Rearrangement in Brown Coal Microstructure upon Drying As Measured by Ultrasmall-Angle Neutron Scattering

Small-angle scattering can supply information about the microporosity of wet materials, unlike techniques such as gas sorption and mercury porosimetry. In this study, we applied the ultrasmall-angle neutron scattering (USANS) technique to compare the microstructure of brown coals before and after drying and after rewetting the dried coal, in the size range from 25 nm to 10 mu m. We found that scattering of neutrons from brown coal is substantially greater when dried than when wet. Because we found that materials that shrink uniformly on all size scales must scatter neutrons to a lesser extent when dried than when wet, the microstructure of brown coal must be different when it is wet from when it is dry. This difference in the brown coal microstructure is on sizes of <100 nm. The change in the microstructure is probably due to clustering of the molecules making up the brown coal into locally denser regions when the coal is dried, which is consistent with the gel structure model for wet brown coal. Brown coal that had been heat-treated wet at 300 C showed no evidence of such a restructure after being dried. We also found that D2O extensively penetrates pores of brown coal, even in the brown coal that had been heat-treated, and that the nature of the pores in the wet coal varies with the pore size.