Electrode for electrolytic processes

Abstract

An electrode on valve metal substrate suitable for the evolution of oxygen in electrolytic processes is provided with a coating having a catalytic layer containing platinum group metals and one or more protective layers based on tin oxide modified with a doping element selected from bismuth, antimony or tantalum and with a small amount of ruthenium. The electrode is useful in processes of non-ferrous metal electrowinning.

Claims

1. Electrode suitable for oxygen evolution in electrolytic processes comprising a valve metal substrate provided with a coating, said coating comprising a catalytic layer and at least one protective layer external to said catalytic layer, said protective layer consisting of a mixture of oxides having a weight composition referred to the metals containing 89-97% of tin, 2-10% of at least one doping element selected from the group consisting of bismuth, antimony and tantalum and 1-9% of ruthenium.

2. The electrode according to claim 1 wherein said at least one protective layer consists of a mixture of oxides having a weight composition referred to the metals containing 89-97% of tin, 2-10% of bismuth and 1-9% of ruthenium.

3. The electrode according to claim 1, wherein said at least one protective layer has a thickness of 1 to 5 m.

4. The electrode according to claim 1, wherein said a catalytic layer is contact with said protective layer, said catalytic layer comprising a mixture of oxides having a weight composition referred to the metals containing 40-46% of platinum group metals, 7-13% of at least one element selected from the group consisting of bismuth, antimony, niobium and tantalum and 47-53% of tin, said catalytic layer having a thickness of 2.5 to 5 m.

5. The electrode according to claim 4, wherein said catalytic layer comprises a mixture of oxides having a weight composition referred to the metals containing 40-46% of iridium, 7-13% of bismuth and 47-53% of tin, said catalytic layer having a thickness of 2.5 to 5 m.

6. The electrode according to claim 4, wherein said catalytic layer consists of a mixture of oxides having a weight composition referred to the metals containing 47-53% of tin, 7-13% of bismuth, 40-46% as the sum of ruthenium and iridium, said catalytic layer having a thickness of 2.5 to 5 m.

7. The electrode according to claim 6 wherein the weight ratio referred to the metals of iridium to ruthenium in said sum of iridium and ruthenium ranges between 60:40 and 40:60.

8. The electrode according to claim 4 comprising at least two of said protective layers, said catalytic layer being interposed between said at least two protective layers.

9. Process of cathodic electrodeposition of metals from an aqueous solution comprising the anodic evolution of oxygen on the surface of an electrode according to claim 1.

Description

EXAMPLE 1

(1) A 1.65 M solution of Sn hydroxyacetochloride complex (SnHAC) was prepared according to the procedure described in WO 2005/014885.

(2) Two distinct 0.9 M solutions of hydroxyacetochloride complexes of Ir and Ru (IrHAC and RuHAC) were prepared according to the procedure described in WO2010055065. A solution containing 50 g/l of bismuth was prepared by dissolving 7.54 g of BiCl.sub.3 at room temperature under stirring in a beaker containing 60 ml of 10% by weight HCl, then bringing the volume to 100 ml with 10% by weight HCl upon observing that a transparent solution had been obtained, indicating that the dissolution was completed.

(3) 5.11 ml of the 1.65 M SnHAC solution, 0.23 ml of the 9 M RuHAC solution and 0.85 ml of the 50 g/l Bi solution were added into a beaker kept under stirring. The stirring was prolonged for 5 minutes. 18.57 ml of 10% by weight acetic acid were then added.

(4) The solution was applied to a sample of the pretreated titanium mesh by brushing in 6 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(5) In this way, an internal protective layer with a Sn:Bi:Ru weight ratio of 94:4:2, a thickness of 4 m and a specific Sn loading of about 9 g/m.sup.2 was obtained.

(6) 10.15 ml of the 1.65 M SnHAC solution, 10 ml of the 0.9 M IrHAC solution and 7.44 ml of the 50 g/l Bi solution were added into a second beaker kept under stirring. The stirring was prolonged for 5 minutes. 20 ml of 10% by weight acetic acid were then added.

(7) The solution was applied over the previously obtained internal protective layer by brushing in 13 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(8) In this way, a catalytic layer with an Ir:Sn:Bi weight ratio of 42:49:9, a thickness of 4.5 m and a specific loading of Ir of about 10 g/m.sup.2 was obtained.

