LASER DIFFRACTION MEASUREMENT OF FINES IN SMELTER-GRADE ALUMINA

The potential of the new generation laser diffraction equipment to measure the size distribution of fines in smelter-grade alumina has been evaluated. The earlier equipment did not have sufficient resolution to measure small amounts of -20 µm material in bulk samples. Extensive testwork has shown that the new generation laser equipment can readily discriminate 1% of -20 µm material in a bulk smelter-grade alumina sample. The equipment can give repeatable size information down to lum in size. The equipment tested has proved to be very reliable. Comparison of data from different manufacturers of such equipment has shown good agreement. For comparative analysis the equipment is ideal, being fast and giving repeatable data providing the correct protocol is followed. For example, a control alumina sample run for over 18 months has given a value for the -20 µm fraction of 4.8% ± 0.35% (3σ). Several difficulties arise in trying to assign absolute values to the data, especially at fine sizes. These include; the measure is volume based, not weight; the output is sensitive to particle shape and the refractive index of the particle. Consequently, size data for the fines are not identical to those from sieving. Comparison of data between different vendors' equipment would be enhanced by the use of industry wide alumina sizing standards.