LASER-INDUCED MACROFLUORESCENCE OF COAL - SPECTRA, RANK, MINERAL AND MACERAL DEPENDENCY

Samples from Australian coal seams were examined by laser-induced fluorescence. Large areas (12.5 mm2) of the samples were illuminated with 325, 442, 488 and 514 nm laser beams. The 325 nm laser was more efficient in exciting fluorescence than the longer wavelength lasers. Spectral shifts in the fluorescence spectra with rank were similar to those observed elsewhere by microscopic fluorescence techniques. Sample preparation was much simpler than that employed in microscopic techniques and the method is suited to the automated measurement of coal rank. The red/green colour intensity ratio of the macrofluorescence of coal is highly correlated with rank (R(v) %mean max., r2 = 0.94) but not correlated with ash content (r2 < 0.2). The residues from radio-frequency ashing of coal and kaolinite both fluoresced much brighter than the coal, as did mudstone layers in the bulk coal samples. Hand-picked samples high in liptinite yielded higher fluorescence intensities than run-of-the-mine coal. Oven drying the coal increased the fluorescence intensity.