Investigation of the unseeded nucleation of gibbsite, Al(OH) 3, from synthetic bayer liquors

Abstract

This paper presents the novel application of multi-angle laser light scattering (MALLS) together with transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and In-lens field emission scanning electron microscopy (IFESEM) for studying the unseeded nucleation of gibbsite from synthetic Bayer liquor (sodium aluminate). The effects of supersaturation in the absence or presence of a known inhibitor (sodium gluconate) were examined. In each series of experiments, both homogeneous and heterogeneous primary nucleation mechanisms were observed. For the unpoisoned liquor used in this work the nucleation appeared to be homogeneous for supersaturation ratios greater than 2.51±0.07 and heterogeneous for lower values. From the induction period measurements, the interfacial tension at 60°C was determined to be 45±6 mJ m −2, and used to predict the critical nucleus size of 1.2±0.1 nm. This was then compared to the measured particle sizes from samples examined by TEM (10–40 nm) and IFESEM (∼50 nm). A crystallite size was determined by XRD to be 22±6 nm. The presence of sodium gluconate inhibitor extended the region over which homogeneous nucleation is dominant, but did not affect the measured value of the interfacial tension and hence the critical nucleus size. This is consistent with the poison inhibiting processes on the solid phase and not the solution phase.