Precipitation from supersaturated aluminate solutions. II. Role of temperature

Abstract

The effect of temperature on the precipitation of aluminum hydroxide from dilute potassium aluminate solutions (CAl(OH)4 = 4 × 10−3 M) was studied in acid titration and pH-stat experiments. The precipitation sequence is largely dictated by the supersaturation (II) and follows the Ostwald rule of stages: amorphous → pseudoboehmite → bayerite → gibbsite. The precipitation boundaries, -log aAl(OH)4−/aOH−, of the amorphous phase (−1.9) and pseudoboehmite (−1.2) evaluated by interpretations of relaxation time curves are temperature insensitive. The interpretation of relaxation curves in the supersaturation region where pure bayerite forms is based on a surface nucleation mechanism and allows estimates of the free energy of formation of the critical nucleus (20 kJ/mole), interfacial tension and two activation energies (−30 kJ/mole, +95 kJ/mole). The preferred formation of pseudoboehmite at high pOH is shown to be the result of a lower interfacial energy and the retarding effect it exerts at all temperatures on the competing growth of bayerite crystals. The growth rate of bayerite is shown to be proportional to the available surface and the square of the supersaturation and is characterized by an activation energy of about 60–80 kJ/mole.