CA1208601A - Electrode with lead base and method of making same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Disclosed is an anode for oxygen evolution in acid electrolytes such as are used in processes for anode electro-winning of metals. The anode comprises a base of lead or lead alloy and an active layer of catalytic particles partially embedded in the base. The active layer comprises valve metal particles such as particles of titanium sponge impregnated with an active coating comprising ruthenium and manganese oxides. Also disclosed is a method of making the oxygen evolving anode. The process comprises impregnating titanium sponge particles with ruthenium and manganese compounds, and converting these compounds to oxides. The resulting particles are pressed and partly embedded into a lead or lead alloy base.

Description

ELECTRODE WITH LEAD 8ASE AND M~HOD OF MAKlNG SAME

Technical Field The present invention relates to dimensionally stable electrodes, and more particularly to anodes for oxygen evolution :n an acid electrolyte, such as are used e.g. in processes for electrowinning metals from acid electrolytes.

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Backqround Art Lead or lead alloy anodes have been widely used in processes for electrowinning metals from sulphate solutions. They nevertheless have important limitations, such as a high oxygen overvoltage and loss of the anode material leading to contamination of the electrolyte, as well as the metal product obtained on the cathode.
Anodes of lead-silver alloy provide a certain decrease of the oxygen overvoltage and improvement of the current efficiency, but they still have the said 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 subject to more or less rapid passivation and oxidation of the titanium base.
It hss also been proposed to provide the titanium base with a protective undercoating compri~ing a platinurn 91 oup metal berleath the ', 36~3L

outer coating, but such coatings do generally not provide sufficient prDteCtion of the titsniun base to justify the high.cost of using precious metals.
s Metal electrowinning cells generally require a large anode surface and operate at a low current density in order to ansure an even electrodeposition of metal on the cathode, ~o that the cost of u~ing a titanium base becomes rela~ively important and rnust also be taken into accour)t.
Dimensionally stable anodes with mixed oxide coating~ comprising platinum group metals and valve metals are described in U.S. Pat. 3 632 498.
An exampls of this patent relates to the preparation of a fine Ti-Pd mixed ~` oxide powder which is then applied by rolling or hammering into a rod of soft-quality titanium. However, the amount of precious metal incorporated in the mixed oxide powder and applied to the electrode in this manner could be prohibitive for various industrial applications. Thus, when the electrode surface is to be substantially covered with the mixed oxide powder, and more particularly when the electrode is intended for operation at a relatively low current density such as is used in metal electrowinning, the cost of precious metal thus applied in the form of a mixed oxide may be especially prohibitive.
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1: isclosure of the Invention .

An object of the invention is to provide an improved anode for evolving oxygen in an acid electrolyte.
Another object of the invention i8 to provide an anode with a base of lead or lead alloy with improved electrochemical p0rformance for anodically evolving oxygen in an scid electrolyte, 80 as to substantlally a~,Did 10s8 of the snode material and the~r sai~ 1~oitati~ns of can~tianal lead or leat alloy anodea A further object of the invention i~ to provids a sirnple method of making ~uch an electrode wlth Improved perforrnance.
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These objects are essentially met by the invention as set forth in the claims.
The electrochemical performance of the anode is improved in accordance with the invention by providing the aoode with titanium parti-cles which are catalytically activated by means of ruthenium in oxide form and are partly embedded at the surface uf the anode base of lead or lead alloy, so that they are firmly anchored and electrically connected to the base. The remaining, non-embedded part of said catalytic particles thus projects from said surface of the anode ba~e, and thereby can præsent a surface for oxyqen evolutiDn which can be considerably larger than the underlying surface of the anode base of lead or lead alloy.
Said partly embedded catalytic particles are preferably arran~ed accordin~ to the invention, so that they substantially cover the entire surface of the lead or lead alloy base, present a maximu~ surface for oxygen evolution, and thereby more especially provide a substantially uniform distribution of the anode current density.
The use of ruthenium to catalytically activate titanium particles in accordance with the inventiDn is parSicularly advantageous since ruthenium can provide an excellent electro-catalyst for Dxygen evolution at a relatively low cost with respect to othsr metals of the platinum group.
The catalytic particles applied according to the invention advantageously consist of titanium sponge and may have a size Iying in the range between 150 and 1250 micron~, and preferably in the range of about 300-lD00 micron~.
The amount or loading of said catalytic particles applied according to the invention per unit area of the anode base should generally be adequate to ~ubstantially cover the anode base.
It has now been found th~t relatively high particle loadings corres-ponding to more than 400 g/m2 are generally necessary for the manufacture of electrodes with sati~factory performance. Higher loadings up to 1000 g/m2 or mors have likewlse besn found to be advsntEIgeoua Th~ catalytic partlcles advantageously comprl~e a~ ~ve1y smal1 ~unt o~ ruthenium, corresponding to at most 6 % by weight of the tltanium of said particles, evenly distrlbuted on a very large surfacæ.
The high loading~ of catalytic particle~ indicated above B~9.

