Enhancement of magnetic separability in coal liquefaction residual solids

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Abstract
The conversion of coal to a clean fuel through liquefaction requires the physical separation of undissolved sulfur-rich mineral matter. The hydrogenation-liquefaction reaction produces residual solids in which the original coal impurity pyrite, FeS2, has largely been converted to pyrrhotite, Fe1−xS, which is a complex non-stoichiometric sulfide. Using thermomagnetic analysis and Mössbauer spectroscopy we show that the iron sulfide in coal liquefaction residues corresponds to an intermediate pyrrhotite. Striking transformations between various magnetic and non-magnetic states are induced by thermal treatments in inert or sulfidation atmospheres. The kinetics of the transformations are strongly influenced by prior thermal and atmosphere history. Transformation to a high magnetization state has practical application wherein magnetic separation techniques become attractive for the liquid-solids separation step in coal liquefaction.

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