A titanium anode coated continuously or discretely by textured platinum is described in Soviet Union Patent No. 1,683,360. Such an electrode may be successfully used to drive electrochemical processes in acidic media. When used in electrodialysis reversal (EDR) processes, however, the coating breaks and crumbles due to frequently alternating oxidation and reduction processes. A general drawback of platinum-coated electrodes is a high oxygen discharge overvoltage exceeding that of iridium by 1 V.

Also known is an iridium-titanium electrode whose surface is stabilized by a phase oxide [J. Appl.

Electrochemistry 17, 737 (1987)] and an iridium-titanium electrode obtained by electroplating iridium on the titanium substrate from ammonium hexachloridate solution [J. Electroanalytical Chemistry 279, 283 (1990)] . These electrodes may operate under EDR conditions, provided the polarity reversal period is at least a few tens of hours, but the high porosity of iridium and its generally loose adhesion to titanium drastically limits the operating life of these electrodes.

The closest technical solution chosen as a prototype is a composition electrode consisting of electrically isolated anode and cathode areas [U.S. Patent No. 4,461,693]. These electrodes cannot be used in EDR systems when the reversal

period is short, that is, from a few seconds to a few minutes.

Under these conditions, the operating life of electrode-membrane stacks is also short. Disposition of anodic and cathodic areas separated by a minimal gap within the same plane, on the same substrate, and in the same electrolytic medium, does not fully protect the surface oxide of the anodic platinum coating from reduction, in spite of applying a protective potential to it when the electrode has cathodic polarization. Similarly, the material of the cathodic area of the composition electrode cannot be protected from oxidation.

Thus, the prior art does not provide an electrode which can ensure stable operation under EDR conditions for a long period of time, i.e. be insensitive to polarity reversal.

SUMMARY An object of the invention is to reduce corrosion losses of noble metals used for electrode coating, to make the electrodes insensitive to high-frequency polarity reversals in order to increase the operating life of the electrodes, to simplify the electrode production technology, and to apply a less expensive material for electrodes in order to reduce operational losses.

To achieve this objective, the combined electrodes may be applied in the electrodialyzer. At each end of membrane stacks composing the electrodialysis system, having one or several desalination stages, there are one anode and one cathode placed in parallel and electrically isolated.

The electrodes are made of metallic foil or sheet fifty millimeters to five hundred millimeters thick. The material of the cathode is titanium or another rectifying

metal, iron, nickel, their alloys or stainless steel. The anode is made of iridium-coated titanium or another rectifying metal (platinum, or other platinoid metals and their alloys, may replace iridium).

The electrode thickness is chosen to ensure a leak-proof assembly of the membrane stack and the individual sets of stacks used to reach the required degree of desalination and/or reduce the supply voltage to the value meeting the conditions of EDR technology and operation safety.

The iridium anode coating is textured along one of the basic crystallographic axes oriented at right angles to the plane of the base. Such texture is achieved by magnetron evaporation of iridium in argon-containing atmosphere with argon pressure of 0.3 to 5 Pa. The base surface is preliminarily etched with argon ions. During the evaporation, the plane of the base is oriented at an angle of 0 to 45" to the plane of the target. The coating is 0.5 millimeters thick. The anodic depreciation rate is equal to 3.5 to 3.8 mg/A hour.

Consequently, assuming the operational current density to be equal to 5 mA/cm2, the reduction in coating thickness will not exceed 0.07 millimeters per year. Application of the proposed iridium-titanium electrodes makes it possible to reduce power consumption in a water desalination process by 12% to 15% owing to a decrease in the overvoltage of oxygen discharge reactions, and to reduce iridium losses by 60% to 70% by curbing corrosion effects.

The electrode production costs are also substantially reduced because expensive platinum is replaced by iridium as a coating material for the titanium anode and no noble metals whatsoever are used for the cathode manufacturing.

The anode can have uniformly located perforations of any shape. The perforations may constitute 30% to 80% of the total anode surface area. The perforations provide a channel for charge transfer allowing a higher current density and favor stabilization of the oxide on the anode coating surface.

The power is fed to the anode and cathode located at the opposite ends of the membrane stack. When the potential is applied to the anode or cathode, the other electrode located at the same end of the membrane stack is not connected with a conductor of the first kind to the power supply source. Each anode and cathode have their own local chamber providing for electrical insulation of the electrodes and separated from the other chambers by ion exchange membranes.

On applying the positive potential to the anode, an acid is synthesized within the anodic chamber. After the reversal of the supply current, this anode is disconnected from the power supply so that no ionic discharge takes place on the anodic surface. Besides, since the anodic chamber is limited from the two sides by cation exchange membranes, an equilibrium cation current is set in the chamber in the direction of the cathode placed in the adjacent cathodic chamber and having the negative potential, i.e., the acid medium remains in the anodic chamber during this period. This circumstance prevents emergence of conditions favorable for oxidation and reduction of the anodic coating and its eventual dissipation during the polarity reversal of supply current of the electrodialyzer.

Another object of the present invention is to provide electrodialyzer membrane stacks representing individual electrodialysis stages that can be assembled into a single apparatus. Each end of the electrodialyzer membrane stack

accommodates one cathode and one anode, each with its own working chamber. Both electrodes are electrically and hydraulically isolated.

Preferably, the planar anode and cathode located at one end of the stack are parallel to each other and are separated from each other and from the working chambers by cation exchange membranes.

In one aspect of the invention, the cathode represents a one-piece plate. The anode has openings of arbitrary shape, uniformly distributed over the surface and having the combined area equal to 30% to 80% of the total anode area.

