge. The mushroom

was then heat treated for 5.40 h at 480°C under an external air flow to convert it to its respective oxidn.

3.5 g of the oxidized Ti sponge produced in this way was then activated as described in example 1, but with the only difference that an intermediate heat treatment was carried out at 250°C instead of at 200°C after each activation. The Mn and Ru quantities per g sponge was respectively 32.8 and 25.8 mg.

The pre-oxidized and activated Ti sponge was then pressed in two stages, first at 230 kg/cm 2 and then at 29 0 kg/cm 2 and yielded Mn and Ru amounts of respectively. 21.1 and 16.6 g/m<2>. The amount of the oxidized Ti sponge was 643 g/m 2

The Mn and Ru amounts in the Ti oxide before the final activation were

the total Mn and Ru amounts respectively 25.3 and 19.9 g/m<2>.

The electrode has been investigated in 150 g H-jSO^pr. litres

<y>ed 500 A/m<2>, and its voltage after use for 275 days was 1.6 5 V in relation to NHE.

Example 10

Two activation solutions were prepared with a higher Mn/Ru ratio than described in Example 1.

Solution A: 0.537 g RuCl3.aq and 2.0819 g Mn (NO3)2-aq in 3.75 ml n-butyl alcohol.

Solution B: 0.537 g RuCl3-aq and 4.6844 g Mn(NO3)2.aq in 3.75 ml n-butyl alcohol.

Both solutions A and B were diluted with three times their amount of n-butyl alcohol before application. Solution A had a molar ratio Mn02/Ru02= 4 and solution B a molar ratio Mn02/Ru02= 9.

4.27 g of Ti sponge (particle size 315-630 µm) was impregnated with the diluted activation solution A.

y

After each impregnation, the titanium sponge was dried for approx. 1 hour at 100°C. A heat treatment was then carried out for 14 minutes at 250°C and finally for approx. 14 minutes at 400°C under an external air flow. This activation method was performed 3 times. The amounts of Ru and Mn obtained in this way were 29.3 mg Ru/g Ti and 63.8 mg Mn/g Ti.

4.16 g of Ti sponge (particle size 315-630 µm) was diluted with the diluted activation solution B. The activation was carried out in the same way as with the activation solution A. The amounts of Ru and Mn thus obtained were 19.9 mg Ru/g Ti and 9 7.4 mg Mn/g Ti.'

The activated Ti sponge particles were then pressed onto a lead plate specimen. The larger particles (larger than 630 µm) which had been activated as described in Example 8 were first pressed at 230 kg/cm 2 and yielded Ti, Mn and Ru quantities per area unit of the lead sheet of respectively 449, 12.0 and 9.4 g/m 2. Then smaller, activated (with the diluted solution A) Ti particles (315-630/lim) were pressed at 350 kg/cm 2 and gave Ti-, Mn- and Ru amounts of respectively 399,

25.5 and 11.7 g/m<2>.

An electrode sample (L 164) was prepared in this way with a lead base uniformly covered with Ru-Mn oxide-activated titanium sponge particles in an amount corresponding to 848.0 g/m<2>Ti sponge, 20.8 g/m< 2>Ru and 37.5 g/m2 Mn.

This electrode sample was examined as an oxygen-winding anode in 150 g of H2SO4pr. litres. Its voltage at a current density of 500 A/m<2>was 1.5 V in relation to NHE after use for 36 days.

For comparison, another anode sample (L 161) was prepared by directly pressing smaller particles of activated Ti sponge (with the dilute solution A) onto lead at 320 kg/cm 2 giving"*a Ti^-, Ru- and Mn amounts corresponding to 531.0, 15.6 and 34.0 g/m<2.> respectively

This electrode test piece L 161 has been examined under identical conditions and showed a voltage of 1.6V in relation to NHE after use for 70 days.

In another series of experiments, Ti sponge particles with a

size above 630/Um and activated as described in example

8 first pressed at 230 kg/cm<2> which gave Ti, Mn and Ru amounts per area unit of the lead sheet of respectively 4 28.0, 11.5 and 9.0 g/m<2>Smaller activated titanium sponge particles (size 315-630µm) prepared with the activation solution B were then pressed at 350 kg/cm<2>which gave Ti -, Mn and Ru quantities of respectively

493.0, 48.0 and 9.8 g/m<2>. An electrode sample (L 163) was prepared in this way with a lead base uniformly covered with Ru-Mn oxide-activated titanium sponge particles in an amount corresponding to 921.0 g/m<2>Ti, 59.5 g/m<2> Mn and 18.8 g/m2 Ru.

