A comparison of methods for the determination of the pyritic sulfur content of coal

Abstract The accurate determination of the concentration of inorganic sulfur in coal is of major environmental importance. With few exceptions, unless the coal is weathered, the inorganic sulfur present within the coal is iron disulfide minerals, chiefly pyrite and to a lesser extent, marcasite. The accuracy of the presently used ASTM procedure for the determination of iron disulfide sulfur can be questioned because various quantities of the mineral grains encapsulated by the coal matrix are not available to the acid-leaching reaction utilized in the procedure. The goal of this study was to test this hypothesis and to devise a procedure which would give analytical results in agreement with a matrix-independent analytical procedure, namely Mossbauer spectroscopy. The concentration of inorganic sulfur was determined for a suite of coal samples varying in sulfur abundance by the ASTM D2492 procedure on two particle sizes of coal, −60 mesh and −220 mesh. The coal samples were subjected to low-temperature ashing and the resultant ashes were leached with 0.5 N hydrochloric acid. The total weight loss from the ashing process was made up by adding sulfur-free spectrographic grade carbon and the sulfur content of the mixture was determined using a LECO 1R32 sulfur analyzer. The inorganic sulfur contents as determined by the three methods were then compared to the sulfur concentrations as calculated from the iron disulfide abundance as determined by Mossbauer spectroscopy. The data show that the sulfur analysis using ASTM D2492 and the −60 mesh coal sample is unsatisfactory. Acceptable sulfur data can, however, be determined using the ASTM method and −220 mesh coal. The best comparison of data was obtained with the sulfur content obtained by the LECO analysis of the acid-leached low-temperature ash. These results strongly suggest a significant error in inorganic sulfur analysis due to the encapsulation of iron disulfide grains within the organic matrix of the coal when a procedure is used based upon the leaching of the coal sample. The problem can be minimized by utilizing fine-grained coal (−325 mesh) and essentially eliminated by utilizing an hydrochloric acid-leached low-temperature ash.