Self-nucleation and kinetic behavior of nanocolloidal sodalite particles in highly caustic liquors

Abstract

Sodium aluminosilicate (SAS) crystallization from supersaturated caustic liquors is of high industrial significance with regards to intractable heat exchanger precipitation fouling issues in Bayer process alumina refining and high level nuclear waste processing. In the present work, isothermal (65 °C) homogeneous nucleation behavior of nanocolloidal SAS particles in optically clear solutions as a function of time was investigated by dynamic light scattering (DLS). At a high SiO₂ relative supersaturation (σ) of 12, the solution was perennially metastable, reflecting a long nucleation induction time of 12 h. Upon ephemeral preheating at 100 °C, and/or a 25− 40% increase in σ, the DLS analysis showed that rapid nucleation and moderate particle growth occurred in the optically clear solutions. Furthermore, evolution uni-, bi-, and trimodal particle size distributions in the range of 20−1800 nm with time was observed, accompanied by significant time-dependent distribution broadening effects. For growth mechanisms, both nanoparticle aggregation and surface integration of ionic growth units are revealed, manifesting in polydispersed, low mass density-contrast agglomerates in optically clear liquors. The SAS solid product observed after prolonged crystallization was high carbonatesodalite crystals, comprising agglomerates of nanoparticles. The pivotal roles played by SiO₂ supersaturation and temperature in the early stages of sodalite nucleation and growth are demonstrated by the results.