This electrode has been investigated as an oxygen-evolving anode in 150 g I^SO^ per liter at 500 A/m 2. Its voltage after use for 33 days was 1.57 V in relation to NHE.

Example 11

For comparison (with sample L 163 according to Example 10), another anode sample (L 162) was prepared by directly pressing smaller particles (315-630 µm) of activated Ti sponge (with the dilute solution B) onto lead at 290 kg/cm 2 which gave amounts of Ti, Ru and Mn corresponding to respectively 652.0, 13.0 and 63.6 g/m<2.>

The electrode has been tested at 500 A/m 2 in 150 g H2SO4pr. liter and showed a voltage of 1.74 V in relation to NHE after use for 18 days (430 hours) under these conditions.

Example 12

An activating solution was prepared by dissolving 0.54 g of RuCl3.aq (38% Ru) and 0.12 g of PdCl2 in 15 ml of butyl alcohol. The solution was stirred until all the salt had dissolved, and 1.84 g of butyl titanate was added.

3.5 g of titanium sponge with a particle size of 315-630 µm was impregnated with this activation solution, dried at 140°C for 20 minutes, fired at 250°C for 15 minutes and finally fired for another 15 minutes at 450°C. All these heating steps were carried out in air. After cooling

the impregnation, drying and firing steps were repeated six times. The amounts of Ru and Pd on the particles obtained in this way were 30 mg Ru/g Ti and 11 mg Pd/g Ti. The activated titanium sponge particles were pressed onto a lead plate test piece at a pressure of 250 kg/cm 2 yielding the following amounts:

500.0 g/m<2>Ti sponge, 15.0 g/m<2>Ru, 5.5 g/m<2>Pd.

This electrode sample was examined as an oxygen-evolving anode in H2S04 (150 g/litre) at 500 A/m 2. The electrode voltage (oxygen half-cell voltage) was 1.78 V in relation to NHE after use for 208 days.

Example 13

7 g of tyranniformes (particle size 315-630 µm) were impregnated with 1.4 ml of a solution containing 1 mg of Ir per ml in the form of IrCl^.aq.. dissolved in isopropyl alcohol.

After impregnation, the titanium sponge was dried for 15 minutes at 140°C, fired for 10 minutes at 250°C and again fired for 10 minutes at 450°C, all of these steps being carried out in air.

The activated titanium sponge particles were pressed onto a lead plate test piece at e:-t pressure of 250 kg/cm 2. The amount of particles was chosen so that a titanium and iridium amount of respectively. 700 g/m 2 and 1 g/m 2 were obtained.

Another activation solution was then applied to the electrode sample in the following manner. A solution was prepared by dissolving 5.0 g Mn (NO 3 ) 2.4 H 2 O and 0.32 g Co (NO 3 ) 2.6 H 2 O and 0.5 ml ethanol. This solution was applied to the electrode surface, dried for 15 minutes at 140°C and fired for 10 minutes at 250°C in air. After cooling, the application, drying and firing steps were repeated five times so that a final amount of 240 g/m 2 MnO 2 and 12 g/m 2 cobalt oxide (calculated as Co^O^) was obtained.

This electrode sample was examined as an oxygen-evolving anode in H2S04 (150 g/litre). The electrode voltage (oxygen half-cell voltage) at a current density ay 500 A/m<2> was 1.78 V in relation to NHE after use for seven months.

Example 14

The electrode sample was prepared as described in Example 13, except that IrCl-^-aq was replaced by RuCl^-aq (14 mg/ml Ru) and that the impregnation step was repeated twice so that a ruthenium amount of 4 g/m 2 for a titanium sponge amount of 7 00 g/m 2.

When the electrode sample was examined under the same conditions as described in Example 13, the oxygen half-cell voltage was 1.80 V relative to NHE after use for 6.5 months.

