Contribution of Mineral Matter to Water Associated with Bituminous Coals

The effect of mineral matter on the moisture-binding capacity of samples from two Australian bituminous coal seams was investigated by gravimetric sorption and nuclear magnetic resonance (NMR) techniques. The water uptake contributions from the mineral matter (predominantly clay) and organic components of the coal samples were determined from precise isothermal sorption measurements at relative pressures up to 0.9 using a Hiden intelligent gravimetric analyzer. Also, water interacting with the coal samples, such that it is unfrozen at −3 °C during heating, was quantified using a proton NMR technique. Water uptake (per unit mass of dry solids) at a relative pressure of 0.9 by the mineral matter in the coal samples studied was assessed to be 2.3−2.8 times the uptake by the organic material. Coal mineral matter containing 26 wt % of montmorillinite, a swelling type clay, had more than twice the water uptake per unit surface area when compared to mineral matter in which clay was present only as kaolinite. For two coal samples from the same seam differing in both mineral-matter content and maceral composition, the differences in water uptake at relative pressures up to 0.9 could be accounted for completely by the mineral-matter content. This indicates that the large difference in maceral composition between these two samples had no effect on the moisture-holding capacity. The NMR results for the amount of water in the coal samples that was unfrozen at −3 °C support the findings of the water sorption measurements.