The nature of solids accumulated during solvent refining of coal

Solids accumulated in the reactor of a solvent-refined coal (SRC-1) pilot plant during processing of three coals were studied using optical microscopy and X-ray diffraction. A classification system was devised for each of the two groups of components: organic materials and mineral matter. The various organic components were classified by the extent of change from the original coal macerals, and by optical properties of different semi-cokes and other organic phases. Minerals were divided into four groups: those unchanged from the original coal; minerals which were physically degraded; minerals which were chemically or crystallographically transformed; and those minerals formed during processing of a subbituminous coal. Gold-tube carbonization experiments were performed on SRC to determine the conditions under which retrogressive reactions occur to form mesophase semi-coke. Autoclave experiments were designed to investigate the recrystallization of pyrite as pyrrhotites, and to determine the causes of carbonate-mineral formation in the reactor. Calcium carbonate was found to crystallize from the interaction of ion-exchangeable calcium and carbon dioxide, which are available when low-rank coals are processed.