Example 15

An activation solution was prepared by dissolving

0.44 g RuCl3.aq (38 wt% Ru), 0.090 g SnCl2.2H20 + 0.52 g Mn (N03)2.4H20 in 4 ml butyl alcohol.

2.5 g of titanium sponge (particle size 315-630 µm) was impregnated with this activation solution as follows: 0.77 ml of the solution was uniformly applied to the titanium sponge, dried for 15 minutes at 14 0°C, fired for 10 minutes at 250°C and burned for 10 minutes at 4-20°C, all drying and burning steps being carried out in air. After cooling, the titanium sponge was again activated twice and each time with 0.5 1 activation solution, and dried and burned as described above.

The activated titanium particles were pressed onto the surface

of a sample of lead-calcium alloy (0.06% Ca) at 250 kg/cm 2 which gave the following amounts: Ti 700 g/m 2 , Ru 20

g/m<2>, Sn 5.8 g/m<2> and Mn 13.7 g/m<2>.

This electrode sample was tested as an oxygen evolving anode in H2S04 (150 g/liter) at 500A/m<2>. The electrode voltage was 1.67 V in relation to NHE after use for 7 months.

It is clear from the above examples that an anode according to the invention can be produced in a simple way and used for longer-term development of oxygen at a voltage that is significantly lower than the anode voltage that corresponds to oxygen development on lead or lead alloy under otherwise similar working conditions.

It should be noted that no loss of lead from the base could be observed when the anode test pieces according to the invention were examined, as described in the above examples, while a clear loss of lead could be observed in the electrolyte when lead or lead alloy comparison test pieces were examined under the same conditions.

It has also been shown that a simultaneous application of heat and pressure when the valve metal particles are partially embedded in the lead or lead alloy on the surface of the anode base, can facilitate their locking while preventing the particles from being completely embedded in and/or flattened on the base.

It should also be noted that further improvements can very well be expected in relation to the above examples by determining the best conditions for providing anodes according to the invention with optimal, stable, electrochemical performance characteristics and while achieving a maximum economy in terms of precious metals.

It will be understood that the catalytic particles can be applied and anchored to the lead or lead alloy base of the anode not only by means of a press, as in the above examples, but also by means of any other device, such as e.g. pressure rollers, which may be suitable for the essential advantages of the present invention to be achieved.

It has also been shown that the application of heat (e.g.

a temperature of approx. 2 50°C) under the pressing step can promote a partial embedding of the catalytic particles in the lead or lead alloy surface.

The present invention offers various advantages of which the following can be mentioned as an example: (a) the anode according to the invention can be used at a significantly reduced voltage which is well below the voltage for ordinary lead or lead alloy anodes which are currently used in industrial cells for electricity extraction a< y>metals from acidic solutions. The cell voltage and thus the energy costs for electroextraction of metals can thus be reduced accordingly. (b) Contamination of the electrolyte and of the cathodic deposit as a result of materials which write from the anode can be essentially avoided as it has been established by experiment that oxygen is evolved on the catalytic particles at a reduced voltage at which the lead or lead alloy of the anode base is effectively protected against corrosion. (c) Dendrite formation on the cathode, which can lead to a short circuit with the anode and thereby burn holes in the anode, will not lead to any serious deterioration of the performance characteristics of the anode according to the invention, as it works with oxygen evolution on the catalytic particles at a reduced voltage at which a any part of the lead or lead base that is exposed is not subject to appreciable corrosion. (d) Ordinary lead or lead alloy anodes can be easily converted into the improved anodes according to the invention, and it is thus possible to re-equip industrial cells for the electroextraction of metals in a particularly simple and affordable way to achieve improved operating characteristics. (e) The reduced cell voltage achieved by using anodes according to the invention can be easily monitored to quickly detect any noticeable increase in the anode voltage.

The catalytic particles on the lead or lead alloy base

can therefore be easily reactivated or replaced should this become necessary.

(f) Ruthenium can be used as a catalyst in an unusually economical way by combining this in a very small proportion with titanium sponge particles which are applied in a several times greater quantity to the anode base of lead or lead alloy. The costs for ruthenium can thus be justified by the achieved improvement in the anode's performance characteristics. (g) Ruthenium can thus be used in very limited quantities and in combination with less expensive, stable materials. (h) Minor short-circuiting could be observed in electroextraction. plant for copper which was equipped with anodes according to the invention. This led to an improved cathode current yield, whereby the energy savings already achieved due to the reduced cell voltage resulting from the use of the anode according to the invention at a reduced oxygen half-cell voltage could be further increased.

