抄録
Solvent extraction is a promising technology for removing ash from coal. The resultant coal extract (Hypercoal) usually has ash content around 0.1wt% or less. Among the eluted metals, Fe is the most prevalent, having a mass percentage ≥ 50%. Coal rank and solvent polarity affect little. This highlights the importance for studying the structures of Fe, especially organic-Fe complexes in coal extract and the raw coal as well. In this paper, a novel technique, electron spin resonance (ESR) spectroscopy was employed for direct measurement of Fe. The results indicate that, a few amount of metals in a coal are even insoluble in HNO_3 (2 M) or HF (48%), among which, Fe is the most prevalent. It has two major configurations as detected by ESR at liquid-nitrogen temperature (77K): high-spin Fe3* oxidation state in which Fe is coordinated by an octahedron of oxygen in a rhombic asymmetry, and high/low-spin Fe^<3+> porphyrin in which Fe is coordinated by tetrapyrrolic nitrogen. Due to their highly organic affinity, these species are inevitably eluted during coal extraction, which are also detected in coal extract.
