Abstract:
Without the ability to model performance, one can only react to a system as opposed to manage it. The aim of this study was to investigate aluminosilicate scaling in bauxite refineries in order to improve the management of the high temperature digestion heat recovery operations. Scaling is a major operational problem in modern high productivity alumina refineries. In particular, desilication product (DSP), an aluminosilicate, precipitates on hot heat transfer surfaces in Bayer liquors supersaturated with respect to silica. The scope explored in this study ranged from the chemistry of DSP precipitation to optimised refinery operation. Scale composition in digester, vapour lines, and heaters was found to vary significantly with bauxite ore composition and operational stability. Second order kinetics were confirmed for DSP precipitation. A model derived in this study of DSP deposition on hot heat transfer surfaces agrees well with observed laboratory and refinery measurements. With the building blocks of liquor chemistry, desilication kinetics, mass and energy balances, and equipment configuration, the Digestion Heat Recovery Model was proposed. This is the first such model in the industry to predict global heat recovery in a bauxite refinery digestion area. Excellent agreement between global performance and model prediction was observed for a high temperature digestion / 900MW heat recovery operation. Successful application of this model has allowed energy / yield optimisation, identified the cost of silica in liquor, predicted optimum equipment configurations, and indicated the energy demands at high liquor productivities.
