Secondary nucleation of gibbsite crystals from synthetic Bayer liquors: effect of alkali metal ions

The affect of alkali metal ions (Na+ versus K+) on secondary nucleation of gibbsite (gamma -Al(OH)(3)) from synthetic Bayer liquors has been investiga ted under seeded, isothermal, batch crystallisation conditions. The nucleat ion kinetics showed a fourth-order dependence upon AI(III) relative supersa turation and a strong temperature effect. An activation energy of 132 kJ mo l(-1), which was independent of alkali metal ion, was calculated. Secondary nucleation and subsequent crystal growth rates however, were greater in so dium than in potassium aluminate solution. The Arrhenius, pre-exponential f actor was at least three times larger in sodium than in potassium aluminate solutions at equivalent crystal surface area, similar supersaturation and temperature. The results indicated that secondary nucleation of Al(OH)(3) i s a chemical reaction-controlled process which is alkali metal ion-mediated . Na+ ions provide a more favourable pathway than potassium ions for the fo rmation of Al(III)-containing clusters, higher collision frequency between the species and crystal surface, and faster growth of potential secondary n uclei in the solutions. (C) 2000 Elsevier Science B.V. All rights reserved.