Improved precipitation of gibbsite from sodium aluminate solution by adding methanol

A new method is explored to improve the precipitation of gibbsite from sodium aluminate solution by adding methanol to the pregnant liquor. The stability of sodium aluminate solution decreases in the presence of methanol, leading to faster crystallisation than the normal seeded precipitation process. The effects of methanol amount, caustic concentration, reaction time and temperature, stirring and feeding mode are systematically investigated. Considering possible industrial application, the optimum conditions are 1:1 volume ratio of methanol to sodium aluminate solution, ~ 170 g/L Na2O and 1.55:1 caustic to aluminate ratio at 60 °C. The feeding mode and feeding rate have no significant effect on the final yield when moderately stirred. Under these conditions, 70% precipitation can be reached within 24 h, which is about 20% higher than the Bayer seeded precipitation practice of at least 56 h. Characterization of the crystals using XRD, SEM, and PSD shows a typical gibbsite phase composed of super-thin sheets with an average diameter of ~ 90 µm. IR and Raman spectra analyses suggest that added methanol captures water molecules from the aluminate ion hydration sphere thus catalyzing the crystallisation process.