Chemical Characterization and Thermodynamic Investigation of Anode Crust Used in Aluminum Electrolysis Cells

The anode cover material (ACM) is necessary to control the top heat losses of aluminum electrolysis cells, to decrease the fluoride fumes emissions and to prevent air burn of carbon anodes. In order to understand the behavior of the ACM and anode crust at operating conditions, samples of these materials are taken from an industrial electrolysis cell in the side channel, center channel and between anodes. Their chemical compositions are analyzed by X-ray diffraction while the temperatures inside the crust are validated by temperature measurements. The reactions occurring in the ACM and anode crust are then determined by thermodynamic equilibrium calculations. Concentration gradients of chiolite (Na5Al3F14), cryolite (Na3AlF6), Na2Ca3Al2F14, and Al2O3 are observed in the anode crust, implying a variation of the cryolite ratio and melting temperature of the crust. The thermodynamic analysis describes the state and behavior of the anode crust in aluminum electrolysis cells.