USE OF LASER DIFFRACTION TECHNIQUES FOR ALUMINA

Laser diffraction particle sizing is widely used in the Bayer process, especially for quantifying the -20mm material in product alumina. There are many issues in the analysis such as what is actually measured, the errors in sub-sampling within the equipment and the algorithms. Data based on twelve years experience at Alcoa of Australia Limited using the second generation laser sizing equipment is presented. Sub-sampling within the equipment is not necessarily as accurate or as precise as that required for the alumina industry for –20 mm measurement.

Comparisons between dry and wet sizing indicated little difference in sizing provided due attention is paid to potential attrition during dry sample presentation. Advantage was made of the dry feeder’s propensity to attrite by using it as an attrition tester. Attrition index information could be obtained in a matter of minutes and for different levels of attrition.

The previously proposed use of the light intensity information to obtain fractals, and thereby surface morphology information (AQW Darwin, 1996), was tested using hydrates with extreme morphology differences. Surface morphology differences were not distinguishable using the proposed technique. However the use of fractals to obtain shape information, especially on the fines, is possible (the fines must be measured separately). Consequently, for –20 mm analysis, the potential exists of determining a conversion factor based on the fractal derived shape factor to convert the laser sizing data to equivalent wet sieve or spherical volume size information.