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500-1000 g/m may nevertheless necessitate quite high ruthenium costs. Consequently, it is particularly important to reduce the loss of ruthenium during anodic operation as far as possible.
I~ has now been established experimentally that activating the titanium particles with manyanese as well as ruthenium in oxide form increases the stability of the catalyst with respect to ruthenium dioxide alone or in other combinations.
This improved electrocatalytic performance and stability of the Ru-Mn oxide system under the conditions of oxygen evolution in acid media constitutes a particularly advantageous feature of the catalytically activated titanium particles used on a lead base according to the present in~ention.
It has also been found that the formation of titanium oxide by thermal decomposition on the activate~ particles provides a further impro~ement of the stability of the particles.
It has moreover been established that a more efficient use of the ~uthenium is achieved when larger activated particles are first pressed into the lead anode base and this is ~ollowed ~y pressing smaller particles, which may advantageously haYe a higher proportion of ruthenium than the large~ particles. This

2-step pressing procedure has been found to improve the contact with ~he lead base as well as the long-term stability of the catalytically acti~ated particles.
It has moreover been found that an additional pressing step to apply non-activated particles of a ~alve metal or a valve metal oxide, more particularly zirconium dioxide, can further increase the stability of the activated particles. This is especially important in processes for electrowinning metals from electrolytes containing Mn2+ ions, where the deposition of poorly conducting MnO2 can be detrimental for anode performance.
Thus, and in accordance with the present teachings, a catalytic anode is provided or evolving oxygen in an acid electrolyte comprising an anode base of lead or lead alloy and titanium particles catalytically activated with a minor amount of ruthenium in oxide form, the particles being uniformly dis-tributed, partly embedded and firmly anchored in the surface o~

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--4a-the base so that oxygen can be anodically evolved on the particles at a reduced potential at which the underlying lead or lead alloy at the surface of the anode base remains electrochemically inactive, the activated titanium particles comprising a minor amount of ruthenium and manganese in oxide form obtained by thermal decomposi-tion of corresponding compounds, and the cafalytically actiYated particles comprise more than 400 grams of titanium per square meter of anode base surface, the particles having a size greater than 300 microns.
In accordance with a further aspect of the present teachings, a method is provided of making an oxygen evolved anode by the steps of:
a) catalytically acti~ating titani~m sponge particles having a size greater than 300 microns by impregnating the particles with an activating solution containing thermally decomposable ruthenium and manganese compounds and thermally converting the compounds in an oxidizing atmosphere into ruthenium and manganese oxide;
b) uniformly distributing the activated particles obtained by step a) over the surface of the anode base of lead or lead alloy, pressing, partly embedding and tnereby anchoring the activated particles in the surface of the anode base so that the amount of titanium in the catalytically activated, partly embedded particles correspond to more than 400 grams per square ~eter of the anode base surface.
The following examples serve to illustrate different modes of carrying out the present invention.

Example 1 An activating solution was prepared by dissolving 0.57 g RuC13.aq and 1 33 g Mn(N03~2 aq in 4 ml l-butylalcohol The solution was then ~¢, ,~

diluted with six times its weight of l-butylalcohol.

