Pristine coals, elemental sulfur

A variety of solvents including hexane, acetone, tetrahydrofuran, toluene and perchloroethylene have been used in these studies together with four analytical strategies. No sulfur has been detected. This feature is well illustrated in Figure 1, which displays the contrasting results for the pristine Wyodak sample, APCSP-2, and another exposed sample of this low sulfur coal. Elemental sulfur at the 0.002% level is easily detected in the exposed sample, none is observed in the pristine sample. 

Figure 1. Chromatographic observations for pristine and exposed samples of low sulfur Wyodak coal. Elemental sulfur emerges at 38.3 minutes under the conditions of the analysis. A. Unconcentrated extract of APCSP-2. B. The extract of APCSP-2 after 20-fold concentration. C. Unconcentrated extract of exposed Wyodak coal. D. The same extract after 20-fold concentration. (Adapted with permission from ref. 6. Copyright 1985 Fuel.)...

Elemental Sulfur. In 1942, Chatterjee (44) reported the presence of elemental sulfur in weathered Indian coal but not in fresh samples. He suggested that, during weathering, pyrite is first oxidized to ferrous and ferric sulfates, and that then ferric sulfate oxidizes pyrite to elemental sulfur. The presence of elemental sulfur in U.S. coals was confirmed recently by Richard et al. (45) and White and Lee (46). Duran et al. (47) used extraction and gas chromatographic analysis to determine elemental sulfur in a suite of U.S. coals. They found that elemental sulfur (0.03-0.17%) is present in coal that has been exposed to the atmosphere, but is absent in pristine samples that have been processed and sealed under a nitrogen atmosphere. These data support Chatteijee s discovery that elemental sulfur in coal is a weathering product. 

Many coals contain elemental sulfur. It is clearly a ubiquitous constituent of bituminous coals that have been exposed to the atmosphere. In contrast, three pristine samples obtained from the Premium Sample Program at the Argonne National Laboratory are free of elemental sulfur within the detection limit of four sensitive analytical methods. Exposure of the pristine coals to the atmosphere leads to the production of elemental sulfur. These observations strongly suggest that sulfur is not a natural constituent of coal, but rather is produced after exposure to the atmosphere by chemical or bacteriological action. 

Table V. Elemental Sulfur Content of Pristine Coals after Intentional Exposure to the Atmosphere...

The notion that naturally occurring organic polysulfides in coal decompose to form elemental sulfur has also been tested in another way. Buchanan and his associates have shown that the 32S/34S ratios of the elemental sulfur and the pyrite in another Illinois Basin Coal Sample Program coal are similar and different from the 32S/34S ratio for the organic material in the same coal (Buchanan, D., private communication, 1989). This result infers that pyrite is the source of elemental sulfur. Thus, we conclude that oxidative chemical and bacteriological processes convert pyrite to elemental sulfur when pristine coals are exposed to the atmosphere. 

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