ELECTRODE FOR ELECTROCHLORINATION PROCESSES AND METHOD OF MANUFACTURING THEREOF

Abstract

The invention relates to an electrode suitable for electrolytic treatments of dilute solutions of sodium chloride even at low temperatures. The electrode can be used in the generation of active chlorine-based biocidal agents in ballast water for marine applications. The electrode has a titanium substrate, an inner catalytic coating containing oxides of tantalum, ruthenium and iridium, and an outer catalytic coating containing oxides of titanium, ruthenium and of at least one of nickel, iron and cobalt.

Claims

1. An electrode in electrochlorination cells comprising: a titanium substrate an inner catalytic coating applied onto said substrate containing a mixture of oxides of tantalum, ruthenium and iridium and an outer catalytic coating applied on top of said inner catalytic coating containing a mixture of oxides of titanium, ruthenium and of at least one element selected from the group consisting of nickel, iron and cobalt.

2. The electrode according to claim 1 wherein said titanium substrate is characterized by a value of average roughness R.sub.a of 4 to 10 μm.

3. The electrode according to claim 1 wherein said inner catalytic coating has a total loading of noble metal expressed as the sum of ruthenium and iridium of 1 to 5 g/m.sup.2.

4. The electrode according to claim 1, wherein the weight composition of said outer catalytic coating comprises 30-60% Ru, 35-70% Ti and 1-8% as the sum of Fe, Co and Ni.

5. The electrode according to claim 1, wherein the weight ratio of ruthenium content in said outer catalytic coating to noble metal content expressed as the sum of ruthenium and iridium in said inner catalytic coating is 3 to 10.

6. A method for the production of an electrode according to claim 1 comprising the following sequential steps: etching the titanium substrate in an acid solution until imparting a controlled roughness profile; applying a solution of tantalum, ruthenium and iridium compounds to the etched substrate, with subsequent thermal decomposition at a temperature above 400° C. and formation of an inner catalytic coating; and applying a solution of compounds of titanium, ruthenium and at least one element selected from the group consisting of nickel, iron and cobalt to the inner catalytic coating, with subsequent thermal decomposition at a temperature above 400° C. and formation of an outer catalytic coating.

7. The method according to claim 6 wherein said acid solution comprises 20-30% by weight sulphuric acid and said etching step is carried out at 80-90° C. until a weight loss of said substrate between 150 and 250 g/m.sup.2 is obtained.

8. A process of biocidal treatment on an aqueous sodium chloride solution comprising electrolyzing the solution in an electrolytic cell equipped with a pair of electrodes according to claim 1 and forming of active chlorine.

9. The process according to claim 8, further comprising the periodic reversal of the polarity of the electrodes of said pair.

10. The process according to claim 8 in which said aqueous solution of sodium chloride is a ballast water for marine applications.

Description

EXAMPLE 1

[0010] A 1 mm-thick solid sheet of titanium grade 1, with a total area of 0.5 m.sup.2, was etched in 27% by weight H.sub.2SO.sub.4 at 87° C. in cycles of 15 minutes for a total of five cycles, until observing a weight loss of 175.5 g/m.sup.2. The resulting roughness profile was characterized by valleys localized at the grain boundary, as shown by a SEM investigation, and the average roughness value R.sub.a was found to be comprised between 8.6 and 10 μm as determined with a profilometer through measurements at various points of the surface. The substrate thus obtained was subdivided into samples of 130 mm×110 mm. The different samples were provided with catalytic coatings according to various formulations, the most significant of which are reported in Table 1. For all the reported samples, the inner catalytic coating was deposited by application of an aqueous solution, acidified with hydrochloric acid, of RuCl.sub.3, H.sub.2IrCl.sub.6 and TaCl.sub.5 in 5 coats, with intermediate drying at 50° C. for 5 minutes and thermal decomposition at 480° C. for 15 minutes after each coat. The outer catalytic coating was deposited with the same procedure, in a number of coats ranging between 25 and 40, selecting the precursors of the corresponding hydrochloric acid-acidified aqueous solution between RuCl.sub.3, TiCl.sub.3, Fe(NO.sub.3).sub.3, NiCl.sub.2 and CoCl.sub.2.

TABLE-US-00001 Precious metal Precious Inner layer loading Outer layer composition metal loading Sample # composition (wt %) (g.sub.[Ru+ Ir]/m.sup.2) (wt %) (g.sub.[Ru]/m.sup.2) A1 Ru 32.5, 32.5 Ir, 3.27 Ru 46.25, 50 Ti, Fe 2.5, 12.1 Ta 35 Ni 1.25 A2 Ru 32.5, 32.5 Ir, 3.25 Ru 46.25, 50 Ti, Fe 2.5, 16.1 Ta 35 Ni 1.25 A3 Ru 32.5, 32.5 Ir, 3.27 Ru 46.25, 50 Ti, Fe 2.5, 31.4 Ta 35 Ni 1.25 A4 Ru 32.5, 32.5 Ir, 3.20 Ru 40, Ti 54.5, 3.5 Fe, 18.3 Ta 35 Co 2 A5 Ru 32.5, 32.5 Ir, 3.18 Ru 38, 60.7 Ti, Fe 1.3 11.0 Ta 35 A6 Ru 32.5, 32.5 Ir, 3.20 Ru 58, Ti 35.5, Ni 6.5 10.7 Ta 35 C1 Ru 32.5, 32.5 Ir, 3.29 Ru 45, Ti 55 15.7 Ta 35 C2 Ru 32.5, 32.5 Ir, 3.11 Ru 38, Ti 62 9.8 Ta 35

EXAMPLE 2

[0011] The samples of Example 1 were subjected to a standard test of electrodic activity, as a measure of potential corrected via Frequency Response Analysis (FRA) in 220 g/l NaCl, at a temperature of 85° C. and pH 2. All samples turned out to be active towards chlorine evolution, with anodic potentials between 1.35 and 1.36 V at 1000 A/m.sup.2. The same samples were subjected to a standard faradic efficiency test in NaCl at 17 g/l, at a temperature of 15° C. and at a current density of 1200 A/m.sup.2.

[0012] Samples A1, A2, A3, A4, A5 and A6 all showed an efficiency between 86 and 87%, versus values of 81.8% for sample C1 and 83.6% for sample C2.

[0013] The characteristics of duration of the same samples were also measured using a standard accelerated test, providing their operation in 17 g/l NaCl at a temperature of 15° C. and a current density of 2500 A/m.sup.2, reversing the polarity every 12 hours starting with anodic operation. The electrode is considered deactivated when its anode potential is 1 V higher than the initial anodic potential.

[0014] Samples numbered A1 to A6 showed durations between 1200 hours (sample A4) and 1500 hours (A3), while samples C1 and C2 showed durations respectively of 500 and 460 hours.

[0015] The foregoing 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.

[0016] 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.