Formation of ledge in aluminum electrolyzer

Abstract

A model unit simulating the actual conditions of electrolytic aluminum production was used to conduct an experimental study of ledge to determine its dynamic behavior (formation/dissolution) depending on the electrolyte overheating temperature, lining thermal resistance and cryolite-alumina electrolyte composition. A window was mounted in the front wall of the unit housing to change the lining material. Ledge is formed due to the heat flow generated by the temperature difference between the electrolyte and electrolyzer walls. The electrolyte cryolite ratio (CR) varied in the range of 2.1–2.5. The alumina concentration in the electrolyte did not exceed 4.5 wt.%. Shape change in the electrolyzer working space during electrolysis was determined by the thickness of the formed ledge on the walls and bottom. The dynamic ledge formation in the experimental cell begins at the overheating of 3–4 degrees. It was found that with a decrease in the thermal resistance of the lining material from 16 to 14 m2/W at the same overheating temperature, the side ledge with a greater thickness was formed, however, the decrease in the thermal resistance hardly affected its thickness when the ledge has been already formed. As in the industrial electrolyzer, the ledge profile formed in the experimental cell can be conditionally divided into three zones: bottom ledge, metal/electrolyte interface ledge and side ledge. The dynamic behavior of the side ledge was different from the bottom ledge: the higher the CR, the thicker the side ledge and the thinner the bottom ledge. Chemical analysis of components in the dry knockout showed that the CR and Al2O3 concentration increase throughout the cell height from top to bottom. It was concluded that the side ledge has a heterogeneous composition depending on the electrolyte composition and cooling rate.