EP0005674B1 - Process of manufacturing a dimensionally stable anode - Google Patents

Description

The present invention relates, in general, to electrolysis cells and relates more particularly to an anode electrode stable in dimensions, of the type comprising a core of electroconductive material and an electrocatalytic conductive coating. The invention also relates to a method of manufacturing such an electrode.

We know, next to conventional electrolytic cells using graphite anodes, electrolytic cells in which the anode is constituted by a core of a metal called stop metal, of the type used for rectifiers with stop layer . Such stop metals, which include in particular titanium, tantalum, niobium, zirconium, molybdenum and tungsten, are expensive and require difficult and expensive machining. The core of the anode can also be produced from oxides of stop metals.

It is also known to coat an anode of the aforementioned type using an electrocatalytic conductive layer constituted for example by a mixture of metallic oxides of noble metals or of stop metals.

Anode electrodes of the aforementioned type, consisting almost entirely of stop metals, noble metals or oxides of these metals, are particularly stable in size over time. However, in addition to the high cost of the base material constituting such anodes, a drawback results from the difficulty of treatment, in particular of mechanical treatment, of such materials. However, for the anodes to resist, during the operation of the electrolysis, in particular thermal and mechanical voltages, it is essential that the geometrical configuration of the anodes is determined with precision. Anodes of the known type do not allow complex geometric configurations to be easily carried out.

One of the aims of the present invention consists precisely in enabling anodes to be produced at a reduced cost and in a simplified manner, while offering the possibility of improving the shape of the anodes constructed, without appreciably affecting their electrical properties.

These objects are achieved by a method of manufacturing an anode electrode stable in dimensions, of the type comprising a core of electroconductive material and an electrocatalytic conductive coating, characterized in that an electroconductive core is first produced by molding. from a common metal melting at high temperature, such as iron, an iron alloy, aluminum, copper, in that said core is then introduced into a first electrolytic bath melted at high temperature, consisting of a mixture of alkali chlorides and stop metal chlorides, to form a first conductive coating on the core, and in that said core provided with its first coating is then introduced into a second electrolytic bath comprising noble metals to form a second electrocatalytic conductive coating on the core provided with its first conductive coating.

The second electrolytic bath can also optionally consist of an aqueous solution of noble metal, such as platinum, ruthenium, iridium, palladium.

Thus, unlike anodes stabilized in dimensions of the conventional type, an inexpensive metal or alloy of metals is used for the core of the anode and is easily worked by molding. The manufacturing cost is thus very reduced, while the manufacturing flexibility is greatly improved.

Surprisingly, if the core is made of iron or ferrous alloy, aluminum or aluminum alloy, copper or copper alloy, a coating of stop metals or oxides of stop metals , then a second coating of noble metals such as platinum or oxides of these noble metals or of noble metals associated with stop metals such as titanium, the mechanical and electrical qualities of the anode remain excellent compared to the electrodes anodic whose core is made entirely of stop metal such as titanium or tantalum.

The advantageous characteristic is the formation of the molded core based on iron or ferrous alloy by molding molten metals.

The second electrocatalytic conductive coating can be produced from a mixture of alkali halides and either halides of a noble metal or of several noble metals, or halides of noble metals and stop metals.

The term "stop metal" should be understood to mean a metal of the type used for rectifiers with a stop layer. Titanium, tantalum, niobium, zirconium, molybdenum and tungsten belong to this family.

According to a characteristic of the invention, the operations of coating the electroconductive core are carried out in an inert atmosphere such as an argon or helium atmosphere. The temperature of the molten electrolytic baths is between 300 and 1000 ° C, this temperature naturally depending on the type of core and the metals to be treated. The electrode provided with its two coatings is then subjected to an oxidation heat treatment.

A particular non-limiting example of embodiment will be described below to better explain the invention.

First an anode core is produced by melting and molding inexpensive common metals, such as iron or an iron alloy, aluminum, copper or alloys of these metals. The core could also be made from a metal such as iron, for example semi-worked. However, the molding technique makes it easier to produce anodes of very different geometries.

The core of the anode then receives a cathodic electrolytic deposit of a stop metal, such as titanium, tantalum or tungsten.

This deposit makes it possible to coat the core of the anode with a layer of stop metal, the thickness of which can be very reduced, for example less than 3 mm.

The quantities of stop metal used are thus very reduced, although the electrode is coated over its entire surface and therefore has interesting electrical and mechanical qualities. The molten electrolytic bath allowing the deposition of a stop metal is a high temperature bath comprising a mixture of alkali chlorides, such as sodium chloride, potassium chloride, lithium chloride, or alkaline fluorides, such than sodium fluoride and potassium fluoride or fluorides and chlorides of stop metals or compounds of stop metals, such as tungsten compounds.

