Effect of Convective Heat Transfer and Phase Change on the Stability of Aluminium Smelting Cells zy This paper focuses on two aspects of smelting operation that are highly dependent on the rate of heat transfer in the molten bath. As a consequence of the elevated temperature and the corrosive nature of the fluoride mixture, the cell sidewalls are designed so that some elec- trolyte will freeze onto them, protecting the carbon from erosion and at the same time insulating the cell against excessive heat zyxwvu loss. The thick- ness and dynamic variation of this frozen ledge are determined by the convective heat transfer from the bath to the freeze surface. Another situation where the rate of convective heat flow exerts a sub- tle influence is the dissolution of aluminium oxide powder in the bath. The localized feeding of large quantities of the powder causes transient electrolyte freezing on some of the added material, hindering its disso- lution and causing the formation of an alumina sludge below the molten aluminium pad. zyxwvutsrq M. P. Taylor, B. J. Welch, R. McKibbin School of Engineering, University of Auckland Auckland. New Zealand Introduction The electrochemical reduction of aluminium oxide to alu- minium is performed in a cryolite-based electrolyte contained within carbon-lined cells. Because ionic mobility is required for the electrolysis reaction, it is necessary that the ohmic heat gen- erated between the anode and cathode be sufficient to maintain the electrolyte above its melting point-that it be “superheated” in the parlance of industry. However the degree of superheat is low in practice and varies continuously as a result of electro- chemical and energy changes associated with the depletion, addition, and dissolution of the feedstock, alumina (Taylor et al. 1984). The background to this study is the dynamic variation and thermal instability associated with smelting cell operation. This instability is strongly linked with electrochemical inefficiency in the process (Kent, 1970) and actual cell degradation (Taylor et al., 1983a). Its basic cause is the batchwise addition and removal of reactants and products in the reaction. These actions give rise to rapid shifts in the energy balance, although electro- lysis is occurring continuously in the molten mixture. The net effect is a cyclic fluctuation in operating parameters (Schmidt- Hatting, 1975), and process control becomes a question of limit- ing the amplitude of the cycles to an acceptable level. The pres- ent paper is indirectly concerned with the determination of a stability threshold for these variations, although its bias is mainly experimental. The approach has been to investigate a fundamental characteristic of a fusible material elevated above its melting point: its tendency to give up sensible heat to the sur- roundings and to freeze. If the evolution of latent heat accom- panying this phase change meets a temporary energy demand such as the preheating and dissolution of a cooler material added to the molten salt, it is probable that the freezing will be fol- lowed by remelting of the solid phase. On the other hand, a steady heat loss from the walls containing the melt (as occurs in a smelting cell where the heating is developed by the passage of current in the electrolyte) may result in a permanent frozen layer adhering to these sidewall surfaces (Grjotheim and Welch, 1980). In both cases, the quantity of material that freezes and the speed at which freezing and melting take place, depend heavily on the rate of sensible heat transport from the bulk of the liquid to the phase change interface. When a large quantity of alumina powder is dropped into the molten electrolyte, the heat capacity of the cooler solid, combined with its endothermic enthalpy of dissolution, gives rise to localized cooling of melt in the vicinity of the addition (Bratland et al., 1976). Subsequent freezing of cryolite around aggregates of undissolved alumina (Jain et al., AIChE Journal September zyxwvu 1986 Vol. 32, No. 9 1459