CHARGE CONTRAST IMAGING – APPLICATIONS TO THE BAYER PROCESS

Roach, G.I.D., Cornell, J.B., Griffin, B.J. and Baroni, T.C.

The new and exciting technique of charge contrast imaging developed at the Centre for Microscopy and Microanalysis in conjunction with Alcoa of Australia Limited utilises the Environmental Scanning Electron Microscope. The technique gives valid and detailed information on the growth of a variety of materials in the Bayer process. Imaging of gibbsite particles from the precipitation process gives information on their growth history such as the number of times a particle cycles through precipitation, at what stage particles agglomerated together and relative growth rates on different crystal faces. Hydrates with varying impurity levels were grown to determine if these impurities were the cause of the observed growth rings. Similar structures were observed with and without the impurities. Improved imaging with the atomic force microscope revealed topographical features at the ring positions.

The technique has also been applied to a range of scale samples including gibbsite and DSP scales and can give useful insights into their growth. For alumina, improved imaging of cracks is obtained as well as the appearance of textural differences which are still awaiting an explanation. The technique is now finding wide application in areas such as medicine (examination of kidney stones), semi-conductors, mineralogy and ceramics.