Influence of Electrolyte Overheating and Composition on the Sideledge of an Aluminum Bath

Abstract

The effect of the electrolyte chemical composition and overheating on the size of a sideledge formed in an aluminum-smelting bath is investigated theoretically. Three electrolyte compositions are chosen: sodium cryolite with the cryolite ratio (CR) = 2.7, cryolite (CR) = 2.7 + 5 wt % CaF₂, and cryolite (CR) = 2.7 + 5 wt % CaF₂ + 5 wt % Al₂O₃. The electrolyte liquidus overheating temperatures are 5, 10, 15 and 20^oC. The calculations are performed using the finite-element method. A simplified design of an aluminum cell with a prebaked anode is used. To calculate the temperature field, a mathematical model in the Boussinesq approximation is used. The model contains the Navier–Stokes equation, the thermal conductivity equation, and the incompressibility equation. The key role of electrolyte overheating on the sideledge formation is established. The resulting sideledge profile depends on the heat transfer coefficients and thermal properties of materials. The smallest sideledge thickness with the same electrolyte overheating is observed in cryolite with CR = 2.7, 5 wt % CaF₂, and 5% by weight of Al₂O₃, and formed sideledge profiles for cryolite with KO = 2.7 and cryolite with KO = 2.7 and 5 wt % CaF₂ almost coincide. The thickness of the sideledge formed with overheating of 5 K is from 7 cm or more, and the difference in temperature between the sideledge touching the electrolyte and airborne block wall is 20–25 K. Almost complete sideledge disappearance occurs when the electrolyte liquidus is overheated by 20 K.