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NEW MATERIALS FOR ALUMINIUM ELECTROLYSIS CELLS PRODUCED BY SELF PROPAGATING HIGH TEMPERATURE SYNTHESIS (SHS)

Objective


The main objectives of the project were achieved. An aluminium wettable TiB2 coating was successfully developed by the Consortium for applying onto carbon substrates. The wettable cathode enabled the operation of a 5 kA pilot cell at Alures in the drained mode with no loss in current efficiency. The pilot cell tests demonstrated that the TiB2 coating is aluminium wettable, stable, abrasion resistant and can be incorporated into drained cell designs. The 5 kA pilot cell tests required extensive instrumentation for data acquisition, monitoring and management. A cell monitoring system was specially designed and installed by ENEA.

Self-propagating high temperature synthesis (SHS), also known as combustion synthesis, was found to be an economical and efficient method for producing advanced materials such as intermetallics and ceramics. Green bodies are formed using conventional powder metallurgical processing and are subsequently brought to reaction, "combustion", by introducing them into a preheated furnace. The heat released by the exothermic reaction causes simultaneous sintering of the combusted products, thereby avoiding long sintering times at high temperature as in conventional ceramic technology. Using SHS processing methods, TiB2/Al2O3 composite tiles and Ni-Al-Fe-Cu (nickel intermetallic alloy) anodes were successfully fabricated as new non-carbon electrodes by Heraeus.

The TiB2/Al2O3 composite tiles produced by SHS had improved thermal shock resistance and mechanical strength compared to bulk TiB2 material by conventional ceramic processing. However, its thermal and mechanical properties were not adequate for use as wettable cathodes in an aluminium cell, without further improvements to the processing techniques. In the absence of an effective adhesive for fixing the tiles to the carbon cell bottom, further development was suspended.

A mathematical model was developed by ENEA to describe the SHS reaction to form nickel aluminides and its alloys, which were investigated as anode candidates during the project. The model fits the experimental results very well and may be easily implemented for complex anode geometry.

The nickel intermetallic alloy was found to be sufficiently corrosion resistant to function as a stable anode to replace carbon anodes in aluminium electrolysis cells. Testing of the anode was limited to laboratory 10 A and 100 A cells at Heraeus. Scaling up the size of the anode was less of a problem than increasing the complexity of the anode structure because the anodes were formed by cold isostatic pressing (CIP). As material development took longer than planned and the parallel development of a joining technology for this material was outside the scope of the project, the operation of the pilot cell with stable anodes could not be realised. However, the anode wear rates measured in the 100 A cells were sufficiently low to warrant further development and testing in larger cells where constant conditions can be maintained over long periods.

Pitch-free cell lining materials were prepared by SHS at Heraeus and evaluated at Alures, but material development did not proceed beyond lab specimens due to re-allocation of resources. The pitch binder of carbon sidewalls can be replaced by an inorganic material produced by the reaction of TiO2, B2O3 and Al to form TiB2 and Al2O3. This pitch-free carbon material was more resistant against air oxidation and sodium penetration than conventional carbon sidewall blocks.
The proposed research is directed at substituting the existing carbon components presently used in hall-heroult cells for aluminium production with non-carbon components produced by the SHS process. The SHS process is a method using a mixture of powdered precursor material which is pressed to yield a green body or is applied as a coating on top of a substrate. The reactor heat sinters the reaction products to form a body or a coating provided the compounds have sufficiently high heat of formation like carbides, borides and aluminide. The research is based on the following stages :

- identification and selection of suitable materials;
- optimization and control of the SHS process and manufacturing of small components : testing on small scale;
- scaling-up of manufacturing capability;
- testing on pilot cell scale of new components;
- evaluation of benefits to the aluminium industry.

The research will comprise the operation of a newly designed pilot cell for at least six months with non-carbon electrodes to produce aluminium with an industrially acceptable purity standard. The successful completion will result in a reduction of 20% in the electric energy consumption and a saving of 100 ecu per ton of aluminium produced due to the cost reduction of the electrodes used. In addition there will be a greatly reduced environmental impact due to the elimination of the CO2 and fluoro-carbon emissions which are the consequence of the consumption of the carbon anodes presently used (consumption rate of 450 kg of carbon anodes per ton of aluminium produced).

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

HERAEUS ELEKTROCHEMIE GMBH
Address
Heramsstraße 12-14
6450 Hanau
Germany

Participants (2)

Alures ScpA
Italy
Address
Località Portovesme
09010 Portoscuso Cagliari
Ente per le Nuove Tecnologie l'Energia e l'Ambiente (ENEA)
Italy
Address
Cre Saluggia
13040 Saluggia Vercelli