FI63604C - STABIL ELEKTROD FOER ELEKTROKEMISKA TILLAEMPNINGAR - Google Patents

Description

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Canada (CA) 290531 (71) Noranda Mines Limited, P.O. Box 1 * 5, Suite U500, Commerce Court West,

Toronto, Ontario, Canada (CA) (72) Raouf 0. Loutfy, Napierville, Illinois, USA, Paul P.K. The present invention relates to an electrode for use in electrochemical processes, in particular as a dimensionally stable anode with good electrical conductivity. , low oxygen and chlorine overvoltage, and low corrosion rate under acidic electrolysis conditions.

An anode material suitable for use under electrolytic conditions must have the following characteristics: (a) it must have a good electrical conductivity and the ability to continue to conduct current to the electrolyte for long periods of time without becoming passivated; (b) it must be an efficient electrocatalyst for the evolution of oxygen and chlorine, i.e. it must have a low oxygen and chlorine overvoltage; and (c) it must be able to withstand corrosion in a highly corrosive electrolysis environment, and the catalyst material must remain firmly adhered to the metal substrate so as to withstand mechanical damage.

2 63604 In practice, very few ingredients are able to satisfy the latter of these requirements. The most common anode material in industrial use is lead dioxide, which is formed on lead and lead alloys. When used to separate metal from ore by electrolysis as sulfate, lead dioxide is very electrochemically stable when anodically polarized, but reacts to form an insoluble lead sulfate coating when the anode potential decreases or ceases. When the current is subsequently passed through again, this lead sulphate coating tends to peel off, and this can cause considerable contamination of the separated metal. This problem is particularly difficult when copper is separated by electrolysis, so that little lead contamination can be tolerated. Thus, alternative anode materials are being vigorously sought for this application, although none of them have so far become widely used in industry.

A wide variety of electrodes based on the use of valve metal substrates such as titanium, tantalum, tungsten, zirconium or these alloys have been proposed. Of these, titanium is particularly suitable compared to other valve metals due to its low price.

When polished as an anode in an acidic sulfate or chloride solution, titanium is passivated due to the formation of a thin layer of rutile-titanium dioxide, which prevents the use of uncoated titanium as an anode material. However, many manufacturers have produced electrodes based on titanium substrates coated with thin layers of platinum, platinum-iridium alloys or other precious metals. These electrodes qn have the mechanical strength and non-reactivity of the noble metal coating in solution, and in addition have the advantage that the substrate is cheap. In addition, if the substrate is exposed due to mechanical damage or mechanical damage or chemical wear to the precious metal coating, passivation will occur, preventing serious deterioration.

Precious metal coated titanium anodes represent a significant advance in the art, but numerous experiments have shown that in sulfuric acid electrolysis, at high temperatures, the wear rate of their noble metal coatings is too high to consider this type of electrode as an economical option for metal separation from ore.

n 63604

An alternative and more economical type of inert electrode material has become widely used in recent years following the development of a mixed oxide or solid solution type electrode coating, which was published in Canadian Patent Publication No. 932,700.

Electrodes based on this development comprise a substrate of valve metal, usually titanium, coated with a coating consisting of a solid solution of a valve metal oxide and a noble metal oxide, typically titanium dioxide and ruthenium dioxide. The electrode thus obtained has very good electrical conductivity and low oxygen and chlorine overvoltage due to the electrocatalytic properties of ruthenium dioxide. Such electrodes are now widely used in the chlor-alkali industry as anodes in the manufacture of chlorine.

However, RuCl 2 / TiCl 2 electrodes are not well suited for the separation of metal from ore by acid electrolysis because they have relatively high corrosion rates and passivation occurs after 200-1000 hours of operation. The use of a RuO 2 / Ta 2+ mixed oxide coating improves the life of the anode to some extent, but not sufficiently for commercial use. IrO 2 / TiO 2 coatings or IrO 2 / Ta 2 O 2 coatings proposed in Canadian Patent Publication No. 989,773 have significantly poorer electrocatalytic activities except for coatings where the ratio of iridium to tannal is high. However, the price of the noble metal oxide IrO 2 is considerably higher than that of RuC> 2, and the large relative amount of this noble metal oxide required, typically between 20 and 40%, results in a high total cost of these electrodes.

An electrode has now been developed using a coating composition which allows a significantly lower level of noble metal oxide, i. RuO 2 ~ or IrO 2 content without compromising the catalytic performance of the coating and without causing premature electrode passivation under conditions where both oxygen and chlorine are evolved. The proposed coating composition also has a low electrode corrosion rate compared to existing electrodes.

