FORMATION OF HUMIC ACIDS IN LIGNITES AND SUBBITUMINOUS COALS BY DRY AIR OXIDATION

A study of dry oxidation of coals by air has been performed. Four Spanish coals, ranging from lignite to subbituminous, were studied at three moderate temperatures for two different periods of time. The main goal with respect to possible use as fertilizers was to increase the humic acid content of the above-mentioned coals. The best results for humic acid production were obtained at the highest temperatures and after longer times. Lower-rank coals yielded the highest humic acid contentrations, but due to the severe loss of weight of these coals, the nett increase was lower than that obtained from subbituminous coals. A detailed study of the elemental composition of the coals, the generation of oxygenated functional groups and the molecular size of humic acids is reported. The evolution of sulphur forms during the process is also described. © 1990.