In another aspect of the invention, the cathode is made of a foil or plate that is fifty millimeters to five hundred millimeters thick. The material may be titanium or other rectifying metal, iron, nickel, their alloys or stainless steel.

In a preferred embodiment of the invention, the anode is made of a titanium foil or plate, fifty millimeters to five hundred millimeters thick, and coated with textured iridium, platinum, or ruthenium, or their alloys and compounds.

The textured iridium coating may be deposited by magnetron evaporation on a metal base in an argon-containing atmosphere with argon pressure of from 0.3 to 5 Pascals (Pa). The base is preliminarily etched with argon ions. During the evaporation, the plane of the base is oriented at an angle of from 0 to 45" to the plane of the target.

In another aspect of the invention, electric power is fed from a dc voltage source to an anode and a cathode

located at opposite ends of a membrane stack, while neighboring electrodes are disconnected from the voltage source. To reverse polarity, the voltage is fed to the other anode-cathode pair while the electrodes to which potential was initially applied are disconnected from the voltage source.

To secure the oxidizing medium in the anodic chamber and intensify the acid synthesis, the anodic chamber may be confined by cation exchange membranes.

Other objects and features of the present invention will be apparent from the following detailed description and drawings which illustrate preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a desalination system with a membrane stack.

FIG. 2 is an exploded perspective view of the electrodialyzer membrane stack of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, where like elements are designated by like reference numerals, there is shown in FIG. 1 an electrodialyser apparatus with an electrode-membrane stack 16 and electrode switching circuitry. The stack 16 has an electrode pair on each end: the cathode 5 and anode 6 on one end and the cathode 8 and anode 7 on the other end. Two electrodes are simultaneously connected to the dc voltage source 17, for example, the anode 7 to the positive terminal of the source 17 and the cathode 5 to the negative one. The electrodes 8 and 6 are disconnected from the source 17.

To reverse polarity, the anode 6 and cathode 8 are connected to the source 17 and the electrodes 7 and 5 disconnected from the source 17. Thus, at any moment of time each anode 6, 7 carries the positive potential from the dc voltage source 17 or is totally disconnected from it, while each cathode 5, 8 carries the negative potential from the source 17 or is totally disconnected from it.

This implies that actual electrode polarity reversal is in fact excluded.

FIG. 2 illustrates the electrode-membrane stack of FIG.

1, which may be used as an EDR water desalination apparatus. The stack 16 consists of the cathodes 5, 8 located at the opposite ends of the stack and sealed on the outside by adjacent rubber spacers 1, cation exchange membranes 2, 2A, 14 and anion exchange membranes 15. The cathodes 5, 8 and the anodes 6, 7 have current leads 4.

The cathode 5, the cation exchange membrane 2, and a plastic spacer (not shown) placed between them to shape the electrolyte stream, form a cathodic chamber 9.

Identically, the cathode 8, the cation exchange membrane 2, and a plastic spacer (not illustrated) form the cathodic chamber 11. The anodes 6, 7, the cation exchange membranes 2A, and plastic spacers (not shown) placed between them, form anodic chambers 10, 12.

The cation exchange membrane 2A and the anion exchange membrane 15, as well as the alternating cation exchange membranes 14 and the anion exchange membranes 15 separated by plastic spacers (not shown) form the working chambers for brine and desalted water whose functions depend on the direction of the electric field crossing the stack at right angles to the plane of the membranes 2A, 15, 14. The number of working chambers is governed by the conditions of the electrodialysis process.

The spacers (not shown) may be for example of the type shown in U.S. Patent No. 4,461,693, which are used to provide flow passages within the stack. The entire disclosure of U.S. Patent No. 4,461,693 is incorporated herein by reference.

The electrodialyzer may have one or several stacks. In the latter case, the individual stacks are placed in series, with the rubber spacer 1 becoming a common element for two adjacent stacks. Large-diameter metal plates (not shown) with peripheral openings for tightening pins are pressed against the outside rubber spacers 1 of the extreme stacks. The tightening of the whole assembly prevents any inter-chamber flows and leakage from the stack.

Each cathode 5, 8 may be in the form of a one-piece plate. The anodes 7, 6 each have openings 20 of arbitrary shape, uniformly distributed over the surface and having the combined area equal to 30% to 80% of the total anode area.

The cathodes 5, 8 may each be made of a foil or plate that is fifty millimeters to five hundred millimeters thick. The material may be titanium or other rectifying metal, iron, nickel, their alloys or stainless steel.

In a preferred embodiment of the invention, the anodes 7, 6 are each made of a titanium foil or plate, fifty millimeters to five hundred millimeters thick, and coated with textured iridium, platinum, or ruthenium, or their alloys and compounds. The textured iridium coating may be deposited by magnetron evaporation on a metal base in an argon-containing atmosphere with argon pressure of from 0.3 to 5 Pascals (Pa). The base is preliminarily etched with argon ions. During the evaporation, the plane of the base is oriented at an angle of from 0 to 450 to the plane of the target.

To secure the oxidizing medium in the anodic chambers 10, 12 and intensify the acid synthesis, the anodic chambers 10, 12 may be confined by cation exchange membranes 2, 2A.

The entire disclosure of U.S. Provisional Patent Application No. 60/028,850 is incorporated herein by reference.

The above description and drawings illustrate preferred embodiments which achieve the objects, features and advantages of the present invention. It is not intended that the present invention be limited to the illustrated embodiments. Any modification of the present invention which comes within the spirit and scope of the following claims should be considered part of the present invention.

What is claimed is: