Interaction between current efficiency and energy balance in aluminum reduction cells

Abstract

In aluminum reduction cells the current efficiency interacts with the energy balance through both the thermodynamics and the hydrodynamics of the reduction process. The overall enthalpy for the reduction reaction is endothermic and increases with current efficiency due to lower heat evolution from the re-oxidation reaction. Changes in current efficiency due to changed mass transfer conditions will lead to an energy deficit or surplus. This will occur predominantly in the electrolyte channels where the largest proportion of mass transfer-driven current efficiency loss occurs. Other sources of current efficiency loss such as electronic loss of faradaic current will further reduce the endothermic contribution of the reaction enthalpy and may cause localized energy surpluses. The energy balance directly affects the electrolyte temperature and composition. It also affects the volume of the liquid electrolyte and the shape and thickness of side freeze and crust, which determine the shape of the operating cavity. Bath composition and temperature determine the physical properties of the electrolyte which, along with the geometry of the flow cavity, influence the turbulence of the bubble-driven circulation and the disturbance to the electrolyte-metal interface. Both factors influence the mass transfer and hence, metal reoxidation rate.