Defects in Carbon Cathode Blocks

Carbon cathode bottom blocks have big dimensions. Unfortunately, ditfing the fabrication process—during pressing or during baking—defects may appear in carbon cathode blocks—cracks, cavities, impurities, and inclusions (Fig. 2.30). If cracks or cavities are on the surface of blocks, they are more or less clearly seen, and during the visual inspection at the producer s or customer s plant in the A1 industry, blocks with defects are rejected as defective and sorted out. They may be crushed and then the crushed material reused for production. The same may be said about metallic or calcite inclusions. 

The problem is that not all defects may be seen on the surface. Only one crack in the bottom block—that is, inside the block, not seen on the surface— might be the cause of the sudden shutdown of the cell during startup or during early service. Unfortunately, such internal defects are not rare, and they may occasirMially appear. One example of the appearance of such an internal defect may be when graphite 

2 Refractories and Carbon Cathode Materials for Aluminium Reduction Cells 

Another reason for exfoliation might be insufficient calcination of anthracite. 

Cracks and chips oti the edges and comers of cathode blocks and other geometrical deviations are less dangerous, but are still not desirable. They are due to the following causes  

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Fig. 2.30 Inner defects in carbon cathode blocks that may be revealed at cutting (a) inner oack (b) calcite grain (c) ferrous contamination (it is almost undetectable visually, but magnet sticks to it) (d) calcite contamination...

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