3.25 9 of Ti sponge (p~rticle size greater than 630 microns) was degreased with trichlorethylene, dried and impregnated with the activating solution. A~ter each impregnation, the titanium sponge was dried at 100C
fDr about 1 h. A heat treatment was then effected at 200C for 10 minutes and finally at 400C under an external air flow for about 10 minutes. This activation procedure was carried out 5 times. The Ru and Mn loadings thus obtained amounted to 28.4 mg Ru/g Ti and 36.0 mg Mn/g Ti.
The same activating solution was used also on 4.9 9 Ti sponge (particle size 315-630 micronsj. The temperatures for drying and heating as well as the number of impregnations were identical to those applied to the larger particles. However, the duration of the heat treatment at 400~C was 12 minutes. The Ru and Mn loadings in this case amounted to ~7 mg Ru/g Ti sponge and 34 mg Mn/g Ti sponge.
The activated titanium sponge particles were then pressed onto a lead sheet coupon. The larger particles size ( greater than 630 microns ) were pressed first at 290 kg/cm2 to give Ti, Mn and Ru loadings per unit lead-sheet area of 322, 11.5 and 9.1 g/m2 respectively. Subsequentlyt smaller activated titanium particles (315-630 microns) were then pressed at 36û kg/cm2 tG give Ti, Mn and Ru loadings of 40û, 13.7 and 1~.8 g/m2 respectively.
An electrode sample (L 62) was thus obtained with a lead base uniformly covered with F~u Mn oxide activated titanium sponge particles in an amount corresponding to 722 g/m2 Ti sponge, 19.9 g/m2 Ru and 25.2 g/m2 Mn.
This electrode sample was tested as an oxygen evolving anode in H2SO4 (150 gpl). The electrode potential (oxygen half-cell potential) at a current density of 500 A/m2 amounted to 1.57 V V5. NHE after 68 days, 1.59 V after 194 days, and 1 75 V after 210 days of anodic operation.
For comparison, another anode sample (L 61), which was obtained by directly pressing smaller particles of activated Ti sponge on lead, wlth higher Ru and Mn loadings corre~panding to 27.9 and 35.4 g/rn2 respectlvely, exhibited anode potential of 1.62 V after 69 day~ 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 like L 62 but the larger particles were only activated 4 times instead of 5. The overall Ru and Mn loadings amounted in this case to 22.1 and 28.0 g/m2 respectively, The anode was tested under identical conditions and showed a potential of 1.5 vs NHE after 22 days and 1.8 V after 140 days of operation.

Example 2 An anode sample (L 64) was prepared like L 62 of Example 1 but with higher Ru and Mn loadings of 2~.1 and 29.~ g/m2 respectively. The anode was tested in a Zn electrowinning solution eontaining Mn2~ as a major impurity.
Its potential after 60 h and 120 h of operation as an oxygen evolving anode in this medium, amounted respectively to 1.6a V and 1,73 V vs. NHE.
The current density was 400 A/m2. No deposit of,Mn-oxide occured during this period.
For comparison, lead samples comprising either only large activated particles (size greater than 630 microns ) or only smaller ones (size 315 -630 microns), with overall Rù and Mn loadings corresponding to 19-20 and 24-25 g/m2 respectively, showed a higher anode potential of about 1.72 -1,75 V vs. NHE after 60 h of operation. A thick anodic deposit of Mn oxide was observed in both cases.
Example 3 Ti sponge (particle size 315- 630 microns ) was activated like in Example 1. It was then pressed onto lead at 270 kg/cm2 to give a loading of Ti, Mn and Ru corresponding to 427, 15.1 and 11.9 g/m2 respectively. Finally particulate ZrO2 tparticle size 150-500 microns ) was pressed with a pressure of about 410 kg/cm2 on top of the Ti sponge to give a ZrO2 loading corresponding to 248 g/m2.
The electrode sample thus obtainsd (L 82) was test~d as an oxygen evolving anode in H2SO4 (159 gpl). The electrode potential at a current density of 500 A/m2, amounted to 1.50 V V3 NHE after 150 h of anodic operation. It amounted to 1.59 V after 293 days, and is still opE~rating. This ~2~

corresponds to a voltage saving of 410 mV with respect to pure, untreated lead.

Exam~le 4 Ti sponge (particle size 315 - 630 microns ) was activated first with a Ru and Mn containing solution as described in Example 1.
The activation method was also identical to the one described in Example 1.
Following this activation, a top-coating was applied by impregnation with a solution containing Ti-butoxide whlch was prepared by dissolving 1.78 9 Ti-butoxide in 3.75 ml l-butylalcohol and 0.25 ml HCl.
The impregnated sponge was dried at 100C for about 1 h. A heat treatment was then effected at 250C for 12 minutes and finally at 400C
under an external air flow for about 12 minutes.
The resulting activated titanium particles were then pressed on lead at about 250 kg/cm2. The electrode sample (L B4) was thus obained with a lead base uniformly covered with Ru^Mn oxide activated titanium sponge particles "topcoated" with Ti-oxide in amounts corresponding to 13.3 9 Ru/m2, 16.9 9 Mn/m2, 5.8 9 Ti/m2 and 515 9 Ti sponge/m2.
This electrode sample was tested as an oxygen evolving anode in H2SO4 (150 gpl). Its potential at a current density of 500 A/m2 amGunted to 1.49 V vs NHE after 130 h of anodic operation. This corresponds to a 510 mV
saving over untreated lead. The anode potential amounted to 1.64 V after 128 days, which corresponds to a 360 mV saving over untreated lead.

