There are many techniques currently available for the examination of particulate material. Several of these have been applied to hydrate and alumina at the Research and Development Department of Alcoa of Australia Limited. The techniques enable the inter- and intra-particle variations in both the morphology and the distribution of impurities to be determined. Techniques which have been used include optical microscopy, scanning and transmission electron microscopy, the electron microprobe, image analysis and energy dispersive spectrometry.
Examples of the use of these techniques, the difficulties in their application (especially of sample preparation), and the problems of data interpretation are given. Four particular areas are highlighted. Optical microscopy of hydrate immersed in a suitable refractive index liquid enables rapid assessment of the internal structure. Examination of polished sections of hydrate and alumina reveal their detailed internal microstructure. Soda analysis of hydrate using the electron microprobe presents major problems because of soda migration; the migration can be minimised by appropriate sample preparation. Examination of thin sections of hydrate and alumina in an analysing transmission electron microscope provides both crystalline, morphological and chemical information. The information provided by such techniques is vital for understanding the fundamental mechanisms involved in the precipitation and calcination processes, and for improving product quality.