Chemical properties of superfine pulverized coals. Part 2. Demineralization effects on free radical characteristics

Inorganic matter in coal significantly influences its chemical properties and potential utilizations. Electron paramagnetic resonance (EPR) characteristics can be affected by inorganic species, and various groups of paramagnetic centers behave differently after demineralization procedures. In this paper, EPR and the power saturation technique were applied to investigate thoroughly the free radical characteristics of acid-treated superfine pulverized coals. Systematic studies were conducted to explore the behaviors of different groups of paramagnetic centers in demineralized and pyrite-free coals. Focus was directed on comparisons of free radical characteristics of coal samples with different particle sizes, indicating that coals with higher maturities and larger particle sizes are much more easily saturated. Compared to raw coals, the acid-treated samples are more susceptible to saturation. Furthermore, the power saturation technique also served as a quantitative analysis method to study the line-broadening mechanism. This technique revealed that the components in coals with higher molecular weights contain more homogeneous components. Worth noticing is that the concentrations of free radicals residues in the acid-treated coals are determined by two competing mechanisms. The oxidation effects would increase the amounts of free radicals while the removal process of certain components in coals has decreasing effects. Multi-component spectra studies reflect that EPR spectra of acid-treated coals are superimposed by one Gaussian and three Lorentzian lines.