The electrode according to the invention comprises an electrically conductive substrate made of a material resistant to the electrolyte and its electrolytic product, which is at least partially coated on its surface with a material containing 30-70% by weight of an acid-insoluble metal tungstate, 15-60% by weight of tantalum pentoxide and 5- 15% by weight of a noble metal oxide.

The preferred metal tungstate is MnWO 4, C 6 WO 4, or ZrfWO 4, because these metal tungstates have been found to be highly acid resistant compounds. The precious metal oxide is also preferably ruthenium or iridium oxide.

The substrate is preferably made of a valve metal selected from the group consisting of titanium, tantalum, tungsten, zirconium, niobium and alloys thereof. However, other electrically conductive metals which are not affected by the electrolyte and the products formed during electrolysis can be used, such as metal electrodes clad with the above-mentioned valve metals.

After undergoing conventional degreasing and cleaning treatments, the substrates are preferably coated with a base layer rich in noble metal oxides, more preferably with a Ta2 ° 5 3a RuC> 2 mixed crystal layer of the former 0-80% and the latter 100-20%. The purpose of this base layer is to prevent possible passivation of the electrode due to oxidation of the substrate under the coating, which causes the growing, insulating oxide film to interrupt the flow of current. This phenomenon is avoided by redistributing the oxide content of the noble metal oxide in the electrode coating by depositing a base layer rich in the noble metal oxide before depositing the surface coating. The primary base layer contains about 50% Ta 2 O and 50% noble metal oxide.

The base layer is preferably deposited by a known mixed crystal technique. This technique involves co-precipitating all the materials in the coating layer so that the crystal lattice of each material in the coating composition is intertwined with the crystal lattice of the other materials that make up the coating. This result can be achieved by various methods. One method that has been used involves preparing a solution containing the substances to be co-precipitated and a suitable solvent. This solution is then deposited on the substrate by brushing it on the substrate or immersing the substrate 5 63604 in it. The solvent is then evaporated, after which the residue remaining on the substrate after evaporation is heated in air to obtain a mixed crystal coating.

The surface coating is also deposited by the same co-precipitation technique, but only Ta 2 and noble metal oxide are deposited as a mixed oxide. The metal tungstate precipitates as a mechanical mixture with the mixed crystal structure of Ta 2 and the noble metal oxide.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a polarization curve of an electrode according to the invention compared to a lead dioxide electrode for oxygen evolution; and Figure 2 shows the polarization curve of an electrode according to the invention compared to a carbon electrode for chlorine evolution.

Fine sand sandblasted titanium substrates having a size of 50 mm x 80 mm for electrode conductive substrate were etched with 10% oxalic acid at 90 ° C for 4 hours. After etching the titanium substrates and rinsing thoroughly with acetone and distilled water, they were painted with various base layer solutions of a thermally decomposable compound of tantalum and a thermally decomposable compound of a precious metal such as ruthenium or iridium to give a base layer with a composition of 80% Ta 2 O 5 and between 100-20% noble metal oxide, preferably about 50% Ta 2 O and 50% noble metal oxide after evaporation of the solvent and heating of the dried electrode substrate in the presence of an oxygen-containing gas.

The heating step was performed at 400-550, preferably at 500 ° C for 5-10 minutes, in a muffle furnace. This procedure was repeated several times to increase the basis weight of the coating to level 2 from 0.5 to 2 mg / cm.

After the base layer of the coating was deposited, the electrode was painted with a surface coating solution of an acid-insoluble metal tungstate and a thermally decomposable tantalum compound and a thermally decomposable compound of a noble metal such as ruthenium or iridium to produce electrodes having a coating size of % noble metal oxide and 15-60% Ta 2 O 5 ', preferably 40-60% metal tungstate, 5-15% noble metal oxide and 35-55% Ta 2 O 4, after evaporation of the solvent and heating of the dried electrode in an oxygen-containing gas. To obtain the above-mentioned total composition, the surface coating composition was 30-90% metal tungstate, 0-15% noble metal oxide and 5-65% Ta 2 O 2,

The heating step was still performed at 400-550, preferably 500 ° C, for 5-10 minutes in a muffle furnace. The painting procedure was repeated several times to achieve the desired final composition of the electrode coating composition. For example, if the composition of the base layer was 50% RuO 2, the coating had a basis weight of 1 mg / cm, and the surface coating contained 5% RuCl 2, and the total coating had a RuCl 2 content of 10%, then solving the equation 50 x 1 + 5 xx = 10 (1 + χ) 2 gives a basis weight χ of 8 mg / cm.

The method of manufacturing the electrode described above ensures that the electrode has a base layer rich in precious metal oxide before the surface coating is deposited. This base layer prevents passivation of the electrode by preventing oxidation of the titanium substrate under the coating. As a result of this redistribution of the noble metal oxide content into the two coating layers, a low corrosion rate of the electrodes is also achieved. This is due to the low concentration level of the precious metal oxide in the surface coating.

