The equilibrium approach to causticisation for optimising liquor causticisity

Abstract

The causticisation reaction using lime to convert sodium carbonate to sodium hydroxide, with the formation of calcium carbonate, has a thermodynamic equilibrium that depends upon liquor composition. The causticisation reaction has been studied from both a theoretical and practical standpoint. The reaction involves an intermediate that is formed almost instantaneously with the approximate composition 3CaO.Al2O3.CaCO3.11H2O (commonly called monocarbonate or hydrocalumite). The kinetics of decomposition of the monocarbonate species to either calcium carbonate or tricalcium aluminate were studied. The equilibrium data have been used in carbonate balance models to help optimise causticisation in Alcoa World Alumina Refineries. Opportunities to increase plant causticity were identified, especially for Western Australian Refineries, where the majority of the carbonate added to the liquor circuit is in the bauxite and the causticity of the liquor is relatively low. This basic understanding indicated the potential use of the monocarbonate to achieve higher than equilibrium ratios and ways to achieve higher lime efficiencies.