Example 5 Ti sponge (particle size 315-630 microns) was activated first with a Ru-containing solution prepared by dissolving 134 9 of RUcl3H2o per liter of butylalcohol. The Ti sponge was impregnated with the Ru containing solution, heated at 120C during 20 minutes in order to evaporate the solvent, heat treated at 250C for 15 min. and finally at 450C for another 15 min. This impregnation, drying and baking was repeated four times. The ruthenium loading thus obtained amounted to 30 mg/g of Ti sponge.
Following this activation, a top-coating of TiO2 was applied on the ~z~

activated particles by impregnation with a solution obtained by mixing 1.8 9 of titanium butoxide with 3.75 ml of b~ltylalcohol. The drying~ heating and baking steps were the same as mentioned above for the Ru containing activating solution. These steps were repeated twice to give a loading of titanium, applied as TiO2, amounting to 5 mg/g Ti sponge.
The activated titanium particles were then pressed on lead at 250 kg/cm2 onto a lead sheet coupon, with a particle loading of 500 g/m2 corresponding to 15 9/rn2 Ru and 2.5 g/m2 of Ti applied to the particles uniformally distributed on the lead surface.
This electrode sample was tested as an oxygen evolving anode in H2S04 (150 gpl) The electrode potential at a current density of 500 A/m2 amounted to 1.66 V vs NHE after 200û hours of anodic operation.

Example 6 TiO2 rutiie particles having a size ranging from 315 to 630 microns are activated by impregnation with the following solution :0.54 9 RuC13.H20; 1.8 9 butyltitanate; 0.25 ml HCl; and 3.75 ml butylalcohol.
After impregnation, the particles are dried at lOO~C in air and baked at 440C for lD minutes under air flow. This procedure is repeated 4 times.
The resulting particles are activated with Ruo2-Tio2.
The particles are then pressed onto a lead sheet coupon by applying a pressure of 250 kg¦cm2. The particle loading amounted to 400 g/m2 corresponding to a Ru and Ti loading of 15 and 16 g/m2 respectively(applied as RUo2-Tio2).
The obtained activated lead electrode was tested as an anode in an aqueous solution containing 150 gpl H2S04 at room temperature. The applied anode current density applied amounted to 500 A/m2. An oxygen half cell potential of 1.75 V Y8 NHE was obtained after 300 hours of operation. After 1000 hours, the anode potential reached the same value as that of an anode of pure, untreated lead.
Example 7 An activating solution was prepared as described in Exarnple 1, but instead of dlluting it 8iX time~ (exarnple 1), it wa~ dllut~d with only three times its amoun~ of n-butylalcohol.

4.11 9 of Ti sponge (particle size 400-630 microns), was impregnated with the activating solution After each impregnation, the titanium sponge was dried at 100C for about 1 hour. A heat treatmlent was then effected at 250C for about 10 minutes and finally at 400C under an external air flow for about 10 minutes. This activation procedure was carried out 3 times. The Ru and Mn loadings thus obtained amounted ~o 36.2 mg Ru/g Ti and 45.8 mg Mn/g Ti.

The activation procedure, described in Example 1 for the Ti sponge with a particle size larger thant 630 microns, was applied also in this case for the larger particles (greater than 630 microns). However, the activation was carried out only 4 times. The Ru and Mn loadings thus obtained amounted to 2~.5 mg Ru/g Ti and 29.9 mg Mn/g Ti.

The activated titanium sponge particles were then pressed and partly embedded at the surface of a lead sheet coupon. The larger particles ( size greater than 63D microns) were pressed first at 240 kg/cm2 to give Ti, Mn and Ru loadings per unit lead sheet areea of 350, 10.5 and 8.3 g/m2 respectively. An electrode sample (L95) was thus obtained with a lead base uniforrnly covered with F'cu-Mn oxide activated titanium sponge particles in an amount corresponding to 760 g/m2 Ti sponge, 23.Z g/m2 Ru and 29.3 g/m2Mn. This electrode s~mple was tested as an oxygen evolving anode in H25O4 (150 gpl). The electrode potential, at a current density of 500 A/m2, amounted to 1.65 V vs NHE after 2B7 days of anodic operation.
For comparison, another anode sample (L93), which was obtained by directly pressing smaller particles of activated Ti sponge at 2B0 kg/cm2 on lead, with Ru and Mn loadings corresponding to 15.4 and 19.5 g/m2 respectively, was tested under identical conditions. The electrode potential, after 289 days, was 1.78 V vs NHE.
A further anode sample (L92) was prepared like L95 but the smaller particles (400-630 microns) were activated like in Example 1 (L62). The overall Ti, Mn and Ru loadings amounted in this case to 726, 22.5 and 17.7 glm2 respectively. Pressing of the larger particles and smaller particles was carried out at 290 kg/cm2 and 41D kg/cm2 respectively. The anode has bePn tested under identical conditions and showed a potential of 1.78 V vs NHE
after 289 days of operation.

