Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-04-08
2001-04-17
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S290030, C204S290060, C204S290090, C204S290120, C204S290130, C204S290140
Reexamination Certificate
active
06217729
ABSTRACT:
BACKGROUND OF THE INVENTION
Anodes have been used commercially for many years in electrolytic processes for the preparation of various chemicals such as chlorine, bromine and hydrogen peroxide, for the electrodeposition of metals such as chromium, copper and zinc, as well as for high speed electroplating such as electrogalvanizing.
The conventional electrolytic anode consists of a substrate made of a valve metal, such as titanium, niobium, tantalum or zirconium or an alloy of these metals, and an electrocatalytic coating of a precision metal(s) or precious metal oxide(s), where the precious metal is usually a platinum group metal, such as iridium, platinum, rhodium or ruthenium. The precious metal or metal oxide coating is often mixed with the oxides of the valve metals. Typically, the valve metal substrate is also subjected to a surface treatment such as chemical etching, mechanical gritblasting and/or the application of a wash coat, prior to the electrocatalytic coating. The electrocatalytic coating is also typically applied by either electrodeposition or thermal deposition methods. Also, with the development of new high speed electrogalvanizing processes, where extremely low pH, high current densities and elevated temperatures are employed, a barrier layer has been introduced to protect the valve metal substrate from its passivation.
For example, U.S. Pat. No. 4,203,810 to Warne discloses an anode for use in an electrolytic process comprising a substrate of titanium, tantalum, or niobium over which a barrier layer containing platinum or platinum-iridium alloy is formed by painting a chemical compound containing platinum and iridium over the substrate, the painted substrate subsequently being heat treated. A layer of a precious metal is applied over the anode by an electroplating process.
Similarly, U.S. Pat. No. 4,331,528 to Beer discloses an anode having a film forming substrate of titanium, tantalum, zirconium, etc. over which a thin barrier layer is formed. The barrier constitutes a surface oxide film grown up from substrate that also incorporates rhodium or iridium metal or their compounds in an amount of less than 1 g/m
2
(as metal). The anode is then thermally coated with an electrocatalytic coating comprising at least one platinum-group metal or metal oxide possibly mixed with other metal oxides, in an amount of at least about 2 g/m
2
.
Additionally, U.S. Pat. No. 4,528,084 to Beer discloses an anode having a barrier layer formed over a substrate from a solution containing a thermo-decomposable compound of a platinum-group metal and also a halide which attacks the substrate which purportedly results in increased performance.
U.S. Pat. No. 4,913,973 to Geusic discloses an anode comprised of a valve metal substrate over which a barrier layer consisting of at least 150 &mgr;inches of electroplated platinum is formed. The barrier layer is subsequently heated at high temperatures to reduce the porosity of the barrier layer. A second thermally deposited coating of iridium oxide is subsequently deposited over the barrier layer.
U.S. Pat. No. 5,672,394 to Hardee describes an anode with a surface roughness of at least 250 microinches (6 microns) and an average surface peaks per inch of at least 40 that has a ceramic barrier layer followed by a thermally deposited electrocatalytic coating composed of a mixture of iridium and tantalum oxides.
SUMMARY OF THE INVENTION
The present invention provides an anode having an improved service life when used in electrolytic processes characterized by, for example, low pH and/or high temperature and or high current density. The anode of the present invention comprises: (a) a valve metal substrate; (b) a first layer comprising at least one platinum-group metal or platinum-group metal oxide and at least one valve metal or valve metal oxide formed on the valve metal substrate; (c) a second layer comprising a platinum-group metal formed on the first layer; and (d) a third layer comprising at least one platinum-group metal or platinum-group metal oxide and at least one valve metal or valve metal oxide formed on the second layer.
The present invention also provides a method for preparing a anode comprising the steps of: (a) forming a first layer comprising at least one platinum-group metal or platinum-group metal oxide and at least one valve metal or valve metal oxide on a valve metal substrate; (b) forming a second layer of a platinum-group metal on the first layer; and (c) forming a third layer comprising at least one platinum-group metal or platinum-group metal oxide and at least one valve metal or valve metal oxide on the second layer.
DETAILED DESCRIPTION OF THE INVENTION
In the anode of the present invention, the valve metal substrate may include at least one valve metal such as titanium, niobium, tantalum, or zirconium. Preferably, the valve metal substrate is made of titanium. Prior to the formation of the first layer onto the substrate, the surface of the substrate may be cleaned using conventional procedures including but not limited to vapor degreasing, alkaline cleaning, and the like. Preferably, the surface is cleaned using a commercial alkaline cleaning bath for 20-30 minutes at 50-60° C. After the surface is cleaned, the surface is preferably roughened using conventional mechanical or chemical means, such as, for example, by grit blasting or acid etching. Preferably, the surface is roughed using an aluminum oxide grit. It is preferred that the surface have a roughness Rq of 2-12 &mgr;m, and more preferably an Rq of 3-6 &mgr;m, and most preferably an Rq of 4-5 &mgr;m as measured using the SURFTEST 212 surface roughness tester (Mitutoyo, Japan). After the surface of the substrate is roughed, it may be further subjected to thermal oxidation by heating the surface at an elevated temperature in an oxygen containing atmosphere for 1-3 hours. The temperature of such treatment is preferably 350-600° C., and more preferably 400-500° C.
The first layer to be formed on the substrate includes at least one platinum-group metal or platinum-group metal oxide and at least one valve metal or valve metal oxide. Suitable platinum-group metals and oxides thereof include ruthenium, osmium, rhodium, iridium, palladium, platinum, ruthenium oxide, osmium oxide, rhodium oxide, iridium oxide, palladium oxide, and platinum oxide. Suitable valve metals and valve metal oxides include but are not limited to tantalum, tantalum oxide, titanium, titanium oxide, zirconium, and zirconium oxide. In the preferred embodiment of the invention, the first layer includes iridium oxide and tantalum oxide.
The first layer is formed on the substrate using conventional procedures such as applying one or more coatings of a solution containing the selected metal salts or other compounds onto the substrate until the total loading of the first layer, after suitable thermal treatment, is 0.5-2.5 g/m
2
, and more preferably 1.8-2.2 g/m
2
. The coating may be prepared by combining the selected metal salts or other compounds with an aqueous or alcohol solution. In the preferred embodiment, the substrate is painted with a n-butanol solution containing salts of iridium and tantalum. The ratio of iridium to tantalum in the solution is also preferably about 65% to 35% by weight. After each coating is applied, it is desirable to let the coating air dry which typically takes approximately 20 minutes. After each coating is air dried, the coating is heated in an oxygen containing atmosphere to permit the components to decompose into their respective stable metal or oxide form. The duration of heat treatment will depend upon the temperature of the heat treatment. The inventors have found that a heat treatment at a temperature of approximately 500° C. for approximately 20-30 minutes is sufficient to form an iridium oxide/tantalum oxide composite coating. However, the actual temperature and duration of treatment may be different if other metals are used and can be determined by the skilled artisan. The process of painting and heat treating the titanium substrate is repeated as necessary in or
Blum David B.
Geusic Mark J.
Ivanter Irina
Zolotarsky Vadim
Bell Bruce F.
United States Filter Corporation
Wolf Greenfield & Sacks P.C.
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