(9) The electrode was labelled EX1.

COUNTEREXAMPLE 1

(10) A protective layer based on titanium and tantalum oxides in a 80:20 molar ratio, with an overall loading of 1.3-1.6 g/m.sup.2 referred to the metals (corresponding to 1.88-2.32 g/m.sup.2 referred to the oxides) was applied to a titanium mesh sample. The application of the protective layer was carried out by painting in four coats a precursor solutionobtained by addition of an aqueous solution of TaCl.sub.5, acidified with HCl, to an aqueous solution of TiCl.sub.4with subsequent thermal decomposition at 515 C. 10.15 ml of the 1.65 M SnHAC solution, 10 ml of the 0.9 M IrHAC solution and 7.44 ml of the 50 g/l Bi solution were added into a beaker kept under stirring. The stirring was prolonged for 5 minutes. 20 ml of 10% by weight acetic acid were then added.

(11) The solution was applied over the previously obtained protective layer by brushing in 14 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(12) In this way, a catalytic layer with an Ir:Sn:Bi weight ratio of 42:49:9, a thickness of 4.5 m and a specific loading of Ir of about 10 g/m.sup.2 was obtained.

(13) The electrode was labelled CE1.

COUNTEREXAMPLE 2

(14) A protective layer based on titanium and tantalum oxides in a 80:20 molar ratio, with an overall loading of 7 g/m.sup.2 referred to the metals (10.15 g/m.sup.2 referred to the oxides) was applied to a titanium mesh sample. The application of the protective layer was carried out by painting in four coats a precursor solutionobtained by addition of an aqueous solution of TaCl.sub.5, acidified with HCl, to an aqueous solution of TiCl.sub.4with subsequent thermal decomposition at 515 C. 10.15 ml of the 1.65 M SnHAC solution, 10 ml of the 0.9 M IrHAC solution and 7.44 ml of the 50 g/l Bi solution were added into a beaker kept under stirring. The stirring was prolonged for 5 minutes. 20 ml of 10% by weight acetic acid were then added.

(15) The solution was applied over the previously obtained protective layer by brushing in 14 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(16) In this way, a catalytic layer with an Ir:Sn:Bi weight ratio of 42:49:9, a thickness of 4.5 m and a specific loading of Ir of about 10 g/m.sup.2 was obtained.

(17) The electrode was labelled CE2.

EXAMPLE 2

(18) Some coupons of 20 mm50 mm area were cut-out from the electrodes of the above example and counterexamples to be subjected to the detection of their anodic potential under oxygen evolutionmeasured with a Luggin capillary and a platinum probe as known in the artin a 150 g/l H.sub.2SO.sub.4 aqueous solution at 50 C. The data reported in Table 1 (CISEP) represent the values of potential detected at the current density of 500 A/m.sup.2. Table 1 also shows the lifetime displayed in an accelerated life test (ALT) in a 150 g/l H.sub.2SO.sub.4 aqueous solution, at a current density of 30 kA/m.sup.2 and a temperature of 60 C.

(19) The results of these tests show how providing an internal protective layer according to the invention allows obtaining a significant increase in the duration accompanied by an improvement of the oxygen evolution potential compared to internal protective layers according to the prior art consisting of a mixture of titanium and tantalum oxides.

(20) Similar results were obtained by varying the nature of the doping element and the concentrations of the constituents of the protective layer as set out in the appended claims.

(21) TABLE-US-00001 TABLE 1 CISEP/V ALT/h (500 A/m.sup.2 in H.sub.2SO.sub.4 (30 kA/m.sup.2 in H.sub.2SO.sub.4 sample # 150 g/l, 50 C.) 150 g/l, 60 C.) EX1 1.522 1385 CE1 1.534 900 CE2 1.583 960

EXAMPLE 3

(22) 5.11 ml of the 1.65 M SnHAC solution, 0.23 ml of the 9 M RuHAC solution and 0.85 ml of the 50 g/l Bi solution were added into a beaker kept under stirring. The stirring was prolonged for 5 minutes. 18.57 ml of 10% by weight acetic acid were then added. The solution was applied to a sample of the pretreated titanium mesh by brushing in 6 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(23) In this way, an internal protective layer with a Sn:Bi:Ru weight ratio of 94:4:2, a thickness of 4 m and a specific Sn loading of about 9 g/m.sup.2 was obtained.

