Abstract
The efficiency of the Hall-Heroult process is influenced by the formation of solid or solid–liquid deposits on the cathode block surface. These deposits have a high electrical and thermal resistivity. Hence, they disturb the electrical current path at the aluminum and cathode block interface. The phase diagrams of deposits with a typical composition were calculated by thermodynamic equilibrium, and the results were confirmed by differential scanning calorimetry measurements. The ledge toe is mainly composed by Na3AlF6(ss), Al2O3(s), and liquid bath in industrial cells. The liquid fraction in the deposits at temperature lower than 933 °C is due to the presence of Na5Al3F14, Na2Ca3Al2F14, and NaCaAlF6 and depends on [Al2O3], [CaF2] and the cryolite ratio (CR). The side ledge in industrial cell contains liquid phase when the CR is lower than 2.8. The sludge contains Al2O3(s) and liquid bath. The ledge toe grows when the local temperature is lower than the solidification temperature of Na3AlF6.
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