Potentiostatic characterization of the electrodes prepared according to the above was performed in 200 g / l sulfuric acid at 60-65 ° C and in brine at 28% concentration and 80 ° C. The electrodes were also tested by continuous electrolysis in 200 g / l sulfuric acid (18%) at a current density of 20 mA / cm ^ 60-65 ° C and determining the anode potential of each anode as a function of the duration of electrolysis. In addition, the electrodes were tested by continuous electrolysis in 28% sodium chloride solution at a current density of 200 mA / cm 2 at 80 ° C, and the anode potential was measured as a function of the electrolysis duration. The following are some examples of these electrodes:

II

63604 7

Example I

50 mm x 80 mm titanium plates were sandblasted with fine sand and etched with 10% oxalic acid at 90 ° C for 4 hours, and then washed thoroughly with acetone and water. The tiles were then coated by painting them with a solution of typical composition:

TaCl5 0.2027 g

RuCl3 - 3H2O 0.27 g n-butyl alcohol 3-15 ml HCl (36%) 0.3-1.5 ml

After painting each layer, the tiles were dried and then heated for 5 minutes at 500 ° C in a muffle furnace. The composition of the bottom layer thus obtained was about 50% Ta 2 O 5 and 50% RuO 2. The basis weight of this layer was typically 1-2 mg / cm 2. After depositing this base layer, the tiles were painted with a solution having the following composition:

MnWO 4 0.445 g

TaCl5 0.8836 g

RuCl 3 · 3H 2 O 0.0216 g n-butyl alcohol 5-15 ml HCl (36%) 0.5-15 ml

After painting each layer, the tiles were dried and then heated for 5 minutes at 500 ° C in a muffle furnace. The composition of the surface coating thus obtained was 1-5% RuCl 2, 40-60% Ta 2 O 3 and

40-60% MnW0 .. The surface weight of this surface coating was typically 2 H

7-9 mg / cm.

For example, to prepare an electrode with a total coating composition of 12% RuO 2, 43% Ta 2 O 4 and 45% MnWCl 4, the base layer o had a composition of 50% RuO 2 and 50% Ta 2 O 4 and a basis weight of 2 mg / cm 3, and a surface coating composition of 3 , 6% RuO 2, λ 2 41.4% Ta 2 O 3 and 55% MnV 3 O 4, and basis weight 9 mg / cm. The total basis weight of coating 2 was 11 mg / cm.

The values of the initial anode potential, the average anode potential after electrolysis and the duration of electrolysis in sulfuric acid solution for the various manganese tungstate anodes are summarized in Table I, together with the values of the anodes prepared in the same way but without manganese tungstate.

TABLE I

Coating Anode Potential, Y

total (NHE) at 20 mA / cm2 Electrolysis

Anode compositions, x. . duration

Initial-% value of electrolysis after (h) 1 RUO2 100% 1.55 1.55 -> 10 Passivates in 216 h 2 Ru02 50% 1.55 1.55 -> 10 Passivates in 312 h

Ta205 50% 3 Ru02 10%

Ta20 (. 90% 1.55 1.55 -> 10 Passivates in 48 h 4 MnW04 45% RUO2 12%

Ta205 43% 1.55 1.55> 7000 5 MnW04 40%

Ru02 6%

Ta 2 O 5 54% 1.56 1.51> 7000

The results shown in Table I show that the electrodes of this invention have high electrocatalytic activity despite the low noble metal oxide content reported in Canadian Patent 989,773, where the ratio of noble metal oxide to other components is on the order of 0.50: 1.0 or 1.5: 1. 0 and consists of a noble metal oxide present in 30-60% of the coating.

It can also be seen from the results in Table I that when the coating did not contain MnWO 4, the electrodes with a low RuO 2 content (10%) became passivated even after a very short electrolysis period. Even electrodes with a high RuO 2 content but no MnWO 4 are rapidly passivated in electrolysis.

A typical potentiostatic characteristic of these manganese tungstate electrodes as oxygen electrodes is shown in Figure 1. Lead electrodes (Pb - 6% 5b) preconditioned by metal electrolysis according to standard separation plant practice are also shown in Figure 1. overvoltage up to 100 mA / cm2. A reduction of 450-500 mV in breakdown overvoltage could be achieved by using these electrodes compared to lead anodes, by electrolysis of the metal at a conventional current density of about 20 mA / cm.

