Inert electrode material in nanocrystalline powder form

Abstract

The invention relates to an inert electrode material in powder form comprising particles having an average particle size of 0.11 to 100 mum and each formed of an agglomerate of grains of a ceramic material and grains of a metal or alloy with each grain of ceramic material comprising a nanocrystal of the ceramic material and each grain of metal or alloy comprising a nanocrystal of the metal or alloy. Alternatively, each particle can be formed of an agglomerate of grains with each grain comprising a nanocrystal of a single phase ceramic material, a metal or an alloy. The electrode material in powder form according to the invention is useful for the manufacture of inert electrodes having improved thermal shock and corrosion resistance properties.

Description

[0001] The present invention pertains to improvements in the field of electrodes for metal electrolysis. More particularly, the invention relates to an inert electrode material in nanocrystalline powder form for use in the manufacture of such electrodes.[0002] Aluminum is produced conventionally in a Hall-Hroult reduction cell by the electrolysis of alumina dissolved in molten cryolite (Na.sub.3AlF.sub.6) at temperatures of up to about 950.degree. C. A Hall-Hroult cell typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining made of prebaked carbon blocks contacting the molten constituents of the electrolyte. The carbon lining acts as the cathode substrate and the molten aluminum pool acts as the cathode. The anode is a consumable carbon electrode, usually prebaked carbon made by coke calcination. Typically, for each ton of aluminum produced, 0.5 ton of carbon anode is required.[0003] During electrolysis in Hall-Hroult cells, ...

AI Technical Summary

Benefits of technology

[0009] It is therefore an object of the invention to overcome the above drawbacks and to provide an electrode material in powder form for use in the manufacture of inert electrodes having improved thermal shock and corrosion resistance properties.

[0014] The term "nanocrystal" as used herein refers to a crystal having a size of 100 nanometers or less. The nanocrystalline microstructure considerably favors densification, even without sintering aids, when the electrode material in powder form according to the invention is compacted and sintered to produce dense electrodes. Nanocrystalline powders also minimize grain growth since sintering can be effected at lower temperatures. The sintering time is also much shorter than that required for densification of the conventional coarse-grained (about 10 .mu.m) powder mixtures for a same densification level. Thus, the overall cost of the sintering process is considerably decreased.

[0015] Since the time and the temperature of the sintering are considerably low, the resulting electrode has a fine microstructure. The finer the microstructure, the higher the toughness and the resistance to thermal shock, and consequently the longer the electrode life time.

Problems solved by technology

A sodium intercalation in the graphitic structure occurs, which causes swelling and deformation of the cathode carbon blocks.

The increase of voltage between the electrodes adversely affects the energy efficiency of the process.

In spite of intensive efforts of more than 20 years to produce inert anodes, to date, no fully acceptable inert anode materials have been found.

Ceramics are generally brittle and do not resist to the thermal shocks during start-up and operation of a Hall-Hroult cell.

Metal oxide ceramics are generally resistant to oxidation, but they are not good electrical conductors.

Metals, however, are very good conductors but the corrosion rate of metallic anodes in cryolite is very high.

Fabrication process of such a cermet is complex and consists of several steps.

Although the above-mentioned cermet seems to be a promising material for inert anode applications, several disadvantages are associated with its production and the characteristics of the final product.

The process is complex and requires several steps, which results in a product having a high cost.

The sintering and densification rates of ceramic and metal powders having an average particle size of about 10 microns are slow so that it is very difficult to obtain a highly dense cermet.

A small amount of porosity is present in the cermet obtained, resulting in a decrease of mechanical properties.

Thus, an anode made of such a cermet is easily destroyed when subjected to repeated thermal shocks.

Using high sintering temperature results in an excessive grain growth and an increase in the final cost of the product.

Segregation is a serious problem when powders having a large average particle size are mixed together.

This results in a non-homogeneous powder mixture and, consequently, in a non-homogeneous sintered anode.

Since the conductivity of the ceramic phase is much lower than that of the metal phase, any non-homogeneity results in a non-homogeneous current density during use of the anode.

Therefore, any non-homogeneity results in an excessive local degradation of the anode.

When powders having a large average particle size are used as starting material, densification is slow and in order to obtain higher densities, the sintering temperature and / or time must be increased.

This results in a cermet with a coarse microstructure which decreases the thermal shock resistance of the cermet.

Coarse structured cermets also exhibit low mechanical properties and non-homogeneous corrosion rates.