Microstructural distribution of sulfur during the retorting and oxidation of oil shale

We report here the first direct observations of the mineralogical and chemical changes in oil shale caused by retorting and combustion. Electron microprobe and scanning electron microscopy techniques were used to examine a portion of the surface area of a sample in the raw shale state, the same area again after the sample had been retorted, and the same area yet again after the sample had been oxidized. Of particular interest was the effect on the sulfur in the oil shale. Retorting was found to cause almost no change in the distribution of the sulfur. Throughout the retorting process, the sulfur remained in its original location, in grains of pyrite (FeS2). During combustion, however, all the sulfur left the pyrite grains and spread nearly uniformly into areas containing carbonate minerals, reacting with the carbonates to form sulfates. The former pyrite grains maintained their original shape, but now their composition had changed to Fe2O3 . These same changes that were observed on the surface of the samples were also found to have occurred in the interior.