Example 8 An activating solution was prepared as dsscribed in Example 7 4.22 9 of larger particles (particle size above 630 microns) was activated twice under the conditions specified in Exarnple 7 to give 21.5 mg Ru/g Ti and 27.4 mg Mn/g Ti.
Another activating solution was appiied to Ti sponge with a smaller particle size ranging from 400-630 microns. This activation solution corresponds to the one described in Example 7 with the difference that it was diluted with only twice its amount of l-butylalcohol. Two activations were carried out in accordance with Example 7. The Ru and Mn loadings per gram Ti amounted to 25.9 and 32.9 mg respectively.
An anode sample (L 12û) was prepared by pressing the larger particles first at 210 kg/cm2 to give Ti, Mn and Ru loadings of 360~ 9.8 and 7.7 g/m~
respectively. Smaller activated titanium particles (400-63û microns) were then pressed at 320 kg/cm2 to give Ti, Mn and Ru loadings of 420, 13.9 and lD.9 g/m2 respectively. The overall Ti, Mn and-Ru loadings thus obtained amounted to 780, 2}.7 and 18.6 g/m2 respectively.
The electrode sample was tested as an oxygen evolving anode in H2SO4 (150 gpl). The electrodes potential, at a current density of 500 A/rn2, amounted to 1.58 V vs NHE after 218 days of anodic operation.

Example 9 Titanium sponge (40~630 microns) was oxidized as ~ollows, prior to activation with Ru-Mn oxide.
4,7b, 9 of titanium sponge was activated once with the activation ~olution described in Exampl~ 1. The heat treatment was carried out at 400~C for 1~ minutes under an external air flow, after subjecting the Ti sponge to drying at 100C. The Ru and Mn loadln~s were 5.2 and 6.6 mg/g Ti Qponge respectively. The sponge wa~ then subjected to heat treatment For 4S
h at 480C undsr an e%ternal air flow to convert it into its respectjve oxide.

3.5 9 of the oxidized Ti sponge thus obtained was then activated as described in Example 1 wlth the only difference that an intermediate heat treatment was carried out at 250C instead of 200"C after each activation.
The Mn and Ru loadings per g sponge amounted to 32 8 and 25.~ mg respectively.
The preoxidized and activated Ti sponge was then pressed in two steps, first at 230 kg/cm2 and then at 29û kg/cm2 to give Mn and Ru loadings of 21.1 and 16.6 g/rn2 respectively. The loading of the oxidized Ti sponge amounted to 643 g/m2. Considering the Mn and Ru loadings in the Ti-oxide, prior to final activation, the overall Mn and Ru loadings amount to 25.3 and 19.9 g/m2 respectively.
The electrode has been tested in 150 gpl H2504 at 500 A/m2 and its potential after 275 days of operation amounted to 1.65 V vs NHE.

