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SELF REGULATING SYSTEM TO CONTROL THE SODIUM CONTENT OF NON-FERROUS METALS

Objective


Construction of a coulometric titration device
The solid electrolyte chosen has been taken from the sodium beta-alumina family of materials which has already been developed for use in secondary battery applications based on sodium. The early stages of the project were dogged with problems relating to the use of metallic sodium within the titration assembly. The breakthrough came when sodium-based molten salts were used to replace metallic sodium. Although not without problems of compatibility with certain materials, much more reliable devices were able to be constructed and sodium has been successfully titrated into aluminium alloys.

Assessment of the device
Once reliable operation had been achieved, devices were tested for their responsiveness and to find the limits of the system. Experiments have been in hand to ascertain the maximum current density achievable with our existing set-up. At present, an upper limit has not been reached: the amount of sodium transported at moderate current levels is such that the concentration in the laboratory scale crucibles of aluminium very quickly builds up to a level where combustion takes place. However, the current densities achieved to date (1 A.cm{-2}) are sufficiently high to ensure that the number of devices required for sodium addition on an industrial scale will not be excessive.

Comparison of the pollution and efficiency with the conventional method
Sodium additions to aluminium have been made using the conventional method and by coulometric titration. Samples of the metal have been taken and analysed for their sodium content. In these laboratory tests, when sufficient sodium is added as sodium-aluminium alloy, to theoretically fix a concentration of 900 ppm, the maximum value achieved does not exceed 300 ppm, the remainder being lost by evaporation and oxidation. By titrating the sodium gradually into the melt, the sodium level increases with less loss of material, resulting in a more efficient process. This also leads to the absence of fumes associated with the initial sodium addition.
It is extremely difficult to control the sodium content of non-ferrous alloys due to its high volatility and rate of oxidation in air. As a result sodium has mainly been replaced by strontium as a method of modifying aluminium-silicon alloys, although strontium is far more expensive than sodium. This proposal addresses the problem of controlled addition or removal of sodium by coulometric titration through a beta-alumina electrolyte tube. This will allow accurate changes in the sodium content with a reduction in the production of polluting and, in some cases, toxic drosses. If successful the technique can be applied to the modification of aluminium-silicon alloys, the addition of sodium to zinc to remove arsenic and antimony and the removal of sodium from aluminium and alumium-magnesium alloys.

The application of this technology will improve both the quality of aluminium alloys and zinc and the working environment. Furthermore, the quantity of polluting and toxic drosses for disposal will be reduced.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

University of Leeds
Address

LS2 9JT Leeds
United Kingdom

Participants (4)

HOOGOVENS ALUMINIUM NV
Belgium
Address
A Stockletlaan 87
2570 Duffel
LEMMERZ ALU PARTS
Belgium
Address
Lage Weg 392
2660 Hoboken
PROTEM MARKT-EN TECHNOLOGIE ADV BV
Netherlands
Address
Po Box 140 Markt 210
7550 AC Hengelo
VITO
Belgium
Address
Boeretang 200
2400 Mol