(24) 10.15 ml of the 1.65 M SnHAC solution, 10 ml of the 0.9 M IrHAC solution and 7.44 ml of the 50 g/l Bi solution were added into a second beaker kept under stirring. The stirring was prolonged for 5 minutes. 20 ml of 10% by weight acetic acid were then added.

(25) The solution was applied over the previously obtained internal protective layer by brushing in 13 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(26) In this way, a catalytic layer with an Ir:Sn:Bi weight ratio of 42:49:9, a thickness of 4.5 m and a specific loading of Ir of about 10 g/m.sup.2 was obtained.

(27) 5.11 ml of the 1.65 M SnHAC solution, 0.23 ml of the 9 M RuHAC solution and 0.85 ml of the 50 g/l Bi solution were added into a third beaker kept under stirring. The stirring was prolonged for 5 minutes. 18.57 ml of 10% by weight acetic acid were then added.

(28) The solution was applied over the previously obtained layers by brushing in 4 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(29) In this way, an external protective layer with a Sn:Bi:Ru weight ratio of 94:4:2, a thickness of 3 m and a specific loading of Sn of about 6 g/m.sup.2 was obtained.

(30) The electrode was labelled EX3.

EXAMPLE 4

(31) 5.11 ml of the 1.65 M SnHAC solution, 0.23 ml of the 9 M RuHAC solution and 0.85 ml of the 50 g/l Bi solution were added into a beaker kept under stirring. The stirring was prolonged for 5 minutes. 18.57 ml of 10% by weight acetic acid were then added.

(32) The solution was applied to a sample of the pretreated titanium mesh by brushing in 6 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(33) In this way, an internal protective layer with a Sn:Bi:Ru weight ratio of 94:4:2, a thickness of 4 m and a specific Sn loading of about 9 g/m.sup.2 was obtained.

(34) 10.15 ml of the 1.65 M SnHAC solution, 10 ml of the 0.9 M IrHAC solution and 7.44 ml of the 50 g/l Bi solution were added into a second beaker kept under stirring. The stirring was prolonged for 5 minutes. 20 ml of 10% by weight acetic acid were then added.

(35) The solution was applied over the previously obtained internal protective layer by brushing in 13 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(36) In this way, a catalytic layer with an Ir:Sn:Bi weight ratio of 42:49:9 and a specific loading of Ir of about 10 g/m.sup.2 was obtained.

(37) 5 ml of the 1.65 M SnHAC solution and 15 ml of 10% by weight acetic acid were then added into a third beaker kept under stirring.

(38) The solution was applied over the previously obtained layers by brushing in 6 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(39) In this way, an external protective layer with a specific loading of Sn of about 9 g/m.sup.2 was obtained.

(40) The electrode was labelled EX4.

EXAMPLE 5

(41) A protective layer based on titanium and tantalum oxides in a 80:20 molar ratio, with an overall loading of 1.3-1.6 g/m.sup.2 referred to the metals (corresponding to 1.88-2.32 g/m.sup.2 referred to the oxides) was applied to a titanium mesh sample. The application of the protective layer was carried out by painting in four coats a precursor solutionobtained by addition of an aqueous solution of TaCl.sub.5, acidified with HCl, to an aqueous solution of TiCl.sub.4with subsequent thermal decomposition at 515 C. 10.15 ml of the 1.65 M SnHAC solution, 10 ml of the 0.9 M IrHAC solution and 7.44 ml of the 50 g/l Bi solution were added into a beaker kept under stirring. The stirring was prolonged for 5 minutes. 20 ml of 10% by weight acetic acid were then added.

(42) The solution was applied over the previously obtained protective layer by brushing in 14 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(43) In this way, a catalytic layer with an Ir:Sn:Bi weight ratio of 42:49:9 and a specific loading of Ir of about 10 g/m.sup.2 was obtained.

(44) 5.11 ml of the 1.65 M SnHAC solution, 0.23 ml of the 9 M RuHAC solution and 0.85 ml of the 50 g/l Bi solution were added into a second beaker kept under stirring. The stirring was prolonged for 5 minutes. 18.57 ml of 10% by weight acetic acid were then added.