Example 10 Two activating solutions were prepared with a larger Mn/Ru ratio than described in Example 1.
Solution A: û.537 9 RuC13.aq and 2.0819 9 Mn ~N03)2.aq in 3.75 ml n-butylalcohol Solution B: C.537 9 RuC13.aq and 4.6844 g Ivm(N03)2. aq in 3.75 ml n-butylalcohol Both solutions A and B were diluted with 3 times their amount of n-butylalcohol prior to application. Solution A corresponds to a moiar ratio of MnO2/RuO2 = 4 and solution B corresponds to a molar ratio of MnO2/RuO2 =
9.
4.27 9 of Ti sponge (particle size 315-63~ microns) was impregnated with diluted activation solution A. After each impregnation, the titanium sponge was dried at 100C for about 1 h~ A heat treatment was then effected at 250C for 14 minutes and finally at 400C under an external air flow for about 14 minutes. Thia activation procedure was carried out 3 times. The Ru and Mn loadings thus obtained amounted to 29.3 mg Ru/g Ti and 63.8 mg Mn/g Ti.
4.16 9 of Ti sponge (particle size 315-630 microns) was irnpregnated with diluted activation solution B. The activatlon was carried out in the same manner as with activating solution A. The Ru and ~In loadings thus obtained amounted to 19.9 mg Ru/g Ti and 97.4 rng Mn/g Ti.
The activated Ti sponge particles were then pressed onto a lead sheet coupon. The larger particles (greater than 630 microns), activated as in Example 8, were pressed first at 230 kg/cm2 to give Ti, Mn and Ru loadings per unit lead-sheet area of 449, 12.0 and '3.4 g/m2 respectively.
Subsequently smaller activated (with diluted solution A) Ti particles (315-630 microns) were pressed at 350 kg/cm2 to give Ti, Mn and Ru loadings of 399, 25.5 and 11.7 g/m2 respectively.
An electrode sample (L 164) was thus obtained with a lead base uniformly covered with Ru-Mn oxide activated titanium sponge particles in an amount corresponding to 848 g/rn2 Ti sponge, 20.8 g/m2 Ru and 37.5 glm2 Mn.
This electrode sample was tested as an oxygen evolYing anode in 15û
gpl H2S04. Its po~ential,at a current density of 500 A/m2, amounted to 1.50 V vs NHE after 36 days of anodic operation.
For comparison~ another anode sample (L 161), was obtained by directly pressing smaller particles of activated Ti sponge (with diluted solution A) at 32û kg/cm2 on lead, with Ti, Ru and Mn loadings corresponding to 531, 15.6 and 34.0 g/m2 respectively.
This 01ectrode L 161 has been tested under identical conditions and showed a potential of 1.60 V vs NHE after 70 days of operation.
In another set of experiments, Ti sponge particles larger than 630 microns, activated as in Example 8, were pressed first at 230 kg/cm2 to give Ti, Mn and Ru loadings per unit lead-sheet area of 428, 11.5 and 9.0 g/m2 respectively. Smaller activated titanium sponge particles (size 315-630 microns), obtained with activating solution B, were then prassed at 350 kg/cm2 to give Ti, Mn and Ru loadings of 493, 48.0 and 9.8 g/m2 respectively. An electrode sample (L 163) was thus obtained with a lead base uniformly covered with Ru-Mn oxide activated titanium sponge particles in an amount corresponding to 921 g/m2 Ti, 59.5 g/m2 Mn and 18.8 g/m2 Ru.
This electrode has been tested as an oxygen evolving anode in 150 gpl H2S04 at 500 A/m2. Its potential after 33 days of operation amounted to 1.57 V vs NHE.

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Example 11 For comparison (with L 163 in Example 10), another anode sample (L
162) was obtained by directly pressing smaller particles (315-630 microns~ of activated Ti sponge (with diluted solution B~ at 290 kg/cm2 on lead with Ti, Ru and Mn loadings corresponding to 652,13~0 and 63.6 g/m2 respectively.
The electrode has been tested at 5ûû A/m2 in 150 gpl H25D4 and shows a potential of 1.74 V vs NHE after 18 days (430 h~ of operation under these conditions.
Exam~le 12 An activating solution was prepared by dissolving 0.54 9 RuCl3- aq (38 % Ru) and 0.12 9 PdC12 in 15 ml of butyl-alcohol. Th~ solution was stirred until all the salts were dissolved and 1.84 9 of buty~titanate was added.
~ .5 9 of titanium sponge having a particle SiZ8 ranging From 315 to 63~ microns was impregnated with this activated solution, dried at 140~C
for 20 minutes, fired at 250C for 15 minutes and finally fired for another period of 15 mirutes at 45ûC. All these heating steps were carried out in air. After cooiing, the impregnating, drying and flring operations were repeated six times. The Ru and Pd loadings thus obtained on the particles amounted to ~0 mg Ru/g Ti and 11 mg Pdtg Ti. The activated titanium sponge particles were pressed onto a lead sheet coupon with a pressure of 250 kg/cm2 in order to get the respective loadings: 5ûû g/m2 Ti sponge, 15 glm2 Ru, 5.5 g/m2 Pd.
This electrode sample was tested as an oxygen evolving anode in H2504 (151~ gpl) at 500 A/m2. The electrode potential (oxygen half-cell potential) amounted to 1.78 V vs NHE after 2û8 days of anodic operation.

Example 13 7 g of titanium ~ponge (particle 3ize 315-630 m;crans) wa3 impregnated with 1.4 ml of a solution containing 7 mg/ml of Ir in the form of IrCI~ aq, di~solved in isopropyl-alcohol. After impregnation, the titanium sponge was dried at 140C for 15 rnin., fired at 250C during 10 min. and fired again at 450C for 10 rnin., all of the~e ~tep~ being carried out in air.

