Thermal, chemical and microstructural characterization of anode crust formed in aluminum electrolysis cells

The anode cover material (ACM) of aluminum electrolysis cells reacts with the liquid bath and vapor penetrating inside this powdered material to form a denser material, which is called the anode crust. The thermal property of the anode crust affects the top heat losses of aluminum electrolysis cells. The operation and design of electrolysis cells can be controlled through a better understanding of the thermochemical evolution of the anode crust. The typical anode crust is found either in the solid state up to 696 °C or in the solid-liquid state at higher temperatures. Its thermal conductivity is determined from 100 °C up to 950 °C on the samples extracted from an industrial electrolysis cell. For example, the thermal conductivity of the lower part of the anode crust decreases from 1.38 W m−1 °C−1 at 100 °C to 0.89 W m−1 °C−1 at 950 °C. The specific heat capacity, enthalpy, bulk density and thermal diffusivity are also determined from experimental measurements. The crystalline phases composing the anode crust are quantified with a Rietveld method performed on X-ray diffraction patterns. The thermochemical behavior of the industrial anode crust is investigated with differential scanning calorimetry and thermodynamic equilibrium calculations. The Na5Al3F14, α-Al2O3 and Na3AlF6 are the main phases composing the anode crust. The chemical analysis also demonstrates a significant amount of amorphous alumina and a low amount of calcium cryolite compounds. From a temperature of 717 °C, the α-Al2O3(s), Na3AlF6(s) solution and liquid bath solution compose the anode crust according to the thermodynamic equilibrium results. Furthermore, the thermal property of each sample is given with the corresponding chemical composition.