(45) The solution was applied to the previously obtained catalytic layer by brushing in 6 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(46) In this way, an external protective layer with a Sn:Bi:Ru weight ratio of 94:4:2, a thickness of 4 m and a specific Sn loading of about 9 g/m.sup.2 was obtained.

(47) The electrode was labelled EX5.

EXAMPLE 6

(48) 5.11 ml of the 1.65 M SnHAC solution, 0.23 ml of the 9 M RuHAC solution and 0.85 ml of the 50 g/l Bi solution were added into a beaker kept under stirring. The stirring was prolonged for 5 minutes. 18.57 ml of 10% by weight acetic acid were then added.

(49) The solution was applied to a sample of the pretreated titanium mesh by brushing in 6 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(50) In this way, an internal protective layer with a Sn:Bi:Ru weight ratio of 94:4:2, a thickness of 4 m and a specific Sn loading of about 9 g/m.sup.2 was obtained.

(51) 5.15 ml of the 1.65 M SnHAC solution, 2.5 ml of the 0.9 M IrHAC solution, 4.75 ml of the 0.9 M RuHAC solution and 3.71 ml of the 50 g/l Bi solution were added into a second beaker kept under stirring. The stirring was prolonged for 5 minutes. 21.7 ml of 10% by weight acetic acid were then added.

(52) The solution was applied over the previously obtained internal protective layer by brushing in 9 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(53) In this way, a catalytic layer with an Ir:Ru:Sn:Bi weight ratio of 21:21:49:9, a thickness of 3.5 m and a specific loading of Ir+Ru of about 7 g/m.sup.2 was obtained.

(54) 5.11 ml of the 1.65 M SnHAC solution, 0.23 ml of the 9 M RuHAC solution and 0.85 ml of the 50 g/l Bi solution were added into a third beaker kept under stirring. The stirring was prolonged for 5 minutes. 18.57 ml of 10% by weight acetic acid were then added.

(55) The solution was applied to the previously obtained layers by brushing in 4 coats, with a drying step at 60 C. for 10 minutes after each coat and a subsequent thermal decomposition step at 520 C. for 10 minutes.

(56) In this way, an external layer with a Sn:Bi:Ru weight ratio of 94:4:2, a thickness of 3 m and a specific loading of Sn of about 6 g/m.sup.2 was obtained.

(57) The electrode was labelled EX6.

EXAMPLE 7

(58) Some coupons of 20 mm50 mm area were cut-out from the electrodes of the above examples to be subjected to the detection of their anodic potential under oxygen evolutionmeasured with a Luggin capillary and a platinum probe as known in the artin a 150 g/l H.sub.2SO.sub.4 aqueous solution at 50 C. The data reported in Table 2 (CISEP) represent the values of potential detected at the current density of 500 A/m.sup.2. Table 2 also shows the lifetime displayed in an accelerated life test (ALT) in a 150 g/l H.sub.2SO.sub.4 aqueous solution, at a current density of 30 kA/m.sup.2 and a temperature of 60 C.

(59) TABLE-US-00002 TABLE 2 CISEP/V ALT/h (500 A/m.sup.2 in H.sub.2SO.sub.4 (30 kA/m.sup.2 in H.sub.2SO.sub.4 sample # 150 g/l, 50 C.) 150 g/l, 60 C.) EX3 1.518 1421 EX4 1.526 1394 EX5 1.549 996 EX6 1.506 1424

(60) The results show how an external protective layer containing tin oxides allows increasing the operational lifetime of electrodes, at the expense of an increase in their anodic overpotential. However, if the protective external layer containing tin oxides is a protective layer according to the invention, the increase in the operational lifetime is further enhanced, probably due to the stabilisation of iridium at the start-up and during the first hours of operation, while the anodic potential remains low.

(61) Similar results were obtained by varying the nature of the doping element and the concentrations of the constituents of the protective layer as set out in the appended claims.

(62) The previous description shall not be intended as limiting the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is solely defined by the appended claims.

(63) Throughout the description and claims of the present application, the term comprise and variations thereof such as comprising and comprises are not intended to exclude the presence of other elements, components or additional process steps. The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.