Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1990-12-24
2001-07-03
Tung, T. (Department: 1743)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S196010, C204S196360, C204S196380, C204S290130, C204S290140, C205S724000, C205S734000, C205S738000
Reexamination Certificate
active
06254743
ABSTRACT:
BACKGROUND OF THE INVENTION
The most important development in electrolysis electrodes in recent years has been the advent of dimensionally stable electrodes following the teachings of U.S. Pat. Nos. 3,771,385 and 3,632,498. These dimensionally stable electrodes consist of a base or substrate of a valve metal, typically titanium, carrying an electrocatalytic coating such as a mixed oxide of platinum group metal and a valve metal forming a mixed crystal or solid solution. Many different coating formulations have been proposed.
The major use of these dimensionally stable electrodes has been as anodes in chlor-alkali production in mercury cells, diaphragm cells and more recently in membrane cells. Other uses have been as oxygen-evolving anodes for metal electrowinning processes, for hypochlorite and chlorate production, as metal plating anodes and so on. Use as an anode in cathodic protection has also been proposed and as cathodes in certain processes.
Depending on the use, these dimensionally stable valve metal electrodes have been proposed with various configurations such as rods, tubes, plates and complex structures such as an array of rods or blades mounted on a supporting current conducting assembly as well as a mesh of expanded valve metal typically having diamond shaped voids mounted on a supporting current conducting assembly which provides the necessary rigidity.
Electrodes in the form of platinized valve metal wire are known for cathodic protection, but in practically every other application rigidity and dimensional stability of the electrode are critical factors for successful operation. For example, many electrolytic cells are operated with an inter-electrode gap of only a few millimeters and the flatness and rigidity of the operative electrode face are extremely important.
For most applications, the dimensionally stable electrodes operate at relatively high current densities, typically 3-5 KA/m
2
for membrane cells, 1-3 KA/m
2
for diaphragm ceris and 6-10 KA/m
2
for mercury cells. These high current densities, combined with the requirements of planarity/rigidity, necessitate valve metal structures of substantial current carrying capacity and strength.
Typical known valve metal electrodes of the type with expanded titanium mesh as operative face use a mesh having an expansion factor of 1.5 to 4 times providing a void fraction of about 30 to 70 percent. Such titanium sheets may be slightly flexible during the manufacturing processes but the inherent elasticity of the sheet is restrained, e.g. by welding it to a current conductive structure, typically having one or more braces extending parallel to the SWD dimension of the diamond-shaped openings. Such electrode sheets typically have a current-carrying capacity of 2-10 KA/m
2
of the electrode surface.
Other electrode configurations are known for special purposes, e.g., a rigid cylindrical valve metal sheet mounted in a linear type of anode structure for cathodic protection (see U.S. Pat. No. 4,515,886).
Manufacture of the known electrodes usually involves assembly of the electrode valve metal structure, e.g. by welding, followed by surface treatment such as degreasing/etching/sandblasting and application of the electrocatalytic coating by various methods including chemi-deposition, electroplating and plasma spraying. Chemi-deposition may involve the application of a coating solution to the electrode structure by dipping or spraying, followed by baking usually in an oxidizing atmosphere such as air.
SUMMARY OF THE INVENTION
It has now been found that titanium and other valve metals, e.g., tantalum and zirconium, can be greatly-expanded to a pattern of substantially diamond-shaped voids having an extremely high void fraction. Having been expanded in this way material cost becomes acceptable and they form an ideal structure for cathodic protection, e.g., of reinforcing steel in concrete as has been more particularly discussed in the U.S. Pat. NO. 4,900,410. Moreover the greatly expanded mesh is flexible and coilable and uncoilable about an axis along the LWD dimension. Thus the expanded metal can be supplied in the form of large rolls which can be easily unrolled onto a surface to be protected, such as a concrete deck or a concrete substructure. The pattern of voids in the mesh is defined by a continuum of valve metal strands interconnected at nodes and carrying on their surface an electrocatalytic coating. These multiplicity of strands provide redundancy for current flow in the event that one or more strands become broken during shipping or installation. The metal mesh is desirably stretchable along the SWD dimension of the pattern units whereby a coiled electrode roll of the mesh can be uncoiled on, and stretched over, a supporting substrate and into an operative electrode configuration, as more particularly described in the U.S. Pat. No. 4,900,410. This application and the other application mentioned above are herein incorporated by reference.
The electrode system of the present invention satisfies all of the requirements for cathodic protection of reinforcing steel in concrete. It consists of the highly expanded valve metal which is activated by an electrocatalytic coating. Current can be distributed to the expanded valve metal by a welded contact of the same valve metal. A multitude of current paths in the expanded metal structure provide for redundancy of current distribution and hence the distribution of current to the reinforcing steel is excellent. Installation is simple since an electrode of greater than 100 square meters can be quickly rolled onto the surface of a concrete deck or-easily wrapped around a concrete substructure.
The electrocatalytic coating used in the present invention is such that the anode operates at a very low single electrode potential, and may have a life expectancy of greater than 20 years in a cathodic protection application. Unlike other anodes used heretofore for the cathodic protection of steel in concrete, it is completely stable dimensionally and produces no carbon dioxide or chlorine from chloride contaminated concrete. It furthermore has sufficient surface area such that the acid generated from the anodic reaction will not be detrimental to the surrounding concrete. The preferred coating operation for applying the electrocatalytic coating has been more particularly described in the U.S. Pat. No. 4,708,888.
In its broadest aspect, the present invention is directed to an electrode for electrochemical processes comprising a valve metal mesh having a pattern of substantially diamond-shaped voids having LWD and SWD dimensions for units of the pattern, the pattern of voids being defined by a continuum of valve metal strands interconnected at nodes and carrying on their surface an electrochemically active coating, wherein the mesh of valve metal is a flexible mesh with strands of thickness less than 0.125 cm and having a void fraction of at least 80%, said flexible mesh being coilable and uncoilable about an axis along the LWD dimension of the pattern units and being stretchable by up to about 10% along the SWD dimension of the pattern units and further being bendable in the general plane of the mesh about a bending radius in the range of from 5 to 25 times the width of the mesh, whereby said electrode can be uncoiled from a coiled configuration onto a supporting surface on which the mesh can be stretched to an operative electrode configuration.
In other important aspects the invention is directed to greatly expanded valve metal mesh as well as to a method for preparing such greatly expanded mesh.
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patent: 425524
Bennett John E.
Mitchell Thomas A.
Pohto Gerald R.
Eltech Systems Corporation
Hudak & Shunk Co. L.P.A.
Tung T.
Tyrpak Michele M.
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