INVESTIGATION OF HUMIC SUBSTANCES FROM NATIVE AND OXIDISED BROWN COALS BY ANALYTICAL SCANNING ELECTRON MICROSCOPY

The samples of native and naturally oxidised brown coals from the Tisul deposit of the Kansk-Achinsk coal basin, as well as the components isolated from them (humic acids (HA), fulvic acids (FA) and residual coal (RC)) were studied using the method of analytical scanning electron microscopy, proximate and ultimate analysis. It has been found that oxidised brown coal is characterised by an increased yield of HA, FA and volatile substances, ash content, oxygen, nitrogen and sulphur content, with lower concentrations of carbon and hydrogen in comparison with native coal. The HA fractions isolated from coal have a higher carbon content in comparison to FA fractions, but lower oxygen, nitrogen and sulphur content. Electron microscopic studies have shown that natural oxidation significantly affects the morphology of coal particles through their destruction, grinding, rounding and the formation of surface defects. Similar changes in morphology can be partially traced in the particles of HA, FA and RC powders isolated from the oxidized form of brown coal. The surface of oxidised brown coal is saturated with oxygen and enriched with mineral components, mainly consisting of Si, Ca, Fe and Al, in the form of a separate phase and inclusions in the organic mass of coal. For native coal, the content of these elements is significantly lower. The surface of HA, FA and RC powders contains mineral components inherited from original brown coal, as well as large NaCl crystals, which are a by-product of the alkaline extraction process. The RC surface has been determined to contain the increased content of silicon oxides and clay minerals, which are extracted from initial coals at the stages of HA and FA isolation only in insignificant amounts, thus being mainly deposited in RC. In general, the surface of coal derivatives (HA, FA and RC) obtained from oxidised raw materials is characterised by an increased content of oxygen and mineral components in comparison with the surface of native analogues.