The activated titanium sponge particles were pressed onto a l~ad sheet ooupon by applying a pressure of 25D kg/cm2~ The amount of particles was chosen so as to obtain a titanium and iridium loading of 700 g/m2 and 1 g/m2respectively.
A second activating solution was the applied to the electrDde sample in the following manner. A solution is prepared by dissolving 5.û g of Mn (N3)2 4 H20 and 0.32 9 Co ~Na3)2 6 H2û and D.5 ml ethanol. This solution is applied to the electrode surface, dried for 15 minutes at 140~C and baked at 250~C (10 min.) in air. After cooling, the painting, drying and baking steps were repeated five times so as to get a final loading of 24û g/m2 Mr~02 and 12 g/m2 cobalt oxide (calculated as Co304).
This electrode sample was tested as an oxygen evolving anode in H2504 (15û gpl). The electrode potential (oxygen half-cell potential) at a current density of 500 A/m2 amounted to 1.78 Volts (vs NHE) after seven months of anodic operation.
Example 14 The electrode sample was prepared as in Example 13, except that IrCI3 aq was replaced by RuU3 aq (14 mg/ml of Ru) and that the impregnation step was repeated twice so as to get a ruthenium loading of 4 g/m2 for a titanium sponge loading of 700 g/m2-When tested under the same conditions as in example 13, the oxygenhalf cell potential amounted to 1.80 V (vs NHE) after 6 1/2 rnonths of operation.

Example 15 An activating solution was prepared by dissolving 0.44 9 RuC13 aq (~8 weight % Ru), O.û90 9 SnClz.2H20 + 0.52 9 Mn (N03)2.4H20 in four ml of butyl-alcohol.
Z.5 g of titanium sponge (particle size 315-630 microns) was impregnated with this activating solutlon in the followlng rnanner: 0.77 ml of solution wa~ uniformly applied to th~ titanium sponge, drisd at 14ûC
during 15 rnin., baked at 25ûC for 10 min. and at 420C for 10 min., all drying and baking steps in air. After cooling, the titanium sponge was 12~860~L

activated twice again, each time with 0.5 ml of activating solution, dried and baked as mentioned above.
The activated titanium particles were pressed onto the surface of a lead-calcium alloy (0.06 % Ca) coupon at 25û kg/cm2 so as to get the following respective loadings: Ti 70û g/m2, Ru 20 g/m2, Sn 5.8 g/m2 and Mn 13.7 9/m20 This electrode sample was tested as an oxygen evolving anode in H2SO4 (15~ gpl) at 500 A/m2. The electrode potential amounted to 1.67 V vs NHE after 7 months of operation.
As may be seen from the above examples, an anode according to the invention can be fabricated in a simple manner and be used for prolonged evolution of oxygen at a potential which is significantly lower than the anode potential corresponding to oxygen evolution on lead or lead alloy under otherwise similar operating conditions.
It may be noted, that no loss of lead from the base could be o~served when testing anode samples according to the invention, as described in the above examples, whereas a notable lead loss could be observed in the electrolyte when testing lead or lead alloy reference samples under the same conditions.
It has moreover been found that simultaneously applying heat and pressure, when partly embedding the valve metal particles in the lead or lead alloy at the surface of the anode baæ, can facilitate their fixation, while preventing the particles from being completely ernbedded in and/or flattened on the base.
It may also be noted that further improvements may well be expected with respect to the above examples by determining the best conditions for providing anodes according to the invention with optimum, stable el~ctrochemical performance with maximum economy of precious metals.
It is understood that the catalytic particles may be applied and anchored 'co the lead or lead alloy base of the anode, not only by means of a pre~s as in the examples described above, but also by any other n-eans such as pres~ure rollers for example, which rnay be ~ultable for providlng the essential advantages of the invention.
It has alBO been found that the application of heat (e.g at about 250C) during the pressing step can prornots partial embedmenc of the ~8~