Anodic corrosion of the stop metal and of the mixture of molten chlorides can thus take place with the intervention of tetrachloride of the stop metal and the intervention of fluorinated compounds of the stop metal.

The coating operation of the iron-based core is carried out in an inert atmosphere of rare gases such as argon or helium.

The melting temperature of the electrolytic bath can be between approximately 300 and 1000 ° C. The density of the anode currents can reach 10 kA / m ² using for the cathodic deposition of stop metal a soluble anode having the same composition as that of the stop metal to be deposited.

The deposit is obtained at anode and cathode potentials controlled and at anode and cathode current density always controlled with cell voltages between 0 and 7 volts. The current densities can be, for example, from 100 to 700 Nm2 at the cathode and from 10 to 90 A / m ² at the anode.

The anode based on a cheap metal, such as iron, aluminum, copper, provided with its first coating of stop metal, then receives a second electrocatalytic coating consisting of noble metals or a mixture of noble metals and stop metals, or intermetallic compounds of noble metals and stop metals or even compounds of stop metals such as tungsten carbide. The second coating can be carried out using the same technique as for the first coating, that is to say using a molten electrolytic bath at high temperature, of a mixture of alkali halides and noble metal halides, which may optionally be combined with stop metal halides.

The noble metal can also be deposited by aqueous baths, that is to say galvanic baths.

It will also be noted that, according to an alternative embodiment, after deposition by electrolysis, by means of a molten bath of a first conductive electro-cathodic or electroanodic coating of stop metal, such as titanium, on a base common metal, such as steel, it is possible to produce, by electrolysis according to the same technique, a second conductive coating of stop metal, such as tungsten, then to carry out at high temperature a carburation of this latter coating, in which case it is not then necessary to deposit a noble metal.

The present invention lends itself well to the manufacture of anodes of very diverse shapes, in particular of anodes provided with several electrode connections coupled to the anode barrier, since in particular the molding technique makes it possible to form cores with the most varied shapes. .

An anode produced according to the present invention has excellent mechanical and chemical resistance, especially to corrosion, including for temperatures above normal.

Claims (8)

1. A process for manufacturing an anodic electrode, stable in dimensions, of the type comprising a core made of electroconducting material and an electrocatalytic conducting coating, characterized in that an electroconducting core is firstly made from a common metal melting at high temperature, such as iron, an iron alloy, aluminium, an aluminium alloy, copper, a copper alloy, said core is then introduced into a first molten high temperature electrolytic bath constituted by a mixture of alkali metal chlorides and of chlorides of valve metals, to form a first conducting coating on the core, said core provided with its first coating is then introduced into a second electrolytic bath comprising noble metals, to form a second electrocatalytic conducting coating on the core provided with its first conducting coating.

2. A process according to claim 1, characterized in that the second electrolytic bath is constituted by an aqueous solution of one or more noble metals.

3. A process according to claim 1 or 2, characterized in that the operations for coating the electroconducting core are effected in an inert atmosphere such as an atmosphere of argon or helium.

4. A process according to any one of claims 1 to 3, characterized in that the temperature of the molten electrolytic baths is between 300 and 1 000 °C.

5. A process according to any one of claims 1 to 4, characterized in that the electrode provided with its two coatings is subjected to a thermal oxidation treatment.

6. A process according to any one of claims 1 to 5, characterized in that upon formation of a conducting coating on the core of the electrode to be manufactured, the deposit of metal is obtained at controlled anodic and cathodic potentials and at controlled anodic and cathodic current density, with cell voltages of between 0 and 7 volts.

7. A process according to claim 6, characterized in that, upon formation of a conducting coating on the core of the electrode to be manufactured, the current densities are included between 100 and 700 A/m² at the cathode and 10 and 90 A/m² at the anode.

8. A process of manufacturing an anodic electrode, stable in dimensions, of the type comprising a core made of electroconducting material and an electrocatalytic conducting coating, characterized in that : an electroconducting core is firstly made from a common metal melting at high temperature, such as iron, an iron alloy, aluminium, an aluminium alloy, copper, a copper alloy, said core is then introduced into a first molten high temperature electrolytic bath constituted by a mixture of alkali chlorides and of chlorides of valve metals, to form a first conducting coating on the core, said core provided with its first coating is then introduced into a second electrolytic bath constituted by a mixture of alkali halides and of halides of valve metals, to form a second conducting coating on the core provided with its first conducting coating, and a carburization of the second coating is effected at high temperature.