Direct Determination of Pyrite Content in Argonne Premium Coals by the Use of Sulfur X-ray Near Edge Absorption Spectroscopy (S-XANES)

Abstract

Argonne premium coal samples are used by researchers worldwide as standards in coal research. The set consists of a suite of eight samples of varying rank from the United States. The sulfur X-ray near edge absorption spectroscopy (S-XANES) third-derivative analysis method uses a well-defined library of model compounds to curve fit each sample spectrum and enables sulfur speciation to within about 10 mol % for materials such as coals and kerogens. This direct, non-destructive characterization technique, used in conjunction with others, such as X-ray photoelectron spectroscopy, can provide valuable information about chemical and thermal sulfur transformations. The S-XANES third-derivative analysis method provides quantitative results for organic sulfur species in coal but has not been used to quantify inorganic sulfur forms to date. In general, the direct determination of the pyrite content, a metal sulfide, has been problematic. Through wet chemical methods, several of the Argonne premium coal samples are known to exceed 50 mol % pyrite but only show a weak pyrite feature in the S-XANES absorbance and third-derivative spectrum. We show that particle-size effects are responsible for attenuating the pyrite signal for high-pyrite-containing Argonne premium coals. Grinding techniques are discussed that decrease the particle size and produce spectra and results that are in better agreement with those from established wet chemical methods for pyrite determination.