catalytic particles into the lead or lead alloy surface.
The invention provides various advantages of whieh the following may be mentioned for example:
(a) The anode ac~ording to the im~ntion c~n be operated at a significantly reduced potential, well b low that oF cDnventional anDdes of lead or lead alJoy currently uæd in industrial cells for electrowinning metals from acid solutions. The cell voltage and hence the energy costs for electrowinning metals may thus be decresæd accordingly.
(b) ~ontamination of the electrDlyte and the cathodic deposit by materials coming from the anode l~an be substantially avoided, since it has been experimentally established that oxygen is eYolved on the catalytic particies at a reduced potential, at which the lead or Jead alloy of the anode base is effectively protected from corrosion.
(c) Dendrite formation on the cathode which may lead to short circuits with the anode and can thereby burn holes into the anode, will nevertheless lead to no serious deterioration of the performance of the anode according to ~he invention, since it operates with oxygen evolution on the catalytic particles at a reduced potential, at which any part of the lead or lead base which is exposed does not undergo noteble eorrosion.
(d) Conventional lead or lead alloy anodes may be readily converted ~nto improved anodes according to the invention and it thus becomes possible to retrofit industrial cells for electrowinning metals in a psrticularly simple and inexpensive mannsr to provide improved performance.
(e) The reduced cell voltage obtained with anodes according to the invention can be readily monitored SDtllat c~e is ~ible t~ rapidly dç~tect a~y notable rise which may occur in-the anode potential. The eatalytic particles on the lead or lead alloy base may thus be readily reactivated or replaced whenever this should become necessary.
(f) Ruthenium can be used as catalyst in an extremely ecor.omical manner, by combining it in a very small proportion with titaniurn spDnge particles applied in a many timea larger amount to the anode base of lead or lead alloy~ The cost of ruthsnium can thus be Justified by the resulting improvement in anode performance.
(g) Rutheniurn can thus be used in very restricted smounts and combined with less expensive stable materials.
(h) Decreased short-circuits could be observed in copper electrowinning plants equipped with anodes according to the invention. This resulted in an improved cathodic current efficiency, thereby further increasing the energy savings already achieved by the reduced cell voltage due to operation of the anode for the invention at a reduced oxygen half-cell potential.

Anodes according to the invention may be advantageously applied instead of currently used anodes of lead or lead alloy, in order to reduce the energy costs required for industrially electrowinning metals such as zinc, copper, cobalt, and nickel and to imprDve the purity of the metal produced on the cathode.
Such anodes may be usefully applied to various processes where oxygen evolution at a reduced overvoltage is required.

Claims (15)

CLAIMS:

1. A catalytic anode for evolving oxygen in an acid electrolyte, comprising an anode base of lead or a lead alloy and titanium particles catalytically activated with a minor amount of ruthenium in oxide form, said par-ticles being uniformly distributed, partly embedded and firmly anchored in the surface of said base so that oxygen may be anodically evolved on said particles at a reduced potential at which the underlying lead or lead alloy at said surface of the anode base remains electrochemically in-active, characterized in that said activated titanium part-icles comprise a minor amount of ruthenium and manganese in oxide form obtained by thermal decomposition of corres-ponding compounds, and in that said catalytically activated particles comprise more than 400 grams of titanium per square meter of said anode base surface, said particles having a size greater than 300 microns.

2. The anode of Claim 1, characterized in that said particles are titanium sponge particles.

3. The anode of Claim 1, characterized in that said activated titanium particles further comprise titanium in oxide form.

4. The anode of Claim 1, characterized in that it further comprises particles of valve metal and/or valve metal oxide, in addition to said activated particles.

5. The anode of Claim 4, characterized in that said valve metal oxide is ZrO2.

6. A method of making an oxygen evolving anode character-ized by the steps of:
(a) catalytically activating titanium sponge particles having a size greater than 300 microns by impregnating said particles with an activating solution containing thermally decomposable ruthenium and mang-anese compounds and thermally converting said compounds in an oxidizing atmosphere into ruthenium and manganese oxide;
(b) uniformly distributing the activated particles obtained in step (a) over the surface of the anode base of lead or lead alloy, pressing, partly embedding and thereby anchoring said activated particles in said surface of the anode base, so that the amount of tit-anium in said catalytically activated, partly embedded particles corresponds to more than 400 grams per sqaure meter of said anode base surface.

7, The method of Claim 6 characterized in that large activated particles are first pressed into the surface of the anode base and smaller activated particles are then pressed into said anode base surface.

8, The method of Claim 7 characterized in that said smaller particles are provided with a greater amount of ruthenium than said large particles.

9, The method of Claim 7 or 8 characterized in that said large particles have a size greater than 600 microns and said smaller particles have a size from 300 to 600 microns.

10. The method of Claim 6 characterized In that titanium oxide is further formed on said catalytically activated particles by thermal decomposition of a titanium compound applied after forming the ruthenium and manganese in oxide form on said particles.

11. The method of Claim 6 characterized in that particles of valve metal and/or valve metal oxide are further pressed into the anode base after partly embedding said activated particles.

12. The method of Claim 11 characterized in that particles of zirconium dioxide are pressed and likewise fixed to the anode base after said catalytically activated titanium particles have been partly embedded.

13. The method of claim 6 wherein said compounds contained in the activating solution are manganese nitrate and ruthenium chloride.

14. The method of claim 13, wherein said activating solution comprises a solvent consisisting of an alcohol for dissolving said componds.

15. The method of claim 13 or 14, wherein said compounds in the activating solution used for impregnating in step (a) provide a larger amount of manganese than ruthenium on said catalytically activated titanium particles.