Sammendrag
The study shows the distribution and evolution of melt components in carbon cathodes from 16 industrial aluminium electrolysis cells shut down after 10 to 2534 days. The return of Na3AlF6 between 800 and 1000 days was probably due to filler and binder graphitization causing increased porosity and melt exchange, and also reduced sodium absorption and chemical reactivity. After 1000 days the cryolite ratio was similar all through the carbon cathode, becoming less basic with time. This is attributed to further filler shrinkage and, with time, probably also binder corrosion. The phase distribution in the top 50 mm, the bulk and the bottom 50 mm was quite similar, and very different from the electrolyte. This meant intruding electrolyte reacted completely near the top and stayed nearly unchanged downwards. Beyond 1000 days a fairly Na3AlF6-rich melt passed throught the carbon cathode and entered the refractory lining, independent of the type of carbon block. This meant that the refractories were exposed to a fairly Na3AlF6-rich melt also from anthracitic linings, and more use of semi-graphitized blocks will not change the existing situation. The graphitic content had a significant influence on the chemical reactivity and melt evolution, with initially more graphitic blocks giving less chemical reactivity and possibly better thermal stability due to less variation in the position of the freezing isotherm. Several phases that can precipitate showed considerable accumulations as peaks in the melt distribution. Especially formation of NaAlO2(s) gave evidence of precipitation, and it is suggested that this might cause crack initiation; due to variation in the rate of electrolyte intrusion it is suggested that the most likely crack initiation point is where the sideledge frequently moves to cover and uncover the carbon.
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