Cryolite block metals

A cell for the Hall—H roult process is shown in Fig. 4.1, and a block diagram of the whole process is shown in Fig. 4.2. Cell design is determined largely by the need to contain molten cryolite at high temperatures and to withstand attack by molten aluminium and also by sodium and fluorine formed as minor products at the cathode and anode respectively. Hence the cell is a strong steel box lined first with alumina to act as a refractory, thermal insulator and then with carbon. In fact the base of the tank is lined with prebaked carbon blocks which are inlaid with steel bars to reduce their electrical resistance and which act as current carriers to the molten aluminium cathode. The sides are lined with partially graphitized anthracite in coal tar pitch. The process is then run so that there remains a layer of solid cryolite and alumina at the sides of the cell and a soHd crust on the surface. This acts as a further barrier to corrosion and also to reduce the heat loss from the cell. The cell also has facilities for the periodic addition of alumina through the crust and for the removal of aluminium metal by suction. It is hooded with an extractor... 

Inclusions may have cmisiderable dimensions. The source of the metallic inclusions are mechanical parts of equipment (including calcinators and crushers and disintegrators). Mineral inclusions are quite rare in graphitic and graphitized blocks. The source of mineral inclusions is usually calcite layers in anthracite deposits. During purification, some parts of calcite may remain in the anthracite. Calcite inclusions in carbon cathode blocks are rapidly dissolved by cryolite and the cavities filled by bath they appear in the block (Fig. 2.30b). 

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