INFLUENCE OF WEATHERING PROCESS ON THE FLOTATION RESPONSE OF COAL

The floatability of a Spanish coal from a seam (Adolfa) which is simultaneously surface mined (ASM) and underground mined (AUM) has been studied. The mineral composition of AUM and ASM coal samples, as well as changes in both mineral and organic constituents due to chemical weathering, were characterized by X-ray diffraction, i.r. and Mossbauer spectroscopies and X-ray fluorescence spectrometry. The effect of pH and surface charge on the floatability of AUM and ASM samples was investigated. The formation of humic substances in the surface of ASM coal and the presence of Fe(III) species in its mineral matter, both a consequence of weathering, together with the high content of mineral impurities (especially aluminosilicates) provide an explanation for the flotation behaviour of this coal. Basic pHs that led to humic acids dissolution and to iron(III) hydroxide precipitate desorption from the surface improved ASM coal floatability up to values similar to those obtained with unoxidized AUM coal. The formation of humic acids-mineral impurities complexes at basic pHs renders clay particles hydrophobic. As a consequence, the improvement of ASM coal floatability at pHs higher than neutral was accompanied by a low ash rejection. Mineral impurities had a large influence on the surface charge of ASM coal.