Looking to a cure to the cathode cooling cracks problem:
In the previous study [Reference 4], it
was suggested that since it is the metal pad that is responsible for the generating of the reversed
vertical gradient in the cathode blocks, tapping the metal pad as quickly as possible after the power
shutdown should reduce the risk of cooling cracks formation. This conclusion was based on the
assumption that the tension stress and the corresponding cooling cracks are generated to compensate
for the fact that the cathode panel is not free to bend down. Under that assumption reducing the
intensity of the reversed vertical thermal gradient did significantly reduced the intensity of the top
surface tension stress. Yet that intensity was already 4 times less than required to produce cooling
cracks!
The new assumption is that the cathode panel as a whole, but more so the top section want to shrink
but the collector bars are anchoring the bottom section of the cathode panel preventing it to move.
Under those conditions the only option left to the cathode panel is to generate cooling cracks. This
was confirmed by model results.
In a way, the obvious cure the cooling cracks problem have already been indentify in the previous
study [reference 4], the cell lining design needs to be modify in order to prevent the collector bars to
be anchored in the pier region. A third study could demonstrate that, assuming that there is a
practical solution to this new lining design requirement which is far from being obvious.
Conclusions
It was demonstrated that it is possible to explain the cooling cracks formation by modeling. The
cooling cracks formation can only be explained by the fact that the cathode panel as a whole want to
shrink but the collector bars are preventing it to do so. The fact that the metal pad is cooling faster
the top section of the cathode panel is compounding the problem but is not the main factor.
In that context, only a cell lining design change can be expected to provide a cure. The aim of the
cell lining design change being to prevent the pier to rigidly anchoring the collector bars.
References
1. A.T. Tabereaux, Light Metals Age, Electrical Power Interruptions: An Escalating Challenge for
Aluminum Smelters", February 2010, pp. 16-20.
2. G. D'Amours, M. Fafard, A. Gakwaya, and A.A. Mirchi, "Mechanical Behavior of Carbon
Cathode: Understanding, Modeling and Identification", Light Metals 2003, pp.633-640.
3. M. Sorlie and H.A. Oye, "Cathodes in Aluminium Electrolysis," Aluminium-Verlag Marketing
& Kommunikation GmbH, 3rd Edition 2010.
4. M. Dupuis and A. Tabereaux, Modeling Cathode Cooling Due to Power Interruption, TMS,
(2012), to be published.
5. K. F. Lalonde, W. Cotton and R. M. Beeler, Rate of Metal Cooling in Aluminum Reduction Cell
removed from Line Curreny Method and Model, TMS, (2006), 291-295.
6. J. Hop, A. Store, T. Foosnaes and H.A. Oye, Chemical and Physical Changes of Cathode
Carbon by Aluminium Electrolysis, VII International Confeence on Molten Slags Fluxes and
Salts, The South African Institute of Mining and Metallurty, (2004), 775-781.