Industrie11 utilization e

Anodes according to the invention can advantageously be used instead of the currently used anodes of lead or lead alloy in order to reduce the energy costs necessary for industrial electroextraction of metals, such as zinc, copper, cobalt or nickel, and to improve the purity of the metal produced on the cathode.

Such anodes can be advantageously used in various processes where it is necessary to develop oxygen at a reduced overvoltage.

Claims (20)

1. Anode for developing oxygen in an acidic electrolyte, comprising an anode base of lead or a lead alloy, characterized in that (a) catalytically activated titanium particles are uniformly distributed and partially embedded on the surface of the base such that they are firmly anchored to and electrically connected to the base; and that (b). the catalytically activated titanium particles comprise a smaller amount of ruthenium in oxide form. 2. Anode according to claim 1, characterized in that the particles consist of titanium sponge. 3. Anode according to claim 1 or 2, characterized in that the particles also comprise manganese in oxide form. 4. Anode according to claim 2 or 3, characterized in that catalytically activated titanium sponge particles with different size ranges are arranged on the base. 5. Anode according to claim 4, characterized in that smaller, catalytically activated titanium sponge particles comprise a greater amount of ruthenium than the larger particles within the aforementioned size ranges. 6. Anode according to claims 1-5, characterized in that the activated titanium particles also comprise titanium in oxide form, 7. Anode according to claims 1-6, characterized in that the activated particles comprise titanium in an amount corresponding to over 400 g/m 2 of the surface of the anode base. 8. Anode according to claims 1-7, characterized in that it also comprises particles of valve metal and/or valve metal oxide, preferably ZxO 2 , in addition to the aforementioned activated particles. 9. Method for the production of an anode for the evolution of oxygen in an acidic electrolyte, comprising an anode base of lead or a lead alloy, characterized by that (a) catalytically activated titanium sponge particles comprising a minor amount of ruthenium and manganese in oxide form obtained by thermal decomposition of ruthenium and manganese compounds applied to the titanium sponge particles are uniformly distributed on the surface of the anode base of lead or lead alloy, and that (b) the catalytically activated particles are partially embedded in the lead or lead alloy for the anode base. 10. Method according to claim 9, characterized in that large, activated particles are first pressed into the surface of the anode base and that smaller, activated particles are then pressed into the surface of the anode base. 11. Method according to claim 10, characterized in that smaller particles are used which are provided with a larger amount of ruthenium than the large particles. 12. Method according to claim 10 or 11, characterized in that large particles with a size of over 600 µm and smaller particles with a size of 300-600 µm are used. 13. - Method according to claims 9-12, characterized in that activated particles are used which comprise titanium in an amount corresponding to over 400 g/m 2 of the surface of the anode base. 14. Method according to claims 9-13, characterized in that catalytically activated particles are used with titanium dioxide on them and formed by thermal decomposition of a titanium compound which has been applied after the ruthenium and manganese in oxide form have been formed on the particles. 15. Method according to claims 9-14, characterized in that particles of valve metal and/or valve metal oxide are also pressed into the anode base after the activated particles have been partially embedded. 16. Method according to claim 15, characterized in that particles of zirconium dioxide are pressed and likewise attached to the anode base after the catalytically activated titanium particles have been partially embedded. 17. Electrode comprising a base of lead or lead alloy and a catalyst for carrying out an electrochemical reaction, characterized in that catalytic particles consisting of an inert refractory oxide activated by means of a catalyst for the desired reaction are evenly distributed and partially embedded on the surface of the base so that they are firmly anchored to and electrically connected to the base. 18. Electrode according to claim 17, characterized in that the inert, refractory oxide particles are activated with a catalyst comprising a precious metal, 19. Electrode according to claim 17 or 18, characterized in that the refractory oxide is titanium oxide in rutile form. 20. Use of an electrode according to claims 17-19 as an anode for the development of oxygen in an acidic electrolyte.