Example II

50 mm x 80 mm titanium plates were sandblasted with fine sand and etched in 10% oxalic acid at 90 ° C for four hours and then washed thoroughly with acetone and water. The tiles were then coated with a preliminary base layer by brushing them with an aqueous solution containing iridium chloride and tantalum pentachloride in a ratio to give a coating composition of about 50% IrO 2 and 50% Ta 2 O 4 after evaporation of the solvent and thermal decomposition of the metal chlorides. These compounds were layered in 2-5 layers, always occasionally heated at 500 ° C for 5 minutes in a muffle furnace. The tiles were then coated with a surface layer of a solution containing manganese tungstate, iridium chloride and tantalum pentachloride in such proportions that the final total composition became that shown in Table II after evaporation of the solvent and thermal decomposition of the metal chlorides. The compounds were layered in 5-10 layers, always with occasional heating at 500 ° C for 5 minutes. The anode potentials were determined as in Example I and the results are reported in Table II together with the results obtained with electrodes prepared without manganese tungstate.

TABLE II

Coating Anode Potential, V

Anode total (NHE) at 20 mA / cm 2 Electrolysis compositions _i, · duration

Initial-% value of electrolysis after (h) 6 Ir02 25% 1.62 1.67 -> 10 Passivates in 120 h

Ta 2 O 5 75% 7 MnWO 4 43% 1.59 1.60> 2000

Ir02 11%

Ta205 46%

The results shown in Table II show that the electrodes of the present invention 6604 have a high electrocatalytic activity, despite the relatively low iridium oxide content. It also shows that when the coating composition did not include MnWC> 4, the electrodes rich in IrC> 2 (25%) became passivated even after a short electrolysis period.

Example III

The titanium plates were treated and coated as in Example I, except that the manganese tungstate was replaced by cobalt tungstate in the surface layer of the coating. The total compositions and anode potentials of the electrode coatings are shown in Table III together with the results of the corresponding electrodes prepared without CoW04.

TABLE III Anode Potential of Coating, V

total (NHE) at 20 mA / cm 2 Duration of electrolysis

Anode composition ... ,, Ί

Initial Electrolysis ...

% value after '' 8 Ru02 12% 1.55 1.55 -> 10 52

Ta2 ^ 5 88% 9 CoWO4 42.5% 1.56 1.52> 2500

Ru02 12.0%

Ta205 45.5%

The results in Table III show that the presence of CoWO 2 in the electrode coating prevents anode passivation.

Example IV

The titanium plates were treated and coated as in Example I to give a total coating composition of 53% MnV704-7% RuO2-40% Ta2O2. The electrodes were tested by continuous electrolysis in brine having a sodium chloride content of 28% at an electrolyte temperature of 80 ° C. A typical potentistic characteristic of these manganese tungstate electrodes as a chlorine electrode is shown in Figure 2. The polarization of the graphite anode conventionally used for chlorine evolution is also shown in Figure 2. The electrodes made according to the invention have very low 63604 932 700 for the described electrode type. A 500-600 mV reduction in chlorine overvoltage could be achieved using these electrodes, compared to the use of graphite anodes for chlorine evolution at a conventional current density of 100-250 mA / cm 2.

X-ray diffraction analysis of all electrodes revealed the presence of tantalum pentoxide and noble metal oxide as one component of the metal tungstate on the electrode surface.

Other metal tungstates such as CdWCO> 4, CaWO 4 and Al 2 (WO 4) 3 were tested, but it was found that these metal tungstates were not sufficiently acid resistant for use in metal electrolysis separation solutions.

Claims (7)

63604 12 An electrode for use in an electrolysis process, which electrode comprises an electrically conductive substrate made of said of a material resistant to the electrolyte and its electrolysis products, wherein the electrode is at least partially coated on its surface, characterized in that the coating arranged in one or two layers is a substance containing 30-70% by weight of an acid-insoluble metal tungstate, 15-60 wt% Ta2Oj and 5-15 wt% of a noble metal oxide. Electrode according to Claim 1, characterized in that the metal tungstate is selected from the group consisting of MnWO 2, Zr (WC> 4) 2 and CoWO 2. Electrode according to Claim 1, characterized in that the noble metal oxide is ruthenium or iridium oxide. An electrode according to claim 1, characterized in that the substrate is coated with a coating having said overall composition and divided into two layers of a noble metal oxide-rich base layer and a surface coating which together form a combined protective coating. An electrode according to claim 4, characterized in that said base layer of the coating is made of a mixed crystal material containing Ta 2 O 3 and RuO 2, the concentration of the former being between 0 and 80 and the concentration of the latter being between 100 and 20% by weight. An electrode according to claim 5, characterized in that said coating base layer contains about 50% Ta 2 O 5 50% RuO 2 7a, that said surface coating contains 30-90% MnWO 4, 0-15% RuO 2 and 5-65% Ta 2 O 4. An electrode according to claim 6, characterized in that said combined coating contains 40-60% MnWO 4, 5-15% RuO 2 and 35-55% Ta 2 O 2.