A THERMODYNAMIC MODEL FOR GIBBSITE SOLUBILITY IN BAYER LIQUORS

A model which accurately predicts the equilibrium solubility of gibbsite in Bayer liquors is a very useful tool for supersaturation control in the alumina refinery, and is fundamental to kinetic expressions for gibbsite precipitation. Although several alumina solubility correlations have been published, many have been based on statistical correlations, and are of limited validity, especially at elevated temperatures. The effect of industrially important impurities, such as sulphate or organic carbon, is often ignored. Others require knowledge of the formation constants of aluminium-containing species, some of which are of questionable existence. This paper describes a thermodynamically-based model for alumina solubility that is based on the equation:

                                              Al(OH)₃ + OH^- <-> Al(OH)₄^-

The influence of impurities such as sulphate, chloride and organic carbon are accounted for by their effect upon activity coefficients. The model has been validated for liquors ranging from synthetics to plant liquors, over caustic concentrations ranging from 4 to 400 g/L (expressed as Na₂CO3) and over temperatures of 55 to 175^oC, with a typical standard deviation of better than 6 A/C ratio points.