Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Reexamination Certificate
2002-06-27
2004-08-31
Valentine, Donald R. (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
C205S230000, C205S381000, C205S383000
Reexamination Certificate
active
06783655
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the production of refractory boride coatings and bodies in particular for use in cells for the electrowinning of aluminium by the electrolysis of alumina dissolved in a molten electrolyte such as cryolite or other fluoride-based electrolytes. The invention more specifically relates to a slurry for producing refractory boride coatings and bodies, as well as bodies coated with or made of refractory boride for use as components of aluminium electrowinning cells.
BACKGROUND OF THE INVENTION
Aluminium is produced conventionally by the Hall-Heroult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures up to around 950° C. A Hall-Heroult reduction cell typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate forming the cell bottom floor. The cathode substrate is usually an anthracite based carbon lining made of prebaked cathode blocks, joined with a ramming mixture of anthracite, coke, and coal tar, or with glue.
It has long been recognized that it would be desirable to make (or coat or cover) the cathode of an aluminium electrowinning cell with a refractory boride such as titanium diboride that would render the cathode surface wettable to molten aluminium which in turn would lead to a series of advantages. Many difficulties were encountered in producing refractory boride coatings which meet up to the rigorous conditions in an aluminium electrowinning cell. However, as described in the following patents, such coatings have recently been successfully introduced.
U.S. Pat. No. 5,310,476 (Sekhar et al.) discloses a method of producing a protective refractory coating on a substrate of, inter alia, carbonaceous materials by applying to the substrate a micropyretic reaction layer from a slurry containing particulate reactants in a colloidal carrier, and initiating a micropyretic reaction. The micropyretic slurry optionally also contains some preformed refractory material, and the micropyretic slurry may be applied on a non-reactive sub-layer.
U.S. Pat. No. 5,364,513 (Sekhar et al.) discloses a method of producing a protective refractory coating by applying to the substrate a reactive or non-reactive layer from a slurry containing particulate reactants and/or preformed particulate refractory materials in a colloidal carrier, and initiating a micropyretic reaction or non-reactive sintering. The colloidal carrier was selected from the group consisting of colloidal alumina, yttria, ceria, thoria, zirconia, magnesia, lithia, monoaluminum phosphate, cerium acetate and mixtures thereof. Various colloid mixtures were disclosed, such as colloidal alumina-colloidal silica and colloidal alumina-monoaluminium phosphate.
U.S. Pat. No. 5,651,874 (de Nora et al.) discloses applying a protective coating of a refractory boride such as TiB
2
to a carbon component of an aluminium electrowinning cell, by applying thereto a slurry of particulate boride in a colloid in several layers with drying between each layer.
These coatings have shown outstanding performance compared to previous attempts to apply refractory coatings to components of aluminium electrowinning cells. These aluminium-wettable refractory boride coated bodies can be used in conventional cells with a deep aluminium pool and also permit the elimination of the thick aluminium pool required to partially protect the carbon cathode, enabling the cell to operate with a drained cathode.
These refractory boride coated bodies have the following attributes: excellent wettability by molten aluminium, inertness to attack by molten aluminium and cryolite, low cost, environmentally safe, ability to absorb thermal and mechanical shocks, durability in the environment of an aluminium production cell, and ease of production and processing. The boride coating also acts as a barrier to sodium penetration.
The refractory boride coated bodies find many applications on account of their excellent resistance, protection, and stability when exposed to the corrosive conditions existing in the cell even when the temperature of operation is low as in the Low Temperature electrolysis process for the production of aluminium, see for example U.S. Pat. No. 4,681,671 (Duruz).
For most applications, thin coatings (less than 1 mm thick) are required and the methods for applying such thin coatings in one or more layers from a slurry in a colloidal carrier have proven to be essentially problem-free. Nevertheless, for some special applications, for example coating certain types of drained-cathode surfaces, it would be desirable to have thicker coatings.
When, however, it was attempted to produce thicker coatings, some problems were encountered, in particular undesirable mud-cracking was found to occur in the finished, dried coating when the coating thickness exceeded about 1 mm or even only 0.8 mm depending on the coating formulation. Mud-cracking involves cracks ranging in width from 0.1 mm to 1 mm. Hairline cracks, typically less than 0.1 mm, (generally from 0.03 to 0.09 mm) are also formed but are more acceptable. Despite this problem of mud-cracking as the thickness increases, the coatings of U.S. Pat. No. 5,651,874 (de Nora et al) remain outstandingly the best available industrially-applicable coatings.
In WO 97/08114 (Sekhar et al.) it has been proposed to make bodies of refractory boride, for example by slip casting or pressing a slurry of particulate boride in a colloid.
SUMMARY OF THE INVENTION
The invention sets out to improve the production of refractory boride coatings or bodies for use as cell components for aluminium electrowinning cells, especially for use as cathodes.
One object of the invention is to provide an improved slurry for the production of bodies or coatings of refractory boride.
Another object of the invention is to produce refractory boride coated bodies and refractory boride bodies starting from readily available and relatively inexpensive commercial particulate borides or their precursors in a specific colloidal carrier.
A specific object of the invention is to produce bodies coated with refractory borides with coating thicknesses equal to or greater than those previously possible, without mud-cracking, the coatings being aluminium-wettable and electrically conductive So they can serve as coatings for cathodes or other cell components of aluminium production cells.
In the production of “thick” refractory boride coatings, it has been discovered that the mud-cracking problem can be alleviated by utilizing specific colloidal carriers. Moreover, it has been observed that these specific colloidal carriers are also advantageous when producing normal “thin” coatings and even more advantageous when producing bodies of refractory borides, as opposed to coatings.
According to one main aspect of the invention, a slurry for the production of bodies or coatings of refractory boride comprises particulate preformed refractory boride and/or particulate precursors of refractory boride in a colloidal carrier which comprises colloidal particles with a non-gaussian particle size distribution, comprising at least two distinct fractions of colloidal particles having mean particle sizes which differ from one another.
In contrast to the known colloid mixtures of different colloidal components (colloidal alumina and colloidal silica, for example) this colloidal carrier according to the invention is obtainable by mixing two different “grades” of the same colloid, producing a desirable particle size distribution.
“Grade” is to be understood as the mean particle size of the colloidal particles, with at least 50% of the particles of that particular particle size. Commercially available colloids have a predominant particle size with generally gaussian particle size distribution.
Use of this bi- or multigrade colloidal carrier, advantageously in combination with an
Duruz Jean-Jacques
Liu James Jenq
Sekhar Jainagesh Akkaraju
Deshmukh Jayadeep R.
Moltech Invent S.A.
Valentine Donald R.
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